Excited States Of Quantum Dots Research Articles

Chiral-Induced Spin Selectivity Modulated Time-Correlated Single-Photon Counting for DNA Hybridization Detection.

The chiral-induced spin selectivity (CISS) effect can distinguish between the spin of electrons as they pass through chiral molecules by backscattering one of the spin components. Herein, we explore the role of the CISS effect in time-correlated single-photon counting measurements to detect DNA hybridization. We observe that the average lifetime of optical excited states of quantum dots attached to double-stranded DNA (dsDNA) varies with directions of the applied magnetic field. Specifically, the difference in the nonradiative average decay lifetime for the two orientations of the applied magnetic field is 2.21 ns in the case of hybridized strands, which is 130 times higher than that observed with quantum dots attached to single-strand DNA. Additionally, we investigate the application of Fourier transform infrared (FTIR) spectroscopy for detecting double-stranded DNA in the presence of a magnetic field, establishing a theoretical framework to substantiate the experimental evidence of magnetic field-dependent FTIR spectroscopy for dsDNA.

Two-state lasing in a quantum dot racetrack microlaser.

The peculiarities of two-state lasing in a racetrack microlaser with an InAs/GaAs quantum dot active region are investigated by measuring the electroluminescence spectra at various injection currents and temperatures. Unlike edge-emitting and microdisk lasers, where two-state lasing involves the ground and first excited-state optical transitions of quantum dots, in racetrack microlasers, we observe lasing through the ground and second excited states. As a result, the spectral separation between lasing bands is doubled to more than 150 nm. A temperature dependence of threshold currents for lasing via ground and second excited states of quantum dots was also obtained.

Ultrafast and Electrically Tunable Rabi Frequency in a Germanium Hut Wire Hole Spin Qubit

Hole spin qubits based on germanium (Ge) have strong tunable spin-orbit interaction (SOI) and ultrafast qubit operation speed. Here we report that the Rabi frequency (fRabi) of a hole spin qubit in a Ge hut wire (HW) double quantum dot (DQD) is electrically tuned through the detuning energy (ϵ) and middle gate voltage (VM). fRabi gradually decreases with increasing ϵ; on the contrary, fRabi is positively correlated with VM. We attribute our results to the change of electric field on SOI and the contribution of the excited state in quantum dots to fRabi. We further demonstrate an ultrafast fRabi exceeding 1.2 GHz, which indicates the strong SOI in our device. The discovery of an ultrafast and electrically tunable fRabi in a hole spin qubit has potential applications in semiconductor quantum computing.

Selectively-Grown Quantum Dot Active Region Lasers

Nanopatterns formed by dense nanoscale (20–30nm diameter) block copolymer lithography and subsequent selective area MOVPE were utilized to realize room temperature operation of quantum dot (QD) active region laser diodes on InP substrates, by employing a carrier-injection layer adjacent to the QDs. Initial studies focused on InGaAs QDs grown on GaAs substrates, which exhibit relatively broad photoluminescence (PL) emission linewidths (FWHM~93meV at RT). Atomic force microscopy measurements reveal that a wide distribution of QD thickness variations correlates well to the observed broad PL linewidths, based on the simulated range of QD transitions. The fabricated edge-emitting devices on InP substrate exhibit lasing emission near <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.57\sim 1.67~\mu \text{m}$ </tex-math></inline-formula> at room temperature with a threshold current density as low as 1.6 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 4mm-long devices. The observed dependency of device characteristics, such as threshold current density and lasing wavelength, on the cavity length and on the heat sink temperature suggests the dominant lasing transition energy corresponds to the QD excited state, except for the longest cavity length devices (L = 4mm) which operate on the QD ground state transition near room temperature. We believe these data represent the only reports of lasers in this wavelength region employing QDs formed by diblock co-polymer lithography and selective area MOVPE growth. These data also suggest that QD height variations need to be further reduced, through MOVPE growth optimization, to achieve higher optical gain from the QD ground state transition.

Defective Nature of CdSe Quantum Dots Embedded in Inorganic Matrices

Quantum dots (QDs) embedded in inorganic matrices have been extensively studied for their potential applications in lighting, displays, and solar cells. While a significant amount of research studies focused on their experimental fabrication, the origin of their relatively low photoluminescence quantum yield has not been investigated yet, although it severely hinders practical applications. In this study, we use time-dependent density functional theory (TDDFT) to pinpoint the nature of excited states of CdSe QDs embedded in various inorganic matrices. The formation of undercoordinated Se atoms and nonbridging oxygen atoms at the QD/glass interface is responsible for the localization of a hole wave function, leading to the formation of low-energy excited states with weak oscillator strength. These states provide pathways for nonradiative processes and compete with radiative emission. The photoluminescence performance is predicted for CdSe QDs in different matrices and validated by experiments. The results of this study have significant implications for understanding the underlying photophysics of CdSe QDs embedded in inorganic matrices that would facilitate the fabrication of highly luminescent glasses.

Manipulation of the quantum dots photostability using gold nanoparticles

The effect of plasmonic films containing gold nanoparticles of different shape (nanospheres and nanorods) on the photostability of InP/ZnSe/ZnSeS/ZnS and CdSe/ZnCdS/ZnS quantum dots with core/shell structure has been determined. Gold nanospheres increase the photostability of InP/ZnSe/ZnSeS/ZnS quantum dots when excited by blue LED radiation when reducing the average lifetime of the excited state of quantum dots and, accordingly, when reducing the probability of Auger processes. An increase in the average lifetime of the excited state of CdSe/ZnCdS/ZnS quantum dots in complexes with gold nanorods leads to a decrease in the photostability upon excitation at 449 and 532 nm.

Evaluation of In(Ga)As capping in a multilayer coupled InAs quantum dot system: Growth strategy involving the same overgrowth percentage

Strain-coupled multilayer Quantum Dot (QD) structures draw a great attention these days because of their superior optical and device performance. However, these coupled multilayer QD structures have limitations such as non-uniform QD size distribution, defects, and dislocations due to the propagation of strain in QD layers. Furthermore, the carrier relaxation lifetime must be improved in these coupled QD structures for sought-after device performance. This study serves for three main purposes. Firstly, we have utilized a growth strategy to maintain the overgrowth percentage such that the Stranski-Krastanov (SK) QD dimensions in every layer is same for all the multilayer structures. Secondly, these multilayer SK QDs are made to electronically couple with the Sub-monolayer (SML) QDs in such a way that the carrier relaxation lifetime can be increased, that could ameliorate the photoconductive gain and the responsivity. Lastly, the amount of cumulative strain generated inside QDs can be reduced by the incorporation of In0.15Ga0.85As strain reducing layer (capping layer) in a multilayer structure. Five different multilayer structures with single (x1), bi (x2), penta (x5), hepta (x7), and ten-layers (x10) of SK QDs electronically coupled to six stacks of SML QDs are used in this study. Photoluminescence (PL) studies reveal that the SK QD dimensions is maintained to be the same in all multilayer structures due to the proposed growth strategy, which is also observed through transmission electron microscopy (TEM) images. Photoluminescence excitation (PLE) analysis shows the match of excited states of SK QDs to the ground state of the SML QDs, aiding the carrier tunnel phenomena. The ω/2Ө measurements from high resolution X-ray diffraction (HRXRD) is carried out to calculate the overall strain in the grown heterostructures. The optical properties and the strain components obtained from the nextnano simulation tool are in good agreement with the experimental results. Thus, coupled multilayer QD structures with a growth strategy in the current study would be more suitable for the realization of quantum dot infrared photodetectors and intermediate band solar cell applications accommodated with high device efficiency.

High-Throughput Time-Resolved Photoluminescence Study of Composition- and Size-Selected Aqueous Ag–In–S Quantum Dots

Brightly luminescent, composition- and size-selected aqueous Ag–In–S (AIS) and Zn-diffused core/shell AIS/ZnS (ZAIS) quantum dots (QDs) were studied by high-throughput time-resolved photoluminescence (TRPL). Both QD size and QD composition effects on the photophysical properties of AIS (ZAIS) were probed independently by using an automated high-throughput TRPL setup. Linear relationships were found between the average PL lifetimes of AIS and ZAIS QDs and their respective bandgaps (Eg) indicating that the dynamics of the radiative recombination is governed mostly by the energy of the QD excited state irrespective of which factor is varied affecting the QD bandgap, i.e., their size or composition. The rate constants of radiative and nonradiative recombination were evaluated as a function of Eg forming continuous dependences for the entire AIS and ZAIS QD families. The rate of nonradiative recombination increases steeply for smaller QDs with Eg > 2.2 eV for both core AIS and core/shell ZAIS QDs, indicating that the interfacial electron transfer is the major contributor to the observed dependence. A strong decrease of the PL lifetime of AIS (and ZAIS) QDs in dense QD films or QDs incorporated into polyvinylpyrrolidone films was observed as compared to colloidal solutions. A reversible character of this effect upon polymer film dissolution shows that such behavior originates from energy (or electron) transfers among the QDs brought into close contact in the films. This phenomenon is expected to be of significance for the application of ternary QDs as a light-harvesting material.

The role and growth of strain – reducing layer by molecular -beam epitaxy in a multi – stack InAs/(In,Ga)As sub - monolayer quantum dot heterostructure

Abstract The present study focuses on a growth strategy to achieve a uniform (and larger) dot size distribution in a multi-stack InAs/InGaAs sub-monolayer (SML) quantum dot (QD) heterostructure by capping with a varying InxGa1-xAs (x = 0.15) layer thickness (Y). The InxGa1-xAs layer serves both as matrix medium for the dot growth and strain - reducing layer (SRL) to preserve the QD morphology from decomposition. The dots capped by an optimum SRL or capping layer (CL) thickness will alleviate from dot segregation or intermixing, surface diffusion and dot inhomogeneity effects, thus paving way to grow a nearly defect - free quantum dot heterostructure. The experimental calculations on low – temperature (19 K), high – excitation power (1.1 kW/cm2) photoluminescence (PL) revealed ground – state (GS) emission energies at 1.12, 1.14, 1.09 eV for 2, 4, and 6 ML SRL thickness respectively. While, the presence of QD excited state (ES) transitions has been analysed using PL - excitation (PLE) spectroscopy, as the CL will highly alter the band offsets as well as strain inside QDs. The interfacial structural quality, strain relaxation in QDs due to SRL was inspected through high-resolution X – ray diffraction (HRXRD) and Raman measurements. The self – consistent Schrodinger – Poisson solver based on 8 – band k.p formulation was used to evaluate the PL energy and strain profile as a comparison to experimental data. Hence, the sample containing 2 ML CL thickness showed superior results in terms of PL emission and strain, which leads towards the growth of uniform QDs for photodetector (PD) applications operating in the short – wave infrared (SWIR) regime (λ: 0.9 μm–2.5 μm, Energy: 1.377 eV - 0.495eV).

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.

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.

Andreev bound states probed in three-terminal quantum dots

Andreev bound states (ABSs) are well-defined many-body quantum states that emerge from the hybridization of individual quantum dot (QD) states with a superconductor and exhibit very rich and fundamental phenomena. We demonstrate several electron transport phenomena mediated by ABSs that form on three-terminal carbon nanotube (CNT) QDs, with one superconducting (S) contact in the center and two adjacent normal-metal (N) contacts. Three-terminal spectroscopy allows us to identify the coupling to the N contacts as the origin of the Andreev resonance (AR) linewidths and to determine the critical coupling strengths to S, for which a ground state (or quantum phase) transition in such S-QD systems can occur. In addition, we ascribe replicas of the lowest-energy ABS resonance to transitions between the ABS and odd-parity excited QD states, a process we call excited state ABS resonances. In the conductance between the two N contacts we find a characteristic pattern of positive and negative differential subgap conductance, which we explain by considering two nonlocal processes, the creation of Cooper pairs in S by electrons from both N terminals, and a transport mechanism we call resonant ABS tunneling, possible only in multiterminal QD devices. In the latter process, electrons are transferred via the ABS without effectively creating Cooper pairs in S. The three-terminal geometry also allows spectroscopy experiments with different boundary conditions, for example by leaving S floating. Surprisingly, we find that, depending on the boundary conditions and the device parameters, the experiments either show single-particle Coulomb blockade resonances, ABS characteristics, or both in the same measurements, seemingly contradicting the notion of ABSs replacing the single-particle states as eigenstates of the QD. We qualitatively explain these results as originating from the finite time scale required for the coherent oscillations between the superposition states after a single-electron tunneling event. These experiments demonstrate that three-terminal experiments on a single complex quantum object can also be useful to investigate charge dynamics otherwise not accessible due to the very high frequencies.

Design of Split Ring Resonators for Enhanced Two-Color Operation of Quantum Dot Infrared Photodetectors

In this paper, we propose using metamaterials for enhanced operation of quantum dot infrared photodetectors (QDIPs) in one- and two-color detection and theoretically calculate its absorption and responsivity. To do so, first, a split ring resonator (SRR) is designed and proposed to resonate at the wavelength of intersubband transition from ground state to first excited state of quantum dots, which is around 9.8 $\mu \text{m}$ . The results show enhanced absorption and responsivity by a factor of 1.5. In addition, another SRR is proposed to support two operation wavelengths and designed to resonate with the two peak absorption wavelengths of QDIP. The results of simulation confirm enhanced absorption and responsivity for both the wavelengths at 9.8 and $6~\mu \text{m}$ by a factor of 1.5.

Bandgap Inhomogeneity of a PbSe Quantum Dot Ensemble from Two-Dimensional Spectroscopy and Comparison to Size Inhomogeneity from Electron Microscopy

Femtosecond two-dimensional Fourier transform spectroscopy is used to determine the static bandgap inhomogeneity of a colloidal quantum dot ensemble. The excited states of quantum dots absorb light, so their absorptive two-dimensional (2D) spectra will typically have positive and negative peaks. It is shown that the absorption bandgap inhomogeneity is robustly determined by the slope of the nodal line separating positive and negative peaks in the 2D spectrum around the bandgap transition; this nodal line slope is independent of excited state parameters not known from the absorption and emission spectra. The absorption bandgap inhomogeneity is compared to a size and shape distribution determined by electron microscopy. The electron microscopy images are analyzed using new 2D histograms that correlate major and minor image projections to reveal elongated nanocrystals, a conclusion supported by grazing incidence small-angle X-ray scattering and high-resolution transmission electron microscopy. The absorption bandgap inhomogeneity quantitatively agrees with the bandgap variations calculated from the size and shape distribution, placing upper bounds on any surface contributions.

Chemiluminescence resonance energy transfer between CdS quantum dots and lucigenin and its sensing application

Weak chemiluminescence (CL) of lucigenin was obtained in alkaline solution in the absence of H2O2, which could be apparently enhanced in the presence of CdS, CdSe, and CdTe quantum dots (QDs), and the most intense CL signal was obtained in the CdS QDs involved CL system. The CL spectrum revealed that the light emitter of lucigenin/CdS CL system is the excited state of CdS QDs. The overlap of the lucigenin CL spectrum and the UV–vis absorption spectrum of CdS QDs suggested that CL resonance energy transfer (CRET) could occur between them. The excited state of lucigenin can act as energy donor, and transfer its energy to CdS QDs, resulting in the strong CL emission. Ascorbic acid (AA) could further increase CL intensity, and the increased CL intensities were proportional to the concentrations of AA in the range of 1.0×10−10molL−1 to 5.0×10−6molL−1 with the detection limit of 6.7×10–11molL−1. The proposed CRET system could be successfully applied in the sensitive detection of the level of AA in juice samples.

Giant photocurrent enhancement by transition metal doping in quantum dot sensitized solar cells

A huge enhancement in the incident photon-to-current efficiency of PbS quantum dot (QD) sensitized solar cells by manganese doping is observed. In the presence of Mn dopants with relatively small concentration (4 at. %), the photoelectric current increases by an average of 300% (up to 700%). This effect cannot be explained by the light absorption mechanism because both the experimental and theoretical absorption spectra demonstrate several times decreases in the absorption coefficient. To explain such dramatic increase in the photocurrent we propose the electron tunneling mechanism from the LUMO of the QD excited state to the Zn2SnO4 (ZTO) semiconductor photoanode. This change is due to the presence of the Mn instead of Pb atom at the QD/ZTO interface. The ab initio calculations confirm this mechanism. This work proposes an alternative route for a significant improvement of the efficiency for quantum dot sensitized solar cells.

Theory of photovoltaic characteristics of semiconductor quantum dot solar cells

We develop a comprehensive rate equations model for semiconductor quantum dot solar cells (QDSCs). The model is based on the continuity equations with a proper account for quantum dots (QDs). A general analytical expression for the total current density is obtained, and the current-voltage characteristic is studied for several specific situations. The degradation in the open circuit voltage of the QDSC is shown to be due to strong spontaneous radiative recombination in QDs. Due to small absorption coefficient of the QD ensemble, the improvement in the short circuit current density is negligible if only one QD layer is used. If spontaneous radiative recombination would be suppressed in QDs, a QDSC with multiple QD layers would have significantly higher short circuit current density and power conversion efficiency than its conventional counterpart. The effects of photoexcitation of carriers from discrete-energy states in QDs to continuum-energy states are discussed. An extended model, which includes excited states in QDs, is also introduced.

Electronic Processes within Quantum Dot-Molecule Complexes.

The subject of this review is the colloidal quantum dot (QD) and specifically the interaction of the QD with proximate molecules. It covers various functions of these molecules, including (i) ligands for the QDs, coupled electronically or vibrationally to localized surface states or to the delocalized states of the QD core, (ii) energy or electron donors or acceptors for the QDs, and (iii) structural components of QD assemblies that dictate QD-QD or QD-molecule interactions. Research on interactions of ligands with colloidal QDs has revealed that ligands determine not only the excited state dynamics of the QD but also, in some cases, its ground state electronic structure. Specifically, the article discusses (i) measurement of the electronic structure of colloidal QDs and the influence of their surface chemistry, in particular, dipolar ligands and exciton-delocalizing ligands, on their electronic energies; (ii) the role of molecules in interfacial electron and energy transfer processes involving QDs, including electron-to-vibrational energy transfer and the use of the ligand shell of a QD as a semipermeable membrane that gates its redox activity; and (iii) a particular application of colloidal QDs, photoredox catalysis, which exploits the combination of the electronic structure of the QD core and the chemistry at its surface to use the energy of the QD excited state to drive chemical reactions.

Output power of a quantum dot laser: Effects of excited states

A theory of operating characteristics of quantum dot (QD) lasers is discussed in the presence of excited states in QDs. We consider three possible situations for lasing: (i) ground-state lasing only; (ii) ground-state lasing at first and then the onset of also excited-state lasing with increasing injection current; (iii) excited-state lasing only. The following characteristics are studied: occupancies of the ground-state and excited-state in QDs, free carrier density in the optical confinement layer, threshold currents for ground- and excited-state lasing, densities of photons emitted via ground- and excited-state stimulated transitions, output power, internal and external differential quantum efficiencies. Under the conditions of ground-state lasing only, the output power saturates with injection current. Under the conditions of both ground- and excited-state lasing, the output power of ground-state lasing remains pinned above the excited-state lasing threshold while the power of excited-state lasing increases. There is a kink in the light-current curve at the excited-state lasing threshold. The case of excited-state lasing only is qualitatively similar to that for single-state QDs—the role of ground-state transitions is simply reduced to increasing the threshold current.

Electrogenerated chemiluminescence of quantum dots with lucigenin as coreactant for sensitive detection of catechol

Electrogenerated chemiluminescence (ECL) of quantum dots (QDs), including CdS, CdSe, CdTe, and CdSe@ZnS QDs, was comparatively studied in neutral lucigenin solution without coreactants. Cathodic ECL signals were obtained at −1.2V at different QDs modified gold electrodes (QDs/GEs) and the strongest ECL emission was obtained at the CdSe@ZnS QDs modified GE, which is nearly 7-times larger than other QDs modified GEs. The electrochemical results and the ECL spectrum revealed that the luminophor of the cathodic ECL is the excited state of QDs but not lucigenin, revealing that lucigenin can act as coreactant to generate cathodic ECL with QDs. Several impact factors, such as the amount of QDs, the supporting electrolytes, and the pH were investigated. Under the optimal condition, catechol exhibited apparent inhibiting effect on the cathodic ECL signal, based on which a new ECL sensor was developed and applied in the sensitive detection of catechol. The inhibited ECL intensity varied linearly with catechol concentration in the range of 5–1000nM with a detection limit of 2nM (S/N=3). The ECL sensor exhibited satisfied stability, repeatability, and selectivity. The mechanism of cathodic ECL was also proposed.