Jiaxu Yan

Self‐Assembly of Honeycomb‐like MoS2 Nanoarchitectures Anchored into Graphene Foam for Enhanced Lithium‐Ion Storage

Honeycomb-like MoS2 nanoarchitectures anchored into 3D graphene foam are successfully fabricated as a high-performance positive electrode for enhanced Li-ion storage. The unique 3D interpenetrating honeycomb-like structure is the key to the high performance. High reversible capacity, superior high-rate capability, and excellent cycling stability are demonstrated.

A flexible alkaline rechargeable Ni/Fe battery based on graphene foam/carbon nanotubes hybrid film.

The development of portable and wearable electronics has promoted increasing demand for high-performance power sources with high energy/power density, low cost, lightweight, as well as ultrathin and flexible features. Here, a new type of flexible Ni/Fe cell is designed and fabricated by employing Ni(OH)2 nanosheets and porous Fe2O3 nanorods grown on lightweight graphene foam (GF)/carbon nanotubes (CNTs) hybrid films as electrodes. The assembled f-Ni/Fe cells are able to deliver high energy/power densities (100.7 Wh/kg at 287 W/kg and 70.9 Wh/kg at 1.4 kW/kg, based on the total mass of active materials) and outstanding cycling stabilities (retention 89.1% after 1000 charge/discharge cycles). Benefiting from the use of ultralight and thin GF/CNTs hybrid films as current collectors, our f-Ni/Fe cell can exhibit a volumetric energy density of 16.6 Wh/l (based on the total volume of full cell), which is comparable to that of thin film battery and better than that of typical commercial supercapacitors. Moreover, the f-Ni/Fe cells can retain the electrochemical performance with repeated bendings. These features endow our f-Ni/Fe cells a highly promising candidate for next generation flexible energy storage systems.

Engineering the Electronic Structure of Graphene

Graphene exhibits many unique electronic properties owing to its linear dispersive electronic band structure around the Dirac point, making it one of the most studied materials in the last 5-6 years. However, for many applications of graphene, further tuning its electronic band structure is necessary and has been extensively studied ever since graphene was first isolated experimentally. Here we review the major progresses made in electronic structure engineering of graphene, namely by electric and magnetic fields, chemical intercalation and adsorption, stacking geometry, edge-chirality, defects, as well as strain.

Stacking-Dependent Interlayer Coupling in Trilayer MoS2 with Broken Inversion Symmetry

The stacking configuration in few-layer two-dimensional (2D) materials results in different structural symmetries and layer-to-layer interactions, and hence it provides a very useful parameter for tuning their electronic properties. For example, ABA-stacking trilayer graphene remains semimetallic similar to that of monolayer, while ABC-stacking is predicted to be a tunable band gap semiconductor under an external electric field. Such stacking dependence resulting from many-body interactions has recently been the focus of intense research activities. Here we demonstrate that few-layer MoS2 samples grown by chemical vapor deposition with different stacking configurations (AA, AB for bilayer; AAB, ABB, ABA, AAA for trilayer) exhibit distinct coupling phenomena in both photoluminescence and Raman spectra. By means of ultralow-frequency (ULF) Raman spectroscopy, we demonstrate that the evolution of interlayer interaction with various stacking configurations correlates strongly with layer-breathing mode (LBM) vibrations. Our ab initio calculations reveal that the layer-dependent properties arise from both the spin-orbit coupling (SOC) and interlayer coupling in different structural symmetries. Such detailed understanding provides useful guidance for future spintronics fabrication using various stacked few-layer MoS2 blocks.

Plasma Modified MoS2 Nanoflakes for Surface Enhanced Raman Scattering

Though the SERS effect based on pristine MoS2 is hardly observed, however, the plasma treated MoS2 nanoflakes can be used as an ideal substrate for surface enhanced Raman scattering. It is proved that the structural disorder induced generation of local dipoles and adsorption of oxygen on the plasma treated MoS2 nanosheets are the two basic and important driven forces for the enhancement of Raman signals of surface adsorbed R6G molecules.

Repeated microwave-assisted exfoliation of expandable graphite for the preparation of large scale and high quality multi-layer graphene

The increasing demand for graphenes industrial application requires a new route for its mass production with high quality. Here, we report a facile, green, highly efficient and cost effective method for preparing a large amount of high quality graphene flakes, which is by the repeated microwave assisted exfoliation of expandable graphite (EG). The successful exfoliation of graphite is realized through the intercalation and decomposition of eco-friendly chemicals with the assistance of a microwave source. The chemical morphology and electrocatalytic performance analyses reveal that the graphene flakes are of high quality with little degradation.

Molecular adsorption induces the transformation of rhombohedral- to Bernal-stacking order in trilayer graphene

The Bernal (ABA)-stacked graphene trilayer is presumed to be thermodynamically more stable than the rhombohedral (ABC) counterpart. However, the thermal transformation from ABC to ABA domains does not occur at a temperature lower than 1,000 °C. Here we report that ABC-stacked trilayers are transformed to ABA-stacked layers after an organic molecule triazine is evaporated onto graphene surfaces at 150 °C. The transformation is found to always initiate at the ABA-ABC domain boundaries. Simulations based on density function theory considering the van der Waals interaction suggest that after triazine decoration the energy difference between ABA and ABC domains is larger, providing a driving force for stacking transformation. The molecular dynamics simulation results further suggest that the triazine decoration on the wrinkles at the ABC-ABA domain boundary activates the wrinkle sliding toward the ABC domains, leading to the stacking transformation from ABC to ABA.

High Mobility 2D Palladium Diselenide Field‐Effect Transistors with Tunable Ambipolar Characteristics

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe2 ), a noble-transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field-effect transistors (FETs) based on ultrathin PdSe2 show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe2 to exhibit electron-dominated transport with high mobility (µe (max) = 216 cm2 V-1 s-1 ) and on/off ratio up to 103 . Hole-dominated-transport PdSe2 can be obtained by molecular doping using F4 -TCNQ. This pioneer work on PdSe2 will spark interests in the less explored regime of noble-TMDCs.

Valley polarization in stacked MoS2 induced by circularly polarized light

Manipulation of valley pseudospins is crucial for future valleytronics. The emerging transition metal dichalcogenides (TMDs) provide new possibilities for exploring the interplay among the quantum degrees of freedom, including real spin, valley pseudospin, and layer pseudospin. For example, spin–valley coupling results in valley-dependent circular dichroism in which electrons with particular spin (up or down) can be selectively excited by chiral optical pumping in monolayer TMDs, whereas in few-layer TMDs, the interlayer hopping further affects the spin–valley coupling. In addition to valley and layer pseudospins, here we propose a new degree of freedom—stacking pseudospin—and demonstrate new phenomena correlated to this new stacking freedom that otherwise require the application of external electrical or magnetic field. We investigated all possible stacking configurations of chemical-vapor-deposition-grown trilayer MoS2 (AAA, ABB, AAB, ABA, and 3R). Although the AAA, ABA, 3R stackings possess a sole peak with lower degree of valley polarization than that in monolayer samples, the AAB (ABB) stackings exhibit two distinct peaks, one similar to that observed in monolayer MoS2 and an additional unpolarized peak at lower energy. Our findings provide a more complete understanding of valley quantum control for future valleytronics.

High‐Pressure‐Induced Comminution and Recrystallization of CH3NH3PbBr3 Nanocrystals as Large Thin Nanoplates

High pressure (HP) can drive the direct sintering of nanoparticle assemblies for Ag/Au, CdSe/PbS nanocrystals (NCs). Instead of direct sintering for the conventional nanocrystals, this study experimentally observes for the first time high-pressure-induced comminution and recrystallization of organic-inorganic hybrid perovskite nanocrystals into highly luminescent nanoplates with a shorter carrier lifetime. Such novel pressure response is attributed to the unique structural nature of hybrid perovskites under high pressure: during the drastic cubic-orthorhombic structural transformation at ≈2 GPa, (301) the crystal plane fully occupied by organic molecules possesses a higher surface energy, triggering the comminution of nanocrystals into nanoslices along such crystal plane. Beyond bulk perovskites, in which pressure-induced modifications on crystal structures and functional properties will disappear after pressure release, the pressure-formed variants, i.e., large (≈100 nm) and thin (<10 nm) perovskite nanoplates, are retained and these exhibit simultaneous photoluminescence emission enhancing (a 15-fold enhancement in the photoluminescence) and carrier lifetime shortening (from ≈18.3 ± 0.8 to ≈7.6 ± 0.5 ns) after releasing of pressure from 11 GPa. This pressure-induced comminution of hybrid perovskite NCs and a subsequent amorphization-recrystallization treatment offer the possibilities of engineering the advanced hybrid perovskites with specific properties.