The fabrication of nanogap electrodes using nanoskiving

Het doel van dit onderzoek is om een beschrijving te geven van een simpele fabricagemethode om moleculaire tunnelingkoppelingen te bouwen. We hebben een methode ontwikkeld om electroden te fabriceren met daartussen een spleet in de range van nanometers, daarbij gebruik makend van “nanoskiving”, een vorm van lithografie. Deze krachtige, nieuwe techniek maakt het mogelijk om de beperkingen die er zijn in het veld van de moleculaire electronica te omzeilen. Nu kunnen we electroden fabriceren waarbij we controle hebben over de grootte van de nanospleet; bovendien kunnen we nu hele series electroden produceren. We hebben laten zien dat deze tunnelingapparaatjes gefabriceerd kunnen worden met een snelheid van één per seconde, terwijl er tegelijkertijd controle is over alle dimensies van de gefabriceerde electroden. Met behulp van deze bottom-up benadering zijn zogenaamde “SAM-templated addressable nanogap (STAN; SAM = zelf-assemblerende monolagen) electroden gefabriceerd. De spleetgrootte van sub-3 nanometer wordt bepaald door de moleculen die fungeren als matrijs voor de spleet. Omdat we alkaandithiolen gebruiken als matrijs voor de nanospleet kunnen we een resolutie halen op het Angströmniveau, de grootte van een C-C binding. Bovendien kunnen we door de hoge aspectratio van de STANs deze direct adresseren en verbinden met sondes voor hun elektrische karakterisatie. Wij geloven dat onze simpele, snelle en goedkope techniek een veelbelovende aanpak is die ons in staat stelt om apparaatjes op aanvraag te fabriceren om de tunnelingstroom te meten van willekeurige moleculen of SAMs. Verder kan de constructie van tunnelingkoppelingen uit willekeurige moleculen bereikt worden door de uitwisseling van dithiolen in de nanospleet met dithiolen in oplossing. Het inbouwen van willekeurige, symmetrische dithiolen in de STANs door middel van uitwisseling voorziet in een hoge doorvoer en generaliseerbare methode welke leidt tot een platform voor de meting van moleculen met een verscheidenheid aan electrodematerialen

Optical modulation of nano-gap tunnelling junctions comprising self-assembled monolayers of hemicyanine dyes

Light-driven conductance switching in molecular tunnelling junctions that relies on photoisomerization is constrained by the limitations of kinetic traps and either by the sterics of rearranging atoms in a densely packed monolayer or the small absorbance of individual molecules. Here we demonstrate light-driven conductance gating; devices comprising monolayers of hemicyanine dyes trapped between two metallic nanowires exhibit higher conductance under irradiation than in the dark. The modulation of the tunnelling current occurs faster than the timescale of the measurement (∼1 min). We propose a mechanism in which a fraction of molecules enters an excited state that brings the conjugated portion of the monolayer into resonance with the electrodes. This mechanism is supported by calculations showing the delocalization of molecular orbitals near the Fermi energy in the excited and cationic states, but not the ground state and a reasonable change in conductance with respect to the effective barrier width

Directly Addressable Sub-3 nm Gold Nanogaps Fabricated by Nanoskiving Using Self-Assembled Monolayers as Templates

This paper describes the fabrication of electrically addressable, high-aspect-ratio (>10000:1) nanowires of gold with square cross sections of 100 nm on each side that are separated by gaps of 1.7-2.2 nm which were defined using self-assembled monolayers (SAMs) as templates. We fabricated these nanowires and nanogaps without a clean room or any photo- or electron-beam lithographic processes by mechanically sectioning sandwich structures of gold separated by a SAM using an ultramicrotome. This process is a form of edge lithography known as Nanoskiving. These wires can be manually positioned by transporting them on drops of water and are directly electrically addressable; no further lithography is required to connect them to an electrometer. Once a block has been prepared for Nanoskiving (which takes less than one day), hundreds of thousands of nanogaps can be generated, on demand, at a rate of about one nanogap per second. After ashing the organic components with oxygen plasma, we measured the width of a free-standing gap formed from a SAM of 16-mercaptodohexanoic acid (2.4 nm in length) of 2.6±0.5 nm by transmission electron microscopy. By fitting current-voltage plots of unashed gaps containing three alkanedithiolates of differing lengths to Simmons' approximation, we derived a value of β = 0.75 Å-1 (0.94 nC-1) at 500 mV. This value is in excellent agreement with literature values determined by a variety of methods, demonstrating that the gap-size can be controlled at resolutions as low as 2.5 Å (i.e., two carbon atoms).

Directly Addressable Sub-3 nm Gold Nanogaps Fabricated by Nanoskiving Using Self-Assembled Monolayers as Templates

This paper describes the fabrication of electrically addressable, high-aspect-ratio (>10000:1) nanowires of gold with square cross sections of 100 nm on each side that are separated by gaps of 1.7–2.2 nm which were defined using self-assembled monolayers (SAMs) as templates. We fabricated these nanowires and nanogaps without a clean room or any photo- or electron-beam lithographic processes by mechanically sectioning sandwich structures of gold separated by a SAM using an ultramicrotome. This process is a form of edge lithography known as Nanoskiving. These wires can be manually positioned by transporting them on drops of water and are directly electrically addressable; no further lithography is required to connect them to an electrometer. Once a block has been prepared for Nanoskiving (which takes less than one day), hundreds of thousands of nanogaps can be generated, on demand, at a rate of about one nanogap per second. After ashing the organic components with oxygen plasma, we measured the width of a free-standing gap formed from a SAM of 16-mercaptodohexanoic acid (2.4 nm in length) of 2.6 ± 0.5 nm by transmission electron microscopy. By fitting current–voltage plots of unashed gaps containing three alkanedithiolates of differing lengths to Simmons’ approximation, we derived a value of β = 0.75 Å^–1 (0.94 <i>n</i>_C^–1) at 500 mV. This value is in excellent agreement with literature values determined by a variety of methods, demonstrating that the gap-size can be controlled at resolutions as low as 2.5 Å (<i>i.e.</i>, two carbon atoms)

Thiol-containing polymeric embedding materials for nanoskiving

<p>This paper describes the characterization of new embedding resins for nanoskiving (ultramicrotomy) that contain thiols. Nanoskiving is a technique to produce nanoscale structures using an ultramicrotome to section thin films of materials (e. g., gold) embedded in polymer. Epoxies are used typically as embedding resins for microtomy. Epoxies, however, do not adhere well to gold or other smooth metallic structures that are used commonly for nanoskiving. Thiol-ene and thiol-epoxy polymers provide improved adhesion to gold due to the thiol functional group. In addition, the thiol-ene polymers can be prepared within minutes using photopolymerization, which allows for rapid prototyping. Two commercial thiol-containing adhesives were evaluated as resins in addition to several formulations of commercially available monomers. The important physical and mechanical properties for microtomy of these unconventional embedding resins were characterized and the properties were compared to commercial epoxy resins. Gold nanowires were fabricated using nanoskiving of gold films embedded in these unconventional resins. These studies show that a 3 : 4 mixture of thiol (pentaerythritol tetra(3-mercaptopropionate)) and ene (triallyl-1,3,5-triazine-2,4,6-trione) works very well as a resin for nanoskiving and provides improved adhesion and reduced preparation time relative to epoxies.</p>

Bisecting Microfluidic Channels with Metallic Nanowires Fabricated by Nanoskiving

This paper describes the fabrication of millimeter-long gold nanowires that bisect the center of microfluidic channels. We fabricated the nanowires by nanoskiving and then suspended them over a trench in a glass structure. The channel was sealed by bonding it to a complementary poly(dimethylsiloxane) structure. The resulting structures place the nanowires in the region of highest flow, as opposed to the walls, where it approaches zero, and expose their entire surface area to fluid. We demonstrate active functionality, by constructing a hot-wire anemometer to measure flow through determining the change in resistance of the nanowire as a function of heat dissipation at low voltage (<5 V). Further, passive functionality is demonstrated by visualizing individual, fluorescently labeled DNA molecules attached to the wires. We measure rates of flow and show that, compared to surface-bound DNA strands, elongation saturates at lower rates of flow and background fluorescence from nonspecific binding is reduced