Single- and multilayer graphene films formed on the surface of a number of metals (Rh, Pt, Ir, and Re) at the thermal decomposition of benzene molecules have been studied. A surprising phenomenon has been revealed: these films with thicknesses up to 15 layers do not absorb light emitted by the surface and affect neither brightness nor true temperatures of the surface, which contradicts the fundamental results obtained by A. Geim and K. Novoselov. At larger thicknesses, absorption occurs in agreement with the commonly accepted concepts

Gall, N.

Rut'kov, E.

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ISSN 00213640, JETP Letters, 2014, Vol. 100, No. 10, pp. 625–628. © Pleiades Publishing, Inc., 2014.Original Russian Text © E.V. Rut’kov, N.R. Gall, 2014, published in Pis’ma v Zhurnal Eksperimental’noi i Teoreticheskoi Fiziki, 2014, Vol. 100, No. 10, pp. 708–711.625INTRODUCTIONSurprising optical properties of graphene have beenstudied in detail in the last decade and A. Geim andK. Novoselov, who discovered these properties, wereawarded with the Nobel Prize. As was shown in [1, 2],a multilayer graphene film absorbs visible light welland the degree of absorption of singlelayer grapheneis determined only by a combination of fundamentalconstants and is about 2.3% [3–6]. Since interlayercoupling is weak, it can be assumed that each of thelayers in a multilayer film absorbs light independently;for this reason, the optical absorption coefficient ofsuch a system can be calculated directly by Lambert’scosine law.We and other groups have studied the formation ofgraphene and graphite on the surface of metals for thelast four decades [7, 8]. We recently discovered anddescribed a phase transition of a twodimensionalgraphene film to threedimensional graphite film onthe surface of a carbonized metal (Rh) [9]. Thedetailed consideration of its kinetics, where a changein the brightness temperature of the surface plays adecisive role, stimulated detailed investigations of thisproblem because it seemed that the behavior of realsystems on the surface of a metal is different from predictions based on the above concepts.EXPERIMENTSThe experiments were performed in high vacuum(p ~ 10–10 Torr) with an Auger spectrometer with a resolution of ΔE/E ~ 0.1% [10]. The samples under studywere 40 × 0.02 × 1mm metallic Rh ribbons, whichwere textured by direct ac heating and were cleanedfrom impurities by aging of heated ribbons in an oxygen atmosphere and in ultrahigh vacuum. After cleaning, the orientation of the surface of a ribbon in the(111) direction was 99.8%. The average size of grainswas 40–50 μm. The surface was uniform in work function within an accuracy of 0.05 eV. In the chamber ofthe instrument, a unit was assembled for measuringthe thermionic emission from heated samples. Augerspectra can be recorded directly from the samplesheated up to 2000 K.The samples were carbonized by the standard procedure of the aging of heated ribbons in the isothermalregime in a benzene atmosphere until carbon saturated the bulk of the sample to the extreme solubilityand a graphene monolayer was formed on the surfaceof the sample. The carbonization procedure was discussed in detail in [11] for rhodium and in [8] for othermetals (Re, Pt, Ni, Ir, etc.). Since a ribbon was thin,diffusion processes were fast and real thermodynamicequilibrium at temperatures above 1100–1200 K wasreached in about several seconds. After that, benzenevapors were evacuated and a strictly monatomicgraphene layer remained on the surface of the metal.In the process of carbonization, carbon atoms penetrate into the bulk of the ribbon and can generallychange its electric resistance and, thereby, the releasedpower at a constant filament current. However, owingto a low extreme solubility of carbon atoms in Rh, thiseffect is beyond the measurement accuracy. In particular, in the carbonization temperature range of 1200–1800 K, a power increase owing to a change in theresistance was less than 0.5% and the brightness temperature of the ribbon remained constant with anaccuracy that was ensured by an optical pyrometer(~4–5 K). The specific heat of the holders of the ribbon was chosen as low as possible. In this case, its middle part with a length of ~35 mm in the pyrometricUnusual Optical Properties of Graphene on an Rh SurfaceE. V. Rut’kova and N. R. Galla–ca Ioffe Physical Technical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russiaemail: gall@ms.ioffe.ru b Institute of Analytical Instrumentation, Russian Academy of Sciences, Rizhskii pr. 26, St. Petersburg, 190103 Russiac St. Petersburg State Polytechnic University, ul. Politekhnicheskaya 29, St. Petersburg, 194021 RussiaReceived October 7, 2014Single and multilayer graphene films formed on the surface of a number of metals (Rh, Pt, Ir, and Re) at thethermal decomposition of benzene molecules have been studied. A surprising phenomenon has beenrevealed: these films with thicknesses up to 15 layers do not absorb light emitted by the surface and affect neither brightness nor true temperatures of the surface, which contradicts the fundamental results obtained byA. Geim and K. Novoselov. At larger thicknesses, absorption occurs in agreement with the commonlyaccepted concepts.DOI: 10.1134/S0021364014220159626JETP LETTERS  Vol. 100  No. 10  2014RUT’KOV, GALLregion was uniform in temperature with the sameaccuracy.GROWTH KINETICS OF GRAPHENE AND GRAPHITE LAYERSIf the temperature is abruptly reduced below thecarbonization temperature TC, the solid solution ofcarbon in a metal becomes supersaturated and excesscarbon rises on the surface, forming graphene andgraphite layers. The layerbylayer growth of a graphitefilm was previously observed in metals Re [10], Pt[12], and Ni [13]. Both single and multilayer filmshave the graphene (graphite) structure, which is confirmed by scanning tunnel microscopy and Ramanscattering measurements.The thickness (the number of layers f) in a grapheneor graphite film formed on the surface at the reductionof the temperature is determined by the differencebetween the extreme solubilities of C in Rh at the carbonization temperature (nC(TC)) and at the filmgrowth temperature (nC(T)):f = [nC(TC) – nC(T)]NRh/2NCML, (1)where NRh is the number of rhodium atoms in the sample with the volume V = Sh = 1 cm2 × 2 × 10–3 cm =2 × 10–3 cm3, NCML = 3.86 × 1015 atoms/cm2 is theconcentration of carbon atoms in the graphene monolayer, and a coefficient of 1/2 is due to the separationof carbon simultaneously on both sides of the ribbonsample. The quantity nC(T) was presented in [14, 15]and was carefully measured in our work [16]. The calculations are in good agreement with the Auger spectroscopy data at the growth of a thin film (f < 6).(Auger spectroscopy is insensitive to thicker films.)The variation kinetics of the Auger signals from thefilm (E = 272 eV) and substrate (E = 302 eV) at therelease of carbon from the bulk onto the surface ofrhodium carbonized at TC = 1340 K and aged at T =1260 K is shown in Fig. 1. The Auger signal of carbonincreases and is saturated at the thickness of the filmthat is several times larger than the mean free path ofAuger electrons with a given energy (λ ~ 5 Å) [17].Simultaneously, the signal from the substrate is nolonger detected because of its screening by carbon.The calculation by Eq. (1) yields f ~ 5–6 layers. Itis known that a graphene monolayer reduces the Augersignal from the substrate by a factor of 1.6 at the energyof electrons of 150–300 eV [17]. As can be seen inFig. 1, the Auger signal from rhodium decreases by afactor of about 16, which corresponds to the indicatedthickness of the film. At TC > 1400 K and T = 1260 K,a film thicker than eight layers is formed and no Augersignal from the substrate is detected. The shape of theAuger signal from carbon is typical of graphite.According to Fig. 1, the time of a decrease in thesignal by a factor of 1.6 is almost the same throughoutthe entire curve. This indicates the layerbylayergrowth of the graphite film, as was previously observedfor the Ni–C system [13]. The constant growth time ofeach layer shows that diffusion processes in the bulkare quite fast and growth is limited by the appearanceof carbon atoms on the surface and nucleation ofa new graphene layer [8].MEASUREMENT OF OPTICAL PROPERTIESFigure 2 shows variation of the brightness and truetemperatures of the sample at the growth of grapheneand graphite. Because of the avalanche release of carbon at the reduction of the temperature, which wasdescribed in our work [9], these curves cannot berecorded in a direct experiment.Fig. 1. Auger signals from (1) carbon and (2) Rh versus theheating time at 1260 K for the rhodium ribbon carbonizedat TC = 1360 K. Arrows mark the times of the formation ofthe respective graphene layer.Fig. 2. (1) Brightness and (2) true temperature shifts of thegraphene film grown on the Rh(111) surface versus thethickness of the film.JETP LETTERS  Vol. 100  No. 10  2014UNUSUAL OPTICAL PROPERTIES OF GRAPHENE ON AN Rh SURFACE 627To record these curves, the experiment was performed as follows. The sample was successively carbonized with an increase in the temperature in therange of 1260–1800 K. After each carbonizationcycle, benzene vapors were evacuated and the currentflowing through the sample was abruptly reduced to avalue corresponding to the formation of the film withthe necessary thickness calculated by Eq. (1). (At ahigher TC value, the grown film appeared to bethicker.) After reaching a steady state, the measuredtemperatures were plotted. We note that the electricpower released in the sample after the growth of thefilm always coincided with the power scattered in theuncarbonized ribbon at the same heating current.The brightness temperature was measured by a Pirocolor pyrometer at a wavelength of 580 nm. As can beseen in Fig. 2, it does not change at the growth of thegraphite film with a thickness of up to 15 layers. At thetime corresponding to 20 layers, it begins to decreaseslightly. At 120 layers, this decrease corresponds toabout 300 K. Then, the curve is stabilized and the temperature shift does not change with a further increasein the thickness of the film.The variation of the true temperature was determined from a decrease in the thermal electron currentat a constant electric power released in the bulk of theribbon. (In the process of growth of the graphite film,this current decreases by five orders of magnitude!) Weused the Richardson formula:, (2)where I– is the current of thermal electrons, S is thecollection area, A = 175 A cm–2 s–1 is the Richardsonconstant for this surface, k is the Boltzmann constant,and eϕ = 4.7 eV is the work function of multilayergraphite, which we determined by the Richardson linemethod. The true temperature can be determined witha high accuracy if the collection area S is constant andthe work function is known.To measure the work function for a thick film, wetook f > 100, where the brightness temperature nolonger depends on the thickness of the film. Assumingthat the grayness value is ε = 0.8 [18], we calculated thetrue temperature from the formula [19]. (3)With the use of this temperature, the Richardson curvecan be plotted for the multilayer film and the workfunction can be calculated with a high accuracy (eϕ =4.7 eV).Line 2 in Fig. 2 shows the true temperature shift inthe sample with an increase in the thickness of thegraphite film. Direct experimental data can beobtained only for f < 30 and f > 120. In the intermediate states, the current is too low. We note that the truetemperature at the maximum thickness of the filmI– SAT2 eϕ/kT–[ ]exp=1/TBr 1/T+ 4.62 105–1/ε( )ln×=decreases by ΔT ~ 430 K, as was previously observed in[18].We now describe how data are obtained. Let thecarbonization temperature be TC = 1680 K, nC =0.19 at %, and a graphene monolayer exist on the surface. We abruptly reduce the heating current from 3.5to 3.0 A, which would correspond to the temperatureT1 = 1480 K in the case of uncarbonized ribbon.Excess carbon from the bulk is segregated on the surface and forms the multilayer graphene film. Thisleads to an increase in the grayness value and radiativeheat absorption and, thereby, to a decrease in thebrightness and true temperatures, as well as in thethermal electron emission current. After the end of theprocess, we calculate the new true temperature byEq. (2), T2 = 1380 K, and determine the corresponding extreme solubility nC(1380 K) = 0.07 at %. Thenumber of layers in the graphene film can be easilyestimated by Eq. (1) as f = 33. In this case, ΔT = T1 –T2 = 1480 – 1380 = 100 K. In order to obtain a thickerfilm, the carbonization temperature should be higher.Thus, neither the true nor brightness temperaturechanges for films thicker than 15 layers on the Rh substrate.GRAPHENE ON OTHER SUBSTRATESIn view of the results obtained, it was necessary totest how graphene films behave on other metallic substrates. Graphene on Ir(111), Pt(111), and Re(1010)was grown as on rhodium by the chemical vapor deposition (CVD) of benzene molecules. The maximumnumber of layers on iridium was f = 1 and on platinumwas f = 10 because of a low solubility. Multilayer filmscan be formed on rhenium, but diffusion processes inthis case are much slower than those in the case ofrhodium [8, 10].Neither the brightness nor true temperaturechanges for films with thicknesses f < 15 on all substrates under study at a constant electric powerreleased in the samples.DISCUSSION OF THE RESULTSWe now discuss the results. In the hightemperature region under consideration, heat loss on holderscan be neglected owing to the temperature uniformityof the central part of a sample. Consequently, the electric power released in the ribbon is completely emittedfrom the surface; therefore,IU = εσT4, (4)where I and U are the current flowing through the ribbon and the voltage drop on it, respectively; ε is thegrayness value in the region of maximum radiation(~580 nm); σ is the Stefan–Boltzmann constant; andT is the true temperature. The lefthand side of Eq. (4)(released power) was maintained constant in the628JETP LETTERS  Vol. 100  No. 10  2014RUT’KOV, GALLexperiment. As graphite grew, the emissivity variedfrom ε = 0.24 [19], which corresponds to a pure metal,to ε = 0.8 [19], which corresponds to graphite. Thebrightness temperature TBr, which is related to the truetemperature T by Eq. (3), varied simultaneously.It seems surprising that the appearance of grapheneand graphite on the surface at the initial stage of theirgrowth affects neither brightness nor true temperature.Within the simplest equilibrium model, it would beexpected that graphene layers forming on the surface,which have the same temperature as the substrate,absorb light emitted by the metal with the coefficientε = 0.2–0.3 and reemit it with ε = 0.8. This model satisfactorily describes effects in thick films. However, itis inapplicable to thin films when both brightness andtrue temperatures remain unchanged.This is possible under the assumption thatgraphene on the metal surface does not absorb light fora certain reason and the multilayer film remains optically transparent up to a certain thickness; after that, itbegins to absorb and emit light. This phenomenon isapparently quite general. It is probably due to a changein the electronic properties of graphene because of theinteraction with the metallic substrate. At present, wecannot propose any model to describe the observedphenomenon.REFERENCES1. A. K. Geim, Science 324, 1530 (2009).2. M. M. Orlita, C. M. Faugeras, P. M. Plochocka,P. K. Neugebauer, G. M. Martinez, D. K. Maude,A. M. Barra, M. A. Sprinkle, C. M. Berger, W. A. deHeer, and M. M. Potemski, Phys. Rev. Lett. 101,267601 (2009).3. R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, andA. K. Geim, Science 320, 1308 (2008).4. B. Kuzmenko, E. van Heumen, F. Carbone, and D. vander Marel, Phys. Rev. Lett. 100, 117401 (2008).5. V. G. Kravets, A. N. Grigorenko, R. R. Nair, P. Blake,S. Anissimova, K. S. Novoselov, and A. K. Geim, Phys.Rev. B 81, 155413 (2010).6. X. Zhao, C. M. Wei, L. Yang, and M. Y. Chou, Phys.Rev. Lett. 103, 186802 (2009).7. N. R. Gall, E. V. Rut’kov, and A. Ya. Tontegode, Int. J.Mod. Phys. 11, 1865 (1997).8. E. V. Rut’kov and N. R. Gall, in Physics and Applications of Graphene: Experiments, Ed. by S. Mikhailov (InTech, Rijeka, Croatia, 2011).9. E. V. Rut’kov, A. V. Kuz’michev, and N. R. Gall, JETPLett. 93, 151 (2011).10. N. R. Gall, S. N. Mikhailov, E. V. Rut’kov, andA. Ya. Tontegode, Surf. Sci. 191, 185 (1987).11. E. V. Rut’kov, A. V. Kuz’michev, and N. R. Gall, Phys.Solid State 53, 1092 (2011).12. E. V. Rut’kov and A. Ya. Tontegode, Phys. Solid State38, 351 (1996).13. E. V. Rut’kov, A. Ya. Tontegode, and M. M. Usufov, Izv.Akad. Nauk, Ser. Fiz. 58, 102 (1994).14. E. Fromm and E. Gebhardt, Gase und Kohlenstoff inMetallen (Springer, Berlin, 1976).15. Y. J. Bhatt, R. Venkataramani, and S. P. Garc, J. Less.Common. Met. 132, L214 (1987).16. E. V. Rut’kov and N. R. Gall, Appl. Surf. Sci. 300, 1087(2014).17. J. P. Biberian and G. A. Somorjai, Appl. Surf. Sci. 2,352 (1979).18. A. Ya. Tontegode, Prog. Surf. Sci. 38, 201 (1991).19. L. Michalski, Temperature Measurement, 2nd ed.(Wiley, New York, 2001), p. 162.Translated by R. Tyapaev

Unusual optical properties of graphene on an Rh surface

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