Excited Semiconductor Clusters: Magic Source

In this paper, we review the preparation and properties of CdS-like clusters as well as their important applications in several nanostructures. The clusters created by the reaction of S^2? ions generated by the decomposition of thioglycolic acid (TGA) and Cd²⁺ ions play important roles for the construction of novel nanocrystals (NCs), the assembly of the NCs, and the formation of nanostructures including 1D solid and tubal fibers, 2D dendritic morphology, and 3D crystals. The formation and property of the clusters depended strongly on the molar ratio of TGA and Cd²⁺ ions in solutions. When aqueous CdTe NCs coated with a hybrid SiO₂ shell containing the clusters, they revealed a drastic increase in photoluminescence (PL) efficiency (from 28 to 80 %) and temperature-dependent PL. These excellent PL properties were ascribed to the clusters very closed to CdTe cores. This is confirmed through the observation of a lengthening of the Auger recombination lifetime by a factor of ~3.5 in the presence of the clusters. The size of the clusters determined the PL properties of the hybrid SiO₂-coated NCs and those fibers. Because of their high PL, these biofunctional materials could provide a platform for various applications.     

Size‐ and Halide‐Dependent Auger Recombination in Lead Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) hold strong promise for a variety of light‐harvesting, emitting, and detecting applications, all of which, however, could be complicated by multicarrier Auger recombination. Therefore, complete documentation of the size‐ and composition‐dependent Auger recombination rates of these NCs is highly desirable, as it can guide system design in many applications. Herein we report the synthesis and Auger measurements of monodisperse APbX₃ (A=Cs and FA; X=Cl, Br, and I) NCs in an extensive size range (ca. 3–9 nm). The biexciton Auger lifetime of all the NCs scales linearly with the NC volume. The scaling coefficient is virtually independent of the cation but rather depends sensitively on the anion, and is 0.035, 0.085, and 0.142 ps nm^?3 for Cl, Br, and I, respectively. In all of these nanocrystals the Auger recombination is much faster than in standard CdSe and PbSe NCs (ca. 1 ps nm^?3).     

Size- and Halide-Dependent Auger Recombination in Lead Halide Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) hold strong promise for a variety of light-harvesting, emitting, and detecting applications, all of which, however, could be complicated by multicarrier Auger recombination. Therefore, complete documentation of the size- and composition-dependent Auger recombination rates of these NCs is highly desirable, as it can guide system design in many applications. Herein we report the synthesis and Auger measurements of monodisperse APbX₃ (A=Cs and FA; X=Cl, Br, and I) NCs in an extensive size range (ca. 3–9 nm). The biexciton Auger lifetime of all the NCs scales linearly with the NC volume. The scaling coefficient is virtually independent of the cation but rather depends sensitively on the anion, and is 0.035, 0.085, and 0.142 ps nm⁻³ for Cl, Br, and I, respectively. In all of these nanocrystals the Auger recombination is much faster than in standard CdSe and PbSe NCs (ca. 1 ps nm⁻³).     

Interplay between Auger and ionization processes in nanocrystal quantum dots

We study the interplay between Auger effects and ionization processes in the limit of strong electronic confinement in core/shell CdSe/ZnS semiconductor nanocrystal quantum dots. Spectrally resolved fluorescence decay measurements reveal a monotonic increase of the photoluminescence decay rate on excitation density. Our results suggest that Auger recombination accelerates ionization processes that lead to the occupation of dark, nonemissive nanocrystal states. A model is proposed in the quantized Auger regime describing these experimental observations and providing an estimate of the Auger assisted ionization rates.     

H2S ultrafast dissociation probed by energy-selected resonant Auger electron-ion coincidence measurements

We have studied the ultrafast dissociation of the H2S molecule upon S 2p3/2-->6a1 inner-shell excitation by combining high-resolution resonant Auger spectroscopy and energy-selected Auger electron-ion coincidence measurements. Auger final states have been correlated to the different fragmentation pathways (S+, HS+, and H2S+ ions). As an original result, we evidence a three-step mechanism to describe the resonant production of S+: the Auger recombination in the HS* fragment is followed for the A 3Pi and c 1Pi states by the S++H fragmentation mechanism.     

Localized versus delocalized excitations just above the 3d threshold in krypton clusters studied by Auger electron spectroscopy

We present Auger spectroscopy studies of large krypton clusters excited by soft x-ray photons with energies on and just above the 3d(52) ionization threshold. The deexcitation spectra contain new features as compared to the spectra measured both below and far above threshold. Possible origins of these extra features, which stay at constant kinetic energies, are discussed: (1) normal Auger process with a postcollision interaction induced energy shift, (2) recapture of photoelectrons into high Rydberg orbitals after Auger decay, and (3) excitation into the conduction band (or "internal" ionization) followed by Auger decay. The first two schemes are ruled out, hence internal ionization remains the most probable explanation.     

Auger Ionization Beats Photo‐Oxidation of Semiconductor Quantum Dots: Extended Stability of Single‐Molecule Photoluminescence

Despite the bright and tuneable photoluminescence (PL) of semiconductor quantum dots (QDs), the PL instability induced by Auger recombination and oxidation poses a major challenge in single‐molecule applications of QDs. The incomplete information about Auger recombination and oxidation is an obstacle in the resolution of this challenge. Here, we report for the first time that Auger‐ionized QDs beat self‐sensitized oxidation and the non‐digitized PL intensity loss. Although high‐intensity photoactivation insistently induces PL blinking, the transient escape of QDs into the ultrafast Auger recombination cycle prevents generation of singlet oxygen (¹O₂) and preserves the PL intensity. By the detection of the NIR phosphorescence of ¹O₂ and evaluation of the photostability of single QDs in aerobic, anaerobic, and ¹O₂ scavenger‐enriched environments, we disclose relations of Auger ionization and ¹O₂‐mediated oxidation to the PL stability of single QDs, which will be useful during the formulation of QD‐based single‐molecule imaging tools and single‐photon devices.     

Mechanisms for photogeneration and recombination of multiexcitons in semiconductor nanocrystals: implications for lasing and solar energy conversion

One consequence of strong spatial confinement of electronic wave functions in ultrasmall semiconductor nanocrystals is a great enhancement of carrier-carrier interactions, which has a dramatic effect on the spectral and dynamical properties of both single and multiexciton states. Strong carrier-carrier interactions open new nanocrystal-specific energy relaxation and recombination channels associated, e.g., with electron-hole energy transfer and ultrafast nonradiative Auger recombination. Further, they lead to extremely efficient direct photogeneration of multiple electron-hole pairs (excitons) by single photons known as carrier (or exciton) multiplication. This review focuses on the effect of Coulomb interactions on carrier recombination and photogeneration mechanisms in nanocrystals based on II-VI (e.g., CdSe) and IV-VI (e.g., PbSe) compounds. The specific topics discussed here include the fine structure of the band-edge optical transitions and its effect on temperature-dependent single-exciton recombination dynamics, Auger recombination of multiexcitons in size- and shape-controlled nanocrystals with a specific emphasis on optical-gain properties of nanocrystalline materials (including quantum rods and multicomponent core-shell heterostructures), and the direct generation of multiple excitons via carrier multiplication and its implications in photovoltaic technologies.     

Electron-ion recombination in dense gaseous and liquid argon: effects due to argon cation clusters allow to explain the experimental data

The role of cation clusters in the bulk electron-ion recombination in dense gaseous and liquid argon is investigated. The size and structure of cation clusters in those systems are determined by a Monte Carlo simulation. Then, the rate constants of electron-ion recombination are calculated by another simulation method that takes into account the presence of cation clusters in the considered systems. A good agreement with experiment for both dense gaseous and liquid argon is obtained.     

Transient excitation behavior of a donor–acceptor–acceptor Auger molecule in a semiconductor host

Under the particular situation of highly doped and almost compensated semiconductors, a new kind of bound state happens at high external excitation levels, which is formed of a close donor–acceptor molecule and a neighboring second donor or acceptor. The de-excitation behavior of such a bound state resembles characteristics known from Auger transitions and for this reason it is called an Auger molecule. The existence region of Auger molecules is determined in silicon-doped Ga_{1− x} Al _x As by electron-beam excited luminescence measurements at low temperature. The main peak position and the luminescence intensity of the donor–acceptor recombination channel turn out to be affected in a characteristic manner by the existence of Auger molecules at high excitation levels. An analysis of corresponding rate coefficients for the reproduction of experimental results is also presented. Received: 14 September 1999 / Accepted: 3 February 2000 / Published online: 21 June 2000     

High-efficiency carrier multiplication and ultrafast charge separation in semiconductor nanocrystals studied via time-resolved photoluminescence

We demonstrate novel methods for the study of multiple exciton generation from a single photon absorption event (carrier multiplication) in semiconductor nanocrystals (or nanocrystal quantum dots) that are complementary to our previously reported transient absorption method. By monitoring the time dependence of photoluminescence (PL) from CdSe nanocrystals via time-correlated single photon counting, we find that carrier multiplication is observable due to the Auger decay of biexcitons. We compare these data with similar studies using transient absorption and find that the two methods give comparable results. In addition to the observation of dynamical signatures of carrier multiplication due to the Auger decay, we observe spectral signatures of multiple excitons produced from the absorption of a single photon. PL spectra at short times following excitation with high-energy photons are red-shifted compared to the single-exciton emission band, which is consistent with previous observations of significant exciton-exciton interactions in nanocrystals. We then show using a combination of transient absorption and time-resolved PL studies that charge transfer between a nanocrystal and a Ru-based catalyst model compound takes place on a time scale that is faster than Auger recombination time constants, which points toward a possible design of donor-acceptor assemblies that can be utilized to take advantage of the carrier multiplication process.     

Observation of resonant interatomic coulombic decay in Ne clusters

We have measured the electron spectra of Ne clusters after excitation with photon energies around the 2s inner valence threshold. At two photon energies below threshold, a resonantly enhanced surplus of low kinetic-energy electrons is observed. The kinetic energy of the peak does not vary with the photon energy and is slightly larger than the transition energy of Interatomic Coulombic Decay (ICD) above threshold. This leads us to assume that an ICD-like process is present. In analogy to the Auger and the resonant Auger decay this new phenomenon is termed resonant ICD.     

Multicarrier recombination in colloidal quantum dots

The rates of multicarrier recombination are measured in colloidal quantum dots with and without hole surface trapping and with hole extraction in type II core/shell systems. We report that hole trapping or the physical separation of electronic and hole wavefunctions into different semiconductor domains of a type II system have an insignificant effect on the multicarrier recombination dynamics. These observations are inconsistent with the accepted Auger transition mechanism.     

Nonergodicity in electron capture dissociation investigated using hydrated ion nanocalorimetry

Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H(2)O)(n)(2+), Pathway I occurs exclusively for n >or= 30 whereas Pathway II occurs exclusively for n 相似文献   

Carrier relaxation dynamics in lead sulfide colloidal quantum dots

We report transient absorption saturation measurements on lead sulfide colloidal nanocrystals at the first and second exciton energies and fit the results to a model incorporating intraband and interband relaxation processes. We study in detail the Auger recombination from the first excited state, which takes place when more than one electron-hole pair is excited in a dot. We find an Auger coefficient of 4.5 x 10(-30) cm6/s for dots of 5.5 nm diameter, and observe saturation of the absorption bleaching when the (8-fold degenerate) first level is filled. We develop a model for the absorption dynamics using Poisson statistics and find a good fit with our experimental measurements.     

Photoionization and Auger decay in free atoms,molecules and small clusters

A brief review on photoionization and subsequent Auger decay in atoms, small molecules and clusters is given. Some illustrative examples are presented in order to show the progress in the field, emphasized by the recent high-resolution measurements of Oulu electron spectroscopy group at the MAX synchrotron radiation facility.     

Radiative dielectronic recombination and correlated processes

A new quasi-resonant radiative dielectronic recombination (RDR) process is formulated as an off-shell extension of the dielectronic recombination. It is an electron-ion recombination process in which dielectronic-excitation capture is accompanied by simultaneous radiation emission, and can occur at all continuum energies of the electron being captured. The RDR can be the dominant mode of recombination at off-resonance energy regions, and may provide an important distortion correction to the direct recombination. The distinct signature of the RDR is the quasi-discrete low energy x-rays emitted during the capture, shifted down in energy by the excitation energies of the core electrons. The analog of the RDR must also take place in the ion-atom collisions, as radiative transfer excitation (XTE). Evaluation of the RDR cross section is difficult because of the correlated nature of the transition operator, but detailed qualitative discussions of the RDR are presented. A crude estimate of the cross section is obtained and compared with the other recombination modes. The radiative Auger effect is roughly an inverse of RDR, and the charge dependence of its cross section is improved, using a correlated vertex.     

Measuring the extent and width of internal energy deposition in ion activation using nanocalorimetry

The recombination energies resulting from electron capture by a positive ion can be accurately measured using hydrated ion nanocalorimetry in which the internal energy deposition is obtained from the number of water molecules lost from the reduced cluster. The width of the product ion distribution in these experiments is predominantly attributable to the distribution of energy that partitions into the translational and rotational modes of the water molecules that are lost. These results are consistent with a singular value for the recombination energy. For large clusters, the width of the energy distribution is consistent with rapid energy partitioning into internal vibrational modes. For some smaller clusters with high recombination energies, the measured product ion distribution is narrower than that calculated with a statistical model. These results indicate that initial water molecule loss occurs on the time scale of, or faster than energy randomization. This could be due to inherently slow internal conversion or it could be due to a multi-step process, such as initial ion-electron pair formation followed by reduction of the ion in the cluster. These results provide additional evidence for the accuracy with which condensed phase thermochemical values can be deduced from gaseous nanocalorimetry experiments.     

An XPS study of microporous and mesoporous titanosilicates

In this contribution we report on an XPS study of microporous and mesoporous titanosilicates, in particular microporous titanium silicalite TS‐1, ordered mesoporous Ti‐MCM‐41 and [Ti]‐MCM‐41 and amorphous mesoporous silica–titania (MST) catalysts. Our aim was to obtain both photoemission and x‐ray‐excited Auger data for Ti species on these catalysts and use them in a Ti Wagner plot to rationalize the dependence of the local electronic structure on the atomic environment. Isolated Ti(IV) species coordinated to four and six oxygen anions and segregated TiO₂ clusters were detected on all catalysts by a curve‐fitting procedure of Ti 2p, O 1s and related peaks. The presence of the Si 2p peak excited by an O Kα ghost makes the detection of Ti LMM Auger transitions in mesoporous samples impossible due to the low Ti loadings and its homogeneous distribution in the silica matrix. Small TiO₂ clusters are eventually segregated within the mesopores of the catalysts and not at their external surface. On TS‐1 microporous catalysts with similar Ti loadings to the mesoporous catalysts we were able to detect Ti LMM Auger transitions, and by the Ti Wagner plot we clearly identify the presence of octahedrally coordinated Ti(IV) species. Thus, it is suggested that on TS‐1 the in‐framework (? O)₄Ti species are easily changed to (? O)₄(H₂O)₂Ti species by insertion of water molecules from the atmosphere. Small TiO₂ clusters (diameter <5 nm), eventually present on samples with Ti loading >2 wt.%, are segregated at their external surface and present spectroscopic features similar to (? O)₄(H₂O)₂Ti species. Copyright © 2004 John Wiley & Sons, Ltd.     

Magic Numbers in Boson 4He Clusters: The Auger Evaporation Mechanism

The absence of magic numbers in bosonic ⁴He clusters predicted by all theories since 1984 has been challenged by high-resolution matter-wave diffraction experiments. The observed magic numbers were explained in terms of enhanced growth rates of specific cluster sizes for which an additional excitation level calculated by diffusion Monte Carlo is stabilized. The present theoretical study provides an alternative explanation based on a simple independent particle model of the He clusters. Collisions between cluster atoms in excited states within the cluster lead to selective evaporation via an Auger process. The calculated magic numbers as well as the shape of the number distributions are in quite reasonable agreement with the experiments.