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J. Phys. Chem. B 2001, 105, 4062-4064
Synthetic Control of the Diameter and Length of Single Crystal Semiconductor Nanowires Mark S. Gudiksen, Jianfang Wang, and Charles M. Lieber* Department of Chemistry and Chemical Biology, HarVard UniVersity, 12 Oxford Street, Cambridge, Massachusetts 02138 ReceiVed: February 12, 2001; In Final Form: March 21, 2001
A general synthetic method has been developed to control both the diameter and the length of nanowires during growth. This approach exploits monodisperse nanocluster catalysts to define both the nanowire diameter and the initiation of nanowire elongation during growth by a vapor-liquid-solid mechanism. To demonstrate this new approach, crystalline indium phosphide (InP) nanowires have been synthesized using a laser catalytic growth (LCG) process combined with gold nanocluster catalysts. InP nanowires with nearly monodisperse diameters of 10, 20, and 30 nm were grown from nanocluster catalysts having diameters of 10, 20, and 30 nm, respectively. High-resolution transmission electron microscopy studies show that the InP nanowires prepared in this manner are single crystals with a [111] growth direction. In addition, studies of nanowire growth as a function of growth time have shown that nanowire length is directly proportional to growth time and have enabled the preparation of InP nanowires with narrow length distributions centered at 2, 4, 6, and 9 µm. The new level of synthetic control afforded by our approach should enable better-defined fundamental studies of nanowires and open up new opportunities for the assembly of functional nanodevices.
Introduction Interest in low-dimensional systems, such as zero-dimensional (0D) nanoclusters and one-dimensional (1D) nanowires, has been sparked by a desire to tune the fundamental optical and electronic properties of materials through rational control of their physical size.1,2 To realize the potential impact of these materials in nanoscale chemistry and physics, from both fundamental and applied viewpoints, demands materials of well-defined size, structure, and composition. One-dimensional materials,1 in contrast to 0D nanoclusters,2 have been relatively unexplored, primarily due to the synthetic challenge of producing highquality materials of controlled size. The utility of nanowire materials in a myriad applications from devices and interconnects3,4 for molecular computing to scanning probe microscopy tips5 emphasizes a definite need for high-quality nanowires. The importance of “bottom-up” chemical approaches to highquality, free-standing nanowires cannot be overstated. “T-wires”6 and “V-groove”7 wires are both examples of high-quality nanowires produced via a combination of lithographic and epitaxial growth techniques. These approaches are intrinsically limited in that the wires remain embedded in the substrates, precluding assembly into complex devices or new tools. Others have grown nanowires via template approaches,8 giving rise to nanowires of well-defined diameters and lengths. Nonetheless, these approaches have the drawback that they often produce polycrystalline materials, which are less suitable for both fundamental and applied studies. Our laboratory has made significant steps toward the development of a general synthetic methodology for semiconductor nanowires via the laser catalytic growth (LCG) method.9,10 Early experiments utilized the laser ablation of a solid target to generate simultaneously semiconductor reactants and nanocluster catalysts, which produce nanowires via a vapor-liquid-solid (VLS)11 growth mechanism. This approach has yielded a wide range of group IV, III-V, and II-VI nanowires,9,10 including control over n- and p-type doping.4,12 In addition, we recently
demonstrated that well-defined gold colloids could be exploited as catalysts for the growth of GaP nanowires with diameter distributions defined by those of the nanocluster catalysts.13,14 Here, we significantly extend this methodology by demonstrating that nanocluster catalysts can be used to control both the diameter and the length of semiconductor nanowires. The conceptual ideas underlying diameter- and lengthcontrolled growth of nanowires are illustrated in Figure 1. This approach exploits monodisperse nanocluster catalysts to define both nanowire diameter (Figure 1a) and initiation of nanowire elongation during growth by a vapor-liquid-solid mechanism. By varying the ablation time in LCG, we were able to grow selectively nanowires of a given length (Figure 1b). We report the growth of indium phosphide (InP) nanowires using predefined gold colloids as the nanocluster catalyst but note that the method is applicable to all semiconductor nanowires previously synthesized.9,10 InP is an intriguing material because of its optoelectronic properties as a direct band gap semiconductor with strong photoluminescence, and thus, controlled diameter and length could open up a number of opportunities for fundamental research and applications. Experimental Section InP nanowires were grown using the laser catalytic growth process described previously.9,10,13 Substrates for nanowire growth were made by functionalizing the surface of a silicon substrate (silicon with 600 nm of thermal oxide, Silicon Sense) for 1 min with a solution of 0.1% poly-L-lysine (Ted Pella). Gold nanocluster solutions (BBI International) were diluted to concentrations of 1010-1011 particles/mL and were dispersed onto the substrates and then the substrates were quickly (
