Increasing Quantum Dots Size Research Articles

Quantifying the Size‐Dependent Exciton‐Phonon Coupling Strength in Single Lead‐Halide Perovskite Quantum Dots

AbstractOptimizing the performance of semiconductors in both classical and quantum applications, not only requires a solid understanding of elementary excitations such as electrons, holes, or bound electron–hole pairs (excitons), but also of their interaction with the host material's vibrational states (phonons). Exciton‐phonon coupling is particularly relevant in quantum dots (QDs) of APbX3 lead‐halide perovskite (where “A” can be Cs, formamidinium (FA), or methylammonium (MA), and X can be Cl, Br, or I), a new class of semiconductors with a soft crystal structure. Here, they quantify the strength of coupling to interband transitions for both FAPbBr3 and CsPbBr3 QDs, via the magnitude of phonon replicas in their photoluminescence (PL) spectra at cryogenic temperatures. CsPbBr3 QDs exhibit weaker exciton‐phonon coupling than similarly sized FAPbBr3 QDs. While the phonon energies are size‐independent, the exciton‐phonon coupling strength decreases with increasing QD size due to the decreased coupling of the transition to low‐energy surface‐enhanced phonon modes, consistent withab initio molecular‐dynamics (AIMD) simulations. Furthermore, within the harmonic approximation, the size‐dependent PL linewidth at room temperature can coarsely be estimated from the low‐temperature phonon replica spectrum, highlighting the crucial role of anharmonic effects. These findings contribute to realizing perovskite QD‐based devices with narrow and coherent emission for quantum technologies.

Tunable Mid-Infrared Interband Emission from Tensile-Strained InGaAs Quantum Dots.

We demonstrate the ability to tailor self-assembled growth of In0.5Ga0.5As quantum dots (QDs) on GaSb(111)A surfaces by molecular beam epitaxy. Spontaneous formation via the Volmer-Weber growth mode produces QDs with excellent structural and optical quality. By harnessing tensile strain to reduce their band gap energy, these QDs are characterized by light emission that extends into the midwave infrared wavelength range of 3.2-3.9 μm (0.318-0.388 eV). As we increase QD size, we can tune the band alignment from type-III to type-II, where light emission occurs due to interband recombination between quantum confined electrons in the InGaAs QDs and holes in the GaSb barriers. Of particular interest is an unusual blue-shift in emission wavelength with increasing QD size, which we attribute to the incorporation of Sb into the InGaAs QDs from the GaSb barriers. By expanding this approach to produce tensile-strained QDs from other narrow band gap semiconductors, we anticipate the development of a range of highly tunable mid-infrared light sources.

A Novel and Cost-Effective CsVO3 Quantum Dots for Optoelectronic and Display Applications.

Quantum dots (QDs) have an unparalleled ability to mimic true colors due to their size-tunable optical and electronic properties, which make them the most promising nanoparticles in various fields. Currently, the majority of QDs available in the market are cadmium, indium, and lead-based materials but the toxicity and unstable nature of these QDs restricts their industrial and practical applications. To avoid using heavy metal ions, especially cadmium, the current research is focused on the fabrication of perovskite and vanadate QDs. Herein, we report the facile synthesis of a novel and cost-effective CsVO3 QDs for the first time. The sizes of the CsVO3 QDs produced were tuned from 2 to 10 nm by varying the reaction temperature from 140 to 190 °C. On increasing QD size, a continuous red shift was observed in absorption and emission spectra, signifying the presence of quantum confinement. In addition, along with CsVO3 QDs, the CsVO3 nanosheets self-assembled microflower-like particles were found as residue after the centrifugation; the X-ray diffraction indicated an orthorhombic structure. Under 365 nm excitation, these CsVO3 microflower-like particles exhibited broad emission with CIE coordinates in the white emission region. The acquired results suggest that CsVO3 QDs may represent a new class of cadmium-free materials for optoelectronic and biomedical applications.

A comparison of optical properties of disc-like and spherical quantum dots

The optical properties of quantum dots (QDs) are of interest to many researchers. In this study, the optical coeefficients analyzed include the optical absorption, the refractive index; both the linear and nonlinear cases together with the electric field effect. The electronic structures of the disc-like and spherical QDs are calculated by using the effective-mass approximation. The results show that the total change in the refractive index and the optical absorption increase with increasing QD size, the peaks are shifted with the changing QD size. The nonlinear optical properties increase with the external electric field and the optical intensity. Comparison of QD types shows that the peaks are red-shifted for the disc-like QD and lower than those in the spherical QD case.

Influence of CdTe thickness on structural and optical properties of CdTe/ZnTe quantum dots on Si substrates

We have investigated the influence of CdTe thickness on the structural and optical properties of CdTe/ZnTe quantum dots (QDs) on Si substrates using molecular beam epitaxy and atomic layer epitaxy (ALE). Atomic force microscopy images reveal that CdTe QDs are formed after being embedded in an undoped ZnTe matrix, which was controlled by the ALE method. As the size of QDs increases, the photoluminescence (PL) peak shifts towards lower energy and the full-width-at-halfmaximum increases. Through temperature-dependent PL, the increase in activation energy with increasing QD size was confirmed. From this study, the different structural and optical properties of QDs deposited on Si substrates could be understood.

Size Effects on the Interaction of QDs with the Mitochondrial Membrane In Vitro.

The mitochondrial toxicity induced by GSH-CdTe Quantum dots (QDs) of different sizes was investigated. The decreases in absorbance and transmission electron microscopy images show that QDs induce the swelling of mitochondria. Results of flow cytometry indicate that QDs cause a reduction of mitochondrial membrane potential (MMP). A remarkable increase in fluidity of protein regions of mitochondrial membrane is observed, whereas the lipid regions are not obviously affected. Cyclosporin A (CsA) effectively prevents the QD-induced mitochondrial swelling. On the basis of these results, it is proposed that QDs induce mitochondrial permeability transition (MPT). Moreover, with increasing QDs size, a pronounced MPT is observed. The difference between the membrane fluidity induced by QDs and Cadmium ion and the ineffective protective effects of EDTA suggests that the mitochondrial toxicity of QDs cannot be only attributed to the release of metal ion. The protective effects of HSA indicate that the interaction of QDs with pore-forming protein gives rise to the increase in membrane fluidity. This hypothesis is demonstrated by the interaction of QDs with model membranes and proteins using differential scanning calorimetry and isothermal titration microcalorimetry. In conclusion, as the size of QDs increases, the binding affinity of QDs with membrane protein increases, and therefore causes a pronounced mitochondrial damage.

Cu2O quantum dots emitting visible light grown by atomic layer deposition

This paper reports the fabrication of the Cu2O quantum dots (QDs) emitting a controlled wavelength in the visible spectral range prepared by atomic layer deposition (ALD). Cu2O thin film layers formed on the Al2O3 surface showed large density of islands via Volmer–Weber growth mode, which resulting in QD formation. As the number of ALD cycles was increased from 60 to 480, the spatial density and mean diameter of the Cu2O QDs increased systematically from 4.02 × 1011/cm2 to 2.56×1012/cm2 and from 2.1 to 3.2nm, respectively. The absorption spectral results indicated that the electron energy transition in the Cu2O QDs was a direct process with the optical band gaps decreasing from 2.71 to 2.15eV with increasing QD size from 2.1 to 3.2nm because of the quantum confinement effect. The Cu2O QDs showed broad emission peaks composed of multiple elementary emission spectra corresponding to the Cu2O QD ensembles with a different size distribution. As the size of Cu2O QDs decreased, the shoulder peaks at the higher energy side developed due to the quantum confinement effect.

Growth and properties of the MOVPE GaAs/InAs/GaAsSb quantum dot structures

This review paper summarizes some of results achieved during last years of our quantum dot (QD) research.We show that the QD shape (aspect ratio and elongation) significantly influences the QD photoluminescence (PL) spectrum. Magnetophotoluminescence can be used for determination of the anisotropy of QDs. While the calculated shifts in magnetic field of the energies of higher radiative transitions are found to be sensitive to the lateral elongation, the shift of the lowest transition is determined mainly by the exciton effective mass. This behavior can be used for determining both the effective mass and the elongation fairly reliably from the magnetophotoluminescence spectra displaying at least two resolved bands.Lateral shape of InAs/GaAs QDs in vertically correlated structures was also studied. We found the ways to control the QD elongation and consequently the energy difference between PL transitions by adjusting properly the spacer layer thickness.The main goal was to redshift QD PL emission towards telecommunication wavelengths of Metal Organic Vapor Phase Epitaxy prepared InAs/GaAs QDs using InGaAs or GaAsSb covering strain reducing layer (SRL). Our results proved that GaAsSb SRL improves the QD PL properties and the type I or type II band alignment can be controlled by both, GaAsSb composition and QD size. Maintaining the type I heterostructure is important for high luminescence efficiency and emission wavelength stability of QD structure. The simulation of electron structure in InAs QDs covered with GaAsSb SRL and experimental results reveal the importance of increasing QD size for obtaining a longer wavelength PL from the type I heterostructure.The type II structure of InAs/GaAs QDs covered by GaAsSb SRL with Sb content near 30% enabled us to achieve extremely long emission wavelength at 1.8µm. The high amount of antimony in the SRL causes the preservation of QD size, and increased QD size prolongs the PL wavelength. The type II heterostructures with ground state electrons confined in InAs QDs and ground state holes in GaAsSb SRL have a strong potential in detector and solar cell applications as is demonstrated by photocurrent measurement.The dependence of built in electric field on QD PL properties is presented since this knowledge is necessary for application of QDs in different kind of electronic structures.We have experimentally demonstrated that the type I/type II transition of band structure of InAs QD with GaAsSb SRL does not depend only on the composition of the SRL, but also on other structure parameters, like QD size or intensity of electric field.

Increased Quantum Dot Loading by pH Control Reduces Interfacial Recombination in Quantum-Dot-Sensitized Solar Cells.

The power conversion efficiency of quantum-dot-sensitized solar cells (QDSSCs) hinges on interfacial charge transfer. Increasing quantum dot (QD) loading on the TiO2 anode has been proposed as a means to block recombination of electrons in the TiO2 to the hole transport material; however, it is not known whether a corresponding increase in QD-mediated recombination processes might lead to an overall higher rate of recombination. In this work, a 3-fold increase in PbS QD loading was achieved by the addition of an aqueous base to negatively charge the TiO2 surface during Pb cation deposition. Increased QD loading improved QDSSC device efficiencies through both increased light absorption and an overall reduction in recombination. Unexpectedly, we also found increased QD size had the detrimental effect of increasing recombination. Kinetic modeling of the effect of QD size on interfacial charge transfer processes provided qualitative agreement with the observed variation in recombination lifetimes. These results demonstrate a robust method of improving QD loading, identify the specific mechanisms by which increased QD deposition impacts device performance, and provide a framework for future efforts optimizing the device architecture of QDSSCs.

Photoinduced Electron Transfer in CdSe/ZnS Quantum Dot–Fullerene Hybrids

Photoinduced electron transfer (ET) in CdSe/ZnS core–shell quantum dot (QD) – fullerene (COOH–C60) hybrids was studied by the means of time-resolved emission and absorption spectroscopy techniques. A series of four QDs with emission in the range 540–630 nm was employed to investigate the dependence of the electron transfer rate on the QD size. Emission of the QDs is quenched upon hybrid formation, and the quenching mechanism is identified as photoinduced electron transfer from the QD to the fullerene moiety due to the fullerene anion signature observed in transient absorption. In order to obtain quantitative information on the ET reaction, several kinetic data analysis techniques were used, including a conventional multiexponential fitting and a maximum entropy method for emission decay analysis, as well as a distributed decay model based on the Poisson distribution of fullerenes in the hybrids. The latter gradually simplifies the interpretation of the transient absorption spectra and indicates that the spectra of QD cations are essentially similar to those of neutral QDs, differing only by a minor decrease in the intensity and broadening. Furthermore, only a minor decrease in the ET rate with the increasing QD size was observed, the time constants being in the range 100–200 ps for all studied QDs. The charge recombination is extended to 10 ns or longer for all hybrids.

The Role of Groove Periodicity in the Formation of Site-Controlled Quantum Dot Chains.

Structural and optical properties of InAs quantum dot (QD) chains formed in etched GaAs grooves having different periods from 200 to 2000 nm in [010] orientation are reported. The site-controlled QDs were fabricated by molecular beam epitaxy on soft UV-nanoimprint lithography-patterned GaAs(001) surfaces. Increasing the groove periods decreases the overall QD density but increases the QD size and the linear density along the groove direction. The effect of the increased QD size with larger periods is reflected in ensemble photoluminescence measurements as redshift of the QD emission. Furthermore, we demonstrate the photoluminescence emission from single QD chains.

Optical properties of ionized donor-bound excitons confined in strained wurtzite ZnO/MgxZn1−xO quantum dots**Project supported by the National Natural Science Foundation for Young Scientists of China (No. 11102100), the Program for New Century Excellent Talents in Fujian Province University (No. JA14285) and the Program for Young Top-Notch Innovative Talents of Fujian Province of China.

Within the framework of the effective-mass approximation and the dipole approximation, considering the three-dimensional confinement of the electron and hole and the strong built-in electric field (BEF) in strained wurtzite ZnO/Mg0.25Zn0.75O quantum dots (QDs), the optical properties of ionized donor-bound excitons (D+, X) are investigated theoretically using a variational method. The computations are performed in the case of finite band offset. Numerical results indicate that the optical properties of (D+, X) complexes sensitively depend on the donor position, the QD size and the BEF. The binding energy of (D+, X) complexes is larger when the donor is located in the vicinity of the left interface of the QDs, and it decreases with increasing QD size. The oscillator strength reduces with an increase in the dot height and increases with an increase in the dot radius. Furthermore, when the QD size decreases, the absorption peak intensity shows a marked increment, and the absorption coefficient peak has a blueshift. The strong BEF causes a redshift of the absorption coefficient peak and causes the absorption peak intensity to decrease remarkably. The physical reasons for these relationships have been analyzed in depth.

The impact of disorder on charge transport in three dimensional quantum dot resonant tunneling structures

Efficient iso-entropic energy filtering of electronic waves can be realized through nanostructures with three dimensional confinement, such as quantum dot resonant tunneling structures. Large-area deployment of such structures is useful for energy selective contacts but such configuration is susceptible to structural disorders. In this work, the transport properties of quantum-dot-based wide-area resonant tunneling structures, subject to realistic disorder mechanisms, are studied. Positional variations of the quantum dots are shown to reduce the resonant transmission peaks while size variations in the device are shown to reduce as well as broaden the peaks. Increased quantum dot size distribution also results in a peak shift to lower energy which is attributed to large dots dominating transmission. A decrease in barrier thickness reduces the relative peak height while the overall transmission increases dramatically due to lower “series resistance.” While any shift away from ideality can be intuitively expected to reduce the resonance peak, quantification allows better understanding of the tolerances required for fabricating structures based on resonant tunneling phenomena.

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.

Study of the stability of quantum dot sensitized solar cells

Quantum dot sensitized solar cells have been constructed using photoanodes bearing nanocrystalline titania and CdS, CdSe and ZnS quantum dots. The combined CdSe/CdS system provides enhanced photon absorption in a large range of the visible spectrum, while ZnS plays a passivation and stabilization role. This was verified by micro-Raman scattering studies as a function of light soaking of successively sensitized photoanodes. Annealing of photoanodes resulted in increasing quantum dot size as verified by both UV-vis diffuse reflectance and Raman spectroscopy. It was also found that a composite CdSxSe1−x species was formed at the interface of CdS and CdSe layer, which constitutes a stabilization factor and is further stabilized by annealing, that influences the electric characteristics of the sensitized photoanodes. Single CdS sensitizers, when annealed gave increased photocurrents but suffered a fast decay. Annealing of fully sensitized photoanodes gave a diverse behavior, markedly stabilizing photocurrents under N2 atmosphere. All cells worked well by using a Pt-free cathode made by chemically growing Cu2S on a brass foil. The employed innovative cell structure provided liberty in designing and optimizing the device geometry.

Spectroscopy of single InAs quantum dots

Ensembles of InAs quantum dots with a very low density (∼106 cm−2) are grown by molecular beam epitaxy, which allows the spectral characteristics of emission of single quantum dots to be studied by the method of cryogenic microphotoluminescence. With increasing quantum dot size, the splitting of exciton states is demonstrated to increase steadily to ∼102 µeV. In the exciton energy range of 1.3–1.4 eV, the magnitude of this splitting is comparable with the natural width of the exciton lines. This result is important for the development of emitters of entangled photon pairs based on InAs quantum dots.

Exciton Annihilation and Dissociation Dynamics in Group II–V Cd3P2 Quantum Dots

Semiconductor quantum dots (QDs) have emerged as a new class of light harvesting materials for solar energy conversion due to their unique size-dependent properties. Most recent studies have focused on II-VI group (such as CdX, X = S, Se, and Te) QDs and lead salt (such as PbS, PbSe, and PbTe) QDs. In this paper, we investigate exciton dissociation and annihilation dynamics of Cd3P2 QDs, a low bulk band gap (0.55 eV) II-V group material, to explore their potential application as a light harvesting component for photoreduction systems. For Cd3P2 QDs with 1S exciton band at 650 nm, a long-lived single exciton state with lifetime of 259 ns and a high emission quantum yield of 65% were observed. In Cd3P2 QD-rhodamine B (RhB, an electron acceptor) complexes, excitons in QDs could be dissociated by ultrafast electron transfer to RhB (6.2 ps), and the charge separated state had a long lifetime (31 ns). Although the photoinduced electron transfer rate in QD-RhB complexes decreased with increasing QD size, electron transfer was observed in QDs with 1S exciton bands at wavelength as long as 1050 nm. Compared with CdSe and PbS, Cd3P2 QDs with both more strongly reducing excited states and broader absorption in the visible and near IR region can be readily achieved, making them potential photosensitizers for photodriven water or CO2 reduction reactions.

First-principles study of single-layer C-terminated BN quantum dots

We present a first-principles study of the structural, electronic and magnetic properties of single-layer C-terminated BN quantum dots (QDs) under different hydrotreating conditions. The morphologies of QDs with fully hydrogenated edges change slightly. For the fully bared cases, the edged C–C bonds become short and protrudent edged C atoms relax inwards therefore edged zigzag C chain tends to a straight line. The cases of the partially passivated by hydrogen atom at apex, the apex C atom is not relaxed inwards and a new C–C bond is formed. The fully hydrogenated QDs especially N-rich cases are energetically more favorable than those with bared or partially hydrogenated ones. The C-terminated BN-QDs have no magnetic moment when their protrudent edged C atoms are fully passivated by hydrogen atoms, while those with bared or partially hydrogenated edges possess magnetic moments and especially for N-rich cases their magnetic moments increase with increasing QD size n for either bared or partially hydrogenated edges. The band gap of the fully hydrogenated QDs decreases oscillatorily with increasing QD size n. Moreover, for the same size n, the energy gap is wider under N-rich condition than under B-rich condition.

Multiscale simulation of the electronic structure of silicon nanoclusters

An interpolation scheme is proposed for determination of the spectrum of silicon quantum dots 2 to 10 nm in diameter involving the bulk silicon spectrum and the results of our first-principle calculations of silicon clusters 0.7 to 1.5 nm in diameter. In the interpolation we allowed for the discrete character of the electron spectrum of clusters, the law of conservation of the number of electrons in the system, and imperfection of the shape of the quantum dot. Comparison of the results of interpolation with direct first-principle calculations is indicative of sufficiently high accuracy of the scheme, which increases with increasing quantum dot size.

Electronic and optical properties of ZnO quantum dots under hydrostatic pressure

In the present work, we studied the electronic and optical properties of ZnO quantum dots (QDs) subjected to externally applied hydrostatic pressure. Our single-particle calculations are based on the empirical pseudopotential method and the excitonic effects are considered by employing the configuration interaction approach. The optical band gap, Stokes shift, and optical emission polarization have been investigated as a function of the applied pressure. It is found that the applied pressure causes a linear increase in the optical band gap. The pressure coefficient appears to be highly size dependent, exhibiting a monotonic increase with increasing QD size. In contrast to this monotonic behavior, the applied pressure induces a nonmonotonic Stokes shift which presents a minimum value at a critical pressure. For pressures larger than this critical value, the optical emission polarization exhibits a sharp transition from in-plane to out-of-plane polarization. Finally, it is found that the critical pressure at which the crossing takes place strongly depends on the QD size, showing larger values for larger QD sizes. Beyond this crossing point, the lowest optically bright exciton state mainly originates from one Slater determinant, where both the single-particle electron and hole states have an $S$-type envelope function and the hole state originates mainly from the bulk Bloch C band.