Incorporating Carbon Nanotubes in SolâGel Synthesized Indium Tin

Subscriber access provided by SETON HALL UNIV

Article

Incorporating Carbon Nanotubes in Sol-Gel Synthesized Indium Tin Oxide Transparent Conductive Films Mohammad Reza Golobostanfard, Saeed Mohammadi, Hossein Abdizadeh, and Mohammad Amin Baghchesara Langmuir, Just Accepted Manuscript • Publication Date (Web): 09 Sep 2014 Downloaded from http://pubs.acs.org on September 13, 2014

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

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

Page 1 of 26

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

Langmuir

Incorporating Carbon Nanotubes in Sol-Gel Synthesized Indium Tin Oxide Transparent Conductive Films Mohammad Reza Golobostanfard1,*, Saeed Mohammadi1, Hossein Abdizadeh1,2, Mohammad Amin Baghchesara3 1

School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box: 14395-553, Tehran, Iran 2

3

Center of Excellence for High Performance Materials, University of Tehran, Tehran, Iran

Department of Metallurgy and Materials Engineering, Masjed Soleyman Branch, Islamic Azad University, Masjed Soleyman, Iran

*

Address correspondence to: [email protected]

ACS Paragon Plus Environment

1

Langmuir

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

Page 2 of 26

KEYWORDS. Indium tin oxide, Sol-gel, Dip coating, Carbon nanotube, Optoelectrical properties. ABSTRACT. Carbon nanotubes (CNT) are incorporated in indium tin oxide (ITO) films through a sol-gel dip coating method. CNT incorporation has no effect on lattice parameter and preferential orientation of ITO matrix. Raman spectrum confirms the presence of CNTs in ITO matrix. The surface roughness is decreased by CNT addition. Moreover, the optical transmittance of the ITO-CNT films is more than that of ITO films in IR region. The higher transmittance region of the ITO-CNT films compared to that of ITO films is extended to visible region with increasing the thickness of the films. In terms of electrical resistance, the ITO film shows slightly lower sheet resistance compared to ITO-CNT film. However, the sheet resistance of ITO-CNT film is further reduced by thickness and reaches to the minimum amount in 15-layer deposited sample, which is lower than that of ITO samples. It seems that the CNT incorporation in ITO matrix can dramatically reduce the microcrack formation, especially in higher thicknesses. Furthermore, the sensitivity of ITO films to thermal cycling is dramatically enhanced with CNT addition. In addition, the figure of merit of ITO film is strongly improved in Vis-NIR region with CNT incorporation.


2

Page 3 of 26

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

Langmuir

INTRODUCTION The emerging application of transparent conductive films (TCF) in different devices such as liquid crystal displays, touch panels, e-papers, thin film solar cells, and flexible displays causes tremendous attention on TCF research.1-4 Among TCFs, tin doped indium oxide (ITO) is widely used due to its high transparency, low electrical resistivity, and proper chemical stability.5-7 However, expensiveness and brittleness are the major drawbacks of ITO films.8,9 The expensiveness of these films can be reduced by reducing the indium usage and increasing the deposition efficiency. The brittleness of the films, which results in microcracks formation under low strain and hence causes rapid decrease in conductivity (σ), can be improved by introducing a second tough conductive phase in the ITO matrix. ITO films can be fabricated by magnetron sputtering,10 RF sputtering,11 electron beam evaporation,12 spray pyrolysis,13 chemical vapor deposition,14 molecular beam epitaxy,15 screen printing,16 pulsed laser deposition,17 and sol-gel method.18,19 The sol-gel method with the advantages of inexpensiveness, high yield, ability to form composite structure, and non-vacuum apparatus is widely used for ITO preparation in recent years.20 Other TCF materials have been introduced in recent years including carbon nanotubes (CNT),21 graphene,22 conducting polymers,23 and metal grids.24 Although the graphene sheet has very low resistivity and very high mobility (µ), it should be fully reduced from graphene oxide to show its appropriate electrical properties.25 The problem for conducting polymers is their electrical instability of doped states when exposed to chemical or thermal stresses, which can rapidly decrease the σ.26 The major bottlenecks for metal grids are their surface sensitivity to oxidation and high photon scattering.27 Among them, CNTs with outstanding chemical, optical, electrical, and mechanical properties can be a promising candidate for TCFs. However, the


3

Langmuir

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

Page 4 of 26

relatively low visible transparency of CNT films, low density of CNT films, and charge transport barrier at the CNT/CNT junctions limit their application as TCF.28 Although CNTs have much lower carrier concentration than ITO films (~1017/cm3 for CNT and 1020/cm3 for ITO), they show substantially higher µ.29 The σ of a film is directly related to the number of charge carriers and their µ. Since most of the researches on increasing the σ of ITO films are based on increasing the number of charge carriers by introducing different dopant atoms,30-32 improving the σ by enhancing the µ with addition of CNTs in ITO matrix would be beneficial. The figure of merit for TCF film can be described by σ/α ratio (α is absorption coefficient). Since increasing the number of charge carriers can strongly increase the probability of scattering, improving the charge mobility could be more efficient in σ enhancement. Most studies on CNT composites with another TCF are focused on conductive polymers such as PEDOT:PSS.33-35 Making a composite based on ITO and CNT is rarely reported in which the CNT film was deposited on substrate and then the ITO was deposited on CNT layer, resulting in a multilayer composite.36 However, incorporation of CNTs in ITO thin film has never been reported. In this paper, CNTs are incorporated in sol-gel synthesized ITO thin films, which can potentially introduce ballistic transport route for charge carriers as well as improve the mechanical stability of ITO brittle films.

EXPERIMENTAL SECTION Material. Anhydrous indium chloride (InCl3, Alfa Aesar, 99.99%), tin chloride (SnCl2.2H2O, Merck, 98%), acetylacetone (AcAc, Merck, 99%), absolute ethanol (EtOH, Merck, 99%), sulfuric acid (H2SO4, Merck, 98%), nitric acid (HNO3, Merck, 68%), and multi-walled CNT (MWCNT, outer diameter 40-60 nm, 97% purity, 130–160 m2/g, Shenzen Nanotech.) were used


4

Page 5 of 26

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

Langmuir

as received without further purification. Deionized water (DIW, 18.2 MΩ) was used in all experiments. CNT functionalization. For CNT functionalization, MWCNTs were subjected to ultrasonication in concentrated HNO3 : H2SO4 (3 : 1 vol.%) for 10 min. Then, the mixture was refluxed at 80 °C for 1 h. After dilution, the mixtures were washed with DIW and centrifuged several times to ensure removal of anions. The product was dried at 60 °C and kept dried until use. The FTIR spectrum of functionalized CNTs is shown in Figure S1 (supplementary file). ITO/CNT sol preparation. ITO sol preparation was previously reported.37 Briefly, InCl3 was dissolved in AcAc (0.2 M) and the resultant solution was refluxed at 85 °C for 1 h. Then, the required amount of SnCl2 was dissolved in EtOH. The InCl3 solution was added to the SnCl2 solution and refluxed at 85 °C for another 1 h. For ITO-CNT sol, 0.28 wt% MWCNT was added to SnCl2 solution and sonicated for 30 min. The other steps were similar to ITO sol preparation. All the sols were aged for 24 h before deposition. The Sn/(In+Sn) ratio was considered as 6% in atomic ratio. Various amounts of MWCNT of 0.14, 0.28, and 0.56 wt% (of calcined ITO powder) were considered to investigate the effect of CNT content on optoelectrical properties of the composites. ITO/CNT film deposition. The prepared sols were deposited by means of dip coating method on soda lime glass substrates which were precleaned ultrasonically in DIW and EtOH. The withdrawal speed was 120 mm/min. Then, the films were dried at 350 °C for 10 min. The dip coating and drying processes were repeated several times to reach the desired thickness. Finally, the films were annealed at 600 °C for 1 h. Characterization. The thermal analyses of the samples were performed on DTA/TG (SII Nanotechnology, A6300) in the range of 25-900 °C at 10 °C/min under air atmosphere. X-ray


5

Langmuir

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

Page 6 of 26

diffraction (XRD), Philips X-pert pro PW1730, Cu-kα, was used to study the phase structure of the films. Field emission scanning electron microscopy (FESEM), Hitachi S4160, was utilized to investigate the morphology and thickness of the films. High resolution transmission electron microscope (HRTEM), Philips CM30, 300 kV, equipped with selected area electron diffraction pattern (SAED) was employed to study the CNT debundling and interfaces. Atomic force microscopy (AFM), Dual Scope DS 95-200/50, was performed in contact mode to achieve surface roughness of the films. Raman spectra were recorded using Bruker (SETERRA, spectral resolution < 3 cm-1) micro Raman spectrometer equipped with the confocal microscope. The excitation wavelength was 785 nm line of argon laser operating at the power of 25 mW. The infrared spectra were recorded using Fourier-transformed infrared (FTIR) spectrophotometer, Bruker TENSOR27, in transmittance mode at 400–4000 cm-1 with KBr as blank. The transmission spectra and optical band gap in the UV–Vis range were determined on a UV–Vis Spectrophotometer PG instrument, model T80+. The resistivity of the films was measured by four-point probe method Keithly model 196 Sys-DMM2 at room temperature. The Hall mobility measurements were performed using van der Pauw method at room temperature.

RESULTS AND DISCUSSION The DTA/TG curves of ITO-CNT powder are shown in Figure 1. The TG curve can be divided into five weight loss regions: less than 200 °C, 200-270 °C, 270-380 °C, 380-510 °C, and more than 510 °C. The first region of weight loss corresponds to loss of adsorbed water. The second region in TG curve is associated with an endothermic peak and an exothermic peak in DTA curve. The endothermic peak can be due to the dissociation and removal of organic residues and the exothermic peak can be related to the decomposition of indium and tin chlorides.16 Removal of organic residues continues in the third region until the exothermic peak occurs at around 440


6

Page 7 of 26

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

Langmuir

°C in fourth region, which can be related to the ITO crystallization temperature. The small exothermic peak at around 670 °C in fifth region may be related to loss of remaining chlorides.38 The large endothermic peak at around 740 °C can be due to the formation of tin oxide phase. The weight loss of the combustion of CNTs could not be truly detected due to the very low amount of CNTs and superposition with ITO weight loss in fifth region.

Figure 1. DTA/TG curves of ITO-CNT sample. The XRD patterns of ITO and ITO-CNT samples are shown in Figure 2. The In2O3 with cubic bixbyite structure is formed in both samples according to JCPDS 06-0416. The incorporation of CNTs in ITO matrix has no effect on diffraction peaks. Also, the lattice parameter of cubic structure and preferred orientation along (222) plane do not dramatically change with incorporation of CNTs.


7

Langmuir

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

Page 8 of 26

Figure 2. XRD patterns of 10-layer ITO and ITO-CNT films after calcination at 600 °C. The Raman spectrum of ITO-CNT film is demonstrated in Figure 3. Cubic In2O3 belongs to the , space group. The structure contains two kinds of cations: 8 In3+ with side symmetry of S6 and 24 In3+ with point symmetry of C2. The 48 oxygen atoms in the body centered cell are on general positions with C1 site symmetry. For such a structure, the following vibration modes are predicted: 4Ag+4Eg+14Tg+5Au+5Eu+16Tu. The vibrations with symmetry Ag, Eg, and Tg are Raman active and infrared inactive, while the Tu vibrations are infrared active and Raman inactive. The Au and Eu vibrations are inactive in both infrared and Raman measurements.39 The peaks at 110, 131, 312, 368, 462, 555, and 611 cm-1 belong to cubic In2O3. The small peaks at 149, 176, 216, and 240 cm-1 belong to the Sn-O vibrations in the ITO structure.40 The peaks at 1291 and 1578 cm-1 are D- and G-band of MWCNT.41 Raman spectrum of MWCNT after acid treatment is shown in Figure S2 (supplementary file). The I2D/ID and IG/ID of 0.21 and 4.34 could be calculated for these nanotubes, respectively, which show high purity of CNTs. The ratio of IG/ID is slightly decreased after incorporation in ITO matrix due to the destruction of some nanotubes after high temperature annealing.


8

Page 9 of 26

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

Langmuir

Figure 3. Raman spectrum of ITO-MWCNT sample. Figure 4 represents FESEM micrographs of ITO and ITO-CNT films before and after thermal cycling treatment. The ITO and ITO-CNT films consist of particles with average size of about 30 and 15 nm, respectively. The microcracks are formed after 50 thermal cycle treatment of the films at 300 °C in both samples. However, the size and amount of microcracks in ITO film is more than that of ITO-CNT sample. The HRTEM micrograph of MWCNTs after impregnation in ITO sol is revealed in Figure S3. Debundling of CNTs occurs after acid treatment and functionalization.


9

Langmuir

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

Page 10 of 26

Figure 4. FESEM micrographs of (a) ITO, (b) ITO-CNT, (c) ITO subjected to 50 thermal cycles, and (c) ITO-CNT subjected to 50 thermal cycles. The AFM micrographs of ITO and ITO-CNT films are indicated in Figure 5. The root mean square roughness and average roughness parameters can be calculated to be 14.5 and 16.6 nm for ITO film, respectively. However, these parameters are decreased to 6.5 and 8.8 in ITO-CNT film. It seems that the CNTs can reduce the porosity of the film during gelation, decrease the surface roughness, and enhance the film continuity.


10

Page 11 of 26

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

Langmuir

Figure 5. AFM images of 10-layer (a) ITO and (b) ITO-CNT films. The UV-Vis-NIR spectra of ITO and ITO-CNT films with 10-layer deposition are compared in Figure 6a. The films show transmittance of approximately 92% and 90% at wavelength of 550 nm for ITO and ITO-CNT, respectively. However, the transmittance of the films in NIR region is slightly changed and the transmittance of ITO-CNT is higher than that of ITO film in this region. The Tauc plots of the ITO and ITO-CNT films with 10-layer deposition are shown in inset Figure 6a. It could be seen that the band gap of ITO film (3.95 eV) is red shifted to lower energies (3.88 eV) with incorporation of CNTs. The band gap of ITO film is blue shifted to higher energies compared to the band gap of undoped In2O3 (~3.75 eV) due to the Burstein-Moss effect, which causes band gap widening in heavily doped semiconductors.42 However, incorporation of conductive CNTs in ITO matrix may result in charge distribution and hence band gap narrowing. The UV-Vis-NIR spectra of ITO and ITO-CNT films after 15-layer deposition are also illustrated in Figure 6b. With increasing the film thickness, the transmittance of ITO-CNT film is slightly higher than that of ITO film in wavelengths higher than 550 nm. The changes in transmittance of ITO films with increasing the thickness (from 93% to 89% at wavelength of 700 nm) is more than those of ITO-CNT films (from 93% to 92% at wavelength of 700 nm), which is mainly due to the positive role of CNTs on reducing the microcracks formation during drying. These microcracks can dramatically affect the optical transmission and act as scattering centers.4,43


11

Langmuir

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

Page 12 of 26

Figure 6. UV-Vis spectra of ITO and ITO-CNT films (a) 10-layer deposition and (b) 15-layer deposition. Insets show the Tauc plots of ITO and ITO-CNT samples for each set. The sheet resistances of the ITO and ITO-CNT films as a function of deposition layer are shown in Figure 7a. The sheet resistance of ITO films is slightly lower than ITO-CNT films. However, the sheet resistance of ITO-CNT films intersects that of ITO films in higher thicknesses and reaches the minimum value of 41.9 Ω/sq. The crystallinity and grain size are enhanced with increasing deposition layer in ITO film, which improve mean free path of charge carriers and reduce electron scattering, and hence decrease sheet resistance. However, the oxygen vacancies are strongly decreased with further increase in deposition layer due to the increase in drying times (the films were dried at 350 °C after each deposition), which increases sheet resistance.44 On the other hand, CNTs can act as transport channels in ITO-CNT films. As a matter of fact, the electrons in ITO are injected to CNTs and transported through nanotube one dimensional structure as schematically shown in Figure 8. If the content of CNTs is high enough


12

Page 13 of 26

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

Langmuir

to provide connected bridges, the charge carriers can be transported through the CNTs with only low resistances at the junctions. Nevertheless, high amount of CNTs dramatically reduces the optical transmittance. Thus, separated bridges of CNTs are formed in 0.28 wt% CNT used in this study, which causes dead ends in transport routes of charge carriers (the process of charge injection from CNT to ITO matrix is not energetically favorable).45 In lower film thickness, the charge carriers prefer to transport through ITO matrix. In higher film thickness, the number of grain boundaries, interlayer regions, and the most importantly microcracks are raised. Therefore, the charge carriers prefer to transport through CNTs (Figure 8). The CNT incorporation can also reduce the microcrack formation in ITO matrix. This can be seen from Figure 7b. The sheet resistances of ITO and ITO-CNT films with 15-layer deposition as a function of thermal heating cycle (heating to 350 °C and cooling to room temperature) are shown in this figure. The ITO sheet resistance increases with increasing heating cycles due to the elimination of oxygen vacancies in ITO matrix, diffusion of Na atoms from substrate, and formation of microcracks.46,47 Incorporation of CNTs in ITO matrix not only decreases the microcracks formation, but also reduces the elimination of oxygen vacancies due to its strong reduction ability. The sheet resistance of the 10-layer ITO and ITO-CNT samples after heat treatment at 500, 600, and 700 °C is shown in Figure S4 (supplementary file). It could be seen that the sheet resistances of these films entirely depend on the heat treatment temperature, especially in case of ITO-CNT film. CNTs burn out after heat treatment at 700 °C and affect the charge conductivity. It should be mentioned that the ITO-CNT has the sheet resistance of about 760 Ω/sq before calcination. However, this value for ITO film before calcination is 4780 Ω/sq. This shows the beneficial role of CNTs in charge transport without the assistance of ITO matrix.


13

Langmuir

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

Page 14 of 26

The sheet resistance of the 10-, 12-, and 15-layer deposited ITO-CNT films as a function of CNT content is revealed in Figure 7c. The sheet resistance is decreased with CNT content in 10and 15-layer film thicknesses at the expense of optical transmittance. The sheet resistance remains unchanged in 12-layer thickness film. The reason for improving the sheet resistance in higher CNT content is formation of connected bridges in these samples. The sheet resistance is lower than bare ITO film only in 15-layer samples as expected. The Hall mobility was performed on 15-layer ITO and ITO-CNT samples. The carrier concentration and charge mobility for ITO film are 3.1±0.6 × 1020 cm-3 and 5.02±1.2 cm2/V.s, respectively. These values for ITO-CNT sample are 3.2±0.7 × 1020 cm-3 and 7.74±1.0 cm2/V.s, respectively. Therefore, the charge mobility is enhanced after incorporation of CNTs in ITO matrix, especially in higher thicknesses.


14

Page 15 of 26

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

Langmuir

Figure 7. Sheet resistance of the ITO and ITO-CNT films as a function of (a) number of deposition layers, (b) number of thermal cycling, and (c) CNT content in different thicknesses.


15

Langmuir

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

Page 16 of 26

Figure 8. Schematic representation of charge injection from ITO matrix to CNT and charge transfer in ITO-CNT samples with two different thicknesses. In lower thickness, the charge carriers are transported through ITO matrix. In higher thickness, the charge carriers prefer to transport through CNT bridges due to the existence of interface regions and microcracks. The thickness (t), sheet resistance (Rs), resistivity (ρ), transmittance (T), and figure of merit (φH) values of ITO and ITO-CNT films are tabulated in table 1. The figure of merits of ITO-CNT films, which can be calculated from φH = T10/Rs, are entirely enhanced for both visible and NIR regions with CNT addition in high thicknesses. This improvement is more tangible in NIR region. To achieve a TCO of given sheet resistance, it is desirable to increase the mobility and decrease the carrier concentration in order to reduce the optical absorption. The figure of merit can be determined by:

.

4

(1)


16

Page 17 of 26

where σ is conductivity (S/cm), ε0 is dielectric constant in vacuum, ε∞ is dielectric constant, c (m/s) is speed of light, τ (s) is average time between collisions, and λ (nm) is wavelength.48 The only fundamental material property that most strongly controls the figure of merit is τ. Since τ = µ.me*/e (µ is mobility, me* is electron effective mass, and e fundamental charge), the mobility can play a major role in figure of merit improvement. Thus, the CNTs with high mobility efficiently act as transport route in ITO matrix in higher thicknesses and result in appreciable figure of merit, which is comparable with other reported values.49 Table 1. Number of layers (d), thickness (t), sheet resistance (Rs), resistivity (ρ), transmittance (T) at 550 and 700 nm, and figure of merit (φH) at 550 and 700 nm of ITO and ITO-CNT films. Samples

d

t

Rs

ρ

(nm)

(Ω/sq)

(Ω.cm)

ITO

× 10-3

ITO-CNT

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

Langmuir

T at

T at

550 nm

700 nm

(%)

(%)

φH at

φH at

550 nm

700 nm

(Ω-1)

(Ω-1)

× 10-2

× 10-2

5

340

91.32

3.10

93.3

92.7

0.55

0.51

8

410

78.82

3.23

93.0

92.4

0.61

0.58

10

480

67.56

3.24

92.3

92.1

0.66

0.65

12

520

52.52

2.73

91.4

89.3

0.77

0.61

15

580

69.21

4.01

89.9

89.1

0.50

0.46

5

350

127.57

4.47

91.5

92.3

0.32

0.35

8

430

92.21

3.97

91.1

92.2

0.43

0.48

10

490

81.14

3.98

90.2

92.1

0.44

0.54

12

535

54.29

2.90

90.0

92.1

0.64

0.81

15

600

41.93

2.52

89.9

92.1

0.82

1.05


17

Langmuir

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

Page 18 of 26

CONCLUSIONS The ITO-CNT films were successfully synthesized by sol-gel dip coating method. The incorporation of CNTs in ITO matrix has no effect on crystalline phases, lattice constant, and preferential orientation. The surface roughness of ITO film is enhanced by CNT addition. The optical transmittance of ITO-CNT film is slightly lower than that of ITO film in visible region. However, the transmittance of ITO-CNT crosses the transmittance of ITO film in 700 nm and is higher than that of ITO in NIR region. The cross point of transmittance curves of ITO and ITOCNT films shifts towards lower wavelengths with increasing the thickness due to the formation of microcracks in higher thicknesses in bare ITO film, which strongly scatter the incident light. The sheet resistance of ITO film is slightly lower than that of ITO-CNT film in low thicknesses. However, the sheet resistance of ITO-CNT film is decreased in higher thicknesses and reaches to the lower values than that of ITO film. It seems that the CNTs can act as bridges in interlayer regions and microcracks in higher thicknesses, which assist the charge transfer process. On the other hand, the charge carriers prefer to transfer through ITO matrix without injection to CNT tunnels in lower thicknesses. Moreover, CNTs not only reduce the sensitivity of ITO films to thermal cycle, but also dramatically improve the figure of merit of ITO films.

ASSOCIATED CONTENT Supporting Information. The FTIR and Raman spectra of MWCNTs after acid treatment, TEM image of functionalized CNT after impregnation in ITO sol, and sheet resistance curves of ITO and ITO-CNT films as a function of heat treatment temperature. This material is available free of charge via the Internet at http://pubs.acs.org.


18

Page 19 of 26

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

Langmuir

AUTHOR INFORMATION Corresponding Author * Tel.: +98 9122300382; Fax: +98 21 88006076. E-mail address: [email protected] (M.R. Golobostanfard). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript and contributed equally. ACKNOWLEDGMENT We wish to thank The Iran National Science Foundation for supporting this research. The Iran Nanotechnology Initiative Council is gratefully acknowledged for partially supporting this research. The authors also thank MaharFan Abzar Co. for AFM images. ABBREVIATIONS ITO, indium tin oxide; CNT, carbon nanotube; MWCNT, multi-walled carbon nanotube. REFERENCES 1.

Minami, T. Transparent Conducting Oxide Semiconductors for Transparent Electrodes. Semicond. Sci. Technol. 2005, 20, 35–44.

2.

Hosono, H.; Ohta, H.; Orita, M.; Ueda, K.; Hirano, M. Frontier of Transparent Conductive Oxide Thin Films. Vacuum 2002, 66, 419–425.

3.

Kumar, A.; Zhou, C. The Race To Replace Tin-Doped Indium Oxide: Which Material Will Win? ACS Nano 2010, 4, 11–14.


19

Langmuir

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

4.

Page 20 of 26

Hecht, D. S.; Hu, L.; Irvin, G. Emerging Transparent Electrodes Based on Thin Films of Carbon Nanotubes, Graphene, and Metallic Nanostructures. Adv. Mater. 2011, 23, 1482– 1513.

5.

Minami, T. Present Status of Transparent Conducting Oxide Thin-Film Development for Indium-Tin-Oxide (ITO) Substitutes. Thin Solid Films 2008, 516, 5822–5828.

6.

Hammad, T.M. Effect of Annealing on Electrical, Structural and Optical Properties of Sol– Gel ITO Thin Films. Phys. Status Solidi A 2009, 206, 2128–2132.

7.

Lewis, B. G.; Paine, D. C. Applications and Processing of Transparent Conducting Oxides. Mater. Res. Bull. 2000, 25, 22–27.

8.

Green, M. A. Estimates of Te and In Prices from Direct Mining of Known Ores, Prog. Photovolt: Res. Appl. 2009, 17, 347–359.

9.

Cairns, D. R.; Witte, R. P.; Sparacin, D. K.; Sachsman, S. M.; Paine, D. C.; Crawford, G. P.; Newton, R. R. Strain-Dependent Electrical Resistance of Tin-Doped Indium Oxide on Polymer Substrates. Appl. Phys. Lett. 2000, 76, 1425–1427.

10.

Latz, R.; Michael, K.; Scherer, M. High Conducting Large Area Indium Tin Oxide Electrodes for Displays Prepared by DC Magnetron Sputtering. Jpn. J. Appl. Phys. 1991, 30, 149–151.

11.

Lien, S. Y.; Nautiyal, A.; Lee, S. J. Optoelectronic Properties of Indium–Tin Oxide Films Deposited on Flexible and Transparent Poly(dimethylsiloxane) Substrate. Jpn. J. Appl. Phys. 2013, 52, 115801.


20

Page 21 of 26

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

Langmuir

12.

George, J.; Menon, C. S. Electrical and Optical Properties of Electron Beam Evaporated ITO Thin Films. Surf. Coat. Tech. 2000, 132, 45–48.

13.

Bisht, H.; Eun, H. T.; Mehrtens, A.; Aegerter, M. A. Comparison of Spray Pyrolyzed FTO, ATO and ITO Coatings for Flat and Bent Glass Substrates. Thin Solid Films 1999, 351, 109–114.

14.

Maruyama, T.; Fukui, K. Indium Tin Oxide Thin Films Prepared by Chemical Vapor Deposition. Thin Solid Films 1991, 203, 297–302.

15.

O'Dwyer, C.; Szachowicz, M.; Visimberga, G.; Lavayen, V.; Newcomb, S. B.; Sotomayor Torres, C. M. Bottom-up Growth of Fully Transparent Contact Layers of Indium Tin Oxide Nanowires for Light-Emitting Devices. Nat. Nanotechnol. 2009, 4, 239–244.

16.

Bühler, G.; Thölmann, D.; Feldmann, C. One-Pot Synthesis of Highly Conductive Indium Tin Oxide Nanocrystals. Adv. Mater. 2007, 19, 2224–2227.

17.

Ohta, H.; Orita, M.; Hirano, M.; Tanji, H.; Kawazoe, H.; Hosono, H. Highly Electrically Conductive Indium–Tin–Oxide Thin Films Epitaxially Grown on Yttria-Stabilized Zirconia (100) by Pulsed-Laser Deposition. Appl. Phys. Lett. 2000, 76, 2740–2742.

18.

Biswas, P. K.; De, A.; Pramanik, N. C.; Chakraborty, P. K.; Ortner, K.; Hock, V.; Korder, S. Effects of Tin on IR Reflectivity, Thermal Emissivity, Hall Mobility and Plasma Wavelength of Sol–Gel Indium Tin Oxide Films on Glass. Mater. Lett. 2003, 57, 2326– 2332.


21

Langmuir

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

19.

Page 22 of 26

Alam, M. J.; Cameron, D. C. Investigation of Annealing Effects on Sol–Gel Deposited Indium Tin Oxide Thin Films in Different Atmospheres. Thin Solid Films 2002, 420, 76– 82.

20.

Mohammadi, S.; Abdizadeh, H.; Golobostanfard, M. R. Opto-electronic Properties of Molybdenum Doped Indium Tin Oxide Nanostructured Thin Films Prepared via Sol–Gel Spin Coating. Ceram. Int. 2013, 39, 6953–6961.

21.

Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Transparent, Conductive Carbon Nanotube Films. Science 2004, 305, 1273–1276.

22.

Li, X.; Zhang, G.; Bai, X.; Sun, X.; Wang, X.; Wang, E.; Dai, H. Highly Conducting Graphene Sheets and Langmuir–Blodgett Films. Nat. Nanotechnol. 2008, 3, 538–542.

23.

Crispin, X.; Jakobsson, F. L. E.; Crispin, A.; Grim, P. C. M.; Andersson, P.; Volodin, A.; Van Haesendonck, C.; Van der Auweraer, M.; Salaneck, W. R.; Berggren, M. The Origin of the High Conductivity of Poly(3,4 ethylene dioxythiophene) – Poly (styrene sulfonate) (PEDOT−PSS) Plastic Electrodes. Chem. Mater. 2006, 18, 4354–4360.

24.

Wu, H.; Hu, L.; Rowell, M. W.; Kong, D.; Cha, J. J.; McDonough, J. R.; Zhu, J.; Yang, Y.; McGehee, M. D.; Cui, Y. Electrospun Metal Nanofiber Webs as High-Performance Transparent Electrode. Nano Lett. 2010, 10, 4242–4248.

25.

Wang, X.; Zhi, L.; Müllen, K. Transparent, Conductive Graphene Electrodes for DyeSensitized Solar Cells. Nano Lett. 2008, 8, 323–327.


22

Page 23 of 26

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

Langmuir

26.

Argun, A. A.; Cirpan, A.; Reynolds, J. R. The First Truly All-Polymer Electrochromic Devices. Adv. Mater. 2003, 15, 1338–1341.

27.

Ghosh, D. S.; Chen, T. L.; Pruneri, V. High Figure-of-Merit Ultrathin Metal Transparent Electrodes Incorporating a Conductive Grid. Appl. Phys. Lett. 2010, 96, 041109.

28.

Li, Z.; Kandel, H. R.; Dervishi, E.; Saini, V.; Xu, Y.; Biris, A. R.; Lupu, D.; Salamo, G. J.; Biris, A. S. Comparative Study on Different Carbon Nanotube Materials in Terms of Transparent Conductive Coatings. Langmuir 2008, 24, 2655–2662.

29.

Hu, L.; Hecht, D. S.; Grüner, G. Infrared Transparent Carbon Nanotube Thin Films. Appl. Phys. Lett. 2009, 94, 081103.

30.

Fortunato, E.; Ginley, D.; Hosono, H.; Paine, D. C. Transparent Conducting Oxides for Photovoltaics. MRS Bull. 2007, 32, 242–247.

31.

Hadj Tahar, R. B.; Ban, T.; Ohya, Y.; Takahashi, Y. Tin Doped Indium Oxide Thin Films: Electrical Properties. J. Appl. Phys. 1998, 83, 2631–2645.

32.

Calnana, S.; Tiwari, A. N. High Mobility Transparent Conducting Oxides for Thin Film Solar Cells. Thin Solid Films 2010, 518, 1839–1849.

33.

Li, J.; Liu, J. C.; Gao, C. J. On the Mechanism of Conductivity Enhancement in PEDOT/PSS Film Doped with Multi-Walled Carbon Nanotubes. J. Polym. Res. 2010, 17, 713–718.

34.

Antiohos, D.; Folkes, G.; Sherrell, P.; Ashraf, S.; Wallace, G. G.; Aitchison, P.; Harris, A. T.; Chen, J.; Minett, A. I. Compositional Effects of PEDOT-PSS/Single Walled Carbon


23

Langmuir

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

Page 24 of 26

Nanotube Films on Supercapacitor Device Performance. J. Mater. Chem. 2011, 21, 15987– 15994. 35.

Fan, B.; Mei, X.; Sun, K.; Ouyang, J. Conducting Polymer/Carbon Nanotube Composite as Counter Electrode of Dye-Sensitized Solar Cells. Appl. Phys. Lett. 2008, 93, 143103.

36.

Li, J.; Hu, L.; Liu, J.; Wang, L.; Marks, T. J.; Grüner, G. Indium Tin Oxide Modified Transparent Nanotube Thin Films as Effective Anodes for Flexible Organic Light-Emitting Diodes. Appl. Phys. Lett. 2008, 93, 083306.

37.

Mohammadi, S.; Abdizadeh, H.; Golobostanfard, M. R. Effect of Niobium Doping on Opto-Electronic Properties of Sol–Gel based Nanostructured Indium Tin Oxide Thin Films. Ceram. Int. 2013, 39, 4391–4398.

38.

Lia, S.; Qiao, X.; Chen, J.; Wang, H.; Jia, F.; Qiu, X. Effects of Temperature on Indium Tin Oxide Particles Synthesized by Co-Precipitation, J. Cryst. Growth 2006, 289, 151–156.

39.

Wang, C. Y.; Dai, Y.; Pezoldt, J.; Lu, B.; Kups, T.; Cimalla, V.; Ambacher, O. Phase Stabilization and Phonon Properties of Single Crystalline Rhombohedral Indium Oxide. Cryst. Growth Des. 2008, 8, 1257–1260.

40.

Berengue, O. M.; Rodrigues, A. D.; Dalmaschio, C. J.; Lanfredi, A. J. C.; Leite, E. R.; Chiquito, A. J. Structural Characterization of Indium Oxide Nanostructures: a Raman Analysis. J. Phys. D: Appl. Phys. 2010, 43, 045401.

41.

Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Raman Spectroscopy of Carbon Nanotubes. Phys. Rep. 2005, 409, 47–99.


24

Page 25 of 26

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

Langmuir

42.

Mryasov, O. N.; Freeman, A. J. Electronic Band Structure of Indium Tin Oxide and Criteria for Transparent Conducting Behavior. Phys. Rev. B 2001, 64, 233111.

43.

Park, J. M.; Wang, Z. J.; Kwon, D. J.; Gu, G. Y.; DeVries, K. L. Electrical Properties of Transparent CNT and ITO Coatings on PET Substrate Including Nano-Structural Aspects. Solid State Electron. 2013, 79, 147–151.

44.

Gross, M.; Winnacker, A.; Wellmann, P. J. Electrical, Optical and Morphological Properties of Nanoparticle Indium–Tin–Oxide Layers. Thin Solid Films 2007, 515, 8567– 8572.

45.

King, P. D. C.; Veal, T. D. Conductivity in Transparent Oxide Semiconductors. J. Phys. Condens. Matter 2011, 23, 334214.

46.

Hamasha, M. M.; Dhakal, T.; Alzoubi, K.; Albahri, S.; Qasaimeh, A.; Lu, S.; Westgate, C. R. Stability of ITO Thin Film on Flexible Substrate Under Thermal Aging and Thermal Cycling Conditions. J. Disp. Technol. 2012, 8, 385–390.

47.

Park, J.; Lee, A.; Yim, Y.; Han, E. Electrical and Thermal Properties of PEDOT:PSS Films Doped with Carbon Nanotubes. Synthetic Met. 2011, 161, 523–527.

48.

Luque, A.; Hegedus, S. Handbook of Photovoltaic Science and Engineering, 2nd Ed., John Wiley & Sons Ltd.: New York, 2011; pp 736–741.

49.

Chen, Z.; Li, W.; Li, R.; Zhang, Y.; Xu, G.; Cheng, H. Fabrication of Highly Transparent and Conductive Indium−Tin Oxide Thin Films with a High Figure of Merit via Solution Processing. Langmuir 2013, 29, 13836−13842.


25

Langmuir

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

39x27mm (300 x 300 DPI)


Page 26 of 26

Incorporating Carbon Nanotubes in SolâGel Synthesized Indium Tin

Recommend Documents

Incorporating Carbon Nanotubes in SolâGel Synthesized Indium Tin