Rocksalt CdO Research Articles

Structural and Photocatalytic Analysis of Nanostructured CdO, ZnO and their Composite Useful to Remove Textile Dyes Waste from the Drainage System

In recent years, the fabrication of semiconductor nanostructures has increasingly been adopted as the stimulating mechanism in nanoscience and nanotechnology. The comprehensive investigation and environmental impact of CdO, ZnO, and their nanocomposite executed in this direction with the help of a profitable co-precipitate approach at room temperature. In the prepared sample, the diffractogram is having rock salt CdO and wurtzite ZnO crystalline phase while the composite sample has combinedpeaks of both.We have investigated the modified intensity, FWHM, crystalline size, and microstrain present in the synthesized samples. The texture of the surface is evaluated by SEM micrographs for prepared nanocrystallites. In the present study, Rhodamine B dye is being selected for decomposition and investigated the catalytic efficiency of prepared samples under visible light.

Amorphous CdO-In2O3 alloy thin films with high conductivity and transparency synthesized by sol-gel method

The growth of CdO-In2O3 amorphous oxide semiconductor alloys with high mobility in a wide composition range, suitable for many optoelectronic applications, has been previously reported using room temperature sputtering. In the present work, we synthesized Cd1−xInxO1+δ alloy thin films across the entire composition range using the non-vacuum, low-cost sol-gel spin coating method. Structural, electrical and optical properties of these alloy films were studied using a variety of analytical tools. We find that the alloy films become amorphous within the In composition range of 0.2 <x < 0.7, while films with x < 0.2 and x > 0.7 have the rocksalt CdO and bixbyite In2O3 structure, respectively. As-grown amorphous alloys have resistivity in the ranging from high 10−3 to low 10−2 Ω-cm due to their relatively low mobility of<10 cm2/Vs. Electrical properties of these amorphous films are improved after annealing at ~400 °C with a mobility of ~10 cm2/Vs and a low resistivity in the low 10−3 Ω-cm range. The optical gap varies from 2.54 to 3.52 eV with increasing In mole fraction, so that transmittance> 85% over a wide transmission window of ~400–1600 nm can be achieved. The low temperature, low-cost non-vacuum processing, wide bandgap tunability, and low resistivity, make these sol-gel alloy films promising materials for optoelectronic applications, especially on organic layers and/or flexible substrates.

Crystallographic and electro-optic analysis of pure and Cu/Mn-doped Cd0.6Zn0.4O ternary alloy: Role of the defect states and imperfection density

Pure and Cu/Mn-doped Cd0.4Zn0.6O (0 ≤ x ≤ 1) ternary alloy is prepared by co-precipitation and further the variation of dopant concentration has been studied. The diffractogram of the undoped sample has peaks of both rocksalt (RS) CdO and wurtzite (WZ) ZnO phase. The individual and simultaneous doping of Cu and Mn elements has significantly altered the intensity, FWHM, and developments of defect states in Cd0.4Zn0.6O alloy. TEM microscope has investigated the crystalline size and interplanar spacing that is slightly decreased with dopant concentration which lies within the range ~ 38–30 nm. UV–Vis spectroscopy analyzed a decreasing trend in bandgap energy with dopant concentration. IN PL, the near band edge and deep trap state emission is observed at 382, 417, 448, 520 and 580 nm wavelengths. The resistivity, carrier concentration, and mobility are also computed which proposed that Cu/Mn-doped Cd0.4Zn0.6O alloy could be the better TCO with new opportunities for material science researchers.

Zinc content (x) induced impact on crystallographic, optoelectronic, and photocatalytic parameters of Cd1-xZnxO (0 ≤ x ≤ 1) ternary nanopowder

We have synthesized Cd1-xZnxO (0 ≤ x ≤ 1) nanopowder by co-precipitation method for its comprehensive study. The Crystalline phase is changed from rocksalt CdO to wurtzite ZnO with an increase in ‘x’. The existence of H2O, COO–, CdO, and ZnO group are confirmed by FTIR. SEM analysis investigated the spherical structure for x = 0.0 & 0.2 and flakes morphology for x = 0.4 & 0.6 and hexagonal block structure for x = 0.8 & 1.0. UV–Vis spectroscopy insured the increasing trend in bandgap energy within the range 2.5 ≤ Eg ≤ 3.5 eV. PL results revealed a blue shift in excitonic and band edge emission. Methyl blue dye is being selected for decomposition using different samples of Cd1-xZnxO (0 ≤ x ≤ 1) nanopowder. It is found that sample with x = 0.8 has the highest catalytic efficiency under UV light. Moreover, higher efficiency of the ternary system is managed by its highly crystalline structure, less microstrain, large surface/volume ratio, and optical bandgap.

Exploring novel phases of Cd–O system at ambient pressure

First-principles evolutionary searches are used to explore stable Cd–O compounds at ambient pressure. Besides the well-known rock-salt CdO, a new cubic thermodynamically stable phase CdO2 with space group Pa3 and two metastable compounds: zinc-blende and wurtzite phases of CdO have been discovered at ambient pressure. Among these, CdO2 was successfully synthesized with perfect structural agreement to our theoretical predictions. The global stability of the well known rock-salt phase of CdO, our calculations of elastic constants and phonon dispersion curves demonstrate that the Pa3, zinc-blende and wurtzite structures are mechanically and dynamically stable. Finally, the state-of-the-art LDA-1/2 methods reveal that at ambient conditions, zinc-blende and wurtzite phases are semiconducting with a direct band gap (Γ–Γ) of 0.89 eV and 0.97 eV respectively; whereas the semiconducting cubic-Pa3 structure shows direct band gap (Γ–Γ) of ∼2.93 eV and an indirect band gap of ∼2.58 eV agreeing well with the experimental value of ∼2.4 eV.

Influence of Cd doping on structural and optical properties of (Cd, Al)-codoped ZnO powders synthesized via sol–gel method

The effect of Cd doping on structural and optical properties of 1at.% Al-doped Zn1−xCdxO (x=0–8%) powders prepared by sol–gel method was systematically investigated. X-ray diffraction (XRD) patterns revealed the powders retained a hexagonal wurtzite structure of ZnO below x=6%, while cubic rocksalt CdO appeared when x=8%. The bandgap and near band emission (NBE) energies determined from ultraviolet–visible (UV–vis) absorbance and photoluminescence (PL) spectra, respectively, decreased linearly with increasing Cd content up to x=6%, it seemed that the Cd worked effectively on ZnO bandgap engineering. However, the Stokes shift unexpectedly decreased from 136meV to 33meV with increasing Cd content. According to the fluctuation of integrated intensity of Cd 3d5/2 X-ray photoelectron spectroscopy (XPS), and redshift for x⩽4% and then blueshift for x>4% of the broadening A1(LO)/E1(LO) Raman mode, it was concluded that CdO impurity might also exist in the sample of x=6% at least. The redshift of bandgap and NBE could not be simply ascribed to bandgap shrinkage of Zn1−xCdxO powders for x>4%, but also due to the existence of CdO impurity with narrower bandgap of 2.3eV.

How Robust are Semiconductor Nanorods? Investigating the Stability and Chemical Decomposition Pathways of Photoactive Nanocrystals

Anisotropic II–VI semiconductor nanostructures are important photoactive materials for various energy conversion and optical applications. However, aside from the many available surface chemistry studies and from their ubiquitous photodegradation under continuous illumination, the general chemical reactivity and thermal stability (phase and shape transformations) of these materials are poorly understood. Using CdSe and CdS nanorods as model systems, we have investigated the behavior of II–VI semiconductor nanorods against various conditions of extreme chemical and physical stress (acids, bases, oxidants, reductants, and heat). CdSe nanorods react rapidly with acids, becoming oxidized to Se or SeO2. In contrast, CdSe nanorods remain mostly unreactive when treated with bases or strong oxidants, although bases do partially etch the tips of the nanorods (along their axis). Roasting (heating in air) of CdSe nanorods results in rock-salt CdO, but neither CdSe nor CdO is easily reduced by hydrogen (H2). Another reductant, n-BuLi, reduces CdSe nanorods to metallic Cd. Variable temperature X-ray diffraction experiments show that axial annealing and selective axial melting of the nanorods precede particle coalescence. Furthermore, thermal analysis shows that the axial melting of II–VI nanorods is a ligand-dependent process. In agreement with chemical reactivity and thermal stability observations, silica-coating experiments show that the sharpest (most curved) II–VI surfaces are most active against heterogeneous nucleation of a silica shell. These results provide valuable insights into the fate and possible ways to enhance the stability and improve the use of II–VI semiconductor nanostructures in the fields of optics, magnetism, and energy conversion.

Fermi level stabilization and band edge energies in CdxZn1−xO alloys

We have measured the band edge energies of CdxZn1−xO thin films as a function of composition by three independent techniques: we determine the Fermi level stabilization energy by pinning the Fermi level with ion irradiation, measure the binding energy of valence band states and core levels by X-ray photoelectron spectroscopy, and probe shifts in the conduction band and valence band density of states using soft X-ray absorption and emission spectroscopy, respectively. The three techniques find consensus in explaining the origin of compositional trends in the optical-bandgap narrowing upon Cd incorporation in wurtzite ZnO and widening upon Zn incorporation in rocksalt CdO. The conduction band minimum is found to be stationary for both wurtzite and rocksalt alloys, and a significant upward rise of the valence band maximum accounts for the majority of these observed bandgap changes. Given these band alignments, alloy disorder scattering is found to play a negligible role in decreasing the electron mobility for all alloys. These band alignment details, combined with the unique optical and electrical properties of the two phase regimes, make CdZnO alloys attractive candidates for photoelectrochemical water splitting applications.

Pressure-induced instability in CdO

The lattice dynamics of rocksalt CdO is studied as a function of pressure using density-functional linear-response theory. The predicted static phase-transition pressure from rocksalt (B1) to CsCl (B2) structures is 76.12 GPa. Analysis of the phonon calculations suggests that the pressure-induced instabilities at p = 78.26 GPa of the transverse acoustic (TA) modes in a large portion of the Brillouin zone are responsible for the phase transition from B1 → B2 phases of CdO. Moreover, the calculated partial phonon density of states at the precursor phase transition of p = 70 GPa reveals strong reduction of the longitudinal acoustic Cd-like modes and the existence of a large peak related to the longitudinal optic O-like mode.

Electronic, transport, optical, and structural properties of rocksalt CdO

We report electronic, optical, and structural properties of rocksalt CdO as obtained from first-principle calculations with both the Tran-Blaha modified Becke-Johnson potential using linearized augmented planewave method in WIEN2k and local density approximation (LDA) potential using the LDA Bagayoko-Zhao-Williams-Ekuma-Franklin (BZW-EF) method in implementing the linear combination of Gaussian orbitals. The results are discussed in relation to existing experimental data, particularly to the Burstein–Moss effect.

Electronic and Optical Properties of Rocksalt CdO: A first-Principles Density-Functional Theory Study

The structural, electronic and optical properties of rocksalt CdO have been studied using the plane-wave-based pseudo-potential density functional theory within generalized gradient approximation. The calculated lattice parameters are in agreement with previous experimental work. The band structure, density of states, and Mulliken charge population are obtained, which indicates that rocksalt CdO having the properties of a halfmetal due to an indirect band gap of -0.51eV. The mechanical properties show that rocksalt CdO is mechanically stable, isotropic and malleable. Significantly, we propose a correct value for e1(0) of about 4.75, which offers theoretical data for the design and application for rocksalt CdO in optoelectronic materials.

Anisotropic epitaxial ZnO/CdO core/shell heterostructure nanorods

Various surface structures and polarities of one-dimensional nanostructures offer additional control in synthesizing heterostructures suitable for optoelectronic and electronic applications. In this work, we report synthesis and characterization of ZnO-CdO nanorod-based heterostructures grown on a-plane sapphire. The heterojunction formed on the sidewall surface of the nanorod shows that wurtzite ZnO {1010} planes are interfaced with rocksalt CdO {100}. This is evidently different from the heterojunction formed on the nanorod top surface, where a ZnO (0001) top plane is interfaced with a CdO (111) plane. Such anisotropic heterostructures are determined by different surface structures of the nanorods and their polarities. Revelation of such anisotropic heterojunctions will provide a clue for understanding charge transport properties in electronic and optoelectronic nanodevices.

Annealing of ion implanted CdZnO

We have studied the effect of the Cd content on the recovery of ion-induced damage in wurtzite CdxZn1−xO (x ⩽ 0.05) films and compared with that in pure wurtzite ZnO and rock-salt CdO. 200 keV Au+ and 55 keV Ar+ ion implants were performed at room temperature in the dose range of 5 × 1014–6.5 × 1015 cm−2. Rutherford backscattering/channelling spectrometry was used to characterize the damage evolution in the course of annealing (600–900 °C in air). A complex defect annealing behaviour is revealed in CdZnO as a function of annealing temperature, Cd content and ion dose. In particular, defects in the low dose implanted CdZnO films can be effectively removed at 800 °C, while the high dose implantation results in the formation of defects stable at least up to 900 °C. Moreover, annealing of the CdZnO films is accompanied by Cd loss at the surface for temperatures exceeding 800 °C. In contrast, CdO exhibits a typical damage accumulation behaviour for metals and semiconductors with high degree of ionicity, resulting in saturation and extended defect formation at high ion doses. These extended defects in pure ZnO and CdO, formed either directly during implantation or by reconstruction during post-implant annealing, are substantially more stable compared with small defects which can be efficiently removed at 700 °C and 600 °C for ZnO and CdO, respectively.

Valence-band electronic structure of CdO, ZnO, and MgO from x-ray photoemission spectroscopy and quasi-particle-corrected density-functional theory calculations

The valence-band density of states of single-crystalline rock-salt CdO(001), wurtzite $c$-plane ZnO, and rock- salt MgO(001) are investigated by high-resolution x-ray photoemission spectroscopy. A classic two-peak structure is observed in the VB-DOS due to the anion $2p$-dominated valence bands. Good agreement is found between the experimental results and quasi-particle-corrected density-functional theory calculations. Occupied shallow semicore $d$ levels are observed in CdO and ZnO. While these exhibit similar spectral features to the calculations, they occur at slightly higher binding energies, determined as 8.8 eV and 7.3 eV below the valence band maximum in CdO and ZnO, respectively. The implications of these on the electronic structure are discussed.

Electronic structure of single-crystal rocksalt CdO studied by soft x-ray spectroscopies andab initiocalculations

Soft x-ray emission spectroscopy (XES) and x-ray absorption spectroscopy (XAS) are employed to investigate the occupied and unoccupied electronic structures in rocksalt-phase single-crystal CdO. Resonant XES at the $\mathrm{O}K$ edge reveals a clear $\mathrm{Cd}\phantom{\rule{0.2em}{0ex}}4d--\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ hybridized peak and momentum-dependent coherent contributions to the resonant emission spectra. Good agreement is obtained between the above-threshold XES and XAS spectra, and the calculated $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ local partial density of states (PDOS). Calculation of the $\mathrm{O}\phantom{\rule{0.2em}{0ex}}2p$ PDOS was performed within the $GW$ framework of many-body perturbation theory.

Influence of shallow core-level hybridization on the electronic structure of post-transition-metal oxides studied using soft X-ray emission and absorption

The influence of shallow core-level hybridization on the electronic structure of the post-transition metal oxides ZnO, CdO, In 2 O 3 , and SnO 2 has been investigated using high-resolution soft x-ray emission and absorptionspectroscopies. Synchrotron radiation excited O K α emission spectra provide a direct measure of the O 2p partial density of states and shallow core-level hybridization for this series of transparent conducting materials and reveal significant mixing of O 2p and shallow-core metal d states for ZnO, CdO, and In 2 O 3 . The experimental data are compared with local density approximation and tight-binding band structure calculations and with previous experimental determinations of direct and indirect band gaps. Rocksalt CdO, bixbyite In 2 O 3 , and rutile SnO 2 all adopt structures with metal cations in sites with locally centrosymmetric coordination. This prevents hybridization of O 2p states with metal 4d states at the zone center, but mixing away from Γ leads to indirect band gaps for CdO and In 2 O 3 . A revised value for the lowest indirect band gap in CdO is proposed and the overall trends in the band gap are discussed in terms of the separation between O 2p and metal 4d states. The experimental investigation has been extended to study the effects of Sn doping in In 2 O 3 .

Structural properties of CdO layers grown on GaAs (0 0 1) substrates by metalorganic molecular beam epitaxy

Transmission electron microscopy (TEM) and transmission electron diffraction (TED) examination has been made to investigate the microstructural behaviour of rock-salt CdO layers grown on (0 0 1) GaAs substrates with two different buffer layers of ZnS and ZnO/ZnS. It is shown that the CdO layer grown on the ZnS buffer layer has epitaxial relationship to the GaAs substrate, even though a low density of CdO nano-crystallites (18–25 nm across) are formed at the CdO/ZnS interface region. TEM and TED results show that the CdO layer grown on the ZnO/ZnS buffer layer is polycrystalline. TED results also show that the ZnO buffer layer is polycrystalline with a mixture of wurtzite and zinc-blende phases, which results in the formation of polycrystalline CdO layers.

CdO epitaxial layers grown on (0 0 1) GaAs surfaces by metalorganic molecular-beam epitaxy

Atomically flat CdO thin layers were grown on GaAs(0 0 1) substrates with ZnS buffer layers. The grown layers were evaluated by reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), X-ray rocking curve (XRC), and transmission electron diffraction (TED) measurements. The RHEED observations of CdO thin layers showed two-dimensional growth mode revealed by streaky patterns with excellent homogeneity through [1 1 0] and [ 1 ̄ 1 0] directions. This streaky pattern is the reflection of the flat surface morphology in atomic level assessed by AFM measurements with the surface root mean square roughness of ∼1 nm. The crystalline structure as well as quality of the thin CdO layers was investigated by X-ray diffraction and XRC measurements which indicated the single-crystalline rocksalt CdO thin layers with minimum full-width at half maximum under the optimal growth condition. The TED observation showed the epitaxial relationship between the rocksalt CdO and zincblende GaAs substrate as [1 1 0] CdO∥[1 1 0] GaAs, [1 1 ̄ 0] CdO∥ [1 1 ̄ 0] GaAs, and [0 0 1] CdO∥[0 0 1] GaAs.

Single-crystalline rocksalt CdO layers grown on GaAs (001) substrates by metalorganic molecular-beam epitaxy

In this letter, we report the growth of single-crystalline rocksalt CdO layers on (001) GaAs substrates using ZnS buffer layers. The growth processes of CdO layers were studied by reflection high-energy electron diffraction (RHEED), and the grown CdO layers were evaluated with atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements. After an initial growth delay, the formation of polycrystalline CdO was observed in RHEED measurements during the initial growth of very thin CdO layers. With the increase of the CdO layer thicknesses, streaky RHEED patterns were observed, which indicate the formation of single-crystalline cubic-phase CdO layers. Surface morphology of the CdO layers observed by AFM was atomically flat with root-mean-square roughness of ∼1 nm. The crystalline structures were elucidated from XRD measurements by the determination of the lattice constant to be 4.686±0.001 Å, indicating the single-phase rocksalt CdO structure.

Room-temperature compressibilities of MnO and CdO: further examination of the role of cation type in bulk modulus systematics

Room-temperature volume measurements up to 8.1 GPa reveal that rock-salt structured MnO and CdO have identical compressibility in the pressure range studied. In the plot of bulk modulus vs unit-cell volume, CdO plots well above the trend of the 3d transition metal monoxides, a behavior that deviates from empirical predictions of constant K 0 V 0. The present observations are in favor of our earlier suggestion that, for isostructural solids, the empirically predicted bulk modulus-volume relationship may be limited to their subsets that share the same valence electron character (i.e., s vs 3d vs 4d). For cations forming transition metal monoxides, variations of Pauling electronegativity with ionic radius show differences that are qualitatively similar to the observed trends of bulk modulus, suggesting that bond covalency differences may contribute to the different behaviors between the 3d and 4d transition metal monoxides.