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Abstract The discovery and synthesis of atomically precise low‐dimensional inorganic materials have led to numerous unusual structural motifs and nascent physical properties. However, access to low‐dimensional van der Waals (vdW)‐bound analogs of bulk crystals is often limited by chemical considerations arising from structural factors like atomic radii, bonding or coordination, and electronegativity. Using single‐walled carbon nanotubes (SWCNTs) as confinement templates, we demonstrate the synthesis of a short‐wave infrared‐absorbing quasi‐1D (q‐1D) chain polymorph of Sb₂Te₃([Sb₄Te₆]_n) that is structurally and electronically distinct from its 2D counterpart. It is found that the q‐1D chain polymorph has both three‐ and five‐coordinate Sb atoms covalently bonded to Te and is thermodynamically stabilized by the electrostatic interaction between the encapsulated chain and the model SWCNT. The complementary experimental and computational results demonstrate the synthetic advantage of vdW nanotube confinement in the discovery of low‐dimensional polytypes with drastically altered physical properties and potential applications in energy conversion processes. 

Physical states in nanoscale solids are tied to their crystalline order, morphology, and size. However, deterministically accessing different nanocrystal morphologies from a single phase usually involves complex synthetic routes, catalysts, or multi-step lithographic techniques. Here, we demonstrate the catalyst-free synthesis of nanosheets and nanowires based on the luminescent 2D van der Waals (vdW) phase, GaTe, as a model phase that manifests atomic precision and a highly anisotropic quasi-1D substructure. We program the size and morphology of the resulting nanostructures by varying the relative rates of precursor deposition and diffusion, achieving dense, uniform, and widespread growth. Ultrathin nanowires resulting from this synthesis exhibit strikingly enhanced low-temperature luminescence with narrow near-infrared (NIR) emission bandwidths. These spectral characteristics arise from defect-bound states confined within a nanowire morphology that acts as a deep sub-wavelength optical cavity, making them suitable as optical emitters with small footprints either as stand-alone structures or coupled with other vdW crystals. 

The recent rediscovery of 1D and quasi-1D (q-1D) van der Waals (vdW) crystals has laid foundation for the realization of emergent electronic, optical, and quantum-confined physical phenomena in both bulk and at the nanoscale. Of these, the highly anisotropic q-1D vdW crystal structure and the visible-light optical/ optoelectronic properties of antimony trisulfide (Sb2S3) have led to its widespread consideration as a promising building block for photovoltaic and non-volatile phase change devices. However, while these applications will greatly benefit from well-defined and sub-nanometer-thick q-1D structures, little has been known about feasible synthetic routes that can access single covalent chains of Sb2S3. In this work, we explore how encapsulation in single or multi-walled carbon nanotubes (SWCNTs or MWCNTs) and visible-range transparent boron nitride nanotubes (BNNTs) influences the growth and phase of Sb2S3 nanostructures. We demonstrate that nanotubes with smaller diameters had a more pronounced effect in the crystallographic growth direction and orientation of Sb2S3 nanostructures, promoting the crystallization of the guest structures along the long-axis [010]-direction. As such, we were able to reliably access wellordered few to single covalent chains of Sb2S3 when synthesized within defect-free SWCNTs with sub2 nm inner diameters. Intriguingly, we found that the degree of crystalline order of Sb2S3 nanostructures was strongly influenced by the presence of defects and discontinuities along the Sb2S3-nanotube interface. We show that amorphous nanowire domains of Sb2S3 form around defect sites in larger, multi-walled nanotubes that manifest inner wall defects and discontinuities, suggesting a means to manipulate the crystallization dynamics of confined sub-10 nm-thick Sb2S3 nanostructures within nanotubes. Lastly, we show that ultranarrow amorphous Sb2S3 can impart functionality onto isolable BNNTs with photocurrent generation in the pA range which, alongside the dispersibility of the Sb2S3@BNNTs, could be leveraged to easily fabricate photoresistors only a few nm in width. Altogether, our results serve to solidify the understanding of how q-1D vdW pnictogen chalcogenides crystallize within confined synthetic platforms and are a step towards realizing functional materials from ensembles of encapsulated heterostructures