Electron–Phonon Interaction and Double-Resonance Raman Studies in Monolayer WS₂

Atomically thin layers of 2D WS₂ offer a realization of novel valley-selective electronics and power-efficient optoelectronic device fabrication due to large spin splitting at the top of the valence band and high quantum efficiency. However, the synthesis of the large-area monolayer WS₂ film through chemical vapor deposition (CVD) method is in a rudimentary stage. Here we report a modified CVD method to synthesize high-crystalline monolayer WS₂ (1L) with uniform size distribution over a large area. The intensity of the second-order Raman modes in 1L WS₂ is enhanced, particularly the overtone of the acoustic mode LA­(M), when the excitation wavelength is in the vicinity of B exciton. The variation in the intensity profile of the first-order Raman modes for 1L and bulk WS₂ in (laser-energy-dependent) resonant Raman scattering processes is discussed within the third-order perturbation theory

Temperature-Dependent Raman Studies and Thermal Conductivity of Few-Layer MoS₂

We report on the temperature dependence of in-plane E_2g and out-of-plane A_1g Raman modes in high-quality few-layer MoS₂ (FLMS) prepared using a high-temperature vapor-phase method. The materials obtained were investigated using transmission electron microscopy. The frequencies of these two phonon modes were found to vary linearly with temperature. The first-order temperature coefficients for E¹_2g and A_1g modes were found to be (1.32 and 1.23) × 10^–2 cm^–1/K, respectively. The thermal conductivity of the suspended FLMS at room temperature was estimated to be ∼52 W/mK

In Situ Raman Studies of Electrically Reduced Graphene Oxide and Its Field-Emission Properties

Electric-field-dependent in situ Raman studies have been carried out on chemically prepared graphene oxide. The Raman spectra show significant changes with increase in the applied electric field; in particular, the intensity of the G peak decreases with electric field. This behavior is typical for chemically or thermally reduced graphene oxide. To understand the nature of reduction, we compared the temperature-dependent and electric-field-dependent Raman spectra of graphene oxide and found that the evolutions of Raman spectra are not in agreement with each other, except the intensity of the G peak that decreases in both cases. The D peak broadens significantly with increase in temperature, whereas it sharpens in the case of applied electric field. The electron-field-emission properties of the electrically reduced graphene oxide were also carried out, and the turn-on field was found to be 9.1 V/μm

Bipolar Resistive Switching in TiO₂ Artificial Synapse Mimicking Pavlov’s Associative Learning

Memristive devices are among the most emerging electronic elements to realize artificial synapses for neuromorphic computing (NC) applications and have potential to replace the traditional von-Neumann computing architecture in recent times. In this work, pulsed laser deposition-manufactured Ag/TiO2/Pt memristor devices exhibiting digital and analog switching behavior are considered for NC. The TiO2 memristor shows excellent performance of digital resistive switching with a memory window of order ∼103. Furthermore, the analog resistive switching offers multiple conductance levels supporting the development of the bioinspired synapse. A possible mechanism for digital and analog switching behavior in our device is proposed. Remarkably, essential synaptic functions such as pair-pulse facilitation, long-term potentiation (LTP), and long-term depression (LTD) are successfully realized based on the change in conductance through analog memory characteristics. Based on the LTP-LTD, a neural network simulation for the pattern recognition task using the MNIST data set is investigated, which shows a high recognition accuracy of 95.98%. Furthermore, more complex synaptic behavior such as spike-time-dependent plasticity and Pavlovian classical conditioning is successfully emulated for associative learning of the biological brain. This work enriches the TiO2-based resistive random-access memory, which provides information about the simultaneous existence of digital and analog behavior, thereby facilitating the further implementation of memristors in low-power NC

Thermally Driven Multilevel Non-Volatile Memory with Monolayer MoS₂ for Brain-Inspired Artificial Learning

The demands of modern electronic components require advanced computing platforms for efficient information processing to realize in-memory operations with a high density of data storage capabilities toward developing alternatives to von Neumann architectures. Herein, we demonstrate the multifunctionality of monolayer MoS2 memtransistors, which can be used as a high-geared intrinsic transistor at room temperature; however, at a high temperature (>350 K), they exhibit synaptic multilevel memory operations. The temperature-dependent memory mechanism is governed by interfacial physics, which solely depends on the gate field modulated ion dynamics and charge transfer at the MoS2/dielectric interface. We have proposed a non-volatile memory application using a single Field Effect Transistor (FET) device where thermal energy can be ventured to aid the memory functions with multilevel (3-bit) storage capabilities. Furthermore, our devices exhibit linear and symmetry in conductance weight updates when subjected to electrical potentiation and depression. This feature has enabled us to attain a high classification accuracy while training and testing the Modified National Institute of Standards and Technology datasets through artificial neural network simulation. This work paves the way toward reliable data processing and storage using 2D semiconductors with high-packing density arrays for brain-inspired artificial learning

Surface Energy Engineering for Tunable Wettability through Controlled Synthesis of MoS₂

MoS₂ is an important member of the transition metal dichalcogenides that is emerging as a potential 2D atomically thin layered material for low power electronic and optoelectronic applications. However, for MoS₂ a critical fundamental question of significant importance is how the surface energy and hence the wettability is altered at the nanoscale in particular, the role of crystallinity and orientation. This work reports on the synthesis of large area MoS₂ thin films on insulating substrates (SiO₂/Si and Al₂O₃) with different surface morphology via vapor phase deposition by varying the growth temperatures. The samples were examined using transmission electron microscopy and Raman spectroscopy. From contact angle measurements, it is possible to correlate the wettability with crystallinity at the nanoscale. The specific surface energy for few layers MoS₂ is estimated to be about 46.5 mJ/m². Moreover a layer thickness-dependent wettability study suggests that the lower the thickness is, the higher the contact angle will be. Our results shed light on the MoS₂–water interaction that is important for the development of devices based on MoS₂ coated surfaces for microfluidic applications

Spin-Polarized Tunneling through Chemical Vapor Deposited Multilayer Molybdenum Disulfide

The two-dimensional (2D) semiconductor molybdenum disulfide (MoS₂) has attracted widespread attention for its extraordinary electrical-, optical-, spin-, and valley-related properties. Here, we report on spin-polarized tunneling through chemical vapor deposited multilayer MoS₂ (∼7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5–2% has been observed, corresponding to spin polarization of 5–10% in the measured temperature range of 300–75 K. First-principles calculations for ideal junctions result in a TMR up to 8% and a spin polarization of 26%. The detailed measurements at different temperature, bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS₂ spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomena that control their performance