Ultrathin (4-6 nm) single-crystal Bi(2)Se(3) nanodiscs and nanosheets were synthesized through a simple and quick solution process. The growth mechanism was investigated in detail. Crystal seeds grew via 2D self-attachment of small nanoparticles followed by epitaxial recrystallization into single crystals. The lateral dimension of the nanodiscs increased and their shape changed from circles to hexagons as the reaction temperature increased. Positively charged polymer surfactants greatly enlarged the lateral dimension to produce nanosheets with uniform thickness.

The wafer-scale synthesis of MoS2 layers with precise thickness controllability and excellent uniformity is essential for their application in the nanoelectronics industry. Here, we demonstrate the atomic layer deposition (ALD) of MoS2 films with Mo(CO)6 and H2S as the Mo and S precursors, respectively. A self-limiting growth behavior is observed in the narrow ALD window of 155-175 °C. Long H2S feeding times are necessary to reduce the impurity contents in the films. The as-grown MoS2 films are amorphous due to the low growth temperature. Post-annealing at high temperatures under a H2S atmosphere efficiently improves the film properties including the crystallinity and chemical composition. An extremely uniform film growth is achieved even on a 4 inch SiO2/Si wafer. These results demonstrate that the current ALD process is well suited for the synthesis of MoS2 layers for application in industry.

Detection of gas-phase chemicals finds a wide variety of applications, including food and beverages, fragrances, environmental monitoring, chemical and biochemical processing, medical diagnostics, and transportation. One approach for these tasks is to use arrays of highly sensitive and selective sensors as an electronic nose. Here, we present a high performance chemiresistive electronic nose (CEN) based on an array of metal oxide thin films, metal-catalyzed thin films, and nanostructured thin films. The gas sensing properties of the CEN show enhanced sensitive detection of H2S, NH3, and NO in an 80% relative humidity (RH) atmosphere similar to the composition of exhaled breath. The detection limits of the sensor elements we fabricated are in the following ranges: 534 ppt to 2.87 ppb for H2S, 4.45 to 42.29 ppb for NH3, and 206 ppt to 2.06 ppb for NO. The enhanced sensitivity is attributed to the spillover effect by Au nanoparticles and the high porosity of villi-like nanostructures, providing a large surface-to-volume ratio. The remarkable selectivity based on the collection of sensor responses manifests itself in the principal component analysis (PCA). The excellent sensing performance indicates that the CEN can detect the biomarkers of H2S, NH3, and NO in exhaled breath and even distinguish them clearly in the PCA. Our results show high potential of the CEN as an inexpensive and noninvasive diagnostic tool for halitosis, kidney disorder, and asthma.

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Two-dimensional (2-D) metal chalcogenides have received great attention because of their unique properties, which are different from bulk materials. A challenge in implementing 2-D metal chalcogenides in emerging devices is to prepare a well-crystallized layer over large areas at temperatures compatible with current fabrication processes. Tin monosulfide, a p-type layered semiconductor with a high hole mobility, is a promising candidate for realizing large-area growth at low temperatures because of its low melting point. However, tin sulfides exist in two notable crystalline phases, SnS and SnS2. Therefore, it is imperative to control the oxidation state of Sn to achieve a pure SnS film. Here, the synthesis of SnS thin films by atomic-layer-deposition (ALD) is demonstrated using bis­(1-dimethylamino-2-methyl-2-propoxy)­tin­(II) and H2S as Sn and S sources, respectively, over a wide temperature window (90–240 °C). Impurities such as carbon, oxygen, and nitrogen were negligibly detected. The morphological evolution of plate-like orthorhombic SnS grains was observed above 210 °C. Moreover, properties of thin film transistors and gas sensors using SnS films as the active layers were investigated. The SnS ALD process would provide promising opportunities to exploit the intriguing properties of the 2-D metal chalcogenides for realizing emerging electronic devices.