Nanomaterials Growth and Synthesis - Development of Next Generation Nanoscale Heterostructures

Owing to their diverse functionality, high surface-to-volume ratio, and unique size-dependent properties, nanostructures are of immense importance for chemical and biological sensors, medical devices, catalysts, photovoltaics, and nanodevices.^1-5 A wide range of material choices coupled with different synthetic strategies result in different morphological versions such as nanometer scale thin films, nanowires, nanotubes, nanobelts, nanoparticles, and nanoporous structures.^2,5,6

Numerous methods to synthesize nanostructures have been reported including, chemical synthesis, electrodeposition, chemical vapor deposition (CVD), plasma synthesis, laser-based synthesis, physical vapor deposition (PVD), mechanical alloying, nano/microfabrication methods.^2,5,6,7 Inspite of several studies,^2,6 nanostructure growth manipulation and direct integration into devices is yet to be achieved.^8-10

Another challenge is to impart multifunctionality to these nanostructures. Towards this end, heterostructuring of nanostructures by combining two nanostructures of same or different materials is of significant interest.⁷ Thus, Professor Chopra's research combines expertise in nano/microfabrication, nanostructure growth, materials chemistry, and characterization and spectroscopic techniques to develop novel nanoscale heterostructures. Professor Chopra's current focus is on developing graphene encapsulated metal nanoparticles (a core/shell geometry) and hierarchical heterostructures such as nanowires coated with nanoparticles.^11-15 In a recent reports, Professor Chopra has demonstrated some unique growth techniques for fabricating such heterostructures through very simple and surfactant-free synthetic routes. Such an approach makes them readily available for their future applications.

Graphene Encapsulated Gold Nanoparticles

By utilizing our CVD design expertise, Professor Chopra was able to develop CVD methods that have the capability to grow CNTs, nanowires, or graphene shells. A typical CVD process employs a feed of chemical precursors undergoing a reaction at high temperature to result in a precipitated nano, micro, or macro structures.

In order to grow a graphene shell around a metal nanoparticle such as gold (generally, considered as an inert), our approach utilizes patterned arrays of gold nanoparticles on a silicon wafer as a catalyst for the growth of graphene shell in the presence of a hydrocarbon source at temperatures between 600 - 700 °C.¹¹ The grown graphene shells encapsulating gold nanoparticles were characterized using high resolution electron microscopy (Figure 1).¹¹

Figure 1. A) Schematic of graphene encapsulated gold nanoparticles, B) TEM images of gold nanoparticles encapsulated in graphene shells (a, b, and c). High resolution image (c) shows interplanar spacing of graphene shell (3.5 A) and encapsulated gold nanoparticle (2.3 A). Figure 1B is reprinted ("Adapted" or "in part") with permission from Chopra et al., Chemistry of Materials, 2009, 21, 1176-1178. Copyright 2009, American Chemical Society.

The graphene shell thickness as low as ~1 nm and its growth rate of ~8 nm/h indicates versatile tunability of our CVD growth method. As compared to previous approaches used for encapsulating gold nanoparticles inside a graphene shell, our novel approach demonstrates that CVD method can lead to scalable growth of such core/shell nanoparticles. Such novel nanoparticles have immense potential for advanced chemical and biological analysis and medical devices.

Hierarchical Heterostructures

An interesting approach recently reported by us involves a simple surfactant-free thermal route to develop CuO nanowires coated with Co₃O₄ nanoparticles.¹³ Our fundamental studies of their crystal structure and interfaces showed a unique interfacial relationship between monoclinic CuO nanowires and spinel Co₃O₄ nanoparticles, leading the nanoparticles to uniformly disperse on the nanowires.

Professor Chopra also performed a comprehensive study pertaining to the morphological evolution of Co₃O₄ nanoparticles on CuO nanowires. This part of the study uniquely considers thermodynamics of the growth process that facilitated the surface migration of Co₃O₄ nanoparticles from base of nanowires along their length (Figure 2).

Figure 2 A) Schematic illustrating the formation of CuO nanowire-Co3O4 nanoparticle heterostructures. B) TEM image showing the heterostructures.

Such a study relating different phenomena as listed in figure 2 for the development of hierarchical heterostructures is unique. These kinds of multifunctional and multicomponent hierarchical heterostructures are extremely useful and will definitely impact our lives in many ways from automobiles to nanoelectronics. The challenge is to take them to the next level of innovation, which we are consistently striving for.

References

1. Bruchez M., Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor nanocrystals as fluorescent biological labels. Science 1998, 281, 2013.
2. Meyyappan, M. Carbon nanotubes: Science and applications. 2005, CRC Press LLC, Boca Raton, FL.
3. Venkatachalam, K.; Arzuaga, X.; Chopra, N.; Gavalas, V.; Xu, J.; Bhattacharya, D.; Hennig, B.; Bachas, L.G. Reductive dechlorination of polychlorinated biphenyl (PCB) using palladium nanoparticles and assessment of the toxic potency in vascular endothelial cells. Journal of Hazardous Materials 2008, 159, 483.
4. Hinds, B.J.; Chopra, N.; Rantell, T.; Andrews, R. Gavalas, V.; Bachas, L.G. Aligned multiwalled carbon nanotube membranes. Science 2004, 303, 62.
5. Chopra, N.; Gavalas, V. G.; Hinds, B. J.; Bachas, L. G. Functional one-dimensional nanomaterials: Applications in nanoscale biosensors. Analytical Letters 2007, 40, 2067.
6. Wang, Z. L. Nanowires and Nanobelts: Materials, properties and devices - Nanowires and nanobelts of functional materials Volume II, 2003, Springer Sciences, New York, NY.
7. Chopra, N. Multifunctional and multicomponent heterostructured one-dimensional nanostructures: advances in growth, characterisation, and applications, Materials Technology: Advanced Performance Materials 2010, 25, 212.
8. Bohr, M. T. Nanotechnology goals and challenges for electronic applications. IEEE Transaction on Nanotechnology 2002, 1, 56.
9. Stupp, S. I.; Braun, P. V. Molecular manipulation of microstructures: Biomaterials, ceramics, and semiconductors. Science 1997, 277, 1242.
10. Huang, Y.; Lieber, C. M. Integrated nanoscale electronics and optoelectronics: Exploring nanoscale science and technology through semiconductor nanowires. Pure and Applied Chemistry 2004, 76, 2051.
11. Chopra, N.; Bachas, L. G.; Knecht, M. Fabrication and Biofunctionalization of Carbon-Encapsulated Au Nanoparticles, Chemistry of Materials 21, 2009, 1176.
12. Wu, J.; Chopra, N. Graphene Encapsulated Gold Nanoparticles and their Characterization, Ceramic Transactions 223, 2010.
13. Shi W.; Chopra, N. Surfactant-free synthesis of novel copper oxide (CuO) nanowire-cobalt oxide (Co₃O₄) nanoparticle heterostructures and their morphological control, Journal of Nanoparticle Research 2010, In press.
14. Chopra, N.; Majumder, M.; Hinds, B.J. Bi-functional carbon nanotubes by sidewall protection. Advanced Functional Materials 2005, 15, 858.
15. Chopra, N.; Claypoole, L. Bachas, L. G. Formation of Ni/NiO core/shell nanostructures and their attachment on carbon nanotubes. Technical Proceedings of 2009 Nanotech 2009, 1, 187.