Valleytronics warms up with 2D materials

Researchers at the Center for Functional Nanomaterials (CFN), an Office of Science user facility at the US Department of Energy (DOE)’s Brookhaven National Laboratory, and Northrop Grumman, a multinational aerospace and defense technology company, have found a way to maintain valley polarization at room temperature using novel materials and techniques. Their discovery, reported in a paper in Nature Communications, could lead to devices that store and process information in novel ways without the need to keep them at ultra-low temperatures.

One of the paths being explored to achieve these devices is a relatively new field called ‘valleytronics’. A material’s electronic band structure – the range of energy levels in each atom’s electron configurations – can dip up or down. These peaks and troughs are known as ‘valleys’. Some materials have multiple valleys with the same energy. An electron in a system like this can occupy any one of these valleys, presenting a unique way to store and process information based on which valley the electron occupies.

One challenge, however, has been the effort and expense of maintaining the low temperatures needed to keep such valley polarization stable. Without this stability, devices would begin to lose information. To make a technology like this feasible for practical, affordable applications, experts would need to find a way around this constraint.

Transition metal dichalcogenides (TMDs) are layered materials that can be, at their thinnest, only a few atoms thick. Each layer in the material consists of a two-dimensional (2D) sheet of transition metal atoms sandwiched between chalcogen atoms. While the metal and the chalcogen are strongly bound by covalent bonds in a single layer, adjacent layers are only weakly bound by van der Waal’s interactions. The weak bonds that hold these layers together allow TMDs to be exfoliated down to a monolayer that’s only one ‘molecule’ thick. These are often referred to as 2D materials.

The team at CFN synthesized single crystals of chiral lead halide perovskites (R/S-NEAPbI₃). Chirality describes a set of objects, like molecules, that are a mirror image of each other but can’t be superimposed. It is derived from the Greek word for ‘hands’, a perfect example of chirality. The two shapes are identical, but if you put one hand on top of the other, they will not align. This asymmetry is important for controlling valley polarization.

The researchers layered flakes of R/S-NEAPbI₃, roughly 500nm thick, onto a monolayer of the TMD molybdenum disulfide (MoS₂) to create what is known as a heterostructure. By combining different 2D materials with properties that affect the charge transfer at the interface between the two materials, these heterostructures open up a world of possibility.

After creating and characterizing this heterostructure, the team was eager to see how it behaved.

“TMDs have two valleys with the same energy,” explained Shreetu Shrestha, a postdoctoral research associate at CFN and author of the paper. “An electron can be in one valley or the other, which gives it an additional degree of freedom. Information can then be stored based on which valley an electron occupies.”

To get a better picture of the material’s behavior, the researchers leveraged tools at CFN’s Advanced Optical Spectroscopy and Microscopy facility. They used a linearly polarized laser to excite the heterostructure and then measured the light that was emitted from the molybdenum disulfide with a confocal microscope. They performed the same process on a TMD without the chiral lead halide perovskite layer.

During these advanced experiments, the researchers noticed something interesting about the way light was emitted. The heterostructure had a lower emission than the bare TMD. The researchers attributed this behavior to the charge transferred from the TMD to the perovskite in the heterostructure. Using ultrafast spectroscopy, the researchers found that the charge transfers very quickly – only a few trillionths of a second.

The team also found that the intensity of the left and right circularly polarized components of the emitted light depends on the handedness of the chiral perovskite. The chiral nature of the perovskite acted like a filter for electrons with different spins. Depending on the handedness of the chiral perovskite, electrons that spin either up or down were preferentially transferred from one valley over electrons with the opposite spin in the other valley. This phenomenon could allow researchers to selectively populate valleys and use their occupation in the same way current transistors on computers store the 1s and 0s of binary bits.

“An important point to highlight in this experiment is that these results were realized at room temperature, which is where the whole field should move,” said Mircea Cotlet, a materials scientist at Brookhaven Lab and the principal investigator of the project. “Keeping hardware at the low temperatures that were being used is so much more complex and costly. It’s encouraging to see these kinds of material properties at room temperature.”

While valleytronics research is still at an early stage, researchers have already been thinking about possible applications. This technology could improve existing devices in surprising ways, expanding the capabilities of classical computers, but it could also be a component in the hardware of the future.

“This would help make classical computing more efficient,” said Shrestha, “but this technology could also be harnessed for quantum information science, which includes quantum computing, or even quantum sensing. These atomically thin materials have unique quantum properties, which we should be able to take advantage of.”

This story is adapted from material from Brookhaven 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.