Microscopic Mechanism of Van der Waals Heteroepitaxy in the Formation of MoS2/hBN Vertical Heterostructures
- Mitsuhiro OkadaMitsuhiro OkadaNanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, JapanMore by Mitsuhiro Okada,
- Mina MaruyamaMina MaruyamaGraduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, JapanMore by Mina Maruyama,
- Susumu OkadaSusumu OkadaGraduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, JapanMore by Susumu Okada,
- Jamie H. WarnerJamie H. WarnerWalker Department of Mechanical Engineering, The University of Texas at Austin, 204 East Dean Keeton Street, Austin, Texas 78712, United StatesMaterials Graduate Program, Texas Materials Institute, The University of Texas at Austin, 204 East Dean Keeton Street, Austin, Texas 78712, United StatesMore by Jamie H. Warner,
- Yusuke KureishiYusuke KureishiDepartment of Chemistry, Nagoya University, Nagoya, Aichi 464-8602, JapanMore by Yusuke Kureishi,
- Yosuke UchiyamaYosuke UchiyamaDepartment of Chemistry, Nagoya University, Nagoya, Aichi 464-8602, JapanMore by Yosuke Uchiyama,
- Takashi TaniguchiTakashi TaniguchiInternational Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, JapanMore by Takashi Taniguchi,
- Kenji WatanabeKenji WatanabeResearch Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, JapanMore by Kenji Watanabe,
- Tetsuo ShimizuTetsuo ShimizuNanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, JapanMore by Tetsuo Shimizu,
- Toshitaka KuboToshitaka KuboNanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, JapanMore by Toshitaka Kubo,
- Masatou IshiharaMasatou IshiharaNanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, JapanMore by Masatou Ishihara,
- Hisanori ShinoharaHisanori ShinoharaDepartment of Chemistry, Nagoya University, Nagoya, Aichi 464-8602, JapanMore by Hisanori Shinohara, and
- Ryo Kitaura*Ryo Kitaura*Email: [email protected]Department of Chemistry, Nagoya University, Nagoya, Aichi 464-8602, JapanMore by Ryo Kitaura
Copyright © 2020 American Chemical Society
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Abstract
Recent studies have revealed that van der Waals (vdW) heteroepitaxial growth of 2D materials on crystalline substrates, such as hexagonal boron nitride (hBN), leads to the formation of self-aligned grains, which results in defect-free stitching between the grains. However, how the weak vdW interaction causes a strong limitation on the crystal orientation of grains is still not understood yet. In this work, we have focused on investigating the microscopic mechanism of the self-alignment of MoS2 grains in vdW epitaxial growth on hBN. Using the density functional theory and the Lennard–Jones potential, we found that the interlayer energy between MoS2 and hBN strongly depends on the size and crystal orientation of MoS2. We also found that, when the size of MoS2 is several tens of nanometers, the rotational energy barrier can exceed ∼1 eV, which should suppress rotation to align the crystal orientation of MoS2 even at the growth temperature.
Introduction
ARTICLE SECTIONS
Two-dimensional (2D) materials have been attracting much attention due to their fascinating properties and possible applications in nanoelectronics and photonics.(1,2) 2D group-VI transition metal dichalcogenides (TMDs: MoS2, WS2, MoSe2, WSe2, etc.), in particular, are outstanding because they can have a sizable bandgap (1.5–2.0 eV) that is absent in graphene.(3,4) In addition, TMDs with the 2H phase, at the monolayer limit, possess a direct gap,(4,5) where the spin and valley degrees of freedom are coupled due to inversion symmetry breaking.(6) These properties make TMDs excellent platforms for exploring the fundamental physics at the 2D limit and for next-generation optoelectronic devices.(7)
One of the unique growth modes of 2D materials is van der Waals (vdW) heteroepitaxial growth, where the vdW force significantly limits the crystallographic orientation of 2D materials grown on crystalline substrates.(8,9) In the past few years, chemical vapor deposition (CVD) growth of monolayer TMDs on various crystalline substrates, including Au (111), Al2O3, graphene, graphite, and hexagonal boron nitride (hBN), has been reported.(10−20) Although there is a significant lattice mismatch between TMDs and the substrates (e.g., ∼26% in the case of MoS2 and hBN), the substrate–TMD interaction, vdW force, precisely aligns the TMD domains, limiting the stacking angles to 0 and 60°, that is, vdW heteroepitaxial growth.(11−13) The vdW heteroepitaxial growth is a universal phenomenon, which has widely been observed not only in the growth of TMDs but also in the growth of other layered materials, such as hBN and graphene.(21,22)
“How the vdW force can strongly restrict the crystal orientation in vdW heteroepitaxial growth?” is one of the mysteries in the growth of 2D materials. In the heteroepitaxial growth of three-dimensional (3D) systems such as III–V semiconductor compounds, chemical bonds between a grown material and a corresponding substrate determine the crystallographic orientation between them.(8) In this case, it is natural that strong chemical bonds perfectly define the crystal orientation to form commensurate heterostructures.(9) On the other hand, the weak nonbonding vdW force determines the stacking angle in the formation of incommensurate stackings in the vdW heteroepitaxial growth.(8,9) Because there is a significant lattice mismatch in most vdW heteroepitaxial growth, atoms in the growing layer cannot always locate at stable positions; the configuration of atoms between a substrate and a growing layer gradually and continuously changes due to the difference in lattice constants.(18) In this case, the stabilization energy would not simply increase depending on the size of a growing layer, and the optimum crystal orientation is not trivial. Moreover, growing layers are expected to move and rotate easily because there are no direct bonds between layers and substrates.(23)
In this work, we focus on the microscopic mechanism of the substrate-induced alignment in the CVD growth of monolayer MoS2 on hBN substrates. Detailed analyses on the relative orientation of MoS2 crystals and hBN substrates by electron diffraction have confirmed that relative orientations between CVD-grown MoS2 and the underlying hBN substrate are limited to 0 and 60°. To elucidate the mechanism of self-alignment, we have calculated cluster-size-dependent interlayer-energy landscapes. In addition to the density functional theory (DFT), we have also employed Lennard–Jones (LJ) pair potential to calculate the interlayer-energy landscape of the MoS2/hBN system because the LJ potential has allowed us to treat a system containing over 104 atoms, which is far beyond the scope of the DFT calculation. In both cases, we found that the interaction energy shows significant stacking angle and cluster size dependence. Although the stacking angle of 0/60° is not always the most stable configuration, we found, up to the size of Mo972S1944 (an edge length of 5.7 nm), that the most stable configurations with the stacking angle of 0/60° appear periodically, whose period is close to the moiré period between MoS2 and hBN. We also found that the rotational energy barrier (the difference between the interaction energy at 0° and the angle corresponding to the nearest local maximum to 0°) is comparable to thermal energy at growth temperatures when the cluster size reaches several tens of nanometers. These indicate that small clusters, at the early stage of growth, rotate to change the stacking angle, and then, the rotational degree of freedom is gradually lost as the cluster size increases. This finding provides a basis to get deep insights into the growth mechanism of 2D films onto these crystalline substrates.
Results and Discussion
ARTICLE SECTIONS
To grow MoS2 onto hBN, we used the multifurnace CVD method with elemental sulfur and molybdenum trioxide as precursors. The hBN substrate is obtained using the mechanical exfoliation method with a bulk hBN crystal grown using the high-pressure, high-temperature method.(34) Details of the growth procedure are shown in the Methods section and a previous report.(24)Figure 1a shows a typical optical image of MoS2 grown on hBN (MoS2/hBN), where the hexagonal-shaped contrasts correspond to single-layer MoS2.(24,35) The grown MoS2 grains with a typical size of ∼5 μm are placed randomly on the hBN substrate; a schematic image of MoS2/hBN is shown in Figure 1b. To confirm the layer number of MoS2, we measured the Raman spectra at room temperature with an excitation energy of 2.33 eV. Figure 1c shows a Raman spectrum, where two characteristics peaks (383.7 and 404.7 cm–1) originating from in-plane (E′ mode) and out-of-plane vibrational modes (A′1 mode) exist.(36) The frequency difference between these two peaks is 21.1 cm–1, which is slightly larger than the reported value of monolayer MoS2.(37)Figure 1d shows a typical photoluminescence (PL) spectrum of MoS2/hBN at room temperature. As seen in Figure 1d, a single-peak PL emission centered at 1.884 eV is observed, consistent with previous reports.(38,39) The full width at half maximum of the obtained PL peak is about 43 meV, which is significantly smaller than that of monolayer MoS2 grown onto a SiO2/Si substrate (55 meV),(39) indicating that inhomogeneous broadening arising from substrates is suppressed in MoS2/hBN.(24) Atomic force microscopy (AFM) observations shown in Figure S1 are also consistent with monolayer MoS2.(12)
Figure 1
Figure 1. (a) Optical image of CVD-grown MoS2 on hBN; (b) schematic of MoS2/hBN; (c) typical Raman spectrum, (d) PL spectrum of the MoS2 crystal shown in the upper left side of Figure 1a; and (e) a typical SAED pattern of MoS2/hBN. Green and blue arrows indicate diffraction spots from MoS2 and hBN in MoS2/hBN, respectively.
As shown in Figure 1a, we can see that all MoS2 domains have only two orientations, where the 60° rotation of one orientation matches the other orientation. The observed limitation in the orientation of MoS2 results from a strict relationship in the crystallographic orientation between MoS2 and hBN. In fact, a typical selective-area electron diffraction (SAED) pattern shown in Figure 1e clearly shows two sets of six-fold-symmetric spots with almost the same orientation; the larger (smaller) six-fold-symmetric pattern originates from MoS2 (hBN).(10,11) This means that the relative crystallographic angle between MoS2 and hBN is limited to two orientations, 0 and 60°, and statistics on the frequency of the two stacking angles are summarized in Table 1 (a corresponding optical image is shown in Figure S2). As clearly seen, the ratio between the two stacking angles is nearly 1:1, which is consistent with previous studies.(11) The slight deviation from 1:1 probably originates from statistical error or defect-controlled nucleation of MoS2 on hBN.(40)
Table 1. Distribution of MoS2 Grain Orientation
| structure | aligned—0° | aligned—60° | others |
|---|---|---|---|
| number of grains | 60 | 46 | 0 |
To investigate the mechanism of the vdW heteroepitaxial growth of MoS2/hBN, we calculated the cluster-size-dependent interaction-energy landscapes by DFT. We calculated the interaction-energy landscapes for four MoS2/hBN with different cluster sizes; the corresponding models are shown in Figure 2a. The detailed calculation method is provided in the Methods section. Figure 2b shows the total energy of MoS2 on hBN as a function of the stacking angle. As clearly seen, the energy is sensitive to the cluster size and the stacking angle. For the smallest MoS cluster, the cluster prefers a stacking angle of approximately 15°, reflecting the local atomic arrangement between MoS and hBN. The increase of the cluster size causes additional energy minima, owing to increased preferential interlayer atomic arrangements arising from the lattice mismatch. Indeed, for the largest cluster, we found two global minima at the angles of 10 and 60° in addition to the two local minima, still reflecting the local atomic arrangement between them. Thus, with the further increase in the flake size, the MoS cluster exhibits some particular orientations by the averaged vdW interaction. Note that the stacking structures with a stacking angle of 0 or 60° do not correspond to the most stable structures since we have not optimized the relative position of MoS and hBN, such as interlayer distances and locations of the center of gravity.
Figure 2
To have a more in-depth insight into the mechanism of the vdW-mediated epitaxial growth of MoS2 onto hBN, we extended the calculation of interaction-energy landscapes to larger clusters with the LJ pair potential. A simple pair potential, such as the LJ potential, allows us to calculate the interaction energy between a cluster of MoS2 and hBN, which are composed of a large number of atoms. In this work, we calculated large MoS2/hBN-stacked structures up to Mo972S1944/B5400N5400, which is beyond the scope of DFT calculations. In the LJ pair potential, the interaction energy between a TMD cluster and an hBN substrate, Einter, is described as a summation of the LJ potential.
(1)NTMD and NhBN are the number of the atoms in TMDs and hBN, respectively. εij and σij are the LJ parameters corresponding to interactions between the ith and jth atom in the system, and rij is the distance between the ith and jth atom in the system. We calculated the stacking angle-dependent energy landscapes with various sizes of MoS2. Note that MoS2 and hBN are treated as rigid bodies, and intralayer energy, such as bending chemical bonds, is ignored. The LJ parameters in eq 1 were estimated with the Lorentz–Berthelot combining rules, where σ and ε between different kinds of atoms, σAB and εAB, were calculated with those between the same kind of atoms, σAA, σBB, εAA, and εBB, as follows.
(2a)
(2b)
(1)NTMD and NhBN are the number of the atoms in TMDs and hBN, respectively. εij and σij are the LJ parameters corresponding to interactions between the ith and jth atom in the system, and rij is the distance between the ith and jth atom in the system. We calculated the stacking angle-dependent energy landscapes with various sizes of MoS2. Note that MoS2 and hBN are treated as rigid bodies, and intralayer energy, such as bending chemical bonds, is ignored. The LJ parameters in eq 1 were estimated with the Lorentz–Berthelot combining rules, where σ and ε between different kinds of atoms, σAB and εAB, were calculated with those between the same kind of atoms, σAA, σBB, εAA, and εBB, as follows.(2a)(2b)The LJ parameters of Mo–Mo, S–S, B–B, and N–N (σMoMo, σSS, σBB, σNN, εMoMo, εSS, εBB, and εNN), which are needed to calculate σMoB, σMoN, σSB, σSN, εMoB, εMoN, εSB, and εSN, are taken from references;(41,42)Table 2 shows the obtained LJ parameters.
| interaction | ε (meV) | σ (Å) |
|---|---|---|
| Mo–B | 58.73 | 3.002 |
| Mo–N | 72.56 | 2.958 |
| S–B | 15.79 | 3.411 |
| S–N | 19.51 | 3.367 |
To confirm the validity of the LJ parameters obtained, we have checked the consistency between the interaction-energy landscapes calculated with the LJ pair potential and the ab initio DFT calculation. Figure S3 shows the stacking angle dependencies of interaction energy derived with the LJ pair potential; a large hexagonal bilayer hBN (B2700N2700 × 2, an edge length of 7.5 nm) and MoS clusters used in DFT calculations are employed in this calculation. As clearly seen, calculations based on the LJ potential qualitatively reproduce the DFT results (Figure 2b). Having reproduced the DFT results successfully, we conducted LJ potential-based calculations to obtain the interaction energy of MoS2/hBN up to Mo972S1944/B5400N5400.
Figure S4 shows the scheme to calculate the interaction-energy landscapes of MoS2/hBN. First, we put a hexagonal Mo3n2S6n2 (n = 1, 2, ..., 18; the relationships between n and the edge length of each cluster are shown in Table S1) cluster on a hexagonal bilayer hBN (B2700N2700 × 2) with a certain stacking angle (Figure S4 (i)), and then, we search for the ground minimum configuration at the stacking angle using the simulated annealing (SA) method (Figure S4 (ii, iii)). The parameters in the SA calculations are lateral positions and interlayer distances. The SA-based ground-minimum search has been repeated with various stacking angles to calculate an interaction-energy landscape for a certain cluster size of MoS2 (Figure S4 (iv)). This process has been repeated with various sizes of clusters and the obtained interaction-energy landscapes for 18 different clusters of MoS2 are shown in Figure 3a; the most stable configurations found for a stacking angle of 0° are shown in Figure S5 and Table S1.
Figure 3
Figure 3. (a) Cluster-size and stacking-angle evolutions of interaction energy; (b) cluster-size dependencies of absolute values of interaction energy (i.e., absolute values of the difference between the maximum energy and the minimum energy) calculated with a stacking angle of 0 and 60° and the most stable stacking angle of each cluster; (c) map showing element-decomposed interaction energies of a MoS2 cluster (an edge length of 1.6 nm) with a stacking angle of 0°. Yellow circles correspond to S2 pairs; and (d) corresponding structure used to calculate the element-decomposed interaction energy in (c). Purple lines indicate the moiré superlattice period. Element coloring is the same as that of Figure 1b.
As shown in Figure 3a, the interaction-energy landscapes show that several local minimums locate at different stacking angles, and the different cluster sizes of MoS2 give different positions of local minimums. Notably, structures with stacking angles of 0 or 60° do not always correspond to the ground minimum in the corresponding interaction-energy landscapes. The cluster-size dependency of the interaction energy at stacking angles of 0 and 60° oscillates periodically (Figure 3b); clusters with an edge length of 0.3, 1.6, 2.8, 3.8, and 5.1 nm have considerable interaction energy compared with those of clusters with a similar size. It should be noted that the periodicity of oscillations in interaction energy seen in Figure 3b is roughly close to that of the moiré superlattice in MoS2/hBN with stacking angles of 0 and 60°; the periodicity of the moiré superlattice is 1.196 nm; the moiré lattice constant λ and the relative rotation angle θ of the moiré pattern with respect to the hBN lattice can be calculated with the following equations(43)
(3)
(4)where δ, a, and ϕ represent the lattice mismatch between MoS2 and hBN, the hBN lattice constant, and the relative rotation angle, respectively. This can be understood by visualizing the contribution of each atom in MoS2 clusters to Einter. Figure 3c shows the contribution of each S2 pairs to Einter in the cluster with an edge length of 1.6 nm; the contribution is evaluated with the sum of the LJ potential energy between a S2 pair in the cluster with the total atoms in hBN. As shown in Figure 3c,d, S2 pairs that contribute to enlarge Einter appear with the moiré superlattice periodicity because the stable configuration between S2 pair and hBN, which significantly contributes to Einter, appears with the moiré superlattice period.
(3)(4)where δ, a, and ϕ represent the lattice mismatch between MoS2 and hBN, the hBN lattice constant, and the relative rotation angle, respectively. This can be understood by visualizing the contribution of each atom in MoS2 clusters to Einter. Figure 3c shows the contribution of each S2 pairs to Einter in the cluster with an edge length of 1.6 nm; the contribution is evaluated with the sum of the LJ potential energy between a S2 pair in the cluster with the total atoms in hBN. As shown in Figure 3c,d, S2 pairs that contribute to enlarge Einter appear with the moiré superlattice periodicity because the stable configuration between S2 pair and hBN, which significantly contributes to Einter, appears with the moiré superlattice period.In the calculation of the interlayer-energy landscapes, we searched for the most stable configuration at each stacking angle. If there is a substantial energy barrier between the most stable configuration and configurations corresponding to a local minimum, MoS2 clusters cannot find the most stable configuration. To see this, the interaction-energy landscape on the position of a cluster (an edge length of 5.1 nm) was calculated (Figure S7). As clearly seen in the figure, the potential barrier does not exist for the translational motion of the cluster, and this means that the MoS2 cluster can move to find the most stable configuration. This conclusion can be extended to larger MoS2 clusters, which contain a larger number of moiré units because interaction energy can be roughly proportional to the number of moiré units, and cluster size is not expected to alter the shape of the potential profile (related discussion is given in the next paragraph). The significantly small energy barrier for translational motion in large systems, called superlubricity, has been observed in incommensurate systems, such as twisted graphite flakes,(44,45) and large MoS2 clusters are also expected to move on an hBN substrate. These results strongly suggest that, at the early stage of the growth, small MoS2 clusters would rotate and move to find the most stable configuration. Further discussions about the energy landscape, which include results on typical triangular-shaped clusters, are provided in the Supporting Information (Figure S8 and discussion therein).
As discussed above, MoS2 clusters on hBN move and rotate to find the most stable configuration. The next question is that “when does a MoS2 cluster stop rotating and align?” To address this question, we focused on cluster-size-dependent evolution of the rotational energy barrier at fixed stacking angles; rotational energies were calculated as the difference between the interaction energy at 0o and the angle corresponding to the nearest local maximum to 0°. As shown in Figure 3b, the stable and unstable atomic configurations appear periodically with the moiré period, and hence, rotational energy barriers can roughly be divided into the moiré contribution (Emoiré × number of moiré unit cell) and the edge contribution. While the edge contribution cannot be neglected compared to the moiré contribution when the cluster size is small, the moiré contribution becomes more dominant when the cluster size becomes large; the moiré/edge contribution should depend quadratically/linearly on the cluster size. Therefore, the rotational energy barriers of large MoS2 clusters and hBN can be estimated by the moiré contribution alone. Figure 4a shows the stacking angle dependence on the total energy of a MoS2 cluster with an edge length of 2.8 nm. As shown in the figure, there is a global minimum and a local maximum at 0 and 4.5°, respectively. Corresponding mappings of the interaction energy are shown in Figure 4b,c, and we extracted 10 moiré unit cells for each cluster; moiré unit cells are shown as dotted lines in Figure 4b,c. In this case, we can express the interaction energy as 10Emoiré + (the edge contribution), and Emoiré has successfully been extracted as ∼0.1 meV/(MoS2 unit cell). Based on the obtained Emoiré of ∼0.1 meV, we have estimated the rotational energy barrier of a large cluster containing ∼1 × 104 MoS2 unit cells (a cluster size of ∼40 nm or an edge length of ∼20 nm) as ∼1 eV. Compared with the thermal energy of a typical CVD growth temperature (∼100 meV), the rotational energy barrier is large enough, thereby suppressing the rotation of clusters.
Figure 4
Figure 4. (a) Stacking angle-dependent interaction energy of a Mo243S486 cluster (the interaction energy at the most stable stacking angle is set to zero) and (b, c) maps showing element-decomposed interaction energies of S2 pairs in a MoS2 cluster with an edge length of 2.8 nm with stacking angles of 0° and 4.5°. Yellow circles correspond to positions of S2 pairs and magenta dotted lines correspond to the moiré superlattice period of each structure.
A possible discrepancy between the calculations based on LJ potential and the experimental results is local/global energy minimums around 10–20 and 40–50° in the potential landscapes (as shown in Figure 3b). However, these stacking angles have not been experimentally observed as shown in Figure 1a and Table 1. This discrepancy is probably explained by the variability of the stable stacking angle and the translational position. As shown in Figure 5a, the stacking angles at the local/global minimums around 10–20 and 40–50o significantly change depending on the cluster size. Furthermore, the cluster size evolution also alters the stable translational position of MoS2 on hBN. To see this, we have calculated the stacking angle-dependent interaction energy evolution of the MoS2 cluster with fixed center positions; the center of the MoS2 clusters locates on B or N atoms (Figure 5b). As clearly demonstrated, both the stable stacking angle and translational position vary depending on the cluster size, while the stacking angle of 0/60° always corresponds to the local or global minimum (Figure 3a). Even when a cluster is trapped at the stacking angles of 10–20 and 40–50°, the energy minimum disappears when the cluster size slightly increases. For example, a cluster with an edge length of 2.5 nm can be trapped at a stacking angle of 16°, but the stacking angle does not correspond to one of the energy minimums anymore when the cluster size increased to have an edge length of 2.8 nm (Figure 5b). The cluster, therefore, is forced to move and rotate to find a stable configuration. Because the cluster keeps moving and rotating during a growth process unless the stacking angle locates at 10–20° and 40–50°, the cluster probably escapes the energy minima at 10–20° and 40–50°, leading to falling into the experimentally observed stacking angle of 0/60°. Note that the lateral positions of stable structures with a stacking angle of 0/60° also change as the cluster size increases (Table S1). Clusters are, however, expected to find the stable lateral position easily because of the absence of a translational energy barrier in this case (Figure S7). Furthermore, the stacking angle of 0/60° always corresponds to, at least, one of the local minimums, and there is no significant local energy minimum nearby the stacking angle of 0/60° (Figure 3a). In conjunction with the sizeable rotational energy barrier (∼1 eV in a cluster size of ∼40 nm), these characteristics of cluster-size- and stacking-angle-dependent energy landscape probably make the experimentally observed stacking angle of 0/60° stable.
Figure 5
Figure 5. (a) Cluster-size-dependent stable stacking angle evolutions around 40–50° (upper panel) and 10–20° (lower panel); (b) stacking angle and structure-dependent interaction-energy evolution of clusters with an edge length of 2.5, 2.8, and 3.2 nm. The left and right panels show the results calculated with stacking angles of 12–21° and 39–48°, respectively. Curves labeled as “SA result” are the same as the curves shown in Figure 3a. Black dotted lines correspond to an energy minimum position of a cluster with an edge length of 2.5 nm.
Conclusions
ARTICLE SECTIONS
In summary, we investigate the mechanism of the vdW heteroepitaxy of MoS2/hBN through the calculation of interaction-energy landscapes. We found that the interaction-energy landscapes strongly depend on the size and stacking angle of clusters. We also found that the stacking angle of 0/60° always corresponds to a local or the ground minimum configuration. The energy barrier for the 0/60° configuration evolves as the cluster size grows, reaching ∼1 eV when the cluster size is several tens of nanometers. These findings suggest that (1) at the very early stage of the growth, a small MoS2 cluster can rotate to find stable configurations and (2) the rotational degree of freedom is gradually suppressed as the cluster grows and finally stops when the cluster size reaches several tens of nanometers. A similar phenomenon can be observed in other vdW heteroepitaxy when there is a lattice mismatch between a growing layer and a substrate. In lattice-mismatched systems, the interaction-energy landscape should show strong cluster-size dependence with a periodicity of the corresponding moiré lattice constant. Also, the interaction-energy landscape is expected to show significant stacking-angle dependence. These cluster-size and stacking-angle dependent landscapes should lead to the alignment of growing layers as already seen in the growth of TMDs onto graphite or the sapphire (0001) surface. Our results can be a basis of controlling and understanding the vdW heteroepitaxy of 2D films onto crystalline substrates.
Methods
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CVD Growth of MoS2 on hBN
We have grown monolayer MoS2 onto exfoliated hBN flakes using the CVD growth method.(24) hBN flakes were prepared using the mechanical exfoliation method on a quartz substrate. As precursors, we used molybdenum trioxide (MoO3, Sigma-Aldrich, 99.5%) and elemental sulfur (Sigma-Aldrich, 99.98%). Sulfur powder and a quartz substrate with hBN flakes were loaded into a quartz tube with an inner diameter of 26 mm. MoO3 was placed in a quartz tube with an inner diameter of 10 mm, and the quartz tube was placed inside the larger diameter quartz tube to avoid unwanted reactions between S and MoO3, and then, under an Ar flow of 200 sccm, we heated the quartz tubes with a three-zone electric furnace at 200, 750, and 1100 °C for S, MoO3, and the substrate, respectively. The typical growth time is 20 min.
MoS2 Characterization
Optical images were taken with a typical optical microscope (Leica DM 2500 M and Nikon Eclipse ME600). Raman and PL measurements were performed using a confocal Raman microscope (Renishaw inVia) with an excitation energy of 2.33 eV. AFM measurement was performed using a standard AFM (Park Systems NX10) operating in a tapping mode. We used a transmission electron microscope (JEOL 2100) operating at 200 keV to obtain a SAED pattern. MoS2/hBN was transferred onto a copper grid using the standard polymer-based transfer method.
DFT Calculation
Theoretical calculations were performed using DFT(25,26) as implemented in the program package Simulation Tool for Atom TEchnology (STATE).(27) We used the generalized gradient approximation with the Perdew–Burke–Ernzerhof functional(28,29) to describe the exchange–correlation potential energy among interacting electrons. The weak dispersive interaction between TMD flakes and hBN was treated using the vdW-DF2 with the C09 exchange–correlation functional.(30,31)
Ultrasoft pseudopotentials generated using the Vanderbilt scheme were adopted as the interaction between electrons and nuclei.(32) Valence wavefunctions and the deficit charge density were expanded in terms of plane-wave basis sets with cutoff energies of 25 and 225 Ry, respectively. The atomic structures of TMDs were fully optimized until the force acting on each atom was less than 1.33 × 10–3 HR/au. We consider the triangular MoS flakes, Mo6S14, Mo10S24, Mo15S36, and Mo21S50, with S edges as the structural model of MoS flakes, which are adsorbed on monolayer hBN with a lateral supercell with the sizes of 5 × 5, 7 ×7, 8 × 8, and 9 × 9, respectively, possessing the lattice parameters of a0 = 1.256, 1.758, 2.009, and 2.226 nm (Figure 2a). To exclude the unphysical dipole interaction due to the electrostatic potential difference between MoS and hBN, we imposed an open boundary condition normal to the hBN sheet using the effective screening medium method,(33) with sufficiently large normal lattice parameters in which TMD/hBN is separated from adjacents by a 1.3 nm vacuum spacing. Integration over the Brillouin zone was carried out using an equidistance mesh of 2 × 2 × 1 k points. The calculation was performed as follows: (i) calculating stable MoS cluster structures; (ii) putting a MoS cluster on hBN at a certain position with a stacking angle of 0°; (iii) optimizing the interlayer distance between MoS and hBN; and (iv) calculating the stacking-angle dependence of the total energy by fixing the center of gravity of the MoS cluster and the interlayer distance.
ARTICLE SECTIONS
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04168.
AFM image and the corresponding height profile of MoS2/hBN (Figure S1), an optical image of MoS2/hBN (Figure S2), stacking-angle dependent total energy of the MoS2 cluster using LJ potential (Figure S3), a schematic of the method for calculating interaction energy (Figure S4), schematic images of the stable MoS2/hBN structure at a stacking angle of 0° (Figure S5), element-decomposed interlayer energy mapping of Mo48S96 (Figure S6), an in-plane interaction-energy map of a Mo768S1536 cluster (an edge length of 5.1 nm) with a stacking angle of 0° (Figure S7), list of n and the corresponding number of Mo atoms, edge length, and the most stable center position of MoS2 clusters at a stacking angle of 0° (Table S1), additional discussion on the effect of the MoS cluster shape in the vdW epitaxy, and LJ result on triangular-shaped MoS clusters (Figure S8) (PDF)
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Author Information
ARTICLE SECTIONS
- Ryo Kitaura - Department of Chemistry, Nagoya University, Nagoya, Aichi 464-8602, Japan;
http://orcid.org/0000-0001-8108-109X;
- Mina Maruyama - Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan;http://orcid.org/0000-0002-2872-5543
- Jamie H. Warner - Walker Department of Mechanical Engineering, The University of Texas at Austin, 204 East Dean Keeton Street, Austin, Texas 78712, United States; Materials Graduate Program, Texas Materials Institute, The University of Texas at Austin, 204 East Dean Keeton Street, Austin, Texas 78712, United States;http://orcid.org/0000-0002-1271-2019
- Takashi Taniguchi - International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan;http://orcid.org/0000-0002-1467-3105
- Kenji Watanabe - Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan;http://orcid.org/0000-0003-3701-8119
- Masatou Ishihara - Nanomaterials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan;http://orcid.org/0000-0002-5016-3356
The authors declare no competing financial interest.
Acknowledgments
ARTICLE SECTIONS
This work was supported by JSPS KAKENHI Grant numbers JP20H05664, JP19K15403, JP16H06331, JP16H03825, JP16H00963, JP15K13283, and JP25107002 and JST CREST Grant Number JPMJCR16F3. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHI Grant Numbers JP20H00354, and the CREST(JPMJCR15F3), JST.
References
ARTICLE SECTIONS
This article references 45 other publications.
- 1Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666– 669, DOI: 10.1126/science.1102896[Crossref], [PubMed], [CAS], Google Scholar1https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXos1Kqt70%253D&md5=488da13500bf24e8fc419052dc1a9e84Electric Field Effect in Atomically Thin Carbon FilmsNovoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A.Science (Washington, DC, United States) (2004), 306 (5696), 666-669CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)The authors describe monocryst. graphitic films, which are a few atoms thick but are nonetheless stable under ambient conditions, metallic, and of remarkably high quality. The films are a two-dimensional semimetal with a tiny overlap between valence and conductance bands, and they exhibit a strong ambipolar elec. field effect such that electrons and holes in concns. up to 1013 per square centimeter and with room-temp. mobilities of ∼10,000 square centimeters per V-second can be induced by applying gate voltage.
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- 4Kumar, A.; Ahluwalia, P. K. Electronic Structure of Transition Metal Dichalcogenides Monolayers 1H-MX2 (M= Mo, W; X= S, Se, Te) from ab-initio Theory: New Direct Band Gap Semiconductors. Eur. Phys. J. B 2012, 85, 186, DOI: 10.1140/epjb/e2012-30070-x[Crossref], [CAS], Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVynsr%252FO&md5=d70bba0d95359b078cd767aa48505bb8Electronic structure of transition metal dichalcogenides monolayers 1H-MX2 (M = Mo, W; X = S, Se, Te) from ab-initio theory: new direct band gap semiconductorsKumar, A.; Ahluwalia, P. K.European Physical Journal B: Condensed Matter and Complex Systems (2012), 85 (6), 186, 7 pp.CODEN: EPJBFY; ISSN:1434-6028. (Springer)We report 1st principles calcns. of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical AO basis sets based methods within the local d. approxn. Electronic band structure and d. of states calcns. found that the states around the Fermi energy are mainly due to metal d states. From partial d. of states we find a strong hybridization between metal d and chalcogen p states below the Fermi energy. All studied compds. have emerged as new direct band gap semiconductors. The electronic band gap is found to decrease as one goes from sulfides to the tellurides of both Mo and W. Reducing the slab thickness systematically from bulk to monolayers causes a blue shift in the band gap energies, resulting in tunability of the electronic band gap. The magnitudes of the blue shift in the band gap energies are found to be 1.14 eV, 1.16 eV, 0.78 eV, 0.64, 0.57 eV and 0.37 eV for MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, resp., as we go from bulk phase (indirect band gap) to monolayer limit (direct band gap). This tunability in the electronic band gap and transitions from indirect to direct band make these materials potential candidates for the fabrication of optoelectronic devices.
- 5Kuc, A.; Zibouche, N.; Heine, T. Influence of Quantum Confinement on the Electronic Structure of the Transition Metal Sulfide TS2. Phys. Rev. B 2011, 83, 245213, DOI: 10.1103/PhysRevB.83.245213[Crossref], [CAS], Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXosVGgtro%253D&md5=80d3805141f9bab4e9799a8275124380Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2Kuc, A.; Zibouche, N.; Heine, T.Physical Review B: Condensed Matter and Materials Physics (2011), 83 (24), 245213/1-245213/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)Bulk MoS2, a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. Reducing its slab thickness to a monolayer, MoS2 undergoes a transition to the direct band semiconductor. We support this exptl. observation by first-principle calcns. and show that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure. We further studied the properties of related TS2 nanolayers (T= W, Nb, Re) and show that the isotopol. WS2 exhibits similar electronic properties, while NbS2 and ReS2 remain metallic independent of the slab thickness.
- 6Xiao, D.; Liu, G.-B.; Feng, W.; Xu, X.; Yao, W. Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides. Phys. Rev. Lett. 2012, 108, 196802, DOI: 10.1103/PhysRevLett.108.196802[Crossref], [PubMed], [CAS], Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVWhsrjO&md5=dd81a9860c9bf7f16dc2e95256e98549Coupled spin and valley physics in monolayers of MoS2 and other Group-VI dichalcogenidesXiao, Di; Liu, Gui-Bin; Feng, Wanxiang; Xu, Xiaodong; Yao, WangPhysical Review Letters (2012), 108 (19), 196802/1-196802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)We show that inversion symmetry breaking together with spin-orbit coupling leads to coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides, making possible controls of spin and valley in these 2D materials. The spin-valley coupling at the valence-band edges suppresses spin and valley relaxation, as flip of each index alone is forbidden by the valley-contrasting spin splitting. Valley Hall and spin Hall effects coexist in both electron-doped and hole-doped systems. Optical interband transitions have frequency-dependent polarization selection rules which allow selective photoexcitation of carriers with various combination of valley and spin indexes. Photoinduced spin Hall and valley Hall effects can generate long lived spin and valley accumulations on sample boundaries. The physics discussed here provides a route towards the integration of valleytronics and spintronics in multivalley materials with strong spin-orbit coupling and inversion symmetry breaking.
- 7Zhang, Y. J.; Oka, T.; Suzuki, R.; Ye, J. T.; Iwasa, Y. Electrically Switchable Chiral Light-Emitting Transistor. Science 2014, 344, 725– 728, DOI: 10.1126/science.1251329[Crossref], [PubMed], [CAS], Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXotVaht7c%253D&md5=8f98422c65e6a4e54bd7394408414947Electrically Switchable Chiral Light-Emitting TransistorZhang, Y. J.; Oka, T.; Suzuki, R.; Ye, J. T.; Iwasa, Y.Science (Washington, DC, United States) (2014), 344 (6185), 725-728CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Tungsten diselenide (WSe2) and related transition metal dichalcogenides exhibit interesting optoelectronic properties owing to their peculiar band structures originating from the valley degree of freedom. Although the optical generation and detection of valley polarization was demonstrated, it was difficult to realize active valley-dependent functions suitable for device applications. The authors report an elec. switchable, circularly polarized light source based on the material's valley degree of freedom. The authors' WSe2-based ambipolar transistors emit circularly polarized electroluminescence from p-i-n junctions electrostatically formed in transistor channels. This phenomenon can be explained qual. by the electron-hole overlap controlled by the in-plane elec. field. The authors' device demonstrates a route to exploit the valley degree of freedom and the possibility to develop a valley-optoelectronics technol.
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- 10Yan, A.; Velasco, J.; Kahn, S.; Watanabe, K.; Taniguchi, T.; Wang, F.; Crommie, M. F.; Zettl, A. Direct Growth of Single- and Few-Layer MoS2 on h-BN with Preferred Relative Rotation Angles. Nano Lett. 2015, 15, 6324– 6331, DOI: 10.1021/acs.nanolett.5b01311[ACS Full Text
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- 12Dumcenco, D.; Ovchinnikov, D.; Marinov, K.; Lazić, P.; Gibertini, M.; Marzari, N.; Sanchez, O. L.; Kung, Y.-C.; Krasnozhon, D.; Chen, M.-W.; Bertolazzi, S.; Gillet, P.; Fontcuberta, i.; Morral, A.; Radenovic, A.; Kis, A. Large-Area Epitaxial Monolayer MoS2. ACS Nano 2015, 9, 4611– 4620, DOI: 10.1021/acsnano.5b01281[ACS Full Text], [CAS], Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXlvV2jtbY%253D&md5=7b7dcc0c954684fbb906fe1f49581aa1Large-Area Epitaxial Monolayer MoS2Dumcenco, Dumitru; Ovchinnikov, Dmitry; Marinov, Kolyo; Lazic, Predrag; Gibertini, Marco; Marzari, Nicola; Sanchez, Oriol Lopez; Kung, Yen-Cheng; Krasnozhon, Daria; Chen, Ming-Wei; Bertolazzi, Simone; Gillet, Philippe; Fontcuberta i Morral, Anna; Radenovic, Aleksandra; Kis, AndrasACS Nano (2015), 9 (4), 4611-4620CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to elec., optical, and mech. properties of MoS2 and other 2-dimensional semiconductors. Here, the authors report on the growth of high-quality monolayer MoS2 with control over lattice orientation. The monolayer film is composed of coalescing single islands with limited nos. of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment via van der Waals interaction, the authors can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in the authors' film are well connected, with interfaces that do not degrade the elec. cond. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80 μm.
- 13Okada, M.; Sawazaki, T.; Watanabe, K.; Taniguch, T.; Hibino, H.; Shinohara, H.; Kitaura, R. Direct Chemical Vapor Deposition Growth of WS2 Atomic Layers on Hexagonal Boron Nitride. ACS Nano 2014, 8, 8273– 8277, DOI: 10.1021/nn503093k[ACS Full Text], [CAS], Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht12gtLnM&md5=06f9f5db0b57645fc945e0fc777d155dDirect chemical vapor deposition growth of WS2 atomic layers on hexagonal boron nitrideOkada, Mitsuhiro; Sawazaki, Takumi; Watanabe, Kenji; Taniguch, Takashi; Hibino, Hiroki; Shinohara, Hisanori; Kitaura, RyoACS Nano (2014), 8 (8), 8273-8277CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Atomically thin transition metal dichalcogenides (TMDCs) have attracted considerable interest owing to the spin-valley coupled electronic structure and possibility in next-generation devices. Substrates are one of the most important factors to limit phys. properties of at.-layer materials, and among various substrates so far investigated, hexagonal boron nitride (hBN) is the best substrate to explore the intrinsic properties of at. layers. Here we report direct chem. vapor deposition (CVD) growth of WS2 onto high-quality hBN using a 3-furnace CVD setup. Triangular-shaped WS2 grown on hBN have shown limited crystallog. orientation that is related to that of the underlying hBN. Photoluminescence spectra of the WS2 show an intense emission peak at 2.01 eV with a quite small fwhm of 26 meV. The sharp emission peak indicates the high quality of the present WS2 at. layers with high crystallinity and clean interface.
- 14Kobayashi, Y.; Sasaki, S.; Mori, S.; Hibino, H.; Liu, Z.; Watanabe, K.; Taniguchi, T.; Suenaga, K.; Maniwa, Y.; Miyata, Y. Growth and Optical Properties of High-Quality Monolayer WS2 on Graphite. ACS Nano 2015, 9, 4056– 4063, DOI: 10.1021/acsnano.5b00103[ACS Full Text], [CAS], Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXlt1Cls7c%253D&md5=c4b5455f1171913ae6886b39663e414eGrowth and Optical Properties of High-Quality Monolayer WS2 on GraphiteKobayashi, Yu; Sasaki, Shogo; Mori, Shohei; Hibino, Hiroki; Liu, Zheng; Watanabe, Kenji; Taniguchi, Takashi; Suenaga, Kazu; Maniwa, Yutaka; Miyata, YasumitsuACS Nano (2015), 9 (4), 4056-4063CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)At.-layer transition metal dichalcogenides (TMDCs) have attracted appreciable interest due to their tunable band gap, spin-valley physics, and potential device applications. The quality of TMDC samples available still poses serious problems, such as inhomogeneous lattice strain, charge doping, and structural defects. The growth of high-quality, monolayer WS2 onto exfoliated graphite by high-temp. CVD is reported. Monolayer-grown WS2 single crystals present a uniform, single excitonic luminescence peak with a Lorentzian profile and a very small full-width at half-max. of 21 meV at room temp. and 8 meV at 79 K. Also, in these samples, no addnl. peaks are obsd. for charged and/or bound excitons, even at low temp. These optical responses are completely different from the results of previously reported TMDCs obtained by mech. exfoliation and CVD. The combination of high-temp. CVD with a cleaved graphite surface is an ideal condition for the growth of high-quality TMDCs, and such samples will be essential for revealing intrinsic phys. properties and for future applications.
- 15Aljarb, A.; Cao, Z.; Tang, H.-L.; Huang, J.-K.; Li, M.; Hu, W.; Cavallo, L.; Li, L.-J. Substrate Lattice-Guided Seed Formation Controls the Orientation of 2D Transition-Metal Dichalcogenides. ACS Nano 2017, 11, 9215– 9222, DOI: 10.1021/acsnano.7b04323[ACS Full Text], [CAS], Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1ylsrjK&md5=e63954216304b2aca824871b60ffb393Substrate lattice-guided seed formation controls the orientation of 2D transition-metal dichalcogenidesAljarb, Areej; Cao, Zhen; Tang, Hao-Ling; Huang, Jing-Kai; Li, Mengliu; Hu, Weijin; Cavallo, Luigi; Li, Lain-JongACS Nano (2017), 11 (9), 9215-9222CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Two-dimensional (2D) transition-metal dichalcogenide (TMDC) semiconductors are important for next-generation electronics and optoelectronics. Given the difficulty in growing large single crystals of 2D TMDC materials, understanding the factors affecting the seed formation and orientation becomes an important issue for controlling the growth. Here, we systematically study the growth of molybdenum disulfide (MoS2) monolayer on c-plane sapphire with chem. vapor deposition to discover the factors controlling their orientation. We show that the concn. of precursors, i.e., the ratio between sulfur and molybdenum oxide (MoO3), plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS2 monolayers. High S/MoO3 ratio is needed in the early stage of growth to form small seeds that can align easily to the substrate lattice structures, while the ratio should be decreased to enlarge the size of the monolayer at the next stage of the lateral growth. Moreover, we show that the seeds are actually cryst. MoS2 layers as revealed by high-resoln. transmission electron microscopy. There exist two preferred orientations (0° or 60°) registered on sapphire, confirmed by our d. functional theory simulation. This report offers a facile technique to grow highly aligned 2D TMDCs and contributes to knowledge advancement in growth mechanism.
- 16Bignardi, L.; Lizzit, D.; Bana, H.; Travaglia, E.; Lacovig, P.; Sanders, C. E.; Dendzik, M.; Michiardi, M.; Bianchi, M.; Ewert, M.; Buß, L.; Falta, J.; Flege, J. I.; Baraldi, A.; Larciprete, R.; Hofmann, P.; Lizzit, S. Growth and Structure of Singly Oriented Single-Layer Tungsten Disulfide on Au(111). Phys. Rev. Mater. 2019, 3, 014003 DOI: 10.1103/PhysRevMaterials.3.014003[Crossref], [CAS], Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFGrsrfK&md5=654266d2cf11ff642fab0467450cc9fbGrowth and structure of singly oriented single-layer tungsten disulfide on Au(111)Bignardi, Luca; Lizzit, Daniel; Bana, Harsh; Travaglia, Elisabetta; Lacovig, Paolo; Sanders, Charlotte E.; Dendzik, Maciej; Michiardi, Matteo; Bianchi, Marco; Ewert, Moritz; Buss, Lars; Falta, Jens; Flege, Jan Ingo; Baraldi, Alessandro; Larciprete, Rosanna; Hofmann, Philip; Lizzit, SilvanoPhysical Review Materials (2019), 3 (1), 014003CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)A singly oriented, single layer of tungsten disulfide (WS2) was epitaxially grown on Au(111) and characterized at the nanoscale by combining photoelectron spectroscopy, photoelectron diffraction, and low-energy electron microscopy. Fast XPS revealed that the growth of a single cryst. orientation is triggered by choosing a low W evapn. rate and performing the process with a high temp. of the substrate. Information about the single orientation of the layer was obtained by acquiring x-ray photoelectron diffraction patterns, revealing a 1H polytype for the WS2 layer and, moreover, detg. the structural parameters and registry with the substrate. The distribution, size, and orientation of the WS2 layer were further ascertained by low-energy electron microscopy.
- 17Wang, S.; Wang, X.; Warner, J. H. All Chemical Vapor Deposition Growth of MoS2: h-BN Vertical van der Waals Heterostructures. ACS Nano 2015, 9, 5246– 5254, DOI: 10.1021/acsnano.5b00655[ACS Full Text], [CAS], Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXmvVyntLg%253D&md5=66402ee0f3dabbc68605456feda7ba81All Chemical Vapor Deposition Growth of MoS2:h-BN Vertical van der Waals HeterostructuresWang, Shanshan; Wang, Xiaochen; Warner, Jamie H.ACS Nano (2015), 9 (5), 5246-5254CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Vertical van der Waals heterostructures are formed when different 2D crystals are stacked on top of each other. Improved optical properties arise in semiconducting transition metal dichalcogenide (TMD) 2D materials, such as MoS2, when they are stacked onto the insulating 2D hexagonal boron nitride (h-BN). Most work to date has required mech. exfoliation of at least one of the TMDs or h-BN materials to form these semiconductor:insulator structures. Here, we report a direct all-CVD process for the fabrication of high-quality monolayer MoS2:h-BN vertical heterostructured films with isolated MoS2 domains distributed across 1 cm. This is enabled by the use of few-layer h-BN films that are more robust against decompn. than monolayer h-BN during the MoS2 growth process. The MoS2 domains exhibit different growth dynamics on the h-BN surfaces compared to bare SiO2, confirming that there is strong interaction between the MoS2 and underlying h-BN. Raman and photoluminescence spectroscopies of CVD-grown MoS2 are compared to transferred MoS2 on both types of substrates, and our results show directly grown MoS2 on h-BN films have smaller lattice strain, lower doping level, cleaner and sharper interfaces, and high-quality interlayer contact.
- 18Pollmann, E.; Morbec, J. M.; Madauß, L.; Bröckers, L.; Kratzer, P.; Schleberger, M. Molybdenum Disulfide Nanoflakes Grown by Chemical Vapor Deposition on Graphite: Nucleation, Orientation, and Charge Transfer. J. Phys. Chem. C 2020, 124, 2689– 2697, DOI: 10.1021/acs.jpcc.9b10120
- 19Liu, X.; Balla, I.; Bergeron, H.; Campbell, G. P.; Bedzyk, M. J.; Hersam, M. C. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene. ACS Nano 2015, 10, 1067– 1075, DOI: 10.1021/acsnano.5b06398
- 20Liu, X.; Balla, I.; Bergeron, H.; Hersam, M. C. Point Defects and Grain Boundaries in Rotationally Commensurate MoS2 on Epitaxial Graphene. J. Phys. Chem. C 2016, 120, 20798– 20805, DOI: 10.1021/acs.jpcc.6b02073[ACS Full Text], [CAS], Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xks1eiu7o%253D&md5=2028387051abdd1ab1a14fbf981490caPoint Defects and Grain Boundaries in Rotationally Commensurate MoS2 on Epitaxial GrapheneLiu, Xiaolong; Balla, Itamar; Bergeron, Hadallia; Hersam, Mark C.Journal of Physical Chemistry C (2016), 120 (37), 20798-20805CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)With reduced degrees of freedom, structural defects are expected to play a greater role in 2-dimensional materials in comparison to their bulk counterparts. Mech. strength, electronic properties, and chem. reactivity are strongly affected by crystal imperfections in the atomically thin limit. Ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS2 grown on epitaxial graphene (EG) at the at. scale. Five types of point defects are obsd. with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect d. is lower than MoS2 grown on other substrates and is likely attributed to the van der Waals epitaxy of MoS2 on EG. Grain boundaries (GBs) with 30 and 60° tilt angles resulting from the rotational commensurability of MoS2 on EG are more easily resolved by STM than at. force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap redn. to ∼0.8 and ∼0.5 eV is obsd. with STS for 30 and 60° GBs, resp. At. resoln. STM images of these GBs agree with proposed structure models. This work offers quant. insight into the structure and properties of common defects in MoS2 and suggests pathways for tailoring the performance of MoS2/graphene heterostructures via defect engineering.
- 21Vlassiouk, I.; Smirnov, S.; Regmi, M.; Surwade, S. P.; Srivastava, N.; Feenstra, R.; Eres, G.; Parish, C.; Lavrik, N.; Datskos, P.; Dai, S.; Fulvio, P. Graphene Nucleation Density on Copper: Fundamental Role of Background Pressure. J. Phys. Chem. C 2013, 117, 18919– 18926[ACS Full Text], [CAS], Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtlSqtL%252FI&md5=9acab02b51539ef2bb75e8548222c441Graphene Nucleation Density on Copper: Fundamental Role of Background PressureVlassiouk, Ivan; Smirnov, Sergei; Regmi, Murari; Surwade, Sumedh P.; Srivastava, Nishtha; Feenstra, Randall; Eres, Gyula; Parish, Chad; Lavrik, Nick; Datskos, Panos; Dai, Sheng; Fulvio, PasqualeJournal of Physical Chemistry C (2013), 117 (37), 18919-18926CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)In this paper we discuss the effect of background pressure and synthesis temp. on the graphene crystal sizes in chem. vapor deposition (CVD) on copper catalyst. The authors quant. demonstrate a fundamental role of the background pressure and provide the activation energy for graphene nucleation in atm. pressure CVD (9 eV), which is substantially higher than for the low pressure CVD (4 eV). The authors attribute the difference to a greater importance of copper sublimation in the low pressure CVD, where severe copper evapn. likely dictates the desorption rate of active carbon from the surface. At atm. pressure, where copper evapn. is suppressed, the activation energy is assigned to the desorption energy of carbon clusters instead. The highest possible temp., close to the m.p. of copper, should be used for large single crystal graphene synthesis. Using these conditions, the authors have synthesized graphene single crystals with sizes over 0.5 mm. Single crystal nature of synthesized graphene was confirmed by LEED. The authors also demonstrate that CVD of graphene at temps. below 1000 °C shows higher nucleation d. on (111) than on (100) and (101) copper surfaces, but there is no identifiable preference at higher temps.
- 22Stehle, Y.; Meyer, H. M.; Unocic, R. R.; Kidder, M.; Polizos, G.; Datskos, P. G.; Jackson, R.; Smirnov, S. N.; Vlassiouk, I. V. Synthesis of Hexagonal Boron Nitride Monolayer: Control of Nucleation and Crystal Morphology. Chem. Mater. 2015, 27, 8041– 8047, DOI: 10.1021/acs.chemmater.5b03607[ACS Full Text], [CAS], Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsl2ht7nL&md5=5da09c8eb6da825ad863243caa23da05Synthesis of Hexagonal Boron Nitride Monolayer: Control of Nucleation and Crystal MorphologyStehle, Yijing; Meyer, Harry M.; Unocic, Raymond R.; Kidder, Michelle; Polizos, Georgios; Datskos, Panos G.; Jackson, Roderick; Smirnov, Sergei N.; Vlassiouk, Ivan V.Chemistry of Materials (2015), 27 (23), 8041-8047CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Monolayer hexagonal B nitride (hBN) attracts significant attention due to the potential to be used as a complementary two-dimensional dielec. in fabrication of functional 2-dimensional heterostructures. The authors study the growth stages of the hBN single crystals and show that hBN crystals change their shape from triangular to truncated triangular and further to hexagonal depending on Cu substrate distance from the precursor. Probably the obsd. hBN crystal shape variation is affected by the ratio of B to N active species concns. on the Cu surface inside the CVD reactor. Strong temp. dependence reveals the activation energies for the hBN nucleation process of ∼5 eV and crystal growth of ∼3.5 eV. Also the resulting h-BN film morphol. is strongly affected by the heating method of borazane precursor and the buffer gas. Elucidation of these details facilitated synthesis of high quality large area monolayer hexagonal B nitride by atm. pressure CVD on Cu using borazane as a precursor.
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- 24Okada, M.; Kutana, A.; Kureishi, Y.; Kobayashi, Y.; Saito, Y.; Saito, T.; Watanabe, K.; Taniguchi, T.; Gupta, S.; Miyata, Y.; Yakobson, B. I.; Shinohara, H.; Kitaura, R. Direct and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS2/MoS2/hBN. ACS Nano 2018, 12, 2498– 2505, DOI: 10.1021/acsnano.7b08253[ACS Full Text], [CAS], Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjtlOmsbo%253D&md5=c6aa6a3534087d0377c455895d69adbcDirect and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS2/MoS2/hBNOkada, Mitsuhiro; Kutana, Alex; Kureishi, Yusuke; Kobayashi, Yu; Saito, Yuika; Saito, Tetsuki; Watanabe, Kenji; Taniguchi, Takashi; Gupta, Sunny; Miyata, Yasumitsu; Yakobson, Boris I.; Shinohara, Hisanori; Kitaura, RyoACS Nano (2018), 12 (3), 2498-2505CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. Luminescence (PL) from a vdW heterostructure, WS2/MoS2, deposited on hexagonal BN (hBN) flakes was studied. PL spectra from the fabricated heterostructures obsd. at room temp. show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS2 or MoS2 monolayers alone. The low-energy PL peaks the authors obsd. can be decompd. into 3 distinct peaks. Through detailed PL measurements and theor. anal., including PL imaging, time-resolved PL measurements, and calcn. of dielec. function ε(ω) by solving the Bethe-Salpeter equation with G0W0, the 3 PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.
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- 27Morikawa, Y.; Iwata, K.; Terakura, K. Theoretical Study of Hydrogenation Process of Formate on Clean and Zn Deposited Cu(111) Surfaces. Appl. Surf. Sci. 2001, 169-170, 11– 15, DOI: 10.1016/S0169-4332(00)00631-0[Crossref], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXhtFKlsL8%253D&md5=b79e2e6c93fdf6b5c27610930aa72e26Theoretical study of hydrogenation process of formate on clean and Zn deposited Cu(1 1 1) surfacesMorikawa, Y.; Iwata, K.; Terakura, K.Applied Surface Science (2001), 169-170 (), 11-15CODEN: ASUSEE; ISSN:0169-4332. (Elsevier Science B.V.)We have studied the effect of Zn on hydrogenation of formate to dioxymethylene on the Cu(1 1 1) surface by a d. functional theory-generalized gradient approxn. (DFT-GGA)-pseudopotential method. Substitutionally adsorbed Zn changes the stability of intermediate states and the activation barrier of the hydrogenation process only slightly. On the other hand, the Zn atom adsorbed on the Cu surface stabilizes all formate, transition state, and dioxymethylene relative to the gas-phase mols. The results support a previously proposed reaction scheme that the adsorption state of Zn changes from substitutional to on-surface adsorption during the methanol synthesis.
- 28Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865– 3868, DOI: 10.1103/PhysRevLett.77.3865[Crossref], [PubMed], [CAS], Google Scholar28https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK28XmsVCgsbs%253D&md5=55943538406ee74f93aabdf882cd4630Generalized gradient approximation made simplePerdew, John P.; Burke, Kieron; Ernzerhof, MatthiasPhysical Review Letters (1996), 77 (18), 3865-3868CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)Generalized gradient approxns. (GGA's) for the exchange-correlation energy improve upon the local spin d. (LSD) description of atoms, mols., and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental consts. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential.
- 29Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple [Phys. Rev. Lett. 77, 3865 (1996)]. Phys. Rev. Lett. 1997, 78, 1396– 1396, DOI: 10.1103/PhysRevLett.78.1396[Crossref], [CAS], Google Scholar29https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2sXht1Gns7o%253D&md5=ecdb6e129b112a3a10e08cba26a083aeGeneralized gradient approximation made simple. [Erratum to document cited in CA126:51093]Perdew, John P.; Burke, Kieron; Ernzerhof, MatthiasPhysical Review Letters (1997), 78 (7), 1396CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)The errors were not reflected in the abstr. or the index entries.
- 30Lee, K.; Murray, É. D.; Kong, L.; Lundqvist, B. I.; Langreth, D. C. Higher-Accuracy van der Waals Density Functional. Phys. Rev. B 2010, 82, 081101 DOI: 10.1103/PhysRevB.82.081101[Crossref], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtFWlurnN&md5=21a4573d23bb686441ae1609cb4e6809Higher-accuracy van der Waals density functionalLee, Kyuho; Murray, Eamonn D.; Kong, Lingzhu; Lundqvist, Bengt I.; Langreth, David C.Physical Review B: Condensed Matter and Materials Physics (2010), 82 (8), 081101/1-081101/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)We propose a second version of the van der Waals d. functional of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate semilocal exchange functional and the use of a large-N asymptote gradient correction in detg. the vdW kernel. The predicted binding energy, equil. sepn., and potential-energy curve shape are close to those of accurate quantum chem. calcns. on 22 duplexes. We anticipate the enabling of chem. accurate calcns. in sparse materials of importance for condensed matter, surface, chem., and biol. physics.
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- 33Otani, M.; Sugino, O. First-Principles Calculations of Charged Surfaces and Interfaces: A Plane-Wave Nonrepeated Slab Approach. Phys. Rev. B 2006, 73, 115407, DOI: 10.1103/PhysRevB.73.115407[Crossref], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XjsV2rs7s%253D&md5=c71d929e8fdf09c5e14d8d8bab8260e1First-principles calculations of charged surfaces and interfaces: A plane-wave nonrepeated slab approachOtani, M.; Sugino, O.Physical Review B: Condensed Matter and Materials Physics (2006), 73 (11), 115407/1-115407/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)A new first-principles computational approach to a charged surface/interface is presented. The surface is modeled as a slab imposed with boundary conditions to screen the excess surface charge. To treat this model, which is nonperiodic in the surface normal direction, a std. pseudopotential plane-wave scheme is modified at the Poisson solver part with the help of the Green's function technique. Benchmark calcns. are done for Al/Si(111) with the bias voltage applied between the surface and the model scanning tunneling microscopy (STM) tip, the model back gate, or the model soln. The calcns. are found to be efficient and stable, and their implementation is found to be easy. Because of the flexibility, the scheme is considered to be applicable to more general exptl. situations.
- 34Taniguchi, T.; Watanabe, K. Synthesis of High-Purity Boron Nitride Single Crystals under High Pressure by Using Ba–BN Solvent. J. Cryst. Growth 2007, 303, 525– 529, DOI: 10.1016/j.jcrysgro.2006.12.061[Crossref], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXksl2gtL4%253D&md5=89a6f05bbe1adc1ab83469c96b60c575Synthesis of high-purity boron nitride single crystals under high pressure by using Ba-BN solventTaniguchi, T.; Watanabe, K.Journal of Crystal Growth (2007), 303 (2), 525-529CODEN: JCRGAE; ISSN:0022-0248. (Elsevier B.V.)High-purity cubic boron nitride (cBN) and hexagonal boron nitride (hBN) single crystals were synthesized at 4.5 GPa and 1500 °C using barium boron nitride as a solvent. Secondary ion mass spectrometry was used to analyze impurities in the crystals. Fine cBN and hBN crystals, whose oxygen and carbon concns. were <1018 atoms/cm3, were obtained, and their band-edge optical properties were measured by cathodoluminescence spectroscopy. High-purity hBN single crystals exhibited intense UV emission, demonstrating their promise for use as deep UV-light emitters.
- 35Wang, S.; Rong, Y.; Fan, Y.; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371– 6379, DOI: 10.1021/cm5025662[ACS Full Text], [CAS], Google Scholar35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvVGlsbzE&md5=8f5503c1db3a41197f6ba69aa5545972Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor DepositionWang, Shanshan; Rong, Youmin; Fan, Ye; Pacios, Merce; Bhaskaran, Harish; He, Kuang; Warner, Jamie H.Chemistry of Materials (2014), 26 (22), 6371-6379CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Atm.-pressure CVD was used to grow monolayer MoS2 two-dimensional crystals at elevated temps. on silicon substrates with a 300. nm oxide layer. The authors' CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace sep. from the MoO3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. The authors intentionally establish a sharp gradient of MoO3 precursor concn. on the growth substrate to explore its sensitivity to the resultant MoS2 domain growth within a relatively uniform temp. range. The shape of MoS2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve the authors' understanding of the factors that influence the growth of MoS2 domains and their shape evolution.
- 36Terrones, H.; Del Corro, E.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A.; Elías, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M. New First Order Raman-Active Modes in Few Layered Transition Metal Dichalcogenides. Sci. Rep. 2014, 4, 4215[Crossref], [PubMed], [CAS], Google Scholar36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht1WjurzK&md5=de104025c96fe5a282568df3b385e2a9New first order raman-active modes in few layered transition metal dichalcogenidesTerrones, H.; Del Corro, E.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A. T.; Elias, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M.Scientific Reports (2014), 4 (), 4215/1-4215/9CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calcns. carried out. We calcd. phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the no. of layers increase, the E', E" and E2g modes shift to lower frequencies, and the A'1 and A1g modes shift to higher frequencies. Second, new high frequency first order A'1 and A1g modes appear, explaining recently reported exptl. data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A'1 and A1g is found which explains those obsd. in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.
- 37Wu, S.; Huang, C.; Aivazian, G.; Ross, J. S.; Cobden, D. H.; Xu, X. Vapor-Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley Polarization. ACS Nano 2013, 7, 2768– 2772, DOI: 10.1021/nn4002038[ACS Full Text], [CAS], Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXivVyisbY%253D&md5=00349bba3010813d8e9cf4f351752cdbVapor-Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley PolarizationWu, Sanfeng; Huang, Chunming; Aivazian, Grant; Ross, Jason S.; Cobden, David H.; Xu, XiaodongACS Nano (2013), 7 (3), 2768-2772CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-gap semiconductors with potential applications in nanoelectronics, optoelectronics, and electrochem. sensing. Recent theor. and exptl. efforts suggest that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. For example, Dirac valley polarization was demonstrated in mech. exfoliated monolayer MoS2 samples by polarization-resolved photoluminescence, although polarization has rarely been seen at room temp. Here the authors report a new method for synthesizing high optical quality monolayer MoS2 single crystals up to 25 μm in size on a variety of std. insulating substrates (SiO2, sapphire, and glass) using a catalyst-free vapor-solid growth mechanism. The technique is simple and reliable, and the optical quality of the crystals is extremely high, as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35% even at room temp., suggesting a virtual absence of defects. This will allow greatly improved optoelectronic TMDC monolayer devices to be fabricated and studied routinely.
- 38Gong, Y.; Lin, Z.; Ye, G.; Shi, G.; Feng, S.; Lei, Y.; Elías, A. L.; Perea-Lopez, N.; Vajtai, R.; Terrones, H.; Liu, Z.; Terrones, M.; Ajayan, P. M. Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2 Monolayers. ACS Nano 2015, 9, 11658– 11666, DOI: 10.1021/acsnano.5b05594[ACS Full Text], [CAS], Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslWjs73L&md5=a662d87448c4b2dc095b48c163287538Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2 MonolayersGong, Yongji; Lin, Zhong; Ye, Gonglan; Shi, Gang; Feng, Simin; Lei, Yu; Elias, Ana Laura; Perea-Lopez, Nestor; Vajtai, Robert; Terrones, Humberto; Liu, Zheng; Terrones, Mauricio; Ajayan, Pulickel M.ACS Nano (2015), 9 (12), 11658-11666CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Chem. vapor deposition (CVD) is a scalable method able to synthesize MoS2 and WS2 monolayers. In this work, we reduced the synthesis temp. by 200°C by introducing tellurium (Te) into the CVD process. The as-synthesized MoS2 and WS2 monolayers show high phase purity and crystallinity. The optical and elec. performance of these materials is comparable to those synthesized at higher temps. We believe this work will accelerate the industrial synthesis of these semiconducting monolayers.
- 39Ling, X.; Lee, Y. H.; Lin, Y.; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J. Role of the Seeding Promoter in MoS2 growth by Chemical Vapor Deposition. Nano Lett. 2014, 14, 464– 472, DOI: 10.1021/nl4033704[ACS Full Text], [CAS], Google Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVOjtrg%253D&md5=4c39b50e8bbadb22be52d267915e064aRole of the Seeding Promoter in MoS2 Growth by Chemical Vapor DepositionLing, Xi; Lee, Yi-Hsien; Lin, Yuxuan; Fang, Wenjing; Yu, Lili; Dresselhaus, Mildred S.; Kong, JingNano Letters (2014), 14 (2), 464-472CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)The thinnest semiconductor, molybdenum disulfide (MoS2) monolayer, exhibits promising prospects in the applications of optoelectronics and valleytronics. A uniform and highly cryst. MoS2 monolayer in a large area is highly desirable for both fundamental studies and substantial applications. Here, using various arom. mols. as seeding promoters, a large-area, highly cryst., and uniform MoS2 monolayer was achieved with CVD at a relatively low growth temp. (650°). The dependence of the growth results on the seed concn. and on the use of different seeding promoters is further studied. Also an optimized concn. of seed mols. is helpful for the nucleation of the MoS2. The newly identified seed mols. can be easily deposited on various substrates and allows the direct growth of monolayer MoS2 on Au, hexagonal boron nitride (h-BN), and graphene to achieve various hybrid structures.
- 40Zhang, F.; Wang, Y.; Erb, C.; Wang, K.; Moradifar, P.; Crespi, V. H.; Alem, N. Full Orientation Control of Epitaxial MoS2 on hBN Assisted by Substrate Defects. Phys. Rev. B 2019, 99, 155430, DOI: 10.1103/PhysRevB.99.155430[Crossref], [CAS], Google Scholar40https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXpslWhsL4%253D&md5=9107da3734433c186a0d22e61ee8646bFull orientation control of epitaxial MoS2 on hBN assisted by substrate defectsZhang, Fu; Wang, Yuanxi; Erb, Chad; Wang, Ke; Moradifar, Parivash; Crespi, Vincent H.; Alem, NasimPhysical Review B (2019), 99 (15), 155430CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)Inversion asymmetry in two-dimensional materials grants them fascinating properties such as spin-coupled valley degrees of freedom and piezoelectricity, but at the cost of inversion domain boundaries if the epitaxy of the grown two-dimensional (2D) layer, on a polar substrate, cannot adequately distinguish what are often near-degenerate 0° and 180° orientations. We employ first-principles calcns. to identify a method to lift this near degeneracy: the energetic distinction between eclipsed and staggered configurations during nucleation at a point defect in the substrate. For monolayer MoS2 grown on hexagonal boron nitride, the predicted defect complex can be more stable than common MoS2 point defects because it is both a donor-acceptor pair and a Frenkel pair shared between adjacent layers of a 2D heterostack. Orientation control is verified in expts. that achieve ∼90% consistency in the orientation of as-grown triangular MoS2 flakes on hBN, as confirmed by aberration-cor. scanning/transmission electron microscopy. This defect-enhanced orientational epitaxy could provide a general mechanism to break the near-degeneracy of 0/180° orientations of polar 2D materials on polar substrates, overcoming a long-standing impediment to scalable synthesis of single-crystal 2D semiconductors.
- 41Varshney, V.; Patnaik, S. S.; Muratore, C.; Roy, A. K.; Voevodin, A. A.; Farmer, B. L. MD Simulations of Molybdenum Disulphide (MoS2): Force-Field Parameterization and Thermal Transport Behavior. Comput. Mater. Sci. 2010, 48, 101– 108, DOI: 10.1016/j.commatsci.2009.12.009[Crossref], [CAS], Google Scholar41https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXitlegsL8%253D&md5=6ff1f4336ada2b48d0cc4663373c08b5MD simulations of molybdenum disulphide (MoS2): force-field parameterization and thermal transport behaviorVarshney, Vikas; Patnaik, Soumya S.; Muratore, Chris; Roy, Ajit K.; Voevodin, Andrey A.; Farmer, Barry L.Computational Materials Science (2010), 48 (1), 101-108CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)In this article, we have investigated the anisotropic nature of thermal transport in molybdenum disulfide using mol. dynamics simulations. At first, a force field has been validated with respect to crystal structure and exptl. vibrational spectra of MoS2. Thereafter, non-equil. MD simulations have been performed in two perpendicular directions (along as well as across the basal planes) to study thermal transport behavior. At room temp., our results show an anisotropic factor of ∼4 in the values of thermal cond. along two studied directions, which is in good agreement with recent expts. on MoS2 thin films. However, the predicted values of thermal cond. are about an order of magnitude higher with respect to expts. The reasoning behind these differences has been discussed in terms of layer disorder and the large no. of grain boundary interfaces in exptl. thin films, which consisted of nano-cryst. MoS2 grains with a predominant parallel or perpendicular basal plane orientation. Incorporation of phonon scattering via structure disorder and boundary interfaces were identified as further directions for the model refinement.
- 42Won, C. Y.; Aluru, N. R. Water Permeation through a Subnanometer Boron Nitride Nanotube. J. Am. Chem. Soc. 2007, 129, 2748– 2749, DOI: 10.1021/ja0687318[ACS Full Text], [CAS], Google Scholar42https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhvVWjurY%253D&md5=f8bee49e0a9fdb7f097b00470b11221aWater Permeation through a Subnanometer Boron Nitride NanotubeWon, Chang Y.; Aluru, N. R.Journal of the American Chemical Society (2007), 129 (10), 2748-2749CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)B nitride nanotubes (BNNTs) possess many excellent phys. properties, including thermal and mech. properties. The authors report on superior H2O permeation properties of BNNTs. Specifically, using mol. dynamics simulations, H2O mols. permeate through the (5,5) B nitride nanotube, while a (5,5) C nanotube (CNT) of approx. the same diam. does not conduct H2O. The relatively strong interactions between the nitride atoms of the BNNT and H2O mols. play a key role in the continuous wetting behavior of the BNNT. The properties of H2O, such as the axial diffusion coeff. and the hydrogen bonding, inside the (5,5) BNNT are comparable to those inside the (6,6) CNT, even though the diam. of the (5,5) BNNT is ∼1.3 Å smaller than that of the (6,6) CNT.
- 43Yankowitz, M.; Xue, J.; Cormode, D.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; Jacquod, P.; LeRoy, B. J. Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride. Nat. Phys. 2012, 8, 382, DOI: 10.1038/nphys2272[Crossref], [CAS], Google Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XksVegtLo%253D&md5=5a462366ee8201e802b62be2ec86f600Emergence of superlattice Dirac points in graphene on hexagonal boron nitrideYankowitz, Matthew; Xue, Jiamin; Cormode, Daniel; Sanchez-Yamagishi, Javier D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, Pablo; Jacquod, Philippe; LeRoy, Brian J.Nature Physics (2012), 8 (5), 382-386CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)The Schroedinger equation dictates that the propagation of nearly free electrons through a weak periodic potential results in the opening of bandgaps near points of the reciprocal lattice known as Brillouin zone boundaries. However, in the case of massless Dirac fermions, it has been predicted that the chirality of the charge carriers prevents the opening of a bandgap and instead new Dirac points appear in the electronic structure of the material. Graphene on hexagonal boron nitride exhibits a rotation-dependent moire pattern. Here, we show exptl. and theor. that this moire pattern acts as a weak periodic potential and thereby leads to the emergence of a new set of Dirac points at an energy detd. by its wavelength. The new massless Dirac fermions generated at these superlattice Dirac points are characterized by a significantly reduced Fermi velocity. Furthermore, the local d. of states near these Dirac cones exhibits hexagonal modulation due to the influence of the periodic potential.
- 44Dienwiebel, M.; Verhoeven, G. S.; Pradeep, N.; Frenken, J. W. M.; Heimberg, J. A.; Zandbergen, H. W. Superlubricity of Graphite. Phys. Rev. Lett. 2004, 92, 126101, DOI: 10.1103/PhysRevLett.92.126101[Crossref], [PubMed], [CAS], Google Scholar44https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXis1Wrt7o%253D&md5=2a61504146d5881e9d4469a148752efaSuperlubricity of GraphiteDienwiebel, Martin; Verhoeven, Gertjan S.; Pradeep, Namboodiri; Frenken, Joost W. M.; Heimberg, Jennifer A.; Zandbergen, Henny W.Physical Review Letters (2004), 92 (12), 126101/1-126101/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resoln. down to 15 pN, the authors have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring at.-scale friction as a function of the rotational angle between two contacting bodies, the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of superlubricity [M. Hirano and K. Shinjo, (1990)].
- 45Wang, W.; Shen, J.; He, Q. C. Microscale Superlubricity of Graphite under Various Twist Angles. Phys. Rev. B 2019, 99, 054103 DOI: 10.1103/PhysRevB.99.054103[Crossref], [CAS], Google Scholar45https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXnvFWhtbg%253D&md5=4f151b083bbbde205ee1df856b288d82Microscale superlubricity of graphite under various twist anglesWang, Wen; Shen, Jian; He, Q.-C.Physical Review B (2019), 99 (5), 054103CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)The relative rotation and translation of graphene and graphite layers lead to remarkable phys. and mech. phenomena. One such phenomenon for graphite corresponds to the ultralow static and dynamic friction between incommensurate graphene layers, referred to as superlubricity. Even though many studies have been dedicated to this promising phenomenon in recent years, an exptl. characterization and a quant. detn. of the effect of relative twist angles on microscale superlubricity are still lacking. The present paper investigates the superlubric properties of microscale graphite under different twist angels by shearing graphite with respect to a substrate. Exptl., it is surprisingly found that the superlubricity of microscale graphite is almost invariant within a wide range of bicrystal twist angles (6o≤θ≤59o). This result is confirmed by carrying out mol. dynamics simulations. Further, the influences of twist angles and normal load on the incommensurate-to-commensurate transition are revealed. The estd. crit. transition angle is less than 0.1 °. These results allow a better understanding of mesoscopic scale superlubricity and extend its application field.
Abstract
Figure 1
Figure 1. (a) Optical image of CVD-grown MoS2 on hBN; (b) schematic of MoS2/hBN; (c) typical Raman spectrum, (d) PL spectrum of the MoS2 crystal shown in the upper left side of Figure 1a; and (e) a typical SAED pattern of MoS2/hBN. Green and blue arrows indicate diffraction spots from MoS2 and hBN in MoS2/hBN, respectively.
Figure 2
Figure 3
Figure 3. (a) Cluster-size and stacking-angle evolutions of interaction energy; (b) cluster-size dependencies of absolute values of interaction energy (i.e., absolute values of the difference between the maximum energy and the minimum energy) calculated with a stacking angle of 0 and 60° and the most stable stacking angle of each cluster; (c) map showing element-decomposed interaction energies of a MoS2 cluster (an edge length of 1.6 nm) with a stacking angle of 0°. Yellow circles correspond to S2 pairs; and (d) corresponding structure used to calculate the element-decomposed interaction energy in (c). Purple lines indicate the moiré superlattice period. Element coloring is the same as that of Figure 1b.
Figure 4
Figure 4. (a) Stacking angle-dependent interaction energy of a Mo243S486 cluster (the interaction energy at the most stable stacking angle is set to zero) and (b, c) maps showing element-decomposed interaction energies of S2 pairs in a MoS2 cluster with an edge length of 2.8 nm with stacking angles of 0° and 4.5°. Yellow circles correspond to positions of S2 pairs and magenta dotted lines correspond to the moiré superlattice period of each structure.
Figure 5
Figure 5. (a) Cluster-size-dependent stable stacking angle evolutions around 40–50° (upper panel) and 10–20° (lower panel); (b) stacking angle and structure-dependent interaction-energy evolution of clusters with an edge length of 2.5, 2.8, and 3.2 nm. The left and right panels show the results calculated with stacking angles of 12–21° and 39–48°, respectively. Curves labeled as “SA result” are the same as the curves shown in Figure 3a. Black dotted lines correspond to an energy minimum position of a cluster with an edge length of 2.5 nm.
References
ARTICLE SECTIONSThis article references 45 other publications.
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- 2Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-layer MoS2 transistors. Nat. Nanotechnol. 2011, 6, 147– 150, DOI: 10.1038/nnano.2010.279[Crossref], [PubMed], [CAS], Google Scholar2https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXislCjsro%253D&md5=555366539a8a87d074a69674aafaf315Single-layer MoS2 transistorsRadisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A.Nature Nanotechnology (2011), 6 (3), 147-150CODEN: NNAABX; ISSN:1748-3387. (Nature Publishing Group)Two-dimensional materials are attractive for use in next-generation nanoelectronic devices because, compared to 1D materials, it is relatively easy to fabricate complex structures from them. The most widely studied 2D material is graphene, both because of its rich physics and its high mobility. However, pristine graphene does not have a bandgap, a property that is essential for many applications, including transistors. Engineering a graphene bandgap increases fabrication complexity and either reduces mobilities to the level of strained Si films or requires high voltages. Although single layers of MoS2 have a large intrinsic bandgap of 1.8 eV, previously reported mobilities in the 0.5-3 cm2 V-1 s-1 range are too low for practical devices. Here, we use a HfO2 gate dielec. to demonstrate a room-temp. single-layer MoS2 mobility of at least 200 cm2 V-1 s-1, similar to that of graphene nanoribbons, and demonstrate transistors with room-temp. current on/off ratios of 1 × 108 and ultralow standby power dissipation. Because monolayer MoS2 has a direct bandgap, it can be used to construct interband tunnel FETs, which offer lower power consumption than classical transistors. Monolayer MoS2 could also complement graphene in applications that require thin transparent semiconductors, such as optoelectronics and energy harvesting.
- 3Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: a New Direct-Gap Semiconductor. Phys. Rev. Lett. 2010, 105, 136805, DOI: 10.1103/PhysRevLett.105.136805[Crossref], [PubMed], [CAS], Google Scholar3https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXht1Chs7zL&md5=f29a2e9692fc341d1b921f7862cf4c2aAtomically Thin MoS2. A New Direct-Gap SemiconductorMak, Kin Fai; Lee, Changgu; Hone, James; Shan, Jie; Heinz, Tony F.Physical Review Letters (2010), 105 (13), 136805/1-136805/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)The electronic properties of ultrathin crystals of MoS2 consisting of N = 1, 2,...,6 S-Mo-S monolayers were investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photocond. spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by >0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 104 compared with the bulk material.
- 4Kumar, A.; Ahluwalia, P. K. Electronic Structure of Transition Metal Dichalcogenides Monolayers 1H-MX2 (M= Mo, W; X= S, Se, Te) from ab-initio Theory: New Direct Band Gap Semiconductors. Eur. Phys. J. B 2012, 85, 186, DOI: 10.1140/epjb/e2012-30070-x[Crossref], [CAS], Google Scholar4https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhsVynsr%252FO&md5=d70bba0d95359b078cd767aa48505bb8Electronic structure of transition metal dichalcogenides monolayers 1H-MX2 (M = Mo, W; X = S, Se, Te) from ab-initio theory: new direct band gap semiconductorsKumar, A.; Ahluwalia, P. K.European Physical Journal B: Condensed Matter and Complex Systems (2012), 85 (6), 186, 7 pp.CODEN: EPJBFY; ISSN:1434-6028. (Springer)We report 1st principles calcns. of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical AO basis sets based methods within the local d. approxn. Electronic band structure and d. of states calcns. found that the states around the Fermi energy are mainly due to metal d states. From partial d. of states we find a strong hybridization between metal d and chalcogen p states below the Fermi energy. All studied compds. have emerged as new direct band gap semiconductors. The electronic band gap is found to decrease as one goes from sulfides to the tellurides of both Mo and W. Reducing the slab thickness systematically from bulk to monolayers causes a blue shift in the band gap energies, resulting in tunability of the electronic band gap. The magnitudes of the blue shift in the band gap energies are found to be 1.14 eV, 1.16 eV, 0.78 eV, 0.64, 0.57 eV and 0.37 eV for MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, resp., as we go from bulk phase (indirect band gap) to monolayer limit (direct band gap). This tunability in the electronic band gap and transitions from indirect to direct band make these materials potential candidates for the fabrication of optoelectronic devices.
- 5Kuc, A.; Zibouche, N.; Heine, T. Influence of Quantum Confinement on the Electronic Structure of the Transition Metal Sulfide TS2. Phys. Rev. B 2011, 83, 245213, DOI: 10.1103/PhysRevB.83.245213[Crossref], [CAS], Google Scholar5https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXosVGgtro%253D&md5=80d3805141f9bab4e9799a8275124380Influence of quantum confinement on the electronic structure of the transition metal sulfide TS2Kuc, A.; Zibouche, N.; Heine, T.Physical Review B: Condensed Matter and Materials Physics (2011), 83 (24), 245213/1-245213/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)Bulk MoS2, a prototypical layered transition-metal dichalcogenide, is an indirect band gap semiconductor. Reducing its slab thickness to a monolayer, MoS2 undergoes a transition to the direct band semiconductor. We support this exptl. observation by first-principle calcns. and show that quantum confinement in layered d-electron dichalcogenides results in tuning the electronic structure. We further studied the properties of related TS2 nanolayers (T= W, Nb, Re) and show that the isotopol. WS2 exhibits similar electronic properties, while NbS2 and ReS2 remain metallic independent of the slab thickness.
- 6Xiao, D.; Liu, G.-B.; Feng, W.; Xu, X.; Yao, W. Coupled Spin and Valley Physics in Monolayers of MoS2 and Other Group-VI Dichalcogenides. Phys. Rev. Lett. 2012, 108, 196802, DOI: 10.1103/PhysRevLett.108.196802[Crossref], [PubMed], [CAS], Google Scholar6https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XhtVWhsrjO&md5=dd81a9860c9bf7f16dc2e95256e98549Coupled spin and valley physics in monolayers of MoS2 and other Group-VI dichalcogenidesXiao, Di; Liu, Gui-Bin; Feng, Wanxiang; Xu, Xiaodong; Yao, WangPhysical Review Letters (2012), 108 (19), 196802/1-196802/5CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)We show that inversion symmetry breaking together with spin-orbit coupling leads to coupled spin and valley physics in monolayers of MoS2 and other group-VI dichalcogenides, making possible controls of spin and valley in these 2D materials. The spin-valley coupling at the valence-band edges suppresses spin and valley relaxation, as flip of each index alone is forbidden by the valley-contrasting spin splitting. Valley Hall and spin Hall effects coexist in both electron-doped and hole-doped systems. Optical interband transitions have frequency-dependent polarization selection rules which allow selective photoexcitation of carriers with various combination of valley and spin indexes. Photoinduced spin Hall and valley Hall effects can generate long lived spin and valley accumulations on sample boundaries. The physics discussed here provides a route towards the integration of valleytronics and spintronics in multivalley materials with strong spin-orbit coupling and inversion symmetry breaking.
- 7Zhang, Y. J.; Oka, T.; Suzuki, R.; Ye, J. T.; Iwasa, Y. Electrically Switchable Chiral Light-Emitting Transistor. Science 2014, 344, 725– 728, DOI: 10.1126/science.1251329[Crossref], [PubMed], [CAS], Google Scholar7https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXotVaht7c%253D&md5=8f98422c65e6a4e54bd7394408414947Electrically Switchable Chiral Light-Emitting TransistorZhang, Y. J.; Oka, T.; Suzuki, R.; Ye, J. T.; Iwasa, Y.Science (Washington, DC, United States) (2014), 344 (6185), 725-728CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)Tungsten diselenide (WSe2) and related transition metal dichalcogenides exhibit interesting optoelectronic properties owing to their peculiar band structures originating from the valley degree of freedom. Although the optical generation and detection of valley polarization was demonstrated, it was difficult to realize active valley-dependent functions suitable for device applications. The authors report an elec. switchable, circularly polarized light source based on the material's valley degree of freedom. The authors' WSe2-based ambipolar transistors emit circularly polarized electroluminescence from p-i-n junctions electrostatically formed in transistor channels. This phenomenon can be explained qual. by the electron-hole overlap controlled by the in-plane elec. field. The authors' device demonstrates a route to exploit the valley degree of freedom and the possibility to develop a valley-optoelectronics technol.
- 8Koma, A. Van der Waals Epitaxy—a New Epitaxial Growth Method for a Highly Lattice-Mismatched System. Thin Solid Films 1992, 216, 72– 76, DOI: 10.1016/0040-6090(92)90872-9[Crossref], [CAS], Google Scholar8https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK38Xmt1ehtL0%253D&md5=bfcec8c109b25d86116d266ab1196359Van der Waals epitaxy - a new epitaxial growth method for a highly lattice-mismatched systemKoma, AtsushiThin Solid Films (1992), 216 (1), 72-6CODEN: THSFAP; ISSN:0040-6090.A review with 22 refs. Van der Waals epitaxy can be applied to the epitaxial growth of a layered material on an ordinary 3-dimensional material substrate, if regular termination of the surface dangling bonds is accomplished.
- 9Koma, A.; Sunouchi, K.; Miyajima, T. Fabrication and Characterization of Heterostructures with Subnanometer Thickness. Microelectron. Eng. 1984, 2, 129– 136, DOI: 10.1016/0167-9317(84)90057-1[Crossref], [CAS], Google Scholar9https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaL2MXks1akurs%253D&md5=5c463b12ad56c6424d50187de07f6767Fabrication and characterization of heterostructures with subnanometer thicknessKoma, Atsushi; Sunouchi, Kazumasa; Miyajima, TakaoMicroelectronic Engineering (1985), 2 (1-3), 129-36CODEN: MIENEF; ISSN:0167-9317.Good quality heterostructures with subnanometer thickness were successfully grown using van der Waals epitaxy. The van der Waals epitaxy can be realized in the materials having no dangling bonds on clean surfaces on which epitaxial growth proceeds by the van der Waals force. Ultrathin Se films were grown on a cleaved face of Te and ultrathin NbSe2 films were grown on a cleaved face of 2H-MoS2. Although there was lattice mismatching as large as 20% between those materials forming the heterostructure, the grown films proved to be of good single cryst. quality.
- 10Yan, A.; Velasco, J.; Kahn, S.; Watanabe, K.; Taniguchi, T.; Wang, F.; Crommie, M. F.; Zettl, A. Direct Growth of Single- and Few-Layer MoS2 on h-BN with Preferred Relative Rotation Angles. Nano Lett. 2015, 15, 6324– 6331, DOI: 10.1021/acs.nanolett.5b01311[ACS Full Text], [CAS], Google Scholar10https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsVWntr3M&md5=e976a09ba0126395dd9e8a6f87cab783Direct Growth of Single- and Few-Layer MoS2 on h-BN with Preferred Relative Rotation AnglesYan, Aiming; Velasco, Jairo; Kahn, Salman; Watanabe, Kenji; Taniguchi, Takashi; Wang, Feng; Crommie, Michael F.; Zettl, AlexNano Letters (2015), 15 (10), 6324-6331CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)Monolayer molybdenum disulfide (MoS2) is a promising two-dimensional direct-bandgap semiconductor with potential applications in atomically thin and flexible electronics. An attractive insulating substrate or mate for MoS2 (and related materials such as graphene) is hexagonal boron nitride (h-BN). Stacked heterostructures of MoS2 and h-BN have been produced by manual transfer methods, but a more efficient and scalable assembly method is needed. Here we demonstrate the direct growth of single- and few-layer MoS2 on h-BN by chem. vapor deposition (CVD) method, which is scalable with suitably structured substrates. The growth mechanisms for single-layer and few-layer samples are found to be distinct, and for single-layer samples low relative rotation angles ( < 5°) between the MoS2 and h-BN lattices prevail. Moreover, MoS2 directly grown on h-BN maintains its intrinsic 1.89 eV bandgap. Our CVD synthesis method presents an important advancement toward controllable and scalable MoS2-based electronic devices.
- 11Yu, H.; Yang, Z.; Du, L.; Zhang, J.; Shi, J.; Chen, W.; Chen, P.; Liao, M.; Zhao, J.; Meng, J.; Wang, G.; Zhu, J.; Yang, R.; Shi, D.; Gu, L.; Zhang, G. Precisely Aligned Monolayer MoS2 Epitaxially Grown on h-BN basal Plane. Small 2017, 13, 1603005, DOI: 10.1002/smll.201603005
- 12Dumcenco, D.; Ovchinnikov, D.; Marinov, K.; Lazić, P.; Gibertini, M.; Marzari, N.; Sanchez, O. L.; Kung, Y.-C.; Krasnozhon, D.; Chen, M.-W.; Bertolazzi, S.; Gillet, P.; Fontcuberta, i.; Morral, A.; Radenovic, A.; Kis, A. Large-Area Epitaxial Monolayer MoS2. ACS Nano 2015, 9, 4611– 4620, DOI: 10.1021/acsnano.5b01281[ACS Full Text], [CAS], Google Scholar12https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXlvV2jtbY%253D&md5=7b7dcc0c954684fbb906fe1f49581aa1Large-Area Epitaxial Monolayer MoS2Dumcenco, Dumitru; Ovchinnikov, Dmitry; Marinov, Kolyo; Lazic, Predrag; Gibertini, Marco; Marzari, Nicola; Sanchez, Oriol Lopez; Kung, Yen-Cheng; Krasnozhon, Daria; Chen, Ming-Wei; Bertolazzi, Simone; Gillet, Philippe; Fontcuberta i Morral, Anna; Radenovic, Aleksandra; Kis, AndrasACS Nano (2015), 9 (4), 4611-4620CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Two-dimensional semiconductors such as MoS2 are an emerging material family with wide-ranging potential applications in electronics, optoelectronics, and energy harvesting. Large-area growth methods are needed to open the way to applications. Control over lattice orientation during growth remains a challenge. This is needed to minimize or even avoid the formation of grain boundaries, detrimental to elec., optical, and mech. properties of MoS2 and other 2-dimensional semiconductors. Here, the authors report on the growth of high-quality monolayer MoS2 with control over lattice orientation. The monolayer film is composed of coalescing single islands with limited nos. of lattice orientation due to an epitaxial growth mechanism. Optical absorbance spectra acquired over large areas show significant absorbance in the high-energy part of the spectrum, indicating that MoS2 could also be interesting for harvesting this region of the solar spectrum and fabrication of UV-sensitive photodetectors. Even though the interaction between the growth substrate and MoS2 is strong enough to induce lattice alignment via van der Waals interaction, the authors can easily transfer the grown material and fabricate devices. Local potential mapping along channels in field-effect transistors shows that the single-crystal MoS2 grains in the authors' film are well connected, with interfaces that do not degrade the elec. cond. This is also confirmed by the relatively large and length-independent mobility in devices with a channel length reaching 80 μm.
- 13Okada, M.; Sawazaki, T.; Watanabe, K.; Taniguch, T.; Hibino, H.; Shinohara, H.; Kitaura, R. Direct Chemical Vapor Deposition Growth of WS2 Atomic Layers on Hexagonal Boron Nitride. ACS Nano 2014, 8, 8273– 8277, DOI: 10.1021/nn503093k[ACS Full Text], [CAS], Google Scholar13https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht12gtLnM&md5=06f9f5db0b57645fc945e0fc777d155dDirect chemical vapor deposition growth of WS2 atomic layers on hexagonal boron nitrideOkada, Mitsuhiro; Sawazaki, Takumi; Watanabe, Kenji; Taniguch, Takashi; Hibino, Hiroki; Shinohara, Hisanori; Kitaura, RyoACS Nano (2014), 8 (8), 8273-8277CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Atomically thin transition metal dichalcogenides (TMDCs) have attracted considerable interest owing to the spin-valley coupled electronic structure and possibility in next-generation devices. Substrates are one of the most important factors to limit phys. properties of at.-layer materials, and among various substrates so far investigated, hexagonal boron nitride (hBN) is the best substrate to explore the intrinsic properties of at. layers. Here we report direct chem. vapor deposition (CVD) growth of WS2 onto high-quality hBN using a 3-furnace CVD setup. Triangular-shaped WS2 grown on hBN have shown limited crystallog. orientation that is related to that of the underlying hBN. Photoluminescence spectra of the WS2 show an intense emission peak at 2.01 eV with a quite small fwhm of 26 meV. The sharp emission peak indicates the high quality of the present WS2 at. layers with high crystallinity and clean interface.
- 14Kobayashi, Y.; Sasaki, S.; Mori, S.; Hibino, H.; Liu, Z.; Watanabe, K.; Taniguchi, T.; Suenaga, K.; Maniwa, Y.; Miyata, Y. Growth and Optical Properties of High-Quality Monolayer WS2 on Graphite. ACS Nano 2015, 9, 4056– 4063, DOI: 10.1021/acsnano.5b00103[ACS Full Text], [CAS], Google Scholar14https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXlt1Cls7c%253D&md5=c4b5455f1171913ae6886b39663e414eGrowth and Optical Properties of High-Quality Monolayer WS2 on GraphiteKobayashi, Yu; Sasaki, Shogo; Mori, Shohei; Hibino, Hiroki; Liu, Zheng; Watanabe, Kenji; Taniguchi, Takashi; Suenaga, Kazu; Maniwa, Yutaka; Miyata, YasumitsuACS Nano (2015), 9 (4), 4056-4063CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)At.-layer transition metal dichalcogenides (TMDCs) have attracted appreciable interest due to their tunable band gap, spin-valley physics, and potential device applications. The quality of TMDC samples available still poses serious problems, such as inhomogeneous lattice strain, charge doping, and structural defects. The growth of high-quality, monolayer WS2 onto exfoliated graphite by high-temp. CVD is reported. Monolayer-grown WS2 single crystals present a uniform, single excitonic luminescence peak with a Lorentzian profile and a very small full-width at half-max. of 21 meV at room temp. and 8 meV at 79 K. Also, in these samples, no addnl. peaks are obsd. for charged and/or bound excitons, even at low temp. These optical responses are completely different from the results of previously reported TMDCs obtained by mech. exfoliation and CVD. The combination of high-temp. CVD with a cleaved graphite surface is an ideal condition for the growth of high-quality TMDCs, and such samples will be essential for revealing intrinsic phys. properties and for future applications.
- 15Aljarb, A.; Cao, Z.; Tang, H.-L.; Huang, J.-K.; Li, M.; Hu, W.; Cavallo, L.; Li, L.-J. Substrate Lattice-Guided Seed Formation Controls the Orientation of 2D Transition-Metal Dichalcogenides. ACS Nano 2017, 11, 9215– 9222, DOI: 10.1021/acsnano.7b04323[ACS Full Text], [CAS], Google Scholar15https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXht1ylsrjK&md5=e63954216304b2aca824871b60ffb393Substrate lattice-guided seed formation controls the orientation of 2D transition-metal dichalcogenidesAljarb, Areej; Cao, Zhen; Tang, Hao-Ling; Huang, Jing-Kai; Li, Mengliu; Hu, Weijin; Cavallo, Luigi; Li, Lain-JongACS Nano (2017), 11 (9), 9215-9222CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Two-dimensional (2D) transition-metal dichalcogenide (TMDC) semiconductors are important for next-generation electronics and optoelectronics. Given the difficulty in growing large single crystals of 2D TMDC materials, understanding the factors affecting the seed formation and orientation becomes an important issue for controlling the growth. Here, we systematically study the growth of molybdenum disulfide (MoS2) monolayer on c-plane sapphire with chem. vapor deposition to discover the factors controlling their orientation. We show that the concn. of precursors, i.e., the ratio between sulfur and molybdenum oxide (MoO3), plays a key role in the size and orientation of seeds, subsequently controlling the orientation of MoS2 monolayers. High S/MoO3 ratio is needed in the early stage of growth to form small seeds that can align easily to the substrate lattice structures, while the ratio should be decreased to enlarge the size of the monolayer at the next stage of the lateral growth. Moreover, we show that the seeds are actually cryst. MoS2 layers as revealed by high-resoln. transmission electron microscopy. There exist two preferred orientations (0° or 60°) registered on sapphire, confirmed by our d. functional theory simulation. This report offers a facile technique to grow highly aligned 2D TMDCs and contributes to knowledge advancement in growth mechanism.
- 16Bignardi, L.; Lizzit, D.; Bana, H.; Travaglia, E.; Lacovig, P.; Sanders, C. E.; Dendzik, M.; Michiardi, M.; Bianchi, M.; Ewert, M.; Buß, L.; Falta, J.; Flege, J. I.; Baraldi, A.; Larciprete, R.; Hofmann, P.; Lizzit, S. Growth and Structure of Singly Oriented Single-Layer Tungsten Disulfide on Au(111). Phys. Rev. Mater. 2019, 3, 014003 DOI: 10.1103/PhysRevMaterials.3.014003[Crossref], [CAS], Google Scholar16https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXhtFGrsrfK&md5=654266d2cf11ff642fab0467450cc9fbGrowth and structure of singly oriented single-layer tungsten disulfide on Au(111)Bignardi, Luca; Lizzit, Daniel; Bana, Harsh; Travaglia, Elisabetta; Lacovig, Paolo; Sanders, Charlotte E.; Dendzik, Maciej; Michiardi, Matteo; Bianchi, Marco; Ewert, Moritz; Buss, Lars; Falta, Jens; Flege, Jan Ingo; Baraldi, Alessandro; Larciprete, Rosanna; Hofmann, Philip; Lizzit, SilvanoPhysical Review Materials (2019), 3 (1), 014003CODEN: PRMHBS; ISSN:2475-9953. (American Physical Society)A singly oriented, single layer of tungsten disulfide (WS2) was epitaxially grown on Au(111) and characterized at the nanoscale by combining photoelectron spectroscopy, photoelectron diffraction, and low-energy electron microscopy. Fast XPS revealed that the growth of a single cryst. orientation is triggered by choosing a low W evapn. rate and performing the process with a high temp. of the substrate. Information about the single orientation of the layer was obtained by acquiring x-ray photoelectron diffraction patterns, revealing a 1H polytype for the WS2 layer and, moreover, detg. the structural parameters and registry with the substrate. The distribution, size, and orientation of the WS2 layer were further ascertained by low-energy electron microscopy.
- 17Wang, S.; Wang, X.; Warner, J. H. All Chemical Vapor Deposition Growth of MoS2: h-BN Vertical van der Waals Heterostructures. ACS Nano 2015, 9, 5246– 5254, DOI: 10.1021/acsnano.5b00655[ACS Full Text], [CAS], Google Scholar17https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXmvVyntLg%253D&md5=66402ee0f3dabbc68605456feda7ba81All Chemical Vapor Deposition Growth of MoS2:h-BN Vertical van der Waals HeterostructuresWang, Shanshan; Wang, Xiaochen; Warner, Jamie H.ACS Nano (2015), 9 (5), 5246-5254CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Vertical van der Waals heterostructures are formed when different 2D crystals are stacked on top of each other. Improved optical properties arise in semiconducting transition metal dichalcogenide (TMD) 2D materials, such as MoS2, when they are stacked onto the insulating 2D hexagonal boron nitride (h-BN). Most work to date has required mech. exfoliation of at least one of the TMDs or h-BN materials to form these semiconductor:insulator structures. Here, we report a direct all-CVD process for the fabrication of high-quality monolayer MoS2:h-BN vertical heterostructured films with isolated MoS2 domains distributed across 1 cm. This is enabled by the use of few-layer h-BN films that are more robust against decompn. than monolayer h-BN during the MoS2 growth process. The MoS2 domains exhibit different growth dynamics on the h-BN surfaces compared to bare SiO2, confirming that there is strong interaction between the MoS2 and underlying h-BN. Raman and photoluminescence spectroscopies of CVD-grown MoS2 are compared to transferred MoS2 on both types of substrates, and our results show directly grown MoS2 on h-BN films have smaller lattice strain, lower doping level, cleaner and sharper interfaces, and high-quality interlayer contact.
- 18Pollmann, E.; Morbec, J. M.; Madauß, L.; Bröckers, L.; Kratzer, P.; Schleberger, M. Molybdenum Disulfide Nanoflakes Grown by Chemical Vapor Deposition on Graphite: Nucleation, Orientation, and Charge Transfer. J. Phys. Chem. C 2020, 124, 2689– 2697, DOI: 10.1021/acs.jpcc.9b10120
- 19Liu, X.; Balla, I.; Bergeron, H.; Campbell, G. P.; Bedzyk, M. J.; Hersam, M. C. Rotationally Commensurate Growth of MoS2 on Epitaxial Graphene. ACS Nano 2015, 10, 1067– 1075, DOI: 10.1021/acsnano.5b06398
- 20Liu, X.; Balla, I.; Bergeron, H.; Hersam, M. C. Point Defects and Grain Boundaries in Rotationally Commensurate MoS2 on Epitaxial Graphene. J. Phys. Chem. C 2016, 120, 20798– 20805, DOI: 10.1021/acs.jpcc.6b02073[ACS Full Text], [CAS], Google Scholar20https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xks1eiu7o%253D&md5=2028387051abdd1ab1a14fbf981490caPoint Defects and Grain Boundaries in Rotationally Commensurate MoS2 on Epitaxial GrapheneLiu, Xiaolong; Balla, Itamar; Bergeron, Hadallia; Hersam, Mark C.Journal of Physical Chemistry C (2016), 120 (37), 20798-20805CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)With reduced degrees of freedom, structural defects are expected to play a greater role in 2-dimensional materials in comparison to their bulk counterparts. Mech. strength, electronic properties, and chem. reactivity are strongly affected by crystal imperfections in the atomically thin limit. Ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) and spectroscopy (STS) are employed to interrogate point and line defects in monolayer MoS2 grown on epitaxial graphene (EG) at the at. scale. Five types of point defects are obsd. with the majority species showing apparent structures that are consistent with vacancy and interstitial models. The total defect d. is lower than MoS2 grown on other substrates and is likely attributed to the van der Waals epitaxy of MoS2 on EG. Grain boundaries (GBs) with 30 and 60° tilt angles resulting from the rotational commensurability of MoS2 on EG are more easily resolved by STM than at. force microscopy at similar scales due to the enhanced contrast from their distinct electronic states. For example, band gap redn. to ∼0.8 and ∼0.5 eV is obsd. with STS for 30 and 60° GBs, resp. At. resoln. STM images of these GBs agree with proposed structure models. This work offers quant. insight into the structure and properties of common defects in MoS2 and suggests pathways for tailoring the performance of MoS2/graphene heterostructures via defect engineering.
- 21Vlassiouk, I.; Smirnov, S.; Regmi, M.; Surwade, S. P.; Srivastava, N.; Feenstra, R.; Eres, G.; Parish, C.; Lavrik, N.; Datskos, P.; Dai, S.; Fulvio, P. Graphene Nucleation Density on Copper: Fundamental Role of Background Pressure. J. Phys. Chem. C 2013, 117, 18919– 18926[ACS Full Text], [CAS], Google Scholar21https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhtlSqtL%252FI&md5=9acab02b51539ef2bb75e8548222c441Graphene Nucleation Density on Copper: Fundamental Role of Background PressureVlassiouk, Ivan; Smirnov, Sergei; Regmi, Murari; Surwade, Sumedh P.; Srivastava, Nishtha; Feenstra, Randall; Eres, Gyula; Parish, Chad; Lavrik, Nick; Datskos, Panos; Dai, Sheng; Fulvio, PasqualeJournal of Physical Chemistry C (2013), 117 (37), 18919-18926CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)In this paper we discuss the effect of background pressure and synthesis temp. on the graphene crystal sizes in chem. vapor deposition (CVD) on copper catalyst. The authors quant. demonstrate a fundamental role of the background pressure and provide the activation energy for graphene nucleation in atm. pressure CVD (9 eV), which is substantially higher than for the low pressure CVD (4 eV). The authors attribute the difference to a greater importance of copper sublimation in the low pressure CVD, where severe copper evapn. likely dictates the desorption rate of active carbon from the surface. At atm. pressure, where copper evapn. is suppressed, the activation energy is assigned to the desorption energy of carbon clusters instead. The highest possible temp., close to the m.p. of copper, should be used for large single crystal graphene synthesis. Using these conditions, the authors have synthesized graphene single crystals with sizes over 0.5 mm. Single crystal nature of synthesized graphene was confirmed by LEED. The authors also demonstrate that CVD of graphene at temps. below 1000 °C shows higher nucleation d. on (111) than on (100) and (101) copper surfaces, but there is no identifiable preference at higher temps.
- 22Stehle, Y.; Meyer, H. M.; Unocic, R. R.; Kidder, M.; Polizos, G.; Datskos, P. G.; Jackson, R.; Smirnov, S. N.; Vlassiouk, I. V. Synthesis of Hexagonal Boron Nitride Monolayer: Control of Nucleation and Crystal Morphology. Chem. Mater. 2015, 27, 8041– 8047, DOI: 10.1021/acs.chemmater.5b03607[ACS Full Text], [CAS], Google Scholar22https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhsl2ht7nL&md5=5da09c8eb6da825ad863243caa23da05Synthesis of Hexagonal Boron Nitride Monolayer: Control of Nucleation and Crystal MorphologyStehle, Yijing; Meyer, Harry M.; Unocic, Raymond R.; Kidder, Michelle; Polizos, Georgios; Datskos, Panos G.; Jackson, Roderick; Smirnov, Sergei N.; Vlassiouk, Ivan V.Chemistry of Materials (2015), 27 (23), 8041-8047CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Monolayer hexagonal B nitride (hBN) attracts significant attention due to the potential to be used as a complementary two-dimensional dielec. in fabrication of functional 2-dimensional heterostructures. The authors study the growth stages of the hBN single crystals and show that hBN crystals change their shape from triangular to truncated triangular and further to hexagonal depending on Cu substrate distance from the precursor. Probably the obsd. hBN crystal shape variation is affected by the ratio of B to N active species concns. on the Cu surface inside the CVD reactor. Strong temp. dependence reveals the activation energies for the hBN nucleation process of ∼5 eV and crystal growth of ∼3.5 eV. Also the resulting h-BN film morphol. is strongly affected by the heating method of borazane precursor and the buffer gas. Elucidation of these details facilitated synthesis of high quality large area monolayer hexagonal B nitride by atm. pressure CVD on Cu using borazane as a precursor.
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- 24Okada, M.; Kutana, A.; Kureishi, Y.; Kobayashi, Y.; Saito, Y.; Saito, T.; Watanabe, K.; Taniguchi, T.; Gupta, S.; Miyata, Y.; Yakobson, B. I.; Shinohara, H.; Kitaura, R. Direct and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS2/MoS2/hBN. ACS Nano 2018, 12, 2498– 2505, DOI: 10.1021/acsnano.7b08253[ACS Full Text], [CAS], Google Scholar24https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXjtlOmsbo%253D&md5=c6aa6a3534087d0377c455895d69adbcDirect and Indirect Interlayer Excitons in a van der Waals Heterostructure of hBN/WS2/MoS2/hBNOkada, Mitsuhiro; Kutana, Alex; Kureishi, Yusuke; Kobayashi, Yu; Saito, Yuika; Saito, Tetsuki; Watanabe, Kenji; Taniguchi, Takashi; Gupta, Sunny; Miyata, Yasumitsu; Yakobson, Boris I.; Shinohara, Hisanori; Kitaura, RyoACS Nano (2018), 12 (3), 2498-2505CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)A van der Waals (vdW) heterostructure composed of multivalley systems can show excitonic optical responses from interlayer excitons that originate from several valleys in the electronic structure. Luminescence (PL) from a vdW heterostructure, WS2/MoS2, deposited on hexagonal BN (hBN) flakes was studied. PL spectra from the fabricated heterostructures obsd. at room temp. show PL peaks at 1.3-1.7 eV, which are absent in the PL spectra of WS2 or MoS2 monolayers alone. The low-energy PL peaks the authors obsd. can be decompd. into 3 distinct peaks. Through detailed PL measurements and theor. anal., including PL imaging, time-resolved PL measurements, and calcn. of dielec. function ε(ω) by solving the Bethe-Salpeter equation with G0W0, the 3 PL peaks originate from direct K-K interlayer excitons, indirect Q-Γ interlayer excitons, and indirect K-Γ interlayer excitons.
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- 27Morikawa, Y.; Iwata, K.; Terakura, K. Theoretical Study of Hydrogenation Process of Formate on Clean and Zn Deposited Cu(111) Surfaces. Appl. Surf. Sci. 2001, 169-170, 11– 15, DOI: 10.1016/S0169-4332(00)00631-0[Crossref], [CAS], Google Scholar27https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD3MXhtFKlsL8%253D&md5=b79e2e6c93fdf6b5c27610930aa72e26Theoretical study of hydrogenation process of formate on clean and Zn deposited Cu(1 1 1) surfacesMorikawa, Y.; Iwata, K.; Terakura, K.Applied Surface Science (2001), 169-170 (), 11-15CODEN: ASUSEE; ISSN:0169-4332. (Elsevier Science B.V.)We have studied the effect of Zn on hydrogenation of formate to dioxymethylene on the Cu(1 1 1) surface by a d. functional theory-generalized gradient approxn. (DFT-GGA)-pseudopotential method. Substitutionally adsorbed Zn changes the stability of intermediate states and the activation barrier of the hydrogenation process only slightly. On the other hand, the Zn atom adsorbed on the Cu surface stabilizes all formate, transition state, and dioxymethylene relative to the gas-phase mols. The results support a previously proposed reaction scheme that the adsorption state of Zn changes from substitutional to on-surface adsorption during the methanol synthesis.
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- 30Lee, K.; Murray, É. D.; Kong, L.; Lundqvist, B. I.; Langreth, D. C. Higher-Accuracy van der Waals Density Functional. Phys. Rev. B 2010, 82, 081101 DOI: 10.1103/PhysRevB.82.081101[Crossref], [CAS], Google Scholar30https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXhtFWlurnN&md5=21a4573d23bb686441ae1609cb4e6809Higher-accuracy van der Waals density functionalLee, Kyuho; Murray, Eamonn D.; Kong, Lingzhu; Lundqvist, Bengt I.; Langreth, David C.Physical Review B: Condensed Matter and Materials Physics (2010), 82 (8), 081101/1-081101/4CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)We propose a second version of the van der Waals d. functional of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate semilocal exchange functional and the use of a large-N asymptote gradient correction in detg. the vdW kernel. The predicted binding energy, equil. sepn., and potential-energy curve shape are close to those of accurate quantum chem. calcns. on 22 duplexes. We anticipate the enabling of chem. accurate calcns. in sparse materials of importance for condensed matter, surface, chem., and biol. physics.
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- 33Otani, M.; Sugino, O. First-Principles Calculations of Charged Surfaces and Interfaces: A Plane-Wave Nonrepeated Slab Approach. Phys. Rev. B 2006, 73, 115407, DOI: 10.1103/PhysRevB.73.115407[Crossref], [CAS], Google Scholar33https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XjsV2rs7s%253D&md5=c71d929e8fdf09c5e14d8d8bab8260e1First-principles calculations of charged surfaces and interfaces: A plane-wave nonrepeated slab approachOtani, M.; Sugino, O.Physical Review B: Condensed Matter and Materials Physics (2006), 73 (11), 115407/1-115407/11CODEN: PRBMDO; ISSN:1098-0121. (American Physical Society)A new first-principles computational approach to a charged surface/interface is presented. The surface is modeled as a slab imposed with boundary conditions to screen the excess surface charge. To treat this model, which is nonperiodic in the surface normal direction, a std. pseudopotential plane-wave scheme is modified at the Poisson solver part with the help of the Green's function technique. Benchmark calcns. are done for Al/Si(111) with the bias voltage applied between the surface and the model scanning tunneling microscopy (STM) tip, the model back gate, or the model soln. The calcns. are found to be efficient and stable, and their implementation is found to be easy. Because of the flexibility, the scheme is considered to be applicable to more general exptl. situations.
- 34Taniguchi, T.; Watanabe, K. Synthesis of High-Purity Boron Nitride Single Crystals under High Pressure by Using Ba–BN Solvent. J. Cryst. Growth 2007, 303, 525– 529, DOI: 10.1016/j.jcrysgro.2006.12.061[Crossref], [CAS], Google Scholar34https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXksl2gtL4%253D&md5=89a6f05bbe1adc1ab83469c96b60c575Synthesis of high-purity boron nitride single crystals under high pressure by using Ba-BN solventTaniguchi, T.; Watanabe, K.Journal of Crystal Growth (2007), 303 (2), 525-529CODEN: JCRGAE; ISSN:0022-0248. (Elsevier B.V.)High-purity cubic boron nitride (cBN) and hexagonal boron nitride (hBN) single crystals were synthesized at 4.5 GPa and 1500 °C using barium boron nitride as a solvent. Secondary ion mass spectrometry was used to analyze impurities in the crystals. Fine cBN and hBN crystals, whose oxygen and carbon concns. were <1018 atoms/cm3, were obtained, and their band-edge optical properties were measured by cathodoluminescence spectroscopy. High-purity hBN single crystals exhibited intense UV emission, demonstrating their promise for use as deep UV-light emitters.
- 35Wang, S.; Rong, Y.; Fan, Y.; Pacios, M.; Bhaskaran, H.; He, K.; Warner, J. H. Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor Deposition. Chem. Mater. 2014, 26, 6371– 6379, DOI: 10.1021/cm5025662[ACS Full Text], [CAS], Google Scholar35https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvVGlsbzE&md5=8f5503c1db3a41197f6ba69aa5545972Shape Evolution of Monolayer MoS2 Crystals Grown by Chemical Vapor DepositionWang, Shanshan; Rong, Youmin; Fan, Ye; Pacios, Merce; Bhaskaran, Harish; He, Kuang; Warner, Jamie H.Chemistry of Materials (2014), 26 (22), 6371-6379CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)Atm.-pressure CVD was used to grow monolayer MoS2 two-dimensional crystals at elevated temps. on silicon substrates with a 300. nm oxide layer. The authors' CVD reaction is hydrogen free, with the sulfur precursor placed in a furnace sep. from the MoO3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. The authors intentionally establish a sharp gradient of MoO3 precursor concn. on the growth substrate to explore its sensitivity to the resultant MoS2 domain growth within a relatively uniform temp. range. The shape of MoS2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve the authors' understanding of the factors that influence the growth of MoS2 domains and their shape evolution.
- 36Terrones, H.; Del Corro, E.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A.; Elías, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M. New First Order Raman-Active Modes in Few Layered Transition Metal Dichalcogenides. Sci. Rep. 2014, 4, 4215[Crossref], [PubMed], [CAS], Google Scholar36https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXht1WjurzK&md5=de104025c96fe5a282568df3b385e2a9New first order raman-active modes in few layered transition metal dichalcogenidesTerrones, H.; Del Corro, E.; Feng, S.; Poumirol, J. M.; Rhodes, D.; Smirnov, D.; Pradhan, N. R.; Lin, Z.; Nguyen, M. A. T.; Elias, A. L.; Mallouk, T. E.; Balicas, L.; Pimenta, M. A.; Terrones, M.Scientific Reports (2014), 4 (), 4215/1-4215/9CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)Although the main Raman features of semiconducting transition metal dichalcogenides are well known for the monolayer and bulk, there are important differences exhibited by few layered systems which have not been fully addressed. WSe2 samples were synthesized and ab-initio calcns. carried out. We calcd. phonon dispersions and Raman-active modes in layered systems: WSe2, MoSe2, WS2 and MoS2 ranging from monolayers to five-layers and the bulk. First, we confirmed that as the no. of layers increase, the E', E" and E2g modes shift to lower frequencies, and the A'1 and A1g modes shift to higher frequencies. Second, new high frequency first order A'1 and A1g modes appear, explaining recently reported exptl. data for WSe2, MoSe2 and MoS2. Third, splitting of modes around A'1 and A1g is found which explains those obsd. in MoSe2. Finally, exterior and interior layers possess different vibrational frequencies. Therefore, it is now possible to precisely identify few-layered STMD.
- 37Wu, S.; Huang, C.; Aivazian, G.; Ross, J. S.; Cobden, D. H.; Xu, X. Vapor-Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley Polarization. ACS Nano 2013, 7, 2768– 2772, DOI: 10.1021/nn4002038[ACS Full Text], [CAS], Google Scholar37https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXivVyisbY%253D&md5=00349bba3010813d8e9cf4f351752cdbVapor-Solid Growth of High Optical Quality MoS2 Monolayers with Near-Unity Valley PolarizationWu, Sanfeng; Huang, Chunming; Aivazian, Grant; Ross, Jason S.; Cobden, David H.; Xu, XiaodongACS Nano (2013), 7 (3), 2768-2772CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Monolayers of transition metal dichalcogenides (TMDCs) are atomically thin direct-gap semiconductors with potential applications in nanoelectronics, optoelectronics, and electrochem. sensing. Recent theor. and exptl. efforts suggest that they are ideal systems for exploiting the valley degrees of freedom of Bloch electrons. For example, Dirac valley polarization was demonstrated in mech. exfoliated monolayer MoS2 samples by polarization-resolved photoluminescence, although polarization has rarely been seen at room temp. Here the authors report a new method for synthesizing high optical quality monolayer MoS2 single crystals up to 25 μm in size on a variety of std. insulating substrates (SiO2, sapphire, and glass) using a catalyst-free vapor-solid growth mechanism. The technique is simple and reliable, and the optical quality of the crystals is extremely high, as demonstrated by the fact that the valley polarization approaches unity at 30 K and persists at 35% even at room temp., suggesting a virtual absence of defects. This will allow greatly improved optoelectronic TMDC monolayer devices to be fabricated and studied routinely.
- 38Gong, Y.; Lin, Z.; Ye, G.; Shi, G.; Feng, S.; Lei, Y.; Elías, A. L.; Perea-Lopez, N.; Vajtai, R.; Terrones, H.; Liu, Z.; Terrones, M.; Ajayan, P. M. Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2 Monolayers. ACS Nano 2015, 9, 11658– 11666, DOI: 10.1021/acsnano.5b05594[ACS Full Text], [CAS], Google Scholar38https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhslWjs73L&md5=a662d87448c4b2dc095b48c163287538Tellurium-Assisted Low-Temperature Synthesis of MoS2 and WS2 MonolayersGong, Yongji; Lin, Zhong; Ye, Gonglan; Shi, Gang; Feng, Simin; Lei, Yu; Elias, Ana Laura; Perea-Lopez, Nestor; Vajtai, Robert; Terrones, Humberto; Liu, Zheng; Terrones, Mauricio; Ajayan, Pulickel M.ACS Nano (2015), 9 (12), 11658-11666CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)Chem. vapor deposition (CVD) is a scalable method able to synthesize MoS2 and WS2 monolayers. In this work, we reduced the synthesis temp. by 200°C by introducing tellurium (Te) into the CVD process. The as-synthesized MoS2 and WS2 monolayers show high phase purity and crystallinity. The optical and elec. performance of these materials is comparable to those synthesized at higher temps. We believe this work will accelerate the industrial synthesis of these semiconducting monolayers.
- 39Ling, X.; Lee, Y. H.; Lin, Y.; Fang, W.; Yu, L.; Dresselhaus, M. S.; Kong, J. Role of the Seeding Promoter in MoS2 growth by Chemical Vapor Deposition. Nano Lett. 2014, 14, 464– 472, DOI: 10.1021/nl4033704[ACS Full Text], [CAS], Google Scholar39https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhsVOjtrg%253D&md5=4c39b50e8bbadb22be52d267915e064aRole of the Seeding Promoter in MoS2 Growth by Chemical Vapor DepositionLing, Xi; Lee, Yi-Hsien; Lin, Yuxuan; Fang, Wenjing; Yu, Lili; Dresselhaus, Mildred S.; Kong, JingNano Letters (2014), 14 (2), 464-472CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)The thinnest semiconductor, molybdenum disulfide (MoS2) monolayer, exhibits promising prospects in the applications of optoelectronics and valleytronics. A uniform and highly cryst. MoS2 monolayer in a large area is highly desirable for both fundamental studies and substantial applications. Here, using various arom. mols. as seeding promoters, a large-area, highly cryst., and uniform MoS2 monolayer was achieved with CVD at a relatively low growth temp. (650°). The dependence of the growth results on the seed concn. and on the use of different seeding promoters is further studied. Also an optimized concn. of seed mols. is helpful for the nucleation of the MoS2. The newly identified seed mols. can be easily deposited on various substrates and allows the direct growth of monolayer MoS2 on Au, hexagonal boron nitride (h-BN), and graphene to achieve various hybrid structures.
- 40Zhang, F.; Wang, Y.; Erb, C.; Wang, K.; Moradifar, P.; Crespi, V. H.; Alem, N. Full Orientation Control of Epitaxial MoS2 on hBN Assisted by Substrate Defects. Phys. Rev. B 2019, 99, 155430, DOI: 10.1103/PhysRevB.99.155430[Crossref], [CAS], Google Scholar40https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXpslWhsL4%253D&md5=9107da3734433c186a0d22e61ee8646bFull orientation control of epitaxial MoS2 on hBN assisted by substrate defectsZhang, Fu; Wang, Yuanxi; Erb, Chad; Wang, Ke; Moradifar, Parivash; Crespi, Vincent H.; Alem, NasimPhysical Review B (2019), 99 (15), 155430CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)Inversion asymmetry in two-dimensional materials grants them fascinating properties such as spin-coupled valley degrees of freedom and piezoelectricity, but at the cost of inversion domain boundaries if the epitaxy of the grown two-dimensional (2D) layer, on a polar substrate, cannot adequately distinguish what are often near-degenerate 0° and 180° orientations. We employ first-principles calcns. to identify a method to lift this near degeneracy: the energetic distinction between eclipsed and staggered configurations during nucleation at a point defect in the substrate. For monolayer MoS2 grown on hexagonal boron nitride, the predicted defect complex can be more stable than common MoS2 point defects because it is both a donor-acceptor pair and a Frenkel pair shared between adjacent layers of a 2D heterostack. Orientation control is verified in expts. that achieve ∼90% consistency in the orientation of as-grown triangular MoS2 flakes on hBN, as confirmed by aberration-cor. scanning/transmission electron microscopy. This defect-enhanced orientational epitaxy could provide a general mechanism to break the near-degeneracy of 0/180° orientations of polar 2D materials on polar substrates, overcoming a long-standing impediment to scalable synthesis of single-crystal 2D semiconductors.
- 41Varshney, V.; Patnaik, S. S.; Muratore, C.; Roy, A. K.; Voevodin, A. A.; Farmer, B. L. MD Simulations of Molybdenum Disulphide (MoS2): Force-Field Parameterization and Thermal Transport Behavior. Comput. Mater. Sci. 2010, 48, 101– 108, DOI: 10.1016/j.commatsci.2009.12.009[Crossref], [CAS], Google Scholar41https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3cXitlegsL8%253D&md5=6ff1f4336ada2b48d0cc4663373c08b5MD simulations of molybdenum disulphide (MoS2): force-field parameterization and thermal transport behaviorVarshney, Vikas; Patnaik, Soumya S.; Muratore, Chris; Roy, Ajit K.; Voevodin, Andrey A.; Farmer, Barry L.Computational Materials Science (2010), 48 (1), 101-108CODEN: CMMSEM; ISSN:0927-0256. (Elsevier B.V.)In this article, we have investigated the anisotropic nature of thermal transport in molybdenum disulfide using mol. dynamics simulations. At first, a force field has been validated with respect to crystal structure and exptl. vibrational spectra of MoS2. Thereafter, non-equil. MD simulations have been performed in two perpendicular directions (along as well as across the basal planes) to study thermal transport behavior. At room temp., our results show an anisotropic factor of ∼4 in the values of thermal cond. along two studied directions, which is in good agreement with recent expts. on MoS2 thin films. However, the predicted values of thermal cond. are about an order of magnitude higher with respect to expts. The reasoning behind these differences has been discussed in terms of layer disorder and the large no. of grain boundary interfaces in exptl. thin films, which consisted of nano-cryst. MoS2 grains with a predominant parallel or perpendicular basal plane orientation. Incorporation of phonon scattering via structure disorder and boundary interfaces were identified as further directions for the model refinement.
- 42Won, C. Y.; Aluru, N. R. Water Permeation through a Subnanometer Boron Nitride Nanotube. J. Am. Chem. Soc. 2007, 129, 2748– 2749, DOI: 10.1021/ja0687318[ACS Full Text], [CAS], Google Scholar42https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2sXhvVWjurY%253D&md5=f8bee49e0a9fdb7f097b00470b11221aWater Permeation through a Subnanometer Boron Nitride NanotubeWon, Chang Y.; Aluru, N. R.Journal of the American Chemical Society (2007), 129 (10), 2748-2749CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)B nitride nanotubes (BNNTs) possess many excellent phys. properties, including thermal and mech. properties. The authors report on superior H2O permeation properties of BNNTs. Specifically, using mol. dynamics simulations, H2O mols. permeate through the (5,5) B nitride nanotube, while a (5,5) C nanotube (CNT) of approx. the same diam. does not conduct H2O. The relatively strong interactions between the nitride atoms of the BNNT and H2O mols. play a key role in the continuous wetting behavior of the BNNT. The properties of H2O, such as the axial diffusion coeff. and the hydrogen bonding, inside the (5,5) BNNT are comparable to those inside the (6,6) CNT, even though the diam. of the (5,5) BNNT is ∼1.3 Å smaller than that of the (6,6) CNT.
- 43Yankowitz, M.; Xue, J.; Cormode, D.; Sanchez-Yamagishi, J. D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, P.; Jacquod, P.; LeRoy, B. J. Emergence of Superlattice Dirac Points in Graphene on Hexagonal Boron Nitride. Nat. Phys. 2012, 8, 382, DOI: 10.1038/nphys2272[Crossref], [CAS], Google Scholar43https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XksVegtLo%253D&md5=5a462366ee8201e802b62be2ec86f600Emergence of superlattice Dirac points in graphene on hexagonal boron nitrideYankowitz, Matthew; Xue, Jiamin; Cormode, Daniel; Sanchez-Yamagishi, Javier D.; Watanabe, K.; Taniguchi, T.; Jarillo-Herrero, Pablo; Jacquod, Philippe; LeRoy, Brian J.Nature Physics (2012), 8 (5), 382-386CODEN: NPAHAX; ISSN:1745-2473. (Nature Publishing Group)The Schroedinger equation dictates that the propagation of nearly free electrons through a weak periodic potential results in the opening of bandgaps near points of the reciprocal lattice known as Brillouin zone boundaries. However, in the case of massless Dirac fermions, it has been predicted that the chirality of the charge carriers prevents the opening of a bandgap and instead new Dirac points appear in the electronic structure of the material. Graphene on hexagonal boron nitride exhibits a rotation-dependent moire pattern. Here, we show exptl. and theor. that this moire pattern acts as a weak periodic potential and thereby leads to the emergence of a new set of Dirac points at an energy detd. by its wavelength. The new massless Dirac fermions generated at these superlattice Dirac points are characterized by a significantly reduced Fermi velocity. Furthermore, the local d. of states near these Dirac cones exhibits hexagonal modulation due to the influence of the periodic potential.
- 44Dienwiebel, M.; Verhoeven, G. S.; Pradeep, N.; Frenken, J. W. M.; Heimberg, J. A.; Zandbergen, H. W. Superlubricity of Graphite. Phys. Rev. Lett. 2004, 92, 126101, DOI: 10.1103/PhysRevLett.92.126101[Crossref], [PubMed], [CAS], Google Scholar44https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2cXis1Wrt7o%253D&md5=2a61504146d5881e9d4469a148752efaSuperlubricity of GraphiteDienwiebel, Martin; Verhoeven, Gertjan S.; Pradeep, Namboodiri; Frenken, Joost W. M.; Heimberg, Jennifer A.; Zandbergen, Henny W.Physical Review Letters (2004), 92 (12), 126101/1-126101/4CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)Using a home-built frictional force microscope that is able to detect forces in three dimensions with a lateral force resoln. down to 15 pN, the authors have studied the energy dissipation between a tungsten tip sliding over a graphite surface in dry contact. By measuring at.-scale friction as a function of the rotational angle between two contacting bodies, the origin of the ultralow friction of graphite lies in the incommensurability between rotated graphite layers, an effect proposed under the name of superlubricity [M. Hirano and K. Shinjo, (1990)].
- 45Wang, W.; Shen, J.; He, Q. C. Microscale Superlubricity of Graphite under Various Twist Angles. Phys. Rev. B 2019, 99, 054103 DOI: 10.1103/PhysRevB.99.054103[Crossref], [CAS], Google Scholar45https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1MXnvFWhtbg%253D&md5=4f151b083bbbde205ee1df856b288d82Microscale superlubricity of graphite under various twist anglesWang, Wen; Shen, Jian; He, Q.-C.Physical Review B (2019), 99 (5), 054103CODEN: PRBHB7; ISSN:2469-9969. (American Physical Society)The relative rotation and translation of graphene and graphite layers lead to remarkable phys. and mech. phenomena. One such phenomenon for graphite corresponds to the ultralow static and dynamic friction between incommensurate graphene layers, referred to as superlubricity. Even though many studies have been dedicated to this promising phenomenon in recent years, an exptl. characterization and a quant. detn. of the effect of relative twist angles on microscale superlubricity are still lacking. The present paper investigates the superlubric properties of microscale graphite under different twist angels by shearing graphite with respect to a substrate. Exptl., it is surprisingly found that the superlubricity of microscale graphite is almost invariant within a wide range of bicrystal twist angles (6o≤θ≤59o). This result is confirmed by carrying out mol. dynamics simulations. Further, the influences of twist angles and normal load on the incommensurate-to-commensurate transition are revealed. The estd. crit. transition angle is less than 0.1 °. These results allow a better understanding of mesoscopic scale superlubricity and extend its application field.
Supporting Information
ARTICLE SECTIONSThe Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04168.
AFM image and the corresponding height profile of MoS2/hBN (Figure S1), an optical image of MoS2/hBN (Figure S2), stacking-angle dependent total energy of the MoS2 cluster using LJ potential (Figure S3), a schematic of the method for calculating interaction energy (Figure S4), schematic images of the stable MoS2/hBN structure at a stacking angle of 0° (Figure S5), element-decomposed interlayer energy mapping of Mo48S96 (Figure S6), an in-plane interaction-energy map of a Mo768S1536 cluster (an edge length of 5.1 nm) with a stacking angle of 0° (Figure S7), list of n and the corresponding number of Mo atoms, edge length, and the most stable center position of MoS2 clusters at a stacking angle of 0° (Table S1), additional discussion on the effect of the MoS cluster shape in the vdW epitaxy, and LJ result on triangular-shaped MoS clusters (Figure S8) (PDF)
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