Fachun Lai

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.

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.

Determination of optical constants and thickness of Nb 2 O 5 optical films from normal incidence transmission spectra

The optical constants and thickness of Nb2O5 optical thin film deposited by a reactive magnetron sputtering technique have been retrieved by all measurement data of the normal incidence transmittance. A computer program written in FORTRAN for determining the refractive index, extinction coefficient and thickness of this film has been developed and tested on two examples. This method can also calculate the optical constants and thickness of the film with a thickness thinner than a quarter wavelength optical thickness. Moreover, the optical constants calculated by this method are more accurate than those calculated by the envelope method. We also suggest that this method should be able to apply to other optical films, such as TiO2, SiO2 and Ta2O5.

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.

DESIGN AND ANALYSIS OF COLORED SOLAR SELECTIVE ABSORBING FILMS WITH THREE-LAYER STRUCTURE

Using glasses with 200-nm Cu films as substrates, the colored solar energy selective absorbing films with three-layer structure had been designed by optical multilayer design software. The three layers materials were Fe doped AlN and AlN. The optical constants of Fe doped AlN films with different Fe volume fractions were calculated by the effective medium theories. The color of the absorbing films includes orange yellow, purplish pink, pink, purple, and green. The solar absorptance of each colored film is between 0.89 and 0.96. The thermal emittance is lower than 0.1. The green film has the highest brightness. The purplish pink film has the highest absorptance, while the orange yellow and purple films have the lowest emittance. The color, absorptance, emittance, and brightness of the green film at different angles of light incidence had been analyzed. When the incident angle increases from 0° to 50°, the color of the green film changes from green to blue, and its absorptance and emittance decrease.

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.

Effects of substrate temperature and annealing on the structure and optical properties of ZnS film

Effects of substrate and annealing temperature on the microstructure, morphology, and optical properties of ZnS films were investigated. ZnS films were deposited on glass substrates by an electron beam evaporation system at different substrate temperatures and annealed at different temperatures in air. The structure and morphology of the film were studied by X-ray diffraction and atomic force microscopy. Transmittances of film were measured by spectrophotometer. Refractive indices and extinction coefficients were calculated from all transmittance data. Experimental results show that the as-deposited ZnS film exhibiting cubic structure, and the crystallinity is apparently improved with the increase of substrate temperature or annealing temperature. It is also found that film surface changes to ZnO after the film is annealed at 500 °C. The average surface grain size and root mean square surface roughness increase with the increase of annealing and substrate temperatures. Additionally, the increase of substrate temperature or annealing temperature will increase the pores in films, which results in the decrease of refractive indices and increase of extinction coefficients of the film.

OPTICAL PROPERTIES OF THE OXIDATION OF Cu THIN FILMS PREPARED BY THERMAL EVAPORATION

Copper films with 87 nm thickness were deposited on quartz substrates by thermal evaporation. In order to investigate the oxidation process, the Cu films were oxidized at different temperatures in air with different durations to obtain the complete and uncompleted copper oxide films. The structure and optical properties of the samples were studied by X-ray diffraction, scanning electron microscopy and spectrophotometer, respectively. It is found that the sample oxidized at 220°C for 200 min is Cu2O film with 106 nm thickness. Both the transmittance and reflectance of the samples increase with the increase of oxidation duration. The optical constants of the Cu film and the Cu2O film were retrieved by simulating the reflectance or transmittance based on the optical dielectric models. The optical constants of the Cu and Cu2O mixed layer with different composition were calculated by the effective medium theories. Adjusting the mixed layers composition and thickness, and Cu2O layer thickness, the transmittance and reflectance of the uncompleted oxidation films were simulated by optical multilayer design software. The results show that the uncompleted oxidation films consist of the Cu and Cu2O mixed layers and Cu2O layer. According to the parabolic rate law, the increase rate of Cu2O layer thickness for the uncompleted oxide films at 200°C is 1.6 nm s-1/2.

EFFECT OF PASSING ELECTRIC CURRENT ON THE ELECTRICAL AND OPTICAL PROPERTIES OF ITO FILMS IN AIR

The effect of passing electric current on the electrical and optical properties of indium tin oxide (ITO) films in air has been studied. Resistivities of the ITO films deposited by direct current magnetron sputtering on glass substrates were about 10.5 × 10-4Ωcm (group A) and 4.3 × 10-4Ωcm (group B). A constant electric current i between 30 and 150 mA was applied to the films in air for 3 h. The results show that there is a critical current (icri) for each group sample. icri of group A is 60 mA and icri of group B is 90 mA. When i > icri, the electrical performance declines and optical transmittance increases for the electric annealed sample. However, both the electrical and optical performances may be improved for the annealed film when i ≤ icri. It is found that the variations in electrical and optical properties of the films strongly depend on the electric power applied to ITO films.

SILVER NANOPARTICLES FILMS DEPOSITED ON AAO TEMPLATES BY THERMAL EVAPORATION FOR SURFACE-ENHANCED RAMAN SCATTERING OF R6G

Silver nanoparticles (NPs) films with different nominal thicknesses were deposited on the anodic aluminum oxide (AAO) templates with pore diameter about 200 nm by thermal evaporation. These NPs films were used as the surface enhanced Raman scattering (SERS) active substrates. The microstructure and surface morphology of the films were studied by X-ray diffraction and scanning electron microscopy, respectively. The SERS activity of the films was investigated by Raman scattering of adsorbed rhodamine 6G (R6G) at different concentrations. The results revealed that the average diameter of Ag NPs in different samples is 58 nm, 75 nm, 93 nm and 108 nm, respectively. Ag NPs film on AAO template is very suitable as a SERS active substrate and can detect R6G molecules with a concentration of 10-8 M. The intensity of the SERS spectra is related to both the Ag NPs size and interparticle distance. The sample with an average particle diameter of 93 nm has the highest SERS enhancement factor (1.3 × 106).