Quantum Dot Nanostructures Research Articles

ZrO2 Nanoflowers Decorated with Graphene Quantum Dots for Electrochemical Immunosensing

In the present work, hybrid nanostructures of graphene quantum dots decorated zirconia (GQDs@ZrO2) were synthesized by a facile one-pot hydrothermal method and utilized as an efficient electrochemi...

Spatially structured transparency and transfer of optical vortices via four-wave mixing in a quantum-dot nanostructure

We propose a scheme to generate structured light using a medium consisting of semiconductor quantum dots via four-wave mixing (FWM). We consider a four-level quantum-dot structure where two strong electromagnetic fields couple upper-level biexciton transitions, whereas a weak field is applied to one of the ground-level exciton transitions. A weak signal field is generated, corresponding to a second ground-level exciton transition; thus the overall configuration transforms into a closed-loop diamond-type shape. To realize a spatially dependent modulating quantum-dot medium, we consider strong coupling fields as structured lights and investigate different regions of spatially structured electromagnetically induced transparency via absorption of the generated signal field. The azimuthal phase-dependent modulation of the absorption profile of the generated signal field is the key factor behind the creation of structured light. In addition, we also demonstrate transfer of orbital angular momentum (OAM) of the control beam to the generated beam through the FWM process and study the effect of phase mismatch on efficiency of the OAM transfer.

The Complexity of the Homeopathic Healing Response Part 2: The Role of the Homeopathic Simillimum as a Complex System in Initiating Recovery from Disease.

Evidence indicates that homeopathic medicines are complex self-organizing nano-scale systems that generate unique low-intensity electromagnetic signals and/or quantum coherence domains. In Part 1, we reviewed relevant concepts from complex adaptive systems science on living systems for the nature of homeopathic healing. In Part 2, we discuss the complex-system nature of homeopathic medicines. The aim is to relate the evidence on the nature and properties of homeopathic medicines to the complex systems model for homeopathic healing. The work is a narrative review, with complexity model development for the nature of homeopathic medicines. Studies suggest that homeopathic manufacturing generates nano-structures of source material, silica and silicon quantum dots if succussed in glassware or including botanical source materials; or carbon quantum dots if succussed in plastic or including any organic source materials, as well as solute-induced water nano-structures carrying medicine-specific information. On contact with physiological fluids (e.g., blood plasma), there is evidence that nano-structures additionally adsorb individualized patterns of the recipient's own proteins on to their surfaces to create a unique protein corona coat (shell). Thus, the simillimum may generate a personalized biological identity upon administration. Consequently, a medicine can serve as an individually salient, self-similar information carrier, whose protein corona constituent pattern reflects the individual's current internal state of health/disease. Homeopathic medicine complexity emerges from interactions of the component parts from source, silica from glassware or carbon from plastic containers, solvents (lactose, water, ethanol), adsorbed biomolecule layers from plant or animal sources, and adsorbed biomolecules of the recipient. Low doses of these complex medicines can act as biological signaling agents to initiate hormesis via a network-wide pattern of adaptive responses by the recipient complex adaptive system, rather than as conventional pharmaceutical drugs. Biological mediators of adaptive responses include inter-connected network elements of the cell danger/damage defense system: for example, gene expression, reactive oxygen species, heat shock proteins, cytokines, macrophages, T-cells, and associated brain-immune system mediator pathways. Every homeopathic medicine is a complex nano-scale system involving multiple inter-connected, interacting components, and emergent properties. Simillimum individualization derives from formation of a unique personalized protein corona shell adsorbed to the reactive surface of the homeopathic nano-structures on contact with the recipient's body fluids. Low doses of such complex nano-structures initiate the adaptive processes of hormesis to mobilize endogenous healing of a disease state. The capacity for self-organization and self-similarity in complex systems is the key to future research on the nature of homeopathic medicines and systemic healing during individualized homeopathic treatment.

Two-Dimensional Phosphorene, Arsenene, and Antimonene Quantum Dots: Anomalous Size-Dependent Behaviors of Optical Properties

Employing size effect to tailor photoabsorption and photoemission properties of quantum dot (QD) nanostructures is a widely used method for optoelectronic and biomedical applications. In this work,...

Nonlinear Seebeck effect of SU(N) Kondo impurity

We develop a theoretical framework to study the influences of coupling asymmetry on the thermoelectrics of a strongly coupled SU($N$) Kondo impurity based on a local Fermi liquid theory. Applying non-equilibrium Keldysh formalism, we investigate charge current driven by the voltage bias and temperature gradient in the strong coupling regime of an asymmetrically coupled SU($N$) quantum impurity. The thermoelectric characterizations are made via non-linear Seebeck effects. We demonstrate that the beyond particle-hole (PH) symmetric SU($N$) Kondo variants are highly desirable with respect to the corresponding PH symmetric setups in order to have significantly improved thermoelectric performance. The greatly enhanced Seebeck coefficients by tailoring the coupling asymmetry of beyond PH symmetric SU($N$) Kondo effects are explored. Apart from presenting the analytical expressions of asymmetry dependent transport coefficients for general SU($N$) Kondo effects, we make a close connection of our findings with the experimentally studied SU(2) and SU(4) Kondo effects in quantum dot nano structures. Seebeck effects associated with the theoretically proposed SU(3) Kondo effects are discussed in detail.

Nanoscale Patterning of Colloidal Nanocrystal Films for Nanophotonic Applications Using Direct Write Electron Beam Lithography.

The small size of colloidal nanocrystal quantum dots (QDs) leads to a variety of unique optical properties that are well-suited to nanophotonics, including bright, tunable photoluminescence (PL). However, exploring the properties of solid QD assemblies at the nanoscale has proven challenging because of the limitations in the nanoscale QD patterning methods. Generally, the precise placement of QD solids is difficult to achieve, especially for tall structures with multiple QD layers, and when it is achieved the patterns often cannot withstand the further processing steps required for final device construction. Direct electron beam lithography of QDs has emerged as a straightforward patterning process that does not require ligand exchange and results in structures that retain bright PL. Here, we demonstrate that direct patterning QD films on substrates treated with a self-assembled monolayer of octadecyltrichlorosilane allows us to create feature sizes as thin as 30 nm with heights of multiple layers and characterize the pattern resolution, robustness, and placement accuracy. These structures withstand sonication in a variety of solvents, and the structures are placed within 20 nm of their intended location nearly 100% of the time. We further show how this patterning method can be applied to nanophotonics by measuring the complex refractive index of the QD materials to model the absorption and scattering cross sections of QD structures of various sizes and shapes. These simulations reveal that edge effects arising from the finite shape of the QD nanostructure lead to substantial absorption enhancement when compared to an equivalent volume region taken from a continuous QD film. Finally, we explore more complex structures by patterning QD arrays, multilayer QD structures, and QD disks inside plasmonic resonators.

Control of Self-Kerr Nonlinearity in a Driven Coupled Semiconductor Quantum Dot–Metal Nanoparticle Structure

The self-Kerr nonlinear optical response of a probe field is investigated in the presence of a near-resonant pump field in coupled metal nanoparticle (MNP)–semiconductor quantum dot (SQD) nanostructures. The study of the spectra is based on the solution of the resulted density matrix equations for the case of a colloidal CdSe-based SQD–(Au)MNP molecule, as well as for an epitaxial SQD structure coupled to an Au nanoparticle. It is demonstrated that below a critical value of the interparticle distance the susceptibility of each distinct component of the structure exhibits a characteristic single-resonance spectrum, whereas above this value, the spectrum has three resonances. This phenomenon can be understood in terms of the theory of exciton–plasmon quantum metastates. In addition, the conditions under which optical transparency occurs with a concurrent positive self-phase-modulation coefficient are also identified.

Branched Polyethylenimine-Modified Upconversion Nanohybrid-Mediated Photoelectrochemical Immunoassay with Synergistic Effect of Dual-Purpose Copper Ions

This work developed a near-infrared (near-IR) light-activated non-enzymatic signal-off photoelectrochemical (PEC) immunoassay for the ultrasensitive detection of α-fetoprotein (AFP) on the basis of branched polyethylenimine (BPEI)-modified upconversion nanoparticle (UCNP)@CdTe quantum dot (QD) nanostructures by coupling with the synergistic effect of dual-purpose copper ions. Emission light originated from NaYF4:Yb,Er UCNP was directly utilized through the electrostatic bonding of CdTe QDs to excite the separation of electron-hole pairs, resulting in the generation of obvious photocurrent under a 980 nm laser. By using polyclonal antibody-labeled cupric oxide nanoparticle as the secondary antibody, the nanolabel was introduced into the monoclonal anti-AFP antibody-modified microplates in the presence of target AFP. After treatment with acid, the as-released copper ion decreased the photocurrent through the synergistic effect with two issues: one was initially to form coordination with BPEI on the surface of UCNP, and then the near-IR excitation light and upconversion luminescence were attenuated due to the internal filter effect; another was to snatch the electrons flowing from the valence band of CdTe QD as the exciton trapping sites. Under optimal conditions, the dual-purpose Cu2+-activated signal-off PEC immunosensing platform exhibited a dynamic linear range from 10 pg mL-1 to 50 ng mL-1, accompanying the decreasing photocurrent with the increment of AFP concentration at an experimental detection limit of 1.2 pg mL-1. Importantly, good accuracy was achieved by this method in comparison with the results with human AFP ELISA kit for analysis of human serum samples. This dual-purpose Cu2+-activated PEC immunoassay brings a promising, enzyme-free and innovative thinking for the detection of low-abundance biomarkers.

Eco-friendly AgInS2/ZnS quantum dot nanohybrids with tunable luminescent properties modulated by pH-sensitive biopolymer for potential solar energy harvesting applications

Semiconductor quantum dots (QDs) are very interesting candidates for the development of green-energy based nanomaterials and devices. However, currently they present serious concerns because the large majority of efficient QDs have been synthesized using organometallic routes with toxic heavy metals. Herein, we designed and produced novel fluorescent ternary and quaternary QD nanostructures based on AgInS2 (AIS) core and ZnS (ZAIS) shell stabilized with carboxymethyl cellulose (CMC) polymer ligand for potential applications in green solar energy harvesting. Colloidal AgInS2 QDs were prepared by co-precipitation process using a one-pot aqueous green route directly stabilized by CMC at room temperature and varying pH conditions. Then, an outlayer of ZnS was grown and thermally annealed to improve and tune their optical properties and split the emission range, leading to core–shell alloyed nanostructures. Their physicochemical and optical properties were extensively characterized, demonstrating that photoluminescent monodispersed AIS and ZAIS QDs were produced with size typically ranging from 8.5 ± 2.4 nm and 3.2 ± 1.1 nm under acidic or alkaline media, respectively. Therefore, by adjusting the parameters of synthesis, the QD nanohybrids showed tunable optical properties ranging from UV to NIR, with maxima absorption/emission in the visible-range of the light spectrum (from λ = 650 to 750 nm). More importantly, the optical properties demonstrated remarkable enhancement of quantum yield of over one order of magnitude due to the combination of processing parameters and engineered core–shell nano-architecture of AgInS2–ZnS. Thus, these luminescent nanomaterials offer great perspectives for developing innovative broad spectral converters for a wide range of applications in solar energy harvesting.

Facile Synthesis of quantum dots SnO2/Fe3O4 hybrid composites for superior reversible lithium-ion storage

Quantum dots SnO2/Fe3O4 hybrid composites were synthesized by galvanic replacement reaction between Fe3+ ions and Sn and were utilized as anodes for lithium-ion batteries. The composites at few-nanometer scale showed different compositions of SnO2 and Fe3O4 with different reaction time. The composites exhibited significantly enhanced electrochemical performance; for instance, SnO2/Fe3O4-18h electrode delivered a reversible capacity of 1300mAhg−1 after 100 cycles and a rate capacity of 507mAhg−1 at 10Ag−1. The notable performance was ascribed to the presence of quantum dots nanostructures and inactive phases and to the generation of a short Li+-ion diffusion pathway.

High Tunability of Size Dependent Optical Properties of ZnO@M@Au (M = SiO2, In2O3, TiO2) Core/Spacer/Shell Nanostructure

This theoretical work presents a comparative study of high tunability size dependent optical properties of quantum dot/wire triple layered core shell nanostructure based on the quasi-static approximation of classical electrodynamics embedded in a fixed dielectrics function of host matrix. In this paper, local field enhancement factor (LFEF), refractive index and optical absorbance of nanocomposite are analyzed by varying core size, thickness of spacer and shell as well as dielectrics function of the spacer for the size of the nanocomposite with the range of 20 nm to 40 nm. For both quantum dot and quantum wire triple layered core shell nanostructure (CSNS), there are two resonances in visible and near/in infrared spectral region with high tunability. When the shell thickness increase and therefore increasing the gold content, the surface plasmon resonance (SPR) at the outer interface shifts to higher energy (blue-shifted) and at the inner interface weak peaks and shifted to lower energy (red-shifted). All of three optical properties, depend on core size, dielectrics and thickess of spacer, thickness of shell, shape of composite and filling factor. For the same thickness of spacer and shell of the two configurations, cylindrical triple layered CSNS less pronounced and shifted to infrared red (IR) spectral region which is recommendable for biological and photocatalysis application.

Excitonic properties in an asymmetric quantum dot nanostructure under combined influence of temperature and lateral hydrostatic pressure

Abstract Simultaneous influences of temperature and lateral hydrostatic pressure combined to axial asymmetric confinement potential and size effects on the behavior of the exciton in 0D GaAs / Ga ( 1 - x j ) A l ( x j ) asymmetric cylindrical quantum dot (QD) nanostructure are investigated. Our calculations are performed in the framework of the effective mass approximation by using a variational method with a robust trial wave function through taking into account the dependence of exciton properties on the QD sizes, the axial asymmetric confinement potential, the temperature and the pressure. Variation of the excitonic binding energy, photoluminescence energy transition and band gap energy are discussed according to temperature and hydrostatic pressure for different confinement regimes and various potential wells. Our numerical findings show that the applied pressure favors the attraction between electron and hole while the temperature tends to decrease the exciton binding energy. Furthermore, the photoluminescence energy is decreasing function of the temperature and increasing function of the pressure. The opposing influences caused by temperature and lateral hydrostatic pressure with contribution of variation of axial asymmetric confinement potential reveal a significant practical interest and offer an alternative manner to the tuning of the excitonic transition in optoelectronic devices.

Role of Surface Morphology on Exciton Recombination in Single Quantum Dot-in-Rods Revealed by Optical and Atomic Structure Correlation.

The physical structure of colloidal quantum dot (QD) nanostructures strongly influences their optical and electronic behavior. A fundamental understanding of this interplay between structure and function is crucial to fully tailor the performance of QDs and their assemblies. Here, by directly correlating the atomic and chemical structure of single CdSe-CdS quantum dot-in-rods with time-resolved fluorescence measurements on the same structures, we identify morphological irregularities at their surfaces that moderate photoluminescence efficiencies. We find that two nonradiative exciton recombination mechanisms are triggered by these imperfections: charging and trap-assisted nonradiative processes. Furthermore, we show that the proximity of the surface defects to the CdSe core of the core-shell structures influences whether the charging or trap-assisted nonradiative channel dominates exciton recombination. Our results extend to other QD nanostructures and emphasize surface roughness as a crucial parameter when designing colloidal QDs with specific excitonic fates.

Fundamental exciton transitions in SiO2/Si/SiO2 cylindrical core/shell quantum dot

This paper reports the numerical investigation of the quantum confinement effects on excitons in a Si cylindrical core/shell quantum dots. Using the effective-mass approximation and considering a variational technique, we have calculated the exciton ground state binding energy as functions of the shell size in order to study the behavior of the confined exciton into the region of a thin shell with rigid walls. Our results show that the core-shell sizes have a remarkable influence on the electron-hole interaction, which leads to a significant impact on the binding and the optical photoluminescence energies of exciton. Also, we found that the exciton binding energy is higher for the small shell sizes. The effects caused by quantum confinement in such quantum dot nanostructures offer an alternative way of tuning the excitonic transitions in optoelectronic semiconducting devices.

Controlling Goos-Hänchen shifts due to the surface plasmon effect in a hybrid system.

We have theoretically studied the Goos-Hänchen (GH) shifts of both the reflected and transmitted probe beams emerging from a cavity consisting of a hybrid system of a coupled quantum dot (QD) nanostructure and a metallic nanoparticle (MNP). It is realized that the GH shifts in the transmitted and reflected light beams can be enhanced due to the surface plasmon effect in the MNP. Also, it is shown that by adjusting the distance between QD and MNP and polarization control between probe field and major axis of the hybrid system, the simultaneous negative and positive GH shifts in reflected and transmitted light beams can occur. Moreover, the effects of the intensity and detuning of the coupling light on the GH shift properties of the reflected and transmitted lights have been discussed. We have found that under different parametric conditions of the hybrid system, the GH shifts of the reflected and transmitted light beams can be adjusted by tuning the intensity and controlling the detuning of the coupling field. The results show that our proposed model may be used for future optical sensor devices based on MNP hybrid systems.

Lasing without population inversion in a four-level Y-type configuration in double quantum dot system

This work discusses lasing without inversion in Y-scheme in a double quantum dot nanostructure. This new type of lasing, which results from the quantum interference of spontaneous emission components, was not discussed earlier in quantum dot nanostructures. It is found that both pumping and cycling fields control the laser emission. The decrease of the cycling detuning increases the possibility of lasing. Probe detuning controls the width of the absorption bath (electromagnetic-induced transparency window) of this structure. This phenomenon can have an interesting application for developing sources of coherent radiation in a region of electromagnetic spectrum where the implementation of traditional laser schemes is difficult.

Modelling of metamorphic quantum dots for single photon generation at long wavelength

In this work we present the modelling of InxGa1-xAs/InzGa1-zAs/InyGa1-yAs metamorphic quantum dot nanostructures. These are of great recent interest as the active materials in single photon devices emitting in the telecom windows. We have calculated: (i) strain-dependent energy band profiles, (ii) ground energy levels for electrons and heavy holes, (iii) carrier probability densities and (iv) oscillation strenghts for radiative recombination for structures where the In composition in the metamorphic buffer x and in the cap layer y were varied. The modelization has been carried out using the TiberCAD software on the basis of realistic parameters and results have been compared against experimental photoluminescence data: such a comparison shows that the calculated and experimental values agree within a 10 meV interval. Simulations show that in the metamorphic quantum dot (QD) system, a Type-II quantum confinement regime can occurr for particular values of x and y, a feature not reported so far for In(Ga)As QDs. Furthermore, we have been able to identify an optimized design for 10 K light emission at 1.55 μm with the highest emission efficiency. These results will be useful to improve the design and the preparation of QD nanostructures for single photon emission in the telecom windows, a topic of high relevance for the upcoming quantum technology revolution.

Slow light propagation in carbon nanotube quantum dot with spin-orbit interaction

Group velocity and slow down factor are discussed by adjusting the transverse magnetic field in the presence or absence of spin-orbit interaction SOI. The transparency window of the probe light can be changed due to presence of SOI. Moreover, the transparency window of the probe light becomes large by enhancing the value of transverse magnetic field. Our proposed model is may be useful for construction of optical buffer based on carbon nano-tube (CNT) quantum dot nanostructure controlled by magnetic field.

Analysis of quantum-dot systems under thermal loads based on gradient elasticity

Since quantum-dot (QD) nanostructures in many applications are subjected to cyclic electrical and thermal loads, it is very important to analyze such nanostructures under transient thermal loads that affect the induced strains which in turn affects the response of the QD system. When the dimensions of the QDs are of the same order of magnitude as the material length scale, gradient elasticity theory should be used to account for the size-dependent behavior of such nano-sized QDs. In this work, a new finite element formulation for gradient-based thermo-electro-mechanical three-dimensional model of strained QDs in piezoelectric matrix under both static and dynamic thermal loads is developed. The proposed formulation requires only C0 shape functions since the original fourth order partial differential equation (PDE) is split into two PDEs of lower order. A unit cell of Indium Arsenide QD in a finite sized Gallium Arsenide (GaAs) substrate is analyzed. Results under dynamic loads demonstrate that the magnitude of the strain component in the thermal loading direction increases significantly with the thermal shock time. On the other hand, at a fixed time, the magnitude of the induced field quantities, such as the strain, decreases remarkably with increasing the size effect parameter. Given that large lattice mismatch induced strain fields are located in or near the QD regions, these newly observed features need to be carefully considered when designing such QD nanostructures.

Planar fault-tolerant quantum cellular automata full adder

In ‘More-than-Moore’ technological trends, designing new devices with quantum dot nanostructure has become an emerging domain of research. Quantum dot cellular automata are one of the major paradigm shifts in the field of nanoscale computing. Quantum cellular automata (QCA) are possible alternatives to technology based on complementary metal-oxide semiconductors (CMOS) due to their ultralow power and high speed of operation up to terahertz frequency. Furthermore, QCA technology takes up a smaller area than CMOS and is based on the charge confinement protocol, rather than the charge dissipation protocol. A planar design of a QCA-based full-adder circuit has been designed in this study. The proposed full adder is achieved with a delay of only 0·25 clock cycles, which is remarkably shorter than that for available designs. The functionality of the proposed full adder is verified using the QCA Designer software tool. Further, the defect analysis of the circuit confirms the robustness of the design. Moreover, the proposed QCA design is highly fault tolerant, which is proved by the stuck-at-fault analysis performed on the device.