Group II-VI Semiconductor Research Articles

Self-Assembly of Chiral Ligands on 2D Semiconductor Nanoplatelets for High Circular Dichroism.

Group II-VI semiconductor nanoplatelets (NPLs) with atomically defined thicknesses and extended atomically flat (001) facets are used for ligand binding and chiro-optical effects. In this study, we demonstrate that tartrate ligands, anchored by two carboxylate groups, chelate the (001) facets of NPLs at an average ratio of one tartrate molecule to two cadmium (Cd) surface atoms. This assembly of chiral molecules on inorganic nanocrystals generates a circular dichroism g-factor as high as 1.3 × 10-2 at the first excitonic transition wavelength of NPLs. Tartrate ligands induce an orthorhombic distortion of the initially "cubic" crystal structure, classifying the NPLs within the 222-point group. Unlike spherical nanocrystals, where it is difficult to discern whether chiral ligands affect only the surface atoms or the entire crystal structure, our findings unequivocally show that the crystal structure of NPLs is modified due to their thinness and atomically precise thickness. The in-plane lattice parameters experience compressive and tensile stresses, significantly splitting the heavy-hole and light-hole bands. Additionally, tartrate ligands adopt different conformations on the NPL surface over time, resulting in dynamic changes in the circular dichroism signal, including an inversion of its sign.

(Invited) Dynamics of Photoluminescence in Mn-Doped Pb-Halide Perovskite Nanocrystals

Nanocrystals of lead halide perovskites have been shown to have extraordinary photoluminescence properties, spanning the entire visible range achieved by the composition tuning via solid solutions at the halide site. Taking cues from Mn doping of traditional Group II-VI semiconductor quantum dots, Mn doping of the lead halide perovskites has been extensively explored by the community, establishing intense Mn-dopant induced emission that is significantly Stokes shifted from the excitonic emissions of the host. While all reports agree on the essential features of this emission, such as the peak position and the width of the Mn photoluminescence, there appears to be little agreement on the microscopic processes by which the host sensitizes Mn following its photoexcitation and the steps that control the deexcitation leading to the Mn emission. There are even reports of disagreement between different reports on the temperature dependence of the Mn emission and its origin. I shall discuss the present status of this field and establish, with the help of new experimental data as well as the already reported ones, what appears to be the microscopic processes controlling the excited dynamics in the Mn-doped lead halide perovskites.

Resonant tunneling in a colloidal CdS semiconductor quantum-dot single-electron transistor based on heteroepitaxial-spherical Au/Pt nanogap electrodes.

Semiconductor quantum dots (QDs) have unique discrete energy levels determined by the particle size and material. Therefore, they have potential applications as novel optical and electronic devices. Among those, colloidal group II-VI semiconductor quantum dots stand out for their facile synthesis and band gaps aligned with the visible light spectrum. However, the electrical characterization studies of an individual quantum dot necessitate the size of nanogap electrodes being equal to the size of the quantum dot, which has conventionally been evaluated using techniques such as scanning tunneling microscopy (STM) and nanogaps fabricated by electromigration. The complexity of device fabrication has restricted research in this area. Here, we present a pioneering approach for the electrical characterization of single-QD: heteroepitaxial-spherical (HS) Au/Pt nanogap electrodes. We fabricated transistors through chemisorption, an anchoring colloidal CdS QD (3.8 nm) between the HS-Au/Pt nanogap electrodes (gap separation: 4.5 nm). The resulting device functions as a quantum-dot single-electron transistor (QD-SET), showing resonant tunneling-an inherent characteristic of the QD. A steep current increase was observed at a negative voltage, apart from the theoretical single-electron tunneling current by Coulomb blockade phenomena, which agreed with the theoretical resonant tunneling current through a discrete energy level of the QD. This underscores the promise of HS-Au/Pt nanogap electrodes in realizing single-QD devices, offering a pathway toward unlocking their full potential.

First-principles calculations to investigate the optoelectronic, and thermoelectric nature of zinc based group II-VI direct band semiconductors

The electronic structure, optical, and thermoelectric properties of novel zinc-based group II-VI semiconductors are investigated using first-principles calculations based on density functional theory. The ternary type materials' structural parameters and electronic band profiles were computed with the advanced TB-mBJ potential. Our computations indicated that the materials are direct band-type semiconductors, with energy gaps of 1.65 eV for MgZnO2 and 2.54 eV for MgZnS2, respectively. If the adjacent valence band is retained, the motion linked with conduction band minima around the Fermi level would most likely reproduce the function in the band gap opening, being the major reason for the decrease in the band gap value from MgZnS2 to MgZnO2. For these materials, the estimated total density of states showed a progressive move along with the energy value. To investigate the optical transitions in these materials, linear optical characteristics such as the complex dielectric function, loss functions, absorption coefficients, reflectivity spectra, and refractive indices were computed and discussed. Furthermore, the thermoelectric properties of these materials were calculated indicating that these materials are competent for applications involving thermoelectric devices.

Synthesis of Group II-VI Semiconductor Nanocrystals via Phosphine Free Method and Their Application in Solution Processed Photovoltaic Devices.

Solution-processed CdTe semiconductor nanocrystals (NCs) have exhibited astonishing potential in fabricating low-cost, low materials consumption and highly efficient photovoltaic devices. However, most of the conventional CdTe NCs reported are synthesized through high temperature microemulsion method with high toxic trioctylphosphine tellurite (TOP-Te) or tributylphosphine tellurite (TBP-Te) as tellurium precursor. These hazardous substances used in the fabrication process of CdTe NCs are drawing them back from further application. Herein, we report a phosphine-free method for synthesizing group II-VI semiconductor NCs with alkyl amine and alkyl acid as ligands. Based on various characterizations like UV-vis absorption (UV), transmission electron microscope (TEM), and X-ray diffraction (XRD), among others, the properties of the as-synthesized CdS, CdSe, and CdTe NCs are determined. High-quality semiconductor NCs with easily controlled size and morphology could be fabricated through this phosphine-free method. To further investigate its potential to industrial application, NCs solar cells with device configuration of ITO/ZnO/CdSe/CdTe/Au and ITO/ZnO/CdS/CdTe/Au are fabricated based on NCs synthesized by this method. By optimizing the device fabrication conditions, the champion device exhibited power conversion efficiency (PCE) of 2.28%. This research paves the way for industrial production of low-cost and environmentally friendly NCs photovoltaic devices.

Nonradiative Carrier Recombination Enhanced by Vacancy Defects in Ionic II-VI Semiconductors

Nonradiative-recombination-related defects are significant for optoelectronic semiconductor devices. Here, we analyze nonradiative-recombination processes in ionic semiconductors using first-principles density-functional theory. In ionic group II-VI semiconductors, we find that large lattice relaxations of anion vacancies caused by strong Coulomb interactions between different charged defect states can significantly enhance recombination processes through a two-level recombination mechanism. Specifically, we show that the defect level of the 2+ charged anion vacancy $\mathrm{(}{V}_{\mathrm{Se}}^{2+})$ in group II-VI $\mathrm{Zn}\mathrm{Se}$ is close to the conduction-band minimum and easily captures an electron to form a metastable 1+ charged state $\mathrm{(}{V}_{\mathrm{Se}}^{+})$; then, the large lattice relaxation, on account of the change in Coulomb interactions locally in the different charged states, rapidly changes this metastable state to a stable one and simultaneously move the defect level of ${V}_{\mathrm{Se}}^{+}$ closer to that valence-band maximum, and thus, increases the hole-capture rate. Compared with the Shockley-Read-Hall nonradiative-recombination theory based on a single defect level, this two-level recombination mechanism involving anion vacancies can greatly increase the nonradiative-recombination rate in ionic group II-VI semiconductors. This understanding is expected to be useful for the study of the nonradiative-recombination process in ionic semiconductors for applications in the field of optoelectronic devices.

(Invited) Theory of Silicon-Based Light Emitting

The indirect band gap preclude the silicon from making light-emitting devices, which is, however, a quest for exploration of large-scale integration of both electronic and photonic devices on a single microchip. Why silicon and germanium are both indirect band gap and most of group III-V and all group II-VI semiconductors are direct band gap is a long standing puzzle. In this talk, I will first present our recently developed unified theory, based on occupied d orbitals in cations, for explaining why diamond, Si, Ge, and Al-containing group III-V semiconductors have indirect band gap, and the remaining group III-V and II-VI semiconductors, except GaP, have direct band gap. This theory also unravels that it is impossible to make allotropes of Si and compounds consisting by Si mixed together with other elements high efficient direct bandgap light emitting. Interface and quantum confinement in low-dimensional Si nanostructures are two knobs to relax the momentum conservation law of optical transition. Our theory confirms that the light emission efficiency of Si quantum dots increases exponentially as reducing dot size, but its highest reachable efficiently is still two orders of magnitude smaller than that of direct bandgap InP and GaAs. Fortunately, we discovered that Si/Ge magic sequence superlattices exhibit orders more efficient at emission light than their existing counterpart records and approach more than 10% brightness of real direct gap materials, such as GaAs. Furthermore, regarding the direct bandgap is only 140 meV above the indirect bandgap in Germanium (Ge), it is well established that a tiny (1-2%) extensile strain could render an indirect-to-direct bandgap transition and makes it direct bandgap. However, there is no feasible way to create such extensive strain on Ge since the lattice parameter of Ge is even 4.3% larger than Si. We propose insert external atoms into interstitial sites of Ge, which grows on Si substrate through SiGe buffer layer, to induce an effective tensile uniaxial strain on Ge, a new pathway toward direct band gap Ge. Because our proposed scheme is highly compatible with CMOS fabrication process, our presented new pathway would be a solution for on-chip lasers.

(Invited) Theory of Silicon-Based Light Emitting

Why silicon and germanium are both indirect band gap and most of group III-V and all group II-VI semiconductors are direct band gap is a long standing puzzle. In this talk, I will first present our recently developed unified theory to explain why diamond, Si, Ge, and Al-containing group III-V semiconductors have indirect band gap, and the remaining group III-V and II-VI semiconductors, except GaP, have direct band gap. We excluded Si QDs as a candidate for on-chip lasers. We also discovered that Si/Ge magic sequence superlattices exhibit orders more efficient at emission light than their existing counterpart records and approach more than 10% brightness of real direct gap materials, such as GaAs. We further propose to insert external atoms into interstitial sites of Ge to induce an effective tensile uniaxial strain on Ge toward direct band gap Ge. This scheme is highly compatible with CMOS fabrication process.

О доминирующем механизме безызлучательного возбуждения ионов марганца в II-VI полумагнитных полупроводниках

Doping of group II-VI semiconductors and low-dimensional structures based on them with manganese leads to effective quenching of electro-and photoluminescence provided that the electron excitation energy of the crystal exceeds the energy of the intracenter transition of Mn2+ EMn и 2.1 eV. Quenching involves efficient energy transfer from the photoexcited crystal to Mn2 + ions. There are three possible mechanisms of this non-radiative energy transfer: dipole-dipole, exchange and related mechanism associated with sp-d mixing. Although the dipole-dipole mechanism is thought to be ineffective due to the forbidden intracenter transition in Mn2+, and the spin-dependent exchange mechanism is dominant, not all experimental facts accumulated to date supports this conclusion. The article discusses two experimental approaches to identify the dominant mechanism of energy transfer to Mn2 + ions and to evaluate the partial contributions of the mechanisms. One of these approaches is related to optically detectable magnetic resonance at single diluted magnetic semiconductor quantum dots (QD), the second - to plasmon amplification of energy transfer to Mn2 + ions by means of dipole-dipole interaction.

Investigation of Structural Properties of a Ternary Semiconductor Compound SrxCd1-xO (x= 0.00, 0.25, 0.50, 0.75 and 1.00)

The present work is based upon the investigation of structural properties of technologically important compound SrxCd1-xO (x= 0.00, 0.25, 0.50, 0.75 and 1.00) of group II-VI semiconductors. The Linear Combination of Atomic Orbital method has been used within density functional theory as embodied in CRYSTAL code. The Hybrid scheme B3LYP is used for making the Kohn-Sham Hamiltonian. The effect of the composition on lattice constant, bulk modulus is investigated. The deviations of the lattice constant from the Vegard’s law were observed for three cases

Гидрохимический синтез халькогенидов металлов. Часть 41. Гидрохимическое осаждение тонких пленок селенида кадмия селеносульфатом натрия

The group II-VI semiconductor materials including Cadmium Selenide (CdSe) thin films are widely used in many fields of science and technology, in particular in optoelectronics, nanoelectronics and solar energy. Chemical bath deposition (CBD) represents the simplest and the most available technique for deposition of semiconducting layers. CBD is characterized by deletion of toxic gaseous precursors, operation at low temperature and using of inexpensive equipment. The ionic equilibriums in reaction mixture «CdCl2 – L − Na2SeSO3» (L− NH4OH or Na3C6H5¬O7 or mixture of NH4OH and Na3C6H5¬O7 ) were calculated in present work. The prevailing cadmium complex compounds were determined in appropriate for CBD of cadmium selenide films pH range. The main complex compounds inhibiting fast formation of cadmium selenide are Cd(OH)Cit^(2-) complex (in citrat- and ammonia-citrat mixtures) and 〖Cd(NH_3)〗_5^(2+) complex (in ammonia mixture). Also the boundary conditions of forming CdSe and Cd(OH)2 in reaction mixture were determined by thermodynamic calculation based on crystallization factor to estimate the formation conditions of main (CdSe) and impurity (Cd(OH)2) phases. The results of the calculations show that the solid phase of cadmium selenide is possible to form in pH range from 10 to 14. CdSe films were grown by chemical bath deposition on glass substrates at a temperature of 353 K. The thickness of films ranges from 100 to 220 nm. The grain size of films is about 30 nm which was determined by electron microscopic investigations. The elemental composition of cadmium selenide was defined by energy dispersive analysis; the ratio of cadmium and selenium is 1.03 : 1.16. The conductivity of n-type was determined by the sign of thermoelectromotive force.

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

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.

Optical Cooling of CdSe/ZnS Quantum Dots Embedded in PMMA

In 1929, the German physicist Peter Pringsheim proposed that anti-Stokes emission can lead to the cooling of bulk matter. Anti-Stokes emission results in photons of shorter wavelength (higher photon energy) than that of the exciting light due to thermal absorption. In solids, the thermal energy is mostly due to the vibrational modes (phonons) of the lattice. Thus, using lasers to cool a solid, one has to irradiate a sample with laser light in the red tail of the absorption spectrum. The material to be cooled would then absorb a photon and absorb extra energy from a phonon to emit a blue-shifted photon of higher energy. By removing these phonons, the material is cooled. Today, this technique is known as laser cooling of solids or optical refrigeration. It can be used to achieve an all-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids. Laser cooling also allows for the possibility of portable lasers that require no or smaller external cooling systems because the pump wavelength can be adjusted such that spontaneous anti-Stokes luminescence cooling compensates for the stimulated quantum defect heating. A thermally balanced laser such as this would not suffer from thermal defocusing or heat damage; therefore, such solid-state-lasers could achieve higher output powers. Unfortunately, for cooling to occur in solids, the quantum efficiency of the material must be high and nearly all the anti-Stokes luminescence must leave the material without being reabsorbed. Recently, much research has been performed on rare-earth ions for laser cooling. However, the cooling efficiency of rare-earth ions approaches zero as the temperature decreases. As a result, they have a theoretical cooling limit about 90 K. On the other hand, semiconductors provide more efficient pump light absorption, the potential for lower temperatures of 10 K, and the ability to directly integrate the material into electronic and photonic devices. This coupled with the strong electron-phonon coupling of Group II-VI semiconductors and core-shell quantum dot structure minimizing the reabsorption of the anti-Stokes luminescence, makes direct bandgap semiconductors viable laser cooling candidate materials. Owing to the possibility for near-unity quantum efficiency of CdSe/ZnS, we have chosen to investigate CdSe/ZnS quantum dots for laser cooling applications. Recently, we have utilized photothermal deflection techniques to show that CdSe/ZnS can be cooled at a microscopic level. Moreover, we developed a new photothermal technique that is capable of discerning heating or cooling inside a material using nothing but the probe deflection thereby eliminating the need for a Stokes and anti-Stokes laser. Using the information gained from the photothermal results, we have begun to cool CdSe/ZnS polymers. This talk will go over the cooling results as well as our new photothermal technique.

Defect-Rich Dopant-Free ZrO2 Nanostructures with Superior Dilute Ferromagnetic Semiconductor Properties.

Control of the spin degree of freedom of an electron has brought about a new era in spin-based applications, particularly spin-based electronics, with the potential to outperform the traditional charge-based semiconductor technology for data storage and information processing. However, the realization of functional spin-based devices for information processing remains elusive due to several fundamental challenges such as the low Curie temperature of group III-V and II-VI semiconductors (<200 K), and the low spin-injection efficiencies of existing III-V, II-VI, and transparent conductive oxide semiconductors in a multilayer device structure, which are caused by precipitation and migration of dopants from the host layer to the adjacent layers. Here, we use catalyst-assisted pulsed laser deposition to grow, for the first time, oxygen vacancy defect-rich, dopant-free ZrO2 nanostructures with high TC (700 K) and high magnetization (5.9 emu/g). The observed magnetization is significantly greater than both doped and defect-rich transparent conductive oxide nanomaterials reported to date. We also provide the first experimental evidence that it is the amounts and types of oxygen vacancy defects in, and not the phase of ZrO2 that control the ferromagnetic order in undoped ZrO2 nanostructures. To explain the origin of ferromagnetism in these ZrO2 nanostructures, we hypothesize a new defect-induced bound polaron model, which is generally applicable to other defect-rich, dopant-free transparent conductive oxide nanostructures. These results provide new insights into magnetic ordering in undoped dilute ferromagnetic semiconductor oxides and contribute to the design of exotic magnetic and novel multifunctional materials.

Laser Cooling of CdSe/ZnS Quantum Dots

In 1929, the German physicist Peter Pringsheim proposed that anti-Stokes emission can lead to the cooling of bulk matter. Anti-Stokes emission results in photons of shorter wavelength (higher photon energy) than that of the exciting light due to thermal absorption. In solids, the thermal energy is mostly due to the vibrational modes (phonons) of the lattice. Thus, using lasers to cool a solid, one has to irradiate a sample with laser light in the red tail of the absorption spectrum. The material to be cooled would then absorb a photon and absorb extra energy from a phonon to emit a blue-shifted photon of higher energy. By removing these phonons, the material is cooled. Today, this technique is known as laser cooling of solids or optical refrigeration. It can be used to achieve an all-solid-state cryocooler that is compact, contains no moving parts, has a high reliability, and does not require the use of cryogenic fluids. Laser cooling also allows for the possibility of portable lasers that require no or smaller external cooling systems because the pump wavelength can be adjusted such that spontaneous anti-Stokes luminescence cooling compensates for the stimulated quantum defect heating. A thermally balanced laser such as this would not suffer from thermal defocusing or heat damage; therefore, such solid-state-lasers could achieve higher output powers. Unfortunately, for cooling to occur in solids, the quantum efficiency of the material must be high and nearly all the anti-Stokes luminescence must leave the material without being reabsorbed. Recently, much research has been performed on rare-earth ions for laser cooling. However, the cooling efficiency of rare-earth ions approaches zero as the temperature decreases. As a result, they have a theoretical cooling limit of 100 K. On the other hand, semiconductors provide more efficient pump light absorption, the potential for lower temperatures of 10 K, and the ability to directly integrate the material into electronic and photonic devices. This coupled with the strong electron-phonon coupling of Group II-VI semiconductors and core-shell quantum dot structure minimizing the reabsorption of the anti-Stokes luminescence, makes direct bandgap semiconductors viable laser cooling candidate materials. Owing to the possibility for near-unity quantum efficiency of CdSe/ZnS, we have chosen to investigate CdSe/ZnS quantum dots for laser cooling applications. Recently, we have determined that the optimum laser excitation wavelength for CdSe/ZnS anti-Stokes emission is 647 nm. This wavelength indicates that the second-harmonic longitudinal optical phonon (2LO) is strongly coupled to the laser photons. Using this wavelength, we were able to cool a 4-mL colloidal solution containing 2 mg of CdSe/ZnS per 1 mL of toluene by 2.3°C from room temperature. However, it took several hours to cool the liquid owing to the convectional cooling produced by the small-diameter laser beam, which was only cooling 3% of the volume of the solution. To reduce this problem, we added a beam expander to irradiate the entire sample area and speed up the cooling process. In addition, we split the beam to simultaneously measure the changes in temperature of CdSe/ZnS colloid and of pure toluene. Photothermal techniques have also been explored to accurately measure the temperature of the CdSe/ZnS material. In addition, polymer hosts have been prepared for the CdSe/ZnS quantum dots.

Characterization, Thermal Effect on Optical Band Gap Energy and Photoluminescence in Wurtzite ZnO:Er Nanocrystallites

Zinc Oxide is an extensively studied group II-VI Semiconductor with optical properties that permits stable emission at room temperature, having immense applications in sensors, field emission and photonic devices. It exhibits a wide variety of morphologies in the nano regime. XRD, FTIR, EDAX and TEM spectra have been used for characterizing the material. The percentage of doping material is confirmed from the EDAX spectrum. The XRD results indicated that the particle size of nano ZnO: Er is much small as compared to that of pure ZnO and decreases with the Erbium loading. XRD has shown that as temperature increases, particle size also increases. The UV Absorption spectra showed blue shift towards 208nm due to doping with Er. From the position of the absorption spectra is shifted towards the lower wavelength side or known as blue-shifted relative to undoped ZnO. The band gap energy of Er-doped ZnO nanoparticles at 120 and 200oC are determined using Tauc plot and found to be in the range 3.321eV. It was found that energy band gap Eg increases with doping of Er. As temperature increases the band gap energy decreases. The Photo-Luminescence study was carried out on this material and observed longer wavelength shift for emission peaks on calcination. The analysis of optical properties shows that ZnO: Er is a promising material and has potential application in optoelectronic devices. Doping of Er changed the free carrier concentration in the ZnO nanoparticles.

(Invited) Stabilizing Semiconductor-Solution Interfaces Via Chemically Stable but Electronically Defective Coatings

Recently, a photoelectrode protection strategy, which uses an amorphous “leaky” TiO2 coating to form a complex semiconductor/TiO2/Ni structure, has shown to stabilize semiconductor-solution interfaces against photoanodic corrosion. Particularly, stabilizing semiconductor-aqueous solution interfaces for water oxidation is important because water oxidation is required for all solar fuel strategies, e.g. H2 production and CO2 reduction. We show that this TiO2protection strategy has stabilized a number of 1.1 – 1.9 eV band gap light absorbers including Si, group III-V, group II-VI semiconductors, their tandem structures and wire structures for up to 2000 hours continuous water oxidation in 1 M KOH(aq). Because of this, various otherwise unstable materials can be brought back to the table again for building photoelectrochemical devices. A water splitting reactor prototype of ~10% solar-to-hydrogen efficiency was also built. The n-doped Si/TiO2/Ni structure also operates as a high barrier height junction for solar energy conversion. To further understand the mechanism of “leaky” TiO2 protection and the role of Ni electrocatalyst overlayers, we used Si/TiO2/Ni structures as a model system to study solid-solution interfaces. TiO2 coatings of 3 – 143 nm in thickness introduce two interfaces: a TiO2/liquid interface and a TiO2/n-Si heterojunction. We have employed operando ambient-pressure X-Ray photoelectron spectroscopy (AP-XPS) to study TiO2/Ni/liquid and TiO2/liquid interfaces, and directly observed their ohmic and rectifying junction behavior, respectively. We also extensively investigated n-Si/TiO2 heterojunction interfaces by employing photoelectrochemical, solid-state electrical, and photoelectron spectroscopic techniques. The distinctive electrical behavior of n-Si/TiO2 heterojunctions will be compared with conventional semiconductor/metal and semiconductor/liquid junctions. Because electronic defect states exist in band gaps of “leaky” TiO2, their effects on the formation of n-Si/“leaky”TiO2 heterojunctions will be discussed. This interfacial study reveal several strategies for improving the performance of thin-layer protected photoelectrodes as well as for further expanding selection of “leaky” protective coatings.

Stable Solar-Driven Water Oxidation to O2(g) By Multifunctional Electrocatalysts Coated Small Band Gap Semiconductors

Technologically important small band gap (<2 eV) semiconductors must be stabilized against corrosion or passivation in aqueous electrolytes before they can be used as photoelectrodes that directly produce fuels from sunlight. In addition, incorporation of electrocatalysts on the surface of the photoelectrodes is required for efficient oxidation of H2O to O2(g) and reduction of H2O or H2O and CO2 to fuels. Stabilization of technologically important semiconductors against photocorrosion and photopassivation would have a significant impact on photoelectrochemical energy conversion, and could enable the development of a new generation of robust integrated devices for efficient solar-driven water splitting and solar-driven CO2 reduction. Previous efforts have been extensively dedicated on elongating the lifetime of semiconductors under harsh fuel forming reaction conditions especially during the water oxidation half reaction. To date, the energy conversion performances and stability were limited on these systems, obscuring the realization of integrated solar fuel devices. In this work, we presented our recent effort on preparation of a multifunctional coating using Ni oxide, which provides multiple important functions on semiconductor photoelectrodes surfaces, including chemical/corrosion protection, electrically conducting, optical transparent/antireflective, and inherent electrocatalytic activity. The combination of the extraordinary film properties has resulted benchmark performances and stability on Si photoanodes, as well as other group II-VI and group III-V semiconductors that have never been able to be protected in relevant water oxidation conditions before. Results presented here indicated a viable strategy that is a promising approach to achieve the long-term stability of semiconducting photoanodes for use in solar fuel applications.

Characterization and Elastic properties of wurtzite ZnO: Ce Nanocrystallites

Zinc Oxide is an extensively studied group II-VI semiconductor with optical properties that permits stable emission at room temperature having immense application in sensors, field emission and photonic devices. It exhibits a wide variety of morphologies in the nano regime that can be grown by tuning the growth habit of the ZnO crystal. ZnO nano materials doped with Cerium ions XRD, SEM, FTIR, TEM and EDS characterized the samples. The percentage of doping material is confirmed from the EDS spectra. The average crystal size of the prepared ZnO nanopowder is determined by XRD. The size of the particles increased as the annealing temperature was increased. The crystallite size varied from 13nm to 16 nm as the calcination temperature increased, are synthesized by chemical route technique. Ultrasonic velocity through doped and undoped sample is measured and compressibility is computed. The compressibility is found to be increased. Also variation of compressibility of ZnO: Ce nanofluid with various grain size have been carried out and found that compressibility increases with decrease of particle size. To study the elastic properties of the nano fluid extensively, certain important physical parameters such as adiabatic compressibility, specific acoustic impedance, relative association, intermolecular free length, Rao's constant, Wada's constant etc. are evaluated using ultrasonic velocity and density.