Quenching of the Resonant States of Single Carbon Vacancies in

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Quenching of the Resonant States of Single Carbon Vacancies in Graphene/Pt(111) Hyo Won Kim, JiYeon Ku, Wonhee Ko, Yeonchoo Cho, Insu Jeon, and Sungwoo Hwang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08161 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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The Journal of Physical Chemistry

Quenching of the Resonant States of Single Carbon Vacancies in Graphene/Pt(111) Hyo Won Kim‡*, JiYeon Ku‡, Wonhee Ko, Yeonchoo Cho, Insu Jeon, and Sung Woo Hwang Samsung Advanced Institute of Technology, Suwon 16678, Korea

‡ These authors contributed equally to this work. * Corresponding author. Tel: +82-10-6414-9226, E-mail: [email protected].

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ABSTRACT: For more than a decade, investigations of single carbon vacancies in graphene have sought to increase the fundamental understanding of the local electronic, magnetic, and mechanical properties of such vacancies. The single C vacancy in graphene has been known to generate a resonant state through the integration of π orbitals near the missing C atom. Here, we examine single C vacancies in graphene/Pt(111) to explore the effects of graphene– substrate interactions on the local electronic properties of imperfect graphene. Our scanning tunneling microscopy, scanning tunneling spectroscopy, and related density functional theory calculations show the resulting modifications, including the complete disappearance of the resonant state attributable to strong graphene–substrate coupling near the vacancy. The different relative positions of single C vacancies corresponding to the Pt atoms lead both to varying C–Pt bonding structures and strengths and to corresponding changes in the local density of states.

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1. INTRODUCTION Graphene shows promise as a material for next-generation electronic devices because it displays extraordinary electronic and mechanical properties resulting from the twodimensional honeycomb arrangement of its carbon atoms.1,2 Working graphene-based devices, however, show evidence of structural defects inevitably introduced during graphene growth or device fabrication. These defects locally disturb the atomic and electronic structures. Structural defects include zero-dimensional point defects, such as vacancies and interstitial atoms,3-12 one-dimensional grain boundaries, and stacking fault defects.13-15 Even the simplest single C vacancy defect is known to cause dramatic changes in the local electronic,6,7,16,17 magnetic,18-21 and mechanical10 properties of graphene. Because these defects may modify the expected properties of graphene, they may play a crucial role in the creation of new functionalities of graphene. In fact, many groups have studied the basic properties of various intrinsic defects in graphene. Such defects even have been artificially introduced using noblegas ion-irradiation methods to enable systematic studies of graphene.4,6,7,10,11,22 Studies of a single C vacancy in graphene, the simplest possible defect in this material, have been shown to generate a resonant state at the Dirac point of graphene. 6,7,9,16,17,20

The resonant state is a collective phenomenon of the π orbitals near the

vacancy.23 Therefore, the resonant state depends on the coupling of the C atoms with the atoms of the underlying substrate. A single C vacancy in graphite shows a sharp electronic resonance at the Fermi energy.6 In the graphene/Pt(111) system, the localized states decay more quickly than similar states in graphite; in turn, because the interaction and the resulting couplings between carbon atoms and substrate Pt atoms are stronger in graphene,24 its resonant peak has a broader shape compared to the 3



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resonant peak displayed by graphite.7 The complete disappearance of localized vacancy states, which may be possible with even stronger graphene–substrate interactions, however, has never been observed. Here, we present the quenching of the resonant state of single-carbon vacancies in graphene grown on a Pt(111) surface. The quenching of the resonant state was induced by the formation of different bonds between C and Pt atoms near the vacancies and corresponding changes in the local density of states (LDOS), which was investigated using experimental scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and calculations based on density functional theory (DFT). Rather than using a simplified model of graphene/Pt(111), such as 2 × 225 and 3 × 37 moiré periodicity, our calculations used a model with a large moiré periodicity of 9 × 9 in which various types of bonds could be formed.

2. EXPERIMENTAL AND COMPUTATIONAL METHODS An experiment was performed on graphene grown on a Pt(111) substrate. The Pt(111) substrate was cleaned by repeated cycles of Ar+-ion sputtering and annealing under ultrahigh vacuum. Graphene was grown by exposing the clean Pt(111) surface to ethylene, followed by annealing at 1100 K.26,27 Single C vacancies were created by glancing incidence ion irradiation of the graphene on the Pt(111) with 150 eV Ar+ ions at ~ 7 × 10-6 Torr, corresponding to an ion current of 8 µA and room temperature (The yield for the creation of defects is approximately 10-2 number/nm2 with a 5-minute irradiation. (Figure S1)).7 Next, the sample was annealed at 1000 K to remove additional impurities adsorbed on the graphene surface.6,7 STM and STS measurements were performed using a UNISOKU low-temperature STM (UNISOKU Co., Ltd., Osaka, Japan) at 2.8 K, and the STS measurements were 4


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performed using a conventional lock-in technique with a modulation-bias voltage with an amplitude of 10 mV at a frequency of 1 kHz. Our DFT-based calculations employed the Perdew-Burke-Ernzerhof generalized gradient approximation using the projector-augmented-wave method, as implemented in the Vienna ab initio simulation package (VASP).28-30 The electronic wave functions were expanded in a plane-wave basis set with a cutoff energy of 273.9 eV, and the long-range dispersion corrections were included by using the DFT-D3 method of Grimme et al.

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with

Becke–Johnson damping.32 The 7 × 7 Г-centered uniform grid was used to sample k-points. Constant-current STM images were calculated based on the optimized structures using the Tersoff–Hamann approach.33 The presented simulated images map the height of constant electron density of 0.03 e nm–3 for interstitials and 0.06 e nm–3 for vacancy defects integrated over an energy range between the Fermi energy EF and EF + eVb, where the bias voltage Vb was set to 0.5 V. The color scale from dark to bright corresponds to variations in corrugation height over a range of ~1 Å.

3. RESULTS AND DISCUSSION 3.1 Creation of C Vacancies. In Figure 1a, the pristine graphene grown on Pt(111) is visualized with various moiré patterns resulting from the lattice mismatch between graphene and the underlying metal lattice. A beam of Ar+ ions was irradiated on the pristine graphene to introduce defects (Figure 1b). Various point defects, such as interstitial defects4,11,22 and vacancies, 6,7,34 may be created by controlling the energy and angle of Ar+ ion irradiation. Here, we used glancing irradiation with 150 eV Ar+ ions to specifically create single C vacancies. Figures 1c and 1d show the STM images of two different vacancies 5



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created with contrasting brightness. In general, single C vacancies in graphene manifest themselves as bright features in STM images owing to the creation of vacancy states.6,7,17,21 Figure 1c shows a representative STM image of a single C vacancy in graphene; the dI/dV spectrum (blue line) in Figure 1e reveals the clear resonant behavior near the Dirac point of graphene on Pt(111), which, as expected, was approximately at +320 meV.7 However, the C vacancy in Figure 1d shows completely different characteristics, a depression in the STM image, and there was no resonant state near the Dirac point in the dI/dV spectrum (red line in Figure 1e). The depression in the STM image of graphene is attributed to the disappearance of the localized resonant state resulting from the strong interaction between C and Pt atoms near the vacancy and not to other possible origins, such as adatom adsorption,35 substitution36 in vacancy sites, or impurities under graphene.37 In our study, we carefully excluded the possibility of adatoms by annealing the sample after Ar+-ion sputtering. We also verified the elimination of impurities under the graphene layer through examination by STS. Contrary to a recent STM study of graphene on Cu(111) with such impurities, which reported a similar depression in an STM image, the existence of impurities in the graphene/Pt(111) system was indicated not by such features in STM images but by specific patterns in the dI/dV map.25 Therefore, we conclude that C vacancies did induce the unusual STM result shown in Figure 1d, where the depression on the surface was caused by an interaction between C and Pt atoms that was stronger than previously reported.7

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Figure 1. (a) STM topograph of pristine graphene grown on Pt(111) (Vs = 0.5 V, It = 0.5 nA, scale bar = 4 nm). (b) Schematic of defect formation by Ar+-ion irradiation. (c, d) STM topographs obtained after Ar+-ion irradiation ((c) Vs = 0.4 V, It = 30 nA, scale bar = 1 nm; (d) Vs = 0.3 V, It = 1 nA, scale bar = 1 nm). (e) Differential conductance dI/dV spectra obtained at the vacancies in (c) (blue line), (d) (red line), and in graphene (black line). 3.2 Changes of the Interaction Strength between C and Pt Atoms near a C Vacancy.

In fact, results of our DFT calculations clearly revealed that the interaction

strength between C and Pt atoms near a C vacancy varied depending on the position of the vacancy relative to the Pt atoms. For our DFT calculations, we used a 9 × 9 graphene supercell atop a three-layer slab of 8 × 8 Pt(111), which produced a moiré pattern that 7



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matched that of our STM experiment in Figure 1d, with a vacuum spacing of ~10 Å. First, the periodic boundary conditions were optimized (Figure 2a), which reproduced the experimentally observed moiré pattern in Figure 1d. Next, a single C atom was manually removed to form a vacancy. The atomic positions of the model structures were relaxed until residual forces were < 0.02 eV Å–1. Among various possible vacancy sites, the results of three representative model systems, V1, V2, and V3, are shown here (Additional models in Figure S2). V1, V2, and V3 reveal that depending on the relative positions of C vacancies, the formation of bonds between Pt atoms and those C atoms near the vacancies changed (Figures 2c, 2g, and 2k). Compared to V1 in Figure. 2c, V2 and V3 show that the number of bonds between C and Pt was greater (Figures 2g and 2k), indicating stronger coupling between graphene and the substrate. Different strengths of coupling near C vacancies for V1, V2, and V3 also are distinctly compared using electronic interactions plotted on the isosurfaces of the charge density difference, as Figures 2d, 2e, 2h, 2i, 2l, and 2m illustrate. These figures show charge accumulations near the bonds between Pt atoms and C atoms near the vacancy, indicating the formation of covalent bond. Charge accumulations for V2 and V3 show wider distributions because of the increased number of covalent bonds near the C vacancies, as compared to V1 in Figures 2d and 2e. 3.3 Quenching of the Resonant States. Changes in the interaction strength between C and Pt atoms near a C vacancy produce different electronic properties, such as the projected density of states (PDOS) and the simulated STM images. As the DFT-optimized structures in Figure 2 indicate, we obtained the PDOS and the simulated STM images of V1, V2, and V3. In the case of V1, a resonant peak for the vacancy appears in the PDOS (blue line in Figure 3b); this result agrees well with both the experimental dI/dV spectrum in Figure 1e and previously reported results.7 The bright feature in the simulated STM image of V1 in 8


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Figure 3c appears as an effect of the resonant state near the C vacancy. In the cases of V2 and V3, however, the model systems yielded different PDOS and simulated STM images. The PDOS plot of V2 shows a peak near ~850 mV, and the PDOS plot of V3 shows a slope change only near the Dirac point. The PDOS of V3, in particular, is considerably close to the experimental dI/dV spectrum of the vacancy appearing in Figure 1d and plotted by the red line in Figure 1e. The simulated STM image of V3 in Figure 3e closely resembles as the STM image in Figure 1d as well. Larger area images of the vacancies in Figures 3d and 3e (Figure S3) also show a threefold spatial modulation around the vacancy observed in Figure 1d. Therefore, we carefully conclude that the depressions in the STM images (Figure 1d) originate from the disappearance of vacancy resonant states due to relatively strong coupling between C and Pt atoms resulting from increased bond formation.

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Figure 2. DFT calculations depending on the relative position of the vacancy with respect to the Pt atoms. (a) Top view of the optimized structure of 9 × 9 graphene supercell atop a threelayer slab of 8 × 8 Pt(111) supercell placed in a periodically replicated simulation cell. Pt atoms in the layer closest to graphene are colored yellow in (a) to show their relative positions with respect to C atoms. The blue, green, and red dots indicate positions of C vacancies created by manual removal, named V1, V2, and V3, respectively. (b, c, f, g, j, k) DFT-optimized structure (top and side views) of single carbon vacancies, V1, V2, and V3 in graphene on Pt(111). (d, e, h, i, l, m) Isosurfaces of the charge density difference (top and side

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views) near single carbon vacancies, V1, V2 and V3 plotted for isovalues of ± 0.04 e/Å3 (Red/blue). Pt atoms in the layer closest to graphene are colored orange.

Figure 3. DFT-calculated density of state (DOS) for V1, V2 and V3. (a) Top view of the optimized structure of 9 × 9 graphene supercell atop a three-layer slab of 8 × 8 Pt(111) supercell. (b) DFT-calculated DOS projected on C atoms of pristine graphene on Pt(111) (black dotted line) and LDOS from 12 closest C atoms to the single-vacancy sites of V1 (blue line), V2 (green line), and V3 (red line). The Fermi energy level was zero. (c, d, e) Simulated STM images of the single-vacancy models in Figures 2b, f and j, respectively. See Supporting information for the images in a larger zoom. We achieved additional clarification about the quenching of the resonant states of V2 and V3 by plotting the spatial distribution of the LDOS with two different energy ranges in Figure 4: (1) 200–600 mV, near the Dirac point (blue area in Figure 3b), and (2) 700– 11



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1100 mV, near the energy of the peak shown in the PDOS of V2 in Figure 3b (green area in Figure 3b). The shape of the spatially distributed LDOS provides information about orbitals of C atoms near the vacancy and allows us to infer the origins of the specific states. In the case of V1, near the Dirac point, the shapes of the LDOS in Figures 4a and 4b resemble those of π orbitals. This observation confirms that the state near the Dirac point is indeed a resonant state induced by the C vacancy. The resonant state disappears as the energy is far from the Dirac point, as shown in Figures 4c and 4d. The shapes of the spatial distributions of LDOS for V2 and V3 are similar to the σ orbitals rather than the π orbitals (Figures 4e–l). Therefore, the peak near 850 mV in the PDOS plot of V2 in Figure 3b is not the resonant peak induced by the π orbitals of C atoms near the vacancy. Rather, it is supposed to have originated from σ orbitals by the recombination of C and Pt atoms near the C vacancy. The nature of the interaction, therefore, between C and Pt, whether it involves the π or σ orbitals, determines the behavior of the resonant state.

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Figure 4. Spatial distributions of the LDOS with different energy ranges, (a, c, e, g, I, k) 200 mV ≤ E ≤ 600 mV and, (c, d, g, h, k, l) 700 mV≤ E ≤ 1100, for V1, V2 and V3. The LDOS is colored red. Pt atoms in the layer closest to graphene are colored orange.

Figure 5. Simulated and experimentally obtained STM topographs of V1, V2, and V3. (a, c, e) and (b, d, f) are simulated STM images using the LDOS ranging from 200 meV to 13



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600 meV and from 700 to 1100 meV, respectively. (g, h) Experimentally obtained STM images ((g) Vs = 0.3 V, It = 30 nA; (h) Vs = 0.9 V, It = 1 nA)). The different LDOS distributions affect the STM images. The bright feature of V1 in the simulated STM image obtained near the Dirac point in Figure 5a changes to a depression in Figure 5b with the bias voltage of 700–1100 mV as a result of the disappearance of the resonance peak. In the cases of V2 and V3, the depressions in Figures 5c and 5e remain unchanged, showing little modification in their detailed structures at the higher bias voltage ranging from 700 to 1100 mV (Figures 5d and 5f). In fact, it was expected that V2 would show a bright feature in the simulated STM image obtained near 850 mV where the peak appears in the PDOS plot of V2 in Figure 3b. However, V2 exhibits the depression in the simulated STM image in Figure 5d. We believe that the geometrical effect is stronger than the enhanced LDOS state. As in the simulation models V2 and V3, the depression observed in experimental STM images (Figures 5g–h) shows little change at different bias voltages of 0.3 V and 0.9 V. Note that the STM image in Figure 5g was taken with the tunneling current of 30 nA instead of 1 nA of Figure 5h in order to clearly show the graphene lattice with atomic resolution. The depression at the vacancy indeed remains unchanged at Vs = 0.3 V and It = 1 nA (Supporting Information Figure S5). 3.4 Moiré Effects on the Resonant States. We found, using DFT calculations and STM observations, only two examples that show quenching of the resonant state. However, graphene on Pt(111) systems has various stable moiré structures; predictions show 22 possible forms; of these, experiments have revealed 10.38 In our DFT study using the moiré structure with 9 × 9 periodicity, at least two cases of single vacancy revealed the quenching of resonant states, and in experiments, such cases of quenching were observed in two 14


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different moiré patterns (Figure S6). We expect that more cases with quenching of the resonant state may exist in different vacancy positions and various moiré structures. We also found evidence from the results of modified resonant states with the C vacancies in our STM images in Figure 6 and the previous study.7 Although typical, single vacancies exhibited bright features in STM images and the corresponding resonant states in the dI/dV spectra, slight differences were also observed in the STM and STS data, as shown in Figure 6. The C vacancies in Figures 6a and c were created in the same graphene island but in different positions. Figure 6b was obtained from a different moiré pattern (Figure S6).

Figure 6. Various C vacancies with little-modified resonant states in graphene/Pt(111). (a–c) STM topographs of single vacancies: (a) Vs = 0.3 V, It = 1 nA; (b) 2 × 2 moiré periodicity, Vs = 0.4 V, It = 1 nA; (c) Vs = 0.3 V, It = 1 nA, (d) differential conductance dI/dV spectra obtained at the centers of vacancies in (a–c). A previous simulation study of C vacancies in graphene using a supercell with 2 × 2 units of the 3 × 3 moiré periodicity reported four different simulated STM results and height changes in resonant peak data, depending on the location of the vacancy relative to the 3 × 3 15



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moiré periodicity.7 Our DFT calculations, which used a moiré periodicity that is different from that reported in Ref. 7, also showed that the precise bonding structure near the C vacancy changed depending on the sites. This change occurred even within the model structure with the number of bonds between C and Pt similar to that in V1 (This calculation yielded a simulated STM image with the typical, bright features.). Given that graphene on Pt(111) is predicted to form 22 stable moiré structures, 10 of which have been confirmed experimentally,38 a variety of similar STM topographs with slight variations in the atomic scale, as shown in Figure 6, may yet be possible . 3.5 Spatial Distribution of the Resonant States/

Figure 7. Spatial distribution of the resonance state. (a) STM topograph of a single vacancy (Vs = 0.4 V, It = 30 nA). (b) dI/dV spectra taken along the dashed green line in (a). To examine the spatial distribution of the resonance states we obtained the dI/dV spectra across the single C vacancy. The dI/dV spectra in Figure 7(b) clearly indicate that the resonance state occurs near the vacancy within approximately 1 nm, which is marked by blue arrows. In previous reports, the resonance state in the C vacancies on graphite surface extends more than 3 nm away from each single C vacancy,6,9 but it decays more quickly in the graphene on Pt(111) due to the relatively strong graphene-metal interaction.7 Our results clearly visualize the rapid decay of the resonance state in graphene on Pt(111). Additional analysis of the spatial distribution of the resonance states also facilitates the determination of 16


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the C vacancy position where the exact position is difficult to identify only with the honeycomb lattice overlaid STM image (Figure S7).

4. CONCLUSION In this study, we demonstrated modifications of the local electronic states in graphene/Pt(111) of a single C vacancy depending on its locations relative to the underlying Pt atoms. By irradiating the graphene with Ar+ ions, single C vacancies with various geometrical and electrical structures were created; these vacancies were observed by STM and STS measurements. DFT calculations showed that the various structures in the STM topographs arose from changes in the number of bonds formed between C atoms in the vicinity of the single vacancy and underlying Pt atoms, depending on the position of the vacancy. Our results provide an important advance in the understanding of the atomic-scale coupling between graphene and its environment in graphene-based applications such as graphene transistor and barristor.

Acknowledgement The authors declare no competing financial interest.

Supporting Information Supporting Information Available: Additional models of possible vacancy sites, the values of C-C and C-Pt bonding distances of V1, V2 and V3, tunneling current dependence of the depression in STM image, STM topographs of various moiré patterns of graphene on Pt(111) with single C vacancies and correspondent FFT images, STM topographs of single vacancies 17



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overlaid with graphene lattice and STM topographs of single vacancies overlaid with graphene lattice. This material is available free of charge via the Internet at http://pubs.acs.org.

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Quenching of the Resonant States of Single Carbon Vacancies in

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