Thin Solid Films 600 (2016) 109–118
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Effect of particle size on various substrates for deposition of NiO film via nanoparticle deposition system Hyungsub Kim a, Seungkyu Yang a, Sung-Hoon Ahn b, Caroline Sunyong Lee a,⁎ a b
Department of Materials Engineering, Hanyang University, 5th Engineering Building, Rm. #312, 55 Hanyangdaehak-ro, Sangrok-gu, Ansan-si, Gyeonggi-do 426-791, South Korea School of Mechanical & Aerospace Engineering, Seoul National University, Building 301, Room 1405, Gwanak-ro 1, Gwanak-Guo, Seoul 151-744, South Korea
a r t i c l e
i n f o
Article history: Received 18 September 2015 Received in revised form 14 January 2016 Accepted 15 January 2016 Available online 19 January 2016 Keywords: Dry deposition Particle NiO Size Substrate Mechanism
a b s t r a c t We report the deposition mechanism of NiO particles using a nanoparticle deposition system. To understand the effects of particle size and substrates on the deposition, nano-, 100-nm-, sub-micro-, and micro-sized NiO particles were deposited on Si wafers, Ni-coated Si wafers, and fluorine-doped tin oxide (FTO)-coated glass. It was found that 100-nm- and nano-sized NiO particles were deposited, forming loosely compacted coating layers, by the breaking up of agglomerates, regardless of the type of substrate. In contrast, sub-micro- and micro-sized NiO particles formed dense and compact coating layers by deformation and fracturing on the Si and Ni-coated Si wafers. Moreover, sub-micro- and micro-sized NiO particles were not deposited on FTO glass; this was likely attributable to the NiO being harder than FTO glass and the micro-sized NiO particles would likely have rebounded on impact, resulting in no deposition. Thus, the deposition mechanism of NiO particles may be greatly related to the relative hardness difference between the NiO particles and the substrate. Moreover, it was found that different particle sizes resulted in different friction and mobility, based on response angle measurements, influencing the deposition mechanism(s), especially at the interface. When the particle size was greater than 100 nm, the deposition was due primarily to deformation and fracturing during the collision with the substrate. In particular, the 100-nm-sized NiO particles showed both mechanisms, a two-step process, with deformation or fracturing at the interface between the substrate and particles, followed by a loosely compacted coating layer forming, preserving the original particle shape. Thus, it was confirmed that the 100-nm-sized NiO particles were at or near a boundary for deposition mechanisms. The effects of particle size and substrate for dry deposition were explained successfully by assessing the deposition behavior using analytical tools. © 2016 Published by Elsevier B.V.
1. Introduction Nickel oxide (NiO) which is crystallized in cubic structure, has been widely used in various scientific and technological applications [1]. NiO is an interesting material that exhibits good chemical stability and useful optical, electrical, and magnetic properties [2]. NiO films have been used in applications such as antiferromagnetic [3] and electrochromic materials [4], chemical sensors [5], and p-type transparent conducting films [6]. Recently, NiO has been used as an electrode material for Li batteries, with favorable electrochemical performance [7]. It can be used in p-type semiconductor films due to the large band gap energy of 3.6–4.0 eV [8]. Most attracting properties of NiO are: (1) excellent durability, (2) low material cost, (3) promising ion storage material in terms of cyclic stability, and (4) possibility of manufacturing via various techniques [9]. Thus, NiO was chosen as a coating material via new deposition method, based on those properties stated above.
⁎ Corresponding author. E-mail address:
[email protected] (C.S. Lee).
http://dx.doi.org/10.1016/j.tsf.2016.01.031 0040-6090/© 2016 Published by Elsevier B.V.
Various processes have been reported to fabricate NiO films, including radio frequency sputtering with a nickel or nickel-oxide target [10], pulsed laser ablation with a nickel oxide target [11], thermal evaporation [12], metal–organic chemical vapor deposition (MOCVD) [13], and electrochemical deposition [14]. However, these processes typically involve complex equipment and are time consuming, leading to high cost. To overcome these limitations, alternative deposition processes are required, such as dry deposition techniques. Dry deposition processes, such as cold spray, the aerosol deposition method (ADM), and nanoparticle deposition systems (NPDS), are known to have high deposition rates, and to proceed at low temperatures [15–17]. Cold spray accelerates micro-sized particles using highpressure gas without any particle melting before impact. The process is used mainly for metallic powders and they achieve supersonic speeds prior to being deposited on a substrate and can deposit cermets and metal matrix composites through a converging and diverging nozzle [17]. ADM, which uses an aerosol by mixing mainly nano-sized ceramic particles with a carrier gas, is a room-temperature process for ceramic powder deposition. The aerosol, with the particles, is accelerated by a carrier gas through a converging nozzle, and the kinetic energy of the particles is used for bonding during the impact [18]. The main
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advantage of these processes is that they can be carried out at low temperatures, so the substrate is not subjected to thermal damage. NPDS is a recently developed dry deposition method that can deposit both nano to sub-micro-sized metal and ceramic particles, rather than only micro-sized particles, which are accelerated at supersonic velocity through a converging–diverging nozzle using compressed air as the carrier gas. The much greater velocity of the powders in NPDS is the major difference from ADM. Moreover, powder deposition occurs at room temperature, considerably lower than the conditions commonly used for cold spray [17,19]. Furthermore, cold spray uses a high-pressure gas, ADM uses medium vacuum, and NPDS uses low vacuum and a low pressure gas. Therefore, NPDS can be considered as a facile and low cost coating process, comparably. With the technique, various metals and ceramic materials, such as Cu, Ni, TiO2, and Al2O3, have been used to form coatings on metals, ceramics, and polymer substrates [17,19–23]. Previously, NPDS was used to deposit single-component materials, and the deposition mechanism with a single-component system has been evaluated by means of experimental and numerical analyses. Results suggested that Al2O3 particles fragment when they impinge onto a substrate with sufficient energy for bonding, resulting in a phase change [12]. Akedo et al. reported the deposition of α-Al2O3 particles with diameters in the range 80–100 nm and PZT particles on silica glass and polycarbonate substrates using ADM. They found that the particles fragmented and deformed plastically when they impinged on the substrate, forming a dense polycrystalline film [18]. However, the model for NPDS and ADM did not clearly explain the deposition mechanism, because the effects of the substrate and particle size were not considered. In this study, the deposition mechanism of NiO was investigated experimentally with various particle sizes and substrates, and the microstructure of the deposited layer was investigated. Here, nano- and micro-sized particles were deposited on Si wafers, Ni-coated Si wafers, and fluorine-doped tin oxide (FTO)-coated glass substrates. The different substrates were selected to vary the relative hardness of the substrate and that of the impinging particles. Moreover, these substrates are widely used in the fabrication of semiconductor and energy-storage devices. We investigated the microstructure of the deposited layers using scanning electron microscopy (SEM), and the microstructure of the NiO particles was observed before and after deposition. The phase behavior of the particles following deposition was investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). 2. Experimental The nano-sized (8–20 nm) and 100-nm-sized NiO were from Alfa Aesar, USA. The sub-micro-sized (200–300 nm) and micro-sized (1– 2 μm) NiO particles were from Tasco, Korea. The substrates used were FTO-coated glass, Si wafers, and Ni-coated Si wafers. For FTO-coated glass, the thickness of FTO on the glass was ~ 500 nm and the Nicoated Si wafer had a Ni thickness of 600 nm. To study the surface characteristics of substrates, contact angle was measured using a dynamic contact angle measurement system (DCA-315, CHAN Instruments Inc., USA). The liquid used for contact angle measurements was distilled water. During the deposition process, the substrate was translated along the x-axis at a speed of 50 μm/s for 60 s. The gas flow rate was 17.5 L/min through a slit nozzle with an outlet cross-section of 0.3 mm × 10 mm. The jet pressure was 3 bar, and the chamber pressure was 0.1 bar. The stand-off distance (SoD) (i.e., the separation between the nozzle and the substrate) was fixed at 4 mm. A schematic diagram of the NPDS is shown in Fig. 1. It consisted of an air compressor for the carrier gas, a powder cartridge, a nozzle, a vacuum chamber, a vacuum pump, and x-, y-, and z-axis translation stages. Pressurized air was supplied via a compressor, and the particles were aerosolized using a fluidized bed powder feeder (3400A, TSI, USA). It is straightforward to apply other materials using a powder cartridge, and the material can be
changed in the fluidized bed powder feeder. The particles were sprayed into the vacuum chamber at supersonic speeds at room temperature, so that they impinged on the substrate. The thickness of the samples was measured using a profiler (ET200, Kosaka, Japan), and the surface morphology and microstructure were characterized using SEM (Mira 3, Tescan, Czech Republic). XRD (Rigaku, TTR II, Japan) with a Cu-Kα source was used to investigate the phase behavior of the NiO before and after deposition over an angle range of 30– 90° using thin film mode. Thin foil samples were fabricated for TEM analysis using focused ion beam (FIB) milling (Lyra, Tescan, Czech Republic), and the microstructure was observed to determine the relationship between the deposited NiO and the substrate following deposition using a TEM (200 kV, 2100F, Jeol, Japan). The hardness values of the Si wafer, the Ni-coated Si wafer, the FTO-coated glass, and the bulk NiO were measured using a micro Vickers hardness tester (402MVD, Wilson-Wolpert, USA) with a force of 100 gf. And, the hardness of coating layer with various particle size was measures using a nano indentation system (Nano indenter XP, MTS, USA) with a force of 10 mN. The apparent densities and response angles of the powders were measured using a Hall flow meter (FLODEX, Acupowder International, USA) according to ISO standard 3923. To calculate the apparent density of the powders, 50 cm3 of the powders were flowed and collected using a Hall flow meter. The mass and volume of the collected powder was measured to calculate the apparent density. The response angle was determined from the tilt angle of the collected powders. Prior to deposition, the microstructure and particle distribution of the various sizes of NiO particles were observed using SEM (Fig. 2). The diameter of the nano-sized NiO particles was ~ 8–20 nm, forming agglomerates up to several microns in diameter (Fig. 2(a)). Moreover, 100-nm-sized NiO powder showed spherical shapes, with sizes ranging from 80 to 120 nm without agglomerates (Fig. 2(b)). The sub-microsized NiO particles were approximately 200–300 nm, with angularshaped particles (Fig. 2(c)). Finally, the micro-sized NiO particles were angular, with diameters ranging from 500 nm to 1.2 μm (Fig. 2(d)). 3. Results and discussion 3.1. Thickness and roughness of the deposited film As shown in Fig. 3, the thickness and roughness of the deposited layer with the three substrates varied considerably, depending on the particle size, which is expressed on a log scale because the particle sizes and deposited thicknesses varied by several orders of magnitude. With the Si substrate (Fig. 3(a)) and Ni-coated Si wafer (Fig. 3(b)), thickness and roughness decreased as the particle size increased. However, the micro-sized and sub-micro-sized NiO particles were not readily deposited on the FTO-coated glass substrate (Fig. 3(c)), resulting in essentially no deposited layer. Reasons for the failure in deposition are discussed in the next section. These findings indicate that particles smaller than 100 nm were deposited on all of the substrates, resulting in layers of similar thickness and roughness. Thus, the size of the NiO particles and the type of substrate are important for deposition. To assess the mechanism of deposition, the contact angle and hardness of the various substrates was measured. To characterize the surface property of substrates, contact angle was measured. The contact angle of FTO glass was measured to be 73.17° which was comparably high. The contact angles of Ni-coated Si wafer and Si wafer were measured to be 69.49° and 46.48°, respectively. Contact angle results indicate that FTO glass and Ni-coated Si wafer has similar surface property, its deposition behavior using sub-micro and micro-sized particles between those two substrates, shown in Fig. 3, were quite different. Fig. 3(b) and (c) show that nano-sized NiO particles were deposited over 3 μm on both FTO glass and Ni-coated Si wafer. However, sub-micro-sized and micro-sized NiO particles were not deposited on the FTO glass substrate (Fig. 3(c)) while they were deposited on Ni-coated Si wafer. Thus, these contact
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Fig. 1. Schematic diagram of the NPDS.
angle results seemed to show no correlation between surface property and deposition trend. To understand the effect of the hardness of substrates, the hardness of substrates were measured. The hardness result (Fig. 4) shows, the Si (10.850 GPa) and Ni-coated Si wafers (8.386 GPa) had higher hardness
values than that of bulk NiO (7.724 GPa). However, FTO glass (6.633) had a lower hardness than NiO. This hardness difference may account for the failed deposition of micro-sized and sub-micro NiO particles on the FTO glass. G. Bae et al. reported the deposition behavior depending on the different physical and mechanical properties between particle
Fig. 2. SEM images and particle distribution of the (a) nano-sized, (b) 100-nm-sized, (c) sub-micro-sized and (d) micro-sized NiO particles before deposition.
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Fig. 4. Hardness of the Si wafer, Ni-coated Si wafer, FTO-coated glass, and NiO.
FTO glass, the micro-sized particles would be likely to rebound, resulting in no deposition. Trompetter et al. reported the effects of substrate hardness during high-velocity thermal spray coating with NiCr alloy powders. When NiCr powders were bombarded on a Si substrate that was harder than the NiCr, the particles deformed during deposition [25]. Therefore, because the hardness of NiO is lower than that of the Si and Ni-coated Si substrates, we predict that those particles deformed during deposition, regardless of particle size. To further investigate the deposition mechanism, microstructural analyses using SEM, TEM, and XRD were performed, as described in the next section. 3.2. Microstructural characterization
Fig. 3. Thickness and roughness of the deposited layer comprising various particle sizes on the (a) Si wafer, (b) Ni-coated Si wafer, and (c) FTO-coated glass.
and substrate in the dissimilar cases. Especially, for the case of hard substrate–soft particle combination, the initial kinetic energy of the particle was mostly dissipated into plastic deformation, flattening the particle with a very slight deformed substrate. Thus, this report shows good agreement with our deposition results as shown in Fig. 11. Moreover, when solid particles impinge on a surface at high velocity, part of the kinetic energy is dissipated in the deformation process, resulting in deposition, and part is converted elastically to the kinetic energy of rebound. If this rebound energy exceeds the adhesion energy, the particles rebound, resulting in no deposition. Such particle rebound is most likely to occur when the particles are larger and harder than the substrate [24]. Thus, because the hardness of NiO was greater than that of the
As shown in Fig. 5(a) and (b), nano-sized and 100-nm-sized NiO particles seemed to preserve their shape during deposition, in comparison with Fig. 2(a) and (b). This indicates that no fracturing or deformation was involved; however, breaking up of agglomerates might have occurred. Moreover, to achieve the nano-sized thin film using nano-sized NiO powder, scan rate was controlled from 50 μm/s to 10,000 μm/s. The nano-sized coating layer was obtained with its minimum thickness of 260 nm at its scan rate of 10,000 μm/s. However, the microstructure of this thin film sample was loosely compacted without deformation or fracture of nano-sized particles themselves, regardless of its scan rate. However, Fig. 5(c) and (d) show the microstructure of a film formed using sub-micro-sized and micro-sized NiO particles on a Si wafer. The micro-sized and sub-micro-sized NiO particles seem to have deformed or fractured considerably, showing no porosity. The microstructure of the particles prior to deposition was significantly different from that of the deposited film (see Fig. 2(c) and (d)). To confirm these observations, the phase change of NiO particles before and after deposition was assessed using XRD. Fig. 6(a) shows the phase of nano-sized NiO particles before and after deposition on a Si wafer. Si peaks were detected with high intensity at the ranges between 51.5 and 55.5° for all the samples corresponding to JCPDS # 01-072-1426 as shown in Fig. 6. Especially, since submicro-sized and micro-sized powders formed very thin coating layer, compared to the other samples, Si peaks were detected with very high intensity, overlapping those two peaks with each other, as shown in Fig. 6(c) and (d). Moreover, as shown in Fig. 6(a), the peak position of the deposited film was almost identical to that of the nano-sized NiO particles (JCPDS # 01-078-4374), with no broadening of the peaks. The nanosized NiO particles were not deformed during deposition, so that the crystallinity of the NiO film was preserved after deposition. As shown in Fig. 6(b), the 100-nm-sized NiO particles were amorphous before and after deposition, with a slight decrease in peak intensity after deposition (JCPDS # 01-074-6700). Fig. 6(c) and (d) show XRD spectra
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Fig. 5. SEM images of deposited film formed using the (a) nano-sized, (b) 100-nm-sized, (c) sub-micro-sized, and (d) micro-sized NiO powders on a Si wafer.
(JCPDS # 01-071-6719) of the micro-sized NiO particles before and after deposition on a Si wafer. The peak intensities of the film using the micro-sized and sub-micro-sized NiO particles differed considerably before and after deposition, showing significant broadening of the peaks with relatively low intensities. Thus, the micro-sized and sub-microsized NiO particles deformed and fractured during deposition, losing crystallinity, with decreases in peak intensity (Fig. 6(c) and (d)).
Through XRD analysis, the fracturing and deformation of particles were observed for the deposition using micro-sized and sub-microsized NiO particles, whereas no change was observed using 100-nmand nano-sized particles. Thus, these XRD results closely match the microstructural changes shown in Fig. 5; i.e., deposition using nano-sized and 100-nm-sized particles showed preservation of the original particle size and shape, and deposition using the micro-sized NiO particles
Fig. 6. XRD spectra of the NiO film before and after deposition on a Si wafer of (a) nano-sized, (b) 100-nm-sized, (c) sub-micro-sized, and (d) micro-sized NiO powders.
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resulted in deformed or fractured particles, producing a more compact layer. Fig. 7 shows the microstructure of the film deposited on the Nicoated Si wafer using various sizes of NiO particles. Nano-sized and 100-nm-sized NiO particles seemed to show preservation of particle shape, with the breaking up of agglomerates and formation of a less compact layer during deposition, as for the Si wafer. In contrast, the sub-micro-sized and micro-sized NiO particles deposited on the Nicoated Si wafer seemed to have deformed or fractured considerably, having a more compact microstructure with no porosity, similar to the Si wafer. Thus, the deposition behavior of the various sized NiO particles was very similar for the Si and Ni-coated Si wafers. Finally, Fig. 8 shows the microstructure of the film deposited on FTO glass. The microstructure of the film on FTO-coated glass using nano-sized and 100-nmsized NiO particles seemed to have preserved the pre-deposition particle shape, with the breaking up of agglomerates (see Fig. 2). However, micro-sized particles were not deposited on the FTO-coated glass, probably for the reason discussed in Section 3.1. Since those micro-sized particles are larger and harder than the substrates, they are likely to rebound, instead of fracturing or deforming on the substrate. Based on these observations, there seems to be a clear difference in deposition mechanism depending on particle size. NiO particles greater than 100 nm seem to deform or fracture to form a compact film with no porosity, whereas deposition using nano-sized particles seemed to have resulted in a less compact layer, by breaking up agglomerates, but the pre-deposited particle shape is largely preserved. As mentioned above, this behavior is strongly related to the relative hardness of the particles and substrates. Moreover, the hardness using nano indentation method, was measured to study the relationship between different particle size and the mechanical property of the coating layer. The hardness of the coating layer deposited using nano-sized and 100-nm-sized NiO powder, was measured to be 5 and 12 MPa respectively. The results indicated that
the hardness measured was quite low due to its coating layer deposited by breaking the agglomerated particles, resulting in a loosely compacted (porous) microstructure. This result is consistent with SEM images as shown in Figs. 5(a) and (b), 7(a) and (b), and 8(a) and (b). Contrary to the results of nano- and 100-nm-sized particles, the hardness values of coating layer formed by sub-micro-sized and micro-sized particles, were measured to be 3.88 and 9.31 GPa respectively, These values are higher than that of the coating layer formed by nano and 100-nm-sized particles, due to its densely compacted coating layer formed by deformation and fracturing (see Figs. 5(c) and (d), and 7(c) and (d)). In addition to comparing hardness values, deposition behaviors among NiO particles of different sizes may be explained in terms of friction among particles. That is, if there is friction between particles during deposition, the particles do not pack readily. This friction results in a force that opposes the relative motion of particles, inhibiting the particles from sliding against each other. Surface area, surface roughness, and chemical properties of the particles are significant factors for friction. Therefore, increasing the specific surface area can increase frictional forces and particles with a large specific surface area are expected to exhibit low packing density, as well as low mobility [26]. The relationship between the surface area and friction can be an important factor in dry deposition. To investigate the relationship, the response angle and apparent density of NiO particles were characterized (Table 1). Generally, a large response angle and low apparent or relative density correspond to high friction and low mobility of the particles [26]. The response angles of the nano-sized and 100-nm-sized NiO particles were measured at 43.5° and 42°, respectively, while those of the sub-micro-sized and micro-sized NiO particles were 41.5° and 40°, respectively. The apparent density of the nano-sized NiO particles was 0.704 g/cm3, whereas that of the micro-sized NiO particles was calculated to be 1.515 g/cm3. Table 1 lists these data, together with the apparent density normalized to the density of NiO (6.67 g/cm3, [27]), labeled as ‘relative density’ in the last column. The micro-sized NiO particles had
Fig. 7. SEM images of the films deposited on a Ni-coated Si wafer using (a) nano-sized, (b) 100-nm-sized, (c) sub-micro-sized, and (d) micro-sized NiO particles.
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Fig. 8. SEM images of the films deposited on FTO-coated glass using (a) nano-sized and (b) 100-nm-sized NiO particles.
lower response angles and higher apparent densities than nano-sized NiO particles, which explains the deformation and fracturing due to low friction and high mobility. However, NiO particles with a size less than 100 nm are expected to have high friction and low mobility due to the response angle and relative density. Thus, the breaking up of agglomerated particles, followed by rearrangement and repacking, may explain the formation of a loosely compacted layer, as shown in Figs. 4, 5, and 6. 3.3. TEM analyses To evaluate the deposition mechanism of the various-sized NiO particles on different substrates, TEM was used to observe the interface between the coating layer and the substrates (Fig. 9). Finally, a deposition mechanism with schematics is proposed, as shown in Figs. 9–11. As shown in Fig. 9(a), nano-sized NiO particles formed a uniform coating layer, clearly distinct from the Si substrate. It can be seen that the crystal structure of the nanoparticles was preserved, without significant deformation or fracturing. Moreover, no defects, such as dislocations or stacking faults, were evident within the Si substrate close to the interface. Thus, the deposition of nano-sized NiO particle on a Si substrate seemed to take place with minimal damage. Moreover, Fig. 9(b) and (c) show TEM images of a NiO layer formed on a Ni-coated Si wafer and FTO glass. Much like the case for nano-sized NiO deposition on the Si wafer, the crystal structure of the nanoparticles and its interface seemed to be preserved, showing no apparent deformation or fracturing on the Ni-sputtered substrate or the FTO glass. Therefore, the deposition mechanism of nano-sized NiO particles, involving the breaking up of agglomerates, seemed to be similar on all substrates (Fig. 9(d)). Before deposition, the nano-sized NiO particles formed agglomerates; however, during collisions, the breaking up of these agglomerates, instead of deformation or fracturing, dissipated the excess kinetic energy. Thus, the resulting film showed a loosely compacted microstructure with some porosity. This result is consistent with the XRD analysis, the response angle and apparent density analysis, and SEM images, as shown in Figs. 5(a) to 7(a), Table 1, and Fig. 8(a), respectively. Fig. 10 shows the microstructure of the deposited NiO films on various substrates using 100-nm-sized NiO particles. For the Si wafer, it can be see that the interface is less sharp than that in Fig. 9(a), indicating some deformation of NiO particles into the Si wafer during deposition. Table 1 Response angle and apparent density of NiO powders of various particle sizes.
Nano-sized NiO 100-nm-sized NiO Sub-micro-sized NiO Micro-sized-NiO
Response angle
Apparent density (g/cm3)
Relative density (apparent density/density of bulk NiO)
43.5° 42° 41.5° 40°
0.704 0.218 1.178 1.515
10.55% 3.27% 17.66% 22.71%
Moreover, the inset to Fig. 10(a) shows that some NiO particles at the interface have changed from the original spherical to an oval shape, providing further evidence of deformation. Furthermore, as distance from the interface increased, 100-nm-sized NiO particles formed a film with a loosely compacted structure, preserving the particle shape and size. Because NiO particles are larger than nano-sized NiO particles, these particles bombard the substrate with higher kinetic energy, which may explain the slightly deformed versus the sharp interface shown in Fig. 9(a). Because the 100-nm-sized NiO particles have a higher response angle than those of the sub-micro- and micro-sized NiO particles (Table 1), they would be expected to have higher friction and lower mobility, and so a lower kinetic energy. Thus, the film using 100-nm-sized NiO particles shows the appearance of a loosely compacted layer without deformation or fracturing of particles. In summary, the deposition of the 100-nm-sized NiO particles on the Si wafer substrate occurred in a two-step process. When the NiO particles bombard the Si wafer at the interface, slight deformation or fracturing at the surface of the Si wafer is observed during deposition, due to the particle size. Then, subsequent particles are deposited in a loosely compacted manner with some porosity in the coating layer due to the decreased kinetic energy. Moreover, Fig. 10(b) and (c) show TEM images of a film deposited using 100-nm-sized NiO particles on the Ni-coated Si wafer and FTO glass, respectively. Similar to the case for the Si wafer, a two-step process for NiO film deposition was observed for both substrates. That is, slight deformation was observed at the surface of the substrates, because the interface is less sharp, but coating with subsequent layers resulted in a loose, compact layer including some porosity, while largely maintaining the original shape of the NiO particles. Therefore, the mechanism of deposition of the NiO film on the Ni-coated Si wafer and the FTO glass is similar to that on the Si wafer. Fig. 10(d) shows a schematic of the deposition mechanism using 100-nm-sized NiO particles. The schematic clearly shows deformed particles at the interface, followed by loosed packed layers with porosity, illustrating the two-step process. Fig. 10(a), (b), and (c) indicate that 100-nm-sized NiO particles tended to deposit similarly on the three substrates. This result is also consistent with the response angle analysis shown in Table 1, and the SEM images in Figs. 5(b) to 7(b), and 8(b), with decreasing kinetic energy due to lower mobility and higher friction, and loosely packed layers on the top surface. Fig. 11 shows TEM images of the deposition of sub-micro-sized and micro-sized NiO particles on Si and Ni-coated Si wafers. A NiO film with a thickness of 20–50 nm was formed regardless of the substrate. This thickness is less than those of the films obtained by deposition of nano-sized and 100-nm-sized NiO particles (Fig. 3). Moreover, in contrast to the films deposited using nano- and 100-nm-sized NiO particles, Fig. 11(a) and (d) show deposition of a dense coating layer with no apparent porosity. The density of the films was such that the interface between the substrate and film was unclear, but smooth. Furthermore, the insets to Fig. 11(a) and (d) show internal defects at the interface, as visualized using a two-beam TEM configuration. Dislocations with g (111)
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Fig. 9. TEM images of NiO film deposited on (a) Si wafer (inset: high-magnification TEM image close to the interface), (b) Ni-coated Si and (c) FTO-coated glass, and (d) schematic of the deposition mechanism of nano-sized NiO particles showing breaking up of agglomerates.
Fig. 10. TEM images of NiO film deposited on (a) Si wafer (inset: high-magnification TEM image close to the interface), (b) Ni-coated Si wafer and (c) FTO-coated glass, and (d) a schematic of the deposition mechanism, using 100-nm-sized NiO particles.
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Fig. 11. TEM images of NiO films deposited on (a) Si wafers (inset—dislocations with g (111) right at the interface) and (b) Ni-coated Si wafer, and (c) schematic of the mechanism of deposition of sub-micro-sized NiO particles. TEM images of NiO films deposited on (d) Si wafers (inset—dislocations with g (111) at the interface) and (e) Ni-coated Si wafer (inset—high-angle annular dark-field image at the interface), and (f) schematic of the mechanism of deposition of micro-sized NiO particles.
were observed at the interface between the NiO film and the Si wafer for both the sub-micro-sized and micro-sized NiO particles. These dislocations suggest damage between the Si wafer and the NiO film. It seems that those films were deposited with deformation and fracturing at the interface. Subsequently, a dense coating layer was formed by consolidation of fractured particles. The formation of a dense film by deformation and fracturing may be due to the sub-micro-sized and microsized NiO particles having less response angle than the nano- and 100-nm-sized NiO particles (Table 1), as well as higher kinetic energy due to their size. Moreover, sub-micro- and micro-sized particles have excess kinetic energy for deformation and fracturing, together with low friction and high mobility. Similar to the case for the Si wafer, Fig. 11(b) shows that sub-microsized NiO particles were fractured and deformed severely on the Nicoated Si wafer during deposition, so that valleys formed at the interface, and the interface was unclear. Moreover, the hardness of the Nicoated Si wafer was lower than that of the Si wafer, so that the damage to the substrate was more severe than that on the Si wafer as it mentioned before. Because the interface between the NiO film and the Nicoated Si wafer surface was difficult to distinguish, a large-angle annular dark-field image was used to clearly differentiate the Ni layer from the NiO coating layer (see the inset to Fig. 11(e)). The micro-sized NiO particles formed a densely compacted coating layer due to deformation and fracturing, resulting in a severely deformed Ni coating layer. Finally, the deposition mechanism of the sub-micro-sized and micro-sized NiO particles is described in schematics (Fig. 11(c) and (f)), showing that the particles were fractured and deformed on the substrates during deposition. This result is consistent with the response angle analysis (Table 1), SEM images in Fig. 5(c) and (d), and Fig. 7(c) and (d), with excess kinetic energy due to the high mobility and low friction, showing densely formed coating layers on the top surfaces. Based on the TEM analysis, micro- and sub-micro-sized NiO particle are deposited mainly by deformation or fracturing, followed by
consolidation. In contrast, nano-sized NiO particles seem to be deposited by the breaking up of agglomerates, resulting in formation of a loosely compacted layer. The deposition of the 100-nm-sized NiO particles seems to involve both types of deposition, with deformation and fracturing at the interface followed by formation of a loosely compacted layer. Initially, particles are fractured and deformed, forming a thin layer on the surface. Thereafter, particles are deposited loosely on top of this thin layer to produce a degree of porosity, preserving particle crystallinity. Therefore, NiO particles with a size of ~ 100 nm seem to be at the boundary between two dominant deposition mechanisms; i.e., the breaking up of agglomerates, forming a loosely compacted layer; versus deformation and fracturing, forming a thin, dense layer. In summary, in dry deposition the size of the particles and the type of substrate are key factors determining the deposition mechanism. In order to understand the velocity of particles which influence the deposition, Chun's work [28] was used to show the CFD (computational Fluid Dynamics) analysis results about changing the impact velocity of particles with varying its particle size and concerning the density of the particle. Based on the analysis results, the maximum impact velocity of sub-micro-sized and micro-sized particles was calculated to be 425 m/s (1.2 Mach) and 445 m/s (1.3 Mach), respectively. However, it was reported that smaller size of particles, such as nano- and 100-nmsize, show the highest velocity during flow due to its small size of particles, but the maximum impact velocity extremely decreased under 300 m/s at the moment of impact on the substrate due to strong bow shock near the substrate. Thus, it is closely related to the results that micro-sized particles which have the highest impact velocity, can nicely deposited by deformation and fracturing of particles, forming the densely compact coating layer. Contrary to that of large sized particles, particles under 100-nm-size, were deposited by breaking up of agglomerates and formed the loosely compact coating layer due to its low impact velocity. Therefore, the particle size and substrates are important factors for effective dry deposition.
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4. Conclusions We investigated experimentally the deposition mechanism of NiO particles of different sizes on various substrates using NPDS. NiO films formed loosely compacted coating layers when NiO particles less than 100 nm in diameter was used, regardless of the substrate. In contrast, when NiO particles greater than 100 nm in size were deposited, a dense and compact coating layer was formed on substrates harder than NiO (i.e., Si and Ni-coated Si), but not on FTO-coated glass. For nano- and 100-nm-sized NiO, the crystal structure of the NiO particles was preserved within the loosely compacted structure, and the particles did not fracture or deform significantly. Use of NiO particles of less than 100 nm diameter produced coating layers with no damage to the substrate. This is attributable to the formation of agglomerates, which broke apart during the collision with the surface, as well as the absorption of excess kinetic energy. In contrast, deposition of submicro- and micro-sized NiO particles resulted in formation of compact and dense layers with a thickness of 20–50 nm, mainly by deformation and fracturing of the particles. Moreover, deposition of NiO particles of N100-nm diameter resulted in significant deformation or fracturing of particles in the coating layer, so that numerous dislocation defects at the interface as well as deformation of the Ni coating layer were observed. The excess kinetic energy associated with deposition of microsized NiO particles was dissipated by deformation of the substrate. Moreover, not only the effect of substrate hardness but also differences in deposited particle size, shape, and condition explain why nanometersized particles formed a film on the FTO-coated glass but micro-sized particles did not. Thus, 100-nm-sized NiO particles are at or near the boundary of a shift in the deposition mechanism. The effects of particle size and substrate on dry deposition were successfully demonstrated experimentally using various analytical tools. Acknowledgments This work was supported by the Energy Efficiency of Resources Core Technology Program (No. 20142020103730) and Human Resources Development Program (No. 20154030200680) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (NRF-2013R1A1A2074605). We thank Mr. Joon-Pil Choi for the useful discussions. References [1] A. Surca, B. Orel, B. Pihlar, Characterisation of redox states of Ni(La)-hydroxide films prepared via the sol–gel route by ex situ IR spectroscopy, J. Solid State Electron. 2 (1998) 38–49. [2] H.L. Chen, Y.M. Lu, W.S. Hwang, Characterization of sputtered NiO thin films, Surf. Coat. Technol. 198 (1–3) (2005) 138–142. [3] E. Fujii, A. Tomozawa, H. Torii, R. Takayama, Preferred orientations of NiO films prepared by plasma-enhanced metalorganic chemical vapor deposition, Jpn. J. Appl. Phys. 35 (1996) L328–L330.
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