In Situ Raman Spectroscopy of Bias-Induced Structural Changes in Nitroazobenzene Molecular Electronic Junctions

Abstract

Carbon/nitroazobenzene (NAB)/titanium/gold molecular electronic junctions with active thicknesses of 7-8 nm were constructed having partially transparent Ti/Au top contacts, which permitted in situ monitoring of molecular structure with Raman spectroscopy for applied biases between +3 and -3 V. Deposition of the Ti/Au top contacts resulted in spectral changes similar to those accompanying NAB reduction in a conventional spectroelectrochemical experiment. Upon application of +3 V (carbon relative to Ti), the spectrum indicated reoxidation of the NAB reduction product, and this redox cycle could be repeated at least three times. When a voltage excursion to -2 or -3 V occurred, the spectra indicated irreversible loss of the nitro group, and a dramatic but reversible decrease in Raman intensity over the entire shift range examined. Negative applied voltage causes formation of reduced NAB and a high oxidation state of titanium, while positive voltage forms oxidized NAB and injects electrons into the titanium oxide layer. The spectral changes were correlated with current/voltage curves in order to probe the mechanism of rectification and conductance switching reported previously. Overall, the combination of spectroscopic and voltammetric results implies a conduction mechanism involving both NAB and titanium oxide, possibly mediated by the injection of carriers into the semiconducting titanium oxide, or by reduction of an insulating titanium oxide to a more conductive form.

A 160-kilobit molecular electronic memory patterned at 1011 bits per square centimetre

“…The amount of rectification is dependent upon the amount of titanium oxidation that occurs at the molecule/Ti interface which, in turn, depends upon the vacuum level of the metal deposition system 33 . For the devices reported here, the Ti was deposited at a pressure of approximately 5e-7 …”

Analytical Challenges in Molecular Electronics

“…Photo and schematic of a carbon/nitroazobenzene/ TiO 2 /gold molecular junction with lateral dimensions of 0.5 ϫ 1 mm and an active thickness of 50 Å. Probes in photograph connect the carbon and gold contacts with external electronics to obtain current as a function of applied voltage (37,49,53 Our line of investigating molecular electronics involves the many-molecule approach, in which ~10 11 molecules are bonded to a conducting carbon substrate, and a top contact of metal or metal oxide is applied (Figure 2; 36 -45). The core of the device is an oriented layer of molecules covalently bonded to the carbon substrate and a conducting top contact, which may also interact covalently with the molecular layer.…”

“…The spectra were acquired for 20 s each, from top to bottom (37,53). Richard L. McCreery, a professor at Ohio State University, conducts research in molecular electronics, electrochemistry, and surface analysis.…”

Direct structural observation of a molecular junction by high-energy x-ray reflectometry

“…Hipps et al have used both inelastic electron tunneling spectroscopy (2) and surface Raman spectra (3) to obtain structural information of buried junctions, but without achieving angstrom resolution. Raman and Fourier transform infrared measurements of similar resolution levels also have been carried out (4,5). We present here an angstrom resolution direct determination of the structure of such buried junctions comprising a single or a double organic molecular layer at the interface between two electrodes: mercury and silicon.…”