Layered graphene with twist displays unique 2D quantum confinement effects

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Understanding how electrons move in 2-D layered material systems could lead to advancements in quantum computing and communication.

Scientists are studying two different bilayer configurations graphene—The two-dimensional (2-D), atom-thin form of carbon – detected electronic and optical interlayer resonances. In these resonant states, electrons bounce between the two atomic planes of the 2D interface at the same frequency. By characterizing these states, they found that twisting one of the graphene layers 30 degrees relative to the other, instead of stacking the layers directly on top of each other, shifts the resonance to a lower energy. .

From this result, which has just appeared in Physical examination letters, they deduced that the distance between the two layers increased significantly in the twisted configuration, compared to the stacked configuration. As this distance changes, the interlayer interactions also change, influencing how electrons move through the bilayer system. An understanding of this movement of electrons could inform the design of future quantum technologies for more powerful computing and more secure communication.

Quantum Materials Press (QPress) Installation

Scientist Jurek Sadowski (left) and postdoctoral fellow Zhongwei Dai at the Quantum Material Press (QPress) facility at the Center for Functional Nanomaterials (CFN) at Brookhaven National Laboratory. The large circular room is the QPress central robot, with various modules attached to the sides for sample annealing, film deposition, plasma cleaning, and sample libraries. The complete QPress system, still under development, will automate the stacking of 2D materials into layered structures with exotic properties for quantum applications. Credit: Brookhaven National Laboratory

“Today’s computer chips are based on our knowledge of how electrons move in semiconductors, especially silicon,” said lead author and co-author Zhongwei Dai, post-doctoral fellow at the Interface Science and Catalysis group at the Center for Functional Nanomaterials (CFN) at the Brookhaven National Laboratory of the US Department of Energy (DOE). “But the physical properties of silicon are reaching a physical limit in terms of how small transistors are made and how many can fit on a chip. If we can understand how electrons move on a small scale of a few nanometers in the reduced dimensions of 2D materials, we may be able to discover another way of using electrons for quantum information science. “

At a few nanometers, or billionths of a meter, the size of a material system is comparable to that of the wavelength of electrons. When electrons are confined in a space with the dimensions of their wavelength, the electronic and optical properties of the material change. These quantum confinement effects are the result of wave motion in quantum mechanics rather than classical mechanical motion, in which electrons move through a material and are scattered by random defects.

Chang-Yong Nam, Jurek Sadowski, Zhongwei Dai, Samuel Tenney, Nikhil Tiwale and Ashwanth Subramanian

(Clockwise from left to right) Chang-Yong Nam team members Jurek Sadowski, Zhongwei Dai, Samuel Tenney, Nikhil Tiwale and Ashwanth Subramanian outside the Functional Nanomaterials Center. Credit: Brookhaven National Laboratory

For this research, the team selected a simple material model, graphene, to study the effects of quantum confinement, by applying two different probes: electrons and photons (particles of light). To probe both electronic and optical resonances, they used a special substrate onto which graphene could be transferred. CFN Interface Science and Catalysis Group co-correspondent and scientist author Jurek Sadowski previously designed this substrate for the Quantum Material Press (QPress). The QPress is an automated tool under development in the CFN Materials Synthesis and Characterization facility for the synthesis, processing and characterization of layered 2D materials. Conventionally, scientists exfoliate “flakes” of 2D material from 3D parent crystals (eg, graphene from graphite) onto a silicon dioxide substrate several hundred nanometers thick. However, this substrate is insulating, and therefore electron-based interrogation techniques do not work. So Sadowski and CFN scientist Chang-Yong Nam and Stony Brook University graduate student Ashwanth Subramanian deposited a conductive layer of titanium oxide only three nanometers thick on the silicon dioxide substrate.

“This layer is transparent enough for optical characterization and determination of the thickness of exfoliated flakes and stacked monolayers while being sufficiently conductive for electron microscopy or synchrotron spectroscopy techniques,” Sadowski explained.

In the Charlie Johnson group at the University of Pennsylvania – Rebecca W. Bushnell, Charlie Johnson professor of physics and astronomy, postdoctoral fellow Qicheng Zhang and former postdoctoral fellow Zhaoli Gao (now assistant professor at Chinese University of Hong Kong) – the graphene grew on metal foils and transferred them to the titanium oxide / silicon dioxide substrate. When graphene is grown in this way, all three domains (single layer, stacked, and twisted) are present.

Electron and photon scattering experiment

(a) Diagrams of the experimental set-up for the scattering of electrons and photons. (b) An atomic model of the unit formed by the crystal structure of twisted bilayer graphene (30 ° -tBLG). (c) A low energy electron microscope image of a typical sample area containing 30 ° -tBLG, stacked bilayer graphene (AB-BLG) and monolayer graphene (SLG). (d) A low energy electron diffraction pattern over an area of ​​30 ° -tBLG. Credit: Brookhaven National Laboratory

Next, Dai and Sadowski designed and performed experiments in which they projected electrons into the material with a low-energy electron microscope (LEEM) and detected the reflected electrons. They also shot photons from a laser light microscope with a spectrometer into the material and analyzed the spectrum of the backscattered light. This Raman confocal microscope is part of the QPress cataloguer, which, together with image analysis software, can pinpoint the locations of areas of interest in samples.

“The QPress Raman microscope allowed us to quickly identify the target sample area, speeding up our research,” Dai said.

Their results suggest that the spacing between the layers in the twisted graphene configuration increased by about six percent compared to the untwisted configuration. Calculations by theorists at the University of New Hampshire verified the unique electronic resonance behavior in the twisted configuration.

“Devices made from spinning graphene can have some very interesting and unexpected properties due to the increased interlayer spacing in which electrons can move,” Sadowski said.

Next, the team will make devices with twisted graphene. The team will also build on initial experiments conducted by CFN scientist Samuel Tenney and CFN postdocs Calley Eads and Nikhil Tiwale to explore how adding different materials to the layered structure affects its electronic properties and optics.

“In this initial research, we chose the simplest 2D material system that we can synthesize and control to understand the behavior of electrons,” Dai said. “We plan to continue these types of foundational studies, hopefully shedding light on how to manipulate materials for quantum computing and communications.”

This research was supported by the DOE Office of Science and used resources from CFN and the National Synchrotron Light Source II (NSLS-II), both of the DOE Office of Science User Facilities at Brookhaven. The LEEM microscope is part of the X-ray Photoemission Electron Microscopy (XPEEM) / LEEM end station of the NSLS-II electron spectroscopy beamline; CFN operates this terminal station through a partner user agreement with NSLS-II. Other funding bodies are the National Science Foundation, the Research Grant Council of Hong Kong Special Administrative Region, and the Chinese University of Hong Kong.

To learn more about this research, read Atomically-thin, twisted graphene has unique properties that could advance quantum computing.

Reference: “Quantum-Well Bound States in Graphene Heterostructure Interfaces” by Zhongwei Dai, Zhaoli Gao, Sergey S. Pershoguba, Nikhil Tiwale, Ashwanth Subramanian, Qicheng Zhang, Calley Eads, Samuel A. Tenney, Richard M. Osgood, Chang-Yong Nam, Jiadong Zang, AT Charlie Johnson and Jerzy T. Sadowski, August 20, 2021, Physical examination letters.
DOI: 10.1103 / PhysRevLett.127.086805


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