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By performing first principles calculations, we investigate the edge reconstruction in free-standing 2D-Xene (X = Si, Ge, Sn) zigzag nanoribbons. Three different periodicities of edge reconstruction (2 a , 3 a , 4 a) are found, in which the reconstruction with 3 a periodicity has the lowest energy and shows non-magnetic ground state. The edge reconstruction can be understood by the reconfiguration of the dangling bond states and edge states at the zigzag edges. Due to the structural buckling, extra bonding states are formed between edge atoms and inner atoms, accompanied with charge transfer from the edge states to the dangling bond states. This results in one-third occupied dangling bond states and a Peierls-like structural reconstruction with 3 a periodicity at the edge which opens a small band gap. With a tight binding model, the reconstruction of the electronic structures at the edges are revealed by the hopping integrals between different edge X-p orbitals. • 2D Xene edge buckling leads to charge transfer from edge states to dangling bond states • 3a edge reconstruction originates from the 1/3 band filling of dangling bond states • Tight binding model reveals the charge transfer and bonding interactions at the edge [ABSTRACT FROM AUTHOR]

Making a metal contact to the two-dimensional semiconductor MoS2 without creating a Schottky barrier is a challenge. Using density functional calculations we show that, although the Schottky barrier for electrons obeys the Schottky-Mott rule for high work function (≳ 4.7 eV) metals, the Fermi level is pinned at 0.1-0.3 eV below the conduction band edge of MoS2 for low work function metals, due to the metal-MoS2 interaction. Inserting a boron nitride (BN) monolayer between the metal and the MoS2 disrupts this interaction, and restores the MoS2 electronic structure. Moreover, a BN layer decreases the metal work function of Co and Ni by ~2 eV, and enables a lineup of the Fermi level with the MoS2 conduction band. Surface modification by adsorbing a single BN layer is a practical method to attain vanishing Schottky barrier heights. [ABSTRACT FROM AUTHOR]

Potential steps exceeding 1 eV are regularly formed at metal∣insulator interfaces, even when the interaction between the materials at the interface is weak physisorption. From first-principles calculations on metal ∣/h-BN interfaces we show that these potential steps are only indirectly sensitive to the interface bonding through the dependence of the binding energy curves on the van der Waals interaction. Exchange repulsion forms the main contribution to the interface potential step in the weakly interacting regime, which we show with a simple model based upon a symmetrized product of metal and h-BN wave functions. In the strongly interacting regime, the interface potential step is reduced by chemical bonding. [ABSTRACT FROM AUTHOR]

The formation of a Schottky barrier at the interface between a metal and hexagonal boron nitride (h-BN) is studied using density functional theory. For metals whose work functions range from 4.2 to 6.0 eV, we find Schottky barrier heights for holes between 1.2 and 2.3 eV. A central role in determining the Schottky barrier height is played by a potential step of between 0.4 and 1.8 eV that is formed at the metal∣/i-BN interface and effectively lowers the metal work function. If h-BN is physisorbed, as is the case on fee Cu, Al, Au, Ag, and Pt(111) substrates, the interface potential step is described well by a universal function that depends only on the distance separating h-BN from the metal surface. The interface potential step is largest when h-BN is chemisorbed, which is the case for hep Co and Ti (0001) and for fee Ni and Pd (111) substrates. [ABSTRACT FROM AUTHOR]

Abstract: Substantial potential steps ∼0.5eV are frequently observed in organic multilayers of donor and acceptor molecules. Often such potential steps depend on the order in which the individual layers are deposited, or on which substrate they are deposited. In this paper we outline a model for these potential steps, based upon integer charge transfer between donors and acceptors, charge equilibration across the multilayer, and simple electrostatics. Each donor, acceptor, or substrate material is characterized by a pinning level, and the potential profile can be deduced from the sequential order of the layers, and the differences between their pinning levels. For particular orderings we predict that intrinsic potential differences lead to electric fields across individual layers, which may falsely be interpreted as band bending. [Copyright &y& Elsevier]

Abstract: The Coulomb interaction between two holes on oligo-thiophene molecules is studied systematically as a function of the oligomer length using first principles density function calculations. The effect of molecular geometry relaxation upon this interaction is found to be small. In contrast, electronic polarization of the molecules that surround the charged oligomer in the crystal lattice reduces the bare Coulomb repulsion between the holes by approximately a factor of 2. In all cases, the effective hole–hole repulsion is much larger than the valence band width, which means that at high doping levels, strong correlation effects should become important. [Copyright &y& Elsevier]

State‐of‐the‐art triple cation, mixed halide perovskites are extensively studied in perovskite solar cells, showing very promising performance and stability. However, an in‐depth fundamental understanding of how the phase behavior in Cs0.05FA0.85MA0.10Pb(I0.97Br0.03)3 (CsMAFA) affects the optoelectronic properties is still lacking. The refined unit cell parameters a and c in combination with the thermal expansion coefficients derived from X‐ray diffraction patterns reveal that CsMAFA undergoes an α–β phase transition at ≈280 K and another transition to the γ‐phase at ≈180 K. From the analyses of the electrodeless microwave photoconductivity measurements it is shown that shallow traps only in the γ‐phase negatively affect the charge carrier dynamics. Most importantly, CsMAFA exhibits the lowest amount of microstrain in the β‐phase at around 240 K, corresponding to the lowest amount of trap density, which translates into the longest charge carrier diffusion length for electrons and holes. Below 200 K a considerable increase in deep trap states is found most likely related to the temperature‐induced compressive microstrain leading to a huge imbalance in charge carrier diffusion lengths between electrons and holes. This work provides valuable insight into how temperature‐dependent changes in structure affect the charge carrier dynamics in FA‐rich perovskites. [ABSTRACT FROM AUTHOR]

The structure and stability of dimers of quaterthiophene radical cations are studied using density functional calculations. Several dimer structures are identified that are metastable in vacuum, in which the cations are stacked coplanarly. Evidence for a weak chemical dimer bonding is found from the binding energy curves and the singlet-triplet splittings. Introducing solvent or solid-state effects by embedding the dimer in a dielectric medium results in stabilizing the dimer, as is observed experimentally. An infinite stack of quaterthiophene cations shows a weak spontaneous dimerization. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Metal oxide/Si heterostructures make up an exciting design route to high‐performance electrodes for photoelectrochemical (PEC) water splitting. By monochromatic light sources, contributions of the individual layers in WO3/n‐Si heterostructures are untangled. It shows that band bending near the WO3/n‐Si interface is instrumental in charge separation and transport, and in generating a photovoltage that drives the PEC process. A thin metal layer inserted at the WO3/n‐Si interface helps in establishing the relation among the band bending depth, the photovoltage, and the PEC activity. This discovery breaks with the dominant Z‐scheme design idea, which focuses on increasing the conductivity of an interface layer to facilitate charge transport, but ignores the potential profile around the interface. Based on the analysis, a high‐work‐function metal is predicted to provide the best interface layer in WO3/n‐Si heterojunctions. Indeed, the fabricated WO3/Pt/n‐Si photoelectrodes exhibit a 2 times higher photocurrent density at 1.23 V versus reversible hydrogen electrode (RHE) and a 10 times enhancement at 1.6 V versus RHE compared to WO3/n‐Si. Here, it is essential that the native SiO2 layer at the interface between Si and the metal is kept in order to prevent Fermi level pinning in the Schottky contact between the Si and the metal. [ABSTRACT FROM AUTHOR]

We explore the adsorption of MoS2 on a range of metal substrates by means of first-principles density functional theory calculations. Including van der Waals forces in the density functional is essential to capture the interaction between MoS2 and a metal surface, and obtain reliable interface potential steps and Schottky barriers. Special care is taken to construct interface structures that have a mismatch between the MoS2 and the metal lattices of <1%. MoS2 is chemisorbed on the early transition metal Ti, which leads to a strong perturbation of its (electronic) structure and a pinning of the Fermi level 0.54 eV below the MoS2 conduction band due to interface states. MoS2 is physisorbed on Au, where the bonding hardly perturbs the electronic structure. The bonding of MoS2 on other metals lies between these two extreme cases, with interface interactions for the late 3d transition metals Co, Ni, Cu and the simple metal Mg that are somewhat stronger than for the late 4d/5d transition metals Pd, Ag, Pt and the simple metal Al. Even a weak interaction, such as in the case of Al, gives interface states, however, with energies inside the MoS2 band gap, which pin the Fermi level below the conduction band. [ABSTRACT FROM AUTHOR]

Low‐dimensional halide perovskites with broad emission are a hot topic for their promising application as white light sources. However, the physical origin of this broadband emission in the sub‐bandgap region is still controversial. This work investigates the broad Stokes‐shifted emission bands in mixed lead‐tin 2D perovskite films prepared by mixing precursor solutions of phenethylammonium lead iodide (PEA2PbI4, PEA = phenethylammonium) and phenethylammonium tin iodide (PEA2SnI4). The bandgap can be tuned by the lead‐tin ratio, whereas the photoluminescence is broad and significantly Stokes‐shifted and appears to be fairly insensitive to the relative amount of Pb and Sn. It is experimentally observed that these low‐dimensional systems show substantially less bandgap bowing than their 3D counterpart. Theoretically, this can be attributed to the smaller spin–orbit coupling effect on the 2D perovskites compared to that of 3D ones. The time‐resolved photoluminescence shows an ultrafast decay in the high‐energy range of the spectra that coincides with the emission range of PEA2SnI4, while the broadband emission decay is slower, up to the microsecond range. Sub‐gap photoexcitation experiments exclude exciton self‐trapping as the origin of the broadband emission, pointing to defects as the origin of the broadband emission in 2D Sn/Pb perovskite alloys. [ABSTRACT FROM AUTHOR]

Carbon-based molecular semiconductors are explored for application in spintronics because their small spin-orbit coupling promises long spin lifetimes. We calculate the electronic transport from first principles through spin valves comprising bi- and tri-layers of the fullerene molecules C60 and C70, sandwiched between two Fe electrodes. The spin polarization of the current, and the magnetoresistance depend sensitively on the interactions at the interfaces between the molecules and the metal surfaces. They are much less affected by the thickness of the molecular layers. A high current polarization (CP>90%) and magnetoresistance (MR>100%) at small bias can be attained using C70 layers. In contrast, the current polarization and the magnetoresistance at small bias are vanishingly small for C60 layers. Exploiting a generalized Jullière model we can trace the differences in spin-dependent transport between C60 and C70 layers to differences between the molecule-metal interface states. These states also allow one to interpret the current polarization and the magnetoresistance as a function of the applied bias voltage. [ABSTRACT FROM AUTHOR]

Devising ways of opening a band gap in graphene to make charge-carrier masses finite is essential for many applications. Recent experiments with graphene on hexagonal boron nitride (h-BN) offer tantalizing hints that the weak interaction with the substrate is sufficient to open a gap, in contradiction of earlier findings. Using many-body perturbation theory, we find that the small observed gap is what remains after a much larger underlying quasiparticle gap is suppressed by incommensurability. The sensitivity of this suppression to a small modulation of the distance separating graphene from the substrate suggests ways of exposing the larger underlying gap. [ABSTRACT FROM AUTHOR]

We calculate the spin-polarized electronic transport through a molecular bilayer spin valve from first principles, and establish the link between the magnetoresistance and the spin-dependent interactions at the metal-molecule interfaces. The magnetoresistance of a Fe∣bilayer-C70∣Fe spin valve attains a high value of 70% in the linear-response regime, but it drops sharply as a function of the applied bias. The current polarization has a value of 80% in linear response and also decreases as a function of bias. Both these trends can be modeled in terms of prominent spin-dependent Fe∣C70 interface states close to the Fermi level, unfolding the potential of spinterface science to control and optimize spin currents. [ABSTRACT FROM AUTHOR]

The energy level alignment in metal-organic and organic-organic junctions of the widely used materials tris-(8-hydroxyquinoline)aluminum (Alq3) and 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTCDA) is investigated. The measured alignment schemes for single and bilayer films of Alq3 and NTCDA are interpreted with the integer charge transfer (ICT) model. Single layer films of Alq3 feature a constant vacuum level shift of ∼0.2-0.4 eV in the absence of charge transfer across the interface. This finding is attributed to the intrinsic dipole of the Alq3 molecule and (partial) ordering of the molecules at the interfaces. The vacuum level shift changes the onset of Fermi level pinning, as it changes the energy needed for equilibrium charge transfer across the interface. [ABSTRACT FROM AUTHOR]

We study how the Fermi energy of a graphene monolayer separated from a conducting substrate by a dielectric spacer depends on the properties of the substrate and on an applied voltage. An analytical model is developed that describes the Fermi level shift as a function of the gate voltage, of the substrate work function, and of the type and thickness of the dielectric spacer. The parameters of this model, that should describe the effect of gate electrodes in field effect devices, can be obtained from density functional theory (DFT) calculations on single layers or interfaces. The doping of graphene in metaIIdielectric graphene structures is found to be determined not only by the difference in work function between the metal and graphene and the dielectric properties of the spacer but potential steps that result from details of the microscopic bonding at the interfaces also play an important role. The doping levels predicted by the model agree very well with the results obtained from first-principles DET calculations on metaIIdielectric graphene structures with the metals Al, Co, Ni, Cu, Pd, Ag, Pt, or Au, and a h-BN or vacuum dielectric spacer. [ABSTRACT FROM AUTHOR]

The atomic structure of interfaces between conducting electrodes and molecular organic materials varies considerably. Yet experiments show that pinning of the Fermi level, which is observed at such interfaces, does not depend upon the structural details. In this letter, we develop a general model to explain Fermi level pinning, and formulate simple expressions for the pinning levels, based upon integer charge transfer between the conductor and the molecular layer. In particular, we show that DFT calculations give good values for the pinning levels. [ABSTRACT FROM AUTHOR]

The semiconducting two-dimensional transition metal dichalcogenides MX2 show an abundance of one-dimensional metallic edges and grain boundaries. Standard techniques for calculating edge states typically model nanoribbons, and require the use of supercells. In this paper, we formulate a Green's function technique for calculating edge states of (semi-)infinite two-dimensional systems with a single well-defined edge or grain boundary. We express Green's functions in terms of Bloch matrices, constructed from the solutions of a quadratic eigenvalue equation. The technique can be applied to any localized basis representation of the Hamiltonian. Here, we use it to calculate edge states of MX2 monolayers by means of tight-binding models. Aside from the basic zigzag and armchair edges, we study edges with a more general orientation, structurally modifed edges, and grain boundaries. A simple three-band model captures an important part of the edge electronic structures. An 11-band model comprising all valence orbitals of the M and X atoms is required to obtain all edge states with energies in the MX2 band gap. Here, states of odd symmetry with respect to a mirror plane through the layer of M atoms have a dangling-bond character, and tend to pin the Fermi level. [ABSTRACT FROM AUTHOR]

Structurally ordered interfaces between ferromagnetic electrodes and graphene or graphite are of great interest for carbon spintronics, since they allow spin-filtering due to k-vector conservation. By solid phase epitaxy of amorphous/nanocrystalline CoFeB at elevated temperatures, the feasibility of fabricating crystalline interfaces between a 3d ferromagnetic alloy and graphite is demonstrated, without suffering from the unwetting problem that was commonly seen in many previous studies with 3d transition metals. The films fabricated on graphite in this way are found to have a strong body-centered-cubic (110) texture, albeit without a unique, well-defined in-plane epitaxial relationship with the substrate lattice. Using various X-ray spectroscopic techniques, it is shown that boron suppresses the formation of CoFe-O during CoFeB deposition, and then diffuses out of the CoFe lattice. Segregation of B occurred exclusively to the film surface upon in situ annealing, and not to the interface between CoFeB and graphite. This is favorable for obtaining a high spin polarization at the hybrid CoFe/graphite crystalline interface. The Co and Fe spin moments in the crystalline film, determined by X-ray magnetic circular dichroism, are found to be bulk-like, while their orbital moments show an unusual giant enhancement which has yet to be understood. [ABSTRACT FROM AUTHOR]

Abstract: In this short review, we will give examples on how photoelectron spectroscopy (PES) assisted by models on interface energetics can be used to study properties important to bulk heterojunction type organic photovoltaic devices focusing on the well-known bulk heterojunction blend of poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) and its model system P3HT:C60. We also will discuss some of the limitations of PES as applied to organic semiconductors (OS) and photovoltaic devices and finish with reviewing recent theoretical advances that now enable calculation of relevant parameters at (hybrid) interfaces measured by PES. [Copyright &y& Elsevier]

We develop an integer charge transfer model for the potential steps observed at interfaces between donor and acceptor molecular semiconductors. The potential step can be expressed as the difference between the Fermi energy pinning levels of electrons on the acceptor material and holes on the donor material, as determined from metal-organic semiconductor contacts. These pinning levels can be obtained from simple density functional theory calculations. [ABSTRACT FROM AUTHOR]

Recently, incorporating guanidium (GA) cations into organolead halide perovskites is shown to effectively improve the stability and performance of the solar cells. However, the underlying mechanisms that govern the GA incorporation have remained unclear. Here, FAPbI3 is used as a basic framework to investigate experimentally and theoretically the role of cesium (Cs) and bromine (Br) substitutions in GA+ incorporation. It is found that simultaneous introduction of the small‐size Cs+ and Br– in the FAPbI3 lattice is critical to create sufficient space for the large GA+ and that the presence of the Cs+ prevents the formation of a GA‐contained low‐dimensional phase, which both assist GA+ incorporation. Upon entering the perovskite lattice, the GA+ can stabilize the lattice structure via forming strong hydrogen bonds with their neighboring halide ions. Such structure modification suppresses halide vacancy formation, thus leading to improved material properties. Compared to the GA‐free perovskite reference samples, the optimal system GA0.05Cs0.15FA0.8Pb(I0.85Br0.15)3 exhibits substantially improved thermal and photothermal stability, as well as increased photocarrier lifetime. Solar cells fabricated with the optimal material system show an excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 21.3% and an open circuit voltage of 1.229 V. [ABSTRACT FROM AUTHOR]