Deep Trap Emission Research Articles

Regulating the Fluorescence Emission of CdSe Quantum Dots Based on the Surface Ligand Exchange with MAA

Cadmium selenide (CdSe) quantum dots (QDs) is a promising semiconductor and fluorescence nanocrystal with quantum size effect. To address the problem of fluorescence emission during the process of the ligand transformation of colloidal cluster CdSe QDs from nonpolar organic solvents to polar inorganic aqueous solutions, the surface ligand exchange with mercapto acetic acid (MAA) was proposed. It was attributed to the Cd‐O bond breaking and the Cd‐S bond formation. The intensity and wavelength of fluorescence emission spectra could be adjusted by controlling the concentration of MAA. The photoluminescence (PL) spectra and UV‐Vis absorption spectra of colloidal cluster CdSe QDs indicated that the electronic properties of the interface varied with the exchange of the surface ligands. Moreover, the band‐edge emission quenching occurred and the maximum redshift of the PL spectrum could reach 28 nm. The enhancement of the deep trap emission peaks of CdSe QDs appeared in 600‐900 nm region, and the maximum blue shift of the defect peak in PL spectrum could reach 80 nm with the increase concentration of MAA. The absorption spectra and PL spectra on the surface of porous alumina thin film were used to further analyze the charge separation and capture of charge carriers. This novel organic‐inorganic ligand exchange method provides an effective strategy for regulating fluorescence and electron transfer, which could be used on biological, solar cells or other practical applications.

Exciton-Phonon Interaction and Role of defect/trap states in CdSe Quantum Dots

Semiconductor quantum dots have become potential candidate for application in LEDs, diode lasers, solar cells and biological labelling. Basic understanding of their photo physical processes affecting their optical transition characteristics is required for their efficient use in different applications. We report here role of phonons and defect/trap states on the photoluminescence intensity inferred from temperature dependence of integrated intensity of main excitonic peak in the temperature range 30-300 K. Clear demarcation of two photoluminescence quenching processes operative in low and high temperature regions has been observed. We find the thermal activation energy of the trap states to be ∼ 7 meV for main excitonic emission peak and ∼ 10.5 meV for deep trap emission. Thermal escape is assisted by three phonons inferred from both emission peak analysis.

Electronic structure and chemical nature of oxygen dopant states in carbon nanotubes.

We performed low temperature photoluminescence (PL) studies on individual oxygen-doped single-walled carbon nanotubes (SWCNTs) and correlated our observations to electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the E11 bright exciton peak. Our simulation suggests an association of these peaks with deep trap states tied to different specific chemical adducts. In addition, oxygen doping is also observed to split the E11 exciton into two or more states with an energy splitting <40 meV. We attribute these states to dark states that are brightened through defect-induced symmetry breaking. While the wave functions of these brightened states are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with our experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from interaction between pinned excitons and one-dimensional phonons. Exciton pinning also increases the sensitivity of the deep traps to the local dielectric environment, leading to a large inhomogeneous broadening. Observations of multiple spectral features on single nanotubes indicate the possibility of different chemical adducts coexisting on a given nanotube.

Improvement of the luminescent properties of cadmium sulfide quantum dots by a post-synthesis modification

Here the improvement of the luminescent properties of CdS quantum dots by a post-synthesis modification with aqueous solutions of NaOH at different concentrations is presented. The CdS quantum dots were synthesized by a microwave-assisted method using citrate ions as stabilizer. The addition of the hydroxide ions increased the intensity of the orange-red emission by about 80%. Besides, a violet-blue emission was achieved by means of this post-synthesis modification. The hydroxide ions control the precipitation equilibria of the CdS and Cd(OH)2, dissolving and precipitating the surface of the quantum dots. The NaOH treatment increases the number of traps, which produces less band-edge and more deep-trap emission, which explains the decrease and increase in the intensity of the violet-blue and orange-red emissions, respectively.

Shallow and deep trap emission and luminescence quenching of TiO2 nanoparticles on Cu doping

TiO2 nanoparticles with 2 and 4 % Cu are synthesized by sol–gel method. The crystalline phase and size of the nanoparticles are investigated with X-ray diffraction and transmission electron microscope. Cu-doped TiO2 has an extended absorption ranging from UV to visible region. Doping of Cu disturbs the arrangement of oxygen ions around Ti4+ and generates oxygen vacancies. These oxygen vacancies capture electrons and form some ionized oxygen vacancy centers or F centers. These F centers form subband states extending from shallow to the deep level in the band gap of TiO2. The visible emission peaks of pure and doped TiO2 are mainly associated with self-trapped excitons (STEs) and F centers. We have observed that Auger type nonradiative recombination is responsible for the quenching of the UV and STE emission peak in the doped samples. The intense visible emission peaks in pure TiO2 are due to shallow type centers whereas deep trap emission is predominant in doped samples. The intensity of UV and visible emission peaks are quenched with the increase in the doping level of Cu. Defects, Cu d-states, band structure of TiO2 and low mobility of the carriers are responsible for the quenching of the emission peaks.

Optical and magnetic properties of Cu-doped ZnO nanoparticles

Cu-doped ZnO nanoparticles were synthesized by a simple chemical method at low temperature with Cu:Zn atomic ratio from 0 to 5 %. The synthesis process was based on the hydrolysis of zinc acetate dehydrate and copper acetate tetrahydrate heated under reflux to 65 °C using methanol as a solvent. X-ray diffraction (XRD) analysis reveals that the Cu-doped ZnO crystallize in a wurtzite structure with a change of crystal size from 12 nm for undoped ZnO to 5 nm for Cu-doped ZnO. These nano size crystallites of Cu doped ZnO self-organized into microspheres. The XRD patterns, Scanning electron microscopy and transmission electron microscopy micrographs of doping of Cu in ZnO confirmed the formation of microspheres and indicated that the Cu2+ is successfully substituted into the ZnO host structure of the Zn2+ site. Cu doping shifts the absorption onset to blue from 373 to 350 nm, indicating an increase in the band gap from 3.33 to 3.55 eV. A relative increase in the intensity of the deep trap emission of Cu-doped ZnO is observed when increasing the concentration of Cu. Magnetic measurements indicate that Cu-doped ZnO samples are ferromagnetic at room temperature except pure ZnO.

Strategy for Synthesizing Quantum Dot-Layered Double Hydroxide Nanocomposites and Their Enhanced Photoluminescence and Photostability

Layered double hydroxide-quantum dot (LDH-QD) composites are synthesized via a room temperature LDH formation reaction in the presence of QDs. InP/ZnS (core/shell) QD, a heavy metal free QD, is used as a model constituent. Interactions between QDs (with negative zeta potentials), decorated with dihydrolipoic acids, and inherently positively charged metal hydroxide layers of LDH during the LDH formations are induced to form the LDH-QD composites. The formation of the LDH-QD composites affords significantly enhanced photoluminescence quantum yields and thermal- and photostabilities compared to their QD counterparts. In addition, the fluorescence from the solid LDH-QD composite preserved the initial optical properties of the QD colloid solution without noticeable deteriorations such as red-shift or deep trap emission. Based on their advantageous optical properties, we also demonstrate the pseudo white light emitting diode, down-converted by the LDH-QD composites.

Near-Infrared Luminescence of Cadmium Pigments: In Situ Identification and Mapping in Paintings

A comprehensive study of the luminescence properties of cadmium pigments was undertaken to determine whether these properties could be used for in situ identification and mapping of the pigments in paintings. Cadmium pigments are semiconductors that show band edge luminescence in the visible range and deep trap luminescence in the red/infrared range. Emission maxima, quantum yields, and excitation spectra from the band edge and deep trap emissions were studied for sixty commercial cadmium pigments that span the color range from yellow to red (reflectance transition 470 to 660 nm). For paints containing cadmium pigments, luminescence from deep traps was more readily observable than that from the band edge, although the yield varied widely from zero to around 4.5%. Optimal excitation for emission is found to be in the visible for both pigments in powder form and mixed with a medium. The maxima of the deep trap emission shift with the band gap energy, providing a potentially useful way to assign pigment type even when used in pigment mixtures. The usefulness of the results of the study on mockups was demonstrated by the mapping of cadmium pigments of different hues with the aid of calibrated luminescence imaging spectroscopy in a painting by Edward Steichen, entitled Study for 'Le Tournesol' (1920). Analysis of the luminescence image cube reveals at least six unique spectral components, associated with emission from white pigments, paint binder, and cadmium red and yellow pigments. The results were compared with those from X-ray fluorescence spectrometry (XRF) and fiber-optic reflection spectroscopy (FORS) and the results obtained on paint samples containing cadmium pigments. These results show that, when present, the emission from traps can be used as an analytical tool to identify cadmium pigments, to distinguish among cadmium sulfide, cadmium zinc sulfide, and cadmium sulfoselenide, and to map cadmium pigments, even in mixtures.

Tuning the Emission of CdSe Quantum Dots by Controlled Trap Enhancement

Ligand exchange with 3-mercaptopropionic acid (MPA) has been successfully used to tune the emission intensity of trioctylphosphineoxide/dodecylamine-capped CdSe quantum dots. Addition of 3-mercaptopropionic acid (MPA) to CdSe quantum dot suspension enhances the deep trap emission with concurrent quenching of the band edge emission. The smaller sized quantum dots, because of larger surface/volume ratio, create a brighter trap emission and are more easily tuned. An important observation is that the deep trap emission which is minimal after synthesis is brightened to have a quantum yield of 1-5% and can be tuned based on the concentration of MPA in solution with the quantum dots. Photoluminescence decay and transient absorption measurements reveal the role of surface bound MPA in altering the photophysical properties of CdSe quantum dots.

Morphological tuning, self-assembly and optical properties of indium oxide nanocrystals

In this paper, weak acids, weak bases or their mixtures were used as reaction media/coordinating ligands to achieve systematic morphological control over amphoteric indium oxide nanostructures. Different indium/oleic acid molar ratios from 1 : 0, 1 : 1, 1 : 2, 1 : 3, 1 : 6 and 1 : 15 in non-coordinating, weakly coordinating, strongly coordinating and their mixed media were adopted to prepare irregular aggregated nanoparticles and uniform regular/truncated octahedra, etc. In addition to their strong size-dependent absorption, single-crystalline indium oxide octahedra also gave a strong band-edge emission while irregular indium oxide aggregated nanoparticles only exhibited a weak deep-trap emission. Meanwhile, the truncated octahedra were self-assembled into either zigzag lines or pentagram patterns, and the regular octahedra and truncated cubes were self-assembled into hexagonally packed nanocrystal arrays. In addition, the formation mechanism of the various nanostructures under different conditions was investigated in detail.

Effects of surface passivation on the exciton dynamics of CdSe nanocrystals as observed by ultrafast fluorescence upconversion spectroscopy

The exciton dynamics of CdSe nanocrystals are intimately linked to the surface morphology. Photo-oxidation of the selenium surfaces of the nanocrystal leads to an increase in radiative decay efficiency from both the band edge and deep trap emission states. The addition of the primary amine hexadecylamine curtails nonradiative excitonic decay attributed to the dangling surface selenium orbitals by passivation of those trap sites by the methylene protons on the amine, leading to enhanced band edge emission and the absence of deep trap emission. Furthermore, CdSeZnSe core/shell nanocrystals are not immune from contributions from surface states because of the alignment of the band structures of the core and shell materials.

Synthesis and Characterization of Zinc-Blende CdSe-Based Core/Shell Nanocrystals and Their Luminescence in Water

Zinc-blende CdSe cores are used as a substrate for the synthesis of core/shell nanocrystals, such as CdSe/ZnS, CdSe/ZnSe/ZnS, and CdSe/CdS/ZnS. Only two monolayers of shell coverage for each material suffice to enhance the photoluminescence (PL) quantum efficiency and achieve ∼50% PL efficiency in water from all core/shell nanocrystals after ligand exchange with 3-mercaptopropionic acid. Powder X-ray diffraction (XRD) patterns and high-resolution transmission electron microscopy images confirm the coherent epitaxial growth of the zinc-blende shell for core/shell nanocrystals. The PL spectra obtained at 5 K illustrate the effects of the shell composition on deep-trap emission, which manifests the role of hole-trapping surface defects. The spectral shift in both the first absorption maximum and PL band varies with the shell composition following the simple band-offset picture. The shell-to-shell variation of the spectral shift and changes in XRD patterns suggests that the contraction of CdSe lattice occurs ...

Growth of ZnS microfans and nanosheets: Controllable morphology and phase

Abstract Zinc sulfide with different phases and morphologies were solvothermally synthesized in diethylenetriamine (DETA) or a mixed solution of DETA and water at 180 °C. Wurtzite ZnS microfans composed of nanobelts with thickness of 30 nm were formed in pure DETA. Sphalerite ZnS nanosheets with thickness of 20 nm were produced in a mixed solvents of DETA and water with a volume ratio of 3:1. As the volume ratio of DETA to water was more than 3:1, the products were also nanosheets, while these nanosheets were in the mixed phases of wurtzite and sphalerite. UV–vis absorption spectrum of the wurtzite ZnS microfans showed a strong blue shift of 27 nm relative to the band gap of bulk wurtzite ZnS, which was attributed to the quantum size effect. Photoluminescence (PL) spectrum of wurtzite ZnS microfans presented a strong emission at 432 nm and a weak emission at 412 nm, which are in accordance with the reported data. The sphalerite ZnS nanosheets had a PL peak at 424 nm, indicating a weak deep-trap emission feature.

ZnO and Mg xZn 1−xO nanocrystals grown by non-hydrolytic route

ZnO and Mg x Zn 1− x O nanocrystals with Mg concentration of 2−30 at% were grown with non-hydrolytic route employing metal cupferronate complex as precursors. ZnO nanocrystals are spherical and their sizes are independent of Zn precursor concentration, but incorporation of Mg results in Mg x Zn 1− x O nanocrystal morphology and size dependence on Mg precursor concentration. Mg x Zn 1− x O exciton emission is dominant when Mg concentration is less than 10 at%. With a further increase in Mg concentration, deep trap emission becomes dominant. Mg x Zn 1− x O nanocrystal band gap enlarges obviously with an increase in Mg concentration from 2 to 15 at%. With a further increase in Mg concentration, Mg x Zn 1− x O band gap enlarges slowly. Mg x Zn 1− x O band gap can be tuned from 3.30 to 3.84 eV with an increase in Mg concentration from 2 to 30 at%.

Optical processes in the formation of stimulated emission from ZnO nanowires

This paper studies power dependent photoluminescence spectra, the stimulated emission occurring at ultraviolet (UV) band instead of the green emission band of ZnO nanowires, which are prepared with a chemical reduction method. The dynamics of the UV emission and green emission is given to demonstrate the reason of stimulated emission occurring at UV band but not the green emission band under high excitation, which indicates that the slow decay rate of trap state makes it easy to be fully filled and saturated, while the fast decay rate of near-band-edge exciton state makes the UV emission dominate the radiative recombination under high excitation. The UV emission, as well as the corresponding stimulated emission, occurs in competition with the green deep-trap emission. In addition, when pump fluence further increases, the multiple lasing modes appear. The dependence of these lasing modes on the pump fluence is first discussed. This diagram should be helpful to understand and design the optical nanodevices of ZnO nanowires.

Generation and Optical Properties of Monodisperse Wurtzite-Type ZnS Microspheres

Monodisperse wurtzite-type ZnS microspheres have been prepared by using glutathione (GSH) as a sulfur source at low reaction temperatures ranging from 160 to 210 degrees C. The diameter of the ZnS microspheres can be tuned from approximately 254 to approximately 597 nm by changing the reaction parameters such as temperature, molar ratio of reactants (GSH/Zn2+), and reaction medium (ethylenediamine or ammonia). Our results demonstrate that monodentate amines (ammonia) play the same role as that of bidentate amines (ethylenediamine) in the formation of the wurtzite-type ZnS microspheres. The formation process of the monodisperse ZnS microspheres consists of a GSH-dominated nucleation process and an amine-dominated assembly process. The as-synthesized monodisperse ZnS microspheres readily self-assemble into ordered hexagonal patterns and thus have potential applications as colloidal crystalline materials. Blue fluorescence emission peaks at 415 and 466 nm in wavelength, attributed to deep-trap emission, are observed at room temperature.

Modulation of the Morphology of ZnO Nanostructures via Aminolytic Reaction: From Nanorods to Nanosquamas

Various diversified morphology-modulated ZnO nanostructures including nanorods, nanotetrahedrons, nanofans, nanodumbbells, and nanosquamas have been successfully prepared via an effective aminolytic reaction of zinc carboxylates with oleylamine in noncoordinating and coordinating solvents. Their shape- and structural defect-dependent optical properties have been investigated as well. Highly crystalline defect-free nanotetrahedrons/nanorods have a sharp band-edge emission, and highly defective nanodumbbells/nanosquamas show a very broad deep-trap emission, resulting from the radiative recombination of electrons with holes in singly ionized oxygen vacancies.

Synthesis of high-quality CdS, ZnS, and ZnxCd1 − xS nanocrystals using metal salts and elemental sulfur

High-quality CdS, ZnS and alloyed ZnxCd1 − xS nanocrystals are synthesized in the commonly available coordinating solvent, high boiling-point long-chain amines, using the corresponding metal salts (such as metal stearate, acetate, chloride, sulfate etc.) and elemental sulfur as the precursors. The obtained CdS and alloyed ZnxCd1 − xS nanocrystals possess quantum-confinement wavelength-tunable optical absorption and band-edge emission with photoluminescence quantum yields of 25–45%. The photoluminescence spectra are dominated by the band-edge emission without deep-trap emission. Particular interests in this synthetic approach are that cubic zinc-blende CdS and ZnxCd1 − xS nanocrystals are formed at high temperatures, and the rapid nucleation and instant termination of the crystal growth process is observed. This nucleation and growth feature makes the reproducible and controllable synthesis of nanocrystals with specific emission wavelengths feasible. The alloy structure of the ZnxCd1 − xS nanocrystals is proved by the optical spectra and XRD diffractograms, in which the absorption onset and emission peak shift to shorter wavelength; the diffraction peaks systematically shift toward larger angles with an increase of the Zn molar ratio in the material.

Molecular Clusters of Binary and Ternary Mercury Chalcogenides: Colloidal Synthesis, Characterization, and Optical Spectra.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Temperature dependence of electronic energy transfer from a polymer host to a triplet emitter in light emitting diode materials

We investigate the temperature dependence of electronic energy transfer between a polymer host, poly( N-vinylcarbazole) blended with 2-(4-biphenylyl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole, and the triplet emitter tris[9,9-dihexyl-2-(phenyl-4 ′-(pyridin-2 ′′-yl))fluorene] iridium(III) (Ir(DPPF) 3). The photophysics of the blend are similar to those of neat poly( N-vinylcarbazole), with two emitting species corresponding to shallow and deep excimer traps. When the blend is doped with Ir(DPPF) 3, the deep trap emission is preferentially quenched. This quenching cannot be explained by simple Förster energy transfer to the dopant. Instead, the data are consistent with a rapid, temperature-dependent partitioning of the energy between the two host species and the Ir(DPPF) 3, each of which decays independently.