Cadmium Selenide Nanocrystals Research Articles

Band gap engineering of cadmium selenide nanocrystals using 120 MeV Ag7+ swift heavy ions, alongside theoretical evidence through PBE+U analysis

Abstract The interaction of 120 MeV Ag7+ swift ions with the chemically prepared cadmium selenide nanocrystals have been studied in this work through XRD data analysis. Atomic force microscopic study provides the information that with SHI irradiation, average grain size increases and surface modification takes place. Thermal spike model gives an idea about the changes in the lattice structure due to SHI irradiation, which induces lattice geometry reorientation. Such changes in lattice geometry result in modification of band structure as well as density of states of the bands and therefore modification in the band gap of the prepared sample takes place. Theoretical study of the band gap tuning has been performed using first principles based on Quantum espresso Code through PBE + U analysis, which supports the results as obtained from experimental diffuse reflectance spectra and Photoluminescence spectral study. Hence, both the experimental and theoretical study confirm the band gap engineering of CdSe nanocrystals through SHI irradiation.

120 MeV Ni10+ swift heavy ions irradiation on CdSe nanocrystals induces cubic to hexagonal phase transformation - A study of microstructural modification

In the present work, chemically synthesized cadmium selenide (CdSe) nanocrystals, which have a cubic lattice structure as per X-ray diffraction (XRD) analysis, has been irradiated with 120 MeV Ni10+ swift heavy ions (SHI) for the study of their structural modification. A significant change in the lattice structure has been found with different ion fluence of 120 MeV Ni10+ irradiation, which may be due to the high electronic energy loss of SHI. AFM analysis also shows changes in morphological properties with ion irradiation. The evolution of lattice structure with different fluence has been studied using double ion impact model. Further, SHI induced cubic to hexagonal transformation effect the microstructural parameters such as intrinsic strain, dislocation density of CdSe nanocrystals. SHI induced microstructural modifications have also been studied with the help of Variance model.

Role of surface defects in colloidal cadmium selenide (CdSe) nanocrystals in the specificity of fluorescence quenching by metal cations

This study aimed to answer the question as whether crystal defects at the surface of soluble capped CdSe nanocrystals (or quantum dots, QDs) in water colloidal suspension are involved in the mechanism of fluorescence quenching induced by metal cations. Nanocrystals of CdSe were synthesized by an aqueous protocol, varying the ratio between the CdSe precursors and the grafted ligand mercaptosuccinic acid (MSA). Changing the MSA/CdSe ratio during synthesis impacts the crystal nucleation growth, which plays an important role in surface construction of CdSe QDs and changes the surface state. In this way, we could modulate the crystal surface defects of CdSe, as verified by analysis of the individual bands which constitute the emission spectra and are associated with different relaxation processes. We found that the various tested metal cations, which interact in solution with the MSA ligand grafted on the QDs, quench their fluorescence differently, depending on the MSA/CdSe ratio used in synthesis. The crystal defects modulate the excitonic relaxation in CdSe and we demonstrated here that the surface defects intervene in the quenching of QDs induced by the binding of cations.

Optimization of hybrid P3HT:CdSe photovoltaic devices through surface ligand modification

Within the last few years, renewable energy devices have been gaining growing interest and the quest for environmentally friendly efficient photovoltaic cells continues. Amongst commonly used designs, bulk heterojunctions consist of blending a conjugated polymer donor with semiconducting nanocrystals (NCs) as acceptors. These nanocrystals are promising substitutes for fullerene normally used as acceptor, offering better electron mobility, tunable band gap and higher dielectric constant to prevent recombination of charge carrier into excitons. In this work, cadmium selenide nanocrystals (CdSe) capped with different phosphine oxides (R3PO: R = Me2N, Et2N, pyrrolidinyl (Pyrr), piperidinyl (Pip) or octyl groups) were synthesized and characterized using powder XRD and Transmission Electron Microscopy (TEM) Then, we investigated the surface structure of blended poly-3-hexylthiophene (P3HT) and CdSe active layer using Atomic Force Microscopy (AFM). Subsequently, absorption spectroscopy (UV–Vis) and photoluminescence (PL) were used for further optical property characterization. Significant quenching of PL spectra along with a larger absorption band were observed. Importantly, the capping ligand effect on the morphology and the fluorescence quenching was investigated in order to high light the role of ligand modification on the photovoltaic device performance. We reported a performance dependence of hybrid devices on the bulkiness of capping molecules. Moreover, the conversion efficiencies of the elaborated cells show an increase from 0.046% for pure P3HT to 0.46% for the hybrid composite containing Pyrr3PO-capped NCs.

Phase transformation of CdSe nanocrystals at high fluence irradiation of 120 MeV swift Ni10+ and Ag7+ ions – X-ray diffraction and Raman spectral analysis

In this work, cadmium selenide (CdSe) nanocrystals have been chemically synthesized and subsequently characterized by SEM, XRD and Raman Study. XRD pattern of pristine CdSe nanocrystals show cubic lattice structure with average particle size of 34 nm approximately. CdSe nanocrystals have been irradiated by 120 MeV swift Ni10+ and Ag7+ heavy ions (SHI) at different ion fluences of 1 × 1011, 1 × 1012 and 3 × 1013 ions/cm2. XRD patterns reveal a structural phase transformation from cubic to hexagonal phase in the SHI irradiated CdSe nanocrystals at highest fluence in both the cases of Ni10+ and Ag7+ ions. Since the deposited energy loss of Ni10+ and Ag7+ ions is different, the evolution of hexagonal phase with the ion fluences is also different. Here, a comparative analysis of the effect of Ni10+ and Ag7+ ion on CdSe nanocrystals have been studied using Garvie and Nicholson model to understand the mechanism of the phase transformation. Further, the effect of both the swift ion on the structural properties of CdSe nanocrystals has also been studied with the help of Raman Spectral analysis.

Dimensional characterization of cadmium selenide nanocrystals via indirect Fourier transform evaluation of small-angle X-ray scattering data

The correlation of single-particle imaging and absorption spectroscopy made the development of sizing curves possible and enabled rapid size determination of semiconductor nanocrystals based solely on optical properties. The increasing demand and production of such materials has resulted in a question of comparability between existing models and adequate volume-weighted size-determining measurement techniques. Small-angle X-ray scattering (SAXS) is a well-established method for obtaining nanostructural information from particle systems while operating sample quantities up to a commercial scale with a large amount of statistically based data. This work utilizes laboratory SAXS to characterize cadmium selenide nanocrystals with band edge energies between 1.97 and 3.08 eV. The evaluation of the scattering patterns is based on an indirect Fourier transformation (IFT), while dimensional parameters are derived from the model-free pair distance distribution functions (Dmode and Dg), as well as the modeled volume (Dv) and number (Dn)-weighted size-density distributions. We find that comparable data from Dn agree well with existing X-ray diffraction (XRD) and with transmission electron microscopy (TEM) results described in literature; this qualifies SAXS as an equivalent integral characterization method. Although based on an estimate, the radius of gyration yields equivalent accurate results. Additionally, corresponding volume-weighted data are shown that can be useful when transferring information to other techniques. Dmode parametrization represents the largest estimated size of the sample and implies that particles interact and deviate from the spherical morphology, whereas Dv demonstrates results not considering such effects. A full set of the parameters discussed quantifies the quality of a sample.

Measurement of ligand coverage on cadmium selenide nanocrystals and its influence on dielectric dependent photoluminescence intermittency

Photoluminescent quantum dots are used in a range of applications that exploit the unique size tuneable emission, light harvesting and quantum efficient properties of these semiconductor nanocrystals. However, optical instabilities such as photoluminescence intermittency, the stochastic switching between bright, emitting states and dark states, can hinder quantum dot performance. Correlations between this blinking of emission and the dielectric properties of the nanoenvironment between the quantum dot interface and host medium, suggest surface ligands play a role in modulating on-off switching rates. Here we elucidate the nature of the cadmium selenide nanocrystal surface, by combining magic angle spinning NMR and x-ray photoelectron spectroscopy to determine ligand surface densities, with molecular dynamics simulation to assess net ligand filling at the nanocrystal interface. Results support a high ligand coverage and are consistent with photoluminescence intermittency measurements that indicate a dominant contribution from surface ligand to the dielectric properties of the local quantum dot environment.

Pressure-Induced Emission Enhancements and Ripening of Zinc Blende Cadmium Selenide Nanocrystals

Zinc blende cadmium selenide nanocrystals (Zb-CdSe NCs) were found to exhibit excellent photostability and high quantum yield compared with wurtzite (Wz) CdSe NCs, although Wz-CdSe NCs are more sta...

Controlling defect state emission in ultra-small sized tellurium doped CdSe nanocrystals via two-phase synthesis method

The utilization of semiconductor nanocrystals as white light emission source draws intense attention since semiconductor nanocrystals are promising candidates to fabricate new generation light emitting diodes. However, to synthesize high quality white light emitting semiconductor nanocrystals under mild reaction conditions at single step is a challenge. Here, we synthesized tellurium doped cadmium selenide (Te-doped CdSe) nanocrystals by using two phase synthesis method under mild reaction conditions at single step. Random localization of tellurium atoms in cadmium selenide nanocrystals caused emergence of defect state emission. The defect state emission was controlled by tuning initial Te:Se molar ratio in aqueous phase of the two phase system. Luminescence behaviour of Te-doped CdSe nanocrystals was affected by defect state emission and therefore, dual light emission (composition of blue-green light, whitish color) was observed. The Te-doped CdSe had cubic structure with Te composition at most 10% with size ranging around 1.8 nm. The CRI 1931 coordinates of the Te-doped CdSe nanocrystals revealed that Te-doped CdSe nanocrystals can be a powerful constituent for new generation light emitting diodes.

Effects of Ag doping on the electronic and optical properties of CdSe quantum dots.

Cadmium selenide (CdSe) nanocrystals are important photoelectric materials. Doping heterovalent impurities such as silver (Ag) in CdSe nanocrystal quantum dots (QDs) can provide additional charge carriers, which can significantly enhance the performance of CdSe QDs for their potential applications in high-efficiency photovoltaic devices. Using density functional theory (DFT) based calculations with the Heyd-Scuseria-Ernzerhof (HSE06) screened hybrid functional, we demonstrate that Ag doping can affect the structural, electronic and optical properties of CdSe QDs significantly. The location and number of Ag dopant atoms are critical factors for modifying the electronic structure, in particular the change of energy position and shape of the valence and conduction band edges. It is found that doping of Ag atoms into the core region of a CdSe nanoparticle induces metallic-like electronic characteristics with a dense number of electrons emerging at the Fermi level. However, incorporation of Ag dopant into the surface of a CdSe quantum dot introduces some mid-gap states that mainly consist of Se 4p states, and results in a new sub-bandgap electronic transition from mid-gap states to the conduction band. The calculated absorption spectra indicate that doping of just one or two Ag atoms greatly strengthens the absorption in the ultraviolet-visible regime and extends the absorption edges of CdSe QDs into the infrared regime. In particular, the spectra show a high-intensity absorption band between 424 and 600 nm with just 1 Ag atom incorporated into the CdSe QDs. Based on the improved absorption spectra, the present results provide a science-based strategy for designing Ag-doped CdSe QDs with enhanced visible light absorption for their application in high-efficiency photovoltaic devices.

ФОТОЛЮМІНЕСЦЕНЦІЯ НАНОЧАСТИНОК CdSe:Ni ОТРИМАНИХ ХІМІЧНИМ МЕТОДОМ

Cadmium selenide nanocrystals doped with nickel were prepared in water phase chemistry technique with gelatin acting as capping agent. Structures were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), visible absorption and photoluminescence spectroscopies. Influence of component concentrations and technological parameters on nanocrystals average size and properties was studied

Charge Transfer from Upconverting Nanocrystals to Semiconducting Electrodes: Optimizing Thermodynamic Outputs by Electronic Energy Transfer.

Light-harvesting inorganic nanocrystals play an important role in emerging solar energy conversion and optoelectronic devices. We describe here a strategy for a new family of photoelectrodes with upconverting nanocrystal assemblies as the photosensitizer. The assemblies consist of oleic acid-capped cadmium selenide (CdSe) nanocrystals that coordinate directly onto a layer of surface-bound, carboxylic acid-derivatized anthracenes through displacement of the oleic acid capping ligands. Steady-state emission and transient absorption measurements show that the upconverting nanocrystal assemblies, selectively excited by green light, generate singlet excitons that enable efficient charge injection into both the conduction band of TiO2 at the photoanode and the valence band of NiO at the photocathode. The singlet excitons form by sensitized triplet-triplet annihilation within the compact layer of anthracenes on the electrode surfaces. Density of state analysis reveals that the electronic coupling between the anthracene singlet excited states and the oxides provides a thermodynamic basis for light-induced charge transfer. The interplay between the excited-state populations at the surface-bound molecules and the assembled nanocrystals presents new design rules that can potentially overcome the limitations of previous dye-sensitized photoelectrochemical cells for catalytic applications.

Thermochemical Measurements of Cation Exchange in CdSe Nanocrystals Using Isothermal Titration Calorimetry.

Among the various reported post synthetic modifications of colloidal nanocrystals, cation exchange (CE) is one of the most promising and versatile approaches for the synthesis of nanostructures that cannot be directly synthesized from their constitutive precursors. Numerous studies have reported on the qualitative analysis of these reactions, but rigorous quantitative study of the thermodynamics of CE in colloidal nanoparticles is still lacking. We demonstrate using isothermal titration calorimetry (ITC), the thermodynamics of the CE between cadmium selenide (CdSe) nanocrystals and silver in solution can be quantified. We survey the influence of CdSe nanocrystal diameter, capping ligands and temperature on the thermodynamics of the exchange reaction. Results obtained from ITC provide a detailed description of overall thermodynamic parameters-equilibrium constant ( K eq), enthalpy (Δ H), entropy (Δ S) and stoichiometry ( n)-of the exchange reaction. We compared the free energy change of reaction (Δ G) between CdSe and Ag+ obtained directly from ITC for both CdSe bulk and nanoparticles with values calculated from previously reported methods. While the calculated value is closer to the experimentally obtained Δ G rxn for bulk particles, nanocrystals show an additional Gibbs free energy stabilization of ∼-14 kJ/mol Se. We discuss a thermochemical cycle elucidating the steps involved in the overall cation exchange process. This work demonstrates the application of ITC to probe the thermochemistry of nanoscale transformations under relevant solution conditions.

Overcoming the Complex Excited-State Dynamics of Colloidal Cadmium Selenide Nanocrystals Involved in Energy Transfer Processes

Semiconductor nanocrystals are often characterized by complex excited-state dynamics which reflect the inhomogeneous character of ensemble of nanocrystals. A new hybrid inorganic–organic donor–acceptor system involving CdSe nanocrystals and paramagnetic nitronyl nitroxide free radicals is shown to lead to efficient Forster (dipolar) resonance energy transfer. This transfer process, which is monitored by steady-state and time-dependent photoluminescence quenching experiments, occurs on a time scale similar to that of the intrinsic recombination in CdSe nanocrystals, allowing to unravel some of the complexity associated with the excited-state photophysics of semiconductor nanocrystals. A Stern–Volmer formalism that can handle the multiexponential nature of the time-dependent excited-state kinetics of CdSe nanocrystals is developed, leading to excellent agreement between steady-state and time-dependent photoluminescence data when a log-normal distribution model is used for the intrinsinc recombination rate c...

High-Resolution Electron Microscopy Investigations of Structure and Morphology of Cadmium Selenide Nanocrystals

The paper presents high-resolution electron microscopy investigations of the structure and morphology of cadmium selenide nanocrystals and nanoplatelets, and nanoplatelets having cadmium selenide core/crown nanostructure. These nanocrystals and nanoplatelets are obtained by the colloid technology and are deposited onto carbon substrates using the Langmuir–Blodgett method. It is shown that two-dimensional structures of nanocrystals and nanoplatelets can be formed on carbon substrates. The quantity specifications of these structures are determined in this paper.

Comparison of the observed size-dependent melting point of CdSe nanocrystals to theoretical predictions

Cadmium selenide nanocrystals were observed to have a size-dependent melting point which was depressed relative to the bulk melting temperature. The observed size-dependent melting point ranged from 500-1478 K, while a model based on the surface area to volume ratio predicted that is should range between 774-1250 K. The nanocrystals were heated in situ in the electron microscope, and the melting point was almost immediately followed by the vaporization of the CdSe nanocrystals, allowing for straightforward determination of the melting temperature. The differences between the observed melting point of CdSe nanocrystals and the values predicted by the surface area to volume ratio model indicates that additional factors are involved in the melting point depression of nanocrystals.

Structural and optical characterization of CdSe nanocrystals (NCs) embedded into a porous silicon nanostructure

The aim of this work is to study the effect of cadmium selenide nanocrystals (CdSe NCs) incorporation into porous silicon (PS) on its structural and opto-electronic properties using a simple method. We focused our investigation on the effect of thermal annealing after the deposition of CdSe into the (PS) layer. Obtained results prove the beneficial role of this treatment and its dependence with temperature. The CdSe NCs was incorporated into the PS layer by spin coating method and then annealed in ambient atmosphere for 30 min at different temperatures between 150 and 300 °C. The effect of thermal annealing on the structural and optical properties of the treated PS with CdSe NCs was investigated using Fourier transmission infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscope (SEM), UV-vis-NIR spectrometer, WTC-120 lifetime tester and PL spectroscopy. The X-ray diffraction results clearly revealed the phase transformation of the deposited CdSe from metastable nanocrystalline cubic (zinc blende type) to a mixture of cubic and hexagonal (wurtzite type), and finally into stable hexagonal. A decrease in the reflectivity was obtained after incorporating the CdSe NCs onto the PS layers, before and after annealing. The samples PS treated with CdSe NCs show an important enhancement in the minority carrier lifetime (τeff) indicating an improved surface quality in comparison with the untreated sample (reference). The photoluminescence intensity shows an obvious dependence with the variation of the annealing temperature compared to the untreated porous silicon layer.

Synthesis, Characterization and Optical Properties of CdSe and ZnSe Quantum Dots

In this paper, we present an organic synthesis method used for the synthesis of high quality Cadmium Selenide (CdSe) and Zinc Selenide (ZnSe) semiconductor nanocrystals. The mixture of hexadecylamine (HDA)-tetradecylphosphonic acid (TDPA), trioctylphosphine oxide (TOPO)-trioctylphosphine (TOP) has been used as stabilizing solvent for synthesis of high quality CdSe and ZnSe nanocrystals. These QDs have been characterized using various techniques like X-ray diffraction, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) for size and morphology of samples. Fourier transform infrared spectroscopy has been used for the identification of functional groups, UV-visible absorption and Fluorescence spectroscopy for optical characteristics. These studies show that the particle size is in 2-5 nm range with cubic crystalline phase. The band gap of 3.5 and 4.5 eV has been calculated for CdSe and ZnSe nanocrystals by using Tauc relation. Effective mass approximation calculation and blue shift of photoluminescence peak also confirms the narrow size of particles. The objective of this work is to produce size controlled CdSe and ZnSe nanocrystals with organic passivated surface having high luminescence for use in different optoelectronic applications. Keywords: Semiconductor nanocrystals, organic synthesis, scanning electron microscopy, crystal structure.

Tight Binding of Carboxylate, Phosphonate, and Carbamate Anions to Stoichiometric CdSe Nanocrystals.

To completely displace the carboxylate surface ligands from cadmium selenide nanocrystals, oleic acid impurities are first removed using dimethylcadmium or diethylzinc. In addition to metal carboxylate and methane coproducts, reactions with CdMe2 produce surface bound methyl groups (δ = 0.4 ppm, 0.04-0.22 nm-2) that photolytically dissociate to methyl radicals and n-doped nanocrystals. Without oleic acid impurities, cadmium carboxylate can be completely displaced from the surface using n-alkylamines (NH2R', R' = n-butyl, n-hexyl, n-octyl) (≤0.01 carboxylates nm-2). Colloidal dispersions of amine bound nanocrystals (CdSe-NH2R') are indefinitely stable at amine concentrations of 0.1 M or higher and slowly aggregate at lower concentrations. Dissociation and evaporation of the amine ligands in 4-ethylpyridine, tri-n-butylphosphine, or molten tri-n-octylphosphine oxide solution results in nanocrystal aggregation. CdSe-NH2R' reacts with oleic acid, n-octadecylphosphonic acid, or carbon dioxide to form surface bound n-alkylammonium oleate, phosphonate, and carbamate ion pairs that bind with greater affinity than primary n-alkylamines. The results indicate that nanocrystal dispersions solely stabilized by neutral donor ligands are relatively unstable compared to those stabilized by adsorbed metal carboxylate or phosphonate complexes or by ion pairs. The challenge of differentiating between the neutral ligand bound form and adsorbed ion pairs is discussed.

Synthesis of cubic CdSe nanocrystals and their spectral properties

The cadmium selenide nanocrystals are prepared by colloidal chemistry under mild conditions. X-ray diffraction and high-resolution transmission electron microscopy measurements indicate that as-prepared cadmium selenide nanocrystals are zinc blende cubic structure. We carry out an analysis of quantum size effect in the Raman spectra of cadmium selenide nanocrystals performed by utilizing the chemical bond theory of Raman peak shift developed recently. It is revealed that the shifts of Raman peaks in cadmium selenide nanocrystals result from the overlapping of the quantum effect shifts and surface effect shifts. The sizes of the as-prepared cadmium selenide nanocrystals obtained by employing the Raman peak shift theory are in good agreement with the nanocrystal sizes determined by high-resolution transmission electron microscopy.