Graphene-Diamond Junctions Mimic Human Perception, Memory; Leads Way for New Class of Computers

A new study led by Japanese researchers managed to configure graphene-diamond junctions to mimic certain functions of the human brain, opening possibilities for more complex computing devices.

Scientists from Nagoya University in Japan demonstrated how applying controlled pulses on vertically aligned graphene-diamond junctions can make the specialized material function as optoelectronically-controllable synapses. Synapses are parts of the human brain that act as connections between neurons, serving fundamental functions in memory building.

The design and verification of these junctions as artificial analogs to optoelectronically-controllable synapses in the human brain are presented in the latest Carbon journal article, "Optoelectronic Synapses Using Vertically Aligned Graphene/Diamond Heterojunctions."

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Biological Synapses and Emulating Them

According to Neuroscience in the 21st Century, synapses are the sites of contact between neurons, converting electrical signals into chemical information readable by the nerve cells.

In humans, biological synapses allow the flow of electric charges, called excitatory postsynaptic current (EPSC), allow people to form memories, to transition between short-term memory (STM) to long-term memory (LTM), or something called paired-pulse facilitation (PPF), which refers to short-term, activity-dependent synaptic plasticity.

This synaptic plasticity is the trait in neuronal links to adapt in response to changing activity levels, whether these levels rise or fall. This particular characteristic has been believed to be the key in developing the next level of "neuromorphic" computer architectures, or computing devices based on the structure, function, and network used by the human brain.

By using the brain as the template, researchers believe that humankind can still push the power of computing devices that has been physically limited on several fronts, such as miniaturization.

So far, efforts toward increasingly neuromorphic computing devices have led to the discovery and fabrication of specialized materials that work like transistors or memristors, the latter being electronic devices that can "store" electrical resistance.

More recently, efforts have led to the development of the photomemristor, or light-actuated memristors, that responds to light and can even serve as a non-volatile memory device, opening a new class of materials that can work as artificial optoelectronically-controllable synapses.

Using Graphene-Diamond Junctions as Artificial Optoelectronically-Controllable Synapses

These advancements inspired the Nagoya University research team to design a material that can also mimic the characteristics and capabilities of biological synapses and the memory functions that come with it, ushering the potential for a new class of image sensing memory devices.

Led by Dr. Kenji Ueda, researchers created junctions made from vertically aligned graphene (VG) and diamond nanoparticles. According to a 2017 study, also from the Carbon journal, vertically aligned graphene nanosheets are also known as carbon nanowalls (CNW), a 3D material based on graphene arranged perpendicularly against a substrate hence its vertical configuration.

The fabricated graphene-diamond junctions were able to exhibit behavior similar to biological synapses. It can generate an EPSC, the electrical charge induced by neurotransmitters across the synapse when optical pulses are applied to it.

Similarly, the material has also shown functions copying basic brain processes such as the transition between short-term memory and long-term memory.

"Our brains are well-equipped to sieve through the information available and store what's important," explains Dr. Ueda in a Nagoya University press release. "We tried something similar with our VG-diamond arrays, which emulate the human brain when exposed to optical stimuli."

He additionally explains that the current breakthrough was driven by a 2016 discovery when they found a significant change in conductivity in graphene-diamond junctions when light was applied to it.

Also, as humans better understand the mechanisms behind synapses, we can better create technologies that mimic them, leading to better devices.