Letter pubs.acs.org/NanoLett
van der Waals Epitaxial Growth of Atomically Thin Bi2Se3 and Thickness-Dependent Topological Phase Transition Shuigang Xu,† Yu Han,† Xiaolong Chen,† Zefei Wu,† Lin Wang,†,‡ Tianyi Han,† Weiguang Ye,† Huanhuan Lu,† Gen Long,† Yingying Wu,† Jiangxiazi Lin,† Yuan Cai,† K. M. Ho,† Yuheng He,† and Ning Wang*,† †
Department of Physics and the William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China ‡ Department of Condensed Matter Physics, University of Geneva, 24 Quai Ernest Ansermet, CH1211 Geneva, Switzerland S Supporting Information *
ABSTRACT: Two-dimensional (2D) atomic-layered heterostructures stacked by van der Waals interactions recently introduced new research fields, which revealed novel phenomena and provided promising applications for electronic, optical, and optoelectronic devices. In this study, we report the van der Waals epitaxial growth of high-quality atomically thin Bi2Se3 on single crystalline hexagonal boron nitride (h-BN) by chemical vapor deposition. Although the inplane lattice mismatch between Bi 2 Se 3 and h-BN is approximately 65%, our transmission electron microscopy analysis revealed that Bi2Se3 single crystals epitaxially grew on hBN with two commensurate states; that is, the (1̅21̅0) plane of Bi2Se3 was preferably parallel to the (1̅100) or (1̅21̅0) plane of hBN. In the case of the Bi2Se3 (2̅110) ∥ h-BN (11̅00) state, the Moiré pattern wavelength in the Bi2Se3/h-BN superlattice can reach 5.47 nm. These naturally formed thin crystals facilitated the direct assembly of h-BN/Bi2Se3/h-BN sandwiched heterostructures without introducing any impurity at the interfaces for electronic property characterization. Our quantum capacitance (QC) measurements showed a compelling phenomenon of thickness-dependent topological phase transition, which was attributed to the coupling effects of two surface states from Dirac Fermions at/or above six quintuple layers (QLs) to gapped Dirac Fermions below six QLs. Moreover, in ultrathin Bi2Se3 (e.g., 3 QLs), we observed the midgap states induced by intrinsic defects at cryogenic temperatures. Our results demonstrated that QC measurements based on h-BN/Bi2Se3/h-BN sandwiched structures provided rich information regarding the density of states of Bi2Se3, such as quantum well states and Landau quantization. Our approach in fabricating h-BN/Bi2Se3/h-BN sandwiched device structures through the combination of bottom− up growth and top−down dry transferring techniques can be extended to other two-dimensional layered heterostructures. KEYWORDS: Topological insulators, van der Waals epitaxy, Moiré pattern, quantum capacitance, midgap states, topological phase transition
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beam epitaxy, are more efficient methods for controlling the thickness of TIs.4,7 However, selecting an appropriate substrate has become extremely critical because surface and interface degradation8 and external strain9 effects often influence the intrinsic properties of TIs grown by bottom−up processes. Single crystalline hexagonal boron nitride (h-BN) is an excellent dielectric material and substrate for fabricating twodimensional (2D) devices with atomically thin structures. Owing to its atomically flat surface and lack of charged impurities,10−12 h-BN has facilitated investigations on graphene intrinsic properties, such as extremely high carrier mobility,10,13 fractional quantum hall effects,14 and Hofstadter’s butterfly15,16 behavior of electrons. Epitaxial growth of single-domain
topological insulator (TI) displays a new state of quantum matter with insulating bulk and nontrivial gapless states on its surface because of strong spin−orbit coupling.1 In experiments, observation of the exotic surface phenomena of TIs (e.g., quantum spin hall effects, Majorana Fermions, and topological superconductors1,2) has largely relied on the insulating state of the bulk component. As a result of unintentionally induced doping effects, for example, in Bi2Se3, Bi2Te3, Sb2Te3,3,4 residual bulk conductance invariably overpowers surface conductance.5,6 One of the effective methods to minimize bulk conductance involves fabricating atomically thin TIs by mechanical exfoliation technique, which has been extensively employed for sample preparation of graphene and transition metal dichalcogenides. However, the brittleness of TIs limits the preparation of ultrathin flakes by mechanical exfoliation.5 In comparison, bottom−up synthesis processes, such as chemical vapor deposition and molecular© XXXX American Chemical Society
Received: January 21, 2015 Revised: February 22, 2015
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DOI: 10.1021/acs.nanolett.5b00247 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Schematics of the fabrication process of van der Waals h-BN/Bi2Se3/h-BN heterostructures. (a) van der Waals epitaxy growth of Bi2Se3 on h-BN. (b) Atomic structure of Bi2Se3. (c) Mechanical transfer of the top h-BN to as-grown Bi2Se3/h-BN. (d) Perspective view of the fabricated h-BN encapsulated Bi2Se3. (e) Final device for capacitance measurements.
Figure 2. Optical images of van der Waals grown Bi2Se3 on h-BN. (a) A typical image of the as-grown sample. Bi2Se3 mainly presents two kinds of alignments on h-BN, as marked by A and B. Rotating A by 30° turns into B. The regular edges (armchair and zigzag) of exfoliated h-BN are also marked by dash lines. (b) Optical contrasts of two QL Bi2Se3 on h-BN substrates with various thicknesses. (c−k) Optical images of three QLs to fifteen QLs on h-BN. The scalar bars in panels b to k are all 5 μm.
graphene11 and related 2D materials on h-BN thin flakes have been realized recently. Bi2Te2Se nanoplates grown on h-BN through van der Waals epitaxial methods showed increased mobility of surface state carriers.17 Our current investigation has shown that few-layer Bi 2 Se 3 (each unit layer is approximately 1 nm thick and called a quintuple layer (QL), which consists of five covalently bonded sheets as shown in Figure 1b) presents interesting van der Waals epitaxial growth behavior on h-BN. By transferring an atomically thin h-BN (as the dielectric layer without introducing charged trapping centers18,19) precisely on the as-grown Bi2Se3 to form a capacitance device with the Bi2Se3 encapsulated, we successfully probed the electronic properties of Bi2Se3 thin layers through the quantum capacitance (QC) measurement technique. A noteworthy transition has been observed from gapless to gap structures of Dirac Fermions at six QLs. Below six QLs, an energy gap emerged due to the interaction between the bottom and top surfaces of Bi2Se3. At six QLs, the gap closes and the Dirac point emerges. Notably, midgap states in the Dirac gap
emerge and override the surface states at high temperatures for Bi2Se3 samples containing sufficient intrinsic defects. Comparatively, Landau quantization is observed in the surface states under different magnetic fields for the sample with few defects. Few-layer Bi2Se3 was grown on mechanically exfoliated h-BN by catalyst-free chemical vapor deposition.7 Bi2Se3 possesses a layered rhombohedral crystal structure. The neighboring layers are weakly bonded through van der Waals interaction. Therefore, the van der Waals epitaxy growth provides the possibility of growing strain-free Bi2Se3/h-BN heterostructures despite the large in-plane lattice mismatch between these two structures. Figure 2 shows the typical optical images of our asgrown Bi2Se3 on h-BN. Few-layer Bi2Se3 formed on h-BN normally have triangle or hexagonal shapes. Virtually no visible Bi2Se3 formed directly on the SiO2 substrate because its growth rate on SiO2 is extremely low.7 The thickness of the as-grown Bi2Se3 is controlled by growth duration. The optical contrasts of the samples with different thicknesses (from 2 to 15 QLs, as confirmed by atomic force microscopy (AFM)) are shown in B
DOI: 10.1021/acs.nanolett.5b00247 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. TEM analysis of van der Waals grown Bi2Se3 on h-BN. (a) A typical real-space image. The bare h-BN is marked with P1. Bi2Se3 samples marked with P2 and P3 present two kinds of alignments that are coincident with the optical images. The indices of the regular edges of h-BN (marked with dash lines) are obtained from SAED. Detailed characterizations of positions P1, P2, and P3 are shown in panels b−i, where panel d is for P1, panels b, e, and h are for P2, and panels c, f, g, and i are for P3. Panels b−d are the corresponding SAED. Panels e and f are the corresponding HRTEM images, where two kinds of Moiré patterns can be clearly observed. Panels h and i are the corresponding simulated Moiré patterns. The red vectors denote the superlattice structures. The image sizes are 5 nm × 5 nm for panel h and 15 nm × 15 nm for panel i. Panel g is the dark-field image when an objective aperture is applied at the yellow circle in panel c. Atomic structure configurations are illustrated in panel j for sample P2 and panel k for sample P3.
contrast, all Bi2Se3 thin layers are terminated with the {1̅100} planes, indicating that in-plane growth along the {11̅ 00} planes with low surface energy is favorable. The two precise alignments of Bi2Se3 have been identified from the SAED patterns in Figure 3b,c. The inner diffraction spots in Figure 3b are attributed to the double diffraction effect (see Figure S6 in Supporting Information). For sample P2, the (1̅21̅0) plane of Bi2Se3 rotates for exactly 30° with respect to the (011̅0) plane of h-BN (Figure 3b). Evidently, the crystallographic epitaxial relations between Bi2Se3 and h-BN are Bi2Se3 (11̅00) ∥ h-BN (110̅ 0) and Bi2Se3(0001) ∥ h-BN(0001). For sample P3, the (1̅21̅0) plane of Bi2Se3 is parallel to the (1̅100) plane of h-BN. In this case, the epitaxial relations are Bi2Se3 (2̅110) ∥ h-BN (11̅00) and Bi2Se3(0001) ∥ h-BN(0001). The Moiré patterns formed by the two stacking crystals are shown in the high-resolution TEM (HRTEM) images in Figure 3e,f, which match fairly well with both simulated patterns (Figure 3h,i) and SAED. Although the lattice parameters of Bi2Se3 and h-BN are extremely different (aBN = 0.2504 nm and aBi2Se3 = 0.414 nm), the lattice plane spacing of Bi2Se3 {2̅110} is extremely close to the that of h-BN {1̅100} (dBN{110̅ 0} = 0.216 nm and dBi2Se3{2̅110} = 0.207 nm). The wavelength in the Moiré pattern generated from Bi2Se3 (2̅110) ∥ h-BN (11̅00) (Figure 3f) is substantially larger than that from Bi2Se3 (11̅00) ∥ h-BN (11̅00) (Figure 3e). The long period of the Moiré pattern in sample P3 is easily visible when an objective aperture is applied at the yellow position in Figure 3c. The corresponding dark
Figure 2b−k. These contrasts originate from the interference of the reflected light from different interfaces in the Bi2Se3/h-BN/ SiO2/Si structures, similar to the optical contrasts in graphene/ SiO2/Si structures.20,21 Figure 2b illustrates the contrast variation of 2 QL Bi2Se3 on h-BN flakes with different thicknesses. The colors of 2 QLs Bi2Se3 appear blue, cyan, viridian, yellowish, orange, and pink, which correspond to h-BN flakes with thickness of 7, 15, 18, 36, 60, and 130 μm h-BN, respectively. The triangular-shaped Bi2Se3 single crystals formed on h-BN substrates present two kinds of specific orientations (marked with “A” and “B”) with their edges parallel to two kinds of regular h-BN edges (the armchair and zigzag edges), respectively, as shown in Figure 2a. By transferring the Bi2Se3 sample onto a carbon holey film based on the polymer-free technique,7,22 detailed crystalline structures and growth mechanisms of the two kinds of Bi2Se3 growth are investigated using transmission electron microscopy (TEM). The selectedarea electron diffraction (SAED) patterns along the [0001] zone axis from the positions marked by “P1”, “P2”, and “P3” in Figure 3a are shown in Figure 3 panels d, b, and c, respectively. The innermost diffractions from h-BN are generated from the {1̅100} planes, whereas the innermost diffractions from Bi2Se3 are from the {1̅21̅0} planes.21 By correlating the SAED patterns obtained from h-BN crystals with their morphology, we identified two regular cleaved edges [the {1̅100} (zigzag) and {21̅ 10} (armchair) planes] of mechanically exfoliated h-BN. By C
DOI: 10.1021/acs.nanolett.5b00247 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. Thickness-dependent Raman spectra of Bi2Se3 on different substrates: (a) on h-BN; (b) on SiO2. The green curves are the Lorentz fitting results. The arrow indicates the appearance of infrared active mode A2u. (c) The A21gand E2g mode frequencies versus thickness at different substrates. (d) The intensity ratio of I(A2u)/I(A21g) as the function of thickness. (e) Schematic of the observed lattice vibration modes.
(located at ∼173 cm−1) are conventionally used as the two characteristic peaks for bulk Bi2Se3. We observed that the interface between Bi2Se3 and substrates (h-BN or SiO2) significantly influences the Raman spectrum that reveals the different strength of interactions (as shown in Figure 4). On the h-BN substrate, both in-plane mode E2g and out-of-plane mode A21g present no observable changes down to three QLs. For the two QL sample, however, the in-plane E2g mode develops a pronounced redshift (∼5 cm−1). Meanwhile, the out-of-plane mode A21g presents a blue shift (∼4.5 cm−1). These results indicate that during the epitaxial growth the two QL Bi2Se3 is under significant in-plane tensile strain and strong out-of-plane van der Waals contact from h-BN.25,26 The inplane strain results in the commensurate van der Waals epitaxial growth and mitigates as thickness increases above three QLs. The thickness-dependent Raman spectra of Bi2Se3 on SiO2/Si substrate has also been measured for comparison (Figure 4b). Different from Bi2Se3 grown on h-BN substrates, both E2g and A21g show observable shifts in or below four QLs. However, the shifts are considerably smaller (∼3.6 cm−1 at E2g and ∼3.3 cm−1 at A21g in two QLs) compared with those in two QL Bi2Se3 on h-BN. The results are summarized in Figure 4c. We believe that the interface interaction on SiO2 surface is not as homogeneous as that on h-BN surfaces because of the surface roughness and dangling bonds on SiO2. On the other hand, the surface roughness of SiO2 leads to difficulty in screening the strain in Bi2Se3. More evidence can be found in the infrared active A2u mode, which only appears in extremely thin samples because of inversion symmetry breaking as periodicity in perpendicular
field image is shown in Figure 3g. The wavelength of Moiré pattern is calculated according to λ = (1 + δ)a)/[2(1 + δ)(1 − cos θ) + δ2]1/2, where a is the lattice constant of h-BN, δ = (b − a)/a the mismatch of Bi2Se3 and h-BN, b = aBi2Se3/ 3 the effective lattice constant, and θ is the rotation angle between Bi2Se3 and h-BN.23 The Moiré pattern wavelength measured from Figure 3g for sample P3 and Figure 3e for sample P2 are 5.47 and 0.47 nm, respectively, which match the calculation results fairly well (5.40 nm for P3 and 0.47 nm for P2). On the basis of these HRTEM results, the atomic stacking order of samples P2 and P3 on h-BN are illustrated in Figure 3j,k, respectively. To investigate the van der Waals interaction between Bi2Se3 and h-BN, the Raman spectra evolution of Bi2Se3 at different substrates were systematically compared as the thickness decreases to a few layers. Figure 4 shows the Raman spectra of Bi2Se3 with thickness down to two QLs on different substrates (the wavelength of the excitation laser = 633 nm). The spectra have been normalized for comparison because the Raman intensity of Bi2Se3 are significantly influenced by the thickness of h-BN due to optical interference (detailed analysis can be found in the Supporting Information). According to group theory, Bi2Se3 has two Raman active modes of 2A1g, 2Eg symmetry and two infrared active modes of 2A2u , 2Eu symmetry.24 The odd-parity phonons (IR active) A2u do not appear in the Raman spectra for bulk samples as long as the crystal retains its symmetry. The in-plane vibrational mode E2g (located at ∼131 cm−1) and out-of-plane vibrational mode A21g D
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Figure 5. Total capacitances of the h-BN/Bi2Se3/h-BN devices with different thicknesses of Bi2Se3: (a) 3; (b) 5; (c) 6; and (d) 16 QLs. The top insets are the corresponding optical images. The insets at the bottom right corners are the schematics of the corresponding energy level diagrams. In panel b, the bulk quantum well states are marked by arrows. In panel d, the capacitance can be divided into two parts: depletion capacitance and surface Dirac states marked with the virtual box.
direction is lost by the two different interfaces.27 The evolution of intensity ratio I(A2u)/I(A21g) shown in Figure 4d is consistent with that of the peak positions in Figure 4c. Crystal asymmetry can be screened only above four QLs on SiO2/Si substrate, whereas this asymmetry is effectively weakened from three QLs on h-BN. The Raman spectroscopy data suggest that the van der Waals interaction between h-BN and Bi2Se3 is strong in short-range but weak in long-range. This interaction results in the commensurate van der Waals growth of Bi2Se3 on h-BN. During growth, the van der Waals interaction between h-BN and Bi2Se3 is too weak to cause any change on the Bi2Se3 crystal structure but is capable of guiding Bi2Se3 growth in the energetically favored orientation. In the initial nucleation stage, given the lack of dangling bonds formed at the interface, the nuclei of Bi2Se3 can move and rotate on the h-BN surface to achieve the favored direction driven by short-range van der Waals interaction. The Bi and Se source atoms can then attach to the edges of the well-aligned nuclei, leading to the in-plane growth of Bi2Se3. Notably, the in-plane growth is scarcely influenced by the h-BN terrain and the crystal can grow across the step edges of h-BN (see Figures S3 and S4 in Supporting Information). Similar van der Waals growth mechanisms have been recently illustrated in the growth of graphene and WS2 on h-BN substrate.11,12,28 The high-quality interface structure and specific crystallography alignment in our Bi2Se3/h-BN samples facilitate the fabrication of capacitance devices for probing the intrinsic electronic properties of atomically thin Bi2Se3. A top h-BN layer can be simply transferred to the as-grown Bi2Se3/h-BN heterostructures by mechanical transferring process conducted under an optical microscope (see Figure 1c,d). Bi 2 Se 3 encapsulated between h-BN layers provide an ideal device structure (ultraflat interfaces and large dielectric breakdown
(>0.7 V/nm)) for QC measurements. The electrodes are prepared using standard electron beam lithography, followed by Cr/Au deposition on h-BN-encapsulated Bi2Se3 as source and top gate (Figure 1e). The measured total capacitances versus applied gate voltages for different thicknesses of Bi2Se3 are shown in Figure 5. For extremely thin samples, the total capacitance consists of a series connection of the geometric capacitance and the QC of Bi2Se3, which is similar to graphene/ h-BN samples.18,29 For extremely thin h-BN flakes (
