Photoluminescence Spectral Shape Research Articles

Revealing the internal luminescence quantum efficiency of perovskite films via accurate quantification of photon recycling

The internal luminescence quantum efficiency ($Q_\mathrm{i}^\mathrm{lum}$) provides an excellent assessment of the optoelectronic quality of semiconductors. To determine $Q_\mathrm{i}^\mathrm{lum}$ of perovskite films from the experimentally accessible external luminescence quantum efficiency ($Q_\mathrm{e}^\mathrm{lum}$) it is essential to account for photon recycling, and this requires knowledge of the photon escape probability ($p_\mathrm{e}$). Here, we establish an analysis procedure based on a curve fitting model that accurately determines $p_\mathrm{e}$ of perovskite films from photoluminescence (PL) spectra measured with a confocal microscope and an integrating sphere setup. We show that scattering-induced outcoupling of initially-trapped PL explains commonly observed red-shifted and broadened PL spectral shapes and leads to $p_\mathrm{e}$ being more than 10% higher in absolute terms compared to earlier assumptions. Applying our model to CH$_3$NH$_3$PbI$_3$ films with exceptionally high $Q_\mathrm{e}^\mathrm{lum}$ up to 47.4% sets a real benchmark for $Q_\mathrm{i}^\mathrm{lum}$ at $78.0 \pm 0.5\%$, revealing there is beyond a factor of two more scope for reducing non-radiative recombination than previously thought.

Effect of A-Site Cation on Photoluminescence Spectra of Single Lead Bromide Perovskite Nanocrystals.

Lead halide perovskite (APbX3) nanocrystals exhibit photoluminescence (PL) with both wide wavelength tunability and high quantum efficiency. While the Pb-X6 octahedra mainly determines the near-band-edge optical properties and the A-site cation affects the structural stability, the role of the A-site cation in determining the optical properties is still unclear. Here, we report the PL properties of three types of lead bromide perovskite APbBr3 nanocrystals with different cations [A = HC(NH2)2+, CH3NH3+, and Cs+], as revealed by single-dot spectroscopy, and discuss the influence of the A-site cation on the PL spectrum. The nanocrystal size dependences of the PL energy and lifetime show no large variation with the species of the A-site cation. We find that the size of the A-site cation determines the coupling strength between electrons and longitudinal-optical phonons in the nanocrystal and thus affects the PL spectral shape, especially the low-energy tail.

Photoluminescence Quantum Yield of Single-Wall Carbon Nanotubes Corrected for the Photon Reabsorption Effect.

Photoluminescence (PL) from single-wall carbon nanotubes (SWCNTs) enables structural identification, but to derive the content rate of the specific chirality species it is necessary to know the quantum yield of each chirality. However, in the PL of SWCNTs, because the Stokes shift is small, the photon reabsorption effect is dominant and the apparent PL spectral shape and emission intensity are greatly modified depending on the concentration. This problem makes quantitative identification of SWCNTs by PL difficult. In this study, the concentration dependence of the PL of SWCNTs separated into a few chiralities was analyzed in detail, including the effect of reabsorption. It is clear that all changes in the PL spectrum occurring in the high concentration range can be explained simply by the reabsorption effect, and additional effects such as Coulomb interactions between SWCNTs can be negligible. Furthermore, a reliable quantum yield was derived from the emission intensity corrected for the reabsorption effect. The PL quantum yield varied with SWCNT chirality and exhibited a clear "family pattern". This is consistent with the theoretical report showing that the chirality-dependent PL quantum yield is dominated mainly by relaxation by optical phonons from E22 to E11.

Temperature dependent photoluminescence of anatase and rutile TiO2 single crystals: Polaron and self-trapped exciton formation

We propose an analysis of the emission properties of anatase and rutile titanium dioxide (TiO2) that emphasizes the role of the strong electron-phonon interaction. We performed measurements of photoluminescence (PL) spectra of bulk monocrystals under continuous wave-laser excitation and of their temperature dependence. We show that in both anatase and rutile, weakly bound self-trapped excitons are actually made out from carrier polarons and give rise to a broad emission band in the visible spectral range. The thermal activation of carrier motion allows their hopping to distant sites that leads to the observed quenching of luminescence. In the specific case of rutile TiO2, the PL spectral shape and its intensity-quenching scenario reveal the presence of dark trap states. Moreover, an additional narrow line structure shows up at low temperatures. The latter is due to localized impurity states that can be attributed to oxygen vacancies and can be fitted with a large Huang-Rhys parameter S = 2.5 within a Franck-Condon model. Both phases show thus a very strong interaction between the photogenerated carriers and the lattice.

Effect of a pick-and-drop process on optical properties of a CVD-grown monolayer tungsten disulfide

We systematically investigated changes of a photoluminescence (PL) spectral shape by picking a tungsten disulfide monolayer (1L-${\mathrm{WS}}_{2}$) grown on a sapphire (SA) substrate and dropping it on various substrates such as SA, ${\mathrm{SiO}}_{2}$, and hexagonal boron nitride (hBN). An inhomogeneous contribution to the exciton PL linewidth was reduced by picking the CVD-grown 1L-${\mathrm{WS}}_{2}$, meaning potential energy disorder lowering in the 1L-${\mathrm{WS}}_{2}$. In addition, a defect-bound exciton peak was not seen at low temperature for all the samples with the pick-and-drop process although it was clearly observed for the as-grown sample. It was thought that the as-grown 1L-${\mathrm{WS}}_{2}$ was covalently bonded on the SA substrate surface due to the existence of crystal imperfections in the ${\mathrm{WS}}_{2}$ and the covalent bonds formed midgap states. A PL spectrum had to be carefully treated in order to fairly evaluate a crystal quality in a CVD-grown 1L-${\mathrm{WS}}_{2}$ since PL spectral shapes were determined by complexes of the defect density with other mechanisms such as strain fluctuation and a connection with the substrate.

Nd3+‐Doped Silicate Glasses as an Efficient Color Filter to Improve the Color Gamut of a White LED

AbstractNd3+‐doped silicate glass (Nd‐glass) was employed as a color filter for a white LED based on red and green phosphor (RG‐LED), to manipulate the photoluminescence spectral shape and thus to provide a wider color gamut. The hypersensitive transition of Nd3+:4I9/2→4G5/2,2G7/2 was adjusted via glass composition and Nd concentration, and improved absorbance as well as reduced the absorption bandwidth. The effective absorption of the Nd‐glass at ~580 nm reduced the spectral linewidth of the green and red emissions, improving the color reproduction range. The color gamut of the RG‐LED was improved from 75.3% to 81.6% NTSC by the introduction of Nd‐glass as a color filter. Reliability under high operating current and high temperature were also examined and discussed.

Energy Transfer in Silicon Nanocrystal Solids Made from All-Inorganic Colloidal Silicon Nanocrystals.

Energy transfer between silicon (Si) nanocrystals (NCs) in Si-NC solids was demonstrated by photoluminescence (PL) spectroscopy. Clear differences of PL spectra and the decay rates between solutions and solids of Si-NCs were observed. The change in the PL properties caused by the formation of solids could be explained by the energy transfer from small to large NCs in the size distribution. In order to obtain further evidence of NC-to-NC energy transfer, the size distribution was intentionally modified by mixing solutions of NCs with different size distributions. NC solids made from the mixed solutions exhibited significantly different PL spectral shape and decay rates from those made from unmixed solutions, providing clear evidence of NC-to-NC energy transfer in Si-NC solids.

Photoluminescence and terahertz generation in InGaN/GaN multiple quantum well light‐emitting diode heterostructures under laser excitation

In this paper the results of experiments on terahertz generation from nitride light‐emitting diode heterostructures under two‐photon excitation by femtosecond laser pulses are reported. Dependencies of the photoluminescence and terahertz spectra on structural properties of the samples and intensity of laser pulses have been studied. It was found that the terahertz pulse amplitude increases and its spectrum shifts towards higher frequencies with an increasing number of quantum wells in the heterostructures. Photoluminescence spectral shape change at high excitation intensities was observed. The probable mechanisms explaining the observed experimental dependencies are discussed.

Enhanced Thermal Sensitivity of Silicon Nanoparticles Embedded in (Nano-Ag/)SiNx for Luminescent Thermometry

Steady-state photoluminescence of silicon nanoparticles embedded in solid-state (nano-Ag/)SiNx thin films at above room temperature is studied and compared to silicon nanoparticles dispersed in low-polar liquids. Roles of local surface plasmons as well as general mechanisms responsible for the temperature-dependent photoluminescence are pointed out. Thermal sensitivities of photoluminescence spectral shape, maximum position, and full width at half-maximum are estimated and application of the (nano-Ag/)SiNx layers as photoluminescent thermal screens is proposed.

Hydrogen plasma induced modification of photoluminescence from a-SiNx:H thin films

Low temperature (250–350 °C) hydrogen plasma annealing (HPA) treatments have been performed on amorphous hydrogenated silicon nitride (a-SiNx:H) thin films having a range of compositions and subsequent modification of photoluminescence (PL) is investigated. The PL spectral shape and peak positions for the as deposited films could be tuned with composition and excitation energies. HPA induced modification of PL of these films is found to depend on the N/Si ratio (x). Upon HPA, the PL spectra show an emergence of a red emission band for x ≤ 1, whereas an overall increase of intensity without change in the spectral shape is observed for x > 1. The emission observed in the Si rich films is attributed to nanoscale a-Si:H inclusions. The enhancement is maximum for off-stoichiometric films (x ∼ 1) and decreases as the compositions of a-Si (x = 0) and a-Si3N4 (x = 1.33) are approached, implying high density of non-radiative defects around x = 1. The diffusion of hydrogen in these films is also analyzed by Elastic Recoil Detection Analysis technique.

Surface/interface-related optical properties in Si nanodisks fabricated by neutral-beam etching using bio-templates

The authors study surface/interface-related optical properties in Si nanodisks (NDs) fabricated by neutral-beam etching using bio-templates. Photoluminescence (PL) spectra and the transient properties are compared between different barrier materials of SiO2 and SiC. The PL spectral shape depends on the barrier material, indicating effects of oxide formation and/or oxygen atoms on the surface of the Si ND. The transient PL shows nanosecond-scale transition of carriers in surface-related quantum states in the ND. Temperature dependences of the PL intensity show that the activation energy responsible for the electron escape from the Si-ND at near room temperature in the case of SiC barrier agrees well with the conduction band offset, whereas that in the case of SiO2 barrier is significantly lower than the expected offset. The latter discrepancy suggests that the electronic band structure at the interface of Si-ND/SiO2 can be largely affected by dielectric screening due to oxygen atoms in the SiO2.

Excitation dependent photoluminescence study of Si-rich a-SiNx:H thin films

We report photoluminescence (PL) investigations on Si-rich amorphous hydrogenated silicon nitride (a-SiNx:H) thin films of different compositions, using three different excitation lasers, viz., 325 nm, 410 nm, and 532 nm. The as-deposited films contain amorphous Si quantum dots (QDs) as evidenced in high resolution transmission electron microscopy images. The PL spectral shape is in general seen to change with the excitation used, thus emphasizing the presence of multiple luminescence centres in these films. It is found that all the spectra so obtained can be deconvoluted assuming Gaussian contributions from defects and quantum confinement effect. Further strength to this assignment is provided by low temperature (300 °C) hydrogen plasma annealing of these samples, wherein a preferential enhancement of the QD luminescence over defect luminescence is observed.

Effects of concentration and thermal annealing on the optical activation of Er implanted into GaN layers

The wide band gap semiconductor, GaN, has emerged as an important host for rare earth-electroluminescence. The annealing behaviour and lattice site location of Er implanted into GaN were studied with the Rutherford Backscattering Spectrometry (RBS)/channelling and photoluminescence (PL) techniques. Also Er site dependence on the annealing temperature and implantation dose has been studied in detail. The optical properties of the Er-doped GaN system, evidencing their dependence on the parameters adopted during the synthesis procedure (Er implantation dose, annealing temperature) have been discussed. RBS/channelling measurements suggested that mostly Er occupy substitutional site and depends on the Er concentration. The main result is the activation of a typical Er giving rise to PL emission in the 1450–1650 nm range, related to radiative 4 I 13/2→4 I 15/2 transitions. Depending on the Er dose, we observe a specific behaviour linked to variation of the annealing temperature that strongly determines PL emission band. We observed a PL spectral shape with the main peak located at 1542 nm and shoulder peak at 1558 nm (and full width at half maximum (FWHM) of 33 nm) with a series of weaker PL structures at 1519, 1572 and 1591 nm, due to the Stark sub-level splitting.

Synthesis and luminescence properties of Eu(III)-doped nanoporous silica spheres

Europium (III) (Eu3+)-doped nanoporous silica spheres were synthesized, and the states of Eu3+ ions in the silica framework structure were investigated. The ordered nanopores were preserved with the doping at the Eu3+ molar concentration to Si up to 10mol%, and the O−Si−O and Si−OH groups in the structures were clearly rearranged with the doping, indicating the interaction of Eu3+ with the O atoms. The significant morphological changes in the spheres were observed with the doping. The photoluminescence spectral shapes due to the transitions of 5D0–7F1 and 5D0–7F2 were indicative of the presence of the Eu3+ in an environment of a low symmetry. It was found that the Eu3+ was located inside the silica framework to electrostatically interact with the environmental O atoms, which would prevent the aggregation among Eu3+ ions to show the efficient luminescence. Therefore, the interactions between the Eu3+ ions and silica framework structures in the spheres were successfully clarified.

Non-blinking semiconductor nanocrystals

The photoluminescence from a variety of individual molecules and nanometre-sized crystallites is defined by large intensity fluctuations, known as 'blinking', whereby their photoluminescence turns 'on' and 'off' intermittently, even under continuous photoexcitation. For semiconductor nanocrystals, it was originally proposed that these 'off' periods corresponded to a nanocrystal with an extra charge. A charged nanocrystal could have its photoluminescence temporarily quenched owing to the high efficiency of non-radiative (for example, Auger) recombination processes between the extra charge and a subsequently excited electron-hole pair; photoluminescence would resume only after the nanocrystal becomes neutralized again. Despite over a decade of research, completely non-blinking nanocrystals have not been synthesized and an understanding of the blinking phenomenon remains elusive. Here we report ternary core/shell CdZnSe/ZnSe semiconductor nanocrystals that individually exhibit continuous, non-blinking photoluminescence. Unexpectedly, these nanocrystals strongly photoluminesce despite being charged, as indicated by a multi-peaked photoluminescence spectral shape and short lifetime. To model the unusual photoluminescence properties of the CdZnSe/ZnSe nanocrystals, we softened the abrupt confinement potential of a typical core/shell nanocrystal, suggesting that the structure is a radially graded alloy of CdZnSe into ZnSe. As photoluminescence blinking severely limits the usefulness of nanocrystals in applications requiring a continuous output of single photons, these non-blinking nanocrystals may enable substantial advances in fields ranging from single-molecule biological labelling to low-threshold lasers.

Optical sensing of NO2 in tin oxide nanowires at sub-ppm level

Tin oxide nanostructures are currently considered as promising materials for gas sensing. Some of their physical properties, such as visible light photoluminescence, are still not fully understood and require further investigations. In this work we report on the quenching of tin oxide nanowires photoluminescence induced by nitrogen dioxide adsorption at the ppm and sub-ppm level. The recombination dynamics was investigated by means of time-resolved photoluminescence, finding non-exponential decay profiles. Neither the photoluminescence spectral shape nor the decay rates are found to depend significantly on the nitrogen dioxide concentration, suggesting that the gas-nanostructure interaction resides in a net reduction of the density of states which are responsible for the radiative transitions.

Anisotropic photoluminescence properties of oriented poly(p-phenylene-vinylene) films: Effects of dispersion of optical constants

We report on the effects of dispersive optical constants on the anisotropic photoluminescence spectra of highly oriented poly($p$-phenylene-vinylene) films. Polarized transmittance, reflectance, and photoluminescence (PL) spectra have been measured over a broad spectral range, at different temperatures. Due to the high degree of chain orientation in the polymer films, the emission properties are highly anisotropic. The PL spectral shape and external quantum efficiency are significantly affected by self-absorption inside the polymer film and by refractive effects at the polymer-air interface. In order to elucidate these aspects we have determined the dispersion of the parallel and perpendicular components of the complex dielectric constant $({\ensuremath{\epsilon}}_{1}+i{\ensuremath{\epsilon}}_{2})$, by which the PL spectra have been corrected according to Fresnel equations. After correction, the PL intensity is found to be higher when the excitation is polarized perpendicular to the stretching direction, a fact that we attribute to charge-induced PL quenching. We have also considered the influence of the refractive index on the PL spontaneous emission rate and on the light extraction from the polymer film. The spectral dependence of the emission anisotropy functions of the corrected PL spectra indicates the presence of two distinct emitting species, which may originate from intramolecular and intermolecular states.

Colloidal CdSe–ZnS core-shell nanoparticles: Dependence of physical properties on initial Cd to Se concentration

Different sets of CdSe–ZnS core-shell colloidal nanoparticles are synthesized from the starting solution containing stoichiometric excess or deficit of metal counterpart (Cd to Se ratio) to study the variation in physical property depending upon the core composition. It is found that the stoichiometric variation in the starting solution affects the microstructure of the nanoparticles and hence the variation in absorption and emission properties. It is found that the band edge emission peak shifted to the lower energy levels with increasing Cd/Se ratio in the starting solution. Broadening of the Photoluminescence spectral shape is found for Cd rich or Se rich samples. The stoichiometric sample shows the brightest emission that can be associated with the lesser defects associated to the nanoparticles in that sample. Tailing of the optical absorption beyond the band edge is found to be least for the stoichiometric sample indicating fewer defects present in the sample.

Instability of 0.4 nm carbon nanotubes in a zeolite template under ultraviolet laser irradiation

The stability of 0.4 nm carbon nanotubes within a zeolite matrix under laser irradiation has been investigated using photoluminescence (PL) spectroscopy. It was found that in the weak excitation power regime the PL spectra have similar line shapes at different excitation wavelengths. When the excitation power is high for ultraviolet light at 325 nm, the PL spectral shape undergoes an irreversible change. This is in contrast to the case with 488 nm laser irradiation and indicates a permanent chemical change in the sample, probably due to photochemical effects.

Anomalous temperature dependence of optical emission in visible-light-emitting amorphous silicon quantum dots

We have investigated optical properties of amorphous silicon quantum dots (a-Si QDs) embedded in a silicon nitride grown on (100) silicon substrates by plasma-enhanced chemical vapor deposition. The photoluminescence (PL) emission colors of red, orange, and blue were shown for the a-Si QDs with average dot sizes of 5.8, 2.4, and 1.3 nm, respectively. The increase in the PL peak energy with decreasing QD size was attributed to the quantum confinement effect in a-Si QDs. An anomalous temperature dependence of the PL intensity and PL peak energy was observed for a-Si QDs. The visible-light PL emission from a-Si QDs is most efficient at near room temperature, and the PL spectral shape and peak energy are almost independent of temperature, which are most favorable for light-emitting device applications. From time-resolved PL experiments, an enhancement of phonon-assisted indirect radiative recombination was proposed to explain the anomalous temperature dependence.