Article pubs.acs.org/JPCC
Confinement of the Pt(111) Surface State in Graphene Nanoislands Hyo Won Kim,†,‡ Seiji Takemoto,†,§ Emi Minamitani,†,∥ Tomonari Okada,†,⊥ Takeshi Takami,†,⊥ Kenta Motobayashi,†,⊥ Michael Trenary,# Maki Kawai,⊥ Nobuhiko Kobayashi,§ and Yousoo Kim*,† †
Surface and Interface Science Laboratory, RIKEN, Wako, Saitama 351-0198, Japan Samsung Advanced Institute of Technology, Suwon 443-803, Korea § Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan ∥ Department of Materials Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan ⊥ Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan # Department of Chemistry, University of Illinois at Chicago, 845 W Taylor Street, Chicago, Illinois 60607, United States ‡
S Supporting Information *
ABSTRACT: We present a combined experimental and theoretical study of electron confinement in graphene nanoislands (GNs) grown on a Pt(111) substrate using scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. We observed standing wave patterns in the STM images of GNs, and the bias dependency of the standing wave pattern was reproduced by considering free electrons with an effective mass of m* ≈ (0.27 ± 0.03)me. Because the effective mass of Pt is m* = 0.28me, our results reveal that the electron confinement is due to the effect of the Pt substrate rather than the massless Dirac electrons of graphene. Our calculated maps of the local density of states (LDOS) for the GNs confirm that the electronic properties of the confinement may be described in terms of electrons with an effective mass. The DFT-calculated charge distribution for graphene on the Pt system also shows a clear hybridization between the pz orbitals of both the first layer of the Pt substrate and the carbon atoms.
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INTRODUCTION
electron confinement of GNs on a Pt(111) substrate has not been examined so far. Here, we report the influence of the underlying metal on the confinement of the electronic states of GNs grown on a Pt(111) substrate through STM, STS, and density functional theory (DFT) calculations. The Pt(111) substrate has been reported to interact very weakly with graphene.14−16 Surprisingly, we found that the confined electronic states reveal the surface states of the underlying Pt(111) substrate rather than those of the Dirac electrons from graphene. The dispersion relations determined from the STS results show a free-electronlike energy dispersion with an effective mass of m* ≈ (0.27 ± 0.03)me. Considering that the effective mass of Pt is 0.28me,17 this result indicates that the confined electrons primarily consist of those from the Pt surface state.
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Graphene, a single layer of two-dimensional sp -bonded carbon atoms, has attracted considerable interest because of its potential as a new material for future electronic devices.1,2 To realize graphene-based nanodevice applications, understanding how the electronic properties of graphene nanostructures are modified by their size and shape in the nanoscale regime is essential. For such study, atomically well-defined graphene nanostructures are required. Very small graphene nanoislands (GNs) can be grown on various transition metals using chemical vapor deposition,3−10 and these GNs can be characterized with scanning tunneling microscopy (STM) and spectroscopy (STS). Nanoscale size effects induce confinement of the electronic states, which are observable as discrete peaks in the local density of states (LDOS) in the STS results and as a standing wave pattern of scattered electrons or holes by spatial mapping of the confined energy levels. Recently, confinement of electronic states of GNs on Ir(111)5−8,11 and Au(111)12,13 substrate has been reported. These studies show that not only nanoscale effects but also the electronic states of the underlying substrate influence the electron confinement. The Ir and Au surface resonances penetrate into the graphene layer, affecting the properties of confined electrons in GNs. However, the © XXXX American Chemical Society
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EXPERIMENTAL AND COMPUTATIONAL METHODS Our Pt(111) sample was cleaned by repeated cycles of Ar ion sputtering and annealing under ultrahigh vacuum (UHV). Received: October 14, 2015 Revised: November 23, 2015
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DOI: 10.1021/acs.jpcc.5b10040 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Graphene nanoislands were created by exposing the clean Pt(111) surface to ethylene, followed by annealing at 1100 K.3,18,19 The STM and STS measurements were performed using a low-temperature STM (Omicron GmbH, base pressure 7.0 × 10−11 Torr) at 4.7 K. For the STM measurements the bias is applied to the sample. The STS measurements were performed using a standard lock-in technique with a bias modulation of 30 mV at 797 Hz. To understand the details of the electronic properties of GNs grown on the Pt(111) substrate, we performed periodic DFT calculations using the Vienna Ab-initio Simulation Package (VASP)20,21 with local density approximation (LDA) for the exchange-correlation function.22 The core electrons were replaced by projector-augmented wave (PAW) pseudopotentials23 expanded in a basis set of plane waves up to a cutoff energy of 400 eV. We used (2 × 2) surface supercells consisting of 16 Pt layers. We used the tetrahedron scheme with 11 × 11 × 1 Monkhorst−Pack k-point grids for the Brillouin zone integrations.
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RESULTS AND DISCUSSION Figure 1a shows GNs grown on the Pt(111) surface. A small hexagonally shaped GN is located on the terrace, and larger
Figure 2. (a) STM image of a GN with a hexagonal shape (Vs = 0.1 V, It = 1.0 nA). (b) The dI/dV spectrum measured from the center of the GN. (c−f) The dI/dV maps. (g−j) Calculated LDOS maps.
V. For higher positive bias voltages, we found three more distinct modulation patterns near 1.4, 2.1, and 2.9 V (Figure 2d,e). A fifth modulation pattern was observed near 3.9 V (see Figure S1, Supporting Information). However, at negative bias voltages no modulation patterns were observed (see Figure S2). The modulation patterns near 1.4, 2.1, and 2.9 V are due to the interference of electron waves in the GN, and as a result, the standing wave patterns inherit the hexagonal shape of this GN. Such behavior has been previously observed in a hexagonal metal island.25−27 To clarify the relationship between the standing wave pattern and the shape of the island, the LDOS of the GN in Figure 2a was calculated using the finite-difference method with an effective Hamiltonian as shown in Figure 2g−j.5 We performed the calculation with the effective mass m* ranging from 0.2me to 0.3me, where me is the bare electron mass. The calculated pattern with an effective mass of 0.2me is in good agreement with the experimentally obtained patterns (Figure 2c−f). In order to study the size and shape dependence of the standing wave patterns in the STM images of GNs, we obtained dI/dV maps (Figure 3b−d) for the larger triangle-shaped GN shown in Figure 3a. The first standing wave appears at a lower voltage of 0.4 V compared to that (0.9 V) obtained for the smaller GN shown in Figure 2. Two additional wave patterns are found at 0.5 and 0.6 V, which are also at low voltages relative to the patterns in Figure 2. The shapes of the standing wave patterns are dominated by the triangle-shaped GN. (In addition, the standing wave patterns of a different shaped GN are shown in Figure S3.) The calculated patterns of the GN (Figure 3e−g) also show good agreement with the experimentally obtained dI/dV maps. The energy levels of the standing waves obtained from the dI/dV mapping experiments were analyzed to study the nature of the confinement. Each standing wave corresponds to an eigenstate of the confined electron characterized by quantum numbers n = 1, 2, and 3. The standing wave energy level is equivalent to the bias voltage. First, we plotted the energy levels of the standing waves as a function of the inverse island size
Figure 1. (a) STM image and (b) dI/dV map of GNs grown on Pt(111) (Vs = 1.5 V, It = 1.0 nA). The inset of (a) shows an atomically resolved graphene (Vs = 0.1 V, It = 1.0 nA; scan area: 1 × 1 nm2).
islands are located at the step edges of the Pt(111) surface. The atomic resolution image of the GN (inset of Figure 1a) shows a honeycomb structure, which is typical in graphene. A dI/dV map was measured at a sample bias voltage of 1.5 V in the same area (Figure 1b). Standing wave patterns were observed on both the GNs and the Pt(111) surface. The former can be derived from electron confinement within the GNs, while the latter is due to the scattering of surface state electrons near step edges and surface defects.24 We confirmed that similar standing wave patterns appear in other GNs (over 30 islands). For a more detailed understanding of the standing wave on the GNs, we focused our study on the small hexagonal GN shown in Figure 2a. To probe the energy levels of the confined electrons, the dI/dV spectra (Figure 2b) were measured at the center of the GN (black line) and at the Pt surface for comparison. The dI/dV spectra measured for the GN (Figure 2b) shows four changes in slope at positive sample bias voltages of approximately 0.9, 1.4, 2.1, and 2.9 V, which are labeled with black arrows. These bias-dependent and spatially modulated dI/dV spectra can be described with electron confinement, which induces a corresponding spatial modulation in the LDOS.25,26 To further clarify these results, we measured the dI/dV maps. The first modulation pattern (Figure 2c) was observed near 0.9 B
DOI: 10.1021/acs.jpcc.5b10040 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Figure 3. (a) STM image (Vs = 2.5 V, It = 1.0 nA). (b−d) dI/dV maps. (e−g) Calculated LDOS maps of a GN with a triangular shape. (h) Energies of n = 1 (red), 2 (blue), and 3 (green) confined states compared to the inverse island area along with linear fits of the data. (i) Dispersion relation extracted from (h). The linear and parabolic lines are the dispersion relations of graphene and Pt,17 respectively.
Ω−1 for n = 1, 2, and 3 (Figure 3h). The energy levels can be linearly fitted by Ω−1. This indicates that the quantized energies, which are obtained by the difference between the energy of the nth state En and the energy of the ground state E0, obey the following equation: En − E0 = ℏ2λn/m*Ω, where m* is the effective mass and the eigenenergies λn are shape parameters,26 as predicted by confinement theory and previously reported results.28,29 Then, we plotted the energy levels as a function of k2 = 2λn/Ω (Figure 3h). We also plotted the dispersion relations of graphene (linear line) and of the Pt surface state (parabolic line) in Figure 3i, which are deduced from the calculated band structure of graphene on Pt (Figure 4a)30 and photoemission on clean Pt(111),17 respectively. We found that the dispersion from graphene on Pt(111) agreed with the dispersion relation of the Pt surface state (Figure 3i). With the parabolic fitting, the effective mass was determined to have a value of m* ≈ (0.27 ± 0.03)me, which is close to the effective mass of Pt (0.28me) estimated by angle-resolved photoemission spectroscopy.17 To clarify E0 and hybridization between graphene and the Pt surface state, DFT calculations were performed to determine the electronic band structure of two-dimensional graphene on Pt(111) (Figure 4a) with the periodically replicated bulk system of 2 × 2 graphene supercell placed on the Pt(111) surface. In our experimental results, GNs have a number of different orientations with respect to the substrate, but the GNs do not show any orientation dependence in the energy dispersion relations, so here we used 2 × 2 graphene/Pt(111) to model the system. Moreover, the charge density distribution of the pz orbitals of the first layer of Pt shows its influence on graphene (Figure 4b). In the calculated electronic band dispersion of graphene on Pt(111), the pz orbital contribution from the first layer of Pt atoms and that from carbon atoms are denoted by red and green dots, respectively. The unoccupied Pt surface states and the Dirac cone are located near the Γ̅ and K̅ points, respectively. The estimated shifts in E0 and the Dirac cone were 0.3 and 0.45 eV, respectively. The calculated E0 of
Figure 4. (a) Calculated band structure of (2 × 2) graphene adsorbed on the Pt (111) surface. The reciprocal space with symmetric points is shown in the inset. The red and green filled dots indicate the components of pz orbitals of the first layer Pt atoms and C atoms. (b) Cross-sectional views of the charge density of the Pt surface state at Γ̅ point and the graphene at K̅ point denoted by the black line in Figure 4a. (c) Charge density distribution depending on z direction of the Pt surface state (blue) and the graphene (red). Charge density of x and y direction was averaged. The transverse axis shows the distance from graphene layer at z = 0 [Å] to STM tip.
graphene/Pt(111) is in good agreement with the E0 of a pristine Pt(111) surface.17 The upward shift of the Dirac cone indicates hole doping into graphene on the Pt surface. The site projected density of states (DOS) of the Pt surface state at the Γ̅ point (denoted by a black arrow near Γ̅ in Figure 4a) was calculated by projecting the wave function of the eigenstate onto the spherical harmonics of spd orbitals within spheres of radii 0.863 Å (C) and 1.455 Å (Pt) around each ion. As a result, we found that the ratio of the C and Pt first layer atoms is 1 to 7.8; the eigenstate primarily consists of electronic states from the first Pt layer. For an intuitive understanding of the hybridization, we plotted the of the Pt surface state at the Γ̅ point and the graphene at the K̅ point, denoted by the black arrow in Figure 4a, as shown in Figure 4b. High accumulation of the charge density in the graphene layer and in the first Pt layer is clear evidence of substantial hybridization between the C and Pt pz orbitals near the Γ̅ point. Accumulation of the charge density of the graphene near the K̅ point is also shown in Figure 4b. However, the charge density of graphene reduces as a function of the distance to the z-direction in the vacuum above the graphene in Figure 4c. Therefore, our DFT C
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calculations support our experimental results: the electronic properties of the GNs are largely determined by the Pt substrate.
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CONCLUSIONS We studied the influence of the substrate on electron confinement in GNs on Pt(111) with STM and DFT calculations. The confined electrons in the GN are observed as standing wave patterns in the dI/dV maps, and their electronic properties are close to those of free electrons in the metal rather than those of Dirac electrons in graphene, as shown in the dispersion relation. The DFT calculations provide further quantitative understanding of the hybridization between the surface states of Pt and graphene.
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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.5b10040. A fifth modulation pattern (dI/dV map, near 3.9 V), dI/ dV maps at negative bias voltages, and the standing wave patterns of a different shaped GN (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (Y.K.). Present Address
K.M.: Department of Engineering Physics, Electronics and Mechanics, Nagoya Institute of Technology, Nagoya, Aichi 466-8555, Japan. Author Contributions
H.W.K. and S.T. contributed equally. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.jpcc.5b10040 J. Phys. Chem. C XXXX, XXX, XXX−XXX
