Formation of QD-porphyrin molecule complexes in aqueous solutions

The spectral and luminescent manifestations of the electrostatic formation of quantum dot (QD)-porphyrin complexes are studied. The QD luminescence in these complexes is found to be efficiently quenched. The luminescence of molecules complexed with QDs is also partially quenched. The luminescence excitation spectra of porphyrin molecules associated with QDs exhibit a contribution of the QD absorption spectrum, which indicates that energy is transferred from QDs to porphyrin. The efficiency of the nonradiative resonant energy transfer from a QD to a porphyrin molecule is estimated. The observed experimental data agree well with a proposed model of formation of complexes of the QD-organic molecule type.     

Photoinduced relaxation processes in self-assembling complexes from CdSe/ZnS water-soluble nanocrystals and cationic porphyrins

Particular features and quenching mechanisms of exciton luminescence of water-soluble nanocomposites that are formed as a result of the interaction of surface charged semiconductor quantum dots (QDs) CdSe/ZnS (d _CdSe = 2.8 nm) and cationic porphyrins (H₂TMPyrP⁴⁺ and ZnTMPyrP⁴⁺) have been studied theoretically and experimentally. It has been found that, in CdSe/ZnS??Porphyrin conjugates, there occurs long-range inductive resonance electronic excitation energy transfer from surface modified (with thioglycolic or mercaptoundecanoic acid) QDs to porphyrins, which is accompanied by quenching of the exciton luminescence of QDs and an increase in the fluorescence intensity of porphyrin. It has been shown that, when mercaptoundecanoic acid is used as a QD shell, the QD luminescence quenching efficiency by porphyrins follows the F?rster-Galanin theory and depends on the overlap integral between the CdSe/ZnS luminescence band and the absorption spectra of free-base porphyrin H₂TMPyrP⁴⁺ and its metal complex ZnTMPyrP⁴⁺. It has been revealed that, as the QDs ? Zn-porphyrin intercenter distance decreases from 39.1 (mercaptoundecanoic acid) to 30.1), a considerable QD luminescence quenching is observed; however, the energy transfer efficiency substantially decreases, from 55% in the former case to 23% in the latter one. Based on the spectral-luminescent data and quantum-chemical calculations, it has been found that the chemical change of H₂TMPyrP⁴⁺ in the structure of the complex with CdSe/ZnS QDs passivated by thioglycolic or mercaptoundecanoic acid is caused by the formation of a metal complex ZnTMPyrP⁴⁺. Based on calculations of the redox-potentials, it has been concluded that the low luminescence quantum yield of CdSe/ZnS QDs passivated by residues of mercaptocarboxylic acids S^?(CH₂) _n COO^? and its dependence on the number of CH₂ groups are related to the possibility of photoinduced electron transfer from the HOMO of passivating molecules to QDs (QD* ? S^?(CH₂)nCOO^? hole transfer). It has been shown that the quenching of the exciton luminescence of QDs in heterogeneous structures CdSe/ZnS(thioglycolic acid)??ZnTMPyrP⁴⁺, which is complementary to the energy transfer, can be caused by the photoinduced electron transfer that involves the participation of the LUMO of the ZnTMPyrP⁴⁺ molecule (QD* ? ZnTMPyrP⁴⁺).     

Luminescent Properties of Hybrid Nanostructures Based on Quantum Dots of CdS,Europium 1,3-Diketonate,and Methylene Blue Molecules

The luminescent properties of hybrid nanostructures constructed from colloidal quantum dots (QDs) of CdS passivated with thioglycolic acid, europium(III) tris(tenoyltrifluoroacetonate), and methylene blue dye molecules are studied. Spectral features typical for the formation of core/shell QDs of the CdS/CdS:Eu³⁺ type are found. It is noted that the adsorption of the europium complex at the QD interfaces and the formation of QDs of the CdS/TGA/Eu³⁺ are probable. Spectral patterns that reveal nonradiative energy transfer from the recombination luminescence centers of CdS QDs to the Eu³⁺ ions in the CdS/CdS:Eu³⁺ and CdS/TGA/Eu³⁺ structures are obtained. This is manifested in quenching the recombination luminescence of QDs and in the ignition of the intracentric luminescence of Eu³⁺, which enhance with an increase in the concentration of the europium complex. When such structures are combined with methylene blue molecules, the half-width of the absorption spectra is found to increase by 10–15% with an unchanged position of the absorption band maximum. With an increase in the concentration of methylene blue molecules, decreases in the intensity of the recombination luminescence band of CdS QDs at a wavelength of 530 nm and in the luminescence intensity of Eu³⁺ ions and simultaneously the rise up of the fluorescence of methylene blue at a wavelength of about 675 nm are observed. At the same time, a decrease in the luminescence lifetime of the bands of QDs and europium ions are observed. It is concluded that the nonradiative excitation energy transfer from both the recombination luminescence centers and Eu³⁺ ions to methylene blue molecules takes place.     

Förster resonance energy transfer in hybrid associates of colloidal Ag<Subscript>2</Subscript>S quantum dots with thionine molecules

Nonradiative resonance energy transfer in hydrophilic hybrid associates of thionine molecules (TH⁺) with colloidal Ag₂S quantum dots (QDs) with average diameter of 3.5 nm was studied. Photoluminescence spectra and its decay shown that for these systems the supplemental photosensitization of recombination luminescence of Ag₂S QDs (1200 nm) from the region of TH⁺ fluorescence (618 nm) is possible. It was found that the average lifetime of TH⁺ molecules luminescence is shortened during their association with Ag₂S QDs. Approximation of luminescence decay by stretched exponent with value of parameter β =?0.5 indicates on the inductive-resonance dipole-dipole (Förster) mechanism of nonradiative energy transfer (FRET). The efficiency of FRET was 0.29–0.41.     

Re-charging the luminescence states in CdSe/ZnS quantum dots at the conjugation to Osteopontin antibodies

The paper presents the original study of photoluminescence (PL) and Raman scattering spectra of core–shell CdSe/ZnS quantum dots (QDs) covered by the amine-derivatized polyethylene glycol (PEG) with luminescence interface states. First commercially available CdSe/ZnS QDs with emission at 640 nm (1.94 eV) covered by PEG polymer have been studied in nonconjugated states. PL spectra of nonconjugated QDs are characterized by a superposition of PL bands related to exciton emission in a CdSe core and to the hot electron–hole recombination via high energy luminescence states. The study of high energy PL bands in QDs at different temperatures has shown that these PL bands are related to luminescence interface states at the CdSe/ZnS or ZnS/polymer interface. Then CdSe/ZnS QDs have been conjugated with biomolecules—the Osteopontin antibodies. It is revealed that the PL spectrum of bioconjugated QDs changed essentially with decreasing hot electron–hole recombination flow via luminescence interface states. It is shown that the QD bioconjugation process to Osteopontin antibodies is complex and includes the covalent and electrostatic interactions between them. The variation of PL spectra due to the bioconjugation is explained on the basis of electrostatic interaction between the QDs and biomolecule dipoles that stimulates re-charging QD interface states. The study of Raman scattering of bioconjugated CdSe/ZnS QDs has confirmed that the antibody molecules have the electric dipoles. It is shown that CdSe/ZnS QDs with luminescence interface states are promising for the study of bioconjugation effects with specific antibodies and can be a powerful technique in biology and medicine.     

Photostimulated formation of sensitized anti-Stokes luminescence centers in AgCl(I) microcrystals

The low-temperature photostimulated activation of sensitized anti-Stokes luminescence in heterogeneous systems based on AgCl(I) microcrystals with adsorbed organic dye molecules and their aggregates is investigated. It is shown that the observed considerable (by more than an order of magnitude) enhancement of the intensity of this luminescence is caused by the formation of silver atoms and few-atom clusters on the surface of AgCl(I) microcrystals, which increase the efficiency of a two-quantum excitation of sensitized anti-Stokes luminescence by optical radiation in the range 630–730 nm with the flux density 10¹³–10¹⁵ quantum cm^?2s^?1. Analysis of all the experimental results indicates that the excitation mechanism of anti-Stokes luminescence is based on successive electron transfer or electron-excitation energy transfer from a dye molecule to an atomic-molecular dispersive silver center.     

Accurate analysis of electron transfer from quantum dots to metal oxides in quantum dot sensitized solar cells

Electron transfer rate from quantum dot (QD) to metal oxide (MO) in quantum dot sensitized solar cells (QDSSCs) has an important role in the efficiency. In this work, we analyse the electron transfer rate from CdSe, CdS and CdTe QDs to TiO₂, ZnO and SnO₂ MOs by extending the related equations with considering various effects, based on the Marcus theory. In this regard, the effects of QD diameter, QD–MO spacing, the crystalline defects, temperature, and the reorganizational energy, on the electron transfer rate are investigated. The results show that, the maximum electron transfer rate is achieved for CdTe QD with the mentioned three MOs. Moreover, in order to direct the designer to reach the appropriate QDs–MOs combinations for obtaining the maximum electron transfer rate, the average electron transfer rate for various combinations is calculated. For the verification of simulation method, a part of work has been compared with the previous experimental and theoretical results, which indicates the correctness of our simulation algorithm.     

Spectral-luminescence study of the formation of QD-sulfophthalocyanine molecule complexes in an aqueous solution

The spectral-luminescence manifestations of the formation of quantum dot (QD)-phthalocyanine complexes as a result of electrostatic interaction have been investigated. Effective QD luminescence quenching has been found in complexes of this type. The luminescence of the molecules associated in complexes with QDs is also partially quenched. A mathematical model of the formation of QD-organic molecule complexes is proposed.     

Anti-stokes luminescence of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed organic dye molecules and few-atom silver clusters

For microcrystals of Zn_0.6Cd_0.4S with adsorbed molecules of a number of organic dyes, we have observed sensitized anti-Stokes luminescence excited by radiation with wavelengths in the range 610–750 nm and flux density 10¹⁴–10¹⁵ photons/cm²·sec. The positions of the bands in the excitation spectra for such luminescence match those of the absorption spectra for the adsorbed dye molecules, which is evidence in favor of a cooperative mechanism for its appearance. We have shown that enhancement of the anti-Stokes luminescence is possible when silver atoms and few-atom clusters appear on the Zn_0.6Cd_0.4S surface in addition to the dye molecules. We hypothesize that its excitation in the latter case occurs as a result of two-photon optical transitions. These transitions occur sequentially, with transfer of an electron or the electronic excitation energy from the dye molecules to silver atoms and few-atom clusters adsorbed on the surface of Zn_0.6Cd_0.4S, creating deep localized states in the bandgap with photoionization energies 1.80–2.00 eV. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 5, pp. 617–621, September–October, 2007.     

Spectral Optical Properties of Polymer Composite Nanomaterials Based on Carbon Nanotubes in a High-Density Polyethylene Matrix

Spectral, kinetic, and nonlinear optical regularities that demonstrate the exchange of electronic excitations between the components of hybrid associates of Ag₂S colloid quantum dots (1.7–1.8 nm) in gelatin with molecules of thiazine dyes (Ds) are found. When the IR luminescence of Ag₂S quantum dots (QDs) is excited by radiation from the thionine absorption region, its enhancement due to nonradiative resonant energy transfer is observed. The association with methylene-blue molecules blocked the IR luminescence of Ag₂S QDs upon its excitation by radiation from the absorption region of the dye due to the transfer of charge carriers. It is demonstrated that the hybrid association of thionine molecules and Ag₂S QDs adversely affects the nonlinear optical properties of the latter, which manifests itself in inverse saturated absorption by the action of 10-ns second-harmonic pulses (532 nm) of a Nd³⁺:YAG laser. For the associates of Ag₂S QDs with methylene-blue molecules, the radiation focusing caused by the transfer of charge carriers from the dye and the change in the population of small traps in nanocrystals is found. It is concluded that the direction of the transfer of electronic excitations and the photophysical processes in these objects are determined by the mutual arrangement of the HOMO–LUMO levels of the dye with respect to the levels of dimensional quantization of the Ag₂S QDs.     

Electronic energy alignment at the PbSe quantum dots/ZnO(101̅0) interface

A number of solar energy conversion strategies depend on exciton dissociation across interfaces between semiconductor quantum dots (QDs) and other electron or hole conducting materials. A critical factor governing exciton dissociation and charge transfer in these systems is the alignment of electronic energy levels across the interface. We probe interfacial electronic energy alignment in a model system, sub-monolayer films of PbSe QDs adsorbed on single crystal ZnO(101?0) surfaces using ultraviolet photoemission spectroscopy. We establish electronic energy alignment as a function of quantum dot size and surface chemistry. We find that replacing insulating oleic-acid capping molecules on the QDs by the short hydrazine or ethanedithiol molecules results in pinning of the valence band maximum (VBM) of QDs to ZnO substrate states, independent of QD size. This is in contrast to similar measurements on TiO₂(110) where the alignment of the PbSe QD VBM to that of the TiO₂ substrate depends on QD size. We interpret these findings as indicative of strong electronic coupling of QDs with the ZnO surface but less with the TiO₂ surface. Based on the measured energy alignment, we predict that electron injection from the 1s_e level in photo-excited PbSe QDs to ZnO can occur with small QDs (diameter ? = 3.4 nm), but energetically unfavorably for larger dots (? = 6.7 nm). In the latter, hot electrons above the 1s_e level are necessary for interfacial electron injection.     

Förster Energy Transfer in Arrays of Epitaxial CdSe/ZnSe Quantum Dots Involving Bright and Dark Excitons

Using time-resolved photoluminescence (PL) spectroscopy, we establish the presence of the Förster energy transfer mechanism between two arrays of epitaxial CdSe/ZnSe quantum dots (QDs) of different sizes. The mechanism operates through dipole–dipole interaction between ground excitonic states of the smaller QDs and excited states of the larger QDs. The dependence of energy transfer efficiency on the width of barrier separating the QD insets is shown to be in line with the Förster mechanism. The temperature dependence of the PL decay times and PL intensity suggests the involvement of dark excitons in the energy transfer process.     

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm⁻¹. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm⁻¹, while the ZnTe LO phonon response completely disappears. The 167 cm⁻¹ mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm⁻¹ mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.     

Structural and optical properties of high-density (>10/cm) InAs QDs with varying Al(Ga)As matrix layer thickness

We have obtained high-density (>10¹¹/cm²) InAs quantum dot (QD) structures by using an Al(Ga)As matrix layer. With increase of the AlAs matrix layer thickness, the density of QDs increases a little and the luminescence intensity emitted from InAs QDs decreases. We have used a thin GaAs insertion layer (IL) for the reason of keeping InAs QDs from an aluminum intermixing towards QDs. As the thickness of GaAs IL increases, the density of QDs decreases slightly due to the reduction of the roughness of an AlAs matrix layer. However, the luminescence intensity increases with increase in the thickness of GaAs IL resulting from the efficient blocking of an aluminum intermixing towards QDs.     

Growth and optical characterization of dense arrays of site-controlled quantum dots grown in inverted pyramids

We report on the growth and optical properties of dense arrays of single GaAs/AlGaAs quantum dot (QD) heterostructures with pitches as small as 300 nm. The samples were grown by organometallic chemical vapor deposition in dense inverted pyramids on {1 1 1}B GaAs substrate pre-patterned using electron beam lithography and wet chemical etching. The growth conditions such as deoxidation and growth temperatures, growth rates, and V/III ratio, had to be chosen quite differently from those employed with micron-size pyramids. Low-temperature micro-photoluminescence and cathodoluminescence spectra of the samples show distinct luminescence from the QDs with a linewidth of less than 1 meV and uniform emission energy for an ensemble of 900 QDs. The possibility of incorporating such QD arrays inside optical microcavity structures is also discussed.     

Sensitized anti-Stokes luminescence centers in microcrystals of Zn<Subscript>0.6</Subscript>Cd<Subscript>0.4</Subscript>S solid solutions with adsorbed dye molecules and few-atomic silver clusters

The sensitized anti-Stokes luminescence excited by radiation with wavelengths from 610 to 750 nm and flux densities of 10¹⁴–10¹⁵ quanta/(cm²·s) is detected for microcrystals of Zn _0.6 Cd _0.4 S solid solutions with adsorbed organic malachite green and methylene blue dye molecules. The position of its excitation spectra coincides with that of the absorption spectra of adsorbed dye molecules, which suggests the cooperative mechanism of its occurrence. The possibility of amplification of the anti-Stokes luminescence by means of adsorption of silver atoms and few-atomic silver clusters, in addition to the dye molecules, on the Zn _0.6 Cd _0.4 S surface is investigated. It is assumed that in the latter case, the anti-Stokes luminescence is excited as a result of two-quantum optical transitions with electron or electron excitation energy transfer from the dye molecules adsorbed on the Zn _0.6 Cd _0.4 S surface to silver atoms and few-atomic silver clusters creating deep local states with photoionization energies of 1.8–2.0 eV in the gap. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 21–26, March, 2008.     

Ab Initio Simulation of Charge Transfer at the Semiconductor Quantum Dot/TiO2 Interface in Quantum Dot‐Sensitized Solar Cells

Quantum dot‐sensitized solar cells (QDSSCs) have emerged as a promising solar architecture for next‐generation solar cells. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO₂ acceptor with size quantization properties of QDs that allows for the modulation of band energies to control photoresponse and photoconversion efficiency of solar cells. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs with different sizes on the TiO₂ substrate are simulated using a rigorous ab initio density functional method. This method capitalizes on localized orbital basis set, which is computationally less intensive. Quite intriguingly, a remarkable set of electron bridging states between QDs and TiO₂ occurring via the strong bonding between the conduction bands of QDs and TiO₂ is revealed. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO₂ acceptor, and may be a general feature for strongly coupled donor/acceptor systems. All the QDs/TiO₂ systems exhibit type II band alignments, with conduction band offsets that increase with the decrease in QD size. This facilitates the charge transfer from QDs donors to TiO₂ acceptors and explains the dependence of the increased charge transfer rate with the decreased QD size.     

Enhancement of spin polarization in asymmetrically coupled CdSe and CdZnMnSe quantum dots in ZnSe matrix

An asymmetrically coupled double quantum dot (QD) system consisting of adjacent CdSe and CdZnMnSe QD layers in a ZnSe matrix was investigated using polarization-selective magneto-photoluminescence (PL). Two well-resolved PL peaks are observed corresponding, respectively, to the CdSe and the CdZnMnSe QDs. The peaks exhibit significant change in the intensity and energy position when a magnetic field is applied. The enhancement of the degree of σ⁻ circular polarization emitted by the non-magnetic CdSe QDs is observed in the double layer system, as compared to that observed in CdSe QDs without the influence of neighboring CdZnMnSe QDs. This behavior was discussed in terms of antiferromagnetic interaction between carrier spins localized in pairs of CdSe and CdZnMnSe QDs that are electronically coupled.     

Study of the Optical Properties of CdZnSe/ZnS-Quantum Dot–Au-Nanoparticle Complexes

The interaction of gold nanoparticles (NPs) and semiconductor alloyed CdZnSe/ZnS quantum dots (QDs) in colloidal solutions is studied. It is shown that the photoluminescence intensity of QDs in a mixture decreases compared to that in the initial QD solution, which is caused by resonance nonradiative energy transfer from QDs to Au NPs in spontaneously formed aggregates. To control the formation of pairs of interacting QDs and Au NPs, we proposed have a method for creating QD–Au NP complexes bound by special molecules—ligands. It is shown that the morphology and optical properties of the samples obtained depend on the method of their preparation, in particular, on the chemical environment of QDs. It is found that the complexes form in the case of addition of hydrophilic Au NPs to hydrophobic QDs and that this almost does not change the optical properties of the latter compared to those of quasi-isolated QDs in colloidal solution.     

Optical and electronic properties of quantum dots with magnetic impurities

The article discusses some of the recent results on semiconductor quantum dots with magnetic impurities. A single Mn impurity incorporated in a quantum dot strongly changes the optical response of a quantum-dot system. A character of Mn-carrier interaction is very different for II-VI and III-V quantum dots (QDs). In the II-VI QDs, a Mn impurity influences mostly the spin-structure of an exciton. In the III-V dots, a spatial localization of hole by a Mn impurity can be very important, and ultimately yields a totally different spin structure. A Mn-doped QD with a variable number of mobile carriers represents an artificial magnetic atom. Due to the Mn-carrier interaction, the order of filling of electronic shells in the magnetic QDs can be very different to the case of the real atoms. The “periodic” table of the artificial magnetic atoms can be realized in voltage-tunable transistor structures. For the electron numbers corresponding to the regime of Hund's rule, the magnetic Mn-carrier coupling is especially strong and the magnetic-polaron states are very robust. Magnetic QD molecules are also very different to the real molecules. QD molecules can demonstrate spontaneous breaking of symmetry and phase transitions. Single QDs and QD molecules can be viewed as voltage-tunable nanoscale memory cells where information is stored in the form of robust magnetic-polaron states. To cite this article: A.O. Govorov, C. R. Physique 9 (2008).