Valley Phenomena and Phase Diagrams of Transition Metal Dichalcogenide Alloys
Dr. Patrick Vora
Assistant Professor | Department of Physics and Astronomy
Director | Quantum Materials Center
George Mason University
Atomically thin materials derived from layered crystals have occupied much of the condensed matter community since the discovery of graphene in 2004. Transition metal dichalcogenides (TMDs) are among the most versatile members in the family of layered materials due to the opportunities for tuning electronic behaviors with chemical composition, layer number, and structural phase. Achieving on-demand transitions between different structural phases could enable a new class of atomically-thin non-volatile memories known has phase change memories (PCMs).1 MoTe2 is an ideal candidate for this technology as it exhibits the smallest energy difference between the 2H semiconducting and 1T′ semi-metallic structural phases.2 However, the energy cost for driving a phase transition could be further reduced by alloying MoTe2 with WTe2, which naturally crystallizes in the 1T′ phase,3 and has led to substantial interest in the properties of TMD alloys.
In this colloquium I will discuss our team’s exploration of TMD alloys that are candidates for PCM applications. I will first present our work on the phase diagram of Mo1-xWxTe2 (x=0..1).4 In this study, we used polarization-resolved Raman spectroscopy to explore the composition-dependent optical properties of MoxW1-xTe2 alloys. These data provided clear signatures of the 2H, 1T′, and Td structural phases and their evolution with W composition x. Combining these results with aberration-corrected transmission electron microscopy and x-ray diffraction measurements allows for the construction of the alloy phase diagram. This interdisciplinary study clarified significant disagreements in the literature regarding the structural phase diagram and has proved foundational in future attempts to create MoxW1-xTe2 – based PCMs. The second half of this talk will focus on a previously unstudied aspect of PCM-candidate TMD alloys: valleytronic behaviors. 5 Here we investigate WSe2(1-x)Te2x alloys which are also potentially useful for PCMs but are more resistant to oxidation. Polarization-resolved photoluminescence measurements show new low energy emission features unique to this alloy system that may originate from buckling due to the large W-Te bond lengths. Despite the significant disorder, we find that valley polarization and coherence in alloys survive at high Te compositions and are larger than in pure WSe2 at elevated temperatures. These findings illustrate the persistence of valley properties in alloys with highly dissimilar parent compounds and suggest a novel class of devices combining PCM characteristics with valleytronics.
(1) Rehn, D. A.; Li, Y.; Pop, E.; Reed, E. J. Theoretical Potential for Low Energy Consumption Phase Change Memory Utilizing Electrostatically-Induced Structural Phase Transitions in 2D Materials. npj Comput. Mater. 2018, 4 (1), 2.
(2) Duerloo, K.-A. N.; Li, Y.; Reed, E. J. Structural Phase Transitions in Two-Dimensional Mo- and W-Dichalcogenide Monolayers. Nat. Commun. 2014, 5 (1), 4214.
(3) Duerloo, K.-A. N.; Reed, E. J. Structural Phase Transitions by Design in Monolayer Alloys. ACS Nano 2016, 10 (1), 289–297.
(4) Oliver, S. M.; Beams, R.; Krylyuk, S.; Kalish, I.; Singh, A. K.; Bruma, A.; Tavazza, F.; Joshi, J.; Stone, I. R.; Stranick, S. J.; et al. The Structural Phases and Vibrational Properties of Mo1−xWxTe2 Alloys. 2D Mater. 2017, 4 (4), 045008.
(5) Oliver, S. M.; Young, J.; Krylyuk, S.; Reinecke, T. L.; Davydov, A. V.; Vora, P. M. Valley Phenomena in the Candidate Phase Change Material WSe2(1-x)Te2x. ArXiv e-prints 2019, 1908.00506.