Photoluminescence Wavelength Research Articles

Morphology and optical properties of p-type porous GaAs(1 0 0) layers made by electrochemical etching

Porous GaAs layers were formed by electrochemical etching of p-type GaAs(1 0 0) substrates in HF solution. A surface characterization has been performed on p-type GaAs samples using X-ray photoelectron spectroscopy (XPS) technique in order to get information about the chemical composition, particularly on the surface contamination. According to the XPS spectra, the oxide layer on as-received porous GaAs substrates contains As 2O 3, As 2O 5 and Ga 2O 3. Large amount of oxygen is present at the surface before the surface cleaning. Compared to untreated GaAs surface, room temperature photoluminescence (PL) investigations of the porous layers reveal the presence of two PL bands: a PL peak at ∼871 nm and a “visible” PL peak at ∼650–680 nm. Both peak wavelengths and intensities varied from sample to sample depending on the treatment that the samples have undergone. The short PL wavelength at 650–680 nm of the porous layers is attributed to quantum confinement effects in GaAs nano-crystallites. The surface morphology of porous GaAs has been studied using atomic force microscopy (AFM). Nano-sized crystallites were observed on the porous GaAs surface. An estimation of the mean size of the GaAs nano-crystals obtained from effective mass theory and based on PL data was close to the lowest value obtained from the AFM results.

Novel nano-structured glasses containing semiconductor quantum dots: controlling the photoluminescence with phonons and photons

Effects of temperatures and excitation intensities on the photoluminescence properties of PbS quantum dots precipitated in the glass were investigated. Peak wavelength of the near-infrared photoluminescence shifted towards the short wavelength side with an increase in temperature and excitation intensity. The largest shift in the peak wavelength of the photoluminescence bands was approximately 90 nm. The temperature coefficient of band gap energy (deduced from the photoluminescence wavelength) of quantum dots varied from 230 to 28 μeV/K under the excitation intensity of 50-600 mW. The integrated photoluminescence intensity also showed similar dependencies on temperature and excitation intensity. The shifts of the photoluminescence with changes in the temperature and excitation intensity were associated with the trapping and re-activation of charge carriers at defect sites located at the QDs/glass interface and inside the glass matrix.

Optimum annealing temperature versus nitrogen composition in InAs/(Ga, In) (N, As) quantum dots

The effect of annealing on long-wavelength emitting InAs/Ga0.85In0.15NxAs1−x (0 ⩽ x ⩽ 0.023) quantum dots (QDs) is studied. For the as-grown sample with x = 0.023, a room temperature photoluminescence (PL) emission wavelength of 1.58 µm is achieved. It is found that the optimum annealing temperature increases when the nitrogen composition in the cap layer increases. After optimum annealing, the QD PL emission exponentially blue-shifts with the nitrogen composition, whereas the as-grown QD emission energy red-shifts with a linear behavior. This implies that to get the longest PL wavelength with the best PL efficiency, the nitrogen composition should not exceed 2%.

Accessing the Triplet Excited State in Perylenediimides

Here, we present a strategy designed to permit access to the PDI triplet manifold that preserves the desirable colorfastness and visible light-absorption properties associated with these dyes. To this end, three new Pt(II) complexes each bearing two PDI moieties tethered to the metal center via acetylide linkages emanating from one of the PDI bay positions have been synthesized, structurally characterized, and thoroughly examined by nanosecond laser flash photolysis. Upon ligation, the bright singlet-state fluorescence of the PDI chromophore is quantitatively quenched, and no long wavelength photoluminescence is observed from the Pt(II)-PDI complexes in deaerated solutions. In each of the Pt-PDI chromophores, quantitatively similar transient absorption difference spectra were obtained; the only distinguishing characteristic is in their single-exponential lifetimes (tau = 246 ns, 1.0 micros, and 710 ns). Triplet-state sensitization experiments of "free" PDI-CCH using thioxanthone confirmed the PDI triplet state assignments in each of the Pt-PDI structures.

Oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation and their effects on the photoluminescence wavelength

Natural oxidation processes of surface hydrogenated silicon nanocrystallites prepared by pulsed laser ablation under various hydrogen gas pressures are discussed by measuring the vibrational frequency of Si–Hn units on the surface and intensity of Si–O–Si stretching vibration. The surfaces of nanocrystallites are predominantly composed of Si–H bonds and oxidation starts from backbonds of these bonds. The deposited nanocrystal films have a porous secondary structure which depends on the background gas pressure. The oxidation rate observed by infrared absorption measurements depended on this porous secondary structure. The oxidation process is discussed by the correlation between oxidation rate and porous structure of nanocrystal film. We found that Si–O bond density increases with covering the surface of the nanocrystallites during the diffusion of oxygen-related molecules through the void spaces in the porous structure. The surface oxidation of each nanocrystallite is not homogeneous; after the coverage of easy-to-oxidize sites, oxidation continues to gradually progress at the post-coverage stage. We point out that the oxidation process at coverage and post-coverage stages result in different photoluminescence (PL) wavelengths. Adsorption of the water molecule before oxidation also affects the PL wavelength. Defect PL centers which have light emission around 600 and 400 nm are generated during the coverage and post-coverage oxidation processes, respectively.

Emissivity-correcting mid-infrared pyrometry for group-III nitride MOCVD temperature measurement and control

We report a pyrometer that operates at a mid-infrared wavelength range (7–8 μm), where sapphire substrates are opaque and the reactant gases are transparent. The pyrometer also employs a novel “self-reflectance” method of emissivity correction, whereby thermal emission from the sample serves as a radiation source to measure its emissivity. The instrument was installed on a multiwafer Veeco D-125 MOCVD system and used to measure the emissivity-corrected temperature at a variety of GaN and InGaN deposition conditions. For a series of InGaN multiquantum well growth, the pyrometer was used to control the surface temperature at the critical InGaN deposition step, resulting in improved control of the photoluminescence wavelength.

Novel Strategy for Synthesis of High Quality CdTe Nanocrystals in Aqueous Solution

A novel aqueous route for the synthesis of high-quality CdTe nanocrystals(NCs) is presented in this article. With both glutathione(GSH) and cysteine[n(GSH):n(cysteine)=1:3] as stabilizers, high-quality CdTe NCs with controllable photoluminescence(PL) wavelength from 500 to 630 nm can be prepared within 4 h. As-prepared CdTe NCs show higher photoluminescence quantum yields(PLQY) compared with CdTe NCs prepared via other aqueous methods. When the fluorescent emission peak appeared in orange-red window, the PLQY reaches 70% or above at room temperature without any post-preparative treatment.

Growth of Si crystalline in SiO x films induced by millisecond rapid thermal annealing using thermal plasma jet

Si crystalline growth in SiO x films is observed by annealing the films with thermal plasma jet (TPJ) at a temperature higher than 1440 K with annealing duration ranging from 0.9 to 1.7 ms. The size of surface granular structure abruptly increases from ∼30 to ∼800 nm by annealing the films at temperatures higher than 1680 K. From the Raman scattering spectra of such large grains, it is confirmed that they have very high crystallinity with the TO phonon band width of 6.9 cm −1 and peak position of 520.3 cm −1. Large grains of ∼10 μm are formed by agglomeration with high temperature annealing. From SiO x films annealed at ∼1200 K, visible red photoluminescence (PL) was observed at room temperature. The peak PL wavelength of 920 nm suggests the formation of nanocrystalline Si with the size of ∼5 nm.

Self-assembled InGaN quantum dots on GaN emitting at 520nm grown by metalorganic vapor-phase epitaxy

Abstract Self-assembled InGaN quantum dots (QDs) have been grown using metalorganic vapor-phase epitaxy (MOVPE), without using anti-surfactant. Using 120 s annealing, InGaN QDs have been successfully formed with a circular base diameter of 40 nm and an average height of 4 nm, with QDs density of 4×10 9 cm −2 . The InGaN QDs have peak photoluminescence (PL) wavelengths of 519 and 509 nm for samples without and with a GaN upper barrier, respectively. The full-width half-maximum (FWHM) of the PL spectra ranges from 56.6 up to 69.6 nm. These results demonstrates that high In-content InGaN QDs can be grown by MOVPE, and can potentially be utilized as the active media for light-emitting diodes (LEDs) and semiconductor laser diodes for green emission.

Synthesis and characterization of efficient orange-red emitting poly( p-phenylenevinylene) derivatives with 1,3,4-oxadiazole unit

A series of the statistical copolymers of tetradecyloxy-3,5-bis-{2-(4-bromo-phenyl)-[1,3,4]oxadiazole}-benzene ( 4), 1,4-didecyloxy-2,5-diiodobenzene ( 5) and 1,4-dodecyloxy-2,5-divinylbenzene ( 6), for efficient light emitting diode were synthesized. As the feed ratio of compound 4 decreased, the absorption band at 320–360 nm decreased in the UV–vis spectra. In addition, as the composition of compound 4 decreased, the absorption intensity ratio at 1541 cm −1 decreased in the FT-IR spectra. The photoluminescence and electroluminescence emission color of P showed green-yellow color ( λ max = 552 nm). However, the maximum wavelength of photoluminescence and electroluminescence of P1 and P2 were 581 and 583 nm, respectively, which were dramatically red-shifted than that of P. Moreover, these values were almost the same as the values of P3 (580 nm). It is concluded that the energy transfer from high energy oxadiazole segments to low energy alkoxy-PPV segments occurred completely. The efficiency of ITO/ P2/Al was 0.0181%, which is the best efficiency among the device based on the copolymers. This is presumably due to that P2 has the best balanced hole- and electron-injection/transporting properties among the copolymers.

Thermal stability of CdZnO∕ZnO multi-quantum-wells

The thermal stability of CdZnO∕ZnO multi-quantum-well (MQW) structures was studied using rapid thermal annealing in nitrogen from 300to750°C. Photoluminescence (PL) emission from the MQWs was studied while varying the annealing temperature and time. For 15min annealings, the PL center wavelength showed a 7nm reduction for temperatures up to 650°C. Above 650°C, the wavelength changed rapidly, with a 50nm reduction at 750°C. Annealing at 700°C for up to 20min produced a systematic reduction in PL wavelength up to 39nm. The data suggest that CdZnO∕ZnO MQWs are relatively stable for nitrogen annealing below 650°C for times up to 15min.

Modification of photoluminescence spectrum of artificial opal under external effects

The effects of washing and heat treatment on photoluminescence (PL) behavior from artificial opals are studied. The PL spectra excited by N 2-laser radiation (337 nm) display a broad maximum at around 435 nm and range from ∼340 to ∼700 nm. It is found that washing of the opal in different solvents and liquids leads to suppression of the long wavelength PL band at ∼500 nm, whereas heat treatment in air and vacuum results in a decrease in intensity of both of the PL bands. The obtained results indicate that the emission centers located on the surface of SiO 2 spheres constructing artificial opal may be responsible for the PL band around ∼500 nm.

Vapor-phase deposition of regioregular and oriented poly(3-hexylthiophene) structures and novel nanostructured composites of interpenetrating poly(3-hexylthiophene) and polyaniline exhibiting full-color wavelength (400–1000nm) photoluminescence

A promising solvent-free technique of electron-beam-assisted vapor-phase codeposition method is presented which allows uniform blending of different conjugated and nonconjugated polymers at the nanoscale. The technique allows direct incorporation of regioregular poly(3-hexylthiophene) (P3HT) polymer with different structural orientations into conventional and semiconducting polymers without fractionation or degradation of P3HT while maintaining the nanoscale morphology of deposited organic films. The results of fabricated novel nanostructured organic composites (∼100–200nm) comprising regioregular and oriented P3HT and different conjugated and nonconjugated polymers including selective assembly of P3HT nanonodules into a copolymer template are presented. We show a typical example of blending of P3HT and polyaniline (PANI) that formed a unique nanoscale morphology comprising interpenetrating networks of different shapes and sizes of nanospherulites (∼100nm) of P3HT in PANI. The so fabricated nanocomposites (∼200nm) exhibited remarkable broadband photoluminescence features covering the entire blue, green, and red wavelength regions between 400 and 1000nm. Such organic nanocomposites might be useful for flexible full-color screen flat panel displays and organic white-light solid-state lighting applications.

Infrared luminescence from spark-processed silicon

The infrared (IR) photoluminescence (PL) emission of spark-processed silicon (sp-Si) was investigated. A broad and strong room temperature PL peak in the 945 nm (1.31 eV) spectral range was observed when sp-Si was excited with an argon laser. This peak is different from the PL commonly reported for anodically etched porous silicon and other silicon-based materials. The PL intensity increases substantially after annealing sp-Si between 350 and 500 °C in air after which it decreases again. The PL wavelength is observed to peak at 1010 nm by annealing sp-Si near 450 °C. It was further found that the most efficient PL occurs for a Si/O ratio of 0.3, for a small spark gap of about 1 mm, and for spark-processing times in the 15–60 min range. A model for the IR PL is proposed which mirrors that for visible PL. Specifically, it is proposed that the electrons which have been pumped by the laser from the ground state into a broad quasi-absorption band (or closely spaced absorption lines between 1.7 and 2.3 eV) revert back to lower IR levels at 1.31 eV by a non-radiative transition from where they revert radiatively to the ground state by emitting the observed 945 nm light.

Modification of CdTe quantum dots as temperature-insensitive bioprobes

CdTe quantum dots (QDs) were synthesized in aqueous solution with 3-mercaptopropionic acid (MPA) as the stabilizer. The photoluminescence (PL) of CdTe QDs (3.5 nm) is found to be temperature-dependent: as the temperature arising from 278 K to 323 K, the PL intensity declines to 50.2% of its original and PL emission peak shows obvious red-shift (∼7 nm). After modification of the QDs surface with denatured ovalbumin, the PL is more temperature-insensitive than before. The PL intensity retains more than 70% of its original and the emission peak shows less red-shift (∼2 nm). Moreover, it is found that the PL intensity and wavelength of denatured ovalbumin coated CdTe QDs are reversible during heating (323 K)–cooling (278 K) cycles. All the studies provide an important theoretical basis for searching temperature-insensitive bioprobes.

LPE growth of InP/InGaAsP/InP heterostructures and separate preparation of high-temperature solutions

We describe a process for separate preparation of high-temperature solutions, which includes a prolonged preannealing (synthesis) at elevated temperatures. This process enables liquid phase epitaxy (LPE) of InGaAsP with highly reproducible physical properties (layer thickness, photoluminescence wavelength, and lattice parameter) and electrical characteristics (current-power curve, dark current, output power, emission wavelength, and threshold and working currents). Using this process, InP/InGaAsP/InP heterostructures can be grown by LPE in a wide temperature range (480–680°C) with highly reproducible parameters, for both photodetectors and light-emitting devices. An original LPE process is proposed which takes advantage of the separate preparation and pouring of high-temperature solutions with the use of graphite equipment.

Growth by molecular beam epitaxy of self-assembled InAs quantum dots on InAlAs and InGaAs lattice-matched to InP

The authors report the results of a detailed study of the effect of growth conditions, for molecular beam epitaxy, on the structural and optical properties of self-assembled InAs quantum dots (QDs) on In0.524Al0.476As. InAs QDs both buried in, and on top of, In0.524Al0.476As were analyzed using photoluminescence (PL) and atomic force microscopy. InAs QD morphology and peak PL emission wavelength both scale linearly with deposition thickness in monolayers (MLs). InAs deposition thickness can be used to tune QD PL wavelength by 170nm/ML, over a range of almost 700nm. Increasing growth temperature from 440to480°C results in a linear decrease in QD size and a blueshift in peak emission wavelength of 3.5nm∕°C. This is a direct result of the temperature dependence of the In-sticking coefficient. InAs deposited on InP-lattice-matched In0.532Ga0.468As forms larger, lower-density features with longer PL wavelength, as expected from a consideration of the effects of In segregation and intermixing on strain and surface roughness. Choice of buffer material is shown to be critical to QD characteristics.

Synthesis and photoluminescent properties of poly(arylene ether)s containing oxadiazole and fluorene moieties

A series of poly(arylene ether)s containing both oxadiazole and fluorene moieties were synthesized by reacting prepared bisphenol fluorenes with 2,5-bis(4-fluorophenyl-1,3,4-oxadiazole) in high yield. These poly(arylene ether)s exhibited very high glass transition temperatures and thermo-oxidative stability. They are very soluble in many common solvents and can be cast into colorless and transparent films. Their photoluminescent properties were fully investigated, and the results showed that the photoluminescent wavelength of these films ranged from 350 nm to 450 nm, respectively in the form of film. These results demonstrated that they could be used as purple–blue electroluminescent materials in purple–blue electroluminescent device.

Blue-emitting Type-II Semiconductor Nanocrystals with High Efficiency Prepared by Aqueous Method

Abstract Forming a type-II structure on alloyed ZnSe core through a facile aqueous method using Cd and S ions resulted in nanocrystals with a photoluminescence wavelength adjusted to the blue region. Their surface was additionally coated by wider band-gap ZnS shell through photodecomposition of the surfactant molecules. Optimizing the conditions for creating the core and shells increased their emission efficiency more than 40%.

Correlation between structural changes and luminescent properties of ZnMgS:Mn thin film phosphor with annealing temperature

With varying rapid thermal annealing temperature, luminescent properties of Zn 0.75 Mg 0.25 S :Mn thin film deposited by RF-magnetron sputtering technique were investigated. Although all samples were deposited from identical source composition, it was found that a main peak wavelength of photoluminescence of Zn 0.75 Mg 0.25 S :Mn depended on RTA temperatures and it shifted toward shorter wavelength upon the increase of RTA temperature. The same dependence of wavelength on RTA temperature was also observed in cathodoluminescence as well as electroluminescence measurements. It is noticeable that Zn 0.75 Mg 0.25 S :Mn thin film phosphor in this study showed more reddish emission than those of the previous studies. It was revealed that changes of the luminescent properties were originated from structural changes in Zn 0.75 Mg 0.25 S :Mn thin film phosphor from cubic to hexagonal phases using X-ray pole figure mapping, and the growing up of hexagonal phase mainly caused cracks and porous morphology on the surface of thin films. It is suggested that the phase transition would be the origin of luminescent property changes with respect to rapid thermal annealing temperature.