Modeling ZnS and ZnO Nanostructures: Structural, Electronic, and Optical Properties (original) (raw)
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Journal of the Korean Physical Society, 2009
The microscopic properties of several ZnO and ZnS surfaces of low Miller index are investigated through rst-principles calculation of the surface energy, crystal structure and electronic structure. Generally, the non-polar surfaces such as (10 10) and (11 20) are found to be more stable than the polar (0001)-Zn surface. The (11 20) surface is found to be the most stable for both ZnO and ZnS semiconductors and the (10 10) surface is found to have a slightly larger surface energy. These are consistent with the abundant observations of ZnO nanostructures having (11 20) and (10 10) surfaces. The near-surface structures and electronic structures are discussed.
Unravelling the origin of the giant Zn deficiency in wurtzite type ZnO nanoparticles
Scientific Reports, 2015
Owing to its high technological importance for optoelectronics, zinc oxide received much attention. In particular, the role of defects on its physical properties has been extensively studied as well as their thermodynamical stability. In particular, a large concentration of Zn vacancies in ZnO bulk materials is so far considered highly unstable. Here we report that the thermal decomposition of zinc peroxide produces wurtzite-type ZnO nanoparticles with an extraordinary large amount of zinc vacancies (>15%). These Zn vacancies segregate at the surface of the nanoparticles, as confirmed by ab initio calculations, to form a pseudo core-shell structure made of a dense ZnO sphere coated by a Zn free oxo-hydroxide mono layer. In others terms, oxygen terminated surfaces are privileged over zinc-terminated surfaces for passivation reasons what accounts for the Zn off-stoichiometry observed in ultra-fine powdered samples. Such Zn-deficient Zn1-xO nanoparticles exhibit an unprecedented pho...
Structural and Electrical Studies on ZnS Nanoparticles Prepared Without Using Capping Agent
In this work, zinc sulfide (ZnS) nanoparticles have been synthesized by simple chemical precipitation method without using any capping agent. The synthesized nanoparticles were characterized using XRD, optical microscopy, UV-Vis, FTIR, and impedance spectroscopy. The X-ray diffraction shows that ZnS particles have cubic sphalerite structure with the crystallite size of 10-20 nm. SEM images of nanopowder samples reveal the presence of nanoflakes and agglomerated nanoparticles. Formation of ZnS has been confirmed through the appearance of 714 cm -1 absorption peak. The thermal stability of synthesized nanoparticles has been checked by annealing the material at different temperatures. Transformation of ZnS into ZnO subsequent to annealing has been evidenced from XRD and FTIR studies. UV-Vis spectra exhibited a red shift in the optical absorption on increase in annealing temperature. Variation in electrical conductivity obtained from impedance measurements at different temperatures has been suitably correlated to Davis-Mott model Keywords: zinc sulfide nanoparticles; II-VI semiconductors; chemical precipitation method; optical band gap; annealing 1 Introduction Currently there is a great deal of interest in optical and structural properties of nanometer sized semiconductor particles . Additionally, such nanoparticles have exhibited applicability as zero-dimensional quantum confinement material besides application in optoelectronics and photonics . Nano semiconductors, including II-VI group semiconductors show significant departures from bulk properties when the scale of confinement approaches to excitonic Bohr radius which sets the length scale for optical process. The photo-emission wavelengths, the band gap and lattice parameter are strongly dependent on the grain size rendering tailorability of these properties as a function of grain size. However, the biggest hurdle in nanotechnology seems to be the control of grain size in a few nanometer range . As a consequence, the development of semiconductor nanocrystals of controlled shape and size possessing desired optoelectronic properties has been the subject of intense research. Within the family of nano-semiconductors, Zinc Sulfide has been extensively investigated due to its wide ranging applications in Photoluminescence (PL), Electroluminescence (EL), Cathodoluminescence (CL) devices, Light Emitting Diodes, reflectors and dielectric filters. It has a wide band gap of 3.5-3.8 eV at room temperature and better chemical stability compared to other chalcogenides. Its band can be tuned in the UV region. [9-13]. Annealing treatment is very common in semiconductor processing. It can be used to remove the defects and to test the stability of the crystals at a given temperature under the given ambient conditions, which is important for device purposes. To the best of our knowledge, there are only few reports on the annealing effects on ZnS . Keeping in view the above, an effort has been made here to study the effect of annealing on the optical, structural and electrical properties of ZnS particles.
Synthesis of wurtzite Zn S nanocrystals at low temperature
Nanocrystallites of wurtzite hexagonal ZnS have been successfully synthesized without using any capping agent by simple chemical precipitation method at a low calcination temperature of 150 °C. It has been found that the size of the synthesized ZnS nanocrystallites decreases as Zn2+:S2− ratio is decreased. The synthesized nanoparticles have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis absorption spectroscopy and M–H characteristics. The XRD patterns have confirmed that the prepared ZnS nanoparticles are of wurtzite hexagonal phase. XRD, SEM and TEM studies have shown the decrease in the particle size with the increase in S2− source. TEM images have clearly shown that size distribution of the particles lie in the range of 5–30 nm. The optical absorption bandgap of the synthesized nanocrystals has been found to be in the range of 3.69–3.74 eV. Magnetization study has shown the ‘diamagnetic’ behavior of synthesized ZnS nanocrystallites with a weak ferromagnetic behavior in the low field regime. The observed weak ferromagnetism has been understood due to the presence of defects in the synthesized ZnS nanoparticles.
CHARACTERISATION AND BAND GAP ENERGY OF WURTZITE ZnO:La 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 Lanthanum ions, with an average particle size of 15-23 nm, are synthesized by chemical route technique. XRD, SEM, FTIR UV-Vis 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 UV absorption spectra revealed the absorption at wavelength <370 nm indicating the smaller size of ZnO nano particle. The absorption spectra obtained by UV-Vis spectrometer determined the optical band gap and found to be in the range 2.94 eV. It was found that energy band gap E g decreases with doping of La.
Analysis of structural and vibrational properties of ZnSe nanostructures: A DFT/TDDFT study
Egyptian Journal of Chemistry, 2019
T HE structural and vibrational properties of Zn n Se n (n=1,3,7,13) nanostructures have been investigated using the Gaussian 09 program, density functional theory (DFT) and time-dependent density functional theory (TDDFT) at the B3LYP level with 6-311G basis functions. The structural properties showed that the rebuilding in surface atoms deviated many bonds from their ideal length, the Zn-Se bond length decreased with the increase in the size of nanostructures and converged to the experimental value. Quantum confinement effect diminution was observed with the growing size of the nanostructures; hence, the energy gap converged to the experimental value of 2.7 eV. Moreover, the binding energy increased with the increase of the structure size, such that wurtziod2c (Zn 13 Se 13) is more stable than smaller structures. The vibrational properties results indicated that the experimental longitudinal optical mode (LO mode) is situated between bare and hydrogen passivated LO modes and very near to the bare case, this gave a good agreement with experimental findings. The presence of hydrogen atoms at the surface caused a several times decrease in vibrational force constant in comparison to the bare case. The IR spectrum for wurtzoid and HP wurtzoid were investigated. The optical edge in UV-Vis spectra of wurtzoid reduced from 4.5 eV to 4.2 eV of wurtzoid2c due to the increase in the size of the nanostructure, while the maximum peak for wurtzoid at 2.88 eV increased to 3.06 eV for wurtzoid2c showing a clear blue shift. These results leads to wide applications in fields such as photoelectronic devices, lasers, sensors, and LEDs.
Structural and Optical Properties of ZnO1– x Sx Nanoparticles
Journal of Nanoelectronics and Optoelectronics, 2012
We describe a method for synthesis of semiconductor ZnO 1−x S x (ZOS) nanoparticles by reaction of hydrogen sulfide with a mixture of ethanol solutions of zinc acetate and sodium hydroxide salts. The nanoparticle composition, structure and optical properties are investigated for different ratios of the salt content in the mixture. Structure of ZOS nanoparticles changes from pure wurtzite (x ≤ 0 1) to sphalerite (0 5 ≤ x ≤ 1). Between x = 0 1 and 0.5 wurtzite and sphalerite phases coexist. According to the transmission electron microscopy as prepared nanoparticles have spherical shape and size of 10-20 nm in diameter. Cathodoluminescence spectra show the emission peaks in the UV-and the visible wavelength regions. The band gap of ZOS nanoparticles nonlinearly depends on the sulfur content as was evaluated from the diffuse reflectance measurements.