Characteristic Band Edge Research Articles

First-principles study of phase-dependent carrier transport mechanism for MASnI3 Sn-based halide perovskite

Organic-inorganic hybrid perovskites have attracted tremendous attentions owing to their excellent properties as next-generation photovoltaic devices. With soft covalent framework, organic-inorganic hybrid perovskites exhibit different phases at different temperatures. The band-edge features of perovskites are mainly contributed by inorganic framework, which means the structural differences between these phases would lead to complex carrier transport. We investigated the carrier transport of Sn-based organic-inorganic hybrid perovskite CH3NH3SnI3 (MASnI3), considering acoustic deformation potential scattering, ionized impurity scattering, and polar optical phonon scattering. It is found that the electron mobility of each phase of MASnI3 is strongly correlated with the Sn–I–Sn bond angle and there is in-plane/out-of-plane anisotropy. The projected crystal orbital Hamilton population analysis suggested that the tilt and rotation of the [SnI6]4− octahedron influence the Sn(p)–I(p) orbital electron coupling and the electron transport, leading to different band-edge features in multiple phases. The carrier mobility with respect to temperature was further calculated for each phase of MASnI3 in respective temperature intervals, showing lower carrier mobility in high temperature. Comparing the contribution of different scattering mechanisms, it was found that the dominant scattering mechanism is polar optical phonon scattering, while multiple scattering mechanisms compete in individual cases.

Trion Formation Resolves Observed Peak Shifts in the Optical Spectra of Transition-Metal Dichalcogenides.

Monolayer transition-metal dichalcogenides (ML-TMDs) have the potential to unlock novel photonic and chemical technologies if their optoelectronic properties can be understood and controlled. Yet, recent work has offered contradictory explanations for how TMD absorption spectra change with carrier concentration, fluence, and time. Here, we test our hypothesis that the large broadening and shifting of the strong band-edge features observed in optical spectra arise from the formation of negative trions. We do this by fitting an ab initio based, many-body model to our experimental electrochemical data. Our approach provides an excellent, global description of the potential-dependent linear absorption data. We further leverage our model to demonstrate that trion formation explains the nonmonotonic potential dependence of the transient absorption spectra, including through photoinduced derivative line shapes for the trion peak. Our results motivate the continued development of theoretical methods to describe cutting-edge experiments in a physically transparent way.

One pot synthesis of Ag-Au/ZnO nanocomposites: a multi-junction component forsunlight photocatalysis

ABSTRACT Plasmonic nanoparticles nanoparticles incorporation increases the photo-physical conversion rate in the semiconductors. Here, we demonstrate the one-pot ZnO/(Ag-Au) nanocomposite for enhanced the visible light photocatalysis. The obtained ZnO nanoparticles are of 10–20 nm size and the deposited metal nanoparticles are of 1–3 nm. The light absorbance spectrum indicated the characteristic band edge absorbance of ZnO (~350 nm) as well as the SPR peaks of Ag (~420 nm) and Au (~520 nm). Enhanced absorption received for the metal nanoparticles deposited ZnO nanocrystals indicate the technological importance of the materials. Emission spectrum of the materials indicated the metal nanoparticles inhibited electron–hole pair recombination process. The photocatalytic methylene blue degradation efficiency of ZnO/(Ag-Au) (87%) nanocomposite is much higher than the ZnO/Ag (65%) and ZnO/Au (57%) nanocomposites. Studies indicated the performance improvement is due to the bimetallic nature over their pristine nature of Ag and Au.

Structural, optical and frequency dependent dielectric studies of nanoscale Na0.5Bi0.5TiO3 processed via non-aqueous route

The present work reports on structural, optical, and dielectric properties of nanoscale Na0.5Bi0.5TiO3 (NBT) derived through a non-aqueous, solid-state cum sintering route. Whereas transmission electron microscopy (TEM) has revealed the formation of nearly spherical particles, x-ray diffraction (XRD) analyses suggest particles crystallize into rhombohedral crystal structure. The band gap of NBT is of direct type, as predicted through first principle calculations. The optical band gaps are also determined from the UV–visible absorption spectra, which showed a declining trend with increase in sintering temperature. The Urbach energy, which accounts for the transitional events between the extended and localized states and featuring long tailing in the absorption spectra is found to be smaller than the Tauc gap by several orders. Moreover, a broad-luminescence response as acquired on Photoluminescence spectroscopy, is discussed in the light of quantum confinement effect, near band-edge features, defect states, and self-trapped excitons. Furthermore, dielectric studies have been carried out in the frequency range, 100 Hz-5 MHz, in which one could observe a decreasing trend of capacitance with increasing frequency. However, the real part of the impedance experiences a slow declining trend initially and abrupt fall eventually beyond 1 MHz thereby suggesting a rise in conductivity at higher frequencies as a greater number of carriers are now able to follow the frequency imposed. At the same time, the single semi-circular peak of the Cole-Cole plot suggests a simple equivalent circuit of the device comprising parallel resistance (grain boundary resistance, Rp) and capacitance (grain boundary capacitance, Cp) network in series with a resistance Rs due to the grain.

The effect of thermal annealing on the optical properties of Mg-doped zincblende GaN epilayers

The effects of thermal annealing on the optical properties of Mg-doped cubic zincblende GaN epilayers grown by metalorganic chemical vapor deposition on 3C-SiC/Si (001) substrates are investigated. The photoluminescence spectra show near band edge features and a blue luminescence band that depend on Mg concentration, temperature, and excitation power density. Annealing the sample in a N2 atmosphere causes the intensity of the blue band to increase by a factor of 5. Power dependent photoluminescence measurements show a reduction in the laser excitation density required for saturation of the blue band after annealing, indicating an increase in the recombination lifetime. Time decay measurements confirm this increase, which is attributed to a reduction in the concentration of non-radiative defects after annealing. The results presented here are compared to those reported previously for Mg-doped hexagonal wurtzite GaN.

Optical Signatures of Transiently Disordered Semiconductor Nanocrystals

The optoelectronic properties of semiconductor nanocrystals (NCs) have led to efforts to integrate them as the active material in light-emitting diodes, solid-state lighting, and lasers. Understanding related high carrier injection conditions is therefore critical as resultant thermal effects can impact optical properties. The physical integrity of NCs is indeed questionable as recent transient X-ray diffraction studies have suggested that nanoscopic particles reversibly lose crystalline order, or melt, under high fluence photoexcitation. Informed by such studies, here, we examine CdSe NCs under elevated fluences to determine the impact of lattice disordering on optical properties. To this end, we implement intensity-dependent transient absorption using both one- and two-pump methods where the latter effectively subtracts out the NC optical signatures associated with lower fluence photoexcitation, especially band-edge features. At elevated fluences, we observe a long-lived induced absorption at a lower energy than the crystalline-NC bandgap across a wide range of sizes that follows power-dependent trends and kinetics consistent with the prior transient X-ray measurements. NC photoluminescence studies provide further evidence that melting influences optical properties. These methods of characterizing bandgap narrowing caused by lattice disordering could facilitate routes to improved optical amplification and band-edge emission at high excitation density.

Wave Function Engineering in CdSe/PbS Core/Shell Quantum Dots.

The synthesis of epitaxial CdSe/PbS core/shell quantum dots (QDs) is reported. The PbS shell grows in a rock salt structure on the zinc blende CdSe core, thereby creating a crystal structure mismatch through additive growth. Absorption and photoluminescence (PL) band edge features shift to lower energies with increasing shell thickness, but remain above the CdSe bulk band gap. Nevertheless, the profiles of the absorption spectra vary with shell growth, indicating that the overlap of the electron and hole wave functions is changing significantly. This leads to over an order of magnitude reduction of absorption near the band gap and a large, tunable energy shift, of up to 550 meV, between the onset of strong absorption and the band edge PL. While the bulk valence and conduction bands adopt an inverse type-I alignment, the observed spectroscopic behavior is consistent with a transition between quasi-type-I and quasi-type-II behavior depending on shell thickness. Three effective mass approximation models support this hypothesis and suggest that the large difference in effective masses between the core and shell results in hole localization in the CdSe core and a delocalization of the electron across the entire QD. These results show the tuning of wave functions and transition energies in CdSe/PbS nanoheterostructures with prospects for use in optoelectronic devices for luminescent solar concentration or multiexciton generation.

Influence of zinc concentration on band gap and sub-band gap absorption on ZnO nanocrystalline thin films sol-gel grown

Abstract ZnO thin films were fabricated on quartz substrates at different zinc acetate molar concentrations using sol-gel spin coating method. The samples were characterized using X-ray diffraction, field emission scanning electron microscope, UV-Vis spectroscopy, FT-IR spectroscopy and photoluminescence spectroscopy. Sub-band gap absorption of ZnO thin films in the forbidden energy region was carried out using highly sensitive photothermal deflection spectroscopy (PDS). The absorption coefficients of ZnO thin films increased in the range of 1.5 eV to 3.0 eV, upon increasing zinc concentration. The optical band gaps were evaluated using Tauc’s plots and found to be in the range of 3.31 eV to 3.18 eV. They showed the red shift in the band edge on increase in zinc concentration. The PL spectra of ZnO thin films revealed the characteristic band edge emission centered at the 396 nm along with green emission centered at the 521 nm.

Structural and luminescence properties of GaN nanowires grown using cobalt phthalocyanine as catalyst

Catalyst free methods have usually been employed to avoid any catalyst induced contamination for the synthesis of GaN nanowires with better transport and optical properties. Here, we have used a catalytic route to grow GaN nanowires, which show good optical quality. Structural and luminescence properties of GaN nanowires grown by vapor-liquid-solid technique using cobalt phthalocyanine as catalyst are systematically investigated as a function of various growth parameters such as the growth temperature and III/V ratio. The study reveals that most of the nanowires, which are several tens of microns long, grow along [101¯0] direction. Interestingly, the average wire diameter has been found to decrease with the increase in III/V ratio. It has also been observed that in these samples, defect related broad luminescence features, which are often present in GaN, are completely suppressed. At all temperatures, photoluminescence spectrum is found to be dominated only by a band edge feature, which comprises of free and bound excitonic transitions. Our study furthermore reveals that the bound excitonic feature is associated with excitons trapped in certain deep level defects, which result from the deficiency of nitrogen during growth. This transition has a strong coupling with the localized vibrational modes of the defects.

Growth of Zn1−x CdxO nanocrystalline thin films by sol-gel method and their characterization for optoelectronic applications

Abstract This paper describes the growth of Cd doped ZnO thin films on a glass substrate via sol-gel spin coating technique. The effect of Cd doping on ZnO thin films was investigated using X-ray diffraction (XRD), UV-Vis spectroscopy, photoluminescence spectroscopy, I–V characteristics and field emission scanning electron microscopy (FESEM). X-ray diffraction patterns showed that the films have preferred orientation along (002) plane with hexagonal wurtzite structure. The average crystallite sizes decreased from 24 nm to 9 nm, upon increasing of Cd doping. The films transmittance was found to be very high (92 to 95 %) in the visible region of solar spectrum. The optical band gap of ZnO and Cd doped ZnO thin films was calculated using the transmittance spectra and was found to be in the range of 3.30 to 2.77 eV. On increasing Cd concentration in ZnO binary system, the absorption edge of the films showed the red shifting. Photoluminescence spectra of the films showed the characteristic band edge emission centred over 377 to 448 nm. Electrical characterization revealed that the films had semiconducting and light sensitive behaviour.

Synthesis and Use of [Cd(Detu)2(OOCCH3)2]·H2O as Single Molecule Precursor for Cds Nanoparticles

Substituted thiourea ligands are of interest because they possess various donor sites for metal ions and their application in separation of metal ions and as antimicrobial agents. The coordination of the sulfur donor atom led to interest in them as precursor for semiconductor nanoparticles. In this study, cadmium(II) complex of diethylthiourea was synthesized and characterized by elemental analysis, FTIR, and X-ray crystallography. Single crystal X-ray structure of the complex showed that the octahedral geometry around the Cd ion consists of two molecules of diethylthiourea acting as monodentate ligands and two chelating acetate ions. The thermal decomposition of the compound showed that it decomposed to give CdS. The compound was thermolysed in hexadecylamine (HDA) to prepare HDA-capped CdS nanoparticles. The absorption spectrum showed blue shifts in its absorption band edges which clearly indicated quantum confinement effect, and the emission spectrum showed characteristic band edge luminescence. The broad diffraction peaks of the XRD pattern showed the materials to be of the nanometric size.

Synthesis and photoluminescence study of poly(ethylene glycol)-capped ZnS:Mn 2+ nanoparticles

Manganese-doped zinc sulphide nanoparticles capped with poly(ethylene glycol) (PEG) had been synthesised in aqueous solution by using PEG as surface modifier. The nanoparticles were prepared by chemical precipitation method at room temperature, which were characterised by the high-resolution analytical transmission electron microscope, X-ray diffraction (XRD), UV–visible and photoluminescence spectroscopy. Photoluminescene studies for all the investigated samples displayed characteristic band edge peak at about 430 nm. The broad peak at about 590 nm was ascribed to Mn incorporation. In addition, XRD and the high-resolution analytical transmission electron microscope studies showed the formation of cubic PEG-ZnS:Mn2+ nanoparticles with an average crystallite size of 3 nm.

Absorption–emission study of hydrothermally grown Al:ZnO nanostructures

The preparation, structural characterization and optical properties of aluminum doped ZnO (Al:ZnO) nanostructures grown under hydrothermal method are reported. One-dimensional (1-D) growth is achieved by the controlled addition of metal nitrate as precursors in the presence of long chain surfactant, poly-ethylene glycol (PEG) at 160 °C for 20 h. The as-synthesized ZnO rods are single crystalline, exhibiting an oriented growth along [001] direction. The Al6 rod has an aspect ratio of 3.2, which can be effectively applied in optoelectronic devices. Comprehensive structural analysis using X-ray diffraction method (XRD) and Energy dispersive X-ray analysis (EDX) indicate that the dopant Al atom occupies Zn sites in ZnO and the elemental composition of Al is consistent with the amount utilized in the hydrothermal synthesis. XRD shows that the Al:ZnO nanostructures from 1 to 9 atomic percent (at.%) has hexagonal wurtzite structure of ZnO. The Al dopant effects on lattice vibration and electronic transitions of the ZnO nanostructures have been investigated by Fourier transform Infrared spectroscopy (FT-IR), Ultraviolet–visible (UV–vis) absorption spectroscopy and photoluminescence (PL) emission recorded at room temperature. The correlation existing between absorption and emission study tell that their characteristic band edge peak of doped ZnO shifts towards higher wavelength side for 3–9 at.% with respect to Al0 thus, exhibiting a red shift phenomenon with decrease in optical bandgap. The observed PL reveals two emission peaks centered at 374 nm and 530 nm. The near band edge (NBE) to defect emission ratio increases with dopant concentration indicating the linear enhancement in crystal quality and declination in zinc vacancies from 3 to 9 at.% of Al.

Characterizing the Ultrafast Charge Carrier Trapping Dynamics in Single ZnO Rods Using Two-Photon Emission Microscopy

Zinc oxide has emerged as an attractive candidate for a variety of optoelectronic and photonic applications, due in part to a large second-order nonlinear susceptibility, its wide band gap, and large exciton binding energy. We have used time-resolved nonlinear two-photon emission microscopy to characterize the excited-state dynamics of individual ZnO rods. Photoluminescence images reveal a rich structure in the spatial distribution of both the band-edge and trap emission, and spectra recorded following excitation at a specific point in the structure show the characteristic band-edge and defect emission. Time-resolved emission spectra reveal a dynamic red shift of the trap emission band in the as-grown structures. Our results suggest that the trap emission is composed of at least two overlapping emission bands. The higher-energy band is assigned to e–h recombination between a conduction band electron and photogenerated hole bound to an acceptor defect lying within the band gap. The lower-energy band is att...

Optical and electrical characterization of AlGaN/GaN high electron mobility transistors irradiated with 5 MeV protons

An AlGaN/GaN HEMT was irradiated with 5 MeV protons at a dose up to 2×10 15/cm 2. Photoluminescence spectra measured at 5 K indicated that GaN lattice was severely damaged by collisions with high energy protons although distinct near band-edge features were still observable. An I DS– V DS measurement showed that the current level was decreased by 43% after proton irradiation, and an I GS– V GS measurement suggested that damage at the metal-contact/AlGaN interface was minimal. Transistor operation was found to be resistant to proton irradiation, which was partially attributed to a self-healing mechanism in proximity to the AlGaN/GaN interface.

Synthesis and characterization of porous ZnO nanoparticles by hydrothermal treatment of as pure aqueous precursor

The presence of the complexing agents in the growth solution poses risk of the unintentional doping in the synthesized product and hence is likely to adversely affect the intrinsic properties. Herein we report the synthesis of ZnO nanoparticles with porous microstructure using pure aqueous precursor. Crystalline ZnO nanoparticles were synthesized by thermal treatment of aqueous solution of zinc acetate in an open bath. The size of the nanocrystals was controlled by changing the initial precursor concentration. The structural and optical properties of the synthesized nanocrystals were analyzed by X-ray diffraction, high resolution transmission electron microscopy, UV–vis absorption and room temperature photoluminescence measurement techniques. The TEM and UV–vis spectral signature analyses confirmed the formation of dispersed single crystalline ZnO nanoparticles. The nanopowders were found to have disordered mesoporous structure. The synthesized nanocrystals exhibited characteristic band edge emission as well as to surface defect related deep level visible luminescence.

Structural and optical properties of Mn doped ZnS semiconductor nanostructures

Manganese doped zinc sulphide nanoparticles were fabricated by adopting an inexpensive solution growth route. Different samples were fabricated by varying Mn concentrations. UV-VIS study reveals blue-shift on the onset of absorption and hence enhancement in the optical band gap upto 0.75 eV, indicating strong quantum confinement. Photoluminescene study for all the samples display characteristic band edge peak at ∼410 nm. The broad peak ∼560–580 nm is ascribed to Mn incorporation. Further, structural investigations were carried out by using X-Ray diffraction and transmission electron microscopy (TEM).

Quantum-dot exciton dynamics with a surface plasmon: Band-edge quantum optics

We examine the spontaneous emission of a two-level emitter, quantum-dot exciton, into surface plasmons propagating on the surface of a cylindrical nanowire. The numerically obtained dispersion relations are found to influence the spontaneous emission rate strongly. At certain values of the exciton band gap, the emission rates can go to infinity due to the band-edge feature of the dispersion relations. Borrowing the idea from the photonic crystals, we model the quantum-dot exciton dynamics with a non-Markovian way and demonstrate that the decay can undergo an oscillatory behavior. In addition, we also show that the remote entangled states can be generated via super-radiance by using the one-dimensional propagating feature of the nanowire surface plasmons.

Carbon enhanced blue–violet luminescence in ZnO films grown by pulsed laser deposition

Five Zinc oxide thin-films were prepared by pulsed-laser deposition (PLD) using a ceramic ZnO target and a KrF excimer laser. The samples were all deposited on c-axis sapphire at different ambient oxygen pressures. They were then characterized using ellipsometry, atomic-force microscopy (AFM), photoluminescence (PL) spectroscopy, Rutherford backscattering spectroscopy (RBS), and nuclear reaction analysis (NRA). Along with the characteristic band-edge (3.36 eV) and deep level (2.3 eV) PL features, strong blue/violet emissions around 3 eV were also observed and are generally attributed to zinc vacancies and zinc interstitials. NRA reveals carbon impurities in all samples and the introduction of more carbon impurities enhanced the blue/violet emissions and inhibited the green luminescence. It is concluded that the carbon impurities promote the zinc related native defects in ZnO and may also be radiative centers in the violet part of the spectrum.

Manufacturing Tolerance Analysis, Fabrication, and Characterization of 3-D Submillimeter-Wave Electromagnetic-Bandgap Crystals

The sensitivity of the characteristic band edge frequencies of three different 500-GHz electromagnetic-bandgap crystals to systematic variations in unit cell dimensions has been analyzed. The structures studied were square bar woodpiles made with dielectric having epsiv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ap12 and epsiv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> =37.5 and two wide bandgap epsiv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> =37.5 crystals designs proposed by Fan and Johnson and Joannopoulos. These epsiv <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> values correspond to high-resistivity silicon and a zirconium-tin-titanate ceramic, respectively. For the woodpiles, the fractional frequency bandgap varied very little for dimensional deviations of up to plusmn5% from the optimum. The bandgaps of the Fan and Johnson and Joannopoulos structures were affected to a greater extent by dimensional variations, particular sensitivity being exhibited to the air-hole radius. For all crystals, the effect of increasing the amount of dielectric in the unit cell was to shift the bandgap edges to lower frequencies. Both silicon and ceramic woodpiles, along with a ceramic Fan structure, were fabricated and dimensionally characterized. Mechanical processing with a semiconductor dicing saw was used to form the woodpiles, while the Fan structure required both dicing and UV laser drilling of circular thru-holes. Good agreement with predicted normal incidence transmissions were found on the low-frequency side of the bandgap in all cases, but transmission values above the upper band edge were lower than expected in the ceramic structures