QD Size Research Articles

Absorption and emission in the visible range by ultra-small PbS quantum dots in the strong quantum confinement regime with S-terminated surfaces capped with diphenylphosphine

The synthesis and characterization of PbS QDs absorbing and emitting in the visible range is a very challenging task, since it requires the QDs to be within the strong quantum confinement regime (QD size<2.5 nm). It implies not only having “small” QDs, but also stable and monodisperse; characteristics that have been elusive to achieve for many researchers. In the current work, ultra-small PbS QDs (size ~2 nm) were synthesized based on a modification of the Hines method, controlling the reaction time, and adding diphenylphosphine (DPP) which serves as a catalyst and a protective agent in the reaction synthesis. Novel ultra-small PbS QDs with S-terminated surfaces were obtained, which formed at the early stages of the synthesis reaction and are stabilized by the DPP; as it was suggested by the TEM, FTIR and Raman results. The ultra-small PbS QDs display a maximum peak of optical absorption at ~532 nm, with a corresponding optical band gap of 1.82 eV; a maximum peak of emission at ~679 nm, which results in a Stokes shift of 119 nm, smaller than the Stokes shift observed in “larger” PbS QDs. These ultra-small QDs displayed an average size of ~2 nm, with a standard deviation of ~0.3 nm, which was the smallest among the synthesized samples, based on TEM measurements. Finally, the LUMO and HOMO levels were measured by means of cyclic voltammetry and optical absorption spectroscopy. The values of the optical band gap and the energies measured for the LUMO and HOMO levels of these ultra-small PbS QDs were affected by their atomistic surface arrangement and the capping ligand interacting with their surface. Producing variations in their values that doesn’t follow the trends established for quantum confinement effects related to size variation only. Thorough physical and chemical characterization of such ultra-small PbS QDs are crucial in understanding the origin of their optoelectronic properties, which will contribute to better delineate possible future applications.

Linear and nonlinear optical characteristics of CsPbBr3 perovskite quantum dots-doped borosilicate glasses

CsPbBr3 perovskite QDs are precipitated in a borosilicate glass matrix, while protects efficiently the QD from photo-induced and chemical degradation. We show that the CsPbBr3 QD doped glasses exhibit strong visible photoluminescence (PL), which is dependence on the concentration that can be controlled by heat treatment conditions. Due to the stabilization by the glass matrix, we are able to determine the nonlinear optical (NLO) properties with a Z-scan technique. We observe a cross-over from saturated absorption (SA) to reverse saturated absorption (RSA) by either increase the pumping intensity or the QD size, reminiscent of quantum size effect in the NLO response. The RSA is associated with two-photon absorption (TPA) that induces strong upconversion luminescence of QD doped glass samples. Our results imply that the glasses containing CsPbBr3 QDs may find potential applications from solid state lighting to ultrafast optical switches.

Charge Transfer from Lead Sulfide Quantum Dots to MoS2 Nanoplatelets

The research interest in the transition metal dichalcogenides (TMD) has been reborn a few years ago. This had happened due to the remarkable properties of monolayered TMD (e.g., high carrier mobility and high exciton binding energy) and due to the development of the exfoliation methods. Photoconductive MoS2-based devices spectral range can be expanded to the NIR by coupling them with PbS QDs. However, this requires extensive knowledge about the charge and energy transfer processes in such systems. In this paper, we investigate charge transfer between PbS QDs and MoS2 nanoplatelets (NPls). Using the PL decay analysis, we show how the charge transfer efficiency changes with the distance between the QDs and NPls, as well as with QD size. Last, we demonstrate that the addition of the MoS2 NPLs increases the photoconductive response for up to an order of magnitude, as compared to the bare QD.

Intrinsic-strain-induced curling of free-standing two-dimensional Janus MoSSe quantum dots

Motivated by the fascinating properties of both two-dimensional transition metal dichalcogenide quantum dots (TMD QDs) and Janus TMD monolayers, we theoretically explore the equilibrium structures of free-standing Janus MoSSe QDs in which atomic asymmetry of chalcogen is introduced. Two distinct types of spontaneous curling are observed by molecular dynamics simulations, and the curling behavior depends on the size of QD. The bowl-like (tube-like) curling occurs in relatively small (large) MoSSe QDs with different shapes (hexagon and triangle) and edge types (zigzag and armchair). The transition between these two curling types occurs at the sizes of around 10 nm and 13 nm for hexagonal and triangular shapes, respectively. By applying equivalent misfit strains into two adjacent sublayers, finite element analysis reproduces similar curling behavior. This confirms the relaxation of intrinsic strain in Janus structure acting as the predominant driving force of spontaneous curling. In addition, the curvatures of Janus TMD QDs increase from MoSSe to MoSeTe to MoSTe, indicating the positive correlation between the curling and misfit.

Study of Interfacial Charge Transfer from an Electron Rich Organic Molecule to CdTe Quantum Dot by using Stern-Volmer and Stochastic Kinetic Models.

Photoinduced electron transfer (PET) from N-methylaniline (NMA) to a photoexcited CdTe quantum dot (QD*) is studied in toluene. The PET mechanism at low to moderate quencher (NMA) concentrations (<0.08 M) remains mostly collisional with some contributions from QD-NMA complex formation. However, at high quencher concentrations (>0.10 M), QDs form larger numbers of static complexes with NMA molecules leading to a steep positive deviation in the steady-state Stern-Volmer curves. An isothermal titration calorimetry (ITC) study confirms the formation of QD-NMA complexes (K∼150 M-1 ) at high quencher concentrations. Fitting our experimental data using a stochastic kinetic model indicates that the number of NMA molecules attached per QD at highest NMA concentration (∼0.16 M) used in this study decreases from ∼0.76 to ∼0.47 with reducing the QD size from ∼5.2 nm to ∼3.2 nm. However, the PET rate increases with decreasing QD size, which is commensurate with the observation that the chemical driving force (ΔG) increases with decreasing the QD particle size. We have analyzed the PET kinetics mainly by using Stern-Volmer fittings. However, in some cases Tachiya's stochastic kinetic model is used for stoichiometric analysis, which seems to be useful only at high quencher concentrations. The measured PET rate coefficients in all the cases are found to be at least an order of magnitude lower when compared to the diffusion-controlled rate of the reaction medium.

Charge Transfer from Lead Sulfide Quantum Dots to MoS-=SUB=-2-=/SUB=- Nanoplatelets-=SUP=-*-=/SUP=-

The research interest in the transition metal dichalcogenides (TMD) has been reborn a few years ago. This had happened due to the remarkable properties of monolayered TMD (e. g. high carrier mobility and high exciton binding energy) and due to the development of the exfoliation methods. Photoconductive MoS2-based devices spectral range can be expanded to the NIR by coupling them with PbS QDs. However, this requires extensive knowledge about the the charge and energy transfer processes in such systems. In this paper, we investigate charge transfer between PbS QDs and MoS2 nanoplatelets (NPls). Using the PL decay analysis, we show how the charge transfer efficiency changes with the distance between the QDs and NPls, as well as with QD size. Last, we demonstrate that the addition of the MoS2 NPLs increases the photoconductive response for up to an order of magnitude, as compared to the bare QD. Keywords: transition metal dichalcogenides, quantum dots, charge transfer, lead sulfide, molybdenum disulfide.

Low-temperature carrier dynamics in MBE-grown InAs/GaAs single- and multi-layered quantum dots investigated via photoluminescence and terahertz time-domain spectroscopy

The photocarrier dynamics in molecular beam epitaxy (MBE)-grown single- (SLQD) and multi-layered (MLQD) InAs/GaAs quantum dots were studied. Photoluminescence (PL) spectroscopy has shown that the MLQD has more uniform QD size distribution as compared to the bimodal SLQD. Correlation between PL and THz-TDS has shown that photocarrier transport is more favored in the MLQD owing to this uniform QD size distribution, resulting to higher THz emission. The THz emission from the QD samples were found to be proportional to temperature. A drift-related photocarrier transport mechanism is proposed, wherein photocarriers generated in the QDs are accelerated by an interface electric field.

Lattice Strain Analysis of a Mn-Doped CdSe QD System Using Crystallography Techniques

In this work, we report on the different sizes of manganese-doped cadmium selenide quantum dots (Mn-doped CdSe QDs) synthesized for 0 to 90 min using a reverse micelle organic solvent method and surfactant having a zinc blende structure, with physical size varying from 3 to 14 nm and crystallite size from 2.46 to 5.46 nm and with a narrow size distribution. At similar reaction times, Mn-doped CdSe QDs displayed the growth of larger QDs compared with the pure CdSe QDs. Due to the implementation of lattice strain owing to the inclusion of Mn atoms in the CdSe QD lattice, the lattice parameter was compressed as the QD size increased. Strain was induced by the particle size reduction, as observed from X-ray diffractometer (XRD) analysis. The analyses of the strain effect on the QD reduction are discussed relative to each of the XRD characteristics.

TiO2 quantum dots loaded sulfonated graphene aerogel for effective adsorption-photocatalysis of PFOA

With the pollution of perfluoroalkyl substances (PFASs) became increasingly serious, the researches focused on removal of PFASs by adsorption-photocatalysis method has attracted considerable attention. To make the catalyst TiO2 disperse uniformly as quantum dots onto hydrophobic surface which was liable to attract perfluorooctanoic acid (PFOA), the surfactant sodium dodecyl sulfate (SDS) were used in this work, which not only connected the hydrophilic TiCl3 to the hydrophobic sulfonated graphene (SG) nanosheets, but also behaved as the molecular template for controlled nucleation and growth of the nanostructured TiO2. After 3D SG-TiO2 QD nanosheets were fabricated, a series of 3D SG-TiO2 QD aerogels were self-assembled by ice-template. TiO2 uniformly distributed on the surface of SG aerogel at QD size level (2–3 nm) and the size of TiO2 could be effectively regulated by concentration of SDS. Compared with aggregated TiO2 material, 3D SG-TiO2 QD aerogels owned higher adsorption and photocatalytic performance. Benefiting from the hydrophobic surface of 3D SG as well as dispersed TiO2 QDs, 3D SG-TiO2 QD could enrich PFOA instantaneously (0.0381/s) and photocatalytic decomposed them effectively (1.898 E−4/s). PFOA degradation by hole and hydroxyl radicals proceeded via a stepwise mechanism. The column made of 3D SG-TiO2 QD could remove PFOA persistently in cycles of permeation. 3D SG-TiO2 QD possessed powerful adsorption-photocatalytic decomposition capability of PFOA and steady reusability performance. The present work highlights the individual roles and synergistic effect of TiO2 QD and 3D SG for effectively removing PFOA.

Luminescence Change of CdS and CdSe Quantum Dots on a Ag Film.

Enhanced luminescence of an emitter on a Ag film is usually ascribed to the resonant surface plasmons. In these studies, the solid cadmium sulfide (CdS) and cadmium selenide (CdSe) quantum dot/silver (QD/Ag) hybrids were prepared, and the luminescence characteristics of these QD/Ag hybrids were measured. It is found that the enhancement of the trap state emission (TSE) is related to the QD size. The TSE features of the annealed QD/Ag hybrids are insensitive to the morphology of the Ag film. We used the wet and dry methods to separate the QD and Ag components and found that the photoluminescence (PL) of the QD component was permanently changed from the initial state. The PL modification is ascribed to the Ag+ doping effect rather than the surface plasmons. This doping method uses pure Ag as the Ag+ ion source. In this case, though the CdS and CdSe QD/Ag hybrids are the solid state, the cation exchange between Ag+ and Cd2+ ions can still occur on the QD surface. Even a small amount of Ag can efficiently influence the luminescence of the QDs embedded in the poly(methyl methacrylate) matrix. A hypothetical model was proposed to explain the PL modification of the QD/Ag hybrid with and without annealing. Using this dry method for doping, the transparent luminescence label can be prepared easily, and the doped QDs can be further doped with Ag+ dopants.

Effect of Lateral Electric Field on the Transition Energies of Heavy Hole State and Light Hole State in a Semiconductor Quantum Dot

Using effective mass approximation, this study theoretically investigated how the ground state of the valence band in an inverse parabolic quantum dot (IPQD) reaches the light hole state by strengthening the confinement in the QD plane through the application of a lateral electric field on IPQDs. We calculated the exact analytical solution in the form of the Heun function associated with a complex eigenvalue for the degenerate and excited energy states of the electron, light hole (LH), and heavy hole (HH), in the IPQD context. Our results showed that, for a large QD size, the inverse parabolic quantum confinement (IPQC) potential removed the electron and hole degeneracy, while in the strong confinement region (small QD size), crossing occurred. In addition, the results offered another way of switching the ground state energy of the valence band inside QDs from the HH to LH state when applying a lateral electric field on a QD with an IPQC potential. Consequently, different optical transitions were established, and from the lateral electric field, we obtained a larger quantum Stark shift in IPQD compared with the parabolic QD (PQD), and the excited state energies showed a linear Stark shift in both the QDs.

Interaction between nitroxyl radicals and CdTe quantum dots: Determination of fluorescence-quenching mechanisms in aqueous solution

The present work characterizes the optical properties of CdTe quantum dots (CdTe QDs) after interaction with nitroxyl radicals based on steady-state and time-resolved fluorescence spectroscopy studies. QDs of different sizes were exposed to 2 different nitroxyl radicals, i.e., TEMPO and 4-amino-TEMPO radicals. A clear dependence of dynamic and static Stern-Volmer constants values, KD and KS, respectively, was observed as a function of the size of QDs used, with a change from a mostly static mechanism (for smaller QD sizes) to a dynamic mechanism predominating as the nanoparticles increase in size. All observed effects are dependent on both the concentration of the radical and the size of the nanoparticles. These results allow causal relationships to be established that could be used as a basis for the design of a potential sensor for this type of reactive species in aqueous media.

Raman study of InAs/GaAs quantum dot solar cells

We report polarized and resonant Raman study of InAs/GaAs quantum dot solar cell (QDSC) structures. Raman spectra obtained from the top surfaces of the samples suggested that the formation of InAs QDs induced tensile strain in the overgrown GaAs layers. Furthermore, a longitudinal optical phonon-plasmon (LPP) coupled modes were observed in the p-type GaAs layers. The tensile strain was increased with an increase in the QD size. The hole concentrations estimated by fitting the individual LPP coupled modes were in the range of 2.4–3.5 × 1018 cm−3. Resonant Raman spectra obtained from the cleaved sides, where the QDs were located, showed a 225 cm−1 mode in parallel polarization configurations. Based on accurate analysis, this mode was identified as the LA(X) phonon of GaAs.

Optimization of size uniformity and dot density of InxGa1−xAs/GaAs quantum dots for laser applications in 1 µm wavelength range

This work reports on growth and characterization of self-assembled InxGa1−xAs quantum dots grown on GaAs (1 0 0) substrate designed to emit at 1030 nm. All structures were grown by molecular beam epitaxy and investigated by photoluminescence (PL) spectroscopy and atomic force microscopy. The growth parameters, such as the growth temperature, QD layer thickness and indium content were changed systematically. A strong reduction in PL linewidth from 54 meV down to 26 meV measured at 10 K for a single QD layer was achieved as the result of a more uniform QD size distribution. Additionally, a correlation between the linewidth and the temperature dependent wavelength shift of the PL peak was found.

Photostimulated control of luminescence quantum yield for colloidal Ag2S/2-MPA quantum dots.

In this paper, we present the results on photoinduced formation of colloidal Ag2S quantum dots with sizes of 1.5–3 nm passivated by 2-mercaptopropionic acid (Ag2S/2-MPA) in the presence of ethylene glycol. The synthetized colloidal Ag2S/2-MPA QDs have NIR recombination luminescence with its maximum near 800 nm. The control of absorption and luminescence properties of the QDs is achieved by photoactivation. It is shown that photoexposure of colloidal solution of Ag2S/2-MPA QDs leads to an increase in the QD size and monodispersity along side with the growth of the luminescence quantum yield from 1% to 7.9%. Enhancement of the luminescence quantum yield is accompanied by an increase in the average luminescence lifetime up to 190 ns, which is due to the blocking of the nonradiative recombination channel with the radiative recombination rate being (3–5.5) × 105 s−1. It is shown that the purification of the Ag2S/2-MPA solution by a dialysis membrane from regenerated cellulose leads to an increase in the sample monodispersity, as well as stops the photoinduced growth of QDs, and also reduces the degradation of their photoluminescence.

Photoluminescence study of InAs/InGaAs sub-monolayer quantum dot infrared photodetectors with various numbers of multiple stack layers

This study investigated the effects of the number of stacking layers (S) on the optical properties of InAs/InGaAs sub-monolayer quantum dot (SML-QD) infrared photodetectors by photoluminescence spectroscopy. As S was increased from two to six, the room temperature PL emission energies were redshifted remarkably (~84 meV) and the full width at half maximum was increased by approximately 10 meV due to a change in the size distribution of SML-QDs. Furthermore, the excitation intensity-dependent PL spectra of the SML-QD (S = 6) showed improved PL integrated intensity caused by a change in the QD distribution and density. With increasing S, the carrier thermal activation energies (Ea) for the InAs QD and InGaAs quantum well (QW) increased by approximately 27 and 8 meV, respectively, due to the QD size effect. These results suggest that the mini-band structure of multiple SML-QDs formed with a very thin spacer thickness (~1 nm) could be tuned by controlling S.

Size Control of CH3NH3PbBr3 perovskite Quantum Dots Via Ostwald Anti-Ripening

「 Introduction 」 Organic-inorganic perovskite crystals have been aggressively applied to perovskite solar cells due to their attractive properties such as long carrier diffusion, am-bipolar conductivity, broad color-tunability, and their small exciton binding energy1. These perovskite crystals have been also applied to various applications like light-emitting diodes (LED), photodetectors, field effect transistors, and lasers due to perovskite crystals can be simply prepared from low-cost precursors with simple solution processes1. These tremendous potential for various applications with high performance are based on perovskite electro-optical features and they are expected to be used as an alternative semiconductor material to silicon and compound materials. Recently, bright luminescence properties from methylammonium lead tri-bromide (MAPbBr3) perovskite quantum dots (PeQDs) have been reported owing to the development of methods for preparing these PeQDs. Zhang et al. developed a ligand-assisted reprecipitation (LARP) inspired by the reprecipitation method for preparing organic and nano / micro crystals2. LARP can simply obtain MAPbX3(X = I, Br, and Cl) PeQDs through mixing a solution dissolving precursors and poor solvent. Although synthesis methods representative for LARP with narrow size-distributions are of particular interest for the successful implementation of PeQDs into LED with narrow emission, size-controlled PeQDs by nanometers have not been developed. In this work, we report on Ostwald ripening as a size-tunable technique for MAPbBr3PeQDs using LARP. 「 Experimental 」 Size-controlled synthesis of MAPbBr3PeQDs, and a simple strategy is as follows; a precursor solution was dropped into poor solvent followed by centrifuged to remove large-sized MAPbBr3crystals. Herein, MAPbBr3PeQD dispersions were aged at the 50℃using water bath to accelerate Ostwald ripening in MAPbBr3PeQD dispersions. Finally, we obtained size-controlled MAPbBr3PeQDs through centrifugation to remove grown MAPbBr3crystals. 「 Results and Discussion 」 MAPbBr3PeQD dispersions can be clearly observed that as aging time increases, PL peaks are blue shifted from 517 nm to 456 nm through aging for 4 hours. The emission colors irradiated UV lamp at 254 nm of MAPbBr3PeQD dispersions also changed from green to cyan and deep blue , while these dispersions close to be color less. When MAPbBr3PeQD dispersion is aged 1 hour, compared with an initial MAPbBr3PeQD dispersion, maximum PL peak was not clearly blue shift (ΔE = 42 meV) because of mean QD sizes were not drastically changed and also these square shape is maintained. In contrast, MAPbBr3PeQD dispersion is aged for 2 hours, a maximum PL peak drastically blue shifted from 508 nm to 489 nm. Because morphologies of samples such as sizes and shapes were completely changed from square to spherical with 7.2 nm size , result in PL energy extremely change (ΔE = 95 meV). Moreover, as further aging times increases, their mean QD sizes down to approach 5.3 nm by aging at the 50℃for 4 hours, in which clearly appear increasing band gap of MAPbBr3PeQDs due to quantum confinement effect. 「 References 」 1) B. R. Sutherland, et al., Nature, 2016, 10, 295-302. 2) F. Zhang, et al., ACS Nano, 2015, 9, 4533-4542.

Modeling of pyramidal shape quantum dot infrared photodetector: the effects of temperature and quantum dot size

Quantum dot base infrared detectors have many advantages such as lower dark current, higher operating temperature, higher photoconductive gain, and broader infrared response. For this reason, we have studied a pyramidal shape quantum-dot infrared photodetector (QDIP) in which the different mechanisms of noise and dark current, the effects of QD size variation, the temperature, and applied electric field are considered. Self-assembled In0.3Ga0.7As / GaAs pyramidal shape quantum dots have been considered and optical properties in the conduction band using effective mass Schrodinger equation by the finite-difference time-domain (FDTD) method have been analyzed. Finally, the detectivity calculated as functions of the applied electrical field and temperature. For example, at the T = 77 ° K and E = 2 kV / cm, the detectivity is 3 × 1013 cmHz − 1/2 / W.

Two-step photocurrent generation enhanced by the fundamental-state miniband formation in intermediate-band solar cells using a highly homogeneous InAs/GaAs quantum-dot superlattice

We studied intermediate-band solar cells (IBSCs) incorporating highly homogeneous InAs/GaAs quantum dot superlattices (QDSLs). The extra photocurrent generated by two-step photon absorption markedly increases at the fundamental state (FS) because the FS miniband has been successfully formed in the QDSL-IBSC by controlling the QD size homogeneity. Here, the carriers excited into the miniband spatially separate in the internal electric field, and the long-lived electrons in the intermediate states of the miniband increase the inter-subband absorption strength. The two-step photocurrent response, therefore, extends toward the longer-wavelength side corresponding to the QDSL-FS at approximately 1.2 µm.

Impact of Dot Size on Dynamical Characteristics of InAs/GaAs Quantum Dot Lasers

The purpose of this research is to study laser dynamics of InAs/GaAs Quantum Dot Lasers (QDLs) by changing QD energy levels. To date, most of the investigations have focused on only one of these circumstances, and hardly the result of change in the energy levels can be seen in lasing. In this work, in the first step, energy levels of lens-shape QDs are investigated by the eight-band k.p method, their variation for different QD sizes are surveyed, and recombination energies of the discrete levels are determined. Then, by representing a three-level InAs/GaAs QD laser, dynamics of such a laser device is numerically studied by rate equations in which homogeneous and inhomogeneous broadenings are taken into account. The lasing process for both Ground State (GS) and Excited States (ES) was found to be much sensitive to the QD size. It was observed that in larger QDs, photon number and bandwidth of the small signal modulation decrease but turn-on delay, maximum output power, and threshold current of gain increase. It was also found that for a good modulation, smaller QDs, and form the point of view of high-power applications, larger QDs seem better.