Accurate thickness measurement of graphene Cameron J Shearer *, Ashley D Slattery, Andrew J Stapleton, Joseph G Shapter and Christopher T Gibson * Centre for NanoScale Science and Technology, School of Chemical and Physical Sciences, Flinders University, Bedford Park, South Australia, 5042 Email: cameron.shearer@flinders.edu.au; christopher.gibson@flinders.edu.au Materials and methods Graphene samples were prepared from the mechanical exfoliation of graphite. Briefly, highly ordered pyrolytic graphite (HOPG, 1 x 1 x 0.1 cm, ZYH, Grade 2, NT-MDT) was pressed firmly onto adhesive tape (SCOTCH TM MAGIC TM Tape, 3M) and then plastic tweezers were pressed along the tape to ensure complete contact to the HOPG. The tape was then peeled slowly off the HOPG over 20 s. The now cleaved graphite on the adhesive tape was again pressed onto a clean piece of adhesive tape and pressed firmly with plastic tweezers followed by slow removal. This process was repeated 5 times until the graphite on the tape was partially transparent. The thin graphite film on adhesive tape was then pressed onto a silicon piece with 100 nm thermal oxide (CZ, 1 20 Ωcm, ABC GmbH, Germany) and peeled away slowly over 20 s transferring graphite onto the surface of the silicon. The SLG sections/areas on the Si wafer were then identified using the optical microscope on a confocal Raman spectrophotometer (Witec Alpha 300RS 100x, 0.9 numerical aperture, working distance 0.23 mm) to find areas of very thin graphite and then mapping the area by Raman spectral imaging with 532 nm laser excitation (with tuneable power up to 30 mw). Raman single spectra were acquired with integration times of 10 s and 3 accumulations. Presented single spectra are normalized to the intensity of the G-band. Raman spectral images were obtained by collecting a series of 50 x 50
single spectra (1.5 s integration per spectrum) over an area of 15 x 15 μm. The plot of the 2D/G band ratio was generated by exporting the values for maximum intensity for the 2D and G bands after using background subtraction (Witec Project 2.1), then computing the value for 2D/G for each position and plotting as a 3D surface matrix (Origin Pro 9.0). To exclude artefacts from the background area (where both 2D and G intensity are ~0 a.u), a minimum intensity value was set for calculation of 125 a.u., which was determined by the noise level of the spectra. Atomic force microscopy was performed using a Bruker Dimension FastScan AFM with Nanoscope V controller, operating in PeakForce Tapping (PFT) mode with the humidity controlled, in the immediate vicinity of the instrument, to less than 35 % using dehumidifying units (CLI-MATE TM model DH2500E). Images were acquired using ScanAsyst-Air probes (silicon tips on silicon nitride lever, Bruker) at a scan rate of 1 Hz at a resolution of 512 pixels and 512 lines. The nominal spring constant of the cantilevers is 0.4 N m -1 but the true value was determined experimentally prior to each scan using established calibration techniques [1, 2]. The deflection sensitivity was determined by measuring the slope in the contact regime of the PeakForce force curves on the hard silicon surface using an automated software function in the Nanoscope control software. The value for PeakForce set point was set prior to capturing an image and calculated by the software using the measured deflection sensitivity and calibrated cantilever spring constant. Engage settings were adjusted manually to ensure the minimal engage force, this was achieved by adjusting the engage setpoint and gain (starting at 0.05 V which equates to approx. 1 nn) until an engage was achieved. This timely process (due to the high number of false engage events) was carried out to ensure the initial engage was as soft as possible to ensure the quality of the AFM probe. Values for feedback gain and Z-limit were adjusted manually for each image. During a scan topography, adhesion, deformation and hardness were all captured concurrently with the calibration of the height sensor verified by scanning silicon calibration grids (Bruker model numbers PG: 1 μm pitch, 110 nm depth and VGRP: 10 μm pitch, 180 nm depth). Presented AFM topography images have been flattened and analysed using either the depth or step analysis features of Nanoscope Analysis 1.4. Single walled carbon nanotube (SWCNT) modified AFM probes were prepared following a previously reported procedure [3]. Briefly, a SWCNT film (arc-discharge, AP quality, Carbon
solutions) was mounted onto a micromanipulator (model MM3A, kleindiek) and an AFM probe (PFTUNA, Pt coated silicon tips on silicon nitride lever, Bruker) was mounted onto the sample stage within a scanning electron microscope (SEM) (Helios, D344 Dualbeam, FEI). An overhanging CNT, from the SWCNT film, was brought into contact with the AFM probe tip. The overhanging CNT was then cut by first injecting water vapour (magnesium sulfate heptahydrate) into the system via a needle located 200 μm from the electron beam and then line imaging the area to be cut with the electron beam at an accelerating voltage of 1 kv for 5 s. The now shortened CNT was then strongly adhered to the AFM probe by introducing a platinum precursor (Trimethyl [(1, 2, 3, 4, 5-ETA)- 1 Methyl 2, 4 Cyclopentadien-1-YL] Platinum) and imaging the CNT area in contact with the AFM probe at an accelerating voltage of 1 kv to deposit ~1 micron of platinum. The following imaging routine was followed for all experiments. The SLG area was cleaned to remove any surface adsorbates by slowly scanning the area with the laser (532 nm) during Raman spectral imaging [4]. The sample was then transferred to the AFM with the humidity set below 35 % and directly imaged with a new AFM probe. After finding the same SLG area, the PeakForce set point was systematically adjusted in intervals with a new image obtained for each peak force set point until the apparent height of the SLG was measured to be ~0.4 nm. Supplemental results To ensure the observed trend of measuring lower SLG height with increasing peak Force set point, the experiment was completed multiple times. Figure S1 and Figure S2 show the results for two further experiments with different AFM probes. The same trend is observed but the rate of change is different, likely due to different tip geometry.
Figure S1. Repeat measurement of graphene height with ScanAsyst-air probe. PeakForce tapping mode AFM images of graphene showing the change in measured height with peak force set point. Below each AFM image is the corresponding histogram depth plot from which the graphene height was determined. Scale bar = 200 nm. Figure S2. Repeat measurement of graphene height with ScanAsyst-air probe. PeakForce tapping mode AFM images of graphene showing the change in measured height with peak force set point. Below each AFM image is the corresponding histogram depth plot from which the graphene height was determined. Scale bar = 200 nm. To show that the observed trend in measured height is not related to imaging artefacts several control experiments were completed. The first was to compare the change in adhesion to the graphene and substrate with peak Force set point. Figure S3 shows adhesion maps with depth histograms obtained concurrently with the PFT topography images shown in Figure 2. The graphene layer does show greater adhesion than the substrate but the difference in adhesion does not vary significantly between peak Force set points. From this, the effect of changing adhesion can be ruled as a possible cause of the varying measured height.
Figure S3. (a, b) PeakForce tapping mode AFM adhesion images of graphene showing the change in measured adhesion with peak Force set point (as indicated in each AFM image). Images were obtained concurrently with the height images shown in Figure 2. (c) Graph of measured adhesion vs peak Force set point. Scale bar = 200 nm. HOPG was measured along with MLG to ensure that an accurate value for graphene layer spacing could be obtained with PFT AFM. Figure S4 shows the results for HOPG where the measured value for a single step was measured to be 0.255 ± 0.155 nm, which is very close to the theoretical value of 0.335 nm.
Figure S4. PeakForce tapping mode AFM topography images of HOPG showing the change in measured height with peak force set point (as indicated in each AFM image). Images were obtained sequentially from 1 nn up to 10 nn. Below each AFM image is the corresponding depth histogram from which the average graphite step height was determined. Scale bar = 200 nm. A second CNT modified AFM probe was used to measure SLG height with varying peak force set point. Shown in Figure S5, the same trend was observed to that reported in the main manuscript, the force required to achieve an accurate value was much lower than for a standard AFM probe. Figure S5. Repeat experiment of PeakForce tapping mode AFM topography images of graphene with a CNT modified AFM probe showing the change in measured height with peak Force set point (as indicated in each AFM image). Images were obtained sequentially for each different Force set point. Below each AFM image is the corresponding histogram depth plot from which the graphene height was determined.. Scale bar = 200 nm.
Figure S6 shows the adhesion maps obtained concurrently with Figure 5 where there is a difference in adhesion between graphene and the substrate but the magnitude of the difference does not change with peak force set point. Figure S6. PeakForce tapping mode AFM adhesion images of graphene using SWCNT-modified tip showing the change in measured adhesion with peak Force set point (as indicated in each AFM image). Images were obtained concurrently with the height images shown in Figure 5. (b) Graph of measured adhesion vs peak Force set point. Scale bar = 200 nm. Figure S7 shows the control experiment to show that imaging multiple graphene steps with a SWCNT modified probe (the probe as used in Figure 5) obtains an accurate value for graphene step height, regardless of applied force. Figure S7. PeakForce tapping mode AFM topography images with a CNT modified AFM probe of multilayer graphene showing the change in measured height with peak force set point (as indicated in each AFM image). Images were obtained sequentially for each Force set point. Below each AFM
image is the corresponding step height plot from which the average graphene step height was determined. Scale bar = 200 nm, grey boxes indicate area where step-height was determined. References [1]. Slattery A D, Blanch A J, Quinton J S, and Gibson C T 2013 Calibration of atomic force microscope cantilevers using standard and inverted static methods assisted by fib-milled spatial markers Nanotechnology 24 015710 [2]. Sader J E, Sanelli J A, Adamson B D, Monty J P, Wei X, Crawford S A, Friend J R, Marusic I, Mulvaney P, and Bieske E J 2012 Spring constant calibration of atomic force microscope cantilevers of arbitrary shape Rev. Sci. Instrum. 83 103705 [3]. Slattery A D, Blanch A J, Quinton J S, and Gibson C T 2013 Efficient attachment of carbon nanotubes to conventional and high-frequency afm probes enhanced by electron beam processes Nanotechnology 24 235705 [4]. Hosoya N, Tanimura M, and Tachibana M 2013 Effect of laser irradiation on few-layer graphene in air probed by raman spectroscopy Trans. Mater. Res. Soc. Jpn. 38 579-583