Unlocking the Potential of Twisted Bilayer Graphene with Spectroscopic Ellipsometry

Significance 

Twisted bilayer graphene is a unique structure formed when two individual layers of graphene are stacked on top of each other with a specific twist angle between them. Graphene is a two-dimensional carbon allotrope consisting of a single layer of carbon atoms arranged in a hexagonal lattice. In twisted bilayer graphene, the relative orientation of the two layers results in a moiré pattern, which arises due to the interference of the overlapping carbon atoms. When the two layers are perfectly aligned, it forms Bernal-stacking, which is the typical arrangement found in regular graphene. However, when the two layers are rotated by a specific angle, usually in the order of a few degrees, interesting electronic properties emerge. The magic angles, in particular, are twist angles that give rise to a special type of twisted bilayer graphene with remarkable properties. The most famous magic angle is approximately 1.1 degrees. At this angle, the twisted bilayer graphene exhibits a flat electronic band structure, where the electronic density of states becomes highly enhanced. This phenomenon leads to the emergence of correlated electronic states, known as “Mott insulator” and “superconductivity” phases, which are not observed in regular graphene or other materials under typical conditions. The study of twisted bilayer graphene has attracted significant attention from researchers due to its unusual electronic properties and potential applications in novel electronic devices. It has been a subject of intense research in condensed matter physics and materials science in recent years. In a recent study published in the peer-reviewed Journal Nano Letters, a team of researchers from the Department of Engineering at the University of Cambridge, Dr. Teja Potočnik, Dr. Oliver Burton, Dr. Marcel Reutzel, Dr. David Schmitt, Dr. Jan Philipp Bange, Dr. Stefan Mathias, Dr. Fabian R. Geisenhof, R. Thomas Weitz, Linyuan Xin, Hannah Joyce, Professor Stephan Hofmann, and Professor Jack Alexander-Webber reported the characterization and applications of twisted bilayer graphene using spectroscopic ellipsometry.

Twisted bilayer graphene exhibits intriguing properties that can be manipulated by adjusting the twist angle (θt) between the two graphene layers. This manipulation leads to various phenomena, such as topological transport, enhanced photocurrent, and correlated insulating phases at specific twist angles. Moreover, the formation of van Hove singularities (vHs) in the electronic density of states enhances the optical absorption of twisted bilayer graphene, making it ideal for optoelectronic devices like wavelength-selective photodetectors. The challenge lies in precisely identifying the twist angle in bilayer graphene and mapping it over large areas efficiently. The researchers employed a technique called spectroscopic ellipsometry, which measures the change in polarized light upon reflection to determine the complex dielectric function (refractive index and extinction parameter) of the material. This technique allows for substrate-agnostic, fast, and large-area mapping of twisted bilayer graphene.

The research team performed spectral ellipsometric contrast microscopy (SECM) on chemical vapor deposition (CVD) grown monolayer, bilayer, and multilayer graphene transferred onto Si/SiO2. SECM provided wavelength-dependent contrast of optically resonant bilayer and multilayer regions, enabling them to extract a map of the twist angle variation. The SECM data were validated by correlating it with Raman mapping and angle-resolved photoelectron emission spectroscopy (ARPES) results.

The authors successfully identified a rich variety of twist angles in bilayer graphene by analyzing the optical absorption spectra and ellipsometric contrast images. Moreover, they achieved accurate twist angle measurements with typical accuracy within 1°. SECM’s versatility and substrate independence allowed for wafer-scale mapping and quick identification of bilayer graphene regions, even on substrates without contrast enhancement.

Additionally, the study shed light on the origins of twist angle variation in bilayer graphene. Some regions exhibited disorder due to graphene folds, while others showed variations away from obvious folds, indicating a combination of processes during growth and post-growth as responsible for twist angle disorder.

To confirm the presence of optical resonances in the material, the authors conducted ARPES experiments to analyze the band structure at different resonant wavelengths. The results correlated well with the SECM data, further affirming the potential of spectroscopic ellipsometry as a powerful and non-destructive tool for material characterization.

The findings of this study hold significant implications for future research and applications of twisted bilayer graphene and other two-dimensional materials. With the accessibility of spectroscopic microellipsometry, SECM opens new possibilities for material characterization and paves the way for diverse research applications in nanotechnology and related fields.  It also provides valuable insights into the band structure and electronic properties of the material, allowing researchers to understand and manipulate its behavior. This knowledge opens up opportunities for designing novel electronic devices and applications based on these tunable properties. Moreover, the authors’ findings demonstrated the potential of using twisted bilayer graphene as a wavelength-selective photodetector, which can have applications in imaging, communication, and other light-based technologies. This could lead to the development of more efficient and high-performance optoelectronic devices. Indeed, SECM offers a non-destructive and substrate-independent method for characterizing twisted bilayer graphene. The ability to map and identify twist angle variations over large areas without damaging the material is crucial for efficient manufacturing and quality control in nanotechnology applications. The application of SECM in studying twisted bilayer graphene showcases the versatility and potential of this technique. It demonstrates that SECM can be extended to other material systems, including two-dimensional materials and heterostructures. This opens up new possibilities for material characterization and provides researchers with a powerful tool to explore and understand various material properties. Furthermore, the study provides insights into the origin of twist angle disorder in bilayer graphene. Understanding the factors that contribute to twist angle variations is essential for optimizing growth processes and improving the quality of twisted bilayer graphene for practical applications.

In conclusion, the study by Dr Jack Alexander-Webber and colleagues highlights the importance of spectroscopic ellipsometry in unlocking the potential of twisted bilayer graphene. The ability to map and characterize bilayer graphene’s twist angle variation in a fast, non-destructive, and substrate-independent manner opens new avenues for the development of advanced optoelectronic devices and other nanotechnological applications. The potential applications of twisted bilayer graphene in optoelectronics, nanoelectronics, and other fields could have significant commercial and industrial implications. The study’s findings may lead to the development of new technologies, products, and industries, contributing to economic growth and technological advancement. As nanotechnology continues to evolve, techniques like SECM will undoubtedly play a crucial role in advancing our understanding and utilization of novel materials with extraordinary properties.

Unlocking the Potential of Twisted Bilayer Graphene with Spectroscopic Ellipsometry - Advances in Engineering
Image Credit: Nano Lett. 2023, 23, 12, 5506–5513

About the author

Professor Stephan Hofmann

Professor in Nanotechnology
Department of Engineering
University of Cambridge

Dr Hofmann’s research explores novel materials, metrology and device architectures. A particular focus thereby lies on nanomaterials, such as graphene, carbon nanotubes and semiconductor nanowires, and the use of in-situ metrology to probe the fundamental mechanisms that govern their growth and functionality.

The nanometer dimensions can lead to extraordinary properties and being able to control and exploit those can have a transformative impact across a wide range of applications, such as information/communication technologies, energy generation, conversion and storage, and environmental and bio-technology. The vision of Dr Hofmann’s research is to unlock this huge technological potential through an unprecedented understanding of material design and functionality on the smallest of size scales.

Dr Hofmann is leading a number of large research projects, funded eg by the ERC and EPSRC, with close industrial links and embedded in a large network of international collaborations.

About the author

Dr Jack Alexander-Webber

Jack Alexander-Webber
Dorothy Hodgkin Research Fellow
University of Cambridge

Jack Alexander-Webber holds a Royal Society Dorothy Hodgkin Research Fellowship. He has previously held a Research Fellowship from the Royal Commission for the Exhibition of 1851 and a Junior Research Fellowship of Churchill College. He graduated from Royal Holloway, University of London in 2009 with an MSci in Physics. After a summer studentship working for the National Physical Laboratory he began his DPhil in the group of Prof Robin Nicholas at the University of Oxford. His doctoral research was on the properties of low-dimensional nanostructures such as graphene, carbon nanotubes and III-V semiconductors with a particular focus on high magnetic field effects studied both in Oxford and at the European Magnetic Field Laboratory facilities in Grenoble and Toulouse. After completing his DPhil, Jack undertook an EPSRC Doctoral Prize fellowship at Oxford. His current research interests lie in exploring the nature of low-dimensional nanomaterials such as graphene, 2D semiconductors, and semiconductor nanowires and exploiting their exceptional properties for electronic and optoelectronic applications.

Reference

Potočnik T, Burton O, Reutzel M, Schmitt D, Bange JP, Mathias S, Geisenhof FR, Weitz RT, Xin L, Joyce HJ, Hofmann S, Alexander-Webber JA. Fast Twist Angle Mapping of Bilayer Graphene Using Spectroscopic Ellipsometric Contrast Microscopy. Nano Lett. 2023;23(12):5506-5513. doi: 10.1021/acs.nanolett.3c00619.

Go to Nano Lett.

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