Synthesis And Flow-electric Coupling Effect Of Two Dimensional Materials

The layered planer structure and unique physico-mechanical properties make two-dimensional atomic crystals become the frontier field of fundamental scientific research with intense attraction.Graphene and hexagonal boron nitride,also called white graphene,are two of the most important components in the family of two-dimensional atomic crystals with complement electric properties to each other.Graphene has shown variety of unique physico-mechanical properties,but its practical applications are still being explored.Hexagonal boron nitride,serving as an ideal atomic thin dielectric material,is challenged by its controlled fabrication.In this thesis,focusing on the functional applications of graphene and controlled fabrication of hexagonal boron nitride,we systematically studied the flow-electrical coupling effect,waving potential and drawing potential in graphene;controlled fabrication and properties of monolayer hexagonal boron nitride and three dimensional hexagonal boron nitride foam.The findings are briefly concluded as following:(1)Flow-electric coupling effect in graphene.Flow-electric coupling effect offers a novel way to construct flow speed sensor and energy conversion device.Due to the coupling of Bernoulli effect and thermo electric effect,a voltage linearly proportional to the square of gas-flow speed is induced across the two ends of graphene when a gas flows over its surface.The flow induced voltage in graphene is more than twenty times that of graphite for its higher Seebeck coefficient.The gas-flow induced voltage in graphene can be modulated by changing its Seebeck coefficient through chemical doping.However,when being immersed in liquid-flow,no electricity can be produced across graphene.The reported liquid-flow induced voltage in graphene is attributed to the interaction between the exposed metal electrodes and the liquid.Once the metal electrodes are isolated from the liquid,no electricity can be produced by the liquid-flow.(2)Waving potential and drawing potential in graphene.According to the electrokinetic theory of streaming potential,no electricity can be induced without pressure gradient.We found that when moving graphene across the surface of ionic solution,a voltage proportional to the moving speed and length of graphene section immersed in the solution could be induced across the immersed graphene.The voltage is also dependent on the kinds and concentration of the ions,and been referred to as waving potential.Moreover,moving droplets of ionic solution on the surface of graphene also produce a voltage,which is proportional to the moving speed and number of liquid droplets,and dependent on the kinds and concentration of ions and size of the droplets.This voltage is referred to as drawing potential.Dynamic forming and vanishing processes of the electric double layer at the gas-liquid-solid boundary contribute to the formation of waving and drawing potential.The circuit models based on the corresponding mechanism explicate the observed phenomena well.The potential applications of the waving and drawing potential are also demonstrated.(3)Controlled fabrication and boundaries of monolayer hexagonal boron nitride.Controlled growth of hexogonal boron nitride of high quality is essiential for the construction of two dimensional materials based electricl devices.Growth of monolayer h-BN is closely related to the configuration and materials of the substrate,partial pressure of the gas precursor and other factors during the chemical vapor deposition.On copper surface,density of h-BN nuclei with random orientation increases with the partial pressure of the gas precursor.The shape of the h-BN domains evaluates from triangle to hexagon when the gap distance between two copper foils is reduced.Through hydrogen etching,the grain boundary of h-BN in large area can be distinguished through optical microscopy.Compared to the random orientation of h-BN on copper,the domain orientations of h-BN grown on signal-crystal germanium wafer are fixed relative to the substrate.The boundaries between h-BN grains of different orientation can be revealed by friction force microscopy.Through first-principle calculations,we reveled the electrical properties of grain boundaries with different ideal structures between oppositely orientated domains.(4)Fabrication and properties of three dimensional h-BN foam.Application of ultralight materials is limited by their dielectric property and thermal stability.Taking nickel foam as templet,porous three dimensional h-BN foam with density of 1.6 mg/cm3 was fabricated through chemical vapor deposition.The foam shows super elasticity,and can recover its deformation even after 70% compression strain.Its dielectric constant is only 1.03 times that of air,and keeps constant within 60% strain.Combined with its intrinstic excellent thermal stability,h-BN foam provides potential application inhibited to current ultralight materials.