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.

We investigate the strain-dependent electronic and magnetic properties of two-dimensional (2D) monolayer and bilayer MoS(2), as well as 1D MoS(2) nanoribbons and nanotubes using first-principles calculations. For 2D monolayer MoS(2) subjected to isotropic or uniaxial tensile strain, the direct band gap of MoS(2) changes to an indirect gap that decreases monotonically with increasing strain; while under the compressive strain, the original direct band gap is enlarged first, followed by gap reduction when the strain is beyond -2%. The effect of isotropic strain is even stronger than that of uniaxial strain. For bilayer MoS(2) subjected to isotropic tensile strain, its indirect gap reduces monotonically to zero at strain about 6%; while under the isotropic compressive strain, its indirect gap increases first and then reduces and turns into direct gap when the strain is beyond -4%. For strained 1D metallic zigzag MoS(2) nanoribbons, the net magnetic moment increases slightly with axial strain from about -5% to 5%, but drops to zero when the compressive strain is beyond -5% or increases with a power law beyond 5%. For 1D armchair MoS(2) nanotubes, tensile or compressive axial strain reduces or enlarges the band gap linearly, and the gap can be fully closed for nanotubes with relatively small diameter or under large tensile strain. For zigzag MoS(2) nanotubes, the strain effect becomes nonlinear and the tensile strain can reduce the band gap, whereas compressive strain can initially enlarge the band gap and then decrease it. The strain induced change in projected orbitals energy of Mo and the coupling between the Mo atom d orbital and the S atom p orbital are analyzed to explain the strong strain effect on the band gap and magnetic properties.

Using atomistic models and molecular dynamics simulations, interlayer corrugation and resistant force in a biwalled carbon nanotube are shown to be strongly dependent upon the morphology combination of the bitube. Consequently, energy dissipation in a commensurate (e.g., armchair/armchair or zigzag/zigzag) bitube oscillator is found to be much larger than that in an incommensurate (e.g., zigzag/armchair) oscillator, resulting in a decay of oscillation amplitude within a few nanoseconds in the commensurate bitube and several tens of nanoseconds in the incommensurate bitube.

Systematic ab initio calculations show that the energy gap of boron nitride (BN) nanoribbons (BNNRs) with zigzag or armchair edges can be significantly reduced by a transverse electric field and completely closed at a critical field which decreases with increasing ribbon width. In addition, a distinct gap modulation in the ribbons with zigzag edges is presented when a reversed electric field is applied. In a weak field, the gap reduction of the BNNRs with zigzag edges originates from the field-induced energy level shifts of the spatially separated edge-states, while the gap reduction of the BNNRs with armchair edges arises from the Stark effect. As the field gets stronger, the energy gaps of both types of the BNNRs gradually close due to the field-induced motion of nearly free electron states. Without the applied fields, the energy gap modulation by varying ribbon width is rather limited.

Dielectrics with ultralow permittivity within 2 times that of air, excellent mechanical performance, and high thermal stability are highly attractive to many applications. However, since the finding of silica aerogels in the 1930s, no alternative ultralight porous dielectric with density below 10 mg/cm(3) has been developed. Here we present three-dimensional hierarchical boron nitride foam with permittivity of 1.03 times that of air, density of 1.6 mg/cm(3), and thermal stability up to 1200 °C obtained by chemical vapor deposition on a nickel foam template. This BN foam exhibits complete recovery after cyclic compression exceeding 70% with permittivity within 1.12 times that of air. Gathering all these exceptional characters, the BN foam should create a breakthrough development of flexible ultralow-permittivity dielectrics and ultralight materials.