Prof. Oded Millo

The Hebrew university 
of Jerusalem 91904 ISRAEL 
Office: Levin   102 
Phone: (972-2) 6585670 
Fax: (972-2) 6584437 
Email:
milode@mail.huji.ac.il

 

 

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.

 

 

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.

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.