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Three basic growth mechanisms of one dimensional semiconductor nano-materials.

1D system is the smallest dimension structures that can be used for efficient transport of electrons and optical excitations. And thus expected to be critical to the function and integration of nanoscale devices.[1]

Due to the high potential for basic studies and applications such as building blocks of electronic, one dimensional nanostrucure semiconductor materials successfully attracted much attention. As far as 1D nanometerials is concerned, the control of morphology, size, growth direction is important.

From the papers which were published recently, there is a tendency that research on 1D nanometerials is expanding rapidly into their assembly to two- (2D) and three-dimensional (3D) ordered superstructures. Once we fully understand the growth mechanism of 1D nanometerials, then it will not be far from successfully assemble 2D and 3D ordered superstructures.[2]

Here just present three basic and widely used growth mechanisms of 1D semiconductor nanometerials.

V(Vapor)-L(Liquid)-S(Solid) Growth Mechanism

The first indications of the VLS growth of semiconductor wires with nanometer-scale diameters were published in 1991 and 1992. [3]

The following are two simplified figures:
 
Figure 1
 
Figure 2: synthesis Si nanocrystals under VLS mechanism

Here, in figure 1, A stand for one reactant. B can be seen as plate, such as Si wafer. Usually, this kind of reaction are kept in furnace. As the T increase to the melting point of A, A turns into liquid. When there is another kind of reatant C approaches to it as gas phase, the reaction starts. As time past, we can get 1D nanometerials from the surface of B.

Of course, this simple growth mechanism also face to a lot of challenges with the improvement of nano-technology. One is that catalytic nanoparticle usually catalyzes the growth of only one nanowire/tube and is attached to one tip of the nanowire/tube after reaction. This will definitely confine the process of later reaction. One batch of catalyst can only catalise one batch of reactants. The amount of catalyst determine the finally amount of products. Also at the same time, catalyst often directs the nucleation and the growth of a nanostructured crystal, and confines the diameter of the crystalline nanowire/tube.[2]

Template Growth Mechanism
When we want to acheive a specific propertity of nanometerials like special morphology or the specific sieze we often try to find a proper template to acheive this goal. To better introduce this growth mechanism, an example will be cited here.
 
Figure 3: Synthesis nanotubes under template mechanism

In the formation process of the CePO4:Tb nanotubes, Ce(OH)CO3:Tb is adopted as a precursors and as both the physical and chemical templates. As H3PO4 is introduced into the system, the following reaction occurs:
Ce(OH)CO3Tb+H3PO4 = CePO4+CO2+2H2O
The CePO4:Tb products are formed by a surface deposition
with a subsequent crystal growth procedure, during which the PO43- is deposited onto the surface of the template Ce(OH)CO3:Tb and then reacts gradually with the inner core to generate the product we want: CePO4:Tb.[4]

S(Solution)-L(Liquid)-S(Solid) Growth Mechanism

It is quite similar to VLS which we just mentioned before. It was a fortuitous find since the study reveal that crystalline whiskers of a semiconductor could be obtained at T as low as 2000C. The following is one example of this. Here the catalyst particle exist as liquid phase and will direct the growth direction and size of semiconductor nanometerials. [3,5]
 
Figure 4: SLS mechanism

Comparison

VLS method is the most general and affordable ways to generate the best crystalline quality nano-meterials in industry. Adopting this growth mechanism, it will always present us NM with large-diameter whiskers(generally greater than 10 nm) and wide diameter distributions.

The SLS method appears to be advantageous for producing the smallest NW diameters(4-10 nm) and for variation and control of surface ligation. The origin of this difference appears to be the growth T. Semiconductors have higher solubility in catalyst droplets at the higher VLS growth T, thus VLS catalyst droplets become more enlarged during the initial alloying stage. Comparing with the VLS, SLS has not afforded high-quality oxide or nitride nanowires. However, SLS is apparently the easiest in implement.

To the template growth mechanism, the apparent simplicity of template-based growth has led it to be used in the synthesis of a wide variety of metal and semiconductor NW. This approach is also popular since it readily yields at room temperature, high aspect ratio NW with narrow diameters ranging from 15 to 350nm.[3,5,6]

Reference:
1. JIANGTAO HU, Chemistry and Physics in One Dimension: Synthesis and Properties of Nanowires and Nanotubes, Acc. Chem. Res. 1999, 32, 435-445
2. Yijun Guo, Fabrication, Characterization, and Strong Exciton Emission of Multilayer ZnTe Nanowire Superstructures, J. Phys. Chem. C 2008, 112, 20307–20311
3. Fudong Wang, Solution-Liquid-Solid Growth of Semiconductor Nanowires, Inorg. Chem. 2006, 45, 7511-7521
4. Guozhu Chen, Template Synthesis and Luminescence Properties of CePO4:Tb Nanotubes, J. Phys. Chem. C 2008, 112, 20217–20221
5. Masaru Kuno, An overview of solution-based semiconductor nanowires: synthesis and optical studies, Phys. Chem. Chem. Phys., 2008, 10, 620–639
6. Hong Jin Fan, Semiconductor Nanowires: From Self-Organization to Patterned Growth, 10.1002/smll.200500495


[[Category:Nanotechnology]]