Vanadium Oxide (VO2) Nanostructures

We investigate novel oxide shape memory films and corresponding nanoactuators based on the reversible monoclinic / tetragonal transformation in vanadium oxide (VO2) and (V-M)O2 compounds. Among the known functional ceramics exhibiting the shape memory effect, VO2 is of special interest, as it combines multifunctional properties like a metal-insulator transition at 68°C, thermoelasticity and superelasticity with a large effect size. Transformation strains of 1% in c-, -0.6% and -0.1% in a- and b-direction of the rutile structure, respectively, and a high Young’s modulus of 140 GPa promise large force and high work density. Nanofabrication processes are developed using these films as base materials to realize free-standing (V-M)O2 nanoactuators with lateral dimensions down to 100 nm. Their coupled electro-thermo-mechanical performance is characterized by novel in-situ methods in order to understand the scaling and size-dependence of structure, shape memory effect and superelasticity.

Schematic cross-section of two designs of the photonic waveguide switch

Figure 1: (a, figure on the left) Schematic cross-section of two designs of the photonic waveguide switch; (b, figure in the middle) FEM simulation of power transmission through a photonic waveguide switch of type 1 in metallic state for the TE mode at 1550 nm. The length L and thickness d of the VO2 film structure are indicated; (c, figure on the right) simulated extinction ratio ER and insertion loss IL of the TE mode at 1550 nm versus VO2 film thickness d (type 2).

The following figure shows a SEM micrograph of a photonic waveguide switch of type 2. We use the fabrication technology comprising of a two-step e-beam lithography (EBL) and etching processes. In the first EBL step, the patterns of waveguides and grating couplers are transferred to the Si device layer of a silicon-on-insulator (SOI) chip. Si is etched anisotropically using a cryogenic RIE process. The second EBL step is used to define the local openings for VO2 deposition. After lift-off, samples are post-annealed at 550 °C [1].

The figure shows a SEM micrograph of a photonic waveguide switch of type 2

Figure 2: Figure on the left: SEM picture of a photonic waveguide switch of type 2. Length and width of VO2 film structure on top of the waveguide are 1 μm  and 0.5 μm  respectively. Figure on the right: Normalized optical transmission of the TE mode at 1550 nm as a function of VO2 film length L at substrate temperatures of 20 and 80 °C.

Publication List
  1. Li, Z.; Rastjoo, S.; Ewy, A.; Wang, X.; Ludwig, A.; Kohl, M. Design and Fabrication of a Photonic Waveguide Switch Based on a VO2 Film. International Conference on Micro- and Nano Engineering (MNE), Turin, Italy, 2021.
  2. Rastjoo, S.; Wang, X.; Ludwig, A.; Kohl  M. Top-down fabrication and transformation properties of vanadium dioxide nanostructures. J. Appl. Phys. 2019; 125 (22): 225104.
  3. Rastjoo, S.; Wang, X.; Ludwig, A.; Kohl  M. Fabrication and Transformation Properties of Vanadium Dioxide Bimorph Nanoactuators. Shape Memory and Superelastic Technology Conference and Exposition (SMST), Konstanz, Germany, 2019.
  4. Rastjoo, S. (2020). Development of Nanodevices Based on VO₂ Thin Films. Dissertation, Karlsruher Institut für Technologie (KIT), Fakultät für Maschinenbau (MACH), DOI: 10.5445/IR/1000120274.
Funding/ Projects

DFG Project: VO2nano

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