An Investigation of Near-Field Optical Scattering by Boron Nitride Nanotubes

by

Maria Marta Karcz

A thesis submitted in conformity with the requirements for the degree of Master of Science

Department of Chemistry University of Toronto

© Copyright by Maria Marta Karcz 2018

ii

An Investigation of Near-Field Optical Scattering by Boron Nitride Nanotubes

Maria Marta Karcz

Master of Science

Department of Chemistry University of Toronto

2018

Abstract A novel infrared (IR) pump-probe continuous wavelength (CW) technique based on scattering

type near-field optical microscopy developed by the Walker group has demonstrated evidence of

coupling between phonon modes and phonon polariton modes in boron nitride nanotubes

(BNNTs). The first experiment presented in this thesis aims to replicate and extend the results

obtained. This paved the way to the creation of an experiment to observe handed light scattering

signals seen in the BNNTs arising from the difference in scattering by phonons of the IR laser

field, oscillating between two different circularly polarized states. The second aim of this thesis

is to present an introduction to an investigation of chirality in BNNTs with near-field optical

scattering experiments with the ‘homemade’ scattering-type scanning near-field optical

microscope (s-SNOM).

iii

Acknowledgments First and foremost, I would like to express my gratitude to my supervisor Prof. Gilbert Walker

for his mentorship and guidance throughout my degree. I would also like to thank Dr. Leonid

Gilburd for the countless hours dedicated to advising me and helping me understand the theory

behind the s-SNOM. I am grateful to the rest of the Walker lab (Cassandra, Hannah, Daniel,

Sam, Kevin and Caroline) for their friendship, advice and discussions. Finally, I would like to

thank my mother Walentyna, my brother Janek as well as my friends who are like a second

family here and my better half, Jorge.

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Table of Contents

Acknowledgments .................................................................................................................... iii

Table of Contents ..................................................................................................................... iv

List of Figures .......................................................................................................................... vi

List of Appendices ................................................................................................................. viii

Chapter 1-Introduction ...............................................................................................................1

Chapter 2- Scattering-type scanning near-field optical microscopy (s-SNOM) ........................3

2.1-Breaking the optical diffraction barrier…………………………………………....3

2.2-Scattering-type near-field microscopy with pointed probes………………………4

2.3-The basic experimental configuration of the s-SNOM……………………………5

2.3.1-Near-field amplification and background reduction…………………….7

2.3.1.1-Homodyne detection…………………………………………...7

2.3.1.2-Heterodyne and pseudo-heterodyne detection…………………9

2.4-Theory of the s-SNOM signal…………………………………………………….10

2.4.1-Two-dimesional IR near-field imaging…………………………………11

2.4.2-Theory of the detected signal in near-field 2D IR spectroscopy………..11

Chapter 3-Boron nitride nanotubes ..........................................................................................14

3.1-Boron nitride nanotubes and phonon-polariton propagation……………………..14

3.2-Boron nitride nanotube synthesis…………………………………………………17

3.3-Structural analysis of boron nitride nanotubes……………………………………19

3.4-Theory of relevant instrumentation used for BNNT sample analysis…………….24

3.4.1-Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-

FTIR)………………………………………………………………………….24

v

3.4.2-Transmission electron microscopy (TEM)…………………………….24

3.5-Isotopically enriched "pure" BNNTs………………………………………….....33

Chapter 4-Pump-probe imaging of surface phonon coupling in boron nitride nanotubes .…..26

4.1-Introduction…………………………………………………………………….…26

4.2-Experimental………………………………………………………………………27

4.3-Results and discussion…………………………………………………………….29

Chapter 5-Assymetric scattering of left and right handed circularly polarized light by boron nitride nanotubes .................................................................................................................33

5.1-Theory………………………………………………………………………..…..33

5.2-Experimental…………………………………………………………………......35

5.3-Results and discussion………………………………………………………..….37

Chapter 6-Conclusions and future work ..................................................................................43

References ................................................................................................................................45

Appendix ..................................................................................................................................56

vi

List of Figures Figure 2.1 A particle illuminated by a light field exhibits both bound and scattered waves 4

Figure 2.2 A simple diagram of the s-SNOM 6

Figure 2.3 Illustration of the image dipole model for the sample response 10

Figure 3.1 A high resolution transmission electron microscopy image showing around 20 walls

on each side of a MWBNNT 16

Figure 3.2 A simple diagram of two MWBNNTs separated by a mirror plane 16

Figure 3.3 The experimental set-up described for the CVD process 18

Figure 3.4 TEM image of a MWBNNT 20

Figure 3.5 TEM image of a bamboo BNNT 21

Figure 3.6 TEM image of a mixed type BNNT 22

Figure 3.7 TEM images of black dots in an ordered arrangement 23

Figure 4.1 A diagram of the summarized ‘homemade’ s-SNOM setup 28

Figure 4.2 Nodal patterns indicating phonon-polariton propagation 29

Figure 4.3 Images of two overlapping BNNTs (pump-probe) 30

Figure 4.4 Images of four overlapping BNNT of various diameters (pump-probe) 30

Figure 5.1 A visual depiction of the zone-folding method in chiral BNNTs 35

Figure 5.2 Diagram of a photoelastic modulator cycle 36

Figure 5.3 Diagram of a PEM modulating a beam of light at λ/4 36

Figure 5.4 Table illustrating the response of the gold substrate to 1532 cm-1 pump frequency 38

Figure 5.5 Table illustrating the response of the BNNT to 1532 cm-1 pump frequency 39

vii

Figure 5.6 Images of 5 overlapping BNNT (PEM configuration) 40

Figure 5.7 Images of 5 overlapping BNNT (Linear polarizer) 41

viii

List of Appendices Appendix 1-Manual for the alignment and troubleshooting of the ‘homemade’ s-SNOM

1

Chapter 1-Introduction Boron nitride nanotubes (BNNTs) caught the attention of the nanoscience world after their initial

synthesis in 1995, when their excellent intrinsic properties came to light1. While carbon

nanotubes (CNTs), which share structural and certain intrinsic properties, have been heavily

researched, boron nitride nanotubes have received significantly less attention. This has mostly

been attributed to issues with the isotopic purity of the boron in the tubes, lack of efficient

synthesis and quality product1,2. Chirality has been studied extensively both theoretically and

experimentally in CNTs however, to the best of my knowledge, rarely in BNNTs. In the world

of nanophotonics, there is interest in using chiral nanomaterials for technological applications

such as super resolution imaging, nanorobotics and ultrathin broadband optical components3.

The aim of this thesis is to present an introduction to an investigation of chirality in BNNTs

using nanoscale circular dichroism experiments with a ‘lab-made’ scattering-type scanning near-

field optical microscope (s-SNOM).

In Chapter 2, I provide an introduction of the scattering-type scanning near-field optical

microscope (s-SNOM) used to obtain the results presented in this thesis and explain its literal

barrier-breaking significance to modern day microscopy. Some relevant s-SNOM signal theory

required to understand the experimental setup and experimental results is presented.

In Chapter 3, I describe the relevance of BNNTs to the world of nanophotonics and describe

their fabrication process using the chemical vapour deposition (CVD) technique. After

fabrication with chemical vapour deposition, boron nitride nanotubes can be notoriously difficult

to purify from surrounding debris such as the iron catalyst and boron reactant1. When imaged

with transmission electron microscopy (TEM), there was evidence that filtration with

hydrochloric acid removes some of the debris. TEM images of BNNTs I synthesized, both with

isotopically pure boron powder and without, are presented.

In Chapter 4, I introduce the pump-probe experiment. I elaborate on the specifics of the pump-

probe spectroscopy setup using the ‘lab-made’ s-SNOM which I used to obtain the results

presented in this thesis. I present data I replicated from previously done experiments by the

2

Walker lab as well as data obtained at wavenumbers that had not be thoroughly investigated and

published.

In Chapter 5, I introduce my handed light scattering signal investigation of BNNTs using the

same experimental configurations of the ‘homemade’ s-SNOM described in Chapter 4 however,

blocking out the probe laser and letting through signal from the pump laser instead of the other

way around. To the best of my knowledge, no such experimental studies on this topic have been

published to date and the findings are assumptions which still require additional experiments to

fully verify.

In Chapter 6, I conclude with a summary of my findings and how they may build a platform for

future investigations of the interaction of BNNTs with circularly polarized light.

3

Chapter 2- Scattering-type scanning near-field optical microscopy (s-SNOM)

2.1-Breaking the Optical Diffraction Barrier

“Even though the classical resolution limits are imposed by physical law…There are loopholes

in the law.” 4

-Mats Gustafsson (1999)

From the beginning of microscopy until 20 years ago, the optical diffraction limit has been

keeping many sub-microscopic curiosities obscured from view5. Where light classically

propagates (or the optical far-field), the spatial resolution can be defined by the Rayleigh or

Abbé criterion shown in equation 2.1 (W),

W=0.61λ/NA [2.1]

where λ is the wavelength of the emitted photons and NA is the numerical aperture of the optical

system6,7. Two point sources are considered “just resolved” when the separation between them is

“W” and going below this value is termed “super-resolution”6. This brings about its own

limitations of low photon flux and strict maintenance of surface-to-aperture distance which is on

a nanometer scale6.

The far-field spatial resolution is generally limited to half-a-wavelength, which in the mid to

long wave infrared can be 5 micrometers6,8. This limitation is resolved with the scanning near-

field optical microscope (s-SNOM) as it probes at less than half a wavelength (the optical near-

field) as opposed to beam focusing6. In near-field optical microscopy, the tip diameter d of the

probe in contact with the sample replaces λ in the Rayleigh criterion7. Now the spatial resolution

is dependent on the size of the photon flux between the probe and the sample surfaces, allowing

for the Rayleigh criterion to be stretched. The idea of using a subwavelength emitter goes back

nearly 100 years.

4

“If one could construct a little cone or pyramid of quartz glass having its point P brought to a

sharpness of order 10-6 cm. One could then coat [it]…with some suitable metal and then remove

the metal from the point until P was just exposed. I do not think such a thing would be beyond the

capabilities of a clever experimentalist.”

-A letter from Synge to Einstein (1928), which is believed to be the first time the idea of using a

sub-wavelength aperture for achieving subwavelength light localization7,9.

2.2-Scattering-type near-field microscopy with pointed probes The use of a subwavelength light scattering particle is another approach, superior in resolution to

the aperture probe mentioned above7. If one considers a particle illuminated by an external

source of light, evanescent fields and propagating fields are generated. Indeed, any illuminated

surface exhibits similar evanescent fields. Therefore, a sharp metal tip can also acts as an antenna

to convert or scatter those evanescent waves at the surface, which contain spatial frequencies that

are smaller than the incident wavelength, into far field7. The evanescent (non-propagating and

exponentially decaying) and propagating fields around an illuminated nanoparticle are illustrated

in the Figure 2.1 below.

Figure 2.1- A particle illuminated by a light field exhibits both bound and scattered waves. These near field (bound) waves are evanescent. In analogy, placing a particle at an illuminated object leads to far field light scattering that reports on the local optical properties of the material. When the particle is smaller than the wavelength of the light, it can convert bound waves at a surface into propagating waves10. This illustrates the basis of scattering type near field optical microscopy (image provided courtesy of Prof. Gilbert Walker).

5

Therefore, the introduction of the pointed probe improves resolution in near field optical

microscopy, which can be used in two different ways: local-scattering and local-excitation7. The

local scattering approach is the type used for the ‘homemade’ s-SNOM described in this thesis,

hence the “scattering-type” part of the name. It is dependent on the pointed probe (such as a tip)

to scatter the evanescent fields near a sample surface. The response is then detected in the far-

field at the wavelength of the incident light (most commonly a laser)7. The tip perturbing the

fields is performs a role similar to a lightening rod, where the metal draws electric fields towards

it7,11. Tips made of metal have been found to give the strongest scattering efficiency and field

enhancement, though tips made of semiconducting and dielectric materials have also been

used12. The strong polarizability of the tip is due to the collective response of free electrons in the

metal which amplify the strength of the electric field generated by the incoming laser

radiation13,14. The resonant coupling of these perturbed metallic electrons and electric field can

be a surface plasmon, which is classified as a polariton. Polaritons are surface-charge oscillations

coupled to electromagnetic fields13,14. The strength of the perturbation is dependent upon the

tip’s electromagnetic properties (which is the most consistent factor), sample’s local dielectric

constant and the probe-sample separation15. The near-field scattering from the probe tip in the

presence of a dielectric sample can be further described with an image dipole model presented

later in Fig.2.3, where the tip apex is a polarizable dipole affected by its material dielectric

function and radius16. Equation 2.2 demonstrates the factors which affect the polarizability of the

dipole17,18:

𝛼 = 4𝜖𝜋𝑟' ()*+(),-

[Eq. 2.2]

where 𝜖. is the vacuum permittivity, r is the radius of the tip, 𝜖/ is the relative dielectric function

of the tip material.

2.3-The basic experimental configuration of the s-SNOM

The s-SNOM, also known as apertureless near-field scanning optical microscopy and pictured in

Figure 2.2, is the most widely used method for surface images at spatial resolutions below 20

6

nanometers19. An atomic force microscope (AFM) is used to control the metal tip used for

scattering in the near-field (the area where the tip approaches and has contact with the sample).

In an AFM, a sharp tip is attached to a cantilever which bends upon interaction with the

sample20. The mode in which the AFM images the sample can be in one of three modes (tapping,

contact or vibrating) however, for the s-SNOM only the tapping mode is relevant. In tapping

mode, the tip only intermittently touches the sample surface21. When not touching the surface,

the tip oscillates freely however upon contact it is affected by long and short range attractive and

repulsive van der Waals forces. This changes the amplitude of the tip’s oscillation which is

measured and then used for the generation of an image22.

Figure 2.2: A simple diagram of the s-SNOM.

The tip in AFM tapping-mode taps at a consistent frequency (Ω) over the sample, simultaneously

recording the topographical information and optical response of the sample13,23. The tip-sample

separation, which is the tapping amplitude, is set to a value that is within the decay length of the

spatial extent of the near field. As mentioned previously, the primary measurable in s-SNOM

experiments is the field perturbed at the apex of the probe at the same wavelength of the incident

photons from the laser which leads to the need of distinguishing between this response and the

background far-field light. Because the scattered light is weak, it is mixed with a reference

optical field to enhance the signal at the detector. This arrangement is called homodyne detection

7

which was used for the s-SNOM experiments presented in this thesis and is one of the detection

methods that can be used for s-SNOM which will be elaborated on later. A lock-in amplifier set

to detect at the tapping-mode frequency is typically used to distinguish between the scattering

from the near field and background scattering from other sources such as parts of the tip, sample

and stray light7,23. Non-near-field scattering responses can be reduced from the detected signal by

operating the lock-in at harmonics of the oscillation at the cantilever, as is described next.7,24.

2.3.1-Near-field amplification and background reduction

One of the biggest problems faced by s-SNOM imaging is a large elastic scattering background

signal which originates from the sample and other parts of the AFM tip25. To extract the

scattering from the pure near-field interaction between the tip and the sample, this background

needs to be suppressed. Two methods have been found to remedy this problem, high harmonic

demodulation (HHD) and pseudo heterodyne detection and are discussed in more detail below25–

27. Using only HHD is generally not enough as the detector measuring the signal will measure

light intensity which is quadratic in the electric field strength so background signal will still

affect the output27. Pseudo-heterodyne detection was originally planned to be implemented into

the system however due to logistical reasons, the current optical interferometric homodyne

amplification method was kept and used for the experiments described in this thesis.

2.3.1.1-Homodyne detection

In homodyned detection, the phase of the reference beam in a Michaelson interferometer

alternates between phases of (ψ) 0 and 90 degrees by virtue of a translating mechanical mirror

that moves between two positions so that there is continuous alternation between recordings15.

There is no frequency offset between the reference beam and the near-field scattered light as they

superimpose coherently with each other. While this detection method allows for faster near-field

signal measurements than the pseudo-heterodyne method, it requires a fixed phase relation

between the reference beam and the near-field scattered light signal which can be managed either

manually or through an automated feedback mechanism25,28. In the setup used for the

experiments obtained in this thesis, the IR radiation from the laser was divided by beam splitter,

8

then half of radiation was reflected to the AFM and the other half to the reference arm. The

AFM-directed light was focused at the probe tip by a parabolic mirror. The scattered light, 𝐸123 ,

was collected by the same mirror then returned to the beam splitter. The reference arm-directed

light is collected and retro-reflected by a piezo stage mirror as 𝐸456 and combined with 𝐸123.

Equation 2.3 demonstrates the light intensity at the IR detector,

𝐼~𝐸- = 𝐸123 2𝑓 + 𝐸456- = 𝐸123 2𝑓 - + 2𝐸123 2𝑓 ∙ 𝐸456 +𝐸-123 [Eq.2.3]

where 𝐸123 is the signal from the sample and the 𝐸456 is the excitation field which is reflected

from the reference arm of the Michaelson interferometer. It is important to note that the scattered

field is weak in comparison to field in the reference arm. Unlike 𝐸123 which is modulated at the

tip tapping frequency, 𝐸456 is not modulated and does not contribute to the demodulated high

harmonics in the detected signal. By this approach, the background field contribution is reduced

a thousand-fold28.

The equation 2.4 relevant to the electronic signal passing through the lock-in amplifier is now:

𝐼 ≈ 𝐸123𝐸∗456 + 𝐸∗123𝐸456 = 2𝑅𝑒(𝐸123𝐸∗456) [Eq.2.4]

where the signal is assumed to be in the form of 𝐸123 = 𝛼𝐸.. If it is assumed that 𝛼𝜖ℂ carries the

information on the response of the sample in contact with the metallic tip and that the reference

field with controlled phase ϕ is 𝐸456~𝐸.𝑒*2E , the following equation 2.5 can be obtained:

F

GHI≈ 2𝑅𝑒(𝛼𝑒*2E) [Eq.2.5]

As mentioned previously, constant phase values need to be obtained ϕ=(0, J-) so the reference

arm piezo mirror positions are set to get constructive or destructive wave interference conditions.

This allows for sensitivity to the local reflectivity and absorption of the material of interest. For

example, ϕ=0 is obtained by determining the position of the moving arm in the interferometer

where the maximum scattered field is obtained over a highly reflective and non-absorbing

surface, e.g., a gold substrate. Once the mirror locations are identified and set to obtain data at

9

only the two phase values, the following equation 2.6 describes the material properties that are

measured by the optical system with the lock-in detection scheme:

F(E)GHI

~2𝑅𝑒 𝛼 , 𝜑 = 02𝐼𝑚 𝛼 , 𝜑 = J

- [Eq. 2.6]

The ϕ= J- phase can be found by shifting the position of the reference piezo mirror until the

non-fundamental demodulation signal output (from a lock-in amplifier) is minimal. The mirror

position must be adjusted before every experiment as this position changes. In the presented

experiments in Chapters 4 and 5, this minimum signal output phase reference was the gold

substrate under the sample which is non-resonant. Tuning the reference phase to ϕ=J- allows for

the amplification and extraction of the imaginary part of the near-field scattering signal (𝜑 =J-→ 𝐼𝑚(𝛼)). At ϕ=0, the sample is highly scattering over a reflective surface but dimmer over

an absorbing surface due to the real part being extracted instead (𝜑 = 0 → 𝑅𝑒(𝛼)). Since the

technique is sensitive to phase, the Michaelson interferometer must be kept stable. Thermal

drifts, high humidity and turbulent air may cause phase instabilities and must be controlled while

experiments are running.

2.3.1.2-Heterodyne and Pseudo-heterodyne detection

The heterodyned detection technique is based on obtaining independent measurements of the

near-field optical signal amplitude and phase simultaneously using a Michaelson

interferometer24,29. Unlike in homodyne detection the reference arm is offset from the near-field

scattered light by sinusoidal phase modulation. First introduced in 2006 by Hillenbrand et al., the

pseudo-heterodyne detection technique can eliminate background interference in the entire near-

UV to far-IR spectral range25. Heterodyne detection is a non-interferometric technique capable of

obtaining independent measurements of the near-field optical signal amplitude and phase

simultaneously using the lock-in amplifier. Recovering the pure near field signal is obtained

through the interference between the scattered signal and the amplitude modulated reference

wave.

10

2.4-Theory of the s-SNOM signal

The illuminated metallic tip behaves like an oscillating electric dipole, where the platinum

dielectric function, εt, and radius (between 10-20 nm), r, determines its polarizability17. Figure

2.3 illustrates the model for coupling to the responding image dipole in the sample, summarizing

how the near-field light at sample scatters from the tip in the presence of dielectric BNNTs. In

the presence of an incident, time-dependent electric field at the tip, E(ω), the induced

polarization of the tip is expressed as P(ω)=αE(ω). The tip electric field at the sample surface is

P(ω)/2π (d+r)3,when the tip apex with tip radius r is at distance d from the surface, and it induces

a dipole in the sample. In other words, a polarization in the dielectric sample is generated by the

electric field of the tip; that polarization is from the distribution of induced charge of an image

dipole in the sample at depth d. The sample’s dielectric function 𝜀 𝜔 which is more specifically

described in Chapter 3 with equation 3.1, is related to the sample’s near-field scattering

susceptibility χ 𝜔 with 𝜀 𝜔 =1+χ 𝜔 . When vibrational resonances are present, they

contribute to χ 𝜔 and therefore affect the β 𝜔 term, β 𝜔 = χ 𝜔 / (χ 𝜔 +2).

Figure 2.3-Illustration of the image dipole model for the sample response

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Only the linear term χ(1) 𝜔 is considered in linear near-field scattering, however it gets more

complicated (and allows for more material properties to be obtained) once additional external

light fields are added. The dipole field at the tip apex located 2(r+d) is coupled to the image

dipole, polarization of system of coupled dipoles as shown in equation 2.7:

P(ω)= α (1 − TU V+WJ 4,X Y)*+E 𝜔 [Eq.2.7]

where P(ω) is essential in defining the linear near-field scattering of a metallic probe over a

vibrationally resonant sample, and E 𝜔 is the external incident field.

2.4.1-Two-dimensional IR near-field imaging

Nonlinear interactions with additional IR incident fields and near-field scattering are essential for

retrieving vibrational mode coupling information. The IR incident fields excite the sample,

generating a coherence between two excited states. How the probe field interacts with the sample

depends on the population and polarization at excited vibrational fields where short-lived

excitation pathways occur.

The linear dielectric function (𝜀 𝜔 ) mentioned previously, or the linear susceptibility 𝜒 + is

affected by the transition frequencies of the vibrational resonances being able to generate a

polarization.

Further prolonged excitation from the probe field can allow further excitation from the singly

excited states to the doubly excited states. In non-linear spectroscopy the excitations, which are

added mathematically to the dielectric function (𝜀 𝜔 ) mentioned previously, can be

represented by 𝜒 [ with n ≥ 2 providing the non-linear susceptibility.

2.4.2-Theory of the detected signal in near-field two-dimensional IR

spectroscopy

A second IR incident field is required to view the third order susceptibility which describes

vibrational mode coupling. In the presence of only the probe field in linear near-field scattering,

12

the polarization of the sample is P(ω) =𝜒 + 𝜔:𝜔' 𝐸' 𝜔' . In near-field 2D scattering, this first

order term as well as the third order term are combined. The overall susceptibility at frequency 𝜔

can therefore be shown as equation 2.8:

𝜒 𝜔 =𝜒 + 𝜔:𝜔') + 𝜒 ' 𝜔:𝜔+, − 𝜔-,𝜔' : 𝐸+ 𝜔+ 𝐸∗- 𝜔- + 𝜒 ' 𝜔:−𝜔+,𝜔-,𝜔' : 𝐸∗- 𝜔+ 𝐸- 𝜔-

[Eq.2.8]

if definitions 𝜒 𝜔 =P(ω)/E(𝜔) and P(ω)=P1(𝜔)+ P(3)(𝜔) are used.

In Figure 2.3, the pump field is 𝐸+ and 𝐸-, while the probe field is represented by 𝐸' . The

scattered near field carrying the response (emitted signal) from the sample is 𝐸^_.

The effective polarization of the near-field 2D scattering signal with the response up to the third

order can be described with equation 2.9:

P 𝜔 = +`− +

a 4,X1 − -

-,b c ,b Y :GcGH*+

𝐸' 𝜔 [Eq.2.9]

Where u(r,d)= 16(r+d)3, 𝜒 is the type of order response and𝑎is the polarizability of the tip

dipole. The near field signal shown in equation 2.9 is linearly proportional to this aforementioned

polarizability. It is important to note that the linear contribution to 𝜒 is typically much larger than

the nonlinear contribution, however due to the presence of the pump field the third order

response, 𝜒 ' , can be treated as a perturbation of the first order response7,28. The difference in

signal between the two responses can then be approximated with the following equation 2.10:

∆𝑆 𝜔 ≈ 2𝑏 𝑢(𝑟, 𝑑) +`− +

a 4,X1 − -

a 4,X -,b c

-(2 + 𝜒 + )-

*+

𝜒 ' : 𝐸+𝐸- [Eq.2.10]

where 𝑏 is the scaling factor which represents the detection coefficient and the overall signal

collection. ∆𝑆 𝜔 is the difference in signal with and without the pump field which is obtained

with a doubly demodulated lock-in amplifier that will be more fully discussed in Chapter 4. This

13

difference in signal is also linearly proportional to 𝜒 ' which carries the mode coupling

information and can be considered the bridge between the s-SNOM and pump-probe

spectroscopy.

14

Chapter 3-Boron Nitride Nanotubes

3.1-Boron nitride nanotubes and surface phonon-polariton propagation

Boron nitride nanotubes (BNNTs) are tubular nanostructures which are composed of a hexagonal

network of boron and nitrogen atoms, or two dimensional (2D) hexagonal boron nitride (h-BN)

sheets. Since BNNTs were first synthesized in 1995, they have demonstrated their potential as

thermal conductors, electrical insulators, neutron shields, oxidation resistors as well as properties

which include excellent mechanical strength and thermal stability30. BNNTs are most commonly

found in multi-walled form as opposed to single-walled, due to the preference for multilayered

structures by the partially ionic B-N bonds. Multi-walled BNNTs (MWBNNTs) are therefore

composed of multiple layers of coaxial cylindrical tubes which are separated by about 0.3-0.4 nm

from each other and can have a total diameter of 20-100 nm31. Their lengths vary by method of

synthesis however; they typically average around 1µm. They have been studied alongside carbon

nanotubes (CNTs) which are simpler to synthesize but can have either metallic or

semiconducting properties, depending on the band gap width, which in turn depends on the tube

diameter and chirality. BNNTs, no matter their chirality or diameter, will always have a constant

band gap of around 5.5eV32.

As h-BN is a polar dielectric material, it has been investigated as a possible nanophotonic

component which in turn allowed BNNTs to gain attention as a “nanowire” to confine and

transport IR energy in the form of surface phonon polaritons (SPhPs)18. Surface phonon

polaritons arise after electromagnetic waves and optical phonons couple. The geometry of

BNNTs can also be controlled to grow to a desired shape or size during synthesis and even bent

BNNTs have been found to propagate SPhPs33. While noble metals have been found to support

surface plasmon polaritons in the visible light range, they typically cannot be supported into the

mid-IR range34. They are also typically not chemically inert or mechanically strong at small

diameters (below 2 nm). As a 2D van der Waals crystal, h-BN is a bi-refractive material, where

its in-plane dielectric constant is different from its out of plane constant. The strong in-plane

covalent bonding is between the nitrogen and boron atoms, while the out of plane bonding is

weak van der Waals interactions between layers35. This allows the material to have two strong

15

resonances in the mid-IR range which allow for negative values of the real part of the dielectric

constant which are called Reststrahlen bands (German: “residual rays”)35–37. BNNTs and h-BN

have an upper and lower Reststrahlen band. The upper Reststrahlen band will be the focus in this

thesis which ranges between 1365-1610 cm-1. The upper of two of these energy bands has

experimentally shown almost 100% reflection from the material due to the change in refractive

index and therefore allows h-BN and BNNTs to exhibit a metal-like behavior17,35. As a result,

they can support mid-IR SPhP due to their strong phonon response in the upper Reststrahlen

band38,39. The lower Reststrahlen band ranges from 746-819 cm-1 however it has different

properties than the upper one, as it corresponds to the out of plane component (𝜀j ≡ 𝜀∥). In the

upper Reststrahlen band, the dielectric function of BNNTs (𝜀m = 𝜀n ≡ 𝜀o) is negative, which

corresponds to the isotropic in-plane component. It provides a good estimation of expected

polaritonic behaviour and can be expressed by equation 3.1:

𝜀o 𝜔 = 𝜀p +1H

VHqr*VH*2Vs [Eq. 3.1]

where the mode frequencies 𝜔tu and 𝛾 can be found using FTIR measurements18,38,40. The

transverse optical (TO) mode is where the real portion of the dielectric function crosses the zero

line (𝜀 = 0) and is considered the lower limit of the upper Reststrahlen band, while the upper

limit is called the longitudinal optical (LO) mode36. As mentioned previously in Chapter 2, this

dielectric function is relevant when considering the extent of the near-field scattering when the

metallic AFM probe is brought towards the tubes. Investigations into SPhP propagation in

BNNTs demonstrated sub-wavelength confined fields in the traverse plane and longitudinal

polariton wavelengths up to 70 times smaller than the free-space wavelength37,38.

Only BNNTs of the multiwalled variety (MWBNNTs) were used for the experiments as SPhP

propagation in bamboo BNNTs had not yet been investigated. Multiwalled BNNTs

(MWBNNTs) are composed of multiple layers of coaxial cylindrical tubes as seen in the

transmission electron microscope (TEM) image from Golberg and co-authors in Figure 3.141.

Bamboo nanotubes, which are seen in Figure 3.5, are composed of short boron nitride tubular

segments with varying interfaces at the bamboo-like joints42.

16

Figure 3.1- A high resolution transmission electron microscopy image showing around 20 walls on each side of a MWBNNT with a hollow channel running between them (Reproduced with permission from Yamaguchi, M. et al. Utilization of multiwalled boron nitride nanotubes for the reinforcement of lightweight aluminum ribbons. Nanoscale Res. Lett. 8, 3 (2013) under the terms of the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited).

The orientation of these tubes in respect to each other influences their chirality as indicated in

Figure 3.243. This differential orientation depends on how the tubes are folded during synthesis

and can only be controlled to an extent using temperature and varying catalyst types44.

Figure 3.2-A simple diagram of two MWBNNTs separated by a mirror plane, one which absorbs left-handed circularly polarized light and the other which absorbs right-handed circularly polarized light which is influenced by the position of the tube layers in respect to each other. For

17

the sake of simplicity, two layers (represented by blue and red) are shown when there are many more in an actual BNNT. 3.2-Boron Nitride Nanotube Synthesis

The BNNTs were synthesized using the chemical vapour deposition (CVD) method for the

experiments presented in this thesis. It is considered one of the most popular methods to

synthesize boron nitride materials due to ease of the experimental setup and control of growth

parameters using catalysts and temperature45. The method involves pumping a gaseous precursor

into a sealed, heated chamber where intermediate species, and later the sample, are formed46. A

variety of metal catalysts such as nickelocene or cobalt and temperature ranges (1000-1500° C)

have been reported for the CVD but the combination described below has been found to be the

most effective1,46,47.

Two different batches of BNNTs were made to compare their structures and to use in the s-

SNOM experiments. One batch was made with isotopically enriched 99.65% 11B powder (Trace

Sciences International, Richmond Hill Ontario) and the other was made with the natural isotope

ratio boron powder (80.1% 11B and 19.9% 10B), however the other reactants and the synthesis

process were the same.

The synthetic procedure is as follows: Four parts boron powder (pure or impure), one part Fe2O3

and one part MgO are combined and ground together with a mortar and pestle. About 10 mg of

the mixed powder is deposited into an alumina combustion boat. The combustion boat is

partially covered with Si/SiO2 substrate. The combustion boat then goes into the end of a quartz

tube, which is placed in the tube furnace (ThermoScientific) so that the boat is exactly in the

middle with the open end facing away from the ammonia gas stream. The gas vacuum is turned

on and then the ammonia gas valve is opened so it streams into the furnace at a flow rate of 200

sccm. The furnace is turned on and the ramp up is set to the final temperature of 1150ºC at a rate

of 10-15ºC which takes about 2 hours. The furnace stays at 1150ºC for two hours and then ramps

down back to room temperature in a span of two hours. The boat remains in the furnace until it

has cooled down to room temperature. The experimental setup described is shown in Figure 3.3.

18

Figure 3.3-The experimental set-up described for the CVD process for the synthesis of BNNTs. The combustion boat (white) is in quartz tube which is placed in the middle of the tube furnace. The ammonia gas is pumped into the setup opposite to the opening of the quartz tube and the vacuum outlet.

After the sample comes out of the furnace, it is a white and gray powder. This powder is then

analyzed using Fourier Transform Infrared Spectroscopy with attenuated total internal reflection

sampling (FTIR-ATR) for composition analysis. Both types of the BNNTs display peaks at ~800

cm-1 and 1500 cm-1 which correspond to A2u, E1u TO modes. There is a sharp peak for the

isotopically enriched boron powder BNNTs (“pure”) typically at 1351 cm-1 and a peak for the

natural isotope ratio BNNTs (“impure”) at around 1367 cm-1 which corresponds to the E1u LO

mode45,48.

The isotopic purity of pure boron isotope BNNTs can be analyzed using X-ray photoelectron

spectroscopy (XPS), however this was not done for this thesis. X-ray photoelectron spectroscopy

(also called electron spectroscopy for chemical analysis or ESCA) is one surface chemical

characterization technique which depends on lower orbital electrons leaving and retrieving a

specific kinetic energy. It is therefore a relatively non-destructive technique as its method of

analysis depends on the ejection of the electrons which leaves the nuclei of the atoms

unchanged49.

Before transmission electron microscope (TEM) analysis on some of the tubes was performed,

the tubes were cleaned with 1.0 M HCl using vacuum filtration50. This aided in removing some

of the iron catalyst debris but it also resulted in the loss of many tubes. The tubes were then

suspended in filtered ethanol, sonicated for 2 hours and deposited on either holey carbon

formvar-coated TEM grids (Ted Pella, Redding, CA) or onto a Si/SiO2 substrate with a 100 nm

thick gold coating and left to dry by air.

19

3.3-Structural Analysis of Boron Nitride Nanotubes

Structural analysis was done with TEM to ensure the tubes are of the multiwalled variety as these

are the only types that have been studied to show a detectable propagation of surface phonon-

polaritons. It took multiple attempts to create a sample of MWBNNTs pictured in Figure 3.4 that

didn’t contain a large amount of bamboo BNNTs, shown in Figure 3.5 and a sample that was

clean without a distracting amount of clumps of debris such as iron catalyst, elemental boron or

intermediate products formed during synthesis. Tubes with too much surrounding debris can be

detrimental to s-SNOM experiments due to the potential sticking of the debris to the AFM tip

which can either destroy it or cause it to track debris through the sample as it is scanning. As is

seen when comparing all images, the BNNTs vary in size and length which is hypothesized to be

attributed to the diameter of the metal catalysts45. Typically the BNNT begins growing from a

catalyst particle however as it changes due to temperature, the type of BNNT (bamboo versus

multiwalled) changes as well which is why mixed BNNT types can be seen in Fig.3.645. Black

dots on the tube in a disordered arrangement, as seen in Figure 3.5, typically demonstrated some

sort of structural defect51. Black dots with a more ordered arrangement with equal separation

between them as seen in Figure 3.7, are caused by the orientation of the tube’s layers parallel to

the incident electron beam52. It is the presence of these dark features that shows the tubes are

multi-walled.

20

Figure 3.4- TEM image of a MWBNNT which can be identified due to the long white channel in the middle of the tube. This channel is not fully seen across the length of the whole tube due to either formation issues or the angle of the TEM imaging. The debris surrounding the tubes may be iron catalyst or elemental boron.

21

Figure 3.5-TEM image of a bamboo BNNT characterized by the interrupted white, bamboo-like channel running through the middle of the tube. A disordered set of defects (black dots) can be seen.

22

Figure 3.6-TEM image of a mixed type BNNT, where the top of the tube is of the bamboo variety and the bottom is of the multiwalled variety.

23

Figure 3.7-TEM images of black dots in an ordered arrangement with equal separation between them (marked by red arrows) are caused by the orientation of the tube’s layers parallel to the incident electron beam which shows the tubes are multi-walled.

24

3.4.-Theory of relevant instrumentation used for BNNT sample analysis

3.4.1- Attenuated total reflectance Fourier transform infrared spectroscopy

(ATR-FTIR)

Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) is technique

that allows for rapid analysis and is non-destructive. FTIR spectrum analysis involves

determining the origins of absorption peaks that show up at different wavelengths and assigning

the modes to determine the chemical identity and structure of the molecule or crystal. For a

molecule to absorb IR radiation, there must be a net dipole moment change in the transition. The

change of the amplitude of the molecular motion can only occur with the radiation’s alternating

electric field if this condition is met53.

Attenuated total internal reflection (ATR) is used when dealing with difficult samples such as

solids with limited solubilities and powders, as is the case with the newly synthesized BNNTs.

The sample is placed on an IR transmitting prism that is illuminated at an angle that admits the

light at one interface but leads to total internal reflection at the interface where the sample is

placed. Absorbance is dependent on the angle of incidence, and on the interaction with the

evanescent field with the first few micrometers of the sample54. As a result of this short effective

penetration depth, this technique can be used on very thin samples. The ATR technique method

of measurement depends on the changes that occur in a totally internally reflected infrared beam

when it encounters the sample54.

3.4.2-Transmission Electron Microscopy (TEM)

In the TEM, a thin beam of electrons is fired from an electron gun and focused by

electromagnetic lenses to hit the ultrathin sample. Since the TEM uses electrons rather than light

to make images, the spatial resolution of images attained by the TEM is many orders of

magnitude better than that of a light microscope as electrons have a smaller wavelength than

light. The bright-field mode, which works much like a light microscope, was used to take high-

resolution images of the samples55.

25

3.5-Isotopically enriched “pure” BNNTs

Initially the pump-probe experiments described in Chapter 4 were meant to be done with both

the isotopically pure and impure BNNTs however, due to time constraints and the circular

dichroism investigations described in Chapter 5, only impure BNNTs were investigated in this

thesis. The motivation behind using pure BNNTs is for the improvement of phonon-polariton

lifetime during propagation and a form of loss control36. Optical losses are a common concern in

the development of nanophotonics, however PhP-based materials have been shown to have long

lifetimes for sub-diffraction modes compared to plasmon-polarition nanophotonics36,56.

The natural isotopic variation of boron (79.9% 11B and 19.9% 10B, while in comparison 14N is

99.6% abundant) is the most common point defect in hexagonal boron nitride (h-BN) and has

been found to be the most dominant mechanism behind optic phonon scattering36. This in turn

translates to shorter SPhP lifetimes, however this has not yet been investigated in BNNTs. The

isotopic enrichment of a material has also been found to change its dielectric function, which in

turn also means shifts to its Reststrahlen bands and phonon frequencies from those of the

naturally abundant material35,36,57. When studied by Caldwell and co-authors in 2018, the TO

mode had been found to shift from 1366 cm-1 in naturally occurring h-BN to 1393 cm-1 98.7% h-10B and to 1357 cm-1 99.2% h-11B. A corresponding shift in the LO mode was observed as well.

While a broader Reststrahlen band was found in 98.7% h-10B, 99.2% h-11B showed a three-fold

improvement in polariton lifetime compared to naturally occurring h-BN36.

An investigation into the effects of purity on the dampening differences due to isotope scattering

vs. field scattering at the surface would be an excellent candidate for a future investigation using

the IR pump-probe s-SNOM setup which will now be introduced in Chapter 4.

26

Chapter 4-Pump-probe imaging of surface phonon coupling in boron nitride

nanotubes

4.1-Introduction

The ability of materials, such as BNNTs, to be used for nanophotonic purposes depends on

couplings between phonon and/or vibrational modes. The couplings lead to losses of energy, and

hence the signal transmission needed for various imagined devices. Hence, we need to have a

method to trace those energy loss pathways, to determine how to design new materials, or even

just understand something new and interesting.

A pump-probe IR spectroscopy set-up can place energy in one mode then allow for the

observation of this energy migrating into another mode. As mentioned earlier, BNNTs contain

two optical phonon bands, the longitudinal optical (LO) and transverse optical (TO) phonon

mode, as well as the surface phonon polariton mode which is energy that travels inside the

material. The range between the LO and TO modes is called a Reststrahlen Band which enables

the BNNT to exhibit a metal-like reflective behaviour. BNNTs have two Reststrahlen bands, the

upper one being between 1367-1610 cm-1 and the lower one being at 746-819 cm-1, due to the

strong resonances mentioned in Chapter 3. Coherently excited phonons, like electrons in highly

reflective metals, induce an opposite response and the far-field reflection has been found to be

almost 100% in some cases35,58. Using the pump-probe coupled to s-SNOM system, the coupling

between the LO and the SPhP modes can be experimentally observed. The pump-excited mode is

the high energy LO phonon mode (in-plane, traverse stretch) which transfers energy to the

probed lower energy phonon-polariton mode. Some of the experimental data reported here

replicates that reported by Gilburd and co-authors who developed the experiment in the Walker

lab in 201638. They demonstrated strong mode coupling at 1532 cm-1 and 1600 cm-1, however

pumping at wavenumbers above and below those wavenumbers had not been thoroughly

investigated38, which is therefore a new aspect of the work reported here.

The range of wavenumbers used for this experiment were selected based on the far-field IR

absorption spectrum of BNNTs and also the previous findings of Gilburd and co-authors18,51,59.

At 1404 cm-1, there is a surface phonon-polariton mode observed while around 1532 cm-1 there is

a tangential vibration of the h-BN network37. At 1600 cm-1 and 1530-1545 cm-1, an orthogonal

27

mode has been reported in 2D h-BN and BNNTs respectively18,60–62.

4.2-Experimental

Pump-probe imaging experiments were done on the ‘homemade’ s-SNOM developed in our

lab28,63. The obtained phase-sensitive probe field needed to be demodulated at the harmonics of

the sum of the photoelastic modulator (PEM) and the AFM tapping frequencies in order to obtain

a response from the sample. The following experimental setup, which is also illustrated in Figure

4.1, remained the same for the experiment described in Chapter 5 except for a few modifications

that will be described later. Further information on the alignment and daily operation of this

system can be found in the appendix of this thesis.

The AFM (Multimode Bruker Nano) was operated in tapping mode where the tip tapped against

the sample surface at the oscillation frequency Ω. The AFM generated the topography image of

the sample, however its tip also acted as an antenna and sensor where it sensed the near-field

then transmitted it to the far-field.

Two quantum cascade continuous wave (CW) IR lasers (QCL, Daylight Solutions) provided

infrared radiation, one exciting the sample (the “pump”) and the other detecting the response

(“the probe”). Quantum cascade lasers are made from layers of semiconductor where ejected

electrons go between the layers, making miniscule energy transitions and emitting light in the

process64. Multiple photons can be generated by one electron as it emits photon tunnels from

each quantum well it goes to, hence the name “quantum cascade”65. The miniscule energy

transitions allow for the lasers to produce terahertz or long-wavelength mid-IR radiation64,66.

The lasers were aligned collinearly and focused on the apex of the AFM tip using an off-axis

parabolic mirror with a numerical aperture of 0.25. The numerical aperture of the parabolic

mirror is especially important in improving the signal to noise ratio of the system38.

28

Figure 4.1-A diagram of the summarized ‘homemade’ s-SNOM setup

The photoelastic modulator (PEM 90, Hinds Instruments) varies at a fixed frequency (or

modulates) the polarization of the laser beam. The PEM frequency was set to 50 kHz (Ω’) and

was used as a half-wave plate (λ/2) , meaning the peak retardation reached half a wavelength of

the laser light passing through and at that point rotated the plane of polarization by 90º67. This

allows the light beam to be modulated at twice the PEM’s frequency (2Ω’) between two

orthogonal, linearly polarized states which are parallel and perpendicular to the AFM tip. This is

necessary as the coupling between the tip near field and the far field are polarization dependent

due to the AFM tip being more polarizable in the vertical direction. When the incident light

polarization is modulated, it simultaneously modulates the induced near-field intensity and

maintains the consistency of the photon flux onto the probe27.

A mercury cadmium telluride (MCT) detector (J15D12, Teledyne-Judson) was used for the

detection of the signal. It is a semiconductor, which allows the electrons present to absorb IR

light and move from the valence to conduction band of the material68. The current generated is

then directly proportional to the IR light intensity. A disadvantage of the detector unfortunately

is that it saturates easily, has a narrow bandwidth and must be cooled with liquid nitrogen every

10 hours of operation otherwise a noisy signal may result69. To obtain only the probe signal, a

double long wave-pass IR filter (LP-6715, Spectrogon) was placed in front of the detector.

A lock-in amplifier (HF2Li Zurich Instrument) demodulated the voltage signal received from the

MCT detector at the third harmonic of the tapping frequency of the AFM tip (Ω) to perform a

Fourier analysis. The demodulated data must be Fourier transformed to produce frequency

29

domain data so it can be processed from the time domain data it receives70. The received probe

field was demodulated at the sideband frequency f=2Ω + 2Ω’ to obtain the coupled response38.

Lock-in amplifiers extract signal amplitudes and phases from noisy environments using the

homodyne technique elaborated on in Chapter 2.3.1. The near-field signal is extracted using a

defined frequency band that surrounds the reference frequency which here is tapping frequency

of the AFM tip (Ω). The integration time was set 20 ms for the pump-probe experiments.

4.3-Results and Discussion

About 25 tubes were investigated for this experiment at the wavenumbers of interest. 1D

experiments, using only the IR probe, were initially carried out to characterize the spatial

distribution of the IR probe field absorbance71. Nodal patterns can be seen on the tubes which

indicates the propagation of SPhP which constructively and destructively interfere, giving the

distinctive crests and nodes38. An example which illustrates this well is in Figure 4.2, which was

first observed and published by Xu and co-authors in 201438. This can also be seen in Figure 4.3

(b).

Figure 4.2-Nodal patterns indicating phonon-polariton propagation is seen in a BNNT. It can be observed that the node separation increases as the frequency decreases. These near-field images were obtained by Xu and co-authors in 2014 during a 1D experiment using the same instrument conditions specified in this chapter (Reproduced with permission from Xu, X. G. et al. One-dimensional surface phonon polaritons in boron nitride nanotubes. Nat. Commun. 5, 4782. (2014), Copyright 2014 Springer Nature38).

It should be noted that rough gold substrate on which the BNNTs were placed (described in

Chapter 3), in addition to being non-absorptive, was previously found to affect the generation

30

and propagation of SPhPs. Rough gold substrate, as opposed to smooth, was found to provide

momentum for their excitation and mitigate loss38.

Figure 4.3-Images of two overlapping BNNT. a) AFM topography image. b) Near-field image at phase π/2 at probe frequency 1404 cm-1(without pump field). Pump-induced probe images probing at 1404 cm-1 and pumping at frequencies ( c )1580 and (d) 1610 cm-1

Figure 4.4-Images of four overlapping BNNT of various diameters a) AFM topography image, near-field images at phase π/2 probing at 1404 cm-1 and pumping at frequencies (b) 1522 (c) 1532 and (d) 1600 (e) 1610 cm-1

31

2D experiments were then carried out, as seen in figures 4.3 and 4.4, where the sample was

pumped at wavenumbers between 1490-1620 cm-1 and probed at 1404 cm-1. The variation of the

pump frequency was done to observe which phonon is initially excited. Gilburd and co-authors

first identified responses at 1532 and 1600 cm-1 which were attributed to the LO modes of

BNNTs and h-BN, respectively. At 1600 cm-1 the coupling is between the walls of BNNTs which

is essentially cross-talk between layers of h-BN38.

The streaking in the images is attributed to phase changes that occur in the middle of the

experiment. These phase changes may be corrected with phase stabilization methods however

they were not implemented into these experiments63.

Strong responses at 1522, 1580 and 1610 cm-1 were observed when the experiment was

replicated for this thesis. While the other wavenumbers are within the upper Reststrahlen band of

BNNTs, 1610 cm-1 is attributed to the LO mode of h-BN. H-BN has a mid-IR Reststrahlen band

from 1395 to 1630 cm-1, which is close to where the real portion of the dielectric function crosses

the zero line, changing in sign17,72. This finding presents further evidence of coupling between h-

BN layers in the BNNTs.

The response observed is the coupled excitation and can only be detected at the π/2 homodyne

phase only as the absorptive profile is dominant only then16.The pump-induced signal is the

strongest at where the BNNT exhibits a highly negative real part of the dielectric function as this

is where phonon-polaritons are supported38,40. The density of states is increased due to the

presence of the SPhP mode, so the down converted energy from the relaxations of the LO mode

is more efficiently collected73. There is also momentum coupling occurring due to the direction

of the LO mode’s electric force and the phonon-polariton38.

The origin of the coupling still requires further study, however there are several accepted

explanations. At around 1404 cm-1, there is a ring-breathing mode which can couple with the

1532 cm-1.38 The spatial variation in the coupling can be due to the differences in separation

between the tube layers and the varying length of each layer48. The orientation the tubes are

wrapped in can also be a major factor; one can imagine rolling a piece of paper into a tube and

having the layers imperfectly aligned if the wrapping is slightly skewed to one side. The layers

32

of multi-walled BNNTs may vary dramatically and several common orientations have been

identified such as zigzag oriented inner layers and arm-chair outer layers59. As discussed later in

Chapter 5, this latter factor also affects the chirality of BNNTs. This can also cause the interlayer

coupling of the TO and LO modes. Structural defects can increase sp3 hybridization and cause

mode shifting which in turn would affect coupling52. Thermal heating as a possible mechanism

was explored by Gilburd and co-authors by comparing probe spectra with and without the pump

field, however it was concluded it does not play a significant role in mode coupling38.

The mode-coupling images, published by Gilburd and co-authors as well as in the extension of

the work mentioned in this chapter, have a lower than ideal signal to noise ratio which cannot be

improved with demodulation at higher harmonics alone. One remedy for this would involve

implementing the pseudo-heterodyne detection method, one of the background suppression

methods mentioned in Chapter 2.3.1.2. This would require vibrating the piezo controlled mirror

in the reference arm of the existing setup to a set amplitude to achieve a desired modulation

depth. While this modification to the experimental setup is not strictly necessary, it would be an

interesting future project which would possibly improve understanding of mode coupling at

wavenumbers that were previously too noisy to investigate.

Returning to the idea raised at the end of Chapter 3.5, assuming the results of Caldwell and co-

authors could be applied to the BNNTs synthesized using 99.65% 11B powder mentioned in

Chapter 3.2, mode coupling results different to those mentioned above could be hypothesized.

The mode couplings would most likely be seen at different wavenumbers due to shifts in the

phonon-polariton and LO modes. Due to fewer point defects, the mode coupling would also be

stronger and be seen to propagate further down the tube due to longer lifetimes.

33

Chapter 5-Asymmetric scattering of left- and right-hand circularly

polarized light by boron nitride nanotubes 5.1-Theory

Chiral molecules are defined as optically active compounds which are non-superimposable with

their mirror image enantiomer (counterpart)74. As a result, these molecules can exist in right (R

or D) or left handed (S or L) forms. Light may also be left (LCP) and right handed circularly

polarized (RCP) due to its vector nature. The rotation of the polarization state of the light occurs

when travelling through a chiral material because LCP and RCP light interprets refractive indices

and absorption coefficients differently when propagating through the material74,75. The difference

in a material’s absorption of right handed circularly polarized light and left handed circularly

polarized light is called circular dichroism (CD)76. To observe this effect, the material must

contain one or more chiral, light-absorbing groups. Strictly speaking, the interaction should

occur over distances greater than the wavelength of the light, in order for a uniformly circularly

polarized field to emerge. If there is a racemic mixture of these chiral molecules in a bulk

solution, we assume equal absorption by each molecules: ΔA=AL-AR, where ΔA=0. If there is an

unequal amount of LCP and RCP molecules however, an absorption signal will be seen75.

Circular dichroism has been commonly used in the pharmaceutical industry to distinguish

between chiral molecules as they can have drastically different effects as well as to study protein

structures to determine functionality76,77. Helical molecules such as glucose have been found to

give some of the strongest responses75. In the case of BNNTs, at the time this thesis was written

and to the best of my knowledge, the nanoscale analog of circular dichroism (CD) had not been

studied experimentally. That analogue is termed here “Asymmetric scattering of left- and right-

hand circularly polarized light.”

The bulk CD experiment does not just measure transitions of the electric dipole; such as would

be measured using linearly polarized light. These would be matrix elements of that transition

dipole operator. Instead, CD measures transitions < 𝜓6⃓𝑟𝑥𝑝⃓𝜓2 > , i.e., it measures

transitions that involve both an electric and a magnetic moment change between vibrational

states77,78. In other words, there needs to be a rotation of charge as well as displacement

34

associated with the transition. Among electronic transitions, a common example is the π-π*

transition of a carbonyl. Among vibrational transitions, common examples are vibrations in

chiral molecules. In the case of BNNT, the tubes involve zone folding of the sheets that form

their walls, where the zone folding can result in chiral (or twisted) tubes79.

There are main three kinds of BNNT folding types: zig-zag, armchair and chiral which can either

spiral in the right direction or to the left2. Tubes with both chiralities, have been rarely observed2.

Electron diffraction experiments have demonstrated the racemic mixture of boron nitride

nanotubes, showing that it is the number of walls rather than the diameter of the tube which

influences its chirality80. Chirality has been found to not affect the band gap of BNNTs, which is

typically between 5.0-6.0 eV and therefore they are always considered electrical insulators81.

This is not the case for carbon nanotubes which can be a semiconductor or a metal depending on

its chirality78. While chirality has been found to affect the modes of carbon nanotubes, BNNTs

have only been found to have one active phonon mode at 1370 cm-1 where the upper Reststrahlen

band begins79. Currently the only way to determine the structure of a BNNT is to take TEM

images of the tubes and examine the tubes ends. Zigzag tubes have been found to have flat ends

and have been seen far more frequently than armchair tubes which have conical ends2. Chiral

tubes have only been distinguished with high-resolution electron diffraction as they have unique

diffraction patterns80,83.

We anticipate that chiral BNNTs will show a differential scattering of left- and right-handed

circularly polarized light, while zigzag and armchair structures do not due to the difference in

how the layers of h-BN are wrapped to make a multi-walled BNNTs and which vibrational and

phonon modes are excited with IR light79. A theoretical investigation in the vibrational properties

of various BNNT structures by Rubio and co-authors further explains this idea with the zone

folding method. The method depends on comparing the point-group symmetry of each tube type,

which then determines which modes are active if IR excited79. Chiral tubes have the low point-

group symmetry Cd.84 Figure 5.1 visually depicts how the zone-folding method can deduce IR-

active modes.

35

Figure 5.1- A visual depiction of the zone-folding method in chiral BNNTs (reprinted figure with permission from Wirtz, L., Rubio, A., de la Concha, R. A. & Loiseau, A. Ab initio calculations of the lattice dynamics of boron nitride nanotubes. Phys. Rev. B 68, (2003). Copyright 2003 by the American Physical Society)(DOI: 10.1103/PhysRevB.68.045425)79.

In the left image of Figure 5.1, the h-BN sheet is rolled so that the tube-axis and translation

vector 𝑇 (which has the length of the 1D unit cell of the tube) are parallel. The phonon wave

vector 𝐾points into the circumferential direction of the tube and its 𝐾o component is quantized.

On the right side image, the 2n discrete steps are taken along the line Γ → 𝑀 → Γ, which is

longer than for the other two types of BNNT folding. The points close to Γ, limit of the Brillouin

zone, giving rise to IR active modes are depicted as E1 and E2.79 As a result, the zone folding

method has shown that chiral tubes can have different IR active modes compared to armchair

and zigzag tubes. Chiral tubes can have a less intense TO mode compared to zigzag tubes but

more intense than armchair tubes79.

In this experiment the ‘homemade’ s-SNOM system described in Chapters 2 and 4 is used to

map the optical energy distribution in the BNNTs in response to the right and left circular

polarized light.

5.2-Experimental The experimental setup was similar as described in chapter 4.2. However, the pump field is

allowed to get to the detector rather than the probe field by the use of filters placed in front of the

MCT detector, and there is a change in the PEM set-up to generate circularly polarized light.

Depending on the experiment, it was either used as a quarter-wave plate (λ/4) as seen in Figure

5.2 or half-wave waveplate (λ/2) as described in Chapter 4.2.

36

Figure 5.2-Diagram of modulator cycle, showing the retardation ±λ/4 vs. time as well as the varying polarization states85.

At λ/4 the peak retardation reaches quarter of a wavelength of the laser light passing through

and at that point rotates the plane of polarization by 45º. In other words, the light coming through

the PEM alternates between two senses of circularly polarized light at the frequency of the photo

elastic modulator. This is described in Figure 5.3.

Figure 5.3- Diagram of a PEM modulating a beam of light with a retardation of λ/4 so that it oscillates between left and right circularly polarized light.

To obtain only the pump signal, a double long wave-pass IR filter (LP-6715, Spectrogon) was

placed in front of the detector. To linearly polarize the light and only obtain the vertical or

horizontal component of the circularly polarized response, a linear polarizer set at either 0º

(vertical) or 90º (horizontal) was placed in front of the detector after the beam splitter which

reconvenes the light from the reference arm and the probe tip.

37

5.3-Results and Discussion

We set out to observe handed scattering signals seen in the BNNTs arising from the difference in

scattering by phonons of an IR laser field, oscillating between two different circularly polarized

states. We began by examining a gold surface, which would not be expected to illustrate

significant scattering loss at these wavelengths. We chose a gold substrate that would usually be

used to support BNNTs.

The gold was scanned using varying parameters of optical excitation and signal processing,

taking far field circular dichroism and birefringence techniques as our initial model as seen in

Figure 5.4. Two different modulation frequencies (Ω’ or 2Ω’) of the PEM were attempted as

were two different PEM modulations (λ/2 and λ/4) in combination with each other as shown in

Figures 5.4 and 5.5. The two phases ϕ=π/2 were also compared with each combination. The Ω’

sideband frequency was altered between 2Ω’ to Ω’ to determine the differences in signal. The

material properties that we intended to measure are given their far field names in column 1 of

figures 5.4 and 5.5. These are: Circular dichroism, optical rotatory dispersion (circular

birefringence), linear birefringence, linear dichroism. These are only approximate descriptions,

because as mentioned above, these terms apply to samples larger than the wavelength of light,

which is not the case here.

38

TypeofSignal Modulation

FrequencyofthePEM

PEMModulation GoldResponse(V)atϕ=π/2

GoldResponse(V)atϕ=π

Circulardichroism/handedinelasticscattering

Ω’ λ/4 2.196 (15.23)

Opticalrotatorydispersion(circularbirefringence)/handedelasticscattering

2Ω’ λ/4 (42.29) 107.87

Linearbirefringence/dipolarelasticscattering

Ω’ λ/2 (24.97) 173.47

Lineardichroism/dipolarinelasticscattering

2Ω’ λ/2 31.02 (41.34)

Figure 5.4-A table illustrating the response of the gold substrate to 1532 cm-1 pump frequency at the third-harmonic of the AFM tip tapping frequency (sideband frequency). Values in brackets are indicated for completeness of the experimental record, but not correspond to the material property listed on the left hand column of the table. These combinations of experimental parameters were then repeated on the tubes to examine the

increase in the obtained signals as seen in Figure 5.5. The signals were the product of the signal

amplification factor registered in the lock-in amplifier during image acquisition and the

absorption signal obtained in the images such as the ones shown in Figure 5.6.

39

Figure 5.5-A table illustrating the response of the BNNT to 1532 cm-1 pump frequency at the third-harmonic of the AFM tip tapping frequency (sideband frequency). Values in brackets are indicated for completeness of the experimental record, but not correspond to the material property listed on the left hand column of the table. A very simplified explanation for the results obtained can be given, as additional investigation is

required to account for factors such as additional stray signals and the mixing of signals. When

the PEM modulation is set at λ/4, the IR light inelastically scattered by the BNNTs is circularly

polarized. Assuming an approximation where the tip acts as an analyzer favoring a single (p)

polarization, in a conventional CD spectrometer, the signal obtained when the PEM frequency is

Ω’ is the circular dichroism signal while at 2Ω’, an optical rotatory dispersion (or circular

birefringence) signal is observed67. Higher signals were obtained when the PEM modulation was

at λ/2 as here the light is modulated between two orthogonal, linearly polarized states (vertical

and horizontal). Under the same approximation stated above for a LD spectrometer, at PEM

frequency Ω’ linear birefringence is presumed to be observed, while at 2Ω’ linear inelastic

dipolar scattering is observed67. This is was found to be the largest signal, and is consistent with

the traditional surface selection rule favoring p-polarized transitions86.

The obtained images at PEM frequency 2Ω’ at λ/4 and λ/2, respectively are shown in Figure 5.6.

The chosen PEM frequency was expected as here the signal to noise ratio decreases however the

PEM peak retardation at λ/4 was expected to give the higher signal as here the light would

TypeofSignal FrequencyofthePEM

PEMModulation

TubeResponse(V)atϕ=π/2

TubeResponse(V)atϕ=π

Circulardichroism/handedinelasticscattering

Ω λ/4 37.44 (121.60)

Opticalrotatorydispersion(circularbirefringence)/handedelasticscattering

2Ω λ/4 (86.20) 129.75

Linearbirefringence/dipolarelasticscattering

Ω λ/2 (180.00) Notdetermined

Lineardichroism/dipolarinelasticscattering

2Ω λ/2 960.96 Notdetermined

40

alternate 45º at two points between the two orthogonal linearly polarized states. This idea was

further explored with the use of a linear polarizer.

Figure 5.6- Images of 5 overlapping BNNT. (a) AFM topography image. Near field images collected at ϕ=π/2 at pump-induced frequency 1532 cm-1 at PEM peak retardation (b) λ/4 (c) λ/2

A linear polarizer set in the vertical or the horizontal direction was used when scanning the tubes

to partially block out the signal as a control experiment and break the signal into two orthogonal

states. While a signal at both polarizations was expected, there was some ambiguity on how to

measure and translate the results seen to the overall signal seen without the linear polarizer. As

the AFM tip is polarized in the vertical direction, the signal was stronger when passed through

the vertical linear polarizer compared to when passed through the horizontal polarizer. This can

be seen in Figure 5.7.

41

Figure 5.7- Images of 5 overlapping BNNT. (a) AFM topography image. Near field images collected at ϕ=π/2 at pump-induced frequency 1532 cm-1 at PEM peak retardation λ/2 with linear polarizer (b) not present (c) vertically positioned d) horizontally positioned The conclusions we draw from the data in Figure 5.4 and Figure 5.5 is that the CD analog signal

from gold is tiny; there appears to be a modest linear birefringence analog signal. The other

signals on gold are not detected. On BNNTs, only the linear dichroism analog signal appears to

be large. The other signals are weak, and may be contaminated by uncontrolled polarization

rotations, though it is clear more effort would be worth applying to this problem, both

theoretically and experimentally.

Hillenbrand and co-authors launched an investigation with a similar experimental setup,

investigating what they termed circular dichroism in chiral meta-materials. They were able to

distinguish between the two enantiomers of the chiral meta-materials by alternating between ϕ=-

42

π/2 and ϕ=+π/2, however, their theoretical model for the response has been challenged75,87. We

hope that by comparing our results with more far field analogs that Hillenbrand did, we can get

deeper insight into the nature of near field material responses. This will guide us to developing a

better theoretical model, as well. In a comparative direction, our investigation could also be

extended further by characterizing the tube structure types using TEM and electron energy loss

spectroscopy, to correlate the differences in their responses.

Studies on how chirality affects the applications and other properties of BNNTs have been few;

however, they all highlight that its effect is not negligible. Using BNNTs for nanofluidic

applications such as for water purification and osmotic energy conversion has been a subject of

great interest88,89. A theoretical experiment studying the friction coefficient of water inside

BNNTs with armchair and zigzag configurations demonstrated that zigzag BNNTs have a much

larger friction coefficient88. Another investigation examined differing piezoelectric responses to

applied loads between BNNTs of varying chiralities90. Such investigations emphasize on the fact

that the folding structures and chiralities of the BNNTs makes a difference in mechanical

properties, and it is expected that natural handed meta-material such as hyperbolic BNNTs could

have exotic applications in imaging including buried interfaces.

43

Chapter 6-Conclusions and Future Work

In electronic devices, geometry, power consumption, efficiency and switching speeds are all

governed by the size of their transistors91. In 1975 it was observed that transistors were shrinking

at exponential rate every two years, which was termed “Moore’s Law.” As smaller silicon

transistors are closer to reaching their functional size limit every year, Moore’s Law may soon

become obsolete92. Nanophotonics and nano-plasmonics have been considered as a suitable

replacement in the race of effort to make transistors even smaller. Their ability to concentrate

and channel sub-wavelength light has been considered for various other applications such as

coupling chemical reactions, as thermal conductors, hyper-lenses and waveguides93.

Investigations have shown that photonic devices tend to outperform plasmonic devices as they

relay on optical phonons rather than free charge carriers and therefore their optical losses are

smaller91,94.

The motivation behind the experiments described in this thesis stems from the fact that

spectroscopic investigations of vibrational and photonic energy transduction in the nanoscale are

still considered rare. These sorts of investigations are instrumental to realizing the potential

extension of Moore’s Law with nanophotonics.

In Chapter 2, the scattering-type scanning near-field optical microscope (s-SNOM) was

introduced and its experimental as well as theoretical operation was discussed. The latest

developments in the s-SNOM has paved the way for many significant findings in various

scientific fields due to its ability to break through the diffraction limit leading to an improved

spatial resolution among other performance benefits. For example, space exploration has also

taken advantage of the s-SNOM’s non-destructive, nanoscale probing ability, for determining the

chemical composition of cometary dust95.

In Chapter 3, various properties, synthesis and applications of BNNTs was discussed. In recent

years, multiple new studies have arisen demonstrating novel uses for BNNTs. The biomedical

sector has considered using BNNTs for drug delivery purposes while the aerospace industry has

investigated using them for lighter space suits due to their mechanical strength and ability to

44

absorb harmful solar radiation96–98. As previously stated, the process behind manufacturing

BNNTs is still relatively inefficient for the commercial scale and purification of the tubes is

difficult30,99–101.

In Chapter 4, the replication and extension of the near-field IR pump-probe imaging of SPhP in

BNNTs experiment and resulting publication by Gilburd and co-authors was described38.

Coupling between a high energy pump-excited phonon mode and a weaker probed phonon-

polariton mode was observed at the wavenumbers attributed to the Reststrahlen bands of

BNNTs. Additional coupling that was not previously discussed by Gilburd and co-authors was

observed at 1522 cm-1, 1580 cm-1, 1610 cm-1. This investigation of cross-talk between modes is

important in the development of nano-devices such as waveguides and thermal conductors as it

relies on understanding the propagation and storage of phonon-polariton energy72,98.

In Chapter 5, the chirality of BNNTs was discussed with circular dichroism experiments and

possible explanations for the signals observed with ‘homemade’ s-SNOM were discussed. This

experimental investigation, to the best of my knowledge, has not been published before and

therefore the results in this chapter were preliminary at best due to uncertainty of what the

presumed optical scattering signals consist of. Further experiments need to be done to understand

the work presented, however it nevertheless opens an investigative avenue into handed light

scattering in BNNTs.

45

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56

Appendix

Appendix 1-Manual for the alignment and troubleshooting of the

‘homemade’ s-SNOM

This is a step-by-step guide that I put together while learning how to align the system. Each step

takes a while to figure out yourself but once you understand how it works, it will be a much

faster process every time you do it. It is typically a good idea to trace out the path of each laser

before beginning your work and alignment so that if something goes wrong, you can go along

the path and see if there is an obstruction where there shouldn’t be. Sometimes other lab

members use the quantum cascade (QC) lasers for the Inspire AFM which is on the same laser

table, so there may be mirrors up along the pathway redirecting one of the lasers as seen in

Figure 1.

Figure 1: Quantum Cascade (QC) lasers 1 (background) and 2 (foreground, marked with a “2”).

The helium laser in the middle is typically blocked with a black glasses case (not pictured) unless

it is being aligned. The mirror next to the QC2 laser may be up if in use for the Inspire setup.

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Putting the AFM tip into the AFM tip holder

1) Remove the AFM tip holder, which is pictured in Figure 2, by detaching the diode laser cable and the two springs on either side of it.

Figure 2: The AFM holder, with the microscope above it and the diode laser cable seen sticking

out.

Lift it slowly off as it is very fragile. Unscrew the side of the tip holder and remove the cantilever

holder (there will be a small grip for pulling it out). Placing the cantilever holder on a flat surface

and pressing on the button at the bottom, use a pair of tweezers to remove the old AFM tip. The

tip that you want to use for this system is the MikroMasch brand tips from spmtips.com with the

descriptor “HQ:NSC14/Pt, 160 kHz, 5.0 N/m, ”as pictured in Figure 3.

Figure 3: The AFM tips from MikroMasch

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Practice a few times moving the AFM tip from the box as sometimes it is difficult to get a good

grip on the tips and dropping them may cause the tip to break. Ensure that the tip is centered and

fully in the indentation in the holder, otherwise it may shift during the alignment. Return the

holder to its place by lowering it very gently onto the sample holder and reattach the metal

springs on each side.

Aligning the diode laser onto the top of the AFM tip

1) Using the two knobs on the top of the AFM tip holder, as seen in Figure 4, position the diode laser so that it is focused on the top of the AFM tip.

Figure 4: The AFM tip holder with the relevant knobs visible.

The red spot of the diode laser should be seen as fainter than when unobstructed but with some

slight diffraction going in all directions to indicate you are on the tip. If you turn the x-direction

knob slightly in either direction, it should immediately be brighter as you are now off the tip. If

you turn the y-direction knob forward, the red spot should be brighter almost immediately but

turning the knob backwards will make it even fainter as you continue moving down the

cantilever. Next use the fine control knobs to align the diode laser even more precisely. The

indicator here is a grey digital circle that can be seen in the mirror behind the AFM apparatus

(not visible in Figure 5).

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Figure 5: The green screen is a reflection off of the mirror in front of the AFM apparatus. The AFM/SPM options can be seen at the bottom right of the image.

Switching between the AFM and SPM setting on the AFM apparatus, as seen in Figure 5, it is

essential that the green bottom screen with numbers seen in the mirror is 0.00 for each setting.

Usually the grey circle being half completed is a good indication that you’ve aligned

successfully.

To check if the tip is intact and properly aligned, a cantilever tune must be performed. This is

done on the AFM software by clicking on the tuning fork icon on the top left part of the screen.

After pressing “autotune”, watch the top panel of the pop-up window and ensure that one single

peak is seen. The following is unacceptable: if there is a shoulder on the peak, if the peak is too

wide or there is another strong peak close to the main peak. If one of these conditions are

observed, remove the AFM tip holder and push the button at the bottom to move it around

slightly or push it with tweezers. Repeat all the above steps. If there is still an anomaly observed

in the cantilever tune, discard the tip. The frequency provided by the cantilever tune should be

between 142 to 179 kHz as the tip manufacturer states that there can be a 10% deviation from the

tip frequency stated on the box (160 kHz).

Aligning the helium laser to the tip using a camera

1) Unblock the helium laser (usually blocked by a glasses case) and put up the “caution fragile” mirror that is near the AFM apparatus and pictured in Figure 6.

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Figure 6: The ‘caution fragile mirror’ which is lowered in this image but needs to be raised when aligning the camera.

Open up the camera software and press the play button on the upper left corner of the program to

turn on the camera. Look down through the microscope that is over the cantilever and ensure it is

visible. If not, focus the microscope. Next ensure the reference arm is blocked (such as with a

piece of paper) and unplug the diode laser cable. Unless the helium laser is focused nearby, the

image under the microscope should be dark with a faint outline of the cantilever.

The parabolic mirror that focuses the helium laser onto the tip is moved around by the three

knobs that surround the AFM apparatus and are pictured in Figure 7.

Figure 7: The three knobs that are needed for moving the parabolic mirror next to the AFM apparatus

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The parabolic mirror is very close to the apparatus so you need to be very careful when moving

the vertical knob so you don’t damage it. Usually it is enough to move each knob very slightly to

have the laser aligned however sometimes the alignment requires more effort, especially if the

AFM tip brand or even box of tips is different or if the person aligning the system is different

(everyone has their own way of placing the tip). This is where you need to keep in mind how the

helium laser reflects off the tip, the cantilever and the surrounding AFM apparatus. If you cannot

see the helium laser light at all, adjust the z-direction knob first to see if the laser reflects

anywhere. If you see the red of the laser, think about what it is reflecting from and go from there.

If you still see nothing, move the x and y knobs slowly until you see red. The following

measurements (numbers on the knobs) I have taken from a few of the alignments I have done

just so that I know I am in the right approximate area and don’t move the knobs too much

(Table 1).

Knob Alignment 1 Alignment 2 Alignment 3 Vertical 24.65 0.65 9.65 Horizontal 10.85 11.85 8.85 Perpendicular 20.53 20.53 21.53

Table 1: Measurements taken from the knobs when aligning the helium laser.

Once you see the red of the helium laser, think what it is reflecting from and move it accordingly

until you see a VERY bright spot reflecting from the cantilever. From there, it is only very small

turns. You can also plug the diode laser back in and look at the screen where the camera is giving

you a side view of the tip and cantilever arm. If you see some red on screen that doesn’t

disappear when you block the helium laser, you’ve found the cantilever arm and you’re very

close to the tip (small only). This red is coming from the diode laser that is reflecting off of the

head of the cantilever. If the red disappears when you block the helium laser, then it is the helium

laser that is reflecting off of the sample surface or the AFM apparatus. You know when you’ve

found the tip when you see a very red triangle with a bit of white in the middle on the screen and

a bit of red above it which doesn’t go away when you block the helium laser (triangle should be

gone).

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Engaging the AFM tip for the first time

1) With the camera software still on, press the “DOWN” button on the left of the AFM

apparatus and watch the red laser ring around the cantilever tighten closely around it.

Once you start seeing a bit of red scattering around the triangle on the screen, stop

manually bringing down the cantilever and use in “Engage” button on the AFM software

as seen in Figure 8. It is also a good idea to do a cantilever tune again before engaging.

Once the tip is engaged, open the Zurich Instruments Lock-In amplifier software, whose

icon is seen in Figure 9. Enter tip frequency that is found in the AFM software (enter all

the digits as well as the “k”) in the “Lock-in MF” tab to the left of where it says

“Internal”.

Figure 8: A screenshot of the NanoScope software program where the cantilever tune, ‘engage’ and ‘withdraw’ buttons can be seen.

Figure 9: Icons on the top screen computer which are essential to the instrument alignment and operation

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Aligning the MCT detector

Fill the detector with liquid nitrogen (usually two liquid nitrogen cups are enough), as pictured in

Figure 10. Turn on Quantum Cascade 2 (QC2) laser by first turning on the chiller next to the

laser then the laser itself, as pictured in Figure 11. Then turn on the laser using the software on

the computer (icon is on the lower screen, pictured like in Figure 9).

Figure 10: The MCT detector is the beige container with the golden lid.

Figure 11: The chiller (left) and the the Quantum Cascade (QC) lasers are on the right. QC1 is on top and QC2 is on the bottom.

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Figure 12: A screenshot of the QC2 laser software. Note that the QC1 laser software looks similar but with a different current and wavenumber range.

It is a good idea to start at 1400 or 1404 cm-1, ensuring the laser is set on “CW” or continuous

wave with current of 650 mA as pictured in Figure 12. On the Lock-in amplifier (LA) software

pictured in Figure 13, select “Zoom FFT” (Fast Fourier Transform), press “restart” and you

should see a big peak.

If there is no big peak, you may need to check if you entered the right tip frequency, if your tip is

tuned and functional and whether the wavenumber of the laser is correct. Center the red cursor

onto your peak by pressing the “center” button to the right of the “Cursor 1” in the “Cursors

section” and move it to the very center of the base of the peak. Next, enter the number you see in

“Cursor 1” into the LA software configuration. Ensure the “k” at the end of the number is there,

otherwise the software will understand a completely different value. Return to the tab in the

software that says “Spectroscope” and select the 1 and 2 (first and second harmonic) on the right

as pictured in Figure 14.

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Figure 13: Zoom FFT function being performed on the LA software. There should be one

solitary peak, as additional noise and/or peaks may indicate a problem.

Here you only move the z and x direction knobs, which are pictured in Figure 10, while watching

the signal coming from “1” on the LA software.

Figure 14: A screenshot of the LA software where the signal fluctuations can be observed. To the right is the “PhaseControl” Labview program where the phase is controlled.

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Do not touch the y direction knob that is pointing away from you, you will misalign and/or crash

the detector against the parabolic mirror. First move one knob and attempt to get the signal the

highest you can. Scan a whole area, in one direction than the other, stopping when you can only

see the same low numbers (1-3 µV, or “uV” on the screen). Then try the next knob and repeat the

process. Repeat two more times. Typically, the first harmonic should be around 1.2 mV when the

reference arm is unblocked. If the signal is the highest you can get, turn one knob slightly in one

direction then scan with the other. If it doesn’t increase after about a 50 µV drop in signal, switch

the direction of the one knob. Ensure that the piezo is in the π phase otherwise the signal will be

very low (π/2, where the substrate reflects the most). This is controlled either manually with the

piezo controller on the laser table pictured in Figure 15 or with a LabView program called

“PhaseControl” pictured in Figure 14. You can adjust either to get the right phase.

Figure 15: The phase can also be controlled manually with the piezo controller on the laser table. Note that here the cable connecting to the piezo is currently connected to the computer at the other end however, switching the cable will allow you to control the piezo.

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Aligning the reference arm

The reference arm mirror sits on a piezo that is controlled by the piezo controller mentioned

above. Here you need to adjust the mirror to ensure you see the highest possible signal from the

reference arm and ‘flipping’ (yellow line flips between a peak and a valley) on the oscilloscope

pictured in Figure 16.

Figure 16: The oscilloscope where the aim is to get the highest amplitude but also ‘flipping’.

In addition to observing flipping and high amplitude signal on the oscilloscope, it is necessary to

see a high signal on the LA software second and third harmonic. Change the cable of piezo

controller on the laser table from the one that connects to the computer to the one that connects

to the piezo itself as seen in Figure 15. You adjust the mirror using a knob and a screwdriver

seen in Figure 17, maximizing one then the other until you get the highest possible signal and

flipping. Switch the piezo controller cable back.

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Figure 17: The reference arm (unblocked), where the mirror attached to the piezo is controlled with a knob as well as the red screwdriver pictured.

Filters

We have three filters currently positioned before the MCT detector as seen in Figure 18. The LP-

3.5 µm filter lets through both the pump and probe laser which is the filter you should use

whenever you do any sort of alignment. It is typically not recommended for experiments

because a lot of noise gets in and can give you noisy images. It It is a single filter while the two

other ones are double filters (hence the “2x”). The 2xLWP6715 filter lets through only probe

light and is typically used for pump-probe experiments to only pick up the pump response (and

not its reflection) with the probe. It lets through 1428.57cm-1 to 800 cm-1. The 2xSP7000 filter

lets through only the pump laser and we have used it only to determine if the pump response was

actually a probe response. It lets through 2000cm-1 to 1470 cm-1.

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Figure 18: The filters that can be used for the experiments. The 2xSP7000 is on the left, the 2xLWP6715 is the one in use and the LP-3.5 is on the right.

Getting your first image

Once you have completed the alignment, you are ready to take your first image and ensure that

everything was aligned properly. Simply press the “scan” button on the AFM software and

ensure that the image looks clean. If your signal is relatively high but the image quality is poor, it

may be that the detector is not cold enough (pour more liquid nitrogen in), that your tip needs to

be retuned (withdraw, then retune), that the Zoom FFT needs to be restarted or that you’re

scanning in an area with a lot of debris where the AFM is going from very high to low tapping

amplitudes and vice versa. If all of this has been taken into account, go back to the reference arm

alignment and ensure there is a high signal and flipping. If you don’t see any improvement,

retrace your steps of the alignment until you find the source of your problem. Only replace your

tip if you observe one of the issues mentioned previously during the cantilever tune and/or if

your image quality cannot be improved in any other way and you have been using it for a while.

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Pump-probe experiment specific instructions

The above instructions are general instructions for aligning the system for any experiment done

on the homemade AFM coupled to s-SNOM. There are additional modifications to the system

and software that must be made in order to carry out a pump-probe experiment. These are

typically carried out after the general alignment has taken place.

Lock-in attachment

The lock-in amplifier (LA) has only two inlets for attaching signal input cables while for pump-

probe three are needed: input from the photo-elastic modulator (PEM) which is pictured in

Figure 19, one from the AFM and one from the piezo. The input from the AFM we can do

without as the tapping frequency can be manually entered from the AFM software into the LA

software, so we can remove this cable (has “tapping frequency” written on it) from the signal

input 2 inlet and plug in the PEM cable instead, pictured in Figure 20.

Figure 19: The photoelastic modulator (PEM)

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When this happens, the frequency of the PEM (typically around 50 kHz) will be locked into the

LA software and a green “locked” light will turn on next to the Signal In 2 (auto) in the Lock-in

MF tab.

Figure 20: The Lock-in amplifier (LA), where the input cable should be switched under the green sticky note.

Lock-in software configurations

Several adjustments need to be made to the lock-in software to get the appropriate modulations,

especially the sidebands. In the auxiliary I/O tab, ensure that Aux 1-3 are all set to “Demod R”

and under the Demodulator they are “Demod 2”, “Demod 3” or Demod 5” respectively. Aux 4

should be set to “Manual” (no voltage window). Ensure you note which Demodulator

corresponds to which input (eg. Demod 2 in my experiments was Input 1). The voltages for all

the demodulations should show 1-2 V and you can bring them up or down by typing in a V/Vrms

value under “Scale.” Typically, a voltage should not exceed 10V.

Under the “Modulation” tab, ensure that “Mod 2” has the frequencies of the AFM tapping

frequency and that of the PEM, both in the 2nd harmonic as pictured in Figure 21. The tapping

frequency should have a demodulation of 4 and the PEM of 5+ and 6(-). For the mode of Mod 2,

select AM Gen + Demod and select “Both” for sideband.

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Figure 21: A screenshot of the modulation tab in the LA software where the sideband modulation is controlled, which is especially important in pump-probe experiments

It is a good idea to save your settings once you know which settings work best for your

experiment. I initially used the settings of previous lab members with similar projects, then

created my own once I found the best ones for me. There is a “save” tab where you can either

“Save settings” or “load settings”.

Performing a pump-probe experiment

Once all of the above has been completed and the system is giving you a good signal and clear

images, you are ready to turn on the Quantum Cascade 1 laser, or the pump laser to carry out a

pump-probe experiment.

1) Ensure the probe has been scanning for at least half an hour. Withdraw the tip, perform a

cantilever tune and Zoom FFT tune.

2) Turn on the PEM box, ensure that it is set to λ/2 (a retardation of 0.500) and units to

“wav” as pictured in Figure 22. Keep in mind that the PEM box only accepts settings in

wavelength so ensure your desired pumping wavenumber is converted beforehand. Set

the wavelength by pressing “set” twice, ensuring the “nM” icon under wavelength is

blinking and moving the arrows. Press “set” again and press the grey button on the right

of “frequency” until “cm-1” lights up, ensuring that you set you desired wavenumber.

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Figure 22: The PEM box with the suggested settings, however the wavelength (top right) is changed according to the wavenumber the QC1 laser is set to.

3) Turn on the pump laser first manually then with the Daylight solutions software on the

upper monitor (range is 1485.02 to 1709.96 cm-1). Ensure the laser mode is set to CW

and current to 500 mA.

4) Ensure you are scanning in the π/2 phase by using the Labview piezo controller program

“Phase_shift”, moving the arrows up and down to get the lowest possible signal while

scanning is paused. Your substrate should appear darker and blend in with your BNNT

tubes.

General troubleshooting

The following are actions I have taken when I was getting a poor image, poor signal or no image

at all. They have all worked on different occasions and when they didn’t, I usually consulted

another lab member with knowledge on the system. It is important to remember that the only

time you should resort to changing the tip is if the cantilever tune shows the anomalies

mentioned previously and/or if you have been using it for more than about a month and the

platinum coating has worn off and no longer reflecting.

1) Ensure you are using the right filter! Typically, only the LP 3.5 µm is used for alignment

and you want to use the LP 6715 filter for most experiments that you perform with this

system.

74

2) Turn off the lock-in with the big power off/on switch in the back, turn off/on the lasers,

try changing the laser wavenumber, retune the tip with the cantilever tune and the zoom

FFT.

3) Reconnect the piezo controller cable and ensure you see flipping on the spectroscope. If

there is no flipping, you will need to adjust realign the reference arm again. If that doesn’t

work, you may need to realign the detector.

4) Ensure that the AFM tapping frequency on the AFM software matches the frequency on

the LA software.