Parallel Fabrication of Self‐Assembled Nanogaps for Molecular Electronic Devices

Abstract

parallel electroless plating, [16] and shadow mask evaporation, [17,18] and the field has recently been reviewed. [19,20] Meanwhile new parallel fabrication strategies for formation of nanogap electrodes are being developed, here recent examples include mole cular crystal lithography, [21] gold nanorod alignment, [22] crack-defined electronic nanogaps, [23] self-limiting electrode growth, [24] the use of graphenebased constrictions, [25] and carbon nanotube electrodes. [26,27] A remaining challenge is scalability, that is, how to position single (or a few) molecules in the nanogap in a parallel way. In this context, an interesting approach is the use of the protodevice concept, where the single (or a few) molecule is isolated in the solution between nanoparticles, and subsequently self-assembled onto prefabricated nanoelectrodes. [28][29][30][31][32][33][34] Templated self-assembly can be used to position nanoparticles at prefabricated surfaces. Examples include positioning of gold nanoparticles and orientation of gold nanorods. [35,36] It is also possible to assemble particles using a meniscus flow over nanostructures, [37,38] chemically activate and passivate parts of a surface prior to deposition of nanoparticles, [39] making use of variations of hydrophobicity on a surface [40] or wet-contact printing. [41] Other methods involve electrostatic trapping of particles attached with molecules. [42][43][44] Here we explore the natural surface charges that different metal/metal oxides acquire in solution to guide bottom-up assembled molecular linked nanoparticle dimers to prefabricated electrodes (Figure 1). It is in this manner possible to position, isolate, and measure electron transport on a single (or a few) molecules. Electron-beam lithography (EBL) was used to fabricate electrode pairs, with optimized geometry and materials to promote the assembly of negatively charged nanoparticle dimers (Figure 2). The electrodes are made in layers of nickel (Ni) and palladium (Pd). Nickel has proven to be able to attract negatively charged citrate stabilized nanoparticles due to the positive surface potential at pH 6.5-7. [45] Pd is sandwiched between the Ni electrodes to improve the conductivity of the electrodes, and to enable an oxide free interface between the electrodes and protodevices. At the same time the top Pd layer reduces assembly of nanoparticles "on top" of the electrodes. Two different molecules, 1,6-hexanedithiol and 1,4-benzenedithiol, were used as test molecules for the construction of Single molecule electronics might be a way to add additional function to nanoscale devices and continue miniaturization beyond current state of the art. Here, a combined top-down and bottom-up strategy is employed to assemble single molecules onto prefabricated electrodes. Protodevices, which are self-assembled nanogaps composed by two gold nanoparticles linked by a single or a few molecules, are guided onto top-down prefabricated nanosized nickel electrodes with sandwiched palladium layers. It is shown that an optimized geo...

Integration of Individual Functionalized Gold Nanoparticles into Nanoelectrode Configurations: Recent Advances

“…We would like to point out that alternative concepts are being explored as well, such as the integration of so‐called “protodevices”, i.e. AuNP pairs linked by a single of few molecules, into prefabricated electrodes utilizing surface charge interactions, [ 67 ] or by combinations of AuNPs and gold nanorods, bridging the gap between nano‐ and microscale, [ 68 ] however, in a non‐directional manner.…”

Direct Deposited Angstrom‐Scale Nanogap Electrodes with Macroscopically Measurable and Material‐Independent Capabilities for Various Applications

“…Sub‐5 nm NGEs have exhibited great superiority in many application fields, such as sensing, optical, molecular, and electronic devices . The gap separations decreasing from the nanometer down to the angstrom scale will revolutionize the existing nanogap‐related research and lead to valuable new physical phenomena such as nonlocal electromagnetic effects, quantum interference, nuclear spins, and electron tunneling, which can unlock the full potential of applications with high scientific and societal impact, including molecular electronics, quantum tunneling, plasmonic nano‐optics, and highly sensitive sequencing …”

“…Despite their importance, fabrication of sub‐5 nm NGEs remains a great technological challenge . Existing NGE‐manufacturing methods can be typically classified into two strategies: physical methods based on planar nanofabrication techniques and chemical methods based on noble metal nanoparticles. Most chemical methods are suitable for creating sub‐1 nm NGEs but limited to a relatively narrow range of applications owing to the contamination induced by linker molecules, the shell‐filled gaps, and/or the restrictions derived from metal nanoparticle size.…”

“…Existing NGE‐manufacturing methods can be typically classified into two strategies: physical methods based on planar nanofabrication techniques and chemical methods based on noble metal nanoparticles. Most chemical methods are suitable for creating sub‐1 nm NGEs but limited to a relatively narrow range of applications owing to the contamination induced by linker molecules, the shell‐filled gaps, and/or the restrictions derived from metal nanoparticle size. Physical methods, including direct patterning techniques, breaking‐/cracking‐based methods, and controllable deposition, are fundamentally limited by the basic planar nanofabrication techniques.…”

“…Compared with the existing NGE‐manufacturing methods, our technology presents several advances which will facilitate the exploitation of the existing NGE‐related applications and enable valuable new potential applications. First, the significant features in terms of facile fabrication, low cost, high efficiency, lithography‐ and etching‐free methods, lack of resist contaminations, and CMOS‐compatibility are naturally enabled by replacing the complicated patterning processes, which conventionally involve equipment‐intensive lithography and etching processing, with only a two‐step specially shadowed evaporation.…”