Epitaxial Growth and Structural Properties of Bi(110) Thin Films on

C: Surfaces, Interfaces, Porous Materials, and Catalysis

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Epitaxial Growth and Structural Properties of Bi(110) Thin Films on TiSe Substrates 2

Xu Dong, Yongkai Li, Ji Li, Xianglin Peng, Lu Qiao, Dongyun Chen, Huixia Yang, Xiaolu Xiong, Qinsheng Wang, Xiang Li, Junxi Duan, Junfeng Han, and Wende Xiao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01923 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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The Journal of Physical Chemistry

Epitaxial Growth and Structural Properties of Bi(110) Thin Films on TiSe2 Substrates Xu Dong,† Yongkai Li,† Ji Li,† Xianglin Peng,† Lu Qiao,† Dongyun Chen,† Huixia Yang,† Xiaolu Xiong,†,‡ Qinsheng Wang,†,‡ Xiang Li,†,‡ Junxi Duan,†,‡ Junfeng Han,†,‡ Wende Xiao,*,†,‡

†Key

Laboratory of Advanced Optoelectronic Quantum Architecture and Measurement (MOE),

School of Physics, Beijing Institute of Technology, Beijing 100081, China

‡Beijing

Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems and Micro-

nano Centre, Beijing Institute of Technology, Beijing 100081, China

ABSTRACT: We report the growth and structural properties of Bi thin films on TiSe2 substrates by using a low-temperature scanning tunneling microscope (LT-STM). Extended Bi(110) thin films are formed on the TiSe2 substrates and adopt a distorted black-phosphorus (BP) structure at room temperature (RT). The diagonal of the Bi(110) rectangular unit cell is parallel to the closepacked direction of the top-layer Se atoms of the TiSe2 substrates, resulting in the formation of a stripe-shaped commensurate moiré pattern with a periodicity of ~ 38.5 Å at RT. Meanwhile, the CDW phase transition of the TiSe2 substrate and the different coefficients of thermal expansion of Bi(110) and TiSe2 lead to the formation of a quasi-hexagonal incommensurate moiré pattern with a periodicity of 14.5 Å at 77 K. In particular, the combination of domains with twisting angles of 1 ACS Paragon Plus Environment

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30 or 60 results in the formation of various domain boundaries. Our work is very helpful for understanding and tuning the structural and electronic properties of epitaxial Bi(110) thin films.

INTRODUCTION

Bismuth (Bi), a typical group-V semimetal, has been studied extensively for decades, because of its unique physical properties, such as highly anisotropic Fermi surface, small effective electron mass and long Fermi wavelength (ca. ~40 nm at room temperature (RT)).1-2 Due to the strong spinorbital coupling (SOC) strength and the quantum size effects,3-4 low-dimensional Bi nanostructures were found to exhibit a variety of novel properties, e.g. semimetal to semiconductor transition,5-6 high thermoelectric efficiency,7-8 surface superconductivity9 and extremely large magnetoresistance.10-11 Despite of intensive efforts, controversy remains about the structural and/or electronic properties of Bi(111) and Bi(110) thin films to date. Bi(111) thin film is predicted to be a two-dimensional (2D) topological insulator (TI) and host topological edge states.12 However, the hybridization of the Bi(111) thin films and the topological surface states of the Bi2Te3 substrates makes it rather complicated to experimentally distinguish the edge states.13 Indeed, Schindler and coworkers recently reported the observation of topologically protected hinge states in Bi(111) nanostructures and proposed that they are high-order topological insulators.14 When other substrates such as Si(111),15 Ge(111),16 W(110),17 graphene,18-19 graphite20-21 and 2 ACS Paragon Plus Environment

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NbSe222 were adopted, (110)-oriented thin films were formed prior to the (111)-oriented ones at the initial stage, because Bi(110) thin films with black phosphorus (BP) structure are energetically more favorable than Bi(111).23 However, the effect of the substrates on the growth and structure of the Bi(110) thin films have rarely addressed. Moreover, domains with different orientations have been widely observed for Bi(110) thin films grown on various substrates,15 but there are few reports on the detailed structure around the domain boundaries to date.

For practical applications, the Bi(110) thin film must be placed or grown on a substrate, which would influence both the structural and electronic properties of the film. As a candidate substrate, the layer-structured transition metal dichalcogenide (TMDC) TiSe2 is a semimetal at RT and undergoes a second-order phase transition to a charge density wave (CDW) state below TCDW ≈ 200 K,24 leading to a commensurate superstructure with a periodicity of 2 × 2.24 This phase transition is associated with a slight contraction of the Ti-Se bond.24 TiSe2 can also exhibit superconductivity with Cu-doping25 or pressure.26 Thus, the epitaxial growth of Bi(110) thin films on TiSe2 substrates may provide a valuable opportunity to study the intriguing interplay of topological states, CDW states and superconductivities.

In this work, we report the epitaxial growth and structural properties of Bi(110) thin films on the TiSe2 substrates by using a low-temperature scanning tunneling microscope (LT-STM). We observe the formation of extended bilayer (BL)-thick Bi thin films. The Bi thin films are (110)oriented and adopt a distorted BP structure. The diagonal of the Bi(110) rectangular unit cell is 3 ACS Paragon Plus Environment


parallel to the close-packed direction of the top-layer Se atoms of the TiSe2 substrates. The symmetry mismatch between the lattices of the Bi(110) thin films and the TiSe2 substrates results in the formation of a stripe-shaped commensurate moiré pattern at RT. Meanwhile, the CDW phase transition of the TiSe2 substrate and the different coefficients of thermal expansion of Bi(110) and TiSe2 lead to the formation of a quasi-hexagonal incommensurate moiré pattern at 77 K. The combination of domains with different orientations results in the formation of various domain boundaries.

EXPERIMENTAL METHODS

TiSe2 single crystals were grown via the chemical vapor transport technique. The high-purity powder of titanium (Alfa Aesar 99.999%) and selenium (Alfa Aesar 99.999%) in a molar ratio of 1:2.1 together with a small amount of iodine was sealed in an evacuated quartz tube. The tube was placed into a furnace at the setting temperature from 895 K to 845 K for one month.

The epitaxial growth of Bi thin films on TiSe2 substrates was carried out in a homemade ultrahigh vacuum (base pressure of 1 × 10-10 mbar) LT-STM and molecular beam epitaxy (MBE) combined system. The as-grown TiSe2 single crystals were cleaved in the MBE chamber and degassed at ∼500 K for several hours. Before deposition of Bi, the surface cleanness of TiSe2 was checked with STM. Bi was deposited via vacuum sublimation from a Knudsen-type at ∼700 K, while the TiSe2 substrates were held at room temperature (RT). The Bi source was thoroughly degassed prior to deposition. The typical deposition rate was ∼0.04 monolayer (ML)/min (1 ML corresponds to 4 ACS Paragon Plus Environment

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a single layer of bulk Bi(110)), as calibrated by LT-STM. All STM images were acquired in the constant-current mode with electrochemically etched tungsten tips at RT or 77 K. The given voltages were referred to the sample.

RESULTS AND DISCUSSION

TiSe2 is a layer-structured semimetal at RT. Figure 1a illustrates a high-resolution STM image of a TiSe2 single crystal acquired at RT. A hexagonal lattice of the top-layer Se atoms of TiSe2 is clearly resolved. The Fast Fourier Transformation (FFT) analysis also reveals a hexagonal pattern in the inset of Figure 1a. The obtained lattice parameter of 3.5  0.1 Å is consistent with previous reports.27-28 Interestingly, Figure 1b shows an additional 2 × 2 superstructure of the TiSe2 crystal after cooling down to 77 K. Two set of spots can be clearly discerned from the FFT analysis, as shown in the inset of Figure 1b. The outer set of spots corresponds to the reciprocal lattice of the 1 × 1 surface structure, while the inner one corresponds to that of the 2 × 2 superstructure. The formation of the 2 × 2 superstructure can be ascribed to the phase transition of TiSe2 to a CDW state below 200 K.24 Notably, Figure 1c displays various defects on the surface of TiSe2, which was classified into four types29-30: Se vacancies (A), I (B) and O (C) substitutions of Se surface atoms, and excess Ti atoms intercalated between TiSe2 single layers (D). These defects may affect the nucleation and growth of Bi thin films.

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Figure 1. Atomically resolved STM topographic image of TiSe2. (a) At RT. Scanning parameters: sample bias (Vs) = 250 mV, tunneling current (It) = 230 pA. The inset shows the corresponding FFT pattern. (b) At T = 77 K. Vs = 180 mV, It = 200 pA. The inset shows the corresponding FFT pattern. (c) At T = 77 K. Vs = 300 mV, It = 250 pA. Four different types of defects are identified. Figure 2a shows a large-scale STM image after deposition of 0.8 ML Bi on the single crystalline TiSe2 substrates at RT. Both small patches and extended islands of Bi can be observed. All islands exhibit a compact shape with similar heights of ~ 6.0 Å with respect to the TiSe2 substrate, corresponding to the BL thickness of bulk Bi(110). With increasing coverage, the Bi islands grow up and merge with each other. At a coverage of ~1.2 ML Bi, we observe the formation of elongated second-BL islands on the extended first-BL islands (see Figure 2b), which indicates a highly anisotropic crystal structure of the Bi islands. Statistical analysis reveals six different orientations of the long edges of the elongated second-BL islands and two different twisting angles of ~30 or 60 between such island edges. Moreover, such unusual growth pathway show that the growth of Bi on TiSe2 is different from the conventional layer-by-layer mode, where a complete layer forms prior to the growth of subsequent layers.31 In order to identify the growth mechanism, we have 6 ACS Paragon Plus Environment

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annealed the sample at about 450 K for an hour. Figure 2c shows the morphology of the sample after annealing. The second-BL Bi islands, which were originally located at the center of the first BL, grow up laterally and even extend to the edge of the first BL, while the area of the first BL shrinks dramatically. This morphology evolution can be well related to the dewetting of the first BL. It implies that the interfacial coupling between the first BL Bi and the TiSe2 substrate is weaker than the interlayer interaction between the two BL Bi. Therefore, we observe that the second-BL Bi islands start to grow before the completion of the first BL Bi. Figure 2d shows a line profile taken along the red solid line in Figure 2c. The apparent heights of the first BL with respect to the substrate and the second BL with respect to the first BL are 6.0  0.1 Å, in good agreement with the thickness of BL Bi(110).20

Figure 2. (a) Large-scale STM image of Bi(110) film on TiSe2 with low coverage at T = 77 K. Vs = 1 V, It = 40 pA. (b) Large-scale STM image of Bi(110) film on TiSe2 with second layer at RT. 7 ACS Paragon Plus Environment


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Vs = 400 mV, It = 250 pA. (c) After sample annealing at ～450 K for 1 h. The white arrow indicates the close-packed direction of the TiSe2 substrate. Vs = 65 mV, It = 300 pA. (d) Line profile along the red line indicated in (c). To further clarify the crystal structure of the Bi islands grown on the TiSe2 substrate, atomicresolution STM images are acquired at RT, as depicted in Figure 3. Figure 3a shows that the lattice of the Bi islands is modulated by a moiré pattern of 1D stripes with a periodicity of 38.5  0.1 Å, akin to previous report of Bi(110) thin films grown on graphite,21, 32 which can be accounted for by the lattice mismatch between the rectangular crystal structure of the Bi islands and the hexagonal lattice of the TiSe2 substrate. The high-resolution STM image in Figure 3b clearly shows a rectangular lattice (indicated by the blue rectangle) of the Bi islands with a = 4.8  0.1 Å and b = 4.5  0.1 Å, respectively, in line with the Bi(110) surface.20 By comparing the atomic resolution images of the Bi(110) islands and the TiSe2 substrates, we find that the diagonal of the Bi(110) rectangular unit cell is parallel to the close-packed direction of the top-layer Se atoms of the TiSe2 substrates (see Figure 3b). Such parallel alignment is probably driven by the lattice commensuration: The diagonal of the Bi(110) unit cell (6.6 Å) is commensurate to that of the TiSe2 unit cell (3.5 Å) with a number ratio of 13:7. These behaviors are very similar to the Bi(110) thin films grown on the Si(111) substrates.15 Moreover, the Bi atom in the center of the unit cell is apparently darker than the corner ones, which reflects the nature of a buckled Bi(110) structure that gives two nonequivalent Bi atomic sites with different heights. As illustrated in Figure 3c, the 8 ACS Paragon Plus Environment

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buckled Bi(110) structure can be viewed as a distorted BP structure.33 Similar buckled Bi(110) structures have been reported in Bi overlayers grown on Si(111),15 graphite,20 Ag(111),34 and graphene/SiC18 substrates. In addition, the long edges of the elongated second-BL islands is parallel to the b direction of the Bi(110) islands.

Figure 3. (a) STM image of Bi(110) on the TiSe2 substrate at RT showing the formation of stripeshaped moiré pattern. The white arrow indicates the close-packed direction of the TiSe2 substrate. Vs = 250 mV, It = 230 pA. (b) Atomically resolved STM topographic image of Bi(110) at RT. The white arrow indicates the close-packed direction of the TiSe2 substrate. Vs = -50 mV, It = 490 pA. (c) Schematic illustration of side- and top-views of the distorted BP structure of the Bi(110) islands. As discussed above, the TiSe2 substrates undergo a CDW phase transition below 200 K and exhibit a 2 × 2 superstructure. Assuming that at 77 K both the lattices of the Bi(110) thin film and the substrate shrink in an identical way with respect to the ones at RT, one would expect an additional moiré pattern of 1D stripes with a doubled periodicity, which, however, has not been observed. Instead, we observe the formation of a quasi-hexagonal moiré pattern, as shown in Figure 4a. Only 9 ACS Paragon Plus Environment


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the corner Bi atoms of the rectangular unit cell, which is modulated by a quasi-hexagonal moiré pattern, can be clearly resolved in this STM image. The zoom-in STM image in Figure 4b illustrates a periodicity of 14.5  0.1 Å for this moiré pattern. Although a 1D stripe-shaped moiré pattern can also be easily recognized under some special tip conditions (Figure 4c), the exact lattice commensuration cannot be distinguished. These behaviors indicate that the Bi(110) lattice is incommensurate to that of the TiSe2 substrate at 77 K. As the TiSe2 substrate undergoes a CDW phase transition and exhibits a 2 × 2 superstructure with slight contracted Ti-Se bonds at 200 K, the interfacial coupling between the Bi(110) overlayers and the TiSe2 substrates might be modified and the strain built up in the Bi(110) overlayers might be relieved at 77 K, resulting in the change of the lattice commensuration. The different coefficients of thermal expansion of Bi(110) and TiSe2 may also contribute to the commensurate to incommensurate transition.35-36 The commensurate to incommensurate transition may further lead to the modification of electronic structures of the Bi(110) thin films. Unfortunately, the tiny variations of the lattice lengths at different sample temperature cannot be probed by STM, due to the limitation of the spatial resolution of STM.


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Figure 4. Atomically resolved STM images of Bi(110) thin film at 77 K. (a) Large-scale image showing the formation of hexagonal moiré pattern. Vs = 150 mV, It = 150 pA. (b) Zoom-in showing the unit cell of the moiré pattern. Vs = 150 mV, It = 50 pA. The unit cell of the moiré pattern is indicated by a blue rhombus. (c) Zoom-in with special tip states showing the stripe-structured moiré pattern. Vs = 250 mV, It = 50 pA.

Figure 5. (a) Possible ways to arrange the diagonal of the Bi(110) rectangular unit cell and the close-packed direction of the top-layer Se atoms of the TiSe2 substrates. For each close-packed direction, there are two mirror-symmetric ways to place the diagonal of the Bi(110) rectangular



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unit cell (L in red and R in blue). (b, c) Magnified STM images of two domain boundaries. (d, e) Sketches of the protrusions in the STM images shown in (b) and (c), respectively. The domain boundaries are indicated by red dashed lines and the unit cells are indicated by red rectangles. The parallel arrangement of the diagonal of the Bi(110) rectangular unit cell and the close-packed direction of the top-layer Se atoms of the TiSe2 substrates is expected to result in the formation of two mirror-symmetric groups (indicated by L and R) of C3-symmetric orientations of Bi(110) domains on the substrates, as schematically illustrated in Figure 5a. The combination of the Bi(110) domains with different orientations will lead to the formation of various domain boundaries. There are essentially three different types of combination for the adjacent domains: (1) L1 and R1, (2) L1 and L2, and (3) L1 and R2. The corresponding twisting angles between the adjacent domains are about 5, 60 and 27, respectively, similar to that of Bi(110) thin films grown on Si(111)15 and graphite37. We have experimentally observed two types of domain boundaries, as shown in Figure 5b and c. The Bi atoms in each domain can be clearly resolved. The structural models of the two domain boundaries are superposed on the STM images (see Figure 5b and c) and also depicted in Figure 5d and e. Careful analysis reveals a tilting angle of 30  3 between the two adjacent domains shown in Figure 5b, consistent with the case (3) discussed above. As a consequence, the bonds of the Bi atoms at the boundary that connect the two adjacent domains are highly distorted. A similar situation can be seen in Figure 5c, where the tilting angle for the two domains is determined to be 60  3, corresponding to the mentioned case (2). The distorted bonds may result 12 ACS Paragon Plus Environment

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in strains around the boundaries and thus modify the electronic structure, as the topological properties of the Bi(110) layers are very sensitively to atomic buckling.20 We note that the domain boundaries of type (1) have not yet been observed so far. Further experiments are under way.

CONCLUSIONS

In summary, we have studied the epitaxial growth and structural properties of Bi(110) thin films on the TiSe2 substrates by STM at RT and 77 K. We observe the formation of extended BL-thick Bi(110) thin films that adopt a distorted BP structure. The diagonal of the Bi(110) rectangular unit cell is parallel to the close-packed direction of the top-layer Se atoms of the TiSe2 substrates, which is driven by the lattice commensuration. The symmetry mismatch between the lattices of the Bi(110) thin films and the TiSe2 substrates leads to the formation of a stripe-shaped moiré pattern with a periodicity of ~ 38.5 Å at RT. Meanwhile, the CDW phase transition of the TiSe2 substrate and the different coefficients of thermal expansion of Bi(110) and TiSe2 lead to the formation of a quasi-hexagonal incommensurate moiré pattern with a periodicity of 14.5 Å at 77 K. The combination of domains with different orientations results in the formation of various domain boundaries. Our work is very helpful for understanding and tuning the structural and electronic properties of epitaxial Bi(110) thin films.

AUTHOR INFORMATION 13 ACS Paragon Plus Environment


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Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300904 and 2016YFA0300600) and the National Science Foundation of China (Grants No. 11734003, 51661135026 and 21773008).

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TOC graphic:


Epitaxial Growth and Structural Properties of Bi(110) Thin Films on

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