Laser Direct Writing of Silver Nanowire with Amino ... - ACS Publications

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Oct 31, 2016 - Yuan-Yuan Zhao,. ‡. Xian-Zi Dong, ... and Zhen-Sheng Zhao*,†. † ..... (15) Zhao, Y. Y.; Zhang, Y. L.; Zheng, M. L.; Dong, X. Z.; Duan, X. M.; Zhao ...


Laser Direct Writing of Silver Nanowire with Amino Acids-Assisted Multiphoton Photoreduction Xue-Liang Ren,†,§ Mei-Ling Zheng,*,† Feng Jin,*,† Yuan-Yuan Zhao,‡ Xian-Zi Dong,† Jie Liu,† Hong Yu,†,§ Xuan-Ming Duan,‡ and Zhen-Sheng Zhao*,† †

Laboratory of Organic NanoPhotonics and CAS Key Laboratory of Bio-Inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, No. 29, Zhongguancun East Road, Beijing 100190, P. R. China ‡ Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, No. 266 Fangzheng Ave, Shuitu Technology Development Zone, Beibei District, Chongqing 400714, P. R. China § University of Chinese Academy of Sciences, No. 29, Zhongguancun East Road, Beijing 100190, P. R. China S Supporting Information *

ABSTRACT: We have demonstrated an approach of silver nanostructure microfabrication by femtosecond laser direct writing of silver ion aqueous solution containing biocompatible amino acids with different molecular architectures. The amino acids-assisted multiphoton photoreduction (MPR) lowers the incident femtosecond laser threshold to 0.32 mW for glycine, 0.72 mW for valine, 0.6 mW for proline, and 0.35 mW for leucine. The line widths of the silver nanowires are varied with the composition of the biocompatible amino acids. The highest resolution of 186 nm has been achieved. Furthermore, the corresponding electrical conductivities of silver nanowires are determined to be 1.48 × 10−6, 4.1 × 10−7, 7.04 × 10−7, and 1.35 × 10−6 Ω m, which have been significantly improved compared to that of the previously reported silver wires fabricated by MPR microfabrication. The proposed protocol for fabricating silver nanostructure would pave the way for fabricating metallic nanostructures and play an extremely important role in the micro/nanofabrication for biological application and the field of photonics.

1. INTRODUCTION Microfabrication of micro/nanostructures and devices by multiphoton absorption using femtosecond laser has attracted extensive research attentions in recent years.1,2 The approach is employed in many cutting-edge fields, e.g., biology,3−6 electronics,7 photonics,8,9 and microfluidics.10 Generally, multiphoton polymerization (MPP) and multiphoton photoreduction (MPR) are two typical multiphoton processes that have been widely applied as powerful tools for fabricating micro/ nanostructures. The feature of MPP is the intrinsic threedimensional (3D) fabrication capability and the nanometerscale resolution,11 owing to the nonlinear effect. MPP is widely used for fabricating fine 3D micro/nanostructures such as photonic crystals,12 micromachines,13,14 Luneburg lens,15 and other arbitrary 3D micro/nanostructures.16−19 Alternatively, MPR has been applied as an effective tool for fabricating metallic micro/nanostructures such as microheater,20 silver cup,21 and silver pyramid.22 In previous studies, the protocols using MPR in both metallic ions doped polymer matrix23 and aqueous solution of metallic salts for fabricating 2D and 3D metallic nanostructures have been reported.24 Jin et al. demonstrated the influence of the laser irradiation time and repeated scanning numbers on the morphology of silver microstructures in MPR technique.25 The ionic liquid (IL)-assisted MPR technique of gold ions aqueous © 2016 American Chemical Society

solutions to fabricate 2D gold nanostructures with excellent electrical and optical properties has been reported.26 Cao et al. reported the microfabrication of metallic microstructures with surfactant-assisted MPR, which indicated a way to achieve small feature sizes in the fabrication of metallic structures with the aid of a surfactant as a metal growth inhibitor.22 Note that this result could only be achieved in the presence of surfactant; otherwise, silver nanowires around several hundreds of nanometers could often be obtained. Sun et al. successfully achieved “multiple arbitrary customizations” of silk-based micro/nanodevices by using femtosecond laser direct writing even in bioenvironments with live bacteria or cells existing nearby.5 The micro/nanoscale structures fabricated with natural biomaterials have their intrinsic advantages, such as low cost, biodegradability, and eco/biocompatibility. On one hand, the nonlinear optical interaction between the carbonyl group and the femtosecond laser pulse induces a metal−ion photoreduction process, and the extra lone pairs in the polar group (such as −COOH and −NH2) restrain silver particle growth through strong affinity toward Ag+ ions.27 On the other hand, the amino group of the molecule coordinating to the silver Received: August 19, 2016 Revised: October 28, 2016 Published: October 31, 2016 26532

DOI: 10.1021/acs.jpcc.6b08395 J. Phys. Chem. C 2016, 120, 26532−26538


The Journal of Physical Chemistry C

Figure 1. (a) Chemical structure of amino acids. (b) Schematic illustration of formation of silver patterns on a substrate. (c) Experimental setup of MPR microfabrication of silver microstructures.

metallic ion can stabilize the silver particles.28,29 Thus, amino acids are promising as a kind of newly developed promoter. Kang et al. employed amino acids as directing agents to synthesize hierarchical silver microspheres with controlled size and surface.30 Meanwhile, the as-prepared Ag particles can be readily used as surfaced-enhanced Raman scattering (SERS) sensor for biological imaging applications.30 Consequently, amino acids are expected to be good photoreducing promoters to achieve metallic nanostructures in MPR process. In this study, we demonstrate the microfabrication of silver nanostructures with amino acids-assisted MPR technique. We introduce four different amino acids including glycine, valine, proline, and leucine with diverse carbon chain lengths and molecular structures into silver ion aqueous solutions for MPR microfabrication. The results indicate that the amino acidsassisted MPR microfabrication could not only decrease the laser threshold of photoreduction but also improve the conductivity of the silver micro/nanostructures. The line width was reduced through either increasing the laser scanning speed or decreasing the laser power. The obtained silver nanostructures have a minimum line width of 186 nm at the laser scanning speed of 10 μm/s and the laser power of 0.35 mW with leucine. To the best of our knowledge, it is the first demonstration of fabricating silver nanowires by MPR with the assistance of biocompatible amino acids, which would offer an efficient protocol for fabricating metallic nanostructures and play an important role in the microfabrication of metallic microstructures for biological and electronic applications.

AgNO3 in the silver precursor is 0.05 M. Figure 1a shows the chemical structures of amino acids. 2.3. MPR Microfabrication of Silver Nanostructures. The experimental setup of MPR microfabrication is illustrated in Figure 1c. A mode-locked Ti:sapphire laser system (SpectraPhysics, Tsunami) with a center wavelength of 780 nm, pulse width of 80 fs, and repetition rate of 80 MHz was used as a light source. The incident laser beam was tightly focused into the sample solution placed on a coverslip by a high numerical aperture (N.A. = 1.45, 100× , Olympus) oil-immersed objective lens. The liquid sample was enclosed in a cell made of coverslips and rubber film as a spacer. Then, the sample was laid up on a 3D piezostage (PI, P-622.ZCL), which can scan along the XYZ direction. After the microfabrication process, the sample on the coverslip was washed by deionized water and dried in the air. The mechanism of this protocol is that the nuclei formation of silver nanoparticles (NPs) will occur when the near-infrared laser beam is introduced into the sample solutions,22 as illustrated in Figure 1b. 2.4. Characterization. The morphology and the component of the silver microstructures were characterized by the scanning electronic microscope (SEM, HITACHI S-4800). Atomic force microscopy (AFM, Bruker, Multimoder 8) images were collected to study the height and cross section of the silver nanostructures. The conductivity of silver line was characterized by the semiconducting parameter analyzer (Keithley, 4200SCS) and a probe station (Lake Shore Cryotronics, CRX-4K). UV-2550 Spectrophotometer (Shimadzu, Japan) was used to study the absorption spectrum of different silver precursor solutions. The cyclic voltammograms was characterized by Potentiolstat/Galvanostat (Princeton Model 263 A) with Pt electrode as working electrode, Pt electrode as counter electrode, and standard Ag/AgCl/KCl (3 M) as reference electrode. The scanning speed is 40 mV/s.

2. EXPERIMENTAL SECTION 2.1. Materials. Silver nitrate (AgNO3, 99.9+%) was purchased from Alfa Aesar Chemical Co. Ltd. Ammonia solution (NH3·H2O, 25 wt %) was purchased from Sinopharm Chemical Reagent Beijing Co. Ltd. Amino acids (glycine (C2H5NO2), valine (C5H11NO2), proline (C5H9NO2), and leucine (C6H13NO2) were purchase from J&K Science and Technology Co. Ltd. All chemicals were used without any purification. Ultrapure deionized water (18.3 MΩ cm) was obtained using a Millipore Milli-Q system. 2.2. Preparation of Silver Precursor. Silver precursor was prepared by mixing ammonium solution, silver nitrate, and amino acids in a dark room. In brief, 25 wt % ammonium solution was mixed with ultrapure water at the proportion of one to four before the microfabrication process. Then, the preprepared 1 mL of AgNO3 aqueous solution and 400 μL of ammonium solution were added into a 5 mL container. The four sample solutions were prepared by mixing silver ion aqueous solutions with 1 mL of amino acid (glycine, valine, proline, and leucine) with a concentration of 0.05, 0.08, 0.099, 0.13, 0.15, and 0.2 M, respectively. The concentration of

3. RESULTS AND DISCUSSION Here, we introduce four kinds of amino acids including glycine, valine, proline, and leucine into the silver ion aqueous solutions in which the amino acids have different carbon chain lengths and molecular structures. Figure 2 shows the UV−vis absorption spectra of pure diamine silver ions (DSI), glycine, and the silver precursor aqueous solutions. As shown in the spectra for both the pure DSI solution and the mixture of DSI with glycine in Figure 2, the absorption band is around 302 nm. However, there is no strong absorption band for pure amino acids at 302 nm. Consequently, this absorption band originates from the DSI itself. In all of the spectra, absorption band is not observed at the laser wavelength of 780 nm. This indicates that the photoinduced reduction reaction is associated with multiphoton absorption process. Similar results were obtained 26533

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silver nanowires fabricated under other circumstances are illustrated in Figures S2−S5. These threshold values are obviously decreased compared to the results in our previous studies.22,25 Furthermore, the mechanism of the dependence of laser threshold on different amino acids have been addressed. Table 1 shows the laser threshold of fabricating silver nanostructures Table 1. Laser Threshold Power and Redox Potential Values Corresponding to Different Amino Acids

Figure 2. UV−vis absorption spectra of DSI, glycine, and the silver precursor solution with glycine.

amino acids





threshold (mW) redox potential (V)

0.32 −0.32

0.72 −0.39

0.6 −0.35

0.35 −0.33

of four DSI aqueous solutions with the involvement of amino acids (0.099 M) as photoreducing promoters. It can be seen that with the same concentration of amino acids presented in sample solutions, the laser threshold is much higher with valineand proline-assisted MPR microfabrication than those with glycine and leucine. It is reasonable that the oxidizability of the silver ion aqueous would affect the threshold value of incident laser. We investigate the cyclic voltammograms of different silver ion aqueous solutions including pure DSI and the mixture solutions of DSI with four kinds of amino acids (Figure 4). The

for silver precursor aqueous solutions with valine, proline, and leucine, respectively, as shown in Figure S1. The laser threshold power dependence on the amino acid concentration in the MPR microfabrication has been investigated first (Figure 3). The threshold power remained

Figure 3. Laser threshold in MPR for silver precursor solutions with the assistance of glycine, leucine, proline, and valine, respectively. The SEM images of silver nanowires fabricated in silver precursor solutions with different amino acids at the concentration of 0.099 M. Figure 4. Cyclic voltammogram curves of silver precursor solutions with different amino acids at the concentration of 0.099 M. The potential measurement range of amino acids is the same as the DSI.

stable while extending the concentration of glycine from 0.05 to 0.2 M. As a comparison, threshold power had a distinct variation when tuning the amino acids to much higher concentration in all of the other three cases. The results indicate that the threshold power would decline along with increasing the concentration of amino acids, but showed relatively negligible variation when the concentration of amino acids reached to a certain value. Note that amino acids with relatively low concentrations such as 0.05 and 0.08 M lead to higher threshold power over 0.6 mW, while the amino acids with comparatively high concentrations, e.g., 0.099 and 0.13 M, result in the threshold power of about 0.3 mW. Thus, further decrease of the laser power could not lead to a continuously complete silver nanowire. Consequently, we demonstrate that the minimum photoreduction power has been successfully lowered to 0.32 mW for glycine, 0.72 mW for valine, 0.6 mW for proline, and 0.35 mW for leucine-assisted MPR due to the existence of amino acids (0.099 M) in the aqueous solutions. The corresponding silver nanowires under corresponding threshold power are demonstrated in the SEM images. The

redox potential of pure DSI solution is −0.59 V, which is the lowest among all of the cases. The lowest redox potential of DSI solution leads to the highest laser threshold power because the reducing capability increases with the increasing of redox potential. This indicates that the higher laser power is needed to achieve nanowires through MPR microfabrication. As a comparison, the threshold power was 0.72 mW when valine was present in the silver precursor solution due to the minimum redox potential of −0.39 V. For silver precursor solutions with glycine and leucine as photoreducing promoters, Figure 4b,e shows that the redox potentials are almost equal, which indicates the threshold power of MPR microfabrication with glycine- and leucine-assisted are almost the same. The laser threshold power and the redox potential values are summarized in Table 1. Consequently, we demonstrate that the minimum photoreduction power are different for the silver 26534

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Figure 5. (a−d) SEM images of silver nanowires patterned in sample solutions with the same concentration (0.13 M) of amino acids (a) glycine, (b) valine, (c) proline, and (d) leucine. The silver nanowires were fabricated under corresponding threshold power and scanning speed changed from 2 to 11 μm/s. The inset shows magnified SEM images of silver nanowires. (e) Dependence of silver line width on the scanning speed in which the silver nanowires were fabricated under corresponding threshold power.

Figure 6. SEM images of the silver dots fabricated in silver precursor solutions under the concentration of glycine of (a) 0.05 M, (b) 0.099 M, and (c) 0.15 M at 0.4 mW and the laser irradiation duration of 3000 ms. (d) Size distribution of silver NPs in the silver dots fabricated in the silver precursor solutions with different concentrations of glycine. (e) Line width dependency on the concentration of glycine amino acids in which the silver nanowires were fabricated at 6 μm/s and corresponding threshold power.

equal and the line width of nanowires shows indistinctive change under this circumstance. Meanwhile, the morphology of silver nanowires becomes rough when the scanning speed increases to a certain value, resulting in the enlarged average line width. The line width of silver nanowires would also be influenced by the amino acids with different chemical structures and properties. Figure 5e clearly shows that in valine- and prolineassisted MPR microfabrication, the average line width was larger than those with glycine and leucine. As we mentioned before, one function of amino acids in the silver precursor aqueous solutions is to inhibit the growth of silver NPs. The amino acids were absorbed on the surface of the Ag NPs and stabilized the Ag NPs. Steric hindrance of the amino acids play crucial roles in the absorbance of amino acids on Ag NPs. Amino acids with bigger steric hindrance will result in the weak absorbance and fast growth of Ag NPs. As a result, the amino acids that have higher steric hindrance would result in larger average line width. Specifically, it can been seen from the chemical structures that the ortho-position of the core carbon atoms that connect carboxyl and amino group are different. For glycine, the side chain consists of only hydrogen and possesses lowest steric hindrance among all of the amino acids used in the

precursor solutions with different amino acids at the concentration of 0.099 M. The laser scanning speed plays an important role in determining the line width of silver nanowires. We have fabricated the silver nanowires by MPR microfabrication with the assistance of different amino acids (0.13 M) with the varied scanning speed from 2 to 11 μm/s (Figure 5a−d). The silver nanowire line widths did not show obvious changes in the glycine-assisted MPR as the scanning speed varied in which the minimum line width was 223 nm (Figure 5e). Differently, the line width declined with the increasing of the scanning speed for other three amino acids-assisted MPR. The minimum line widths achieved were 242 nm for valine, 254 nm for proline, and 207 nm for leucine, respectively. The higher scanning speed as well as the decreased exposure time produced narrower silver nanowires. The line width of silver nanowires under all circumstance are illustrated in Table S1−S4. It is reasonable that shorter exposure time would lead to the less photoreduction of silver ions, resulting in the reduced amount of Ag NPs to aggregate into silver nanowire. However, the exposure time shows indistinctive change when the scanning speed increased to a certain value such as 8 μm/s or even higher, which means the amount of Ag nanoparticles are almost 26535

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The Journal of Physical Chemistry C

Figure 7. (a) AFM images of silver nanowire fabricated with valine as photoreducing promoter. The inset shows the SEM images of the corresponding topography. (b) Height profile of the corresponding silver nanowire. (c) Current−voltage curve of silver nanowire. (d) EDX spectrum of silver nanowire.

that silver nanowire is produced through aggregation of Ag NPs. With valine, proline, and leucine present in the silver precursor solutions, average diameter of Ag NPs exhibits similar variation tendency (Figure S6). The dependence of the silver line width on the concentration of different amino acids is shown in Figure 6e. The line widths are reduced with the increase of the amino acid concentration. Furthermore, the conductivity of the silver nanowires has been carefully characterized. Conductivity of the metal microstructure is extremely significant, which would directly determine the applications in electronics, photonics, etc. We employed a metal mask plate to cover the individual silver nanowire, which prefabricated on the glass substrate, and then we deposited silver electrodes on the end of the silver nanowires. Figure 7a is a representative SEM and AFM images of silver nanowire, which is fabricated by MPR microfabrication under the laser power of 0.6 mW and the scanning speed of 6 μm/s with valine (0.099 M). The line width of silver is 320 nm. The corresponding AFM images and the profile of cross section show that the surface of silver nanowire is rough. Moreover, it can be clearly identified from the height profile (Figure 7b) that the height and cross section of silver nanowire were about 32 nm and 0.01024 μm2. The length of silver nanowires for electrical characterization is 80 μm. The resistivity of silver nanowires (ρs) based on Ohm’s law can be written as follows:31

reaction; therefore, the narrower silver nanowire is achieved in the MPR process assisted by glycine. As a comparison, the side chain of valine and proline are isopropyl and is a 5-membered ring that would result in higher steric hindrance, and thus, the line width is much larger. For leucine, the side chain consists not only of hydrogen but also methylene and isopropyl; however, steric hindrance is mainly affected by hydrogen and methylene, which means narrower line width can be achieved compared to proline and valine cases due to the relatively high reducing capability. Therefore, the average line width were larger with valine and proline than glycine and leucine-assisted MPR as illustrated in Figure 5e. Based on the understanding of the line width as a function of experiment conditions, the silver micro/nanostructures are achieved through Ag NPs aggregation, thus the line width of silver nanowires are mainly determined by the size of Ag NPs. Figure 6a−c shows the magnified SEM images of the Ag dots deposited on coverslips by MPR microfabrication with assistance of glycine under laser power of 0.4 mW and with the laser irradiation duration of 3000 ms. The average diameter of Ag NPs decreased with the increase of concentration of glycine. The size distribution of Ag NPs constructed the silver nanowires are dependent on the concentration of amino acids (Figure 6d). When the concentration of glycine is 0.05 M, the Ag NPs show average size of 41.75 nm with a wide distribution from 30 to 60 nm. As concentration of glycine increases to 0.099 M, the average diameter of Ag NPs is reduced to 25 nm with narrowing of the size distribution between 17 to 30 nm. When the concentration of glycine is further increased to 0.15 M, almost all of the Ag NPs possess average size of 21.3 nm with a narrow distribution from 12 to 31 nm. These results clearly show the evidence that the concentration of amino acids are effective for the size control of Ag NPs in the MPR microfabrication process. Higher concentration of amino acids are propitious for achieving smaller Ag NPs with a narrow distribution, which is easy to obtain narrower line width since

ρs = R

S dh =R L 2L


where S, R, d, h, and L represent the cross section, resistance, line width, height, and length of silver nanowires, respectively. The typical I−V curves of silver nanowires are illustrated in Figure 7c. Thus, the nanowire resistance achieved by valineassisted MPR microfabrication was estimated to be 4.1 × 10−7 Ω m, which is only 25 times larger than that of bulk silver (1.65 × 10−8 Ω m). With glycine, proline, and leucine presented in the silver ion aqueous solutions, the resistances of silver 26536

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The Journal of Physical Chemistry C nanowires are 1.48 × 10−6 Ω m, 7.04 × 10−7 Ω m, and 1.35 × 10−6 Ω m, respectively (Figures S7−S9). The results are significantly improved compared to the results in our previous report.32 To confirm the component of silver nanowires, energy dispersive X-ray spectroscopy (EDX) analysis was characterized. The EDX results uncover the existence of silver resulting from the photoreduction of silver ions as illustrated in Figure 7d. The ability to fabricate subwavelength conductive silver nanostructures using MPR could demonstrate vital development for the micro/nanotechnology. The silver nanowires we fabricated exhibit fabulous electrical performance, which holds momentous potential applications in nanoelectronics and nanophotonics.



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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08395. Absorption spectra of pure diamine silver ions (DSI), amino acids, and the silver precursor aqueous solutions. Silver line width at different scanning speed and concentrations. Size distribution of silver particles fabricated in the sample solutions with different amino acids and concentrations. Resistance of silver nanowires by different amino acids-assisted MPR microfabrication. AFM images and height profile of the silver nanowires achieved with glycine-, proline-, and leucine-assisted MPR microfabrication (PDF)

The authors thank the financial support of National Natural Science Foundation of China (NSFC, Grant Nos. 91323301, 91123032, 61475164, 51473176, 61275171, and 61275048), the Ministry of Science and Technology of China under Basic Science Research Program (2010CB934103), the National Key Research and Development Program of China (Grant No. 2016YFA0200500), CAS-JSPS Joint Research Project (GJHZ1411), and the Key Research Program of the Chinese Academy of Sciences (KGZD-EW-T04).

4. CONCLUSION In summary, we have proposed an approach for fabricating silver nanostructures by using amino acids-assisted MPR microfabrication. The dependence of the silver line width on the concentration of amino acids and scanning speed has been investigated, which are important factors for improving the resolution of silver microstructures. The obtained silver nanowires have a minimum line width of 186 nm. The minimum photoreduction laser power has been successfully decreased to around 0.3 mW in the amino acids-assisted MPR microfabrication. Furthermore, the silver nanowires with valineassisted MPR microfabrication technique maintains a low resistivity of about 4.1 × 10−7 Ω m, which denotes that the electrical conductivity is 25 times larger than that of the silver bulk. The amino acids-assisted MPR microfabrication protocol could be expected to play an important role in the fabrication of metallic micro/nanostructures for the further applications in photonics and electronics. The improved electrical conductivity imparts significant potential for various circuitry and electronic interconnections.



Corresponding Authors

*(M.-L.Z.) E-mail: [email protected] *(F.J.) E-mail: [email protected] *(Z.-S.Z.) E-mail: [email protected] Tel/Fax: +86-1082543596. Notes

The authors declare no competing financial interest. 26537

DOI: 10.1021/acs.jpcc.6b08395 J. Phys. Chem. C 2016, 120, 26532−26538


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DOI: 10.1021/acs.jpcc.6b08395 J. Phys. Chem. C 2016, 120, 26532−26538

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