Effect Of Quantum Dot Size Research Articles

Modeling of light absorption in self-assembled truncated conical quantum dot structures

Quantum Dots have shown a significant potential as a top candidate for infrared photodetection at higher temperatures. In the presented work, a theoretical model for estimating the coefficient of optical absorption of self-assembled truncated conical quantum dot is developed. This model considers both bound-to-continuum and bound-to-bound absorption mechanisms that increase the accuracy of the absorption coefficient estimation. The developed model is based on estimating the bound states by diagonalizing the Hamiltonian matrix, where the density of states is computed using the Non-Equilibrium Greens function and the effective mass theory to obtain the unbound states. The kinetic equation of Green’s function is solved numerically by finite difference method. Besides, the effects of quantum dot size, height, aspect ratio, and density on the coefficient of the optical absorption are investigated. The results of the developed model are contrasted with those of other alternative QD structures where the truncated conical QD structure results in a higher absorption coefficient in infrared range than semispherical and conical QD structures.

First-principles studies the optical properties of defective-GQDs and -fullerene

We demonstrate the size effect of quantum dots and discuss the effect of defect on the luminescence properties of graphene quantum dots (GQDs) and fullerene (C60) in this paper. Results show that the smaller the GQDs, the more difficult it is to prepared. The absorption peak of visible light region can be attained from large-size GQDs and small-size GQDs mainly show the luminescence performance of UV region. The band gap of GQDs become narrower and narrower with increasing size and adjusting symmetry. And then C60 (0.7 nm) with higher symmetry exhibits better optical and electronic conductivity. The defective GQDs and C60 have better adsorption capacity for functional group –OH and -NMP than non-defective structure. And the effect of defect and adsorbed functional group on the optical properties is clearly different, which C defect and functional group adsorption can improve its optical properties for large size GQDs and completely weaken its optical properties for C60.

Optical properties of core/shell spherical quantum dots**Project supported by the Natural Science Foundation of Inner Mongolia Autonomous Region, China (Grant Nos. 2019MS01006 and 2020MS01008), the Science Project of the Higher Education of Inner Mongolia Autonomous Region, China (Grant No. NJZY19047), the Doctoral Starting-up Foundation of Inner Mongolia Agricultural University, China (Grant No. BJ2013B-2), and the Grassland Talent Project, China.

In this study, the effects of quantum dot size on the binding energy, radiative lifetime, and optical absorption coefficient of exciton state in both GaN/AlxGa1−xN core/shell and AlxGa1−xN/GaN inverted core/shell quantum dot structures are studied. For the GaN/AlxGa1−xN core/shell structure, the variation trend of binding energy is the same as that of radiation lifetime, both of which increase first and then decrease with the increase of core size. For AlxGa1−xN/GaN inverted core/shell structure, the binding energy decreases first and then increases with core size increasing, and the trends of radiation lifetime varying with core size under different shell sizes are different. For both structures, when the photon energy is approximately equal to the binding energy, the peak value of the absorption coefficient appears, and there will be different peak shifts under different conditions.

Ligands induced NiS2 quantum dots for synchronous high specific capacity and robust stability of advanced electrochemical energy storage

The structural collapse of transition metal sulfides in repeated charge and discharge process restricts their cyclic performance. How to effectively remedy stability without sacrifice of capacity still remains a daunting challenge. Herein, ultra-small NiS2 quantum dots (QDs) are assembled with pyridine ligands containing heterocyclic nitrogen atoms of high electronegativity. The induction of electron cloud of heterocyclic nitrogen shortens the NiS bond with stronger structural stability. Besides, the localized lone pair of electrons near nickel atoms reduces the repulsive force to the diffused OH− ions, thus increasing affinity of the redox reaction. Benefiting from the tailor-made of ligands and size effect of QDs, a great promotion in both specific capacity of 651.8 C∙g−1 at 1 A∙g−1 and stability of 94.7% retention at 5 A∙g−1 over 8000 cycles is achieved as compared to ordinary bulk materials. These results demonstrate that ligands induced QDs is a promising strategy for precisely regulating metal sulfide electrode materials of advanced energy storage devices.

Amino‐Functionalized Graphene Quantum Dots as Cathode Interlayer for Efficient Organic Solar Cells: Quantum Dot Size on Interfacial Modification Ability and Photovoltaic Performance

AbstractIn this work, a series of amino‐functionalized graphene quantum dots (AF‐GQDs) with different quantum dot sizes are prepared successfully and employed as the cathode interfacial layers (CILs) for fabrication of efficient organic solar cells (OSCs). The effect of quantum dot size on the interfacial modification ability and photovoltaic performance is investigated in detail. By varying the particle size of AF‐GQDs, the work function of the cathode and the conductivity of CIL can be finely tuned. M‐GQD with the medium size achieves the best balance of interfacial modification and conductivity, giving rise to the best power conversion efficiencies both in fullerene and nonfullerene OSCs (poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7):[6,6]‐phenyl‐C71‐butylic acid methyl ester (PC71BM) blend: 10.14%; poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene)‐co‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐c:4,5‐c′]dithiophene‐4,8‐dione)] (PBDB‐T):3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d′]‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene) (ITIC) blend: 11.87%; and poly[(4,8‐bis(5‐(tripropylsilyl)thiophen‐2‐yl)benzo[1,2‐b:4,5‐b′]dithiophene)‐co‐(5,6‐difluoro‐2‐(2‐hexyldecyl)‐4,7‐di(thiophen‐2‐yl)‐2H‐benzo[d][1,2,3]triazole)] (J71):ITIC blend: 12.81%). This work not only provides a new type of high‐performance CIL materials but also demonstrates a simple way for fine‐tuning the interfacial modification ability and the conductivity via just controlling the quantum dot size of AF‐GQDs.

Effect of size and indium-composition on linear and nonlinear optical absorption of InGaN/GaN lens-shaped quantum dot**Project supported by the Ministry of Higher Education and Scientific Research in Iraq, Ibnu Sina Institute and Physics Department of Universiti Teknologi Malaysia (UTM RUG Vote No. 06-H14).

Based on the Schrödinger equation for envelope function in the effective mass approximation, linear and nonlinear optical absorption coefficients in a multi-subband lens quantum dot are investigated. The effects of quantum dot size on the interband and intraband transitions energy are also analyzed. The finite element method is used to calculate the eigenvalues and eigenfunctions. Strain and In-mole-fraction effects are also studied, and the results reveal that with the decrease of the In-mole fraction, the amplitudes of linear and nonlinear absorption coefficients increase. The present computed results show that the absorption coefficients of transitions between the first excited states are stronger than those of the ground states. In addition, it has been found that the quantum dot size affects the amplitudes and peak positions of linear and nonlinear absorption coefficients while the incident optical intensity strongly affects the nonlinear absorption coefficients.

Phonon bottleneck in p-type Ge/Si quantum dots

We study the effect of quantum dot size on the mid-infrared photo- and dark current, photoconductive gain, and hole capture probability in ten-period p-type Ge/Si quantum dot heterostructures. The dot dimensions are varied by changing the Ge coverage and the growth temperature during molecular beam epitaxy of Ge/Si(001) system in the Stranski-Krastanov growth mode. In all samples, we observed the general tendency: with decreasing the size of the dots, the dark current and hole capture probability are reduced, while the photoconductive gain and photoresponse are enhanced. Suppression of the hole capture probability in small-sized quantum dots is attributed to a quenched electron-phonon scattering due to phonon bottleneck.

Suppression of hole relaxation in small Ge/Si quantum dots

We study the effect of quantum dot size on the mid-infrared photocurrent, photoconductive gain, and hole capture probability in ten-period p-type Ge/Si quantum dot heterostructures. The dot dimensions are varied by changing the Ge coverage during molecular beam epitaxy of Ge/Si(001) system in the Stranski–Krastanov growth mode while keeping the deposition temperature to be the same. A device with smaller dots is found to exhibit a lower capture probability and a higher photoconductive gain and photoresponse. The integrated responsivity in the mid-wave atmospheric window (λ = (3–5) μm) is improved by a factor of about 8 when the average in-plane dot dimension changes from 18 to 11 nm. The decrease in the dot size is expected to reduce the carrier relaxation rate due to phonon bottleneck by providing strong zero-dimensional quantum mechanical confinement.

The effect of quantum dot size and poly(ethylenimine) coating on the efficiency of gene delivery into human mesenchymal stem cells

Quantum dot (QDs) have been employed as bioimaging agents and delivery vehicles for gene therapeutics in several types of cells. In this study, we fabricated multiple QD bundled nanoparticles (NPs) to investigate the effect of QD size and poly(ethylenimine) (PEI) coating on the efficiency of gene delivery into human mesenchymal stem cells (hMSCs). Several types of QDs, which exhibit different ranges of particle size and fluorescence when employed, were coated with PEI to alter their negative charges and to enable them to be bundled into larger particles. Using specific wavelengths of QDs for bioimaging, gene-complexed QD bundled NPs were easily detected in the hMSCs using several different methods such as fluorescence-activated cell sorter, confocal laser scanning microscopy, and in vivo optical imaging. These PEI-coated, bundled QD NPs exhibited significantly higher gene transfection efficacy than single-type QDs. Particularly, the largest QD bundled NPs examined, QD655, had a much higher uptake capability and greater gene expression ability than the other QD NPs (QD525, QD565, and QD605). We believe that our findings help to enrich knowledge of design considerations that will aid in the engineering of QD NPs for stem cell application in the future.

Size-Dependent Charge Transfer Yields in Conjugated Polymer/Quantum Dot Blends

We investigate the effect of quantum dot size on photocurrent and photoinduced charge transfer yields in blends of the conjugated polymer, poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b′)dithiophene-2,6-diyl)(2-((dodecyloxy)carbonyl)thieno(3,4-b)thiophenediyl)) (PTB1), with PbS nanocrystal quantum dots (QDs). These hybrid solar cells exhibit external quantum efficiencies of over 70% and power conversion efficiencies of up to 2.8%. We use photoinduced absorption (PIA) spectroscopy and device EQE measurements to probe long-lived charge transfer at the polymer/QD interface as a function of QD size. We observe that both the PIA signal associated with charge formation on the polymer, as well as the external quantum efficiency of the hybrid photovoltaic devices decrease in magnitude with increasing quantum dot size, despite the broader absorption spectrum of the larger dots. We interpret these results as evidence that PTB1/PbS blends behave at least partially as bulk heterojunction (BHJ) solar cells, and conclude that the long-lived charge transfer yield is diminished at larger dot sizes as the energy level offset at the polymer/quantum dot interface is changed through decreasing quantum confinement.

Realisation of semiconductor ternary quantum dot cellular automata

Quantum dot cellular automata (QCA) have become a strong alternative for nanoscale computing. After a decade of exhaustive research on binary QCA, the newly introduced ternary QCA (tQCA) for implementing ternary logic needs further research to actually fabricate the tQCA cell. In this Letter, a quantum mechanical approach is presented for calculating the diverse parameters required for the fabrication of the tQCA. The effect of quantum dot size, the size of tQCA cell as well as the effect of temperature on polarisation for the tQCA cell are examined. The significant result for fabrication of the tQCA cell is tunnelling resistance, which must be greater than the quantum Hall resistance, that is, Rt ≫ 25.813 kΩ. The tQCA needs a quantum dot size which is < 6.36 nm to operate at room temperature and the distance of separation between two dots must be 5 nm. The steady-state behaviour of the tQCA cell using the Hubbard model and the Schrodinger time-independent equation is also explored.

Size-dependent optical absorption of silicon nanocrystals embedded in SiO2/Si3N4 hybrid matrix

Optical constants of silicon quantum dots (Si QDs) in SiO2/Si3N4 hybrid matrix on quartz substrates have been determined in the wavelength range of 350–2500nm. Forouhi–Bloomer (F-B) dispersion model is applied to describe the optical properties of silicon nanocrystals and the mismatch due to global fitting is fixed point by point. To extract the specific optical bandgap of silicon nanocrystal samples, Tauc-plots with γ=1/2 and γ=1/3 are used and discussed for the spectral range of 2eV–3eV. The effect of quantum dot size on the optical bandgap is explored. The results from both Tauc-plot and photoluminescence show the trend that the optical bandgap increases with decreasing quantum dot size, which agrees with the prediction of quantum confinement.

Effect of elemental composition and size on electron confinement in self-assembled SiC quantum dots: A combinatorial approach

A high level of control over quantum dot (QD) properties such as size and composition during fabrication is required to precisely tune the eventual electronic properties of the QD. Nanoscale synthesis efforts and theoretical studies of electronic properties are traditionally treated quite separately. In this paper, a combinatorial approach has been taken to relate the process synthesis parameters and the electron confinement properties of the QDs. First, hybrid numerical calculations with different influx parameters for Si1−xCx QDs were carried out to simulate the changes in carbon content x and size. Second, the ionization energy theory was applied to understand the electronic properties of Si1−xCx QDs. Third, stoichiometric (x=0.5) silicon carbide QDs were grown by means of inductively coupled plasma-assisted rf magnetron sputtering. Finally, the effect of QD size and elemental composition were then incorporated in the ionization energy theory to explain the evolution of the Si1−xCx photoluminescence spectra. These results are important for the development of deterministic synthesis approaches of self-assembled nanoscale quantum confinement structures.