Publications

The realization of extreme optical anisotropy is foundational for nanoscale light manipulation. The van der Waals (vdW) crystal MoOCl₂ has emerged as a promising candidate for this quest, hosting hyperbolic plasmon polaritons in the visible and near-infrared wavelengths. However, the fundamental anisotropic dielectric tensor governing this behavior has remained elusive. Here, we resolve this problem by providing the first experimental determination of the full dielectric tensor of hyperbolic vdW MoOCl₂. Via spectroscopic ellipsometry, Mueller matrix, and reflectance measurements, we quantify the material's optical duality: a metallic optical response (<i>ε</i>₁ < 0) along the crystallographic <i>a</i>-axis and a dielectric response (<i>ε</i>₁ > 0) along the orthogonal directions. This dichotomy drives an epsilon-near-zero (ENZ) condition at ≈512 nm and results in a giant in-plane birefringence of Δ<i>n</i> ≈ 2.2 for MoOCl₂. As a result, our work provides the critical missing experimental parameters for MoOCl₂, establishing it as a benchmark hyperbolic and ENZ material.

We present a recyclable perovskite–graphene heterostructure that demonstrates ultrahigh X-ray detection sensitivities over 108 μC/Gyair·cm2 for medical imaging applications. The high mobility of the graphene pixel is preserved to over 1200 cm2/V·s after perovskite deposition and enables large conversion efficiency for ultrahigh sensitivity. Increasing the operational bias of the graphene channel increased the X-ray detection signal-to-noise ratio from 30 to over 200. The perovskite can be washed off by an organic solvent at room temperature without damaging the graphene. Redepositing the perovskite layer retains the detectors' high gain, making our heterostructure X-ray detector a recyclable device. The perovskite–graphene device exhibits robust operation given 10,000 gate sweeps and multicycle X-ray irradiations. Here we have demonstrated a high-performance, low-cost, plug-and-play solution with a recyclable design that could significantly reduce the manufacturing and maintenance costs associated with X-ray cameras in medical imaging.

One of the fundamental properties of spin 1/2 particles is their interaction with magnetic fields. The exploration of this coupling can be quite elusive, for example in the case of neutrinos. Graphene has been shown to be a condensed matter platform for the study of such ultrarelativistic particles, with its neutrino-like charge carriers having a spin-like degree of freedom called pseudospin. Here we show that in analogy to the spin alignment of a fermion in a magnetic field, the pseudo-spin of graphene can be oriented by a strain induced pseudo-magnetic field through the Zeeman term. We reveal this pseudo-spin polarization as a sublattice symmetry breaking by tunably straining graphene using the tip of a scanning tunnelling microscope. The observed pseudo-spin polarization scales with the lifting height of the strained deformation and therefore with the pseudo-magnetic field strength. Its magnitude is quantitatively reproduced by analytic and tight-binding models. This adds a key ingredient to the celebrated analogy of graphene's charge carriers to ultra-relativistic Dirac fermions. Furthermore, the deduced fields of about 1000 T could provide an effective THz valley filter, as a basic element of valleytronics.

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 3.2 times more water molecules than in its freestanding state. From experimental Van’t Hoff estimation and 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.

Graphene oxide (GO) based multi-layered membranes have shown outstanding molecular-sieving properties for gas separation, surpassing the upper bound for polymeric membranes especially for hydrogen decarbonization. At the same time, the mechanism of gas permeation through such 2D GO membranes is very different in comparison to the traditional polymeric membranes due to multilayer, laminated nature of the former. For strategical design of novel membranes based on two-dimensional materials, it is important to understand the mechanism and to be able to measure two key parameters for gas transport: solubility and diffusivity. Such measurements are well established for the characterization of transport properties of bulk polymeric membranes. However, it is still a challenge to measure gas diffusion coefficients directly and accurately in ultra-thin multi-layered membranes. The lack of characterization limits our understanding of the mechanisms of gas transport though such membranes. In this work, we applied a time lag method to determine the diffusivities for He, H2, O2, N2, CH4, CO2 and equimolar mixture by on-line mass spectrometry. In contrast to polymeric membranes, the diffusivity and diffusion activation energy for all gases in 2D membranes are exponentially dependent on membrane thickness. Thus, in 2D membrane we can use an easy strategy to precisely regulate permeability and selectivity by the adjustment of the number of nanolayers, which is not possible using traditional polymeric membranes. This study is important for both the characterization and the standardization of gas transport properties of multi-layered membranes, and the design of novel membranes based on 2D materials.

No abstract is provided for this article.

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 disk ultramicroelectrodes, remain unverified. Herein, we present a study of electron tunneling across a hexagonal boron nitride acting as a barrier between a graphite electrode and redox mediators in a liquid solution. This was achieved by the fabrication of disk ultramicroelectrodes with a typical diameter of 5 μm. Analysis of voltammetric measurements, using two common outer-sphere redox mediators, yielded several electrochemical parameters, including the electron transfer rate constant, limiting current, and transfer coefficient. They depart significantly from the Butler-Volmer kinetics and instead show behavior previously predicted by the Marcus-Hush theory of electron transfer. In addition, our system provides a noteworthy 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.

We investigate the electronic properties of ultrathin hexagonal boron nitride (h-BN) crystalline layers with different conducting materials (graphite, graphene, and gold) on either side of the barrier layer. The tunnel current depends exponentially on the number of h-BN atomic layers, down to a monolayer thickness. Conductive atomic force microscopy scans across h-BN terraces of different thickness reveal a high level of uniformity in the tunnel current. Our results demonstrate that atomically thin h-BN acts as a defect-free dielectric with a high breakdown field. It offers great potential for applications in tunnel devices and in field-effect transistors with a high carrier density in the conducting channel.

We observe a series of sharp resonant features in the differential conductance of graphene-hexagonal boron nitride-graphene tunnel transistors over a wide range of bias voltages between 10 and 200 mV. We attribute them to electron tunneling assisted by the emission of phonons of well-defined energy. The bias voltages at which they occur are insensitive to the applied gate voltage and hence independent of the carrier densities in the graphene electrodes, so plasmonic effects can be ruled out. The phonon energies corresponding to the resonances are compared with the lattice dispersion curves of graphene–boron nitride heterostructures and are close to peaks in the single phonon density of states.Received 8 December 2015DOI:https://doi.org/10.1103/PhysRevLett.116.186603This article is available under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsGraphenePhotonsTransistorsTechniquesMethods in transportCondensed Matter, Materials & Applied Physics

2D semiconductors with atomically thin body thickness have attracted tremendous research interest for high-performance nanoelectronics and optoelectronics. Most of the 2D semiconductors grown by chemical vapor deposition (CVD) methods suffer from rather low carrier mobility, small single-crystal size, and instability under ambient conditions. Here, we develop an improved CVD method with controllable reverse-gas flow to realize the direct growth of quality Bi2O2Se 2D single crystals on a mica substrate. The applied reverse flow significantly suppresses the random nucleation and thus promotes the lateral size of 2D Bi2O2Se crystals up to ∼750 μm. The Bi2O2Se field-effect transistors display high-room-temperature electron mobility up to ∼1400 cm2·V-1·s-1 and a well-defined drain current saturation. The on/off ratio of the Bi2O2Se transistor is larger than 107, and the sub-threshold swing is about 90 mV·dec-1. The responsivity, response time, and detectivity of Bi2O2Se photodetectors approach up to 60 A·W-1, 5 ms, and 2.4 × 1010 Jones at room temperature, respectively. Our results demonstrate large-size and high-quality Bi2O2Se grown by reverse-flow CVD as a high-performance channel material for next-generation transistors and photodetectors.

The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in the device fabrication.

A class of compounds sharing the properties of 2D materials and electrolytes, namely 2D electrolytes is described theoretically and demonstrated experimentally. 2D electrolytes dissociate in different solvents, such as water, and become electrically charged. The chemical and physical properties of these compounds can be controlled by external factors, such as pH, temperature, electric permittivity of the medium, and ionic concentration. 2D electrolytes, in analogy with polyelectrolytes, present reversible morphological transitions from 2D to 1D, as a function of pH, due to the interplay of the elastic and Coulomb energies. Since these materials show stimuli‐responsive behavior to the environmental conditions, 2D electrolytes can be considered as a novel class of smart materials that expand the functionalities of 2D materials and are promising for applications that require stimuli‐responsive demeanor, such as drug delivery, artificial muscles, and energy storage.

No abstract is provided for this article.

No abstract is provided for this article.

We present a systematic experimental and theoretical study of the two-phonon (2D) Raman scattering in graphene under uniaxial tension. The external perturbation unveils that the 2D mode excited with 785 nm has a complex line-shape mainly due to the contribution of two distinct double resonance scattering processes (inner and outer) in the Raman signal. The splitting depends on the direction of the applied strain and the polarization of the incident light. The results give new insight into the nature of the 2D band and have significant implications for the use of graphene as reinforcement in composites since the 2D mode is crucial to assess how effectively graphene uptakes an applied stress or strain.

During the last years, giant optical anisotropy demonstrated its paramount importance for light manipulation which resulted in numerous applications ranging from subdiffraction light guiding to switchable nanolasers. In spite of recent advances in the field, achieving continuous tunability of optical anisotropy remains an outstanding challenge. Here, we present a solution to the problem through chemical alteration of the ratio of halogen atoms (X = Br or Cl) in single-crystal CsPbX$_3$ halide perovskites. It turns out that the anisotropy originates from an excitonic resonance in the perovskite, which spectral position and strength are determined by the halogens composition. As a result, we manage to continually modify the optical anisotropy by 0.14. We also discover that the halide perovskite can demonstrate optical anisotropy up to 0.6 in the visible range -- the largest value among non-van der Waals materials. Moreover, our results reveal that this anisotropy could be in-plane and out-of-plane, depending on perovskite shape -- rectangular and square. Hence, it can serve as an additional degree of freedom for anisotropy manipulation. As a practical demonstration, we created perovskite anisotropic nanowaveguides and show a significant impact of anisotropy on high-order guiding modes. These findings pave the way for halide perovskites as a next-generation platform for tunable anisotropic photonics.