Interfacial Carriers Dynamics of CdS Nanoparticles

Klimov V.I., Haring-Bolivar P., Kurz H., Karavanskii V.A.

Abstract Application of femtosecond photoluminescence and nonlinear transmission techniques allows us to separate electron and hole relaxation paths in CdS nanocrystals (NCs) and to observe a strong difference in optical nonlinearities in NCs formed by direct- and indirect-gap phases of copper sulfide.

Klimov V., Bolivar P.H., Kurz H.

The dynamics of band-edge photoluminescence (PL) in CdS nanocrystals (NC's) dispersed in a glass matrix are studied with the femtosecond up-conversion technique. The time-resolved PL spectra exhibit several discrete features (three of them are in the NC energy band gap) which are not pronounced in a cw PL spectrum. The initial stage of a PL decay is governed by a depopulation of the lowest extended states due to carrier trapping (localization) on the time scale of 1 ps. The low-energy bands originating from the extended-to-localized state transitions exhibit extremely fast buildup dynamics (rise time is 400--700 fs) which is explained by the preexisting occupation of the localized states. \textcopyright{} 1996 The American Physical Society.

Burfeindt B., Hannappel T., Storck W., Willig F.

Modified perylene chromophores were adsorbed with virtually constant reaction distance on the surface of a spongelike TiO2 electrode. Interfacial electron transfer was probed with femtosecond resolution in ultrahigh vacuum via transient absorption and fluorescence up-conversion measurements. Identical time constants were measured for the decay of the reactant and rise of the product states. The dominant fast time constant was 190 fs. It remained constant between 300 and 22 K.

Rehm J.M., McLendon G.L., Nagasawa Y., Yoshihara K., Moser J., Grätzel M.

The charge injection dynamics of dye sensitization from a surface-bound dye (coumarin 343 (C343)) to the conduction band (CB) of the TiO2 is reported here for the first time. Ultrafast fluorescence dynamics demonstrate that the charge injection from the C343 dye to the CB of the TiO2 occurs on a time scale of ca. 200 fs. The charge injection efficiency is attributed to strong electronic coupling between the dye and TiO2 energy levels. The results yield a rate of injection of 5 × 1012 s-1.

Hagfeldt A., Graetzel M.

A review with 156 refs. on interfacial electron transfer reactions in colloidal semiconductor solns. and thin films and their application for solar light energy conversion and photocatalytic water purifn. Some of the topics discussed include; optical and electronic properties of colloidal semiconductor particles, quantum size effects in the photoluminescence of colloidal semiconductors, light-induced charge sepn., dynamics of interfacial charge transfer processes, properties and prepn. of nanocryst. semiconductor electrodes, energetics and operations of the nanoporous solar cell.

Zhang J.Z., O'Neil R.H., Roberti T.W., McGowen J.L., Evans J.E.

We report the direct measurements of the dynamics of trapped electrons at the liquid—solid interface of aqueous CdS colloids on the femtosecond time scale. The trapped electrons are found to be generated in less than 100 fs. The majority of the trapped electrons decays in 10 ps through predominately electron—hole recombination, while a small fraction lives for longer than 1 ns.

Hässelbarth A., Eychmüller A., Weller H.

Stationary and time-resolved quenching measurements of the excitonic and trapped fluorescence of quantum sized CdS colloids were carried out. We found that nitromethane is only capable of quenching the excitonic fluorescence nearly completely, whereas a fraction of the trapped fluorescence, the amount of which depends on the particle size, always remains. Methylviologen, with a slightly more positive acceptor potential, is able to quench both types of fluorescence nearly completely in all the investigated samples. The findings are explained straightforwardly by a fluorescence mechanism involving shallow electron traps and deep hole traps located on the surface of the nanocrystalline particles.

Barzykin A.V., Fox M.A.

A study of the dynamics of the electronic states of CdS semiconductor clusters grown in situ in reversed micelles via picosecond pump-probe technique reveals efficient transient bleaching of the excitonic absorption. This effect is attributable mainly to hole localization near the trapped electron, with no evidence of electron ejection (forming a solvated electron) detectable despite the apparent proximity of the cluster to the included water pool. Bleaching efficiency is cluster-size dependent, and recovery kinetics are governed by the nature of the particle surface. Similar decay behavior is observed in cluster fluorescence. Generally, the decay is characterized by two components, the faster attributed to recombination of a mobile, detrapped electron with a trapped hole, and the slower to an exchange interaction between trapped carriers. The observed strong multiexponentionality of the decay results from a distribution in trap energy levels, as well as from a distribution of distances between traps on the cluster surface. By varying preparation conditions, particles of different sizes with different surface defects can be grown in reversed micelles in a reproducible way so that their electronic properties can be controlled.

Misawa K., Yao H., Hayashi T., Kobayashi T.

We investigated the size dependence of picosecond dynamics of the band-edge emission from CdS microcrystallites. Excitonic superradiance was observed from CdS microcrystallites with larger diameters than 50 Å, while microcrystallites smaller than 35 Å yield only shallow-trap emission from surface-trapping states. The superradiant decay rate increases with particle diameter, is nearly constant below a threshold temperature, and decreases above that temperature. The threshold temperatures of microcrystallites of 25 and 29 Å in radius are 45 and 30 K, respectively, in good agreement with the calculated radial-compression-mode frequencies of 43.2 and 37.2 K, using the elastic-sphere model.

Eychmüller A., Hässelbarth A., Katsikas L., Weller H.

The fluorescence properties of surface modified quantum-sized CdS colloids were investigated by means of static and time-resolved low temperature fluorescence spectroscopy. All samples investigated had high fluorescence quantum yields already at room temperature and exhibited either exclusively excitonic or a mixture of excitonic and trapped fluorescence. Mechanisms are presented which explain both the static and time-resolved behaviour of both the excitonic and trapped fluorescence. The excitonic fluorescence is described as delayed fluorescence occuring by detrapping of electrons. Extremely shallow traps having trap depths of a few meV are discussed.

Wang Y., Suna A., McHugh J., Hilinski E.F., Lucas P.A., Johnson R.D.

We studied the optical transient bleaching of ∼40 Å, ammonia-passivated CdS clusters in a polymer with nanosecond and picosecond pump-probe techniques. The transient bleaching spectra behave differently in different time regimes. Within the 30-ps pump laser pulse width, we tentatively attribute the bleaching to the exciton-exciton interaction, and the magnitude can be enhanced by surface passivation. On time scales of tens of picoseconds and longer following the pump pulse, when only trapped electron-hole pairs remain from the pump excitation, the bleaching is due to the interaction between such a trapped electron-hole pair and a bound exciton produced by the probe light. Experimentally we determined that roughly one trapped electron-hole pair can bleach the excitonic absorption of the whole CdS cluster. We developed a theoretical model which considers the effects of the trapped electron-hole pair on the energy of the exciton transition and its oscillator strength. We found that, when a trapped electron and hole are present, the lowest exciton absorption is red-shifted from the original exciton absorption, and this transition has a weak oscillator strength, which explains the observed efficient bleaching. The model also predicts that a trapped electron is more efficient than a trapped hole for bleaching the excitonic absorption of CdS clusters in the size regime considered here. This is confirmed by pulse radiolysis results. Finally, we discuss the possible effects of charged surface defects on the linear absorption spectra of semiconductor clusters.

Steigerwald M.L., Brus L.E.

O'Neil M., Marohn J., McLendon G.

The kinetics of radiative electron-hole pair recombination in CdS and Cd{sub 3}As{sub 2} clusters (where the radius of the cluster is smaller than the de Broglie wavelength of photogenerated excitons) were studied with picosecond photon counting luminescence decay measurements over wide temperature and energy ranges. The decay profiles were quantitatively examined with several models. The decays are composed of two distinct time regimes, each with very different temperature and emission energy dependence. The first (fast) regime is attributed to an unusually efficient thermal repopulation mechanism. The second (slow) component is well described by a distributed kinetic model. The kinetic behavior of wide (CdS) and narrow (Cd{sub 3}As{sub 2}) band gap materials was remarkably similar when composed of clusters in the quantum confined regime.

O'Neil M., Marohn J., McLendon G.

The formation of deep traps in 30 A CdS semiconductor clusters has been time resolved using picosecond single-photon counting. The trapping rates, measured by the risetime of recombinate or emission, vary inversely with trap depth ranging from ≈ 30 ps above 500 nm to

Henglein A.

Chauhan S., Watson D.F.

Naz B., Mubeen M., Mukhtar M., Khalid M.A., Huma Z.E., Waseem S., Rashid Z., Iqbal A.

Photocatalytic CO 2 reduction has sparked significant interest in the generation of chemical fuels through solar energy harvesting. However, the activity, stability and selectivity of products are heavily dependent upon the efficiencies of light harvesting ability, charge migration and surface reactions. Herein, colloidal CdS quantum dots (QDs) have been interfacially engineered with four different capping ligands viz. mercaptoacetic acid (MAA), 3-mercaptopropanoic acid (MPA), l -cysteine (LC) and oleic acid (OA). Compared to other ligands, MAA functionalization introduces deep trap states by developing interactions with the surface Cd atoms. These states enhance the lifetime of photogenerated electrons for substantial CO 2 reduction with an activity of approximately 2120 mol g −1 h −1 . Computational analysis suggests the rate of electron transfer to CO 2 from CdS QDs is much faster as compared to the nuclear degrees of freedom, and the bending angle of CO 2 enhances electron transfer. The maximum electron transfer efficiency was observed at the bent angle of 130°. The bent geometry is supported by the surface proton shutter provided by the acid group of the thiol/acid bifunctional ligand. This work tries to comprehend the crucial effect of ligand-induced deep trap states and change in the bending angle of CO 2 , which makes the C-Cd distance shorter and significantly promotes the photocatalytic reduction of CO 2 to formaldehyde.

Fan K., Sergeeva K.A., Sergeev A.A., Zhang L., Chan C.C., Li Z., Zhong X., Kershaw S.V., Liu J., Rogach A.L., Wong K.S.

Rempel Andrey A., Ovchinnikov Oleg V., Weinstein Ilya A., Rempel Svetlana V., Kuznetsova Yulia V., Naumov Andrei V., Smirnov Mikhail S., Eremchev Ivan Yu., Vokhmintsev Alexander S., Savchenko Sergey S.

Quantum dots are the most exciting representatives of nanomaterials. They are synthesized using advanced methods of nanotechnology pertaining to both inorganic and organic chemistry. Quantum dots possess unique physical and chemical properties; therefore, they are used in very different fields of physics, chemistry, biology, engineering and medicine. It is not surprising that the Nobel Prize in chemistry in 2023 was given for discovery and synthesis of quantum dots. This review addresses modern methods for the synthesis of quantum dots and their optical properties and practical applications. In the beginning, a short insight into the history of quantum dots is given. Many gifted scientists, including chemists and physicists, were engaged in these studies. The synthesis of quantum dots in solid and liquid matrices is described in detail. Quantum dots are well-known owing to their unique optical properties; that is why the attention in the review is focused on the quantum-size effect. The causes for fascinating blinking of quantum dots and techniques for observation of a single quantum dot are considered. The last part of the review describes mportant applications of quantum dots in biology, medicine and quantum technologies.The bibliography includes 772 references.

Li Q., Wu K., Zhu H., Yang Y., He S., Lian T.

Cai M., Tong X., Zhao H., Liao P., Pan L., Li G., Wang Z.M.

Understanding and manipulating intragap states in semiconductors may enable superior solar-to-hydrogen energy conversion. The effect of intragap states on photocatalysis usually remains unclear and is sometimes contradictory. Quantum-confined colloidal quantum dots (QDs) provide a unique platform to tune the density and distribution of intragap states due to their discrete energy levels. Herein, intragap active domains, composed of Cu vacancies (VCu′) and high-valent Cu (Cu⁎) defect states, are constructed in copper-deficient Zn-doped CuInS2 QDs. Note that these intragap states mainly exist at in-facet and on-edge defects in QDs, being away from the valence band maximum and close to Fermi level. Steady and transient optical spectra indicate that photoactivated Cu⁎ states serving as photoinduced absorption centers can facilitate the generation of long-lived hot electrons (ca. 85 ps) as a manifestation of phonon bottleneck. Synergistically, the VCu′ states enable the holes capture and electron-hole pairs decoupling to suppress ultrafast Auger-like hot carrier cooling (ca. 178 fs). Moreover, the on-edge defects are demonstrated to play an active role in mediating proton reduction kinetics through density functional calculation. As a result, the QDs exhibit an outstanding hydrogen generation rate of 50.4 mmol g-1 h-1 without any noble metal, meanwhile, various molecule oxidation and polymer degradation can be integrated with the hydrogen generation process.

Wang C., Chen Z.Z., Xiao S., He J.

The carrier dynamics of Cu1.81S nanoplates, Cu1.81S-CdS heteronanoplates and CdS nanoplates were investigated, and the results indicate that photogenerated holes may be transferred from the CdS phase to the Cu1.81S phase under 400 nm excitation.

Zhang M., Liu Z., Wang J., Chen Z., Jiang G., Zhang Q., Li Z.

Wang L., Shi B., Yao C., Wang Z., Wang X., Jiang C., Feng L., Song Y.

Wu W., Liu W., Han Q., Gao Y., Kong D., Yang Q.

With increasing x in core/shell Cd1−xZnxS/ZnS QDs, the contribution of short-lived band-edge excitonic to the PL decay increases and the trapping state at the core/shell interface can trap more holes that are excited by multi-exciton Auger process.

Zhao G., Ma W., Yu S., Zhang J., Wu K.

Hu A., Ye J., Ren G., Qi Y., Chen Y., Zhou S.

Bio-nano hybrids with methanogens and nano-semiconductors provide an innovative strategy for solar-driven CO2-to-CH4 conversion; however, the efficiency mismatch between electron production and utilisation results in low quantum yield and CH4 selectivity. Herein, we report the integration of metal-free polymeric carbon nitrides (CNx) decorated with cyanamide (NCN) groups and Methanosarcina barkeri (M. b). The self-assembled M. b-NCNCNx exhibited a quantum yield of 50.3 % with 92.3 % CH4 selectivity under illumination, which outperforms other reported bio-nano hybrid systems and photocatalytic systems for CO2 reduction. This excellent performance was attributed to the distinct capacitance and conductive effects of NCNCNx, which promoted electron storage and redistribution at the biotic–abiotic interface to alleviate recombination losses and side reaction. This study provides new design guidelines for bio-nano hybrids for the sustainable photocatalytic reduction of CO2 into fuels.

Hu A., Ye J., Ren G., Qi Y., Chen Y., Zhou S.

The efficient reduction of CO2 to CH4 by a designed bio-nano hybrid with high product selectivity and record quantum yield is reported. In this process, distinct capacitance and conductive effects promote electron storage and redistribution. This work sheds light on the development of sustainable CO2-to-fuel conversion to meet environmental and energy requirements.

Keene J.D., Freymeyer N.J., McBride J.R., Rosenthal S.J.

SummarySemiconductor nanocrystals have become ubiquitous both in scientific research and in applied technologies related to light. When a nanocrystal absorbs a photon an electron-hole pair is created whose fate dictates whether the nanocrystal will be suitable for a particular application. Ultrafast spectroscopy provides a real-time window to monitor the evolution of the electron-hole pair. In this review, we focus on CdSe nanocrystals, the most-studied nanocrystal system to date, and also highlight ultrasmall nanocrystals, "standard nanocrystals" of different binary composition, alloyed nanocrystals, and core/shell nanocrystals and nanorods. We focus on four time-resolved spectroscopies used to interrogate nanocrystals: pump-probe, fluorescence upconversion, time-correlated single photon counting, and non-linear spectroscopies. The basics of the nanocrystals and the spectroscopies are presented, followed by a detailed synopsis of ultrafast spectroscopy studies performed on the various semiconductor nanocrystal systems.

Jana B., Ghosh S., Dutta A., Baranov A.V., Fedorov A.V., Patra A.

The study of carrier dynamics of QDs is extremely important for developing efficient light-harvesting systems. Here, we investigate size-dependent hole transfer from CdSe QDs to 4-aminothiophenol (4-ATPh) ligand using ultrafast spectroscopy. The photoluminescence (PL) quenching is found to be 99% and 77% for 4-ATPh ligand capped 2.9 nm and 4.3 nm QDs, respectively. We propose a stochastic model for analyzing time-resolved fluorescence decay curves of QDs to estimate the average number of ligands around QDs and it is found that the larger number of ligands are attached with smaller QDs. The analysis of TA spectroscopy data reveals that the kinetics of hole transfer from 2.9 nm QDs to ligand is faster than 4.3 nm CdSe QDs, depending on the offset between valence band (VB) of the CdSe QD and the HOMO of 4-ATPh ligands, and the average number of 4-ATPh ligands attached to each CdSe QD. The fundamental study of ligand-induced charge transfer processes in QDs is important for QD-based solar cell applications. • Quantum dot (QD)-based light-harvesting systems are found to be potentially interesting for solar cell and optoelectronic device applications because of efficient charge carrier generation right after photoexcitation. • We have investigated the ligand-induced hole transfer process from QDs to ligands which depends on the size of QDs and the number of ligands on the surface of QDs using steady-state and time-resolved ultrafast spectroscopic techniques. • A stochastic kinetic model is proposed to understand the influence of the size of QDs on the number of ligands and trap states. • The analysis of TA spectroscopy data reveal that the bleaching recovery dynamics of the smaller sized CdSe QDs becomes slower in presence of the hole-withdrawing ligand, 4-ATPh. • The fundamental understanding of ligand-induced charge transfer processes in QDs is important for QD-based solar cell applications.