Publications

2007
Uri Banin. 2007. “Nanocrystals: Tiny seeds make a big difference.” Nature materials, 6, 9, Pp. 625. Publisher's Version Abstract

A study was to demonstrate a seeded growth approach that provides shape-controlled bimetallic nanocrystals, generating new ways for selecting new nanoscale building blocks. It was demonstrated that semiconductor and oxide nanowires were grown, using metal seeding approach that acted as catalysts. Gold rod growth in solution was also carried out by seeding with small gold particles, while semiconductor rods were grown using semiconductor nanoparticles as seeds. The new approach also used faceted metal nanocrystals as nucleation centers for the overgrowth of a secondary metal. It was also observed that the ability of the new approach to synthesize binary-metal nanoparticles has significant implications for diverse applications in memory devices, electrical contacts, biological applications, and catalysis.

Miri Kazes, Dan Oron, Itzhak Shweky, and Uri Banin. 2007. “Temperature dependence of optical gain in CdSe/ZnS quantum rods.” The Journal of Physical Chemistry C, 111, 22, Pp. 7898-7905. Publisher's Version Abstract

We studied the optical gain characteristics of CdSe/ZnS core/shell colloidal quantum rods, investigated their temperature dependence, and compared the gain properties with quantum dots (QD). The gain was measured systematically for close-packed films of rods and dots under quasi-CW nanosecond optical pumping, using the variable stripe length method measuring the amplified spontaneous emission (ASE). Tunable ASE can be achieved by changing the rod diameter. Optical gain factors of up to 350 cm-1 at a temperature range of 10−120 K were measured for quantum rods. Above 120 K, the gain decreased sharply, but by increasing the pump power, ASE was easily achieved also at room temperature. The temperature dependence was assigned to the Auger heating process and phonon assisted thermal relaxation. QD of similar diameters as the rods showed much smaller gain values (∼50 cm-1) and a sharp decrease in gain at lowered temperatures (∼50 K), and ASE could not be detected at room temperature even at high pump powers. The significantly improved gain values in quantum rods as compared with dots were attributed to the slower Auger relaxation rates, the higher absorption cross-section, and the reduced self-absorption due to the larger Stokes shift. The temperature dependence of the threshold power for the quantum rods, used to characterize the thermal insensitivity of the system, showed two distinct temperature regions. In the low-temperature region, a very high T0 value of 3500 K was measured, as predicted for a low-dimensional quantum confined system.

Dan Oron, Miri Kazes, and Uri Banin. 2007. “Multiexcitons in type-II colloidal semiconductor quantum dots.” Physical Review B, 75, 3, Pp. 035330. Publisher's Version Abstract

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The spectroscopy and dynamics of multiple excitations on colloidal type-II CdTe∕CdSe core-shell quantum dots (QDs) are explored via quasi-cw multiexciton spectroscopy. The charge separation induced by the band offset redshifts the exciton emission and increases the radiative lifetime. In addition, we observe a significant modification of multiexciton properties compared with core-only or type-I QDs. In particular, the Auger recombination lifetimes are significantly increased, up to a nanosecond time scale. While in type-I QDs the Auger lifetime scales with the volume, we find for type-II QDs a scaling law that introduces a linear dependence also on the radiative lifetime. We observe a blueshift of the biexciton emission and extract biexciton repulsion of up to 30meV in type-II QDs. This is assigned to the dominance of the Coulomb repulsion as the positive and negative charges become spatially separated, which overwhelms the correlation binding term. Higher electronic excited states can remain type I even when the lowest transition is already type II, resulting in a different size dependence of the triexciton emission. Finally, we discuss the possibilities of “multiexciton band gap engineering” using colloidal type-II QDs.

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The invention comprises a composite material comprising a host material in which are incorporated semiconductor nanocrystals. The host material is light-transmissive and/or light-emissive and is electrical charge-transporting thus permitting electrical charge transport to the core of the nanocrystals. The semiconductor nanocrystals emit and/or absorb light in the near infrared spectral range. The nanocrystals cause the composite material to emit/absorb energy in the near infrared (NIR) spectral range, and/or to have a modified dielectric constant, compared to the host material. The invention further comprises electro-optical devices composed of this composite material and a method of producing them. Specifically described are light emitting diodes that emit light in the NIR and photodetectors that absorb light in the same region.