Adam S. Foster

Magnetic properties of vacancies in graphene and single-walled carbon nanotubes

Spin-polarized density functional theory has been used to study the properties of vacancies in a graphene sheet and in single-walled carbon nanotubes (SWNTs). For graphene, we find that the vacancies are magnetic and the symmetry of the sheet is broken by the distortion of an atom next to the vacancy site. We also studied vacancies in four armchair SWNTs from (3,3) to (6,6) and six zigzag SWNTs from (5,0) to (10,0). Our calculations demonstrate that vacancies can change the electronic structure of SWNTs, converting some metallic nanotubes to semiconductors and vice versa. Metallic nanotubes with vacancies exhibit ferro- or ferrimagnetism, whereas some semiconducting nanotubes with vacancies show an antiferromagnetic order. The magnetic properties depend on chiralities of the tubes, the configuration of the vacancy and the concentration of the vacancies.

Atomically controlled substitutional boron-doping of graphene nanoribbons

Boron is a unique element in terms of electron deficiency and Lewis acidity. Incorporation of boron atoms into an aromatic carbon framework offers a wide variety of functionality. However, the intrinsic instability of organoboron compounds against moisture and oxygen has delayed the development. Here, we present boron-doped graphene nanoribbons (B-GNRs) of widths of N=7, 14 and 21 by on-surface chemical reactions with an employed organoboron precursor. The location of the boron dopant is well defined in the centre of the B-GNR, corresponding to 4.8 atom%, as programmed. The chemical reactivity of B-GNRs is probed by the adsorption of nitric oxide (NO), which is most effectively trapped by the boron sites, demonstrating the Lewis acid character. Structural properties and the chemical nature of the NO-reacted B-GNR are determined by a combination of scanning tunnelling microscopy, high-resolution atomic force microscopy with a CO tip, and density functional and classical computations.

Recent Trends in Surface Characterization and Chemistry with High‐Resolution Scanning Force Methods

The current status and future prospects of non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM) for studying insulating surfaces and thin insulating films in high resolution are discussed. The rapid development of these techniques and their use in combination with other scanning probe microscopy methods over the last few years has made them increasingly relevant for studying, controlling, and functionalizing the surfaces of many key materials. After introducing the instruments and the basic terminology associated with them, state-of-the-art experimental and theoretical studies of insulating surfaces and thin films are discussed, with specific focus on defects, atomic and molecular adsorbates, doping, and metallic nanoclusters. The latest achievements in atomic site-specific force spectroscopy and the identification of defects by crystal doping, work function, and surface charge imaging are reviewed and recent progress being made in high-resolution imaging in air and liquids is detailed. Finally, some of the key challenges for the future development of the considered fields are identified.

The role of nitrogen-related defects in high-k dielectric oxides: Density-functional studies

Using ab initio density-functional total energy and molecular-dynamics simulations, we study the effects of various forms of nitrogen postdeposition anneal (PDA) on the electric properties of hafnia in the context of its application as a gate dielectric in field-effect transistors. We consider the atomic structure and energetics of nitrogen-containing defects which can be formed during PDA in various N-based ambients: N2, N2+, N, NH3, NO, and N2O. We analyze the role of such defects in fixed charge accumulation, electron trapping, and in the growth of the interface SiO2 layer. We find that nitrogen anneal of the oxides leads to an effective immobilization of native defects such as oxygen vacancies and interstitial oxygen ions, which may inhibit the growth of a silica layer. However, nitrogen in any form is unlikely to significantly reduce the fixed charge in the dielectric.

Towards an accurate description of the capillary force in nanoparticle-surface interactions

We present a method to numerically calculate the exact (non-circular) meniscus profile from the Kelvin equation, and compare the results of the obtained capillary force with different previous approximations and experiments. We show that a circular meniscus profile gives correct results in most cases. We also compare different models of pull-off behaviour and show that the often used approximation of humidity independent capillary force is viable for spherical particles above 1 µm radius, but below that there is a strong humidity dependence, as seen in experiments. At the same length scale the direct surface tension force component becomes important. We also discuss the vanishing of the capillary force at very low humidity, the effect of small initial separation between the particle and the surface and the effects of different particle shapes and contact angles on the capillary force. Finally, calculated results are compared with experimental measurements of the capillary force. (Some figures in this article are in colour only in the electronic version)

An atomistic introduction to anisotropic etching

This review-oriented paper presents a simplified model of anisotropic etching of crystalline silicon for the three principal orientations (1 1 1), (1 1 0) and (1 0 0), including their vicinal surfaces. The model combines pit nucleation and step flow with micromasking and diffusion phenomena in order to explain the major morphologic features and their changes with concentration. It also qualitatively explains the orientation and concentration dependence of the etch rate. We conclude that the shallow round pits on (1 0 0) and the elongated zigzag structures on (1 1 0), each of which constitutes the basic morphology of the corresponding surface, are actually the result of the same physical phenomenon, diffusion, disguised by a different underlying symmetry. It is also shown that the formation of hillocks on the two surfaces at different concentrations is a related process. We also describe and support the idea that the rotation of the triangular pits on (1 1 1) is due to a selective blocking mechanism by the etchant cations and explain how the formation of polygonal steps and/or step bunches on miscut (1 1 1) surfaces can occur as a result of diffusion phenomena and not only due to micromasking. Finally, the particular features of Cu as a micromasking agent are explained.

Chemical identification of point defects and adsorbates on a metal oxide surface by atomic force microscopy

Atomic force microscopy in the non-contact mode (nc-AFM) can provide atom-resolved images of the surface of, in principle, any material independent of its conductivity. Due to the complex mechanisms involved in the contrast formation in nc-AFM imaging, it is, however, far from trivial to identify individual surface atoms or adsorbates from AFM images. In this work, we successfully demonstrate how to extract detailed information about defects and the chemical identity of adsorbates on a metal oxide surface from nc-AFM images. We make use of the observation that the apex of the AFM tip can be altered to expose either a positive or negative tip termination. The complementary set of images recorded with the two tip terminations unambiguously define the ionic sub-lattices and reveal the exact positions of oxygen vacancies and hydroxyl (OH) defects on a TiO(2) surface. Chemical specificity is extracted by comparing the characteristic contrast patterns of the defects with results from comprehensive AFM simulations. Our methodology of analysis is generally applicable and may be pivotal for uncovering surface defects and adsorbates on other transition metal oxides designed for heterogeneous catalysis, photo-electrolysis or biocompatibility.

Reactions and clustering of water with silica surface

The interaction between silica surface and water is an important topic in geophysics and materials science, yet little is known about the reaction process. In this study we use first-principles molecular dynamics to simulate the hydrolysis process of silica surface using large cluster models. We find that a single water molecule is stable near the surface but can easily dissociate at three-coordinated silicon atom defect sites in the presence of other water molecules. These extra molecules provide a mechanism for hydrogen transfer from the original water molecule, hence catalyzing the reaction. The two-coordinated silicon atom is inert to the water molecule, and water clusters up to pentamer could be stably adsorbed at this site at room temperature.

Contrast formation in atomic resolution scanning force microscopy on CaF2(111): experiment and theory

We investigate mechanisms of contrast formation in atomic resolution imaging of flat terraces on the CaF2(111) surface with scanning force microscopy operated in the dynamic mode. Experimental results are interpreted with a theory based on atomistic modelling. Experiments reveal characteristic contrast features in the form of triangles that can be explained by theory as being due to the interaction of a positively terminated tip with fluorine ions from two different sublattices. Results for a tip with negative termination are found not to be compatible with experiments. We demonstrate that theory correctly predicts the trend in contrast changes when varying the tip-surface distance but is also limited in quantitative agreement due to the non-ideal atomic structure of real tips. In a distance range where such peculiarities do not play a major role, however, we find good quantitative agreement between theoretical predictions and experimental results. The validity of the comparison between theory and experimental scan lines is discussed in detail using an extensive statistical image analysis.

Topography and work function measurements of thin MgO(001) films on Ag(001) by nc-AFM and KPFM

The surface topography and local surface work function of ultrathin MgO(001) films on Ag(001) have been studied by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). First principles calculations have been used to explain the contrast formation of nc-AFM images. In agreement with literature, thin MgO films grow in islands with a quasi rectangular shape. Contrary to alkali halide films supported on metal surfaces, where the island heights can be correctly measured, small MgO islands are either imaged as depressions or elevations depending on the electrostatic potential of the tip apex. Correct island heights therefore cannot be given without knowing the precise contrast formation discussed in this paper. KPFM shows a silver work function which is reduced by the MgO islands. The values for the work function differences for one and two layer thin films are -1.1 and -1.4 eV, respectively, in good agreement with recent calculations and experiments.