Combining electrochemical desorption and metal deposition on patterned self-assembled monolayers

Two-Step Nanoscale Approach for Well-Defined Complex Alkanethiol Films on Au Surfaces

Controlling the molecular organization of organic self-assembled monolayers (SAM) is of utmost importance in nanotechnology, molecular electronics, and surface science. Here we propose two well-differentiated approaches, double printing based on microcontact printing (μ-cp) and molecular backfilling adsorption, to produce complex alkanethiol films. The resulting films on model Au surfaces were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. Double printing alkanethiols results in clear coexisting regions where no molecular displacement is observed, highlighting the slow diffusion rates of long alkanethiols and large attractive interaction between long alkyl chains. Exposing a single-print μ-cp Au substrate to an additional alkanethiol solution yields the formation of differently ordered domain boundaries with different thickness and micrometer lateral size. The high order is a result of enhanced molecular mobility and restructuring during solution backfilling. The formed molecular assemblies constitute an excellent testing ground for nanoscale phenomena that strongly depend on the nanoscale geometrical and chemical features of the surface such as designed functionality or corrosion initiation and inhibition.

Double-Sided Opportunities Using Chemical Lift-Off Lithography

We discuss the origins, motivation, invention, development, applications, and future of chemical lift-off lithography, in which a specified pattern of a self-assembled monolayer is removed, i.e., lifted off, using a reactive, patterned stamp that is brought into contact with the monolayer. For Au substrates, this process produces a supported, patterned monolayer of Au on the stamp in addition to the negative pattern in the original molecular monolayer. Both the patterned molecular monolayer on the original substrate and the patterned supported metal monolayer on the stamp are useful as materials and for further applications in sensing and other areas. Chemical lift-off lithography effectively lowers the barriers to and costs of high-resolution, large-area nanopatterning. On the patterned monolayer side, features in the single-nanometer range can be produced across large (square millimeter or larger) areas. Patterns smaller than the original stamp feature sizes can be produced by controlling the degree of contact between the stamp and the lifted-off monolayer. We note that this process is different than conventional lift-off processes in lithography in that chemical lift-off lithography removes material, whereas conventional lift-off is a positive-tone patterning method. Chemical lift-off lithography is in some ways similar to microtransfer printing. Chemical lift-off lithography has critical advantages in the preparation of biocapture surfaces because the molecules left behind are exploited to space and to orient functional(ized) molecules. On the supported metal monolayer side, a new two-dimensional material has been produced. The useful important chemical properties of Au (vis-à-vis functionalization with thiols) are retained, but the electronic and optical properties of bulk Au or even Au nanoparticles are not. These metal monolayers do not quench excitation and may be useful in optical measurements, particularly in combination with selective binding due to attached molecular recognition elements. In contrast to materials such as graphene that have bonding confined to two dimensions, these metal monolayers can be straightforwardly patterned-by patterning the stamp, the initial monolayer, or the initial substrate. Well-developed thiol-Au and related chemistries can be used on the supported monolayers. As there is little quenching and photoabsorption, spectroscopic imaging methods can be used on these functionalized materials. We anticipate that the properties of the metal monolayers can be tuned by varying the chemical, physical, and electronic connections made by and to the supporting molecular layers. That is, the amount of charge in the layer can be determined by controlling the density of S-Au (or other) connections and the molecular backbone and functionality, which determine the strength with which the chemical contact withdraws charge from the metal. This process should work for other coinage-metal substrates and additional systems where the binding of the outermost layers to the substrate is weaker than the molecule-substrate attachment.

1-Adamantanethiol as a versatile nanografting tool

Strategies to regulate the self-assembly of adsorbates to create surface structures with molecular-scale features and organization are of broad interest to nanoscience, biochemistry, and engineering. One approach utilizes molecules with tailored intermolecular interaction strengths and topologies to direct molecular self-assembly as exemplified by the adsorption of 1-adamantanethiol molecules on Au{111} substrates. 1-Adamantanethiolate self-assembled monolayers exhibit decreased packing densities and weaker intermolecular interaction strengths than n-alkanethiolate self-assembled monolayers, which result in their complete displacement upon exposure to n-alkanethiol molecules. Herein, we explore the capabilities of the atomic force microscopy-based lithographic technique, nanografting, to fabricate chemical patterns comprised of 1-adamantanethiolate monolayers. Positive 1-adamantanethiolate patterns are generated by nanografting 1-adamantanethiol molecules into preexisting n-alkanethiolate self-assembled monolayers, and negative 1-adamantanethiolate patterns are created by nanografting n-alkanethiol molecules into preexisting 1-adamantanethiolate self-assembled monolayers. The patterned 1-adamantanethiolate regions are displaced upon exposure to solutions of n-alkanethiol molecules. This two-step nanografting-displacement strategy minimizes pattern dissolution as 1-adamantanethiol molecules do not intercalate into the preexisting self-assembled monolayer during nanografting. 1-Adamantanethiol can be utilized create high-resolution sacrificial chemical patterns with feature sizes beyond those afforded other 1-adamantanethiol patterning strategies for applications such as resists for metallic and organic structures.

Parallel- and serial-contact electrochemical metallization of monolayer nanopatterns: A versatile synthetic tool en route to bottom-up assembly of electric nanocircuits

Contact electrochemical transfer of silver from a metal-film stamp (parallel process) or a metal-coated scanning probe (serial process) is demonstrated to allow site-selective metallization of monolayer template patterns of any desired shape and size created by constructive nanolithography. The precise nanoscale control of metal delivery to predefined surface sites, achieved as a result of the selective affinity of the monolayer template for electrochemically generated metal ions, provides a versatile synthetic tool en route to the bottom-up assembly of electric nanocircuits. These findings offer direct experimental support to the view that, in electrochemical metal deposition, charge is carried across the electrode-solution interface by ion migration to the electrode rather than by electron transfer to hydrated ions in solution.

A bipolar electrochemical approach to constructive lithography: metal/monolayer patterns via consecutive site-defined oxidation and reduction

Experimental evidence is presented, demonstrating the feasibility of a surface-patterning strategy that allows stepwise electrochemical generation and subsequent in situ metallization of patterns of carboxylic acid functions on the outer surfaces of highly ordered OTS monolayers assembled on silicon or on a flexible polymeric substrate. The patterning process can be implemented serially with scanning probes, which is shown to allow nanoscale patterning, or in a parallel stamping configuration here demonstrated on micrometric length scales with granular metal film stamps sandwiched between two monolayer-coated substrates. The metal film, consisting of silver deposited by evaporation through a patterned contact mask on the surface of one of the organic monolayers, functions as both a cathode in the printing of the monolayer patterns and an anodic source of metal in their subsequent metallization. An ultrathin water layer adsorbed on the metal grains by capillary condensation from a humid atmosphere plays the double role of electrolyte and a source of oxidizing species in the pattern printing process. It is shown that control over both the direction of pattern printing and metal transfer to one of the two monolayer surfaces can be accomplished by simple switching of the polarity of the applied voltage bias. Thus, the patterned metal film functions as a consumable "floating" stamp capable of two-way (forward-backward) electrochemical transfer of both information and matter between the contacting monolayer surfaces involved in the process. This rather unusual electrochemical behavior, resembling the electrochemical switching in nanoionic devices based on the transport of ions in solid ionic-electronic conductors, is derived from the nanoscale thickness of the water layer acting as an electrolyte and the bipolar (cathodic-anodic) nature of the water-coated metal grains in the metal film. The floating stamp concept introduced in this report paves the way to a series of unprecedented capabilities in surface patterning, which are particularly relevant to nanofabrication by chemical means and the engineering of a new class of molecular nanoionic systems.

Nanosieving of anions and cavity-size-dependent association of cyclodextrins on a 1-adamantanethiol self-assembled monolayer

In this paper, we studied charge transfer through a self-assembled monolayer (SAM) of 1-adamantanethiol on gold. Charge transfer through the 1-adamantanethiol SAM depended on the type of anion present when [Fe(CN)6]3- was used as a redox probe. The sluggish charge transfer process was monitored by cyclic voltammetry using the relatively large and hydrophobic perchlorate and hexafluorophosphate ions as the supporting electrolyte. In contrast, the charge transfer kinetics were nearly identical to those measured on bare gold with chloride, sulfate, and nitrate ions as the supporting electrolyte. We investigated the adsorption of alpha- and beta-cyclodextrin on the 1-adamantanethiol SAM via a host-guest interaction. The 1-adamantanethiol SAM could not bind beta-cyclodextrin via a host-guest interaction, probably due to the proximity of neighboring adamantine molecules on the surface. Immobilization of alpha-cyclodextrin by formation of an exterior complex with the SAM suppressed charge transfer. The adsorbed alpha-cyclodextrin was quantified using faradaic impedance experiments. The obtained adsorption isotherm was in good agreement with the Langmuir isotherm with a binding constant of 39.53 M(-1).

The influence of a single thiol group on the electronic and optical properties of the smallest diamondoid adamantane

At the nanoscale, the surface becomes pivotal for the properties of semiconductors due to an increased surface-to-bulk ratio. Surface functionalization is a means to include semiconductor nanocrystals into devices. In this comprehensive experimental study we determine in detail the effect of a single thiol functional group on the electronic and optical properties of the hydrogen-passivated nanodiamond adamantane. We find that the optical properties of the diamondoid are strongly affected due to a drastic change in the occupied states. Compared to adamantane, the optical gap in adamantane-1-thiol is lowered by approximately 0.6 eV and UV luminescence is quenched. The lowest unoccupied states remain delocalized at the cluster surface leaving the diamondoid's negative electron affinity intact.

Site-specific dual ink dip pen nanolithography

The ability to deposit different materials with nanoscale precision at user-specified locations is a very important attribute of dip pen nanolithography (DPN). However, the potential of DPN goes beyond simple deposition since DPN used in conjunction with lateral force microscopy (LFM) allows site-specific investigations of nanoscale properties. In this work, we use two different inks, 16-mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT) to show site-specific dual ink DPN enabled exclusively by our proprietary software. A diamond-dot pattern was created by using a layer-to-layer alignment (LLA) algorithm, which enables a MHA pattern (diamond) to be written concentric with another ODT (central dot) pattern. This simple demonstration of multi-ink DPN is not specific to alkanethiol ink systems, but is also applicable to other multi-material patterning, interaction, and exchange studies.