Elucidation of two giants: challenges to thick-shell synthesis in CdSe/ZnSe and ZnSe/CdS core/shell quantum dots.

Core/thick-shell giant quantum dots (gQDs) possessing type II electronic structures exhibit suppressed blinking and diminished nonradiative Auger recombination. We investigate CdSe/ZnSe and ZnSe/CdS as potential new gQDs. We show theoretically and experimentally that both can exhibit partial or complete spatial separation of an excited-state electron-hole pair (i.e., type II behavior). However, we reveal that thick-shell growth is challenged by competing processes: alloying and cation exchange. We demonstrate that these can be largely avoided by choice of shelling conditions (e.g., time, temperature, and QD core identity). The resulting CdSe/ZnSe gQDs exhibit unusual single-QD properties, principally emitting from dim gray states but having high two-exciton (biexciton) emission efficiencies, whereas ZnSe/CdS gQDs show characteristic gQD blinking suppression, though only if shelling is accompanied by partial cation exchange.

Reaction dynamics of O(¹D) + HCOOD/DCOOH investigated with time-resolved Fourier-transform infrared emission spectroscopy.

We investigated the reaction dynamics of O((1)D) towards hydrogen atoms of two types in HCOOH. The reaction was initiated on irradiation of a flowing mixture of O3 and HCOOD or DCOOH at 248 nm. The relative vibration-rotational populations of OH and OD (1 ≦ v ≦ 4, J ≤ 15) states were determined from time-resolved IR emission recorded with a step-scan Fourier-transform spectrometer. In the reaction of O((1)D) + HCOOD, the rotational distribution of product OH is nearly Boltzmann, whereas that of OD is bimodal. The product ratio [OH]/[OD] is 0.16 ± 0.05. In the reaction of O((1)D) + DCOOH, the rotational distribution of product OH is bimodal, but the observed OD lines are too weak to provide reliable intensities. The three observed OH/OD channels agree with three major channels of production predicted with quantum-chemical calculations. In the case of O((1)D) + HCOOD, two intermediates HOC(O)OD and HC(O)OOD are produced in the initial C-H and O-D insertion, respectively. The former undergoes further decomposition of the newly formed OH or the original OD, whereas the latter produces OD via direct decomposition. Decomposition of HOC(O)OD produced OH and OD with similar vibrational excitation, indicating efficient intramolecular vibrational relaxation, IVR. Decomposition of HC(O)OOD produced OD with greater rotational excitation. The predicted [OH]/[OD] ratio is 0.20 for O((1)D) + HCOOD and 4.08 for O((1)D) + DCOOH; the former agrees satisfactorily with experiments. We also observed the v3 emission from the product CO2. This emission band is deconvoluted into two components corresponding to internal energies E = 317 and 96 kJ mol(-1) of CO2, predicted to be produced via direct dehydration of HOC(O)OH and secondary decomposition of HC(O)O that was produced via decomposition of HC(O)OOH, respectively.

Visible to near-infrared sensitization of silicon substrates via energy transfer from proximal nanocrystals: further insights for hybrid photovoltaics.

We provide a unified spectroscopic evidence of efficient energy transfer (ET) from optically excited colloidal nanocrystal quantum dots (NQDs) into Si substrates in a broad range of wavelengths: from visible (545 nm) to near-infrared (800 nm). Chemical grafting of nanocrystals on hydrogenated Si surfaces is achieved via amine-modified carboxy-alkyl chain linkers, thus ensuring complete surface passivation and accurate NQD positioning. Time-resolved photoluminescence (PL) has been measured for a set of CdSe/ZnS and CdSeTe/ZnS NQDs of various sizes and compositions grafted on Si and SiO2 substrates. The measured acceleration of the PL decays on Si substrates is in good agreement with theoretical expectations based on the frequency-dependent dielectric properties of Si and NQD-Si separation distances. A comparative analysis reveals separate contributions to ET coming from the nonradiative (NRET) and radiative (RET) channels: NRET is a dominant mechanism for proximal NQDs in the middle of the visible range and becomes comparable with RET toward near-infrared wavelengths. The broad range over which the ET efficiency is estimated to be at the level of ∼90% further supports the concept that hybrid nanocrystal/silicon thin-film photovoltaic devices could efficiently harvest solar energy across the entire spectrum of wavelengths.

Dynamics of the reactions of O(1D) with CD3OH and CH3OD studied with time-resolved Fourier-transform IR spectroscopy.

We investigated the reactivity of O((1)D) towards two types of hydrogen atoms in CH(3)OH. The reaction was initiated on irradiation of a flowing mixture of O(3) and CD(3)OH or CH(3)OD at 248 nm. Relative vibration-rotational populations of OH and OD (1 ≤ v ≤ 4) states were determined from their infrared emission recorded with a step-scan time-resolved Fourier-transform spectrometer. In O((1)D) + CD(3)OH, the rotational distribution of OD is nearly Boltzmann, whereas that of OH is bimodal; the product ratio [OH]/[OD] is 1.56 ± 0.36. In O((1)D) + CH(3)OD, the rotational distribution of OH is nearly Boltzmann, whereas that of OD is bimodal; the product ratio [OH]/[OD] is 0.59 ± 0.14. Quantum-chemical calculations of the potential energy and microcanonical rate coefficients of various channels indicate that the abstraction channels are unimportant and O((1)D) inserts into the C-H and O-H bonds of CH(3)OH to form HOCH(2)OH and CH(3)OOH, respectively. The observed three channels of OH are consistent with those produced via decomposition of the newly formed OH or the original OH moiety in HOCH(2)OH or decomposition of CH(3)OOH. The former decomposition channel of HOCH(2)OH produces vibrationally more excited OH because of incomplete intramolecular vibrational relaxation, and decomposition of CH(3)COOH produces OH with greater rotational excitation, likely due to a large torque angle during dissociation. The predicted [OH]/[OD] ratios are 1.31 and 0.61 for O((1)D) + CD(3)OH and CH(3)OD, respectively, at collision energy of 26 kJ mol(-1), in satisfactory agreement with the experimental results. These predicted product ratios vary weakly with collision energy.

Efficient radiative and nonradiative energy transfer from proximal CdSe/ZnS nanocrystals into silicon nanomembranes.

We demonstrate efficient excitonic sensitization of crystalline Si nanomembranes via combined effects of radiative (RET) and nonradiative (NRET) energy transfer from a proximal monolayer of colloidal semiconductor nanocrystals. Ultrathin, 25-300 nm Si films are prepared on top of insulating SiO(2) substrates and grafted with a monolayer of CdSe/ZnS nanocrystals via carboxy-alkyl chain linkers. The wet chemical preparation ensures that Si surfaces are fully passivated with a negligible number of nonradiative surface state defects and that the separation between nanocrystals and Si is tightly controlled. Time-resolved photoluminescence measurements combined with theoretical modeling allow us to quantify individual contributions from RET and NRET. Overall efficiency of ET into Si is estimated to exceed 85% for a short distance of about 4 nm from nanocrystals to the Si surface. Effective and longer-range radiative coupling of nanocrystal's emission to waveguiding modes of Si films is clearly revealed. This demonstration supports the feasibility of an advanced thin-film hybrid solar cell concept that relies on energy transfer between strong light absorbers and adjacent high-mobility Si layers.

Kinetic study of the 2-naphthyl (C10H7) radical reaction with C2H2.

The kinetics for the gas-phase reaction of 2-naphthyl radical with acetylene has been measured by monitoring the C10H7O2 radical in the visible region employing cavity ringdown spectrometry (CRDS) using 2-C10H7Br as a radical source photolyzing at 193 nm in the presence of a small fixed amount of O2 at 40 Torr pressure with Ar as a diluent. Absolute rate constants measured at temperatures between 303 and 448 K can be expressed by the following Arrhenius equation: k(T) = (3.36 +/- 0.63) x 10(11) exp[-(817 +/- 34)/T] cm3 mol(-1) s(-1). Theoretically, the potential energy surfaces (PESs) for the reactions of acetylene with 1- and 2-C10H7 radicals have been calculated with the G2MS//B3LYP/6-311+G(d,p) method. The PESs show that the reactions of 1- and 2-C10H7 with C2H2 occur first by forming adducts with 2.6 and 2.9 kcal/mol barriers, respectively. The rate constants for the stabilization and decomposition of the adducts have been predicted by RRKM/ME calculations. The mechanisms for the decomposition of the two adducts were predicted to be distinctively different under experimental conditions; the excited 1-C10H7C2H2 radical produces primarily acenaphthylene because of its low formation barrier, while the excited 2-C10H7C2H2 radical can be effectively stabilized by collisional quenching due to its high exit barrier. The predicted rate constant for the 2-C10H7 reaction with C2H2 is in reasonable agreement with the experimental values under the conditions employed.

Ab initio study on the oxidation of NCN by OH: prediction of the individual and total rate constants.

The mechanism for the reaction of NCN with OH has been investigated by ab initio molecular orbital and transition-state theory calculations. The potential energy surface (PES) was calculated by the highest level of the modified GAUSSIAN-2 (G2M) method, G2M(CC1). The barrierless association process of OH + NCN --> OH...NCN (van der Waals, vdw) was also examined at the UCCSD(T)/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) and CASPT2(13,13)/ANO-L//B3LYP/6-311+G(d,p) levels. The predicted heats of reaction for the production of H + NCNO, HNC + NO, HCN + NO, and N(2) + HOC, 7.8, -53.2, -66.9, and -67.7, respectively, are in excellent agreement with the experimental values, 8.2 +/- 1.3, -52.3 +/- 1.7 (or 55.7 +/- 1.7), -66.3 +/- 0.7, and -68.1 +/- 0.7 kcal/mol. The kinetic results indicate that, in the temperature range of 300-1000 K, the formation of trans,trans-HONCN (LM2) is dominant. Over 1000 K, formation of H + NCNO is dominant, while the formation of HCN + NO becomes competitive. The rate constants for the low-energy channels given in units of cm(3) molecule(-1) s(-1) can be represented by the following: k(1)(LM2) = 1.51 x 10 (15)T(-8.72) exp(-2531/T) at 300-1500 K in 760 Torr N(2); k(2)(H+NCNO) = 5.54 x 10 (-14)T(-0.97) exp(-3669/T) and k(3)(HCN+NO) = 7.82 x 10 (-14)T(0.44) exp(-2013/T) at 300-2500 K, with the total rate constant of k(t) = 3.18 x 10 (2)T(-4.63) exp(-740/T), 300-1000 K, and k(t) = 2.53 x 10 (-14)T(1.13) exp(-489/T) in the temperature range of 1200-2500 K. These results are recommended for combustion modeling applications.