‘Exciton’ new way to measure the band gap of 2D materials

Two-dimensional (2D) materials are atomically thin and can exhibit radically different electronic and optical properties than their thicker, more conventional forms, so researchers are flocking to this fledgling field to find ways to tap these exotic traits.

Applications for 2D materials range from microchip components to super-thin and flexible solar panels and display screens, among a growing list of possible uses. But because their fundamental structure is inherently tiny, 2D materials can be tricky to manufacture and measure, and to match with other materials. So while research into 2D materials is on the rise, there are still many unknowns about how to isolate, enhance and manipulate their most desirable qualities.

Now, a science team at the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has precisely measured some previously obscured properties of moly sulfide, a 2D semiconducting material also known as molybdenum disulfide (MoS₂). The team also revealed a powerful tuning mechanism and an interrelationship between moly sulfide’s electronic and optical properties.

To best incorporate such monolayer materials into electronic devices, engineers want to know their ‘band gap’. This is the minimum energy level it takes to jolt electrons away from the atoms they are coupled to, so that they flow freely through the material as electric current flows through a copper wire. Supplying sufficient energy to the electrons by irradiating them with light, for example, can convert monolayer materials into an electrically conducting state.

As reported in a paper in Physical Review Letters, the researchers measured the band gap of a monolayer of moly sulfide, which has proved difficult to accurately predict theoretically, and found it to be about 30% higher than expected based on previous experiments. They also quantified how the band gap changes with electron density – a phenomenon 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 paper. "That provides very important guidance to all of the optoelectronic device engineers," who need to know what the band gap is in orderly to properly connect the 2D material with other materials and components in a device.

Obtaining the direct band gap measurement is made challenging by the so-called ‘exciton effect’ in 2D materials. This is produced by a strong pairing between electrons and electron ‘holes’ – vacant positions around an atom where an electron can exist. The strength of this effect can mask measurements of the band gap.

Nicholas Borys, a project scientist at Berkeley Lab's Molecular Foundry, said the study also resolves how to tune optical and electronic properties in a 2D material. "The real power of our technique, and an important milestone for the physics community, is to discern between these optical and electronic properties," he said.

The team used several tools at the Molecular Foundry, a facility that is open to the scientific community and specializes in the creation and exploration of nanoscale materials. These included photoluminescence excitation (PLE) spectroscopy, which promises to bring new applications for the material within reach, such as ultrasensitive biosensors and tinier transistors, and also shows promise for similarly pinpointing and manipulating properties in other 2D materials, the researchers said.

The scientists measured both the exciton and band gap signals, and then detangled these separate signals. They observed how light was absorbed by electrons in the moly sulfide sample as they adjusted the density of electrons crammed into the sample, which they did by changing the electrical voltage on a layer of charged silicon that sat below the moly sulfide monolayer.

The researchers noticed a slight ‘bump’ in their measurements that they realized was a direct measurement of the band gap. Through a slew of other experiments, they used this 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 James Schuck, director of the Imaging and Manipulation of Nanostructures facility at the Molecular Foundry during this study and now at Columbia University. "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. It turns out all of these properties are dependent on one another."

Moly sulfide, Schuck also noted, is "extremely sensitive to its local environment," which 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 also translate incoming light into electronic signals and vice versa.

Schuck said the team now hopes to use a suite of 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. "It turns out no one yet knows the band gaps for some of these other materials," he said.

The team also has expertise in the use of a nanoscale probe to map the electronic behavior across a given sample. "We certainly hope this work seeds further studies on other 2D semiconductor systems," says Borys.

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.