Strong Coulomb Interaction Research Articles

Tough hydrogel diodes with tunable interfacial adhesion for safe and durable wearable batteries

Flexible Zn-based batteries with intrinsic safety, non-toxicity and biocompatibility are promising power sources for wearable electronics. To meet practical applications, they need to be durable, robust and stable. However, inevitable self-discharge and interface delamination between electrodes and hydrogel electrolytes often cause serious capacity fading or even failure for flexible Zn-based batteries. Here, inspired from nature, we report hydrogel diodes with strong and tunable interfacial adhesion by attaching oppositely charged hydrogels together. The high adhesion energy is stemmed from strong coulombic interaction and effectively dissipating energy of hydrogel matrix. The adhesion is facilely tuned by narrowing the depletion region in hydrogel diodes. Subsequently we design Hook&Loop-like Cu-Zn batteries by using hydrogel diodes as electrolytes, which are robust, stable and durable and capable for powering wearable electronics. The batteries can eliminate self-discharge and maintain their capacity by splitted into two half cells during long-term storage and easily reattaching together at working stage. Further, the batteries are ultra-stable during 2000 stretching and bending cycles because of their robust interfacial adhesion. It is believed the hydrogel-electrolyte platforms can be utilized to harness issues including interface contact, stability, durability and safety for flexible power sources.

Colloquium : Excitons in atomically thin transition metal dichalcogenides

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here we review recent progress in our understanding of the excitonic properties in monolayer TMDs and lay out future challenges. We focus on the consequences of the strong direct and exchange Coulomb interaction, discuss exciton-light interaction and effects of other carriers and excitons on electron-hole pairs in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

Can the Dielectric Constant of Fullerene Derivatives Be Enhanced by Side-Chain Manipulation? A Predictive First-Principles Computational Study.

The low efficiency of organic photovoltaic (OPV) devices has often been attributed to the strong Coulombic interactions between the electron and hole, impeding the charge separation process. Recently, it has been argued that by increasing the dielectric constant of materials used in OPVs, this strong interaction could be screened. In this work, we report the application of periodic density functional theory together with the coupled perturbed Kohn–Sham method to calculate the electronic contribution to the dielectric constant for fullerene C60 derivatives, a ubiquitous class of molecules in the field of OPVs. The results show good agreement with experimental data when available and also reveal an important undesirable outcome when manipulating the side chain to maximize the static dielectric constant: in all cases, the electronic contribution to the dielectric constant decreases as the side chain increases in size. This information should encourage both theoreticians and experimentalists to further investigate the relevance of contributions to the dielectric constant from slower processes like vibrations and dipolar reorientations for facilitating the charge separation, because electronically, enlarging the side chain of conventional fullerene derivatives only lowers the dielectric constant, and consequently, their electronic dielectric constant is upper bound by the one of C60.

КОМПЬЮТЕРНОЕ МОДЕЛИРОВАНИЕ ЭЛЕКТРОННОЙ СТРУКТУРЫ ГЕМАТИТА

Представлены результаты компьютерного моделирования из первых принципов электронной структуры объемного гематита α-Fe2O3 при нормальных условиях. Расчеты проводились методом линеаризованных присоединенных плоских волн (ЛППВ) в рамках теории функционала плотности (density functional theory, DFT) и с помощью DFT, модифицированной постоянной Хаббарда U. Были получены спектры полной и локальных парциальных плотностей электронных состояний, зонные структуры. Путем сопоставления рассчитанных и известных экспериментальных данных найден метод адекватного описания электронной структуры α-Fe2O3.
 
 Расчеты были проведены на вычислительных мощностях Центра Обработки Данных (ЦОД) Воронежского государственного университета

Nonadiabatic Molecular Dynamics Simulation of Charge Separation and Recombination at a WS2/QD Heterojunction

Two-dimensional transition metal dichalcogenides (TMDs), such as WS2, are appealing candidates for optoelectronics and photovoltaics. The strong Coulomb interaction in TMDs is however known to prevent electron–hole pairs from dissociating into free electron and hole. The experiment demonstrates that combination of WS2 and quantum dots (QDs) can achieve efficient charge separation and enhance photon-to-electron conversion efficiency. Using real-time time-dependent density functional theory combined with nonadiabatic molecular dynamics, we model electron and hole transfer dynamics at a WS2/QD heterojunction. We demonstrate that both electron and hole transfer are ultrafast due to strong donor–acceptor coupling. The photoexcitation of the WS2 leads to a 75 fs electron transfer, followed by a 0.45 eV loss within 90 fs. The photoexcitation of QD results in 240 fs hole transfer, but loses only 0.15 eV of energy within 1 ps. The strong charge–phonon coupling and a broad range of phonon modes involved in electron...

Role of Hole Trap Sites in MoS2 for Inconsistency in Optical and Electrical Phenomena

Because of strong Coulomb interaction in two-dimensional van der Waals-layered materials, the trap charges at the interface strongly influence the scattering of the majority carriers and thus often degrade their electrical properties. However, the photogenerated minority carriers can be trapped at the interface, modulate the electron-hole recombination, and eventually influence the optical properties. In this study, we report the role of the hole trap sites on the inconsistency in the electrical and optical phenomena between two systems with different interfacial trap densities, which are monolayer MoS2-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO2 substrates. Electronic transport measurements indicate that the use of h-BN as a gate insulator can induce a higher n-doping concentration of the monolayer MoS2 by suppressing the free-electron transfer from the intrinsically n-doped MoS2 to the SiO2 gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS2/SiO2 is heavier than that in MoS2/h-BN, manifested by the relative red shift of the A1g Raman peak. The inconsistency in the evaluation of the electron concentration in MoS2 by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence band edge of the monolayer MoS2 caused by the local strain from the SiO2/Si substrate. This photoinduced electron doping in MoS2/SiO2 is further confirmed by the development of the trion component in the power-dependent photoluminescence spectra and negative shift of the threshold voltage of the FET after illumination.

Suppression of Na interstitials in Na-F codoped ZnO

Controlling the formation of interstitial Na (Nai) self-compensating defects has been a long-term physics problem for effective Na doping in ZnO. Herein, we present an experimental approach to the suppression of Nai defects in ZnO via Na and F codoping under an oxygen-rich condition during the molecular beam epitaxy growth process. It is found that the incorporation of such large numbers of Na and F dopants (∼1020 cm−3) does not cause an obvious influence on the lattice parameters. Hall-effect measurements demonstrate that F doping efficiently raises the Fermi level (EF) of ZnO films, which is expected to make the formation energy of Nai and NaZn increase and decrease, respectively. Most of the Na atoms occupy the substitutional Zn sites, and the formation of Nai is suppressed consequently. Secondary ion mass spectrometry measurements reveal that F and Na atoms are tightly bonded together due to their strong Coulomb interaction. The enhanced deep level emission (DLE) in ZnO:Na-F is ascribed to the considerable amount of isolated Zn vacancy (VZn) defects induced by the elevated EF and the formation of neutral FO+−NaZn−0 complexes. On the other hand, formation of FO+−VZn2−− complexes in ZnO:F exhausts most of the isolated Zn vacancies, leading to the disappearance of the DLE band.

High pressure RAFT of sterically hindered ionic monomers. Studying relationship between rigidity of the polymer backbone and conductivity

The synthesis of poly(ionic liquid)s (PILs), a new class of polymers with numerous possible applications and tailored thermorheological properties, is quite a challenging task. To achieve that goal, different strategies have been proposed and developed. However, in the majority of cases macromolecules with relatively low molecular weights and high dispersities were produced, probably due to the strong intermolecular coulombic interactions that determine the behavior of monomeric ionic liquids. In this paper, we proposed a completely new approach that relies on the pressure-induced reversible addition fragmentation chain transfer polymerization (RAFT) to produce PILs of desired properties. For this purpose, a series of model imidazolium-based ionic monomers, with different lengths of aliphatic side chains as additional steric hindrances, have been successfully polymerized under high pressure (p = 250, 500 and 800 MPa). In contrast to results obtained at ambient pressure, all monomers yielded high molecular weight polymers (degrees of polymerization DPn ≤ 10 000) with narrow dispersities (Ð∼1.10). From the kinetic data obtained at various thermodynamic conditions, the rate of polymerization, Rp, and overall activation volumes, ΔV, were estimated, which in the limit of low pressures varied as follows −16.7, −18.1, −32.6 and −35.6 cm mol−1 for [MVIM][NTf2], [EVIM][NTf2], [BVIM][NTf2] and [OVIM][NTf2], respectively. An unexpected significant jump in ΔV can be correlated with the nanostructure organization that, accordingly to the literature, starts to dominate in the latter two monomers. It was also demonstrated that below p = 500 MPa, the termination reaction is almost completely suppressed, independently on the sample. On the other hand, above that pressure both the polymerization rate and the control over the reaction decreased due to the high viscosity preventing diffusion of the monomers. Moreover, taking advantage of the high pressure polymerization, we had a unique opportunity of exploring and better understanding a correlation among molecular weight, Mn, the glass transition temperature, Tg, and the dc conductivity, σdc, for a very wide range of Mn (up to 430 kg/mol) polymers of various backbone rigidity. We observed that the evolution of Tg with Mn follows a typical Fox-Flory relation. Nevertheless, Tg decreases with an increase in the size of the monomer. Additionally, a similar chemical structure dependence was observed for the dc conductivity, which seems to strongly depend on the rigidity of the produced polymers. It can be seen that the higher rigidity of the polymer (longer alkyl chain of side group), the higher σdc. We believe the results reported herein offer an easy alternative way to synthesis of well-defined polyelectrolytes of moderate/high conductivity in shorter reaction times, and expand our knowledge of their properties and the correlations among them.

Dynamical heterogeneities of rotational motion in room temperature ionic liquids evidenced by molecular dynamics simulations.

Room temperature ionic liquids (RTILs) have been shown to exhibit spatial heterogeneity or structural heterogeneity in the sense that they form hydrophobic and ionic domains. Yet studies of the relationship between this structural heterogeneity and the ∼picosecond motion of the molecular constituents remain limited. In order to obtain insight into the time scales relevant to this structural heterogeneity, we perform molecular dynamics simulations of a series of RTILs. To investigate the relationship between the structures, i.e., the presence of hydrophobic and ionic domains, and the dynamics, we gradually increase the size of the hydrophobic part of the cation from ethylammonium nitrate (EAN), via propylammonium nitrate (PAN), to butylammonium nitrate (BAN). The two ends of the organic cation, namely, the charged Nhead-H group and the hydrophobic Ctail-H group, exhibit rotational dynamics on different time scales, evidencing dynamical heterogeneity. The dynamics of the Nhead-H group is slower because of the strong coulombic interaction with the nitrate counter-ionic anions, while the dynamics of the Ctail-H group is faster because of the weaker van der Waals interaction with the surrounding atoms. In particular, the rotation of the Nhead-H group slows down with increasing cationic chain length, while the rotation of the Ctail-H group shows little dependence on the cationic chain length, manifesting that the dynamical heterogeneity is enhanced with a longer cationic chain. The slowdown of the Nhead-H group with increasing cationic chain length is associated with a lower number of nitrate anions near the Nhead-H group, which presumably results in the increase of the energy barrier for the rotation. The sensitivity of the Nhead-H rotation to the number of surrounding nitrate anions, in conjunction with the varying number of nitrate anions, gives rise to a broad distribution of Nhead-H reorientation times. Our results suggest that the asymmetry of the cations and the larger excluded volume for longer cationic chain are important for both the structural heterogeneity and the dynamical heterogeneities. The observed dynamical heterogeneities may affect the rates of chemical reactions depending on where the reactants are solvated in ionic liquids and provide an additional guideline for the design of RTILs as solvents.

Ultrafast quantum beats of anisotropic excitons in atomically thin ReS2

Quantum beats, periodic oscillations arising from coherent superposition states, have enabled exploration of novel coherent phenomena. Originating from strong Coulomb interactions and reduced dielectric screening, two-dimensional transition metal dichalcogenides exhibit strongly bound excitons either in a single structure or hetero-counterpart; however, quantum coherence between excitons is barely known to date. Here we observe exciton quantum beats in atomically thin ReS2 and further modulate the intensity of the quantum beats signal. Surprisingly, linearly polarized excitons behave like a coherently coupled three-level system exhibiting quantum beats, even though they exhibit anisotropic exciton orientations and optical selection rules. Theoretical studies are also provided to clarify that the observed quantum beats originate from pure quantum coherence, not from classical interference. Furthermore, we modulate on/off quantum beats only by laser polarization. This work provides an ideal laboratory toward polarization-controlled exciton quantum beats in two-dimensional materials.

Dark excitons in transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap, since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.

Interplay between strong correlation and adsorption distances: Co on Cu(001)

Adsorbed transition metal atoms can have partially filled $d$- or $f$-shells due to strong on-site Coulomb interaction. Capturing all effects originating from electron correlation in such strongly correlated systems is a challenge for electronic structure methods. It requires a sufficiently accurate description of the atomistic structure (in particular bond distances and angles), which is usually obtained from Kohn-Sham density functional theory (DFT), which due to the approximate nature of the exchange-correlation functional may provide an unreliable description of strongly correlated systems. To elucidate the consequences of this popular procedure, we apply a combination of DFT with the Anderson impurity model (AIM), as well as DFT+U for a calculation of the potential energy surface along the Co/Cu(001) adsorption coordinate, and compare the results with those obtained from DFT. The adsorption minimum is shifted towards larger distances by applying DFT+AIM, or the much cheaper DFT+U method, compared to the corresponding spin-polarized DFT results, by a magnitude comparable to variations between different approximate exchange-correlation functionals (0.08 to 0.12 Angstrom). This shift originates from an increasing correlation energy at larger adsorption distances, which can be traced back to the Co 3$d_{xy}$ and 3$d_{z2}$ orbitals being more correlated as the adsorption distance is increased.

Photo-induced excitonic structure renormalization and broadband absorption in monolayer tungsten disulphide.

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS2) was investigated considering different excitonic transitions. WS2 illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS2 opens up new applications in broadband saturable absorbers and ultrafast photonic devices.

The half-metallicity of Co2FeGe full Heusler alloy in (001) thin film: First principles study

The electronic and magnetic properties of the Co2FeGe full Heusler alloy in (001) thin film are investigated using the first-principles electronic structure calculations within the density functional theory. We employ various exchange correlation functionals including the local density approximation (LDA), the generalized gradient approximation (GGA), and the additional +U corrections for strong on-site Coulomb interaction of transition metal 3d states, aiming to examine the correlation effect on the electronic structures which determine the spin gap and thus the half-metallicity. Our results reveal that the Co2FeGe thin film is metallic in both LDA and GGA, while the +U correction opens up the spin gap for spin minority channel in GGA+U but not in LDA+U in contrast to its bulk alloy which is predicted to be half-metallic in both LDA+U and GGA+U approaches with total spin magnetic moment of 6 μ B . It is found that the surface states developed around the Fermi level and the enhanced 3d e g -t2g band splitting for the spin minority channel due to the correlation effect play critical roles to determine the emergence of the half-metallicity.

Interface and phase transition between Moore-Read and Halperin 331 fractional quantum Hall states: Realization of chiral Majorana fermion

We consider an interface separating the Moore-Read state and Halperin 331 state in a half filled Landau level, which can be realized in a double quantum well system with varying inter-well tunneling and/or interaction strength. We find in the presence of electron tunneling and strong Coulomb interaction across the interface, all charge modes localize and the only propagating mode left is a chiral Majorana fermion mode. Methods to probe this neutral mode are proposed. Quantum phase transition between the Moore-Read and Halperin 331 states is described by a network of such Majorana fermion modes. In addition to a direct transition, they may also be separated by a phase in which the Majorana fermions are delocalized, realizing an incompressible state which exhibits quantum Hall charge transport and bulk heat conduction.

Van der Waals Layered Materials: Opportunities and Challenges.

Since graphene became available by a scotch tape technique, a vast class of two-dimensional (2D) van der Waals (vdW) layered materials has been researched intensively. What is more intriguing is that the well-known physics and chemistry of three-dimensional (3D) bulk materials are often irrelevant, revealing exotic phenomena in 2D vdW materials. By further constructing heterostructures of these materials in the planar and vertical directions, which can be easily achieved via simple exfoliation techniques, numerous quantum mechanical devices have been demonstrated for fundamental research and technological applications. It is, therefore, necessary to review the special features in 2D vdW materials and to discuss the remaining issues and challenges. Here, we review the vdW materials library, technology relevance, and specialties of vdW materials covering the vdW interaction, strong Coulomb interaction, layer dependence, dielectric screening engineering, work function modulation, phase engineering, heterostructures, stability, growth issues, and the remaining challenges.

Lithium halide monolayer sheets: First-principles many-body calculations

The geometric, electronic and optical properties of two-dimensional lithium halide (LiF, LiCl, LiBr) monolayers are systematically explored by using ab initio density functional theory, the HSE06 functional, the GWA method and Bethe–Salpeter equation calculations. The stabilities of these structures have been further evaluated by cohesive energy, phonon spectra, mechanical stability analysis and ab initio molecular dynamics. The structural results exhibit that the buckled configurations of these monolayer systems are stable; therefore, it is possible to synthesize these structures in experiments. The electronic properties were analyzed by HSE06 functional and quasi-particle many-body perturbation theory (MBPT) via GW approach. It is found that all these nanostructures are indirect band gap insulators. The HSE06 and G0W0 gave remarkably larger band gaps than the PBE functional. The optical properties and excitonic effects of these materials are investigated in independent-particle, independent-quasiparticle and including excitonic effects (BSE). The formation of first exciton peaks at 8.48, 7.7, and 6.92 eV with large binding energy of 2.92, 2.13 and 1.70 eV was observed for LiF, LiCl and LiBr monolayers, respectively. The strong Coulomb interaction between electrons and holes in these compounds leads to the large binding energy and small exciton Bohr radius. The interesting stability, electronic and optical properties of these monolayers are potentially useful for future electronic and optoelectronic device design such as VUV transmitter and X-ray monochromator plates.

Exciton fission in monolayer transition metal dichalcogenide semiconductors

When electron-hole pairs are excited in a semiconductor, it is a priori not clear if they form a plasma of unbound fermionic particles or a gas of composite bosons called excitons. Usually, the exciton phase is associated with low temperatures. In atomically thin transition metal dichalcogenide semiconductors, excitons are particularly important even at room temperature due to strong Coulomb interaction and a large exciton density of states. Using state-of-the-art many-body theory, we show that the thermodynamic fission–fusion balance of excitons and electron-hole plasma can be efficiently tuned via the dielectric environment as well as charge carrier doping. We propose the observation of these effects by studying exciton satellites in photoemission and tunneling spectroscopy, which present direct solid-state counterparts of high-energy collider experiments on the induced fission of composite particles.

Real-time diagrammatic approach to current-induced forces: Application to quantum-dot based nanomotors

During the last years there has been an increasing excitement in nanomotors and particularly in current-driven nanomotors. Despite the broad variety of stimulating results found, the regime of strong Coulomb interactions has not been fully explored for this application. Here we consider nanoelectromechanical devices composed by a set of coupled quantum dots interacting with mechanical degrees of freedom taken in the adiabatic limit and weakly coupled to electronic reservoirs. We use a real-time diagrammatic approach to derive general expressions for the current-induced forces, friction coefficients, and zero-frequency force noise in the Coulomb blockade regime of transport. We prove our expressions accomplish with Onsager's reciprocity relations and the fluctuation-dissipation theorem for the energy dissipation of the mechanical modes. The obtained results are illustrated in a nanomotor consisting of a double quantum dot capacitively coupled to some rotating charges. We analyze the dynamics and performance of the motor as function of the applied voltage and loading force for trajectories encircling different triple points in the charge stability diagram.

Lateral Graphene-Contacted Vertically Stacked WS2 /MoS2 Hybrid Photodetectors with Large Gain.

A demonstration is presented of how significant improvements in all-2D photodetectors can be achieved by exploiting the type-II band alignment of vertically stacked WS2 /MoS2 semiconducting heterobilayers and finite density of states of graphene electrodes. The photoresponsivity of WS2 /MoS2 heterobilayer devices is increased by more than an order of magnitude compared to homobilayer devices and two orders of magnitude compared to monolayer devices of WS2 and MoS2 , reaching 103 A W-1 under an illumination power density of 1.7 × 102 mW cm-2 . The massive improvement in performance is due to the strong Coulomb interaction between WS2 and MoS2 layers. The efficient charge transfer at the WS2 /MoS2 heterointerface and long trapping time of photogenerated charges contribute to the observed large photoconductive gain of ≈3 × 104 . Laterally spaced graphene electrodes with vertically stacked 2D van der Waals heterostructures are employed for making high-performing ultrathin photodetectors.