Nondestructive Contact Deposition for Molecular Electronics: Si-Alkyl//Au Junctions

Abstract

One of the major problems in molecular electronics is how to make electronically conducting contact to the "soft" organic and biomolecules without altering the molecules. As a result, only a small number of metals can be applied, mostly by special deposition methods with severe limitations. Transferring a predefined thin metal leaf onto a molecular layer provides a nondestructive, noninvasive contacting method that is, in principle, applicable to many types of metal and a variety of metal/molecules combinations. Here we report a modification of our earlier lift-off, float-on (LOFO) method, using as a basis its offspring, the polymer-assisted lift-off (PALO) method, where a backing polymer enables simultaneous deposition of multiple contacts as well as reduces wrinkles in the thin metal leaf. The modified PALO (MoPALO) method, reported here, adds lithography steps to obviate the need to punch through the polymer, as is done to complete PALO contacts. Morphological characterization of the electrodes indicates highly uniform, wrinkle-free contacts of negligible roughness. The good electrical performance of the MoPALO contacts was proven with metal/organic-monolayer/ semiconductor (MOMS) junctions, which are known to be very sensitive to molecular degradation and metal penetration. We also show how MoPALO contacts enabled us to compare the effect of varying the metal work function and contact area on the current-voltage characteristics of MOMS devices.

Reversible Soft Top‐Contacts to Yield Molecular Junctions with Precise and Reproducible Electrical Characteristics

“…Table is not comprehensive, but we included data obtained by large‐area SAM‐based junctions that contain large numbers of molecules, and techniques based on scanning probes that contain small numbers of molecules or even single molecules. In typical scanning tunneling microscope (STM) measurements, the air or vacuum gap between the tip and the molecules complicates evaluating the true conductance of the molecules .…”

“…Other techniques have avoided metal deposition by using liquid‐metal top‐electrodes (Hg or GaO x /EGaIn) which deform and conform to, rather than penetrate, the SAM once brought into contact with the SAM. Others have deposited solid electrodes from solution or used bending wires to form junctions …”

“…Many fabrication methods use protective layers (to protect the SAMs during the metal deposition process to form the top‐contacts) that are deposited by solution based processes, or on the deposition of the electrode from solution . We believe that our method to form electrical contacts to SAMs of n‐alkanethiolates can be readily extended to other types of SAMs, monolayers of biomolecules, or other types of materials that may not be compatible with direct deposition methods of metals, or exposure to solvents, to form high quality junctions in good yields with high reproducibility.…”

Molecules on Si: Electronics with Chemistry

“…Atomic layer deposition (ALD) is at present the most promising alternative to non-vacuum deposition methods that are non-invasive, [188] such as Hg, [63,133,154,189] LOFO, [64,87,190] PALO, [191] MoPALO. [20,192] Chemical reactivity across the monolayer explains also why Hg works well with Si or Ag substrate but not with, e.g., Au (amalgamation). Use of InGa or Ga as metallic liquid contact [193] instead of Hg requires working in inert atmosphere to avoid uncontrolled metal oxidation, as InGa or Ga are much less noble than Hg.…”

Symmetric Large‐Area Metal‐Molecular Monolayer‐Metal Junctions by Wedging Transfer

“…To our knowledge, only one technique allows the fabrication of large‐area metal‐molecule‐metal junctions with a relatively high yield, without posing restrictions on the end group of the monolayer 25–27. The latest iteration of this technique, permanent modified polymer‐assisted lift‐off (PeMoPALO),27 introduced electrical contacting away from the molecular junction, removed the restriction on the junction area, and introduced the possibility to make permanent contacts via wire bonding. The system was used to study Si‐alkyl‐Au junctions (covalently attached monolayers of alkenes with a H‐passivated Si surface).…”