Nanofabrication

We aim to explore the possibilities of self-assembly and supramolecular chemistry in bottom-up nanofabrication to obtain devices that are organized at the molecular level. We use non-classical approaches that are compatible with the relatively fragile molecules to fabricate molecular electronic devices for applications in plasmonics, molecular electronics, and biomolecular electronics. Below we give a brief overview of some of the fabrication techniques we use besides common clean room based techniques such as photolithography at Centre for Advanced 2D Materials (CA2DM). To achieve new design of molecular electronic and plasmonic devices, electron beam lithography (EBL) is introduced into our nanofabrication with the collaboration at the cleanroom of CA2DM (GRC).

Template-stripping

Since we work at molecular length-scales, it is important to minimize defects in the metal surfaces that support the SAMs. For instance, many types of common defects, such as grain boundaries, easily exceed molecular dimensions. Apart from that, the metal surfaces have to be clean as physisorbed of chemisorbed materials may hamper the formation of well-defined molecular assemblies. Therefore, it is important to have access to clean metal surfaces that are available on demand (that is without additional cleaning steps) in ordinary laboratory conditions that are ultra-flat.
It is well-known that template-stripping of metal from a metal coated Si/SiO2 wafer produces ultra flat surfaces with low rms values (<1 nm over 1 x 1 μm2) large grains. By simply gluing glass supports against the metal followed by template stripping, we have access to clean ultra-flat surface on demand. Recently we showed that a soft annealing step prior to template stripping removes small grains leaving a surface that consist of only large grains ( see Thin Solid Films 2015593, 26–39). These ultra-flat surfaces support SAMs of good quality and allow for detailed physical organic studies of charge transport, and structural characterization of the SAMs.

Fig 1: The 2D AFM images and height profiles of AgTS surfaces annealed at 200 °C for 15 min (A and E), 30 min (B and F), 45 min (C and G) and 60 min (D and H). The white line dashed lines indicate the location where the height profiles were recorded. On the right, the template stripping process is shown: by simply gluing glass supports to the metal surface one can obtain clean metal surfaces by just stripping off the glass/glue/metal composite from the template (here a Si/SiO2 wafer).

Fabrication of arrays of SAM based junctions

The reproducibility of the electrical characteristics of molecular junctions has been notoriously low. We devised a method to construct tunnel junctions based on SAMs by forming reversible electrical contacts to SAMs using top-electrodes of EGaIn stabilized in a microfluidic-based device. A single top-electrode could be used to form up to 15-25 different junctions (see Adv. Funct. Mater. 2014, 24, 4442–4456).

Fig 2: The fabrication process of the top-electrode used in forming arrays of SAM-based junctions.

Recently, we showed that arrays of junctions can be formed by fabricating the top electrode as shown in the above figure. These arrays were utilised to demonstrate diode based Boolean logic (see Nanoscale 20157, 19547-19556). Unlike other approaches that rely on cross-bar or nano/micro-pore configurations, our method does not require patterning of the bottom-electrodes and is compatible with ultra-flat template-stripped (TS) surfaces. This compatibly with non-patterned electrodes is important for three reasons. i) No edges of the electrodes are present at which SAMs cannot pack well. ii) Patterning requires photoresist that may contaminate the electrode and complicate SAM formation. iii) TS-surfaces contain large grains, have low rms values of roughness, and can be obtained and used within a few seconds to minimize contamination.

Nanobeams

We use nanoskiving, a method developed by the Whitesides group, to cut large metal crystals into smaller ones to fabricate plasmonic structures. We use this method to investigate the photoluminiscence properties well-defined crystalline nanostructures (see ACS Photonics 2015, 2, 1348−1354)

Fig 3: Gap plasmon modulated photoluminescence of a single gold nanobeam. (a-f) Flowchart of steps involved in Nanoskiving, a method we use to fabricate plasmonic structures. (g) The SEM image of the Ag NW-Au NB structure. (h) The photoluminescence image of the Ag NW-Au NB structure with an excitation laser of 457 nm.