II-VI Semiconductor Research Articles

Colloidal Atomic Layer Deposition with Stationary Reactant Phases Enables Precise Synthesis of "Digital" II-VI Nano-heterostructures with Exquisite Control of Confinement and Strain.

In contrast to molecular systems, which are defined with atomic precision, nanomaterials generally show some heterogeneity in size, shape, and composition. The sample inhomogeneity translates into a distribution of energy levels, band gaps, work functions, and other characteristics, which detrimentally affect practically every property of functional nanomaterials. We discuss a novel synthetic strategy, colloidal atomic layer deposition (c-ALD) with stationary reactant phases, which largely circumvents the limitations of traditional colloidal syntheses of nano-heterostructures with atomic precision. This approach allows for significant reduction of inhomogeneity in nanomaterials in complex nanostructures without compromising their structural perfection and enables the synthesis of epitaxial nano-heterostructures of unprecedented complexity. The improved synthetic control ultimately enables bandgap and strain engineering in colloidal nanomaterials with close to atomic accuracy. To demonstrate the power of the new c-ALD method, we synthesize a library of complex II-VI semiconductor nanoplatelet heterostructures. By combining spectroscopic and computational studies, we elucidate the subtle interplay between quantum confinement and strain effects on the optical properties of semiconductor nanostructures.

Photoluminescence Characteristics of Chemically Synthesized Ag Doped Zns Nanoparticles

The study of group II-VI semiconductor nanomaterials have shown widespread interest due to their potential application. Amongst this group, Zinc sulfide (ZnS), which has further improved optical or magnetic properties when doped with one or more tran

Semiconductor Quantum Dots: An Emerging Candidate for CO2 Photoreduction.

As one of the most critical approaches to resolve the energy crisis and environmental concerns, carbon dioxide (CO2 ) photoreduction into value-added chemicals and solar fuels (for example, CO, HCOOH, CH3 OH, CH4 ) has attracted more and more attention. In nature, photosynthetic organisms effectively convert CO2 and H2 O to carbohydrates and oxygen (O2 ) using sunlight, which has inspired the development of low-cost, stable, and effective artificial photocatalysts for CO2 photoreduction. Due to their low cost, facile synthesis, excellent light harvesting, multiple exciton generation, feasible charge-carrier regulation, and abundant surface sites, semiconductor quantum dots (QDs) have recently been identified as one of the most promising materials for establishing highly efficient artificial photosystems. Recent advances in CO2 photoreduction using semiconductor QDs are highlighted. First, the unique photophysical and structural properties of semiconductor QDs, which enable their versatile applications in solar energy conversion, are analyzed. Recent applications of QDs in photocatalytic CO2 reduction are then introduced in three categories: binary II-VI semiconductor QDs (e.g., CdSe, CdS, and ZnSe), ternary I-III-VI semiconductor QDs (e.g., CuInS2 and CuAlS2 ), and perovskite-type QDs (e.g., CsPbBr3 , CH3 NH3 PbBr3 , and Cs2 AgBiBr6 ). Finally, the challenges and prospects in solar CO2 reduction with QDs in the future are discussed.

Influence of Al, Ta Doped ZnO Seed Layer on the Structure, Morphology and Optical Properties of ZnO Nanorods

Background:Zinc oxide (ZnO) is one of the most attractive II-VI semiconductor oxide material, because of its direct wide band gap (3.37 eV) and large binding energy (60 meV). Zinc oxide (ZnO) is a promising semiconductor due to its optimised optical properties. Among semiconductor nanostructures, the vertically aligned one-dimensional ZnO nanorods are very important for nano device application.Methods:Vertically aligned ZnO nanorod arrays were grown on ZnO, aluminum doped ZnO (ZnO:Al), tantalum doped ZnO (ZnO:Ta) and aluminum and tantalum co-doped ZnO (ZnO:Al,Ta) seed layer by hydrothermal method.Results:The X-Ray Diffraction (XRD) investigation indicated the presence of hexagonal phase for the both seed layers and nanorods. The Scanning Electron Microscope (SEM) images of ZnO and doped ZnO seed layer thin-films show spherical shaped nanograins organized into wave like morphology. The optical absorption spectra revealed shift in absorption edge towards the shorter wavelength (blue shifted) for ZnO nanorods grown on ZnO:Al, ZnO:Ta and ZnO:Al,Ta seed layer compared to ZnO nanorods grown on ZnO seed layer.Conclusion:The increase in band gap value for the ZnO nanorods grown on doped ZnO seed layers due to the decrease in crystallite size and lattice constant as evidenced from XRD analysis. The unique property of Al, Ta doped ZnO can be used to fabricate nano-optoelectronic devices and photovoltaic devices, due to their improved optical properties.

Role of Surface Reduction in the Formation of Traps in n-Doped II-VI Semiconductor Nanocrystals: How to Charge without Reducing the Surface.

The efficiency of nanocrystal (NC)-based devices is often limited by the presence of surface states that lead to localized energy levels in the bandgap. Yet, a complete understanding of the nature of these traps remains challenging. Although theoretical modeling has greatly improved our comprehension of the NC surface, several experimental studies suggest the existence of metal-based traps that have not yet been found with theoretical methods. Since there are indications that these metal-based traps form in the presence of excess electrons, the present work uses density functional theory (DFT) calculations to study the effects of charging II–VI semiconductor NCs with either full or imperfect surface passivation. It is found that charge injection can lead to trap-formation via two pathways: metal atom ejection from perfectly passivated NCs or metal–metal dimer-formation in imperfectly passivated NCs. Fully passivated CdTe NCs are observed to be stable up to a charge of two electrons. Further reduction leads to charge localization on a surface Cd atom and the formation of in-gap states. The effects of suboptimal passivation are probed by charging NCs where an X-type ligand is removed from the (100) plane. In this case, injection of even one electron leads to Cd-dimerization and trap-formation. Addition of an L-type amine ligand prevents this dimer-formation and is suggested to also prevent trapping of photoexcited electrons in charge neutral NCs. The results presented in this work are generalized to NCs of different sizes and other II–VI semiconductors. This has clear implications for n-doping II–VI semiconductor NCs without introducing surface traps due to metal ion reduction. The possible effect of these metal ion localized traps on the photoluminescence efficiency of neutral NCs is also discussed.

Effects of temperature on properties of ZnS thin films deposited by pulsed laser deposition

The Ⅱ-Ⅵ semiconductors are well-known materials applied as a window layer for thin film solar cells, but the fabrication details still require significant improvement. Compared with CdS, ZnS is eco-friendly and has a wider bandgap, which makes it a promising window layer candidate. In this work, ZnS thin films were deposited at different substrate temperatures with the same thickness on SnO2: F coated glasses by pulsed laser deposition. The structural, morphological and optical properties of ZnS films were studied. The results show ZnS thin films have a face-centered cubic phase. At 350 ℃, ZnS has the highest degree of crystallinity and roughest surface. Grain size increases from 24 nm to 39 nm and the optical bandgap increases from 3.38 eV to 3.63 eV when temperature rises from RT to 450 ℃. The ratio of sulfur to zinc atoms is about 1.04–1.07, close to the stoichiometry of ZnS.

Halide Ligands To Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness.

Zinc blende II-VI semiconductor nanoplatelets (NPLs) are defined at the atomic scale along the thickness of the nanoparticle and are initially capped with carboxylates on the top and bottom [001] facets. These ligands are exchanged on CdSe NPLs with halides that act as X-L-type ligands. These CdSe NPLs are costabilized by amines to provide colloidal stability in nonpolar solvents. The hydrogen from the amine can participate in a hydrogen bond with the lone pair electrons of surface halides. After ligand exchange, the optical features are red-shifted. Thus, ligand tuning is another way, in addition to confinement, to tune the optical features of NPLs. The improved surface passivation leads to an increase in the fluorescence quantum efficiency of up to 70% in the case of bromide. However, for chloride and iodide, the surface coverage is incomplete, and thus, the fluorescence quantum efficiency is lower. This ligand exchange is associated with a decrease in stress that leads to unfolding of the NPLs, which is particularly noticeable for iodide-capped NPLs.

Effect of Bond Dispersion on Raman Spectra Shift in II-VI Semiconductor Nanocrystals.

To understand Raman spectra shifts of nanocrystals, the top-down phonon confinement approach and the bottom-up quantum chemical approach were developed. The former is suitable for large-sized nanocrystals, and the latter is suitable for clusters containing fewer atoms. Here, we find that a simpler chemical bond model based on the bond dispersion feature can demonstrate Raman spectra shift either in normal size II-VI semiconductor nanocrystals or in atomically precise clusters. According to the bond dispersion model, the Raman spectral line of the II-VI semiconductor nanocrystal (AIIBVI) is expressed as the sum of the Lorentz subpeaks of the AII( i)BVI( j) bonds with different coordinates i and j. The calculated Raman lines of CdSe, CdS, CdTe, ZnS, and ZnSe nanocrystals are in agreement with the measured Raman spectral lines. The origin of the red shift and asymmetric broadening of the peak position of nanocrystals may be revealed as well. Results provide insight into how different bonds contribute to different vibrational spectra.

DFT calculations of metal-organic I-III-VI semiconductor clusters: Benchmark of exchange-correlation functionals and localized basis sets

The primary objective of this work was to find the numerical methods suitable for DFT calculations of ternary I-III-VI semiconductor quantum dots emerging as new functional materials which demonstrate an increasing need to comprehend their complex physicochemical properties. The benchmarking analysis of 8 exchange-correlation functionals and 11 basis sets including all-electron and effective core potential ones was performed. Four metal-organic molecules, widely used as a single-precursor in the synthesis of A-In-X2 semiconductor quantum dots (A = Cu, Ag; X = S, Se) were considered as simple representatives of ternary semiconductor quantum dots. The geometrical parameters of the optimized structures were compared to the X-ray diffraction data. The hybrid PBE0 and B3PW91 functionals were found to be the best performing methods especially when connected with cc-PVDZ or Def2-SVP basis sets. The methods widely used in the previous calculations of II-VI semiconductor quantum dots, namely B3LYP functional and LANL2DZ or SBKJC basis sets, in this case resulted in lower agreement with the experimental data.

Direct Measurement of Hyperfine Shifts and Radio Frequency Manipulation of Nuclear Spins in Individual CdTe/ZnTe Quantum Dots.

We achieve direct detection of electron hyperfine shifts in individual CdTe/ZnTe quantum dots. For the previously inaccessible regime of strong magnetic fields B_{z}≳0.1 T, we demonstrate robust polarization of a few-hundred-particle nuclear spin bath, with an optical initialization time of ∼1 ms and polarization lifetime exceeding ∼1 s. Nuclear magnetic resonance spectroscopy of individual dots reveals strong electron-nuclear interactions characterized by Knight fields |B_{e}|≳50 mT, an order of magnitude stronger than in III-V semiconductor quantum dots. Our studies confirm II-VI semiconductor quantum dots as a promising platform for hybrid electron-nuclear spin qubit registers, combining the excellent optical properties comparable to III-V dots and the dilute nuclear spin environment similar to group-IV semiconductors.

Synthesis and characterization of high-efficiency low-cost solar cell thin film

Abstract Polycrystalline chalcogenide semiconductors play a vital role in solar cell applications due to their outstanding electrical and optical properties. Among the chalcogenide semi-conductors, CdZnS is one kind of such important material for applications in various modern solid state devices such as solar cells, light emitting diode, detector etc. Due to their applications in numerous electro-optic devices, group II-VI semiconductors have been studied extensively. In recent years, major attention has been given to the study of electrical and optical properties of CdZnS thin films. In this work, Cd1−xZnxS thin films were prepared by chemical bath deposition technique. Phase purity and surface morphology properties were analyzed using field emission scanning electron microscope (FE-SEM) and X-ray diffraction (XRD) studies. Chemical composition was studied using energy dispersive spectrophotometry (EDS). Optical band gap property was investigated using UV-Spectroscopy. Electrical conductivity studies were performed by two probe method and thermoelectric power setup (TEP) to determine the type of the material. This work reports the effect of Zn on structural, electrical, microstructural and optical properties of these films.

Magnetogyrotropic reflection from quantum wells induced by bulk inversion asymmetry

Bulk inversion asymmetry (BIA) of III-V and II-VI semiconductor quantum wells is demonstrated by reflection experiments in magnetic field oriented in the structure plane. The linear in the magnetic field contribution to the reflection coefficients is measured at oblique incidence of $s$ and $p$ polarized light in vicinity of exciton resonances. We demonstrate that this contribution to the reflection is caused by magnetogyrotropy of quantum wells, i.e. by the terms in the optical response which are linear in both the magnetic field strength and light wavevector. Theory of magnetogyrotropic effects in light reflection is developed with account for linear in momentum BIA induced terms in the electron and hole effective Hamiltonians. Theoretical estimates agree with the experimental findings. We have found the electron BIA splitting constant in both GaAs and CdTe based quantum wells is about three times smaller than that for heavy holes.

The Middle Road Less Taken: Electronic-Structure-Inspired Design of Hybrid Photocatalytic Platforms for Solar Fuel Generation

The development of efficient solar energy conversion to augment other renewable energy approaches is one of the grand challenges of our time. Water splitting, or the disproportionation of H2O into energy-dense fuels, H2 and O2, is undoubtedly a promising strategy. Solar water splitting involves the concerted transfer of four electrons and four protons, which requires the synergistic operation of solar light harvesting, charge separation, mass and charge transport, and redox catalysis processes. It is unlikely that individual materials can mediate the entire sequence of charge and mass transport as well as energy conversion processes necessary for photocatalytic water splitting. An alternative approach, emulating the functioning of photosynthetic systems, involves the utilization of hybrid systems wherein different components perform the various functions required for solar water splitting. The design of such hybrid systems requires the multiple components to operate in lockstep with optimal thermodynamic driving forces and interfacial charge transfer kinetics. This Account describes a new class of nanoscale heterostructures comprising M xV2O5 nanowires, where M is a p-block cation with a ( n - 1) d10 ns2 np0 electronic configuration characterized by a stereoactive lone pair of electrons and x is its stoichiometry, interfaced with II-VI semiconductor quantum dots (QDs). Photocatalytic water splitting involves the transfer of excited-state holes from QDs to mid-gap states (derived from the stereoactive lone pairs of p-block cations) of nanowires, hole transport through nanowires, the reduction of protons at a QD-immobilized catalyst, and water oxidation at an anode. The M xV2O5/QD architectures provide a vast design space for evolutionary optimization of function with considerable tunability of composition and structure of the individual components as well as of the interfacial structure, thereby facilitating programmability of absorption spectra, energetic offsets, and charge-transfer reactivity. The available design space spans choice of the p-block cation M, its stoichiometry x, the composition and size of various QDs, and the nature of the nanowire/QD interface. This multivariate parameter space has been navigated by integrating first-principles modeling, diversified synthesis, spectroscopic measurements, and catalytic evaluation to facilitate the rational design of several generations of heterostructures and the systematic improvement of their photocatalytic performance. The electronic structures of the target heterostructures are predicted by DFT calculations in light of the revised lone pair model of stereoactive structural distortions and evaluated by hard X-ray photoelectron spectroscopy such as to systematically tune the interfacial band offsets. Central to this approach is the development of a topochemical "etch-a-sketch" intercalation approach that allows for facile installation of p-block cations in metastable polymorphs of V2O5. The interfacial charge transfer kinetics of M xV2O5/QD heterostructures is further evaluated by transient absorption spectroscopy to measure excited-state charge-transfer dynamics and is found to depend sensitively on interfacial structure and the thermodynamic driving forces in accordance with Marcus theory. The integration of theory and experiment has allowed for the design of viable photocatalytic architectures exemplified by the exceptional catalytic performance of β-Pb xV2O5/CdX (X= S, Se) architectures, which has subsequently been elaborated to other lone-pair M xV2O5 compounds, demonstrating the effective exploitation of the opportunities for programmability available in the design space.

Giant Modal Gain Coefficients in Colloidal II-VI Nanoplatelets.

Modal gain coefficient is a key figure of merit for a laser material. Previously, net modal gain coefficients larger than a few thousand cm-1 were achieved in II-VI and III-V semiconductor gain media, but this required operation at cryogenic temperatures. In this work, using pump-fluence-dependent variable-stripe-length measurements, we show that colloidal CdSe nanoplatelets enable giant modal gain coefficients at room temperature up to 6600 cm-1 under pulsed optical excitation. Furthermore, we show that exceptional gain performance is common to the family of CdSe nanoplatelets, as shown by examining samples having different vertical thicknesses and lateral areas. Overall, colloidal II-VI nanoplatelets with superior optical gain properties are promising for a broad range of applications, including high-speed light amplification and loss compensation in plasmonic photonic circuits.

RO7112689, a pH-switch anti-C5 mAb, for efficient cross-species C5 purification and inhibition of classical and reactive lysis

In view of realizing the economic viability, we fabricate a solar cell device containing low band gap and easily processable polymer 5-yl-8-(thiophene-2,5-diyl)-2,3-bis(3-(octyloxy)phenyl) quinoxaline (TQ1) and CdSe nanoparticles (NPs) and investigate its charge transport properties. When the TQ1 is combined with the CdSe NPs a strong photoluminescence quenching and shortening of photoluminescence lifetime of the TQ1 is observed indicating exciton transfer from TQ1 to the CdSe NPs. The time-resolved photoluminescence further reveals that the exciton transfer from the polymer to CdSe NPs is very efficient (68%) and it occurs in <1 ns. The exciton transfer from TQ1 to the NPs and electron-hole pair separation followed by hole transfer from the NPs to the TQ1 at the interface indeed increases the lifetime of the charge carriers. This in turn increases the efficiency of the solar cell as compared to polymer only device. These observations suggest the importance of other II-VI semiconductor NPs to achieve higher efficiency for photovoltaic devices containing TQ1 polymer.

Large and persistent photoconductivity due to hole-hole correlation in CdS

Large and persistent photoconductivity (LPPC) in semiconductors is due to the trapping of photo-generated minority carriers at crystal defects. Theory has suggested that anion vacancies in II-VI semiconductors are responsible for LPPC due to negative-U behavior, whereby two minority carriers become kinetically trapped by lattice relaxation following photo-excitation. By performing a detailed analysis of photoconductivity in CdS, we provide experimental support for this negative-U model of LPPC. We also show that LPPC is correlated with sulfur deficiency. We use this understanding to vary the photoconductivity of CdS films over nine orders of magnitude, and vary the LPPC characteristic decay time from seconds to 10^4 seconds, by controlling the activities of Cd^{2+} and S^{2-} ions during chemical bath deposition. We suggest a screening method to identify other materials with long-lived, non-equilibrium, photo-excited states based on the results of ground-state calculations of atomic rearrangements following defect redox reactions, with a conceptual connection to polaron formation.

Tuning Exciton-Mn2+ Energy Transfer in Mixed Halide Perovskite Nanocrystals.

Doping nanocrystals (NCs) with luminescent activators provides additional color tunability for these highly efficient luminescent materials. In CsPbCl3 perovskite NCs the exciton-to-activator energy transfer (ET) has been observed to be less efficient than in II–VI semiconductor NCs. Here we investigate the evolution of the exciton-to-Mn2+ ET efficiency as a function of composition (Br/Cl ratio) and temperature in CsPbCl3–xBrx:Mn2+ NCs. The results show a strong dependence of the transfer efficiency on Br– content. An initial fast increase in the relative Mn2+ emission intensity with increasing Br– content is followed by a decrease for higher Br– contents. The results are explained by a reduced exciton decay rate and faster exciton-to-Mn2+ ET upon Br– substitution. Further addition of Br– and narrowing of the host bandgap make back-transfer from Mn2+ to the CsPbCl3–xBrx host possible and lead to a reduction in Mn2+ emission. Temperature-dependent measurements provide support for the role of back-transfer as the highest Mn2+-to-exciton emission intensity ratio is reached at higher Br– content at 4.2 K where thermally activated back-transfer is suppressed. With the present results it is possible to pinpoint the position of the Mn2+ excited state relative to the CsPbCl3–xBrx host band states and predict the temperature- and composition-dependent optical properties of Mn2+-doped halide perovskite NCs.

Photoluminescence and electrical properties from CdO/Cd-nanocrystallites on Cd foil

CdO, which is an n-type II-VI semiconductor compound, plays an increasingly important role in the optoelectronic field. In this paper, CdO micro-rods have been synthesized through using Cd foil as the Cd2+ source and the substrate by a solvothermal method, which are approximately perpendicular to the substrate. Meanwhile, Cd nanocrystallites (nc-Cd) have been fabricated, which are observed to locate at the bottom of CdO micro-rods. Photoluminescence at 10 K has been measured and shown multiple peak emissions. Through analyzing the temperature-dependent photoluminescence and the Varshni formula, the peak located at ~ 487 nm has been confirmed as the emission of band gap. The peaks of ~ 501 nm and ~ 588 nm are disappeared with the increasing of temperature, which are ascribed to the emissions from Cd interstitials to valence band and excitonic transitions, respectively. The peak located at ~ 715 nm is attributed to the emission of surface defects and shows blue shift with increasing temperature. It may be due to the influence of the existence of nc-Cd located at the bottom of CdO micro-rods and/or the interface between CdO and nc-Cd. The current density vs voltage (I-V characterization) from CdO/nc-Cd on Cd foil shows the obvious rectifying effect. Through analyzing the I-V characterization, it is indicated that there are a lot of defects in the CdO to impede the performance. It is believed that through optimizing the preparation process CdO/nc-Cd on Cd foil will be potentially applied in the future optoelectronic field.

Optical properties and exciton binding energy and related parameters of CdTe: Pressure-induced effects

The present paper reports on the hydrostatic pressure dependence of the direct and indirect energy band-gaps, refractive index, static and high-frequency dielectric constants, exciton banding energy and exciton Bohr radius for CdTe in the zinc-blende structure. The applied pressure is taken in the range 0–30 kbar. The calculations are performed using a pseudopotential approach. At zero pressure, our results show a good accord with experiment for most studied optical properties. Nevertheless, for exciton binding energy and Bohr radius the use of Adachi's expression [S. Adachi, Properties of Group-IV, III–V, and IIVI Semiconductors, Wiley, Chichester, 2005] has given poor results as compared to experiment. In this respect, a modified Adachi's expression formula have been proposed and found to give meaningful accord with experiment. Upon compression up to 30 kbar, the material of interest is found to remain a direct (Г-Г) band-gap semiconductor. All optical features being studied here have shown a monotonic behavior under compression. The present study can be useful for infrared applications.

(Cd,Zn,Mg)Te-based microcavity on MgTe sacrificial buffer: Growth, lift-off, and transmission studies of polaritons

Opaque substrates precluded, so far, transmission studies of II-VI semiconductor microcavities. This work presents the design and molecular beam epitaxy growth of semimagnetic (Cd,Zn,Mn)Te quantum wells embedded into a (Cd,Zn,Mg)Te-based microcavity, which can be easily separated from the GaAs substrate. Our lift-off process relies on the use of a MgTe sacrificial layer which stratifies in contact with water. This allowed us to achieve a II-VI microcavity prepared for transmission measurements. We evidence the strong light-matter coupling regime using photoluminescence, reflectivity, and transmission measurements at the same spot on the sample. By comparing a series of reflectance spectra before and after lift-off, we prove that the microcavity quality remains high. Thanks to Mn content in quantum wells we show the giant Zeeman splitting of semimagnetic exciton-polaritons in our transmitting structure.