Accessing the intrinsic functionality of molecules for electronic applications1,2,3, light emission4 or sensing5 requires reliable electrical contacts to those molecules. A self-assembled monolayer (SAM) sandwich architecture6 is advantageous for technological applications, but requires a non-destructive, top-contact fabrication method. Various approaches ranging from direct metal evaporation6 over poly(3,4-ethylenedioxythiophene) polystyrene sulfonate7 (PEDOT:PSS) or graphene8 interlayers to metal transfer printing9 have been proposed. Nevertheless, it has not yet been possible to fabricate SAM-based devices without compromising film integrity, intrinsic functionality or mass-fabrication compatibility. Here we develop a top-contact approach to SAM-based devices that simultaneously addresses all these issues, by exploiting the fact that a metallic nanoparticle can provide a reliable electrical contact to individual molecules10. Our fabrication route involves first the conformal and non-destructive deposition of a layer of metallic nanoparticles directly onto the SAM (itself laterally constrained within circular pores in a dielectric matrix, with diameters ranging from 60 nanometres to 70 micrometres), and then the reinforcement of this top contact by direct metal evaporation. This approach enables the fabrication of thousands of identical, ambient-stable metal–molecule–metal devices. Systematic variation of the composition of the SAM demonstrates that the intrinsic molecular properties are not affected by the nanoparticle layer and subsequent top metallization. Our concept is generic to densely packed layers of molecules equipped with two anchor groups, and provides a route to the large-scale integration of molecular compounds into solid-state devices that can be scaled down to the single-molecule level.
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We acknowledge technical support from M. Tschudy, S. Reidt, M. Sousa, U. Drechsler, A. Zulji and M. Bürge, as well as strategic support from B. Michel, W. Riess and A. Curioni. This work was funded by the NCCR MSE and SNF NRP 62.
Extended data figures and tables
a, Three example curves of an open circuit. b, Four example curves with different ratios of resistance at 50 mV and 300 mV bias. The top two (orange and blue lines) are categorized as linear, whereas the lower two (yellow and grey lines) are categorized as nonlinear.
a, AFM scan of a 2.7-μm-diameter pore. b, The surface at the bottom of the pore with a roughness of 0.39 nm RMS. c, A similar pore after film growth and nanoparticle deposition. d, Scan of the surface inside the pore, showing circular particles.
a, TEM images of part of a C10 device lamella. Both the platinum bottom electrode and the gold top electrode are visible as bright layers, separated by the SAM on the left side and the SiNx dielectric on the right. The top electrode is warped in some regions. b, Zoom into a warped region: the bottom electrode, SAM region and top electrode are clearly visible, as well as the electron-deposited platinum protection layer for extraction of the lamella (the grainy area at the top of the image). c, d, High-resolution TEM images of two different areas of the SAM region. Although crystal lattices are visible, no 3-nm-sized features can be identified that would correspond to nanoparticles.
Energy-dispersive X-ray spectrum of a region located approximately 2 nm above the SAM. Three elements can be identified: carbon, gold and copper.
a, Arrhenius plot for two devices (solid black, 790 nm diameter; dashed blue, 5.5 μm), showing a very slight temperature dependence. b, c, Preliminary results of current density for C8 and C10 monolayers with two different nanoparticle diameters: 3 nm (b) and 5 nm (c). The current density decreases for larger nanoparticles.
a, Histogram of the current ratio between upward and downward sweeps at 0.5 V, with no substantial change between the two sweeps. b, Histogram of the current ratio between the initial sweep and the sweep taken 136 days later, at 0.5 V. A large part of the devices did not change, or changed by only a small amount.
A comparison of the full width half maximum (FWHM) current density deviation σJ against active device area for the literature quoted in Extended Data Table 1. We obtain similar or lower deviation than the literature samples over a considerably increased range in device area.
a–d, Histograms of I versus A. e–h, Device categorization for different values of A. i–l, Histograms of J versus A. The molecular length increases from left to right (a, e, i, C4; b, f, j, C6; c, g, k, C8; d, h, l, C10).