Reduced interface effect of proton beam irradiation on the electrical properties of WSe2/hBN field effect transistors.

Two-dimensional transition metal dichalcogenide (TMDC) semiconductors are emerging as strong contenders for electronic devices that can be used in highly radioactive environments such as outer space where conventional silicon-based devices exhibit nonideal characteristics for such applications. To address the radiation-induced interface effects of TMDC-based electronic devices, we studied high-energy proton beam irradiation effects on the electrical properties of field-effect transistors (FETs) made with tungsten diselenide (WSe2) channels and hexagonal boron-nitride (hBN)/SiO2gate dielectrics. The electrical characteristics of WSe2FETs were measured before and after the irradiation at various proton beam doses of 1013, 1014, and 1015cm-2. In particular, we demonstrated the dependence of proton irradiation-induced effects on hBN layer thickness in WSe2FETs. We observed that the hBN layer reduces the WSe2/dielectric interface effect which would shift the transfer curve of the FET toward the positive direction of the gate voltage. Also, this interface effect was significantly suppressed when a thicker hBN layer was used. This phenomenon can be explained by the fact that the physical separation of the WSe2channel and SiO2dielectric by the hBN interlayer prevents the interface effects originating from the irradiation-induced positive trapped charges in SiO2reaching the interface. This work will help improve our understanding of the interface effect of high-energy irradiation on TMDC-based nanoelectronic devices.

Enhanced Thermoelectric Power Factor in Carrier-Type-Controlled Platinum Diselenide Nanosheets by Molecular Charge-Transfer Doping.

2D transition metal dichalcogenides (TMDCs) have revealed great promise for realizing electronics at the nanoscale. Despite significant interests that have emerged for their thermoelectric applications due to their predicted high thermoelectric figure of merit, suitable doping methods to improve and optimize the thermoelectric power factor of TMDCs have not been studied extensively. In this respect, molecular charge-transfer doping is utilized effectively in TMDC-based nanoelectronic devices due to its facile and controllable nature owing to a diverse range of molecular designs available for modulating the degree of charge transfer. In this study, the power of molecular charge-transfer doping is demonstrated in controlling the carrier-type (n- and p-type) and thermoelectric power factor in platinum diselenide (PtSe2 ) nanosheets. This, combined with the tunability in the band overlap by changing the thickness of the nanosheets, allows a significant increase in the thermoelectric power factor of the n- and p-doped PtSe2 nanosheets to values as high as 160 and 250 µW mK-2 , respectively. The methodology employed in this study provides a simple and effective route for the molecular doping of TMDCs that can be used for the design and development of highly efficient thermoelectric energy conversion systems.

Proton irradiation effects on mechanochemically synthesized and flash-evaporated hybrid organic-inorganic lead halide perovskites.

A hybrid organic-inorganic halide perovskite is a promising material for developing efficient solar cell devices, with potential applications in space science. In this study, we synthesized methylammonium lead iodide (MAPbI3) perovskites via two methods: mechanochemical synthesis and flash evaporation. We irradiated these perovskites with highly energetic 10 MeV proton-beam doses of 1011, 1012, 1013, and 4 × 1013protons cm-2and examined the proton irradiation effects on the physical properties of MAPbI3perovskites. The physical properties of the mechanochemically synthesized MAPbI3perovskites were not considerably affected after proton irradiation. However, the flash-evaporated MAPbI3perovskites showed a new peak in x-ray diffraction and an increased fluorescence lifetime in time-resolved photoluminescence under high-dose conditions, indicating considerable changes in their physical properties. This difference in behavior between MAPbI3perovskites synthesized via the abovementioned two methods may be attributed to differences in radiation hardness associated with the bonding strength of the constituents, particularly Pb-I bonds. Our study will help to understand the radiation effect of proton beams on organometallic halide perovskite materials.

Enhanced Photodetection Performance of an In Situ Core/Shell Perovskite-MoS2 Phototransistor.

While two-dimensional transition metal dichalcogenides (TMDCs)-based photodetectors offer prospects for high integration density and flexibility, their thinness poses a challenge regarding low light absorption, impacting photodetection sensitivity. Although the integration of TMDCs with metal halide perovskite nanocrystals (PNCs) has been known to be promising for photodetection with a high absorption coefficient of PNCs, the low charge mobility of PNCs delays efficient photocarrier injection into TMDCs. In this study, we integrated MoS2 with in situ formed core/shell PNCs with short ligands that minimize surface defects and enhance photocarrier injection. The PNCs/MoS2 heterostructure efficiently separates electrons and holes by establishing type II band alignment and consequently inducing a photogating effect. The synergistic interplay between photoconductive and photogating effects yields a high responsivity of 2.2 × 106 A/W and a specific detectivity of 9.0 × 1011 Jones. Our findings offer a promising pathway for developing low-cost, high-performance phototransistors leveraging the advantages of two-dimensional (2D) materials.

Seamless Capacitive Body Channel Wireless Power Transmission Toward Freely Moving Multiple Animals in an Animal Cage.

Unstable wireless power transmission toward multiple living animals in an animal cage is one of the significant barriers to performing long-term and real-time neural monitoring in preclinical research. Here, seamless capacitive body channel (SCB) wireless power transmission (WPT) along with power management integrated circuit (PMIC) is designed using a standard 65 nm CMOS process. The SCB WPT enables stable wireless power transmission toward multiple 35 mm×20 mm×2 mm sized receivers (RXs) attached to freely moving animals in a 600 mm×600 mm×120 mm sized animal cage. By utilizing fringe-field capacitance and a body channel for wireless power link between the cage and RXs, the maximum difference in all measured power efficiencies in diverse scenarios is only 6.66 % with a 20 mW load. Even with a 90 ° RX rotation against the cage, power efficiency marks 17.76 %. Furthermore, an in-vivo experiment conducted with three untethered rats demonstrates the capability of continuous long-term power delivery in practical situations.

Channel-Length-Modulated Avalanche Multiplication in Ambipolar WSe2 Field-Effect Transistors.

Recently there has been growing interest in avalanche multiplication in two-dimensional (2D) materials and device applications such as avalanche photodetectors and transistors. Previous studies have mainly utilized unipolar semiconductors as the active material and focused on developing high-performance devices. However, fundamental analysis of the multiplication process, particularly in ambipolar materials, is required to establish high-performance electronic devices and emerging architectures. Although ambipolar 2D materials have the advantage of facile carrier-type tuning through electrostatic gating, simultaneously allowing both carrier types in a single channel poses an inherent difficulty in analyzing their individual contributions to avalanche multiplication. In ambipolar field-effect transistors (FETs), two phenomena of ambipolar transport and avalanche multiplication can occur, and both exhibit secondary rise of output current at high lateral voltage. We distinguished these two competing phenomena using the method of channel length modulation and successfully analyzed the properties of electron- and hole-initiated multiplication in ambipolar WSe2 FETs. Our study provides a simple and robust method to examine carrier multiplication in ambipolar materials and will foster the development of high-performance atomically thin electronic devices utilizing avalanche multiplication.

Reduced dopant-induced scattering in remote charge-transfer-doped MoS2 field-effect transistors.

Efficient doping for modulating electrical properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is essential for meeting the versatile requirements for future electronic and optoelectronic devices. Because doping of semiconductors, including TMDCs, typically involves generation of charged dopants that hinder charge transport, tackling Coulomb scattering induced by the externally introduced dopants remains a key challenge in achieving ultrahigh mobility 2D semiconductor systems. In this study, we demonstrated remote charge transfer doping by simply inserting a hexagonal boron nitride layer between MoS2 and solution-deposited n-type dopants, benzyl viologen. A quantitative analysis of temperature-dependent charge transport in remotely doped devices supports an effective suppression of the dopant-induced scattering relative to the conventional direct doping method. Our mechanistic investigation of the remote doping method promotes the charge transfer strategy as a promising method for material-level tailoring of electrical and optoelectronic devices based on TMDCs.

Molecular Dopant-Dependent Charge Transport in Surface-Charge-Transfer-Doped Tungsten Diselenide Field Effect Transistors.

The controllability of carrier density and major carrier type of transition metal dichalcogenides(TMDCs) is critical for electronic and optoelectronic device applications. To utilize doping in TMDC devices, it is important to understand the role of dopants in charge transport properties of TMDCs. Here, the effects of molecular doping on the charge transport properties of tungsten diselenide (WSe2 ) are investigated using three p-type molecular dopants, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 -TCNQ), tris(4-bromophenyl)ammoniumyl hexachloroantimonate (magic blue), and molybdenum tris(1,2-bis(trifluoromethyl)ethane-1,2-dithiolene) (Mo(tfd-COCF3 )3 ). The temperature-dependent transport measurements show that the dopant counterions on WSe2 surface can induce Coulomb scattering in WSe2 channel and the degree of scattering is significantly dependent on the dopant. Furthermore, the quantitative analysis revealed that the amount of charge transfer between WSe2 and dopants is related to not only doping density, but also the contribution of each dopant ion toward Coulomb scattering. The first-principles density functional theory calculations show that the amount of charge transfer is mainly determined by intrinsic properties of the dopant molecules such as relative frontier orbital positions and their spin configurations. The authors' systematic investigation of the charge transport of doped TMDCs will be directly relevant for pursuing molecular routes for efficient and controllable doping in TMDC nanoelectronic devices.

Ultrasensitive Photodetection in MoS2 Avalanche Phototransistors.

Recently, there have been numerous studies on utilizing surface treatments or photosensitizing layers to improve photodetectors based on 2D materials. Meanwhile, avalanche breakdown phenomenon has provided an ultimate high-gain route toward photodetection in the form of single-photon detectors. Here, the authors report ultrasensitive avalanche phototransistors based on monolayer MoS2 synthesized by chemical vapor deposition. A lower critical field for the electrical breakdown under illumination shows strong evidence for avalanche breakdown initiated by photogenerated carriers in MoS2 channel. By utilizing the photo-initiated carrier multiplication, their avalanche photodetectors exhibit the maximum responsivity of ≈3.4 × 107 A W-1 and the detectivity of ≈4.3 × 1016 Jones under a low dark current, which are a few orders of magnitudes higher than the highest values reported previously, despite the absence of any additional chemical treatments or photosensitizing layers. The realization of both the ultrahigh photoresponsivity and detectivity is attributed to the interplay between the carrier multiplication by avalanche breakdown and carrier injection across a Schottky barrier between the channel and metal electrodes. This work presents a simple and powerful method to enhance the performance of photodetectors based on carrier multiplication phenomena in 2D materials and provides the underlying physics of atomically thin avalanche photodetectors.