Princip of SNOM-Can

7/30/2019 Princip of SNOM-Can

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Scanning Probe Microscopy

(SPM)

Real-Space

Surface Microscopic Methods


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SPM Principle

Probes that are nanosized(accomplishedmicrolithographically),

scanning and feedback mechanisms

that are accurate to the subnanometerlevel (achieved with piezoelectricmaterial), and

highly sophisticated computer controls(obtained with fast DACs (digital analogconverters, etc.).

Consists of


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Schematic of SPM Principle


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Resolution Comparison


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3 Axis Cylindrical Piezo


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SPM Tree Conventional STMTopography of conductive surfaces, I-V spectroscopy (e.g., local band gaps)

Ballistic Electron Emission Microscopy and Spectroscopy

Subsurface investigations, e.g., of metal/semiconductor interfaces

STM Scanning Tunneling PotentiometryScanning Tunneling Microscopy Surface potential studies (e.g., study of grain boundaries)

Photovoltaic and Photoassisted Tunneling SpectroscopySurface electron-hole pair recombination during photo-excitation

Inelastic Electron Tunneling

STM induced photon emissions (study of heterostructures)

Conventional SFM (atomic force microscopy AFM)

Topography of mainly non-conducting surfaces, force spectroscopy

(S)LFM (Lateral force mapping of surfaces)

SFM Friction studies, local material distinction ("Chem. Force Microscopy")Scanning Force Microscopy (S)EFM (Electrostatic Force Microscopy)

Non-contact electrostatic force mapping, (e.g., study of charge decay)

SPM (S)MFM (Magnetic Force Microscopy)Scanning Probe Microscopy Contact and non-contact technique used to study magnetic domains

Rheological Force Microscopy

Contact sinusoidal modulation (distance or force) methods

(S)UFM (Ultrasonic Force Microscopy)

non-linear surface effects (e.g., true non-contact interactions, or rheology)(S)PFM (Pulsed Force Microscopy); rheology and adhesion force mapping

SCAM (Scanning Capacitance Microscopy); measuring of trapped charges

SECM (Scanning Electrochemical Microscopy); spatial variations of Faradaic currents or potential changes)

SNOM (Scanning Near-field Optical Microscopy); optical properties, luminescence

SMM (Scanning Micropipette Microscopy); local ion concentration (e.g., transport processes in membranes)

SCM (Scanning Calorimetric Microscopy); local heat transfer coefficients and transition temperatures

SNAM (Scanning Near-field Acoustic Microscopy); topography and rheology


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The Three Basic SPM Systems

Scanning Tunneling Microscope (STM) Scanning Force Microscope (SFM)

Scanning Nearfield

Optical Microscope (SNOM)


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Scanning Tunneling Microscopy (STM)

Signal: Tunnel CurrentThe tunnel current depends

on the tip-sample distance,

the barrier height, and the

bias voltage. Studying the

bias dependence provides

important spectroscopic

information on the occupiedand unoccupied electronic

states (-> local LDOS

studies).

FT

FS

FS

FT

Positive sample bias: Net tunnelingcurrent arises from electrons that tunnel from

occupied states of the tip into unoccupied

states of the sample

Negative sample bias: Net tunneling

current arises from electrons that tunnel fromoccupied states of the sample into unoccupied

states of the tip.

The tunnel current is strongly distance, Dz, dependent

zAexpVI2/1

biasF

A = const.


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Conventional STM

Tunneling Current, I

Bias Voltage, V

Conductive Sample

STM Tip

Piezo Scanner


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STM Modes of Operations

Examples:

Constant height imaging or variable current mode (fast scan mode)The scan frequency is fast compared to the feedback response, whichkeeps the tip in an average (constant) distance from the sample surface.Scanning is possible in real-time video rates that allow, for instance, thestudy of surface diffusion processes.

Differential tunneling microscopyTip is vibrated parallel to the surface, and the modulated current signal isrecorded with lock-in technology.

Tracking tunneling microscopyScanning direction is guided by modulated current signal (e.g., steepestslope).

Scanning noise microscopyUse current noise as feedback signal at zero bias. Nonlinear alternating-current tunneling microscopy

Conventionally, STM is restricted to non-conducting surfaces. A highfrequency AC driving force causes a small number of electrons to tunnelonto and off the surface that can be measured during alternative half-cycles(third harmonics).


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Scanning Force Microscopy (SFM)

Sample

SFM Tip

Piezo Scanner

z

Force: FN = kN* z

Ppring constant: kNSpring deflection: z

Interaction or force

dampening field

Contact Method: Non-Contact Method:

Sample

Input Modulation Cantilever Response

Scan

Cantilever

4-Quadrant

PhotodiodeLaser

fully elastic viscoelastic

Friction

Topography

50/50 PS/PMMA blend annealed at 180 oC for 1 week

Spinodal Decomposition of PS/PMMA Blend

PSPMMAPMMA

PS

10 m

complex flow pattern over time

SFM Topography SFM Lateral Force

2D spinodal decomposition

different from bulkNote: The bright spots (PS phase/lateral force image) represent

spinodal frustration points of PMMA.

50/50 PS/PMMA blend annealed at 180 oC for 1 week

Spinodal Decomposition of PS/PMMA Blend

PSPMMAPMMA

PS

10 m

PSPMMAPMMA

PS

10 m10 m

complex flow pattern over time

SFM Topography SFM Lateral ForceSFM Topography SFM Lateral Force

2D spinodal decomposition

different from bulkNote: The bright spots (PS phase/lateral force image) represent

spinodal frustration points of PMMA.


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Rheological SFM

Sample

SFM Tip

Piezo sinusoidallymodulated either in x or z

z

Load:

FN = kN*z

Lateral Force:

FL = kL*x

x

Input Modulation Signal

Response Modulation

Signal

Amplitude

TimeTime Delay


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Topography Modes of SPM

Constant deflection (contact mode)Analog to the constant current STM mode. The deflection of the cantileverprobe is used as the feedback signal and kept constant.

Constant dampening (AM detection, intermittent contact mode in air orliquid)The response amplitude of sinusoidally modulated cantilevers allow

feedback in the pseudo-non-contact regime (intermittent contact) due tofluid dampening.

Constant frequency shift (FM detection, non-contact mode in ultrahighvacuum)Similar to the FM radio, the frequency is measured and frequency shifts areused as feedback system. This approach works only in vacuum where fluid-dampening effects can be neglected.

Variable deflection imaging (contact mode)Analog to the variable current STM (constant height) mode. Uses fast scanrates compared to the force deflection feedback (close to zero). Sensitive tolocal force gradients such as line defects. Improved high resolutioncapability (atomic resolution).


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SFM Force Spectroscopy

Sample

F(D) forces acting on the tip

linearly ramped voltage

applied to piezo

D = Do - vt

F(D)

0

D

jump in contact

jump out of contact


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Cantilevers Probes for SFM


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Scanning Near-field OpticalMicroscopy (SNOM)

SNOM Principle (Pohl et al. 1984): A tinyaperture, illuminated by a laser beam from the

rear side, is scanned across a samle surface,

and the intensity of the light transmitted throughthe sample is recorded. To achieve high lateral

resolution (first experiments provided already

tens of nanometer resolution), the aperture had

to be nanometer sized, and maintained at ascanning distance of less than 10 nm from the

sample surface (i.e., within the evanescent field).


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SNOM Schematic Examples

Small aperture

Evanescent Field

Regime

Illumination

Objective

Detector

Sample

Illumination

Objective

Detector

Sample

Illumination Mode Reflection Mode


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SNOM

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