RESUMEN
It has recently been demonstrated that N-heterocyclic carbenes (NHCs) form self-assembled monolayers (SAMs) on metal surfaces. Consequently, it is important to both characterize and understand their binding modes to fully exploit NHCs in functional surface systems. To assist with this effort, we have performed first-principles total energy calculations for NHCs on Au(111) and simulations of X-ray absorption near edge structure (XANES). The NHCs we have considered are N,N-dimethyl-, N,N-diethyl-, N,N-diisopropylbenzimidazolylidene (BNHCX, with X = Me, Et, and iPr, respectively) and the bis-BNHCX-Au complexes derived from these molecules. We present a comprehensive analysis of the energetic stability of both the BNHCX and the complexes on Au(111) and, for the former, examine the role of the wing group in determining the attachment geometry. Further structural characterization is performed by calculating the nitrogen K-edge X-ray absorption spectra. Our simulated XANES results give insight into (i) the relationship between the BNHCX/Au geometry and the N(1s) â π*/σ*, pre-edge/near-edge, absorption intensities, and (ii) the contributions of the molecular deformation and molecule-surface electronic interaction to the XANES spectrum. These simulated spectra work not only as a map to the BNHCX conformation, but also, combined with electronic structure calculations, provide a clear understanding of recent experimental XANES findings on BNHCX/Au.
RESUMEN
The B/Si(1 1 1)- (â3 x â3)R30° surface reconstruction has recently been used as a platform for supramolecular assembly. However, our understanding of the native defects in this delta-doped system and their corresponding scanning tunnelling microscopy (STM) signatures is incomplete. So we have studied this system using ab initio total energy calculations and scanning tunneling microscopy. We find that although perturbations to the equilibrium geometry of the surface are in general weak, the perturbations to the electronic structure can be quite strong due to the presence of dangling bonds composed of Si-3p(z) orbitals. Additionally, we propose a possible structure for a previously unidentified defect that appears in positive bias constant-current STM images as an equilateral triangular arrangement of Si adatoms with attenuated intensity.
RESUMEN
Because of their compact form factor and rigidity, piezoelectric motors are used in scanning probe microscopes that operate at low temperature and high magnetic field. Here we present detailed information to facilitate the assembly, operation, and characterization of inertial motors. Specifically, a model of the motor is developed and used to identify different regions of operation. Drive electronics with high slew rate and large output current are described and a step-by-step procedure for assembling piezoelectric shear stacks is detailed. Additionally, a novel reflective object sensor is described and used to characterize a Pan-style inertial motor that was designed and assembled using the concepts presented in this paper.
RESUMEN
The registry of bismuth dimers, integral components of the bismuth nanoline on Si(001), is examined. In contrast to the currently accepted view, the bismuth dimers are found to be in registry with the two-dimensional lattice created by the silicon dimers. The consequences of this finding are briefly explored.
RESUMEN
The equilibrium geometry, electronic structure and energetic stability of Bi nanolines on clean and hydrogenated Si(001) surfaces have been examined by means of ab initio total energy calculations and scanning tunnelling microscopy. For the Bi nanolines on a clean Si surface the two most plausible structural models, the Miki or M model (Miki et al 1999 Phys. Rev. B 59 14868) and the Haiku or H model (Owen et al 2002 Phys. Rev. Lett. 88 226104), have been examined in detail. The results of the total energy calculations support the stability of the H model over the M model, in agreement with previous theoretical results. For Bi nanolines on the hydrogenated Si(001) surface, we find that an atomic configuration derived from the H model is also more stable than an atomic configuration derived from the M model. However, the energetically less stable (M) model exhibits better agreement with experimental measurements for equilibrium geometry. The electronic structures of the H and M models are very similar. Both models exhibit a semiconducting character, with the highest occupied Bi-derived bands lying at approximately 0.5 eV below the valence band maximum. Simulated and experimental STM images confirm that at a low negative bias the Bi lines exhibit an 'antiwire' property for both structural models.
RESUMEN
It is known that the vibrational spectra of beetle-type scanning tunneling microscopes with a total mass of approximately 3-4 g contain extrinsic 'rattling' modes in the frequency range extending from 500 to 1700 Hz that interfere with image acquisition. These modes lie below the lowest calculated eigenfrequency of the beetle and it has been suggested that they arise from the inertial sliding of the beetle between surface asperities on the raceway. In this paper we describe some cross-coupling measurements that were performed on three home-built beetle-type STMs of two different designs. We provide evidence that suggests that for beetles with total masses of 12-15 g all the modes in the rattling range are intrinsic. This provides additional support for the notion that the vibrational properties of beetle-type scanning tunneling microscopes can be improved by increasing the contact pressure between the feet of the beetle and the raceway.