Improved Stability of Metal Nanowires via Electron Beam Irradiation

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Improved Stability of Metal Nanowires via Electron Beam Irradiation Induced Surface Passivation Beibei Luo, Yunsheng Fang, Jia Li, Zhen Huang, Bin Hu, and Jun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00875 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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ACS Applied Materials & Interfaces

Improved Stability of Metal Nanowires via Electron Beam Irradiation Induced Surface Passivation Beibei Luo,† Yunsheng Fang,† Jia Li,† Zhen Huang,† Bin Hu†,‡,* Jun Zhou†

†Wuhan

National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan, 430074, China ‡Shenzhen

Huazhong University of Science and Technology Research Institute,

Shenzhen, 518057, China *E-mail: [email protected]

ABSTRACT: Suppressing the corrosion of nanoscaled metal materials is a critical issue for various devices. Herein, we demonstrate the electron beam irradiation can be a simple and efficient method to realize silver/copper nanowires protection by transforming the original organic capping agents into dense carbonaceous shells. Single nanowire tests prove the significant stability improvement from 4 days to 20 days for silver nanowire and from 20 hours to at least 1 week for copper nanowire. The comprehensive advantages such as solution/pollution-free and continuous process with high precision offer this method substantial potential applications in bottom-up assembled electronic and optoelectronic devices.

KEYWORDS: silver nanowire, copper nanowire, electron beam irradiation, surface passivation, stability, organic ligands

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The corrosion of metal materials significantly degrades the mechanical strength and electrical conductivity performance.1 As the dominated electric contacts materials, the stability of silver and copper is critical for device reliability. Nevertheless, both of them are vulnerable to O2, H2O, and sulfide, etc, especially as the sizes down to nanoscale, higher surface activities make the chemical stability issue more challenging.1-3 Coating passive layers are a common method for bulk metal protection in industry, and has also been implemented in nanoscaled metal materials protection such as silver nanowires (AgNWs) and copper nanowires (CuNWs), which would propel their practical applications in flexible transparent conductors,4-6 nanodevice interconnects, 7 neural electrodes, etc.8 Hitherto, various materials have been employed as the passive coatings for AgNWs/CuNWs stability improvement but still have limitations. Specifically, dense and stable metal oxides shells provide satisfactory protection but at the expense of electrical conductivity along the radial direction limiting their applications in optoelectronic devices.9-10 Noble metals are conductive but require relatively thick thickness to achieve pore-free layer for sufficient protection11-12 and their cost needs to be concerned for mass production.13 Carbon materials, especially graphene and graphene oxide with highly impermeable to gases and excellent chemical inert14-15, have also been used as anticorrosion barriers.16-17 However, ensuring uniform and welldefined coverage of these 2D carbon sheets on the whole nanowires is required to avoid accelerated corrosion on the uncovered area but remains a challenge.18 Organic precursors such as glucose have been introduced for carbon layer encapsulation during 2


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metal nanowires synthesis but are always thick and porous.19 And all of these solutionbased processes produce chemical wastes or by-products inevitably. Therefore, developing a simple, clean and reliable way is still needed to passivate the AgNWs/CuNWs without scarifying their electrical properties. Utilizing AgNWs/CuNWs their native structural features for surface passivation is a promising strategy such as surface alloying.20 Transmission electron microscope (TEM) images in Figure 1a shows a typical AgNW/CuNW structure with polyvinylpyrrolidone (PVP)/hexadecylamine (HDA) ligands capped on the surface through tight Ag-O/Cu-N bondsfor morphology control.21-22 These insulating organic layers are capable of suppressing the corrosion of AgNW/CuNW to some extent but also create high contact resistance,22-23 and the long molecular chain with steric effect induced lower graft density makes the surface passivation insufficient, the corrosion would still occur gradually especially under the high humidity condition.24-25 Converting these native ligands as precursors into a protective dense shell can be an effective way for AgNWs/CuNWs stability improvement. We employed e-beam as a powerful tool for this processing which have demonstrated its capability in modification of various polymers.26-27 And very recently e-beam irradiation has also shows its ability in Mg alloy protection by turning a natural or corroded surface into anti-corrosion MgCO3 coating in excited CO2 at room temperature.28 The e-beam of scanning electron microscope (SEM) is used as radiation source in our experiment and the radiation area and dose can be controlled precisely.29-30 As schematically shown in Figure 1c, when focused e-beam scans across AgNW/CuNW, e-beam energy would 3



interact with the organic ligands and re-arrange the molecular structure through chain scission and crosslinking, even produce free radicals and carbonize the organic ligands for creating a carbonaceous shell.31-32 The rapid and effective process avoids using any additives, and the sweeping irradiation mode ensures the complete transformation of the organic shell. SEM images in Figure 1c show a scanned AgNW by short period of e-beam with the electron dose around 106 nC·μm-2 (pixel dwell time of 20 μs at 10 kV accelerating voltage and 158 μA current, detailed calculation is summarized in Supporting Information 1).24 By virtue of ultrathin thickness of the formed shell, no obvious change in morphology can be observed. In some specimens, the occasionally increased shell thickness with the irradiation time can be attributed to the carbon contamination in the chamber or sample preparation process (Figure S1).33 To identify the composition evolution of the organic shell when subjected to ebeam irradiation, confocal Raman spectrum were compared as shown in Figure 2a. The peak intensity at 231 cm-1 belongs to featured Ag-O stretching vibration between Ag and capped PVP decreased, which indicates the interface bonding was significantly broken.[38,39] The characteristic absorption peaks of PVP (mainly: ν(C-C) at 837 cm-1 and ν(C=O) at 1630 cm-1) were weakened as well, and the decomposed hydrophilic functional group (ν(N-C=O)) at 673 cm-1 plays an important role for the stability improvement of metal nanowires in humidity condition. Meanwhile, two intense bands at 1390 cm-1 and 1578 cm-1 emerged after irradiation, which attributes to the characteristic signals of D and G peaks of carbon, respectively, and the relatively low G/D content confirmed the amorphous carbon structure. Other three sets of data is shown in Figure S2 to prove the 4


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reliability of the results. The change of higher energy of electron energy loss spectroscopy (EELS) of the AgNWs with the irradiation is compared to investigate the composition evolution of the organic shell for further evidence as seen in Figure 2c. The absorption K edge of carbon (C-K edge) and nitrogen (N-K edge) located at 283 eV and 400 eV, respectively (enlarged spectrum in Figure S3).34 The result shows that content of carbonaceous component significantly increased after irradiation according to the intensity increase of C-K edge. The peak height ratio π*/σ*, defined as the ratio of sp2-bonded carbon to sp3-bonded carbon, decreased from 0.78 to 0.54, indicating part of the pyrryl sp2 hybridized carbon in PVP chain transformed into amorphous carbonand formed a sp2–sp3 hybrid carbonaceous shell.35-36 Previous report by Ruoff et al. prepared carbon film using e-beam-induced deposition by introducing hydrocarbon molecules into SEM chamber and proved that the deposition rate of carbon strongly depends on the distance between the hydrocarbon source and the target surface.37 In our case, it is probably that the scission of partial molecular chain of PVP/HDA occurred in early stages under e-beam bombarding, and the formed dangling bonds induced intermolecular cross-linking and achieved a higher sp3 content which mainly composed by saturating C=C bonds. These crosslinked fragments would redeposit onto the surface and become a thin but denser carbonaceous shell.38 Due to the organic ligands cover the whole sidewalls of metal nanowires,39 the produced carbonaceous layers can be uniform barriers to against the invasion of oxygen, sulfur dioxide and water. The tentative structures change of the shell from original ligands to a dense carbonaceous layer is illustrated in Figure 2c and the possible process 5



of the deterioration of AgNWs/CuNWs are depicted in Supplementary note 2. The stability of the AgNW before and after e-beam irradiation was firstly investigated through individual nanowire test, which can exhibit the inherent properties in corrosion resistance of AgNW compared to the previous works.40-41 To guarantee the comparability, a single AgNW spanned three Au microelectrodes was divided into two segments as shown in the upper inset in Figure 3a, and the marked yellow shadow represents the e-beam irradiated area while the other area kept initial state. The accelerated aging test of the AgNW was conducted in a harsh environment with high temperature and high humidity (75 oC, 100% RH) and the striking stability difference of these two areas can be observed. For the unirradiated part, the resistance of AgNW increased rapidly in less than 4 days and became infinity after then, while the irradiated part can maintain stable up to 20 days. These electrical behavior are consisted with the morphology changes as seen in inset in Figure 3a. The significant surface corrosion occurred after 3 days on the unirradiated part, while the irradiated part maintained the smooth surface as the pristine state even after 20 days aging as shown in Figure S4. More severe sulfuration test, which is considered as main causes of corrosion of silver,2 was conducted using 1 wt% of Na2S aqueous solution as oxidative reagent to further investigate the chemical stability, and the SEM images are compared in Figure 3b. Upon chemical attack about 5 min, the unirradiated AgNW became rough and discontinuous. In contrast, no corrosion indication was observed in the irradiated part which also confirmed by the energy-dispersive spectra (EDS). The demonstrated marked contrast in chemical stability give a convincing evidence that irradiated induced 6


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carbonaceous shell can be a reliable encapsulation for AgNW protection. Highly conductive AgNWs based network is an idea current collector for electrochemical and printed optoelectronic devices but suffers from corrosion issue in solution conditions, and we extended the e-beam irradiation method to the nanowires network protection and demonstrated the galvanic reaction of AgNWs can be effectively suppressed by the carbonaceous shell. Figure 3c shows the AgNWs network with selectively irradiated area marked by the yellow dotted box. The treated sample was immersed in the electrolyte as an anode with graphite plate served as a cathode as schematically shown in Figure 3c. After 40 s galvanic corrosion under 0.2 mA, the morphology comparison of AgNWs network is shown in Figure 3d and the enlarged images in red box is shown in Figure 3c. We can see that the unirradiated area were severely etched and turned into fragments but the irradiated area maintained smooth surface as the initial state. The significant morphologic difference across the irradiation boundary indicates that the hydrophobic carbonaceous shell could isolate the AgNW from the electrolyte to avoid surface corrosion. It's worth mentioning that the selective irradiation process can also be an ideal approach for AgNWs pattern. Compared to the laser ablation or chemical etching that completely removes the unwanted AgNWs for patterning, e-beam method can retain the corroded AgNWs in unirradiated area which can greatly decrease the pattern visibility for display applications. Improving the stability of copper is more desired because it is more abundant and 100 times less expensive than silver. However, CuNWs are far more vulnerable and high susceptibility to oxidize, and various surface passivation methods have been 7



developed to address this knotty problem.42 E-beam irradiation can be a general approach and implanted onto CuNWs for surface protection as well. The single CuNW stability test was carried out as AgNW and the conductivity evolutions of two parts of CuNW are plotted in Figure 4a. Similarly, the resistance of the unirradiated part rose sharply, and most of the samples in our experiment became disconnected after 20 h (28 oC,

60% RH). In contrast, the irradiated CuNW showed much slower degradation with

a slight increase in resistance after 1 week. This improvement largely enhanced the reliability of the sample which is not only helpful to the high yield in the manufacturing process, but also to the lifetime in practical applications after packaging. The inserted SEM images in Figure 4a show the morphology changes of two parts of CuNW, and the difference between severely damaged unirradiated part and intact irradiated part are consistent with the electrical properties. Larger area irradiation of the CuNWs network in Figure 4b also gives evidence about the improved stability, which shows the striking morphology contrast on both sides of the irradiation boundary of a bundle of CuNWs after aging. Moreover, the controllable irradiation intensity only produced ultrathin carbonaceous protective layers, and had no damage to the intrinsic conductivity of AgNW/CuNW as summarized in Table S1 and S2, nor to the optical properties unlike the gold or nickel coating. By virtue of precision positioning of e-beam irradiation, the passivated area of AgNW/CuNW can be well controlled, and we demonstrated a hardware-encrypted information storage based on AgNWs as shown in Figure 5, in which the e-beam was used as a pen that can precisely write the encrypted information on AgNWs by 8


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irradiation. Six AgNWs on a chip as schematically shown in Figure 5a were used as information storage medium, in which the white ones represent the irradiated AgNWs and other three black ones represent the pristine state. To enhance the visibility, each AgNW as an electric lead in the circuit connected to a light-emitting diode (LED), and all the LEDs can be lit up in the beginning indicating the information cannot be read out. The dark and bright LED as "0" and "1" respectively, which carried the signal of “111111” in upper image in Figure 5b. After aging 6 days in (75 oC, 100% RH), the corroded unirradiated AgNWs can break the circuits and the encrypted message written by e-beam can be read out as “011001” (middle image in Figure 5b), and this message can preserve for at least 20 days. Due to the rapid processing of e-beam, this demonstrated time-delayed hardware-encryption can be scaled up to protect sensitive information with higher data storage density.43 In summary, a highly efficient and reliable e-beam irradiation method is presented to improve the stability of AgNWs and CuNWs significantly. This method utilizes the capped organic ligands on AgNW/CuNW as the precursors and transit them to dense carbonaceous shells by e-beam irradiation. Single AgNW/CuNW tests prove that this irradiation-produced passivate layer could effectively protect the vulnerable metal nanowires from corrosion under harsh environment. This method also has a potential of universal applicability to various inorganic/organic core/shell nanomaterials. Moreover, by virtue of rapid process, high precision of e-beam processing, it can combined with roll-to-roll processes for mass production of transparent electrodes, or construct the reliable micro/nano circuit using metal nanowires as building blocks. 9



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Synthesis of metal nanowires, chemical stability test of nanowire, parameter and dose calculation of electron irradiation, corrosion process of silver nanowires and copper nanowires, morphology change of AgNW with irradiation time; Raman and EELS of AgNW; morphology change of AgNWs in harsh environment, electrical conductivities of e-beam irradiated/unirradiated AgNWs and CuNWs.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (61674064, 61434001), Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20170818170143363), and the China Postdoctoral Science Foundation (2018M640693). We wish to thank the support facilities at the Center for Nanoscale Characterization & Devices, WNLO of the Huazhong University of Science and Technology (HUST) and the Analytical and Testing Center of HUST.

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Figure 1. Characterization of AgNWs and CuNWs. TEM images of (a) thin PVP layer (colored by green) capped AgNW and (b) thin HDA layer (colored by yellow) capped CuNW. (c) Conceptual illustration of surface protection of the metal nanowire by ebeam irradiation, and the insets show the SEM images of a single AgNW before and after irradiation.

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Figure 2. Characterization of the organic shells before and after e-beam irradiation. (a) Raman spectra of the transition from organic ligands to a carbonaceous shell. (b) Corelevel EELS spectrum of carbon and nitrogen K edges recorded from the AgNW before and after irradiation. (c) Schematic diagram of corrosion resistance improvement of AgNW caused by transition from loose shell into a dense carbonaceous shell by e-beam irradiation.

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Figure 3. Stability improvement of AgNW by e-beam irradiation. (a) Resistance evolution of two parts of an AgNW with and without irradiation during 20 days (75 oC, 100% RH). Left inset: SEM image of the AgNW with irradiated part colored by yellow. Right inset: morphology of AgNW two parts after 13 days. (b) Sulfidation test of an AgNW. Partially irradiated AgNW before sulfidation (Upper image), and the boundary morphology of the AgNW after sulfidation (Lower image) with corresponding EDS spectra. (c) Schematic of the electrochemical. (d) SEM image of AgNWs network before and (e) after electrochemical corrosion, and the dotted lines represent the irradiation boundary.

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Figure 4. Stability improvement of CuNW by e-beam irradiation. (a) Resistance evolution of two parts of a CuNW with and without irradiation for 1 week (28 oC, 60% RH). Upper inset: SEM image of the CuNW with irradiated part colored by yellow. Lower inset: the morphology of two parts of the CuNW after 1 week. (b) SEM image of CuNWs bundles after ~90 h aging (28 oC, 60% RH).

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Figure 5. E-beam writing for information encryption using AgNWs as a storage medium. (a) Schematic diagram of irradiated (white) and unirradiated (black) AgNWs with different stability manipulated by e-beam. The inset shows each AgNW bonded on the chip is connected to a LED. (b) Optical image of the encrypted information can display after 6 days and still preserve after 20 days. Dark LED is denoted by “0” and the bright LED is denoted as “1”.

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Table of contents

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Improved Stability of Metal Nanowires via Electron Beam Irradiation

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