Article pubs.acs.org/JPCC
Graphene Nanomesh Formation by Fluorine Intercalation Swee Liang Wong,†,∥,⊥ Khoong Hong Khoo,†,‡,§ Su Ying Quek,*,†,‡,§ and Andrew Thye Shen Wee*,†,‡ †
Department of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117551, Singapore Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117542, Singapore § Institute of High Performance Computing, Agency for Science Technology and Research, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632, Singapore ∥ Institute of Materials Research and Engineering, Agency for Science Technology and Research, 2 Fusionopolis Way, Innovis #08-03, Singapore 138634, Singapore ‡
S Supporting Information *
ABSTRACT: Graphene nanomeshes are mainly produced through top-down lithography, resulting in unavoidable defects or contamination. In this article, we demonstrate a bottom-up approach through partial intercalation of fluorine between the carbon buffer layer and the underlying SiC(0001) substrate by low-temperature annealing of a deposited molecular layer of fluorinated fullerenes C60F48. Due to the inherent periodicity of the bonding between the buffer layer and the underlying SiC(0001) substrate, selective fluorination and partial intercalation take place. Using scanning tunneling microscopy and spectroscopy as well as density functional theory calculations, the existence of a graphene nanomesh with the local atomic arrangement of a graphene sheet and surface corrugation of long-range periodicity is revealed. Surprisingly, the nanomesh exhibits electronically an intermediate state between the conventional buffer layer and quasi-free-standing graphene. Specifically, unlike the buffer layer, which is bonded covalently to the SiC(0001) surface so that the characteristic graphene π network about the K point of the Brilluoin zone is destroyed, this intermediate state retains the wave function characteristics of graphene, but a two-peak structure in the local density of states (LDOS) is introduced about the K point. This graphene nanomesh with a two-peak LDOS structure about the K point presents another playground for the study of transport properties in supported two-dimensional materials.
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INTRODUCTION Graphene, a monatomic layer of carbon, has been extensively studied over the past decade due to its wide range of electronic, optical, mechanical, and thermal properties.1−4 Graphene-based devices show electron mobilities of up to 1 000 000 V cm s−1 while maintaining a high degree of stability.5 There are many different methods in producing graphene, such as mechanical/ liquid exfoliation, chemical-vapor-based deposition, and chemical reduction of graphene oxides,6 among which, epitaxial graphene grown from thermal annealing of silicon carbide (SiC) has attracted much attention due to the possibility of large-area graphene growth and wafer scale device fabrication.1,7 The growth of epitaxial graphene takes place through a bottomup process. During thermal annealing, a buffer layer, which is a precursor to epitaxial graphene grown from the Si face of hexagonal SiC substrates, is first developed during silicon sublimation of the SiC(0001) surface. This buffer layer is structurally similar to graphene,8−13 but it lacks the characteristic graphene π network because the buffer layer is split into lattice-mismatched and lattice-matched regions where the carbon atoms are covalently bonded to surface silicon atoms. Eventually, the buffer layer is converted into epitaxial graphene © 2015 American Chemical Society
during the growth process, but a new buffer layer is developed under the newly formed epitaxial graphene. Interaction of the epitaxial graphene with the underlying buffer layer results in high intrinsic electron doping as well as reduction in charge mobility compared to that of mechanically exfoliated graphene. To reduce this effect, intercalation of the buffer layer has been carried out in recent studies where epitaxial graphene on SiC(0001) is exposed to foreign elements such as hydrogen14,15 and fluorine16 and even molecular species such as methane,17 typically in gaseous forms. Intercalation between graphene and their metal growth substrate has also been demonstrated.18,19 During intercalation, these intercalants may diffuse into the interfacial region between the buffer layer and the bulk substrate, reacting with and breaking the bonds holding the buffer layer to the SiC surface, causing the conversion of the buffer layer to quasi-free-standing graphene. For these processes, complete intercalation of the buffer layer takes place, and a pristine quasi-freestanding graphene is Received: October 28, 2015 Revised: December 10, 2015 Published: December 10, 2015 29193
DOI: 10.1021/acs.jpcc.5b10582 J. Phys. Chem. C 2015, 119, 29193−29200
Article
The Journal of Physical Chemistry C
periodic boundary conditions, a vacuum layer of more than 10 Å is included along the direction perpendicular to the slabs. For our DFT calculations, we employ norm-conserving pseudopotentials to represent the ionic cores along with a pseudoatomic basis set as implemented in the SIESTA package34 that has been thoroughly tested to reproduce previous literature.8,10,35 The local density approximation36 was used for exchangecorrelation, and we found it sufficient to sample the Γ-point in the Brillouin zone owing to the large size of our supercell (31.9 Å × 31.9 Å) for structural optimization purposes. Structural optimization was carried out until the residual forces on the atoms were converged to less than 0.02 eV/Å. STM images are simulated using the Tersoff−Hamann approximation.37 The projections of wave functions onto those of pristine graphene (Figure 5) were performed by carefully ensuring that the kpoint in the folded Brillouin zone, corresponding to the respective k-point in the unfolded Brillouin zone of primitive graphene, is included in the calculation.
produced without observation of any intermediate state. This is most likely due to the use of fast intercalation processes through direct introduction of H214,15 or XeF2.16 The quasifreestanding graphene exhibits improved electronic transport properties compared to conventional epitaxial graphene grown through thermal decomposition of SiC(0001).20,21 In our previous work, we have also shown that a C60F48 molecular source may be employed as a source for fluorine intercalation.22 Recent studies have focused on top-down patterning techniques and covalent modification of graphene such as lithography or etching to produce graphene nanoribbons23−25 or nanomeshes26−29 to modify its electronic and physical properties, inadvertently introducing atomic defects or contamination. The latest bottom-up procedures involve growth of graphene nanoribbons through chemical reactions between organic molecules deposited on specific substrates such as Au30 or Ge31 with limited dimensions. In this article, we propose an alternative bottom-up strategy in the production of graphene nanomeshes by exploiting the inherent periodicity as well as the variation of the bonding between the buffer layer and the underlying SiC(0001) substrate.8−11 This is achieved through partial fluorine intercalation when a C60F48 covered buffer layer undergoes a series of low-temperature anneals. As fullerenes can adopt different packing structures on graphene, SiC, and other semiconducting surfaces and have complex interactions with the substrate surface,32,33 low-temperature scanning tunneling microscopy (LT-STM) is used to study the partial intercalation process. In addition, LT-STM is also employed to perform in situ physical and electronic studies of the as-produced graphene nanomesh after the process has taken place, while density functional theory calculations are used to understand its formation mechanism as well as the measured electronic properties.
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RESULTS AND DISCUSSION Figure 1a shows a large-scale STM image of the sample surface where all molecules have desorbed after annealing at 150 °C. Presence of a deposited molecular layer is confirmed through STM prior to annealing (refer to Figure S1). At −2.0 V tip bias, the buffer layer is displayed as a rough surface without distinct features, while the graphene layer is observed as a smooth surface. As explained in our previous work,22 the pre-existing epitaxial graphene was unaffected by the intercalation process due to the weaker method of intercalation employed as compared to conventional gas-related methods,14−16 as well as the lower adsorption energy of the molecules on epitaxial graphene compared to the buffer layer,22 which limits fluorine migration from molecule to interface prior to desorption. We observe that there now exists a third surface which occupies the bulk of the image and has a rippled appearance made up of quasiperiodic valleys and ridges as seen in the higher-resolution STM image of Figure 1b. We note the difference in appearance between this rippled surface in Figure 1c and the pre-existing buffer layer on the substrate as described in Figure 1d. The well-defined trimer features that we observed for the buffer layer38−40 (inset of Figure 1d) are not present on the rippled surface. Instead, the rippled surface is made up of a series of distinct irregular polygons which are also not observed in the buffer layer. At lower tip bias (−1.0 V), structures resembling incomplete honeycomb structures are revealed along the elevated ridges in Figure 1d. This differs from previous reports of STM studies where the buffer layer is observed as a complete graphene-like carbon sheet41 or with nodal-like structures39,42 at low tip biases (≤1 V), further indicating that a surface unlike that of the buffer layer is attained after our annealing process. Fourier transformation was performed for the large-scale STM images of the buffer layer (Figure 1e) and the graphene nanomesh (Figure 1f), respectively, and we note that, despite differences in their topographical details as described earlier, the 6-fold symmetry and direction of unit cell vectors between the two surfaces are preserved. The unit cell that is reflected in the Fourier-transformed images is outlined in Figure 1b and c. Similar observations to that in Figure 1b have been reported by Cranney et al. for gold-intercalated epitaxial graphene on SiC(0001) and are attributed to standing waves formed due to gold nanoclusters present between graphene and the buffer layer.43 In contrast to their STM results in which a continuous
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METHODS Experiment. A sample having both epitaxial monolayer graphene and a buffer layer surface was grown via partial graphitization of 4H-SiC(0001) substrates under ultrahigh vacuum (UHV) conditions. The cleanliness of the sample was checked prior to molecular deposition. Initial experimental procedures are similar to our previous work.22 C60F48 molecules (95% purity, Term USA) were first thermally evaporated from an effusion cell held at 110 °C onto the sample that was kept at room temperature. Formation of the C60F48 adlayer was confirmed under STM (Figure S1 in Supporting Information). A direct current was then passed through the sample for an initial annealing at 120 °C for 1 h before increasing to 150 °C for an additional hour for partial intercalation to take place. LTSTM and STS experiments were performed to measure the electronic and morphological properties of the partially decoupled graphene nanomesh. In situ LT-STM experiments were carried out after every stage of the procedure in a custombuilt multichamber ultrahigh-vacuum system with base pressure lower than 1.0 × 10−10 mbar and housing an Omicron LTSTM.22 STM imaging was carried out at 77 K in constant current mode with a chemically etched tungsten tip. Theory. In our theoretical simulations, the 6√3 × 6√3 R30° unit cell of the buffer layer is chosen, and our system consists of a 4H-SiC(0001) substrate modeled using four alternating silicon and carbon atomic layers topped by the buffer layer.8 F-intercalants are then added between the buffer layer and the SiC surface. In order to avoid spurious interactions between slabs from different supercells under 29194
DOI: 10.1021/acs.jpcc.5b10582 J. Phys. Chem. C 2015, 119, 29193−29200
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The Journal of Physical Chemistry C
annealing of a C60F48-covered buffer layer. Since we have previously demonstrated that a C60F48 molecular source may be employed as a source for fluorine intercalation, 22 we hypothesize that the surface observed is a semi-intercalated graphene nanomesh, consisting of both elevated ridges (generally 1 nm in width) where fluorine has intercalated under the carbon layer and bonded to the underlying Si atoms and lower-lying valleys where carbon atoms remain bonded to the substrate. To test the hypothesis that the observed surface is that of a partially F-intercalated buffer layer, we perform DFT calculations, starting with a relaxed buffer layer,8 using the 6√3 × 6√3 R30° unit cell of the buffer layer and modeling the 4H-SiC(0001) substrate using four alternating silicon and carbon atomic layers (see Methods for details). In the absence of F-intercalants, the interplay of lattice mismatch and binding at the graphene−SiC interface splits the 6√3 × 6√3 R30° supercell into hexagonal lattice-matched regions, where carbon atoms in the buffer layer are covalently bonded to the surface Si atoms. These regions are separated by boundaries where C and Si atoms are misaligned and exhibit no covalent bonding. Fintercalants introduced into the graphene−SiC interface are expected to prefer the nonlattice matched regions, as interface Si atoms provide unsaturated dangling bonds for binding to F. By placing F-intercalants above Si at various distances from the center of the lattice-matched regions (see Figure 2e) and calculating their binding energies, we do indeed see significantly larger binding energies for regions further away (beyond ∼6 Å) from the centers of the lattice-matched regions (Table 1). The main intercalation element is attributed to fluorine atoms because on further annealing at higher temperatures of 800 °C we observed the same fully decoupled quasi-freestanding graphene as reported in our previous work.22 In Figure 2a−d, we show the relaxed structures obtained from DFT calculations for a buffer layer on (0001) SiC at various levels of F-intercalation, binding at the energetically most favorable sites. These intercalants result in significant rippling in the buffer layer in agreement with experiment. The rippling can be quantified by tbuffer, defined as the difference between the maximum and minimum z-coordinates of buffer layer atoms. tbuffer increases from 0.89 Å before F-intercalation (Figure 2a) to 2.04 Å with 15 F-intercalants (Figure 2b) and 2.22 Å with 51 F-intercalants (Figure 2c). This is due to F atoms pushing up nearby buffer layer atoms, while other parts of the buffer that are strongly bound to SiC remain unchanged. However, when the number of F-intercalants is increased to 69 (Figure 2d), tbuffer decreases to 2.01 Å as the entire sheet starts becoming delaminated and the buffer layer begins acquiring the characteristics of pristine (quasi-free-standing) graphene. The system with 51 F atoms corresponds to that where all Si sites more than 8.0 Å from the centers of the lattice-matched regions are passivated with F, leading to intercalation with 51 F atoms (Figures 2c and 2f). Henceforth we use this system with 51 F atoms per unit cell as our model for a partially intercalated buffer layer. Notably, the STM images simulated using this structure reproduce the incomplete honeycomb features observed in Figure 1c (see Figure 1d), thus providing evidence that the experimentally observed images correspond to a partially F-intercalated buffer layer. However, we note that the randomness in the polygonal motifs (Figure 1b and d) is not reflected in the simulated image. We attribute the random polygonal shapes in the ridges and valleys of the semiintercalated graphene in the experimental STM images to the
Figure 1. (a) 70 × 70 nm2 STM image of the semi-intercalated graphene surface observed after thermal annealing (Vtip = −2.0 V, I = 100 pA). (b) High-resolution 20 × 20 nm2 STM image of semiintercalated graphene (Vtip = 2.0 V, I = 100 pA). (c) 30 × 30 nm2 STM image of the buffer layer (Vtip = 2.0 V, I = 100 pA). Inset: Zoomed in 11.5 × 11.5 nm2 STM image showing trimers in the buffer layer. Dashed lines indicate unit cell reflected in Fourier-transformed images. (d) 10 × 10 nm2 STM image of semi-intercalated graphene (Vtip = −1.0 V, I = 150 pA). Incomplete honeycomb structure is circled. Fourier transformed image of (e) semi-intercalated graphene with indicated lattice vectors (0.65 nm−1) and (f) buffer layer with indicated lattice vectors (0.59 nm−1). Scale bars are 0.5 (1/nm) for (e) and (f). The represented unit cells are indicated by dashed lines in (b) and (c). (g) Simulated STM image of the partially F-intercalated graphene−SiC surface shown in Figure 2c. This image was obtained by taking an isosurface of the integrated wave function charge density for states with energies between EF and EF + 1.0 eV.
graphene sheet can be imaged above these clusters at appropriate tip biases within a small range of voltage, the network of ridges and valleys of this rippled surface is observed continuously across a significant range of voltage bias (−2 V to +2 V). This indicates that we are not imaging a continuous graphene sheet but rather that the images correspond to the buffer layer that has been modified by low-temperature 29195
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Figure 2. Structures obtained from DFT calculations of a graphene buffer layer on (0001) SiC at various levels of F-doping. The systems in (a), (b), (c), and (d) contain 0, 15, 51, and 69 F atoms, respectively, intercalated between the topmost SiC and graphene buffer layer, out of a total of 108 Si atoms in the surface unit cell of the buffer layer. C, F, Si, and H atoms are represented by yellow, gray, large blue, and small blue balls. The quantity tbuffer, defined as the difference between the maximum and minimum z-coordinates of buffer layer atoms, is indicated in (c). (e) Ball-and-stick model of the interface SiC layer. Green atoms denote center of lattice matched regions, and white rings indicate F-intercalant positions for binding energy calculations. (f) Ball-and-stick model of the interface SiC layer with 51 F-intercalants. Si atoms with and without F-intercalants on top are colored light yellow and blue, respectively.
valley peak height is 2.2 ± 0.1 Å at a tip bias of 0.3 V (Figure 3a) and the smallest contrast of 0.5 ± 0.1 Å at a tip bias of 1.5 V (Figure 3e). The same trend can also be observed for the reverse tip polarity (not shown). As the height difference does not remain constant with bias, we conclude that the observed quasi-periodic motifs are not solely topographical but also have electronic origins. In addition, we can also exclude the possibility of the fluorinated/nonfluorinated fullerenes being intercalated as their molecular heights of ∼1 nm are much larger than the height variation observed in both our STM measurements (Figure 3) and DFT calculations (Figure 2). The variation in height difference with tip bias is thus attributed to the difference in electronic density of states close to the Fermi level of the ridges and valleys. To understand this bias dependence better, we also simulated the tip trajectories at different biases for our F-intercalated system, using the Tersoff−Hamann approximation by taking an isosurface of the sum of wave function densities over the bias window. The tip trajectories for the partially intercalated structure at tip bias voltages of 0.3 and 2.0 V are simulated by contours of constant integrated wave function density in Figures 4b and 4c, respectively, using the calculation cell shown in Figure 4a. It is clear that the contours become smoother with smaller variations in height as the bias voltage is increased, consistent with experimental data in Figure 3f. The smoothing of the tip trajectory at high bias can thus be understood using the following argument. At low bias, the STM tip follows a density contour that is highly characteristic of electron states at the
Table 1. Binding Energy of F-Intercalant as a Function of Distance from the Si Atom to the Center of the LatticeMatched Region distance from Si to the center of lattice-matched region (Å)
binding energy (eV)
0.0 3.0 6.1 8.0 9.2
2.8 3.0 2.8 3.9 4.4
randomness of the intercalation process in experiment, which results in different regions of the surface having a varying number of F-intercalants. Such randomness may be related to the probabilistic nature of individual molecular desorption during annealing as well as that of fluorine dissociation from the fluorinated fullerenes. Other factors may include the presence and location of defects in the buffer layer which vary the likelihood of intercalation. In order to ascertain whether the observed valleys and ridges are of topographical or electronic origin, a series of STM images of different biases are taken over the same region and shown in Figure 3. The corresponding height profiles taken along the lines indicated are shown in Figure 3f. We note that the height difference between the ridges and valleys is more pronounced at lower tip biases, while the height undulation reduces significantly at higher tip biases. The maximum ridge to 29196
DOI: 10.1021/acs.jpcc.5b10582 J. Phys. Chem. C 2015, 119, 29193−29200
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The Journal of Physical Chemistry C
Figure 3. Series of 10 × 10 nm2 STM images with I = 90pA and bias of (a) 0.3 V, (b) 0.6 V, (c) 0.9 V, (d) 1.2 V, and (e) 1.5 V. (f) Height profiles taken along the indicated green line for each STM image. Maximum peak to valley heights of 2.2 Å (0.3 V), 1.2 Å (0.6 V), 0.8 Å (0.9 V), 0.6 Å (1.2 V), and 0.5 Å (1.5 V) are measured. Lines are offset for clarity, and bias of the associated image is indicated.
areas of the semi-intercalated graphene and is presented in Figure 5a. Between each measurement, the tip condition is checked for consistency by performing STS over the remaining buffer layer that has not been intercalated (Figure S2, Supporting Information). In contrast to the STS collected over the buffer layer (Figure S2, Supporting Information) the STS spectrum for the graphene nanomesh has two shoulders at 850 ± 10 and 570 ± 10 meV above the Fermi level (Figure 5a). The energy difference between them is measured to be 280 ± 20 meV. Since the spectrum also includes contribution from the bulk substrate, the shoulders correspond to local conductance maxima in the graphene nanomesh. The graphene nanomesh formed through fluorine intercalation is also p-doped, with the “Dirac point” approximated at 710 ± 10 meV above the Fermi level (here, the “Dirac point” is defined to be the average of the voltage biases where the two conductance maxima are located). The p-doping is expected as fluorine is highly electron withdrawing. In order to ascertain the nature of the states at the two observed conductance maxima, wave functions from our graphene−SiC DFT calculations with varying levels of Fintercalation are analyzed by projection onto pristine graphene wave functions about the K point of the graphene Brilluoin zone (BZ) as indicated in Figure 5c. Systems with 15, 51, and 69 F-intercalants (Figures 2b−d) are generated by passivating all Si sites more than 9.2, 8.0, and 6.1 Å from the centers of the lattice-matched regions, respectively. The magnitude squared of the projection is plotted as a function of the graphene−SiC wave function eigenenergy in Figure 5b. For systems with 0 or 15 F-intercalants, strong hybridization between the buffer layer and SiC substrate causes the projection on pure graphene states to be very small. Increasing the number of F-intercalants to 51 passivates a sufficient number of interface Si to allow a recovery of graphene states about the Fermi level, as evidenced by larger projections in Figure 5b. For this system, the projection onto graphene states at the K-point yields two peaks centered about −0.035 eV, separated by 0.43 eV (black curve; Figure 5b). Projection onto pristine graphene states away from the K-point results in one major peak centered at approximately the same energy as the pristine graphene state and one minor peak closer
Figure 4. (a) Top view of the calculation cell for partially Fintercalated graphene on SiC with the red line indicating the cut plane where integrated wave function density is displayed. Color map of integrated wave function density from (b) E = EF − 0.3 eV to EF and (c) E = EF − 2.0 eV to EF to simulate tip trajectory over semiintercalated graphene. Similar results are obtained for the integrated wave function density from E = EF to EF + 0.3 eV and E = EF to EF + 2.0 eV, corresponding to opposite tip polarities.
Fermi level. Upon increasing the tip bias, however, the electrons can tunnel into a larger set of states spanning a bigger range of orbital characters and localizations. These high bias contributions tend to complement the low bias states and even out spatially the density of available tunneling states, leading to smoother tip trajectories and smaller tip height variations. STS, which reflects the electronic local density of states (LDOS), is also performed to investigate the electronic structure of the graphene nanomesh by perfoming local spectroscopy on random points above the ridges as well as in the center of the valleys. A lock-in voltage modulation of 600 Hz and 6 mV amplitude is applied to achieve the conductance measurement. There was no observable difference between STS taken from the valleys or the ridges of the semi-intercalated graphene. Hence, an average of 50 spectra is taken over random 29197
DOI: 10.1021/acs.jpcc.5b10582 J. Phys. Chem. C 2015, 119, 29193−29200
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Figure 5. (a) Averaged STS spectra of semi-intercalated graphene, taken with initial bias of −0.50 V and tunneling current of 100 pA. Red arrows indicate the location of the conductance peaks. (b) Density of graphene−SiC states for various levels of F-intercalation weighted by projection onto pristine graphene π and π* states at and about the Dirac point. (c) Schematic band structure of pristine graphene about the Dirac point highlighting states used for obtaining (b).
substrates. This graphene nanomesh has the local atomic arrangement of a graphene sheet but electronically is an intermediate state between the conventional buffer layer and quasi-free-standing graphene. Specifically, graphene electronic states are recovered (unlike in the buffer layer), but the local density of states shows a two-peak structure about the “Dirac point”. Unlike previous methods for production of graphene nanomesh, no ex-situ lithographic procedures were required, thus minimizing the number of defects produced in the graphene layer. The unique electronic properties for this graphene nanomesh suggest possibilities for further study. For instance, the presence of local maxima in the LDOS about the “Dirac point” may be exploited through negative differential resistance related devices.45
to the Fermi level (red and green curves). Analysis of the states in the graphene nanomesh indicates that these states tend to have density localized away from regions where the graphene sheet is commensurate with and strongly bound to the SiC substrate. As the number of F atoms is increased to 69, the projection onto graphene states at the K-point yields a single peak due to the recovery of the Dirac point crossing, which is shifted up in energy to 0.41 eV due to strong p-type doping by the F atoms. Our calculations show clearly that the system with 51 Fintercalants (47.2% intercalation), our model for the partially intercalated nanomesh, is an intermediate state between the conventional buffer layer (with no graphene-like character) and a carbon layer that closely resembles free-standing graphene (69 F-intercalants). In this unique intermediate system, the graphene electronic structure is recovered but modified by the presence of two peaks, likely due to interaction with the substrate. Other graphene-like states close to the K point are also retained but broadened. These features are consistent with the two-peak feature observed in STS in Figure 5a. We acknowledge that in order to have a comprehensive theoretical comparison against the experimental STS results simulation of the complete STS-DOS of our theoretical system is required. However, to achieve that, a very dense sampling of k-points is required so that features about the K-point are adequately represented in our calculations. In light of this, we have opted to perform a wave function projection procedure to study the states about the Fermi level, as this analysis provides similar physical information about the electronic structure while avoiding the computational effort associated with dense kpoint sampling. We also note that the theoretically observed graphene-like features are closer to EFermi than the peaks observed in the STS data, indicating a larger p-doping effect in the experiment. One possibility for this, as suggested by earlier DFT studies,44 is that some F atoms may be covalently bound to the graphene sheet in the experiment.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10582. STM image showing the deposited C60F48 molecular monolayer on substrate prior to annealing procedures. A STS spectrum of buffer layer is also presented with comparison to the STS spectrum of graphene nanomesh. Details of theoretical calculations used to generate Figure 5b are also included (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Present Address ⊥
S.L.W. Institute of Materials Research and Engineering, Agency for Science Technology and Research, 2 Fusionopolis Way, Innovis #08−03, Singapore 138634.
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Author Contributions
CONCLUSION In summary, we have demonstrated using a combination of experiment and theory that a graphene nanomesh with modified electronic states can be achieved through partial intercalation of the conventional buffer layer on SiC(0001)
S.L.W. performed the STM and STS experiments. K.H.K. carried out DFT calculations and simulations. A.T.S.W. supervised the experiments, and S.Y.Q. supervised the calculations. K.H.K. and S.Y.Q. discussed the theoretical interpretation of experimental results. The manuscript was 29198
DOI: 10.1021/acs.jpcc.5b10582 J. Phys. Chem. C 2015, 119, 29193−29200
Article
The Journal of Physical Chemistry C
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written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS KHK and SYQ gratefully acknowledge S. Kim and Y.-W. Son for providing the relaxed atomic coordinates for the buffer layer, as well as support from the Singapore National Research Foundation under grant NRF-NRFF2013-07. Computations were performed on the NUS Graphene Research Centre cluster. We acknowledge support from the Singapore National Research Foundation, Prime Minister’s Office, under its medium-sized centre program.
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