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Synthesizing silicon and germanium quantum dots and nanowires

Nanocrystals and nanowires for use as active elements in electronic and optical applications can be created using novel, solution-based synthetic methods.
11 September 2006, SPIE Newsroom. DOI: 10.1117/2.1200607.0203

Semiconductor nanostructures exhibit unique size-dependent properties. These include energy-level quantization, single-electron charging, blue-shifted optical absorption, and enhanced luminescence and photocatalytic activity.1 Simply by changing structure size, one can tune material properties for specific applications.

Nanostructure fabrication methods are of two types. ‘Top-down’ approaches use patterning followed by etching or deposition, while ‘bottom-up’ synthesis converts chemical reactants to nanomaterials. Although both approaches provide control over size, shape, composition, and surface chemistry, the bottom-up approach has more advantages. It facilitates the large-scale, low-cost manufacturing of dispersible nanomaterials for low-temperature processing, deposition on plastic substrates, mixing with molecular and polymeric materials in coatings and composites, and even interfacing with living organisms for medical applications.

Solution-based (colloidal) synthetic methods can produce narrow size and shape distributions of many kinds of nanometer-size crystalline particles with coated surfaces and useful optical and electronic properties.2 Moving away from the trial-and-error development of ‘recipes,’ the field of nanomaterials chemistry has evolved a fundamental understanding of synthesis with control of size, shape, and properties. For the last several years, my research group has focused on the challenges of controlling nanocrystal shapes—which include nanorods,3 nanodisks,4,5 and nanowires6–8—and solution-grown nanostructures made from crystalline Group IV elements, including carbon (C),9 silicon (Si),10 and germanium (Ge).11

Crystalline Si nanostructures are extremely challenging to synthesize in solution, because they require relatively high temperatures to crystallize. For example, chemical-vapor deposition processes are carried out at temperatures exceeding ∼400°C, but silane (SiH4), which is relatively inert, does not thermally decompose below ∼350°C. The problem, then, is how to carry out reactions in a solvent at temperatures between 350°C and 500°C. In 1999, we began to explore high-temperature, high-pressure (i.e., supercritical) organic solvents as reaction media to produce Si and Ge nanocrystals.12 Our first attempts used diphenylsilane as a Si reactant in hexane at ∼450°C and ∼200 bar,6,10,13 and capping reagents such as octanol and dodecanethiol that were added to the system. The results were Si nanocrystals less than 5nm in diameter with relatively stable photoluminescence similar to porous silicon.10 Such crystals are sufficient for single-particle spectroscopy,14 electrogenerated chemiluminescence,15 and electroluminescence in LEDs (see Figure 1). Still, this synthetic route faces problems. Significant amounts of oligomeric byproducts form, which greatly limits the reaction yield, and the particle-passivation chemistry is poorly understood. These challenges must be resolved before Si nanocrystals can be produced in commercial quantities.

Figure 1. Silicon (Si) nanocrystals can be synthesized in supercritical hexane. At top, transmission electron micrographs (TEMs) show crystalline cores that are consistent with diamond-cubic Si. The LED shown at the bottom was built by sandwiching a thin layer of Si particles between a conducting indium-tin oxide glass substrate and an aluminum-electrode layer. The LED emits visible light, but with relatively low (less than 0.1%) electroluminescence. (The TEMs are courtesy of Lindsay Pell, and the LED images are courtesy of R. Chris Doty.)

In contrast, Si nanowires can be made with very high yields in solution by adding metal seed-particles to promote Si crystallization.13 Gold (Au) forms a liquid Au:Si alloy at ∼360°C in equilibrium with solid, crystalline Si—a eutectic—that promotes Si crystallization at low temperature.16 Au nanocrystals, injected as crystallization seeds into supercritical hexane at 450°C with diphenylsilane, produce crystalline nanowires with a diameter close to that of the seed particle.6 This synthetic process is called ‘supercritical fluid-liquid-solid’ (SFLS) growth (see Figure 2). Reactants are fed into a supercritical fluid to produce Si that dissolves into a liquid Au:Si alloy and crystallizes into solid Si nanowires.16 The SFLS process is quite general, and we have grown Si,6,13,16,17 Ge,18,19gallium arsenide (GaAs),20 and gallium phosphorous (GaP)21 nanowires (see Figure 3). We have also produced up to 1g of wires in a single reaction.22

Figure 2. In supercritical fluid-liquid-solid (SFLS) nanowire synthesis, solutions of organic monolayer-coated metal nanocrystals such as the gold (Au) at the upper left, are fed with organometallic reactants, such as diphenylgermane (Ph)2GeH2, into a reactor at a temperature and pressure exceeding the solvent critical point. The precursor thermally decomposes to the semiconductor, which crystallizes into nanowires, as shown in the scanning electron micrograph in the lower right. (The Au nanocrystal TEM is courtesy of Aaron Saunders, and the SEM of the germanium nanowires is courtesy of Tobias Hanrath.)

Figure 3. These SEMs and TEMs (insets) show four nanowires—Ge, Si, gallium arsenide (GaAs), and gallium phosphorous (GaP)—synthesized by Au nanocrystal-seeded SFLS. (The figures are courtesy of Doh C. Lee, Tobias Hanrath, and Forrest M. Davidson.)

Nanowires are being explored for applications such as chemical sensing, photovoltaics, and computation, where the nanowires serve as conductive channels between electrodes (see Figure 4). We have begun to assess the electronic properties of SFLS-synthesized nanowires.20,22–24 Surface chemistry is very important in this regard, and chemical-surface passivation strongly influences the carrier mobility and field response of the nanowires.22–24

Figure 4. The Ge nanowire at top connects to four electrodes on a substrate. Below, a Si nanowire is shown suspended between scanning tunneling microscopy tips. (The Ge nanowire image is courtesy of Tobias Hanrath, and the Si nanowire image is courtesy of Damon Smith.)

The metal seed can also influence properties. Au, for instance, traps carriers in Si and is probably not a good choice for electronics applications. We recently showed that cobalt (Co) and nickel (Ni) nanocrystals work well as seeds for Si and Ge nanowires.25,26 This is encouraging since both materials are commonly used in the semiconductor industry to make source/drain contacts. Interestingly, Co- and Ni-seeded nanowires grow at temperatures (∼450°C) hundreds of degrees lower than the bulk Ni:Si and Co:Si eutectic temperatures, indicating that these nanowires grow by a solid-phase seeding process from a silicide.25,26

Furthermore, Ni and Co induce nanowire formation, presumably by catalyzing precursor decomposition, from reactants like octylsilane and trisilane that do not form nanowires in the presence of Au.25,26 We recently applied the concept of solid-phase seeded catalytic growth to multiwall carbon nanotubes made in supercritical toluene at ∼640°C using metallocene catalysts.9 The metallocene degrades in situ to metal particles, which then catalyze toluene decomposition to C and promote graphitization and nanotube formation. Without the catalyst, toluene does not decompose and therefore serves as both solvent and carbon source.

High-pressure and -temperature reactors, although very useful, have safety concerns. One of our long-term goals has been to synthesize Si and Ge nanowires under milder conditions on the bench in high-boiling solvents like long-chain ethers. Low-melting metals, such as bismuth (Bi), form eutectics with Si and Ge at a relatively low temperature. For example Bi:Si and Bi:Ge exhibit eutectics at ∼280°C. We recently demonstrated Bi nanocrystal-seeded nanowire growth at relatively low temperatures (300–350°C) using Ge27 nanowires in trioctylphosphine, as well as with Group III-V compounds28 such as GaAs, indium arsenide (InAs), GaP, and InP. We are currently working on Si nanowires.

Challenges remain for the synthesis process, including high-yield solution methods for Si nanocrystals, a benchtop colloidal synthesis of Si nanowires, and a solution route to single-wall carbon nanotubes. Commercialization is also one important goal for the nanomaterials field. High-payoff opportunities exist, such as Si nanowire electronics for memory, logic, photovoltaics, and display driver electronics, but these have very high development costs and risks.

There are also smaller, short-term opportunities with lower costs. For instance, we are developing Si nanowires to serve as the critical dimension metrology standards for the Si-CMOS industry that will be needed to support next-generation chips. Commercialization requires robust and scalable synthetic processes. It also needs broad intellectual property that covers processes, applications, device structures, and even composition of matter, applications, or devices that are reliable and cheap to implement. There must also be realistic business opportunities. There are several relatively mature (founded less than 4 years ago) start-up companies—including Nanomix, Nanosys, Qdot (which was recently acquired by Invitrogen), and Innovalight. They are trying to exploit the synthetic advances made during the last decade. Nonetheless, these companies are still searching for the right combination of business, innovation, and functional nanotechnology needed to break into the marketplace.

This work has been funded in part by the National Science Foundation, the Robert A. Welch Foundation, the Advanced Materials Research Center in collaboration with International SEMATECH, the Advanced Processing and Prototyping Center (A2PC; DARPA: HRD011-06-1-0015), and the Office of Naval Research (N00014-05-1-0857).

Brian A. Korgel
Department of Chemical Engineering, University of Texas at Austin
Austin, TX
Brian A. Korgel is associate professor and Frank A. Liddell Jr. Fellow in Chemical Engineering at the University of Texas at Austin. He has more than 90 papers in nanomaterials chemistry and engineering. His Science article on silicon nanowires is the second-highest cited paper written by a chemical engineer since 2000.

1. V. I. Klimov,
Semiconductor and Metal Nanocrystals: Synthesis and Electronic Properties,
2. G. A. Ozin, A. C. Arsenault,
Nanochemistry: A Chemical Approach to Nanomaterials,
2005. doi:10.1333/s00897061001a
3. F. Shieh, A. E. Saunders, B. A. Korgel, General shape control of colloidal CdS, CdSe, CdTe quantum rods and quantum rod heterostructures,
J. Phys. Chem. B,
Vol: 109, pp. 8538-8542, 2005. doi:10.1021/jp0509008
4. M. B. Sigman, A. Ghezelbash, T. Hanrath, A. E. Saunders, F. Lee, B. A. Korgel, Solventless synthesis of monodisperse Cu2S nanorods, nanodisks, and nanoplatelets,
J. Am. Chem. Soc.,
Vol: 125, pp. 16050-16057, 2003. doi:10.1021/ja037688a
5. A. Ghezelbash, B. A. Korgel, Nickel sulfide and copper sulfide nanocrystal synthesis and polymorphism,
Vol: 21, pp. 9451-9456, 2005. doi:10.1021/la051196p
6. J. D. Holmes, K. P. Johnston, R. C. Doty, B. A. Korgel, Control of thickness and orientation of solution-grown silicon nanowires,
Vol: 287, pp. 1471-1471, 2000. doi:10.1126/science.287.5457.1471
7. M. B. Sigman, B. A. Korgel, Solventless synthesis of Bi2S3 (bismuthinite) nanorods, nanowires, and nanofabric,
Chem. Mater.,
Vol: 17, pp. 1655-1660, 2005. doi:10.1021/cm0478733
8. M. B. Sigman, B. A. Korgel, Strongly birefringent Pb3O2Cl2 nanobelts,
J. Am. Chem. Soc.,
Vol: 127, pp. 10089-10095, 2005. doi:10.1021/ja051956i
9. D. C. Lee, F. V. Mikulec, B. A. Korgel, Carbon nanotube synthesis in supercritical toluene,
J. Am. Chem. Soc.,
Vol: 126, pp. 4951-4957, 2004. doi:10.1021/ja031522s
10. J. D. Holmes, K. J. Ziegler, R. C. Doty, L. E. Pell, K. P. Johnston, B. A. Korgel, Highly luminescent silicon nanocrystals with discrete optical transitions,
J. Am. Chem. Soc.,
Vol: 123, pp. 3743-3748, 2001. doi:10.1021/ja002956f
11. X. M. Lu, K. J. Ziegler, A. Ghezelbash, K. P. Johnston, B. A. Korgel, Synthesis of germanium nanocrystals in high temperature supercritical fluid solvents,
Nano Lett.,
Vol: 4, pp. 969-974, 2004. doi:10.1021/nl049831j
12. P. S. Shah, T. Hanrath, K. P. Johnston, B. A. Korgel, Nanocrystal and nanowire synthesis and dispersibility in supercritical fluids,
J. Phys. Chem. B,
Vol: 108, pp. 9574-9587, 2004. doi:10.1021/jp049827w
13. D. C. Lee, T. Hanrath, B. A. Korgel, The role of precursor-decomposition kinetics in silicon-nanowire synthesis in organic solvents,
Angew. Chem. Intl. Ed.,
Vol: 44, pp. 3573-3577, 2005. doi:10.1002/anie.200463001
14. D. S. English, L. E. Pell, Z. H. Yu, P. F. Barbara, B. A. Korgel, Size tunable visible luminescence from individual organic monolayer stabilized silicon nanocrystal quantum dots,
Nano Lett.,
Vol: 2, pp. 681-685, 2002. doi:10.1021/nl025538c
15. Z. F. Ding, B. M. Quinn, S. K. Haram, L. E. Pell, B. A. Korgel, A. J. Bard, Electrochemistry and electrogenerated chemiluminescence from silicon nanocrystal quantum dots,
Vol: 296, pp. 1293-1297, 2002. doi:10.1126/science.1069336
16. T. Hanrath, B. A. Korgel, Supercritical fluid-liquid-solid (SFLS) synthesis of Si and Ge nanowires seeded by colloidal metal nanocrystals,
Adv. Mater.,
Vol: 15, pp. 437-440, 2003. doi:10.1002/adma.200390101
17. X. M. Lu, T. Hanrath, K. P. Johnston, B. A. Korgel, Growth of single crystal nanowires in supercritical solution from tethered gold particles on a silicon substrate,
Nano Lett.,
Vol: 3, pp. 93-93, 2003. doi:10.1021/nl0202307
18. T. Hanrath, B. A. Korgel, Nucleation and growth of germanium nanowires seeded by organic monolayer-coated gold nanocrystals,
J. Am. Chem. Soc.,
Vol: 124, pp. 1424-1429, 2002. doi:10.1021/ja016788i
19. T. Hanrath, B. A. Korgel, Crystallography and surface faceting of germanium nanowires,
Vol: 1, pp. 717-727, 2005. doi:10.1002/smll.200500033
20. F. M. Davidson, A. D. Schricker, R. J. Wiacek, B. A. Korgel, Supercritical fluid-liquid-solid synthesis of gallium arsenide nanowires seeded by alkanethiol-stabilized gold nanocrystals,
Adv. Mater.,
Vol: 16, pp. 646-649, 2004. doi:10.1002/adma.200306284
21. F. M. Davidson, R. Wiacek, B. A. Korgel, Supercritical fluid-liquid-solid synthesis of gallium phosphide nanowires,
Chem. Mater.,
Vol: 17, pp. 230-233, 2005. doi:10.1021/cm0486262