Pz States Research Articles

Band order reversal and valley engineering driving to dual high electron and hole mobility in strained monolayer BS

Strain engineering is an effective method to tune the physical properties of materials. In this work, we found that applying 5% biaxial compression to monolayer boron sulfur can simultaneously induce a band order reversal of valence bands and valley engineering of conduction bands. This change significantly enhances the room temperature intrinsic mobility, boosting it from 2 to 260 cm2 V−1 s−1 for holes and 63 to 543 cm2 V−1 s−1 for electrons. The high hole mobility surpasses that of bulk GaN and silicon. We further demonstrate that the valence band order reversal not only lifts the px/y state above the pz states, giving to a more dispersive highest valence band, and thus smaller electron scattering channels and hole effective mass, but also shifts the stronger σz bonding to weaker π-bonding characteristics, resulting in smaller electron–phonon coupling strength. Those combined effects results in a substantial increase in hole mobility.

Graphene-Molecule Hybridization at a Ferromagnetic Interface.

The introduction of a graphene (Gr) buffer layer between a ferromagnetic substrate and a metallorganic molecule is known to mediate the magnetic coupling between them, an effect attributed to a weak hybridization between graphene and molecule. In this paper, we present experimental evidence of this effect through a detailed investigation of the frontier electronic properties of iron phthalocyanine deposited on cobalt-supported graphene. Despite being physisorbed, the molecular adsorption on Gr/Co induces a sizeable charge transfer from graphene to the molecular macrocycle leading to the partial occupation of the LUMO and the appearance of an energetically localized hybrid state, which can be attributed to the overlap between the graphene pz state and the molecular macrocycle. Graphene is not inert either; the adsorption of the molecule induces doping and alters the Fermi velocity of both the hybrid minicone state and the Dirac cone. Similar effects are observed when the molecular periphery is decorated with fluorine atoms, known for their electron-withdrawing properties, with minimal changes in the energy alignment.

Effects of N-doped concentration in graphene on CO2 adsorption

Solving the problem of the environment related to CO2 emission is an urgent and challenging task to prevent the escalation of climate change, which is due to increasing energy demand from fossil fuels and industrial activities. Using graphene to capture CO2 gas is of great interest to mitigate global warming, where CO2 capture by doped graphene has been shown to significantly improve the CO2 adsorption capacity compared to that of pure graphene. In particular, N-doped graphene is a unique structure that was suggested for CO2 capture by the adsorption phenomenon. However, there is no research available to clarify nitrogen doping concentration in graphene on CO2 adsorption. Therefore, this work has been devoted to elucidating the problem using the density functional theory method considering van der Waals interaction. We showed that increasing the doping content of nitrogen by 3.1, 6.3, 9.4, and 12.5% will increase the CO2 adsorption energy, monotonically for N content below 9.4% and significantly for 12.5%. The most favorable adsorption configuration of the CO2 molecule should be parallel to the surface of the N-doped graphene. The structure of the substrate at 12.5% N distorted upon the CO2 adsorption; therefore, the substrate is more active for CO2 capture. The physical meaning underlying the interaction of the N@graphene system is physisorption, which is due to the contribution of the N pz state and the CO2 pz state along the surface normal of the substrate. Increasing the N-doping content shifts down the unoccupied states of the N pz orbital in the conduction band across the Fermi level to the valence band. Hence, the population of the occupied state at the Fermi level increases as increasing the N-doping concentration, leading to the stronger interaction of the CO2 molecule with the N@graphene substrate. The results should be useful for the rational design of suitable N@graphene substrates for CO2 capture and storage applications.

Interaction of graphene with Au n clusters: a first-principles study

The interaction between Au n (n = 1–6) clusters and graphene is studied using first-principles simulations, based on density functional theory. The computed binding energy between Au n and graphene depends on the number of atoms in the cluster and lies between −0.6 eV and −1.7 eV, suggesting (weak) chemisorption of the clusters on graphene, rather than physisorption. Overall, the electronic properties, spin–orbit interaction and spin texture, as well as the transport properties of graphene strongly depend on the precise size of the Au n clusters. Doping of graphene is predicted for clusters with an odd number of Au atoms, due to overlap between Au s and carbon pz states close to the Fermi level. On the other hand, there is no charge transfer between even size Au clusters and graphene, but a gap is formed at the Dirac cone, due to the breaking of the pseudo spin inversion symmetry of graphene’s lattice. The adsorbed Au n clusters induce spin–orbit interactions as well as spin and pseudo spin interactions in graphene, as indicated by the splitting of the electronic band structure. A hedgehog spin texture is also predicted for adsorbed clusters with an even number of Au atoms. Ballistic transport simulations are performed to study the influence of the adsorbed clusters on graphene’s electronic transport properties. The influence of the cluster on the electron transmission across the structure depends on the mixing of the valence orbitals in the transport energy window. In the specific case of the Au3/graphene system, the adsorbed clusters reduce the transmission and the conductance of graphene. The Au3 clusters act as ‘scattering centers’ for charge carriers, in agreement with recent experimental studies.

A Computational Approach to Nontraditional Intrinsic Luminescence: Vibrationally Resolved Absorption and Fluorescence Spectra of DABCO.

Recently, so-called "nontraditional intrinsic luminescence" has been reported in several macromolecular systems. Although DABCO (1,4-diazabicyclo[2.2.2]octane) is the first system in which the effect was observed, a thorough analysis of the optical properties of the molecule, which would reveal the origin of this mysterious effect, is still pending. We perform an advanced post-Hartree-Fock treatment of the low-lying electronic states of this molecule, which need to be described with care because of their pronounced Rydberg character. We take a deeper look into the low-lying electronic transitions of DABCO targeting the explanation of the complex vibronic structures of its absorption and fluorescence spectra. Two electronic states, the 1E'(n+3pxy) and 1A2″(n+3pz) states, contribute to the absorption spectrum in the 39000-46000 cm-1 spectral range. We also reveal the spectroscopic signature of the 1A2″(n+3pz) state. The analyses of the contributions of individual vibrational normal modes allowed the identification of those giving rise to the complex vibronic structures of the spectra. Fluorescence emission arises from the vibronic coupling of the one-photon forbidden transition between the 1A1'(n+3s) state and the electronic ground state. The spectrum, which can be interpreted in terms of populating a few vibrational normal modes, is shifted toward visible wavelengths mostly due to the forced interaction of the lone pair electrons of the two nitrogen atoms. Our work on DABCO may help to rationalize the luminescence of more complex systems containing tertiary amine groups.

P‐doped g‐C3N4 as an efficient photocatalyst for CO2 conversion into value‐added materials: a joint experimental and theoretical study

AbstractThe photocatalytic yield of g‐C3N4 for CO2 reduction was modified by phosphorus doping. Possible reaction pathways for CO2 reduction on the P‐doped g‐C3N4 (PCN) surface were investigated by density function theory calculations for the first time. The experimental results showed that P doping increases the carriers' lifetime, which improves the production of CH4 through the increase in the driving force of the electrons. The partial density of states of the PCN showed that the valence band maximum and conduction band minimum are composed of px, py, and, s orbitals of the N atoms and pz states of carbon, nitrogen, and phosphorus, respectively. Mechanism studies confirm that formic acid, formaldehyde, methanol, and methane are the most probable products. Methane, having positive adsorption energy, can be easily desorbed from the PCN surface, and the Gibbs activation energy of the final step is 1.98 eV. The formation of H2COOH is the rate‐determining step.

Studies of surface states in Bi2Se3 induced by the BiSe substitution in the crystal subsurface structure

We present a systematic theoretical study of bismuth selenide electronic structure in the presence of spin-orbit interaction. It is confirmed that the inversion of Bi pz and Se pz states in the bulk band structure is induced by spin-orbit coupling. The energy gap in the bulk electronic structure is closed by pz states associated mainly with Bi atoms from the first quintuple layer. We elucidate the influence of Bi substitution in the fifth atomic layer on the local density of electronic states in the top quintuple layer. Finally, we describe how it develops into the additional p states in the surface electronic structure, which produce the triangular protrusion observed in Scanning Tunnelling Microscopy measurements and characteristic narrow features in Scanning Tunnelling Spectroscopy results.

Pressure-Induced Structural Phase Transition and Superconductivity in NaSn5

The structural and electronic properties of the tin-rich compound NaSn5 were investigated under pressures of up to 10 GPa on the basis of the evolutionary algorithm (EA) technique coupled with first-principles total energy calculations. Upon compression, the known metallic tetragonal P4̅21m phase transforms into a metallic hexagonal P6/mmm phase at 1.85 GPa accompanied by an unusual change in the existing form of Sn atoms. The P6/mmm phase can be interpreted as a quasi-layered sandwich structure with two Sn layers and one sodium layer. The presence of softening phonon modes and the existence of Fermi pockets together with the obvious Fermi surface nesting indicate a strong electron-phonon coupling (EPC) and thus potential superconductivity in the P6/mmm phase. The strong EPC in the P6/mmm phase is mainly attributed to the phonons from Sn1 atoms together with electrons from the Sn1 py and Sn1 pz states. The calculated superconducting critical temperature Tc of the P6/mmm phase is 5.91 K at 1.85 GPa. This study provides a new clue for designing intercalated compounds with superconductivity.

Tunable electronic structures, half-metallicity and optical properties in Fe-NM (NM=B, C, N) co-doped monolayer 2H–MoSe2

Exploring two-dimensional (2D) graphene-like materials with magnetic characteristics has attracted intense research interest. Here, based on spin-polarized density functional theory calculations, the tunable electronic structures, half-metallicity and optical properties can be realized in the 2D monolayer MoSe2 by Fe-NM (NM=B, C, N) codoping. The results indicate that the Fe-doping and Fe–B codoping exhibit a robust magnetic half-metallicity, which can be attributed to that the electrons partially occupy the bonding state consisting of Fe e1 and Fe a1 states at the Femi level (EF). The Fe–N codoping exhibits a magnetic semiconductor character due to the strong hybridization between Fe dxy, dz2 states and N pz state. However, the Fe–C codoping shows a non-magnetic ground state due to the effective charge compensation between Fe and C atoms. The Fe-NM codoping can effectively tune the optical properties of monolayer MoSe2 by the introducing of impurity levels near the EF. A stronger and wider absorption in the visible region of 1.8–3.2 eV is observed, which is in favor of the visible light devices and potential solar absorbers. The distinctive magnetic half-metallicity, electronic and optical properties for the Fe-NM co-doped monolayer MoSe2 have great prospects in the fields of spintronics, opto-electronics and magneto-optics devices.

First principles study of graphene on metals with the SCAN and SCAN+rVV10 functionals.

Integrating graphene into electronic devices requires support by a substrate and contact with metal electrodes. Ab initio calculations at the level of density functional theory are performed on graphene-fcc-metal(111) [Gr/M(111)] (M = Ni, Cu, Au) systems. The strongly constrained and appropriately normed (SCAN) and SCAN with the revised Vydrov-van Voorhis (SCAN+rVV10) functionals are relatively new approximations to the exchange-correlation (xc) energy shown to account for van der Waals (vdW) interactions which many non-empirical semi-local functionals fail to include. Binding energies and distances as well as electronic band structures are calculated with SCAN, SCAN+rVV10, Perdew-Burke-Ernzerhof (PBE), and PBE-D3 with and without Becke-Johnson damping, Bayesian error estimation functional with van der Waals correlation (BEEF-vdW), and optB86b-vdW. SCAN and SCAN+rVV10 succeed in describing chemisorption and physisorption in the Gr/Ni(111) system and physisorption in the Gr/Cu(111) and Gr/Au(111) systems. Incorrectly, the physisorption is found to be more favorable than chemisorption in the Gr/Ni(111) system with SCAN, but the result is reversed when the experimental bulk Ni lattice parameter is used as opposed to the SCAN calculated lattice parameter. The SCAN+rVV10 functional produces binding energies and distances comparable to those calculated using the random phase approximation as well as the experiment. The SCAN based functionals produce the highest spin magnetic moments in the bulk Ni and Gr/Ni(111) systems compared to the rest of the functionals investigated, overestimating the experiment by at least ∼0.18 μB. Also, in contrast to the rest of the functionals, the induced spin magnetic moment in graphene is found to be larger in magnitude in the physisorption region than the chemisorption region. The pristine graphene band structure is preserved in the physisorbed systems but with a shift in the Dirac point away from the Fermi energy causing graphene to become n-doped in the Gr/Cu(111) system and p-doped in the Gr/Au(111) system. Chemisorption occurs in the Gr/Ni(111) system where carbon pz states mix with the nickel d states causing a gap to form at the K point, destroying the Dirac point and conical dispersion.

Detecting electronic structure evolution of semiconductor nanocrystals by magnetic circular dichroism spectroscopy.

The evolution of electronic states of nanocrystals under shape variation is hardly detected by conventional optical and electronic instruments due to the condensed electronic levels of nanocrystals. Herein, we demonstrate that magnetic circular dichroism (MCD) spectroscopy is a high-resolution method to monitor this delicate progress on account of the sensitive Zeeman response to electronic states. In particular, the MCD intensity of the first excitonic transition exponentially decreases with the shape changing from quantum dots to quantum rods owing to the increased density of valence pz state with elongation in the z direction, which contributes much less to MCD intensity compared with p±. This work provides a simple but effective strategy for understanding the electronic state evolution in various semiconductor nanomaterials.

Theoretical Study on the Quantum Capacitance Origin of Graphene Cathodes in Lithium Ion Capacitors

Quantum capacitance (QC) is a very important character of the graphene cathode in lithium ion capacitors (LIC), which is a novel kind of electrochemical energy conversion and storage device. However, the QC electronic origin of the graphene cathode, which will affect the electrochemical reaction at the electrode/electrolyte interface, is still unclear. In this article, the QC of various kinds of graphene cathode is investigated systematically by DFT calculation. It was found that the value and origin of QC strongly depend on the defects and alien atoms of graphene. Graphene with pentagon defects possesses a higher QC than pristine graphene due to the contribution from the electronic states localized at the carbon pentagon. The introduction of graphitic B can contribute to QC, while graphitic N and P does not work in the voltage range of the LIC cathode. Single vacant defect graphene and pyrrolic N-doped graphene demonstrate very high QC due to the presence of states associated with the σ orbital of unbonded carbon atoms. However, pyridinic graphene shows an even higher QC because of the states from the N atom. For the residual O in graphene, its QC mainly originated from the pz states of carbon atoms and the effect of O, especially the O in bridged oxygen functional group (–COC–), is very limited. These results provide new insight into further study of the catalytic behavior and the design of a high performance graphene cathode for LIC.

Band Modulation of Black Phosphorus and Molybdenum Disulfide van der Waals Heterojunction: Twist and Electric Field Effects

The understanding of band modulation and related band alignment in van der Waals heterojunctions (vdW HJs) is of great importance to their electronic properties and potential applications in the field of nanoelectronic devices. In our work, we investigate the band evolution and band alignment of black phosphorus (BP) and molybdenum disulfide (MoS2) vdW HJs under the approaches of twist and applied electric field using first-principles calculations. It is found that the positions and orbital contributions of conduction band minimum (CBM) and valence band maximum (VBM) in bilayer BP/MoS2 HJs will alter as the composition twists relative to each other, while the major orbital contributions that are dz2 states of Mo atoms and pz states of P atoms do not change. When applying an electric field, the band structure of bilayer BP/MoS2 HJs can experience type-II/I/II, indirect/direct/indirect, and semiconductor/metal transitions, respectively. The underlying mechanism on the twist effect and electric field modulat...

Electronic and Magnetic Structure of Intercalated Graphene Films

Ab initio calculations of the electron spectrum of the graphene–cobalt–nickel system were performed in the slope of the spin density functional theory (SDFT). Dispersion curves Eσn(k) are presented; they were used to determine partial and total densities of valence electron states, and also magnetic moments of all atoms in the supercell. Energy position of the “Dirac cone” defined by pz states in graphene is shown to depend only slightly on the number of Co layers intercalated into the gap between the cobalt and graphene layers.

Nitrogen-rich 1T'-MoS2 layered nanostructures using alkyl amines for high catalytic performance toward hydrogen evolution.

The imminent global energy crisis and current environmental issues have stimulated considerable research on high-performance catalysts for sustainable hydrogen energy generation. Two-dimensional layered MoS2 has recently drawn worldwide attention because of its excellent catalytic properties for the hydrogen evolution reaction (HER). In the present work, we prepared nitrogen (N)-rich 1T' (distorted 1T) phase MoS2 layered nanostructures using different alkyl amines with 1-4 nitrogen atoms (methylamine, ethylenediamine, diethylenetriamine, and triethylenetetramine) as intercalants. The amine molecules intercalate at 10 atomic%, and simultaneously supply the N atoms that substitute the S atoms to produce the N-doped MoS2, whose composition is MoS2(1-x)Nx, where x = 0.1-0.26. MoS2 prepared with amines having more N atoms has enhanced catalytic HER performance: a Tafel slope of 36 mV dec-1 and 10 mA cm-2 at -160 mV (vs. RHE). First-principles calculations showed that the amine intercalation and N doping increase the density of states near the Fermi level in a narrow range and bring about an effective overlap of the dz2(Mo), pz(S), and pz(N) states. These factors in turn increase the carrier (electron) concentration and mobility for improved HER. The calculation also predicted that the most active site is S vacancies. The present work illustrates how the HER catalytic performance of 1T' phase MoS2 can be effectively controlled by the amine molecules.

Novel 3D metallic boron nitride containing only sp2 bonds

As the closest isoelectronic analogue of carbon, boron nitride (BN) shares a similar structure with carbon from 1D nanotubes, 2D nanosheets, and 3D diamond structures. However, most BN structures are insulators, which limits their application. In this work, under the inspiration of the sp2 hybridized carbon honeycomb, we propose a hexagonal phase of BN consisting of only sp2 bonds, which exhibits intriguingly intrinsic metallicity. First-principles calculations confirm that this phase is both thermally and dynamically stable. Moreover, the calculations on the band structure, partial density states and electron localization function suggest that the metallic behavior is attributable to the delocalized B-2p electrons, leading to second-neighbor interaction between the pz states of sp2-bonded B atoms in adjacent layers. Our findings not only enrich the BN allotrope family with 3D structures but also stimulate further experimental interest in applications of metallic BN in electronic devices.

Coulomb interactions and screening effects in few-layer black phosphorus: a tight-binding consideration beyond the long-wavelength limit

Coulomb interaction and its screening play an important role in many physical phenomena of materials ranging from optical properties to many-body effects including superconductivity. Here, we report on a systematic study of dielectric screening in few-layer black phosphorus (BP), a two-dimensional material with promising electronic and optical characteristics. We use a combination of a tight-binding model and rigorously determined bare Coulomb interactions, which allows us to consider relevant microscopic effects beyond the long-wavelength limit. We calculate the dielectric function of few-layer BP in the random phase approximation and show that it exhibits strongly anisotropic behavior even in the static limit. We also estimate the strength of effective local and non-local Coulomb interactions and determine their doping dependence. We find that the pz states responsible for low-energy excitations in BP provide a moderate contribution to the screening, weakening the on-site Coulomb interaction by less that a factor of two. Finally, we calculate the full plasmon spectrum of few-layer BP and discuss the effects beyond long-wavelengths.

First-principles and angle-resolved photoemission study of lithium doped metallic black phosphorous

First principles calculations demonstrate the metallization of phosphorene by means of Li doping filling the unoccupied antibonding pz states. The electron–phonon coupling in the metallic phase is strong enough to eventually lead to a superconducting phase at Tc = 17 K for LiP8 stoichiometry. Using angle-resolved photoemission spectroscopy we confirm that the surface of black phosphorus can be chemically functionalized using Li atoms which donate their 2s electron to the conduction band. The combined theoretical and experimental study demonstrates the semiconductor-metal transition indicating a feasible way to induce a superconducting phase in phosphorene and few-layer black phosphorus.

Quantum anomalous Hall effect in stable dumbbell stanene

Topological property of the dumbbell (DB) stanene, more stable than the stanene with a honeycomb lattice, is investigated by using ab initio methods. The magnetic DB stanene demonstrates an exotic quantum anomalous Hall (QAH) effect due to inversion of the Sn spin-up px,y and spin-down pz states. The QAH gap is found to be opened at Γ point rather than the usual K and K′ points, beneficial to observe the effect in experiments. When a 3% tensile strain is applied, a large nontrivial gap (∼50 meV) is achieved. Our results provide another lighthouse for realizing QAH effects in two-dimensional systems.

Impurities and electronic localization in graphene bilayers

We analyze the electronic properties of bilayer graphene with Bernal stacking and a low concentration of adatoms. Assuming that the host bilayer lies on top of a substrate, we consider the case where impurities are adsorbed only on the upper layer. We describe non-magnetic impurities as a single orbital hybridized with carbon's pz states. The effect of impurity doping on the local density of states with and without a gated electric field perpendicular to the layers is analyzed. We look for Anderson localization in the different regimes and estimate the localization length. In the biased system, the field induced gap is partially filled by strongly localized impurity states. Interestingly, the structure, distribution and localization length of these states depend on the field polarization.