Ben Q. Li

Continuous-wave ultraviolet laser action in strongly scattering Nd-doped alumina

We report electrically pumped, cw laser action near 405 nm from Nd(3+) -doped delta -alumina nanopowders. To our knowledge, this is the first report of stimulated emission from the high-lying F(2) -excited states, achieved through feedback from strong elastic scattering of light over transport path lengths shorter than half a wavelength.

Molecular dynamics simulation of nanosized water droplet spreading in an electric field.

Molecular dynamics (MD) simulations are performed for the spreading of a nanosized water droplet on a solid substrate subject to a parallel electric field. A combined electrostatic and Lennard-Jones potential is employed to represent the intermolecular interactions. Results show that in response to the applied field, polar water molecules realign themselves and this microscopic reorientation of molecular dipoles combines with the intermolecular forces to produce a macroscopic deformation of a free spherical water droplet into an ellipsoid. The applied field has a strong effect on the spreading of the water droplet on a solid substrate. For a weaker parallel field, the droplet spreading is asymmetric with the leading contact angle being greater than the trailing contact angle. With an increase in field strength, this asymmetry continues to increase, culminates, and then decreases until it disappears. The symmetric spreading remains with a further increase in the field strength until the saturation point is reached. This transition from the asymmetric to symmetric spreading is a manifestation of the interaction of the electric field with polar water molecules and the intermolecular forces within the droplet and between the water and solid; the interaction also leads to a change in hydrogen bonds along the droplet surface. The dynamics of the droplet spreading is entailed by the electrically induced motion of molecules along the liquid surface toward the solid substrate and is controlled by a competing mechanism among the electric, water-water, and water-solid intermolecular forces.

Facile construction of ultrathin standing α-Ni(OH)2 nanosheets on halloysite nanotubes and their enhanced electrochemical capacitance

One-dimensional nanostructures of ultrathin standing α-Ni(OH)2 nanosheets@halloysite nanotubes are synthesized through one-step facile precipitation method. The nanocomposites exhibit high capacitance (1677 F g−1) and excellent cycling stability (100% capacity retention after 2000 cycles) due to their ultrathin and standing nanosheets and intense cation/anion exchange performance of halloysite nanotubes. The remarkable electrochemical performance will undoubtedly make the hybrid structures attractive for high-performance supercapacitors.

A numerical study of nanoscale electrohydrodynamic patterning in a liquid film

A computational model is developed to study the manufacturing of micro/nanostructures by electrohydrodynamic (EHD) patterning processes. The computational methodology is based on the iterative coupling of the discontinuous boundary element method for electric field with the finite element method for free surface deformation. The model is capable of modeling fully nonlinear free surface deformations induced by an electric field. Geometric discontinuity resulting from the contact of the liquid film with the template is fully accounted for by introducing the short-range molecular forces. The critical voltage for liquid film instability is found by tracing the asymptote of the structural height vs. applied voltage curve, followed by a confirmation by the existence of an internal minimum of the total free energy. Computer codes are verified using the analytical solutions and available measurements. An important finding from the numerical simulations is that steady state structures can be electrohydrodynamically patterned when the applied voltage is either below or above a critical value. While linear analyses are useful, they may significantly over-predict the critical voltage, a crucial parameter for an EHD patterning process. The highly nonlinear phenomena of wetting the template above the critical voltage were considered to be unstable within the framework of linear perturbation. However, these phenomena can be very well predicted by the nonlinear boundary/finite element model enhanced by the short-range molecular forces. Finally, the computational model is flexible and may be modified with ease to analyze an EHD-patterning process for an air–polymer–polymer tri-layered system or other multiple-layered films for fabricating more complex nanostructures.

3D Printing of Carbon Nanotubes-Based Microsupercapacitors

A novel 3D printing procedure is presented for fabricating carbon-nanotubes (CNTs)-based microsupercapacitors. The 3D printer uses a CNTs ink slurry with a moderate solid content and prints a stream of continuous droplets. Appropriate control of a heated base is applied to facilitate the solvent removal and adhesion between printed layers and to improve the structure integrity without structure delamination or distortion upon drying. The 3D-printed electrodes for microsupercapacitors are characterized by SEM, laser scanning confocal microscope, and step profiler. Effect of process parameters on 3D printing is also studied. The final solid-state microsupercapacitors are assembled with the printed multilayer CNTs structures and poly(vinyl alcohol)-H3PO4 gel as the interdigitated microelectrodes and electrolyte. The electrochemical performance of 3D printed microsupercapacitors is also tested, showing a significant areal capacitance and excellent cycle stability.

Electroless fabrication and supercapacitor performance of CNT@NiO-nanosheet composite nanotubes

Composite nanostructures consisting of porous NiO nanosheets on carbon nanotubes (CNTs) are fabricated using a facile and low-cost electroless plating method. The CNTs, modified by a polymer, are adopted as the template upon which porous Ni nanosheets are grown using electroless plating. This is followed by removal of the polymer layer and oxidation of the Ni by controlled thermal annealing. The effect of reductant concentration on the morphology of the NiO nanosheets is studied. The electrochemical characteristics of the nanostructures are measured using chronopotentiometry. Experimental measurements show that the NiO nanosheet covered CNT composite nanostructures exhibit a relatively high specific capacitance of 1177 F g(-1) at a discharge current density of 2 A g(-1), while retaining 89.2% of its initial capacitance at a current density of 2 A g(-1) after 1000 cycles.

Molecular dynamics simulation of the electrically induced spreading of an ionically conducting water droplet

Molecular dynamics simulations are applied to study the spreading behavior of a nanosized water droplet that contains freely moving Na(+)/Cl(-) ions subject to an imposed electric field parallel to a solid surface. Results show that the positive and negative ions move relatively freely in response to an applied electric field, whereas polar water molecules realign themselves. These localized behaviors of the ions and the polar molecules are affected by both the applied electric field strength and the ion concentration, which in turn determine the deformation and spreading of the droplet on a solid substrate. The presence of the freely moving ions causes the ion-containing droplet to spread differently from a droplet of pure water. In a weak electric field of 0.05 V/Å, a droplet of a lower ion concentration spreads asymmetrically and the spreading asymmetry is considerably smaller than that associated with a pure water droplet of the same size. In a stronger field of 0.1 V/Å, a droplet of a higher ion concentration spreads symmetrically and completely wets the solid surface whereas a less ionically conducting droplet undergoes an asymmetric-to-symmetric transition in spreading until it reaches equilibrium.

Electrical generation of stationary light in random scattering media

In recent years there has been great interest in controlling the speed of propagation of electromagnetic waves. In gases and crystals, coherent techniques have been applied to alter the speed of light without changing the physical or chemical structure of the medium. Also, light transmitted by highly disordered solids has exhibited signatures of Anderson localization, indicating the existence of a regime of “stopped” light that is mediated by random elastic scattering. However, to date, light has not been generated in a random medium as a pointlike excitation that is fixed in space from the outset. Here we report experimental evidence for the electrical generation and confinement of light within nanosized volumes of a random dielectric scattering medium in which a population inversion has been established, and discuss the properties of these novel light sources.

Dynamic modelling of micro/nano-patterning transfer by an electric field

A computational electrohydrodynamic (EHD) model is presented for the 3D electrically induced fluid motion and free surface morphology evolution during EHD patterning transfer of micro/nano-structures. The model entails a finite difference solution of the electric field equation with a leaky dielectric model to account for polymer behaviour, the Navier–Stokes equations for electrically driven flows and the phase field equation for the free surface deformation. These equations are fully coupled and represent a very large complex numerical system. Once discretized, the intensive computation is alleviated with the use of parallel computing algorithms. Computed results are presented that illustrate the transient development of 3D micro/nano-structures under an electric field. Two classical templates are considered in this study, for both of which the polymeric structures conform well to the template, resulting in the micro/nano-patterning transfer from the template to polymer film. The model is a useful tool to explore optimal conditions for scalable manufacturing of large scale nanostructures using the EHD patterning processes.

Step-Controllable Electric-Field-Assisted Nanoimprint Lithography for Uneven Large-Area Substrates.

Large-area nanostructures are widely used in various fields, but fabrication on large-area uneven substrates poses a significant challenge. This study demonstrates a step-controllable electric-field-assisted nanoimprint lithography (e-NIL) method that can achieve conformal contact with uneven substrates for high fidelity nanostructuring. Experiments are used to demonstrate the method where a substrate coated with liquid resist is brought into contact with a flexible template driven by the applied electric field. Theoretical analysis based on the elasticity theory and electro-hydrodynamic theory is carried out. Effective voltage range and the saturation voltage are also discussed. A step-controllable release of flexible template is proposed and demonstrated to ensure the continuous contact between the template and an uneven substrate. This prevents formation of air traps and allows large area conformal contact to be achieved. A combination of Vacuum-electric field assisted step-controllable e-NIL is implemented in the developed prototype. Finally, photonic crystal nanostructures are successfully fabricated on a 4 in., 158 μm bow gallium nitride light-emitting diode epitaxial wafer using the proposed method, which enhance the light extraction property.