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We show that the shift current in bulk photovoltaics originates from a light-induced shift of charge centers to the many-body excited states. This many-body shift vector is expressed, using a sum-rule, in terms of pairs of nearly-degenerate optically-active excitons overlapping in $k$-space. The shift current can thus be enhanced by nearly-degenerate optically-active and tightly-bound excitons, spread out in $k$-space. $Ab$ $initio$ results for the many-body shift current and shift vector are presented for CdS and MoS$_2$, and exciton-phonon coupling is included through the exciton-phonon self-energies.

Terahertz technologies are important for a number of emerging applications, such as for next generation communications. We predict that transition metal substitutional defects in two-dimensional transition metal dichalcogenides (TMDs) can serve as quantum defects for terahertz technologies. Central to this prediction is the finding that the zero field splittings between spin sublevels in such defects are typically in the sub-terahertz to terahertz range due to the large spin-orbit coupling in these systems. As a proof of concept, we consider different transition metal impurities from across the periodic table, in prototypical TMDs, MoS2 and WSe2. Using first principles calculations, we demonstrate that selected spin triplet defects can potentially serve as qubits operating in the terahertz regime. We also propose defects that can potentially be quantum sources of terahertz radiation. Our research broadens the scope of advancements in quantum information science and lays a foundation for their integration with THz technologies.

We show that excitonic resonances and interexciton transitions can enhance the probability of spontaneous parametric down-conversion, a second-order optical response which generates entangled photon pairs. We benchmark our ab initio many-body calculations using experimental polar plots of second harmonic generation in NbOI$_2$, clearly demonstrating the relevance of excitons in the nonlinear response. A strong double-exciton resonance in 2D NbOCl$_2$ leads to giant enhancement in the second order susceptibility. Our work paves the way for the realization of efficient ultrathin quantum light sources.

Using high-throughput first-principles calculations to search for layered van der Waals materials with the largest piezoelectric stress coefficients, we discover NbOI2 to be the one among 2940 monolayers screened. The piezoelectric performance of NbOI2 is independent of thickness, and its electromechanical coupling factor of near unity is a hallmark of optimal interconversion between electrical and mechanical energy. Laser scanning vibrometer studies on bulk and few-layer NbOI2 crystals verify their huge piezoelectric responses, which exceed internal references such as In2Se3 and CuInP2S6. Furthermore, we provide insights into the atomic origins of anti-correlated piezoelectric and ferroelectric responses in NbOX2 (X = Cl, Br, I), based on bond covalency and structural distortions in these materials. Our discovery that NbOI2 has the largest piezoelectric stress coefficients among 2D materials calls for the development of NbOI2-based flexible nanoscale piezoelectric devices.

Single photon emitter (SPE) sources are important building blocks for photonics-based quantum technologies. Recently, the highly bright and versatile SPEs from the two-dimensional insulator material hexagonal boron nitride (hBN) have attracted significant research interest. However, due to the variability of emitter species and properties, an exact correlation between the underlying atomistic structures and their photo-physical properties is still lacking. In this work, we study six boron vacancy-derived defect centers in hBN ($V_B^-$, $V_B$+$H$, $V_{B2}$, $V_BC_N$, $V_BC_N^-$, and $V_BC_NC_N$) using advanced first principles techniques, characterizing their quasiparticle defect levels, optical spectra and excitation energies, and magneto-resonance properties. These defects have been chosen because of their relatively low formation energies, and, importantly, because they are amenable to intentional creation under experimental conditions. We establish the correlation between the underlying defect atomic structure and their photo-physical properties, thus facilitating the identification of SPEs that have already been observed in experiments. Our results lead to clear insights into very recent experiments where hBN SPEs can be controlled intentionally. On the other hand, our results also serve as guidelines for the bottom-up design of defect emitter centers in hBN for target applications that require specific defect properties, such as emission in the telecom wavelength, optical addressability and high radiative decay rates. This work thus provides a comprehensive understanding of the photo-physical characteristics of $V_B$-derived defect emitting centers, aiding in their identification and manipulation for tailored applications., Comment: 28 pages including supplementary information

Surface functionalization of two-dimensional (2D) materials with organic electron donors (OEDs) is a powerful method to modulate the electronic properties of the material. However, our fundamental understanding of the doping mechanism is largely limited to the categorization of molecular dopants as n- or p-type based on the relative position of the molecule's redox potential in relation to the Fermi level of the 2D host. Our limited knowledge about the impact of factors other than the redox properties of the molecule on doping makes it challenging to controllably use molecules to dope 2D materials and design new OEDs. Here, we functionalize monolayer MoS$_2$ using two molecular dopants, Me- and $^t$Bu-OED, which have the same redox potential but different steric properties to probe the effects of molecular size on the doping level of MoS$_2$. We show that, for the same functionalization conditions, the doping powers of Me- and $^t$Bu-OED are 0.22 - 0.44 and 0.11 electrons per molecule, respectively, demonstrating that the steric properties of the molecule critically affect doping levels. Using the stronger dopant, Me-OED, a carrier density of 1.10 +/- 0.37 x 10$^{14}$ cm$^{-2}$ is achieved in MoS$_2$, the highest doping level to date for MoS$_2$ using surface functionalization. Overall, we establish that tuning of the steric properties of the dopant is essential in the rational design of molecular dopants., Comment: 5 figures and 1 table

Carrier-doped transition metal dichalcogenide (TMD) monolayers are of great interest in valleytronics due to the large Zeeman response (g-factors) in these spin-valley-locked materials, arising from many-body interactions. We develop an \textit{ab initio} approach based on many-body perturbation theory to compute the interaction-enhanced g-factors in carrier-doped materials. We show that the g-factors of doped WSe$_2$ monolayers are enhanced by screened exchange interactions resulting from magnetic-field-induced changes in band occupancies. Our interaction-enhanced g-factors $g^*$ agree well with experiment. Unlike traditional valleytronic materials such as silicon, the enhancement in g-factor vanishes beyond a critical magnetic field $B_c$ achievable in standard laboratories. We identify ranges of $g^*$ for which this change in g-factor at $B_c$ leads to a valley-filling instability and Landau level alignment, which is important for the study of quantum phase transitions in doped TMDs. We further demonstrate how to tune the g-factors and optimize the valley-polarization for the valley Hall effect.

Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical non-linearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, we demonstrate that the electronic, superconducting and optical properties of air-stable two-dimensional metals can be controllably tuned by the formation of alloys. Environmentally robust large-area two-dimensional InxGa1-x alloys are synthesized by Confinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Mixed-dimensional magnetic heterostructures are intriguing, newly available platforms to explore quantum physics and its applications. Using state-of-the-art many-body perturbation theory, we predict the energy level alignment for a self-assembled monolayer of cobalt phthalocyanine (CoPc) molecules on magnetic VSe 2 monolayers. The predicted projected density of states on CoPc agrees with experimental scanning tunneling spectra. Consistent with experiment, we predict a shoulder in the unoccupied region of the spectra that is absent from mean-field calculations. Unlike the nearly spin-degenerate gas phase frontier molecular orbitals, the tunneling barriers at the interface are spin-dependent, a finding of interest for quantum information and spintronics applications. Both the experimentally observed shoulder and the predicted spin-dependent tunneling barriers originate from many-body interactions in the interface-hybridized states. Our results showcase the intricate many-body physics that governs the properties of these mixed-dimensional magnetic heterostructures, and suggests the possibility of manipulating the spin-dependent tunneling barriers through modifications of interface coupling.

We demonstrate that it is possible to rationally incorporate both an isolated flat band, and the physics of zero dimensions (0D), one dimension (1D), and two dimensions (2D) in a single 2D material. Such unique electronic properties are present in a recently synthesized 2D covalent organic framework (COF), where "I"-shaped building blocks and "T"-shaped connectors result in quasi-1D chains that are linked by quasi-0D bridge units arranged in a stable 2D lattice. The lowest unoccupied conduction band is an isolated flat band, and electron-doping gives rise to novel quantum phenomena, such as magnetism and Mott insulating phases. The highest occupied valence band arises from wave functions in the quasi-1D chains. Examples of mixed dimensional physics are illustrated in this system. The strong electron-hole asymmetry in this material results in a large Seebeck coefficient, while the quasi-1D nature of the chains leads to linear dichroism, in conjunction with strongly bound 2D excitons. We elucidate strategies to design and optimize 2D COFs to host both isolated flat bands and quantum-confined 1D subsystems. The properties of the 2D COF discussed here provide a taste of the intriguing possibilities in this open research field., Comment: preprint version; published in Nanoscale

Hydrogen adatoms on graphene are investigated using DFT and analytical approaches. We demonstrate that the level of lattice deformation due to the hydrogen adsorption does not substantially change the coupling between the graphene $p_{z}$ orbitals. The hybridization primarily takes place between the adsorbate's s orbital and the graphene $p_{z}$ orbitals. We also show that the impurity interaction with the graphene atoms is limited to only a few nearest neighbors, allowing us to construct a compact TB model for the impurity-graphene system with an arbitrary impurity distribution. The complexity of our model scales with the number of impurities, not their separation, making it especially useful in the study of low impurity concentrations., Comment: 10 pages, 7 figures

Surface functionalization with organic electron donors (OEDs) is an effective doping strategy for two-dimensional (2D) materials, which can achieve doping levels beyond those possible with conventional electric field gating. While the effectiveness of surface functionalization has been demonstrated in many 2D systems, the doping efficiencies of OEDs have largely been unmeasured, which is in stark contrast to their precision syntheses and tailored redox potentials. Here, using monolayer MoS2 as a model system and an organic reductant based on 4,4-bipyridine (DMAP-OED) as a strong organic dopant, we establish that the doping efficiency of DMAP-OED to MoS2 is in the range of 0.63 to 1.26 electrons per molecule. We also achieve the highest doping level to date in monolayer MoS2 by surface functionalization and demonstrate that DMAP-OED is a stronger dopant than benzyl viologen, which was the previous best OED dopant. The measured range of the doping efficiency is in good agreement with the values predicted from first-principles calculations. Our work provides a basis for the rational design of OEDs for high-level doping of 2D materials., Comment: 19 pages, 4 figures, 1 table

Two-dimensional (2D) materials are increasingly being used as active components in nanoscale devices. Many interesting properties of 2D materials stem from the reduced and highly non-local electronic screening in two dimensions. While electronic screening within 2D materials has been studied extensively, the question still remains of how 2D substrates screen charge perturbations or electronic excitations adjacent to them. Thickness-dependent dielectric screening properties have recently been studied using electrostatic force microscopy (EFM) experiments. However, it was suggested that some of the thickness-dependent trends were due to extrinsic effects. Similarly, Kelvin probe measurements (KPM) indicate that charge fluctuations are reduced when BN slabs are placed on SiO$_2$, but it is unclear if this effect is due to intrinsic screening from BN. In this work, we use first principles calculations to study the fully non-local dielectric screening properties of 2D material substrates. Our simulations give results in good qualitative agreement with those from EFM experiments, for hexagonal boron nitride (BN), graphene and MoS$_2$, indicating that the experimentally observed thickness-dependent screening effects are intrinsic to the 2D materials. We further investigate explicitly the role of BN in lowering charge potential fluctuations arising from charge impurities on an underlying SiO$_2$ substrate, as observed in the KPM experiments. 2D material substrates can also dramatically change the HOMO-LUMO gaps of adsorbates, especially for small molecules, such as benzene. We propose a reliable and very quick method to predict the HOMO-LUMO gap of small physisorbed molecules on 2D and 3D substrates, using only the band gap of the substrate and the gas phase gap of the molecule., Comment: 24 pages, 5 figures, Supplementary Information

This paper presents a theoretical description of both the valley Zeeman effect (g-factors) and Landau levels in two-dimensional H-phase transition metal dichalcogenides (TMDs) using the Luttinger-Kohn approximation with spin-orbit coupling. At the valley extrema in TMDs, energy bands split into Landau levels with a Zeeman shift in the presence of a uniform out-of-plane external magnetic field. The Landau level indices are symmetric in the $K$ and $K'$ valleys. We develop a numerical approach to compute the single band g-factors from first principles without the need for a sum over unoccupied bands. Many-body effects are included perturbatively within the GW approximation. Non-local exchange and correlation self-energy effects in the GW calculations increase the magnitude of single band g-factors compared to those obtained from density functional theory. Our first principles results give spin- and valley-split Landau levels, in agreement with recent optical experiments. The exciton g-factors deduced in this work are also in good agreement with experiment for the bright and dark excitons in monolayer WSe$_2$, as well as the lowest-energy bright excitons in MoSe$_2$-WSe$_2$ heterobilayers with different twist angles.

Interaction between adsorbed atoms in graphene is studied using a combination of DFT and the path integral formalism. Our results reveal a complex non-monotonic interaction profile. We show that the strength and sign of the interaction are dictated by the arrangement of impurities, as well as the system doping. These findings can be used to interpret the complex behavior of impurities in experimentally realized systems, as well as other classes of impurities, such as C substitutions in graphene., Comment: 9 pages, 8 figures

A combination of numerical and analytical methods is employed to study a one-dimensional chain of identical atoms with adsorbates. We show that the electron-mediated interaction energy between two impurities can change sign and magnitude depending on the adatom-adatom separation, as well as the system doping. By focusing on this simple system, we provide insight into the bulk-mediated interaction for more complex materials., Comment: 6 pages, 5 figures

We present a fully ab initio approach based on many-body perturbation theory in the GW approximation, to compute the quasiparticle levels of large interface systems without significant covalent interactions between the different components of the interface. The only assumption in our approach is that the polarizability matrix (chi) of the interface can be given by the sum of the polarizability matrices of individual components of the interface. We show analytically, using a two-state hybridized model, that this assumption is valid even in the presence of interface hybridization to form bonding and anti-bonding states, up to first order in the overlap matrix elements involved in the hybridization. We validate our approach by showing that the band structure obtained in our method is almost identical to that obtained using a regular GW calculation for bilayer black phosphorus, where interlayer hybridization is significant. Significant savings in computational time and memory are obtained by computing chi only for the smallest sub-unit cell of each component, and expanding (unfolding) the chi matrix to that in the unit cell of the interface. To treat interface hybridization, the full wavefunctions of the interface are used in computing the self-energy. We thus call the method XAF-GW (X: eXpand-chi, A: Add-chi, F: Full wavefunctions). Compared to GW-embedding type approaches in the literature, the XAF-GW approach is not limited to specific screening environments or to non-hybridized interface systems. XAF-GW can also be applied to systems with different dimensionalities, as well as to Moire superlattices such as in twisted bilayers. We illustrate the generality and usefulness of our approach by applying it to self-assembled PTCDA monolayers on Au(111) and Ag(111), and PTCDA monolayers on graphite-supported monolayer WSe2, where good agreement with experiment is obtained., Comment: More detailed proof of Add-Chi for hybridized states added in this version

We present a detailed first principles density functional theory study of intrinsic and extrinsic point defects in monolayer (ML) WSe2. Among the intrinsic point defects, Se vacancies (Sevac) have the lowest formation energy (disregarding Se adatoms that can be removed with annealing). The defects with the next smallest formation energies (at least 1 eV larger) are SeW (Se substituting W atoms in an antisite defect), Wvac (W vacancies) and 2Sevac (Se divacancies). All these intrinsic defects have gap states that are not spin-polarized. The presence of a graphite substrate does not change the formation energies of these defects significantly. For the extrinsic point defects, we focus on O, O2, H, H2 and C interacting with perfect WSe2 and its intrinsic point defects. The preferred binding site in perfect WSe2 is the interstitial site for atomic O, H and C. These interstitial defects have no gap states. The gap states of the intrinsic defects are modified by interaction with O, O2, H, H2 and C. In particular, the gap states of Sevac and 2Sevac are completely removed by interaction with O and O2. This is consistent with the significantly larger stability of O-related defects compared to H- and C-related defects. The preferred binding site for O is Sevac, while that for H is SeW. H bonded to SeW results in spin-polarized gap states, which may be useful in defect engineering for spintronics applications. The charge transition levels and ionization energies of these defects are also computed. H in the interstitial site is an effective donor, while all the other defects are deep donors or acceptors in isolated WSe2 ML.

Identifying the point defects in 2D materials is important for many applications. Recent studies have proposed that W vacancies are the predominant point defect in 2D WSe2, in contrast to theoretical studies, which predict that chalcogen vacancies are the most likely intrinsic point defects in transition metal dichalcogenide semiconductors. We show using first principles calculations, scanning tunneling microscopy (STM) and scanning transmission electron microscopy experiments, that W vacancies are not present in our CVD-grown 2D WSe2. We predict that O-passivated Se vacancies (O_Se) and O interstitials (Oins) are present in 2D WSe2, because of facile O2 dissociation at Se vacancies, or due to the presence of WO3 precursors in CVD growth. These defects give STM images in good agreement with experiment. The optical properties of point defects in 2D WSe2 are important because single photon emission (SPE) from 2D WSe2 has been observed experimentally. While strain gradients funnel the exciton in real space, point defects are necessary for the localization of the exciton at length scales that enable photons to be emitted one at a time. Using state-of-the-art GW-Bethe-Salpeter-equation calculations, we predict that only Oins defects give localized excitons within the energy range of SPE in previous experiments, making them a likely source of previously observed SPE. No other point defects (O_Se, Se vacancies, W vacancies and Se_W antisites) give localized excitons in the same energy range. Our predictions suggest ways to realize SPE in related 2D materials and point experimentalists toward other energy ranges for SPE in 2D WSe2.

Due to the chemical inertness of 2D hexagonal-Boron Nitride (h-BN), few atomic-layer h-BN is often used to encapsulate air-sensitive 2D crystals such as Black Phosphorus (BP). However, the effects of h-BN on Schottky barrier height, doping and contact resistance are not well known. Here, we investigate these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co) contacts. In sharp contrast to directly Co contacted p-type BP devices, we observe strong n-type conduction upon insertion of the h-BN at the Co/BP interface. First principles calculations show that this difference arises from the much larger interface dipole at the Co/h-BN interface compared to the Co/BP interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN contacts exhibit low contact resistances (~ 4.5 k-ohm), and are Schottky barrier free. This allows us to probe high electron mobilities (4,200 cm2/Vs) and observe insulator-metal transitions even under two-terminal measurement geometry.

Hybridized molecule/metal interfaces are ubiquitous in molecular and organic devices. The energy level alignment (ELA) of frontier molecular levels relative to the metal Fermi level (EF) is critical to the conductance and functionality of these devices. However, a clear understanding of the ELA that includes many-electron self-energy effects is lacking. Here, we investigate the many-electron effects on the ELA using state-of-the-art, benchmark GW calculations on prototypical chemisorbed molecules on Au(111), in eleven different geometries. The GW ELA is in good agreement with photoemission for monolayers of benzene-diamine on Au(111). We find that in addition to static image charge screening, the frontier levels in most of these geometries are renormalized by additional screening from substrate-mediated intermolecular Coulomb interactions. For weakly chemisorbed systems, such as amines and pyridines on Au, this additional level renormalization (~1.5 eV) comes solely from static screened exchange energy, allowing us to suggest computationally more tractable schemes to predict the ELA at such interfaces. However, for more strongly chemisorbed thiolate layers, dynamical effects are present. Our ab initio results constitute an important step towards the understanding and manipulation of functional molecular/organic systems for both fundamental studies and applications., Comment: main text - first 22 pages

Edge contact geometries are thought to yield ultralow contact resistances in most non-ferromagnetic metal-graphene interfaces owing to their large metal-graphene coupling strengths. Here, we examine the contact resistance of edge- versus surface-contacted ferromagnetic metal-graphene interfaces (i.e. nickel- and cobalt-graphene interfaces) using both single-layer and few-layer graphene. Good qualitative agreement is obtained between theory and experiment. In particular, in both theory and experiment, we observe that the contact resistance of edge-contacted ferromagnetic metal-graphene interfaces is much lower than that of surface-contacted ones, for all devices studied and especially for the single-layer graphene systems. We show that this difference in resistance is not due to differences in the metal-graphene coupling strength, which we quantify using Hamiltonian matrix elements. Instead, the larger contact resistance in surface contacts results from spin filtering at the interface, in contrast to the edge-contacted case where both spins are transmitted. Temperature-dependent resistance measurements beyond the Curie temperature TC show that the spin degree of freedom is indeed important for the experimentally measured contact resistance. These results show that it is possible to induce a large change in contact resistance by changing the temperature in the vicinity of TC, thus paving the way for temperature-controlled switches based on spin., Comment: 20 pages, 5 figures

Controlling the threshold voltage (Vth) of a field-effect transistor is important for realizing robust logic circuits. Here, we report a facile approach to achieve bidirectional Vth tuning of molybdenum disulfide (MoS2) field-effect transistors. By increasing and decreasing the amount of sulfur vacancies in the MoS2 surface, the Vth of MoS2 transistors can be left- and right-shifted, respectively. Transistors fabricated on perfect MoS2 flakes are found to exhibit two-fold enhancement in mobility and a very positive Vth. More importantly, our elegant hydrogen treatment is able to tune the large Vth to a small value without any performance degradation simply by reducing the atomic ratio of S:Mo slightly; in other words, creating a certain amount of sulfur vacancies in the MoS2 surface, which generate defect states in the band gap of MoS2 that mediate conduction of a MoS2 transistor in the subthreshold regime. First-principles calculations further indicate that the edge and width of defect band can be tuned according to the vacancy density. This work not only demonstrates for the first time the ease in tuning the Vth of MoS2 transistors, but also offers a process technology solution that is critical for further development of MoS2 as a mainstream electronic material.

Bulk black phosphorus (BP) consists of puckered layers of phosphorus atoms. Few-layer BP, obtained from bulk BP by exfoliation, is an emerging candidate as a channel material in post-silicon electronics. A deep understanding of its physical properties and its full range of applications are still being uncovered. In this paper, we present a theoretical and experimental investigation of phonon properties in few-layer BP, focusing on the low-frequency regime corresponding to interlayer vibrational modes. We show that the interlayer breathing mode A3g shows a large redshift with increasing thickness; the experimental and theoretical results agreeing well. This thickness dependence is two times larger than that in the chalcogenide materials such as few-layer MoS2 and WSe2, because of the significantly larger interlayer force constant and smaller atomic mass in BP. The derived interlayer out-of-plane force constant is about 50% larger than that in graphene and MoS2. We show that this large interlayer force constant arises from the sizable covalent interaction between phosphorus atoms in adjacent layers, and that interlayer interactions are not merely of the weak van der Waals type. These significant interlayer interactions are consistent with the known surface reactivity of BP, and have been shown to be important for electric-field induced formation of Dirac cones in thin film BP., Comment: Nano Letters, 2015

2D layered materials have recently attracted tremendous interest due to their fascinating properties and potential applications. The interlayer interactions are much weaker than the intralayer bonds, allowing the as-synthesized materials to exhibit different stacking sequences (e.g. ABAB, ABCABC), leading to different physical properties. Here, we show that regardless of the space group of the 2D material, the Raman frequencies of the interlayer shear modes observed under the typical configuration blue shift for AB stacked materials, and red shift for ABC stacked materials, as the number of layers increases. Our predictions are made using an intuitive bond polarizability model which shows that stacking sequence plays a key role in determining which interlayer shear modes lead to the largest change in polarizability (Raman intensity); the modes with the largest Raman intensity determining the frequency trends. We present direct evidence for these conclusions by studying the Raman modes in few layer graphene, MoS2, MoSe2, WSe2 and Bi2Se3, using both first principles calculations and Raman spectroscopy. This study sheds light on the influence of stacking sequence on the Raman intensities of intrinsic interlayer modes in 2D layered materials in general, and leads to a practical way of identifying the stacking sequence in these materials., Comment: 30 pages, 8 figures

2D materials are well-known to exhibit interesting phenomena due to quantum confinement. Here, we show that quantum confinement, together with structural anisotropy, result in an electric-field-tunable Dirac cone in 2D black phosphorus. Using density functional theory calculations, we find that an electric field, E_ext, applied normal to a 2D black phosphorus thin film, can reduce the direct band gap of few-layer black phosphorus, resulting in an insulator-to-metal transition at a critical field, E_c. Increasing E_ext beyond E_c can induce a Dirac cone in the system, provided the black phosphorus film is sufficiently thin. The electric field strength can tune the position of the Dirac cone and the Dirac-Fermi velocities, the latter being similar in magnitude to that in graphene. We show that the Dirac cone arises from an anisotropic interaction term between the frontier orbitals that are spatially separated due to the applied field, on different halves of the 2D slab. When this interaction term becomes vanishingly small for thicker films, the Dirac cone can no longer be induced. Spin-orbit coupling can gap out the Dirac cone at certain electric fields; however, a further increase in field strength reduces the spin-orbit-induced gap, eventually resulting in a topological-insulator-to-Dirac-semi-metal transition., Comment: 8 Pages and 8 figures in the main text + 8 supplementary figures

In recent years, bottom-up synthesis procedures have achieved significant advancements in atomically-controlled growth of several-nanometer-long graphene nanoribbons with armchair-shaped edges (AGNRs). This greatly encourages us to explore the potential of such well-defined AGNRs in electronics and spintronics. Here, we propose an AGNR based spin valve architecture that induces a large magnetoresistance up to 900%. We find that, when an AGNR is connected perpendicularly to zigzag-shaped edges, the AGNR allows for long-range extension of the otherwise localized edge state. The huge magnetoresistance is a direct consequence of the coupling of two such extended states from both ends of the AGNR, which forms a perfect transmission channel. By tuning the coupling between these two spin-polarized states with a magnetic field, the channel can be destroyed, leading to an abrupt drop in electron transmission.

Graphene nanoribbons with armchair edges (AGNRs) have bandgaps that can be flexibly tuned via the ribbon width. A junction made of a narrower AGNR sandwiched between two wider AGNR leads was recently reported to possess two perfect transmission channels close to the Fermi level. Here, we report that by using a bias voltage to drive these transmission channels into the gap of the wider AGNR lead, we can obtain a negative differential resistance (NDR) effect. Owing to the intrinsic properties of the AGNR junctions, the on-set bias reaches as low as ~ 0.2 V and the valley current almost vanishes. We further show that such NDR effect is robust against details of the atomic structure of the junction, substrate and whether the junction is made by etching or by hydrogenation., Comment: The following article has been submitted to Applied Physics Letters (http://scitation.aip.org/content/aip/journal/apl). Copyright (2014) Author(s). This article is distributed under a Creative Commons Attribution 3.0 Unported License

We report an approach to achieve low-resistance contacts to MoS2 transistors with the intrinsic performance of the MoS2 channel preserved. Through a dry transfer technique and a metal-catalyzed graphene treatment process, nickel-etched-graphene electrodes were fabricated on MoS2 that yield contact resistance as low as 200 ohm-um. The substantial contact enhancement (~2 orders of magnitude) as compared to pure nickel electrodes, is attributed to the much smaller work function of nickel-graphene electrodes, together with the fact that presence of zigzag edges in the treated graphene surface enhances tunneling between nickel and graphene. To this end, the successful fabrication of a clean graphene-MoS2 interface and a low resistance nickel-graphene interface is critical for the experimentally measured low contact resistance. The potential of using graphene as an electrode interlayer demonstrated in this work paves the way towards achieving high performance next-generation transistors.

A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as $\gamma_0 = 3.1$ eV, $\gamma_1 = 0.39$ eV, and $\gamma_4 = 0.22$ eV., Comment: 12 pages, 4 figures

We report the observation and interpretation of new Raman peaks in few-layer tungsten diselenide (WSe2), induced by the reduction of symmetry going from 3D to 2D. In general, Raman frequencies in 2D materials follow quite closely the frequencies of corresponding eigenmodes in the bulk. However, while the modes that are Raman active in the bulk are also Raman active in the thin films, the reverse is not always true due to the reduced symmetry in thin films. Here, we predict from group theory and density functional calculations that two intra-layer vibrational modes which are Raman inactive in bulk WSe2 in our experimental configuration become Raman active in thin film WSe2, due to reduced symmetry in thin films. This phenomenon explains the Raman peaks we observe experimentally at ~310 cm-1 and 176 cm-1 in thin film WSe2. Interestingly, the bulk mode at ~310 cm-1 that is Raman inactive can in fact be detected in Raman measurements under specific wavelengths of irradiation, suggesting that in this case, crystal symmetry selection rules may be broken due to resonant scattering. Both theory and experiment indicate that the and modes blue-shift with decreasing thickness, which we attribute to surface effects. Our results shed light on a general understanding of the Raman/IR activities of the phonon modes in layered transition metal dichalcogenide materials and their evolution behavior from 3D to 2D.

Organic spintronics is a promising emerging field, but the sign of the tunneling magnetoresistance (TMR) is highly sensitive to interface effects, a crucial hindrance to applications. A key breakthrough in molecular electronics was the discovery of amine-Au link groups that give reproducible conductance. Using first principles calculations, we predict that amine-Au links give improved reproducibility in organic spintronics junctions with Au-covered Fe leads. The Au layers allow only states with sp character to tunnel into the molecule, and the flexibility of amine-Au links results in a narrow range of TMR for fixed number of Au layers. Even as the Au thickness changes, TMR remains positive as long as the number of Au layers is the same on both sides of the junction. Since the number of Au layers on Fe surfaces or Fe nanoparticles can now be experimentally controlled, amine-Au links provide a route towards robust TMR in organic spintronics.

The layered dichalcogenide MoS2 has many unique physical properties in low dimensions. Recent experimental Raman spectroscopies report an anomalous blue shift of the in-plane E2g1 mode with decreasing thickness, a trend that is not understood. Here, we combine experimental Raman scattering and theoretical studies to clarify and explain this trend. Special attention is given to understanding the surface effect on Raman frequencies by using a force constants model based on first-principles calculations. Surface effects refer to the larger Mo-S force constants at the surface of thin film MoS2, which results from a loss of neighbours in adjacent MoS2 layers. Without surface effects, the frequencies of both out-of-plane A1g and in-plane E2g1 modes decrease with decreasing thickness. However, the E2g1 mode blue shifts while the A1g mode red shifts once the surface effect is included, in agreement with the experiment. Our results show that competition between the thickness effect and the surface effect determines the mechanical properties of two-dimensional MoS2, which we believe applies to other layered materials.

Bi2Se3 and Bi2Te3 are layered compounds of technological importance, being excellent thermoelectric materials as well as topological insulators. We report density functional theory calculations of the atomic, electronic and thermoelectric properties of strained bulk and thin film Bi2Se3 and Bi2Te3, focusing on an appropriate description of van der Waals interactions. The calculations show that the van der Waals Density functional with Cooper's exchange can reproduce closely the experimental interlayer distances in unstrained Bi2Se3 and Bi2Te3. Interestingly, we predict atomic structures that are in much better agreement with the experimentally determined structure from Nakajima than that obtained from Wyckoff, especially for Bi2Se3 where the difference in atomic structures qualitatively changes the electronic band structure. The band structure obtained using the Nakajima structure, and the theoretically optimized structure, are in much better agreement with previous reports of photoemission measurements, than that obtained using the Wyckoff structure. Fully optimizing atomic structures of bulk and thin film Bi2Se3 and Bi2Te3 under different in-plane and uniaxial strains, we predict that the electronic band gap of both the bulk materials and thin films decreases with tensile in-plane strain and increases with compressive in-plane strain. We also predict, using the semiclassical Boltzmann approach, that the magnitude of the n-type Seebeck coefficient of Bi2Te3 can be increased by the compressive in-plane strain, while that of Bi2Se3 can be increased with tensile in-plane strain. Further, the in-plane power factor of n-doped Bi2Se3 can be increased with compressive uniaxial strain, while that of n-doped Bi2Te3 can be increased by compressive in-plane strain. ..., Comment: Please see manuscript for full abstract. Published in PRB - http://link.aps.org/doi/10.1103/PhysRevB.86.184111

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have recently attracted tremendous interest as potential valleytronic and nano-electronic materials, in addition to being well-known as excellent lubricants in the bulk. The interlayer van der Waals (vdW) coupling and low frequency phonon modes, and how they evolve with the number of layers, are important for both the mechanical and electrical properties of 2D TMDs. Here we uncover the ultra-low frequency interlayer breathing and shear modes in few-layer MoS2 and WSe2, prototypical layered TMDs, using both Raman spectroscopy and first principles calculations. Remarkably, the frequencies of these modes can be perfectly described using a simple linear chain model with only nearest-neighbour interactions. We show that the derived in-plane (shear) and out-of-plane (breathing) force constants from experiment remain the same from two-layer 2D crystals to the bulk materials, suggesting that the nanoscale interlayer frictional characteristics of these excellent lubricants should be independent of the number of layers.

Using density functional theory and many-body perturbation theory within a GW approximation, we calculate the electronic structure of a metal-molecule interface consisting of benzene diamine (BDA) adsorbed on Au(111). Through direct comparison with photoemission data, we show that a conventional G$_0$W$_0$ approach can underestimate the energy of the adsorbed molecular resonance relative to the Au Fermi level by up to 0.8 eV. The source of this discrepancy is twofold: a 0.7 eV underestimate of the gas phase ionization energy (IE), and a 0.2 eV overestimate of the Au work function. Refinements to self-energy calculations within the GW framework that account for deviations in both the Au work function and BDA gas-phase IE can result in an interfacial electronic level alignment in quantitative agreement with experiment.

Molecular-scale components are expected to be central to nanoscale electronic devices. While molecular-scale switching has been reported in atomic quantum point contacts, single-molecule junctions provide the additional flexibility of tuning the on/off conductance states through molecular design. Thus far, switching in single-molecule junctions has been attributed to changes in the conformation or charge state of the molecule. Here, we demonstrate reversible binary switching in a single-molecule junction by mechanical control of the metal-molecule contact geometry. We show that 4,4'-bipyridine-gold single-molecule junctions can be reversibly switched between two conductance states through repeated junction elongation and compression. Using first-principles calculations, we attribute the different measured conductance states to distinct contact geometries at the flexible but stable N-Au bond: conductance is low when the N-Au bond is perpendicular to the conducting pi-system, and high otherwise. This switching mechanism, inherent to the pyridine-gold link, could form the basis of a new class of mechanically-activated single-molecule switches.

The measured conductance distribution for single molecule benzenediamine-gold junctions, based on 59,000 individual conductance traces recorded while breaking a gold point contact in solution, has a clear peak at 0.0064 G$_{0}$ with a width of $\pm$ 40%. Conductance calculations based on density functional theory (DFT) for 15 distinct junction geometries show a similar spread. Differences in local structure have a limited influence on conductance because the amine-Au bonding motif is well-defined and flexible. The average calculated conductance (0.046 G$_{0}$) is seven times larger than experiment, suggesting the importance of many-electron corrections beyond DFT.