B. J. Chen

Blueshift of optical band gap in ZnO thin films grown by metal-organic chemical-vapor deposition

The optical band gap of ZnO thin films deposited on fused quartz by metal-organic chemical-vapor deposition was studied. The optical band gap of as-grown ZnO blueshifted from 3.13to4.06eV as the growth temperature decreased from 500to200°C. After annealing, the optical band gap shifted back to the single-crystal value. All the ZnO thin films studied show strong band-edge photoluminescence. X-ray diffraction measurements showed that samples deposited at low temperatures (<450°C) consisted of amorphous and crystalline phases. The redshift of the optical band gap back to the original position after annealing was strong evidence that the blueshift was due to an amorphous phase. The unshifted photoluminescence spectra indicated that the luminescence was due to the crystalline phase of ZnO, which was in the form of nanocrystals embedded in the amorphous phase.

Field emission from gallium-doped zinc oxide nanofiber array

Gallium-doped nanostructural zinc oxide fibers have been fabricated by vapor-phase transport method of heating the mixture of zinc oxide, gallium oxide, and graphite powders in air. The zinc oxide fibers grew along [002] direction, forming a vertically aligned array that is predominantly perpendicular to the substrate surface. With a gallium doping concentration of 0.73 at. %, the corresponding carrier concentration and resistivity were 3.77×1020 cm−3 and 8.9×10−4 Ω cm, respectively. The field emission of these vertically aligned ZnO fiber arrays showed a low field emission threshold (2.4 V/μm at a current density of 0.1 μA/cm2), high current density, and high field enhancement factor (2317). The dependence of emission current density on the electric field followed Fowler–Nordheim relationship. The enhanced field emission is attributed to the aligned structure, good crystal quality, and especially, the improved electrical properties (increased conductivity and reduced work function) of the nanofibers due ...

Nanostructural zinc oxide and its electrical and optical properties

Nanostructural zinc oxide fibers have been fabricated by a simple vapor transport method of heating the mixture of zinc oxide, gallium oxide, and carbon powders in air. The zinc oxide nanofibers showed cauliflower-like, disordered, vertically and horizontally aligned morphologies in different temperature regions. The aligned nanofibers were composed of hexagonal zinc oxide with good crystallinity. Gallium was doped into zinc oxide with a concentration of 0.73 at. %. The growth process and the characteristics can be interpreted by a vapor-liquid-solid mechanism. The field emission of the vertically aligned zinc oxide fiber array showed a low field emission threshold, high current density, rapid surge, and high field enhancement factor. The threshold electric field is about 2.4 V/μm at a current density of 0.1 μA/cm−2. The field enhancement factor was 2991. The emission current density and the electric field followed the Fowler–Nordheim relationship.

Room-Temperature Ultraviolet Lasing from Zinc Oxide Microtubes

Prismatic zinc oxide microtubes have been fabricated by vapor transport. Room-temperature ultraviolet lasing action has been demonstrated in these microtube arrays. The ZnO microtubes, mainly appearing in tapped bell-mouthed shape, form natural laser cavities along the length direction. The hexagon diagonal and length of the microtube vary from 1 µm to 20 µm and 10 µm to a few hundred µm respectively. Under 355 nm optical excitation, lasing action is observed at room-temperature around 393 nm. Multi-longitudinal modes are also observed with significantly narrowed emission linewidth.

Ultraviolet amplified spontaneous emission from self-organized network of zinc oxide nanofibers

Self-organized zinc oxide (ZnO) nanofiber network with six-fold symmetry was fabricated on ZnO-buffered (0001) sapphire substrate with patterned gold catalyst by vapor-phase transport method. From the ZnO buffer layer, hexagonal ZnO nanorods with identical in-plane structure grew epitaxially along [0001] orientation to form vertical stems. The nanofiber branches grew horizontally from six side-surfaces of the vertical stem along [011¯0] and other equivalent directions. The aligned network structure constructed a waveguide array with optical gain. Ultraviolet amplified spontaneous emission was observed along the side-branching nanofibers when the aligned ZnO network was excited by a frequency-tripled Nd:YAG laser.

Cluster coarsening in zinc oxide thin films by postgrowth annealing

Postgrowth annealing was carried out on ZnO thin films grown by metal-organic chemical-vapor deposition. It was found from the scanning electron microscopy and atomic force microscopy measurements that the morphology of the thin films changed drastically after annealing. The as-grown thin films consist of fine nanoscale-sized sheets with random orientation. Upon annealing at 800°C, the ZnO nanosheets changed to three-dimensional nanoneedles. The different types of the mass transport mechanisms are discussed and correlated with the experimental results. A coarsening kinetics developed by Lifshitz and Slyozov [J. Phys. Chem. Solids 19, 35 (1961)] and Wagner [Z. Elektrochem. 65, 581 (1961)] was used to estimate the activation energy of the coarsening process. The activation energy of the Ostwald ripening in ZnO films was estimated in the first attempt, and the value is at around 1.33eV. Hall effect and photoluminescence measurements were carried out to investigate the effect of coarsening on electrical and o...

Organic light-emitting devices with a hole-blocking layer inserted between the hole-injection layer and hole-transporting layer

A hole-blocking layer (HBL), 2,9-dimethyl-4, 7-diphenylphenanthroline (BCP), was incorporated between the hole-transporting layer (HTL) and hole-injection layer for a tris-(8-hydroxyqunoline) aluminum-based organic light-emitting device. Such a structure helps to reduce the hole-leakage to the cathode resulting in improved current efficiency. Optimum BCP thickness of around 3nm was observed to produce a current efficiency of 3.25cd∕A, which corresponds to a 30% improvement compared to that of the standard device without BCP (2.5cd∕A). Low operating voltage was also achieved by minimizing the thickness of the HTL. Both operating voltage and efficiency can be tuned by varying the thickness of HTL and HBL, respectively.

Organic light-emitting devices with in situ postgrowth annealed organic layers

A comparative study of in situ postgrowth annealing of organic layers before metal cathode was conducted on tris-(8-hydroxyqunoline) aluminum (Alq3)-based organic light-emitting devices (OLEDs). The devices were fabricated in the same run with a standard device without annealing for comparison, with an identical structure of indium tin oxide (ITO)/copper phthalocyanine (CuPc) (10nm)∕N,N′-di(naphthalene-l-yl)-N,N′-diphenyl-benzidine (NPB) (90nm)∕Alq3(90nm)∕Mg:Ag(200nm)∕Ag(20nm). The annealing temperature used was 60, 80, and 100°C, respectively. It was found that, in situ postgrowth annealing improves the device performance, and annealing near the glass transition temperature of NPB (99.7°C), improves device performance drastically. Power efficiency and current efficiency increase significantly with the annealing temperature, except the current efficiency for device annealed at 100°C is slightly lower than that of the standard device. The voltage and current density for 100cd∕m2 luminance are 5.6V and 4.4m...

Influences of central metal ions on the electroluminescene and transport properties of tris-(8-hydroxyquinoline) metal chelates

A series of tris-(8-hydroxyquinoline) metal chelates with central metal ions of Al3+, Ga3+, and In3+ was synthesized, characterized, and used in organic electroluminescent devices. The ionization potential and optical band gap of the three chelates were measured by ultraviolet photoelectron spectroscopy and ultraviolet-visible spectrum, respectively. Two types of devices, with configurations of indium tin oxide (ITO)/N,N′-diphenyl-N,N′-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) (80 nm)/Mq3 (80 nm)/Mg:Ag (200 nm) and ITO/TPD (60 nm)/TPD:rubrene (3%) (20 nm)/Mq3 (80 nm)/Mg:Ag (200 nm) (M=Al, Ga, or In), were fabricated and characterized based on these metal chelates. In the first type of configuration, the metal chelates were used as both the emitter and the electron-transporting layer. In the second type of configuration, the metal chelates were believed to only act as electron-transporting layer. As the central ion changed, both the luminescence and the carrier transport properties of the metal che...

Network array of zinc oxide whiskers

A zinc oxide (ZnO) whisker network array with sixfold symmetry was fabricated on ZnO-buffered (0001) sapphire substrate by the vapour-phase transport method using a mixture of zinc oxide and graphite powders as source materials and patterned gold as catalyst. From the ZnO buffer layer, hexagonal ZnO nanorods with identical in-plane structure grew epitaxially along the [0001] orientation to form vertical stems. The branches grew horizontally from six side-surfaces of the vertical stem along and other equivalent directions. Most whiskers were confined along the six preferential orientations and interconnected with each other to form a regular network structure. The growth mechanism is discussed.