Journal of Science: Advanced Materials and Devices (2017) 99e104 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Carbon nanotubes length optimization for preparation of improved transparent and conducting thin film substrates Mansoor Farbod*, Amir Zilaie, Iraj Kazeminezhad Department of Physics, Shahid Chamran University of Ahvaz, Ahvaz, Islamic Republic of Iran a r t i c l e i n f o a b s t r a c t Article history: Received November 2016 Received in revised form 18 February 2017 Accepted 19 February 2017 Available online 27 February 2017 Transparent and conductive thin films of multiwalled carbon nanotubes (MWCNTs) with different lengths were prepared on glass substrates by the spin coating method In order to reduce the MWCNTs length, they were functionalized The initial length of MWCNTs (10e15 mm) was reduced to 1200, 205 and 168 nm after 30, 60 and 120 refluxing time, respectively After post annealing at 285 C for 24 h, the electrical and optical properties were greatly improved for functionalized MWCNT thin films They strongly depend on the length of CNTs The optical transmittance of the film prepared using 30 reflux CNTs was 2.6% and 6.6% higher than that of the 60 and 120 refluxed samples respectively The sheet resistance of this film showed reductions of 45% and 80% as well The film also exhibited the least roughness The percolative figure of merit, which is proportional to the transparency and disproportional to the sheet resistance, was found to be higher for the sample with 30 refluxed MWCNTs © 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Transparent and conductive films Multi-walled carbon nanotubes Spin coating Figure of merit Substrate Introduction Transparent and conductive thin films have attracted considerable interest due to their importance in the fundamental researches and potential industrial applications in optoelectronic devices, transparent films, automobile glasses and smart windows Indium tin oxide (ITO) is a superior material which is used to fabricate the transparent conducting films [1e3] For these materials, the main disadvantage is related to its applicability to flexible substrates The small strains of these materials cause a reduction in their electrical conductivity and so in their performance as a good conducting film Currently, carbon nanotubes are the materials of everincreasing interest due to their excellent electronic, physical and chemical properties and are of major research interest for their outstanding behaviours in practical applications [4e8] It has been shown that the single wall carbon nanotubes' performance is comparable to ITO for many applications such as solar cells, smart windows and sensors [9,10] The high electrical conductivity and mechanical strength of carbon nanotubes (CNTs) make them a good replacing candidate for the ITO materials to make transparent conducting films The CNTs transparent conducting films can be prepared on flexible substrates so they may have many applications in different types of electronic, optoelectronic, solar cell and sensor systems [10] Different conductivities have been reported for CNTs because several factors can affect the conductivity of CNTs such as sample purity, metallic to semiconducting volume ratio and doping level of the semiconducting CNTs [10] In order to investigate the relation between the optical and electrical properties of CNTs' films, some parameters need to be introduced It has been shown that the below relation is held between transmittance (T), optical conductivity (sop) and film thickness (t) [11]: T¼ Z0 sop t À2 (1) where the Z0 is the free space impedance (377 U) This relation can be converted to a relation between T and sheet resistance (Rs) as Tẳ * Corresponding author Fax: ỵ98 6133331040 E-mail addresses: Farbod_m@scu.ac.ir (M Farbod), amirzilaie@gmail.com (A Zilaie), I.Kazeminezhad@scu.ac.ir (I Kazeminezhad) Peer review under responsibility of Vietnam National University, Hanoi 1ỵ 1ỵ Z0 sop 2Rs sDC:B 2 (2) where sDC.B is the bulk DC conductivity For a thin film, DC conductivity is thickness dependent and is proportional to tn [12e15], where t is the film thickness and n is the percolation http://dx.doi.org/10.1016/j.jsamd.2017.02.005 2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 100 M Farbod et al / Journal of Science: Advanced Materials and Devices (2017) 99e104 exponent Using a simplified model, De et al have found another relation between T and Rs of transparent conductors in percolative regime at which the conductivity is thickness dependent as [15]: T¼ !2 Z 1ỵn 1ỵQ Rs (3) here, P is a dimensionless parameter called the percolative figure of merit and its higher values mean tmin higher T and lower Rs In order to rate the performance of different films, we applied these equations to the films prepared by different lengths CNTs and found different behaviours that will be presented and discussed in this paper We believe that the length is an important factor which can have a significant impact on the conductivity and transparency of CNTs thin films In particular, in the present study, MWCNTs thin films with different CNTs lengths were fabricated by spin coating method without using any surfactant The sheet resistance and optical transmittance of the films were measured and the findings were interpreted based on the CNTs lengths 3:1) Details of acid treatment have been described elsewhere [16] The chemical functionalization of CNTs is a technique to improve their dispersibility and homogeneity in organic solvents and water 2.2 Surface modification of the glass substrate In order to have better adhesion between carbon nanotubes and the glass substrates, 3-aminopropyltriehtoxysilane (APTES) solution was used to modify the substrate's surface The glass substrate was cleaned by DI water, acetone and isopropanol alcohol respectively in an ultrasonic bath for 15 then dried in an oven at 100 C for 30 The dried substrate then was immersed into a prepared aqueous solution including 0.5 ml of APTES mixed with 47 ml deionized water for 24 h [17,18] It was found that without using APTES, the adhesion between glass surface and CNTs was very weak and the CNTs film could easily peeled-off from the substrate 2.3 Film fabrication method Experimental 2.1 Materials CVD synthesized pristine MWCNTs with the length of 10e15 mm and diameters of less than 10 nm were purchased (Shenzhen Co., China) Commercial round glasses with 15e19 mm in diameter were also used as substrate Purification and functionalization of the MWCNTs were carried out into a mixture of concentrated sulphuric and nitric acids (95% H2SO4, 65% HNO3; Four kinds of acid-treated MWCNTs which were refluxed for different times of 15, 30, 60 and 120 were used to make a suspension One milligram of such MWCNTs was dispersed into ml of pure ethanol by an ultrasonic bath (42 kHz, 100 W) for 15 The suspension then centrifuged for a few minutes to remove the possible existing particles and large bundles in the suspension The spin coating method was employed as a fast technique to prepare the films A final speed of 4900 rpm for 25 s was chosen The films then were annealed at 285 C for 24 h Fig Typical SEM images of films prepared using different refluxed times of MWCNTs and a typical 135 nm thin film on glass substrate M Farbod et al / Journal of Science: Advanced Materials and Devices (2017) 99e104 101 2.4 Characterization A field emission scanning electron microscope (FESEM: MIRA, TESCAN- Czech Republic) was used to study the surface morphology and thickness of MWCNTs films To measure the film's thickness, the glass substrates were tilted for a better view for imaging Sheet resistance of the films was measured by 4-point probe technique at room temperature The optical transmittance was recorded be means of a UVeVis spectrophotometer (Cintra 101, GBC e Australia) The roughness of the films was measured using an atomic force microscope (SPM: DME, 95-50E- Denmark) Results and discussion The SEM images of films prepared using 30, 60 and 120 refluxing time and a typical 135 nm thickness on glass substrate are shown in Fig As can be observed, by increasing the refluxing time, the lengths of CNTs become smaller During the functionalization, the carbon nanotubes break apart and their length decreased which was depending on the refluxing time It was found from a “computer software measurement” that the initial length of CNTs (10e15 mm) is reduced to 1200, 205, 168 nm after 30, 60 and 120 refluxing time respectively A circuit set up to light an LED with a battery, using 30 refluxed CNTs films with various thicknesses in series with a battery and LED is presented in Fig The optical transmittance of the films prepared using different refluxed CNTs with different thicknesses is illustrated in Fig As can be observed, the transmittance increases with decreasing the thickness for all the films In order to describe the figures' differences, the transmittances of the films at 550 nm were plotted versus their thickness and are shown in Fig It is clear seen from the figure that by increasing the films' thicknesses, the transmittance decreases The decrease rate is nearly the same for the films prepared using CNTs refluxed for higher than 30 A faster decrease, however, is observed for films prepared by 15 refluxed CNTs For all thicknesses, the transmittance is almost highest for 30 refluxed CNTs films Indeed, the optical transmittance of the 30 refluxed films was 2.6% and 6.6% higher than that of for 60 and 120 ones This means that besides the thickness, the length of CNTs can affect the transmittance of the films Due to the fact that the 15 refluxed CNTs films exhibited unsuitable behaviours, their data will not be presented here after Fig Optical transmittance of thin films prepared using different length functionalized MWCNTs and different thicknesses The higher refluxing time means the length of CNTs are shorter Fig Transparency of a typical film and a circuit set up to light an LED, using the film in series with a battery The sheet resistances of the films were calculated using IeV measurements Fig shows a typical IeV graph of different films with the same thickness and different CNTs lengths The sheet resistance deduced using such measurements are plotted versus their thickness and shown in Fig As can be seen from the figure, the sheet resistance reduces with increasing the thickness exponentially for the all samples The sheet resistance value of 30 reflux CNTs films showed 45% and 80% reduction compared to the 60 and 120 refluxed films, respectively Also the sheet 102 M Farbod et al / Journal of Science: Advanced Materials and Devices (2017) 99e104 Fig Transmittance versus thickness for the films prepared using different refluxed time CNTs Raman study of functionalized CNTs, it was found that after h refluxing, the ratio of ID/IG was kept constant This indicated that the difference in above mentioned properties is independent on the defect levels, but can be affected by the length of CNTs As already mentioned in Section 2.3, the films were annealed at 285 C after being spin-coated We found that the post heat treatment has a remarkable influence on the reduction of sheet resistance and the improvement of the films' optical transmittance By post heat treatment the sheet resistance was decreased at least one order of magnitude It seems that before annealing, the potential barrier between the CNTs junctions is high and the free carriers are impeded so the electrical conductivity is poor By post annealing the contacts and fusion between the CNTs is improved resulting in the reduction of the sheet resistance Fig shows the plot of the optical transmittance at 550 nm versus sheet resistance of the films prepared using CNTs with different lengths As it was expected, the films with higher transparency typically showed higher sheet resistance It is desirable to have films with high transparency and low sheet resistance From the Fig it is clear that the transparency of the films with the same sheet resistance depends on the length of CNTs and the films which were prepared using 30 refluxed CNTs show higher transparency It seems that the settlement of CNTs on the substrate during the film coating is arranged in a manner that Fig Typical IeV graph of films prepared using different length CNTs and the same thickness of about 240 nm The higher slope means lower resistance Fig Optical transmittance at 550 nm versus sheet resistance for the films prepared using different lengths CNTs Fig Sheet resistance versus thickness for films prepared using differently refluxed CNTs resistance shows no significant thickness dependence when the thickness is higher than 300 nm In addition, the sheet resistance of such film showed 45% and 80% reduction as well Although the samples are exposed to the difference in refluxing time, the difference in optical and electrical properties can not be connected to the different density of defects Based on our previous work [16] on Fig Average roughness versus the thickness for films prepared using differently refluxed CNTs M Farbod et al / Journal of Science: Advanced Materials and Devices (2017) 99e104 103 Fig A percolation path from A to B can be achieved by a thinner layer but longer length carbon nanotubes shorter CNTs allow for less light transmission Such observation is in agreement with the film roughness measurements Fig shows the average roughness versus the film's thickness which were measured using AFM images One can observe that by increasing the films' thickness their roughness decrease As it is clear from the figure, the roughness for the films which were prepared using 30 refluxed CNTs is lower than that of the others Further analysis of CNTs films was carried out by studying the percolation behaviour of the network By drawing a vertical line at a certain thickness in Fig one can observe that at the same thickness, the sheet resistance is lesser for the films which were prepared using 30 refluxed CNTs As mentioned above, by increasing the refluxing time from 30 to 120 the initial length of CNTs reduced to 1200, 205 and 168 nm Therefore, for the films with the same thicknesses, the sheet resistance is higher when the CNTs lengths are shorter According to Hecht et al [19] for a CNTs network two kinds of resistance sources are existed One is the resistance along the MWCNT itself (RNT) and the other is due to the CNTseCNTs junction (Rjnt) When RNT >> Rjnt, the sheet resistance or conductivity should be independent of the length of CNTs but if Rjnt >> RNT the sheet resistance of the network should be dependent on the number of CNTseCNTs junctions So, using shorter CNTs in film fabrication means that the number of CNTseCNTs junctions increased, which leads the sheet resistance enhancement It has been reported that for a network of Ag nanowires with a given length (L), the critical number of nanowires (Nc) required for percolation is given by the below equation [18]: Nc L2 ¼ 5:71 If we apply such an equation to CNTs thin films, one can conclude that an electrical percolation path can be achieved by less number of CNTs but with longer length If more CNTs with shorter length are used, more connections will be formed resulting in higher sheet resistance and lower transmittance Fig shows schematically that a percolation path can be achieved by a thinner layer but longer carbon nanotubes sop Based on our data, sop of the films and the sDC:B ratio were calculated using the Eqs (1) and (2) respectively In order to calculate the films percolative figure of merite (P) and n the percolation exponent, the Eq (3) was employed They were deduced using the logelog plot of the (TÀ0.5 À1) versus Rs The calculated parameters are listed in Table The results show that the percolative figure of merit is higher for the films with longer CNTs This means utilizing 30 refluxed CNTs for transparent thin film fabrication can give better results So we suggest being careful in using refluxed CNTs for fabrication of transparent conducting films Table Values of sDC.B/sOp, P and n found from fitting in curves of (TÀ0.5 À1) versus Rs for different films Q Refluxed time (min) sDC.B/sop sop(S/m) n 30 60 120 37 30 29 7427.1 10079.6 9018.6 0.9 1.0 1.2 4.5 3.3 2.5 Conclusion Carbon nanotube thin films with different thickness and CNTs lengths were fabricated Their sheet resistance and optical transmittance were investigated It was observed that the sheet resistance and optical transmittance of MWCNTs films were extremely dependent on the CNTs average length The optical transmittance of the films prepared using 30 reflux CNTs was 2.6% and 6.6% higher than that of for 60 and 120 refluxed films with the same thickness, respectively Also the sheet resistance of 30 reflux CNTs films showed 45% and 80% reduction compared to the 60 and 120 refluxed films, respectively Such films showed the least roughness among the other samples The percolative figure of merit was 4.5, 3.3 and 2.5 for the films prepared using 30, 60 and 120 refluxed CNTs respectively It proposes a possibility to achieve percolation conducting paths by longer length carbon nanotubes at lower thickness and therefore higher transmittance Acknowledgments The authors acknowledge Shahid-Chamran University of Ahvaz for the financial support of this work and also Khuzestan Water and Power Authority for their helps to use their laboratory References [1] H.E Unalan, G Fanchini, A Kanwal, A.D Pasquier, C Manish, Design criteria for transparent single-wall carbon nanotube thin film transistors, Nano Lett (2006) 667e682 [2] T Minami, Present status of transparent conducting oxide thin-film development for Indium-Tin-Oxide (ITO) substitutes, Thin Solid Films 516 (2008) 5822e5828 [3] X.W Sun, H.C Huang, H.S Kwok, On the initial growth of indium tin oxide on glass, Appl Phys Lett 68 (1996) 2663e2665 [4] V.H.R Souza, S Husmann, E.G.C Neiva, F.S Lisboa, L.C Lopes, R.V Salvatierra, A.J.G Zarbin, Flexible, transparent and thin films of carbon nanomaterials as electrodes for electrochemical applications, Electrochim Acta 197 (2016) 200e220 [5] P Liu, Y.F Tan, D.C.M Hu, D Jewell, H.M Duong, Multi-property enhancement of aligned carbon nanotube thin films from floating catalyst method, Mater Des 108 (2016) 754e760 €der, P H€ [6] T Abendroth, H Althues, G Ma artel, S Kaskel, E Beyer, Selective absorption of carbon nanotube thin films for solar energy applications, Sol Energy Mater Sol C 143 (2015) 553e556 [7] S Mallakpour, E Khadem, Carbon nanotubeemetal oxide nanocomposites: fabrication, properties and applications, Chem Eng J 302 (2016) 344e367 [8] X Zheng, J.S Kim, C.W Park, Multiwall carbon nanotube/copper porous coating for heat transfer application, Surf Eng 31 (2015) 723e732 [9] H Hanaei, M.K Assadi, R Saidur, Highly efficient antireflective and selfcleaning coatings that incorporate carbon nanotubes (CNTs) into solar cells: a review, Renew Sustain Energy Rev 59 (2016) 620e635 [10] M Kaempgen, S Roth, Transparent and flexible carbon nanotube/polyaniline pH sensors, J Electroanal Chem 586 (2006) 72e76 [11] M Dressel, G Grüner, Electrodynamics of Solids: Optical Properties of Electrons in Matter, Cambridge University press, Cambridge, 2002 [12] B Dan, G.C Irvin, M Pasquali, Continuous and scalable fabrication of transparent conducting carbon nanotube films, ACS Nano (2009) 835e843 [13] L Hu, D.S Hecht, G Gruner, Percolation in transparent and conducting carbon nanotube networks, Nano Lett (2004) 2513e2517 [14] D.S Stauffer, A Aharony, Introduction to Percolation Theory, Taylor, Francis, London, 1994 [15] S De, P.J King, P.E Lyons, U Khan, J.N Coleman, Size effects and the problem with percolation in nanostructured transparent conductors, ACS Nano (2010) 7064e7072 [16] M Farbod, S Khajehpour-Tadavani, A Kiasat, Surface oxidation and effect of electric field on dispersion and colloids stability of multiwalled carbon nanotubes, Colloids Surf A Physicochem Eng Asp 384 (2011) 685e690 104 M Farbod et al / Journal of Science: Advanced Materials and Devices (2017) 99e104 [17] A Gomathi, S.R.C Vivekchand, A Govindaraj, C.N.R Rao, Chemically bonded ceramic oxide coatings on carbon nanotubes and inorganic nanowires, Adv Mater 17 (2005) 2757e2761 [18] T.H Joong, Y.K Sun, S.W Jong, L Geon-Woong, Transparent, conductive and super hydrophobic films from stabilized carbon nanotube/silane sol mixture solution, Adv Mater 20 (2008) 3724e3727 [19] D Hecht, L Hu, G Grüner, Conductivity scaling with bundle length and diameter in single walled carbon nanotube networks, Appl Phys Lett 89 (2006) 113112e113113 ... CNTs for transparent thin film fabrication can give better results So we suggest being careful in using refluxed CNTs for fabrication of transparent conducting films Table Values of sDC.B/sOp, P and. .. for 30 refluxed CNTs films Indeed, the optical transmittance of the 30 refluxed films was 2.6% and 6.6% higher than that of for 60 and 120 ones This means that besides the thickness, the length of. .. status of transparent conducting oxide thin- film development for Indium-Tin-Oxide (ITO) substitutes, Thin Solid Films 516 (2008) 5822e5828 [3] X.W Sun, H.C Huang, H.S Kwok, On the initial growth of