A crosslinked SAM of 5,5′-bis (mercaptomethyl)-2,2′-bipyridine-Ni2+ (BPD-Ni2+) has been prepared on top of the pre-patterned Au bottom contacts. Then the top Au contacts were evaporated. A two-electrode probe station

was used to assess the fidelity of the molecular junctions. Additionally, to elucidate the molecular transport in the device junctions, temperature-dependent I-V examinations were performed. Methods Fabrication of the crossbar molecular devices Fabrication of the bottom electrode Lithography of bottom electrodes was accomplished by starting with a clean single-side polished SiO2 substrate. Photoresist TPCA-1 mw PMMA 950 was spin-coated on SiO2 at 2,000 rpm for 90 s and baked at 180°C for 3 min (Figure 1a). Then, to avoid the charge-up of PMMA, 15 nm of conductive polymer (ESPACER 300Z; Showa Denko K.K., Minato, Tokyo, Japan) was spin-coating on the top of the PMMA at 2,000 rpm for 60 s. RO4929097 The

100-nm bar patterns were fabricated using an electron beam lithography system (50 kV, 100 mC/cm2; Elionix Co. Ltd., Hachioji, Tokyo, Japan). The resist was developed in MIBK methyl isobutyl ketone + IPA isopropanol 1:3 solution (MIBK-IPA) for 30 to 40 s to remove the irradiated zones and to form a pattern for the bottom electrode bars (Figure 1b). Finally, using electron-beam deposition, 10 nm of titanium and 150 nm of gold were deposited on the photoresist-patterned wafer. The wafer was immersed in acetone to remove the photoresist and the excess metal which adhered on the resist (Figure 1c). Figure 1 Scheme process flow for fabrication of crossbar molecular devices. (a) Photoresist patterning for bottom contacts on SiO2. (b) The 100-nm bar patterns were created

using electron beam lithography. (c) Deposition of 10 nm of Ti and 150-nm Au over patterned substrate and lift-off excess Au with photoresist removal. (d) Deposition of SAM over the entire substrate. (e) Preparation and deposition of top electrodes. Preparation of the crosslinked BPD-Ni2+ SAM The SAM of BPD films was fabricated in the following manner: 5,5′-bis(mercaptomethyl)-2,2′-bipyridine was purchased from Aldrich and used as received. The SAM of 5,5′-bismercaptomethyl-2,2′-bipyridine (BPD) was prepared by Carnitine palmitoyltransferase II immersing the bottom electrodes in freshly prepared 1-mM solution of n-hexane for 1 h at 60°C. Solutions were well-degassed using Ar. All preparation steps were performed in the absence of ambient light, which is the same as the process in our previous studies [4, 6]. Subsequently, the bottom gold bar was modified with a layer of BPD and immersed for 3 h in a 50-mM aqueous solution of NiCl2 (see Figure 2a,b). Figure 2 Preparation of the cross-linked BPD-Ni 2+ SAM. (a) Preparation of the BPD SAM. (b) Encapsulation of Ni on the BPD SAM. (c) A BPD-Ni system was employed as a negative resist for e-beam lithography. Microscope image of etched BPD-Ni/Au template, preliminary patterned by electrons in proximity printing geometry using a metal mesh as mask.