catalyst-free direct growth of triangular nano-graphene on all substrates

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Published: June 28, 2011 r2011 American Chemical Society 14488 dx.doi.org/10.1021/jp2017709 | J. Phys. Chem. C 2011, 115, 1448814493 ARTICLE pubs.acs.org/JPCC Catalyst-Free Direct Growth of Triangular Nano-Graphene on All Substrates Ki-Bum Kim, Chang-Mook Lee, and Jaewu Choi* Department of Information Display, Kyung Hee University, 1 Hoegi-Dong Dondaemoon-Gu, Seoul 130-701, Korea INTRODUCTION Chemical vapor deposition (CVD) methods have been uti- lized for thin lm development because it is advantageous for the development of large-scale devices. Graphene also has been grown by CVD methods; typically metallic catalysts such as transition and noble metals are employed. 1,2 However, in grow- ing graphene on various metal lms on substrates, the sand- wiched metallic layer becomes a serious obstacle for diverse device developments because the metallic catalyst should be removed before utilizing the unique physical properties of graphene for device applications, or the grown graphene should be transferred to desirable substrates, such as semiconducting silicon, insulating silicon dioxide, or transparent substrates for the fabrication of electronic and optical devices. Removing the underlying metallic catalyst layer through the narrow gap be- tween the graphene and the substrates requires the time-con- suming wet-chemical etching processes. Additionally, it is unrealistic to apply transfer technology to large-scale applications even though large-scale graphene can be grown on the metal substrate because the required time for the wet-chemical etching proportionally increases with the size of the growth pads. Graphene is a one atomic layer hexagonal structure of carbon atoms. 3 5 To grow graphene layers, a metal catalyst may not be needed because monolayer graphene lms do not require a higher growth rate perpendicular to the surface even though a higher growth rate along the surface may be desirable. Whenever a gas molecule touches the surface of a solid, at least there is a van der Waals attractive interaction, but the attractive interaction strength may vary between molecules over a wide range according to the origin of the interaction between them. As a result, gases adsorb on the surface as long as the sticking coecient is not zero. This suggests that graphene can be grown on the surface of any material including insulators, semiconductors, and metals even though a high temperature may be required to enhance surface reactivity as well as diusivity to increase the lateral growth rate and the extent of order. This is supported by the recently published work. 6 8 Even with excellent physical properties of graphene, such as quasi-particle behavior near the Dirac point with zero-eective mass 4,5,9 11 and extremely high carrier mobility, 5,12 the intrinsic semimetallic properties of graphene with the lack of an intrinsic energy band gap have become a critical hurdle for the develop- ment of devices such as electronic switches, sensors, optical devices, and logic gates because these devices generally require semiconducting materials with a substantial energy band gap compared to environmental thermal energy. 13 18 Various eorts have been made to open the energy band gap of graphene by controlling the interaction between graphene and its substrates 19 or among graphene layers 20 and by controlling the size and edge of the graphene, which has been demonstrated by employing chemical methods, 21 unzipping carbon nanotubes, 22 using atomic force microscopy 23 and metal nanoparticles, 24 and shaping with costly labor-intensive, time-consuming physiochemical e-beam lithographical methods. 25 Additionally, the large energy band gap was obtained by transforming the sp 2 graphene network to the sp 3 hybridization by hydrogenation or oxidation, but this leads to a large distortion into the nonplanar structure. 26 Band gap opening by controlling the size and the edge of graphene at nanoscale is promising and practical because the planner structure of nanographenes may maintain the intrinsically excellent physical property of graphene while the energy band gap can be opened up to a few electronvolts. 21,27 Received: February 22, 2011 Revised: June 28, 2011 ABSTRACT: To epitaxially grow graphene, metallic catalysts or carbon containing silicon carbide have been typically utilized. The embedded metallic catalyst between graphene and the substrate as well as the expensive silicon carbide substrate create hurdles in the development of graphene-based devices. However, what is inevitably necessary is not a metallic catalyst but a at plane able to hold the carbon species and to mediate their interaction on the plane. The plane needs neither to hold a large amount of carbon species nor be a highly ecient catalyst because one monolayer of carbon on the plane may be enough to grow graphene. In this study, graphene was grown directly on various substrates such as transparent substrates, insulators, and semiconductors without any catalyst. The directly grown graphene is triangular nano- graphene with sides of 100 200 nm in length. This study suggests that graphene can be directly grown on all substrates.

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Published: June 28, 2011

r 2011 American Chemical Society 14488 dx.doi.org/10.1021/jp2017709 | J. Phys. Chem. C 2011, 115, 14488–14493

ARTICLE

pubs.acs.org/JPCC

Catalyst-Free Direct Growth of Triangular Nano-Grapheneon All SubstratesKi-Bum Kim, Chang-Mook Lee, and Jaewu Choi*

Department of Information Display, Kyung Hee University, 1 Hoegi-Dong Dondaemoon-Gu, Seoul 130-701, Korea

’ INTRODUCTION

Chemical vapor deposition (CVD) methods have been uti-lized for thin film development because it is advantageous for thedevelopment of large-scale devices. Graphene also has beengrown by CVD methods; typically metallic catalysts such astransition and noble metals are employed.1,2 However, in grow-ing graphene on various metal films on substrates, the sand-wiched metallic layer becomes a serious obstacle for diversedevice developments because the metallic catalyst should beremoved before utilizing the unique physical properties ofgraphene for device applications, or the grown graphene shouldbe transferred to desirable substrates, such as semiconductingsilicon, insulating silicon dioxide, or transparent substrates for thefabrication of electronic and optical devices. Removing theunderlying metallic catalyst layer through the narrow gap be-tween the graphene and the substrates requires the time-con-suming wet-chemical etching processes. Additionally, it isunrealistic to apply transfer technology to large-scale applicationseven though large-scale graphene can be grown on the metalsubstrate because the required time for the wet-chemical etchingproportionally increases with the size of the growth pads.

Graphene is a one atomic layer hexagonal structure of carbonatoms.3�5 To grow graphene layers, a metal catalyst may not beneeded because monolayer graphene films do not require ahigher growth rate perpendicular to the surface even though ahigher growth rate along the surface may be desirable.

Whenever a gas molecule touches the surface of a solid, at leastthere is a van der Waals attractive interaction, but the attractiveinteraction strength may vary between molecules over a widerange according to the origin of the interaction between them. Asa result, gases adsorb on the surface as long as the stickingcoefficient is not zero. This suggests that graphene can be grownon the surface of any material including insulators,

semiconductors, and metals even though a high temperaturemay be required to enhance surface reactivity as well as diffusivityto increase the lateral growth rate and the extent of order. This issupported by the recently published work.6�8

Even with excellent physical properties of graphene, such asquasi-particle behavior near the Dirac point with zero-effectivemass4,5,9�11 and extremely high carrier mobility,5,12 the intrinsicsemimetallic properties of graphene with the lack of an intrinsicenergy band gap have become a critical hurdle for the develop-ment of devices such as electronic switches, sensors, opticaldevices, and logic gates because these devices generally requiresemiconducting materials with a substantial energy band gapcompared to environmental thermal energy.13�18

Various efforts have beenmade to open the energy band gap ofgraphene by controlling the interaction between graphene andits substrates19 or among graphene layers20 and by controlling thesize and edge of the graphene, which has been demonstrated byemploying chemical methods,21 unzipping carbon nanotubes,22 usingatomic force microscopy23 and metal nanoparticles,24 and shapingwith costly labor-intensive, time-consuming physiochemical e-beamlithographical methods.25 Additionally, the large energy band gap wasobtained by transforming the sp2 graphene network to the sp3hybridization by hydrogenation or oxidation, but this leads to a largedistortion into the nonplanar structure.26

Band gap opening by controlling the size and the edge ofgraphene at nanoscale is promising and practical becausethe planner structure of nanographenes may maintain theintrinsically excellent physical property of graphene while theenergy band gap can be opened up to a few electronvolts.21,27

Received: February 22, 2011Revised: June 28, 2011

ABSTRACT: To epitaxially grow graphene, metallic catalysts or carboncontaining silicon carbide have been typically utilized. The embeddedmetallic catalyst between graphene and the substrate as well as theexpensive silicon carbide substrate create hurdles in the development ofgraphene-based devices. However, what is inevitably necessary is not ametallic catalyst but a flat plane able to hold the carbon species and tomediate their interaction on the plane. The plane needs neither to hold alarge amount of carbon species nor be a highly efficient catalyst becauseone monolayer of carbon on the plane may be enough to grow graphene.In this study, graphene was grown directly on various substrates such astransparent substrates, insulators, and semiconductors without any catalyst. The directly grown graphene is triangular nano-graphene with sides of 100�200 nm in length. This study suggests that graphene can be directly grown on all substrates.

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Further, the control of the edge structure allows for the tuning ofthe physical properties of the nanographene. Thus, the control ofthe size and edge structure of graphene at nanoscale becomesimportant in device applications.

Among the methods for the size and shape control ofgraphene, the mostly practical one is by cutting graphene on asubstrate using e-beam lithography.25 Before cutting, graphenemust be transferred to the desired substrates from highlyoriented graphite,25 from graphene grown on a metallic catalystlayer,1 or from graphene grown at high temperatures such as∼1500 �C on polar crystals such as expensive silicon carbide.28

These transferring processes require multiple, time-consuming,and costly procedures that risk damaging the graphene during theprocess and are inadequate for the deposition of graphene onvarious technologically important substrates for the large scaledevice application of graphene.

This study shows catalyst free growth of nanographene ontechnologically important representative substrates such as sili-con (semiconductor), silicon oxide (insulator), and quartz(transparent substrate). The direct synthesis of graphene ontothe desired substrates without using any metallic catalyst is freefrom the hurdle related to the metallic catalysts as well as is ableto control the size of the nanographene by self-assemblingmethods. This will open the opportunity to fully utilize graphenein the development of various types of devices including thin filmtransistors, light emitting diodes, solar cells, touch pads,displays, etc.

’EXPERIMENTAL METHODS

Graphene was grown directly on technologically importantsubstrates, such as semiconducting 500 μm thick silicon (100)wafers, 300 nm thick insulating silicon dioxide on Si(100) wafers,and 1000 μm thick transparent quartz plates. Before thesesubstrates were loaded into the growth system, they weresequentially cleaned by acetone, isopropyl alcohol, and deionizedwater. The cleaned substrates were inserted into a 3 in. quartzreaction tube of a thermal chemical vapor deposition (CVD)system. The system was pumped at 1 mTorr, and then the

reaction tube was heated to the desired growth temperatures.Once the pressure reached 1 mTorr at the growth temperature,acetylene (C2H2) (25 sccm) with argon (50 sccm) was intro-duced. Graphene growth was conducted by varying the tempera-ture (800�1100 �C), pressure (2�100 Torr), and growth period(1.5�60 min.).

The sheet resistance was measured by a four-probe setup witha Keithley 2400 sourcemeter, and the transmittance was mea-sured by a Scinco S-4100 UV�vis spectrometer. The topographyof graphene was investigated by a Bruker AXS N8 NEOS atomicforce microscope. The Raman spectra were obtained from thedirectly grown graphene samples on the various substrates usinga JY LabRamHRmicro Raman spectrometer with backscatteringgeometry using a 514.5 nm wavelength Ar-ion laser.

’RESULTS AND DISCUSSION

Without using any metallic catalyst, graphene directly grewnot only on quartz but also on silicon (100) and silicon oxide(300 nm)/silicon (100) using acetylene as a carbon source attemperatures of 800�1100 �C.

Optical images of the graphene-transparent electrodes, whichconsist of graphene directly grown on quartz substrates at800 (a), 900 (b), 1000 (c), and 1000 �C (d) for 1 h at 10 Torr,are shown in Figure 1. With increasing growth temperature, thegraphene-transparent electrodes become increasingly darkerdue to the thicker graphene film grown on the substrates. Graphenewas directly grown simultaneously on both sides of the quartz.

The experimental data of transmittance (T) vs sheet resistance(Rs) were taken from graphene directly grown on quartz byvarying temperature (800�1100 �C), pressure (2�500 Torr),and growth time (1.5�60 min). The transmittance and the sheetresistance decreased with growth temperature, pressure, andtime. Figure 1e shows the single side transmittance of gra-phene-transparent electrodes versus sheet resistance. The singleside transmittance of graphene grown on the quartz was ex-tracted from the measured transmittance using a simple relation-ship of T = T1T2 with assumption of T1 = T2, and plotted inFigure 1e. The sheet resistance at the transmittance of 80% was

Figure 1. Optical images of graphene directly grown on quartz substrates at growth temperatures of (a) 800, (b) 900, (c) 1000, and (d) 1100 �C at 10Torr for 1 h. (e) The sheet resistance vs the transmittance of graphene grown on quartz substrates at various temperatures (800�1100 �C) and pressures(2�100 Torr) for various growth periods (1.5�60 min).

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∼4 kΩ. The sheet resistance is 1 order of magnitude higher thanthe large-scale graphene itself but is lower than the samplesprepared by spin coating or Langmuir�Blodgett monolayer de-posited graphene films from graphene dispersed in solutions.29,30

The transmittance data of Figure 1e were fitted with a functionof T(%) = (1� R) exp(�A/RS)� 100 and shown by a solid linewhere R and A are reflectance and a coefficient related to theresistivity and absorption coefficient of the material,31 and theyare∼0.027 and 0.90Ω, respectively. The reflectance of∼0.027 isvery close to the quantum optical opacity rather than thereflectance.32

Atomic force microscopy (AFM) images of graphene grownon silicon (100) and quartz are shown in Figures 2 and 3,respectively. The AFM images (Figure 2a�f) of graphene grownon silicon at the growth temperature range of 800�1000 �Cshow nanoscale triangle-shaped planar graphitic carbon struc-tures, triangle nanographenes (TNGs). However, at 1100 �C,spherical nonplanar carbon clusters were grown as shown inpanels g and h of Figure 2.

The AFM topographical (Figure 2a) and phase images(Figure 2b) of the TNGs grown at the growth temperature of800 �C, which are shown in panels a and b of Figure 2,

Figure 2. AFM images of nanographene directly grown on silicon substrates for 1 h at 800 �C (a and b) and at 900 �C (c and d), 1000 �C (e and f), and1100 �C (g and h) using the topographymode except (b) (phase mode). The scan size is 5μm� 5μm for (a), (c), (e), and (g) and 1 μm� 1 μm for (b),(d), (f), and (h).

Figure 3. AFM topographical images of nanographene grown on quartz substrates at 900 �C (a and b) at 50 Torr for 5 min, and at 900 �C (c and d),1000 �C (e and f), and 1100 �C (g and h) at 10 Torr for 1 h. The insets of panels a and b are line profiles. The scan size is 5 μm� 5 μm for (a), (c), (e),and (g) and 1 μm � 1 μm for (b), (d), (f), and (h).

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respectively, show that the whole silicon surface was covered withthe TNGs, which are right triangles. The sides of TNGs are∼80and ∼95 nm, respectively. The phase image (Figure 1b) of theTNGs shows rough edges, which indicate that the edge may benonplanar. The nonplanar structure of edges can originate fromthe sp3 hybridization with hydrogen, which likely happened dueto the relatively low growth temperature in the hydrocarbonenvironment.

The right triangular shape of nanographene is anisotropic anda good indication that TNGs are crystalline rather than amor-phous because the crystal growth largely depends on the crystal-line orientation owing to orientation-dependent formationenergy. This was supported by their Raman spectra showingthe crystalline sp2 characteristics of nano-graphene with a highand sharp G band at 1600 cm�1 and a D band at 1350 cm�1

features as shown in Figure 4a.The sides of the right triangle could be highly symmetric

structures such as armchair (A) or zigzag (Z). If one of the sides isarmchair, the perpendicular side should be zigzag. Therefore, thepossible highly symmetric edge structure of the sides perpendi-cular to each other is armchair (A)�zigzag (Z) while thecrystalline structure of the hypotenuse depends on the angle orthe length ratio between neighboring edges. When the 30� angleis between a side and the hypotenuse of the triangle, the side isarmchair (zigzag) and the hypotenuse is zigzag (armchair).When the angle is 60� between a side and the hypotenuse, theside and the hypotenuse are either armchair or zigzag. However,the measured angles between the side and the hypotenuse areneither 30� nor 60�, but they are close to these. This suggests thatthe hypotenuse of the right triangle could be the combination ofthe zigzag and armchair.

However, two types of TNGs were observed to have grown onsilicon at 900 �C: (right triangles and isosceles triangles) whilethe majority of TNGs are right triangles as shown in panels c andd of Figure 2. This is evidence that thermal energy plays animportant role in determining the shape of the nano-graphenesby competition between the orientation-dependent formation

energy and the thermal energy. The typical perpendicular sides ofTNGs grown at 900 �C are 100 and 135 nm, respectively. Thesize of the TNGs became larger (80 nm � 95 nm) than that at800 �C, but the population of the TNGs became less comparedto that at 800 �C. This indicates that the surface diffusion wasenhanced by the relatively high growth temperature.

As the growth temperature increased to 1000 �C, TNGsbecame larger and the perpendicular sides of the TNGs are160 and 160 nm, respectively, as shown in panels e and f ofFigure 2. They are of the isosceles right triangular nano-grapheneon silicon, and the number density of the triangles is even lower.This strongly suggests that isosceles triangle formation on siliconis more favorable as the growth temperature increases. Thus, athigh growth temperatures, the growth depends less on thediffusion than on the crystal orientation. In the range of800�1000 �C, the size of TNGs increases with temperaturewhile the number density of nanographene becomes lower withtemperature.

However, at the growth temperature of 1100 �C, the carbonstructure grown on silicon was no longer planar but a sphericalcluster with the diameter of∼200 nm as shown in panels g and hof Figure 2. That carbon cluster growth on silicon was morefavorable than the planar structure growth at the high growthtemperature can be attributed to the high thermal stress driven bythe large thermal expansion coefficient (TEC) with a significantlattice mismatch between graphene and silicon.33,34 It is knownthat the TEC of silicon is relatively higher than the in-plane TECof graphene.

Unlike TNG growth at 800�1000 �C and spherical clustergrowth at 1100 �C on crystalline silicon substrates, the growthbehavior of graphene on thick thermal oxide or quartz substrateswas distinct in several aspects as shown in Figure 3. First, theAFM studies show that carbon films grown on quartz and siliconoxide on silicon at the low growth temperature of 800 �C werecontinuous (not shown here). However, the Raman spectrumshown in Figure 4b indicates that the continuous carbon filmscorrespond to nano-graphene rather than a large-scale graphenesheet. This suggests that the continuous graphitic carbon film canbe considered as a mosaic of nanographenes rather than a singledomain large-scale film.

The graphene grown on quartz at 900 �C for 5 min showsTNG islands as well as vacancies as shown in panels a and b ofFigure 3. The nanographenes grown on quartz at 900 �C for5 min are isosceles right triangular nano-graphene in shape and190 nm � 190 nm in size. TNG islands and vacancies resembleeach other in shape and size. The typical thickness of the TNGislands and vacancies are 1�2 nm as shown by the line profile inthe inset of panels a and b of Figure 3. This strongly suggests thatTNG islands were detached from the continuous layer and leftTNG vacancies, indicating how the growth mode transitionoccurred from the continuous-mosaic growth mode to discretegrowth mode at 900 �C.

The one corner of the isosceles right TNGs became roundedwith the longer growth time as shown in panels c and d ofFigure 3 (at 900 �C for 1 h). In particular, by increasing thegrowth temperature to 1000 �C, all three corners of the isoscelesright TNGs became rounded as shown in panels e and f ofFigure 3. Additionally, a dent was observed in the middle of eachrounded triangle nanographene as in panels e and f of Figure 3(1000 �C). The dent might correspond to a nucleation site.

At 1100 �C (quartz), the number of prominent triangles wasreduced, but TNGs were merged as shown in panels g and h of

Figure 4. The Raman spectra taken from nano-graphene grown for 1 hat 800, 900, 1000, and 1100 �C on (a) silicon and (b) quartz substrates,respectively. (c) Raman signal intensity of substrate Si (975 cm�1) andquartz (435 cm�1) as a function of temperature. (d) The ratio of the Dband intensity to the G-band intensity grown on silicon (blue) andquartz (red), respectively.

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Figure 3 by enhanced diffusion at higher growth temperature.Unlike the spherical formation on silicon substrate, the mergingof TNGs is attributed to the relatively low TEC of silicon oxidecompared to that of silicon.

Panels a and b of Figure 4 show temperature-dependentRaman spectra taken from the nano-graphene grown on siliconand quartz substrate, respectively. Features that originated fromsubstrates (Si features at 520, 980 cm�1 and quartz feature at480 cm�1) decrease in intensity with growth temperaturecompared to graphitic carbon features at 1350 (D), 1600 (G),2710 (2D), 2950 (D + G), and 3200 (2G) cm�1. Over theemployed growth temperature region, the position of the G bandis located at 1600 cm�1, which is higher than the typical large-scale graphene or graphite (1580 cm�1), and this is the typicalfingerprint of the nano-graphene.35

As mentioned above, with an increase in the growth tempera-ture, panels a and b of Figure 4 show that the Raman intensity ofsilicon or quartz features from substrates exponentially decayswith temperature because the graphitic carbon film is thicker withhigher growth temperature. The exponential decaying intensityvariation with temperature is clearly shown in Figure 4c. Thesilicon signal intensity decays faster than the quartz signalintensity with growth temperature. This indicates that the growthrate of graphitic carbon film on silicon is higher than that onquartz because the substrate signal intensity is inversely propor-tional to the growth rate for the fixed growth period. It is believedthat the higher density of the dangling bond on silicon comparedto that on quartz plays an important role in enhancing the growthrate. On silicon, the relatively low growth rate at 900 �Ccompared to that at 1000 �C suggests that the silicon danglingbond may not be fully activated at 900 �C caused by thedimerization of the dangling bonds. However, on quartz, thequartz signal intensity decays monotonically and exponentiallywith temperature because the density of the dangling bondincreases with temperature, but the dimerization of the silicondangling bond on quartz did not occur due to spareddangling bonds.

The ratio of the intensity of the D band (ID) to the intensity ofthe G band (IG) in Figure 4d can represent the crystallineproperty of nano-graphene.28 The intensity ratio for nanogra-phene grown on silicon slightly increases with the growthtemperature until 1000 �C. This indicates that the crystallineproperty of graphene grown on silicon becomes worse withhigher growth temperature even though the size of the nano-graphene becomes larger with temperature as shown in Figure 2.This implies that defect density became larger with growthtemperature. It is believed that the higher TEC of silicon playeda significant role in the increase of defect density.

However, at 1100 �C, the ratio of the intensity of the D band(ID) to the intensity of the G band (IG) was recovered as shownin Figure 4d. This suggests that the thermal stress relaxation byforming spherical graphitic clusters reduces the defect formationunlike the case for the planar structure.

However, the crystalline property of graphene grown onquartz is significantly improved with growth temperature. Atlow temperature, the ratio is relatively high due to the loweractivity and dangling bonds. This indicates that the nano-graphene grown at low temperature has a large number of defectsdue to the lack of nucleation centers. The density of the activesilicon dangling bonds acting as a nucleation center becomes lessat lower growth temperature. This can cause large defects.However, as the temperature increases, the number and activity

of the dangling bond on the quartz becomes higher, and this leadsto the growth of a highly crystalline nano-graphene. Overall, thecrystalline property grown on silicon is better than that grown onquartz over the employed growth temperature region exceptat 1000 �C. The Raman study clearly indicates that TNGscorrespond to the sp2 two-dimensional graphitic carbon.

The temperature-dependent growth behavior of nano-graphene on silicon was distinct from that on silicon oxide(both polycrystalline quartz and amorphous thick thermal oxideon silicon). These distinct behaviors of graphene growth on twotypes of substrate can be summarized as follows: discrete (on Si)versus semicontinuous (on SiO2) at 800 �C, decreasing (on Si)versus increasing (on SiO2) the number density of TNGs in thegrowth temperature range of 800�1000 �C, increasing the size ofnanographene (on Si) versus smaller in size and rounded inshape (on SiO2) (800�1000 �C), and clustering (on Si) versusripening (on SiO2) at 1100 �C. The crystalline properties ofTNGs are worse on silicon but better on quartz with growthtemperatures. The growth rate increases with temperature exceptat 900 �C on silicon due to the dimerization of dangling bonds.The clear growth mode transition on silicon oxide was observedat 900 �C by the simultaneous formation of TNG islands andvacancies. The growth behavior of graphene on amorphoussilicon oxide was similar to that on polycrystalline quartz. Thisindicates that the dominant parameter determining the graphenegrowth behavior is the nucleation density rather than the crystal-line property itself.

’CONCLUSIONS

Catalyst-free direct growth of triangular nano-graphenes onsemiconductors (silicon), insulators (silicon oxide on silicon),and transparent substrates (quartz) using a simple thermalchemical vapor deposition method was demonstrated. Thissuggests that nano-graphene can grow on any kind of substrateand the direct growth method can be free from the time-consuming, costly, labor-intensive multiple processes requiredfor graphene transfer and size control.

The shape of the directly grown nano-graphene is an isoscelesright triangle, and its size depends on the growth temperatureand substrates. This unique growth behavior can be utilized tocontrol the size, shape, and edge of nano-graphene by varyinggrowth temperature and substrates. Thus, it is not difficult topredict that these direct growing methods and graphenes grownon various technologically important substrates can be widelyutilized for the development of passive components as well asactive components in the development of transparent electrodes,electronic devices, photonics devices, sensors, and spintronicdevices.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

We thank Professor H. K. Park and his research group forallowing us to utilize the AFM facilities. This work was supportedby the Korea Research Foundation Grant funded by the KoreanGovernment (KRF-2008-313-C00316) and by the Basic ScienceResearch Program through the National Research Foundation of

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Korea (NRF) funded by the Ministry of Education, Science andTechnology (MEST) (2010-0005706).

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