Photoluminescence Wavelength Research Articles

Fabrication and luminescent properties of core-shell InGaN/GaN multiple quantum wells on GaN nanopillars

Core-shell InGaN/GaN multiple quantum wells (MQWs) on GaN nanopillars were fabricated by top-down etching followed by epitaxial regrowth. The regrowth formed hexagonal sidewalls and pyramids on the nanopillars. The cathodoluminescence of MQWs blue shifts as the location moves from top to bottom on both the pillar sidewalls and pyramid facets, covering a spectral linewidth of about 100 nm. The MQWs on the pillar sidewalls have a higher InN fraction than those on the pyramid facets. The photoluminescent wavelength is stable over two orders of carrier density change due to the smaller quantum confined Stark effect on the nanopillar facets.

Effect of Silver Ion‐Exchange on the Precipitation of Lead Sulfide Quantum Dots in Glasses

This article reports the effect of Ag+ ion‐exchange on the precipitation of PbS quantum dots (QDs) in silicate glasses. The glasses were subjected to ion‐exchange in AgNO3 salt melts at 260°C for 60 s. Ag nanoparticles (NPs) and PbS QDs precipitated after heat treatment at temperatures of 440°C–460°C for 10 h. Photoluminescence wavelength λPL of PbS QDs was longer (~1525 ≤ λPL ≤ ~1580 nm) in Ag+ ion‐exchanged regions than in Ag+‐free regions (~1030 ≤ λPL ≤ ~1200 nm). Average diameter D of PbS QDs estimated by calculation and TEM images was larger in Ag+ ion‐exchanged regions (~5.2 ≤ D ≤ ~5.5 nm) than in Ag+‐free regions (~2.4 ≤ D ≤ ~3.3 nm). Ag NPs that formed during heat treatment provided the sites for heterogeneous nucleation of PbS QDs and promoted the precipitation of PbS QDs in glasses.

Post-growth annealing of type-II GaSb/GaAs quantum dots grown with different V/III ratios

Abstract We report the influence of V/III beam-equivalent-pressure ratios and post-growth annealing on the photoluminescence of GaSb quantum dots grown on GaAs(1 0 0) by molecular beam epitaxy. Increasing the V/III beam-equivalent-pressure ratio from 3 to 5 and then to 7 results in decreased photoluminescence intensity and redshifts the photoluminescence wavelength. The post-growth annealing blueshifts the quantum dot photoluminescence emission and decreases the full-width-at-half-maximum of the photoluminescence peak when annealing temperature is increased above 800 °C. The blueshift behavior is found to be independent on the V/III ratios indicating a similar atomic interdiffusion mechanism for all investigated samples regardless of the quantum dot properties. The photoluminescence intensities of the three samples experience an increase after moderate annealing. Whereas the intensity of the sample with the highest V/III ratio further increases, the intensity of the sample with lower V/III ratios decreases again upon higher annealing steps above 900 °C. Furthermore, temperature- and power dependent photoluminescence measurements are performed on as-grown and 870 °C annealed samples with V/III ratios of 3 and 7 in order to study the reduced quantum dot confinement in more detail.

Photoexcited carrier dynamics in AlInN/GaN heterostructures

Photoexcited carrier dynamics and localization potentials in Al0.86In0.14N/GaN heterostructures have been examined by time-resolved and scanning near-field photoluminescence (PL) spectroscopy. The large GaN and AlInN PL intensity difference, and the short AlInN PL decay and GaN PL rise times indicate efficient photoexcited hole transfer from AlInN to GaN via sub-band-gap states. These states are attributed to extended defects and In clusters. Near-field PL scans show that diameter of the localization sites and the distance between them are below 100 nm. Spatial variations of the GaN PL wavelength have been assigned to the electric field inhomogeneities at the heterostructure interface.

Effects of Inhomogeneous Shell Thickness in the Charge Transfer Dynamics of ZnTe/CdSe Nanocrystals

ZnTe/CdSe core/shell nanocrystals with varying ZnTe core sizes and CdSe shell thickness have been synthesized. The absorption onset and photoluminescence (PL) wavelengths vary from the mid-visible to the near-infrared range. The conduction and valence band energies as well as bandgap and electron–hole overlap are accurately described by an effective mass approximation model. These particles are type-II, with the hole confined to the ZnTe core. PL quenching dynamics of ZnTe/CdSe nanocrystals having various shell thicknesses by hole acceptors are studied. A model has been developed to describe the interfacial charge transfer of the core-confined holes to adsorbed acceptors. This model considers a Poisson distribution of acceptor numbers and includes shell thickness variability. The shell thickness variability occurs on each particle and corresponds to surface roughness. The shell local thickness is assumed to have a Poisson distribution of numbers of layers.

Amorphous carbon-based films with surface-plasmon-enhanced full-color photoluminescence

A series of amorphous carbon-based films were deposited on the nanostructured Ag layers to observe surface plasmon (SP) enhanced photoluminescence (PL). The dependence of PL peak wavelength and intensity on the film composition and the nanostructure of the Ag layers were systematically investigated. The PL wavelength was tuned from 442nm to 635nm by varying the carbon content of the as-deposited carbon-based films. The nanostructure of Ag layers varied from nanoparticles (NPs) to continuous films via process control. With the SPs generated at the carbon-based film/Ag layer interface, the PL intensity was found to be significantly enhanced with a peak enhancement factor of 6, and the light emission range of the composite films was extended to 434–653nm. The dependence of PL intensity on the spectral overlap between the carbon-based films and plasmon resonant Ag layers, Ag surface morphology and the internal quantum efficiency (IQE) of the carbon-based films was discussed. The redshift of SP resonance with the increasing refractive index of the upper carbon-based films was observed.

Site-selective assembly of quantum dots on patterned self-assembled monolayers fabricated by laser direct-writing

A simple and efficient route for quantum dot (QDs) patterning using self-assembled monolayers (SAMs) as templates is described. By means of a laser direct-writing (LDW) technique, SAMs of octadecylphosphonic acid formed by adsorption on native oxide layer of titanium film were patterned through laser-induced ablation of the SAM molecules. This technique allows the creation of chemical-specific patterns accompanied by slight change in the topography. Using atomic force microscopy and friction force microscopy, the dependence of feature size and characteristics on the irradiation dose was demonstrated. Upon immersion of a substrate with patterned SAMs bearing thiol as the terminal group into a dispersion of QDs resulted in the assembly of QDs on the specific thiol-terminated areas. Patterns of QDs with different photoluminescent wavelength were generated. The LDW technique, which is convenient and flexible due to its path-directed and maskless fabrication process, provided a new powerful approach for patterning materials on surfaces for various applications.

Porosity-induced relaxation of strains in GaN layers studied by means of micro-indentation and optical spectroscopy

We report the fabrication of porous GaN nanostructures using UV-assisted electroless etching of bulk GaN layer grown on c-plane sapphire substrate in a solution consisting of HF:CH3OH:H2O2. The morphology of the porous GaN nanostructures was characterized for different etching intervals using high resolution scanning electron microscopy. The geometry and size of resultant pores do not appear to be affected by the etching time; however, the pore density was augmented for longer etching time. Micro-indentation tests were carried out to quantify the indentation modulus for different porous GaN nanostructures. Our results reveal a relationship between the elastic properties and the porosity kinetics, i.e., a decrease of the elastic modulus was observed with increasing porosity. The photoluminescence (PL) and Raman measurements carried out at room temperature for the etched samples having a high degree of porosity revealed a strong enhancement in intensity. Also, the peak of the PL wavelength was shifted towards a lower energy. The high intensity of PL was correlated to an increase of scattered photons within the porous media and to the reduction of the dislocation density.

Producing Photoluminescent Species from Sp2 Carbons

The treatment of sp2 carbon materials, including micrographite, nanographite, HOPG, onion-like-carbon, and single-walled carbon nanotubes, in a 3:1 sulfuric to nitric acid mixture produced photoluminescent reaction solutions. These colloidal, aqueous solutions appeared photoluminescently stable under a UV lamp and ranged in color from red to blue. The photoluminescent wavelength shifted to shorter wavelength with increasing reaction time or increasing reaction temperature. Raman spectroscopy showed evidence of defect structures in graphitic residue, and transmission electron microscopy showed unusual structures present in the supernatant including graphitic balls.

Simple Tuning of the Optoelectronic Properties of IrIII and PtII Electrophosphors Based on Linkage Isomer Formation with a Naphthylthiazolyl Moiety

AbstractBy tuning the substitution position of a thiazolyl group on the naphthalene system (α or β site), the distinctive electronic structures associated with these functional ligands have a substantial influence on both the photophysical behavior and electroluminescent (EL) performance of the resulting linkage isomers for two series of IrIII and PtII phosphorescent emitters. The photoluminescence wavelength can be redshifted by ca. 23 nm for the linkage isomers upon replacing the β‐substituted thiazolyl‐based ligand with its α‐substituted counterpart in the homoleptic series of IrIII phosphors. Furthermore, the bathochromic effect can be as much as ca. 42 nm for the heteroleptic IrIII phosphors and ca. 59 nm for the PtII compounds. Similarly, metallophosphors that bear β‐substituted ligands exhibit a different EL performance with respect to that of their linkage isomers with α‐substituted ligands. The best EL results associated with the triplet emitters chelated with β‐substituted ligands show a maximum brightness (Lmax) of 22563 cd m–2, an external quantum efficiency (ηext) of 12.88 %, a luminance efficiency (ηL) of 30.84 cd A–1, and a power efficiency (ηp) of 26.17 lm W–1, whereas the EL performance of their α‐counterparts was characterized by a peak Lmax of 8653 cd m–2, ηext of 6.18 %, ηL of 8.55 cd A–1, and ηp of 6.54 lm W–1. Owing to its unique electronic structure, the thiazolyl group is a good alternative to the pyridyl moiety to improve the EL performance of the metallophosphor. We have also demonstrated a simple and useful route to tune the functional properties of cyclometalated triplet emitters for EL applications.

One-Step Synthesis and Photoluminescence Evaluation of Cadmium-Containing Quantum Dots

Abstract We have developed a simple one-step process for synthesis of ternary quantum dots (ZnCdSe, MgCdSe) with photoluminescence wavelengths ranging from the red to the blue region of the visible spectrum. The primary aim of this work was to develop a synthesis for the preparation of Cd-containing quantum dots using a Cd precursor with lower toxicity than those used in common syntheses. This synthesis makes use of Cd(acac)2 which is significantly less toxic than precursors such as CdO and CdCl2. We have studied the effect of solvent boiling point, precursors and reaction time on the photoluminescence properties of the ternary quantum dots. Ternary quantum dots synthesized from Cd(acac)2 in low boiling point solvents have photoluminescence wavelengths in the blue region, while those synthesized in high boiling point solvents have photoluminescence wavelengths in the red region.

Photoluminescence from In0.5Ga0.5As/GaP quantum dots coupled to photonic crystal cavities

We demonstrate room temperature visible wavelength photoluminescence from In0.5Ga0.5As quantum dots embedded in a GaP membrane. Time-resolved above band photoluminescence measurements of quantum dot emission show a biexpontential decay with lifetimes of ~200 ps. We fabricate photonic crystal cavities which provide enhanced outcoupling of quantum dot emission, allowing the observation of narrow lines indicative of single quantum dot emission. This materials system is compatible with monolithic integration on Si, and is promising for high efficiency detection of single quantum dot emission as well as optoelectronic devices emitting at visible wavelengths.

The saturation density property of (B)InAs/GaAs quantum dots grown by metal-organic chemical vapor deposition

The density property of InAs/GaAs quantum dots (QDs) grown by metal-organic chemical vapor deposition (MOCVD) was investigated in detail. The saturation density of InAs/GaAs QDs was experimentally demonstrated. The InAs deposition thickness (InAs coverage) when the saturation density reached was also found. Furthermore, we systemically analyzed the influences of growth temperature and growth rate of InAs/GaAs QDs on the saturation density. Finally, BInAs/GaAs QDs with room-temperature photoluminescence (RT-PL) wavelength of 1320nm and full width at half maximum (FWHM) of 40mev were achieved for the first time, and the effects of boron incorporation on InAs/GaAs QDs were studied.

Wavelength-Controlled Etching of Silicon Nanocrystals

A detailed investigation of the photochemical etching of silicon nanocrystals (Si-NCs) in aqueous hydrofluoric acid (HF) mixtures is described. By increasing the HF concentration and lowering the pH upon addition of hydrochloric acid (HCl), oxide-embedded Si-NCs were rapidly etched, and photoluminescence (PL) from the resulting hydride-terminated Si-NCs tailored from the near-IR to the yellow/green spectral region by irradiating the reaction mixture at the desired PL wavelength. These results are consistent with a hole-driven etching pathway rather than the chemically induced oxidation/etch pathway commonly exploited for Si nanostructures. The relationship among NC size, polydispersity, and PL was investigated using small-angle X-ray scattering. We suggest the defect density of Si-NCs is a crucial parameter for effective size control via photochemical exciton-mediated HF etching. Improved HF etching methods are expected to enable Si-NC applications through the realization of narrow polydispersity and PL b...

Comparisons of the structural and optical properties of o-AgInS2, t-AgInS2, and c-AgIn5S8 nanocrystals and their solid-solution nanocrystals with ZnS

Cubic AgIn5S8 (c-AgIn5S8) as well as orthorhombic AgInS2 (o-AgInS2) and tetragonal AgInS2 (t-AgInS2) nanocrystal (NC) luminophores are successfully prepared by adjusting the stoichiometric [Ag] : [In] ratio at reaction temperatures ranging from 100 to 180 °C. The photoluminescence (PL) quantum yield (QY) of c-AgIn5S8 NCs reached 18.8%, which is much higher than that of o-AgInS2 and t-AgInS2. In this study, the identification and characterization of efficient c-AgIn5S8 luminophore NCs are reported for the first time. Broad PL bands, large Stokes shifts and long PL lifetimes indicate that the PL of c-AgIn5S8 as well as o-AgInS2 and t-AgInS2 NCs can be attributed to donor–acceptor (D–A) pair recombinations. The PL peak shifts and changes in the PL intensity depending on the excitation intensity and temperature confirm that the c-AgIn5S8 NCs comply with D–A pair recombinations. The PL emission of the NCs can be enhanced via a solid-solution with ZnS and the PL wavelength can be tuned by varying the stoichiometric [Ag] : [In] ratio and the preparation temperature. The best c-AgIn5S8–ZnS sample presents a PL emission in the range of 552–587 nm with a maximum QY of 61.3%. The luminous efficacy and color rendering index (CRI) of these AgIn5S8–ZnS NC-based white light-emitting diodes (LEDs) were respectively 53 lm W−1 and 74 at 3700 K under 60 mA. The development of an efficient AgIn5S8–ZnS NC to be applied as a color-converting material in white LEDs as demonstrated in this work provides the possibility of various potential applications in the fields of optic, optoelectronic and bio-related devices.

Study of multiple-stacking growth of 1.55 μm InAs columnar quantum dots with modulated tensile-strained InGaAsP barrier layers

An investigation was performed of columnar InAs quantum dots (CQDs) with modulated tensile-strained InGaAsP barriers in which the amount of tensile strain in the upper parts was higher than in the lower parts, the dots being deposited on an InP substrate grown by metalorganic vapor phase epitaxy. The smaller tensile strain of the barrier layers in the lower parts made the photoluminescence (PL) wavelength longer while the larger tensile strain of the barrier layers in the upper parts increased the strain compensation of the CQDs. Compared to CQDs with uniformly tensile-strained barriers, 1.55 μm emission was obtained at a higher average strain of barrier layers. By utilizing modulated tensile-strained barriers, triple-stacking of 12-fold CQDs with a PL wavelength of 1.55 μm using 30-nm-thick spacer layers was achieved with good crystallinity, indicating suitability for fabrication of high density CQDs.

Reduction of internal polarization fields in InGaN quantum wells by InGaN/AlGaN ultra-thin superlattice barriers with different indium composition

The internal well polarization field in InGaN quantum wells (QWs), surrounded by strain-compensated (InxGa1−xN)/(Al0.065Ga0.935N) ultra-thin superlattice (SC-SL) barriers with different indium composition, is investigated. The indium composition of InGaN constituent of superlattice barriers has been varied in the range from 0.04 to 0.18. It is observed that the increase of indium composition of InGaN into the barrier results in a strong blue-shift of the peak wavelength of the room-temperature photoluminescence (RT-PL) and the significant increase in the intensity of the luminescence emission until too much indium is added into InGaN layers of superlattice barriers. From the bias-dependent photoluminescence measurements, it is determined that the blue-shift and intensity increase of the emission are caused by the decrease of well polarization field as the indium composition in InGaN of SC-SL barrier increases. In case of In0.16Ga0.84N containing SC-SL barriers, the well internal polarization field is greatly reduced to −0.33 MV/cm from −1.5 MV/cm with respect to typical GaN barriers, indicating that the internal field reduction similar to that obtained in semi-polar InGaN/GaN quantum wells can be obtained by applying the strain-compensating barrier to polar substrates.

Electrochromic performances and photoluminescence characteristics of versatile N-vinylimidazole-based hybrid hydrogels

We report the synthesis and multi-sensitive performances of a novel type of poly(N-vinylimidazole-co-N-methylolacrylamide) (poly(VI-co-NMA)) hydrogels. These hydrogels behave excellent electric-sensitive properties and reversible bending behaviors. The influence of various parameters such as VI concentration, electric voltage, pH, and ionic strength on the electric-sensitive properties was thoroughly investigated. To obtain electrochromic hydrogels, we further prepared colloidal crystal (CC)-loaded hydrogels in which the color of the CC can be tuned by the bending behavior of the hydrogels. Moreover, these as-prepared hydrogels present excellent adsorption performances toward quantum dots, and the photoluminescence (PL) of CdTe-doped poly(VI-co-NMA) hydrogels was investigated. We found that the PL wavelengths of the CdTe-doped hydrogels could be switched reproducibly by altering the process of swelling and deswelling. These multi-functional hydrogels coupling with the characteristics of electrochromism and PL response confer them a variety of potential applications in sensors.

Stabilization of Excited State Using Through-Space Interaction between Independent π-Systems Mediated by a peri-Substituted Hydroxy Group in 1-Arylnaphthalenes: Unexpected Blue Emission of 1,3,5-Tris(peri-hydroxynaphthyl)benzene

Abstract The control of photoluminescence wavelength is important for the design of emitting materials, and compounds bearing aryl–aryl bonds are considered to be useful candidates for the synthesis of these materials. However, the biaryl system lacks an efficient linkage of independent conjugated moieties because of a structure that, rather than planar, is made up of almost perpendicular dihedral angles due to steric effect. In this paper, we report a novel method to link independent π-systems in biaryl compounds. The oxy-substituents at the peri-position in 1-arylnaphthalene systems, which are biaryl compounds, showed interesting and unexpected photophysical properties of a bathochromic shift of emission. In particular, peri-OH-substituted arylnaphthalenes showed a large bathochromic shift that was induced by polarization based on intramolecular OH–aryl interaction. Quantum chemical (TDDFT) calculations accurately reproduced the large bathochromic shifts of peri-OH-substituted arylnaphthalenes. The through-space interaction between the peri-substituted OH and aryl groups generated an unexpected extended conjugation system. The substituting position of OH in 1-arylnaphthalenes is quite critical, because the hydroxy group mediates the independent π-systems to expand conjugation. Based on this concept, 1,3,5-tris(peri-hydroxynaphthyl)benzene had characteristic photophysical properties, giving an unexpected blue emission.

Electrical control of photoluminescence wavelength from semiconductor quantum dots in a ferroelectric polymer matrix

We report controllable tuning of the room temperature photoluminescence band of CdSe semiconductor quantum dots embedded in thin films of ferroelectric copolymer of vinylidene fluoride and trifluoroethylene made by Langmuir-Blodgett deposition. The high breakdown strength of the polymer permits the application of electric fields of up to 400 MV/m and results in a shift in the photoluminescence peak by up to 9 nm, nearly half the fluorescence band width. Moreover, we found that at these high electric fields, the Stark effect exhibits unusual fourth power dependence.