Electrical spectroscopy of defect states and their hybridization in monolayer MoS(2)

Deterministic grayscale nanotopography to engineer mobilities in strained MoS2 FETs

Field-effect transistors (FETs) based on two-dimensional materials (2DMs) with atomically thin channels have emerged as a promising platform for beyond-silicon electronics. However, low carrier mobility in 2DM transistors driven by phonon scattering remains a critical challenge. To address this issue, we propose the controlled introduction of localized tensile strain as an effective means to inhibit electron-phonon scattering in 2DM. Strain is achieved by conformally adhering the 2DM via van der Waals forces to a dielectric layer previously nanoengineered with a gray-tone topography. Our results show that monolayer MoS2 FETs under tensile strain achieve an 8-fold increase in on-state current, reaching mobilities of 185 cm²/Vs at room temperature, in good agreement with theoretical calculations. The present work on nanotopographic grayscale surface engineering and the use of high-quality dielectric materials has the potential to find application in the nanofabrication of photonic and nanoelectronic devices.

CVD-Grown Monolayer MoS2 and GaN Thin Film Heterostructure for a Self-Powered and Bidirectional Photodetector with an Extended Active Spectrum

Photodetector technology has evolved significantly over the years with the emergence of new active materials. However, there remain trade-offs between spectral sensitivity, operating energy, and, more recently, an ability to harbor additional features such as persistent photoconductivity and bidirectional photocurrents for new emerging application areas such as switchable light imaging and filter-less color discrimination. Here, we demonstrate a self-powered bidirectional photodetector based on molybdenum disulfide/gallium nitride (MoS2/GaN) epitaxial heterostructure. This fabricated detector exhibits self-powered functionality and achieves detection in two discrete wavelength bands: ultraviolet and visible. Notably, it attains a peak responsivity of 631 mAW-1 at a bias of 0V. The device's response to illumination at these two wavelengths is governed by distinct mechanisms, activated under applied bias conditions, thereby inducing a reversal in the polarity of the photocurrent. This work underscores the feasibility of self-powered and bidirectional photocurrent detection but also opens new vistas for technological advancements for future optoelectronic, neuromorphic, and sensing applications.

Ordered Vacancies as Sodium Ion Micropumps in Cu-Deficient Copper Indium Diselenide to Enhance Sodium Storage

Unordered vacancies engineered in host anode materials cannot well maintain the uniform Na+ adsorbed and possibly render the local structural stress intense, resulting in electrode peeling and battery failure. Here, the indium is first introduced into Cu2Se to achieve the formation of CuInSe2. Next, an ion extraction strategy is employed to fabricate Cu0.54In1.15Se2 enriched with ordered vacancies by spontaneous formation of defect pairs. Such ordered defects, compared with unordered ones, can serve as myriad sodium ion micropumps evenly distributing in crystalline host to homogenize the adsorbed Na+ and the generated volumetric stress during the electrochemistry. Furthermore, Cu0.54In1.15Se2 is indeed proved by the calculations to exhibit smaller volumetric variation than the counterpart with unordered vacancies. Thanks to the distinct ordered vacancy structure, the material exhibits a highly reversible capacity of 428 mAh g-1 at 1 C and a high-rate stability of 311.7 mAh g-1 at 10 C after 5000 cycles when employed as an anode material for Sodium-ion batteries (SIBs). This work presents the promotive effect of ordered vacancies on the electrochemistry of SIBs and demonstrates the superiority to unordered vacancies, which is expected to extend it to other metal-ion batteries, not limited to SIBs to achieve high capacity and cycling stability.

A Molecular Coordination Strategy for Regulating the Interface of MoS2 Field Effect Transistors

Chemically modifying monolayer two-dimensional transition metal dichalcogenides (TMDs) with organic molecules provides a wide range of possibilities to regulate the electronic and optoelectronic performance of both materials and devices. However, it remains challenging to chemically attach organic molecules to monolayer TMDs without damaging their crystal structures. Herein, we show that the Mo atoms of monolayer MoS2 (1L-MoS2) in defect states can coordinate with both catechol and 1,10-phenanthroline (Phen) groups, affording a facile route to chemically modifying 1L-MoS2. Through the design of two isomeric molecules (LA2 and LA5) comprising catechol and Phen groups, we show that attaching organic molecules to Mo atoms via coordinative bonds has no negative effect on the crystal structure of 1L-MoS2. Both theoretical calculation and experiment results indicate that the coordinative strategy is beneficial for (i) repairing sulfur vacancies and passivating defects; (ii) achieving a long-term and stable n-doping effect; and (iii) facilitating the electron transfer. Field effect transistors (FETs) based on the coordinatively modified 1L-MoS2 show high electron mobilities up to 120.3 cm2 V-1 s-1 with impressive current on/off ratios over 109. Our results indicate that coordinatively attaching catechol- or Phen-bearing molecules may be a general method for the nondestructive modification of TMDs.

In Operando Study of Charge Modulation in MoS2 Transistors by Excitonic Reflection Microscopy

Monolayers of transition metal dichalcogenides (2D TMDs) experience strong modulation of their optical properties when the charge density is varied. Indeed, the transition from carriers composed mostly of excitons at low electron density to a situation in which trions dominate at high density is accompanied by a significant evolution of both the refractive index and the extinction coefficient. Using optical interference reflection microscopy at the excitonic wavelength, this (n, κ)-q relationship can be exploited to directly image the electron density in operating TMD devices. In this work, we show how this technique, which we call XRM (excitonic reflection microscopy), can be used to study charge distribution in MoS2 field-effect transistors with subsecond throughput, in wide-field mode. Complete maps of the charge distribution in the transistor channel at any drain and gate bias polarization point (VDS, VGS) are obtained, at ∼3 orders of magnitude faster than with scanning probe techniques such as KPFM. We notably show how the advantages of XRM enable real-time mapping of bias-dependent charge inhomogeneities, the study of resistive delays in 2D polycrystalline networks, and the evaluation of the VDS vs VGS competition to control the charge distribution in active devices.

Improving the Operational Lifetime of Metal-Halide Perovskite Light-Emitting Diodes with Dimension Control and Ligand Engineering

Perovskite light-emitting diodes (LEDs) have emerged as one of the most propitious candidates for next-generation lighting and displays, with the highest external quantum efficiency (EQE) of perovskite LEDs already surpassing the 20% milestone. However, the further development of perovskite LEDs primarily relies on addressing operational instability issues. This Perspective examines some of the key factors that impact the lifetime of perovskite LED devices and some representative reports on recent advancements aimed at improving the lifetime. Our analysis underscores the significance of "nano" strategies in achieving long-term stable perovskite LEDs. Significant efforts must be directed toward proper device encapsulation, perovskite material passivation, interfacial treatment to address environment-induced material instability, bias-induced phase separation, and ion migration issues.

Excitonic Rydberg States in a Trilayer to Monolayer H2-Aided CVD-Grown Large-Area MoS2 Film with Excellent UV to Visible Broad Band Photodetection Applications

The diverse nature of optoelectronic properties of few-layer or monolayer MoS2 is generally dominated by A and B excitons. Occasionally, strong Coulombic interactions within the 2D monolayer led to the creation of hydrogen-like Rydberg states of excitons in MoS2 similar to other 2D monolayers. In this paper, a simple process is used to convert trilayer MoS2 films to a monolayer by introducing H2 gas during chemical vapor deposition. Remarkably, alongside the usual A, B excitons, and A- trion, the appearance of the Rydberg states is evidenced by photoluminescence spectra even at room temperature; also, there is an increase in their areal percentage with an increase in H2 content. The s-type excited Rydberg states up to the fourth order (n = 5) and third order (n = 4) of A and B excitons, respectively, have been probed from the photoluminescence spectra at 93 K. Unprecedentedly, the first-order derivative of room-temperature photocurrent spectrum reveals the Rydberg states concurrently and elaboratively. Furthermore, the large-area MoS2 films exhibit photoresponse in a broad UV to visible region with excellent photosensitivity (∼102) toward both UV and visible lights. Not only does this provide a profound understanding of the excitonic Rydberg states but also highlights the considerable potential of large-area monolayer MoS2 overcoming the difficulty of tiny flake-related 2D device endeavors.

Cesium Cyclopropane Acid-Aided Crystal Growth Enables Efficient Inorganic Perovskite Solar Cells with a High Moisture Tolerance

While all-inorganic halide perovskites (iHPs) are promising photovoltaic materials, the associated water sensitivity of iHPs calls for stringent humidity control to reach satisfactory photovoltaic efficiencies. Herein, we report a moisture-insensitive perovskite formation route under ambient air for CsPbI2 Br-based iHPs via cesium cyclopropane acids (C3 ) as a compound introducer. With this approach, appreciably enhanced crystallization quality and moisture tolerance of CsPbI2 Br are attained. The improvements are attributed to the modified evaporation enthalpy of the volatile side product of DMA-acid initiated by Cs-acids. As such, the water-involving reaction is directed toward the DMA-acids, leaving the target CsPbI2 Br perovskites insensitive to ambient humidity. We highlight that by controlling the C3 concentration, the dependence of power conversion efficiency (PCE) in CsPbI2 Br devices on the humidity level during perovskite film formation becomes favorably weakened, with the PCEs remaining relatively high (>15 %) associated with improved device stability for RH levels changed from 25 % to 65 %. The champion solar cells yield an impressive PCE exceeding 17 %, showing small degradations (<10 %) for 2000 hours of shell storage and 300 hours of 85/85 (temperature/humidity) tests. The demonstrated C3 -based strategy provides an enabler for improving the long-sought moisture-stability of iHPs toward high photovoltaic device performance.

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

Two-dimensional transition metal dichalcogenides (TMDs) offer fascinating opportunities for fundamental nanoscale science and various technological applications. They are a promising platform for next generation optoelectronics and energy harvesting devices due to their exceptional characteristics at the nanoscale, such as tunable bandgap and strong light-matter interactions. The performance of TMD-based devices is mainly governed by the structure, composition, size, defects, and the state of their interfaces. Many properties of TMDs are influenced by the method of synthesis so numerous studies have focused on processing high-quality TMDs with controlled physicochemical properties. Plasma-based methods are cost-effective, well controllable, and scalable techniques that have recently attracted researchers' interest in the synthesis and modification of 2D TMDs. TMDs' reactivity toward plasma offers numerous opportunities to modify the surface of TMDs, including functionalization, defect engineering, doping, oxidation, phase engineering, etching, healing, morphological changes, and altering the surface energy. Here we comprehensively review all roles of plasma in the realm of TMDs. The fundamental science behind plasma processing and modification of TMDs and their applications in different fields are presented and discussed. Future perspectives and challenges are highlighted to demonstrate the prominence of TMDs and the importance of surface engineering in next-generation optoelectronic applications.

Auxiliary Diagnostic Signal for Deep-Level Detection

We propose and demonstrate that temperature-dependent curve-fitting error values of the Schottky diode I-V curve in the forward regime can be an auxiliary diagnostic signal as the temperature-scan Capacitance DLTS (CDLTS) signals and helps to work time-efficiently with high accuracy when using the Laplace Transform (LT)-DLTS or Isothermal Capacitance transient spectroscopy (ICTS) method. Using Be-doped GaAs showing overlapping DLTS signals, we verify that the LT-DLTS or ICTS analysis within a specific temperature range around the characteristic temperature Tpeak coincides well with the results of the CDLTS and Fourier Transform DLTS performed within the whole temperature range. In particular, we found that the LT-DLTS signals appeared intensively around Tpeak, and we confirmed it with the ICTS result. The occurrence of the curve fitting error signal is attributed to the relatively increased misfit error by the increased thermal emission from the deep-level trap in the case near the Tpeak, because the applied transport model excludes defect characteristics.

Room-Temperature Photoluminescence Mediated by Sulfur Vacancies in 2D Molybdenum Disulfide

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS₂ by annealing at 600 °C in an argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoelectron spectroscopy shows that a Mo 3d_5/2 core peak at 230.1 eV emerges in the annealed MoS₂ associated with nonstoichiometric MoS_x (0 < x < 2), and Raman spectroscopy shows an enhancement of the ∼380 cm^-1 peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ∼1.8 × 10¹⁴ cm^-2, we observe a defect peak at ∼1.72 eV (referred to as LX_D) at room temperature in the photoluminescence (PL) spectrum. The LX_D peak is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LX_D emission is longer than that of band edge excitons, both at room and low temperatures (∼2.44 ns at 8 K). The LX_D peak can be suppressed by annealing the defective MoS₂ in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emissions in MoS₂ are influenced by sulfur vacancies at room and low temperatures.