Research paperFabrication and microanalysis of 3D-shell-like chiral nanostructure based on micro-sphere assembled technology
Introduction
Chiral structures that can't congruence with its mirror image by translating or rotating operation can interact differently with left circular polarization (LCP) light and right circular polarization (RCP) light, leading to observable chiroptical effect, include circular dichroism (CD) [1] and optical activity (OA) [2], [3], [4]. Chiral structures widely exist in nature, and most of the biomolecules, such as DNA, various proteins and polysaccharide, are chiral. However, the chiroptical effect from these natural chiral molecules is too weak, which both limits the detection sensitivity of chiral molecules and their ability to control the polarization state of light. Recently, artificial metallic chiral nanostructures (AMCN) have been attracted a great amount of interest for their excellent chiroptical effect, include giant chiroptical signal and circular transition dichroism, and their ability to achieve some novel physical effects, such as circular transition dichroism, repulsive Casimir force [5], [6] unusual spin Hall Effect [7], and negative refractive index [8], [9], [10]. Particularly, a kind of super-chiral field can be created near AMCN when excited by circular polarization lights, which has been demonstrated to be an effective method to enhance the chiroptical effect of proteins and improve the detection sensitivity of about 106 for the secondary and tertiary structures of proteins with respect to the traditional chirally sensitive spectroscopic techniques [27], [28]. These advantages make AMCN as one of the most hot research topics in the nanophotonic field.
The electromagnetic coupling is a basic requirement for the achievement of chiroptical effect for AMCN. This usually requires aspiral-like topological structure in AMCN, where a magnetic field will be formed when the circular electric current is excited by light. This requirement restricts AMCN with complicated geometrical morphology. And the fabrication methods for this kind AMCN usually is very expensive and low efficiency, especially for the AMCN working in visible region. For example, the direct laser writing technique has been used to fabricate the spiral-like AMCN that working in mid-infrared region [11]; the electron beam lithography usually should be used in the fabrication of planar chiral nanostructures working in visible region [12], [13]. This complicated geometrical morphology together with the expensive and low efficiency fabrication methods limits the related research and applications of AMCN. The self-assembled technologies based on biomolecules, such as DNA [14], [15], have been proved to be low-cost and high-efficiency methods. And many chiral nanostructures have been successfully achieved, such as nanoribbons [16], nanotubules [17] and gyroid [18]. However, the chiroptical effect from the obtained AMCN is usually very weak.
The micro-sphere assembled technology is one of the recently-developed methods, which has been demonstrated to a low-cost and high-efficient method, and a great number of nanostructures have been fabricated by this method, such as nanohole [19], nanoparticle [20] and nanoshell [21], [22]. Our previous work demonstrated that a kind of shell-like chiral nanostructures (SLCS) also could be fabricated by the micro-sphere assembled technology together with the step-wise angle-dependent material deposition technology [23]. However, the CD signal from these SLCS bulk samples is very weak due to the coexistence of left-hand and right-hand AMCN in one sample, which will create chiroptical signal of opposite sign and cancel each other in bulk samples. In this paper, we discuss the fabrication of SLCS in microdomains through single material-deposition on the self-assembled micro-sphere array monolayer. The spectra analysis reveals that a giant CD signal of about 9° can be obtained from the SLCS in micro-domains, indicating the potential application prospect of SLCS. The in-depth shadow effect and theoretical analysis further verifies this giant chiroptical effect, and the weak CD signal from bulk sample should be attributed to the cancellation effect originating from the coexistence of left-hand and right-hand SLCSs. The fabrication method can be further optimized and the obtained SLCS have potential applications in many fields, such as integrated photonics and enantiomer sensor.
Section snippets
Experiment
The fabrication process of SLCS is shown in Fig. 1, include glass substrate cleaning, microsphere array assembled on substrate, angle dependent material deposition, SLCS coated micro-sphere array transferring to PDMS surface and the microsphere etching. The detailed fabrication steps are shown as below:
Firstly, the K9 glass substrates were immerged into the 3:1 H2SO4/30%–H2O2 mixed solution under 80 °C for 1 h; Then the substrates were washed by deionized water for several times. To enhance the
Result and discussion
In fact, the morphologies of SLCS are affected by the shadow effect from the neighboring spheres in the material deposition process, which depends mainly on the angles θ and φ as shown in Fig. 1(b). The deposition angle θ is defined as the material vapor beam with respect to the substrate normal, and the azimuth angle φ is defined as the projection of material vapor beam in substrate surface with respect to the reference line, as shown in Fig. 2(a). In this paper, θ is fixed at 60°. And we just
Conclusion
In summary, the SLCS have been successfully fabricated by a simple, low-cost and high-efficient method, the micro-sphere self-assembled technique. The indepth theoretical and experimental analysis indicate that SLCS owns a giant CD signal of about 9° in visible region. While the weak chiroptical effect in bulk sample should be attributed to the coexistence of the left- and right-hand SLCS in one sample. A home-built measured system, 3D geometrical model and FDTD method have used to verify the
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Nanomanufacturing—Perspective and applications
2017, CIRP Annals - Manufacturing TechnologyCitation Excerpt :By projecting two different 2D patterns within one 3D mask on the two inclined surfaces in a single step, a mutual alignment between the patterns is ensured (Fig. 18) [82]. Through single-material deposition on the self-assembled microsphere array monolayer, 3D-shell-like artificial chiral nanostructures, as shown in Fig. 19, have been fabricated; they have potential applications in integrated photonics and enantiomeric sensors [24]. The printing method is one specific additive nanomanufacturing process, while the traditional methods with precision limited include thermal and piezoelectric printing as well as electrostatic printing.
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