Excited Quantum Dots Research Articles

Electron Transfer Mediated by Iron Carbonyl Clusters Enhance Light-Driven Hydrogen Evolution in Water by Quantum Dots.

Photocatalytic water splitting has become a promising strategy for converting solar energy into clean and carbon‐neutral solar fuels in a low‐cost and environmentally benign way. Hydrogen gas is such a potential solar fuel/energy carrier. In a classical artificial photosynthetic system, a photosensitizer is generally associated with a co‐catalyst to convert photogenerated charge into (a) chemical bond(s). In the present study, assemblies consisting of CdSe quantum dots that are coupled with one of two molecular complexes/catalysts, that is, [Fe2S2(CO)6] or [Fe3Te2(CO)9], using an interface‐directed approach, have been tested as catalytic systems for hydrogen production in aqueous solution/organic solution. In the presence of ascorbic acid as a sacrificial electron donor and proton source, these assemblies exhibit enhanced activities for the rate of hydrogen production under visible light irradiation for 8 h in aqueous solution at pH 4.0 with up to 110 μmol of H2 per mg of assembly, almost 8.5 times that of pure CdSe quantum dots under the same conditions. Transient absorption and time‐resolved photoluminescence spectroscopies have been used to investigate the charge carrier transfer dynamics in the quantum dot/iron carbonyl cluster assemblies. The spectroscopic results indicate that effective electron transfer from the molecular iron complex to the valence band of the excited CdSe quantum dots significantly inhibits the recombination of photogenerated charge carriers, boosting the photocatalytic activity for hydrogen generation; that is, the iron clusters function as effective intermediaries for electron transfer from the sacrificial electron donor to the valence band of the quantum dots.

Two-colour Three-pulse Spectrally Resolved Nonlinear Spectroscopy for Studies of Dynamics in Semiconductor Si Quantum Dots

We demonstrate that a two-colour three-pulsenonlinear spectroscopy can be used to study the dynamics of excited carriers inSi quantum dot structures embedded in SiN. Decays of the transverse optical phonon population and the transverse acoustic phonon population are measured and discussed. A simple theoretical modelis also used to support interpretation of our experimental observations.

Electron extraction from excited quantum dots with higher order coulomb scattering

The electron kinetics in nanowire-based hot-carrier solar cells is studied, where both relaxation and extraction are considered concurrently. Our kinetics is formulated in the many-particle basis of the interacting system. Detailed comparison with simplified calculations based on product states shows that this includes the Coulomb interaction both in lowest and higher orders. While relaxation rates of 1 ps are obtained, if lowest order processes are available, timescales of tens of ps arise if these are not allowed for particular designs and initial conditions. Based on these calculations we quantify the second order effects and discuss the extraction efficiency, which remains low unless an energy filter by resonant tunnelling is applied.

Nonlinear optical properties of colloidal CdSe/ZnS quantum dots in PMMA

The colloidal CdSe/ZnS quantum dots (QDs) in the PMMA polymer film with different QDs concentrations were fabricated. The influence of QDs concentration and excitation pump energy on nonlinear optical (NLO) properties of PMMA capped CdSe/ZnS QDs was investigated by the Z-scan technique with nanosecond laser pulses in the near-infrared spectral band. A large effective nonlinear absorption coefficient (βeff ∼ −10−13 esu) due to the saturable absorption was observed. It was found that the appropriate concentration could lead to the reinforcement of NLO effect. In addition, the impact of the excitation energy on the nonlinear refractive index n2, real and imaginary parts of the third-order nonlinear optical susceptibility was also performed. This study involving the light–matter interactions in the colloidal quantum dots will benefit potential NLO-based applications of optoelectronics, optical modulation and photonics.

Spin-Controlled Charge-Recombination Pathways across the Inorganic/Organic Interface.

Charge transfer and recombination across the inorganic/organic interface in nanocrystal or quantum dot (QD)-molecule hybrid materials have been extensively studied. Principles of controlling charge transfer and recombination via energetics and electronic coupling have been established. However, the use of electron spin to control transfer and recombination pathways in such systems remains relatively underexplored. Here we use CdS QD-alizarin (AZ) as a model system to demonstrate this principle. Using time-resolved spectroscopy, we found that the charge-separated states (QD--AZ+) created by selectively exciting AZ molecules mostly recombined to regenerate ground-state complexes, whereas apparently the "same" charge separated states created by exciting QDs recombined to produce AZ molecular triplet states. Such a difference can be traced to the distinct spin configurations between excited QDs (QD*, with an ill-defined spin) and AZ (1AZ*, spin singlet) and the asymmetric electron and hole spin-flip rates in II-VI group QDs. The transferability of such a principle was confirmed by similar observations obtained for CdS QD-tetracene complexes. Opening an avenue for controlling charge transfer and recombination pathways via electron spin is potentially important for applications such as artificial photosynthesis.

Retraction: Upconversion luminescent logic gates and turn-on sensing of glutathione based on two-photon excited quantum dots conjugated with dopamine.

Retraction of 'Upconversion luminescent logic gates and turn-on sensing of glutathione based on two-photon excited quantum dots conjugated with dopamine' by Rijun Gui et al., Chem. Commun., 2014, 50, 14847-14850, DOI: .

Luminescence Excitation Spectroscopy of InAs/InGaAs/GaAs Quantum-Dot Arrays in the Temperature Range between 20 and 300 K

Evolution of photoluminescence (PL) and PL excitation spectra in structures containing InAs/InGaAs/GaAs quantum-dot (QD) arrays is investigated in detail as a function of detection energy in the temperature range from 20 to 300 K. Detailed analysis of PL excitation spectra enabled identification of peaks corresponding to excited QD states. The transition probability from the latter to the ground state is higher than in states the transition probability from which to the ground state is low but considerably increases due to effective carrier relaxation involving LO phonons. The dependence of the energy difference of the spectral position of the peaks corresponding to the ground and excited states on ground-state energy (i.e., on QD size) that is characteristic for QDs is found to be violated at 140–160 K, the temperature at which transport of carriers between QDs becomes activated.

Color conversion efficiency enhancement of colloidal quantum dot through its linkage with synthesized metal nanoparticle on a blue light-emitting diode.

Four surface-modified and, hence, positively charged metal nanoparticles (NPs) of different localized surface plasmon (LSP) resonance wavelengths are synthesized for linking with negatively charged, red-emitting colloidal CdZnSeS/ZnS quantum dots (QDs) on the top surface of a blue-emitting InGaN/GaN quantum well (QW) light-emitting diode (LED) through electro-static force. The metal NP-QD linkage leads to a short distance between them for producing their strong surface plasmon (SP) coupling, such that QD absorption and emission can be enhanced. Meanwhile, the small p-GaN thickness in the LED results in strong SP coupling between the LSP resonance of metal NP and the QWs of the LED, leading to enhanced QW emission and, hence, stronger QD excitation. All those factors together result in the increase of the color conversion efficiency of the QD.

A frequency-tunable nanomembrane mechanical oscillator with embedded quantum dots

Hybrid systems consisting of a quantum emitter coupled to a mechanical oscillator are receiving increasing attention for fundamental science and potential applications in quantum technologies. In contrast to most of the presented works in this field, in which the oscillator eigenfrequencies are irreversibly determined by the fabrication process, we present here a simple approach to obtain frequency-tunable mechanical resonators based on suspended nanomembranes. The method relies on a micromachined piezoelectric actuator, which we use both to drive resonant oscillations of a suspended Ga(Al)As membrane with embedded quantum dots and to fine-tune their mechanical eigenfrequencies. Specifically, we excite oscillations with frequencies of at least 60 MHz by applying an AC voltage to the actuator and tune the eigenfrequencies by at least 25 times their linewidth by continuously varying the elastic stress state in the membranes through a DC voltage. The light emitted by optically excited quantum dots is used as a sensitive local strain gauge to monitor the oscillation frequency and amplitude. We expect that our method has the potential to be applicable to other optomechanical systems based on dielectric and semiconductor membranes possibly operating in the quantum regime.

Role of Emissive and Non-Emissive Complex Formations in Photoinduced Electron Transfer Reaction of CdTe Quantum Dots.

Bimolecular photoinduced electron transfer (PET) from excited state CdTe quantum dot (QD*) to an electron deficient molecule 2,4-dinitrotoluene (DNT) is studied in toluene. We observed two types of QD-DNT complex formations; (i) non-emissive complex, in which DNT is embedded deep inside the surface polymer layer of QD and (ii) emissive complex, in which DNT molecules are attached to QDs but approach to the QD core is shielded by polymer layer. Because of its non-emissive nature, the lifetime of QD is not affected by dark complex formation, though the steady-state emission is greatly quenched. However, emissive complex formation causes both, lifetime and steady-state emission quenching. In our fitting model, consideration of Poisson distribution of the attached quencher (DNT) molecules at QD surface enables a comprehensive fitting to our time resolved data. QD-DNT complex formation was confirmed by an isothermal titration calorimetry (ITC) study. Fitting to the time resolved data using a stochastic kinetic model shows moderate increase (0.05 ns-1 to 0.072 ns-1 ) of intrinsic quenching rate with increasing the QD particle size (from ≈3.2 nm to ≈5.2 nm). Our fitting also reveals that the number of DNT molecules attached to a single QD increases from ≈0.1-0.2 to ≈1.2-1.7, as the DNT concentration is increased from ≈1 mm to 17.5 mm. Complex formation at higher quencher concentration assures that the observed PET kinetics is a thermodynamically controlled process where solvent diffusion has no role on it.

Highly indistinguishable single photons from incoherently excited quantum dots

Semiconductor quantum dots are converging towards the demanding requirements of photonic quantum technologies. Among different systems, quantum dots with dimensions exceeding the free-exciton Bohr radius are appealing because of their high oscillator strengths. While this property has received much attention in the context of cavity quantum electrodynamics, little is known about the degree of indistinguishability of single photons consecutively emitted by such dots and on the proper excitation schemes to achieve high indistinguishability. A prominent example is represented by GaAs quantum dots obtained by local droplet etching, which recently outperformed other systems as triggered sources of entangled photon pairs. On these dots, we compare different single-photon excitation mechanisms, and we find (i) a "phonon bottleneck" and poor indistinguishability for conventional excitation via excited states and (ii) photon indistinguishablilities above 90% for both strictly resonant and for incoherent acoustic- and optical-phonon-assisted excitation. Among the excitation schemes, optical phonon-assisted excitation enables straightforward laser rejection without a compromise on the source brightness together with a high photon indistinguishability.

Accessing the bath information in open quantum systems with the stochastic c -number Langevin equation method

In traditional open quantum systems, the baths are usually traced out so that only the system information is left in the equations of motion. However, recent studies reveal that using only the system degrees of freedom can be insufficient. In this work, we develop a stochastic c-number Langevin equation method which can conveniently access the bath information. In our method, the studied quantities are the expectation values of operators which can contain both system operators and bath operators. The dynamics of the operators of interest is formally divided into separate system and bath parts, with auxiliary stochastic fields. After solving the independent stochastic dynamics of the system part and the bath part, we can recombine them by taking the average over these stochastic fields to obtain the desired quantities. Several applications of the theory are highlighted, including the pure dephasing model, the spin-boson model, and an optically excited quantum dot coupled to a bath of phonons.

Efficient fluorescence quenching of CdSe quantum dots on epitaxial GaAs nanostructures

Interaction of CdSe quantum dots (QDs) with epitaxially grown GaAs nanostructures has been studied using photoluminescence (PL) technique. Highly fluorescent CdSe QDs of size 3.9 nm were synthesized by colloidal method and coated over GaAs nanostructures grown on GaAs (111)B substrate by metal organic vapor phase epitaxy (MOVPE) using self-assembled Ga droplets as catalyst. Effect of conditions like catalyst growth time and temperature on the growth of GaAs nanostructures has also been studied. Highly uniform tapered hexagonal nanostructures of height around 500 nm were obtained in nearly 100% yield at 420 °C using Ga droplets grown for 10 s. The fluorescence emission of the CdSe QDs on sample bearing the GaAs nanostructures was measured by steady state and time-resolved photoluminescence (TRPL) techniques and compared with the one obtained on bare substrate. Enhanced quenching of the fluorescence of QDs has been witnessed on the sample in which GaAs nanostructures were present. It has been attributed to more efficient defect-related non-radiative relaxation of excited QDs on the surface of six {110} side facets of the GaAs nanostructures that were not present in the bare substrate. The detailed analysis of the TRPL characteristics of the samples has suggested Forster-like resonance energy transfer (FRET)-based relaxation of CdSe QDs on GaAs nanostructures through shallow traps with significantly reduced average lifetime and the high quenching efficiency.

Cell Imaging Using Two-Photon Excited CdS Fluorescent Quantum Dots Working within the Biological Window.

In recent years, two-photon excited semiconductor quantum dots (QDs) have been the subject of intense investigation due to their long excitation wavelength which helps to achieve deeper penetration and higher image resolution in optical bioimaging. In this paper, water-soluble CdS QDs were synthesized using a hydrothermal method and applied to human liver hepatocellular carcinoma (HepG2) cells. The first-principles calculation suggested that the S-rich defected structure contributes to a narrower band gap compared to the pristine structure. The resulting fluorescence wavelength was significantly red shifted, which was attributed to the deep defect states emission. The large Stokes shifts (> 200 nm) of the QDs can eliminate the possible cross-talk between the excitation light and the emission light. Two-photon induced red fluorescence emission can avoid overlapping with the autofluorescence emission of biological samples. The uptake and cell viability measurements of the HepG2 cells showed a good biocompatibility and a low toxicity of CdS QDs. Two-photon excited scanning microscopy images revealed that the HepG2 cells incubated with CdS QDs emitted bright red upconversion fluorescence and the fluorescence brightness was 38.2 times of that of the control group. These results support CdS QDs as a good candidate for application in cellular imaging.

Coupling O2 and K2S2O8 Dual Co‐reactant with Fe‐N‐C Modified Electrode for Ultrasensitive Electrochemiluminescence Signal Amplification

AbstractThe sensitivity of electrochemiluminescence (ECL) sensor relies on the co‐reactant and the electrode modification materials. Herein, dual co‐reactants (O2 and K2S2O8) and porous carbon with doped N and Fe3O4 (Fe‐N−C) materials modified electrode were proposed for ECL signal amplification in a quantum dots (QDs) ECL system. Firstly, Fe‐N−C materials were synthesized and the effect of pyrolysis temperature on Fe‐N−C materials was evaluated towards ECL intensity. The developed ECL sensor exhibited high sensitivity and specificity towards oxytetracycline (OTC) detection with a detection limit of 0.27 pM (≈ 0.12 ng L−1). Doped N in porous Fe‐N−C materials not only promoted the O2 reduction reaction (ORR) but also accelerated the conversion of Fe2+/Fe3+ in Fe3O4, which catalyzed co‐reactants to their relevant free radicals (SO4⋅− and ⋅OH). Moreover, the decomposition efficiency of K2S2O8 and generation of free radical were promoted through the reaction between OH⋅ and K2S2O8, as well as SO4⋅− and H2O. Therefore, the amounts of excited QDs were increased via the energy and electron transfer process between QDs and free radicals, resulting in a highly enhanced ECL signal. The proposed signal amplification strategy offered an efficient way to improve the catalytic activity of ECL reagents and to boost the sensitivity of ECL sensors.

Imaging of Carbon Nanotube Electronic States Polarized by the Field of an Excited Quantum Dot.

Efficient heat dissipation and large gate capacitance have made carbon nanotube field-effect transistors (CNT FETs) devices of interest for over 20 years. The mechanism of CNT FETs involves localization of the electronic structure due to a transverse electric field, yet little is known about the localization effect, nor has the electronic polarization been visualized directly. Here, we co-deposit PbS quantum dots (QDs) with CNTs and optically excite the QD so its excited-state dipolar field biases the local environment of a CNT. Using single-molecule absorption scanning tunneling microscopy, we show that the electronic states of the CNT become transversely localized. By nudging QDs to different distances from the CNT, the magnitude of the localization can be controlled. Different bias voltages probe the degree of localization in different CNT excited states. A simple tight-binding model for the CNT in an electrostatic field provides a semiquantitative model for the observed behavior.

Metal-enhanced fluorescence in polymer composite films with Au@Ag@SiO2 nanoparticles and InP@ZnS quantum dots.

For white light-emitting diode (LED) applications, semiconductor quantum dots (QDs) have been widely utilized as efficient down-converters to change the blue color of the light source into different emission colors. Because QDs offer spectral tunability over the entire visible light range, as well as improved color purity, they have rapidly replaced conventional phosphor-based white LEDs. However, for the sustainable growth of QD-mediated LEDs, the amount of QDs required must be reduced by enhancing the color-conversion efficiency. For this purpose, we prepared poly(lauryl methacrylate) (PLMA) composite films by the photo-crosslinking polymerization of lauryl methacrylate monomers in the presence of Au@Ag@SiO2 nanoparticles (NPs) and InP@ZnS QDs. In the PLMA composites, the Au@Ag NPs not only amplified the blue light source but also modified the relaxation of the excited QDs via localized surface plasmon resonance. This resulted in a maximum 12.9-fold enhancement in the QD fluorescence. Because the blue light source in this study can be easily replaced by blue LEDs, the enhanced efficiency of QD emissions via the plasmonic effect could potentially increase the performance of QDs for display applications.

Manganese-Doped Zinc Sulfide Quantum Dots for Methane Detection in Aqueous Media

A simple aqueous based synthesis technique at room temperature was performed for preparation manganese-doped zinc sulfide quantum dots. Under 4 eV excitation quantum dots show photoluminescence bands at 2.11 and 3.1 eV corresponded to Mn2+and intrinsic ZnS emission respectively. ZnS quantum dots were used as the luminescent sensing element for methane detection in aqueous media. The luminescent sensor response occurs due to photoinduced electron transfer from QDs to methane molecule resulting in QD luminescence quenching.

Optical properties of bilayer quantum dot models based on coronene and its BN analogues with a BODIPY dye: Theoretical TD-CAM-B3LYP-D3 investigation.

Quantum dots (QD) belong to a class of materials considered technologically important for their tunable absorption and emission properties and a huge application potential in optoelectronic technologies. To date, only simplified monolayer models of QDs and their dimers have been considered when modeling their absorption and/or emission spectra and effects of edge functionalization. Here, we analyze the optical properties of new type electron donor-acceptor bilayer quantum dot models based on coronene and its boron-nitride analogues (electron donors) with a 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dye (Bdp, an electron acceptor) dye by using time-dependent density functional theory. To understand the nature of transition shifts in electronic spectra, we use BN models with strong modification of the HOMO and LUMO: m-BNC (middle hexagonal ring CC bonds are substituted by BN), p-BNC (all peripheral CC bonds are substituted by BN), and f-BNC (all C atoms are replaced by B and N in an alternate manner). Adiabatic ionization potentials of all coronenes were also calculated. The obtained results show that significant interlayer charge transfer (CT) on excitation of bilayer QDs in S1, S2, and S3 states from strong electron donors, coronene (6.93 eV) and m-BNC (6.52 eV), to Bdp will occur. As a result, one can observe only very weak absorption of CT-character in the low-energy spectrum region as well as strong fluorescence quenching of Bdp. On the contrary, for the weak electron donors, p-BNC (7.22 eV) and f-BNC (8.22 eV), strong Bdp-like absorption and fluorescence bands of local excitation character in the low-energy region are expected. Correlation of optical properties of bilayer QDs with ionization potentials of coronene monolayers has been founded. The obtained results can be useful for future graphene-based optoelectronic applications.

Mechanism of Efficient Viologen Radical Generation by Ultrafast Electron Transfer from CdS Quantum Dots

Artificial photosynthetic systems consisted of nanocrystal light absorbers, molecular redox mediators, and catalysts are one of the most promising and flexible approaches for solar fuel generation because their constituents can be independently tuned. In this work, we investigate the photoreduction of three viologen derivatives, one of the most widely investigated molecular redox mediators, of different redox potentials, 7,8-dihydro-6H-dipyrido[1,2-a:2′,1′-c][1,4]diazepinediium (PDQ2+), methyl viologen (MV2+), and benzyl viologen (BV2+), using CdS quantum dots (QDs) as the light absorber and mercaptopropionic acid as a sacrificial electron donor in aqueous (pH = 7) solution. Under continuous 405 nm light-emitting diode illumination, the steady-state radical generation quantum yield (QY) follows the order of PDQ•+ (15.99%) > MV•+ (12.61%) > BV•+ (6.56%). Transient absorption spectroscopy studies show that while the rates of initial electron transfer (ET) from the excited QD conduction band to the mediators...