We report ultra-efficient multiple exciton generation (MEG) for single photon absorption in colloidal PbSe and PbS quantum dots (QDs). We employ transient absorption spectroscopy and present measurement data acquired for both intraband as well as interband probe energies. Quantum yields of 300% indicate the creation, on average, of three excitons per absorbed photon for PbSe QDs at photon energies that are four times the QD energy gap. Results indicate that the threshold photon energy for MEG in QDs is twice the lowest exciton absorption energy. We find that the biexciton effect, which shifts the transition energy for absorption of a second photon, influences the early time transient absorption data and may contribute to a modulation observed when probing near the lowest interband transition. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs. We present experimental and theoretical values of the size-dependent interband transition energies for PbSe QDs, and we also introduce a new model for MEG based on the coherent superposition of multiple excitonic states.

We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.

Quantum dots (QDs) of InP strongly adsorb onto transparent, porous, nanocrystalline TiO2 electrodes prepared by sintering 200−250 Å diameter TiO2 colloidal particles. The interparticle space of the TiO2 electrodes is large enough to permit deep penetration of 65-Å InP QDs into the porous TiO2 film. The absorption of light increases linearly with the thickness of the TiO2 film indicating that the InP QDs are adsorbed homogeneously on the TiO2 surface. We found that large particles adsorb better than smaller ones probably due to less hindrance by the stabilizer. The solid films exhibit strong photoconductivity in the visible region indicating photosensitization of TiO2 by InP QDs. The photocurrent action spectrum of the TiO2/InP QD film at a potential of +1 V is consistent with the absorption spectrum of the InP QDs. A photoelectrochemical cell was formed that consisted of p-type InP QDs loaded on TiO2, which was immersed in a I-/I3 - or hydroquinone/quinone acetonitrile solution, and a Pt counter electrode. These photoelectrochemical experiments show that electron transfer from InP QD into TiO2 nanoparticles occurs. p-Type InP/TiO2 electrodes are stable during illumination while n-type photocorrodes in an electrochemical cell.

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Surface complexation of colloidal titanium dioxide nanoparticles (40−60 Å) with cysteine was investigated by electron paramagnetic resonance (EPR) and infrared (diffuse reflectance infrared Fourier transform−DRIFT) spectroscopies. Cysteine was found to bind strongly to the TiO2 surface, resulting in formation of new trapping sites where photogenerated electrons and holes are localized. Illumination of cysteine-modified TiO2 at 77 K resulted in formation of cysteine radicals with the unpaired electron localized on the carboxyl group. Upon warming to 150 K, these radicals are transformed into sulfur-centered radicals as observed by EPR spectroscopy. We have demonstrated the existence of two surface Ti(III) centers on cysteine-modified TiO2 particles having different extents of tetragonal distortion of the octahedral crystal field. Upon addition of lead ions, a new complex of cysteine that bridges surface titanium atoms and lead ions was detected by IR spectroscopy. Illumination of lead/cysteine-modified TiO2 did not result in the formation of sulfur-centered radicals. Instead, a symmetrical, lattice defect type EPR signal for trapped holes was observed. Addition of methanol to this system resulted in the formation of a ·CH2OH radical at 8.2 K. After the temperature was raised to 120 K, doubling of the signal associated with electrons trapped at the particle surface (Ti(III)surf) was observed. On further increase of the temperature to 200 K, the EPR signal for trapped electrons disappeared due to the reduction of Pb2+ ions, and metallic lead precipitated at room temperature. Conversion of photogenerated holes in the presence of methanol into trapped electrons can lead to the doubled quantum efficiency of metallic lead precipitation.