In Situ Generation of Photosensitive Silver Halide for Improving the

Subscriber access provided by UNIVERSITY OF CALGARY

Article

In-situ Generation of Photosensitive Silver Halide for Improving the Conductivity of Electrically Conductive Adhesives Chaowei Li, Qiulong Li, Xiaoyang Long, Taotao Li, Jingxin Zhao, Kai Zhang, Songfeng E, Jun Zhang, Zhuo Li, and Yagang Yao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07045 • Publication Date (Web): 07 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

In-situ Generation of Photosensitive Silver Halide for Improving the Conductivity of Electrically Conductive Adhesives Chaowei Li a,b, Qiulong Li a, Xiaoyang Long a, Taotao Li a, Jingxin Zhao a, Kai Zhang a, Songfeng E a,b , Jun Zhang a, Zhuo Li c*,Yagang Yao a,b* a

Division of Advanced Nanomaterials, Key Laboratory of Nanodevices and

Applications, CAS Center for Excellence in Nanoscience, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215123, China b

School of Nano Technology and Nano Bionics, University of Science and

Technology of China, Hefei, 230026, China

c

Department of Materials Science, Fudan University, Shanghai 200433, China

*Corresponding Author’s Email: [email protected]; [email protected].

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Electrically conductive adhesives (ECAs) can be regarded as one of the most promising materials to replace Sn/Pb solder. However, relatively low conductivity seriously restricts their applications. In the present study, we develop an effective method to decrease the bulk electrical resistivity of ECAs. KI or KBr is added to replace the lubricant and silver oxide layer on silver flakes and to form photosensitive silver halide. After exposing to sunlight, silver halide can photo-decompose into silver nanoparticles that will sinter and form metallic bonding between/among flakes during the curing process of ECAs, which would remarkably reduce the resistivity. The modified micro silver flakes play a crucial role in decreasing the electrical resistivity of the corresponding ECAs, exhibiting the lowest resistivity of 7.6×10-5 Ω·cm for 70 wt% loaded ECAs. The obtained ECAs can have wide applications in electronics industry, where high conductance is required. Keywords: surface photosensitization, silver flakes, electrically conductive adhesives, low electrical resistivity, sintering phenomenon

1. INTRODUCTION Electrically conductive adhesives (ECAs) have many advantages over traditional Sn/Pb-solder such as more environmentally friendly, much lower processing temperature, and higher resolution for fine pitch inter-connection capability.1,2 Therefore, ECAs have been regarded as one of the most promising materials to replace Sn/Pb solder in applications such as multilayer printed circuits, coupled resonators, thin film electronics and flexible electronic devices.3-5 Although ECAs offer many advantages, the complete replacement of the traditional solders still has a


Page 2 of 28

Page 3 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


long way to go.6 The main obstacle is their lower electrical conductivity than that of the eutectic Pb-based solders, which limits their widespread application in microelectronics.7 Therefore, their electrical conductivity should be improved in order to ensure their competitiveness in the electronics industry.8 The electrical conductivity of ECAs is tightly associated with the filler surface status.9 For silver filled ECAs, the electrical conductivity is affected by the silver oxide and the lubricant on silver surface.10 The resistivity of silver oxide and lubricant is much higher than the resistivity of pure silver. Even a thin layer of silver oxide or lubricant on the surface of silver fillers can dramatically increase the resistivity of ECAs. Unfortunately, most of the commercially available silver flakes are covered with a long-chain fatty acid, forming a silver carboxylate compound layer.11 The surfactant layers help to avoid agglomeration and facilitate the dispersion of silver flakes in ECAs.12 However, they are insulating and form an energy-barrier for electron tunneling between/among neighboring silver flakes. In consequence, many methods reported to decrease the resistivity of ECAs,13 such as addition of reducing agents like aldehyde,14,15 NaBH4 to reduce metal oxides into conductive metallic particles,16 incorporating short-chain diacids such as succinic acid17 and adipic acid18,19 to in-situ replace the long chain acids, partially thermal decomposition of surfactants with high temperature,20-22 increasing the shrinkage of matrix resin,23 and addition of low-melting-point metal to form metallurgical bonding between adjacent silver flakes.24-26 However, the electrical conductivity still has yet to be enhanced. Recently, Wong’s group developed an iodination approach to treat silver



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 28

flakes. As-made ECAs with modified silver content of 27.5 wt% display the resistivity can decrease to 4.81×10−4 Ω⋅cm, which has a strong competitiveness to solders. They believe that the formation of Ag/AgI nano-islands could avoid the oxidization of silver flakes at the curing temperature.27 However, little attention has been paid to how to make full use the silver oxide and lubricant layer to increase the ECAs’ electrical conductivity. In this work, we develop an effective silver surface halogenation strategy to enhance the ECAs’ electrical conductivity and disclose the modification mechanism. After the silver filler treatment by soluble halides, the oxidation and lubricant layer on the metal surface are turned into insoluble silver halides. Subsequent sunlight exposure of the treated silver flakes convert silver halides to conductive silver metals for the conductivity enhancement. Thus, through the halogenation modification of the filler, the oxidation and lubricant layer on the silver surface are fully utilized, which manifests that the surface halogenation approach can remarkably decrease the electrical resistivity of silver fillers based ECAs.

2 EXPERIMENTAL SECTION 2.1 Materials Micro silver flakes (SF-01 about 1~10 µm, Chengdu Banknote Printing Complex, China), the curing regent (Hexahydro-4-methylphthalic anhydride, HMMPA, Huicheng Chemicals Co., Ltd), epoxy resin (diglycidyl ether of biphenyl A, DGEBA, Epon

828,

Jiangsu

Terachemical

Co.,

Ltd)

and

the

catalyst

(1-cyanoethyl-2-ethyl-4-methy imidazole, 2E4MZ-CN, Shell Chemical Company) are


Page 5 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


used

as

received

without

any

treatment.

Acetone,

antioxidant

reagent

(Tris-(nonylphenyl)-phosphite, TNPP), potassium bromide (KBr), potassium iodide (KI), stearic acid, silver stearate and silver oxide (Ag2O) are purchased from Sinopharm Chemical Reagents Company. These chemicals are analytical grade. 2.2 Halogenation Treatment of Micro Silver Flakes The micro silver flakes (0.5 g) are added to the ethanol and water mixture (Vol/Vol~50/1) with different volumes (100, 200, 300, 400 and 500 µL, respectively) of 1×10-2 mol/L KI or KBr and stir for 3 h. Then, the silver flakes are washed with ethanol and water by centrifugation to remove the remaining KI or KBr and dried for 12 h under vacuum. 2.3 Exposure of Halogenated Micro Silver Flakes to Sunlight The modified silver powders (0.5 g) are placed in the round glass bowl (Φ14×8 cm) and then are exposed to the sunlight for 5, 15, 30 and 60 mins, respectively, under the oscillation of moderate intensity. 2.4 Preparation of Micro Silver Flakes Filled ECAs The preparation of ECAs is based on previous literature.14 The epoxy resin, HMMPA, antioxidant reagent, and acetone are blended in a weight ratio of 1:0.8:0.05:0.05 and mixed for 15 minutes under ultrasonic and stirring (IKA VORTEX GENIUS3 Model: VG 3S025). Then, different types of silver flakes are added to the viscous pastes and the acquired compounds are mixed for another 15 minutes. Lastly, the catalyst is added to the compounds to prompt curing (the weight ratio of catalyst to resin is 1:100). Then, the ECA is cured at 150 oC for 1h.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 28

2.5 Characterizations The bulk resistance (R) of cured ECAs is obtained by a Keithley 4200 multimeter. Bulk electrical resistivity, ρ, is calculated from the equation: ρ= Rtw/l, where R, t, w, and l are the resistance, thickness, width, and length of the sample, respectively. Five parallel samples are prepared for each treatment condition. The thermal decomposition behavior of silver oxide, lubricant and silver fillers in air is studied by a thermogravimetric analyzer (TGA, NETZSCH Instruments, model 209 F1 Libra). The temperature is increased from 25 to 600 oC (the ramping rate is 10 o

C/min). The micro-structures and morphologies of the untreated and treated silver

flakes are observed by field emission scanning electron microscopy (SEM, Quanta 400 FEG). The X-ray photoelectron spectroscopies (XPS) are collected by an ESCALab MKII X-ray photoelectron spectrometer with nonmonochromatized Mg Ka X-ray as the excitation source. LabRAM ARAMIS Raman confocal microscope (HORIBA Jobin Yvon) with a 532 nm diode pumped solid state laser (DPSSL) is used to collect the Raman spectrum. The silver flakes are placed on the glass slides for Raman measurements. The pH values are obtained from extensive pH indicator paper (Aokangxingainian). Optical studies are carried out using a UV-Vis spectrophotometer V-660

(Jasco

Corporation,

Japan,

wavelength

coverage:

187~900nm) with high purity of BaSO4 as standard baseline reagent.

3. RESULTS AND DISCUSSION Here, we provide a possible mechanism of the modification: (a) partial removal of silver surfactants and reduction of silver oxide on the micro silver flakes; (b) in-situ


Page 7 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


generation of photosensitive Ag-halides, and then photo-decomposition into Ag nanoparticles. Those silver nanoparticles can sinter forming the connected bridges during the curing process of ECAs and therefore enhance the conductivity of ECAs. Figure 1 shows the schematic diagram of the modification process and growth mechanism of silver nanoparticles.

Figure1. Schematic illustration of the modification process and growth mechanism of silver nanoparticles. 3.1 Partial Removal of Silver Surfactants and Reduction of Silver Oxide on the Surface of Micro Silver Flakes The commercially available micro silver flakes are generally obtained by ball milling process. In order to avoid the aggregation of silver flakes and facilitate the ball-milling process, an organic lubricant, usually a long chain fatty acid is added and forms a silver salt layer.12 In addition, the ball milling process could generate heat and lead to the oxidation of silver flakes. Therefore, the silver oxide layer and lubricant layer on the surface become a major disadvantage for the overall contact resistance. Owing to the high resistivity of silver oxide and organic lubricant, removing the surfactant layer and silver oxide becomes one important way to obtain highly conductive ECAs.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 28

When we use KI and KBr to modify the micro silver flakes, KI and KBr not only can react with the silver oxide (as evidenced by the increased pH value after treatment, Figure S1) but also substitute the lubricant on the silver flakes surface. (1)

Ag O + H O + 2KI → 2AgI + 2KOH

(2)

Ag O + H O + 2KBr → 2AgBr + 2KOH R − COOAg + KBr → R − COOK + AgBr R − COOAg + KI → R − COOK + AgI

(3) (4)

The surface lubricant on the silver flakes can be replaced by Br- and I-, because they have higher affinity and lower solubility than that of silver carboxylate.28 Because the unit cell constant of AgI (6.495Å)29 and AgBr (5.774 Å)29,30 are much shorter than the length of long chain fatty acid molecule (27 Å),18,31 the partial substitution can significantly reduce the distance between the adjacent silver flakes, enhancing the electron tunneling. The above substitution reactions can be demonstrated by the XPS, Raman, TGA and bulk electrical resistivity data of ECAs. The surface elemental compositions of silver flakes before and after modification are investigated by XPS and related element content is shown in Table S1. The XPS spectra of survey, 1s region of oxygen and carbon, 3d regions of silver and iodine/bromide on the treated and untreated silver flakes surface are shown in Figure 2. It can be seen that there is no signal of iodine or bromine in the untreated silver flakes (Figure 2(a)), while the presence of iodine and bromine can be confirmed in the silver flakes treated with iodine and bromine, respectively (Figure 2(a), 2(e) and 2(f)). This is the direct evidence that AgI and AgBr substances formed on the silver flakes


Page 9 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


after the modification process. The presence of trace carbon and oxygen in the XPS survey spectrum can be assigned to surface lubricant and metal oxide of silver flakes. The O 1s XPS peak of untreated silver flakes at 530.37 eV is shown in Figure 2(b), but the peak shifted to higher binding energy of 532.30 eV after treated with KBr and KI. The O1s curve can be separated to two symmetrical peaks (530.46 and 532.25 eV, respectively) as illustrated in Figure S2 (a), (b) and (c). These two peaks are associated with the oxygen of Ag2O and the functional group COO- of surface lubricant, respectively.32,33 After KI and KBr treatment, the peak at 530.46 eV of O1s is notably weakened, which implies that the Ag2O is decreased in content. Figure 2(c) shows the peak at 284.6 eV, which can be attributed to the C1s peak of surface lubricant from the sample. This peak could be separated into three peaks at 284.5 eV, 285.3 and 286.5 eV, corresponding sp2 hybridized carbon, sp3 hybridized carbon and C-O, respectively (The peak separation results are shown in Figure S2(d), (e), and (f)). After KI and KBr halogenation, the intensity of C 1s decreases, which suggests the surface lubricant is partially removed.32,34,35 Figure 2(d) shows the peaks of the 3d levels of silver, which show the spin orbit doublet with the binding energies around 368 and 374 eV, with a separation of 6 eV. Figure S2(g), (h) and (i) show the peak separation results.36,37 Figure 2(e) shows Br 3d XPS spectra for KBr treated silver flakes. The peak at 68.8 eV belongs to Br 3d of AgBr.38 Figure 2(f) shows curve-fitted I 3d XPS spectra for KI treated silver flakes. The peaks appeared at 619.4 and 631 eV belong to I 3d5/2 and I 3d3/2 which matches the literature well with values of 619.4 eV of AgI.39 Figure



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2(e) and 2(f) confirm the formation of AgBr and AgI respectively after halogenation treatment.

Figure 2. XPS of the untreated, 100 µL KBr and 100 µL KI modified silver flake. (a) survey scan of all samples, (b) O 1s regions of all samples, (c) C 1s regions of all samples, (d) Ag 3d regions of all samples, (e) Br 3d region of KBr treated sample, (f) I 3d region of KI treated sample. Because of the presence of a thin lubricant layer (long chain of fatty acid) on commercial silver flakes, the treated and untreated silver flakes show similar Raman spectra as exhibited in Figure 3(a), (b). The peak at 927 cm-1 is from CH2–COO-,40,41


Page 10 of 28

Page 11 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the peaks at 1098 and 1128 cm-1 are attributed to the C–C bond,42 and the peak of the twist and scissor modes of methylene groups is located at 1290 and 1431 cm-1, respectively.43 The appearance of silver carboxylate (surfactant)of silver flakes is verified by the symmetric (Vs(COO-)) stretching mode (1402 cm-1) and asymmetric (Vas(COO-)) stretching mode (1571 cm-1), indicating that the lubricant layer on micro silver flakes is not free fatty acid, but silver carboxylate.44,45 After the fillers treatment, an obvious decrease of Raman intensity can be seen. Moreover, as the volume of KBr and KI increases, the intensity of Raman decreases; when treated with 500 µL of modification reagent, the signal of lubricant on the silver flakes nearly disappeared indicating that the amount of lubricant on the surface of silver flakes has been greatly reduced.

Figure 3. Raman of untreated, KBr treated and KI treated with silver flakes. (a) KBr treated and untreated silver flakes and (b) KI treated and untreated silver flakes. The TGA results of untreated and treated micro silver flakes with different volumes of KI and KBr, respectively, are shown in Figure 4(a), (b) (the accurate data of weight loss are shown in Table S2 and Table S3). The weight loss of pristine silver flakes at temperatures lower than 300 oC is attributed to the thermal decomposition of lubricant and Ag2O can be decomposed above 300 oC (the TGA of weight loss of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

silver oxide and lubricant is shown in Figure S3). When the amount of KBr or KI is enough, the KBr or KI can substitute the surface silver oxide completely and no decomposition is observed above 300 oC. Also the content of surfactant on the silver flakes decreases with the volume of KBr or KI increases. The part of silver flakes without lubricant can be connected with each other and result in the conductivity improvement during ECA curing. This is supported by the following bulk resistivity measurements of ECAs.

Figure 4. TGA of (a) KBr treated and untreated silver flakes and (b) KI treated and untreated silver flakes in air atmosphere. The bulk resistivity of the as-made ECAs (70 wt%) is shown in Figure 5 and decreases with increasing the volume of KBr and KI. The 70 wt% ECAs filled with micro silver fillers treated with 500 µL KBr or KI can reach the lowest bulk resistivity to 3.98×10-4 Ω·cm and 1.08×10-4 Ω·cm, respectively. When continue to increase the content of the modifying agent, the as-made ECAs have a high viscosity, bringing great difficulty in mixing and processing of ECAs. The AgI or AgBr molecules are much shorter than that of fatty acid chains, therefore, the substitution of lubricant and Ag2O with AgI or AgBr can enhance electron tunneling between/among neighboring micro silver flakes, leading to increased electric conductivity of ECAs.19 In addition,


Page 12 of 28

Page 13 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the electrical resistivity of AgI and AgBr is lower than that of Ag2O and the surface lubricant, which is another reason leading to the decreased bulk resistivity of as-prepared ECAs after surface modification (the bulk electrical resistivity of Ag2O, AgBr and AgI are shown in Figure S4). This result demonstrates that by the halogenation treatment of silver flakes, the ECAs have over an order of magnitude decrease in bulk resistivity compared to those without treatment (7.6×10-3 Ω·cm).

Figure 5. Bulk resistivity of 70 wt% as-made ECAs filled with untreated and different treated silver flakes. Further characterizations of untreated and different treated flakes are conducted to explore the possible reasons for the decrement of electrical resistivity. We also utilize SEM to investigate the morphology changes of silver flakes after the treatment. From Figure 6 and Figure 7, we can learn that the treated silver flakes and untreated silver flakes exhibit similar morphology, which is irregular shape between 1~10 µm in length (Figure 6, Figure 7, Figure S5 and Figure S6) and about ~100 nm in thickness, which suggests the treatment has little effect on the size of silver flakes. The only difference is the degree of roughness as shown in Figure 6, Figure 7, Figure S5 and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure S6. The untreated silver flakes have a clean and smooth surface whereas the treated silver flakes have many particles and show roughened surface. What’s more, it is clear that the number of particles increases with KI or KBr volume (Figure S5 and Figure S6). In addition, the size distribution of untreated silver flakes (Figure S7) demonstrates the multimodal size distribution, which can improve the packing density.

Figure 6. Morphologies of untreated silver flakes (a) and treated silver flakes with different amount of KBr (b) 100, (c) 200, (d) 300, (e) 400 and (f) 500 µL. Scale bars are 5 µm.


Page 14 of 28

Page 15 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Morphologies of untreated silver flakes (a) and treated silver flakes with different amount of KI (b) 100, (c) 200, (d) 300, (e) 400 and (f) 500 µL. Scale bars are 5 µm. 3.2 The Photo-decomposition of Silver Halide into Silver Nanoparticles Figure 8 and Figure 9 show SEM to characterize the microstructures of metal flakes treated by KBr and KI with different exposure time of sunlight, respectively. We can see that after the exposure with sunlight, the number of small particles increases with the extension of exposure time. This may be caused by the decomposition of AgBr and AgI on the surface of silver fillers into silver nanoparticles shown in equations (5) and (6), respectively.

2 2Ag +

2 2Ag +

(5) (6)

It is known that Ag nanoparticles can sinter in the curing process of ECAs, forming metallic bonding between adjacent silver flakes, which is beneficial for the electron transport. The contact situations in ECAs filled untreated and treated silver fillers after exposed with sunlight are investigated (Figure S8). The in-situ generated Ag nanoparticles on the surface of treated silver sintered after curing process, forming metallic bonds between neighboring silver flakes. We also prove that Ag nanoparticles don’t separate with silver flakes as shown in Figure S9. Therefore, the bulk resistivity of ECAs further decreases after the halogenated silver flakes are exposed to sunlight and the resistivity decreases with increasing exposure time. ECAs with fillers treated by KI and KBr exposed for 60 mins show the lowest resistivity of



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7.6×10-5 Ω·cm and 1.05×10-4 Ω·cm, respectively (Figure 10). The difference of the bulk electrical resistivity of ECAs filled treated silver flakes by KI and KBr may be caused by the difference in photosensitivity of AgI and AgBr. Furthermore, we also investigate the dispersion of the different silver powder in the epoxy resin shown in Figure S10. We can find that the silver flakes dispersed in resin homogeneously without aggregation even in those ECAs filled with the highest silver flakes volume. Besides electrical resistivity, we also evaluate the shear strength of ECAs as shown in Figure S11. Even though the shear strength of as-prepared ECA decreases with increasing the KI or KBr volume, the lowest shear strength of ECA after surface modification of fillers, can still reach 6 MPa.

Figure 8. Morphologies of 500 µL KBr treated silver flakes with different exposure time: (a) 5, (b) 15, (c) 30 and (d) 60 mins. Scale bars are 5 µm.


Page 16 of 28

Page 17 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Morphologies of 500 µL KI treated silver flakes with different exposure time: (a) 5, (b) 15, (c) 30 and (d) 60 mins. Scale bars are 5 µm.

Figure 10. Bulk resistivity of 70 wt% as-made ECAs with modified silver flakes exposed with different exposure time. Adopting the ultraviolet-visible spectrophotometer with high purity of BaSO4 as standard baseline reagent, we can determine the absorption properties of untreated, 100 µL KI treated and 100 µL KBr treated silver flakes in the spectrum of 200



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nm~800 nm. As displayed in Figure 11, all silver flakes have large absorption near 200 nm, and show a strong absorption band at ~318 nm, which is assigned to the bulk Ag metal and the surrounding environment.46,47 As the wavelength increases, even though all the absorption has a decreasing trend, the absorption of untreated Ag flakes decreases most and is much weaker than the KBr and KI treated Ag flakes. In other words, the photosensitivity of Ag flakes is increased by in-situ generated photo sensitive AgI30,46 and AgBr29,48 after the KBr and KI treatment, respectively. What’s more, the absorption intensity of KI treated silver flakes is stronger than that of the KBr treated ones, which explains the resistivity of KI treated ECA lower than that of KBr treated ones. After exposed with sunlight, both KBr treated and KI treated silver flakes show similar absorption spectra (Figure S12).

Figure 11. The reflection-mode UV-vis absorption spectra of the untreated silver flakes, 100 µL KI treated silver and 100 µL KBr treated silver flakes.

4. CONCLUSIONS In summary, we develop an effective and facile method to enhance the electrical conductivity of silver based-ECAs, which can fully utilize the oxidation and lubricant


Page 18 of 28

Page 19 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layer on the silver surface through the halogenation modification. KI and KBr act as a substituting agent for the lubricant layer and silver oxidation on silver flakes and form photosensitive silver halide (AgI and AgBr), which can further photo-decompose into silver nanoparticles under the exposure of sunlight. Subsequently, the in-situ generated silver nanoparticles would sinter during the curing process, enhancing the metallic contact between silver flakes and increasing the conductivity. This method can be used to prepare ECAs for various applications in consumer electronics, where high conductance of ECAs is required.

ASSOCIATED CONTENT Supporting Information Tables, SEM images, size distribution, dispersion, pH value, the peak separation of XPS, sintered result, adhesion strength by lap shear test, electrical resistivity, and UV-vis absorption spectra of the silver flakes after light exposure are included in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Y. Yao, E-mail:[email protected]. *Z. Li, E-mail:[email protected]. ORCID Yagang Yao: 0000-0001-7566-3085 Notes



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The authors declare that there is no competitive economic benefit.

ACKNOWLEDGMENTS This work was funded by the National Key R&D Program of China (No. 2017YFB0406000), the Key Research Program of Chinese Academy of Sciences (No. QYZDB-SSW-SLH031), the Transformation of Scientific and Technological Achievements in Jiangsu Province (No. BA2016026), the Natural Science Foundation of Jiangsu Province, China (BK20140392 and Nos. BK20160399), Shanghai Pujiang Program (17PJ1400400), the Postdoctoral Foundation of Jiangsu Province (No. 1601065B) and the Science and Technology Project of Suzhou, China (ZXG201428, Nos. SZS201508 and ZXG201401).

REFERENCES (1) Li, Y.; Moon, K.-s.; Wong, C.P., Electronics without Lead. Science 2005, 308, 1419-1420. (2)Yang, C.; Wong, C. P.; Yuen, M. M., Printed Electrically Conductive Composites: Conductive Filler Designs and Surface Engineering. J. Mater. Chem. C 2013, 1, 4052-4069. (3) Qiao, W.; Bao, H.; Li, X.; Jin, S.; Gu, Z., Research on Electrical Conductive Adhesives Filled with Mixed Filler. Int. J. Adhes. Adhes. 2014, 48, 159-163. (4)Chen, C.-Y.; Hsiao, P.-H., Position-dependent Sensing Performance of Coupled Resonators with Symmetry Breaking. J. Phys. D: Appl. Phys. 2016, 49, 135101(1-7), DOI:10.1088/0022-3727/49/13/135101. (5) Ren, H. M.; Guo, Y.; Huang, S. Y.; Zhang, K.; Yuen, M. M. F.; Fu, X. Z.; Yu, S.


Page 20 of 28

Page 21 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H.; Sun, R.; Wong, C. P., One-Step Preparation of Silver Hexagonal Microsheets as Electrically Conductive Adhesive Fillers for Printed Electronics. ACS Appl. Mater. Interfaces 2015, 7, 13685-13692. (6)Yim, M. J.; Li, Y.; Moon, K. S.; Paik, K. W.; Wong, C. P., Review of Recent Advances in Electrically Conductive Adhesive Materials and Technologies in Electronic Packaging. J. Adhes. Sci. Technol. 2008, 22, 1593-1630. (7) Amoli, B. M.; Hu, A.; Zhou, N. Y.; Zhao, B., Recent Progresses on Hybrid Micro-Nano Filler Systems for Electrically Conductive Adhesives (ECAs) Applications. J. Mater. Sci.: Mater. Electron. 2015, 26, 4730-4745. (8)Ji, Y.-H.; Liu, Y.; Huang, G.-W.; Shen, X.-J.; Xiao, H.-M.; Fu, S.-Y., Ternary Ag/Epoxy Adhesive with Excellent Overall Performance. ACS Appl. Mater. Interfaces 2015, 7, 8041-8052. (9)Chatterjee, K.; Banerjee, S.; Chakravorty, D., Metal-to-Insulator Transition in Silver Nanolayers Grown on Silver Oxide Nanoparticles. Europhys. Lett. 2004, 66, 592. (10) Sivaramakrishnan, S.; Chia, P.-J.; Yeo, Y.-C.; Chua, L.-L.; Ho, P. K.-H., Controlled Insulator-to-Metal Transformation in Printable Polymer Composites with Nanometal Clusters. Nat. Mater. 2007, 6, 149-155. (11)Wong, C.; Lu, D.; Tong, Q. Lubricants of Silver Fillers for Conductive Adhesive Applications. Adhes. Joining Coat. Technol. Electron. Manuf., Proc. Adhes. 3rd, Int. Conf., IEEE. 1998, 184-192, DOI: 10.1109/ADHES.1998.742025. (12)Lu, D.; Tong, Q. K.; Wong, C., A Study of Lubricants on Silver Flakes for



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Microelectronics Conductive Adhesives. IEEE Trans. Compon. Packag. Technol. 1999, 22, 365-371. (13)Mir, I.; Kumar, D., Recent Advances in Isotropic Conductive Adhesives for Electronics Packaging Applications. Int. J. Adhes. Adhes. 2008, 28, 362-371. (14)Li, C.; Gong, X.; Tang, L.; Zhang, K.; Luo, J.; Ling, L.; Pu, J.; Li, T.; Li, M.; Yao, Y., Electrical Property Enhancement of Electrically Conductive Adhesives through Ag-coated-Cu Surface Treatment by Terephthalaldehyde and Iodine. J. Mater. Chem. C 2015, 3, 6178-6184. (15)Li, Y.; Moon, K.-S.; Whitman, A.; Wong, C.P., Enhancement of Electrical Properties of Electrically Conductive Adhesives (ECAs) by Using Novel Aldehydes. IEEE Trans. Compon. Packag. Technol. 2006, 29, 758-763. (16)Yang, C.; Lin, W.; Li, Z.; Zhang, R.; Wen, H.; Gao, B.; Chen, G.; Gao, P.; Yuen, M. M.; Wong, C. P., Water-Based Isotropically Conductive Adhesives: Towards Green and Low-Cost Flexible Electronics. Adv. Funct. Mater. 2011, 21, 4582-4588. (17)Luo, J.; Cheng, Z.; Li, C.; Wang, L.; Yu, C.; Zhao, Y.; Chen, M.; Li, Q.; Yao, Y., Electrically Conductive Adhesives Based on Thermoplastic Polyurethane Filled with Silver Flakes and Carbon Nanotubes. Compos. Sci. Technol. 2016, 129, 191-197. (18)Li, Y.; Moon, K.-S.; Wong, C.P., Electrical Property Improvement of Electrically Conductive Adhesives through In-Situ Replacement by Short-Chain Difunctional Acids. IEEE Trans. Compon. Packag. Technol. 2006, 29, 173-178. (19)Li, Y.; Moon, K.-S.; Li, H.; Wong, C.P., Conductivity Improvement of Isotropic Conductive Adhesives with Short-Chain Dicarboxylic Acids. Electron. Compon.


Page 22 of 28

Page 23 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Technol.

Conf.,

Proc.54th,

IEEE.

2004,

1959-1964,

DOI:

10.1109/ECTC.2004.1320395. (20)Zhang, R.; Moon, K.-S.; Lin, W.; Agar, J. C.; Wong, C.P., A Simple, Low-Cost Approach to Prepare Flexible Highly Conductive Polymer Composites by In-Situ Reduction of Silver Carboxylate for Flexible Electronic Applications. Compos. Sci. Technol. 2011, 71, 528-534. (21)Li, C.; Li, Q.; Cheng, L.; Li, T.; Lu, H.; Tang, L.; Zhang, K.; Songfeng, E.; Zhang, J.; Li, Z., Conductivity Enhancement of Polymer Composites Using High-Temperature Short-Time Treated Silver Fillers. Composites Part A 2017, 100, 64-70. (22)Gao, H.; Liu, L.; Liu, K.; Luo, Y.; Jia, D.; Lu, J., Preparation of Highly Conductive Adhesives by In-Situ Generated and Sintered Silver Nanoparticles during Curing Process. J. Mater. Sci.: Mater. Electron. 2012, 23, 22-30. (23)Lu, D. Q.; Wong, C. P., Effects of Shrinkage on Conductivity of Isotropic Conductive Adhesives. Int. J. Adhes. Adhes. 2000, 20, 189-193. (24)Dickey, M. D.; Chiechi, R. C.; Larsen, R. J.; Weiss, E. A.; Weitz, D. A.; Whitesides, G. M., Eutectic Gallium-Indium (EGaIn): aLiquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature. Adv. Funct. Mater. 2008, 18, 1097-1104. (25)Zhang, R.; Moon, K.-S.; Lin, W.; Wong, C.P., Preparation of Highly Conductive Polymer Nanocomposites by Low Temperature Sintering of Silver Nanoparticles. J. Mater. Chem. 2010, 20, 2018-2023.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 28

(26)Zhang, Z.; Chen, X.; Xiao, F., The Sintering Behavior of Electrically Conductive Adhesives Filled with Surface Modified Silver Nanowires. J. Adhes. Sci. Technol. 2011, 25, 1465-1480. (27)Yang, C.; Xie, Y.-T.; Yuen, M. M.-F.; Xu, B.; Gao, B.; Xiong, X.; Wong, C. P., Silver Surface Iodination for Enhancing the Conductivity of Conductive Composites. Adv. Funct. Mater. 2010, 20, 2580-2587. (28)Bell, S. E. J.; Sirimuthu, N. M. S., Surface-Enhanced Raman Spectroscopy as a Probe of Competitive Binding by Anions to Citrate-Feduced Silver Colloids. J. Phys. Chem. A 2005, 109, 7405-7410. (29)Bi, Y. P.; Ouyang, S. X.; Cao, J. Y.; Ye, J. H., Facile Synthesis of Rhombic Dodecahedral AgX/Ag3PO4 (X= Cl, Br, I) Heterocrystals with Enhanced Photocatalytic Properties and Stabilities. Phys. Chem. Chem. Phys. 2011, 13, 10071-10075. (30)Wang, P.; Huang, B. B.; Zhang, X. Y.; Qin, X. Y.; Jin, H.; Dai, Y.; Wang, Z. Y.; Wei, J. Y.; Zhan, J.; Wang, S. Y.; Wang, J. P.; Whangbo, M. H., Highly Efficient Visible-Light Plasmonic Photocatalyst Ag@Ag(Br,I).

Chem. - Eur. J. 2009, 15,

1821-1824. (31)Li, Y.; Moon, K.-S.; Wong, C.P., Effects of Reducing Agents on the Electrical Properties of Electrically Conductive Adhesives, Proc. 11th, Int. Symp. Adv. Packag. Mater.:

Processes,

Prop.

Interfaces,

IEEE.

2006,

179-183,

DOI:

10.1109/ISAPM.2006.1666041. (32)Johansson, E.; Nyborg, L., XPS Study of Carboxylic Acid Layers on Oxidized


Page 25 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Metals with Reference to Particulate Materials. Surf. Interface Anal. 2003, 35, 375-381. (33)Lopez, G. P.; Castner, D. G.; Ratner, B. D., Xps O 1s Binding-Energies for Polymers Containing Hydroxyl, Ether, Ketone and Ester Groups. Surf. Interface Anal. 1991, 17, 267-272. (34)Guan, X. H.; Chen, G. H.; Shang, C., ATR-FTIR and XPS Study on the Structure of Complexes Formed upon the Adsorption of Simple Organic Acids on Aluminum Hydroxide. J. Environ. Sci. 2007, 19, 438-443. (35)Merel, P.; Tabbal, M.; Chaker, M.; Moisa, S.; Margot, J., Direct Evaluation of the sp3 Content in Diamond-like-Carbon Films by XPS. Appl. Surf. Sci. 1998, 136, 105-110. (36)Zhang, H.; Wang, G.; Chen, D.; Lv, X. J.; Jinghong, U. H., Tuning Photoelectrochemical

Performances

of

Ag-TiO2

Nanocomposites

via

Reduction/Oxidation of Ag. Chem. Mater. 2008, 20, 6543-6549. (37)Wang, P.; Huang, B. B.; Zhang, Q. Q.; Zhang, X. Y.; Qin, X. Y.; Dai, Y.; Zhan, J.; Yu, J. X.; Liu, H. X.; Lou, Z. Z., Highly Efficient Visible Light Plasmonic Photocatalyst Ag@Ag(Br,I). Chem. - Eur. J. 2010, 16, 10042-10047. (38)Cao, J.; Luo, B. D.; Lin, H. L.; Xu, B. Y.; Chen, S. F., Visible Light Photocatalytic Activity Enhancement and Mechanism of AgBr/Ag3PO4Hybrids for Degradation of Methyl Orange. J. Hazard. Mater. 2012, 217, 107-115. (39)Zheng, Z.; Liu, A. R.; Wang, S. M.; Huang, B. J.; Ma, X. M.; Zhao, H.; Li, D. P.; Zhang, L. Z., In-Situ Fabrication of AgI Films on Various Substrates. Mater. Res. Bull.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 28

2008, 43, 2491-2496. (40)Genin, F.; Quiles, F.; Burneau, A., Infrared and Raman Spectroscopic Study of Carboxylic Acids in Heavy Water. Phys. Chem. Chem. Phys. 2001, 3, 932-942. (41)Li, Z.; Zhang, R.; Moon, K. -S.; Liu, Y.; Hansen, K.; Le, T.; Wong, C.P., Highly Conductive, Flexible, Polyurethane-Based Adhesives for Flexible and Printed Electronics. Adv. Funct. Mater. 2013, 23, 1459-1465. (42)Moskovits,

M.;

Suh,

J.

S.,

Conformation

of

Mono-Carboxylic

and

Dicarboxylic-Acids Adsorbed on Silver Surfaces. J. Am. Chem. Soc. 1985, 107, 6826-6829. (43)Yamamoto, S.; Fujiwara, K.; Watarai, H., Surface-Enhanced Raman Scattering From Oleate-Stabilized Silver Colloids at a Liquid/Liquid Interface. Anal. Sci. 2004, 20, 1347-1352. (44)Li, Z.; Hansen, K.; Yao, Y.; Ma, Y.; Moon, K.-S.; Wong, C.P., The Conduction Development Mechanism of Silicone-Based Electrically Conductive Adhesives. J. Mater. Chem. C 2013, 1, 4368-4374. (45)Zhang, R.; Lin, W.; Moon, K.-S.; Wong, C.P., Fast Preparation of Printable Highly Conductive Polymer Nanocomposites by Thermal Decomposition of Silver Carboxylate and Sintering of Silver Nanoparticles. ACS Appl. Mater. Interfaces 2010, 2, 2637-2645. (46)Yang, C.; Xie, Y. T.; Yuen, M. M. F.; Xiong, X. M.; Wong, C. P., A Facile Chemical Approach for Preparing a SERS Active Silver Substrate. Phys. Chem. Chem. Phys. 2010, 12, 14459-14461.


Page 27 of 28

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(47)Hu, C.; Peng, T. W.; Hu, X. X.; Nie, Y. L.; Zhou, X. F.; Qu, J. H.; He, H., Plasmon-Induced Photodegradation of Toxic Pollutants with Ag-Agl/Al2O3 under Visible-Light Irradiation. J. Am. Chem. Soc. 2010, 132, 857-862. (48) Wang, D.; Duan, Y.; Luo, Q.; Li, X.; Bao, L, Visible Light Photocatalytic Activities of Plasmonic Ag/AgBr Particles Synthesized by a Double Jet Method. Desalination 2011, 270, 174-180.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents (TOC) Graphic


Page 28 of 28

In Situ Generation of Photosensitive Silver Halide for Improving the

Recommend Documents