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Silver nanowires for anti-counterfeiting
Journal Pre-proof Silver nanowires for anti-counterfeiting Yan Wang, Ningning Bai, Junlong Yang, Zhiguang Liu, Gang Li, Minkun Cai, Lingyu Zhao, Yuan Zhang, Jianming Zhang, Chunhua Li, Yunlong Zhou, Chuan Fei Guo PII:
S2352-8478(19)30180-7
DOI:
https://doi.org/10.1016/j.jmat.2020.01.008
Reference:
JMAT 269
To appear in:
Journal of Materiomics
Received Date: 30 September 2019 Revised Date:
9 December 2019
Accepted Date: 15 January 2020
Please cite this article as: Wang Y, Bai N, Yang J, Liu Z, Li G, Cai M, Zhao L, Zhang Y, Zhang J, Li C, Zhou Y, Guo CF, Silver nanowires for anti-counterfeiting, Journal of Materiomics (2020), doi: https:// doi.org/10.1016/j.jmat.2020.01.008. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
Graphical Abstract
Silver nanowires for anti-counterfeiting
Yan Wang a,b, Ningning Bai a,b, Junlong Yang b, Zhiguang Liu b, Gang Li b, Minkun Cai b, Lingyu Zhao b, Yuan Zhang b, Jianming Zhang b, Chunhua Li c, Yunlong Zhou c, Chuan Fei Guo b,*
a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin
150001, China b
Department of Materials Science & Engineering, and Centers for Mechanical Engineering
Research and Education at MIT and SUSTech, Southern University of Science and Technology, Shenzhen 518055, China c
Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment
Devices of Zhejiang Province, Wenzhou Institute of Biomaterials and Engineering, University of Chinese Academy of Sciences, Wenzhou, 325000, PR China. * Corresponding author. E-mail address: [email protected].
1
Abstract Silver nanowire (AgNW) films have been widely used as flexible transparent electrodes due to the high electrical conductance and high transmittance in the visible range. The infrared (IR) properties of AgNW films, however, are seldom discussed. Here, we show that ultrathin AgNWs present high transmittance in the visible range (~95%) and high reflectance (60-70%) in the IR range of 8-14 µm. Such a significant difference makes AgNW films invisible to naked eyes but visible under an IR camera, being an ideal selection for anti-counterfeit technologies, while no excitation is required. We have demonstrated that AgNW films can be spray-coated on either flat or microstructured surfaces with high conformability and high scratch-resistance, and thus these anti-counterfeiting materials can be applied to a variety of applications. The AgNW-based anti-counterfeiting may be helpful for combating counterfeiting crimes in the fields of medicines, artwork, banknotes, brand luxuries, industrial parts, and so on.
Keywords:
silver
nanowires;
anti-counterfeiting;
infrared
light;
excitation-free;
conformability
1. Introduction Counterfeit and shoddy products flood the market worldwide, causing great risk to human health and national security [1,2]. Accordingly, various anti-counterfeiting methods and products have been emerged [3-5]. The most popular anti-counterfeiting technology is based on colorless or light-colored luminescent materials that emit light under the excitation of ultraviolet (UV) or near-infrared (NIR) light to reveal different colors [6-9]. Luminescent anti-counterfeiting technology relies highly on luminescent materials, including lanthanide nanomaterials [6,7,9], quantum dots [10,11], and metal-organic frameworks [12], which can be homogeneously dispersed into ink medium and coated on target substrates. Such materials have received great attention for the virtue of a wide absorption wavelength range, a narrow 2
emission wavelength range, high fluorescence efficiency, and tunable luminescent properties by changing their composition, particle size and shape, and ways of coating [3]. In order to enhance the luminescence signals, metal nanowires or nanoparticles have been randomly dispersed into a fluorescent dye to form a plasmonic mode, leading to the concentration of the incident light [13-15]. Despite the developments in providing approaches for fabricating multifunctional nanomaterials, most luminescent materials are toxic, difficult to disperse and synthesize in an oily medium, and excitation sources are needed. The major challenge in the present scenario is the full exploitation of luminescent nanomaterials which are free of these constraints. Silver nanowires (AgNWs), as a widely used material for transparent flexible electrodes, have attracted much attention in flexible electronics due to their cost-effective preparation, ultra-flexibility, high transparency in the visible range, together with high conductivity [16-19]. Especially, AgNW with a diameter of ~25 nm or smaller and a length to diameter aspect ratio over 1000 is almost fully transparent [20]. While the high transparency in the visible range is well studied, the optical properties of AgNWs in infrared (IR) range has caused little attention. In this work, we found that ultrathin AgNW films exhibit an exceptionally high transmittance (~95%) in the visible range but high reflectance (60-70%) in the IR range of 8-14 µm, corresponding to the room temperature black body radiation. That is, at room temperature, the ultrathin AgNW film is almost invisible by naked eyes while being well visible under an IR camera. Herein, we utilized ultrathin AgNWs for anti-counterfeiting through spray coating. Because of the high aspect ratio and small diameter of the AgNWs (~ 20 nm), the film can be well conformable to surface microstructures, and the high conformability on rough substrates allows the film to be highly scratch-resistant. When coated on a stretchable substrate, the anti-counterfeiting effect maintains at a strain up to 100%. The AgNW films are expected to have a variety of applications in high-value merchandise, documents, pharmaceuticals, artworks, and banknotes.
2. Materials and methods Materials: AgNWs dispersion with different nanowire diameters (~20 nm, ~30 nm, ~ 40 3
nm, ~60 nm) were purchased from HeFei Vigon Material Technology Co., Ltd, China. Hydroxypropyl methyl cellulose (HPMC, viscosity of 2600~5600 cP) was purchased from Sigma, USA. The polydimethylsiloxane (PDMS) elastomer (Sylgard 184) was purchased from Dow Corning Co., Ltd, USA. Fabrication of Microstructures: Different materials (such as matte papers, abrasive papers, fresh Calathea zebrina leaves, etc.) with microstructures were cut into rectangular shapes and fixed on a glass substrate using Scotch tapes (3M Co., USA). PDMS with a base to curing agent volumetric ratio of 10:1 was prepared and placed into a vacuum drying chamber until bubbles disappeared, then curing at 70 °C for 1 h and peeled off from the microstructured substrate to get an inverse structure. To weaken the adhesion during the second molding process, the inverse structure was treated by a plasmon cleaning machine for 2 min. The second molding process was the same as the first molding process to achieve the PDMS replica. Spray coating of AgNWs: AgNWs (0.25 mg/ml) were dispersed with 0.05 wt% HPMC working as the spraying solution. Receiver substrates were heated at 80 °C during spraying to ensure that the droplets evaporate rapidly without streaming. By using an automatic spray gun (Ho-CN, ANEST IWATA, Japan) operating by an air pump (IS-51, ANEST IWATA, Japan), the spraying density were controlled to 0.3 µg/cm2, 0.8 µg/cm2, 1.6 µg/cm2, 3.1 µg/cm2, 6.3 µg/cm2, 9.4 µg/cm2. Characterization. The negative plastic masks were cut by using a CO2 laser cutting machine (WE-640, Xiamen Waner, China). The SEM images were taken by using a scanning electron microscope (TESCAN MIRA3, TESCAN, Czech Republic). The transmittance and reflectance of the samples were recorded with a spectrometer (Lambda 950, PerkinElmer, USA) and FTIR (ImageIR 7300, Bruke, Germany ). A commercial IR camera (226s, Fotric, USA) was used to receive IR radiation (8~14 µm).
3. Results and discussions Preparation of AgNWs based anti-counterfeiting. The AgNW film was prepared by using spray coating and a shadow mask was used to get form desired patterns (Fig. 1A). 4
Compared to direct dropping and drying, spray coating often results in a much uniform distribution of the AgNWs (Fig. S1). Due to the high conformability of the AgNWs (~20 nm diameter and ~30 µm length), the film can well be coated on either flat or microstructured surfaces. Transmittance spectra of AgNW films in the wavelength range of 400-800 nm with different loading densities are shown in Fig. S2. Scanning electron microscopy (SEM) observations indicate that AgNWs become denser with the increasing of loading density from 0.3 µg/cm2 to 9.4 µg/cm2 (Fig. 1B-G), and correspondingly, optical images taken by using an IR camera get brighter and clearer (Fig. 1H). Optical properties of AgNW films. Ultrathin AgNW films with a diameter of less than 25 nm have been applied as highly transparent electrodes. While our AgNW network exhibits a high transmittance (~95% at 6.3 µg/cm2 spraying density of Fig. 1E) and a tiny reflectance, while the reflectance significantly increases to larger than 60% in the wavelength range of 8-14 µm. Since the room temperature black body radiation is centered at ~10 µm, the AgNW film would be well visible under IR camera (Fig. 2A, and optical properties of Fig. 2B). The result indicates that at room temperature the AgNW film is invisible by naked eyes but can be detected by using an IR camera, and therefore it might be used for anti-counterfeiting. An ideal anti-counterfeiting should be fully transparent in the visible range but fully reflective in the IR range, as indicated by the red solid line (Fig. 2A). Here for the AgNW film, further increase of nanowire density will lead to a higher reflectance in the IR range, but also a decrease in transmittance in visible range, therefore a tradeoff between the two is required, and we chose a loading density of 3.1 µg/cm2 with a detectable IR reflectance in this study. The huge change in reflectance under visible and IR range can also be confirmed by our simulation based on finite element modeling (FEM). The simulation is based on an Ag network that mimics the network contours of an SEM image (3.1 µg/cm2) shows that the transmittance is 99.1% at 550 nm wavelength and the reflectance is 23.2% over 10 µm wavelength, in good consistency with our experimental data of 97.4% and 26.4%, respectively (Fig. 2C). When such a patterned AgNW film is coated on polymers and ceramics which exhibits low IR reflectance, the pattern will show a significant contrast to the surroundings. 5
Scratch
resistance,
temperature
stability,
and
flexibility
of
AgNW-based
anti-counterfeiting. An anti-counterfeiting material needs to be robust and stable, for example, by hand touching, or by wearing with abrasive paper in some extreme conditions. In our experiment, an AgNW pattern deposited on a piece of abrasive paper still maintains the contour after a few times of scratches with another abrasive paper, even though the surface of the sprayed area was fully scratched (Fig. 3A). AgNW is also a material that is stable by heating at an elevated temperature up to 200 °C that ensures long-time readout without photobleaching effect. Fig. 3B shows the recognition of a star pattern that was heated at 200 °C for 2 h under air atmosphere. In addition to this, the anti-counterfeiting patterns can also be clearly observed under different temperatures, from room temperature to 80 °C and to 200 °C (Fig. S3). However, the IR image of the AgNW pattern is negative at temperatures (e.g., 80 °C and 200 °C) higher than that of the environment, this is because AgNWs plays the role of a mirror that reflects IR radiation from the hot side. In some special scenarios, anti-counterfeiting patterns are required to be coated on curved surfaces, and spray coating is capable of depositing AgNWs or various curved surfaces. Fig. S4 shows an AgNW pattern deposited on a plastic cylinder with a curvature of 26 mm, which can also be well observed using an IR camera. AgNWs have been widely used as a flexible and stretchable electrode [21,22], and here we deposited AgNWs on PDMS substrate, and the IR pattern can be observed under a large strain of 100% (Fig. 3C). Such a large stretchability allows AgNWs to be applied to different flexible substrates. Compared with other anti-counterfeiting materials, AgNWs present advantages in several aspects. First, AgNW synthesis is simple and cost-effective, and the spray process is also simple and can be applied to various substrates. By contrast, most anti-counterfeiting inks are difficult to disperse and synthesize, and the ink is often toxic and expensive oily medium and cannot tolerate high temperature[2]. Second, silver nanowires (which often has a polyvinyl pyrrolidone on the surface) has also been proven to be chemically stable both in solution and in air, and stable in mildly elevated temperatures up to 200 °C [23,24]. Third, AgNW films are often used as transparent and flexible electrodes, and this allows the anti-counterfeiting design to be combined with flexible electronic applications, although we did not demonstrate 6
any electronic devices or circuits in this work. Application of AgNW-based anti-counterfeiting. AgNWs can easily be sprayed on flat surfaces and the distributed morphology is shown in Fig. 4A with a lightweight spraying density of 3.1 µg/cm2 (> 97% transparency, Fig. S2), which is dense enough to be readout by using an IR camera (detecting wavelength 8~14 µm). Here we show a few cases that AgNWs might be applied for anti-counterfeiting. Fig. 4B shows an AgNW pattern of ‘FLEX’ on a piece of paper currency with a size of 75 mm×25 mm. Because of the high transmittance, the pattern could not be perceived by naked eyes, but the corresponding IR image well reflects the pattern. Such an anti-counterfeiting function can also be extended to other situations, and we have shown a few other anti-counterfeiting patterns that are taken by using a cell phone and an IR camera (Fig. 4C and F). Especially, a grey image is observed with different brightness when the loading density of AgNWs is changed at different parts for bars (Fig. 4E). We expect that by using a high-resolution ink printer, even more complicated grey scale IR patterns can be prepared. We also show that the anti-counterfeiting pattern of a QR code, which might be used in electronic-commerce and internet security (Fig. 4F). In addition to spray on flat surfaces, the AgNWs anti-counterfeiting can also form a good conformability on different rough surfaces. As shown in Fig. 4H to J, AgNWs form a perfect cladding on microstructured substrates such as matte papers, abrasive paper (roughness of no. 10000 #), PDMS replica of abrasive paper (roughness of no. 1000 #), and PDMS replica of Calathea zebrina leaf that shows a microconed surface. The conformability of AgNWs is attributed to the low buckling force with the ultrathin diameter and high aspect ratio [25,26]. Fig. S5 shows that AgNWs with larger diameters (30 nm, 40 nm, and 60 nm) could not well conform to microconed surfaces. In addition, when removing HPMC in the AgNW solution, even the AgNW with a diameter of ~20 nm cannot well comply with the rough surface (Fig. S6). Other common metal networks. We need to point out that the AgNW film is not the only metal network that can be used for anti-counterfeiting applications. Other metal networks with a high transparency may also be used for anti-counterfeiting purposes. Here, we demonstrate that Au nanomesh [27,28] pattern made by using grain boundary lithography 7
with a transparency of ~83% was easily authenticated (shown in Fig. S7). 4. Conclusion In conclusion, we have demonstrated a novel anti-counterfeiting technique based on ultra-transparent AgNWs. The AgNWs network is highly transparent (~95% at 6.3 µg/cm2 spraying density) in the visible range, while exhibiting a high reflectance in the middle and far-infrared range, making AgNW patterns an ideal selection for IR anti-counterfeit. Due to the ultrathin diameter and high-aspect-ratio property, the AgNWs can easily be sprayed onto the rough surface with a good conformability, and thus can be applied not on flat but also rough surfaces. Such AgNW-based anti-counterfeiting has enormous potential applications in medicines, artwork, banknotes, brand luxuries, industrial parts, and so on.
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work was financially supported by the funds of the National Natural Science Foundation of China (no. 51771089 & U1613204), the “Guangdong Innovative and Entrepreneurial Research Team Program” under contract no. 2016ZT06G587, the “Science Technology and Innovation Committee of Shenzhen Municipality” (grant no. JCYJ20160613160524999), the Shenzhen Sci-Tech Fund (no. KYTDPT20181011104007).
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[4] Heo Y, Kang H, Lee JS, Oh YK, Kim SH. Lithographically encrypted inverse opals for anti-counterfeiting applications. Small 2016; 12:3819-3826. [5] Chen FF, Zhu YJ, Zhang QQ, Yang RL, Qin DD, Xiong ZC. Secret paper with vinegar as an invisible security ink and fire as a decryption key for information protection. Chem-Eur J 2019; 25:10918-10925. [6] Andres J, Hersch RD, Moser JE, Chauvin AS. A new anti-counterfeiting feature relying on invisible luminescent full color images printed with lanthanide-based inks. Adv Funct Mater 2014; 24:5029-5036. [7] Kaczmarek AM, Liu YY, Wang C, Laforce B, Vincze L, et al. Lanthanide "chameleon" multistage anti-counterfeit materials. Adv Funct Mater 2017; 27(20). [8] De Cremer G, Sels BF, Hotta JI, Roeffaers MBJ, Bartholomeeusen E, et al. Optical encoding of silver zeolite microcarriers. Adv Mater 2010; 22: 957. [9] Zhang Y, Zhang L, Deng R, Tian J, Zong Y, Jin D, Liu X. Multicolor barcoding in a single upconversion crystal. J Am Chem Soc 2014; 136:4893-4896. [10] Bao B, Li M, Li Y, Jiang J, Gu Z, et al. Patterning fluorescent quantum dot nanocomposites by reactive inkjet printing. Small 2015; 11:1649-1654. [11] Kalytchuk S, Wang Y, Polakova K, Zboril R. Carbon dot fluorescence-lifetime-encoded anti-counterfeiting. Acs Appl Mater Inter 2018; 10:29902-29908. [12] Da Luz LL, Milani R, Feix JF, Ribeiro IRB, Talhavini M, et al. Inkjet printing of lanthanide-organic frameworks for anti-counterfeiting applications. Acs Appl Mater Inter 2015; 7:27115-27123. [13] Fukuoka T, Yamaguchi A, Hara R, Matsumoto T, Utsumi Y, Mori Y. Application of gold nanoparticle self-assemblies to unclonable anti-counterfeiting technology. 2015 International Conference on Electronics Packaging and iMAPS All Asia Conference (ICEP-IAAC). IEEE, 2015: 432-435. [14] Park K, Jung K, Kwon SJ, Jang HS, Byun D, et al. Plasmonic nanowire-enhanced upconversion luminescence for anticounterfeit devices. Adv Funct Mater 2016; 26:7836-7846. [15] Smith AF, Skrabalak SE. Metal nanomaterials for optical anti-counterfeit labels. J Mater Chem C 2017; 5:3207-3215. [16] De S, Higgins TM, Lyons PE, Doherty EM, Nirmalraj PN, et al. Silver nanowire networks as flexible, transparent, conducting films: extremely high DC to optical conductivity ratios. Acs Nano 2009; 3:1767-1774. [17] Spechler JA, Arnold CB. Direct-write pulsed laser processed silver nanowire networks for transparent conducting electrodes. Appl Phys a-Mater 2012; 108:25-28. [18] Hsu PC, Liu X, Liu C, Xie X, Lee HR, et al. Personal thermal management by metallic nanowire-coated textile. Nano Lett 2015; 15:365-371. [19] Lee SM, Cho Y, Kim DY, Chae JS, Choi KC. Enhanced light extraction from 9
mechanically flexible, nanostructured organic light-emitting diodes with plasmonic nanomesh electrodes. Adv Opt Mater 2015; 3:1240-1247. [20] Li B, Ye S, Stewart IE, Alvarez S, Wiley BJ. Synthesis and purification of silver nanowires to make conducting films with a transmittance of 99%. Nano Lett 2015; 15:6722-6726. [21] Liang J, Tong K, Pei Q. A water-based silver-nanowire screen-print ink for the fabrication of stretchable conductors and wearable thin-film transistors. Adv Mater 2016; 28:5986-5996. [22] Liu Y, Zhang J, Gao H, Wang Y, Liu Q, et al. Capillary-force-induced cold welding in silver-nanowire-based flexible transparent electrodes. Nano Lett 2017; 17:1090-1096. [23] Zhu Q, Zhang ZJ, Sun Z, Cai B, Cai WJ. Preparation of transparent and conductive silver nanowires films by screen printing method. Chin J Inorg Chem 2016; 32:782-788. [24] Madaria AR, Kumar A, Ishikawa FN, Zhou C. uniform, highly conductive, and patterned transparent flms of a percolating silver nanowire network on rigid and flexible substrates using a dry transfer technique. Nano Res 2010; 3: 564-573. [25] Heidelberg A, Ngo LT, Bin W, Phillips MA, Sharma S, et al. A generalized description of the elastic properties of nanowires. Nano Lett 2006; 6:1101-1106. [26] Inaba K, Saida K, Ghosh P, Matsubara K, Subramanian M, et al. Determination of Young's modulus of carbon nanofiber probes fabricated by the argon ion bombardment of carbon coated silicon cantilever. Carbon 2011; 49:4191-4196. [27] Guo CF, Sun T, Liu Q, Suo Z, Ren Z. Highly stretchable and transparent nanomesh electrodes made by grain boundary lithography. Nat Commun 2014; 5:3121. [28] Wang Y, Liu Q, Zhang J, Hong T, Sun W, et al. Giant poisson's effect for wrinkle-free stretchable transparent electrodes. Adv Mater 2019; 31 (35).
10
11
Figures
Fig. 1. Fabrication AgNW patterns. (A) Schematic of the spraying process of AgNW patterns. (B-G) SEM images AgNWs films (adding with 0.05 wt% adhesive) with different loading densities. (H) The intensity of reflected infrared (IR) light of the AgNW pattern increases with the loading density. The scale bars are 5 µm.
12
Fig. 2. Optical properties of AgNW films. (A) Transmittance and reflectance spectra in the wavelength range from 0.4-16 µm for AgNW films with loading densities of 3.1 µg/cm2 and 6.3 µg/cm2.Reflectance spectra of the AgNW film over 0.4-16 µm. A normalized black body radiation spectrum of 298 K is also shown with the green dashed line. (B) Schematic of AgNW-based anti-counterfeiting. (C) FEM results for the AgNW film with a density of 3.1 µg/cm2 showing that the transmittance is 99.1% at 550 nm and the reflectance is 23.2% over 10 µm, which in good consistency with our experimental data of 97.4% and 26.4%, respectively.
13
Fig.
3. Properties
of AgNW-based anti-counterfeiting. (A) IR
images for an
anti-counterfeiting pattern on abrasive paper before and after scratching by another abrasive paper. (B) IR images for an anti-counterfeiting pattern on flat PDMS before and after being heated at 200 °C for 2 h. (C) IR images for an anti-counterfeiting pattern on PDMS before and after stretching (strain=100%). The scale bars are 5 mm.
14
Fig. 4. Application of AgNW-based anti-counterfeiting. (A) SEM image of an AgNW network sprayed on a flat PDMS substrate. (B-F) Photographic and IR images of different anti-counterfeiting patterns on different flat surfaces (paper currency for Figure B, PDMS for Figure C, D, E, and glass for Figure F, respectively). (G-J) SEM images, and corresponding normal photographic and IR images of anti-counterfeiting patterns on different rough surfaces (matte papers for Figure G, abrasive paper (roughness of no. 10000 #) for Figure H, PDMS replica of abrasive paper (roughness of no. 1000 #) for Figure I, and PDMS replica of Calathea zebrina leaf for Figure J). The scale bars are 10 mm. The spray density is 3.1 µg/cm2 except for 1.6 µg/cm2 for the lighter stripes and 6.3 µg/cm2 for the brighter strips in Figure E.
15
Silver nanowires for anti-counterfeiting
Yan Wang a,b, Ningning Bai a,b, Junlong Yang b, Zhiguang Liu b, Gang Li b, Minkun Cai b, Lingyu Zhao b, Yuan Zhang b, Jianming Zhang b, Chunhua Li c, Yunlong Zhou c, Chuan Fei Guo b,*
a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin
150001, China b
Department of Materials Science & Engineering, and Centers for Mechanical Engineering
Research and Education at MIT and SUSTech, Southern University of Science and Technology, Shenzhen 518055, China c
Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment
Devices of Zhejiang Province, Wenzhou Institute of Biomaterials and Engineering, University of Chinese Academy of Sciences, Wenzhou, 325000, PR China. * Corresponding author. E-mail address: [email protected].
16
Fig. S1. SEM images of AgNWs on silicon substrate deposited by direct dispensing process and spray process.
Fig. S2. Transmittance spectra of AgNW films with different loadings.
Fig. S3. The AgNWs pattern can be detected not only at room temperature, but also at high temperatures at 80 °C and 160 °C. 17
Fig. S4. The AgNWs patterns deposited on a plastic bottle with a curvature radius of 26 mm, and a corresponding IR image.
Fig. S5. SEM images of AgNWs with different diameters (20, 30, 40, and 60 nm) coated on a microcone array. The scale bars are 10 µm.
Fig. S6. AgNWs with a diameter of 20 nm do not well comply with the rough microsurface after removing HPMC from the dispersion.
18
Fig. S7. Other transparent metal networks (such as an Au nanomesh, ~83% transparency) can also be used for IR anti-counterfeiting.
19
Highlights
1. A new anti-counterfeiting technology based on high reflectance in the infrared range for silver nanowire films. 2. The ultrathin silver nanowires can be applied to a variety of substrates with different roughness or rigidity. 3. The silver nanowire-based anti-counterfeiting patterns exhibits high flexibility and high mechanical stability.
Author Biography Chuan Fei Guo is an associate professor in the Department of Materials Science & Technology, the Southern University of Science and Technology, China. He received his Ph.D. degree in condensed matter physics from the National Center for Nanoscience and Technology, (NCNST), Chinese Academy of Sciences, China. From 2011 to 2016, Dr. Guo worked as a postdoctoral fellow and research associate at Boston College and the University of Houston. He has ten years of experience in flexible electronics and advanced manufacturing.
Author Photo
S2352-8478(19)30180-7
DOI:
https://doi.org/10.1016/j.jmat.2020.01.008
Reference:
JMAT 269
To appear in:
Journal of Materiomics
Received Date: 30 September 2019 Revised Date:
9 December 2019
Accepted Date: 15 January 2020
Please cite this article as: Wang Y, Bai N, Yang J, Liu Z, Li G, Cai M, Zhao L, Zhang Y, Zhang J, Li C, Zhou Y, Guo CF, Silver nanowires for anti-counterfeiting, Journal of Materiomics (2020), doi: https:// doi.org/10.1016/j.jmat.2020.01.008. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. All rights reserved.
Graphical Abstract
Silver nanowires for anti-counterfeiting
Yan Wang a,b, Ningning Bai a,b, Junlong Yang b, Zhiguang Liu b, Gang Li b, Minkun Cai b, Lingyu Zhao b, Yuan Zhang b, Jianming Zhang b, Chunhua Li c, Yunlong Zhou c, Chuan Fei Guo b,*
a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin
150001, China b
Department of Materials Science & Engineering, and Centers for Mechanical Engineering
Research and Education at MIT and SUSTech, Southern University of Science and Technology, Shenzhen 518055, China c
Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment
Devices of Zhejiang Province, Wenzhou Institute of Biomaterials and Engineering, University of Chinese Academy of Sciences, Wenzhou, 325000, PR China. * Corresponding author. E-mail address: [email protected].
1
Abstract Silver nanowire (AgNW) films have been widely used as flexible transparent electrodes due to the high electrical conductance and high transmittance in the visible range. The infrared (IR) properties of AgNW films, however, are seldom discussed. Here, we show that ultrathin AgNWs present high transmittance in the visible range (~95%) and high reflectance (60-70%) in the IR range of 8-14 µm. Such a significant difference makes AgNW films invisible to naked eyes but visible under an IR camera, being an ideal selection for anti-counterfeit technologies, while no excitation is required. We have demonstrated that AgNW films can be spray-coated on either flat or microstructured surfaces with high conformability and high scratch-resistance, and thus these anti-counterfeiting materials can be applied to a variety of applications. The AgNW-based anti-counterfeiting may be helpful for combating counterfeiting crimes in the fields of medicines, artwork, banknotes, brand luxuries, industrial parts, and so on.
Keywords:
silver
nanowires;
anti-counterfeiting;
infrared
light;
excitation-free;
conformability
1. Introduction Counterfeit and shoddy products flood the market worldwide, causing great risk to human health and national security [1,2]. Accordingly, various anti-counterfeiting methods and products have been emerged [3-5]. The most popular anti-counterfeiting technology is based on colorless or light-colored luminescent materials that emit light under the excitation of ultraviolet (UV) or near-infrared (NIR) light to reveal different colors [6-9]. Luminescent anti-counterfeiting technology relies highly on luminescent materials, including lanthanide nanomaterials [6,7,9], quantum dots [10,11], and metal-organic frameworks [12], which can be homogeneously dispersed into ink medium and coated on target substrates. Such materials have received great attention for the virtue of a wide absorption wavelength range, a narrow 2
emission wavelength range, high fluorescence efficiency, and tunable luminescent properties by changing their composition, particle size and shape, and ways of coating [3]. In order to enhance the luminescence signals, metal nanowires or nanoparticles have been randomly dispersed into a fluorescent dye to form a plasmonic mode, leading to the concentration of the incident light [13-15]. Despite the developments in providing approaches for fabricating multifunctional nanomaterials, most luminescent materials are toxic, difficult to disperse and synthesize in an oily medium, and excitation sources are needed. The major challenge in the present scenario is the full exploitation of luminescent nanomaterials which are free of these constraints. Silver nanowires (AgNWs), as a widely used material for transparent flexible electrodes, have attracted much attention in flexible electronics due to their cost-effective preparation, ultra-flexibility, high transparency in the visible range, together with high conductivity [16-19]. Especially, AgNW with a diameter of ~25 nm or smaller and a length to diameter aspect ratio over 1000 is almost fully transparent [20]. While the high transparency in the visible range is well studied, the optical properties of AgNWs in infrared (IR) range has caused little attention. In this work, we found that ultrathin AgNW films exhibit an exceptionally high transmittance (~95%) in the visible range but high reflectance (60-70%) in the IR range of 8-14 µm, corresponding to the room temperature black body radiation. That is, at room temperature, the ultrathin AgNW film is almost invisible by naked eyes while being well visible under an IR camera. Herein, we utilized ultrathin AgNWs for anti-counterfeiting through spray coating. Because of the high aspect ratio and small diameter of the AgNWs (~ 20 nm), the film can be well conformable to surface microstructures, and the high conformability on rough substrates allows the film to be highly scratch-resistant. When coated on a stretchable substrate, the anti-counterfeiting effect maintains at a strain up to 100%. The AgNW films are expected to have a variety of applications in high-value merchandise, documents, pharmaceuticals, artworks, and banknotes.
2. Materials and methods Materials: AgNWs dispersion with different nanowire diameters (~20 nm, ~30 nm, ~ 40 3
nm, ~60 nm) were purchased from HeFei Vigon Material Technology Co., Ltd, China. Hydroxypropyl methyl cellulose (HPMC, viscosity of 2600~5600 cP) was purchased from Sigma, USA. The polydimethylsiloxane (PDMS) elastomer (Sylgard 184) was purchased from Dow Corning Co., Ltd, USA. Fabrication of Microstructures: Different materials (such as matte papers, abrasive papers, fresh Calathea zebrina leaves, etc.) with microstructures were cut into rectangular shapes and fixed on a glass substrate using Scotch tapes (3M Co., USA). PDMS with a base to curing agent volumetric ratio of 10:1 was prepared and placed into a vacuum drying chamber until bubbles disappeared, then curing at 70 °C for 1 h and peeled off from the microstructured substrate to get an inverse structure. To weaken the adhesion during the second molding process, the inverse structure was treated by a plasmon cleaning machine for 2 min. The second molding process was the same as the first molding process to achieve the PDMS replica. Spray coating of AgNWs: AgNWs (0.25 mg/ml) were dispersed with 0.05 wt% HPMC working as the spraying solution. Receiver substrates were heated at 80 °C during spraying to ensure that the droplets evaporate rapidly without streaming. By using an automatic spray gun (Ho-CN, ANEST IWATA, Japan) operating by an air pump (IS-51, ANEST IWATA, Japan), the spraying density were controlled to 0.3 µg/cm2, 0.8 µg/cm2, 1.6 µg/cm2, 3.1 µg/cm2, 6.3 µg/cm2, 9.4 µg/cm2. Characterization. The negative plastic masks were cut by using a CO2 laser cutting machine (WE-640, Xiamen Waner, China). The SEM images were taken by using a scanning electron microscope (TESCAN MIRA3, TESCAN, Czech Republic). The transmittance and reflectance of the samples were recorded with a spectrometer (Lambda 950, PerkinElmer, USA) and FTIR (ImageIR 7300, Bruke, Germany ). A commercial IR camera (226s, Fotric, USA) was used to receive IR radiation (8~14 µm).
3. Results and discussions Preparation of AgNWs based anti-counterfeiting. The AgNW film was prepared by using spray coating and a shadow mask was used to get form desired patterns (Fig. 1A). 4
Compared to direct dropping and drying, spray coating often results in a much uniform distribution of the AgNWs (Fig. S1). Due to the high conformability of the AgNWs (~20 nm diameter and ~30 µm length), the film can well be coated on either flat or microstructured surfaces. Transmittance spectra of AgNW films in the wavelength range of 400-800 nm with different loading densities are shown in Fig. S2. Scanning electron microscopy (SEM) observations indicate that AgNWs become denser with the increasing of loading density from 0.3 µg/cm2 to 9.4 µg/cm2 (Fig. 1B-G), and correspondingly, optical images taken by using an IR camera get brighter and clearer (Fig. 1H). Optical properties of AgNW films. Ultrathin AgNW films with a diameter of less than 25 nm have been applied as highly transparent electrodes. While our AgNW network exhibits a high transmittance (~95% at 6.3 µg/cm2 spraying density of Fig. 1E) and a tiny reflectance, while the reflectance significantly increases to larger than 60% in the wavelength range of 8-14 µm. Since the room temperature black body radiation is centered at ~10 µm, the AgNW film would be well visible under IR camera (Fig. 2A, and optical properties of Fig. 2B). The result indicates that at room temperature the AgNW film is invisible by naked eyes but can be detected by using an IR camera, and therefore it might be used for anti-counterfeiting. An ideal anti-counterfeiting should be fully transparent in the visible range but fully reflective in the IR range, as indicated by the red solid line (Fig. 2A). Here for the AgNW film, further increase of nanowire density will lead to a higher reflectance in the IR range, but also a decrease in transmittance in visible range, therefore a tradeoff between the two is required, and we chose a loading density of 3.1 µg/cm2 with a detectable IR reflectance in this study. The huge change in reflectance under visible and IR range can also be confirmed by our simulation based on finite element modeling (FEM). The simulation is based on an Ag network that mimics the network contours of an SEM image (3.1 µg/cm2) shows that the transmittance is 99.1% at 550 nm wavelength and the reflectance is 23.2% over 10 µm wavelength, in good consistency with our experimental data of 97.4% and 26.4%, respectively (Fig. 2C). When such a patterned AgNW film is coated on polymers and ceramics which exhibits low IR reflectance, the pattern will show a significant contrast to the surroundings. 5
Scratch
resistance,
temperature
stability,
and
flexibility
of
AgNW-based
anti-counterfeiting. An anti-counterfeiting material needs to be robust and stable, for example, by hand touching, or by wearing with abrasive paper in some extreme conditions. In our experiment, an AgNW pattern deposited on a piece of abrasive paper still maintains the contour after a few times of scratches with another abrasive paper, even though the surface of the sprayed area was fully scratched (Fig. 3A). AgNW is also a material that is stable by heating at an elevated temperature up to 200 °C that ensures long-time readout without photobleaching effect. Fig. 3B shows the recognition of a star pattern that was heated at 200 °C for 2 h under air atmosphere. In addition to this, the anti-counterfeiting patterns can also be clearly observed under different temperatures, from room temperature to 80 °C and to 200 °C (Fig. S3). However, the IR image of the AgNW pattern is negative at temperatures (e.g., 80 °C and 200 °C) higher than that of the environment, this is because AgNWs plays the role of a mirror that reflects IR radiation from the hot side. In some special scenarios, anti-counterfeiting patterns are required to be coated on curved surfaces, and spray coating is capable of depositing AgNWs or various curved surfaces. Fig. S4 shows an AgNW pattern deposited on a plastic cylinder with a curvature of 26 mm, which can also be well observed using an IR camera. AgNWs have been widely used as a flexible and stretchable electrode [21,22], and here we deposited AgNWs on PDMS substrate, and the IR pattern can be observed under a large strain of 100% (Fig. 3C). Such a large stretchability allows AgNWs to be applied to different flexible substrates. Compared with other anti-counterfeiting materials, AgNWs present advantages in several aspects. First, AgNW synthesis is simple and cost-effective, and the spray process is also simple and can be applied to various substrates. By contrast, most anti-counterfeiting inks are difficult to disperse and synthesize, and the ink is often toxic and expensive oily medium and cannot tolerate high temperature[2]. Second, silver nanowires (which often has a polyvinyl pyrrolidone on the surface) has also been proven to be chemically stable both in solution and in air, and stable in mildly elevated temperatures up to 200 °C [23,24]. Third, AgNW films are often used as transparent and flexible electrodes, and this allows the anti-counterfeiting design to be combined with flexible electronic applications, although we did not demonstrate 6
any electronic devices or circuits in this work. Application of AgNW-based anti-counterfeiting. AgNWs can easily be sprayed on flat surfaces and the distributed morphology is shown in Fig. 4A with a lightweight spraying density of 3.1 µg/cm2 (> 97% transparency, Fig. S2), which is dense enough to be readout by using an IR camera (detecting wavelength 8~14 µm). Here we show a few cases that AgNWs might be applied for anti-counterfeiting. Fig. 4B shows an AgNW pattern of ‘FLEX’ on a piece of paper currency with a size of 75 mm×25 mm. Because of the high transmittance, the pattern could not be perceived by naked eyes, but the corresponding IR image well reflects the pattern. Such an anti-counterfeiting function can also be extended to other situations, and we have shown a few other anti-counterfeiting patterns that are taken by using a cell phone and an IR camera (Fig. 4C and F). Especially, a grey image is observed with different brightness when the loading density of AgNWs is changed at different parts for bars (Fig. 4E). We expect that by using a high-resolution ink printer, even more complicated grey scale IR patterns can be prepared. We also show that the anti-counterfeiting pattern of a QR code, which might be used in electronic-commerce and internet security (Fig. 4F). In addition to spray on flat surfaces, the AgNWs anti-counterfeiting can also form a good conformability on different rough surfaces. As shown in Fig. 4H to J, AgNWs form a perfect cladding on microstructured substrates such as matte papers, abrasive paper (roughness of no. 10000 #), PDMS replica of abrasive paper (roughness of no. 1000 #), and PDMS replica of Calathea zebrina leaf that shows a microconed surface. The conformability of AgNWs is attributed to the low buckling force with the ultrathin diameter and high aspect ratio [25,26]. Fig. S5 shows that AgNWs with larger diameters (30 nm, 40 nm, and 60 nm) could not well conform to microconed surfaces. In addition, when removing HPMC in the AgNW solution, even the AgNW with a diameter of ~20 nm cannot well comply with the rough surface (Fig. S6). Other common metal networks. We need to point out that the AgNW film is not the only metal network that can be used for anti-counterfeiting applications. Other metal networks with a high transparency may also be used for anti-counterfeiting purposes. Here, we demonstrate that Au nanomesh [27,28] pattern made by using grain boundary lithography 7
with a transparency of ~83% was easily authenticated (shown in Fig. S7). 4. Conclusion In conclusion, we have demonstrated a novel anti-counterfeiting technique based on ultra-transparent AgNWs. The AgNWs network is highly transparent (~95% at 6.3 µg/cm2 spraying density) in the visible range, while exhibiting a high reflectance in the middle and far-infrared range, making AgNW patterns an ideal selection for IR anti-counterfeit. Due to the ultrathin diameter and high-aspect-ratio property, the AgNWs can easily be sprayed onto the rough surface with a good conformability, and thus can be applied not on flat but also rough surfaces. Such AgNW-based anti-counterfeiting has enormous potential applications in medicines, artwork, banknotes, brand luxuries, industrial parts, and so on.
Conflict of interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work was financially supported by the funds of the National Natural Science Foundation of China (no. 51771089 & U1613204), the “Guangdong Innovative and Entrepreneurial Research Team Program” under contract no. 2016ZT06G587, the “Science Technology and Innovation Committee of Shenzhen Municipality” (grant no. JCYJ20160613160524999), the Shenzhen Sci-Tech Fund (no. KYTDPT20181011104007).
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[4] Heo Y, Kang H, Lee JS, Oh YK, Kim SH. Lithographically encrypted inverse opals for anti-counterfeiting applications. Small 2016; 12:3819-3826. [5] Chen FF, Zhu YJ, Zhang QQ, Yang RL, Qin DD, Xiong ZC. Secret paper with vinegar as an invisible security ink and fire as a decryption key for information protection. Chem-Eur J 2019; 25:10918-10925. [6] Andres J, Hersch RD, Moser JE, Chauvin AS. A new anti-counterfeiting feature relying on invisible luminescent full color images printed with lanthanide-based inks. Adv Funct Mater 2014; 24:5029-5036. [7] Kaczmarek AM, Liu YY, Wang C, Laforce B, Vincze L, et al. Lanthanide "chameleon" multistage anti-counterfeit materials. Adv Funct Mater 2017; 27(20). [8] De Cremer G, Sels BF, Hotta JI, Roeffaers MBJ, Bartholomeeusen E, et al. Optical encoding of silver zeolite microcarriers. Adv Mater 2010; 22: 957. [9] Zhang Y, Zhang L, Deng R, Tian J, Zong Y, Jin D, Liu X. Multicolor barcoding in a single upconversion crystal. J Am Chem Soc 2014; 136:4893-4896. [10] Bao B, Li M, Li Y, Jiang J, Gu Z, et al. Patterning fluorescent quantum dot nanocomposites by reactive inkjet printing. Small 2015; 11:1649-1654. [11] Kalytchuk S, Wang Y, Polakova K, Zboril R. Carbon dot fluorescence-lifetime-encoded anti-counterfeiting. Acs Appl Mater Inter 2018; 10:29902-29908. [12] Da Luz LL, Milani R, Feix JF, Ribeiro IRB, Talhavini M, et al. Inkjet printing of lanthanide-organic frameworks for anti-counterfeiting applications. Acs Appl Mater Inter 2015; 7:27115-27123. [13] Fukuoka T, Yamaguchi A, Hara R, Matsumoto T, Utsumi Y, Mori Y. Application of gold nanoparticle self-assemblies to unclonable anti-counterfeiting technology. 2015 International Conference on Electronics Packaging and iMAPS All Asia Conference (ICEP-IAAC). IEEE, 2015: 432-435. [14] Park K, Jung K, Kwon SJ, Jang HS, Byun D, et al. Plasmonic nanowire-enhanced upconversion luminescence for anticounterfeit devices. Adv Funct Mater 2016; 26:7836-7846. [15] Smith AF, Skrabalak SE. Metal nanomaterials for optical anti-counterfeit labels. J Mater Chem C 2017; 5:3207-3215. [16] De S, Higgins TM, Lyons PE, Doherty EM, Nirmalraj PN, et al. Silver nanowire networks as flexible, transparent, conducting films: extremely high DC to optical conductivity ratios. Acs Nano 2009; 3:1767-1774. [17] Spechler JA, Arnold CB. Direct-write pulsed laser processed silver nanowire networks for transparent conducting electrodes. Appl Phys a-Mater 2012; 108:25-28. [18] Hsu PC, Liu X, Liu C, Xie X, Lee HR, et al. Personal thermal management by metallic nanowire-coated textile. Nano Lett 2015; 15:365-371. [19] Lee SM, Cho Y, Kim DY, Chae JS, Choi KC. Enhanced light extraction from 9
mechanically flexible, nanostructured organic light-emitting diodes with plasmonic nanomesh electrodes. Adv Opt Mater 2015; 3:1240-1247. [20] Li B, Ye S, Stewart IE, Alvarez S, Wiley BJ. Synthesis and purification of silver nanowires to make conducting films with a transmittance of 99%. Nano Lett 2015; 15:6722-6726. [21] Liang J, Tong K, Pei Q. A water-based silver-nanowire screen-print ink for the fabrication of stretchable conductors and wearable thin-film transistors. Adv Mater 2016; 28:5986-5996. [22] Liu Y, Zhang J, Gao H, Wang Y, Liu Q, et al. Capillary-force-induced cold welding in silver-nanowire-based flexible transparent electrodes. Nano Lett 2017; 17:1090-1096. [23] Zhu Q, Zhang ZJ, Sun Z, Cai B, Cai WJ. Preparation of transparent and conductive silver nanowires films by screen printing method. Chin J Inorg Chem 2016; 32:782-788. [24] Madaria AR, Kumar A, Ishikawa FN, Zhou C. uniform, highly conductive, and patterned transparent flms of a percolating silver nanowire network on rigid and flexible substrates using a dry transfer technique. Nano Res 2010; 3: 564-573. [25] Heidelberg A, Ngo LT, Bin W, Phillips MA, Sharma S, et al. A generalized description of the elastic properties of nanowires. Nano Lett 2006; 6:1101-1106. [26] Inaba K, Saida K, Ghosh P, Matsubara K, Subramanian M, et al. Determination of Young's modulus of carbon nanofiber probes fabricated by the argon ion bombardment of carbon coated silicon cantilever. Carbon 2011; 49:4191-4196. [27] Guo CF, Sun T, Liu Q, Suo Z, Ren Z. Highly stretchable and transparent nanomesh electrodes made by grain boundary lithography. Nat Commun 2014; 5:3121. [28] Wang Y, Liu Q, Zhang J, Hong T, Sun W, et al. Giant poisson's effect for wrinkle-free stretchable transparent electrodes. Adv Mater 2019; 31 (35).
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Figures
Fig. 1. Fabrication AgNW patterns. (A) Schematic of the spraying process of AgNW patterns. (B-G) SEM images AgNWs films (adding with 0.05 wt% adhesive) with different loading densities. (H) The intensity of reflected infrared (IR) light of the AgNW pattern increases with the loading density. The scale bars are 5 µm.
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Fig. 2. Optical properties of AgNW films. (A) Transmittance and reflectance spectra in the wavelength range from 0.4-16 µm for AgNW films with loading densities of 3.1 µg/cm2 and 6.3 µg/cm2.Reflectance spectra of the AgNW film over 0.4-16 µm. A normalized black body radiation spectrum of 298 K is also shown with the green dashed line. (B) Schematic of AgNW-based anti-counterfeiting. (C) FEM results for the AgNW film with a density of 3.1 µg/cm2 showing that the transmittance is 99.1% at 550 nm and the reflectance is 23.2% over 10 µm, which in good consistency with our experimental data of 97.4% and 26.4%, respectively.
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Fig.
3. Properties
of AgNW-based anti-counterfeiting. (A) IR
images for an
anti-counterfeiting pattern on abrasive paper before and after scratching by another abrasive paper. (B) IR images for an anti-counterfeiting pattern on flat PDMS before and after being heated at 200 °C for 2 h. (C) IR images for an anti-counterfeiting pattern on PDMS before and after stretching (strain=100%). The scale bars are 5 mm.
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Fig. 4. Application of AgNW-based anti-counterfeiting. (A) SEM image of an AgNW network sprayed on a flat PDMS substrate. (B-F) Photographic and IR images of different anti-counterfeiting patterns on different flat surfaces (paper currency for Figure B, PDMS for Figure C, D, E, and glass for Figure F, respectively). (G-J) SEM images, and corresponding normal photographic and IR images of anti-counterfeiting patterns on different rough surfaces (matte papers for Figure G, abrasive paper (roughness of no. 10000 #) for Figure H, PDMS replica of abrasive paper (roughness of no. 1000 #) for Figure I, and PDMS replica of Calathea zebrina leaf for Figure J). The scale bars are 10 mm. The spray density is 3.1 µg/cm2 except for 1.6 µg/cm2 for the lighter stripes and 6.3 µg/cm2 for the brighter strips in Figure E.
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Silver nanowires for anti-counterfeiting
Yan Wang a,b, Ningning Bai a,b, Junlong Yang b, Zhiguang Liu b, Gang Li b, Minkun Cai b, Lingyu Zhao b, Yuan Zhang b, Jianming Zhang b, Chunhua Li c, Yunlong Zhou c, Chuan Fei Guo b,*
a
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin
150001, China b
Department of Materials Science & Engineering, and Centers for Mechanical Engineering
Research and Education at MIT and SUSTech, Southern University of Science and Technology, Shenzhen 518055, China c
Engineering Research Center of Clinical Functional Materials and Diagnosis & Treatment
Devices of Zhejiang Province, Wenzhou Institute of Biomaterials and Engineering, University of Chinese Academy of Sciences, Wenzhou, 325000, PR China. * Corresponding author. E-mail address: [email protected].
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Fig. S1. SEM images of AgNWs on silicon substrate deposited by direct dispensing process and spray process.
Fig. S2. Transmittance spectra of AgNW films with different loadings.
Fig. S3. The AgNWs pattern can be detected not only at room temperature, but also at high temperatures at 80 °C and 160 °C. 17
Fig. S4. The AgNWs patterns deposited on a plastic bottle with a curvature radius of 26 mm, and a corresponding IR image.
Fig. S5. SEM images of AgNWs with different diameters (20, 30, 40, and 60 nm) coated on a microcone array. The scale bars are 10 µm.
Fig. S6. AgNWs with a diameter of 20 nm do not well comply with the rough microsurface after removing HPMC from the dispersion.
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Fig. S7. Other transparent metal networks (such as an Au nanomesh, ~83% transparency) can also be used for IR anti-counterfeiting.
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Highlights
1. A new anti-counterfeiting technology based on high reflectance in the infrared range for silver nanowire films. 2. The ultrathin silver nanowires can be applied to a variety of substrates with different roughness or rigidity. 3. The silver nanowire-based anti-counterfeiting patterns exhibits high flexibility and high mechanical stability.
Author Biography Chuan Fei Guo is an associate professor in the Department of Materials Science & Technology, the Southern University of Science and Technology, China. He received his Ph.D. degree in condensed matter physics from the National Center for Nanoscience and Technology, (NCNST), Chinese Academy of Sciences, China. From 2011 to 2016, Dr. Guo worked as a postdoctoral fellow and research associate at Boston College and the University of Houston. He has ten years of experience in flexible electronics and advanced manufacturing.
Author Photo