Carbon/Molecule/Metal and Carbon/Molecule/Metal Oxide Molecular Electronic Junctions

Abstract

Molecular junctions were fabricated on the basis of a 1.7-4.5 nm thick layer of fluorene (FL) or nitroazobenzene (NAB) covalently bonded to a graphitic pyrolyzed photoresist film (PPF) substrate. The junction was completed with a top contact consisting of metallic Cu, TiO 2 , or aluminum(III) oxide (AlOx) and a final layer of Au. The current/voltage behavior of the junctions depended strongly both on the nature of the metal or metal oxide top layers and on the structure of the molecular layer. PPF/NAB/Cu/ Au and PPF/FL/Cu/Au junctions were highly conducting, with resistances of 0.3-1.7 Ω cm 2 , depending on the identity and thickness of the molecular layer. Substitution of Cu with either AlOx or TiO 2 caused a large increase in junction resistance by 2-4 orders of magnitude, but also yielded rectifying junctions in the case of PPF/NAB(4.5)/TiO 2 (3.1)/Au. For a positive bias (PPF relative to Au) above +2 V, the NAB(4.5)/TiO 2 (3.1) junction became highly conductive, apparently due to injection of electrons into the TiO 2 conduction band. PPF/NAB(4.5)/AlOx(3.3)/Au junctions exhibited symmetric i/V responses with very low currents, and capacitances consistent with those expected for a parallel plate capacitor with two dielectric layers. However, Raman spectroscopy of the NAB/AlOx junctions showed structural changes under negative bias corresponding to reduction of NAB, despite the absence of significant current flow. The changes were reversible and repeatable provided the bias was between -1.5 and +1.0, but partially irreversible when the bias excursion was negative of -1.5 V. Combined with a previous spectroscopic study of PPF/NAB/TiO 2 /Au junctions, the results imply a rectification mechanism based on electron transport through the NAB LUMO and the TiO 2 conduction band, and possibly a Coulombic barrier resulting from reduction of the NAB in the molecular junction.

Molecular Self‐Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin‐Film Transistor Applications

“…[142][143][144][145][146][147][148] As conjugated SAMs become more widely used in (and on) the gate dielectric of TFTs, it is important to consider the consequences that their smaller HOMO-LUMO gaps may have on the conduction mechanism of the SAM. [149] There is abundant literature on conduction mechanisms [84,[150][151][152][153] and on charge transport in SAMs; [70,118,120,144,145,[154][155][156][157][158] therefore we present a summary of some of the relevant conduction mechanisms and the most recent results relating to various SAMs.…”

Analytical Challenges in Molecular Electronics

“…Combined with other evidence, the results indicate a carbon/NAB/copper molecular junction with partial reduction of the NAB centers and formation of covalent copper-nitrogen bonds (57 ). Similar XPS analysis of PPF/fluorene/copper junctions revealed no copper-carbon bonds, nor any detectable oxygen within the junction (37 ). The electronic behavior of several PPF/molecule/copper molecular junctions is shown in Figure 4 (38).…”

“…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.…”

Organic semiconductors for device applications: current trends and future prospects

“…Despite improving the charge carrier transport in each layer used in an organic device, the interface properties of the metal-semiconductor [498][499][500][501][502][503][504][505][506][507][508][509][510][511][512] and insulatorsemiconductor [8,[513][514][515][516][517][518][519][520][521] are not only important from the point of view of influencing the device performance significantly but also assuming the role of a limiting factor while trying to extract the best out of a given configuration. For example, the quality of the interfaces between the OS on one side and source/drain electrode and the insulator on the other ultimately limits the overall performance of the OFETs realized experimentally.…”