Atomic Termination Research Articles

Atomistic Insights into FeF3 Nanosheet: An Ultrahigh-Rate and Long-Life Cathode Material for Li-Ion Batteries.

Iron fluoride with high operating voltage and theoretical energy density has been proposed as a high-performance cathode material for Li-ion batteries. However, the inertness of pristine bulk FeF3 results in poor Li kinetics and cycling life. Developing nanosheet-based electrode materials is a feasible strategy to solve these problems. Herein, on the basis of first-principles calculations, first the stability of FeF3 (012) nanosheet with different atomic terminations under different environmental conditions was systematically studied, then the Li-ion adsorption and diffusion kinetics were thoroughly probed, and finally the voltages for different Li concentrations were given. We found that F-terminated nanosheet is energetically favorable in a wide range of chemical potential, which provide a vehicle for lithium ion diffusion. Our Li-ion adsorption and diffusion kinetics study revealed that (1) the formation of Li dimer is the most preferred, (2) the Li diffusion energy barrier of Li dimer is lower than isolated Li atom (0.17 eV for Li dimer vs 0.22 eV for Li atom), and (3) the diffusion coefficient of Li is 1.06 × 10-6 cm2·s-1, which is orders of magnitude greater than that of Li diffusion in bulk FeF3 (10-13-10-11 cm2·s-1). Thus, FeF3 nanosheet can act as an ultrahigh-rate cathode material for Li-ion batteries. More importantly, the calculated voltage and specific capacity of Li on the FeF3 (012) nanosheet demonstrate that it has a much more stable voltage profile than bulk FeF3 for a wide range of Li concentration. So, few layers FeF3 nanosheet provides the desired long-life energy density in Li-ion batteries. These above findings in the current study shed new light on the design of ultrahigh-rate and long-life FeF3 cathode material for Li-ion batteries.

Edge chemistry controlling effects on electronic structure, carrier mobility and device properties for phagraphene nanoribbons

A new carbon-based mono-layer atomic crystal, phagraphene, was proposed recently, which has received increasing attention. We here consider variations of edge chemistry for a phagraphene nanoribbon, and also explore to expand its functional properties, some typical nonmetallic atoms are used as ribbon-edge terminations. With these edge chemistry, the bandgap of ribbons may be nearly unchanged, increased, or decreased as compared with the bare-edge ribbon, and even a quasi-metal or metal arises, presenting rich and flexibly tunable electronic structures. For these features, new emerging hybridized state subbands upon edge terminations between two intrinsic subbands play an important role. In particular, by several types of termination atoms, ribbons exhibit significantly enhanced carrier mobility, and diverse edge terminations can effectively control the carrier mobility to a difference of three orders of magnitude, which can be used to explain the fact that the various measured mobility is quite different for the same nanomaterial when synthesized by different ways. Furthermore, the introduction of non-hydrogenation terminations can substantially improve the electronic transport features of ribbon, such as a appearance of impressive negative differential resistance phenomena. These studies suggest that resulting structures presented here might possess promising applications in future electronics.

Scalable faceted voids with luminescent enhanced edges in WS2 monolayers.

A scalable approach is needed in the formation of atomically flat edges with specific terminations to enhance local properties for optoelectronic, nanophotonic and energy applications. We demonstrate point defect clustering-driven faceted void formations with luminescent enhanced edges in WS2 monolayers during large-scale CVD growth and controlled annealing. With the aid of aberration-corrected scanning transmission electron microscopy (AC-STEM) high angle annular dark field (HAADF) imaging, we probed atomic terminations of S and W to explain observed luminescence enhancement in alternate edges. Faceted void formation in monolayer WS2 was found to be sensitive to annealing temperature, time, gas environment and precursor supply. Our observations of areal coverage evolution over time revealed competition between monolayer WS2 growth and void formation at 850 °C. While the initial stage was dominated by monolayer growth, defect generation and void growth dominated at later stages and provided an optimum processing window for monolayer WS2 as well as faceted void growth. Growth of faceted voids not only followed the geometry of monolayer facets but also showed similar atomic terminations at the edges and thus enabled local manipulation of photoluminescence enhancement with an order of magnitude increase in intensity. The developed CVD processing enabled multi-fold increase in the luminescent active edge length through the formation of faceted voids within the WS2 monolayer.

Raman spectroscopy in black phosphorus

In this article, we review the current status of Raman spectroscopy in orthorhombic black phosphorus (BP) (for simplicity, henceforth, referred to as BP). BP is a layered semiconductor crystal that recently regained interest because it can be exfoliated down to the single layer, thus exhibiting 2‐D properties. First, we briefly review the crystalline structure and the phonon dispersion relations in BP. Then, the symmetries of the Raman‐active modes are discussed in the light of group theory, and the scattering configurations for observing the different phonon modes are presented. Polarized Raman spectroscopy results are discussed and reveal unusual angular dependence features, which can be ascribed to the linear dichroism and to the complex nature of the electron–phonon matrix elements. Edge phonon effects originated from rearrangements of the atomic terminations are also discussed. Subsequently, Raman modes that emerge from interlayer interaction and that are only visible in the few‐layer regime are presented and discussed. Finally, we outline new perspectives to BP Raman spectroscopy in directions that remain partially or totally unexplored and that can provide potentially important outcomes to problems such as defects, oxidation, doping, strain, stacking order and other BP‐like 2‐D materials. Copyright © 2017 John Wiley & Sons, Ltd.

Oxygen Partial Pressure during Pulsed Laser Deposition: Deterministic Role on Thermodynamic Stability of Atomic Termination Sequence at SrRuO3/BaTiO3 Interface.

With recent trends on miniaturizing oxide-based devices, the need for atomic-scale control of surface/interface structures by pulsed laser deposition (PLD) has increased. In particular, realizing uniform atomic termination at the surface/interface is highly desirable. However, a lack of understanding on the surface formation mechanism in PLD has limited a deliberate control of surface/interface atomic stacking sequences. Here, taking the prototypical SrRuO3/BaTiO3/SrRuO3 (SRO/BTO/SRO) heterostructure as a model system, we investigated the formation of different interfacial termination sequences (BaO-RuO2 or TiO2-SrO) with oxygen partial pressure (PO2) during PLD. We found that a uniform SrO-TiO2 termination sequence at the SRO/BTO interface can be achieved by lowering the PO2 to 5 mTorr, regardless of the total background gas pressure (Ptotal), growth mode, or growth rate. Our results indicate that the thermodynamic stability of the BTO surface at the low-energy kinetics stage of PLD can play an important role in surface/interface termination formation. This work paves the way for realizing termination engineering in functional oxide heterostructures.

Works of separation for (0 0 0 1)ZnO|(1 1 1)ZrO2 interfaces: A first-principle study

In-depth understanding of interfacial adhesive properties is required to access the long-term performance of substrate-film structures. The ZnO|ZrO2 (or ZnO|YSZ) interfaces are of widely interests in experiments, however, the study on their adhesive strength is rare. Works of separation for (0001)ZnO|(111)ZrO2 interfaces have been investigated by using density functional calculations. Four coherent interface configurations were calculated to clarify the influence of atom termination and stacking sequence on the interfacial strength. The coherent interface model indicates that the small values for the work of separation are attributed to the large lattice mismatch between two slabs. Among the four configurations, interface model with ZrZrO2-OZrO2⋯OZnOB-ZnZnOβ stacking sequence has the largest work of separation, followed by the model with ZrZrO2-OZrO2⋯ZnZnOβ-OZnOB stacking sequence. We also construct “semicoherent interface” models to investigate the effect of strain. The ZrZrO2-OZrO2⋯OZnOB-ZnZnOβ sequence brings in some direct electrostatically repulsive anion-anion interactions across the interface, which causes significant distortion for the interface, especially on ZnO slab. Based on the “semicoherent interface” model with ZrZrO2-OZrO2⋯ZnZnOβ-OZnOB sequence, we elucidate that the interfacial adhesive strength for (0001)ZnO|(111)ZrO2 interface is very low.

Topological electronic effects in exfoliated thin films of bismuth telluride

AbstractAn important aspect of band gap engineering in 2D thin films of topological insulators is determining their surface electronic properties at various atomic terminations. In this work, the electronic structure evolution of thin films of bismuth telluride () with varying surface termination is discussed based on density functional theory. The reported electronic effects include closure of the hybridization energy band gap and distinct spatial charge localization of topological surface states on two types of defected surfaces –Te and Bi‐terminated. The observed band gap value changes periodically every five monolayers following the quintuple‐based layered structure. The studied system shares, to a large extent, the main features with thin films of bismuth selenide (), although with a stronger size effect.

Impact of Atomic-Scale Contact Geometry on Andreev Reflection.

Charge transport has been examined in junctions comprising the normal-metal tip of a low-temperature scanning tunneling microscope, the surface of a conventional superconductor, and adsorbed C_{60} molecules. The Bardeen-Cooper-Schrieffer energy gap gradually evolves into a zero-bias peak with decreasing electrode separation. The peak is assigned to the spectroscopic signature of Andreev reflection. The conductance due to Andreev reflection is determined by the atomic termination of the tip apex and the molecular adsorption orientation. Transport calculations unveil the finite temperature and the strong molecule-electrode hybridization as the origin to the surprisingly good agreement between spectroscopic data and the Blonder-Tinkham-Klapwijk model that was conceived for macroscopic point contacts.

First-principles investigations of electronic and magnetic properties of the FeRh/MgO (001) interface

Ab initio calculations of the electronic and magnetic properties of FeRh/MgO (001) interface are reported, focusing on the effects of different atomic terminations. It was found that the Fe-O termination is energetically the most stable. The change in the electronic properties at the interface is analyzed using the spin-polarized layer projected density of states, which shows that the spin polarization at the Fermi level is dominated by the spin-down t2g states. However, the magnetic moment of the interfacial Fe atom is not enhanced significantly compared to that of the inner layers. The difference of total energy between antiparallel (AP) and parallel (P) alignments of the magnetizations of the FeRh electrodes as function of MgO thickness shows that the AP configuration is the most stable and this interlayer exchange coupling decreases exponentially as predicted by the free-electron theory. Finally, Löwdin analysis of the electron density shows that the charge is transferred from the interface MgO layer to that of FeRh and confirm the change of the interface states at the vicinity of the Fermi level.

Electronic and transport features of zigzag boron nitride nanoribbons with nonmetallic atom terminations

Electronic properties of zigzag boron nitride nanoribbons (ZBNNRs) terminated with nonmetallic (NM) atoms H, O, S, P and F are studied systematically. Several possible cases are considered, namely, ZBNNRs with symmetrical edge terminations, asymmetrical edge terminations, and the half-bare edge case. The calculated results show that the ribbons with O, S, P, and F atom edge modifications are more stable than the H-terminated ZBNNR. For ZBNNRs terminated with O and S atoms, they are always metals regardless of the termination cases. While ZBNNR terminated with F, the electronic nature is basically similar to that for H passivated ZBNNR, either a metal or a semiconductor depends on the termination cases. More interestingly, for the ZBNNR terminated with P atom regardless of the termination cases, it exhibits a particular massless Dirac-fermion behavior, i.e., its charge carriers mimic the relativistic quasiparticles with zero rest mass and travel with an effective “speed of light”. In addition, we also investigate the electronic transport properties by constructing the nanodevices consisting of ZBNNRs terminated respectively by S and P at both edges and being fully bare edge. It is found that the IV characteristics are significantly enhanced due to terminations. Our findings indicate that the introduction of NM atoms at edge(s) can effectively modulate the electronic and the transport properties of ZBNNRs, even a particular nature could be achieved.

Oxide Single-Crystal Surfaces: A Playground for Self-assembled Oxide Nanostructures

The different (structural and chemical) properties of oxide single-crystal surfaces that can be exploited for the growth of self-assembled oxide nanostructures are briefly reviewed. A large variety of nanostructures can be obtained, controlled by surface and interface structure and chemistry, which play a predominant role in their formation mechanisms at this nanometer scale. It is reminded that surface atomic order, surface steps, chemical terminations or heteroepitaxial strain can be used to generate various nanostructures such as nanodots, nanowires, nanostripes, with controlled size, morphology and spatial ordering.

Influence of atom termination and stacking sequence on the θ′/Al interfaces from first-principles calculations

First-principles calculations was used to explore the influence of atom termination and stacking sequence on the interface strength and stability between θ′(Al2Cu) precipitate and Al matrix along experimentally observed orientations, (001)θ′/(001)Al and (010)θ′/(010)Al interfaces. Six interfacial structures were modeled, and work of adhesion, bonding characters, number of valence electrons and thermal stability had been studied. Calculated results revealed that the Cu-terminated interface has larger work of adhesion than Al-terminated interface, and hollow site stacking sequence, with stronger bonding, is superior to top site stacking sequence, adhesion strength for coherent (001)θ′/(001)Al interface is better than that for semi-coherent (010)θ′/(010)Al interface. These differences are attributed to the bonding feature and number of valence electrons. Among the six interface models, the Cu-terminated (001)θ′/(001)Al interface with hollow site stacking has the largest work of adhesion and the smallest interface energy, indicating that it has the best mechanical and thermodynamic properties.

Thermodynamic stability, magnetism and half metallicity of Mn2CoAl/GaAs(0 0 1) interface

Interface properties of the heterojunction which is composed of the inverse Heusler alloy MnCoAl and semiconductor GaAs are investigated by employing the first-principles density functional simulations. Two kinds of interface structures, namely the top-type and bridge-type structure by connecting termination of nine MnCoAl layers to the top of the As-terminated GaAs layer and bridge site between interface As atoms are respectively built. Our calculations reveal that, as for the structure with the same interface atoms, different atoms sitting directly on top of the interface As atom will lead to different interface magnetism and electronic structures. The calculated phase diagram reveals that the top-type structure including natural MnCo or MnAl termination is stable only when the interface Mn or interface Al atom directly locates on top of the As atom. Besides, bridge-type and top-type structures containing a pure Mn interface are always thermodynamically accessible regardless of values of the chemical potential of Mn and Co. The atom-resolved spin magnetic moments of most interface magnetic atoms are enhanced due to the rehybridization caused by symmetry breaking at the interface. Further analyses on electronic structures indicate that, owing to the interface effect, the interface half metallicity of all structures are completely destroyed. However, the top-type structure with MnAl termination where the interface Al atom directly sits on top of the As atom preserves the highest interface spin polarization of 80%, indicating that it has more advantages in spintronics application than other atomic terminations.

Structural stability, half-metallicity and magnetism of the CoFeMnSi/GaAs(0 0 1) interface

The ferromagnet/semiconductor interface plays a crucial role in the performance of advanced magnetic tunnel junctions (MTJs) built of ferromagnetic electrodes and semiconductor as a spacer. We investigate the interface character between LiMgPbSb-type Heusler alloy CoFeMnSi and semiconductor GaAs by using the first-principles density functional simulations. In our calculations, we build two kinds of interface structures, namely the top-type and the bridge-type structure by connecting the termination of nine CoFeMnSi layers to the top of the As-terminated GaAs layer and the bridge site between interface As atoms, respectively. The calculated phase diagram indicated that the CoFe-terminated interface is more stable in the bridge-type structure than in the top-type structure, and a favored MnMn- or MnSi-terminated interface will appear in the top-type structure instead of the bridge-type structure under Fe-rich conditions. Besides, our calculation reveals that interface Mn and interface Fe atoms prefer to extend outward and their atom-resolved spin magnetic moments are enhanced due to the rehybridization caused by the symmetry breaking at the interface, while interface Co atoms shrink inward and their moments are decreased compared with the bulk value. Further analysis on DOS and PDOS indicates that owing to the interface effect, the half metallicity of CoFe-, MnSi-, and SiSi-terminated interfaces is completely destroyed. However, the MnMn-terminated interface in the top-type structure preserves 100% spin polarization, indicating that the CoFeMnSi/GaAs heterostructure with the top-type MnMn-terminated interface has more advantages than other atomic terminations in spintronics applications.

Fully spin-polarized two-dimensional electron gas at the <span class="aps-inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mrow><mi mathvariant="normal">CoFe</mi></mrow><mn>2</mn></msub><msub><mrow><mi mathvariant="normal">O</mi></mrow><mn>4</mn></msub><mo>/</mo><msub><mrow><mi mathvariant="normal">MgAl</mi></mrow><mn>2</mn></msub><msub><mrow><mi mathvariant="normal">O</mi></mrow><mn>4</mn></msub></math></span>(001) polar interface

We performed first-principles calculations to show that a fully spin-polarized two-dimensional electron gas can be created at the interface between the polar and insulating spinel oxides ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{MgAl}}_{2}{\mathrm{O}}_{4}$. We give a clear description of the physical parameters (in particular the atomic termination of the interfaces), which favor the formation of this electron gas that is due either to an electric field induced in stoichiometric oxide layers because of their polar character or to a charge reorganization that preserves the global electric neutrality in nonstoichiometric layers. We show that the electric field-induced spin-polarized two-dimensional electron gas can only exist if the thickness of the ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ layer is large enough and that it may be destroyed by intermixing at the interfaces.

Electronic and Quantum Transport Properties of Atomically Identified Si Point Defects in Graphene.

We report high-resolution scanning transmission electron microscopy images displaying a range of inclusions of isolated silicon atoms at the edges and inner zones of graphene layers. Whereas the incorporation of Si atoms to a graphene armchair edge involves no reconstruction of the neighboring carbon atoms, the inclusion of a Si atom to a zigzag graphene edge entails the formation of five-membered carbon rings. In all the observed atomic edge terminations, a Si atom is found bridging two C atoms in a 2-fold coordinated configuration. The atomic-scale observations are underpinned by first-principles calculations of the electronic and quantum transport properties of the structural anomalies. Experimental estimations of Si-doped graphene band gaps realized by means of transport measurements may be affected by a low doping rate of 2-fold coordinated Si atoms at the graphene edges, and 4-fold coordinated at inner zones due to the apparition of mobility gaps.

A first-principles study on interfacial properties of Ni(001)/Ni3Nb(001)

The Ni (001) surface, Ni3Nb (001) surface and Ni (001)/Ni3Nb (001) interfaces were studied using the first-principles pseudopotential plane-wave method. The adhesion work, thermal stability and electronic structure of Ni/Ni3Nb (001) interfaces were calculated to expound the influence of atom termination and stacking sequence on the interface strength and stability. Simulated results indicate that Ni and Ni3Nb (001) surface models with more than eight atomic layers exhibit bulk-like interior. The (Ni+Nb)-terminated interface with hollow site stacking has the largest cohesive strength and critical stress for crack propagation and the best thermal stability among the four models. This interfacial Ni and the first nearest neighbor Nb atoms form covalent bonds across the interface region, which are mainly contributed by Nb 4d and Ni 3d valence electrons. By comparison, the thermal stability of Ni/Ni3Nb (001) interfaces is worse than Ni/Ni3Al (001) interface, implying that the former is harder to form. But the Ni/Ni3Nb interface can improve the mechanical properties of Ni-based superalloys.

Tunable atomic termination in nano-necklace BiFeO3

We investigate the growth kinetics of BiFeO3 (BFO) nano-necklace based on first principles modeling and electrospinning experiments. A two-step layered growth mechanism is proposed to control the atomic termination of each nano-necklace. X-ray diffraction (XRD) results agree well with our model predictions; (1) samples calcined at 500 and 550 °C consist of FeO2 (F-F) terminated nano-necklace, (2) samples calcined at 650 °C consist of either BiO (B-B) or BiO and FeO2 (B-F) terminated ones. The XRD diffraction peaks are located at lower 2θ values with a magnetic moment of 3.0 emu/g for the former and at higher 2θ values with a decrease in the magnetic moment to 0.4 emu/g for the later samples. The activation energy associated with the growth of the nano-necklaces was experimentally determined to be 0.37 eV/atom. This study provides fundamental understanding for the long-sought layer-by-layer assembly of BFO nanostructures.

Ab initio study of the magnetoelectric effect and critical thickness for ferroelectricity in Co2FeSi/BaTiO3 multiferroic tunnel junctions

The interfacial magnetoelectric effect and the critical thickness for the ferroelectric in half-metallic ferromagnet/ferroelectric Co2FeSi/BaTiO3 multiferroic tunnel junctions are investigated using a first-principles density-functional theory. The magnetoelectric effect is derived from the difference of the magnetic moments on interfacial atoms, which are sensitive to the polarization of ferroelectric layers in multiferroic tunnel junctions. Besides this, the coupling is found to be dependent on the interfacial electronic hybridizations and to be affected by the choice of the atomic termination. In addition, the magnetoelectric coefficients calculated for the FeSi/TiO2 and Co2/TiO2 interfaces of the junctions are different from each other. Meanwhile, the critical thickness of ferroelectric state was found to be about 1.7 nm in the Co2/TiO2 interface but vanished in the FeSi/TiO2 interface. The half-metallic ferromagnetic/ferroelectric tunnel junctions may open a promising avenue for designing novel electronic and spintronic devices at nanoscale.

Influence of atomic termination on the LaAlO3/SrTiO3interfacial polar rearrangement

We report the results of second-harmonic optical spectroscopy of LaAlO${}_{3}$ films grown on SrTiO${}_{3}$ (001) substrates having either TiO${}_{2}$ or SrO atomic terminations. The atomic termination is found to have a large effect on the observed spectra, with a strong dependence on the $\ensuremath{\omega}$-$2\ensuremath{\omega}$ light polarizations. These results are analyzed through a model based on symmetry-controlled selection rules and atomic orbital overlaps. The analysis shows that, as suspected from transport measurements, the SrO termination prevents the formation of a mobile interfacial electron gas. Yet, by explicitly probing and identifying the electronic states at the O${}^{2\ensuremath{-}}$--Ti${}^{3+}$ band gap we now obtain unique and direct evidence that this is caused by the absence of any detectable charge injection in the interfacial region rather than by carrier localization.