Exciton Photoluminescence Research Articles

Broadening mechanisms of donor-bound exciton photoluminescence in Ga-doped ZnO nanowires

Donor spins in ZnO NWs have promise for quantum information (QI) applications due to high crystalline quality, narrow excitonic luminescence linewidths, and a direct bandgap of this material. It is important to understand the processes that can lead to inhomogeneous broadening of the excitonic transitions for realization of QI devices. We investigate the effect of Ga dopant concentration on the low temperature photoluminescence (PL) of Ga-doped ZnO nanowires. Spectrometer-resolution-limited donor-bound exciton (D0X) PL lines are observed at low concentrations with linewidths of around 0.1 meV. A clear increase in the Ga D0X line is observed as trace amounts of Ga are added. Above a certain concentration threshold, we observe a strong increase in the lateral growth coupled with a significant tail on the low energy side of the D0X emission, which scales linearly with dopant precursor concentration. We have analyzed this behavior using different models, including a model based on a bound exciton wavefunction overlap with neigbhouring donors and a Stark effect model due to random charged impurities. We rule out both of these models based on PL excitation spectroscopy measurements and show that a simple exponential model of the Urbach form gives the best fit and points to disorder in the more heavily doped shells.

Time-resolved exciton photoluminescence and its linear polarization of an ensemble of CdSe/CdS colloidal nanoplatelets

We present experimental and theoretical studies of the time-resolved photoluminescence and its linear polarization of an ensemble of CdSe/CdS colloidal nanoplatelets in the Faraday magnetic fields up to 6 T. The linearly polarized photoluminescence kinetics stems from the excitons resonantly excited with the linearly polarized light pulse and comprises both the structural contribution and the exciton optical alignment effect. The emission from the bright and dark exciton states is distinguished by their different intensity decay timescales with lifetime at cryogenic temperature of about 1–2ns and 100–200ns, respectively. The longitudinal spin relaxation time of the bright exciton was determined to be from 4 to 6ns depending on the magnetic field. The dark exciton spin relaxation time was estimated to be in the order of 20 to 100ns. The developed theory for the bright exciton polarization kinetics and the modeling of the data allowed us to conclude that spin dephasing times of the bright as well as of the dark exciton are shorter than 0.3ns. The origin of such a strong spin relaxation time anisotropy in the ensemble of nanoplatelets is discussed.

Photon Upconversion of Defect-Bound Excitons in hBN-Encapsulated MoS2 Monolayer.

Atomic defects associated with vacancies in two-dimensional transition metal dichalcogenide monolayers efficiently trap charged carriers and strongly localize excitons. Defects in semiconducting monolayers are seldomly utilized for enhancing optical phenomena, although they may provide resonant intermediate states within the energy band gap for applications with multiphoton excitations, like highly efficient and thermally robust photon upconversion. In an MoS2 monolayer encapsulated by hBN with high defect and resident electron densities, we observe an upconversion of localized exciton (XL) emission with a huge energy gain of up to 290 meV. The upconverted XL emission is robust up to temperatures of about 120 K and exhibits a sublinear or a nearly linear laser power dependence for the energy gain of about 100 meV and above 200 meV, respectively. The upconversion mechanism is explained by a cooperative energy transfer process between the photocreated and resident electrons, in which hybridized pairs of single sulfur vacancies likely act as real intermediate states. Additionally, we find a weak upconversion of the neutral exciton photoluminescence with an energy gain of about 350 meV for quasi-resonant excitation of the XL exciton. It is attributed to a two-step, two-photon absorption.

Electrical Control of Valley Polarized Charged Exciton Species in Monolayer WS2.

Excitons are key to the optoelectronic applications of van der Waals semiconductors, with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation remains challenging due to inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic electrical control of valley polarization in charged excitonic states of monolayer tungsten disulfide, achieving up to a 6-fold increase in the degree of circular polarization under off-resonant excitation. In contrast to the weak direct tuning of excitons typically observed using electrical gating, the charged exciton photoluminescence remains stable, even with increased scattering from electron doping. By exciting at the exciton resonances, we observed the reproducible nonmonotonic switching of the charged state population as the electron doping is varied under gate bias, indicating a resonant interplay between neutral and charged exciton states.

Selecting ns2 electron (Sb3+) and d3 electron (Cr3+) co-doped in lead-free halide double perovskite to achieve blue and near-infrared luminescence

The double-perovskite Cs2NaInCl6 has emerged as a highly favored candidate due to its remarkable stability and eco-friendly properties. However, it is noteworthy that this material does not demonstrate the desired optical performance in the crucial visible and near-infrared (NIR) spectrum, which may limit its potential applications in certain domains. We develop such characteristics in Cs2NaInCl6 by co-doping Sb3+ (s-electron) and Cr3+ (d-electron) ions. Doping with Sb3+ allows for 1S0→3P1 transition absorption, which then emits blue photoluminescence of self-trapped exciton (STE). By co-doping Sb3+ and Cr3+ ions, it causes the transfer of excitation energy from STE to Cr3+, yielding strong NIR emission at 950 nm. Temperature-dependent Photoluminescence (PL), PL excitation, PL decay and Tanabe-Sugano diagram provide insight into photophysical processes. Furthermore, a novel dual-modal optical information encryption based on the integration of blue and NIR luminescence printing patterns using Cs2NaInCl6: Sb3+ and Cs2NaInCl6: Sb3+/Cr3+, respectively, is successfully demonstrated.

Ultrafast photoinduced charge carrier dynamics of L-cysteine and oleylamine stabilized CsPbBr3 perovskite quantum dots coupled with gold nanoparticles.

We have synthesized L-cysteine and oleylamine stabilized CsPbBr3 perovskite quantum dots (PQDs) and coupled them with gold nanoparticles (AuNPs). The PQDs and AuNPs, as well as their hybrid nanostructures (HNS), were characterized using UV-visible (UV-vis) and photoluminescence (PL) spectroscopy. The UV-vis spectra show absorption bands of the HNS at 503 and 520nm, attributed mainly to PQDs and AuNPs, respectively. The PQDs show a strong excitonic PL band peaked at 513nm from PQDs. The HR-TEM results show the formation of hybrid structures between PQDs and AuNPs, which is also supported by the PL quenching of the PQDs by the coupled AuNPs. Ultrafast dynamics of the exciton and charge carriers in the HNS and pristine PQD were studied using femtosecond transient absorption. Multiexponential fitting of the dynamic data revealed the existence of shallow and deep trap states in pristine PQDs and ultrafast electron transfer from PQDs to AuNPs in the HNS. A kinetic model was proposed to account for the key dynamic processes involved and to extract the time for electron transfer from PQDs to AuNPs in the HNS, found to be ∼2ps. Dynamic processes in pristine PQDs are largely unchanged by HNS formation with AuNPs.

Spin chain orientation and magneto-optical coupling in twisted NiPS3 homostructures

Magnetic two-dimensional (2D) materials have garnered significant attention due to their unique electronic, magnetic, and optical properties and their potential applications in next-generation electronic and optoelectronic devices. However, the magneto-optical effects of oligolayer antiferromagnetic materials remain inadequately understood. Here, we investigate the magnetic properties of few-layer nickel phosphorus trisulfide (NiPS3) and its twisted heterostructures, emphasizing the observation of optical phenomena at low temperatures (1.65 K). By stacking few-layer NiPS3 to fabricate twisted homostructures, we probe their magnetic characteristics using photoluminescence (PL) spectroscopy. Our results reveal that sharp exciton peaks emerge at low temperatures and that the spin chain orientation in oligolayer NiPS3 can be discerned through the polarization dependence of exciton PL intensity. Notably, fewer-layered NiPS3 exhibits a significant magneto-optical effect under an applied magnetic field, allowing the modulation of the polarization angle of its exciton PL spectrum. Additionally, the polarization-dependent Raman spectrum of NiPS3 shows substantial changes under the influence of a magnetic field. These findings underscore the potential of few-layer NiPS3 for future magneto-optical device applications.

Chemically Tailored Growth of 2D Semiconductors via Hybrid Metal-Organic Chemical Vapor Deposition.

Two-dimensional (2D) semiconducting transition-metal dichalcogenides (TMDCs) are an exciting platform for excitonic physics and next-generation electronics, creating a strong demand to understand their growth, doping, and heterostructures. Despite significant progress in solid-source (SS-) and metal-organic chemical vapor deposition (MOCVD), further optimization is necessary to grow highly crystalline 2D TMDCs with controlled doping. Here, we report a hybrid MOCVD growth method that combines liquid-phase metal precursor deposition and vapor-phase organo-chalcogen delivery to leverage the advantages of both MOCVD and SS-CVD. Using our hybrid approach, we demonstrate WS2 growth with tunable morphologies─from separated single-crystal domains to continuous monolayer films─on a variety of substrates, including sapphire, SiO2, and Au. These WS2 films exhibit narrow neutral exciton photoluminescence line widths down to 27-28 meV and room-temperature mobility up to 34-36 cm2 V-1 s-1. Through simple modifications to the liquid precursor composition, we demonstrate the growth of V-doped WS2, MoxW1-xS2 alloys, and in-plane WS2-MoS2 heterostructures. This work presents an efficient approach for addressing a variety of TMDC synthesis needs on a laboratory scale.

Hybrid Photoexcitation in Undoped Diamond by Mid-Infrared Femtosecond Laser Pulses

Direct interband and intragap photoexcitation by intense mid-infrared (4.0 and 4.7 μm) femtosecond (fs) laser pulses was explored in ultrapure chemical-vapor deposited (CVD) diamond via acquisition of characteristic ultraviolet photoluminescence of free excitons and A-band photoluminescence of electrons anchored at deep donor–acceptor or dislocation-related traps, respectively. At lower laser intensities (<10 TW/cm2) the excitonic photoluminescence yields exhibit highly nonlinear dependences with Iλ2-scaling and power slopes N ≈ 17 (4.0 μm) and 14 (4.7 μm), still insufficient to cross over the direct bandgap (≥6.5 eV) by ≈1.2 and 2.8 eV, respectively. Similarly high slope of ≈9 (4.7 μm) for intragap (≥3.5 eV) photo-population of donor–acceptor traps is still insufficient for their direct excitation by ≈1 eV. At the intermediate Iλ2-dependent values of the Keldysh parameter γ ∼ 1 such incomplete multiphoton excitation anticipates the hybrid total “multiphoton + tunneling” photoexcitation generally predicted by the Keldysh theory, but never unambiguously experimentally demonstrated. At higher laser intensities (>10 TW/cm2) both the excitonic and A-band photoluminescence yields exhibit (sub)linear slopes, apparently, indicating formation of more strongly absorbing electron–hole plasma. These findings shed light on the hybrid multiphoton + tunneling character of Keldysh photoexcitation at intermediate values γ and pave the way to defect/impurity band engineering of intragap nonlinear optical properties in bulk dielectrics for their precise fs-laser nanomodification.

Superconductivity and Metallic Behavior in Heavily Doped Bulk Single Crystal Diamond and Novel Transportation Behavior at Graphene/Diamond Heterostructure

AbstractOwing to extremely large bandgap of 5.5 eV and high thermal conductivity, diamond is recognized as the most important semiconductor. The superconductivity of polycrystalline diamond has always been reported, but there are also many controversies over the existence of superconductivity in bulk single crystal diamond. Besides, it remains a question whether a metallic state exists for such a large bandgap semiconductor. Herein, a single crystal superconducting diamond with a Hall carrier concentration larger than 3 × 1020 cm−3 is realized by co‐doped of boron and nitrogen, with an extremely low resistivity of 2.07 × 10−3 Ω cm at room temperature and dominated acceptor–donor bounded exciton photoluminescence. Furthermore, it is shown that diamond can transform from superconducting to metallic state under similar carrier concentration with tuned carrier mobility degrading from 9.10 cm2 V−1 s−1 or 5.30 cm2 V−1 s−1 to 2.66 cm2 V−1 s−1 or 1.34 cm2 V−1 s−1. Through integrating graphene on a nitrogen and boron heavily co‐doped diamond, a novel transportation behavior in the monolayer graphene similar with superconducting behavior is realized through combining Andreev reflection and exciton‐mediated superconductivity, which may reveal more interesting superconducting behavior of diamond.

Achieving Inkjet‐Printed 2D Tin Iodide Perovskites: Excitonic and Electro‐Optical Properties

AbstractCurrently, there is a great demand for non‐toxic lead‐free halide perovskites as active materials for solar cells, light‐emitting diodes and other optoelectronic devices. Although an essential progress has been made using tin(II) halide perovskites, still greater efforts are needed to improve their stability and manufacture films and devices under a scalable technology. The first goal of the work is to achieve suitable physical properties of 2D Sn(II) polycrystalline perovskite films obtained by the industrially scalable inkjet printing deposition technique. In the present work, inks of 2D tin(II) halide perovskite 2‐thiopheneethylammonium tin(II) iodide, TEA2SnI4, have been successfully formulated in DMF (toxic) and DMSO (non‐toxic) solutions and using appropriate additives (SnF2 and reducing agents) for improving the stability of the inks and the resulting films. Room‐ and low‐temperature excitonic photoluminescence (PL), charge carrier recombination dynamics and µ‐PL is used to explain the observed two excitonic bands, which are associated to the bulk and edges of perovskite grains nanoplatelet‐like composing the polycrystalline films. Promising electro‐optical properties are also obtained in the TEA2SnI4 films inkjet‐printed from DMSO formulations onto ITO‐interdigitated electrodes, such as low dark currents, ≈10 – 20 nA at 10 V of bias voltage, and high responsivities ≈1–20 A/W.

Tuning Exciton Emission via Ferroelectric Polarization at a Heterogeneous Interface between a Monolayer Transition Metal Dichalcogenide and a Perovskite Oxide Membrane.

We demonstrate the integration of a thin BaTiO3 (BTO) membrane with monolayer MoSe2 in a dual-gate device that enables in situ manipulation of the BTO ferroelectric polarization with a voltage pulse. While two-dimensional (2D) transition metal dichalcogenides (TMDs) offer remarkable adaptability, their hybrid integration with other families of functional materials beyond the realm of 2D materials has been challenging. Released functional oxide membranes offer a solution for 2D/3D integration via stacking. 2D TMD excitons can serve as a local probe of the ferroelectric polarization in BTO at a heterogeneous interface. Using photoluminescence (PL) of MoSe2 excitons to optically read out the doping level, we find that the relative population of charge carriers in MoSe2 depends sensitively on the ferroelectric polarization. This finding points to a promising avenue for future-generation versatile sensing devices with high sensitivity, fast readout, and diverse applicability for advanced signal processing.

Competition between Phonon-Assisted and Exciton Photoluminescence Modulated by Temperature in WSe2/Graphene Nanosheet Heterostructures for Flexible Optoelectronic Sensor Devices

Competition between Phonon-Assisted and Exciton Photoluminescence Modulated by Temperature in WSe₂/Graphene Nanosheet Heterostructures for Flexible Optoelectronic Sensor Devices

Optical Probe of Magnetic Ordering Structure and Spin‐Entangled Excitons in Mn‐Substituted NiPS3

AbstractIn van der Waals magnets, the interplay between magneto‐excitonic coupling and optical phenomena opens avenues for directly probing magnetic ordering structures, manipulating excitonic properties through magnetic fields, and exploring quantum entanglement between electronic and magnetic states. Notably, NiPS3 stands out for its capacity to host spin‐entangled excitons, where the excitonic states are intricately tied with the material's spin configuration. Herein, it is experimentally showcased that the spin‐entangled excitons can be utilized for detecting the magnetic easy axis in Ni1‐xMnxPS3 (x = 0–0.1). The end members of this series exhibit distinct magnetic ordering patterns and easy axes: zigzag ordering with magnetic moments aligned along the a‐axis for NiPS3 versus Néel ordering with the out‐of‐plane easy axis for MnPS3. By combining angle‐resolved exciton photoluminescence with magnetic susceptibility measurements, it is observed that the magnetic easy axis rotates away from the local spin chain direction with increasing Mn content. Moreover, through a comprehensive thermal and substitution study, it is demonstrated that the energy and lifetime of spin‐entangled excitons are governed by two spin‐flip processes and are drastically influenced by disparate electronic states. These findings not only provide optical means to map out magnetic ordering structures but also offer insights into decoherence processes in spin‐exciton entangled states.

Identification and Dynamics of Microsecond Long-Lived Charge Carriers for CsPbBr3 Perovskite Quantum Dots, Featuring Ambient Long-Term Stability.

We analyze the stability and photophysical dynamics of CsPbBr3 perovskite quantum dots (PeQDs), fabricated under mild synthetic conditions and embedded in an amorphous silica (SiOx) matrix (CsPbBr3@SiOx), underscoring their sustained performance in ambient conditions for over 300 days with minimal optical degradation. However, this stability comes at the cost of a reduced photoluminescence efficiency. Time-resolved spectroscopic analyses, including flash-photolysis time-resolved microwave conductivity and time-resolved photoluminescence, show that excitons in CsPbBr3@SiOx films decay within 2.5 ns, while charge carriers recombine over approximately 230 ns. This longevity of the charge carriers is due to photoinduced electron transfer to the SiOx matrix, enabling hole retention. The measured hole mobility in these PeQDs is 0.880 cm2 V-1 s-1, underscoring their potential in optoelectronic applications. This study highlights the role of the silica matrix in enhancing the durability of PeQDs in humid environments and modifying exciton dynamics and photoluminescence, providing valuable insights for developing robust optoelectronic materials.

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.

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.

Uncovering the Effect of A-Site Cations on Localized Excitons Photoluminescence of Manganese-Doped Zinc Chloride Nanocrystals.

Elucidating the key factors that affect the localized excitons (LEs) photoluminescence (PL) in lead-free metal halide nanocrystals (NCs) is important for their optoelectronic applications. However, the effect of A-site cations on LEs based PL is not well understood. Herein, we varied the A-site cation ratio (Rb/Cs) to investigate the influence on LEs based PL in manganese-doped zinc chloride NCs. Through time-resolved photoluminescence (TR-PL) spectra and density functional theory (DFT) calculations, we discovered that Cl vacancy is energetically more favorable in Mn2+-doped Rb3ZnCl5 NCs compared to Mn2+-doped Cs3ZnCl5 NCs. The higher concentration of Cl vacancy increases the nonradiative recombination process in Rb3ZnCl5:Mn2+ NCs, ultimately determining the PL efficiency. This research enhances the understanding of the A-site cation effect on LEs-based PL in lead-free metal halide NCs.

Inverse exciton spin orientation due to trion formation in modulation doped quantum wells

Time-resolved and time-integrated circularly polarized photoluminescence of excitons and trions in external magnetic fields up to 10 T has been studied in undoped and n-type doped quantum well structures based on ZnSe. In an undoped structure, a circular polarization of photoluminescence induced by magnetic fields corresponded to the Boltzmann distribution of excitons on Zeeman sublevels. The inverse spin orientation of excitons is observed in doped samples with a carrier density of 3×1010 cm−2 and higher. Model calculations show that the reason for the inverse spin orientation is the effective depletion of the lowest-exciton Zeeman sublevel as a result of the spin-dependent formation of trions. The trion formation time as a result of exciton-electron binding was determined as 2 ps. This is noticeably shorter than the characteristic time of the exciton-photon interaction. The observed effect can be considered as a way of spin manipulation by electric fields.

Surface optical phonon replica in photoluminescence spectroscopy of nitride nanostructures: Crystal structure and surface effects

The surface optical (SO) phonon replica in photoluminescence (PL) spectroscopy of nitride nanowires (NWs) was theoretically investigated in this study. The dispersive relationships of SO phonon mode in anisotropic wurtzite (WZ) and isotropic zinc-blende (ZB) crystal structure NWs with circular and square cross sections (CSs) were derived within the framework of the dielectric continuum model. Based on the energy and momentum conservation laws, a constraint relationship between the frequency and wave-number was constructed for SO phonon-assisted excitonic PL spectra in the NW structure. By combining the dispersive and constraint relationships, the frequency and wave-number of the SO phonon replica in the PL spectra could be determined. The WZ and ZB crystal structures of nitride semiconductor were considered. The influences of surface factors including the CS shape, dielectric medium, and environment temperature on the frequency and photon wavelength of the band-edge emission of the SO phonon replica were studied in detail. Numerical results reveal that the crystal structure, surface factors, and environment temperature greatly affect the frequency and photon wavelength of the band-edge emission of the SO phonon replica. The calculated results for the photon wavelength agree well with the experimental values of the SO phonon replica in AlN NWs. The results of the dielectric effect obtained here are also supported by previous experimental and theoretical results for nitrides and other semiconductor NWs. The present theoretical scheme and numerical results can be used to analyze and design the SO phonon replica in PL spectra of nanostructures.