Multiexciton Absorption Cross Sections of CdSe Nanocrystals at Band-Edge Energy (original) (raw)
Multiexciton Absorption Cross Sections of CdSe Quantum Dots Determined by Ultrafast Spectroscopy
The Journal of Physical Chemistry Letters, 2013
Picosecond transient absorption signals of two kinds of cadmium selenide quantum dots were measured at various excitation intensities. The average number of excitons per quantum dot was calculated from a Poisson model, which together with kinetic parameters was used to determine exciton population kinetics. Exciton and multiexciton absorption cross sections were determined and analyzed in terms of the electronic states of the quantum dots.
Re-examination of the Size-Dependent Absorption Properties of CdSe Quantum Dots
The Journal of Physical Chemistry C, 2009
We investigate the size-dependent optical absorption coefficients of CdSe nanocrystals at both the band-edge and high within the absorption profile. The absorption properties in both of these regions must be selfconsistent to ensure accuracy of the measured coefficients. By combining transmission electron microscopy and inductively coupled plasma-optical emission spectroscopy, we map out the optical absorption properties and establish reliable size-dependent band-edge calibration curves. The measured absorption properties are compared to a simple 0D confinement model, to classical theory based on light absorption by small particles in a dielectric medium and to state-of-the-art atomistic semiempirical pseudopotential modeling. The applicability of these newly established calibration curves is demonstrated by analyzing the nucleation and growth kinetics of CdSe nanocrystals in solution.
Three-photon absorption for nanosecond excitation in cadmium selenide quantum dots
Optical Engineering, 2007
We experimentally observe saturable absorption and dramatic three-photon absorption in various colloidal solutions of cadmium selenide quantum dots, with a strong size dependence witnessed for these properties. We also develop a model for the electronic portion of the nonlinearities that illustrates well the trends exhibited by our experimental data. The model incorporates six bands ͑three each in the valence and conduction bands͒.
Small, 2006
The tuning of CdSe quantum dot (QDs) sizes, and consequently their corresponding two-photon absorption (TPA) cross section, has been systematically investigated. As the size (diameter) of the quantum dots increases, the TPA cross section is found to be empirically related via a power-law proportionality of 3.5 AE 0.5 and 5.6 AE 0.7 to the diameters of CdSe and CdTe QDs, respectively. The results are tentatively rationalized via a theoretical model of two-photon excitation properties in a system incorporating excitons and defects.
Spectroscopic study of oscillator strength and radiative decay time of colloidal CdSe quantum dots
Optical and Quantum Electronics, 2018
Characterization of samples of cadmium selenide quantum dots (CdSe) QDs dissolved in toluene colloidal solutions at a concentration of 1.4 mg/ml was carried out through UV-Vis absorption and photoluminescence (PL) spectroscopy. The size-dependent absorption and red-shifted PL emission peak wavelengths could be tuned between 510-576 and 545-606 nm respectively. Optical absorption spectral measurements yielded CdSe QDs having diameters about * 2.44-3.69 nm with energy gaps 2.32-2.08 eV which are higher than the bulk CdSe (1.74 eV) reminiscent of quantum confinement. This is found to be in good agreement with the semi-empirical pseudopotential model. In addition, the first excitonic absorption transition 1S (e) 1S 3/2(h) oscillator strength and the corresponding fluorescence radiative decay time of CdSe QDs are assessed using relevant Einstein relations for absorption and emission in a two-level system. The elaborated calculations would anticipate that the transition oscillator scale with the CdSe QD radius as * R 2.54. Correspondingly, the calculated radiative decay times decrease from 56.4 to 23.2 ns which scale with CdSe QDs radius as * R-2.155 in fairly good agreement with experimental values reported in the literature.
Band-Edge Exciton in CdSe and Other II–VI and III–V Compound Semiconductor Nanocrystals − Revisited
Nano Letters, 2018
In this letter we summarize major corrections to the dark/bright exciton theory [Al. L. Efros et al, Phys. Rev B 54, 4843-4856 (1996)] which should be used for quantitative description of the band edge exciton in II-VI and III-V compound quantum dot nanocrystals (NCs). The theory previously did not take into account the long-range exchange interaction, resulting in the underestimation of the splitting between the upper bright and lower dark or quasi-dark exciton, as reported by several experimental groups. Another type of correction originates from the closeness in energy of the ground, 1S 3/2 , and the first excited, 1P 3/2 , hole levels in a spherical NC resulting in significant energetic overlap of the levels from the 1S 3/2 1S e and 1P 3/2 1S e exciton manifolds connected with the ground 1S e electron level. The thermal occupation of the optically forbidden 1P 3/2 1S e exciton levels changes the radiative decay time of the NCs both at helium and at room temperatures. We demonstrate the role of both effects in CdSe NCs, and compare our predictions with available experimental data.
Exciton−Trion Transitions in Single CdSe–CdS Core–Shell Nanocrystals
ACS Nano, 2009
We report on the observation of an intermediate state in the blinking of single CdSe/CdS core؊shell nanocrystals. This state has a low quantum yield and connects the "on" and "off" states commonly observed in the photoluminescence blinking of individual nanocrystals. We find that the transitions between these two emitting states follow nearly single-exponential statistics. The transitions from the "on" state to this intermediate state result from changes in the surface passivation of the nanocrystal. The data are consistent with photoinduced, adsorption/desorption events that take place at the surface of the nanocrystals. The trion state leads to a reduction in photoluminescence in nanocrystals.
Size Dependence of the Multiple Exciton Generation Rate in CdSe Quantum Dots
ACS Nano, 2011
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 average much higher excess energy. We also find, contrary to earlier studies, that the effective Coulomb coupling governing carrier multiplication is energy dependent. We show that smaller dots result in a decrease in the carrier multiplication rate for a given absolute photon energy. However, if the photon energy is scaled by the volume dependent optical gap, then smaller dots exhibit an enhancement in carrier multiplication for a given relative energy.
Role of Core–Shell Interfaces on Exciton Recombination in CdSe–Cd x Zn 1– x S Quantum Dots
The Journal of Physical Chemistry C, 2014
The shell thickness and composition of CdSe-Cd x Zn 1-x S core-shell quantum dots (QDs) are defining parameters for the efficiency of such materials as light emitters. In this work we present a detailed study into the optical absorption and fluorescence properties of CdSe-CdS, CdSe-Cd 0.5 Zn 0.5 S, and CdSe-ZnS QDs as a function of shell thickness. Moreover, the single-exciton recombination dynamics of these systems are analyzed by means of a time-correlated single-photon counting technique and directly related to the specific core-shell interfaces of the various QDs studied using a phenomenological kinetic model. The findings from this model highlight the strong role of the core-shell interface on both steady state photoluminescence and exciton recombination dynamics in these systems.
Chemistry of Materials, 2018
Semiconductor nanocrystals are often characterized by complex excited-state dynamics which reflect the inhomogeneous character of ensemble of nanocrystals. A new hybrid inorganic-organic donoracceptor system involving CdSe nanocrystals and paramagnetic nitronyl nitroxide free radicals is shown to lead to efficient Förster (dipolar) resonance energy transfer. This transfer process, which is monitored by steady-state and time-dependent photoluminescence quenching experiments, occurs on a timescale similar to that of the intrinsic recombination in CdSe nanocrystals, allowing to unravel some of the complexity associated with the excited-state photophysics of semiconductor nanocrystals. A Stern-Volmer formalism that can handle the multi-exponential nature of the time-dependent excited-state kinetics of CdSe nanocrystals is developed, leading to excellent agreement between steady-state and time-dependent photoluminescence data when a log-normal distribution model is used for the intrinsinc recombination rate constant and a Poisson distribution for the number of bound quencher per emitter.
Journal of Chemical Physics, 2008
The exciton dynamics of CdSe nanocrystals are intimately linked to the surface morphology. Photo-oxidation of the selenium surfaces of the nanocrystal leads to an increase in radiative decay efficiency from both the band edge and deep trap emission states. The addition of the primary amine hexadecylamine curtails nonradiative excitonic decay attributed to the dangling surface selenium orbitals by passivation of those trap sites by the methylene protons on the amine, leading to enhanced band edge emission and the absence of deep trap emission. Furthermore, CdSe/ ZnSe core/shell nanocrystals are not immune from contributions from surface states because of the alignment of the band structures of the core and shell materials.
Electric-field and exciton structure in CdSe nanocrystals
Physical Review B, 2004
Quantum Stark effect in semiconductor nanocrystals is theoretically investigated, using the effective mass formalism within a 4 × 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 (EQD) 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 lower excitonic states and their symmetry properties in CdSe nanocrystals are studied. It is found that the dependence of the exciton binding energy upon the applied field is strongly reduced for small quantum dot radius. Optical selection rules for absorption and luminescence are obtained. The electric-field induced quenching of the optical spectra as a function of EQD is studied in terms of the exciton dipole matrix element. It is predicted that photoluminescence spectra present anomalous field dependence of the emission lines. These results agree in magnitude with experimental observation and with the main features of photoluminescence experiments in nanostructures.
Optical absorption of CdSe quantum dots on electrodes with different morphology
AIP Advances, 2013
We have studied the optical absorption of CdSe quantum dots (QDs) adsorbed on inverse opal TiO 2 (IO-TiO 2 ) and nanoparticulate TiO 2 (NP-TiO 2 ) electrodes using photoacoustic (PA) measurements. The CdSe QDs were grown directly on IO-TiO 2 and NP-TiO 2 electrodes by a successive ionic layer adsorption and reaction (SILAR) method with different numbers of cycles. The average diameter of the QDs was estimated by applying an effective mass approximation to the PA spectra. The increasing size of the QDs with increasing number of cycles was confirmed by a redshift in the optical absorption spectrum. The average diameter of the CdSe QDs on the IO-TiO 2 electrodes was similar to that on the NP-TiO 2 ones, indicating that growth is independent of morphology. However, there were more CdSe QDs on the NP-TiO 2 electrodes than on the IO-TiO 2 ones, indicating that there were different amounts of active sites on each type of electrode. In addition, the Urbach parameter of the exponential optical absorption tail was also estimated from the PA spectrum. The Urbach parameter of CdSe QDs on IO-TiO 2 electrodes was higher than that on NP-TiO 2 ones, indicating that CdSe QDs on IO-TiO 2 electrodes are more disordered states than those on NP-TiO 2 electrodes. The Urbach parameter decreases in both cases with the increase of SILAR cycles, and it tended to move toward a constant value.
Ukrainian Journal of Physical Optics, 2010
Peculiarities of the absorption and photoluminescence (PL) spectra of colloidal solution of CdSe nanoparticles have been studied in the process of size-selective photoetching. The values of homogeneous broadening and lifetime of resonantly excited exciton states in nanoparticles of definite sizes have been estimated. A model explaining the nature of Stokes shift has been suggested, which takes into account inhomogeneity of energy of the electron-hole pair in the volume of nanoparticle. The Stokes shift values have been determined for different samples. Different changes in the PL spectra observed during the process of photoetching could be caused by the influence of different initial defect structures of the samples.
Wavefunction Engineering of Type-I/Type-II Excitons of CdSe/CdS Core-Shell Quantum Dots
Scientific Reports
Nanostructured semiconductors have the unique shape/size-dependent band gap tunability, which has various applications. The quantum confinement effect allows controlling the spatial distribution of the charge carriers in the core-shell quantum dots (QDs). Upon increasing shell thickness (e.g., from 0.25-3.25 nm) of core-shell QDs, the radial distribution function (RDF) of hole shifts towards the shell suggesting the confinement region switched from Type-I to Type-II excitons. As a result, there is a jump in the transition energy towards the higher side (blue shift). However, an intermediate state appeared as pseudo Type II excitons, in which holes are co-localized in the shell as well core whereas electrons are confined in core only, resulting in a dual absorption band (excitation energy), carried out by the analysis of the overlap percentage using the Hartree-Fock method. The findings are a close approximation to the experimental evidences. Thus, the understanding of the motion of e-h in core-shell QDs is essential for photovoltaic, LEDs, etc. Quantum dots primarily exhibits the confinement effect that leads to the spatial enclosure of the electronic charge carriers within the nanocrystals (NCs) 1. The effect is present when the dimensions of the semiconductor are roughly below the Bohr's exciton radius of that specific enclosure. This quantizes the conduction band (CB) and the valence band (VB) energy levels, nearly continuous for bulk semiconductors. The spacing between the valence band and conduction band can be varied by changing the size or shape of the nanocrystals which gives more monochromatic emission and near unity quantum yield by increasing shell thickness. At strong confinement regime (size below the Bohr's exciton radius) allows for controlling the spatial distribution of carrier wave functions across various domains of a hetero-nanostructure QDs which shows size-dependent absorption and photoluminescence (PL) spectra 2,3 .Within a study carried out, the calculated exciton's binding energy approximates to the reported Bohr's exciton radius of CdS single crystal (2.5 nm) 4 at the shell thickness of 2.4 nm, as obtained in our model. The spatial separation of the electron and hole wave functions within such semiconductor heterostructure 5,6 results in a prolonged charge transfer (CT) states which favours desirable characteristics for applications, such as light emitters, lasers, photo-catalysts and photovoltaic devices 7-12. The charge separation state can be achieved either by direct excitation of the CT state or by photoexcitation of an exciton followed by hole transfer through the core-shell boundary. Investigating the formation and relaxation of the CT state provides information about the excitation and de-excitation processes 13. Recently, the hetero-structuring idea has been implemented by various groups for actuating NCs capability of emitting multi-colour light because of radiative recombination of excitons confined into core/shell domains. These are DiB's (Dot in bulk) semiconductors, which is due to the large size of the shell as compared to the core exhibits dual emission. The primary reason for dual colour emission is believed to be the size of the shell, the less non-radiative transfer of the shell's hole towards the core, enhancing the probability of radiative recombination of the shell's exciton into dual emission 14. Also, the geometric separation of excitons in the nanostructures results in inhibited relaxation processes (exciton cooling and multi-exciton recombination), which gives dual-emission 15. Materials which has small lattice mismatch, the strain-induced localization of carriers are less efficient, as the energy levels in the CB and VB shifts insignificantly 16. This is the case with CdSe/CdS NCs, which are structures having specific properties such as mono-dispersity, narrow emission bandwidth, and high quantum yield 17,18. In shape altered QDs (nano-rods), it has been reported that the smaller CB and VB offset of CdSe favour localization
Nanoscale Research Letters, 2007
Time Resolved Photoluminescence (TRPL) measurements on the picosecond time scale (temporal resolution of 17 ps) on colloidal CdSe and CdSe/ZnS Quantum Dots (QDs) were performed. Transient PL spectra reveal three emission peaks with different lifetimes (60 ps, 460 ps and 9–10 ns, from the bluest to the reddest peak). By considering the characteristic decay times and by comparing the energetic separations among the states
Absorption characteristics of semiconductor quantum dots
1999
Density matrix approach has been employed to study analytically the absorption spectra of small semiconductor quantum dots under strong confinement regime. The results are obtained for a single quantum dot (SQD) as well as for inhomogeneous distribution of quantum dots (IQD) with Gaussian distribution of quantum dot sizes. A numerical analysis has been made for SQD and IQD of CdS crystal with data taken from recent experimental work. A negative change in the absorption coefficient occurs in the shorter pump wavelength side of the spectrum due to the biexcitonic contribution. The wavelength at which crossover from positive to negative values of the change in absorption coefficient occurs is found to depend upon both, the QD size as well as the excitation intensity. The results agree satisfactorily with the experimental observations in small CdS quantum dots.
Fine structure and size dependence of exciton and biexciton optical spectra in CdSe nanocrystals
2010
Abstract: Theory of electronic and optical properties of exciton and biexciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic sp3d5s* 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.
ExcitonâExciton Interaction and Optical Gain in Colloidal CdSe/CdS Dot/Rod Nanocrystals
Advanced Materials, 2009
Semiconductor colloidal nanocrystals have been proposed as optically-active media for solution-processable optoelectronic devices, because they combine inexpensive, wet-chemistry synthesis with high photoluminescence quantum yield, large oscillator strength and size tuneability of optical transitions. Key to the success of nanocrystal-based devices is the possibility to design and consistently synthesize nanocrystals with desired properties. Size uniformity can be usually controlled within less than 5% uncertainty; surface capping, passivation and core/shell structures can lead to photoluminescence quantum yields exceeding 50%, optical gain and lasing. A new frontier in nanocrystal design has appeared with heterostructures allowing spatial separation of electron and hole wavefunctions, like in type-II CdSe/CdTe core/shell nanocrystals, through staggered conduction and valence band offsets. Charge separation inside nanocrystals is useful in photodetector and photovoltaic devices, quantum optics and low-threshold lasers. Exciton nonlinearities also depend on the degree of separation of electron and hole wavefunction. In type-II heterostructures, it has been demonstrated that charge separation can lead to a large repulsive exciton-exciton interaction. The resulting blueshift of the exciton-to-biexciton transition suppresses to a large extent resonant re-absorption of stimulated emission from single-exciton states, allowing net optical gain and lasing at excitations corresponding to less than one electron-hole pair per nanocrystal. In this regime, losses inherent to multiexciton recombinations are avoided, resulting in optical gain with a much longer lifetime, an essential step towards the demonstration of lasing under continuous wave operation.