Bias-tunable two-dimensional magnetic and topological materials
In this work, we proposed a strategy for using fullerenes as building blocks for the synthesis of novel 2D materials.
36 MATERIALS SCIENCE↗
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Low-Loss Charge Transfer Plasmons in Graphene/α-RuCl 3 Heterostructures Below 40 K
Charge transfer at material interfaces governs a wide range of physical properties, from electronic band structures to emergent collective excitations. In two-dimensional (2D) material heterostructures, charge transfer phenomena play important roles in enabling novel quantum phases, proximity effects, and tunable plasmonic responses. One representative charge transfer interface is formed between α-RuCl 3 , a van der Waals material with high electron affinity, and graphene. Significant charge transfer across this interface induces the formation of charge-transfer plasmon polaritons (CPPs), hybrid excitations between light and charge oscillations. However, previous studies found that as the charge transfer process takes place, α-RuCl 3 becomes lossy, which limits the quality factor of CPPs. Here, we investigate CPPs down to 10 K using a home-built scattering-type scanning near-field optical microscope (s-SNOM) optimized for low-temperature measurements. Our study reveals a dramatic suppression of plasmon loss channels below 40 K, contributing to a significant enhancement in the plasmonic quality factor. This reduction in loss is likely attributed to the blue shift of the correlation-induced Mott gap in α-RuCl 3 with decreasing temperature, along with the reduction of phonon scattering at low temperature. In conclusion, our results highlight the potential of using s-SNOM and CPPs to study complex 2D interfaces and reveal correlated electron dynamics in the underlying material.
36 MATERIALS SCIENCE↗
Van der Waals heterostructures
The integration of dissimilar materials into heterostructures has become a powerful tool for engineering interfaces and electronic structure. The advent of two-dimensional (2D) materials brought unprecedented opportunities for novel heterostructures in the form of van der Waals stacks, laterally stitched 2D layers, and more complex layered and 3D architectures. This Primer provides an overview of state-of-the-art methodologies for producing such van der Waals heterostructures, focusing on the two fundamentally different strategies, top-down deterministic assembly and bottom-up synthesis. For both approaches, successful techniques, advantages, and limitations are discussed. As important as the fabrication itself is the characterization of the resulting engineered materials, for which a range of analysis techniques covering structure, composition, and emerging functionality are highlighted. Examples of the properties of artificial van der Waals structures include optoelectronics and plasmonics, twistronics, and unique functionality arising from the generalization of van der Waals assembly from 2D- to 3D-crystalline components. Lastly, current issues of reproducibility, limitations, and opportunities for future breakthroughs in terms of enhanced homogeneity, interfacial purity, feature control, and ultimately orders of magnitude increased complexity of van der Waals heterostructures are discussed.
36 MATERIALS SCIENCE↗
3D-Printable Inks for Energy and Environmental Applications
Partnering additive manufacturing (3D printing) with functional nanomaterial-based inks has the potential to push the properties and performance of advanced materials beyond previous capabilities. This is particularly true in energy and environmental applications. Graphene, 2D materials, and ceramic-based inks, when coupled with 3D printing technologies like Direct Ink Writing (DIW), also called “robocasting,” have shown promise to greatly enhance the performance of batteries, supercapacitors, reactors, and separator devices. Robocasting enables the printing of large-scale electrodes for energy applications with micron resolution by using high concentration viscous inks.
36 MATERIALS SCIENCE↗
Charge density waves in two-dimensional transition metal dichalcogenides
Charge density wave (CDW is one of the most ubiquitous electronic orders in quantum materials. While the essential ingredients of CDW order have been extensively studied, a comprehensive microscopic understanding is yet to be reached. Recent research efforts on the CDW phenomena in two-dimensional (2D) materials provide a new pathway toward a deeper understanding of its complexity. This review provides an overview of the CDW orders in 2D with atomically thin transition metal dichalcogenides (TMDCs) as the materials platform. We mainly focus on the electronic structure investigations on the epitaxially grown TMDC samples with angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy as complementary experimental tools. Finally, we discuss the possible origins of the 2D CDW, novel quantum states coexisting with them, and exotic types of charge orders that can only be realized in the 2D limit.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Intrinsic ferromagnetism and restrictive thermodynamic stability in MA$_{2}$N$_{4}$ and Janus VSiGeN$_{4}$ monolayers
The seminal experimental discovery of the remarkably stable MoSi 2 N 4 monolayer has led to a handful of predicted magnetic two-dimensional (2D) materials in the MA 2 Z 4 family (M = transition metals, A = Si, Ge, and Z = N, P, As). These magnetic monolayers were predicted to be dynamically stable, but none of them has been synthesized to date. In this Research Letter, from first-principles thermodynamic stability analysis, we demonstrate that only the nitrides are thermodynamically stable and this occurs under N-rich conditions. Based on this finding, we propose two ferromagnetic, semiconducting Janus monolayers in the family: VSiGeN 4 and VSiSnN 4 . They are both dynamically and thermally stable, but only the former is thermodynamically stable. Intriguingly, Janus VSiGeN 4 and VSiSnN 4 monolayers show weak in-plane anisotropy compared with the VSi 2 N 4 monolayer. Furthermore, these two emerging Janus magnetic semiconductors offer opportunities for studying 2D magnetism and spin control for spintronics applications.
2-dimensional systems↗
Enhanced valley splitting of WSe 2 in twisted van der Waals WSe 2 /CrI 3 heterostructures
Van der Waals (vdW) heterostructures composed of different two-dimensional (2D) materials offer an easily accessible way to combine properties of individual materials for applications. Owing to the discovery of a set of unanticipated physical phenomena, the twisted 2D vdW heterostructures have gained considerable attention recently. Here, we report enhanced valley splitting in twisted 2D vdW WSe 2 /CrI 3 heterostructures. In particular, the splitting can be 1200% (or 5.18 meV) of the value for a non-twisted heterostructure. According to the $k ∙ p$ model, this value is equivalent to a ~20T external magnetic field applied perpendicular to the 2D sheet. The thermodynamic stability of 2D vdW WSe 2 /CrI 3 heterostructures, on the other hand, depends linearly on the interlayer twisting angle.
36 MATERIALS SCIENCE↗
Universal slow plasmons and giant field enhancement in atomically thin quasi-two-dimensional metals
Plasmons depend strongly on dimensionality: while plasmons in three-dimensional systems start with finite energy at wavevector q = 0, plasmons in traditional two-dimensional (2D) electron gas disperse as ω<!-- ω --> p ∼<!-- ~ --> q . However, besides graphene, plasmons in real, atomically thin quasi-2D materials were heretofore not well understood. Here we show that the plasmons in real quasi-2D metals are qualitatively different, being virtually dispersionless for wavevectors of typical experimental interest. This stems from a broken continuous translational symmetry which leads to interband screening; so, dispersionless plasmons are a universal intrinsic phenomenon in quasi-2D metals. Moreover, our ab initio calculations reveal that plasmons of monolayer metallic transition metal dichalcogenides are tunable, long lived, able to sustain field intensity enhancement exceeding 10 7 , and localizable in real space (within ~20 nm) with little spreading over practical measurement time. This opens the possibility of tracking plasmon wave packets in real time for novel imaging techniques in atomically thin materials.
36 MATERIALS SCIENCE↗
Trace Chemical Detection Using Intercalated MXenes as a Signal Enhancing Substrate in Optical Probes
MXenes are 2D materials composed of layered transition metal nitrides or carbides. These materials are synthesized by HF exfoliation from MAX phases (Ti{sub 3}AlC{sub 2}). The 2D nanomaterial was synthesized by the removal of the 'A' element, resulting in a Mxene product (Ti{sub 3}C{sub 2}). MXenes have the general formula M{sub n+1}X{sub n}T{sub x}, where M is an early transition metal, X is Carbon and/or Nitrogen, and T accounts for surface terminated functional groups such as Fluoride, hydroxyl, and oxygen. These materials have very unique properties, similar to graphene, that allows them to be applied in a variety of trace detection techniques including surface-enhanced Raman spectroscopy (SERS). MXenes have also been demonstrated to selectively uptake uranyl ion, UO{sub 2}{sup 2+}. If this property can be combined with SERS or fluorescence detection, it may be possible to use MXenes as the basis for an alternative method to kinetic phosphorescence analysis (KPA) for trace uranyl measurements. Objectives: To confirm that MXene Nano materials are suitable substrates for SERS and sensor development by enhancing Raman signaling. To determine if certain MXene preparation methods yield materials that are more suitable for trace sensing methods. To determine uranyl uptake properties of these MXene materials and test them for analytical signals. Sample Preparation: Preparation of Ti{sub 3}C{sub 2}MXene (at FSU). MXenes were prepared by etching Al from Ti{sub 3}AlC{sub 2} (MAX phase)material. Two etching techniques yield different MXene products: LiF/HCl: Milder reaction, larger MXene flakes. HF: Harsher reaction, smaller flakes, larger layer separation. Products washed to remove etchant, vacuum filtered, and dried. Dried MXene flakes are air-stable. Film preparation for sensor testing (at SRNL): Suspend powder in diH{sub 2}O, purge with Ar, sonicate for 30 min. Centrifuge and collect supernate with suspended particles. Observed LiF-etched Mxene yielded a higher density of particles and darker collected solution. Drop-cast (4 ml) supernate onto slides and dried with Ar. For Rhodamine B (RhB) testing, drop-cast 4 ml drops onto Mxene spots and dried with Ar. Scanning Electron Microscopy conditions: 10 kV Beam energy, high vacuum; Working distance of 8 mm; beam penetration depth appx. 4 microns, beam spot size appx. 2 nanometers. Results: Detection of aluminum correlates with bright spots on image. Presence of aluminum shows that LiF/HCl etching was less thorough than HF etching. Trace Cl detection in LiF images suggests incomplete rinsing. HF has smaller feature size, more layer structure, and increased homogeneity, consistent with expectations. Macroscopic Raman spectroscopy measurements: 532 nm excitation, ∼50 mW with a ∼100 micron spot size (InPhotonics RPB probe). Kaiser Optical Holospec f/1.8 spectrometer with cooled (-60 deg.C) Andor iDus OE420 CCD. LiF 1x supernate showed good signal for trace measurements of Rhodamine B. HF and 1/4x LiF supernates showed little Mxene or Rhodamine B signal. Low deposition densities led to excess background signal from glass slides. For LiF film, response is linear with Rhodamine B concentration over range tested. Will retest with Raman microscope (∼1 micron spot size) to characterize SERS of more dilute LiF and HF etched Mxenes. Conclusions: The LiF etched material was more suitable for macroscopic SERS measurements because it was more concentrated, resulting in a thicker film than the HF etched Mxene and diluted LiF sample. However, the other materials may give greater SERS enhancements, which we hope to determine from measurements with the Raman microscope. From characterization with SEM we concluded that the HF etched Mxene is more uniform/homogenous and has smaller particle size than the LiF etched Mxene. There is still aluminum present in both samples indicating that etching wasn't complete, but the removal of the aluminum was more efficient in the HF method. Path Forward: Observe SERS with Raman microscopy, to obtain better signals for the more diluted samples and be able to compare enhancement effects for the different MXenes. Characterize uranyl sorption into MXene films and test Raman and fluorescence signals. Revisit the etching conditions to improve removal of aluminum. FSU and SRNL will continue to collaborate to create and characterize different Mxene materials and test their usefulness for sensor applications.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Excitonic Anisotropy in Single‐Crystalline 2D Silver Phenylchalcogenides
2D materials exhibiting in‐plane anisotropy enable new applications in directional energy transport and polarized optical response. Silver phenylchalcogenides (AgEPh) – including mithrene (AgSePh), tethrene (AgTePh), and thiorene (AgSPh) – represent an exciting new addition to this family, with optical response spanning the visible to near‐UV. Here, excitonic anisotropy is predicted and characterized in this family of materials using a combination of ab initio theory and optical micro‐spectroscopy of single‐crystalline flakes. Using density functional theory and GW with the Bethe–Salpeter equation calculations, it is revealed that all AgEPh compounds exhibit anisotropic electronic band structure and host multiple delocalized excitons with in‐plane anisotropy. Room‐temperature polarization‐resolved optical micro‐spectroscopy shows that orthogonally polarized excitons with similar energy lead to nearly isotropic absorption in AgSPh, whereas energy separation between excitonic resonances in AgSePh and AgTePh leads to strong absorption and emission anisotropy. Cryogenic reflectance micro‐spectroscopy further reveals exciton fine structure in AgSePh, reconciling the discrepancies between room‐temperature experiments and theoretical predictions. Finally, it is demonstrated that the optical response of thicker AgEPh crystals is influenced by photonic effects arising from finite crystal size. Overall, this work advances the understanding of the relationship between anisotropic structure, composition, and excitonic properties in AgEPh, providing a foundation for technological integration.
2D↗
Liquid-like spin dynamics in a hybrid Heisenberg-Ising antiferromagnet
Two-dimensional (2D) materials and their heterostructures enable unconventional electronic properties and functionalities not accessible in their bulk counterparts. This approach is now being extended to magnetic materials to engineer their spin structures and magnetic fields produced by them. However, spin dynamics of 2D magnetic heterostructures remain largely unexplored. Here, we demonstrate that heterointerfacing Heisenberg square-lattice antiferromagnet (AF) Sr 2 IrO 4 with its bilayer variant Ising AF Sr 3 Ir 2 O 7 in a superlattice leads to liquid-like spin dynamics in the former, characterized by slow recovery of the AF order after its transient suppression by an optical pump, and complete absence of spin waves except in an immediate vicinity of the ordering wavevector. Instead, the spin excitation spectra are dominated by isotropic continua, which in previous works have been interpreted as fractional spin excitations, or spinons, that extends to unprecedentedly low energies. Thus, our results provide a pathway to frustrated magnetism in square lattices by heterointerfacing two distinct types of AFs.
RIXS↗
Mechanisms of adsorbing hydrogen gas on metal decorated graphene
Hydrogen is a key player in global strategies to reduce greenhouse gas emissions. In order to make hydrogen a widely used fuel, we require more efficient methods of storing it than the current standard of pressurized cylinders. An alternative method is to adsorb H 2 in a material and avoid the use of high pressures. Among many potential materials, layered materials such as graphene present a practical advantage as they are lightweight. However, graphene and other 2D materials typically bind H 2 too weakly to store it at the typical operating conditions of a hydrogen fuel cell, meaning that high pressure would still be required. Modifying the material, for example by decorating graphene with adatoms, can strengthen the adsorption energy of H 2 molecules, but the underlying mechanisms are still not well understood. In this work, we systematically screen alkali and alkaline-earth metal decorated graphene sheets for the static thermodynamic adsorption of hydrogen gas from first principles and focus on the mechanisms of binding. We show that there are three mechanisms of adsorption on metal decorated graphene and each leads to distinctly different hydrogen adsorption structures. The three mechanisms can be described as weak van der Waals physisorption, metal adatom facilitated polarization, and Kubas adsorption. Among these mechanisms, we find that Kubas adsorption is easily perturbed by an external electric field, providing a way to tune H 2 adsorption. In conclusion, this work is foundational and builds our understanding of H 2 adsorption under idealized conditions.
36 MATERIALS SCIENCE↗
Low-dimensional solid-state single-photon emitters
Solid-state single-photon emitters (SPEs) are attracting significant attention as fundamental components in quantum computing, communication, and sensing. Low-dimensional materials-based SPEs (LD-SPEs) have drawn particular interest due to their high photon extraction efficiency, ease of integration with photonic circuits, and strong coupling with external fields. The accessible surfaces of LD materials allow for deterministic control over quantum light emission, while enhanced quantum confinement and light–matter interactions improve photon emissive properties. This perspective examines recent progress in LD-SPEs across four key materials: zero-dimensional (0D) semiconductor quantum dots, one-dimensional (1D) nanotubes, two-dimensional (2D) materials, including hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDCs). We explore their structural and photophysical properties, along with techniques such as spectral tuning and cavity coupling, which enhance SPE performance. Finally, we address future challenges and suggest strategies for optimizing LD-SPEs for practical quantum applications.
42 ENGINEERING↗
Erbium-Implanted WS 2 Flakes with Room-Temperature Photon Emission at Telecom Wavelengths
Optically addressable spin impurities in crystals along with device engineering provide an attractive route to realizing quantum technologies in the solid state, but reconciling disparate emitter and host material constraints for a given target application is often challenging. Rare-earth ions in two-dimensional (2D) materials could mitigate this problem given the atomic-like transitions of the emitters and the versatile nature of van der Waals systems. Here we combine ion implantation, confocal microscopy, and ab initio calculations to examine the photon emission of Er-doped WS 2 flakes. Optical spectroscopy reveals narrow, long-lived photoluminescence lines in the telecom band, which we activate after low-temperature thermal annealing. Spectroscopic and polarization-selective measurements show a uniform response across the ensemble, while the fluorescence brightness remains mostly unchanged with temperature, suggesting nonradiative relaxation channels are inefficient. Our results create opportunities for novel solid state devices coupling 2D-hosted, telecom-band emitters to photonic heterostructures separately optimized for photon manipulation.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Ionically Active MXene Nanopore Actuators
Reversible electrochemical intercalation of cations into the interlayer space of 2D materials induces tunable physical and chemical properties in them. In MXenes, a large class of recently developed 2D carbides and nitrides, low intercalation energy, high storage capacitance, and reversible intercalation of various cations have led to their improved performance in sensing and energy storage applications. In this work, a coupled nanopore-actuator system where an ultrathin free-standing MXene film serves as a nanopore support membrane and ionically active actuator is reported. In this system, the contactless MXene membrane in the electric field affects the cation movement in the field through their (de)intercalation between individual MXene flakes. This results in reversible swelling and contraction of the membrane monitored by ionic conductance through the nanopore. This unique nanopore coupled to a mechanical actuation system could provide new insights into designing single-molecule biosensing platforms at the nanoscale.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Defect identification in simulated Bragg coherent diffraction imaging by automated AI
X-ray Bragg coherent diffraction imaging is a powerful technique for operando and in situ materials characterization and provides a unique means of quantifying the influence of one-dimensional (1D) and two-dimensional (2D) material defects on material response. However, obtaining full images from raw x-ray diffraction data is nontrivial and computationally intensive, precluding real-time experimental feedback. Here, we present a machine learning approach to identify the presence of crystalline line defects (edge and screw) in samples from the raw, 2D, coherent diffraction data without the need for image reconstruction through iterative phase retrieval. Further, we compare different approaches to designing neural networks for this application and demonstrate the potential of automated ML (autoML) approaches.
36 MATERIALS SCIENCE↗
Robust Topological Nodal-Line Semimetals from Periodic Vacancies in Two-Dimensional Materials
A nodal-line semimetal (NLSM) is suppressed in the presence of spin–orbit coupling unless it is protected by a nonsymmorphic symmetry. We show that two-dimensional (2D) materials can realize robust NLSMs when vacancies are introduced on the lattice. As a case study we investigate borophene, a boron honeycomb-like sheet. While the Dirac cones of pristine borophene are shown to be gapped out by spin–orbit coupling and by magnetic exchange, robust nodal lines (NLs) emerge in the spectrum when selected atoms are removed. We propose an effective 2D model and a symmetry analysis to demonstrate that these NLs are topological and protected by a nonsymmorphic glide plane. Finally, our findings offer a paradigm shift to the design of NLSMs: instead of searching for nonsymmorphic materials, robust NLSMs may be realized simply by removing atoms from ordinary symmorphic crystals.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Acoustic cavities in 2D heterostructures
Two-dimensional (2D) materials offer unique opportunities in engineering the ultrafast spatiotemporal response of composite nanomechanical structures. In this work, we report on high frequency, high quality factor ( Q ) 2D acoustic cavities operating in the 50–600 GHz frequency ( f ) range with f × Q up to 1 × 10 14 . Monolayer steps and material interfaces expand cavity functionality, as demonstrated by building adjacent cavities that are isolated or strongly-coupled, as well as a frequency comb generator in MoS 2 /h-BN systems. Energy dissipation measurements in 2D cavities are compared with attenuation derived from phonon-phonon scattering rates calculated using a fully microscopic ab initio approach. Phonon lifetime calculations extended to low frequencies (<1 THz) and combined with sound propagation analysis in ultrathin plates provide a framework for designing acoustic cavities that approach their fundamental performance limit. These results provide a pathway for developing platforms employing phonon-based signal processing and for exploring the quantum nature of phonons.
36 MATERIALS SCIENCE↗