Article pubs.acs.org/JPCC
Spatially Resolved Modification of Graphene’s Band Structure by Surface Oxygen Atoms Colin Harthcock, Abdolreza Jahanbekam, Yi Zhang, and David Y. Lee* Department of Chemistry and Materials Science & Engineering Program, Washington State University, Pullman, Washington 99164, United States ABSTRACT: We present the spatially resolved modification of the topography and electronic properties of monolayer graphene by a low dosage of atomic oxygen on the nanometer scale. Using the combination of an ultrahigh-vacuum scanning tunneling microscope and a gas beam of oxygen atoms, we show that the surface O-atoms, even at a low coverage of O/C = ∼1/150, serve as p-type dopants that leads to site-dependent partial and full graphene band modifications up to a gap of a few hundred millielectronvolts. The degree of modification and the number of O-atom-induced charge-holes per area are inversely proportional to the distance between the measuring position and the location of the nearest adsorbate. However, the number of holes contributed per oxygen atom is found to be a site-independent constant of 0.15 ± 0.05. For a small population of adsorbates taller than 4 Å, the graphene energy bands are no longer resolved; instead, our tunneling spectra show very spatially localized but highly dense states over a wide potential range, which indicates a sole tunneling contribution from the tall stacks of the electron-rich O-atoms and a complete decoupling from the graphene bands.
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INTRODUCTION Graphene is a highly symmetric sheet of crystalline carbon that is well-known to possess excellent electron mobility and other unique physical properties,1−3 many of which have made graphene a potentially exciting 2-D material for applications such as field-effect transistors, highly sensitive detectors, and organic photovoltaic cells.4−7 However, the lack of an electronic band gap has led to restrictions on broader applications of graphene in advanced nanoelectronic devices, and this problem has precipitated considerable research emphasis in recent years on modifying graphene’s band structure.8−12 It has been experimentally shown that a single nonmetal element (such as hydrogen, oxygen, nitrogen, and boron) addition both into and onto the graphene lattice can be effective in changing graphene’s surface chemical composition in order to disrupt its high lattice symmetry.13−21 This indicates that the band structure of graphene can potentially be custom-changed as a function of coverage of the foreign species, making functionalized graphene a better candidate for future lightweight nanoelectronic devices. In terms of probing the effect of doping on graphene’s band structure, one needs to consider the local modification based on the nanometer-scale spatial distribution of the foreign species as well as the corresponding ensemble effect to the entire sample in the millimeter-scale. Angle-resolved photoemission spectroscopy (ARPES) has been demonstrated to be the ideal technique for ensemble measurements of the occupied states in the momentum domain, and high angular resolution can be achieved using synchrotron radiation.8 On the other hand, scanning tunneling microscopy and spectroscopy (STM and STS) can measure both the occupied and unoccupied states with high, albeit very localized, angstrom-scale spatial resolution.22 © XXXX American Chemical Society
The effect of hydrogen atom chemisorption on the band structure of graphene has been theoretically calculated by Lu and co-workers to be a complex and nonlinear relationship, with the graphene band gap not opened until there is a relatively high surface coverage of >65%.23 For ensemble measurements, Hornekær and co-workers have performed ARPES investigation of a hydrogen-induced band gap opening with an at least 0.5 eV gap at 21% surface coverage on a graphene/Ir(111) system.24 For localized STS measurements, Stroscio and co-workers have reported the creation of a 1.5 eV band gap by exposing graphene/SiC samples to atomic hydrogen,14 while Hu and co-workers have observed highly ordered H atom chemisorption on both sides of a graphene monolayer on copper with a band gap up to 1.2 eV.21 For oxygen, the direct effect of O-atom chemisorption on the graphene band structure has been calculated by Luo and coworkers: a band gap of 1.19 eV can be created with a low O/C = 1/16 surface compositional ratio,25 and a wider gap of ∼3.4 eV can be formed with a O/C ratio of 1/2.26 Experimentally, chemisorption of atomic oxygen on graphene has been studied with single-atom resolution by Hersam and co-workers using ultrahigh vacuum (UHV) STM.16 Supported by density functional theory calculations, they demonstrated that each O-atom preferred to chemisorb between two carbon atoms in forming a surface epoxy structure and induced a large (∼1.2 nm in diameter) surface buckling of graphene around the chemisorption site. Using a UHV STM and a low-flux gas beam of atomic oxygen,27 we report that the graphene band structure can be Received: June 16, 2017 Revised: August 9, 2017 Published: August 10, 2017 A
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next). In addition, Moiré patterns with a periodicity of ∼6 Å (indicated by the yellow dots in Figure 1e) due to the graphene−copper interaction can also be observed. The cleanliness of the graphene sample shown in Figure 1a becomes obvious when being compared to the after-dosage surface topographies in images c−e. Since our samples have nearly uniform graphene coverage on copper (according to the manufacturer), we have neither observed the distinguishable “stripe” and “rhombic” phases nor the graphene-induced Cu surface depressions that have been previously reported for graphene islands on Cu foils.28,29 However, we have frequently observed linear ripples and wrinkles of graphene on the edges of crystalline Cu areas,30 and one example of such tall graphene defect-like structures is shown in the upper right-hand corner of Figure 1c. We do not refer to our O-atom dosed graphene as “graphene oxide”, since this term is commonly used for graphene surfaces containing other types of functional groups (such as OH and COOH) in addition to epoxides.31 It is clear from Figure 1c−e that the Oatom distribution on graphene is completely random and has no preference for residing either on flat areas or at the Cu stepping edges, and the O-atom adsorbates have formed surface clusters of various sizes. This observation of adsorbate distribution, in general, agrees with the images previously reported by Hersam and co-workers.16 Particularly in the highresolution images of Figure 1d and 1e, most of the graphene surfaces remain free of O-atoms while some areas contain bright features aggregating close to one another, and all the surface features are similar in height (4 Å). The population of these tall clusters, among all the adsorbate features, is estimated to be about 3%. This significant height difference was not previously reported in nitrogen-doped and boron-doped graphene when the graphene monolayers were grown in the presence of the doping reagents; as a result the doping atoms were found residing within the graphene lattice plane with measured heights of 0.6 ± 0.2 and 1.0 ± 0.1 Å, respectively.15,19 In addition, no direct relationship between areas and heights of the oxygen clusters can be concluded, as the tall cluster shown in Figure 1d has a smaller area than the two large-area but shorter ones indicated by the arrows in Figure 1c. More detailed topographical information about the lightly dosed graphene is presented in Figure 1f, which is an illustration mapping the exact graphene honeycomb lattice (shown in the gray background) and the adsorbate areas (shown as white bubbles) of Figure 1e. In order to precisely define the adsorbate area and elemental coverage of our dosed graphene sample, it is necessary to take into account the previously reported buckling effect of O-atoms on graphene16 since the smallest feature we have observed has a radius of ∼5 Å (∼20 Å2 in area), which is significantly larger than the radius of a single oxygen atom. Furthermore, the O-atom coverage can differ between various monitoring areas due to the random spatial distribution and clustering. For Figure 1e the percent adsorbate-modified versus total surface area, i.e., the percent area occupied by the white bubbles in Figure 1f, is about 25%. However, since all the surface features in this image have similarly low heights, we readily adapt the conclusion of the Oatom buckling effect on graphene from ref 16 and attribute each surface feature (the white bubble) to the occupancy of a single epoxide or equivalently one oxygen atom. This assignment is illustrated by a red dot in each white bubble in Figure 1f.
immediately altered with an extremely low surface O-atom coverage. Our STM images indicate that, at this low coverage (with a O/C ratio of ∼1/150), O-atoms distribute themselves randomly, instead of evenly, on graphene. Using STS, we are able to show that the degree of modification on the graphene’s energy band is highly dependent on the local O-atom coverage. While each O-atom contributes ∼0.15 charge-hole to graphene, surface clustering of these adsorbates can also introduce rare but unique and localized electronic properties to graphene.
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EXPERIMENTAL SECTION Gas−surface interaction experiments were performed on chemically vapor deposited (CVD) monolayer graphene supported on copper foil (Graphenea Inc.., with >95% coverage) samples that had been annealed under UHV condition at a mild temperature of ∼600 K for about 6 h. Pt0.8/Ir0.2 tips (Nanoscience Instruments) were used after being either mechanically cut or electrochemically etched. A beam of low-flux oxygen atoms was produced using a gas cracker with an Ir capillary tube and an Ir filament (Mantis Deposition) and was directly aligned to the STM tip−sample junction.27 We typically avoided graphene on top of noncrystalline copper surfaces that were difficult for high-resolution characterizations, and all STM and STS measurements were conducted on top of crystalline Cu substrate areas at room temperature.
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RESULTS AND DISCUSSION Figure 1a represents the graphene sample before being loaded into the UHV chamber, and Figure 1b is an atomic-scale image
Figure 1. (a) This STM image shows the cleanliness of pristine monolayer graphene on copper (2500 Å × 2500 Å, 1.0 V, 100 pA) at ambient conditions. (b) The graphene honeycomb lattice can be routinely observed on top of a crystalline copper surface (20 Å × 20 Å, 400 mV, 200 pA) after being annealed >600 K at UHV. (c) A larger area (428 Å × 405 Å, 700 mV, and 200 pA) STM image of a graphene surface after a light dosage of atomic oxygen. Parts d and e show higher-resolution images (121 Å × 76 Å and 123 Å × 167 Å, both of which were collected at 500 mV and 200 pA) of the dosed sample with a resolved graphene lattice structure. (f) An illustration depicting the O-atom coverage and the adsorbate-modified regions of the dosed graphene surface shown in panel e.
of the graphene monolayer taken under UHV conditions after annealing. Figure 1c shows the O-atom distribution over a relatively large graphene surface area after being lightly dosed with atomic oxygen, while Figure 1d and 1e show smaller area after-dosage images with resolved graphene honeycomb lattice and an O/C coverage ratio of ∼1/150 (to be discussed in detail B
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dopant-induced shift of Dirac energy has also been reported in Figure 3a of ref 19 that illustrated the p-type effect of borondoped graphene as well as in Figure 3d of ref 15 that showed the n-type doping effect of graphitic nitrogen in graphene. Using the similar analysis method, we assign the Ed value of this spectrum to be ∼120 meV. Note that, for chemically modified graphene, the band structure (especially when a gap opens) is no longer linear. This assignment of Ed should be considered as an estimation. Third, the density of states up to +100 meV above Ef is probed to be low (but not zero, as compared to the value at Ef), so no band gap can be clearly defined in this spectrum. Lastly, the depression of the spectrum at Ef remains to be well-defined “V”, instead of “U”, shaped, which indicates a direct (without phonon-assisted inelastic) tunneling process.34 The dark blue ○ trace in Figure 2 corresponds to STS measurements taken on a region with slightly higher O-atom coverage that is indicated by the same dark blue ○ symbol in Figure 1e. When being compared to the previously discussed spectra, the e−ph couplings are no longer observed at this position which is ∼2 nm away from the nearest O-atom adsorbate, possibly due to the reduced graphene planar symmetry by the surrounding O-atom buckling. There are two depressions in this spectrum, and the first one with lower energy can be readily assigned as Ef. Although there is residual (very low but still nonzero) LDOS between the depressions, this ○ spectrum indicates that a band gap is about to open on the graphene surface ∼2 nm away from the O-atom adsorbates. The local charge-carrier or hole density per area, n, of a specific location on the dosed graphene surface can be calculated using
Therefore, the O/C ratio for Figure 1e is measured to be around 1/150, which corresponds to a value of 0.67% for elemental coverage. We rely on STS differential conductance measurements (dI/ dV, which is proportional to the local density of states, LDOS) to directly probe the site-dependent alteration of graphene’s band structure induced by the random distribution and clustering of the surface O-atoms. Spectroscopic derivatives arise from a lock-in amplifier set to 922 Hz with a 10 mV ac modulation applied to the bias of the same frequency. A sample time of 3.5 ms was used along with 7.0 ms for the presampling delay. A setting of 500 mV and 500 pA was used to define the tip−sample distance for all spectra; samples and tips were allowed to equilibrate at room temperature for at least an hour before STS data were collected to ensure stability. All reported STS data are composed of at least 30 spectra that have been averaged. A typical dI/dV spectrum on pristine graphene before Oatom dosage is shown by the black ● trace in Figure 2. This
n=
Ed2 π (ℏvF)2
,15,19 where vF is the Fermi velocity of 106 m/s, with
the observed Ed from STS measurements. The measured Ed of +260 meV in this spectrum indicates a charge-hole density of 5.0 × 1012 per cm2, which can be compared with previously reported charge-carrier density values of (5.4 ± 0.8) × 1012 per cm2 on a N-atom-doped graphene/Cu sample15 and 1.8 × 1013 per cm2 on a N-atom-doped graphene/SiC sample,18 as well as a charge-hole density value of (9.1 ± 5.5) × 1011 per cm2 on a B-atom-doped graphene/Cu sample.19 All of these values were based on the same Fermi velocity of 106 m/s and with the same 2 nm distance from the corresponding adsorbate. Around this ○ STS measuring position, we have counted seven O-atoms within 3 nm and six O-atoms within 2 nm radius (2 nm distance to the edge of the nearest adsorbate feature plus 1 nm adsorbate buckling diameter), which render a value of 0.15 ± 0.02 holes contributed per O-atom. This value is slightly lower but comparable with 0.40 ± 0.24 charge-holes per B atom19 and 0.42 ± 0.07 charge-carriers per N atom.15 Note that these authors observed equal Ed values measured both on top of and in a distance of 2 nm away from the N or B dopants that resided “in” instead of “on” the graphene lattice. For our Oatom dosed graphene, the estimated Ed value of 480 meV (derived from the green △ spectrum in Figure 3; to be discussed later in detail) measured directly on top of an adsorbate is different from the one measured 2 nm away and would correspond to a higher value of 1.7 × 1013 charge-hole density per cm2. In addition, going back to the red □ trace with an estimated Ed value of 120 meV, the measured charge-hole density is 1.06 × 1012 per cm2 for this less crowded area that is 4.75 nm away from the nearest adsorbate. The fourth light blue ◇ trace in Figure 2 corresponds to STS measurements taken in the middle of a close-lying but isolated O-atom adsorbate pair that is indicated by the same light blue ◇ symbol in Figure
Figure 2. STS differential conductance measurements taken on pristine monolayer graphene (black ●), on a graphene surface ∼4.75 nm away from the nearest O-atom adsorbate (red □), on a graphene surface ∼2 nm away from the nearest O-atom adsorbate (blue ○), and in the middle of a close-lying but isolated O-atom adsorbate pair (light blue ◇). All the positions are denoted by the corresponding symbols in Figure 1c and 1e.
“V” shaped spectrum directly indicates a zero band gap with the Dirac energy (Ed) and the Fermi energy (Ef) overlapping on one another, and the observed shoulder-like features at ∼± 60 meV can be attributed to the electron−phonon (e−ph) interactions.32 The red □ trace in Figure 2 corresponds to STS measurements taken on a region of low O-atom coverage that is indicated by the same red □ symbol in Figure 1c. We have vertically shifted the relative dI/dV values for this and the rest of the spectra downward relative to the before-dosage ● trace in Figure 2 for the purpose of easy visual comparison among the spectra. As compared to the before-dosage ● spectrum, the band structure of graphene measured at this position, which is ∼4.7 nm away from the nearest O-atom adsorbate, has been significantly modified. First, the e−ph couplings can still be identified but now are located at −60 and +170 meV. Second, due to the electron withdrawing, or p-type doping, character of O-atoms on graphene,33 we expect the Ed of dosed samples to be raised relative to Ef and attribute the depression at which the dI/dV value is a minimum to Ef. This C
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Cu metal surface states.39 Essentially, we attribute these constant nonzero “Ohmic” measurements (that a constant dI/dV indicates I ∝ V) to the aggregation of electron-rich Oatoms with sufficient heights of >4 Å that can provide widerange and constant LDOS and subsequently shield the tunneling between the STM tip and graphene. This tallcluster-induced high differential conductance, however, is rare (3%) as compared to the partial and full band gap opening processes induced by the majority of surface O-atom adsorbates of low height. Therefore, for ensemble measurements, such as ARPES, this anomalous effect may not be observed. The comprehensive charge-hole contribution of the chemisorbed O-atoms to graphene is summarized in Figure 4, where
Figure 3. STS spectra collected on top of a ∼1 Å tall adsorbate (green △) and of three O-atom clusters that are at least 4 Å in height (black ).
1e. Neither e−ph nor depression features can be seen in this spectrum; instead, a small but well-defined ∼300 meV band gap is detected with an estimated Ed value of 370 meV and chargehole density of 1.0 × 1013 per cm2. Besides vertically separating the minimum dI/dV value of each spectrum in Figure 2 for easy visual comparison, we have kept the individual relative dI/dV scale the same from each set of data, so that the band separation process from pristine graphene (●) to graphene surfaces in close vicinity between two O-atom adsorbates (◇) can be faithfully compared. As expected, the degree of graphene band modification is inversely proportional to the distance between the STS measurement position and the nearest O-atom adsorbate. Due to the uneven adsorbate distribution caused by O-atom clustering, Figure 2 illustrates that different local graphene band structures can be readily observed on the same sample surface that has undergone the same amount of O-atom dosage. We further investigate the graphene band structure directly on top of the O-atom adsorbates, and the results are shown in Figure 3. The green △ trace represents an STS measurement taken on top of a ∼1 Å tall adsorbate that is indicated by the same green △ symbol in Figure 1e. As expected, a wider ∼500 meV band gap than all the measurements in Figure 2 is observed. However, three traces (black solid lines) in Figure 3 represent dI/dV measurements on top of adsorbate features that are taller than 4 Å and show a completely different type of LDOS distribution. This type of featureless spectrum has been observed for several samples, with different tips and on different days. The “V” or “U” shaped STS curve for graphene is no longer observed, and as a consequence no distinguishable Ef or Ed can be identified. Furthermore, these anomalous spectra illustrate a nearly constant nonzero LDOS (even at zero sample bias), and this wide and featureless extension from −500 to +800 mV cannot be explained by the previously reported van Hove singularity models (either the point or the extended type) for graphene.35−37 Our measurements directly indicate that the tunneling processes on top of these tall clusters are fully decoupled from the graphene lattice. Even though epoxide formation could potentially increase the graphene−substrate interaction,38 the possibility of tunneling directly into the copper substrate can also be excluded because of the tall-cluster heights as well as of the absence of a sharp rise in differential conductance corresponding to the onset of tunneling into the
Figure 4. Simultaneous illustrations of charge-hole density per area (red ○) and the number of charge-holes contributed by each oxygen atom (blue ■) as a function of distance between the STS measurement location to the nearest O-atom adsorbate.
site-dependent values of the number of charge-holes per area are plotted as a function of distance r to the nearest adsorbate in red ○. As discussed above, these values were obtained from the STS data shown in Figures 2 and 3, and using the STM image shown in Figure 1c and 1e, the number of charge-holes contributed per O-atom can be readily calculated by multiplying each per area value by the circular area with the radius equal to the distance to the nearest adsorbate, followed by dividing the total number of O-atoms located on the perimeter of this area. The solid red curve in Figure 4 represents a simple 1/r relation, and the close proximity of our charge-holes per area measurements to the 1/r curve can be interpreted such that the electron-withdrawing effect of each O-atom is inversely proportional to r2, while the number of O-atoms being encountered from each measuring site is directly proportional to r. On the contrary, the charge-hole contribution per O-atom for each site is plotted in blue ■ in Figure 4 and appears to be spatially independent. The error bars for large r are mostly contributed by the complexity in accurately counting the number of O-atoms located on the edge of the radius r, since the effective (buckling) areas of two closely located O-atoms can overlap up to 1 nm. For the cases of smaller r the uncertainty for this 1 nm buckling diameter to accurately include the O-atom location becomes significant relative to r, while the numbers of O-atoms to be included can be more precisely determined. Nevertheless, the trend is clear that the number of charge-holes contributed per O-atom to the monolayer graphene is about 0.15 with the uncertainly of D
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CONCLUSION With high-quality graphene becoming commercially available and affordable, full understanding of the postsynthesis modification becomes important for researchers interested in producing graphene-based materials with customizable properties. In summary, we demonstrate here that the combination of STM and STS is a powerful method to monitor the process of functionalizing pristine graphene with simultaneous nanometerscale spatial and millielectronvolt-scale band energy resolutions. While ensemble techniques such as ARPES measure graphene property changes as a whole, this localized effect of an individual adsorbate on graphene also needs to be carefully considered. Our experimental results illustrate that oxygen atoms produce an uneven graphene surface structure of various adsorbate heights and local band alterations. Even at a low coverage level, O-atoms can immediately modify graphene’s band structure with a contribution value of 0.15 charge-hole per O-atom. It is thus possible for one to effectively approach a customizable materials property via carefully controlling the degree of O-atom exposure on pristine graphene.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
David Y. Lee: 0000-0002-0494-9809 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding for this project comes from the Washington State University new faculty start-up support.
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REFERENCES
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DOI: 10.1021/acs.jpcc.7b05938 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jpcc.7b05938 J. Phys. Chem. C XXXX, XXX, XXX−XXX
