Modulating the Electronic Properties of Graphene by Self-Organized

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Modulating the Electronic Properties of Graphene by Self-Organized S Identical Nanoclusters and Atomic Superlattices Confined at Interface Donglin Ma, Zhongqiu Fu, Xuelei Sui, Keke Bai, Jiabin Qiao, Chao Yan, Yu Zhang, Jingyi Hu, Qian Xiao, Xinrui Mao, Wenhui Duan, and Lin He ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04874 • Publication Date (Web): 25 Sep 2018 Downloaded from http://pubs.acs.org on September 26, 2018

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Modulating the Electronic Properties of Graphene by Self-Organized S Identical Nanoclusters and Atomic Superlattices Confined at Interface ⊥ Donglin Ma†,‡,§, Zhongqiu Fu†,§, Xuelei Sui , Keke Bai†, Jiabin Qiao†, Chao Yan†, Yu ⊥ † † † Zhang , Jingyi Hu , Qian Xiao , Xinrui Mao†, Wenhui Duan , and Lin He†,*

†

Center for Advanced Quantum Studies, Department of Physics, Beijing Normal

University, Beijing, 100875, People’s Republic of China ‡

Department of Physics, Capital Normal University, Beijing, 100048, People’s

Republic of China ⊥

State Key Laboratory of Low-Dimensional Quantum Physics and Collaborative

Innovation Center of Quantum Matter, Department of Physics, Tsinghua University, Beijing 100084, People’s Republic of China

ABSTRACT Ordered atomic-scale superlattices on surface hold great interest both for basic science and for potential applications in advanced technology. However, controlled fabrication of superlattices down to atomic scale has proven exceptionally challenging. Here we develop a segregation method to realize self-organization of S superlattices at the interface of graphene and S-rich Cu substrates. Via scanning tunneling microscope measurements, we directly image well-ordered identical nanoclusters superlattices and atomic superlattices under the cover of graphene. Scanning tunneling spectra show that the superlattices in turn could modulate the electronic structure of top-layer graphene. Importantly, a special-ordered S monoatomic superlattice commensurate with graphene lattice is found to drive semi-metal graphene into a symmetry-broken phase— —the electronic Kekulé distortion (KD) phase, which opens a bandgap of ~245 meV. KEYWORDS: graphene, interface, nanocluster superlattices, atomic superlattices, Kekulé distortion, scanning tunneling microscope

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Creation and control of periodic atomic-scale structures to build nanodevices and nanosystems meet the emerging need of nanoelectronics and data storage.1–3 Thanks to the invention of scanning tunneling microscope (STM),4–6 it becomes possible to fabricate atomic superlattices on metal surfaces by manipulation of STM tip.7–12 However, the drawback of tip manipulation is obvious: the atomic-scale precision is almost impossible to realize on a large scale. Another promising route is self-organized growth of periodic adatoms on designer surfaces,13–18 which has advantages in fabrication of large-area atomic superlattices with tunable size and periodicity.19 However, there are still two severe difficulties that need to overcome for the application of the atomic superlattices. One is that the atomic superlattices are usually only stable at low temperature and become unstable at high temperature due to thermal disturbance. The other is that the atomic superlattices on surface are easy to be destroyed because of absorption of other different atoms/molecules. Therefore, most of these delicately-built periodic atomic structures were realized in cryogenic environment and ultrahigh vacuum (UHV), which in principle hinder their potential application. The idea of self-organization of atomic superlattice at the interface between graphene and the supporting substrate naturally overcome the above-said two shortcomings of the atomic superlattices at the surface. Interface could be seen as a Z-confined two-dimensional space compared to the open space of surface. The confined space provides extra stability against thermal disturbance and protects the "trapped" atoms or molecules at the interface from contaminations.20 Intercalation of different atoms into the graphene-substrate interface have been reported in several graphene supported systems.21–41 However, the intercalation process has to be accomplished below the etching temperature of graphene (600~1000 K in UHV), which hinder the further evolution of intercalated atoms into large-area atomic superlattices.


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In this letter, we adopt an alternative strategy to introduce adatoms into graphene-substrate interface. Adatoms are designed to segregate from the bulk of substrate under the cover of graphene at graphene growth temperature (~1300 K), which process occurs above the temperature of the intercalation process reported previously. With the assist of high temperature, we realize self-organized S nanoclusters superlattices and atomic superlattices at interface between graphene and S-rich Cu substrate. Scanning tunneling spectra show that the superlattices in turn could modulate the electronic structure of top-layer graphene. Most intriguingly, our experiment demonstrates that the S monoatomic superlattice in special order can tune the band structure of graphene, driving graphene into the electronic Kekulé distortion (KD) phase with sizeable bandgap.

RESULTS AND DISCUSSION Figure 1 schematically shows chemical vapor deposition (CVD)-based growth process of graphene monolayer on Cu substrates. To introduce S adatoms confined at the interface between graphene and the substrates, we use S-rich Cu foils as the supporting substrates.16,42–44 In our experiment, the S-rich Cu substrates were annealed at high temperature as the first step to activate the S atoms both on the metal surface and in the bulk. In the second step, carbon sources were introduced into the system for graphene growth, then the sample was slowly cooled to room temperature (~20 ℃/min). The S atoms segregate from the Cu foils during the growth process, which is confirmed by our X-ray photoelectron spectroscopy (XPS) measurement (Fig. S1), and form two types of ordered S superlattices at the interface, as demonstrated subsequently in Fig. 2 and Fig. 4. The as-grown sample was delivered into UHV chamber from atmosphere and then annealed at ~650 K for degassing. All STM measurements were performed at 4.2 K unless otherwise noted. Figure 2 presents STM characterizations of the first type of S superlattice at the interface. As shown in Fig. 2a, large-area square-ordered superlattice of the intercalated S atoms over several hundreds of nanometers can be observed in our STM


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measurement. The unit cell of the superlattice is rectangle with anisotropic periodicities of ~1.2 nm and ~1.5 nm respectively. In the zoom-in image (white dashed box in Fig. 2a), typical honeycomb lattices of the topmost graphene sheet were observed with blurred undercover features (Fig. 2b). The Fourier transform (FT) of the image (inset in Fig. 2b) clearly reflects the detailed periodicity of graphene lattice (the outer black-circled points) and the square-like S nanocluster superlattice (the inner red-circled points), as well as their spatial relationship that the superlattice is in line (or 90°) with graphene lattice in one crystalline direction. At low tunneling bias, atomic-resolved STM image of the intercalated S nanometer was obtained as shown in Fig. 2c. The nanometer cluster looks like the ordered assembly of S adatoms in a special “dice-five” shape (red circles in white box of Fig. 2c) with a size of ~0.5 nm, which is similar as the structure of S clusters on Cu(100).45 Such a result, along with the result obtained by our XPS measurement, demonstrates explicitly that the studied sample is the graphene monolayer on Cu substrate and the interface is intercalated with S atoms. It is also noted that the apparent height of S clusters is only ~30 pm, consistent with the apparent height of S atoms on Cu(100) (Height profile seen in Fig. S2).46 Based on the above analysis, detailed DFT calculations including the Cu(100) substrate were carried out to unravel the atomic-scale structures of nanoclusters. After an extensive structural search and comparison with high-resolution STM images, an energetically favorable model is found as shown in Fig. 2d. From this DFT relaxed model, we distinguish that the nanocluster could be a c(2×2)-like S5 assembly. The simulated STM image of this proposed structure (Fig. 2e) show identical “dice-five” topography, which fits well with the experiment. It is worth to mention that the observed S nanocluster superlattice on Cu(100) are topologically alike to p(2×2)-S reconstruction or clover-like √17 × √1714° -S reconstruction in previous literatures,45–49 but quite different in symmetry, periodicity and fine structures. It indicates that graphene templating effect take place in the formation of this new S nanocluster superlattice. All above evidences show that segregated S adatoms spontaneously assemble themselves into identical nanometer


clusters, and

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self-organize into large-area hierarchical superlattice at the interface between graphene and Cu substrate. Importantly, the obtained nanoclusters superlattice is quite robust and still stable after several cycles of air exposure and degas annealing (~650 K), as shown in Fig. S3. To further explore mechanism of the superlattice formation at interfaces, we carefully studied structures around boundaries of the superlattice. Figure 2f exhibits typical STM morphology around the edges of the nanocluster superlattices. It is surprisingly found that the isolated S nanocluster “vacancies” (red squares in Fig. 2f) or even individual S nanoclusters (Fig. S4) at the edges still strongly follow the rectangle symmetry of the superlattice. This observation indicates that the Cu(100) surface exerts a predominant templating effect on the periodic position of S nanoclusters. It should be noted that interaction between nanoclusters play role in the formation of the superlattice. We additionally conduct an analysis on nearest-neighboring average pair distances ̅ compared with the noninteracting adatom random separation distribution ( ) for nanocluster superlattice.16 As shown in Fig. S5, the ̅ (~1.3 nm) obviously shift towards larger value compared with the peak position (~0.5 nm) in ( ). The results show that long-range repulsive interaction exists in-between S nanoclusters, which also stabilizes the superlattice. Thus, it concludes that the substrate templating effect together with repulsive interaction in-between nanoclusters determine the periodicity of the nanocluster superlattice to a large extent. However, these two factors could only induce “square”-like nanocluster superlattice following the symmetry of Cu(100) substrate, which cannot explain the observed anisotropic periodicity of the superlattice (rectangle symmetry). Figure 2g shows a typical STM image around the boundary of the superlattice (indicated by blue lines). On the left region of the boundary, we do not observe the S nanoclusters superlattice. Instead, a typical one-dimensional moiré pattern generated by the lattices of graphene and Cu(100) facet is clearly observed. Interestingly, the one-dimensional moiré pattern on the left region is in line with the S superlattice on the right region (indicated by white dashed lines). This strongly indicates that the periodicity of the nanocluster superlattice is


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possibly modulated by the moiré pattern of graphene and the underlying Cu facets. In Fig. 2h, we schematically model the three-layer structure of graphene, S superlattice and Cu(100). The graphene and Cu(100) is stacked in alignment (or 90°) and the moiré pattern could be visualized through the model simulation. The short-axis periodicity of the moiré supercell (~1.2 nm) is in coincidence with that of the S nanoclusters superlattice (white dashed lines). This observation implies that the one-dimensional graphene moiré pattern plays an extra templating role in stabilizing more condensed S nanoclusters arrays in parallel direction while no (or weak) templating effect on the vertical direction. In all, the substrate templating effect, repulsive interaction in-between nanoclusters together with graphene moiré pattern templating effect jointly determine the formation of S nanocluster superlattice. In turn, the underlying S nanocluster superlattices could modulate the electronic properties of on-top graphene. Figure 3a shows the tunneling spectra on graphene/nanocluster superlattice regions which is quite different from pristine regions (seen in Fig. S6). The Dirac point shifts upward to -160 meV, and a new dip arises at 350 meV. The dip is assigned as the emergence of new superlattice Dirac points at the conduction band, which are generated by the period potential of S nanocluster superlattice. The absence or faint feature of superlattice Dirac points at the valence band (theoretically at -670 meV) is possibly due to electron-hole asymmetry. The Line profile of tunneling spectra along the nanocluster superlattices (Fig. 3b) demonstrates that the energies of Dirac points and superlattice Dirac points remain constant whatever on and off the nanoclusters, proving the fact the superlattice potential generated by nanoclusters is a global effect. It is worth noting that the spectrum intensities along the nanocluster superlattice show periodic oscillation as indicated by the white dashed curve. Through tunneling spectra mappings (Fig. 3c, d), it could be clearly seen that the regions of nanoclusters always show a lower local density of states (LDOS) compared to around regions. This phenomenon indicates nanoclusters also have local effect on graphene which suppress electron density on the spot. But not like the global effect induced by superlattices, the local effect didn’t change the electronic band structure of


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graphene. Figure 4 presents STM characterizations of the second type of S superlattices at the interface. Figure 4a shows the main feature of the sample that a large amount of dispersed atomic protrusions with a density of (5.5±0.2) × 1014 cm-2 could be visualized on graphene lattices. These protrusions were attributed to individual S adatoms trapped at the interface. The S adatoms locally exhibit square or hexagonal order (inset of Fig. 4a). However, there is no long-range order in large area. Such a result is further confirmed by FT of the image (Fig. 4d), which shows scattering-ring feature (red dashed rings) besides the set of clear reciprocal lattice of graphene (black circles). We define the average radius of the rings as 1/ , where corresponds to the average pair distance of the S adatoms and is estimated to be 0.5±0.15 nm. A relatively high kinetic energy or the short relaxation time of the S adatoms may be reason that limit the atoms forming long-range order in large area. This is especially true for areas with high coverage of S adatoms. For areas with low coverage of S adatoms, it is relatively easy to observe ordered S adatoms superlattices confined at the interface. As shown in Fig. 4b and 4c, ordered S adatoms superlattices with a S density of (2.6±0.2) × 1014 cm-2 and (3.5±0.2) × 1014 cm-2 were clearly observed at the area. The FT of the images (Fig. 4e, f) exhibit clear patterns of both the graphene lattice (black circles) and the S adatoms superlattice (first-order points marked by red circles). The FT patterns of superlattices show the symmetries of stripped hexagonal lattice and square lattice respectively, indicating of the diversity of atomic superlattices in the interface. Furthermore, the positions of the S adatoms have strong correlation with the high-symmetry points of graphene lattice, i.e., most of which are right underneath carbon atoms (red cycles in Fig. 4a inset). It indicates that the graphene may play a notable role of atomic templating in the formation of the S adatoms superlattices. Analysis on nearest-neighboring average pair distances ̅ compared with the noninteracting adatom random separation distribution ( ) for atomic superlattices were also conducted in these regions (Fig. S7). The data also strongly indicates that long-range repulsive interaction exists in-between atoms in the superlattices.


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It was proposed that specific-ordered exotic adatoms could tune the electronic properties of graphene into new quantum phases beyond the semimetal state of pristine graphene.50–58 One possible candidate of novel quantum phases in graphene is the Kekulé distortion phase.59–62 Exotic adatoms on graphene could scatter electrons between two non-equivalent Dirac cones (valleys) at the corners of the Brillouin zone (BZ) (Fig. 5a). The so-called intervalley scattering generates the Friedel oscillations (FO)—the electronic ripples in total charge density—in the vicinities of the adatoms. When the arrangement of the adatoms on graphene follows an (√3 × √3)R30° (R3) order (indicated with three different colors in inset of Fig. 5a), the induced FO of individual adatoms could be in a coherent phase and behave as the RKKY-type interaction between the adatoms. The coherence of the FO can significantly strengthen oscillations of LDOS and lead to the emergence of the KD phase in graphene. Fig. 4b shows a special area of herringbone-like morphology. The one-dimensional strips (red dashed lines marked in Fig. 5b) are likely to be the corrugation of underlying substrate. Figure 5c (white box in Fig. 5b) shows clear honeycomb lattices of graphene emerging over the whole area (white hexagons in Fig. 5c). Another important feature is that hexagonal contrasts of graphene lattice show a typical R3-ordered atomic feature (red hexagons in Fig. 5c), which is exactly alike to the predicted KD phase. The simultaneously-acquired dI/dV mapping gives more clearly evidence of the R3-ordered nature of the CDW state (Fig. 5d). Above results indicate that a typical KD phase was observed and the stripped underlay may be the origin of the special symmetry-broken phase of graphene. To elucidate the spatial configuration of graphene and substrate, FT of Fig. 5c is shown in Fig. 5e. Three sets of ordered patterns are marked in the image: the outer one corresponds to the reciprocal lattice of the graphene lattice (Qg, black circles); the intermediate pattern corresponds to the reciprocal lattice of the typical R3 superlattice, which is assigned to the KD phase (Qk, blue circles); the rest inner scattering peaks (red circles) can be seen as a largely stretched hexagonal pattern of underlying “substrate”, which is attributed to the reciprocal pattern of the ordered S adatoms superlattice, as


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demonstrated in Fig. 5d. Figure 5f schematically shows the atoms of the “substrate” (red balls) in real space superimposed on graphene lattice by extracting the details of the pattern from the FT. The atoms of the “substrate”, which is attributed to the S adatoms, are commensurate with graphene lattice and can be described by two unequal integer sums of graphene vectors = − 5" (~1.13 nm) and # = + " (~0.43 nm), respectively. By further coloring the graphene lattice into three-color Kekulé texture (Fig. 5f upper right), it is surprisingly to find that the ordered adatoms sit in the same-color grid, which follow a hidden Kekulé order. We employed a phenomenological model of the KD phase to further confirm the proposed configuration (left panel of Fig. 5g). The simulated STM image (right panel of Fig. 5g) reproduces well the experimental topography shown in Fig. 5b and Fig. 5c, demonstrating the validity of the proposed configuration of graphene and adatoms superlattice. This well explains the origin of such strong KD phase in this area. The symmetry of graphene may play an important role in the formation of ordered adatoms superlattices at interface since that the adatoms superlattice to a large extent follows the order of graphene lattice. The KD phase is expected to possess a gapped band structure which depends on strength of intervalley scattering induced by adatoms. As characterized by our STS measurements (Fig. 5h), the spectra with different tip-sample distances (tuned by setpoints) show similar asymmetric bandgap of 245±5 meV ranging from -100 meV (top of the valence band) to 145 meV (bottom of the conduction band), which is totally different from the gapless feature of pristine graphene. Bandgap opening of graphene is of central importance for electronics applications of graphene. There are three general scenarios for the bandgap opening: (i) bandgap induced by chemical absorption; (ii) bandgap resulted from local strain;63,64 (iii) bandgap generated by strong interaction between graphene and substrate. We can easily rule out scenarios (i) and (ii) because there is neither chemical absorption nor local strain according to our STM measurements (strain analysis seen in Fig. S8). In scenario (iii), both experiment and theory demonstrated explicitly that there is no


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observable bandgap for graphene on Cu substrate due to their weak interactions.65–68 Considering the observation of the unique R3 CDW morphology and special-ordered S superlattice in this region, it is reasonable to conclude that the bandgap opening originates from the KD of graphene. The origin of the gap in the KD phase was attributed to inequivalent electrons hopping t1 and t2 between the nearest sublattices (inset in Fig. 5h). According to the simulation, a similar gap would open at Γ point considering ~5% difference between t1 and t2, creating massive Dirac fermions with effective mass of mD= 0.28±0.02 me (see supplementary method and Fig. S9 for details of calculation). Compared with previous work, the observation of bandgap opening here may be attributed to two reasons: one is that a more ordered atomic array was formed under graphene compared to a hidden-Kekulé order, which profoundly enhance the coherence of FO; the other one is that the S atoms may have a stronger coupling with graphene lattices compared to Cu vacancies, which may strengthen the FO in the vicinity of atoms.

CONCLUSION In conclusion, our experiments describe a method to create ordered S nanoclusters superlattice and adatoms superlattice through self-organization at the interface of graphene and Cu substrate. Both the symmetries of graphene and Cu substrate affect the self-organization process, leading to various self-organized nanostructures. The superlattice with specific periodicity in turn modulate the electronic properties of graphene into the KD phase. The reported self-organization of atomic superlattices at interfaces could be extended to other systems, which may provide a route to realize exotic electronic states in graphene and other two-dimensional materials.

METHODS CVD preparation of graphene. The 25-µm Cu foil was purchased from Alfa Aesar. Before growth, Cu foil was first electropolished at 1.5 V DC voltage for 60 min, using


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a mixture of phosphoric acid and ethylene glycol (volume ratio = 3:1) as the electrolyte. The pre-treated Cu foil was loaded into a 2-inch quartz tube of low-pressure chemical vapor deposition furnace for sample growth. The Cu foil was first heated from room temperature to 1,000 °C in 30 min and kept for another 30 min, with 50 sccm (standard cubic centimeter per minute) H2 and 50 sccm Ar as carrier gas. In the second step, the furnace temperature was set to 1000°C and then 5 sccm CH4 was introduced into the furnace to grow graphene for 10 min. After all growth, the sample was cooled down to room temperature slowly (~20 ℃/min). STM and STS measurements. STM/STS characterizations were performed in ultrahigh vacuum scanning probe microscopes (USM-1500 and USM-1400) from UNISOKU. The STM tips were obtained by chemical etching from a wire of Pt/Ir (80/20%) alloys. Lateral dimensions observed in the STM images were calibrated using a standard graphene lattice and a Si (111)-(7×7) lattice and Ag (111) surface. The dI/dV measurements were taken with a standard lock-in technique by turning off the feedback circuit and using a 793-Hz 5mV A.C. modulation of the sample voltage.

SUPPORTING INFORMATION The supporting information involves DFT calculations, tight-binding calculations, XPS data, supplementary STM images and strain analysis et al. The supporting information is available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION * Corresponding Author E-mail: [email protected] § Donglin Ma and Zhongqiu Fu contributed equally to this work. The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant ACS Paragon Plus Environment

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Nos. 11674029, 11422430, 11374035), the National Basic Research Program of China (Grants Nos. 2014CB920903, 2013CBA01603), the program for New Century Excellent Talents in University of the Ministry of Education of China (Grant No. NCET-13-0054), the China Postdoctoral Science Foundation (No. 212400207). L.H. also acknowledges support from the National Program for Support of Top-notch Young Professionals, support from “the Fundamental Research Funds for the Central Universities”, and support from “Chang Jiang Scholars Program”.

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Figure 1. A general synthetic method for self-organized growth of sulfur superlattices at interface between graphene and substrates. In the first step, the growth substrates were annealed at high temperature to dissolve and activate S atoms in the bulk. In the following, carbon sources were introduced into the system for graphene growth, then the samples were slowly cooled to room temperature. During the slow cooling process, vast of S atoms begun to segregate from the bulk onto the metal surfaces due to the decrease of solubility. At this stage, the S adatoms had sufficient energy and time to rearrange beneath the as-grown graphene sheet to form kinds of ordered assemblies.


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Figure 2. Self-organization of S nanoclusters superlattice at interface. (a) STM topography of large-area square-ordered superlattices (Vb = 800 mV, I = 400 pA). (b) Zoom-in image clearly exhibiting honeycomb lattices of graphene (Vb = -450 mV, I = 200 pA) Inset: Fourier transform (FT) of the large-version STM image of (b).. (c) Atomic-resolved STM image of the superlattice under graphene (Vb = 50 mV, I = 150 pA). The nanoclusters are the ordered assembly of adatoms in a special “dice-five” shape, as illustrated by red circles in white box. (d) S5 nanocluster on Cu(100) as determined by DFT calculations. Sulfur atoms are indicated by yellow balls, while copper atoms are the orange balls. (e) Simulated STM image of the S5 nanocluster on Cu(100). (f) Typical STM morphology (Vb = 150 mV, I = 310 pA) around the edges of the nanocluster superlattices shows S nanocluster “vacancies” (red squares) at the edges. (g) A boundary of the nanoclusters superlattice (Vb = 50 mV, I = 200 pA). One-dimensional moiré patterns are indicated by red dashed lines. The missing-row


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defects and adding-row defects of the superlattice around the boundary were marked in white dashed oval and rectangle, respectively. (h) Schematics of the three layer configuration of graphene, S superlattice and Cu(100) substrate. The moiré pattern generated by the misalignment of graphene and Cu(100) is illustrated by white dashed rectangle. The unit cell of S superlattice is illustrated by red dashed rectangle. The scale bars are 6 nm in (a), 1 nm in (b), 4 nm-1 in inset of (b), 3 nm in (c, f, g).

Figure 3. The electronic properties of graphene modulated by the S nanoclusters superlattice. (a) Tunneling spectra on nanocluster region (black) and off that (red) respectively. (b) Line profile of tunneling spectra along the nanocluster superlattices. (c, d) Representative STS mapping of nanocluster superlattices at 91 meV and -111 meV. The scale bars are 2 nm in (c, d).


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Figure 4. Self-organization of S adatoms superlattice at the interface. (a) STM topography of large-area graphene lattices with a large amount of dispersed adatoms underneath graphene (Vb = 200 mV, I = 250 pA). Inset shows a small area where the S adatoms locally exhibit a weak square order. (b, c) Large-area ordered S adatoms superlattice with clear graphene lattice (Vb = 650 mV, I = 250 pA; Vb = -500 mV, I = 200 pA). (d, e, f) Corresponding FT images of (a, b, c). The scale bars are 4 nm in (a), 3 nm in (b), 2 nm in (c), 4 nm-1 in (d, e, f).

Figure 5. Realization of Kekulé distortion (KD) phase on graphene modulated by the S adatoms superlattice. (a) Brillouin zone and real-space schematics (upper left) of graphene in the KD phase. (b) STM topographic image of strong herringbone-like charge-density wave (CDW) morphologies (Vb = 300 mV, I = 200 pA). (c) Low-bias


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image (white box area in (a)) showing honeycomb lattice of graphene and R3-ordered atomic features, as marked by white and red hexagons (Vb = 30 mV, I = 200 pA). (d) The simultaneously-acquired dI/dV mapping (Vb = 218 mV, I = 200 pA). (e) Fourier transform of (c). (f) Schematic illustrations of underneath adatoms superimposed on color-coded graphene lattice. (g) Phenomenological model of the KD phase on the configuration of (f). (h) Scanning tunnelling spectra on the KD phase with different tunneling current setpoints (from 200 to 1000 pA). Edges of valence band and conduction band are indicated by arrows. Inset is the honeycomb model for calculation, with inequivalent electrons hoppings t1and t2 between nearest sublattices. The scale bars are 2 nm in (b), 1 nm in (c, d), 4 nm-1 in (e).


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Modulating the Electronic Properties of Graphene by Self-Organized

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