Turning over a new leaf in resistive switching devices

Common household plant, Golden Pothos, has an unusual electrical response that could be harnessed in resistive memory devices, according to researchers at Colgate University [Adhikari et al., Applied Materials Today 24 (2021) 101077, https://doi.org/10.1016/j.apmt.2021.101077].

“[We] study electronic properties of biological materials and became curious about the electrical response of broad flat – or ‘dicot’ – leaves,” explains Ramesh Y. Adhikari, who led the project. “Our initial goal was to create leaf-based devices with the outer surfaces – or epidermis – acting as a supporting structure and the inner mesophyll region, [which is] rich in air gaps, filled with conducting polymers as the active region.”

Adhikari, along with undergraduate researchers Nicole E. Harmon and K. Paige Williams, noticed that the current response of Golden Pothus leaves show high current/low resistance and low current/high resistance regimes when swept through a voltage range. Ionic charge transport within the leaves produces an interesting electrical response without the need for additional conducting polymers.

“We realized we could construct electrical devices embedded in the leaves solely by taking advantage of their intrinsic transport mechanism,” he says.

The resistive devices consist of samples from freshly cut leaves with two probes piercing the upper epidermis layer as electrical contact points. When the voltage is swept from +10 V to -10 V, the current response approaches zero before peaking at -3 V. As the voltage sweeps back to +10 V, the current peaks again at +3 V. This behavior is typical of bipolar switching devices.

“The current response to applied voltages in these devices is highly reproducible,” says Adhikari. “We experimented with leaves of various ages and carried out measurements at different locations on the same leaf. The high and low resistance signature of the current response to applied voltage followed the same behavior in all cases.”

In this device, the ON-state can be set at 10 V and the reading voltage at -3 V, where the current is at a maximum (i.e. the device is in a low resistance state). Conversely, the OFF-state voltage can be set at -10 V and the reading voltage at -3 V, where the current is at a minimum indicating the device is in a high resistance state. The ON/OFF ratio is sufficient for states to be read without ambiguity and the devices can withstand hundreds of ON/OFF cycles, so information can be stored and cleared repeatedly.

“The novel aspect of our findings is that… we were able to take advantage of intrinsic ion transport in the leaves of Golden Pothos and convert them into resistive memory devices,” points out Adhikari.

Electronic devices like this could be used to heal, monitor, or even control living systems and the researchers now plan to explore the potential for neuromorphic computing.