Identifying the Non-Identical Outermost Selenium Atoms and

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Identifying the Non-Identical Outermost Selenium Atoms and Invariable Band Gaps across the Grain Boundary of Anisotropic Rhenium Diselenide Min Hong,†,‡,∥ Xiebo Zhou,†,‡,∥ Nan Gao,§,∥ Shaolong Jiang,†,‡ Chunyu Xie,†,‡ Liyun Zhao,† Yan Gao,† Zhepeng Zhang,†,‡ Pengfei Yang,†,‡ Yuping Shi,†,‡ Qing Zhang,† Zhongfan Liu,‡ Jijun Zhao,*,§ and Yanfeng Zhang*,†,‡ †

Department of Materials Science and Engineering, College of Engineering and ‡Center for Nanochemistry (CNC), Beijing Science and Engineering Center for Nanocarbons, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China S Supporting Information *

ABSTRACT: Rhenium diselenide (ReSe2) is a unique transition-metal dichalcogenide (TMDC) possessing distorted 1T structure with a triclinic symmetry, strong inplane anisotropy, and promising applications in optoelectronics and energy-related fields. So far, the structural and physical properties of ReSe2 are mainly uncovered by transmission electron microscopy and spectroscopy characterizations. Herein, by combining scanning tunneling microscopy and spectroscopy (STM and STS) with firstprinciples calculations, we accomplish the on-site atomicscale identification of the top four non-identical Se atoms in a unit cell of the anisotropic monolayer ReSe2 on the Au substrate. According to STS and photoluminescence results, we also determine the quasiparticle and optical band gaps as well as the exciton binding energy of monolayer ReSe2. In particular, we detect a perfect lattice coherence and an invariable band gap across the mirror-symmetric grain boundaries in monolayer and bilayer ReSe2, which considerably differ from the traditional isotropic TMDCs featured with defect structures and additional states inside the band gap. Such essential findings should deepen our understanding of the intrinsic properties of two-dimensional anisotropic materials and provide fundamental references for their applications in related fields. KEYWORDS: ReSe2, anisotropic structure, scanning tunneling microscopy and spectroscopy, grain boundaries, electronic properties

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properties of monolayer or few-layer ReX2 are essential issues. Controllable synthesis of high-quality 2D ReX2 samples is also imperative. Several theoretical calculations revealed that bulk ReX2 behaved as electronically and vibrationally decoupled monolayers due to very weak interlayer coupling.7,10,14 The bulk state possessed a nearly identical band structure to the monolayer counterpart. In experiment, angle-resolved photoemission spectroscopy (ARPES) was utilized to acquire the

wo-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) such as MoS2 and WS2, have attracted worldwide interest as the promising functional materials for the next-generation optoelectronics and electronics due to their versatile physical properties.1−4 Unlike the conventional hexagonal TMDCs, 2D rheniumbased dichalcogenides (ReX2, X = S or Se) crystallize in a distorted 1T (1T′) phase with a triclinic symmetry, thus presenting strong in-plane anisotropy.5,6 The unique crystal structure affords 1T′-ReX2 exceptional anisotropic optical5,7,8 and electrical9,10 properties, enabling brand new applications in field-effect transistors,11,12 integrated digital inverters,10 and polarization-sensitive photodetectors,13 etc. For these applications, in-depth investigations of the structural and electronic © XXXX American Chemical Society

Received: June 27, 2018 Accepted: September 18, 2018 Published: September 18, 2018 A

DOI: 10.1021/acsnano.8b04872 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Anisotropic lattice feature of ReSe2. (a) Schematic illustration of the ReSe2 lattice. The top and side views of a ReSe2 unit cell are presented at the bottom panel. (b) SEM image of parallelogram-shaped ReSe2 flakes on the Au foil substrate. (c) High-resolution STEM image showing the anisotropic ReSe2 lattice with the observation of Re chains. (d) Large-scale STM image of monolayer ReSe2 flakes on Au (Vsample = 3.0 V, It = 0.2 nA, 50 nm × 50 nm, T = 78 K). (e, f) Atomic-scale STM images of monolayer ReSe2/Au at Vsample values of 400 and 20 mV, respectively, showing subunit-cell resolution of the four non-identical Se atoms (400 mV, 0.8 nA; 20 mV, 200 pA, 5 nm × 5 nm, 78 K).

occupied band structure (below the Fermi level) of ReS2.15−17 However, the on-site correlations between the atomic-scale structures [especially the superstructures, defects, and grain boundaries (GBs)] and the local electronic properties are still challenging due to the limited spatial resolution of the current analysis method. Recently, several groups have made efforts to synthesize ultrathin ReX2 flakes on diverse substrates by chemical vapor deposition (CVD).13,18−21 However, numerous GBs were universally observed in the polycrystalline ReX2 domains according to angle-resolved Raman spectroscopy and lowenergy electron microscopy and diffraction (LEEM and LEED) characterizations,18,19 similar to the CVD-synthesized MoS222−24 and graphene.25,26 Particularly, according to theoretical calculations and experimental characterizations, e.g., transmission electron microscopy (TEM) and scanning tunneling microscopy and spectroscopy (STM and STS), both tilt GBs and mirror-symmetric GBs were identified in monolayer MoS2, as featured with arrays of dislocation cores (composed of non-six-membered rings),22,27−29 which induced midgap states in the band gap of MoS2 and strongly affected its transport and optical properties. In view of this, the on-site characterization technique of STM and STS should be reliable for clarifying the related issues in the 2D anisotropic material of ReX2. Moreover, the enhanced excitonic effect has been intensively explored in conventional isotropic monolayer TMDCs.24,30,31 This effect sheds light on the many-body physics in 2D materials and is essential for the electronic and photonic applications.31,32 Specifically, a large exciton binding energy of ∼0.87 eV was theoretically predicted in monolayer ReSe2,6,8 while no any experimental result has been reported so far, due to the requirement of determining both the optical and the quasiparticle band gaps.

Herein, we report the on-site investigations of the structural and electronic properties of CVD-grown monolayer and bilayer ReSe2 on a conductive substrate of Au foil. The highresolution STM/STS technique is introduced for establishing the correlation between the atomic-scale structure and local electronic properties, which cannot be achieved by traditional TEM and ARPES routes. Particularly, the subunit-cell structure of the anisotropic ReSe2 featured by the outermost nonidentical Se atoms is clearly unveiled by bias-dependent STM imaging and density functional theory (DFT) calculations. Meanwhile, the quasiparticle band gap and exciton binding energy of monolayer ReSe2 are determined by complementary STS and photoluminescence (PL) data. More importantly, the atomic-scale patching behavior of the mirror-symmetric ReSe2 GBs, as well as the related electronic properties are unambiguously demonstrated. This work should propel the explorations of the fundamental properties and the device applications of the 2D anisotropic ReX2 materials.

RESULTS AND DISCUSSION Recently, high-quality monolayer ReSe2 has been successfully synthesized on Au foils by our group through an ambient pressure CVD (APCVD) route.18 As depicted in the schematic image in Figure 1a, the CVD-grown ReSe2 is stabilized in the 1T′ structure, in which the in-between Re atoms are arranged into Re4 diamond-shaped clusters and linked into onedimensional chains along the b[010] direction6−8 (denoted by the red arrow). Such structural distortion results in a reduced crystal symmetry and a two-times-enlarged unit cell (indicated by a black dashed parallelogram), which consists of four Re atoms and eight Se atoms. Notably, the buckling of Se layers makes the four Se atoms in top (or down) layers in a unit cell not locate in the same plane,7,33,34 which can be numbered as Se1, Se2, Se3, and Se4, ranked by the gradually B

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Figure 2. Bias-dependent STM images and the simulated results for monolayer ReSe2 on Au. (a) Calculated LDOS of the 4px and 4py orbitals for the four non-identical Se atoms in a unit cell. (b−e) STM images of monolayer ReSe2 on Au at various Vsample values (20 mV, 0.2 nA; 80 mV, 1 nA; 340 mV, 0.25 nA; 400 mV, 1.2 nA; 4 nm × 4 nm, 78 K). (b′−e′) Corresponding simulated STM images under the biases 0.5, 1.0, 1.5, and 2.0 V, respectively. (f) Section views captured along the A−B−C directions of the triangle denoted in panel e, showing varying surface undulations in the three typical directions. (g) Schematic structure of ReSe2 based on the STM image in panel e. Only the upperlayer Se atoms are presented for clarity.

bright spots corresponding to the outermost layer of Se atoms. However, the apparent spacing between bright spots measured in three typical directions are ∼0.68, 0.69, and 0.71 nm, respectively (as denoted in Figure 1e), which are very close to the reported lattice parameters of ReSe2.33,35 A unit cell is thus defined and marked by a black dashed parallelogram, and the top four non-identical Se atoms are marked by yellow balls. Notably, only one of the Se atom can be seen in this STM image. Intriguingly, when Vsample was reduced to 20 mV (Figure 1f), the four non-identical Se atoms can be observed concurrently with slight contrast differences. Nevertheless, the lattice orientations of ReSe2 in the STM image cannot be directly determined as those in TEM images. Bias-dependent STM characterizations were further performed to establish a direct correlation between the applied Vsample and the number of visible Se atoms in a unit cell. Specifically, at a Vsample of 20 mV (Figure 2b), four Se atoms are noticeable with inhomogeneous STM contrasts (as denoted by the colored dots), suggesting their slightly inequivalent contributions to the tunneling currrent.36 With increasing Vsample from 20 mV to 400 mV, the visible Se atoms in a unit cell are gradually decreased from four to one (Figure 2b−e). Such STM morphological evolutions should originate from the disparate electronic states of the outermost Se atoms, which have not been detected in the conventional isotropic TMDCs like MoS224,37 and MoSe2.30 To elucidate this, DFT calculations were then conducted on a five-layer Au slab model (Figure S1a) to calculate the local density of states (LDOS) of 4px, 4py (Figure 2a), and 4pz (Figure S1b) orbitals of the top four non-identical Se atoms in a unit cell (i.e., Se1, Se2, Se3, and Se4, shown in the inset of

decreased apparent height with respect to the basal plane, as shown in the bottom of Figure 1a. Moreover, an anisotropic growth behavior, i.e., faster growth along the b[010] direction has been reported,18 which leads to the formation of parallelogram-shaped single-crystal ReSe2 flake, as displayed by the scanning electron microscopy (SEM) image in Figure 1b. Previously, the atomic structure of ReSe2 is commonly characterized by high-resolution scanning transmission electron microscopy (STEM). The Re atoms are clearly seen with brighter contrasts due to their larger atomic number than Se atoms (Figure 1c). The superimposed structural model highlights the diamond-shaped Re4 chains, i.e., the b[010] direction. Accordingly, the a[100] axis showing an intersection angle ∼118.5° with that of b[010] axis is easily determined. The larger spacing between Re chains (∼0.37 nm) than that between two vicinal Re4 clusters in a chain (∼0.32 nm) is also consistent with the previous results.9,18 Although TEM can easily obtain the atomic structure and the lattice orientation of ReSe2, the inevitable sample transfer and the electron beam irradiation processes are likely to damage the surface structure and induce various defects.9,19 On the contrary, in situ STM characterization provides a noninvasive pathway to explore the intrinsic structure of CVDgrown sample. As shown in the large-scale STM image in Figure 1d, the as-grown monolayer ReSe2 flake preserves the typical trapezoidal shape over the terraced Au substrate. The shrunken domain size and irregular edge morphology may arise from the pre-annealing process in ultrahigh-vacuum (UHV) system. The atomic-resolved STM image of ReSe2 captured at a sample bias (Vsample) of 400 mV (Figure 1e) presents periodic C

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Figure 3. Identifying the different interfacial coupling between monolayer or bilayer ReSe2 and Au or graphite substrates by STM and STS measurements. (a) STM image showing coexisting monolayer and bilayer ReSe2 on the Au substrate (1.8 V, 180 pA, 50 nm × 50 nm, 78 K). (b) STS spectra taken on bilayer (black curve) and monolayer (red curve) ReSe2 regions shown in panel a (1.1 V, 0.35 nA; 1.5 V, 0.2 nA, Vrms = 10 mV, f = 932 Hz, 78 K). (c) STM image of a transferred monolayer ReSe2 on graphite (1.23 V, 500 pA, 12 nm × 12 nm, 78 K). (d) STS spectrum of transferred ReSe2/graphite (2.0 V, 0.8 nA, 10 mV, 932 Hz, 78 K). (e) PL spectra of monolayer ReSe2 on SiO2/Si acquired at 78 K (red) and RT (blue), respectively. (f) Schematic illustration showing the correlation between the measured electronic band gap (Eg), optical band gap (Eopt), and exciton binding energy (Eb).

In this regard, the top four non-identical Se atoms in a unit cell of ReSe2 are clearly recognized by the bias-dependent STM images, which can be used to identify the lattice orientations because the Se1 and Se2 atoms are arranged along the Re chain direction, i.e., the b[010] direction (as denoted by the red arrows in Figure 2b−d). However, for the more commonly observed STM image with the visibility of only Se1 atoms (Figure 2e), it is challenging to directly identify the lattice orientation. Another identification method is developed accordingly. As denoted by the solid lines in Figure 2e, three special directions (A−B, B−C, and A−C) were chosen to plot the height profiles. Consequently, a minor peak is found between two adjacent bright spots in both A−C and B−C profiles, while it is nearly invisible in the A−B profile (as indicated by solid and dashed arrows in Figure 2f). Through correlating this STM image with the ReSe2 structure model and the LDOS calculations, the minor peaks should correspond to the invisible Se atoms between the bright Se1 atoms, and the least-obvious one in the A−B profile should be Se4 atom due to its lowest LDOS among four non-identical Se atoms. Once the positions of Se1 and Se4 are determined, the b[010] and a[100] directions can be identified concurrently, as shown in Figure 2e,g. This simple identification method should be reliable for marking the lattice orientations of single-crystal ReSe2 domain and the multidomain GBs. In the CVD-grown ReSe2/Au sample, bilayer regions are occasionally evolved and coexisted with the monolayer film, as enclosed by the dashed line in Figure 3a. Corresponding height profile shown in the inset reveals a relative height of ∼0.8 nm with the ground layer, consistent with the thickness of a monolayer ReSe2.7,18 The prominent contrasts at the edge of bilayer ReSe2 with regard to the domain interior can be attributed to the decoration of adsorbents at this open edge site, which is more evidence of the existence of bilayer region.

Figure 2a) because the tunneling current contributed by each Se atom is proportional to the total intensity of the LDOS. Moreover, the corresponding STM morphologies of ReSe2 are simulated at different energy ranges (Figure 2b’-e’). For the positive Vsample, the tunneling electrons are injected from tip to the conduction band of ReSe2. Hereby, the LDOS near the conduction band minimum (CBM) is critical to the STM imaging. Notably, the evolution trend of the simulated STM images with increasing bias perfectly coincide with the experimental observations (Figure 2b−e), although the absolute set biases are not one-to-one correspondence. This variation tendency can be explained by the calculated LDOS results shown in Figure 2a, which manifest obviously different LDOS intensity distributions for the Se1−Se4 atoms in the conduction band region. Specifically, at a simulated bias voltage of 0.5 V, the tunneling contributions (total intensity of LDOS) from Se1−Se4 atoms have a minor difference, leading to slightly different contrasts in the simulated STM image (denoted by differently colored dots in Figure 2b′). When the bias voltage was set to 1.0 V, the total LDOS intensity of Se4 (blue curve in Figure 2a) becomes much smaller than the other three, resulting in depressed contrast of Se4 atoms in the simulated STM image, while the rest three Se atoms exhibit brighter contrasts (Figure 2c′). By that analogy, the invisibility of Se3 and Se4 at 1.5 V (Figure 2d′), and the observation of only Se1 at 2.0 V (Figure 2e′) can be well understood. Notably, the large energy discrepancy between the experimental data and the simulation results partly arises from the specific tip− sample coupling effect that affects the STM imaging,38,39 which is not included in the DFT calculations. Moreover, the number of layers of Au atoms used in the structural model also influences the energy range of the LDOS distributions and the STM simulations (see Figure S2 for comparison between fivelayer and three-layer slab models). D

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Figure 4. STM and STS characterizations of the structural and electronic properties of a mirror-symmetric GB in monolayer ReSe2. (a) STM image of a ReSe2 GB with mirror symmetry on the Au substrate (0.7 V, 180 pA, 20 nm × 20 nm, 78 K). (b) Zoomed-in STM image over the GB in the black rectangle in panel a (0.7 V, 180 pA, 8 nm × 8 nm, 78 K). (c) A series of STS spectra captured at the marked positions across the GB in panel b (1.5 V, 0.2 nA, 10 mV, 932 Hz, 78 K). (d) Structural model and simulated STM image of the mirror-symmetric ReSe2 GB. (e) Calculated DOS for the monolayer ReSe2 GB and pristine ReSe2.

MoSe230,42 overlayers. The STM image of transferred monolayer ReSe2/graphite (Figure 3c) presents distinct ReSe2 lattices, indicating the rather high crystal quality of the transferred sample. Bias-dependent STM morphological evolutions of the ReSe2 lattice are also observed on the graphite substrate (Figure S6), indicating its universality on different substrates. The corresponding STS spectrum (Figure 3d) exhibits an Eg ≈ 1.85 eV, with the VBM and CBM located at ∼−1.50 and 0.35 V, respectively. Notably, this band gap value is moderately larger than that of bilayer ReSe2/Au and agrees well with the theoretical band gap of monolayer ReSe2 (Figure S3). Moreover, a minor band gap decrease is observed in bilayer ReSe2 on graphite (Figure S7), which differs from the dramatic band gap decrease for conventional TMDCs (like MoS2) from monolayer to few layers.43,44 The not-obvious thickness dependence of band gap reconfirms the very weak interlayer coupling and the minor quantum confinement effect in ReSe2.45,46 According to this comparison, a strong interlayer coupling induced band gap reduction is clearly verified in the ReSe2/Au system. More specifically, the CBM of ReSe2/ graphite is pinned at the same position with that of monolayer/bilayer ReSe2 on Au, along with a band gap variation deriving from the VBM shift. In other words, the Au donor states contribute greatly to the variation of the valence band edge of ReSe2. Additionally, the optical band gap (Eopt) of monolayer ReSe2 was measured by PL spectroscopy on the transferred monolayer ReSe2 on SiO2/Si. As shown in Figure 3e, the PL peak corresponding to Eopt of ReSe2 is located at ∼1.42 eV at room temperature (RT), consistent with the previous reports.47 At a low temperature of 78 K, the peak position is shifted to ∼1.45 eV. Comparing the value of 1.85 eV for Eg and 1.45 eV for Eopt at 78 K, the exciton binding energy (Eb) of ∼0.40 eV is thus deduced for monolayer ReSe2 (Figure 3f).

To achieve the electronic property of CVD-grown ReSe2 on Au substrate, STS measurements were performed on these two typical regions, as shown in Figure 3b. Consequently, monolayer ReSe2 possesses a quasiparticle band gap (Eg) of ∼0.90 eV, with the valence band maximum (VBM) and CBM locating at ∼−0.52 and 0.38 V, respectively (red curve). This result is substantially smaller than the theoretical value of isolated monolayer ReSe2 (∼1.83 eV) (Figure S3), as is likely due to the strong interfacial interaction between ReSe2 and Au, which induces abundant Au donor states in the band gap of ReSe2. Further charge density difference analysis indicates that an evident charge transfer occurs at the interface of ReSe2 and Au (Figure S4), which primarily causes the band gap reduction of monolayer ReSe2. Similar substrate-induced band gap reduction was also reported in monolayer MoS2/Au.37,40 However, the STS spectrum captured from bilayer ReSe2 region manifests an abnormal band gap variation (black curve in Figure 3b). Compared to monolayer ReSe2, the CBM of bilayer is pinned at the same position of ∼0.38 V, while the VBM is downshifted to ∼−1.27 V, yielding an increased Eg ≈ 1.65 eV, which is unexpectedly close to the theoretical band gap of monolayer ReSe2 (Figure S3). DFT calculations also confirm the abnormal band gap modulations for monolayer and bilayer ReSe2 by the Au substrate (Figure S5), which is mainly mediated by the very weak interlayer coupling in ReSe2. The bilayer ReSe2 can be considered as two independently stacked monolayers,6,14 where the first ReSe2 layer serves as a buffer layer for effectively weakening the strong interfacial interaction between the second ReSe2 layer and Au substrate, leading to a bulk-like band gap for bilayer ReSe2. To further explore the substrate effect on the electronic property of ReSe2, the as-grown ReSe2 was transferred onto graphite, which is known as a weakly interacting substrate for its retaining the relatively intrinsic band gap of MoS224,41 and E

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Figure 5. STM and STS characterizations of the structural and electronic properties around a mirror-symmetric GB in the bilayer ReSe2. (a) Large-scale STM image of a mirror-symmetric GB in the bilayer ReSe2 (2.0 V, 300 pA, 30 nm × 30 nm, 78 K). (b) Zoomed-in STM image from the black rectangle in panel a (1.0 V, 350 pA, 6 nm × 6 nm, 78 K). (c) A series of STS spectra taken at the marked positions across the GB in panel b, showing the invariable band gap across the GB (within a range of ∼3 nm) (1.1 V, 0.35 nA, 10 mV, 932 Hz, 78 K). (d) A series of STS spectra taken at the marked positions along the GB in panel b, showing the invariable band gap at the GB (1.1 V, 0.35 nA, 10 mV, 932 Hz, 78 K). (e, f) dI/dV mapping around the GB region under the biases of 0.3 and 1.0 V, respectively.

Notably, this Eb is smaller than the theoretical value ∼0.87 eV,6,8 probably due to the substrate screening effect.24 Even though, the obtained Eb of monolayer ReSe2 is still comparable with that of MoS224 and MoSe2,30 verifying that a large exciton binding energy is a common characteristic for 2D semiconducting TMDCs. For the CVD-grown polycrystalline monolayer ReSe2, GBs are inevitably evolved along the patched boundaries, as shown in the SEM image in Figure S8. Figure 4a depicts a typical GB region of ReSe2, and the detailed structure of the two adjacent domains and boundary site is presented in the magnifying STM image in Figure 4b. Because only the brightest Se atom (Se1) in each unit cell is observable in both left and right domains, the domain orientations can be identified by the method discussed in Figure 2e−g, as also shown in Figure S9. As a result, the a[100] direction of left and right domains are along the same perpendicular direction in the image, while the b[010] direction has a rotation angle of ∼120°, suggesting the formation of a mirror-symmetric GB. Intriguingly, the ReSe2 lattices present a perfect coherence at the GB, avoiding the evolution of any apparent defects or dislocations (Figure 4b). It is therefore fascinating to know the electronic property at this unique patching interface. A series of STS spectra were then taken across the GB. As shown in Figure 4c, the results acquired from the left (i.e., points 1−3) and right (i.e., points 5−7) domains show the same band gap ∼0.90 eV with the VBM and CBM located at −0.52 and 0.38 V, respectively, in line with the results discussed above. Surprisingly, the STS of GB (point 4) manifests the same VBM and CBM positions as well as an invariable band gap value with regard to the domain interior. Similar band gap invariance is also observed at the ReSe2 GB on the graphite substrate (Figure S10). In this regard, the structural and electronic features of ReSe2 GB are in sharp contrast to the conventional isotropic 2D materials, such as graphene,26,48 hexagonal boron nitride,49 and MoS2,22,23,28 where various non-six-membered rings were

formed at the tilt and mirror-symmetric GBs. These dislocation cores were reported to induce midgap states or additional LDOS peaks in the energy bands of MoS2 or graphene, followed by abnormal optical and transport properties.22,25,29,50 The seamlessly stitched structure and the invariable electronic property are unique for the predominantly evolved mirror-symmetric ReSe2 GB. To clarify the underlying mechanism, DFT calculations were performed with the simulated structural model shown in Figure 4d, which is generated by stitching two perfect ReSe2 domains with sufficiently large supercells together (one is derived by rotating 180° along the a[100] axis with another) (Figure S11). Hereby, the b[010] directions of the two ReSe2 domains have an in-plane rotation angle of 120°, while their a[100] directions are identical. In this manner, the boundary site can be integrated well if the two ReSe2 domains are as close as possible. The simulated STM image at the energy of 3.5 V agrees well with the experimental result (Figure 4b). Furthermore, the calculated DOS at the ReSe2 GB and the pristine ReSe2 region present nearly the same semiconducting band gap of 1.25 eV. The theoretical result coincides well with the STS data, although the DFT calculation with Perdew− Burke−Ernzerhof (PBE) functional underestimates the band gap of ReSe2. Briefly, the invariable band gap can be attributed to the perfect structural coherence at the mirror-symmetric GB site. Specifically, the Re−Re and Re−Se bond lengths maintain the original value 2.86−3.0 and 2.45−2.60 Å at the GB interface and the coordination numbers of Re and Se atoms retains 8 and 3, respectively. The bond surroundings resemble the pristine ReSe2 possessing effectively saturated chemical bonds. In this regard, the transport behavior along such GB is expected to be perfectly maintained. Similar mirror-symmetric GBs can also be observed in the bilayer ReSe2 on the Au substrate, as shown in Figure 5a,b, with more related STM images presented in Figure S12. Due to the large imaging bias, only the brightest Se atom (Se1) in F

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Transfer of ReSe2. For sample transfer, the ReSe2/Au samples were first spin-coated with poly(methyl methacrylate) (PMMA) at 2500 rpm for 1 min, resulting in a uniform polymer film on the sample surfaces. The stack was then baked at 120 °C for 15 min. Next, the PMMA film, together with ReSe2 flakes, was detached from the Au foil using an electrochemical bubbling method.51 The PMMAsupported ReSe2 was then washed with deionized water several times, transferred onto SiO2(300 nm)/Si, graphite substrates, and TEM grids and dried on a hot plate at 80 °C. Finally, the PMMA was removed using acetone and isopropanol. Characterization of ReSe2. The synthesized ReSe2 flakes were systematically characterized using SEM (Hitachi S-4800, 2 kV), Raman spectroscopy (HORIBA iHR550 with an excitation light wavelength of 532 nm), and STEM (JEOL JEM-ARM200CF, acceleration voltage of 200 kV). UHV low-temperature STM/STS systems were also utilized for the atomic-scale structural characterization under a base pressure of better than 10−10 mbar. The STS spectra were acquired by recording the output of a lock-in system with the manually disabled feedback loop. A modulation signal of 10 mV at 932 Hz was selected under the specific tunneling conditions. DFT Calculations. DFT calculations were performed with the Vienna ab initio simulation package (VASP),52 by using the planewave basis set with energy cutoff of 500 eV, the projector augmented wave (PAW) potentials,53,54 and the generalized gradient approximation (GGA) parametrized by PBE for the exchange-correlation functional.55 Due to the intrinsic flexibility of distorted 1T phase of monolayer ReSe2, we constructed the GBs model of rectangular supercells with a lateral distance of 25 Å to avoid the interaction between the GB and its replica. A vacuum region of 18 Å was applied for the vertical direction to eliminate the interactions between periodic images. The 2D Brillouin zones of the supercell were sampled by a uniform k-point mesh with spacing of 0.03/Å. The model structures were fully optimized for electronic and ionic degrees of freedom with thresholds for the total energy of 10−5 eV and the forces on each atom of 0.02 eV/Å. The LDOS calculations of the top four non-identical Se atoms in a unit cell of ReSe2 and the related STM simulations were performed on both three-layer and five-layer slab model of Au(111) for comparison, while the other calculations on the ReSe2/Au system were performed on the three-layer slab model for simplification. During the relaxation of ReSe2 on Au(111) surface, the bottom layer of Au atoms in the three-layer or five-layer slab model was fixed to simulate a semi-infinite metal substrate. The STM images of GB and perfect ReSe2 on Au superstructures were simulated using the Tersoff-Hamann approximation56 with a constant height of 2 Å above the uppermost Se atoms.

each unit cell is visible in the two neighboring ReSe2 domains. The domain orientations are then identified with the developed method (Figure S13). Moreover, a perfect coherence of the ReSe2 lattice is also noticeable at the boundary site in the atomic-scale STM image (Figure 5b), further confirming the perfect patching of the two ReSe2 domains with a mirror symmetry. Spatial-resolved STS spectra were also captured across and along the GB, respectively (Figure 5c,d). Clearly, the band gap retains a fixed value of ∼1.65 eV, with the VBM and CBM located at ∼−1.27 and 0.38 V, respectively. These band gap features are consistent with those of bilayer ReSe2/Au, indicating that the mirror-symmetric GB in bilayer ReSe2 also has the same VBM and CBM positions as well as gap value with regard to the neighboring domain interior. In this regard, the invariable band gap of ReSe2 GB should be independent of the number of ReSe2 layers. However, the STS spectra taken at bilayer GB (curves e and f in Figure 5c) present slightly larger DOS than that of domain internal at the energy outside the gap. The subsequent dI/dV mapping at different energy regions clearly visualize the DOS difference between GB and domain interior (Figure 5e,f). At the energy right inside the gap (0.3 V), the mapping image presents a very uniform contrast in the whole region, while the mapping image captured outside the gap (1.0 V) exhibits a brighter contrast at the boundary site, corresponding to a larger DOS contribution at the GB.

CONCLUSIONS In summary, we have realized the first on-site identification of the outermost four non-identical Se atoms in a unit cell of the anisotropic monolayer ReSe2 on a solid conductive surface, with the aid of in situ STM and STS techniques. We have established a facile method for determining the lattice orientation of the anisotropic ReSe2 lattice based on the STM imaging at a subunit-cell scale. We have also depicted the electronic properties of monolayer and bilayer ReSe2, and the adlayer−substrate interaction from the viewpoint of band gap variation. Of particular significance, we observe that the commonly observed mirror-symmetric GBs in CVD-grown ReSe2 possess invariable electronic property with regard to the domain interior. This work should provide a key reference to the essential properties of 2D anisotropic ReSe2 and the abnormal structural and electronic properties at the GBs, which should propel their practical applications in electronic and photonic devices as well as the energy-related fields.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b04872. Additional details and figures regarding DFT calculations, STM results, and STS spectra of ReSe2 on Au and graphite substrates (PDF)

METHODS Synthesis of ReSe2 on Au Foil. The synthesis of singlecrystalline monolayer ReSe2 on Au foil was conducted in a multitemperature-zone APCVD tube furnace equipped with a quartz tube (1 in. in diameter). Before growth, the Au foil (ZhongNuo Advanced Material Technology Co., Ltd., 30 μm thick, 99.99% purity) was annealed at 970 °C for 8 h to reduce its surface roughness. A total of 5 mg of ReO3 powder (Alfa Aesar, 99.9% purity) and 1 g of Se powder (Alfa Aesar, 99+% purity) were utilized as Re and Se precursors for the CVD growth. The Au foil with an area of 1 cm × 1 cm was placed in a quartz boat at the center of the furnace. To expel air, the tube furnace was first flushed with ultrahigh-purity Ar gas. The furnace was then heated to 750 °C over 30 min and maintained at 750 °C for 15, 30, or 60 min for growth under a constant flow rate of Ar gas of 50 sccm and H2 gas of 10 sccm at atmospheric pressure. After the synthetic procedure, the furnace was naturally cooled to room temperature.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhepeng Zhang: 0000-0002-9870-0720 Qing Zhang: 0000-0002-6869-0381 Zhongfan Liu: 0000-0003-0065-7988 Jijun Zhao: 0000-0002-3263-7159 Yanfeng Zhang: 0000-0003-1319-3270 Author Contributions ∥

G

M.H., X.Z., and N.G. contributed equally to this work. DOI: 10.1021/acsnano.8b04872 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano Notes

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 51861135201, 51290272, 51472008, and 61774003), the National Key Research and Development Program of China (grant nos. 2016YFA0200103, 2017YFA0304600, and 2017YFA0205700), and the Open Research Fund Program of the State Key Laboratory of LowDimensional Quantum Physics (grant no. KF201601). We acknowledge the use of computing resources from the Supercomputing Center of Dalian University of Technology. REFERENCES (1) Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L.-J.; Loh, K. P.; Zhang, H. The Chemistry of Two-Dimensional Layered Transition Metal Dichalcogenide Nanosheets. Nat. Chem. 2013, 5, 263−275. (2) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 147−150. (3) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699− 712. (4) Jia, Z.-Y.; Song, Y.-H.; Li, X.-B.; Ran, K.; Lu, P.; Zheng, H.-J.; Zhu, X.-Y.; Shi, Z.-Q.; Sun, J.; Wen, J.; Xing, D.; Li, S.-C. Direct Visualization of a Two-Dimensional Topological Insulator in the Single-Layer 1 T′− WTe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2017, 96, 041108. (5) Chenet, D. A.; Aslan, O. B.; Huang, P. Y.; Fan, C.; van der Zande, A. M.; Heinz, T. F.; Hone, J. C. In-Plane Anisotropy in Monoand Few-Layer ReS2 Probed by Raman Spectroscopy and Scanning Transmission Electron Microscopy. Nano Lett. 2015, 15, 5667−5672. (6) Zhong, H.-X.; Gao, S.; Shi, J.-J.; Yang, L. Quasiparticle Band Gaps, Excitonic Effects, and Anisotropic Optical Properties of the Monolayer Distorted 1 T Diamond-Chain Structures ReS2 and ReSe2. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 115438. (7) Wolverson, D.; Crampin, S.; Kazemi, A. S.; Ilie, A.; Bending, S. J. Raman Spectra of Monolayer, Few-Layer, and Bulk ReSe2: An Anisotropic Layered Semiconductor. ACS Nano 2014, 8, 11154− 11164. (8) Arora, A.; Noky, J.; Drüppel, M.; Jariwala, B.; Deilmann, T.; Schneider, R.; Schmidt, R.; Del Pozo-Zamudio, O.; Stiehm, T.; Bhattacharya, A.; Kruger, P.; de Vasconcellos, S. M.; Rohlfing, M.; Bratschitsch, R. Highly Anisotropic In-Plane Excitons in Atomically Thin and Bulklike 1T′-ReSe2. Nano Lett. 2017, 17, 3202−3207. (9) Lin, Y.-C.; Komsa, H.-P.; Yeh, C.-H.; Bjorkman, T.; Liang, Z.-Y.; Ho, C.-H.; Huang, Y.-S.; Chiu, P.-W.; Krasheninnikov, A. V.; Suenaga, K. Single-Layer ReS2: Two-Dimensional Semiconductor with Tunable In-Plane Anisotropy. ACS Nano 2015, 9, 11249−11257. (10) Liu, E.; Fu, Y.; Wang, Y.; Feng, Y.; Liu, H.; Wan, X.; Zhou, W.; Wang, B.; Shao, L.; Ho, C.-H.; Huang, Y.-S.; Cao, Z.; Wang, L.; Li, A.; Zeng, J.; Song, F.; Wang, X.; Shi, Y.; Yuan, H.; Hwang, H. Y.; Cui, Y.; Miao, F.; Xing, D. Integrated Digital Inverters Based on TwoDimensional Anisotropic ReS2 Field-Effect Transistors. Nat. Commun. 2015, 6, 6991. (11) Corbet, C. M.; McClellan, C.; Rai, A.; Sonde, S. S.; Tutuc, E.; Banerjee, S. K. Field Effect Transistors with Current Saturation and Voltage Gain in Ultrathin ReS2. ACS Nano 2015, 9, 363−370. (12) Gao, N.; Zhou, S.; Liu, N.; Bai, Y.; Zhao, J. Selecting Electrode Materials for Monolayer ReS2 with an Ohmic Contact. J. Mater. Chem. C 2018, 6, 6764−6770. (13) Zhang, E.; Wang, P.; Li, Z.; Wang, H.; Song, C.; Huang, C.; Chen, Z.-G.; Yang, L.; Zhang, K.; Lu, S.; Wang, W.; Liu, S.; Fang, H.; Zhou, X.; Yan, H.; Zou, J.; Wan, X.; Zhou, P.; Hu, W.; Xiu, F. Tunable Ambipolar Polarization-Sensitive Photodetectors Based on H

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DOI: 10.1021/acsnano.8b04872 ACS Nano XXXX, XXX, XXX−XXX