Publications

Vertical stacking of atomically thin layered materials opens new possibilities for the fabrication of heterostructures with favorable optoelectronic properties. The combination of graphene, hexagonal boron nitride and semiconducting transition metal dichalcogenides allows fabrication of electroluminescence (EL) devices, compatible with a wide range of substrates. Here, we demonstrate a full integration of an electroluminescent van der Waals heterostructure in a monolithic optical microcavity made of two high reflectivity dielectric distributed Bragg reflectors (DBRs). Owing to the presence of graphene and hexagonal boron nitride protecting the WSe2 during the top mirror deposition, we fully preserve the optoelectronic behaviour of the device. Two bright cavity modes appear in the EL spectrum featuring Q-factors of 250 and 580 respectively: the first is attributed directly to the monolayer area, while the second is ascribed to the portion of emission guided outside the WSe2 island. By embedding the EL device inside the microcavity structure, a significant modification of the directionality of the emitted light is achieved, with the peak intensity increasing by nearly two orders of magnitude at the angle of the maximum emission compared with the same EL device without the top DBR. Furthermore, the coupling of the WSe2 EL to the cavity mode with a dispersion allows a tuning of the peak emission wavelength exceeding 35 nm (80 meV) by varying the angle at which the EL is observed from the microcavity. This work provides a route for the development of compact vertical-cavity surface-emitting devices based on van der Waals heterostructures.

Abstract Thanks to their record high refractive index and giant optical anisotropy, van der Waals (vdW) materials have accelerated the development of nanophotonics. However, traditional high refractive index materials, such as titanium dioxide (TiO 2 ), still dominate in the most important visible range. This is due to the current lack of transparent vdW materials across the entire visible spectrum. In this context, we propose that germanium disulfide (GeS 2 ) could offer a significant breakthrough. With its high refractive index, negligible losses, and biaxial optical anisotropy across the whole visible range, GeS 2 has the potential to complement TiO 2 and close the application gap of vdW materials in the visible spectrum. The addition of GeS 2 could have a profound impact on the design of van der Waals nanophotonic circuits for any operation wavelength from ultraviolet to infrared, emphasizing the significance of the potential impact of GeS 2 on the field of nanophotonics.

Topological Spin Textures Implementation of magnets with low-damping, minimum Joule heating and tunable topological spin properties consist one of the main goals towards novel information storage technologies. In article number 2311949, Sergey Grebenchuk, Elton J. G. Santos, Maciej Koperski, and co-workers report that a wide variety of topological magnetic textures can be created and controlled on-demand on CrBr3 van der Waals magnet insulators at different thicknesses, temperatures and external excitations, such as energy-efficient optical excitations.

Marcus-Hush theory of electron transfer is one of the pillars of modern electrochemistry with a large body of supporting experimental evidence presented to date. However, some predictions, such as the electrochemical behavior at microdisk electrodes, remain unverified. Herein, we present a study of electron tunneling across a hexagonal boron nitride barrier between a graphite electrode and redox levels in a liquid solution. This was achieved by the fabrication of microdisk electrodes with a typical diameter of 5 µm. Analysis of voltammetric measurements, using two common redox mediators, yielded several electrochemical parameters, including the electron transfer rate constant, limiting current, and transfer coefficient. They show a significant departure from the Butler-Volmer behavior in a clear manifestation of the Marcus-Hush theory of electron transfer. In addition, our system provides a novel experimental platform, which could be applied to address a number of scientific problems such as identification of reaction mechanisms, surface modification, or long-range electron transfer.

Strongly anisotropic media where principal components of the dielectric tensor have opposite signs are called hyperbolic. These materials permit highly directional, volume-confined propagation of slow-light modes at deeply sub-diffractional size scales, leading to unique nanophotonic phenomena. The realization of hyperbolic materials within the optical spectral range has been achieved primarily through the use of artificial structures typically composed of plasmonic metals and dielectric constituents. However, while proof-of-principle experiments have been performed, the high plasmonic losses and inhomogeneity of the structures limit most advances to the laboratory. Recently, hexagonal boron nitride (hBN) was identified as a natural hyperbolic material (NHM), offering a low-loss, homogeneous medium that can operate in the mid-infrared. We have exploited the NHM response of hBN within periodic arrays of conical nanoresonators to demonstrate ‘hyperbolic polaritons,’ deeply sub-diffractional guided waves that propagate through the volume rather than on the surface of a hyperbolic material. We have identified that the polaritons are manifested as a four series of resonances in two distinct spectral bands that have mutually exclusive dependencies upon incident light polarization, modal order, and aspect ratio. These observations represent the first foray into creating NHM building blocks for mid-infrared to terahertz nanophotonic and metamaterial devices. This talk will also discuss potential near-term applications stemming from these developments.

No abstract is provided for this article.

Commensurability effects play a crucial role in the formation of electronic properties of novel layered heterostructures. The interest in these moiré superstructures has increased tremendously since the recent observation of a superconducting state (Nature 2018, 556, 43–50) and metal–insulator transition (Nature 2018, 556, 80–84) in twisted bilayer graphene. In this regard, a straightforward and efficient experimental technique for detection of the alignment of layered materials is desired. In this work, we use optical second harmonic generation, which is sensitive to the inversion symmetry breaking, to investigate the alignment of graphene/hexagonal boron nitride heterostructures. To achieve that, we activate a commensurate–incommensurate phase transition by a thermal annealing of the sample. We find that this structural change in the system can be directly observed via a strong modification of a nonlinear optical signal. Unambiguous interpretation of obtained results reveals the potential of a second harmonic generation technique for probing of structural changes in layered systems.

Semiconducting transition metal dichalcogenides (TMDCs) are very promising materials for quantum dots and spin-qubit implementation. Reliable operation of spin qubits requires the knowledge of Land\'e g-factor, which can be measured by exploiting the discrete energy spectrum on a quantum dot. However, the quantum dots realized in TMDCs has yet to reach the required quality for reliable measurement of g-factor. Quantum dot sizes reported in TMDCs so far are not small enough to observe discrete energy levels on them. Here, we report on electron transport through discrete energy levels of quantum dot in a single layer MoS2. The quantum dot energy levels are separated by few (5-6) meV such that the ground state and the excited state transitions are clearly visible. This well resolved energy separation allows us to accurately measure the ground state g-factor of ~5 in MoS2 quantum dots. We observe a spin filling sequence in our quantum dot under perpendicular magnetic field. Such a system offers an excellent testbed to measure the key parameters for evaluation and implementation of spin-valley qubits in TMDCs, thus accelerating the development of quantum systems in two dimensional semiconducting TMDCs.

Abstract Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy was recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS 3 and h-BN crystals. This inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Walls interaction. To do this, we carried out a correlative far- and near-field characterization validated by first-principle calculations that reveals an unprecedented birefringence of 1.5 in the infrared and 3 in the visible light for MoS 2 . Our findings demonstrate that this outstanding anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.

In thermodynamic equilibrium, current in metallic systems is carried by electronic states near the Fermi energy whereas the filled bands underneath contribute little to conduction. Here we describe a very different regime in which carrier distribution in graphene and its superlattices is shifted so far from equilibrium that the filled bands start playing an essential role, leading to a critical-current behavior. The criticalities develop upon the velocity of electron flow reaching the Fermi velocity. Key signatures of the out-of-equilibrium state are current-voltage characteristics resembling those of superconductors, sharp peaks in differential resistance, sign reversal of the Hall effect, and a marked anomaly caused by the Schwinger-like production of hot electron-hole plasma. The observed behavior is expected to be common for all graphene-based superlattices.

No abstract is provided for this article.

Water molecules at the solid-liquid interface display intricate behaviours sensitive to small changes. The presence of different interfacial components, such as cations or functional groups, shape the physical and chemical properties of the hydrogen bond network. Understanding such interfacial hydrogen-bond networks is essential for a large range of applications and scientific questions. To probe the interfacial hydrogen-bond network, atmospheric water capture is a powerful tool. Here, we experimentally observe that a calcium ion on a calcium-intercalated graphene oxide aerogel (Ca-GOA) surface captures 2.7 times more water molecules than in its freestanding state. From density functional theory (DFT) calculations, we uncover the synergistically enhanced hydrogen-bond network of the calcium ion-epoxide complex due to significantly larger polarizations and hydrogen bond enthalpies. This study reveals valuable insights into the interfacial water hydrogen-bond network on functionalized carbon-cation complexed surfaces and potential pathways for future atmospheric water generation technologies.

Nature serves as an inexhaustible source of inspiration for advanced material design. While nature-inspired nonliving materials exhibit exceptional properties, they typically lack the dynamic functionalities of living systems, such as self-healing and environmental responsiveness. To bridge this gap, living materials, which integrate living cells (e.g., bacteria, fungi, algae) within abiotic matrices, have emerged as transformative platforms. These materials harness cellular functions (e.g., biomineralization, programmable metabolism) to achieve unprecedented adaptability and sustainability. In this review, we categorized living materials into two distinct types based on the role of the cells: (1) cells acting as platforms for material synthesis and (2) cells integrated as components of materials for functionalization. We summarized the characteristics of living and nonliving materials inspired by nature, with applications of living materials in energy, medicine, catalysis, concrete, and soft robotics. We further discussed advanced manufacturing techniques for living materials. We envision that the design principles of living materials will advance health, energy, and sustainability.

In this paper, we demonstrate realization of printable radio frequency identification (RFID) antenna by low temperature processing of graphene ink. The required ultra-low resistance is achieved by rolling compression of binder-free graphene laminate. With compression, the conductivity of graphene laminate is increased by more than 50 times compared to that of as-deposited one. Graphene laminate with conductivity of 4.3 × 104 S/m and sheet resistance of 3.8 Ω/sq (with thickness of 6 μm) is presented. Moreover, the formation of graphene laminate from graphene ink reported here is simple and can be carried out in low temperature (100 °C), significantly reducing the fabrication costs. A dipole antenna based on the highly conductive graphene laminate is further patterned and printed on a normal paper to investigate its RF properties. The performance of the graphene laminate antenna is experimentally measured. The measurement results reveal that graphene laminate antenna can provide practically acceptable return loss, gain, bandwidth, and radiation patterns, making it ideal for low cost printed RF applications, such as RFID tags and wearable wireless sensor networks.