Ni induced few-layer graphene growth at low temperature by pulsed laser deposition

Ni induced few-layer graphene growth at low temperature by pulsed laser deposition K. Wang,1,a G. Tai,1,2 K. H. Wong,1 S. P. Lau,1 and W. Guo2 1Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong SAR, China 2Institute of Nanoscience, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, China


I. INTRODUCTION
5][6][7] Utilization of all these attractive and inspiring properties of graphene for practical purposes, however, relies on the availability of good and controllable fabrication technique.Historically, graphene was produced by micromechanical cleavage. 5At present, graphene syntheses mainly use thermal decomposition of a C containing material, such as SiC and C 60 , and metal induced graphitization. 8,9 ecent research focus is on developing preparation technique to produce graphene with controllable few-layer thickness.Graphitization of amorphous C (a-C) by using catalytic transition metals, such as cobalt (Co), 10 platinum (Pt), 10,11 ruthenium (Ru), 12 iridium (Ir) 13 and nickel (Ni), 14,15 is the most popular way to produce graphene.It has been known for many years that various forms of carbon were observed on metal surfaces after thermal treatment at elevated temperatures. 16,17 he dominant mechanism involves the dissolution and precipitation of a-C in metal, which is similar to metal-mediated crystallization of silicon and germanium. 18,19 he number of graphene layers thus formed relies on how much C precipitates from C-metal solid solution.As a result, a fine control of the thickness ratio of C to metal is crucial for the few-layer graphene formation.Up-to-date, chemical vapor deposition (CVD) is the most successful method to produce high quality graphene.It involves passing through hydrocarbon gases in a tube held at elevated temperature and a chemical reaction on the active metal surface.In most cases, the processing temperature needs to be as high as 1000 • C. 8,9,20 With the current fast growing interests in graphene-based nanotechnology, there are great demands on high quality few-layer graphene.We have developed a simple and fast processing technique based on pulsed laser deposition (PLD) to grow few-layer graphene at reduced temperature a Author to whom correspondence should be addressed.Electronic mail: wangkai369@hotmail.com of 650 • C on Ni thin film.These days, Ni, together with copper (Cu), have received the most attention as substrate materials for graphene growth.We chose Ni because it is inexpensive and is a standard material for electronic applications.In contrast to conventional PLD growth of C films, in which the concerns are mainly on the deposited C layer thickness, our work focuses on the fine control of the relative thicknesses of the deposited C and Ni films, and adatom diffusion.PLD is known to produce energetic atomic species with energy up to a few keV. 21It is therefore expected to promote C adatom diffusion into Ni at reduced substrate temperature.Furthermore, the C to Ni film thickness ratio can be controlled easily in PLD by altering the laser ablation time under fixed repetition rate or by using different laser irradiation fluences for C and Ni respectively.In these respects, controllable few-layer graphene growth can be obtained by PLD method at relatively low temperature.

II. EXPERIMENTAL
In the experiment, monocrystalline Si wafers coated with 300 nm SiO 2 were used as substrates.They were cut into 1x1 cm 2 pieces and cleaned with acetone, ethanol and de-ionized water.PLD was carried out in a stainless steel chamber evacuated to a base pressure of 2×10 -6 Torr.A KrF laser (λ = 248 nm) operated at 4 Hz repetition rate was used throughout.For both Ni and C targets, the separation between the targets and the substrate was set to be 35 mm.The Ni thin films were deposited onto SiO 2 with pulsed laser fluence of 5.43 J/cm 2 (220 mJ) at room temperature.Immediately after the Ni deposition, the substrate temperature was raised to 650 • C without breaking the vacuum for 1 hour in order to enlarge the average grain size of Ni film.Afterward, a rotating graphite target was then ablated by the same laser with lower laser fluence of 4.40 J/cm 2 (180 mJ).The as-prepared sample was cooled down naturally to room temperature under vacuum without considering quick quenching of the substrate.All samples were studied by micro-Raman spectroscopy using a 488 nm laser as the excitation source.Graphene samples with and without lifting from the Ni films were examined by High Resolution Transmission Electron Microscopy (HRTEM) for definitive evaluation of few-layer graphene formation.For cross section TEM determination, a graphene sample was cut into two small pieces.Both of them are stacked together firmly by protective epoxy.Afterward, the sample went through the normal polishing, dimpling and ion milling processes.The transfer of the PLD derived graphene on copper grid for TEM plane view was achieved by spin coating a very thin poly[methyl methacrylate] (PMMA) layer and wet-etching the Ni thin film with an aqueous HCL solution (5%) for 24 hours.After the sample has been closely attached to the TEM copper grid, the PMMA can be dissolved by exposing to acetone vapor for 4 to 5 hours.This transfer process allows graphene to maintain its continuity. 22In addition, Atomic Force Microscopy (AFM) was used to evaluate the height profile of graphene after it was transferred on SiO 2 /Si substrate.

III. RESULTS AND DISCUSSION
It has been widely accepted that Raman spectroscopy is the most reliable, non-destructive and quick inspection for graphene. 9,23 igure 1 shows the Raman spectra of sample structure, Si/SiO 2 /Ni (120 nm)/C (30 nm), at 5 different deposition temperatures of C. The thickness ratio of C to Ni was randomly chosen in order to investigate Ni induced graphitization temperature for PLD.From Figure 1(a) to 1(e), three remarkable peaks, around 1350 cm -1 , 1580 cm -1 and 2700 cm -1 , are observed for samples fabricated at higher temperatures [Fig.1(e)].The Raman peak shown at about 1350 cm -1 is due to defected graphite and it is usually called disorder-induced D band. 24It originates from unorganized carbon domains and small graphite crystal size.The intensity of D band is associated with non-sp 2 bonding.Since Raman fundamental selection rule can not applied for zone-boundary phonons, the D band is not seen in the first order Raman spectra of defect-free graphite.The peak located at 1580 cm -1 is graphitic G band.It represents the crystalline quality of graphite and evidences the formation a hexagonal lattice in graphite.For mono-crystalline graphite, only single line appears at 1580 cm -1 and the G band is highly symmetric. 25Thus, G denotes the symmetry-allowed graphite band.For the presence of amorphous C (a-C), the full width at half maximum (FWHM) of the G band is broad and the peak shifts to about 1540 cm -1 .The profile shown around 2700 cm -1 is the graphite-like G band, which is a double resonance of D band.we found the G-band tends to become narrow and symmetric.The result reflects higher temperature for carbon gives rise to relatively good crystallinity.In additional to this, the separation for both D-and G-bands tend to become distinguishable.In additional to this, we found a broad D-band for sample fabricated at 650 • C still exists, which gives a result of amount of defect.This reason is primarily due to non-sp 2 bond formation of C atoms.For the thickness ration of C to Ni which we have chosen here is not sufficient to demonstrate the reduction of D band.Therefore, we chose 650 • C as an appropriate temperature for C deposition in the following investigations.
Figure 2 shows the corresponding XRD and TEM images of the sample fabricated at 650 • C. In Figure 2 In order to investigate the effect of thickness ratio of C to Ni on graphene growth, C with different thicknesses (or different laser ablation time) were deposited onto 25 nm Ni thin films.The corresponding Raman spectra are shown in Figure 3.Despite those three bands which we have discussed previously, another band appears at 2900 cm -1 and it is regarded as a combination of D and G peaks, or sometime called S3 peak. 26Conventionally, the ratio, I D /I G , is used as a measure for the non-sp 2 to sp 2 bonding characteristics.Some previous reports have also suggested that the number of graphene layers is sensitive to the ratio I 2D /I G . 9,27 or this set of our samples with different amount of C depositions, their intensities of D band are almost the same as those of G band of crystalline phase C.Moreover, their ratios I 2D /I G , which are plotted in the inset, are all around 1/4.This is a clear indication that multi-layer graphene (more than 5 layers) occurs in all samples.Judging from Figure 3, the sample with the least C deposition shows the strongest Raman signal.We believe that this is primarily due to the presence of non-dissolved a-C, which shields off some of the  17 Further evaluation of graphene layer was obtained by studies of the relative Raman shift of the 2D bands for bulk graphite, multi-layer graphene and bi-layer graphene.As seen from Figure 5(a)-5(c), the 2D band shifts from approximately 2739 cm -1 down to 2686 cm -1 .Both the Raman 2D profiles of bulk graphite and bi-layer graphene are asymmetric.Raman scattering is a fourth order process involving electron-phonon scattering.Due to the interaction of graphene planes, the 2D peak of bi-layer graphene splits into 4 components. 28,29 s indicated in Figure 5(d), the 2D peak of our bi-layer graphene sample is, indeed, composed of four components, 2D 1B , 2D 1A , 2D 2A and 2D 2B .Among these four, 2D 1A and 2D 2A have relatively higher intensities than the other two.The result can be directly compared with those of previous Raman spectroscopic study for bi-layer graphene. 23fter the catalytic Ni thin film was dissolved in FeCl 3 acid, the graphene layer coated PMMA was transferred out. Figure 6  0.57, which confirm the presence of bi-and tri-layer graphene on the SiO 2 /Si substrates.Further evaluation of the number of graphene layer was conducted using cross section HRTEM images in the following part.

IV. CONCLUSION
In summary, we have thus far demonstrated that few-layer graphene can be fabricated on Ni thin film at 650 • C by PLD.The crystalline graphene layer growth is based on Ni induced crystallization method.Both XRD and TEM reveal good crystallinity of graphitic layer and graphene.The number of graphene layers relies on thickness ratio of C to Ni, which can be controlled conveniently by tuning the laser ablation time.The non-destructive Raman spectroscopy has also revealed the distinct features of 2D band among bi-layer graphene, multi-layer graphene and bulk graphite.Therefore, our few-layer graphene fabrication technique based on PLD is expected to be very useful for graphene research.
TABLE I. Raman intensities for I D , I G , I 2D , I D /I G and I 2D /I G of Fig. 4. Ni Ablation time (min) Ni thickness (nm) I D (a.u.)I G (a.u.)I 2D (a.u.)I D /I G I 2D /I G
(a), a sharp and intense Ni peak appears at 44.78 • .Moreover, the Ni grains are all well aligned in the (111) direction after annealing at 650 • C. The XRD peak shown at 26.62 • indicates hexagonal arrangement of graphitic (0002) lattice planes.Apart from this, the (10-10) and (0004) oriented lattice planes were also detected.Figure 2(b) and 2(c) show the top view of low magnification TEM images of graphitic layer.Smooth and homogeneous surfaces are clearly seen.The corresponding Selected Area Electron Diffraction (SAED) spot marked by the white dotted circle in Figure 2(d) proves the (0002) crystalline orientation of graphitic layers.

FIG. 6 .
FIG. 6.(a) Photographic image for graphene transfer.(b) Graphene is transferred on a clean SiO 2 /Si substrate.(c) AFM image for bi-layer graphene.(d) and (e) Height profiles taken along the blue and red solid lines for bi-layer graphene.(f) AFM image for tri-layer graphene.(g) and (h) Height profiles taken along the blue and red solid lines for tri-layer graphene.

FIG. 7 .
FIG. 7. (a) The corresponding Raman spectrum for bi-layer graphene on SiO 2 /Si substrate (the same sample shown in Figure 6(c)).(b) The corresponding Raman spectrum for tri-layer graphene on SiO 2 /Si substrate (the same sample shown in Figure 6(f)).
Figure5(a)-5(c), the 2D band shifts from approximately 2739 cm -1 down to 2686 cm -1 .Both the Raman 2D profiles of bulk graphite and bi-layer graphene are asymmetric.Raman scattering is a fourth order process involving electron-phonon scattering.Due to the interaction of graphene planes, the 2D peak of bi-layer graphene splits into 4 components.28,29As indicated in Figure5(d), the 2D peak of our bi-layer graphene sample is, indeed, composed of four components, 2D 1B , 2D 1A , 2D 2A and 2D 2B .Among these four, 2D 1A and 2D 2A have relatively higher intensities than the other two.The result can be directly compared with those of previous Raman spectroscopic study for bi-layer graphene.23After the catalytic Ni thin film was dissolved in FeCl 3 acid, the graphene layer coated PMMA was transferred out.Figure6(a) shows the photographic image of graphene coated with PMMA detached from Ni thin film/SiO 2 /Si.The solution is diluted HCl, which can be used to further clean the residual Ni flakes on the graphene surface.Figure6(b) displays the same graphene layer attached to a new SiO 2 /Si substrate.The PMMA was removed by dipping the sample into acetone.From this image, few layer graphene coated on SiO 2 exhibits blue color.Two different samples followed the same transferring process were used for AFM examination.In Figure6(c), two large pieces of graphene layers are shown in a 2 μm×2 μm area of AFM image.The break is due to scratch of large graphene for height profile measurement.Two different positions along each graphene layer were chosen.The corresponding height profiles are shown in Figure6(d) and 6(e).Owing to the average thickness is around 1nm, both measurements indicate bi-layer graphene formation.Same method was applied for another sample and the AFM image is shown in Figure6(f).The green dotted circle shows the present of graphene wrinkle.Its formation is due to the thermal expansion coefficient difference between Ni and graphene.8From the height profiles of Figure6(g) and 6(h), a slight increase in the thickness of graphene layer by approximately 1nm indicates 3 or 4 layers graphene formation.Furthermore, for both samples examined by AFM in Figure6(c) and 6(f), the corresponding Raman spectra are displayed in Figure7(a) and 7(b) respectively.Obviously, highly symmetric G bands indicate our transferred graphene samples have excellent crystallinity.The Raman intensity ratios of I 2D /I G for both Raman spectra in Figure 7(a) and 7(b) are 0.63 and Figure5(a)-5(c), the 2D band shifts from approximately 2739 cm -1 down to 2686 cm -1 .Both the Raman 2D profiles of bulk graphite and bi-layer graphene are asymmetric.Raman scattering is a fourth order process involving electron-phonon scattering.Due to the interaction of graphene planes, the 2D peak of bi-layer graphene splits into 4 components.28,29As indicated in Figure5(d), the 2D peak of our bi-layer graphene sample is, indeed, composed of four components, 2D 1B , 2D 1A , 2D 2A and 2D 2B .Among these four, 2D 1A and 2D 2A have relatively higher intensities than the other two.The result can be directly compared with those of previous Raman spectroscopic study for bi-layer graphene.23After the catalytic Ni thin film was dissolved in FeCl 3 acid, the graphene layer coated PMMA was transferred out.Figure6(a) shows the photographic image of graphene coated with PMMA detached from Ni thin film/SiO 2 /Si.The solution is diluted HCl, which can be used to further clean the residual Ni flakes on the graphene surface.Figure6(b) displays the same graphene layer attached to a new SiO 2 /Si substrate.The PMMA was removed by dipping the sample into acetone.From this image, few layer graphene coated on SiO 2 exhibits blue color.Two different samples followed the same transferring process were used for AFM examination.In Figure6(c), two large pieces of graphene layers are shown in a 2 μm×2 μm area of AFM image.The break is due to scratch of large graphene for height profile measurement.Two different positions along each graphene layer were chosen.The corresponding height profiles are shown in Figure6(d) and 6(e).Owing to the average thickness is around 1nm, both measurements indicate bi-layer graphene formation.Same method was applied for another sample and the AFM image is shown in Figure6(f).The green dotted circle shows the present of graphene wrinkle.Its formation is due to the thermal expansion coefficient difference between Ni and graphene.8From the height profiles of Figure6(g) and 6(h), a slight increase in the thickness of graphene layer by approximately 1nm indicates 3 or 4 layers graphene formation.Furthermore, for both samples examined by AFM in Figure6(c) and 6(f), the corresponding Raman spectra are displayed in Figure7(a) and 7(b) respectively.Obviously, highly symmetric G bands indicate our transferred graphene samples have excellent crystallinity.The Raman intensity ratios of I 2D /I G for both Raman spectra in Figure 7(a) and 7(b) are 0.63 and

Figure 8 (
a) displays the low magnification TEM image of the cross section of the graphene/Ni/SiO 2 /Si heterostructure.During laser ablation of the C target, the ejected C atoms were deposited on and adsorbed by the Ni layer.In lowering the substrate temperature, the amount of C segregates from Ni depends on the initial saturation status of C-Ni solid solution.The bi-and tri-layer graphene are clearly revealed in the HRTEM images of Figure 8(b) and 8(c).Clean and sharp interfaces with long range order between graphene layers and C-doped Ni thin films are indicated by those arrows.Both bi-and tri-layer graphene lie flat on the C-doped Ni top surface.
Figure 8(d) displays the HRTEM image of multi-layer graphene grown on 25 nm Ni film.The HRTEM results thus strongly support our view that PLD is an excellent and simple technique for few-layer graphene growth.