The field of molecular electronics mainly entails the assessment of electrical response of single molecules or assemblies of molecules in parallel between two electrode contacts, referred to as a molecular junction. Recent focus in this field has been motivated by the need to develop novel electronic functions in solid-state devices. This has proven to be quite a daunting challenge with the common conventional electronic materials. It is well known that light interactions with molecular devices not only offer vital diagnostics of structure and internal energy levels, but also possess the capability to be exploited for photodetection and light emission. Recent studies have shown that a good comprehension of the interaction between the structure, electric behavior and orbital energies of both single and multilayer molecular junction is paramount to engineering novel molecular electronics devices for a potentially broad range of electronic functions. Unfortunately, the mono-component molecular junctions studied to date have exhibited relatively weak photo-effects due to, in part, the weak optical absorption by the thin molecular layer.
Recently, research conducted by Professor Richard McCreery and postdoctoral fellow Dr. Scott Smith from the Department of Chemistry at University of Alberta examined the photo-effects in molecular bilayers. Their main motivation was the fact that the relationships between internal energy levels of both the molecules and the associated contacts in completed, functioning molecular junctions could be revealed by the photo-response of the molecular junctions. Secondly, the fact that sensitivity to light might be a potentially useful function in molecular electronics, therefore, bilayer devices would be excellent candidates for photodetectors. Their work is currently published in the research journal, Advanced Electronic Materials.
In brief, the research method employed commenced with material purchase and preparation where for the latter, bottom electrode contacts were patterned on diced, fused quartz chips by sequential electron beam evaporation through a shadow mask. Next, the layers were fabricated and characterized by Raman spectroscopy. Density functional theory calculations were later performed for the purposes of predicting the lowest unoccupied and highest occupied molecular orbitals levels of bisthienylbenzene and anthraquinone as free molecules. Lastly, molecular layer thicknesses were determined near molecular junctions through an atomic force microscopy scratching technique.
The authors observed that the bilayer structure not only introduced asymmetry into the molecular junction by juxtaposing two sets of molecular orbital energies but also enabled optical transitions which directly transferred charge between the two molecular layers. Additionally, they noted that the covalent bond at the organic-organic interface created a distinct molecule with different optical and electronic properties from its components, and which has the potential to be exploited for enhancing the observed photocurrents and changing its polarity.
In summary, University of Alberta scientists presented the fabrication and utilization of bilayer donor-acceptor molecular junctions having an active layer thickness of less than 15 nm to study the mechanism of photo-induced charge transport. In general, it was observed that the direction of rectification and the photocurrents polarity depended on the relative energies of the two molecular layers, and not on illumination direction or differences in the contact/molecule interfaces. Altogether, in addition to providing probes of internal energy levels and charge transport in molecular junctions, photo-induced currents and voltages may potentially be useful in light-weight, durable, and ﬂexible photodetectors.
Scott R. Smith, Richard L. McCreery. Photocurrent, Photovoltage, and Rectification in Large-Area Bilayer Molecular Electronic Junctions. Advanced Electronic Material 2018, volume 4, 1800093.Go To Advanced Electronic Material
Tefashe, U. M.; Nguyen, Q. V.; Lafolet, F.; Lacroix, J.-C.; McCreery, R. L., Robust Bipolar Light Emission and Charge Transport in Symmetric Molecular Junctions. J. Am. Chem. Soc. 2017, 139, 7436-7439.Go To J Am Chem Soc.
Ivashenko, O.; Bergren, A. J.; McCreery, R. L., Monitoring of Energy Conservation and Losses in Molecular Junctions through Characterization of Light Emission. Advanced Electronic Materials 2016, 2, 1600351.Go To Advanced Electronic Material