Dense Network of One-Dimensional Midgap Metallic Modes in MonolayerMoSe2and Their Spatial Undulations

Abstract

We report the observation of a dense triangular network of one-dimensional (1D) metallic modes in a continuous and uniform monolayer of MoSe 2 grown by molecular-beam epitaxy. High-resolution transmission electron microscopy and scanning tunneling microscopy and spectroscopy studies show that these 1D modes are midgap states at inversion domain boundaries. Scanning tunneling microscopy and spectroscopy measurements further reveal intensity undulations of the metallic modes, presumably arising from the superlattice potentials due to the moiré pattern and the quantum confinement effect. A dense network of the metallic modes with a high density of states is of great potential for heterocatalysis applications. The interconnection of such midgap 1D conducting channels may also imply new transport behaviors distinct from the 2D bulk. DOI: 10.1103/PhysRevLett.113.066105 PACS numbers: 68.37.Ef, 68.55.-a, 73.20.Fz, 73.21.-b Layered transition metal dichalcogenides (TMDs), with the common formula of MX 2 (M ¼ Mo, W; X ¼ S, Se), are of great current interest for their potential electronic, optoelectronic, and catalytic applications [1][2][3]. Remarkable intrinsic properties of these two-dimensional (2D) crystals have been discovered recently by optical and transport experiments [1,[4][5][6][7][8][9][10][11][12][13][14]. At smooth edges or domain boundaries of the TMD monolayers (MLs), one-dimensional (1D) metallic states are found in the bulk gap, which have also attracted considerable interest [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32]. These midgap states can affect the optical, transport, and magnetic properties of the 2D materials [30][31][32]. Owing to their large density of states (DOS), the midgap metallic modes in TMDs are catalytically active for hydrodesulfurization (HDS) and hydrogen evolution reaction (HER) and are actively explored for catalytic applications [16][17][18][19][20][21][22][23][24][25][26][27][28][29]. To improve catalytic efficiency, networks of the edge sites have been widely adopted for HDS and HER applications [21,26,29].In this Letter, we report the observation of a dense triangular network of midgap 1D metallic modes in uniform and continuous ML MoSe 2 grown by molecular-beam epitaxy (MBE). Scanning tunneling microscopy and spectroscopy (STM and STS) and high-resolution transmission electron microscopy (HRTEM) reveal that these 1D modes are associated with inversion domain boundary (IDB) defects. Moreover, we find intensity undulations of these 1D modes in STM and STS micrographs and attribute them to the superlattice potential of the moiré pattern and quantum confinement effect. The experiments of MBE growth, STM and STS, HRTEM, and density functional theory calculations are presented in the Supplemental Material, Secs. 1-3 [33].Figures 1(a) and 1(b) show two STM images of the same surface under different bias conditions. The MoSe 2 film was grown on highly oriented pyrolytic graphite (HOPG) with the nominal thickness of 1.4 MLs. The terrace-and-step morpho...

Spin-resolved photoemission study of epitaxially grown MoSe2and WSe2thin films

“…Our MBE grown samples are bound to have a mixture of the domains with triangle point up (uptriangle domain) and its mirror twin domain pointing downwards (down-triangle domain), since they are energetically degenerate. Although such mirror twin domain gives interesting properties at the boundary [45,46], it also acts to reduce the measured polarization in SARPES since the signal from K-point always mixes with that from K'-point.…”

Engineering the Electronic Properties of Two‐Dimensional Transition Metal Dichalcogenides by Introducing Mirror Twin Boundaries

“…[29,30,34] This was suggested to originate from the chalcogen-deficient stoichiometry of the sample coupled with the low formation energy of MTBs in comparison to the isolated chalcogen vacancies. That is, the chalcogen deficiency is accommodated by forming MTBs rather than isolated vacancies.…”

“…[26,27] Importantly, mirror twin boundaries (MTBs) with particular atomic structures between regions with 60° relative orientation have been repeatedly reported. [28][29][30] These were observed either upon flake merging, inside grains to accommodate for the nonstoichiometry, or produced a posteriori by electron-beam or thermal-annealing treatments. [28,[30][31][32] The latter approach led to formation of triangular domains bounded by MTBs.…”

“…Since chalcogens are the outer atoms in the TMD monolayer, and because they are bonded to only 3 neighbors, and often lighter than the metal atoms, the probability for sputtering of chalcogen [14,19,28] grain merge, [28] annealing, [31] annealing [31] e-beam [38] --MoSe 2 growth, [29,30,34] --e-beam [30,31] e-beam [31] e-beam [32] MoTe 2 growth [37] --WS 2 --e-beam [32] WSe 2 --e-beam [32] …”

Quantum Effects and Phase Tuning in Epitaxial Hexagonal and Monoclinic MoTe2 Monolayers

“…Figure 2b reveals a dense triangular network of the bright lines, which resembles that in epitaxial MoSe 2 grown by MBE. [25] These bright lines in MoSe 2 epifilms were identified to be inversion domain boundary defects, and the triangular network of the lines reflects the very symmetry of 2H MX 2 lattice. STS measurements at the center of the triangle (i.e., away from the lines) reveal a DOS gap, as seen in Figure 2c, confirming that it is a semiconductor.…”