Table of contents

Graphene structures of finite size are expected to reveal exceptional electronic and magnetic properties which are highly attractive for future nano-technological applications. In this study we have looked at the edge-states in graphene nanoribbons (GNR) grown by self-assembly on mesa structured SiC(0001) templates. By means of a 4-tip STM/SEM system, both local spectroscopy and lateral transport have been performed in situ on the same nanostructures. The conductance in these structures was found to be e²/h for temperatures up to 400 K. Scanning tunneling spectroscopy clearly reveals edge-localized states on these ribbons. The local bonding of these ribbons to their support turns out to be essential in order to preserve the metallicity of the edge-states.

Hematite, α-Fe₂O₃, is an attractive narrow gap oxide for consideration as an efficient visible light photocatalyst, with significant potential for band gap engineering via doping. We examine optical absorption in α-(Fe_1−xCr_x)₂O₃ epitaxial films and explain the observed excitations, and the nature of the band gap dependence on x, through first-principles calculations. The calculated and measured optical band gap becomes smaller than that of bulk α-Fe₂O₃ and reaches a minimum as the Cr cation fraction increases to 50%. The lowest energy transitions in the mixed-metal alloys involve electron excitation from occupied Cr 3d orbitals to unoccupied Fe 3d orbitals, and they result in a measurable photocurrent. The onset of α-Fe₂O₃ photoconductivity can be reduced by nearly 0.5 eV (to 1.60 eV) through addition of Cr.

A methodology for calculating the contribution of charged defects to the configurational free energy of an ionic crystal is introduced. The temperature-independent Wang–Landau Monte Carlo technique is applied to a simple model of a solid electrolyte, consisting of charged positive and negative defects on a lattice. The electrostatic energy is computed on lattices with periodic boundary conditions, and used to calculate the density of states and statistical-thermodynamic potentials of this system. The free energy as a function of defect concentration and temperature is accurately described by a regular solution model up to concentrations of 10% of defects, well beyond the range described by the ideal solution theory. The approach, supplemented by short-ranged terms in the energy, is proposed as an alternative to free energy methods that require a number of simulations to be carried out over a range of temperatures.

The Sb(111) surface was studied with helium atom scattering (HAS). Elastic HAS at different energies of the incident helium beam (15.3, 21.9, 28.4 meV) was applied for structural investigations. The lattice constants derived from the positions of the observed diffraction peaks up to third order were found to be in perfect agreement with previous structure determinations of Sb(111). The observed diffraction patterns with clear peaks up to second order were used to model the electronic surface corrugation with the GR method. As an estimation for the attractive part of the interaction potential a well depth of (4.0 ± 0.5) meV was found. Best fit results were obtained with a corrugation height of 12–13% of the lattice constant, which is rather large compared to other surfaces with metallic character. Intensity measurements of the specular peak as a function of incident energy were analysed to determine the distribution of terraces on the surface. The results show a quite flat Sb(111) surface and a step height of 3.81 Å of the remaining terraces.

Nickel has been proposed as a low-cost alternative to silver for contacting in high-performance solar cells. Nickel at a crystalline silicon surface can form a number of silicide phases, depending on fabrication conditions. Using density functional theory calculations we calculate the Schottky barrier height (SBH) at the different possible interfaces. Depending on the silicide phase, crystallographic orientation and doping the SBH at the interface with Si can range from 0.39 to 0.70 eV. These calculations demonstrate which of the nickel (silicide) phases have potential use as contacting materials for silicon based solar cells. Furthermore, we explain the origin of the SBH tuning effect of P dopant atoms as being due to a dipole formed at the interface, demonstrating the linear relationship between the charge transfer at the interface upon doping and the concomitant modulation of the SBH.

Ab initio molecular dynamics is used to study defect production and interactions from overlapping atomic recoil events in thoria. The pre-existing defects, charge redistribution, and structural distortion from an initial recoil event significantly affect the dynamics of defect production processes that occur from a subsequent overlapping recoil event. The final defect configurations and increase in system energy are dependent on the incident directions and sequence of the recoils. A linear relationship between system potential energy and charge transfer at the distance of closest approach between the recoil and atomic nuclei demonstrates the important role of charge transfer in the response of thoria to single and overlapping recoils.

Magnetic domains in ultrathin films form domain patterns, which strongly depend on the magnetic anisotropy. The magnetic anisotropy in Co/Ni multilayers changes with the number of layers. We provide a model to simulate the experimentally observed domain patterns. The model assumes a layer-dependent magnetic anisotropy. With the anisotropy parameter estimated from experimental data, we reproduce the magnetic domain patterns.

Structural transformations at the Pb/Si(111) surface occurring upon C₆₀ adsorption onto Pb/Si(111)1 × 1 phase at room temperature and Pb/Si(111)$\sqrt{7}\times \sqrt{3}$ at low temperatures between 30 and 210 K, have been studied using scanning tunneling microscopy and low-energy electron diffraction observations. Typically, C₆₀ fullerenes agglomerate into random molecular islands nucleated at the surface defects. C₆₀ island formation is accompanied by expelling Pb atoms to the surrounding surface area where more dense Pb/Si(111) phases form. Productivity of C₆₀-induced expelling of Pb atoms is controlled by surface defects and is suppressed dramatically when regular ('crystalline') C₆₀ islands self-assemble at the defect-free Pb/Si(111) surface. When Pb atoms are ejected by the random C₆₀ islands, extended structural transformations involving reordering of numerous Pb atoms are fully completed at the surface within the shortest possible time (a few dozen seconds) to reapproach and image the surface after C₆₀ deposition. Estimations show that the observed transformations cannot be controlled by random walk diffusion of Pb adatoms, which implies a highly correlated motion of the Pb atom displacements within the layer.

Scanning tunneling microscopy and x-ray photoemission spectroscopy on a polygrain icosahedral (i-) Al–Pd–Re quasicrystal (QC) show the formation of the twofold surfaces with symmetry and composition expected from the bulk. The predominant occurrence of the twofold surface on the polygrain i-QC having random grain orientation, as well as preferential formation of terrace edges, kinks and voids along the twofold axes, consistently indicates that the twofold surface, which has the highest atomic density, is the most stable among all the crystallographic planes.

Graphene under strain exhibits new fascinating properties. In this work, I show that lattice strain introduced by uniform expansion of unit cells can strongly modify the chemical properties of graphene. By employing density functional theory calculations I found that strain enhances the bonding between atomic oxygen and graphene. Strain also increases the diffusion energy barrier of atomic oxygen on graphene; however, it reduces the activation energy for oxygen migrating through the graphene sheet. Strong stability enhancement of atomic oxygen on graphene induced by strain would also change molecular oxygen dissociation reactions from endothermic to exothermic.

By solving the two-component spinor equation for massless Dirac fermions, we show that graphene under a periodic external magnetic field exhibits a unique energy spectrum. At low energies, Dirac fermions are localized inside the magnetic region with discrete Landau energy levels, while at higher energies, Dirac fermions are mainly found in non-magnetic regions with continuous energy bands originating from wavefunctions analogous to particle-in-box states of electrons. These findings offer a new methodology for the control and tuning of massless Dirac fermions in graphene.

Although sheets of layered van der Waals solids offer great opportunities to custom-design nanomaterial properties, their weak interlayer adhesion challenges structural stability against mechanical deformation. Here, bending-induced delamination of multilayer sheets is investigated by molecular dynamics simulations, using graphene as an archetypal van der Waals solid. The simulations show that delamination of a graphene sheet occurs when its radius of curvature decreases roughly below R_c = 5.3 nm × (number of layers)^3/2 and that, as a rule, one-third of the layers get delaminated. These clear results are explained by a general and transparent model, a useful future reference for guiding the design of nanostructured van der Waals solids.

We show that two nonlinear resonant cavities aligned between two parallel waveguides can support self-induced bound states in the continuum (BSCs). These BSCs are symmetrical relative to an inversion of the waveguides and to inversion of the transport axis. Due to this BSCs can drop an incident wave from one waveguide to another with very high efficiency. We show also that the frequency of the efficient channel dropping can be tuned by injecting power. All these results are in good agreement with numerical solutions of the Maxwell equations in a two-dimensional photonic crystal of GaAs rods holding two parallel waveguides and two defects made of a Kerr medium.

We present first-principles studies of the optical absorbance of the group IV honeycomb crystals graphene, silicene, germanene, and tinene. We account for many-body effects on the optical properties by using the non-local hybrid functional HSE06. The optical absorption peaks are blueshifted due to quasiparticle corrections, while the influence on the low-frequency absorbance remains unchanged and reduces to a universal value related to the Sommerfeld fine structure constant. At the Dirac points spin–orbit interaction opens fundamental band gaps; parabolic bands with a very small effective mass emerge. Consequently, the low-frequency absorbance is modified with a spin–orbit-induced transparency region and an increase of the absorbance at the fundamental absorption edge.

We report here the results of a study to understand the formation mechanism of single crystals of the transition metal chalcogenide, CuS, at the water–toluene interface through an interfacial reaction. Systematic measurements carried out using synchrotron x-ray scattering, electron microscopy, atomic force microscopy and calorimetric techniques clearly show that nano-crystallites of CuS form within a few minutes at the interface as the reagents are brought from the organic (upper) and aqueous (lower) layers to the interface, then crystallization of CuS proceeds over a few hours only by reorganization, despite the large excess available in both upper and lower liquid phases. The interface confinement and passivation by organics is critical here in the formation of single crystals having sizes of 6 and 200 nm along the normal and in-plane directions of the liquid–liquid interface.

Based on the arrangement of two-dimensional 'melon', we construct a unit cell for polymeric carbon nitride (PCN) synthesized via thermal polycondensation, whose theoretical diffraction powder pattern includes all major features measured in x-ray diffraction. With the help of this unit cell, we describe the process-temperature-induced crystallographic changes in PCN that occur within a temperature interval between 510 and 610 °C. We also discuss further potential modifications of the unit cell for PCN. It is found that both triazine- and heptazine-based g-C₃N₄ can only account for minor phases within the investigated synthesis products.

The zeolitic imidazolate framework ZIF-4 undergoes an amorphization transition at about 600 K, and then transforms at about 700 K to ZIF-zni, the densest of the crystalline ZIFs. This series of long-range structural rearrangements must give a corresponding series of changes in the local structure, but these have not previously been directly investigated. Through analysis of neutron total diffraction data by reverse Monte Carlo modelling, we assess the changes in flexibility across this series, identifying the key modes of flexibility within ZIF-4 and the amorphous phase. We show that the ZnN₄ tetrahedra remain relatively rigid, albeit less so than SiO₄ tetrahedra in silicates. However, the extra degrees of freedom afforded by the imidazolate ligand, compared to silicate networks, vary substantially between phases, with a twisting motion out of the plane of the ligand being particularly important in the amorphous phase. Our results further demonstrate the feasibility of reverse Monte Carlo simulations for studying intermolecular interactions in solids, even in cases, such as the ZIFs, where the pair distribution function is dominated by intramolecular peaks.

From first-principles calculations, we proposed a silicon germanide (SiGe) analog of silicene. This SiGe monolayer is stable and free from imaginary frequency in the phonon spectrum. The electronic band structure near the Fermi level can be characterized by Dirac cones with the Fermi velocity comparable to that of silicene. The Ge and Si atoms in SiGe monolayer exhibit different tendencies in binding with hydrogen atoms, making sublattice-selective hydrogenation and consequently electron spin-polarization possible.

We have developed empirical interatomic potentials for studying radiation defects and dislocations in tungsten. The potentials use the embedded atom method formalism and are fitted to a mixed database, containing various experimentally measured properties of tungsten and ab initio formation energies of defects, as well as ab initio interatomic forces computed for random liquid configurations. The availability of data on atomic force fields proves critical for the development of the new potentials. Several point and extended defect configurations were used to test the transferability of the potentials. The trends predicted for the Peierls barrier of the $\frac{1}{2}\langle 1 1 1\rangle$ screw dislocation are in qualitative agreement with ab initio calculations, enabling quantitative comparison of the predicted kink-pair formation energies with experimental data.

Since the discovery of ferrocene, many one-dimensional metallic sandwich molecular wires have been identified. However, most of the known systems are assembled from organic molecules. Suffering from many drawbacks has, however, hampered their widespread applications. With the goal of breaking this logjam, we provide a blueprint for the designing of a variety of novel sandwich molecular wires ([(P)₅TM]∞, TM = Ti, V, Cr, Mn, Fe, and Co) assembled from ferrocene-like inorganic molecules (P)₅TM, offering evidence of the existence of inorganic molecular wires in this class. We present first-principles calculations to investigate systematically the electronic and magnetic properties of such novel inorganic sandwich molecular wires. Compared with the organic molecular wires, all the inorganic [(P)₅TM]∞ wires are of large magnetic moment. Among them, we find that [(P)₅V]∞, [(P)₅Cr]∞ and [(P)₅Mn]∞ display ferromagnetic character, while for [(P)₅Ti]∞, [(P)₅Fe]∞ and [(P)₅Co]∞, the magnetic coupling is antiferromagnetic. More remarkably, the TM atoms distributed in these wires show regular docking and lead to structures with ordered spin signals, which is a long-term dream of spintronics. We propose that the difference in magnetic coupling for the studied systems is related to the competition between two exchange interactions of TM atoms. Specifically, we propound that the general mechanism for the formation of stable 1D [(P)₅TM]∞ involves the transfer of one electron from the TM atom to the P5 ligand forming $(\mathrm{P})_{5}^{-}$ and TM⁺ alternating structure.

We report on the far- and mid-infrared reflectivity of NdMnO₃ from 4 to 300 K. Two main features are distinguished in the infrared spectra: active phonons in agreement with expectations for the orthorhombic ${\mathrm{D}}_{2\mathrm{h}}^{1 6}$–Pbnm (Z = 4) space group remaining constant down to 4 K and a well defined collective excitation in the THz region due to e_g electrons in a d-orbital fluctuating environment. We trace its origin to the NdMnO₃ high-temperature orbital disordered intermediate phase not being totally dynamically quenched at lower temperatures. This results in minute orbital misalignments that translate into randomized non-static e_g electrons within orbitals yielding a room-temperature collective excitation. Below T_N ∼ 78 K, electrons gradually localize, inducing long-range magnetic order as the THz band condenses into two modes that emerge pinned to the A-type antiferromagnetic order. They harden simultaneously down to 4 K, obeying power laws with T_N as the critical temperature and exponents β ∼ 0.25 and β ∼ 0.53, as for a tri-critical point and Landau magnetic ordering, respectively. At 4 K they match known zone center spin wave modes. The power law dependence is concomitant with a second order transition in which spin modes modulate orbital instabilities in a magnetoelectric hybridized orbital–charge–spin–lattice scenario. We also found that phonon profiles also undergo strong changes at T_N ∼ 78 K due to magnetoelasticity.

Structural refinement, lattice micro-strain and spontaneous strain analyses have been carried out on selected members of the La(Fe_1−xRu_x)AsO system using high-resolution neutron and synchrotron powder diffraction data. The obtained results indicate that the character of the tetragonal to orthorhombic structural transition changes from first order for x = 0.10, possibly to tricritical for x = 0.20, up to second order for x = 0.30; for x ≥ 0.40 symmetry breaking is suppressed, even though a notable increase of the lattice micro-strain develops at low temperature. By combining structural findings with previous muon spin rotation data, a phase diagram of the La(Fe_1−xRu_x)AsO system has been drawn. Long-range ordered magnetism occurs within the orthorhombic phase (x ≤ 0.30), whereas short-range magnetism appears to be confined within the lattice strained region of the tetragonal phase up to x < 0.60. Direct comparison between the magnetic and structural properties indicates that the magnetic transition is always associated with structural symmetry breaking, although confined to a local scale at high Ru contents.

The in-plane longitudinal and Hall resistivities, ρ_xx and ρ_xy, of superconducting NaFe_1−xCo_xAs (NFCA) single crystals with x = 0.022 and 0.0205 in the mixed state and the normal state were measured to study the electrical transport properties in nearly optimum-doping iron-based superconductors. The resistivities under magnetic fields show thermally activated behavior and a power law magnetic field dependence of activation energy has been obtained. Due to the weak flux pinning, there is no sign reversal of Hall resistivities observed for NFCA with either x = 0.022 or 0.0205. The correlation between longitudinal and Hall resistivities shows that the scaling behavior of |ρ_xy| ∝ (ρ_xx)^β with the exponent β ≈ 2.0 is in agreement with theoretical predictions for weak-pinning superconductors. Anisotropic upper critical fields and coherence lengths with an anisotropy ratio of γ ≈ 1.63 have been deduced. Furthermore, the normal-state transport properties show that the anomalies of the linear-T resistivity, the T²-dependent cotangent of the Hall angle, the linear-T-like Hall number, and the magnetoresistance, which can be scaled by the modified Kohler rule, are analogous to those observed on optimally doped high-T_c superconducting cuprates and other pnictides. The longitudinal resistivity can be understood within a widely accepted scenario of the spin density-wave quantum critical point, while the transverse resistivity requires some further explanation. It is suggested that all the transport anomalies should be simultaneously taken into account when developing theory.

We investigate the interdependent processes of strain and diffusion in the formation of holes and atolls obtained by rapid annealing of Ge/Si(111) islands at T ≈ 970 °C. We show that the shape evolution from islands to atolls and holes is closely captured by an analytical model including strain-driven diffusion. In the model, strain profiles obtained by finite element solutions of continuum elasticity equations are introduced in the diffusion equation as the source of a diffusion flux driven by the strain gradient. When the shape of the elastic field in Ge/Si(111) islands is coupled to diffusion, the morphology of the SiGe nanostructures observed after annealing is reproduced.

Conductance fluctuations have been seen in semiconductors and graphene for quite some time. It has generally been believed that a universality existed in which the conductance variance was the same for variations in energy and magnetic field, although some experiments have questioned this view. Here, we use numerical simulations to show that fluctuations in magneto-conductance are typically smaller than those in energy by as much as a factor of 3. Moreover, the amplitude of the fluctuations in each case varies with the strength of the random potential.

Using first-principles density-functional theory calculations, we systematically investigate the magnetic anisotropy of the multilayer system Cu/(FePt)_n/MgO, a promising spintronics structure. Particularly, we have studied the influence of the epitaxial strain, thickness of the ferromagnetic layer, and different interfaces on the magnetic anisotropy energy (MAE) of the system. It is found that the thickness of FePt has slight influence on the MAE, while the increase of the in-plane lattice constant a, or tensile strain, can significantly reduce and even change the sign of the MAE. The calculated density of states shows that the occupation number of the minority spin channel of Fe d_x²−y² orbital decreases with the increase of a, which leads to the reduction of the orbital moment anisotropy of the Fe atom and therefore the decrease of MAE. We also consider the influence of the Cu/FePt and FePt/MgO interfaces on the MAE, and find that both interfaces can reduce the MAE. Especially, the effect of the Cu/FePt interface is more pronounced due to the increased occupation number of the minority spin channel of Fe d_z² orbital.

Rare-earth materials, due to their unique magnetic properties, are important for fundamental and technological applications such as advanced magnetic sensors, magnetic data storage, magnetic cooling and permanent magnets. For an understanding of the physical behaviors of these materials, first principles techniques are one of the best theoretical tools to explore the electronic structure and evaluate exchange interactions. However, first principles calculations of the crystal field splitting due to intra-site electron–electron correlations and the crystal environment in the presence of exchange splitting in rare-earth materials are rarely carried out despite the importance of these effects. Here we consider rare-earth dialuminides as model systems and show that the low temperature anomalies observed in these systems are due to the variation of both exchange and crystal field splitting leading to anomalous intra-site correlated-4f and itinerant-5d electronic states near the Fermi level. From calculations supported by experiments we uncover that HoAl₂ is unique among rare-earth dialuminides, in that it undergoes a cubic to orthorhombic distortion leading to a spin reorientation. Calculations of a much more extended family of mixed rare-earth dialuminides reveal an additional degree of complexity: the effective quadrupolar moment of the lanthanides changes sign as a function of lanthanide concentration, leading to a change in the sign of the anisotropy constant. At this point the quadrupolar interactions are effectively reduced to zero, giving rise to lattice instability and leading to new phenomena. This study shows a clear picture that accurate evaluation of the exchange, crystal field splitting and shape of the charge densities allows one to understand, predict and control the physical behaviors of rare-earth materials.