Thermally Driven Crossover from Indirect toward Direct Bandgap in 2D Semiconductors: MoSe₂ versus MoS₂

Abstract

Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe 2 , which shows stark contrast from the well-explored MoS 2 where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe 2 is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe 2 , identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices. KEYWORDS: 2D-Semiconductors, MoSe 2 , MoS 2 , photoluminescence, bandgap, temperature dependence T wo-dimensional (2D) materials have attracted much interest mainly owing to their exotic physical properties that are strikingly different from their three-dimensional (bulk) counterparts. Even though graphene, the most famous member of the 2D material family, possesses extraordinary properties 1 and is readily integrated in various applications, 2−4 the lack of a native bandgap in graphene has led to a broad search for other 2D semiconducting materials. More recently, the transitionmetal dichalcogenide (TMD) semiconductor MoS 2 has been focused on and has shown great potential in the field; singlelayer MoS 2 has been used as an integral part of transistors, 5−8 sensors, 9 and magnetic materials. 10 However, beyond MoS 2 , other layered TMDs offer a large variety of 2D materials with distinct properties.In this work we studied, for the first time, single-layer MoSe 2 mechanically exfoliated onto SiO 2 /Si. 11 Single-layer MoSe 2 displays good thermal stability with a 1.55 eV direct bandgap as determined from photoluminescence (PL) measurements. The PL peak intensity is enhanced dramatically from few-layer to single-layer as a result of the crossover from indirect bandgap in the bulk limit to direct bandgap in the quantum (2D) limit, similar to the behavior of MoS 2 . 12−14 More interestingly, we find that few-layer MoSe 2 flakes posssess a nearly degenerate indirect and direct bandgap, and an increase in temperature can effectively push the system toward the quasi-2D limit by thermally reducing the coupling between the layers. This response in f...

Origin of Indirect Optical Transitions in Few-Layer MoS2, WS2, and WSe2

“…The phonon effects appear to enhance the temperature dependent shift beyond the prediction by the ground state DFT results in agreement with the results by Tongay et al 10 The indirect emission peak shift in WSe 2 shows an unexpected trend below 173 K where the temperature dependence is reversed (Fig 3f). We attribute this to emergence of a new indirect emission peak which originates from Λ→Γ transition.…”

“…Similarly, the conduction band valley at the Λ point (midpoint between K and Γ) shifts downward in the presence of interlayer interaction as indicated by calculations 7,16,17,[20][21][22] . In contrast, the states near the K point are comparatively less susceptible to the number of layers 4,7,9,10 .…”

“…For instance, monolayer graphene exhibits linear dispersion at low energies while Bernal-stacked bilayer graphene shows parabolic dispersion. 1,2,8 Similarly, direct band gap of single layer MoS 2 is in contrast to indirect band gap of their multilayer counterparts 4,7,9,10 . In bilayers and thicker multilayers, interlayer interaction, geometrical confinement, and crystal symmetry play a collective role in defining their electronic structures.…”

“…The indirect-to-direct crossover in these MX 2 compounds (M=Mo,W and X=S,Se) results from local shift of valence band hills and conduction band valleys in the Brillouin zone 4,7,9,10,[16][17][18][19] . In single layers, the conduction band minimum (CBM) and valence band minimum (VBM) coinciding at the K point, making them direct gap semiconductors.…”

Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors

“…Furthermore, the 1T phase MoS 2 is thermally unstable above 100 o C. The availability of a variety of semiconducting TMDs such as MoSe 2 , WS 2 and WSe 2 with different band structures and charge neutrality levels offers additional distinct properties and opportunities for device applications. 5,6,8,9,13,[26][27][28][29][30][31][32][33][34][35][36][37] However, the variation of electron affinity, band gap, and band alignments also presents significant challenges to contact engineering. To unlock the full potential of TMDs as channel materials for high-performance thin-film transistors, highly effective and versatile contact strategies for making low-resistance ohmic contacts are needed.…”