Room Temperature Photoluminescence Emission Research Articles

Indium incorporation and emission wavelength of polar, nonpolar and semipolar InGaN quantum wells

InGaN quantum wells were grown by metal organic vapor-phase epitaxy on polar (0 0 0 1), nonpolar (1 0 0) and on semipolar (1 0 2), (1 1 2), (1 0 1) as well as (2 0 1) oriented GaN substrates. The room-temperature photoluminescence (PL) and electroluminescence (EL) emission energies for quantum wells grown on different crystal orientations show large variations of up to 600 meV. The following order of the emission energy was found throughout the entire range of growth temperatures: (1 0 1) < (1 1 2) = (0 0 0 1) < (2 0 1) < (1 0 0) = (1 0 2). In order to differentiate between the effects of strain, quantum-confined stark effect (QCSE) and indium incorporation the experimental data were compared to k.p theory-based calculations for differently oriented InGaN QWs. The major contribution to the shift between (1 0 0) and (0 0 0 1) InGaN quantum wells can be attributed to the QCSE. The redshift between (1 0 0) and the semipolar (1 0 2) and (2 0 1) QWs can be attributed to shear and anisotropic strain affecting the valence band structure. Finally, for (1 1 2) and (1 0 1) the emission energy shift could be attributed to a significantly higher indium incorporation efficiency.

α- to β-[C 6H 4(NH 3) 2] 2Bi 2I 10 reversible solid-state transition, thermochromic and optical studies in the p-phenylenediamine-based iodobismuthate(III) material

α-[C 6H 4(NH 3) 2] 2Bi 2I 10, which is a new material containing low-dimensional iodobismuthate anions, was synthesized and through its single crystal X-ray diffraction measurements, was proven to crystallize at room temperature in the centrosymmetric space group P2 1 /c. It consists of a p-phenylenediammonium dication and a discrete (0-D) anion built up of edge-sharing bioctahedron. Due to the hydrogen bonds and the interatomic distances (Bi–I, I⋯I and π–π) changes, α-phase was transformed into the corresponding centrosymmetric β-phase, β-[C 6H 4(NH 3) 2] 2Bi 2I 10, through a single-crystal to single-crystal transformation occurring upon cooling to −28/−26 °C. Below the transition temperature, β-[C 6H 4(NH 3) 2] 2Bi 2I 10 crystallizes in the monoclinic system, centrosymmetric space group P2 1 /n. Besides, the optical transmission measurements on α-[C 6H 4(NH 3) 2] 2Bi 2I 10 thin films have revealed two absorption bands at 2.47 and 3.01 eV. Finally, two room temperature photoluminescence emissions attributed to excitons radiative recombinations confined within the bioctahedra Bi 2I 10 4−, were observed in the red spectral range at 1.9 and 2.05 eV energy.

Effect of manganese doping on the photoluminescence characteristics of chemically synthesized zinc sulfide nanoparticles

The studies on luminescent II-VI semiconducting nanomaterials have attracted widespread attention recently, due to their potential applications in optoelectronic and biophotonic devices. Amongst other II-VI semiconductor nanoparticles (NPs), Mn2+-doped ZnS NPs having large exciton binding energy and wide direct band gap at room temperature have drawn considerable attention for exploring its interesting optical properties. However, in this report, water-soluble Mn2+-doped ZnS (ZnS:Mn) NPs with Mn2+ concentration varying between 1.5 and 5% (wt%) have been synthesized by chemical co-precipitation method at room temperature. X-ray diffraction (XRD) studies and the analysis of the selected area electron diffraction (SAED) pattern, obtained from transmission electron microscopy (TEM), confirmed the formation of zinc blende structure in all the synthesized samples. The particle sizes of the samples, as obtained from the optical absorption studies, varies between 2.2 and 2.7 nm with the increase of Mn2+ concentration between 1.5 and 5%. The room temperature photoluminescence (PL) emission measurements revealed the presence of yellow-orange emission band in all the Mn2+-doped samples which is attributed to Mn incorporation in ZnS. The Gaussian fittings of the measured PL spectra of all the samples show the presence of four PL peaks. Amongst the four PL peaks three peaks appeared at 445, 476, and 520 nm in all the samples but the fourth yellow-orange emission peak suffered a red shift from 593 to 600 nm with increasing Mn2+ concentration from 1.5 to 5%. In this report no quenching of yellow-orange emission peak is observed up to 5% Mn2+ doping concentration in ZnS. The synthesized water-soluble ZnS:Mn NPs can be further functionalized for using them as biolabels.

Thermal evaporation and condensation synthesis of metallic Zn layered polyhedral microparticles

Metallic zinc layered polyhedral microparticles have been fabricated by thermal evaporation and condensation technique using zinc as precursor at 750 °C for 120 min and NH 3 as a carrier gas. The zinc polyhedral microparticles with oblate spherical shape are observed to be 2–9 μm in diameter along major axes and 1–7 μm in thickness along minor axes. The structural, compositional and morphological characterizations were performed by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). A vapour–solid (VS) mechanism based growth model has been proposed for the formation of Zn microparticles. Room temperature photoluminescence (PL) emission spectrum of the product exhibited a strong emission band at 369 nm attributed to the radiative recombination of electrons in the s, p conduction band near Fermi surface and the holes in the d bands generated by the optical excitation.

Effect of boron doping on optical properties of sol–gel based nanostructured zinc oxide films on glass

Boron doped zinc oxide thin films (∼80 nm) were deposited onto pure silica glass by sol–gel dip coating technique from the precursor sol/solution of 4.0 wt.% equivalent oxide content. The boron concentration was varied from 0 to 2 at.% w.r.t. Zn using crystalline boric acid. The nanostructured feature of the films was visualized by FESEM images and the largest cluster size of ZnO was found in 1 at.% boron doped film (B1ZO). The presence of mixed crystal phases with hexagonal as major phase was identified from XRD reflections of the films. Particle size, optical band gap, visible specular reflection, room temperature photoluminescence (PL) emissions (3.24–2.28 eV), infra-red (IR) and Raman active longitudinal optical (LO) phonon vibration were found to be dependent on dopant concentration. For the first time, we report the room temperature fine structured PL emissions as phonon replicas originated from the LO phonon (both IR and Raman active) in 1 at.% boron doped zinc oxide film.

Enhanced 1.3 µm luminescence from InGaAs self-assembled quantum dots with a GaAsN strain-compensating layer

We have studied room temperature (RT) photoluminescence (PL) emission and strain characteristics of InGaAs quantum dots (QDs) grown on GaAs using metal organic vapor phase epitaxy. The InGaAs QDs were covered by a 10 nm thick GaAsN layer having a nitrogen composition of either 0.3% or 0.9% and a following 50 nm thick GaAs layer. The RT-PL emission intensity of InGaAs QDs has been increased with reduced linewidth for 1.3 µm device application by using GaAsN strain-compensating layers. The Raman spectra of InGaAs QDs covered by GaAsN0.009 show the InGaAs QD phonon frequency shift toward that of InGaAs surface QDs, which gives clear evidence for strain compensation.

Directed self-assembly of InAs quantum dots on nano-oxide templates

We describe the growth and characterization of InAs quantum dots on SiO2 patterned GaAs by metal organic chemical vapor deposition. Arrays of quantum dots with densities as high as 1.8×1010 cm−2 fabricated by electron beam lithography are demonstrated. A process consisting of dry and wet etching to minimize etch damage is developed. As the mask diameter increases, the nucleation transitions from single dots to multidot clusters. We achieve more uniform size and shape distributions of dots on patterned regions relative to unpatterned dots as revealed by structural characterization and room temperature photoluminescence emission spectra.

Down-converting lanthanide doped TiO 2 photoelectrodes for efficiency enhancement of dye-sensitized solar cells

Lanthanide (Ln 3+) doped TiO 2 down-conversion photoelectrodes (Ln 3+ = Eu 3+ and Sm 3+ ions) are used to enhance the photovoltaic efficiency of dye-sensitized solar cells (DSSC). We report on achieving fill factors of 0.67 and 0.69 and efficiencies of 5.81% and 5.16% for Sm 3+ and Eu 3+, respectively. This is compared to the 4.23% efficiency for the undoped-titania photoelectrodes. This enhancement is probably due to the improved UV radiation harvesting via a down-conversion luminescence process by the lanthanide ions. The structure, optical and photoluminescence properties of the down-converting photoelectrode are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDX) and room temperature photoluminescence excitation and emission spectrofluorimetric measurements.

Optimization of InGaAsN(Sb)/GaAs quantum dots for optical emission at 1.55 μm with low optical degradation

Low optical degradation in GaInAsN(Sb)/GaAs quantum dots (QDs) p–i–n structures emitting up to 1.55μm is presented in this paper. We obtain emission at different energies by means of varying N content from 1 to 4%. The samples show a low photoluminescence (PL) intensity degradation of only 1 order of magnitude when they are compared with pure InGaAs QD structures, even for an emission wavelength as large as 1.55μm. The optimization studies of these structures for emission at 1.55μm are reported in this work. High surface density and homogeneity in the QD layers are achieved for 50% In content by rapid decrease in the growth temperature after the formation of the nanostructures. Besides, the effect of N and Sb incorporation in the redshift and PL intensity of the samples is studied by post-growth rapid thermal annealing treatments. As a general conclusion, we observe that the addition of Sb to QD with low N mole fraction is more efficient to reach 1.55μm and high PL intensity than using high N incorporation in the QD. Also, the growth temperature is determined to be an important parameter to obtain good emission characteristics. Finally, we report room temperature PL emission of InGaAsN(Sb)/GaAs at 1.4μm.

Structural and optical changes induced by incorporation of antimony into InAs/GaAs(001) quantum dots

We present experimental evidence of Sb incorporation inside InAs/GaAs001 quantum dots exposed to an antimony flux immediately before capping with GaAs. The Sb composition profile inside the nanostructures as measured by cross-sectional scanning tunneling and electron transmission microscopies show two differentiated regions within the quantum dots, with an Sb rich alloy at the tip of the quantum dots. Atomic force microscopy and transmission electron microscopy micrographs show increased quantum-dot height with Sb flux exposure. The evolution of the reflection high-energy electron-diffraction pattern suggests that the increased height is due to changes in the quantum-dot capping process related to the presence of segregated Sb atoms. These structural and compositional changes result in a shift of the room-temperature photoluminescence emission from 1.26 to 1.36 m accompanied by an order of magnitude increase in the room-temperature quantum-dot luminescence intensity.

Host Sensitized White Luminescence of Dy3 + Activated GdPO4 Phosphors

White light phosphors have been synthesized by solid-state reaction. The phosphors are characterized by X-ray diffraction patterns, room temperature photoluminescence excitation spectra, and room temperature photoluminescence (PL) emission spectra. Under host sensitization and direct 4f–4f excitation, the activator exhibits bright white light. The dependence of PL spectra on the activator doping concentration (from ) and calcining temperature (800, 900, 1000, 1100, and ) are also studied. The study suggests that the phosphor annealed at for with host sensitization has the strongest white light. The calculated chromaticity coordinates with radiation of all the phosphors fall well in the white region of 1931 Commission International de I’Eclairage diagram. These experimental results indicate that the activated is a promising white light material and has potential applications in lighting technology.

1.52 μ m photoluminescence emissions from InAs quantum dots grown on nanopatterned GaAs buffers

InAs patterned quantum dots (PQDs) are preferentially formed on faceted GaAs pyramidal buffers using selective-area epitaxy (SAE) by metalorganic chemical vapor deposition. Photoluminescence (PL) wavelength is adjustable through a single parameter, the growth time, and strong room-temperature PL emissions from 1.3 μm to over 1.5 μm are demonstrated, with linear polarization from PQD’s asymmetric geometries. The long wavelength emission is attributed to the large PQD size and the reduced strain within PQDs enabled by SAE. It is thus a viable technique to independently control PL wavelength while maintaining the QD density, and to address single QDs for device applications.

Low-Temperature Growth of ZnO Nanowire Arrays on p-Silicon (111) for Visible-Light-Emitting Diode Fabrication

We report on the successful growth of homogeneous and well-covering ZnO nanowire arrays at a low temperature (90 °C) directly on a p-type Si(111) wafer by an electrochemical method. The wires were self-standing and vertically oriented. Room-temperature micro-Raman and photoluminescence emission analyses showed the high global structural and optical quality of the material with a low density of deep defects. The ZnO nanowires of the heterostructure were contacted with a transparent ITO electrode, and a light-emitting diode was fabricated. The device current−voltage curve had a rectification magnitude of about 20 at 2.5 V. The threshold forward voltage was low at 1.4 V. The device emitted a broad visible band centered at 590 nm at room temperature under forward bias. According to the energy band diagram of the junction, the emission has been assigned possibly to Si hole injection to near-interfacial ZnO deep levels, followed by the radiative recombination with electrons of the ZnO conduction band.

Size Dependence in Hexagonal Mesoporous Germanium: Pore Wall Thickness versus Energy Gap and Photoluminescence

A series of hexagonal mesoporous germanium semiconductors with tunable wall thickness is reported. These nanostructures possess uniform pores of 3.1-3.2 nm, wall thicknesses from 1.3 to 2.2 nm, and large internal BET surface area in the range of 404-451 m(2)/g. The porous Ge framework of these materials is assembled from the templated oxidative self-polymerization of (Ge(9))(4-) Zintl clusters. Total X-ray scattering analysis supports a model of interconnected deltahedral (Ge(9))-cluster forming the framework and X-ray photoelectron spectroscopy indicates nearly zero-valence Ge atoms. We show the controllable tuning of the pore wall thickness and its impact on the energy band gap which increases systematically with diminishing wall thickness. Furthermore, there is room temperature photoluminescence emission which shifts correspondingly from 672 to 640 nm. The emission signal can be quenched via energy transfer with organic molecules such as pyridine diffusing into the pores.

Influence of thickness on the microstructural, optoelectronic and morphological properties of nanocrystalline ZnSe thin films

Abstract ZnSe films of different thicknesses have been deposited on glass substrates at 150 °C substrate temperature. The films exhibited cubic structure with preferential orientation in the (1 1 1) direction. The crystallinity improved and the grain size increased with increase of thickness. A very high value of absorption coefficient (104 cm−1) is observed. Band gap values are found to decrease from about 2.92 to 2.69 eV with increase of the film thickness. The average refractive index value is in the range of 2.39–2.45 for the films with different thickness. It is observed that the conductivity increases continuously with temperature. Laser Raman studies show the presence of peaks at 140.8, 247.2 and 205.3 cm−1 corresponding to 2TA, LO phonon and TO phonon respectively. TEM study confirms the formation of well crystallized ZnSe nanoparticles. Room temperature photoluminescence emission peaks were observed at 426.9, 485 and 518 nm.

Molecular beam epitaxial (MBE) growth and spectroscopy of dilute nitride InAsN:Sb for mid-infrared applications

The molecular beam epitaxial (MBE) growth and spectroscopy of dilute nitride InAsN:Sb epilayers are presented. Nitrogen incorporation in InAsN epilayers grown by radio-frequency plasma-assisted MBE was investigated as a function of growth conditions. High-quality InAsN epilayers containing up to 2.5% nitrogen were successfully grown using optimal growth conditions. The optical properties of InAsN were studied by photoluminescence (PL). Intense PL emission at 4 K was observed with double-peak features, which were attributed to free carrier recombination and localised carrier recombination. Strong room temperature PL emission extending up to a wavelength of 4.5 µm was obtained and a bandgap reduction of 63 meV for 1% N was deduced. The electronic properties of InAsN such as residual carrier concentration and mobility were also studied by Hall effect measurements. Further improvement of InAsN by addition of Sb during growth is also discussed. The authors observed that the introduction of Sb flux dramatically enhances nitrogen incorporation and significantly improves optical properties. In addition, Sb incorporation is enhanced with the presence of nitrogen. The authors also report the realisation of InAsN:Sb/InAs mid-infrared light emitting diodes operating at 4.0 µm at 4 K.

Optical properties of 1.3 μm InAs/GaAs bilayer quantum dots with high areal density

InAs/GaAs bilayer quantum dots (BQDs) are interesting structures for long wavelength emission due to its ability to tune the areal density and dot size separately. However, the need for two sets of growth rate and temperature for the respective QD layers complicates the growth procedures. Furthermore, the highest areal density reported for BQDs with 1.3 μm emission is only in the low 1010 cm−2. In this letter, we investigated the effect of GaAs spacer thickness and monolayer coverage of the active QDs on the optical properties of InAs/GaAs BQDs grown with constant growth rate and temperature. Consequently, high areal density (∼1.2×1011 cm−2) and room temperature photoluminescence emission at 1304 nm with spectral width of 24 meV was obtained.

Two-Dimensional Excitons and Photoluminescence Properties of the Organic/Inorganic (4-FC6H4C2H4NH3)2[PbI4] Nanomaterial

Single crystals of a hybrid organic/inorganic material with the formula (4-FC6H4C2H4NH3)2[PbI4] were synthesized and studied by X-ray diffraction, ellipsometry, and photoluminescence. The crystals consist of a self-assembled multilayer structure with a P21/a space group. The inorganic semiconductor sublattice is built up from corner-sharing PbI6 octahedra forming infinite two-dimensional quantum wells. The organic (4-FC6H4C2H4NH3)+ chains form the insulator barriers. A strong room temperature photoluminescence emission, due to excitons confined in the PbI quantum wells, is found around 2.34 eV. The exciton binding energy 540 meV is determined from the temperature dependence of the photoluminescence intensity and using a modified Arrhenius model, which accounts for the two-dimensional electronic structure. Calculations of the exciton binding energy based on a variational method indicate that the dielectric confinement effect is enhanced by the low value of the organic barrier dielectric constant. Moreover, it is shown that the temperature variation of the photoluminescence peak energy is well described by Varshni’s model, thus indicating the free character of the two-dimensional excitons. The temperature dependence of the photoluminescence line width is also analyzed and interpreted in terms of homogeneous broadening due to exciton−phonons interaction.

Effect of the plasma composition on the structural and electronic properties of as-grown SiOx/Si heterolayers deposited by reactive sputtering

We report the effects of varying the oxygen partial pressure (OPP) on the structural and electronic properties of SiOx/Si heterolayers grown by RF reactive sputtering. The produced samples present silicon poly-crystalline characteristics for low values of OPP. The crystallinity decreases as the OPP increases due to oxygen interdiffusion until the silicon crystal structure becomes amorphous. The results of infrared and Raman spectroscopies show higher deviation from stoichiometry and an increment of structural disorder for samples grown with higher values of OPP. Room temperature photoluminescence (PL) is present in all as-grown samples. The PL spectra show two bands, around 1.87 and 2.16 eV, for all the samples, while a third broad band at lower energy shows up and shifts to the red as OPP increases. Our results indicate that silicon-related room temperature PL emission is correlated with the stoichiometry of the SiOx and to the formation of silicon crystals embedded in a silicon dioxide matrix.

On the effects of thermal treatment on the composition, structure, morphology, and optical properties of hydrogenated amorphous silicon-oxycarbide

The composition, structure, morphology, and optical characteristics of hydrogenated amorphous silicon-oxycarbide (a-SiCxOyHz) materials were investigated as a function of experimental processing conditions and post-deposition thermal treatment. Thermal chemical vapor deposition (TCVD) was applied to the growth of three different types of a-SiCxOyHz films, namely, SiC-like (SiC1.08O0.07H0.21), Si-C-O (SiC0.50O1.20H0.22), and SiO2-like (SiC0.20O1.70H0.24). The resulting films were subsequently annealed at temperatures ranging from 500 °C to 1100 °C for 1 h in an argon atmosphere. The composition, structure, and morphology of as-deposited and post-annealed films were characterized by Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS), nuclear-reaction analysis (NRA), and scanning electron microscopy. Corresponding optical properties were assessed by spectroscopic ultraviolet-visible ellipsometry (UV-VIS-SE). These studies led to the identification of an optimized process window for the growth of Er doped silicon oxycarbide (SiC0.5O1.0:Er) thin film with strong room-temperature photoluminescence emission measured around 1540 nm within a broad (460 nm to 600 nm) wavelength band. Associated modeling studies showed that the effective cross section for Er excitation in the SiC0.5O1.0:Er matrix was approximately four orders of magnitude larger than its analog for direct optical excitation of Er ions.