The effect of temperature controlled annealing on the confined valence electron states in CdSe nanocrystal arrays, deposited as thin films, was studied using two-dimensional angular correlation of annihilation radiation (2D-ACAR). A reduction in the intensity by ~35% was observed in a feature of the positron annihilation spectrum upon removal of the pyridine capping molecules above 200 degrees Celsius in a vacuum. This reduction is explained by an increased electronic interaction of the valence orbitals of neighboring nanocrystals, induced by the formation of inorganic interfaces. Partial evaporation of the nanoporous CdSe layer and additional sintering into a polycrystalline thin film was observed at a relatively low temperature of ~486 degrees Celsius.
We have demonstrated the fabrication of highly continuous and smooth CdSe semiconductor films containing self-assembled nanocrystals (NCs) using a simple, low cost solution-processed deposition technique. The impact of thermal annealing and ethanedithiol (EDT) treatment on oleate capped CdSe NCs films is illustrated. Post deposition EDT treatment enhances strong electron coupling between NCs by reducing the inter-particle distance, which enhances four orders of magnitude of photocurrent in the pn-device. Mild thermal annealing of NC films cause large redshift and significant broadening. Our fi…
We studied the rate of spontaneous emission from colloidal CdSe and CdTe nanocrystals at room temperature. The decay rate, obtained from luminescence decay curves, increases with the emission frequency in a supra-linear way. This dependence is explained by the thermal occupation of dark exciton states at room temperature, giving rise to a strong attenuation of the rate of emission. The supra-linear dependence is in agreement with the results of tight-binding calculations.
We report here investigations on the instability in luminescence of bare (TOPO-stabilized) and CdS- capped CdSe particles under infrared radiation. During photo-thermal annealing the formation of oxide layers on the surfaces of the particles create defect states. Consequently there is a reduction in particle size. These two effects control the light output from the samples. We make a quantitative comparison of the stability of bare CdSe and core-shell type CdSe-CdS particles under photo-annealing. Using diffusion theory, we show that the volume of the oxide layer, adhered to the crystallites, …
The distribution of rates of carrier multiplication (CM) following photon absorption is calculated for semiconductor nanocrystals (NCs). The NC electronic structure is described using a screened pseudopotential method known to give reliable description of NC excitons. The rates of biexciton generation are calculated using the Fermi golden rule with all relevant Coulomb matrix elements, taking into account proper selection rules. In CdSe and InAs NCs we find a broad distribution biexciton generation rates depending strongly on the exciton energy and size of the NC. The process becomes inefficie…
Optical and vibrational properties of bare and CdS shelled CdSe nanocrystalline particles are investigated. To confirm the formation of such nanocrystals in our samples we estimate their average particle sizes and size distributions using TEM measurements. From the line profile analysis of the images the core-shell structure in the particles has been confirmed. The blue shift in optical absorption spectra, analyzed using theoretical estimates based on the effective bond order model, establishes the electron confinement in the nanoparticles. Unique characteristics of the nanocrystals (which are…
We report on polarization-resolved resonant photoluminescence (PL) spectroscopy of bright (spin-1) and dark (spin-2) excitons in colloidal CdSe nanocrystal quantum dots. Using high magnetic fields to 33 T, we resonantly excite (and selectively analyze PL from) spin-up or spin-down excitons. At low temperatures (<4K) and above ~10 T, the spectra develop a narrow, circularly polarized peak due to spin-flipped bright excitons. Its evolution with magnetic field directly reveals a large (1-2 meV), intrinsic fine structure splitting of bright excitons, due to anisotropic exchange. These findings are…
Insights to the mechanism of CdSe nanoparticle attachment to carbon nanotubes following the hot injection method are discussed. It was observed that the presence of water improves the nanotube coverage while Cl containing media are responsible for the shape transformation of the nanoparticles and further attachment to the carbon lattice. The experiments also show that the mechanism taking place involves the right balance of several factors, namely, low passivated nanoparticle surface, particles with well-defined crystallographic facets, and interaction with an organics-free sp2 carbon lattice.…
Accurate energy-size dependence of excitonic transitions in semiconductor nanocrystals in the strong confinement regime using classical theoretical approaches such as effective mass approximation, tight binding, or empirical pseudo-potential is difficult. We propose a simple empirical expression with three fitting parameters that accurately relates the size dependence of most known excitonic transitions in CdSe and in InAs nanocrystals. We show that this empirical expression can be deduced from a phase jump approach if the charge carriers are considered to travel on the atomic lattice of the n…
We report an optically gated transistor composed of CdSe nanocrystals (NCs), sensitized with the dye Zinc beta-tetraaminophthalocyanine for operation in the first telecom window. This device shows a high ON/OFF ratio of six orders of magnitude in the red spectral region and an unprecedented 4.5 orders of magnitude at 847 nm. By transient absorption spectroscopy, we reveal that this unexpected infrared sensitivity is due to electron transfer from the dye to the CdSe NCs within 5 ps. We show by time-resolved photocurrent measurements that this enables fast rise times during near-infrared optical…
In an effort to elucidate the spin (rather than charge) degrees of freedom in colloidal semiconductor nanocrystal quantum dots, we report on a series of static and time-resolved photoluminescence measurements of colloidal CdSe quantum dots in ultra-high magnetic fields up to 45 Tesla. At low temperatures (1.5 K - 40 K), the steady-state photoluminescence (PL) develops a high degree of circular polarization with applied magnetic field, indicating the presence of spin-polarized excitons. Time-resolved PL studies reveal a marked decrease in radiative exciton lifetime with increasing magnetic fiel…
Emission from single CdSe nanocrystals in PMMA was investigated. A fraction of the nanocrystals exhibiting switching between two energy states, which have similar total intensities, but distinctly different spectra were observed. We found that the spectral shift characteristic frequency increases with the pump power. By using the dynamic shift in the spectral position of emission peaks, we were able to correlate peaks from the same nanocrystal. The measured correlation is consistent with assignment of low energy lines to phonon replicas.
Although poorly understood, cation-exchange reactions are increasingly used to dope or transform colloidal semiconductor nanocrystals (quantum dots). We used density-functional theory and kinetic Monte Carlo simulations to develop a microscopic theory that explains structural, optical, and electronic changes observed experimentally in Ag-cation-exchanged CdSe nanocrystals. We find that Coulomb interactions, both between ionized impurities and with the polarized nanocrystal surface, play a key role in cation exchange. Our theory also resolves several experimental puzzles related to photolumines…
Quantum Stark effect in semiconductor nanocrystals is theoretically investigated, using the effective mass formalism within a $4\times 4$ Baldereschi-Lipari Hamiltonian model for the hole states. General expressions are reported for the hole eigenfunctions at zero electric field. Electron and hole single particle energies as functions of the electric field ($\mathbf{E}_{QD}$) are reported. Stark shift and binding energy of the excitonic levels are obtained by full diagonalization of the correlated electron-hole Hamiltonian in presence of the external field. Particularly, the structure of the l…
We study the low-temperature magneto-photoluminescence (PL) from individual CdSe nanocrystals. Nanocrystals having a small "bright" exciton fine structure splitting ($<$0.5 meV) exhibit a conventional left- and right-circularly polarized Zeeman PL doublet in applied magnetic fields. In contrast, nanocrystals with large fine structure splitting ($>$1 meV) show an anomalous magneto-PL polarization, wherein the lower-energy peak becomes circularly polarized with increasing field, while the higher-energy peak remains linearly polarized. This unusual behavior arises from strong mixing between the a…
We present a theoretical description of excitons and positively and negatively charged trions in "giant" CdSe/CdS core-shell nanocrystals (NCs). The developed theory provides the parameters describing the fine structure of excitons in CdSe/CdS core/thick shell NCs as a function of the CdSe/CdS conduction band offset and the CdSe core radius. We have also developed a general theory describing the fine structure of positively charged trions created in semiconductor NCs with a degenerate valence band. The calculations take into account the complex structure of the CdSe valence band and inter-part…
Engineering semiconductors to enhance carrier multiplication (CM) could lead to increased photovoltaic cell performance and a significant widening of the materials range suitable for future solar technologies. Semiconductor nanocrystals (NCs) have been proposed as a favourable structure for CM enhancement, and recent measurements by transient absorption have shown evidence for highly efficient CM in lead chalcogenide and CdSe NCs. We report here an assessment of CM yields in CdSe and CdTe NCs by a quantitative analysis of biexciton and exciton signatures in transient photoluminescence decays. …
Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures provide great flexibility via growth-controlled `engineering' of electron and hole wavefunctions within individual nanocrystals. Here we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manga…
CdSe nanocrystals and aggregates of an aryleneethynylene derivative are assembled into a hybrid thin film with dual fluorescence from both fluorophores. Under continuous excitation, the nanocrystals and the molecules exhibit anti-correlated fluorescence intensity variations, which become periodic at low temperature. We attribute this to a structure-dependent aggregation induced emission of the aryleneethynylene derivative, which impacts the rate of excitation energy transfer between the molecules and nanocrystals. Energy transfer also affects the electric transport properties of the hybrid mat…
We report the synthesis and optical characterization of fully inorganic gradientshell CdSe/CdZnS nanocrystals (NCs) with high luminescence quantum yield (QY, 50 percent), which were obtained by replacing native oleic-acid (OA) ligands with halide ions (Br and Cl). Absorption, photoluminescence excitation (PLE) and photoluminescence (PL) spectra in solution were unaffected by the ligand-exchange procedure. The halide-capped NCs were stable in solution for several weeks without modification of their PL spectra; once deposited as unprotected thin films and exposed to air, however, they did show s…
The kinetics and thermodynamics of structural transformations under pressure depend strongly on particle size due to the influence of surface free energy. By suitable design of surface structure, composition, and passivation it is possible, in principle, to prepare nanocrystals in structures inaccessible to bulk materials. However, few realizations of such extreme size-dependent behavior exist. Here we show with molecular dynamics computer simulation that in a model of CdSe/ZnS core/shell nanocrystals the core high pressure structure can be made metastable under ambient conditions by tuning th…
We report the growth of an unstable shell-like gold structure around dihexagonal pyramidal CdSe nanocrystals in organic solution and the structural transformation to spherical domains by two means: i) electron beam irradiation (in situ) and (ii) addition of a strong reducing agent during synthesis. By varying the conditions of gold deposition, such as ligands present or the geometry of the CdSe nanocrystals, we were able to tune the gold domain size between 1.4 nm to 3.9 nm and gain important information on the role of surface chemistry in hetero nanoparticle synthesis and seed reactivity, bot…
The interaction between excitons and phonons in semiconductor nanocrystals plays a crucial role in the exciton energy spectrum and dynamics, and thus in their optical properties. We investigate the exciton2 phonon coupling in giant-shell CdSe/CdS core-shell nanocrystals via resonant Raman spectroscopy. The Huang-Rhys parameter is evaluated by the intensity ratio of the longitudinal-optical (LO) phonon of CdS with its first multiscattering (2LO) replica. We used four different excitation wavelengths in the range from the onset of the CdS shell absorption to well above the CdS shell band edge to…
Surface traps and associated emission in quantum dots (QDs) sought a lot of research attention because of the fundamental interests apart from their influence on the emission characteristics. In ref [Acc. Chem. Res. Vol 43 (2) pp 190, 2010] the hole traps (h-traps) are depicted close to the conduction band (CB) for CdSe QDs while discussing the emission mechanism. However, notably electron traps (e-traps) are close to CB and h-traps are supposedly close to the valance band, especially in anion rich CdSe QDs. Although such emission (so called deep-trap) is a well known phenomenon, the energetic…
Theory of electronic and optical properties of exciton and bi-exciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells ($s$, $p$, ...), whilst the …
The design of efficient electrocatalysts for electrochemical water splitting with minimal amount of precious metal is crucial to attain renewable and sustainable energy conversion. Here, we report the use of a network of CdSe branched colloidal nanocrystals, made of a CdSe core and eight CdSe pods (so-called octapods), able to host on their pods Pt particles, and thus catalyzing water splitting reactions. Thanks to the octapod shape, the resulting Pt-hosting network is mechanically trapped onto carbon nanotube buckypaper, providing mechanically flexible and binder-free electrodes. We found tha…
We study how shape affects multiexciton generation (MEG) rates in a semiconducting nanocrystal by considering CdSe nanorods with varying diameters and aspect ratios. The calculations employ an atomistic semiempirical pseudopotential model combined with an efficacious stochastic approach applied to systems containing up to 20,000 atoms. The effect of nanorod diameter and aspect ratio on multiexciton generation rates is analyzed in terms of the scaling of the density of trion states and the scaling of the Coulomb couplings. Both show distinct scaling from spherical nanocrystals leading to a surp…
We report semi-empirical pseudopotential calculations of emission spectra of charged excitons and biexcitons in CdSe nanocrystals. We find that the main emission peak of charged multiexcitons - originating from the recombination of an electron in an s-like state with a hole in an s-like state - is blue shifted with respect to the neutral mono exciton. In the case of the negatively charged biexciton, we observe additional emission peaks of lower intensity at higher energy, which we attribute to the recombination of an electron in a p state with a hole in a p state.
We present a theory of highly excited interacting carriers confined in a semiconductor nanostructure, incorporating Auger coupling between excited states with different number of excitations. The Coulomb matrix elements connecting exciton, bi-exciton and tri-exciton complexes are derived and an intuitive picture of breaking neutral multi-exction complexes into positively and negatively charged multi-exciton complexes is given. The general approach is illustrated by analyzing the coupling of biexciton and exciton in CdSe spherical nanocrystals. The electron and hole states are computed using at…
We present an experimental demonstration of strong coupling between a surface plasmon propagating on a planar silver substrate, and the lowest excited state of CdSe nanocrystals. Variable-angle spectroscopic ellipsometry measurements demonstrated the formation of plasmon-exciton mixed states, characterized by a Rabi splitting of $\sim$ 82 meV at room temperature. Such a coherent interaction has the potential for the development of plasmonic non-linear devices, and furthermore, this system is akin to those studied in cavity quantum electrodynamics, thus offering the possibility to study the reg…
The temperature and electrical field dependent conductivity of n-type CdSe nanocrystal thin films is investigated. In the low electrical field regime, the conductivity follows G ~ exp(-(T*/T)^0.5) in the temperature range 10K<T<120K. At high electrical field, the conductivity is strongly field dependent. At 4K, the conductance increases by eight orders of magnitude over one decade of bias. At very high field, conductivity is temperature-independent with G ~ exp(-(E*/E)^0.5). The complete behavior is very well described by variable range hopping with Coulomb gap.
A brief review is given of recent positron studies of metal and semiconductor nanocrystals. The prospects offered by positron annihilation as a sensitive method to access nanocrystal (NC) properties are described and compared with other experimental methods. The tunability of the electronic structure of nanocrystals underlies their great potential for application in many areas. Owing to their large surface-to-volume ratio, the surfaces and interfaces of NCs play a crucial role in determining their properties. Here we focus on positron 2D angular correlation of annihilation radiation (2D-ACAR) …
Core-shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that makes them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties, poses a major challenge. This work demonstrates how core-shell CdSe/CdS quantum dot-in-rods can be transferred …
Full Quantum and Mixed Quantum--Classical Dynamics of Hot Exciton Cooling in Semiconductor Nanocrystals
University of California, Berkeley（加州大学伯克利分校）；University of Freiburg（弗赖堡大学）；Universität Heidelberg（海德堡大学）；The Hebrew University of Jerusalem（耶路撒冷希伯来大学）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）
The structure of the electron quantum size levels in spherical nanocrystals is studied in the framework of an eight--band effective mass model at zero and weak magnetic fields. The effect of the nanocrystal surface is modeled through the boundary condition imposed on the envelope wave function at the surface. We show that the spin--orbit splitting of the valence band leads to the surface--induced spin--orbit splitting of the excited conduction band states and to the additional surface--induced magnetic moment for electrons in bare nanocrystals. This additional magnetic moment manifests itself …
Benchmarking Machine-Learning Interatomic Potentials for Dynamical Stability in Inorganic Semiconductor Nanocrystals: A CdSe Case Study
BCMaterials, Basque Center for Materials, Applications, and Nanostructures, UPV/EHU（巴斯克材料、应用与纳米结构中心（BCMaterials））；University of the Basque Country (UPV/EHU)（巴斯克大学）；Ikerbasque Basque Foundation for Science（伊克拉巴斯克科学基金会）
Charge carrier dynamics essentially determine the performance of various optoelectronic applications of colloidal semiconductor nanocrystals. Among them, two-dimensional nanoplatelets provide new adjustment freedom for their unique core/crown heterostructure. Herein, we demonstrate that by fine-tuning the core size and the lateral quantum confinement, the charge carrier transfer rate from the crown to the core can be varied by one order of magnitude in CdSe/CdSeS core/alloy-crown nanoplatelets. In addition, the transfer can be affected by a carrier blocking mechanism, i.e., the filled carriers…
Colloidal cadmium chalcogenide nanoplates are two-dimensional semiconductors that have shown great application prospect for light-emitting technologies. Self-trapped state (STS), a special localized state originated from strong electron-phonon coupling (EPC), has great potential in one-step white light luminance owing to its broadband emission linewidth. However, achieving STS in cadmium chalcogenide nanocrystals is extremely challenging due to their intrinic weak EPC nature. By building hybrid superlattice (SL) structures via self-assembly of colloidal CdSe nanoplates (NPLs), we demonstrated …
The dephasing time of the lowest bright exciton in CdSe/ZnS wurtzite quantum dots is measured from 5 K to 170 K and compared with density dynamics within the exciton fine structure using a sensitive three-beam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the ~10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin-flip from the lowest bright state to dark exciton states. This is…
We report magneto-photoluminescence studies of strongly quantum-confined "0-D" diluted magnetic semiconductors (DMS), realized in Mn$^{2+}$-doped ZnSe/CdSe core/shell colloidal nanocrystals. In marked contrast to their 3-D (bulk), 2-D (quantum well), 1-D (quantum wire), and 0-D (self-assembled quantum dot) DMS counterparts, the ubiquitous yellow emission band from internal \emph{d-d} ($^4T_1 \rightarrow ^6A_1$) transitions of the Mn$^{2+}$ ions in these nanocrystals is \emph{not} suppressed in applied magnetic fields and \emph{does} become circularly polarized. This polarization tracks the Mn$…
We report the synthesis of colloidal Mn2+-doped ZnO (Mn2+:ZnO) quantum dots and the preparation of room-temperature ferromagnetic nanocrystalline thin films. Mn2+:ZnO nanocrystals were prepared by a hydrolysis and condensation reaction in DMSO under atmospheric conditions. Synthesis was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopies. Zn(OAc)2 was found to strongly inhibit oxidation of Mn2+ by O2, allowing the synthesis of Mn2+:ZnO to be performed aerobically. Mn2+ ions were removed from the surfaces of as-prepared nanocrystals using dodecylamine to …
Optical and X-ray Photo-emission Spectroscopies of Core/Shell Colloidal CdSe/CdS Quantum Dots: Modeling and Experimental Determination of Band Alignment
Colloidal quantum dots (cQDs) are now a mature nanomaterial with optical properties customizable through varying size and composition. However, their use in optical devices is limited as they are not widely available in convenient forms such as optical fibers. With advances in polymerization methods incorporating nanocrystals, nanocomposite materials suitable for processing into high quality hybrid active fibers can be achieved. We demonstrate a plastic optical fiber fabrication method which ensures homogeneous dispersion of cQDs within a polymer core matrix. Loading concentrations between 10$…
The surface ligands on colloidal nanocrystals (NCs) play an important role in the performance of NCs based optoelectronic devices such as photovoltaic cells, photodetectors and light emitting diodes (LEDs). On one hand, the NC emission depends critically on the passivation of the surface to minimize trap states that can provide non-radiative recombination channels. On the other hand, the electrical properties of NC films are dominated by the ligands that constitute the barriers for charge transport from one NC to its neighbor. Therefore, surface modifications via ligandexchange have been emplo…
Low power photon upconversion (UC) based on sensitized triplet-triplet annihilation (sTTA) is considered as the most promising upwards wavelength-shifting technique to enhance the light harvesting capability of solar devices by recovering the low-energy tail of the solar spectrum. Semiconductor nanocrystals (NCs) with conjugated organic ligands have been proposed as broadband sensitizers for extending the light-harvesting capability of sTTA-UC molecular absorbers. Key to their functioning is efficient energy transfer (ET) from the NC to the triplet state of the ligands that sensitizes the trip…
