Large-Area Highly Conductive Transparent Two ... - ACS Publications

Letter pubs.acs.org/JPCL

Large-Area Highly Conductive Transparent Two-Dimensional Ti2CTx Film Yajie Yang,† Sima Umrao,† Shen Lai,† and Sungjoo Lee*,†,‡ †

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University (SKKU), Suwon 440-746, Korea School of Electronics and Electric Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Korea

‡

S Supporting Information *

ABSTRACT: We report a simple and scalable method to fabricate homogeneous transparent conductive thin films (Ti2CTx, one of the MXene) by dip coating of an Al2O3 substrate in a colloidal solution of largearea Ti2CTx thin flakes. Scanning electron microscopy and atomic force microscopy images exhibit the wafer-scale homogeneous Ti2CTx thin film (∼5 nm) covering the whole substrate. The sheet resistance is as low as 70 Ω/sq at 86% transmittance, which corresponds to the high figure of merit (FOM) of 40.7. Furthermore, the thickness of the film is tuned by a SF6+Ar plasma treatment, which etches Ti2CTx film layer by layer and removes the top oxidized layer without affecting the bottom layer of the Ti2CTx flake. The resistivity of plasma-treated Ti2CTx film is further decreased to 63 Ω/sq with an improved transmittance of 89% and FOM of 51.3, demonstrating the promise of Ti2CTx for future transparent conductive electrode application. materials for large-scale TCE films with improved transmittance and conductivity that can be fabricated at low cost. Recently, inspired by the discovery of various attractive 2D materials, MXene, a new class of 2D transition-metal carbides and carbonitrides, has shown great promise in supercapacitors,17,18 Li-ion batteries,19,20 as well as some biological applications.21 MXene is synthesized by the selective etching out of the A layers from the MAX phases (Mn+1AXn), where M is an early transition metal (Ti, Zr, Nb, V, Ta, or Mo), A is an A-group element (mostly Al or Si), X is C and/or N, and n is 1, 2, or 3. The surfaces of MXene nanosheets (Mn+1XnTx) are mainly terminated by T functional groups (such as−O, −OH, −F, etc.) due to etching of the A layer from the MAX phase in acidic solutions, which make the hydrophilic nature of MXene.22 With its high conductivity, MXene can be a promising candidate for TCE application. In that regard, Ti3C2Tx films were synthesized by using spin-coating23 and spray-coating.24 Although those films demonstrated high transparency, excellent flexibility, and high mechanical strength, the reported sheet resistance was high as 437 Ω/sq at 77% transmittance and 0.5−8.0 kΩ/sq at 40−90% transmittance, respectively. Therefore, there is a need to develop a transparent MXene film with a low sheet resistance comparable to that of ITO. In this work, we studied Ti2CTx, another type of MXene, as a TCE. Previous studies showed that 2D Ti2CTx layers prepared by thermal and mechanical exfoliation processes have high

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ecently, there have been significant efforts to develop transparent conductive electrodes (TCEs), which are a very important component of electronic devices such as solar cells, liquid-crystal displays, sensors, and touch panels. Indium tin oxide (ITO) is the most-used TCE, with 90% transmittance and a sheet resistance (Rsh) of 10−25 Ω/square.1−3 However, there are increasing requirements for the next generation of TCEs, including not only high conductivity (low resistivity) but also flexibility, low cost, and ease of processing. The high cost and brittleness of ITO limits its further applications for future advanced optoelectronic devices. However, researchers are making continuous efforts to replace ITO with alternative materials, such as conducting polymers,4 metals,5 and carbonbased nanostructures.6−8 Since 2D graphene was discovered,9 which has received much attention in the field of TCE material because of outstanding electronic properties such as relatively low sheet resistance with high transmittance, Park et al.10 and Pang et al.11 have demonstrated that pristine graphene synthesized by chemical−vapor deposition can be used as a TCE in organic photovoltaic device applications. Since then, various solution processes have been developed to fabricate a large thin film of reduced graphene oxide (rGO), such as in spray coating, spin coating, and dip coating. The rGO film is transparent, with ∼80% transmittance; however, it exhibits high resistance (1−1000 kΩ/sq), probably because of defects induced by oxygen functional groups.12−15 Recently, Kang et al. developed a carbon nanotube−graphene composite transparent film with Rsh ≈ 533 Ω/sq at 88% transmittance; it has been used as TCE in GaN-based light-emitting diodes.16 However, there remains a big challenge to develop new © 2017 American Chemical Society

Received: December 30, 2016 Accepted: February 3, 2017 Published: February 3, 2017 859

DOI: 10.1021/acs.jpclett.6b03064 J. Phys. Chem. Lett. 2017, 8, 859−865

Letter

The Journal of Physical Chemistry Letters

Figure 1. (a) Schematic illustration of synthesis of Ti2CTx thin film and plasma treatment. (b) OM image of Ti2CTx thin film on Al2O3. The scale bar of the main figure is 100 μm and 5 μm for the inserted OM image. (c) SEM image of Ti2CTx thin film on Al2O3. Scale bar is 10 μm. (d) AFM image of Ti2CTx thin film deposited on the Al2O3 substrate.

carrier mobility ∼104 cm2 V−1 s−1 at room temperature25 and can be used as source/drain electrodes in FET devices when integrated with 2D semiconducting materials, such as WSe2 and MoS2. Low resistance and high work function (4.98 eV) has been exhibited, which makes Ti2CTx suitable for electrode applications.26 Here we report the use of Ti2CTx solutionprocessed large-area thin films for TCE applications using a liquid exfoliation method. We have fabricated wafer-scale thin (∼5 nm) Ti2CTx films covering the whole Al2O3 substrate by simple dip coating and have obtained sheet resistance as low as ∼70 Ω/sq at 86% transmittance. Our results show that uniform full-coverage Ti2CTx film was achieved on the Al2O3 substrate but not on the SiO2 substrate. Furthermore, by using SF6+Ar plasma treatment, it was found that Ti2CTx film can be etched layer-by-layer down to a monolayer. Physical and chemical analyses show that the oxidized top layer on Ti2CTx film, formed during the solution process, was etched after optimized plasma treatment, without affecting the bottom layer, resulting in improved conductivity (63 Ω/sq), transmittance (89%), and FOM (51.3). Our results pave the way to develop new 2D TCEs for future optoelectronic device application. Figure 1a illustrates the process of getting MXene (Ti2CTx) thin film from the MAX phase and coating Ti2CTx on the Al2O3 substrate. (The detailed process can be found in the Experimental Methods.) The colloidal solution of Ti2CTx flakes was synthesized by exfoliation of Ti2AlC under an acidic condition (HCl + LiF) for 24 h, after which a 10% HF solution was added for 10 min and washing with DI water. The addition of the HF solution resulted in very thin and large size flakes (5−10 μm), most of which were bilayer (∼2 nm), as shown in Figure S1d. Low-resolution transmission electron microscopy (TEM) images (Figure S2a) show a typical large-area Ti2CTx flake of several micrometers scale. High-resolution TEM of a folded part of the Ti2CTx film and of the corresponding simulated structure are shown in Figure S2b,c, respectively. The interlayer distance (∼1.5 nm) was found to agree well with calculated data.27 Energy-dispersive spectrometry (EDS) analysis of each element of Ti2CTx flake (Figure S3) shows the presence of Ti, C, F, O, and Cl, indicating that the MXene

edge would be terminated with electron-withdrawing and electron-donating group such as −F, −Cl and −O, −OH.28−30 The Ti2CTx films are fabricated by simple dip coating of Al2O3 substrates into the colloidal solution of Ti2CTx flakes for ∼2 min. Optical microscopy (OM) images (Figure 1b) and scanning electron microscopy (SEM) images (Figure 1c) show the homogeneous large-area Ti2CTx film deposited on the Al2O3 substrate on a millimeter scale. A high-resolution OM image (inset of Figure 1b) shows the continuous Ti2CTx film with full coverage of the substrate. Similar results were obtained from the different spots. An atomic force microscopy (AFM) image of the Ti2CTx film on the Al2O3 substrate (Figure 1d) shows a film of Ti2CTx that is ∼5 nm in average thickness. Remarkably, full-coverage continuous thin films of Ti2CTx have been found by sample dip-coating on Al2O3 substrate, but they are not on SiO2 substrate. Ti2CTx flakes distribute separately on SiO2 substrate, as shown in Figure S1b. This may be attributed to the fact that the SiO2 has a negatively charged surface, so the strong electrostatic repulsion occurs between the negatively charged Ti2CTx and the SiO2 surface during the coating process, which prevents the depositing of a continuous Ti2CTx thin film. On the contrary, Al2O3 has a positively charged surface, so the Ti2CTx flakes easily covered its surface by van der Waals interaction,31,32 so that a wafer-scale continuous thin film of Ti2CTx can be formed. The synthesized Ti2CTx films were etched by Ar+SF6 plasma to further study the optoelectronic properties of thin Ti2CTx films and to achieve controllability of the number of layers of 2D Ti2CTx films for TCE application. Few-layer Ti2CTx has been used for characterizing plasma condition, which was treated by the Ar+SF6 plasma treatment for various durations to achieve layer-controlled Ti2CTx films on the Al2O3 substrate. Under optimized condition (a mild inductively coupled plasma power ∼25 W with a fixed pressure of 30 mTorr), layer-by-layer etching of Ti2CTx flakes was obtained by adjusting plasma treatment time. The etching reactions used in this process are as follows33,34 SF6 + e → SF5+ + F* + 2e 860


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Figure 2. (a−d) AFM image of Ti2CTx under different etching time. Scale bar is 1 μm. (e) Surface profile scans from white lines in panels a−d. (f) Etched Ti2CTx thickness as a function of etching time. (g) Raman peaks of Ti2AlC powder and Ti2CTx films before and after plasma treatment.

Figure 3. (a) HRTEM image of Ti2CTx flake before plasma treatment. (b) HRTEM image of Ti2CTx flake after 100s plasma treatment. (c) Detailed Ti 2p spectrum of Ti2CTx flakes before and after 100s plasma treatment. (d) Detailed C 1s spectrum of Ti2CTx flakes before and after 100s plasma treatment.

Ti2CTx film before and after the plasma treatment. AFM images show that after each plasma treatment the surface of Ti2CTx flakes becomes clean, presumably because of etching of the top surface of Ti2CTx, which was oxidized in the synthesis process. Figure 2e shows the thickness profiles of the same Ti2CTx film (taken from the white lines in each image) without plasma treatment and after plasma treatments for 300, 400, and 500 s. The thickness of the Ti2CTx films was reduced from 5.1

SF5+ + O + e → SOF4 ↑ +F* Ti 2CTx + (2y + z)F * → 2TiFz ↑ + CFz ↑ +Tx

The byproducts SOF4, TiFy, and CF4 are volatile, so residuals were not left on the etched surface. The morphologies and thicknesses of the Ti2CTx layers were examined by AFM measurement. Figure 2a−d exhibits the morphology of the 861


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Figure 4. (a) Sheet resistance mapping of the synthesized Ti2CTx thin film (unit: Ω/sq). (b) Statistic data of sheet resistance shown in panel a. (c) Transmission spectra of Ti2CTx thin film with different plasma etching time.

the structure, suggesting regaining of the crystallinity and symmetry of the Ti2CTx phase after the plasma treatment. Furthermore, to investigate chemical composition and effect of plasma treatment, we performed XPS measurements of Ti2CTx thin films before and after the plasma treatment for 100 s. Detailed XPS survey spectra show the presence of titanium (Ti), carbon (C), oxygen (O), fluorine (F), and chlorine (Cl) before and after the plasma treatment with (Figure S4). The high-resolution Ti 2p core-level spectra for Ti2CTx are shown in Figure 3c, in which 2p1/2−2p3/2 splitting energy is 5.7 eV and the intensity ration is ∼2:1. Ti 2p3/2 region of Ti2CTx could be deconvoluted into four components Ti−C, Ti(II) (Ti−O, Ti− Cl), Ti(III) (Ti2O3, TiO2−x), and Ti(IV) (TiO2), which were located at 455.1, 456.2, 457.3, and 458.9 eV respectively.38,39 However, the Ti 2p1/2 region of Ti2CTx could also be deconvoluted into four same components, but Ti−F bonds are coexisting with Ti(II) at ∼462.1 eV. Obviously, in the case of as-synthesized Ti2CTx thin film, TiO2 and other oxide peaks dominated in comparison with Ti−C bonds. However, for plasma-treated Ti2CTx thin film, the dominated peak is corresponding to Ti−C bond, and the intensity of TiO2 peak decreased because of the reduction of surface oxygen,37,40,41 which is consistent with TEM and AFM results. The highresolution C 1s XPS spectra before and after the plasma treatment (100 s) are shown in Figure 3d. Each C 1s spectrum was fitted in three components located at 281.9, 284.7 and 289.0 eV corresponding to C−Ti, C−C, and (C−F, C−OC) bonds, respectively. After 100 s of plasma treatment, the area ratio between C−C bond and C−Ti bond decreased from 5.03 to 2.65, which further confirms that the surface oxidation on the Ti2CTx film was reduced and can increase conductivity.37 The electrical and optical properties of the Ti2CTx films for application to a TCE were evaluated. On the Al2O3/Si substrate, ∼4 cm large-area Ti2CTx films were deposited with full coverage of the substrate, as shown in Figure 4a. The covered area with the dashed red line is the Al2O3 substrate, which was covered by poly(methyl methacrylate), which was removed by acetone after coating. Sheet resistance was measured with a four-probe sheetresistance meter at different positions; measured values are marked on the OM image in Figure 4a. The statistical analysis of Rsh values, presented in Figure 4b, clearly shows that most dominant Rsh values fall in the range of 70−75 Ω/sq with good uniformity. After the plasma treatment for 100 s (approximately one layer etching), Rsh value decreased from 70 to 63 Ω/sq, but further plasma treatments increase the Rsh values. Transparency is another key factor of a TCE, so, to study the transparency performance of our Ti2CTx film, we measured transmittance

to 2.6 nm after 300 s of plasma treatment. After another two plasma treatments at 100 s intervals, the thickness was reduced to 1.9 and 1.1 nm, respectively. Considering the interlayer distance and the presence of the surface groups, the obtained results indicate that layer-by-layer etching of the Ti2CTx film was achieved with the optimized plasma treatment. The etching rate (∼0.8 nm/100 s) and corresponding surface roughness variation are shown in Figure 2f. As plasma treatment time increases, the etched thickness increases steadily and the corresponding roughness value decreases. The decrease in roughness can be attributed to the removal of surface oxidation and impurities, resulting in better transmittance and conductivity (Figure 4c). Figure 2g displays the Raman spectra of the Ti2AlC powder, the synthesized Ti2CTx film, and the plasma-treated Ti2CTx film, respectively. The Raman spectrum of bulk Ti2AlC (black curve) exhibited three intense peaks at ∼150, 265, and 365 cm−1, corresponding to the ω1, ω2, ω3, and ω4 Raman-active phonon vibration modes.35 In the Raman spectra of the synthesized Ti2CTx (red curve), all Raman vibration bands ω1, ω2, and ω3 are blue-shifted, with reduced peak intensity, but the ω4 peak remains in the same position with reduced intensity. After blue-shifting, the peaks ω2 and ω3 are merged with the Si peak at ∼300 cm−1, and the broadening of peaks ω1 and ω4 is increased, as shown in Figure 2g. Such upshifting and broadening in the Raman vibrational mode of Ti2CTx is due to introduced interlayer adsorbents and delamination by HF solution etching.25 After the plasma treatment of Ti2CTx (blue curve), the intensities of the ω1 and ω4 peaks decrease relative to those of the ω2 and ω3 mode merged with the Si peak because of thinning of the film by plasma treatment. To further investigate the atomic structure and crystallinity of Ti2CTx films before and after the plasma treatment, highresolution TEM measurement and selective-area energy diffraction (SAED) were performed. Figure 3a shows the TEM image of the as-synthesized Ti2CTx film; the image clearly displays the lattice planes of Ti2CTx and some surface defects with structural variation, probably caused by strain induced by functional groups and oxidation on the MXene surface.36 The SAED pattern of the as synthesized Ti2CTx film (inset of Figure 3a) shows the hexagonal symmetry with diffraction rings, which may correspond to the Ti2CTx phase and titanium oxide. However, the TEM image of the plasmatreated Ti2CTx film (Figure 3b) shows the single-crystalline structure with few intrinsic defects because of the removal of surface functional groups and impurities after the plasma treatment.37 The SAED pattern of the plasma-treated Ti2CTx film (inset of Figure 3b) shows only the hexagonal symmetry of 862


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With most optimized plasma treatments, Rsh reached 63 Ω/sq and transmittance of 89%. The FOM of our 2D Ti2CTx film (40.7 for as-synthesized and 51.3 for plasma-treated) as a TCE is highest among the reported results. Our results show that 2D Ti2CTx is a very promising candidate for next-generation TCEs for various electronic and photonic devices.

using UV−vis spectroscopy before and after the plasma treatment. Figure 4c shows the transmittance of the Ti2CTx films as-synthesized and after plasma treatment for various periods. The plasma-treated (for 100 s) Ti2CTx film showed high transmittance ∼89% at 550 nm, in comparison with an assynthesized Ti2CTx film ∼86%, which is caused by thinning of the film and by the reduction of impurities and oxidation. With continuous layer-by-layer plasma etching, transmittance is increased. We compare the transmittance of our Ti2CTx film with other reported transparent conductive materials with their Rsh values, as shown in Table 1. We notice that our plasma-

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EXPERIMENTAL METHODS 1 g of Ti2AlC powders (3-ONE-2, Voorhees, NJ) was added to a premixed solution of LiF (1.8 g) and 12 M HCl (30 mL), and the mixture was kept in ice water for 30 min to avoid overheating. Then, the mixture was held at 37 °C for 24 h in a water bath. The resulting mixture was immersed in 10% HF solution for 10 min. Then, we washed the mixture solution with deionized (DI) water through ∼16 cycles (4000 rpm × 5 min for each cycle) until the pH of the supernatant reached ∼6. After that, MXene and DI water were sonicated for 10 min and centrifuged at 1000 rpm for 10 min to remove bulk Ti2CTx, and further supernatant was centrifuged at 6000 rpm for 10 min to get the Ti2CTx byproduct. A 15 nm thick Al2O3 substrate, grown by atomic layer deposition (ALD), and thermally grown SiO2 were washed using acetone and IPA for 15 min, respectively. The Ti2CTx flakes were dispersed in distilled water (1 g MXene per 10 mL of DI water), and the clean Al2O3 substrate was immersed in the Ti2CTx solution for 120s, washed with DI water, and blowdried. OM (100× magnification, BX51M, Olympus) and SEM (FESEM; JSM7401F, JEOL) were used to examine the morphologies of the samples. Thickness of MXene flakes was measured by AFM (NTEGRA Spectra, NT-MDT), and Raman spectroscopy was performed with a laser micro-Raman spectrometer (Kaiser Optical Systems Model RXN, 532 nm excitation wavelength). Thin flakes were examined by using TEM (JEM 2100 F, JEOL) with an accelerating voltage of 300 kV. Electrical measurements were performed with a Keithley 4200 parameter analyzer.

Table 1. Comparison of the Optoelectronic Properties of Various TCEs material ITO Ag NW solution-processed graphene reduced-GO Ti3C2Tx Ti2CTx (as synthesized) Ti2CTx (plasma treated)

transmittance % at 550 nm

sheet resistance Ω/sq

σdc/σoc

ref

90 77 83.7

46 48 1150

75.7 72 1.81

1−3 42 43

80 77 86

1000 437 70

1.6 3.1 40.7

89

63

51.3

12−15 23 this work this work

treated (for 100 s) films have the lowest Rsh value (63 Ω/sq) with good transmittance ∼89%. In fact, Ti2CTx film that is plasma treated shows higher transmittance (∼91%) than the previously reported value for 2D Ti3C2Tx film as well as a lower Rsh value.24 Transmittance of our plasma-treated Ti2CTx film is comparable to that of pressed rGO (∼91%), but the Rsh value of pressed rGO is higher than that of our plasma-treated Ti2CTx film.12−15 All of these comparative investigations demonstrate that our Ti2CTx film, both as-synthesized and after plasma treatment, performs better than other solutionprocessed 2D films. To further characterize the quality of Ti2CTx thin film as a TCE, we calculated the figure of merit (FOM). The FOM is the ratio of direct conductivity (σdc) to the optical conductivity (σoc), and a higher (σdc/σoc) value indicates better optoelectronic properties. The FOM is described in the following formula ⎛ 1 T = ⎜⎜1 + 2R sh ⎝

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b03064. Statistical analysis of as-synthesized Ti2CTx flakes and corresponding OM image of Ti2CTx flakes on SiO2 substrate. HRTEM image, elements mapping, and Raman survey spectra of Ti2CTx flake. FET devices of Ti2CTx flake before and after plasma treatment. (PDF)

−2 μ 0 σoc ⎞ ⎟⎟ ε0 σdc ⎠

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where T is the optical transmittance, μ0 is the vacuum permeability, and ε0 is the vacuum permittivity ( (μ 0/ε0) ≈376 Ω). The FOM of as-synthesized Ti2CTx thin film is 40.7, which is higher than that of previously reported other solutionprocessed thin films. Furthermore, the Rsh of Ti2CTx film was reduced from 70 to 63 Ω/sq with high FOM (51.3) by SF6+Ar soft plasma treatment. We report large-area continuous 2D Ti2CTx film, which was created by a simple and scalable method of dip coating of an Al2O3 substrate, for TCE application. The synthesized Ti2CTx film exhibited excellent optoelectronic properties, with a low Rsh ≈ 70Ω/sq at transmittance of 86%. Furthermore, by using an SF6+Ar plasma-treated layer-by-layer etching process, we could control the thickness of Ti2CTx down to a monolayer.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sungjoo Lee: 0000-0003-1284-3593 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Global Frontier Program through the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078873) of the National Research Foundation of Korea (NRF) and the Pioneer Research Center 863


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Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (2014M3C1A3053024), and Basic Science Research Program through the National Research Foundation of Korea funded by the Korean government (MSIP) (grant numbers: 2015R1D1A1A09057297 and 2015M3A7B7045496).

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