Tunable 1D van der Waals Nanostructures by Vapor-Liquid-Solid Growth

Vapor-liquid-solid (VLS) growth using molten metal catalysts has traditionally been used to synthesize nanowires from different 3D-crystalline semiconductors. With their anisotropic structure and properties, 2D/layered semiconductors create additional opportunities for materials design when shaped into 1D nanostructures. In contrast to hexagonal 2D crystals such as graphene, h-BN, and transition metal dichalcogenides, which tend to roll up into nanotubes, VLS growth of layered group III and group IV monochalcogenides produces diverse nanowire and nanoribbon morphologies that crystallize in a bulk-like layered structure with nanometer-scale footprint and lengths exceeding tens of micrometers. In this Account, we discuss the achievable morphologies, the mechanisms governing key structural features, and the emerging functional properties of these 1D van der Waals (vdW) architectures. Recent results highlight rich sets of phenomena that qualify these materials as a distinct class of nanostructures, far beyond a mere extension of 3D-crystalline VLS nanowires to vdW crystals.The main difference between 3D- and vdW crystals, the pronounced in-plane/cross-plane anisotropy of layered materials, motivates investigating the factors governing the layer orientation. Recent research suggests that the VLS catalyst plays a key role, and that its modification via the choice of chalcogens or through modifiers added to the growth precursor can switch both the nanostructure morphology and vdW layering. In many instances, ordinary layered structures are not formed but VLS growth is dominated by morphologies-often containing a crystal defect-that present reduced or vanishing layer nucleation barriers, thus achieving fast growth and emerging as the principal synthesis product. Prominent defect morphologies include vdW bicrystals growing by a twin-plane reentrant process and chiral nanowires formed by spiral growth around an axial screw dislocation. The latter carry particular promise, e.g., for twistronics. In vdW nanowires, Eshelby twist-a progressive crystal rotation caused by the dislocation stress field-translates into interlayer twist that is precisely tunable via the wire diameter. Projected onto a helicoid vdW interface, the resulting twist moires not only modify the electronic structure but also realize configurations without equivalent in planar systems, such as continuously variable twist and twist homojunctions. 1D vdW nanostructures derive distinct functionality from both their layered structure and embedded defects. Correlated electron microscopy methods including imaging, nanobeam diffraction, as well as electron-stimulated local absorption and luminescence spectroscopies combine to an exceptionally powerful probe of this emerging functionality, identifying twist-moire induced electronic modulations and chiral photonic modes, demonstrating the benign nature of defects in optoelectronics, and uncovering ferroelectricity via symmetry-breaking by single-layer stacking faults in vdW nanowires. Far-reaching possibilities for tuning crystal structure, morphology, and defects create a rich playground for the discovery of new functional nanomaterials based on vdW crystals. Given the prominence of defects and extensive prospects for controlling their character and placement during synthesis, 1D vdW nanostructures have the potential to cause a paradigm shift in the science of electronic materials, replacing the traditional strategy of suppressing crystal imperfections with an alternative philosophy that embraces the use of individual defects with designed properties as drivers of technology.