Contents 1. Introduction 4367 2. Elimination Lithography 4369 2.1. STM Modification under Ambient Conditions 4370 2.2. STM Modification in UHV 4372 2.3. Modification with AFM (Mechanical and Electrical) 4374 2.4. Scanning Probe Lithography with SECM 4380 2.5. Scanning Probe Lithography with NSOM 4380 3. Addition Lithography 4384 3.1. Historical Precedent for Addition Lithography 4384 3.2. Fundamentals of DPN 4384 3.3. Variation of the Ink and Substrate 4388 3.4. DPN Structures as Templates 4391 3.5. Mechanistic Aspects underlying DPN 4393 4. Substitution Lithography 4397 4.1. Substitution Initiated by Removal of SAM under Elevated Bias 4398 4.2. Substitution Initiated by Removal of SAM via Mechanical Desorption 4401 4.3. Substitution Followed by ex-Situ Addition to Patterns 4404 4.4. Substitution via Tip-Induced Terminal Group Modification 4406 5. Conclusions and Outlook 4413 6. Acknowledgments 4414 7. Glossary 4414 8. References 4415

Nanometer-sized metal particles (e.g., gold and silver) are certain to be important fundamental building blocks of future nanoscale electronic and optical devices. However, there are numerous challenges and questions which must be addressed before nanoparticle technologies can be implemented successfully. For example, basic capping ligand chemistrysnanoparticle electronic function relationships must be addressed in greater detail. New methods for assembling nanoparticles together into higher-order arrays with more complex electronic functions are also required. This review highlights our recent progress toward characterizing electron transport in gold nanoparticles as a function of capping ligand charge state. These studies have shown that single electron tunneling energies can be manipulated predictably via pH-induced charge changes of surfacebound thiol capping ligands. We also show that rigid phenylacetylene molecules are useful bridges for assembling gold and silver nanoparticles into arrays of two, three, and four particles with psuedo D ∞h , D 3h , and T d symmetries. These nanoparticle "molecules" interact electromagnetically in a manner qualitatively consistent with dipole coupling models.

The phenomenon of negative differential resistance (NDR) is potentially very useful in molecular electronics device schemes. Here, it is shown that NDR can be observed in self-assembled monolayers composed of electroactive thiols on gold. Furthermore, these monolayers can be patterned using a scanning probe lithography technique described earlier to form a basis for potential molecular electronic device construction.

Complex mesostructures showing gradient‐type surface coverage with ω‐substituted alkanthiols can be generated by STM replacement lithography. Variations of the replacement bias, lithographic scan rate, or raster line spacing create the gradient. The Figure (see also front cover) shows an L‐shaped gradient structure where Au(111) is separately thiol‐covered in the corner, and highly covered on both ends.

The negative differential resistance (NDR) peak current observed in redox active self-assembled monolayer-based molecular junctions has been attenuated by controlling the composition of the molecular junction. Two approaches studied here include capping the electroactive ferrocenyl groups with beta-cyclodextrin and functionalizing the scanning tunneling microscope tip used to probe the self-assembled monolayer (SAM) with n-alkanethiols of different lengths. These are the first examples of systematic modification of the magnitude of the NDR response in a molecule-based system.

A method of chemically well-defined, scanning tunneling microscope-based lithography is presented in which one thiolate in a self-assembled monolayer is removed and replaced with a second thiol. This method is distinguishable from other lithographic replacement processes on SAMs in that a nonpolar solution and an uncoated tip can be employed. Elevated relative humidity was important in the facility of this process, suggesting an electrochemical mechanism for replacement. The resolution of features written with this process is ca. 10−15 nm. In nonpolar solution, the apparent heights of self-assembled decanethiolate and dodecanethiolate monolayers are reversed compared to those observed in images obtained in air. When the thiol solution was exchanged after the first replacement, writing with two different thiols was demonstrated.

The relative conductance of two different electroactive thiol molecules (containing ferrocene and viologen headgroups) inserted into an n-alkanethiolate background SAM was tracked over time using a scanning tunneling microscope. Both types of inserted molecules exhibited stochastic variation in their conductance. This phenomenon of “blinking” thus appears to be quite general, despite the fact that these two molecules are structurally different from one another and from molecules in which this phenomenon had been studied previously. This behavior is most simply rationalized as conformational and/or orientational changes of one or a small collection of molecules over time.

During blood vessel injury, fibrinogen is converted to fibrin, a polymer that serves as the structural scaffold of a blood clot. The primary function of fibrin is to withstand the large shear forces in blood and provide mechanical stability to the clot, protecting the wound. Understanding the biophysical forces involved in maintaining fibrin structure is of great interest to the biomedical community. Previous reports have identified the "A-a" knob-hole interaction as the dominant force responsible for fibrin's structural integrity. Herein, biochemical force spectroscopy is used to study knob-hole interactions between fibrin fragments and variant fibrinogen molecules to identify the forces occurring between individual fibrin molecules. The rupture of the "A-a" knob-hole interaction results in a characteristic profile previously unreported in fibrin force spectroscopy with two distinct populations of specific forces: 110 +/- 34 and 224 +/- 31 pN. In the absence of a functional "A" knob or hole "a", these forces cease to exist. We propose that the characteristic pattern represents the deformation of the D region of fibrinogen prior to the rupture of the "A-a" knob-hole bond.