Dimensional Quantum Dots Research Articles

Exploring quantum confinement signature in nitrogen-functionalized graphene quantum dots: Effective mass approximation (EMA) model insights from computational and experimental analyses

A well-established relationship between bandgap energy and Quantum Dots (QDs) dimension, predicted by Effective Mass Approximation (EMA) models, is a cost-effective and reliable method to evaluate the bandgap energy of QDs. The EMA model sheds light on linearity between bandgap energy (Egap) to their inverse square diameter (1/dn), as a signature of the quantum confinement, with the n value reflecting the dimension of the quantum confinement. This study systematically investigated the power parameter "n" dependence in the bandgap energy of Nitrogen-functionalized Graphene Quantum Dots (N-GQDs) with distinct C–N configurations, i.e., Pyridinic-N, Pyrrolic-N, Graphitic-N based on computational and experimental data. The results showed that the calculated bandgap energy values reveal a notable weakening of the size dependence in N-GQDs compared to pristine GQDs, particularly in Pyrrolic-N GQDs, indicative of a unique geometric dimensionality of confinement. This systematic exploration advances our understanding of "n" dependence and bandgap energy in N-GQDs, laying a solid groundwork for modulating the bandgap energy of N-GQDs for optoelectronics and quantum device applications.

PbI2 Passivation of Three Dimensional PbS Quantum Dot Superlattices Toward Optoelectronic Metamaterials.

Lead chalcogenide colloidal quantum dots are one of the most promising materials to revolutionize the field of short-wavelength infrared optoelectronics due to their bandgap tunability and strong absorption. By self-assembling these quantum dots into ordered superlattices, mobilities approaching those of the bulk counterparts can be achieved while still retaining their original optical properties. The recent literature focused mostly on PbSe-based superlattices, but PbS quantum dots have several advantages, including higher stability. In this work, we demonstrate highly ordered 3D superlattices of PbS quantum dots with tunable thickness up to 200 nm and high coherent ordering, both in-plane and along the thickness. We show that we can successfully exchange the ligands throughout the film without compromising the ordering. The superlattices as the active material of an ion gel-gated field-effect transistor achieve electron mobilities up to 220 cm2 V-1 s-1. To further improve the device performance, we performed a postdeposition passivation with PbI2, which noticeably reduced the subthreshold swing making it reach the Boltzmann limit. We believe this is an important proof of concept showing that it is possible to overcome the problem of high trap densities in quantum dot superlattices enabling their application in optoelectronic devices.

Theoretical Investigation and Improvement of Characteristics of InAs/GaAs Quantum Dot Intermediate Band Solar Cells by Optimizing Quantum Dot Dimensions

Quantum dot (QD)-based solar cells have been the focus of extensive research. One of the critical challenges in this field is optimizing the size and placement of QDs within the cells to enhance light absorption and overall efficiency. This paper theoretically investigates InAs/GaAs QD intermediate band solar cells (QD-IBSC) employing cylindrical QDs. The goal is to explore factors affecting light absorption and efficiency in QD-IBSC, such as the positioning of QDs, their dimensions, and the spacing (pitch) between the centers of adjacent dots. Achieving optimal values to enhance cell efficiency involves modifying and optimizing these QD parameters. This study involves an analysis of more than 500 frequency points to optimize parameters and evaluate efficiency under three distinct conditions: output power optimization, short-circuit current optimization, and generation rate optimization. The results indicate that optimizing the short-circuit current leads to the highest efficiency compared to the other conditions. Under optimized conditions, the efficiency and current density increase to 34.3% and 38.42 mA/cm2, respectively, representing a remarkable improvement of 15% and 22% compared to the reference cell.

Synthesis, characterization and dielectric relaxation studies of graphene quantum dots prepared by hydrothermal treatment method

Zero dimensional graphene quantum dots (GQDs) exhibit fascinating chemical and physical properties discovered by Pan et al., in 2010, due to the quantum confinement and edge effect. GQDs have been synthesized by hydrothermal treatment method from graphite powder as low cost, easy process, and large amount with high production yield. The functional groups of GQDs on the surface sites of carbon atoms have been studied through Fourier transform infrared spectroscopy (FTIR) analysis. X-ray diffraction (XRD) pattern of GQDs shows the broad diffraction at 2 θ = 24.47 ° with 0.36 nm an interlayer distance confirming the formation of the GQDs. Raman analysis reveals that GQDs show both D and G bands mostly the armchair edges. UV–visible spectra show absorption peak at 287 and 330 nm credited to π → π * and n → π * transitions due to the presence of conjugated carbon–carbon bonds and carbonyl groups. The particle size of GQDs is found below 5 nm from high- resolution transmission electron microscopy (HR-TEM) image. Scanning electron microscopy shows the surface morphology of GQDs are crumpled and aggregated thin sheets. Dielectric properties of synthesized GQDs have been studied in the range frequency between 0.01 and 105 Hz at different temperatures (25, 50, 75, and 100 °C). The dielectric relaxation mechanism is explained in the framework of dielectric loss tangent (tanδ), permittivity and electrical conductivity. The dielectric loss tangent increases with frequency and maximum peak decreases with frequency at all different temperatures due to present of polar groups in GQDs. Dielectric permittivity decreases with frequency at all different temperatures due to charge orientation and rotation of dipole moments. The electrical conductivity increases with frequency at all different temperature due to the GQDs is highly conductive nature. So synthesized GQDs can be used in electrical and electronic materials.

Zero dimensional Graphene Quantum Dots self-assembled collagen 3D bio-matrices for soft tissue regeneration

The current study focuses on self-assembly of collagen fibrils with Graphene Quantum Dots (GQD) and its tissue regenerative applications. GQDs were synthesised by the liquid exfoliation method using ethyl acetoacetate (EAA) as the solvent. The photoluminescence property of the GQD was established by the UV-Vis absorption spectrum. Characteristic increase in thermal stability, enhanced tensile strength was found after the self-assembly of collagen with GQD. The GQDs self-assembled collagen bio-matrices showed 3D surface topology mimicking the extracellular matrix. The GQDs self-assembled collagen bio-matrices did not show toxicity in the structural cells namely fibroblast cells and exhibited increased biocompatibility as indicated by enhanced cell adhesion, proliferation and cell migration of fibroblast cells. Angio-proliferative effect as indicated by an enhanced blood vessel formation and intact unruptured blood vessels was observed in the GQD self-assembled collagen. The study opens a way forward for the use of zero-dimensional graphene based 3-dimensional self-assembled collagen biomatrices for soft tissue regenerative applications.

Cu2O quantum dots modified α-Ga2O3 nanorod arrays as a heterojunction for improved sensitivity of self-powered photoelectrochemical detectors

Long distance charge transfer will lead to the recombination of carriers, which seriously limits the practical application of photoelectrochemical (PEC) detector. In this work, a self-powered solar-blind photodetector based on a mixed dimensional α-Ga2O3 nanorod arrays and Cu2O quantum dots has been prepared by a simple hydrothermal method and a two-part impregnation method. Cu2O quantum dots can form type Ⅱ heterojunction with α-Ga2O3 and effectively reduce the photoinduced electron and hole recombination due to their short carrier transport distances. Just because of this, α-Ga2O3 photodetectors modified by Cu2O quantum dots present special self-power optical response at 254 nm, with optical flow densities of 13.88 μA/cm2, large responsivity of 4.57 mA/W, high detection sensitivity of 2.107 × 109 Jones and fast photoresponse time of 0.815 s/0.978 s. This kind of thought provides a simple and feasible method for the preparation of α-Ga2O3 and other semiconductor oxides based on PEC photodetectors.

Graphene/III–V Quantum Dot Mixed-Dimensional Heterostructure for Enhanced Radiative Recombinations via Hole Carrier Transfer

Fabrication of high quantum efficiency nanoscale device is challenging due to increased carrier loss at surface. Low dimensional materials such 0D quantum dots and 2D materials have been widely studied to mitigate the loss. Here, we demonstrate a strong photoluminescence enhancement from graphene/III-V quantum dot mixed-dimensional heterostructures. The distance between graphene and quantum dots in the 2D/0D hybrid structure determines the degree of radiative carrier recombination enhancement from 80% to 800% compared to the quantum dot only structure. Time-resolved photoluminescence decay also shows increased carrier lifetimes when the distance decreases from 50 to 10 nm. We propose that the optical enhancement is due to energy band bending and hole carrier transfer, which repair the imbalance of electron and hole carrier densities in quantum dots. This 2D graphene/0D quantum dot heterostructure shows promise for high performance nanoscale optoelectronic devices.

Strain and magnetic field effect on thermodynamic properties of a two dimensional GaAs quantum dot

The strain and magnetic field effect on thermodynamic properties of a two dimensional GaAs quantum dot (QD) is investigated within the effective mass approximation. To this end, first we have obtained the wave functions and energy levels of the system in the presence of Bychkov-Rashba and Dressalhaus terms. Then using the obtained energy levels, we have computed the partition function by the Poisson summation formalism. Afterward, we have deduced some thermodynamic properties such as mean energy, entropy, specific heat and free energy using the canonical ensemble approach. Finally, the thermodynamic properties for a two dimensional GaAs QD have been discussed in detail.

Physical probing of quantum energy levels in a single indium arsenide (InAs) quantum dot.

Indium arsenide (InAs) quantum dots (QDs) grown by molecular beam epitaxy (EBM) on gallium arsenide (GaAs) substrates have exhibited quantized charge-trapping characteristics. An electric charge can be injected in a single QD by a gold-coated AFM nano-probe placed directly on it using a conductive-mode atomic force microscope (C-AFM). The results revealed separate current-voltage (I-V) curves during consecutive measurements, where the turn-on voltages measured at the subsequent voltage sweeps are incrementally lower than that at the initial sweep. We demonstrate that the charge state of the QD can change over a long enough time by measuring the I-V data on the same QD at different time intervals. Discrete energy states (here, five states) have been observed due to the quantized charge leakage from the QD into the surrounding materials. These quantum states with five energy levels have been verified using quantum theory analysis of the quantum-well with the help of a numerical simulation model, which depends on the QD dimensions. The size of the quantum-well in the model is in good agreement with the actual QD size, whose lateral dimension is confirmed using a scanning electron microscope. At the same time, the height is estimated from the atomic force microscope topography.

Wannier excitons confined in hexagonal boron nitride triangular quantum dots

With the ever-growing interest in quantum computing, understanding the behavior of excitons in monolayer quantum dots has become a topic of great relevance. In this paper, we consider a Wannier exciton confined in a triangular quantum dot of hexagonal boron nitride. We begin by outlining the adequate basis functions to describe a particle in a triangular enclosure, analyzing their degeneracy and symmetries. Afterwards, we discuss the excitonic Hamiltonian inside the quantum dot and study the influence of the quantum dot dimensions on the excitonic states.

Syndiotactic hexamer peptide nanodots.

Spatial confinement of excitons in the nano-crystalline region of semiconducting nanostructures differ significantly from the optoelectronic properties exhibited by the bulk material. We report spike-like absorption observed in the UV spectrum of a phenylalanine hexamer peptide [(Ff)3-OH] nano-assembly, which may be attributed to the spatial confinement of electrons to the dimension of quantum dots. Interdependency of the UV and PLE spectrum of the peptide confirms the existence of quantum confinement in (Ff)3-OH nano-assemblies.

Analytical Model and Experimental Analysis to Estimate the Interdiffusion and Optoelectronic Properties of Coupled InAs Quantum Dots Post Rapid Thermal Processing

The stoichiometric variations in molecular beam epitaxy (MBE)-grown In(Ga)As quantum dots (QDs) after rapid thermal processing (RTP) is estimated from an analytical model assisted by transmission electron microscopy (TEM) images. A systematically grown multilayer (bi-, penta-, hepta-, and deca-layer) InAs Stranski–Krastanov (SK) QDs coupled electronically to submonolayer (SML) QDs are used in this study. The As-grown (ASG) samples are treated with RTP at temperatures of 650 °C, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$700~^{\circ }\text{C}$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$750~^{\circ }\text{C}$ </tex-math></inline-formula> , and 800 °C for 30 s. Our model estimates a significant diffusion of In atoms in the growth direction compared with the in-plane direction altering the QD dimensions and composition. To check its reliability, we incorporated an eight-band k.p model on all the RTP-treated strained In(Ga)As QD heterostructures. Photoluminescence (PL) measurements reveal a strong correlation with the simulation results; furthermore, a strong PL blueshift (56 nm) and narrow linewidth from 67 nm (ASG) to 33 nm ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$800~^{\circ }\text{C}$ </tex-math></inline-formula> ) are seen due to an increase in the uniformity of QDs supporting the atomic force microscopy (AFM) results. AFM images reveal the formation of highly uniform and dense QDs with increased annealing temperature. The activation energy (AE) shows a decreasing trend due to the reduction in carrier confinement with RTP, agreeing with the analytical model. This is probably the first report that presents the effect of RTP on SK-SML coupled QD devices both in terms of theoretical and experimental analyses. This analytical model can thus be extended to any annealing study and for any material with certain known parameters and is helpful for futuristic studies.

Investigation of two-electron quantum dot in a magnetic field by using of quasi-exact-solvable method

We study the non-relativistic particles of a two dimensional two-electron quantum dot. We consider the system in the presence of harmonic plus linear terms as well as a term related to spin interaction. The solution of the system is obtained by using of Quasi-Exactly-Solvable (QES) method. Next, we calculate the energy eigenstates and wave functions of the system in the presence of an external magnetic field.

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.

Quantum information measures of the Dirichlet and Neumann hyperspherical dots

$\mathtt{d}$-dimensional hyperspherical quantum dot with either Dirichlet or Neumann boundary conditions (BCs) allows analytic solution of the Schr\"{o}dinger equation in position space and the Fourier transform of the corresponding wave function leads to the analytic form of its momentum counterpart too. This paves the way to an efficient computation in either space of Shannon, R\'{e}nyi and Tsallis entropies, Onicescu energies and Fisher informations; for example, for the latter measure, some particular orbitals exhibit simple expressions in either space at any BC type. A comparative study of the influence of the edge requirement on the quantum information measures proves that the lower threshold of the semi-infinite range of the dimensionless R\'{e}nyi/Tsallis coefficient where one-parameter momentum entropies exist is equal to $\mathtt{d}/(\mathtt{d}+3)$ for the Dirichlet hyperball and $\mathtt{d}/(\mathtt{d}+1)$ for the Neumann one what means that at the unrestricted growth of the dimensionality both measures have their Shannon fellow as the lower verge. Simultaneously, this imposes the restriction on the upper value of the interval $[1/2,\alpha_R)$ inside which the R\'{e}nyi uncertainty relation for the sum of the position $R_\rho(\alpha)$ and wave vector $R_\gamma\left(\frac{\alpha}{2\alpha-1}\right)$ components is defined: $\alpha_R$ is equal to $\mathtt{d}/(\mathtt{d}-3)$ for the Dirichlet geometry and to $\mathtt{d}/(\mathtt{d}-1)$ for the Neumann BC. Some other properties are discussed from mathematical and physical points of view. Parallels are drawn to the corresponding properties of the hydrogen atom and similarities and differences are explained based on the analysis of the associated wave functions.

Synthesis and characterization of a new water-soluble non-cytotoxic mito-tracker capped silicon quantum dot

Allyl triphenylphosphonium bromide based mito-tracker capped silicon quantum dot ( Mito-SiQDs ) has been synthesized through an inverse micelle process. It was then fully characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, dynamic light scattering techniques and X-ray photoelectron spectroscopic method. Energy dispersive X-ray spectroscopy analyses of the quantum dots confirm the presence of carbon, silicon, phosphorous and bromine atoms in Mito-SiQDs . Morphological study by transmission electron microscopy experiment showed the formation of the particles of size 11-12 nm of quantum dot dimension. The high negative zeta potential value of –23.7 mV calculated from dynamic light scattering study indicates the high stability of the circumvent agglomeration of Mito-SiQDs . The mito-tracker capped silicon quantum dot has blue emission at 400 nm wavelength upon excitation at 327 nm. Mito-SiQDs has not shown any significant cytotoxic effect with 10 to 50 μL/mL concentration on HeLa cell line for at least up to 12 h of its treatment. The ito - iQDs would be useful a possible fluorescent marker to visualize mitochondrial subcellular compartment in living cell through fluorescence imaging study.

Encapsulation study of MOVPE grown InAs QDs by InP towards 1550 nm emission

The three-dimensional carrier confinement in Quantum Dots (QDs) is the key to achieve superior properties (electronic and optical) compared to the Quantum Well (QWL) for optoelectronic applications, such as semiconductor lasers, photodiodes. After the growth of QDs, the encapsulation is the next crucial step to confine carriers in QDs and achieve the targeted wavelength emission. In this work, we have studied the InP capping of Stranski-Krastanov (S-K) grown InAs QDs on InP(001) substrate by MOVPE. During the encapsulation, the P/As exchange is a vital process which either transforms the QDs into a 2D layer or reduces QDs’ dimensions. This study shows that a control of Phosphorous concentration on the surface during InP capping facilitates to obtain the expected QDs dimension for 1550 nm wavelength. The emitted photoluminescence peak shifts following the preserved average QDs’ dimensions. By combining simulation with optical response we have proved the Phosphorous incorporation into the InAs structure (QDs or 2D layer) during the encapsulation step. As expected, the carrier lifetime reveals the superior quality of the preserved QDs in InP barrier compared to the 2D layer. Finally, the reduction of the non-radiative recombination sources, e.g. dangling bonds, by passivation treatment demonstrates a further increment in carrier lifetime.

Influence of quantum dot shape on energy spectra of three-dimensional quantum dots superlattices

The band spectrum of quantum dots superlattices of different shapes at points of high symmetry is determined. Cubic, cylindrical and spherical quantum dots are considered. The width of the minizone is calculated. The dependences of the minizones on the geometric dimensions of quantum dots and their concentration are established.

Landau levels, edge states, and gauge choice in 2D quantum dots

We examine the behavior of a charged particle in a two dimensional quantum dot in the presence of a magnetic field. Emphasis is placed on the high magnetic field regime. Compared to free space geometry, confinement in a dot geometry provides a more realistic system where edge effects arise naturally. It also serves to remove the otherwise infinite degeneracy due to the magnetic field; nonetheless, as described in this paper, additional ingredients are required to produce sensible results. We treat both circular and square geometries, and in the latter, we explicitly demonstrate the gauge invariance of the energy levels and wave function amplitudes. The characteristics of bulk states closely resemble those of free space states. For edge states, with sufficiently high quantum numbers, we achieve significant differences in the square and circular geometries. Both circular and square geometries are shown to exhibit level crossing phenomena, similar to parabolic dots, where the confining potential is a parabolic trap. Confinement effects on the probability current are also analyzed; it is the edge states that contribute non-zero current to the system. The results are achieved using straightforward matrix mechanics, in a manner that is accessible to novices in the field. On a more pedagogical note, we also provide a thorough review of the theory of single electron Landau levels in free space and illustrate how the introduction of surfaces naturally leads to a more physically transparent description of a charged particle in a magnetic field.

Comparison of static and dynamic characteristics of 1550 nm quantum dash and quantum well lasers.

Compared to quantum well (QW) lasers, lower dimensional quantum dot (QD) or quantum dash (QDash) devices demonstrate superior performances, owing to their quantized energy levels and increased carrier confinement. Here, we report the systematic comparison of static and dynamic properties of long wavelength (1550 nm) QDash and QW lasers. For the QDash lasers, a higher maximum operating temperature and lower temperature dependence was achieved for long cavities, although the threshold current densities were larger than the QW reference devices. The lasing characteristics for QDashes are significantly improved following the application of a high reflectance (HR) coating on the rear facets. The QDash lasers also exhibit three orders lower dark current, of 45 µA/cm2 under -1 V reverse bias. Small signal modulation on the 4 × 550 µm2 Fabry-Perot cavities yields a modulation efficiency of 0.48 GHz/√mA and a maximum 3-dB bandwidth of 7.4 GHz for QDashes, slightly larger than that for the QW devices. Meanwhile, a stronger damping effect was observed for the QDash lasers due to their lower differential gain.