Electron-hole Binding Research Articles

Nonbosonic Moiré Excitons.

Optical excitations in moiré transition metal dichalcogenide bilayers lead to the creation of excitons, as electron-hole bound states, that are generically considered within a Bose-Hubbard framework. Here, we demonstrate that these composite particles obey an angular momentum commutation relation that is generally nonbosonic. This emergent spin description of excitons indicates a limitation to their occupancy on each site, which is substantial in the weak electron-hole binding regime. The effective exciton theory is accordingly a spin Hamiltonian, which further becomes a Hubbard model of emergent bosons subject to an occupancy constraint after a Holstein-Primakoff transformation. We apply our theory to three commonly studied bilayers (MoSe_{2}/WSe_{2}, WSe_{2}/WS_{2}, and WSe_{2}/MoS_{2}) and show that in the relevant parameter regimes their allowed occupancies never exceed three excitons. Our systematic theory provides guidelines for future research on the many-body physics of moiré excitons.

Optical transmission and photoconductivity of chemical bath-deposited CdS thin films for optoelectronic applications

This paper presents a comprehensive study of the optical transmission, photoconductivity, and morphology of CdS thin films deposited via the chemical bath deposition (CBD) method. The films deposited for 60 min exhibit an optical energy band gap value of 2.42 eV and the highest optical transmission of 75 % in the wavelength range of 500–900 nm. For these films, the transmission electron microscopy imaging shows a distribution of particle sizes around 10 nm. The wavelength dependence of the photoconductivity, extracted from photo resistivity data, shows that the maximum photoconductivity occurs at 492 nm wavelength. This corresponds to an electrical energy band gap of 2.52 eV, which is greater than the optically measured energy gap of 2.42 eV. Based on these results, the electron-hole pair binding energy for CdS films is estimated as 100 meV. According to the electrical resistivity vs. temperature measurements, the activation energy is 1.26 eV which agrees with the electrical energy band gap of 2.52 eV. CdS films annealed under nitrogen gas at 200 °C for one hour exhibited an energy gap of 2.32 eV. These films displayed a conductivity of 60 × 10−4 Scm−1, a carrier concentration of 6.38 × 1014 cm−3, and a mobility of 7.46 cm2 V−1 s−1. These impressive characteristics suggest the suitability of CBD-grown CdS films annealed at 200 °C in nitrogen gas, to be used as the window material in CdS/CdTe thin film solar cells and other optoelectronic applications.

MXene modified by ZnTCPP for sonodynamic treatment of bladder cancer

Sonodynamic therapy (SDT) is a promising non-invasive treatment strategy for site-specific therapy of deep penetration tumours using ultrasound. Herein, we report the design and preparation of ZnTCPP/MXene organic–inorganic hybrid materials for enhanced SDT of bladder cancer. The assembly of porphyrin-metal complex ZnTCPP on the surface of MXene effectively inhibits electron-hole pair binding and enhances sonodynamic performance. Under ultrasound stimulation, the ZnTCPP/MXene hybrid material exhibited remarkable ability to remove bladder cancer cells in just 15 min through the combined action of SDT and Zn2+. Our findings may provide new insights into the treatment of bladder tumours and offer a promising approach for SDT-enhanced cancer therapy.

Efficient photosynthesis of hydrogen peroxide by triazole-modified covalent triazine framework nanosheets

Two-dimensional (2D) polymeric semiconductors, especially covalent triazine framework (CTF) nanosheets with aromatic triazine linkages are emerging as attractive metal-free photocatalysts owing to their predictable structures, good semiconducting properties, and high stability. However, the quantum size effect and ineffective electron screening of 2D CTF nanosheets cause an enlargement of electronic band gap and high excited electron-hole binding energies, which lead to low-level enhancements in photocatalytic performance. Herein, we present a novel triazole groups functionalized CTF nanosheet (CTF-LTZ) synthesized by facile combination of ionothermal polymerization and freeze-drying strategy from the unique letrozole precursor. The incorporation of the high-nitrogen-containing triazole group effectively modulates the optical and electronic properties, resulting in narrowed bandgap from 2.92 eV for unfunctionalized CTF to 2.22 eV for CTF-LTZ and dramatically improved charge separation, as well as highly-active sites for O2 adsorption. As a result, CTF-LTZ photocatalyst exhibits excellent performance and superior stability in H2O2 photosynthesis, with a high H2O2 production rate of 4068 μmol h−1 g−1 and a remarkable apparent quantum efficiency of 4.5 % at 400 nm. This work provides a simple and effective approach for rational design highly-efficient polymeric photocatalysts for H2O2 production.

Quantum control of optoelectronic and thermodynamic properties of dopamine molecule in external electric field : A DFT and TD-DFT study

The effect of external electric field on molecular properties of dopamine is investigated using DFT and TD-DFT methods with B3LYP functional and 6-311++G(d,p) basis set. Optoelectronic properties (energy, polarizability, molecular orbitals, HOMO-LUMO gap, optical gap, electron–hole binding energy, UV-visible and electroabsorption spectra), and thermodynamic properties (zero-point vibrational energy, thermal energy, entropy, and specific heat capacity) were investigated. These properties are useful in developing molecular sensing devices, and understanding molecule receptor binding interaction. Our results using TD-DFT for excited states show that when an external electric field of magnitude F = 0.020 a.u. is applied along the x-axis, there is a shift in the absorption spectra of dopamine from the ultraviolet to the visible region which will be useful in optogenetic stimulation using visible light. The tunneling ionization rate increases and tunneling ionization time decreases with increase in electric field. These studies have potential applications in many of the emerging technologies in neurobiology such as deep brain stimulation and iontophoresis.

Shift of the superconducting dome induced by a conduction band of small occupancy: Enhanced d -wave pairing in the overdoped regime

Motivated by the presence of an extremely small electron pocket in the electronic band structure of infinite-layer nickelate superconductors, we systematically explore the superconducting (SC) properties of a two-dimensional Hubbard model influenced by an additional conduction band with small occupancy (dry Fermi sea). Our dynamic cluster quantum Monte Carlo calculations demonstrated the unusual impact of this additional dry metallic layer, which shifts the SC dome of the correlated Hubbard layer towards the overdoped regime, namely the $d$-wave SC can be enhanced (suppressed) in the overdoped (underdoped) regime. Simultaneously, the pseudogap crossover line shifts to the same higher doping regime as well. Our analysis revealed the interplay of the effective pairing interaction, pair-field susceptibility, and antiferromagnetic spin fluctuation in the underlying nonmonotonic physics. We postulate on the physical picture of the SC dome shift in terms of the Kondo-type electron-hole binding in the case of a dry additional conduction band. Our investigation provided valuable insights on tuning the SC properties via external dry bands.

Low-cost novel X-shaped hole transport materials for efficient perovskite solar cells: Molecular modelling of the core and schiff base effects on photovoltaic and photophysical properties

Eight molecules with D-π-D molecular motifs coded HTM(1-4)a,b were designed as efficient symmetric hole transporting materials for perovskite solar cells. These HTMs are composed of different core moieties, such as spiro [fluorene-9,9′-xanthene]-diol (SFO), spiro [fluorene-9,9′-xanthene]-dimethoxy (SFM), benzo [c][1,2,5]thiadiazole (BTD), and biphenyl (BP) and are gaining lot of attention because of their low-cost, reproducible electrical and optical properties in device performance. Detailed information about the energetic of molecular levels (GSOP/ESOP), first singlet excitation energy (E0-0), equilibrium molecular geometry, charge separation, charge transfer, reorganization energies, polarizability and hyperpolarizability, density of states (DOS), solubility and stability of these molecules was attained by systematically performing molecular modeling calculations using density functional theory (DFT) and time dependent-DFT calculations utilizing hybrid density functional B3LYP using the basis set 6–31g(d,p) level of theory as a successful method for predicting the photophysical and photovoltaic properties of these conjugated systems. The results showed that not only the core moieties, but also Schiff-base linkage extended conjugation, have an effect on the photophysical and photovoltaic properties of the proposed HTMs. It was demonstrated that HTMs incorporating SFO (HTM1a,b), SFM (HTM2a,b), and BP (HTM4a,b) achieved better charge transport, high stability values (ηa = 2.11–2.40 eV), low electron-hole binding energy (Eb = 0.16–0.21 eV) than HTM3a,b with BTD core, which improves hole mobility and decreases recombination, all of which improved device photocurrent, which can be attributed to the extended π-conjugation in SFO, SFM and BP cores. These findings are strikingly similar to those of Spiro-OMeTAD (ηa = 2.45 eV, Eb = 0.16 eV), indicating that these HTMs are promising candidates for efficient and cost-effective PSCs. Interestingly, when the stability of HTM(1-4)a and HTM(1-4)b was compared, it was clear that HTM(1-4)a has lower stability than HTM(1-4)b, which could be attributed to the presence of Schiff base CN bonds in HTM(1-4)b, which reduces the molecules stability when compared to HTM(1-4)b, owing to a strong C–C linkage. Meanwhile, HTM(1-4)a outperforms HTM(1-4)b in terms of electronic performance and hole mobility.

Hybridization and deconfinement in colloidal quantum dot molecules.

The structural, electronic, and optical properties of CdSe/CdS core-shell colloidal quantum dot molecules, a new class of coupled quantum dot dimers, are explored using atomistic approaches. Unlike the case of dimers grown by molecular beam epitaxy, simulated strain profile maps of free-standing colloidal dimers show negligible additional strain resulting from the attachment. The electronic properties of the relaxed dimers are described within a semiempirical pseudopotential model combined with the Bethe-Salpeter equation within the static screening approximation to account for electron-hole correlations. The interplay of strain, hybridization (tunneling splitting), quantum confinement, and electron-hole binding energies on the optical properties is analyzed and discussed. The effects of the dimensions of the neck connecting the two quantum dot building blocks, as well as the shell thickness, are studied.

Size and Solvation Effects on Electronic and Optical Properties of PbS Quantum Dots.

PbS quantum dots (QDs), among the most mature nanocrystals obtained by colloidal chemistry, are promising candidates in optoelectronic applications at various operational frequencies. QD device performances are often determined by charge transport, either carrier injection before photoemission or charge detection after photoabsorption, which is significantly influenced by the dielectric environment. Here, we present the electronic structure and the optical gap of PbS QDs versus size for various solvents calculated using ab initio methods including the many-body perturbation approaches. This study highlights the importance of the dielectric environment, pointing out (1) the non-negligible shift of the electronic structure due to the ground state polarization and (2) a substantial impact on the electronic bandgap. The electron-hole binding energy, which varies largely with the QD size and solvent, is well-described by an electrostatic model. This study reveals the fundamental physics of size and solvation effects, which could be useful to design PbS QD-based optoelectronic devices.

Electron-Hole Binding Governs Carrier Transport in Halide Perovskite Nanocrystal Thin Films.

Two-dimensional halide perovskite nanoplatelets (NPLs) have exceptional light-emitting properties, including wide spectral tunability, ultrafast radiative decays, high quantum yields (QY), and oriented emission. Due to the high binding energies of electron-hole pairs, excitons are generally considered the dominant species responsible for carrier transfer in NPL films. To realize efficient devices, it is imperative to understand how exciton transport progresses therein. We employ spatially and temporally resolved optical microscopy to map exciton diffusion in perovskite nanocrystal (NC) thin films between 15 °C and 55 °C. At room temperature (RT), we find the diffusion length to be inversely correlated to the thickness of the nanocrystals (NCs). With increasing temperatures, exciton diffusion declines for all NC films, but at different rates. This leads to specific temperature turnover points, at which thinner NPLs exhibit higher diffusion lengths. We attribute this anomalous diffusion behavior to the coexistence of excitons and free electron hole-pairs inside the individual NCs within our temperature range. The organic ligand shell surrounding the NCs prevents charge transfer. Accordingly, any time an electron-hole pair spends in the unbound state reduces the FRET-mediated inter-NC transfer rates and, consequently, the overall diffusion. These results clarify how exciton diffusion progresses in strongly confined halide perovskite NC films, emphasizing critical considerations for optoelectronic devices.

Energetics of the charge generation in organic donor-acceptor interfaces.

Non-fullerene acceptor materials have posed new paradigms for the design of organic solar cells , whereby efficient carrier generation is obtained with small driving forces, in order to maximize the open-circuit voltage (VOC). In this paper, we use a coarse-grained mixed quantum-classical method, which combines Ehrenfest and Redfield theories, to shed light on the charge generation process in small energy offset interfaces. We have investigated the influence of the energetic driving force as well as the vibronic effects on the charge generation and photovoltaic energy conversion. By analyzing the effects of the Holstein and Peierls vibrational couplings, we find that vibrational couplings produce an overall effect of improving the charge generation. However, the two vibronic mechanisms play different roles: the Holstein relaxation mechanism decreases the charge generation, whereas the Peierls mechanism always assists the charge generation. Moreover, by examining the electron-hole binding energy as a function of time, we evince two distinct regimes for the charge separation: the temperature independent excitonic spread on a sub-100fs timescale and the complete dissociation of the charge-transfer state that occurs on the timescale of tens to hundreds of picoseconds, depending on the temperature. The quantum dynamics of the system exhibits the three regimes of the Marcus electron transfer kinetics as the energy offset of the interface is varied.

Optimizing binding energy and electron-hole pair binding distance for efficient organic solar cells with low voltage loss

The energy loss during exciton dissociation is one of key reasons for organic solar cells (OSCs) with inferior performance. Herein, a theoretical and experimental guidance for highly efficient OSCs with outstanding exciton dissociation rate is introduced via fine-tuning the binding energy (Eb) and the electron-hole pair binding distance (a) in the charge transfer (CT) state. Results suggest that decreasing the Eb appropriately is favorable for higher photovoltaic conversion efficiency (PCE). Hence, we attempt to modulate the Eb of PM6:Y6 photoactive layer by changing additive and their corresponding concentration. Investigations show that the Eb of PM6:Y6 changes slightly with different additives, and the chloronaphthalene (CN) with appropriate concentration affords an Eb of 0.14 eV, and thus supporting the excellent efficiency of 16.14%. Meanwhile, the influences of the internal spacing of electron and hole in CT state on device performance were explored for further elucidating carrier micro-transport mechanism and instructing further experimental investigation.

Fabrication of a stable light-activated and p/n type AgVO3/V2O5-TiO2 heterojunction for pollutants removal and photoelectrochemical water splitting

In this study, we fabricated TiO2 nanorod arrays (TiO2), which were decorated with AgVO3 quantum dots (QDs) on and a layer of V2O5 film, forming a heterojunction material for removal pollutants and photoelectrochemical (PEC) water splitting. The AgVO3/V2O5-TiO2 nanorod arrays (AgVO3/V2O5-TiO2) synthesized by the secondary hydrothermal method were grown on conductive glass to form one-dimensional (1D) nanorod and p-n junction structures. Through experimental characterisation and testing, optical, morphological photocatalysis and PEC characteristics of the materials were measured and studied. The TiO2 modified with AgVO3 and V2O5 can significantly improve the visible light optical absorption, the reduce the electron-hole pair binding rate and shorten the band gap (3.07–1.41 eV) of TiO2. The resulting photocurrent density (116 uA/cm2) and photodegradation efficiency (rate constant, k = 0.025 min−1) of AgVO3/V2O5-TiO2 are approximately 20 (6 uA/cm2) and 5 times (0.005 min−1) higher than those of bare TiO2, respectively. The AgVO3/V2O5-TiO2 achieved a current density of 10 mA at an overpotential of 246.2 mV and exhibited excellent oxygen evolution reaction (OER) performance. The systematic PEC experiments concluded that the optimized of the TiO2 interface by AgVO3 and V2O5 could promote the separation and transport of charge carriers.

Accurate modeling of electron-hole binding in CuCl. II. Biexciton wavefunction

We present an approach to calculate the biexciton ground state including the electron-longitudinal-optical-phonon coupling and an accurate variational function to describe the Coulomb correlations in the biexciton. We apply this method to the long-standing problem of biexciton binding energy in CuCl and obtain a binding energy of 32.6 meV. We also discuss the effect of electron-hole ($e$-h) exchange interaction on the biexciton binding energy. Including correction due to $e$-h exchange, the theoretical binding energy is 28.8 meV, in good agreement with the experimental value of 32 meV. Details of the biexciton wavefunction are presented in the form of correlation functions with respect to two-particle separations, which show how the particles in the biexciton are distributed.

Accurate modeling of electron-hole binding in CuCl. I. Exciton states

We present an accurate variational approach for calculating the ground and excited exciton states in CuCl, and provide detailed comparison with measured polariton energies of excited exciton states. Computed exciton energies and oscillator strengths allow us to reproduce the polariton dispersion of up to four exciton states with unprecedented accuracy. A reinterpretation of the observed $1s$-exciton binding energy shows that the actual Coulomb energy in the exciton ground state is more than 50% larger than the observed binding energy, with important consequences for calculation of exciton complexes such as the biexciton.

Compact Real-Space Representation of Excited States Using Frequency-Dependent Explicitly Correlated Electron-Hole Interaction Kernel.

We present the frequency-dependent geminal-screened electron-hole interaction kernel (FD-GSIK) method for describing electron-hole correlation in electronically excited many-electron systems. The FD-GSIK is a parameter-free, first-principles method derived from excited-state wave function that was both frequency-dependent and r12-explicitly correlated. The FD-GSIK avoids using unoccupied orbitals for kernel construction by performing an infinite-order summation of particle-hole excitation and representing it as a compact real-space operator. It bypasses the computationally demanding steps of evaluation, storage, and transformation of atomic-orbital integrals by directly evaluating molecular orbital integrals in real space using the stratified Monte Carlo method. We demonstrate and discuss the advantages of this method by presenting excitation and electron-hole binding energies of large nanoparticles including Pb140S140, Pb140Se140, Cd144Se144, and Cd72S72.

Non-Coulomb strong electron-hole binding in Ta2NiSe5 revealed by time- and angle-resolved photoemission spectroscopy

We reveal an ultrafast purely electronic phase transition in \TNS, which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks with enhancing the pump pulse, and above a critical pump fluence, a photo-excited semimetallic state is experimentally identified with the absence of ultrafast structural transition. In addition, the bare valence and conduction bands and also the effective exciton binding energy in \TNS~are determined. These findings and detailed analysis suggest a bare nonequilibrium semimetallic phase in $Ta_2NiSe_5$ and the strong electron-hole binding cannot be exclusively driven by Coulomb interaction.

Neutral excitation density-functional theory: an efficient and variational first-principles method for simulating neutral excitations in molecules

We introduce neutral excitation density-functional theory (XDFT), a computationally light, generally applicable, first-principles technique for calculating neutral electronic excitations. The concept is to generalise constrained density functional theory to free it from any assumptions about the spatial confinement of electrons and holes, but to maintain all the advantages of a variational method. The task of calculating the lowest excited state of a given symmetry is thereby simplified to one of performing a simple, low-cost sequence of coupled DFT calculations. We demonstrate the efficacy of the method by calculating the lowest single-particle singlet and triplet excitation energies in the well-known Thiel molecular test set, with results which are in good agreement with linear-response time-dependent density functional theory (LR-TDDFT). Furthermore, we show that XDFT can successfully capture two-electron excitations, in principle, offering a flexible approach to target specific effects beyond state-of-the-art adiabatic-kernel LR-TDDFT. Overall the method makes optical gaps and electron-hole binding energies readily accessible at a computational cost and scaling comparable to that of standard density functional theory. Owing to its multiple qualities beneficial to high-throughput studies where the optical gap is of particular interest; namely broad applicability, low computational demand, and ease of implementation and automation, XDFT presents as a viable candidate for research within materials discovery and informatics frameworks.

Quasiparticle energies and excitonic effects of chromium trichloride: From two dimensions to bulk

Layered van der Waals (vdW) magnetic materials have attracted significant research interest to date. In this work, we employ the first-principles many-body perturbation theory to calculate excited-state properties of a prototype vdW magnet, chromium trichloride (CrCl3), covering monolayer, bilayer, and bulk structures. Unlike usual non-magnetic vdW semiconductors, in which many-electron interactions and excited states are sensitive to dimensionality, many-electron interactions are always enhanced and dominate quasiparticle energies and optical responses of both two-dimensional and bulk CrCl3. The electron-hole (e-h) binding energy can reach 3 eV in monolayer and remains as high as 2 eV in bulk. Because of the cancellation effect between self-energy corrections and e-h binding energies, the lowest-energy exciton (optical gap) is almost not affected by the change of dimensionality. Besides, for the excitons with similar e-h binding energies, their dipole oscillator strength can differ by a few orders of magnitude.Our analysis shows that such a big difference is from a unique interference effect between complex exciton wavefunctions and interband transitions. Finally, we find that the interlayer stacking sequence and magnetic coupling barely change quasiparticle band gaps and optical absorption spectra of CrCl3. Our calculated low-energy exciton peak positions agree with available measurements. These findings give insight into the understanding of many-electron interactions and the interplay between magnetic orders and optical excitations in vdW magnetic materials.

Designing dithienothiophene (DTT)-based donor materials with efficient photovoltaic parameters for organic solar cells.

Three acceptor-donor-acceptor (A-D-A)-type small donor molecules (M1, M2 and M3) were evaluated for optoelectronic properties through density functional theory calculations. These designed molecules consist of a dithieno [2,3-b:3,2-d] thiophene (DTT) donor group linked with 2-(3-methyl-4-oxothiazolidin-2-ylidine)malononitrile acceptor through three different bridge groups. The effect of the donor and three different bridge spacer groups on the designed molecules for opto-electronic properties was evaluated in comparison with the reference molecule R. The reorganization energies of the designed donor molecules suggest very good charge mobility property. The lower value of hole mobility (λh), as compared to electron mobility (λe), revealed that the three designed molecules are best for hole mobility. Frontier molecular orbital (FMO) surfaces confirm the transfer of charge from donor to acceptor unit during excitation. The designed molecules show relatively low HOMO values (in the range of -2.19 to -2.36eV), with strong absorption in UV-Visible region in the range of 459nm to 500nm in chloroform solvent. Electron-hole binding energy results indicate that the designed molecule M2 contains the highest amount of charge, which may dissociate into separate charge easily. Among all the studied molecules, the highest open circuit voltage (Voc) of 3.01eV (with respect to HOMOdonor-LUMOPC61BM) was shown by M3. The open circuit voltages (Voc) of R, M1, M2 and M3 were 2.91eV, 3.01eV, 2.77eV and 3.02eV, respectively. Graphical abstract Three newly acceptor donor acceptor (A-D-A)-based donor small molecules (M1, M2 and M3) were designed by taking dithieno [2,3-b:3,2-d]thiophene (DTT) as a donor group linked with acceptor 2-(3-methyl-4-oxothiazolidin-2-ylidine)malononitrile through three different bridge groups. All the designed molecule were compared with the well-known reference compound R. Optical properties, electronic properties, photophysical and excited state energy were calculated and compared with the well-known, and recently published, reference molecule R . All the newly designed molecules shows good optoelectronic properties with respect to R.