Preparation of single crystalline AlN thin films on ZnO nanostructures by atomic layer deposition at low temperature.

The growth of hetero-epitaxial ZnO-AlN core-shell nanowires (NWs) and single crystalline AlN films on non-polar ZnO substrate at temperature of 380 °C by atomic layer deposition (ALD) was investigated. Structural characterization shows that the AlN shells have excellent single-crystal properties. The epitaxial relationship of [0002]ZnO//[0002]AlN, and [10-10]ZnO//[10-10]AlNbetween ZnO core and AlN shell has been obtained. The ZnO NW templates were subsequently removed by annealing treatment in forming gas, resulting in ordered arrays of AlN single-crystal nanotubes. The impact factors on the epitaxial growth of AlN films are thoroughly investigated. It turned out that the growth parameters including lattice mismatch between substrate and AlN, growth temperature, and the polarity of ZnO substrate play important roles on the growth of single-crystal AlN films by ALD. Finally, non-polar AlN films with single-crystalline structure have been successfully grown onm-plane ZnO (10-10) single-crystal substrates. The as-grown hollow AlN nanotubes arrays and non-polar AlN films with single-crystalline structures are suggested to be highly promising for applications in nanoscale devices. Our research has developed a potential method to obtain other inorganic nanostructures and films with single-crystalline structure at fairly low temperature.

FeCl3-promoted tandem 1,4-conjugate addition/6-endo-dig cyclization/oxidation of propargylamines and benzoylacetonitriles/malononitriles: direct access to functionalized 2-aryl-4H-chromenes.

An efficient and concise procedure has been developed for the synthesis of functionalized 2-aryl-4H-chromenes based on a tandem reaction of propargylamines and benzoylacetonitriles/malononitriles in the presence of FeCl3 as an environmentally friendly promoter. This reaction involves a highly efficient tandem sequence consisting of 1,4-conjugate addition, 6-endo-dig cyclization, and oxidation. This protocol tolerates a variety of functional groups, thereby providing a practical and efficient method for the fabrication of 2-aryl-4H-chromene skeletons.

FeCl3-Mediated One-Pot Domino Reactions for the Synthesis of 9-Aryl/9-Arylethynyl-2,3,4,9-tetrahydro-1H-xanthen-1-ones from Propargylic Amines/Diaryl Amines and 1,3-Cyclohexanediones.

An efficient, environmentally friendly and one-pot route to new 9-aryl/9-arylethynyl-2,3,4,9-tetrahydro-1H-xanthen-1-one derivatives from inexpensive starting materials has been developed. This method proceeded by a domino nucleophilic-substitution/intramolecular cyclization/dehydration sequence of propargylic amines/diaryl amines and 1,3-cyclohexanediones under the promotion of FeCl3, which involved the formation of two new σ (C-C and C-O) bonds in a single operation for the construction of novel tetrahydroxanthene skeletons in 68-95% yields.

FeCl3-Mediated Synthesis of ß-Alkynyl Ketones via Domino Nucleophilic-Substitution/Intramolecular-Cyclization/Reverse Claisen Condensation of N-Cyclohexyl Propargylamines and 1,3-Diketones.

The synthesis of ß-alkynyl ketones was achieved in good to excellent yields by an iron-catalyzed domino reaction of N-cyclohexyl propargylamines and 1,3-diketones. A plausible mechanism involving nucleophilic substitution, intramolecular cyclization, and reverse Claisen condensation for this process is proposed.

Wavelength Modulation Characteristics of Metal Gratings on Si-Based Blocked-Impurity-Band (BIB) Terahertz Detectors.

In this work, the wavelength selection characteristics of metal gratings on Si-based blocked-impurity-band (BIB) detectors in the terahertz band were studied by performing experiments and a finite difference time domain (FDTD) simulation. The transmission spectra of metal gratings with different periods on 130 µm intrinsic Si substrates were measured. When the metal grating period increased from 16 to 20 to 32 µm, the peak position of the spectrum moved from 21.71 to 24.50 to 36.59 µm, which is in good agreement with the FDTD simulation results. The structure with the period of 32 µm shows the best wavelength selective transmission characteristics. Then, the bare Si-based BIB devices and metal grating/Si-based BIB hybrid devices with different thicknesses of blocking layers of 2 and 5 µm were fabricated. By covering different periods of metal gratings for the devices with a thicker blocking layer of 2 µm, we obtained more effective wavelength selection characteristics and stronger response spectra enhancement ratios that were about 1.3, 2.4, or 1.9 times. This was mainly due to the localized optical field enhancement effect of the plasmons resonance in metal gratings, which decays exponentially in a vertical direction. Our results demonstrate a new approach for the Si-based BIB detector to realize multiband selective detection applications.

Fabrication of 1D Te/2D ReS2 Mixed-Dimensional van der Waals p-n Heterojunction for High-Performance Phototransistor.

The superior optical and electronic properties of the two-dimensional (2D) rhenium disulfide (ReS2) makes it suitable for nanoelectronic and optoelectronic applications. However, the internal defects coupled with with the low mobility and light-absorbing capability of ReS2 impede its utilization in high-performance photodetectors. Fabrication of mixed-dimensional heterojunctions is an alternative method for designing high-performance hybrid photodetectors. This study proposes a mixed-dimensional van der Waals (vdW) heterojunction photodetector, containing high-performance one-dimensional (1D) p-type tellurium (Te) and 2D n-type ReS2, developed by depositing Te nanowires on ReS2 nanoflake using the dry transfer method. It can improve the injection and separation efficiency of photoexcited electron-hole pairs due to the type II p-n heterojunction formed at the ReS2 and Te interface. The proposed heterojunction device is sensitive to visible-light sensitivity (632 nm) with an ultrafast photoresponse (5 ms), high responsivity (180 A/W), and specific detectivity (109), which is superior to the pristine Te and ReS2 photodetectors. As compared to the ReS2 device, the responsivity and response speed is better by an order of magnitude. These results demonstrate the fabrication and application potential of Te/ReS2 mixed-dimensional heterojunction for high-performance optoelectronic devices and sensors.

Fabrication of a Nb-Doped ß-Ga2O3 Nanobelt Field-Effect Transistor and Its Low-Temperature Behavior.

For the first time, we report the successful fabrication of well-behaved field-effect transistors based on Nb-doped ß-Ga2O3 nanobelts mechanically exfoliated from bulk single crystals. The exfoliated ß-Ga2O3 nanobelts were transferred onto a purified surface of the 110 nm SiO2/Si substrate. These Nb-doped devices showed excellent electrical performance such as an ultrasmall cutoff current of ∼10 fA, a high current on/off ratio of >108, and a quite steep subthreshold swing (SS, ∼120 mV/decade). Furthermore, we investigated the temperature dependence down to 200 K, providing insightful information for its operation in a harsh environment. This work lays a foundation for wider application of Nb-doped ß-Ga2O3 in nano-electronics.

Precise preparation of WO3@SnO2 core shell nanosheets for efficient NH3 gas sensing.

Development of high-performance ammonia (NH3) sensor is imperative for monitoring NH3 in the living environment. In this work, to obtain a high performance NH3 gas sensor, structurally well-defined WO3@SnO2 core shell nanosheets with a controllable thickness of SnO2 shell layer have been employed as sensing materials. The prepared core shell nanosheets were used to obtain a miniaturized gas sensor based on micro-electro-mechanical system (MEMS). By tuning the thickness of SnO2 layer via atomic layer deposition, a series of WO3@SnO2 core-shell nanosheets with tunable sensing properties were realized. Particularly, the sensor base on the fabricated WO3@SnO2 nanosheets with 20-nm SnO2 shell layer demonstrated superior gas sensing performance with the highest response (1.55) and selectivity toward 15 ppm NH3 at 200 °C. This remarkable enhancement of NH3 sensing ability could be ascribed to the formation of unique WO3-SnO2 core-shell heterojunction structure. The detailed mechanism was elucidated by the heterojunction-depletion model with the help of specific band alignment.

Modification of 1D TiO2 nanowires with GaOxNy by atomic layer deposition for TiO2@GaOxNy core-shell nanowires with enhanced photoelectrochemical performance.

As a well-known semiconductor that can catalyse the oxygen evolution reaction, TiO2 has been extensively investigated for its solar photoelectrochemical water properties. Unmodified TiO2 shows some issues, particularly with respect to its photoelectrochemical performance. In this paper, we present a strategy for the controlled deposition of controlled amounts of GaOxNy cocatalysts on TiO2 1D nanowires (TiO2@GaOxNy core-shell) using atomic layer deposition. We show that this modification significantly enhances the photoelectrochemical performance compared to pure TiO2 NW photoanodes. For our most active TiO2@GaOxNy core-shell nanowires with a GaOxNy thickness of 20 nm, a photocurrent density up to 1.10 mA cm-2 (at 1.23 V vs. RHE) under AM 1.5 G irradiation (100 mW cm-2) has been achieved, which is 14 times higher than that of unmodified TiO2 NWs. Furthermore, the band gap matching with TiO2 enhances the absorption of visible light over unmodified TiO2 and the facile oxygen vacancy formation after the deposition of GaOxNy also provides active sites for water activation. Density functional theory studies of model systems of GaOxNy-modified TiO2 confirm the band gap reduction, high reducibility and ability to activate water. The highly efficient and stable systems of TiO2@GaOxNy core-shell nanowires with ALD deposited GaOxNy demonstrate a good strategy for the fabrication of core-shell structures that enhance the photoelectrochemical performance of readily available photoanodes.

Enhanced photoelectrochemical properties of nanocrystalline TiO2 electrode by surface sensitization with CuxO quantum dots.

Nanoporous anatase TiO2 films were fabricated by a screen-printing method, and CuxO quantum dots (QDs) were deposited on the TiO2 films through successive ionic layer adsorption and reaction (SILAR). The amount of CuxO QDs on the TiO2 films are controlled by changing the number of SILAR cycles. The morphology, microstructure, optical, and photoelectrochemical properties of different CuxO sensitized TiO2 films (CuxO/TiO2) were investigated in detail. The nanoporous TiO2 film offers a large surface area for anchoring QDs. QD deposited samples exhibited a significant improvement in photoelectrochemical performance than the bare of TiO2. CuxO/TiO2, prepared with 7 SILAR cycles, showed the best photoelectrochemical properties, where the photocurrent density was enhanced to 500.01 µA/cm2 compared with 168.88 µA/cm2 of bare TiO2 under visible light. These results indicate that the designed CuxO/TiO2 structure possesses superior charge separation efficiency and photoelectrochemical properties.