Observed Photoluminescence Emission Research Articles

Crystallization of Germanium-SiO2 composite films via nanosecond laser pulse irradiation

Germanium (Ge)-SiO2 composite films fabricated by using radio frequency magnetron co-sputtering were subjected to nanosecond (ns) laser pulse irradiation. The intense laser irradiation effects on as deposited composite films lead to the modification and the formation of Ge nanocrystals (NCs) embedded in SiO2. The as-deposited and laser treated samples have been characterized with X-ray diffraction (XRD) and micro-Raman spectroscopy to understand the formation of Ge NCs due to laser irradiation. The respective Ge NCs related peaks in XRD pattern as well as the Ge-Ge related optical phonon vibrational peaks in micro-Raman spectra confirm the formation of Ge NCs. Transmission electron microscopy images indicate the formation of Ge NCs after the laser irradiation. The effects of laser pulse energy and the irradiation time play a vital role in the crystallization of the as-deposited films. The changes in the Ge NCs size related shifts in the peak position of Ge optical phonons from the Raman spectra of the respective samples are explained with Ge-Ge optical phonon vibrational model. The observed photoluminescence emission in the green and red region of the visible spectrum from the laser treated samples are explained using the phenomenon of creation of Ge NCs in the Ge+SiO2 composite films and the oxygen defects created in SiO2 during the ns laser treatment.

Properties of Erbium-Doped Silicon Oxycarbide Thin Films

This research paper presents findings on the properties of erbium ions incorporated within an amorphous silicon oxycarbide host matrix. A special analysis is made on photoluminescence emission. The experimental samples were prepared using tetraethoxysilane and erbium oxide as reagents via Catalytic chemical vapor deposition. Notably, a unique preparation method was employed for thin films obtention, avoiding plasma damage, which had not been utilized for this purpose previously. One of the most important accomplishments of this study consists in achieving a broad band PL emission centered at 580 nm, which is attributed to the incorporation of erbium ions into a silicon oxycarbide matrix. The obtained results indicate a direct correlation between the photoluminescence emission evolution and the presence of erbium oxide used during the deposition process. The observed photoluminescence emission is attributed to the formation of erbium-silica-based complexes that facilitate energy transfer to the erbium ions. This research opens new possibilities in areas such as optoelectronics, sensing, and telecommunications. The findings obtained have numerous potential applications, particularly in advancing the design of LEDs, lasers, and waveguides.

Enhancement of UV luminescence in Zn/ZnO nanocomposites synthesized by controlled thermal oxidation of Zn nano-octahedrals

Defect-free photoluminescence (PL) emission is always desirable for the practical applications of functional materials. The presence of metal nanostructures near the semiconductor can result in a reduction of defect emission, if present. Here, the enhancement of ultraviolet (UV) PL emission of Zn/ZnO nanocomposites (NCs) has been demonstrated with a corresponding reduction of defect-mediated PL emission. Zn/ZnO NCs have been synthesized by annealing of Zn nano-octahedral in the air (at 500 °C for 5 h). Annealing at 800 °C has been found to result in the complete oxidation of Zn and the formation of ZnO nanoparticles (NPs). UV and visible PL emissions have been observed from Zn nano-octahedral, Zn/ZnO NCs, and ZnO NPs. The observed PL emissions from Zn have been argued as due to transitions of electrons from sp band to the upper states of 3 d band. For ZnO and Zn/ZnO, UV emission is due to near-band-edge emission of ZnO and visible PL emissions are due to the presence of defect states in ZnO. About 2.15-fold enhancement of UV PL emission of Zn/ZnO NCs has been argued as due presence of metallic Zn in the Zn/ZnO NCs. Thus, the present study can open up a new avenue for Zn/ZnO NCs for possible applications in UV regions.

Influence of external factors on photoluminescence of Ca(AlxGa1-x)2S4:Eu2+ solid solutions

The effect of γ-irradiation, concentration of Eu2+ ions, cationic substitution of components (Ga→Al) on photoluminescence (PL) of Ca(AlxGa1-x)2S4:Eu2+ solid solutions was studied. It is shown that the PL intensity of the samples increases in two times with increasing the gamma irradiation dose (D) in the range of 0–1000 krad, as well as the PL intensity increase three times with increasing the concentration of Eu2+ ions from 3% to 7%. There are observed 44 nm blueshift in PL maxima when partial substitutions of Ga ions with Al ions, which enables efficient luminescence emission in the green-yellow region of the optical spectrum. It has been determined that observed PL emission is due to the electronic transitions of Eu2+ ions from excited 4f65d levels to the ground 4f7 level.

The importance of surface states in N-doped carbon quantum dots

Nitrogen-doped carbon quantum dots are synthesized by a one-step atmospheric pressure microplasma process. The origin of the observed photoluminescence emission and its relationship with nitrogen doping is studied using a range of optical and chemical measurements along with verification by theoretical calculations. Nitrogen doping into the core and functionalization of surface states with nitrogen and oxygen groups gives rise to a hybrid structure which is responsible for the luminescence with quantum yields up to 33%. Carrier multiplication is observed as a step-like enhancement in the quantum yield. The analysis of visible-light emission suggests that the emission originates for the most part from surface states and not due to recombination within the quantum dot core. The role of surface functional groups is dominant over quantum confinement in determining the optical properties.

Band gap engineering, quantum confinement, defect mediated broadband visible photoluminescence and associated quantum States of size tuned zinc oxide nanostructures

A simple wet chemical method has been used to synthesize ZnO nanostructures. The ZnO nanocrystals exhibit hexagonal unit cell structure as revealed from the XRD pattern. The intensity difference of various diffraction peaks indicates that the growth of the ZnO nanostructure is anisotropic. Various lattice parameters, dislocation density, strain, particle size, and Zn-O bond length were also calculated. The band gap of the synthesized ZnO nanostructures was determined from the UV–vis absorption spectra using Tauc equation and the band gap of the synthesized ZnO varies from 3.34 eV to 3.57 eV by varying the growth duration. Crystallite sizes also decreased with increased growth duration. This variation of size leads to the change in band gap of the nanostructures. Thus, it is possible to tune the band gap of the nanostructures by varying the growth duration. The synthesized ZnO nanostructures exhibit visible photoluminescence due to deep level transitions. A simple weak quantum confinement model has been used to designate the observed photoluminescence emission peak associated with such transitions.

TEMPERATURE DEPENDENT CHARACTERISTICS OF SrAl2O4:Dy 3+PHOSPHOR

We report the detailed temperature-dependent characteristics of Dy3+ doped SrAl2O4 phosphor. The phosphor synthesised using precipitation method was annealed at three different temperatures, specifically 800ºC, 900ºC, and 1000ºC. The three samples were analyzed using XRD, SEM, TEM, RAMAN, and FTIR. The observed photoluminescence emissions were consisting of peaks arising from the host SrAl2O4 as well as the dopant Dy3+. The crystallite sizes were found to be 27.22nm, 29.74nm, and 31.24nm, respectively, with the increase in annealing temperature. SEM, TEM images showed nearspherical, rod-like shapes of the crystals, and SAED confirmed the crystals were single-crystal. CIE analysis results showed that the colour coordinate was found to be very close to white colour in the three annealing temperatures, which is an advantage in the field of technology development.

Study of isolation method of green fluorescent pigments contained in cherry tomatoes using column chromatography

We investigated a method to isolate the green fluorescent pigments contained in cherry tomatoes using column chromatography. To determine the best developing solvent for the column chromatography, we tested several solvents using a thin-layer chromatography and adopted a solution containing hexane and 2-propanol in a ratio of 99:1 because its Rf value can be adjusted to 0.3. Some pigments were extracted by the column chromatography and identified by evaluating their absorption spectra: β-carotene, phytoene, phytofluene, lycopene and ζ-carotene. Among them, we observed photoluminescence emissions from the solutions containing phytofluene and lycopene. In particular, lycopene reveals a green emission around 542 nm. However, the solution containing lycopene reveals another undesired red emission around 674 nm that originates from the chlorophylls. By performing the second column chromatography with a different developing solvent, only lycopene was successfully isolated to produce a pure green emission.

Optical transitions in lysozyme mediated zinc oxide nanoparticles probed by deep UV photoluminescence

Lysozyme mediated zinc oxide nanoparticles (L:ZnO) synthesized by sol-gel method were characterized at room temperature using x-ray diffraction, scanning electron microscopy and absorption spectroscopy for structural, morphological and optical properties, respectively. A deep UV photoluminescence (PL) spectroscopy was employed to probe the optical transitions in the L:ZnO nanoparticles at low temperature (15 K). In the excitation power dependent PL measurements, we observed band edge emissions at 3.318, 3.359, and 3.374 eV due to recombination of a free electron from the conduction band to a hole in the acceptor level (FA), donor bound exciton (DoX) and free exciton (FX), respectively. A dominant emission peak around 2.40 eV is confirmed to be a donor acceptor pair (DAP) from the power dependent PL experiment. Based on the observed PL emission peaks, energy level diagram is constructed for the optical transitions in the L:ZnO particles and the DAP is assigned to the electronic transition from a deep donor level to the acceptor level related to the FA transition.

Structural, optical, electrochemical, and antibacterial features of ZnS nanoparticles: incorporation of Sn

Incorporation of Sn into ZnS nanoparticles was performed by simple co-precipitation method and was analyzed for various parameters by X-ray diffraction (XRD), Transmission electron microscope (TEM), energy dispersive X-rays (EDX), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), UV–visible spectroscopy, photoluminescence (PL) spectra, and electrochemical and antimicrobial studies. XRD results showed that there was no structural, geometrical alteration in Sn: ZnS and they remained in their cubic structure. Crystallite size was calculated using Debye–Scherrer method and it was ranged from 2 to 3 nm. UV–visible absorption intensity was increased for the increase of Sn concentration. Band-gap values were red shifted from that of the bulk ZnS value. The observed PL emission at 360 nm was due to transition of electrons from the shallow states near the conduction band to the sulfur vacancies present near the valence band in the ZnS lattice. Electrochemical analysis proved that the Sn = 4% composition showed a better electrical response. Since this composition of Sn had good electrical conductivity, the material can be useful to energy material applications. Antibacterial activity of Sn: ZnS nanoparticles was also discussed. A better antibacterial behavior was exhibited by 6% Sn-incorporated sample and this composition may be useful for biological applications.

Comprehensive physical studies on nanostructured Zn-doped CdSe thin films

Electrochemically deposited un- and Zn-doped CdSe nanostructures have been examined in this study. An aqueous solution having 3CdSO4·8H2O, SeO2, and various amounts of ZnSO4·7H2O was used to deposition un- and Zn-doped CdSe samples. The X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) with energy dispersive X-ray analysis (EDXA) attachment, atomic force microscopy (AFM), room temperature photoluminescence (PL), and UV–Vis spectroscopy have been served to study the impact of Zn-dopant on the physical characteristics of CdSe thin films. All samples had a cubic crystalline system and their crystallite size was 7–9 nm. The lattice constant (a), unit cell volume (V), and the interplanar distance of CdSe samples were reduced after Zn-doping. FESEM images presented grain-like morphology for all samples. According to AFM results, the surface smoothness of Zn-doped CdSe samples was improved compared to the undoped CdSe sample. Two observed PL emission peaks at 564 and 690 nm were associated with the recombination of electron-hole and the near band emission (NBE) of CdSe, respectively. The intensity of PL spectra of CdSe samples was decreased after Zn-doping, which related to the generation of non-radiative relaxation of electron transition. According to UV–Vis spectroscopy results, the Eg of undoped CdSe sample was 1.73 eV that it reduced to 1.67 eV for Zn-doped CdSe samples. The optical- and electrical conductivity of CdSe thin films have been improved due to Zn-doping. Accordingly, the obtained results confirmed that the Zn dopant plays a key role in the physical properties of nanostructured CdSe thin films.

Photodegradation of phenanthrene catalyzed by rGO sheets and disk like structures synthesized using sugar cane juice as a reducing agent

In the present report, rGO sheets (rG1) and disk (rG2) like structures of reduced graphene oxide (rGO) were synthesized using sugar cane juice as green reducing agent. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV–vis.) spectroscopy and photoluminescence (PL) spectroscopy techniques. The transition of electrons localized in different sized sub-domain of the sp2 bonded carbons having different values of highest occupied molecular orbital (HOMO) -lowest unoccupied molecular orbital (LUMO) gap may likely to be responsible for the observed PL emission in rG1 and rG2 at different excitation wavelengths. The rG1 and rG2 were also used as photocatalyst materials for the degradation of phenanthrene (PHE) under the UV irradiation. The rG2 shows better photocatalytic degradation compared to rG1 by degrading the PHE up to 30%.

Effect of Surface and Defect Chemistry on the Photocatalytic Properties of Intentionally Defect-Rich ZnO Nanorod Arrays.

Due to the abundance of intrinsic defects in zinc oxide (ZnO), the material properties are often governed by same. Knowledge of the defect chemistry has proven to be highly important, especially in terms of the photocatalytic degradation of pollutants. Given the fact that defect-free materials or structures exhibiting only one type of defect are extremely difficult to produce, it is necessary to evaluate what influence various defects may have when present together in the material. In this study, intentionally defect-rich ZnO nanorod (NR) arrays are grown using a simple low-temperature solution-based growth technique. Upon changing the defect chemistry using rapid thermal annealing (RTA) the material properties are carefully assessed and correlated to the resulting photocatalytic properties. Special focus is put on the investigation of these properties for samples showing strong orange photoluminescence (PL). It is shown that intense orange emitting NR arrays exhibit improved dye-degradation rates under UV-light irradiation. Furthermore, strong dye-adsorption has been observed for some samples. This behavior is found to stem from a graphitic surface structure (e.g., shell) formed during RTA in vacuum. Since orange-luminescent samples also exhibit an enhancement of the dye adsorption a possible interplay and synergy of these two defects is elucidated. Additionally, evidence is presented suggesting that in annealed ZnO NRs structural defects may be responsible for the often observed PL emission at 3.31 eV. However, a clear correlation with the photocatalytic properties could not be established for these defects. Building on the specific findings presented here, this study also presents some more general guidelines which, it is suggested, should be employed when assessing the photocatalytic properties of defect-rich ZnO.

Optical properties of Sr3B2O6:Dy3+/PMMA polymer nanocomposites

The paper presents a facile way to synthesize luminescent polymer nanocomposite (PNC) films consisting of nanophosphors (NPs) of rare earth ions doped alkaline earth borates (Sr3B2O6:Dy3+) dispersed in a polymer (PMMA) matrix via a solution casting method and the results of their detailed structural and optical properties measurements. The PNC films were characterized using X-ray diffraction (XRD), Photoluminescence (PL), and differential scanning calorimetry (DSC). The crystallinity of the dispersed NPs did not suffer on account of being dispersed in the PMMA. The Rhombohedral structure and the formation of a single phase of Sr3B2O6:Dy3+ were confirmed by the XRD data of both the NP powders and the PNC films with an average particle size of 43nm. Also, the observed PL emission and excitation spectra of the PNC films amply suggested that embedding of the nanophosphors in the PMMA matrix preserves their typical luminescence emission. The chromaticity coordinates (x = 0.37, y = 0.39) of the PNC films also validated the yellowish white emission of the nanophosphor. DSC scans on the PMMA only and the Sr3B2O6:Dy3+/PMMA films suggested an increase in the thermal stability of the PNC films as compared to pure PMMA although no significant change in the glass transition temperature was observed.

(Invited) Strongly-Bound Interlayer Exciton States in Twisted Bilayer Materials

When two sheets of a 2D material such as graphene are stacked at a twisted angle, the 2p orbitals rehybridize, giving angle-tunable absorption resonances. By comparing the ultrafast intra-band and multi-photon transient absorption spectra, our results agree best with a recent theoretical model that predict that overlapping interlayer 2p orbitals interfere to produce symmetrized bound exciton states that are decoupled from lower lying continuum states. We further map out the excited state manifold of exciton fine states using 2-photon photoluminescence microscopy. Our spectral analysis suggests the observed photoluminescence emission is a bright exciton state that is thermally populated by a lower-lying, long lived dark-exciton state. For this dark state, both our ultrafast transient absorption and two-photon photoluminescence studies support a large binding energy of the order of ~0.6 eV. If resonantly excited, we believe twisted bilayer graphene is the first 2D metallic material that can form stable, strongly bound excitons. Our results support that twisted bilayer graphene and similar materials are novel example of 2D hybrid material, where resonantly excited excitons are strongly-bound and quasi-stable from inherently weak coupling to the continuum metallic states. The formation mechanism of this dark state is very general, and holds exciting promise for other twisted stacked 2D materials. [1] R. W. Havener, Y. Liang, L. Brown, L. Yang, J. Park, Van Hove Singularities and Excitonic Effects in the Optical Conductivity of Twisted Bilayer Graphene, Nano Letters, 14, 3353-3357 (2014). [2] Y. Liang, R. Soklaski, S. Huang, M. W. Graham, R. Havener, J. Park, L. Yang, Strongly Bound Excitons in Gapless Two-dimensional Structures, Phys Rev. B, 90, 115418-3 (2014). [3] H. Patel, R. Havener, L. Brown, Y. Liang, L. Yang, J. Park, M.W. Graham, Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons, Nano Letters, 15, 5932-7 (2015). Figure 1

Electron energy transfer effect in Au NS/CH3NH3PbI3-xClx heterostructures via localized surface plasmon resonance coupling.

Localized surface plasmon resonance coupling effects (LSPR) have attracted much attention due to their interesting properties. This Letter demonstrates significant photoluminescence (PL) enhancement in the Au NS/CH3NH3PbI3-xClx heterostructures via the LSPR coupling. The observed PL emission enhancement is mainly attributed to the hot electron energy transfer effect related to the LSPR coupling. For the energy transfer effect, photo-generated electrons will be directly extracted into Au SPs, rather than relaxed into exciton states. This energy transfer process is much faster than the diffusion and relaxation time of free electrons, and may provide new ideas on the design of high-efficiency solar cells and ultrafast response photodetectors.

Optical properties of type-II AlInAs/AlGaAs quantum dots by photoluminescence studies

We report photoluminescence (PL) characterization and model simulation of AlInAs/AlGaAs type-II quantum dots (QDs). A thorough and precise determination of the band parameters for QD and matrix materials is given, focusing on the effects of alloy composition and strain state on the electronic properties. Origins of experimentally observed PL emission peaks are identified through a comparison with the band lineup theoretically determined in this work. We interpret the QD emission as originating from indirect type-II transitions involving electrons in the barrier X valley and heavy holes with S and P symmetry.

A facile synthesis of graphene oxide–ZnS/ZnO nanocomposites and observations of thermal quenching of visible photoluminescence emission and nonlinear optical properties

Here we have reported a facile synthesis of graphene oxide–ZnS/ZnO nanocomposite and the temperature dependent photoluminescence (PL) emissions in the synthesized materials, which are scarcely been available in the literature. In the present work PL emission in GO and its composites with ZnS and ZnO semiconductor quantum dots (QDs) have been measured at variable temperatures in 283–353K temperature region. From the measured results it has been found that quenching of PL emission has been taken place in the composite sample and it has been proposed that as the temperature is increased, the excited electrons in the localized states formed by the sp2 clusters in GO can migrate to the nearby sp3 defects states, thereby the intensity of PL emission is reduced. Nonlinear Optical (NLO) properties as well as the optical limiting (OL) properties has also been studied by using an indigenously developed Z-scan technique with a 10ns laser pulse at 1064nm laser radiation. Two photon absorptions (2PA) behavior have been found to be the dominant mechanism in the synthesized samples. A suitable energy level scheme has been proposed to explain the observed PL emission behavior as well as the 2PA mechanism. The present report will open up a lot of prospects for synthesizing GO-semiconductor nanocomposites with semiconductor materials as well as for potential applications in future luminescent devices.

Spatially indirect radiative recombination in InAlAsSb grown lattice-matched to InP by molecular beam epitaxy

A photoluminescence (PL) spectroscopy study of the bulk quaternary alloy InAlAsSb is presented. Samples were grown lattice-matched to InP by molecular beam epitaxy and two different growth temperatures of 450 °C and 325 °C were compared. Interpolated bandgap energies suggest that the development of this alloy would extend the range of available direct bandgaps attainable in materials lattice-matched to InP to energies as high as 1.81 eV. However, the peak energy of the observed PL emission is anomalously low for samples grown at both temperatures, with the 450 °C sample showing larger deviation from the expected bandgap. A fit of the integrated PL intensity (I) to an I∝Pk dependence, where P is the incident power density, yields characteristic coefficients k = 1.05 and 1.18 for the 450 °C and 325 °C samples, respectively. This indicates that the PL from both samples is dominated by excitonic recombination. A blue-shift in the peak emission energy as a function of P, along with an S-shaped temperature dependence, is observed. These trends are characteristic of spatially-indirect recombination associated with compositional variations. The energy depth of the confining potential, as derived from the thermal quenching of the photoluminescence, is 0.14 eV for the 325 °C sample, which is consistent with the red-shift of the PL emission peak relative to the expected bandgap energy. This suggests that compositional variation is the primary cause of the anomalously low PL emission peak energy. The higher energy PL emission of the 325 °C sample, relative to the 450 °C sample, is consistent with a reduction of the compositional fluctuations. The lower growth temperature is therefore considered more favorable for further growth optimization.

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D0) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.