Exciton Formation Research Articles

Regulating Exciton De-Trapping of Te4+ -Doped Zero-Dimensional Scandium-Halide Perovskite for Fluorescence Thermometry with Record High Time-Resolved Thermal Sensitivity.

Fluorescence thermometry has been propelled to the forefront of scientific attention due to its high spatial resolution and remote non-invasive detection. However, recent generations of thermometers still suffer from limited thermal sensitivity (Sr ) below 10% change per Kelvin. Herein, this work presents an ideal temperature-responsive fluorescence material through Te4+ -doped 0D Cs2 ScCl5 ·H2 O, in which isolated polyhedrons endow highly localized electronic structures, and the strong electron-phonon coupling facilitates the formation of self-trapped excitons (STEs). With rising temperature, the dramatic asymmetric expansion of the soft lattice induces increased defects, strong exciton-phonon coupling, and low thermal activation energy, which evokes a rapid de-trapping process of STEs, enabling several orders of magnitude changes in the fluorescence lifetime over a narrow temperature range. After regulating the de-trapping process with different Te4+ doping, a record-high Sr (27.36% K-1 ) of fluorescence lifetime-based detection is achieved at 325 K. The robust stability against multiple heating/cooling cycles and long-term measurements enables a low temperature uncertainty of 0.067 K. Further, the developed thermometers are demonstrated for the remote local monitoring of operating temperature on internal electronic components. It is believed that this work constitutes a solid step towards building the next generation of ultrasensitive thermometers based on low-dimensional metal halides.

Chiral induction and Sb3+ doping in indium halides to trigger second harmonic generation and circularly polarized luminescence

Recently, organic-inorganic hybrid metal halides (HMHs) have attracted extensive attention as promising multifunctional materials by virtue of their structural diversity and tunable photophysical properties. However, it remains a challenge to design HMHs with specific functions on demand. Herein, by introducing R/S-methylbenzylamine (R/S-MBA) and doping Sb3+, we have achieved both second harmonic generation (SHG) and circularly polarized luminescence (CPL) properties in lead-free indium halides. The introduction of chiral organic cations can break the symmetry and induce the indium halides to crystallize in the chiral space group. The Sb3+ with ns2 electronic configuration can serve as the dopants to promote the formation of self-trapped excitons, so as to activate highly efficient luminescence. As a result, the as-prepared Sb3+ doped (R/S-MBA)3InCl6 show not only SHG responses but also CPL signals with luminescence dissymmetry factor of −5.3 × 10−3 and 4.7 × 10−3. This work provides a new inspiration for the exploitation of chiral multifunctional materials.

Origin of Broad Emission Induced by Rigid Aromatic Ditopic Cations in Low-Dimensional Metal Halide Perovskites.

The development of broadband emitters based on metal halide perovskites (MHPs) requires the elucidation of structure-emission property correlations. Herein, we report a combined experimental and theoretical study on a series of novel low-dimensional lead chloride perovskites, including ditopic aromatic cations. Synthesized lead chloride perovskites and their bromide analogues show both narrow and broad photoluminescence emission properties as a function of their cation and halide nature. Structural analysis shows a correlation between the rigidity of the ditopic cations and the lead halide octahedral distortions. Density functional theory calculations reveal, in turn, the pivotal role of octahedral distortions in the formation of self-trapped excitons, which are responsible for the insurgence of broad emission and large Stokes shifts together with a contribution of halide vacancies. For the considered MHP series, the use of conventional octahedral distortion parameters allows us to nicely describe the trend of emission properties, thus providing a solid guide for further materials design.

Orange-emitting zero-dimensional Te4+-Doped Cs2ScCl5·H2O perovskite microcrystals

Low-dimensional metal halide perovskites have gained the favor of many researchers for their potential applications in solid-state lighting due to their excellent optoelectronic properties. Herein, Te4+ doped Cs2ScCl5·H2O, a novel orange-emitting zero-dimensional scandium halide perovskite microcrystal was synthesized by hydrothermal method. Cs2ScCl5·H2O is isomorphic to Cs2InCl5·H2O with the space group Pnma. Under UV excitation, Cs2ScCl5·H2O:Te4+ exhibits a bright orange emission at 620 nm with a full width at half maximum (FWHM) of 170 nm, which is related to the formation of self-trapping excitons (STEs) in the zero-dimensional Te4+ doped Cs2ScCl5·H2O soft lattice. The fluorescence lifetime of Cs2ScCl5·H2O:Te4+ microcrystals are highly temperature-dependent with significant changes from microseconds (5.352 μs) at 110 K to nanoseconds (252 ns) at 298 K. The microcrystal-based white LED and thin film were initially explored for their potential application. These findings provide new insights into the excited state dynamics of Te4+ in Cs2ScCl5·H2O and demonstrate the potential application prospect for lead-free scandium halides doped by ns2 metal ions.

Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe_{2}.

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe_{2} with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe_{2} layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K^{'} valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.

Efficient Intramolecular Singlet Fission in Spiro-Linked Heterodimers.

We investigate intramolecular singlet fission (iSF) of spiro-linked azaacene heterodimers by time-resolved spectroscopy and quantum chemical calculations. Combining two different azaacenes through a nonconjugated linker using condensation chemistry furnishes azaacene heterodimers. Compared to their homodimers, iSF quantum yields are improved at an extended absorption range. The driving force of iSF, the energy difference ΔEiSF between the S1 state and the correlated triplet pair 1(TT), is tuned by the nature of the heterodimers. iSF is exothermic in all of the herein studied molecules. The overall quantum yield for triplet exciton formation reaches approximately 174%. This novel concept exploits large energy differences between singlet electronic states in combination with spatially fixed chromophores, which achieves efficient heterogeneous iSF, if the through-space interaction between the chromophores is minimal.

The Influence of Halide Ion Substitution on Energy Structure and Luminescence Efficiency in CeBr2I and CeBrI2 Crystals.

This study aims to determine the optimum composition of the CeBr1-xIx compound to achieve the maximum light output. It is based on calculations of the band energy structure of crystals, specifically taking into account the characteristics of the mutual location of local and band 5d states of the Ce3+ ions. The band energy structures for CeBr2I and CeBrI2 crystals were calculated using the projector augmented wave method. The valence band was found to be formed by the hybridized states of 4p Br and 5p I. The 4f states of Ce3+ are located in the energy forbidden band gap. The conduction band is formed by the localized 5d1 states, which are created by the interaction between the 5d states of Ce3+ and the 4f0 hole of the cerium ion. The higher-lying delocalized 5d2 states of Ce3+ correspond to the energy levels of the 5d states of Ce3+ in the field of the halide Cl0 (Br0) hole. The relative location of 5d1 and 5d2 bands determines the intensity of 5d-4f luminescence. The bottom of the conduction band is formed by localized 5d1 states in the CeBr2I crystal. The local character of the bottom of the conduction band in the CeBr2I crystal favors the formation of self-trapped Frenkel excitons. Transitions between the 5d1 and 4f states are responsible for 5d-4f exciton luminescence. In the CeBrI2 crystal, the conduction band is formed by mixing the localized 5d1 and delocalized 5d2 states, which leads to quenching the 5d-4f luminescence and a decrease in the light output despite the decrease in the forbidden band gap. CsBr2I is the optimum composition of the system to achieve the maximum light output.

The Mechanism of the Photodimerization of [60]Fullerene Derivatives: It is Still a Charge Separation Followed by a Back Transfer!

AbstractHerein, we present a systematic study of the photochemical behavior of a set of fullerene compounds bearing various organic addends attached to the fullerene cage. It has been shown that the yield of photodimerization products strongly depends on the structure and electronic properties of the material, primarily the electron affinity (acceptor strength) of the modified fullerene core. This finding being inconsistent with the conventional [2+2]cycloaddition mechanism and pointed to an alternative pathway. This proposed pathway involves photoinduced charge separation followed by back charge transfer events leading to the efficient generation of triplet excitons, which are responsible for the formation of dimerized species. The light‐induced charge separation pathway was confirmed by electron spin resonance spectroscopy and was supported by theoretical calculations. The revealed mechanism allows one to control the photochemical behavior of the fullerene derivatives via rational structural engineering: some of the compounds were shown to be fully resistant to photodimerization, which makes them promising candidates in the development of stable organic photovoltaics. Furthermore, this approach could be used to suppress back charge transfer in donor‐acceptor blends and hinder the formation of triplet excitons which represents one a major energy loss channel in the current generation of organic solar cells.

Charge Separation from Triplet Excited States in Nonfullerene Acceptor‐Based Organic Solar Cells

Suppressing charge recombination is pivotal for further improving the power conversion efficiency of organic solar cells (OSCs). The bimolecular recombination of free charge carriers leads to the formation of both singlet and triplet charge transfer (CT) states at a ratio of 1:3 when considering simple spin statistics, meaning that charge recombination via the triplet excited state is the main deactivation channel for OSCs. Although the formation of local triplet excitons through back charge transfer from triplet CT states is thought to be a terminal loss process, it is shown that charge separation from triplet excitons can occur in nonfullerene acceptor (NFA)‐based OSCs. To reveal the triplet exciton dynamics, a triplet sensitizer PtTPBP is doped into an OSC consisting of PM6 and Y6 as an electron donor and acceptor, respectively. Upon photoexcitation of PtTPBP, triplet excitons are formed, which subsequently transfer their energy to Y6, resulting in Y6 triplet excitons formation. Based on transient absorption and external quantum efficiency measurements, clear experimental evidence of charge photogeneration from Y6 triplet excitons at the PM6:Y6 interface is provided. This study highlights the importance of minimizing the energy difference between the singlet and triplet excited states of NFAs to suppress charge recombination.

The Impact of Spacer Size on Charge Transfer Excitons in Dion-Jacobson and Ruddlesden-Popper Layered Hybrid Perovskites.

Organic materials can tune the optical properties in layered (2D) hybrid perovskites, although their impact on photophysics is often overlooked. Here, we use transient absorption spectroscopy to probe the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) 2D perovskite phases. We show the formation of charge transfer excitons in DJ phases, resulting in a photoinduced Stark effect which is shown to be dependent on the spacer size. By using electroabsorption spectroscopy, we quantify the strength of the photoinduced electric field, while temperature-dependent measurements demonstrate new features in the transient spectra of RP phases at low temperatures resulting from the quantum-confined Stark effect. This study reveals the impact of spacer size and perovskite phase configuration on charge transfer excitons in 2D perovskites of interest to their advanced material design.

Modeling the impacts of energetic disorder and charge transport on the exciton formation zone in organic light-emitting diodes

The width of exciton formation zone plays a vital role in determining the long-term stability of organic light-emitting diodes (OLEDs). Here, based on the general model of carrier device lifetimes, the dependences of the width and location of exciton formation zone on energetic disorder and charge transport have been simulated in OLEDs with emissive layer featuring hole and electron transport on host and guest molecules, respectively. The width of exciton formation zone increases with emissive layer’s energetic disorder increasing. The emissive layer with properly imbalanced carrier mobilities enables larger width of exciton formation zone than that with balanced carrier mobilities, due to the different hole and electron trap densities. Moreover, increasing hole transport layer’s hole mobility or decreasing electron transport layer’s electron mobility helps increase the width of exciton formation zone. There is no width of exciton formation zone obtained, when hole (electron) transport layer’s hole (electron) mobility is smaller than a certain value. The location of exciton formation relies on not only emissive layer’s carrier mobilities but also hole (electron) transport layer’s hole (electron) mobility. The general model provides the comprehensive picture to elucidate the roles of energetic disorder and charge transport in enhancing the stability of OLEDs, beneficial to improve the OLEDs designs towards the high stability at high luminance.

Improving the efficiency of metal ions doped Fe2O3 nanoparticles: Photocatalyst for removal of organic dye from aqueous media

The metal ion-based nanocomposite photocatalysts were accepted to exhibit a wide range of photocatalytic and biological applications. In this paper, we synthesize bare Fe2O3, 1 wt% metal (Ag, Co, and Cu) doped Fe2O3 nanoparticles (NPs) using a simple hydrothermal process and wet impregnation method. The as-prepared nanomaterials crystalline structure, shape, optical characteristics, and elemental composition were determined by using X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscope (SEM), Energy-dispersive X-ray (EDS) and Transmission electron microscopy (TEM) techniques. Furthermore, the synthesized nanocomposites were utilized as a photosensitizer for the degradation of reactive red (RR120) and orange II (O-II) dyes under sunlight irradiation. The synthesized 1 wt% Ag–Fe2O3 (AgF) NPs samples exhibit a more exceptional catalytic performance of RR120 and O-II dyes (98.32%) within 120 min than the existing Fe2O3, 1 wt% Co–Fe2O3, and Cu–Fe2O3 NPs. The effect of parameters such as exciton formation under solar irradiation, charge recombination rate, and surface charge availability. The metal oxide-doped nanocomposite economic relevance is revealed by their long-term durability and recyclability in photodegradation reactions. The photocatalytic investigations show that the active species O2∙−, HO∙ and h+ play an important role in the dye degradation process. This research might pave the opportunity for the sustainable development of greater photocatalysts for photodegradation and a wide range of environmental applications.

Giant excitonic effects in vacancy-ordered double perovskites

Using first-principles GW plus Bethe-Salpeter equation calculations, we identify anomalously strong excitonic effects in several vacancy-ordered double perovskites Cs2MX6 (M = Ti, Zr; X = I, Br). Giant exciton binding energies about 1 eV are found in these moderate-gap, inorganic bulk semiconductors, pushing the limit of our understanding of electron-hole (e-h) interaction and exciton formation in solids. Not only are the exciton binding energies extremely large compared with any other moderate-gap bulk semiconductors, but they are also larger than typical 2D semiconductors with comparable quasiparticle gaps. Our calculated lowest bright exciton energy agrees well with the experimental optical band gap. The low-energy excitons closely resemble the Frenkel excitons in molecular crystals, as they are highly localized in a single [MX6]2- octahedron and extended in the reciprocal space. The weak dielectric screening effects and the nearly flat frontier electronic bands, which are derived from the weakly bonded [MX6]2- units, together explain the significant excitonic effects. Spin-orbit coupling effects play a crucial role in red-shifting the lowest bright exciton by mixing up spin-singlet and spin-triplet excitons, while exciton-phonon coupling effects have minor impacts on the strong exciton binding energies.

Simple synthesis of Bi3+ and Fe3+ co-doped Cs2Ag0.6Na0.4InCl6 double perovskite and its dual emission mechanism

The lead-free double perovskite with low toxicity and good performance stability has attracted much attention in optoelectronic device applications. But their photoluminescence quantum yields (PLQY) are low, which limits their applications. Co-doping and alloying are the most valuable methods to improve its PLQY and stability. In this study, we successfully synthesized Bi3+ and Fe3+ co-doped Cs2Ag0.6Na0.4InCl6 double perovskite using the hydrothermal method, and the emission mechanism was further investigated. Under UV-light excitation, the Cs2Ag0.6Na0.4InCl6: Bi3+/Fe3+ shows dual emission. This emission results from the modification effect of self-trapped exciton (STE) formation, and Fe3+ induces longer wavelength emission. By changing the doping contents of Bi3+ and Fe3+, the revolution of double perovskite color from yellow to orange-red can be observed, and its PLQY is up to 39.95%. The studies of the PL emission mechanism reveal that energy transfer occurs between the STE and Fe3+, and different doping contents can affect this transfer mechanism. This work has implications for producing double perovskite for optoelectronic semiconductors and provides direction for designing new lead-free double perovskites.

Peculiarities of exciton formation in 2D semiconductor nanostructures induced by quantum electromagnetic field under nonlinear self-phase modulation

Specific features of the excitation of a 2D semiconductor nanosystem by non-classical light in a solid state micro-resonator are found under the impact of the Kerr phase nonlinearity in the cavity. Different regimes of excitation are revealed in dependence on the efficiency of the nonlinearity. A negative impact of the nonlinear self-phase modulation on the excitation is found to be compensated by proper choice of the field frequency detuning. The possibility to controllably enhance a certain excitation channel by varying field frequency detuning is demonstrated and the value of the optimal detuning is found analytically. The effect of transfer of non-classical features from the field to the electronic subsystem is revealed. The formation of the photon-like non-classical excitonic states is demonstrated. The obtained results seem to be a basis for the creation of the light–matter interface and development of quantum information algorithms in solid state nanosystems.

Dual‐Phosphorescent Heteroleptic Silver(I) Complex in Long‐Lasting Red Light‐Emitting Electrochemical Cells

AbstractThe design of red‐emitting silver(I) complexes and their implementation in thin‐film lighting are still challenging as (i) their high ligand‐field splitting energy leads to high‐energy emissions with a controversial mechanism (thermally activated delayed fluorescence vs fluorescence/phosphorescence), and (ii) their low electrochemical stability leads to the formation of silver nanoclusters, limiting device stability to a few seconds. Herein, a thoughtful complex design [Ag(xantphos)(deebq)]PF6 combining a large‐bite angle diphosphine ligand (xantphos), a rigid, sterically hindered, π‐extended biquinolin (deebq) is reported. In contrast to prior‐art, this complex possesses (i) efficient red‐emission (λem = 660 nm; photoluminescence quantum yield of 42%) assigned to a thermally equilibrated dual‐phosphorescent emission based on spectroscopic/theoretical studies and (ii) stable reduction behavior without forming silver nanoclusters. This results in the first red light‐emitting electrochemical cells featuring (i) improved stability of two orders of magnitude compared to prior‐art (from seconds to hours) at irradiances of 20 µW cm−2, and (ii) a new degradation mechanism exclusively related to p‐doping as confirmed by electrochemical impedance spectroscopy analysis. Indeed, a multi‐layered architecture to decouple hole injection/transport and exciton formation enables a further 2‐fold enhanced irradiance/stability. Overall, this work illustrates that deciphering the rules for silver(I) complex design for lighting is tricky, but worthwhile.

Transition from Diffusive to Superdiffusive Transport in Carbon Nanotube Networks via Nematic Order Control.

The one-dimensional confinement of quasiparticles in individual carbon nanotubes (CNTs) leads to extremely anisotropic electronic and optical properties. In a macroscopic ensemble of randomly oriented CNTs, this anisotropy disappears together with other properties that make them attractive for certain device applications. The question however remains if not only anisotropy but also other types of behaviors are suppressed by disorder. Here, we compare the dynamics of quasiparticles under strong electric fields in aligned and random CNT networks using a combination of terahertz emission and photocurrent experiments and out-of-equilibrium numerical simulations. We find that the degree of alignment strongly influences the excited quasiparticles' dynamics, rerouting the thermalization pathways. This is, in particular, evidenced in the high-energy, high-momentum electronic population (probed through the formation of low energy excitons via exciton impact ionization) and the transport regime evolving from diffusive to superdiffusive.

Linker Independent Regioselective Protonation Triggered Detoxification of Sulfur Mustards with Smart Porous Organic Photopolymer.

The development of efficient metal-free photocatalysts for the generation of reactive oxygen species (ROS) for sulfur mustard (HD) decontamination can play a vital role against the stockpiling of chemical warfare agents (CWAs). Herein, one novel concept is conceived by smartly choosing a specific ionic monomer and a donor tritopic aldehyde, which can trigger linker-independent regioselective protonation/deprotonation in the polymeric backbone. In this context, the newly developed vinylene-linked ionic polymers (TPA/TPD-Ionic) are further explored for visible-light-assisted detoxification of HD simulants. Time-resolved-photoluminescence (TRPL) study reveals the protonation effect in the polymeric backbone by significantly enhancing the life span of photoexcited electrons. In terms of catalytic performance, TPA-Ionic outperformed TPD-Ionic because of its enhanced excitons formation and charge carrier abilities caused by the donor-acceptor (D-A) backbone and protonation effects. Moreover, the formation of singlet oxygen (1 O2 ) species is confirmed via in-situ Electron Spin Resonance (ESR) spectroscopy and density functional theory (DFT) analysis, which explained the crucial role of solvents in the reaction medium to regulate the (1 O2 ) formation. This study creates a new avenue for developing novel porous photocatalysts and highlights the crucial roles of sacrificial electron donors and solvents in the reaction medium to establish the structure-activity relationship.

Comparison of Frenkel and Excimer Exciton Diffusion in Perylene Bisimide Nanoparticles.

Exciton migration is an important process for light harvesting with organic systems and often the bottleneck. Especially the formation of trap states hinders the mobility considerably. Although excimer excitons are often referred to as traps, their mobility has been demonstrated while their nature is still unclear. Here, we compare the mobility of singlet and excimer excitons in nanoparticles consisting of the same type of perylene bisimide molecules. By changing the preparation conditions, nanoparticles with different intermolecular coupling strengths are prepared. Femtosecond transient absorption spectroscopy reveals the formation of excimer excitons from Frenkel excitons. The mobility of both exciton types is determined by evaluating exciton-exciton annihilation processes. In the lower coupling regime, singlet mobility is observed, whereas for stronger coupling the dynamics is dominated by a 10-fold increased excimer mobility. The excimer mobility can thus even be higher than the singlet mobility and is affected by the intermolecular electronic coupling.

Direct formation of interlayer excitons in MoSSe/WSSe van der Waals heterobilayer

In respect to the van der Waals structures composed of two-dimensional transition metal dichalcogenides, the interlayer excitonic physics plays a determinative role in the exciting new phenomena and applications in such as photonics, optoelectronics and valleytronics. In this work, beyond the well-accepted, conventional indirect two-step process, we proved that the large interlayer polarization can cause the direct formation of interlayer excitons in MoSSe/WSSe. In MoSSe/WSSe, the interlayer exciton with a sizable oscillator strength is located at 1.49 eV, well below the characteristic intralayer excitons, with a significantly reduced exciton binding energy of 0.28 eV and an improved exciton lifetime of 2.25 ns.