ABSTRACT
We present a theoretical description of negative ion conversion of the grazing scattering of neutral hydrogen atoms on LiF(100) surface. Here, in addition to the capture of a valence band electron near a surface F- site by an incident H atom, the Coulomb repulsive barrier tunneling behavior is also considered to treat the detachment of the affinity electron from the formed H- ion to vacuum through its interaction with the surface F- site. With incorporation of the image-attraction-induced increase on the vertical component of the projectile energy and the collective dielectric screening effect of surrounding anions and cations on the charge of surface F- site which participates in the electron detachment, the image attractive interaction was revealed to obviously increase the electron detachment probability, in turn obviously decreasing the final negative ion yield. Moreover, the sub-eV order of the energy defect between the H- affinity level and the unoccupied image state induced by the potential field of the H- image charge in LiF leads to an additional H- destruction channel in which H- affinity electron transfers to the unoccupied image state without electron emission by a nearly resonant charge transfer manner. A clear picture of this electron loss process is also presented. Our present results well reproduce the experimental observation in the whole velocity range and the high fraction of H- ion yield as an alternative solution can be used on the implantation of ITER device.
ABSTRACT
Graphene bubbles (GBs) are of significant interest owing to their distinguished electrical, optical, and magnetic properties. GBs can also serve as high-pressure reaction vessels to numerous chemical reactions. However, previous strategies to produce GBs are relatively elaborate and random. Therefore, their potential applications are severely restricted. Here, a facile and effective protocol is proposed to construct position-controllable GBs in liquid nitrogen (LN) with the assistance of laser and graphene wrinkles. Specifically, a film of graphene mounted on a SiO2 substrate (G@SiO2) is subjected to irradiation by a low-power laser in LN and then many GBs emerge from the surface of G@SiO2. Most impressively, the domain where GBs arise is the position of the laser beam spot. Hence, we demonstrated that the high collimation of laser facilitates the position definition of GBs. The microscopic results indicate that some GBs split into three parts when they were subjected to irradiation by an electron. Meanwhile, some GBs degenerate into pores with a diameter of 500 nm when they are exposed to air. To grasp the properties of GBs in depth, the molecular dynamics (MD) simulations are performed, and the corresponding results indicate that temperature has very little impact on the GBs' shape. A phase transition process of the substance inside GBs is also revealed. Moreover, a two-dimensional (2D) solid nitrogen is discovered by MD simulations. The simplicity of our protocol paves the way to engineer high-pressure microreaction vessels and fabricate porous graphene membranes.
ABSTRACT
The use of nanoporous graphene for separation has attracted increasing interest over the past few years. Nanopore size, temperature, pressure and functionalization are vital for selectivity, but the influence of nanopore density is not known. Thus, we designed a monolayer nanoporous graphene membrane and revealed the influence of nanopore density on its ethylene/acetylene separation performance by employing molecular dynamics (MD) simulations. Our results indicate that an optimal nanopore density exists for permeation flux and selectivity. The MD simulation results were confirmed by density functional theory (DFT) and the kinetic theory of gases. The interactions between ethylene/acetylene and nanopores were also investigated, and van der Waals (vdW) interactions with slight steric repulsion were detected.
