Self‐Assembly for Creating Vertically‐Aligned Graphene Micro Helices with Monolayer Graphene as Chiral Metamaterials

Graphene's emergence enables creating chiral metamaterials in helical shapes for terahertz (THz) applications, overcoming material limitations. However, practical implementation remains theoretical due to fabrication challenges. This paper introduces a dual‐component self‐assembly technique that enables creating vertically‐aligned continuous monolayer graphene helices at microscale with great flexibility and high controllability. This assembly process not only facilitates the creation of 3D microstructures, but also positions the 3D structures from a horizontal to a vertical orientation, achieving an aspect ratio (height/width) of ≈2700. As a result, an array of vertically‐aligned graphene helices is formed, reaching up to 4 mm in height, which is equivalent to 4 million times the height of monolayer graphene. The benefit of these 3D chiral structures made from graphene is their capability to infinitely extend in height, interacting with light in ways that are not possible with traditional 2D layering methods. Such an impressive height elevates a level of interaction with light that far surpasses what is achievable with traditional 2D layering methods, resulting in a notable enhancement of optical chirality properties. This approach is applicable to various 2D materials, promising advancements in innovative research and diverse applications across fields.


Introduction
Nature's breathtaking helical (spiral) structures, ranging from minuscule DNA strands to vast galaxies, captivate researchers with their distinctive 3D forms and their unique physical properties such as precession process, [1] Gaussian surface [2] and optical vortex. [3]This fascination drives comprehensive investigations across diverse fields, including mathematics, [2] materials with the light propagation direction in parallel, and the height of helix (meaning the length that couples with the light in parallel) should be at least a few hundred nanometers. [23]However, it is only limited to numerical analysis due to the fabrication difficulty.In reality, transfering graphene monolayer thousands of times at an exact specific position to satisfy the helix scaffold with hundreds of nanometer thick is not practical.
To address the difficulty of the twisted stacking fabrication approach, [24] an initially seemingly impossible idea is generated: is it possible to let the monolayer graphene, as a separate whole, self-twist to the 3D graphene helix?16c,25] In detail, because the propagation of the circularly polarized light (CPL) occurs through the torsion of the field vectors, [10b] the parallel direction between the incident light and the graphene helix greatly extend the interaction distance between each other.Meanwhile, this vertically-aligned direction largely decoupled the side effect from the substrate. [26]Besides, to fully utilize its inherent optoelectronic properties, it is crucial to employ a continuous, complete, and uniform monolayer of graphene.Using a continuous graphene layer instead of connected pieces provides an uninterrupted pathway that can continuously support the surface current transporting through the surface and for photo-generated carriers, reducing energy losses that can occur at junctions or boundaries. [27]Moreover, it is imperative that the whole graphene surface, not just a portion, is aligned parallel to the direction of the incident light.This condition is key to enhancing interaction, achieving a consistent and reliable optical response, and avoiding interference.Therefore, to satisfy the aforementioned requirement for better chiral sensitivity, a new fabrication approach for 3D verticallyaligned monolayer graphene micro helix is under significant demand.
Numerous fabrication methods have emerged that can produce vertically-aligned 3D structures.Of these contemporary techniques, 3D printing is a promising avenue for fabricating the antenna-shaped graphene helix. [28]However, fabrica-tion of graphene helix with only a monolayer thick configuration cannot be realized.Moreover, the prepared graphene dispersion with aim at preserving its pristine property with a few layers thick mainly relies on sonication with cavitationbubbles' mechanism. [29]This method provides the best exfoliation of graphene, but it still remains a challenge to obtain monolayer graphene. [30]Furthermore, the large defects brought to the graphene during the long-time sonication are still under investigation. [30]More importantly, the printed 3D helix does not consist of continuous graphene layers, and the graphene surface inside the printed shape is randomly oriented [31] which weakens the overall optical effect. [32]To address the above issue, in this report, rather than using printing to produce vertically-aligned 3D graphene, we develop a dual-component self-assembly method with a continuous single layer graphene transferred on a silicon wafer.The first portion shapes 3D structures (cylindrical helix), simultaneously, the second positions 3D structures from a horizontal to a vertical orientation, resulting in a vertically-aligned graphene helix array (Figure 1).Through meticulous numerical analysis, we've designed the geometrical parameters essential for crafting the 3D vertically-aligned graphene helix using this method.Furthermore, to explore the chiral properties of the 3D graphene helix, simulations have been executed, with the results distinctly emphasizing its unique chiral characteristics (Figure 1a).This 3D self-assembly approach enables the positioning of continuous monolayer graphene onto 3D microstructures while preserving its pristine physical/material properties.This fusion of 3D fabrication with 2D materials could potentially harness the distinct physical characteristics of magic angle graphene [33] and the 3D graphene molecular sieve. [34]The approach presented in this paper is pioneering, as it is the first to allow continuous monolayer graphene to self-assemble into a vertically-aligned helix structure for optical chirality instead of commonly reported twisted stacking, [24] which is not a continuous layer and very time consuming.Moreover, this innovative fabrication method can be applied to additional 2D materials, such as hBN and MXene, significantly enhancing the research on these new vertically-aligned 3D structures made from 2D materials and promoting a wide range of applications in various fields.In situ monitored self-assembly process of SU-8 tubular structures.h-j) Schematic and a real experimental image of a micro scorpion h) before and i) after self-assembled to 3D j) in large array.k) 3D tubular structures intentionally designed in vertical direction with graphene, l) in large array, m) with a well-controlled angle.n) Tubular structures with different lengths in water.o) A 1.5 cm long vertically-aligned tube.

Results and Discussion
To create the microscale vertically-aligned monolayer graphene helix, two distinct patterns were applied in regions 1 and 2 (Figure 1b).Specifically, region 1 is designated for helix creation, while region 2 transforms the helix to a vertically-aligned setup and these processes happen simultaneously.The graphene was prepatterned on a sacrificial layer (copper surface) under region 1 only (Figure 1b), followed by SU-8 coating and baking process.Next, the photolithography process with ultraviolet (UV) light at a wavelength of 365 nm was carefully designed, where all areas in both regions 1 and 2 were first less exposed, then some areas with pre-defined shape were fully exposed (dark blue color in Figure 1b) with separate photomasks.After a developing process, region 1 was released from the substrate by etching the copper layer underneath (Figure 1b).The SU-8 released in region 1, serving as the primary driving force (as detailed in Figure 2), undergoes a twist and morphs into a 3D cylindrical helix in conjunction with graphene.Simultaneously, the less exposed area in region 2 (light blue color, Figure 1b), acting as the micro "hinge", drives the graphene helix gradually from the x-y 2D plane to the vertical direction (Figure 1c).Constrained by the geometrical design (less/fully UV exposed area in region 2), the graphene helix finally stops in a vertically-aligned direction, resulting in vertically-aligned graphene helix (Figure 1d,e).An array of the self-assembled vertically-aligned helices can be simultaneously realized on a large scale with a high assembly yield (Figure 1f), as evidenced by the successful fabrication of vertically-aligned helical structures on a four-inch wafer (later will be discussed).The step-by-step details of the fabrication procedures and experimental operations are described in Supporting Information (Figure S1, Supporting Information).
A previous study reported that, by using the solvent (acetone), low-dose exposed SU-8 can be curved into 3D structures. [35]pecifically, acetone was used to remove an uncross-linked part of the SU-8 near its bottom surface, followed by immersing the rest of the SU-8 in water.The SU-8 film undergoes great contraction toward the less-cross-linked region, thus curving downward. [35]he method for curving SU-8 presented in this paper, however, is based on a distinct working mechanism.Our method involves a self-assembly where, by exposing monolayer photoresist (SU-8) to a heat source with temperature of 70 °C, the SU-8 can be made to curve upward.A SU-8 film exposed with low dose (E≈170 mJ cm −2 for a ≈11 μm thick film) forms a cross-linking gradient along the thickness direction, in other words, from the matrix polymerization perspective, larger shrinkage occurs near the top of the film while comparably weaker shrinkage occurs near the bottom [36] (Figure 2a, see Figure S2, Supporting Information attenuated total reflectance measurement for proof).The variation of the volume changes along the depth generates stress gradient forming curving forces (F c ), resulting in curvature curving up (Figure 2b,c).However, due to the elastic restoring force (F r ) of SU-8 (F r > F c ), little deformation in the 2D pattern can be observed after being released from the sacrificial layer.The force (F c ) arising from the stress gradient, intended to curve the SU-8 film, is inadequate to induce a lasting curvature.The primary resistance to this curving is the elastic restoring force of the SU-8.To induce curvature, it is essential to either reduce the elastic restoring force (F r ↓) or enhance the curving force (F c ↑) created by the stress gradient due to cross-linkage, resulting in F r < F c .Yet, given that the stress produced by cross-linkage is not sufficient to surpass the elastic restoring force under room temperature.Therefore, we managed to reduce the SU-8's elastic restoring force (F r ) by softening the SU-8 using heat, either in air or water.The softened SU-8 diminishes its elastic restoring force (F r ↓). [37]With this modification, the stress generated by cross-linkage becomes adequate to overcome the now-reduced elastic restoring force (F r < F c ), leading to an upward curvature of the SU-8 (Figure 2b).Once the flat SU-8 film is converted into 3D shapes, the sample is picked up from the water for cooling down immediately.As the temperature decreases rapidly, the elastic restoring force (F r ) of SU-8 (essentially a thermally sensitive epoxy, aiming to preserve deformed shapes) [37] strengthens and surpasses the curving force (F c ).Consequently, the 3D shapes become permanently set or "frozen".As proof of concept, an array of tubular structures was fabricated (Figure 2d-g).It is worth mentioning that the mechanism of this fabrication approach is not due to polymer swelling or liquid diffusion in SU-8 which were heavily investigated in previous studies. [35,38]ur control experiment indicates that the substrate-free 2D pattern, when being heated up in the air, also results in self-curving, which confirms the lack of swelling and liquid diffusion mechanism (see Video S1, Supporting Information and the Supporting Information file for the video's detailed description).The detailed mechanisms presented in this paper will be elaborated upon in a subsequent publication.
By selectively offering the low/high dose exposure, resulting in less/full cross-linkage in different exposure regions, diverse 3D microstructures can be realized (Figure 2c-o).Several advantages of this fabrication approach should be clarified here.First, many stimuli-responsive materials cannot maintain their 3D shape after the stimulus source is removed, [39] but ours, as aforementioned, can be "locked" in its 3D form.Second, compared with 3D printing, [40] this photolithography-based method can easily make wafer-scale 2D patterns and then curve everything up in minutes with ultrathin film thickness (A 1.8 μm thick film was tested, which also has curving behavior under proper conditions).Third, different patterns or designs with other materials can be created on the surface of this photosensitive material, and the desired dose condition can be easily applied.Moreover, multidirectional self-assembly process can be pre-programed and then easily realized (Figure 2c,h-o).A 3D scorpion was fabricated as an example (Figure 2h-j).By varying the exposure levels (≈170 mJ cm −2 for less exposure and 390 mJ cm −2 for full exposure) on different regions of the 2D pattern (Figure 2h, with a film thickness of ≈11 μm), we could simultaneously create 1800 submillimeter-sized scorpions with parts that curve or bend in various directions.Remarkably, the entire fabrication process, from spin-coating photoresist to curving everything up, takes less than 30 min (Figure 2i,j, see Figure S3a, Supporting Information for details).With a diameter of ≈500 μm, the tail is capable of achieving a curve of ≈170°, which shows excellent mechanical properties of SU-8.Owing to the mechanical stability, by incorporating some active materials functioning as an actuator onto scorpions, micro robotics might be realized.The substantial angular deformation seen in a scorpion's tail motivates us to refine this method, enabling the conversion of horizontally aligned tubular structures (Figure 2g) into vertically-aligned graphene-based tubular configurations (Figure 2k).The UV dose combined with the pre-determined design of 2D patterns enables precise manipulation of the angle in the z-direction to achieve a specific, targeted value.(Figure 2k-m, details are shown in Figure 3).By utilizing the buoyancy force, vertically-aligned micro tubular structures with an extremely high aspect ratio of ≈2700 (having lengths of 30 mm width −1 of ≈11 μm) were achieved in water (Figure 2n).Additionally, a structure with an aspect ratio of about 1400 was able to maintain its vertical orientation in air even after the water drying process, which typically induces strong capillary forces and may collapse such tubular structures (Figure 2o and Figure S3b, Supporting Information).
The dual-component self-assembly method facilitates setting all necessary parameters on a 2D planar surface in advance (Figure 3).These parameters are crucial for defining the structure of vertically-aligned graphene helices, which are types of chiral metamaterials.As previously reported, [23] obtaining a strong Circular Dichroism (CD) signal should satisfy three geometric conditions.First, a large graphene surface area should be aligned with the incident CPL propagation direction in parallel. [23]Second, the length of graphene surface along the light propagation direction should be at least a few hundred nanometers. [23]Third, for optimal interaction with incident CPL, the helix's geometric parameters must be matched with the physical characteristics of the CPL. [23]Our assembly process is tailored to facilitate the easy achievement of specific geometric parameters, such as radius, pitch, total length, helix angle, and vertical angle.This precision in geometry is made possible through the strategic design of 2D net ribbons and the careful control of cross-linkage depth before proceeding to the 3D self-assembly.
The creation of helical structures in this process begins with a 2D ribbon subjected to a lower degree of cross-linking.Subsequently, those sections that are completely cross-linked, shown in dark blue in Figure 3a-c, are intentionally patterned with a misorientation angle (ϕ).This step is crucial for creating the helical structures.The parts that are fully cross-linked are quite rigid and resist curving, causing the ribbon to naturally curve toward the direction of the misorientation ϕ (Figure 3c).This process concludes with the self-assembly of the 2D pattern into a 3D helix.Simultaneously, this indicates that, by varying the angle ϕ, the helix connecting with a large anchor would deflect to different angles between the anchor and the helix (as indicated by  in Figure 3e-g).Therefore, to realize the perpendicular direction between the helix and the anchor ( = 90°) which is subsequently purposed for vertical direction, when ϕ is set to be 45°, a pre-set angle  (shown in Figure 3b) must be precisely designed (Figure 3e-g).Notably, the misorientation angle ϕ is also a determining factor for the pitch, radius, and number of turns for the helix (Figure 3h-j).Thus, achieving vertically-aligned helices necessitates careful consideration and design of the angles of ϕ and .With a preset misorientation angle ϕ = 45°but varying pre-designed angle , the angle  is obtained after self-assembly (Figure 3e-g) and the measurement and predicted angles are plotted (Figure 3k).The  value predicted and the one obtained from measurement display a high level of agreement.This result indicates that when ϕ = 45°,  should be pre-set as 45°, then the helix becomes perpendicular to the large 2D anchor, representing  = 90°and would finally curve upward without incline.From another perspective, by fixing the angle  but varies ϕ, the deflection angle can also be changed (Figure 3h-j).When two extreme cases are given: which are ϕ equal to 90°or 0°, it can be predicted that the 2D pattern would self-curve to a long tubular structure or form a ring with large radius, respectively.Within the range of these two extreme misorientation angles, the radius and pitch of the helix can be finely tuned to different geometric shapes.The outcomes of these adjustments are detailed in Figure 3l.This data underscores the ability to precisely manipulate the geometry of the helix by altering the angles  and ϕ.It is important to highlight that these helices created using our method can be broadly regarded as cylindrical helices with zero Gaussian curvature. [41]his aspect greatly simplifies mathematical analysis for the prediction of certain geometrical parameters like helix angle, pitch, and radius.For a more comprehensive numerical analysis and detailed derivation, refer to Figure S4 and S5, Supporting Information.
As 2D patterns transform into 3D cylindrical helices, they simultaneously undergo vertical transformation owing to the selfcurving of a less cross-linked anchor section in region 2, as indicated in Figures 1b and 3d.The vertical angle between the helix and the substrate is adjustable either by altering the width (W) of the less cross-linked anchor in region 2, or through the application of different UV doses (Figure 3m-p).Under the same dose (D 1 = D 2 ) but with different widths (W 1 < W 2 ), longer beam can curve with a larger curving angle compared with the shorter beam under the same stress and stiffness, despite with the same radius of curvature (R 1 = R 2 ) (Figure 3d).On the contrary, two beams with the same width (W 1 = W 3 ) but the one that received lower dose (D 3 ) curves more compared with the one that received high UV dose (D 1 ) (Figure 3d) because of a higher stress gradient formed under less dose exposure.The correlation between width (W) and UV dose in relation to the folding angle implies multiple methods for achieving a specific vertical angle through various combinations of width (W) and UV dose.For example, based on our experimental data, to achieve a 90°folding angle with an 11 μm thick SU-8, certain combinations like W = 100 μm with a dose of ≈134 mJ cm −2 , W = 160 μm with ≈150 mJ cm −2 , or W = 190 μm with ≈174 mJ cm −2 are necessary (Figure 3m,n).Note that the unusual angle saturated at ≈110°when dose is 134 mJ cm −2 with larger width is due to our geometric design, where the helix at both sides of the anchor (Figure 3a) self-folds over 90°and touches each other.This results in the folding angle to be stopped at ≈110°(Figure 3m).Additionally, a curve-fitting plot depicting width versus dose at a 90°vertical (folding) angle, along with its corresponding equation shown in Figure 3o, provides clear guidance for selecting the right parameters to achieve the desired vertical orientation.Expanding on this concept, the 3D plot displayed in Figure 3p provides a clear and detailed visual depiction of the interconnected relationship between width, dose, and folding angle.This visual aid significantly contributes to a deeper comprehension of their dynamic interaction.
From the characterization of curving behaviors depicted in Figure 3, designs for 2D nets of vertically-aligned helical structures can be implemented on a substrate.The curving processes for both vertical and helical formation were observed in realtime, correlating with temperature changes (Figure 4a-l, the realtime video is available in Video S2, Supporting Information).As the temperature rose from 50 to 68 °C over a period of 10 min, ribbons exhibited both twisting and upward folding along the z-direction (Figure 4a-j).In the initial 4 min, the water temperature increases rapidly (3.3 °C min −1 ) to 64 °C, causing the SU-8 photoresist to soften quickly and curve with high angular velocity due to reduced stiffness and the conversion of a large amount of stored energy originating from stress gradient to kinetic (Figure 4k).After 4 min, from 64 °C onward, the temperature rise slows considerably (0.7 °C min −1 ) over the next 4 min because the hotplate temperature closely approaches the set temperature.This means the reduction of stiffness is not as obvious as the first 4 min. [37]Moreover, the released film stress is not as large as the initial stage.These factors result in the maximum angular velocity occurring around 4 min.During the entire process, the released film stress and moment of inertia compete with the heat-affected stiffness, eventually stabilizing at a 90°equilibrium.It is important to note that, near the end of this upward folding process, the folding angle does not significantly increase further even though the temperature continues to rise, as the SU-8 has already softened enough and is not able to exert film stress for shape change.The heat source was subsequently removed and a large heat exchange from the sample to the air drastically increased the stiffness of the SU-8, causing the 3D shapes to become permanently "frozen".As anticipated from the folding angle analysis in Figure 3m, the folding angular velocity of a sample (190 μm in width with a dose of ≈174 mJ cm −2 ) slows down fairly at a 90°folding angle, eventually stabilizing at this final folding angle (Figure 4k).Concurrently, the transition of the self-assembly process from a ribbon pattern into a cylindrical helix reaches its completion (Figure 4f-j,l).To visualize the twisting behavior with a better perspective, the upward folding direction was constrained, and the twisting behavior was closely monitored (Figure 4f-j,l-n, the real-time video is available in Video S3, Supporting Information).As depicted in Figure 3, the misorientation angle ϕ is one of the keys in defining certain geometrical aspects of a helix, like its pitch.Furthermore, the twisting behavior, which influences the pitch and the number of turns, can be effectively controlled through the UV exposure dose (Figure 4m) and the patterns of the exposed area (Figure 4n).A higher dose results in reduced internal stress but increased stiffness, resulting in a decrease in the number of turns and an increase in pitch value (Figure 4m).Similarly, the width ratio of less/fully cross-linked sections of a pitch P (illustrated as A:B in Figures 3b and 4n) also plays a crucial role in determining the overall stress of an entire ribbon.A higher ratio (A:B) implies a greater width of the stressdriven part compared to the width of the rigid section.When this ratio changes from 1:1 to 4:1, while keeping the dose constant, the number of turns escalates from 0.6 to 3.2, and the pitch diminishes from 2400 μm to half of that value (Figure 4n).
By implementing dual-component self-assembly, which incorporates both twisting and vertical standing techniques, the creation of vertically-aligned helical structures was effectively achieved.These structures were created in versions both incor-porating a monolayer of graphene (Figure 5a-d) and without it (Figure 5e).Notably, this self-assembly approach provides considerable stability in the fabrication of vertically-aligned 3D structures with high aspect ratios.An ≈11 μm thick layer of SU-8 supports a 6.5 mm tall hollow helix, achieving an aspect ratio of ≈600:1 (Figure 5e).It is important to point out that our assembly process enables the creation of vertically-aligned monolayer graphene helix up to ≈4 mm in height (Figure 5d).This means the structure is 4 million times (4 × 10 6 ) taller than a standard graphene monolayer, which is typically 1 nm thick.It is believed that further optimization experiments can extend this height to the centimeter level.Such an impressive height elevates a level of interaction with light that far surpasses what is achievable with traditional 2D layering methods which is typically defined on a substrate in 2D like graphene nanodisks [23] and aligned at a 90°a ngle to the incoming light.Augmented interaction with light is essential for achieving enhanced optical chirality (as detailed in Figure 6).Furthermore, it is worth mentioning that the fabricated helices can maintain their original shape for years without undergoing any deformation or collapse.
A shortcoming of standard confocal bright field microscopy is its insufficiency for detailed observation of transparent graphene/SU-8.To address this issue, scanning electron microscopy (SEM) images were captured to observe the opaque surface of helices in both perpendicular (Figure 5f) and side view (Figure 5g).Besides, the curving/anchor part at the bot-tom is also clearly visualized (Figure 5f inset).It can be directly observed that the fully cross-linked part acts as the anchor while the less cross-linked part curves up and stops almost vertically, together with the helix structure connected with it (Figure 5f,g).This strongly supports our theoretical hypothesis shown in Figure 3d-m.Close-up views from SEM images reveal a smooth exterior on the helix's surface (Figure 5h and Figure S6, Supporting Information).This smoothness suggests the mechanical stability of the structure, emphasizing the absence of wrinkles and crumples in the graphene, as well as no delamination of graphene from the SU-8 during the self-assembly process.Additionally, UV fluorescent images were also captured to identify potential debris or residues, such as copper etchant, which might not be visible in SEM images. [42]The fluorescent images confirm the pristine condition of both the inner and outer surfaces of the helices (Figure 5i,j), verifying their cleanliness and absence of contamination.Conversely, this method revealed copper etchant contamination on the substrate, which is observable under the UV source (Figure 5k).
Our assembly technique is notable for its scalability and can be effectively applied on a wafer scale through a parallel process, offering both stability and control.In a span of just 10 min, it is possible to self-assemble hundreds of helices into a verticallyaligned orientation on a 4-inch wafer with an impressive yield of over 90% (Figure 5l, the red circle highlights the position held by tweezers, which caused some breakage of helices near it.This can further be solved by using a critical point dryer).Following the self-assembly process, the properties of the graphene helix were analyzed by Raman spectroscopy on the curved surface (Figure 5m-o).The helix with monolayer graphene displays a strong and prominent graphene 2D peak at 2690 cm −1 (Figure 5m).Contrary to the sample of SU-8/graphene, the bare SU-8 photoresist shows no Raman peak at this wavenumber.This absence indicates that the observed peak at 2690 cm −1 is characteristic of the 2D band of graphene.On the other hand, observing another G peak is challenging due to its overlapping with the chemical peaks of SU-8.The outcomes from the Raman measurements offer clear insights into the quality of the graphene, serving as direct evidence that our selfassembly method effectively preserves graphene's inherent physical and material properties.Moreover, to assess the uniformity of graphene distribution across the entire helix surface, the Raman laser was moved along the curved surface of the graphene helix, from the base to the top.Throughout this scanning process, the position and its corresponding intensity of the 2D peak signal after normalization were recorded (Figure 5n).Besides, the curve fit plot based on the coordinates of these scatter points was generated and reconstructed with "the least squares method" (Figure 5n,o, see Supporting Information for details).Throughout the entire 3D graphene helix, strong 2D peaks were detected, which robustly indicates that the graphene is uniformly distributed across the whole surface (Figure 5o).To further strengthen this argumentation, the SEM charging experiment was performed to irradiate in situ monitored electron beam (e-beam) on the non-conductive material (SU-8) coated with/without graphene, resulting in an obvious difference in the charging effect.Details are shown in Figure S6, Supporting Information.These results of Raman measurement and SEM test strongly confirm that the self-assembly process applied for the formation of vertically-aligned graphene helix effectively retains the original properties of graphene, ensuring that the monolayer graphene is uniformly applied over the entire surface of the 3D helix.
In addition to self-assembling graphene helices on the Si substrate, this fabrication approach can be well applicable to other substrates such as CaF 2 (Figure 6a), which is optically transparent in the THz range.The optical chirality property of vertically-aligned graphene helices was examined in the THz range through the simulator (Figure 6b-h).Compared with 2D objects, 3D objects are widely believed to have a stronger optical chirality effect because of the original reason from chirality. [8]mong 3D objects, the vertically-aligned helix has been predicted and confirmed to exhibit strong optical chirality.The main reason for this is that CPL has a more effective interaction with the helix structure, [8] due to both its geometric resemblance and the prolonged optical path that facilitates interaction.However, to the best of our knowledge, vertically-aligned helix with continuous monolayer 2D materials on it has not been constructed or proposed from both numeral analysis and experimental verification perspective.Here, toward the vertically-aligned monolayer graphene helix, we fabricated, simulation using COMSOL Multiphysics was carried out.Under left-and right-handed circularly polarized light (LCP and RCP), the optical response of the same right-handed vertically-aligned graphene helix with a diameter of 100 μm was simulated (Figure 6b, see Figure S7, Supporting Information for detailed modeling).Under the illumination of LCP and RCP light respectively, the RCP light couples more effectively with the designed right-handed graphene (R-HG) helix, causing lower transmission, in other words, more absorption at their resonant frequency (Figure 6b).This optical chirality performance was further analyzed and understood by depicting electric field enhancement and surface current density at each resonant frequency shown in Figure 6b inset and c.Compared with LCP coupling result with the highest surface current density at the top and then forming a decreasing gradient toward the bottom, the RCP that coupled with R-HG helix displays more uniform surface current distribution.This is due to the geometric similarity and indicates both adequate and better interaction than LCP at the resonant frequency.The resonant frequency of a verticallyaligned graphene helix can be effectively modulated by varying its diameter (Figure 6d).A helix with 120 μm diameter is shown in the inset.By optimizing some parameters in the future work, such as reducing the thickness and width of SU-8 pattern, the size of the helix can be reduced to tens of micrometers.To further reduce the size, it could be possible to fabricate via nano self-assembly techniques that first introduced by our research group. [43]To underscore the significance of a vertically-aligned orientation, simulations were performed by altering the angle between the helix and the substrate.These simulations assessed the CD and g-Factor (dissymmetric factor, with further details in the Supporting Information) in relation to the angle.When the orientation of the helix changes from parallel to perpendicular in relation to the substrate, there is a significant increase in CD (Figure 6e) and the g-Factor (Figure 6f).This enhancement is due to the longer path length of graphene helix for light interaction in the parallel orientation created by 90 °angle between the helix and the substrate.In comparison to a graphene helix with the same dimensions and geometries, operating within the same frequency range, a helix composed entirely of SU-8 structure demonstrates a markedly reduced CD response (Figure S8, Supporting Information).The superiority of vertically-aligned, graphene-based 3D chiral structures lies in their ability to achieve significantly greater heights than conventional 2D layering techniques, such as stacking graphene disks on a substrate. [23]This vertical alignment facilitates a unique interaction with incident light, something beyond the reach of conventional 2D layering.It enables a longer path length for light interaction within the graphene helix, thereby enhancing asymmetric absorption.This, in turn, leads to improved CD and g-Factor.In further substantiation of this evidence, a simulation was performed to assess the correlation between the CD/g-Factor property and the number of turns, indicative of varying heights in the vertical direction.The simulation results indicate that with an increase in height, there is a corresponding rise in the CD and g-Factor value (Figure 6g).This can be attributed to two key factors.First, the increased height offers a longer distance, enhancing the interaction with light, and second, a higher number of turns leads to a more pronounced disruption of symmetry, resulting in elevated CD values and higher g-Factor.
One significant advantage of this self-assembly method over 3D printing is its ability to first transfer a monolayer of highquality pristine graphene onto a 2D surface (substrate), which then self-assembles vertically along with the driving layer.This ensures that the graphene remains a continuous monolayer across the entire helical surface, unlike the fragmented pieces created by sonicated precursors in 3D printing. [29]The critical importance of this unbroken layer was tested by applying random defects in the vertically-aligned graphene helix.Each defect scenario is simulated three times to minimize discrepancies (details provided in the Supporting Information).When the level of defects approaches roughly 20%, there is a marked reduction in both CD and g-Factor, with CD values dropping 94% and g-Factor decreasing 73% (Figure 6h,i), underscoring the importance of a continuous graphene layer covering entire surface of the helix without defect.
We should point out that the optical chirality effect is not that strong compared with conventional metal-based MEMS [44] and graphene-metal hybrid system, [22] However, our system consists of only a monolayer graphene and then self-assembles to a 3D vertically-aligned helix without any auxiliary from the metal for enhanced optical response, although the later one is commonly known to have extremely strong dispersion and absorption in the THz range. [45]Even though, the future outlook should be briefly discussed here.First, the 3D graphene-metal hybrid system may reinforce the optical response compared with 2D system which is worth testing.46a] The next step in our research involves the real experimental characterization and further device fabrication.

Conclusion
A dual-component self-assembly method has been developed to create vertically-aligned monolayer graphene helices at the microscale.This assembly process not only enables the creation of 3D microstructures with a high aspect ratio of around 2700 but also provides substantial flexibility for 3D design.It achieves this by allowing the pre-setting of parameters on a 2D pattern, which then evolves into intricate 3D structures.This method is particularly effective in forming micro graphene helices, positioning them vertically in a way that preserves their material integrity and significantly enhances their optical chirality response.
The potential applications of this fabrication method extend beyond graphene.It could be adapted for use with other 2D materials, such as hBN and MXene, opening up new possibilities for research into these novel 2D materials-based, vertically-aligned 3D micro and nanostructures.This approach promises to drive forward the exploration and development in this exciting field of 2D materials science.

Figure 1 .
Figure 1.Schematic design of 3D vertically-aligned micro graphene helix.a) Conceptual drawing of vertically-aligned graphene helix that coupled with circularly polarized light.b) Schematic of exposed SU-8 2D pattern with graphene layer underneath after developing process.c) Halfway and d) verticallyaligned 3D graphene helix with e) its zoomed-in region.f) Vertically-aligned graphene helices in a large array.

Figure 2 .
Figure 2. Design and fabrication of different 3D architectures with SU-8 photoresist via heating up in water.a,b) Conceptual schematic analysis of the curving behavior of the SU-8 photoresist when being heated up in water.c) Design of multiple exposures with different doses at different locations.d-g)In situ monitored self-assembly process of SU-8 tubular structures.h-j) Schematic and a real experimental image of a micro scorpion h) before and i) after self-assembled to 3D j) in large array.k) 3D tubular structures intentionally designed in vertical direction with graphene, l) in large array, m) with a well-controlled angle.n) Tubular structures with different lengths in water.o) A 1.5 cm long vertically-aligned tube.

Figure 3 .
Figure 3. Mechanism and design of 3D vertically-aligned graphene helix.a) Conceptual schematic of the designed 2D pattern (half) before curving up, with b) its zoomed-in image.c) 2D ribbon before and after curves to a 3D helix.d) Schematic of approaches to realize vertically-aligned curving direction.e-l) Experimental images and their corresponding data plot of the relationship between designed angle  versus angle , and designed misorientation angle ϕ versus radius and pitch of the helix.m,n) Data plot of the relationship of width and dose versus different folding angles.o) A curve fit with the equation given for finding the optimal width and dose for structures aligned vertically with 90 °. p) A 3D plot to give a direct visualization of the relationship among width, dose, and the folding angle.

Figure 4 .
Figure 4. Investigation on critical parameters during the 3D self-assembly process.a-j) In situ monitored folding and k,l) twisting behavior of the helix with the data plot.m) Pitch and number of turns versus different doses and n) versus ratio of the less/fully cross-linked area on the SU-8 2D pattern.

Figure 5 .
Figure 5. 3D vertically-aligned graphene micro helices array with characterization.a,b) Schematic drawing of designing vertically-aligned graphene helices array and c,d) its real fabrication images.e) Longer 3D helices with more turns in an array.f) SEM image of vertically-aligned helices array in perpendicular view (inset shows the curving part on the anchor on the substrate and its zoomed-in image).g) SEM image of vertically-aligned graphene helices array in the side view and h) its zoomed-in image.i) UV image of vertically-aligned graphene micro helices array with j) clean surface of a zoomed-in part.k) Contamination can be easily visualized under UV light illumination.l) A four-inch wafer scale vertically-aligned helices array.m) Raman measurement of the SU-8 with/without graphene.n) A 3D helix-type curve fit plot (left) based on graphene Raman signal intensity measured along the o) 3D graphene helix from the beginning to the end (right).The helix lies on the substrate.

Figure 6 .
Figure 6.Optical chirality performance based on simulation.a) The vertically and horizontally aligned graphene helix on the CaF 2 substrate.b) Different optical response of a graphene helix under LCP and RCP light, respectively.Inset shows the electric field distribution at the resonant peak, which is 0.5 THz and 0.9 THz for LCP and RCP, respectively.c) Surface current density distribution by LCP and RCP illumination at the resonant peak.d) Simulation result of the diameter of the graphene helix versus resonant frequency peak under the illumination of RCP light.e,f) CD and g-Factor responses of the graphene helix from laying down on the substrate to vertically-aligned from the substrate.The inset shows the CD and g-Factor versus frequency range.g) CD and g-Factor responses with the height of the graphene helix that coupled with the light.h,i) CD and g-Factor response versus percentage of defects on the vertically-aligned graphene helix.