Publications

The recent advent of atomically-thin ferromagnetic crystals has allowed experimental studies of two-dimensional (2D) magnetism that not only exhibits novel behavior due to the reduced dimensionality but also often serves as a starting point for understanding of the magnetic properties of bulk materials. Here we employ ballistic Hall micromagnetometry to study magnetization of individual 2D ferromagnets. Our devices are multilayer van der Waals (vdW) heterostructures comprising of an atomically-thin ferromagnetic crystal placed on top of a Hall bar made from encapsulated graphene. 2D ferromagnets can be replaced repeatedly, making the graphene-based Hall magnetometers reusable and expanding a range of their possible applications. The technique is applied for the quantitative analysis of magnetization and its behavior in atomically thin CrBr3. The compound is found to remain ferromagnetic down to a monolayer thickness and exhibit high out-of-plane anisotropy. We report how the critical temperature changes with the number of layers and how domain walls propagate through the ultimately thin ferromagnets. The temperature dependence of magnetization varies little with thickness, in agreement with the strongly layered nature of CrBr3. The observed behavior is markedly different from that given by the simple 2D Ising model normally expected to describe 2D easy-axis ferromagnetism. Due to the increasingly common usage of vdW assembly, the reported approach offers vast possibilities for investigation of 2D magnetism and related phenomena.

Graphene-based printable inks are advanced and promising candidates for flexible printed electronic devices. To develop large-scale graphene-based highly conductive electronics on the basis of existing printing approaches of the material, it is imperative to scale up its ink production methods and optimize the electrical properties of the printed films. Here, a sustainable approach for preparing high-conductive screen-printing graphene ink by recycling cellulose solvent is reported. The ink uses Cyrene solvent in circulation and produces highly concentrated and conductive graphene ink to conquer the limitation of graphene inks in massive industrial production brought by the high cost of the cellulose solvents. A systematic study of the sheet resistance variation with the number of cycles has been carried out. The average conductivity of the ink can exceed 3×104 S m−1, with only 2-hour shear exfoliation process, which exhibits about the same conductivity of recent works (using toxic NMP solvent, complex centrifugation, ultrasonication, etc.), but with much simpler and cost- effective fabrication process. The formulation of graphene inks allows flexible metal-free far- field tag antenna to be screen printed and integrated with off-the-shelf sensory chip to achieve battery-free wireless UHF RFID far field temperature sensing. The high responsiveness, accuracy and sensitivity of the RFID sensing tag were observed with instantaneous detection, accurate and reliable wireless data transmission.

No abstract is provided for this article.

We demonstrate a simple method to tune the electrical properties of graphene synthesized by chemical vapor deposition (CVD) with dual-side chemical doping – the bottom-side doping<italic>via</italic>substrate modification with NH₂-SAMs and the top-side doping with diethylenetriamine (DETA) molecules.

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: diffusivity and solubility. 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 H2/CO2 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 pathway length. Thus, in 2D membrane we can use an easy strategy to precisely regulate permeability and selectivity by the adjustment of the number of nanolayers and the size of 2D flakes, 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.

Investigated resonant tunneling through defect levels in h-BN barrier van der Waals heterostructures. The effect of multiplication of tunnel resonances through these levels was found, due to the influence of a high degree of defectiveness of the structure of the neighboring layer of graphene, created by intentionally processing in plasma. Various mechanisms of such influence are discussed.

Smart and sustainable natural fiber‐based composites are of great interest due to their biodegradability, recyclability, and environmental benefits over synthetic fiber composites. In addition, the environmental impact of plastics and synthetic fibers are widespread and substantial, as they can stay in the environment for hundreds of years and contribute significantly to global carbon emissions. Natural fibers such as jute can potentially replace synthetic fibers to manufacture environmentally sustainable, biodegradable, and lightweight composites with improved properties, good thermal and acoustic insulation, and a smaller carbon footprint. However, natural jute fiber‐based composites suffer not only from poor mechanical properties but also being inherently electrically insulating, which limits their applications as multifunctional composites. Here multi‐functional and environmentally sustainable smart composites of graphene‐based natural jute fibers with excellent tensile and interfacial properties are reported. The reduced graphene oxide‐based natural jute fiber enhance the Young's modulus of the composites by ≈450%, and tensile strength by ≈183% after physical and chemical treatment. Such high‐performance composites can also be used as multifunctional smart composites, as demonstrated by effective electro‐magnetic interference shielding performance. This may lead to manufacturing of next generation smart, strong, and sustainable natural fiber composites for high performance engineering applications without conferring environmental problems.

In this work, we demonstrate record low contact resistance at low temperatures. Moreover, we carefully engineer the device to leverage the sample inhomogeneity to demonstrate confinement in the quantum limit.

No abstract is provided for this article.

The quantum capacitance of graphene can be negative when the graphene is placed in a strong magnetic field, which is a clear experimental signature of positional correlations between electrons. Here we show that the quantum capacitance of graphene is also strongly affected by its dielectric polarizability, which in a magnetic field is wave-vector dependent. We study this effect both theoretically and experimentally. We develop a theory and numerical procedure for accounting for the graphene dielectric response, and we present measurements of the quantum capacitance of high-quality graphene capacitors on boron nitride. Theory and experiment are found to be in good agreement.

Weak interlayer interactions in van der Waals crystals facilitate their mechanical exfoliation to monolayer and few-layer two-dimensional materials, which often exhibit striking physical phenomena absent in their bulk form. Here we utilize mechanical exfoliation to produce a two-dimensional form of a mineral franckeite and show that the phase segregation of chemical species into discrete layers at the sub-nanometre scale facilitates franckeite's layered structure and basal cleavage down to a single unit cell thickness. This behaviour is likely to be common in a wider family of complex minerals and could be exploited for a single-step synthesis of van der Waals heterostructures, as an alternative to artificial stacking of individual two-dimensional crystals. We demonstrate p-type electrical conductivity and remarkable electrochemical properties of the exfoliated crystals, showing promise for a range of applications, and use the density functional theory calculations of franckeite's electronic band structure to rationalize the experimental results.

We uncover the constitutive relation of graphene and probe the physics of its optical phonons by studying its Raman spectrum as a function of uniaxial strain. We find that the doubly degenerate E(2g) optical mode splits in two components: one polarized along the strain and the other perpendicular. This splits the G peak into two bands, which we call G(+) and G(-), by analogy with the effect of curvature on the nanotube G peak. Both peaks redshift with increasing strain and their splitting increases, in excellent agreement with first-principles calculations. Their relative intensities are found to depend on light polarization, which provides a useful tool to probe the graphene crystallographic orientation with respect to the strain. The 2D and 2D(') bands also redshift but do not split for small strains. We study the Gruneisen parameters for the phonons responsible for the G, D, and D(') peaks. These can be used to measure the amount of uniaxial or biaxial strain, providing a fundamental tool for nanoelectronics, where strain monitoring is of paramount importance.

Devices made from graphene encapsulated in hexagonal boron-nitride exhibit pronounced negative bend resistance and an anomalous Hall effect, which are a direct consequence of room-temperature ballistic transport at a micrometer scale for a wide range of carrier concentrations. The encapsulation makes graphene practically insusceptible to the ambient atmosphere and, simultaneously, allows the use of boron nitride as an ultrathin top gate dielectric.

No abstract is provided for this article.

We report on the damage caused to mechanically exfoliated monolayer graphene, bound to silicon dioxide substrate, upon contact with liquids. This phenomenon is of significant importance for a wide range of applications where monolayer graphene sheets are used with liquids, especially as an electrode material in electrochemical applications such as energy storage and conversion. Liquid-induced damage to SiO2-bound graphene was previously observed with a range of solvents. A recently developed microdroplet system, used for a detailed examination of this behaviour, reveals that few-layer graphene flakes down to a bi-layer are stable with respect to aqueous electrolyte droplet formation, but the stability of these droplets is significantly reduced on monolayer graphene and irreversible rupture of the underlying graphene flake occurs. This damage, which we attribute to the presence of nanoscale defects and high adhesion between the graphene and the substrate, seems specific to plasma-cleaned SiO2 substrates and is not observed on flakes transferred to other substrates. Furthermore, the introduction of impurities, in the form of both polymer residues and native impurities between the flake and the SiO2 substrate, significantly enhance graphene's immunity to external strain as shown by optical microscopy, atomic force microscopy, and Raman spectroscopy.

Screen-printed graphene is integrated with multiple-input multiple-output (MIMO) technology to conquer the most concerned surge in electronic waste caused by the mass deployment of Internet of things (IoT) applications. A flexible MIMO antenna is implemented with simple fabrication process suitable for large-scale production by screen printing graphene highly conductive ink on paper substrate, ensuring high-speed 5G mass data wireless transmission without damaging the ecological environment. This environmental-friendly, low-cost, flexible and conformal MIMO antenna with orthogonal polarization diversity employs co-planar waveguide feed and planar pattern for achieving high space utilization and better integration in most scenarios, for instance, body centric networks and monitoring systems. Excellent performance has been achieved due to the high conductivity of the graphene: the fabricated antenna exhibits an average sheet resistance of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>1.9</mml:mn> <mml:mrow> <mml:mi mathvariant="normal">Ω</mml:mi> </mml:mrow> <mml:mrow> <mml:mtext>s</mml:mtext> </mml:mrow> <mml:mrow> <mml:msup> <mml:mrow> <mml:mtext>q</mml:mtext> </mml:mrow> <mml:mrow> <mml:msup> <mml:mi/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> . The bandwidth of the antenna ranges from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2.22</mml:mn> <mml:mrow> <mml:mtext> GHz to </mml:mtext> </mml:mrow> <mml:mn>3.85</mml:mn> <mml:mrow> <mml:mtext> GHz</mml:mtext> </mml:mrow> </mml:math> (53.71% fractional bandwidth), covering <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>4</mml:mn> <mml:mrow> <mml:mtext>G</mml:mtext> </mml:mrow> </mml:math> long term evolution, sub-6 GHz <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>5</mml:mn> <mml:mrow> <mml:mtext>G</mml:mtext> </mml:mrow> </mml:math> mobile communication networks, 2.5 and 3.5 GHz WiMAX, and 2.4 and 3.6 GHz WLAN. Within this range, the antenna exhibits effective radiation, also its envelope correlation coefficient remains below <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>0.2</mml:mn> <mml:mo>×</mml:mo> <mml:mrow> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> , manifesting outstanding signal transmission quality in a variety of wireless networks. This work illustrates a novel aggregation of MIMO technology and graphene printing electronics, enabling cheap accessible and green MIMO antennas to be massively integrated in IoT applications.