A 17 GHz molecular rectifier

Abstract

Molecular electronics originally proposed that small molecules sandwiched between electrodes would accomplish electronic functions and enable ultimate scaling to be reached. However, so far, functional molecular devices have only been demonstrated at low frequency. Here, we demonstrate molecular diodes operating up to 17.8 GHz. Direct current and radio frequency (RF) properties were simultaneously measured on a large array of molecular junctions composed of gold nanocrystal electrodes, ferrocenyl undecanethiol molecules and the tip of an interferometric scanning microwave microscope. The present nanometre-scale molecular diodes offer a current density increase by several orders of magnitude compared with that of micrometre-scale molecular diodes, allowing RF operation. The measured S₁₁ parameters show a diode rectification ratio of 12 dB which is linked to the rectification behaviour of the direct current conductance. From the RF measurements, we extrapolate a cut-off frequency of 520 GHz. A comparison with the silicon RF-Schottky diodes, architecture suggests that the RF-molecular diodes are extremely attractive for scaling and high-frequency operation.

Figure 1. Description/characterization of the RF molecular rectifier.

(a) Schematic representation of the molecular junction composed of a gold nanocrystal, Ferrocenyl undecanethiol (FcC₁₁) molecules enabling rectification properties, and a Pt tip. Nanocrystals form an ohmic contact to a highly doped silicon substrate. The Pt tip is biased (through a bias-T) to both d.c. and HF (1–18 GHz) excitation simultaneously. (b) Picture of a 1 cm × 1 cm array of gold nanocrystals used for X-ray photoemission spectroscopy (XPS) or Cyclic Voltammetry measurements. The picture is taken just after dipping the sample into HF (for removal of the SiO₂ covering dots), the gold nanoarray area being identified through an hydrophilic/hydrophobic contrast. (c) Gold nanodot array imaged by Scanning Electron Microscope (SEM). Scale bar, 200 nm. (d) XPS measurements for SAMs of FcC₁₁ grafted on gold nanocrystals (∼1 billion dots fabricated by high-speed lithography) showing the presence of a Fe doublet related with ferrocene at 707.8 eV and 720.7 eV (Fe 2p_3/2 and Fe 2p_1/2, respectively), anda Fe doublet related to ferricenium at at 710.6 eV at 723.9 eV (2p_3/2 and 2p_1/2, respectively). The XPS signal for the bare Au nanoarray is also shown as a reference. Inset: schematic representation of the SAM with Ferricenium molecules located at dots borders due to the presence of a negatively charged silica. (e) Cyclic voltammetry measurements supports the presence of ferrocenyl molecules on the nanodots with a double peak at E=0.34 V and 0.37 V versus Ag/AgCl as a reference electrode in agreement with previous studies. (f) 2D histogram (normalized to 1) showing the I–V (and J–V) curve from one hundred junctions on 20 nm gold nanoparticles. The voltage step is 0.1 V, and the 2D histogram is obtained by the contour plot function (Originlab). The applied load was 18 nN (see Supplementary Note 4 for a detailed discussion on the tip load).

Figure 2. Sketch of the energetic level of the molecular device.

(a) Sketch of the energetic level of the molecular device when negative (−1 V) is applied on the Pt tip (see details in Supplementary Information). (b) Same as a with 0 V applied on the tip. (c) Same as a with −1 V applied on the tip. (d) Same as a with +1 V applied on the tip. We considered a coupling parameter of 0.8 to the Fc from the gold atom (80% of the potential drop occurs in the alkyl chain) to calculate the energy shift of the HOMO levels in the junction (see Supplementary Fig. 6 for details).

Figure 3. Demonstration of a molecular rectifier at 3.78 and 17.8 GHz.

(a–c) iSMM images at 0.8 V d.c. and 3.78 GHz measured simultaneously by the amperometer (Resiscope) for the d.c. (a) and vectorial network analyser (VNA) for amplitude (b) and phase (c) S₁₁ parameters. Scale bars, 500 nm. (d,e) 2D |S₁₁| histogram (normalized to one) versus tip bias (V) and ϕ(S₁₁)–V 2D histograms (normalized to one) generated from one hundred molecular rectifier junctions. The voltage step was 0.05 V and the contour plot generated automatically (Originlab). The applied load was 18 nN. (f) 2D d.c. I–V histogram from one hundred of ferrocenyl undecanethiol gold nanojunctions with a 17.8 GHz RF input signal. The d.c. reference current (solid line) when no RF input signal was added is shown for comparison. It was obtained from the average I–V from a 2D histogram, without RF power. The voltage step was 0.1 V and the contour plot generated automatically (Originlab). (g,h) 2D |S₁₁| versus voltage curve from one hundred of ferrocenyl undecanethiol gold nanojunctions at 17.8 GHz and related ϕ(S₁₁) versus V curve. The voltage step was 0.1 V and the contour plot generated automatically (Originlab).

Figure 4. Perspectives for RF-molecular rectifiers.

(a) Equivalent circuit representation of the device. Molecules can be decomposed as a conductance in parallel with a capacitance (G_mol and C_mol). Also, a fringe capacitance (C_p) between the iSMM tip and the sample has to be considered. (b) Conductance G_mol estimated from both the d.c. measurement (δI/δV) —red curve—(technically obtained after multi-exponential fit with 200 points of the d.c. I–V curve: 21 points), and from S₁₁ parameters (equation1)—blue curve—(see Supplementary Information, section 8 for fitting details). The error bar in log scale is considered to be the same as that of full width half maximum in current histograms. (c) Similar curves as in b at 17.8 GHz. The error bar in log scale is considered to be the same as that of full width half maximum in current histograms. (d) Schematic cross section of the RF Schottky diode architecture. The high resistivity epitaxial layer is thin (few nm) so as to to tune R_j up to R_s, but not too thin to avoid a large C_j. The substrate is highly doped (resistivity ρ_s=0.001 Ω cm). Its resistance scales as A^1/2 where A is the junction area. (e) Schematic cross section of the proposed RF molecular rectifier. The molecular layer plays the role of the diode with a small dielectric constant ρ_r. Similar to Schottky diodes, the molecular diode is connected to a highly doped silicon substrate. (f) Graph illustrating the theoretical (ideal) f_T and resistance (R_s or R_mol) for both the RF-molecular rectifier and the RF-Schottky diode architectures shown in d,e. The dash curve corresponds to R_s=ρ_s/2d with ρ_s=1 mΩ·cm (ref. 78). C_j/A=6.2 μF cm⁻² from refs , and C_mol/A=0.9–1.4 μF cm⁻² based on a dielectric constant of 2–3 for the monolayer and a monolayer thickness of 1.9 nm (the length of the molecule). A current density failure limitation of 3 × 10⁸ A cm⁻² from refs , , induces a saturation of f_T in the graph. Measured R_mol at +1 V and estimated f_T are also indicated. The error bar is related to the conductance dispersion from 2D I–V histograms.