(Supporting Information)

Transcription

1 Atomic and Electronic Structure of Graphene-Oxide (Supporting Information) K. Andre Mkhoyan, 1,2 * Alexander W. Contryman, 1 John Silcox, 1 Derek A. Stewart, 3 Goki Eda, 4 Cecilia Mattevi, 4 Steve Miller, 4 and Manish Chhowalla, 4 1 School of Applied and Engineering Physics, Cornell University, Ithaca, New York , 2 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455, 3 Cornell Nanoscale Facility, Cornell University, Ithaca, New York 14853, 4 Department of Material Science and Engineering, Rutgers University, Piscataway, New Jersey * Corresponding authors. mkhoyan@umn.edu 1

2 To ensure that this synthesis based on Hummers method (for a detailed description see supporting materials in Ref. [1]) produces good quality oxidized graphene films, several samples were analyzed by energy dispersive X-ray (EDX) spectroscopy to identify most of the elements present in the film. The analysis was carried out using the same Cornell VG HB-501 dedicated STEM. The microscope is also equipped with a Link LZ-5 Model 5493 EDX spectrometer. Several EDX spectra were obtained from graphene oxide (GO) sheets by exposing the samples to the 100 kev electron beam of the STEM. One such spectrum is presented in Fig. S1. Strong Cu signal in the spectrum comes from the TEM grid itself, which is made of copper. The Ta signal comes from unavoidable exposure of the different parts of microscope column and cartridge, where Ta is present, to the electron beam. The Ta signal was also measured by EDX in STEM even without any sample being present in the microscope. The peaks characteristic to C and O were measured only when GO films were exposed to the STEM electron beam. The low intensity of the C K shell peak at 0.28 KeV in these EDX spectra, relative to O K-shell peak at 0.53 KeV is due to the low x-ray fluorescence yield for C [2]. The trace amounts of the sulfur detected in these measurements are from residual H 2 SO 4 used in synthesis of these GO films. Figure S1: EDX spectrum from the claster of GO films showing presence of O and C in the sample. A small amount of residual S from sample preparation is also detected. Additional Cu and Ta measured in the spectrum are due to exposure of the parts of the grid, cartridge and microscope column to probe electrons. The presence of H in these GO films was checked with low-loss EELS measurements. Since hydrogen s characteristic peak near 13 ev [3], was absent in EELS measurements (see Fig. 4 in main text) the amount of H in the films is therefore negligible. It should be noted here that typically H atoms 2

3 are sensitive to intense 100 kev electron beam and can be easily removed from the surface of the specimen by knock-on process. The Mott cross-section describing the probability of such surface sputtering of H is estimated to be about 90 barns [4]. In the low-loss EELS measurements conducted here, the beam current was kept at about 0.2 pa, which is in order of 102 times lower than standard operational condition. Under these conditions, H sputtering from GO surface should be negligible. However, we believe that some residual H is still likely be present in these samples. A careful analysis of the fine structure of the O K-edge (see Fig. 3(b) in main text) indicates that oxygen in GO films is primarily in a form of single atoms covalently bonded to C. If there are any peroxi radical ( C-O-O-) or links ( C-O-O-C ), in these films, their amounts should be negligible as well. These radicals and links manifest themselves in the appearance of pre-peaks in the fine structure of the O K-edge spectrum (similar to 531 ev peak in molecular O 2 [5]), which was not observed in corelevel EELS measurements [6]. To understand the effects of the oxygen on the ADF imaging of graphene and appearance of the unusual strong intensity variations in experimentally recorded STEM-ADF images, several atomicresolution ADF images of such structures were simulated. Here the oxygen atoms were considered to be bonded randomly to the different possible sites of the graphene honeycomb (in the same way as simulations presented in the main text). The algorithm for calculation of the ADF-STEM images is based on the multislice method [7, 8]. The code developed by Kirkland [9] was implemented. First, a STEM focused electron probe was generated using the experimental probe parameters. The wave function of the STEM probe located at point x r p can be expressed as: where λ = α max max kmax r r r r r ( x) = A exp[ iχ( k ) 2πik ( x x )] 2 ψ p p p d k, (S1) 0 k is the maximum angle allowed by the objective aperture, ( k r ) r χ is the aberration function and A p is a normalization constant. The incident electron beam then propagates through the entire thickness of the specimen by alternately passing through thin layers of the specimen and 3

4 propagating between the layers. The ADF intensity at each point is calculated by summing up all of the electrons that are scattered from the specimen into the conical solid angle of the ADF detector. The final image is generated by scanning the probe position across a small area of the model specimen [9]. For simulations presented here, a STEM 2 Å probe was generated by using the following electron optical parameters: 100 kv acceleration voltage, spherical aberration of Cs=1.3 mm, objective angle of 11.4 mrad, and defocus of 850 Å. The inner and outer angles of the ADF detector were 54 and 330 mrads. These numbers are also comparable to the optical conditions of other STEMs with similar resolutions. The effects of thermal vibrations of the atoms (or phonons) are included in the calculation by randomly displacing atoms from their sites using a Gaussian distribution function with the corresponding Debye-Waller factors [10]. The graphene was viewed along the direction perpendicular to the sheet. The ADF images were obtained by scanning the probe across a Å 2 area in the middle of the supercell with pixels. A slice thickness of 1.24 Å was used in all simulations. One of these simulated ADF images is presented in Fig. S2. For direct comparison with experimentally recorded images (as presented in Fig. 2(a)) the simulated image was first rescaled to match the pixel size with recorded data and then was overlaid on top of the image as shown Fig. 2(b). Good correlation can be observed, which confirms that the strong variation in contrast in experimental ADF images of single GO film can indeed be attributed to the random-site-oxidation of the graphene sheet. It should be noted here that in Multislice simulations, differences between using of simple model of O atoms attached to graphene and a more accurate model based on the atomic structure obtained by ab initio calculations (see Fig. 3(e) in main text) are negligible. In simulations, when GO is sliced into layers, O atoms are in the first or third slice (depending if they are on top or bottom surface) and C atoms are in the second slice. Inside each slice, the atomic potentials are averaged over the slice thickness and, therefore, the actual height-positions of the atoms inside the slice are not essential [9]. Additionally, since we consider 2 Å STEM probe, small corrections to the bond-lengths ( Å) predicted by ab initio calculations and critical for our understanding of the structure of GO films will be washed out in ADF simulations. 4

5 Figure S2: (a) Experimental STEM-ADF image of the single GO film showing amorphous-like contrast variations. This is the same image as in Fig. 1(d) in the main text. (b) Lower-right section of the image in (a) (inside white borders) with overlaid simulated ADF image. Simulated image is indicated by arrow. Images are individually scaled to fill the available grayscale. To enhance the fine structure of the C K-edge recorded from GO, graphite and a-c, a Lorentz function fitted to the measured zero-loss energy spread function was removed from the measured K-edge EELS data using a simple deconvolution routine. Use of a fitted Lorentzian avoided the addition of noise to the final spectrum. The results are presented in Fig. S3 and as can be seen, the fine structure of all three C K-edges are now significantly pronounced. For determination of the fractions of a-c and graphene-like contributions in C K-edge of GO films, these data were used in a least-square-curve fitting algorithm for linear superposition of two reference spectra (a-c and graphite) to fit the spectrum from GO (see Fig. 3(c) in main text). Figure S3: The EELS spectra of the C K-edges from GO films, graphite and a-c. These are the spectra presented in Fig. 3(a) of the main text (a) before and (b) after removing the STEM probe function. The curves are vertically shifted for clarity. 5

6 METHODS STEM Experiments and Multislice Simulations The experimental STEM-ADF images and EELS measurements were carried out in the Cornell VG HB kv STEM that produces a focused electron probe of about 2 Å with about 11 mrad convergent angle and a beam current of ~15 pa. The microscope is equipped with a cold field emission gun, single-electron sensitive imaging detectors and a parallel electron energy loss spectrometer. ADF images were recorded using single-electron counting yttrium aluminum perovskite scintillatorphotomultiplier system and the dark counts were adjusted to be above zero by appropriately setting the discriminator level. The signal counts for ADF imaging was monitored and was kept below the saturation limit. Small volumes of the solution with GO flakes were dropped on standard holey-carbon film covered copper grids and air-dried for minutes. The grids then were loaded into the preparation chamber of the microscope and baked for a few hours at about 100 C in 10 8 Torr vacuum in order to minimize contamination. Samples then were loaded into STEM column for observations. Spectra of C and O K-edges as well as low-loss EELS data were measured from the same single GO sheets for consistency. All EELS data were recorded in area mode for reduction of the beam dose. To minimize the noise level in the spectrum many spectra were recorded from different areas of the same sheet and averaged. Due to the very low count rate of the low-loss EELS the final spectra from GO films were obtained by subtracting EELS data obtained by positioning the beam over the hole from the data recorded from the sample. This also eliminates the effects of the detector dark current. Systematic study of the alterations in ADF-STEM images and EELS data, including C and O K-edges, showed that neither electron-beam-induced damage nor contamination can be detected under the STEM operational condition and electron doses used in these experiments. For Multislice calculations, a 2 Å probe was generated by using the following STEM electron optical parameters: 100 kv acceleration voltage, spherical aberration of Cs=1.3 mm, objective angle of αobj=11.4 mrad, and defocus of f=850 Å. The inner and outer angles of the ADF detector were 54 and 330 mrads. The effects of thermal vibrations of the atoms are included into the calculation by randomly 6

7 displacing atoms from their sites using a Gaussian distribution with the corresponding Debye-Waller factors. The size of the graphene supercell used in the calculations was Å 2 and it was viewed along the direction perpendicular to the sheet. The ADF images were obtained by scanning the probe only over a small area in the middle of the supercell. A slice thickness of 1.24 Å was used in all simulations. DFT Calculations For this work, we employ a standard density functional package Quantum Espresso [11] that uses a plane wave basis set and ultrasoft pseudopotentials to describe the graphene structures. All calculations were performed in the local density approximation (LDA) with Perdew-Zunger parameterization for the exchange and correlation functional. A plane wave cutoff of 45 Ryd was found to be sufficient to converge the system energy. Investigations of a single sheet of graphene used a Monkhorst- Pack k-point grid for Brillouin zone integrations in the self consistent calculations and a Monkhorst-Pack grid for density of states calculations. We found a high k-point sampling was essential to accurately represent the density of states near the Fermi energy. The impact of an oxygen atom on the graphene sheet was evaluated by using a 5 5 supercell of graphene (50 C atoms) with a single oxygen atom initially located above a C-C bond in the center of the supercell. The atomic coordinates of the system were relaxed based on the Hellmann-Feynman forces until the individual force components on all atoms were reduced to less than Ryd/bohr. A Monkhorst-Pack grid of was used for all supercell calculation. In all calculations, the supercell was chosen so that there was over 14 Å distance between the graphene sheet and periodic images. References [1] G. Eda, G. Fanchini, and M. Chhowalla, Nature Nanotechnology 2008, 3, 270. [2] L. Reimer, Scanning Electron Microscopy: Physics of Image Formation and Microscopy (Springer Series in Optical Sciences, Vol. 45) (Springer-Verlag, 1985). [3] C.C. Ahn and O.L. Krivanek, EELS Atlas (Gatan Inc. 1983). [4] K.A. Mkhoyan, J. Silcox, M.A. McGuire, and F.J. DiSalvo, Phil. Mag. 2006, 86,

8 [5] Y. Ma, C.T. Chen, G. Meigs, K. Rendall, and F. Sette, Phys. Rev. A 1991, 44, [6] K.A. Mkhoyan, J. Silcox, A. Ellison, D. Ast, and R. Dieckmann, Phys. Rev. Lett. 2006, 96, [7] J.M. Cowley, A.F. Moodie, Acta. Cryst. 1957, 10, 609. [8] E.J. Kirkland, R.F. Loane and J. Silcox, Ultramicroscopy 1987, 23, 77. [9] E.J. Kirkland, Advanced Computing in Electron Microscopy (Plenum Press, 1998). [10] R.F. Loane, P. Xu and J. Silcox, Acta. Cryst. 1991, A47, 267. [11] P. Giannozzi et al. 8