S. P. Lau

Photoluminescence study of ZnO films prepared by thermal oxidation of Zn metallic films in air

Zinc oxide (ZnO) films were synthesized by thermal oxidation of metallic zinc films in air. The influence of annealing temperatures ranging from 320 to 1000 °C on the structural and optical properties of ZnO films is investigated systematically using x-ray diffraction and room temperature photoluminescence (PL). The films show a polycrystalline hexagonal wurtzite structure without preferred orientation. Room temperature PL spectra of the ZnO films display two emission bands, predominant excitonic ultraviolet (UV) emission and weak deep level visible emission. It is observed that the ZnO film annealed at 410 °C exhibits the strongest UV emission intensity and narrowest full width at half maximum (81 meV) among the temperature ranges studied. The excellent UV emission from the film annealed at 410 °C is attributed to the good crystalline quality of the ZnO film and the low rate of formation of intrinsic defects at such low temperature. The visible emission consists of two components in the green and yellow ...

Infrared Photodetectors Based on CVD‐Grown Graphene and PbS Quantum Dots with Ultrahigh Responsivity

Infrared photodetectors based on single-layer CVD-grown graphene and PbS quantum dots, which are fabricated by solution processing, show ultrahigh responsivities of up to 10(7) A/W under infrared light illumination. The devices fabricated on flexible plastic substrates have excellent bending stability. The photoresponse is attributed to the field-effect doping in graphene films induced by negative charges generated in the quantum dots.

Exceptional tunability of band energy in a compressively strained trilayer MoS2 sheet

Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic, and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of the band gap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed an electromechanical device that can apply biaxial compressive strain to trilayer MoS2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence and Raman characterizations show that the direct band gap can be blue-shifted for ~300 meV per 1% strain. First-principles investigations confirm the blue-shift of the direct band gap and reveal a higher tunability of the indirect band gap than the direct one. The exceptionally high strain tunability of the electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.

The Application of Highly Doped Single-Layer Graphene as the Top Electrodes of Semitransparent Organic Solar Cells

A single-layer graphene film with high conductance and transparency was realized by effective chemical doping. The conductance of single-layer graphene was increased for more than 400% when it was doped with Au nanoparticles and poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid). Then semitransparent organic solar cells based on poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were fabricated with single-layer graphene and indium tin oxide (ITO) as the top and bottom electrodes, respectively. The performance of the devices was optimized by tuning the active layer thickness and doping the single-layer graphene electrodes. The maximum efficiency of 2.7% was observed in the devices with the area of 20 mm(2) illuminated from graphene electrode under the AM1.5 solar simulator. It is notable that all of the devices showed higher efficiency from the graphene than ITO side, which was attributed to the better transmittance of the graphene electrodes. In addition, the influence of the active area of the organic solar cell on its photovoltaic performance was studied. We found that, when the active areas increased from 6 to 50 mm(2), the power conversion efficiencies decreased from 3% to 2.3% because of the increased series resistances and the decreased edge effect of the devices.

Deep Ultraviolet to Near-Infrared Emission and Photoresponse in Layered N-Doped Graphene Quantum Dots

Material that can emit broad spectral wavelengths covering deep ultraviolet, visible, and near-infrared is highly desirable. It can lead to important applications such as broadband modulators, photodetectors, solar cells, bioimaging, and fiber communications. However, there is currently no material that meets such desirable requirement. Here, we report the layered structure of nitrogen-doped graphene quantum dots (N-GQDs) which possess broadband emission ranging from 300 to >1000 nm. The broadband emission is attributed to the layered structure of the N-GQDs that contains a large conjugated system and provides extensive delocalized π electrons. In addition, a broadband photodetector with responsivity as high as 325 V/W is demonstrated by coating N-GQDs onto interdigital gold electrodes. The unusual negative photocurrent is observed which is attributed to the trapping sites induced by the self-passivated surface states in the N-GQDs.

Comprehensive study of ZnO films prepared by filtered cathodic vacuum arc at room temperature

Room temperature deposition of high crystal quality zinc oxide (ZnO) films was realized by the filtered cathodic vacuum arc (FCVA) technique. Detrimental macroparticles in the plasma as byproducts of arcing process are removed with an off-plane double bend magnetic filter. The influence of oxygen pressure on the structural, electrical and optical properties of ZnO films were investigated in detail. The crystal structure of ZnO is hexagonal with highly c-axis orientation. Intrinsic stress decreases with an increase of chamber pressure, and near stress-free film was obtained at 1×10−3 Torr. Films with optical transmittance above 90% in the visible range and resistivity as low as 4.1×10−3 Ω cm were prepared at pressure of 5×10−4 Torr. Energetic zinc particles in the cathodic plasma and low substrate temperature enhance the probability of formation of zinc interstitials in the ZnO films. The observation of strong ultraviolet photoluminescence and weak deep level emission at room temperature manifest the high ...

Random laser action in ZnO nanorod arrays embedded in ZnO epilayers

Random laser action with coherent feedback has been observed in ZnO nanorod arrays embedded in ZnO epilayers. The sample was fabricated by depositing a MgO buffer layer and followed by a layer of ZnO thin film onto a vertically well-aligned ZnO nanorod arrays grown on sapphire substrate. Under 355 nm optical excitation at room temperature, sharp lasing peaks emit at around 390 nm with a linewidth less than 0.4 nm has been observed in all directions. In addition, the dependence of the lasing threshold intensity on the excitation area is shown in good agreement with the random laser theory. Hence, it is demonstrated that random laser action can also be supported in ZnO nanorod arrays.

UV Raman characteristics of nanocrystalline diamond films with different grain size

Abstract Nanocrystalline diamond films with different size were characterized by ultraviolet (UV) (244 nm) Raman spectroscopy. It was found that a diamond peak at 1333 cm −1 was enhanced, while the D and G peak of graphite as well as photoluminescence was suppressed, compared with that measured by visible (514.5 nm) Raman. With decreasing the particle size from 120 to 28 nm, the diamond peak shifts from 1332.8 to 1329.6 cm −1 , the line width of the peak becomes broader, the intensity ratio of diamond and G peak decreases. The down shift and broadening of the diamond peak depending on the particle size by UV Raman measurements are consistent with the phonon confinement model.

Stretchable all-solid-state supercapacitor with wavy shaped polyaniline/graphene electrode

A stretchable electronic device can retain its functionalities during high-level mechanical deformation, and stimulates the applications in the field of wearable and bio-implantable electronics. Efficient energy storage devices are an indispensable component in stretchable electronic systems. To integrate power supplies together with electronic devices that are mechanically flexible and stretchable, we demonstrate a new kind of stretchable all-solid-state supercapacitor, which consists of two slightly separated polyaniline/graphene electrodes in a wavy shape, with a phosphoric acid/polyvinyl alcohol gel as the solid-state electrolyte and separator. The as-fabricated wavy shaped supercapacitor was encapsulated in an elastomeric substrate which can be stretched to a large extent without mechanical degradation. The supercapacitor exhibited a maximum specific capacitance of 261 F g−1. Electrochemical cycling testing with the supercapacitor showed 89% capacitance retention over 1000 charge–discharge cycles at a current density of 1 mA cm−2. The bending and stretching tests showed that the supercapacitor maintained high mechanical strength and high capacitance simultaneously, even under a strain of 30%. This stretchable all-solid-state supercapacitor shows great potential as an energy storage device for stretchable electronic systems.

Zinc oxide thin-film random lasers on silicon substrate

Room-temperature ultraviolet lasing is demonstrated in mirrorless zinc oxide thin-film waveguides on (100) silicon substrate. Laser cavities, due to closed-loop optical scattering from the lateral facets of the irregular zinc oxide grains, are generated through the post-growth annealing of high-crystal-quality zinc oxide thin films obtained from the filtered cathodic vacuum arc technique. It is found that the lasing wavelength and linewidth of the zinc oxide random lasers under 355 nm optical excitation are around 390 nm and less than 0.4 nm, respectively. In addition, the lasing threshold characteristics are in good agreement with the random laser theory.