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August 2017

New Results Reveal High Tunability of 2-D Material

Two-dimensional materials are a sort of a rookie phenom in the scientific community. They are atomically thin and can exhibit radically different electronic and light-based 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 2-D materials range from microchip components to superthin and flexible solar panels and display screens, among a growing list of possible uses. But because their fundamental structure is inherently tiny, they can be tricky to manufacture and measure, and to match with other materials. So while 2-D materials R&D 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 Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has precisely measured some previously obscured properties of moly sulfide, a 2-D semiconducting material also known as molybdenum disulfide or MoS2. The team also revealed a powerful tuning mechanism and an interrelationship between its electronic and optical, or light-related, properties.

Obtaining the direct band gap measurement is challenged by the so-called “exciton effect” in 2-D materials that 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.

The Molecular Foundry technique that researchers adapted for use in studying monolayer moly sulfide, known as photoluminescence excitation (PLE) spectroscopy, 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 2-D materials, researchers said.

The research team measured both the exciton and band gap signals, and then detangled these separate signals. Scientists observed how light was absorbed by electrons in the moly sulfide sample as they adjusted the density of electrons 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, and through a slew of other experiments used their discovery to study how the band gap was readily tunable by simply adjusting the density of electrons in the material.

Read the full press release.