Eutectic gallium-indium (EGaIn): a moldable liquid metal for electrical characterization of self-assembled monolayers.

Herein we describe the formation of conformal electrodes from the fluid metal eutectic, Ga–In (which we abbreviate “EGaIn” and pronounce “e-gain”; 75% Ga, 25% In by weight, m.p.= 15.5 8C), and their use in studying charge transport across self-assembled monolayers (SAMs). Although EGaIn is a liquid at room temperature, it does not spontaneously reflow into the shape with the lowest interfacial free energy as do liquids such as Hg and H2O: as a result, it can be formed into metastable, nonspherical structures (e.g., cones, and filaments with diameters 1 mm). This behavior, along with its high electrical conductivity (3.4 4 10 Scm ) and its tendency to make low contact-resistance interfaces with a variety of materials, makes EGaIn useful for forming electrodes for thin-film devices. We discuss the convenience and precision of measurements of current density (J, Acm ) versus applied voltage (V, V) through SAMs of n-alkanethiolates on template-stripped, ultraflat Ag (Ag–SCn Ag–SCnH2n+1, n= 10, 12, 14, 16) using EGaIn. An ideal electrode for physical-organic studies of SAMs would 1) make conformal, but nondamaging, physical contacts, 2) readily form small-area (micrometer diameter) contacts, to minimize the contribution of defects in the SAM to J, 3) form without specialized equipment, and 4) be nontoxic. Point 3 is particularly important: elimination of procedures such as evaporating metals or lithographic patterning would allow a wide range of laboratories—including those without access to clean rooms—to survey relationships between structure and electrical conductivity. There are currently three general techniques for forming top contacts for large-area (i.e., more than a few molecules) electrical measurements on SAMs of organic molecules: 1) The direct deposition of metals such as Au or Ti by using electron-beam or thermal evaporation ensures atomic-level contact, but results in low yields of devices owing to damage to the organic monolayer by reaction with hot metal vapors, and in the formation of metal filaments that short the junctions. 2) The installation of an electrically conducting polymer between the SAM and a metallic top contact inhibits formation of metal filaments, but the instability of SAMs of alkanethiolates to the temperatures required to anneal most electroactive polymers limits the broad application of this approach. 3) The use of Hg allows formation of conformal contacts at room temperature, but Hg is toxic, amalgamates with metals, tends to form junctions that short, is difficult to form into small contacts, and measurements with Hg must be performed under a solvent bath. EGaIn does not flow until it experiences a critical surface stress (0.5 Nm ), at which point it yields (i.e., flows). EGaIn 1) makes conformal, nondamaging contacts at room temperature, 2) can be molded into nonspherical shapes with micrometer-scale (or larger) dimensions, 3) is commercially available, 4) can be deposited with a pipette or syringe without high temperatures or vacuum, 5) has a low vapor pressure, and 6) is nontoxic. The work function of EGaIn (4.1–4.2 eV) is close to that of Hg (4.5 eV), but EGaIn does not alloy with many metals. It is therefore an ideal replacement for Hg, especially in devices that incorporate SAMs (which are generally formed on Au or Ag). Auger spectroscopy on samples of EGaIn in air show that its surface is principally composed of oxides of Ga (see the Supporting Information); gallium oxide is an n-type semiconductor. There is undoubtedly an adsorbed film of water on this surface, as EGaIn has a high surface free energy (ca. 630 dynescm ), as do oxides formed from similar metals. During our measurements, there were no observable changes in the average magnitude or range of J when EGaIn was allowed to sit in air for extended periods before we deposited it on the SAM, or when we performed the measurements using the same drop of EGaIn to form between three and five junctions, or while we flowed dry N2 over the sample: therefore the contribution of the surface oxide to J was probably constant for the duration of the experiments. We formed EGaIn electrodes by suspending a drop of EGaIn from a metal 26s-gauge needle affixed to a 10-mL syringe, bringing the drop into contact with the bare surface of a sacrificial film of Ag using a micromanipulator, and retracting the needle slowly (ca. 50 mms ); the EGaIn adhered to both the needle and the Ag (Figure 1). The drop of EGaIn pinched into to an hour-glass shape until it bifurcated into two structures, one attached to the syringe (a cone approximately 0.05 mL in volume) and one (which was discarded) attached to the Ag. We produced conical tips of EGaIn with diameters ranging from less than 1 mm to 100 mm; the larger the bore of the needle, and the more rapidly we [*] Dr. R. C. Chiechi, Dr. E. A. Weiss, Dr. M. D. Dickey, Prof. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University 12 Oxford St., Cambridge, MA 02138 (USA) Fax: (+1)617-495-9857 E-mail: gwhitesides@gmwgroup.harvard. edu