AFM can provide new insights into 2D materials to better understand their potential applications.
What are the key characteristics and importance of 2D materials like graphene and hexagonal boron nitride (hBN)?
2D materials, such as graphene and hexagonal boron nitride (hBN), have unusual features because they can exist as single-layer crystals. Graphene, for example, has remarkable electrical conductivity, mechanical strength, and flexibility, making it a game-changing substance in nanotechnology.
On the other hand, hexagonal boron nitride is a superb insulator with a largely flat surface, which is essential when combined with graphene or other 2D materials to form heterostructures.
These materials also create superlattices, especially when stacked at specific angles, resulting in phenomena like moiré patterns. These qualities are critical for developing new electrical devices, such as transistors and quantum computer components, as well as investigating fundamental physics in low dimensions.
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How does atomic force microscopy (AFM) help study 2D materials and their heterostructures?
AFM is important in examining 2D materials because it can analyze mechanical, electrical, and topographical properties at the nanoscale. Surface adhesion and elasticity can be mapped using techniques such as PeakForce QNM, which provides detailed insights into mechanical properties.
Tapping mode AFM allows us to scan surfaces without harming delicate 2D crystals while getting high-resolution images. Conductive AFM monitors electrical current across a sample and is particularly useful for investigating faults or the performance of devices manufactured from these materials, such as resistive switching devices.
These AFM techniques help us comprehend anything from atomic structures to mechanical stresses in heterostructures.
What are Moiré patterns, and how are they related to twisted bilayer graphene?
Moiré patterns appear when two layers of two-dimensional materials, such as graphene, are stacked with a tiny twist in their crystal lattices. This mismatch results in a superlattice, which can be observed using various imaging techniques, including AFM.
Moiré patterns are especially important in twisted bilayer graphene because they generate fascinating physical phenomena, such as superconductivity, when the layers are twisted at a precise "magic angle" of approximately 1.1 degrees.
These patterns also help us understand orbital ferromagnetism, which happens when graphene aligns with hBN. Thus, moiré patterns are critical for understanding the complicated interactions in layered 2D materials.
What are the advantages of torsional force microscopy (TFM) for imaging superlattices in 2D materials?
Torsional Force Microscopy (TFM) is a highly successful technique for imaging superlattices and atomic lattices in 2D materials.
One significant advantage is its ability to capture high-fidelity photographs of moiré patterns in atomic-scale detail. By activating torsional piezos and monitoring lateral deflection, we can precisely visualize both the moiré and the atomic lattice.
This approach enabled us to repeatedly photograph complicated structures in twisted bilayer graphene and hBN, bypassing the constraints of standard AFM modes. It is extremely adaptable, operating in ambient settings, making it suitable for everyday tasks.
What role do mechanical properties play in studying 2D materials using AFM?
Mechanical properties are critical for investigating 2D materials with AFM. Torsional resonance in TFM can disclose the frictional response between the AFM tip and the material, which is necessary for imaging atomic lattices and moiré patterns.
We can alter the vertical loading force or torsional drive amplitude to improve image contrast and detect these structures more accurately. Friction and modulus mapping properties provide information about materials' stress, strain, and deformation, allowing us to better understand how they perform under various conditions.
This information is critical for building devices where mechanical stability is just as crucial as electrical performance.
What are the challenges in using CVD-grown hexagonal boron nitride (hBN) for nanoelectronics?
CVD-grown hexagonal boron nitride shows promise, but it is not without obstacles. One key concern is the occurrence of imperfections, such as grain boundaries, wrinkles, and pinholes, which can affect the performance of hBN-based devices, particularly when used as a gate dielectric.
These flaws cause variances in thickness and lattice distortions, which are undesirable in high-precision electronics. Despite these flaws, CVD-grown hBN has shown promise for application in resistive switching devices, where the defects may play a role in the device's switching behavior. Improving the quality of CVD-grown hBN is still an area of current research.
How does AFM contribute to the development of resistive switching devices?
AFM, particularly conductive AFM, is useful in researching resistive switching devices, especially those constructed of materials such as hBN. We can identify faults or anomalies that affect switching behavior by mapping the current flow across a device.
Conductive AFM enables us to visualize these current routes and observe how they change under various situations, such as changing voltages. This technique assists researchers in identifying important regions responsible for switching events and understanding how material features, particularly the presence of flaws, affect device performance.
What are the main limitations and future potential of using AFM for characterizing 2D materials?
While AFM is incredibly useful for nanoscale characterization, it does have limitations. One difficulty is tip degradation, which can diminish imaging quality and produce inconsistent findings. Furthermore, sample variability, such as the existence of flaws or contamination, might make it difficult to obtain consistent results.
Despite these challenges, the future of AFM appears promising, with advances in nanomanipulation and novel modes such as TFM that allow us to get deeper insights into material properties. As we improve AFM technology, we expect it to remain indispensable for investigating the next generation of 2D materials and their applications in electronics and quantum devices.
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About the Speakers
Dr. Bede Pittenger is a Senior Staff Development Scientist in the AFM Unit of Bruker's Nano Surfaces Business. He received his PhD in Physics from the University of Washington (Seattle, WA) in 2000, but has worked with scanning probe microscopes for 25 years, building systems, developing techniques, and studying properties of materials at the nanoscale. His work includes more than thirty publications and three patents on various techniques and applications of scanning probe microscopy. Dr. Pittenger's interests span topics from interfacial melting of ice, to mechanobiology of cells and tissues, to the nanomechanics of polymers and composites.
Mihir Pendharkar is a Q-FARM Bloch Postdoctoral Fellow at Stanford University working in the group of Prof. David Goldhaber-Gordon focusing on making 2D materials stacking more uniform, reproducible and repeatable. Mihir was previously a postdoctoral researcher at UC Santa Barbara where he also received his PhD and MS in Electrical and Computer Engineering specializing in epitaxy of superconductor-semiconductor heterostructures for applications in topological quantum computation.
Mario Lanza got the PhD in Electronic Engineering (with honors) in 2010 at the Autonomous University of Barcelona. In 2010-2011 he was NSFC postdoctoral fellow at Peking University, and in 2012-2013 he was Marie Curie postdoctoral fellow at Stanford University. In October 2013 he joined Soochow University as Associate Professor, and in March 2017 he was promoted to Full Professor. Since October 2020 he is an Associate Professor of Materials Science and Engineering at the King Abdullah University of Science and Technology (KAUST, in Saudi Arabia), where he leads a group formed by 10 PhD students and postdocs. His research focuses on how to improve electronic devices and circuits using 2D materials, with special emphasis on resistive switching applications.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces and Metrology.
For more information on this source, please visit Bruker Nano Surfaces and Metrology.
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