Nanowires, Nanoclusters, and Surface-Enhanced Raman Spectroscopy

Growth and Surface Properties of Semiconductor and Metal Oxide Nanowires

Dr. Sharka Prokes, sharka.prokes@nrl.navy.mil

One-dimensional structures, such as carbon nanotubes and semiconductor nanowires, are currently of great interest due to their unique physical properties and potential applications, including nanoscale devices and sensors. We have been investigating a number of nanowire systems, from the perspective of growth mechanism, surface properties, as well as potential applications, especially to sensing. We have reported on a novel stress-driven growth mechanism for Si nanowires, which does not use any catalyst, nor any Si vapor, resulting in high densities of Si nanowires, 100s of microns in length. A model for this growth mode is shown in Figure 1. We have also developed a simple and inexpensive system for the growth of high quality InAs, InSb and other III-V materials systems, allowing the growth of epitaxial III-V nanowires of specific diameters, shown in Figure 2. This growth system has also allowed us to investigate a novel Si cluster assisted growth of InAs nanowires, in which phase separation of a thin surface Si suboxide serves as the source of the Si clusters, which act as nanowire nucleation sites.

Stress driven model for the growth of Si NWs, in which the Si source is provided by the substrate due to enhanced diffusion

SEM images of high quality InAs and InSb NWs produced by VLS in simple closed system.

Another nanowire system we have been investigating is monoclinic gallium oxide (β-Ga₂O₃), a wide band gap semiconductor (E_g= 4.9 eV), which exhibits n-type conduction and luminescence properties, and which may have applications in opto-electronics and high temperature gas sensing. We have produced nanowires, nanobelts and nano-sheets β-Ga₂O₃ by the vapor liquid solid (VLS) growth mechanism, with Au, Ni or oxygen as a catalyst. We have also investigated the effects of defects, hydrogen gas and temperature on the stability of these nanowires, which is critical for high temperature applications. These results are shown in Figure 3. As can be seen, although bulk Ga₂O₃ is a very stable high temperature material, this is not the case for the nanowire system, which exhibits significant drop in conduction properties at temperatures above 900°C, due to enhanced etching of the nanowires, which is a result of their high surface area.

2010

• Interaction of functionalized Ga₂O₃ NW-based room temperature gas sensors with different hydrocarbons, Sensors and Actuators-B Chemical, Vol 151 Issue 1, pp 114-120 (2010).

2009

• Controlled growth of parallel oriented ZnO arrays on Ga₂O₃ nanowires, Crystal Growth & Design 9 (2009) 1164-1169.

2008

• Study of defect behaviour in Ga₂O₃ nanowires and nano-ribbons under reducing gas annealing conditions: applications to sensing, Int. J. Nanotechnol., Vol. 5, Nos. 4/5, 2008, 475.

2007

• Growth of high quality, epitaxial InSb nanowires, J. Cryst. Growth, 304, 399 (2007).
• Study of the initial nucleation and growth of catalyst-free InAs and Ge nanowires, Appl. Phys. Lett. 90, 203104 (2007).

2006

• Si-assisted growth of InAs nanowires, Appl. Phys. Lett., 89, 223125 (2006).

2005

• Stress-driven formation of Si nanowires, Appl. Phys. Lett., 86, #193105 (2005).
• Growth of epitaxial InAs nanowires in a simple closed system, Appl. Phys. Lett. 87, #063110 (2005).

Application of dielectric/metal nanowire composites to Surface Enhanced Raman sensing.

Dr. Sharka Prokes, sharka.prokes@nrl.navy.mil

Optically based sensing provides advantages over electronic sensing because optical spectra can uniquely finger print a chemical compound, significantly reducing false alarms and simplifying the detection process. In Raman scattering (RS), light is scattered from a chemical of interest and the vibrational modes in the chemical red shift the frequency of the scattered light, producing a spectrum of lines that are characteristic of that molecule. The Raman signal can be enhanced by many orders of magnitude by the use of metal nano particles, referred to as surface enhanced Raman scattering (SERS). The SERS enhancement of molecules adsorbed on the roughened metal surface is caused by local electromagnetic fields that are created by the laser excitation of surface plasmons at the metal surface. To obtain a better understanding of the basic mechanism of the SERS effect, we have developed dielectric/Ag metal shell nanowires, which exhibit high SERS sensitivity due to their plasmonic coupling, which makes them ideal for basic studies of "hot" spots in the electric fields, as well as ideal for applications in low concentration sensing.

The growth of the Ga₂O₃ nanowires was performed by the vapor-liquid-solid (VLS) growth in a tube furnace, and the Ag shell coating was deposited via e-beam evaporation under high vacuum conditions. An example of the random nanowire composite array is shown in Figure 1. A comparison of the SERS signal from 0.2 picograms of Rh6G for the nanowire composite substrates and a commercially available SERS substrate from Mesophotonics (Klarite) is shown in Figure 2. As can be seen, no SERS signal is evident in the case of the commercial Mesophotonics sample, while a strong SERS signal is clearly seen for the nanowire composites. The most intriguing result from this work indicates that randomly crossed wires increase the SERS enhancement in the vicinity of the regions where wires cross, which can be modeled using a finite element Comsol simulation of the electric field near two 45 nm diameter Ag crossed wires in response to light polarized in the x-direction (Figure 3). The crossing of the nanowires leads to coupled plasmonic behavior that spatially extends the sensitivity of the nanowires to encompass the regions between the wires and significantly beyond the wires. This would not only enhance the SERS effect due to the strong coupling, but allow more molecules to enter this high electric field region, thereby enhancing the SERS sensitivity.

This research is being performed in collaboration with Code 6880.

2011

• Synthesis and Oxidation of Silver Nano-particles, Proceedings of the SPIE 7947, 79470Y (2011).
• Growth of ZnO nanowires on retroreflector microspheres and the resulting light channeling and plasmonic properties, Proceedings of the SPIE 8106, 8106K (2011).

2010

• Plasmonic Coupling on dielectric Nanowire Core-Metal Sheath Composites, Nanotechnology 21 (2010) 085705.
• The effect of size and size distribution on the oxidation kinetics and plasmonics of nanoscale Ag particles, Nanotechnology 21 (2010) 215706.

2009

• Effect of Crossing Geometry on the plasmonic behavior of dielectric core/metal sheath nanowires, Appl. Phys. Lett. 94 (2009) 093105.
• Plasmonic behavior of Ag/dielectric nanowires and the effect of geometry, J. Vac. Sci. Technol. B 27, (2009).

2007

• Surface-enhanced Raman spectroscopy of dielectric/metal nanowire composites, Applied Physics Letters 90, 093105 (2007).
• Highly Efficient SERS Nanowire/Ag composites, NRL Review (2007).
• Formation of ordered and disordered dielectric/metal nanowire arrays and their plasmonic behavior, Proceedings of the SPIE, vol. 6768 67680-E1 (2007).
• Dielectric and Geometric Properties of plasmonics in metal/dielectric nanowire composites used in SERS, Proc. SPIE 6768, 676801 (2007).

2006

• Growth and Characterization of single crystal InAs nanowire arrays and their application to plasmonics, Proceedings of the SPIE, v. 6370, 63700Q-1, (2006).

2005

• Growth and Characterization of single crystal Ga₂O₃ nanowires and nano-ribbons for sensing applications, Proceedings of the SPIE, volume 6008, 60080C-1 (2005).

Self-Assembly of Gold Nanocluster Devices

Dr. Mario Ancona, mario.ancona@nrl.navy.mil

Schematic of gold nanocluster

(a) Schematic of cluster-based SET (b) Simulated switching characteristics at 300K

This project is a collaboration between researchers in NRL's Electronics S&T Division, Chemistry Division and Center for Biomolecular Science & Engineering.

Ultra-small gold nanoclusters (2-3nm in diameter) like that depicted in Fig. 1 could provide the foundation for new electronics/sensor technologies that are scalable to the few nanometer regime and would operate at room temperature. The physical basis for these possibilities is the strong Coulomb blockade effect exhibited by such nanoclusters. If configured in a structure like the single-electron transistor (SET) shown schematically in Fig. 2a, numerical simulation indicates that the strong switching behavior illustrated in Fig. 2b would be expected to occur.

Thiol-assembled nanoclusters in a 15nm gap between electrodes

TEM micrograph of gold nanoclusters "sequenced" using a DNA template

The main thrust of this project has been to develop fabrication methods suitable for assembling the nanoclusters into useful configurations, and ultimately into structures like that of Fig. 2. Three strategies are being pursued, one using simple thiol-based self-assembly chemistry, one using rigid-rod polymer templates, and one using DNA templates. An example of the first of these is in the TEM micrograph in Fig. 3 that shows thiol-assembled clusters in an e-beam-defined electrode gap. The use of DNA to organize linear chains of clusters by exploiting base-pair complementarity is depicted in Fig. 4.

One final aspect of the project relates to the same gold nanoclusters and their use in chemical vapor sensors. The scaling properties of such sensors in both the micron- and nanometer-scale regimes have been studied in detail.

2009

• Fluorine-labeling as a diagnostic for thiol-ligand and gold nanocluster self-assembly, ANALYST Volume 134, Issue 9 (1790-1801), 2009.

2008

• Selective DNA-mediated assembly of gold nanoparticles on electroded substrates, LANGMUIR. Vol. 24 Iss. 18, (10245-10252), 2008.

2006

• Scaling properties of gold nanocluster chemiresistor sensors, IEEE Sensors Journal, 6, 1403 (2006).

2004

• Isolation and characterization of trioxyethylene-encapsulated gold nanoclusters functionalized with a single DNA strand, Nano Letts. 4, 737 (2004).

2002

• Thiol-terminated Di-, Tri-, and tetraethylene oxide functionalized gold nanoparticles: A water-soluble, charge-neutral cluster, Chem. Mater. 14, 2401 (2002).

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