Laboratory researchers may have found a way to improve Raman spectroscopy
as a tool for identifying substances in extremely low concentrations. Potential
applications for Raman spectroscopy include medical diagnosis, drug/chemical
development, forensics and highly portable detection systems for national security.
The ability to identify molecules at low concentrations with great specificity
and provide non-invasive, nondestructive measurements has led to the increasing
use of Raman spectroscopy as an accepted analytical technique. But a shortcoming
of this technique has been its lack of sensitivity and reliability at extremely
Raman spectroscopy consists of observing the scattering of light, usually from
a laser, by molecules of a transparent substance. The difference in the wavelength
of scattered light and incident light can provide detailed information about
the nature of the substance.
"Raman scattering provides a nice fingerprint of materials of interest
for national security," said Tiziana Bond of LLNL's
Center for Micro and Nano Technology.
Bond and her group develop surface-enhanced Raman spectroscopy (SERS), a method
that increases sensitivity orders of magnitude by improving signals. While showing
great potential, the substrates used for SERS, typically roughened metal surfaces,
have yielded variable signals considered, as yet, unreliable. The roughened
surface enhances the interaction of the molecule with the metal. The challenge
has been to find a way to create a substrate with uniform topographic features
that yield consistent signal enhancements.
Some of this work is described in a paper published in the September 2010 edition
of Nanotechnology entitled "Rigorous Surface Enhanced Raman Spectral Characterization
of Large-Area, High-Uniformity, Silver-Coated Tapered Silica Nanopillar Arrays,"
which was published by Bond and her group in collaboration with researchers
from the University of Illinois at Urbana-Champaign.
Improved nano-engineering techniques and semiconductor manufacture methods
have enabled the production of SERS substrates -- the base layer or texture
on 4- to 6-inch wafers -- that are more reliable. The key is substrates with
"reproducibility" sufficient for reliable analysis. LLNL researchers
have worked on several techniques to achieve a more robust and uniform substrate
that maintains high sensitivity and reproducibility.
Electromagnetic and chemical enhancements are two factors that affect SERS
total enhancement (with respect to Raman). The first is stronger and accounts
for 106-108 magnitude improvements, while the second is typically responsible
for 10-100 factors. To exploit the electromagnetic effects, the metallic nanostructures
need to be properly designed.
In an article entitled "Plasmon Resonant Cavities in Vertical Nanowire
Arrays" published in Nano Letters earlier this year, Bond's group, investigate
an innovative design using a vertical a gold-coated nanowire array substrate
that would provide strong and controllable enhancement. The LLNL team's innovation
is the fabrication of "tunable" plasmon resonant cavities in the vertical
wire arrays -- cavities are the space between the vertical wires. Mihail Bora,
a postdoc that joined Bond's group a year ago, is heavily involved in this part
of the project and explains that surface plasmons are electromagnetic waves
similar to light, except they are confined on metallic surfaces. Tuning of plasmon
resonance is achieved by controlling the geometrical dimensions of the cavity.
They introduce the smallest optical resonant cavity that is thousands of times
smaller than wavelength of light and showed that it is possible to go beyond
this diffraction limit by using surface plasmons. Resonant cavities are currently
used for surface enhanced Raman spectroscopy to detect chemical analytes (concentration).
"By confining the light in such tight spaces we are able to create intense
fields that are useful in increasing the spectroscopy signal," Bond said.
These design features offer a number of advantages. For example, it allows
the sensitivity of the substrates to be tuned, or adapted, to different wavelengths
offering researchers greater versatility.
Among possible application extensions of the plasmonic substrate beyond the
enhancement of SERS are enabling the demonstration of sub-wavelength plasmonic
lasers, and broadband nanoantenna arrays for photovoltaics by playing with geometry
The group's work has been funded by Defense Advanced Research Projects Agency
(DARPA) and LLNL's Laboratory Directed Research and Development (LDRD) program.