1.Photoformation of Nanoparticles in Aerogels: Aerogels are interesting materials due to their high porosity, low Young’s modulus, low refractive indices, and low thermal and electrical conductivities. They are used as optical materials and catalyst supports. Currently, in collaboration with Professor William Risen's group at Brown University, we are investigating the structural and spectral characteristics of the gold nanoparticles generated upon irradiation with UV light in the solid matrix of novel gold-chitosan-silica aerogel materials. Certain questions that need to be answered are:

a. What is the mechanism behind the generation of nanoparticles in a solid aerogel matrix upon UV irradiation?
b. Can we control the size and properties of the nanoparticles by changing the UV wavelength, intensity, and exposure time?
c. Does the presence of surrounding neighboring molecules that target the gold particles affect the size of nanoparticles?
d. Does the size of the nanoparticles affect the optical, thermal, and elastic properties of the material?
e. Does the aerogel material behave in the same way in the bulk and in thin films?

Au(III)-Chitosan-Silica aerogels are transparent yellow porous materials with low refractive index. Upon exposing to 320 nm, its color changes from yellow to reddish-brown. Photoacoustic spectra of 320nm-exposed sample revealed a new peak around 526 nm, which corresponds to the plasmon resonance band of gold nanoparticles. The plasmon peak is found to shift to the higher energy region upon irradiating the sample for a longer time interval. Most nanoparticles are formed at the surface of the material as a result of light absorption.


2.Carbon nanotubes and bismuth nanorods: Conventional spectroscopic studies on the identification of electrically distinct metallic, semimetallic and semiconducting single-walled carbon nanotubes (SWNTs) and multi-walled naotubes (MWNTs) are not straightforward, and require extensive sample preparation techniques such as chemical functionalization. There is always a need for simple, efficient, and straightforward technique to obtain the optical absorption spectra of as-received individual SWNTs to distinguish between metallic, semimetallic and semiconducting nanotubes. We are in the process of developing a photoacoustic based technique, the Fourier Transform Infrared photoacoustic spectroscopy (FTIR PAS), to analyze such samples.  Since this technique essentially relies on the heat generated as a result of optical absorption, it is immune to any noise resulting from scattered and reflected light. Also, the photoacoustic signal is directly proportional to the incident photon energy, the signal intensity can be considerably increased by increasing the power of the light source. This is particularly important while dealing with nanotubes with smaller diameters because the absorption cross-section decreases upon decreasing the tube diameter.  The current research along this area includes (1) the investigation on the diameter dependence of the optical absorption frequencies, (2) analysis of the composition of bulk as-received SWNTs multi-walled carbon nanotubes (MWNTs), and (3) Time-resolved photoluminescence studies.

Another area of research interest is the study of bismuth nanorods. Caompared to the bulk form, Bi nanostructures form interesting mechanical and optical properties. A semimetal to semiconductor transition occurs in Bi nanostructures due to quantum confinement effects. The low-dimensional version of Bi shows several interesting properties including nonlinear optical effects. We have recently observed nonlinear optical scattering and absorption in bismuth nanorod suspensions using Z-scan measurements (Sivaramakrishnan, V.S.Muthukumar, Si.Sivasankara Sai, K. Venkataramaniah, J. Reppert, Apparao Rao, M. Anija, Reji Philip, and Narayanan Kuthirummal, Appl. Phys. Lett., 91, 1, 2007). Current research in my lab is more targetted toward obtaining absorption and excited state time-resolved tudies in bismuth as-received nanorods in solid state. This research is done in collaboration with Dr. A.R. Rao's research group at Clemson University.

UV-visible (upper trace) absorption spectrum of Bi nanorod suspension. The surface plasmon peak at around 302 nm is clearly visible. Crystalline planes (012) of triagonal Bi are shown in inset b. The photoacoustic spectra of solid as-received samples (lower traces) show additional features around 600 nm. Obviously, the PA spectra are well-resolved becuase of its immunity to scattered and reflected light.
3.Rare Earth Nanoparticles: There has been a significant increase in rare earth doped nanoparticle (NP) research due to the numerous applications to which they may be applied. The rare earth ions, typically trivalent though sometimes di- or tetra-valent, can exhibit luminescent emissions ranging from 172 nm to over 7 mm providing a suitable host of high intrinsic transparency. Due to such a broad spectral range of potential emissions, active NPs are being considered for practical use in LEDs, solar cell energy conversion, lasers and amplifiers, and biological assaying. One particular benefit of using NPs versus bulk materials is that NPs of differing spectroscopic properties can be incorporated into a single host to form optically multifunctional nanocomposites. The present technology allows multiple lanthanides to be doped within a single LaF₃ NP through the use of complex core-shell architectures. Terbium (Tb³⁺) and europium (Eu³⁺) were chosen as dopants to study the degree of energy transfer within a single nanoparticle because both exhibit a strong, easily measured, visible photoluminescence and Tb³⁺ is known to act as a sensitizer to Eu³⁺. In the aforementioned research, particles are grown with a Eu⁺³ core and three additional outer shells. These shells were either undoped LaF₃ or Tb⁺³ doped LaF₃. Current research interests include the preparation and characterization of novel multiply-doped core-shell nanoparticles involving Er, Ho, Eu, Tb, and Nd with different shell structures that permit new strategies for designing luminescent materials. As such, this is the introduction to a new and enabling technology that will allow for advances in many fields of photonics by controlling the energy transfer between lanthanides through shell positions and thicknesses. This research is being carried out with active support from Dr. John Ballato's group at Clemson University.


4.Quantum Dots: This is a relatively new research area in my lab. Quantum Dots are particles on the nanometer scale with about 10⁵to 10⁵ atoms. Unlike bulk materials, quantum dots shows interesting size-dependent optical and electrical properties because of the confinement of electrons in small spaces (quantum boxes). In a quantum dot, for example semiconducting CdSe, the average distance between an electron and a hole is called the Bohr Exciton Radius. When the size of CdSe falls below the Bohr Exciton Radius, the semiconductor is called a quantum dot.  The energy gap the can be tuned by altering the number of atoms. Studies along this mainly concentrate on the nonradiatice deexcitation characteristics of CdSe quatum dots.