Musical molecules: the molecular junction as an active component in audio distortion circuits.

Molecular junctions that have a non-linear current-voltage characteristic consistent with quantum mechanical tunneling are demonstrated as analog audio clipping elements in overdrive circuits widely used in electronic music, particularly with electric guitars. The performance of large-area molecular junctions fabricated at the wafer level is compared to currently standard semiconductor diode clippers, showing a difference in the sound character. The harmonic distributions resulting from the use of traditional and molecular clipping elements are reported and discussed, and differences in performance are noted that result from the underlying physics that controls the electronic properties of each clipping component. In addition, the ability to tune the sound using the molecular junction is demonstrated. Finally, the hybrid circuit is compared to an overdriven tube amplifier, which has been the standard reference electric guitar clipped tone for over 60 years. In order to investigate the feasibility of manufacturing molecular junctions for use in commercial applications, devices are fabricated using a low-density format at the wafer level, where 38 dies per wafer, each containing two molecular junctions, are made with exceptional non-shorted yield (99.4%, representing 718 out of 722 tested devices) without requiring clean room facilities.

Spatially resolved Raman spectroelectrochemistry of solid-state polythiophene/viologen memory devices.

A three terminal molecular memory device was monitored with in situ Raman spectroscopy during bias-induced switching between two metastable states having different conductivity. The device structure is similar to that of a polythiophene field effect transistor, but ethylviologen perchlorate was added to provide a redox counter-reaction to accompany polythiophene redox reactions. The conductivity of the polythiophene layer was reversibly switched between high and low conductance states with a "write/erase" (W/E) bias, while a separate readout circuit monitored the polymer conductance. Raman spectroscopy revealed reversible polythiophene oxidation to its polaron form accompanied by a one-electron viologen reduction. "Write", "read", and "erase" operations were repeatable, with only minor degradation of response after 200 W/E cycles. The devices exhibited switching immediately after fabrication and did not require an "electroforming" step required in many types of memory devices. Spatially resolved Raman spectroscopy revealed polaron formation throughout the polymer layer, even away from the electrodes in the channel and drain regions, indicating that thiophene oxidation "propagates" by growth of the conducting polaron form away from the source electrode. The results definitively demonstrate concurrent redox reactions of both polythiophene and viologen in solid-state devices and correlate such reactions with device conductivity. The mechanism deduced from spectroscopic and electronic monitoring should guide significant improvements in memory performance.

Wet-etching of structures with straight facets and adjustable taper into glass substrates.

Wet etching of glass by hydrofluoric acid is widely used in microfabrication, but is limited by the isotropic nature of the process that leads to rounded sidewalls and a 90 degrees angle between the etch front and the surface of the substrate. For many applications such as microvalving, or for further processing such as spin-coating, well controlled, gently sloping sidewalls are often preferred. Here, we present a new approach for forming straight facets and for adjusting the sidewall angle in wet-etched glass substrates by controlling the lateral dissolution of an etch mask during etching. The etch mask comprises a Ti-Au bilayer where Au serves to protect the Ti. During isotropic etching of glass by HF the Ti layer is etched away laterally at the same time, which leads to straight, gently sloping sidewalls. We introduce two methods for controlling the sidewall angle. The first one is based on adjusting the thickness of Ti which controls the lateral etch rate, and thus the angle; the thinner the Ti, the slower its lateral etch rate and the steeper the angle in the etched glass. The second method involves a cathodic bias applied to the Ti-Au etch mask which again regulates the dissolution rate of Ti; the more negative the bias the slower the lateral etch rate. Both methods offer accurate control of the sidewall angle over a wide range, can be readily integrated into existing fabrication processes, and will be particularly useful for making channels with trapezoidal cross-sections, valve seats with gentle profiles, or for patterning electrodes across and inside of microfluidic channels.