Design and Fabrication of Three-Dimensional Chiral Nanostructures ...

Article pubs.acs.org/Langmuir

Design and Fabrication of Three-Dimensional Chiral Nanostructures Based on Stepwise Glancing Angle Deposition Technology Yidong Hou,† Shuhong Li,† Yarong Su,† Xia Huang,† Yu Liu,† Li Huang,† Yin Yu,‡ Fuhua Gao,† Zhiyou Zhang,*,† and Jinglei Du*,† †

Department of Physics, Sichuan University, Chengdu 610064, China Department of Electronics and Information Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

‡

ABSTRACT: The chiral structures have displayed some inevitable and fascinating properties in many research fields, such as chemistry, biology, mathematics, and physics. In this Article, we report the use of stepwise glancing angle deposition technology to produce the 3D chiral nanostructures. Through the optimization of deposition parameters (such as the orientation angle of poly styrene spheres (PSs) array, the deposition angle, thickness, and number), a great number of chiral structures have been achieved, and their size depends on the diameter of PS spheres. These chiral structures all can be simulated and predesigned through the use of a 3D geometrical model, which greatly improves the efficiency of this method. In addition, the circular dichroism spectrum shows that these chiral structures own an obvious Cotton effect, indicating their potential application as 3D chiral metamaterials.

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INTRODUCTION Chirality is a widespread phenomenon in nature, such as our left and right hands. The 2D/3D chiral structures are defined as the structures that cannot be brought into congruence with their mirror image with the rotation or translation operations in 2D/3D space. These chiral structures, together with their special properties, are a fascinating issue in many fields, such as chemistry, biology, mathematics, and physics. In recent years, 2D/3D chiral metamaterials have attracted attention in the physical fields, which are usually composed of small chiral or achiral building blocks, such as U-shaped,1 cross-shaped,2−4 rose-shaped,5,6 and sphere-shaped blocks.7−10 Previous research indicates that these chiral metamaterials own many amazing properties, such as optical activity,2,3,5 circular dichroism,11 negative refraction,4,6,12 repulsive Casimir force,13 and spin Hall effect.14 These properties have huge application potential (such as chirality-based biosensor15) in many fields. Particularly, some chiral metamaterials can achieve negative refractive index without requiring simultaneously negative permittivity and negative permeability,12 making it a potential candidate for conventional metamaterial.16 In the previous work, many methods have been developed for the fabrication of chiral metamaterials, such as the conventional lithography and the focused ion/electron beam etching technology for 2D chiral structures,7,8 and the polymerase chain reaction and DNA-based self-assembly technologies for 3D chiral structures.9,10 In addition, chiral metamaterials also can be created through embedding achiral plasmonic nanostructures in the chiral molecular layers,17 where significant chiroptical effect can be induced. However, the development of chiral metamaterials is still © 2012 American Chemical Society

a challenge, and the mentioned fabrication methods are usually material-depended, expensive, or operation complex methods. In this Article, we propose the use of stepwise glancing angle deposition technology to achieve the simulation and fabrication of a kind of 3D chiral nanostructures based on the PS sphere assembly technology. Through employing different fabrication parameters (such as deposition angle, thickness, number), a great number of chiral structures can be easily achieved, and any thermal evaporation materials can be used in the deposition process. In addition, a 3D geometrical model has been developed for the predesign of these structures, which can greatly reduce the cost and improve the fabrication efficiency. These 3D chiral structures display an obvious Cotton effect, indicating their enormous application prospect as chiral metamaterials. The PS sphere assembly technology has been widely used for the fabrication of 2D nanostructures.18−22 In the fabrication process, the material deposition on the close-packed PS array monolayer also can lead to the formation of PSs with a patterned surface,23 which are so-called “Janus particles” or “patchy particles”24,25 and a research hotspot in chemistry. In fact, these patterns can be further peeled off from the PSs to form new nanostructures, shell-like structures, as shown in Figure 1, right, and their size and morphology depend on the diameter D (unit: nm) of PSs, the angle θ and φ, and the deposition thickness k, where θ is defined as the incidence angle of the material vapor beam measured from the substrate side, φ is the Received: October 17, 2012 Revised: December 30, 2012 Published: December 31, 2012 867

dx.doi.org/10.1021/la304122f | Langmuir 2013, 29, 867−872

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structures can be applied for the development of additional new structures.

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EXPERIMENTAL SECTION

The fabrication process is shown in Figure 3.

Figure 1. Left: Schematic illustration of the incident material vapor beam above a hexagonally close-packed sphere monolayer. θ is the incidence angle measured from the substrate side, and the angle φ is related to the monolayer orientation. Right: The simulation pictures and the related SEM images of the basic structures with θ = 75° and φ = 0°, 15°, 30°, or 45°.

parameter to describe the PSs array monolayer orientation, and k (unit: nm) is defined as the deposition thickness on the plane perpendicular to the vapor beam (as shown in Figure 1, left). Figure 1, right shows the images of the simulated and fabricated structures with single deposition parameters of D = 750 nm, θ = 75°, and φ = 0°, 15°, 30°, or 45°. These images indicate that the simulated results meet well with the fabrications, and the structures with φ = 15° and 45° are chiral. These four structures will be used as basic blocks for the development of chiral structures in the following discussions. The twice or multiple depositions on the same PSs array will lead to the combination of basic structures and thus the formation of new structures, as shown in Figure 2, where new

Figure 3. Schematic illustration of the technological process: (a) selfassembly of PSs monolayer on glass substrate; (b) stepwise material deposition; (c and d) the transfer of PSs array from glass substrate to PDMS stamp; and (e) the wet chemical etching of PSs. The upperright image is the obtained new 3D chiral nanostructure array through the combination of two different basic structures with parameters of 80[75°, 0°] and 80[75°, 285°], respectively. First, a monolayer of PSs array with hexagonal symmetry on glass substrate is obtained through the method reported in ref 26. The monolayer orientation is confirmed by the scanning electron microscope (SEM). The orientation along line 1 shown in Figure 3a is set as a reference monolayer orientation, that is, φ = 0°, and for all of the other orientations, φ is measured through the anticlockwise rotation of the domain with respect to the reference orientation; Second, the silver deposition on the closely packed PSs array monolayer is performed inside a vacuum thermal evaporation system, which is improved with a collimation component as reported in ref 27. The base pressure, the deposition rate, and the temperature are 4 × 10−4Pa, 0.1 nm/s, and 30 °C, respectively. For a single deposition, we just need to control the deposition time (i.e., deposition thickness k) and the angle θ and φ, which can be easily adjusted by tilting and rotating the samples in the vacuum chamber, respectively. For stepwise depositions, the operational process is the repeat of the single deposition, as shown in Figure 3b. Third, the silver-coated PSs array monolayer is placed on the top (flat) surface of a polydimethylsiloxane (PDMS) stamp with a uniform pressure of about 500 g/cm2 (Figure 3c). After being lifted up, this particle monolayer is transferred to the surface of the PDMS stamp, due to the larger van der Walls force with the PDMS stamp as compared to the silicon substrate (Figure 3d). Finally, the PDMS stamp together with the silver-coated PSs array monolayer is immersed in tetrahydrofuran for 3 min to etch away the PSs. This stamp is then dried in a fume hood at room temperature. After 6 h, a new nanoparticle array is obtained on the PDMS stamp (Figure 3e). The upperright image in Figure 3 is the obtained new nanoparticle array with deposition parameters of D = 750 nm and f 2 = 80[75°, 15°] + 80[75°, 45°]. Certainly, these arrays can also be fabricated on any other desired substrate as indicated in ref 28. The chiroptical effect measurement is performed with the use of circular dichroism (CD) for the characterization of these chiral structures, where CD is the difference in absorbance (or extinction, in the more general case) between left- and right-circularly polarized light in the forward scattering direction (light collection angle of ∼10°). The CD

Figure 2. The schematic illustration of the combination of three identical basic structures with [75°, 0°]. Bottom right corner: The simulated pictures of the obtained structures.

structures have been successfully achieved through the depositions performed three times with parameters of f 2 = 80[75°, 0°] + 80[75°, 120°] + 80[75°, 240°] (80[75°, 0°] means θ = 75°, φ = 0°, and k = 80 nm). Because of the hexagonal symmetry of the PSs array monolayer, this new structure can also be regarded as the combination of the three same basic structures with [75°, 0°]. Certainly, different basic 868


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signal is usually the evidence for the existence of chiral structures, and the intensity is proportional to the ellipticity angle of the out-coming light polarization. Here, the CD measurement is performed with a J-810 circular dichroism spectrometer (made by Jasco Corp.). The wavelength range is from 200 to 900 nm, and the step is 1 nm. To avoid oxidation of the obtained silver structures, the samples for measurement are embedded in PDMS. Figure 4 is the CD spectrum of samples with (blue curve) and without (red curve) chiral structures (fabrication parameters: D = 750 nm, f 2 = 87[75°, 15°]). The sample with chiral structures displays obvious wavelength-dependent absorbance differences for the L- and R-circularly polarized light, while the samples without chiral structures do not. This indicates that these chiral structures own an obvious chiroptical effect, and can

be used as 3D chiral metamaterials for the manipulation of light polarization.

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RESULTS AND DISCUSSION

As shown in Figure 2, the multiple depositions of materials on PSs array will lead to the combination of basic structures, that is, the formation of new structures. Because of the hexagonal symmetry of the PSs array, the same basic structures can be used six times in the fabrication process of one new structure. For the use of four basic structures, up to 24 basic structures can be used, which will lead to the formation of a great number of new nanostructures. In this case, the use of the predesign and simulation to select the desired structures from the multitudinous new structures becomes particularly important. In the following discussion, we apply a 3D geometrical model to achieve the design and simulation of new 3D chiral nanostructures. There are three steps in the development process of chiral structures: first, the selection of proper basic structures through the geometrical analysis based on the 3D geometrical model; second, the design and simulation of new structures through the combination of basic structures; and, finally, the fabrication of these designed structures. The last step has been described in the Experimental Section. For the first step, four basic structures with parameters of 80[75°, 0°], 80[75°, 15°], 80[75°, 30°], and 80[75°, 45°] have been chosen, which have obvious different shapes from each other, as shown in Figure 1, right. The 3D geometrical model of these four basic structures is shown in Figure 5, where O is the center of the related PSs, and A, B, C, and D are the catastrophe points of the edge of their inner surface. Obviously, the thicknesses of these structures at points A and D are all zero, and the thicknesses at points C and D and some other parameters are shown in Table 1. The thickness at point A is defined as the thickness of the structures on the direction indicated by the radial OA. The structures with parameters of 80[75°, 15°] and 80[75°, 45°] are chiral structures and also the mirror images of each other. In fact, all of the basic structures with φ ≠ 30° are chiral structures, when 0° < φ < 60° (i.e., in one period Tφ = 60°). In addition, the basic structures with φ around 30° are all mirror images of each other, like the structures with parameters of 80[75°, 15°] and 80[75°, 45°]. In the second step, the design and simulation of new chiral structures are very complex, for up to 24 basic structures can be used in the development of one new structure. Here, we just show the designed results with the combination of two or three basic structures, as shown in Figures 6, 7, and 8. The parameter φ1 or 2 of the basic structures is shown in the left column. Through the clockwise rotation of one (φ = φ2) or two (φ = φ2 and φ = φ3) of the basic structures with rotation angles of 0°, 60°, 120°, ...240°, a lot of new structures have been achieved.

Figure 4. CD spectrum of samples with (blue curve) and without (red curve) chiral structures (D = 750 nm, f 2 = 87[75°, 15°]).

Figure 5. The oblique view of the simulated basic structures with parameters of [75°, 0°], [75°, 15°], [75°, 30°], and [75°, 45°].

Table 1. Related Parameters of Basic Structures A-point thickness B-point thickness C-point thickness D-point thickness ∠AOBa ∠BOC ∠COD ∠DOA a

[75°, 0°]

[75°, 15°]

[75°, 30°]

[75°, 45°]

0 0.973105*k 0.973105*k 0 78.6202° 18.1597° 78.6202° 113.6074°

0 0.848428*k 0.921731*k 0 60.9274° 10.3851° 108.2861° 124.7822°

0 0.583639*k 0.583639*k 0 37.9911° 105.2442° 37.9911° 139.2378°

0 0.921731*k 0.848428*k 0 108.2861° 10.3851° 60.9274° 124.7822°

∠AOB is the angle between the radial OA and OB. 869


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For the combination of two basic structures, we get 48 new structures, and 36 of them are chiral (Figures 6 and 7); for the combination of three identical basic structures, we get 16 new

structures, and 12 of them are chiral (Figure 8). Thus, 48 chiral structures have been achieved. Certainly, more chiral structures can be obtained if more basic structures are used. From these designed results, we can expediently select and fabricate the desired 3D chiral nanostructures.

Figure 6. The simulated pictures of structures through the combination of two identical basic structures with parameters of k[75°, φ1 or 2]. The inset yellow rectangles mean that these structures are chiral, and the inset green rectangles mean that these structures can be transformed to chiral structures.

Figure 8. The simulated pictures of new 3D chiral nanostructures through the combination of three identical basic structures with parameters of k[75°, φ1 or 2]. The inset yellow rectangles mean that these structures are chiral, and the inset green rectangles mean that these structures can be transformed to chiral structures.

Figure 7. The simulated pictures of structures through the combination of two different basic structures with parameters of k[75°, φ1 or 2]. The inset yellow rectangles mean that these structures are chiral, and the inset green rectangles mean that these structures can be transformed to chiral structures. 870


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the surface tension coming from the organic solvents in the PSs-etching process, or other factors. The CD spectrum (Figure 4) shows that the chiral structure with parameters of D = 750 nm and f 2 = 87[75°, 15°] owns obvious wavelengthdependent absorbance differences for the L- and R-circularly polarized light, that is, chiroptical effect, which can be used to manipulate the polarizaiton state and the ellipticity of outcoming light. This indicates that these 3D chiral structures mentioned in this Article have enormous application prospect, such as 3D chiral metamaterials. Certainly, this chiroptical effect is related to many factors, such as the structure parameters (including the diameter D of PSs, the angle θ and φ, the deposition thickness k, and number) and environment index. Further work is in progress.

The designed structures discussed above are all based on the combination of basic structures with fixed deposition angle (θ) and thickness (k). In fact, θ and k are also the adjustable parameters in the design and fabrication process of new structures. Particularly, some achiral structures shown in Figures 6, 7, and 8 can be effectively transformed to chiral structures. For example, if the different colors shown in Figure 2 represent different deposition thicknesses or materials, the obtained structure will be a chiral structure. Through this way, 14 of the 16 achiral structures shown in Figure 6, 7, and 8 can be transformed to chiral structures or structures with chiral physicochemical properties, where the structure with chiral physicochemical property is defined as the structure that cannot be brought into congruence with their mirror image from the point of the physicochemical property. Therefore, the vast majority of new developed 3D structures by the stepwise glancing angle deposition technology are chiral or can be transformed to chiral structures or chiral structures with chiral physicochemical property, and this provides a solid foundation for the related research and applications based on the chiral macro-/nanostructures. In experiment, the designed structures can be easily fabricated through the fabrication process as described in the Experimental Section. Figure 9 shows the SEM images of some

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CONCLUSIONS In this Article, we successfully achieve the development of new 3D chiral nanostructures through the use of a 3D geometrical model and the stepwise glancing angle deposition technology. The monolayer orientation angle φ of PSs array, the number of basic structures used in the development of one new structure, the deposition angle θ, and thickness k are all adjustable parameters in the design and fabrication process, and this will lead to the formation of a great number of new 3D chiral nanostructures. In addition, most of the obtained achiral structures can be transformed to chiral structures or structures with chiral physicochemical property through the use of different deposition thicknesses or materials. These chiral structures display a giant optical activity effect, indicating their potential application as 3D chiral metamaterials.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 (028) 85412983. Fax: +86 (028) 85412983. E-mail: [email protected] (J.D.); [email protected] (Z.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the technological support of the Analysis and Testing Center, Huazhong University of Science and Technology.

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REFERENCES

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Figure 9. The SEM images of the fabricated new 3D chiral nanostructures through the combination of (a) two identical basic structures with [75°, 45°]; (b) two identical basic structures with [75°, 30°]; (c) two different basic structures with [75°, 0°] and [75°, 30°]; (d) two different basic structures with [75°, 30°] and [75°, 15°]; (e) three identical basic structures with [75°, 15°]; and (f) three identical basic structures with [75°, 0°]. The insets are the related simulated structures with the same parameters.

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