Publications

The presented work summarizes various aspects of uniaxial deformation in monolayer graphene studied by means of Raman spectroscopy. Graphene flakes were subjected to tension - compression uniaxial loading using the cantilever beam technique. The evolution of the Raman single-resonance (G) and doubleresonance (2D) bands was monitored at strain levels < 1%. The position of all peaks redshifts under tension and blueshifts under compression. The G peak splitting into two sub-bands (G and G + ) which is caused by symmetry lowering, is observed in both strain directions. The sub-bands’ intensities are used to calculate the crystal lattice orientation of the measured graphene flakes with respect to the strain axis. The nature and splitting of the 2D band even in the unstrained flakes, when excited by the 785 nm (1.58 eV) laser line, is interpreted as the interplay between two distinct double resonance scattering processes.

The high electrical conductivity and low dimensionality of graphene is essential for the development of novel lightweight electrodes for bioenergy technologies with a zero CO2 footprint. However, the integration of graphene in ecosystems presents a formidable challenge, especially because the surface energy of graphene is not compatible with living matter. Here we propose a sustainable chemical control method to balance surface hydrophilicity and conductivity of graphene nanowalls to form a graphene-based lightweight sponge bioreactor. The few-nanometer--thick conductive nanowalls create biocompatible hydrophilic microconfinements to harvest the biomass density of electrogenic Shewanella Oneidensis MR-1. The graphene-based bioreactor shows a stable and rapid bio-current response with a steady-state bio-current density of 135.35 mA·m-2 realized within a few hours. Our novel and sustainable graphene-based material provides a revolutionarily energy opportunity for the establishment of new energy-related graphene industries as well as facilitates many startups.

A micromanipulator combined with a microinjection system was employed to inject micro-droplets of aqueous solutions containing redox-active species on selected areas of graphene. Chronoamperometric and voltammetric measurements were performed in order to investigate the diffusion regime established within the micro-droplets. The heterogeneous electron transfer rate for two model redox couples, Fe(CN)63− and IrCl62− were estimated on various regions of chemical vapour deposited monolayer and turbostratic bilayer graphene on the basis of the voltammetric responses. New insights are thus obtained into the electron transfer properties of graphene, which is of primary importance for its exploitation as an electrode material.

Cascade transition between energy levels has important applications, such as in quantum information protocols and quantum cascade lasers. In two-dimensional heterostructure, the moiré superlattice potential can result in multiple interlayer exciton (IX) energy levels. We demonstrate the cascade transitions between such moiré IXs by performing time- and energy-resolved photoluminescence measurements. We show that the lower-energy moiré IX can be excited to higher-energy ones, facilitating IX population inversion.

The 2D structure and excellent electronic properties of graphene have generated worldwide interest, and offer the potential to overcome the limitations of silicon electronics.1 Early graphene-based devices consisted of a single atomic layer, but the possibility of tailoring electronic properties by stacking several different 2D crystals atop each other has since been discovered.2–5 In the assembly of these multilayer heterostructures, unwanted materials can become trapped between layers, adversely affecting the device quality. Understanding the cause of performance variations and thereby optimizing the properties of these devices is an extremely challenging task.

The identification of single- and few-layer boron nitride is described. Its optical contrast is much smaller than that of graphene, but even monolayers are discernable by optimizing viewing conditions. The number of layers in thicker crystals can be counted by exploiting an integer-step increase in the Raman intensity and optical contrast.

Exfoliation of large-area monolayers is important for fundamental research and technological implementation of transition-metal dichalcogenides. Various techniques have been explored to increase the exfoliation yield, but little is known about the underlying mechanism at the atomic level. Here, we demonstrate gold-assisted mechanical exfoliation of monolayer molybdenum disulfide, up to a centimeter scale. Detailed spectroscopic, microscopic, and first-principles density functional theory analyses reveal that strong van der Waals (vdW) interaction between Au and the topmost MoS2 layer facilitates the exfoliation of monolayers. However, the large-area exfoliation promoted by such strong vdW interaction is only achievable on freshly prepared clean and smooth Au surfaces, while rough surfaces and surfaces exposed to air for more than 15 min result in negligible exfoliation yields. This technique is successfully extended to MoSe2, WS2, WSe2, MoTe2, WTe2, and GaSe. In addition, electrochemical characterization reveals intriguing interactions between monolayer MoS2 and Au. A subnanometer-thick MoS2 monolayer strongly passivates the chemical properties of the underlying Au, and the Au significantly modulates the electronic band structure of the MoS2, turning it from semiconducting to metallic. This could find applications in many areas, including electrochemistry, photovoltaics, and photocatalysis.

Twisted bilayer graphene (tBLG) has recently attracted growing interest due to its unique twist-angle-dependent electronic properties. The preparation of high-quality large-area bilayer graphene with rich rotation angles would be important for the investigation of angle-dependent physics and applications, which, however, is still challenging. Here, we demonstrate a chemical vapor deposition (CVD) approach for growing high-quality tBLG using a hetero-site nucleation strategy, which enables the nucleation of the second layer at a different site from that of the first layer. The fraction of tBLGs in bilayer graphene domains with twist angles ranging from 0° to 30° was found to be improved to 88%, which is significantly higher than those reported previously. The hetero-site nucleation behavior was carefully investigated using an isotope-labeling technique. Furthermore, the clear Moiré patterns and ultrahigh room-temperature carrier mobility of 68,000 cm 2 V −1 s −1 confirmed the high crystalline quality of our tBLG. Our study opens an avenue for the controllable growth of tBLGs for both fundamental research and practical applications.

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.

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.

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.