Zongyou Yin

Single-Layer MoS2 Phototransistors

A new phototransistor based on the mechanically exfoliated single-layer MoS(2) nanosheet is fabricated, and its light-induced electric properties are investigated in detail. Photocurrent generated from the phototransistor is solely determined by the illuminated optical power at a constant drain or gate voltage. The switching behavior of photocurrent generation and annihilation can be completely finished within ca. 50 ms, and it shows good stability. Especially, the single-layer MoS(2) phototransistor exhibits a better photoresponsivity as compared with the graphene-based device. The unique characteristics of incident-light control, prompt photoswitching, and good photoresponsivity from the MoS(2) phototransistor pave an avenue to develop the single-layer semiconducting materials for multifunctional optoelectronic device applications in the future.

Graphene-based materials: synthesis, characterization, properties, and applications.

Graphene, a two-dimensional, single-layer sheet of sp(2) hybridized carbon atoms, has attracted tremendous attention and research interest, owing to its exceptional physical properties, such as high electronic conductivity, good thermal stability, and excellent mechanical strength. Other forms of graphene-related materials, including graphene oxide, reduced graphene oxide, and exfoliated graphite, have been reliably produced in large scale. The promising properties together with the ease of processibility and functionalization make graphene-based materials ideal candidates for incorporation into a variety of functional materials. Importantly, graphene and its derivatives have been explored in a wide range of applications, such as electronic and photonic devices, clean energy, and sensors. In this review, after a general introduction to graphene and its derivatives, the synthesis, characterization, properties, and applications of graphene-based materials are discussed.

Single‐Layer Semiconducting Nanosheets: High‐Yield Preparation and Device Fabrication

The common featureof these materials is that the bulk material forms are layeredstructures with strong covalent bonding in each layer andweak van der Waals forces between the layers. Therefore,single or few-layer nanosheets of these materials can beobtained by using adhesive tapes for mechanical cleavage.

Fabrication of Single‐ and Multilayer MoS2 Film‐Based Field‐Effect Transistors for Sensing NO at Room Temperature

Single- and multilayer MoS(2) films are deposited onto Si/SiO(2) using the mechanical exfoliation technique. The films were then used for the fabrication of field-effect transistors (FETs). These FET devices can be used as gas sensors to detect nitrous oxide (NO). Although the single-layer MoS(2) device shows a rapid response after exposure to NO, the current was found to be unstable. The two-, three-, and four-layer MoS(2) devices show both stable and sensitive responses to NO down to a concentration of 0.8 ppm.

Synthesis of Few-Layer MoS2 Nanosheet-Coated TiO2 Nanobelt Heterostructures for Enhanced Photocatalytic Activities

MoS(2) nanosheet-coated TiO(2) nanobelt heterostructures--referred to as TiO(2)@MoS(2)--with a 3D hierarchical configuration are prepared via a hydrothermal reaction. The TiO(2) nanobelts used as a synthetic template inhibit the growth of MoS(2) crystals along the c-axis, resulting in a few-layer MoS(2) nanosheet coating on the TiO(2) nanobelts. The as-prepared TiO(2)@MoS(2) heterostructure shows a high photocatalytic hydrogen production even without the Pt co-catalyst. Importantly, the TiO(2)@MoS(2) heterostructure with 50 wt% of MoS(2) exhibits the highest hydrogen production rate of 1.6 mmol h(-1) g(-1). Moreover, such a heterostructure possesses a strong adsorption ability towards organic dyes and shows high performance in photocatalytic degradation of the dye molecules.

Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS2 and WSe2 Nanosheets

Although great progress has been achieved in the study of graphene, the small current ON/OFF ratio in graphene-based field-effect transistors (FETs) limits its application in the fields of conventional transistors or logic circuits for low-power electronic switching. Recently, layered transition metal dichalcogenide (TMD) materials, especially MoS2, have attracted increasing attention. In contrast to its bulk material with an indirect band gap, a single-layer (1L) MoS2 nanosheet is a semiconductor with a direct band gap of ~1.8 eV, which makes it a promising candidate for optoelectronic applications due to the enhancement of photoluminescence and high current ON/OFF ratio. Compared with TMD nanosheets prepared by chemical vapor deposition and liquid exfoliation, mechanically exfoliated ones possess pristine, clean, and high-quality structures, which are suitable for the fundamental study and potential applications based on their intrinsic thickness-dependent properties. In this Account, we summarize our recent research on the preparation, characterization, and applications of 1L and multilayer MoS2 and WSe2 nanosheets produced by mechanical exfoliation. During the preparation of nanosheets, we proposed a simple optical identification method to distinguish 1L and multilayer MoS2 and WSe2 nanosheets on a Si substrate coated with 90 and 300 nm SiO2. In addition, we used Raman spectroscopy to characterize mechanically exfoliated 1L and multilayer WSe2 nanosheets. For the first time, a new Raman peak at 308 cm(-1) was observed in the spectra of WSe2 nanosheets except for the 1L WSe2 nanosheet. Importantly, we found that the 1L WSe2 nanosheet is very sensitive to the laser power during characterization. The high power laser-induced local oxidation of WSe2 nanosheets and single crystals was monitored by Raman spectroscopy and atomic force microscopy (AFM). Hexagonal and monoclinic structured WO3 thin films were obtained from the local oxidization of single- to triple-layer (1L-3L) and quadruple- to quintuple-layer (4L-5L) WSe2 nanosheets, respectively. Then, we present Raman characterization of shear and breathing modes of 1L and multilayer MoS2 and WSe2 nanosheets in the low frequency range (<50 cm(-1)), which can be used to accurately identify the layer number of nanosheets. Magnetic force microscopy was used to characterize 1L and multilayer MoS2 nanosheets, and thickness-dependent magnetic response was found. In the last part, we briefly introduce the applications of 1L and multilayer MoS2 nanosheets in the fields of gas sensors and phototransistors.

Electrochemical deposition of ZnO nanorods on transparent reduced graphene oxide electrodes for hybrid solar cells

Monocrystalline ZnO nanorods (NRs) with high donor concentration are electrochemically deposited on highly conductive reduced graphene oxide (rGO) films on quartz. The film thickness, optical transmittance, sheet resistance, and roughness of rGO films are systematically studied. The obtained ZnO NRs on rGO films are characterized by X-ray diffraction, transmission electron microscopy, photoluminescence, and Raman spectra. As a proof-of-concept application, the obtained ZnO NRs on rGO are used to fabricate inorganic-organic hybrid solar cells with layered structure of quartz/rGO/ZnO NR/poly(3-hexylthiophene)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (P3HT/PEDOT:PSS)/Au. The observed power conversion efficiency (PCE, eta), approximately 0.31%, is higher than that reported in previous solar cells by using graphene films as electrodes. These results clearly demonstrate that rGO films with a higher conductivity have a smaller work function and show a better performance in the fabricated solar cells.

Three-dimensional graphene materials: preparation, structures and application in supercapacitors

Three-dimensional (3D) graphene materials (3DGMs) are of great importance due to their unique properties and practical applications. A number of 3DGMs with novel structures have been developed in recent years. This review presents the current progress of 3DGMs. After introducing the preparation strategies of 3DGMs, we summarize the reported 3DGMs based on their different structures, and then focus on the description of their preparation methods, properties and applications. Lastly, the applications of 3D graphene-based materials in supercapacitors are described.

Fabrication of Flexible MoS2 Thin‐Film Transistor Arrays for Practical Gas‐Sensing Applications

By combining two kinds of solution-processable two-dimensional materials, a flexible transistor array is fabricated in which MoS(2) thin film is used as the active channel and reduced graphene oxide (rGO) film is used as the drain and source electrodes. The simple device configuration and the 1.5 mm-long MoS(2) channel ensure highly reproducible device fabrication and operation. This flexible transistor array can be used as a highly sensitive gas sensor with excellent reproducibility. Compared to using rGO thin film as the active channel, this new gas sensor exhibits much higher sensitivity. Moreover, functionalization of the MoS(2) thin film with Pt nanoparticles further increases the sensitivity by up to ∼3 times. The successful incorporation of a MoS(2) thin-film into the electronic sensor promises its potential application in various electronic devices.

Centimeter-Long and Large-Scale Micropatterns of Reduced Graphene Oxide Films: Fabrication and Sensing Applications

Recently, the field-effect transistors (FETs) with graphene as the conducting channels have been used as a promising chemical and biological sensors. However, the lack of low cost and reliable and large-scale preparation of graphene films limits their applications. In this contribution, we report the fabrication of centimeter-long, ultrathin (1-3 nm), and electrically continuous micropatterns of highly uniform parallel arrays of reduced graphene oxide (rGO) films on various substrates including the flexible polyethylene terephthalate (PET) films by using the micromolding in capillary method. Compared to other methods for the fabrication of graphene patterns, our method is fast, facile, and substrate independent. In addition, we demonstrate that the nanoelectronic FETs based on our rGO patterns are able to label-freely detect the hormonal catecholamine molecules and their dynamic secretion from living cells.