Nano-fabrication of molecular electronic junctions by targeted modification of metal-molecule bonds

Abstract

Reproducibility, stability and the coupling between electrical and molecular properties are central challenges in the field of molecular electronics. The field not only needs devices that fulfill these criteria but they also need to be up-scalable to application size. In this work, few-molecule based electronics devices with reproducible electrical characteristics are demonstrated. Our previously reported 5 nm gold nanoparticles (AuNP) coated with ω-triphenylmethyl (trityl) protected 1,8-octanedithiol molecules are trapped in between sub-20 nm gap spacing gold nanoelectrodes forming AuNP-molecule network. When the trityl groups are removed, reproducible devices and stable Au-thiol junctions are established on both ends of the alkane segment. The resistance of more than 50 devices is reduced by orders of magnitude as well as a reduction of the spread in the resistance histogram is observed. By density functional theory calculations the orders of magnitude decrease in resistance can be explained and supported by TEM observations thus indicating that the resistance changes and strongly improved resistance spread are related to the establishment of reproducible and stable metal-molecule bonds. The same experimental sequence is carried out using 1,6-hexanedithiol functionalized AuNPs. The average resistances as a function of molecular length, demonstrated herein, are comparable to the one found in single molecule devices.

Figure 1. Schematic drawing of (a) dielectrophoretically trapped ω-trityl protected 1,8-octanedithiol-coated gold nanoparticles, and (b) the junction after removal of the trityl protective groups by usage of an acidic deprotection solution resulting in formation of chemisorbed junctions at both ends of 1,8-octanedithiol.

Figure 2

(a) Scanning electron image of ω-trityl protected 1,8-octanedithiol coated nanoparticles trapped in between nanoelectrode setup, and (b) current-voltage (I–V) response of the same device after (1) trapping and (2) after removal of trityl protective groups from the ω-end of 1,8-octanedithiol. The zero offset in curve 1 is due to the instrument and measurement system.

Figure 3

(a) Log-normal resistance histogram of devices having ω-trityl protected 1,8-octanedithiol in nanoparticle-nanoelectrode bridge platform (b) Linear scale resistance histogram (bin size = 2 GΩ) of 1,8-octanedithiol in nanoparticle-nanoelectrode bridge platform after removal of trityl protecting group and establishing a chemisorbed metal-molecule junction (inset log-normal distribution of the measured resistances). The shaded line indicates the number of devices that have demonstrated resistances lower than 100 MΩ after removal of protecting trityl groups. Inset: log-normal resistance histogram of the devices after deprotection. (c) Log-normal resistance histogram of devices before removal of trityl group from ω end of 1,8-octanedithiol in nanoparticle-nanoelectrode bridge platform , that shown resistance variation between 0.1 GΩ and 10 GΩ after removal of protection groups and formation of chemisorbed junctions at both ends of ODT (inset).

Figure 4

(a) Change in conductivity of devices upon removal of trityl protective group, after removal of trityl protective group chemisorbed junctions established at both ends of the 1,8-octanedithiol, and (b) atomic configuration considered in theoretical simulation I) 1,8-octanedithiol chemisorbed at two nearby gold surfaces II) ω-trityl protected 1,8-octanedithiol chemisorbed at one end and physisorbed at other end. III) ω-trityl protected 1,8-octanedithiol attached to the both gold surfaces in presence of surface layer of backbiting 1,8-octanedithiol on both gold surfaces IV) ω-trityl protected 1,8-octanedithiol attached to both electrode surface at maximum separation, and (c) zero-bias transmission for the four considered setups.

Figure 5. Transmission electron microscope images of (a) ω-trityl protected 1,8-octanedithiol coated nanoparticles, scale bar: 20 nm (Inset: high resolution image with a scale bar of 5 nm) and (b) after removal of the trityl protective groups from ω-end of 1,8-octanedithiol-coated nanoparticles in solution, scale bar: 20 nm (Inset: high resolution image of gold nanoparticles with a scale bar of 5 nm).

Figure 6

(a) Current-voltage (I–V) response of 1,6-hexandithiols coated gold nanoparticles in between nanoelectrode (inset: SEM of resultant device, scale bar:50 nm) after (1) trapping and (2) removal of trityl protective groups from ω-end of 1,6-hexanedithiol. (b) Linear scale resistance histogram (bin size = 200 MΩ) of 1,6-hexanedithiol in nanoparticle-nanoelectrode bridge platform after removal of trityl protecting group and establishing a chemisorbed metal-molecule junction.