Exciton Binding Energy Research Articles

Tailoring the dielectric screening in WS2–graphene heterostructures

The environment contributes to the screening of Coulomb interactions in two-dimensional semiconductors. This can potentially be exploited to tailor material properties as well as for sensing applications. Here, we investigate the tuning of the band gap and the exciton binding energy in the two-dimensional semiconductor WS2 via the external dielectric screening. Embedding WS2 in van der Waals heterostructures with graphene and hBN spacers of thicknesses between one and 16 atomic layers, we experimentally determine both energies as a function of the WS2-to-graphene interlayer distance and the charge carrier density in graphene. We find that the modification to the band gap as well as the exciton binding energy are well described by a one-over-distance dependence, with a significant effect remaining at several nanometers distance, at which the two layers are electrically well isolated. This observation is explained by a screening arising from an image charge induced by the graphene layer. Furthermore, we find that the effectiveness of graphene in screening Coulomb interactions in nearby WS2 depends on its doping level and can therefore be controlled via the electric field effect. We determine that, at room temperature, it is modified by approximately 20% for charge carrier densities of 2 × 1012 cm−2.

A Flexible Sensitive Visible‐NIR Organic Photodetector with High Durability

AbstractLow‐cost, deformable visible‐near‐infrared (visible‐NIR) photodetectors can be configured into non‐planar shapes to meet emerging optical needs. However, it remains challenging to develop low‐cost flexible visible‐NIR photosensor with excellent mechanical deformability and high optical detectivity. Herein, we report a flexible visible‐NIR organic photodetector (OPD) based on a blend film of D18:BTP‐4F as the active layer on a polyethylene naphthalate substrate. Owing to the low exciton binding energy of BTP‐4F and the preferred orientation of the D18:BTP‐4F film, the as‐fabricated OPD exhibits excellent self‐powered optoelectronic performances with a high responsivity, high detectivity, and fast response time of 206 mA W−1, 6.45 × 1012 Jones (1 Jones = 1 cm Hz1/2 W−1), and 9 µs at the wavelength of 800 nm and bias voltage of 0 V, respectively. After 1000 bending/releasing cycles with an angle of 150° at a radius of curvature of 3 mm, the current‐voltage curves remain stable both measured under dark and light illumination, indicating obvious device operation robustness. The reliable linear photosensitivity of the as‐obtained OPD is demonstrated by the photoresponse under a range of different incident light intensities and its practical application for wearable health monitoring such as heart rate measurement is well demonstrated. Furthermore, a flexible visible‐NIR image sensor for imaging acquisition is shown by demonstrating 5 × 5 OPD array under 850 nm light irradiation. The collective results on blend film design, flexible device fabrication and characterization, and imager development shed light on sought‐after visible‐NIR photosensing and imaging systems.

The optical properties of strontium manganite thin films prepared by novel phototreatment technique

SrMnO3 thin films were successfully prepared by spin coating the viscous solution obtained from the autocombustion synthesis. To ensure the transformation of the precursor solution into the manganite phase, conventional thermal treatment and novel phototreatment were applied. The thermal treatment resulted in stable hexagonal SrMnO3, while phototreatment yielded cubic SrMnO3 phase which is usually not stable at ambient conditions. The optical properties of the prepared materials were measured revealing the optical gaps of 1.5 eV for thermally treated material and about 1.3 – 1.4 eV for the phototreated samples. Comparison with ab initio result shows a higher theoretical value of 1.9 eV, ascribed to high exciton binding energy. Since these values are similar to the optical gap (1.48 eV) of widely studied formamidium lead iodide thin films, SrMnO3 thin films are promising candidates for active layers in inorganic perovskite solar cells.

Localized Bound Multiexcitons in Engineered Quasi-2D Perovskites Grains at Room Temperature for Efficient Lasers.

Reducing the excitation threshold to minimize the Joule heating is critical for the realization of perovskite laser diodes. Although bound excitons are promising for low threshold laser, how to generate them at room temperature for laser applications is still unclear in quasi-2D perovskite-based devices. In this work, via engineering quasi-2D perovskite PEA2 (CH3 NH3 )n -1 Pbn Br3 n +1 microscopic grains by the anti-solvent method, room-temperature multiexciton radiative recombination is successfully demonstrated at a remarkably low pump density of 0.97 µJ cm-2 , which is only one-fourth of that required in 2D CdSe nanosheets. In addition, the well-defined translational momentum in quasi-2D perovskite grains can restrict the Auger recombination which is detrimental to radiative emission. Furthermore, the quasi-2D perovskite grains are favorable for increasing binding energies of excitons and biexcitons and so as the related radiative recombination. Consequently, the prepared <n= 8> phase quasi-2D perovskite film renders a threshold of room-temperature stimulated emission as low as 13.7 µJ cm-2 , reduced by 58.6% relative to the amorphous counterpart with larger grains. The findings in this work are expected to facilitate the development of solution-processable perovskite multiexcitonic laser diodes.

Low Temperature Optical Properties of Novel Lead‐Free Cs2NaFeCl6 Perovskite Single Crystals

Lead‐free double perovskites have attracted much attention as possible alternatives to lead halide based perovskites in photovoltaic applications. However, to date only few double perovskites have been successfully employed in optoelectronic device prototypes. Therefore, the search for stable and lead‐free materials is ongoing. Here, we present the successful growth of high‐quality Cs2NaFeCl6 single crystals and their temperature‐dependent structural and optical properties. By combining electron paramagnetic resonance (EPR), crystal structure analysis and density functional theory (DFT) we could determine a cubic crystal structure with a spin of 5/2 for this material, showing strongly spin polarized character. Furthermore, combining photoluminescence (PL) and optical absorption measurements we find a bandgap of approximately 2.1 eV at room temperature as well as the presence of excitonic states. Using Elliot's formula, we are able to extract the temperature‐dependent behavior of the bandgap as well as an estimated exciton binding energy of only 20 meV at 80 K.

Structure, functional groups, and optical properties of Al doped ZnO nanorods thin films

Zinc oxide (ZnO) in the form of thin films is a semiconductor that has a direct energy gap of 3.37 eV and an exciton binding energy of 60 meV. These attractive properties make ZnO widely used as a transparent conducting material application in liquid crystal displays, solar cells, etc. Undoped and doped ZnO thin films are widely used in transparent conductor layers due to their better thermal resistance and resistance. In this study, thin films of Al doped ZnO (Zn1-xAlxO) nanorods (x = 1 wt% and 3 wt%) were synthesized by hydrothermal method. The effect of Al doping on microstructure, binding vibration, and optical properties was characterized by XRD (X-Ray Diffraction), SEM (Scanning Electron Microscopy), FTIR (Fourier Transform Infra Red) and UV–Vis (Ultraviolet–visible light) Spectroscopy. Furthermore, the (002) peak of ZnO was shifted because of Al doped ZnO. The diameter of hexagonal is around 78 nm. The binding vibrations found are Zn-O (531, 512, 985, 911 cm−1), Al-O (812 cm−1), Zn-hydroxyl group (1132, 1107 cm−1), CH3 group (2167, 2837, 2861 cm−1), and the hydroxyl group (3560, 3547 cm−1). The energy gap had a moderate drop from 2.97 eV to 2.95 eV.

Decyl disulfide surface treatment improved photoluminescence quantum yield and stability of blue-emitting CsPbBr3 nanoplatelets

Improving the photoluminescence quantum yields (PL QYs) and stability of blue-emitting CsPbBr3 nanoplatelets (NPLs) is essential for the application in high-quality displays. Herein, highly luminescent CsPbBr3 NPLs with PL peak at 462 nm were prepared by a modified supersaturated recrystallization synthesis strategy at room temperature. The resulting blue CsPbBr3 NPLs show high PL QY of 98% and a full width at half maximum of 13 nm, benefiting from the decyl disulfide (DF) surface treatment. Besides, the DF-treated CsPbBr3 NPLs show lower Urbach energy (15 meV) and more outstanding water, acetone, and UV-light resistance stabilities than the pristine ones, suggesting that the surface ligand treatment can reduce Br− vacancy defects and inhibit the non-radiative recombination process. Based on the temperature-dependent PL, the exciton binding energy of CsPbBr3 NPLs is found to increase from 109 to 201 meV after DF treatment. Finally, light-emitting diodes based on the treated CsPbBr3 NPLs were fabricated.

Fluence dependent dynamics of excitons in monolayer MoSi2Z4 (Z = pnictogen)

Reduced dielectric screening in two-dimensional materials enables bound excitons, which modifies their optical absorption and optoelectronic response. Here, we demonstrate the existence of excitons in the bandgap of the monolayer family of the newly discovered synthetic (Z=N, P, and As) series of materials. All three monolayers support several bright and strongly bound excitons with binding energies varying from 1 eV to 1.35 eV for the lowest energy exciton resonances. We show that on increasing the pump fluence or photo-excited carrier density, the lowest energy exciton first undergoes a redshift followed by a blueshift, due to the renormalized exciton binding energies. The exciton binding energy varies as a Lennard-Jones-like potential as a function of the inter-exciton spacing. This establishes an atom-like attractive and repulsive interaction between excitons depending on the inter-exciton separation. Our study shows that the series of monolayers offer an exciting test-bed for exploring the physics of strongly bound excitons and their non-equilibrium dynamics.

Strong Electron–Phonon Coupling Induced Self‐Trapped Excitons in Double Halide Perovskites

Double halide perovskites exhibit impressive potential for the self‐trapped exciton (STEs) luminescence. However, the detailed mechanism of the physical nature during the formation process of STEs in double perovskites is still ambiguous. Herein, theoretical research on a series of double halide perovskites Cs2B1B2Cl6 (B1 = Na+, K+; B2 = Al3+, Ga3+, In3+) regarding their electronic structures, exciton characteristics, electron–phonon coupling performances, and geometrical configuration is conducted. These materials have flat valence band edges and thus possess localized heavy holes. They also show high exciton binding energies, and their short exciton Bohr radius indicates that the spatial size of their excitons is comparable to the dimension of their single lattice. Based on the Fröhlich coupling constant and Feynman polaron radius, the stronger electron–phonon coupling strength in Ga‐series double halide perovskites is revealed. In particular, Cs2NaGaCl6 shows a high and effective Huang–Rhys factor of 36.21. The phonon characteristics and vibration modes of Cs2NaGaCl6 are further analyzed, and the Jahn–Teller distortion of the metal–halogen octahedron induced by hole‐trapping after excitation is responsible for the existence of STEs. This study strengthens the physical understanding of STEs and provides effective guidance for the design of advanced solid‐state phosphors.

Intrinsic Dipole Arrangement to Coordinate Energy Levels for Efficient and Stable Perovskite Solar Cells.

Despite great progress in perovskite photovoltaics, it should be noted that the intrinsic disorder dipolar cations in organic-inorganic hybrid perovskites exert negative effects on the energy band structure as well as the carrier separation and transfer dynamics. However, oriented polarization achieved by applying an external electric field may cause irreversible damage to perovskites. Herein, a unique and efficient strategy is developed to modulate the intrinsic dipole arrangement in perovskite films for high-performance and stable perovskite solar cells (PSCs). The spontaneous reorientation of dipolar cation methylamine is triggered by a polar molecule, constructing a vertical polarization during crystallization regulation. The oriented dipole determines a gradient energy-level arrangement in PSCs and more favorable energetics at interfaces, effectively enhancing the built-in electric field and suppressing the nonradiative recombination. Besides, the dipole reorientation induces a local dielectric environment to remarkably reduce exciton binding energy, leading to an ultralong carrier diffusion length of up to 1708nm. Accordingly, the n-i-p PSCs achieve a significant increase in power conversion efficiency, reaching 24.63% with negligible hysteresis and exhibiting outstanding stabilities. This strategy also provides a facile route to eliminate the mismatched energetics and enhance carrier dynamics for other novel photovoltaic devices.

Tuning the exciton binding energy of covalent organic frameworks for efficient photocatalysis

Owing to the large exciton binding energy (>100 meV) of most organic materials, the process of exciton dissociation into free electrons and holes is seriously hindered, which plays a key role in the photocatalytic system. In this study, a series of chalcogen (S, Se)-substituted mesoporous covalent organic frameworks (COFs) have been synthesized for enhanced photocatalytic organic transformations. Photoelectrochemical measurements indicate that the introduction of semi-metallic Se atom and the enlargement of conjugation degree can not only reduce the exciton binding energy accelerating the charge separation, but also reduce the band gap of COFs. As a result, the COF-NUST-36 with the lowest exciton binding energy (39.5 meV) shows the highest photocatalytic performance for selective oxidation of amines (up to 98% Conv. and 97.5% Sel.). This work provides a feasible method for designing COFs with high photocatalytic activity by adjusting exciton binding energy.

Influence of π-linker on pyrone-based hole transporting materials in perovskite solar cells

ABSTRACT In this study, we designed four pyrone-based hole transport materials (HTMs) P1-P4 in perovskite solar cells (PSCs) and studied the effects of the benzene and thiophene groups on their performance. Based on density functional theory (DFT), we investigated the geometry, frontier molecular orbitals (FMOs), density of states (DOS), solvation free energy (ΔG sol), absolute hardness, electrostatic potential (ESP), and hole transport rates of all designed molecules. Time-dependent density functional theory (TD-DFT) was used to analyse the absorption spectra, charge density difference diagrams (CDD), heat map, D index, H index, S r index, and exciton binding energy (E coul) of HTMs P1-P4 to examine their optical and electronic excitation features. The simulation findings demonstrate that the HTMs P1-P4 molecular energy levels match with the energy level of perovskite (MAPbI3). Additionally, all designed molecules have good stability and high hole transport rates. The UV-visible absorption spectra show that the designed HTMs can broaden the optical absorption range of PSCs in the visible light region. In addition, by increasing the length of π-linker can significantly improve the photoelectric properties of the HTMs. The designed molecules exhibit great electronic character, optical character, hole transport rates, and stability, which provide ideas for the future design of high-efficiency HTMs.

Creating a Nanoscale Lateral Junction in a Semiconductor Monolayer with a Large Built-in Potential

The ability to engineer atomically thin nanoscale lateral junctions is critical to lay the foundation for future two-dimensional (2D) device technology. However, the traditional approach to creating a heterojunction by direct growth of a heterostructure of two different materials constrains the available band offsets, and it is still unclear if large built-in potentials are attainable for 2D materials. The electronic properties of atomically thin semiconducting transition metal dichalcogenides (TMDs) are not static, and their exciton binding energy and quasiparticle band gap depend strongly on the proximal environment. Recent studies have shown that this effect can be harnessed to engineer the lateral band profile of a monolayer TMD to create a lateral electronic junction. Here we demonstrate the synthesis of a nanoscale lateral junction in monolayer MoSe2 by intercalating Se at the interface of an hBN/Ru(0001) substrate. The Se intercalation creates a spatially abrupt modulation of the local hBN/Ru work function, which is imprinted directly onto an overlying MoSe2 monolayer to create a lateral junction with a large built-in potential of 0.83 ± 0.06 eV. We spatially resolve the MoSe2 band profile and work function using scanning tunneling spectroscopy to map out the nanoscale depletion region. The Se intercalation also modifies the dielectric environment, influencing the local band gap renormalization and increasing the MoSe2 band gap by ∼0.26 ± 0.1 eV. This work illustrates that environmental proximity engineering provides a robust method to indirectly manipulate the band profile of 2D materials outside the limits of their intrinsic properties.

Modulation of Excitonic Confinement of TiO2 in Molten Salts for Overall CO2 Photoreduction†

Comprehensive SummaryExcitonic confinement greatly determines the charge carrier transport of photocatalysts. A molten salt modulation of excitonic confinement is herein demonstrated as formation of ultrafine carbon‐doped anatase TiO2 with grafted graphitic carbon nitride, which is rationalized as an excellent catalyst for overall CO2 photoreduction. Compared with bulk TiO2, the carbon‐doped TiO2 (M‐TiO2) possesses a weaker excitonic confinement to decrease exciton binding energy from 99 to 58 meV, consequently enhancing free‐charge‐carrier generation and transportation. Effective Z‐scheme electron transfer from M‐TiO2 to C3N4 is built, enhancing the CO2 conversion via the synchronous optimization of redox ability, CO2 activation, and *COOH generation. This work highlights the unique chemistry of excitonic dissociation on facilitating separation of electron and hole, and also extends the scope of molten salt‐mediated modulation of photocatalysis materials.

Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials.

The relaxation of the above-gap ("hot") carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier-carrier and carrier-phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump-push-probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak size effect on the cooling dynamics. In contrast, two-dimensional systems show suppression of the hot phonon bottleneck effect common in bulk perovskites. The proposed kinetic model describes the intrinsic and density-dependent cooling times accurately in all studied perovskite systems using only carrier-carrier, carrier-phonon, and excitonic coupling constants. This highlights the impact of exciton formation on carrier cooling and promotes dimensional confinement as a tool for engineering carrier-phonon and carrier-carrier interactions in LHP optoelectronic materials.

Reducing dielectric confinement effect in ionic covalent organic nanosheets to promote the visible-light-driven hydrogen evolution

Ultra-thin two-dimensional (2D) organic semiconductors are promising candidates for photocatalysts because of the short charge diffusion pathway and favorable exposure of active sites plus the versatile architecture. Nonetheless, the inherent dielectric confinement of 2D materials will induce a strong exciton effect hampering the charge separation. Herein, we demonstrated an effective way to reduce the dielectric confinement effect of 2D ionic covalent organic nanosheets (iCONs) by tailoring the functional group via molecular engineering. Three ultra-thin CONs with different functional groups and the same ionic moieties were synthesized through Schiff base condensation between ionic amino monomer triaminoguanidinium chloride (TG) and aldehyde linkers. The integration of the hydroxyl group was found to significantly increase the dielectric constant by enhancing the polarizability of ionic moieties, and thus reduced the dielectric confinement and the corresponding exciton binding energy (Eb). The champion hydroxyl-functional iCON exhibited promoted exciton dissociation and in turn a high photocatalytic hydrogen production rate under visible-light irradiation. This work provided insights into the rationalization of the dielectric confinement effect of low-dimensional photocatalysts.

Improved Optical Properties of Lead‐Free Double Perovskite Cs2AgBiBr6 Nanocrystals via Na Ions Doping

AbstractLead‐free double perovskite nanocrystals (NCs) are a promising alternative to lead‐based perovskite NCs for optoelectronic materials due to their nontoxic and good moist‐thermal stability. However, most of the lead‐free double perovskite NCs exhibit unsatisfying photoluminescence quantum yield (PLQY); the metal ions doping is an effective method to improve the optical properties. Here, monodisperse, homogeneous cubic morphology of Na+‐doped Cs2AgBiBr6 NCs is prepared via a colloidal synthesis strategy. When Na+ is doped into Cs2AgBiBr6 NCs, the inversion‐symmetry‐induced parity‐forbidden transition is broken. So, the wave function parity of the self‐trapped excitons at Ag atoms is changed, which allows the radiative transition to occur. This leads to a higher PLQY for Na+‐doped Cs2AgBiBr6 NCs (9.08%) than that for Cs2AgBiBr6 NCs (0.40%), and the exciton lifetime is extended from 1.87 ns for Cs2AgBiBr6 NCs to 17.15 ns for Na+‐doped Cs2AgBiBr6 NCs. The Na+ doping elevates the longitudinal optical phonon energy and exciton binding energy, which results in weaker nonradiative transitions during exciton recombination and improved NC thermal stability. So, the Na+‐doped Cs2AgBiBr6 NCs maybe possess potentially valuable applications in optoelectronic devices.

Prominent Free Charges Tunneling Through Organic Interlayer of 2D Perovskites.

The diversity of organic cations greatly enriches the species of 2D perovskites; traditional 2D Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) perovskites are synthesized by two different organic amines. Here, according to the difference in pKa values between conjugated acids of monoprotonated and biprotonated 4-(2-Aminoethyl)pyridine (4AEPy) ions, the 2D perovskites of RP (4AEPy)2 PbI4 and DJ (4AEPy)PbI4 from same organic amine is reported, which can realize reversible transformation under the treatment of HI and NH3 , respectively. The interaction of N-H···N hydrogen bond between adjacent organic molecules in (4AEPy)2 PbI4 leads to the bending conformation of ethylamine groups, which results in a 2.4 Å reduction in layer spacing compared to typical phenylethylamine lead iodine ((PEA)2 PbI4 ) 2D perovskite. Besides, the ethylamine groups of organic layers in (4AEPy)PbI4 are deeply inserted into octahedral cavities and directly participate in the construction of the conduction band minimum, which leads to a small exciton binding energy of 27.3 meVtogeneratefreecharges. The stronger coupling between the organic and inorganic layers and the minor exciton binding energy can promote the DJ phase to possess a more stable structure and better optoelectronic properties. Thusthe (4AEPy)PbI4 device displays better light response and X-ray detection capability with a high sensitivity of 5627 µC Gyair -1 cm-2 and the lowest detectable dose rate of 20 nGyair s-1 .

Sub-Nanosecond 2D Perovskite Scintillators by Dielectric Engineering.

Perovskite materials have demonstrated great potential for ultrafast scintillators with high light yield. However, the decay time of perovskite still cannot be further minimized into sub-nanosecond region, while sub-nanosecond scintillators are highly demanded in various radiation detection, including high speed X-ray imaging, time-of-flight based tomography or particle discrimination, and timing resolution measurement in synchrotron radiation facilities, etc. Here, a rational design strategy is showed to shorten the scintillation decay time, by maximizing the dielectric difference between organic amines and Pb-Br octahedral emitters in 2D organic-inorganic hybrid perovskites (OIHP). Benzimidazole (BM) with low dielectric constant inserted between [PbBr6 ]2- layers, resulting in a surprisingly large exciton binding energy (360.3 ± 4.8 meV) of 2D OIHP BM2 PbBr4 . The emitting decay time is shortened as 0.97ns, which is smallest among all the perovskite materials. Moreover, the light yield is 3190photonsMeV-1 , which is greatly higher than conventional ultrafast scintillator BaF2 (1500photonsMeV-1 ). The rare combination of ultrafast decay time and considerable light yield renders BM2 PbBr4 excellent performance in γ-ray, neutron, α-particle detection, and the best theoretical coincidence time resolution of 65.1ps, which is only half of the reference sample LYSO (141.3ps).

Structural Reconstruction Modulated Physical Properties of Titanium Oxide at the Monolayer Limit

To suppress surface dangling bonds, monolayer oxides derived from non-layered bulks usually undergo a pronounced structural reconstruction. It remains challenging to resolve these structural reconstructions and the induced distinct modulation of intrinsic properties. In this study, the structural reconstruction-modulated electronic, polaronic, and exitonic properties of a non-layered oxide at the monolayer limit are unraveled. Based on first-principles calculations and tight-binding simulations for a stable titanium dioxide (TiO2) monolayer, we show that its distinct surface Kagome sublattices host a topologically nontrivial flat band at the valence band edge. The strong electron–hole interaction in this monolayer oxide gives rise to a large exciton binding energy of around 2.49 eV. Interestingly, the monolayer TiO2 also exhibits strong electron–lattice coupling, which favors the formation of small electron polarons and thus greatly reduces its band gap energy into the visible light range. This work could be useful to understand the structural reconstruction-induced modulation of exotic physical and chemical properties for a broad range of non-layered oxides at the monolayer limit.