Room Temperature <i>Cmcm</i> Phase of Ca_<i>x</i>Sn_1–<i>x</i>Se for Thermoelectric Energy Conversion

While SnSe has been known as a high figure of merit (ZT) thermoelectric material at high temperatures, it has a low ZT at room temperature. SnSe has the β (Cmcm) phase at high temperature but the α (Pnma) phase at room temperature. In the present work, we first investigate the phase-transition temperature Tc between the α and β phases of SnSe based on density functional theory calculations and obtain 740 K, which is close to the experimental value of about 800 K. We then consider Ca alloying in SnSe and calculate Tc between the α and β phases of CaxSn1–xSe. It is found that the Ca alloying lowers Tc down to 220 K as the Ca content x increases up to x = 0.1875. For x > 0.14, CaxSn1–xSe is obtained to have the β phase at room temperature, allowing it to be suggested as a room-temperature high figure of merit thermoelectric material

Self-Aligned Top-Gate Amorphous Indium Zinc Oxide Thin-Film Transistors Exceeding Low-Temperature Poly-Si Transistor Performance

Thin-film transistor (TFT) is a key component of active-matrix flat-panel displays (AMFPDs). These days, the low-temperature poly silicon (LTPS) TFTs are to match with advanced AMFPDs such as the active matrix organic light-emitting diode (AMOLED) display, because of their high mobility for fast pixel switching. However, the manufacturing process of LTPS TFT is quite complicated, costly, and scale-limited. Amorphous oxide semiconductor (AOS) TFT technology is another candidate, which is as simple as that of conventioanl amorphous (a)-Si TFTs in fabrication but provides much superior device performances to those of a-Si TFTs. Hence, various AOSs have been compared with LTPS for active channel layer of the advanced TFTs, but have always been found to be relatively inferior to LTPS. In the present work, we clear the persistent inferiority, innovating the device performaces of a-IZO TFT by adopting a self-aligned coplanar top-gate structure and modifying the surface of a-IZO material. Herein, we demonstrate a high-performance simple-processed a-IZO TFT with mobility of ∼157 cm² V^–1 s^–1, SS of ∼190 mV dec^–1, and good bias/photostabilities, which overall surpass the performances of high-cost LTPS TFTs

Visualization 1: Columnar deformation of human red blood cell by highly localized fiber optic Bessel beam stretcher

Irreversible stretching Originally published in Biomedical Optics Express on 01 November 2015 (boe-6-11-4417

J‑MISFET Hybrid Dual-Gate Switching Device for Multifunctional Optoelectronic Logic Gate Applications

High-performance and low operating voltage are becoming increasingly significant device parameters to meet the needs of future integrated circuit (IC) processors and ensure their energy-efficient use in upcoming mobile devices. In this study, we suggest a hybrid dual-gate switching device consisting of the vertically stacked junction and metal–insulator–semiconductor (MIS) gate structure, named J-MISFET. It shows excellent device performances of low operating voltage (<0.5 V), drain current ON/OFF ratio (∼4.7 × 105), negligible hysteresis window (<0.5 mV), and near-ideal subthreshold slope (SS) (60 mV/dec), making it suitable for low-power switching operation. Furthermore, we investigated the switchable NAND/NOR logic gate operations and the photoresponse characteristics of the J-MISFET under the small supply voltage (0.5 V). To advance the applications further, we successfully demonstrated an integrated optoelectronic security logic system comprising 2-electric inputs (for encrypted data) and 1-photonic input signal (for password key) as a hardware security device for data protection. Thus, we believe that our J-MISFET, with its heterogeneous hybrid gate structures, will illuminate the path toward future device configurations for next-generation low-power electronics and multifunctional security logic systems in a data-driven society

Selective Dispersion of Highly Pure Large-Diameter Semiconducting Carbon Nanotubes by a Flavin for Thin-Film Transistors

Scalable and simple methods for selective extraction of pure, semiconducting (s) single-walled carbon nanotubes (SWNTs) is of profound importance for electronic and photovoltaic applications. We report a new, one-step procedure to obtain respective large-diameter s- and metallic (m)-SWNT enrichment purity in excess of 99% and 78%, respectively, via interaction between the aromatic dispersing agent and SWNTs. The approach utilizes <i>N</i>-dodecyl isoalloxazine (FC12) as a surfactant in conjunction with sonication and benchtop centrifugation methods. After centrifugation, the supernatant is enriched in s-SWNTs with less carbonaceous impurities, whereas precipitate is enhanced in m-SWNTs. In addition, the use of an increased centrifugal force enhances both the purity and population of larger diameter s-SWNTs. Photoinduced energy transfer from FC12 to SWNTs is facilitated by respective electronic level alignment. Owing to its peculiar photoreduction capability, FC12 can be employed to precipitate SWNTs upon UV irradiation and observe absorption of higher optical transitions of SWNTs. A thin-film transistor prepared from a dispersion of enriched s-SWNTs was fabricated to verify electrical performance of the sorted sample and was observed to display p-type conductance with an average on/off ratio over 10⁶ and an average mobility over 10 cm²/V·s

Interband Transitions in Monolayer and Few-Layer WSe₂ Probed Using Photoexcited Charge Collection Spectroscopy

Transition-metal dichalcogenides are currently under rigorous investigation because of their distinct layer-dependent physical properties originating from the corresponding evolution of the band structure. Here, we report the highly resolved probing of layer-dependent band structure evolution for WSe2 using photoexcited charge collection spectroscopy (PECCS). Monolayer, few-layer, and multilayer WSe2 can be probed in top-gate field-effect transistor platforms, and their interband transitions are efficiently observed. Our theoretical calculations show a great coincidence with the PECCS results, proving that the indirect Γ–K and Γ–Λ transitions as well as the direct K–K transition are clearly resolved in multilayer WSe2 by PECCS

Black Phosphorus–Zinc Oxide Nanomaterial Heterojunction for p–n Diode and Junction Field-Effect Transistor

Black phosphorus (BP) nanosheet is two-dimensional (2D) semiconductor with distinct band gap and attracting recent attention from researches because it has some similarity to gapless 2D semiconductor graphene in the following two aspects: single element (P) for its composition and quite high mobilities depending on its fabrication conditions. Apart from several electronic applications reported with BP nanosheet, here we report for the first time BP nanosheet–ZnO nanowire 2D–1D heterojunction applications for p–n diodes and BP-gated junction field effect transistors (JFETs) with n-ZnO channel on glass. For these nanodevices, we take advantages of the mechanical flexibility of p-type conducting of BP and van der Waals junction interface between BP and ZnO. As a result, our BP–ZnO nanodimension p–n diode displays a high ON/OFF ratio of ∼10⁴ in static rectification and shows kilohertz dynamic rectification as well while ZnO nanowire channel JFET operations are nicely demonstrated by BP gate switching in both electrostatics and kilohertz dynamics

Photostable Dynamic Rectification of One-Dimensional Schottky Diode Circuits with a ZnO Nanowire Doped by H during Passivation

For the first time, we demonstrated photostable and dynamic rectification in ZnO nanowire (NW) Schottky diode circuits where two diodes are face-to-face connected in the same ZnO wire. With their properties improved by H-doping from atomic layer deposited Al2O3 passivation, our ZnO NW diode circuits stably operated at a maximum frequency of 100 Hz displaying a good rectification even under the lights. We thus conclude that our results promisingly appoached one-dimensional nanoelectronics

Anisotropic Electron Mobility and Contact Resistance of β‑Ga₂O₃ Obtained via Radio Frequency Transmission Line Methods on Schottky Devices

Monoclinic semiconducting β-Ga2O3 has drawn attention, particularly because its thin film could be achieved via mechanical exfoliation from bulk crystals, which is analogous to van der Waals materials’ behavior. For the transistor devices with exfoliated β-Ga2O3, the channel direction becomes [010] for in-plane electron transport, which changes to vertical [100] near the source/drain (S/D) contact. Hence, anisotropic transport behavior is certainly worth to study but rarely reported. Here we achieve the vertical [100] direction electron mobility of 4.18 cm2/(V s) from Pt/β-Ga2O3 Schottky diodes with various thickness via radio frequency-transmission line method (RF-TLM), which is recently developed. The specific contact resistivity (ρc) could also be estimated from RF-TLM, to be 4.72 × 10–5 Ω cm2, which is quite similar to the value (5.25 × 10–5 Ω cm2) from conventional TLM proving the validity of RF-TLM. We also fabricate metal–semiconductor field-effect transistors (MESFETs) to study anisotropic transport behavior and contact resistance (RC). RC-free [010] in-plane mobility appears as high as maximum ∼67 cm2/(V s), extracted from total resistance in MESFETs

Impact of Organic Molecule-Induced Charge Transfer on Operating Voltage Control of Both n‑MoS₂ and p‑MoTe₂ Transistors

Since transition metal dichalcogenide (TMD) semiconductors are found as two-dimensional van der Waals materials with a discrete energy bandgap, many TMD based field effect transistors (FETs) are reported as prototype devices. However, overall reports indicate that threshold voltage (Vth) of those FETs are located far away from 0 V whether the channel is p- or n-type. This definitely causes high switching voltage and unintended OFF-state leakage current. Here, a facile way to simultaneously modulate the Vth of both p- and n-channel FETs with TMDs is reported. The deposition of various organic small molecules on the channel results in charge transfer between the organic molecule and TMD channels. Especially, HAT-CN molecule is found to ideally work for both p- and n-channels, shifting their Vth toward 0 V concurrently. As a proof of concept, a complementary metal oxide semiconductor (CMOS) inverter with p-MoTe2 and n-MoS2 channels shows superior voltage gain and minimal power consumption after HAT-CN deposition, compared to its initial performance. When the same TMD FETs of the CMOS structure are integrated into an OLED pixel circuit for ambipolar switching, the circuit with HAT-CN film demonstrates complete ON/OFF switching of OLED pixel, which was not switched off without HAT-CN