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Al-doped ZnO (AZO) is a promising candidate as a transparent conducting electrode. However, the electrical properties of AZO deteriorate greatly after exposing it to excessive heat. This limits the applications of AZO in devices that experience a demanding operation environment. It has been shown that a ZnO cap layer with proper morphology is capable to dramatically improve the thermal stability of AZO. However, the detailed mechanism is not yet clear. A comparison study of the electrical properties of AZO with a ZnO cap layer prepared by magnetron sputtering (MS) at low substrate temperature (70 °C) and chemical vapor deposition (CVD) at high substrate temperature (600 °C) indicates that MS-prepared ZnO is much less effective in protecting AZO from an oxidizing environment under elevated temperature than the CVD-prepared ZnO. The morphology and crystal structures of two types of ZnO/AZO, investigated by a scanning electron microscope and x-ray diffraction, are relatively similar, whereas the atomic structures (e.g., defects) revealed by Raman spectroscopy are rather different. The results suggest that it is difficult to improve the thermal stability of electrical properties of AZO without a proper restructuring process and a ZnO cap layer that could sacrifice its own structural order. The discoveries offer a novel approach to improve the performance of other transparent conducting oxides. [ABSTRACT FROM AUTHOR]

Steadily, continuously, and precisely adjusting infrared emissivity of materials is of great importance for various applications, such as thermal camouflage. Herein, a method is proposed to fine-tune the emissivity of multilayer graphene (MLG) by ion intercalation with assistance of pulse voltage. The dynamic ion intercalation process in MLG-based emissivity modulators is investigated by a home-made electrical-optical synchronizing test system, which consists of a potentiostat and a thermal camera. It is observed that the apparent temperature of MLG is closely correlated with pulse duration and residual charges. Based on a second-order RC model, the ion transportation and charge transfer in the modulator are likely affected by three factors (i.e., internal resistance, electric field, and concentration gradient), which changes the doping level of MLG. Hence, the emissivity of MLG can be finely adjusted by regulating the duration of the voltage pulse. This also leads to a wavelength dependence of emissivity modulation, which is confirmed by Fourier transform infrared spectroscopy. The observations offer a promising venue for precise and continuous adjustment of MLG's emissivity, which is of great significance for many practical applications, such as simulating complex and subtle thermal images. [ABSTRACT FROM AUTHOR]

Actively controlling the infrared emissivity of materials is critical for many applications, such as radiative cooling and thermal camouflage. Multilayer graphene (MLG) has shown great potential as a functional material with a tunable infrared emissivity. However, the spatial homogeneity in adjusting MLG's emissivity through ion intercalation is rather poor, which limits its practical applications. Here, we study how mixing various molecular components (e.g., ethanol) with ionic liquids affects the performance of MLG-based infrared emissivity modulators. The results indicate that introducing a moderate concentration of molecular components not only significantly improves spatial homogeneity in tuning emissivity but also enhances modulation depth, dynamic response, and the stability of the modulators. This is likely due to the increasing conductivity of the mixture and the improved wetting properties between the mixture and the separator, which leads to more effective charge transfer from ions in ionic liquids to carbons in graphene. The microscopic structure of MLG, revealed by Raman spectroscopy, confirms that the improvement in modulator performance is mostly due to homogeneous transportation of ions in the modulators. This not only sheds light on the underlying physics of the modulator but also greatly expands the options when selecting ionic liquids, thereby opening a new route for the modulator design. [ABSTRACT FROM AUTHOR]

Graphene photodetectors are highly attractive owing to its ultra-fast and wide-range spectral response from visible to infrared benefit from the superior carrier mobility and the linear dispersion with zero bandgap of graphene. The application of graphene photodetectors however is seriously limited by the low intrinsic responsivity in the order of ∼10 mA/W. Here, we demonstrate photogating field-effect transistors based on pure monolayer graphene with simple device structures. The light absorption in the heavily n-doped silicon/silicon oxide (Si/SiO2) substrate generates an additional photovoltage that effectively modulates the conductance of graphene, leading to room temperature graphene photodetectors with high responsivity of ∼500 A/W for 450 nm light and ∼4 A/W for 1064 nm light, respectively. The generated photocurrent changes with applied gate voltage and shows a strongly nonlinear power dependence. Meanwhile, the photoresponse of graphene exhibits a cut-off wavelength of ∼1100 nm, confirming the dominance photogating effect caused by light absorption in Si/SiO2 substrate. Considering the great compatibility of graphene to Si technology, our result paves a way for high-performance chip-integrated photodetectors. [ABSTRACT FROM AUTHOR]

In this letter, we report an oxygen-assisted chemical vapor deposition method to synthesize uniform large-area high-quality multilayer hexagonal boron nitride (hBN) films (denoted by O-hBN). Nonvolatile bipolar resistive switching (RS) of resistive random access memories (RRAMs) based on O-hBN films is presented. These RRAMs exhibit enhanced RS performance with lower cycle-to-cycle variability, lower set voltage, and higher current on/off ratio. The enhancement is benefited from the clean and smooth surface of O-hBN films and the reduction of grain boundaries which serve as an energetically favored path for ion migration. This scalable approach to synthesize hBN films could facilitate practical applications of hBN-based RRAMs. [ABSTRACT FROM AUTHOR]

2H phase Molybdenum ditelluride (MoTe2) is a layered two-dimensional (2D) semiconductor that has recently gained extensive attention for its intriguing properties, demonstrating great potential for nanoelectronics and optoelectronics. Optimizing the electric contacts to MoTe2 is a critical step for realizing high performance devices. Here, we demonstrate Co/hBN tunnel contacts to few-layer MoTe2. In sharp contrast to the p-type conduction of Co contacted MoTe2, Co/hBN tunnel contacted MoTe2 devices show clear n-type transport properties. Our first principles calculation reveals that the inserted few-layer hBN strongly interacts with Co and significantly reduces its work-function by ∼1.2 eV, while MoTe2 itself has a much weaker influence on the work-function of Co. This allows us to build MoTe2 diodes using the mixed Co/hBN and Co contact architecture, which can be switched from p-n type to n-p type by changing the gate-voltage, paving the way for engineering multi-functional devices based on atomically thin 2D semiconductors. [ABSTRACT FROM AUTHOR]