Showing papers on "Potential well published in 2019"

Triplet Energy Transfer from CsPbBr3 Nanocrystals Enabled by Quantum Confinement

Abstract: The spectral properties of lead halide perovskite nanocrystals (NCs) can be engineered by tuning either their sizes via the quantum confinement effect or their compositions using anion and/or cation exchange. To date, the latter is more frequently adopted, primarily because of the ease of ion exchange for lead halide perovskites, making the quantum confinement effect seemingly redundant for perovskite NCs. Here we report that quantum confinement is required for triplet energy transfer (TET) from perovskite NCs to polycyclic aromatic hydrocarbons (PAHs). Static and transient spectroscopy measurements on CsPbBr3 NC-pyrene hybrids showed that efficient TET occurred only for small-sized, quantum-confined CsPbBr3 NCs. The influences of the size-dependent driving force and spectral overlap on the TET rate were found to be negligible. Instead, the TET rate scaled linearly with carrier probability density at the NC surface, consistent with a Dexter-type TET mechanism requiring wave function exchange between the NC donors and pyrene acceptors. Efficient TET funnels the excitation energy generated in strongly light-absorbing perovskite NCs into long-lived triplets in PAHs, which may find broad applications such as photon upconversion and photoredox catalysis.

Wavefunction Engineering of Type-I/Type-II Excitons of CdSe/CdS Core-Shell Quantum Dots.

Abstract: Nanostructured semiconductors have the unique shape/size-dependent band gap tunability, which has various applications. The quantum confinement effect allows controlling the spatial distribution of the charge carriers in the core-shell quantum dots (QDs). Upon increasing shell thickness (e.g., from 0.25–3.25 nm) of core-shell QDs, the radial distribution function (RDF) of hole shifts towards the shell suggesting the confinement region switched from Type-I to Type-II excitons. As a result, there is a jump in the transition energy towards the higher side (blue shift). However, an intermediate state appeared as pseudo Type II excitons, in which holes are co-localized in the shell as well core whereas electrons are confined in core only, resulting in a dual absorption band (excitation energy), carried out by the analysis of the overlap percentage using the Hartree-Fock method. The findings are a close approximation to the experimental evidences. Thus, the understanding of the motion of e-h in core-shell QDs is essential for photovoltaic, LEDs, etc.

Manipulating efficient light emission in two-dimensional perovskite crystals by pressure-induced anisotropic deformation.

Abstract: The hybrid nature and soft lattice of organolead halide perovskites render their structural changes and optical properties susceptible to external driving forces such as temperature and pressure, remarkably different from conventional semiconductors. Here, we investigate the pressure-induced optical response of a typical two-dimensional perovskite crystal, phenylethylamine lead iodide. At a moderate pressure within 3.5 GPa, its photoluminescence red-shifts continuously, exhibiting an ultrabroad energy tunability range up to 320 meV in the visible spectrum, with quantum yield remaining nearly constant. First-principles calculations suggest that an out-of-plane quasi-uniaxial compression occurs under a hydrostatic pressure, while the energy is absorbed by the reversible and elastic tilting of the benzene rings within the long-chain ligands. This anisotropic structural deformation effectively modulates the quantum confinement effect by 250 meV via barrier height lowering. The broad tunability within a relatively low pressure range will expand optoelectronic applications to a new paradigm with pressure as a tuning knob.

Flame-formed carbon nanoparticles exhibit quantum dot behaviors.

Abstract: We examine the quantum confinement in the photoemission ionization energy in air and optical band gap of carbon nanoparticles (CNPs). Premixed, stretched-stabilized ethylene flames are used to generate the CNPs reproducibly over the range of 4-23 nm in volume median diameter. The results reveal that flame-formed CNPs behave like an indirect band gap material, and that the existence of the optical band gap is attributed to the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap in the polycyclic aromatic hydrocarbons comprising the CNPs. Both the ionization energy and optical band gap are found to follow closely the quantum confinement effect. The optical band gaps, measured both in situ and ex situ on the CNPs prepared in several additional flames, are consistent with the theory and the baseline data of CNPs from stretched-stabilized ethylene flames, thus indicating the observed effect to be general and that the particle size is the single most important factor governing the variation of the band gap of the CNPs studied. Cyclic voltammetry measurements and density functional theory calculations provide additional support for the quantum dot behavior observed.

Exciton-Driven Chemical Sensors Based on Excitation-Dependent Photoluminescent Two-Dimensional SnS

Abstract: Excitation wavelength-dependent photoluminescence (PL) in two-dimensional (2D) transition-metal chalcogenides enables a strong excitonic interaction for high-performance chemical and biological sensing applications. In this work, we explore the possible candidates in the domain of post-transition-metal chalcogenides. Few-layered 2D p-type tin monosulfide (SnS) nanoflakes with submicrometer lateral dimensions are synthesized from the liquid phase exfoliation of bulk crystals. Excitation wavelength-dependent PL is found, and the excitonic radiative lifetime is more than one order enhanced compared to that of the bulk counterpart because of the quantum confinement effect. Paramagnetic NO2 gas is selected as a representative to investigate the exciton-driven chemical-sensing properties of 2D SnS. Physisorption of NO2 results in the formation of dipoles on the surface of 2D SnS, causing the redistribution of photoexcited charges in the body and therefore modifying PL properties. For practical sensing applications, 2D SnS is integrated into a resistive transducing platform. Under light irradiation, the sensor exhibits excellent sensitivity and selectivity to NO2 at a relatively low operating temperature of 60 °C. The limit of detection is 17 parts per billion (ppb), which is significantly improved over other previously reported 2D p-type semiconductor-based NO2 sensors.

Incorporation of rubidium cations into blue perovskite quantum dot light-emitting diodes via FABr-modified multi-cation hot-injection method.

Abstract: Solution-processed lead halide perovskite quantum dots (QDs) are emerging as one of the most promising candidates for emissive display application. Although perovskite QDs with a full spectrum of visible light emissions have been realized for years, realizing the efficient electroluminescence of blue perovskites at room temperature still faces severe challenges. Herein, we demonstrate both the efficient photoluminescence and electroluminescence of the blue perovskite QDs via a simple FABr-modified multi-cation hot-injection (FMMHI) method. The FMMHI method is unique in both the addition of FABr into the PbBr2 precursor solution and the incorporation of small rubidium (Rb+) into the blue perovskite QDs light-emitting diodes (QLEDs). The addition of FABr into the precursor solution can realize strong quantum confinement effect, large exciton binding energy and high-quality perovskite QD films. Besides, the bandgap can be enlarged by the Rb+-induced perovskite octahedral distortion and strong quantum confinement effect. Excellent PLQYs of 64.5% and 49.8% were achieved for the developed greenish-blue QDs (Rb0.33Cs0.67)0.42FA0.58PbBr3 and deep-blue QDs (Rb0.33Cs0.67)0.42FA0.58PbCl1.25Br1.75 in solid film state. Moreover, maximum external quantum efficiencies (EQEs) of 3.6% and 0.61% were also achieved with an electroluminescence peak wavelength at 502 and 466 nm, respectively, indicating that the perovskite QDs incorporated with Rb+ possess great potential for the development of high-performance blue perovskite electroluminescence diodes.

One-Step Vapor-Phase Synthesis and Quantum-Confined Exciton in Single-Crystal Platelets of Hybrid Halide Perovskites.

Abstract: To investigate the quantum confinement effect on excitons in hybrid perovskites, single-crystal platelets of CH3NH3PbBr3 are grown on mica substrates using one-step chemical vapor deposition. Photoluminescence measurements reveal a monotonous blue shift with a decreasing platelet thickness, which is accompanied by a significant increase in exciton binding energy from approximately 70 to 150 meV. Those phenomena can be attributed to the one-dimensional (1D) quantum confinement effect in the two-dimensional platelets. Furthermore, we develop an analytical model to quantitatively elucidate the 1D confinement effect in such quantum wells with asymmetric barriers. Our analysis indicates that the exciton Bohr radius of single-crystal CH3NH3PbBr3 is compressed from 4.0 nm for the thick (26.2 nm) platelets to 2.2 nm for the thin (5.9 nm) ones. The critical understanding of the 1D quantum confinement effect and the development of a general model to elucidate the exciton properties of asymmetric semiconductor quantum wells pave the way toward developing high-performance optoelectronic heterostructures.

Efficient and Tunable Electroluminescence from In Situ Synthesized Perovskite Quantum Dots

Abstract: Semiconductor quantum dots (QDs) are among the most promising next-generation optoelectronic materials. QDs are generally obtained through either epitaxial or colloidal growth and carry the promise for solution-processed high-performance optoelectronic devices such as light-emitting diodes (LEDs), solar cells, etc. Herein, a straightforward approach to synthesize perovskite QDs and demonstrate their applications in efficient LEDs is reported. The perovskite QDs with controllable crystal sizes and properties are in situ synthesized through one-step spin-coating from perovskite precursor solutions followed by thermal annealing. These perovskite QDs feature size-dependent quantum confinement effect (with readily tunable emissions) and radiative monomolecular recombination. Despite the substantial structural inhomogeneity, the in situ generated perovskite QDs films emit narrow-bandwidth emission and high color stability due to efficient energy transfer between nanostructures that sweeps away the unfavorable disorder effects. Based on these materials, efficient LEDs with external quantum efficiencies up to 11.0% are realized. This makes the technologically appealing in situ approach promising for further development of state-of-the-art LED systems and other optoelectronic devices.

Investigation of high contrast and reversible luminescence thermochromism of the quantum confined Cs4PbBr6 perovskite solid

Abstract: Thermochromism of organic/inorganic halide perovskites has attracted particular interest due to their potential applications as photoluminescence (PL)-based temperature sensors. However, despite the outstanding PL characteristics, their use as a thermochromic material in practical temperature ranges has been limited because of their poor thermal stability. In this study, we used the quantum confinement effect and exceptional PL quantum efficiency of the Cs4PbBr6 perovskite to demonstrate their high on/off ratio (20) and reversible PL thermochromism in the solid state in practical temperature ranges including room temperature (RT). Systematic photophysical and optical characterization studies, including exciton-phonon scattering, exciton binding energy, exciton decay dynamics, and crystal structure change, were performed to investigate the origin of this unique thermochromic PL property. The results showed that the efficient and highly reversible thermochromic PL emission of the Cs4PbBr6 perovskite is due to its desirable optical properties such as highly luminescent emission, efficient PL quenching at high temperatures, and thermally reversible structural changes.

Blue emitting CsPbBr 3 perovskite quantum dot inks obtained from sustained release tablets

Abstract: Blue emitting perovskite ink obtained from cesium lead halide quantum dots bearing chlorine (CsPbClxBr3−x, 0 < x ≤ 3) suffers from the low photoluminescence quantum yield and poor stability. Cesium lead bromine (CsPbBr3) quantum dots free of chlorine have more stable crystal structure and fewer crystal defects. Precise control of crystal sizes and surface passivation components of CsPbBr3 quantum dots is crucial for the best use of quantum confinement effect and blueshift of emission wavelength to blue region. Here, by polymerizing acrylamide under UV-light irradiation to form polymer gel networks in dimethyl sulfoxide (DMSO) with CsPbBr3 precursors and passivating agents trapped, we successfully prepared novel sustained release tablets with different shapes and sizes. Thanks to the limitation of the polymer networks on solvent releasing, the resulting CsPbBr3 quantum dots have the average size of 1.1 ± 0.2 nm. On the basis of the excellent quantum confinement effect and optimized surface passivation, the obtained PQD ink can emit high quality blue light for more than 6 weeks. This work elucidates a new and convenient technique to prepare blue emission perovskite quantum dots ink with high stability and photoluminescence quantum yield and provides a great potential technology for the preparation of perovskite optoelectronic devices.

MOF-Confined Sub-2 nm Stable CsPbX3 Perovskite Quantum Dots.

Abstract: The metal halide with a perovskite structure has attracted significant attention due to its defect-tolerant photophysics and optoelectronic features. In particular, the all-inorganic metal halide perovskite quantum dots have potential for development in future applications. Sub-2 nm CsPbX3 (X = Cl, Br, and I) perovskite quantum dots were successfully fabricated by a MOF-confined strategy with a facile and simple route. The highly uniform microporous structure of MOF effectively restricted the CsPbX3 quantum dots aggregation in a synthetic process and endowed the obtained sub-2 nm CsPbX3 quantum dots with well-dispersed and excellent stability in ambient air without a capping agent. The photoluminescence emission spectra and lifetimes were not decayed after 60 days. The CsPbX3 quantum dots maintained size distribution stability in the air without any treatment. Because of the quantum confinement effect of CsPbX3 quantum dots, the absorption and photoluminescence (PL) emission peak were blue shifted to shorter wavelengths compare with bulk materials. Furthermore, this synthetic strategy provides a novel method in fabricating ultra-small photoluminescence quantum dots.

Band-edge Exciton Fine Structure and Exciton Recombination Dynamics in Single crystals of Layered Hybrid Perovskites

Abstract: Two-dimensional (2D) perovskite materials have recently re-attracted intense research interest for applications in photovoltaics and optoelectronics. As a consequence of the dielectric and quantum confinement effect, they show strongly bound and stable excitons at room temperature. In this report, the band-edge exciton fine structure and in particular its exciton and biexciton dynamics in high quality crystals of (PEA)2PbI4 are investigated. A comparison of bulk and surface exciton lifetimes yields a room temperature surface recombination velocity of 2x10^3cm/s and an intrinsic lifetime of 185ns. Biexciton emission is evidenced at room temperature, with binding energy of about 45meV and a lifetime of 80ps. At low temperature, exciton state splitting is observed, which is caused by the electron-hole exchange interaction. Transient photoluminescence resolves the low-lying dark exciton state, with a bright/dark splitting energy estimated to be 10meV. This work contributes to understand the complex scenario of the elementary photoexcitations in 2D perovskites.

Identification of the Band Gap Energy of Two-dimensional (OA)2(MA)n-1PbnI3n+1 Perovskite with up to 10 Layers.

Abstract: Two-dimensional (2D) perovskites are attracting broad attention for their stability and wavelength tunability. However, random crystallization of sample preparation makes it difficult to obtain 2D perovskites with pure structure, especially when the number of layers is large. Herein, we prepared 2D perovskite (C8H17NH3)2(MA)n−1PbnI3n+1 with different layers (n = 1–10). For the first time, we experimentally identified the band gap energy Eg of 2D perovskite (C8H17NH3)2(MA)n−1PbnI3n+1 with layers up to 10 by investigating specific pieces of crystal with pure emission spectra using fluorescence microscopy. Intriguingly, the relationship between Eg and n perfectly fits an exponential function rather than the pure quantum confinement effect in good agreement with the theoretical calculation based on first principles. Our results suggest that the band gap of the 2D perovskite is determined not only by quantum confinement effect, but other factors including chemical components also give significant contribution.

Synthesis and characterization of CdSe nanoparticles

Abstract: CdSe capped with TSC nanoparticles was prepared by chemical method at room temperature. Spin coating method has been used to deposited thin film on the glass substrates at room temperature. The morphological, structural and optical properties of the thin film are characterized by transmission electron microscope, atomic force microscope and optical absorption and Photoluminescence measurements. The average roughness of CdSe thin film is 0.935 nm and the average diameter is 44 nm. The direct optical band gap (Eg) has been determined from the absorption coefficient which is equal to 2.2 eV. The increase in the band gap value of our thin films as compared to bulk is due to the quantum confinement effect. These nanoparticles were found to display strong photoluminescence with emission peaks at the green region. Increases of excitation wavelength lead to an increase in emission intensity.

Engineering a topological quantum dot device through planar magnetization in bismuthene

Abstract: The discovery of quantum spin Hall materials with huge bulk gaps in experiment, such as bismuthene, provides a versatile platform for topological devices. We propose a topological quantum dot (QD) device in bismuthene ribbon in which two planar magnetization areas separate the sample into a QD and two leads. At zero temperature, peaks of differential conductance emerge, demonstrating the discrete energy levels from the confined topological edge states. The key parameters of the QD, the tunneling coupling strength with the leads and the discrete energy levels, can be controlled by the planar magnetization and the sample size. Especially, different from the conventional QD, we find that the angle between two planar magnetization orientations provides an effective way to manipulate the discrete energy levels. Combining the numerical calculation and the theoretical analysis, we identify that such manipulation originates from the unique quantum confinement effect of the topological edge states. Based on such a mechanism, we find the spin transport properties of QDs can also be controlled.

Strain-Engineered Asymmetrical Superlattice Si/Si 1– x Ge x Nano-ATT $\langle$ p++-n-n − -n++ $\rangle$ Oscillator: Enhanced Photo-Sensitivity in Terahertz Domain

Abstract: This paper proposes a strained Si-based single drift nano-mixed tunnel avalanche transit time (MITATT) oscillator capable of generating high RF power in the terahertz regime, inaccessible by conventional Si devices. The authors have developed a quantum modified classical drift-diffusion model to study the nanoscale properties of strained Si oscillator. The study reveals that 14.72-GW/m2 RF power could be generated from 0.715-THz strain-engineered Si oscillator. Such performance enhancement of the Si device is due to selective incorporation of trace amounts of Ge in the Si active region. This generates in-plane biaxial strain which in turn degrades the in-plane electron mobility, hence unsuitable for conventional MOSFETs. However, such strain remarkably enhances the out-of-plane mobility, and uniqueness of the current research is the utilization of in-plane strain to boost up the out-of-plane mobility depending on geometry and dimensions of the oscillator’s active region. The model incorporates parasitic series resistance and elevates junction temperature effects on terahertz properties of the device. Incorporation of the strained layer in the active region reduces the parasitic series resistance effect on high frequency properties of the oscillator. Validity of the developed model is established by comparing simulated data with the corresponding experimental observations. The authors have further studied photo irradiation effects on terahertz characteristics of the new class of devices. The structure-induced quantum confinement effect has made the Si-based active region to behave as a partially direct bandgap material, and therefore, a significant improvement is observed in optical sensitivity. To the best of the authors’ knowledge, this is the first report on strain-engineered asymmetrically doped Si/SiGe based avalanche photosensor in the high-frequency terahertz domain.

Effects of energy and hydrogen peroxide concentration on structural and optical properties of CuO nanosheets prepared by pulsed laser ablation

Abstract: Laser ablation of copper targets using Ce:Nd:YAG laser with pulses of 1064 nm wavelengths at 10 ns pulse width and with different laser pulse energies and in different concentrations of H2O2 solution is investigated experimentally. Cu/CuO flake-like nanostructures with visible band gap and the extension of its light absorption by changing the synthesis parameters such as laser pulse energy and concentration of H2O2 solution were observed. Characterization and comparison of the obtained suspensions are exploited by X-ray diffraction (XRD), UV–vis spectroscopy and field emission scanning electron microscopy (FE-SEM). The XRD analysis data show that in 10 vol% H2O2 solution CuO phase has been synthesized, while with decreasing the H2O2 concentration, Cu phase is also observed. The mean crystallite sizes of CuO were estimated using XRD patterns and were between 2.1 and 3.3 nm. The comparison of the UV–visible absorption spectra indicates that with decreasing pulse energy and decreasing H2O2 concentration up to 5 vol% the band gap of nanoparticles increased from 2.19 eV to 2.34 eV, which is accordant with the quantum confinement effect. FESEM image of all the synthesized samples illustrate the formation of flake-like nanostructure with the thickness of about 12–15 nm.

Quantum Size Effect in Superconducting Aluminum Films

Abstract: High-quality aluminum films on GaAs substrates are studied experimentally. The critical temperature of superconductivity is found to increase markedly with decreasing the film thickness. The observed phenomenon is considered as a manifestation of the quantum confinement effect, which affects both the density of states and the electron–phonon interaction.

Crystal orientation dependence of alternating current photo-assisted (ACPEC) porous silicon for potential optoelectronic application

Abstract: Porous silicon (PS) was successfully fabricated using an alternating current photo-assisted electrochemical etching (ACPEC) technique. This study aims to compare the effect of different crystal orientation of Si n(100) and n(111) on the structural and optical characteristics of the PS.,PS was fabricated using ACPEC etching with a current density of J = 10 mA/cm2 and etching time of 30 min. The PS samples denoted by PS100 and PS111 were etched using HF-based solution under the illumination of an incandescent white light.,FESEM images showed that the porous structure of PS100 was a uniform circular shape with higher density and porosity than PS111. In addition, the AFM indicated that the surface roughness of porous n(100) was less than porous n(111). Raman spectra of the PS samples showed a stronger peak with FWHM of 4.211 cm−1 and redshift of 1.093 cm−1. High resolution X-ray diffraction revealed cubic Si phases in the PS samples with tensile strain for porous n(100) and compressive strain for porous n(111). Photoluminescence observation of porous n(100) and porous n(111) displayed significant visible emissions at 651.97 nm (Eg = 190eV) and 640.89 nm (Eg = 1.93 eV) which was because of the nano-structure size of silicon through the quantum confinement effect. The size of Si nanostructures was approximately 8 nm from a quantized state effective mass theory.,The work presented crystal orientation dependence of Si n(100) and n(111) for the formation of uniform and denser PS using new ACPEC technique for potential visible optoelectronic application. The ACPEC technique has effectively formed good structural and optical characteristics of PS.

Quantum Confined Tomonaga-Luttinger Liquid in MoSe2 Twin Domain Boundaries

Abstract: There have been conflicting reports on the electronic properties of twin domain boundaries (DBs) in MoSe2 monolayer, including the quantum well states, charge density wave, and Tomonaga-Luttinger liquid (TLL). Here we employ low-temperature scanning tunneling spectroscopy to reveal both the quantum confinement effect and signatures of TLL in the one-dimensional DBs. The data do not support the CDW at temperatures down to ~5 K.

Screened shallow impurity properties of quantum well heterosystems with high-κ dielectric barrier environment

Abstract: The effect of high-κ dielectric mismatch on the impurity states screened by free electron gas is investigated theoretically in high-κ dielectric barrier/quantum well/high-κ dielectric barrier (hh-κ) and low-κ dielectric barrier/quantum well/high-κ dielectric barrier (lh- κ) types of quasi two-dimensional (Q2D) heterostructures . A restriction of the characteristic long-range nature of the impurity Q2D screened interaction due to the high-κ dielectric mismatch has been revealed here - atypical of the Q2D dielectrically homogeneous systems. Accordingly, for the hh -κ type heterostructure, an impurity screened interaction potential at moderately large (relative to the quantum well width values) in-plane distances behaves as the Q2D Debye-Huckel potential, while at rather large distances falls off on the background of the high-κ dielectric constant of the barrier medium. The screened impurity ground state energy in the framework of the Q2D Debye-Huckel potential has been investigated variationally for the first time. A role of high-κ dielectric mismatch is exposed when comparing the results for the discussed heterostructures based on the realistic InSb/HfO 2 interface. The characteristic ranges where the quantum confinement effect, enhancing the Coulomb interaction, balances the joint weakening influence of the impurity screening and high-κ dielectric mismatch effects are revealed. The received admissible values of the in-plane density/temperature ratio parameter for the hh-κ type heterostructure typically refer to non-degenerate electron gas while for the lh-κ type both electron gas statistics are significant. Due to the high-κ dielectric mismatch a binding energy sufficient decline with reducing of the quantum well width has been established. In the hh-κ type heterostructure an impurity binding is quite sizable at the wide quantum wells (d ∼8 ÷ 10 nm). Meanwhile, for the lh-κ-type heterostructure, an impurity binding energy is enough large (∼10 meV) in both wide and narrow quantum well cases.