Atomically precise inorganic helices with a programmable irrational twist.

Helicity in solids often arises from the precise ordering of cooperative intra- and intermolecular interactions unique to natural, organic or molecular systems. This exclusivity limited the realization of helicity and its ensuing properties in dense inorganic solids. Here we report that Ga atoms in GaSeI, a representative III-VI-VII one-dimensional (1D) van der Waals crystal, manifest the rare Boerdijk-Coxeter helix motif. This motif is a non-repeating geometric pattern characterized by 1D face-sharing tetrahedra whose adjacent vertices are rotated by an irrational angle. Using InSeI and GaSeI, we show that the modularity of 1D van der Waals lattices accommodates the systematic twisting of a periodic tetrahelix with a 41 screw axis in InSeI to an infinitely extending Boerdijk-Coxeter helix in GaSeI. GaSeI crystals are non-centrosymmetric, optically active and exfoliable to a single chain. These results present a materials platform towards understanding the origin and physical manifestation of aperiodic helicity in low-dimensional solids.

Porous lanthanide metal-organic frameworks with metallic conductivity.

Metallic charge transport and porosity appear almost mutually exclusive. Whereas metals demand large numbers of free carriers and must have minimal impurities and lattice vibrations to avoid charge scattering, the voids in porous materials limit the carrier concentration, provide ample space for impurities, and create more charge-scattering vibrations due to the size and flexibility of the lattice. No microporous material has been conclusively shown to behave as a metal. Here, we demonstrate that single crystals of the porous metal-organic framework Ln1.5(2,3,6,7,10,11-hexaoxytriphenylene) (Ln = La, Nd) are metallic. The materials display the highest room-temperature conductivities of all porous materials, reaching values above 1,000 S/cm. Single crystals of the compounds additionally show clear temperature-deactivated charge transport, a hallmark of a metallic material. Lastly, a structural transition consistent with charge density wave ordering, present only in metals and rare in any materials, provides additional conclusive proof of the metallic nature of the materials. Our results provide an example of a metal with porosity intrinsic to its structure. We anticipate that the combination of porosity and chemical tunability that these materials possess will provide a unique handle toward controlling the unconventional states that lie within them, such as charge density waves that we observed, or perhaps superconductivity.

Bonding-Directed Crystallization of Ultra-Long One-Dimensional NbS3 van der Waals Nanowires.

The rediscovery of one-dimensional (1D) and quasi-1D (q-1D) van der Waals (vdW) crystals ushered the realization of nascent physical properties in 1D that are suitable for applications in photonics, electronics, and sensing. However, despite renewed interest in the creation and understanding of the physical properties of 1D and q-1D vdW crystals, the lack of accessible synthetic pathways for growing well-defined nanostructures that extend across several length scales remains. Using the highly anisotropic 1D vdW NbS3-I crystal as a model phase, we present a catalyst-free and bottom-up synthetic approach to access ultralong nanowires, with lengths reaching up to 7.9 mm and with uniform thicknesses ranging from 13 to 160 nm between individual nanowires. Control over the synthetic parameters enabled the modulation of intra- and interchain growth modalities to selectively yield only 1D nanowires or quasi-2D nanoribbons. Comparative synthetic and density functional theory (DFT) studies with a closely related nondimerized phase, ZrS3, show that the unusual preferential growth along 1D can be correlated to the strongly anisotropic bonding and dimeric nature of NbS3-I.

GaSI: A Wide-Gap Non-centrosymmetric Helical Crystal.

The complex non-centrosymmetric and chiral nature of helical structures endow materials that possess such motifs with unusual properties. However, despite their ubiquity in biological and organic systems, there is a severe lack of inorganic crystals that display helicity in extended lattices, where these unusual properties are expected to be most pronounced. Here, we report a new inorganic helical structure, gallium sulfur iodide (GaSI), within the exfoliable class of III-VI-VII (1:1:1) one-dimensional (1D) van der Waals (vdW) crystals. Through detailed structural analyses, including single-crystal X-ray diffraction, electron microscopy, and density functional theory (DFT), we elucidate the apparent noncrystallographic screw axis and the first example of an atomic scale helical structure bearing a "squircular" cross-section in GaSI. Crystallizing in the non-centrosymmetric P4̅ space group, we found that GaSI crystals exhibit pronounced second-harmonic generation. From diffuse reflectance spectroscopy, GaSI displays a sizeable bandgap of 3.69 eV, owing tostrong covalent interactions arising from the smaller sulfur atoms within the helix core. These results position GaSI as a promising exfoliable nonlinear optical material across a broad optical window.

Anisotropy-Driven Crystallization of Dimensionally Resolved Quasi-1D Van der Waals Nanostructures.

Unusual behavior in solids emerges from the complex interplay between crystalline order, composition, and dimensionality. In crystals comprising weakly bound one-dimensional (1D) or quasi-1D (q-1D) chains, properties such as charge density waves, topologically protected states, and indirect-to-direct band gap crossovers have been predicted to arise. However, the experimental demonstration of many of these nascent physics in 1D or q-1D van der Waals (vdW) crystals is obscured by the highly anisotropic bonding between the chains, stochasticity of top-down exfoliation, and the lack of synthetic strategies to control bottom-up growth. Herein, we report the directed crystallization of a model q-1D vdW phase, Sb2S3, into dimensionally resolved nanostructures. We demonstrate the uncatalyzed growth of highly crystalline Sb2S3 nanowires, nanoribbons, and quasi-2D nanosheets with thicknesses in the range of 10 to 100 nm from the bottom-up crystallization of [Sb4S6]n chains. We found that dimensionally resolved nanostructures emerge from two distinct chemical vapor growth pathways defined by diverse covalent intrachain and anisotropic vdW interchain interactions and controlled precursor ratios in the vapor phase. At sub-100 nm nanostructure thicknesses, we observe the hardening of phonon modes, blue-shifting of optical band gaps, and the emergence of a new high-energy photoluminescence peak. The directional growth of weakly bound 1D ribbons or chains into well-resolved nanocrystalline morphologies provides opportunities to develop ordered nanostructures and hierarchical assemblies that are suitable for a wide range of optoelectronic and quantum devices.

Nanoparticle-directed bimodal crystallization of the quasi-1D van der Waals phase, Bi4I4.

Anisotropy often yields unexpected structures and properties in the solid state. In van der Waals (vdW) solids comprised of 1D or quasi-1D (q-1D) building blocks, anisotropy in both intra- and inter-chain directions results in an abundance of crystalline packing motifs and drastically altered physical states. Among these, structurally and chemically complex 1D/q-1D vdW solids that display topologically protected states, unique optical properties, and enhanced electrical transport properties in 1D are sought after owing to their potential as building blocks for next-generation quantum devices that approach the sub-nanometer regime. Yet, the access to such facet- and edge-specific physical states is still limited by the stochastic nature of micromechanical exfoliation. Here, we demonstrate that the representative Bi4I4 phase, an established pnictohalide q-1D vdW topological insulator in the bulk, can be crystallized from the vapor phase either into well-defined nanowires or quasi-2D nanosheets. We find that gold nanoparticles (Au NPs) on the growth substrate, in conjunction with the highly anisotropic structure of Bi4I4 common to many q-1D vdW crystals, direct the dimensionality of high-purity Bi4I4 nanostructures. Systematic variation of Au NP diameters, Bi : I precursor ratios, and growth-deposition temperatures reveal that Au NPs generally act as nucleation sites for vapor-solid (VS) growth of Bi4I4 nanowires. Strikingly, post-synthesis analyses of the elemental composition of 20 nm Au NPs on the substrate surface show an equisotichiometric 1 : 1 ratio of Bi to I within the Au NP that triggers the vapor-liquid-solid (VLS) growth of [001]-oriented quasi-2D nanosheets comprised of laterally-ordered [Bi4I4]n chains along the perpendicular [100] direction. We rationalize the observed bimodal growth pathways and the morphologically distinct nanostructures based on crystallization habits and orientations of the nanostructures, Bi : I ratios in the resulting Au NPs post-synthesis, and the orientation of stereochemically active Bi lone pairs between adjacent chains. We anticipate that these growth pathways are adaptable to the synthesis of emergent halide- and chalcogen-based 1D vdW nanocrystals with diverse physical and quantum properties.

Single Quasi-1D Chains of Sb2Se3 Encapsulated within Carbon Nanotubes.

The realization of stable monolayers from 2D van der Waals (vdW) solids has fueled the search for exfoliable crystals with even lower dimensionalities. To this end, 1D and quasi-1D (q-1D) vdW crystals comprising weakly bound subnanometer-thick chains have been discovered and demonstrated to exhibit nascent physics in the bulk. Although established micromechanical and liquid-phase exfoliation methods have been applied to access single isolated chains from bulk crystals, interchain vdW interactions with nonequivalent strengths have greatly hindered the ability to achieve uniform single isolated chains. Here, we report that encapsulation of the model q-1D vdW crystal, Sb2Se3, within single-walled carbon nanotubes (CNTs) circumvents the relatively stronger c-axis vdW interactions between the chains and allows for the isolation of single chains with structural integrity. High-resolution transmission electron microscopy and selected area electron diffraction studies of the Sb2Se3@CNT heterostructure revealed that the structure of the [Sb4Se6]n chain is preserved, enabling us to systematically probe the size-dependent properties of Sb2Se3 from the bulk down to a single chain. We show that ensembles of the [Sb4Se6]n chains within CNTs display Raman confinement effects and an emergent band-like absorption onset around 600 nm, suggesting a strong blue shift of the near-infrared band gap of Sb2Se3 into the visible range upon encapsulation. First-principles density functional theory calculations further provided qualitative insight into the structures and interactions that could manifest in the Sb2Se3@CNT heterostructure. Spatial visualization of the calculated electron density difference map of the heterostructure indicated a minimal degree of electron donation from the host CNT to the guest [Sb4Se6]n chain. Altogether, this model system demonstrates that 1D and q-1D vdW crystals with strongly anisotropic vdW interactions can be precisely studied by encapsulation within CNTs with suitable diameters, thereby opening opportunities in understanding dimension-dependent properties of a plethora of emergent vdW solids at or approaching the subnanometer regime.

Sensitive Thermochromic Behavior of InSeI, a Highly Anisotropic and Tubular 1D van der Waals Crystal.

Thermochromism, the change in color of a material with temperature, is the fundamental basis of optical thermometry. A longstanding challenge in realizing sensitive optical thermometers for widespread use is identifying materials with pronounced thermometric optical performance in the visible range. Herein, it is demonstrated that single crystals of indium selenium iodide (InSeI), a 1D van der Waals (vdW) solid consisting of weakly bound helical chains, exhibit considerable visible range thermochromism. A strong temperature-dependent optical band edge absorption shift ranging from 450 to 530 nm (2.8 to 2.3 eV) over a 380 K temperature range with an experimental (dEg/dT)max value extracted to be 1.26 × 10-3 eV K-1 is shown. This value lies appreciably above most dense conventional semiconductors in the visible range and is comparable to soft lattice solids. The authors further seek to understand the origin of this unusually sensitive thermochromic behavior and find that it arises from strong electron-phonon interactions and anharmonic phonons that significantly broaden band edges and lower the Eg with increasing temperature. The identification of structural signatures resulting in sensitive thermochromism in 1D vdW crystals opens avenues in discovering low-dimensional solids with strong temperature-dependent optical responses across broad spectral windows, dimensionalities, and size regimes.

Functionalization and Structural Evolution of Conducting Quasi-One-Dimensional Chevrel-Type Telluride Nanocrystals.

Interfacing organic molecular groups with well-defined inorganic lattices, especially in low dimensions, enables synthetic routes for the rational manipulation of both their local or extended lattice structures and physical properties. While appreciably studied in two-dimensional systems, the influence of surface organic substituents on many known and emergent one-dimensional (1D) and quasi-1D (q-1D) crystals has remained underexplored. Herein, we demonstrate the surface functionalization of bulk and nanoscale Chevrel-like q-1D ionic crystals using In2Mo6Te6, a predicted q-1D Dirac semimetal, as the model phase. Using a series of alkyl ammonium (-NR4+; R = H, methyl, ethyl, butyl, and octyl) substituents with varying chain lengths, we demonstrate the systematic expansion of the intrachain c-axis direction and the contraction of the interchain a/b-axis direction with longer chain substituents. Additionally, we demonstrate the systematic expansion of the intrachain c-axis direction and the contraction of the interchain a/b-axis direction as the alkyl chain substituents become longer using a combination of powder X-ray diffraction and Raman experiments. Beyond the structural modulation that the substituted groups can impose on the lattice, we also found that the substitution of ammonium-based groups on the surface of the nanocrystals resulted in selective suspension in aqueous (NH4+-functionalized) or organic solvents (NOc4+-functionalized), imparted fluorescent character (Rhodamine B-functionalized), and modulated the electrical conductivity of the nanocrystal ensemble. Altogether, our results underscore the potential of organic-inorganic interfacing strategies to tune the structural and physical properties of rediscovered Chevrel-type q-1D ionic solids and open opportunities for the development of surface-addressable building blocks for hybrid electronic and optoelectronic devices at the nanoscale.

Lattice-guided assembly of optoelectronically active π-conjugated peptides on 1D van der Waals single crystals.

The deployment of organic molecules in high-performance devices strongly relies on the formation of well-ordered domains, which is often complicated by the dynamic and sensitive nature of supramolecular interactions. Here, we engineered the assembly of water-processable, optoelectronic π-conjugated peptides into well-defined organic-inorganic heterointerfaced assemblies by leveraging the long-range anisotropic ordering of 1D van der Waals (vdW) crystals composed of subnanometer-thick transition metal sulfide chains (MS3; M = Nb, Ta) as assembly templates. We found that the monomers can readily form 1D supramolecular assemblies onto the underlying crystal surface, owing to the structural correspondence between the π-π interactions of the quaterthiophene (4T)-based peptide units (DDD-4T) and sulfur atom ordering along the NbS3 (100) surface. The heterointerfaced assemblies exhibited substantially red-shifted photoluminescence and enhanced visible-range photocurrent generation compared to solution-assembled films. Our results underscore the role of lattice matching in forming ordered supramolecular assemblies, offering an emergent approach to assembling organic building blocks endowed with improved physical properties.

Encapsulation of crystalline and amorphous Sb2S3 within carbon and boron nitride nanotubes.

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 well-ordered few to single covalent chains of Sb2S3 when synthesized within defect-free SWCNTs with sub-2 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.

Modulation doping and charge density wave transition in layered PbSe-VSe2 ferecrystal heterostructures.

Controlling charge carrier concentrations remains a major challenge in the application of quasi-two-dimensional materials. A promising approach is the modulation doping of transport channels via charge transfer from neighboring layers in stacked heterostructures. Ferecrystals, which are metastable layered structures created from artificial elemental precursors, are a perfect model system to investigate modulation doping, as they offer unparalleled freedom in the combination of different constituents and variable layering sequences. In this work, differently stacked combinations of rock-salt structured PbSe and VSe2 were investigated using X-ray photoelectron spectroscopy. The PbSe layers act as electron donors in all heterostructures, with about 0.1 to 0.3 donated electrons per VSe2 unit cell. While they initially retain their inherent semiconducting behavior, they themselves become metallic when combined with a larger number of VSe2 layers, as evidenced by a change of the XPS core level lineshape. Additional analysis of the valence band structure was performed for selected stacking orders at different sample temperatures to investigate a predicted charge density wave (CDW) transition. While there appear to be hints of a gap opening, the data so far is inconclusive and the application of spatially resolved techniques such as scanning tunneling microscopy is encouraged for further studies.