Chemical Vapor Deposition method for Synthesis of Nanomaterials
W. Cao
Skyspring Nanomaterials, Inc.
http://www.ssnano.com, wcao@ssnano.com
In this approach, vapor phase precursors are brought into a hot-wall reactor under conditions that favor nucleation of particles in the vapor phase rather than deposition of a film on the wall. It is called chemical vapor synthesis or chemical vapor condensation in analogy to the chemical vapor deposition (CVD) processes used to deposit thin solid films on surfaces. This method has tremendous flexibility in producing a wide range of materials and can take advantage of the huge database of precursor chemistries that have been developed for CVD processes. The precursors can be solid, liquid or gas at ambient conditions, but are delivered to the reactor as a vapor (from a bubbler or sublimation source, as necessary).
When a mixture of gas reactants are delivered into a reaction chamber, the chemical reactions among the gas molecules are induced by an input of energy such as resistant heating, laser, and plasma. Chlorides are popular reactants for the formation of oxides because of their generally low vaporization temperature and low cost . The typical reaction is as following:
SnCl4 (gas) + 2H2O (gas) → SnO2 (solid) + 4HCl (gas) (1)
Another key feature of chemical vapor synthesis is that it allows formation of doped or multi-component nanoparticles by use of multiple precursors. Schmechel et al. prepared nanocrystalline europium doped yttria (Y2O3:Eu3+)from organometallic yttrium and europium precursors. Senter et al. incorporated erbium into silicon nanoparticles using disilane and an organometallic erbium compound as precursors. Srdic et al. prepared zirconia particles doped with alumina. Brehm et al. synthesized nanoparticles of indium oxide, tin oxide, and indium oxide doped with tin oxide (ITO) by chemical vapor synthesis for the applications as transparent conducting oxides, catalysts and gas sensors. The powders exhibit a narrow size distribution with an average size of about 5 nm.
Recently, Wang’s group successfully synthesized a series of binary semiconducting oxide nanobelts (or nanoribbons), such as ZnO, In2O3, Ga2 O3, CdO and PbO2 and SnO2 by simply evaporating the source compound. Condensed or powder source material is vaporized in a tube furnace at an elevated temperature and the resultant vapor phase codense under certain conditions (temperature, pressure, substrate, etc.) to form the desired product. The assynthesized oxide nanobelts are pure, structurally uniform, single crystalline and most of them free from defects and dislocations; they have a rectangular-like cross-section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The belt-like morphology appears to be a unique and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts are an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts. Wang’s group has recently applied the nanobelt materials to make the world’s first field effect transistor and single wire sensors. The latest breakthrough of Wang’s group is the success of first piezoelectric nanobelts and nanorings for applications as sensors, transducers and actuators in microand nano-electromechanical systems. A typical SEM image of as-synthesized ZnO nanorings is shown in the following figure. Owing to the positive and negative ionic charges on the zinc- and oxygen-terminated ZnO basal planes, respectively, a spontaneous polarization normal to the nanobelt surface is induced. As a result, helical nanosprings/nanocoils are formed by rolling up single crystalline nanobelts. The mechanism for the helical growth is suggested for the first time to be a consequence of minimizing the total energy contributed by spontaneous polarization and elasticity. The nanobelts have widths of 10–60 nanometers and thickness of 5–20 nanometers, and they are free of dislocations. The polar surface dominated ZnO nanobelts and helical nanosprings are likely to be an ideal system for understanding piezoelectricity and polarization induced ferroelectricity at nano-scale.
(A) Low-magnification SEM image of the as-synthesized ZnO nanorings. (B) High magnification SEM image of a freestanding single-crystal ZnO nanoring, showing uniform and perfect geometrical shape. The ring diameter is 1 to 4 µm, the thickness of the ring is 10 to 30 nm, and the width of the ring shell is 0.2 to 1 µm.
These metal oxide nanobelts, nanowires, nanodiskettes, and nanoribbons are highly promising candidates for gas sensing materials.
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