Time-resolved Photoluminescence Spectroscopy Research Articles

Controlling the Orientation-Dependent Second Harmonic Generation in Hybrid Germanium Perovskite Crystals.

Manipulating the crystal orientation plays a crucial role in the conversion efficiency during second harmonic generation (SHG). Here, we provide a new strategy in controlling the surface-dependent anisotropic SHG with the precise design of (101) and (20) MAGeI3 facets. Based on the SHG measurement, the (101) MAGeI3 single crystal exhibits larger SHG (1.3 × (20) MAGeI3). Kelvin probe force microscopy imaging shows a smaller work function for the (101) MAGeI3 compared with the (20), which indirectly demonstrates the stronger intrinsic polarization on the (101) surface. X-ray photoelectron spectroscopy confirms the band bending within the (101) facet. Temperature-dependent steady-state and time-resolved photoluminescence spectroscopy show shorter lifetime and wider emission band in the (101) MAGeI3 single crystal, revealing the higher defect states. Additionally, powder X-ray diffraction patterns show the (101) MAGeI3 possesses larger in-plane polar units [GeI3]- density, which could directly enhance the spontaneous polarization in the (101) facet. Density functional theory (DFT) calculation further demonstrates the higher intrinsic polarization in the (101) facet compared with the (20) facet, and the larger built-in electric field in the (101) facet facilitates surface vacancy defect accumulation. Our work provides a new angle in tuning and optimizing hybrid perovskite-based nonlinear optical materials.

Impacts of vacancy complexes on the room-temperature photoluminescence lifetimes of state-of-the-art GaN substrates, epitaxial layers, and Mg-implanted layers

For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τPLRT) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τPLRT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τPLRT in n-GaN samples are compared with those of p-GaN, in which τPLRT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively.

Ultrafast Exciton Dynamics of CH3NH3PbBr3 Perovskite Nanoclusters.

Exciton dynamics of perovskite nanoclusters has been investigated for the first time using femtosecond transient absorption (TA) and time-resolved photoluminescence (TRPL) spectroscopy. The TA results show two photoinduced absorption signals at 420 and 461 nm and a photoinduced bleach (PB) signal at 448 nm. The analysis of the PB recovery kinetic decay and kinetic model uncovered multiple processes contributing to electron-hole recombination. The fast component (∼8 ps) is attributed to vibrational relaxation within the initial excited state, and the medium component (∼60 ps) is attributed to shallow carrier trapping. The slow component is attributed to deep carrier trapping from the initial conduction band edge (∼666 ps) and the shallow trap state (∼40 ps). The TRPL reveals longer time dynamics, with modeled lifetimes of 6.6 and 93 ns attributed to recombination through the deep trap state and direct band edge recombination, respectively. The significant role of exciton trapping processes in the dynamics indicates that these highly confined nanoclusters have defect-rich surfaces.

Temperature-dependent excitonic emission characteristics of highly crystallized carbon nitride nanosheets

Highly-crystallized carbon nitride (HCCN) nanosheets exhibit significant potential for advancements in the field of photoelectric conversion. However, to fully exploit their potential, a thorough understanding of the fundamental excitonic photophysical processes is crucial. Here, the temperature-dependent excitonic photoluminescence (PL) of HCCN nanosheets and amorphous polymeric carbon nitride (PCN) is investigated using steady-state and time-resolved PL spectroscopy. The exciton binding energy of HCCN is determined to be 109.26 meV, lower than that of PCN (207.39 meV), which is attributed to the ordered stacking structure of HCCN with a weaker Coulomb interaction between electrons and holes. As the temperature increases, a noticeable reduction in PL lifetime is observed on both the HCCN and PCN, which is ascribed to the thermal activation of carrier trapping by the enhanced electron–phonon coupling effect. The thermal activation energy of HCCN is determined to be 102.9 meV, close to the value of PCN, due to their same band structures. Through wavelength-dependent PL dynamics analysis, we have identified the PL emission of HCCN as deriving from the transitions: σ*–LP, π*–π, and π*–LP, where the π*–LP transition dominants the emission because of the high excited state density of the LP state. These results demonstrate the impact of high-crystallinity on the excitonic emission of HCCN materials, thereby expanding their potential applications in the field of photoelectric conversion.

Cypate-sensitized upconversion nanoprobes for intracellular and in-vivo ATP ratiometric detection

As energy currency of life, adenosine triphosphate (ATP) is highly dynamic in living organisms. It is crucial yet challenging to probe intracellular or in-vivo ATP precisely without destruction of the living biological systems. Herein, we have designed novel ATP-responsive ratiometric nanoprobes by employing upconversion nanoparticles (UCNPs) with cypate as antennas. Nuclear magnetic resonance spectroscopy was used to analyze the interfacial interactions among lanthanide nanocrystal, cypate and adenosine phosphates, disclosing the origin of detection specificity. ATP induced interruption of energy transfer pathway in cypate-sensitized UCNPs was further investigated by femtosecond transient absorption and time-resolved photoluminescence spectroscopy. The nanoprobes with dual near-infrared (NIR)-excited ratiometric UCL were successfully applied for quantification of ATP level in living cells under vegetatively growing, starving, and glycolysis-inhibiting states, respectively. Furthermore, the increase of available energy by feeding for zebrafish larvae was confirmed through monitoring the in-vivo ATP variation with the nanoprobes. Such nanoprobes showed great potential in ATP-associated biological research and clinical applications. These findings may gain deep insights into the surface science and photophysics in dye-lanthanide nanocomposites, and open new avenues for the design and bioapplications of lanthanide nanoprobes.

Functional Pt(II) and Re(I) Complexes with CO- and β-Carboline-Based Coligands: From Time-Resolved Photoluminescence Spectroscopy and Evaluation of 1O2 Photosensitization Efficiency toward in vitro (Photo)cytotoxicity

Functional Pt(II) and Re(I) Complexes with CO- and β-Carboline-Based Coligands: From Time-Resolved Photoluminescence Spectroscopy and Evaluation of ¹O₂ Photosensitization Efficiency toward <i>in vitro</i> (Photo)cytotoxicity

Halide Engineering in Mixed Halide Perovskite-Inspired Cu2AgBiI6 for Solar Cells with Enhanced Performance.

Cu2AgBiI6 (CABI) is a promising perovskite-inspired absorber for solar cells due to its direct band gap and high absorption coefficient. However, the nonradiative recombination caused by the high extrinsic trap density limits the performance of CABI-based solar cells. In this work, we employ halide engineering by doping bromide anions (Br-) in CABI thin films, in turn significantly improving the power conversion efficiency (PCE). By introducing Br- in the synthetic route of CABI thin films, we identify the optimum composition as CABI-10Br (with 10% Br at the halide site). The tailored composition appears to reduce the deep trap density as shown by time-resolved photoluminescence and transient absorption spectroscopy characterizations. This leads to a dramatic increase in the lifetime of charge carriers, which therefore improves both the external quantum efficiency and the integrated short-circuit current. The photovoltaic performance shows a significant boost since the PCE under standard 1 sun illumination increases from 1.32 to 1.69% (∼30% relative enhancement). Systematic theoretical and experimental characterizations were employed to investigate the effect of Br- incorporation on the optoelectronic properties of CABI. Our results highlight the importance of mitigating trap states in lead-free perovskite-inspired materials and that Br- incorporation at the halide site is an effective strategy for improving the device performance.

Ultrahigh Photo-Responsivity and Detectivity in 2D Bismuth Sulfide Photodetector for Vis-NIR Radiation.

Bismuth sulfide (Bi2S3) exhibits a direct energy bandgap and an exceptional optical absorption capability over a broadband radiation, thus presents a novel class of 2D photodetector material. The field effect transistor (FET) photodetector device is fabricated from 2D Bi2S3. An anomalous variation in the transport characteristics of 2D Bi2S3 is observed with the variation in temperature. The electrical resistance reduces by 99.26% at 10 K compared to the response at 300 K. Defects due to the bismuth and sulfur vacancies play a critical role in the dramatic behavior, which is confirmed using photoluminescence, time-resolved photoluminescence, Hall measurements, and energy dispersive X-ray spectroscopy. The density functional theory calculations provide a significant insight into the thermodynamic properties of intrinsic defects in Bi2S3. Moreover, the effect of gate bias on responsivity additionally confirms its invariance at low temperature. The Bi2S3 based FET photodetector achieves ultrahigh responsivity in the order of ≈106 A W-1 and detectivity of ≈1014 Jones. Moreover, the external quantum efficiency of ≈107% is measured in a wide spectrum of optical illumination (532 to 1064nm) with a noise-equivalent power of 3.5 × 10-18W/√Hz at a bias of 0.2V. The extraordinary performance of Bi2S3 photodetector outstands 2D photodetectors.

Time-Resolved Spectroscopy for Dynamic Investigation of Photoresponsive Metal-Organic Frameworks.

Photoresponsive MOFs with precise and adjustable reticular structures are attractive for light conversion applications. Uncovering the photoinduced carrier dynamics lays the essential foundation for the further development and optimization of the MOF material. With the application of time-resolved spectroscopy, photophysical processes including excimer formation, energy transfer/migration, and charge transfer/separation have been widely investigated. However, the identification of distinct photophysical processes in real experimental MOF spectra still remains difficult due to the spectral and dynamic complexity of MOFs. In this Perspective, we summarize the typical spectral features of these photophysical processes and the related analysis methods for dynamic studies performed by time-resolved photoluminescence (TR-PL) and transient absorption (TA) spectroscopy. Based on the recent understanding of excited-state properties of photoresponsive MOFs and the discussion of challenges and future outlooks, this Perspective aims to provide convenience for MOF kinetic analysis and contribute to the further development of photoresponsive MOF material.

Effective redox reaction in a three-body smart photocatalyst through multi-interface modulation of organic semiconductor junctioned with metal and inorganic semiconductor

Forming metal/semiconductor heterojunctions is a well-known strategy to enhance photo-reaction through interface band bending. However, challenges remain in simultaneously minimizing fast charge carrier recombination and avoiding potential photo-corrosion in photocatalysts for high efficiency and long lifetime application in energy, environment, and green chemistry. We here design and propose a new strategy by which multi-interface band structure modulation occurs in a three-body photocatalyst, exemplified by Ag/PDA@Ag2S (PDA: polydopamine). Experimental analyses including in-situ x-ray photoelectron spectroscopy (XPS) and time-resolved photoluminescence (TRPL) spectroscopy together with projected density of states studies suggest an S-scheme heterojunction formation in the PDA@Ag2S, which greatly enhances the separation of photogenerated carriers. Consequently, we demonstrate that this multibody photocatalyst exhibits superior stability and selective oxidation of a series of ionic dyes under solar-simulated irradiation. The hydrogen generation efficiency (reduction) is also remarkably enhanced compared to virgin Ag/Ag2S photocatalysts. We believe this work brings a new insight into catalyst design.

The effect of solvent on the formation of low-dimensional metal halides and their self-trapped exciton emission

Low-dimensional organic–inorganic hybrid metal halides, with broadband luminescence, have attracted much attention for optoelectronic applications due to their rich in structural diversity and solution processibility. However, it is still unclear about how the solvent molecules influence the optoelectronic properties of the solution processed low-dimensional metal halides. Here, we prepared five different antimony-based crystal structures, [SbCl6]3- as the metal halide octahedron and 4, 4-difluoropiperidine (DFPD+) as organic cation, by using different solvents: hydrochloric acid (HCl) aqueous solution and four organic solvents (dimethylformamide (DMF), methanol (MeOH), acetonitrile (ACN) and dimethylacetamide (DMAC)). We revealed the relation between their crystal structures and optical properties, and we found the participation of organic molecules in the crystal structure causes significant lattice distortions, which is beneficial for achieving self-trapped exciton (STE) emission. Among them, (DFPD)6SbCl9·2DMAC exhibits a remarkable photoluminescence (PL) quantum yield of approximately 90 %. The STE dynamics in (DFPD)6SbCl9·2DMAC were characterized by femtosecond transient absorption and time-resolved PL spectroscopies. Simultaneously, this study also provides new directions for expanding the application of low-dimensional luminescent metal halides: in addition to UV-LEDs, efficient and rapid detection of methanol or acetonitrile can be achieved using the raw materials, while also realizing the potential application of multi-level optical anti-counterfeiting.

Electrochemical control of photoluminescence in pure wurtzite CdSe/CdS core/shell nanocrystals

Spectroelectrochemical methods are employed to systematically investigate the impacts of external charge filling in three pure wurtzite CdSe/CdS core/shell nanocrystals (NCs) with different shell thicknesses. We observed that electrons can penetrate the thick CdS shell and inject into the conduction band with an abnormal brightening, while holes are blocked. The formed negative trions result in a different spectral shift according to the size of the NCs, with a redshift of 3.7 nm for 12.5 nm NCs and almost no movement for 23.4 nm NCs. Meanwhile, due to the formation of biexcitons, a variational spectral blueshift with the NCs' size can be determined in time-resolved photoluminescence spectroscopy. This indicates that the binding energy of both biexcitons and negatively charged excitons can be regulated by controlling the shell thickness. Extra charges can also be used to activate/passivate trap states, thereby confirming that the 12.5 nm sample has better surface/interface quality.

Dual Emissive Zn(II) Naphthalocyanines: Synthesis, Structural and Photophysical Characterization with Theory-Supported Insights towards Soluble Coordination Compounds with Visible and Near-Infrared Emission.

Metal phthalocyaninates and their higher homologues are recognized as deep-red luminophores emitting from their lowest excited singlet state. Herein, we report on the design, synthesis, and in-depth characterization of a new class of dual-emissive (visible and NIR) metal naphthalocyaninates. A 4-N,N-dimethylaminophen-4-yl-substituted naphthalocyaninato zinc(II) complex (Zn-NMe2Nc) and the derived water-soluble coordination compound (Zn-NMe3Nc) exhibit a near-infrared fluorescence from the lowest ligand-centered state, along with a unique push-pull-supported luminescence in the visible region of the electromagnetic spectrum. An unprecedentedly broad structural (2D-NMR spectroscopy and mass spectrometry) as well as photophysical characterization (steady-state state and time-resolved photoluminescence spectroscopy) is presented. The unique dual emission was assigned to two independent sets of singlet states related to the intrinsic Q-band of the macrocycle and to the push-pull substituents in the molecular periphery, respectively, as predicted by TD-DFT calculations. In general, the elusive chemical aspects of these macrocyclic compounds are addressed, involving both reaction conditions, thorough purification, and in-depth characterization. Besides the fundamental aspects that are investigated herein, the photoacoustic properties were exemplarily examined using phantom gels to assess their tomographic imaging capabilities. Finally, the robust luminescence in the visible range arising from the push-pull character of the peripheral moieties demonstrated a notable independence from aggregation and was exemplarily implemented for optical imaging (FLIM) through time-resolved multiphoton micro(spectro)scopy.

Effect of Single-Crystal TiO2/Perovskite Band Alignment on the Kinetics of Electron Extraction.

The kinetics of electron extraction at the electron transfer layer/perovskite interface strongly affects the efficiency of a perovskite solar cell. By combining transient absorption and time-resolved photoluminescence spectroscopy, the electron extraction process between FA0.83Cs0.17Pb(I0.83Br0.17)3 and TiO2 single crystals with different orientations of (100), (110), and (111) were probed from subpicosecond to several hundred nanoseconds. It was revealed that the band alignment between the constituents influenced the relative electron extraction process. TiO2(100) showed the fastest overall and hot electron transfer, owing to the largest conduction band and Fermi level offset compared to FA0.83Cs0.17Pb(I0.83Br0.17)3. It was found that an early electron accumulation in these systems can have an influence on the following electron extraction on the several nanosecond time scale. Furthermore, the existence of a potential barrier at the TiO2/perovskite interface was also revealed by performing excitation fluence-dependent measurements.

Tunneling-Driven Marcus-Inverted Triplet Energy Transfer in a Two-Dimensional Perovskite.

Quantum tunneling, a phenomenon that allows particles to pass through potential barriers, can play a critical role in energy transfer processes. Here, we demonstrate that the proper design of organic-inorganic interfaces in two-dimensional (2D) hybrid perovskites allows for efficient triplet energy transfer (TET), where quantum tunneling of the excitons is the key driving force. By employing temperature-dependent and time-resolved photoluminescence and pump-probe spectroscopy techniques, we establish that triplet excitons can transfer from the inorganic lead-iodide sublattices to the pyrene ligands with rapid and weakly temperature-dependent characteristic times of approximately 50 ps. The energy transfer rates obtained based on the Marcus theory and first-principles calculations show good agreement with the experiments, indicating that the efficient tunneling of triplet excitons within the Marcus-inverted regime is facilitated by high-frequency molecular vibrations. These findings offer valuable insights into how one can effectively manipulate the energy landscape in 2D hybrid perovskites for energy transfer and the creation of diverse excitonic states.

Defects in epitaxial 4H-SiC revealed by exciton recombination

In this work, we conducted an analysis of 4H-SiC epitaxial layer grown on two distinct 4H-SiC substrates (both 6 inches in diameter) using non-invasive techniques such as micro-Raman spectroscopy, steady-state absorption spectroscopy and time-resolved photoluminescence spectroscopy. We have shown that despite the doping homogeneity, confirmed by micro-Raman and steady-state absorption spectroscopy, the carrier lifetime, assessed by monitoring the excitonic band at 3.2 eV by time-resolved photoluminescence spectroscopy, depends on the position on the wafer. This variability is attributed to the presence of defects, such as impurities or point defects, which are not uniformly distributed on the epitaxial layer and that, in addition to extended defects, affect the charge carrier recombination. Additionally, it is found that interactions with the underlying substrate could contribute to these effects as evidenced in regions of the substrate characterized by differences of doping.

Chiral Bay-Alkynylated Tetraazaperylenes: Photophysics and Chiroptical Properties.

Fully bay-alkynylated octaazaperopyrene dioxide (OAPPDO) derivatives were accessible through Stille cross coupling reaction of the corresponding bay-chlorinated derivatives. This steric congestion of the bay area led to helically chiral fluorophores, and chiral resolution of two derivatives allowed the investigation of their chiroptical properties as well as their kinetics of enantiomerization and the related thermodynamic parameters depending on the size of the terminal alkynyl substituent. An increase of the latter resulted in stable OAPPDO atropisomers at room temperature. The dynamics of the photoexcited states of two of the OAPPDO derivatives were investigated by transient absorption (TA) and time-resolved photoluminescence (tr-PL) spectroscopy.

Time-resolved mid-infrared photoluminescence spectroscopy of an undoped InAs substrate

Time-resolved mid-infrared photoluminescence (PL) spectroscopy of an undoped InAs substrate has been achieved with wavelength upconversion and time-correlated single photon counting methods. The substrate exhibits multiple PL peaks at photon energies of around 0.415 eV, and the peak positions and intensities change as the temperature is varied from 3.7 to 80 K. The dominant PL peaks are attributed to free and donor-bound excitons and radiative recombination between electrons at the Fermi edge in the conduction band and holes in the valence band edge. The PL lifetime of the excitons is 12 ns, which is four times longer than that of GaAs. The band edge electron–hole recombination has a longer PL lifetime of 60 ns at 20 K. The unveiling of luminescence dynamics in narrow bandgap semiconductors will contribute to the development of mid-infrared light-emitting devices.

β -rays induced displacement damage on epitaxial 4H-SiC revealed by exciton recombination

One of the most interesting wide-bandgap semiconductor is 4H-SiC that has an indirect wide-bandgap of 3.3 eV. This material holds great potential to develop power devices that find applications in the field of high-voltage and high-temperature electronics and harsh environments. In this study, we employed complementary noninvasive characterization techniques, including micro-Raman, optical absorption, steady-state, and time-resolved photoluminescence spectroscopy, to investigate the characteristics of a 12 μm thick epitaxial layer of 4H-SiC grown on 4H-SiC. Furthermore, we explored the impact of ionizing radiation on this material, utilizing β-rays and two x-ray sources. The doses are in the range of 1–100 kGy for electrons with energy of 2.5 MeV, 16 kGy for the first x-ray source (an x-ray tube with a W target operating at an anode bias voltage of 28 kV), and 100 kGy for the second x-ray source (an x-ray tube with a W target operating at an anode bias voltage of 100 kV). When exposed to the electron beam, the excitonic band at 3.2 eV exhibits a reduction in its lifetime as the deposited dose increases. In particular, in samples characterized by a greater amount of native defects, both extended and point defects, this effect becomes evident at lower deposited doses. Conversely, in the samples subjected to x-ray irradiation, these effects are not observed. These findings indicate that electron beam irradiation triggers the formation of defects associated with atomic displacement. Ultimately, we have examined the impact of thermal treatments in air, ranging from 100 to 900 °C, to investigate the recovery characteristics of 4H-SiC.

Synergistic Zinc(II) and Formate Doping of Perovskites: Thermal Phase Stabilization of α-FAPbI3 and Enhanced Photoluminescence Lifetime of FA0.8MA0.2PbI3 up to 3.7 µs.

Adding zinc (II) cations and formate anions improves the thermal phase stability of α-FAPbI3 materials, and the spin-coated thin films of such doped FAPbI3 (produced using MACl) show an increased emission lifetime of up to 3.7 μs on quartz (for FA0.8MA0.2PbI3). This work investigates the effects of zinc and formate on the phase stability and time-resolved photoluminescence of FAPbI3 perovskites for solar cell applications. Perovskite samples with varying concentrations of zinc and formate were made by incorporating different amounts of zinc formate and zinc iodide and were characterized with XRD. Doping levels of 1.7% Zn(II) and 1.0% formate (relative to Pb) seem optimal. The thermal phase stability of the doped perovskite powders (FAPbI3) and thin films (FA0.8MA0.2PbI3) was assessed. XRD of the thin films after 6 months shows only the alpha-phase. The time-resolved photoluminescence spectroscopy of the doped spin-coated perovskite samples (FA0.8MA0.2PbI3 produced using MACl) is reported. The results show that synergy between an anionic and a cationic dopant can take place, making the perovskite thermally more phase-stable (not converting to the yellow delta-phase) with a longer charge carrier lifetime. In order to produce good thin films by spin coating, the use of MACl was essential.