Atomic Termination Research Articles

Electronic and transport properties of zigzag phosphorene nanoribbons with nonmetallic atom terminations.

Using the first-principles method based on density-functional theory and nonequilibrium Green's function, electronic properties of zigzag phosphorene nanoribbons (ZPNRs) terminated with nonmetallic (NM) atoms such as H, C, F, N, O, S and Si, as well as a pristine case, are studied systematically. Three possible cases are considered, namely, ZPNRs with symmetrical edge terminations, asymmetrical edge terminations, and the half-bare edge case. It is shown that the pristine ZPNRs show metallic behavior. For ZPNRs terminated with C, O, S and Si atoms, they are always metals regardless of the termination cases. For ZPNR terminated with H, F, and N, the electronic structure is either a metal or a semiconductor, which depends on the termination cases. The results from the calculated edge formation energy show that the ribbons with C, F, N, O, S and Si atom edge modifications are more stable than the H-terminated ZPNR. Moreover, an applied external transverse electric field can effectively modulate the bandgaps of ZPNRs terminated with H, F and N, especially reducing the gap with the increase of the applied external transverse electric field strength. The ZPNRs terminated with N undergo a semiconductor-to-metal transition. We also investigate the electronic transport properties in nano devices consisting of the ZPNRs terminated respectively by O and S at both edges and with the fully bare edge. It is found that O and S terminated ZPNR devices have a good linear response on bias, and the current is bigger than the pristine case. The results indicate that the introduction of NM atoms at the edge(s) can effectively modulate the electronic and transport properties of ZPNRs. These novel electronic properties suggest that PNRs are a promising candidate for future nanoelectronic and optoelectronic applications.

In Situ Preparation of Pr1-xCaxMnO3 and La1-xSrxMnO3 Catalysts Surface for High-Resolution Environmental Transmission Electron Microscopy

The study of changes in the atomic structure of a catalyst under chemical reaction conditions is extremely important for understanding the mechanism of their operation. For in situ environmental transmission electron microscopy (ETEM) studies, this requires preparation of electron transparent ultrathin TEM lamella without surface damage. Here, thin films of Pr1-xCaxMnO3 (PCMO, x = 0.1, 0.33) and La1-xSrxMnO3 (LSMO, x = 0.4) perovskites are used to demonstrate a cross-section specimen preparation method, comprised of two steps. The first step is based on optimized focused ion beam cutting procedures using a photoresist protection layer, finally being removed by plasma-etching. The second step is applicable for materials susceptible to surface amorphization, where in situ recrystallization back to perovskite structure is achieved by using electron beam driven chemistry in gases. This requires reduction of residual water vapor in a TEM column. Depending on the gas environment, long crystalline facets having different atomic terminations and Mn-valence state, can be prepared.

Ultrahigh spin transport performance in Ti2CoGe based magnetic tunnel junction

Abstract The magnetic tunnel junction (MTJ) device employing inverse Heusler alloy as electrode has great potential for application in spintronics. In this work, we performed first-principles calculations combined with nonequilibrium Green’s function to study the spin transport properties of Heusler alloy based Ti2CoGe/MgO/Ti2CoGe MTJ with various atomic terminations. The calculated interface free energy (γ) indicates that TiCo-terminated interface could be easily formed than TiGe-terminated interface, and the interface containing pure Ge atom is the most favorable owing to its lowest γ. Spin transport calculations reveal that tunnel magnetoresistance (TMR) ratio in MTJ with natural TiCo terminated interface (1.58 × 104) is higher than that with natural TiGe-terminated interface (2.99 × 103). When the interfacial Ti atom in TiCo-terminated interface is substituted by Co atom, the transport ability of majority spin electrons is promoted. Moreover, our calculation indicates that the MTJ with the interface containing pure Co atom possesses the largest TMR ratio of 9.51 × 105. Our work paves the way for the fabrication of Ti2CoGe based MTJ for practical application in spintronics.

First-principles study on the structure, magnetism, and electronic properties in inverse Heusler alloy Ti2FeAl/GaAs(100) heterojunction

Abstract The interface structure stability, atomic displacement, atom-resolved magnetism, electronic properties, and spin polarization of 12 possible atomic terminations in Ti2FeAl/GaAs(100) heterojunction are comprehensively investigated by implementing the first-principles calculations. All of the lowest-energy interface structures are theoretically favorable to stability from interface formation energy examinations. The interface Fe atoms will shrink inwards, whereas the interface Ti atom will be moved outwards inversely. Therefore, an uneven interface atomic layer is observed in the heterojunction, which can lead to further spin-electronic scattering and reduction of the spin polarization. The competition among antiferromagnetic coupling between Fe and Ti atoms, d-electronic localization, and the relaxing effect of the interface metal atoms plays an important role in the atomic magnetic moment at the interface. The ideal half-metallicity observed in bulk has been destroyed by the undesirable interface states. In the TiAl-Ti-Ga-Top interface structure, the spin-polarization ratio reaches 77.2%, which suggests that the tunnel magnetoresistance value can be close to 300% at low temperature. The TiAl-Ti-Ga-Top interface structure with Ti-Ga atoms top-connected termination is a promising candidate for applications in magnetic tunnel junction devices.

Phase-field simulations on the electrocaloric properties of ferroelectric nanocylinders with the consideration of surface polarization effect

Due to the symmetry breaking on surfaces of a ferroelectric crystal, the polarizations on surfaces are different from those inside the crystal. To describe this surface polarization effect which depends on the surface atomic termination, the extrapolation length is incorporated in the phase-field method based on the time-dependent Ginzburg-Landau equation to investigate different surface polarizations and their influence on the electrocaloric properties in PbTiO3 nanocylinders. The simulation results show that the extrapolation length can alter the maximum value of the adiabatic temperature change and shift its temperature location.

Editorial overview: Food chemistry and biochemistry

Based on the first-principles calculations, the influence of different geometry models on the theoretical results about the electronic properties of LaAlO3/SrTiO3 perovskite heterostructures has been studied. The results show that the superlattice model can indicate n-type conductivity of interfaces, but it is limited to describe the insulator-metal transition of heterostructures. The sandwich and thin film models with the vacuum layer can take into account the surface termination effect on the electronic properties of the interface. The AlO2 or LaO atomic layer as the outmost surface termination is selected during constructing the model, leading to the distinct electronic properties of the heterostructures. The build-in electric filed appears in the AlO2-terminated structure, whereas it does not exist in LaO-terminated structure. Therefore, the AlO2-terminated structure can give a clear description about the insulator-metal transition of heterostructure and the charge transfer mechanism. The build-in electric filed, the atomic rumpling and surface termination effect of perovskite materials codetermine the conductivity of interfaces. The results would be helpful to construct theoretical model and understand the novel electronic properties during investigating other perovskite heterostructures.

Magnetic and electronic properties of zigzag boron nitride nanoribbons with nonmetallic atom asymmetric passivation

In this paper, we modify the magnetic and electronic properties of zigzag boron nitride nanoribbons (ZBNNRs) with asymmetric edge passivation nonmetallic (NM) atoms. The 12 possible cases of the XN-ZBNNR-BY systems (X, Y=O, S, F or P) can be divided into four types: the PN-ZBNNR-BY (Y=O, S or F), SN-ZBNNR-BY (Y=F, O or P), ON-ZBNNR-BY (Y=F, S or P) and FN-ZBNNR-BY (Y=O, P or S). The results show that all four types of the XN-ZBNNR-BY systems display the metallic properties in both channels mainly due to termination atoms O, S and P with 4, 4 and 3 p electrons, but only the ON-ZBNNR-BY (Y=F, S or P) systems display the magnetic properties. Furthermore, the total magnetic moments of the ON-ZBNNR-BY (Y=F, S or P) systems are contributed constantly by the termination O atoms and edge N atoms, regardless of termination Y atoms. Finally, the contributions to the total magnetic moments are very small for the inner N and B atoms.

Comparison of geometry models in the study of perovskite heterostructures

Based on the first-principles calculations, the influence of different geometry models on the theoretical results about the electronic properties of LaAlO3/SrTiO3 perovskite heterostructures has been studied. The results show that the superlattice model can indicate n-type conductivity of interfaces, but it is limited to describe the insulator-metal transition of heterostructures. The sandwich and thin film models with the vacuum layer can take into account the surface termination effect on the electronic properties of the interface. The AlO2 or LaO atomic layer as the outmost surface termination is selected during constructing the model, leading to the distinct electronic properties of the heterostructures. The build-in electric filed appears in the AlO2-terminated structure, whereas it does not exist in LaO-terminated structure. Therefore, the AlO2-terminated structure can give a clear description about the insulator-metal transition of heterostructure and the charge transfer mechanism. The build-in electric filed, the atomic rumpling and surface termination effect of perovskite materials codetermine the conductivity of interfaces. The results would be helpful to construct theoretical model and understand the novel electronic properties during investigating other perovskite heterostructures.

Spectral mapping of thermal transport across SiC-water interfaces

In this investigation, the thermal transport across 3C-type silicon carbide (SiC) and water interfaces was analyzed by means of nonequilibrium molecular dynamics (NEMD) simulations. To understand the details of the mechanisms involved in the transport of energy across hard-soft interfaces, spectral mapping methods were implemented. It was observed that the phonon density of states (DOS) at the interface is affected by the atomic surface termination, crystallographic plane, and the wetting conditions for both phases, indicating that different vibrational modes contribute to the interfacial heat transfer process based on the interface configuration. Low-frequency modes were found to contribute the most for the C- and Si-terminated SiC(1 0 0) planes and the C-terminated SiC(1 1 1) plane, while a noticeable contribution from high-frequency modes was observed for the Si-terminated SiC(1 1 1) plane. Out-of-plane modes significantly contributed to the heat transfer in all the analyzed surfaces, while the heat flux composition by in-plane modes was notably smaller, particularly for the SiC(1 1 1) plane. The in-plane modes lower contribution to the interfacial heat flux was related to the interfacial bonding strength and liquid structuring formed at the interface. An agreement was found between the thermal boundary conductance dependence on the DOS overlap and the interfacial liquid structure, while the interfacial bonding strength did not conclusively inform on the thermal transport behavior across these interfaces.

Structure, Magnetism, and Electronic Properties of Inverse Heusler Alloy Ti2CoAl/MgO(100) Herterojuction: The Role of Interfaces

In this study, the interface structures, atom-resolved magnetism, density of states, and spin polarization of 10 possible atomic terminations in the Ti2CoAl/MgO(100) heterojunction were comprehensively investigated using first-principle calculations. In the equilibrium interface structures, the length of the alloy–Mg bond was found to be much longer than that of the alloy–O bond because of the forceful repulsion interactions between the Heusler interface atoms and Mg atoms. The competition among d-electronic hybridization, d-electronic localization, and the moving effect of the interface metal atoms played an important role in the interface atomic magnetic moment. Unexpected interface states appeared in the half-metallic gap for all terminations. The “ideal” half-metallicity observed in the bulk had been destroyed. In TiAl–Mg and AlAl–O terminations, the maximal spin polarization of about 65% could be reserved. The tunnel magnetoresistance (TMR) value was deduced to be lower than 150% in the Ti2CoAl/MgO(100) heterojunction at low temperature.

Faceting of local droplet-etched nanoholes in AlGaAs

Nanoholes, drilled in the (001) surface of AlGaAs by local Al droplet etching, are shown to consist of faceted inner walls. The most prominent facets of the inverted pyramidlike nano-sized etch pits are the {111}A and {$1\overline{1}1$}B surfaces, which differ in their atomic surface terminations. In the [110] direction, the {111} facets change to {112} and/or {113}, which are both stepped surfaces with (111)A terraces. Etching-temperature-dependent data indicate that this facet transition seems kinetically hindered up to etch temperatures above 660 \ifmmode^\circ\else\textdegree\fi{}C, at which point the walls along $[1\overline{1}0]$ have already evolved completely towards {$1\overline{1}1$}B facets. The redeposited ring material outside the nanohole develops facets with indices of (115) and higher, thereby forming relatively flat structures. The facets and their indices are unraveled by a combination of atomic force microscopy, scanning electron microscopy, and x-ray diffraction, which is performed on an ensemble as well as on a single hole using nanodiffraction. These results imply that this nanoconfined etching process can be largely understood in a vapor-liquid-solid scheme, which includes the bulk thermodynamics in the Al-Ga-As system, the surface energies of low index facets, and their etch rates and surface terminations.

Factors limiting ferroelectric field-effect doping in complex oxide heterostructures

Ferroelectric field-effect doping has emerged as a powerful approach to manipulate the ground state of correlated oxides, opening the door to a new class of field-effect devices. However, this potential is not fully exploited so far, since the size of the field-effect doping is generally much smaller than expected. Here we study the limiting factors through magneto-transport, scanning transmission electron and piezo-response force microscopy in ferroelectric/superconductor (YBa2Cu3O7-{\delta} /BiFeO3) heterostructures, a model system showing very strong field-effects. Still, we find that they are limited in the first place by an incomplete ferroelectric switching. This can be explained by the existence of a preferential polarization direction set by the atomic terminations at the interface. More importantly, we also find that the field-effect carrier doping is accompanied by a strong modulation of the carrier mobility. Besides making quantification of field-effects via Hall measurements not straightforward, this finding suggests that ferroelectric poling produces structural changes (e.g. charged defects or structural distortions) in the correlated oxide channel. Those findings have important consequences for the understanding of ferroelectric field-effects and for the strategies to further enhance them.

Magnetic and electronic properties of zigzag boron nitride nanoribbons with nonmetallic atom terminations from first-principles

In this paper, we modify the magnetic and electronic properties of zigzag boron nitride nanoribbons (ZBNNRs) by adopting the method of edge-modification, which is achieved by passivating the edges of ZBNNRs with nonmetallic (NM) atoms H, O, S, F and P. Five different cases for XN + N-ZBNNR-B + BH, where X = H, O, S, F and P nonmetallic (NM) atoms are studied systematically. The calculated results indicate that for only X = P, the XN + N-ZBNNR-B + BH displays the half-metallic feature while the others exhibit metallic properties. The metallic feature of the XN + N-ZBNNR-B + BH (X = H, O, S, and F) systems is due to the contribution of both termination atoms and edge atoms to the total density of states at the Fermi level. The XN + N-ZBNNR-B + BH (X = H, O, S, F and P) are all magnetic materials. In addition, the half-metallic feature of the PN + N-ZBNNR-B + BH material is a new style of nano-material and a perfect candidate for spintronic applications.

Thermal Transport across SiC-Water Interfaces.

Thermal transport across interfaces made of 3C-type silicon carbide (SiC) and water was investigated by means of nonequilibrium classical molecular dynamics. The effects of different crystallographic planes and atomic surface terminations were studied, as it pertains to interfacial heat transfer. Hydrophilic and hydrophobic conditions were analyzed by modifying the interfacial bonding strength between the solid and liquid phases. The formation of structures in the liquid molecules close to the solid substrate was observed and found that such structures are sensitive to the uppermost atomic layer termination, the wettability condition, and the temperature of the system. It was found that the interfacial heat transfer and the wetting properties are not universally related and to obtain a more comprehensive description, it is required to include the structuring observed in the liquid phase at the interface. A reconciliation of the thermal boundary conductance calculations was found after the density depletion length was utilized as the descripting parameter.

DFT Studies of Single Lithium Adsorption on Coronene

Abstract Density functional theory (DFT) calculations were performed to study the adsorption of neutral and cationic forms of single lithium (Li) on representative original and each of oxygen/sulfur-terminated coronene monolayer surfaces. First, the monolayers of coronene structures were prepared. Next, Li/Li+ adsorptions were investigated on the surfaces of the already optimised coronene models. The results indicate that the singular coronene models can be considered as appropriate surfaces for Li/Li+ adsorption, with stronger Li+ adsorption. Localisations of LI/Li+ species were carefully examined at the central carbon zone of the monolayer surface; however, only one model showed discrepancy by getting localised at the monolayer edge. Energy levels and distribution patters for the molecular orbitals indicate the effects of atomic terminations and Li/Li+ adsorptions, in which the singular and Li+-adsorbed models reveal identical results. Atomic-scale nuclear quadrupole resonance (NQR) properties were also evaluated, with the results indicating that the atomic properties can determine the major electronic properties for applying the coronene structure for specific applications.

Electronic properties and carrier mobility for penta-graphene nanoribbons with nonmetallic-atom -terminations

Abstract Penta-graphene has been widely studied recently due to its superior properties for promising applications. We here theoretically investigate electronic property and carrier mobility for four kinds of penta-graphene nanoribbons (P-GNRs) terminated with nonmetallic atom (H, N, P, O, S and F) at two edges (one part of P-GNR surface). The calculations show that these ribbon possess a favorable energetic stability and thermal stability. Interestingly, three kinds of ribbons (ZZ-, ZA-, and AA-types) with these terminations have a basically similar electronic structure, only with terminations O and S, they can be semiconductors, whereas other are metals or quasi-metals. The remaining ribbon (SS-type) can maintained to be semiconductors for all types of termination situations. These behaviors are intimately related to the suppression of the edge states of edge C atoms due to the coupling effects of termination atoms. In particular, these ribbons also hold a rich carrier mobility feature from ∼101 to ∼104 cm2 V−1 s−1 at room temperature depending on termination atoms. The S atom termination can lead to a significant carrier polarity for all the ribbons, and size effects of ribbons on carrier mobility also is clearly observed as well. These findings suggests that the functionalized P-GNRs might hold promising potential applications in nanoelectronic devices.

Investigation on the Wetting Behavior of 3C-SiC Surfaces: Theory and Modeling

Silicon carbide (SiC) is a promising material for high-power and high-frequency electronics due to its wide band gap, large breakdown field, and high thermal conductivity. Several applications of micro- and nanoelectronics are found in aqueous environments; thus, it is important to understand the atomic-scale interactions between SiC and water, as these interactions govern the transport processes at solid–liquid interfaces. In an effort to characterize the solid–liquid interactions, the wetting behavior of 3C-SiC was numerically investigated. The wettability of two crystallographic planes ((100) and (111)) was characterized, allowing to have silicon or carbon terminations. It was found that the crystallographic planes as well as the atomic surface terminations play an important role in the wetting behavior of 3C-SiC. Higher hydrophilicity was observed for the Si-terminated surfaces as well as for the SiC(111) crystallographic plane. A combination of a mean-field model of wettability and an analysis of the...

Different Atomic Terminations Affect the Photocatalytic Nitrogen Fixation of Bismuth Oxybromide: A First Principles Study.

We have systematically investigated the electronic structures and activation capacities of BiOBr {001} facets with different atomic terminations by means of DFT methods. Our calculations reveal that oxygen vacancies (OVs) give a significant boost in band edges of the O-terminated BiOBr {001} facets, and excess electrons induced by OVs could exceed the reduction potentials of high-energy N2 intermediates. Interestingly, the Bi-terminated BiOBr {001} facets may be good candidates for photocatalytic nitrogen fixation due to the stronger activation ability of N2 molecules comparing with O-terminated BiOBr {001} facets with OVs. Moreover, the Bi-terminated BiOBr {001} facets may tend to yield NH3 instead of N2 H4 .

A scalable distance-1 vertex coloring algorithm for power-law graphs

We propose a distributed, unordered, label-correcting distance-1 vertex coloring algorithm, called Distributed Control (DC) coloring algorithm. DC eliminates the need for vertex-centric barriers and global synchronization for color refinement, relying only on atomic operations and local termination detection to update vertex color. We implement our DC coloring algorithm and the well-known Jones-Plassmann algorithm in the AM++ AMT runtime and compare their performance. We show that, with runtime support, the elimination of waiting time of vertex-centric barriers and investing this time for local ordering results in better execution time for power-law graphs with dense local subgraphs.

Anti-adhesive properties of nanocrystalline diamond films against Escherichia coli bacterium: Influence of surface termination and cultivation medium

Recent interest in industrial and medical usage of carbon nanomaterials arises the need for approved testing protocols for characterizing their interaction with living systems. Nanocrystalline diamond (NCD) films were chosen as a promising material for such investigation due to their unique mechanical properties, chemical stability and biocompatibility. In this study, we examined the anti-adhesive properties of differently terminated NCD films deposited by plasma-enhanced CVD on glass or silicon against Escherichia coli bacterium. We focused on the influence of cultivation medium in this process and compared bacterial adhesion to H-, O- and F-terminated NCD films in complex Luria-Bertani (LB) and mineral (M9) cultivation media that substantially differ in the content of organic molecules and thus in the potential of conditioning film formation. We found the properties of NCD films to be independent of the base substrate, as the amount of biofilm was comparable for both NCD films deposited on glass and on silicon. The anti-adhesive effect was observed only in the mineral medium where hydrogen and fluorine terminated NCD films reduced bacterial adhesion by ca. 50%. In complex medium we did not observe the reduction of bacterial adhesion. These differences are attributed to the passivation of the H- or F-terminated NCD films by organic molecules adsorbed from the complex medium. For O-terminated NCD films, no anti-adhesive effect was observed, regardless of the cultivation medium. Our results show that the growth of Escherichia coli in mineral cultivation medium can be affected by the diamond atomic termination.