Research in the Weiss Group

Self Assembly in the Weiss Group

Control and stabilization of molecular assemblies at the nanometer scale are crucial steps in the fabrication of molecular-scale devices. For ultrahigh resolution, the "top down" approach of producing patterns has been shown to have fundamental limits and is yielding to the "bottom up" approach of fabrication, where atomic and molecular building blocks are hierarchically assembled (often through self-, directed, and positional assembly techniques) to make gradually larger structures. Current techniques such as photolithography or electron beam lithography and “soft lithography” are limited in their resolution and cannot reproducibly achieve patterns with dimensions at the nanometer scale. On the other end of the spectrum, single-molecule manipulation has been successfully demonstrated using scanning probe microscopy, but is unable to produce devices in parallel and is still too time-consuming to be practical as a fabrication technique.

We anticipate the need to combine the speed and versatility of lithographic techniques with the resolution of single-molecule manipulation in order to construct commercially viable molecular devices. We and others have developed and utilized methods that exploit the inherent chemical, physical, and thermodynamic properties of molecules for facile means to pattern surfaces at the molecular level using self-assembly techniques. Self-assembly is a natural phenomenon that can be observed in many biological, chemical, and physical processes. This method has been explored recently as a means to produce supramolecular assemblies in a straightforward manner.


Schematic of a decanethiol SAM on Au(111).	STM image of a decanethiol SAM on Au(111) showing high spatial order, defect sites, and domain boundaries.

The most commonly studied and well-characterized self-assembling systems are alkanethiolate self-assembled monolayers (SAMs) on Au{111}. Alkanethiolate SAMs form spontaneously on Au{111} through chemisorption of the S head group to the Au surface. The monolayers interact on the surface through van der Waals forces that occur amongst adjacent alkyl chains. The origin of the stability of SAMs is thus two-fold, due to the covalent S-Au bond and the attractive van der Waals forces between the methylene groups. As a result of the intrinsic stability of these systems, SAMs are known to have a low defect density and resist degradation in air. The process of self-assembly lends itself naturally to controlling the local placement of molecules. In particular, SAMs have been used as model systems for fabricating structures with controlled geometries, as well as essential components in the actual device.

Multi-component SAMs formed by codeposition of two or more adsorbates from solution have been investigated for their patterning potential. When more than one adsorbate is considered, it is necessary to account for the interactions between the different adsorbates and whether these will favor mixing or separation. Self-assembled films with increasing degrees of complexity can be created through their formation with multiple adsorbates, especially when exerting control over the relative concentrations of adsorbates in the film as well as their local spatial distributions.

Naturally occurring defects within SAMs, such as domain boundaries, substrate vacancies, and lattice vacancies, can be utilized as "placeholders" for insertion of individual molecules. At these features within the film, there can be vapor or solution access to the substrate and the exchange of thiolates is favorable. These locations can thus be exploited to insert or exchange additional molecules preferentially. These extensively studied SAMs can be host matrices for individual guest molecules to be isolated and inserted for modification or characterization.

Schematic of the "molecular ruler" process.

We have developed a method that combines conventional lithographic techniques and chemical self-assembly processes to create gaps and nanostructures with nanometer-scale resolution. We are thus able to scale down the spacings between nanofabricated structures. The chemical process creates a three-dimensional molecular resist of desired thickness composed of alternating organic molecule and metal ion coordinated multilayers. The molecules composing the self-assembled multilayers can be used as "molecular rulers" to control precisely and quantitatively the size of the created structures. These multilayers are attached to lithographically defined "parent" structures to form a lift-off mask for subsequent metal deposition. Once the "daughter" metal has been evaporated, the mask (molecular resist) is then chemically removed leaving behind only the newly created daughter structure and the lithographic parent structure from which it was formed. The precision with which the scaling can be achieved is limited only by the length of the molecule used within the multilayer, which allows the structure's proportions to be tuned by fractions of a nanometer.

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