Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin films.pdf
a SpringerOpen Journal Piazzi et al. SpringerPlus 2012, 1:52 http://www.springerplus.com/content/1/1/52 RESEARCH Open Access Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin ﬁlms Marco Piazzi1,2*, Luca Croin1,3, Ettore Vittone2 and Giampiero Amato1 Abstract The outstanding electrical and mechanical properties of graphene make it very attractive for several applications, Nanoelectronics above all. However a reproducible and non destructive way to produce high quality, large-scale area, single layer graphene sheets is still lacking. Chemical Vapour Deposition of graphene on Cu catalytic thin ﬁlms represents a promising method to reach this goal, because of the low temperatures (T < 950°C−1000°C) involved during the process and of the theoretically expected monolayer self-limiting growth. On the contrary such self-limiting growth is not commonly observed in experiments, thus making the development of techniques allowing for a better control of graphene growth highly desirable. Here we report about the local ablation eﬀect, arising in Raman analysis, due to the heat transfer induced by the laser incident beam onto the graphene sample. Keywords: CVD graphene, Copper, Laser induced etching, Heating ablation eﬀects PAC Codes: 61.80.Ba, 81.15.Gh, 61.48.Gh, 81.05.ue Background Graphene (a single bidimensional layer of carbon atoms arranged in an hexagonal lattice) has attracted a major interest in the last few years because of its astonishing electrical (Castro Neto et al. 2009; Peres 2010; Peres et al. 2006), mechanical (Lee at al. 2008) and chemical properties (Elias et al. 2009; Wang et al. 2009a), that make it a good candidate for the future development of nanoelectronics devices. Although the main properties of this material are nowadays well known from a theo- retical point of view, an eﬃcient and highly reproducible method to grow high quality, large-scale area, single layer graphene ﬁlms, suitable for practical applications, is still lacking. For this reason, several techniques have been developed in the last years in order to achieve this goal: the most important are the epitaxial growth of graphene by thermal sublimation of SiC (de Heer et al. 2007; Emtsev et al. 2009; Hass et al. 2008; Sprinkle et al. 2009; Varchon *Correspondence: m.piazzi@inrim.it 1Quantum Research Laboratory, Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Turin, Italy 2Department of Physics, NIS Centre of Excellence and CNISM, University of Turin, Via Pietro Giuria 1, 10125 Turin, Italy Full list of author information is available at the end of the article et al. 2007), the Chemical Vapour Deposition (CVD) synthesis of graphene on various metal catalysts (Reina et al. 2009; Lee et al. 2010; Liu et al. 2010; Kim et al. 2012; Nandamuri et al. 2010; Somani et al. 2006; Li et al. 2009a; 2009b; Tao et al. 2012) and the chemical reduction of graphene oxide (Gilje et al. 2007; Lee at al. 2009; Paredes et al. 2008; Schniepp et al. 2006). Among these, CVD technique seems to be one of the most promising meth- ods because of the reported possibility (Liu et al. 2010) of obtaining highly uniform, defect-free graphene ﬂakes as large as ∼100 μm2 in a reproducible, highly accessible and inexpensive way. Since CVD synthesis needs a catalyst to activate the chemical decomposition of the carbon precursor (usually methane or ethylene) used for graphene growth at low temperatures (T < 950°C – 1000°C), the use of many metals (Ir (Coraux et al. 2008), Ru (Martoccia et al. 2008), Pt (Sasaki et al. 2000; Starr et al. 2006), Fe (Kondo et al. 2010), Ag (Di et al. 2008), Ni (Liu et al. 2010; Kim et al. 2009; Obraztsov et al. 2007), Cu (Bae et al. 2010; Li et al. 2009a; Tao et al. 2012) as catalysts during the process has been reported in litera- ture. Cu is one of the most promising catalyst (Mattevi et al. 2011) because of the low C solid solubility in it © 2012 Piazzi et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Piazzi et al. SpringerPlus 2012, 1:52 Page 2 of 10 http://www.springerplus.com/content/1/1/52 (0.001 −0.008 weight % at ∼1084°C): this property brings to the formation of only soft (not covalent) bonds between the π electrons of 2pz orbitals of sp2 hybridized C atoms and the 4s electrons of Cu, without formation of any carbide phase during the growth process. As a con- sequence, formation of graphene should stop after one single layer has been formed: this makes CVD growth of graphene on Cu very attractive. Nonetheless, many exper- iments show that actually such a self-limiting behaviour is hardly observed, since few-layered graphitic structures are usually grown on Cu substrates. For this reason, to obtain monolayer graphene, several post processing techniques have been proposed to selec- tively etch atomic graphene layers. Among the various approaches (e.g. heat-induced etching by oxygen (Liu et al. 2008), e-beam lithography assisted technique (Novoselov et al. 2004; Zhang et al. 2005), graphene cutting by carbon- soluble metals (Campos et al. 2009; Datta et al. 2008) the thinning of atomic carbon multilayers by laser irra- diation (Han et al. 2011) can be a promising method to obtain monolayer graphene. In this last work, authors show how the central rˆole in graphene etching is held both by the laser irradiation used for the Confocal Raman Spec- troscopy performed on the samples, and by the SiO2/Si substrate on top of which few-layered graphene has been transferred: the heat produced by the irradiation “burns” (in presence of oxygen) locally the outermost C layers, while the innermost layer (the one bound to the substrate) is left unetched because of the presence of the SiO2 layer acting as heat sink. Cu, being a metal, has thermal conductivity higher than SiO2 and can represent therefore an enhanced heat sink, so it can be expected to observe a similar behaviour also for graphene grown by CVD on it. Here we report the change in shape and position of the G and 2D peaks observed in Raman spectra acquired at diﬀerent time intervals on the same spot of a graphene sample synthesized by CVD on Cu thin ﬁlms: the evolution of the spectra, indicating that the structure of graphene is changing during the exposure to the laser used during Raman analysis, is compati- ble with a decrease in the number of graphene layers present on the substrate. This decrease may be attributed to the same laser-induced etching eﬀect observed on graphene deposited onto SiO2/Si substrates. If this is the case, an eﬃcient and easy graphene etching technique can be developed and a promising way to obtain high uniform, large-scale area, monolayer graphene can be envisaged. Methods Cu deposition The samples subjected to CVD process have been pre- pared by e-beam evaporation of 500 nm Cu thin ﬁlm on top of a p-type (100) oriented Si wafer (∼1 cm2) with ∼300 nm thermal SiO2. The deposition has been carried out in a load-lock chamber at a base pressure of ∼10−8 mbar and deposition pressure of ∼10−6 mbar, with an average growth rate of 3 −5 ˚A/s. The thickness d of deposited Cu ﬁlm has been chosen in order to limit the known problem (Mattevi et al. 2011) of dewetting occurring on very thin ﬁlms (d < 500 nm) at temperatures ≳800°C. Scanning Electron Microscopy (SEM) images of the samples (taken with a FEI InspectF Scanning Electron Microprobe) after Cu deposition show a uniform cover- age of the SiO2 surface characterized by a Cu polycrys- talline structure with grains of ∼90 nm as typical size (Figure 1(a)). Moreover, Scanning Tunneling Microscopy (STM) scanning an area of ∼10−2 μm2 allowed us to eval- uate the roughness of the Cu surface (Figure 1(b)). The resulting root mean square (RMS) of ∼2 nm (an order of magnitude higher than single layer graphene thickness, ∼ 3.3 ˚A), together with the topographic behaviours obtained for certain scanning directions (Figure 1(c)), showing among others height variations as small as few angstroms, makes ineﬀective the use of Atomic Force Microscopy (AFM) to detect any change in the number of graphene layers eventually present on Cu: the changes in height pro- duced by the latter eﬀect would be hardly distinguishable from topographic changes due to the roughness of the substrate’s surface. CVD growth process Before undergoing CVD process, samples have been care- fully cleaned in acetone and isopropanol. CVD has been then performed in a Rapid Thermal Annealing (RTA) sys- tem (Jipelec JetFIRST 100) suitable for depositions in Low Vacuum conditions (pmin ∼10−2 mbar) up to Tmax ∼ 1300°C. The system is equipped with four gas lines con- trolled through mass ﬂow meters and it is characterized by a small heat capacity allowing for fast cooling-down processes, up to ∼300°C/min (see Figure 2). The temperature of the system is controlled by a pyrom- eter, exposed to the back of the sample holder (a 4” Si wafer) and calibrated by means of a thermocouple in con- tact with it (Figure 2). The pyrometer sets the power of the lamps. Since the Cu sample undergoing the CVD process is directly exposed to the lamps, the temperature reading by the pyrometer (correct in absence of Cu) is probably lower than the temperature reached by the surface during the process. Therefore, the real deposition temperature may be underestimated. We are performing some studies on this topic in order to solve this ambiguity in the future. However, at the temperature reached during the process as read by the pyrometer dissociation of CH4 and subse- quent graphene formation take place, while the undesired Cu dewetting eﬀect is prevented. Piazzi et al. SpringerPlus 2012, 1:52 Page 8 of 10 http://www.springerplus.com/content/1/1/52 Figure 9 Phenomenological model for local overheating and etching of multilayer graphene. Schematic model representing the possible origin of local overheating and etching of outermost graphene layers grown on Cu ﬁlms. The heat provided by the focused laser beam is dissipated through in-plane (horizontal arrows) and out-of-plane (vertical arrows) channels. Though the in-plane channel is dominant in graphene, the ﬁnite size of outer layers makes out-of-plane dissipation signiﬁcant and reduces the in-plane contribution, thus causing local overheating and subsequent etching of the layers. The innermost layer is instead prevented from etching by the presence of the Cu substrate acting as heat sink. with respect to the case of transferred graphene onto SiO2. At such photon energies other mechanisms, like molec- ular desorption of chemical species, cannot be a priori excluded, but they can hardly aﬀect the Raman signature and they can be detected through electrical measurements (Sun et al. 2012). The fact that the G peak position is almost unaﬀected by the laser irradiation can be explained as a result of two competing eﬀects: the local enhancement of temperature (due to overheating), bringing to a redshift of the G peak (Cai et al. 2010; Calizo et al. 2007) and the decrease in the number of graphene layers (due to etching), resulting instead in a blueshift of the G peak (Wang et al. 2009b). The unexpected unaltered intensity of the G peak (that should decrease as the number of graphene layers decreases) can be explained in two ways. A ﬁrst eﬀect, applying to Raman measurements performed both on transferred and not transferred graphene samples, relies on the increase in temperature of graphene layers upon laser irradiation, likely resulting in an eﬀect similar to what observed in experiments concerning the evolution of graphene Raman signature upon controlled annealing at high temperatures (Ni et al. 2008a). As pointed out in this work, the G peak intensity is not changing between few- and monolayer graphene sheets after the annealing process, meaning that in these experimental conditions G peak intensity cannot be regarded as a ﬁngerprint to distinguish number of graphene layers. A second reason- ing, applying only on graphene samples over Cu, relies on the roughness of the Cu surface, determining a light trap- ping eﬀect close to the substrate’s surface that results in an enhanced number of multiple reﬂections of the laser light between Cu and graphene layers: as a consequence, the number of C atoms detected by the unfocused beam is always comparable to the number of C atoms present in multilayer graphene, although the number of graphene layers is decreasing. As reported in (Ni et al. 2008b) the G band intensity for a number of graphene layers exceeding ∼15 is in this case decreasing by increasing number of layers and then constant, as observed in our spectra. The origin of the prominent D peak (increasing in intensity as the number of acquisitions increases) is not completely clear yet: it can be attributed to the acquisition of the spectra on a point of the sample lying on a grain boundary of the Cu substrate (resulting in a change of the crystallographic orientation of graphene ﬂake through it) or to defects (terrace boundaries) produced in graphene crystal structure by the laser etching. Raman analysis performed on transferred graphene samples using the same experimental setup (laser at 442 nm, power P = 1 mW and exposure time t = 80 s) con- ﬁrms the behaviour observed in the spectra acquired on graphene/Cu substrates. A similar evolution in shape and position of the peaks is obtained for subsequent spectrum acquisitions, as shown in Figure 7(b) and Figure 8(b). Conclusions In summary, a possible way to locally etch graphene lay- ers based on laser heating released during Raman analysis, has been presented. Graphene structure (crystallization degree and number of layers) evolution can be monitored and inferred by looking at the Raman spectra acquired on the sample. The technique is suitable in particular for etching layers of graphene grown by CVD on a metal catalyst. In our case we have reported results obtained with Cu: by lightening the sample with incident laser light at quite high power (≳1mW) for short time periods (∼80 s) a clear sharpening and lowering of the 2D peak position is observed in Raman spectra, together with a decrease in the IG/I2D ratio. These results are compatible with a decrease in the number of graphene layers grown on the metallic substrate. D peak increases in intensity as function of the laser exposure, meaning an increasing of defects in the graphene structure. We believe that the method can be easily applied to other metallic substrates: since the technique deeply relies Piazzi et al. SpringerPlus 2012, 1:52 Page 9 of 10 http://www.springerplus.com/content/1/1/52 on the dispersion of the heat provided by laser irradia- tion through the substrate, the most important feature required for the substrate is its high thermal conductivity. Although some catalysts, like Cu, are very promising for CVD synthesis of graphene because of the expected self- limited mono-layered growth of this material on them, a few-layered structure is often found in experiments. Laser etching here reported can therefore provide an in- situ technique to get rid of this problem. However, laser can also have an active rˆole in inducing unwanted defects, such as vacancies in pristine graphene ﬁlms: for this rea- son the proposed method shall be further developed. We ﬁnally envisage the application of the photo-etching pro- cess here reported to large areas if eﬃcient and uniform illumination conditions, as those used in our RTA system, are employed. Abbreviations CVD: Chemical vapour deposition; SEM: Scanning electron microscopy; STM: Scanning tunneling microscopy; RMS: Root mean square; AFM: Atomic force microscopy; RTA: Rapid thermal annealing; PMMA: poly(methyl-methacrylate); XRD: X-Ray diﬀraction. Competing interests The authors declare that they have no competing interests. Authors’ contributions MP has been responsible for data analysis and modeling and for Cu e-beam evaporation, helping also in the theoretical interpretation of the results. LC coordinated all the experimental work, in particular the Cu e-beam evaporation and the CVD process, participating also in Raman analysis. EV and GA coordinated all the work and have been responsible for the theoretical interpretation of the results. All authors read and approved the ﬁnal manuscript. Acknowledgements The authors acknowledge the Nanofacility Piemonte and the Compagnia di San Paolo for ﬁnancial supports. Thin ﬁlms evaporation, graphene deposition and SEM analysis on the samples have been performed at Quantum Laboratories - Nanofacility Piemonte present at I.N.Ri.M. - Turin. Raman characterization has been performed with the help of A. Damin at the NIS Centre of Excellence - University of Turin. The help of A. Battiato and E. Olivetti with XPS and XRD analyses is gratefully acknowledge. Author details 1Quantum Research Laboratory, Istituto Nazionale di Ricerca Metrologica, Strada delle Cacce 91, 10135 Turin, Italy. 2Department of Physics, NIS Centre of Excellence and CNISM, University of Turin, Via Pietro Giuria 1, 10125 Turin, Italy. 3Department of Applied Science and Technology, Politecnico of Turin, Corso Duca deli Abruzzi 24, 10129 Turin, Italy. 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Nature 438: 201–204 doi:10.1186/2193-1801-1-52 Cite this article as: Piazzi et al.: Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin ﬁlms. SpringerPlus 2012 1:52. Submit your manuscript to a  journal and beneﬁ t from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the ﬁ eld 7 Retaining the copyright to your article     Submit your next manuscript at 7 springeropen.com
Title: Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin films ​
Authors: Marco Piazzi, Luca Croin, Ettore Vittone, Giampiero Amato ​
Publisher: SpringerOpen
Date: 27 November 2012 ​
Abstract: The outstanding electrical and mechanical properties of graphene make it very attractive for several applications, Nanoelectronics above all. ​ However, a reproducible and non-destructive way to produce high-quality, large-scale area, single-layer graphene sheets is still lacking. ​ Chemical Vapour Deposition of graphene on Cu catalytic thin films represents a promising method to reach this goal, because of the low temperatures (T < 950°C−1000°C) involved during the process and of the theoretically expected monolayer self-limiting growth. ​ On the contrary, such self-limiting growth is not commonly observed in experiments, thus making the development of techniques allowing for a better control of graphene growth highly desirable. ​ Here we report about the local ablation effect, arising in Raman analysis, due to the heat transfer induced by the laser incident beam onto the graphene sample. ​