We describe the synthesis of ZnS-capped CdSe semiconductor nanocrystals using organometallic reagents by a two-step single-flask method X-ray photoelectron spectroscopy, transmission electron microscopy and optical absorption are consistent with nanocrystals containing a core of nearly monodisperse CdSe of 27−30 A diameter with a ZnS capping 6 ± 3 A thick The ZnS capping with a higher bandgap than CdSe passivates the core crystallite removing the surface traps The nanocrystals exhibit strong and stable band-edge luminescence with a 50% quantum yield at room temperature

Synthesis and Characterization of Strongly Luminescing ZnS-Capped CdSe Nanocrystals

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

FOLLOWING the discovery1 of electroluminescence in poly(p-phenylenevinylene) (PPV), considerable effort has been directed towards the realization of optoelectronic devices based on semiconducting conjugated polymers of this type2–6. But the viability of these materials for such applications depends critically on the nature of the photoexcited states—in particular, whether they are predominantly non-emitting interchain species7–9 or emitting intrachain species10. One way to study this fundamental issue is in a device structure known as a microcavity11, which offers the possibility of using quantum electrodynamic effects to alter (and hence probe the nature of) spontaneous and stimulated emission in these materials12–17. Here we make use of such a structure to demonstrate optically driven laser activity in devices based on solid films of PPV. This demonstration of lasing provides direct support for a model10 in which the main photoexcitation in PPV is an emissive intrachain species, and opens the possibility of electrically driven polymer-based lasers.

Lasing from Conjugated-Polymer Microcavities

Observations of intermolecular excimers in several pi-conjugated polymers and exciplexes of these polymers with tris(p-tolyl) amine are reported. It is shown that the luminescence of conjugated polymer thin films originates from excimer emission and that the generally low quantum yield is the result of self-quenching. Thus, in sufficiently dilute solution, the "single-chain" emission has a quantum yield of unity. Exciplex luminescence and exciplex-mediated charge photogeneration have much higher quantum yields than the excimer-mediated photophysical processes. These results provide a basis for understanding and controlling the photophysics of conjugated polymers in terms of supramolecular structure and morphology.

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Excimers and Exciplexes of Conjugated Polymers

In semiconductor particles of nanometer size, a gradual transition from solid-state to molecular structure occurs as the particle size decreases. Consequently, a splitting of the energy bands into discrete, quantized levels occurs. Particles that exhibit these quantization effects are often called “Q-particles” or, generally, quantized material. The optical, electronic and catalytic properties of Q-particles drastically differ from those of the corresponding macrocrystalline substance. The band gap, a substance-specific quantity in macrocrystalline materials, increases by several electron volts in Q-particles with decreasing particle size. In Q-particles there are approximately as many molecules on the surface as in the interior of the particle. Therefore, the nature of the surface as well as the particle size is also largely responsible for the physico-chemical properties of the particle. Q-particles of many materials can be prepared in the form of colloidal solutions or embedded in porous matrices and are stable over a long period of time. In sandwich colloids, in which Q-particles of different materials are coupled, as well as in porous semiconductor electrodes containing Q-particles in the pores, very efficient primary charge separation is observed. As a result, sandwich colloids have greatly enhanced photocatalytic activity relative to the individual particles, while electrodes modified with Q-particles show high photocurrents. This article deals with the size quantization effect, the synthesis and characterization of Q-particles, as well as with the spectroscopic, electrochemical, and electron-microscopic investigation of these particles.

Colloidal Semiconductor Q‐Particles: Chemistry in the Transition Region Between Solid State and Molecules

We use transient optical hole burning and photoluminescence to investigate the static and dynamic electronic properties of 32-\AA{} CdSe quantum dots. We observe a number of discrete electronic transitions, resolve LO-phonon progressions, and obtain homogeneous linewidths and electron--LO-phonon couplings. We find that the band-gap luminescence is not from the exciton state, but from a surface trapped state. Rapid (\ensuremath{\sim}160 fs) trapping into these surface states results in long-lived (\ensuremath{\sim}10--100 ns) bleach and induced-absorption features in pump-probe experiments.

Electronic structure and photoexcited-carrier dynamics in nanometer-size CdSe clusters.

We report results of time-resolved photoinduced absorption experiments on oriented poly(2-methoxy-1,4-phenylene vinylene) films using probe energies from 0.2 to 2.3 eV. We observe two photoinduced absorption bands centered near 0.5 and 1.5 eV which form within our time resolution (1 ps) and decay with a fast (ps) and a slow (ns) component. The relaxation dynamics for the two bands are identical, indicating they are from the same excited species. On the basis of the spectra, the dynamics, and the pump intensity and polarization dependences, we conclude that these absorption features are due to bound polaron pairs and not to singlet or triplet excitons as previously suggested.

Assignment of the picosecond photoinduced absorption in phenylene vinylene polymers.

We describe the generation of 350-fs pulses tunable between 2.3 and 5.5 microm by seeded optical parametric amplification. These pulses are synchronized with visible pulses for time-resolved absorption and reflectivity in semiconductors with 500-fs resolution.

Tunable femtosecond radiation in the mid-infrared for time-resolved absorption in semiconductors.

We report on the first observation of a Raman echo, using spin-flip Raman scattering at 4905 \AA{} of bound donors in CdS to create a coherent spin state. The free-induction decays associated with each pulse in the sequence of two $\frac{\ensuremath{\pi}}{2}$ pulses as well as the echo are probed by spin-flip Raman scattering at 4880 \AA{}.

Spin-Flip Raman Echo in n -Type CdS

Ultrashort mid-infrared pulses at a repetition rate of 82 MHz have been generated by difference-frequency mixing at the output of a single-cavity, two-color femtosecond Ti:sapphire laser. The center wavelengths of the midinfrared pulses are tunable from 7.5 to 12.5 microm. Their pulse durations range from 450 to 650 fs, as a function of the center wavelength.

T. M. Jedju

Papers

Electronic structure and photoexcited-carrier dynamics in nanometer-size CdSe clusters.

Assignment of the picosecond photoinduced absorption in phenylene vinylene polymers.

Tunable femtosecond radiation in the mid-infrared for time-resolved absorption in semiconductors.

Spin-Flip Raman Echo in n -Type CdS

High-repetition-rate femtosecond mid-infrared pulse generation.