Research
Interests
My group focuses on
local probe measurements of the electronic properties of nanoparticles and
systems that exhibit inhomogeneous electrical properties on the nano-scale level.
In particular, we employ cryogenic Scanning Tunneling Microscopy and
Spectroscopy (STM and STS) and Conductance Atomic Force Microscopy (C-AFM),
along with other AFM techniques. These measurements enable the
characterization of the electronic properties of such systems, in
correlation with their structural and mechanical properties, with extremely
high spatial resolution. In parallel to the local measurements,
global measurements of transport, magnetization and optical spectroscopy
are carried out. This combination of local and global characterization
techniques is very effective in the study of nanostructured inhomogeneous
disordered systems, since the global measurements average out the local
behavior, while these local properties and their special variations
determine, in fact, the macroscopic behavior.
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Our investigations
of single nanoparticles focused in recent years mainly on semiconductor
nanocrystals, Quantum Dots (QDs) and Quantum Rods (QRs), in collaboration
with Prof. Banin (Chemistry). Here we studied the quantized level
structure and single electron tunneling effects in these particles. In
our research of nanostructured macroscopic systems we have addressed
issues such as the proximity effect and the superconductor-to-insulator
transition in nonhomogeneous superconductors, as well as the conduction
percolation network in metal/insulator composites and disordered
nano-crystalline silicon arrays (in collaboration with Prof. I.
Balberg). Our research is aimed at providing fundamental
microscopic insight into the complex electronic and transport properties
in nanostructured and disordered systems, but it may also contribute to
the development of nano-technological applications.
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Nanostructured Systems
In the following, we briefly describe some
of the research efforts mentioned above.
Semiconductor
nanoparticles – By combining tunneling spectroscopy on single
nanocrystalses with optical spectroscopy on an ensemble, we succeeded to
map the discrete energy levels of spherical semiconductor QDs and elongated
QRs, as a function of their size and shape. In particular, we showed that
the QD level structure is atomic-like, revealing s- and p-like orbitals. We
have also directly imaged, with the STM, the electronic wavefunctions of
these states. The QR research focuses also on the transition between the
zero-dimensional and the one-dimensional regimes.
Nanostructured
Systems.
The STM
results are well correlated with the optical spectra measured by the Banin
group as well as with numerical simulations performed in our group. In
addition to the basic interest in understanding of the transition from the
atomic and molecular scale to the macroscopic bulk regime, this research is
also of technological importance, since semiconductor nanocrystals may have
important applications, such as in nano opto-electronic devices.
Nanostructured superconductors – The
non-homogeneity of the superconductors can be monitored via the spatial
variations of the superconductor energy gap. This, for example, allowed us
to follow the spatial evolution of the superconductor order parameter
across the interfaces between normal and superconductor regions, and thus
locally characterize the superconductor proximity effect. We were
also able to directly portray the microscopic nature of the
superconductor-to-insulator transition, showing that it takes place in a
percolative-granular fashion, even in nominally homogeneous systems.
Currently, we are focusing on issues related to the symmetry of the order
parameter in the high-temperature superconductor YBCO. We have
demonstrated a clear correlation between the local tunneling spectra and
the surface nano-morphology, manifesting the d-wave nature of the order
parameter at the nano-scale. We also found evidence for a doping
driven phase-transition to a state of broken time-reversal symmetry that
has a complex order parameter. The d-wave order parameter is also expected
to have a unique signature in the superconductor proximity effect, another
issue that we are currently studying.
Metal/insulator composites -
Conductance properties of materials made of a random mixture of conducting
and insulating phases have been investigated extensively in the past using
macroscopic techniques. These materials serve as a model system for the
study of percolation in a continuous medium and other mesoscopic
phenomena. C-AFM allows for the direct two-dimensional mapping of
three-dimensional percolation paths. A statistical geometric analysis of
the current map enabled us to determine various parameters, such as the
fractal dimension of the three-dimensional conduction percolation cluster
and the local transport properties of specific conduction paths. One
interesting system to which this approach was applied is the
technologically important carbon black/polymer composite. Here, we
were able to provide significant insight into two open question related to
this system: the nature of the electro-thermal switching effect, and
the reason for the genuine percolation behavior observed for their
electrical properties while an “infinite” percolation cluster never forms
geometrically.
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