Publications

Magnetic 2D materials and heterostructures

No abstract is provided for this article.

Electronic phase separation in multilayer rhombohedral graphite

No abstract is provided for this article.

Metal-Graphene Interaction Studied via Atomic Resolution Scanning Transmission Electron Microscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Topological phase singularities in atomically thin high-refractive-index materials

Atomically thin transition metal dichalcogenides (TMDCs) present a promising platform for numerous photonic applications due to excitonic spectral features, possibility to tune their constants by external gating, doping, or light, and mechanical stability. Utilization of such materials for sensing or optical modulation purposes would require a clever optical design, as by itself the 2D materials can offer only a small optical phase delay - consequence of the atomic thickness. To address this issue, we combine films of 2D semiconductors which exhibit excitonic lines with the Fabry-Perot resonators of the standard commercial SiO2/Si substrate, in order to realize topological phase singularities in reflection. Around these singularities, reflection spectra demonstrate rapid phase changes while the structure behaves as a perfect absorber. Furthermore, we demonstrate that such topological phase singularities are ubiquitous for the entire class of atomically thin TMDCs and other high-refractive-index materials, making it a powerful tool for phase engineering in flat optics. As a practical demonstration, we employ PdSe2 topological phase singularities for a refractive index sensor and demonstrate its superior phase sensitivity compared to typical surface plasmon resonance sensors.

Modeling crystal defects using defect-informed neural networks

<title>Abstract</title> Machine learning has revolutionized the study of crystalline materials forenabling rapid predictions and discovery. However, most AI-for-Materialsresearch to date has focused on ideal crystals, whereas real-world materialsinevitably contain defects that play a critical role in modern functionaltechnologies. The defects and dopants break geometric symmetry andincrease interaction complexity, posing particular challenges for traditionalML models. Addressing these challenges requires models that areable to capture sparse defect-driven effects in crystals while maintainingadaptability and precision. Here, we introduce Defect-Informed EquivariantGraph Neural Network (DefiNet), a model specifically designedto accurately capture defect-related interactions and geometric configurationsin point-defect structures. Trained on 14,866 defect structures,DefiNet achieves highly accurate structural predictions in a single step,avoiding the time-consuming iterative processes in modern ML relaxationmodels and possible error accumulation from iteration. We furthervalidates DefiNet’s accuracy by using density functional theory (DFT) relaxation on DefiNet-predicted structures. For most defect structures,regardless of defect complexity or system size, only 3 ionic steps arerequired to reach the DFT-level ground state. Finally, comparisonswith scanning transmission electron microscopy (STEM) images confirmDefiNet’s scalability and extrapolation beyond point defects, positioningit as a groundbreaking tool for defect-focused materials research.

Reconstructing critical current density in Josephson junctions with phase nonlinearity

In this Letter, we show that the standard Dynes-Fulton analysis, commonly used to reconstruct the critical current density from interference patterns, breaks down in Josephson junctions with nonlinear phase distributions, leading to non-physical artifacts. To address this, we developed a simple iterative reconstruction algorithm and validated it both numerically and experimentally using a planar Josephson junction model. Unlike conventional approaches based on the logarithmic Hilbert transform, the proposed method allows for incorporating prior knowledge about the system and addresses the fundamental issue of ambiguity in reconstructing the critical current density from interference patterns.

From Layered Crystals to Permselective Membranes: History, Fundamentals, and Opportunities

Membranes are crucial in permselective processes, such as water purification, gas sequestration, ion separation, membrane distillation and pervaporation, and energy conversion and storage. The two-decade development of two-dimensional (2D) membranes has shifted the membrane transport from solution-diffusion in the polymer matrix to size-dependent migration in Ångström-to-nanoscale slit-like channels, delivering distinctive fundamentals within nanoconfinements and potential implementations. In this review, we delve into the comprehensive information that facilitates the transformation of layered crystals into continuous thin-film architectures boasting well-defined nanopores. We trace the evolutionary history of these technologies, engage in debates on their underlying principles, and reveal obstacles that demand attention. Starting with the preparation of "single-layer carbon," we explore an array of protocols designed for the sizable production of 2D nanosheets. Our focus then shifts toward unveiling the solutions available for membrane fabrication and control, specifically focusing on the chemical and structural properties of 2D membranes that dictate their mass transportation behaviors in various scenarios. This is followed by a brief comparative discussion on the development of atomically thin and 2D-based mixed matrix membranes. Finally, we conclude by outlining the challenges and limitations associated with streamlining the interlocked exfoliation-reconstruction-control processes to consolidate their practicalities for use in environment and energy-related applications.

The Structure of Suspended Graphene

No abstract is provided for this article.

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

Lightweight acoustic hyperbolic paraboloid diaphragms with graphene through self-assembly nanoarchitectonics

The paper presents a study on the fabrication of a lightweight acoustic hyperbolic paraboloid (HyPar) diaphragm using self-assembly nanoarchitectonics. The diaphragm is composed of a polyacrylonitrile (PAN) network combined with graphene oxide (GO) nanolayers. Spray coating is employed as a fabrication method, providing a simple and cost-effective approach to create large-scale curved diaphragms. The results demonstrate that the PAN/GO diaphragm exhibits acoustic performance comparable to a commercially available banana pulp diaphragm while significantly reducing weight and thickness. Notably, the graphene-based diaphragm is 15 times thinner and 8 times lighter than the commercial banana pulp diaphragm. This thinner and lighter nature of the graphene-based diaphragm offers advantages in applications where weight and size constraints are critical, such as in portable audio devices or acoustic sensors.

Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer

We have studied temperature dependences of electron transport in graphene and its bilayer and found extremely low electron-phonon scattering rates that set the fundamental limit on possible charge carrier mobilities at room temperature. Our measurements show that mobilities higher than 200 000 cm2/V s are achievable, if extrinsic disorder is eliminated. A sharp (thresholdlike) increase in resistivity observed above approximately 200 K is unexpected but can qualitatively be understood within a model of a rippled graphene sheet in which scattering occurs on intraripple flexural phonons.

Degradation Chemistry and Kinetic Stabilization of Magnetic CrI₃

The discovery of the intrinsic magnetic order in single-layer chromium trihalides (CrX3, X = I, Br, and Cl) has drawn intensive interest due to their potential application in spintronic devices. However, the notorious environmental instability of this class of materials under ambient conditions renders their device fabrication and practical application extremely challenging. Here, we performed a systematic investigation of the degradation chemistry of chromium iodide (CrI3), the most studied among CrX3 families, via a joint spectroscopic and microscopic analysis of the structural and composition evolution of bulk and exfoliated nanoflakes in different environments. Unlike other air-sensitive 2D materials, CrI3 undergoes a pseudo-first-order hydrolysis in the presence of pure water toward the formation of amorphous Cr(OH)3 and hydrogen iodide (HI) with a rate constant of kI = 0.63 day-1 without light. In contrast, a faster pseudo-first-order surface oxidation of CrI3 occurs in a pure O2 environment, generating CrO3 and I2 with a large rate constant of kCr = 4.2 day-1. Both hydrolysis and surface oxidation of CrI3 can be accelerated via light irradiation, resulting in its ultrafast degradation in air. The new chemical insights obtained allow for the design of an effective stabilization strategy for CrI3 with preserved optical and magnetic properties. The use of organic acid solvents (e.g., formic acid) as reversible capping agents ensures that CrI3 nanoflakes remain stable beyond 1 month due to the effective suppression of both hydrolysis and oxidation of CrI3.

Nonlocal Response and Anamorphosis: The Case of Few-Layer Black Phosphorus

Few-layer black phosphorus was recently rediscovered as a narrow-bandgap atomically thin semiconductor and has already attracted unprecedented attention due to its interesting properties. One feature of this material that sets it apart from other atomically thin crystals is its structural in-plane anisotropy which manifests in strongly anisotropic transport characteristics. However, traditional angle-resolved conductance measurements present a challenge for nanoscale systems such as black phosphorus, calling for new approaches in precision studies of transport anisotropy. Here we show that the nonlocal response, being exponentially sensitive to the anisotropy value, provides a powerful tool for determining the anisotropy. This is established by combining measurements of the orientation-dependent nonlocal resistance response with the analysis based on the anamorphosis relations. We demonstrate that the nonlocal response can differ by orders of magnitude for different crystallographic directions even when the anisotropy is at most order-one, allowing us to extract accurate anisotropy values.

Detecting topological currents in graphene superlattices

A hidden advantage of van der Waals materials for overcoming limitations in photonic integrated circuitry

With the advance of on-chip nanophotonics, there is a high demand for high refractive index, low-loss materials. Currently, this technology is dominated by silicon, but van der Waals (vdW) materials with high refractive index can offer a very advanced alternative. Still, up to now it was not clear if the optical anisotropy perpendicular to the layers might be a hindering factor for the development of vdW nanophotonics. Here, we studied WS2-based waveguides in terms of their optical properties and, particularly, in terms of possible crosstalk distance. Surprisingly, we discovered that the low refractive index in the direction perpendicular to the atomic layers improves the characteristics of such devices, mainly due to expanding the range of parameters at which single-mode propagation can be achieved. Thus, using anisotropic materials offers new opportunities and novel control knobs when designing the nanophotonic devices.

The Intricate Love Affairs between MoS₂ and Metallic Substrates

Mechanical exfoliation yields high‐quality 2D materials but is challenging to scale up due to the small lateral size and low yield of the exfoliated crystals. Gold‐mediated exfoliation of macroscale monolayer MoS 2 and related crystals addresses this problem. However, it remains unclear whether this method can be extended to other metals. Herein, mechanical exfoliation of MoS 2 on a range of metallic substrates is studied. It is found that Au outperforms all the other metals in their ability to exfoliate macroscale monolayer MoS 2 . This is rationalized by gold's ability to resist oxidation, which is compromised on other metals and leads to a weakened binding with MoS 2 . An anomalously high monolayer yield found for Ag suggests that the large interfacial strain in the metal–MoS 2 heterostructures measured by Raman spectroscopy also is a critical factor facilitating the exfoliation, while the relative differences in the metal–MoS 2 binding play only a minor role. These results provide a new incentive for investigations of 2D material‐substrate combinations applicable where high‐quality 2D crystals of macroscopic dimensions are of importance.

Long-range ballistic transport of Brown-Zak fermions in graphene superlattices

In quantizing magnetic fields, graphene superlattices exhibit a complex fractal spectrum often referred to as the Hofstadter butterfly. It can be viewed as a collection of Landau levels that arise from quantization of Brown-Zak minibands recurring at rational (p/q) fractions of the magnetic flux quantum per superlattice unit cell. Here we show that, in graphene-on-boron-nitride superlattices, Brown-Zak fermions can exhibit mobilities above 106 cm2 V-1 s-1 and the mean free path exceeding several micrometers. The exceptional quality of our devices allows us to show that Brown-Zak minibands are 4q times degenerate and all the degeneracies (spin, valley and mini-valley) can be lifted by exchange interactions below 1 K. We also found negative bend resistance at 1/q fractions for electrical probes placed as far as several micrometers apart. The latter observation highlights the fact that Brown-Zak fermions are Bloch quasiparticles propagating in high fields along straight trajectories, just like electrons in zero field.

Gain modulation by graphene plasmons in aperiodic lattice lasers

Tunable lasers Lasers emit coherent light at wavelengths that are well defined. These wavelengths are usually fixed once the device has been fabricated. Now, Chakraborty et al. have combined an atomically thin graphene sheet with terahertz quantum cascade lasers to realize a terahertz laser that can be tuned via the carrier doping level of the graphene layer (see the Perspective by Polini). The demonstration opens up the possibility of reversible control over the laser emission through the integration of graphene waveguides. Science , this issue p. 246 ; see also p. 229