Philip Moriarty

Manipulation of C60 molecules on a Si surface

We have used the tip of a scanning tunneling microscope to position individual C60 molecules on a Si(111) surface. It is possible to form simple patterns of molecules at room temperature using this technique.

Mapping the force field of a hydrogen-bonded assembly

Hydrogen bonding underpins the properties of a vast array of systems spanning a wide variety of scientific fields. From the elegance of base pair interactions in DNA to the symmetry of extended supramolecular assemblies, hydrogen bonds play an essential role in directing intermolecular forces. Yet fundamental aspects of the hydrogen bond continue to be vigorously debated. Here we use dynamic force microscopy (DFM) to quantitatively map the tip-sample force field for naphthalene tetracarboxylic diimide molecules hydrogen-bonded in two-dimensional assemblies. A comparison of experimental images and force spectra with their simulated counterparts shows that intermolecular contrast arises from repulsive tip-sample interactions whose interpretation can be aided via an examination of charge density depletion across the molecular system. Interpreting DFM images of hydrogen-bonded systems therefore necessitates detailed consideration of the coupled tip-molecule system: analyses based on intermolecular charge density in the absence of the tip fail to capture the essential physical chemistry underpinning the imaging mechanism.

Translation, rotation and removal of C60 on Si(100)-2 × 1 using anisotropic molecular manipulation

We have investigated the interactions of C60 molecules adsorbed on Si(100)-(2 × 1) through their response to manipulation induced by a scanning tunneling microscope operating at room temperature. Intramolecular features are resolved which vary as a molecule is displaced showing that C60 undergoes rotation during tip-induced displacement. For translation to and from certain bonding sites, the apparent size of a molecule may change following lateral manipulation. Furthermore, reversible changes in dimer buckling are observed as a molecule is moved across the surface. Our experimental observations show that the C60Si(100)-(2 × 1) interaction is dominant over the C60C60 interaction and attempts to move a molecular pair result in the transfer of one molecule across a dimer row due to barrier lowering by the intermolecular interaction. We also show that transfer of a molecule from sample to tip (or vice versa) changes both the imaging and manipulation properties of the tip.

Front instabilities in evaporatively dewetting nanofluids

Various experimental settings that involve drying solutions or suspensions of nanoparticles-often called nanofluids-have recently been used to produce structured nanoparticle layers. In addition to the formation of polygonal networks and spinodal-like patterns, the occurrence of branched structures has been reported. After reviewing the experimental results we use a modified version of the Monte Carlo model first introduced by Rabani [Nature 426, 271 (2003)] to study structure formation in evaporating films of nanoparticle solutions for the case that all structuring is driven by the interplay of evaporating solvent and diffusing nanoparticles. After introducing the model and its general behavior we focus on receding dewetting fronts which are initially straight but develop a transverse fingering instability. We analyze the dependence of the characteristics of the resulting branching patterns on the driving effective chemical potential, the mobility and concentration of the nanoparticles, and the interaction strength between liquid and nanoparticles. This allows us to understand the underlying instability mechanism.

Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces

The structural and the optical properties of InAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (QDs) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QD coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) spectra of GaAs (100) and (311) samples show typical QD features with PL peaks in the energy range 1.15-1.35 eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70K was observed. These results suggest that the high index substrates are promising candidates for production of high quality self-assembled QD materials for application to photonics.

Deposition of Fe clusters on Si surfaces

We have used an ultrahigh vacuum scanning tunneling microscope to investigate the deposition of mesoscopic iron clusters from a gas aggregation source. The size of the clusters was found to be in the range 1–7 nm. The effect of exposure of the iron clusters to the atmosphere results in a significant increase in cluster size, which we believe is consistent with total oxidation of the clusters. A specially designed quadrupole mass filter is incorporated inside the cluster source. We have investigated the sizes of the clusters deposited using different quadrupole settings and find that it is possible to size select the clusters prior to deposition. Finally we have studied the effect of sample surface reactivity and annealing on cluster distribution.

Coerced mechanical coarsening of nanoparticle assemblies

Coarsening is a ubiquitous phenomenon that underpins countless processes in nature, including epitaxial growth, the phase separation of alloys, polymers and binary fluids, the growth of bubbles in foams, and pattern formation in biomembranes. Here we show, in the first real-time experimental study of the evolution of an adsorbed colloidal nanoparticle array, that tapping-mode atomic force microscopy (TM-AFM) can drive the coarsening of Au nanoparticle assemblies on silicon surfaces. Although the growth exponent has a strong dependence on the initial sample morphology, our observations are largely consistent with modified Ostwald ripening processes. To date, ripening processes have been exclusively considered to be thermally activated, but we show that nanoparticle assemblies can be mechanically coerced towards equilibrium, representing a new approach to directed coarsening. This strategy enables precise control over the evolution of micro- and nanostructures.

Chemical and structural studies of the interactions of molecular sulfur with the GaAs(111)A and GaAs(111)B surfaces

The interactions of sulfur with the GaAs(111)A and (111)B surfaces has been investigated by synchrotron radiation photoelectron spectroscopy. A scanning tunnelling microscopy study has been performed on the sulfur treated GaAs(111)B surface. Clean GaAs(111) surfaces, prepared in UHV by the thermal removal of an As cap, were exposed in situ to a molecular beam of sulfur. Changes in the surface chemistry and the Fermi level position were monitored as a function of annealing temperature. While this treatment results in complex surface chemistry which displays evidence of both surface and sub-surface sulfur-arsenic exchange reactions, the degree of Fermi level movement would indicate that neither of these surfaces are effectively electronically passivated. The implications of this for surface passivation are considered.

Chemical bonding and structure of the sulfur treated GaAs(111)B surface

We have investigated the atomic structure and chemical bonding present at sulfur exposed GaAs(111)B‐(2×2) surfaces using both scanning tunneling microscopy (STM) and synchrotron radiation core‐level photoemission. Exposure of the (2×2) surface to a molecular beam of sulfur leads to the appearance of a (1×1) low‐energy electron diffraction pattern which becomes increasingly well defined as the sample is annealed. However, at no stage of the annealing process does the surface display an ordered (1×1) ideal termination. Both the photoemission data and STM images show that a large proportion of the As trimer units of the clean (2×2) surface remain after sulfur exposure and annealing to 450 °C with strong evidence of sulfur substituting for As in atomic layers below the surface. The effect of these reactions is to increase the surface band‐bending from that of the clean (2×2) surface.

Room temperature manipulation of the heterofullerene C59N on Si(100)-2×1

The absorption of the heterofullerene C59N on the Si(100)-2×1 surface has been investigated using scanning tunneling microscopy (STM) under ultrahigh vacuum conditions. The molecules are adsorbed in monomer form in the troughs between silicon dimer rows. It is possible to use the tip of the STM to manipulate the molecules parallel and perpendicular to the dimer rows in a controlled fashion at room temperature. To determine the stability of the C59N monomer we have examined the response of pairs of molecules to STM manipulation and found that the Si(100)-2×1 surface inhibits conversion to (C59N)2 dimers.