Improving the optoelectronic properties of monolayer MoS2field effect transistor through dielectric engineering.

The reduced dielectric screening in atomically thin two-dimensional materials makes them very sensitive to the surrounding environment, which can be modulated to tune their optoelectronic properties. In this study, we significantly improved the optoelectronic properties of monolayer MoS2by varying the surrounding environment using different liquid dielectrics, each with a specific dielectric constant ranging from 1.89 to 18. Liquid mediums offer the possibility of environment tunability on the same device. For a back-gated field effect transistor, the field effect mobility exhibited more than two-order enhancement when exposed to a high dielectric constant medium. Further investigation into the effect of the dielectric environment on the optoelectronic properties demonstrated a variation in photoresponse relaxation time with the dielectric medium. The rise and decay times were observed to increase and decrease, respectively, with an increase in the dielectric constant of the medium. These results can be attributed to the dielectric screening provided by the surrounding medium, which strongly modifies the charged impurity scattering, the band gap, and defect levels of monolayer MoS2. These findings have important implications for the design of biological and chemical sensors, particularly those operating in a liquid environment. By leveraging the tunability of the dielectric medium, we can optimize the performance of such sensors and enhance their detection capabilities.

Surface Functionalization of Black Phosphorus via Potassium toward High-Performance Complementary Devices.

Two-dimensional black phosphorus configured field-effect transistor devices generally show a hole-dominated ambipolar transport characteristic, thereby limiting its applications in complementary electronics. Herein, we demonstrate an effective surface functionalization scheme on few-layer black phosphorus, through in situ surface modification with potassium, with a view toward high performance complementary device applications. Potassium induces a giant electron doping effect on black phosphorus along with a clear bandgap reduction, which is further corroborated by in situ photoelectron spectroscopy characterizations. The electron mobility of black phosphorus is significantly enhanced to 262 (377) cm2 V-1 s-1 by over 1 order of magnitude after potassium modification for two-terminal (four-terminal) measurements. Using lithography technique, a spatially controlled potassium doping technique is developed to establish high-performance complementary devices on a single black phosphorus nanosheet, for example, the p-n homojunction-based diode achieves a near-unity ideality factor of 1.007 with an on/off ratio of ∼104. Our findings coupled with the tunable nature of in situ modification scheme enable black phosphorus as a promising candidate for further complementary electronics.

Enhancement of electrophoretic mobility of microparticles near a solid wall--experimental verification.

Although the existing theories have predicted enhancement of electrophoretic mobility of microparticles near a solid wall, the relevant experimental studies are rare. This is mainly due to difficulties in experimentally controlling and measuring particle-wall separations under dynamic electrophoretic conditions. This paper reports an experimental verification of the enhancement of electrophoretic mobility of a microparticle moving near the wall of a microchannel. This is achieved by balancing dielectrophoretic and lift forces against gravitational force acting on the microparticle so as to control the gap of particle-wall separation. A simple experimental setup is configured and a fabrication method is developed to measure such separation gap. The experiments are conducted for various particle sizes under different electric field strengths. Our experimental results are compared against the available theoretical predictions in the literature.

Nonconventional Strain Engineering for Uniform Biaxial Tensile Strain in MoS2 Thin Film Transistors.

Strain engineering has been employed as a crucial technique to enhance the electrical properties of semiconductors, especially in Si transistor technologies. Recent theoretical investigations have suggested that strain engineering can also markedly enhance the carrier mobility of two-dimensional (2D) transition-metal dichalcogenides (TMDs). The conventional methods used in strain engineering for Si and other bulk semiconductors are difficult to adapt to ultrathin 2D TMDs. Here, we report a strain engineering approach to apply the biaxial tensile strain to MoS2. Metal-organic chemical vapour deposition (MOCVD)-grown large-area MoS2 films were transferred onto SiO2/Si substrate, followed by the selective removal of the underneath Si. The release of compressive residual stress in the oxide layer induces strain in MoS2 on top of the SiO2 layer. The amount of strain can be precisely controlled by the thickness of oxide stressors. After the transistors were fabricated with strained MoS2 films, the array of strained transistors was transferred onto plastic substrates. This process ensured that the MoS2 channels maintained a consistent tensile strain value across a large area.

X-ray based tools for the investigation of buried interfaces in organic electronic devices.

X-ray reflectivity combined with grazing incidence diffraction is a valuable tool for investigating organic multilayer structures that can be used in devices. We focus on a bilayer stack consisting of two materials (poly-(3-hexylthiophene)) (P3HT) and poly-(4-styrenesulfonic acid) (PSSA) spin cast from orthogonal solvents (water in the case of PSSA and chloroform or toluene for P3HT). X-ray reflectivity is used to determine the thickness of all layers as well as the roughness of the organic-organic hetero-interface and the P3HT surface. The surface roughness is found to be consistent with the results of atomic force microscopy measurements. For the roughness of P3HT/PSSA interface, we observe a strong dependence on the solvent used for P3HT deposition. The solvent also strongly impacts the texturing of the P3HT crystallites as revealed by grazing incidence diffraction. When applying the various PSSA/P3HT multilayers in organic thin-film transistors, we find an excellent correlation between the determined interface morphology, structure and the device performance.

Defect Compensation Weakening Induced Mobility Enhancement in Thermoelectric BiTeI by Iodine Deficiency.

Carrier mobility (weighted mobility more specifically) of thermoelectrics fundamentally determines its power factor, representing a new cut-in point to optimize the thermoelectric performance. However, researches on enhancing the carrier mobility to improve power factor has been overlooked. In present work, we highlight a significant mobility enhancement in BiTeI by introducing I deficiency, which improves the power factor and final ZT value. A defect compensation weakening mechanism is adopted that the induced I vacancies reduce the concentration of intrinsic I Te • and Te I ' antisite defects, which weakens the donor-acceptor defect compensation and suppresses the defects-induced carrier scattering. As a result, the carrier mobility is obviously enhanced in I-deficient samples, which ensures an effectively improved power factor and final ZT. A maximum ZT of 0.57 is achieved at 570 K perpendicular to the pressing direction, which is superior to pristine BiTeI and among the highest values reported for bulk BiTeI-based thermoelectric materials. Present work opens up a new avenue for thermoelectric optimization mainly by mobility enhancement.

Participatory design and validation of mobility enhancement robotic wheelchair.

The design of the mobility enhancement robotic wheelchair (MEBot) was based on input from electric powered wheelchair (EPW) users regarding the conditions they encounter when driving in both indoor and outdoor environments that may affect their safety and result in them becoming immobilized, tipping over, or falling out of their wheelchair. Phase I involved conducting a participatory design study to understand the conditions and barriers EPW users found to be difficult to drive in/over. Phase II consisted of creating a computer-aided design (CAD) prototype EPW to provide indoor and outdoor mobility that addressed these conditions with advanced applications. Phase III involved demonstrating the advanced applications and gathering feedback from end users about the likelihood they would use the advanced applications. The CAD prototype incorporated advanced applications, including self-leveling, curb climbing, and traction control, that addressed the challenging conditions and barriers discussed with EPW users (n = 31) during the participatory design study. Feedback of the CAD design and applications in phase III from end users (n = 12) showed a majority would use self-leveling (83%), traction control (83%), and curb climbing (75%). The overall design of MEBot received positive feedback from EPW users. However, these opinions will need to be reevaluated through user trials as the design advances.