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Graphene P-type Doping and Stability by Thermal Treatments in Molecular Oxygen Controlled Atmosphere Aurora Piazza, Filippo Giannazzo, Gianpiero Buscarino, Gabriele Fisichella, Antonino La Magna, Fabrizio Roccaforte, Marco Cannas, Franco Mario Gelardi, and Simonpietro Agnello J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07301 • Publication Date (Web): 10 Sep 2015 Downloaded from http://pubs.acs.org on September 12, 2015

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The Journal of Physical Chemistry

Graphene P-Type Doping and Stability by Thermal Treatments in Molecular Oxygen Controlled Atmosphere A.Piazza1,2,3, F. Giannazzo1, G. Buscarino2, G. Fisichella1, A. La Magna1 , F. Roccaforte1, M. Cannas2, F.M. Gelardi2, S. Agnello2,*. 1

CNR-IMM, Strada VIII, 5, Zona Industriale, 95123 Catania, Italy.

2

Department of Physics and Chemistry, University of Palermo, Via Archirafi 36, 90143 Palermo,

Italy. 3

Department of Physics and Astronomy, University of Catania, Via Santa Sofia, 64, 95123 Catania

Italy. * Via Archirafi 36, Palermo, Italy, phone: +39 091 23891703, Department of Physics and Chemistry, University of Palermo, e-mail: [email protected]

ABSTRACT. Doping and stability of monolayer low defect content graphene transferred on a silicon dioxide substrate on silicon is investigated by micro-Raman spectroscopy and atomic force microscopy (AFM) during thermal treatments in oxygen and vacuum controlled atmosphere. The exposure to molecular oxygen induces graphene changes as evidenced by a blue-shift of the G and 2D Raman bands, together with the decrease of I2D/IG intensity ratio, which are consistent with a high p-type doping (~1013cm-2) of graphene. The successive thermal treatment in vacuum does not affect the induced doping showing this latter stability. By investigating the temperature range 140350°C and the process time evolution, the thermal properties of this doping procedure are characterized and an activation energy of ~56 meV is estimated. These results are interpreted on the 1 ACS Paragon Plus Environment


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basis of molecular oxygen induced ~1013 cm-2 p-type doping of graphene with stability energy > 49 meV and post-doping reactivity in ambient atmosphere due to reaction of air molecules with oxygen trapped between graphene and substrate.

Keywords: Graphene doping, Oxygen Thermal Treatments, Chemical Vapour Deposition (CVD), µ-Raman Spectroscopy, Atomic Force Microscopy (AFM). I.

INTRODUCTION

The recent interest on graphene (Gr) electronic properties has stimulated the development of advanced methods for large-area Gr growth on different kinds of substrates1,2,3. The most promising, low cost and very accessible method for Gr production is the chemical vapour deposition (CVD) on metal substrates (such as Cu, Ni), since with this approach high quality samples can be obtained3. The Gr film is grown directly on the substrate surface that acts as a catalyst in the process. In the case of Cu substrates, the resulting film is predominantly composed by single layer Gr domains4. For applicative aims the Gr film grown on metal is then transferred on another insulator substrate, such as silica (amorphous SiO2), through a transfer procedure consisting firstly in covering the surface of Gr by a polymer, usually PMMA (poly methyl methacrylate), in a successive chemical etching to remove the metal substrate and in the final transfer on the end substrate chosen4,5,6. The removal of the protective polymer resist, that could leave residues and strains on Gr introducing defects and impurities, and the deformation induced by the final substrate are among the most critical steps in this preparation process7,8,9. These features are of particular concern for electronic applications since they affect the mobility of charge carriers and favour the reaction with ambient molecules, able themselves to change the electronic characteristics of Gr10,11,12. One of the most promising applications of large area and high quality graphene grown by CVD is as transparent conducting electrode for touch-screens as well as for new generation photovoltaic cells13. In these fields graphene could compete with commonly used transparent 2 ACS Paragon Plus Environment

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conductive oxides (such as Indium Tin Oxide, ITO), thanks to its high optical transparency (~97.7% for a single layer) in a wide wavelength range, and to its mechanical robustness and flexibility (opening the way to flexible devices applications). In this context, the main limitation of graphene with respect to ITO is represented by the relatively high sheet resistance (in the order of few hundreds Ohms) of unintentionally doped single layers grown by CVD. Several strategies are currently under consideration to reduce the sheet resistance either by substitutional doping of donor or acceptor species during CVD growth14 or by post deposition treatments, including adsorption or physical bonding of specific atoms and molecules on Gr15,16,17,18,19. In this context, it is known that p-type doping of Gr can be obtained by thermal treatments in oxygen. Notwithstanding the doping by thermal treatments is challenging because the Gr could be damaged by the high temperature annealing, and the doping effects could be unstable20,21,22,23,24. The studies carried out both theoretically and experimentally have estimated a molecular oxygen binding energy between 40 and more than 100 meV.11,22,25 This large discrepancy stems in part from the adopted calculus and experimental procedures and in part from the uncertainties due to the reversibility of doping, arising from the atmosphere of treatments. Nevertheless, it has been suggested that dry oxygen doping predominantly occurs on basal Gr, by contrast with the edge doping.26 This is a relevant point for applications since it permits large area doping, even if its stability and energetic properties have not been fully clarified26. The above reported relevance of oxygen for both doping and stability of Gr devices pushes to further explore the doping efficiency of single layer Gr and to evaluate its stability. In this work we report an experimental study on the effects of thermal treatments in molecular oxygen controlled atmosphere at temperatures lower than 350°C. The study as a function of the treatment time enabled to evaluate the kinetics of the doping mechanisms and to clarify the stability by opportune thermal treatments in vacuum. The experiments allowed to determine a stable thermally activated positive doping of ~1013cm-2 and to estimate a limit value for the binding energy of O2.

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EXPERIMENTAL TECHNIQUES

II.

Gr samples consisting of single crystalline monolayer domains were grown on copper (Cu) by Chemical Vapour Deposition (CVD) and were transferred on a 300 nm thick SiO2 layer on Si by using the PMMA procedure. Raman measurements were performed with a Bruker SENTERRA µRaman confocal equipment using a laser-diode source at 532 nm and having maximum spectral resolution of 9 cm-1. Laser power ( 140°C and is completed at T ≈ 250°C. Furthermore, the Raman bands changes for the G and 2D shifts and for their intensity ratio modification occur in a good correlated temperature range, suggesting that all these features reflect modification of Gr by the same process. The AFM analysis, reported in Figure 4c, shows that at T > 300°C Gr begins to deteriorate, indeed the flakes recorded after 350°C treatment are smaller than the native ones and those recorded at 300°C (see Figure 2a), or are completely destroyed.



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Figure 4. Peak positions (a) and intensity ratio (b) of the Gr µ-Raman characteristics spectral features after thermal treatments of 2h in oxygen pressure of 10 bar at various treatment temperatures. The error bars originate from a statistic of measurements on different flakes of graphene. (c) AFM morphology image after thermal treatment of 2h in oxygen pressure of 10 bar at 350°C. 12 ACS Paragon Plus Environment

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Once determined the temperature range of major effectiveness in the doping of graphene by treatment in fixed pressure of O2, the kinetics of the process has been studied as a function of time. The temperature 250°C, when the process is completed, and 190°C, when the process is not completed, were chosen to compare these kinetics aspects and two different samples were directly treated at each temperature. As shown in Figure 5, for both temperatures, a blue shift of G and 2D bands, and a decrease of the I2D/IG ratio are found already after 30 minutes. After 1 hour of treatment no more changes can be observed by the Raman spectra for the given temperature suggesting that the process arrives at thermodynamic equilibrium. Furthermore, the changes of the spectral features differ for the different temperatures suggesting different doping effects. It has been found, also in these cases, that the induced changes are stable by successive thermal treatments in vacuum.. These kinetics aspects enable to conclude that the data previously reported in Figure 4 are related to equilibrium states specific of each given temperature since each of those experiments were done in 2h, a time long enough to reach a stationary state, and that many different equilibrium states exists for oxygen doping.



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Figure 5. Evolution of the peak position (a) and of the intensity ratio (b) of the Gr µ-Raman spectral features induced by thermal treatments at two different temperatures (190°C, squares, and 250°C, circles) in oxygen pressure of 10 bar by varying the treatment time. The dotted lines mark the equilibrium time. The error bars originate from a statistic of measurements on different flakes of graphene.

It has been previously shown that oxygen is preferentially trapped between graphene and substrate20. From the results reported in Figure 4 and Figure 5 we can assume an energy distribution of stable graphene/substrate configurations where oxygen can be allocated. In particular, considering that at each temperature we get a specific equilibrium population, the differential changes in the Raman features deduced from fig.4a,b reflect the differences in these equilibrium states density and suggest a possibility to schematize the population of the configurations. In 14 ACS Paragon Plus Environment

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details, no stable doping population is expected for treatments below 140°C nor increment of doping above 250°C, since at low temperature no spectral changes are induced, whereas at high temperature the process is completed and again no spectral changes occur. By contrast some destroying occurs above 300°C, as above reported by AFM. At the same time the maximum changes are observed at about 200°C, where it can be theorized that the maximum density of stable doping configurations is located. To schematize these features a distribution of stable configurations as a function of configuration energy can be pictorially suggested as reported in Figure 6a. This distribution can be explored by the thermal equilibrium population change. In details, below 140°C no stable states can be populated, at about 250°C the population density decreases and between these temperatures a maximum density is found.

Figure 6. (a) Schematic distribution of stable doping states population as a function of energy. The temperatures reported in the scheme represent the onset and completion process temperature determined experimentally. (b) Schematic representation of the doping state energy as a function of a generalized configuration coordinate, a barrier of activation and a trap are theorized. The reported temperatures refer to the equivalent limit of activation energy and of de-trapping, the activation energy has been determined as explained in the text.

The temperature dependence of the process of doping reconstructed through the Raman measurements and reported in Figure 4 suggests that this is an energy activated process. By the analysis of hole concentration versus temperature derived from the data reported in Figure 4, and using the doping concentration estimation from Raman features suggested in literature30, an 15 ACS Paragon Plus Environment


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Arrhenius plot can be constructed. As reported in Figure 7, a quite good linear dependence of hole concentration on T-1 is found. From the best fit curve of these data an activation energy for the doping process Ea = (56 ± 8) meV is found. Basing on previous observation that oxygen is trapped at the interface between SiO2 and Gr,20 the observed process is then compatible with a surface adsorption effect at the interface between SiO2 and Gr, reaching a specific equilibrium value for each given temperature in the explored range15,35,36.

Figure 7. Arrhenius plot of the hole concentration estimated from the Raman features reported in Figure 4. The best fit line gives an activation energy for the doping process Ea = (56 ± 8) meV. Even if the details of the process could be quite complex, as suggested in literature15, the stability observed here by vacuum treatments up to 300°C enables to estimate an equivalent binding energy of molecular oxygen and, as a consequence, of the obtained doping, larger than about 49 meV by considering that the energy difference between the bottom of the valley in Figure 6b and the freed state should be equivalent to the Boltzmann kT energy relative to 573K. This value is compatible with those reported in literature for O2 bonding with pristine (not defective) graphene11,22,25. By contrast the instability of doping reported in literature,20 and observed also here in preliminary experiments (not reported) as a weak reduction of the thermally induced effects by prolonged exposure to atmospheric gas at room temperature on a timescale of few weeks, suggests that the doping obtained in static pressure is related to oxygen bond that could successively react with 16 ACS Paragon Plus Environment

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atmospheric gases and isconverted into other species. In fact, oxygen bond or trapped during doping is not released below 300°C in vacuum up to 15 h and as a consequence it is not expected to escape at room temperature in the week timescale. The instability should then be attributed to reaction of oxygen involving doped sites with other species reaching them. In preliminary tests N2 and CO2 gases were found to be ineffective in modifying the doping suggesting a major role of water molecules usually contained in normal ambient atmosphere together with the previous molecules. It is worth to notice that previously it has been suggested that, on the contrary, water favors the doping strengthening its effect,26 further work is therefore needed to try to clarify the role of ambient molecules. IV.

CONCLUSION

In this work we have shown the possibility to stably dope single layer low defect content graphene deposited on SiO2/Si, without destroying it, with thermal treatments in oxygen atmosphere at temperature lower than 300°C. A p-type doping of ~1013cm-2 has been attained. Furthermore we have shown that the effects of doping on graphene are stable even after thermal treatment in vacuum of several hours. By isothermal treatments, made at different temperatures, it is seen that the equilibrium configuration depends on the temperature indicating a distribution of doping states. Finally, an activation energy of 56 meV of the doping process has been estimated together with a stability or bonding energy larger than 49 meV. ACKNOWLEDGMENTS The authors would like to thank people of the LAMP group (http://www.unipa.it/lamp/) at the Department of Physics and Chemistry at University of Palermo, and P. Fiorenza and R. Lo Nigro from CNR-IMM, Catania for useful discussions and comments. G. Tricomi and G. Napoli from Department of Physics and Chemistry at University of Palermo are acknowledged for the assistance during the thermal treatments in controlled atmosphere. S. Di Franco from CNR-IMM is acknowledged for the skillful assistance with clean-room equipment and in the sample preparation. 17 ACS Paragon Plus Environment


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