Zhendong Hu

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.

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.

Violet luminescence in phosphorus-doped ZnO epitaxial films

A violet luminescence band at 3.1099eV was observed at 12.5K in phosphorus-doped ZnO epitaxial films deposited by O2 plasma-assisted pulsed laser ablation. The band results from a transition between a shallow donor and a deep acceptor induced by phosphorus doping. The activation energy of the acceptor varies with the phosphorus concentration [P] and is 0.34eV when [P] is 1.7×1019cm−3. Under oxygen-rich conditions, the dominant acceptor in P2O5-doped ZnO may be the zinc vacancy, in agreement with recent first-principles calculations.

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.