Chi-Yuan Lin

Platinum-graphene counter electrodes for dye-sensitized solar cells

This paper describes the photovoltaic performance of dye-sensitized solar cells (DSSCs) containing graphene-incorporated counter electrodes (CEs). The location and thickness of graphene in CEs are optimized to improve the photovoltaic performance of DSSCs, compared with typical Pt CEs. The DSSC, with a Pt/few-layer graphene (FLG) CE, achieved 8% in short-circuit current density and 13% in power conversion efficiency (PCE). Electrochemical impedance spectroscopy shows that the DSSC, with a Pt/FLG CE, exhibits a series resistance lower than that with a Pt CE. The lower series resistance is attributed to the contact resistance at the interface of platinum and fluorine doped tin oxide. The contact resistance is reduced by the formation of the thin platinum-carbon composite layer. It is demonstrated that the consumption of Pt could be reduced with a Pt/FLG CE. However, graphene/Pt CEs resulted in a slow charge-transfer process and consequently a worse photovoltaic performance of DSSCs.

Surface-enhanced Raman scattering of graphene with photo-assisted-synthesized gold nanoparticles.

This paper presents a convenient and reliable method to prepare gold nanoparticles (AuNPs) on graphene. Photo-assisted synthesis (PAS) was employed to grow AuNPs in AuCl(4)(-) electrolyte on graphene. The size of AuNPs could be as large as 130 nm. This optical method had a steady growth rate of AuNPs. The distribution of AuNPs was well controlled by focusing the laser for PAS. The minimum diameter of the distribution was approximately 1 μm. Surface-enhanced Raman scattering of graphene due to AuNPs was observed. Electrical fields near AuNPs calculated by the finite-difference time-domain algorithm ensured that the Raman enhancement was attributed to the localized surface plasmons of AuNPs.

Fabrication of spatial modulated second order nonlinear structures and quasi-phase matched second harmonic generation in a poled azo-copolymer planar waveguide

This work demonstrates that arbitrary types of spatially modulated second-order susceptibility (chi((2)) structures such as 1D and 2D, periodic and quasi-periodic structures can be obtained by using the combination of corona poling and direct laser writing (DLW) techniques. The fabrication technique is based on the photodepoling of azo-dye molecules caused by one-photon or two-photon absorption during the DLW process. Polarization and second harmonic generation (SHG) images of the fabricated structures were measured by electrostatic force microscope and SHG mapping techniques, respectively. Furthermore, quasi-phase-matched (QPM) enhanced SHG from a 1D periodically poled azo-copolymer planar waveguide is demonstrated using an optical parametric oscillator laser by scanning wavelength from 1500 to 1600 nm. The resonant wavelength of the QPM enhanced SHG is peaked at 1537 nm with FWHM is congruent to 2.5 nm.

Molecular beam epitaxy regrowth using a thin In layer for surface passivation

We report a molecular beam epitaxy regrowth technique using a thin In layer for surface passivation. X‐ray photoemission spectroscopy measurements show that an In layer as thin as a few tenths of an angstrom is adequate for the effective protection of the underlying III‐V epilayers from carbon and oxygen contamination, while still providing exposure to the atmosphere. C–V depth profilings of the regrown pseudomorphic high electron mobility transistors (PHEMTs) reveal no significant residual charge carriers near the regrowth interface. The regrown PHEMTs with 1 μm gate length have a transconductance as high as 330 mS/mm and ft over 23 GHz.

Optical controlled graphene-based nonvolatile ternary-logic transistor with azobenzene copolymer

We demonstrated optical-controlled graphene-based nonvolatile transistors incorporated with azobenzene copolymer. The transistor was gated by the quasi remnant polarization of azobenzene copolymer, which was built by photo-assisted poling and erased by photo-depoling at room temperature. By taking the nature of polymer electret of azobenzene copolymer, the graphene-based device can perform ternary logic, and the resistance change ratio of the written status “±1” to the erased status “0” was ∓60%. The device can retain its statuses against an electric field as high as 0.2 MV/cm because the azobenzene molecules were frozen in copolymer at room temperature.

The effect of thermal annealing on the optical and electrical properties of ZnO epitaxial films grown on n-GaAs (001)

Wurtzite ZnO epitaxial layers grown on n-type GaAs (001) by pulsed laser deposition (PLD) exhibited n-type conductivity. Post-growth annealing leads the conversion of carrier type from electron to hole, as revealed by Hall effect measurements, although only moderate structural improvement was observed. The carrier type conversion is attributed to thermally activated arsenic diffusion from the substrate, confirmed by secondary ion mass spectrometry and photoluminescence. The surface electrical properties of both the as-deposited n-type and annealed p-type ZnO epitaxial layers were thoroughly characterized by Kelvin force microscopy (KFM) and electrostatic force microscopy (EFM). The results indicated the existence of a high density of surface states close to the ZnO midgap with a density of a few 1014 cm−2 eV−1. The Fermi levels (EF) of n- and p-type ZnO epitaxial layers were found to be 1.06 eV below the conduction-band minimum (CBM) and 1.612–1.769 eV above the valence-band maximum (VBM), respectively. The small EF difference between the n- and p-type ZnO epitaxial layers implies Fermi level pinning at the surface of both n- and p-type ZnO epitaxial layers.

High performance non-alloyed pseudomorphic high electron mobility transistors

Abstract We have studied the InAs/GaAs strained layer superlattice (SLS) structures as non-alloyed contacts applied to pseudomorphic high electron mobility transistors (PHEMTs) grown by the molecular beam epitaxy (MBE) system. Transmission line measurements with four point configurations showed that specific contact resistances between 6.6 × 10 −7 and 2.6 × 10 −6 Ω cm 2 were obtained for the as-grown devices, with a linear correlation coefficient of 0.997. D.c. measurements of the as-grown PHEMT with 1 μ m gate showed a transconductance of 240 mS mm −1 . Microwave measurements on the same device showed a cut-off frequency of 19 GHz and a maximum power gain frequency of 42 GHz. These results are comparable with those of devices with the conventional contact scheme. Meanwhile, since the non-alloying process maintains excellent surface morphology and sharp metal pad edges, AuGeNi type metals can be used in small dimension devices. In contrast, the conventionally alloyed devices showed rough surface morphology and zigzag edges, which may affect device processing and limit the usefulness of AuGeNi metal as the ohmic contact in small dimension devices.