Oded Millo

Heavily Doped Semiconductor Nanocrystal Quantum Dots

Impurities can be added into semiconductor nanoparticles to control their electronic and optical properties. Doping of semiconductors by impurity atoms enabled their widespread technological application in microelectronics and optoelectronics. However, doping has proven elusive for strongly confined colloidal semiconductor nanocrystals because of the synthetic challenge of how to introduce single impurities, as well as a lack of fundamental understanding of this heavily doped limit under strong quantum confinement. We developed a method to dope semiconductor nanocrystals with metal impurities, enabling control of the band gap and Fermi energy. A combination of optical measurements, scanning tunneling spectroscopy, and theory revealed the emergence of a confined impurity band and band-tailing. Our method yields n- and p-doped semiconductor nanocrystals, which have potential applications in solar cells, thin-film transistors, and optoelectronic devices.

Tunneling and percolation in metal-insulator composite materials

In many composites, the electrical transport takes place only by tunneling between isolated particles. For a long time, it was quite a puzzle how, in spite of the incompatibility of tunneling and percolation networks, these composites conform well to percolation theory. We found, by conductance atomic force microscopy measurements on granular metals, that it is the apparent cut off of the tunneling to non-nearest-neighbors that brings about this behavior. In particular, the percolation cluster is shown to consist of the nearest-neighbors subnetwork of the full tunneling network.

Size-dependent tunneling and optical spectroscopy of CdSe quantum rods

Photoluminescence excitation spectroscopy and scanning-tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero-dimensional to a one-dimensional quantum-confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the diameter of the rod and not its length. With increasing diameter, the band gap and the excited state level spacings shift to the red. The level structure was assigned using a multiband effective-mass model, showing a similar dependence on rod dimensions.

Determination of Band Offsets in Heterostructured Colloidal Nanorods Using Scanning Tunneling Spectroscopy

The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.

Tuning energetic levels in nanocrystal quantum dots through surface manipulations.

We demonstrate tuning of the electronic level positions with respect to the vacuum level in colloidal InAs nanocrystals using surface ligand exchange. Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect. The implications of affecting the electronic system of nanocrystal through its capping are illustrated through prototype devices.

Imaging and Spectroscopy of Artificial-Atom States in Core/Shell Nanocrystal Quantum Dots

Current imaging scanning tunneling microscopy is used to observe the electronic wave functions in InAs/ZnSe core/shell nanocrystals. Images taken at a bias corresponding to the s conduction band state show that it is localized in the central core region, while images at higher bias probing the p state reveal that it extends to the shell. This is supported by optical and tunneling spectroscopy data demonstrating that the s-p gap closes upon shell growth. Shapes of the current images resemble atomlike envelope wave functions of the quantum dot calculated within a particle in a box model.

Single electron tunneling and level spectroscopy of isolated C60 molecules

The interplay between single electron tunneling effects and the discrete molecular levels of C60 molecules is studied using scanning tunneling microscopy. Isolated C60 molecules were deposited onto a gold substrate, covered by a thin insulating layer. The tunneling current–voltage characteristics of isolated molecules, both at room temperature and at 4.2 K, exhibit rich structures, resulting from the interplay between charging effects and the electronic spectrum. In particular, we observe degeneracy lifting within the C60 molecular orbitals, probably due to the Jahn–Teller effect and local electric fields.

Energy level tunneling spectroscopy and single electron charging in individual CdSe quantum dots

We directly show the evolution of the electronic structure of semiconductor quantum dots (QDs) with QD size in the strong confinement regime by employing low-temperature tunneling current–voltage spectroscopy to individual electrodeposited CdSe QDs. From the spectra we measure the values of Eg, map discrete energy levels in both valence and conduction “bands,” and show the occurrence of single-electron tunneling effects for isolated QDs of different sizes (2, 3, and 4.5 nm diameter). Since tunneling is not limited by optical selection rules, we are able to directly measure energy level spacings not necessarily accessible by optical spectroscopy. Our spectra demonstrate the interplay between charging and energy level spacing, resulting in a rich and controllable structure that forms a basis for QD nanoelectronic devices.

Tunneling spectroscopy and magnetization measurements of the superconducting properties of MgB{sub 2}

Cryogenic scanning tunneling microscopy and magnetization measurements were used to study the superconducting properties of MgB_2. The magnetization measurements show a sharp superconductor transition onset at T_c = 38.5 K, in agreement with previous works. The tunneling spectra exhibit BCS gap structures, with gap parameters in the range of 5 to 7 meV, yielding a ratio of 2delat/KT_c ~ 3-4. This suggests that MgB_2 is a conventional BCS (s-wave) superconductor, either in the weak-coupling or in the `intermediate-coupling` regime

Scanning tunneling spectroscopy of SmFeAsO 0.85 : Possible evidence for d -wave order-parameter symmetry

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) have been examined for an Al nanocluster periodic array grown on p-type silicon substrate. Local density of states at Al and Si sites within the nanocluster has been extracted from site-resolved STS spectra by taking into account tip-induced band bending (TIBB) effect. Besides, it has been clarified that the surface-potential- energy shift caused by TIBB effect directly influences the tunneling current spectra. Consequently, a good correspondence has been found between the experimental spectra and the theoretical local density of states except for the energy gap. The energy gap has been experimentally determined as 1.7 eV, which is much larger than the predicted value. This discrepancy could be ascribed to the local-density approximation rather than the TIBB effect.