Nanostructured Quantum Dots Research Articles

Unraveling Origins of EPR Spectrum in Graphene Oxide Quantum Dots.

Carbon nanostructures are utilized in a plethora of applications ranging from biomedicine to electronics. Particularly interesting are carbon nanostructured quantum dots that can be simultaneously used for bimodal therapies with both targeting and imaging capabilities. Here, magnetic and optical properties of graphene oxide quantum dots (GOQDs) prepared by the top-down technique from graphene oxide and obtained using the Hummers’ method were studied. Graphene oxide was ultra-sonicated, boiled in HNO3, ultra-centrifuged, and finally filtrated, reaching a mean flake size of ~30 nm with quantum dot properties. Flake size distributions were obtained from scanning electron microscopy (SEM) images after consecutive preparation steps. Energy-dispersive X-ray (EDX) confirmed that GOQDs were still oxidized after the fabrication procedure. Magnetic and photoluminescence measurements performed on the obtained GOQDs revealed their paramagnetic behavior and broad range optical photoluminescence around 500 nm, with magnetic moments of 2.41 µB. Finally, electron paramagnetic resonance (EPR) was used to separate the unforeseen contributions and typically not taken into account metal contaminations, and radicals from carbon defects. This study contributes to a better understanding of magnetic properties of carbon nanostructures, which could in the future be used for the design of multimodal imaging agents.

Quantum dots of molybdenum nitride embedded in continuously distributed polyaniline as novel electrode material for supercapacitor

Molybdenum nitride (Mo2N) and molybdenum nitride/polyaniline composites (Mo2N@PANI) have been prepared by simple static precipitation and in situ growth methods. The morphological and structural properties and chemical composition of the prepared materials have been analyzed by transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The obtained results illustrate that in molybdenum nitride/polyaniline composites, polyaniline has been allocate continuously embedded with the quantum dots of molybdenum. Therefore, the formation of quantum dots of molybdenum and its uniform distribution in polyaniline bulk suggested that the synthesized materials could be used as an electrode material of supercapacitors. From the results of electrochemical analysis, the mass ratio capacitance of molybdenum nitride/polyaniline composites have been found to be 111.8 F/g at 0.5 A/g, while the mass ratio capacitance of pure molybdenum nitride is only 47.5 F/g at the same current density. Hence, obtained results clearly confirm that molybdenum nitride/polyaniline composites nanostructured quantum dot material is one of the best and novel electrode materials for high performed supercapacitors which exhibits excellent electrochemical properties.

Quantum dots for light conversion, therapeutic and energy storage applications

In this review paper, we highlight the utilization of nanostructured quantum dots (QDs) in light-emitting diodes, biomedical, and energy-related applications. We discuss different preparation methods, cation-doping effects, and the optical applications of perovskite QDs. Cadmium selenide QDs are semiconductor materials with narrow bandgaps; therefore, their optical properties and electronic structures can be tuned. They can absorb photons (light energy) and convert multiple electron-hole pairs efficiently via multiple exciton generations. These effective light absorption properties are suitable for solar-driven water electrolysis processes and efficient photo-electrochemical lithium-air batteries. We focus on the utilization of upconverting nanoparticles in the field of biomedical applications. Suitable bandgap position, efficient charge separation, transportation, and photo-stability are the advantages of QD nanostructured materials. Hence, they are efficient and challenging candidates for the future.

Potential effect of CuInS2/ZnS core-shell quantum dots on P3HT/PEDOT:PSS heterostructure based solar cell

Nanostructured quantum dots (QDs) are quite promising in the solar cell application due to quantum confinement effect. QDs possess multiple exciton generation and large surface area. The environment friendly CuInS2/ZnS core-shell QDs were prepared by solvothermal method. Thus, the 3 nm average sized CuInS2/ZnS QDs were employed in the bulk heterojunction device and the active blend layer consisting of the P3HT and CuInS2/ZnS QDs was investigated. The energy level information of CuInS2/ZnS QDs as an electron acceptor was explored by ultra violet photoelectron spectroscopy. Bulk heterojunction hybrid device of ITO/PEDOT:PSS/P3HT: (CuInS2/ZnS QDs)/ZnO/Ag was designed by spin coating approach and its electrical characterization was investigated by solar simulator. Current density – voltage characteristics shows the enhancement in power conversion efficiency with increasing concentration of CuInS2/ZnS QDs in bulk heterojunction device.

Recent Advances in Manganese Doped II‐VI Semiconductor Quantum Dots

The introduction of dopants into II‐VI semiconductor quantum dots (QDs) allows for the realization of materials with novel electro‐optical properties. Motivated by its vast potential applications, Manganese doping into II‐VI semiconductor QDs has been studied by several groups. Studies have focused on applications of manganese doped QDs as phosphors, diluted magnetic semiconductors as well as more recently their potential in light harvesting. Herein we review recent advances in understanding the emissive properties and electronic structure of Mn doped QDs. We survey synthetic approaches that have been proposed as well as advantages and disadvantages of conventional approaches to Mn doping. In particular, we discuss the advantages of reduced self‐absorption, as well as the problem of emission tunability. Effects of pressure on manganese emission are discussed. We then summarize recent efforts to tune Mn emission by generating pressure and strain in nanostructured QDs.

A nonlinear eigenvalue problem arising in a nanostructured quantum dot

In this paper we investigate a minimization problem related to the principal eigenvalue of the s-wave Schrödinger operator. The operator depends nonlinearly on the eigenparameter. We prove the existence of a solution for the optimization problem and the uniqueness will be addressed when the domain is a ball. The optimized solution can be applied to design new electronic and photonic devices based on the quantum dots.

Spontaneous emission from large quantum dots in nanostructures: Exciton-photon interaction beyond the dipole approximation

We derive a rigorous theory of the interaction between photons and spatially extended excitons confined in quantum dots in inhomogeneous photonic materials. We show that, beyond the dipole approximation, the radiative decay rate is proportional to a non-local interaction function, which describes the interaction between light and spatially extended excitons. In this regime, light and matter degrees of freedom cannot be separated and a complex interplay between the nanostructured optical environment and the exciton envelope function emerges. We illustrate this by specific examples and derive a series of important analytical relations, which are useful for applying the formalism to practical problems. In the dipole limit, the decay rate is proportional to the projected local density of optical states and we obtain the strong and weak confinement regimes as special cases.

Effect of pressure on the energy band gaps of wurtzite GaN and AlN and electronic properties of their ternary alloys AlxGa1−xN

Abstract Using the Empirical Pseudopotential Method (EPM) combined with an improved Virtual Crystal Approximation (VCA), where the effect of compositional disorder is included as an effective periodic potential, we have calculated the electronic band structure of GaN and AlN under hydrostatic pressure up to 200 kbar and 160 kbar, respectively. The behavior of electronic properties of their alloys AlxGa1−xN in the wurtzite structure have been predicted at zero pressure, for the entire range of alloy concentrations. The improved VCA correction is found to be essential in predicting the value of the band-gap bowing parameters. The electron effective masses of the Γ valley were also investigated. The agreement between our results and the available experimental data is generally satisfactory. Thus, the calculated EPM parameters can be incorporated into existing device simulators code to study nano-structured quantum dots and light-emitting diodes.

Existence of an extremal ground state energy of a nanostructured quantum dot

This paper is concerned with two rearrangement optimization problems. These problems are motivated by two eigenvalue problems which depend nonlinearly on the eigenvalues. We consider a rational and a quadratic eigenvalue problem with Dirichlet’s boundary condition and investigate two related optimization problems where the goal function is the corresponding first eigenvalue. The first eigenvalue in the rational eigenvalue problem represents the ground state energy of a nanostructured quantum dot. In both the problems, the admissible set is a rearrangement class of a given function.

Graphene gates

A nanostructured quantum dot with tunable electronic properties can be carved from a single sheet of graphene with a reactive ion etching process

SO phonons in spherical nanostructured quantum dots

AbstractA theoretical model for a nanostructured spherical Quantum Dot (QD) is considered, where to a core sphere of semiconductor “1” it is added a concentric spherical layer of semiconductor “2”, while this structure is assumed as imbedded in an infinite medium made of semiconductor “1”. Using a long‐wave continuum approach, the so‐called dielectric continuum approach, we study the surface optical (SO) phonons associated with the two spherical interfaces of the nanostructure. The possible SO phonon frequencies are analyzed as a function of both the sizes of the structure and the angular momentum quantum number l = 1, 2, …. We also calculate the electric potential provided by the SO phonons and derive an analytical expression for the electron–phonon interaction Hamiltonian. Different possible cases for semiconductors “1” and “2” are discussed, comparisons with previously published works on similar subjects are done and the experimental techniques allowing the possible determination of this kind of phonons are invoked. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of Single-Electron Transport in Nanostructured Quantum Dots

Resonant tunneling via discrete energy states in single-electron electronic systems have been observed in many nanostructured configurations. The nodes of such circuits may assume either a continuum of energy states or a set of discrete states with different degeneracy factors. This paper presents a general-purpose simulator intended to provide a unique platform that is able to simulate such circuits. The model is described and then used to study some typical simple circuits. It is shown that even for simple circuits, the interplay between energy spectrum, sensitivity to voltage variations and Coulomb blockade conditions might lead to very complicated current-voltage characteristics.

Semiclassical density functional theory: Strutinsky energy corrections in quantum dots

We develop a semiclassical density functional theory in the context of quantum dots. Coulomb blockade conductance oscillations have been measured in several experiments using nanostructured quantum dots. The statistical properties of these oscillations remain puzzling, however, particularly the statistics of spacings between conductance peaks. To explore the role that residual interactions may play in the spacing statistics, we consider many-body systems that include electron-electron interactions through an explicit density functional. First, we develop an approximate series expansion for obtaining the ground state using the idea of the Strutinsky shell correction method. Next, we relate the second-order semiclassical corrections to the screened Coulomb potential. Finally, we investigate the validity of the approximation method by numerical calculation of a one-dimensional model system, and show the relative magnitudes of the successive terms as a function of particle number.

Raman spectroscopy of dry etched SiSi 1−xGe x quantum dots

SiSi 1−xGe x (x=0.2∼0.3) quantum dots of 50∼60nm in diameter fabricated by using electron beam lithography and reactive ion etching were characterized by Raman spectroscopy. Clear evidence of process-induced strain relief was found in addition to the observation of features from enhanced multi-phonons and SiSi 1−xGe x intermixing arising from the process-induced crystal symmetry breakdown in the nanostructured quantum dots.