Jin Yu

Generating electricity by moving a droplet of ionic liquid along graphene

Since the early nineteenth century, it has been known that an electric potential can be generated by driving an ionic liquid through fine channels or holes under a pressure gradient. More recently, it has been reported that carbon nanotubes can generate a voltage when immersed in flowing liquids, but the exact origin of these observations is unclear, and generating electricity without a pressure gradient remains a challenge. Here, we show that a voltage of a few millivolts can be produced by moving a droplet of sea water or ionic solution over a strip of monolayer graphene under ambient conditions. Through experiments and density functional theory calculations, we find that a pseudocapacitor is formed at the droplet/graphene interface, which is driven forward by the moving droplet, charging and discharging at the front and rear of the droplet. This gives rise to an electric potential that is proportional to the velocity and number of droplets. The potential is also found to be dependent on the concentration and ionic species of the droplet, and decreases sharply with an increasing number of graphene layers. We illustrate the potential of this electrokinetic phenomenon by using it to create a handwriting sensor and an energy-harvesting device.

Top–down fabrication of sub-nanometre semiconducting nanoribbons derived from molybdenum disulfide sheets

Developments in semiconductor technology are propelling the dimensions of devices down to 10 nm, but facing great challenges in manufacture at the sub-10 nm scale. Nanotechnology can fabricate nanoribbons from two-dimensional atomic crystals, such as graphene, with widths below the 10 nm threshold, but their geometries and properties have been hard to control at this scale. Here we find that robust ultrafine molybdenum-sulfide ribbons with a uniform width of 0.35 nm can be widely formed between holes created in a MoS2 sheet under electron irradiation. In situ high-resolution transmission electron microscope characterization, combined with first-principles calculations, identifies the sub-1 nm ribbon as a Mo5S4 crystal derived from MoS2, through a spontaneous phase transition. Further first-principles investigations show that the Mo5S4 ribbon has a band gap of 0.77 eV, a Young’s modulus of 300GPa and can demonstrate 9% tensile strain before fracture. The results show a novel top–down route for controllable fabrication of functional building blocks for sub-nanometre electronics.

Water-evaporation-induced electricity with nanostructured carbon materials

Water evaporation is a ubiquitous natural process that harvests thermal energy from the ambient environment. It has previously been utilized in a number of applications including the synthesis of nanostructures and the creation of energy-harvesting devices. Here, we show that water evaporation from the surface of a variety of nanostructured carbon materials can be used to generate electricity. We find that evaporation from centimetre-sized carbon black sheets can reliably generate sustained voltages of up to 1 V under ambient conditions. The interaction between the water molecules and the carbon layers and moreover evaporation-induced water flow within the porous carbon sheets are thought to be key to the voltage generation. This approach to electricity generation is related to the traditional streaming potential, which relies on driving ionic solutions through narrow gaps, and the recently reported method of moving ionic solutions across graphene surfaces, but as it exploits the natural process of evaporation and uses cheap carbon black it could offer advantages in the development of practical devices.

Waving potential in graphene

Nanoscale materials offer much promise in the pursuit of high-efficient energy conversion technology owing to their exceptional sensitivity to external stimulus. In particular, experiments have demonstrated that flowing water over carbon nanotubes can generate electric voltages. However, the reported flow-induced voltages are in wide discrepancy and the proposed mechanisms remain conflictive. Here we find that moving a liquid-gas boundary along a piece of graphene can induce a waving potential of up to 0.1 V. The potential is proportional to the moving velocity and the graphene length inserted into ionic solutions, but sharply decreases with increasing graphene layers and vanishes in other materials. This waving potential arises from charge transfer in graphene driven by a moving boundary of an electric double layer between graphene and ionic solutions. The results reveal a unique electrokinetic phenomenon and open prospects for functional sensors, such as tsunami monitors.

Boron Nitride Nanostructures: Fabrication, Functionalization and Applications.

Boron nitride (BN) structures are featured by their excellent thermal and chemical stability and unique electronic and optical properties. However, the lack of controlled synthesis of quality samples and the electrically insulating property largely prevent realizing the full potential of BN nanostructures. A comprehensive overview of the current status of the synthesis of two-dimensional hexagonal BN sheets, three dimensional porous hexagonal BN materials and BN-involved heterostructures is provided, highlighting the advantages of different synthetic methods. In addition, structural characterization, functionalizations and prospective applications of hexagonal BN sheets are intensively discussed. One-dimensional BN nanoribbons and nanotubes are then discussed in terms of structure, fabrication and functionality. In particular, the existing routes in pursuit of tunable electronic and magnetic properties in various BN structures are surveyed, calling upon synergetic experimental and theoretical efforts to address the challenges for pioneering the applications of BN into functional devices. Finally, the progress in BN superstructures and novel B/N nanostructures is also briefly introduced.

Tunable electronic and magnetic properties of two‐dimensional materials and their one‐dimensional derivatives

Low‐dimensional materials exhibit many exceptional properties and functionalities which can be efficiently tuned by externally applied force or fields. Here we review the current status of research on tuning the electronic and magnetic properties of low‐dimensional carbon, boron nitride, metal‐dichalcogenides, phosphorene nanomaterials by applied engineering strain, external electric field and interaction with substrates, etc, with particular focus on the progress of computational methods and studies. We highlight the similarities and differences of the property modulation among one‐ and two‐dimensional nanomaterials. Recent breakthroughs in experimental demonstration of the tunable functionalities in typical nanostructures are also presented. Finally, prospective and challenges for applying the tunable properties into functional devices are discussed. WIREs Comput Mol Sci 2016, 6:324–350. doi: 10.1002/wcms.1251

Two-Dimensional Hexagonal Beryllium Sulfide Crystal.

We report a new two-dimensional hexagonal beryllium sulfide (h-BeS) sheet with exceptional properties by extensive first-principles calculations. The h-BeS sheet presents an indirect energy gap of 4.26 eV and an outstanding thermodynamic stability up to 1000 K. Armchair-edged nanoribbons of h-BeS are wide-energy-gap semiconductors with a giant Stark effect, while the zigzag-edged ones are metals with spin glass state. Especially, the ferromagnetic zigzag nanoribbons exhibit a net magnetic moment of nearly 1.15 μB. These interesting electronic and magnetic properties suggest the promise of the h-BeS crystal for potential applications and should inspire experimental enthusiasm.

A New Paradigm to Half-Metallicity in Graphene Nanoribbons

In contrast to the well-recognized transverse-electric-field-induced half-metallicity in zigzag graphene nanoribbons, here, we demonstrate by first-principles calculations that zigzag graphene nanoribbons sandwiched between hexagonal boron nitride nanoribbons or sheets can be tuned into half-metal simply by a bias voltage or a moderate compressive strain. The half-metallicity is attributed to an enhanced coupling effect of spontaneous polarization and asymmetrical exchange correlation along the ribbon width. The findings should open a viable route for efficient spin-resolved band engineering in graphene-based devices that are compatible with the current technology of the semiconductor industry.

Electronic properties of graphene nanoribbons stacked on boron nitride nanoribbons

Hexagonal boron nitride sheet has been shown to be the best insulating substrate for graphene electronics. Using first-principles calculations, we here show that BN nanoribbons (BNNRs) can not only serve as a desirable substrate but also bring new properties into the supported graphene nanoribbons (GNRs). In particular, zigzag GNRs on zigzag BNNRs become a spin-relevant semiconductor that can be easily tuned into a half-metal, thanks to polar character of the BNNRs. In contrast, armchair GNRs can basically have all their electronic properties survived from the interaction of armchair BNNRs. Our findings provide helpful guide for developing hybrid BN-graphene nanodevices.

Strain tunable electronic and magnetic properties of pristine and semihydrogenated hexagonal boron phosphide

Tunable electromagnetic properties of pristine two-dimensional boron phosphide (h-BP) nanosheet and its semihydrogenated structure were studied by density functional theory computations. In sharp contrast to previously reported tensile strain-induced red shift in two-dimensional semiconductors, the direct gap of h-BP undergoes blue shift under biaxial tensile strain. Once semihydrogenated, the h-BP not only transform from the nonmagnetic semiconductor into metal which is spin-resolved but also exhibits linear response between the magnetic moment and biaxial strain with a slope up to 0.005 μB/1%. These findings provide a simple and effective route to tune the electronic and magnetic properties of h-BP nanostructures in a wide range and should inspire experimental enthusiasm.