Semiconductor nanocrystals of different formulations have been extensively studied for use in thin-film photovoltaics. Materials used in such devices need to satisfy the stringent requirement of having large absorption cross sections. Hence, type-II semiconductor nanocrystals that are generally considered to be poor light absorbers have largely been ignored. In this article, we show that type-II semiconductor nanocrystals can be tailored to match the light-absorption abilities of other types of nanostructures as well as bulk semiconductors. We synthesize type-II ZnTe/CdS core/shell nanocrystals. This material is found to exhibit a tunable band gap as well as absorption cross sections that are comparable to CdTe. This result has significant implications for thin-film photovoltaics, where the use of type-II nanocrystals instead of pure semiconductors can improve charge separation while also providing a much needed handle to regulate device composition.
Conventional solids are prepared from building blocks that are conceptually no larger than a hundred atoms. While van der Waals and dipole−dipole interactions also influence the formation of these materials, stronger interactions, referred to as chemical bonds, play a more decisive role in determining the structures of most solids. Chemical bonds that hold such materials together are said to be ionic, covalent, metallic, dative, or otherwise a combination of these. Solids that utilize semiconductor nanocrystal quantum dots as building units have been demonstrated to exist; however, the interparticle forces in such materials are decidedly not chemical. Here we demonstrate the formation of charge transfer states in a binary quantum dot mixture. Charge is observed to reside in quantum confined states of one of the participating quantum dots. These interactions lead to materials that may be regarded as the nanoscale analog of an ionic solid. The process by which these materials form has interesting parallels to chemical reactions in conventional chemistry.S emiconductor nanocrystals with physical dimensions smaller than the size of the Bohr exciton exhibit strong quantum confinement of the carriers and are consequently are referred to as quantum dots (QDs). Due to their spheroidal confinement potential, electrons and holes confined in these materials occupy states with separable radial and angular components. Similar to atoms, the angular components of the electronic states are given by spherical harmonics. 1 QDs are sometimes referred to as "artificial atoms" to highlight this similarity. Although QDs can indeed be assembled into solids in the same manner as atoms, the full realization of artificial materials based on these building blocks requires the demonstration of atomlike "chemical" interactions between individual QDs.Here we show the existence of charge transfer states 2 between two formulations of QDs. These states act as nanoscale analogs of ionic bonds that are observed in inorganic compounds such as sodium chloride.Conventionally, ionic bonds have been known to exist in ionic solids as well as between atomic clusters as in Zintl Phases. 3 More recently, ionic bonding has been demonstrated to exist in solids formed from fullerenes and alkali metal or organometallic donors. 4−7 Chemical bonding in the above examples leads typically to structures that correspond to thermodynamic minima of the system. In contrast to these examples, the diffusion coefficients of QDs are several orders of magnitude smaller than those of atoms. While solids formed out of QDs with weak interparticle interactions 8−13 do exhibit crystalline order, stronger interparticle interactions could in principle prevent the system from arriving at a thermodynamic minimum. Solids formed out of chemically interacting QDs are therefore expected to be kinetically frozen, unlike the other examples described above.In order to achieve electron transfer between QDs, we adopt a charge transfer doping approach. Divalent ions from the first transition ...
Semiconductor quantum dots have replaced conventional inorganic phosphors in numerous applications. Despite their overall successes as emitters, their impact as laser materials has been severely limited. Eliciting stimulated emission from quantum dots requires excitation by intense short pulses of light typically generated using other lasers. In this Letter, we develop a new class of quantum dots that exhibit gain under conditions of extremely low levels of continuous wave illumination. We observe thresholds as low as 74 mW/cm(2) in lasers made from these materials. Due to their strong optical absorption as well as low lasing threshold, these materials could possibly convert light from diffuse, polychromatic sources into a laser beam.
Photonic devices stand to benefit from the development of chromophores with tunable, precisely controlled spontaneous emission lifetimes. Here, we demonstrate a method to continuously tune the radiative emission lifetimes of a class of chromophores by varying the density of electronic states involved in the emission process. In particular, we examined the peculiar composition-dependent electronic structure of copper doped CdZnSe quantum dots. It is shown that the nature and density of electronic states involved with the emission process is a function of copper inclusion level, providing a very direct handle for controlling the spontaneous lifetimes. The spontaneous emission lifetimes are estimated by examining the ratios of emission lifetimes to absolute quantum yields and also measured directly by ultrafast luminescence upconversion experiments. We find excellent agreement between these classes of experiments. This scheme enables us to tune spontaneous emission lifetimes by three orders of magnitude from ∼15 ns to over ∼7 µs, which is unprecedented in existing lumophores.
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