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Xenon Flash Lamp-Induced Ultrafast Multilayer Graphene Growth

Tae Hong Im, Dae Yong Park, Hwan Keon Lee, Jung Hwan Park, Chang Kyu Jeong, Daniel J. Joe, and Keon Jae Lee*

T. H. Im, D. Y. Park, H. K. Lee, J. H. Park, Dr. D. J. Joe, Prof. K. J. LeeDepartment of Materials Science and EngineeringKorea Advanced Institute of Science and Technology (KAIST)291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of KoreaE-mail: [email protected]. C. K. JeongKAIST Institute for NanoCentury (KINC)Daejeon 34141, Republic of Korea

DOI: 10.1002/ppsc.201600429

demonstrated with large scale and high quality, it is incapable of mass produc-tion for practical electronic applications due to multistep time-consuming process, including vacuum pumping, annealing, and cooling process. Each step takes from several minutes up to a few hours, and inevitably requires high-temperature pro-cess up to 1000 °C, results in substantial energy consumption during synthesis.

Alternatively, optically induced annealing methods have been considered as a promising candidate to solve some of the critical issues of the CVD method for graphene growth.[15–17] Focused laser beams utilizing various types of oscilla-tion including pulsed or continuous wave (CW) have been received investigated as

novel techniques for rapid graphene synthesis. Laser-assisted methods can provide sufficiently high temperature on target substrates to realize domain growth of a metal catalyst, and the decomposition of carbon precursors for graphene growth at room temperature. In spite of their focused local heating and rapid temperature rise, laser-assisted method has critical drawbacks with regard to large-scale production due to its small focused beam size, which typically ranges from a few micro-meters to millimeters. Moreover, graphene grown by pulsed laser methods exhibits different physical properties at each position because of variations in the energy density of each laser shot, which is an intrinsic problem of the pulsed laser equipment. Optical flash lamp annealing, using a broad spec-trum from ultraviolet to infrared, has also been studied due to its fast processing, large-scale annealing, and compatibility with roll-to-roll manufacturing.[18] Our research group has also demonstrated flash light-induced photothermal annealing on nanomaterials.[19] In particular, this technique has been widely applied to control biochemical reactions, improve the adhesion and conductivity of transparent electrodes, and manipulate the growth of nanostructures. Although several research groups have developed hydrocarbon photolysis using a xenon flash lamp,[20,21] only a few studies[22] involving the synthesis of 2D carbon materials have been reported.

Herein, we demonstrate a novel synthesis method of multilayer graphene (MLG) using xenon flash lamp with intense and broad spectrum light. A single irradiation of the flash light generates high temperature to a Ni thin film inducing decomposition of carbon precursor and segregation of the carbon atoms within milliseconds. The flash light intensity for MLG synthesis was optimized by a finite-element method

Optically induced annealing technology has provided promising strategies to synthesize graphene for future flexible electronics. Focused laser-assisted methods have the capability of direct graphene growth, but have significant drawbacks such as small beam size, and locally different intrinsic properties caused by energy variations of each pulsed shot. Herein, a novel synthesis method of multilayer graphene (MLG) via xenon flash lamp with intense and achromatic light is presented. An extremely high temperature (over 1500 °C) by a single flash irradiation induces the MLG with about 15 layers in milliseconds time scale. Experimental and theoretical studies reveal characteristics of the MLG with I2D/IG of 0.65 and average sheet resistance of 3.272 × 103 Ω sq−1 as well as photothermal interactions between flash and metal catalyst. The synthe-sized MLG is successfully transferred onto plastic substrates, demonstrating a new feasibility of the flash lamp system for next-generation flexible electronics.

Graphene

Graphene, a sp2-bonded monolayer of carbon atoms arranged in a 2D honeycomb crystal lattice, is one of the most widely studied nanomaterials for future flexible electronics including integrated circuits,[1] transparent conductors,[2] solar cells,[3] and smart sensors[4] due to its outstanding intrinsic properties.[5–7] In order to realize graphene-based electronic devices, it is first necessary to synthesize high-quality, high-throughput, and large-scale graphene. Ever since the electric field effect of gra-phene was measured in 2004,[8] there have been many efforts to synthesize graphene by mechanical/chemical exfoliation,[9,10] and epitaxial growth on a SiC substrates.[11] However, these methods have critical problems such as limitation of large-scale synthesis (mechanical exfoliation), inferior intrinsic properties (chemical exfoliation), and difficulty of graphene transfer proce-dure (epitaxial method), respectively.

Among current graphene synthesis methods, chemical vapor deposition (CVD) has received significant attention as an advantageous fabrication method.[12,13] Using hydrocarbon pre-cursors (methane, acetylene, etc.) and transition metal catalysts (Ni, Cu, and Co), low defect graphene can be synthesized by thermal annealing.[14] Although CVD-grown graphene has been

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(FEM) to avoid ablation of the metal catalyst that causes defects to the graphene during the xenon light process. Finally, the syn-thesized MLG was successfully transferred onto not only SiO2 (300 nm)/Si rigid substrate for verification of the dispersion property but also plastic substrates for flexible electronic device applications.[22] These results may open up a new feasibility of the xenon flash lamp system for mass production of graphene associating with highly productive roll-to-roll process.

Figure 1a presents a schematic illustration of the millisecond level ultrafast synthesis of MLG on metal thin film based on photothermal annealing by intense flash light. A Ni thin film was deposited on a quartz wafer and placed into a customized vacuum chamber to ensure suitable pressure and gas condi-tions. Ni film was selected as the metal catalyst due to its high carbon solubility (>0.1 at%), which leads to easier decomposi-tion of the hydrocarbon feedstock and dissolution of carbon into the Ni forming chemically stable NiC compound.[23] Acet-ylene (C2H2) gas was used as the hydrocarbon precursor due to its relatively low binding energy between carbon and hydrogen compared to methane (CH4) gas.[24] This lower binding energy results in a high pyrolysis rate, which is more suitable to the ultrafast graphene synthesis process. Argon (Ar) gas was flowed in the chamber to remove residual oxygen and water vapor in the chamber. Hydrogen (H2) gas was also used as a diluent gas, which plays important roles such as supporting decomposition of C2H2 gas, and passivation of defect sites in the substrate.[25] After the chamber was filled up with a mixture of Ar, H2, and C2H2 gas, the flash light was locally irradiated to the Ni thin film under room temperature. This light exposure induced extremely high and subsequent rapid heating/quenching pro-cess within a few tens of milliseconds, the C2H2 photolysis, and ultrafast carbon precipitation for the MLG synthesis. Detailed explanation of the synthesis process is described in the Experi-mental Section.

Figure 1b shows a photograph of the MLG synthesized on Ni film under the Ar, H2, and C2H2 gas environment by intense flash light irradiation, which had a pulse duration of 15 ms and light energy density of 30 J cm−2. The regions on the Ni sur-face exposed to the flash light exhibit a noticeable color change due to the extremely rapid melting and solidification of the Ni film, and simultaneous MLG growth. A scanning electron

microscopy (SEM) image, shown in the inset of Figure 1b, illus-trates the synthesized MLG grain and Ni thin film containing defects such as holes and vacancies as a result of damage from the intense light. The SEM image indicates relatively irregular shaped graphene grains with average sizes of 800 nm, which is typically considered to be an evidence of multilayered graphene with armchair or zigzag termination.

For practical applications of synthesized MLG, it needs to be transferred onto an insulator and/or specific substrate. Therefore, it is essential to transfer synthesized graphene on to arbitrary rigid/flexible substrates. Figure 1c presents a successfully transferred MLG to various substrates including SiO2 (300 nm)/Si, glass, and flexible polyethylene terephtha-late (PET) substrates (thickness of 50 µm) using conventional wet transfer method. In this transfer method, polymethyl-methacrylate (PMMA) was spin-coated and cured on the MLG/Ni film as a support layer, and the Ni film was then entirely etched away. After placing the PMMA/MLG layer on a target substrate, a proper amount of PMMA solution was dropped onto the PMMA/MLG/substrate. This process redis-solves the precoated PMMA layer, and improves the quality of the transferred graphene by increasing contact between the graphene and target substrate due to relaxation of the under-lying graphene.[26]

Since the qualities of graphene grown on a metal catalyst are determined by conditions of the metal catalyst surface, it is essential to identify the characteristics of the metal surface after flash lamp irradiation including uniformity, grain bound-aries, and domain orientation. Figure 2a shows an electron backscatter diffraction (EBSD) orientation mapping image of Ni thin film (thickness of 30 nm) after intense light irradia-tion with a pulse duration of 15 ms and light energy density of 30 J cm−2. These results indicate that the Ni film grew as a polycrystalline structure with an average domain size of 3.5 µm and a dominant (111) orientation direction. The formation of Ni (111) film is especially important for growing structurally homogeneous graphene due to the great lattice match between the Ni (111) surface and graphene, where the hexagonal lat-tice constant is 2.46 Å for graphene and 2.497 Å for Ni (111), respectively.[23] Compared to the 30 nm thick Ni film, the ori-entation of 100 nm thick Ni was analyzed by EBSD as shown


Figure 1. a) Schematic illustration of the process of synthesizing multilayer graphene (MLG) induced by xenon flash light. The inset shows a magnified illustration of MLG grown on Ni thin film (30 nm) deposited onto quartz substrate. b) Photograph and surface SEM image (inset) of the synthesized MLG with an island shape on the Ni film. The small black spots in the SEM image indicate crystalline defects. c) Photograph of the transferred MLG onto PET substrate (thickness of 50 µm). The flash light-induced MLG was also transferred onto SiO2 (300 nm)/Si (right top inset) and transparent glass (right bottom inset) substrates.



in Figure S1 (Supporting Information). As the thickness of the metal film increased, the number of (111) oriented domains clearly decreased while the number of (001) and (101) oriented domains increased. The X-ray diffraction (XRD) pattern shown in Figure S2 (Supporting Information) also exhibits the domi-nant orientation of domains with regard to the Ni film thick-ness after flash irradiation. This clearly represents that the 30 nm thick Ni film is preferred for homogeneous graphene growth.

In order to estimate the photothermal effect of the xenon flash light, we theoretically simulated temperature distribu-tion of the Ni thin film using finite element method (FEM). Figure 2b shows the temperature profile of the Ni film surface versus time after flash light irradiation. Heat flux and tempera-ture distribution were calculated by solving the unsteady heat transfer equation as the following[27,28]

1 , expoCT

tR I x y z T

x y

ρ απσ σ

α κ( )( ) ( ) ( )∂∂

= − − + ∇ ⋅ ∆ (1)

where ρ is the density (kg m3), C is the thermal capacity (J kg−1 K−1), R is the reflection coefficient, α is the absorp-tion coefficient of the Ni film, κ is the thermal conductivity (W m−1 K−1), and I0 is the input power of the flash light. We assumed that the flash light source had a single wavelength of 830 nm, with the highest intensity peak of the light source, as shown in Figure S3 (Supporting Information). The light source was modeled as a Gaussian function as follows

I x yx x y y

x y

, exp2 2

o0

2

2

02

2σ σ( ) ( )( )= − − −

−

(2)

where σx is the semi-x-axis and σy is the semi-y-axis of the Gaussian beam of xenon flash light. Within 15 ms after the light irradiation, the calculated temperature of the Ni surface increased exponentially up to 1530 °C. This high temperature induced by flash light is sufficient to not only melt and solidify the Ni film for (111) oriented domain growth but also decom-pose the carbon precursor for graphene synthesis. Because the xenon flash lamp allows large-area processability, the overall surface area of the Ni thin film uniformly reaches to over 1500 °C, as shown in the inset of Figure 2b. Additionally, the full width at half maximum (FWHM) of the calculated tempera-ture distribution is correlated with that of the actual flash light spectrum, which is the pulse duration of the xenon flash lamp (see Figure S4, Supporting Information). The remarkably high and rapid heating process of the flash lamp system enables the ultrafast and effective synthesis of MLG under room tempera-ture conditions.

The MLG grown on Ni thin film by flash lamp annealing was verified by Raman spectroscopy. Raman spectroscopy is extensively used in graphene research because it provides an unambiguous and nondestructive technique for determining the structure and electronic properties of graphene.[29,30] Figure 2c indicates the Raman spectrum of flash light-induced MLG under various H2 and C2H2 gas conditions. All six of


Figure 2. a) EBSD orientation mapping of the Ni thin film (thickness of 30 nm) after a single irradiation of intense flash light. b) A simulated tem-perature profile of the surface of the Ni film induced by flash light (pulse duration of 15 ms and light energy density of 30 J cm−2). The inset shows a 3D temperature distribution of the Ni (30 nm)/quartz substrate. c) Raman spectrum results of synthesized MLG on Ni thin film under different C2H2 (left column) and H2 (right column) gas flow conditions. d) The ratio of D to G peak intensity (ID/IG, red line) and 2D to G peak intensity (I2D/IG, blue line) under different gas flow conditions (top panel is H2 flow and bottom panel is C2H2 flow).



the Raman spectrum results exhibit three major peaks of gra-phitic materials corresponding to a defect-induced D peak, a primary in-plane vibrational G peak, and a two-phonon scat-tered 2D peak at ≈1360, ≈1581, and ≈2675 cm−1, respectively. The Raman peak intensity of the MLG shows similar results at different C2H2 and H2 gas flow rates. As the flow rate of C2H2 (left column) and H2 (right column) increased, the inten-sity of the D peak decreased and that of the 2D peak increased up to 10 sccm of C2H2 and 4 sccm of H2 gas, respectively. As additional amounts of each gas were supplied, however, the tendency of characteristic peak intensity was reversed that the D peak increased and 2D peak decreased, respectively.[29] Figure 2d displays the ratio of D and 2D to G peak intensity (ID/IG and I2D/IG) related to the graphene defects as well as the number of graphene layers. At 10 sccm of C2H2 and 4 sccm of H2, the ID/IG and I2D/IG values were 0.34 and 0.65, respec-tively. The value of I2D/IG confirms that the MLG was formed on the Ni film by the flash light annealing. At the same time, we could confirm the formation of MLG by the asymmetric and broader FWHM of 2D band (65 cm−1) compared to mon-olayer graphene (≈30 cm−1). The slightly higher value of ID/IG implies that the synthesized MLG contains a fewer number of crystalline defects such as vacancies, dislocations, and dangling bonds.[30] It is thought that defects are caused by photodamage and ablation of the metal catalyst due to poor adhesion between the Ni film and the quartz substrate, as shown in Figure S5 (Supporting Information). To reduce the number of crystalline defects, chrome (Cr) thin film was selected as an adhesion layer because its melting temperature of Cr (≈1900 °C) is higher than that of the Ni film, thus preventing exfoliation during the intense flash light process. Figure S6 (Supporting Information) provides the Raman spectrum results for different Cr layer thickness. As the thickness of adhesion layer increased, D peak intensity decreased and exhibited the lowest value (the ID/IG of 0.009) at 7 nm thick Cr layer, which is comparable to previously reported results. These results represent that a single irradia-tion by our flash lamp can melt the metal catalyst and precipi-tate carbon atoms for MLG synthesis on the metal film, which can be applied to mass production of graphene.

Figure 3a indicates X-ray photoelectron spectroscopy (XPS) spectrum which was obtained to characterize chemical bonding

and atomic composition of the flash light-induced MLG. The XPS survey spectrum present major peaks at binding energy (BE) of 285 eV for the C1s, 853 eV for the Ni2p3, and 532.5 eV for O1s corresponding to main element of graphene, Ni sub-strate, and adsorbed oxygen on the surface of MLG, respec-tively, as shown in the inset of Figure 3a.[31] High-resolution spectrum of C1s peak describes the degree of oxidation and functional groups of carbon atoms. The C1s peak of synthe-sized MLG is primarily characterized by four major compo-nents: a nonoxygenated CC or CH peak at BE of 284.5 eV, which corresponds to the sp2 carbon bonding in aromatic rings, defective graphitic structure, and CC sp3 bonding structure at 285.4 eV (CO or free radical) due to the change in localized electronic states. The small peak at 286.2 eV suggests the sub-stantial incorporation of nitrogen bonding on the MLG to be sp2 or sp3 C atoms, and carboxylate carbon groups (OCO) at 289.4 eV, respectively.[32] The oxygen-related peaks originate from crystalline impurities or oxygen in the reaction chamber during the synthesis procedure.

To determine the precise thickness and morphological characteristics of the MLG, atomic force microscopy (AFM) analysis was performed to obtain accurate lateral dimensions, surface distribution, morphology, and thickness of the MLG. The upper image of Figure 3b demonstrates the topography of the MLG synthesized by xenon flash light annealing. The relatively dark region corresponds to the SiO2/Si substrate, and bright region exhibits synthesized MLG layer. The thick-ness of MLG, indicated by the height histograms shown in the bottom image of Figure 3b, was found to have an average step height of MLG about 5.5 ± 0.5 nm. Based on the inter-layer distance of graphene (around 0.335 nm), it can be con-firmed that about 15–20 graphene layers are formed on the Ni film by the flash lamp annealing.[33] In order to examine the remaining organic residues and flatness of the surface, the root-mean-square roughness (Rrms) of the MLG on the SiO2/Si substrate was investigated. The measured Rrms value was about 0.9 ± 0.2 nm, indicating that the transferred MLG has suffi-ciently smooth and clean surface.[34] In order to analyze optical and electrical properties of the synthesized MLG, transmittance and sheet resistance were investigated as shown in Figure S7 (Supporting Information). The flash-induced MLG presented


Figure 3. a) XPS spectrum of the flash light-induced MLG. The C 1s peak of the MLG is separated into four peaks at 284.5, 285.4, 286.2, and 289.4 eV, corresponding to CC sp2, CC sp3, CN sp2, and OCO, respectively. The inset shows the XPS survey analysis of the MLG grown on Ni thin film. b) AFM image (top) and histograms of height distribution (bottom) of the MLG transferred onto SiO2 (300 nm)/Si substrate. The average thickness of MLG is 5.5 ± 0.5 nm. The inset in the bottom panel shows a topographical image of the MLG. c) Cross-section TEM image of the synthesized MLG on Ni substrate. The upper inset shows a magnified image of the MLG. The lower inset represents the FFT patterns.



high transmittance of ≈86% at a wavelength of 550 nm, which is inconsistent with the thickness of MLG measured by AFM analysis and theoretical value.[35] This mismatched result with theory was caused by the fact that the PET substrate more contributed to optical transmittance than small-sized gra-phene islands with average value of 800 nm (see the insets of Figure 2b and Figure S7a, Supporting Information). In addi-tion, the flash-induced MLG has a relatively high average sheet resistance of 3.272 × 103 Ω sq−1 due to the thick graphene with defects. The characteristics of the MLG could be improved by optimizing the lamp system with high power density, uniform light irradiation, and adjustable cooling rate. Finally, high- resolution transmission electron microscopy (HRTEM) analysis was conducted to confirm the thickness of the synthesized MLG, as shown in Figure 3c. The upper inset image displays a magnified cross-sectional TEM image of the flash light-induced MLG on a Ni thin film. The measured thickness of the MLG was about 6 nm, indicating about 15–20 layers of graphene. These results are also consistent with the MLG thickness meas-ured by the AFM height histogram. The fast Fourier transform (FFT) pattern of the TEM image shown in the bottom inset of Figure 3c indicated that the synthesized MLG has uniform lat-tice spacing (0.335 nm) along the zone axis, which agrees with the interlayer distance of the graphite.[33]

In summary, we have demonstrated a novel and ultrafast synthesis method of MLG using a xenon flash lamp system. A Ni thin film with a thickness of 30 nm was deposited on a quartz substrate as a metal catalyst for graphene growth. C2H2 gas, which has a higher photolysis rate than CH4 gas, was used as the carbon precursor. The FEM simulation theoretically con-firmed that a single exposure of flash light can instantaneously raise the substrate temperature up to 1530 °C within 15 ms, which enables the ultrafast graphene synthesis. Raman analysis showed that the synthesized MLG had ID/IG and I2D/IG values of 0.34 and 0.65, respectively, which indicate that the graphene was grown with about 15 layers and quite defects. The number of crystalline defects in MLG was reduced by additional deposi-tion of the Cr layer as an adhesive layer, which was confirmed by the low D peak intensity of the Raman spectrum results. The XPS analysis showed that CC sp2 bonding was the dominant peak compared to low value peaks related to impurities, which clearly indicates the formation of MLG. The accurate and pre-cise characteristic of MLG layers and topography was also veri-fied by AFM histogram distribution and TEM analysis, and the results were consistent with our other material analysis results. The flash light system can open up a new platform for synthe-sizing 2D materials, which can be applied to next-generation flexible and stretchable electronics.

Experimental SectionMultilayer Graphene Synthesis via Flash Lamp System: A quartz wafer

with a thickness of 700 µm and (100) orientation was initially cleaned by sequential ultrasonic baths in acetone, isopropyl alcohol (IPA), and deionized (DI) water. Subsequently, a thin Ni thin film (thickness of 30 nm) was deposited as a metal catalyst for graphene growth on the quartz wafer by e-beam evaporator with a deposition rate of 0.1 Å s−1. Afterward, the prepared sample was placed in a customized vacuum chamber containing a light window with a diameter of 4 cm, and pumped down to 10 mTorr to remove ambient contamination such as

adsorbed oxygen and nitrogen. Thereafter, 100 sccm of Ar, 4 sccm of H2, and 10 sccm of C2H2 gas were sufficiently flowed into the vacuum chamber. After adjusting the process pressure to 100 Torr, a xenon flash light (pulse duration of 15 ms and charging voltage of 100 V corresponding to the light energy density of 30 J cm−2) was exposed to the Ni (30 nm)/quartz wafer at room temperature.

Analysis of Characteristic of the MLG: Raman spectroscopy (LabRAM Aramis, Horiba Jobin Yvon) was performed at room temperature using an Ar ion laser (514 nm) focused on MLG with a spot size of 1 µm. XPS analysis was conducted using a monochromatic Al Kα excitation under the pressure of 5 × 10−9 Torr (Sigma Probe, Thermo VG Scientific, Inc.). XRD analysis of the Ni thin film (30 and 100 nm) was performed by θ/2θ scanning of the surface using parallel beam optics (Ultima IV, RIGAKU). AFM (XE-70, Park Systems) analysis was conducted by contact mode using an Au-coated silicon nitride tip, and the scan condition included dimensions of [(X:Y = 3:3) µm] with a scan rate of 0.1 Hz at resolution of 1024 pixels. The optical and electrical properties were measured using UV–vis–NIR spectrophotometer (Solid spec-3700, Shimudzu) and Hall effect measurement (HMS-3000, Ecopia), respectively. For the cross-sectional TEM analysis, the sample was coated with amorphous carbon as a capping layer and prepared using a focused ion beam (FIB, Helios Nanolab 450 F1, FEI Company). The TEM image was observed using Tecnai F30 ST (FEI Company) at 300 kV.

Finite-Element Method Simulation: COMSOL Multiphysics 5.2 was used to calculate the thermal effect of the flash lamp light on the Ni thin film. The absorption and reflection coefficients of the Ni thin film (30 nm) at wavelength of 830 nm were measured by a UV–vis–NIR spectrophotometer (SolidSpec-3700, Shimadze). Heat diffusion of the completed substrate was calculated by solving an unsteady heat transfer equation. The entire structure was divided into 21,375 elements.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsT.H.I. and D.Y.P. contributed equally to this work. This work was supported by Nano Material Technology Development Program through the National Research Foundation (NRF) funded by the Ministry of Science, ICT and Future Planning (grant code: NRF-2016M3A7B4905609), and by Creative Materials Discovery Program through the NRF funded by the Ministry of Science, ICT and Future Planning (grant code: NRF-2016M3D1A1900035).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsphotothermal effect, ultrafast graphene synthesis, xenon flash lamp

Received: December 28, 2016Revised: March 7, 2017

Published online:

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xenon flash lamp‐induced ultrafast multilayer graphene growthfand.kaist.ac.kr/attach/ppsc -...

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