Generation Of Quantum Dots Research Articles

Enhanced THz Generation From Interdigitated Quantum Dot Based Photoconductive Antenna Operating in a Quasi-Ballistic Regime

In this paper, we present a new approach for the enhancement of pulsed terahertz (THz) generation in quantum dot (QD) based photoconductive antennas (PCA). We demonstrate the benefits of the combination of a QD substrate based PCA and an interdigitated electrodes topology which allows the photocarriers to reach the antenna terminals in a quasi-ballistic regime and immediately contribute to the THz emission. A 50-fold increase in the generated THz power is observed. Such enhancement is made possible by unique combination of QD substrate properties, such as very high electric and thermal breakdown ruggedness, high carrier mobility, and yet short carrier lifetimes, compared to typical low temperature grown materials. We expect this solution to become favourable for development of powerful compact THz emitters.

Ultrafast electron dynamics of graphene quantum dots: High harmonic generation

We study theoretically nonlinear optical properties of graphene quantum dots placed in a field of a short and strong linearly polarized optical pulse. We address the problem of high harmonic generation in quantum dots and how such nonlinear effect is affected by dephasing processes in a quantum dot. The dephasing makes the ultrafast electron dynamics more irreversible with a large residual population of the excited quantum dot levels. In relation to the high-harmonic spectrum, with increasing the dephasing time, the intensities of the low-frequency harmonics increase while the cutoff energy decreases. The dependence of the cutoff energy on the amplitude of the optical pulse is also sensitive to the frequency of the pulse. When the frequency of the optical pulse is much less than the quantum dot band gap, this dependence is almost linear, but when the frequency of the pulse is comparable to the band gap, the cutoff energy shows saturation behavior at large field amplitude, $>0.4$ V/\AA{}.

Generation of quantum dots at the semiconductor surface for photoreforming of biomass to CO

In this issue of Chem Catalysis , Wang et al. proposed a new approach by incorporating CdO quantum dots (QDs) to CdS surface by partial oxidation of CdS surface. The catalyst demonstrates efficient biomass reforming to CO by the promotion of separation of electrons and holes at the CdO-QDs/CdS intimate heterojunction. In this issue of Chem Catalysis , Wang et al. proposed a new approach by incorporating CdO quantum dots (QDs) to CdS surface by partial oxidation of CdS surface. The catalyst demonstrates efficient biomass reforming to CO by the promotion of separation of electrons and holes at the CdO-QDs/CdS intimate heterojunction.

Role of metallic leads and electronic degeneracies in thermoelectric power generation in quantum dots

The heat-to-energy conversion efficiency of a thermoelectric nanodevice - which can be quantified by the power factor - is substantially influenced by intrinsic device characteristics in zero-dimensional systems. Using an electroburned graphene quantum dot, the authors demonstrate that controlling the spin degeneracy of the quantum dot and its coupling to the electrodes can increase the power factor considerably. In addition, the adverse effect of non-ideal heat exchanging contacts which suppress the power factor is studied.

Ultrafast multiexciton Auger recombination of CdSeS

Multiexciton generation is a process where multiple excitons are generated by absorbing single photons. Efficient multiexciton generation in quantum dots may be a revolutionary discovery, because it provides a new method to improve the solar-to-electric power conversion efficiency in quantum dots-based solar cells and to design novel quantum dots-based multielectron or hole photocatalysts. However, the mechanism of ultrafast multiexciton generation and recombination remain unclear. In this paper, alloy-structured quantum dots, CdSeS, are prepared by the hot injection method. The generation and recombination mechanism of charge carriers in quantum dots samples are discussed in detail. The bivalent band structure of alloy-structured quantum dots is determined by ultraviolet-visible absorption spectra. It is found that the 1S<sub>3/2</sub>(h)-1S(e) (or 1S), 2S<sub>3/2</sub>(h)-1S(e) (or 2S) and 1P<sub>3/2</sub>(h)-1P(e) (or 1P) exciton absorption bands of these quantum dots are at 510 nm, 468 nm and 430 nm, respectively. Femtosecond transient absorption spectroscopy and nanosecond time-resolved photoluminescence spectroscopy are used to investigate the ultrafast exciton generation and recombination dynamics in the alloy-structured quantum dots. By fitting the transient kinetics of 1S exciton bleach, an average biexciton decay time is obtained to be about 80 ps, which is almost twice the decay time of traditional quantum dots (less than 50 ps). Combined with the recently developed ultrafast interface charge separation technology that can extract multiple excitons before their annihilation, it will have a promising application prospect. Moreover, there is a hole relaxation on a the time scale of 5-6 ps via a phonon coupling pathway to lower-energy hole states in addition to the above-described ultrafast exciton-exciton annihilation process in 2S and 1P excitons. Furthermore, by nanosecond time-resolved photoluminescence spectroscopy, it can be concluded that the charge separated state is long-lived (200 ns). Our findings provide a valuable insight into the understanding of ultrafast multiexciton generation and recombination in quantum dots. These results are helpful to understand the intrinsic photo-physics of multiexciton generation in quantum dots, to implement the photovoltaic and optoelectronic applications, and to ascertain the exciton relaxation dynamics of quantum dots.

High Harmonic Generation in Quantum Dots in Short Pulse

We consider high harmonic generation from a quantum dot (QD) in the presence of chirped laser pulse. The system dynamics is described by coupled equations which are solved numerically for various material parameters of QD and external laser field. Results show the effective controlling effect on the generation of High Harmonic Generation by changing the Chirping parameter of the laser pulse and the size of the QD which otherwise is not possible in other systems.

Bulk Heterojunction Solar Cells Based on Blends of Conjugated Polymers with II⁻VI and IV⁻VI Inorganic Semiconductor Quantum Dots.

Bulk heterojunction solar cells based on blends of quantum dots and conjugated polymers are a promising configuration for obtaining high-efficiency, cheaply fabricated solution-processed photovoltaic devices. Such devices are of significant interest as they have the potential to leverage the advantages of both types of materials, such as the high mobility, band gap tunability and possibility of multiple exciton generation in quantum dots together with the high mechanical flexibility and large molar extinction coefficient of conjugated polymers. Despite these advantages, the power conversion efficiency (PCE) of these hybrid devices has remained relatively low at around 6%, well behind that of all-organic or all-inorganic solar cells. This is attributed to major challenges that still need to be overcome before conjugated polymer–quantum dot blends can be considered viable for commercial application, such as controlling the film morphology and interfacial structure to ensure efficient charge transfer and charge transport. In this work, we present our findings with respect to the recent development of bulk heterojunctions made from conjugated polymer–quantum dot blends, list the ongoing strategies being attempted to improve performance, and highlight the key areas of research that need to be pursued to further develop this technology.

Photovoltaic Properties of CdSe Quantum Dot Sensitized Inverse Opal TiO<sub>2</sub> Solar Cells: The Effect of TiCl<sub>4</sub> Post Treatment

Recently, semiconductor quantum dot (QD) sensitized solar cells (QDSSCs) are expected to achieve higher conversion efficiency because of the large light absorption coefficient and multiple exciton generation in QDs. The morphology of TiO2 electrode is one of the most important factors in QDSSCs. Inverse opal (IO) TiO2 electrode, which has periodic mesoporous structure, is useful for QDSSCs because of better penetration of electrolyte than conventional nanoparticulate TiO2 electrode. In addition, the ordered three dimensional structure of IO-TiO2 would be better for electron transport. We have found that open circuit voltage Voc of QDSSCs with IO-TiO2 electrodes was much higher (0.2 V) than that with nanoparticulate TiO2 electrodes. But short circuit current density Jsc was lower in the case of IO-TiO2 electrodes because of the smaller surface area of IO-TiO2. In this study, for increasing surface area of IO-TiO2, we applied TiCl4 post treatment on IO-TiO2 and investigated the effect of the post treatment on photovoltaic properties of CdSe QD sensitized IO-TiO2 solar cells. It was found that Jsc could be enhanced due to TiCl4 post treatment, but decreased again for more than one cycle treatment, which indicates excess post treatment may lead to worse penetration of electrolyte. Our results indicate that the appropriate post treatment can improve the energy conversion efficiency of the QDSSCs.

Distarch Phosphate as a Matrix for the Generation of Quantum Dots

Generation of ZnS, CdS, CaS and MgS quantum dots (QDs) was attempted in the matrix of distarch phosphate, an anionic polysaccharide. Based on the analysis of FTIR spectra, thermograms and Differential Scanning Calorimetry, interactions of QDS with the matrix could be excluded. On generation of MgS QDs and also, to a certain extent, CaS QDs, hydrolysis of the salts generating these QDs took place. That process had an impact on the partial destruction of the matrix. Molecular weight and radii of gyration of the products showed that the generation of QDs was dependent on the properties of the matrix polysaccharide. The elasticity, plasticity and viscosity of the gels containing QDs were significantly lower than those of plain distarch phosphate. Generally these values changed with time similarly to gels free of QDs.

The role of the fano resonance in multiple exciton generation in quantum dots

The phenomenon of interference between two pathways of electron transfer from the valence to the conduction band at a quantum dot is considered. The first way is the conventional “valence band–conduction band” transition, while the second is the transition via a virtual two-electron state on the Tamm level in a quantum dot (QD) followed by the Auger effect, which ejects one electron from the Tamm level to the conduction band. In the case of a coherent addition of these ionization pathways, the Fano resonance can take place, this leading to an increase in the coefficient of photon absorption. This results in increasing internal efficiency of light conversion and can provide a basis for increasing the efficiency of solar cells due to the phenomenon of multiple exciton generation.

Multiple exciton generation in quantum dots versus singlet fission in molecular chromophores for solar photon conversion.

Both multiple exciton generation (MEG) in semiconductor nanocrystals and singlet fission (SF) in molecular chromophores have the potential to greatly increase the power conversion efficiency of solar cells for the production of solar electricity (photovoltaics) and solar fuels (artificial photosynthesis) when used in solar photoconverters. MEG creates two or more excitons per absorbed photon, and SF produces two triplet states from a single singlet state. In both cases, multiple charge carriers from a single absorbed photon can be extracted from the cell and used to create higher power conversion efficiencies for a photovoltaic cell or a cell that produces solar fuels, like hydrogen from water splitting or reduced carbon fuels from carbon dioxide and water (analogous to biological photosynthesis). The similarities and differences in the mechanisms and photoconversion cell architectures between MEG and SF are discussed.

Third harmonic generation in quantum dot with Rashba spin orbit interaction

Here we have investigated the influence of magnetic field and confinement potential on nonlinear optical property, third harmonic generation (THG) of a parabolically confinement quantum dot in the presence of Rashba spin orbit interaction. We have used density matrix formulation for obtaining optical properties within the effective mass approximation. The results are presented as a function of confining potential, magnetic field, Rashba spin orbit interaction strength and photon energy. Our results indicate that an increase of Rashba spin orbit interaction coefficient produces strong effect on the peak positions of THG. The role of confinement strength and spin orbit interaction strength as control parameters on THG have been demonstrated.

Advance of Ge/Si Quantum Dot Infrared Photodetector

The generation of quantum dots (QDs), the advantages and disadvantages of quantum dot infrared photodetector (QDIP) are briefly reviewed. Typical techniques for fabricating ordered Ge/Si QDs, the application of Ge/Si QDIP in optical communication and thermal imaging and the structure optimization are described. Finally, the key problems for improving the properties of Ge/Si QDs and Ge/Si QDIP, future trends and prospects are discussed.

High-fidelity biexciton generation in quantum dots by chirped laser pulses

We present a detailed theoretical analysis of biexciton state generation in InAs-GaAs quantum dots by strong, chirped laser pulses. Specifically, we derive an accurate analytical expression, which not only provides a clear physical picture of the process, but also allows identifications of laser parameter regimes where efficient biexciton generation should be possible, even at temperatures up to 80 K. The results are confirmed by numerical simulations, in very good agreement with the model proposed. A clear choice of parameters is proposed, which might pave the way towards the optimal design of high-fidelity sources of entangled photon pairs based on individual quantum dots.

Spectroscopic insights into the performance of quantum dot light-emitting diodes

Abstract

Simple Method for Sub-Diffraction Resolution Imaging of Cellular Structures on Standard Confocal Microscopes by Three-Photon Absorption of Quantum Dots

This study describes a simple technique that improves a recently developed 3D sub-diffraction imaging method based on three-photon absorption of commercially available quantum dots. The method combines imaging of biological samples via tri-exciton generation in quantum dots with deconvolution and spectral multiplexing, resulting in a novel approach for multi-color imaging of even thick biological samples at a 1.4 to 1.9-fold better spatial resolution. This approach is realized on a conventional confocal microscope equipped with standard continuous-wave lasers. We demonstrate the potential of multi-color tri-exciton imaging of quantum dots combined with deconvolution on viral vesicles in lentivirally transduced cells as well as intermediate filaments in three-dimensional clusters of mouse-derived neural stem cells (neurospheres) and dense microtubuli arrays in myotubes formed by stacks of differentiated C2C12 myoblasts.

Heat generation by electric current in quantum dot system responded by ac magnetic fields

We investigate the heat generation in quantum dot (QD) coupled to normal leads irradiated with a rotating magnetic field and an oscillating magnetic field by employing nonequilibrium Green's function technique. The heat generation under the influence of ac magnetic fields is very different compared with the system without perturbation of ac magnetic fields. The heat generation is suppressed by ac magnetic fields, but the changes of current is not very obvious. An ideal region for device operation is found. The threshold of source drain bias decreases due to the absorption of photons which caused by ac magnetic field. This photon absorption and emission processes also bring several mini steps in heat generation. The symmetric structure of heat generation is destroyed by the spin flip effect. As the heat generation Q is related to bessel function Jn(Λ), the heat generation displays attenuation oscillation behavior. The heat generation Q is also influenced by the linewidth function Γ.

InAs Quantum Dot Growth on a Thin GaNP Buffer Layer on GaP by Metalorganic Chemical Vapor Deposition

InAs quantum dots (QDs) on a 2-nm-thick GaNP buffer layer on GaP were investigated using low-pressure metalorganic chemical vapor deposition. The InAs QDs density was significantly increased from 2.6×109 to 2.0×1010 cm-2 by increasing the nitrogen composition from 0 to 4.5% in GaNP. The formation characteristics of InAs QDs were also investigated at various InAs supply amounts from 0.7 to 2.5 monolayers (ML). The generation of QDs on the GaNP and GaP buffer layers started at the InAs supplies of 1.0 and 1.1 ML, respectively. Since the lattice mismatches were large, these supply amounts were definitely smaller than that on GaAs.

Multiple Exciton Generation in Semiconductor Quantum Dots.

Multiple exciton generation in quantum dots (QDs) has been intensively studied as a way to enhance solar energy conversion by utilizing the excess energy in the absorbed photons. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron-phonon coupling that cools hot excitons in bulk semiconductors. However, variations in the reported enhanced quantum yields (QYs) have led to disagreements over the role that quantum confinement plays. The enhanced yield of excitons per absorbed photon is deduced from a dynamical signature in the transient absorption or transient photoluminescence and is ascribed to the creation of biexcitons. Extraneous effects such as photocharging are partially responsible for the observed variations. When these extraneous effects are reduced, the MEG efficiency, defined in terms of the number of additional electron-hole pairs produced per additional band gap of photon excitation, is about two times better in PbSe QDs than that in bulk PbSe. Thin films of electronically coupled QDs have shown promise in simple photon-to-electron conversion architectures. If the MEG efficiency can be further enhanced and charge separation and transport can be optimized within QD films, then QD solar cells can lead to third-generation solar energy conversion technologies.

Sol–gel thin films for optics and photonics

Thin films have wide applications in the area of optics and photonics. Conventional thin film processing is usually followed for deposition, but the sol–gel route is unique as it can be applied very easily in a cost effective way on desired substrates of any shape for specific applications. In this review, the basic optical designs of antireflection, high reflection, different types of optical filters, coloured coatings, etc. have been discussed with some typical examples of sol–gel products. The importance of coloured coatings and low thermal emissivity coatings on window glass has also been highlighted. In addition, the use of sol–gel processing for different types of film formation which are effective for photonic applications such as non-linearity in optics, ferromagnetism in transparent dilute magnetic semiconductors (DMS), generation of quantum dots as phosphor, grating coupler waveguide in optical sensors are discussed. The basic characterizations of a few sol–gel products which may be used for photonic applications have also been highlighted.