Silicon nanospring films for multifunctional interfaces

The mechanical behavior of thin films comprised of dense arrays of Si nanosprings and nanocolumns, and the mechanical and chemical lithiation properties of individual structures isolated from these films were investigated in this dissertation research. The nanostructured films, fabricated via Glancing Angle Deposition (GLAD), were comprised of 10 μm high Si columns or springs that were either unseeded or seeded with of 900 nm or 1500 nm spacing. The Si columns were 130-430 nm in diameter while the springs had 4 or 10 coil turns along their length and were either free at their top end, or capped with 1 μm thick solid Si cap that terminated the top coil turn. The aforementioned geometrical and seeding parameters resulted in six different types of Si spring films which were subjected to compression in ambient conditions via a cylindrical flat punch attached to a nanoindenter, and in situ compression inside an SEM with the aid of a custom loading device. The applied normal pressure was varied from 0.5 MPa to 50 MPa in increments of 5 MPa. The experiments showed pronounced stiffening with increased applied pressure, with the lowest compressive film stiffness measured in the range of 13±0.2 MPa to 151±15 MPa for capped 4-turn springs with 900 nm seed spacing, and uncapped 10-turn springs with 1500 nm seed spacing, respectively, at 0.5 MPa applied pressure. Capped films showed higher resistance to permanent deformation: at 15 MPa the permanent deformation of capped 4-turn springs with 900 nm seed spacing was only 1%, compared to 9.5% for the same uncapped films. Of all types of films, uncapped Si coils with 4 turns were the most compliant at all values of applied pressure, reaching a maximum stiffness of 384±2 MPa and permanent compression of 22% at 50 MPa. All uncapped films were subject to permanent set at pressures lower than the capped films: the film cap provided a higher restoring force that prevented bending and “buckling” of individual springs in random directions, which was the case in uncapped films. Notably, capped seeded springs with 4 turns and 900 nm seed spacing outperformed the unseeded counterparts at stresses as high as 15 MPa. In situ SEM compression experiments showed that increased spring intertwining, i.e. springs with larger pitch and coil radius, provided more resistance to bending and “buckling” and less cap damage under compression. The experimental results at the film level were compared to microscale experiments of uniaxial tension/compression and bending of individual Si springs, which were carried out with the aid of MEMS devices: Individual springs with 4 coil turns, exhibited the smallest axial stiffness of 7.3±2.1 N/m and the best agreement with film-level measurements, and analytical and Finite Element (FE) calculations of the axial stiffness. Estimates of the film stiffness based on individual spring data showed best agreement at 10 MPa applied stress, which implies that not all springs in a film are fully engaged and interacting at lower stresses, as well as off-axis bending even at small loads. Microscale compression and bending experiments with pairs of 4-turn springs confirmed the stiffening effect of neighboring springs, and the stabilizing nature of lateral spring interactions which, in the case of axial compression, resulted in a linearized mechanical response of a pair of springs. In general, the axial spring stiffness as computed via FE was up to 40% larger than the experimental values. This discrepancy was due to the uncertainty in (a) the measurement of exact dimensions of the elliptical cross-section of the wire comprising the seeded Si coils, and (b) the elastic material constants. Finally, individual Si columns and springs were subjected to in situ chemical lithiation inside an SEM. 1-D longitudinal lithiation proceeded in Si columns at 13.1±2.3 nm/s, which is 5 times faster than the value reported before in literature for the electrochemical lithiation of crystalline Si nanowires. The fast lithiation rates were attributed to the dendritic and fibrillar microstructure of the columnar and spring Si structures, respectively, which was revealed by TEM imaging. The lithiation of open coil springs with 4 turns had a clear advantage over Si columns and 10-turn springs which exhibited irreversible localized “buckling”. Furthermore, springs with the smallest wire diameter, associated with seed spacing of 900 nm, did not form surface cracks, while isolated cracks were observed on the surface of thicker wire coils but not on Si columns. These isolated cracks did not hinder the lithiation process. Upon full lithiation, Si springs with 4 coil turns (900 nm seed spacing) extended by 19% in length and 29% in coil diameter, which averted “buckling” stresses and lateral deflection due to lithiation, thus making the particular type of Si nanosprings the most advantageous both mechanically and electrochemically