Table of contents

In organic and hybrid photovoltaic devices, the asymmetry required for charge separation necessitates the use of a donor and an acceptor material, resulting in the formation of internal interfaces in the device active layer. While the core objective of these interfaces is to facilitate charge separation, bound states between electrons and holes may form across them, resulting in a loss mechanism that diminishes the performance of the solar cells. These interfacial transitions appear in organic systems as charge transfer (CT) states and as bound charge pairs (BCP) in hybrid systems. Despite being similar, the latter are far less investigated. Herein, we employ photothermal deflection spectroscopy and pump-push-probe experiments in order to determine the characteristics and dynamics of interfacial states in two model systems: an organic P3HT:PCBM and hybrid P3HT:ZnO photovoltaic layer. By controlling the area of the internal interface, we identify CT states between 1.4 eV and 1.8 eV in the organic bulk-heterojunction (BHJ) and BCP between 1.1 eV and 1.4 eV in the hybrid BHJ. The energetic distribution of these states suggests that they not only contribute to losses in photocurrent, but also significantly limit the possible maximum open circuit voltage obtainable from these devices.

We demonstrate a novel method for the excitation of sizable magneto-optical effects in Au by means of the laser-induced injection of hot spin-polarized electrons in Au/Fe/MgO(0 0 1) heterostructures. It is based on the energy- and spin-dependent electron transmittance of Fe/Au interface which acts as a spin filter for non-thermalized electrons optically excited in Fe. We show that after crossing the interface, majority electrons propagate through the Au layer with the velocity on the order of 1 nm fs⁻¹ (close to the Fermi velocity) and the decay length on the order of 100 nm. Featuring ultrafast functionality and requiring no strong external magnetic fields, spin injection results in a distinct magneto-optical response of Au. We develop a formalism based on the phase of the transient complex MOKE response and demonstrate its robustness in a plethora of experimental and theoretical MOKE studies on Au, including our ab initio calculations. Our work introduces a flexible tool to manipulate magneto-optical properties of metals on the femtosecond timescale that holds high potential for active magneto-photonics, plasmonics, and spintronics.

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoO_x (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoO_x. Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoO_x molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoO_x/BHJ. The high recombination resistance (~5 kΩ cm²) and electron lifetime (~70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

We present a theoretical investigation of the structural and vibrational properties of ordered 2D phases formed by the Li, Na and K atoms on the Cu surface. The lattice relaxation, phonon dispersions and polarization of vibrational modes as well as the local density of states are calculated using the embedded-atom method. The obtained structural parameters and vibrational frequencies are in close agreement with available experimental results.

In this study, we conduct molecular dynamics simulations to investigate the effect of interface atomic structure on the deformation mechanisms of Ti₂AlN/TiAl composite under nanoindentation. It is found that the interface atomic structure has a remarkable effect on the deformation behavior of Ti₂AlN/TiAl composite due to significantly different interface-dislocation interactive mechanisms. For the Ti₂AlN(0 0 0 1)/TiAl(1 1 1) coherent interface system, although plenty of dislocations nucleate and propagate through the TiAl layer during the indentation process, there are no dislocations transmitting across the coherent interface. The formation of stair-rod dislocation and dislocation tangle is the major mechanism for blocking the slip motion of dislocations by the coherent interface. Thus, the Ti₂AlN(0 0 0 1)/TiAl(1 1 1) coherent interface is beneficial for the strengthening effect of Ti₂AlN/TiAl composite. For the Ti₂AlN(1 0  3)/TiAl(1 1 1) incoherent interface system, the incoherent interface has some ability to annihilate the dislocations nucleated from the TiAl during the indentation process, but it cannot simultaneously annihilate plenty of dislocations. These dislocations accumulate in the incoherent interface and cause the stress concentration, providing the driving force for dislocation nucleation in Ti₂AlN from the poor matching regions in the Al atomic arrays at the incoherent interface. Therefore, the incoherent interface can provide the access of dislocation transmission from TiAl to Ti₂AlN, which benefits the ductility of Ti₂AlN/TiAl composite.

Janus MoSSe and WSSe as new members to the family of transitional metal dichalcogenides (TMDCs) present intriguing properties that absent in its parent MX₂ (M  =  Mo, W; X  =  S, Se, Te) monolayers due to the out-of-plane mirror asymmetry. For WSSe/MoSSe van der Waals (vdW) heterostructures, intralayer/interlayer potential drops lead to significantly larger band offset than MX₂ heterobilayers, ensuring the long lifetimes for valley polarized interlayer excitons. Regard to the spin–valley–layer locking effects in WSSe/MoSSe vdW heterostructures, the band offset larger than the Zeeman-type spin splitting guarantees effective interlayer hopping and, therefore, large degree of valley polarization. Rashba-type spin splitting can coexist with the valley spin splitting and thus add the carrier transport paths, and intralayer/interlayer potential drops show obvious effects on the Rashba parameter. According to these results, WSSe/MoSSe vdW heterostructures manifest themselves the most promising candidates for spintronics and valleytronics with superiorities to the MX₂ counterparts.

Linear coefficient of thermal expansion is calculated for a mixture of poly(3,4-ethylenedioxythiophene) and polystyrene sulfonate (PEDOT:PSS) using density functional theory and the Debye–Grüneisen model. The linear coefficient of thermal expansion is a key factor in thermal management (thermal conductivity, thermal stress and thermal fatigue) of microelectronic and energy devices, being common applications of the conjugated polymeric PEDOT:PSS system. The obtained value of 53  ×  10⁻⁶ K⁻¹ at room temperature can be rationalised based on the electronic structure analysis. The PEDOT and PSS units are bonded by a dipole–dipole interaction between S in PEDOT and H in PSS. A C–C bond in a benzene ring (PSS) or thiophene (PEDOT) is up to 13 times stronger than the S–H bond. By adjusting the population of the S–H bonds by deprotonating PSS, the linear coefficient of thermal expansion can be enhanced by 57%. This allows for tuning the thermal properties of PEDOT:PSS in cutting-edge devices.

Here we report on photo-isomerization of azobenzene containing surfactants induced during irradiation with near-infrared (NIR) light in the presence of upconversion nanoparticles (UCNPs) acting as mediator. The surfactant molecule consists of charged head group and hydrophobic tail with azobenzene group incorporated in alkyl chain. The azobenzene group can be reversible photo-isomerized between two states: trans- and cis- by irradiation with light of an appropriate wavelength. The trans-cis photo-isomerization is induced by UV light, while cis-trans isomerization proceeds either thermally in darkness, or can be accelerated by exposure to illumination with a longer wavelength typically in a blue/green range. We present the application of lanthanide doped UCNPs to successfully switch azobenzene containing surfactants from cis to trans conformation in bulk solution using NIR light. Using Tm³⁺ or Er³⁺ as activator ions, the UCNPs provide emissions in the spectral range of 450 nm  <  λ_em  <  480 nm (for Tm³⁺, three and four photon induced emission) or 525 nm  <  λ_em  <  545 nm (for Er³⁺, two photon induced emission), respectively. Especially for UCNPs containing Tm³⁺ a good overlap of the emissions with the absorption bands of the azobenzene is present. Under illumination of the surfactant solution with NIR light (λ_ex  =  976 nm) in the presence of the Tm³⁺-doped UCNPs, the relaxation time of cis-trans photo-isomerization was increased by almost 13 times compared to thermally induced isomerization. The influence of thermal heating due to the irradiation using NIR light was shown to be minor for solvents not absorbing in NIR spectral range (e.g. CHCl₃) in contrast to water, which shows a distinct absorption in the NIR.

By employing particle-swarm optimization (PSO) and first-principles computations, we theoretically predicted five stable phases of graphene-like borocarbonitrides (g-BCN) with the stoichiometric ratio of 1:1:1 and uniformly distributed B, C, N atoms, which are the isoelectronic analogues of graphene. These g-BCN monolayers are effectively stabilized by their relatively high proportion of robust C–C or B–N bonds and strong partial ionic-covalent B–C and C–N bonds within them, leading to pronounced thermal and kinetic stability. The visible-light absorption and high carrier mobility of the investigated g-BCN monolayers indicate their possible applications in high-efficiency photochemical processes and electronic devices. Our computations could provide some guidance for designing the graphene-like materials with earth-abundant elements, as well as some clues for the experimental synthesis and practical applications of ternary BCN nanosheets.

We consider bias-induced circular current in a molecular ring junction. It is natural to define circular current as a component of ring current that acts as a sole source of magnetic flux induced in the ring. Alternatively, the bias-induced circular current can also be determined from the magnetic response of the ring junction to an external flux in the zero-flux limit. This leads to determination of bias-induced circular current without actually calculating the bond currents. We also explore the possibility of circular current-induced force rupturing the covalent bonds in the ring leading to ultimate breakdown of the ring junction. Our calculations underscore the reliability problem posed by the current magnification effect in the molecular ring structures.

Thermoelectric properties of hybrid systems composed of graphene nanoribbons (GNRs) coupled to rectangular rings or functionalized with aromatic carbon molecules are theoretically addressed here. Graphene-based nanostructures are designed with the purpose of enhancing thermopower responses compared to the thermal performance of pristine GNRs. The electronic transport is calculated using standard tight binding models and the Landauer transport formalism. We found that both semiconducting and metallic armchair nanoribbons coupled to rings exhibit a pronounced enhancement of the thermoelectric responses with comparable intensities, due to Fano antiresonance and Breit–Wigner-like resonances in the electronic transport. As expected, details of the ring geometry and ribbons are important in determining the precise chemical potential values for optimal performance. Different configurations of attached aromatic molecules (single and double molecules) at the graphene nanoribbon edges are addressed. Our findings show that the presence of a molecule induces a gap formation in the metallic pristine GNRs, and a pronounced peak of the Seebeck coefficient is revealed for low chemical potential values, independent of the molecule length. Other features on the Seebeck spectra are found to depend on the electronic nature of the GNRs and on the molecule length and distribution. We have shown that by playing with them, it is possible to design better thermoelectric devices based on GNRs.

The substitution of helium atom inside BCC Fe results in (1) lattice distortion, (2) the resonance phonon modes, and (3) the scattering center, enhancing the anharmonicity of phonon and magnon. The ferromagnetic effects are analyzed by comparing results in the ferromagnetic and non-magnetic systems using spin-lattice dynamics simulations based on quantum fluctuation dissipation relation. The ferromagnetic effects are subtracted into static and dynamic contributions, where the former is found to be dominant. Neglecting ferromagnetic effects would bring with unexpected large error in the description of kinetics of helium in metals.

We propose a model of two-leg ladder topological insulator in which the spin–orbit couplings are presented in both intra-chain and inter-chain interactions. The topological phase supports four fractional charged edge states localized at opposite ends of the ladder, which belongs to the chiral symplectic (CII) class protected by time-reversal symmetry and chiral symmetry. In our model, the presence of time-reversal and chiral symmetry generates fourfold degeneracy for the edge states, and the two edge states with same chirality at one end of the ladder each carries half charge. In contrast to the two edge states spatially localized at one end of the ladder being not distinguished, these two edge states can be detected by the momentum density. The experimental scheme for realizing our model with cold atoms in optical lattice is discussed. By introducing a magnetic field in the x direction, the system is driven from CII class to AIII class. In AIII class, there exist two distinct topological phases that exhibit four degenerate edge states and two degenerate edge states in the gap, respectively. As same as the system in CII class, each edge state carries a half charge in AIII class.

Strain engineering applied to carbon monosulphide monolayers allows to control the bandgap, controlling electronic and thermoelectric responses. Herein, we study the semiconductor–metal phase transition of this layered material driven by strain control on the basis of first-principles calculations. We consider uniaxial and biaxial tensile strain and we find a highly anisotropic electronic and thermoelectonic responses depending on the direction of the applied strain. Our results indicate that strain-induced response could be an effective method to control the electronic response and the thermoelectric performance.

We investigate the topological pumping effect in Weyl semimetals, subject to the modulation of two ac electric fields along y  and z directions, respectively. We show that the pumping effect originates from the anomalous velocity related to the Berry curvature. The direction of the pumping current is dependent on the chirality of the Weyl fermions. While the total particle current is vanishing because the Weyl points of opposite chirality always come in pairs in Weyl semimetals, the pump gives rise to a net chirality current or valley current. The noiseless valley current generated can be useful in valleytronic applications.

Under a magnetic field perpendicular to an monolayer graphene, the existence of a two-dimensional periodic scatter array can not only mix Landau levels of the same valley for displaying split electron–hole Hofstadter-type energy spectra, but also couple two sets of Landau subbands from different valleys in a bilayer graphene. Such a valley mixing effect with a strong scattering strength has been found observable and studied thoroughly in this paper by using a Bloch-wave expansion approach and a projected effective Hamiltonian including interlayer effective mass, interlayer coupling and asymmetrical on-site energies due to a vertically-applied electric field. For bilayer graphene, we find two important characteristics, i.e. mixing and interference of intervalley scatterings in the presence of a scatter array, as well as a perpendicular-field induced site-energy asymmetry which deforms severely or even destroys completely the Hofstadter-type band structures due to the dependence of Bloch-wave expansion coefficients on the applied electric field.

Using functional renormalization group method, we studied the phase diagram of the one-dimensional extended Hubbard model at different dopings. At half filling, variety of states strongly compete with each other. These states include spin-density wave, charge-density wave, s-wave and p -wave superconductivity, phase separation, and an exotic bond-order wave. By doping, system favours superconductivity more than density waves. At 1/8 doping, a new area of extended s-wave superconductivity emerges between charge density wave and bond-order wave regions. If the system is doped to 1/4-doping, a new area of p -wave superconductivity emerges between charge-density wave and spin-density wave regions.

SrRuO₃ is a popular material extensively used as a bottom electrode in various applications, however, a few problems which will certainly change the interface band structure and greatly alter the device's property are still not fully understood, such as the change of carrier types at a certain temperature and the quasiparticle scattering for non-Fermi liquid behavior below ferromagnetic transition temperature. In this study, magnetic, transport (electrical and thermal) properties and x-ray photoemission spectra have been used to understand the role of quasiparticle interactions in the SrRuO₃ bulk system. At the Fermi level, the hybridization of Ru4dt_2g  ↓  and O2p  bands form a typical two band system. In order to explain the problems as mentioned, our present work reveals that there must be an impurity band that couples with the bands around Fermi level and serves as a charge reservoir. In the present case, the impurity is attributed to the Ru vacancies. As a result, the conduction electrons scatter strongly with the Ru vacancies and couple with the Ru magnons to give rise to a dominant electron-magnon coupling that overwhelms the electron-phonon coupling in the temperature range of 90–150 K.

The crystalline electric field (CEF) level scheme and magnetic structure of a tetragonal antiferromagnet CePd₅Al₂ with K and K were studied by neutron scattering, magnetization and magnetoresistance measurements. Inelastic neutron scattering measurements on the powder sample revealed CEF excitations at 21.3 and 22.4 meV. The derived wave functions of the CEF ground state for the Ce³⁺ ion consist primarily of under the tetragonal symmetry. By means of single-crystal neutron diffraction, magnetic Bragg peaks characterized by a propagation vector were observed at . Our analysis indicates a sinusoidally modulated magnetic structure with amplitude of 2.0(1) /Ce, where the magnetic moments point to the -axis. The intensity of the third-order harmonic at 0.8 K is 1/30 as small as that expected for an antiphase structure, suggesting that the modulated structure remains at least down to 0.8 K. Both the magnetization and magnetoresistance show several anomalies in the magnetically ordered phase, indicating field-induced successive changes of the magnetic structure.

Due to potential applications in nano- and opto-electronics, two-dimensional (2D) materials have attracted tremendous interest. Their thermal transport properties are closely related to the performance of 2D materials-based devices. Here, the phonon transports of monolayer GeC with a perfect planar hexagonal honeycomb structure are investigated by solving the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). Without inclusion of Born effective charges (Z^*) and dielectric constants (), the lattice thermal conductivity () decreases almost linearly above 350 K, deviating from the usual law. The underlying mechanism is because the contribution to from high-frequency optical phonon modes increases with increasing temperature, and the contribution exceeds one from acoustic branches at high temperature. These can be understood by huge phonon band gap caused by large difference in atom mass between Ge and C atoms, which produces important effects on scattering process involving high-frequency optical phonon. When considering Z^* and , the phonon group velocities and phonon lifetimes of high-frequency optical phonon modes are obviously reduced with respect to ones without Z^* and . The reduced group velocities and phonon lifetimes give rise to small contribution to from high-frequency optical phonon modes, which produces the the traditional relation in monolayer GeC. Our works highlight the importance of Z^* and to investigate phonon transports of monolayer GeC, and motivate further theoretical or experimental efforts to investigate thermal transports of other 2D materials.

Ni₃V₂O₈, regarded as an S  =  1 kagome staircase lattice antiferromagnet, possesses a novel magnetic field–temperature phase diagram. Specifically, a half plateau region is observed in the high field magnetization curve for magnetic fields in the range of 11–19 T. This experimental observation is theoretically unexpected for a standard kagome lattice antiferromagnet, and consequently, the underlying magnetic structure is still unclear. Multi-frequency electron spin resonance results in this study strongly support a collinear magnetic arrangement at the half plateau region. The resonant modes can be well fit by only considering the antiferromagnetic interactions on a four-spin sublattice of the spine sites.

Spin wave dispersion in the frustrated fcc type-III antiferromagnet MnS₂ has been determined by inelastic neutron scattering using a triple-axis spectrometer. Existence of multiple spin wave branches, with significant separation between high-energy and low-energy modes highlighting the intrinsic magnetic frustration effect on the fcc lattice, is explained in terms of a spin wave analysis carried out for the antiferromagnetic Heisenberg model for this S  =  5/2 system with nearest and next-nearest-neighbor exchange interactions. Comparison of the calculated dispersion with spin wave measurement also reveals small suppression of magnetic frustration resulting from reduced exchange interaction between frustrated spins, possibly arising from anisotropic deformation of the cubic structure.

We report on the nature of the magnetism in Ru substituted MnNiGe using the combined results of x-ray diffraction, dc-magnetization, ac-susceptibility and ab initio calculations. Mn_0.7Ru_0.3NiGe crystallizes in Ni₂In-type hexagonal structure (P6₃/mmc) at room temperature with lattice parameters a  =  b  =  4.099 and c  =  5.367 . From the dc-magnetization; a broad peak around 46.55 K, separation between zero-field cooled and field-cooled warming state and non-saturating isothermal magnetization with typical S-type hysteresis indicate glassy behavior. A cusp in is observed to shift toward high temperatures with increasing frequency. Mydosh parameter (), single-relaxation time ( s) obtained through critical slowing-down analysis, from the Vogel–Fulcher law and Tholence criterion , confirm that Mn_0.7Ru_0.3NiGe belongs to the short-range interaction spin-glass systems with strong coupling between the magnetic clusters. LSDA+U calculations confirmed the competing exchange interactions between large magnetic moments of the Mn ions in Mn_0.7Ru_0.3NiGe compound resulting in the formation of spin-glassy characteristics.

A Phase Field model is developed combining the Orientation Field approach to modeling solidification with the Kim, Kim, Suzuki method of modeling binary alloys. These combined methods produce a model capable of simulating randomly oriented second phase dendrites with discrete control of the solid–liquid interface energy and thickness. The example system of carbon in a liquid uranium (U) melt is used as a test for the model. The formation of uranium carbide within a liquid U melt is simulated for isothermal conditions and compares well with experiments.