Colm Durkan

Electronic spin detection in molecules using scanning-tunneling- microscopy-assisted electron-spin resonance

By combining the spatial resolution of a scanning-tunneling microscope (STM) with the electronic spin sensitivity of electron-spin resonance, we show that it is possible to detect the presence of localized spins on surfaces. The principle is that a STM is operated in a magnetic field, and the resulting component of the tunnel current at the Larmor (precession) frequency is measured. This component is nonzero whenever there is tunneling into or out of a paramagnetic entity. We have succeeded in obtaining spectra from free radical molecules from which the g factor of a spin entity may be inferred. For the molecules studied here, α,γ-bisdiphenylene-β-phenylallyl, g was found to be 2±0.1.

Analysis of failure mechanisms in electrically stressed Au nanowires

An analysis of polycrystalline Au thin film interconnects of widths ranging from 850 to 25 nm, and lengths ranging from 1.0 μm to 20 nm which have been electrically stressed to the point of failure is presented. For the longer wires (widths 60–850 nm), the failure current density is typically found to be 1012 A m−2, essentially independent of the wire width, and then rapidly approaching zero for thinner wires. For the wider wires, failure occurs at the end towards the negative electrode; for narrow wires, failure tends to occur towards the center of the wire, as observed using scanning electron microscopy and atomic force microscopy. The mean time to failure for fixed current density is seen to decrease with decreasing wire width. The failure current density for a given wire width increases as the length decreases. An analysis of the temperature profile based on calculations of a simple model is presented which shows that this width-dependent behavior of narrow lines is not anticipated from the assumption...

A review and outlook for an anomaly of scanning tunnelling microscopy (STM): superlattices on graphite

Since its invention in 1981, scanning tunnelling microscopy (STM) is well-known for its supreme imaging resolution enabling one to observe atomic-scale structures, which has led to the flourishing of nanoscience. As successful as it is, there still remain phenomena which are observed using STM but are beyond our understanding. Graphite is one of the surfaces which have been most extensively studied using STM. However, there are a number of unusual properties of graphite surfaces. First reported in the 1980s, superlattices on graphite have since been observed many times and by many groups, but as yet our understanding of this phenomenon is quite limited. Most of the observed superlattice phenomena are widely believed to be the result of a Moire rotation pattern, arising from the misorientation between two graphite layers, as verified experimentally. A Moire pattern is a lattice with larger periodicity resulting from the overlap of two lattices with smaller periodicities. As graphite layers are composed of hexagonal lattices with a periodicity of 0.246 nm, as observed using STM, when there are misoriented graphite layers overlapping each other, a Moire pattern with larger periodicity, depending on the misorientation angle, will be produced and appear as a superperiodic hexagonal structure on top of the graphite atomic lattice of the topmost surface layer. It is important to study graphite superlattices because, firstly, knowledge of this phenomenon will enable us to properly interpret STM images; secondly, it helps us to understand the correlation between electronic structures and atomic-structure rearrangement of graphite which is of tremendous aid for engineering material properties; thirdly, and perhaps most importantly, the observation of the phenomenon exhibits the capability of STM to produce images indicating the nature of internal defects which are below the surface. Over recent years, experimental and modelling techniques have been developed to study this anomalous regime of STM; however, there is a lack of a systematic classification of this scattered information. This review article thus serves the purpose of organizing all these results so as to enable a more comprehensive understanding of this phenomenon. We review the discovery of graphite superlattices, the observation of the associated properties, and the research efforts on this subject. An effort is made to envision the future experimental and theoretical research possibilities to unveil the mystery of this anomaly of STM. Applications of graphite superlattices are also proposed.

Observation of magnetic domains using a reflection mode scanning near-field optical microscope

It is demonstrated that it is possible to image magnetic domains with a resolution of better than 60 nm with the Kerr effect in a reflection-mode scanning near-field optical microscope. Images taken of tracks of thermomagnetically prewritten bits in a Co/Pt multilayer structure magnetized out-of plane showed optical features in a track pattern whose appearance was determined by the position of an analyzer in front of the photomultiplier tube. These features were not apparent in the topography, showing this to be a purely magneto-optic effect.

Tailoring the local interaction between graphene layers in graphite at the atomic scale and above using scanning tunneling microscopy.

With recent developments in carbon-based electronics, it is imperative to understand the interplay between the morphology and electronic structure in graphene and graphite. We demonstrate controlled and repeatable vertical displacement of the top graphene layer from the substrate mediated by the scanning tunneling microscopy (STM) tip-sample interaction, manifested at the atomic level as well as over superlattices spanning several tens of nanometers. Besides the full-displacement, we observed the first half-displacement of the surface graphene layer, confirming that a reduced coupling rather than a change in lateral layer stacking is responsible for the triangular/honeycomb atomic lattice transition phenomenon, clearing the controversy surrounding it. Furthermore, an atomic scale mechanical stress at a grain boundary in graphite, resulting in the localization of states near the Fermi energy, is revealed through voltage-dependent imaging. A method of producing graphene nanoribbons based on the manipulation capabilities of the STM is also implemented.

Investigations into local piezoelectric properties by atomic force microscopy

We describe nanoscale characterization of sol-gel produced ferroelectric thin films of lead–zirconate–titanate. We have performed quantitative localized measurements of surface polarization charge density using atomic force microscopy techniques in conjunction with electric field calculations. We show that domains with radii of 40 nm may by written and subsequently characterized, and we analyze the dependence of domain size on write voltage and write time, and show that surface contaminants influence the formation of domains.

Controlled fabrication of 1–2nm nanogaps by electromigration in gold and gold-palladium nanowires

The authors report the electrical characterization of gold and gold-palladium nanowires failed by electromigration. Nanogaps 1–2nm in size are reliably made from metal nanowires by controlling the electromigration failure process, opening up the possibility of using these metal nanowires with nanogaps for molecular conduction studies and large-scale molecular junction device fabrication. Nanogaps are formed by applying a voltage sweep to the wires at a ramp rate of 4mV∕s. The interplay between Joule heating and electromigration means that reliable nanogaps can be formed without the need of a feedback circuit, rendering the technique relatively simple to implement.

Analysis of failure mechanisms in electrically stressed gold nanowires

An analysis of polycrystalline Au thin-film interconnects of widths ranging from 850 to 25 nm, and lengths ranging from 1 microm to 20 nm which have been electrically stressed to the point of failure is presented. A new method for testing failure of interconnects is proposed, based on a quantity we call the failure current density. The mean time to failure for fixed current density and also the failure current density are seen to decrease with decreasing wire width contrary to expectations. The failure current density for a given wire width increases as the length decreases. An analysis of the temperature and stress profiles based on calculations of a simple model is presented which shows that the length dependence is due to thermal stresses rather than electromigration, and the width dependence is due to enhanced electromigration due to surface diffusion.

Nanometer resolution piezoresponse force microscopy to study deep submicron ferroelectric and ferroelastic domains

Understanding ferroelectricity at the deep submicron regime is desirable in utilizing it for next generation nonvolatile memory devices, medical imaging systems, and rf filters. Here we show how piezoresponse force microscopy can be enhanced (1 nm resolution). Using this method, we have investigated ferroelectric and ferroelastic domains at the deep submicron regime in polycrystalline lead zirconium titanate thin films. We demonstrate that in the clamped films, periodic pairs of 90° domains are stable even at 10 nm width, challenging recent predictions of minimum domain size, and suggesting ferroelectricity for high-density storage devices (≥10 Tbyte/in2).

Investigation of the physical mechanisms of shear-force imaging

It is shown that shear‐force imaging, as is commonly used for distance regulation in scanning near‐field optical microscopy, is not a reliable technique for accurate topographic measurements. This is because different materials experience different shear‐force damping. Results of the shear‐force damping characteristics are presented for a number of different materials, and some consequences of the different dampings for different materials are demonstrated. It is also shown that there are at least two distinct shear force damping mechanisms. Results of imaging small conducting islands on a glass substrate show that the damping characteristics depend on the islands’ size.