Gold Nanowire Chiral Ultrathin Films with ... - Wiley Online Library

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International Edition: DOI: 10.1002/anie.201701512 German Edition: DOI: 10.1002/ange.201701512

Chiral Films

Gold Nanowire Chiral Ultrathin Films with Ultrastrong and Broadband Optical Activity Jiawei Lv, Ke Hou, Defang Ding, Dawei Wang, Bing Han, Xiaoqing Gao, Man Zhao, Lin Shi, Jun Guo, Yonglong Zheng, Xi Zhang, Chenguang Lu, Ling Huang,* Wei Huang, and Zhiyong Tang* Abstract: An ultrastrong and broadband chiroptical response is key but remains challenging for many device applications. A simple and cost-effective bottom-up method is introduced to fabricate large-area long-range ordered chiral ultrathin films with the Langmuir–Schaeffer technique using gold nanowires as building blocks. Significantly, as-prepared ultrathin films display giant optical activity across a broad wavelength range covering visible and near infrared regions with an anisotropic factor of up to 0.285, which is the record value for bottom-up techniques. Detailed experimental result and theoretical analysis disclose that such remarkable optical activity originates from birefringence and dichroism of the well-aligned Au nanowire layers in the ultrathin films. The universality of this facile strategy for constructing chiral ultrathin films is further demonstrated with many other one-dimensional nanomaterials.

Chiral materials of large optical activity, which respond

distinctly differently to left and right circularly polarized light, have attracted much scientific and technical interest, owing to not only their key role in understanding chirality evolution in nature but also promising applications in optical devices. Unfortunately, as for conventional chiral molecules, their optical activity is rather weak in a narrow wavelength band. As a comparison, recently developed chiral inorganic nanostructures exhibit orders of magnitude enhanced and tunable optical activity, making them promising alternatives to chiral molecules.[1] Various methods have been adopted to fabricate these chiral nanostructures, which can be generally categorized into two major groups: top-down methods and bottomup methods. Top-down methods are good at constructing small-scale and well-defined 3D chiral geometries with large

optical activity.[1, 2] However, the procedures involved are sophisticated and time-consuming and expensive equipment is always needed, making them difficult to be widely accepted. In contrast, bottom-up methods are usually solution based and more convenient for large-scale fabrication, in which various chiral templates are utilized to produce optically active inorganic nanomaterials.[3] Despite the great potentials of bottom-up methods, the product quality obtained is far from satisfaction. On one hand, compared with top-down methods, chiral nanostructures prepared with current bottomup methods usually are long range disordered, giving rise to relatively weak optical activity. On the other hand, until now no any efforts have been made by bottom-up methods to acquire broadband chiroptical response, though it is highly desired in many applications. Herein, a facile bottom-up assembly method based on Langmuir–Schaeffer (LS) technique is developed for preparing long-range ordered chiral inorganic films. The resulting chiral thin films display many impressive performances including giant optical activity (ca. 6500 mdeg) with an anisotropic factor (g-factor) of up to 0.285 and ultra-broadband response across 300–2000 nm wavelength range, which make a big step forward towards the real application of chiral nanomaterials. Figure 1 outlines the procedure of preparing Au nanowire chiral ultrathin films. In brief, ultrathin Au

[*] J. Lv, X. Zhang, Prof. L. Huang, Prof. W. Huang Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NJTECH) 30 South Puzhu Road, Nanjing 211816 (P.R. China) E-mail: [email protected] J. Lv, K. Hou, D. Ding, Dr. D. Wang, Dr. B. Han, X. Gao, Dr. M. Zhao, L. Shi, J. Guo, Y. Zheng, Prof. C. Lu, Prof. Z. Tang CAS Key Laboratory for Nanosystem and Hierarchy Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology Beijing 100190 (P.R. China) E-mail: [email protected] Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.201701512. Angew. Chem. Int. Ed. 2017, 56, 5055 –5060

Figure 1. Fabrication procedure of Au nanowire chiral ultrathin films. a) Dispersing Au nanowire solution at the air–liquid interface in the Langmuir trough. b) Compressing the barrier to obtain aligned Au nanowire film at the air–liquid interface followed by transferring the assembled film onto a quartz substrate with a horizontal transfer approach. c) Rotating the substrate with angle q for a second transfer. d) Representation of chiral films with opposite handedness. e) Optical activity of the resulting chiral films.

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Communications nanowires floating at the air–liquid interface in the Langmuir trough are slowly compressed between parallel barriers and then spontaneously become aligned. Afterwards, the aligned Au nanowires are horizontally transferred to a substrate. Before transferring the subsequent layers, the substrate is rotated clockwise or anticlockwise with a pre-designed angle to obtain left- or right-handed chiral ultrathin films, respectively. The relative angle and layer number, which determine the chiroptical property of as-fabricated chiral ultrathin films, can be easily controlled as expected. The critical step of preparing chiral ultrathin films is to macroscopically align one-dimensional nanowires at the air– liquid interface. The Langmuir–Blodgett (LB) or Langmuir– Schaeffer (LS) technique has been generally adopted to align a variety of nanowires at the air–water interface.[4] In fact, the LB technique was previously utilized to assemble Au nanowires;[5] however, the acquired films did not show the longrange alignment needed for chiral arrangement. We have also tried commonly used water as the sub-phase, and asassembled Au nanowires with diameters of circa 2 nm are randomly oriented with many voids and openings between them (Supporting Information, Figures S1, S2). A reasonable explanation is that as the excessive oleylamine molecules is not soluble in water, they will remain at the interface after solvent evaporates and occupy some surface area, making it difficult for the Au nanowires to form continuous and compact films. As a result, the long range alignment is not good. We also notice that it is not feasible with pre-removal of oleylamine molecules in Au nanowire solution before LS assembly, because as-cleaned nanowires prefer aggregating and do not disperse well at the interface. To solve this problem, we have modified the traditional LS method by changing the sub-phase from water to ethylene glycol (EG). The improved alignment of Au nanowires originates from two factors. First, oleylamine is more soluble in EG than in water.[6] As a result, the excessive oleylamines will dissolve in the sub-phase, making the film more continuous and compact. Second, the surface tension of EG (46.69 mN m@1 at 20 8C) is much lower than water (72.75 mN m@1 at 20 8C). As for Au nanowires at the air–liquid interface, two forces affect the assembled structure, namely inter-nanowire Van der Waals forces and the nanowire–interface interaction related to surface tension. The inter-nanowire Van der Waals forces make the nanowires tend to form parallel bundles with each other, whereas the surface tension tends to pin the nanowires onto the surface and draw them apart from each other. As the surface tension of EG is much smaller than water, which is validated by p–A curve measurement during the assembling process (Supporting Information, Figure S2c and S3e), the inter-nanowire Van der Waals forces will dominate in the two forces, and the parallel nanowire bundles are easily formed during the assembling process (Supporting Information, Figure S3a–d). Finally, more than 90 % of Au nanowires are well oriented within small deviation angles of : 1088 (Supporting Information, Figure S3f). Moreover, the alignment of Au nanowires can span over very large area that is only limited by the Langmuir trough size. Figure 2 a represents three layers of Au nanowire assembly transferred on the quartz substrate, which is uniform over ca. 4 cm X 4 cm without visible defects.

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Figure 2. a) Chiral ultrathin film containing three layers of Au nanowire assembly on quartz substrate placed on top of the acronyms. b) AFM images of the aligned films on silicon substrate. c) SEM images of one layer of Au nanowire assembly on silicon substrate. d) Magnified view of (c).

The scanning electron microscopy (SEM) and atomic force microscopy (AFM) images further confirm the excellent alignment of Au nanowires inside the layer (Figure 2 b–d). The fact that Au nanowires are aligned parallel with the barriers offers the opportunity to control the relative angles between different layers in the prepared ultrathin films with reference of the barrier direction. Accurate angle control over two layers of Au nanowire assembly of left-handed chirality is demonstrated in the Supporting Information, Figure S4. Both AFM measurement and SEM cross-section (Supporting Information, Figure S5a–c) indicate that one layer of Au nanowire assembly is about 90 nm thick, and the film thickness grows linearly with layer number (Supporting Information, Figure S5c–f). It needs to be stressed that herein, one layer refers to one transferred LS film rather than one monolayer of nanowire. The chirality of asfabricated films is investigated with diffused transmission circular dichroism (DTCD). To exclude the possible angledependent effect, every sample is rotated perpendicular to the optical path of the spectrometer at each step of 4588, and eight measurements at different rotation angles are averaged to obtain the CD response for following analysis (Supporting Information, Figure S6). We also measured the DTCD by illuminating the sample from the front side and the back side to verify that the present structure is a 3D chiral material (Supporting Information, Figure S7). Besides, diffused reflection circular dichroism (DRCD) confirms that the chiroptical response is mainly attributed to absorption rather than reflection (Supporting Information, Figure S7). Figure 3 a–c show the CD response of chiral ultrathin films composed of two layers of Au nanowire assembly with different inter-layer angles. Evidently, the films of the left-handed structure show the opposite CD sign but the similar CD intensity with respect to those of the right-handed structure. Furthermore, at the same optical absorption density (Figure 3 b), the films with an inter-layer angle of 4588 possess the largest CD response (Figure 3 a) and the biggest g-factor (Figure 3 c), whereas


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Figure 3. a)–c) CD response, UV/Vis absorption, and g-factor of Au nanowire chiral ultrathin films with different inter-layer angles and handedness for the two-layer case. LH 15 = left-handed structure with an inter-layer angle of 1588. d)–f) CD response, UV/Vis absorption, and g-factor of chiral films with different layer number with a fixed inter-layer angle of 4588. LH 2 = left-handed structure with two layers of Au nanowire assembly. g) Measured transmission spectra for left and right circular polarized light (LCP and RCP) passing through left-handed chiral ultrathin films containing three layers of Au nanowire assembly. h) CD response derived from transmission spectrum in (g). i) Corresponding g-factor in near-IR region.

there is no CD response at angles of 088 or 9088 as these two configurations are achiral. Except for the angle dependence, the relationship between the layer number and the CD response is also investigated. As indicated in Figure 3 d–f, CD response nearly doubles when the layer number increases from 2 to 3 with a fixed inter-layer angle of 4588. However, further increase of the number of layers would not give rise to obvious enhancement of the optical activity. As for the ultrathin film containing three layers of Au nanowire assembly, the largest CD value approaches 4700 mdeg at 900 nm and the corresponding g-factor reaches 0.25, which are much larger than the previously reported values (Supporting Information, Table S1) for self-assembled chiral structures. Another prominent merit of Au nanowire chiral ultrathin films is the broadband optical activity ranging from visible to near infrared (NIR) wavelength. Figure 3 g–i present the transmission spectra and the corresponding CD curves of left-handed chiral ultrathin film containing three layers of Au nanowires. The broadband feature expands to 2000 nm, which is much wider than any reported results based on selfassembly methods. Also, the CD response remains quite strong over the whole spectrum range with a peak intensity of 6500 mdeg at 1250 nm and a g-factor of 0.285 at 1100 nm, Angew. Chem. Int. Ed. 2017, 56, 5055 –5060

which are so far the highest values among the reported chiral assemblies (Supporting Information, Table S1). Such large and broadband CD response makes the Au nanowire chiral ultrathin films very promising in the applications such as broadband circular polarizers. The well-defined and tunable structure of our chiral films makes it possible to quantitatively study the mechanism of optical activity. Previously, many mechanisms have been developed to understand the observed optical activity of chiral molecule or nanostructures. As for molecular materials, exciton coupling well explains the optical activity;[7] while with regard to chiral-structured plasmonic nanomaterials, plasmonic coupling is a generally accepted mechanism.[2b, 3a,b] For other inorganic chiral nanostructures, some researchers adopted the transition coupling mechanism.[3g–i] Nevertheless, those optical activities are all assigned to near-field coupling between neighboring components. Unfortunately, for our system, none of the above mechanisms can interpret the experimental result. For example, the measured CD intensity varies little even when the gap between two thin layers increases to 1 mm (Supporting Information, Figure S8), which excludes the possible near field coupling.[2b, 3a,b] So another mechanism needs to be explored to elucidate the observed


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Communications CD result. About 150 years ago, the Reusch model for optical rotation of macroscale objects was established based on helical stacked mica plates.[8] Recently, this theory was utilized for clarifying the observed circular birefringence oscillations in various types of spherulites,[8, 9] in which the helical stacked crystal plates with linear birefringence resulted in strong circular birefringence. It is known that the aligned nanowire systems possess ultra-large birefringence,[10] which well fits the Reusch model. Furthermore, one layer of the aligned Au nanowire assembly is explored to possess giant linear dichroism (Figure 4 c). Hence, as for one layer of the Au nanowire assembly with linear dichroism and birefringence simultaneously, we can treat it as the combination of a partial linear polarizer and a retarder (Figure 4 a). For convenience, herein we only exemplify the calculation model based on the left-handed structure (Figure 4 b). The top layer is considered as a horizontal retarder and vertical polarizer, while the rotated bottom layer is regarded as a retarder with an angle of q relative to the horizontal axis as well as a polarizer with an angle of 9088 + q. Then, the experimental

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result is simulated by passing circular polarized light through this series of optical devices. With regard to the anisotropic absorption coefficient ko and ke, we calculate it from the absorption and linear dichroism (LD) of one layer of aligned Au nanowire assembly (Figure 4 c,d). As for the birefringence parameter (Dn = no@ne) of a single layer of Au nanowire assembly (Figure 4 e), it is derived from the experimental CD response of left-handed ultrathin film containing two layers of Au nanowire assembly with an inter-layer angle of 4588 (pink curve in Figure 3 a) in combination with as-achieved anisotropic absorption coefficient. Finally, using the derived parameters of anisotropic absorption and birefringence, one could easily calculate the CD response of chiral ultrathin films of varied configurations such as different inter-layer angles, handedness, and layer numbers. Figure 4 e and f highlight that the calculation result agrees well with the experimental observation (Figure 3 a and d), confirming the validity of the proposed model and analysis. It should be noted that the Au nanowire film is not very stable[11] even in ambient conditions, and the Supporting

Figure 4. a) Illustration of anisotropic optical parameters of aligned Au nanowires in ultrathin film. no and ko refer, respectively, to the refraction and absorption coefficients of linear polarized light vertical to the alignment direction; ne and ke refer, respectively, to the refraction and absorption coefficients of linear polarized light along with the alignment direction. b) Calculation model of the left-handed film containing two layers of Au nanowire assembly with an inter-layer angle of q. c) UV/Vis absorption and linear dichroism spectra of one layer of Au nanowire assembly. d) Anisotropic absorption coefficients of one layer of Au nanowire assembly derived from linear dichroism spectrum. e) Birefringence derived from CD response of two layers of Au nanowire assembly with an inter-layer angle of 4588. f) Calculated CD response of two layers of Au nanowire assembly with different inter-layer angle and handedness. g) Calculated layer number dependence of CD response with a fixed inter-layer angle of 4588.

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Communications Information, Figure S9 shows that its optical activity diminishes dramatically after 2 weeks exposed in air. Nevertheless, this mechanism involved with linear dichroism and birefringence of the aligned nanowire assemblies offers a facile and universal designing rule for chiral nanostructures with ultrastrong optical activity. Therefore, many systems composed of different nanowires have been fabricated and examined (Supporting Information, Figures S10 and FS11). In the Supporting Information, Figure S11a and S11b display the CD response of the assembled chiral ultrathin films of W18O49 and NiMoO4·x H2O nanowires, respectively. Interestingly enough, the chiral assembly might be further used as a template to acquire other chiral structures with unique optical activity. As shown in the Supporting Information, Figure S11c, chiral Au nanorod assembly with the assistance of the aligned W18O49 ultrathin nanowires (Supporting Information, Figure S10g–i) exhibits the characteristic CD response at its surface plasmon resonance wavelength. Evidently, regardless of the type of building blocks such as metals,[12] metal oxides, inorganic semiconductors, and even organic materials, all of them would become candidates for building chiral structures with characteristic optical activity as long as they have a one-dimensional shape with a large aspect ratio. In summary, a general and simple method to make Au nanowire chiral ultrathin films is developed using the Langmuir–Schaeffer technique. The obtained chiral films exhibit giant CD response and span across visible and near infrared wavelengths, both of which are the best performance among those chiral nanostructures prepared with bottom-up techniques (Supporting Information, Table S1). Muller matrix calculation reveals that the strong optical activity originates from the helical stacking of anisotropic Au nanowire assembly layers with large linear dichroism and birefringence simultaneously. Significantly, this work opens an avenue to construct a wide variety of unprecedented and rich chiral structures with unique optical activities, which are hardly achieved by conventional bottom-up or top-down techniques. What is more, with the scale of assembly building blocks approaching the molecular scale, we expect that the achieved chiral films will not only possess outstanding chiroptical properties, but also find many applications in chiral recognition, separation, and catalysis.

Acknowledgements The authors acknowledge financial support from National Key Basic Research Program of China (2014CB931801 and 2016YFA0200700, Z.Y.T.), National Natural Science Foundation of China (21475029 and 91427302, Z.Y.T.), Frontier Science Key Project of the Chinese Academy of Sciences (QYZDJ-SSW-SLH038, Z.Y.T.), Instrument Developing Project of the Chinese Academy of Sciences (YZ201311, Z.Y.T.), CAS-CSIRO Cooperative Research Program (GJHZ1503, Z.Y.T.), “Strategic Priority Research Program” of Chinese Academy of Sciences (XDA09040100, Z.Y.T.), National Natural Science Foundation of China (Grant 21401103), National Key Basic Research Program of China Angew. Chem. Int. Ed. 2017, 56, 5055 –5060

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(973 Program, Grant 2015CB932200), National Natural Science Foundation of China (Grant 21371095, L.H.), Natural Science Foundation of Jiangsu Province (Grants BK20131404 and, BL2014075, L.H.) and Synergetic Innovation Center for Organic Electronics and Information Displays.

Conflict of interest The authors declare no conflict of interest. Keywords: chirality · gold · Langmuir–Blodgett films · nanowires · self-assembly How to cite: Angew. Chem. Int. Ed. 2017, 56, 5055 – 5060 Angew. Chem. 2017, 129, 5137 – 5142

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