Optical and X-ray Photo-emission Spectroscopies of Core/Shell Colloidal CdSe/CdS Quantum Dots: Modeling and Experimental Determination of Band Alignment
Damien Simonot, Céline Roux-Byl, Xiangzhen Xu, Willy Daney de Marcillac, Corentin Dabard, Mathieu G Silly, Emmanuel Lhuillier, Thomas Pons, Simon Huppert, Agnès Maître
Optical properties of multilayer semi-conductor nano-emitters are crucially dependent on the relative energy levelsof their different components. For core/shell quantum dots, the relative energy difference between conduction bandedge of core and shell materials induces, depending on its value, either a confinement of the electron within the coreor a delocalization of its wave function within the whole quantum dot. This results in drastic consequences on theenergy and the oscillator strength of the fundamental transition. Surprisingly, the literature currently lacks a definitivevalue for the energy difference between CdSe and CdS conduction band edges as most of the experimental studiesprovide values corresponding to specific geometries of quantum dots. Here, we develop a full theoretical modelexpressing energy levels considering core/shell interface pressure, ligands and enabling the accurate prediction ofthe bandgap value with the nanocrystal size. It allows to reliably determine the energy difference between theconduction band edge of CdSe and CdS materials, known as the conduction band offset, in such a way that this valuecan later be used to model quantum dots of any geometry. This value is determined using our model and two differentexperimental methods: optical spectroscopy and X-ray photoemission (XPS) experiments.
Atomically flat semiconductor nanostructures have sharp photoluminescence emission, short radiative lifetimes and a well-defined planar structure. However, these nanostructures lack the optical and electronic fine-tuning that justify extensive research in colloidal quantum dots. Here, we propose a heterostructure based on CdS nanoplatelets with laterally grown CdSe. Samples of two different thickness and varying ring width are synthesized by a one-pot method during which CdSe ring width is controlled by Se precursor amount. The CdSe dimensionality is continuously tuned from a 0D array of CdSe …
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, …
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…
The size-dependent structures and optical properties of CdSeS nanoclusters in water medium are investigated. The stability of different size-dependent Cd$_n$Se$_m$S$_p$ nanoclusters (up to n=6) is studied using density functional theory/time-dependent density functional theory (DFT/TDDFT). The computed results for ground (S$_0$) and excited (S$_1$, S$_2$, S$_3$) states are experimentally verified through UV-Vis spectroscopy. Computed ab initio results suggest that CdSeS clusters are significantly more hyperpolarizable compared to CdX (X= S, Se, Te) clusters. Structure dependent response proper…
We present a method to realize active optical tips for use in near-field optics that can operate at room temperature. A metal-coated optical tip is covered with a thin polymer layer stained with CdSe nanocrystals or nanorods at low density. The time analysis of the emission rate and emission spectra of the active tips reveal that a very small number of particles - possibly down to only one - can be made active at the tip apex. This opens the way to near-field optics with a single inorganic nanoparticle as a light source.
The vibrational spectra of CdSe/CdS superlattices (SLs) with different layer thicknesses are calculated from first principles within the density functional theory. It is shown that, along with folded acoustic and confined optical modes, a number of confined acoustic modes appear in SLs. In structures with a minimum thickness of one of the layers, microscopic interface modes similar to local and gap modes in crystals appear. An analysis of projections of the eigenvectors of vibrational modes in SLs onto the orthonormal basis of normal modes in binary compounds enables to establish the details o…
Unveiling the local elemental arrangements across the interfaces inside CdSe/Cd1-xZnxS core-shell and CdSe/CdS/ Cd1-xZnxS core-crown-shell quantum wells
We report on the synthesis of CdSe quantum dots on a graphene surface by an electrochemical deposition method. By using a mesoporous silica film formed on the graphene surface as a template and a potential equalizer between the edge/defect sites and the basal plane of the graphene, CdSe quantum dots can be grown on the basal plane into a regular hexagonal array.
The investigation of the possibility of optical refrigeration (OR) on zinc-blende cadmium selenide/cadmium sulfide (CdSe/CdS) core/shell structure quantum dots (QDs) has been carried out. Quality samples were synthesized in our lab, and significant energy up-conversion photoluminescence (UCPL) was observed in these samples, showing the potential of generating net cooling effects. To better understand and predict the UCPL characteristics of the QDs, a semi-empirical model has been developed, showing good agreement with our experimental results. The model takes into account the corresponding qua…
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…
Semiconducting nanoparticles are the building blocks of optical nanodevices as their electronic states, and therefore light absorption and emission, can be controlled by modifying their size and shape. CdSe is perhaps the most studied of these nanoparticles, due to the efficiency of its synthesis, the high quality of the resulting samples, and the fact that the optical gap is in the visible range. In this article, we study light absorption of CdSe nanostructures with sizes up to 1.5 nm within density functional theory. We study both bulk fragments with wurtzite symmetry and novel fullerene-lik…
Metal sulfides are emerging as an important class of materials for photocatalytic applications, because of their high photo responsive nature in the wide visible light range. CdS in this class of materials, have a direct band gap of 2.4 eV, have gained special attention due to the relative position of its conduction band minimum, which is very close to the energies of the reduced protons. However, the photogenerated holes in the valence band of CdS are prone to oxidation and destroy its structure during photocatalysis. Thus constructing a CdS based heterostructure would be an effective strateg…
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 have used the variable stripe technique and pump-probe spectroscopy to investigate both gain and the dynamics of amplified spontaneous emission from CdSe quantum dot structures. We have found modal gain coefficients of 75 and 32 1/cm for asymmetric and symmetric waveguide structures, respectively. Amplified spontaneous emission decay times of 150 and 300 ps and carrier capture times of 15 and 40 ps were measured for the structures with high and low material gains respectively. The difference in the capture times are related to the fact that for the symmetric waveguide, carriers diffuse into…
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 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 study experimentally the photoconductivity of CdS-CdSe sintered granular films obtained by the screen printing method. We mostly focus on the dependences of photoconductivity on film's microstructure, which varies with changing heat-treatment conditions. The maximum photoconductivity is found for samples with compact packing of individual grains, which nevertheless are separated by gaps. Such a microstructure is typical for films heat-treated during an intermediate (optimal) time. In order to understand whether the dominant mechanism of charge transfer is identical with the one in monocryst…
CdSe nanoplatelets show perfectly quantized thicknesses of few monolayers. They present a situation of extreme, yet well defined quantum confinement. Due to large dielectric contrast between the semiconductor and its ligand environment, interaction between carriers and their dielectric images strongly renormalize bare single particle states. We discuss the electronic properties of this original system in an advanced tight-binding model, and show that Coulomb interactions, including self-energy corrections and enhanced electron-hole interaction, lead to exciton binding energies up to several hu…
Junction between metal, superconductor and semiconductor materials seems to be promising for their applications as the electronic devices, microelectronic devices and high power transmission. E-mail shpawar_phy@unishivaji.ac.in.
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 …
The electronic structure of CdSe/CdS core/shell seeded nanorods of experimentally relevant size is studied using a combination of molecular dynamics and semiempirical pseudopotential techniques, with the aim to address the transition from type-I to a quasi-type-II band alignment. The hole is found to be localized in the core region regardless of its size. The overlap of the electron density with the core region depends markedly on the size of the CdSe core: For small cores, we observe little overlap, consistent with type-II behavior. For large cores, significant core-overlap of a number of exc…
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$…
We report fluorescence blinking statistics measured from single CdSe nanorods (NRs) of seven different sizes with aspect ratio ranging from 3 to 11. This study included core/shell CdSe/ZnSe NRs and core NRs with two different surface ligands producing different degrees of surface passivation. We compare the findings for NRs to our measurements of blinking statistics from spherical CdSe core and CdSe/ZnS core/shell nanocrystals (NCs). We find that for both NRs and spherical NCs, the off-time probability distributions are well described by a power law, while the on-time probability distributions…
Using micro- and nano-scale resonantly excited PL and PLE, we study the excitonic structure of CdSe/ZnSe and CdTe/ZnTe self assembled quantum dots (SAQD). Strong resonantly enhanced PL is seen at one to four optic phonon energies below the laser excitation energy. The maximum enhancement is not just one phonon energy above the peak energy distribution of QDs, but rather is 50 meV (for CdSe dots) or 100 meV (for CdTe) above the peak distribution. We interpret this unusual result as from double resonances associated with excited state to ground state energies being commensurate with LO phonons. …
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.
CdSe-Au networks were synthesized by a colloidal chemistry technique. They entail CdSe nanorods with a diameter of ~10 nm and a length of ~40 nm, which are joined together by Au domains (~5 nm). Individual networks were positioned by AC dielectrophoresis between bow-tie electrodes with a gap of ~100 nm and their conductivity as well as the photoelectrical properties were investigated. Nanorod networks, with multiple Au domains on the nanorod surface, displayed stable conductivity that was not sensitive to blue laser light illumination. Such nanostructures were transformed by thermal annealing …
The vibrational spectroscopy of semiconductor nanostructures can provide important information on their structure. In this work, experimental Raman and infrared spectra are compared with vibrational spectra of CdSe/CdS core/shell nanoplatelets calculated from first principles using the density functional theory. The calculations confirm the two-mode behavior of phonon spectra of nanostructures. An analysis of the experimental spectra reveals the absence of modes with a high amplitude of vibrations of surface atoms, which indicates their strong damping. Taking into account the difference in the…
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 evap…
The size-dependent structure of CdSe nanoparticles, with diameters ranging from 2 to 4 nm, has been studied using the atomic pair distribution function (PDF) method. The core structure of the measured CdSe nanoparticles can be described in terms of the wurtzite atomic structure with extensive stacking faults. The density of faults in the nanoparticles ~50% . The diameter of the core region was extracted directly from the PDF data and is in good agreement with the diameter obtained from standard characterization methods suggesting that there is little surface amorphous region. A compressive str…
Using a first-principle approach beyond density functional theory we calculate the electronic and optical properties of small diameter CdSe nanowires.Our results demonstrate how some approximations commonly used in bulk systems fail at this nano-scale level and how indispensable it is to include crystal local fields and excitonic effects to predict the unique optical properties of nanowires. From our results, we then construct a simple model that describes the optical gap as a function of the diameter of the wire, that turns out to be in excellent agreement with experiments for intermediate an…
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…
We study the excitonic structure of CdSe/ZnSe self assembled quantum dots (SAQD) by magneto-photoluminescence (PL) spectroscopy. Using fixed 200 nm apertures through a metal film, we are able to probe single narrow (200 micro-eV) spectroscopic lines emitted from CdSe quantum dots at 2K. Using linear polarization analysis of these lines, we find that approximately half of the quantum dots are elliptically elongated along the [110] direction. The other half of the QDs are symmetric, with emission lines which are completely unpolarized at zero field and exhibit no doublet structure. Using an appl…
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…
The exciton-exciton interaction energy of Tellurium doped CdSe colloidal quantum dots is experimentally investigated. The dots exhibit a strong Coulomb repulsion between the two excitons, which results in a huge measured biexciton blue shift of up to 300 meV. Such a strong Coulomb repulsion implies a very narrow hole wave function localized around the defect, which is manifested by a large Stokes shift. Moreover, we show that the biexciton blue shift increases linearly with the Stokes shift. This result is highly relevant for the use of colloidal QDs as optical gain media, where a large biexci…
We report electroluminescence (EL) measurements carried out on three-terminal devices incorporating individual n-type CdSe nanowires. Simultaneous optical and electrical measurements reveal that EL occurs near the contact between the nanowire and a positively biased electrode or drain. The surface potential profile, obtained by using Kelvin probe microscopy, shows an abrupt potential drop near the position of the EL spot, while the band profile obtained from scanning photocurrent microscopy indicates the existence of an n-type Schottky barrier at the interface. These observations indicate that…
We show that the finite lateral sizes of ultrathin CdSe nanoplatelets strongly affect both their photoluminescence and optical absorption spectra. This is in contrast to the situation in quantum wells, in which the large lateral sizes may be assumed to be infinite. The lateral sizes of the nanoplatelets are varied over a range of a few to tens of nanometers. For these sizes excitons experience in-plane quantum confinement, and their center-of-mass motion becomes quantized. Our direct experimental observation of the discretization of the exciton center-of-mass states can be well understood on t…
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…
We investigate the time-resolved photocurrent response of CdSe quantum dot (QD) thin films sensitized with zinc beta-tetraaminophthalocyanine (Zn4APc) on three different substrates, namely silicon with 230 nm SiO2 dielectric, glass as well as polyimide. While Si/SiO2(230 nm) is not suitable for any transient photocurrent characterization due to an interfering photocurrent response of the buried silicon, we find that polyimide substrates invoke the larger optical bandwidth with 85 kHz vs. 67 kHz for the same quantum dot thin film on glass. Upon evaluation of the transient photocurrent, we find …
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…
Fluorophores emitting in the NIR-IIb wavelength range (1.5 micron - 1.7 micron) show great potential for bioimaging due to their large tissue penetration. However, current fluorophores suffer from poor emission with quantum yields ~2% in aqueous solvents. In this work, we report the synthesis of HgSe/CdSe core/shell quantum dots emitting at 1.7 microns through the interband transition. Growth of a thick shell led to a drastic increase in the photoluminescence quantum yield, with a value of 55% in nonpolar solvents. The quantum yields of our QDs and other reported QDs are explained well by a mo…
We calculate the optical properties of a series of passivated non-stoichiometric CdSe clusters using two first-principles approaches: time-dependent density functional theory within the local density approximation, and by solving the Bethe-Salpeter equation for optical excitations with the GW approximation for the self-energy. We analyze the character of optical excitations leading to the first low-energy peak in the absorption cross-section of these clusters. Within time-dependent density functional theory, we find that the lowest-energy excitation is mostly a single-level to single-level tra…
The multiplication rates of hot carriers in CdSe quantum dots are quantified using an atomistic pseudopotential approach and first order perturbation theory. Both excited holes and electrons are considered, and electron-hole Coulomb interactions are accounted for. We find that holes have much higher multiplication rates than electrons with the same excess energy due to the larger density of final states (positive trions). When electron-hole pairs are generated by photon absorption, however, the net carrier multiplication rate is dominated by photogenerated electrons, because they have on avera…
Having already achieved near-unity quantum yields, with promising properties for light-emitting diode, lasing, and charge separation applications, colloidal core/shell quantum dots have great technological potential. The shell thickness and band alignment of the shell and core materials are known to influence the efficiency of these devices. In many such applications, a key to improving the efficiency requires a deep understanding of multiexcitonic states. Herein, we elucidate the shell thickness and band alignment dependencies of the biexciton Auger recombination lifetime for quasi-type-II Cd…
Excitation energies for wurtzite spherical ZnS and CdSe quantum dots in the range of 40-4000 atoms were calculated using empirical pseudopotentials and a real-space basis. The energies are compared to experiments and other pseudopotential models. For ZnS quantum dots, squared transition dipole sums were computed efficiently, without the need for full wave functions of the excited states; and some transition dipole calculations include the effects of an approximate electron-hole Coulomb potential. Squared transition dipole sums from the highest energy linear dipole like valence states to the lo…
We present a combined experimental and theoretical approach for the determination of the low-temperature valence band offset (VBO) at CdSe/ZnTe heterojunctions with underlying zincblende crystal structure. On the experimental side, the optical transition of the type II interface allows for a precise measurement of the type II band gap. We show how the excitation-power dependent shift of this photoluminescence (PL) signal can be used for any type II system for a precise determination of the VBO. On the theoretical side, we use a refined empirical tight-binding parametrization in order to accura…
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…
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…
We study spin dynamics of excitons confined in self-assembled CdSe quantum dots by means of optical orientation in magnetic field. At zero field the exciton emission from QDs populated via LO phonon-assisted absorption shows a circular polarization of 14%. The polarization degree of the excitonic emission increases dramatically when a magnetic field is applied. Using a simple model, we extract the exciton spin relaxation times of 100 ps and 2.2 ns in the absence and presence of magnetic field, respectively. With increasing temperature the polarization of the QD emission gradually decreases. Re…
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…
A complex InAs/CdSe/ZnSe Core/Shell1/Shell2 (CSS) structure is synthesized, where the intermediate CdSe buffer layer decreases strain between the InAs core and the ZnSe outer shell. This structure leads to significantly improved fluorescence quantum yield as compared to previously prepared core/shell structures and enables growth of much thicker shells. The shell growth is done using a layer-by-layer method in which the shell cation and anion precursors are added sequentially allowing for excellent control and a good size distribution is maintained throughout the entire growth process. The CSS…
CdSe quantum dots (QDs) with different organic linker molecules are attached to ZnO nanowires (NWs) to study the luminescence dynamics and the electron tunneling from the QDs to the nanowires in time-resolved photoluminescence (PL) and photoconductivity measurements. The PL transients of the QD luminescence indicate two different recombination channels: the direct recombination inside the QD core and the recombination via QD surface defect states. After linking the QDs to the ZnO NW surface, photo-induced electron tunneling from an excited state of the QD into the conduction band of the nanowi…
Inverted type-I heterostructure core/crown quantum rings (QRs) are quantum-efficient luminophores, whose spectral characteristics are highly tunable. Here, we study the optoelectronic properties of type-I core/crown CdS/CdSe QRs in the zincblende phase - over contrasting lateral size and crown width. For this we inspect their strain profiles, transition energies, transition matrix elements, spatial charge densities, electronic bandstructure, band-mixing probabilities, optical gain spectra, maximum optical gains and differential optical gains. Our framework uses an effective-mass envelope funct…
Fulfilling the potential of the colloidal semiconductor quantum dots (QDs) in electrically driven applications remains a challenge largely since operation of such devices involves charged QDs with drastically different photo-physical properties compared to their well-studied neutral counterparts. In this work, the full picture of excited state dynamics in charged CdSe QDs at various time-scales has been revealed via transient absorption spectroscopy combined with electrochemistry as direct manipulation tool to control the negative charging of CdSe QDs. In trions, excited states of single charg…
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…
In this work, we have investigated the optical properties of two samples of CdSe quantum dots by using submicro-photoluminescence spectroscopy. The effect of vicinal-surface GaAs substrates on their properties has been also assessed. The thinner sample, grown on a substrate with vicinal surface, includes only dots with a diameter of less than 10 nm (type A islands). Islands of an average diameter of about 16 nm (type B islands) that are related to a phase transition via a Stranski-Krastanow growth process are also distributed in the thicker sample grown on an oriented substrate. We have studie…
