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SnO hybrid graphene for thermal interface material and interconnections with Sn hybrid carbon nanotubes
Materials Science & Engineering B 253 (2020) 114485
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Sn/SnO hybrid graphene for thermal interface material and interconnections with Sn hybrid carbon nanotubes
T
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Jagjiwan Mittala,b, , Kwang Lung Lina a b
Department of Material Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China Amity Institute of Nanotechnology, Amity University Uttar Pradesh, Noida Sector 125, Uttar Pradesh 201301, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Few layer graphene Hybrid few layer graphene Sn/SnO Thermal conductivity Tuna current Interconnects
Few layer graphene (FLG) was successfully coated with Sn/SnO after reacting with SnCl2 and reducing under H2/N2 gas. X ray diffraction and X ray photoelectron spectroscopy showed the formation of Sn/SnO on the surface. The thermal diffusivity and conductivity of ~1 mm thick pallet of Sn/SnO coated hybrid FLG was surged to 7.91 mm2/s, 14.41 W/m.K from 2.17 mm2/s and 3.27 W/m.K of the pristine FLG, respectively. The increase in thermal properties was attributed to the formation of compact structures in the film and reducing the air gaps by joining hybrid graphene due to Sn. Current-voltage measurements using tuna probe in atomic force microscopy demonstrated the higher number of negative charge carriers in the hybrid FLG compared to pristine FLG because of electron transfer from graphene to Sn. Study using transmission electron microscopy showed the development of interconnection using Sn/SnO hybrid graphene with Sn coated and filled multiwalled carbon nanotubes.
1. Introduction Reducing chip size and increasing performance are among the most important research aspects for today’s electronics industries. Besides high thermal conductivity, the required material for small sized chip should have good electrical properties. Large amount of heat is generated in this process because of high circuit density. The life span and reliability of the gadgets also depend on the dissipation of heat [1]. Thermal interface material (TIM) provides a conductive path and improves heat flow across the interface [2]. Existing TIMs are found to be not effective for heat dissipation [3]. Poor or very high anisotropy in the thermal conductivity is a problem [4–10]. Therefore, search is going on for improving the thermal property of TIM. Thermal conductivity of single layer graphene (SLG) at room temperature is ~5000 W/m.K [11,12] which is much higher than the thermal conductivity of bulk graphite ≈500 W/m.K [13]. High thermal conductivity of graphene [11], due to large phonon mean free path in strong carbon sp2 bond network, renders graphene possible application for thermal management in future ULSI circuits. [12]. Few layer graphene (FLG) is preferred for application because of its lower cost. However, thermal conductivity ~1600 W/m.K of FLG is lower than SLG. This is due to the cross-plane coupling of the low-energy phonons and corresponding changes in the phonon Umklapp scattering [14]. Graphene layers are easy to attach to heat sinks, which can solve the
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thermal contact resistance problems, Further, flat plane geometry of graphene is also suitable for integration with Si CMOS circuits for thermal management. However, this high thermal conductivity value is reduced drastically when used in bulk. It has been observed in the current study that the average thermal conductivity of graphene with the film thickness from 1 mm is measured around 3.27 W/m K at room temperature. The reasons are very low thermal conductivity across the axis, existence of substantial defects and large gaps due to no interaction between the graphene. These problems restrict the applications of graphene as TIM. Therefore, graphene is studied as thermal interface material using formation composites with other materials like polymers and boron nitrides. Reducing thermal conductivity of low-k dielectrics and increasing current density demands from small dimension interconnects raise the reliability concerns for Cu based interconnects [15]. The rise in metal temperature and electromigration (EM) exponentially degrades interconnect lifetime [16]. There are also environmental and economic concerns for the current copper-based technology in electronics. Due to exceptional electronic and other properties carbon nanotubes and graphene are considered to be the best materials for very small dimension interconnects [17–19]. CNTs are less sensitive to electromigration [20], and possible alternative interconnect material because of their high current carrying capacity and thermal conductivity [21,22]. In our
Corresponding author at: Amity Insitute of Nanotechnology, Amity University Uttar Pradesh, Noida Sector 125, Uttar Pradesh 201301, India. E-mail address: [email protected] (J. Mittal).
https://doi.org/10.1016/j.mseb.2019.114485 Received 30 October 2018; Received in revised form 22 October 2019; Accepted 11 December 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 253 (2020) 114485
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Table 1 Bulk thermal properties of pristine and Sn/SnO coated graphene. Material
Bulk Density g/cm3
Cp J/g/K
Thermal diffusivity (α) mm2/s
Thermal conductivity (K) (W/m.K)
Few layer Graphene Sn/SnO- few Layer Graphene
1.130 2.165
1.334 0.840
2.171 7.916
3.277 14.416
measured on the pellet prepared after compressing the materials using 68 MPa pressure. Size of pellet was kept at 13.1 mm diameter and 1 mm thickness. Pallets were then heated at 200 °C in tube furnace for 1 h under 5% H2/95% N2 gas. Thereafter, pallets were cooled down to room temperature. The pellets of hybrid FLGs and pristine FLGs were analyzed by laser flash analysis (LFA) [32] for determining thermal conductivity using the equation (1)
previous study, we showed the formation of interconnects using the Sn hybrid single multiwalled carbon nanotube on the copper substrate [23]. However, Cu involved in this interconnect also leads to the formation of Cu6Sn5 inter-metallic compound during the formation of connection [23]. Besides high thermal conductivity, graphene also shows extremely [24] low electrical resistivity of 7.5 × 10−7 Ω m along the plane so it is highly suitable for interconnect applications. However, high resistance in the vertical direction to the plane and very low interactions among the graphene are big hindrance for their applications. One possibility for using the FLGs for both as thermal interface material and interconnect applications is to modify them by coating, doping, substitution or functionalisation [25–28]. Sn metal is used for soldering purpose because of its high thermal, electrical conductivity, and availability. Coating and filling of CNTs with Sn have been studied for the bulk thermal conductivity and interconnect applications [23,29] but few studies has been undertaken for Sn-graphene. In the present work, a new way is used the FLGs as thermal interface material and interconnect applications. FLGs was coated with Sn/SnO using a easy method and studied for bulk thermal conductivity and electrical properties. Study also reports a possibility of forming an interconnect between Sn coated and filled single carbon nanotubes and Sn coated graphene.
K = ρ × α × Cp
(1)
Here, K, ρ, α Cp are thermal conductivity, density, thermal diffusivity and specific heat of the samples, respectively. Specific heat and thermal diffusivity were provided by the instrument and density of materials was determined using the weight/volume ratio of the pallets. Experiments for thermal conductivity measurements were repeated three times and average values are presented in Table 1.
2.4. Electrical measurements The AFM can be operated in different modes depending on the sample properties that user wants to determine. The TUNA module senses the current passing through the sample to the AFM tip. This has been done in this study using the Peak Force Tunneling AFM (PF TUNA). Peak Force Tapping, the maximum force on the tip (peak force) is maintained at a constant value for each individual cycle. By maintaining a constant tip-sample force, topographic and current images are generated simultaneously. The peak current is the current detected when the AFM tip exerts maximum force on the sample surface (peak force) and TUNA current is the average current detected in one full tapping cycle (approach and retract). Local I-V spectra can be measured in the Peak Force TUNA. To extract the I-V curves, the current passing across the sample is plotted against the applied DC bias. Current-voltage (I-V) study on the pellets of pristine and hybrid FLGs was investigated using tuna probe in atomic force microscopy (AFM). Local I-V spectra is measured in the Peak Force TUNA by keeping tip in a fixed location and the sample bias is ramped up and down either once or continuously. The current passing across the sample is plotted against the applied DC bias. In this study, both PF TUNA imaging mode and I-V spectroscopy mode have been used to investigate the electrical properties of graphene sheets. It was reported in the earlier study [33] that the I-V studies using TUNA probe produces the same results as using two probe method.
2. Experimental 2.1. Development of hybrid few layer graphene The commercial few layer graphene (FLG), produced using chemical vapor deposition method, were used in the present study. The FLGs were heated in O2 at 250 °C for 15 min for surface oxidation. The oxidized FLGs were then stirred with SnCl2 solution in small amount of HCl at 70 °C for half an hour for coating of SnO2 on the surface of FLGs, similar to the earlier studies on carbon nanotubes [30,31]. After the reaction, hybrid FLGs-SnO2 material was separated from solution using centrifugation and dried. The hybrid sample was heated isothermally at 250 °C for 30 min in a furnace under 5% H2/N2 gas flow and thereafter cooled down to room temperature for formation of FLGs-Sn. 2.2. Characterization of hybrid few layer graphene Pristine FLG and hybrid FLGs were investigated using scanning electron microscope (SEM) under 10 keV beam energy and energy dispersive spectroscopy (EDS) using silicon drift detectors. The materials were also examined by high resolution transmission electron microscope (HRTEM) with 200 keV beam energy and < 0.19 nm point resolution. The HRTEM micrographs were further analyzed using Fast Fourier Transform (FFT) and inverse FFT (IFFT). Crystal structures of the samples were determined using electron diffraction (ED) with convergent angle of 1.5–2.0 mrad in HRTEM and X-ray diffraction (XRD) using the 1.5408A CuKα radiation. X-ray photoelectron spectroscopy (XPS) was used to identify different elements, their compositions and oxidation states in the pristine FLGs and hybrid FLGs. All peaks in the XPS spectra were standardized using the C 1s peak.
2.5. Interconnect applications with Sn coated nanotubes The commercial MWCNTs having outer diameter 110–170 nm was coated and filled with SnOx using SnCl2 solution, according to the procedure applied in earlier study [30,31]. The SnOx coated and filled MWCNT were sprayed, after dispersing and sonicating in the alcohol solution, on the Sn/SnOx hybrid graphene settled on the holy carbon grid. The grids containing the SnOx coated and filled nanotubes with Sn/SnO hybrid graphene were heated in a furnace to 250 °C at the rate of 10 °C/min and isothermally at 250 °C for one hour in 5%H2/95%N2 atmosphere and then cooled down to room temperature. Interconnections between hybrid graphene and hybrid MWCNTs were characterized using HRTEM.
2.3. Thermal properties measurements Bulk thermal properties of hybrid FLGs and pristine FLGs were 2
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structure of graphene (Fig. 2h). EDS area scan in Fig. 2d–f and respective weight and at.% of elements are displayed in table (Fig. 2c). At.% of elements in pristine graphene revealed the presence of mostly carbon 97.48% and some scattered oxygen 1.56%. This low amount of oxygen coupled with XRD study reveals the high purity of pristine FLGs. On the other hand, EDS area scan in Fig. 2j–m and at.% in attached table (Fig. 2i) established the presence of Sn, carbon and oxygen elements in the hybrid film. Carbon is also the main elements with 69.44 at.% in the hybrid graphene film. Sn with at.%17.40 is uniformly distributed in the graphene like carbon. The concentration of oxygen is much higher in hybrid graphene (13.15 at.%) than pristine graphene (1.56 at%). However, it is mostly scattered and may link with Sn as well as carbon. Higher concentrations of Sn than oxygen also suggest the presence of metallic Sn-Sn bonds in the hybrid graphene, but the presence of C-Sn bonds is ruled out because of low temperature synthesis of hybrid graphene [37]. 3.1.3. HRTEM study HRTEM micrograph in Fig. 3a displays a pristine few layer graphene on the grid. High magnification study in Fig. 3b showed 12 graphene layers with interplanner distance of 3.38 Å in the pristine graphene, as calculated by XRD. ED in Fig. 3c revealed the presence of (0 0 2) crystal plane of graphite among other crystal planes such as (1 0 0), (01¯1¯0), etc. of graphene, as described in previous studies [38]. The HRTEM micrograph in Fig. 3d demonstrates a hybrid graphene where coating on the surface appears darker than the graphene. Higher magnification study in Fig. 3e&f revealed the covering of entire sheet by coating of Sn/SnO. Interplanner distances in HRTEM micrograph revealed the presence of (1 1 0) crystal plane of SnO. The presence of the SnO in the hybrid film was also confirmed by an ED investigation (Fig. 3g) and Xray diffraction in Fig. 1. During the reduction the formation reduction of tin oxide into Sn/ SnO, graphene joins with each other at the edges, as shown by the HRTEM study in Fig. 3h, Details of this study is provided in the Section 3.4.3 using Fig. 10a–e.
Fig. 1. X-ray diffraction of (a) pristine few layer graphene and (b) Sn/SnO-few layer graphene.
3. Results and discussion 3.1. Characterization of pristine few layer graphene and Sn hybrid few layer graphene films 3.1.1. Crystal structure studies X-ray diffraction of pristine FLGs in Fig. 1 displays only two peaks. The major peak at 2θ = 26.2° denoted as (G0 0 2) is due to (0 0 2) of graphite with the interplanner distance of 3.38 Å. The minor peak centered at 2θ = 54.3° with an interplanner distance of 1.69 Å is attributed to (G0 0 4). These peaks indicate the existence of multilayer structure in graphene. The interplanner distances of (0 0 2) and (0 0 4) peaks are consistent with previous work [34]. Broadness of G0 0 2 peak shows the existence of small irregularity in distances between the planes. Higher interplanner distance than graphite (3.353 Å) suggest lower crystallization of FLGs. Contrary to pristine FLGs, XRD of hybrid FLGs in Fig. 1 exhibits various peaks. The major peak G002 with an interplanner distance of 3.38 Å is same as in pristine FLGs. This indicates that the crystal structure of graphene is undamaged during processing. Interestingly, peak is sharper and narrower in hybrid FLGs than pristine FLGs. Although the exact mechanism is not clear, but the heat treatment of the sample at 250 °C in H2/N2 may be responsible for this reduction in peak broadness. Beside the major peak, XRD also displays two small bands centered at 2θ = 33.3° and 2θ = 51.08° with interplanner distance of 2.68 Å and 1.786 Å, respectively. These bends are due to (1 1 0) and (1 1 2) of SnO [35,36] XRD of hybrid FLG also shows a small peak corresponding to pure Sn at 2θ = 32.9°. This implies that Sn is also present in the sample.
3.1.4. XPS study XPS survey scan spectra of the pristine FLGs in Fig. 4a establishes the presence of carbon and oxygen. Detailed analysis of carbon (C1s) XPS in Fig. 4b reveals three minor peaks at 284.4 eV, 285.2 and 285.6 binding energies. These peaks belong to C-C, C-OH and and C-O bonds in graphene, respectively. Presence of C-O and C-OH bonds are also shown by oxygen (O1s) XPS in Fig. 4c by the presence of C-O bonds are present in graphene as -CO or -COOH bonds. In contrast, XPS survey scan spectra of the hybrid FLGs shown in Fig. 5a reveals the presence of Sn, carbon and oxygen. Similar to pristine FLGs, analysis of C1s XPS in Fig. 5b demonstrates two oxidation states of carbon. The major peak at 284.4 eV is due to C-C bonds of the graphene [39] and minor peak at285.6 eV is due to C-O bonds [40,41]. Sn 3d XPS spectrum in Fig. 5c displays two oxidation states. In the band due to Sn 3d3/5, the major peak at 486.6 eV is attributed to the +2oxidation state of Sn as Sn2+ while minor peak at 484.7 eV indicates the presence of 0 oxidation state of Sn or Sn metal [42]. Similar results are obtained in the band due to Sn 3d2/5 where peaks at 495.2 and 493.1 binding energies are due to Sn2+ and Sn. This confirms the formation of SnO along with Sn at the surface of FLGs. Interestingly, no peak belongs to C-Sn bonds is observed either in the C1s or Sn3d XPS peak. This indicates that the carbon did not make any bond with Sn during the process. O1s XPS of hybrid FLGs in Fig. 5d shows the existence of two types of oxygen at 530.6 eV (with Sn) and 531.6 eV (with C) binding energies [43]. Above XPS results disclose the formation of FLG-Sn/SnOhybrid during the reaction in which the linkage between carbon (of FLGs) and Sn was through oxygen. For determining the exact composition of compound at surface and below the surface, depth profile study of hybrid FLGs film was studied
3.1.2. FESEM study FESEM micrograph in Fig. 2a&d demonstrates flat sheets in various sizes of pristine FLGs. Shapes and sizes of graphene sheets are mostly irregular. Some graphene sheets are larger in sizes than others. The folding at the edges of graphene sheets which is observed as sharp edges are quite visible at various places in the micrograph. Higher magnification study in Fig. 2b revealed mostly flat morphology of pristine FLGs sheets. However, clinks in the middle and folding at the edges of the flat sheets are noticeable. In comparison, hybrid FLGs displayed flat morphology having few sharp edges (Fig. 2g&j). The individual sheets of the hybrid graphene were observed as agglomerated whereas sheets of pristine graphene are observed as separated from each other. A high magnification study of the hybrid-graphene film revealed the flat 3
Materials Science & Engineering B 253 (2020) 114485
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Fig. 2. SEM micrographs of sintered pallets showing pristine few layer graphene in (a) ×1000 magnification (b) ×5000 high magnification (c) EDS results showing elemental composition in the pristine few layer graphene (d–f) SEM micrograph and corresponding EDX mapping demonstrating elemental analysis of C and O in pristine graphene. SEM micrographs of and Sn/coated few layer graphene in (g) ×1000 magnification (h) ×5000 magnification. (i) EDS results showing elemental composition in the Sn/SnO coated few layer graphene, (j-m) SEM micrograph and corresponding EDX mapping showing elemental analysis of C, Sn and O in Sn/SnO coated few layer graphene.
current flow (shown in bright yellow color) of 5.1 nA at most of the places. However, at some places the current flow is lower at 1.6 nA as evidenced by black color spots. This may be due to bigger height difference at a few places. Tuna current flow behavior in the pristine FLGs sample in Fig. 7d with same bias of 0.500 mV is similar to the peak current. However, the current flow is lower as evidenced with dark color. There are more black spots with lower current of zero at some places. On the other hand, surface topography of 10 µm × 10 µm hybrid FLG film in AFM micrograph [Fig. 8a] shows flattish structure. Threedimensional analysis in the Fig. 8b reveals the height difference of 117.2 nm between different places which is less than the pristine FLGs. This shows the better compression of the hybrid FLGs during film making and sintering. Peak current generated after the 0.500 mV bias in Fig. 8c shows the almost similar current flow (shown in bright yellow color) of 5.1 nA at most the places. At some places the current flow is lower at 1.6 nA, as evident by black color spots but these spots are not centered at one place as in case of pristine FLGs. Interestingly, TUNA current flow behavior in the sample with the AFM tip of 0.500 mV in Fig. 8d shows more current flow than the peak current. This shows higher number of charge carriers in the case of hybrid FLGs than pristine FLGs.
using XPS. Fig. 6a shows the depth profile of the three elements Sn, O and C below the surface of the hybrid film. As visualized in the Fig. 6a, oxygen amount is more on the surface than below the surface whereas the amount of carbon and Sn are more in the bulk. Since there is no CSn bonding, there is higher C-O and Sn-O bonding on the surface than the bulk. As shown in Fig. 6b, presence of 286.4 eV peaks in C1s XPS proves the C-O bonds on the surface. However, C-O bonds are either absent or in much smaller amount below the surface. This is further proven by the O1s XPS in Fig. 6c where peak belong to C-O at 531.4 is observed only at the surface with no indication below the surface. Analysis of Sn 3d peaks in the Fig. 6d indicates the presence of Sn-O (486.6–486.8 keV) and absence of Sn at the surface. However, Sn (485 keV) increases with respect to SnO in the inner layers.
3.2. Electrical properties using TUNA probe in AFM AFM micrograph in Fig. 7a shows the surface topography of pristine FLGs on dimensions 10 µm × 10 µm FLG film. Three-dimensional analysis represented in the Fig. 7b reveals the height difference of 195.3 nm between different places. Peak current generated after the 0.500 mV bias giving to AFM tip in Fig. 7c shows the almost similar 4
Materials Science & Engineering B 253 (2020) 114485
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Fig. 3. HRTEM micrographs showing (a) pristine few layer graphene in low and (b) high magnifications (c) electron diffraction (ED) of pristine graphene in (b) (d) Sn/SnO hybrid graphene in low and (e.f) high magnifications showing Sn and SnO planes (g) electron diffraction (ED) of Sn/SnO hybrid graphene in (e) (h) joining of two graphene.
Fig. 4. Survey scan spectra (a), C1s (b) and O1s (c) XPS spectrum of pristine few layer graphene.
increase in bias either given to sample or tip. This is a particular behavior of the graphene [44]. Hybrid FLGs on the other hand show the negative current flow of 600 nA when no bias is given to the probe or the substrate [Fig. 8e]. This shows that electron charge carriers are much higher in the hybrid FLGs which leads the flow of charge from sample to tip. This negative current flow is increased with the increase of the negative bias. It is increased to −600 nA when the bias to sample is increased to −250 mV. However, the decrease is slowed down and only −710 nA current is generated with the increase in the negative bias to −500 mV. During positive bias to the probe, negative current flows of −541 nA is
I-V curves using Tuna probe in AFM in Fig. 8e show the behavior of charge carriers on the surface of pallets of pristine FLGs and hybrid FLGs after the bias from −500 mV to 1000 mV. Pristine FLGs show no flow of current when there is no voltage bias to the probe. With negative bias up to 250 mV to the sample (−250 mV) flow of current is increased to −23 nA from sample to tip. Current is increased to −150 nA when bias to sample is increase to −500 mV bias. When 250 mV bias to given tip (+250 mV), the current flow is increased to 33 nA. This current is increased to 78 nA when the bias to tip is increased to 500 mV. This current is further increased to 310 nA with positive bias of 1000 mV. This shows increase in current with the 5
Materials Science & Engineering B 253 (2020) 114485
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Fig. 5. Survey scan spectra (a), C1s (b) Sn3d (c) O1s (d) XPS spectrum of Sn/SnO few layer graphene.
Fig. 6. XPS study of Sn/SnO-few layer graphene showing depth profile of (a) of C, Sn and O (b) C1s spectra (c) Sn3d spectra (d) O1s spectra.
Since in TUNA experiment set up, the length L and width W are not well defined, it is difficult to calculate the resistivity of the graphene sheet in this study. However, comparison of the resistances of the pristine FLG and Sn-FLGs using TUNA probe with same scanning area. Pristine FLG shows the resistance of 4.44 × 10−6 Ω whereas resistance observed by Sn-FLGs is 2.26 × 10−6 Ω. Higher current flow in the SnFLGs is also observed by brightness of yellow color during TUNA current analysis in Fig. 8d when compared with Fig. 7d. This means that
observed even when voltage bias of above +250 mV is given to the probe. When the positive bias to the tip is increased to 1000 mV, negative current flow is reduced to −1 nA. This shows that the higher number of charge carriers are present in the hybrid FLGs due to Sn/SnO coating on the graphene. As shown by the results in the X-ray and XPS the Sn/SnO are connected to graphene through oxygen. Therefore, d orbitals of the Sn accepts the electrons from graphene through oxygen and produce enough charge carriers. 6
Materials Science & Engineering B 253 (2020) 114485
J. Mittal and K.L. Lin
Fig. 7. AFM study of pristine few layer graphene showing (a & b) micrograph and its three-dimensional projection (c & d) peak current and Tuna current during Tuna probe study.
Fig. 8. AFM study of Sn/SnO few layer graphene showing (a & b) micrograph and its three-dimensional projection (c & d) peak current and Tuna current during Tuna probe study (e) Current voltage behavior of the pristine few layer graphene as well as Sn-few layer graphene. 7
Materials Science & Engineering B 253 (2020) 114485
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Table 2 Comparison of the thermal conductivity of various materials in literature and our material. S.N
Material
Thermal conductivity (TC)
Comments
Ref
1 2 3 4 5 6 7 8 9.
Flexible PMIA/hexagonal boron nitride composites Graphene/Silicon carbon nanorods PVA/Boron nitride nanosheets 30%Graphene-Perfluoroalkoxy Nanocomposite Porous boron nitride (BN)/epoxy composite Graphene/epoxy composite Sn coated and filled CNT Cellulose nanofibers/boron nanotube nanocomposites Sn/SnO coated graphene
0.94 W/m.K 10.9 W/m.K 1.9 W/m.K In plane 2.39 W/m.K through plan 25.57 W/m.K in-plane (5.19 W/m.K) out-of-plane (3.48 W/m.K) 5.1 W/m.K 5.1 W/m.K In plan 21.39 W/m.K 14.41 W/m.K
Poor TC
[5] [6] [4] [7] [8] [9] [29] [10]
Poor TC Highly anisotropic Anisotropic Low TC Low TC Anisotropic Current work
4SnCl2+2H2O→Sn4(OH)2Cl6+2HCl
(1)
Sn4(OH)6Cl2→4SnO+2HCl+2H2O
(2)
2SnCl2 (HCl)+2H2O+O2→2SnO2+4HCl
(3)
either the resistance is lowered or smoothen the surface by the addition of Sn. It may be due to less defects and/or higher conductivity. According to the 3-dimension analysis in Figs. 7b and 8b, surface is smoothened after the addition of Sn. 3.3. Thermal properties
FLGs
Table 1 lists the thermal properties of pristine as well as Sn/SnO coated FLGs after sintering. Density of pristine FLGs is low but magnifies in hybrid FLGs. As observed in the second column of Table 1, density of pristine FLGs is increased from 1.13 g/cm2 to 2.16 g/cm2. This high increase in density is due to the incorporation of heavy element Sn and oxygen via coating. On the other hand, specific heat (Cp) of hybrid nanotubes in the fourth column is reduced. It is reduced from 1.334 J/g/K to 0 0.840 J/g/K. This means that less heat is required for raising the temperature of hybrid FLGs. This is because of the addition of metal on the surface of graphene. Thermal diffusivity (α) is increased from 2.17 mm2/s to 7.91 mm2/s. The thermal conductivity estimated from equation (1) gives rise to 14.41 W/m.K of hybrid FLGs as compared to 3.277 W/m.K of pristine FLGs.
SnO/SnO2
Air, 250°C
→
Surface functionalized FLGs
H2 /N2, 250°C
→
Sn/SnO + 2H2 O
(4) (5)
As the reduction process is carried out at 250 °C which is above the melting point of the Sn (232 °C), Sn on the surface of FLGs melted and the hybrid FLGs in proximity join each other. 3.4.2. Thermal conductivity of few layer graphene and hybrid few layer graphene Contrary to metals, thermal conductivity of carbon materials is dominated by phonons [47]. Thermal transport in graphene is also carried by the intrinsic properties of the strong sp2 lattice, rather than by phonon scattering on boundaries or by disorder, giving rise to extremely high K values [48,49]. Thermal conductivity of few-layer graphene strongly depends on the number of atomic planes. The reduction of the ability of few-layer graphene to conduct heat is attributed to the crossover from 2D graphene to 3D graphite. It is assumed that decrease in thermal conductivity is mainly due to the shrinking of high frequency phonon induced by the cross-layer coupling. The phonon group velocity in graphene is higher than that in CNTs, which leads to larger thermal conductivity [50,51]. However, in the samples, the heat loss to inter air gaps among the graphene sheet reduces the thermal conductivity. The small thermal conductivity for bulk graphene samples may also result from low thermal conductivity in the perpendicular direction of graphene, high density of defects which efficiently scatter the flow of phonons [52]. Although thermal conductivity in metals is much higher in bulk but heat is conducted by the electrons. Therefore, pure metals cannot be used as thermal interface material because of leakage of current. Possible oxidation of metals is also another reason.
3.3.1. Comparison of the thermal conductivity of Sn/SnO hybrid graphene with other materials Table 2 compares the thermal conductivity of Sn/SnO hybrid graphene with other materials studied in the literature. As observed, only graphene-based material 30% Graphene-Perfluoroalkoxy nanocomposite and cellulose nanofibers/boron nanotube composites showed high thermal conductivity than current material. However, these values are highly anisotropic. 3.4. Mechanisms 3.4.1. Synthesis of hybrid FLGs It was shown in earlier studies [30] that during hydrolysis of SnCl2 in water Sn4(OH)2Cl6 is formed (reaction 1). In the subsequent reaction, SnO (reaction 2) is formed. But addition of small amount of HCl in SnCl2 solutions suppresses the hydrolysis of SnCl2 and the SnO2 is formed (reaction 3). A sketch of the process for the coating of FLGs is shown in Fig. 9. Due to highly inert surface, pristine FLGs do not disperse in the water solution of SnCl2. Therefore, surface of pristine FLGs cannot be coated with Sn or SnO on its surface without modification. In the present study, we oxidized the surface of pristine FLGs by heating in air at 250 °C (reaction (4)). Presence of O2 in air oxidizes the surface of pristine FLGs at high temperature [45,46] and creates functional groups such as carbonyl, alcoholic, carboxylic etc. Because of the generation of functional groups at the surface, oxidized FLGs are easily dispersed in SnCl2 solution, similar to earlier study for MWCNTs [30]. Subsequently, the graphene is coated with tin oxides on its surface. When tin oxide coated FLGs are heated under H2/N2 gas at 250 °C in the last step (reaction (5)), tin oxide coated on the surface are converted into Sn.
3.4.3. Joining of graphene with each other Coating of solder metals on the surface of FLGs helps in joining of FLGs with each other [Fig. 10a&b]. This is caused by the formation of Sn from SnO during reduction by H2/N2 treatment at the edges and subsequent joining of the graphene using Sn-Sn bonding, as observed in carbon nanotubes in our earlier study [30]. High magnification HRTEM study at the joint between graphene in Fig. 10d&e reveals the presence of Sn (2 0 0). This shows that Sn is responsible for the joining of the graphene. Joining of nanotubes reduces the inter air gaps between graphene. Combination of phonon and electron carriers in FLGs and Sn are responsible for the increase in the thermal conductivity. 3.4.4. Current voltage study of the FLGs and hybrid FLGs It has been shown that graphene has high electrical conductivity [53,54]. For FLGs, carrier mobility ≤10,000 cm2/Vs was found at 300 K [54] and electron can cover micrometer-long distances without scattering [55]. FLGs can sustain very high current densities in ambient 8
Materials Science & Engineering B 253 (2020) 114485
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Fig. 9. (a) Sketch showing the outline of Sketch showing the outline of synthesis of Sn/SnO-few layer graphene Sketch showing the outline of synthesis of Sn/SnOfew layer graphene Sketch showing the outline of synthesis of Sn/SnO-few layer graphene Sketch showing the outline of synthesis of Sn/SnO-few layer graphene Sketch showing the outline of synthesis of Sn/SnO-few layer graphene (b) proposed chemical mechanisms for the synthesis of Sn-few layer graphene.
3.5. Interconnections between Sn coated graphene and Sn coated/filled MWCNTs
air [56]. Flow of current in graphene is affected by the presence of defects like cracks, and grain boundaries [57]. Presence of defects increases the electrical resistance of graphene and lowers the electrical conductivity. Hurdles in current flow in some areas of the graphene sheet lower current in some areas unexpected as seen by the black spots in Fig. 7d. The reasons for hindrance in current are the interaction between the sample surface and the AFM tip, and the contribution of graphenemetal contact resistance to the total resistance [58,59] because of charge carriers at the interface. The presence of SnO also increases the resistance of the graphene. On the other hand, graphene can accept from donors or donate the electron to acceptors [60–62]. Studies on graphite intercalation compounds showed that accepting the electrons from donor elements increases the charge carriers in graphene [63,64] Sn is a metal having large amount of the electrons at the valence band [65]. Adding Sn to graphene increases the charge carriers in graphene because of the donation of electrons from Sn. Therefore, higher number of charge carriers in the Sn-graphene is due to contribution from Sn to graphene and higher amount of charge accumulated at the interface of SnO/Sn-graphene.
3.5.1. Interconnection of hybrid nanotube with hybrid graphene at both ends Fig. 11a shows the formation of an interconnection between Sn coated FLG and Sn coated/filled MWCNT. Here, hybrid nanotube is joined with hybrid graphene at both ends. It is pertinent to mention that this interconnect was developed intentionally but formed on the HRTEM grid when both hybrid MWNCT and hybrid graphene was heated together. Fig. 11b–d confirms that hybrid nanotubes are connected with hybrid graphene at the graphene. High magnification micrograph of joint at one end of CNT in Fig. 11e shows that the CNT is filled and coated at the end and joined with hybrid graphene using Sn. Presence of only Sn and C at the joint was shown by EDX studies. Not only to two hybrid graphene at the ends, as observed in Fig. 11f, tips of both sides of hybrid nanotubes can made the connections with single hybrid graphene. Presence of Sn (2 0 0) at the joint between CNT and graphene in the high magnification micrograph in Fig. 11g, indicate the Sn is responsible for making the joint.
3.5.2. Linkage of hybrid graphene at the side of hybrid CNT HRTEM micrographs in Fig. 12a&b shows joining of a hybrid graphene with Sn coated carbon nanotube. Presence of Sn at the joint in high magnification study in Fig. 12c&d confirms the joining is through Sn present at the surfaces of CNT and graphene. Sn on the surface of nanotubes and adjacent graphene melted during 9
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Fig. 10. HRTEM micrographs of Sn hybrid FLGs showing (a–c) joining with each other in (a&b) low magnifications (c) high magnifications (d&e) presence of Sn planes at the joint in high magnification.
3.5.3. Mechanism When Sn/SnO coated and filled carbon nanotubes are heated in N2/ H2 (reaction (5)) along with Sn/SnO graphene at the TEM grid, SnO present on the graphene and nanotubes was reduced by the H2 and formed the Sn. Heating above 232 °C, Sn melted and formed the connections between CNT and graphene. Due to unavailability of the suitable manipulation device and
this heating and joined each other. In the present study, we used a Sn coated and filled MWCNTs for developing the interconnection. However, inter-connection can be made using the partially Sn filled MWNCTs at their tips with Sn coated graphene [Fig. 11h]. Possibility of formation of partially Sn filling nanotube without coating was shown in our earlier studies [31].
Fig. 11. TEM micrographs showing (a) interconnection of Sn coated and filled nanotubes attached between two Sn-few layer graphene (b) the attachment of Sn coated and filled CNT with one Sn coated graphene (c) another interconnection of Sn coated and filled nanotubes attached between two Sn-few layer graphene (d&e) in high magnification of attachment of Sn coated and filled nanotubes with Sn coated few layer graphene. (f) Connection of both ends of the hybrid carbon nanotube with single hybrid graphene (g) high magnification micrograph showing presence of Sn at the joint (h) sketch showing the possibility to develop an interconnection between the formation of interconnection between partially Sn filled MWNCTs at their tips with Sn coated graphene. 10
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Lin, Bulk thermal conductivity studies of Sn/SnO coated and filled multiwalled carbon nanotubes for thermal interface material, Fullerene Nanotubes
Fig. 12. (a, b) TEM micrographs showing joining of Sn coated carbon nanotube attached to Sn-few layer graphene (c, d) high magnified image of the joint in (b) shows the presence of Sn.
characterization techniques, we could not develop the desired interconnections and measure the electrical properties on this inter-connects. It is believed that electromigration effect will not take place in these connections during heating because of non-formation of Cu-Sn intermetallic compounds. 4. Conclusions A simple method was used for coating the few layer graphene with Sn/SnO. Bulk thermal conductivity of the Sn/SnO hybrid FLGs pellet was surged > 4 times than that of pristine graphene pallet. Study demonstrates the possibility of using Sn/SnO hybrid graphene as thermal interface materials. When compared, the thermal conductivity value of 14.41 Wm−1 K−1 of was higher than the existing thermal interface materials. Possibility of further possibility of improving this thermal conductivity is there by better compression of pallet and sintering conditions, Presence of Sn/SnO on surface of the graphene also increased the negative charge carriers in the graphene Heating with Sn filled and coated carbon nanotubes, hybrid graphene showed the formation of interconnection between them, This interconnection is between graphene and CNT using Sn without using copper. However, more studies are required to analyze the performance of Sn/SnO hybrid graphene for using them as thermal interface materials or interconnections in packaging devices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The support of this study by the Ministry of Science and Technology (MOST) of the Republic of China (Taiwan) under Grant NSC101-2221E006-117-MY3 is gratefully acknowledged. One of the authors (J. Mittal) is grateful to MOST for his financial support during the course of this work under Grant NSC102-2811–E-006-048. 11
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Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb
Sn/SnO hybrid graphene for thermal interface material and interconnections with Sn hybrid carbon nanotubes
T
⁎
Jagjiwan Mittala,b, , Kwang Lung Lina a b
Department of Material Science and Engineering, National Cheng Kung University, Tainan 70101, Taiwan, Republic of China Amity Institute of Nanotechnology, Amity University Uttar Pradesh, Noida Sector 125, Uttar Pradesh 201301, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Few layer graphene Hybrid few layer graphene Sn/SnO Thermal conductivity Tuna current Interconnects
Few layer graphene (FLG) was successfully coated with Sn/SnO after reacting with SnCl2 and reducing under H2/N2 gas. X ray diffraction and X ray photoelectron spectroscopy showed the formation of Sn/SnO on the surface. The thermal diffusivity and conductivity of ~1 mm thick pallet of Sn/SnO coated hybrid FLG was surged to 7.91 mm2/s, 14.41 W/m.K from 2.17 mm2/s and 3.27 W/m.K of the pristine FLG, respectively. The increase in thermal properties was attributed to the formation of compact structures in the film and reducing the air gaps by joining hybrid graphene due to Sn. Current-voltage measurements using tuna probe in atomic force microscopy demonstrated the higher number of negative charge carriers in the hybrid FLG compared to pristine FLG because of electron transfer from graphene to Sn. Study using transmission electron microscopy showed the development of interconnection using Sn/SnO hybrid graphene with Sn coated and filled multiwalled carbon nanotubes.
1. Introduction Reducing chip size and increasing performance are among the most important research aspects for today’s electronics industries. Besides high thermal conductivity, the required material for small sized chip should have good electrical properties. Large amount of heat is generated in this process because of high circuit density. The life span and reliability of the gadgets also depend on the dissipation of heat [1]. Thermal interface material (TIM) provides a conductive path and improves heat flow across the interface [2]. Existing TIMs are found to be not effective for heat dissipation [3]. Poor or very high anisotropy in the thermal conductivity is a problem [4–10]. Therefore, search is going on for improving the thermal property of TIM. Thermal conductivity of single layer graphene (SLG) at room temperature is ~5000 W/m.K [11,12] which is much higher than the thermal conductivity of bulk graphite ≈500 W/m.K [13]. High thermal conductivity of graphene [11], due to large phonon mean free path in strong carbon sp2 bond network, renders graphene possible application for thermal management in future ULSI circuits. [12]. Few layer graphene (FLG) is preferred for application because of its lower cost. However, thermal conductivity ~1600 W/m.K of FLG is lower than SLG. This is due to the cross-plane coupling of the low-energy phonons and corresponding changes in the phonon Umklapp scattering [14]. Graphene layers are easy to attach to heat sinks, which can solve the
⁎
thermal contact resistance problems, Further, flat plane geometry of graphene is also suitable for integration with Si CMOS circuits for thermal management. However, this high thermal conductivity value is reduced drastically when used in bulk. It has been observed in the current study that the average thermal conductivity of graphene with the film thickness from 1 mm is measured around 3.27 W/m K at room temperature. The reasons are very low thermal conductivity across the axis, existence of substantial defects and large gaps due to no interaction between the graphene. These problems restrict the applications of graphene as TIM. Therefore, graphene is studied as thermal interface material using formation composites with other materials like polymers and boron nitrides. Reducing thermal conductivity of low-k dielectrics and increasing current density demands from small dimension interconnects raise the reliability concerns for Cu based interconnects [15]. The rise in metal temperature and electromigration (EM) exponentially degrades interconnect lifetime [16]. There are also environmental and economic concerns for the current copper-based technology in electronics. Due to exceptional electronic and other properties carbon nanotubes and graphene are considered to be the best materials for very small dimension interconnects [17–19]. CNTs are less sensitive to electromigration [20], and possible alternative interconnect material because of their high current carrying capacity and thermal conductivity [21,22]. In our
Corresponding author at: Amity Insitute of Nanotechnology, Amity University Uttar Pradesh, Noida Sector 125, Uttar Pradesh 201301, India. E-mail address: [email protected] (J. Mittal).
https://doi.org/10.1016/j.mseb.2019.114485 Received 30 October 2018; Received in revised form 22 October 2019; Accepted 11 December 2019 0921-5107/ © 2019 Elsevier B.V. All rights reserved.
Materials Science & Engineering B 253 (2020) 114485
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Table 1 Bulk thermal properties of pristine and Sn/SnO coated graphene. Material
Bulk Density g/cm3
Cp J/g/K
Thermal diffusivity (α) mm2/s
Thermal conductivity (K) (W/m.K)
Few layer Graphene Sn/SnO- few Layer Graphene
1.130 2.165
1.334 0.840
2.171 7.916
3.277 14.416
measured on the pellet prepared after compressing the materials using 68 MPa pressure. Size of pellet was kept at 13.1 mm diameter and 1 mm thickness. Pallets were then heated at 200 °C in tube furnace for 1 h under 5% H2/95% N2 gas. Thereafter, pallets were cooled down to room temperature. The pellets of hybrid FLGs and pristine FLGs were analyzed by laser flash analysis (LFA) [32] for determining thermal conductivity using the equation (1)
previous study, we showed the formation of interconnects using the Sn hybrid single multiwalled carbon nanotube on the copper substrate [23]. However, Cu involved in this interconnect also leads to the formation of Cu6Sn5 inter-metallic compound during the formation of connection [23]. Besides high thermal conductivity, graphene also shows extremely [24] low electrical resistivity of 7.5 × 10−7 Ω m along the plane so it is highly suitable for interconnect applications. However, high resistance in the vertical direction to the plane and very low interactions among the graphene are big hindrance for their applications. One possibility for using the FLGs for both as thermal interface material and interconnect applications is to modify them by coating, doping, substitution or functionalisation [25–28]. Sn metal is used for soldering purpose because of its high thermal, electrical conductivity, and availability. Coating and filling of CNTs with Sn have been studied for the bulk thermal conductivity and interconnect applications [23,29] but few studies has been undertaken for Sn-graphene. In the present work, a new way is used the FLGs as thermal interface material and interconnect applications. FLGs was coated with Sn/SnO using a easy method and studied for bulk thermal conductivity and electrical properties. Study also reports a possibility of forming an interconnect between Sn coated and filled single carbon nanotubes and Sn coated graphene.
K = ρ × α × Cp
(1)
Here, K, ρ, α Cp are thermal conductivity, density, thermal diffusivity and specific heat of the samples, respectively. Specific heat and thermal diffusivity were provided by the instrument and density of materials was determined using the weight/volume ratio of the pallets. Experiments for thermal conductivity measurements were repeated three times and average values are presented in Table 1.
2.4. Electrical measurements The AFM can be operated in different modes depending on the sample properties that user wants to determine. The TUNA module senses the current passing through the sample to the AFM tip. This has been done in this study using the Peak Force Tunneling AFM (PF TUNA). Peak Force Tapping, the maximum force on the tip (peak force) is maintained at a constant value for each individual cycle. By maintaining a constant tip-sample force, topographic and current images are generated simultaneously. The peak current is the current detected when the AFM tip exerts maximum force on the sample surface (peak force) and TUNA current is the average current detected in one full tapping cycle (approach and retract). Local I-V spectra can be measured in the Peak Force TUNA. To extract the I-V curves, the current passing across the sample is plotted against the applied DC bias. Current-voltage (I-V) study on the pellets of pristine and hybrid FLGs was investigated using tuna probe in atomic force microscopy (AFM). Local I-V spectra is measured in the Peak Force TUNA by keeping tip in a fixed location and the sample bias is ramped up and down either once or continuously. The current passing across the sample is plotted against the applied DC bias. In this study, both PF TUNA imaging mode and I-V spectroscopy mode have been used to investigate the electrical properties of graphene sheets. It was reported in the earlier study [33] that the I-V studies using TUNA probe produces the same results as using two probe method.
2. Experimental 2.1. Development of hybrid few layer graphene The commercial few layer graphene (FLG), produced using chemical vapor deposition method, were used in the present study. The FLGs were heated in O2 at 250 °C for 15 min for surface oxidation. The oxidized FLGs were then stirred with SnCl2 solution in small amount of HCl at 70 °C for half an hour for coating of SnO2 on the surface of FLGs, similar to the earlier studies on carbon nanotubes [30,31]. After the reaction, hybrid FLGs-SnO2 material was separated from solution using centrifugation and dried. The hybrid sample was heated isothermally at 250 °C for 30 min in a furnace under 5% H2/N2 gas flow and thereafter cooled down to room temperature for formation of FLGs-Sn. 2.2. Characterization of hybrid few layer graphene Pristine FLG and hybrid FLGs were investigated using scanning electron microscope (SEM) under 10 keV beam energy and energy dispersive spectroscopy (EDS) using silicon drift detectors. The materials were also examined by high resolution transmission electron microscope (HRTEM) with 200 keV beam energy and < 0.19 nm point resolution. The HRTEM micrographs were further analyzed using Fast Fourier Transform (FFT) and inverse FFT (IFFT). Crystal structures of the samples were determined using electron diffraction (ED) with convergent angle of 1.5–2.0 mrad in HRTEM and X-ray diffraction (XRD) using the 1.5408A CuKα radiation. X-ray photoelectron spectroscopy (XPS) was used to identify different elements, their compositions and oxidation states in the pristine FLGs and hybrid FLGs. All peaks in the XPS spectra were standardized using the C 1s peak.
2.5. Interconnect applications with Sn coated nanotubes The commercial MWCNTs having outer diameter 110–170 nm was coated and filled with SnOx using SnCl2 solution, according to the procedure applied in earlier study [30,31]. The SnOx coated and filled MWCNT were sprayed, after dispersing and sonicating in the alcohol solution, on the Sn/SnOx hybrid graphene settled on the holy carbon grid. The grids containing the SnOx coated and filled nanotubes with Sn/SnO hybrid graphene were heated in a furnace to 250 °C at the rate of 10 °C/min and isothermally at 250 °C for one hour in 5%H2/95%N2 atmosphere and then cooled down to room temperature. Interconnections between hybrid graphene and hybrid MWCNTs were characterized using HRTEM.
2.3. Thermal properties measurements Bulk thermal properties of hybrid FLGs and pristine FLGs were 2
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structure of graphene (Fig. 2h). EDS area scan in Fig. 2d–f and respective weight and at.% of elements are displayed in table (Fig. 2c). At.% of elements in pristine graphene revealed the presence of mostly carbon 97.48% and some scattered oxygen 1.56%. This low amount of oxygen coupled with XRD study reveals the high purity of pristine FLGs. On the other hand, EDS area scan in Fig. 2j–m and at.% in attached table (Fig. 2i) established the presence of Sn, carbon and oxygen elements in the hybrid film. Carbon is also the main elements with 69.44 at.% in the hybrid graphene film. Sn with at.%17.40 is uniformly distributed in the graphene like carbon. The concentration of oxygen is much higher in hybrid graphene (13.15 at.%) than pristine graphene (1.56 at%). However, it is mostly scattered and may link with Sn as well as carbon. Higher concentrations of Sn than oxygen also suggest the presence of metallic Sn-Sn bonds in the hybrid graphene, but the presence of C-Sn bonds is ruled out because of low temperature synthesis of hybrid graphene [37]. 3.1.3. HRTEM study HRTEM micrograph in Fig. 3a displays a pristine few layer graphene on the grid. High magnification study in Fig. 3b showed 12 graphene layers with interplanner distance of 3.38 Å in the pristine graphene, as calculated by XRD. ED in Fig. 3c revealed the presence of (0 0 2) crystal plane of graphite among other crystal planes such as (1 0 0), (01¯1¯0), etc. of graphene, as described in previous studies [38]. The HRTEM micrograph in Fig. 3d demonstrates a hybrid graphene where coating on the surface appears darker than the graphene. Higher magnification study in Fig. 3e&f revealed the covering of entire sheet by coating of Sn/SnO. Interplanner distances in HRTEM micrograph revealed the presence of (1 1 0) crystal plane of SnO. The presence of the SnO in the hybrid film was also confirmed by an ED investigation (Fig. 3g) and Xray diffraction in Fig. 1. During the reduction the formation reduction of tin oxide into Sn/ SnO, graphene joins with each other at the edges, as shown by the HRTEM study in Fig. 3h, Details of this study is provided in the Section 3.4.3 using Fig. 10a–e.
Fig. 1. X-ray diffraction of (a) pristine few layer graphene and (b) Sn/SnO-few layer graphene.
3. Results and discussion 3.1. Characterization of pristine few layer graphene and Sn hybrid few layer graphene films 3.1.1. Crystal structure studies X-ray diffraction of pristine FLGs in Fig. 1 displays only two peaks. The major peak at 2θ = 26.2° denoted as (G0 0 2) is due to (0 0 2) of graphite with the interplanner distance of 3.38 Å. The minor peak centered at 2θ = 54.3° with an interplanner distance of 1.69 Å is attributed to (G0 0 4). These peaks indicate the existence of multilayer structure in graphene. The interplanner distances of (0 0 2) and (0 0 4) peaks are consistent with previous work [34]. Broadness of G0 0 2 peak shows the existence of small irregularity in distances between the planes. Higher interplanner distance than graphite (3.353 Å) suggest lower crystallization of FLGs. Contrary to pristine FLGs, XRD of hybrid FLGs in Fig. 1 exhibits various peaks. The major peak G002 with an interplanner distance of 3.38 Å is same as in pristine FLGs. This indicates that the crystal structure of graphene is undamaged during processing. Interestingly, peak is sharper and narrower in hybrid FLGs than pristine FLGs. Although the exact mechanism is not clear, but the heat treatment of the sample at 250 °C in H2/N2 may be responsible for this reduction in peak broadness. Beside the major peak, XRD also displays two small bands centered at 2θ = 33.3° and 2θ = 51.08° with interplanner distance of 2.68 Å and 1.786 Å, respectively. These bends are due to (1 1 0) and (1 1 2) of SnO [35,36] XRD of hybrid FLG also shows a small peak corresponding to pure Sn at 2θ = 32.9°. This implies that Sn is also present in the sample.
3.1.4. XPS study XPS survey scan spectra of the pristine FLGs in Fig. 4a establishes the presence of carbon and oxygen. Detailed analysis of carbon (C1s) XPS in Fig. 4b reveals three minor peaks at 284.4 eV, 285.2 and 285.6 binding energies. These peaks belong to C-C, C-OH and and C-O bonds in graphene, respectively. Presence of C-O and C-OH bonds are also shown by oxygen (O1s) XPS in Fig. 4c by the presence of C-O bonds are present in graphene as -CO or -COOH bonds. In contrast, XPS survey scan spectra of the hybrid FLGs shown in Fig. 5a reveals the presence of Sn, carbon and oxygen. Similar to pristine FLGs, analysis of C1s XPS in Fig. 5b demonstrates two oxidation states of carbon. The major peak at 284.4 eV is due to C-C bonds of the graphene [39] and minor peak at285.6 eV is due to C-O bonds [40,41]. Sn 3d XPS spectrum in Fig. 5c displays two oxidation states. In the band due to Sn 3d3/5, the major peak at 486.6 eV is attributed to the +2oxidation state of Sn as Sn2+ while minor peak at 484.7 eV indicates the presence of 0 oxidation state of Sn or Sn metal [42]. Similar results are obtained in the band due to Sn 3d2/5 where peaks at 495.2 and 493.1 binding energies are due to Sn2+ and Sn. This confirms the formation of SnO along with Sn at the surface of FLGs. Interestingly, no peak belongs to C-Sn bonds is observed either in the C1s or Sn3d XPS peak. This indicates that the carbon did not make any bond with Sn during the process. O1s XPS of hybrid FLGs in Fig. 5d shows the existence of two types of oxygen at 530.6 eV (with Sn) and 531.6 eV (with C) binding energies [43]. Above XPS results disclose the formation of FLG-Sn/SnOhybrid during the reaction in which the linkage between carbon (of FLGs) and Sn was through oxygen. For determining the exact composition of compound at surface and below the surface, depth profile study of hybrid FLGs film was studied
3.1.2. FESEM study FESEM micrograph in Fig. 2a&d demonstrates flat sheets in various sizes of pristine FLGs. Shapes and sizes of graphene sheets are mostly irregular. Some graphene sheets are larger in sizes than others. The folding at the edges of graphene sheets which is observed as sharp edges are quite visible at various places in the micrograph. Higher magnification study in Fig. 2b revealed mostly flat morphology of pristine FLGs sheets. However, clinks in the middle and folding at the edges of the flat sheets are noticeable. In comparison, hybrid FLGs displayed flat morphology having few sharp edges (Fig. 2g&j). The individual sheets of the hybrid graphene were observed as agglomerated whereas sheets of pristine graphene are observed as separated from each other. A high magnification study of the hybrid-graphene film revealed the flat 3
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Fig. 2. SEM micrographs of sintered pallets showing pristine few layer graphene in (a) ×1000 magnification (b) ×5000 high magnification (c) EDS results showing elemental composition in the pristine few layer graphene (d–f) SEM micrograph and corresponding EDX mapping demonstrating elemental analysis of C and O in pristine graphene. SEM micrographs of and Sn/coated few layer graphene in (g) ×1000 magnification (h) ×5000 magnification. (i) EDS results showing elemental composition in the Sn/SnO coated few layer graphene, (j-m) SEM micrograph and corresponding EDX mapping showing elemental analysis of C, Sn and O in Sn/SnO coated few layer graphene.
current flow (shown in bright yellow color) of 5.1 nA at most of the places. However, at some places the current flow is lower at 1.6 nA as evidenced by black color spots. This may be due to bigger height difference at a few places. Tuna current flow behavior in the pristine FLGs sample in Fig. 7d with same bias of 0.500 mV is similar to the peak current. However, the current flow is lower as evidenced with dark color. There are more black spots with lower current of zero at some places. On the other hand, surface topography of 10 µm × 10 µm hybrid FLG film in AFM micrograph [Fig. 8a] shows flattish structure. Threedimensional analysis in the Fig. 8b reveals the height difference of 117.2 nm between different places which is less than the pristine FLGs. This shows the better compression of the hybrid FLGs during film making and sintering. Peak current generated after the 0.500 mV bias in Fig. 8c shows the almost similar current flow (shown in bright yellow color) of 5.1 nA at most the places. At some places the current flow is lower at 1.6 nA, as evident by black color spots but these spots are not centered at one place as in case of pristine FLGs. Interestingly, TUNA current flow behavior in the sample with the AFM tip of 0.500 mV in Fig. 8d shows more current flow than the peak current. This shows higher number of charge carriers in the case of hybrid FLGs than pristine FLGs.
using XPS. Fig. 6a shows the depth profile of the three elements Sn, O and C below the surface of the hybrid film. As visualized in the Fig. 6a, oxygen amount is more on the surface than below the surface whereas the amount of carbon and Sn are more in the bulk. Since there is no CSn bonding, there is higher C-O and Sn-O bonding on the surface than the bulk. As shown in Fig. 6b, presence of 286.4 eV peaks in C1s XPS proves the C-O bonds on the surface. However, C-O bonds are either absent or in much smaller amount below the surface. This is further proven by the O1s XPS in Fig. 6c where peak belong to C-O at 531.4 is observed only at the surface with no indication below the surface. Analysis of Sn 3d peaks in the Fig. 6d indicates the presence of Sn-O (486.6–486.8 keV) and absence of Sn at the surface. However, Sn (485 keV) increases with respect to SnO in the inner layers.
3.2. Electrical properties using TUNA probe in AFM AFM micrograph in Fig. 7a shows the surface topography of pristine FLGs on dimensions 10 µm × 10 µm FLG film. Three-dimensional analysis represented in the Fig. 7b reveals the height difference of 195.3 nm between different places. Peak current generated after the 0.500 mV bias giving to AFM tip in Fig. 7c shows the almost similar 4
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Fig. 3. HRTEM micrographs showing (a) pristine few layer graphene in low and (b) high magnifications (c) electron diffraction (ED) of pristine graphene in (b) (d) Sn/SnO hybrid graphene in low and (e.f) high magnifications showing Sn and SnO planes (g) electron diffraction (ED) of Sn/SnO hybrid graphene in (e) (h) joining of two graphene.
Fig. 4. Survey scan spectra (a), C1s (b) and O1s (c) XPS spectrum of pristine few layer graphene.
increase in bias either given to sample or tip. This is a particular behavior of the graphene [44]. Hybrid FLGs on the other hand show the negative current flow of 600 nA when no bias is given to the probe or the substrate [Fig. 8e]. This shows that electron charge carriers are much higher in the hybrid FLGs which leads the flow of charge from sample to tip. This negative current flow is increased with the increase of the negative bias. It is increased to −600 nA when the bias to sample is increased to −250 mV. However, the decrease is slowed down and only −710 nA current is generated with the increase in the negative bias to −500 mV. During positive bias to the probe, negative current flows of −541 nA is
I-V curves using Tuna probe in AFM in Fig. 8e show the behavior of charge carriers on the surface of pallets of pristine FLGs and hybrid FLGs after the bias from −500 mV to 1000 mV. Pristine FLGs show no flow of current when there is no voltage bias to the probe. With negative bias up to 250 mV to the sample (−250 mV) flow of current is increased to −23 nA from sample to tip. Current is increased to −150 nA when bias to sample is increase to −500 mV bias. When 250 mV bias to given tip (+250 mV), the current flow is increased to 33 nA. This current is increased to 78 nA when the bias to tip is increased to 500 mV. This current is further increased to 310 nA with positive bias of 1000 mV. This shows increase in current with the 5
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Fig. 5. Survey scan spectra (a), C1s (b) Sn3d (c) O1s (d) XPS spectrum of Sn/SnO few layer graphene.
Fig. 6. XPS study of Sn/SnO-few layer graphene showing depth profile of (a) of C, Sn and O (b) C1s spectra (c) Sn3d spectra (d) O1s spectra.
Since in TUNA experiment set up, the length L and width W are not well defined, it is difficult to calculate the resistivity of the graphene sheet in this study. However, comparison of the resistances of the pristine FLG and Sn-FLGs using TUNA probe with same scanning area. Pristine FLG shows the resistance of 4.44 × 10−6 Ω whereas resistance observed by Sn-FLGs is 2.26 × 10−6 Ω. Higher current flow in the SnFLGs is also observed by brightness of yellow color during TUNA current analysis in Fig. 8d when compared with Fig. 7d. This means that
observed even when voltage bias of above +250 mV is given to the probe. When the positive bias to the tip is increased to 1000 mV, negative current flow is reduced to −1 nA. This shows that the higher number of charge carriers are present in the hybrid FLGs due to Sn/SnO coating on the graphene. As shown by the results in the X-ray and XPS the Sn/SnO are connected to graphene through oxygen. Therefore, d orbitals of the Sn accepts the electrons from graphene through oxygen and produce enough charge carriers. 6
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Fig. 7. AFM study of pristine few layer graphene showing (a & b) micrograph and its three-dimensional projection (c & d) peak current and Tuna current during Tuna probe study.
Fig. 8. AFM study of Sn/SnO few layer graphene showing (a & b) micrograph and its three-dimensional projection (c & d) peak current and Tuna current during Tuna probe study (e) Current voltage behavior of the pristine few layer graphene as well as Sn-few layer graphene. 7
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Table 2 Comparison of the thermal conductivity of various materials in literature and our material. S.N
Material
Thermal conductivity (TC)
Comments
Ref
1 2 3 4 5 6 7 8 9.
Flexible PMIA/hexagonal boron nitride composites Graphene/Silicon carbon nanorods PVA/Boron nitride nanosheets 30%Graphene-Perfluoroalkoxy Nanocomposite Porous boron nitride (BN)/epoxy composite Graphene/epoxy composite Sn coated and filled CNT Cellulose nanofibers/boron nanotube nanocomposites Sn/SnO coated graphene
0.94 W/m.K 10.9 W/m.K 1.9 W/m.K In plane 2.39 W/m.K through plan 25.57 W/m.K in-plane (5.19 W/m.K) out-of-plane (3.48 W/m.K) 5.1 W/m.K 5.1 W/m.K In plan 21.39 W/m.K 14.41 W/m.K
Poor TC
[5] [6] [4] [7] [8] [9] [29] [10]
Poor TC Highly anisotropic Anisotropic Low TC Low TC Anisotropic Current work
4SnCl2+2H2O→Sn4(OH)2Cl6+2HCl
(1)
Sn4(OH)6Cl2→4SnO+2HCl+2H2O
(2)
2SnCl2 (HCl)+2H2O+O2→2SnO2+4HCl
(3)
either the resistance is lowered or smoothen the surface by the addition of Sn. It may be due to less defects and/or higher conductivity. According to the 3-dimension analysis in Figs. 7b and 8b, surface is smoothened after the addition of Sn. 3.3. Thermal properties
FLGs
Table 1 lists the thermal properties of pristine as well as Sn/SnO coated FLGs after sintering. Density of pristine FLGs is low but magnifies in hybrid FLGs. As observed in the second column of Table 1, density of pristine FLGs is increased from 1.13 g/cm2 to 2.16 g/cm2. This high increase in density is due to the incorporation of heavy element Sn and oxygen via coating. On the other hand, specific heat (Cp) of hybrid nanotubes in the fourth column is reduced. It is reduced from 1.334 J/g/K to 0 0.840 J/g/K. This means that less heat is required for raising the temperature of hybrid FLGs. This is because of the addition of metal on the surface of graphene. Thermal diffusivity (α) is increased from 2.17 mm2/s to 7.91 mm2/s. The thermal conductivity estimated from equation (1) gives rise to 14.41 W/m.K of hybrid FLGs as compared to 3.277 W/m.K of pristine FLGs.
SnO/SnO2
Air, 250°C
→
Surface functionalized FLGs
H2 /N2, 250°C
→
Sn/SnO + 2H2 O
(4) (5)
As the reduction process is carried out at 250 °C which is above the melting point of the Sn (232 °C), Sn on the surface of FLGs melted and the hybrid FLGs in proximity join each other. 3.4.2. Thermal conductivity of few layer graphene and hybrid few layer graphene Contrary to metals, thermal conductivity of carbon materials is dominated by phonons [47]. Thermal transport in graphene is also carried by the intrinsic properties of the strong sp2 lattice, rather than by phonon scattering on boundaries or by disorder, giving rise to extremely high K values [48,49]. Thermal conductivity of few-layer graphene strongly depends on the number of atomic planes. The reduction of the ability of few-layer graphene to conduct heat is attributed to the crossover from 2D graphene to 3D graphite. It is assumed that decrease in thermal conductivity is mainly due to the shrinking of high frequency phonon induced by the cross-layer coupling. The phonon group velocity in graphene is higher than that in CNTs, which leads to larger thermal conductivity [50,51]. However, in the samples, the heat loss to inter air gaps among the graphene sheet reduces the thermal conductivity. The small thermal conductivity for bulk graphene samples may also result from low thermal conductivity in the perpendicular direction of graphene, high density of defects which efficiently scatter the flow of phonons [52]. Although thermal conductivity in metals is much higher in bulk but heat is conducted by the electrons. Therefore, pure metals cannot be used as thermal interface material because of leakage of current. Possible oxidation of metals is also another reason.
3.3.1. Comparison of the thermal conductivity of Sn/SnO hybrid graphene with other materials Table 2 compares the thermal conductivity of Sn/SnO hybrid graphene with other materials studied in the literature. As observed, only graphene-based material 30% Graphene-Perfluoroalkoxy nanocomposite and cellulose nanofibers/boron nanotube composites showed high thermal conductivity than current material. However, these values are highly anisotropic. 3.4. Mechanisms 3.4.1. Synthesis of hybrid FLGs It was shown in earlier studies [30] that during hydrolysis of SnCl2 in water Sn4(OH)2Cl6 is formed (reaction 1). In the subsequent reaction, SnO (reaction 2) is formed. But addition of small amount of HCl in SnCl2 solutions suppresses the hydrolysis of SnCl2 and the SnO2 is formed (reaction 3). A sketch of the process for the coating of FLGs is shown in Fig. 9. Due to highly inert surface, pristine FLGs do not disperse in the water solution of SnCl2. Therefore, surface of pristine FLGs cannot be coated with Sn or SnO on its surface without modification. In the present study, we oxidized the surface of pristine FLGs by heating in air at 250 °C (reaction (4)). Presence of O2 in air oxidizes the surface of pristine FLGs at high temperature [45,46] and creates functional groups such as carbonyl, alcoholic, carboxylic etc. Because of the generation of functional groups at the surface, oxidized FLGs are easily dispersed in SnCl2 solution, similar to earlier study for MWCNTs [30]. Subsequently, the graphene is coated with tin oxides on its surface. When tin oxide coated FLGs are heated under H2/N2 gas at 250 °C in the last step (reaction (5)), tin oxide coated on the surface are converted into Sn.
3.4.3. Joining of graphene with each other Coating of solder metals on the surface of FLGs helps in joining of FLGs with each other [Fig. 10a&b]. This is caused by the formation of Sn from SnO during reduction by H2/N2 treatment at the edges and subsequent joining of the graphene using Sn-Sn bonding, as observed in carbon nanotubes in our earlier study [30]. High magnification HRTEM study at the joint between graphene in Fig. 10d&e reveals the presence of Sn (2 0 0). This shows that Sn is responsible for the joining of the graphene. Joining of nanotubes reduces the inter air gaps between graphene. Combination of phonon and electron carriers in FLGs and Sn are responsible for the increase in the thermal conductivity. 3.4.4. Current voltage study of the FLGs and hybrid FLGs It has been shown that graphene has high electrical conductivity [53,54]. For FLGs, carrier mobility ≤10,000 cm2/Vs was found at 300 K [54] and electron can cover micrometer-long distances without scattering [55]. FLGs can sustain very high current densities in ambient 8
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Fig. 9. (a) Sketch showing the outline of Sketch showing the outline of synthesis of Sn/SnO-few layer graphene Sketch showing the outline of synthesis of Sn/SnOfew layer graphene Sketch showing the outline of synthesis of Sn/SnO-few layer graphene Sketch showing the outline of synthesis of Sn/SnO-few layer graphene Sketch showing the outline of synthesis of Sn/SnO-few layer graphene (b) proposed chemical mechanisms for the synthesis of Sn-few layer graphene.
3.5. Interconnections between Sn coated graphene and Sn coated/filled MWCNTs
air [56]. Flow of current in graphene is affected by the presence of defects like cracks, and grain boundaries [57]. Presence of defects increases the electrical resistance of graphene and lowers the electrical conductivity. Hurdles in current flow in some areas of the graphene sheet lower current in some areas unexpected as seen by the black spots in Fig. 7d. The reasons for hindrance in current are the interaction between the sample surface and the AFM tip, and the contribution of graphenemetal contact resistance to the total resistance [58,59] because of charge carriers at the interface. The presence of SnO also increases the resistance of the graphene. On the other hand, graphene can accept from donors or donate the electron to acceptors [60–62]. Studies on graphite intercalation compounds showed that accepting the electrons from donor elements increases the charge carriers in graphene [63,64] Sn is a metal having large amount of the electrons at the valence band [65]. Adding Sn to graphene increases the charge carriers in graphene because of the donation of electrons from Sn. Therefore, higher number of charge carriers in the Sn-graphene is due to contribution from Sn to graphene and higher amount of charge accumulated at the interface of SnO/Sn-graphene.
3.5.1. Interconnection of hybrid nanotube with hybrid graphene at both ends Fig. 11a shows the formation of an interconnection between Sn coated FLG and Sn coated/filled MWCNT. Here, hybrid nanotube is joined with hybrid graphene at both ends. It is pertinent to mention that this interconnect was developed intentionally but formed on the HRTEM grid when both hybrid MWNCT and hybrid graphene was heated together. Fig. 11b–d confirms that hybrid nanotubes are connected with hybrid graphene at the graphene. High magnification micrograph of joint at one end of CNT in Fig. 11e shows that the CNT is filled and coated at the end and joined with hybrid graphene using Sn. Presence of only Sn and C at the joint was shown by EDX studies. Not only to two hybrid graphene at the ends, as observed in Fig. 11f, tips of both sides of hybrid nanotubes can made the connections with single hybrid graphene. Presence of Sn (2 0 0) at the joint between CNT and graphene in the high magnification micrograph in Fig. 11g, indicate the Sn is responsible for making the joint.
3.5.2. Linkage of hybrid graphene at the side of hybrid CNT HRTEM micrographs in Fig. 12a&b shows joining of a hybrid graphene with Sn coated carbon nanotube. Presence of Sn at the joint in high magnification study in Fig. 12c&d confirms the joining is through Sn present at the surfaces of CNT and graphene. Sn on the surface of nanotubes and adjacent graphene melted during 9
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Fig. 10. HRTEM micrographs of Sn hybrid FLGs showing (a–c) joining with each other in (a&b) low magnifications (c) high magnifications (d&e) presence of Sn planes at the joint in high magnification.
3.5.3. Mechanism When Sn/SnO coated and filled carbon nanotubes are heated in N2/ H2 (reaction (5)) along with Sn/SnO graphene at the TEM grid, SnO present on the graphene and nanotubes was reduced by the H2 and formed the Sn. Heating above 232 °C, Sn melted and formed the connections between CNT and graphene. Due to unavailability of the suitable manipulation device and
this heating and joined each other. In the present study, we used a Sn coated and filled MWCNTs for developing the interconnection. However, inter-connection can be made using the partially Sn filled MWNCTs at their tips with Sn coated graphene [Fig. 11h]. Possibility of formation of partially Sn filling nanotube without coating was shown in our earlier studies [31].
Fig. 11. TEM micrographs showing (a) interconnection of Sn coated and filled nanotubes attached between two Sn-few layer graphene (b) the attachment of Sn coated and filled CNT with one Sn coated graphene (c) another interconnection of Sn coated and filled nanotubes attached between two Sn-few layer graphene (d&e) in high magnification of attachment of Sn coated and filled nanotubes with Sn coated few layer graphene. (f) Connection of both ends of the hybrid carbon nanotube with single hybrid graphene (g) high magnification micrograph showing presence of Sn at the joint (h) sketch showing the possibility to develop an interconnection between the formation of interconnection between partially Sn filled MWNCTs at their tips with Sn coated graphene. 10
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Lin, Bulk thermal conductivity studies of Sn/SnO coated and filled multiwalled carbon nanotubes for thermal interface material, Fullerene Nanotubes
Fig. 12. (a, b) TEM micrographs showing joining of Sn coated carbon nanotube attached to Sn-few layer graphene (c, d) high magnified image of the joint in (b) shows the presence of Sn.
characterization techniques, we could not develop the desired interconnections and measure the electrical properties on this inter-connects. It is believed that electromigration effect will not take place in these connections during heating because of non-formation of Cu-Sn intermetallic compounds. 4. Conclusions A simple method was used for coating the few layer graphene with Sn/SnO. Bulk thermal conductivity of the Sn/SnO hybrid FLGs pellet was surged > 4 times than that of pristine graphene pallet. Study demonstrates the possibility of using Sn/SnO hybrid graphene as thermal interface materials. When compared, the thermal conductivity value of 14.41 Wm−1 K−1 of was higher than the existing thermal interface materials. Possibility of further possibility of improving this thermal conductivity is there by better compression of pallet and sintering conditions, Presence of Sn/SnO on surface of the graphene also increased the negative charge carriers in the graphene Heating with Sn filled and coated carbon nanotubes, hybrid graphene showed the formation of interconnection between them, This interconnection is between graphene and CNT using Sn without using copper. However, more studies are required to analyze the performance of Sn/SnO hybrid graphene for using them as thermal interface materials or interconnections in packaging devices. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The support of this study by the Ministry of Science and Technology (MOST) of the Republic of China (Taiwan) under Grant NSC101-2221E006-117-MY3 is gratefully acknowledged. One of the authors (J. Mittal) is grateful to MOST for his financial support during the course of this work under Grant NSC102-2811–E-006-048. 11
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