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Two types of nanoridge domains oriented with each other with an angle ranging between 109° and 124° were measured by scanning tunneling microscopy on the α-W film sputter deposited on an oxidized Si surface. Each domain contains nanoridges with a period of 7.5 ± 1.0 nm. No such domains were observed on the β-W film surface. We argued that due to the anisotropy of the W(1 1 0) surface, the impinging W atoms diffuse faster along the 〈1 1 1〉 directions on the surface to form the nanoridge structure. There are two equivalent 〈1 1 1〉 directions, which give rise to two orientational domains with an angle of ∼110°. An isotropic β-W(1 0 0) phase has no preferred diffusion direction for the impinging W atoms and therefore, no nanoridge domain structure was observed. [Copyright &y& Elsevier]

Density of sputter deposited silicon (Si) thin films was changed by a simple working gas pressure control process, and its effects on the cycling performance of Si films in Li-ion batteries as anodes was investigated. Higher gas pressure results in reduced film densities due to a shadowing effect originating from lower mean free path of sputter atoms, which leads to a wider angular distribution of the incoming flux and formation of a porous film microstructure. Si thin film anodes of different densities ranging from 2.27 g/cm 3 (film porosity ~ 3%) down to 1.64 g/cm 3 (~ 30% porosity) were fabricated by magnetron sputtering at argon pressures varying from 0.2 Pa to 2.6 Pa, respectively. High density Si thin film anodes of 2.27 g/cm 3 suffered from an unstable cycling behavior during charging/discharging depicted by a continuous reduction in specific down to ~ 830 mAh/g at the 100th cycle. Electrochemical properties of lower density films with 1.99 g/cm 3 (~ 15% porosity) and 1.77 g/cm 3 (~ 24% porosity) got worse resulting in only ~ 100 mAh/g capacity at 100th cycle. On the other hand, as the density of anode was further reduced down to about 1.64 g/cm 3 (~ 30% porosity), cycling stability and capacity retention significantly improved resulting in specific capacity values ~ 650 mAh/g at 100th cycle with coulombic efficiencies of > 98%. Enhancement in our low density Si film anodes are believed to mainly originate from the availability of voids for volumetric expansion during lithiation and resulting compliant behavior that provides superior mechanical and electrochemical stability. [ABSTRACT FROM AUTHOR]

The high volumetric expansion/contraction of silicon (Si) anodes in Li-ion batteries by about ∼400% during lithiation/delithiation causes considerably high stress followed by cracking, pulverization, and the loss of electrical contact; and finally results in capacity fading and failing. In this work, we present a new density modulated multilayer Si thin film anode approach, which can provide a robust high capacity electrode for Li-ion batteries. Alternating high and low density layers have been achieved by simply changing the working gas pressure between low and high values, respectively during magnetron sputter deposition of Si thin film anodes. Our results reveal that density modulated Si films can provide a high coulombic efficiency up to 99% and reversible specific capacity as high as ∼1700 mAh g −1 after 50 cycles. Low-density layers are believed to be performing as compliant layers during volume change making the films more durable compared to conventional Si film anodes. The results of this work can lead to Si thin film anode materials with superior capacity and mechanical stability compared to conventional Si anodes. [ABSTRACT FROM AUTHOR]