Zinc-blende Nanocrystals Research Articles

Interplay of luminescence and magnetic phenomena in Mn-doped ZnS zinc blende nanocrystals: Influence of magnetic doping

This research comprehensively investigates the interplay between luminescence and magnetic phenomena in the zinc blende phase of manganese-doped and undoped zinc sulfide. The study involves the synthesis of zinc sulfide nanocrystals through a soft chemical method, followed by an in-depth experimental characterization. Theoretical studies were conducted using a 2 × 2 × 2 supercell model with 64 atoms to explore the impact of native defects and manganese doping on zinc sulfide's the electronic, magnetic, and optical properties. The research findings offer a detailed physical description of magnetic and optic phenomena observed, underpinned by a strong correlation between theoretical predictions and experimental results. An essential contribution of this work is developing a depiction that elucidates the relationship between optical transitions and spin-exchange in the context of these phenomena.

Influence of Structural Properties of Oleic Acid-Capped CdSe/ZnS Quantum Dots in the Detection of Hg2+ Ions.

Oleic acid-capped CdSe/ZnS quantum dots (QDs) were used to investigate their photoluminescence (PL) response to Hg2+ ions as a function of the surface properties of QDs. Three distinctly-size CdSe/ZnS QDs were obtained by varying the molar ratio of shell precursors, which were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), Fourier-Transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), absorption spectroscopy, and time-resolved fluorescence spectroscopy. Results revealed the obtention of zinc blende nanocrystals with sizes ranging from 2.7 to 3.2nm (± 0.5) and ZnS thickness between 0.3 and 1.0 monolayer (ML). The variation of the [S]/[Zn] molar ratio introduced chemical species that act as traps, affecting the PL properties differently. Depending on the thickness of the shell and chemical speciation on surface, Hg2+ ions could induce quenching or enhancement of PL. Detection of mercury ions was evaluated in terms of Stern-Volmer equation, where the limit of detection (LOD) for the PL quenching system was 11.2 nM, while for the PL enhancing systems were 8.98 nM and 10.7 nM. Results demonstrate the performance of oleic acid-capped CdSe/ZnS QDs to detect Hg2+ and their capacity to turn the PL on/off depending on surface properties.

Excitonic fine structure of zinc-blende and wurtzite colloidal CdSe nanocrystals and comparison to effective mass results

We study the excitonic fine structure of CdSe nanocrystals (also known as quantum dots) using atomic effective pseudopotentials in combination with the screened configuration interaction method, and we obtain excellent agreement with experiment. The direct comparison between atomistic and effective mass results reveals qualitative differences that become apparent almost as soon as confinement effects start to play a role, i.e., for the experimentally most relevant sizes. We proceed with a comparison of the wurtzite and zinc-blende fine structure as a function of size and ellipticity. We show that the absorption of significantly oblate zinc-blende nanocrystals can be very similar to ``spherical'' wurtzite nanocrystals. On the other hand, if the nanocrystals have no ellipticity but different crystal structures, the fine structure differs significantly. Conversely, structures with high ellipticity, either oblate or prolate, with different crystal structures show similar fine structures.

The effect of ambient temperature on the optical properties and crystalline quality of ZnSe and ZnSe:Cu NCs grown by rapid microwave irradiation

ZnSe nanocrystals (NCs) of size about 1.8 nm were prepared in aqueous media and were grown using microwave irradiation, which is simple and inexpensive method in which it needs very short reaction time. In this method, using selenium powder and NaBH4 the source of Se ions has been provided and then, the present solution has been injected to Zn ions source. Also solutions of sodium hydroxide (NaOH) and thioglycolic acid were used to adjust the pH of samples and capping agent, respectively. To study the effect of transition elements on the properties of these nanoparticles, Cu ions were used as impurity. After the completion of nucleation and formation of nanoparticles, the ultimate solution was microwave irradiated for 6 min. To investigate the nanoparticles morphology and their optical properties, X-ray diffraction (XRD) and UV–Visible (UV–Vis) optical spectroscopy were used. UV–Vis spectra were recorded at different ambient temperatures. XRD analysis demonstrated cubic zinc blende NCs and field emission scanning electron microscopy (FESEM) image indicated round shape NCs. Using the UV–Vis outputs, the optical band gap (Eg DASF) and also the exact nature of charge carriers optical transitions were studied and evaluated in ZnSe and ZnSe:Cu nanocrystals, by employing an accurate new method [called in literatures as derivation of absorption spectrum fitting (DASF)]; this method only requires the measurement of the absorbance spectrum of the sample, avoiding any needs to film thickness or any other parameters. According to the UV–Vis analysis, band gap of the nanoparticles was determined and was in the range of 3.350–3.652 eV depending on the temperature and Cu dopant, showing the decreasing trend of energy gap with increasing ambient temperature and impurity. The decreasing trend of Eg DASF with ambient temperature, confirm the semiconducting nature of the present samples. Also, the direct gap nature of their optical transitions was justified. moreover, the band tailings (Urbach energy: ETail) was estimated and was within the range of 0.052–0.106 eV, which was so small comparing to the corresponding band gap values; these small ETail valus confirm the sharpness of valence and conduction band edges, and so good degree of crystallinity of the studied samples. Furthermore, optical transition index (m), dielectric constant (e), size of the nanoparticles and the refractive index (n) were evaluated.

Atomistic tight-binding calculations of near infrared emitting CdxHg1−xTe nanocrystals

I present the structural and optical calculations for CdxHg1−xTe zinc-blende nanocrystals with the experimentally synthesized Cd compositions (x) in the framework of atomistic tight-binding model (TB) and configuration interaction description (CI). This atomistic tight-binding model incorporates the sp3s∗ orbital and the first neighbouring interaction, in addition to the non-linear dependence of compositions on the band gaps the bowing factor is involved into this model through the extended virtual crystal approximation (VCA). The information of DOS highlights that the contribution of the conduction and valence bands is subjected by cation and anion atoms, respectively. The improvement of the optical band gaps in CdxHg1−xTe nanocrystals is presented with the increasing Cd compositions. The optical band gaps can be used to tune their optical properties across a technologically useful range from 688nm to 2755nm which can be implemented for the near infrared emitting devices. In addition, the enhancement of the optical property is reported with the increasing Cd contents. With the increasing Cd compositions, the atomistic electron-hole Coulomb interaction is mainly increased, whereas the atomistic electron-hole exchange interaction is reduced. The Stokes shift and fine structure splitting are progressively reduced with the increasing compositions and diameters. The application of fine structure splitting is utilized to implement as a source of entangled photon pairs in the quantum information. Finally, the comprehensive computations of alloy CdxHg1−xTe nanocrystals effectively determine the composition- and size-dependent structural and optical properties which render these nanocrystals promising candidates as the near infrared emitting devices and optical amplifiers.

Publisher’s Note: “Number series of atoms, interatomic bonds and interface bonds defining zinc-blende nanocrystals as function of size, shape and surface orientation: Analytic tools to interpret solid state spectroscopy data” [AIP Advances 6, 085306 (2016)

Publisher’s Note: “Number series of atoms, interatomic bonds and interface bonds defining zinc-blende nanocrystals as function of size, shape and surface orientation: Analytic tools to interpret solid state spectroscopy data” [AIP Advances <b>6</b>, 085306 (2016)

Number series of atoms, interatomic bonds and interface bonds defining zinc-blende nanocrystals as function of size, shape and surface orientation: Analytic tools to interpret solid state spectroscopy data

Semiconductor nanocrystals (NCs) experience stress and charge transfer by embedding materials or ligands and impurity atoms. In return, the environment of NCs experiences a NC stress response which may lead to matrix deformation and propagated strain. Up to now, there is no universal gauge to evaluate the stress impact on NCs and their response as a function of NC size dNC. I deduce geometrical number series as analytical tools to obtain the number of NC atoms NNC(dNC[i]), bonds between NC atoms Nbnd(dNC[i]) and interface bonds NIF(dNC[i]) for seven high symmetry zinc-blende (zb) NCs with low-index faceting: {001} cubes, {111} octahedra, {110} dodecahedra, {001}-{111} pyramids, {111} tetrahedra, {111}-{001} quatrodecahedra and {001}-{111} quadrodecahedra. The fundamental insights into NC structures revealed here allow for major advancements in data interpretation and understanding of zb- and diamond-lattice based nanomaterials. The analytical number series can serve as a standard procedure for stress evaluation in solid state spectroscopy due to their deterministic nature, easy use and general applicability over a wide range of spectroscopy methods as well as NC sizes, forms and materials.

Synthesis of ZnSe nanocrystals (NCs) using a rapid microwave irradiation method and investigation of the effect of copper (Cu) doping on the optical properties

ZnSe nanocrystals (NCs) were synthesized using a microwave activated method. Synthesized NCs were characterized by means of X-ray diffraction (XRD), UV–visible (UV–vis) optical spectroscopy and photoluminescence (PL). XRD analysis demonstrated cubic zinc blende NCs. TEM image indicated round shape NCs and most of the particles had diameters of about 3nm. Band gap of the NCs was obtained about 3.15eV and PL spectra indicates a broad emission with two peaks located about 415 and 500nm related to band edge and trap state respectively. For ZnSe:Cu NCs, PL intensity of band gap emission of ZnSe NCs at 415nm decreased gradually with the increase in the concentration of Cu dopant ions and for precursor ratio of Cu:Zn 1% band gap emission at 415nm disappeared completely. At the same conditions, PL QY was obtained about 2% and 8% for ZnSe and ZnSe:Cu (1%) NCs, respectively.

Nanocrystal grain growth and device architectures for high-efficiency CdTe ink-based photovoltaics.

We study the use of cadmium telluride (CdTe) nanocrystal colloids as a solution-processable "ink" for large-grain CdTe absorber layers in solar cells. The resulting grain structure and solar cell performance depend on the initial nanocrystal size, shape, and crystal structure. We find that inks of predominantly wurtzite tetrapod-shaped nanocrystals with arms ∼5.6 nm in diameter exhibit better device performance compared to inks composed of smaller tetrapods, irregular faceted nanocrystals, or spherical zincblende nanocrystals despite the fact that the final sintered film has a zincblende crystal structure. Five different working device architectures were investigated. The indium tin oxide (ITO)/CdTe/zinc oxide structure leads to our best performing device architecture (with efficiency >11%) compared to others including two structures with a cadmium sulfide (CdS) n-type layer typically used in high efficiency sublimation-grown CdTe solar cells. Moreover, devices without CdS have improved response at short wavelengths.

Photoluminescence and Photo-Catalytic Activity of Synthesized Nanocrystals

Abtract Intrinsic and extrinsic semiconductor nanostructures have attracted great attention due to their size tunable photo-physical and photo-chemical properties. In the present paper, polyvinyl pyrrolidone (PVP) capped Zn 1-x Eu x S (0.00001≤x≤0.1) nanocrystals have been synthesized by means of a facile chemical synthesis method. Crystallography and morphology of synthesized materials have been deliberated using X-ray diffraction (XRD) and transmission electron microscope (TEM), respectively. Diffraction and electron micrograph studies reveal that the synthesized materials are zinc blende nanocrystals having average particle size ~3nm. Elemental and compositional analyses of the nanocrystals have been done using energy dispersive X-ray fluorescence (EDXRF) technique. Steady state photoluminescence spectra have been recorded for optical characterization of synthesized nanomaterials. Photocatalytic activity potential of synthesized nanomaterials under UV radiation exposure has been investigated using methylene blue (MB) dye as a test contaminant in aqueous media. Photo-physical and photo-chemical behaviour dependence on doping concentration has been described in detail. Moreover, the sophistication of competition between charge carrier recombination and charge carrier trapping followed by the competition between recombination of trapped carriers and interfacial charge transfer processes have been presented in a fantastic and elaborative way by comparative study of photoluminescence and photo-catalytic activity results.

Solvothermal Synthesis of Zincblende and Wurtzite CuInS2 Nanocrystals and Their Photovoltaic Application

We report a simple solvothermal synthesis approach to the growth of CuInS(2) nanocrystals with zincblende- and wurtzite-phase structures. Zincblende nanocrystals with particle sizes of 10-20 nm were produced using oleylamine as the solvent. When ethylenediamine was used as the solvent, similarly sized wurtzite nanocrystals with some degree of particle aggregation were formed. Use of a mixture of these solvents gave products with mixed phases including some polyhedral nanostructures. The crystal phases of these nanocrystals were carefully determined by X-ray diffraction and transmission electron microscopy analysis. All the samples exhibit strong absorption from the entire visible light region to the near-infrared region beyond 1300 nm. Pure-phase zincblende and wurtzite CuInS(2) nanocrystals were employed as ink in the fabrication of solar cells. The spray-coated nanocrystal layer was subjected to a selenization process. A power conversion efficiency of ~0.74% and a good external quantum efficiency profile over broad wavelengths have been measured. The results demonstrate that wurtzite and zincblende CuInS(2) nanocrystals may be attractive precursors to light-absorbing materials for making efficient photovoltaic devices.

First-Principles Study of the Effect of Organic Ligands on the Crystal Structure of CdS Nanoparticles

We show with the aid of first-principles electronic structure calculations that suitable choice of the capping ligands may be an important control parameter for crystal structure engineering of nanoparticles. Our calculations on CdS nanocrystals reveal that the binding energy of model trioctylphosphine molecules on the (001) facets of zincblende nanocrystals is larger compared to that on wurtzite facets. Similarly, the binding energy of model cis-oleic acid is found to be dominant for the (1010) facets of wurtzite structure. As a consequence, trioctylphosphine as a capping agent stabilizes the zincblende structure while cis-oleic acid stabilizes the wurtzite phase by influencing the surface energy, which has a sizable contribution to the energetics of a nanocrystal. Our detailed analysis suggests that the binding of molecules on the nanocrystalline facets depends on the surface topology of the facets, the coordination of the surface atoms where the capping molecule is likely to attach, and the conformati...

Chemical substitution of Cd ions by Hg in CdSe nanorods and nanodots: Spectroscopic and structural examination

► We studied cadmium-by-mercury chemical substitution in CdSe nanocrystals. ► Zinc blende CdSe quantum dots can be easily converted to isostructural Cd x Hg 1− x Se. ► Wurtzite CdSe QDs require longer time to convert to a zinc blende Cd x Hg 1− x Se. ► Wurtzite CdSe nanorods transform to nanoheterogeneous luminescent Cd x Hg 1− x Se rods. The chemical substitution of cadmium by mercury in colloidal CdSe quantum dots (QDs) and nanorods has been examined by absorption, photoluminescence and Raman spectroscopy. The crystalline structure of original CdSe QDs used for Cd/Hg substitution (zinc blende versus wurtzite) shows a strong impact on the optical and structural properties of resultant Cd x Hg 1− x Se nanocrystals. Substitution of Cd by Hg in isostructural zinc blende CdSe QDs converts them to ternary Cd x Hg 1− x Se zinc blende nanocrystals with significant NIR emission. Whereas, the wurtzite CdSe QDs transformed first to ternary nanocrystals with almost no emission followed by slow structural reorganization to a NIR-emitting zinc blende Cd x Hg 1− x Se QDs. CdSe nanorods with intrinsic wurtzite structure show unexpectedly intense NIR emission even at early Cd/Hg substitution stage with PL active zinc blende Cd x Hg 1− x Se regions.

Formation mechanisms of spatially-directed zincblende gallium nitride nanocrystals

We report on the spatially selective formation of GaN nanocrystals embedded in GaAs. Broad-area N+ implantation followed by rapid thermal annealing leads to the formation of nanocrystals at the depth of maximum ion damage. With additional irradiation using a Ga+ focused ion beam, selective lateral positioning of the nanocrystals within the GaAs matrix is observed in isolated regions of increased vacancy concentration. Following rapid thermal annealing, the formation of zincblende GaN is observed in the regions of highest vacancy concentration. The nucleation of zincblende nanocrystals over the wurtzite phase of bulk GaN is consistent with the predictions of a thermodynamic model for the nanoscale size-dependence of GaN nucleation.

Synthesis and Control of ZnS Nanodots and Nanorods with Different Crystalline Structure from an Identical Raw Material Solution and the Excitonic UV Emission

ABSTRACTWe synthesized ZnS nanocrystals from identical raw material solution by the thermal decomposition of an amine complex. The shapes of products were changed by simply varying heating rate. At higher heating rate, we obtained the isotropic zincblende nanocrystals. At the lower heating rate, the nanorods were formed and the length was increased with the decrease of heating rate. The nanorods had wurtzite structure below 175 °C, and consequently transformed to zincblende phase during a temperature rise to 200 °C. These particle shapes and phases were related to the adsorption properties of amine ligands. Additionally, the synthesized ZnS nanodots and nanorods exhibited predominantly band-edge emission in fluorescence spectra.

N-type doping via avoiding the stabilization ofDXcenters in InP quantum dots

We demonstrate that it is preferable to dope III-V semiconductor nanocrystals by $n$-type anion substitution as opposed to cation substitution. Specifically, we show the dopability of zinc-blende nanocrystals is more efficient when the dopants are placed at the anion site as quantified by formation energies and the stabilization of $DX$-like defect centers. Our results are based on first-principles calculations of InP quantum dots by using a real-space implementation of density-functional theory and pseudopotentials.

Amorphous nature of small CdS nanoparticles: Molecular dynamics simulations

The semiconductor CdS is generally found in the wurtzite structure. Prior experimental and theoretical results confirm that the semiconductor CdS nanoparticles maintain a wurtzite structure for diameters greater than 6 nm. There is disagreement in the literature for sizes smaller than 6 nm. We use the density-functional theory FIREBALL code and perform finite-temperature molecular dynamics simulations on nanoparticles that are approximately 2 nm in diameter, considering different sized structures and different simulation temperatures. To determine the structure of the nanoparticles we analyze the radial distribution of the atoms about the center of the nanoparticle, the nearest-neighbor and next-nearest-neighbor bond lengths, and the radial distribution function about individual atoms. Comparing the molecular dynamics simulations of relaxed nanocrystals against bulklike wurtzite and zinc-blende nanocrystals, we find that small uncapped CdS nanoparticles are not nanocrystals but are amorphous.

Interface and confined optical phonons in wurtzite nanocrystals

We derive within the dielectric-continuum model an integral equation that defines interface and confined polar optical-phonon modes in nanocrystals with a wurtzite crystal structure. It is demonstrated theoretically that, while the frequency of confined polar optical phonons in zinc-blende nanocrystals is equal to that of the bulk crystal phonons, the confined polar optical phonons in wurtzite nanocrystals have a discrete spectrum of frequencies different from those of the bulk crystal. The calculated frequencies of confined polar optical phonons in wurtzite ZnO nanocrystals are found to be in an excellent agreement with the experimental resonant Raman-scattering spectra of spherical ZnO quantum dots.

Interface and confined polar optical phonons in spherical ZnO quantum dots with wurtzite crystal structure

We derive analytically the interface and confined polar optical-phonon modes for spherical quantum dots with wurtzite crystal structure. While the frequency of confined optical phonons in zincblende nanocrystals is equal to that of the bulk crystal phonons, the confined polar optical phonons in wurtzite nanocrystals are shown to have a discrete spectrum of frequencies different from those in bulk crystal. The calculated frequencies of confined polar optical phonons in wurtzite ZnO quantum dots are found to be in excellent agreement with experimental resonant Raman scattering data. The derived analytical expression for phonon modes can facilitate interpretation of experimental data obtained for ZnO quantum dots. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Pair-state formation in a nanocrystal: a theoretical perspective

ABSTRACTSimulations of dopant pair-state distributions are presented for zincblende nanocrystals with different radii and for different dopant fractions. The probability of finding at least one pair-state and the concentration of pair-states were calculated on the basis of a statistical average of 105simulations per crystal size and dopant concentration. The distribution of nanocrystal lattice positions over the surface and the bulk of the crystal is computed. A mathematical description of the distributions, valid in any crystal lattice, is discussed. This removes the need for further simulations.