Effective Descriptor for Screening Single-Molecule Conductance Switches.

The adsorption-type molecular switch exhibits bistable states with an equivalently long lifetime at the organic/inorganic interface, promising reliable switching behavior and superior assembly ability in the electronic circuits at the molecular scale. However, the number of reported adsorption-type molecular switches is currently less than 10, and exploring these molecular switches poses a formidable challenge due to the intricate interplay occurring at the interface. To address this challenge, we have developed a model enabling the identification of diverse molecular switches on metal surfaces based on easily accessible physical characteristics. These characteristics primarily include the metal valency electron concentration, the work function of metal surfaces, and the electronegativity difference of molecules. Using this model, we identified 56 new molecular switches. Employing the gradient descent algorithm and statistical linear discriminant analysis, we constructed an explicit descriptor that establishes a relationship between the interfacial structure and chemical environment and the stability of molecular switches. The model's accuracy was validated through density functional theory calculations, achieving a 90% accuracy for aromatic molecular switches. The conductive switching behaviors were further confirmed by nonequilibrium Green's function transport calculations.

Effective Screening Descriptor for MXenes to Enhance Sulfur Reduction in Lithium-Sulfur Batteries.

MXenes, two-dimensional transition (2D) metal carbides/nitrides, have shown promise as cathodic catalysts for accelerating the conversion of lithium polysulfides (LiPSs) in lithium-sulfur (Li-S) batteries due to their diverse redox-active sites and rapid electron transfer. However, efficiently screening the optimal cathodic catalysts out of thousands of MXenes is challenging. To address this, we developed a model that accurately predicts the thermodynamic energy barrier of the rate-limiting step in Li-S batteries. Our model relates the local chemical reactivity of the MXene sites to the p-band center of the terminations and the electronegativity of subsurface transition metals. The accuracy of the model was verified through density functional theory calculations and contrast experiments in pure and Zn-doping MXenes qualitatively. By utilizing this model, we screened a large library of MXenes (27 types of five-atom-layer MXenes) and identified Ti2CS2, Mo2CS2, and W2CS2 as potential cathodic catalysts for Li-S batteries.

Surface Strain-Induced Collective Switching of Ensembles of Molecules on Metal Surfaces.

A central difficulty in the design of molecular electronics is poor control of the contact state between the molecule and metal electrode, which may induce instability and noise in logic and memory devices and even destroy the intrinsic functionality of the device. Here, we theoretically propose a simple and effective strategy for realizing full control of the contact state of organic molecules coated on the metal surface by applying homogeneous surface strain. As exemplified by pyrazine molecules on Cu(111), application of compressive (tensile) strain causes the molecules to uniformly adopt the physisorbed (chemisorbed) state. Within the framework of non-equilibrium Green's function calculations, we show that the two distinct contact states yield simultaneous rectification and switching behaviors. Because the contact states of all surface-bound molecules are transformed uniformly via surface strain perturbations, fully controlled collective switching and rectification effects can be simultaneously achieved in this contact system.

Asymmetric Schottky Contacts in van der Waals Metal-Semiconductor-Metal Structures Based on Two-Dimensional Janus Materials.

Physical and electronic asymmetry plays a crucial role in rectifiers and other devices with a directionally variant current-voltage (I-V) ratio. Several strategies for practically creating asymmetry in nanoscale components have been demonstrated, but complex fabrication procedures, high cost, and incomplete mechanistic understanding have significantly limited large-scale applications of these components. In this work, we present density functional theory calculations which demonstrate asymmetric electronic properties in a metal-semiconductor-metal (MSM) interface composed of stacked van der Waals (vdW) heterostructures. Janus MoSSe has an intrinsic dipole due to its asymmetric structure and, consequently, can act as either an n-type or p-type diode depending on the face at the interior of the stacked structure (SeMoS-SMoS vs. SMoSe-SMoS). In each configuration, vdW forces dominate the interfacial interactions, and thus, Fermi level pinning is largely suppressed. Our transport calculations show that not only does the intrinsic dipole cause asymmetric I-V characteristics in the MSM structure but also that different transmission mechanisms are involved across the S-S (direct tunneling) and S-Se interface (thermionic excitation). This work illustrates a simple and practical method to introduce asymmetric Schottky barriers into an MSM structure and provides a conceptual framework which can be extended to other 2D Janus semiconductors.

Lateral InSe p-n Junction Formed by Partial Doping for Use in Ultrathin Flexible Solar Cells.

Two-dimensional InSe possesses good electrical conductivity, intrinsic and structural flexibility, high chemical stability, and a tunable band gap, enabling it to be a promising candidate for flexible and wearable solar cells. Here we construct a lateral p-n junction by partially doping molybdenum trioxide (MoO3) at the surface of the InSe monolayer. Our density functional theory calculations reveal that the strong hybridization between MoO3 and InSe induces a lateral built-in electric field in the partially doped substrate and promotes the effective separation of carriers. Under a large range of external stains, the doped InSe can maintain the direct band gap, and the lateral structure device exhibits power conversion efficiencies over 5% and high carrier mobility around 1000 cm2 V-1 s-1. In particular, a power conversion efficiency of 20.7% can be achieved with 10% compressive strain. The partially doped InSe monolayer is expected to be used as an ultrathin flexible solar cell.

Deep Molecular Orbital Driven High-Temperature Hydrogen Tautomerization Switching.

Hydrogen tautomerization molecular switches, a promising class of molecular components for the construction of complex nanocircuits, have been extensively studied using low-temperature scanning tunneling microscopy. However, these molecules are generally only reliably controllable in cryogenic environments, obstructing their utility in real devices. Here, we use dispersion-inclusive density functional theory and systematically investigate the adsorption and tautomerization behaviors of porphycene on six transition-metal surfaces. Among these surfaces, we found that hydrogen tautomerization on the Pt(110) surface corresponds to the largest switching barrier, allowing a controllable transition at high temperature. The switching behavior is closely related to the exceptional degree of charge transfer in the HOMO-2 orbital, illustrating the important role of deep orbital-surface interactions in porphycene molecular switching. Our work provides an in-depth understanding of the porphycene tautomerization mechanism and highlights new research avenues toward the practical application of molecular switches.

Hydrogen-Location-Sensitive Modulation of the Redox Reactivity for Oxygen-Deficient TiO2.

Hydrogenated black TiO2 is receiving ever-increasing attention, primarily due to its ability to capture low-energy photons in the solar spectrum and its highly efficient redox reactivity for solar-driven water splitting. However, in-depth physical insight into the redox reactivity is still missing. In this work, we conducted a density functional theory study with Hubbard U correction (DFT+U) based on the model obtained from spectroscopic and aberration-corrected scanning transmission electron microscopy (AC-STEM) characterizations to reveal the synergy among H heteroatoms located at different surface sites where the six-coordinated Ti (Ti6C) atom is converted from an inert trapping site to a site for the interchange of photoexcited electrons. This in-depth understanding may be applicable to the rational design of highly efficient solar-light-harvesting catalysts.

External strain-enhanced cysteine enantiomeric separation ability on alloyed stepped surfaces.

Using density functional theory with an accurate treatment of van der Waals interactions, we investigate the enantioselective recognition and separation of chiral molecules on stepped metal surfaces. Our calculations demonstrate that the separation ability of metal substrates can be significantly enhanced by surface decoration and external strain. For example, applying 2% tensile strain to the Ag-alloyed Au(532) surface leads to a dramatic increase (by 89%) in cysteine enantioselectivity as compared to that of pristine Au(532). Analysis on the computed binding energies shows that the interaction energy is the predominant factor that affects the separation efficiency in strongly bound systems. Our study presents a new strategy to modify the enantioselectivity of stepped metal surfaces and paves the way for exploring high efficiency chiral separation technology in pharmaceutical industry.

van der Waals Stacking Induced Transition from Schottky to Ohmic Contacts: 2D Metals on Multilayer InSe.

Incorporation of two-dimensional (2D) materials in electronic devices inevitably involves contact with metals, and the nature of this contact (Ohmic and/or Schottky) can dramatically affect the electronic properties of the assembly. Controlling these properties to reliably form low-resistance Ohmic contact remains a great challenge due to the strong Fermi level pinning (FLP) effect at the interface. Herein, we employ density functional theory calculations to show that van der Waals stacking can significantly modulate Schottky barrier heights in the contact formed between multilayer InSe and 2D metals by suppressing the FLP effect. Importantly, the increase of InSe layer number induces a transition from Schottky to Ohmic contact, which is attributed to the decrease of the conduction band minimum and rise of the valence band maximum of InSe. Based on the computed tunneling and Schottky barriers, Cd3C2 is the most compatible electrode for 2D InSe among the materials studied. This work illustrates a straightforward method for developing more effective InSe-based 2D electronic nanodevices.

Switchable Schottky Contacts: Simultaneously Enhanced Output Current and Reduced Leakage Current.

Metal-semiconductor contacts are key components of nanoelectronics and atomic-scale integrated circuits. In these components Schottky diodes provide a low forward voltage and a very fast switching rate but suffer the drawback of a high reverse leakage current. Improvement of the reverse bias characteristics without degrading performance of the diode at positive voltages is deemed physically impossible for conventional silicon-based Schottky diodes. However, in this work we propose that this design challenge can be overcome in the organic-based diodes by utilizing reversible transitions between distinct adsorption states of organic molecules on metal surfaces. Motivated by previous experimental observations of controllable adsorption conformations of anthradithiophene on Cu(111), herein we use density functional theory simulations to demonstrate the distinct Schottky barrier heights of the two adsorption states. The higher Schottky barrier of the reverse bias induced by a chemisorbed state results in low leakage current, while the lower barrier of the forward bias induced by a physisorbed state yields a larger output current. The rectifying behaviors are further supported by nonequilibrium Green's function transport calculations.

Ultrahigh Conductivity in Two-Dimensional InSe via Remote Doping at Room Temperature.

Conductivity of two-dimenstional (2D) materials, which largely determines the efficiency and reliability of nanodevices, is proportional to the product of carrier concentration and mobility. Conventional doping, such as ionic substitution or introduction of vacancies, increases carrier concentration and decreases carrier mobility due to the scattering or trapping of carriers. We propose a remote-doping strategy that enables the simultaneous enhancement of both parameters. Density functional theory calculations in 2D InSe reveal that adsorbing the molecule tetrathiafulvalene (TTF) and applying a 4% external tensile strain leads to an increase in the carrier concentration of the TTF-InSe system that is 13 orders of magnitude higher than that of the pristine counterpart, whereas the carrier mobility is enhanced by 35% compared with the InSe monolayer. As a consequence of the synergetic role of molecule doping and strain engineering, ultrahigh conductivity of 1.85 × 105 S/m is achieved in the TTF-InSe system at room temperature.

Bipolar magnetic semiconductors among intermediate states during the conversion from Sc2C(OH)2 to Sc2CO2 MXene.

MXenes represent a new family of two-dimensional materials that have attracted considerable attention in recent years. Because of the remarkably different structures of Sc2C(OH)2 and Sc2CO2 MXene and their recently reported properties, this study explored the structural evolution and mechanism of chemical conversion between these two MXenes. Using first-principles density functional theory (DFT), the mechanism for dehydrogenation/hydrogenation is investigated by gradually removing/adding surface hydrogen atoms for Sc2C(OH)2/Sc2CO2 supercells. Employing three different supercells (2 × 2 × 1, 3 × 3 × 1 and 4 × 4 × 1), intermediate states Sc2C(OH)xO2-x with varying hydrogen content x (0.0625≤x ≤ 1.94) are obtained. The results show that the trend is to minimize the difference in the number of hydrogen atoms and the distance between them on the two sides of the monolayer. This feature is found to be generally applicable to other functional groups of MXenes during surface conversion. Analysis of these structures shows that all the oxygen, carbon and scandium atoms remain in essentially the same locations as in Sc2C(OH)2 until atoms rearrange in the carbon layer at sufficiently low x. Regarding the electronic properties, the behavior of the rearranged configurations is found to depend on the structure, moving beyond the conventional model of p-type doping induced by dehydrogenation. Bipolar magnetic semiconductors (BMSs) are identified from these rearranged configurations by the inhomogeneous distribution of hydrogen atoms on the different sides and x values approximately in the range of 0.188 ≤ x ≤ 0.812. Findings from this study suggest that the intrinsic spin-polarized semiconducting characteristics of Sc2C(OH)xO2-x are expected to be experimentally observable if samples are prepared as nanoscale flakes. The current results indicate that Sc-based MXene may be a promising material for nanoscale spintronic devices.

Phonon-driven electron scattering and magnetothermoelectric effect in two-dimensional tin selenide.

The bulk tin selenide (SnSe) is the best thermoelectric material currently with the highest figure-of-merit due to strong phonon-phonon interactions. We investigate the effect of electron-phonon coupling (EPC) on the transport properties of a two-dimensional (2D) SnSe sheet. We demonstrate that EPC plays a key role in the scattering rate when the constant relaxation time approximation is deficient. The EPC strength is especially large in contrast to that of pristine graphene. The scattering rate depends sensitively on the system temperatures and the carrier densities when the Fermi energy approaches the band edge. We also investigate the magnetothermoelectric effect of the 2D SnSe. It is found that at low temperatures there is enormous magnetoelectrical resistivity and magnetothermal resistivity above 200%, suggesting possible potential applications in device design. Our results agree qualitatively well with the experimental data.

Enhancement of spin polarization induced by Coulomb on-site repulsion between localized pz electrons in graphene embedded with line defects.

It is well known that the effect of Coulomb on-site repulsion can significantly alter the physical properties of the systems that contain localized d and/or f electrons. However, little attention has been paid to the Coulomb on-site repulsion between localized p electrons. In this study, we demonstrated that Coulomb on-site repulsion between localized pz electrons also plays an important role in graphene embedded with line defects. It is shown that the magnetism of the system largely depends on the choice of the effective Coulomb on-site parameter Ueff. Ueff at the edges of the defect enhances the exchange splitting, which increases the magnetic moment and stabilizes a ferromagnetic state of the system. In contrast, Ueff at the center of the defect weakens the spin polarization of the system. The behavior of the magnetism is explained with the Stoner criterion and the charge accumulation at the edges of the defect. Based on the linear response approach, we estimate reasonable values of Ueff to be 2.55 eV (2.3 eV) at the center (edges) of the defects. More importantly, using a DFT+U+J method, we find that exchange interactions between localized p electrons also play an important role in the spin polarization of the system. These results imply that Coulomb on-site repulsion is necessary to describe the strong interaction between localized pz electrons of carbon related materials.

Strong slip-induced anomalous enhancement and red-shifts in wide-range optical absorption of graphite under uniaxial pressure.

Natural graphite shows little optical response. Based on first-principles calculations, we demonstrate, for the first time, that an in-plane pressure-induced slip between atomic layers causes a strong anomalous enhancement and large red-shifts in the infrared and far infrared optical absorption by graphite. Specifically, a slip along the armchair direction induces an absorption feature that redshifts from ∼ 3 eV to ∼ 0.15 eV, while its intensity increases by an order of magnitude, due to an electron density delocalization effect with slip. Our results provide a way to detect and measure the magnitude of the in-plane slip of graphite under compression and also open up potential applications in electronics and photonics.