Showing papers on "Potential well published in 2017"

Solvothermal Synthesis of High-Quality All-Inorganic Cesium Lead Halide Perovskite Nanocrystals: From Nanocube to Ultrathin Nanowire

Abstract: Recently, all-inorganic cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals have drawn much attention because of their outstanding photophysical properties and potential applications. In this work, a simple and efficient solvothermal approach to prepare CsPbX3 nanocrystals with tunable and bright photoluminescent (PL) properties, controllable composition, and morphology is presented. CsPbX3 nanocubes are successfully prepared with bright emission high PL quantum yield up to 80% covering the full visible range and narrow emission line widths (from 12 to 36 nm). More importantly, ultrathin CsPbX3 (X = Cl/Br, Br, and Br/I) nanowires (with diameter as small as ≈2.6 nm) can be prepared in a very high morphological yield (almost 100%). A strong quantum confinement effect is observed in the ultrathin nanowires, in which both the absorption and emission peaks shift to shorter wavelength range compared to their bulk bandgap. The reaction parameters, such as temperature and precursors, are varied to investigate the growth process. A white light-emitting device prototype device with wide color gamut covering up to 120% of the National Television System Committee standard has been demonstrated by using CsPbBr3 nanocrystals as the green light source. The method in this study provides a simple and efficient way to prepare high-quality CsPbX3 nanocrystals.

Structural, optical and magnetic investigation of Gd implanted CeO2 nanocrystals

Abstract: Gadolinium implanted cerium oxide (Gd-CeO2) nanocomposites is an important candidate which have unique hexagonal structure and high K- dielectric constant. Gd-CeO2 nanoparticles were synthesized using hydrothermal method. X-ray diffraction (XRD) results showed that the peaks are consistent with pure phase cubic structure the XRD pattern also confirmed crystallinity and phase purity of the sample. Nanocrystals sizes were found to be up to 25 nm as revealed by XRD and SEM. It is suggested that Gd gives an affirmative effect on the ion influence behavior of Gd-CeO2. XRD patterns showed formation of new phases and SEM micrographs revealed hexagonal structure. Photoluminescence measurement (PL) reveals the systematic shift of the emission band towards lower wavelength thereby ascertaining the quantum confinement effect (QCE). The PL spectrum has wider broad peak ranging from 390 nm to 770 nm and a sharp one centered on at 451.30 nm which is in tune with Gd ions. In the Raman spectra showed intense band observed between 460 cm−1 and 470 cm−1 which is attributed to oxygen ions into CeO2. Room temperature ferromagnetism was observed in un-doped and Gd implanted and annealed CeO2 nanocrystals. In the recent studies, ceria based materials have been considered as one of the most promising electrolytes for reduced temperature SOFC (solid oxide fuel cell) system due to their high ionic conductivities allowing its use in stainless steel supported fuel cells. CeO2 having an optical bandgap 3.3 eV and n-type carrier density which make it a promising candidate for various technological application such as buffer layer on silicon on insulator devices.

Anomalous Size Dependence of Optical Properties in Black Phosphorus Quantum Dots

Abstract: Understanding electron transitions in black phosphorus nanostructures plays a crucial role in applications in electronics and optoelectronics. In this work, by employing time-dependent density functional theory calculations, we systematically study the size-dependent electronic, optical absorption, and emission properties of black phosphorus quantum dots (BPQDs). Both the electronic gap and the absorption gap follow an inversely proportional law to the diameter of BPQDs in conformity to the quantum confinement effect. In contrast, the emission gap exhibits anomalous size dependence in the range of 0.8–1.8 nm, which is blue-shifted with the increase of size. The anomaly in fact arises from the structure distortion induced by the excited-state relaxation, and it leads to a huge Stokes shift in small BPQDs.

Two-Dimensional GeSe as an Isostructural and Isoelectronic Analogue of Phosphorene: Sonication-Assisted Synthesis, Chemical Stability, and Optical Properties

Abstract: Monochalcogenides of germanium (or tin) are considered as isoelectronic and isostructural analogues of black phosphorus. Here, we demonstrate the synthesis of atomically thin GeSe by direct sonication-assisted liquid phase exfoliation (LPE) of bulk microcrystalline powders in organic solvents. The thickness of the GeSe sheets is dependent on the exfoliation conditions, and highly crystalline few-layer GeSe sheets of 4–10 layer stacks with lateral sizes over 200 nm were obtained. In ambient atmosphere, the LPE sheets deposited on the substrate demonstrate strong resistance against degradation, while decomposition into elemental Ge and Se nanostructures occurs at a moderate rate for ethanol dispersions. Density functional theory calculation together with optical characterizations confirm the blue-shifted bandgap for the GeSe sheets as a result of strong quantum confinement effect. In addition, we show that the few-layer GeSe sheets with favorable optical bandgap allow for efficient solar light harvesting fo...

Size-dependent bandgap and particle size distribution of colloidal semiconductor nanocrystals.

Abstract: A new analytical expression for the size-dependent bandgap of colloidal semiconductor nanocrystals is proposed within the framework of the finite-depth square-well effective mass approximation in order to provide a quantitative description of the quantum confinement effect. This allows one to convert optical spectroscopic data (photoluminescence spectrum and absorbance edge) into accurate estimates for the particle size distributions of colloidal systems even if the traditional effective mass model is expected to fail, which occurs typically for very small particles belonging to the so-called strong confinement limit. By applying the reported theoretical methodologies to CdTe nanocrystals synthesized through wet chemical routes, size distributions are inferred and compared directly to those obtained from atomic force microscopy and transmission electron microscopy. This analysis can be used as a complementary tool for the characterization of nanocrystal samples of many other systems such as the II-VI and III-V semiconductor materials.

Size-dependent bandgap and particle size distribution of colloidal semiconductor nanocrystals

Abstract: A new analytical expression for the size-dependent bandgap of colloidal semiconductor nanocrystals is proposed within the framework of the finite-depth square-well effective mass approximation in order to provide a quantitative description of the quantum confinement effect. This allows one to convert optical spectroscopic data (photoluminescence spectrum and absorbance edge) into accurate estimates for the particle size distributions of colloidal systems even if the traditional effective mass model is expected to fail, which occurs typically for very small particles belonging to the so-called strong confinement limit. By applying the reported theoretical methodologies to CdTe nanocrystals synthesized through wet chemical routes, size distributions are inferred and compared directly to those obtained from atomic force microscopy and transmission electron microscopy. This analysis can be used as a complementary tool for the characterization of nanocrystal samples of many other systems such as the II-VI and III-V semiconductor materials.

Atomically thin cesium lead bromide perovskite quantum wires with high luminescence.

Abstract: We report a room-temperature colloidal synthesis of few-unit-cell-thick CsPbBr3 QWs with lengths over a hundred nanometers. The surfactant-directed oriented attachment growth mechanism was proposed to explain the formation of such CsPbBr3 QWs. Owing to the strong quantum confinement effect, the photoluminescence (PL) emission peak of few-unit-cell-thick CsPbBr3 QWs blue-shifted to 430 nm. The ensemble PL quantum yield (PLQY) of the few-unit-cell-thick CsPbBr3 QWs increased to 21.13% through a simple heat-treatment process. The improvement of PLQY was ascribed to the reduction of the density of surface trap states and defect states induced by the heat-treatment process. Notably, the dependence of the bandgap on the diameter with different numbers of unit cells was presented for the first time in 1-D CsPbBr3 QWs on the basis of the produced few-unit-cell-thick CsPbBr3 QWs.

Optical Properties of Colloidal CH3NH3PbBr3 Nanocrystals by Controlled Growth of Lateral Dimension

Abstract: Organometal halide perovskites become important in the photovoltaic and light emitting devices due to the compositional flexibility with AMX3 formula (A is a monovalent organic ammonium cation; M is a metal ion; X is a halogen atom), imposing a significant demand to develop a synthetic route toward new types of nanocrystals. Although chemical pathways for perovskites nanoparticles were developed on the basis of the reprecipitation method, poor control of the nucleation and growth process results in a large size polydispersity that induces the ambiguities associated with a quantum confinement effect depending on their size. Here, a modified reprecipitation method is presented for the synthesis of CH3NH3PbBr3 perovskite nanoparticles with a controlled nanoparticle size by systematically tuning the feed ratio of the precursors. Fine control of the nanocrystal size provides new insights into the quantum confinement effect observed in microscale and nanoscale perovskite materials, where their energy bandgap is...

Synthesis of Cr-doped SnO2 quantum dots and its enhanced photocatalytic activity

Abstract: Chromium (Cr)-doped SnO2 quantum dots (QDs) with various doping concentration have been successfully synthesized using a simple combustion method. The QDs have been characterized by using various techniques. The X-ray diffraction patterns revealed that the prepared QDs have monophasic tetragonal rutile-type crystallite structure with an average crystallite size of ∼3 nm. The optical band gap energies of QDs increased with increasing the Cr ion concentration. The XPS spectra reveal the presence of Sn4+, Cr3+, and O respectively. Cr (0.03 mol%)-doped SnO2 QDs are exhibited the greater photocatalytic activity compared with other photocatalysts, which is attributed mainly due to quantum confinement effect and an increase in specific surface area. The existence of a Cr3+ as shown from XPS spectra, act as an electron acceptor and/or hole donor, which enhancing longer survived charge carrier separation in Cr-doped SnO2 QDs as confirmed by PL spectroscopy.

Spectral Anomaly in Raman Scattering from p-Type Silicon Nanowires

Abstract: An anomalous nature of Raman spectral asymmetry has been reported here from silicon nanowires (SiNWs) prepared from a heavily doped p-type Si wafer using a metal induced etching technique. Raman spectra of SiNWs prepared from two p-type Si wafers with different doping levels show different behaviors in terms of asymmetry as characterized by the asymmetry ratio. The SiNWs prepared from high doped p-type wafer show an anomaly in asymmetry in addition to the red shift and broadening of the Raman line shape due to the presence of the “FAno-quaNTUM” (FANTUM) effect. The heavy doping in the wafer provides a continuum of energy states to be available to interact with confined optic phonons which results in electron–phonon interaction. SiNWs prepared from low doped p-type wafer show a red shift and asymmetric broadening due to the quantum confinement effect alone. Careful analysis has been provided to clearly understand the role of Fano and quantum effects in p-type SiNWs with high doping and their relative contr...

Quantum confinement effect of two-dimensional all-inorganic halide perovskites

Abstract: Quantum confinement effect (QCE), an essential physical phenomenon of semiconductors when the size becomes comparable to the exciton Bohr radius, typically results in quite different physical properties of low-dimensional materials from their bulk counterparts and can be exploited to enhance the device performance in various optoelectronic applications. Here, taking CsPbBr3 as an example, we reported QCE in all-inorganic halide perovskite in two-dimensional (2D) nanoplates. Blue shifts in optical absorption and photoluminescence spectra were found to be stronger in thinner nanoplates than that in thicker nanoplates, whose thickness lowered below ~7 nm. The exciton binding energy results showed similar trend as that obtained for the optical absorption and photoluminescence. Meanwile, the function of integrated intensity and full width at half maximum and temperature also showed similar results, further supporting our conclusions. The results displayed the QCE in all-inorganic halide perovskite nanoplates and helped to design the all-inorganic halide perovskites with desired optical properties.

First Principles Study of Molybdenum Disulfide Electronic Structure

Abstract: The study of two dimensional material has gain interest due to unique properties which are different from the bulk precursors. Mono- and few-layered of Transition metal dichalcogenides (TMDCs) has band gap properties between 1-2 eV that suitable of FET devices or any optoelectronic devices. Among TMDCs, Molybdenum Disulfide (MoS2) has gain interest due to its promising band gap-tuning and transition between direct to indirect band gap properties depends on its thickness. First principles calculation by Density Functional Theory has been performed to study the characteristic of MoS2 electronic structure. Indirect band gap of MoS2 lies between point Γ to Γ-K in first brillouine zone, while the direct bandgap lies in point-K. The indirect band gap became larger while the number of layer decreased due to quantum confinement effect in c axis direction. In monolayer MoS2, the indirect band gap become larger than direct one, band gap properties transitioned from indirect to direct. The unique bandgap properties of MoS2 can lead into better application in energy devices such as solar cell [11], FET [10], and photoluminescence device.

High performance infrared photodetectors up to 28 µm wavelength based on lead selenide colloidal quantum dots

Abstract: The strong quantum confinement effect in lead selenide (PbSe) colloidal quantum dots (CQDs) allows to tune the bandgap of the material, covering a large spectral range from mid- to near infrared (NIR). Together with the advantages of low-cost solution processability, flexibility and easy scale-up production in comparison to conventional semiconductors especially in the mid- to near infrared range, PbSe CQDs have been a promising material for infrared optoelectronic applications. In this study, we synthesized monodisperse and high purity PbSe CQDs and then demonstrated the photodetectors working at different wavelengths up to 2.8 µm. Our high quality PbSe CQDs show clear multiple excitonic absorption peaks. PbSe CQD films of different thicknesses were deposited on interdigitated platinum electrodes by a simple drop casting technique to make the infrared photodetectors. At room temperature, the high performances of our PbSe CQD photodetectors were achieved with maximum responsivity, detectivity and external quantum efficiency of 0.96 A/W, 8.13 × 109 Jones and 78% at 5V bias. Furthermore, a series of infrared LEDs with a broad wavelength range from 1.5 μm to 3.4 μm was utilized to demonstrate the performance of our fabricated photodetectors with various PbSe CQD film thicknesses.

Facile microwave approach to controllable boron nitride quantum dots

Abstract: Boron nitride quantum dots (BNQDs), as promising metal-free quantum dots with unique photoelectric properties, have been controllably fabricated by a facile and high-efficiency microwave irradiation technique in this work. Though a number of attempts have been reported so far, it remains challenging to explore an effective approach to synthesize high-quality BNQDs with uniform size, well dispersion and high quantum yield (QY). Microwave irradiation strategy is identified as an advanced and beneficial method not only for high-efficiency energy inputting but also time-saving in comparison with the reported solvothermal process. Encouragingly, the particle size and QY of BNQDs can be well controlled by adjusting microwave reaction temperature as well as duration time. The average diameter of the as-prepared blue luminescent BNQDs ranges from 1.98 to 7.05 nm with QY up to 23.44%. Furthermore, attributed to the unique nanostructure, quantum confinement effect, and high dielectric loss, the as-prepared BNQDs exhibits an optimal reflection loss of −19.6 dB at 8.9 GHz with a broad effective absorption bandwidth of 5.02 GHz in the frequency range of 2–18 GHz, demonstrating as potential microwave absorption material in electromagnetic interference field.

Size-dependent donor and acceptor states in codoped Si nanocrystals studied by scanning tunneling spectroscopy

Abstract: The electrical and optical properties of semiconductor nanocrystals (NCs) can be controlled, in addition to size and shape, by doping. However, such a process is not trivial in NCs due to the high formation energy of dopants there. Nevertheless, it has been shown theoretically that in the case of B and P (acceptor/donor) codoped Si-NCs the formation energy is reduced relative to that of single type doping. Previous comprehensive measurements on ensembles of such codoped Si-NCs have pointed to the presence of donor and acceptor states within the energy gap. However, such a conjecture has not been directly verified previously. Following that, we investigate here the electronic properties of B and P codoped Si-NCs via Scanning Tunneling Spectroscopy. We monitored the quantum confinement effect in this system, for which the energy gap changed from ∼1.4 eV to ∼1.8 eV with the decrease of NC diameter from 8.5 to 3.5 nm. Importantly, all spectra showed two in-gap band-states, one close to the conduction band edge and the other to the valence band edge, which we attribute to the P and B dopant levels, respectively. The energy separation between these dopants states decrease monotonically with increasing NC diameter, in parallel to the decrease of the conduction-to-valence bands separation. A fundamental quantity that is derived directly for these Si-NCs is the intrinsic like position of the Fermi energy, a non-trivial result that is very relevant for understanding the system. Following the above results we suggest an explanation for the character and the origin of the dopants bands.

Effects of functionalization and side defects on single-photon emission in boron nitride quantum dots

Abstract: Boron nitride quantum dots (BNQDs) functionalized with chemical ligands exhibit intriguing optoelectronic properties due to the quantum confinement effect. This paper presents peculiar insights on the effect of side defects on the electronic structure and optical properties of BNQDs functionalized with different chemical bonds including hydrogen (H), nitrogen (N), hydroxyl (OH), amine (${\mathrm{NH}}_{2}$), and thiol groups (inspired by experimental reports of functionalized BN nanosheets and nanotubes) Weng et al., Chem. Soc. Rev. 45, 3989 (2016). Hybrid density functional simulations and Green's function calculations indicate an intriguing coexistence of two different Peierls-like distortions in the functionalized low-dimensional material. The presence of side defects increases the side strain and creates interband electronic states. As a result, the band gap of BNQDs could vary in a wide range depending on the type of chemical bonds and surface disorders. Enhanced edge states also improve the photoluminescence emission of the quantum dots. These side-defect enriched states in BNQDs create optical and electrical responses which could offer unprecedented potential for large scale nanophotonics such as photovoltaic, bioimaging, and quantum communication.

Linear and non-linear optical properties of Ag doped ZnS thin film

Abstract: Un-doped and Silver (Ag) doped zinc sulfide (ZnS) thin films were deposited on glass substrate by thermal evaporation method in the vacuum chamber with different doping concentrations. The structural properties of the thin films were analyzed by X-ray diffraction patterns. There is no peaks corresponding to impurities and Ag2O were detected which suggests that Ag ions were well incorporated in the ZnS lattice structure. On the other hand the intensity of the diffraction peaks increases with the increasing dopant concentration. This might be due the change in electronic density in the crystallographic position of Ag doped ZnS. UV measurement shows a blue shift due to the quantum confinement effect by incorporation of Ag2+ ions in ZnS thin films. The band gap values of ZnS and Ag doped ZnS nanostructures were calculated from transmission data. Photoluminescence (PL) measurements at room temperature show a PL peak at around 450 nm for all thin films. The PL intensity decreases with increasing Ag2+ ions concentration. The nonlinear optical measurements were carried out using Z-scan technique. The results reveal that the films exhibit self-defocusing nonlinearity. There is an increment in the nonlinear refractive index with increasing Ag2+ ions concentration in ZnS thin films. Open aperture Z-scan measurement shows two-photon absorption within the medium. The measurements confirm that the more concentration of Ag dopant also makes it possible to increase the nonlinear absorption coefficient. These results show that ZnS:Ag thin films are promising candidate for various potential applications in the field of nonlinear optics.

Electronic, Mechanical, and Dielectric Properties of Two-Dimensional Atomic Layers of Noble Metals

Abstract: We present density functional theory-based electronic, mechanical, and dielectric properties of monolayers and bilayers of noble metals (Au, Ag, Cu, and Pt) taken with graphene-like hexagonal structure. The Au, Ag, and Pt bilayers stabilize in AA-stacked configuration, while the Cu bilayer favors the AB stacking pattern. The quantum ballistic conductance of the noble-metal mono- and bilayers is remarkably increased compared with their bulk counterparts. Among the studied systems, the tensile strength is found to be highest for the Pt monolayer and bilayer. The noble metals in mono- and bilayer form show distinctly different electron energy loss spectra and reflectance spectra due to the quantum confinement effect on going from bulk to the monolayer limit. Such tunability of the electronic and dielectric properties of noble metals by reducing the degrees of freedom of electrons offers promise for their use in nanoelectronics and optoelectronics applications.

Single-step synthesis of crystalline h-BN quantum- and nanodots embedded in boron carbon nitride films

Abstract: Herein we report on the synthesis and characterization of novel crystalline hexagonal boron nitride (h-BN) quantum- and nanodots embedded in large-area boron carbon nitride (BCN) films. The films were grown on a Cu substrate by an atmospheric pressure chemical vapour deposition technique. Methane, ammonia, and boric acid were used as precursors for C, N and B to grow these few atomic layer thick uniform films. We observed that both the size of the h-BN quantum/nanodots and thickness of the BCN films were influenced by the vaporization temperature of boric acid as well as the H3BO3 (g) flux over the Cu substrate. These growth conditions were easily achieved by changing the position of the solid boric acid in the reactor with respect to the Cu substrate. Atomic force microscope (AFM) and TEM analyses show a variation in the h-BN dot size distribution, ranging from nanodots (∼224 nm) to quantum dots (∼11 nm) as the B-source is placed further away from the Cu foil. The distance between the B-source and the Cu foil gave an increase in the C atomic composition (42 at% C-65 at% C) and a decrease in both B and N contents (18 at% B and 14 at% N to 8 at% B and 7 at% N). UV-vis absorption spectra showed a higher band gap energy for the quantum dots (5.90 eV) in comparison with the nanodots (5.68 eV) due to a quantum confinement effect. The results indicated that the position of the B-source and its reaction with ammonia plays a significant role in controlling the nucleation of the h-BN quantum- and nanodots. The films are proposed to be used in solar cells. A mechanism to explain the growth of h-BN quantum/nanodots in BCN films is reported.

Photo physical studies of PVP arrested ZnS quantum dots

Abstract: Monodispersed polyvinylpyrrolidone (PVP) arrested ZnS quantum dots (QDs) having diameter in range ~2-5 nm are synthesized by a colloidal precipitation method using PVP as the stabilizing agent. X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selective area electron diffraction (SAED) and Fourier transform infrared (FT-IR) spectroscopy are probed to investigate the structural information. The optical properties are studied using diffuse UV-visible reflectance and photoluminescence (PL) spectroscopy techniques. TEM images as well as XRD reflection peak broadening indicate the nanometer size particles formation with cubic (sphalerite) phase within the polymer matrix. Optical absorbance studies reveal an excitonic peak at around ~310 nm dictates the effect of quantum confinement effect in the ZnS QDs. PL emission spectra for ZnS QDs in PVP exhibit four emission peaks at ~382 nm, ~414 nm, ~480 nm and ~527 nm are observed. These excitonic emissions from ZnS QDs are caused by the interstitial sulfur/Zn vacancies and surface states.

Multiband Hot Photoluminescence from Nanocavity-Embedded Silicon Nanowire Arrays with Tunable Wavelength.

Abstract: Besides the well-known quantum confinement effect, hot luminescence from indirect bandgap Si provides a new and promising approach to realize monolithically integrated silicon optoelectronics due to phonon-assisted light emission. In this work, multiband hot photoluminescence is generated from Si nanowire arrays by introducing trapezoid-shaped nanocavities that support hybrid photonic-plasmonic modes. By continuously adjusting the geometric parameters of the Si nanowires with trapezoidal nanocavities, the multiband hot photoluminescence can be tuned in the range from visible to near-infrared independent of the excitation laser wavelength. The highly tunable wavelength bands and concomitant compatibility with Si-integrated electronics enable tailoring of silicon-based light sources suitable for next-generation optoelectronics devices.

Analytical model of photon reabsorption in ZnO quantum dots with size and concentration dependent dual-color photoluminescence

Abstract: We investigate the concentration and size dependent UV/green photoluminescence properties of the ZnO quantum dots (QDs) with sizes in the strong confinement regime. The luminescence characteristics of an ensemble of colloidal semiconductor QDs with quantum confinement effect depend sensitively on particle concentration but this has only been qualitatively understood. By taking ZnO QDs as an ideal prototype, we construct a material-independent theoretical model to study the photon reabsorption phenomenon. The theoretical result agrees well with the experiment. This model can be used to quantitatively study the concentration-dependent luminescence properties of any collection of QDs with considerable size dispersion. On the other hand, the origin of green emission in ZnO QDs remains debated. The comparative study of the size dependence of UV and green emissions in conjunction with the effective-mass approximation calculation suggests that the green emission in the ZnO QDs originates from the conduction band to the deep level transition.

Quantum confinement effect in 6H-SiC quantum dots observed via plasmon–exciton coupling-induced defect-luminescence quenching

Abstract: The quantum confinement effect is one of the crucial physical effects that discriminate a quantum material from its bulk material. It remains a mystery why the 6H-SiC quantum dots (QDs) do not exhibit an obvious quantum confinement effect. We study the photoluminescence of the coupled colloidal system of SiC QDs and Ag nanoparticles. The experimental result in conjunction with the theoretical calculation reveals that there is strong coupling between the localized electron-hole pair in the SiC QD and the localized surface plasmon in the Ag nanoparticle. It results in resonance energy transfer between them and resultant quenching of the blue surface-defect luminescence of the SiC QDs, leading to uncovering of a hidden near-UV emission band. This study shows that this emission band originates from the interband transition of the 6H-SiC QDs and it exhibits a remarkable quantum confinement effect.