Valley phenomena in the candidate phase change material WSe2(1-x)Te2x.

Alloyed transition metal dichalcogenides provide an opportunity for coupling band engineering with valleytronic phenomena in an atomically-thin platform. However, valley properties in alloys remain largely unexplored. We investigate the valley degree of freedom in monolayer alloys of the phase change candidate material WSe2(1-x)Te2x. Low temperature Raman measurements track the alloy-induced transition from the semiconducting 1H phase of WSe2 to the semimetallic 1Td phase of WTe2. We correlate these observations with density functional theory calculations and identify new Raman modes from W-Te vibrations in the 1H-phase alloy. Photoluminescence measurements show ultra-low energy emission features that highlight alloy disorder arising from the large W-Te bond lengths. Interestingly, valley polarization and coherence in alloys survive at high Te compositions and are more robust against temperature than in WSe2. These findings illustrate the persistence of valley properties in alloys with highly dissimilar parent compounds and suggest band engineering can be utilized for valleytronic devices.

Growth Kinetics and Atomistic Mechanisms of Native Oxidation of ZrSxSe2-x and MoS2 Crystals.

A thorough understanding of native oxides is essential for designing semiconductor devices. Here, we report a study of the rate and mechanisms of spontaneous oxidation of bulk single crystals of ZrSxSe2-x alloys and MoS2. ZrSxSe2-x alloys oxidize rapidly, and the oxidation rate increases with Se content. Oxidation of basal surfaces is initiated by favorable O2 adsorption and proceeds by a mechanism of Zr-O bond switching, that collapses the van der Waals gaps, and is facilitated by progressive redox transitions of the chalcogen. The rate-limiting process is the formation and out-diffusion of SO2. In contrast, MoS2 basal surfaces are stable due to unfavorable oxygen adsorption. Our results provide insight and quantitative guidance for designing and processing semiconductor devices based on ZrSxSe2-x and MoS2 and identify the atomistic-scale mechanisms of bonding and phase transformations in layered materials with competing anions.

Increased Transfer Efficiency from Molecular Photonic Wires on Solid Substrates and Cryogenic Conditions.

Molecular photonic wires (MPWs) are tunable nanophotonic structures capable of capturing and directing light with high transfer efficiencies. DNA-based assembly techniques provide a simple and economical preparation method for MPWs that allows precise positioning of the molecular transfer components. Unfortunately, the longest DNA-based MPWs (∼30 nm) report only modest transfer efficiencies of ∼2% and have not been demonstrated on solid-state platforms. Here, we demonstrate that DNA-based MPWs can be spin-coated in a polymer matrix onto silicon wafers and exhibit a 5-fold increase in photonic transfer efficiency over solution-phase MPWs. Cooling these MPWs to 5 K led to further efficiency increases ranging from ∼40 to 240% depending on the length of the MPW. The improvement of MPW energy transport efficiencies advances prospects for their incorporation in a variety of optoelectronics technologies and makes them an ideal test bed for further exploration of nanoscale energy transfer.

The structural phases and vibrational properties of Mo1-xWxTe2 alloys.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T' semimetallic phase at high temperatures. Alloying MoTe2 with WTe2 reduces the energy barrier between these two phases, while also allowing access to the T d Weyl semimetal phase. The Mo1-x WxTe2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe2-WTe2 system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk Mo1-xWxTe2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T', and T d structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe2-WTe2 system, including single-phase 2H, 1T', and T d regions, as well as a two-phase 1T' + T d region. Disorder arising from compositional fluctuations in Mo1-xWxTe2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T'-MoTe2 mode and the enhancement of a double-resonance Raman process in 2H-Mo1-x WxTe2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo1-xWxTe2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.