Electronic structure of ZnS, ZnSe, ZnTe, and their pseudobinary alloys

Limitations and Progresses in Carbon-Based Cesium Lead Halide Perovskite Solar Cells

Inorganic cesium lead halide perovskites (CsPbIxBr3-x, 0≤x≤3) are promising alternatives with great thermal stability. Additionally, the choice of moisture-resistive and dopant-free carbon as the electrode material can simultaneously solve the problems of stability and cost. Therefore, carbon electrode-based inorganic PSCs (C-IPSCs) represent a promising candidate for commercialization, yet both the efficiencies and stability of related devices demand further progress. This article reviews the recent advancement of C-IPSCs and then unravels the distinctive merits and limitations in this field. Subsequently, our perspective on various modification strategies is analyzed on a methodological level. Finally, this article outlooks the promising research contents and the remaining unresolved issues in this field. We believe that understanding and analyzing the related problems in this field are instructive to stimulate the future development of C-IPSCs.

Electronic Properties of Zn2V(1-x)NbxN3 Alloys to Model Novel Materials for Light-Emitting Diodes

We propose the Zn2V(1-x)NbxN3 alloy as a new promising material for optoelectronic applications, in particular for light-emitting diodes (LEDs). We perform accurate electronic-structure calculations of the alloy for several concentrations x using density-functional theory with meta-GGA exchange-correlation functional TB09. The band gap is found to vary between 2.2 and 2.9 eV with varying V/Nb concentration. This range is suitable for developing bright LEDs with tunable band gap as potential replacements for the more expensive Ga(1-x)In(x)N systems. Effects of configurational disorder are taken into account by explicitly considering all possible distributions of the metal ions within the metal sublattice for the chosen supercells. We have evaluated the band gap's nonlinear behavior (bowing) with variation of V/Nb concentration for two possible scenarios: (i) only the structure with the lowest total energy is present at each concentration and (ii) the structure with minimum band gap is present at each concentration, which corresponds to experimental conditions when also metastable structures are presents. We found that the bowing is about twice larger in the latter case. However, in both cases, the bowing parameter is found to be lower than 1 eV, which is about twice smaller than that in the widely used Ga(1-x)In(x)N alloy. Furthermore, we found that both crystal volume changes due to alloying and local effects (atomic relaxation and the V-N/Nb-N bonding difference) have important contributions to the band gap bowing in Zn2V(1-x)NbxN3.

In2Se3, In2Te3, and In2(Se,Te)3 Alloys as Photovoltaic Materials

In its lowest-energy three-dimensional (3D) hexagonal crystal structure (γ phase), In₂Se₃ has a direct band gap of ∼1.8 eV and displays high absorption coefficient, making it a promising semiconductor material for optoelectronics. Incorporation of Te allows for tuning the band gap, adding flexibility to device design and extending the application range. Here we report results of hybrid density functional theory calculations to assess the electronic and optical properties of γ-In₂Se₃, γ-In₂Te₃, and γ-In₂(Se_1-xTe_x)₃ alloys, and initial experiments on the growth and characterization of γ-In₂Se₃ thin films. The predicted band gap of 1.84 eV for γ-In₂Se₃ is in good agreement with the absorption onset derived from transmission and reflection spectra of thin films. We show that incorporation of Te gives γ-In₂(Se_1-xTe_x)₃ alloys with a band gap ranging from 1.84 eV down to 1.23 eV, thus covering the optimal band gap range for single-junction solar cells. In addition, the γ-In₂Se₃/γ-In₂(Se_1-xTe_x)₃ bilayer could be employed in tandem solar-cell architectures absorbing at E_g ≈ 1.8 eV and at E_g ≤ 1.4 eV, toward overcoming the ∼33% efficiency set by the Shockley-Queisser limit for single junction solar cells. We also discuss band gap bowing and mixing enthalpies, aiming at adding γ-In₂Se₃, γ-In₂Te₃, and γ-In₂(Se_1-xTe_x)₃ alloys to the available toolbox of materials for solar cells and other optoelectronic applications.

Synergetic Effects of Zn Alloying and Defect Engineering on Improving the CdS Buffer Layer of Cu2ZnSnS4 Solar Cells

The inferior electrical properties at the interface of the Cu₂ZnSnS₄/CdS (CZTS/CdS) heterojunction resulting in the severe loss of open-circuit voltage (V_oc) highly restrict the photovoltaic efficiency of CZTS solar cell devices. Here, first-principles calculations show that the Zn-alloyed CdS buffer layer reverses the unfavorable cliff-like conduction band offset (CBO) of CZTS/CdS to the desirable spike-like CBO of CZTS/Zn_0.25Cd_0.75S, which suppresses carrier nonradiative recombination and blocks electron backflow. In addition, the weakened n-type conductivity of Zn_0.25Cd_0.75S can be enhanced by In, Ga, and Cl doping without the introduction of detrimental deep-level defects and severe band-tail states, which improves the V_oc of CZTS solar cells by promoting strong band bending and large quasi-Fermi-level splitting at the absorber side of the CZTS/Zn_0.25Cd_0.75S heterojunction. This study finds that the synergetic effects of Zn alloying and defect engineering on the CdS buffer layer are promising for overcoming the long-standing issue of the V_oc deficit in CZTS solar cells, and understanding the optimized interfacial electrical properties provides theoretical guidance for improving the efficiency of semiconductor devices.

Dynamics of the nanocrystal structure and composition in growth solutions monitored by in situ lab-scale X-ray diffraction

In situ characterization of nanoparticle (NP) growth has become the state-of-the-art approach for studying their growth mechanisms; there is broad consensus on the reliability and precision of in situ characterization techniques compared to more traditional ex situ ones. Nonetheless, most of the currently available methods require the use of sophisticated setups such as synchrotron-based X-ray sources or an environmental liquid transmission electron microscopy (TEM) cell, which are expensive and not readily accessible. Herein, we suggest a new approach to study NP growth mechanisms: using a commercially available heating chamber for time-resolved X-ray diffraction (TR-XRD) measurements of NP growth in solution. We demonstrate how this lab-scale in situ XRD can be used to study NP growth mechanisms when complemented by standard ex situ techniques such as TEM and UV-vis spectroscopy. TR-XRD reveals the crystallographic phase and real-time evolution of NP size, shape, and composition. A detailed analysis allows determining the growth mechanism and measuring the alloying kinetics of multinary nanocrystals, demonstrated herein for a colloidal Cd_xZn_1-xS system. This approach proves itself as a promising strategy for NP growth research and could be expanded to related fields that study dynamic changes as the formation and evolution of crystalline materials in solutions.

Interface Engineering of Cu(In,Ga)Se2 Solar Cells by Optimizing Cd- and Zn-Chalcogenide Alloys as the Buffer Layer

The optimization of band alignment at the buffer/absorber interface is realized by tuning compositions of Cd and Zn chalcogenides as the buffer layer toward high-efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. Using the special quasi-random structure (SQS) approach, we construct randomly disordered Zn_xCd_1-xS_ySe_1-y alloys and ZnS_xO_1-x alloys as alternatives to the traditional CdS buffer layer. The compositional dependence of formation energies, lattice parameters, band-gap energies, and band alignments of Zn_xCd_1-xS_ySe_1-y and ZnS_xO_1-x alloys is investigated by first-principles density functional theory calculations. For quaternary Zn_xCd_1-xS_ySe_1-y alloys, we find that the miscibility temperatures and the bandgap bowing coefficients are proportional to the lattice mismatch between the mixing elements. The linear dependence of lattice parameters, trinomial dependence of band-gap energies and band-edge positions on the alloy-composition of Zn_xCd_1-xS_ySe_1-y alloys are established. For ZnS_xO_1-x alloys, we find the lattice parameters also exhibit a linear dependence on its composition. Because of the large lattice mismatch and the chemical disparity between ZnO and ZnS, the bowing coefficient for the bandgap energies of ZnS_xO_1-x alloys is composition dependent, and is larger for dilute ZnS_xO_1-x alloys. With the optimization criteria of moderate spike-like conduction band offset, large valance band offset, sufficiently wide bandgap, and lattice match with respect to the CIGS absorber, we illustrate the optimal composition range of both Zn_xCd_1-xS_ySe_1-y alloys and ZnS_xO_1-x alloys as the buffer layer of the CIGS solar cells. Our work demonstrates that Zn_xCd_1-xS_ySe_1-y alloys and ZnS_xO_1-x alloys are promising buffer layers for high-efficiency CIGS solar cells.

Identification of Potential Optoelectronic Applications for Metal Thiophosphates

Metal thiophosphates are a large family of compounds that received far less attention than conventional chalcogenides. Recently, however, metal thiophosphates arouse research interest in regard of energy harvesting and conversion due to their structural and chemical diversity. Nevertheless, there remain many unexplored metal thiophosphates. Here, we performed a comprehensive investigation on the electronic and optoelectronic properties of a series of metal thiophosphates using first-principles calculations and identified several highly promising compounds as p-type transparent conductors, photovoltaic absorbers, and single visible-light-driven photocatalysts for water splitting. Our investigation reveals the intrinsic features of a series of typical metal thiophosphates, identifies their new optoelectronic applications, and validates that metal thiophosphates are promising materials deserving exploration.

Copper sulfide nano-globules reinforced electrodes for high-performance electrochemical determination of toxic pollutant hydroquinone

A high-performance electrochemical sensing platform based on CuS nano-globules is efficiently developed.

Exploring different photocatalytic behaviors of CdxZn1−xSeyS1−y gradient-alloyed quantum dots via composition regulation

Different photocatalytic behaviors of CdxZn1−xSeyS1−y gradient alloyed quantum dots via composition regulation.

Enhancement of photovoltaic efficiency in CdSe x Te1-x (where 0 ⩽ x ⩽ 1): insights from density functional theory

Recent advancements in CdTe photovoltaic efficiency have come from selenium grading, which reduces the band gap and significantly improves carrier lifetimes. In this work, density functional theory calculations were performed to understand the structural and electronic effects of Se alloying. Special quasirandom structures were used to simulate a random distribution of Se anions. Lattice parameters decrease linearly as Se concentration increases in line with Vegard's Law. The simulated band gap bowing shows strong agreement with experimental values. Selenium, by itself, does not introduce any defect states in the band gap and no significant changes to band structure around the [Formula: see text] point are found. Band offset values suggest a reduction of recombination across the CdSeTe/MgZnO interface at [Formula: see text], which corresponds with the Se concentration used experimentally. Band structure analysis of two cases [Formula: see text] and x  =  0.4375, shows a change from dominant Cd/Te contributions in the conduction band minimum to Cd/Se contributions as Se concentration is increased, hinting at a change in optical transition characteristics. Further calculations of optical absorption spectra suggest a reduced transition probability particularly at higher energies, which confirms experimental predictions that Se passivates the non-radiative recombination centres.

Giant defect emission enhancement from ZnO nanowires through desulfurization process

Zinc oxide (ZnO) is a stable, direct bandgap semiconductor emitting in the UV with a multitude of technical applications. It is well known that ZnO emission can be shifted into the green for visible light applications through the introduction of defects. However, generating consistent and efficient green emission through this process is challenging, particularly given that the chemical or atomic origin of the green emission in ZnO is still under debate. In this work we present a new method, for which we coin term desulfurization, for creating green emitting ZnO with significantly enhanced quantum efficiency. Solution grown ZnO nanowires are partially converted to ZnS, then desulfurized back to ZnO, resulting in a highly controlled concentration of oxygen defects as determined by X-ray photoelectron spectroscopy and electron paramagnetic resonance. Using this controlled placement of oxygen vacancies we observe a greater than 40-fold enhancement of integrated emission intensity and explore the nature of this enhancement through low temperature photoluminescence experiments.

Orbital hybridization-induced band offset phenomena in NixCd1-xO thin films

Herein, we present the cationic impurity-assisted band offset phenomena in NixCd1-xO (x = 0, 0.02, 0.05, 0.1, 0.2, 0.4, 0.8, and 1) thin films and further discuss them based on orbital hybridization modification. The compositional and structural studies revealed that the cationic substitution of Cd2+ by Ni2+ ions leads to a monotonic shift in the (220) diffraction peak, indicating the suppression of lattice distortion, while the evolution of local strain with an increase in Ni concentration is mainly associated with the mismatch in the electronegativity of the Cd2+ and Ni2+ ions. In fact, Fermi level pinning towards the conduction band minimum takes place with an increase in the Ni concentration at the cost of electronically compensated oxygen vacancies, resulting in the modification of the distribution of carrier concentration, which eventually affects the band edge effective mass of the conduction band electrons and further endorses band gap renormalization. Besides, the appearance of a longitudinal optical (LO) mode at 477 cm-1, as manifested by Raman spectroscopy, also indicates the active involvement of electron-phonon scattering, whereas modification in the local coordination environment, particularly anti-crossing interaction in conjunction with the presence of satellite features and shake-up states with Ni doping, was confirmed by X-ray absorption near-edge and X-ray photoelectron spectroscopy studies. These results manifest the gradual reduction of orbital hybridization upon the incorporation of Ni, leading to a decrement in the band edge effective electron mass. Finally, the molecular dynamics simulation reflected a 13% reduction in the lattice parameter for the NiO thin film compared to the undoped film, while the projected density of states calculation further supports the experimental observation of reduced orbital hybridization with an increase in Ni concentration.

DFT Computed Dielectric Response and THz Spectra of Organic Co-Crystals and Their Constituent Components

Terahertz (THz) spectroscopy has been put forth as a non-contact, analytical probe to characterize the intermolecular interactions of biologically active molecules, specifically as a way to understand, better develop, and use active pharmaceutical ingredients. An obstacle towards fully utilizing this technique as a probe is the need to couple features in the THz regions to specific vibrational modes and interactions. One solution is to use density functional theory (DFT) methods to assign specific vibrational modes to signals in the THz region, coupling atomistic insights to spectral features. Here, we use open source planewave DFT packages that employ ultrasoft pseudopotentials to assess the infrared (IR) response of organic compounds and complex co-crystal formulations in the solid state, with and without dispersion corrections. We compare our DFT computed lattice parameters and vibrational modes to experiment and comment on how to improve the agreement between theory and modeling to allow for THz spectroscopy to be used as an analytical probe in complex biologically relevant systems.

Bandgap Engineering of Lead-Free Double Perovskite Cs2 AgBiBr6 through Trivalent Metal Alloying

The double perovskite family, A₂ M^I M^III X₆ , is a promising route to overcome the lead toxicity issue confronting the current photovoltaic (PV) standout, CH₃ NH₃ PbI₃ . Given the generally large indirect band gap within most known double perovskites, band-gap engineering provides an important approach for targeting outstanding PV performance within this family. Using Cs₂ AgBiBr₆ as host, band-gap engineering through alloying of In^III /Sb^III has been demonstrated in the current work. Cs₂ Ag(Bi_1-x M_x )Br₆ (M=In, Sb) accommodates up to 75 % In^III with increased band gap, and up to 37.5 % Sb^III with reduced band gap; that is, enabling ca. 0.41 eV band gap modulation through introduction of the two metals, with smallest value of 1.86 eV for Cs₂ Ag(Bi_0.625 Sb_0.375 )Br₆ . Band structure calculations indicate that opposite band gap shift directions associated with Sb/In substitution arise from different atomic configurations for these atoms. Associated photoluminescence and environmental stability of the three-metal systems are also assessed.

Tunable interaction between metal clusters and graphene

Using first principles calculations we find that the interaction between small transition metal clusters and graphene follows the d-band model.

Size-confined fixed-composition and composition-dependent engineered band gap alloying induces different internal structures in L-cysteine-capped alloyed quaternary CdZnTeS quantum dots

The development of alloyed quantum dot (QD) nanocrystals with attractive optical properties for a wide array of chemical and biological applications is a growing research field. In this work, size-tunable engineered band gap composition-dependent alloying and fixed-composition alloying were employed to fabricate new L-cysteine-capped alloyed quaternary CdZnTeS QDs exhibiting different internal structures. Lattice parameters simulated based on powder X-ray diffraction (PXRD) revealed the internal structure of the composition-dependent alloyed CdxZnyTeS QDs to have a gradient nature, whereas the fixed-composition alloyed QDs exhibited a homogenous internal structure. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) analysis confirmed the size-confined nature and monodispersity of the alloyed nanocrystals. The zeta potential values were within the accepted range of colloidal stability. Circular dichroism (CD) analysis showed that the surface-capped L-cysteine ligand induced electronic and conformational chiroptical changes in the alloyed nanocrystals. The photoluminescence (PL) quantum yield (QY) values of the gradient alloyed QDs were 27-61%, whereas for the homogenous alloyed QDs, the PL QY values were spectacularly high (72-93%). Our work demonstrates that engineered fixed alloying produces homogenous QD nanocrystals with higher PL QY than composition-dependent alloying.

Isoelectronically doped CdSe/Te nanoalloys as alternative solar cell materials: insight from computational analysis

CdSe/Te nanoalloy as a solar energy harvesting material.

Resonant Raman scattering of ZnSxSe1−x solid solutions: the role of S and Se electronic states

A combined theoretical and experimental study of the enhancement in the Raman mode intensities of ZnSSe compounds, under various resonant conditions, is presented, leading to more detailed insights into the role of chalcogen electronic states in the photon–matter interaction.

First-principles calculations of structural, electronic, and thermodynamic properties of monolayer Si1−xGexC sheet

The bowing coefficient of structural parameters is calculated. A band gap transition is also observed. The T–x phase diagram is calculated and shows a critical temperature of 187.4 K.

Gradient band gap engineered alloyed quaternary/ternary CdZnSeS/ZnSeS quantum dots: an ultrasensitive fluorescence reporter in a conjugated molecular beacon system for the biosensing of influenza virus RNA

Composition-dependent alloyed CdZnSeS/ZnSeS QDs were synthesized and used as a fluorescent reporter in a molecular beacon assay to detect influenza virus RNA.

Hydrazine solution processed Sb2S3, Sb2Se3 and Sb2(S(1-x)Se(x))3 film: molecular precursor identification, film fabrication and band gap tuning

Sb₂(S_1−xSe_x)₃ (0 ≤ x ≤ 1) compounds have been proposed as promising light-absorbing materials for photovoltaic device applications. However, no systematic study on the synthesis and characterization of polycrystalline Sb₂(S_1−xSe_x)₃ thin films has been reported. Here, using a hydrazine based solution process, single-phase Sb₂(S_1−xSe_x)₃ films were successfully obtained. Through Raman spectroscopy, we have investigated the dissolution mechanism of Sb in hydrazine: 1) the reaction between Sb and S/Se yields [Sb₄S₇]^2-/[Sb₄Se₇]^2- ions within their respective solutions; 2) in the Sb-S-Se precursor solutions, Sb, S, and Se were mixed on a molecular level, facilitating the formation of highly uniform polycrystalline Sb₂(S_1−xSe_x)₃ thin films at a relatively low temperature. UV-vis-NIR transmission spectroscopy revealed that the band gap of Sb₂(S_1−xSe_x)₃ alloy films had a quadratical relationship with the Se concentration x and it followed the equation , where the bowing parameter was 0.118 eV. Our study provides a valuable guidance for the adjustment and optimization of the band gap in hydrazine solution processed Sb₂(S_1−xSe_x)₃ alloy films for the future fabrication of improved photovoltaic devices.

Compositional effects and optical properties of CdSeXTe1−X alloyed nanotube arrays

The compositional effects on the cell constant, absorption and Raman scattering were demonstrated for well-aligned CdSe_XTe_1−X (0 ≤ X ≤ 1) nanotube arrays on ITO.

Tunable Electronic Properties of Two-Dimensional Transition Metal Dichalcogenide Alloys: A First-Principles Prediction

We investigated the composition-dependent electronic properties of two-dimensional transition-metal dichalcogenide alloys (WxMo1-xS2) based on first-principles calculations by applying the supercell method and effective band structure approximation. It was found that hole effective mass decreases linearly with increasing W composition, and electron effective mass of alloys is always larger than that of their binary constituents. The different behaviors of electrons and holes in alloys are attributed to the fact that metal d-orbitals have different contributions to conduction bands of MoS2 and WS2 but almost identical contributions to valence bands. We examined the conduction polarity of WxMo1-xS2 monolayer alloys with four metal electrode materials (Au, Ag, Cu, and Pd). It suggests the main carrier type for transport in transistors could change from electrons to holes as W composition increases if high work function metal contacts were used. The tunable electronic properties of two-dimensional transition-metal dichalcogenide alloys make them attractive for electronic and optoelectronic applications.

Ternary alloy nanocrystals of tin and germanium chalcogenides

Sn_xGe_1−xS, Sn_xGe_1−xSe, GeS_xSe_1−x, and SnS_xSe_1−x alloy nanocrystals were synthesized by novel gas-phase laser photolysis. Their composition-dependent lattice parameters and band gap were thoroughly characterized. The Sn_xGe_1−xS and SnS_xSe_1−x nanocrystals exhibit higher photoconversion efficiency as compared with the end members.

Facile synthesis of ternary homogeneous ZnS1−xSex nanosheets with tunable bandgaps

Homogeneous ternary ZnS_1−xSe_x nanosheets were easily fabricated through thermal decomposition of lamellar inorganic–organic hybrid precursors; their complete composition and bandgap tunability are demonstrated.

Near infrared absorption of CdSe(x)Te(1-x) alloyed quantum dot sensitized solar cells with more than 6% efficiency and high stability

CdSe0.45Te0.55 alloyed quantum dots (QDs) with excitonic absorption onset at 800 nm and particle size of 5.2 nm were prepared via a noninjection high-temperature pyrolysis route and used as a sensitizer in solar cells. A postsynthesis assembly approach with use of bifunctional linker molecule mercaptopropionic acid (MPA) capped water-soluble QDs, obtained via ex situ ligand exchange from the initial oil-dispersible QDs, was adopted for tethering QDs onto mesoporous TiO2 film. With the combination of high loading of the QD sensitizer and intrinsic superior optoelectronic properties (wide absorption range, high conduction band edge, high chemical stability, etc., relative to their constituents CdSe and CdTe) of the adopted CdSe0.45Te0.55 QD sensitizer, the resulting CdSexTe1-x alloyed QD-based solar cells exhibit a record conversion efficiency of 6.36% (Jsc = 19.35 mA/cm(2), Voc = 0.571 V, FF = 0.575) under full 1 sun illumination, which is remarkably better than that of the reference CdSe and CdTe QD based ones. Furthermore, the solar cells with Cu2S counter electrodes based on eletrodeposition of Cu on conductive glass show long-term (more than 500 h) stability.

One-step preparation and assembly of aqueous colloidal CdS(x)Se(1-x) nanocrystals within mesoporous TiO2 films for quantum dot-sensitized solar cells

In the field of quantum dots (QDs)-sensitized solar cells, semiconductor QDs sensitizer with a moderate band gap is required in order to sufficiently match the solar spectrum and achieve efficient charge separation. At present, changing the size of QDs is the main method used for adjusting their band gap through quantum size effect, however, the pore sizes of mesoporous TiO2 film set a limit on the allowed size of QDs. Therefore, the tuning of electronic and optical properties by changing the particle size could be limited under some circumstances. In this paper, high-quality aqueous CdS(x)Se(1-x) QDs sensitizer is successfully synthesized and effectively deposited on a mesoporous TiO2 film by a one-step hydrothermal method. In addition to size, alloy QDs provide composition as an additional dimension for tailoring their electronic properties. The alloy composition and band gap can be precisely controlled by tuning the precursor (Se/Na2S·9H2O) ratio while maintaining the similar particle size. By using such CdS(x)Se(1-x) sensitized TiO2 films as photoanodes for solar cell, a maximum power conversion efficiency of 2.23% is achieved under one sun illumination (AM 1.5 G, 100 mW cm(-2)).

Cu(In(1-x)Ga(x))S2 nanocrystals and films: low-temperature synthesis with size and composition control

We demonstrate a single-step X-ray irradiation process that yields high-quality Cu(In1-xGax)S2 nanocrystals in colloidal solutions, with complete control of size and composition. Thin films produced by drop-casting exhibit high-quality photoresponse, confirming that our process is suitable for microelectronics applications.

Study of surface and bulk electronic structure of II-VI semiconductor nanocrystals using Cu as a nanosensor

Efficiency of the quantum dots based solar cells relies on charge transfer at the interface and hence on the relative alignment of the energy levels between materials. Despite a high demand to obtain size specific band offsets, very few studies exist where meticulous methods like photoelectron spectroscopy are used. However, semiconductor charging during measurements could result in indirect and possibly inaccurate measurements due to shift in valence and conduction band position. Here, in this report, we devise a novel method to study the band offsets by associating an atomic like state with the conduction band and hence obtaining an internal standard. This is achieved by doping copper in semiconductor nanocrystals, leading to the development of a characteristic intragap Cu-related emission feature assigned to the transition from the conduction band to the atomic-like Cu d state. Using this transition we determine the relative band alignment of II-VI semiconductor nanocrystals as a function of size in the below 10 nm size regime. The results are in excellent agreement with the available photoelectron spectroscopy data as well as the theoretical data. We further use this technique to study the excitonic band edge variation as a function of temperature in CdSe nanocrystals. Additionally, surface electronic structure of CdSe nanocrystals have been studied using quantitative measurements of absolute quantum yield and PL decay studies of the Cu related emission and the excitonic emission. The role of TOP and oleic acid as surface passivating ligand molecules has been studied for the first time.

Structural and electronic contributions to the bandgap bowing of (In,Ga)P alloys

The variation of atomic configurations and their corresponding structural parameters, such as first nearest neighbor distances, is a characteristic feature of ternary semiconductor alloys with zincblende structure. It has a strong influence on important material properties, most prominently the bandgap energy, and can contribute to a nonlinear behavior with changing alloy composition. Using (In,Ga)P as a model system, the atomic-scale structure has been modeled for all possible first nearest neighbor configurations based on experimentally determined structural parameters. While the average position of the P anion corresponds to the ideal lattice site, the average distance from this ideal lattice site does not vanish even in the random alloy. Based on this average anion displacement, the contribution to the bandgap bowing caused by structural relaxation of the alloy was calculated over the whole compositional range. Furthermore, the bowing contribution arising from the overall change of the In-P and Ga-P distances with respect to the binary values was determined. Thus, a clear distinction between the bandgap bowing caused by structural effects on the one hand and by charge redistribution on the other hand is possible.

Controlled synthesis of compositionally tunable ternary PbSe(x)S(1-x) as well as binary PbSe and PbS nanowires

High-quality compositionally tunable ternary PbSe(x)S(1-x) (x = 0.23, 0.39, 0.49, 0.68, and 0.90) nanowires (NWs) and their binary analogues have been grown using solution-liquid-solid growth with lead(II) diethyldithiocarbamate, Pb(S(2)CNEt(2))(2), and lead(II) imido(bis(selenodiisopropylphosphinate)), Pb((SeP(i)Pr(2))(2)N)(2), as single-source precursors. The alloyed nature of PbSe(x)S(1-x) wires was confirmed using ensemble X-ray diffraction and energy dispersive X-ray spectroscopy (EDXS). Single NW EDXS line scans taken along the length of individual wires show no compositional gradients. NW compositions were independently confirmed using inductively coupled plasma atomic emission spectroscopy. Slight stoichiometric deviations occur but never exceed 13.3% of the expected composition, based on the amount of introduced precursor. In all cases, resulting nanowires have been characterized using transmission electron microscopy. Mean diameters are between 9 and 15 nm with accompanying lengths that range from 4 to 10 μm. Associated selected area electron diffraction patterns indicate that the PbSe(x)S(1-x), PbSe, and PbS NWs all possess the same <002> growth direction, with diffraction patterns consistent with an underlying rock salt crystal structure.

Arrays of ZnO/Zn(x)Cd(1-x)Se nanocables: band gap engineering and photovoltaic applications

Arrays of ZnO/Zn(x)Cd(1-x)Se (0 ≤ x ≤ 1) core/shell nanocables with shells of tunable compositions have been synthesized on fluorine-doped tin oxide glass substrates via a simple ion-exchange approach. Through the effects of stoichiometry and type II heterojunction, optical absorptions of the nanocable arrays can be controllably tuned to cover almost the entire visible spectrum. Lattice parameters and band gaps of the ternary Zn(x)Cd(1-x)Se shells were found to have respectively linear and quadratic relationships with the Zn content (x). These ZnO/Zn(x)Cd(1-x)Se nanocable arrays are further demonstrated to be promising photoelectrodes for photoelectrochemical solar cells, giving a maximum power conversion efficiency up to 4.74%.

Tuning the synthesis of ternary lead chalcogenide quantum dots by balancing precursor reactivity

We report the synthesis and characterization of composition-tunable ternary lead chalcogenide alloys PbSe(x)Te(1-x), PbS(x)Te(1-x), and PbS(x)Se(1-x). This work explores the relative reaction rates of chalcogenide precursors to produce alloyed quantum dots (QDs), and we find the highly reactive bis(trimethylsilyl) (TMS(2))-based precursors allow for the homogeneous incorporation of anions. By varying the Pb to oleic acid ratio, we demonstrate size control of similar composition alloys. We find the resulting QDs are Pb-rich but the Pb/anion ratio is size- and composition-dependent in all alloyed QD as well as in PbSe, PbTe, and PbS QDs and is consistent with the reaction rates of the anion precursors. A more reactive anion precursor results in a lower Pb/anion ratio.

Simple point-ion electrostatic model explains the cation distribution in spinel oxides

The A2BO4 spinel oxides are distinguished by having either a normal (N) or an inverse (I) distribution of the A, B cations on their sublattices. A point-ion electrostatic model parametrized by the oxygen displacement parameter u and by the relative cation valencies Z{A} vs Z{B} provides a simple rule for the structural preference for N or I: if Z{A}>Z{B} the structure is normal for u>0.2592 and inverse for u<0.2578, while if Z{A}<Z{B} the structure is normal for u<0.2550 and inverse for u>0.2578. This rule is illustrated for the known spinel oxides, proving to be ∼98% successful.