Growth of ultra thin ZnSe nanowires
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1144-LL01-04
Growth of ultra thin ZnSe nanowires Tai-Lun Wong, Yuan Cai, Siu-Keung. Chan, Iam-Keong Sou and Ning Wang Department of Physics and the William Mong Institue of Nano Science and Technology, the Hong Kong University of Science and Technology, Hong Kong, China ABSTRACT We report here the growth of ultra thin ZnSe nanowires at low temperatures by Aucatalyzed molecule beam epitaxy and structural characterization of the nanowires. ZnSe nanowires may contain a high density of stacking faults and twins from low temperature growth and show a phase change from cubic to hexagonal structures. Ultra thin ZnSe nanowires can grow at a temperature below the eutectic point, and the relationship between the growth rates and nanowire diameters is V= 1/dn+ C0 (C0 is a constant and n is a fitting parameter). The growth rate of the ultra thin nanowires at low temperatures can be elucidated based on the model involving interface incorporation and diffusion, in which the catalyst is solidified, and the nanowire growth is controlled through the diffusion of atoms into the interface between the catalyst and nanowire. The growth rate of ZnSe ultra thin nanowires has been simulated. INTRODUCTION In the classical vapor-liquid-solid (VLS) model [1,2], it is believed that the metal catalyst is in molten state which absorbs the source materials to form a supersaturated liquid droplet. The precipitation of the source atoms occurs at the droplet-whisker interface, and the precipitation rate is mainly determined by the supersaturation of the droplet. Givargizov et al. [1,2] determined the whisker growth rate as a function of the driving force of supersaturation (∆µ/kT) and first empirically described the growth rate by n
∆µ dL 4Ωσ (1), V= = b o − dt k BT dkBT where d is the nanowire diameter, T is the growth temperature, kB is Boltzmann's constant ∆µ0 is the effective difference between the chemical potentials of source element in the nutrient phase and in solid phase, Ω is the atomic volume source element, σ is the specific of the nanowire surface, b and n (~2) were empirical fitting parameters, . According to Equation (1), the larger the whisker diameter, the faster is its growth rate. This growth phenomenon is attributed to the well-known Gibbs-Thomson effect, i.e., the decrease of supersaturation as a function of the whisker diameter. [1, 2]
Due to the change of the driving force, Si whiskers with small diameters (< 100 nm) grow very slowly. Obviously, there is a critical diameter at which ∆µ = 0 and the whisker growth stops completely. Those whiskers with diameters less than the critical diameter (about 50nm) should stop growing. However, in recent years, both experimentalists and theorists [4-6] have showed that semiconductor nanowires with diameter smaller than 50 nm can grow and show interesting growth behaviors. For example, in the growth of thin
Si and ZnSe nanowires catalyzed by Au particles, thinner nanowires grow faster than thicker ones,[5,6] and most ultra thin nanowires grow at relatively low temperature
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