Yong Che

Nanoparticle generation in ultrafast pulsed laser ablation of nickel

The process of particle generation during ultrafast pulsed laser ablation of nickel is investigated. Two types of particles with different sizes depending on the laser fluence are found, indicating different particle generation mechanisms. By limiting the laser fluence below a threshold of strong plasma formation, the large dropletlike particles can be eliminated. In addition, by supplying different background gases, various crystalline structures are obtained for the particles, including Ni∕NiO core/shell spheres and NiO cubes. This study provides evidence that ultrafast laser ablation can be a room temperature physical method for generating nanocrystals with a narrow particle size distribution.

Microstructure and properties of epitaxial antimony-doped p-type ZnO films fabricated by pulsed laser deposition

Antimony-doped p-type ZnO films epitaxially grown on (0001) sapphire substrates were fabricated by pulsed laser deposition at 400–600°C in 5.0×10−2Torr oxygen without postdeposition annealing. The films grown at 600°C have among the highest reported hole concentration of 1.9×1017cm−3 for antimony doping, Hall mobility of 7.7cm2∕Vs, and resistivity of 4.2Ωcm. Transmission electron microscopy reveals that the p-type conductivity closely correlates to the high density of defects which facilitate the formation of acceptor complexes and the compensation of native shallow donors. The thermal activation energy of the acceptor was found to be 115±5meV and the corresponding optical ionization energy is ∼158±7meV.

Epitaxial ZnO films on (111) Si substrates with Sc2O3 buffer layers

Epitaxial (0001) ZnO films were grown on (111) Si substrates using epitaxial (111) Sc2O3 buffer layers. The quality of the ZnO epilayers is manifested by a Hall mobility of 77 cm2/V s at room temperature, x-ray diffraction rocking curve full widths at half maximum of 300–400 arc sec, and optical properties comparable to ZnO single crystals. Transmission electron microscopy studies reveal that a thin layer of SiOx was formed at the Sc2O3/Si interface not during the Sc2O3 growth, but during the growth of the ZnO films. The thermal-mismatch induced residual strain in the films causes an energy shift of the exciton resonances in the photoluminescence spectrum. The redshifts are smaller than those of GaN films, indicating that the optical properties of ZnO are less strain sensitive.

Burst-Mode Femtosecond Pulsed Laser Deposition for Control of Thin Film Morphology and Material Ablation

We introduce an alternative approach of pulsed laser deposition (PLD) using groups of closely time spaced (20 ns) femtosecond laser pulses, namely burst-mode fs-PLD. This approach enables a broad and continuous tunability over the material morphologies ranging from nanoparticle aggregates to epitaxial thin films with completely droplet-free and atomically smooth surfaces. The tunability of materials is realized by simply tuning laser parameters. An unusual phenomenon of laser-matter interaction is revealed in terms of the breakdown of nanoparticles, the enhancement of plasma ionization, and the decrease of ablation threshold during the burst-mode fs-ablation. A TiO2 film, a wide band gap semiconductor, was deposited with as low as 50 nJ of pulse energy. This approach and the phenomenon are applicable to many other materials.

Temperature-dependent Hall and photoluminescence evidence for conduction-band edge shift induced by alloying ZnO with magnesium

This work discusses the effect of conduction-band edge shift induced by alloying ZnO with magnesium. Temperature-dependent Hall and temperature-dependent photoluminescence measurements are used to characterize the epitaxial Zn1−xMgxO thin films grown on (111) Si using intervening epitaxial Lu2O3 buffer layers, which prove that the addition of Mg in ZnO shifts the conduction-band edge to higher energy, thus increasing the activation energy of the defect donor states and reducing the n-type background carrier concentration.

Ultrafast pulsed laser ablation for synthesis of nanocrystals

Near infrared ultrafast pulsed laser is used to ablate pure metal and metal alloy targets in a vacuum chamber. We find that by optimizing the ablation conditions, as a direct result of ultrafast laser ablation, crystalline nanoparticles can be abundantly produced without intermediate nucleation and growth processes. Combining with different background gases, versatile structural forms can also be obtained for the nanocrystals. Using metal nickel as a sample material, we have produced Ni/NiO core/shell nanospheres and NiO nanocubes. We also study the production of alloy nanoparticles, which has been challenging in fabrication. We demonstrate production of nanoparticles containing up to three metal elements using ultrafast laser ablation. The laser ablation process is investigated using an ion probe in real-time. Nanoparticle samples are examined using atomic force microscopy and high resolution transmission electron microscopy for morphological, structural, and chemical analysis. This study provides a simple physical method for generating nanoparticles with a narrow particle size distribution, a high particle yield, versatile chemical compositions and structural forms.

Ultrafast pulsed laser micro-deposition printing on transparent media

Ultrafast pulsed laser ablation is employed in laser-induced backward transfer for printing on transparent media. By combining a high pulse repetition rate of 1 MHz and an ultrashort pulse duration of 700 fs in an ultrafast fiber laser, we demonstrate printing of bitmap images and vector graphics with nearly continuous gray scales and high linear printing speeds up to 10 m/s. In addition, we find that the printing process preserves several original functional properties of the target material, and as an example of functional printing, we demonstrate printing of phosphorescent images.

Stable gold nanocolloids with controllable surface modification and functionalization

This paper demonstrates a new method for fabrication of stable gold nanoparticle-poly(ethylene glycol) (PEG) conjugates with a defined number of PEG molecules. The PEG molecules are directly bound to the surface of gold nanoparticles with almost 100% conjugation efficiency and the PEG surface coverage is tunable between values of 0 and 100%. Gold nanoparticles for the nanoparticle PEG conjugates are prepared by femtosecond laser ablation in liquid of a gold bulk target in deionized water. This method for fabrication of nanoparticles creates gold surfaces which are negatively charged and chemically clean. This facilitates uniform and controlled binding of thiolated PEG molecules to the surface of the gold nanoparticles. The method used to bind PEG to gold can be used with other biomolecules to prepare gold nanoparticles with single or mixed monolayers of biologically important molecules.

Effects of defects on the electrical and optical properties of ZnO thin films

Zinc oxide is a wide band gap semiconductor with potential applications in optoelectronic devices. The greatest challenge for these applications is to fabricate reliable and stable p-type ZnO thin films. There exist many reports of p-type conductivity in ZnO films doped with group V elements.Ü However, little understanding of the role of defects, either native or induced by doping, on the onset of p-type conductivity and on the degradation of optical properties. In this talk, we will demonstrate the effects of crystal defects on the optoelectronic properties of p-type ZnO epitaxial thin films. We will show that the interplay between dopants and extended defects such as dislocations and stacking faults is fundamentally important for the fabrication of p-type ZnO. ZnO thin films doped with nitrogen, phosphorus and antimony were grown by pulsed laser ablation. We will show that dislocations can, under certain conditions, aid the formation of shallow acceptors. ThÜe role of dislocations on the dopant solubility, electrical transport properties and photoluminescence will be addressed. We will also show that depending on the oxygen chemical potential, different defect levels can be introduced in the band-gap. These defects result in various optical transitions that can provide useful information on the donors and acceptors formed. Finally, issues of homoepitaxy and p-n junction fabrication will be presented.Ü From these studies, we conclude that the fabrication of p-type ZnO films is trivial, however, a p-type epitaxial thin film with high structural quality is challenging.Ü