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Vol. 8. Issue 5.
Pages 3752-3763 (September - October 2019)
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Vol. 8. Issue 5.
Pages 3752-3763 (September - October 2019)
Original Article
DOI: 10.1016/j.jmrt.2019.06.035
Open Access
Poly(methyl methacrylate)-derived graphene films on different substrates using rapid thermal process: a way to control the film properties through the substrate and polymer layer thickness
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Kathalingam A.a, Hafiz Muhammad Salman Ajmalb, Sivalingam Rameshc, Heung Soo Kimc, Sam-Dong Kimb, Soo Ho Choid, Woochul Yangd, Ki Kang Kime, Hyun-Seok Kimb,
Corresponding author
hyunseokk@dongguk.edu

Corresponding author.
a Millimeter-wave Innovation Technology (MINT) Research Center, Dongguk University-Seoul, Seoul, 04620, Republic of Korea
b Division of Electronics and Electrical Engineering, Dongguk University-Seoul, Seoul, 04620, Republic of Korea
c Department of Mechanical, Robotics and Energy Engineering, Dongguk University-Seoul, Seoul, 04620, Republic of Korea
d Department of Physics, Dongguk University-Seoul, Seoul, 04620, Republic of Korea
e Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul, 04620, Republic of Korea
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Abstract

Increasing interest and applications for graphene and carbon-based films emphasize the need for economical synthesizing techniques. We report a facile and novel synthesis method to prepare graphene, graphitic carbon, graphitic carbon nitride composite layers depending upon the spin-coated poly(methyl methacrylate) (PMMA) polymer layer and substrate used. Few and multilayer graphene sheets were formed on SiO2 covered Si substrate using a simple rapid thermal annealing process. We examined hot plate and rapid thermal annealing using a nickel capping layer and found that the rapid thermal process converted PMMA into graphene efficiently. The resultant graphitic films were characterized using FESEM, HRTEM, XRD and Laser Raman. Current–voltage response of the prepared graphene layers was analyzed fabricating as two terminal devices. The thickness of the formed layer depended on PMMA layer thickness, and the metal capping layer was crucial for converting PMMA into graphene. This polymer conversion method to fabricate graphene layers will be attractive for many graphene applications due to its versatility.

Keywords:
Graphene
Graphitic carbon
Graphitic carbon nitride
Poly(methyl methacrylate)
Polymer
Rapid thermal annealing
Full Text
1Introduction

Graphene is a two-dimensional (2D) layered material with sp2 hybridized carbon atoms arranged in a hexagonal honeycomb structure, providing large surface area, and has many exciting applications since it exhibits outstanding mechanical, thermal, and electrical properties, used in wide variety of fields with excellent performs [1–5]. It can potentially be applied to many devices from electronic devices to bio, chemical sensors, and energy devices due to its multidimensional properties. Although graphene was only practically realized in 2004, it has been widely explored. However, despite extensive exploration and wide application, challenges remain regarding production strategies to produce high quality layer selected graphene sheets at low cost.

Many methods are currently used to prepare graphene sheets, including chemical vapor deposition (CVD) [6,7], thermal and chemical exfoliation [8], micromechanical cleavage [9,10], graphene oxide reduction, scotch tape, direct ultrasound sonication, vacuum thermal annealing, etc. [11–13]. Each method has its own merits and demerits, but large area production limits their application. A suitable method to easily and economically produce large graphene sheets remains urgently required, capable of large scale graphene sheet production with specific properties suitable for particular applications.

The widely used CVD method is highly expensive, requiring considerable resource to produce the controlled environment for graphene film preparation. Most CVD based methods physically transfer the grown graphene layers onto pre-patterned substrates to fabricate devices, which can cause adverse effects on the films leading to poor performance. Hence the transfer process requires particular care and consumes significant time and energy. Scotch tape and other mechanical exfoliation methods cannot effectively control the number of layers and can also damage the material, which cannot be rectified. Epitaxial growth methods also face problems to control layer numbers and transfer to patterned substrates. Therefore, methods to directly construct graphene layers on the required substrate are urgent required to fabricate graphene based devices.

Graphene is generally formed by evaporating carbon based organic precursors and controlled deposition onto the substrate [14]. Poly(methyl methacrylate) (PMMA) polymer is a commonly employed precursor for graphene and carbonaceous material preparation, and is also commonly used as a photoresist for electron beam and scanning probe lithography, and as a polymer matrix to prepare nanomaterial composites [15,16]. The PMMA polymer chain can be separated and rearranged into new forms with different properties at high temperatures. Heating PMMA above its thermal decomposition temperature (310 °C) produces chemical bonds dissociation, causing sp2 hybridization of carbon atoms, forming graphene sheets. In contrast to most other materials used in CVD methods, PMMA is inexpensive and can be easily prepared as films by spin coating, where coating thickness can also be adjusted as desired by varying the solution concentration or by spin coating speed.

We formed carbonaceous films from PMMA by thermal conversion using hot plate heating with and without a metal capping layer. However, hot plate heating did not yield useful results. Therefore, we tried rapid thermal annealing (RTP) and found very good graphene formation. The RTP process comprised annealing spin coated PMMA layers with a Ni capping layer at different temperatures and durations. We selected Ni for the capping material since it is a good catalyst. Thus, graphene is grown directly on the insulating dielectric material, following the desired device pattern, avoiding any subsequent physical transfer. In contrast to current graphene production methods, RTP is a very short process, hence minimizes power requirements for vacuum and heating. The recently proposed microwave heating method is a fast process at low temperature, but is only suitable to help prepare metal and graphene hybrid powder samples and cannot be used to prepare thin films [1]. The RTP technique is a simple process that can produce large area graphene sheets directly onto pre-patterned substrate at low cost.

To the best of our knowledge, this is the first report to propose converting PMMA directly into graphene using RTP annealing. Although Byun et al. investigated converting PMMA into graphene using high temperature annealing in a vacuum furnace [17], they only reported a Raman study of the prepared graphene, and did not consider the metal capping layer etching details and PMMA thickness variation. Similarly, Sun et al. also reported on PMMA-derived graphene using high temperature annealing for 10 min. In which, they not used metal capping layer, instead they used Cu metal as catalytic layer below the PMMA layer. Moreover, they used high temperature furnace with H2/Ar atmosphere, and tuned the graphene layer numbers by controlling gas flow pressure, temperature and time. As the graphene was formed on Cu metal layer unlike ours formed on SiO2 layer, it requires a separate transfer process to fabricate devices [18]. Hence, we report direct PMMA conversion into graphene sheets using a fast and facile technique, where final properties were ascertained using various characterization techniques, and depend on initial PMMA thickness, and annealing temperature and time.

2Experimental section

This study employed poly(methyl methacrylate) with molecular weight 850,000 g/mol, purchased from Sigma-Aldrich. We used silicon with 300 nm SiO2 layer as the substrate to form graphene layers. Three PMMA concentration solutions were prepared by dissolving PMMA in toluene at 1:4, 1:8, and 1:12 volume ratios. We then cleaned 2 × 2 cm areas of the SiO2/Si substrates by sonication in acetone and isopropanol for 10 min each, rinsed with de-ionized water, and finally dried by nitrogen air blow. The prepared PMMA solution was then spin cast onto cleaned substrate at 3000 rpm for 30 s, and the coated PMMA films were baked at 110 °C for 10 min to remove excess solvents. Spin coated PMMA film thickness measure using α-step200 Surface profiler (Tencor Instruments) was found to be ˜15, 10, and 5 nm, respectively, for the three solutions. After preparing the PMMA layers, we coated a ˜50 nm thick nickel (Ni) layer as a capping over the PMMA layer. The resultant layers were annealed at 350 °C and 500 °C using a hot plate for 1 h with and without Ni capping. The remaining PMMA films were annealed using various RTP temperature and duration regimes. Finally, the Ni capping layer was removed by wet chemical etching using HNO3, HCl, and DI water solution with 5:5:90 volume ratio, respectively.

The formed graphene layer structural and chemical properties were examined using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDAX), high resolution tunneling electron microscopy (HRTEM), and laser Raman studies. We used a Hitachi Ultrahigh Resolution SEM (S-4800) with attached EDAX module for SEM and EDAX measurements. Laser Raman spectra were collected using a LabRam Aramis (Horiba Jobin Yvon) device with 514 nm excitation wavelength argon ion laser. TEM images were obtained after transferring the graphene layers onto a TEM Cu grid. For thick PMMA layers (>20 nm) with thinner Ni capping (˜25 nm), the Ni layer was detached from the substrate with formed graphene while dipping into 1 M HNO3 solution. Then, the Ni was completely dissolved leaving the graphene layer floating on the solution, which was then transferred onto a Cu grid for TEM measurement. The converted graphene layers were also analyzed for current–voltage response using in two-terminal configuration. For the fabrication of two-terminal devices silver (Ag) paste was used as contacts on the Ni layer without etching.

3Results and discussion

Fig. 1 shows the proposed fabrication procedure adapted in this work. The spin coated PMMA layer was converted as graphene using RTP annealing at high temperatures.

Fig. 1.

Proposed process to directly convert PMMA into graphene sheets.

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Different thicknesses PMMA films coated on SiO2 covered silicon substrates by spin coating method were annealed at different temperatures using hot plate and RTP heating. E-beam evaporated Ni metal capping layer was applied over the PMMA layer to investigate the effect of Ni on PMMA conversion into carbonaceous material. Samples heated to 350 °C and 500 °C for 1 h using a hot plate in open atmosphere without Ni capping did not show any trace of carbonaceous material, whereas Ni capped PMMA layers showed significant carbon traces in their Raman spectrum, discussed in Section 3.5 below.

3.1Optical microscopy

Consequently, films were annealed using RTP under vacuum with and without metal capping layer for different temperatures and durations. We first annealed the PMMA layers at 250 °C for 30 s. This did not show appreciable conversion, rather PMMA was largely evaporated leaving some residue. Fig. 2(a) shows a typical microscope view of the resultant substrate, with a closer magnification in the inset. We then increased RTP temperature to 500 °C for 5 s duration, with no significant improvement, although some PMMA material remained, shown as dots on the substrate in Fig. 2(b).

Fig. 2.

Typical optical microscope images for poly(methyl methacrylate) films after rapid thermal annealing at (a) 250 °C for 30 s and (b) 500 °C for 5 s.

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This outcome indicated that the PMMA was evaporated, leaving only a small residue due to the relatively short evaporation time. Therefore, we applied a Ni capping layer over the PMMA layer(s) to avoid evaporation. The metal capped PMMA layers were annealed at 350 °C and 500 °C for 1 h using a hot plate, showing some carbon traces in their Raman spectra. Finally, the Ni capped PMMA layers were annealed at 500 °C for 10 s, 800 °C for 10 s, and 1000 °C for 2 s using RTP heating. Figs. 3 and 4 show optical microscope images for RTP films at 800 °C for 10 s and 1000 °C for 2 s, respectively. Fig. 3(c) inset shows an enlarged view of the graphene sheet transferred onto a substrate, where the graphene sheet exhibits wrinkled transparent silk veil-like morphology. Similarly, Fig. 4(c) shows a thick graphitic layer after etching.

Fig. 3.

Typical optical microscope images for 5 nm thick PMMA layer after (a) RTP annealing at 800 °C for 10 s, (b) etching for 25 min; and (c) close view of the etched portion.

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Fig. 4.

Typical optical microscope images for 20 nm thick PMMA layer after (a) RTP annealing at 1000 °C for 2 s, (b) etching for 10 min, and (c) etching for 25 min.

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3.2X-ray diffraction

The formed graphene layers were investigated under x-ray diffraction (XRD) to analyze their structural properties. Fig. 5 shows XRD peaks for 15 nm thick PMMA layer after annealing at 1000 °C for 2 s using RTP heating. Although the Si peaks are dominant (Fig. 5(a)), closer view (Fig. 5(b)) shows peaks at approximately 26.8°, 44.75°, 52.40°, 62°, and 76.85°, corresponding to (002), (101), (004), (103), and (110) planes of hexagonal graphite (JCPDS file no. 41-1487) [13,19–21]. The peak at 26.8° (002) corresponds to typical graphite, indicating multilayer graphene formation with 3.25 Å interplanar distance. The other peaks are from the Si substrate [22], with the wide peak around 20° most likely due to the silicon oxide layer.

Fig. 5.

X-ray diffraction (XRD) pattern for (a) 15 nm thick PMMA layer annealed at 1000 °C for 2 s using RTP heating and (b) enlarged view of the XRD pattern.

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3.3Electron microscopy

We used SEM, TEM, and AFM to analyze prepared graphene layer morphological features. Fig. 6a that 50 nm Ni layer surface over 15 nm PMMA layer after RTP annealing at 800 °C for 10 s exhibited an island-like Ni layer. However, after etching for 10 min, Fig. 6(b) shows that the Ni layer has mostly dissolved, revealing graphite sheets below the Ni layer and between unetched agglomerated Ni clusters. Further 20 min etching reveals sheet-like graphite layers where their thickness depends on the 15, 10, and 5 nm pre coated PMMA thickness (Fig. 6(c)–(e)), respectively. The thicker layers (Fig. 6(c) and (d)) show flake-like graphite sheets, whereas the thinnest layer (Fig. 6(e)) exhibits very thin irregular shapes distributed throughout the surface, indicating the presence of few layer graphene.

Fig. 6.

Typical scanning electron microscope images for 50 nm Ni layer coated over 15 nm poly(methyl methacrylate) (PMMA) layer after (a) rapid thermal annealing at 800 °C for 10 s, (b) etching for 10 min; and etching for 20 min with initial PMMA layer thickness (c) 15 nm, (d) 10 nm, and (e) 5 nm.

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Fig. 7 shows typical SEM images for PMMA (5 nm)/Ni (50 nm) layers annealed at 1000 °C for 2 s and subsequently etched for 5, 15 and 25 min (Fig. 7(a)–(c), respectively). The appearance flakes in all surfaces and between the Ni clusters indicates graphene layer formation with strong sp2 bonding between carbon atoms within the PMMA polymer.

Fig. 7.

Typical scanning electron microscope images for a 5 nm thick poly(methyl methacrylate) with 50 nm Ni layer after (a) annealing at 1000 °C for 2 s and subsequently etched for 5 min; and etched for (b) 15 min, and (c) 25 min.

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Thicker PMMA layers with 25 nm capping layer, annealed at 800 °C for 10 s was detached from substrate while immersing in 1 M HNO3 solution for Ni etching. After dissolving the Ni, the formed graphene was clearly observed floating on the etching solution. This detached graphene was transferred onto a substrate for TEM characterization, where optical microscopic images of detached graphene layers is shown in Fig. 3(c), inset.

Fig. 8 shows typical TEM images of graphene sheets, with (a) transparent and (b) wrinkled sheet-like (b) graphene layers. Graphene self-assembles into sheets to form a stack of layers. The visual black spots are due to residual undissolved Ni or some impurities introduced while transfer of graphene to the Cu grid. The observed fringe patterns are due to misaligned or folded of graphene sheets. Inset of Fig. 8(b) shows SEAD patterns of the graphene sheet, indicating formation of crystalline graphene layers. These results suggest that the formed graphene layers are oriented randomly as wrinkled and folded sheets.

Fig. 8.

Typical transmission electron microscopy images for (a) 15 nm poly(methyl methacrylate) layer annealed at 800 °C for 10 s, and (b) SEAD image and pattern.

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3.4Atomic force microscopy

Fabricated graphene layer topology was also investigated using atomic force microscopy (AFM), providing precise graphene layer thickness, as shown in Fig. 9(a)–(e) for different shape and size of the graphene layers formed below the Ni capping layer. Fig. 9(a) and (b) shows the substrate with and without Ni capping layer, respectively, over the 15 nm PMMA layer after RTP annealing. Samples exhibit irregularly arranged graphite flake layers with different shape and sizes. Comparing Ni layer unetched and etched samples (Fig. 9(a)), we can infer that the PMMA layer under the Ni layers was wholly converted to graphene. Fig. 9(c) AFM height profile of the sample Fig. 9(b) indicates graphene layer thickness ≈12 nm. Similarly, Fig. 9(d) and (e) shows samples prepared with 10 nm and 5 nm PMMA layers, respectively, and etched with different Ni etching times. Fig. 9(e) shows a relatively smooth surface, without Ni layer except remnants aside from some small dots spread over the substrate when etched was extended (25 min). Apparent layer thickness of formed graphene sheets is indicated in attached line profile of each AFM image. The variation in thickness of the graphene layers confirms the PMMA layer dependent thickness of the formed graphene.

Fig. 9.

Typical atomic force microscope (AFM) images for 15 nm poly(methyl methacrylate) (PMMA) layer after rapid thermal annealing (RTP) at 1000 °C (a) before and (b) after etching the Ni capping; and (c) the corresponding line profile; (d) 10 nm PMMA layer etched for 10 min and (e) 5 nm PMMA layer etched for 25 min, with respective line profiles.

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3.5Raman spectroscopy

Raman spectroscopy is a very useful nondestructive technique to analyze chemical composition and quality for graphene and other carbon based materials, and also allows investigating disorder. Generally, three peaks centered at 1350, 1580, and 2690 cm−1 were obtained, corresponding to graphene D, G, and 2D bands [23]. The disorder peak D was assigned to A1g symmetry breathing mode K phonons, and indicates the level of defects in the graphene layer. The graphitic peak G confirms the graphitic nature of the carbon material denoting the E2g phonon mode of first order scattering from sp2 hybridized carbon. The 2D band denotes second order Raman spectrum from the hexagonal carbon networks [24].

We captured Raman spectra for different thickness PMMA films, annealed at various temperatures and times, as shown in Fig. 10, where Fig. 10(b) shows enlarged portions (1100–1600 cm−1) of the figures in (a). Spin coated PMMA films annealed at 350 °C and 500 °C using hot plate for 1 h under atmospheric conditions did not exhibit significant Raman signal for graphene aside from some trace of amorphous carbon. On the other hand, the PMMA film annealed at 350 °C by covering e-beam coated 50 nm Ni layer also showed some trace of amorphous carbon, but only the G peak, as shown in Fig. 10(b), with similar outcomes for annealing at 500 °C. This inspired us to further study thermal conversion of PMMA at increased temperature. Therefore, we explored PMMA conversion under controlled RTP annealing for different temperatures and durations.

Fig. 10.

Typical Raman spectra for poly(methyl methacrylate) (PMMA) layer (a) annealed using a hot plate at different temperatures with and without Ni layer, and (b) enlarged view of (a) in the range 1100–1600 cm−1.

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The PMMA films annealed at 500 °C using RTP for 10 s did not exhibit peaks related to graphene, but rather exhibited two broad peaks at 1350 and 1580 cm−1 (D and G bands, respectively) without a 2D band peak ∼2700 cm−1 (Fig. 11(a)). These broad and overlapping D and G bands indicate the appearance of amorphous graphitic carbon, possibly sp2 and/or sp3 hybridized carbon bonds, which can be identified using Raman characteristic peaks. Fig. 11(b) shows the deconvoluted Raman spectra from Fig. 11a (500 °C, 10 s) exhibits five bands (G, D1 (D), D2, D3 and D4), where the G and D2 peaks were due to graphitic lattices. The D3 band originated from amorphous carbon present in the distorted carbon lattice. These peaks confirm amorphous carbon formation when annealing PMMA at 500 °C for 10 s, and the observed D4 band confirms CC and CC stretching vibrations of graphitic carbon [8,25].

Fig. 11.

(a) Raman peaks of poly(methyl methacrylate) (PMMA) films following rapid thermal annealing (RTP) at various temperature and time, (b) deconvoluted D and G Raman peaks for PMMA annealed at 500 °C for 10 s; PMMA layer annealed at 800 °C for 10 s deconvoluted (c) G peak and (d) 2D peak.

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We then increased the annealing temperature to 800 °C for 2 s, with impact on Raman spectra as shown in Fig. 11(a). This increased temperature retains amorphous carbon as the 500 °C annealed sample. However, annealing at 800 °C for 10 s produced a multilayer graphene-like peak pattern showing all three peaks: D, G, and 2D, corresponding to 1350, 1580 and 2685 cm−1, respectively, as shown in Fig. 11(a). These frequencies show good agreement with the three characteristic graphitic material peaks. Analysis of individual peaks can further clarify the nature of the graphene formed. Fig. 11(c) and (d) shows deconvoluted G and 2D peaks respectively, confirming the crystalline graphitic structure. The Lorentzian dissolved G peak comprised two peaks (G1, G2), as shown in Fig. 11(c). Generally, the G peak should be symmetric for perfect crystalline graphene, hence the current outcome strongly indicates graphitic carbon production [26].

Fig. 11(d) shows that the 2D peak’s full width at half maximum ≈69 cm−1, which is comparable to CVD prepared graphene layers [1], and hence confirms graphene growth. The de-convoluted 2D peak shows two Lorentzian peaks with 16 cm−1 separation. This further confirms formation of few to multilayer graphene. In addition, the intensity ratios of the peaks (ID/IG = 0.79), (I2D/IG = 0.89) also confirm few layer graphene formation [4].

Fig. 12(a) shows Raman spectra for graphene formed by different thicknesses PMMA layers after RTP at 1000 °C for 2 s. Graphene sheet thickness is strongly dependent on the original spin coated PMMA layer thickness, and increased PMMA layer thickness altered peak intensity without position shifts. Thus, there is only thickness dependence in the crystalline arrangement without extra impurity defect. However, the shoulder peak (G) for the 15 nm PMMA layer indicates more defect formation for thicker PMMA layers: the thinnest PMMA layer exhibited the most uniform graphene layers throughout the surface, whereas thicker PMMA layers exhibited relatively poor film uniformity.

Fig. 12.

(a) Raman spectra for graphene layers formed by different thickness poly(methyl methacrylate) (PMMA) layers after rapid thermal annealing (RTP) at 1000 °C for 2 s,(b) Raman spectra for different graphene locations in the sample (inset: locations considered) converted by annealing 10 nm poly(methyl methacrylate) (PMMA) after rapid thermal annealing (RTP) at 1000 °C for 2 s.

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Raman spectra were obtained from three different location for the 10 nm thick PMMA layer after RTP at 1000 °C for 2 s without removing the Ni capping layer, as shown in Fig. 12(b). Although all three peaks occur (D, G, and 2D), their intensities are substantially different indicating significant film inhomogeneity. Fig. 12(b) inset visually confirms that the film surface is not uniform thickness, and highlights the necessity for thin and uniform initial PMMA and capping layers.

Samples exhibited similar Raman peaks before and after Ni layer removal, confirming graphene presence under the Ni metal layer; whereas thicker PMMA layers exhibited different Raman spectra peak shapes and intensities with and without the capping layer removed. Therefore, some variation remains in the PMMA conversion to graphene, and it is largely the top layer, which is in direct contact with the Ni layer, that has converted to quality graphene due Ni catalytic effects; with PMMA close to the SiO2 layer forming relatively poor quality graphene. This suggests that thinner PMMA films are more likely to produce higher quality graphene layers.

We also used SiN covered silicon substrate to investigate substrate dependent effects on PMMA conversion into carbon-based materials. XRD and Raman spectra for 15 nm PMMA layer coated onto SiN/Si substrate after RTP at 1000 °C for 2 s exhibited mixed graphitic carbon phases (g-C), graphitic carbon nitride (g-C3N4) phases, and Ni peaks (Fig. 13). As shown in Fig. 13(a), XRD peaks located at 26.8, 44.48, 53.42 and 70.08° corresponds to (002), (004), (110) and (112) planes of graphitic carbon, respectively, whereas, the peaks 13.03° and 28.65° attributes (100) and (002) planes of g-C3N4, respectively. Raman spectrum (Fig. 13(b)) displays broad peak combination of D (1355 cm−1) and G (1594 cm−1) bands with a weak 2D band (2687 cm−1) indicating defective graphitic carbon film. The SiN/Si substrate has produced higher Raman spectra D peaks compared with SiO2/Si substrates, indicating the presence of more defects and sp3 content, similar to diamond like carbon (DLC). This outcome warrants further study to explore substrate dependent properties for PMMA derived carbon, because at high temperature the substrate can also play role in deciding quality and nature of converted carbon materials.

Fig. 13.

(a) XRD and (b) Raman profile (inset: deconvoluted D and G Raman peaks) for 15 nm thick PMMA layer on SiN and annealed at 1000 °C for 2 s using RTP heating.

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In order to analyze the electrical properties of the graphene films, the prepared graphene layers were subjected for current–voltage (I–V) characterization using silver dots on to the Ni layer as electrical contacts. Fig. 14 shows the I–V plots of the device fabricated with 5 nm graphene layer for different biases at dark condition. The inset of the figure displays the schematic representation of the fabricated graphene two terminal device. This I–V plots indicate the asymmetric response of the device irrespective of the applied voltages which could be aroused from the Schottky junction formed between the Ni and Graphene layers. The Fig. 14(b,c) displays the cyclic sweep of applied voltage at different bias. Invariably, all the biases have resulted a hysteresis-like I–V nature. Whereas, the devices prepared without PMMA showed only a normal I–V curve as shown in Fig. 16(d) without any hysteresis for the cyclic scan. It attributes the graphene-induced response on the I–V nature of PMMA incorporated devices, which indicates the inherent property of the graphene.

Fig. 14.

Current–voltage plots of 5 nm thick graphene layer two terminal device, (a) for different applied biases; inset schematic of the fabricated device, (b) cyclic sweep at 2 V, (c) cyclic sweep at 5 V and (d) cyclic sweep at 10 V.

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Graphene is quite a different material, has ambipolar and zero band gap in contrast to common semiconductors, hence it shows unexpected and new properties. Its carrier mobility is also high compared to other semiconductors. Moreover, graphene is very sensitive to atmosphere and its adjoining metal atoms or molecules causing hysteresis nature in I–V curve [27]. Hence, this hysteresis can be ascribed by several factors such as adsorbates, trapped charges and interaction with adjacent layers or substrates [28]. As shown in the figures, the hysteresis nature of the graphene depends on the applied potential strength and surrounding condition such as illumination. An applied bias of 5 V has produced a significant hysteresis loop both in positive and negative voltages compared to other biases either for low (2 V) or high (10 V). It attributes that the hysteresis effect is increased with applied potential upto certain optimum range, beyond this range of potential it is reduced due to the potential-induced polarization effects.

Similarly, the I–V nature of the devices was also obtained under different illuminated conditions. Fig. 15(a) shows the I–V plot of 5 nm graphene device observed at different wavelengths of illumination. No appreciable change is obtained for the illumination of the device with light of different wavelengths. The Fig. 15(b) presents the effect of 700 nm light irradiation on to the device and its hysteresis nature. It indicates that the hysteresis effect is reduced at illumination compered to dark condition.

Fig. 15.

Current–voltage plots of 5 nm thick graphene layer two terminal device, (a) at different illumination and (b) cyclic sweep at 700 nm wavelength light excitation.

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Fig. 16.

Current–voltage plots of 15 nm thick graphene layer two terminal device, (a) dark I–V plots of the devices with 5 nm and 15 nm graphene layers, (b) dark and illuminated I–V plot of the 15 nm graphene device, (c) cyclic I–V plot of the device at dark condition and at (d) illuminated condition.

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The Fig. 16 shows the I–V plots of both 5 nm and 15 nm thick graphene devices, indicating a high current value of the devices with 15 nm thick graphene layers. However, the 15 nm thick graphene also showed a very feeble effect due to the illumination of light as shown in Fig. 16(b), where it gives I–V plots of the devices under dark and 700 nm wavelength light. Other wavelengths showed very smaller change compared to 700 nm. Similarly, the Fig. 16(c and d) shows the cyclic I–V plots of the device under dark and under 700 nm wavelength light illuminated conditions. There is no noticeable change in the current levels of both forward and reverse scans signifying the reduced hysteresis effect at higher thickness graphene layers. It concludes that the application of high voltage and irradiation of light are found to suppress the hysteresis effect significantly.

4Conclusions

Graphite-like multilayered graphene sheets were successfully fabricated onto SiO2/Si substrate by RTP of spin coated PMMA layers. We found that a Ni metal capping layer was vital for quality PMMA conversion into graphene, to avoid polymer evaporation at high annealing temperatures, and also to act as a catalyst. The number of stacked layers can be tuned simply by controlling the initial PMMA layer thickness. PMMA layer RTP at 800–1000 °C for short spans (2–5 s) can efficiently convert PMMA into carbon based materials. Small temperature and time variations can change fabricated graphene characteristics. Two-terminal current–voltage measurement done on the formed graphene layers showed thickness dependent changes on the variation of light illumination and applied biases. The proposed approach provides a facile and cost effective technique that could be used to fabricate layer number and property selective graphene layers directly onto patterned substrates without requiring physical transfer.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgments

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2017R1D1A1A09000823).

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