Why Phonon Behaviors in Transition Metal Dichalcogenides Matter
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.100
Why Phonon Behaviors in Transition Metal Dichalcogenides Matter Chenzhang Zhou1 and Kofi Adu1,2 1
Pennsylvania State University, University Park, PA 16802, U. S. A.
2
Pennsylvania State University, Altoona, PA, 16601, U. S. A.
Abstract
Phonons are critical in understanding the electronic, optical and optoelectronic properties of transition metal dichalcogenides (TMDs). The interpretation of lineshapes is important for understanding the effect of specific physical processes on the electronic, optical and optoelectronic properties of TMDs. We employ an analytical approach to investigate the influence of the layered effect, the quantum size effect, the Breit-Wigner-Fano effect, and the inhomogeneous heating effect on the phonon lineshape of TMDs, using WS2 and MoS2 as prototypes. We demonstrate the similarities and differences in how individual processes affect the lineshapes and also the effect of combining processes. Such an approach is useful in guiding the interpretation of the phonon lineshapes of TMDs in particular, and nanostructures in general.
INTRODUCTION Transition metal dichalcogenides (TMDs) exhibit unique properties, including strong spin–orbit coupling and thickness-dependent electronic and mechanical properties [1]. TMDs are layered structures of a form MX2, where M is a transition metal atom and X is a chalcogen atom. These atoms are covalently bonded with interlayer Van der Waal forces, similar to that of graphite [2]. However, in TMDs, many physical and chemical properties are layer-dependent, the most prominent being the transition from indirect bandgap in the bulk to direct bandgap in a single-layer [3]. This unique property stems from the modified electronic density of states due to the absence of interlayer coupling. For example, molybdenum disulfide (MoS2) is an indirect semiconductor with a bandgap of 1.29eV in its bulk phase. However, in the single-layer phase, it exhibits a direct
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bandgap of 1.75eV and, as a result, strongly enhanced photoluminescence [4,5]. Furthermore, the two dimensional form of MoS2 is shown to display good mobility (~700 cm2V-1s-1), large optical absorption (~107m-1 in the visible range) and large exciton binding energy [4,5]. Due to these distinctive properties, monolayer MoS 2 sheets have been used for applications in FET sensors, bio-FET for DNA detection, energy storage devices [6-8], just to mention a few. However, one major drawback is their poor electrical conductivity. Phonons are key factors in modifying these interesting properties in TMDs [9] and play a critical role via scattering of the optical and the electronic excitations into different states or decay into vibrational excitations [9-11]. As has been shown in MoS2, phonon scattering due to defo
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