Wei-Fan Lee

GaN Nanowire Arrays for High-Output Nanogenerators

Three-fold symmetrically distributed GaN nanowire (NW) arrays have been epitaxially grown on GaN/sapphire substrates. The GaN NW possesses a triangular cross section enclosed by (0001), (2112), and (2112) planes, and the angle between the GaN NW and the substrate surface is approximately 62 degrees . The GaN NW arrays produce negative output voltage pulses when scanned by a conductive atomic force microscope in contact mode. The average of piezoelectric output voltage was about -20 mV, while 5-10% of the NWs had piezoelectric output voltages exceeding -(0.15-0.35) V. The GaN NW arrays are highly stable and highly tolerate to moisture in the atmosphere. The GaN NW arrays demonstrate an outstanding potential to be utilized for piezoelectric energy generation with a performance probably better than that of ZnO NWs.

Single‐InN‐Nanowire Nanogenerator with Upto 1 V Output Voltage

CTH and JHS contributed equally to the research in this paper. Research was supported by DARPA (Army/AMCOM/REDSTONE AR, W31P4Q-08-1-0009), BES DOE (DE-FG02-07ER46394), KAUST, and NSF (DMS0706436, CMMI 0403671). Thanks are also due to National Science Council of Taiwan, Republic of China for a fellowship to study abroad (C. T. Huang). (NSC97-2917-1-007-110)

Oriented growth of large-scale nickel sulfide nanowire arrays via a general solution route for lithium-ion battery cathode applications

A general solution method for the oriented growth of large-scale Ni3S2nanowire arrays has been developed. The controlled oxidation scheme by combining ethylenediamine-chalcogens and hydrazine in alkali solution has been shown to have great advantages for the fabrication of metal chalcogenides with fewer instrumental limitations. This method is reliable and works in mild template-free conditions for the production of single-crystalline nanowire arrays. It provides a convenient route for the large-scale growth of pure-phase metal chalcogenide nanowire arrays on metal substrates. The electrochemical measurement results of Ni3S2nanowire arrays for lithium-ion battery electrode applications reveal that they have high reversible lithium storage capacity, long cycle life, good cyclic stability and high charge/discharge rate. With the simplicity of fabrication and good electrochemical performance, Ni3S2nanowire arrays are promising cathode materials for lithium-ion batteries.

Growth of CuInSe2 and In2Se3/CuInSe2 nano-heterostructures through solid state reactions.

In(2)Se(3) is an essential phase change material and CuInSe(2) is the fundamental basis of the copper-indium-gallium-diselenide (CIGS) solar energy material. In this paper, we demonstrate the feasibility to transform the phase change material to the solar energy material via the solid state reaction. The In(2)Se(3) nanobelts (NBs) were synthesized via the vapor-liquid-solid mechanism. The chemical composition and the optical properties were investigated by energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and reflectance and photoluminescence spectra. In the in situ observation of the solid state reaction with Cu to form the CuInSe(2) NBs with ultrahigh vacuum transmission electron microscopy, we observed the In(2)Se(3)/CuInSe(2) transformation at atomic scale in real time. The progression of the atomic layer at the interface provided the pertinent information on the kinetic mechanism. In(2)Se(3)/CuInSe(2) nano-heterostructures were also obtained in the present investigation. The approach to the CIGS nanosolar cell was also proposed. This study shall be beneficial in the development of high-performance nanowire solar cells and nanodevices with In(2)Se(3)/CuInSe(2) nano-heterostructures.

Highly luminescent, homogeneous ZnO nanoparticles synthesized via semiconductive polyalkyloxylthiophene template

We report here the preparation of ZnO/polythiophene-based conjugated polymer nanocomposites that retain especially highly luminescent (Φf > 0.5) and semiconductive properties without the addition of capping agents. The as-prepared nanocomposites have been well characterized by UV-Vis, fluorescence, TEM/EDS, etc. This method provides several advantages in terms of homogeneous size distribution, good dispersion and intense, distinctive band-gap emission free from defect trapping. The results thus demonstrate the great potential of alkyloxyl modified polythiophene-based nanocomposites in the field of optoelectronics and microelectronics devices.

Room-temperature ferromagnetism in self-assembled (In, Mn)As quantum dots

Self-assembled In1−xMnxAs quantum dots (0.19⩽x⩽0.45) have been grown on GaAs (100) substrates by low-temperature molecular beam epitaxy. The microstructure analysis revealed that the uniformly distributed In1−xMnxAs dots have a zinc blende structure as x⩽0.38. Furthermore, all samples exhibit ferromagnetic state at 5K, and their Curie temperatures range from 260to340K varying with x. These (In, Mn)As quantum dots are promising for room-temperature spintronic devices.

Low temperature synthesis of copper telluride nanostructures: phase formation, growth, and electrical transport properties

We propose a low cost solution-based approach to synthesize various low dimensional copper telluride (Cu-Te) nanostructures. By precisely controlling different ethylenediamine (EDA) ratios in a reaction solution, we are able to control the phases and morphologies of Cu-Te nanostructures from Te/Cu core–shell nanowires at a low volume fraction of EDA <8%, Cu3Te2 nanowires at the volume fraction of EDA between 8% and 24%, Cu2Te nanowires and nanobelts at the volume fraction of EDA between 24% and 48%, to Cu2Te/Cu core–shell nanobelts at the volume fraction of EDA over 48%. The formation mechanism is attributed to varied tendency of different coordinative copper complexes. In situ heating XRD results and TEM observations of the Cu2Te nanowires reveal the phase transition from hexagonal P3m1, hexagonal P6/mmm to cubic structure at annealing temperatures of 25 °C, 500 °C to 600 °C, respectively. The lack of back gate dependence demonstrates the metallic feature of Te/Cu core–shell nanowire while obvious p-type behavior can be found for Cu2Te nanowire with an on/off ratio of ∼104 and the field effect hole mobility of ∼18 cm2 V−1 s−1. These Cu-Te nanostructures exhibit controllable transport behaviors from metallic to semiconducting natures with different EDA volume fractions and have promising applications in electronics such as nonvolatile memory, photodetectors, and solar cells.

Enhancement of exchange coupling between GaMnAs and IrMn with self-organized Mn(Ga)As at the interface

An atomically flat and uniform reaction layer of Mn(Ga)As was found to self-organize at the (Ga,Mn)As∕IrMn interface by postannealing. The Mn(Ga)As layer exhibits strong ferromagnetic characteristics up to the measured 300K. In particular, the manifested horizontal shift of field-cooled hysteresis loops shows a clear signature of exchange bias attributable to the exchange coupling between IrMn and Mn(Ga)As. Implication from composition analyses, exchange-bias effect, and thickness dependence of the Mn(Ga)As layer versus annealing conditions is also discussed.

Nd-doped silicon nanowires with room temperature ferromagnetism and infrared photoemission

Nd-doped silicon nanowires have been synthesized by a vapor transport and condensation method. The incorporation of neodymium within silicon nanowires was achieved by using NdCl3⋅6H2O powder as the doping source. Ferromagnetism and infrared photoluminescence at room temperature were discovered. The significant variation and versatility of the properties exhibited by the Nd-doped silicon nanowires are promising for exploitation for the advanced silicon-based devices.

Self-assembly and magnetic properties of MnAs nanowires on GaAs(001) substrate

The in-plane aligned MnAs nanowires have been grown by molecular-beam epitaxy on GaAs(001) substrates at high growth temperature (≥450u2009°C). A discontinuous growth with break intervals (50 s’ interval per 10 s’ growth) was employed. The obtained nanowires were identified to be mainly type-B hexagonal MnAs. The influences of growth temperature and As4/Mn flux ratio on the nanowires’ morphology were investigated. Both high growth temperature and high As4/Mn flux ratio are necessary for the growth of uniaxially aligned MnAs nanowires with high aspect ratio. The magnetic anisotropy of the nanowires and their multimodal size distributions contribute to the large coercivity and special shape of the M-H loops along the magnetic easy axis, which is [11¯02]MnAs∥[110]GaAs. However, the longer growth time would lead to the both azimuthal alignments of the MnAs wires and the weakening of the magnetic anisotropy.