Ultrafast Heating for Intrinsic Properties of Atomically Thin Two

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Ultrafast Heating for Intrinsic Properties of Atomically Thin Two-Dimensional Materials on Plastic Substrates Ho Young Kim, Jae-Won Lee, Hye Min Oh, Kang-Jun Baeg, Sunshin Jung, HoSoon Yang, Wonki Lee, Jun Yeon Hwang, Keun Soo Kim, Seung Yol Jeong, Joong Tark Han, Mun Seok Jeong, Geon-Woong Lee, and Hee Jin Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09677 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Applied Materials & Interfaces

Ultrafast Heating for Intrinsic Properties of Atomically Thin Two-Dimensional Materials on Plastic Substrates Ho Young Kim,1,2,‡ Jae-Won Lee,1,3,‡ Hye Min Oh,2 Kang-Jun Baeg,1,† Sunshin Jung,1 Ho-Soon Yang,3 Wonki Lee,4 Jun Yeon Hwang,4 Keun Soo Kim,5 Seung Yol Jeong,1 Joong Tark Han,1 Mun Seok Jeong,2,* Geon-Woong Lee,1,* and Hee Jin Jeong1,* 1

Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute,

Changwon 641-120, Korea. 2

Center for Integrated Nanostructure Physics (CINAP), Institute for Basic Science (IBS),

Sungkyunkwan University, Suwon 440-746, Korea. 3

Department of Physics, Pusan National University, Busan 609-735, Korea.

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Institute of Advanced Composite Materials, Korea Institute of Science and Technology,

Jeonbuk 565-905, Korea. 5

Department of Physics and Graphene Research Institute, Sejong University, Seoul 05006,

Korea.

ACS Paragon Plus Environment

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*Corresponding authors. E-mail: (M. S. Jeong) [email protected], (G.-W. Lee) [email protected], and (H. J. Jeong) [email protected] ‡These authors contributed equally to this work. †

Present address: Department of Graphic Arts Information Engineering, Pukyong National

University, Busan 608-739, Korea.

ABSTRACT

Despite recent progress in producing flexible and stretchable electronics based on twodimensional (2D) nanosheets, their intrinsic properties are often degraded by the presence of polymeric residues that remain attached to the 2D nanosheet surfaces following fabrication. Further breakthroughs are therefore keenly awaited to obtain clean surfaces compatible with flexible applications. Here we report a method that allows the 2D nanosheets to be intrinsically integrated onto flexible substrates. The method involves thermal decomposition of polymeric residues by microwave-induced ultrafast heating of the surface without affecting the underlying flexible substrate. Mapping the C=O stretching mode by Fourier-transform infrared spectroscopy in combination with atomic force microscopy confirms the elimination of the polymeric residues from the 2D nanosheet surface. Flexible devices prepared using microwave-cleaned 2D nanosheets show enhanced electrical, optical, and electrothermal performances. This simple technique is applicable to a wide range of 2D nanomaterials and represents an important advance in the field of flexible devices.

KEYWORDS: microwave, ultrafast heating, 2D nanomaterial, selective heating, flexible device.


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INTRODUCTION The development of the technology required to deposit two-dimensional (2D) nanosheets (such as graphene, hexagonal boron nitride, and transition metal dichalcogenide) on polymeric substrates1–4 has accelerated research on 2D nanosheets-based flexible electronic devices because high flexibility can be achieved without any significant degradation of the associated electrical properties.5–8 Since 2D nanosheets are generally grown by chemical vapor deposition (CVD) on rigid substrates,1,3 a prerequisite for these devices is the transfer of 2D nanosheets from the rigid substrate to the desired flexible substrate. One widely used approach involves etching the rigid substrate and transferring the 2D nanosheets using a mechanical supporting layer mainly composed of polymeric materials.1,2 However, this supporting layer interacts with 2D nanosheets via noncovalent van der Waals bonds and is difficult to remove under even the most vigorous conditions, such that residues often remain on the transferred 2D nanosheets surface. These polymeric residues can act as external scattering centers thereby degrading the transport properties of 2D nanosheets.3,9 Alternatively, thermal release tape,5 an additional metal layer,10,11 or an epoxy layer12 have been used as a supporting or adhesive layer, but impurity residues are similarly problematic. Furthermore, when the 2D nanosheets are integrated into flexible devices by conventional lithography13 – another prerequisite for such applications – resist polymer residues are only partially removed by standard solvents. On the other hand, heat treatment in a clean or reducing environment is effective,13–15 but high-temperature annealing is not suitable for flexible electronic devices whose polymeric substrates have low melting temperatures. The microwave-induced heating of conductive nanomaterials has recently been attracting a lot of interest because it allows the ultrafast heating of nanomaterials and can stimulate a wide range of physical processes and chemical reactions, notably to bind carbon nanotubes (CNTs) to


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polymers, for the self-passivation of CNTs, and for the sintering of silver nanoparticles.16–20 Nanomaterials directly interacting with microwave irradiation can play a significant role as an efficient and immediate source of heat, although the intensity and speed of heat generation are dependent on the electrical conductivity and morphological structure of the nanomaterials. Selective heating of nanomaterials has also been investigated through the microwave irradiation of multi-layered CNTs deposited on polycarbonate (PC) film.21 In contrast with the effects of conventional heating in a convection oven, the CNT films were successfully heat-treated without deforming the PC substrate due to the differences in the microwave-adsorption ability between CNT and PC. In this context, we show in this report how microwave irradiation can be used to make clean surfaces of the 2D nanosheets, especially graphene, transferred or integrated on polymeric substrates. The absorption of microwaves induces a surface current resulting in the immediate ohmic heating of the graphene sheet which can facilitates the thermal decomposition of the polymeric residues. Moreover, we show how this rapid and selective heating technique can be applied to other nanomaterials such as two-dimensional molybdenum disulfide (MoS2) and thin titanium dioxide (TiO2).

RESULTS AND DISCUSSION The experimental setup used for the selective microwave-induced heating of graphene sheets is shown in Figure 1a. Graphene films, transferred on PC substrate using a conventional polymethyl methacrylate (PMMA) method (Figure S1), were irradiated with microwaves inside a rectangular waveguide applicator, within which the microwave electric field was precisely controlled. The graphene films were first inserted through the long narrow slit oriented along the z-axis of the waveguide, then positioned in the yz-plane parallel to the microwave electric field


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(Figure S2). The microwave field intensity in the films was constant in the x-axis direction, which ensured uniform heating throughout the graphene layer. The amplitude of the conduction current density, J, in a conductor decreases exponentially from its value at the surface, Js, according to the depth, d, as follows: J = Jse-d/δ, where δ is the skin depth of the conductor. Moreover, the current density induced on the surface of the conductor by the microwave electric field intensity, EMW, may be described as follows:20 Js = σ EMW, where σ is the electrical conductivity of the conductor. In this experimental configuration, in which the microwave electric field is parallel to the single-atom-thick highly conducting graphene sheet, microwaves induce a fast oscillating current resulting in immediate ohmic heating of the graphene sheet. Although this fast heating could also be accomplished by applying microwave power normal to the graphene sheet, this was found to drastically increase its temperature, which reached 250 °C within 0.5 seconds under 140 W microwave irradiation (Figure S3). In addition to the speed of heating it provides, another useful aspect of this method for applications in flexible graphene devices is the ability to restrict the heating to the graphene sheet. To demonstrate this selective heating, we measured the surface temperatures of the graphene sheet and of the PC substrate (Figure 1b and S4). The microwave irradiation was automatically stopped once the temperature of the graphene sheet had reached a certain value (250 °C in this case) and the sample was cooled down to below 85 °C. The temperature of the graphene sheet increased rapidly as expected, while the temperature measured on the back of the PC substrate rose very slowly up to about 100 °C, much lower than the heat deflection


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temperature (132 °C) of the PC substrate. This slower heating is probably due to the thermal resistance between the graphene and the PC substrate21 that gives rise to the dramatic temperature difference between both sides of the sample. While the low thermal conductivity of the PC substrate certainly slows the transfer of heat from the graphene sheet, the low heating rate of the substrate may also be due to heat dissipating from the graphene sheet through convection. Although graphene has a lower convective heat transfer coefficient than Cr thin film in free air,22 the N2 flow in this setup may considerably enhance convection. To further evaluate this selective heating, we compared its effects with homogeneous heating in a conventional oven, notably in terms of the distortion of the PC substrate, as highlighted in Figure 1c. In a conventional oven, the PC substrate is highly distorted after just 5 s at 200 °C. In contrast, with microwave irradiation of the graphene sheet, the PC substrate is only heated locally in the areas in direct contact with the graphene sheet as described in the scheme. This can thereby be heated to higher temperatures without distorting the PC substrate. These results are in good agreement with a previous report of the effects of microwaves on carbon nanotube films deposited on PC substrates.21 The surface temperature of the graphene sheets as a function of the microwave power is shown in Figure 1d and S3. The surface temperature of the graphene layers increases continuously with the microwave power, reaching 250 °C after 0.5 s microwave irradiation at 140 W. Experiments were not conducted above 250 °C because higher temperatures were found to cause distortions in the PC substrate (Figure S5). The graphene films were irradiated with microwaves at each temperature before their sheet resistance was measured using the four-point probe method, as shown in Figure 1d. As the microwave power is increased, the sheet resistance decreases slightly in the low power regime but more drastically for powers above 100 W,


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reaching its lowest value of 365 Ω/sq at 140 W. This ~19.6 % reduction in the sheet resistance of the graphene film is attributed to the cleanness of the graphene surface. After the PMMAmediated transfer of graphene films to the desired substrate, the PMMA supporting layer is typically removed using acetone, but as explained in the introduction, residues often remain due to the formation of noncovalent van der Walls bonds with graphene (Figure 1e). These PMMA residues can cause cracks and act as charge carrier scattering centers,3,23 resulting in high sheet resistance of the graphene thin film. Here, irradiation with 140 W microwaves heats the graphene films up to 250 °C, which is sufficient to induce the thermal degradation of the PMMA residues thereby cleaning the graphene surface (Figure 1f). To obtain a clean graphene surface, it is well known that PMMA residues are only eliminated by conventional annealing in a vacuum oven at temperatures above 300 °C,15 much higher than the temperatures reported here. In fact, these do not represent actual local temperatures but, because of the limited optical resolution of the infrared sensors, the average temperature over the entire graphene film. It has been reported, albeit controversially, that under microwave irradiation the actual temperatures of heterogeneous materials may differ from the average values.18 More recently, a similar effect was observed for in situ quasi-elastic neutron scattering measurements of the temperature of molecules adsorbed in nanopores heated selectively by microwaves.24 There is therefore evidence to suggest that the actual temperature of the graphene may be significantly different from the measured value. This may explain why the PMMA residues are removed effectively here even at the relatively low (measured) temperature of 250 °C. Fourier transform infrared (FTIR) spectroscopy was used to verify the thermal degradation of the PMMA residues by microwave-induced selective heating. The FTIR spectra in Figure 2a


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show sharp peaks at around 1720, 1470, and 1150 cm−1, corresponding respectively to the C=O, CH2/CH3, and C−O stretching modes of the PMMA residues, whose intensity decreases markedly with increasing microwave powers. This indicates that the concentration of the associated PMMA chemical structures is reduced, most probably by thermal degradation due to the ohmic heating of the graphene films at high microwave powers. Clearer evidence of the efficient removal of the PMMA residues is revealed in the high-resolution images obtained by combined FTIR-AFM (atomic force microscopy). The AFM topology and the associated map of C=O stretching mode intensities are presented in Figure 2b as obtained from a graphene film irradiated with 20 W microwaves. The AFM topology shows a continuous graphene sheet with a few domain boundaries (the inhomogeneous dark brown lines) that correspond to those of the polycrystalline copper substrate. The white spots indicate the presence of nanoparticles tens of nanometers in diameter but it is not clear whether these are PMMA residues or other impurities such as chemical compounds and/or dust. Indeed, these nanoparticles do not appear in the FTIR image. However, although these are not observed by AFM, the FTIR image does show a large number of smaller nanoparticles a few nanometers in diameter. As shown in Figure 2c and S6, these disappear considerably under 140 W microwave irradiation for 1 h, while the AFM topology does not vary. The effects on graphene of this microwave treatment are comparable to those of thermal treatment at 300 °C for 1 h in a conventional vacuum oven (Figure S7). Additional evidence of the effective removal of PMMA residues by microwave-induced selective heating was obtained by C1s X-ray photoemission spectroscopy (XPS), as shown in Figure 2d. Graphene irradiated with 20 W microwaves shows not only a main sp2 peak, originating from graphene, but also a sp3 and oxygen species peaks such as epoxy and carboxyl, originating from the PMMA residues. However, these oxygen-related peaks weaken as the


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microwave power is increased up to 140 W, from 38.1 % to 10.9 % of the integrated intensity in the XPS spectrum, highlighting the gradual removal of PMMA residues. Although minor sp3, epoxy, and carboxyl peaks are still observed, the C1s peaks are comparable in magnitude to those of a bare graphene surface on a copper substrate (shown in the inset of Figure 2d) and may be attributed to Stone-Wales defects and oxygen functional groups. The removal of PMMA residues was also demonstrated using water contact angle analysis. Reported water contact angles of PMMA (a hydrophilic polymer) and hydrophobic graphene are 67–72°, and 86–103°, respectively.25–29 The contact angle of 90.2° shown in Figure S8 confirms the effective removal of the PMMA residues from the graphene surface following irradiation with 140 W microwaves. Raman spectroscopy was used to characterize the doping effect of the PMMA residues on the microwave-treated graphene films and the crystallinity of the latter, as shown in Figure 1e. The G peak at 1590 cm−1 originates from sp2 hybridized carbon, while the 2D peak at ~2680 cm−1 results from two-phonon inelastic scattering. The intensity ratio of the 2D and G peaks (I2D/IG) is 3.21 for the graphene films treated with 140 W microwaves, which is comparable to the values reported for high quality single layered graphene grown by CVD.3 However, the 2D peak shifts to higher wavenumbers (higher energies) and the I2D/IG is significantly lower for the graphene samples treated with lower power microwaves (Figure S9). This is characteristic of p-type doped graphene,30 and PMMA residues have indeed been reported to act as p-type dopants in CVDgrown graphene.15 Thus, phonon hardening by charge transfer from graphene to the PMMA residues can result in an upshift of the Raman 2D peak. This effect disappears on removal of the PMMA residues such that the position and intensity of the 2D peak are retrieved. The differences observed in the Raman spectra in Figure 2e may therefore be attributed to the doping and dedoping effects of the PMMA residues. The same phenomenon is observed in the Raman spectra


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measured with a 632.8 nm laser shown in Figure S9. Moreover, the disappearance of the PMMA peaks31,32 at 2842, 2931, and 2946 cm−1 in the spectrum obtained following irradiation with 140 W microwaves (as highlighted in the inset of Figure S10) underlines the effective removal of these impurity residues. To investigate the electrical transport characteristics of the microwave-treated graphene films, field-effect transistors (FETs) were fabricated. V-shape transfer characteristics are observed for all the FETs (Figure 3a–3d), reflecting the conduction of both electrons and holes. However, the Dirac points decrease as the microwave power used to treat the graphene is increased (Figure S11). This could be the result of de-doping due to the removal of the PMMA residues and supports the Raman analysis described above. In undoped graphene, the Fermi level lies at the boundary between the valence and conduction bands but within the valence band in p-type graphene. The removal of PMMA residues that act as p-type dopants should therefore lead to the Fermi energy returning to its undoped level. This effect was further examined in terms of the carrier mobilities. The hole and electron mobilities increase from 1205 and 802 cm2/(V⋅s) respectively in the FETs prepared using graphene treated with 20 W microwaves, up to 2786 and 2401 cm2/(V⋅s) when 140 W microwaves are used (Figure S11). Meanwhile, differences are apparent in the transfer characteristics of the microwave-treated graphene FETs under forward and reverse gate voltages (Figure 3 and S11). The hysteresis effect clearly observed for the graphene FETs treated with 20 W microwave irradiation, disappears as the microwave power is increased and with 140 W microwave irradiation, the forward and reverse curves become coincident. This current hysteresis is probably the result of charge carriers being trapped by the polymeric residues and absorbents between PMMA and the graphene surface;9,33,34 its disappearance therefore reflects the elimination of the PMMA residues.


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Electrothermal properties of the microwave-treated graphene films were investigated by fabricating graphene-film-based thin film heaters on PC substrates. Four-layered graphene film with a sheet resistance of 131 Ω/sq and an optical transmittance of 89.8 % at 550 nm was fabricated by transferring graphene monolayers one by one, with the microwave treatment applied sequentially (Figure S12). The surface temperatures of the heating devices (2 cm × 4 cm) before and after treatment with 140 W microwaves are shown as a function of time in Figure 3e and 3g, respectively. On application of the external voltage, the surface temperature increases monotonically before reaching a certain steady-state value, which is higher for larger applied voltages, as expected.35 The response times (the time taken to reach the steady-state temperature) of the treated and untreated graphene-film-based heaters are similar, as reported previously,36 however, for a given applied voltage, the performance of the microwave-treated graphene heater is superior. For example, with 18 V applied, the steady-state temperature and average heating rate are 108 °C are 2.04 °C/s respectively in the treated device, compared with 98 °C and 1.83 °C/s for the untreated graphene film heater. (Similar comparisons can be made for all applied voltages.) The infrared thermal images of the graphene films at their steady-state temperatures under an applied voltage of 9 V are shown in Figure 3f. The temperature distribution is more uniform in the microwave-treated graphene film heater. These favourable electrothermal properties are attributed to the higher electrical conductivity and homogeneity of the microwave-treated graphene film.36 An infrared thermal image of a larger (5 cm × 10 cm) microwave-treated graphene film heater bent at an angle of approximately 40º is shown in Figure S13, taken under an applied voltage of 15 V. The uniform temperature distribution observed over the entire graphene surface emphasises the benefits of selective microwave-induced heating. For further examination of the effect of microwave-induced heating, chemically exfoliated


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MoS2 nanosheets prepared on top of the graphene thin films were used. While chemical exfoliation is useful approach to produce monolayer MoS2 with mass quantity,37–40 their phase change (from semiconducting 2H to metallic 1T) occurred during Li intercalation should be reversed by thermal annealing at temperatures over 200 °C for 1 h,39,40 which is not compatible with flexible substrates. We therefore demonstrate that phase recovering of MoS2 nanosheets can be successfully achieved through desorption of intercalated Li ions by microwave-induced selective heating (scheme in Figure 4a). Uniformly distributed, highly crystalline LixMoS2 nanosheets with typical thickness of 1.3–1.5 nm as shown in Figure 4a and S14 were treated with various microwave powers. LixMoS2-deposited graphene films show rapid temperature increase (250 °C for 0.5 sec at 140 W irradiation) similar with the graphene case (Figure S15), The chemical composition of the LixMoS2 nanosheets before and after microwave treatment with various microwave powers was studied by X-ray photoelectron spectroscopy (XPS) (Figure 4b). As shown in the XPS spectrum of bulk sample, the 2H phase MoS2 consists of two doublet Mo 3d (229.1 and 232.2 eV) and S 2p peaks (163.2, 161.9 eV), represented by red plot. However, the 1T phase MoS2 shows additional peaks at lower energy position, represented by green plot, due to Li intercalation during the chemical exfoliation process. These peaks are slowly reduced by the microwave irradiation power of 100 W and rapidly disappeared at 140 W, indicating desorption of the intercalated Li ions by microwave-induced heating. This phase transformation of MoS2 nanosheets is also proved by decrease of Raman characteristic peaks at 229.7 and 303.9 cm–1 of the 1T MoS2 (Figure S16). Unlike the bulk MoS2, single-layer MoS2 is a direct bandgap semiconductor where the lowest interband transition occurs at the K point of the Brillouin zone.41 Figure 4c shows photoluminescence spectra of the LixMoS2 thin films. With increasing microwave power, the intensity of a peak centered at 1.88 eV with a shoulder at 2.02 eV is


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gradually increased, which agrees well with excitonic peaks from the K point of the Brillouin zone. The emission spectrum is consistent with mechanically exfoliated monolayer samples,42 indicating that the observed photoluminescence arises from the intrinsic electronic properties of monolayer MoS2 and not from structural defects or chemical impurities. In order to show additional application of the microwave-induced selective heating on the graphene surface, we measured photocatalytic properties of graphene/TiO2 hybrid thin films. Substantially, anatase TiO2 has been widely used as one of photocatalytic materials due to its fascinating physical and chemical properties with relatively low production cost.43,44 Photocatalytic properties are markedly improved by hybridization of anatase TiO2 with graphene.45,46 Photocatalytic properties were measured on the graphene/TiO2 hybrid structure deposited on indium tin oxide (ITO)-coated PC substrate as shown in the scheme of Figure 4d. After condensation of TiO2 sol with microwave power of 140 W, it is mostly converted to anatase TiO2 which corresponds to X-ray diffraction and Raman spectra (Figure S17).47,48 In term of photocurrent measurement, I–V characteristics for photocatalysts based on the graphene/TiO2 and TiO2 thin films are shown in Figure 4e. Under white light illumination, photocurrent is drastically increased for both the samples. The current ratio before and after illumination is approximately 4.2 × 102 at 5 V for the graphene/TiO2 thin film and 1.9 × 102 for the TiO2 thin film, respectively. The superior photoelectrical properties of graphene/TiO2 thin film might be due to decreased barrier height from the junction area between ITO and graphene/TiO2 as described in the inset of Figure 4e. Moreover, the reproducible photocurrent response with high photocurrent and fast response time (Figure 4f) indicates that the photogenerated electrons can be effectively transported by highly conductive graphene layer without further recombination with holes within the TiO2 layer.45


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Top-gate/bottom-contact structured organic thin-film transistors (OTFTs) were also prepared using source/drain (S/D) electrodes prepared using microwave-treated graphene (Figure 4g). Figure 4h highlights the typical n-channel transfer characteristics of these devices. The OTFTs prepared with microwave-treated graphene S/D electrodes have a higher electron mobility and threshold voltage in the saturation region (at Vd = 40 V), than the devices prepared with untreated graphene, namely 0.0286 cm2/(V⋅s) compared with 0.0091 cm2/(V⋅s) and 12.9 V as opposed to 2.6 V, respectively. This is attributed to the higher electrical conductivity of the microwavetreated graphene electrodes as well as to the lower contact resistance between the organic semiconductor and the clean graphene electrode surface.

CONCLUSION Through a series of experiments, we have shown that microwave-induced heating is an effective method to eliminate polymeric residues from transferred graphene, thereby providing clean surfaces with favorable electrical, optical, and thermal properties. The microwave heating is selective, allowing the preparation of clean graphene surfaces without any distortion of the underlying polymer substrates. These results provide new insights into the benefits of simple microwave treatment for graphene-based flexible or stretchable devices. This microwaveinduced selective heating technique should be applicable to a wide range of graphene-based materials, notably those involving carbon nanotubes, transition metal dichalcogenides, metal nano-wires/particles, thin metal oxides, and quantum dots, to enhance their intrinsic properties or impart multifunctionality.

EXPERIMENTAL SECTION


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Growth and transfer of graphene. Monolayer graphene was grown on a 100-µm-thick polycrystalline copper foil by low-pressure CVD of methane (99.96%) reported previously.32,34 For graphene transfer to the PC substrate, a 200-nm-thick PMMA layer with a molecular weight of 950k was prepared by spin casting and subsequent curing. The sample was then immersed in a 0.2 M ammonium persulfate solution for complete etching of copper foil. After transferring to the PC substrate, the sample was dipped in acetone for 6 h to remove the PMMA layer. Microwave irradiation. Microwave irradiation was used to selectively heat the graphene film on the PC substrates. All samples were sufficiently dried at 80 °C overnight in a vacuum oven before microwave irradiation. The microwave applicator consisted of a moving metal reflector plate and a rectangular waveguide. The microwaves were applied with a fundamental transverse electric (TE10) mode (Ez = 0) at a frequency of 2450 MHz. The moving metal plate reflected the incident microwaves, giving rise to a standing wave in the chamber. A microwave generator (PM740T, Richardson Electronics, Ltd.) supplied 20–180 W microwaves at a frequency of 2450 MHz for 1 h to the graphene films. In all experiments, the microwave irradiation was conducted by the multiple shots of a 0.5 s microwave with a duty cycle of approximately 8.3% under a N2 atmosphere. Microwave power sensor (U2000, Agilent Technologies) was used to measure the microwave power. The surface temperature of the graphene films was measured using an infrared optical sensor (MI3-LT, Raytek). The data was fed directly to a computer that automatically controlled the microwave heating system. Fabrication of graphene-based flexible FETs. As-grown CVD graphene was transferred onto a polyimide substrate. The electrode pattern was obtained using a conventional photolithography process. The source and drain electrodes were prepared by thermal evaporation of gold at 2ⅹ10−6 Torr to a thickness of 100 nm. The channel length and width were 50 and 30


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µm, respectively. The sample was then irradiated with 140 W microwaves. An ion gel gate dielectric was obtained by dropping the ion gel ink over the channel region. The ion gel ink was prepared by mixing poly(styrene-methyl methacrylate-styrene) (PS-PMMA-PS), 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) and ethyl acetate. The Ids vs. Vg curve was observed for gate voltages (Vg) from −50 V to 60 V with a constant drainsource voltage (Vds) of 0.2 V. Fabrication of graphene film-based transparent heaters. A two-terminal side-contact composed of screen-printed silver on the four-layered graphene films on a PC substrate was used for the electrodes of transparent film heaters. The four as-grown graphene monolayers transferred one by one and irradiated with a 140 W microwaves sequentially. DC voltage ranging from 6 V to 18 V was applied to graphene film heaters with dimensions either of 2 cm × 4 cm or 5 cm × 10 cm. The surface temperature of the film was measured using an infrared thermal imager (TH9100ML, NEC San-ei) with a temperature resolution of 0.08 °C at 30 °C, a temperature accuracy of ±2%, a frame time of 1/60 sec, and a spatial resolution of 320 × 240 pixels. Fabrication of graphene/MoS2 thin film. MoS2 nanosheets were prepared by Li intercalation and subsequent exfoliation. Li intercalation was carried out by immersing 1 g of natural MoS2 crystals (Sigma-Aldrich) in 3 mL of 1.6 M butyllithium solution in hexane for 2 days in a flask filled with argon gas. After filtration and washing, LixMoS2 was exfoliated using a homogenizer in water for 2 h and then was centrifuged several times. The graphene/MoS2 thin film was obtained by spin-casting the LixMoS2 nanosheets onto the microwave-treated graphene surface on PC substrate. Finally, Li desorption was achieved by microwave-induced selective heating of graphene with various microwave powers.


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Fabrication of graphene/TiO2 thin film. A TiO2 sol was prepared by stirring the mixture of titanium isopropoxide/acetylacetone (1M/2M), HCl, and water at 60 °C for 10 h. Prepared TiO2 sol was spin-coated at 1000 rpm for 1 min onto the microwave-treated graphene surface on ITO-coated PC substrate. Finally, the graphene/TiO2 thin film with a thickness of 50 nm was irradiated with 140 W microwave for 1 h. For the comparison, TiO2 thin film on the ITO glass substrate was prepared by thermal curing at 300 °C for 1h in a vacuum furnace. The photocurrent measurement was carried out using a solar simulator (Abet Technologies, Sun 2000) with a 450 W Xe source of 365 nm wavelength and a Keithley 2400 source meter. Fabrication of OTFTs. CVD graphene transferred onto a polyimide substrate was micropatterned by conventional photolithography and lift-off processes, and then irradiated with 140 W microwaves. The channel width/length (W/L) was 0.1 mm/10 µm. The n-type semiconducting polymer poly((N,N'-bis2-octyldodecyl-1,4,5,8-naphthalenedicarboximide-2,6-diyl)-alt-5,5'-2,2'bithiophene) P(NDI2OD-T2) (Polyera Corp.), at a concentration of 7 mg/ml in p-xylene, was spin-coated onto the graphene-patterned substrates. A PMMA polymer gate dielectric (80 mg/ml in n-butyl acetate) was spin-coated on top of the semiconductor film. All dielectric materials and solvents were purchased from Sigma-Aldrich and used as received. The semiconductor and dielectric films were annealed respectively at 150 °C and 80 °C for 30 min. The OTFT devices were completed by the vapor deposition of Al gate electrodes (~35 nm thick) through a metal shadow mask. Characterization. The surface morphologies of the graphene and MoS2 thin films were imaged by optical microscopy (Nikon Eclipse LV100), field-emission scanning electron microscopy (Hitachi S4800), and transmission electron microscopy (FEI TECNAI G2 F20) with an acceleration voltage of 120 kV. To confirm the chemical composition of the graphene and


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MoS2 thin films, X-ray photoelectron spectroscopy (Thermo VG Scientific Inc. Multilab2000) was carried out with monochromatized Al Kα radiation as the excitation source. The crystallinity of the graphene, MoS2, and TiO2 layers were examined by confocal Raman spectrometry (NTMDT, NTEGRA SPECTRA), using 532.8 nm and 632.8 nm excitation radiation and a Rayleigh line injection filter with a spectral range of 100–3600 cm−1 to account for the Stokes shift. Water contact angle measurements (SEO, Pheonix 300) were performed to estimate the wettability of the graphene films. High resolution IR-absorption mapping of the thermal degradation of the PMMA residues was performed using a combined Fourier transform IR-atomic force microscope machine (Nano IR, ANASYS Instruments).


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FIGURES

Figure 1. (a) Schematic illustration of the experimental setup for microwave-induced selective heating of the graphene sheet. (b) Surface temperatures of the graphene sheets and of the PC substrates as a function of time for repeated irradiations with 140 W microwaves. (c) Photographs of graphene films transferred onto PC substrates following homogeneous heating in a conventional oven or selective heating by microwave irradiation. (d) Surface temperature (in black) and sheet resistance (in blue) of the graphene surface as a function of the applied microwave power. Optical (left, 50 µm scale bar) and transmission electron (right, 5 nm scale bar) micrographs of the transferred graphene film (e) before and (f) after microwave-induced selective heating.


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Figure 2. (a) Fourier-transform infrared (FTIR) spectra of graphene films treated at various microwave powers. Atomic force micrographs and the corresponding FTIR images of graphene films at microwave powers of (b) 20 W and (c) 140 W. The scale bars represent 2 µm. (d) C1s X-ray photoelectron spectra and (e) Raman of graphene films treated at various microwave powers. The inset in (d) is the C1s X-ray photoelectron spectrum of as-grown graphene on a copper substrate.


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Figure 3. Reverse and forward transfer curves for graphene FETs treated at microwave powers of (a) 20 W, (b) 60 W, (c) 100 W, and (d) 140 W, respectively. Time dependent temperature profiles of (e) untreated and (g) microwave-treated graphene films as a function of the applied voltage. (f) Infrared thermal images of the graphene-based devices taken at an applied voltage of 9 V.


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Figure 4. (a) Schematic illustration of phase transformation of the LixMoS2 by microwaveinduced heating of graphene. SEM and TEM images of the LixMoS2 nanosheets on graphene surface. Scale bars are 5 µm, 200 nm, and 5 nm, respectively. (b) XPS and (c) PL spectra of the


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LixMoS2 films treated with various microwave powers. (d) Schematic illustration of the photocurrent measurement setup using the graphene/TiO2 thin film. (e) Photocurrent-voltage characteristics of the graphene/TiO2 and bare TiO2 film in the dark and under illumination. (f) Time-resolved photocurrent of the graphene/TiO2 and bare TiO2 film measured by white light illumination under an applied bias of 5 V. (g) Schematic illustration and (h) electrical transport properties of OTFTs using either untreated or microwave-treated graphene for the source and drain electrodes.


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AUTHOR INFORMATION Corresponding Author E-mail: *[email protected] E-mail: *[email protected] E-mail: *[email protected] Present Addresses †Department of Graphic Arts Information Engineering, Pukyong National University, Busan 608-739, Korea. ACKNOWLEDGMENT This work was supported by the Center for Advanced Soft-Electronics funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (2016M3A6A5060953), and by Nano Material Technology Development Program through the NRF funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4021151), and by KERI Primary Research Program of MSIP/NST (No. 16-12-N0101-18 and 16-12-N0101-33). K. S. Kim supported from NRF (2010-0020207). J. Y. Hwang also partially supported from NRF (2016M3A7B4900135). ASSOCIATED CONTENT Additional experimental data such as surface temperature, AFM and FTIR mapping, water contact angle, Raman, transfer characteristics, transmittance, and XRD measurement of the graphene and MoS2 thin films. The Supporting Information is available free of charge on the Internet at http://pubs.acs.org.


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Table of Contents Graphic


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Ultrafast Heating for Intrinsic Properties of Atomically Thin Two

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