Temperature-dependent Steady-state Photoluminescence Research Articles

Temperature-dependent excited states for detecting reversible phase transitions in 2D lead(II) iodide perovskites.

Significant interest exists in water-tolerant 2D lead iodide perovskites owing to their stability and proven potential in photovoltaic and photonic applications. These materials have solid-state phase transitions that are accessible below 100 °C. Here, the study witnesses the multiple phase transitions of the last members of a series of organic-inorganic hybrid materials, [(CnH2n+1NH3)2PbI4], with even n as n = 14, 16, and 18, once again. By employing temperature-dependent steady-state photoluminescence (PL) and temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy in the temperature range of -18 to +90 °C and at -196 °C, we explore the thermal responses of these materials. The investigation reveals reversible phase transitions occurring between room temperature (RT) and elevated temperatures, impacting the optical properties and emitting colors of the perovskite compounds. The longer the alkyl chain, the higher the phase transition temperature, attributed to increased conformational disorder and enhanced perovskite symmetry. The decay constants for all compounds are very close in value, which confirms the underlying excited-state dynamics, pointing to contributions primarily from inorganic components across different phases. We anticipate that our results on the detection of phase transitions in 2D perovskites will not only motivate the use of these techniques for detecting phase transitions but also would help to understand their excited states in more details to selectively use them for solar cell and next-generation display technologies.

Pt(II) Complexes with Tetradentate C^N*N^C Luminophores: From Supramolecular Interactions to Temperature-Sensing Materials with Memory and Optical Readouts.

A series of four regioisomeric Pt(II) complexes (PtLa-n and PtLb-n) bearing tetradentate luminophores as dianionic ligands were synthesized. Hence, both classes of cyclometallating chelators were decorated with three n-hexyl (n = 6) or n-dodecyl (n = 12) chains. The new compounds were unambiguously characterized by means of multiple NMR spectroscopies and mass spectrometry. Steady-state and time-resolved photoluminescence spectroscopy as well quantum chemical calculations show that the effect of the regioisomerism on the emission colour and on the deactivation rate constants can be correlated with the participation of the Pt atom on the excited state. The thermal properties of the complexes were studied by DSC, POM and temperature-dependent steady-state photoluminescence spectroscopy. Three of the four complexes (PtLa-12, PtLb-6 and PtLb-12) present an intriguing thermochromism resulting from the responsive metal-metal interactions involving adjacent monomeric units. Each material has different transition temperatures and memory capabilities, which can be tuned at the intermolecular level. Hence, dipole-dipole interactions between the luminophores and disruption of the crystalline packing by the alkyl groups are responsible for the final properties of the resulting materials.

Radiolytically prepared blue photoluminescent Silicon Oxide nanocomposites for highly selective sensing of picric acid: Mechanism and development of paper strip-based detection

Herein, we wish to report highly selective sensing of picric acid (PA), one of the primary explosive ingredients as well as a well-known environmental hazard in 100 % water medium. For the said purpose, radiolytically synthesized blue photoluminescent PEGylated silicon oxide nanocomposites (PEG@SiONCs) with ∼49 % quantum efficiency (QE) were used as fluorescence probe. The limit of detection (LOD) of PA was determined to be ∼0.16 μM in the 2–85 μM range. Substantial reduction in blue photoluminescence of PEG@SiONCs, and shift in the emission colour from blue towards green in presence of specific concentrations of PA, are the interesting highlights of the present work. This colour change can be utilized to confirm the presence of PA in the sample. Mechanistic investigations involving temperature-dependent steady-state photoluminescence, and PA concentration-dependent lifetime studies revealed inner filter effect (IFE) based photoluminescence quenching mechanism. In addition, the interference studies carried out in the presence of 21 other organic/inorganic compounds demonstrated the highly selective sensing of PA by PEG@SiONCs. The PA sensing ability of the nanocomposites was also tested in the pH window ranging from 2 to 12. Meanwhile, paper strips were prepared using PEGylated SiONCs, and samples containing various metal ions and organic compounds were tested to examine their suitability for real-time detection of PA.

Octahedral Distortions Generate a Thermally Activated Phonon-Assisted Radiative Recombination Pathway in Cubic CsPbBr3 Perovskite Quantum Dots.

Exciton-phonon interactions elucidate structure-function relationships that aid in the control of color purity and carrier diffusion, which is necessary for the performance-driven design of solid-state optical emitters. Temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL) reveal that thermally activated exciton-phonon interactions originate from structural distortions related to vibrations in cubic CsPbBr3 perovskite quantum dots (PQDs) at room temperature. Exciton-phonon interactions cause performance-degrading PL line width broadening and slower electron-hole recombination. Structural distortions in cubic PQDs at room temperature exist as the bending and stretching of the PbBr6 octahedra subunit. The PbBr6 octahedral distortions cause symmetry breaking, resulting in thermally activated longitudinal optical (LO) phonon coupling to the photoexcited electron-hole pair that manifests as inhomogeneous PL line width broadening. At cryogenic temperatures, the line width broadening is minimized due to a decrease in phonon-assisted recombination through shallow traps. A fundamental understanding of these intrinsic exciton-phonon interactions gives insight into the polymorphic nature of the cubic phase and the origins of performance degradation in PQD optical emitters.

Polar-Solvent-Free Sonochemical Synthesis of Mn(II)-Doped CsPbCl3 Perovskite Nanocrystals for Dual-Color Emission

We present a facile, one-step, polar-solvent-free sonochemical synthesis of a series of Mn-doped CsPbCl3 perovskite nanocrystals (PNCs) by varying the Pb-to-Mn ratio, after which their structure, morphology, and temperature-dependent photoluminescence (PL) properties are investigated. The partial substitution of Pb2+ with Mn2+ in CsPbCl3 PNCs caused lattice contraction without affecting the morphology and structure. All Mn-doped PNCs exhibited dual-wavelength PL profiles, with a weak violet band at around 410 nm attributed to band-edge exciton and a strong orange band at around 596 nm attributed to Mn2+:4T1→6A1 transition, benefited by the exciton-to-Mn2+ energy transfer. Furthermore, Mn doping increased the PL quantum yield (PLQY) from 5.1% in CsPbCl3 to 41.14% for 11.8% Mn-doped PNCs as well as decay lifetimes from nanoseconds to milliseconds (long-lived emissive states). Owing to the thermally assisted transition from the exciton to 4T1 energy level of Mn2+, the temperature-dependent steady-state PL and time-resolved PL spectra of exciton and Mn2+ in Mn-doped CsPbCl3 PNCs displayed unusual and contradictory trends. The ultrasonication synthesized Mn-doped PNCs exhibited superior long-term stability, retaining 62% (@ RH 75%) of the initial PL without requiring additional encapsulation. To evaluate the suitable application of Mn-doped CsPbCl3 PNCs in the white light-emitting diode (w-LED) as a display system, we built a prototype w-LED using blue-, green-, and orange-emitting CsPb(Cl0.5/Br0.5)3, CsPbBr3, and Mn-doped CsPbCl3-coated glass slices onto a 20 mA UV LED chip. The constructed prototype w-LED generated bright white light, achieving a luminous efficiency of 76.3 lm/W, color rendering index of 81.4, R9 of 80.1, and a wide color gamut of 101.78% NTSC and 76% BT-2020, demonstrating its promising application in w-LEDs.

Room-temperature synthesis of CsPbBr3@CsPb2Br5 microcrystals and their upconversion luminescence

In this work, a convenient and environmental-friendly aqueous-phase method is used to synthesize CsPbBr3@CsPb2Br5 microcrystals (MCs) at room temperature, of which structural and optical properties are characterized by powder X-ray diffraction (PXRD), absorption, and photoluminescence (PL) spectroscopic techniques. Two obvious absorption peaks at 307 and 515 nm are attributed to CsPb2Br5 matrix and CsPbBr3 impurity, respectively. Full-width at half-maximum (FWHM) of PL for MCs is 83.5 meV at room temperature, and its emission peak is observed at 518 nm. With the temperature changed from 80 to 340 K, the fitted value of the exciton binding energy is 84.1 meV from temperature-dependent steady-state PL. Upconversion luminescence (UCL) can be observed obviously under near-infrared femtosecond laser pulse excitation, which is compared with normal luminescence from steady-state and time-resolved PL measurements. The experimental investigations show that CsPbBr3@CsPb2Br5 MCs have excellent stability against thermal, water and light irradiation, which will provide a new choice for the commercial applications to lasering and displaying.

Experimental and modeling analysis of photocarrier dynamics in CZTSSe using temperature-dependent photoluminescence

CZTSSe shows great potential in photovoltaic applications. However, the device efficiency is limited by the high density detrimental defects in the absorber. Therefore, investigating the influence of defects on absorber properties is essential. In this study, temperature-dependent steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements were performed on a CZTSSe film. Four emission components involving defect-associated transitions were obtained by analyzing the PL spectra. Beyond the traditional exponential analysis, a diffusion-recombination model was applied to simulate the TRPL curves, by which the key parameters describing the carrier recombination and diffusion were extracted. The model includes defect-mediated and band-involved recombination rates, surface/interface recombination velocities and their corresponding temperature dependences. The results indicate that the increasing Shockley–Read–Hall recombination rate is the dominant factor that leads to the enhanced PL quenching with increasing temperature. Our results help to further explain the key factors that influence the optoelectronic properties of CZTSSe.

H2O-NH4+-Induced Emission Modulation in Sb3+-Doped (NH4)2InCl5·H2O.

Lead-based metal halide perovskites have received widespread attention for their promising application prospects in the field of lighting and display due to their excellent optical properties. However, the toxicity of lead may hinder their further commercial application. Herein, a zero-dimensional (0D) metal halide (NH4)2InCl5·H2O with an orthorhombic structure and the Pnma space group was produced. With doping with Sb3+, these products exhibit one highly efficient and wide yellow emission band (∼450-850 nm) in their photoluminescence (PL) spectra, which covers almost the entire visible spectral range at room temperature; however, they give two emission bands with long decay lifetimes (microseconds) at low temperature. Temperature-dependent steady-state PL, transient PL spectroscopy, temperature-dependent Raman spectra characterization, and theoretical band structure calculations confirm that the dual-band emission at low temperature originates from the dual vibronic levels of the self-trapped exciton (STE) in the hole-vibration state, whose vibration energy is related to the H2O-NH4+ connection in the valence band. This result proves that the vibronic state in STE formation involves both electrons and holes in the excited states, the opposite of this happens in the electron-vibration band in most perovskite halides. These results provide new insight into the luminescent mechanism of Sb3+ in halide perovskites, especially used for emission color modulation by the temperature-dependent electron- or hole-vibration processes.

Exciton dynamics and photoresponse behavior of the in situ annealed CsSnBr3 perovskite films synthesized by thermal evaporation

The CsSnBr3 photodetectors are fabricated by thermal evaporation and 75 °C in situ annealing, and the effect of in situ annealing on the morphology, structure, exciton dynamics and photoresponse of thermally evaporated CsSnBr3 films are investigated. Especially, temperature dependent steady-state photoluminescence (PL) and transient PL decaying have been analyzed in details for understanding the exciton dynamics. Meanwhile, effect of annealing on the activation energy for trap sites (E a), exciton binding energy (E b), activation energy for interfacial trapped carriers (ΔE), trap densities and carriers mobilities are studied and the annealed (A-CsSnBr3) reveals obviously lower E b and trap density together with notably higher carrier mobility than those of the unannealed (UA-CsSnBr3). Temperature dependence of the integrated PL intensity can be ascribed to the combining effect of the exciton dissociation, exciton quenching through trap sites and thermal activation of trapped carriers. The temperature dependent transient PL decaying analysis indicates that the PL decaying mechanism at low and high temperature is totally different from that in intermediate temperature range, in which combing effect of free exciton and localized state exciton decaying prevail. The beneficial effects of the in situ annealing on the photoresponse performance of the CsSnBr3 films can be demonstrated by the remarkable enhancement of the optimal responsivity (R) after in situ annealing which increases from less than 1 A W−1 to 1350 A W−1 as well as dramatically improved noise equivalent power, specific detectivity D* and Gain (G).

Multiexciton dynamics in CsPbBr3 nanocrystals: the dependence on pump fluence and temperature

Multiexcitons generation is a process of generating electron–hole pairs in nanostructured semiconductors by absorbing a single high-energy photon. The multiexciton process is essential for the performance of optoelectronic devices based on perovskite nanomaterials. In this paper, ultrafast time-resolved transient absorption spectroscopy is used to study the ultrafast dynamics of CsPbBr3 nanocrystals. It is found that the multiexcitons Auger recombination lifetime increases with the decrease of pump fluence, while it is on the contrary for the hot carrier cooling time. The increase in the number of photons absorbed by each nanocrystal under high pump fluence slows down the relaxation of hot carriers to the band edge. The hot carrier cooling lifetime increases from 0.25 to 0.85 ps when the pump fluence increases from 6 to 127 μJ cm−2. Temperature-dependent transient absorption spectroscopy exhibits that the relaxation process of hot carriers slows down sharply when the lattice temperature decreases from 280 to 80 K. Moreover, the exciton binding energy 46 meV of CsPbBr3 nanocrystals is obtained by temperature-dependent steady-state photoluminescence spectroscopy. These findings provide insights for applications such as solar cells and light-emitting devices based on CsPbBr3 nanocrystals.

Photoluminescence lifetime based nickel ion detection by glutathione capped CdTe/CdS core-shell quantum dots

Here we have demonstrated a methodology for detecting nickel ions in aqueous medium using photoluminescence (PL) lifetime of the probe made of glutathione capped CdTe/CdS core-shell quantum dots. The ratio of (τavg)probe/(τavg)probe+Ni2+ has been developed as an analytical signal for detecting Ni2+ ions. The plot of (τavg)probe/(τavg)probe+Ni2+ versus concentration of Ni2+ revealed a linear positive slope and the sensitivity was 0.61 L/mg, which is in good agreement with that determined from conventional Stern-Volmer plot (i.e., 0.64 L/mg). The limit of Ni2+ detection was 23 μg/L. The temperature dependent steady state PL quenching, time-resolved PL decay and photocatalytic dye degradation in absence and in presence of Ni2+ suggested that the PL quenching by Ni2+ is due to pure collision based dynamic quenching with favourable electron transfer process. On the other hand, the present method was not applicable for Cu2+ detection as the ratio of (τavg)probe/(τavg)probe+Cu2+ was non-responsive over the given range of Cu2+ concentration. The PL quenching by Cu2+ comprised of a combination of dynamic quenching and due to interaction of Cu2+ with the probe. It has been concluded that ratio of (τavg)probe/(τavg)probe+analyte can be a promising analytical signal for detecting metal ions, particularly for those exhibiting dynamic PL quenching.

Excitation Wavelength and Intensity-Dependent Multiexciton Dynamics in CsPbBr3 Nanocrystals.

CsPbBr has attracted great attention due to unique optical properties. The understanding of the multiexciton process is crucial for improving the performance of the photoelectric devices based on CsPbBr nanocrystals. In this paper, the ultrafast dynamics of CsPbBr nanocrystals is investigated by using femtosecond transient absorption spectroscopy. It is found that Auger recombination lifetime increases with the decrease of the excitation intensity, while the trend is opposite for the hot-exciton cooling time. The time of the hot-carriers cooling to the band edge is increased when the excitation energy is increased from 2.82 eV (440 nm) to 3.82 eV (325 nm). The lifetime of the Auger recombination reaches the value of 126 ps with the excitation wavelength of 440 nm. The recombination lifetime of the single exciton is about 7 ns in CsPbBr nanocrystals determined by nanosecond time-resolved photoluminescence spectroscopy. The exciton binding energy is 44 meV for CsPbBr nanocrystals measured by the temperature-dependent steady-state photoluminescence spectroscopy. These findings provide a favorable insight into applications such as solar cells and light-emitting devices based on CsPbBr nanocrystals.

Eu3+ optical activation engineering in AlxGa1-xN nanowires for red solid-state nano-emitters

In this work, Eu3+-implanted and annealed AlxGa1-xN (0 ≤ x ≤ 1) nanowires (NWs) grown on GaN NW template on Si (111) substrates by plasma-assisted molecular beam epitaxy are studied by µ-Raman, cathodoluminescence (CL), nano-CL, and temperature-dependent steady-state photoluminescence (PL). The preferential location of the Eu3+-implanted ions is found to be at the AlxGa1-xN top-section. The recovery of the as-grown crystalline properties is achieved after rapid thermal annealing (RTA). After RTA, the red emission of the Eu3+ ions is attained for all the samples with below and above bandgap excitation. The 5D0 → 7F2 transition is the most intense one, experiencing a redshift with increasing AlN nominal content (x) from GaN to AlN NWs. Moreover, AlN nominal content and annealing temperature alter its spectral shape suggesting the presence of at least two distinct optically active Eu3+ centers (Eu1 and Eu2). Thermal quenching of the Eu3+ ions´ luminescence intensity, I, is determined for all the samples from 14 K to 300 K, being the emission of Eu3+-implanted AlN NWs after RTA at 1200 °C the most stable (I300 K/I14 K ~80%). The GaN/AlN interface in this sample is also found to have a key role in the Eu3+ optical activation.

Minority carrier lifetime and photoluminescence of mid-wave infrared InAsSbBi

Time-resolved photoluminescence measurements are reported for InAsSbBi alloys grown by molecular beam epitaxy with Bi mole fractions ranging from 0 to 0.8%, yielding minority carrier lifetimes on the order of hundreds of nanoseconds. The minority carrier lifetimes extracted from the time-resolved photoluminescence measurements are comparable to those of lattice-matched InAsSb grown at the same respective temperatures. Nomarski imaging shows that smooth, droplet-free surface morphologies are obtained in 1 μm thick InAsSbBi epilayers grown at temperatures between 360 and 380 °C. The alloy composition-dependent bandgap energies for the InAsSbBi samples are determined from temperature-dependent steady-state photoluminescence measurements and compared with the tetragonal distortion measured by x-ray diffraction to determine the Sb and Bi mole fractions of each sample. The minority carrier lifetime and the achievable extension of the InAsSb(Bi) cut-off wavelength are analyzed as functions of alloy composition and compared with the performance of InAsSb layers with similar growth parameters.

The correlation between phase transition and photoluminescence properties of CsPbX3 (X= Cl, Br, I) perovskite nanocrystals.

We report a correlation between the structural phase transition of CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) and their temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL). In contrast to CsPbBr3 and CsPbI3 NCs which exhibited a continuous blue-shift in their band gap with increasing temperature, the CsPbCl3 exhibited a blue shift until ∼193 K, followed by a red shift until room temperature. We attribute this change from a blue to a red shift to a structural phase transition in CsPbCl3, which also manifested in the temperature dependent TRPL. This pronounced phase transition in CsPbCl3 NCs is probably due to the condensation of its vibrational modes at low temperature, and the presence of the weak quantum confinement effect. Notably, the exciton recombination lifetimes showed a similar reverse trend due to the phase transition in CsPbCl3, which has not been reported previously.

Manipulating Charge Transfer from Core to Shell in CdSe/CdS/Au Heterojunction Quantum Dots.

The photophysics of charge-transfer and recombination mechanisms in a heterojunction structure of CdSe/CdS/Au quantum dots (QDs) are studied by temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL). We manipulate the charge transfer from core to shell surface by varying the tunneling barrier height resulting from temperature variation and the barrier width resulting from shell thickness variation. The charge-transfer process, which can be described by a tunneling transmission model, is manifested by two competitive recombination processes, an intrinsic exciton emission and a trap emission in the near-infrared (NIR) range. Our study establishes the photophysics foundation for the core/shell/metal application in photocatalyst and optoelectronics.

Multiple optical centers in Eu-implanted AlN nanowires for solid-state lighting applications

A detailed spectroscopic analysis of Eu3+ implanted and annealed AlN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy is presented by using micro-Raman, temperature-dependent steady-state photoluminescence, and time-resolved photoluminescence. Two different annealing temperatures (1000 °C and 1200 °C) were used. Such annealing conditions achieved a recovery of the original AlN crystalline structure as confirmed by Raman analysis. For both samples, the red Eu3+ intra-4f 6 luminescence was demonstrated, where the 5D0 → 7F2 transition at 624 nm is the most intense. Two well-resolved Eu optically active centers were observed in the present AlN NWs and designated as Eu1 and Eu2, due to their similar spectral shape when compared to those observed in GaN layers [Bodiou et al., Opt. Mater. 28, 780 (2006); Roqan et al., Phys. Rev. B 81, 085209 (2010)]. Their behavior was found to depend on the annealing temperature. Photoluminescence studies reveal that at 14 K, Eu2 is dominant for the lower annealing temperature, while Eu1 is dominant for the highest annealing temperature. Moreover, at room temperature, Eu1 center was found to be the dominant for both samples. Indeed, the luminescence intensity of the 5D0 → 7F2 transition exhibits a lower thermal quenching for the samples annealed at the highest temperature (∼80% for the sample annealed at 1200 °C and ∼50% for the sample annealed at 1000 °C) boosting their potential use as efficient red emitters.

Effect of strain on the Curie temperature and band structure of low-dimensional SbSI

Photoferroelectric materials show great promise for developing alternative photovoltaics and photovoltaic-type non-volatile memories. However, the localized nature of the d orbital and large bandgap of most natural photoferroelectric materials lead to low electron/hole mobility and limit the realization of technologically practical devices. Antimony sulpho-iodide (SbSI) is a photoferroelectric material which is expected to have high electron/hole mobility in the ferroelectric state due to its non-local band dispersion and narrow bandgap. However, SbSI exhibits the paraelectric state close to room temperature. In this report, as a proof of concept, we explore the possibility to stabilize the SbSI ferroelectric phase above room temperature via mechanical strain engineering. We synthesized thin low-dimensional crystals of SbSI by chemical vapor deposition, confirmed its crystal structure with electron diffraction, studied its optical properties via photoluminescence spectroscopy and time-resolved photoluminescence spectroscopy, and probed its phase transition using temperature-dependent steady-state photoluminescence spectroscopy. We found that introducing external mechanical strain to these low-dimensional crystals may lead to an increase in their Curie temperature (by ∼60 K), derived by the strain-modified optical phase transition in SbSI and quantified by Kern formulation and Landau theory. The study suggests that strain engineering could be an effective way to stabilize the ferroelectric phase of SbSI at above room temperature, providing a solution enabling its application for technologically useful photoferroelectric devices.

Spectroscopic analysis of the NIR emission in Tm implanted AlxGa1-xN layers

AlxGa1-xN samples, with different AlN molar fractions, x = 0, 0.15, 0.77, and 1, grown by halide vapor phase epitaxy were implanted with Tm ions. Photoluminescence (PL) measurements revealed that after thermal annealing all the samples exhibit intraionic Tm3+ luminescence. In samples with x > 0, the low temperature emission is dominated by the lines that appear in the near infrared (NIR) spectral region, corresponding to the overlapped 1G4 → 3H5 and 3H4 → 3H6 multiplet transitions. A detailed spectroscopic analysis of NIR emission of the thulium implanted and annealed AlxGa1-xN layers is presented by using temperature dependent steady-state PL, room temperature PL excitation, and time resolved PL. The results indicate that the excitonic features sensitive to the alloy disorder are involved in the excitation population processes of the Tm3+ luminescence and the highest thermal stability for the NIR emission occurs for the AlN:Tm sample.

Spectroscopic Analysis of Eu3+ Implanted and Annealed GaN Layers and Nanowires

A detailed spectroscopic analysis of Eu3+ implanted and annealed GaN nanowires (NWs) and layers is presented by using temperature-dependent steady-state photoluminescence, room temperature photoluminescence excitation, and time-resolved photoluminescence. Independently of the used implantation angle and ion fluence, all the studied postimplant annealed samples evidence red 5D0 → 7FJ luminescence transitions of the Eu3+ (4f6) ions. One dominant Eu3+ optical center was found for both GaN NWs and layers, together with the presence of different overlapped Eu3+ minority optical centers. The thermal stability of the intra-4f6 lines was found to be higher for the NWs where the red emission is observed with the naked eye even at room temperature. Besides the lanthanide emission, the photoluminescence spectra of the NWs and layers exhibit a broad yellow luminescence band (YL) differing slightly in the spectral shape and peak position in the different samples. While the YL in the layers is commonly ascribed to a fr...