We investigated the ability of silver thin metal films to enhance photovoltaic conversion efficiency in blends of
poly-3-hexylthiophene (P3HT) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). By varying
the thickness of the silver films and developing a new fabrication routine that involves annealing for long periods of time
at low temperatures, we were able to reproducibly enhance photoconversion in P3HT/PCBM devices. Photovoltaic
conversion efficiency was monitored using internal photon to current conversion efficiency (IPCE) and current-voltage
measurements. We observed that plasmonic materials were able to enhance the conversion efficiencies of organic, bulk
heterojunction devices. The relationship between the surface plasmon resonance wavelength and overall device
performance is also presented with IPCE data. These preliminary studies indicate that plasmonic enhancement in bulk
heterojunction devices show promise to improve the viability of organic solar cells.
Surface enhanced Raman spectroscopy (SERS) has been widely demonstrated to be capable of single molecule
detection. In addition to enhancement of Raman scattering, the substrates used for SERS also display other unique
optical properties such as photoluminescence and blinking. In this work, the photoactivation of Ag thin metal films as it
relates to the mechanism of SERS enhancement and the production of Ag cluster SERS active sites was explored.
Specifically, the photodynamics of SERS-active thin Ag films were qualitatively studied using a combination of optical
imaging and high and low resolution spectroscopy. A key hypothesis tested in this work addressed the role of oxygen in
thin metal film photodynamics. Based on spectroscopic and kinetic differences observed from thin Ag films under both
ambient and nitrogen atmospheres, a simple photochemical mechanism for blinking in optical phenomena was
developed and tested. The proposed mechanism relies on the photoreduction of silver oxide to produce an active
species, which was postulated to be silver clusters.
Noble metal thin films have been used for surface enhanced Raman scattering (SERS) nearly since its inception, but only recently has single molecule detection (indicated by blinking of the Raman signal) been demonstrated on these types of films. It has been widely accepted that thin metal films provide an average enhancement of the Raman signal of only 106. However, with the combination of the use of high magnification objectives and sensitive detection a new view of thin metal films as a SERS substrate is emerging. Bolstered by these results, our lab has endeavored to further study the optical properties of vapor deposited Ag films. A Stokes-shifted blinking optical response has been observed in our lab in the absence of any specific adsorbate on a silver thin metal film surface. The origin of blinking behavior on Ag this films in the presence and absence of adsorbate was investigated under various environmental conditions. It is anticipated that this system will help elucidate the mechanistic relationship between blinking and in SERS.
This proceeding is a summary of our progress in both fundamental studies of surface enhanced Raman scattering (SERS) active surfaces and the design and characterization of nanostructured SERS-active surfaces. Based on the prior demonstration of single molecule SERS (smSERS)-like behavior from vapor deposited thin silver films, we've focused on these substrates as model systems for fundamental studies of the "blinking" phenomenon. Preliminary studies suggest that Stokes-shifted emission "blinking" is more directly associated with metal nanofeatures and less dependent on the nature of the adsorbate. It is anticipated that the insight provided by these fundamental studies will eventually lead to the rational design of nanostructured surfaces capable of smSERS. Toward that goal, preliminary characterization of the optical properties of nanoaperture arrays in silver suggests that these surfaces may exhibit SERS enhancement greater than that of the overlaying thin silver film.
This manuscript is a summary of our progress toward the development of nanoaperture arrays as surface enhanced Raman scattering (SERS) active surfaces. Nanopatterned substrates have been fabricated using electron beam lithography. The substrates were metallized by thermal vapor deposition of silver. The resulting silver films exhibited interesting optical transmission and preliminary results with respect to SERS are encouraging.