Scanning Tunneling Microscope and Photoemission Spectroscopy

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Scanning Tunneling Microscope and Photoemission Spectroscopy Investigations of Bismuth on Epitaxial Graphene on SiC(0001) Han Huang,*,†,‡,§ Swee Liang Wong,‡,∥ Yuzhan Wang,‡ Jia-Tao Sun,⊥ Xingyu Gao,‡ and Andrew Thye Shen Wee*,‡,§,∥ †

Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, No. 605 Lushan South Road, Changsha Hunan 410012, People’s Republic of China ‡ Department of Physics, National University of Singapore, 2 Science Drive 3, 117542 Singapore § Graphene Research Centre, National University of Singapore, Block S14, Level 6 6 Science Drive 2, 117546 Singapore ∥ NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 28 Medical Drive, 117456 Singapore ⊥ Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543 Singapore ABSTRACT: The initial growth of bismuth (Bi) on epitaxial graphene (EG) on SiC(0001) at low deposition rates has been investigated using low temperature scanning tunneling microscopy (LT-STM) and synchrotron-based photoemission spectroscopy (PES). PES measurements reveal an islanding growth mode of Bi on EG due to weak interfacial interactions. LT-STM measurements show that Bi forms onedimensional (1D) 4-monolayer-thick nanoribbons on EG with the orientation relationship of Bi(011̅2) ∥ EG(0001) and Bi⟨112̅0⟩ aligned well with EG⟨112̅0⟩. Scanning tunneling spectroscopy (STS) results reveal the semiconducting nature of such Bi nanoribbons.

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hole doping of EG by simple atomic adsorbates.29 ARPES measurements have shown that Bi is able to shift the Dirac point toward the Fermi level (EF), i.e., reducing the n-type doping from the substrate,29 even though density functional theory (DFT) calculations reveal that the isolated Bi atom is weakly physisorbed on EG.35 Therefore, understanding the interactions between adsorbates and graphene in real space at atomic level is very important for fundamental understanding and possible applications. In this work, the structural and electronic properties of Bi thin films on EG are systematically investigated using low temperature scanning tunneling microscopy (LT-STM) and synchrotron-based photoemission spectroscopy (PES). PES results show an islanding growth mode of Bi on EG due to weak interfacial interactions. LT-STM measurements exhibit atomic scale details of such Bi islands in real space and reveal that one-dimensional (1D) 4-ML-thick (ML, monolayer) Bi nanoribbons form on EG with the orientation relationship of Bi(011̅2) ∥ EG(0001) and Bi ⟨112̅0⟩ in line with EG⟨112̅0⟩. Scanning tunneling spectroscopy (STS) results reveal the local electronic properties of such Bi nanoribbons of four characteristic features and an asymmetric energy gap around EF, indicating their semiconducting nature.

INTRODUCTION Bismuth (Bi) is a group-V semimetal with a charge carrier concentration far less than normal metals, having small effective electron mass and long Fermi wavelength (∼40 nm at room temperature).1−3 Because of quantum and finite size effects, Bi nanostructures exhibit interesting physical properties, such as semimetal to semiconductor transition,4−6 high thermoelectric efficiency,7 and superconductivity.8 Additionally, strong spin− orbit coupling on Bi surfaces results in a significant and anisotropic splitting of the surface-state bands, indicating possible applications in spintronics.9,10 Details of structural and electronic properties of low-index surfaces of bulk Bi can be found in the review by Hofmann.1 As the electronic properties of Bi nanostructures depend significantly on their structural properties, there have been numerous investigations on their growth. For example, on highly ordered pyrolitic graphite (HOPG), Bi forms islands with striped surface features at a preferred height and with the orientation relationship Bi(011̅2) ∥ HOPG(0001) as well as with a preference for Bi⟨1120̅ ⟩ ∥ HOPG⟨101̅0⟩.11−15 Graphene, a single layer of graphite, has attracted great interest in both academia and industry due to its superlative electronic properties16−19 such as high charge carrier mobility even at high charge carrier concentration at room temperature (RT)20 and long spin relaxation length up to micrometer scale at RT.21 Epitaxial graphene (EG) on SiC22−24 has been proposed as a possible platform for the development of graphene-based electronics.23−27 There are numerous investigations of adsorbates on graphene,28−34 for instance, atomic © 2014 American Chemical Society

Received: July 15, 2014 Revised: September 26, 2014 Published: October 2, 2014 24995

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EXPERIMENTAL METHODS All the experiments were carried out in two ultrahigh vacuum (UHV) systems with base pressures better than 2 × 10−10 mbar. Epitaxial graphene (EG) was prepared by annealing chemically etched (10% HF solution) n-type Si-terminated 4HSiC(0001) samples (CREE Inc.) in the multichamber endstation of the Surface, Interface and Nanostructure Science (SINS) beamline, Singapore Synchrotron Light Source,36 where in situ synchrotron-based photoemission experiments were performed. The detailed EG sample preparation method is reported elsewhere.22,34 The in situ LT-STM experiments were carried out in a custom-built multichamber system housing an Omicron LT-STM.37 All STM images were recorded in constant current mode at 77 K using chemically etched tungsten (W) tips. Bismuth was deposited in situ from a Knudsen cell (MBE-Komponenten, Germany) onto EG substrates at RT. During deposition, the pressure was always maintained below 5.0 × 10−10 mbar. The deposition rate was calibrated to be constant at 0.05 ML/min on Ag(111) (1 ML is defined as 1 atomic layer of Bi(011̅2), i.e., 9.3 × 1014 cm−2) using a quartz microbalance and STM in the LT-STM system.38,39

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RESULTS AND DISCUSSION Top and side views of the Bi(011̅2) surface are schematically shown in Figure 1. The surface is reconstructed into zigzag lines along the [112̅0] direction with a unit cell of 4.54 Å × 4.75 Å containing two Bi atoms.15 Figure 2. Synchrotron-based high-resolution PES spectra as a function of Bi thickness: (a) valence band spectra at the low binding energy part; (b) PES spectra at the low kinetic energy part (second electron cut off); (c) C 1s core-level spectra; and (d) Bi 4f7/2 and 4f5/2 corelevel spectra with a separation of 5.2 eV.

peak to increase significantly, while the EG substrate peaks undergo no apparent changes. This is unlike the case of PTCDA on EG,34 where EG substrate peaks are gradually attenuated with PTCDA deposition because PTCDA grows in a layer-by-layer mode. Thus, we can infer that Bi grows in an islanding mode on EG. In a recent report of Bi on HOPG,15 the Bi 6p (3.1 eV) and 6s (1.1 eV) peaks were observed. However, because of the strong states of EG/SiC substrate, we do not observe such peaks. During Bi deposition, we did not observe any new interface states and any shift of vacuum level, as shown in Figure 2a,b, confirming the weak electronic coupling between Bi thin films and EG, consistent with the previously reported physisorbtive nature of single Bi atom on EG by DFT calculations.35 The weak interfacial interaction of Bi on EG is supported by core-level PES measurements. Figure 2c shows the evolution of C 1s peaks as a function of Bi coverage, using photon energy of 350 eV for higher surface sensitivity. The bottom spectrum of clean EG displays a very strong peak at the binding energy of 284.6 eV and a weaker shoulder at higher binding energy of 285.7 eV, consistent with our previous report on EG.34 No apparent change in both intensity and shape for the C 1s peak as a function of the Bi thickness was observed, confirming the islanding growth mode of Bi on EG. Figure 2d shows Bi 4f7/2 and 4f5/2 peaks located at 156.8 and 162.0 eV, respectively. The spin−orbit splitting between these two peaks is 5.2 eV, in line with previous reports on Bi(011̅2).15 While their intensities

Figure 1. Schematic showing top-view and side-view of the Bi(0112̅ ) surface.15

Synchrotron-based high-resolution PES measurements were carried out to investigate the electronic structures at the interface between Bi and EG. In Figure 2a,b, representative PES spectra at the low-binding energy region and the low-kinetic energy region are shown as a function of the nominal thickness of Bi, respectively. To resolve the low kinetic energy cutoff, a −5 V sample bias was applied. The vacuum levels (Evac) were measured by linear extrapolation of the low-kinetic energy onset (secondary electron cutoff) of the PES spectra. The bottom spectrum in Figure 2a displays the electronic structure of a clean EG/SiC surface with a main peak at ∼7.6 eV and a weaker peak at ∼21.6 eV. Upon 0.5 nm Bi deposition, a new peak appears at 23.9 eV, which corresponds to Bi 5d5/2. Increasing Bi coverage to 2.0 nm causes the intensity of this 24996

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Figure 3. STM images of Bi on monolayer EG. (a−c) Large scale images (VT = 1.6 V; scale bar = 12, 20, and 25 nm, respectively) of Bi at coverage of 0.5 ML, 1.0 ML, and 2.0 ML. The red arrows in panel a direct to EG⟨112̅0⟩, the green one to ⟨101̅0⟩. (d) High-resolution image (VT = 0.08 V; scale bar =0.8 nm) shows the atomic structures of Bi and ML EG. The red line highlights Bi⟨1120̅ ⟩ ∥ EG⟨1120̅ ⟩. The green hexagon and rectangle highlight one honeycomb of graphene and one unit cell of Bi film. (e) Proposed model of ultrathin Bi on EG. (f) Zoomed-in STM image (VT = 1.0 V; scale bar = 5 nm) from the red square in panel c. The inset (4 nm × 4 nm; VT = 0.2 V) shows the atomic zigzag structure of Bi(0112̅ ) with Bi atoms overlaid.

the possibility of square-shaped Bi island formation as observed. The atomically resolved STM image of Bi on monolayer EG confirms Bi⟨112̅0⟩ ∥ EG⟨112̅0⟩, as highlighted by the red line in Figure 3d. The green hexagon and rectangle highlight one honeycomb of graphene and one unit cell of Bi film. The measured lattice parameters of Bi film are ∼0.505 nm × 0.503 nm before calibration, and those for graphene are 0.259 nm × 0.479 nm. By calibrating to the lattice constant of graphene (0.246 nm), the unit cell parameters of Bi film on EG are determined to be 0.480 nm × 0.447 nm, in good agreement with the value in Figure 1. A model is proposed as shown in Figure 3e. The evaluation of the morphology of Bi nanostructures as a function of coverage is shown in Figure 3b,3c. Some Bi nanostructures keep the 1D features, but in height up to ∼5 nm (the bright ones in panel c), some appear as 2D islands (at the upper-left corner in panel c). The high-resolution differentiated STM image from the red square in Figure 3f displays the stripe features with averaged separation of ∼0.5 nm on the 2D terrace, consistent with that of the Bi(011̅2) surface. An atomically resolved STM image showing the zigzag structure of the Bi(0112̅ ) is inserted at the lower-left corner of Figure 3f. A schematic of Bi atoms in zigzag chains is overlaid on the inset as a guide. Figure 4a shows a typical 4-ML-thick nanoribbon that extends continuously over the SiC nanomesh and monolayer EG. The part on SiC nanomesh appears dimmer than that on EG. The measured height difference is up to 0.19 nm depending on the scanning conditions. To understand the electronic structure of the Bi nanoribbon on SiC nanomesh and EG, STS measurements were carried out with the tip bias at

grow with increasing Bi coverage, no binding energy shift or new peaks were observed, confirming the weak interactions between Bi and EG. However, this is in contrast with Gierz’s report,29 where the Dirac point of EG clearly shifts toward EF and the band structure remains unaltered with increasing the Bi coverage. More experiments are required to specify this point. In situ LT-STM experiments were performed to image the growth of Bi thin film on EG in real space. At low coverage, as shown in Figure 3a, Bi aggregates into islands oriented Bi(011̅2) ∥ EG(0001) and with apparent height of 1.5 ± 0.1 nm, which is slightly higher than the calculated height of four atomic Bi layers of 1.3 nm. To solve this problem, a wetting layer is proposed in the Bi/HOPG.6,15 However, it is an intriguing unresolved problem because the wetting layer is not directly accessible.6 Besides, the measured height is dependent on the scanning conditions. For clarity, we refer to the island thickness as 4 ML. Most of the islands are 1D nanoribbons, while a few Bi islands are square. Most 1D Bi nanoribbons are aligned with the underlying 6 × 6 reconstruction of SiC as marked by red arrows (oriented ∼60° with respect to one another), that is, Bi⟨112̅0⟩ ∥ EG⟨112̅0⟩. This is consistent with the fact that the misfit of Bi⟨1120̅ ⟩ ∥ HOPG⟨1120̅ ⟩ is smaller in both directions than that of Bi⟨112̅0⟩ ∥ HOPG⟨101̅0⟩.12 However, it is different from the case of Bi on HOPG, where Bi⟨112̅0⟩ ∥ HOPG⟨101̅0⟩ is preferred.10−14 One possible reason is that EG is corrugated and slightly n-doped due to the underlying 6 × 6 reconstructed SiC, which may induce the interaction between Bi and EG to be different with that between Bi and HOPG. Occasionally, nanoribbons oriented along EG⟨101̅0⟩ are observed, as marked by a green arrow, which is oriented 90° to its neighboring red arrow. This allows 24997

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temperature (77 K) high-resolution STM and synchrotronbased PES. PES results show almost no changes in graphenerelated peaks as Bi coverage increases. LT-STM images show a 1D-4 ML-islanding growth mode of Bi on EG at the initial stage with preference for Bi⟨1120̅ ⟩ ∥ EG⟨1120̅ ⟩ orientation, in contrast with Bi on HOPG. STS reveals an asymmetric narrow gap around EF in the Bi nanoribbons due to quantum and size effects as well as the evidence of easy electronic transfer from the substrate to Bi. All the results paint a picture of the physisorptive nature of Bi on EG. This may open up potential applications of Bi such as in spintronics.

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AUTHOR INFORMATION

Corresponding Authors

*(H.H.) E-mail: [email protected]. *(A.T.S.W.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We acknowledge the support from NRF-CRP grants R-143000-360-281 “Graphene and Related Materials and Devices” and R-144-000-295-281 “Novel 2D Materials with Tailored Properties−Beyond Graphene”. H.H. acknowledges the “Shenghua Professorship” startup funding from Central South University and the support from the NSF of China (Grant No. 11304398).

Figure 4. (a) STM (VT = −0.75 V; scale bar = 8 nm) and (b) STS measurements on a typical 4-monolayer-thick Bi nanoribbon partly on monolayer EG (left in panel a) and partly on SiC nanomesh (right in panel a). The boundary between Bi on EG and Bi on SiC nanomesh is highlighted by a green dotted curve.

−0.75 V and the set-point current at 100 and 200 pA for comparison. The spectrum of differential tunneling conductance (dI/dV) as a function of the tip bias (VT), which is correlated with the local electronic density of states, was measured under open feedback conditions by lock-in detection of the AC tunneling current modulated by a 600 Hz, 10 mV signal added to the tunneling bias. Figure 4b shows four typical dI/dV−VT spectra measured on Bi on EG and SiC nanomesh, respectively. Each spectrum was spatially averaged over 60 points. At each point, 20 spectra were taken. The upper two spectra were taken on Bi on EG at 100 pA (blue) and 200 pA (green) and show four peaks α, β, γ, and η located at −0.75 , −0.49 , 0.38, and 0.62 V respectively, as well as an asymmetric gap of ∼0.4 eV with vanishing dI/dV value at EF. The origin of such a gap at low dimensions can be attributed to the strainenergy relaxation-induced edge buckling as discussed in ref 40. The lower two spectra taken on Bi on SiC nanomesh at 100 pA (black) and 200 pA (red) show three peaks located at −0.68, 0.45, and 0.69 V, respectively, and a vanishing shoulder at −0.42 V, as well as a smaller asymmetric gap of ∼0.30 eV. These three peaks are rigidly shifted 0.07 V to the right relative to peaks α, γ, and η for Bi on EG, respectively. This is attributed to more charge transfer from SiC nanomesh to Bi than that from EG due to the different work function between SiC nanomesh and EG. The transferred charge partially fills the unoccupied states corresponding to peak β, lowering the intensity of peak β so that it appears as a vanishing shoulder in the lower two dI/dV−VT spectra.

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CONCLUSIONS In summary, the structural and electronic properties of Bi deposited on EG have been investigated by in situ low24998

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