States In Quantum Dots Research Articles

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.

Flopping-Mode Electric Dipole Spin Resonance in Phosphorus Donor Qubits in Silicon

Single spin qubits based on phosphorus donors in silicon are a promising candidate for a large-scale quantum computer. Despite long coherence times, achieving uniform magnetic control remains a hurdle for scale-up due to challenges in high-frequency magnetic field control at the nanometre scale. Here, we present a proposal for a flopping-mode electric dipole spin resonance qubit based on the combined electron and nuclear spin states of a double phosphorus donor quantum dot. The key advantage of utilizing a donor-based system is that we can engineer the number of donor nuclei in each quantum dot. By creating multidonor dots with antiparallel nuclear spin states and multielectron occupation we can minimize the longitudinal magnetic field gradient, known to couple charge noise into the device and dephase the qubit. We describe the operation of the qubit and show that by minimizing the hyperfine interaction of the nuclear spins we can achieve $\ensuremath{\pi}/2\ensuremath{-}X$ gate error rates of about ${10}^{\ensuremath{-}4}$ using realistic noise models. We highlight that the low charge noise environment in these all-epitaxial phosphorus-doped silicon qubits will facilitate the realization of strong coupling of the qubit to superconducting microwave cavities, allowing for long-distance two-qubit operations.

Quantum Chemical Characterization and Design of Quantum Dots for Sensing Applications.

The ability to tune the optoelectronic properties of quantum dots (QDs) makes them ideally suited for the use as fluorescence sensing probes. The vast structural diversity in terms of the composition and size of QDs can make designing a QD for a specific sensing application a challenging process. Quantum chemical calculations have the potential to aid this process through the characterization of the properties of QDs, leading to their in silico design. This is explored in the context of QDs for the fluorescence sensing of dopamine based upon density functional theory and time-dependent density functional theory (TDDFT) calculations. The excited states of hydrogenated carbon, silicon, and germanium QDs are characterized through TDDFT calculations. Analysis of the molecular orbital diagrams for the isolated molecules and calculations of the excited states of the dopamine-functionalized quantum dots establish the possibility of a photoinduced electron-transfer process by determining the relative energies of the electronic states formed from a local excitation on the QD and the lowest QD → dopamine electron-transfer state. The results suggest that the Si165H100 and Ge84H64 QDs have the potential to act as fluorescent markers that could distinguish between the oxidized and reduced forms of dopamine, where the fluorescence would be quenched for the oxidized form. The work contributes to a better understanding of the optical and electronic behavior of QD-based sensors and illustrates how quantum chemical calculations can be used to inform the design of QDs for specific fluorescent sensing applications.

Demonstration and modeling of time-bin entangled photons from a quantum dot in a nanowire

Resonant excitation of the biexciton state in an InAsP quantum dot by a phase-coherent pair of picosecond pulses allows preparing time-bin entangled pairs of photons via the biexciton–exciton cascade. We show that this scheme can be implemented for a dot embedded in an InP nanowire. The underlying physical mechanisms can be represented and quantitatively analyzed by an effective three-level open system master equation. Simulation parameters including decay and intensity dependent dephasing rates are extracted from experimental data, which in turn let us predict the resulting entanglement and optimal operating conditions.

Characterization of electronic charged states of high density self-aligned Si-based quantum dots evaluated with AFM/Kelvin probe technique

We have characterized charged states of a self-aligned Si quantum dot (QD) structure with an areal density as high as ∼1013 cm−2 by surface potential measurements by using atomic force microscopy (AFM)/Kelvin probe force microscopy (KFM). By line scanning with the electrically-biased AFM tip with respect to the substrate in a contact mode, the corresponding area was negatively charged caused by electron injection from the tip into QDs. Cross-sectional profiles of the surface potential were clearly changed, which depends on the tip bias voltages. Also, a stepwise shape in a relationship between the change of the surface potential (ΔV) and the tip bias voltages was observed, reflecting electron charging into the discrete energy levels due to the quantum confinement effect. By assuming a simple equivalent circuit model for KFM measurements of the self-aligned Si-QDs, calculated results for ΔV are in good agreement with the measured results. The agreements precisely clarified the charged states in Si-QDs.

Thermally Activated Bright-State Delayed Blue Photoluminescence from InP Quantum Dots.

Thermally activated delayed photoluminescence (TADPL) generated from organic chromophore-functionalized quantum dots (QDs) is potentially beneficial for persistent light generation, QD-based PL sensors, and photochemical synthesis. While previous research demonstrated that naphthoic acid-functionalized InP QDs can be employed as low-toxicity, blue-emissive TADPL materials, the electron trap states inherent in these nanocrystals inhibited the observation of TADPL emerging from the higher-lying bright states. Here, we address this challenge by employing the heterocyclic aromatic compound 8-quinolinecarboxylic acid (QCA), whose triplet energy is strategically positioned to bypass the electron trap states in InP QDs. Transient absorption and photoluminescence spectroscopies revealed the generation of bright-state TADPL from QCA-functionalized InP QDs resulting from a nearly quantitative Dexter-like triplet-triplet energy transfer (TTET) from photoexcited InP QDs to surface-anchored QCA chromophores followed by reverse TTET from these bound molecules to the InP QDs. This modification resulted in a 119-fold increase in the average PL intensity decay time with respect to the as-synthesized InP QDs.

Direct transport between superconducting subgap states in a double quantum dot

We demonstrate direct transport between two opposing sets of Yu-Shiba-Rusinov (YSR) subgap states realized in a double quantum dot. This sub-gap transport relies on intrinsic quasiparticle relaxation, but the tunability of the device allows us to explore also an additional relaxation mechanism based on charge transferring Andreev reflections. The transition between these two relaxation regimes is identified in the experiment as a marked gate-induced stepwise change in conductance. We present a transport calculation, including YSR bound states and multiple Andreev reflections alongside with quasiparticle relaxation, due to a weak tunnel coupling to a nearby normal metal, and obtain excellent agreement with the data.

Electronic and optical properties of Janus black arsenic-phosphorus AsP quantum dots under magnetic field

By utilizing the tight-binding method, the electronic spectrum and states distribution of square Janus monolayer black arsenic phosphorus (b-AsP) quantum dots (QDs) in the presence of a perpendicular magnetic field are explored. Strong in-gap states of b-AsP QDs, whose probability densities are distributed on the armchair boundary (armchair edge states) appear in the energy gap of host perfect two-dimensional b-AsP. The corresponding energy levels of the armchair edge states can degenerate to the Landu energy levels upon applying a perpendicular magnetic field. When an in-plane polarized light is introduced, due to the presence of armchair edge states, the edge-to-edge transitions are mainly induced from the armchair edge (hole) states to zigzag edge (electron) states. The optical absorption undergoes blue shift as a function of the magnetic field. Our work suggests tunable optical properties via modulating the armchair edge states of a b-AsP QD and provides a theoretical basis for the design of b-AsP-based optoelectronic devices.

How Valley-Orbit States in Silicon Quantum Dots Probe Quantum Well Interfaces.

The energies of valley-orbit states in silicon quantum dots are determined by an as yet poorly understood interplay between interface roughness, orbital confinement, and electron interactions. Here, we report measurements of one- and two-electron valley-orbit state energies as the dot potential is modified by changing gate voltages, and we calculate these same energies using full configuration interaction calculations. The results enable an understanding of the interplay between the physical contributions and enable a new probe of the quantum well interface.

Experimental investigation of stripe cavity length effect on threshold current density for InP/AlGaInP QD laser diode

InP/AlGaInP QD laser diodes with various cavity lengths grown on GaAs substrate were synthesized by (MOVPE) technique. L-I characteristics at various temperatures (190–390K) were measured for InP/AlGaInP QD laser diodes to investigate the effect of the various stripe cavity length on threshold current density and characteristic temperature. It is found that the threshold current increases with temperature, reaches a peak, then drops as temperature increases, before it increases again at temperatures over 300K due to carrier redistribution in the quantum dot states with temperature. It is found also that the threshold current density decreases with increasing cavity length due to the increasing threshold gain with the cavity length. However, as the cavity length increases beyond certain length, the rate of the decrease of the threshold current with cavity length decreases. One important finding is that the characteristic temperature of threshold current decreases significantly with the cavity length at the operating temperature range from 190K to 360K.

Redshifted biexciton and trion lines in strongly confined (211)B InAs/GaAs piezoelectric quantum dots

The emission lines of strongly confined (211)B InAs/GaAs quantum dots (QDs), embedded in short-period GaAs/AlAs superlattices, are thoroughly characterized by a range of single-dot spectroscopy techniques, including cross correlation photon-counting measurements. Contrary to what is expected for a piezoelectric QD system, the single-dot biexciton line is found redshifted with respect to the exciton one by as many as 6 meV. This comes in striking contrast to previous reports on the same QD system, without additional confinement, where the biexciton lines always showed up at higher energies than the exciton, by 4–13 meV. In addition, two charged exciton lines are identified for the first time in a piezoelectric InAs-based QD. A positively charged (Χ+) and a negatively charged (Χ−) trion line are observed 1.5 and 7.5 meV below the neutral exciton line, respectively. Our results pave the way to an enhanced understanding of the excitonic transitions in (211)B QDs and highlight the possible role of strong confinement and accompanying correlation effects as a means to tailor the transition energies of multi-particle states in semiconductor QDs.

Photoluminescence Enhancement of NIR‐II Emissive Ag2S Quantum Dots via Chloride‐Mediated Growth and Passivation

AbstractAg2S quantum dots (QDs) with high photoluminescence quantum yield (PLQY) in the second near‐infrared (NIR‐II, 1000–1700 nm) window are urgently pursued. Halide atoms (especially chloride) have been studied as inorganic capping ligands on improving the PLQY of Cd‐, Pb‐based chalcogenide QDs; however, it remains a challenge to well replace the organic ligand with chloride ligand from silver chalcogenide QDs. Herein, a novel strategy to prepare Ag2S QDs with significant photoluminescence (PL) enhancement via chloride‐mediated growth and passivation is demonstrated. The oleylamine and oleate ligands on the surface of silver‐rich Ag2S QDs are readily replaced by chloride ligands due to the smaller size and higher binding affinity with Ag+. Upon partial ligand exchange and chloride passivation, the surface trap states in Ag2S QDs are reduced through a decrease of unsaturated surface Ag atoms. Therefore, Ag2S QDs with approximately 300‐fold enhancement of PL intensity, and a PLQY of 47.6% at 1220 nm emission, and a lifetime of 256 ns are obtained because the nonradiative recombination is inhibited. This work reveals that the passivation of Ag2S QDs with chloride ligands is fully feasible, providing new insights into improving the PLQY of silver chalcogenide QDs.

Lifetime enhancement of quasibound states in graphene quantum dots via circularly polarized light

Permanent localization of electrons inside a graphene quantum dot (GQD) is known to be forbidden, as a manifestation of Klein tunneling. However, an electron which scatters on a GQD may be transiently trapped inside and one known practice is the usage of magnetic field. These electronic states discussed here, called quasibound states, are scattering resonances typically characterized by a finite lifetime (trapping time). In this paper, we present a theoretical perspective concerning the opportunity to enhance the lifetime of quasibound states excited in a GQD placed in a uniform magnetic field, using circularly polarized light. Generally speaking, electron trapping inside GQDs is achievable for certain well-defined conditions, for instance, magnetic field intensity. We report here that the trapping time of an electron inside a GQD may be successfully enhanced by adjusting the light intensity while keeping the magnetic field constant.

Coupled cluster theory for the ground and excited states of two-dimensional quantum dots

We present a study of the two-dimensional circular quantum dot model Hamiltonian using a range of quantum chemical ab initio methods. Ground and excited state energies are computed on different levels of perturbation theories, including the coupled cluster method. We outline a scheme to compute the required Coulomb integrals in real space and utilize a semianalytic solution to the integral over the Coulomb kernel in the vicinity of the singularity. Furthermore, we show that the remaining basis set incompleteness error for two-dimensional quantum dots scales with the inverse number of virtual orbitals, allowing us to extrapolate to the complete basis set limit energy. By varying the harmonic potential parameter we tune the correlation strength and investigate the predicted ground and excited state energies.

Quantum Dot Acceptors in Two-Dimensional Epitaxially Fused PbSe Quantum Dot Superlattices.

Oriented attachment of colloidal quantum dots allows the growth of two-dimensional crystals by design, which could have striking electronic properties upon progress on manipulating their conductivity. Here, we explore the origin of doping in square and epitaxially fused PbSe quantum dot superlattices with low-temperature scanning tunneling microscopy and spectroscopy. Probing the density of states of numerous individual quantum dots reveals an electronic coupling between the hole ground states of the quantum dots. Moreover, a small amount of quantum dots shows a reproducible deep level in the band gap, which is not caused by structural defects in the connections but arises from unpassivated sites at the {111} facets. Based on semiconductor statistics, these distinct defective quantum dots, randomly distributed in the superlattice, trap electrons, releasing a concentration of free holes, which is intimately related to the interdot electronic coupling. They act as acceptor quantum dots in the host quantum dot lattice, mimicking the role of dopant atoms in a semiconductor crystal.

Electronic, Optical, and Magnetic Properties of Doped Triangular MoS2 Quantum Dots: A Density Functional Theory Approach

A theoretical investigation on electronic states in triangular MoS2 quantum dots of different sizes is performed within density functional theory first‐principles formalism. Herein, the associated interband optical response from real and imaginary parts of the dielectric function is calculated. The study considers both undoped and Al‐, Si‐, and P‐doped systems. Spin‐related magnetic properties are considered through the evaluation of total magnetic moment. It is revealed that small enough structures have a semiconductor character, but evolve to seemingly half‐metallic with increasing dot size. Nonzero magnetic response is attributed to edge effects. Magnetic behavior of doped systems deviates from the linear dependence of the total magnetic moment with size occurring in the undoped case, producing situations with higher total momentum values. It is found that increasing the doping density may have certain influence over total magnetic moment response of the structures, although edge dominance is kept all the way. Optical properties are not as sensitive to incident light polarization, but they actually do with regard to the particular spin orientation. Calculation for the refraction index in the triangular quantum dots shows values still below the accepted in the MoS2 monolayer case, although a correct trend of variation is reported in this sense.

Investigation the three dimensional bound states in quantum dot nanowire systems

Electronic confined states of a cylindrical InP quantum dot embedded in infinite GaP nanowire is studied theoretically. To this end the Schrodinger equation is solved numerically in the effective mass approximation. The subband separation between the GaP nanowire and InP/GaP core–shell nanowire is utilized to predict the possible existence of the three dimensionally confined states of the system. Using this, the nanowire critical radius is calculated for each channel of angular momentum, the frontier of type I-Type II transition in the InP/GaP quantum dot nanowire system. It is shown that the lower limit of the continuum energy depends on the angular momentum and increases by it, causing discrete bound states of higher angular momentum to lie in the continuum of the lower ones. Surviving the energy states in the presence of external magnetic field reveals stronger confinement for electrons in states with negative angular momentum quantum numbers.

Contributions of exciton fine structure and hole trapping on the hole state filling effect in the transient absorption spectra of CdSe quantum dots.

The optoelectronic properties of quantum confined semiconductor nanocrystals depend critically on the band edge electron and hole levels and their exciton fine structures. Transient absorption (TA) spectroscopy has been widely used to probe the dynamics of photogenerated electrons, holes, and excitons in these materials through their state filling induced bleach of the band edge exciton transition. Such effects, in principle, reflect the band edge fine structures and are well understood for the conduction band electrons. However, the valence band hole state filling signals remain poorly understood due to the complexity of the valence band level structure and the presence of fast hole trapping in many materials. Herein, we report a study of the valence band hole state filling effect by comparing the TA spectra of CdSe quantum dots (QDs) with different degrees of hole trapping and by selective removal of the conduction band electrons to adsorbed methyl viologen molecules. We observe that in CdSe/CdS core/shell QDs with a high photoluminescence quantum yield of 81%, the valence band hole contributes to 22% ± 1% of the exciton bleach, while a negligible hole state filling signal is observed in CdSe core only QDs with a photoluminescence quantum yield of 17%. This hole state filling effect can be explained by a simplified valence band edge hole model that contains two sets of twofold degenerate hole levels that are responsible for the higher energy bright exciton and lower energy dark exciton states, respectively. Our result clarifies the TA spectral features of the valence band holes and provides insights into the nature of single hole states in CdSe-based QDs.

In-plane electric and exchange fields engineered corner state bismuthene quantum dots

The energy spectrum of bismuthene quantum dots in the presence of in-plane electric and exchange fields are investigated by the tight-binding method. In the case of an in-plane exchange field, bismuthine is a second-order topological insulator and supports corner states in triangular and hexagonal quantum dots with zigzag edges. The corner states in hexagonal bismuthene quantum dots are more stable than the corner states in triangular quantum dots. In hexagonal quantum dots, the corner states are degenerate at zero energy as the exchange field h > 0.18 eV. However, the degeneracy of corner states can be destroyed by the electrical fields, and the energy levels of corner states oscillate with respect to the electric field direction. In addition, the on-site disorders will not destroy the corner states but lead to the separation of these states around zero energy.

Mixed magnetic edge states in graphene quantum dots

Graphene quantum dots (GQDs) exhibit abundant magnetic edge states with promising applications in spintronics. Hexagonal zigzag GQDs possess a ground state with an antiferromagnetic (AFM) inter-edge coupling, followed by a metastable state with ferromagnetic (FM) inter-edge coupling. By analyzing the Hubbard model and performing large-scale spin-polarized density functional theory calculations containing thousands of atoms, we predict a series of new mixed magnetic edge states of GQDs arising from the size effect, namely mix-n, where n is the number of spin arrangement parts at each edge, with parallel spin in the same part and anti-parallel spin between adjacent parts. In particular, we demonstrate that the mix-2 state of bare GQDs (C) appears when and the mix-3 state appears when , where N is the number of six-membered-ring at each edge, while the mix-2 and mix-3 magnetic states appear in the hydrogenated GQDs with N = 13 and N = 15, respectively.