Introduction {#sec1}
============

By the scotch tape method, graphite repeatedly stuck and peeled off (in a painful way with poor yield) turned into graphene flakes with typical size of 10 μm.^[@ref1]^ Graphene, a two-dimensional (2D) structure of graphite, has become a new attractive material because of its unique physical properties.^[@ref2]^ Graphene is not only the thinnest and strongest material in all of the resources, but it also shows excellent electrical,^[@ref3]^ thermal, mechanical,^[@ref4]^ and optical properties.^[@ref5],[@ref6]^ These properties of graphene can be of importance in many novel applications, such as field-effect transistors and photodetectors.^[@ref7],[@ref8]^

For large-scale production of graphene, enormous efforts have been devoted to develop alternative methods besides the primitive technique using scotch tape.  The thermal decomposition of SiC^[@ref9],[@ref10]^ requires literally high temperature to sublimate Si atoms to form carbon sheet; furthermore, it is expensive and forms nonuniform structures on the film surface.  By chemical vapor deposition (CVD), a well-established method, graphene uniformly grows on large areas covered with catalytic layers, such as Ni, Pt, Ru, Ir, Cu, and Co, at high temperatures.^[@ref11]−[@ref13]^ Although graphene layers epitaxially grow by the CVD method, both CVD and exfoliation methods require transferring the graphene layers onto another insulating substrate for electronic measurements or device applications.  Other methods, such as nanodiamond conversion^[@ref14],[@ref15]^ and electrochemical deposition, are also used to prepare graphene films.^[@ref4]^ Recently, we have reported a novel but simple and primitive method using a lead pencil and a piece of paper.^[@ref16]^

Pulsed laser deposition (PLD) is one of the versatile methods for growing thin films of insulators, semiconductors, and metals^[@ref17]−[@ref19]^ and even for producing nanoparticles.^[@ref20]^ In contrast to the complex and expensive molecular beam epitaxy (MBE) method,^[@ref21]^ the PLD method is very simple and flexible for the layer-by-layer deposition.  The PLD method is also employed to prepare various carbon materials, such as amorphous carbon and diamondlike carbon (DLC), and even diamond grows in either hydrogen or oxygen atmosphere by the PLD method.^[@ref22],[@ref23]^ Carbon film prepared by the PLD method turns into diamond in a relatively high oxygen pressure.^[@ref23]^ The oxidizing atmosphere consumes extra carbon atoms, such as amorphous carbon, and only diamond remains in the oxygen atmosphere.  With the increase of oxygen pressure and substrate temperature, the oxidative etching becomes predominant over depositing carbon on substrates.  Even diamond cannot remain in an inhospitable environment provided by oxygen atmosphere, and it becomes carbon dioxide on reaction with oxygen atoms. 

Interestingly, stable and chemically nonreactive carbon dioxide has been known to provide oxidizing atmosphere since the mid-19th century,^[@ref24],[@ref25]^ and carbon dioxide oxidizes metals^[@ref26]^ such as magnesium, aluminum, silicon, calcium, and iron by turning itself into carbon or carbon monoxide.  As graphene is made of only one layer of graphite sheet, oxidizing atmosphere provided by oxygen gas is excessive for its growth.  To present a more *gentle and tender* oxidizing atmosphere, we proposed carbon dioxide instead of oxygen to obtain a fine adjustment of the oxidizing atmosphere for graphene growth.  By the PLD method together with CO~2~-oxidizing environment, layer-by-layer growth can be verified on stepped insulating substrates, and layered graphene can be deposited quite rapidly (∼1 min) compared with the CVD method, which usually requires a relatively prolonged period for graphene deposition.^[@ref27]^

Experimental Section {#sec2}
====================

Carbon target was irradiated in nitrogen atmosphere, and oxygen and carbon dioxide at substrate temperature, from room temperature to 800° C, at a wavelength of 532 nm, generated by a slow Q-switched YAG laser.^[@ref21]^ The carbon target was a highly oriented pyrolytic graphite, and the substrates were *c*-plane sapphire and SrTiO~3~(001) substrates.  The distance between the target and the substrate was varied from 20 to 60 mm, and the types of gas atmosphere during depositions were nitrogen, oxygen, and carbon dioxide with pressures between 1 and 43 Pa.

Carbon films were examined by Raman spectroscopy at a wavelength of 785 nm, and the surface morphology was observed by atomic force microscopy (AFM).  Stepped SrTiO~3~(001) was used as a substrate to investigate the initial growth of graphene films.  AFM was again employed to investigate the top surface of monographene, and 2D Fourier transform was used to observe the six-membered carbon ring on the stepped insulating substrates. 

Raman spectroscopy obtained at the wavelength of 785 nm was corrected using Standard Reference Material (SRM) 2241 provided by the National Institute of Science and Technology (NIST).^[@ref28]^ SRM 2241 is a certified spectroscopic standard (optical glass) intended for correction of the relative intensity of Raman spectra observed at the wavelength of 785 nm.  Together with SRM 2242 and 2243, Raman spectra obtained at wavelengths of 785, 532, or 488/514.5 nm can be corrected by coefficients of the certified polynomial and of the confidence curves describing the luminescence spectrum of SRM 2241. 

Results and Discussions {#sec3}
=======================

In nitrogen atmosphere, graphene growth was reported by the PLD method;^[@ref29]^ however, the grown film resembled graphite rather than graphene or multigraphene.  The ratio of Raman intensities of 2D to G peaks, *I*~2D~/*I*~G~, is often employed to estimate the number of graphene layers.^[@ref30]−[@ref32]^ G and D peaks together with 2D peaks are observed on carbon films prepared in nitrogen atmosphere with a high fluence of 6 J/cm^2^. The 2D peak is quite weak, and the intensity ratio (*I*~2D~/*I*~G~) remains less than 0.5.^[@ref29]^ Carbon films prepared in nitrogen atmosphere by the PLD method show a broad peak around the G and D bands of graphite, indicating the growth of diamondlike carbon. 

In this study, diamondlike carbon also grew in nitrogen atmosphere, whereas both G and D peaks were clearly observed on samples prepared in oxygen atmosphere, as shown in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}a.  In oxygen atmosphere, defective carbon cluster, such as amorphous carbon, must be consumed before reaching the substrate during deposition.  The strong oxidizability in oxygen atmosphere did not allow the film growth at high substrate temperature and long distance between the substrate and the target (target distance), that is, more than 40 mm. 

![ Raman spectra of carbon films.  The samples were prepared in (a) nitrogen and oxygen atmosphere at 9 Pa, (b) oxygen atmosphere, and (c) carbon dioxide atmosphere. ( d) Raman peak standardized by relative intensity correction using Standard Reference Materials.  Both G and D peaks were clearly observed on samples prepared in oxidizing environment, and the 2D peak corresponding to the graphene structure was observed on samples prepared in carbon dioxide.](ao-2017-00140b_0003){#fig1}

As shown in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}b, in both G and D peaks clearly observed on samples prepared in oxygen atmosphere, oxygen pressure more than 20 Pa resulted in less intense Raman peaks.  In the figure, the solid gray line is the spectra of high oxygen pressure of 20 Pa showing a small peak around 1330 cm^--1^, which is often observed by nanodiamond crystal.  No film grew under the condition of oxygen pressure more than 20 Pa. There were no remarkable 2D peaks on all of the samples prepared in oxygen atmosphere. 

In oxygen atmosphere, at high temperature of substrate, oxidative etching must be dominant and result in severe oxidization.  The carbon cluster supplied by the PLD intensely reacts with the oxygen atoms reaching the substrate in the deposition chamber.  A more gentle and tender oxidizing atmosphere is required for graphite growth.  Interestingly, chemically unreactive carbon dioxide provides oxidizing atmosphere in vacuum environment and even in atmospheric pressure at high temperature. 

In addition to oxygen, in this study, carbon dioxide was used as oxidative gas for graphite deposition by the PLD method, as shown in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a; the growth of multigraphene was verified by Raman spectroscopy, and the surface morphology was observed by AFM. [ Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}c shows Raman spectra of carbon films prepared in a CO~2~ pressure range of 9--43 Pa. Although the plume in oxygen atmosphere was relatively small and did not reach the substrate with the target distance more than 30 mm, the plume in carbon dioxide fully involved the substrate placed at the target distance of 40 mm.  The intensity of G and 2D peaks increased under optimal conditions at a CO~2~ pressure of 25 Pa, substrate temperature of 800 °C, and the target distance of 40 mm. 

![ Equilibrium process in CO~2~ atmosphere. ( a) Schematic of reactions in CO~2~ atmosphere and (b) the equilibrium relationship for the Boundouard reaction: CO~2~ + C ↔ 2CO.](ao-2017-00140b_0004){#fig2}

Raman spectra excited at the wavelength of 785 nm were different from the ordinal spectra obtained at 532 nm, especially at high wavelengths required for such 2D band of graphene, as the sensitivity of silicon detector exponentially decreased at the region of high wavelength.  Applying the Standard Reference Material 2241 (SRM 2241) correction into the Raman spectra observed at 795 nm, intensity-corrected Raman spectra were obtained, as shown in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}d.  With SRM 2241 correction, the ratio of Raman intensities (*I*~2D~/*I*~G~) was increased to be ∼2, indicating the direct growth of multigraphene on the insulating substrate without any catalyst.  With the MBE method, graphitic film prepared on SrTiO~3~ substrate in nitrogen atmosphere shows no 2D peak in a previous report.^[@ref33]^ The details about the SRM correction can be found in the [Supporting Information](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00140/suppl_file/ao7b00140_si_001.pdf). 

What makes a gentle and tender oxidizing environment?  Carbon dioxide is extremely stable in the environment.  Surprisingly, carbon monoxide becomes much more stable than carbon dioxide at high temperature, such as 900 °C, even in atmospheric environment.  In the high-vacuum environment, carbon monoxide is stable in the following equilibrium stateas shown in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b.  The above reaction was first studied by Deville in 1864,^[@ref24]^ and the effect of temperature on the equilibrium was investigated by Boundouard.^[@ref25]^ In 1910, quantitative investigation by Rhead et al.  showed thatproceeded at a speed of 166 times higher that of the reverse reaction.^[@ref34],[@ref35]^ They also investigated the effect of temperature together with gas pressure on the equilibrium state and showed that the formation of carbon monoxide from carbon dioxide was favored even at 500 °C in vacuum atmosphere (0.001 atm). 

To investigate the initial growth of graphene, the graphene films were prepared on stepped annealed SrTiO~3~ substrates with deposition time from 10 to 120 s. As shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}a, a stepped surface consists of terrace and ledge.  The initial growth possibly starts (1) by sticking carbon to the ledge of step (ledge growth) or (2) from the terrace edge (terrace-edge growth).  AFM was again employed to investigate the top surface by using 2D Fourier transform.  An annealed SrTiO~3~ substrate showed an atomically flat stepped surface, as shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}b,c. Small grains grew on the flat terrace of stepped substrate with height between 0.2 and 0.35 nm ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}d). 

![ Surface morphology of stepped substrate. ( a) Schematic of two types of initial growth on the stepped substrate.  AFM images of (b) annealed stepped SrTiO~3~ substrate in 2D view, (c) annealed stepped SrTiO~3~ in 3D view, and graphene films with deposition times of (d) 30 s and (e) 60 s. The inset in (f) shows six-membered carbon rings observed on the sample deposited for 60 s.](ao-2017-00140b_0001){#fig3}

In general, a thin film often starts to grow from kink or ledge of substrate.  However, the grains grew at the terrace edge of the stepped substrate instead of growing from ledge of terrace.  Graphene grew on the terrace edge in the initial stage, and the terrace showed an atomically flat surface (Ra ∼ 0.02 nm) again after 60 s deposition, as shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}e.  As shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}d, nanoparticle grew at the initial stage of graphene deposition on SrTiO~3~ substrate.  The direct growth of graphene on germanium (Ge) substrate, for instance, is self-limiting, such as the one deposited on copper catalyst via a surface-mediated process rather than a precipitation process.^[@ref27]^ The growth of CVD graphene on Ge does not involve C precipitation but is rather attributed to surface adsorption.  According to the equilibrium phase diagram of the Ge---C system, the constituents in the Ge---C alloy are immiscible under equilibrium in the bulk.  Interestingly, graphene on mica shows ultraflat surface^[@ref36]^ instead of intrinsic rippling with ∼1 nm high corrugations usually found on both freestanding graphene and graphene film deposited on the substrate.  The direct growth of graphene on mica is accomplished by MBE.^[@ref37]^ Mica with no extra dangling bonds on the surface is the preferred substrate for van der Waals (vdW) epitaxy, which successfully grows into a high-quality film with 2D layered materials, such as MoS2~2~^[@ref38]^ and α-alumina.^[@ref39]^ However, graphene does not cover all of the mica surface and tends to agglomerate around the growth centers.^[@ref37]^

Few-layered graphenes are synthesized by the conversion of graphite particle or nanodiamond in an inert or reducing atmosphere at high temperature.^[@ref15],[@ref40]^ Nanodiamond powder yields a few-layered graphene after annealing for 1 h in helium atmosphere at about 2000 °C.  During the direct growth of graphene in CO~2~-oxidizing environment, in this study, the nanoparticle grew in the initial stage of graphene deposition, as shown in AFM images ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}d).  Such a nanoparticle can lead to layered graphene during PLD at a substrate temperature of 800 °C, which is relatively lower than 2000 °C for the transformation of nanodiamond to layered graphene.  It is reasonable that a lower temperature was required to form layered graphene during the deposition process compared to that of the solid solution process, and the graphene converted from nanodiamond shows only a weak 2D peak on Raman spectra.  The formation of crystal surface is often described by the terrace ledge kink model.^[@ref41]^ The deposited atoms adsorb onto a substrate and diffuse along the substrate surface; they are captured by kinks of the terrace ledge to form crystal structure.  However, graphene has a 2D structure with strong covalent bonds and relatively weak vdW-like forces, which are sufficient to keep the stack together.^[@ref42]^ Such a vdW heterostructure might grow differently from the terrace ledge kink model, in which the ordinal crystal structure forms a thin film.  As shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}, the initial growth of graphene, in this study, did not result from the terrace ledge but from the terrace edge and grew by a layer-by-layer process. 

Six-membered carbon rings were observed on the sample with deposition time more than 60 s, as shown in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}f, indicating (1) the start of the layer-by-layer growth of graphene from terrace edge on stepped substrates and (2) the possibility of self-organization of wire graphene on insulating substrates. 

Summary {#sec4}
=======

Diamond, one of the members of the carbon family, grows in oxidizing atmosphere in the presence of oxygen.  Because the chemical processes taking place in the oxygen atmosphere are relentless, especially at high temperature, several carbon clusters will be consumed by oxygen atoms during deposition, and only diamond might remain after deposition.  To provide a modest oxidizing environment, carbon target was irradiated in carbon dioxide at a relatively low substrate temperature.  Raman spectroscopy verified the direct growth of multigraphene on SrTiO~3~ substrates without any catalyst layer.  Graphene growing from the terrace edge covered a partial area on stepped substrate, and the film growth extended to all over the terrace after 60 s deposition, resulting in the recovery of the flat surface; graphene showed "layer-by-layer" growth on stepped structure.  The growth of wire graphene can be expected on the stepped structure by deposition in gentle and tender oxidizing environment produced from carbon dioxide atmosphere. 

The Supporting Information is available free of charge on the [ACS Publications website](http://pubs.acs.org) at DOI: [10.1021/acsomega.7b00140](http://pubs.acs.org/doi/abs/10.1021/acsomega.7b00140).Correction of Raman spectra (S1) ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.7b00140/suppl_file/ao7b00140_si_001.pdf))

Supplementary Material
======================

###### 

ao7b00140_si_001.pdf

The authors declare no competing financial interest. 

The authors are grateful to Hironori Torii at JSW-AFTY for AFM measurement and Shibata and T. Shimizu at Lamda Vision for Raman spectroscopy and useful discussion with Prof. Tokumasu at Tohoku University.  They also thank Yoshitsugu Sato for special assistance in the PLD system.  This study was supported in part by Grants-in-Aid for Scientific Research (KAKENHI), Japan Society for the Promotion of Science (JSPS), under contract no.  26420692.