Science and technology at nanoscale have numerous opportunities both for fundamental research and new applications. Quantum dots, nanocrystals, nanowires, and nanorods are among the most important building blocks of nano-photonic devices. Therefore, the understanding of underlying fundamental physical phenomena in such structures is very important for future progress. We are interested in fundamental properties of wide bad gap nanostructured materials, particularly those with type-II band alignment, with potential application in photo-detection, quantum information, and biomedical field. Type-II heterostructures have several substantial advantages over type-I systems in that that they suppress non-radiative Auger recombination (1, 2) and their emission can be controlled by external means such as intensity of excitation, electric and magnetic fields. We particularly focused on properties of epitaxial ZnTe/ZnSe quantum dot multilayers, type-II colloidal core-shell nanoparticles, and II-VI nanowires.

In the ZnTe-ZnSe systems holes are strongly confined within ZnTe-rich quantum dots, whereas electrons locate in ZnSe barriers, and only weakly attracted to holes via the Coulomb interaction, forming the spatially indirect (type-II) excitons. It is important that QDs in this system coexist with Te_n isoelectronic centers, and a smooth transition between these two different species is indicated by experimental results. Therefore, QDs here are formed by continuing enlargement of Te_n/Se isoelectronic centers. In magnetic field this system exhibits one of the most interesting quantum phenomena - so-called optical Aharonov-Bohm Effect, for which we observed photoluminescence intensity as well as energy oscillations within the same sample, experimentally, for the first time. The samples are grown by Prof. M. C. Tamargo’s group of The City College. We collaborate on the experimental aspects of the project with Prof. McCombe's group at SUNY Buffalo and the theoretical aspects of the project with Prof. A. O. Govorov of Ohio University.

In addition to epitaxial quantum dots we, currently, are working on several aspects of optical properties of colloidal ZnO nanostructures, including core-shell systems. We have shown that quantum size effects can be achieved in ZnO nanorods, and we continue to investigate the role of dielectric confinement on ZnO nanorods optical properties, as it is important for 1-D systems. We investigate also the role of morphology on the optical properties of ZnO nanostrutures, including origin of the green band. This work is in collaboration with Prof. S. O’Brien’s Group of Columbia University.

We are also working in application of type-II structures for high sensitivity biological detectors. This work is in collaboration with Prof. Mourokh and Prof. H. Matsui of Hunter College. The pathogen diagnostics is constrained by a variety of challeneges that compromise essential elements of detection.Our approach uses unique optical properties of type-II colloidal fluorescent quantum dots.

Other interests include general properties of wide band gap (III-N and II-VI) semiconductors (see publications).

Abstact:

Optical emission from type-II ZnTe/ZnSe quantum dots demonstrates large and persistent oscillations in both the peak energy and intensity indicating the formation of coherently rotating states. Furthermore, these Aharonov-Bohm oscillations are shown to be remarkably robust and persist until 180 K. This is at least one order of magnitude greater than the typical temperatures in lithographically defined rings. To our knowledge, this is the highest temperature at which the Aharonov-Bohm effect has been observed in solid-state and molecular nanostructures.

Abstact:

Spectrally and time-resolved photoluminescence of a ZnTe/ZnSe superlattice reveals a smooth transition of the photoluminescence PL lifetime from 100 ns at 2.35 eV to less than a few nanoseconds at 2.8 eV. The significant increase of the lifetime in the low energy region is strong evidence to support the formation of type-II quantum dots QDs, since in these nanostructures the spatial separation of carriers is increased. The shorter lived emission above 2.5 eV is attributed to excitons bound to Te isoelectronic centers in the ZnSe matrix. The smooth transition of the PL lifetime confirms that clusters of these Te atoms evolve into type-II ZnTe/ZnSe QDs.

Abstact:

The properties of multiple type-II ZnTe/ZnSe quantum dots (QDs), which are coexistent with isoelectronic centers formed by Te, grown by migration-enhanced epitaxy, are studied. The samples with a single deposition cycle of Zn-Te-Zn sandwiched between ZnSe barriers are investigated via temperature- and excitationdependent photoluminescence PL as well as magneto-PL measurements. It is found that the PL consists of two broad bands: a “blue” band, which is dominant at low temperatures, and a “green” band, which is observed at T 60 K. Upon increasing the excitation intensity by about 4 orders of magnitude, the peak energy position of the blue band remains nearly the same, whereas the green band exhibits a large blueshift of 50 meV, which suggests that the green band is due to, at least partially, the recombination of excitons bound to type-II QDs. The existence of type-II ZnTe/ZnSe QDs is further supported by the results of magneto-PL, for which the oscillation in the PL intensity as a function of magnetic field is observed. The properties of ZnTe/ZnSe QDs grown under the same Zn/Te flux ratio but with one and three contiguous deposition cycles of Zn-Te-Zn are compared. It is concluded that type-II QDs are formed in both types of samples; however, the density, size, and chemical composition of QDs strongly depend on the deposition of the submonolayer quantities of ZnTe.