Surface nanotopography and cell shape modulate tumor cell susceptibility to NK cell cytotoxicity.

Natural killer (NK) cells are innate cytotoxic lymphocytes exerting cytotoxicity against virally infected cells and tumor cells. NK cell cytotoxicity is primarily determined by biochemical signals received from ligands expressed on target cell surfaces, but it is also possible that biophysical environments of tumor cells, such as nanoscale surface topography typically existing on extracellular matrixes (ECMs) or cell morphology determined by ECM spaces or cell density, regulate NK cell cytotoxicity. In this study, micro/nanofabrication technology was applied to examine this possibility. Tumor cells were plated on flat or nanogrooved surfaces, or micropatterned into circular or elliptical geometries, and the effects of surface topography and tumor cell morphology on NK cell cytotoxicity were investigated. NK cells exhibited significantly higher cytotoxicity against tumor cells on nanogrooved surfaces or tumor cells in elliptical patterns than tumor cells on flat surfaces or tumor cells in circular patterns, respectively. The amounts of stress fiber formation in tumor cells positively correlated with NK cell cytotoxicity, indicating that increased cellular tension of tumor cells, either mediated by nanogrooved surfaces or elongated morphologies, was a key factor regulating NK cell cytotoxicity. These results may provide insight into the design of NK cell-based cancer immunotherapy.

Ambipolar Charge Transport in Two-Dimensional WS2 Metal-Insulator-Semiconductor and Metal-Insulator-Semiconductor Field-Effect Transistors.

Two-dimensional (2D) materials with ambipolar transport characteristics have attracted considerable attention as post-complementary metal-oxide semiconductor (CMOS) materials. These materials allow for electron- or hole-dominant conduction to be achieved in a single channel of the field-effect transistors (FETs) without an extrinsic doping. In this study, all-2D metal-insulator-semiconductor (MIS)-based devices, which were composed of all-2D graphene, hexagonal boron nitride, and WS2, exhibited ambipolar and symmetrical transport characteristics with a low surface state density (Dit, min ≈ 7 × 1011 cm-2·eV-1). Hole- or electron-dominant inversion under the influence of electrostatic doping was obtained in a WS2-based 2D capacitor up to a frequency range of 1 MHz. n- and p-channel conductions with enhancement-mode operations were selectively realized in a single MISFET, which presented a current on/off ratio of >106 and high field-effect mobility (µe = 58-67 cm2/V·s and µh = 19-30 cm2/V·s). Furthermore, a monolithic CMOS-like logic inverter, which employed a single WS2 flake, exhibited a high gain of 78. These results can be used to reduce the footprints of the device architectures and simplify the device fabrication processes of next-generation CMOS integrated circuits.

Monolithically Integrated Enhancement-Mode and Depletion-Mode ß-Ga2O3 MESFETs with Graphene-Gate Architectures and Their Logic Applications.

Ultrawide band gap (UWBG) ß-Ga2O3 is a promising material for next-generation power electronic devices. An enhancement-mode (E-mode) device is essential for designing power conversion systems with simplified circuitry and minimal loss. The integration of an E-mode field-effect transistor (FET) with a depletion-mode (D-mode) FET can build a high-performance logic circuit. In this study, we first demonstrated the realization of an E-mode quasi-two-dimensional (quasi-2D) ß-Ga2O3 FET with a novel graphene gate architecture via a van der Waals heterojunction. Then, we monolithically integrated it with a D-mode quasi-2D ß-Ga2O3 FET, achieving an area-efficient logic circuit. The threshold voltage of the n-channel UWBG ß-Ga2O3 material was controlled by forming a novel architecture of a double-gate graphene/ß-Ga2O3 heterojunction, where both graphene and ß-Ga2O3 were obtained by a mechanical exfoliation method. The fabricated double graphene-gate ß-Ga2O3 metal-semiconductor FET (MESFET) was operated in the E-mode with a positive threshold voltage of +0.25 V, which is approximately 1.2 V higher than that of a single-gate D-mode ß-Ga2O3 MESFET. Both E-/D-modes ß-Ga2O3 MESFETs showed excellent electrical characteristics with a subthreshold swing of 68.9 and 84.6 mV/dec, respectively, and a high on/off current ratio of approximately 107. A ß-Ga2O3 logic inverter composed of E-/D-mode ß-Ga2O3 devices exhibited desired inversion characteristics. The monolithic integration of an E-/D-mode quasi-2D FET with an UWBG channel layer can pave the way for various applications in smart and robust power (nano) electronics.

Heterostructure WSe2-Ga2O3 Junction Field-Effect Transistor for Low-Dimensional High-Power Electronics.

Layered materials separated from each bulk crystal can be assembled to form a strain-free heterostructure by using the van der Waals interaction. We demonstrated a heterostructure n-channel depletion-mode ß-Ga2O3 junction field-effect transistor (JFET) through van der Waals bonding with an exfoliated p-WSe2 flake. Typical diode characteristics with a high rectifying ratio of ∼105 were observed in a p-WSe2/n-Ga2O3 heterostructure diode, where WSe2 and ß-Ga2O3 were obtained by mechanically exfoliating each crystal. Layered JFETs exhibited an excellent IDS- VDS output as well as IDS- VGS transfer characteristics with a high on/off ratio (∼108) and low subthreshold swing (133 mV/dec). Saturated output currents were observed with a threshold voltage of -5.1 V and a three-terminal breakdown voltage of +144 V. Electrical performances of the fabricated heterostructure JFET were maintained at elevated temperatures with outstanding air stability. Our WSe2-Ga2O3 heterostructure JFET paves the way to the low-dimensional high-power devices, enabling miniaturization of the integrated power electronic systems.

Quasi-Two-Dimensional h-BN/ß-Ga2O3 Heterostructure Metal-Insulator-Semiconductor Field-Effect Transistor.

ß-gallium oxide (ß-Ga2O3) and hexagonal boron nitride (h-BN) heterostructure-based quasi-two-dimensional metal-insulator-semiconductor field-effect transistors (MISFETs) were demonstrated by integrating mechanical exfoliation of (quasi)-two-dimensional materials with a dry transfer process, wherein nanothin flakes of ß-Ga2O3 and h-BN were utilized as the channel and gate dielectric, respectively, of the MISFET. The h-BN dielectric, which has an extraordinarily flat and clean surface, provides a minimal density of charged impurities on the interface between ß-Ga2O3 and h-BN, resulting in superior device performances (maximum transconductance, on/off ratio, subthreshold swing, and threshold voltage) compared to those of the conventional back-gated configurations. Also, double-gating of the fabricated device was demonstrated by biasing both top and bottom gates, achieving the modulation of the threshold voltage. This heterostructured wide-band-gap nanodevice shows a new route toward stable and high-power nanoelectronic devices.

Exfoliated ß-Ga2O3 nano-belt field-effect transistors for air-stable high power and high temperature electronics.

This study demonstrated the exfoliation of a two-dimensional (2D) ß-Ga2O3 nano-belt and subsequent processing into a thin film transistor structure. This mechanical exfoliation and transfer method produces ß-Ga2O3 nano-belts with a pristine surface as well as a continuous defect-free interface with the SiO2/Si substrate. This ß-Ga2O3 nano-belt based transistor displayed an on/off ratio that increased from approximately 10(4) to 10(7) over the operating temperature range of 20 °C to 250 °C. No electrical breakdown was observed in our measurements up to VDS = +40 V and VGS = -60 V between 25 °C and 250 °C. Additionally, the electrical characteristics were not degraded after a month-long storage in ambient air. The demonstration of high-temperature/high-voltage operation of quasi-2D ß-Ga2O3 nano-belts contrasts with traditional 2D materials such as transition metal dichalcogenides that intrinsically have limited temperature and power operational envelopes owing to their narrow bandgap. This work motivates the application of 2D ß-Ga2O3 to high power nano-electronic devices for harsh environments such as high temperature chemical sensors and photodetectors as well as the miniaturization of power circuits and cooling systems in nano-electronics.