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Jan Genzer, NCSU
1
Experimental techniques for studying polymer surfaces and interfaces
The immense activity in studying polymer surfaces and interfaces in the past several years has precipitated a renewed interest in advancing existing techniques as well as developing novel methods. The goal is to provide information about the system behavior on a scale comparable with or smaller than dimension of a single polymer chain (≈ 100-500 Å). Here we provide basic characteristics of the experimental techniques used to study the surface and interfacial behavior of polymers.
Depending on the phenomenon studied, one usually distinguishes between i) the experimental techniques used to investigate the surface properties of polymers, and ii) those applied in studying polymer interfacial behavior. It is very hard, if not impossible, to draw a solid line between these two groups. Some of the techniques are truly versatile and can be used to study both cases. Generally, the techniques differ in their principle, provided information, sensitivity, depth or lateral resolution, required contrast, source of the incident and outgoing radiation, etc. All these parameters dictate the applicability of each technique to investigate a particular problem. Every technique has its strengths and weaknesses and in many cases the application of more than just one method is recommended. Tables 1 and 2 list the experimental techniques used most frequently in surface and interfacial polymer science. More detailed discussion about the experimental techniques listed in Tables 1 and 2 are given in the in the provided references. Some general characteristics of the above techniques and discussion about their application in polymer science can also be found in several recently published reviews (see ’General references’).
Objectives in the characterization of the surface properties of polymers include phenomena such as the study of the surface topography, surface chemical composition, molecular orientation, surface roughness, molecular imaging, etc. The techniques in this group include some traditional surface science techniques such as contact angle measurements, X-ray photoelectron spectroscopy, static secondary ion mass spectrometry, infra-red attenuated total reflection, high-resolution electron energy loss spectroscopy, etc. In addition, a vast variety of numerous microscopy techniques (cf. Tab. 1) complement the collection of the techniques for studying the surface properties of polymers. The above techniques usually provide information about polymer behavior at the surface and in the region a few nanometers below the sample surface. The lateral resolution of the surface sensitive techniques spans areas ranging from several Angstroms to several millimeters. As stressed earlier, a complete picture about surfaces of thin polymer films can thus be obtained by simultaneously applying two or more techniques.
While in metals or small molecule systems, the perturbation of the system due to the presence of the surface and/or additional interfaces usually extends only a couple of nanometers beneath the sample surface, in polymer systems the length scales of interest are much larger. This is because the size of a polymer molecule is many times larger than the size of atoms of small molecules. As a result, in polymers the transition from surface to bulk behavior takes place over much broader regions. These can extend up to several hundreds of Angstroms. In addition, many interesting phenomena that involve polymer behavior take place at interfaces that are buried underneath the surface. They involve, for
Jan Genzer, NCSU
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example, the interfaces between various polymeric phases, polymer/non-polymer interfaces, etc. Some of them will be discussed later in the class. Thus in addition to the techniques for investigating the properties of polymers at the free surface, one needs experimental tools that can be used to provide information at various depths in the sample.
The knowledge of polymer behavior in the direction perpendicular to the sample surface can be gained by using depth-profiling techniques. In most cases, the information provided is the polymer concentration as a function of depth, known as the polymer concentration profile. The depth-profiling techniques can be further subdivided into two main groups depending on whether the concentration profiles are obtained in reciprocal or real space. The first category of techniques, called scattering techniques, include various techniques that use light (ellipsometry), X-rays (X-ray reflectivity) or neutrons (neutron reflectivity, small angle neutron diffraction) as the source of incoming radiation (particles). On the other hand, the real space depth-profiling techniques use primarily beam of accelerated ions that are directed onto the sample. The techniques in the second group include dynamic secondary ion mass spectrometry and ion beam techniques.
Crucial parameters that determine the applicability of the depth-profiling techniques are their sensitivity and particularly depth resolution. While the depth resolution of the
scattering techniques is excellent (several Angstroms), their sensitivity is limited to only abrupt changes in the concentration. In addition, the information about the depth distribution of polymers in the sample is obtained in the form of an optical transform. Due to its highly non-linear nature, the information from the reciprocal space cannot be directly converted into the real space. In practice, the data is analyzed by model fitting. Obviously, this evaluation method can lead to some uncertainty in the interpretation of the experimental results. This is because usually more than one model concentration profile can provide the same results. In most cases it is thus mandatory to have independent information about the concentration of the polymers in the sample. This can be achieved by analyzing the sample using real space depth-profiling techniques. In spite of their lower depth resolutions, as compared to the scattering techniques, the techniques based on the interaction of ions with the sample can thus prove useful in the analysis of polymers near surfaces and at the buried interfaces. Because the depth resolution of some of the ion beam techniques is as good as the dimension of a single polymer molecule, they can be used as standalone experimental tools. In addition to their excellent sensitivity (usually below 1 atomic percent), the real space depth-profiling techniques discussed here are not limited to just sharp concentration gradients, such as the scattering techniques.
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In-plane
Long-range
Short-range
Depth
Long-range
Short-range
In-plane
Depth
In-plane
Depth
In-plane
Depth – in conjunction with other methods
Classification of techniques
Order Geometry
Elemental analysis
Chemical state
Physical properties (topography,
hardness, etc.)
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Source: Charles Evans & Associates
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Table 1 Experimental techniques for polymer surface analysis
Technique Acronym Chemical information
Lateral resolution
Information content Note Ref.
Contact angle CA Rough estimate mm’s Surface hydrophobicity
[1]
Optical microscopy OM - > 1 µm Surface image [2]
Scanning near-field optical microscopy
SNOM - > 500 Å Surface topography [3]
Scanning electron microscopy
SEM Quantitative ≈ 50 Å Surface topography, surface image
a), c) [4]
Transmission electron microscopy
TEM In selected cases ≈ 30 Å Two dimensional profile
a), c), d) [5]
Atomic force microscopy AFM Possible ≈ 5 Å Molecular imaging, surface topography
[6]
Scanning tunneling microscopy
STM Possible ≈ 1 Å Molecular imaging, surface topography
a), b) [6,7]
Static secondary ion mass spectrometry
SSIMS Elemental ≈ 1 µm Surface composition a), c) [8,9]
X-ray reflectivity XR - - Surface roughness a), e) [10]
X-ray photoelectron spectroscopy
XPS (ESCA)
Chemical ≈ 10 µm Surface composition a), c) [4]
Near-edge X-ray absorption fine structure
NEXAFS Chemical ≈ 1 mm Surface composition, group orientation
c), e) [11]
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Table 1 Experimental techniques for polymer surface analysis (cont.)
Technique Acronym Chemical information
Lateral resolution
Information content Note Ref.
Photoemission electron microscopy
PEEM Chemical ≈ 50 Å Surface composition, group orientation
c), e) [12]
Infra-red attenuated total reflection
IR-ATR Semi-quantitative
mm’s Surface vibrational spectrum
[1]
High-resolution electron energy loss spectroscopy
HREELS - ≈ 1 µm Surface vibrational spectrum
a), c) [13]
Surface force apparatus SFA Indirect Nm’s Force between two surfaces
[14]
Johnson-Kendall-Roberts technique
JKR Indirect mm’s Force between two surfaces
[15]
Auger electron spectroscopy
AES Elemental ≈ 1 µm Surface composition, comp. topography
a), c), f) [4]
a) beam damage to specimen during the measurement b) requires conductive substrate c) technique requires high vacuum (10-5 - 10-9 torr) d) requires special sample preparation (microtoming, staining, etc.) e) possible sample damage, especially when a synchrotron is used to produce the source of radiation f) in connection with sputtering a depth profile of a chosen element can be obtained
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Table 2 Experimental techniques for polymer interfacial analysis
Technique Acronym Contrast Depth resolution
Information content Note Ref.
Pendant drop technique PDT - - Interfacial tension [16]
Transmition electron microscopy
TEM Staining ≈ 10 Å Cross-sectional view a), b), c) [5]
Atomic force microscopy AFM Different friction coefficients
≈ 5 Å Cross-sectional view [6]
Dynamic secondary ion mass spectrometry
DSIMS Elemental ≈ 90 - 130 Å Composition-depth profile
a), b), d) [17]
Rutherford backscattering spectrometry
RBS Elemental (Heavy atoms)
≈ 80 - 300 Å Marker depth profile a), b), g) [18,19]
Forward recoil spectrometry
FRES Elemental (1H, 2H)
≈ 800 Å 1H and 2H depth profiles
a), b) [20]
Time-of-flight forward recoil spectrometry
TOF-FRES
Elemental (1H, 2H)
≈ 250 Å 1H and 2H depth profiles
a), b) [21]
Low-energy forward recoil spectrometry
LE-FRES Elemental (1H, 2H)
≈ 130 - 400 Å 1H and 2H depth profiles
a), b), g) [22]
Nuclear reaction analysis NRA Elemental (1H, 2H)
≈ 50 – 300 Å 1H and 2H depth profiles
a), b), h) [23]
X-ray reflectivity XR Elemental (heavy atoms)
≈ 5 Å marker depth profile e), f) [10]
Neutron reflectivity NR Elemental (1H, 2H)
≈ 10 Å 1H and 2H depth profiles
f) [10]
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Table 2 Experimental techniques for polymer interfacial analysis (cont.)
Technique Acronym Contrast Depth resolution
Information content Note Ref.
Ellipsometry ELLI Refractive index ≈ 10 Å Film thickness, refr. index profile
f) [24]
X-ray photoelectron spectroscopy
XPS (ESCA)
Chemical ≈ 10 Å Composition vs. depth profile
a), b), d) [9]
Infrared densitometry IR-D Elemental (1H, 2H)
≈ 10 µm Composition vs.depth profile
[25]
Small angle neutron scattering
SANS Elemental (1H, 2H)
≈ 10 Å Composition vs.depth profile
f) [26]
a) beam damage to specimen during the measurement b) technique requires high vacuum (10-3 - 10-5 torr) c) requires special sample preparation (microtoming, staining, etc.) d) provides relative concentration profile only e) possible sample damage, especially when a synchrotron is used to produce the source of radiation f) provides optical transform of the measured profile; the direct conversion into the real space profile is not possible g) The depth resolution depends on the current set of parameters such as the beam type or/and energy, sample type and
geometry of the measurement h) The type of element to be profiled depends upon the applied technique
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References
[1] S. Wu Polymer Interface and Adhesion, Marcel Dekker: New York, 1982; A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press: Boston, 1991.
[2] L. C. Sawyer and D. T. Grubb, Polymer Microscopy, Chapman and Hall: New York, 1987.
[3] H. Bielefeldt, I. Hörsch, G. Krausch, J. Mlynek and O. Marti, Appl. Phys. A59 103 (1994).
[4] D. H. Reneker, in: H.-M. Tong and L. T. Nguyen, (Eds.), New Characterization Techniques for Thin Polymer Films, Wiley: New York, 1990, p. 327; M. C. Davies et al., in: W. J. Feast, H. S. Munro and R. W. Richards, (Eds.), Polymer Surfaces and Interfaces II, Wiley: New York, 1993, p. 203.
[5] E. L. Thomas, in: Encyclopedia of Polymer Science and Engineering, Vol. 5, Wiley: New York, 1986.
[6] S. Magonov and D. H. Reneker, Annu. Rev. Mater. Sci. 27 175 (1997); K. D. Jandt, Mater. Sci. Rep. 21, 221 (1998); S. Sheiko, Adv. Polymer Sci. 151, 61 (2000)
[7] E. Occhiello, G. Marra and F. Garbassi, Polymer News 14 198 (1989); O. Marti, in: O. Marti, M. Amrein (Eds.) STM and SFM in Biology, Academic Press: New York, 1993.
[8] D. Briggs, in: J. Allen and J. C. Bevington, (Eds.), Comprehensive Polymer Science, Vol. 2, Pergamon: Oxford, 1989; M. C. Davies et al., in: W. J. Feast, H. S. Munro and R. W. Richards, (Eds.), Polymer Surfaces and Interfaces II, Wiley: New York, 1993, p. 227.
[9] N. J. Chou, in: H.-M. Tong and L. T. Nguyen, (Eds.), New Characterization Techniques for Thin Polymer Films, Wiley: New York, 1990, p. 289.
[10] T. P. Russell, Mater. Sci. Rep. 5 171 (1990); M. D. Foster, Crit. Rev. Anal. Chem. 179 24 (1993).
[11] J. Stöhr, NEXAFS Spectroscopy; Springer-Verlag: Berlin, 1992.
[12] E. Bauer, Rep. Prog. Phys. 57, 895 (1994).
[13] J. H. Wandass and J. A. Gardella, Jr., Surf. Sci. 150 L107 (1985).
[14] J. Israelachvili, Intermolecular and surfaces forces, Academic Press: London, 1992.
[15] M. K Chaudhury, Mat. Sci. Eng. Rep. 16, 97 (1996).
[16] A. K. Rastogi and L. St. Pierre, J. Coll. Interface Sci. 31 168 (1969); S. H. Anastasiadis, J.-K. Chen, J. T. Koberstein, A. F. Siegel, J. E. Sohn and J. A. Emerson, ibid. 119 55 (1986).
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[15] G. Coulon, T. P. Russell, V. R. Deline and P. F. Green, Macromolecules 22 2581 (1989); S. J. Whitlow and R. P. Wool, ibid. 22 2648 (1989).
[16] W. K. Chu, J. W. Mayer and M. A. Nicolet, Backscattering Spectrometry, Academic Press: New York, 1978; L. C. Feldman and J. W. Mayer, Fundamentals of Surface and Thin Film Analysis, North-Holland: New York, 1986.
[17] E. J. Kramer, Physica B173 189 (1991).
[18] P. Mills, P. F. Green, C. J. Palmstrøm, J. W. Mayer and E. J. Kramer, Appl. Phys. Lett. 45 958 (1984); K. R. Shull, in: Physics of polymer surfaces and interfaces; I. C. Sanchez (Ed.); Butterworth-Heinemann: London, 1992; p.203.
[19] J. Sokolov, M. H. Rafailovich, R. A. L. Jones and E. J. Kramer, Appl. Phys. Lett. 74 590 (1989).
[20] J. Genzer, J. B. Rothman, and R. J. Composto, Nucl. Instrum. Meth. B86, 345 (1994).
[21] U. K. Chaturvedi, U. Steiner, O. Zak, G. Krausch, G. Schatz and J. Klein, Appl. Phys. Lett. 56 1228 (1990); R. S. Payne, A. S. Clough, P. Murphy and P. Mills, Nucl. Instr. Meth. Phys. Res., B42 130 (1989); D. Endisch, F. Rauch, A. Götzelmann, G. Reiter and M. Stamm, Nucl. Instr. Meth. Phys. Res., B62 513 (1992).
[22] R. M. A. Azzam and N. M. Bashara, Ellipsometry and Polarized Light, North-Holland, Amsterdam, 1977; H. G. Tompkins and W. A. McGahan, Spectroscopic ellipsometry and reflectometry – A user’s guide, John Wiley & sons: New York, 1999.
[23] S. L. Hsu, in: J. Allen and J. C. Bevington, (Eds.), Comprehensive Polymer Science, Vol. 2, Pergamon Press: Oxford, 1989, p. 429.
[24] T. P. Russell, et. al., Macromolecules 28 787 (1995).
General references
[1] H. M. Tong and L. T. Nguyen, (Eds.), New Characterization Techniques for Thin Polymer Films, John Wiley: New York, 1990.
[2] T. P. Russell, Annu. Rev. Mater. Sci. 21 249 (1991).
[3] M. Stamm, Adv. Polym. Sci. 100 357 (1992).
[4] M. Tirrell, and E. E. Parsonage, in: R. W. Cahn, P. Haasen and E. J. Kramer, (Eds.), Materials Science and Technology, Vol. 12, E. L. Thomas, Structure and Properties of Polymers, VCH: Weinheim, 1993, p. 653.
[5] N. J. Chou, S. P. Kowalczyk, R. Saraf and H.-M. Tong (Eds.) Characterization of Polymers, Butterworth-Heineman: Boston, 1994.
[6] A. Ulman (Ed.), Characterization of organic thin films, Butterworth-Heinemann: Manning,1995
[7] J. P. Sibilia (Ed.), A guide to materials Characterization and chemical analysis, VCH Publishers: New York, 1996.
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Selected web links on surface science techniques Charles Evans & Associates
http://www.caa.com/
Surface Science Techniques (UK ESCA Users Group) A truly outstanding and a must-visit site!!! http://www.ukesca.org/tech/list.html
Surface Science Western http://www.uwo.ca/ssw/
Surface science links in the UK http://www.chem.qmw.ac.uk/surfaces/
Other surface science techniques links http://www.eng.uc.edu/~vs/surface.html http://www.ags.uci.edu/~kng/surface_science.htm http://www.omicron-instruments.com/surf_web.html
http://www.physics.ucdavis.edu/stm/terms.htm http://www.dmoz.org/Science/Methods_and_Techniques/Microscopy/
plus many more !!!
Experimental probes
Technique Acronym Chemical information Lateral resolution Information content
Contact angle CA Rough estimate mm’s Surface hydrophobicity
By no means a complete list, only some of the most widely probes used…
Contact angle CA Rough estimate mm s Surface hydrophobicity
Optical microscopy OM - > 1 μm Surface image
Scanning near-field optical microscopy
SNOM - > 500 Å Surface topography
Scanning electron microscopy SEM Quantitative ≈ 50 Å Surface topography, surface imagesurface image
Transmission electron microscopy TEM In selected cases ≈ 30 Å 2-dimensional profile
Atomic force microscopy AFM Possible ≈ 5 Å Molecular imaging, surface topography
Scanning tunneling microscopy STM Possible ≈ 1 Å Molecular imaging, Scann ng tunn ng m croscopy S M oss Mo cu ar mag ng, surface topography
Static secondary ion mass spectrometry
SSIMS Elemental ≈1 mm (some are better)
Surface composition
X-ray reflectivity XR - - Surface roughness
X h t l t t XPS (ESCA) Ch i l 10 S f itiX-ray photoelectron spectroscopy XPS (ESCA) Chemical ≈ 10 mm Surface composition
Near-edge X-ray absorption fine structure
NEXAFS Chemical ≈ 1 mm Surface composition, group orientation
Photoemission electron microscopy PEEM Chemical ≈ 50 Å Surface composition, group orientation
Infra-red attenuated total reflection
IR-ATR Semi-quantitative mm’s Surface vibrational spectrum
High-resolution electron energy loss spectroscopy
HREELS Chemical ≈ 1 μm Surface vibrational spectrum
Surface force apparatus SFA Indirect Nm’s Force between two surfacespp
Johnson-Kendall-Roberts technique
JKR Indirect mm’s Force between two surfaces
Auger electron spectroscopy AES Elemental ≈ 1 μm Surface composition, compositional topography
What to start with? The simplest experiment…When thinking a set of surface-science experiments, always start with the simplest one:
wettability.
You’ll be amazed about how much information can be extracted from such an experiment!
The results of your wettability measurements can in fact influence your can n fact nfluence your next decision-making.
What sort of questions do I get answers for?
Is my surface hydrophilic or hydrophobic?Is my surface rough?
answers for?
Is this a chemical of physical roughness?Is the surface stable with time?Other questions?
Wettability measurementsEstablishes the wetting (and ultimately thermodynamic) properties of the substrate. For systems in equilibrium, there is a unique angle of the liquid at the contact line, θ, which follows Young’s equation
SLSVLV )cos( γ−γ=Θ⋅γwater on a superhydrophobic substrate
Rough classification of substrates based on wettability
water on a hydrophilic substrate
Rough classification of substrates based on wettability
hydrophilic substrate: θ < 90°hydrophobic substrate: θ > 90°superhdyrophobic substrate: θ >> 90°
T. Onda & al, Langmuir 12, 2125 (1996)
In reality, we measure two kinds of contact angles:advancing (drop advances on the surface) - θavd CA hysteresis: Δθ = θadv – θrec
Δθ (θ ) (θ )receding (drop recedes from the surface) - θrec
Causes of hysteresis: Chemical, physical heterogeneity, reorientation of surface groups, solubility of surfaces, interaction between the surface and the probing liquid, …
Δθ = cos(θadv) – cos (θrec)
Wettabilities of physically rough surfaces
Wentzel’s equation SLSVr)cos(γ
γ−γ⋅=Θ
r is the ratio between the actual surface area of the rough surface and the projected ( t) LVγ (apparent) area
⎥⎦
⎤⎢⎣
⎡+
γγ−γ
φ+−=Θ 11)cos(LV
SLSVA
φA is the fraction of the surface that remains in contact with the liquid. The remaining fraction (1-φA) is in contact with air
Cassie’s & Baxter’s equation ⎦⎣ ( φA)
Wettabilities of chemically “rough” surfaces
http://www.mysterra.org/Reportage_Cadres_Images/0096C05.htm
( )[ ] ( )[ ] ( )[ ]2222
112
mix cos1cos1cos1 Θ+φ+Θ+φ=Θ+
Israelachvili’s & Gee’s equation
( ) ( ) ( )2211mix coscoscos Θφ+Θφ=Θ
Cassie’s & equation
φ1 and φ2 (=1-φ1) are the fractions of the surface having a contact angle θ1 and θ2, respectively.
Which of these approaches really work? Dunno… it really varies from case to case.
Determining the surface energies from wettability dataPrepare a mixed self-assembled monolayer (SAM) on Au-covered substrate by co-adsorbing HS(CH2)17CH3and HS(CH2)15COOH from THF and measure their wettabilities using two liquids, e.g., water (1) and dii d th (2)diiodomethane (2)
Evaluate the surface energy of the SAM by geometric mean approximation
110120 probing liquid: H
2O 80 )50dispersive component
708090
100110 CH
2I
2
eg) 60
70
AM
(mJ/
m2 )
J/m
2 )
10
20
30
40polar component
3040506070
θ (
de
30
40
50 γ SA
xHS(CH
2)
15COOHγ SA
M
(mJ
0.0 0.2 0.4 0.6 0.8 1.0
0
0.0 0.2 0.4 0.6 0.8 1.0102030
x0.0 0.2 0.4 0.6 0.8 1.0
20
30
xxHS(CH
2)
15COOH
xHS(CH
2)
15COOH
Geometric mean approximation
( )[ ] [ ]ps
pl1
ds
dl1
pl2
dl11 2cos1 γγ+γγ=γ+γΘ+ ( )2SVLVβ-SV e21cosΘ γ−γ⋅
γ+−=
Another EOS…
JG, unpublished
( )[ ] [ ]sl1sl1l2l11 γγγγγγ
( )[ ] [ ]ps
pl2
ds
dl2
pl2
dl22 2cos1 γγ+γγ=γ+γΘ+
LVγ
β=0.0001247 (m2 mJ-1)2
D.Y. Kwok & A.W. Neumann, Coll. Surf. A 161, 49 (2000)
Wettability and Zisman plotSo-called “critical surface tension” (γC) is determined by measuring wettabilities using a homologous series of liquids (typically alkanes), plotting cos(θ) vs. γLV and extrapolating to cos(θ)=1.
1.0 f = 0 f = 0.103 f = 0.202
SAM made of HS(CH2)17CH3 / HS(CH2)15COOH mixtures (f=content of HS(CH2)15COOH)
0.8
0.9
0.6
0.7
γc = 19.75 (mJ/m2) γ
c = 19.15 (mJ/m2)
20 25 30γ
c = 21.74 (mJ/m2)
os(θ
)
20 25 30
0 8
0.9
1.0 f = 0.301
hexacedane 27.6 mJ/m2
dodecane 25.4 mJ/m2
f = 0.401co
0 6
0.7
0.8 dodecane 25.4 mJ/m
undecane 24.8 mJ/m2
nonane 22.9 mJ/m2
heptane 20.2 mJ/m2
2 2
surface tension (mJ/m2)20 25 30
0.6 γc = 23.62 (mJ/m2)
20 25 30γ
c = 24.21 (mJ/m2)
JG, unpublished
EllipsometryA non-destructive metrology technique for thin films and bulk materials. It utilizes polarized light to characterize material properties.
When polarized light is reflected from a surface, it undergoes a change in polarization, r, that is measured by the ellipsometer. This change can be related back to the sample properties (film thickness, refractive index of each material, etc).
The basic ellipsometry measurement is expressed as: ( ) iΔp eΨtanR
ρ ⋅==
where Rp and Rs are the Fresnel reflection coefficients in the p- and s- planes, respectively. This equation illustrates that the parameters measured in ellipsometry (Ψ, Δ) are ratios. Because of this, the ellipsometry measurement is typically very accurate and is much less dependent on the fluctuations in
he bas c ell psometry measurement s expressed as ( )s
eΨtanR
ρ
http://www.future-fab.com/documents.asp?grID=216&d_ID=1251
ellipsometry measurement is typically very accurate and is much less dependent on the fluctuations in absolute intensity (unlike reflectometry). The “delta” term in the ellipsometry measurement is highly sensitive to the surface condition and provides high precision when measuring ultrathin films (0-10 nanometers).
Attenuated total reflectance infrared spectroscopy (ATR/IR)In the mid-infrared, absorption of radiation is related to fundamental vibrations of the chemical bonds. Internal reflection spectrometry provides information related to the presence or absence of
ifi f ti l ll th h i l t t f fspecific functional groups, as well as the chemical structure of surfaces.
Absorption bands are assigned to functional groups (e.g., C=O stretch and C-H bend).
Shifts in the frequency of absorption bands and changes in relative band intensities indicate changes in the chemical structure or changes in the sample environment.
IR radiation is focused onto the end of the internal reflection element (IRE), ZnSe or Ge. Light enters the IRE and reflects down the IRE and reflects down the length of the crystal.
At each internal reflection, the IR radiation penetrates a short distance (~1 μm) from ( μ ) fthe surface of the IRE into the sample. This enables one to obtain infrared spectra of samples placed in contact with the IRE.
http://www.micromemanalytical.com/ATR_Ken/ATR.htm
http://www.spectroscopynow.com/Spy/pdfs/eac10815.pdf
Infrared spectroscopy (IR) vs. Raman spectroscopyBoth techniques belong to the family of vibrationalBoth techniques belong to the family of vibrational
IR spectroscopy Raman spectroscopy
Absorption mode Scattering modeyspectrosopies…
…but there are a few differences…
yspectrosopies…
…but there are a few differences…
Absorption mode Scattering mode
To be visible in IR, vibration must change the molecule DIPOLE MOMENT (or change distribution)
To be visible in Raman, vibration must change the molecule POLARIZABILITY
Detection of polar bonds (C-O, C=O, etc)
Detection of carbon allotropes and polarizable bonds (C-C, C=C,etc)A comparison of IR and Raman spectra
of 2,5-dichloroacetophenoneI t t tiInstrumentation
in.materials.drexel.edu/blogs/280_advanced.../3738.ashx
Scanning probe microscopies T. Kajiyama & al, Prog. Surf. Sci. 52, 1 (1996)
A “probe” tip is brought very close to the specimen surface, and the interaction of the tip with the region of the specimen tip with the region of the specimen immediately below it is measured. The type of interaction measured defines the type of SPM:
Atomic Force Microscopy (AFM)Lateral Force Microscopy (LFM)
AFMLateral Force Microscopy (LFM)Chemical Force Microscopy (CFM)Magnetic Force Microscopy (MFM)Electric Force Microscopy (MFM)Scanning Viscoelastic Microscopy (SVM) Scanning Viscoelastic Microscopy (SVM) (more?)
The KEY to the SFM functionality is the tip
LFM
is the tip
SVM
CFM
Comparison of scanning probe microscopies with other techniques
SPM vs. Scanning Tunneling Microscope (STM)I th l ti f STM i b tt th AFM b f th ti l In some cases, the resolution of STM is better than AFM because of the exponential dependence of the tunneling current on distance. The force-distance dependence in AFM is much more complex when characteristics such as tip shape and contact force are considered. STM is generally applicable only to conducting samples while AFM is applied to b th d t d i l t I t f tilit th AFM i F th th both conductors and insulators. In terms of versatility, the AFM wins. Furthermore, the AFM offers the advantage that the writing voltage and tip-to-substrate spacing can be controlled independently, whereas with STM the two parameters are integrally linked.
SPM vs. Scanning Electron Microscope (SEM)AFM provides extraordinary topographic contrast direct height measurements and unobscured views of surface features (no coating is necessary).
SPM vs. Transmission Electron Microscope (TEM)Three dimensional AFM images are obtained without expensive sample preparation and yield far more complete information than the two dimensional profiles available from cross-sectioned samples.
AFM O ti l Mi (OM)AFM vs. Optical Microscope (OM)AFM provides unambiguous measurement of step heights, independent of reflectivity differences between materials.
A few examples…Consider you can make a surface like this one…
How do you probe in-plane chemistry?
How do you probe in-plane chemistry?
CFM
(A) Optical image after water condensation (A) Optical image after water condensation (on –COOH regions)
(B) SFM topography – flat!
(C) CFM recorded with CH terminated tip (C) CFM recorded with –CH3 terminated tip higher friction with –CH3 surfaces (black)
(D) CFM recorded with –COOH terminated (D) CFM recorded with COOH terminated tip higher friction with –COOH surfaces (white)
“Sky is the limit” for the number of combinations (depending on the tip chemistry)
“Sky is the limit” for the number of combinations (depending on the tip chemistry)
Nanotemplating with BCP films
Single molecule imaginglDNA polymer
J.P. Spatz & al, Macromolecules 33, 150 (2000).
Homopolymer ordering on chemically structured substratesPattern size >> spinodal wavelength Combine AFM + polymer removal using selective
l t solvents
PS/PVP PS/PBrS80 x 80μm2
30 x 30μm2
Substrate:Prepared by μCP of H3C-terminated thiol μm2 μm2Prepared by μCP of H3C terminated thiol on gold
PS removed by cyclohexanePVP removed by methanol
**
n
N
**
n
PS PVPN
*
PS - light gray PBrS - dark gray
* y x nco
M. Böltau & al, Nature 391, 877 (1998).
BrPBrS
Ternary homopolymer blendsPS / PMMA / PVP
(a) PS/PMMA/PVP film (1:1:1) cast from THF onto a SAM-covered surface; (b) after immersion in ethanol to (a) PS/PMMA/PVP film (1:1:1) cast from THF onto a SAM covered surface; (b) after immersion in ethanol to remove PVP; (c) after removal of PS by dissolution in cyclohexane. The PMMA phase forms a quasi-two-dimensional network which separates the PS and PVP domains.
S. Walheim, & al, Langmuir 15, 4828 (1999)
Crystal quartz microbalance (QCM)Real time monitoring of adsorption to surfaces and structural (viscoelastic) properties of thin films provides complete understanding of binding p p g gevents (molecular interaction) and structure of the film.
How does it work?
A quartz is a piezo. By applying a voltage between the 2 electrodes the crystal will oscillate to a defined frequency frequency.
When molecules adsorb on the surface, the resonance f f th t l
Adapted from: http://www.ksvltd.com/sitellite/pdf/brochures/14_QCM-Z500.pdf
frequency of the crystal will decrease, proportionally to the added mass.
Crystal quartz microbalance (QCM)QCM facilitates monitoring of multiple interfacial events
Langmuir 14, 729-734 (1998)
a adsorption of human serum albumin (HAS) – protein in h bl d f (f human blood - on surface (f decreases)
b rinsing the substrate (to HSA)remove excess HSA)
c adding antibody against HAS (f decreases, increase in D i di t f ti f indicates formation of non-rigid complex)
d rinsing with buffer removes l l b d d tib dloosely bonded antibody
Mass sensitivityin air (1 bar) ≈0.2 ng/cm2
in water (25 °C) ≈0.9 ng/cm2
Light incoming onto glass/water interface the from a higher index
Surface plasmon resonance (SPR)
gof refraction medium (glass) is totally reflected under a certain angle.
So-called evanescent wave propagates into the medium (a few tens of nm).
When a thin layer of Au (other materials as well) is placed between glass and water an evanescent glass and water an evanescent plasmon wave propagates along the metal-dielectric interface.
Under certain conditions (angle of incidence, wavelength, polarization, metal thickness) the
Under certain conditions (angle of incidence, wavelength, polarization, metal thickness) the metal thickness) the reflected light is reduced sharply (surface plasmonresonance) due to resonance
metal thickness) the reflected light is reduced sharply (surface plasmonresonance) due to resonance energy transfer between the surface plasmon and the evanescent wave.
energy transfer between the surface plasmon and the evanescent wave.
Surface plasmon resonance (SPR) Measure changes in:refractive index, or layer thickness in the vicinity of
Measure changes in:refractive index, or layer thickness in the vicinity of layer thickness in the vicinity of the metallic interface
monitor as a shift in the resonance curve.
layer thickness in the vicinity of the metallic interface
monitor as a shift in the resonance curve.
Benefits: Di t t f ti l Benefits: Di t t f ti l
Attachment of thiols (R-SH) to gold
Direct measurement of optical constants of the dielectric layer above the metal.
Direct measurement of optical constants of the dielectric layer above the metal.
Measure (indirect) of the change in in mass resulting from adsorption/desorption of biomolecules
Measure (indirect) of the change in in mass resulting from adsorption/desorption of biomoleculesbiomolecules.
Measure binding constants of ligand affinity (i.e.,
biomolecules.
Measure binding constants of ligand affinity (i.e., g y (antigen/antibody)g y (
antigen/antibody)
Read more on http://en.wikipedia.org/wiki/Surface_plasmon_resonance
X-ray photoelectron spectroscopy (XPS)Sometimes also called Electron Spectroscopy for Chemical Analysis (ESCA)
U b i h l d h i f h i i ll i d Uses an x-ray beam to excite the sample and measure the energies of characteristically emitted photoelectrons. Information on the elements and their chemical bonding allows the identification of functional groups and molecular types
Analyzes the outermost two through ten atomic layers, although greater depths can be investigated na yzes the outermost two through ten atom c ayers, a though greater depths can e n est gated through ion etching
Sensitive to as low as 0.1 atom percent and detects elements except H and He. It can be applied to all solid materials, including insulators such as polymers and glasses
The specimen surface is irradiated with x-rays of known energy (most commonly 1486 or 1254 eV), causing electrons to be ejected from the surface as shown above The kinetic energy (KE) surface as shown above. The kinetic energy (KE) of these so-called photoelectrons is measured using a hemispherical analyzer. Binding energy (BE) values are calculated using the following equation:
KE = hν - BE
The binding energy is characteristic of both the electronic shell and the oxidation state of the atom from which the electron was ejected. Therefore, XPS can be used for both elemental and chemical analysis of the sample surface. There is no matrix effect and by applying sensitivity factors a quantitative analysis can be obtained
www.iac.bris.ac.uk/techniques/xps.html
X-ray photoelectron spectroscopy (XPS)http://cmm.mrl.uiuc.edu/techniques/xps.htm
http://www.ktr.co.jp/topics/topi_xps.htm
Here are a few examples of examples of typical XPS
(ESCA) scans
Note the t di extraordinary chemical
selectivity
http://www-ssrl.slac.stanford.edu/nexafs.htmlComparison with other techniques, such as NEXAFS reveals
the complementarity of both techniques. More on this later.
http://nb.engr.washington.edu/
X-ray photoelectron spectroscopy (XPS)
α⋅λ= sin3dProbing depth
α … take-off angleTypical values:Typical values:λ ≈ 3 nmd ≈ 9 nm (α=90°)d ≈ 3 nm (α=20°)
Example: Distribution of fluorocarbon in polymer brushes after post-polymerization reaction
A few words about x-ray spectroscopy When x-rays pass through any sort of material, a proportion of them will be absorbed. Measuring the amount of absorption with increasing x-ray energy reveals so-called edge structures, where the level of absorption suddenly increases. This happens when an x-ray has sufficient energy to free or excite a bound electron within the material. Usually, small oscillations can be seen superimposed on the edge step. These gradually die away as the x-ray energy increases. The oscillations, which occur relatively close to the edge (within about 40 eV), are known as NEXAFS (near edge x-ray absorption fine structure) or XANES (x-ray absorption near edge structure).
Near-edge X-ray absorption fine structure (NEXAFS) spectroscopySoft X-rays (E<1000 eV) - sharp 1s σ* and 1s π* transitions for elements commonly found in organic mater (e.g., C, F, O);
Characteristic electron (Auger, secondary) and fluorescence signals are associated with the 1s→σ* and 1s→π* transitions of a bond
Partial electron yield and fluorescence yield signals provide y f y g pinformation about surface and bulk orientation and chemistry
NEXAFS intensity depends on the
strong NEXAFS depends on the
orientation of the x-ray polarization vector (E) with
NEXAFS signal
weak
probing depth≈2 nm ≈100 nm
respect to a bond’s antibonding orbital
weak NEXAFS
signal
NEXAFS is very sensitive to the bonding environment of the absorbing atom. The NEXAFS spectrum exhibits considerable fine structure above each elemental absorption edge. This fine structure arises from
i i i i d l l bi l I l d i
What can NEXAFS do for us?
excitations into unoccupied molecular orbitals. In a related picture one can also think of the resonances arising from scattering of the excited low-energy photoelectron by the molecular potential. This structure is considerably larger than the higher energy extended x-ray absorption fine structure (EXAFS), which is extremely weak in polymers. EXAFS is due mostly to single scattering events of a high-energy photoelectron due mostly to single scattering events of a high-energy photoelectron off the atomic cores of the neighbors while NEXAFS is dominated by multiple scattering of a low-energy photoelectron in the valence potential set up by the nearest neighbors.
Courtesy of Joachim Stöhr (right)(http://www ssrl slac stanford edu/stohr/)(http://www-ssrl.slac.stanford.edu/stohr/)
Courtesy of Harald Ade (below)
Often one can use a spectral "fingerprint" technique to identify the local bonding environment. The spectra shown here exhibit chemical shifts within each group similar to XPS spectra but more importantly considerably different fine structure p y yfor carbon in different molecular groups. This clearly illustrates the power of NEXAFS to distinguish chemical bonds and local bonding. In many ways it is superior to XPS, which does not provide local structural information.
X
nα
E
σ*
(LEFT) Consider a semifluorinated alkane (molecule where all hydrogens have been replaced with fluorine) and look at the orientation of a single C-F bond.
X-rays<τ> X-rays
θ
<τ>
θ
σ
substrate
7σ*
θ = 90°.u.)
θ
0.8
1.0
α =.u.)
substrate
5
6 σ*
C-C
σC-F θ = 90
θ = 20°
ensi
ty
(a.
0.6
α = 90°
= 0°
sity
sig
nal
(a
3
4
σ*
C-Hon y
ield
int
0.2
0.4
σ* in
tens
1
2
tial
ele
ctro
0 10 20 30 40 50 60 70 80 90
0.0
θ (deg)
285 290 295 300 305 3100Pa
rt
Photon energy (eV)
(RIGHT) This is what the real NEXAFS spectra colleted from this molecule look like. Look at the angular dependence of the C-F and C-C signals.
A few details about NEXAFS: “NEXAFS 101”
1.0 α = Soft X-rays (E 1000 V)
0.6
0.8
α = 0°
α = 90°
ty
(a.u
.)
(E<1000 eV) -sharp 1s σ*and 1s π*transitions for l m ts
0.2
0.4 α = 54.5°
Inte
nsitelements
commonly found in organic mater (e.g., C, F, O);
0 10 20 30 40 50 60 70 80 90
0.0
θ (deg)NEXAFS intensity depends on the orientation of the x-ray polarization vector (E) with respect to b d’ ib di bi l
Characteristic electron (Auger,
d ) d
a bond’s antibonding orbital
strong NEXAFS
secondary) and fluorescence
signals are associated with
h 1 * d
signal
weak the 1s→σ* and 1s→π*
transitions of a bond
probing depth≈2 nm ≈100 nm
weak NEXAFS
signal
6
7σ*
C-F θ = 90°(a.u
.)
<τF8> X-rays <τF8>
A few MORE details about NEXAFS: “NEXAFS 201”
4
5
6 σ*
C-C
C F
θ = 20°
ld in
tens
ity
(X-rays
θ
F8 y
θ
F8
1
2
3 σ*
C-H
l ele
ctro
n yi
elθ
normal incidence geometry
grazing incidence geometry
285 290 295 300 305 3100
1
Part
ia
Photon energy (eV)
• NEXAFS cannot distinguish between all molecules pointing in the same direction and some average molecular gorientation
• We therefore assign an average orientation (<angle>).
vs.
• One exception is when the measured angle is close to 90° (all molecules must point in the same direction)
Secondary ion mass spectrometry (SIMS)
Surface spectroscopy
a solid surface is bombarded by primary ions of a few keV energy. The primary ion energy is transferred to target atoms via atomic collisions; a so-called collision cascade is generated. Part of the energy is transported back to the surface allowing surface atoms p pyPart of the energy is transported back to the surface allowing surface atoms and molecular compounds to overcome the surface binding energy
Rastering
Depth-profiling
http://www.ion-tof.com/html/sims.html
Reflectivity or reflectometry (neutron or X-ray)Reflectivity/reflectometry is a technique for investigating the near-surface structure of materials. It probes the electron density (X-ray) or scattering length density (neutrons) with a depth resolution of less than one nm for depths of up to several hundred nm!than one nm for depths of up to several hundred nm!
The method involves measuring the reflected X-ray (neutron) intensity as a function of X-ray (neutron) incidence angle (typically small angles are used).
I l d i fil hi k d i h d h h l i It can accurately determine films thickness, density, average roughness, and the roughness correlation function.
Specular reflectivity:Th i id t b s i i th s l t s ll The incident beams impinge on the sample at a small angle Θ and the intensity of the specularly reflected X-rays is detected at an angle Θ from the surface; the scattering angle is 2Θ and the scattering vector is normal to the surface. Data are collected as function of Θ or equivalently Q = (4π/λ) sinΘ.
Diffuse (off-specular) reflectivity: The incidence angle is Θ-ω, while the reflected b d t t d t it l f Θ f th beams are detected at an exit angle of Θ+ω from the surface. In this situation, the scattering vector has a component parallel to the surface (Qx= (4π/λ) sinΘ sinω.). Measurements of the intensity as a function of Qx allow a determination of the lateral correlation
http://www-ssrl.slac.stanford.edu/materialscatter/scatter-reflect.html
f Q w m n n f nfunction of roughness or the lateral length scale of surface or interface roughness.
Model fittingfitting
http://www.ncnr.nist.gov/programs/reflect/
Fitting multi-layer films is much more challenging… Results are often ambiguous b/c of the inverse nature of the problem (one cannot typically convert the reflectivity curve from the reciprocal to direct space)of the problem… (one cannot typically convert the reflectivity curve from the reciprocal to direct space)
http://www.ansto.gov.au/ansto/bragg/hifar/nreflect_performance.html
Neutron and X-ray reflectivity
scheme
NR from a single (PS) component film [from T. P. Russell, Mater. Sci. Rep. 5 171 (1990)]
NR from a multicomponent (PS-b-PMMA) film [from S. H. Anastasiadis, J. Chem. Phys. 92 5677 (1990)]
Ion Beam techniques – basic terminology
Energy loss of ions (dE/dx) in materials
Stopping cross section
=ε
xd
dE
N
1
components:
• nuclear (low E)
• electronic (high E)
Scattering cross-section
Yield
SQN)(Y Ωψσ=
σ(ψ) scattering x-sectionΩ detector solid angleQ # of inc. particles NS
# of target atoms
Elastic collision of a projectile and a target
projectile target
MtMp
E1
E0
E2
Kinematic factor
Backscattering (Mp < Mt)
2
tp
p22
p2t
0
1
MM
cosMsinMM
E
E
+
ψ+ψ−=
Forward scattering (Mp ~ Mt)
( ) ξ+
= 22
tp
tp
0
2 cosMM
MM4
EE
ψ
ξ
Rutherford Backscattering Spectrometry (RBS) • Rutherford Backscattering (RBS) is based on collisions between atomic
nuclei • RBS is ideally suited for determining the concentration of trace elements
heavier than the major constituents of the substrate. Its sensitivity for light masses, and for the makeup of samples well below the surface, is poor.
• When a sample is bombarded with a beam of high energy particles, the vast majority of particles
are implanted into the material and do not escape. This is because the diameter of an atomic nucleus is on the order of 1 x 10-15 m while the spacing between nuclei is on the order of 2 x 10-10 m. A small fraction of the incident particles do undergo a direct collision with a nucleus of one of the atoms in the upper few micrometers of the sample. This “collision” does not actually involve direct contact between the projectile ion and target atom. Energy exchange occurs because of Coulombic forces between nuclei in close proximity to each other. However, the interaction can be modeled accurately as an elastic collision using classical physics.
• The energy measured for a particle backscattering at a given angle
depends upon two processes: 1) Particles lose energy while they pass through the sample, both before
and after a collision. The amount of energy lost is dependent on that material's stopping power.
2) A particle will also lose energy as the result of the collision itself. The collisional lost depends on the masses of the projectile the target atoms. The ratio of the energy of the projectile before and after collision is called the kinematic factor.
• The number of backscattering events that occur from a given element in a
sample depend upon two factors: the concentration of the element and the effective size of its nucleus. The probability that a material will cause a collision is called its scattering cross section.
How RBS works…
Kinematic factor
2
tp
22p
2tp
MM)90(cosMM)sin(M
k
+
φ−−+φ=
o
Detected energy
[ ]xSkEE 0det −=
(spectrum taken from the CEA website: www.cea.com)
Polymer interdiffusion through marker movement1
• measure motion of Au particles sandwiched between two polymer films
• From the Au particle motion
determine polymer self-diffusion coefficient
1 E. J. Kramer et al., Polymer 25, 473 (1984).
Diffusion of iodine into polymer photoresist2
• As diffusion time increases, iodine diffuses into polyimid
• Depending on the nature of the
n-iodoalkane (CH3I – left, C5H11I - right), the diffusion is either Fickian (left) or non-Fickian Case II (right)
2 P. J. Mills and E. J. Kramer, Polymer 25, 473 (1984).
Forward recoil spectrometry (FRES) [sometimes called Elastic recoil detection -ERD]
• uses heavy projectiles (Mp) to forward recoil light elements (Mr), Mp > Mt
polymer film
4He+
E0 = 2.7 MeV
4He
1H and 2Hdetector
θ
φ= 150°
Mylar
Si substrate
EdetEfoil
Eout
Kinematic factor
( ) ( )φ−+
⋅⋅= 1cos
MMMM4
k 22
pr
pr
Stopping power @ x
)90sin(S
)90sin(Sk
]S[ rp
φ−θ++
θ−⋅
=
Sp…projectile stopping power Sr…recoil particle stopping power
Detected energy
foil0det Ex]S[EkE ∆−⋅−⋅=
Yield
x
detx
dxdE)90cos(
E),E()x(NeQ
Y
⋅θ−
δ⋅Ω⋅φσ⋅=
image source: courtesy of E. J. Kramer, UC Santa Barbara
Ex.: Study of phase separation in thin polymer films1
Sample geometry FRES Spectrum
subs
trate
poly
mer
B-(
PEP)
poly
mer
A-(
dPEP
)
subs
trate
0.0
0.2
0.4
0.6
0.8
1.0
SAM
-CO
OH
dPEP-rich phasePEP-rich phase
-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.20.0
0.2
0.4
0.6
0.8
1.0
Poly
mer
vol
ume
fract
ion
Depth / Sample thickness
SAM
-CH
3
dPEP-rich phasePEP-rich phase
“conventional” FRES is good to depth profile features ≈ 80 nm
But what if we want to measure something smaller?
1 J. Genzer and E. J Kramer, ", Phys. Rev. Lett. 78, 4946 (1997).
Improvements to the FRES depth resolution2 1. Time-of-flight FRES
carbon foile-
detector
4He+, E0
4He, 1H, 2H
∆x
30°
stop signal
sample
TOF00:00:00
depth resolution of TOF-FRES ≈ 25 nm
2 J. Sokolov, M. H. Rafailovich, R. A. L. Jones and E. J. Kramer, Appl. Phys. Lett. 74
590 (1989).
2. low energy FRES3
• same geometry as conventional FRES with 2 improvements:
1. lower E0 down to 1.3 MeV
2. vary the sample tilt angle θ (glancing geometries)
q
4He+beam
D, H, He
q
D, H, He4He+beam
θθθθ < 75° θθθθ > 75°
sample sample
Glancing geometries
How do we test if this idea works?
Test sample design
deuterated polystyrenepolystyrene
Si substrate
thicknesses of the individual polymer layers ca. 180 - 330 Å
3 J. Genzer, J. B. Rothman and R. J. Composto, Nucl. Instrum. Meth. Phys. Res. B86
345 (1994).
Step 1
Step 2
LE-FRES vs. “conventional” FRES1
800 1000 1200 1400 16000
100
200
300
400
500
600
deuterium
hydrogen
10.3 µm Al stopper foil E0 = 3.0 MeVY
ield
(c
ount
s)
Energy (keV)
Yie
ld
(cou
nts)
Energy (keV)
200 300 400 500 6000
50
100
150
200
250
300
deuterium
hydrogen
4.5 µm Mylar stopper foil E0 = 1.3 Me
50
100
150
200
250
3003.0 µm Mylar
hydrogen
θ = 62.5° (ca.130 Å)θ = 65.0° (ca.190 Å)θ = 70.0° (ca.250 Å)
E0=1.3 MeV
Yie
ld
(cou
nts)
deuterium
depth resolution of 1 J. Genzer, J. B. Rothman and R. J. Composto, Nucl. Instru
Step 1
Step 2
1800
700
V
350 400 450 500 550 600 650 7000
Yie
ld
(cou
nts)
Energy (keV)
Energy (keV)
100 200 300 400 500 600 7000
100
200
300
4004.5 µm Mylar
deuterium
hydrogen
θ = 80.0° (ca.185 Å)θ = 82.5° (ca.160 Å)θ = 85.0° (ca.140 Å)
LE-FRES can be as good as ≈ 13 nm
m. Meth. Phys. Res. B86 345 (1994).
Nuclear reaction analysis (NRA) • We distinguish resonant and non-resonant NRA • Non-resonant nuclear reaction for depth profiling 2H in thin polymer films
Reaction cross-section
quite broad ! non-resonant NRA
adopted from: Möller et al, NIM 168, 111 (1980)
E0
E1
E2
1H
2H
4He
3He
3He + 2H → 4He + 1H + Q Q = 18.352 MeV
Experimental scheme
Reation products: Typical depth resolution when 4He is used as a detection particle: Surface: 14 nm FHWM 100 nm: 25 nm FHWM
(very good!) disadvantages:
• low sensitivity • limited to the above
nuclear reaction
Ex. Diffusion in PS/d-PS system1
"
Resonant NRA:
• For example, detection of 1H using 1H(15N, 4He γ)12C (Ref. 2)
• Better depth resolution (> 5 nm) but rather cumbersome to operate 1 U. K. Chaturvedi, U. Steiner, O. Zak, G. Krausch, G. Schatz and J. Klein, Appl. Phys.
Lett. 56 1228 (1990) 2 D. Endisch, F. Rauch, A. Götzelmann, G. Reiter and M. Stamm, Nucl. Instr. Meth.
Phys. Res., B62 (1992) 513.
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