Surface Reconstruction of Germanium: Hydrogen Intercalation and

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Surface Reconstruction of Germanium: Hydrogen Intercalation and Graphene Protection Dechun Zhou, Zhiqiang Niu, and Tianchao Niu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04965 • Publication Date (Web): 10 Sep 2018 Downloaded from http://pubs.acs.org on September 10, 2018

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The Journal of Physical Chemistry

Surface Reconstruction of Germanium: Hydrogen Intercalation and Graphene Protection Dechun Zhou†, Zhiqiang Niu‡, Tianchao Niu†,* † Herbert Gleiter Institute of Nanoscience, College of Materials Science & Engineering, Nanjing University of Science and Technology, Nanjing 210094, China ‡ Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin 300071, China Abstract Understanding the interfacial properties of functional nanomaterials on semiconductor surfaces is crucial for developing electronics, optoelectronics and other devices. By using graphene on germanium (110) surface as a model, we performed scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to examine the surface reconstructions of Ge (110) under graphene. Two reconstructions, the (1×1) phase (R2) which was previously proposed to survive only 2 2 ቃ superstructure (R1 phase), were determined 5 −1 based on atomically resolved STM images. The R2 phase will transform to R1 after annealing in UHV above 300˚C, while the R1 phase can reversibly change to the (1×1) phase after heating in hydrogen at 700˚C. Finally, we confirmed the presence of interfacial hydrogen that stabilized the (1×1) phase at the initial stage of graphene growth based on control experiments. The zigzag edge of graphene is perpendicular to the close-packed [1-10] direction of Ge(110) which ensures the unidirectional growth of graphene seeds for final merging into single crystal. at high temperature, and a ቂ

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INTRODUCTION The reinvestigation of germanium (Ge) as a promising candidate to replace silicon has stimulated broad research in both industry and research laboratories with the aim for solving the challenges preventing the exploration of Ge in material science, device physics and semiconductor processing1,2,3,4. Among others, the applications of Ge require extensive investigations of its surface properties 5,6 . Furthermore, surface stability is the prerequisite for its broad applications because poor interface of native GeO2 with Ge always results in high density electronic defects7. Therefore, it is vital to protect the Ge surface with an efficient passivation layer from the surface roughness with high defect density to enhance the mobility and reliability8, and deposit an appropriate gate dielectric layer on it with high-quality interface and minor trap density9. Graphene either grown or transferred on Ge exhibits more effective surface protection than the bare germanium, but still has minor difference which is mainly due to the interfacial properties between germanium and graphene10. Since the Ge has been proved to be a suitable substrate for the growth of high quality graphene 11 , 12 , 13 , many efforts have been devoted to revealing the growth mechanism14,15, and exploring the electronic properties16,17,18. Lee et al. demonstrated that the twofold symmetrical hydrogen-terminated Ge (110) surface confined the graphene seeds to unidirectionally align along the [1-10] direction, and finally merge into uniform single-crystal graphene. The interface hydrogen facilitates the etch-free dry transfer of graphene12. Dai et al. found that graphene seeds formed strong covalent bonds with the atomic steps of germanium defined their uniform orientation based on atomic force microscopy (AFM) images and theoretical calculations14. Tesch et al. reported the growth of nearly free-standing graphene via the direct deposition of solid carbon on heated Ge(110)19. Kiraly et al. systematically studied the mechanical and electronic properties of graphene on Ge(110), (111) and (001)16. They found that strain on the graphene has significant impact on the atomic structure of interface, and modifies the mechanical properties of graphene due to the formation of surface reconstructions after annealing. In particular, the Ge(001) surface under graphene can reconstruct into [107] facet. Dabrowski et al. theoretically studied the interactions between graphene and Ge(001), showing that the reconstructed Ge(001) surface introduced extra states into graphene, leading to a 0.1 eV downward shift of the graphene Dirac point while there is no effect from the hydrogen terminated germanium 20 . Thereafter, thanks to the two-fold symmetric Ge(001), graphene nanoribbons (GNRs) with width less than 10 nm and well-defined armchair edges can be fabricated based on the directional and anisotropic growth21. Patterning graphene seeds with their armchair direction aligning with the [110] direction of Ge(001) can 2


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promote the anisotropy growth and the formation of GNRs with high-aspect ratio22. Despite the fruitful investigations on Ge in engineering two-dimensional (2D) materials, there are only few studies on its surface reconstructions and electronic properties 23 , 24 , 25 , 26 , 27 . Understanding the effect of graphene and the hydrogen intercalation on the facet of germanium is important for future developments in graphene/Ge synthesis28,29,30. Ge(110) forms different surface reconstructions which strongly rely on the annealing temperature and cooling sequences31. Short-range zigzag stripes are the typical characters of (16×2) surface reconstruction of Ge(110) which was proposed to evolve from the high-temperature disordered (1×1) phase. The elementary building block of the (16×2) surface reconstruction is a five-membered ring (pentagon) which has been revealed by STM25, low-energy electron diffraction (LEED)23, and theoretical calculations25,26. Herein, using single-crystalline graphene on Ge(110) as a model, we studied the surface reconstructions of Ge(110) under graphene at atomic scale by means of STM and X-ray photoelectron spectroscopy (XPS). Based on the control experiments by annealing graphene/Ge(110) in ultrahigh vacuum and hydrogen, we found a reversible transformation between two 2 2 reconstructions of Ge(110) under graphene, i.e., the (1×1) phase, and a ቂ ቃ 5 −1 superstructure. The former one was detected only after annealing graphene/Ge(110) in hydrogen at 700˚C, while the latter one was formed by annealing in UHV above 300 ˚C. In this work, we addressed the following key points: (i) the presence of hydrogen at the interface of graphene and germanium after graphene growth; (ii) two new surface reconstructions of Ge(110) on the graphene covered areas; (iii) the lattice registry between graphene and hydrogen passivated (1×1) phase of Ge to ensure the single crystal growth of graphene. METHODS Preparation of clean Ge (110): Ge(110) wafer was p-type (500 µm thick, Ga doped, resistivity 0.01Ω·cm), and purchased from WSK tech (Beijing, China). The treatment of germanium is similar with the condition that we have used to clean the metal substrates, i.e., cycles of sputtering and annealing.32,33 The difference is that Ge(110) usually needs lower sputtering energy. In present study, bare Ge(110) samples were cleaned in UHV by repeated cycles of Ar+ sputtering for 15 min, followed by annealing at 700°C for 15min. The sputtering was carried out on SPECS ion source IQE-11A, with energy of 400V and argon pressure of 1.2×10-5 mbar. The surface structure and cleanliness of Ge (110) were characterized by LT-STM. In the control experiments, the graphene samples were annealed in 5%H2+95%Ar gas, the pressure was precisely controlled by using a leak valve (K. J. Lesker). 3



CVD Graphene on Ge(110): The graphene samples were fabricated by chemical vapor deposition (CVD) on Ge(110) single crystal in a horizontal tube furnace as described previously14. Briefly, the Ge(110) substrates were placed in a 2-inch horizontal quartz tube. The quartz tube was evacuated to 10-4 mbar and then filled with 200 standard cubic cm per min (sccm) argon (Ar, 99.9999% purity) and hydrogen (H2, 99.9999% purity). The Ge substrates were pre-annealed in 25 sccm H2 at 910 °C for 30 min, and then introducing 0.5 sccm methane (CH4, 99.99%) to grow graphene film for different durations (from 60 min to 200 min). Afterwards, the CH4 gas was turned off and the furnace was cooled to room temperature under flowing H2 and Ar. The samples were transferred to the STM system through air. They were degassed in the preparation chamber by an E-beam heater. The temperature was monitored by a thermal-couple and a CHINO Infrared thermometer, respectively. UHV LT-STM Experimental Details. STM measurements were performed in a multi-chamber system with pressure better than 5.0×10−11 mbar at 5K, housing a SPECS Joule–Thomson STM interfaced to a Nanonis controller. STM measurements were taken in constant current mode by applying a bias voltage to the sample. Tips used were electrochemically etched from a 0.25 mm-diameter tungsten wire. Color scale bar unit is nm for all STM images. X-ray photoelectron Spectroscopy. The X-ray photoelectron spectroscopy (XPS) measurements were carried out in a custom-built multichamber UHV-XPS system with VSM 125 hemispherical electron analyzer in National University of Singapore. The samples were transferred to the UHV system through air. The XPS peaks were fitted with a mixture of Gaussian and Lorentzian functions. The ratio of Gaussian and Lorentzian was fixed to the same value, and other parameters such as full-width at half-maximum were allowed to change. The presented XPS data have a step size of 0.05 eV, and Shirley background subtractions have been applied. RESULTS AND DISCUSSION

Surface Structure of Germanium under Graphene The graphene samples were grown on Ge (110) substrates through an ambient pressure CVD method, i.e., H2:CH4 = 50:0.1 sccm at a growth temperature of 910°C11 (see details in methods). After growth, the methane (CH4) gas and the furnace were switched off while keeping the sample to be cooled down to room temperature under the H2 and Ar atmosphere. Figure 1a is a large-scale STM image of the bare Ge (110) after cycles of sputtering and annealing, which shows the large terrace with few steps. 4


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Enlarged STM image of Figure 1b presents the typical (16×2) reconstruction of Ge(110) with zigzag stripes (Figure S1)23. After the CVD growth of monolayer graphene on germanium at ~910°C, many terraces appeared due to the etching effects of H2/Ar/methane gases14. The bright lines represent graphene wrinkles that generated during the cooling stage due to the thermal expansion coefficient difference between graphene and germanium34 (Figure 1c). Although there are many steps on the Ge(110) surface, graphene covered terraces are ultra-flat as shown in Figure 1d. Furthermore, domains with different contrasts can be observed on the terraces, i.e., deep colored patches (R1) and white areas (R2) (Figure 1d). The R1 phase has a defined orientation (74˚) with respect to each other. Enlarged STM image (Figure 1e) at empty state shows that the R1 phase is composed of bright dotted arrays with a periodicity of 2.1nm, while the R2 phase is dominated by small

Figure 1. STM images of bare Ge(110) and monolayer graphene on Ge(110) (a) the bare Ge(110) after cycles of sputtering and annealing in UHV; (b) a typical (16×2) reconstruction with zigzag stripes; (c) Fully covered monolayer graphene on Ge(110), bright lines are graphene wrinkles; (d) two different phases indicated by R1 and R2. The R1 phase has a defined orientations with an angle of 74°; (e) bright dotted arrays in R1 and the clusters in R2 at positive bias; (f) different topographies of R1 and R2 phase at negative bias; (g) continuous monolayer graphene on Ge(110) taken from the yellow square in panel f; (h) zoom-in scan showing the honeycomb lattice of graphene. Scanning parameters: (a) 2.5V, 150pA; (b) 1.8V, 90pA; (c) 1V, 200pA; (d) 2V, 200pA; (e) 1V, 200pA; (f) -2.2V, 200pA; (g) and (h) -0.2 V, 400pA. clusters. In the filled state, the R1 phase changes into ellipsoid-shaped arrays, while the R2 phase turns to dotted chains (Figure 1f). These topographies dramatically differ from those of the bare Ge(110). High-resolution STM images in Figure 1g, h show 5



that both the R1 and R2 phases are covered by graphene. Furthermore, bias-dependent STM images demonstrating the evolution processes of the R1 and R2 phase from empty to filled state reveal that these features are also different from the typical moiré patterns (Supporting information Figure S2)35,36. We propose that the R1 and R2 phase would be the reconstructions of Ge(110) under graphene. To illustrate the origins of these reconstructions, we performed control experiments by cutting one Ge(110) wafer covered by 0.6 ML (monolayer, 1ML refers to that the whole substrate is fully covered by graphene) graphene into two pieces (See more information in methods). The first piece was annealed at 700 °C in UHV for 1hr which generated 100% R1 phase in the graphene covered area, as shown in Figure 2 a. Figure 2b demonstrates the typical stripes and the defined orientation of the R1 phase. The second piece under 300 °C heating in UHV for 1hr resulted in a mixed R1 and R2 phase with a ratio of 1:1, as illustrated in Figure 2c. Figure 2d displays the atomic structure in the R2 phase.

Figure 2. (a) and (b) 0.6 ML graphene on Ge(110) after annealing at 700°C in UHV for 1hr, giving rise to 100% R1 phase; (c) 0.6 ML graphene after annealing at 300°C in UHV for 1hr, the R1 phase occupies 50% on the graphene covered areas; (d) and (e) unidirectionally aligned lines of the R2 phase; (f) the R1 phase packed into an ordered pattern which rotated by 53° respect to the red arrow. Scanning parameters: (a) 2V, 200pA; (b) -2V, 200pA; (c) 2V, 200pA; (d) and (e) -1.5V, 1nA; (f) -2V, 500pA. The ellipsoids highlight the short-range ordered chains, while the red dashed arrow 6


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indicates the unidirectionally aligned lines along the [1-10] direction of Ge(110). It is noted that these lines tend to form a linear pattern with an angle of ~53° with respect to the [1-10] direction, as guided by the green arrow in Figure 2d. This orientation is exactly the same with the propagation direction of the R1 stripes. Figure 2e is an atomically resolved STM image of the R2 phase. It is clear that bright spots packed into one-dimensional chains along the [1-10] direction of Ge(110) with a periodicity of ~0.55 nm. The size of these bright spots is comparable with that of Ge adatoms on Ge (111)37 and single Ge atoms in buckled germanene38. Meanwhile, the inter-chain distance is close to the lattice constant of Ge (a=0.56 nm). Furthermore, under a certain scanning condition (Figure 2f), the stripes (R1) are composed of ordered lines (R2) with the same orientation of ~53°. Therefore, the R2 phase transforms into R1 phase after heating the sample at elevated temperature. Correspondingly, the R2 phase can be ascribed to the (1×1) phase. Previous studies demonstrated that the (1×1) phase only survived at high temperature23. In present study, the temperature during the CVD growth of graphene (910°C) is quite close to the melting point of Ge, which can generate the (1×1) phase. Furthermore, the surface germanium atoms can be passivated by H and graphene, resulting in the (1×1) phase after cooling down to RT. This structure has also been found by J. Tesch et al., while their graphene samples were prepared by depositing solid carbon on the heated Ge(110) at 850ºC and 900 ºC17. Although there is no hydrogen gas in their experiments, the substrate temperature is still close to the melting point of germanium, and, thus, the carbon atoms can stabilize the (1×1) phase as well17. In our experiment, heating the sample in UHV released the hydrogen, as a result, the germanium atoms under graphene underwent a surface reconstruction. However, the reconstruction would be different from that of pristine germanium due to the protection from the top graphene layer.39 Before confirming the presence of hydrogen at the interface of graphene and germanium, it is necessary to reveal the atomic structure of the R1 phase.

Structure of the R1 phase The packing structure of germanium adatoms within the R1 phase can be revealed based on atomically resolved bias-dependent STM images. Figure 3a shows a boundary between the R1 (right) and R2 (left) phase, the unidirectional lines in the R2 phase are clearly visible, which can clarify the direction of the R1 phase with respect to the [1-10] direction of Ge(110). As we discussed above, the topographies of both phases exhibit strong bias dependence. At -0.2V, R1 phase is composed of equidistant stripes with a periodicity of ~2.1 nm, and decorated with spindle-shaped bright dots on the ridges. Figure 3b shows the atomic geometry of R1 phase at filled state (-2V). 7



Figure 3. Top row: Bias dependent STM images showing different morphologies of the R1 phase. (a) A boundary between R1 and R2 phase; the periodicity and orientation of R1 phase were highlighted; (b) and (c) zoom-in scan of R1 from panel a showing the atomic structure at different sample biases. Bottom row: schematic model illustrating the adsorption sites and packing geometry of germanium adatoms in R1 phase. (d) proposed model of R1, the parallelograms are corresponding to the aggregations of adatoms, as shown in panel b; red triangle is corresponding to the unit cell in panel b; the atomically resolved STM image showing the Ge adatoms on the R1 phase is shown in the top right; (e) proposed structure of the adatoms in each parallelogram. Light blue circles represent a cluster of Ge adatoms. Green and pink colored balls represent the top and bottom layer germanium of the puckered Ge(110) surface. The sample was annealing at 300ºC in UHV. Scanning parameters: (a) -0.2V, 200pA; (b) and (e) -2V, 1.2nA; (c) 1V, 600pA.

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Two triangles and two dots compose a repeat unit, showing a unit cell of d1=1.3nm, d2=2.1nm, and θ=110° (highlighted by a parallelogram). However, at the empty state (+1V), the center of each repeat unit changed to bright and large spots, while the troughs turned to dim dot arrays within which a triangle rotated by ~20° with respect to the R1 stripe (Figure 3c). Although it is known that a pentagon is the basic building block in the reconstructed Ge(110) surface25,26, the pentagon exhibits a bright dot in STM images at high bias. As the topography of our sample strongly depends on the bias, only the graphene lattice can be observed at low bias. Based on these, the packing geometry of Ge adatoms in the R1 phase was depicted in Figure 3d, each repeat unit is represented by one parallelogram, giving a unit cell of d1=1.38 nm, d2=2.08 nm and θ=110°, which is quite close to the measured value from the STM image (Figure 3b). Furthermore, at the empty state (Figure 3c), the germanium atoms in the zigzag chains of the top layer surface, the small dots in the valley, can be visible due to that there is no perturbation from adatoms. Furthermore, the inter-chain distance along the [1-10] direction of Ge(110) can also be clarified (~5.3Å). As schematically depicted in Figure 3d, the unit cell of Ge(110) was highlighted by dashed triangle. The unit cell of R1 phase rotated by 54º with respect to [1-10] direction. The proposed atomic model within the triangle is highlighted in Figure 3e. Therefore, the R1 phase is defined as a

2 2 ቃ superstructure. Furthermore, G. P. Campbell et al. also performed surface 5 −1 X-ray diffraction (SXRD) study on graphene/Ge(110)18, and revealed a real space 6 ×

ቂ

2 unit cell with the size of a=2.08, b=1.39, γ = 70.5º. This unit cell size is exactly the same with our model.

Hydrogen Intercalation and Reversible Phase transition As shown above, the R2 (1×1) phase transforms to R1 phase after annealing the graphene/Ge(110) in UHV. Graphene was prepared at 910˚C in the presence of hydrogen that can generate hydrogen-terminated germanium, and further be protected by graphene. Therefore, we propose that hydrogen plays a critical role in the reversible phase transition. To validate the proposed hydrogen effects on the surface reconstruction of graphene covered Ge(111), we performed control experiments. Figure 4 are STM images taken on the 0.6ML graphene on Ge(110) heated at different conditions. Figure 4a is a large area STM image of 0.6 ML graphene/Ge(110) sample after annealing in H2/Ar (5×10-7 mbar) at 700°C for 1hr. It is noted that there are minimal steps comparing with the as-prepared CVD graphene without further annealing in hydrogen. This phenomenon is consistent with the recently reported 9



substantial flattening of the nanofacet structure after hydrogen interaction on Gr/Ge(001)29. Deep-colored flat islands with sharp edges are graphene domains, while the white stripes are graphene wrinkles. The R1 and R2 phases exhibit different features in STM topography under the same scanning conditions. Clearly, the graphene covered areas are free from R1 phase as examined by the enlarged STM image (Figure 4b). High resolution STM image shows that the graphene covered areas are the R2 phase with unidirectionally aligned lines (Figure 4c). Therefore, annealing in H2/Ar can lead to the

Figure 4. Annealing 0.6 ML graphene/Ge(110) in 5×10-7 mbar H2/Ar (top) and in UHV (bottom). (a) smooth graphene covered area after annealing in hydrogen; (b) enlarged STM image taken from panel “a” confirming the transformation from R1 to R2 phase; (c) atomically resolved STM image showing that all the graphene covered Ge(110) turns to the R2 phase with unidirectional lines; (d) annealing the same sample at 300 °C for 1hr in UHV regenerates small area of R1 phase. Bright dashed ellipsoids highlight the reconstructed patches; (e) further annealing at 300 °C for long time in UHV leads to large area R1 phase. The arrows highlight the defined orientation of ~74° between reconstructed patches; (f) atomic structure of the marked area in panel “e”. Scanning parameters: (a) -2.2V, 80pA; (b) -2.5V, 500pA; (c) -2.5V, 1.5nA; (d) 2.5V, 85pA; (e) 2.5V, 1nA; (f) -2.5V, 1nA. phase transition from R1 to R2 because of the hydrogen passivation on germanium. Moreover, annealing the hydrogen-treated graphene sample at 300 °C in UHV generated dark-colored lines, as guided by the ellipsoids in Figure 4d. Further 10


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annealing in UHV, these lines became larger and propagated with a specific orientation of 74° (Figure 4e). The filled-state topography of these lines (Figure 4f) is exactly the same with that of the R1 phase as described in Figure 1f and 3b. Annealing this sample with the R1 phase in H2/Ar (5×10-7 mbar) at 700°C for 1hr can reversibly change R1 to R2. Therefore, the reversible phase transition between the R1 phase and R2 (1×1) phase of Ge(110) under graphene is induced by hydrogen desorption and intercalation. Graphene/Germanium Interface interaction

Figure 5. XP-spectra of the C 1s (a) and Ge 3d (b) of 0.6 ML graphene/Ge(110) which was treated under different conditions as marked. The as-loaded sample without outgassing is shown as the reference in the topmost curve.

To gain additional insight into the hydrogen intercalation effect on the interfacial 11



properties, we performed XPS investigations. Figure 5a displays C1s spectra of a 0.6 ML graphene/Ge(110) treated under different conditions. Before the outgassing in UHV, the low binding energy of C 1s at 284.31 eV can be ascribed to surface adsorbate induced doping effect, such as H2O and O240. After degassing the sample at 700°C in UHV to remove the contaminations, the C 1s peak shifted to higher binding energy (284.5eV). However, it is noted that C1 has barely changed after annealing the sample at 700°C in H2. Further heating this sample in UHV to remove the interface hydrogen, the C1s peak still locates at the same position, indicating the minimal effect of interface hydrogen on the electronic properties of graphene, as well as the weak interaction between germanium and graphene41.

Figure 5b shows the Ge 3d core level spectra of 0.6 ML graphene/Ge(110) following the procedures as indicated in Figure 5a. Two distinguishable components appear in the as-loaded sample without outgassing. The peak at higher binding energy (30.38eV) is assigned to the oxidized Ge (GeO)10. After outgassing the sample at 700 °C in UHV for 1hr, the oxidized peak disappeared while the peak of Ge 3d shifted to higher binding energy (29.56 eV), which can be correlated to the formation of R1 phase. Furthermore, annealing the sample in H2 at 700 °C for 1hr leads to a peak shift to even higher binding energy (29.77 eV), indicating the transformation to R2 phase with the formation of H-Ge bond. However, after heating the sample at 300 °C in UHV, the Ge 3d peaks shifted back to 29.56 eV. The reversible shift of Ge 3d peaks under different annealing conditions agrees well with the STM investigations, i.e., the phase transition induced by the hydrogen intercalation and desorption. In present 12


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study, the Ge peak shift distinctly reveals the substantial interaction between hydrogen and germanium under graphene. On the contrary, no peak shift of C1s indicates the ignored perturbation of hydrogen on the graphene layer, proving the weak interaction between graphene and germanium20. Unidirectional alignment of Graphene on Ge (110) Ensuring Single Crystal Growth

Figure 6. Alignment of graphene on Ge(110). (a) A boundary between R1 and R2 phase. The [1-10] direction of Ge(110) is indicated; (b) Image taken from panel “a” at lower bias showing the R1 phase and graphene lattice. The inset is an atomically resolved STM image of graphene lattice; (c) schematic model showing the maximal overlap between graphene and Ge atoms. Scanning parameters: (A) -1V, 200pA; (B) -200mV, 400pA. The three-fold symmetric graphene can grow single crystal on the two-fold symmetric Ge (110) surface14, but there are domain boundaries and multiple orientations of graphene on Ge(111)12. As the substrate plays key role in determining the structure and property of the final product, this is exactly the case for the growth of group VA monoelemental monolayers, such as phosphorene and antimonene. Gao et al. theoretically proposed that substrate with a moderate interaction (about 0.35 eV/P atom) with phosphorus can stabilize the nanoflakes of phosphorus that is vital for the realization of phosphorene42. Nonetheless, there is still no high throughput vapor deposition or epitaxial method to fabricate phosphorene. In the case of the molecular beam epitaxial growth of antimonene, PdTe243, Bi2Se344, Ge(111)45, and Ag(111)46,47 have been used to engineer both the phase and the buckling structure. In current study, 13



we highlighted the presence of hydrogen at the interface between graphene and Ge(110) that stabilizes the (1×1) phase, and, hence, confines the unidirectional alignment of graphene seeds for further merging into single crystal. This concept would be helpful for guiding the fabrication of antimonene and phosphorene. In particular, the H-terminated germanium is able to recover the semiconductor properties of antimonene45. Although the beginning of graphene nucleation process is complicated48, we can capture the correlation of substrate and graphene lattice after the event. As representatively demonstrated in Figure 6a, the right side is the (1×1) phase which can distinctly define the [1-10] direction of Ge(110), as highlighted by the dashed arrow. Figure 6b shows the graphene lattice in company with the visible R1 phase as wide ridges. The inset is the high resolution image showing the honeycomb lattice of graphene. Apparently, the zigzag edge of graphene is perpendicular to the [1-10] direction of Ge(110). This conclusion consists with the SEM results reported by Lee et al12. Such unidirectionally aligned graphene seeds ensures the final merging into single crystal. Several reasons account for such unidirectional alignment (Supporting information Figure S3). First, the matched lattice constant of graphene to the Ge-Ge bond along the zigzag chain. As highlighted in Figure 6c, the unit cell of graphene is 0.25 nm (the next nearest carbon atoms), while the Ge-Ge bond length is 0.25 nm (zigzag chain along the [1-10] direction). Second, hydrogen-terminated Ge(110) stabilizes the (1×1) phase during the graphene growth that can confine the graphene seeds. Finally, graphene will nucleate with such a way that zigzag edge is perpendicular to the [1-10] direction of Ge(110) . CONCLUSION In summary, we have found two kinds of germanium reconstructions on the graphene

2 2 ቃ 5 −1 superstructure. Based on control experiments, we have systematically studied the hydrogen intercalation effects on the phase transformation between the two covered Ge(110), i.e., the hydrogen-passivated (1×1) phase and the ቂ

reconstructions. STM results revealed that the (1×1) phase is stabilized by hydrogen

2 2 interaction between graphene and Ge(110), while the ቂ ቃ superstructure is 5 −1 formed due to the aggregated adatoms after the desorption of hydrogen. Finally, we addressed the unidirectional alignment of graphene on the hydrogen-terminated (1×1) 14


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phase of Ge(110), i.e., the zigzag edge of graphene is perpendicular to the close-packed [1-10] direction of Ge(110), which ensures the unidirectionally aligned graphene seeds to merge into single crystal. This preferential alignment can be ascribed to the matched lattice constant of graphene to the Ge-Ge bond along the zigzag chain of Ge(110). Furthermore, the controllable and reversible hydrogen passivation on Ge(110) may also be useful for tuning the interaction between graphene and substrate. As an inspiration, the hydrogen-terminated Ge and the reversible phase transformation of the surface reconstruction of Ge may provide valuable information to guide the controllable synthesis of other 2D materials, such as phosphorene and antimonene. Supporting information Additional STM images of the surface reconstruction of clean Ge(110) which was annealed in UHV and hydrogen, respectively; Bias dependent STM images showing the topography of the R1 and R2 phase; Schematic models showing the structure of Ge(110) and graphene. XPS data showing that the surface is free from dopant. These materials are available free of charge on the ACS Publications website. Corresponding author * E-mail: [email protected] ACKNOWLEDGEMENTS We greatly acknowledge the final support from Natural Science Foundation of China under Contract Nos. 21403282 and 11227902, and the 111 project (B12015). T. N. thanks Prof. A. Li and Prof. Z. Liu for their great help during the experiments in Shanghai Institute of Microsystem and Information Technology (SIMIT, CAS). Helpful discussion with Dr. M. Ye and Prof. Z. Di is acknowledged. T. N. thanks Dr. J. Zhang and Prof. W. Chen for their help with the XPS experiments. The authors thank Dr. J. Dai, Mr. H. Zhu, Mr. Q. Liu for their help with the sample preparation and characterization. REFERENCES

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Surface Reconstruction of Germanium: Hydrogen Intercalation and

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