Small Quantum Dots Research Articles

(Invited) Surfactant-Directed Quantum Dot Self-Assembly on Unconventional GaAs Surfaces

The pursuit of integrated sources for quantum optics has motivated quantum dot research for decades. However, difficulties in controlling the geometry, symmetry and size-uniformity of self-assembled quantum dots has been a major roadblock in device development. While the Stranski–Krastanov (SK) growth of InAs three-dimensional (3D) islands has been extensively investigated on GaAs(001), the SK growth mode is not observed on GaAs (110) and {111}, the other low index surfaces of GaAs. This is unfortunate as these surfaces have symmetries (Cs and C3v, respectively) which differ from that of the (001) surface, and owing to their low energy they are often present in self-assembled nanostructures such as nanowires. Furthermore, the high symmetry of {111} surfaces is expected to yield quantum dots that are ideal for entangled photon emission.Here we show that using a Bi surfactant to modify surface properties can profoundly influence epitaxial growth on GaAs (110) and (111)A surfaces. On GaAs(110), the Bi surfactant alters the fundamental growth mode of InAs from 2D layer growth to a 3D SK mode. Furthermore, a morphological phase transition can be induced “on-demand” in static strained 2D InAs(110) layers by exposing them to Bi, resulting in a rapid rearrangement of the InAs layer into 3D islands. Small (110) QDs are coherently strained to the substrate. These islands are optically active and exhibit emission that is linearly polarized, showing perspective for polarized single-photon emitters. On GaAs(111)A, GaAs buffer layer growth under a Bi flux results in ultra-smooth surfaces, which are free from the typically-observed morphological defects. Similar to the (110) case, exposing 2D InAs/GaAs(111)A layers to Bi induces the InAs 3D island self-assembly. These findings illustrate how surface-energy-modifying surfactants open the door to QDs synthesis on new substrates and with new materials.

Intrinsic-strain-induced curling of free-standing two-dimensional Janus MoSSe quantum dots

Motivated by the fascinating properties of both two-dimensional transition metal dichalcogenide quantum dots (TMD QDs) and Janus TMD monolayers, we theoretically explore the equilibrium structures of free-standing Janus MoSSe QDs in which atomic asymmetry of chalcogen is introduced. Two distinct types of spontaneous curling are observed by molecular dynamics simulations, and the curling behavior depends on the size of QD. The bowl-like (tube-like) curling occurs in relatively small (large) MoSSe QDs with different shapes (hexagon and triangle) and edge types (zigzag and armchair). The transition between these two curling types occurs at the sizes of around 10 nm and 13 nm for hexagonal and triangular shapes, respectively. By applying equivalent misfit strains into two adjacent sublayers, finite element analysis reproduces similar curling behavior. This confirms the relaxation of intrinsic strain in Janus structure acting as the predominant driving force of spontaneous curling. In addition, the curvatures of Janus TMD QDs increase from MoSSe to MoSeTe to MoSTe, indicating the positive correlation between the curling and misfit.

Can second order nonlinear spectroscopies selectively probe optically "dark" surface states in small semiconductor nanocrystals?

Second order nonlinear responses such as sum frequency and second harmonic generation arise from the response of a material system to the second power of an incident electromagnetic field through the material's first hyperpolarizability or second-order optical susceptibility. These quantities are nonzero only for noncentrosymmetric systems, but different length scales of the noncentrosymmetry give rise to second harmonic or sum frequency radiation with different spatial and coherence characteristics. This perspective discusses the possible contributions to the second-order signal from films of small semiconductor quantum dots and addresses whether such experiments are expected to selectively enhance transitions to surface defects or trap states in such systems. It points out how "surface" and "bulk" contributions to the sum frequency or the second harmonic signal should be distinguishable through their angular dependence in a scattering geometry. It also explores possible mechanisms whereby second order spectroscopies might provide access to surface states that are very weak or absent in other forms of optical spectroscopy.

Quantum size effect and surface defect passivation in size-controlled CsPbBr3 quantum dots

Abstract Luminescent CsPbBr3 quantum dots (QDs) with adjustable size and bandgap were grown by controlling the synthesis temperature in this paper. As the synthesis temperature increases, the QD size becomes larger and then smaller. The corresponding absorption and steady-state photoluminescence spectrum indicate that the QD band gap becomes smaller and then larger, which is a typical quantum confinement effect. Time-resolved photoluminescence spectrum and femtosecond transient absorption spectrum (fs-TAS) show that the non-radiative recombination probability of photocarriers in small QDs is small it has few defects, indicating that the ligand molecules adsorbed on the surface of QDs effectively passivate the surface defects of CsPbBr3. Finally, the hot-phonon bottleneck effect of CsPbBr3 QDs is revealed by the kinetic curves fitting of fs-TAS and the cooling kinetic process of hot carriers is also discussed in detail. This work provides new insights on size-dependent photophysical properties of CsPbBr3 QDs.

High quality silicon: Colloidal quantum dot heterojunction based infrared photodetector

The integration of silicon (Si) and nanomaterials with infrared light harvesting capability is a promising approach to fabricate large area infrared light detecting arrays. However, the construction of a high quality junction between Si and small bandgap colloidal quantum dots (CQDs) remains a challenge, which limited their photodetecting performance in the short wavelength infrared region (1.4 μm–3 μm). Herein, a layer of solution processed ZnO nanoparticles was inserted between silicon and CQDs to passivate the surface dangling bond of silicon. This significantly reduces the carrier recombination between Si and CQDs. Meanwhile, the formation of the Si:CQD heterojunction structure enables effective carrier extraction. As a result, the photodetector shows the detecting range to the short wavelength infrared region (0.8 eV) and achieves a standard detectivity of 4.08 × 1011 Jones at a bias of −0.25 V at room temperature.

Electric dipole spin resonance of two-dimensional semiconductor spin qubits

Monolayer transition metal dichalcogenides (TMDs) offer a novel two-dimensional platform for semiconductor devices. One such application, whereby the added low dimensional crystal physics (i.e. optical spin selection rules) may prove TMDs a competitive candidate, are quantum dots as qubits. The band structure of TMD monolayers offers a number of different degrees of freedom and combinations thereof as potential qubit bases, primarily electron spin, valley isospin and the combination of the two due to the strong spin orbit coupling known as a Kramers qubit. Pure spin qubits in monolayer MoX$_2$ (where X=S or Se) can be achieved by energetically isolating a single valley and tuning to a spin degenerate regime within that valley by a combination of a sufficiently small quantum dot radius and large perpendicular magnetic field. Within such a TMD spin qubit, we theoretically analyse single qubit rotations induced by electric dipole spin resonance. We employ a rotating wave approximation within a second order time dependent Schrieffer-Wolf effective Hamiltonian to derive analytic expressions for the Rabi frequency of single qubit oscilations, and optimise the mechanism or the parameters to show oscilations up to $250\,{\rm MHz}$.

Impact of transition metal ion doping on electron spin relaxation time in CdSe/ZnSe quantum dots

Theoretical calculations of spin relaxation time (SRT) of conduction electrons have been carried out considering the relaxation mediated by acoustic phonons using k.p perturbation theory and envelope function approximation in a transition metal doped II-VI semiconductor quantum dot under the strong confinement regime. In this calculation, we are considering the transitions in the Zeeman sublevels arising due to magnetic impurity doping and applied magnetic field in a Mn doped CdSe/ZnSe quantum dots. The occurrence of spin polarization switching at moderately low applied magnetic field is established in Cd1−xMnxSe/ZnSe quantum dots. The spin relaxation times have been found to be considerably longer with a higher dopant concentration in small magnetic fields (B < 2T) and at very low temperature (T < 50 K) regime. The results may help to demonstrate that, such small quantum dots can successfully be used as polarization switch in different spintronic nano-device.

Phase Transfer and DNA Functionalization of Quantum Dots Using an Easy-to-Prepare, Low-Cost Zwitterionic Polymer.

Small, stable, and bright quantum dots (QDs) are of interest in many biosensing and biomedical imaging applications, but current methodologies for obtaining these characteristics can be highly specialized or expensive. We describe a straightforward, low-cost protocol for functionalizing poly(isobutylene-alt-maleic anhydride) (PIMA) with moieties that anchor to the QD surface (histamine), impart hydrophilicity [(2-aminoethyl)trimethylammonium chloride (Me3N+-NH2)], and provide a platform for biofunctionalization via click chemistry (dibenzocyclooctyne (DBCO)). Guidelines to successfully use this polymer for QD ligand exchange are presented, and an example of biofunctionalization with DNA is shown. Stable QD-DNA conjugates are obtained with high yield and without requiring additional purification steps.

NanoPaint: A Tool for Rapid and Dynamic Imaging of Membrane Structural Plasticity at the Nanoscale.

Single-particle tracking with quantum dots (QDs) constitutes a powerful tool to track the nanoscopic dynamics of individual cell membrane components unveiling their membrane diffusion characteristics. Here, the nano-resolved population dynamics of QDs is exploited to reconstruct the topography and structural changes of the cell membrane surface with high temporal and spatial resolution. For this proof-of-concept study, bright, small, and stable biofunctional QD nanoconstructs are utilized recognizing the endogenous neuronal cannabinoid receptor 1, a highly expressed and fast-diffusing membrane protein, together with a commercial point-localization microscope. Rapid QD diffusion on the axonal plasma membrane of cultured hippocampal neurons allows precise reconstruction of the membrane surface in less than 1 min with a spatial resolution of tens of nanometers. Access of the QD nanoconstructs to the synaptic cleft enables rapid 3D topological reconstruction of the entire presynaptic component. Successful reconstruction of membrane nano-topology and deformation at the second time-scale is also demonstrated for HEK293 cell filopodia and axons. Named "nanoPaint," this super-resolution imaging technique amenable to any endogenous transmembrane target represents a versatile platform to rapidly and accurately reconstruct the cell membrane nano-topography, thereby enabling the study of the rapid dynamic phenomena involved in neuronal membrane plasticity.

Renormalization of the Landé Factor and Effective Mass in Small Spherical Quantum Dots

Using the modified Kane theory, a model has been built for describing the formation of the effective mass and g factor of bound electrons in quantum dots several nanometers in size. It is shown that these values ​​depend critically on the dot size and are significantly different from the corresponding values ​​for the bulk semiconductor. The effective mass and g factor affect the binding energy of an electron on a quantum dot in a magnetic field and are determined by this binding energy, which ultimately forms the local band structure in the vicinity of the quantum dot. In the covariant InAs/AlSb heterostructure, the characteristics have been calculated for which a quantum dot localizes no more than one electron without localizing holes and therefore can serve as a prototype of a solid-state qubit with the controlled g factor.

One step synthesis of graphene quantum dots, graphene nanosheets and carbon nanospheres: investigation of photoluminescence properties

Single layer graphene quantum dots (GQDs), graphene nanosheets (GNSs), and carbon nanospheres (CNSs) were synthesized by an easy, environment-friendly, and low-cost hydrothermal process and the effect of the hydrothermal reaction time on their structural and photoluminescence properties was studied. Four different kinds of samples were prepared only using glucose and deionized water. By varying the hydrothermal reaction time, we obtained excitation-dependent photoluminescent GQDs and GNSs with different photoluminescence quantum yields (PLQYs) and CNSs without any photoluminescence. Atomic force microscopy images showed that the synthesized GQDs are single or double-layered. Transmission electron microscopy (TEM) images indicated that the sizes of the samples are distributed between the small graphene quantum dots (GQDs ≤ 10 nm) for reaction time of 2 h and the homogeneous carbon nanospheres (CNSs ∼ 560 nm) for reaction time of 30 h. Finally, it has shown that the optimized sample exhibits the high PLQY of 22.7% for reaction time of 8 h.

A Low Power Flash ADC using Single-Electron Transistor

This paper proposes an analog-digital converter (ADC) using Single-electron transistor (SET).Single Electron Transistor is Nanodevice having a small quantum dot or island instead of the channel that works on the principle of Coulomb blockadewhich allows one electron tunnelingat a time from source to drain terminal. SET operates at low voltage and consumes lesspower. The proposed Flash ADC consists of SET based priority encoder and comparator circuits. The proposed design offers large input/output voltage swing, ultra-low power, compact circuit block for ADC compared to SET/CMOS hybrid amplifier-based ADC. In this paper, we have designed a 4-bit and 8bitFlash ADC using SET operating at room temperature and the performance estimationwas performed by using the CADENCE VIRTUOSO simulator.

Effects of thiol ligands on the growth and stability of CdS nanoclusters

A DFT calculation of small CdnSn QDs (n = 6) has been achieved in the study of CdS-ligand systems using DFT and TDDFT calculations. We discussed the influence of three thiol ligands, namely, 3-Mercaptopropionic Acid (MPA), Thioglycerol (TG) and 2-Mercaptoethanol (ME), on the size of CdS QDs. In addition, both the carboxyl group of MPA and the hydroxyl group of TG and ME affected the mechanisms of bonding between ligands and the cadmium atom of the cluster. The interaction between CdS QDs and thiol ligands (R–SH) form stable bonds Cd–S. Increasing the number of (R–SH)−Cd bonds using different ligands ensured the good cohesion of the nanoparticle as well as better confinment. Furthermore, TEM and FTIR analyses confirmed the size, shape and morphology calculated using the DFT.

Modification of a plasmonic nanoparticle lifetime by coupled quantum dots

In this study, the interaction between a plasmonic nanoparticle and coupled quantum dots is investigated to identify how the coupled particles can manipulate the plasmonic nanoparticle decay rate. This subject is very important, because most applications of the plasmonic system are restricted due to the nanoparticle decay rate and the related losses. Therefore, in the present work, we try to find out how and by which method the plasmonic nanoparticle decay rate can be manipulated. For this purpose, a plasmonic system containing a nanoparticle coupled to some small quantum dots is designed. The system dynamics of motions are analyzed with Heisenberg-Langevin equations. These equations are analyzed to study the effect of the plasmonic nanoparticles on the quantum dots' decay rate. In the following, as an interesting point, the quantum dot coupling influence on the nanoparticle's decay rate is theoretically analyzed in the transient and steady-state conditions. Additionally, a theoretical formula is derived by which one can explicitly find the dependency of the modified decay rate of the plasmonic nanoparticle on the number of the coupled quantum dots and the coupling strength. The simulation results show that it is possible to effectively control the nanoparticles' decay rate with regard to the application for which they are utilized.

Electrospun Fibers Containing Emissive Hybrid Perovskite Quantum Dots.

We demonstrate a single-step fabrication process of highly stable and luminescent polymer fibers embedded with quantum dots (QDs) of the organic-inorganic hybrid perovskite (OIP) (CH3NH3PbBr3) using the electrospinning process. The fiber (∼2 μm diameter) primarily consists of poly(methyl methacrylate) dispersed with clusters of OIP quantum dots. The OIP clusters are radially distributed, normal to the fiber axis. The photoluminescence quantum yield (PLQY) is high (∼80%) and comparable to that of conventional QDs. The emission maxima are tunable by varying the concentration of OIP precursor in the electrospinning solution. Submicron emission maps show an isotropic and continuous emission along the fiber, suggesting uniform distribution of QD clusters. Temperature-dependent PL response indicates features which are a function of the particle size. For small QDs, the PLQY(T) maxima are close to the ambient temperature, whereas the PLQY(T) maxima shift sizably to T < 50 K for larger QDs. Significant waveguiding of QDs emission and amplified spontaneous emission, a prerequisite for lasing, were observed in the fiber confined OIP system at room temperature.

Time-dependent first-principles study of optical response of BaTiO3 quantum dots coupled with silver nanowires**Project support by the National Key Research and Development Program of China (Grant No. 2017YFA0303600) and the National Natural Science Foundation of China (Grant No. 11474207).

All-inorganic perovskite quantum dots (QDs) have drawn much attention due to their prominent quantum-size effects and highly tunable optical properties. Tuning the size of perovskite QDs is attractive for many potential applications. For instance, smaller QDs exhibit more evident quantum properties than larger QDs, but present a blue-shifted spectrum, which limits their applications. Here, we conduct a systematically theoretical analysis about the optical response and plasmon resonance of comparatively small barium titanate quantum dots (BTO–QDs) coupled with silver (Ag) nanowires based on time-dependent density functional theory (TDDFT). Our results show that the silver nanowires can induce an intense optical response respectively in the infrared and visible region to eliminate the spectrum-shift. Furthermore, the absorption spectrum and plasmon resonance can be effectively modified by either altering the position of the silver nanowires or changing the thickness of the BTO–QDs. More importantly, these two methods can act simultaneously, this maybe provide a new approach to implementing the quantum control.

Ratiometric determination of hydrogen peroxide based on the size-dependent green and red fluorescence of CdTe quantum dots capped with 3-mercaptopropionic acid.

A ratiometric fluorescent nanoprobe is described for the detection of H2O2. It is based on the use of a mixture of green-emitting CdTe quantum dots (GQDs) and red-emitting CdTe QDs (RQDs). The two kinds of QDs have different size and different fluorescence response towards H2O2. The ratio of the emission intensities at 606 and 510nm (under 365nm photoexcitation) can be used as the analytical information. Even without any chemical modification of the surface of the QDs, the probe display high sensitivity and selectivity for H2O2. The fluorescence of small QDs is more effectively quenched by H2O2. Stern-Volmer analysis showed both static and dynamic quenching to occur. The probe works well in the 10~125μM H2O2 concentration range and has a 0.3μM detection limit (3σ/slope). Graphical abstract Schematic presentation of the ratiometric fluorescent nanoprobe composed of green-emitting and red-emitting CdTe QDs. λ, I, and k are the emission wavelength, emission intensity, and quenching/enhancement coefficient, respectively. The subscript 0 and 1 present the green-emitting and red-emitting CdTe QDs, respectively.

Engineered Exosome-Mediated Near-Infrared-II Region V2C Quantum Dot Delivery for Nucleus-Target Low-Temperature Photothermal Therapy.

The limited penetration depth of photothermal agents (PTAs) active in the NIR-I biowindow and the thermoresistance caused by heat shock protein (HSP) significantly limit the therapeutic efficiency of photothermal therapy (PTT). To address the problem, we introduce a strategy of low-temperature nucleus-targeted PTT in the NIR-II region achieving effective tumor killing by combining the vanadium carbide quantum dots (V2C QDs) PTA and an engineered exosomes (Ex) vector. The small fluorescent V2C QDs with good photothermal effect in the NIR-II region were modified with TAT peptides and packaged into Ex with RGD modification (V2C-TAT@Ex-RGD). The resulting nanoparticles (NPs) exhibited good biocompatibility, long circulation time, and endosomal escape ability, and they could target the cell and enter into the nucleus to realize low-temperature PTT with advanced tumor destruction efficiency. The fluorescent imaging, photoacoustic imaging (PAI), and magnetic resonance imaging (MRI) capability of the NPs were also revealed. The low-temperature nucleus-targeted PTT in the NIR-II region provides more possibilities toward successful clinical application of PTT.

The effective Hamiltonian and propagator of a parabolic confined dissipative electron under a perpendicular magnetic field

We study the model of an electron in an isotropic quantum dot in the presence of a perpendicular magnetic field and dissipation. Starting from the system plus bath approach, the quantum Langevin equations are first obtained and consequently the equation of motion. Then the effective Hamiltonian is derived from the equation of motion. We obtained a new effective Hamiltonian for the case when the magnetic field and dissipation are both present. This effective Hamiltonian is quite different from the one used widely for time-dependent harmonic oscillators under a magnetic field in earlier studies. The corresponding propagator for this new effective Hamiltonian is also calculated. As an example of application, the dissipative dynamics of a Gaussian wavepacket confined in small quantum dots in the presence of a perpendicular magnetic field is studied.

Interdot carrier and spin dynamics in a two-dimensional high-density quantum-dot array of InGaAs with quantum dots embedded as local potential minima

Interdot carrier and spin dynamics were studied in a two-dimensional array of high-density small quantum dots (SQDs) of InGaAs with an average diameter of 16 nm and a sheet density of 1.2 , in which 24 nm diametric large QDs (LQDs) were embedded as local potential minima. We observed a delayed photoluminescence (PL) rise from the lower-lying LQD states and a considerably faster PL decay from the higher-lying SQD states, indicating carrier transfer from the two-dimensionally coupled SQDs into the LQDs. In addition, inverse carrier tunneling from the LQDs into the SQDs was thermally induced, which is characterized by the thermal activation energy between the LQDs and SQDs. Moreover, circularly polarized transient PL behavior from the SQD states exhibits a suppression of the spin polarization decay in the initial time region, depending on the excited spin density. This tentatively suppressed spin relaxation can be quantitatively explained by selective interdot transfer of minority-spin electrons from the SQDs into LQDs, when the majority spin states in both QDs are sufficiently populated by excited spins. These findings indicate that the high-density SQDs behave as the main emitters with suppressed spin relaxation, while the scattered LQDs with lower potential behave as the receivers of minority-spin electrons.