| Research Overview
Doping Semiconductor Quantum
Dots
A challenge in the creation of nanometer sized p-n junctions is the ability to control the free carrier concentration in semiconductor nanostructures. Controlling the carrier concentration in quantum dots(QD) can be done via incorporating a foreign atom in the semiconductor (doping). Critical components of this research are to find ways to circumvent challenges and understand the underlying mechanisms of doping quantum dots. We are currently developing combined strategies for doping QDs along with mechanistic studies using terahertz spectroscopic techniques have the potential to significantly further this research.
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. 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 the 'usual' monomer induced growth of CdSe quantum dots. RIGHT figure shows the time evolution of the absorption spectra of CdTe quantum dot solution during the synthesis. The first step is the formation of magic sized CdTe quantum dot, which subsequently undergoes aggregation.
 
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.
Growth of Radial Nanowires
Our approach to grow radial Nanowires is to use create nanocatalysts that are able to catalyze radial nanowire growth. The nanocatalysts are created by selectively 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. The key question is how and under what conditions imprinting of the melted nanocatalysts can take place during the growth of radial nanowires.
  
Magnetic Hyperthermia via Protein/Nanoparticle Complexes
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 superparamagnetic and ferromagnetic particles from AC magnetic field or from 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.
Sponsors: Kansas State University, Department of Chemistry, COBRE (NIH) Center for Cancer Experimental Therapeutics, The Terry C. Johnson Center for Basic Cancer Research, University Small Research Grant, President’s Faculty Development Award, American Chemical Society Doctoral New Investigator
Selected Publications
(The complete list of publications is available here)
•Santanu Roy, C. T., Fadzai Fungura, Pinar Dagtepe, Jacek Jasinski and Viktor Chikan, Progress towards Producing n-type CdSe Quantum Dots: Tin and Indium Doped CdSe Quantum Dots. J. Phys. Chem. C (2009), in press.
•Dahal, N.; Jacek Jasinski; Valerie J. Leppert; Viktor Chikan, Synthesis of Water-Soluble Iron-Gold Alloy Nanoparticles. Chem. Mater., 20 (20), 6389–6395, (2008)
•Dagtepe, P.; Chikan, V., Effect of Cd/Te Ratio on the Formation of CdTe Magic-Sized Quantum Dots during Aggregation. J. Phys. Chem. A:STEPHEN R. LEONE FESTSCHRIFT (2008), 112, (39), 9304-9311.
•Christopher Tuinenga, Jacek Jasinski, Valerie J. Leppert; Takeo Iwamoto, Viktor Chikan, In situ Observation of Heterogeneous Growth of CdSe Quantum Dots; Effect of Indium Doping on the Growth Kinetics, ACS Nano, 2(7), 1411–1421, (2008)
•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., 8(4); 1229-1236, (2008)
•Dagtepe, P., Jacek Jasinski, Valerie J. Leppert; Viktor Chikan, 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. & Viktor Chikan Terahertz Conductivity of n-type (charged) CdSe Quantum Dots. Nano Lett., 7 (8), 2521 -2528, (2007)
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