By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films. Upon visible light excitation, CdSe QDs inject electrons into TiO(2) nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO(2) nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 x 10(9) and 1.95 x 10(11) s(-1). The injected charge carriers in a CdSe-modified TiO(2) film can be collected at a conducting electrode to generate a photocurrent. The TiO(2)-CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO(2)/CdSe interfaces and internal TiO(2) grain boundaries.

Impurity doping has been widely used to endow semiconductor nanocrystals with novel optical, electronic, and magnetic functionalities. Here, we introduce a new family of doped NCs offering unique insights into the chemical mechanism of doping, as well as into the fundamental interactions between the dopant and the semiconductor host. Specifically, by elucidating the role of relative bond strengths within the precursor and the host lattice, we develop an effective approach for incorporating manganese (Mn) ions into nanocrystals of lead-halide perovskites (CsPbX, where X = Cl, Br, or I). In a key enabling step not possible in, for example, II-VI nanocrystals, we use gentle chemical means to finely and reversibly tune the nanocrystal band gap over a wide range of energies (1.8-3.1 eV) via postsynthetic anion exchange. We observe a dramatic effect of halide identity on relative intensities of intrinsic band-edge and Mn emission bands, which we ascribe to the influence of the energy difference between the corresponding transitions on the characteristics of energy transfer between the Mn ion and the semiconductor host.

Organic-inorganic lead-halide perovskites have been the subject of recent intense interest due to their unusually strong photovoltaic performance. A new addition to the perovskite family is all-inorganic Cs-Pb-halide perovskite nanocrystals, or quantum dots, fabricated via a moderate-temperature colloidal synthesis. While being only recently introduced to the research community, these nanomaterials have already shown promise for a range of applications from color-converting phosphors and light-emitting diodes to lasers, and even room-temperature single-photon sources. Knowledge of the optical properties of perovskite quantum dots still remains vastly incomplete. Here we apply various time-resolved spectroscopic techniques to conduct a comprehensive study of spectral and dynamical characteristics of single- and multiexciton states in CsPbX3 nanocrystals with X being either Br, I, or their mixture. Specifically, we measure exciton radiative lifetimes, absorption cross-sections, and derive the degeneracies of the band-edge electron and hole states. We also characterize the rates of intraband cooling and nonradiative Auger recombination and evaluate the strength of exciton-exciton coupling. The overall conclusion of this work is that spectroscopic properties of Cs-Pb-halide quantum dots are largely similar to those of quantum dots of more traditional semiconductors such as CdSe and PbSe. At the same time, we observe some distinctions including, for example, an appreciable effect of the halide identity on radiative lifetimes, considerably shorter biexciton Auger lifetimes, and apparent deviation of their size dependence from the "universal volume scaling" previously observed for many traditional nanocrystal systems. The high efficiency of Auger decay in perovskite quantum dots is detrimental to their prospective applications in light-emitting devices and lasers. This points toward the need for the development of approaches for effective suppression of Auger recombination in these nanomaterials, using perhaps insights gained from previous studies of II-VI nanocrystals.

Electron injection from excited CdSe quantum dots into TiO2 nanoparticles can be easily modulated by controlling the particle size. Femtosecond transient absorption studies indicate that the rate constant for electron transfer from the thermalized s-state of CdSe quantum dots increases with decreasing particle size. The energy difference between the conduction bands of the two semiconductor systems acts as a driving force for the electron transfer in the normal Marcus region. An increase in the interparticle electron transfer rate constant by 3 orders of magnitude (from ∼107 to 1010 s-1) has been achieved by decreasing the CdSe particle diameter from 7.5 to 2.4 nm.

Development of light-emitting diodes (LEDs) based on colloidal quantum dots is driven by attractive properties of these fluorophores such as spectrally narrow, tunable emission and facile processibility via solution-based methods. A current obstacle towards improved LED performance is an incomplete understanding of the roles of extrinsic factors, such as non-radiative recombination at surface defects, versus intrinsic processes, such as multicarrier Auger recombination or electron-hole separation due to applied electric field. Here we address this problem with studies that correlate the excited state dynamics of structurally engineered quantum dots with their emissive performance within LEDs. We find that because of significant charging of quantum dots with extra electrons, Auger recombination greatly impacts both LED efficiency and the onset of efficiency roll-off at high currents. Further, we demonstrate two specific approaches for mitigating this problem using heterostructured quantum dots, either by suppressing Auger decay through the introduction of an intermediate alloyed layer, or by using an additional shell that impedes electron transfer into the quantum dot to help balance electron and hole injection.