We have developed a controlled and highly reproducible method of making nanometer-spaced electrodes using electromigration in ambient lab conditions. This advance will make feasible single molecule measurements of macromolecules with tertiary and quaternary structures that do not survive the liquid-helium temperatures at which electromigration is typically performed. A second advance is that it yields gaps of desired tunnelling resistance, as opposed to the random formation at liquid-helium temperatures. Nanogap formation occurs through three regimes: First it evolves through a bulk-neck regime where electromigration is triggered at constant temperature, then to a few-atom regime characterized by conductance quantum plateaus and jumps, and finally to a tunnelling regime across the nanogap once the conductance falls below the conductance quantum.Electromigration has recently been successfully employed to make nanometer-spaced electrodes for single molecule devices [1,2,3,4]. The typical procedure entails an abrupt break at liquid-helium temperatures that yields a nanogap with a random tunnelling resistance [4,5,6,7,8]. However, this procedure makes gaps at room temperature which are typically too large for molecular measurements [6]. This hinders the application of the typical electromigration procedure to molecules which do not survive a sub-freezing environment, such as macromolecules that feature modest thermodynamic stability of their respective tertiary and quaternary structures.We have developed an electromigration procedure that is completely performed in ambient laboratory conditions and yields a controllable nanogap resistance to within a factor of about three of the target value in the 0.5 MΩ to 1 TΩ range. The electromigration procedure evolves through three regimes. At large conductance (G), local heating increases Au mobility and triggers electromigration in the metallic neck at a critical temperature. When the neck narrows to the few-atom regime it shows jumps and plateaus near multiples of the conductance quantum (G o = 2e 2 /h) and a sharp decrease in the critical temperature. A tunnelling regime is entered once G falls below G o accompanied by formation of a nanogap.We first fabricate two overlapping Au leads (each 8-30 nm thick) using electron-beam lithography and doubleangle evaporation of Au (Fig. 1a). An initial 3 nm thick Cr layer (deposited normal to surface) helps the contact * Electronic Address: drstrach@sas.upenn.edu † Electronic Address: cjohnson@physics.upenn.edu FIG. 1: (a) Field-emission SEM micrograph of electrodes before electromigration. (b) Nanogap after electromigration.pads adhere to the SiO 2 substrate while a final 40 nm thick layer of Au on the contacts reduces the resistance to between 100-200 Ω at room temperature. At room temperature and atmospheric pressure, we perform controlled electromigration with a succession of voltage (V ) ramps while monitoring the current (I) and conductance of the leads (Fig. 2). We make an initial measurement of G and compare this to later mea...

The photoconductivity of nanorods self-assembled from meso-tetrakis(4-sulfonatophenyl)porphine is described. The nanorods are insulating in the dark. Upon illumination with 488 nm light, the nanorods become photoconductive, exhibiting a rapid turn on/off (<100 ms) of the current when the light is turned on/off. This photoconductivity grows over hundreds of seconds with light exposure and decays slowly when the light is off. The nanorods can be trained via an applied bias to exhibit a short-circuit photocurrent (with corresponding open-circuit photovoltage) that flows in the direction opposite that of the training bias. A qualitative model is proposed, in which conduction occurs through the tightly coupled LUMOs of close-packed porphyrin molecules.

Electromigrated nanogaps have shown great promise for use in molecular scale electronics. We have fabricated nanogaps on free-standing transparent SiN(x) membranes which permit the use of transmission electron microscopy (TEM) to image the gaps. The electrodes are formed by extending a recently developed controlled electromigration procedure and yield a nanogap with approximately 5 nm separation clear of any apparent debris. The gaps are stable, on the order of hours as measured by TEM, but over time (months) relax to about 20 nm separation determined by the surface energy of the Au electrodes. A major benefit of electromigrated nanogaps on SiN(x) membranes is that the junction pinches in away from residual metal left from the Au deposition which could act as a parasitic conductance path. This work has implications to the design of clean metallic electrodes for use in nanoscale devices where the precise geometry of the electrode is important.

The excellent properties of transistors, wires, and sensors made from single-walled carbon nanotubes (SWNTs) make them promising candidates for use in advanced nanoelectronic systems. 1 Gas-phase growth procedures such as the high pressure decomposition of carbon monoxide (HiPCO) method 2,3 yield large quantities of small-diameter semiconducting SWNTs, which are ideal for use in nanoelectronic circuits. Asgrown HiPCO material, however, commonly contains a large fraction of carbonaceous impurities that degrade properties of SWNT devices. 4 Here we demonstrate a purification, deposition, and fabrication process that yields devices consisting of metallic and semiconducting nanotubes with electronic characteristics vastly superior to those of circuits made from raw HiPCO. Source-drain current measurements on the circuits as a function of temperature and backgate voltage are used to quantify the energy gap of semiconducting nanotubes in a field effect transistor geometry. This work demonstrates significant progress towards the goal of producing complex integrated circuits from bulkgrown SWNT material.To date most work on nanotube electronics has relied on SWNTs grown directly onto substrates by chemical vapor deposition (CVD). 5-7 CVD-grown SWNTs have clean sidewalls enabling high-quality electrical contact to metal electrodes, 8,9 and they hold promise for accurate placement in integrated circuits using patterned catalysts. However, the use of such material for integrated devices is made difficult by lack of control over whether individual SWNTs are metallic or semiconducting. Moreover, semiconducting SWNTs grown by CVD are typically large diameter (2 -4 nm), leading to an energy band gap much less than 1 eV. This makes them incompatible with designs for nanoelectronic logic gates consisting of SWNT transistors with high ON/OFF ratios.

We have developed a technique for simultaneously fabricating large numbers of nanogaps in a single processing step using feedback-controlled electromigration. Parallel nanogap formation is achieved by a balanced simultaneous process that uses a novel arrangement of nanoscale shorts between narrow constrictions where the nanogaps form. Because of this balancing, the fabrication of multiple nanoelectrodes is similar to that of a single nanogap junction. The technique should be useful for constructing complex circuits of molecular-scale electronic devices.

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Self-assembled block copolymer patterns may render more robust masks for plasma etch transfer through block-selective infiltration with metal oxides, affording opportunities for improved high contrast, high fidelity pattern transfer for sub-15 nm lithography in wafer-scale processes. However, block selective infiltration alters the self-assembled block copolymer latent image by changing feature size, duty cycle, and sidewall profile. The authors systematically investigate the effects of aluminum oxide infiltration of 27 and 41 nm pitch line/space patterns formed using polystyrene-b-poly(methyl methacrylate) block copolymers and evaluate the process compatibility with directed self assembly. The degree of image distortion depends on the amount of infiltrated material, with smaller amounts resulting in complete mask hardening and larger amounts shifting and collapsing pattern features. An attractive feature of the resulting oxide mask is the relatively smooth line edge roughness of the final transferred features into Si with a 3σ = 2.9 nm line edge roughness.

Determination of the three-dimensional order in thin nanostructured films remains challenging. Real-space imaging methods, including electron microscopies and scanning-probe methods, have difficulty reconstructing the depth of a film and suffer from limited statistical sampling. X-ray and neutron scattering have emerged as powerful complementary techniques but have substantial data collection and analysis challenges. This article describes a new method, grazingincidence transmission small-angle X-ray scattering, which allows for fast scattering measurements that are not burdened by the refraction and reflection effects that have to date plagued grazing-incidence X-ray scattering. In particular, by arranging a sample/beam geometry wherein the scattering exits through the edge of the substrate, it is possible to record scattering images that are well described by straightforward (Born approximation) scattering models. ‡ Present address: