Positron Center for Material Characterization (H. Bakhru SUNY- I. Bondarev and B.
Vlahovic NCCU)
North Carolina Central University is leading the project on establishing the High
Intensity Positron National Center (HIPNC) at Jefferson National Laboratory (JLab).
HIPNC will use existing Free Electron Laser accelerator at JLab to generate the world
highest continuous current beam of low-energy positrons (>1010 e+s-1), a few eV to
several tens of keV, for the spectroscopic study of surfaces, nanostructures, and thin film
materials. The goal of the project is to develop the center that will establish itself as the
world’s leading facility for positron multidisciplinary research with an outstanding
scientific and educational program built with strong collaboration with academic,
industrial, government and international partners.
The center has a long term vision for research and innovation. The in house
science program has been under development in collaboration with external users and
will address a wide range of research ranging from the academic intellectual curiosity to
the real world applications. JLab high intensity positron beam will allow fundamental and
applied research in materials characterization and basic materials science, solid state and
surface physics, as well as in atomic physics. These range from the development of a new
generation of materials to study of the nature, concentration, and spatial distribution of
defects. This will be particularly applicable to nanotechnology and nanoscience,
nanopores, nanostructures, optical materials and thin film semiconductors studies. This
facility could significantly extend diagnostic capabilities of the SUNY Nanoscience
Center and could contribute to the ongoing industrial projects at the Nanoscience Center.
Ion Implantation and RBS Characterization (H. Bakhru SUNY- M. Wu and B.
Vlahovic NCCU)
Ion implantation, a mature technology currently used for semiconductor doping,
preparation of silicon-on-insulator (SOI) materials and optical waveguides, has also
recently been used to produce semiconductor quantum dots (QDs). Constituent elements
of QDs are implanted into a host matrix with number densities and depth distributions
that can be tightly controlled through implantation dose and energy. Annealing of these
supersaturated areas drives precipitation of the elements, forming QDs buried within the
host material. QD size, composition and depth distribution can be controlled through the
implantation and annealing conditions. Vlahovic has demonstrated production of InP
QDs through this technique.1 Rutherford backscattering spectroscopy (RBS) is used to
characterize elemental depth distributions before and after annealing, enabling fine tuning
of experimental conditions to optimize QD for solar applications.
Photovoltaics (H. Efstathiadis SUNY- I. Filikhin, I. Bondarev, K. Wang, J. Dutta, Y.
Tang, M. Wu and B. Vlahovic NCCU)
Accurate simulations of the quantum dot electronic structure at finite temperatures
are desirable to understand how to enhance the efficiency of quantum dot based
photovoltaic devices. Computationally intensive ab initio methods2 have been used to
that end, but neither approach repeatably reproduces experimental data. Theoreticians at
NCCU have developed an original single sub-band approach with an energy dependent
electron effective mass to study realistic quantum dot nanostructures with explicit
consideration of the dot size, shape, and substrate material 3 , 4 , 5 , 6 . They have also
developed theoretical models for exciton-phonon coupling in quantum dots7, as well as
an original quantum approach to model near-field electromagnetic phenomena8 (such as
the optical response of single atoms and excitons in atomically doped9,10 and pristine11
carbon nanotubes) very closely related to the Förster-type energy transfer in quantum dot
polymer composites considered currently to be very perspective for photovoltaics.
Theoretical methods developed at NCCU allow one to achieve the high accuracy of
calculations at a very low computational cost – calculations can be completed using a 3
GHz PC with 1 GB of memory in ~30 min., in contrast to ab initio techniques that
require specialized high-performance computers.
The NCCU theoretical group
(Bondarev, Filikhin, Vlahovic, Wang) will be focusing on the following important
issues. (i) How to optimize the quantum dot electronic structure through the dot size and
chemical composition variation for the most efficient Multi-Exciton Generation (MEG)
per single absorbed photon? (ii) Once we have reached an efficient MEG regime, how to
reduce the relaxational cooling of the excitons via the exciton-exciton and/or excitonphonon coupling before they get charge separated? (iii) Theory and simulations of the
Förster-type energy transfer rate in quantum dot polymer composites of different
configurations. All the simulations will be compared with experimental results (Wu,
Dutta, Tang) to provide a better understanding of factors affecting the efficiencies of
MEG and charge separation in hybrid semiconductor/polymer solar cells.
We have also developed experimental tools for fabrication of materials outside of
boundaries imposed by conventional techniques to systematically study new classes of
semiconductor nanostructures that are anticipated to play an important role in the
development of advanced devices. Achievement of the predicted solar power conversion
efficiencies due to quantum dots (QDs) critically depend on the design and growth of
nanostructured materials in device heterostructures. Current technology is centered on
QDs produced by Stranski-Krastanow (S-K) or wet chemical synthesis methods. S-K
techniques require a lattice mismatch between the QD and substrate materials to drive
formation of small islands at high temperatures. This mechanism of growth results in
QDs with large (> 10 nm) lateral sizes that prevent the QDs from exhibiting truly discrete
densities of states, wide bandgap tunabilities, strong absorption cross-sections, long
carrier lifetimes necessary to completely leverage the advantages of QDs for solar
applications. The strain induced growth mechanism also leads to significant strain and
compositional gradients in the growth direction. 12 13 Lattice mismatch and high
temperature growth also place severe restrictions on the choice of substrate for a given
QD material. Wet chemical synthesis produces small QDs with excellent size
uniformities, but the resulting solution based, ligand covered QDs are difficult to
integrate with semiconductor device fabrication methods.
Many of these difficulties can avoided through the use of ultrafast pulsed laser
deposition (PLD) to produce nanostructures through two distinct mechanisms based on
high fluence and low fluence ablation. In the high fluence regime, nucleation and growth
of QDs occurs near the target surface in the high-density plume resulting from ultrafast
laser ablation. 14 15 16 QD growth far from the substrate surface allows QD - substrate
lattice matching, eliminating strain and resulting defects. Lower substrate temperatures
reduce the formation of compositional gradients. QD density can also be controlled
independently of QD size, as the density is controlled through the number of laser shots
applied to the target allowing higher layer densities of QDs and increased device
absorption. Finally, ultrafast PLD preserves the stoichiometry of complex targets,
allowing the formation of largely unexplored ternary or quaternary semiconductor QDs.
Wu has successfully demonstrated the production of Si and InAs QDs by high fluence
PLD with average sizes below 5 nm and size distributions (~15%) comparable to those of
S-K techniques.17 Sizes of the QDs were tuned over the range of 1.5 nm to > 20 nm by
varying the laser fluence. Optical absorption onsets of the QDs were observed to
blueshift with decreasing size, demonstrating the ability of PLD to produce QDs in the
strong quantum confinement regime. Similar effects were observed for the binary
semiconductors InAs and GaAs, with the stoichiometry of InAs QDs determined to be
unity within experimental uncertainty (4%) across a ~ 1 cm substrate by Rutherford
Backscattering spectroscopy. The typical production rate of Si QDs with a laser fluence
of 0.50 J/cm2 was ~ 107 QDs/cm2 per pulse at a target to substrate distance of 5 cm.
Coverages of 1011 QDs/cm2 can thus be obtained in ~ 500 sec using a pulse repetition rate
of 20 Hz.
When target are exposed to lower laser pulse energy densities, ultrafast PLD produces
a pulsed flux of atoms and small clusters to the substrate surface. Low fluence PLD can
produce atomically sharp interfaces and low surface roughness even in the presence of
high backing gas pressures.18 Wu has produced stoichiometric CuInS2 thin films on glass
substrates using low fluence PLD, 19 demonstrating the ability to produce complex
materials using low fluence PLD. PLD can also produce epitaxial semiconductor films
with lower surface roughnesses at low temperatures than more commonly used MBE
techniques.20 Low fluence PLD is thus well suited for QD formation through selective
area deposition using nanoscale substrate masks such as anodic aluminum oxide (AAO)
membranes or nanosphere monolayer, 21 and NW production through vapor – liquid –
solid growth methods. Key advantages of ultrafast PLD in this case include the
preservation of target stoichiometry and the versatility of the PLD technique, as different
nanomaterials can be grown by simply changing the target wafer. Overlap the spatially
inhomogeneous plumes produced by ablation of two spatially separated targets results in
production of combinatorial composition spreads. Combinatorial PLD has been used to
study transition transparent conducting oxides, 22 transition metal oxides for gate
dielectric applications23 and metal alloys,24 allowing rapid synthesis and evaluation of
entire compositional families.
Biochemical Sensor (N.C. Cady and M. A. Carpenter, SUNY- I. Filikhin, I. Bondarev,
M. Wu and B. Vlahovic NCCU)
The biochemical sensor is based on the well understood phenomenon of charge
tunneling between two quantum confined systems. Briefly, we propose to match density
of states in the detector nanostructure (for instance quantum dots, nanowires, nanotubes)
and the density of states (energy levels) in the target molecule (to be detected) to design
highly sensitive, target specific sensors. The fundamental mechanism behind this device
design is that strong charge transfer will occur only between a detector nanostructure and
a target molecule with matching electronic energy levels. This new approach to
biochemical sensing is unique and our group is the only group working in this direction.
Theoretically, this type of sensor is much more selective in detecting biochemical agents
than most current sensors, which rely on changes in conductivity. The signal from a
device based on charge tunneling, e.g. a change in capacitance, will occur only if a
specific molecule, with specific energy levels is present next to a detector nanostructure
with matching energy levels. Changes in conductivity in conventional sensors, however,
can arise from many molecules, even those that are very different in structure.
It is important to note that the exact calculations of energy levels of analyte molecules
or quantum dots (QDs) are not necessary. A sensor with an array of size sorted QDs with
relatively broad variation in energy levels range can be created. The detector could be
easily calibrated by detecting a known material. However, it is better to predict through
quantum calculations which sizes, compositions, and deposition parameters, are required
to form QD with needed energy levels prior to QD deposition, so that the QD energy
levels will not lie outside the range of the analyte levels, which is possible using
uncontrolled formation of QDs.
A possible typical design sensor involves fabrication of QD arrays sandwiched
between parallel metal strips (collectors) bonded to a substrate, which could be for
instance silicon substrate. 5 m wide metal electrodes separated by 10 to 50 m conduct
information to a computerized acquisition system when charge transfer occurs between
the QD and the analyte. The population of QDs sandwiched between the metal strips is a
common group of identical QDs ranging in size from 1 to 100 nm. Each separate QDgroup will contain a QD of different size or shape and, therefore, a different set of energy
levels.
As our primary goal is to use the advantages of the quantum confinement effect for an
efficient analyte single molecule sensing by specially designed quantum dot (QD) arrays,
we will be focusing on the simulations of electronic energy level structures, wave
functions and electronic density of states (DOS) for semiconductor/carbon nanostructures
of different geometries at different temperatures. The geometries will be quasi-0D
nanoparticles of varying shapes (to mimic PLD/S-K produced semiconductor QDs), and
quasi-1D semiconductor nanowires (NWs) with rectangular or cylindrical cross-sections,
including hollow single-/multi-walled cylinders to model carbon nanotubes (CNs).
Electron-phonon relaxation will be included in these simulations as it affects the
electronic response of nanomaterial by shifting and broadening its confined electronic
energy level structure with temperature. Particular care will be given to the development
of the models and simulations of the Förster-type/tunnel energy transfer rates between the
QD nanostructure and analyte molecule for nanostructures of different geometries and
chemical composition, including QD polymer composites, hybrid metal-semiconductor
nanostructures and defective CN films as very promising candidates for biosensor
applications.
1
D. Denmark, A. Ueda, C.L. Shao, M.H. Wu, R. Mu, C.W. White, B. Vlahovic, C.I. Muntele, D. Ila, Y.C.
Liu, Indium phosphide nanocrystals formed in silica by sequential ion implantation, Surf. and Coatings
Tech. 196 123-129(2005).
2
See, e.g.:
H. Fu, and A. Zunger, Local-density-derived semiempirical nonlocal
psuedopotentials for InP with applications to large quantum dots, Phys. Rev. B 55, 1642
(1997); C.Pryor, Eight-band calculations of strained InAs/GaAs quantum dots compared
with one-, four-, and six-band approximations, Phys. Rev. B 57, 7190 (1998).
3
I.Filikhin, V.M.Suslov, M.H.Wu, and B.Vlahovic, Effective approach for strained
InAs/GaAs quantum structures, Physica E 40, 715 (2008).
4
I.Filikhin, V.M.Suslov, and B.Vlahovic, Modeling of capacitance spectroscopy of
InAs/GaAs quantum ring in the non-parabolic approximation, Phys. Rev. B 73, 205332
(2006).
5
I.Filikhin, E.Deyneka, and B.Vlahovic, Non-parabolic model for InAs/GaAs quantum
dot capacitance spectroscopy, Sol. Stat. Commun. 140, 483 (2006).
6
I.Filikhin, V.M.Suslov, and B.Vlahovic, Electron spectral properties of the InAs/GaAs
quantum ring, Physica E 33, 349 (2006).
7
I.V.Bondarev, S.A.Maksimenko, G.Ya.Slepyan, I.L.Krestnikov, and A. Hoffmann,
Exciton-phonon interactions and exciton dephasing in semiconductor quantum-well
heterostructures, Phys. Rev. B 68, 073310 (2003).
8
I.V.Bondarev and Ph.Lambin, Near-field electrodynamics of atomically doped carbon
nanotubes, in: Trends in Nanotubes Research, edited by D.A.Martin (Nova Science, NY,
2006). Ch.6, pp.139-183. [see also PRB70,035407; PRB72,035451].
9
I.V.Bondarev, G.Ya.Slepyan, and S.A.Maksimenko, Spontaneous decay of excited
atomic states near a carbon nanotube, Phys. Rev. Lett. 89, 115504 (2002).
10
I.V.Bondarev and B. Vlahovic, Optical absorption by atomically doped carbon
nanotubes, Phys. Rev. B 74, 073401 (2006). [see also PRB75,033402].
11
I.V.Bondarev, K.Tatur and L.M.Woods, Optical response of small-diameter
semiconducting carbon nanotubes under exciton-surface-plasmon coupling, Optics
Communications, in print (arXiv0708.3262v2).
12
I. Kegel, T. H. Metzger, A Lorke, J. Peisl, J. Stangl, G. Bauer, J. M. Garcia and P. M. Petroff,
Nanometer-Scale Resolution of Strain and Interdiffusion in Self-Assembled InAs/GaAs Quantum
Dots, Phys. Rev. B 85, 1694 (2000).
13
G. Medeiros-Ribeiro and R. Stanley Williams, Thermodynamics of Coherently Strained
GexSi1-x Nanocrystals on Si(001): Alloy Composition and Island Formation, Nano Lett. ASAP
Article (2007).
14
S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, G. Ausanio, V. Ianotti
and L. Lanotte, Generation of silicon nanoparticle via femtosecond laser ablation in vacuum, Appl.
Phys. Lett. 84, 4502 (2004).
15
David B. Geohegan, Alex A. Puretzky, Gerd Duscher and Stephen J. Pennycook, Time
resolved imaging of gas-phase nanoparticle synthesis by laser ablation, Appl. Phys. Lett. 72, 2987
(1998).
16
D. B. Geohegan, A. A. Puretzky, G. Duscher and S. J. Pennycook, Time-resolved imaging of
gas phase nanoparticle synthesis by laser ablation, Appl. Phys. Lett. 72, 2987 (1998).
17
M. H. Wu, R. Mu, A. Ueda and B. Vlahovic, Production of silicon quantum dots for
photovoltaic applications by picosecond pulsed laser ablation, Mat. Sci. Eng. B 116, 273 (2005).
18
See, for example, Ho Nyung Lee, Hans M. Christen, Matthew F. Chisholm, Christopher M.
Rouleau and Douglas H. Lowndes, Strong polarization enhancement in asymmetric threecomponent ferroelectric superlattices, Nature 433, 395 (2004) and A. Tiwari, C. Jin, A. Kvit, D.
Kumar, J. F. Muth and J. Narayan, Structural, optical and magnetic properties of diluted magnetic
semiconducting Zn1−xMnxO films, Sol. State Comm. 121, 371 (2002).
19
R. Mu, M. H. Wu, Y. C. Liu, A. Ueda, D. O. Henderson, A. B. Hmelo, L. C. Feldman, A. Hepp,
Fabrication Of High Quality Copper Indium Disulfide Films On Mo/Glass Substrates Via Pulsed
Laser Ablation, Mat. Res. Symp. Proc. 730, V3.4 (2002).
20
Byungha Shin and Michael J. Aziz, Kinetic-energy induced smoothening and delay of epitaxial
breakdown in pulsed-laser deposition, Phys. Rev. B 76, 085431 (2007).
21
Mi. Jun, Sun-Il-Mho and Hong Lee Park, Long-range-ordered CdTe/GaAs nanodot arrays
grown as replicas of nanoporous alumina masks, Appl. Phys. Lett. 88, 133121 (2006).
22
J. D. Perkins, J. A. del Cueto, J. L. Alleman, C. Warmsingh, B. M. Keyes, L. M. Gedvilas, P. A.
Parilla, B. To, D. W. Readey and D. S. Ginley, Combinatorial studies of Zn-Al-O and Zn-Sn-O
transparent conducting oxide thin films, Thin Solid Films 411, 152 (2002).
23
R. Yasuhara, M. Komatsu, H. Takahashi, S. Toyoda, J. Okabayashi, H. Kumigashira, M.
Oshima, D. Kukuruznyak and T. Chikyow, Composition dependence of band offsets for
(LaAlO3)(1-x)(Al2O3)(x) gate dielectrics determined by photoelectron spectroscopy and x-ray
absorption spectroscopy, Appl. Phys. Lett. 89, 122904 (2006).
24
C. H. Olk, G. G. Tibbetts, D. Simon and J. J. Moleski, Combinatorial preparation and infrared
screening of hydrogen sorbing metal alloys, J. Appl. Phys. 94, 720 (2003).