Experimental Investigation on the effect of surface electric field in the growth of tungsten nano-tendril morphology due to low energy helium irradiation

The mechanisms responsible for and controlling the growth of tungsten nano-tendrils (or "fuzz") under low-energy helium plasma exposure remain unclear. For the first time in nano-tendril experiments, the plasma sheath-produced electric field and the helium (He) ion energy have been decoupled, showing that the sheath electric field has little impact on nano-tendril growth, eliminating a possible cause for tendril growth. The well-established necessary growth conditions for W fuzz were maintained with He ion flux density Gamma_He > 10^21 He m^-2 s^-1, surface temperature T_s = 1273 K, He ion energy E_He = 64 eV, and He ion fluence Phi_He > 10^24 He m^-2. A grid is situated between the tungsten sample and plasma, with the grid and sample potentials independently controlled in order to control the electric field at the surface of the sample while maintaining the same incident He ion energy to the surface. A calculation of the potential profile in the drift space between the grid and sample was used to account for space charge and calculate the electric field at the surface of the sample. Tungsten fuzz formed at all electric fields tested, even near zero electric field. Also, the depth of the resulting W fuzz layer was unaltered by the electric field when compared to the calculated depth determined from an empirical growth model. The conclusion is that the sheath electric field is not necessary to cause the changes in surface morphology.

低能氦等离子体辐照下钨纳米须（tungsten nano-tendrils，又称"绒毛状结构"（fuzz））的生长机制与调控原理目前仍不明确。本研究首次在纳米须相关实验中，将等离子体鞘层（plasma sheath）产生的电场与氦（He）离子能量解耦，结果表明鞘层电场对纳米须生长几乎无影响，从而排除了电场作为须状结构生长的潜在诱因。本实验维持了已被广泛证实的钨绒毛状结构生长必要条件：氦离子通量密度Γ_He > 10^21 He·m^-2·s^-1、表面温度T_s = 1273 K、氦离子能量E_He = 64 eV，以及氦离子注量Φ_He > 10^24 He·m^-2。在钨样品与等离子体之间设置栅极（grid），通过独立调控栅极与样品的电位，实现在维持样品表面入射氦离子能量不变的前提下，精准调控样品表面的电场强度。通过计算栅极与样品之间漂移区（drift space）的电位分布，兼顾空间电荷（space charge）效应，进而推导得到样品表面的实际电场强度。所有测试电场条件下（甚至接近零电场）均成功形成了钨绒毛状结构。此外，与经验生长模型（empirical growth model）计算得到的理论深度相比，所制备的钨绒毛层厚度未受电场调控而发生变化。综上，鞘层电场并非引发钨样品表面形貌（surface morphology）改变的必要条件。