Energy of Step Defects on the TiO₂ Rutile (110) Surface: An ab initio DFT Methodology

We present a novel methodology for dealing with quantum size effects (QSE) when calculating the energy per unit length and step–step interaction energy of atomic step defects on crystalline solid surfaces using atomistic slab models. We apply it to the TiO₂ rutile (110) surface using density functional theory (DFT) for which it is well-known that surface energies converge in a slow and oscillatory manner with increasing slab size. This makes it difficult to reliably calculate step energies because they are very sensitive to supercell surface energies, and yet the surface energies depend sensitively on the choice of slab chemical formula due to the dominance of QSE at computationally practical slab sizes. The commonly used method of calculating surface energies by taking the intercept of a best fit line of total supercell energies against slab size breaks down and becomes highly unreliable for such systems. Our systematic approach, which can be applied to any crystalline surface, bypasses such statistical estimation techniques and cross checks and makes robust what is otherwise a very unreliable process of extracting the energies of steps. We use the calculated step energies to predict island shapes on rutile (110) which compare favorably with published scanning tunneling microscopy (STM) images

Probing the Bonding and Electronic Structure of Single Atom Dopants in Graphene with Electron Energy Loss Spectroscopy

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and ab initio calculations reveal striking electronic structure differences between two distinct single substitutional Si defect geometries in graphene. Optimised acquisition conditions allow for exceptional signal-to-noise levels in the spectroscopic data. The near-edge fine structure can be compared with great accuracy to simulations and reveal either an sp³-like configuration for a trivalent Si or a more complicated hybridized structure for a tetravalent Si impurity

Incisive Probing of Intermolecular Interactions in Molecular Crystals: Core Level Spectroscopy Combined with Density Functional Theory

The α-form of crystalline <i>para</i>-aminobenzoic acid (PABA) has been examined as a model system for demonstrating how the core level spectroscopies X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) can be combined with CASTEP density functional theory (DFT) to provide reliable modeling of intermolecular bonding in organic molecular crystals. Through its dependence on unoccupied valence states NEXAFS is an extremely sensitive probe of variations in intermolecular bonding. Prediction of NEXAFS spectra by CASTEP, in combination with core level shifts predicted by WIEN2K, reproduced experimentally observed data very well when all significant intermolecular interactions were correctly taken into account. CASTEP-predicted NEXAFS spectra for the crystalline state were compared with those for an isolated PABA monomer to examine the impact of intermolecular interactions and local environment in the solid state. The effects of the loss of hydrogen-bonding in carboxylic acid dimers and intermolecular hydrogen bonding between amino and carboxylic acid moieties are evident, with energy shifts and intensity variations of NEXAFS features arising from the associated differences in electronic structure and bonding

Stacking Variants and Superconductivity in the Bi–O–S System

High-temperature superconductivity has a range of applications from sensors to energy distribution. Recent reports of this phenomenon in compounds containing electronically active BiS₂ layers have the potential to open a new chapter in the field of superconductivity. Here we report the identification and basic properties of two new ternary Bi–O–S compounds, Bi₂OS₂ and Bi₃O₂S₃. The former is non-superconducting; the latter likely explains the superconductivity at <i>T</i>_c = 4.5 K previously reported in “Bi₄O₄S₃”. The superconductivity of Bi₃O₂S₃ is found to be sensitive to the number of Bi₂OS₂-like stacking faults; fewer faults correlate with increases in the Meissner shielding fractions and <i>T</i>_c. Elucidation of the electronic consequences of these stacking faults may be key to the understanding of electronic conductivity and superconductivity which occurs in a nominally valence-precise compound

Stacking Variants and Superconductivity in the Bi–O–S System

High-temperature superconductivity has a range of applications from sensors to energy distribution. Recent reports of this phenomenon in compounds containing electronically active BiS₂ layers have the potential to open a new chapter in the field of superconductivity. Here we report the identification and basic properties of two new ternary Bi–O–S compounds, Bi₂OS₂ and Bi₃O₂S₃. The former is non-superconducting; the latter likely explains the superconductivity at <i>T</i>_c = 4.5 K previously reported in “Bi₄O₄S₃”. The superconductivity of Bi₃O₂S₃ is found to be sensitive to the number of Bi₂OS₂-like stacking faults; fewer faults correlate with increases in the Meissner shielding fractions and <i>T</i>_c. Elucidation of the electronic consequences of these stacking faults may be key to the understanding of electronic conductivity and superconductivity which occurs in a nominally valence-precise compound

Jérémie 17,1-4TM, oracle contre ou sur Juda propre au texte long, annoncé en 11,7-8.13 et en 15,12-14

A combination of scanning transmission electron microscopy, electron energy loss spectroscopy, and <i>ab initio</i> calculations is used to describe the electronic structure modifications incurred by free-standing graphene through two types of single-atom doping. The N <i>K</i> and C <i>K</i> electron energy loss transitions show the presence of π* bonding states, which are highly localized around the N dopant. In contrast, the B <i>K</i> transition of a single B dopant atom shows an unusual broad asymmetric peak which is the result of delocalized π* states away from the B dopant. The asymmetry of the B <i>K</i> toward higher energies is attributed to highly localized σ* antibonding states. These experimental observations are then interpreted as direct fingerprints of the expected p- and n-type behavior of graphene doped in this fashion, through careful comparison with density functional theory calculations

Local plasmon engineering in doped graphene

Single atom B or N substitutional doping in single-layer suspended graphene, realised by low energy ion implantation, is shown to induce a dampening or enhancement of the characteristic interband π plasmon of graphene through a high-resolution electron energy loss spectroscopy study in the scanning transmission electron microscope. A relative 16% decrease or 20% increase in the π plasmon quality factor is attributed to the presence of a single substitutional B or N atom dopant respectively. This modification is in both cases shown to be relatively localised, with data suggesting the plasmonic response tailoring can no longer be detected within experimental uncertainties beyond a distance of approximately 1 nm from the dopant. Ab initio calculations confirm the trends observed experimentally. Our results directly confirm the possibility of tailoring the plasmonic properties of graphene in the ultraviolet waveband, at the atomic scale, a crucial step in the quest for utilising graphene’s properties towards the development of plasmonic and optoelectronic devices operating at ultraviolet frequencies. Due to its fascinating properties, graphene is emerging as a highly promising plasmonic material for implementation in devices aimed at applications such as chemical and molecular sensing, ultrafast optical modulation, non-linear optics, photo detection, light sources and quantum optics.1-6 In the terahertz (THz) to mid-infrared (mid-IR) spectral range graphene plasmons are associated with the collective excitation of free charge carriers and exhibit a higher degree of tunability and mode confinement, as well as longer propagation distances than noble metals.1-5 The graphene “charge carrier plasmon” frequency scales as ⁄ . , where EF is the Fermi energy and D is the size of the graphene sheet.1, 4 This means that the plasmon frequency can be tuned by either varying the Fermi level (e.g. through electrostatic gating4, 5, 7, 8 or chemical doping2, 4, 5), modifying the size of the graphene sheet4 (e.g. by making micro to nanoscale graphene ribbons4, 5 or discs3, 5, 9) or a combination of both. Using one or both these “methods” to push the charge carrier plasmon frequency into the near-IR to visible spectrum is of significant current interest in the community.1, 3, 4, 10 Existing graphene-based IR plasmonic devices already exhibit promising properties, such as gate tuneable switching and control of the plasmon wavelength as well as a 40-60 times reduction in plasmon wavelength (as compared to the incident IR illumination).7, 8 While these plasmons can propagate a distance on the order of a few times their own wavelength,7, 8 on par with measurements of plasmons in Au,8, 11 this falls short of that expected for high purity graphene.7, 8 This has been attributed to disorder.12 Indeed, the graphene charge carrier plasmon mobility is expected to decrease to various degrees depending on type and concentration of dopants12, 13 and other defects,14 as well as the specific edge structures of nanoscale ribbons and similar nanoscale geometries.4 Possible strategies for realisation of graphene based plasmonics in the near-IR to visible spectrum, while taking into consideration the above effects (among others), are discussed in Ref. 4 At higher spectral frequencies, in the ultraviolet (UV) range, graphene exhibits interband plasmons resonances attributed to the collective oscillation of π and σ valence electrons.15- 17 These interband plasmons show a remarkable degree of sensitivity to various nano- to atomic-scale structures and defects in graphene: interband plasmon localisation has been attributed to confinement induced by edge states of a ~1.3 nm graphene quantum disc18 and single substitutional Si atoms have been associated with a highly localised enhancement of the interband plasmon response.19 In periodically rippled graphene (on a Ru(0001) surface) the interband π plasmon is confined to ripple “hills” while being significantly dampened in ripple “valleys”.20 Admittedly showing a more limited tunability compared to the charge carrier plasmon,4 the interband π plasmon frequency is nonetheless predicted to progressively red-shift with increasing graphene nanodisc diameter, being the most sensitive to disc diameters below 20 nm.21 However, with the exception of the above studies, reports on other aspects of the interband plasmon response of graphene are lacking in the literature. Such knowledge might open up avenues for future implementation of graphene based plasmonic and optoelectronic devices operating in the UV waveband. With this goal in mind, the present work investigates the modification of the interband plasmon response of graphene associated with two key substitutional dopants, namely boron and nitrogen atoms. The inclusion of B or N atoms in the graphene lattice is the focus of extensive study in the scientific community, with the aim to modify the electronic structure of graphene.22-29 Substitutional B and N atoms have been predicted to induce a shift of the Fermi level,26, 29, 30 resulting in p or n doping akin to that routinely exploited in current semi-conductor technology. Indeed a p and n character has recently been verified in suspended graphene containing single substitutional B and N atom dopants.29 Under certain circumstances, boron and nitrogen doping is also expected to induce a band gap in graphene.24, 25. Key to these proof-of-principle studies, electron energy loss spectroscopy (EELS) in combination with high angle annular dark field (HAADF) imaging in the aberration corrected scanning transmission electron microscope (STEM) are considered particularly useful techniques for identifying individual nano to atomic scale defects in a material and the associated effects on its electronic structure and dielectric response.31 Due to the ideal ‘gentle STEM’32 combination of ultra-high vacuum conditions and low acceleration voltage (which minimises any beam-induced damage to the samples), individual B23, 29 and N23, 29, 33, 34 atom dopants in graphene can be identified directly in an ADF image. So-called “core” EEL spectra (EEL >≈ 50 eV) contain information about the local electronic structure and bonding in graphene,28, 29, 33-35 while “valence” EEL spectra (EEL <≈ 50 eV) contain information about the graphene dielectric response18, 19, 36-41. In combination with simultaneous (STEM) ADF imaging, EEL spectra allow for a direct correlation of defect-induced modifications of the graphene electronic structure28, 29, 33-35 and dielectric response18, 19 with atomic scale structure. These capabilities mean STEM-EELS is an excellent technique for investigating the interband plasmon response induced by individual B and N atom dopants in graphene, as this information can be correlated directly with the atomic structure, all within the same experiment. In the valence loss spectrum of graphene the so-called “π peak” (~ 5 eV) is attributed primarily to the excitation of the π interband plasmon,37, 39-43 superimposed on a sum over π-π* interband transitions.37, 39-42 A recent controversy concerning the interpretation of the π peak41, 44 was addressed in subsequent theoretical studies which show that (for STEMEELS) the π loss peak is most appropriately interpreted as predominantly due to the excitation of the interband π plasmon of both doped17 and dopant free15, 16 graphene. In following with this, we use STEM-EELS to show that a single B or N substitutional atom dopant induces dampening or enhancement of the graphene interband π plasmon, respectively, with an estimated relative 16% decrease (B) or 20% increase (N) in quality factor, and, a ~1 nm localisation in both cases. This effect was observed independently using two separate STEM-EELS systems (with different yet complementary experimental parameters) and is significantly more pronounced than that previously reported for single Si atoms.19 Ab initio calculations were carried out on the largest supercells possible whilst keeping the computational costs tractable to validate the experimental results. While our theoretical spectra broadly reproduce the trends observed experimentally, it is suggested that the supercell sizes used in the present work (in practice limited by associated computational costs) are simply too small to accurately predict the relevant properties of the experimentally probed systems and faithfully reproduce all the details of the loss function.Nevertheless, our combination of state-of-the-art experimental and theoretical results demonstrates that the plasmonic properties of graphene can be tailored at the atomic scale, using an implantation technique already extensively used in semi-conductor industry.23, 29 Atomic scale plasmon engineering of graphene might prove valuable in the quest for utilising graphene’s properties towards the development of plasmonic and optoelectronic devices operating in the UV waveband. Indeed, recent reports propose utilising the π interband plasmon response associated with graphene nanopores as a sensing mechanism for DNA nucleotides.45, 4