Toward sub-20 nm hybrid nanofabrication by combining the molecular ruler method and electron beam lithography

Expanding the molecular-ruler process through vapor deposition of hexadecanethiol

The development of methods to produce nanoscale features with tailored chemical functionalities is fundamental for applications such as nanoelectronics and sensor fabrication. The molecular-ruler process shows great utility for this purpose as it combines top-down lithography for the creation of complex architectures over large areas in conjunction with molecular self-assembly, which enables precise control over the physical and chemical properties of small local features. The molecular-ruler process, which most commonly uses mercaptoalkanoic acids and metal ions to generate metal-ligated multilayers, can be employed to produce registered nanogaps between metal features. Expansion of this methodology to include molecules with other chemical functionalities could greatly expand the overall versatility, and thus the utility, of this process. Herein, we explore the use of alkanethiol molecules as the terminating layer of metal-ligated multilayers. During this study, it was discovered that the solution deposition of alkanethiol molecules resulted in low overall surface coverage with features that varied in height. Because features with varied heights are not conducive to the production of uniform nanogaps via the molecular-ruler process, the vapor-phase deposition of alkanethiol molecules was explored. Unlike the solution-phase deposition, alkanethiol islands produced by vapor-phase deposition exhibited markedly higher surface coverages of uniform heights. To illustrate the applicability of this method, metal-ligated multilayers, both with and without an alkanethiol capping layer, were utilized to create nanogaps between Au features using the molecular-ruler process.

Atomic force microscopy characterization and lithography of Cu-ligated mercaptoalkanoic acid "molecular ruler" multilayers

Hybrid chemical patterning strategies that combine the sophistication of lithography with the intrinsic precision of molecular self-assembly are of broad interest for applications including nanoelectronics and bioactive surfaces. This approach is exemplified by the molecular-ruler process where the sequential deposition of mercaptoalkanoic acid molecules and coordinated metal ions is integrated with conventional lithographic techniques to fabricate registered, nanometer-scale spacings. Herein, we illustrate the capabilities of atomic force microscopy characterization and lithography to investigate the morphology, quality, and local thickness of Cu-ligated mercaptohexadecanoic acid multilayers on Au{111} substrates. These multilayers are a key component utilized in the molecular-ruler process. The rich and varied topographic features of each layer are investigated via contact-mode atomic force microscopy. Using nanoshaving, an atomic force microscopy lithographic strategy that reveals the underlying Au{111} substrate via tip-induced desorption of a molecular film, the local thicknesses of these multilayers are ascertained; these thicknesses are consistent with the anticipated heights for Cu-ligated mercaptohexadecanoic acid multilayers as well as previous ensemble surface analytical measurements. By regulating the force set point utilized during nanoshaving, the upper layer of a Cu-ligated mercaptohexadecanoic acid bilayer is removed, revealing the carboxyl moiety of the lower mercaptohexadecanoic acid layer. This selective nanoshaving demonstrates a simple and practical means to generate three-dimensional multilayers and to reveal buried chemical functionalities within metal-ligated multilayers.

From the bottom up: dimensional control and characterization in molecular monolayers

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.