低维碳材料的热电子发射及其在真空电子学的应用

真空电子器件的微型化是真空电子学的一个重要发展趋势。随着微纳加工技术的发展和新材料的发现与应用,纳米尺度真空沟道三极管取得了一定的进展。当前实现真空三极管微型化的主要手段还是基于场发射的阴极,而场发射对发射部位原子结构的强依赖决定了在集成时不可避免遇到均一性和稳定性差的问题。我们发现了碳纳米管和石墨烯具有不同于传统热电子发射的热发射新机制,基于半导体微加工技术,利用石墨烯和碳纳米管作为电子发射材料,实现了热阴极的微型化、片上化和阵列化,并进一步利用石墨烯微型热阴极实现了一种新结构的石墨烯真空三极管。石墨烯微型热阴极具有工作电压低(2~4V)、响应时间短(<1μs)、发射电流可大幅度调制、可控性好等特点。石墨烯真空三极管的开关比高达106,亚阈值斜率仅为120mV/dec,工作电压不足10V,与石墨烯固态三极管相当。我们还演示了石墨烯真空三极管的片上集成,将两个单极性石墨烯真空三极管集成在一起实现了双极性的真空电子器件。由于石墨烯真空三极管具有开关性能好、工作电压低、可集成等特点,未来有望基于石墨烯真空三极管实现更复杂的器件和真空集成电路。

Sound Velocity in Soap Foams

The velocity of sound in soap foams at high gas volume fractions is experimentally studied by using the time difference method.It is found that the sound velocities increase with increasing bubble diameter,and asymptotically approach to the value in air when the diameter is larger than 12.5 mm.We propose a simple theoretical model for the sound propagation in a disordered foam.In this model,the attenuation of a sound wave due to the scattering of the bubble wall is equivalently described as the effect of an additional length.This simplicity reasonably reproduces the sound velocity in foams and the predicted results are in good agreement with the experiments.Further measurements indicate that the increase of frequency markedly slows down the sound velocity,whereas the latter does not display a strong dependence on the solution concentration.