Enhancement Of Photoluminescence Efficiency Research Articles

Synthesis and Structure-Photophysics Evaluation of 2-N-Amino-quinazolines: Small Molecule Fluorophores for Solution and Solid State.

2-N-aminoquinazolines were prepared by consecutive SN Ar functionalization. X-ray structures display the nitrogen lone pair of the 2-N-morpholino group in conjugation with the electron deficient quinazoline core and thus representing electronic push-pull systems. 2-N-aminoquinazolines show a positive solvatochromism and are fluorescent in solution and in solid state with quantum yields up to 0.73. Increase in electron donor strength of the 2-amino substituent causes a red-shift of the intramolecular charge transfer (ICT) band (300-400 nm); whereas the photoluminescence emission maxima (350-450 nm) is also red-shifted significantly along with an enhancement in photoluminescence efficiency. HOMO-LUMO energies were estimated by a combination of electrochemical and photophysical methods and correlate well to those obtained by computational methods. ICT properties are theoretically attributed to an excitation to Rydberg-MO in SAC-CI method, which can be interpreted as n-π* excitation. 7-Amino-2-N-morpholino-4-methoxyquinazoline responds to acidic conditions with significant increases in photoluminescence intensity revealing a new turn-on/off fluorescence probe.

Enhancement of photoluminescence efficiency in GeSe ultrathin slab by thermal treatment and annealing: experiment and first-principles molecular dynamics simulations

The effect of thermal treatment and annealing under different temperatures from 100 °C to 250 °C on the photoluminescence spectroscopy of the GeSe ultrathin slab is reported. After the thermal treatment and annealing under 200 °C, we found that the photoluminescence intensity of A exciton and B exciton in GeSe ultrathin slab is increased to twice as much as that in untreated case, while is increased by ~84% in the photoluminescence intensity of C exciton. Combined by our experimental work and theoretical simulations, our study confirms the significant role of thermal treatments and annealing in reducing surface roughness and removing the Se vacancy to form more compact and smoother regions in GeSe ultrathin slab. Our findings imply that the improved quality of GeSe surface after thermal treatments is an important factor for the photoluminescence enhancement.

Enhancement of photoluminescence efficiency from semi-polar InGaN/GaN multiple quantum wells with silver metal

We have studied the effect of surface plasmon polariton (SPP) and localized surface plasmon (LSP) on the emission of semi-polar InGaN/GaN light emitting diode (LED) with multi-quantum wells structure. From the photoluminescence (PL) measurement at room temperature, spectrally-integrated enhancements of semi-polar SPP LEDs with 15 and 40-nm-thick Ag films were 1.7 and 2.9, respectively. The absorbance peak of Ag nanoparticles was red-shifted as diameter of Ag nanoparticles increases. However, the absorbance peak of Au nanoparticles was not related with their diameters. Spectrally-integrated enhancement of semi-polar LSP LED with 250-nm-diameter Ag nanoparticles was shown to 1.3. These results showed that the blue emission of semi-polar InGaN/GaN LED can be improved by SPP and LSP.

H− ion implantation induced ten-fold increase of photoluminescence efficiency in single layer InAs/GaAs quantum dots

We demonstrate a ten-fold increase in photoluminescence (PL) efficiency from 50keV H− ion-implanted InAs/GaAs quantum dots (QDs) at a temperature of 8K and/or 145K. Enhancement occurred without post-annealing treatment. PL efficiency increased with increasing implantation fluence from 6×1012ions/cm2 up to an optimum value of 2.4×1013ions/cm2, beyond which PL efficiency decreased drastically (up to a fluence of 2.4×1015ions/cm2). Passivation of non-radiative recombination centres (due to direct interaction of H− ions with lattice defects) and de-excitation of photo-generated carriers to QDs through quantum mechanical tunnelling via H− ion-induced defects (e-traps) that are created near the QD–cap layer interface, resulted in PL efficiency enhancement. Shallow e-traps with activation energy ~90meV and 30meV created near the conduction band of GaAs cap layer for the samples implanted with H− of fluence 6×1012 and 2.4×1013ions/cm2 respectively are identified using low temperature PL study. Contribution of de-trapped electrons from the e-traps to the QDs enhanced the PL efficiency at 145K. Cross-section transmission electron microscopy and X-ray diffraction study revealed that the structural damage created by H− ions at the high fluence level of 2.4×1015ions/cm2, caused the degradation in PL efficiency.

Enhancement of photoluminescence efficiency from GaN(0001) by surface treatments

We investigated the photoluminescence (PL) efficiency of GaN(0001) single crystals with clean and well-defined surfaces using the PL technique in ultrahigh vacuum in situ. We found typical degradation factors: native oxides at the top surface, damaged layers in the subsurface, and hydrogenated non-radiative states inside bulk GaN. By eliminating the degradation factors, a band-to-band PL intensity of approximately 120 times higher than that of the as-received samples was achieved. The PL efficiency enhancement mechanism is discussed, and the role of hydrogen in GaN crystals is proposed.

Ga nanoparticle-enhanced photoluminescence of GaAs

We have examined the influence of surface Ga nanoparticles (NPs) on the enhancement of GaAs photoluminescence (PL) efficiency. We have utilized off-normal focused-ion-beam irradiation of GaAs surfaces to fabricate close-packed Ga NP arrays. The enhancement in PL efficiency is inversely proportional to the Ga NP diameter. The maximum PL enhancement occurs for the Ga NP diameter predicted to maximize the incident electromagnetic (EM) field enhancement. The PL enhancement is driven by the surface plasmon resonance (SPR)-induced enhancement of the incident EM field which overwhelms the SPR-induced suppression of the light emission.

Core–shell nanoparticle of silver coated with light-emitting rubrene: Surface plasmon enhanced photoluminescence

Silver (Ag) nanoparticles (NPs) with a diameter of 50–100nm were fabricated by using a reduction process of silver nitrate with sodium citrate. Using the Ag NPs, hybrid core (Ag)–shell (organic light-emitting rubrene) NPs were prepared through a hydrothermal process. The formation of hybrid core–shell NP of Ag-rubrene was confirmed through transmission electron microscope images. From ultraviolet and visible absorption spectra, the Ag and rubrene characteristic peaks were simultaneously observed for the core–shell NPs. Using a high-resolution laser confocal microscope (LCM), the photoluminescence (PL) intensity of the Ag–rubrene core–shell single NP was about 300 times higher than that of the rubrene single NP. This remarkable enhancement of PL efficiency in the core–shell single NP is due to the energy transfer effect in the surface plasmon resonance coupling between Ag and rubrene materials.

Effect of high energy proton irradiation on InAs/GaAs quantum dots: Enhancement of photoluminescence efficiency (up to ∼7 times) with minimum spectral signature shift

We demonstrate 7-fold increase of photoluminescence efficiency in GaAs/(InAs/GaAs) quantum dot hetero-structure, employing high energy proton irradiation, without any post-annealing treatment. Protons of energy 3–5 MeV with fluence in the range (1.2–7.04) × 10 12 ions/cm 2 were used for irradiation. X-ray diffraction analysis revealed crystalline quality of the GaAs cap layer improves on proton irradiation. Photoluminescence study conducted at low temperature and low laser excitation density proved the presence of non-radiative recombination centers in the system which gets eliminated on proton irradiation. Shift in photoluminescence emission towards higher wavelength upon irradiation substantiated the reduction in strain field existed between GaAs cap layer and InAs/GaAs quantum dots. The enhancement in PL efficiency is thus attributed to the annihilation of defects/non-radiative recombination centers present in GaAs cap layer as well as in InAs/GaAs quantum dots induced by proton irradiation.

Polygermole Nanoaggregate Chemical Sensor for TNT: Enhancement of Photoluminescence Efficiency by Aggregation-Induced Emission

New photoluminescent and nanoscale polygermole nanoaggregates for the detection of 2,4,6-trinitrotoluene (TNT) were developed by using aggregation-induced emission property. Polygermole nanoaggregates displaying diameter of 40 nm exhibited that photoluminescence (PL) intensity of emission band was increased about 730% when the water fraction was increased to 90% by volume. Relative PL efficiency of polygermole nanoaggregates was exponentially increased to the percent of water fraction and particle diameter was dependent on solvent composition. Particle size of polygermole nanoaggregates was tuned by controlling the water fraction by volume. PL intensity of polygermole nanoaggregates was not changed over a month. This indicated that polygermole nanoaggregates showed neither further aggregation nor degradation. Detection of TNT was achieved from the measuring of quenching PL of polygermole nanoaggregates by adding the TNT A linear Stern-Volmer relationship was observed for the detection of TNT.

Silica-coated CdTe Quantum Dots of Unchanged Size with Intense Photoluminescence at Various Wavelengths

A new method for controlling the photoluminescence (PL) wavelength of silica-coated quantum dots (QDs) without changing the sizes of the core QDs is reported. When silica-coated green-emitting CdTe QDs were refluxed in aqueous solution containing Cd2+ and a sulfur-containing surfactant (thioglycolic acid (TGA)), the PL peak wavelength shifted to red with a significant increase in PL efficiency. This spectral shift was attributed to the reduction of the quantum confinement effect by the formation of small CdS-like clusters very close to the QDs in the silica shells. When the silica-coated QDs were refluxed in a solution of higher pH, the extent of this red shift increased. The larger red shift of the PL with pH is attributed to the hydrolysis speed of TGA increasing with pH. There is faster generation of S2− and as a consequence the growth rate of CdS-like clusters is enhanced. When a different sulfur-containing surfactant (thioglycerol (TG)) was used for reflux, the PL spectral shift and enhancement of PL efficiency became very small. This was because TG did not hydrolyze significantly enough to release S2−. These results demonstrate that the reported reflux procedure is a promising method for tuning the PL color of QDs through selection of the solution pH and the surfactant during reflux.

Improved luminescence efficiency of InAs quantum dots grown on atomic terraced GaAs surface prepared with in‐situ chemical etching

AbstractObservation of the enhanced luminescence efficiency of InAs quantum dots (QDs) grown on atomically controlled GaAs surfaces is reported. With the trisdimethylaminoarsenic (TDMAAs) in‐situ surface etching process, formation of atomic steps and terraces on GaAs surfaces were clearly observed. InAs QDs grown on the processed GaAs surfaces showed the clear dependence of QDs size, density and optical characteristics on the surface properties, i.e., the increase of the QDs height and diameter the decrease of the QDs density. About 6‐times enhancement of photoluminescence efficiency which has the peak around 1550‐nm wavelength was observed by growing InAs QDs on atomically controlled GaAs surfaces. This is due to the migration enhancement of InAs during thegrowth the QDs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Solvothermal Synthesis of High-Aspect Ratio Alloy Semiconductor Nanowires: Cd1−xZnxS, a Case Study

High-aspect ratio Cd1−xZnxS (x = 0−1) alloy semiconductor nanowires are reported here for the first time using an ethylenediamine (en)-assisted solvothermal approach. The composition of the en−water (en-w) mixed solvent plays a crucial role in determining the morphology and crystalline phase of the alloy nanowires in the middle range of x (i.e., when x approaches 0.5). A phase transformation from hexagonal to cubic was observed for the middle range of x in an en-dominated solvent (en/w, 5:1). Nanowires with hexagonal phases were formed for the entire range of x with increasing water content of the solvent system (en/w, 2:1). The aspect ratio of the nanowires was found to be dependent on the x value as well as solvent composition. Alloy nanowires exhibit photoresponse properties when illuminated under white light. The synthesis technique is modified to synthesize core−shell Cd1−xZnxS/ZnS nanowires. Enhancement in photoluminescence efficiency is observed with a ZnS shell over the alloy nanowires.

Enhancement of photoluminescence efficiency in binary quantum dot arrays in hybrid organic/inorganic materials

The photoluminescence (PL) quantum efficiency of dense semiconductor colloidal quantum dot (QD) arrays is significantly reduced by the quenching of the optical excitons via defect-induced nonradiative decay channels. This luminescence quenching is facilitated by the rapid migration of excitons between neighboring QDs via resonant Förster transfer processes. We propose to mitigate this quenching by using nonradiative “spacer” quantum dots (SQDs) to separate radiative primary quantum dots (PQDs). We have identified the maximum and minimum values of the PL quantum efficiency (PLQE) for different compositions of spacer-primary QD arrays. Using the kinetic Monte Carlo technique, we have found that for a given composition, the PLQE is highest for randomly distributed spacer and primary QDs, decreasing dramatically when clusters of SQDs and PQDs are formed. We have modeled cluster formation of QDs in binary QD arrays and determined the resultant PL properties. By comparing our simulation results with those we obtained experimentally, we have estimated the magnitude of the attractive pairwise van der Waals interaction between the QDs which is the driving force for QD cluster formation. We have also explored possible strategies to achieve the highest PLQE of the PQDs in hybrid organic/inorganic materials for photoelectronics and we found that lowering the mixing time of the binary QD mixture can improve the PLQE.

Effect of proton-irradiation on photoluminescence emission from self-assembled InAs/GaAs quantum dots

A study of the effects of proton-irradiation of InAs/GaAs quantum dots (QDs) is presented. The samples are simple GaAs-capped, single layer of InAs quantum dots, grown on GaAs substrates by molecular beam epitaxy. Using proton implantation (10keV to 1MeV protons, dose of up to 1×1013/cm2) and photoluminescence (PL) spectroscopy at 25K, we discuss the influence of proton-irradiation upon the photoluminescence degradation or enhancement of quantum dots. In general, we find the PL efficiency enhancement occurs when the dose is about the same order of magnitude as the average QD density and it occurs when the average separation between proton passage is about twice the QD size. The QDs must be annealed of radiation-damage of a size in 15nm range to maximize PL efficiency.

Direct-write composition patterning of InGaN by focused thermal beam during molecular-beam epitaxy

A direct-write patterning of InGaN during molecular-beam epitaxy has been achieved by using in situ focused thermal beam. The surface of growing InGaN is exposed to a 50μm diameter pulse laser beam that is directed to controlled locations. Indium (In) mole fraction is reduced from 0.85 where it is adjacent to laser exposure, and to 0.75 where exposure takes place, whereas it is 0.81 away from exposed regions during a nominal 78nm deposition on a thick InGaN buffer. The effect of local heating increases surface diffusion of In without evaporating the written materials. One additional feature of direct-write patterning is the enhancement of photoluminescence efficiency, which increases by a factor of 7 compared to nonwritten regions. Gray scale features with composition variations are also demonstrated by laser direct write.

Highly Efficient and Stable Photoluminescence of Nanocrystalline Porous Silicon by Combination of Chemical Modification and Oxidation under High Pressure

A combination of high-pressure water vapor annealing (HWA) and chemical modification (CM) was used in order to fully passivate the nanocrystalline silicon surface of porous Si (PS) and electrochemically oxidized PS. The effects of these treatments on the surface chemistry are characterized by Fourier transform infrared spectroscopy in relation to the respective photoluminescence (PL) properties. The use of both HWA and CM enables a higher PL intensity than that of only HWA. The order in which the treatments are implemented greatly affects the final nanostructure and PL characteristics. The marked enhancement in PL efficiency by HWA was confirmed. PL efficiency is further improved by additionally performing CM, owing to further enhancement of surface passivation. The highest PL intensity was obtained when CM was performed before HWA. This may be explained by the enhanced surface passivation together with a better preservation of Si nanocrystals. For all the modified samples, PL stability was considerably enhanced compared with that of the reference sample.

Substantial enhancement of photoluminescence in CdSe nanocrystals by femtosecond pulse illumination

We report on characterisation of the effect of femtosecond laser pulse illumination on photoluminescence (PL) properties of CdSe nanocrystalline films (diameter approx. 5 nm) prepared by chemical bath deposition technique on crystalline silicon substrate. The substantial PL efficiency enhancement (up to 20×) and modification of PL spectrum were dependent on ambient atmosphere and illumination intensity. We discuss the possible microscopic origin of the observed effect and we suggest the rate equation model to reproduce the experimental data.

Engineering carrier confinement potentials in 1.3-μm InAs/GaAs quantum dots with InAlAs layers: Enhancement of the high-temperature photoluminescence intensity

We describe an optical study of structures consisting of an InAlAs-GaAs strained buffer layer and an InAlAs-InGaAs composite strain-reducing layer designed to modify the confining potential of 1.3-μm InAs/GaAs quantum dots (QDs). With increasing (decreasing) InAlAs (InGaAs) thickness in the strain-reducing layer grown above the QDs, the integrated photoluminescence (PL) intensity of the QD ground-state transition increases dramatically and the emission wavelength decreases slightly from 1.36 to 1.31 μm. The enhancement of PL efficiency is temperature dependent, being much greater above 200 K. A maximum enhancement of 450 is achieved at room temperature. This improvement of the high-temperature PL efficiency should lead to a significant improvement in the characteristics of 1.3-μm InAs/GaAs QD lasers.

Photoluminescence and Optical Characterizations of Nanocrystalline Silicon Dots Formed by Plasma-Enhanced Chemical Vapor Deposition

We report the photoluminescence (PL) and optical properties of nanocrystalline silicon (nc-Si) dots grown on thermally grown SiO2 by rf (13.56 MHz) plasma-enhanced chemical vapor deposition of tetrachlorosilane (SiCl4) and H2. The nc-Si dots fabricated at temperatures of ∼120°C show relatively strong emissions in the visible range of 1.3–2.0 eV. The PL peak energy systematically shifts to lower energy from 1.75 eV to 1.45 eV by increasing the deposition period, which is due to the size distribution of nc-Si dots. On the other hand, the PL efficiency depends on both the deposition period and subsequent oxidation methods such as the storage in air, and pure water and HCl acid rinse treatments. The PL efficiency is markedly improved by the pure water rinse rather than by the storage in air, although a prolonged water rinse deteriorates the PL efficiency. On the other hand, the HCl acid rinse results in marked enhancement of PL efficiency with high stability. The effects of deposition conditions and surface treatment on the PL efficiency, stability and optical properties are demonstrated.