Research Interests: Physical chemistry of nanostructures- optical, electrical properties and thermodynamics of doped quantum confined semiconductor systems, magnetic Hyperthermia
Doping - Manipulating Conductivity of Semiconductor Quantum
Dots
Physical properties of the semiconductor materials can be engineered by changing the size in the nanometer size regime, where the Bohr radius of an exciton is comparable to the spatial extent of the particle. Another way of controlling the physical properties of a semiconductor material is doping, by substituting a few atoms in the crystal structure with different elements. If the dopant is electronically different from the replaced atom, the carrier concentration may change, resulting in a p- or n-type semiconductor quantum dot. This research focuses on identifying and understanding the key processes during the doping in order to better control the physical properties of semiconductor quantum dots. New properties are expected as a result of the interaction of the dopant levels and the levels of the three dimensionally quantum confined systems.
Growth Kinetics of Quantum dots
Controlling the growth of semiconductor quantum dot is an important step towards developing materials with well defined optical and physical properties.One challenge of growing semiconductor nanoparticles is to obtain quantum dots with well defined size and narrow size distribution. In a typical semiconductor quantum dot synthesis, the average size and size distribution of QDs is determined by the growth and the dissolution kinetics. There are numerous examples when the size and size distribution of the nanoparticle growth is determined by the thermodynamics of the nanoparticles rather then the kinetics. The thermodynamic control of the nanoparticle growth may lead to the formation of magic sized nanoparticles. Currently, our research focuses on the formation of magic sized CdTe quantum dots and its 'quantized' aggregation into larger quantum dots. LEFT figure shows a high resolution transmission electron microscope image of a 4.5 nm CdTe quantum dot showing the twinning planes and stacking faults from the aggregation of 1.9 nm magic sized quantum dots. ZB and W correspond to zinc blende and wurzite phase, respectively. Right figure shows the time evolution of the absorption spectra of CdTe quantum dots solution at high temperature during the synthesis. The different peaks correspond to different quantum dot sizes
*HRTEM image has been taken by The Imaging and Microscopy Facility at the University of California, Merced
Terahertz Spectroscopy of Nanostructures
Terahertz spectroscopy (Terahertz time domain and terahertz time resolved spectroscopy) is a powerful technique,
which can probe the dynamic changes in the far infrared part of the
electromagnetic spectrum (typically between 10 – 600 cm-1) on
sub-picosecond timescales. The observed signal is related
to the complex dielectric response of the sample, therefore its conductivity. Obtaining the
conductivity of the sample without electrical connections is very
desirable because important conclusions can be drawn from the
efficiency of the active component of a quantum junction based
device. Time-resolved terahertz spectroscopy allows one
to obtain information about the carrier dynamics such as
carrier-carrier interactions, interfacial carrier transport and
carrier relaxation processes on the femtosecond timescale. The schematic of the terahertz time-domain spectrometer built in our lab is shown below.
Nanoscale Ordering of Semiconductors - Core/shell Catalysts for Radial Nanowire Growth
Once the doped quantum dots are created, a second challenge is the creation of quantum junctions. One approach is to use create nanocatalysts that are able to catalyze radial nanowire growth. The nanocatalysts are created by melting core/shell metal nanoparticles on Si surface. Then the core/shell metal nanocatalysts are deposited on a surface and melted to produce radial nanowire structures similar to the image shown above (LEFT image). The middle image shows 5.5 nm Fe/Au core/shell nanoparticles(low resolution TEM image ofthe particles are shown on the right) deposited on Si 111 taken by atomic force microscopy in tapping mode. This research exploring the melting dynamics of the core/shell metal nanoparticles will lead better manipulation of bimetallic nanocatalysts. An important question is how and under what conditions imprinting of the melted nanocatalysts can take place during the growth of radial nanowires.
Magnetic Hyperthermia
Magnetic Hyperthermia represents a one step development towards selective and uniform heating of cancerous tissue by introducing nanometer sized magnetic particles close to a tumor site. The temperature increase of the tissue can significantly contribute to the destruction of the cancerous cells. Heating takes place by power absorption of the nanometer sized particles due to an AC magnetic field or by ultrafast magnetic field. Understanding and controlling the demagnetization process is very important to achieve efficient energy transfer from the magnetic nanoparticles to the surrounding environment. The energy dissipation process of the magnetic nanoparticles will be probed by ultrafast lasers.
Sponsors: Kansas State University, Department of Chemistry, COBRE Center for Cancer Experimental Therapeutics (National Institute of Health), The Terry C. Johnson Center for Basic Cancer Research
•Christopher Tuinenga, Jacek Jasinski, Takeo Iwamoto, Viktor Chikan, In situ Observation of Heterogeneous Growth of CdSe Quantum Dots; Effect of Indium Doping on the Growth Kinetics, ACS Nano, 2008 (ASAP)
•Raj Kumar Dani, Myungshim Kang, Mausam Kalita, Paul E. Smith, Stefan H. Bossmann and Viktor Chikan MspA Porin-Gold Nanoparticle Assemblies: Enhanced Binding through a Controlled Cysteine Mutation. Nano Lett.,2008; 8(4); 1229-1236, (2008)
•Dagtepe, P. & Chikan, V. Quantized Growth of CdTe Quantum Dots; Observation of Magic Sized CdTe Quantum Dots. J. Phys. Chem. C,111 (41), 14977 -14983, (2007)
• Mandal, P. K. & Chikan, V. Terahertz Conductivity of n-type (charged) CdSe Quantum Dots.Nano Lett.,7 (8), 2521 -2528, (2007)
