Ronald P. Andres

Self-Assembly of a Two-Dimensional Superlattice of Molecularly Linked Metal Clusters

Close-packed planar arrays of nanometer-diameter metal clusters that are covalently linked to each other by rigid, double-ended organic molecules have been self-assembled. Gold nanocrystals, each encapsulated by a monolayer of alkyl thiol molecules, were cast froma colloidal solution onto a flat substrate to form a close-packed cluster monolayer. Organic interconnects (aryl dithiols or aryl di-isonitriles) displaced the alkyl thiol molecules and covalently linked adjacent clusters in the monolayer to form a two-dimensional superlattice of metal quantum dots coupled by uniform tunnel junctions. Electrical conductance through such a superlattice of 3.7-nanometer-diameter gold clusters, deposited on a SiO2 substrate in the gap between two gold contacts and linked by an aryl di-isonitrile [1,4-di(4-isocyanophenylethynyl)-2-ethylbenzene], exhibited nonlinear Coulomb charging behavior.

Coulomb Staircase at Room Temperature in a Self-Assembled Molecular Nanostructure

Double-ended aryl dithiols [α,α′-xylyldithiol (XYL) and 4,4′-biphenyldithiol] formed self-assembled monolayers (SAMs) on gold(111) substrates and were used to tether nanometer-sized gold clusters deposited from a cluster beam. An ultrahigh-vacuum scanning tunneling microscope was used to image these nanostructures and to measure their current-voltage characteristics as a function of the separation between the probe tip and the metal cluster. At room temperature, when the tip was positioned over a cluster bonded to the XYL SAM, the current-voltage data showed “Coulomb staircase” behavior. These data are in good agreement with semiclassical predictions for correlated single-electron tunneling and permit estimation of the electrical resistance of a single XYL molecule (∼18 ± 12 megohms).

FABRICATION OF TWO-DIMENSIONAL ARRAYS OF NANOMETER-SIZE CLUSTERS WITH THE ATOMIC FORCE MICROSCOPE

An atomic force microscope tip is used as a vector positioner to manipulate nanometer‐size preformed Au clusters deposited on atomically smooth substrates. Using this technique, two‐dimensional cluster nanostructures can be assembled at room temperature.

A simple, reliable technique for making electrical contact to multiwalled carbon nanotubes

A simple method of making reliable electrical contact to multiwalled carbon nanotubes is described. With these contacts, current in the mA range can be routinely passed through individual multiwalled nanotubes without adverse consequences, thus allowing their resistance to be measured using a common multimeter. The contacts are robust enough to withstand temperature excursions between room temperature and 77 K. I(V) data from different multiwalled nanotubes are presented and analyzed.

Writing nanometer‐scale symbols in gold using the scanning tunneling microscope

The conditions required to electroetch nanometer‐sized craters in flat gold substrates with a scanning tunneling microscope operating in air are identified. Reproducible nanometer‐scale modifications of the substrate are possible. Letters and complex symbols with linewidths as small as 2 nm have been written. Experiments show that a good tunneling tip is not destroyed by the writing process.

Room temperature Coulomb blockade and Coulomb staircase from self‐assembled nanostructures

The self‐assembly of well‐characterized, nanometer‐size Au clusters into ordered monolayer arrays spanning several microns has been achieved. Techniques to insert molecular wires to link adjacent clusters in the self‐assembled array have also been developed. ‘‘Unit cell’’ nanostructures formed from individual Au clusters supported on a self‐assembled monolayer film of the double‐ended thiol molecule p‐xylene‐α,α′‐ dithiol show evidence for reproducible single electron effects at room temperature when studied by scanning tunneling microscopy. From these measurements, estimates for the electrical resistance of a single molecule can be obtained. The experimental values for this resistance are in reasonable agreement with theoretical calculations using the Landauer approach.

Electronic Properties of Metallic Nanoclusters on Semiconductor Surfaces: Implications for Nanoelectronic Device Applications

We review current research on the electronic properties of nanoscale metallic islands and clusters deposited on semiconductor substrates. Reported results for a number of nanoscale metal-semiconductor systems are summarized in terms of their fabrication and characterization. In addition to the issues faced in large-area metal-semiconductor systems, nano-systems present unique challenges in both the realization of well-controlled interfaces at the nanoscale and the ability to adequately characterize their electrical properties. Imaging by scanning tunneling microscopy as well as electrical characterization by current-voltage spectroscopy enable the study of the electrical properties of nanoclusters/semiconductor systems at the nanoscale. As an example of the low-resistance interfaces that can be realized, low-resistance nanocontacts consisting of metal nanoclusters deposited on specially designed ohmic contact structures are described. To illustrate a possible path to employing metal/semiconductor nanostructures in nanoelectronic applications, we also describe the fabrication and performance of uniform 2-D arrays of such metallic clusters on semiconductor substrates. Using self-assembly techniques involving conjugated organic tether molecules, arrays of nanoclusters have been formed in both unpatterned and patterned regions on semiconductor surfaces. Imaging and electrical characterization via scanning tunneling microscopy/spectroscopy indicate that high quality local ordering has been achieved within the arrays and that the clusters are electronically coupled to the semiconductor substrate via the low-resistance metal/semiconductor interface.

Shape of nanometer-size supported gold clusters studied by scanning tunneling microscopy

Abstract The shape of preformed spherical Au clusters with radii varying from 1 to 6 nm has been studied with the scanning tunneling microscope after deposition onto flat Au substrates. The supported clusters are found to resemble spherical caps with radii of curvature greater than and not strongly correlated with the original free-space radii. Measurements revealed that ∼ 80% of the clusters studied have a radius of curvature lying between 10 and 30 nm. A continuum model to interpret this result indicates that clusters with radii less than a critical value, characteristic of the cluster material, are stressed beyond their elastic limit and can deform so as to decrease their surface free energy even at temperatures well below their melting point.

Guided self-assembly of Au nanocluster arrays electronically coupled to semiconductor device layers

We report the controlled deposition of close-packed monolayer arrays of ∼5-nm-diam Au clusters within patterned regions on GaAs device layers, thus demonstrating guided self-assembly on a substrate which can provide interesting semiconductor device characteristics. Uniform nanometer scale ordering of the clusters is achieved by a chemical self-assembly process, while micron scale patterning is provided by a soft lithographic technique. Scanning tunneling microscope imaging and current–voltage spectroscopy indicate the Au nanoclusters are strongly coupled electronically into the underlying semiconductor substrate while exhibiting only weak electronic coupling in the lateral plane.

Substrate induced deformation of nanometer-size gold clusters studied by non-contact AFM and TEM

The ability to perform atomic force and transmission electron microscopy on nanoscale Au clusters supported on the identical α-Al2O3 substrate has been demonstrated. Non-contact atomic force and transmission electron microscope images were used, in a complimentary manner, to determine: (i) the nanoscale structure of the α-Al2O3 substrate, (ii) the size distributions of cluster heights (AFM) and diameters (TEM) on α-Al2O3, (iii) the fraction of Au clusters imaged on different atomically-smooth substrates using non-contact AFM, and (iv) the surface induced deformation of Au clusters on α-Al2O3, MoS2 and HOPG substrates. From these data, information about supported cluster shape and cluster-substrate interactions for these three substrates was obtained.