Photoexcited Quantum Dots Research Articles

Reactive oxygen species generation from photoexcited quantum dot nanoparticles: Type I versus Type II photochemical mechanism

Reactive oxygen species generation from photoexcited quantum dot nanoparticles: Type I versus Type II photochemical mechanism

Unpredicted electron injection in CdS/CdSe quantum dot sensitized ZrO2 solar cells

A quantum dot sensitized solar cell based on a porous ZrO(2) film, sensitized with CdSe quantum dots using CdS as an intermediate layer is presented. We observe electron injection from photo-excited quantum dots into the ZrO(2), which is unexpected due to the much higher conduction band edge (closer to the vacuum level) of bulk ZrO(2) compared to TiO(2).

Mechanisms of manganese-assisted non-radiative recombination in Cd(Mn)Se/Zn(Mn)Sequantum dots

Mechanisms of non-radiative recombination of electron–hole complexes inCd(Mn)Se/Zn(Mn)Se quantum dots accompanied by interconfigurational excitations ofMn2 + ions are analyzed within the framework of the single-electron model of deep 3d levels insemiconductors. In addition to the mechanisms caused by Coulomb and exchangeinteractions, which are related because of the Pauli principle, another mechanism due tosp–d mixing is considered. It is shown that the Coulomb mechanism reducesto long-range dipole–dipole energy transfer from photoexcited quantum dots toMn2 + ions. The recombination due to the Coulomb mechanism is allowed for any states ofMn2 + ions and e–h complexes. In contrast, short-range exchange and sp–d recombinationsare subject to spin selection rules, which are the result of strong lh–hh splittingof hole states in quantum dots. Estimates show that efficiency of the sp–dmechanism can considerably exceed that of the Coulomb mechanism. Thephonon-assisted recombination and processes involving the upper excited states ofMn2 + ions are studied. The increase in PL intensity of an ensemble of quantum dotsin a magnetic field perpendicular to the sample growth plane observed earlieris analyzed as a possible manifestation of the spin-dependent recombination.

Ni 2+-modulated homocysteine-capped CdTe quantum dots as a turn-on photoluminescent sensor for detecting histidine in biological fluids

The high affinity of histidine to Ni 2+ has long been recognized in metal ion affinity chromatography for the separation and purification of histidine-tagged proteins. Besides, such affinity pair has been explored in modern nanotechnology for constructing functional nanoparticle-histidine-tagged protein conjugates. However, the use of Ni 2+-histidine affinity pair in conjunction with optically-active nanomaterials for sensor design, to our knowledge, has not been reported yet. Here we report a turn-on photoluminescent sensor for histidine based on Ni 2+-modulated homocysteine (Hcy)-capped CdTe quantum dots (QDs) by taking the advantages of this well-known Ni 2+-histidine affinity pair and photoluminescent QDs. The photoluminescence of Hcy-capped CdTe QDs can be effectively quenched by Ni 2+ due to the binding of Ni 2+ to the Hcy on the surface of the QDs and the electron transfer from the photoexcited QDs to Ni 2+. The high affinity of histidine to Ni 2+ enables Ni 2+ to be dissociated from the surface of Hcy-capped CdTe QDs to form stable complex with histidine in solution, thereby recovering the photoluminescence of Hcy-capped CdTe QDs. The Ni 2+ induced photoluminescence quenching and subsequent histidine-induced photoluminescence recovery for Hcy-capped CdTe QDs build a solid base for the present QD-based turn-on photoluminescent sensor for detecting histidine. The developed QD-based sensor gives excellent selectivity for histidine over other amino acids with the limit of detection (3 s) of 0.3 μM. The relative standard deviation for 11 replicate detections of 15 μM histidine was 2.7%. The developed sensor was applied to the determination of histidine in human urine samples with recoveries from 94.4% to 106%.

Radiative recombination in photoexcited quantum dots up to room temperature: The role of fine-structure effects

We demonstrate dominant radiative recombination in high-quality self-assembled quantum dots all the way up to room temperature. This allows a proof of the theoretically predicted characteristic doubling of the radiative recombination time with increasing temperature, entirely caused by fine-structure effects. Over the whole temperature range, the transient decay of the photoluminescence can be described by temperature-independent radiative decay rates of dark and bright excitons, respectively, taking into account a thermally activated interplay between dark and bright states and dark-exciton accumulation.

Relaxation of Exciton Confinement in CdSe Quantum Dots by Modification with a Conjugated Dithiocarbamate Ligand

Coordination of phenyldithiocarbamate (PTC) ligands to solution-phase colloidal CdSe quantum dots (QDs) decreases the optical band gap, E(g), of the QDs by up to 220 meV. These values of DeltaE(g) are the largest shifts achieved by chemical modification of the surfaces of solution-phase CdSe QDs and are-by more than an order of magnitude in energy-the largest bathochromic shifts achieved for QDs in either the solution or solid phases. Measured values of DeltaE(g) upon coordination to PTC correspond to an apparent increase in the excitonic radius of 0.26 +/- 0.03 nm; this excitonic delocalization is independent of the size of the QD for radii, R = 1.1-1.9 nm. Density functional theory calculations indicate that the highest occupied molecular orbital of PTC is near resonant with that of the QD, and that the two have correct symmetry to exchange electron density (PTC is a pi-donor, and the photoexcited QD is a pi-acceptor). We therefore propose that the relaxation of exciton confinement occurs through delocalization of the photoexcited hole of the QD into the ligand shell.

Light Harvesting Semiconductor Core−Shell Nanocrystals: Ultrafast Charge Transport Dynamics of CdSe−ZnS Quantum Dots

Capping or functionalization of semiconductor quantum dots (QDs) is unavoidable for their photostability in practical use including sensitizer and biological tagging agents. However, the efficiency of the electron/hole transport from the photoexcited QDs to the external environments across the capping shell is not well-understood. In this study we report on the femtosecond carrier dynamics of core−shell type CdSe−ZnS semiconductor QDs of various sizes. Steady-state spectroscopic studies followed by picosecond-resolved time correlated single photon counting (TCSPC) experiments on the complexation of the QDs with a well-known electron acceptor, benzoquinone (BQ), reveal that the complex is essentially static in nature. Femtosecond-resolved fluorescence upconversion experiments on the complex explore the dynamics of electron transport from core CdSe to BQ via ZnS shell. The dependence of the electron transport dynamics on the core size of the QDs has also been explored. We have also studied the dynamics of e...

Photoluminescence Quenching of CdSe/ZnS Quantum Dots by Molecular Ferrocene and Ferrocenyl Thiol Ligands

Photoluminescence quenching of CdSe/ZnS quantum dots (QDs) by molecular ferrocene and ferrocenyl thiols was investigated by steady-state absorption and emission spectroscopy. Modification of QD surfaces with ferrocenyl thiols exhibiting different lengths of the alkyl chains (6, 8, and 11 carbon units) was performed by ligand-exchange reaction. These alkyl chains function as spacers between the QDs and ferrocene. Diffusion-filtered NMR spectroscopy was applied to characterize the surface of the modified nanoparticles. Ferrocene and ferrocenyl thiols bound to the QD surface efficiently quenched the QD photoluminescence. Charge transfer between the photoexcited QD and ferrocene is ascribed as possible quenching mechanism. At maximum surface coverage the quenching efficiency of ferrocenyl thiols at the QD surface depends on the hydrocarbon spacer length.

Multiplexed energy transfer mechanisms in a dual-function quantum dot for zinc and manganese

Photoexcited quantum dots (QDs) offer a wealth of mechanisms for interactions with the valence band holes or conduction band electrons, influencing electron-hole recombination. The potential to use combinations of these mechanistic pathways to achieve detection of different metal ions with one modified QD system has been tested. Dual-function water-soluble core/shell-modified CdSe/ZnS quantum dot nanoparticles have been created, that exploit two different fluorescence emission pathways for the detection of two heavy metal ions: Zn(2+) and Mn(2+). A QD-zincon system is proposed, which shows a static Perrin-type quenching mechanism, with sphere of action radii 1.1, 1.3 and 1.6 nm respectively, for 500, 540 and 620 nm QD emission. The QD-zincon system was produced using a layer-by-layer approach: mercaptopropionic acid-capped QDs were modified with a positive polyelectrolyte by electrostatic interaction and then a negatively charged chromogenic reagent, zincon, classically used for the determination of metals. QD-zincon is able to coordinate both Zn(2+) and Mn(2+) and, by exploiting two different mechanisms, QD-zincon conjugates can be tailored to respond to Zn(2+) or Mn(2+). Upon coordination of zincon with Mn(2+), a dramatic enhancement of the fluorescence intensity results as the quenching interaction between zincon and QDs is deactivated, thereby 'switching on the fluorescence emission'. The versatility of this system is demonstrated in terms of fluorescent emission wavelength, which could be selected across a wide range, through choice of QDs (examples are shown for lambda(max) = 500, 540 and 620 nm). In contrast, in the case of Zn(2+) detection, the mechanism is based on the radiationless resonance energy transfer (RET) from QDs acting as donor, to the acceptor zincon-Zn(2+), since its absorption spectra offer adequate overlap with the emission spectra of QD(540) and QD(620), producing a useful analytical signal by the RET process. Using these different operating principles, CdSe/ZnS core/shell QD-zincon conjugates showed very good linearity in the range 10-1000 microM and 5-500 microM for Zn(2+) and Mn(2+) nanosensors, respectively, and RSDs about 3% (n = 10). In a study of interferences, the QD-zincon conjugate showed higher selectivity than the corresponding method with zincon in solution. The results from synthetic ionic mixtures suggested very good applicability in the determination of Zn(2+) and Mn(2+) in samples containing other metal ions, with just a small reduction of sensitivity at very high ionic concentration.

Surface Effects on Quantum Dot-Based Energy Transfer

CdSe quantum dot (QD)-phthalocyanine (Pc) conjugates were prepared as energy transfer donor-acceptor pairs, and the efficiency of the energy transfer process in this system was investigated as a function of QD size and under different surface chemistry conditions. The kinetics and efficiency of the energy transfer process were studied by femtosecond time-resolved laser spectroscopy. We observed that the energy transfer efficiency does not follow a linear dependence on spectral overlap integrals as predicted by the Förster theory for molecules. This observation is found to be due to the involvement of QD surface states in the energy transfer process from the photoexcited QDs to the molecular energy acceptor.

Quenching of Photoluminescence in Conjugates of Quantum Dots and Single-Walled Carbon Nanotube

Development of quantum dot (QD) based device components requires controlled integration of QDs into different photonic and electronic materials. In this regard, introduction of methods for regular arrangement of QDs and investigation of properties of QD-based assemblies are important. In the current work we report (1) controlled conjugation of CdSe-ZnS QDs to sidewall-functionalized single-walled carbon nanotube (SWCNT) templates (2) and the effect of conjugation of QDs to SWCNT on the photoluminescence (PL) properties of QDs. We identified that PL intensity and lifetime of QDs are considerably reduced after conjugation to SWCNT. The origin of the quenching of the PL intensity and lifetime was discussed in terms of Förster resonance energy transfer (FRET). FRET involves nonradiative transfer of energy from a photoexcited QD (energy donor) to a nearby SWCNT (energy acceptor) in the ground state. This was examined by varying the density of QDs on SWCNT and conjugating smaller and bigger QDs to the same SWCNT. We estimated the FRET efficiency in QD-SWCNT conjugates from the quenching of the PL intensity and lifetime and identified that FRET is independent of the density and type of QDs on SWCNT but inherent to QD-SWCNT conjugates.

Comment on “Optical characterization of quantum dots entrained in microstructured optical fibers” [Physica E 26 (2005) 377–381

In a recent paper, K.E. Meissner, C. Holton, W.B. Spillman Jr. [Physica E 26 (2005) 377] presented a study of the optical properties of colloidal CdSe/ZnS core-shell quantum dots entrained in a microstructured optical fiber. In particular, the authors claimed to have observed optical gain in this system through tight evanescent coupling between a probe light in the core of the fiber and photo-excited quantum dots in the cladding. I suggest that there are fundamental reasons, which are well investigated experimentally and documented, why such quantum dots cannot exhibit optical gain phenomena under continuous-wave excitation, as the authors suggest, and that the experimental results and interpretation do not address these issues, nor do they adequately justify the conclusions reached.

Exciton Multiplication and Relaxation Dynamics in Quantum Dots: Applications to Ultrahigh-Efficiency Solar Photon Conversion

Huge amounts of carbon-free energy will be required during the coming decades in order to stabilize atmospheric CO2 to acceptable levels. Solar energy is the largest source of non-carbonaceous energy and can be used to produce both electricity and fuel. However, the ratio of the areal cost to the conversion efficiency for devices converting solar photons to electricity or fuel must be reduced by at least 1 order of magnitude from the present values; this requires large increases in the cell efficiency and large reductions in the cost per unit area. We have shown how semiconductor quantum dots may greatly increase photon conversion efficiencies by producing multiple excitons from a single photon. This is possible because quantization of energy levels in quantum dots slows the cooling of hot excitons, promotes multiple exciton generation, and lowers the photon energy threshold for this process. Quantum yields of 300% for exciton formation in PbSe quantum dots have been reported at photon energies 3.8 times the HOMO-LUMO transition energy, indicating the formation of three excitons/photon for all photoexcited quantum dots. Similar high quantum yields have also been reported for PbS quantum dots. A new model for this effect that is based on a coherent superposition of multiple excitonic states has been proposed.

Optical characterization of quantum dots entrained in microstructured optical fibers

We present a study of the basic optical properties of colloidal CdSe/ZnS core/shell quantum dots entrained in a microstructured optical fiber. Quantum dots suspended in heptane were pulled into the holes surrounding the solid core of a microstructured optical fiber of the holey fiber class via capillary action and are found to remain in the fiber. In this experiment, a laser coupled into the fiber photoexcited quantum dots along the length of the fiber. Quantum dot emission was observed to couple into the fiber core and propagate along the fiber. To investigate the use of such a system in fiber-based light generation or amplification, a second laser overlapping the low-energy portion of the quantum dot emission was simultaneously coupled into the fiber. We observed apparent amplification of this light when photoexciting the quantum dots well above their bandedge.

Electron Transfer Dynamics in Quantum Dot/Titanium Dioxide Composites Formed by in Situ Chemical Bath Deposition

We present experimental transient absorption (TA) results on the dynamics of electron relaxation and electron transfer in cadmium sulfide quantum dots (QDs) grown by chemical bath deposition techniques on nanocrystalline oxide substrates. The quantum dots are prepared in situ in nanocrystalline titanium dioxide (TiO2) and zirconium oxide (ZrO2) films. The conduction band offset between the CdS QDs and TiO2 allows for efficient electron injection from the photoexcited QDs into the conduction band of TiO2. An unprecedented peak is seen in the transient absorption spectrum, which may be used to track the spectral response of the QD/TiO2 composites. Dynamic measurements in the visible and mid-IR are used to evaluate the time scale of the electron injection process. The TA dynamics for these systems are found to be multiexponential. A comparison of the TA dynamics for CdS/TiO2 and CdS/ZrO2 composites in the visible and mid-IR region indicates that electron transfer occurs on the time scale of ∼10−50 ps.

Carrier-carrier correlations in an optically excited single semiconductor quantum dot

We applied low-temperature diffraction-limited confocal optical microscopy to spatially resolve and spectroscopically study photoluminescence from single self-assembled semiconductor quantum dots. Using selective wavelength imaging we unambiguously demonstrated that a single photoexcited quantum dot emits light in a few very narrow spectral lines. The measured spectrum and its dependence on the power of either cw or pulsed excitation are explained by taking carrier correlations into account. We solve numerically a many-body Hamiltonian for a model quantum dot, and we show that the multiline emission spectrum is due to optical transitions between confined exciton multiplexes. We furthermore show that the electron-electron and hole-hole exchange interaction is responsible for the typical appearance of pairs in the photoluminescence spectra and for the appearance of redshifted new lines as the excitation power increases. The fact that only a few spectral lines appear in the emission spectrum strongly indicates fast thermalization. This means that a multiexciton relaxes to its ground state much faster than its radiative lifetime.

Long-Lived Delocalized Electron States in Quantum Dots: A Step-Scan Fourier Transform Infrared Study

Visible light-induced IR absorption in colloidal CdSe quantum dots is observed by step-scan FTIR spectroscopy. Size dependence and the effect of surface modification are investigated. The introduction of surface hole trap states enhances and prolongs the IR absorption in photoexcited quantum dots. Our measurements reveal long-lived (>1 ms) delocalized electron states in 6 nm average diameter dots capped with thiocresol.

Optical Spectroscopy of Single Self Assembled Quantum Dots

ABSTRACTWe applied low temperature diffraction limited confocal optical microscopy to spatially resolve, and spectroscopically study photoluminescence from single self-assembled semiconductor quantum dots. Using selective wavelength imaging we unambiguously demonstrated that a single photoexcited quantum dot emits light in a few very narrow spectral lines. By solving numerically a many body Hamiltonian for a model quantum dot, we show that the multi-line emission spectrum is due to optical transitions between confined exciton multiplexes. We explain the measured spectrum and its dependence on the power of either cw or pulsed excitation by analytically solving the coupled rate equations for the excitation relaxation between these exciton multiplexes.