3D Atomistic Tomography of W-based Alloyed 2D Transition Metal

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Functional Nanostructured Materials (including low-D carbon)

3D Atomistic Tomography of W-based Alloyed 2D Transition Metal Dichalcogenides Ju Yeon Seo, Kyo-Jin Hwang, Sung-Il Baik, Su Ryeon Lee, Byungjin Cho, Euihyun Jo, Minseok Choi, Myung Gwan Hahm, and Yoon-Jun Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09604 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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ACS Applied Materials & Interfaces

3D Atomistic Tomography of W-based Alloyed 2D Transition Metal Dichalcogenides Juyeon Seo,

†

†

Kyo-Jin Hwang,

§

Euihyun Jo,

†Department

§

Minseok Choi,

‡

Sung-Il Baik,

Suryeon Lee,

Myung Gwan Hahm,

∗,†

†

Byungjin Cho,

¶

∗,†

and Yoon-Jun Kim

of Materials Science and Engineering, Inha University, 100 Inharo, Nam-Gu, Incheon 22212, Republic of Korea

‡Department

of Materials Science and Engineering, Northwestern University, Evanston, IL

60208, USA, Northwestern University Center for Atom-Probe Tomography (NUCAPT), Evanston, IL 60208, USA

¶Department

of Advanced Materials Engineering, Chungbuk National University, 1

Chungdae-ro, Seowon-gu, Cheongju, Chungbuk 28644, Republic of Korea

§Department

of Physics, Inha University, 100 Inharo, Nam-Gu, Incheon 22212, Republic of Korea

E-mail: [email protected]; [email protected]

Phone: +82 32 8747525; +82 32 8607531

Abstract Increased interest in two-dimensional (2-D) materials and heterostructures for use as components of electrical devices has led to the use of an atomically mixed phase between semiconducting and metallic transition metal dichalcogenides (TMDs) that exhibited enhanced interfacial characteristics. To understand the lattice structure and properties of 2-D materials on the atomic scale, diverse characterization methods such as Raman spectroscopy, high-resolution transmission electron microscopy, and X-ray 1

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photoemission spectroscopy have been applied. However, determination of the exact chemical distribution, which is a critical factor for the interfacial layer, was hindered by limitations of these typical methods. In this work, atom-probe tomography was introduced for the rst time to analyze the three-dimensional atomic distribution and composition variation of the atomic-scale multi-layered alloy structure WxNb(1x)Se2. Composition proles and theoretical calculations for each atom demonstrated the reaction kinetics and stoichiometric inhomogeneity of the WxNb(1x)Se2 layer. The role of the intermediate layer was investigated by fabrication of a WSe2-based eld-eect transistor. Introduction of WxNb(1x)Se2 between metallic NbSe2 and semiconducting WSe2 layers resulted in improved charge transport with lowering of the contact barrier.

Keywords 2-D materials, metallic transition metal dichalcogenides, high resolution transmission electron microscopy, atom-probe tomography, density functional theory

1

Introduction

Interfacial characteristics between two dierent materials are intriguing for scientic study and are of crucial interest in the context of the diverse industrial applications of such materials. Obviously, most devices usually comprise more than one constituent material such as semiconductor, electrodes, interconnect, passivating materials, etc. Therefore, often, they involve a junction between two dierent materials. These junctions bring about several major issues the need to be overcome for the diverse practical applications of the devices, such as the contact resistance and large Schottky barrier height (SBH). 14 Recently, there have been remarkable advances in research involving low-dimensional nanomaterials that can be translated into a wide range of newfangled practical applications. 58 Undoubtedly, atomic-layered transition metal dichalcogenides (TMDs) are primary

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driving forces in the advances. Due to their tunable band gaps, 911 high mobilities, 12,13 outstanding switching capabilities, 14 and low power consumption, 15,16 two-dimensional (2D) TMDs have emerged as key semiconducting materials for promising electronic devices. However, to utilize semiconducting TMDs in many electric devices, metallic electrodes are also indispensable components. Some of the great advantages of atomic-layered TMDs can be achieved because of their tunable electrical characteristics via combination of transition metal and chalcogen atoms, control of polymorphs, and the number of layers. 17,18 The low lattice mismatch between semiconducting and metallic TMDs and absence of dangling bonds at the van der Waals (vdW) junction can reduce their interfacial energy. 1923 Furthermore, a recent urry of activity in this area adopted an atomically mixed phase between semiconducting and metallic TMDs, resulting in enhancement of interfacial characteristics between two dierent layered nanomaterials. 24,25 In the case of insertion of a mixed phase between semiconducting and metallic TMDs, its stoichiometry is a critical factor for the formation of an interface with a low contact resistance and SBH. However, determination of the atomistic spatial distribution is limited by the drawbacks of the characterization methods used, such as high-resolution transmission electron microscopy (HR-TEM) and X-ray photoemission spectroscopy (XPS). In this respect, direct three-dimensional (3-D) identication of the structure of 2-D TMDs, especially atomic-scale alloyed structures, with atomic-scale spatial resolution is highly desirable to understand the behavior of individual atoms at the interface between layers. Therefore, in this work, we examined the linear composition variation of the atomicscale multi-layered alloy structure (WxNb(1x)Se2) to identify the precise atomic distribution and arrangement of Nb atoms throughout the WSe2 layer, employing atom-probe tomography (APT). APT characterization produces a three-dimensional (3-D) reconstruction of the lattice on an atom-by-atom basis, which provides an accurate measurement of the chemical concentration of all the elements in the periodic table with essentially the same detection eciency. 3-D atomic scale reconstruction of WxNb(1x)Se2 was obtained by combining the 3



times-of-ights (TOFs) and x-, y-, and z-positional data for all the eld-evaporated atoms in an analyzed volume at sub-nanoscale spatial resolution. Field-evaporation is aided by employing ultraviolet (wavelength = 355 nm) picosecond laser pulses to dissect a specimen essentially one atom at a time and one atomic-plane at a time, whereby individual atoms on the surface of a nanoscale hemispherical nanotip with a radius of curvature of < 50 nm, can be desorbed as single and molecular ions in dierently charged states. 2628 The atomic layer-by-layer evaporation process yields the highest possible depth spatial resolution along the z-axis of an APT nanotip. Additionally, the TOF mass spectra, based on mass-tocharge state (m/n) ratios, were utilized to obtain quantitative chemical analyses of the 3-D reconstructed sample.

2

Results and discussion

In the rst place, we explored the electronic band structures of semiconducting WSe2, metallic NbSe2, and alloyed WxNb(1x)Se2 via a theoretical approach. Our particular interest is a metal-semiconductor hybrid structure composed of W, Nb, and Se. Figure 1a shows the schematic drawings of the atomic crystal structures and the corresponding electronic band structures of semiconducting WSe2 and metallic NbSe2, calculated using Density Functional Theory (DFT). Showing in Figure 1a, the Fermi level of NbSe2 is positioned across a valence band edge, whereas the valence band edge is below the Fermi level in WSe2. This indicates that NbSe2 and WSe2 are metallic and semiconducting, respectively. In the case of an alloyed structure, Nb atoms take the place of W sites in the atomic crystal structure, as shown in Figure 1a. The change in the atomic bonding structure gives rise to reduction in the band gap of WSe2. To synthesize atomic-layered semiconducting WSe2 and metallic NbSe2, we used direct selenization method using tungsten trioxide (WO3) and niobium pentoxide (Nb2O5) thin lms as precursors and high-purity selenium (Se) powder as a Se source. WO3 and Nb2O5

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Figure 1: Characterization of as-grown semiconducting WSe2 and metallic NbSe2. (a) Sets of atomic crystal structure (right) and calculated electronic band structure (left) using DensityFunctional theory (DFT) for WSe2, WxNb(1x)Se2, and NbSe2. The W, Nb, and Se atoms are denoted by blue, red, and yellow spheres, respectively. The Fermi level is set to zero. (b and d) Top-view high-resolution transmission electron microscopy (HR-TEM) images of assynthesized (b) WSe2 and (d) NbSe2, showing hexagonal atomic lattice structure and Moire pattern. (c and e) Raman spectra of as-grown (c) WSe2 and (e) NbSe2, showing two Raman1 . X-ray photoemission spectroscopy (XPS) spectra of active vibrational modes, A1g and E2g (f) W 4f, (g) Se 3d of WSe2, (h) Nb 3d, and (i) Se 3d core levels of NbSe2. thin lms were prepared on SiO2/Si substrates using a thermal evaporator and then selenized via

chemical vapor deposition (CVD) technique at 1000 ◦ C. (see Experimental Methods for

details) Figure 1b and d show high-resolution transmission electron microscopy (HR-TEM) images taken from the surfaces of as-synthesized WSe2 and NbSe2 atomic layers, respectively. The hexagonal atomic lattice structure and Moiré pattern indicate that the CVD-based selenization process generates multilayered WSe2 and NbSe2. Raman spectra recorded from the surfaces of the as-grown WSe2 and NbSe2 show that the synthesis technique is very eective for growing diverse atomic-layered TMDs. Figure 1c and e exhibit two characteristic peaks

5



1 for the A1g (out-of-plane vibrational mode) and E2g (in-plane vibrational mode) modes of the

WSe2 and NbSe2 layers. 2931 No signicant peak shifts, compared with previously reported spectra of WSe2 and NbSe2, suggest no doping or straining and good crystallinity of both lms. X-ray photoemission spectroscopy (XPS) was conducted on as-grown WSe2 (Figure 1f and g) and NbSe2 (Figure 1h and i) lms. The W binding energies of 31.88 and 34.03 eV correspond to two core levels of W (W 4f7/2 and W 4f5/2, respectively). The Nb 3d spectra show two peaks at 203.11 and 206.08 eV, which correspond to Nb 3d5/2 and Nb 3d3/2, respectively. Two core levels of Se appear in both the semiconducting WSe2 layer (Se 3d5/2: 54.43 eV, Se 3d3/2: 54.64 eV) and the metallic NbSe2 layer (Se 3d5/2 : 53.15 eV, Se 3d3/2 : 54.08 eV) Furthermore, electron energy loss spectroscopy (EELS) analysis was carried out to conrm the crystal structure and chemical composition of the NbSe2 and NbSe2 lms, as shown in Figure S2.

Figure 2: Atomic structure and chemical composition of WxNb(1x)Se2 layer. (a1) Highresolution atomic-resolution annual dark eld (ADF) scanning TEM (STEM) image reveals 1T stacking orientations of WxNb(1x)Se2. (a2) High magnied ADF-STEM image of (a1). W atoms show brighter contrast than Nb atoms owing to dierence in atomic mass and number. (a3) Color-rendered image of (a2). (a4) Corresponding intensity prole across the lattice along green dashed line in (a2). (b) Raman spectra of as-synthesized WxNb(1x)Se2 interfacial layer. (c) Electron energy loss spectroscopy (EELS) spectra, showing existence of Nb-Se-W bonding. (d) XPS spectra (top) W 4f, (middle) Nb 3d, and (bottom) Se 3d of as-grown alloy layer. Next, we turned our attention to the W-based atomic-layer alloyed structure. First, we 6


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synthesized the atomic-scaled alloyed WxNb(1x)Se2 layer formed by sequential deposition of WO3 (3 nm) and Nb2O5 (1 nm) utilizing thermal evaporator and selenization by CVD. The selenization temperature gives rise to thermally activated diusion of Nb atoms into W sites, and it leads to formation of atomic-scale alloyed TMDs composed of W, Nb, and Se. The atomic structure of the thin lm containing three dierent atomic species was visualized by annular dark eld (ADF) scanning TEM (STEM), as shown in Figure 2a1. Figure 2a2 shows the highly magnied image of ADF-STEM. The intensity of the electron beam scattered into the ADF detector is approximately proportional to the square of the atomic number. 32,33 Three dierent atoms-W, Nb, and Se- can be distinguished by image contrast. The substitution of Nb atoms without considerable lattice distortion can be identied in the ADF-STEM image. Figure 2a4 presents the corresponding intensity prole along the normal to the interface passing through both W/Nb and Se sublattices and is consistent with ADF-STEM results. The brightness dierence in the color-rendered image of Figure 2a2 enables visualization of both W and Nb atoms (Figure 2a3). The Raman spectra of the three-atom mixed transition layer, WxNb(1x)Se2, is shown in Figure 2b. Even though the Raman spectra of WxNb(1x)Se2 exhibits broad peaks compared to that of the WSe2 layer, the vibrational mode associated with Nb-Se bonding is seen, as well as the vibrational modes of W-Se. Hence, it can be suggested that the Nb atoms successfully replaced W atoms via thermal diusion. The stoichiometry of the binary alloy layer, WxNb(1x)Se2, was determined by EELS and XPS, as shown in Figure 2c and d. By integrating the intensities of XPS peaks of each atom, we can estimate the 8.1% of Nb atoms occupied the W sites. The calculated W (x) and Nb (1-x) contents were 0.76 and 0.24, respectively. via

diverse characterizations, including ADF-STEM, Raman spectroscopy, EELS, and

XPS, the chemical composition and lattice structure of the alloyed 2D TMD was conrmed on an atomic scale. However, in the case of the such typical characterization techniques, sample preparations for characterization are complicated and only a small area can be inspected. 7



As a result, it is dicult to identify the precise atomic distribution and arrangement of Nb atoms throughout the WSe2 layer. Therefore, to determine the exact chemical distribution and composition variation of each atom, APT was conducted. Figure 3a displays a SEM micrograph of the APT nanotip corresponding to a 3-D atomprobe tomographic reconstruction. The needle-shaped nanotip can be utilized directly for TEM analyses because the apex radius is

3D Atomistic Tomography of W-based Alloyed 2D Transition Metal

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