Building Molecular Electronic Systems with Multiple Functionalities

As manufacturers look to scale down their electronic systems to align with consumer demand, there are often limitations to how small different components can be. While devices have not fallen entirely in line with Moore's Law in recent years, miniaturisation is happening, but the sheer scale of developments over the years means that the scaling down of components has had to slow down slightly.

There is only so far where current technologies can facilitate the scaling down of electronic components. In recent years, there has been a shift from trying to start big and miniaturise to building up from nothing to create ultra-small systems. Some of these have been nanomaterial systems, while others have been molecular electronics.

Molecular electronics is an area of small-scale electronics where electrical circuits are created by individual or ensemble molecules. Molecular electronics offers an alternative route for miniaturising beyond Moore's Law to develop small-scale devices with low energy consumption and useful functionalities.

To date, many molecular electronics have only focused on one functionality that can be utilised in an electronic system. It's believed that the functionalities of molecular electronics can be expanded by coupling the ionic and electronic charges in the devices (something seen in many large-scale electronic components). It's also thought that the types of functionalities developed in molecular electronics could be expanded because of the presence of electrons, holes, anions, and cations within the molecularly thin channels. A team of researchers from Germany and China recently expanded the functionalities of molecular electronics, allowing them to switch between different functionalities.

A Focus on Molecular Junctions

Most molecular devices focus on molecular junctions. In these junctions, the device's functionalities are realised by manipulating both the chemical properties and the electronic structure of the molecules used in the device. Ultrathin molecular layers within these devices are seen as an underused area that could help push the functionalities of molecular electronics beyond what is possible now. It's thought that controlling the ionic and electronic functions within molecular layer devices could offer a way of creating more practical molecular devices.

For molecular devices to become more practical and for these factors to be controlled easily, there are a couple of conditions that will need to be met in molecular devices. These conditions are that there needs to be a reliable way of fabricating sub-10 nm thick molecular devices, and there needs to be an easy way of bringing the mobile ions into the molecular channels.

When it comes to molecular devices, molecular junctions have been the most studied―especially within the sub-10nm domain. While a lot of single-molecule junctions have been fabricated, creating large area junction based on molecular layers offers a much more promising route for scaling up fabrication and making molecular devices more commercially feasible―especially in the realm of integrated nanoelectronics applications.

Expanding the Scope of Molecular Electronics to Perform Multiple Functions

Looking towards developing molecular devices with multiple functionalities―with the help of molecular layers―the team of researchers has created a new integrated molecular device using ultrathin copper phthalocyanine/fullerene hybrid layers. In the case of this molecular device, it was found to possess two distinct functionalities―in the form of memristive and photomultiplication behaviours―and could switch between the two functionalities. This is possible because the mobile ions and electronic charges are controlled in the active channels by rolled-up electrodes, switching between photomultiplication photodiodes and bipolar memristors under different external stimuli.

When it comes to controlling and modulating the molecular channels, an interfacial electric field is utilised between a polymeric PEDOT:PSS electrode and the enclosed molecular channels. The electrical field tunes the carrier injection barrier, which allows the number of ions to be released into the channels, changing the device's function. PEDOT:PSS was chosen as the electrode because it is not only transparent, but its polymeric matrix can store ions and act as an 'ion reservoir' for the switching process.

When the interfacial electric field is low, and the ions are not driven into the molecular channels, photogenerated holes are trapped. These holes act as electronic space charges and can exhibit photomultiplication effects with a high quantum efficiency. The photomultiplication effect occurs under illumination because the photogenerated holes are located near the top electrode in a blocking layer. When illuminated, the holes assist in tunnelling (quantum tunnelling) the electrons from the top electrode resulting in photomultiplication occurring.

On the other hand, when there's a high interfacial electric field, the polarised mobile ions accumulate as ionic space charges around the top and bottom electrodes. This accumulation modifies the both the interfacial barrier and electronic carrier injection properties of the system. This modification results in ferroelectric-like memristive switching that possesses resistive ON/OFF and rectification ratios.

Both the trapped photogenerated holes and the accumulated mobile ions are responsible for the modulation of the electronic carrier injection into the molecular system using an external circuit. This is possible because the electronic band gap of the molecular junction is bent at the interface, which is more effective in these systems because the molecular layers are just as thin as the interface.

It's the first time in the realm of molecular electronics that both the electronic and ionic charge processes have been coupled to modulate the device's function and opens the possibility of expanding on this area to create different molecular devices with multiple functionalities. Building molecular electronic systems with various components that can perform multiple functions (as opposed to single functions) will make them more scalable and feasible for real-world use in the future.

Reference:

Schmidt O. et al., On-chip integrated process-programmable sub-10 nm thick molecular devices switching between photomultiplication and memristive behaviour, 13, (2022), 2875