Femtosecond Laser Used for ZnO Nanowire Synthesis

Daniel S. Burgess

A laser-assisted nanowire growth process reported by scientists at Lawrence Berkeley National Laboratory in Berkeley, Calif., employs a high-power femtosecond source to reduce the formation of microparticulates in the synthesis of crystalline ZnO nanowires and to yield a narrow size distribution of the structures. Because the method involves a nonequilibrium process, the investigators note, it may enable the production of new structural phases and material compositions for the development of nanowire-based devices.

Researchers at Lawrence Berkeley National Laboratory are using a femtosecond laser to produce ZnO nanowires. Nucleation seeds from the pulsed laser deposition process are visible at the tips of the structures. Courtesy of Samuel S. Mao.

Samuel S. Mao, a staff scientist at the laboratory and principal investigator on the project, explained that previous work at Harvard University in Cambridge, Mass., had shown that pulsed laser deposition was an effective alternative to chemical synthesis for the production of silicon and germanium nanowires. In the approach, the laser ablates a target that includes the nanowire material and a catalyst, seeding the substrate upon which the nanowires form and providing the source material. The nanosecond laser conventionally used for materials deposition, however, also forms particulates tens of microns in size that serve as undesired growth sites, increasing the size distribution of the synthesized nanowires.

To overcome these limitations, the laboratory team used a Spectra-Physics Ti:sapphire laser operating at 10 Hz as a source of 100-fs, 10-mJ pulses of 800-nm radiation for ablation. The target was compressed ZnO powder, and gold-coated sapphire or silicon served as the substrate for deposition and nanowire growth, which was performed at a pressure of 1 atmos and a temperature of approximately 900 °C.

Electron microscopy and photoluminescence measurements revealed that the ZnO nanowires were relatively uniform in diameter and displayed good crystalline characteristics, with nanowires grown under low oxygen partial pressure being of the best quality. Pumped by the 266-nm output of an Nd:YAG laser, the nanowires lased at approximately 380 nm.

Although ZnO nanowires show promise for the development of ultraviolet emitters and, Mao emphasized, of optical and chemical sensors, it is unclear at this point how nanowire-based devices could be produced in large numbers. Similarly, scaling the laser deposition technique for manufacturing will be challenging, he said, because it traditionally has been used with wafers smaller than 4 in. in diameter.

Nevertheless, Mao said, innovative engineering of the growth system and the pulsed laser deposition process may help to overcome these obstacles. The researchers currently are investigating the synthesis of various types of nanowires and plan to pursue the development of nanowire-based devices.

Applied Physics Letters, Sept. 26, 2005, 133115.