Temperature-Dependent Phonon Shifts in van der Waals Crystals

Evidence of Higher Order Phonon Anharmonicity in Gray Arsenic Crystal

Phonons play a key role in the heat transport process of quantum materials. The understanding of thermal behaviors of phonons will be beneficial for designing modern electronic devices. In this study, we utilize specific heat, Raman spectroscopy, and first-principles calculations combined with the phonon Boltzmann transport equation to explore the thermal transport of gray arsenic. Our specific heat data indicate the presence of the phonon anharmonicity at high temperature. This is further supported by temperature-dependent Raman data showing evident phonon softening and line width broadening. More interestingly, from the analysis of temperature-dependent Raman modes, we found that the four-phonon scattering process is indispensable for interpreting the line width broadening at high temperatures. Moreover, we evaluate the importance of the four-phonon scattering process in the heat transport of gray arsenic using the moment tensor potential method. Our work sheds light on the importance of a higher order phonon scattering process in heat transport of the materials with moderate thermal conductivity.

Quaternary, layered, 2D chalcogenide, Mo1-xWxSSe: thickness dependent transport properties

Highly oriented, single crystalline, quaternary alloy chalcogenide crystal, MoxW1-xS2ySe2(1-y), is synthesized using a high temperature chemical vapor transport technique and its transport properties studied over a wide temperature range. Field effect transistors (FET) with bottom gated configuration are fabricated using Mo0.5W0.5SSe flakes of different thicknesses, from a single layer to bulk. The FET characteristics are thickness tunable, with thin flakes (1-4 layers) exhibiting n-type transport behaviour while ambipolar transfer characteristics are observed for thicker flakes (>90 layers). Ambipolar behavior with the dominance of n-type over p-type transport is noted for devices fabricated with layers between 9 and 90. The devices with flake thickness ∼9 layers exhibit a maximum electron mobility 63 ± 4 cm2V-1s-1and anION/IOFFratio >108. A maximum hole mobility 10.3 ± 0.4 cm2V-1s-1is observed for the devices with flake thickness ∼94 layers withION/IOFFratio >102-103observed for the hole conduction. A maximumION/IOFFfor hole conduction, 104is obtained for the devices fabricated with flakes of thickness ∼7-19 layers. The electron Schottky barrier height values are determined to be ∼23.3 meV and ∼74 meV for 2 layer and 94 layers flakes respectively, as measured using low temperature measurements. This indicates that an increase in hole current with thickness is likely to be due to lowering of the band gap as a function of thickness. Furthermore, the contact resistance (Rct) is evaluated using transmission line model (TLM) and is found to be 14 kohm.μm. These results suggest that quaternary alloys of Mo0.5W0.5SSe are potential candidates for various electronic/optoelectronic devices where properties and performance can be tuned within a single composition.

Bridging Structure, Magnetism, and Disorder in Iron-Intercalated Niobium Diselenide, FexNbSe2, below x = 0.25

Transition-metal dichalcogenides (TMDs) intercalated with magnetic ions serve as a promising materials platform for developing next-generation, spin-based electronic technologies. In these materials, one can access a rich magnetic phase space depending on the choice of intercalant, host lattice, and relative stoichiometry. The distribution of these intercalant ions across given crystals, however, is less well defined-particularly away from ideal packing stoichiometries-and a convenient probe to assess potential longer-range ordering of intercalants is lacking. Here, we demonstrate that confocal Raman spectroscopy is a powerful tool for mapping the onset of intercalant superlattice formation in Fe-intercalated NbSe₂ (Fe_xNbSe₂) for 0.14 ≤ x < 0.25. We use single-crystal X-ray diffraction to confirm the presence of longer-range intercalant superstructure and employ polarization-, temperature-, and magnetic field-dependent Raman measurements to examine both the symmetry of emergent phonon modes in the intercalated material and potential magnetoelastic coupling. Magnetometry measurements further indicate a correlation between the onset of magnetic ordering and the relative degree of intercalant superlattice formation. These results show Raman spectroscopy to be an expedient, local probe for mapping intercalant ordering in this class of magnetic materials.

Observation of Phonon Cascades in Cu-Doped Colloidal Quantum Wells

Electronic doping has endowed colloidal quantum wells (CQWs) with unique optical and electronic properties, holding great potential for future optoelectronic device concepts. Unfortunately, how photogenerated hot carriers interact with phonons in these doped CQWs still remains an open question. Here, through investigating the emission properties, we have observed an efficient phonon cascade process (i.e., up to 27 longitudinal optical phonon replicas are revealed in the broad Cu emission band at room temperature) and identified a giant Huang-Rhys factor (S ≈ 12.4, more than 1 order of magnitude larger than reported values of other inorganic semiconductor nanomaterials) in Cu-doped CQWs. We argue that such an ultrastrong electron-phonon coupling in Cu-doped CQWs is due to the dopant-induced lattice distortion and the dopant-enhanced density of states. These findings break the widely accepted consensus that electron-phonon coupling is typically weak in quantum-confined systems, which are crucial for optoelectronic applications of doped electronic nanomaterials.

Photoluminescence Enhancement by Band Alignment Engineering in MoS2/FePS3 van der Waals Heterostructures

Single-layer semiconducting transition metal dichalcogenides (2H-TMDs) display robust excitonic photoluminescence emission, which can be improved by controlled changes to the environment and the chemical potential of the material. However, a drastic emission quench has been generally observed when TMDs are stacked in van der Waals heterostructures, which often favor the nonradiative recombination of photocarriers. Herein, we achieve an enhancement of the photoluminescence of single-layer MoS2 on top of van der Waals FePS3. The optimal energy band alignment of this heterostructure preserves light emission of MoS2 against nonradiative interlayer recombination processes and favors the charge transfer from MoS2, an n-type semiconductor, to FePS3, a p-type narrow-gap semiconductor. The strong depletion of carriers in the MoS2 layer is evidenced by a dramatic increase in the spectral weight of neutral excitons, which is strongly modulated by the thickness of the FePS3 underneath, leading to the increase of photoluminescence intensity. The present results demonstrate the potential for the rational design of van der Waals heterostructures with advanced optoelectronic properties.

Temperature-dependent optical phonon shifts and splitting in cubic 10BP, natBP, and 11BP crystals

The infrared reflectance spectrum of cubic boron phosphide (BP) single crystals shows a very narrow Reststrahlen band, indicating a small TO-LO (transverse optical-longitudinal optical) splitting. To study the phonon thermal behavior of the TO(Γ) and the LO(Γ) of ¹⁰BP, ^natBP, and ¹¹BP bulk single crystals, temperature-dependent infrared reflectance spectroscopy in 85-500 K is applied here. As the temperature increases, the Reststrahlen band broadens. The frequencies of the TO(Γ) and the LO(Γ) exhibit nonlinear red shifts, and the TO-LO splitting gradually increases. Our research found that thermal expansion plays a leading role at low temperatures while phonon anharmonicity gradually takes place at high temperatures.