M.S. in Chemistry; University of Szeged (1993-1998)
Ph.D., Kansas State University (1999-2002)
Postdoctoral Fellow, University of California, Berkeley and LBNL (2003-2005)
Research Interests: Physical chemistry of nanostructures- optical, electrical properties and thermodynamics of doped quantum confined semiconductor systems, Magnetic Hyperthermia, Chemical Dynamics
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 the understanding the 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.
Understanding and 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
Measuring Conductivity and Carrier Dynamics in Semicondtuctor Quantum Dots - Terahertz Spectroscopy
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.
Demagnetization Dynamics in Superparamagnetic Nanoparticles - 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.
Chemical Dynamics: Probing State-resolved Dynamics by Time-resolved Fourier Transform Visible Spectroscopy: A methodology towards achieving coherent control of state resolved dynamics of chemical reactions
Understanding the dynamics and spectroscopy of excited electronic
states of radicals and molecules in the gas phase is important for a
variety of practical problems such as combustion and formation of
planetary atmospheres. Fourier Transform Emission Spectroscopy is
powerful tool to obtain high-resolution emission spectra of
molecular fragments from a photodissociation or a reaction. Analysis
of the nascent spectra of these molecular fragments can reveal the
energy disposal of the reaction or photodissociation, which gives
insight to the mechanism of the process. The method offers the possibility to
probe effects of coherent laser control on the state-resolved
dynamics of relatively large systems leading to diatomic fragments.
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
Selected Publications
• 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)
• Chikan, V., Fournier, F., Leone, S. R. & Nizamov, B. State-resolved dynamics of the CH(A(2)Delta) channels from single and multiple photon dissociation of bromoform in the 10-20 eV energy range.J. Phys. Chem. A 110, 2850-2857 (2006).
• Chikan, V, Nizamov, B and Leone, SR, "Time-Resolved Fourier
Transform Infrared Emission Study of The C2H+O(3P) Reaction", J.
Phys Chem A2004, 108(49); 10770
• Chikan, V. and Kelley, D.F., "Carrier Relaxation Dynamics in
GaSe Nanoparticles", Nano Letters2002; 2(9); 1015
• Chikan, V. and Kelley, D.F., "Synthesis of Highly Luminescent
GaSe Nanoparticles", Nano Lett.2002, 2 (2), 141
• Chikan, V. and Kelley, D.F., "Size-Dependent Spectroscopy of
MoS2 Nanoclusters", J. Phys Chem. B2002, 106 (15); 3794
• Chikan, V., Waterland, M.R., Huang, J.M. and Kelley, D.F., "Relaxation and electron transfer dynamics in bare and DTDCI
sensitized MoS2 nanoclusters", J. Chem. Phys. 2000, 113,
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