Function Of Excitation Power Research Articles

Investigation of Exciton Dynamics to Confirm Coupling Effects Using a Comparison between Ensemble and Single Laterally‐Coupled GaAs Quantum Dot Molecules

Herein, time‐integrated and time‐resolved photoluminescence (PL) measurement experiments are performed on coupled GaAs quantum dot (QD) molecules with larger laser spot size and a single‐coupled QD molecule to investigate the dynamics of exciton states. A linearly polarized laser along the coupled direction of a laterally‐coupled GaAs QD molecule [10] is used to determine the coupling effects of the different exciton states. For the ensemble of coupled QD molecules, the exciton PL peak shifts to lower energy (≈70 meV) and shows a linear increase in the PL intensity, demonstrating the properties of exciton states as a function of the laser excitation power. In addition, a power‐dependent redshift is observed in the single‐coupled GaAs QD molecule as a function of the excitation power; however, the two distinguished exciton states become resolved and show different power factors when the laser excitation power is linearly increased. This result clearly indicates the presence of energy transfer between the two different exciton states, X1 and X2. Furthermore, when the polarized laser excitation power is increased, a coupled biexciton consisting of two different localized exciton states (X1 and X2) is clearly observed, and a power‐dependent spectral redshift is detected in the exciton and biexciton states. At a high excitation power (15 I0), the decay times of the two different localized exciton states clearly show different lifetimes; and the decay time difference due to the size difference can be ignored at a low excitation power. Theoretically, the decay times of the two different exciton states, wherein the size deviation can be ignored, should be the same. Based on the results of the time‐integrated and time‐resolved PL measurements and the comparison between the ensemble and single exciton PL peaks, it is concluded that the coupling process in laterally‐coupled GaAs QD molecules is strongly supported against long‐range separation.

Determination of Gallium Concentration in Silicon from Low‐Temperature Photoluminescence Analysis

Increasing the understanding of the electronic properties of gallium (Ga) in silicon (Si) used nowadays to manufacture p‐type Si solar cells is of key technological importance. In this contribution, the results of the effect of Ga concentration on the low‐temperature photoluminescence (PL) spectra in crystalline Si are reported. The Ga‐doped Si samples studied have negligible boron concentrations, which can complicate spectral analysis of the bound exciton (BE) lines. The split Ga BE ground state at T = 10 K is analyzed and the PL intensity ratios for the BE to free exciton peaks are compared. By comparing these to known Ga concentrations, derived from capacitance–voltage measurements, an all‐optical (PL) calibration curve for the quantification of Ga concentration in Si is established. The effects of both temperature and excitation power on the PL intensity ratios are also studied. By combining the temperature‐induced changes in the PL intensity ratios with the calibration curve at 10 K, a calibration function has been determined. It is found that the decay rates of the PL intensity ratios as a function of excitation power are independent of the chosen (split) BE peak. The major benefits of this method and its limitations are discussed.

Electrical control of hybrid exciton transport in a van der Waals heterostructure

Interactions between out-of-plane dipoles in bosonic gases enable the long-range propagation of excitons. The lack of direct control over collective dipolar properties has so far limited the degrees of tunability and the microscopic understanding of exciton transport. In this work we modulate the layer hybridization and interplay between many-body interactions of excitons in a van der Waals heterostructure with an applied vertical electric field. By performing spatiotemporally resolved measurements supported by microscopic theory, we uncover the dipole-dependent properties and transport of excitons with different degrees of hybridization. Moreover, we find constant emission quantum yields of the transporting species as a function of excitation power with radiative decay mechanisms dominating over nonradiative ones, a fundamental requirement for efficient excitonic devices. Our findings provide a complete picture of the many-body effects in the transport of dilute exciton gases, and have crucial implications for studying emerging states of matter such as Bose–Einstein condensation and optoelectronic applications based on exciton propagation.

Discrete Donor–Acceptor Pair Transitions in CH3NH3PbI3 Perovskite Single Crystals

Achieving a better understanding of the physics of defects in halide perovskites (HPs) is a key challenge for improving the efficiency of the devices. Herein, a comprehensive study of the defect emission of CH3NH3PbI3 perovskite single crystals is presented. The emission of the pristine surface of cleaved crystals is systematically investigated based on steady‐state and time‐resolved micro‐photoluminescence (micro‐PL) spectroscopy. Donor–acceptor pair (DAP) recombination is observed due to the presence of native shallow defects. The DAP spectra present an important variability depending on the location on the surface of the crystals due to inhomogeneous defect distribution. A strong blueshift of the emission is measured as a function of excitation power and is explained by fluctuating potential caused by compensated defects. With increasing photocarrier density, a transition from a structureless to a structured DAP emission with several longitudinal optical phonon replicas is observed. The DAP transition is characterized by the redshift of the emission with time and a slow, non‐exponential PL decay. Sharp discrete lines with sub‐meV widths, the most recognizable spectral signature of DAP transition, are revealed. Herein, the apparently contradictory previous observations on defect emission of CH3NH3PbI3 are reconciled by the results and new insights are provided into the properties of defects in HPs.

The trilayer exciton emission in WSe2/WS2/MoS2 van der Waals heterostructures

In this Letter, we present the experimental observation of trilayer exciton emission in the WSe2/WS2/MoS2 heterostructures by photoluminescence spectroscopy at a low temperature of ∼10 K. It is found that the trilayer exciton has a lower exciton binding energy than that in the WSe2/MoS2 heterobilayer due to the insertion of monolayer WS2, which is manifested by the blueshift of this trilayer exciton peak relative to the bilayer exciton. Through the measurements of the exciton peak position as a function of excitation power, the trilayer excitons exhibited the density-dependent repulsive interaction among them, and the reduction of binding energy ∼8 meV is deduced. Moreover, the trilayer exciton has a longer lifetime than the bilayer exciton, resulting from the reduction of electron–hole wavefunction overlap.

Mapping the Energetics of Defect States in Cu2ZnSnS4 films and the Impact of Sb Doping.

The sub-bandgap levels associated with defect states in Cu2ZnSnS4 (CZTS) thin films are investigated by correlating the temperature dependence of the absorber photoluminescence (PL) with the device admittance spectroscopy. CZTS thin films are prepared by thermolysis of molecular precursors incorporating chloride salts of the cations and thiourea. Na and Sb are introduced as dopants in the precursor layers to assess their impact on Cu/Zn and Sn site disorder, respectively. Systematic analysis of PL spectra as a function of excitation power and temperature show that radiative recombination is dominated by quasi-donor–acceptor pairs (QDAP) with a maximum between 1.03 and 1.18 eV. It is noteworthy that Sb doping leads to a transition from localized to delocalized QDAP. The activation energies obtained associated with QDAP emission closely correlate with the activation energies of the admittance responses in a temperature range between 150 K and room temperature in films with or without added dopants. Admittance data of CZTS films with no added dopants also have a strong contribution from a deeper state associated with Sn disorder. The ensemble of PL and admittance data, in addition to energy-filtered photoemission of electron microscopy (EF-PEEM), shows a detailed picture of the distribution of sub-bandgap states in CZTS and the impact of doping on their energetics and device performance.

Mapping Transport Properties of Halide Perovskites via Short-Time-Dynamics Scaling Laws and Subnanosecond-Time-Resolution Imaging

The excellent optoelectronic and transport properties of halide perovskites have led to the rapid development of perovskite-based optoelectronic devices. A fundamental understanding of charge-carrier dynamics, as well as the implementation of physical models able to accurately describe their behaviour, is essential for further improvements in the field. Here, combining advanced modeling and characterization, a method for analyzing the short time dynamics of time-resolved fluorescence imaging (TRFLIM) decays is demonstrated. A theoretical scaling law for the time derivative of transient photoluminescence decays as a function of excitation power is extracted. This scaling law, computed from classical drift-diffusion equations, defines an innovative and simple way to extract quantitative values for several transport parameters, including the external radiative-recombination coefficient. The model is notably applied on a set of images acquired with a temporal shift of 250 ps to map the top-surface recombination velocity of a triple-cation mixed-halide perovskite thin film at the microscale. The development of high-time-resolution imaging techniques coupled with a scaling method for analyzing short time dynamics provides a solid platform for the investigation of local heterogeneities in semiconductor materials and the accurate determination of the main parameters governing their carrier transport.

Down-Shifting in the YAM: Ce3+ + Yb3+ System for Solar Cells.

In this work, we investigate Ce3+ to Yb3+ energy transfer in Y4Al2O9 (YAM) for potential application in solar spectrum down-converting layers for photovoltaic devices. Photoluminescence properties set, of 10 samples, of the YAM host activated with Ce3+ and Yb3+ with varying concentrations are presented, and the Ce3+ to Yb3+ energy transfer is proven. Measurement of highly non-exponential luminescence decays of Ce3+ 5d band allowed for the calculation of maximal theoretical quantum efficiency, of the expected down-conversion process, equal to 123%. Measurements of Yb3+ emission intensity, in the function of excitation power, confirmed the predominantly single-photon downshifting character of Ce3+ to Yb3+ energy transfer. Favorable location of the Ce3+ 5d bands in YAM makes this system a great candidate for down-converting, and down-shifting, luminescent layers for photovoltaics.

Formation and Photoluminescence Properties of ZnO Nanoparticles on Electrospun Nanofibers Produced by Atomic Layer Deposition

The unique combination of optical, chemical, and structural properties of one-dimensional zinc oxide (1D ZnO) makes it one of the most attractive materials in a wide range of research and applications. In the present study, 1D ZnO nanomaterials were fabricated using a combination of two independent methods: electrospinning and atomic layer deposition (ALD). The electrospinning technique was used to produce 1D electrospun fibers consisting of four types of polymers: polylactic acid (PLLA), polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), and polyamide 6 (PA6). The ALD technology, in turn, was selected as an excellent candidate for the synthesis of a ZnO thin layer over polymer fibers for the production of 1D ZnO/polymer nanofiber composites (PLLA/ZnO, PVDF/ZnO, PVA/ZnO, PA6/ZnO). Structural and optical properties of the produced nanofibers were studied by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), diffuse reflectance, and photoluminescence (PL) spectroscopy. It was found that only PVDF/ZnO nanofibers exhibit stable room temperature PL that may be the result of a higher ZnO content in the sample. In addition, PL measurements were conducted as a function of excitation power and temperature in order to establish the main PL mechanisms and parameters for the PVDF/ZnO sample, as a most promising candidate for the biophotonic application.

Photoluminescence Characterization of Fluorescent Sic with High Boron and Nitrogen Concentrations

The dependence of optical properties on impurity concentrations and excitation power was explored. In particular, it was found that the peak shift of photoluminescence (PL) is proportional to the boron concentration. This might be due to donor–acceptor pair (DAP) recombination via double deep acceptor levels (D-centers), where the occupancy of the D*-center increased with the B concentration, and the recombination via the D*-center for longer wavelengths became dominant. Moreover, the relative constants B and C were calculated by BC model fitting from the internal quantum efficiency (IQE) curve as a function of excitation power. The theoretical extrapolation based on BC model predicted that high impurity concentrations are sufficient to suppress the droop phenomenon of efficiency induced by the increased excitation power.

Up-conversion luminescence properties and energy transfer mechanism of SrAl2B2O7: Er3+@Yb3+ phosphors

Abstract The Er3+@Yb3+ co-doped SrAl2B2O7 phosphors were prepared by a sol-gel auto-combustion method. The components, microstructure and up-conversion (UC) properties of samples were investigated systematically. The phosphors emitted red light and green light under the excitation of 980 nm wavelength. The intensity of red emission and green emission nearly linearly decreased with increasing ambient temperatures. The red emission, green emission and their intensity ratio as a function of Er3+ concentrations and Yb3+ concentrations were studied in detail. The results show that 4.0% Er3+ and 8.0% Yb3+ doped samples can obtain the best luminescence intensity while the best light purity cannot be obtained simultaneously. The intensity of red and green emission was measured as a function of excitation power. It indicates that there was no photon avalanche mechanism for SrAl2B2O7: Er3+@Yb3+ due to the absence of sharp change of n values. The UC luminescence corresponded to a two-photon absorption process which was illustrated by the energy transfer mechanism and the steady-state rate equations. Because of the controllable monochromatic luminescence and excellent temperature-dependent luminescence properties, the SrAl2B2O7: Er3+@Yb3+ phosphors have potential application in the fields of non-contact temperature measurement and bio-polychromatic label.

Classification of Transitions in Upconversion Luminescence of Lanthanides by Two-Dimensional Correlation Analysis.

Upconversion luminescence bands from Yb3+/Er3+ codoped into a matrix such as NaGdF4 can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.

Multiple-photon excitation of nitrogen vacancy centers in diamond

We report the first observation of multi-photon photoluminescence excitation (PLE) below the resonant energies of nitrogen vacancy (NV) centers in diamond. The quadratic and cubic dependence of the integrated fluorescence intensity as a function of excitation power indicate a two-photon excitation pathway for the NV- charge state and a three-photon process involved for the neutral NV0 charge state respectively. Comparing the total multi-photon energy with its single-photon equivalent, the PLE spectra follows the absorption spectrum of single photon excitation. We also observed that the efficiency of photoluminescence for different charge states, as well as the decay time constant, was dependent on the excitation wavelength and power.

Potential Fluctuations and Localization Effects in CZTS Single Crystals, as Revealed by Optical Spectroscopy

Reports on Cu $$_2$$ ZnSn(S $$_x$$ Se $$_{1-x}$$ ) $$_4$$ (CZTSSe) solar cell devices all show an open-circuit voltage lower than expected, especially when compared to CuIn $$_x$$ Ga $$_{1-x}$$ (S,Se) $$_2$$ devices, which reduces their power efficiency and delays their development. A high concentration of intrinsic defects in CZTSSe, and their stabilization through neutral complex formation, which induces some local fluctuations, are at the origin of local energy shifts in the conduction and valence band edges. The implied band tail in Cu $$_2$$ ZnSnS $$_4$$ is studied in this work by combining three types of optical spectroscopy data: emission spectra compared to photoluminescence excitation spectroscopy, emission spectra as a function of excitation power, and time-resolved photoluminescence spectra. All these data converge to show that both the bandgap and the band tail of localized states just below are dependent on the degree of order/disorder in the Cu/Zn cation sublattice of the quaternary structure: in the more ordered structures, the bandgap increases by about 50 meV, and the energy range of the band tail is decreased from about 110 to 70 meV.

Downconversion process in Yb3+doped GdAG nanocrystals

The efficient near infrared 2F5/2→2F7/2 downconversion emission of Yb3+ was observed upon excitation of Gd3Al5O12:Yb3+ nanocrystals in UV range. Its integrated intensity was measured as a function of excitation power in the range 1–30mW and compared for a low and high concentration of Yb ions. The model of luminescence dependency on absorbed pump power was applied to investigate the emission mechanism. It was found that the N parameter of that model, interpreted as the order of the process, was dependent on concentration of Yb ions. Moreover, the presence of Yb2+ has been confirmed. The mechanism of energy transfer processes leading to observed Yb3+ emission was discussed in terms of a sequence of successive cross relaxations via a presence of divalent and trivalent Yb ions: Gd3+→Yb2+→ (Yb3+)*.

Luminescence properties ofCu2ZnSn(S,Se)4solar cell absorbers: State filling versus screening of electrostatic potential fluctuations

The power conversion efficiency of ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$ kesterite thin-film solar cells is mainly limited by a low open-circuit voltage ${V}_{\text{OC}}$. In the literature, this low ${V}_{\text{OC}}$ has been amongst others attributed to electrostatic potential fluctuations leading to fluctuating band edges. This assignment was mainly based on the observation of a shift of the photoluminescence (PL) maximum to higher photon energies as a function of excitation power, which was interpreted in terms of a screening of the electrostatic potential fluctuations. However, in this paper, we show evidence that the observed shift of the PL maximum is dominantly caused by state filling rather than screening. Our assignment is based on the investigation of the full PL line shape (not only of the PL maximum) and the PL yield as a function of excitation power and temperature. Further support of our interpretation is given by the observation of additional band-tail emission showing up as a second high-energy PL band at the highest excitation powers. The spectral position of this additional band coincides with the absorption-tail deduced from photoluminescence excitation spectroscopy (PLE).

Polariton-condensation effects on photoluminescence dynamics in a CuBr microcavity

Temporal profiles of photoluminescence (PL) from the cavity polariton at the ground state in a CuBr microcavity at 10K have been investigated from the viewpoint of the cavity-polariton condensation. We measured angle-resolved reflectance spectra to characterize the cavity-polariton dispersions, cavity-photon dispersion, and quality factor from which the intrinsic cavity-polariton lifetime was estimated. The threshold-like changes of the PL intensity, band width, and energy of the lower polariton as a function of excitation power are consistent with previously reported experimental results for the cavity-polariton condensation. It was found that the PL rise and decay times are markedly decreased by the cavity-polariton condensation, which can be attributed to the appearance of the bosonic final state stimulation and intrinsic lifetime of polaritons in the relaxation and decay processes, respectively.

Carrier transfer between confined and localized states in type II InAs/GaAsSb quantum wells

Temperature-resolved photoluminescence studies were performed on tensely-strained AlSb/InAs/GaAsSb W-shaped type II quantum wells. They revealed two emission bands: one at lower energy of localized origin resulting from carrier trapping states at interfaces and dominates at low-temperature; and one corresponding to the fundamental optical transition in the type II quantum well. With the temperature increase to 170–200 K the low-energy emission is quenched and the high-energy band dominates and its intensity increases, indicating carrier transfer processes between the respective states at elevated temperatures. In addition, the integrated photoluminescence intensity was measured as a function of excitation power. At high excitation regime the emission intensity of the low-energy emission band saturated, indicating low density of states, thus confirming its localized nature. The depth of the localization potential at the InAs/GaAsSb interface was determined to be 13–15 meV.

Broadband white emission from Yb3+ doped Sr2CeO4 nanocrystals

Intense broadband anti-Stokes white emission (BASWE) generated from Sr2CeO4:Yb3+ nanocrystals in vacuum upon focused infrared excitation lines (808 and 975 nm) is reported. The samples were prepared via Pechini's technique. The crystal structures of the phosphors were confirmed by X-ray diffraction data measurements. Based on them the average crystallite sizes of the samples were determined by Rietveld refinement to be 72 and 41 nm for 1 and 20% of dopant concentration, respectively. In particular the emission intensity in function of power density and surroundings pressure were investigated. It was found that the emission intensity increases significantly with increasing power excitation for excitation lines. It is also interesting to see that the pressure dependence for both excitation lines are similar. Results of these two measurements exhibit threshold behavior which is characteristic for the avalanche effect. The rise times dependence in function of excitation power were also studied.

O-Doping Profile along ZnTe Nanowires Obtained Using Power-Dependent Microphotoluminescence Measurements

We investigated the O-doping profile along ZnTe nanowires using laser power-dependent microphotoluminescence measurements. ZnTe nanowires were grown by physical vapor transport catalyzed by bismuth. The photoluminescence spectra consisted of band-edge green photoluminescence and red photoluminescence due to a highly luminescent intermediate band formed by unintentional O doping. An obvious color-tuning effect of photoluminescence was observed along the nanowire and was suggestive of the competition between two luminescence bands. A significant variation in the intensity of red luminescence from the catalyst tip to the bottom was observed, and the photoluminescence intensity as a function of excitation power at each sampling spot along the nanowire was analyzed using a power law. The exponent value in the power law, from the sampling spots, was determined to be 0.58 ± 0.11, indicating an identical recombination channel for red luminescence. However, the O-doping density shows a superlinear increase from the catalyst tip to the bottom. Besides the direct impingement of doping species on the side wall, the additional incorporation of the doping species surface-migrating from the substrate may explain the nonlinear O-doping profile along the nanowire.