Dimensionally Resolved Nanostructures of an Atomically Precise and Optically Active 1D van der Waals Helix

Abstract

Inorganic freestanding helices are rare and sought‐after for their unusual physical states endowed by chirality. To this end, III–VI–VII solids have emerged as a distinct class of ternary 1D van der Waals (vdW) crystals which bear atomically precise helical motifs. However, the physical understanding of the intrinsic and size‐dependent properties of these materials is limited by the lack of synthetic strategies to directly access freestanding nanocrystals in high volumes. Using GaSI as a representative phase, a bottom‐up strategy is presented to grow high yields of ultrathin nanostructures based on this helical materials class. With this strategy, it is possible to grow single crystals of 1D nanowires with thicknesses in the 10–100 nm range at high temperature conditions, as well as quasi‐2D nanoribbons at lower temperatures. The bandgap of the nanowires is established in the UV region and demonstrates the persistence of nonlinear optical behavior as evidence of the persistence of the noncentrosymmetric crystal structure of GaSI at the nanoscale. Inspired by these results, the effect of the helical nature of GaSI on the electronic structure of hypothetical single chains is probed from first principles and shows the pronounced handedness‐dependent and helicity‐imposed spin polarization at the single helix regime.

The ability to grow nanostructures based on inorganic helical crystals with long‐range order will enable a platform to realize physical states that arise from chirality. Herein, it is demonstrated that controlled vapor phase deposition of an atomically precise helical crystal, GaSI, into ultrathin 1D nanowires and quasi‐2D nanoribbons. The nanostructures retain the helical and noncentrosymmetric nature of the bulk, absorb in the UV, and manifest nonlinear optical activity.

1. Introduction

Nascent physical properties in solids often result from the complex interplay of their size, crystal structure, and symmetry, especially as they approach the atomic scale. Among recently discovered low‐dimensional solids, 1D van der Waals (vdW) solids, which include MCh₃ transition metal trichalcogenides (M = Nb, Ta, Zr, Hf; Ch = S, Se), main group chalcogenides like the Pn₂Ch₃ class (Pn = Sb, Bi; Ch = S, Se), and bismuth halides like Bi₄X₄ (X = Br, I), have re‐emerged as broad classes of materials that display unusual physical properties due to their exfoliable 1D sub‐structural motifs.^{[ 1} , ² , ³ , ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ , ¹⁰ , ¹¹ , ¹² , ¹³ , ¹⁴ , ^{15 ]} Owing to their highly confined 1D electronic states, these low‐dimensional crystals have been demonstrated to exhibit atypical and highly confined behavior such as higher‐order topological states, high temperature charge density waves, unusual superconductivity, and long‐range shift current bulk photovoltaic effect.^{[ 2} , ¹³ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ^{23 ]} The weak vdW interactions that hold together the crystalline chains within these solids also lead to their straightforward (albeit morphologically random) exfoliability into few‐ or even single‐chain nanowires with stable surfaces.^{[ 24} , ²⁵ , ²⁶ , ^{27 ]} Meanwhile, their inherently anisotropic bonding motifs and long‐range structures give rise to strongly axis‐dependent optical responses.^{[ 28} , ^{29 ]} These structural motifs have led to the seamless integration of vdW solids into emergent nanoscale devices.^{[ 30} , ³¹ , ³² , ^{33 ]} At present, there have been numerous efforts to deterministically control the formation of well‐defined and device‐ready nanostructures of these materials from both top‐down and bottom‐up approaches.^{[ 3} , ⁶ , ¹⁴ , ²⁴ , ²⁷ , ³⁴ , ³⁵ , ³⁶ , ³⁷ , ³⁸ , ³⁹ , ^{40 ]} While such strategies have enabled orientation‐controlled growth and aligned assemblies in 2D vdW systems, analogous substrate‐level directional control of nanocrystal growth have remained inaccessible for the 1D vdW crystal counterparts.^{[ 41} , ⁴² , ⁴³ , ^{44 ]} In 1D vdW crystals, exfoliation and growth still yield largely random orientations, with a few examples of aligned quasi‐1D materials being accessed but bearing uncontrollable thicknesses and widths.^{[ 45 ]} This lack of directional consistency critically highlights the need to first study synthetic routes that can reproducibly generate ultrathin, high‐quality nanowires as a platform for probing their anisotropic and helicity‐enabled properties, for which subsequent studies on alignment or aligned, directional growth can follow.

Among recently discovered 1D vdW crystals that form nanostructures approaching the atomic scale, all‐inorganic 1D vdW materials with rare and highly sought‐after atomically precise helical motifs have gained tremendous attention due to their manifestation of well‐defined helical order in isolable sub‐nanometer‐thin chains.^{[ 26} , ⁴⁶ , ⁴⁷ , ⁴⁸ , ⁴⁹ , ^{50 ]} One such class of helical 1D vdW materials that have emerged is the III‐VI‐VII 1D vdW crystals (e.g., InSeI, GaSeI, GaSI, AlSeI).^{[ 26} , ⁵¹ , ⁵² , ^{53 ]} These highly modular ternary crystals form three concentric tetrahelical chains based on the constituent Group 13, Group 16, and Group 17 elements, which form from a quasi‐tetrahedral [(III)(VI)₃(VII)]_n building unit that propagates in 1D through a screw axis.^{[ 26} , ⁴⁴ , ⁴⁵ , ^{47 ]} Grown at high temperatures, the helices pack with adjacent chains of opposing handedness in non‐centrosymmetric (except InSeI), but achiral space groups and have been shown to manifest rare quasi‐periodic motifs such as the Boerdijk‐Coxeter tetrahelix in GaSeI and an unusual “squircular” (a hybrid between a square and a circle) helical cross‐sections in GaSI. Arising from the helical, spring‐like, and 1D vdW long‐range structure in most of these helical crystals, including other examples such as elemental Te or Se, SnXP (X = Br, I), pronounced nonlinear optical (NLO) properties, sensitive thermochromism, ultrafast photoconductivity, and excellent mechanical properties, especially in nanoscale single crystals, have also been experimentally observed from these phases.^{[ 26} , ⁴⁶ , ⁴⁸ , ⁴⁹ , ⁵¹ , ⁵² , ⁵⁴ , ⁵⁵ , ⁵⁶ , ^{57 ]} Moreover, theory and several calculations have suggested that the intrinsic chirality of the constituent helical chains is hypothetically predicted to host unique physical states such as nonreciprocal phonon modes, topological helical states, and chirality‐induced spin selectivity (CISS).^{[ 58} , ⁵⁹ , ⁶⁰ , ⁶¹ , ⁶² , ⁶³ , ⁶⁴ , ⁶⁵ , ⁶⁶ , ⁶⁷ , ⁶⁸ , ⁶⁹ , ^{70 ]}

While some of these chirality‐driven properties have been observed in chiral assemblies of biological, organic, and hybrid helices, experimental studies in all‐inorganic helical lattices remain limited.^{[ 64 ]} This is due to their typically racemic or achiral packing driven by thermodynamically favored higher‐density chain packing. To access these properties, synthetic strategies must be employed to reliably produce few‐ or single‐chain nanowires, introduce chiral excess, or expose chiral facets. Current top‐down methods, like micro‐mechanical and liquid‐phase exfoliation, are inherently stochastic, leading to a wide array of nanocrystal sizes and dimensionalities. Bottom‐up synthetic strategies offer a pathway to overcome these packing constraints by enabling the controlled growth of ultrathin nanowires, the selective exposure of specific facets, or even the stabilization of new structural phases.^{[ 71} , ⁷² , ⁷³ , ⁷⁴ , ⁷⁵ , ⁷⁶ , ^{77 ]} Therefore, the deterministic bottom‐up strategy to consistently grow ultrathin, few‐chain‐thick nanowires in high density and access novel morphologies of these phases will represent a significant advance and a critical step toward realizing their predicted anisotropic and helicity‐imposed physical states.

Here, we report a facile, vapor‐phase growth approach to access nanowires and nanoribbons of the representative helical crystalline phase from the III‐VI‐VII 1D vdW class, GaSI. To our knowledge, this is the first example of a bottom‐up, vapor‐phase growth of an all‐inorganic freestanding helical structure. We use a catalyst‐free, modified chemical vapor transport method to obtain a dense growth of ultrathin 1D nanowires of GaSI with thicknesses in the range of 10–100 nm that directly correlate to their location on the substrate, temperature, and distance from the precursors. We also demonstrate that, under non‐thermodynamically favored conditions, covalent growth along the 1D axis is suppressed and GaSI chains grow laterally across the vdW direction to form quasi‐2D nanoribbons. We characterized the resulting nanowires using a range of microscopy and spectroscopy techniques to establish their crystallinity, confirm the persistence of non‐centrosymmetric order, and probe the optical activity through second harmonic generation (SHG) microscopy. Owing to the potential of experimentally accessing ultrathin sub‐10 nm chains, we also computationally depict from first principles density functional theory (DFT) calculations that a single helical chain of GaSI has the potential to exhibit helicity‐imposed and handedness‐dependent spin polarization. While such effects are strictly hypothetical at this time and remain to be experimentally validated, these results highlight the broader range of phenomena that could emerge once synthetic access to single‐ or few‐chain regimes is achieved. Broadly, this work positions GaSI and related helical III‐VI‐VII crystals as a new platform of optically active nanoscale materials, marking a step forward in the synthesis and exploration of helical all‐inorganic solids with anisotropy‐ and helicity‐enabled properties, and the realization of helical all‐inorganic solids for chirality‐ and helicity‐enabled device applications.

2. Results and Discussion

2.1. Bottom‐Up Vapor Phase Growth of GaSI Nanostructures

The crystal structure of GaSI forms in the non‐centrosymmetric, tetragonal P $\overline{4}$ space group, with non‐covalently bound helices propagating along the z‐axis and packs with opposite handedness along the basal plane, x‐ and y‐axis, directions (Figure  1A).^{[ 51 ]} Compared to other known helical III‐VI‐VII crystals, the GaSI structure is unique, in that, the cross‐sectional shape of the constituent helices resembles a “squircle,” which is a hybrid between a square and a circle and stabilizes the tetragonal packing of the helices.^{[ 51} , ^{78 ]} The tetragonal packing motif of the helices and equivalent vdW distances between chains along the x‐ and y‐directions indicate that there are equivalent degrees of inter‐chain interactions along these axes. Despite these equivalent inter‐chain interactions, there are a variety of ways in which GaSI nanostructures could be mechanically exfoliated or synthesized depending on the dominant exposed facet or orientation upon exfoliation (Figure 1B). The range of accessible nanostructures, either from top‐down or bottom‐up approaches, include 1D nanowire bundles, quasi‐2D nanoribbons (which may have an exposed chiral facet), or isolated single chains (Figure 1A).^{[ 79 ]}

Figure 1.

Helical crystal structure and accessible packing motifs of GaSI nanostructures. A) Cross‐sectional and long‐axis projection of the helical crystal structure of GaSI chains, shown with different accessible packing motifs arising from the inter‐chain vdW interactions. B) Graphical representation of the two vapor phase routes: intra‐ and inter‐chain directions in GaSI nanostructures. C) Sizeable GaSI crystals grown from a melt‐type reaction, highlighting the radial growth pattern of the crystals. Scale bar, 1 mm. D) Dark‐field optical image of mechanically exfoliated GaSI nanowires on an Si/SiO₂ substrate. Scale bar, 245 µm. E) Dark‐field optical image of GaSI nanowires resulting from liquid‐phase sonication‐assisted exfoliation with isopropanol. Scale bar, 245 µm.

Sizeable single crystals can be grown from a melt‐type synthesis, which induces the radial crystallization of GaSI (Figure 1C). Nanostructures such as nanowire bundles can be accessed via top‐down approaches, where bulk crystals are either exfoliated using micro‐mechanical (Figure 1D) or liquid‐phase sonication‐assisted (Figure 1E) exfoliation routes. Unlike in 2D vdW crystals, which have equivalent inter‐layer interactions between adjacent layers, the multiple inter‐chain axes that can be exfoliated result in a stochastic exfoliation process. As a result, it common to observe a range of nanowire and nanoribbon morphologies from exfoliation and, therefore, hinders the systematic investigation of the evolution of the structure and physical properties with size. To this end, we leverage the anisotropic 1D vdW nature of GaSI to grow well‐defined and dimensionally resolved nanostructures from bottom up. We demonstrate this by precisely controlling the two main growth directions that are accessible for nanostructure crystallization: 1) lengthening the chain through the covalent bonding axis to form nanowires; or 2) via the lateral growth of new chains along the inter‐chain vdW axes to form nanoribbons (Figure 1B).

To predict the likely faceting and nanoscale morphology of the resulting crystals from a bottom‐up, vapor phase route, we calculated the Wulff shape using the experimentally established GaSI single crystal structure as the basis (Figure  2 ). Considering the 1D vdW structure of GaSI, we expect the pronounced 1D morphology to persist in the bottom‐up growth of the nanostructures as evidenced by the calculated Wulff shape, which indicates that the thermodynamically favorable morphology of GaSI crystals is a 1D, wire‐like structure (Figure 2A). As expected, the calculated Wulff shape closely matches the observed wire‐like morphologies of the experimentally synthesized bulk GaSI crystals (Figure 1C). The calculated surface energies also indicate that the most stable surfaces would be those that correspond to the vdW axes and surfaces (Figure 2B; Supporting Discussion). Furthermore, we found from the calculations that the (100) surface has the lowest surface energy, indicating that this would be the easiest facet to cleave or to synthetically express. Intriguingly, both the (100) surface and the (110) surface lie very close in energy, indicating that chiral facets that correspond to the (110) surface in this material may be accessible under distinct synthetic conditions. Based on other known examples, the (110) surfaces have also been experimentally accessed for the structurally similar InSeI phase.^{[ 79 ]} Thus, from these calculations, 1D nanowires that grow along the [001] direction are expected to dominate the morphological distribution of the resulting nanocrystals under thermodynamically favorable conditions.

Figure 2.

Calculated Wulff shape and bottom‐up growth strategy. A) Calculated Wulff shape of GaSI. B) Calculated surface energies of various high‐symmetry surfaces of GaSI. C) Graphical representation and typical postreaction ampule depicting the synthetic approach for the growth of GaSI nanostructures used in this study. D) Typical temperature profile used to grow GaSI nanowires. For (C) and (D), T _hot represents the precursor zone, and T _cold represents the growth zone.

We achieved the synthesis of well‐defined and crystalline GaSI nanostructures using a modified chemical vapor transport approach.^{[ 80 ]} A typical reaction can be seen in Figure 2C, which is set up according to the heating program and placement in Figure 2D and Figure S1 (Supporting Information). More detailed synthesis details are available in the Supporting Information. Postsynthesis, we established the identity and structure of the resulting GaSI nanowires through ensemble diffraction, spectroscopy, and microscopy tools (Figure 3 ; Figures S2–S4, Supporting Information). These results from grazing incidence X‐ray diffraction (XRD) that confirm the matching indexed Bragg reflections with the single crystal structure, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) energy dispersive X‐ray spectroscopy (EDS) indicative of stoichiometric elemental ratios, and Raman spectroscopy of GaSI nanowires confirm that the structure and composition across the substrate are consistent with bulk GaSI. In more detail, we used Raman spectroscopy to directly compare the phonon modes of bulk and nanoscale GaSI crystals which show little to no shifts in the frequency, indicating that the long‐range structure at the single nanowire regime is generally preserved (Figure 3A). We note that, because of the highly anisotropic structure both at the atomic scale and chain‐level and nanowire morphologies, the peak intensities of the modes change significantly based on the orientation of the nanowires with respect to the polarization of the incident laser excitation source. Orientation‐dependent Raman spectra of GaSI bulk crystals were, therefore, taken to show these orientational, polarization‐dependent peak intensity differences at 0°, 45°, and 90° and compare these peak intensity differences qualitatively to our nanowires (Figure S5, Supporting Information). Based on the orientation‐dependent Raman spectra of the bulk as the reference, all the modes are accounted for in our nanowires when compared with the bulk, dependent on orientation. We also used EDS to confirm the relative stoichiometric ratios of the constituent elements, which we found to be close to the expected 1:1:1 ratio (Figures S2–S3 and Table S1, Supporting Information). A typical growth is shown in Figure 3B, which highlights that the nanowires display a strong and well‐defined 1D morphology, are deposited at a high density, and present a relative uniformity in thickness when compared to exfoliated crystals (Figure S6, Supporting Information). To assess the ambient air stability of the nanowires, we also conducted air exposure‐dependent Raman spectroscopy studies (Figure S7, Supporting Information). These results indicate the GaSI nanowires decompose after ≈5 h of air exposure, as extrapolated from our studies, which is within the expected air stability of very thin and low‐dimensional crystals of this type. While suitable for many of our measurements and experiments, all substrates were opened and kept in an inert Ar glove box until use to ensure their pristine quality. This nanosizing‐enhanced degradation is expected for the ultrathin nanowires and is consistent with other materials that similarly become more air sensitive as they are accessed in the nanoscale.^{[ 51} , ⁸¹ , ⁸² , ^{83 ]} We then used high‐resolution transmission electron microscopy (HRTEM) to confirm the local ordering and crystallinity of the resulting GaSI nanowires and resolve the lateral dimensions of the nanowires in greater resolution. As synthesized, we were able to find several isolated micron‐length nanowires which show uniform morphologies across a long spatial range (Figure 3C; Figure S8, Supporting Information). Through the HRTEM imaging, we also found a ≈15 nm‐thick nanowire with a high degree of crystallinity and distinct lattice fringes whose corresponding d‐spacing directly matches the width of a single GaSI chain (8.6 Å) (Figure 3D).

Figure 3.

Structure and uniformity of bottom‐up grown 1D GaSI nanowires. A) Raman spectra of both bulk crystals (bottom, light blue), and bottom‐up grown nanowires (top, orange) of GaSI. B, left) Representative dark‐field optical micrograph of GaSI nanowires grown on a Si/SiO₂ substrate. Scale bar, 245 µm. (B, right) Representative dark‐field optical micrograph at high magnification depicting ultrathin GaSI nanowires grown on an Si/SiO₂ substrate. Scale bar, 25 µm. C) Low‐magnification 200 kV conventional TEM image of a single GaSI nanowire. Scale bar, 250 nm. D) High‐magnification 200 kV conventional TEM image of a nanowire that is ≈15 chains‐thick. The top half of the image is the raw micrograph, while the bottom half corresponds to a FFT filtered image. Scale bar, 10 nm. E) AFM height histogram of GaSI nanowires produced from solution exfoliation in isopropanol (top, pink), as grown in Zone 1 (middle, navy blue), and as grown in Zone 2 (bottom, light blue). F) 200 kV aberration corrected HRTEM image of a bottom‐up grown GaSI nanowire. Scale bar, 2 nm. G) FFT of the micrograph in (F), which shows that the nanowire orientation corresponds to the [701] zone axis.

To guide our synthetic understanding of the growth habits of GaSI from the vapor phase, upon inspection of the substrate post‐reaction, we were able to precisely map the nanostructures that were observed with 1 mm spatial resolution (Figure S9, Supporting Information). From these, the resulting nanowires showed pronounced 1D morphologies, and we were able to systematically map out four distinct growth regions based on the characteristic nanowire sizes and deposition densities that we observed. The two regions with the densest wire growth were found on the region of the substrate that was at a higher temperature and positioned at a closer distance to the precursors. We denoted these into two separate regions, Zone 1 and Zone 2, based on the quantitatively measured nanowire heights, with Zone 1 covering the end of the substrate at the highest temperature and the largest nanowires. To quantitatively measure the sizes of the wires and to compare these to the measured heights from GaSI nanowires obtained from top‐down exfoliation, we performed atomic force microscopy (AFM) of isolated GaSI nanowires derived from the two substrate regions (Zones 1 and 2) and from a liquid‐phase exfoliated nanowires of GaSI in isopropanol (i‐PrOH) (Figure 3E; Figure S10, Supporting Information). The AFM images, and the resulting height histogram, were taken from two substrates of GaSI nanowires exfoliated via sonication in i‐PrOH and from the respective Zones 1 and 2 of three substrates that contain bottom‐up grown GaSI nanowires. From these results, we found that the GaSI crystals that were exfoliated in i‐PrOH showed the widest distribution of nanowire sizes, with a median height of 102 nm. In contrast, the nanowires from Zone 2 had the smallest measured heights and a tighter height distribution, with a median height of 24 nm.

We conducted various electron microscopy experiments to further obtain high‐resolution images of the nanowires, however, it was apparent that GaSI nanostructures undergo significant beam‐induced damage under imaging conditions. While HRTEM imaging resolved the lateral dimensions of the resulting nanowires, we found that thinner nanowires appeared to be significantly more damaged under a 200 kV electron beam compared to thicker GaSI nanowires (Figure S8B, Supporting Information). We also employed 60 kV scanning transmission electron microscopy (STEM) to image these wires, although this condition quickly induced beam damage as well (Figure S11, Supporting Information). While these images corroborate the long‐range order and crystallinity of the GaSI nanowires, the 200 kV aberration corrected HRTEM imaging of a larger GaSI nanowire enabled us to extract the best quality and highest resolution images, which clearly show the helical motif in GaSI approaching the atomic scale (Figure 3F). From this micrograph, we found that the highest symmetry zone axis from the fast Fourier transform (FFT) of the image to be the [701] axis (Figure 3G). While this is an unusual axis that contains both an h (x‐axis) an l (z‐axis) component, when an h‐, k‐, or hk‐dominant orientation is expected, the high h‐index is still indicative that the nanowire lays generally flat on the grid (along the 1D long axis) with a slight tilt in the crystallographic z‐axis direction. To access nanowires with more prominent x‐ and y‐ components and further highlight their 1D helical chain character, we also collected TEM micrographs that were systematically tilted at 40⁰ from the horizontal plane. This tilted zone axis of the nanowire was indexable to the [112] axis that directly matches that of the bulk single crystal, and experimentally shows the 1D, wire‐like nature of GaSI nanowires (Figure S12, Supporting Information).^{[ 51 ]}

Based on the calculated Wulff shape (Figure 2), the observed bottom‐up growth habits and exfoliation behavior (Figures 1 and 3), and the equivalent inter‐chain distances (and interactions), the formation of the target 1D nanostructures was apparent under thermodynamically favored conditions. Typically, higher growth temperatures are considered to induce growth conditions that are within the thermodynamic regime. Thus, we posited that the deviation from such parameters we used to access the 1D nanowires could lead to non‐thermodynamically favored conditions and the growth of atypical (non‐nanowire) morphologies of GaSI. To induce the formation of the non‐thermodynamically favored nanostructures of GaSI, we lowered the maximum set temperature of T _hot and T _cold by 25 °C on each side to reach 475 and 375 °C, respectively (Figure 4 ; Figure S13, Supporting Information). By lowering both the growth and deposition temperatures, we observed the suppression of the growth along the covalent 1D axis and the promotion of the growth of GaSI chains laterally across the vdW direction. These conditions led, for the first time, to the formation of well‐defined quasi‐2D nanoribbons of GaSI (Figure 4A). These quasi‐2D nanoribbons were not observed in any of the reactions at the temperature gradient that was employed to grow the nanowires in Figure 2B. Under this distinct nonthermodynamically preferred growth condition, we found that there are notably fewer nanowires that formed alongside the more dominantly expressed nanoribbons arising from the, now preferred crystallization of the helical chains along the crystallographic y‐axis vdW direction instead of the thermodynamically preferred covalent z‐axis direction (Figure 4A; Figure S14, Supporting Information). Comparing the Raman spectra of the nanoribbons to the bulk enabled us to confirm the persistence of the GaSI crystal structure and long‐range order in these nanocrystals (Figure 4B). Looking into representativeSEM and AFM images, we show that the resulting nanoribbons are typically 3–5 µm wide, while being 50–100 nm thick (Figure 4C,D; Figures S15 and S16, Supporting Information). Using selected‐area electron diffraction (SAED) and HRTEM imaging, we were able to determine the preferred orientation of the nanoribbons, which we found to be the [100] zone axis (Figure 4E,F). To further corroborate this zone axis assignment, we took the high‐magnification HRTEM image of the nanoribbon with its corresponding FFT and found that it is also indexable to the [100] zone axis (Figure 4G,H). Taken together, these results are consistent with the calculated surface energies and Wulff structure of the GaSI crystal, where the (100) facet exhibits the most accessible surface of GaSI. The morphological control presented via the systematic tuning of the growth conditions indicates that further optimization of the growth conditions, including modifying precursor ratios or substrate engineering, could yield other novel and metastable morphologies or chiral enantiomorphic nanostructures of GaSI.

Figure 4.

Structure of bottom‐up grown quasi‐2D GaSI nanoribbons. A) Bright‐field optical micrograph of GaSI nanoribbons grown on a Si/SiO₂ substrate. Scale bar, 245 µm. The right shows a high magnification optical image of a nanoribbon. Scale bar, 6.25 µm. B) Comparative Raman spectra of bulk crystals (bottom, blue) and nanoribbons (top, pink) of GaSI. C) SEM image of a well‐defined GaSI nanoribbon. Scale bar, 5 µm. D) Representative AFM image of a ≈60 nm thick nanoribbon. Scale bar, 4 µm. E) Low‐magnification TEM image of an isolated GaSI nanoribbon. Scale bar, 500 nm. F) Selected‐area electron diffraction pattern of the nanoribbon depicted in (E), indicating the [100] zone axis. Scale bar, 2 nm⁻¹. G) High‐magnification TEM image of the nanoribbon depicted in (E). Scale bar, 5 nm. H) FFT of the TEM image in (G), which corroborates the [100] zone axis orientation of the nanoribbon. Scale bar, 2 nm⁻¹.

2.2. Optical Properties of GaSI Nanowires: Bandgap, Persistent Noncentrosymmetric Order, Structural Anisotropy, and Second Harmonic Generation

To primarily confirm the lack of center of inversion in the packing of the GaSI nanowires based on its parent bulk crystal structure, understand their chain‐level and nanoscale anisotropy, and probe their nanoscale non‐linear optical (NLO) activity in the UV–Vis region as a direct consequence of its crystallographic noncentrosymmetry, we characterized the optical properties of the resulting nanostructures and performed SHG microscopy experiments on the nanowires. First, we employed orientation‐dependent Raman spectroscopy to understand the structural anisotropy of GaSI crystals (Figure S5, Supporting Information). To obtain a large enough crystal with a flat facet, we initially used a bulk GaSI crystal as a reference crystal for these experiments. These measurements allowed for us to qualitatively assess the anisotropic nature of the bonding and the ensuing Raman phonon modes of the GaSI crystals (Supporting Discussion). For the mode at 110 cm⁻¹, we observed an approximate doubling in intensity from 0⁰ to 90⁰, consistent with an interchain vibration that couples more strongly when the laser polarization is perpendicular to the chain axis. In contrast, for the 297 cm⁻¹ mode, we observed an ≈2.5 times decrease in intensity from 0⁰ to 90⁰, indicating that this mode is predominantly intrachain in character. For the highest intensity mode at 245 cm⁻¹, the observed intensity did not significantly change, indicating either a symmetric mode along x‐ and z‐axes or an isotropic Raman mode. While these qualitative observations inform us about the anisotropy of GaSI crystals, the full angle‐ and orientation‐resolved Raman spectroscopy and complementary DFT calculations would be required to assign the phonon modes, which are extremely computationally expensive based on the GaSI unit cell size and are beyond the scope of our study. Similar studies have been done on the similar but higher symmetry InSeI structure, which further highlights the strong anisotropy in these materials.^{[ 84 ]} Taken together, the contrasting angular dependences of the Raman modes highlight the strong anisotropy of the helical 1D GaSI chains along inter‐chain and intra‐chain directions, reflecting the underlying 1D covalent chain structure held by weak vdW interactions.

To establish the persistence of the optical properties and bandgap of GaSI as it evolves from the bulk down to the nanoscale, we used diffuse reflectance spectroscopy (DRS) to determine the approximate bandgap of the GaSI nanowires, which confirms a similar bandgap in the UV‐A region energy as with the reported bulk bandgap of GaSI (Figure  5A). With the accessible visible‐to‐UV spectral window for SHG, we performed visible‐range SHG microscopy on both nanowires derived from micro‐mechanical exfoliation (Figure 5B) and through the bottom‐up synthetic route that we presented herein (Figure 5C; Figure S12, Supporting Information) to critically assess the persistence of the crystalline, non‐centrosymmetric order imparted by the helical order and packing that results in the P $\overline{4}$ space group from the bulk structure. The log–log plot clearly exhibits a slope of 2, consistent with a second‐order non‐linear optical susceptibility, χ⁽²⁾, process (Figure S18, Supporting Information). We found that both nanowires show pronounced, well‐defined, and spatially resolved SHG signals, indicative of the persistence of the noncentrosymmetric crystalline order of GaSI down to the nanoscale regime and, therefore, validating the potential NLO properties of the as‐grown nanowires.

Figure 5.

Optical properties of GaSI nanocrystals. A) DRS spectrum of bottom‐up grown GaSI nanocrystals plotted in terms of the Kubelka–Munk function, F(R_∞). B) Composite SHG micrograph of the forward (yellow) and epi (blue) channels taken at 800 nm excitation (detected at 400 nm) from a micro‐mechanically exfoliated GaSI microcrystallite. The forward (yellow) and epi (blue) channels are shown on the right. Scale bars, 20 µm. C) Composite SHG micrograph of the forward (yellow) and epi (blue) channels from bottom‐up grown GaSI nanowires. Scale bars, 10 µm.

Presented with this platform, we took advantage of the pronounced SHG signal of GaSI and qualitatively compared the signal of our GaSI nanowires with known standards. To the extent that we cover in the scope of our study—that is—to establish the non‐centrosymmetric nature and SHG activity of GaSI nanowires, we collected measurements of our signals with those from β‐Ba(BO₂)₂ (BBO) and lysozyme microcrystals under identical excitation and detection conditions to present a practical qualitative benchmark where the intensity of the SHG signal from GaSI nanowires is qualitatively comparable with these established NLO materials (Figures S19 and S20, Supporting Information). Finally, to further explore the directional dependence of the nonlinear response, we performed qualitative polarization‐resolved SHG imaging, which allows visualization of how the SHG intensity varies with the orientation of the excitation polarization relative to the nanowire axes. From Figure S21 (Supporting Information), we clearly see the strong polarization dependence of the forward SHG signal across many crystal samples. Together with the pronounced structural and optical anisotropy we observed in our polarization‐resolved Raman and SHG measurements, as well as the wide bandgap of GaSI in the UV regime, the literature reports on 1D vdW materials suggest that the helical, noncentrosymmetric nature of the GaSI structure—in the bulk, in nanoscale bundles, and, potentially, at the single chain regime—presents potential applications for nonlinear optical applications across length scales and a wide optical window, warranting further study of this material in this direction.^{[ 85} , ⁸⁶ , ⁸⁷ , ⁸⁸ , ^{89 ]}

2.3. Computational and Structural Signatures of Helicity‐Imposed Spin Polarization in Hypothetically Modeled 1D GaSI Chains

The realization and first demonstration of ultrathin nanostructures of GaSI, albeit not at the single‐chain regime, sets a logical precedence for a possible synthetic route that could lead to such enantiopure single‐chain helices if further optimized. Taking these results as inspiration, we took the opportunity and the knowledge gained from understanding the GaSI structure to theoretically assess how chirality alone could influence the electronic structure and potentially manifest helicity‐driven spin polarization using hypothetical single GaSI chains. We note that this model strictly serves as a conceptual, symmetry‐resolved computational diagnostic model to demonstrate the potential of such materials.

The crystal structure of GaSI comprises four helical chains, each bearing an unusual “squircular” helical cross‐section, and are held by weak vdW interactions.^{[ 51 ]} The squircular structure arises to relieve the repulsive interactions within the core Ga–S tubule and the angular strain introduced by the smaller bridging S atom. It is enabled by the highly reconfigurable corner‐sharing tetrahedral motif and further stabilized by van der Waals (vdW) contacts between adjacent, oppositely handed chains (Supporting Discussion). To understand how this conformational change affects the electronic behavior of GaSI, we computed the electronic states of chiral 1D GaSI chains (only left‐ and right‐handed) using DFT at the generalized gradient approximation with the Perdew‐Burke‐Ernzerhof (PBE) exchange‐correlation functional without the inclusion of spin‐orbit coupling (SOC), which is a similar approach which we also implemented in our previous report on bulk crystals but now applied to single chains (Figure S22, Supporting Information).^{[ 51 ]} By comparing the results from these 1D chains to those of the 3D bulk crystal structure of GaSI, we found that reducing the dimensionality from 3D to 1D induces a transition from a direct to an indirect semiconductor, with the valence band maximum (VBM) and the conduction band minimum (CBM) for the 1D chain to be at the Gamma point and the Z point, respectively. Moreover, the bandgap increases from 1.9 eV in the 3D structure^{[ 51 ]} to 2.55 eV in 1D using DFT‐PBE, and 2.9–3.26 eV using DFT‐HSE (Heyd‐Scuseria‐Ernzerhof hybrid functional) due to quantum confinement effects (Figure S23, Supporting Information). The density of states (DOS) calculations for the single chain indicate that, similarly to the bulk structure, valence bands are dominated by contributions from the p‐orbitals of iodine, while conduction bands are dominated by contributions from s‐orbitals of gallium and the p‐orbitals of iodine and sulfur (Figure S24, Supporting Information). We note that while the computed bandgap values may be underestimated due to the use of generalized gradient approximation, this level of accuracy seems to be sufficient for characterizing the electronic properties qualitatively, as demonstrated in similar materials, such as chiral 1D InSeI.^{[ 69 ]}

Given the lack of inversion symmetry and the presence of heavy elements (like iodine) in chiral 1D GaSI chains, SOC can potentially have a significant influence on their electronic structures. Hence, we calculated the electronic states of the two enantiomorphic configurations of 1D GaSI with left‐ and right‐handedness with the SOC effect included. Under these conditions, single‐chain GaSI remains an indirect bandgap semiconductor, with a slight decrease in its bandgap by 0.11 eV (Figure S25, Supporting Information). Although the effect of SOC on the bandgap is minimal, it leads to band splitting in both the valence and conduction bands (Figure S23, Supporting Information). For the band structure, with the inclusion of the SOC effect, the bands split except at the high symmetry points (Γ and Z).^{[ 89 ]} There is a stronger band splitting in valence bands because of the large contribution from the heavy elements with I p‐orbitals than the conduction bands (Figure S14, Supporting Information).

To determine the chirality dependence on the spin splitting, we computed the spin‐projected band structure for each spin operator (S_x, S_y, and S_z) for L‐ and R‐GaSI. Consistent with prior studies on halogen‐based systems with spin‐orbit coupling‐driven band splitting and InSeI, a related halogenated III–VI helical chain where the PBE functional has been successfully used to computationally capture spin textures and CISS effects, we employ the same functional to compute chirality‐induced energy level splitting and spin textures in GaSI helices.^{[ 69} , ⁷⁹ , ⁹⁰ , ⁹¹ , ^{92 ]} We found that the largest contribution to the spin polarization occurs when the polarized spins are aligned along the chain direction, 〈S_z〉, whereas the spin polarizations within the cross‐sectional area (〈S_x〉 and 〈S_y〉) are negligible. (Figure  6A–D; Figures S25 and S26, Supporting Information). The band structures with the 〈S_z〉, values of left‐ and right‐handed 1D GaSI are plotted in Figure 6A–D. We can see that left‐ and right‐handed 1D GaSI exhibit opposite spin characteristics at the same wavevector and band index, consistent with handedness‐dependent spin textures predicted for such helical systems. Additionally, the wavefunctions for both the valence and conduction bands at the Gamma point were dominated by the p_z ‐orbitals of iodine, which are tilted to each other and rotate along the helical axis (Figure  7 ), yielding a net polarization along the z‐axis.^{[ 70 ]} These wavefunctions, especially for the VBM, are different from their counterparts in the bulk phase, where the iodine p‐orbitals in the VBM span along the cross‐sectional area (xy‐plane) rather than the z‐direction (Figure S27, Supporting Information). As the material transitions from the bulk to the single chain, there is an apparent loss of cross‐sectional electronic interactions, enabling helicity‐imposed and spin‐polarized features to emerge in the chain direction. The spin textures of the first and second valence and conduction bands show that along k_z, the polarization direction for L‐ and R‐GaSI is opposite, suggesting that helicity‐ and handedness‐imposed spin‐polarized phenomena could be realized in single 1D GaSI chains. (Figure 6E,F). We stress that, while these signatures remain theoretical, they underscore the potential of helical III–VI–VII materials to host chirality‐induced physical states once synthetic access to few‐ or single‐chain nanostructures is achieved. The strategy outlined within this work indicates a promising method to access ultrathin nanostructures of these materials and opens the door for synthetic approaches to be further developed and optimized to realize the chiral, single‐chain analogues of GaSI and related phases. In future work, various routes based on this strategy can be implemented for the growth of chiral, enantiopure analogues of these nanostructures. Some future directions of growth include substrate engineering, chiral molecule intercalation, using chiral catalysts, or using applied fields.^{[ 73 ]}

Figure 6.

Chirality‐induced spin splitting in 1D GaSI. Calculated band structures of left‐ A,C) and right‐handed 1D GaSI B,D) with the expectation values of the spin operator in the z‐direction, 〈S_z〉 (in units of ℏ), on the electronic states as shown in the color scale. Energies levels are plotted with reference to the Fermi level (E _F) and considering SOC effects. E,F) Spin textures for the first two valence and conduction bands of L‐GaSI (E) and R‐GaSI (F).

Figure 7.

Isosurface plots for chiral 1D GaSI single chains. Wavefunctions of L‐GaSI at the valence band maximum, VBM A), and the conduction band minimum, CBM B). Wavefunctions of R‐GaSI at the VBM C) and CBM D). Green and orange regions represent positive and negative signs of wavefunctions, respectively.

3. Conclusion

In this work, we present a catalyst‐free growth strategy to demonstrate, for the first time, dimensionally resolved nanostructures of a model structure in the helical III‐VI‐VII class of all‐inorganic 1D vdW crystals. Through the synthesis of 1D nanowires and quasi‐2D nanoribbons of the model phase, GaSI, we highlight the facile route to access various morphologies with ultrathin nanocrystal sizes more reliably than with the usually stochastic top‐down exfoliation methods. The synthesis of highly crystalline and ultrathin GaSI nanostructures enabled our understanding of the persistence of the crystalline order, lack of center of inversion, helicity, and electronic structure in this phase, down to the nanoscale, and how it can be potentially utilized as a highly anisotropic nanoscale optical and optoelectronic material across a broad optical window. The experimental realization of such nanostructures reaching the sub‐10 nm regime inspired our computational studies to look into the emergence of helicity‐imposed physical states, leading to our computational and conceptual depiction of handedness‐dependent spin polarization in hypothetical single chains of GaSI. Altogether, the synthetic breakthrough and computational studies that we demonstrated herein lays the foundation toward the realization of chiral, enantiopure nanocrystals from bottom‐up routes, the systematic probing of size dependent properties, and the potential of realizing helicity‐driven physical states in this underexplored class of helical crystals.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Dold K. G., Spencer J., Milligan G. M., et al. “Dimensionally Resolved Nanostructures of an Atomically Precise and Optically Active 1D van der Waals Helix.” Adv. Mater. 38, no. 8 (2026): e10230. 10.1002/adma.202510230

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.