Quantum Dot Levels Research Articles

Plasmon-assisted two-photon Rabi oscillations in a semiconductor quantum dot – metal nanoparticle heterodimer

Tho-photon Rabi oscillations hold potential for quantum computing and quantum information processing, because during a Rabi cycle a pair of entangled photons may be created. We theoretically investigate the onset of this phenomenon in a heterodimer comprising a semiconductor quantum dot strongly coupled to a metal nanoparticle. Two-photon Rabi oscillations in this system occur due to a coherent two-photon process involving the ground-to-biexciton transition in the quantum dot. The presence of a metal nanoparticle nearby the quantum dot results in a self-action of the quantum dot via the metal nanoparticle, because the polatization state of the latter depends on the quantum state of the former. The interparticle interaction gives rise to two principal effects: (i) - enhancement of the external field amplitude and (ii) - renormalization of the quantum dot's resonance frequencies and relaxation rates of the off-diagonal density matrix elements, both depending on the populations of the quantum dot's levels. Here, we focus on the first effect, which results in interesting new features, in particular, in an increased number of Rabi cycles per pulse as compared to an isolated quantum dot and subsequent growth of the number of entangled photon pairs per pulse. We also discuss the destructive role of radiative decay of the excitonic states on two-photon Rabi oscillations for both an isolated quantum dot and a heterodimer.

Three-terminal normal-superconductor junction as thermal transistor

We propose a thermal transistor based on a three-terminal normal-superconductor (NS) junction with superconductor terminal acting as the base. The emergence of heat amplification is due to the negative differential thermal conductance (NDTC) effect for the NS diode in which the normal side maintains a higher temperature. The temperature dependent superconducting energy gap is responsible for the NDTC. By controlling quantum dot levels and their coupling strengths to the terminals, a huge heat amplification factor can be achieved. The setup offers an alternative tuning scheme of heat amplification factor and may find use in cryogenic applications.

A consolidated account of electrochemical determination of band structure parameters in II-VI semiconductor quantum dots: a tutorial review.

Probing absolute electronic energy levels in semiconductor quantum dots (Q-dots) is crucial for engineering their electronic band structure and hence for precise design of composite nano-structure based devices. The use of electrochemistry has allowed us to investigate size, shape and composition dependent band structure parameters viz. the conduction band edge, valence band edge & quasi-particle gap and to establish novel charge induced phenomena in colloidal semiconductor Q-dots. The electrochemical behavior is also of special importance for the prediction of the stability of Q-dots in biological environments as well as for precise design of composite nanohetero-structures for opto-electronic (light emitting diodes) and photovoltaic (solar cells) applications. Several researchers have contributed to probing and predicting the positions of absolute energy levels of band edges and surface states as well as to the establishment of a potential window of stability for a wide variety of Q-dots both in aqueous media and in organic solution. The crucial point about these studies is that unlike spectroscopic methods, no unified approach has been followed and a variety of methods and protocols have been developed to carry out these measurements either on diffusing or thin films of Q-dots in different electrolyte media viz. aqueous, organic and ionic liquids, each having their own advantages over the others. However, a consolidated account of these methods and protocols is not available in the literature. The aim of this tutorial review is therefore to consolidate and compare the studies related to the determination of the band structure of II-VI semiconductor Q-dots through electrochemical measurements. A brief introduction to electrochemical techniques, especially cyclic voltammetry, is given, followed by a summary of experimental methods developed for these measurements. Finally, a concise protocol that can be easily applied universally and is attractive for other users dealing with semiconductor Q-dot based devices is discussed.

Electron g -factor of valley states in realistic silicon quantum dots

We theoretically model the spin-orbit interaction in silicon quantum dot devices, relevant for quantum computation and spintronics. Our model is based on a modified effective mass approach with spin-valley boundary conditions, derived from the interface symmetry under presence of perpendicular to the interface electric field. The g-factor renormalization in the two lowest valley states is explained by the interface-induced spin-orbit 2D (3D) interaction, favoring intervalley spin-flip tunneling over intravalley processes. We show that the quantum dot level structure makes only negligible higher order effects to the g-factor. We calculate the g-factor as a function of the magnetic field direction, which is sensitive to the interface symmetry. We identify spin-qubit dephasing sweet spots at certain directions of the magnetic field, where the g-factor renormalization is zeroed: these include perpendicular to the interface magnetic field, and also in-plain directions, the latter being defined by the interface-induced spin-orbit constants. The g-factor dependence on electric field opens the possibility for fast all-electric manipulation of an encoded, few electron spin-qubit, without the need of a nanomagnet or a nuclear spin-background. Our approach of an almost fully analytic theory allows for a deeper physical understanding of the importance of spin-orbit coupling to silicon spin qubits.

Characterization of Nanoparticle Intestinal Transport Using an In Vitro Co-Culture Model.

We aimed to obtain a tunable intestinal model and study the transport of different types of nanoparticles. Caco-2/HT29-MTX co-cultures of different seeding ratios (7:3 and 5:5), cultured on Transwell® systems, were exposed to non-cytotoxic concentration levels (20 μg/mL) of silicon quantum dots and iron oxide (α-Fe2O3) nanoparticles. Transepithelial electric resistance was measured before and after exposure, and permeability was assessed via the paracellular marker Lucifer Yellow. At regular intervals during the 3 h transport study, samples were collected from the basolateral compartments for the detection and quantitative testing of nanoparticles. Cell morphology characterization was done using phalloidin-FITC/DAPI labeling, and Alcian Blue/eosin staining was performed on insert cross-sections in order to compare the intestinal models and evaluate the production of mucins. Morphological alterations of the Caco-2/HT29-MTX (7:3 ratio) co-cultures were observed at the end of the transport study compared with the controls. The nanoparticle suspensions tested did not diffuse across the intestinal model and were not detected in the receiving compartments, probably due to their tendency to precipitate at the monolayer surface level and form visible aggregates. These preliminary results indicate the need for further nanoparticle functionalization in order to appropriately assess intestinal absorption in vitro.

Quantum Dot Size Effect on the Frontier Molecular Orbital Energies in the Presence of Different Aquatic Environmental Ligands

One of the challenging tasks of the century is to clean up the contaminants of the environment by ecofriendly, sustainable and economically adoptable technologies. The application of quantum dots (QDs) is growing rapidly in to the field of nanotechnology. The electronic properties such as the energy of the highest occupied molecular orbital (EHOMO), the lowest unoccupied molecular orbital (ELUMO), the energy gap ΔE are correlated to the size of quantum dots (QDs) such as CdSe in the presence of different ligands. Linear regression equations were developed through statistical analysis using SPSS. In terms of correlation coefficient R2, both the size and nature of organic ligand capping affect the EHOMO, ELUMO and ΔE levels of QDs. Among all QDs, CdSe has the strongest correlation between its frontier molecular orbital energy levels with the QDs size. The R2 value is greater than 0.63 for a regression equation with a coefficient value of −0.17 and a constant of 2.9. This result implies that a unit decrease of size in the range lower than 7 nm would increase the band gap ΔE by 17%. Potential environmental applications of QDs are presented.

Interplay of structural design and interaction processes in tunnel-injection semiconductor lasers

Tunnel-injection lasers promise various advantages in comparison to conventional laser designs. In this paper, the physics of the tunnel-injection process is studied within a microscopic theory in order to clarify design requirements for laser structures based on quantum dots as active material and an injector quantum well providing excited charge carriers. We analyze how the electronic states of the injector quantum-well and quantum-dot levels should be aligned and in which way their coupling through the tunnel-injection barrier should be adjusted for optimal carrier injection rates into the quantum-dot ground state used for the laser transition. Our description of the tunnel-injection process combines two main ingredients: the tunnel coupling of the wave functions as well as the phonon- and Coulomb-assisted transition rates. For this purpose, material-realistic electronic state calculations for the coupled system of injector quantum well, tunnel barrier, and quantum dots are combined with a many-body theory for the carrier scattering processes. We find that the often assumed longitudinal-optical-phonon resonance condition for the level alignment has practically no influence on the injection rate of carriers into the quantum-dot states. The structural design should provide optimal hybridization of the injector quantum-well states with excited quantum-dot states.

Unraveling the Origin of Operational Instability of Quantum Dot Based Light-Emitting Diodes.

We investigate the operational instability of quantum dot (QD)-based light-emitting diodes (QLEDs). Spectroscopic analysis on the QD emissive layer within devices in chorus with the optoelectronic and electrical characteristics of devices discloses that the device efficiency of QLEDs under operation is indeed deteriorated by two main mechanisms. The first is the luminance efficiency drop of the QD emissive layer in the running devices owing to the accumulation of excess electrons in the QDs, which escalates the possibility of nonradiative Auger recombination processes in the QDs. The other is the electron leakage toward hole transport layers (HTLs) that accompanies irreversible physical damage to the HTL by creating nonradiative recombination centers. These processes are distinguishable in terms of the time scale and the reversibility, but both stem from a single origin, the discrepancy between electron versus hole injection rates into QDs. Based on experimental and calculation results, we propose mechanistic models for the operation of QLEDs in individual quantum dot levels and their degradation during operation and offer rational guidelines that promise the realization of high-performance QLEDs with proven operational stability.

Renormalization of the quantum dot g -factor in superconducting Rashba nanowires

We study analytically and numerically the renormalization of the $g$-factor in semiconducting Rashba nanowires (NWs), consisting of a normal and superconducting section. If the potential barrier between the sections is high, a quantum dot (QD) is formed in the normal section. For harmonic (hard-wall) confinement, the effective $g$-factor of all QD levels is suppressed exponentially (power-law) in the product of the spin-orbit interaction (SOI) wavevector and the QD length. If the barrier between the two sections is removed, the $g$-factor of the emerging Andreev bound states is suppressed less strongly. In the strong SOI regime and if the chemical potential is tuned to the SOI energy in both sections, the $g$-factor saturates to a universal constant. Remarkably, the effective $g$-factor shows a pronounced peak at the SOI energy as function of the chemical potentials. In addition, if the SOI is uniform, the $g$-factor renormalization as a function of the chemical potential is given by a universal dependence which is independent of the QD size. This prediction provides a powerful tool to determine experimentally whether the SOI in the whole NW is uniform and, moreover, gives direct access to the SOI strengths of the NW via $g$-factor measurements. In addition, it allows one to find the optimum position of the chemical potential for bringing the NW into the topological phase at large magnetic fields.

Pressure dependent optical properties of quantum dot with spin orbit interaction and magnetic field

In this paper, a detailed investigation of the effects of the interplay of temperature, hydrostatic pressure, Rashba spin orbit interaction, and external magnetic field on the optical properties for quantum dot is presented. We obtained the exact wavefunctions and energy levels of a harmonic potential quantum dots taking Rashba spin orbit interaction into account. We have obtained the linear and nonlinear absorption coefficients and relative refractive index change for varying incident photon energies using density matrix theory. Counter intuitive red shift in the absorption coefficient peaks with increase in hydrostatic pressure is demonstrated. The role of temperature and hydrostatic pressure on variation of effective charge carrier mass is investigated.

Polarization switch in an elliptical micropillar – quantum dot system induced by a magnetic field in Faraday configuration

In this paper we study the effect of applying a magnetic field on an elliptical microcavity pillar with quantum dots embedded, in the presence of external laser excitation. To obtain the system dynamics we use the matrix density formalism, taking into account realistic parameters and including losses. Our results show that it is possible to use the magnetic field strength to control the polarization of the photons inside the cavity, making our system behave like a photon polarization switch. We also report the best set of parameters where this is possible. Our results also indicate that we can use the polarization of the cavity photons to look into the fine structure of the energy levels of quantum dots.

Curvature and vacancies in graphene quantum dots

The optical properties and the geometric characteristics of multivacancy graphene quantum dots systems are studied using Density Functional Theory calculations, in a systematic way, in relation to the number of carbon atoms extracted and the relative configurations that could be present in the cluster. The way to characterize these configurations was using the complementary figure model -i.e. the figure formed by the carbon atoms extracted in order that the vacancy could be created-. The number of pentagonal or square figures formed after structure optimization, as well as their relative location, is found to be very relevant to determine the curvature level of the graphene quantum dot, but the occurrence of higher order cycles have a non-negligible role and cannot be ruled out. However, this occurrence is much dependent on the specific multivacancy to be studied and prevents to reach more accurate models that could allow the prediction of the magnitude of the curvature. We found unexpected instability in dendritic-like vacancies and we suggest that the non-periodic character of the graphene quantum dot, in contrast to graphene, would prevent such vacancies to be relaxed in a higher extension, causing this kind of systems to be less stable than, for example, zigzag complementary figures systems.

Theory of non-Markovian dynamics in resonance fluorescence spectrum

We present a detailed theoretical study of non-Markovian dynamics in the fluorescence spectrum of a driven semiconductor quantum dot (QD), embedded in a cavity and coupled to a three-dimensional (3D) acoustic phonon reservoir. In particular, we investigate the effect of pure dephasing on one of the side-peaks of the Mollow-triplet spectrum, expressed in terms of the off-diagonal element of the reduced system operator. The QD is modeled as a two-level system with an excited state representing a single exciton, and ground state represents the absence of an exciton. Coupling to the radiative modes of the cavity is treated within usual Born-Markov approximation, whereas dot-phonon coupling is discussed within non-Markovian regime beyond Born approximation. Using an equation-of-motion technique, the dot-phonon coupling is solved exactly and found that the exact result coincides with that of obtained within Born approximation. Furthermore, a Markov approximation is carried out with respect to the phonon interaction and compared with the non- Markovian lineshape for different values of the phonon bath temperature. We have found that coupling to the phonons vanishes for a resonant pump laser. For a non-resonant pump, we have characterzied the effect of dot-laser detuning and temperature of the phonon bath on the lineshape. The sideband undergoes a distinct narrowing and aquires an asymmetric shape with increasing phonon bath temperature. We have explained this behavior using a dressed-state picture of the QD levels.

Interdot spin transfer dynamics in laterally coupled excited spin ensemble of high-density InGaAs quantum dots

Interdot spin transfer dynamics is studied in a laterally coupled excited spin ensemble of high-density InGaAs quantum dots (QDs). We observe a rise time of the photoluminescence intensity of ∼100 ps and a simultaneous increase in the spin polarization of the excited spin ensemble, indicating spin injection from higher-energy levels in smaller QDs. Moreover, this coupled ensemble exhibits decay properties of the spin polarization that vary with the excited spin density. This phenomenon can be quantitatively understood by considering interdot spin transfer into lower-energy levels of the surrounding QDs, where the transfer rate depends on the degree of state filling of each QD level.

The influence of the hole transport layers on the performance of blue and color tunable quantum dot light‐emitting diodes

AbstractThe performance of the blue quantum dot light‐emitting diodes (QLEDs) is largely affected by the hole transport layers (HTLs). As a consequence of the deep valance band level of blue quantum dots (QDs), hole injection is relatively difficult in blue QLEDs. To favor the hole injection, HTLs with high hole mobility and deep‐lying highest occupied molecular orbital level are desired. In this work, various HTLs and their influence on the performance of blue QLEDs are demonstrated. Devices with poly(N‐vinylcarbazole) (PVK) HTL exhibit the highest external quantum efficiency while devices with poly[9,9‐dioctylfluorene‐co‐N‐(4‐(3‐methylpropyl))‐diphenylamine] (TFB) exhibit the lowest driving voltage. By combining the advantages of PVK and TFB, the blue QLEDs with TFB/PVK bilayered HTL simultaneously exhibit a low driving voltage of 2.6 V and a high external quantum efficiency of 5.9%. Moreover, the exciplex emission at the interface of HTL/QDs is also observed, and the emission intensity can be tuned by modulating the hole injection. By utilizing PVK doped with 25% poly(3‐hexylthiophene) (P3HT) as HTL, exciplex emission is significantly enhanced at low driving voltage while QD emission is dominant at high driving voltage. By combining the exciplex emission and the QD emission, the emission color can be effectively tuned from red to blue as the driving voltage changing from 2 to 10 V.

Bi quantum dots obtained via in situ photodeposition method as a new photocatalytic CO2 reduction cocatalyst instead of noble metals: Borrowing redox conversion between Bi2O3 and Bi

Metal Bi is applied as the cocatalyst instead of noble metals, for the first time, in photocatalytic CO2 reduction and exhibits significant increase in CO yield compared to that of pristine photocatalyst, about 4.8 times. In situ photodeposition method is used to prepare metal Bi. Surprisingly, the average size of metal Bi obtained is ca. 5 nm and can be considered as quantum dots level, which is very difficult to be realized for noble metals via in situ photodeposition. Unfortunately, such small Bi is unstable in air, making it very difficult for the preservation. Another fortunate phenomenon is discovered, the unstable metal Bi can be stored via using the form of Bi/Bi2O3 quantum dots (Bi2O3 is main body of white Bi/Bi2O3 composites). When carried out the photocatalytic CO2 reduction, the white Bi/Bi2O3 can be easily transformed into grey Bi/Bi2O3 composites (the main body of grey Bi/Bi2O3 composites is metal Bi) to improve the photocatalytic CO2 reduction rate. This reason is that CB level of Bi2O3 quantum dots has shifted to a more negative position due to quantum confinement effect compared to the standard redox potential of Bi2O3/Bi (0.37 eV), leading to Bi2O3 easily to be reduced to metal Bi under the effect of photogenerated electrons derived from TiO2. This work demonstrated the mutual conversion between storage and utilization may offer an attractive approach for the application of unstable quantum dot materials or single atom materials.

Two-dimensional WS2-based nanosheets modified by Pt quantum dots for enhanced room-temperature NH3 sensing properties

As a typical two-dimensional (2D) layered transition metal dichalcogenides (TMDs), tungsten disulfide (WS2) has been considered as a promising sensing material for room-temperature NH3 detection. However, the bulk WS2-based room-temperature NH3 sensors can hardly recover to its initial state after turning off gas. Although the recovery rate of bulk WS2 was accelerated by thinning method, the response of few- or monolayer WS2 nanosheets (NSs) to NH3 was sharply decreased. Here, in premise of keeping fast recovery rate, few- or monolayer WS2 NSs modified with Pt quantum dots (QDs) were prepared for room-temperature NH3 detection, which exhibited significantly enhanced sensing properties with fast recovery speeds. Especially, the response of nanocomposite to 250 ppm NH3 is nearly 10 times than that of WS2 NSs, which could be attributed to the significantly decreased initial conductivity caused by electrons flowing from higher Fermi level of Pt QDs to that of WS2 NSs and the higher catalytic activity. Furthermore, the PtS bonds confirmed by XPS results could benefit electrons transfer between the interface. We hope that the 0D/2D heterostructure system in this work could provide a direction to improve sensing properties of 2D TMDs-based room-temperature sensors.

Spin polarized current in a quantum dot connected to a spin-orbit interacting Fermi sea

In this study, we investigate the transport properties of a device comprising a quantum dot (QD) connected to a Rashba spin-orbit interacting nanoribbon, which is spontaneously magnetically polarized or deposited on a ferromagnetic substrate. The QD is as well connected to two other lateral contacts along which the electronic current circulates. One contact comprising the source of electrons is ferromagnetic, whereas the other contact comprising , the drain, could be a non-magnetic or a ferromagnetic metal with anti-parallel magnetization relative to the source contact polarization. The connection of the QD local state to the Fermi sea under the effect of spin-orbit coupling (SOC) creates a spin-flip mechanism that affects the electronic spins circulating along the device. This process is tuned by the QD gate voltage and it is strongly enhanced when the Fermi energy of the system is in the neighborhood of a Van-Hove singularity of the SOC Fermi sea band. The spin-flip of the incident polarized current can be controlled by manipulating the gate potential of the QD. We show that this system possesses all the requirements necessary to operate as a spin transistor.Due to the local character of the QD level, the Coulomb interaction plays an important role. It enhances the parameter region within which the spin transistor can operate. The effects of the intra-dot Coulomb interaction are analyzed within the context of the alloy analogy Hubbard III approximation, which provides an adequate understanding of the Coulomb blockade transport properties above the Kondo temperature.

Crystal Growth, Exponential Optical Absorption Edge, and Ground State Energy Level of PbS Quantum Dots Adsorbed on the (001), (110), and (111) Surfaces of Rutile-TiO2

It is important to investigate the dependencies of the optical absorption and the ground state energy level on the size of the semiconductor quantum dots (QDs) on fully studied single crystal TiO2 surfaces. The present study focuses on the systems comprising PbS QDs on (001), (110), and (111) surfaces of single crystal rutile-TiO2. By the optical absorption characterization, the average diameter of PbS QDs on a (001) surface is independent of the number of adsorption cycles, although those on (110) and (111) surfaces increase with the number of cycles. The rate of adsorption of PbS QDs on a (001) surface is higher than those grown on (110) and (111) surfaces. The results suggest that the crystal growth is caused by the difference of the surface energy of the substrate. The exponential optical absorption edge suggests that the structural disorder of PbS QDs on (001) and (110) surfaces increases as the number of adsorption cycles increases. On the other hand, that on a (111) surface decreases as the number ...

Enhanced visible-light photocatalysis via back-electron transfer from palladium quantum dots to perylene diimide

The electronic-coupling interaction between noble-metal cocatalysts and host semiconductor nanocrystals has been found to be effective for the utilization of the solar energy. However, electron transfer (ET) mechanism between noble metals and self-assemblies has not been elucidated clearly. Here, we revealed a mechanism of back-electron-transfer-enhanced photocatalysis, which contributed to the visible-light photocatalytic improvement of perylene diimide (PDI) assembly for phenolic degradation and hydrogen generation. By compared with the energy level of PDI and Pd quantum dots (QDs), it seemed to be disadvantageous for ET from PDI assembly to Pd QDs, but surface photovoltage spectra showed that this can come true under visible-light irradiation, indicative of formation of a new Schottky barrier resulting from other light-induced transition species. Absorption spectra showed that an additional electronic state existed above the conduction band of PDI assembly originating from PDI radical anions. Such active species provided the light-driven dissipative structure with stable energy and electron flow through two opposite types of ET pathway: one is ET from light-excited PDI anions to Pd QDs and the other is direct electron injection from plasma Pd QDs accumulated with high-energy electrons to PDI neutral molecules. It has been found that continuous ET from PDI assembly to Pd QDs caused the plasma resonance of the Pd QDs to be higher in energy to overcome interfacial energy barrier. And then back-ET from Pd QDs to neutral PDI molecules occurred and led to formation of more PDI radical anions that were important for the improvement of the photocatalytic activity. These findings provide a new strategy for the development of highly efficient visible-light photocatalysts based on self-assembly for energy production.