Phase‐Dependent Band Gap Engineering in Alloys of Metal‐Semiconductor Transition Metal Dichalcogenides

Abstract

Bandgap engineering plays a critical role in optimizing the electrical, optical and (photo)‐electrochemical applications of semiconductors. Alloying has been a historically successful way of tuning bandgaps by making solid solutions of two isovalent semiconductors. In this work, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe2 with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, the 1T and 2H phases, are produced each with emergent electronic structure. At room temperature, it is observed that the 1T and 2H phases are semiconducting and metallic, respectively. For the 1T structure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps in the range of 0.42–0.58 at 77 K. Electron diffraction patterns of the 1T structure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions that result in an uncommon 4 × 4 supercell, rotated approximately 4° from the lattice. Density‐functional‐theory calculations confirm that local distortions, such as those in a NCCDW, can open up a band gap in 1T‐Mo0.5Nb0.5Se2, but not in the 2H phase. This work expands the boundaries of alloy‐based bandgap engineering by introducing a novel technique that facilitates CDW phases through alloying.

Review of strategies toward the development of alloy two-dimensional (2D) transition metal dichalcogenides

“…Hence, it highlighted the importance of controlling stoichiometric ratio in-band tuning ( Xie et al., 2020 ). Another approach ( Wang et al., 2020a ) explained the phase-dependent characteristics of Nb alloyed in Mo site, Mo (1-x) Nb x Se 2 , where 2H phase exhibiting metallic behavior and 1T phase with distortion cause semiconductor behavior with a bandgap of ∼0.4 eV. Figure 3 A shows the phase mixture state of Mo (1-x) Nb x Se 2 .…”

“… (A) The atomic-resolution HAADF of Mo (1–x) Nb x Se 2 (left, scale bar: 1nm), and diffraction patterns from 2H (top, scale bar: 2 nm −1 ) and 1T Mo (1–x) Nb x Se 2 flakes (bottom, scale bar: 2 nm −1 ), respectively (right) ( Wang et al., 2020a ). …”

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices

“…Thus far, CVT has been successfully exploited to synthesize a host of layered material alloys such as Mo 1− x W x S 2 , [ 109 , 110 ] Mo 1− x Nb x Se 2 , [ 111 ] Nb x Re 1− x Se 2 , [ 112 ] MoSe 2 x Te 2(1− x ) , [ 89 , 113 ] WSe 2(1− x ) Te 2 x , [ 114 ] ZrS x Se 2− x , [ 115 ] HfS x Se 2− x , [ 116 ] HfS 2(1− x ) Te 2 x , [ 117 ] TaSe 2− x S x , [ 118 ] Sn(S x Se 1− x ) 2 , [ 119 ] In x Sn 1− x S 2 , [ 120 ] V x Sn 1− x S 2 , [ 121 ] Pb x Sn 1− x Se 2 , [ 122 ] Ga 1− x In x Se, [ 123 ] MnPS 3− x Se x [ 124 ] and Nb 1− x Ti x S 3 , [ 125 ] just to name a few, demonstrating the broad generalizability of this approach. In addition, the composition of the CVT‐derived alloys can be facilely customized in a wide range, since the source materials are sealed in an enclosed environment, which can effectively prevent the unintentional volatilization.…”

Optoelectronic Properties of Atomically Thin MoxW(1−x)S2 Nanoflakes Probed by Spatially-Resolved Monochromated EELS

“…Concerning the electronic properties of 2D materials, tunable band gaps seem to have become crucial for the further development of electronic applications [24][25][26][27][28][29][30][31][32][33]. Most available 2D materials offer limited and rigid band gap values to work with (5.8 eV for monolayer BN [34][35][36][37] and 0 eV for monolayer graphene [38], for instance) and offer a limited versatility for electronic applications.…”

“…Diverse approaches have been tried for band gap tuning in 2D materials, and TMDS have been no exception. Band gap tuning of 2D TMDs by means of functionalisation [30,39,40], dielectric screening [41], doping [42][43][44], straining [29,45] and phase engineering [46,47], alloying [48][49][50][51][52][53], as well as combinations of these methods [26], have been on the spotlight of 2D material research.…”