Decay Of Quantum Dots Research Articles

Optical temperature sensing of four modes by using CsPb(Cl/Br)3 quantum dots and Tb3+ ions co-doping glass

We have fabricated CsPb(Cl/Br)3 quantum dots (QDs) and Tb3+ co-doping glass and comparatively investigated four-mode temperature sensing of fluorescence intensity ratio (FIR), full width at half maximum (FWHM), lifetime and chromatic coordinate. Temperature dependent photoluminescence (PL) spectra are measured. The Tb3+ ions and CsPb(Cl/Br)3 QDs have independent luminescence and FIR of Tb3+ to QDs has obvious decrease with temperature, which is used for FIR temperature sensing. The PL and PL decay of QDs are used for FWHM and lifetime temperature sensing. The change of FIR with temperature leads to the change of luminous color and chromatic coordinate with temperature. A new chromatic temperature sensing method is established, which utilizes linear relation of chromatic coordinate and temperature. Temperature dependence for FIR, FWHM, lifetime and chromatic coordinate has been fitted. Absolute sensitivity, relative sensitivity, temperature accuracy and repeatability have been evaluated. Among the four sensing modes, the FIR and chromatic coordinate modes have large sensitivity and small accuracy. The FIR mode is better than chromatic coordinate mode in absolute sensitivity and relative sensitivity, whereas the chromatic coordinate mode is better in its good linear relation. The four sensing modes have excellent repeatability due to the stability of QDs-Tb3+ co-doping glass.

Lattice-plasmon effect on entanglement behavior in a plasmonic system

In this study, we designed a plasmonic system containing an array of nanoparticles (NPs) coupled to a quantum dot (QD) to generate entangled photons. The interaction of incoming light with the array of NPs generates a modified plasmon resonance, so-called lattice-plasmon (interaction of the NPs near-field with the photonic mode). Due to its unique optical properties, the lattice-plasmon strongly manipulates the output modes’ entanglement behavior. This is mainly because of the influence of the lattice-plasmon on the transition and dephasing rates of the quantum dot. It is shown that it can be possible to manipulate the QD decay rates via the optical properties of the lattice-plasmon. Also, to manage the output modes entanglement, the emphasis is put on the retarded field effect, which dramatically impacts the lattice-plasmon optical properties. It is theoretically found that engineering the optical properties of the lattice-plasmon facilitates the manipulation of the entanglement behavior.

Excitation wavelength-dependent photoluminescence decay of single quantum dots near plasmonic gold nanoparticles.

Changing the excitation wavelength is a simple but effective strategy to modulate the photophysical cha racteristics of colloidal quantum dots (QDs) near plasmonic nanostructures. It has been observed that the photoluminescence (PL) decay of QDs near plasmonic nanostructures differs when the excitation wavelength is varied, but the exact mechanism is still unclear today. Here, we studied the excitation wavelength dependence of the PL decay of CdSe/CdS core/shell QDs near plasmonic gold nanoparticles at the single QD level. With the aid of statistical science, we demonstrated that the PL decay of a single QD near gold nanoparticles is generally faster when the QD is excited spectrally close to the localized surface plasmon resonance of gold nanoparticles. This excitation wavelength dependence is mainly caused by the varied proportion of photons coming from biexciton emission, which is the result of different local electric field enhancement by gold nanoparticles upon excitation.

Simulation of open quantum systems by automated compression of arbitrary environments

The central challenge for describing the dynamics in open quantum systems is that the Hilbert space of typical environments is too large to be treated exactly. In some cases, such as when the environment has a short memory time or only interacts weakly with the system, approximate descriptions of the system are possible. Beyond these, numerically exact methods exist, but these are typically restricted to baths with Gaussian correlations, such as non-interacting bosons. Here we present a numerically exact method for simulating open quantum systems with arbitrary environments which consist of a set of independent degrees of freedom. Our approach automatically reduces the large number of environmental degrees of freedom to those which are most relevant. Specifically, we show how the process tensor -- which describes the effect of the environment -- can be iteratively constructed and compressed using matrix product state techniques. We demonstrate the power of this method by applying it to problems with bosonic, fermionic, and spin environments: electron transport, phonon effects and radiative decay in quantum dots, central spin dynamics, anharmonic environments, dispersive coupling to time-dependent lossy cavity modes, and superradiance. The versatility and efficiency of our automated compression of environments (ACE) method provides a practical general-purpose tool for open quantum systems.

Quenched or alive quantum dots: The leading roles of ligand adsorption and photoinduced protonation

The fluorescence emission of water-soluble CdTe quantum dots (QDs) capped with mercaptocarboxylic acids (MCAs) is known to be pH-dependent. However, this behaviour is quite different from a study to another, so that literature suffers from a lack of coherence. Here we assume that the QD fluorescence efficiency is actually driven by the acid-base equilibrium of MCA thiol groups, and that light-excited QDs open a non-radiative relaxation path through photoinduced protonation. We address this issue by examining colloidal CdTe QDs with (time-resolved) fluorescence spectroscopy under various conditions of acidity and light excitation. It appears that the emission of QDs is quenched below a critical pH value of 6.87, and that light excitation power strengthens this quenching. We thus demonstrate the existence of an additional photochemical process and developed a mathematical modeling accounting for all our experimental results. With only three parameters, it is possible to accurately predict the fluorescence decay of QDs over time, at any pH. Further, we also related the critical pH value of 6.87 to QD surface properties, explaining why observations may differ from a study to another and making the literature much more coherent.

Photocatalytic properties of hybrid structures based on Titania nanoparticles and semiconductor quantum dots

Titania nanoparticles (NPs) demonstrate the highest photocatalytic activity among metal oxides nanoparticles. A combination of Titania nanoparticles with brightly luminescent semiconductor quantum dots (QDs) makes it possible to obtain highly efficient photocatalytic systems activated by visible light due to photoinduced electron transfer. Multilayered Titania NPs/QDs hybrid structures have been formed using Langmuir–Blodgett technique. Photoluminescent properties of the structures have been analyzed taking into account multiexponential decay of QDs’ photoluminescence and their nonuniform distribution in the structures. It has been shown that luminescent fractions of QDs are strong electron donors for Titania NPs with electron transfer rate $$k_{ET}\ge 4\cdot 10^{10}\, \hbox{s}^{-1}$$. It allows for electron transfer with 85% efficiency. At the same time, it was observed that average electron transfer efficiency in the structures is $$55\pm 5\%$$ because of presence of a dark fraction in QDs ensemble. Our results clearly demonstrate that Titania NPs/QDs hybrid structures are prospective generator of reactive oxygen species under visible light and reducing QDs’ dark fraction should increase average electron transfer efficiency in the structures.

Plasmonic metaresonances: harnessing nonlocal effects for prospective biomedical applications

Metal nanoparticles (MNPs) possess optical concentration capabilities that can amplify and localize electromagnetic fields into nanometer length scales. The near-fields of MNPs can be used to tailor optical response of luminescent semiconductor quantum dots (QDs), resulting in fascinating optical phenomena. Plasmonic metaresonances (PMRs) form a class of such optical events gaining increasing popularity due to their promising prospects in sensing and switching applications. Unlike the basic excitonic and plasmonic resonances in MNP-QD nanohybrids, PMRs occur in the space/time domain. A nanohybrid experiences PMR when system parameters such as QD dipole moment, MNP-QD centre separation or submerging medium permittivity reach critical values, resulting in the plasmonically induced time delay of the effective Rabi frequency experienced by the QD asymptotically tending to infinity. Theoretical analyses of PMRs available in the literature utilize the local response approximation (LRA) which does not account for the nonlocal effects of the MNP, and neglect the MNP dependence of the QD decay and dephasing rates which hinder their applicability to QDs in the close vicinity of small MNPs. Here, we address these limitations using an approach based on the generalized nonlocal optical response (GNOR) theory which has proven to yield successful theoretical explanations of experimentally observed plasmonic phenomena. Our results indicate that, omission of the MNP nonlocal response and the associated decay/dephasing rate modifications of the QD tend to raise implications such as significant over-estimation of the QD dipole moment required to achieve PMR, under-estimation of the critical centre separation and prediction of significantly lower near-PMR QD absorption rates, in comparison to the improved GNOR based predictions. In light of our observations, we finally suggest two prospective applications of PMR based nanoswitches, namely, aptamer based in vitro cancer screening and thermoresponsive polymer based temperature sensing. To demonstrate the latter application, we develop and utilize a proof of concept (two dimensional) skin tumor model homogeneously populated by MNP-QD nanohybrids. Our simulations reveal a novel near-PMR physical phenomenon observable under perpendicular illumination, which we like to call the margin pattern reversal, where the spatial absorption pattern reverses when the near-PMR QDs switch from the bright to dark state.

A bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles.

We describe a bulk adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanoparticles. In this method, we apply a many-body scaling function (in similar manner as in the environment-modified total energy based tight-binding method) to the DFT-derived diatomic AO interaction potentials (like in the conventional orbital-based density-functional tight binding approach) strictly according to atomic valences acquired naturally in a bulk structure. This modification, (a) facilitates all atom orbital-based electronic structure calculations of charge carrier dynamics in nanoscale structures with a molecular acceptor, and (b) allows to closely match high-level density functional calculation data (previously adjusted to the available experimental findings) for bulk structures. To advance practical application of the BA-LCAO approach we parameterize the Hamiltonian of wurtzite CdSe by fitting its band structure to a high-level DFT reference, corrected for experimentally measured band edges. Here, unlike in conventional DFTB approach, we: (1) use hydrogen-like AOs for the basis as exact atomic eigenfunctions, while orbital energies of which are taken from experimentally measured ionization potentials, and (2) parameterize the many-body scaling functions rather than the atomic wavefunctions. Development of this approach and parameters is guided by our goals to devise a method capable of simultaneously treating the problems of (i) interfacial electron/hole transfer between finite, variable size nanoparticles and electron scavenging molecules, and (ii) high-energy electronic transitions (Auger transitions) that mediate multi-exciton decay in quantum dots. Electronic structure results are described for CdSe quantum dots of various sizes. © 2018 Wiley Periodicals, Inc.

Effect of phonon coupling on the cooperative two-photon emission from two quantum dots

We predict dominating cooperative two-photon emission from two quantum dots coupled with a single-mode photonic crystal cavity. The cooperative two-photon emission occurs when excitons in two off-resonantly coupled quantum dots decay simultaneously. The interaction with a common cavity field leads to cavity-induced two-photon emission which is strongly inhibited by electron-phonon coupling. The interaction with a common phonon bath produces phonon-induced two-photon emission which increases on increasing temperature. For identical quantum dots, cavity-induced two-photon emission is negligible, but phonon-induced two-photon emission could be large.

Ultrafast emission decay with high emission efficiency of quantum dots in plasmonic–dielectric metasubstrates

We report on the ultrafast decay of quantum dots that reaches nearly 430 ps without reduction in the emission intensity when near a metasubstrate of simple geometry. By implementing a layered structure of amorphous silicon sandwiched between two gold layers, the balance between plasmonic near field enhancement and energy transfer of the quantum dots has been shifted. This is achieved by tailoring the amount of Förster resonance energy transfer (FRET) to the top Au layer and plasmonic near field enhancement by the bottom Au layer. We also study the impact of deposition of Al oxide on the top Au layer, forming a charge barrier junction that can suppress Auger recombination of quantum dots. The results show that such a barrier can increase emission efficiency of the metastructure without significant reduction in the decay rate. We study the impact of formation of small nanoislands to contiguous islands via variation of the thickness of the top Au layer, and discuss how such morphologies determine the amount of FRET, screening of plasmonic field from the bottom Au layer, and enhancement of emission due to their own plasmonic fields.

Carrier dynamics of strain-engineered InAs quantum dots with (In)GaAs surrounding material

The present study reports on the optical properties of epitaxially grown InAs quantum dots (QDs) inserted within an InGaAs strain-reducing layer (SRL). The critical energy states in such QD structures have been identified by combining photoluminescence (PL) and photoluminescence of excitation (PLE) measurements. Carrier lifetime is investigated by time-resolved photoluminescence (TRPL), allowing us to study the impact of the composition of the surrounding materials on the QD decay time. Results showed that covering the InAs QDs with, or embedding them within, an InGaAs SRL increases the carrier dynamics, while a shorter carrier lifetime has been observed when they are grown on top of an InGaAs SRL. Investigation of the dependence of carrier lifetime on temperature showed good stability of the decay time, deduced from the consequences of improved QD confinement. The findings suggest that embedding or capping the QDs with SRL exerts optimization of their room temperature optical properties.

Modifying the Fluorescence of CdTe Quantum Dots by Silica-Coated Gold Nanorods

The influence of Au nanorods coated with a silica layer on the photoluminescence (PL) of CdTe quantum dots (QDs) was investigated. The Au nanorods were prepared using a seed-mediated method. They were coated with a silica layer on the surface and CdTe QDs were connected to the surface of the silica layers. The PL spectra suggested that the quenching effect of the Au nanorods was decreased by the isolation of the silica layer. Through the time-resolved PL measurement, it was found that the PL decay of QDs became faster. The [email protected]2-CdTe complex could potentially be used in biological sensing and thermal therapy.

Quantum dot spontaneous emission control in a ridge waveguide

We investigate the spontaneous emission (SE) of self-assembled InAs quantum dots (QDs) embedded in GaAs ridge waveguides that lay on a low index substrate. In thin enough waveguides, the coupling to the fundamental guided mode is vanishingly small. A pronounced anisotropy in the coupling to non-guided modes is then directly evidenced by normal-incidence photoluminescence polarization measurements. In this regime, a measurement of the QD decay rate reveals a SE inhibition by a factor up to 4. In larger wires, which ensure an optimal transverse confinement of the fundamental guided mode, the decay rate approaches the bulk value. Building on the good agreement with theoretical predictions, we infer from calculations the fraction β of SE coupled to the fundamental guided mode for some important QD excitonic complexes. For a charged exciton (isotropic in plane optical dipole), β reaches 0.61 at maximum for an on-axis QD. In the case of a purely transverse linear optical dipole, β increases up to 0.91. This optimal configuration is achievable through the selective excitation of one of the bright neutral excitons.

Study on the lifetime decay of quantum dots as a function of temperature.

We have fabricated a temperature sensor which is characterized by fluorescence decay of quantum dots. In our previous work, we showed that the photoluminescence intensity of the quantum dots had negative correlation with the temperature. In this paper, the life time dependence of QDs on temperature was studied. Three kinds of Zn(x)Cd(1-x)S alloyed quantum dots have been prepared and the life time of QDs was measured with temperature changed from 33 to 43 °C. As the temperature increased, the fluorescence lifetime of QDs decreased. This shows that temperature sensor based on life time change of QDs can be fabricated.

A Multipulse Time-Resolved Fluorescence Method for Probing Second-Order Recombination Dynamics in Colloidal Quantum Dots

The ability to generate and utilize multiexcited electronic states in colloidal quantum dots (QDs) is key to a growing range of QD technologies, but the factors that control their radiative and nonradiative recombination dynamics are not yet fully resolved. A significant barrier toward a greater understanding of these species is the fact that their spectroscopic signatures are energetically close to dominant exciton transitions, making it difficult to separate their decay contributions in inhomogeneously broadened ensembles. Here we describe a multipulse technique wherein a controllable number of 80 MHz laser pulses are used to generate different excited state populations, which are then monitored using time-resolved fluorescence. By changing the number of pulses and using a general data analysis method we are able to separate second-order emission generated by absorption of two or more laser pulses from first-order contributions generated by just one pulse. Furthermore, we show that it is possible to determine the nature of the multiexcited state by comparing the second-order emission intensity to models of QD decay dynamics. We find that in our sample of CdSe/CdS core/shell QDs the second-order emission is dominated by emissive trion states rather than biexcitons. Our spectroscopic technique offers a powerful new way to study multiexcited QDs, and the insights that will be gained from this and future studies could be an important step toward harnessing multiexcitons and other multiexcited states in new QD technologies.

Detection of the relaxation rates of an interacting quantum dot by a capacitively coupled sensor dot

We present a theoretical study of the detection of the decay time scales for a single-level quantum dot by means of a capacitively coupled sensor dot, which acts as an electrometer. We investigate the measurement back-action on the quantum-dot decay rates and elucidate its mechanism. We explicitly show that the setup can be used to measure the bare quantum-dot relaxation rates by choosing gate pulses that minimize the back-action. Interestingly, we find that besides the charge relaxation rate, also the rate associated to the fermion parity in the dot can be accessed with this setup.

Electron dynamics of interatomic Coulombic decay in quantum dots induced by a laser field

In this paper we investigated the dynamics of an electron in the presence of a time-dependent laser field in a model potential for a two-level single-electron semiconductor quantum dot (QD) that is capable of undergoing interatomic Coulombic decay (ICD) together with an electron bound to a neighboring QD. We demonstrate that ICD can be initiated by coupling the two-level QD to either a continuous or a pulsed moderate to strong laser and we obtain the total and partial decay widths of the resonance excited state in agreement with that from the solely decay of the resonance [A. Bande, K. Gokhberg, and L. S. Cederbaum, J. Chem. Phys. 135, 144112 (2011)]. A detailed discussion of the effects of direct ionization by the laser in single- or multi-photon process as well as Rabi oscillations is furthermore presented.

Power-law photoluminescence decay in indirect gap quantum dots

In a certain contrast to the standard expectation, according to which the photoluminescence intensity decay in quantum dots could have the mathematical form of exponential function, some experiments indicate a slower type of the intensity decay development, namely in the form of the power-law functional dependence. We are presenting a theoretical interpretation of this phenomenon based on the electron–phonon interaction taken into account in an approximation going beyond the limits of the perturbation theory. We use a simple though quite realistic model of two electronic bound states representing the basic features of the electron in the conduction band states of the InAs small quantum dot. Within this model, the photoluminescence intensity is connected with the electronic level occupation up-conversion process. The electron–phonon coupling is taken into account with the help of the quantum kinetic equations developed with the nonequilibrium Green’s functions.

Microscopic theory of phonon-induced effects on semiconductor quantum dot decay dynamics in cavity QED

We investigate the influence of the electron-phonon interaction on the decay dynamics of a quantum dot coupled to an optical microcavity. We show that the electron-phonon interaction has important consequences on the dynamics, especially when the quantum dot and cavity are tuned out of resonance, in which case the phonons may add or remove energy leading to an effective non-resonant coupling between quantum dot and cavity. The system is investigated using two different theoretical approaches: (i) a second-order expansion in the bare phonon coupling constant, and (ii) an expansion in a polaron-photon coupling constant, arising from the polaron transformation which allows an accurate description at high temperatures. In the low temperature regime we find excellent agreement between the two approaches. An extensive study of the quantum dot decay dynamics is performed, where important parameter dependencies are covered. We find that in general the electron-phonon interaction gives rise to a greatly increased bandwidth of the coupling between quantum dot and cavity. At low temperature an asymmetry in the quantum dot decay rate is observed, leading to a faster decay when the quantum dot has a larger energy than to the cavity. We explain this as due to the absence of phonon absorption processes. Furthermore, we derive approximate analytical expressions for the quantum dot decay rate, applicable when the cavity can be adiabatically eliminated. The expressions lead to a clear interpretation of the physics and emphasizes the important role played by the effective phonon density, describing the availability of phonons for scattering, in quantum dot decay dynamics. Based on the analytical expressions we present the parameter regimes where phonon effects are expected to be important. Also, we include all technical developments in appendices.

Effect of Shell Layers on Luminescence of Colloidal CdSe/Zn<SUB>0.5</SUB>Cd<SUB>0.5</SUB>Se/ZnSe/ZnS Core/Multishell Quantum Dots

Colloidal CdSe/Zn(0.5)Cd(0.5)Se/ZnSe/ZnS core/multishell quantum dots (QDs) were synthesized by using the well developed successive ion layer adsorption and reaction (SILAR) technique. The UV-vis and PL spectra, TEM, X-ray diffraction and Raman measurement were performed to investigate the structure and optical properties of prepared QDs during the growth of shell layers, which indicated that the stress in CdSe core became stronger with the increasing shell thickness. Due to the gradual adjustment of the lattice parameters in the radial direction and the radial increase of the respective valence- and conduction-band offsets, the optical measurements show a significant enhancement in the photoluminescence quantum yield (QY) and an expedited radiative decay in QDs overcoated with thicker shell. The temperature-dependent optical spectra were measured, and the relation between the microstructure and the optical properties of these core/multishell quantum dots was discussed.