Thin Solid Films 518 (2010) 6634–6637
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Thin Solid Films
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Eco-friendly synthesis of SiO2 nanoparticles with high purity for digital printing
Sung-Jei Hong a,⁎, Jeong-In Han b
a Display Components and Materials Research Center, Korea Electronics Technology Institute, Republic of Koreab Department of Chemical and Biochemical Engineering Dongguk University, Republic of Korea
⁎ Corresponding author. #68, Yatap, Bundang, SeRepublic of Korea. Tel.: +82 31 789 7431; fax: +82 31
E-mail address: [email protected] (S.-J. Hong).
0040-6090/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.tsf.2010.03.064
a b s t r a c t
a r t i c l e i n f oAvailable online 31 March 2010
Keywords:SiO2 nanoparticlePasteEco-friendly synthesisDigital printingInsulating layerDielectric layerHeat-treatment at low temperature
In this study, SiO2 nanoparticles were synthesized by an eco-friendly method, which excludes harmfulelements, such as NO3
−, and in so doing, lowers the heat-treatment temperature from 500 to 300 °C, leadingto the downsizing of the nanoparticles. The sizes of the SiO2 nanoparticles were less than 20 nm and wereuniformly distributed. In the XRD analysis, the most intense peak was observed at about 23.5° confirmingthat the synthesized nanoparticles have a SiO2 structure. No harmful elements such as NO3
− were found inthe SiO2 nanoparticles, showing that their purity was improved. Also, SiO2 paste was well formulated bydispersing the SiO2 nanoparticles uniformly in a solvent uniformly. The SiO2 paste printed onto a glasssubstrate followed by curing at 200 °C showed good insulating and dielectric properties. The resistance andthe dielectric constant of the printed layer were above 1011 Ω and 4.434, respectively. Those values weresufficient for insulating and dielectric characteristics. Therefore, the SiO2 nanoparticles synthesized by thiseco-friendly method have the potential to be used as the material for insulating and dielectric layers.
ongnam, Gyeonggi, 463-816,789 7439.
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© 2010 Elsevier B.V. All rights reserved.
1. Introduction
SiO2 thin film has a wide range of applications and is used asinsulating and dielectric layers in various microelectronic devicessuch as semiconductor and flat panel displays, due to its suitablecharacteristics [1–3]. It is fabricated using vacuum methods, forexample, thermal oxidation, chemical vapor deposition (CVD),thermal evaporation, sputtering, and the plasma decomposition ofgaseous molecules [3]. Also, wet-type coating methods such as thesol–gel technique are increasingly used due to the increasing use offlexible electronics [4]. Recently, the development of methods whichlimit the use of Si material is increasingly being demanded due to itsexhaust on earth. As an alternative, digital printing is attracting moreand more. Digital printing is used to print the insulating patterndirectly onto the substrate [5,6] and, therefore, the amount of Simaterial used can be considerably decreased. Digital printing has theadvantages of decreasing the number of process steps, input materialsand waste water, leading to the enhancement of the quality of theproducts. For digital printing, an ink pastematerial is needed, which iscomposed of SiO2 nanoparticles, solvent and additives. In order toenhance the function of the digitally printed SiO2 thin film layer,ultrafine sized SiO2 nanoparticles are required. That is, a uniform SiO2
insulating layer can be achieved by using ultrafine nanoparticlesuniformly dispersed in the ink paste. However, in the conventionalsynthetic process, an ultrafine size cannot be achieved, because of the
presence of harmful elements such as NO3−. In order to eliminate these
harmful elements, a high temperature of 500 °C is required to coarsenthe particle [7]. Moreover, the purity of the particles synthesized bythis process is low, resulting in the degradation of the electricallyinsulating properties.
Therefore, in this study, the eco-friendly synthesis of SiO2
nanoparticles for digital printing was attempted. The eco-friendlysynthesis method is employed to fabricate SiO2 nanoparticles withoutany harmful elements such as NO3
−, which are normally used. Byexcluding these harmful elements during the synthesis, the purity ofthe SiO2 nanoparticles can be enhanced, leading to the improvementof the insulating properties of the printed SiO2 layer in the electronicdevices. In addition, by eliminating these harmful elements, theheating temperature of the SiO2 nanoparticle can be lowered from 500to 300 °C. Using the SiO2 nanoparticles synthesized by the eco-friendly method, a SiO2 paste was made. Then, a SiO2 insulating layerwas attempted to fabricate onto the glass substrate in order toinvestigate whether the SiO2 nanoparticles have the potential to beused as a printed insulating and dielectric layer.
2. Experimental details
As the startingmaterial, silicon tetra-acetate was selected, andwasdispersed in methanol to act as the precursor of the SiO2 nanopar-ticles. The raw material was dissolved in the solvent, and the mixedsolution was stirred to evaporate the solvent component. Theremained components were dried at 80 °C, and the precursor wasmade. After that, thermal analyses, such as thermogravimetricanalysis (TGA) and differential thermal analysis (DTA) of the
Fig. 1. Thermal behavior of precursor of SiO2 nanoparticle. (a) DTA. (b) TGA.
Fig. 2. Size and morphology of SiO2 nanoparticle synthesized at 300 °C.
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precursor, were carried out in order to determine the optimaltemperature, that is, the lowest temperature at which the organiccomponents can be burnt out, leaving only Si. The temperature wasraised from 25 to 400 °C at a rate of 5 °C/min and the presence of anythermally decomposed points was observed to determine the optimalheating temperature. Then, the precursor was heated at thattemperature. The physical properties of the synthesized SiO2
nanoparticles were analyzed by high resolution transmission electronmicroscopy (HRTEM, JEOL 300 kV) in conjunction with energydispersion spectroscopy (EDS), X-ray diffraction (XRD, Rigaku Rota-flex D/MAX System), and Brunauer, Emmett & Teller (BET) surfacearea analyzer. Purity of the SiO2 nanoparticle was investigated usingEDS analysis, by detecting impurities such as NO3
−. Also, in order toinvestigate the feasibility of applying them to direct printing, thesynthesized SiO2 nanoparticles were mixed with an eco-friendlysolvent and additives to make 30 wt.% SiO2 paste. The viscosity of theSiO2 paste was less than 1×103 cps, and the paste was directly printedonto a glass substrate with a size of 5×2.5 cm, followed by curing at200 °C. After the fabrication of the SiO2 layer, its insulating propertieswere observed by measuring its resistance by the 2 point probemethod. The resistance of the printed layer was measured at 5randomly selected points with a 1 μm sized tip. Also, for theevaluation of dielectric constant of the layer, a PNA-L networkanalyzer (Agilent Technologies, 5230A)was used tomeasure under anapplication of 1 GHz signal.
3. Results and discussions
The thermal behavior of the precursor is shown in Fig. 1. As shownin Fig. 1(a), a change in the heat flow was observed at about 280 °C.That is, exothermic reaction occurred at that temperature. Thisexothermic behavior is due to the decomposition of the organiccomponent of the precursor. This phenomenon is explained by thethermal weight change of the precursor. That is, as shown in Fig. 1(b),the weight was decreased up to about 280 °C. From the thermalanalysis, it is supposed that the organic components included in theprecursor were burnt out at a temperature of less than 300 °C.Therefore, the optimum heating temperature was determined to be300 °C, and SiO2 nanoparticles were synthesized at that temperature.After that, the physical properties of the synthesized SiO2 nanopar-ticles were analyzed. The sizes of the synthesized particles are lessthan 20 nm, as shown in Fig. 2, and are uniformly distributed. Thespecific surface area of the SiO2 nanoparticles was about 200 m2/g.From the analysis, it is confirmed that ultrafine sized SiO2 nanopar-ticles were synthesized. This is attributed to the suppression ofparticle growth by lowering the heat-treatment temperature. One ofthe mechanisms which can be used to increase the size of thenanoparticles is particle surface migration [8]. According to thetransformation kinetics;
D = exp −ΔG= kTð Þ ð1Þ
where D, ΔG, k and T are the mean particle size, activation energy forparticle surface migration, Boltzmann constant and temperature,respectively. When the activation energy for particle growth is larger,the surface activity of the SiO2 nanoparticles as a function of theirtemperature is lowered. Accordingly, the growth of the nanoparticlescan be suppressed by lowering the heating temperature.
Then, to investigatewhether a temperature of 300 °C is sufficient tosynthesize the SiO2 nanoparticles or not, EDS and XRD analyses wereperformed. In the case of the EDS analysis, as shown in Fig. 3, Si and Oelements were clearly observed. No harmful elements such as NO3
−
were found, thus confirming that the purity of the SiO2 nanoparticleswas improved. The composition ratios of Si and O were about 40 and60 wt.%. The calculated composition ratios of Si and O were 46.7 and53.3 wt.% and, thus, the weight ratio of oxygen is lower than the
measured value. This is attributed to defects in the SiO2 nanoparticles.That is, it is supposed that excess oxygen atoms exist in the matrix ofthe SiO2 nanoparticles [9]. Further work is required to reduce thenumber of defects. Also, in the case of XRD analysis, as shown in Fig. 4,
Fig. 3. Composition ratio of Si and O in SiO2 nanoparticle. (a) Qualitative. (b) Quantitative.
Fig. 4. Preferred orientation of SiO2 nanoparitcle.
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the most intense peak of the nanoparticle was detected at 23.5°. Fromthe analyses, it is confirmed that the synthesized nanoparticles have aSiO2 structure [10]. In addition, full width half maximum (FWHM) ofthe peak is wide. This is attributed to the fact that the SiO2
nanoparticles synthesized at low temperature have an ultrafine crystalstructure according to Scherrer's equation [11]. From the X-raydiffraction peak, the particle size can be calculated by using Scherrer'sformula as follows;
t = 0:9λ = B cos θB ð2Þ
where t, λ, B, and cos θB are the particle size, wavelength (0.1542 nmfor CuKα radiation), FWHM of the peak in radians, and diffractionangle, respectively. In Eq. (2), the intensity of the peak increases withdecreasing peak half width, indicating the growth of the SiO2
nanoparticles. Thus, the widening of the FWHM corresponds to thelowering of the particle size. The particle size calculated using Eq. (2)is about 13 nm. Therefore, it is confirmed that the lowering of thetemperature is required to synthesize smaller sized nanoparticles, and300 °C is a suitable temperature for synthesizing ultrafine SiO2
nanoparticles.To investigate the feasibility of using the SiO2 nanoparticles in an
insulating layer, a SiO2 paste was formulated by dispersing the SiO2
nanoparticles in a solvent. In order to disperse them uniformly, theagglomeration of the nanoparticles has to be prevented. Theagglomeration of the SiO2 nanoparticles is attributed to attractiveinteractions between them which reduce the surface energy. Thesurface area of a nanoparticle is about 105 times that of amicroparticle. Thus, the driving force to reduce the surface area isvery high, so that the aggregation of the nanoparticles occursspontaneously. Therefore, to disperse the nanoparticles, an externalpotential energy should be applied by using organic additives toprevent their aggregation. In the theory of Derjiaguin, Landau, Verweyand Overbeek (DLVO), the potential energies of attraction andrepulsion are summed to provide the total interaction potentialenergy between colloidal particles [12]. The interaction potentialenergies of attraction and repulsion are generated by van der Waalsforces and electrostatic forces, respectively. In the case of nano-dimensional particles, attraction is much larger than repulsion,because of their low electrostatic charge, which is the surface chargedensity times the surface area [13]. Accordingly, the total energyassumes a negative value and the SiO2 nanoparticles start toaggregate. Therefore, in order to disperse the SiO2 nanoparticlesuniformly, the total energy has to be made positive by increasing therepulsion. Thus, dispersing agents were added to the paste to generatea repulsive force. As a result, the SiO2 nanoparticles were disperseduniformly. It is believed that the total energy becomes positive, due tothe dispersing agent surrounding the SiO2 nanoparticles, leading totheir uniform dispersion. After that, the SiO2 paste was printed onto aglass substrate and cured at 200 °C. The curing temperature wasdetermined from the thermal analysis of the paste, and the thicknessof the printed layer was about 5 μm. The thickness was controlled toeliminate the effect of glass substrate. After curing the printed SiO2
layer, its electrically insulating properties were investigated. As aresult, as shown in Fig. 5, the resistance of the printed layer was above1011 Ω. Especially, the resistance values measured at random pointsare very uniform. The deviation of the resistance values is less than10%. This is attributed to the uniform dispersion of the SiO2
nanoparticles with high purity in the layer. That is, as the pure SiO2
nanoparticles is uniformly distributed, the resistance values areuniform. The electrical properties are sufficient for insulation. Thatis, since a value of more than 109 Ω is required for an insulating layer,
Fig. 5. Electrically insulating properties of the printed SiO2 layer.
6637S.-J. Hong, J.-I. Han / Thin Solid Films 518 (2010) 6634–6637
the value obtained in this study is sufficient. Also, in case of dielectricbehavior, 4.434 was achieved from the printed layer. This is asufficient value because that of bulk is 4.2 [14]. Therefore, the SiO2
nanoparticles synthesized by the eco-friendly method have thepotential to be used as an insulating and dielectric layer.
4. Conclusions
In this study, SiO2 nanoparticles were synthesized by an eco-friendly method. By excluding harmful elements, the heat-treatmenttemperature is lowered from 500 to 300 °C, leading to downsizing of
the nanoparticle. The sizes of the SiO2 nanoparticles were less than20 nm and were uniformly distributed. Also, the most intense peak inthe X-ray diffracted pattern was observed at about 23.5°, confirmingthat the synthesized nanoparticles have a SiO2 structure. No harmfulelements such as NO3
− were found in the SiO2 nanoparticles, showingthat their purity was improved. In addtion, SiO2 paste was formulatedand printed onto a glass substrate, followed by curing at 200 °C. Theresistance of the printed layer was above 1011 which is sufficient forinsulation. Also, the dielectric constant of the layer was 4.434.Therefore, the SiO2 nanoparticles synthesized by the eco-friendlymethod have the potential to be used as insulating and dielectriclayers.
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