Thomas Nuckolls, Morgan Sherer, Arthur Barnard
Engineering and tuning the electronic and magnetic properties of vdW heterostructures is vital to using and understanding 2D materials. Stacking engineered ferroelectricity in bilayer hexagonal boron nitride is intrinsically dependent on the stacking order of the monolayers relative to each other and both translations and rotations of one layer relative to the other have shown that domains of ferroelectricity can be manipulated. Similarly the antiferromagnetic properties of CrI$_3$ have been shown to have dependencies on the stacking order and experimental confirmation of irreversible antiferromagnetic to ferromagnetic switching has been reported. Due to the dependence on relative displacements and rotations of each monolayer relative to the others within a device, we hypothesize that shear forces applied to the topmost layer could lead to shear deformations in the device that trigger transitions between different ferroelectric (ferromagnetic) states in hBN (CrI$_3$). To investigate the hypothesis we devise a method of applying shear forces to vdW devices by means of organic microstructures of spherical and cylindrical geometries. The primary polymer under investigation is poly(dimethyl)siloxane (PDMS), due to its rapid production time, optical transparency, tunable stiffness and ease of access. We consider possible production methods to create PDMS microstructures with the desired features. We compare several methods of microstructure fabrication, using a PDMS double casting technique and outline modes of failure based on different parameter values. In addition to fabrication techniques, we also simulate the experimental setup by means of a COMSOL simulation. In the simulation we model the PDMS microstructure and device to probe the stress through the microstructure and the forces applied to the device as a function of lateral pillar displacement.
Engineering and tuning the electronic and magnetic properties of 2D heterostructures is vital to using and understanding 2D materials. Several materials of interest exhibit novel properties dependent on stacking orientations. In addition to twist angle devices, non-twisted devices have been shown to have desirable properties, such as ferroelectricity in parallel stacked bilayer hexagonal boron nitride [1] and ferromagnetism in bilayer CrI$_3$ [3]. In both cases twist-angle devices showed domains that varied in their stacking orders, and thus varied in their properties. By applying a unidirectional shear force we hypothesize that we’ll be able to switch the stacking orders and tune the properties of these devices to better suit experimental goals. Furthermore, we hope to show that this process is reversible, and that the initial stacking order can be recovered by continuing to apply the shear force in the same direction, or reversing the direction.
To better understand the system we employed use of COMSOL Multiphysics software to model the experimental setup. In the lab, the device will be on a silicon substrate and fixed to the bottom of the sample chamber inside of a Janis cryostat cooled to tens of Kelvins. A glass slide with the microstructure will be attached to piezo-stacks for fine motor control and then lowered into contact with the device, where an optical microscope will confirm that contact has been made. Once contact is verified the slide will be displaced up to 10μm by piezos, applying a shear force to the surface of the device.
To simulate this in the COMSOL environment we simplified the setup. First we replaced the glass slide’s motion with a prescribed displacement applied parallel to the plane of the top surface of the cylinder and the silicon substrate was replaced with a fixed constraint applied to the bottom of the device. The translation was simulated in a stationary study with a displacement parameter ranging from 1-10μm. From the simulation stress and displacement plots were extracted, as well as figures relating the prescribed displacement to the force applied by the microstructure to the device.
The microstructures we fabricated were made of polydimethylsiloxane (PDMS) and the fabrication process followed the steps described in Fig. 3. We used an augmented method of PDMS double casting to rapidly prototype mold designs and methods using an SU8 photoresist pillar as the pattern.
The process is inspired by [2], with some modifications borrowed from [3].
We choose PDMS for several reasons:
This process was not without trial, as many molds/castings failed for a variety of reasons. Modes of failure included: