Br-Induced Orientation Control of PbI₂ van der Waals Nanowires and Their Optoelectronics

Abstract

van der Waals (vdW)-layered direct bandgap semiconductor materials such as lead iodide (PbI 2 ) possess highly anisotropic optoelectronic properties that are dependent on their layer stacking orientations. Here, we report an in situ control of PbI 2 layer growth orientation, or stacking axis, via the vapor− liquid−solid (VLS) growth. The PbI 2 vdW nanowires initially grow with transverse stacking relative to the [0001]-oriented growing axis, and the controlled introduction of PbBr 2 induces threedimensional twin formation, which is responsible for the change in the layer stacking axis (i.e., kinking). By varying the duration of the PbBr 2 introduction, two different types of kinking modes are confirmed: Type I kink in parallel with the twin boundary (TB) propagation accompanying two mirrored stacking c-axes and Type II kink, in which the layers are stacked parallel to the growing segment. Optoelectronic analyses using photoluminescence and cathodoluminescence reveal that the kinked PbI 2 nanostructures exhibit asymmetric band-edge excitonic emission profiles, which are distinct from those of the single-oriented nanowires. These results demonstrate a new methodology to modulate the structuredependent optoelectronic/photonic properties, as well as phonon transport, which cannot be achieved using conventional 2D design platforms for the vdW-layered materials.

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

“…Angled nanoribbons are asymmetric bicrystals whose two halves, joined at a central grain boundary, intersect at an angle of ∼19°. Similar modifications to the layering were reported for a different materials system, namely Pb-catalyzed PbI 2 nanowires, where added PbBr 2 switched the layer orientation from [0001] to [2 0] or induced [021]-axis twinning …”

“…Similar modifications to the layering were reported for a different materials system, namely Pb-catalyzed PbI 2 nanowires, where added PbBr 2 switched the layer orientation from [0001] to [2110] or induced [2021]-axis twinning. 31…”

“…Recent work shows VLS growth of vdW nanostructures dominated by fast-growing/defect morphologies, some of which mirror those observed for 3D-crystals: (i) Nanoribbons, ; 1,30,38,39 and (iii) bicrystals growing by a twin-plane reentrant process (Figure 2(c)). 3,31 Compared with defect-free wires, such fast-growing nanostructures achieve a significantly greater length. Hence, the nanostructure selection during VLS growth follows an evolutionary process, where the system initially explores a large number of possible morphologies of which the fastest-growing ones collect most of the incoming vapor and become the predominant synthesis product.…”

Contact Geometry-Dependent Excitonic Emission in Mixed-Dimensional van der Waals Heterostructures

“…The detailed procedures for the preparation of each component are described in the methods section. First, the PbI 2 nanowires were bottom-up grown via the VLS method, capable of controlling the axial orientation of the growing crystals. , The anisotropic nature of the PbI 2 lattice structure (trigonal, P 3 m 1 group in the 2H phase) leads to several favored growth directions of the nanowires, , which depend on the interfacial energy preference between the liquid catalyst and the nanowire growth front. , Here, we synthesized PbI 2 nanowires exhibiting two distinct crystallographic orientations, as shown in Figure S1. Detailed structural analyses using transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirm that these nanowires are oriented along [0001] (hereafter denoted as c-PbI 2 ), where the vdW layers are stacked transversely, and [12̅10] (hereafter denoted as a-PbI 2 ), with vdW layers stacked longitudinally along the axis, as depicted in Figures S2 and S3.…”

“…Comprising various crystallographic planes with different energies, the sidewall facets of a nanowire dictate the overall physical properties, including optical and electronic transport phenomena. , The orientations of these facets can be modulated by the bottom-up vapor–liquid–solid (VLS) growth, , which has also recently been demonstrated for vdW-layered semiconductor materials such as PbI 2 , GeS, and GeSe. − As direct bandgap semiconductors, these vdW-layered materials exhibit anisotropic excitonic behavior that is further enhanced when structured into nanowires, leading to significant emission dependencies on their crystallographic orientations. , Hence, precise control over axial orientation (i.e., layer stacking) in vdW nanowires can enable the creation of interfaces with various atomic configurations upon fabrication of heterostructures, providing opportunities for exploration that were previously inaccessible. These engineered interfaces, whether homo- or heterojunctions, play a crucial role in determining the electronic and optical properties of materials and can significantly optimize device performance, thereby enhancing functionalities across various applications including optoelectronics and energy conversion. , …”