On-chip integrated process-programmable sub-10 nm thick molecular devices switching between photomultiplication and memristive behaviour

Abstract

Molecular devices constructed by sub-10 nm thick molecular layers are promising candidates for a new generation of integratable nanoelectronic applications. Here, we report integrated molecular devices based on ultrathin copper phthalocyanine/fullerene hybrid layers with microtubular soft-contacts, which exhibit process-programmable functionality switching between photomultiplication and memristive behaviour. The local electric field at the interface between the polymer bottom electrode and the enclosed molecular channels modulates the ionic-electronic charge interaction and hence determines the transition of the device function. When ions are not driven into the molecular channels at a low interface electric field, photogenerated holes are trapped as electronic space charges, resulting in photomultiplication with a high external quantum efficiency. Once mobile ions are polarized and accumulated as ionic space charges in the molecular channels at a high interface electric field, the molecular devices show ferroelectric-like memristive switching with remarkable resistive ON/OFF and rectification ratios.

Fig. 1. Schematic showing the concept of functional molecular devices based on electronic-ionic synergistic effects.

a Schematic illustration of electronic/ionic/electronic-ionic transport on the molecular scale under various external stimuli such as electrical and optical excitation. One electrode (e.g., the bottom electrode) also acts as a controllable “ion reservoir” to supply mobile ions. b The concept in a can be realized by integrated molecular devices based on damage-free rolled-up soft contacts. c Based on the configuration in b, a transition between photomultiplication and memristive switching is realized in the on-chip integrated devices by controlling the electronic-ionic transport in the molecule channel.

Fig. 2. Design and microfabrication of the molecular devices.

a Formation of rolled-up soft contacts. The finger-shaped mesa structure provides a platform for the bottom electrode, onto which molecular layers are grown. During the selective etching of the sacrificial layer GeOx by deionized (DI) water, the Au/Ti/Cr nanomembranes release their built-in stress and roll up. After rolling, the rolled-up metallic nanomembrane bonds as a top electrode to the molecule layer below, forming a sandwich structure. b Micrograph of the molecular device array. c–e Typical single device based on a rolled-up soft contact (c). The width of the bottom electrode and the diameter of the rolled-up metallic tube are ~40 μm and ~10 μm, respectively. AFM image of CuPc (3 nm)/CuPc (3 nm) grown on the wafer (d) and the corresponding height profile (e). f, g AFM images of the spin-coated PEDOT:PSS:AgNWs film before (f) and after (g) 30 s wet etching. Schematic illustrations are also given. h, i Raman spectrum (h) and adsorption coefficient profile (i) of the spin-coated PEDOT:PSS:AgNWs film before (denoted as PPA0) and after 30 s wet-etching (denoted as PPA30). j, k XPS spectra (Al-Kα = 1486.6 eV) corresponding to Ag 3d (j) and N 1 s (k) of the etched PEDOT:PSS:AgNWs film.

Fig. 3. Molecular photomultiplication photodiodes based on PPA30/CuPc (x nm)/C₆₀ (x nm)/Au tube.

a–c I-V characteristics of the molecular devices in the dark and under light illumination (a x = 3 nm; b x = 5 nm; c x = 20 nm). The bottom electrode is grounded during the I-V measurement. Both the dark and photo currents increase with the decrease in thickness, while the photo/dark current ratios remain close to 100. d–f Simulation of photogenerated exciton distribution in the molecular devices (d x = 3 nm; e x = 5 nm; f x = 20 nm). g, h Schematic showing the molecular devices with an ultrathin molecular hybrid layer in the dark (g) and under illumination (h). Purple-glowing “e^-” and “h⁺” denote photogenerated free electrons and holes, respectively. Trapped photogenerated holes by the C₆₀ layer are marked as red-glowing “h⁺”. The red gradient area represents the region affected by the trapped photogenerated holes. i Schematic illustration of the molecular devices with a thick molecular hybrid layer under illumination, in which the effect of trapped photogenerated holes cannot reach the top Au electrode, because the trapped holes have a low density and are located relatively far away from the Au tube electrode. j Comparison of EQE (at 1 V or −1 V) of our photomultiplication photodiodes (PPA30/CuPc (3 nm)/C₆₀ (3 nm)/Au tube to 630 nm laser light with a power density of 5 mW cm⁻²) with previously reported results that are based on polymer/organic active materials^,–.

Fig. 4. Molecular bipolar memristors based on PPA60/CuPc (x nm)/C₆₀ (x nm)/Au tube.

a–f I-V characteristics of the molecular devices in different voltage windows ((a) and b, x = 3 nm; c and d, x = 5 nm; e and f, x = 20 nm). In the low applied voltage range, the molecular devices show nearly symmetric I-V curves without a loop. At higher voltages electric hysteresis loops appear. g–i Schematic showing the molecular devices with an ultrathin molecular hybrid layer (g low bias; h high reverse bias; i high forward bias). The red gradient area represents the n-doped region induced by cation accumulation, while the purple gradient area represents the p-doped region induced by anion accumulation. When firstly biased by a high reverse voltage, ion polarization makes the system similar to a diode dominated by the p-n junction (h and Supplementary Fig. 13d). When firstly biased by a high forward voltage, the system can be considered as a diode dominated by the two Schottky junctions (i and Supplementary Fig. 13e). j Schematic illustration of the molecular devices with thick molecular hybrid layers under reverse bias, in which the accumulated ions have a much weaker modulation effect for carrier injection due to the ineffective ion polarization and relatively low ion concentration. k Retention of ON and OFF states of PPA60/CuPc(3 nm)/C₆₀(3 nm)/Au under continuous read-out voltages (at 0.5 V). The device is switched to the ON state by applying a positive voltage pulse at 1.5 V for a few seconds. l Endurance of ON and OFF states of PPA60/CuPc(3 nm)/C₆₀(3 nm)/Au over the first 100 measurement cycles (read at 0.5 V).

Fig. 5. Mechanism of the electric-field-driven transition.

a Both trapped holes and accumulated ions in the vicinity of the electrodes have the same effect, i.e., they induce band bending and enhance the charge injection due to the increase in the local carrier density. Red-glowing “h⁺”, red circle with “+”, and black “e⁻” represent the trapped photogenerated hole, accumulated cation, and injection electron, respectively. b Photoelectron spectra (Al Kα-monochromated, 300 W, 1486.6 eV) of the wet-etched PEDPT:PSS:AgNWs and the proposed band structures (before equilibrium) with 30 s-etching and 60 s-etching. From the photoelectron spectra obtained by using monochromatic Al Kα radiation, the calculated values of the work function for PPA30 and PPA60 are 4.8 eV and 4.5 eV, respectively. c At a low local electric field across the PPA/CuPc interface, electronic transport dominates, resulting in photomultiplication photodiodes as demonstrated in Fig. 3. At a high local electric field across the PPA/CuPc interface, ion migration is activated, resulting in bipolar memristors as demonstrated in Fig. 4. Therefore, by controlling the PPA/CuPc interface electric field, the molecular device can be switched between the photomultiplication photodiode and bipolar memristor.