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Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics
Accepted Manuscript Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics Zhiliang Zhang, Weiyue Zhu PII:
S0925-8388(15)30592-2
DOI:
10.1016/j.jallcom.2015.07.195
Reference:
JALCOM 34889
To appear in:
Journal of Alloys and Compounds
Received Date: 26 May 2015 Revised Date:
29 June 2015
Accepted Date: 20 July 2015
Please cite this article as: Z. Zhang, W. Zhu, Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.07.195. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics Zhiliang Zhang *a,b, Weiyue Zhu a Shandong Provincial Key Laboratory of Fine Chemicals, Qilu university of technology, Jinan 250353,
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a
China b
Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China
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Email: [email protected] Tel: +86-531-89631632
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Abstract
An effective and facile strategy was developed to successfully synthesize nearly uniform silver nanoparticles (AgNPs) with particle size of <10 nm, and demonstrated to achieve the sintering of AgNPs at room temperature for inkjet-printed flexible electronics. In such
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system, a series of different chain-length alkylamines were exploited as capped molecules to controllable synthesis of uniform AgNPs with the mean nanoparticle size in rang of 8.6±0.9,
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8.9±1.2 and 9.2±1.6 nm, and these ultra-small nanoparticles were very favorable to attain an excellent printing fluency. Based on the as-synthesized AgNPs, a sequence of flexible
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electrocircuits was successfully fabricated by ink-jet printing technique. After the dipped treatment, the printed AgNPs were achieved to spontaneous coalescence and aggregation at room temperature induced by preferential dissolution of capped molecules on AgNPs surfaces into methanol solution. These aggregated AgNPs demonstrated superior controllability, excellent stability and low resistivity in the range of 31.6~26.5 µΩ.cm, and would have enormous potential in the application to be tailored for assembly of optoelectronics devices. 1
ACCEPTED MANUSCRIPT Keywords: silver nanoparticles, controllable synthesis, dipping methods, ink-jet printing
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technique, flexible electronics
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ACCEPTED MANUSCRIPT 1. Introduction Flexible electronics has attracted considerable research interests in recent years as it exhibits tremendous potential in optoelectronics such as solar cells [1], LCDs [2], OLED
technique
is
considered
as
an
excellent
alternative
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lighting [3], and touch screen panels devices [4-6]. As a pattern approach, ink-jet printing candidate
to
conventional
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photolithography for fabrication of multifarious conductive patterns on the flexible substrates [7, 8]. Compared with the other fabrication methods, it enables design flexibility
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without the requirement of masks, multilayer deposition of a wide variety of materials due to its elimination of conventionally complex photolithography process. In addition, ink-jet printing technique can be easily scaled up towards industrial roll-to-roll fabrication processes for mass-production of various electrocircuits [9].
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Although ink-jet printing technique provides a promising potential pathway to fabricate various flexible electronics devices for well defined contacts, interconnects, antennas and
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electrodes, there are two major issues that must be resolved before widespread commercial
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application [10]. One of major issues in fabrication of flexible electronics by ink-jet printing technique is to explore an appropriate sintering strategy to achieve highly conductive patterns. Currently, the most commonly used sintering approach is to decompose the capped molecules on nanoparticle surfaces under high temperature and trigger electrical conductivity, wherein the sintering temperatures are typically above 200 °C, which are not compatible with the most flexible polymer substrate due to their low glass-transition temperatures [11-13]. Besides the thermal sintering, some other methods have also been
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ACCEPTED MANUSCRIPT exploited for this purpose, such as microwave [8], laser radiation [14-21], plasma [22], electrical [23] and photonic sintering [24, 25]. These methods could be used to fabricate conductive tracks or sinters these conductive materials selectively without affecting the
pre- or post-treatments for printed flexible electronics [26].
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substrate. However, they involve high-cost equipment and require high energy or complex
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In contrast, the chemical sintering of metallic nanoparticles, especially the solvent sintering strategy is considered as another promising candidate to achieve AgNPs sintering at
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room temperature [9, 27-29]. Compared with aforementioned sintering techniques, this approach can be conducted at low temperature in a facile, low-cost and energy-conserving manner. As a versatile sintering technique, chemical sintering has opened up a new possibility of room-temperature processable metal nanoparticle assemblies. Recently,
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Magdassi et al. proposed to neutralize charges of the AgNPs stabilized by poly(acrylic acid) (PAA) or detach the capped molecules on the surfaces with poly(diallyldimethylammonium
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chloride) (PDAC). After the destabilization process, it achieved to spontaneous coalescence
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of AgNPs at room temperature by tuning the concentration of PDAC [30]. In continuative study, Grouchko et al. demonstrated a built-in sintering mechanism to enable the printed PAA-capped AgNPs with excellent conductivity [31]. Wakuda et al. tried to trigger the sintering of AgNPs at room temperature by dipping the AgNPs paste into several solvent of the organic stabilizer, whereas the synthesis procedure of used AgNPs in the experiments was quite completed and time-consuming [32]. Obviously, it severely restricts the opportunity for obtaining a flexible conductor directly on plastic substrate.
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ACCEPTED MANUSCRIPT Another crucial issue related for ink-jet printed flexible electronics is to develop a facile method to synthesize uniform and ultra-small metal nanoparticles, which is extremely pivotal to attain a fluent printing process and highly stable conductive ink. To date, a wide
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variety of methods, such as microwave irradiation [33], sonochemical reduction [34], microemulsion [35], solvothermal reduction[36] and chemical reduction [37], have been
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employed to synthesize metal nanoparticles. However, it is difficult to obtain uniform nanoparticle with particle size of <10 nm by these methods. Consequently, the ink stability
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based on these synthesized nanoparticles would be likely affected as they aggregate and precipitate out from the solution. More seriously, the printer nozzle would be clogged with these larger nanoparticles or aggregation [38]. Therefore, it is very urgent to exploit a simple and versatile approach to synthesize ultra-small and uniform nanoparticles and achieve
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complete sintering at room temperature for inkjet-printed flexible electronics. In order to overcome these obstacles, following our previous study on AgNPs synthesis
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and printed electronics [39, 40], we here introduced a facile and efficient strategy to fabricate
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flexible conductive electronics directly on poly(ethyleneterephthalate) (PET) substrates by tuning the surface characters of AgNPs. In this work, a series of uniform and ultra-small AgNPs were successfully synthesized with alkylamines as capped molecules. The results showed that the size distribution of the synthesized AgNPs was 8.6±0.9, 8.9±1.2 and 9.2±1.6 nm corresponding to the respective capped molecules, and these ultra-small nanoparticles were very favorable to attain a printing fluency. Based on these as-synthesized AgNPs, a sequence of flexible electrocircuits were fabricated by ink-jet printing technique. After the
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ACCEPTED MANUSCRIPT dipped treatment in methanol solution, the printed AgNPs were achieved to spontaneous coalescence and aggregation induced by preferential dissolution of capped molecules on AgNPs surfaces into methanol solution. More importantly, the spontaneous coalescence
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extent of AgNPs could be precisely regulated by controlling the dipping time in methanol solution at room temperature. These aggregated AgNPs could serve as conductive materials
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and demonstrate super controllability, excellent stability and low resistivity in the range of 31.6~26.5 µΩ.cm. This strategy could be considered as a universal approach to construct
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various conductive microstructures and would have great potential to be tailored for applications in flexible electronics fields. 2. Experimental 2.1 Chemicals acetate
(AgAc),
1-dodecylamine
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Silver
(DDA),
1-tetradecylamine
(TDA),
1-hexadecylamine (HDA), and phenylhydrazine were obtained from American Sigma CO.,
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LTD. Methanol, acetone, toluene and α-terpineol were all purchased from Beijing Chemical
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Co. The other chemicals in the experiments were analytical or high-reagent grade. All of these chemicals and materials were used as received without further purification. The water used throughout the experiments was ultrapure water (18.2 MΩ) produced by a Milli-Q system.
2.2 Synthesis of AgNPs with alkylamine as capped molecules Solution-phase route was an effective approach to synthesize AgNPs with controlled spherical shape and size distribution [41, 42]. The AgNPs used in this work were
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ACCEPTED MANUSCRIPT synthesized by a straightforward, one-phase reaction, and alkylamine were used as capped molecules. In a typical synthesis process of AgNPs, silver acetate (0.51 g) and 1-dodecylamine (1.38 g) in toluene (20 mL) were firstly heated at 60 °C with stirring until
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silver acetate and 1-hexadecylamine were completely dissolved. A solution of phenylhydrazine (0.31 g) in toluene (10 mL) was slowly added into the resulting solution
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with continuous stirring and reacted at 60 °C for 1 h. Subsequently, before the above solution down to 30°C, acetone (30 mL) was added into to precipitate the AgNPs. The
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AgNPs were isolated by centrifugation at 5000 rpm, washed twice with acetone (30 mL), and vacuum-dried at room temperature to obtain a blue-black solid. The AgNPs synthesized with tetradecylamine or hexadecylamine as capped molecules were received respectively with the similar procedure.
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2.3 Fabrication of conductive ink based on synthesized AgNPs The synthesized AgNPs were redissolved into the mixture of cyclohexane and α-terpineol,
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and the concentration was set at 8% by weight for the corresponding suspension dispersed by
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ball milling. The obtained AgNPs ink was filtrated through a 0.45µm syringe filter and filled into the ink cartridge. The α-terpineol was used as a co-solvent to reduce the evaporation rate of solvent and thus avoided blocking at the nozzles of printer as well as provided the ink proper values of viscosity and surface tension for fabricating the ink-jet printed flexible electrocircuits.
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ACCEPTED MANUSCRIPT Fig.1 illustrated our design to fabricate flexible electronics sintered at room temperature by dipping treatment. Briefly, a series of electrocircuits were fabricated by ink-jet printing the AgNPs ink onto PET substrates using Dimatix Fujifilm DMP-2831 printer with 10 pL
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Dimatix materials cartridge. The whole printing process was controlled by the Dimatix Drop Manager software, and the diameter of nozzle was approximately 20 µm. The printing
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frequency was set at 5.0 kHz and a customized waveform was used, which had a maximum voltage of 22 V and a pulse width of 8.5 µs. The substrate temperature was set to 30 °C, and
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the relative humidity (RH) within the printing chamber was 30-40%.
After evaporation of solvent, the printed electrocircuits were dipped into the methanol solution for a certain time and triggered AgNPs sintered at room temperature. The capped layers on AgNPs surfaces were gradually removed and led to the AgNPs coalescence and
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aggregation due to the preferential dissolution of alkylamine in methanol. By regulating the dipping time, the extent of coalescence and aggregation could be conveniently
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controlled, which was very crucial to achieve AgNPs sintering at room temperature.
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After that, the printed electrocircuits were rinsed sufficiently with ethanol and water to remove any capped molecules for conductivity measurements. 2.4 Characterization
The UV-vis and X-ray diffraction (XRD) spectra of AgNPs were collected on a Hitachi U-4100 spectrophotometer and
Bruker D8 X-ray diffractometer respectively. The size
and distribution of synthesized AgNPs were characterized by JEOL JEM-2100F transmission electron microscopy (TEM). The samples were prepared by dripping two or
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ACCEPTED MANUSCRIPT three drops of the dilute AgNPs dispersion in cyclohexane onto carbon-coated copper grids and dried exposed to atmosphere. Size distribution and number-average particle diameters were obtained using the Image ProPlus Image Analysis System. X-ray
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photoelectron spectroscopy (XPS) was recorded on ESCALab 220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation, and the base pressure was
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about 3×10-9 mbar. Nuclear magnetic resonance spectra (1H-NMR) were recorded on Inova 500 MHz spectroscopy. The surface morphology of aggregated AgNPs was
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investigated by Hitachi S-4800 scanning electron microscopy (SEM) and Bruker Nanoscope8 atomic force microscope (AFM). The optical images of printed electrocircuits were obtained by Olympus MX40 optical microscope. The cross-sectional profile of printed pattern was measured in the Kosaka ET4000 surface profiler. The
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resistance of the fabricated electrocircuits was measured on a Keithley 4200-SCS semiconductor analyzer with two point test method at 25 ℃.
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3. Results and discussion
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3.1 UV-vis spectra of the synthesized AgNPs In order to attain a fluent printing process and achieve controllable sintering of AgNPs at room temperature for the printed flexible electronics, the choice of capped molecules was fairly pivotal. Due to the strong bonding to the AgNPs surface, the capped molecules, such as alkanethiol [43], carboxylate [44], ammonium salts [45] and polymer stabilizers [4], were not particularly appropriate as they were difficult to remove at low annealing temperatures. In the case, different chain-length alkylamines were chosen as
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ACCEPTED MANUSCRIPT the capped molecules to prepare AgNPs since they exhibited weaker interactions with AgNPs surface [46]. These weak interactions could potentially be broken up at significantly lower temperatures, thus enabling the “stripped” AgNPs to coalesce to form
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a continuous conductive layer.
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Fig. 2(a) showed UV-vis spectra of the synthesized AgNPs as the capped molecules
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were dodecylamine (416 nm, curve a), tetradecylamine (416 nm, curve b) and hexadecylamine (416 nm, curve c) respectively. According to Mie’s theory, the shape and position of UV-vis absorption attributed to the surface plasmon resonance were strongly dependant on the nature of the metal nanoparticles, such as the size, shape, and
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status of aggregation [47]. From Fig. 2(a), the absorption shapes in curve (a)-(c) were nearly symmetrical, and this revealed that the synthesized AgNPs protected by these
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capped molecules were not agminated. In addition, the half-peak breadth of absorption peak in curve (b and c) exhibited more broad compared with that in curve (a), which
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indicated that the synthesized AgNPs protected by tetradecylamine and hexadecylamine possessed slightly more wide size distribution than those by dodecylamine [48, 49]. These preliminary results suggest that all these different chain-length alkylamines could be used as excellent capped molecules to synthesize nearly uniform and monodisperse AgNPs. As dodecylamine had the the most dissolvability and rate of extraction in methanol solution, it was more advantageous than tetradecylamine and hexadecylamine with 10
ACCEPTED MANUSCRIPT regards to the sintering process. Therefore, the synthetic strategy was focused on the AgNPs with dodecylamine as capped molecules in this study. To further investigate the influence of dodecylamine molecules on the size and distribution, the systems of
synthesis of AgNPs.
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different dodecylamine/ CH3COOAg molar ratio were conducted to controllable From Fig. 2(b), the absorption position for curves (d-f) was
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determined to be at 421, 418, and 414 nm, corresponding to dodecylamine/CH3COOAg molar ratio of 1.5:1, 2:1 and 2.5:1 respectively. With the ratio increase, the absorption
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peak showed a little blue shift and exhibited narrower half-peak breadth, which suggested that the formed AgNPs possessed smaller size and narrower distribution at high molar ratio. All these results were further confirmed by TEM analysis in Fig. 3(a)–(f).
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3.2 TEM images of the synthesized AgNPs
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Fig.3a-Fig.3c showed the TEM images of AgNPs synthesied with dodecylamine, tetradecylamine and hexadecylamine as capped molecules, and the size and distribution
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were 8.6±0.9, 8.9±1.2 and 9.2±1.6 nm respectively. A careful comparison of size distribution, it was clearly demonstrated that the synthesized AgNPs had no obvious difference in average size dimameter. Whereas, the AgNPs in Fig. 3 (b)- Fig. 3 (c) presented
more broad size distribuion compared with Fig. 3a, which was in good
agreement with the trend of plasmon absorption band (Fig.2a). The correlation of the size and distribution with different chain-length alkylamines could be understood by examining the process involved in the build-up to the AgNPs. It has been postulated that 11
ACCEPTED MANUSCRIPT silver ions could coordinate with the terminal groups of capped molecules to form Ag-complex, then the AgNPs formation would generally take place in the internal capped molecules or among the capped molecules. When dodecylamines were used as capped
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molecules, the stereo-hindrance effect was smaller due to shorter molecules chain, so it was easy to form abundant Ag-complex between silver ions and dodecylamines
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molecules, which was very crucial to control the size and distribution during the AgNPs synthesis procedure. As the structure evolved to tetradecylamine, the stereo-hindrance
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effect started to enhance, and more so in hexadecylamine. As a result, the size distribution became broader with the increase of chain-length alkylamines. Moreover, Fig.3(d)-Fig.3(f) showed the AgNPs that were synthesized by regulating the molar ratio between dodecylamine and silver acetate. From these TEM images, all
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AgNPs were almost spherical, uniform, and the size and distribution of the synthesized AgNPs decreased with the increase of dodecylamines/CH3COOAg ratio, which was also
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supported by the plasmon absorption band (Fig.2b). From the above results, nearly
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uniform AgNPs with particle size of <10 nm was sucessfully synthsized by this facile and efficient method, which was very beneficial to attain a printing flucency for ink-jet flexible electronics.
3.3 Morphology changes of the corresponding AgNPs aggregates The surface character of the AgNPs was extremely important to achieve the sintering at room temperature for flexible electronics. It was proved that dodecylamine could form proactive layers on AgNPs surface to efficiently prevent further agglomeration, which
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ACCEPTED MANUSCRIPT was attributed to the Ag-N interaction between AgNPs surface and amino moiety in dodecylamine. Compared with relative weaker Ag-N interaction, dodecylamine molecules were found to have high affinity with methanol [46]. Therefore, the
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detachment could be spontaneously realized and triggered self-sinter at room temperature
of AgNPs was bound to show significant changes.
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when the printed AgNPs were dipped in methanol solution. As a result, the morphology
In order to validate the feasibility our strategy, the as-synthsized AgNPs were ink-jet printed on substrate and subsequently dipped in methanol solution for a certain time. To exhibit the morphology changes, the formed AgNPs under different dipping times were
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investigated by SEM. From Fig. 4, the initial AgNPs were uniformly deposited on the substrate and could be distinguished individually due to the presence of dodecylamine
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protective layers around the AgNPs surfaces (Fig. 4a). After dipped in methanol solution, a portion of the protective layers were gradually removed from the AgNPs surfaces due
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to the preferential dissolution of dodecylamine in methanol and tiggered the detachments of capped molecules from the AgNPs surface. These detachments would greatly broaden the specific surface and increase the energy of the AgNPs. In order to achieve a stable state, the neighboring AgNPs with increased energy would naturally joint together to lower the total surface energy. As a result, the AgNPs would spontaneously contact, coalesce and a lot of necks were emerged among the AgNPs aggregate (Fig. 4b- Fig. 4f). Thus, multiple percolation paths were formed, which was fairly pivotal to the electron 13
ACCEPTED MANUSCRIPT transport among AgNPs aggregate [50]. All these SEM results proved that this strategy could be utilized to enable AgNPs sintered at room temperature and construct metal conductors directly on flexible substrates.
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3.4 The sintering mechanism AgNPs aggregates To further elucidate the sintering mechanism was triggered by the removal of
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dodecylamine protective layers from the AgNPs surfaces, XPS technique was employed to characterize the AgNPs samples before and after dipped in methanol solution, and the
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binding energy was referenced to the standard C1s at 287.60 eV. Form Fig. 5a, the peak signals of C1s at 285.12 ev, which mainly originated from carbon atom of alkyl chain in dodecylamine molecules, demonstrated an obvious decrease after dipping in the methanol solution. Accordingly, the N1s spectrum at 399.56 ev, attributed to the nitrogen atom of
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amino moiety in dodecylamine molecules, also exhibited a distinct reduction after dipping the printed AgNPs in methanol solution (Fig. 5b). The decreases of C1s and N1s
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peak intensity were all ascribed to the removal of dodecylamine molecules from the
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AgNPs surface, which was further supported by the decrease of 1HNMR peak intensity (Fig. S1).
In addition, from the XPS survey spectra of AgNPs sample in Fig. 5c, only the atoms of C, N, and Ag were detected, and no obvious other peaks were found, indicating the high purity of the sample. Fig. 5d also presented the high-resolution binding energy spectra for Ag3d5/2 and Ag3d3/2, identified at 367.85 eV and 373.96 eV respectively. The narrow width 14
ACCEPTED MANUSCRIPT of the peaks suggested that only a single element-silver was present in the system, and provided evidence for the encapsulation of zero valence AgNPs, which was also observed in the XRD spectra (Fig. S2). All these XPS and 1HNMR analysis provided sufficient
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supporting evidence that the protective layers on the AgNPs surfaces could be effectively detached by our dipping method, and consequently caused the AgNPs to coalesce and
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spontaneously be sintered at room temperature. 3.5 Fabrication of ink-jet printed flexible electronics
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In order to investigate the applicability of this fabrication strategy, the synthesized AgNPs were redissolved into the mixture of cyclohexane and α-terpineol to fabricate the
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AgNPs conductive ink for ink-jet printing flexible electrocircuits. Due to the smaller size, uniform shape and narrow distribution, the conductive ink based on these synthesized
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nanoparticles demonstrated excellent printing fluency (Fig.6a), and the printer nozzle would not be clogged with the larger nanoparticles or aggregation, which was very
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advantageous for ink-jet printing technique to fabricate flexible electrocircuits [38]. According to the designs, a subsequence of electrocircuits were fabricated by ink-jet printing the AgNPs ink onto PET film and subsequently dipped into the methanol solution (Fig.S3). From the cross-sectional surface profile, the morphology of printed electrocircuits remained flat and smooth with a relatively uniform thickness (Fig. S4), which indicated that the coffee-ring effect was effectively depressed and beneficial to electron transport among AgNPs aggregate. 15
ACCEPTED MANUSCRIPT It could be expected that the sintering process by dipping the printed electrocircuits in methanol method was a gradual process, since it involved surface desorption of the capped molecules, and coalescence of the AgNPs at the room temperature. As shown in Fig. 6b, the
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change of electrical resistivity was strongly with the dipping time. At the initial time, abundant AgNPs were stacked very densely in the printed areas due to convective flow
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during evaporation of the solvents (Fig. 6c) [51]. However, the residual capped molecule on the AgNPs surfaces would seriously prevent electrons transfer from one nanoparticles to
conducting percolation paths [52].
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another and lead to the relatively high resistivity, owing to the scarcity of effectively
The electrical conductance appeared only within 5 min after the dipping the printed AgNPs into methanol solution, and the resistivity quickly decreased with the extension of
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dipping time up to about 16 min. After that, the resistivity gradually decreased and a constant value was achieved until the dipping time reached 30 min. The resistivity
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remained no obvious changes when dipping time further extended, and the final electrical
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resistivity treated by this method fall in the range of 31.6~26.5 µ Ω.cm. This resistivity was even lower than the electrical resistivity, 46.2 µ Ω.cm, obtained by the same AgNPs after heat treatment at 160 ℃ for 30 min. Moreover, due to the facility nature of the method, the conductivity of printed AgNPs electrocircuits could be easily regulated by controlling the dipping time, which was very favorable to construct multifarious electronic patterns [53]. This decrease in resistivity corresponded to the nanostructure changes, and the
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ACCEPTED MANUSCRIPT morphology of AgNPs after sintering were investigated in detail. From the AFM image (Fig. 6d), the residual capped molecules were effectively removed from the AgNPs surfaces due to the dipping treatment, and the surface morphology exhibited obvious
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changes. It should be noted that the AgNPs changed from the uniform and spherical nanoparticles into bigger and irregular ones. Most nanoparticles of AgNPs have
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connected, coalesced, and abundant of necks were formed among the AgNPs aggregate. According to AFM, the height of printed electrocircuits in Fig. 6c was approximately 50
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nm, and showed a slightly decrease to 46 nm in Fig. 6d after sintered treatment. Consequently, multiple percolation paths continuously emerged in the ink-jet printed electrocircuits, which was fairly pivotal to the electron transmission among AgNPs aggregate [54-56]. These AFM results further proved that our strategy could be explored
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to enable AgNPs sintered at room temperature and construct conductive AgNPs patterns directly on flexible substrates. In addition, the resistance had no obvious change after
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storage for two months under ambient conditions, revealing the high stability of the
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ink-jet printed electrocircuits fabricated by our strategy. 4. Conclusions
In summary, a facile and efficient strategy was developed to successfully synthesize nearly uniform AgNPs with nanoparticle size of <10 nm and demonstrated excellent printing fluency for ink-jet printing flexible electrocircuits. These nanoparticles were capped with easily detachable alkylamine molecules, thus permitting to achieve the sintering of AgNPs at room temperature. A series of flexible electrocircuits were
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ACCEPTED MANUSCRIPT fabricated by ink-jet printing the as-synthesized AgNPs on PET substrates, and exhibited the resistivity in range of 31.6~26.5 µ Ω.cm after dipped treatment. This strategy provided a versatile approach to construct the flexible electrocircuits and promised
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enormous potentials in optoelectronic devices fields. Acknowledgments
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The authors would like to thank the National Nature Science Foundation (21303091 and 21073203), the Promotive Fundation for Excellent Young and Middle-aged
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Scientisits of Shandong Province (BS2013CL002) and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.
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ACCEPTED MANUSCRIPT [50] W.P. Wuelfing, S.J. Green, J.J. Pietron, D.E. Cliffel, R.W. Murray, J. Am. Chem. Soc., 122 (2000) 11465. [51] H. Hu, R.G. Larson, Langmuir., 21 (2005) 3963. [52] J. Perelaer, M. Klokkenburg, C.E. Hendriks, U.S. Schubert, Adv. Mater., 21 (2009)
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ACCEPTED MANUSCRIPT Figure caption Fig.1 Schematic illustration of controllable sintering AgNPs by dipped treatment for flexible
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inkjet-printed electronics
Fig.2 (a) UV-vis spectra of the synthesized AgNPs with different chain-length alkylamines
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as capped molecules: dodecylamine (416 nm, curve a), tetradecylamine (416 nm, curve b) and hexadecylamine (416 nm, curve c) respectively; (b) UV-vis spectra of the synthesized
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AgNPs with different dodecylamine /CH3COOAg molar ratio: 1.5:1 (421 nm, curve d), 2:1(418 nm, curve e) and 2.5:1 (414 nm, curve f) respectively.
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Fig.3 TEM images of the synthesized AgNPs capped with different chain-length alkylamines: (a) dodecylamine, (b) tetradecylamine and (c) hexadecylamine respectively;
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TEM images of the synthesized AgNPs with different dodecylamine/CH3COOAg molar
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ratio: (d) 1.5:1, (e) 2:1 and (f) 2.5:1 respectively.
Fig.4 (a) SEM images of corresponding AgNPs aggregates formed under different dipping time in methanol solution (a) 0, (b) 6, (c) 15, (d) 30, (e) 60, (f) 120 min.
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survey binding energy spectrum and (d) Ag3d binding energy spectrum.
Fig.6 (a) Optical image of the excellent printing fluency; (b) the resistivity changes as a
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topography changes after dipped treatment.
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Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics Zhiliang Zhang *a,b, Weiyue Zhu a a
Tel: +86-531-89631632
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Email: [email protected]
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Shandong Provincial Key Laboratory of Fine Chemicals, Qilu university of technology, Jinan 250353, China b Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
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Highlights
Silver nanoparticles with particle size of <10 nm was controllably synthesized. The sintering of silver nanoparticles was conducted at room temperature.
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The resistivity was reached as low as 26.5 µΩ.cm for flexible electronics.
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Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics Zhiliang Zhang*ab and Weiyue Zhua
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Shandong Provincial Key Laboratory of Fine Chemicals, Qilu university of technology, Jinan 250353, China Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences,
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Beijing 100190, China.
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*E-mail: [email protected]
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Fig.S1 The 1H-NMR of AgNPs with dodecylamines as capped molecules in initial state (a) and after methanol dipping treatment (b)
Fig.S2 XRD spectra of the synthesized AgNPs capped with dodecylamine (curve a), tetradecylamine (curve b) and hexadecylamine (curve c) respectively
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Fig. S3 The optical image of ink-jet printed flexible electrocircuits.
Fig. S4 The cross-sectional profile of ink-jet printed flexible electrocircuits.
S0925-8388(15)30592-2
DOI:
10.1016/j.jallcom.2015.07.195
Reference:
JALCOM 34889
To appear in:
Journal of Alloys and Compounds
Received Date: 26 May 2015 Revised Date:
29 June 2015
Accepted Date: 20 July 2015
Please cite this article as: Z. Zhang, W. Zhu, Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics, Journal of Alloys and Compounds (2015), doi: 10.1016/ j.jallcom.2015.07.195. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
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Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics Zhiliang Zhang *a,b, Weiyue Zhu a Shandong Provincial Key Laboratory of Fine Chemicals, Qilu university of technology, Jinan 250353,
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a
China b
Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100190, China
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Email: [email protected] Tel: +86-531-89631632
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Abstract
An effective and facile strategy was developed to successfully synthesize nearly uniform silver nanoparticles (AgNPs) with particle size of <10 nm, and demonstrated to achieve the sintering of AgNPs at room temperature for inkjet-printed flexible electronics. In such
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system, a series of different chain-length alkylamines were exploited as capped molecules to controllable synthesis of uniform AgNPs with the mean nanoparticle size in rang of 8.6±0.9,
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8.9±1.2 and 9.2±1.6 nm, and these ultra-small nanoparticles were very favorable to attain an excellent printing fluency. Based on the as-synthesized AgNPs, a sequence of flexible
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electrocircuits was successfully fabricated by ink-jet printing technique. After the dipped treatment, the printed AgNPs were achieved to spontaneous coalescence and aggregation at room temperature induced by preferential dissolution of capped molecules on AgNPs surfaces into methanol solution. These aggregated AgNPs demonstrated superior controllability, excellent stability and low resistivity in the range of 31.6~26.5 µΩ.cm, and would have enormous potential in the application to be tailored for assembly of optoelectronics devices. 1
ACCEPTED MANUSCRIPT Keywords: silver nanoparticles, controllable synthesis, dipping methods, ink-jet printing
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technique, flexible electronics
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ACCEPTED MANUSCRIPT 1. Introduction Flexible electronics has attracted considerable research interests in recent years as it exhibits tremendous potential in optoelectronics such as solar cells [1], LCDs [2], OLED
technique
is
considered
as
an
excellent
alternative
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lighting [3], and touch screen panels devices [4-6]. As a pattern approach, ink-jet printing candidate
to
conventional
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photolithography for fabrication of multifarious conductive patterns on the flexible substrates [7, 8]. Compared with the other fabrication methods, it enables design flexibility
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without the requirement of masks, multilayer deposition of a wide variety of materials due to its elimination of conventionally complex photolithography process. In addition, ink-jet printing technique can be easily scaled up towards industrial roll-to-roll fabrication processes for mass-production of various electrocircuits [9].
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Although ink-jet printing technique provides a promising potential pathway to fabricate various flexible electronics devices for well defined contacts, interconnects, antennas and
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electrodes, there are two major issues that must be resolved before widespread commercial
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application [10]. One of major issues in fabrication of flexible electronics by ink-jet printing technique is to explore an appropriate sintering strategy to achieve highly conductive patterns. Currently, the most commonly used sintering approach is to decompose the capped molecules on nanoparticle surfaces under high temperature and trigger electrical conductivity, wherein the sintering temperatures are typically above 200 °C, which are not compatible with the most flexible polymer substrate due to their low glass-transition temperatures [11-13]. Besides the thermal sintering, some other methods have also been
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ACCEPTED MANUSCRIPT exploited for this purpose, such as microwave [8], laser radiation [14-21], plasma [22], electrical [23] and photonic sintering [24, 25]. These methods could be used to fabricate conductive tracks or sinters these conductive materials selectively without affecting the
pre- or post-treatments for printed flexible electronics [26].
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substrate. However, they involve high-cost equipment and require high energy or complex
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In contrast, the chemical sintering of metallic nanoparticles, especially the solvent sintering strategy is considered as another promising candidate to achieve AgNPs sintering at
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room temperature [9, 27-29]. Compared with aforementioned sintering techniques, this approach can be conducted at low temperature in a facile, low-cost and energy-conserving manner. As a versatile sintering technique, chemical sintering has opened up a new possibility of room-temperature processable metal nanoparticle assemblies. Recently,
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Magdassi et al. proposed to neutralize charges of the AgNPs stabilized by poly(acrylic acid) (PAA) or detach the capped molecules on the surfaces with poly(diallyldimethylammonium
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chloride) (PDAC). After the destabilization process, it achieved to spontaneous coalescence
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of AgNPs at room temperature by tuning the concentration of PDAC [30]. In continuative study, Grouchko et al. demonstrated a built-in sintering mechanism to enable the printed PAA-capped AgNPs with excellent conductivity [31]. Wakuda et al. tried to trigger the sintering of AgNPs at room temperature by dipping the AgNPs paste into several solvent of the organic stabilizer, whereas the synthesis procedure of used AgNPs in the experiments was quite completed and time-consuming [32]. Obviously, it severely restricts the opportunity for obtaining a flexible conductor directly on plastic substrate.
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ACCEPTED MANUSCRIPT Another crucial issue related for ink-jet printed flexible electronics is to develop a facile method to synthesize uniform and ultra-small metal nanoparticles, which is extremely pivotal to attain a fluent printing process and highly stable conductive ink. To date, a wide
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variety of methods, such as microwave irradiation [33], sonochemical reduction [34], microemulsion [35], solvothermal reduction[36] and chemical reduction [37], have been
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employed to synthesize metal nanoparticles. However, it is difficult to obtain uniform nanoparticle with particle size of <10 nm by these methods. Consequently, the ink stability
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based on these synthesized nanoparticles would be likely affected as they aggregate and precipitate out from the solution. More seriously, the printer nozzle would be clogged with these larger nanoparticles or aggregation [38]. Therefore, it is very urgent to exploit a simple and versatile approach to synthesize ultra-small and uniform nanoparticles and achieve
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complete sintering at room temperature for inkjet-printed flexible electronics. In order to overcome these obstacles, following our previous study on AgNPs synthesis
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and printed electronics [39, 40], we here introduced a facile and efficient strategy to fabricate
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flexible conductive electronics directly on poly(ethyleneterephthalate) (PET) substrates by tuning the surface characters of AgNPs. In this work, a series of uniform and ultra-small AgNPs were successfully synthesized with alkylamines as capped molecules. The results showed that the size distribution of the synthesized AgNPs was 8.6±0.9, 8.9±1.2 and 9.2±1.6 nm corresponding to the respective capped molecules, and these ultra-small nanoparticles were very favorable to attain a printing fluency. Based on these as-synthesized AgNPs, a sequence of flexible electrocircuits were fabricated by ink-jet printing technique. After the
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extent of AgNPs could be precisely regulated by controlling the dipping time in methanol solution at room temperature. These aggregated AgNPs could serve as conductive materials
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and demonstrate super controllability, excellent stability and low resistivity in the range of 31.6~26.5 µΩ.cm. This strategy could be considered as a universal approach to construct
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various conductive microstructures and would have great potential to be tailored for applications in flexible electronics fields. 2. Experimental 2.1 Chemicals acetate
(AgAc),
1-dodecylamine
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Silver
(DDA),
1-tetradecylamine
(TDA),
1-hexadecylamine (HDA), and phenylhydrazine were obtained from American Sigma CO.,
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LTD. Methanol, acetone, toluene and α-terpineol were all purchased from Beijing Chemical
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Co. The other chemicals in the experiments were analytical or high-reagent grade. All of these chemicals and materials were used as received without further purification. The water used throughout the experiments was ultrapure water (18.2 MΩ) produced by a Milli-Q system.
2.2 Synthesis of AgNPs with alkylamine as capped molecules Solution-phase route was an effective approach to synthesize AgNPs with controlled spherical shape and size distribution [41, 42]. The AgNPs used in this work were
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silver acetate and 1-hexadecylamine were completely dissolved. A solution of phenylhydrazine (0.31 g) in toluene (10 mL) was slowly added into the resulting solution
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with continuous stirring and reacted at 60 °C for 1 h. Subsequently, before the above solution down to 30°C, acetone (30 mL) was added into to precipitate the AgNPs. The
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AgNPs were isolated by centrifugation at 5000 rpm, washed twice with acetone (30 mL), and vacuum-dried at room temperature to obtain a blue-black solid. The AgNPs synthesized with tetradecylamine or hexadecylamine as capped molecules were received respectively with the similar procedure.
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2.3 Fabrication of conductive ink based on synthesized AgNPs The synthesized AgNPs were redissolved into the mixture of cyclohexane and α-terpineol,
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and the concentration was set at 8% by weight for the corresponding suspension dispersed by
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ball milling. The obtained AgNPs ink was filtrated through a 0.45µm syringe filter and filled into the ink cartridge. The α-terpineol was used as a co-solvent to reduce the evaporation rate of solvent and thus avoided blocking at the nozzles of printer as well as provided the ink proper values of viscosity and surface tension for fabricating the ink-jet printed flexible electrocircuits.
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Dimatix materials cartridge. The whole printing process was controlled by the Dimatix Drop Manager software, and the diameter of nozzle was approximately 20 µm. The printing
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frequency was set at 5.0 kHz and a customized waveform was used, which had a maximum voltage of 22 V and a pulse width of 8.5 µs. The substrate temperature was set to 30 °C, and
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the relative humidity (RH) within the printing chamber was 30-40%.
After evaporation of solvent, the printed electrocircuits were dipped into the methanol solution for a certain time and triggered AgNPs sintered at room temperature. The capped layers on AgNPs surfaces were gradually removed and led to the AgNPs coalescence and
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aggregation due to the preferential dissolution of alkylamine in methanol. By regulating the dipping time, the extent of coalescence and aggregation could be conveniently
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controlled, which was very crucial to achieve AgNPs sintering at room temperature.
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After that, the printed electrocircuits were rinsed sufficiently with ethanol and water to remove any capped molecules for conductivity measurements. 2.4 Characterization
The UV-vis and X-ray diffraction (XRD) spectra of AgNPs were collected on a Hitachi U-4100 spectrophotometer and
Bruker D8 X-ray diffractometer respectively. The size
and distribution of synthesized AgNPs were characterized by JEOL JEM-2100F transmission electron microscopy (TEM). The samples were prepared by dripping two or
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photoelectron spectroscopy (XPS) was recorded on ESCALab 220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation, and the base pressure was
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about 3×10-9 mbar. Nuclear magnetic resonance spectra (1H-NMR) were recorded on Inova 500 MHz spectroscopy. The surface morphology of aggregated AgNPs was
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investigated by Hitachi S-4800 scanning electron microscopy (SEM) and Bruker Nanoscope8 atomic force microscope (AFM). The optical images of printed electrocircuits were obtained by Olympus MX40 optical microscope. The cross-sectional profile of printed pattern was measured in the Kosaka ET4000 surface profiler. The
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resistance of the fabricated electrocircuits was measured on a Keithley 4200-SCS semiconductor analyzer with two point test method at 25 ℃.
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3. Results and discussion
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3.1 UV-vis spectra of the synthesized AgNPs In order to attain a fluent printing process and achieve controllable sintering of AgNPs at room temperature for the printed flexible electronics, the choice of capped molecules was fairly pivotal. Due to the strong bonding to the AgNPs surface, the capped molecules, such as alkanethiol [43], carboxylate [44], ammonium salts [45] and polymer stabilizers [4], were not particularly appropriate as they were difficult to remove at low annealing temperatures. In the case, different chain-length alkylamines were chosen as
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a continuous conductive layer.
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Fig. 2(a) showed UV-vis spectra of the synthesized AgNPs as the capped molecules
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were dodecylamine (416 nm, curve a), tetradecylamine (416 nm, curve b) and hexadecylamine (416 nm, curve c) respectively. According to Mie’s theory, the shape and position of UV-vis absorption attributed to the surface plasmon resonance were strongly dependant on the nature of the metal nanoparticles, such as the size, shape, and
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status of aggregation [47]. From Fig. 2(a), the absorption shapes in curve (a)-(c) were nearly symmetrical, and this revealed that the synthesized AgNPs protected by these
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capped molecules were not agminated. In addition, the half-peak breadth of absorption peak in curve (b and c) exhibited more broad compared with that in curve (a), which
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indicated that the synthesized AgNPs protected by tetradecylamine and hexadecylamine possessed slightly more wide size distribution than those by dodecylamine [48, 49]. These preliminary results suggest that all these different chain-length alkylamines could be used as excellent capped molecules to synthesize nearly uniform and monodisperse AgNPs. As dodecylamine had the the most dissolvability and rate of extraction in methanol solution, it was more advantageous than tetradecylamine and hexadecylamine with 10
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synthesis of AgNPs.
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different dodecylamine/ CH3COOAg molar ratio were conducted to controllable From Fig. 2(b), the absorption position for curves (d-f) was
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determined to be at 421, 418, and 414 nm, corresponding to dodecylamine/CH3COOAg molar ratio of 1.5:1, 2:1 and 2.5:1 respectively. With the ratio increase, the absorption
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peak showed a little blue shift and exhibited narrower half-peak breadth, which suggested that the formed AgNPs possessed smaller size and narrower distribution at high molar ratio. All these results were further confirmed by TEM analysis in Fig. 3(a)–(f).
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3.2 TEM images of the synthesized AgNPs
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Fig.3a-Fig.3c showed the TEM images of AgNPs synthesied with dodecylamine, tetradecylamine and hexadecylamine as capped molecules, and the size and distribution
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were 8.6±0.9, 8.9±1.2 and 9.2±1.6 nm respectively. A careful comparison of size distribution, it was clearly demonstrated that the synthesized AgNPs had no obvious difference in average size dimameter. Whereas, the AgNPs in Fig. 3 (b)- Fig. 3 (c) presented
more broad size distribuion compared with Fig. 3a, which was in good
agreement with the trend of plasmon absorption band (Fig.2a). The correlation of the size and distribution with different chain-length alkylamines could be understood by examining the process involved in the build-up to the AgNPs. It has been postulated that 11
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molecules, the stereo-hindrance effect was smaller due to shorter molecules chain, so it was easy to form abundant Ag-complex between silver ions and dodecylamines
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molecules, which was very crucial to control the size and distribution during the AgNPs synthesis procedure. As the structure evolved to tetradecylamine, the stereo-hindrance
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effect started to enhance, and more so in hexadecylamine. As a result, the size distribution became broader with the increase of chain-length alkylamines. Moreover, Fig.3(d)-Fig.3(f) showed the AgNPs that were synthesized by regulating the molar ratio between dodecylamine and silver acetate. From these TEM images, all
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AgNPs were almost spherical, uniform, and the size and distribution of the synthesized AgNPs decreased with the increase of dodecylamines/CH3COOAg ratio, which was also
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supported by the plasmon absorption band (Fig.2b). From the above results, nearly
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uniform AgNPs with particle size of <10 nm was sucessfully synthsized by this facile and efficient method, which was very beneficial to attain a printing flucency for ink-jet flexible electronics.
3.3 Morphology changes of the corresponding AgNPs aggregates The surface character of the AgNPs was extremely important to achieve the sintering at room temperature for flexible electronics. It was proved that dodecylamine could form proactive layers on AgNPs surface to efficiently prevent further agglomeration, which
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detachment could be spontaneously realized and triggered self-sinter at room temperature
of AgNPs was bound to show significant changes.
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when the printed AgNPs were dipped in methanol solution. As a result, the morphology
In order to validate the feasibility our strategy, the as-synthsized AgNPs were ink-jet printed on substrate and subsequently dipped in methanol solution for a certain time. To exhibit the morphology changes, the formed AgNPs under different dipping times were
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investigated by SEM. From Fig. 4, the initial AgNPs were uniformly deposited on the substrate and could be distinguished individually due to the presence of dodecylamine
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protective layers around the AgNPs surfaces (Fig. 4a). After dipped in methanol solution, a portion of the protective layers were gradually removed from the AgNPs surfaces due
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to the preferential dissolution of dodecylamine in methanol and tiggered the detachments of capped molecules from the AgNPs surface. These detachments would greatly broaden the specific surface and increase the energy of the AgNPs. In order to achieve a stable state, the neighboring AgNPs with increased energy would naturally joint together to lower the total surface energy. As a result, the AgNPs would spontaneously contact, coalesce and a lot of necks were emerged among the AgNPs aggregate (Fig. 4b- Fig. 4f). Thus, multiple percolation paths were formed, which was fairly pivotal to the electron 13
ACCEPTED MANUSCRIPT transport among AgNPs aggregate [50]. All these SEM results proved that this strategy could be utilized to enable AgNPs sintered at room temperature and construct metal conductors directly on flexible substrates.
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3.4 The sintering mechanism AgNPs aggregates To further elucidate the sintering mechanism was triggered by the removal of
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dodecylamine protective layers from the AgNPs surfaces, XPS technique was employed to characterize the AgNPs samples before and after dipped in methanol solution, and the
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binding energy was referenced to the standard C1s at 287.60 eV. Form Fig. 5a, the peak signals of C1s at 285.12 ev, which mainly originated from carbon atom of alkyl chain in dodecylamine molecules, demonstrated an obvious decrease after dipping in the methanol solution. Accordingly, the N1s spectrum at 399.56 ev, attributed to the nitrogen atom of
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amino moiety in dodecylamine molecules, also exhibited a distinct reduction after dipping the printed AgNPs in methanol solution (Fig. 5b). The decreases of C1s and N1s
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peak intensity were all ascribed to the removal of dodecylamine molecules from the
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AgNPs surface, which was further supported by the decrease of 1HNMR peak intensity (Fig. S1).
In addition, from the XPS survey spectra of AgNPs sample in Fig. 5c, only the atoms of C, N, and Ag were detected, and no obvious other peaks were found, indicating the high purity of the sample. Fig. 5d also presented the high-resolution binding energy spectra for Ag3d5/2 and Ag3d3/2, identified at 367.85 eV and 373.96 eV respectively. The narrow width 14
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supporting evidence that the protective layers on the AgNPs surfaces could be effectively detached by our dipping method, and consequently caused the AgNPs to coalesce and
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spontaneously be sintered at room temperature. 3.5 Fabrication of ink-jet printed flexible electronics
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In order to investigate the applicability of this fabrication strategy, the synthesized AgNPs were redissolved into the mixture of cyclohexane and α-terpineol to fabricate the
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AgNPs conductive ink for ink-jet printing flexible electrocircuits. Due to the smaller size, uniform shape and narrow distribution, the conductive ink based on these synthesized
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nanoparticles demonstrated excellent printing fluency (Fig.6a), and the printer nozzle would not be clogged with the larger nanoparticles or aggregation, which was very
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advantageous for ink-jet printing technique to fabricate flexible electrocircuits [38]. According to the designs, a subsequence of electrocircuits were fabricated by ink-jet printing the AgNPs ink onto PET film and subsequently dipped into the methanol solution (Fig.S3). From the cross-sectional surface profile, the morphology of printed electrocircuits remained flat and smooth with a relatively uniform thickness (Fig. S4), which indicated that the coffee-ring effect was effectively depressed and beneficial to electron transport among AgNPs aggregate. 15
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change of electrical resistivity was strongly with the dipping time. At the initial time, abundant AgNPs were stacked very densely in the printed areas due to convective flow
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during evaporation of the solvents (Fig. 6c) [51]. However, the residual capped molecule on the AgNPs surfaces would seriously prevent electrons transfer from one nanoparticles to
conducting percolation paths [52].
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another and lead to the relatively high resistivity, owing to the scarcity of effectively
The electrical conductance appeared only within 5 min after the dipping the printed AgNPs into methanol solution, and the resistivity quickly decreased with the extension of
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dipping time up to about 16 min. After that, the resistivity gradually decreased and a constant value was achieved until the dipping time reached 30 min. The resistivity
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remained no obvious changes when dipping time further extended, and the final electrical
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resistivity treated by this method fall in the range of 31.6~26.5 µ Ω.cm. This resistivity was even lower than the electrical resistivity, 46.2 µ Ω.cm, obtained by the same AgNPs after heat treatment at 160 ℃ for 30 min. Moreover, due to the facility nature of the method, the conductivity of printed AgNPs electrocircuits could be easily regulated by controlling the dipping time, which was very favorable to construct multifarious electronic patterns [53]. This decrease in resistivity corresponded to the nanostructure changes, and the
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changes. It should be noted that the AgNPs changed from the uniform and spherical nanoparticles into bigger and irregular ones. Most nanoparticles of AgNPs have
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connected, coalesced, and abundant of necks were formed among the AgNPs aggregate. According to AFM, the height of printed electrocircuits in Fig. 6c was approximately 50
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nm, and showed a slightly decrease to 46 nm in Fig. 6d after sintered treatment. Consequently, multiple percolation paths continuously emerged in the ink-jet printed electrocircuits, which was fairly pivotal to the electron transmission among AgNPs aggregate [54-56]. These AFM results further proved that our strategy could be explored
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to enable AgNPs sintered at room temperature and construct conductive AgNPs patterns directly on flexible substrates. In addition, the resistance had no obvious change after
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storage for two months under ambient conditions, revealing the high stability of the
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ink-jet printed electrocircuits fabricated by our strategy. 4. Conclusions
In summary, a facile and efficient strategy was developed to successfully synthesize nearly uniform AgNPs with nanoparticle size of <10 nm and demonstrated excellent printing fluency for ink-jet printing flexible electrocircuits. These nanoparticles were capped with easily detachable alkylamine molecules, thus permitting to achieve the sintering of AgNPs at room temperature. A series of flexible electrocircuits were
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enormous potentials in optoelectronic devices fields. Acknowledgments
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The authors would like to thank the National Nature Science Foundation (21303091 and 21073203), the Promotive Fundation for Excellent Young and Middle-aged
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Scientisits of Shandong Province (BS2013CL002) and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.
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ACCEPTED MANUSCRIPT Figure caption Fig.1 Schematic illustration of controllable sintering AgNPs by dipped treatment for flexible
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inkjet-printed electronics
Fig.2 (a) UV-vis spectra of the synthesized AgNPs with different chain-length alkylamines
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as capped molecules: dodecylamine (416 nm, curve a), tetradecylamine (416 nm, curve b) and hexadecylamine (416 nm, curve c) respectively; (b) UV-vis spectra of the synthesized
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AgNPs with different dodecylamine /CH3COOAg molar ratio: 1.5:1 (421 nm, curve d), 2:1(418 nm, curve e) and 2.5:1 (414 nm, curve f) respectively.
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Fig.3 TEM images of the synthesized AgNPs capped with different chain-length alkylamines: (a) dodecylamine, (b) tetradecylamine and (c) hexadecylamine respectively;
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TEM images of the synthesized AgNPs with different dodecylamine/CH3COOAg molar
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ratio: (d) 1.5:1, (e) 2:1 and (f) 2.5:1 respectively.
Fig.4 (a) SEM images of corresponding AgNPs aggregates formed under different dipping time in methanol solution (a) 0, (b) 6, (c) 15, (d) 30, (e) 60, (f) 120 min.
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survey binding energy spectrum and (d) Ag3d binding energy spectrum.
Fig.6 (a) Optical image of the excellent printing fluency; (b) the resistivity changes as a
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function of dipping time; (c) AFM image of the printed AgNPs at initial and (d) the surface
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topography changes after dipped treatment.
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Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics Zhiliang Zhang *a,b, Weiyue Zhu a a
Tel: +86-531-89631632
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Email: [email protected]
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Shandong Provincial Key Laboratory of Fine Chemicals, Qilu university of technology, Jinan 250353, China b Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
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Highlights
Silver nanoparticles with particle size of <10 nm was controllably synthesized. The sintering of silver nanoparticles was conducted at room temperature.
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The resistivity was reached as low as 26.5 µΩ.cm for flexible electronics.
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Controllable synthesis and sintering of silver nanoparticles for inkjet-printed flexible electronics Zhiliang Zhang*ab and Weiyue Zhua
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Shandong Provincial Key Laboratory of Fine Chemicals, Qilu university of technology, Jinan 250353, China Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences,
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Beijing 100190, China.
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*E-mail: [email protected]
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Fig.S1 The 1H-NMR of AgNPs with dodecylamines as capped molecules in initial state (a) and after methanol dipping treatment (b)
Fig.S2 XRD spectra of the synthesized AgNPs capped with dodecylamine (curve a), tetradecylamine (curve b) and hexadecylamine (curve c) respectively
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Fig. S3 The optical image of ink-jet printed flexible electrocircuits.
Fig. S4 The cross-sectional profile of ink-jet printed flexible electrocircuits.