3D Electronics Get a Boost from 2D Materials

Scanning probe microscopy image of graphene. (Image courtesy of U.S. Army Materiel Command/Wikimedia Commons.)

Two-dimensional materials, the shape, thickness and size of which are explained on a scale of a few atomic elements, show great promise for improving electronics.

Different from conventional 3D materials, where the connections between the atoms run in three dimensions, atomically thin layers bonded by van der Waals interactions have opened the doors to a whole new world of possibilities for scientists and engineers.

It began when a group of researchers, led by Sir Andre Geim, isolated of a single-atom-thick layer of hexagonal honeycomb latticed carbon (graphene) from graphite and found amazing properties never seen before. Though the apparatus consisted of nothing but a crystal of graphite and some sticky tape (derived in a process called mechanical exfoliation), this super light material was found to be the strongest ever tested, non-flammable, flexible and a much better conductor than copper.

Scott Perry, a professor of material science at the University of Florida, believes 2D materials will soon find their place in transistors as a result of their unique electronic properties. However, this technology is still in its early stages, and research on the uses of 2D materials in optoelectronics is just getting started.

Applications of 2D Materials

Michael Spencer, a professor of electrical and computer engineering at Cornell University, also working on applications of 2D materials, believes that they are “ripe for innovation” with many potential applications. In organic devices (such as OLEDs and organic solar cells), 2D materials can be used as cheaper substitutes for optoelectronic devices which use elements that are too expensive to manufacture, such as indium or arsenic.

Semiconductor properties can be found in a group of 2D materials called transition metal dichalcogenides (TMDs). Being only 3 atoms thick, TMDs emit light when excited by light or electrical energy.

The monolayers fall into the direct bandgap semiconductor category, but the luminescence is indirectly proportional to the number of layers, as thicker TMD sheets (bilayers, trilayers or others) don’t emit as much light. Sheets of individual 2D layers of different materials and different orders can be used to create optoelectronic devices with customized properties.

Researchers at the department of physics at the University Of Sheffield, England, have been focusing their efforts on the development of hybrid photonic devices involving these 2D films.

Schematic of a 2D layer of MoS2. (Image courtesy of Christoper Petoukhoff.)

Christopher Petoukhoff, working in the Femtosecond Spectroscopy Unit at the Okinawa Institute of Science and Technology Graduate University (OIST), studies 2D molybdenum disulfide (MoS₂) with a specific focus on applications in optoelectronics. Common optoelectronic devices include LEDs, solar cells, optical fibers and the photodetectors in automatic doors and hand dryers.

The 2D molybdenum disulfide absorbs the same amount of light used in today’s 50nm silicon-based devices, while being 70 times thinner. Petoukhoff is aiming to improve the absorption power of optoelectronic devices by adding a layer of MoS₂ on organic semiconductors. He and his supervisor, Professor Keshav Dani, have theorized that when using two materials with similar absorption powers, the interaction between the layers would yield a stronger and extremely faster charge transfer. His research shows that this charge transfer would take only tens of femtoseconds, less than a millionth of a millionth of a second.

This figure depicts the organic semiconductor, in this case P3HT:PCBM in red, with a 2-D MoS2 layer on a silver plasmonic metasurface. (Image courtesy of Christopher Petoukhoff.)

Although the research is still in its infancy, its implications for the future of electronics in general and optoelectronic devices in particular, are huge.

For more information, read Petoukhoff’s published research in ACS Nano.