Helicity, a geometric property rendering an object nonsuperimposable on its own mirror image is ubiquitous in Nature. Inorganic crystals can grow into helical form, which is of great interest from the perspective of fundamental material science as well as application. Various inorganic crystals can be grown into helically twisted form on different scales ranging from nanoscale to mesoscale and to macroscale. The natural growth of twisted macroscopic quartz crystals in the bulk form has been documented and studied for centuries. On the nanoscale, quantum dots (QD), nanowires and carbon nanotube into helical structures. Those helical crystals have intriguing optoelectronics properties including rotatory optical activity and circular dichroisms in both absorption and photoluminescence as well as unique stereoselectivity in chemical reactions. These properties render inorganic crystals good potential of applications in polarization optics, chiroptical sensing, enantioselective catalyst and biomedical imaging. The material science responsible for forming these twisted inorganic crystals, nevertheless, remains largely mysterious and elusive.
In recent years, twisted van der Waals materials with rotational stacking of two-dimensional materials have attracted tremendous attention. The twist angle strongly affects the electronic states, excitons and phonons of the twisted structures through interlayer coupling, giving rise to exotic optical, electric, excitonic and spintronic behaviors. In twisted bilayer graphene, at certain twist angles, long-range periodicity associated with moiré patterns introduces flat electronic bands and highly localized electronic states, resulting in Mott insulating behavior and superconductivity. Theoretical studies suggest that these twist-induced phenomena are common to layered materials such as transition-metal dichalcogenides, black phosphorus and germanium monoselenide. In contrast to electronic band structure of the twisted bilayer graphene, unique features such as one-dimensional flat electronic band may emerge in those twisted 2D materials with different structures and symmetries.
The ability to manipulate the twisting topology of van der Waals structures offers a new degree of freedom through which to tailor their electrical and optical properties. Twisted van der Waals materials are usually created using mechanical exfoliation and a transfer-stacking method, but limitations exist to extend this method to a variety of two-dimensional materials. In contrast, bottom-up growth methods could provide an alternative means to create twisted van der Waals structures. This dissertation explores the bottom-up synthesis of twisted van der Waals materials. We demonstrate that the Eshelby twist associated with a screw dislocation (a chiral topological defect), can drive the formation of twisted van der Waals materials.
The Eshelby twist is a continuous crystallographic twist generated by the torsional force of an axial screw dislocation in a one-dimensional structure. It has been shown to result in growth of helically twisted nanowires of various materials. This mechanism potentially provides a means to create twisted van der Waals (vdW) structures. Materials such as germanium sulfide (GeS) can grow into nanowires along the vdW stacking direction (the cross-plane direction), and introducing Eshelby twist into such nanowires naturally leads to twist between the successive layers.
We synthesized twisted GeS crystals at both nanoscale and mesocscale. In the synthesis method, GeS nanowires with axial screw dislocations are first grown along the stacking direction, yielding vdW nanowires with Eshelby twist. These wires possess continuous twists in which the total twist rates are defined by the radii of the nanowires, consistent with Eshelby’s theory. Further radial growth of those twisted nanowires that are attached to the substrate leads to an increase in elastic energy, as the total twist rate is fixed by the substrate. The stored elastic energy can be reduced by accommodating the fixed twist rate in a series of discrete jumps in the twisting profile. This yields mesoscale twisting structures consisting of a helical assembly of nanoplates demarcated by atomically sharp interfaces with a range of twist angles.
The twisting profiles of the structures are tunable. We show that the twisting profile can be tailored by controlling the radial size of the structure. The twisting morphology gradually transitions from initial continuous twisting to intermediate twisting (consisting of both continuous twisting between the twist boundaries and discrete twisting at the boundaries) and eventually to discrete twisting with increasing radial size. This allows us to control the twisting profile and angles at twist interfaces by controlling the radial growth of the structure.
We also demonstrate that the twist rate and period can be tailored by tailoring the radii of the dislocated nanowires first grown in the VLS process. This is achieved by adding GeSe into the growth, which modulates the size of the droplets catalyzing the VLS process, therefore modulating the radii of the nanowires. The chemical modulation demonstrates good potential to tailor the twist rate and period of helical vdW crystals, enabling a new freedom to modulate optoelectronic properties and chiral light-matter interactions.
Last, we explored the anisotropic propagation of surface phonon polaritons in GeS, enabled by the strong in-plane anisotropy of the crystal structure. We demonstrate that GeS thin films at thickness of tens of nanometers support strongly anisotropic surface phonon polaritons in the far-infrared range. The dispersion relations of these phonon polaritons can be tuned by varying the thickness of the GeS film.