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Researchers Discover High Tunability of 2D Materials

28 August 2017

Kaiyuan Yao, Nick Borys and P. James Schuck (from left), seen here at Berkeley Lab's Molecular Foundry, measured a property in a 2D material that could help realize new applications. Source: Marilyn Chung/Berkeley LabKaiyuan Yao, Nick Borys and P. James Schuck (from left), seen here at Berkeley Lab's Molecular Foundry, measured a property in a 2D material that could help realize new applications. Source: Marilyn Chung/Berkeley Lab

2D materials are an up-and-coming material in the scientific community. 2D is atomically thin and can exhibit very different electronics and light-based properties than other thicker and more conventional forms, causing researchers to flock to these materials.

Applications for 2D materials range from microchip components to thin and flexible solar panels and display screens. The list of possible uses is constantly growing. But their fundamental structure is very tiny, so they can be hard to manufacture, measure and match with other materials. While 3D materials R&D is on the rise, there are still many questions about how to isolate, enhance and manipulate their desirable qualities.

A science team at the Department of Engineering’s Lawrence Berkley National Laboratory (Berkeley Lab) has measured some previously obscured properties of moly sulfide, a 2D semiconducting material (molybdenum disulfide or MoS2). The team revealed a powerful tuning mechanism and an interrelationship between its electronics and optical or light-related properties.

In order to incorporate the monolayer materials into electronic devices, engineers want to know the
“band gap,” which is the minimum energy level it takes to jolt electrons away from the atoms they are coupled to. This means that they flow freely through the material as electric current flows through a copper wire. Supplying sufficient energy to the electrons by absorbing light, for example, converts the material into an electrically conducting state.

The researchers measured the band gap for a monolayer of moly sulfide, which has proven difficult to accurately predict, and found it was about 30 percent higher than expected based on previous experiments. They also quantified how the band gap changes with electron density, which is known as “band gap renormalization.”

"The most critical significance of this work was in finding the band gap," said Kaiyuan Yao, a graduate student researcher at Berkeley Lab and the University of California, Berkeley, who served as the lead author of the research paper. “That provides very important guidance to all of the optoelectronic device engineers. They need to know what the band gap is in order to properly connect the 2-D materials and components in a device,” Yao said.

Obtaining the direct gap measurement is challenged by the “excitation effect” in 2D materials that are produced by a strong pairing between electrons and electron “holes” — vacant positions around an atom where an electron can exist. The strength of the effect can mask measurements of the band gap.

The team used tools at the Molecular Foundry, a facility that is open to scientists and specializes in the creation and exploration of nanoscale materials.

The Molecular Foundry technique that researchers adapted for use in studying monolayer moly sulfide, also known as photoluminescence excitation (PLE) spectroscopy, promises to bring new applications for the material within reach, like ultrasensitive biosensors and tinier transistors. This shows promise for similarly pinpointing and manipulating properties in other 2D materials.

The research team measured the excitation and band gap signals and then detangled the separate signals. Scientists observed how light was absorbed by electrons in the moly sulfide sample as they adjusted the density of electronics crammed into the sample by changing the electrical voltage on a layer of charged silicon that sat below the moly sulfide monolayer.

Researchers noticed a slight “bump” in their measurements that they realized was a direct measurement of the band gap. Through a show of other experiments, they used their discovery to study how the band gap was readily tunable by simply adjusting the density of electrons in the material.

"The large degree of tunability really opens people's eyes," said P. James Schuck, who was director of the Imaging and Manipulation of Nanostructures facility at the Molecular Foundry during this study. And because we could see both the band gap's edge and the excitons simultaneously, we could understand each independently and also understand the relationship between them," said Schuck, "It turns out all of these properties are dependent on one another."

Moly sulfide is very sensitive to local environment. This makes it a prime candidate for use in a range of sensors. Because it is highly sensitive to both optical and electronic effects, it could translate incoming light into electronic signals and vice versa.

The team hopes to use many techniques at the Molecular Foundry to create other types of monolayer materials and samples of stacked 2D layers and to obtain definitive band gap measurements for these too.

The team has expertise in the use of a nanoscale probe to map the electronics behavior across a given sample.

A paper on this research was published in Physical Review Letters.

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