Silicon and zinc telluride nanoparticles synthesized by low energy density pulsed laser ablation into ambient gases
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Silicon and zinc telluride nanoparticles synthesized by low energy density pulsed laser ablation into ambient gases Douglas H. Lowndes and Christopher M. Rouleau Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6056
T. G. Thundat Life Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6123
G. Duscher Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6056
E. A. Kenik Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6376
S. J. Pennycook Solid State Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6056 (Received 17 December 1997; accepted 5 May 1998)
The size distributions of Si and ZnTe nanoparticles produced by low energy density ArF (193 nm) pulsed laser ablation into ambient gases were measured as a function of the gas pressure, P, and target-substrate separation, Dts . For both Si and ZnTe, the largest nanoparticles were found closest to the ablation target, and the mean nanoparticle size decreased with increasing Dts . For Si ablation into He, the mean nanoparticle diameter did not increase monotonically with gas pressure but reached a maximum near P 6 Torr. High resolution Z-contrast transmission electron microscopy and energy loss spectroscopy revealed that ZnTe nanoparticles consist of a crystalline core surrounded by an amorphous ZnO shell; growth defects and surface steps are clearly visible in the crystalline core. A pronounced narrowing of the ZnTe nanocrystal size distribution with increasing Dts also was found. The results demonstrate that the size of laser-ablated nanoparticles can be controlled by varying the molecular weight and pressure of an ambient gas and that nanometer-scale particles can be synthesized. Larger aggregates of both ZnTe and Si having a “flakelike” or “weblike” structure were formed at the higher ambient gas pressures; for ZnTe these appear to be open agglomerates of much smaller (,10 nm) particles.
I. INTRODUCTION
Recent pulsed laser deposition (PLD) experiments have demonstrated that by varying the pulsed laser wavelength, intensity, and ambient gas pressure, both the energy distribution and the nature of the ablated flux can be controlled.1,3 –12 Species ranging from highly energetic (>100 eV) atoms and ions up through clusters and nanocrystals can be produced, creating new opportunities for synthesis of novel thin-film materials and control of their properties.1 Cluster and nanocrystal formation are greatly enhanced by ablating a material into a moderatepressure (0.1–10 Torr) ambient gas.2 With increasing gas pressure, the deposition flux changes from primarily atoms and ions to clusters and nanocrystals, the latter typically having diameters of 1 to 20 nm and containing from 102 to 106 atoms, with beneficial or detrimental consequences for film properties. For example, we recently grew p-type, nitrogen-doped
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