Theta Probe: A tool for characterizing ultra thin films and self assembled monolayers using parallel angle resolved XPS (ARXPS)

Theta Probe: A tool for characterizing ultra thin films and self assembled monolayers using parallel angle resolved XPS (ARXPS) C. E. Riley, P. Mack, T. S. Nunney and R. G. White Thermo Fisher Scientific

2 Contents

Introduction Angle Resolved XPS

Introduction ARXPS provides 3 types of non-destructive depth information:- 1. Relative depth plots (RDPs) Using logarithmic ratios 2. Film thickness measurement of single and multiple overlayers Using derivatives of the Beer-Lambert Equation, I = I exp(-d/λcosθ) 3. Reconstructed depth profiles Using Maximum Entropy Methods ARXPS measures electron signals at different angles from sample by sample tilting (regular ARXPS) by parallel angle detection (Theta Probe ARXPS) 4

Attenuation Length, λ, in electron spectroscopy Each data point represents a different element or transition 0.5 λ ~ E Ref.: M. P. Seah and W.A. Dench, Surface and Interface Analysis 1 (1979) 2 Photoelectron peak intensity as a function of depth 65% of the signal from <λ 85% from <2λ 95% from <3λ (typically < 9nm) at zero degrees to sample normal 5

Collection Angle and angle resolved XPS (ARXPS) 0 o 60 o 60 o 4.5 nm 9.0 nm Information depth varies with collection angle I = I exp(-d/λcosθ) 95% intensity from 3λcosθ SiO 2 on Si, gate oxide Alkane thiol SAM on Au Spectra from thin films on substrates are affected by the collection angle 6

Thermo Scientific Theta Probe XPS and PARXPS Monochromated XPS Non-destructive, surface sensitive technique (0-9nm depth) Elemental identification and quantification Chemical bonding identification and quantification Parallel angle resolved XPS (PARXPS) Depth distribution information non-destructively Molecular bonding orientation 60 collection angle (20-80 ) NO tilting the sample Two Dimensional Detector Measures Energy and Angle simultaneously 112 channels for snapshot spectroscopy 96 angle channels 7

Parallel angle resolved XPS Full range of angles collected simultaneously Fast parallel acquisition No sample tilting Advantages for constant transmission No change in analysis area No change is sample height off the tilt axis 2-D Detector snapshot image 112 energy channels - collected simultaneously 96 angle channels - collected simultaneously - banded into 16 Spectra - 3.75 o interval from 20 o to 80 o 8

ARXPS yields depth information non-destructively ARXPS provides 3 types of non-destructive depth information:- 1. Relative depth plots (RDPs) Using logarithmic ratios 2. Film thickness measurement of single and multiple overlayers Using derivatives of the Beer-Lambert Equation, I = I exp(-d/λcosθ) 3. Reconstructed depth profiles Using Maximum Entropy Methods All 3 methods are integrated within Avantage Data System 9

Treatment of ARXPS Data 1. Relative Depth Plot Construction: Collect ARXPS spectra For each element, calculate: I ln I SurfaceAngle BulkAngle Information Reveals the ordering of the chemical species Advantages Fast Model independent, no assumptions Limitation No depth scale Provides Information about layer ordering Surface Increasing depth Bulk Relative depth plot from silicon oxide on Silicon substrate: C 1s on top surface (contaminant) Oxidised Si 2p at surface Elemental Si 2p at substrate 10

Treatment of ARXPS Data 2a. Thickness Calculation Two layer model Signal from A I A = I A[1-exp(-d/λ A,A cos θ)] Signal from B I B = I B exp(-d/λ B,A cosθ) Ratio I I A B = R = R 0 1 exp exp where R 0 = I a/ I b Simplify If λ A,A = λ B,A = λ A then ln[1+r/ R 0 ] = d/(λ A cosθ) d λ cosθ A, A d λ cosθ B, A This assumption is suitable for an oxide on its own metal (e.g. SiO 2 on Si) ln(1+r/r ) 5 4 3 2 1 0 Silicon dioxide on silicon - 6 samples of varying thickness Plot: ln[1+r/ R ] vs. 1/cos(θ) Fit through the origin Gradient = d/λ 9.0 nm 6.4 nm 4.3 nm 3.6 nm 2.3 nm 1.9 nm 0 0.5 1 1.5 2 1/cos( θ ) 11

Thickness Calculation, comparison with ellipsometry SiO 2 on Si Excellent linearity Unity gradient Intercept at 0.9 nm because ellipsometry included AMC* in thickness ARXPS Measurements (nm) 10 8 6 4 2 y = 1.077x - 0.914 R 2 = 0.999 0 0 2 4 6 8 10 Ellipsometry Measurements (nm) *AMC = Airborne molecular contamination 12

XPS Thickness map of Graphene layers on SiO2 ptical Image Trilayer 5-6 4-5 3-4 2-3 1-2 0 Graphene layers Bilayer By using the 2 layer model, the attenuation of the Si signal reveals the thickness of the graphene sheet that the Si2p photoelectrons are passing through. This allows a thickness image of the surface to be generated, showing the number of layers present in each structure 13

14 Substrate n 2 1 The ratio of the ith peak to that of the substrate will be: (λ ij is the attenuation length of photoelectrons characteristic of layer i in layer j) The ratio of peaks between adjacent layers, i and i+1: Knowing R 0 and λ, fit the angle resolved data to obtain thickness of each layer, values for d = = = = = n j j i j j ij j sj j ii i i s i d d d R I I 1 1 1 0 cos 1 exp cos exp 1 λ λ θ θ λ = = = = = + + + + + + i j j i j j ij j j i j i i i ii i i i i i d d d d R R I I 1 1 1 1, 1 1, 1 0 1 0 1 cos 1 exp cos exp 1 cos exp 1 λ λ θ θ λ θ λ Treatment of ARXPS Data 2b. Multi Overlayer Thickness Calculator

Multi Overlayer Thickness Calculation Al 2 O 3 Growth curve in close agreement with TEM Ellipsometry Measured SiO 2 thickness independent of number of ALD cycles C Al 2 O 3 SiO 2 Si 15

Treatment of ARXPS Data 3. Depth Profile Generation Maximum Entropy Method Sample C1s Atomic Concentration (%) 100 80 60 40 20 0 HfO 2 Al 2 O 3 SiO 2 Si Ha4f O1s Al2p Si2pO Si2p Initial RDP (for reference) Non-destructive depth profile O 1s Hf 4f (Oxide) Al 2p (Oxide) Si 2p (Element) Si 2p (Oxide) Generate random trial Profile Take average profile from 5 cycles Hf4f O1s Al2p Si2pO Si2p 0 1 2 3 Depth (nm) Atomic Concentration (%) 100 80 60 40 20 0 O1s Hf4f Al2p Si2p Si2p(O) 0 1 2 3 Depth (nm) Repeat Process 20,000 x. choose most likley profile Determine error between experimental and calculated data Non-destructive depth profile consistent with RDP Relative intensity (arb. unit) 0.7 7 0.6 6 0.5 5 0.4 4 0.3 3 0.2 2 0.1 Hf4f Calculate expected ARXPS data (Beer Lambert Law) T j (θ) = exp(-t/λcosθ) O1s Si2p Al2p Si2p(O) 00 20 40 60 80 Angle ( o ) 16

Treatment of ARXPS Summary of 3 types 1. Relative Depth Plot (RDP) 2. Overlayer Thickness HfO 2 Al 2 O 3 SiO 2 0.7 nm 0.5 nm 0.8 nm Relative Intensity (%) 1. I ln I SurfaceAngle BulkAngle 0.7 ARXPS original data 0.6 0.5 O1s 0.4 0.3 Si2p 0.2 0.1 Al2p Si2p(O) 0 20 40 60 80 Angle ( ) Hf4f 2. Thickness Calculator 3.Maximum Entropy Si 3. Non-destructive Depth Profile Atomic Concentration (%) 100 80 60 40 20 0 O 1s Hf 4f (Oxide) Al 2p (Oxide) Si 2p (Element) Si 2p (Oxide) O1s Al2p Hf4f Si2pO Si2pE 0 1 2 3 Depth (nm) 17

ARXPS analysis of Graphene on SiC

ARXPS analysis of Graphene on SiC Analysis summary Theta Probe analysis summary Experimental The Thermo Scientific Theta Probe was used to analyse a sample of graphene on SiC The sample was mounted on the standard 70 x 70mm Theta Probe sample holder with conductive carbon tape The monochromated X-ray source was used for XPS analysis. This offers a selectable spot size from 15-400µm. The 400µm X-ray spot was used for higher sensitivity and rapid analysis. Angle resolved data was taken from the central point of the sample to obtain depth information from the sample Using the angle resolved data, it is possible to find the thickness of the graphene layer Thermo Scientific Theta Probe 19

Sample Analysis Angle resolved Bulk sensitive Angle resolved Surface sensitive Using the Theta Probe s unique angle resolved capabilities, information can be obtained from the sample non-destructively Angle resolved data was acquired for carbon, oxygen and silicon The 2D detector has 96 angle channels For analysis, these angle channels were binned into 16 discrete angle ranges of 3.75 angular resolution The higher the angle, the more surface sensitive that spectra are The data on the left is an example of how the carbon spectra change throughout the angle range. The highest angle and the lowest angle this can be seen on the next slide Proprietary 20 and confidential

Graphene on SiC As-received C1s spectra Bulk angle vs. Surface angle Bulk angle Surface angle Bulk vs. Surface angle The two spectra for the lowest angle and the highest angle are compared here Spectra normalised for clarity The higher the angle, the more surface sensitive that spectra are Graphene SiC From the bulk angle spectra it is possible to see a similar intensity of SiC to graphene Intensity On the surface angle spectra the intensity of SiC is much lower than the graphene As the ratio of SiC to graphene is much lower on the surface angle; this points to graphene being predominantly on the surface, with SiC the substrate 298 296 294 292 290 288 286 284 282 280 278 Binding Energy (ev) Proprietary 21 and confidential

Graphene on SiC As-received Relative depth plot Relative Depth Plot Peaks (Peaks) O1s C1s Graphene C1s SiC Relative depth plot Avantage software can produce a relative depth plot from ARXPS data Rapid and model free method for describing depth ordering of chemical states and elements Provides qualitative information The plot of the graphene on SiC sample shows clear structure Oxygen on the surface Graphene underneath SiC substrate Proprietary 22 and confidential

Graphene on SiC As-received ARXPS film thickness measurement Integrated Avantage layer thickness calculation software ARXPS film thickness measurement Avantage software for thickness analysis Avantage software, combined with ARXPS analysis on Theta Probe, can be used to measure the thickness of up to three layers on a substrate Example of film thickness recipe shown to left Thickness measurements for graphene Surface oxygen is a very low concentration. This points to there being a small amount of dilute oxygen spread out over the surface A density of 2.27 g/cm 3 was used for graphene. This is a reference density of graphite A band gap of 0.01eV was used for graphene Using these values and the obtained spectra the thickness of graphene can be calculated Graphene = 0.857nm 23

Graphene on SiC As-received ARXPS depth profile Relative Intensity (%) 100 90 80 70 60 50 40 30 20 10 ARXPS profile C graphene O SiC 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Depth (nm) ARXPS non-destructive depth profile Avantage data system allows concentrations of various elements/ chemical states to be constrained Results on previous slide were used to determine the ARXPS depth profile of graphene on SiC The sample was modelled as a mixture of graphene, oxygen and SiC No SiC on surface Small amount of oxygen on surface 0.857nm of graphene The 0.857nm of graphene is the approximate correct distance for two graphene layers on the surface of the sample The reconstructed profile shows the presence of oxygen at the surface, suggesting that there maybe some oxygen content in the first layer 24

ARXPS analysis of a Fluoropolymer Catheter

ARXPS Applications Fluropolymer Catheter Live optical view from Theta Probe camera Fluoropolymer catheter ARXPS from a curved, insulating surface Live optical view for easy alignment of sample Analysis area DOES NOT change as a function of photoemission angle Charge neutralisation conditions DO NOT change as a function of photoemission angle Depth distribution of carbon bonding states 26

ARXPS Applications Fluropolymer Catheter Live optical view from Theta Probe camera Fluoropolymer catheter ARXPS from a curved, insulating surface Live optical view for easy alignment of sample Analysis area DOES NOT change as a function of photoemission angle Charge neutralisation conditions DO NOT change as a function of photoemission angle Depth distribution of carbon bonding states CF 2 C1s spectrum Depth distribution of carbon bonding states Depth integrated carbon chemistry High energy resolution spectrum of C1s region shows carbon bonding states within total XPS sampling depth (~10 nm) Fluorocarbon states easily observed Excellent resolution due to high performance charge neutralisation system CF 3 C-O O-*C=O C-*C=O C-C 27

ARXPS Applications Fluropolymer Catheter Live optical view from Theta Probe camera Fluoropolymer catheter ARXPS from a curved, insulating surface Live optical view for easy alignment of sample Analysis area DOES NOT change as a function of photoemission angle Charge neutralisation conditions DO NOT change as a function of photoemission angle Depth distribution of carbon bonding states ARXPS C1s spectra Depth distribution of carbon bonding states Depth distribution of carbon chemistry ARXPS C1s spectra acquired simultaneously at all angles Constant charge neutralisation conditions at all angles Constant analysis area at all angles ARXPS data was peak fit with the components shown on the previous slide to generate a Relative Depth Plot Bulk Surface 28

ARXPS Applications Fluropolymer Catheter Live optical view from Theta Probe camera Fluoropolymer catheter ARXPS from a curved, insulating surface Live optical view for easy alignment of sample Analysis area DOES NOT change as a function of photoemission angle Charge neutralisation conditions DO NOT change as a function of photoemission angle Depth distribution of carbon bonding states Layer ordering of carbon bonding states Depth distribution of carbon bonding states Depth distribution of carbon chemistry Relative depth plot shows the layer ordering of elements and chemical states Method is model independent Instant conversion of ARXPS data into depth information CF 3 C-*C=O CF 2 C-C O-*C=O C-O Relative Depth Plot 29

Measuring the quality of Self-Assembled Monolayers of alkane thiols on gold

Alkane Thiol SAMs on Au for Biological Applications Functionalising SAMs grown on substrate surfaces Potential for well controlled design of biomaterials Modified functionalised groups for immobilisation of proteins, etc. Wide variety of potential applications eg Biosensors in diagnosis, lab-on-chip, micro-contact printing, etc Need reliable characterisation technique (XPS, ARXPS) Identification and quantification of the functional groups Probe chemistry of overlayer with nano-scale depth resolution Provide information about orientation and structure Au S Au Au Au Au S S S S Protein of interest O O CH 3 O O O O H 2 N SNAP O CH 3 3 NH O O 6 N NH N O CH 3 3 O CH 3 3 O CH 3 3 O CH 3 3 N S O Au Au S S We acknowledge Karlsruhe Institute of technology for the use of the diagram 31

Self-assembled monolayers Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules Theta Probe ARXPS measurement Experimental advantages Data from all angles comes from same analysis point Imaging ARXPS is possible, allowing film uniformity to be studied Rapid snapshot acquisition reduces X-ray spot dwell time Lower X-ray power on sensitive monolayer samples 3 mm Imaging ARXPS of undecane thiol sample damaged in transit 32

Alkane thiol self-assembled monolayers on Au Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules Nonanethiol Undecanethiol Self-assembled monolayer materials used in this work Dodecanethiol Hexadecanethiol Hydroxy undecanethiol 1-mercapto-11-undecyl-tri(ethylene glycol) Images from Asemblon TM, 15340 NE 92nd Street, Suite B, Redmond, WA 98052-3521, USA. www.asemblon.com 33

Alkane thiol self-assembled monolayers on Au Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules 2.5 Non-destructive ARXPS thickness measurement Thickness as a function of organic chain length Film thickness measured on Theta Probe Thickness increases linearly with organic chain length Layer Thickness 2 1.5 1 0.5 0 0 5 10 15 20 Number of Carbon Atoms Theta Probe measured layer thickness 34

Proposed mechanism of SAM growth LOW COVERAGE HIGH COVERAGE Reproduced from Asemblon Self-Assembled Monolayers (SAMs) Handbook At LOW COVERAGE we expect to observe a mixture of SAM bonding modes At HIGH COVERAGE we expect to see one type of bonding mode 35

Coverage versus bonding Atomic concentration maps of C, Au and S C Au Au Concentration of elements varies across sample Carbon / sulphur correlate well Three zones High C, high S, lower Au Mid C, mid S, mid Au Low C, low S, high Au We have a strongly changing coverage of undecanethiol self-assembled monolayer across sample C S S Undecanethiol 36

Coverage versus bonding Au Sulphur chemistry Sulphur spectrum from black shaded area on image [S B ]:[S A ] = 1 : 3.11 S A SAM S A Undecanethiol sulphur chemistry at HIGH COVERAGE XPS image has full sulphur spectrum at each pixel S B Retrospective spectroscopy of sulphur from shaded area Two chemical states of sulphur observed S B Sulphur chemistry diagnostic of SAM bonding modes 168 167 166 165 164 163 162 161 160 159 High proportion of S A compared to S B Binding Energy (ev) 37

Coverage versus bonding Au Sulphur chemistry Sulphur spectrum from black shaded area on image [S B ]:[S A ] = 1 :1.43 S A SAM S A S B Undecanethiol sulphur chemistry at LOW COVERAGE Increased proportion of S B at low coverage region of image PARXPS mapping allows us to acquire full angle resolved datasets at each pixel in the map S B Next slide shows sulphur spectra from LOW COVERAGE zone from bulk and surface sensitive angles 168 167 166 165 164 163 162 161 160 159 Binding Energy (ev) 38

Coverage versus bonding ARXPS analysis of sulphur bonding Angle resolved analysis from LOW COVERAGE zone Qualitative analysis of data indicates S B closer to top surface than S A Sulphur spectrum from bulk sensitive angle [S B ]:[S A ] = 1 : 2.00 S A S A Sulphur spectrum from surface sensitive angle [S B ]:[S A ] = 1 : 1.47 S A S A S B S B S B S B 168 167 166 165 164 163 162 161 160 159 168 167 166 165 164 163 162 161 160 159 Binding Energy (ev) Binding Energy (ev) 39

Coverage versus bonding ARXPS analysis of sulphur bonding Au Relative depth plot for LOW COVERAGE zone S B C SAM Undecanethiol bonding modes Angle resolved XPS information easily summarised as Relative Depth Plot Increasing relative depth O S A Shows molecular orientation for SAM bonding There are at least two bonding modes for undecanethiol at LOW COVERAGE, with thiol group pointing downwards or upwards Au At HIGH COVERAGE, most of the bonding is with thiol pointing downwards 40

Influence of head group Sulphur chemistry Sulphur chemistry with different head groups PEG SAM shows both chemical states of sulphur Sulphur spectrum from hydroxyundecanethiol on Au S A Sulphur spectrum from 1-mercapto-11-undecyltri(ethylene glycol) on Au [S B ]:[S A ] = 1 : 2.35 S A Indicates different bonding modes of PEG SAM Hydroxy SAM shows only one bonding mode S A S A Steric effect of larger PEG head group affects SAM bonding modes S B S B Use mixed PEG / alkanethiol to reduce steric effect 168 167 166 165 164 163 162 161 160 159 168 167 166 165 164 163 162 161 160 159 Binding Energy (ev) Binding Energy (ev) 41

Alkane thiol self-assembled monolayers on Au Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules Non-destructive ARXPS profile of alkanethiol on Au Alkanethiol non-destructive depth profiles 100 C Au Thickness and molecular orientation information Confirms that organic bonds to gold at sulphur at HIGH COVERAGE Relative layer thickness is observed in profiles Concentration/% 80 60 40 20 S Nonanethiol 0 Nonanethiol 0 0.5 1 1.5 Depth / nm 42

Alkane thiol self-assembled monolayers on Au Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules Non-destructive ARXPS profile of alkanethiol on Au Alkanethiol non-destructive depth profiles Thickness and molecular orientation information Confirms that organic bonds to gold at sulphur at HIGH COVERAGE Relative layer thickness is observed in profiles Concentration/% 100 80 60 40 20 C Layer thickness ~ 1.6 nm SAM length ~1.8 nm SAM tilted by 27 o S Au Dodecanethiol 0 Dodecanenanethiol 0 1 2 Depth / nm 43

Alkane thiol self-assembled monolayers on Au Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules Non-destructive ARXPS profile of alkanethiol on Au Alkanethiol non-destructive depth profiles Thickness and molecular orientation information Confirms that organic bonds to gold at sulphur at HIGH COVERAGE Relative layer thickness is observed in profiles Hexadecanethiol Concentration/% 100 80 60 40 20 0 C Hexadecanenanethiol 0 1 2 Depth / nm S Au 44

Alkane thiol self-assembled monolayers on Au Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules Functionalised alkanethiol non-destructive depth profiles Thickness and molecular orientation information Confirms that organic bonds to gold at sulphur Chemical state information is preserved Possible to observe CH 2 OH at top surface, then alkane chain, then thiol group at Au interface Concentration/% 100 80 60 40 20 Non-destructive ARXPS profile of hydroxy undecanethiol on Au CH 2 OH CH 2 S Au 0 0 1 2 3 Depth / nm 45

Alkane thiol self-assembled monolayers on Au Schematic of self-assembled monolayer Self-assembled monolayers Non-destructive depth profiling of single molecule Self-assembled monolayers allow controlled modification of surface properties by controlled functionalisation Possible application in molecular electronics and biomaterials Organo-sulphur chemistry often used to form layers on gold Layer thickness as a function of organic chain length Molecular orientation information and depth profile of single molecules Functionalised alkanethiol non-destructive depth profiles Thickness and molecular orientation information Confirms that organic bonds to gold at sulphur Chemical state information is preserved Concentration/% 100 80 60 40 20 0 Non-destructive ARXPS profile of 1-mercapto-11- undecyl-tri(ethylene glycol) on Au C 2 H 4 O CH 2 OH CH 2 S Au 0 1 2 3 Depth / nm 46

Alkane thiol SAM study - summary Thermo Scientific Theta Probe ARXPS analysis of self-assembled monolayers Conclusion For reliable and complete analysis of SAMs Combination of XPS/PARXPS and mapping should be used Minimises X-ray flux density Thickness measurement of different SAMs possible For a series of alkanethiols, thickness found to be proportional to chain length Dodecanethiol shown to have thickness of 1.6 nm, 27 o tilted Proposed mechanism of SAM growth has been confirmed Low coverage of SAM is associated with two bonding modes of alkanethiols to Au substrate Thiol group or methyl group bound to Au Thiol / Au bonding is predominantly observed at high coverage Non-destructive profiling of SAMs with Theta Probe confirms molecular bonding mode for high coverage Acknowledgement: Thermo Fisher Scientific acknowledge Assemblon Inc., USA and Daniel J. Graham for providing the alkane-thiol samples and images and for helpful discussions 47

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