Direct growth of single-chiral-angle tungsten disulfide nanotubes using gold nanoparticle catalysts.

Transition metal dichalcogenide (TMD) nanotubes offer a unique platform to explore the properties of TMD materials at the one-dimensional limit. Despite considerable efforts thus far, the direct growth of TMD nanotubes with controllable chirality remains challenging. Here we demonstrate the direct and facile growth of high-quality WS2 and WSe2 nanotubes on Si substrates using catalytic chemical vapour deposition with Au nanoparticles. The Au nanoparticles provide unique accommodation sites for the nucleation of WS2 or WSe2 shells on their surfaces and seed the subsequent growth of nanotubes. We find that the growth mode of nanotubes is sensitive to the temperature. With careful temperature control, we realize ~79% WS2 nanotubes with single chiral angles, with a preference of 30° (~37%) and 0° (~12%). Moreover, we demonstrate how the geometric, electronic and optical properties of the synthesized WS2 nanotubes can be modulated by the chirality. We anticipate that this approach using Au nanoparticles as catalysts will facilitate the growth of TMD nanotubes with controllable chirality and promote the study of their interesting properties and applications.

Simulated annealing fitting: a global optimization method for quantitatively analyzing growth kinetics of colloidal Ag nanoparticles.

The involvement of heterogeneous solid/liquid reactions in growing colloidal nanoparticles makes it challenging to quantitatively understand the fundamental steps that determine nanoparticles' growth kinetics. A global optimization protocol relying on simulated annealing fitting and the LSW growth model is developed to analyze the evolution data of colloidal silver nanoparticles synthesized from a microwave-assisted polyol reduction reaction. Fitting all data points of the entire growth process determines with high fidelity the diffusion coefficient of precursor species and the heterogeneous reduction reaction rate parameters on growing silver nanoparticles, which represent the principal functions to determine the growth kinetics of colloidal nanoparticles. The availability of quantitative results is critical to understanding the fundamentals of heterogeneous solid/liquid reactions, such as identifying reactive species and reaction activation energy barriers.

Chemical vapor deposition growth of ReS2 nanowires for high-performance nanostructured photodetector.

Rhenium disulfide (ReS2) is a recently discovered next-generation transition metal dichalcogenides (TMDs) material that exhibits unique properties, which have resulted in its wide use in the fabrication of electronic and optoelectronic devices. Studies on ReS2 have mainly focused on the synthesis and applications of two-dimensional (2D) materials, while studies on one-dimensional (1D) ReS2 have yet to be reported. Herein, 1D single-crystal ReS2 nanowires have been synthesized successfully for the first time via chemical vapor deposition (CVD) and utilized as the active layer in a nanostructured photodetector. The crystal structure, phonon vibration modes, and chemical states of the single-crystal ReS2 nanowires have been investigated. Furthermore, the nanostructured photodetector using single-crystal ReS2 nanowires as the active layer and Ag nanowire networks as the transparent electrodes exhibited excellent performance, including a higher photoresponsivity (5.08 × 105), the highest external quantum efficiency (1.07 × 106), and the largest specific detectivity (6.1 × 1015) among ReS2 nanostructure-based photodetectors reported to date. Furthermore, the photodetector performance under vacuum conditions was investigated, which provides some information on the work mechanism of the ReS2 photodetector.

Self-powered ZnS Nanotubes/Ag Nanowires MSM UV Photodetector with High On/Off Ratio and Fast Response Speed.

In this study, we design and demonstrate a novel type of self-powered UV photodetectors (PDs) using single-crystalline ZnS nanotubes (NTs) as the photodetecting layer and Ag nanowires (NWs) network as transparent electrodes. The self-powered UV PDs with asymmetric metal-semiconductor-metal (MSM) structure exhibit attractive photovoltaic characteristic at 0 V bias. Device performance analysis reveals that the as-assembled PDs have a high on/off ratio of 19173 and a fast response speed (τr = 0.09 s, τf = 0.07 s) without any external bias. These values are even higher than that of ZnS nanostructures- and ZnS heterostructure-based PDs at a large bias voltage. Besides, its UV sensivity, responsivity and detectivity at self-powered mode can reach as high as 19172, 2.56 A/W and 1.67 × 1010 cm Hz1/2 W-1, respectively. In addition, the photosensing performance of the self-powered UV PDs is studied in different ambient conditions (e.g., in air and vacuum). Moreover, a physical model based on band energy theory is proposed to explain the origin of the self-driven photoresponse characteristic in our device. The totality of the above study signifies that the present self-powered ZnS NTs-based UV nano-photodetector may have promising application in future self-powered optoelectronic devices and integrated systems.

One-step fabrication of single-crystalline ZnS nanotubes with a novel hollow structure and large surface area for photodetector devices.

ZnS nanotubes (NTs) were successfully prepared via a one-step thermal evaporation process without using any templates. The resulting NTs were single crystalline and structurally uniform. Based on experimental analysis, a tube-growth vapor-liquid-solid process was proposed as the growth mechanism of ZnS NTs. A metal-semiconductor-metal full-nanostructured ultraviolet (UV) photodetector with ZnS NTs as the active layer, and Ag nanowires of low resistivity and high transmissivity as electrodes, was fabricated and characterized. The ZnS NT-based device displayed a high I on/I off ratio of up to ∼1.56 × 105 with a high response to UV incident light at low operation voltage. This work is a meaningful exploration for preparing other one-dimensional semiconductor NTs, and developing a high-performance and power-saving UV sensor.

High-Performance Fully Nanostructured Photodetector with Single-Crystalline CdS Nanotubes as Active Layer and Very Long Ag Nanowires as Transparent Electrodes.

Long and single-crystalline CdS nanotubes (NTs) have been prepared via a physical evaporation process. A metal-semiconductor-metal full-nanostructured photodetector with CdS NTs as active layer and Ag nanowires (NWs) of low resistivity and high transmissivity as electrodes has been fabricated and characterized. The CdS NTs-based photodetectors exhibit high performance, such as lowest dark currents (0.19 nA) and high photoresponse ratio (Ilight/Idark ≈ 4016) (among CdS nanostructure network photodetectors and NTs netwok photodetectors reported so far) and very low operation voltages (0.5 V). The photoconduction mechanism, including the formation of a Schottky barrier at the interface of Ag NW and CdS NTs and the effect of oxygen adsorption process on the Schottky barrier has also been provided in detail based on the studies of CdS NTs photodetector in air and vacuum. Furthermore, CdS NTs photodetector exhibits an enhanced photosensitivity as compared with CdS NWs photodetector. The enhancement in performance is dependent on the larger surface area of NTs adsorbing more oxygen in air and the microcavity structure of NTs with higher light absorption efficiency and external quantum efficiency. It is believed that CdS NTs can potentially be useful in the designs of 1D CdS-based optoelectronic devices and solar cells.