Ab Initio Study of Aluminum-Phosphorus Co-doped Graphene

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1 Journal of Physical Science and Application 7 () (07) -7 doi: 0.765/ / D DAVID PUBLISHING Ab Initio Study of Aluminum-Phosphorus Co-doped Graphene Víctor Eduardo Comparán Padilla, María Teresa Romero de la Cruz, Carlos Eduardo Rodríguez García, Reyes García Díaz and Gregorio Hernández Cocoletzi 3. Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila, Blvd. Venustiano Carranza, C.P. 580, Saltillo, Coahuila, México. Facultada de Ciencias Físico Matemática, Universidad Autónoma de Coahuila, Blvd. Venustiano Carranza, C.P. 580, Saltillo, Coahuila, México 3. Instituto de Física Luis Rivera Terrazas, Benemérita Universidad Autónoma de Puebla, Av. San Claudio y Blvd. 8 Sur, C.P. 7570, Puebla, Puebla, México Abstract: In the present work it is studied the phosphorus-aluminum co-doping effect on the electronic and structural graphene properties using ab initio calculations in the framework of DFT (density functional theory). The doping of graphene with substituent heteroatoms can modify the band structures as well as the electron transfer, improving the electronic performance that could enhance the sensing ability in gas sensor devices. The incorporation of heteroatoms in the graphene monolayer alters the unit cell. The alteration degree depends on the dopant concentration. Furthermore, the electronic properties were modified by opening the gap up to 0.6 ev produced by the combination of phosphorus and aluminum as dopants. The dopant concentration can be controlled, which causes different degrees of semiconductor behavior on the co-doped graphene. Key words: Density functional theory, graphene, co-doping, band gap, aluminum, phosphorus.. Introduction Graphene, a single-atom-thick layer of sp bonded carbon (C) atoms tightly packed into a D honeycomb lattice [-3], has attracted the scientific and technological attention due to the low production cost and physical/chemical properties such as high surface area, excellent conductivity (thermal and electrical) and mechanical strength. In addition, graphene has been proposed for potential applications in many fields such as electronics, energy, and biotechnology [4-6]. The doping increases the reactivity of carbon nanostructures and provides a mechanism for anchoring molecules and chemical groups to the surface of graphene. It has been shown that the Corresponding author: Víctor Comparán, M.Sc., researcher, research fields: computational chemistry, nanotechnology, materials. chemical activity, electric transport properties, and optical characteristics of graphene can be tailored by adding a suitable dopant [7-9]. The doping of graphene causes an enhancement in the electronic behavior. Theoretical studies have demonstrated that doping graphene could transform the band structure as well as the electron transfer, so the applications of graphene could be significantly improved [0-]. The doping of graphene is in function of the applications. It has been demonstrated that doping graphene with substituent heteroatoms could effectively modulate the electronic characteristics, surface and local chemical features, which is essential for novel device applications [3-5]. Generally, pristine graphene is chemically inactive and hard to react with molecules. However, the sp hybridized structure of graphene would be disturbed

2 Ab Initio Study of Aluminum-Phosphorus Co-doped Graphene by the introduction of heteroatoms, and this may introduce more defect sites into the graphene basal plane for interactions with foreign molecules. For example, phosphorus (P) doped graphene nanosheets show excellent ammonia (NH 3 ) sensing ability at room temperature, since the P atom acts as an active site for NH 3 adsorption [6, 7]. Besides, also it has been studied in other applications such as energy storage, electrocatalysts and fuel cell [8, 9]. On the other hand, it has been reported theoretically the potential application of graphene doping with aluminum (Al) as a gas sensor for formaldehyde and some other highly toxic molecules [0, ]. The co-doping of multiple species of foreign atoms may generate new properties or create synergistic effects on graphene [, ]. For example, boron (B) and nitrogen (N) atoms are similar in size and it has been reported that they produce opposite doping effects on graphene. While being simultaneously doped into graphene, boron nitride (BN) domains tend to form due to the phase separation between carbon and BN. The electronic properties tuning, by varying the concentration of B and N, will provide a tremendous boost for obtaining materials with properties relevant to applications in solid state electronics [3-5]. The P atom has valence electrons located in the third shell and smaller ionization energy compared with the N atom, which could enhance doping capability by increasing the fraction of delocalized electrons per atom that allows for a strong n-doping effect compared to N doping [6, 7]. On the other hand when graphene is doped with Al the electron density decreases near the doping site as induced by the charge transfer from Al to the surrounding carbon atmos. This charge deficiency makes the aluminum site an active for the adsorption of gas molecules [, 4, 8]. In this study, we have performed first-principles DFT calculations to investigate the effect of co-doping of graphene layers with aluminum and phosphorus.. Method The computational approach was based on an ab initio pseudopotential method in the framework of periodic DFT (density functional theory). DFT calculations were carried out using the PWscf (Plane Wave self-consistent field) code of the Quantum ESPRESSO package [9]. In these calculations, we use a plane wave basis set and Vanderbilt pseudopotentials [30] with no-lineal core correction to represent the interaction between ionic cores and valence electrons. Exchange-correlation energies are treated within the generalized gradient approximation (GGA) with the PBE (Perdew-Burke-Ernzerhof) parametrization [3]. We use an energy cutoff of 30 Ry for the plane wave basis used for the wave functions, 40 Ry for that used to represent charge density and 0.04 Ry for value of the gaussian spreading and we consider Van der Waals forces in our calculations. The D layer of graphene was simulated using a supercell geometry, we used a supercell size of 5 5 unit cells, containing 50 carbon atoms (Fig. ). To analyze the electronic properties we have calculated the band structure and (density of states). 3. Results and Discussion Ab initio calculations have been conducted to study pristine graphene, doped graphene (P and Al) and co-doped graphene (P-Al) with the purpose to explore the effect of doping on the graphene structural and electronic properties. Table shows bond lengths and angles for the systems under study. The length (C-C) and angle bond (C-C-C3) obtained for pristine graphene system (G) are.4 Å and 0, respectively. It is consistent with the results reported by other groups using similar calculation methods [3, 33]. Fig. shows the positions of impurity atoms after structural relaxation by total energy minimization for (a) pristine graphene, (b) doped graphene (P or Al), and (c) doped

3 Ab Initio Study of Aluminum-Phosphorus Co-doped Graphene 3 a) b) 3 c) d) Carbon Phosphorous or aluminum Aluminum Phosphorous Fig. Supercell of 5 5 unit cells and doped positions. Table The length and angle bond of systems. System Bond Bond length (Å) Bond angle (Degree) G C -C G-P P -C.6 0. G-P P -C G-P P 3 -C G-Al Al -C G-Al Al -C.69.3 G-Al Al 3 -C G-PAl P -C G-PAl Al 3 -C G-PAl P -C G-PAl P 5 -C G-PAl Al 3 -C G-PAl Al 7 -C and co-doped graphene with two heteroatoms (P and Al), and (d) co-doped graphene (P and Al) with four heteroatoms. For comparison, in the case of pristine graphene and doped graphene with heteroatoms, the impurity atoms have the same position. For the case of graphene doped with an atom of P (system G-P, Table ), the bond length increases 0.9 Å, this effect may be explained by the larger atomic radius of P in comparison with that of C, moreover, the bond angle obtained was similar to C. The system with two P atoms (G-P, Table ) yielded similar structural properties. The graphene doped with an Al atom (G-Al, Table) has larger bond length than that system doped with P, in fact it has the largest bond length, reported in Table, due to its larger atomic radius compared with P and C. The bond angle obtained in G-Al system was related to C and P. Meanwhile, the graphene sheet doped with two Al atoms shows a decrease in the bond lengths and increase in the bond angle, which may be due to relaxation of the unit cell. The co-doped graphene with P and Al (G-PAl, Table ) shows bond lengths similar in comparison with their single doping of graphene, the bond angles are similar to each other but it increases a little in comparison with other systems. This could be due to

4 4 Ab Initio Study of Aluminum-Phosphorus Co-doped Graphene the supercell relaxation by the presence of both atoms. When the co-doping increases to four heteroatoms, two of each one (G-PAl, Table ), the bond lengths and angles decrease, with the exception of the bond angle of Al7-C8-C9 that increase, intensifying the effect of deformation on the graphene to reach a stable structure. Fig. and Fig. 3 show the electronic band structure of pristine and doped graphene, respectively. The pristine system (Fig. ) shows that the Dirac point is consistent with the Fermi level indicating the zero-gap semiconducting behavior; similar results have been obtained by other groups using analogous methods of calculation [33]. On the other hand, the P doped Fig. 3 graphene (Fig. 3-G-P) shows electronic states that cross the Fermi level indicating metallic behavior that is induced by P doping. This behavior is more evident Fig. Electronic band structure of pristine graphene. Electronic band structure of doped and co-doped graphene. Cero energy was set to the Fermi energy.

5 Ab Initio Study of Aluminum-Phosphorus Co-doped Graphene 5 G G C (p) P G P G P P G P P (p) C (p) P G P P (p) P (p) C (p) C (p) G Al G Al P G Al Al (p) P Al (p) G Al Al (p) C (p) C (p) G PAl G PAl P P (p) G PAl Al (p) C (p) C (p) P G PAl P (p) Al (p) C P (p) C Al (p) Fig E EF (ev) Graph of total density state () of pristine, doped and co-doped graphene. The dot red line is the Fermi energy. when the graphene is doped with two P heteroatoms (Fig. 3-G-P). Similar effect is observed in the Al doped graphene (Fig. 3 G-Al and 3 G-Al). However, when the graphene is co-doped with P-Al (Fig. 3 G-PAl) electronic states are generated above and below the Fermi level opening a gap of 0.48 ev, indicating a semiconductor behavior. This effect increases when the co-doping is with four heteroatoms, two P and two Al, (Fig. 3 G-PAl) opening a gap of 0.6 ev. The enhancement of the band gap could be attributed to the synergistic effect of P-Al co-doping and probably it could be tuned changing the concentration of dopant in the graphene. Additionally, we have also studied the total electronic and partial of the systems (Fig. 4). The of the pristine graphene (Fig. 4 G) exhibits electronic states on the conduction and valence bands that do not cross the Fermi level revealing which is related to a zero-gap on the Dirac point, this effect is characteristic for pristine graphene [34, 35]. The of P doped graphene (Fig. 4 G-P) indicates a conductance change near the Femi level, their P shows that this effect is caused by p orbitals of P and C bonded to P, such behavior is more evident with the

6 6 Ab Initio Study of Aluminum-Phosphorus Co-doped Graphene increment of dopant number (Fig. 4 G-P). Similarly, Al doped graphene (Fig. 4 G-Al) shows changes in valence states near the Fermi level produced by p orbitals of Al and C bonded to Al. Similar to P doped graphene when the Al dopant increases the change is more notorious. On the other hand, on the co-doped graphene with Al and P (Fig. 4 G-PAl) the opening of a band gap takes place. This is caused by the interaction of p orbitals of P, Al and the C atoms bonded to them. This effect is more evident when increases the co-doping concentration (Fig. 4 G-PAl) opening a widest gap. 4. Conclusions DFT calculations have been performed in order to investigate the effect of doping and co-doping of graphene with phosphorus and aluminum. Results have shown that the incorporation of heteroatoms in graphene distorts the unit cell changing angles and lengths of bonds, owing to larger atomic radius of the dopants. The distortion is more evident when the dopant concentration is large. The electronic properties of graphene were modified with the co-doping phosphorus-aluminum that forms a synergistic effect, opening a gap (0.6 ev) to generate a semiconductor behavior that could be tuned by the concentration of the dopant. The energy gap was opened to separate conduction and valence bands near the Fermi level caused by p-orbitals of phosphorus and aluminum, respectively. Acknowledgments The authors would like to thank the program of doctorate in materials of the Universidad Autónoma de Coahuila and Reyes García Díaz wants to acknowledge CONACYT postdoc scholarship. References [] Wang, X., Sun, G., Routh, P., Kim, D. H., Huang, W., and Chen, P. 04. Heteroatom-doped Graphene Materials: Syntheses, Properties and Applications. Chem, Soc. Rev. 43: [] Whitener, K. E., and Sheehan, P. E. 04. Graphene Synthesis. Diam. Rel. Mat. 46: [3] Geim, A. K., and Novoselov, K. S The Rise of Graphene. Nat. Mater. 6: [4] Shao, Y., Zhang, S., Engelhard, M., Li, G., Shao, G., Wang, Y., Liu, J., Aksa, I. A., and Lin, Y. 00. Nitrogen-doped Graphene and Its Electrochemical Applications. J. Mater. Chem. 0: [5] Zhu, Y., Murali, S., Cai, W., Li, X., Suk, J. W., Potts, J. R., and Ruoff, R. S. 00. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. : [6] Jo, G., Choe, M., Lee, S., Park, W., Kahng, Y. H., and Lee, T. 0. The Application of Graphene as Electrodes in Electrical and Optical Devices. Nanotechnology 3: -0. [7] Bandosz, T. J. 00. On the Adsorption/Oxidation of Hydrogen Sulfide on Activated Carbons at Ambient Temperatures. Colloid Interface Sci. 46: 00. [8] Liu, H., Liu, Y., and Zhu, D. 0. Chemical Doping of Graphene. J. Mater. Chem. : [9] Wang, Y., Shao, Y., Matson, D. W., Li, J., and Lin, Y. 00. Nitrogen-doped Graphene and Its Application in Electrochemical Biosensing. ACS Nano 4: [0] Shokuhi, R. A. 06. Al-doped Graphene as a New Nanostructure Adsorbent for Some Halomethane Compounds: DFT Calculations. Surf. Sci. 645: 6-. [] Chang, C. K., Kataria, S., Kuo, C. C., Ganguly, A., Wang, B. Y., Hwang, J. Y., Huang, K. J., Yang, W. H., Wang, S. B., Chuang, C. H., Chen, M., Huang, C. I., Pong, K. J, Song, K. J., Chang, S. J., Guo, J. H., Tsujimoto, M., Isoda, S., Chen, C. W., Chen, L. C., and Chen, K. H. 03. Band Gap Engineering of Chemical Vapor Deposited Graphene by in situ BN Doping. ACS Nano 7: [] Denis, P. 00. Band Gap Opening of Monolayer and Bilayer Graphene Doped with Aluminium, Silicon, Phosphorus, and Sulfur. Chem. Phys. Lett. 49: 5-7. [3] Zhao, Y., Hu, C., Hu, Y., Cheng, H., Shi, G., and Qu, L. 0. A Versatile, Ultralight, Nitrogen-doped Graphene Framework. Angew. Chem. Int. 5: [4] Ling, C., and Mizuno, F. 04. Boron-doped Graphene as a Promising Anode for Na-ion Batteries. Phys. Chem. Chem. Phys. 6: [5] Chen, D., Zhang, H., Liu, Y., and Li, J. 03. Graphene and Its Derivatives for the Development of Solar Cells, Photoelectrochemical, and Photocatalytic Applications. Energy Environ. Sci. 6: [6] Niu, F., Tao, L., Deng, Y., Wang, Q., and Song, W. 04. Phosphorus Doped Graphene Nanosheets for Room Temperature NH3 Sensing. New J. Chem. 38: [7] Varghese, S., Lonkar, S., Singh, K. K., Swaminathan, S., and Abdala, A. 05. Recent Advances in Graphene

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