Ge(110)

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Unraveling the Structural and Electronic Properties of Graphene/Ge(110) Hyo Won Kim, Wonhee Ko, Won-Jae Joo, Yeonchoo Cho, Youngtek Oh, JiYeon Ku, Insu Jeon, Seongjun Park, and Sungwoo Hwang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03315 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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The Journal of Physical Chemistry Letters

Unraveling the Structural and Electronic Properties of Graphene/Ge(110) Hyo Won Kim*, Wonhee Ko, Won-Jae Joo, Yeonchoo Cho, Youngtek Oh, JiYeon Ku, Insu Jeon, Sungjun Park, Sung Woo Hwang*

Samsung Advanced Institute of Technology, Suwon 16678, Korea,

Corresponding Author *[email protected], [email protected]

ACS Paragon Plus Environment

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The direct growth of graphene on a semiconducting substrate opens a new avenue for future graphene-based applications. Understanding the structural and electronic properties of the graphene on a semiconducting surface is key for realizing such structures; however, these properties are poorly understood thus far. Here, we provide an insight into the structural and electronic properties of graphene grown directly on a Ge(110) substrate. Our scanning tunneling microscopy (STM) study reveals that overlaying graphene on Ge(110) promotes the formation of a new Ge surface reconstruction, i.e., a (6 × 2) superstructure, which has been never observed for a bare Ge(110) surface. The electronic properties of the system exhibit the characteristics of both graphene and Ge. The differential conductance (dI/dV) spectrum from a scanning


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tunneling spectroscopy (STS) study bears a parabolic structure, corresponding to a reduction in the graphene Fermi velocity, with exhibiting additional peaks stemming from the p-orbitals of Ge. The density functional theory (DFT) calculations confirm the existence of surface states due to the p-orbitals of Ge.

TOC GRAPHICS

KEYWORDS Graphene, Ge surface reconstruction, Ge(110), scanning tunneling microscopy, scanning tunneling spectroscopy, density functional theory calculations

Graphene grown on metal surfaces by chemical vapor deposition (CVD) techniques is generally used for the mass production of graphene-based applications.1-2 However, to integrate graphene into devices, a transfer step is required. During this process,


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undesirable physical damages such as impurities, wrinkles, and cracks are inevitably introduced, degrading the performance of the devices. The direct growth of graphene on a semiconductor substrate such as Si3 and Ge4-10 has emerged as the most promising method to avoid the transfer process altogether. Interestingly, the different crystallographic orientations of a germanium surface can be exploited to control the orientation of graphene with respect to the substrate or to create graphene nanoribbons. The anisotropic twofold symmetry of a germanium (110) surface and the atomic steps promote the formation of uniaxially aligned graphene islands at the early stages8 of growth into single-crystal graphene on hydrogen-terminated Ge(110).5 Two different orientations of graphene grown on Ge(110) have also been obtained by adjusting the partial pressure of CH4 during growth.6 The fabrication of self-aligned graphene nanoribbons on Ge(001) with a 3° rotation from the Ge substrate has been reported.7 Despite the remarkable progress in synthesizing graphene on germanium substrates, a fundamental understanding of the interlayer interaction between graphene and substrate and its influence on the geometric and electronic properties of graphene are still lacking.


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Our study on the structural and electronic properties of graphene grown on Ge(110) is based on scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and density functional theory (DFT) calculations. The STM results confirm that the direct growth of graphene on a Ge(110) substrate induces a new (6 × 2) Ge surface reconstruction with two different (6 × 2) reconstructions, which has been recently observed,10 with the armchair edge of graphene rotated by 3° and 33° from the

[110] direction of the Ge(110) surface. The electronic structure of the graphene/Ge(110) surface exhibits the characteristics of both graphene and Ge. The parabolic-shaped differential conductance (dI/dV) spectrum bears the reduction in the graphene Fermi velocity. At the same time, several additional peaks originating from the p-orbitals of Ge are observed. The existence of surface states due to the p-orbitals of Ge is confirmed from the DFT calculations.

The experiments were carried out on graphene grown on a Ge(110) substrate with two different orientations of graphene, with angles of 3° and 33° between the [110] direction of Ge(110) and the armchair edge of graphene. The different orientations were


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obtained by adjusting the partial pressure of CH4 during the growth (0.05 of CH4/H2 ratio).5-6 The large-scale STM image in Figure 1a shows the representative morphology of graphene grown on Ge(110), clearly revealing the underlying stepped structure of the Ge substrate. A close-up STM image and its fast Fourier transform (FFT) show the hexagonal lattice of graphene with the armchair edge rotated by 3° from the [110] direction of the Ge(110) surface and a newly created (6 × 2) Ge reconstruction with alternating stripes of up and down (110) terraces (Figure 1b,c,e). In fact, the (6 × 2) structure was very recently observed using surface X-ray diffraction and high-resolution X-ray reflectivity measurement with STM.10 The reconstruction is indeed different from previously known (16 × 2), (8 × 2), and c(8 × 10) reconstructions observed on clean Ge(110) surfaces11-13 (Figure 1d) and Supporting Information, Figure S1) or from commonly observed Moiré patterns of graphene on metal substrates.14-15 The corresponding dI/dV spectra of the graphene grown on Ge(110) reveals previously unobserved characteristics, as shown in Figure 1f.


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Figure 1. (a) Large-scale STM topograph of graphene grown on Ge(110) (Vsample = 1 V, I = 1 nA). (b) Close-up STM topograph of graphene grown on Ge(110). (c) STM topograph of graphene grown on Ge(110) with atomic-resolution. (d) FFT patterns obtained from the STM topograph shown in (b). (e) Large-scale STM topograph of clean Ge(110) surface (Vsample = 2 V, I = 0.1 nA). (f) dI/dV spectra obtained for the graphene grown on Ge(110) with the (6 × 2) reconstruction beneath the graphene (black line) and clean Ge(110) surface with the (8 × 2) reconstruction (blue line).


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The STM topographs of graphene grown on Ge(110) with the (6 × 2) reconstruction feature alternating stripes along two directions (indicated using white and black boxes in Figure 2a). The different directions of the stripes occur because of the mirror symmetry of Ge surface atoms with respect to the [110] direction of Ge(110). The parallelograms in Figures 2c and e clearly show the mirror symmetry between the (6 × 2) unit cells from different regions. The unit cells can also be observed in the FFT patterns shown in Figure 2d, f (blue circles). To confirm the structure of the observed reconstructions, we constructed a model system based on the experimental observations. In this model, the armchair edge of graphene is rotated by 3° from the

[110] direction of the Ge(110) surface, and the Bravais lattices of the (6 × 2) structure are overlaid and indicated by blue dots (Figure 2b and Supporting information, Figure S2). Figure 2g shows the FFT patterns obtained for the lattice in the upper right portion of the model; this is in good agreement with the experimental result shown in Figure 2d. The (6 × 2) reconstructions are also found in the experiments on graphene/Ge(110) with the 33° relative rotation (Supporting Information, Figure S3); this result is also in good agreement with the obtained FFT patterns (Figure S3).


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Figure 2. (a) STM topograph of graphene grown on Ge(110) revealing newly created Ge (110) reconstructions (Vsample = 1 V, I = 1 nA). (b) Schematic depicting Ge(110) (6 × 2) reconstructions on graphene grown on Ge. The armchair direction of graphene is rotated by 3° from the [110] direction of the Ge(110) surface. The Bravais lattices of the


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(6 × 2) unit cell are marked by blue dots. (c,e) Close-up STM topographs of graphene on Ge(110) (Vsample = 1 V, I = 1 nA). The unit cells of (6 × 2) structures are marked by blue parallelograms. (d,f) FFT patterns obtained from the STM topographs shown in c and e, respectively. The white, green, and blue circles indicate the signals from graphene, Ge, and reconstruction, respectively. (g) FFT patterns obtained for the upper part of the model shown in (b) (supporting information, Figure S2).

The (6 × 2) reconstruction of the Ge surface has a significant effect on the electronic structure of graphene grown on Ge(110), as observed from the dI/dV spectra shown in Figure 3c. The overall parabolic-shape of the dI/dV spectrum indicate a relatively strong interaction between the graphene and the underlying Ge surface corresponding to a reduction in the graphene Fermi velocity, supporting the model of interaction/epitaxy between the two material surfaces,5-6 and the spectrum exhibits several additional peaks (Figure 3c). We claim that these peaks originate from the surface states of Ge beneath the graphene. These peaks generally do not appear for


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graphene grown on metal surfaces, except for highly strained graphene,16-17 in which case curvature-dependent Landau level (LL) peaks appear because of the straininduced pseudo-magnetic field. We safely eliminated the possible scenario of LL peaks, as the peak positions at positive bias voltages in the dI/dV spectra do not vary with respect to the measurement positions, whereas they do vary at negative bias voltages as observed in the spectroscopic map shown in Figure 3d. We performed DFT calculations for the graphene grown on the Ge(110) substrate to identify the origin of the additional peaks in the dI/dV spectra. To study an appropriate model for graphene on Ge, we examined the systems with the armchair edge of graphene rotated by an angle within 3 ± 1° from the [110] direction of the Ge(110) substrate. We chose for our calculation a representative model with the rotation angle of 2.4° as shown in Figure 3e, which enables efficient and effective computation with 206 atoms in its unit cell. The exact peak positions in the obtained projected density of states (PDOS) plot in Figure 3f slightly differ from those observed in the experiment. However, as in the experimentally measured dI/dV, the parabolic-shape observed for graphene is indeed evident in the obtained projected density of states (PDOS) plot, shown in Figure 3f, and the other


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prominent peaks are clearly due to the p-orbitals of Ge. Although we can verify the origins of several peaks in the dI/dV spectra by DFT calculations, a question on the discrepancies the exact peak position and numbers of the peaks still remains. We speculate the possibilities of relatively strong graphene-substrate interactions (charge density difference in Figure S4) and/or newly created (6 × 2) reconstruction of the Ge(110).

To explore the effect of the reconstructions of Ge surface, we examined the electronic structure of graphene grown on Ge(110) without the (6 × 2) reconstructions (Figures 2a,3g). The dI/dV spectrum (Figure 3h) shows a parabolic-shape but the exact peak position and numbers of the peaks are different from those of the dI/dV spectrum obtained for graphene on Ge(110) with the (6 × 2) reconstruction. In particular, the peaks at negative bias almost disappear. These features are further close to those of the PDOS plot without the reconstruction shown in Figure 3f. The (6 × 2) reconstructions and/or the interactions between graphene and the reconstructions,


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therefore, may produce the peak near – 0.7 V and the changes of the peak positions in the dI/dV spectra obtained for graphene on Ge(110).

Figure 3. (a,b) Applied bias voltage-dependent STM topographs of graphene on Ge(110) obtained at 1.0 V and 0.5 V, respectively. (I = 1 nA). (c) dI/dV spectra obtained for the


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positions indicated by the red and black dots in (a). (d) dI/dV spectra measured along the dashed white line in (a). (e) Top and side views of the optimized structure of graphene atop a four-layer slab of Ge(110). The lattice constant of graphene for the calculation is 2.468 Ǻ, which is very close to the experimentally obtained lattice constant of graphene, 2.40 Ǻ. (f) Total and projected density of states calculated from the cell. (g) STM topograph of graphene grown on Ge(110) with the regions of no reconstructions of Ge (110) (Vsample = 1 V, I = 1 nA). (h) dI/dV spectra obtained for the positions indicated by the red and blue dots in (g). (i) dI/dV spectra measured along the dashed white line in (a).

We examined the electronic structure of a bare Ge(110) surface. The dI/dV spectra (Figures 4c and f) obtained for the clean Ge(110) surface exhibit several peaks because of the surface states of Ge; however, the overall parabolic-shape found in the case of graphene on Ge(110) is missing. In previous reports, the dI/dV spectrum obtained for the clean Ge(001) or Ge(110) surface revealed a few peaks between –1 V


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to 1 V because of the p-orbitals of Ge, which was confirmed from the calculated PDOS of the Ge(100) surface.13,18 The surface states of Ge lead to changes in the STM images depending on the applied bias voltage (Figures 4a,b,d,e, and STM topographs shown in Supporting Information, Figure S5). This characteristic is also observed in the STM topographs of graphene grown on Ge(110) (Figures 3a,b). In addition, the exact positions and the number of peaks between –1 V and 1 V in the dI/dV spectra obtained for different surface reconstructions, i.e., c(8 × 10) and (8 × 2), are different (Figure 4c,f). Similarly, the newly created (6 × 2) reconstructions underlying graphene may affect the positions and the number of peaks in the dI/dV spectra obtained for graphene/Ge.


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Figure 4. (a,b) Applied bias voltage-dependent STM topographs of clean Ge(110) with c(8 × 10) reconstruction obtained at 2.0 V and 1.5 V, respectively. (I = 0.1 nA). The primitive unit cell is indicated with a dotted white line, and the rectangular c(8 × 10) unit cell is indicated with a solid white line. (c) dI/dV spectra obtained for the positions indicated by the red and black dots in (a). (d,e) Applied bias voltage-dependent STM topographs of clean Ge(110) with (8 × 2) reconstruction obtained at 2.0 V and 1.0 V, respectively. (I = 0.1 nA). The (8 × 2) unit cell is indicated with a solid black line. (f) dI/dV spectra obtained for the positions indicated by the blue and black dots in (d).

In conclusion, based on STM, STS, and DFT calculations, we study the geometrical and electronic properties of graphene grown on a Ge(110) substrate. We find that graphene leads to the formation of a new Ge surface reconstruction, i.e., a (6 × 2) superstructure, which has not been previously observed on bare Ge(110) surfaces. The dI/dV spectra of graphene grown on Ge(110) are found to show the characteristics of both graphene and Ge. In particular, the dI/dV spectra exhibit several peaks


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originating from the p-orbitals of Ge. The DFT calculations confirm the existence of surface states due to the p-orbitals of Ge. Our results provide an important understanding of how the interlayer coupling between graphene and a semiconducting substrate affects the structural and electronic properties of graphene.

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI:

Experimental (sample preparation and STM/STS measurements), computational (DFT calculations) methods, LEED data for graphene/Ge, further STM and DFT calculations data.

AUTHOR INFORMATION

Corresponding Author


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*[email protected], [email protected]

Notes The authors declare no competing financial interests.

REFERENCES

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