The High Anisotropy of the Epitaxial Growth of the Well-Aligned Sb2Se3 Nanoribbons on Mica.

One-dimensional semiconductor nanostructures, which are different from those of bulk materials, have attracted considerable interest in either scientific research or practical application. Herein, the Sb2Se3 nanoribbons have been successfully synthesized by the epitaxial growth process on mica using the rapid physical vapor deposition method. The density of the Sb2Se3 nanoribbons increased quickly when the temperature decreased, and finally, the nanoribbons connected to each other and formed a network structure even in film. These nanoribbons were all well aligned along the preferred direction that either is parallel to each other or forms 60° angles. Further structural investigation demonstrated that the Sb2Se3 nanoribbons grew along the [001] directions, which are aligned along the directions [11̅0] and [100] or [100] and [110] on the mica surface. Then, an asymmetric lattice mismatch growth mechanism causing incommensurate heteroepitaxial lattice match between the Sb2Se3 and mica crystal structure was suggested. Furthermore, a polarized photodetector based on the film with the well-aligned Sb2Se3 nanoribbons was constructed, which illustrated strong photosensitivity and high anisotropic in-plane transport either in the dark or under light. The incommensurate heteroepitaxial growth method shown here may provide access to realize well-ordered nanostructures of other inorganic materials and promote the anisotropic photodetector industrialization.

The Role of Hydrogen on the Growth of Graphene Nanostructure Using a Two-Step Method.

Chemical vapor deposition (CVD) is widely applied in synthesizing high quality graphene, whose size, shape and structure are strongly impacted by the hydrogen concentration and, however, its role is not fully understood. In the traditional CVD, the concentration of the hydrogen keeps the constant in whole synthesis process and subsequently the nucleation and growth process are carried out simultaneously, therefore, its roles are usually confused and indistinguishable. In this report, the role of hydrogen on the growth of graphene nanostructure was creatively studied by introducing a two-step method which divided the nucleation and growth process for the first time. In the first step, the hexagonal graphene domain grown with the same conditions was used as precursor to eliminate the impact of the nucleation. In the second step, the role of hydrogen on the growth of graphene nanostructure was investigated by controlling the hydrogen concentration. The evolution behavior of the graphene nanostructure with the hydrogen concentration was systematically investigated. Two roles of the hydrogen, namely growth and etching modes, are clearly disclosed and then a possible mechanism was proposed. The results shown here may provide valuable guidance to understand the graphene growth mechanism and further advance the synthesis of unique graphene nanostructure.

Anisotropic Photoresponse of the Ultrathin GeSe Nanoplates Grown by Rapid Physical Vapor Deposition.

Anisotropic materials, especially two-dimensional (2D) layered materials formed by van der Waals force (vdW) with low-symmetry, have become a scientific hot-spot because their electrical, optical, and thermoelectric properties are highly polarization dependent. The 2D GeSe, a typical anisotropic-layered orthorhombic structure and narrow bandgap (1.1-1.2 eV) semiconductor, potentially meets these demands. In this report, the ultrathin elongated hexagonal GeSe nanoplates were successfully synthesized by the rapid physical vapor deposition method developed here. The ultrathin elongated hexagonal GeSe nanoplates have a zigzag edge in the long edge and an armchair edge in the short edge. In addition, the typical Raman mode exhibited 90° periodic vibration, having its maximum intensity between the zigzag direction or the zigzag and armchair direction, indicating an anisotropic electron-phonon interaction. Furthermore, the field effect transistor devices based on the elongated hexagonal GeSe nanoplates were constructed and exhibited the p-type semiconducting behavior with a high photoresponse characteriscs. Finally, the polarized sensitive photocurrent was identified, further revealing the intrinsically anisotropy of the GeSe nanoplate. The results illustrated here may give a useful guidance to synthesize the 2D-layered anisotropic nanomaterials and further advance the development of the polarized photodetector.

High White Light Photosensitivity of SnSe Nanoplate-Graphene Nanocomposites.

The multi-functional nanomaterial constructed with more than one type of materials has gained a great attention due to its promising application. Here, a high white light photodetector prototype established with two-dimensional material (2D) and 2D nanocomposites has been fabricated. The 2D-2D nanocomposites were synthesized with SnSe nanoplate and graphene. The device shows a linear I-V characterization behavior in the dark and the resistance dramatically decreases under the white light. Furthermore, the photosensitivity of the device is as large as 1110% with a rapid response time, which is much higher than pristine SnSe nanostructure reported. The results shown here may provide a valuable guidance to design and fabricate the photodetector based on the 2D-2D nanocomposites even beyond the SnSe nanoplate-graphene nanocomposites.

Double hexagonal graphene ring synthesized using a growth-etching method.

Precisely controlling the layer number, stacking order, edge configuration, shape and structure of graphene is extremely challenging but highly desirable in scientific research. In this report, a new concept named the growth-etching method has been explored to synthesize a graphene ring using the chemical vapor deposition process. The graphene ring is a hexagonal structure, which contains a hexagonal exterior edge and a hexagonal hole in the centre region. The most important concept introduced here is that the oxide nanoparticle derived from annealing is found to play a dual role. Firstly, it acts as a nucleation site to grow the hexagonal graphene domain and then it works as a defect for etching to form a hole. The evolution process of the graphene ring with the etching time was carefully studied. In addition, a double hexagonal graphene ring was successfully synthesized for the first time by repeating the growth-etching process, which not only confirms the validity and repeatability of the method developed here but may also be further extended to grow unique graphene nanostructures with three, four, or even tens of graphene rings. Finally, a schematic model was drawn to illustrate how the double hexagonal graphene ring is generated and propagated. The results shown here may provide valuable guidance for the design and growth of unique nanostructures of graphene and other two-dimensional materials.

Controllable Growth of the Graphene from Millimeter-Sized Monolayer to Multilayer on Cu by Chemical Vapor Deposition.

As is well established, mastery to precise control of the layer number, stacking order of graphene, and the size of single-crystal monolayer graphene is very important for both fundamental interest and practical applications. In this report, millimeter-sized single-crystal monolayer graphene has been synthesized to multilayer graphene on Cu by chemical vapor deposition. The relationship of the growth process between monolayer graphene and multilayer graphene is investigated carefully. Besides the general multilayer graphene with Bernal stacking order, parts of multilayer graphene with non-Bernal stacking order were modulated under optimized growth conditions. The oxide nanoparticle on the Cu surface derived from annealing has been found to play the key role in nucleation. In addition, the hydrogen concentration impacts significantly on the layer number and shape of the graphene. Moreover, a possible mechanism was proposed to understand the growth process discussed above, which may provide an instruction to graphene growth on Cu by chemical vapor deposition.

Growth and optical properties of ZnO nanorod arrays on Al-doped ZnO transparent conductive film.

ZnO nanorod arrays (NRAs) on transparent conductive oxide (TCO) films have been grown by a solution-free, catalyst-free, vapor-phase synthesis method at 600°C. TCO films, Al-doped ZnO films, were deposited on quartz substrates by magnetron sputtering. In order to study the effect of the growth duration on the morphological and optical properties of NRAs, the growth duration was changed from 3 to 12 min. The results show that the electrical performance of the TCO films does not degrade after the growth of NRAs and the nanorods are highly crystalline. As the growth duration increases from 3 to 8 min, the diffuse transmittance of the samples decreases, while the total transmittance and UV emission enhance. Two possible nanorod self-attraction models were proposed to interpret the phenomena in the sample with 9-min growth duration. The sample with 8-min growth duration has the highest total transmittance of 87.0%, proper density about 75 µm-2, diameter about 26 nm, and length about 500 nm, indicating that it can be used in hybrid solar cells.

Direct synthesis of high-density lead sulfide nanowires on metal thin films towards efficient infrared light conversion.

We report chemical-vapor-deposition (CVD) synthesis of high-density lead sulfide (PbS) nanowire arrays and nano pine trees directly on Ti thin films, and the fabrication of photovoltaic devices based upon the PbS nanowires. The as-grown nanowire arrays are largely vertically aligned to the substrates and are uniformly distributed over a relatively large area. Field effect transistors incorporating single PbS nanowires show p-type conduction and high mobilities. These catalytic metal thin films also serve as photocarrier collection electrodes and greatly facilitate device integration. For the first time, we have fabricated Schottky junction photovoltaic devices incorporating PbS nanowires, which demonstrate the capability of converting near-infrared light to electricity. The PbS nanowire devices are stable in air and their external quantum efficiency shows no significant decrease over a period of 3 months in air. We have also compared the photocurrent direction and quantum efficiencies of photovoltaic devices made with different metal electrodes, and the results are explained by band bending at the Schottky junction. Our research shows that PbS nanowires are promising building blocks for collecting near-infrared solar energy.

Dual wavelengths monitoring for optical coatings.

A new monitoring method based on the use of dual wavelengths monitoring is proposed. Firstly, the sensitivity of each layer in an optical coating for the monitoring wavelength is calculated by admittance equations. Then two appropriate monitoring wavelengths are chosen to make sure that every layer has a sensitive terminal point. The thickness error of the layer can be compensated. For quarter-wave multilayer and nonquarter-wave multilayer optical coatings, the advantage of this new monitoring method has been demonstrated by both the theoretical analyses and experimental results.

Substrate temperature effect on the refractive index and a two-step film method to detect small inhomogeneities in optical films.

Nb2O5 films were deposited by a reactive magnetron sputtering technique. The average refractive index was found to increase with the rise of substrate temperature. Modulated interference transmittance spectra were observed in the two-step films, which were prepared by stopping the deposition process in the middle of the designed sputtering time, and then, after a full cooling down to room temperature, starting the same deposition process again to complete the whole preparation of the films. A linearly graded-index model was used to explain the interference behavior. It was proved that the two-step film method was sensitive to the small inhomogeneity in the films. We also suggest that the inhomogeneity of sputtered films can be minimized by controlling the substrate temperature at a constant value.