Similarity in Band Gap Behavior of Modified Graphene with Different

Dec 26, 2013 - In this paper we summarize the band gap dependence for 2D graphene -based superlattices. We describe a mechanism of altering the electr...
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Similarity in Band Gap Behavior of Modified Graphene with Different Types of Functionalization Leonid A. Chernozatonskii, Dmitry G. Kvashnin, Olga P. Kvashnina, and Nelly A. Konstantinova J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 Dec 2013 Downloaded from http://pubs.acs.org on December 27, 2013

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Dependence of the band gap of partially hydrogenated graphene based on the distance between adsorbed areas. (a) The atomic structure of investigated structure and their classification; (b) dependence of the bang gap on the distance between adsorbed areas along the zigzag graphene direction (Y axis) for (8,m) structure; (c) dependences of the band gap on the distance between the adsorbed areas along armchair graphene direction (X axis) for (n,11) and (n,13) structures. 135x70mm (300 x 300 DPI)

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Dependence of the band gap on the distance between defective areas along graphene zigzag direction (Y axis) for different types of adsorbed and introduced atoms and for a graphene structure with holes. 118x92mm (300 x 300 DPI)

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Similarity in Band Gap Behavior of Modified Graphene with Different Types of Functionalization Leonid A. Chernozatonskii1, Dmitry G. Kvashnin1,2*, Olga P. Kvashnina3, Nelly A. Konstantinova3 1

2

Emanuel Institute of Biochemical Physics RAS, 119334, 4 Kosigin st., Moscow, Russia

National University of Science and Technology MISiS, 119049, 4 Leninskiy prospekt, Moscow, Russian Federation

3

Pirogov Russian National Research Medical University (RNRMU), 117997, 1 Ostrovitianov st., Moscow, Russia

ABSTRACT: In this paper we summarize the band gap dependence for 2D graphene based superlattices. We describe a mechanism of altering the electronic properties of graphene 2D superlattices formed by local hydrogenation, fluorination, substitution with boron nitride or periodically arranged holes. Varying the distance between these regions significantly changes the electronic properties in a manner similar to the band gap behavior of graphene nanoribbons. Characteristic dependences of the band gap on the shifts of the regions along the zigzag and armchair directions (semiconductor m=3p+2 rule) were obtained. The origin of this effect is discussed and explained from an atomistic point of view.

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Graphene is one of the most promising subjects in the field of nanotechnology. 1 Graphene is currently considered one of the main candidates for applications in semiconductor electronics. The main challenge lies in opening the band gap in graphene’s semimetallic electronic structure. Opening the band gap will allow it to be used in the semiconducting field for new lowdimensional nanoelectronic devices and as the basis for solar cells, 2,3 liquid crystal devices, 4 nano-sized transistor prototypes. 5,6 Many attempts have been made to obtain the band gap in graphene. 7-9 We considered opening the band gap by modifying the graphene surface through introducing vacancy defects10,11 ( which increased the band gap to 0.2 - 0.3 eV) and through the chemical adsorption of adatoms 12 (which increased the bandgap 1.25 eV through hydrogen adsorption). The most interesting case is the synthesis of hydrogenated and fluorinated graphene (graphane and fluorographene, respectively) which would lead to a change in the conductivity from a semimetal to an insulator. 13-18 Additionally, the electronic properties of graphene can be changed drastically even by partial hydrogenation. It has been found that the adsorption of a single hydrogen atom onto a graphene sheet about 2 nm2 could open the band gap to 1.25 eV. 12,19 Recently there have been a number of theoretical reports about partially hydrogenated graphene explaining the behavior of its electronic properties. Early it was found that as graphane grows on graphene by increasing the amount of adsorbed hydrogen atoms on the graphene surface, the band gap in the structure increases monotonically. 20-22 However, a series of papers suggests the band gap exhibits more complex behavior. The physical origin of this behavior is still not clear. 23-29

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Such semiconductor structures can be fabricated by fluorine passivation with the same geometric arrangement as hydrogenated graphene. 30 Recently a number of reports about such objects have been published. 23,30 In this work, we predict the drastic transformations in the electronic spectrum of graphene with the presence of periodically arranged small hydrogenated and fluorinated regions with varying distances between the regions. All calculations were made using density functional theory with general gradient approximation implemented in the SIESTA package 31 with periodic boundary conditions. The relaxation was carried out only at the Г-point due to the large unit cell (~ 20×30 Å2) of the structures. In the course of the atomic structure minimization, structural relaxation was carried out until the change in the total energy was less than 10-4 eV, or until the forces acting on each atom were less than 10-3 eV/Å. The adsorption of atoms on both sides of graphene surface locally breaks graphene’s π-system and increases the chemical activity of the neighboring carbon atoms, which will form the covalent bond with further adsorbing atoms. 32,33 Every adsorbed atom can equiprobably connect with carbon atoms belonging to sites of the A or B sublattice of the graphene lattice. In our simulation all adsorbed areas grew from the A graphene sublattice. The geometric scheme of these structures is shown in Figure 1 (a).

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Figure 1. Dependence of the band gap of partially hydrogenated graphene based on the distance between adsorbed areas. (a) The atomic structure of investigated structure and their classification; (b) dependence of the bang gap on the distance between adsorbed areas along the zigzag graphene direction (Y axis) for (8,m) structure; (c) dependences of the band gap on the distance between the adsorbed areas along armchair graphene direction (X axis) for (n,11) and (n,13) structures. At a low concentration, adsorbed atoms (indicated by red color) are covalently bonded to carbon atoms, forming islands on graphene surface 33. Such a structure can be considered as the intersection of zigzag (Y direction) and armchair (X direction) graphene nanoribbons (Figure 1 (a)). Therefore, the structure in Figure 1(a) can be represented by the (13, 12) intersection of graphene nanoribbons. The first index m corresponds to the width of the armchair nanoribbon (along the X direction), and the second index n corresponds to the width of the zigzag nanoribbon (along the Y direction). The classification of these nanoribbons corresponds to the classification proposed previously. 34

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First, let us consider structures with hydrogenated areas. The dependence of the band gap on the geometry arrangement of functionalized islands in both directions is demonstrated. Figure 1(b) shows the dependence of band gap on the distance between the hydrogenated graphene areas along X direction for the system (8, m). This dependence has an unusual behavior and obeys the m=3p+2 rule. If index m of the armchair nanoribbon satisfies m = 3p+2 then the structure displays semiconductor nonzero band gap. Otherwise it demonstrates zero band gap. In the range of m= 9 to 19, two characteristic peaks are seen at m = 11 and 17 that correspond to structures where the armchair nanoribbon has semiconductor properties. This dependence is independent of the distance between the functionalized areas in the perpendicular direction. Figure 1(c) presents the dependence of the band gap on the index n of zigzag nanoribbon along the Y direction for m= 11 and 13. The dependence for system (n, 11) in Figure 1(c) has a monotonous behavior. This is because zigzag graphene nanoribbons demonstrate only metallic behavior of electronic properties, and therefore their contribution to the conductivity of the whole system can’t drastically change the band gap. Figure 1(c) shows the dependence of the band gap on the index n for the system including the armchair graphene nanoribbon with metallic properties (n, 13). We see that for all values of index n the band gap remains zero. For the index m obeying the m=3p+2 rule, the dependence of the band gap on the n shows a decreasing behavior. Otherwise, the band gap is zero. Similar results were obtained in the case of fluorine passivation of graphene surface with the same arrangement of fluorine atoms. The results of changing the electronic properties of graphene superlattices through periodically arranged holes (also called “antidots” or graphene nanomesh (GNM)) were found. The electronic properties of both also obey the m=3p+2 rule (Figure 2).

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It is well known that there is two-dimensional analogic to graphene Boron Nitride with honeycomb structures which has already been successfully synthesized. 35 Boron Nitride in the honeycomb structure is a semiconductor and its ribbons are also semiconductors with large band gaps. 36 The band gap of boron nitride nanoribbons do not change significantly with the width of the nanoribbon. 37 Moreover, graphene and BN both have commensurate lattices. Hence, these two objects are perfect for creating mixed structures. As such, this study also considered the band gap of structures similar to those in Figure 1, but with boron nitride introduced instead of adsorbed areas. Figure 2 summarizes the values of the band gap for the 2D graphene superlattices formed by different functionalization ways: by hydrogen or fluorine adsorption on graphene surface, by introducing the h-BN areas and by making holes in graphene lattice.

Figure 2. Dependence of the band gap on the distance between defective areas along graphene zigzag direction (Y axis) for different types of adsorbed and introduced atoms and for a graphene structure with holes.

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The results agree with the previous work. 38 Producing holes in graphene lattices have great prospects for changing the conductivity properties of recent structures. In Ref. 38 it was shown that the variation of the band gap in depends on the distance between the holes and obeys the Q=3p rule where Q is the number of repeating conventional cells for the perfect graphene along zigzag direction. Structures satisfying this rule exhibit semiconducting properties, and other structures demonstrate semimetallic properties. If we translate these indices following the classification given in our paper, we can conclude that the band gaps of such structures also obey the m=3p+2 rule, where m is the number of atoms along zigzag direction. In summary we can conclude about the similarity in the behavior of the band gap. The band gap only depends on the distance along the zigzag graphene direction (Y axis) between the modified regions and obey the m=3p+2 rules (where m is the distance between the defective areas along the zigzag direction) without any dependence on the types of defective areas. Due to the fact that the boron nitride areas introduce structural tension, we can see strain-induced changes in the values of the band gap of recent structures.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

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This work was supported by the Russian Foundation for Basic Research (projects no. 11-0201453/12, 12-02-31261) and International Research Staff Exchange Scheme “FAEMCAR” (FP7-PEOPLE-2012-IRSES). The computations were performed at the Joint Supercomputer Center of the Russian Academy of Sciences and the Moscow State University “Lomonosov” supercomputer. D.G.K. acknowledges the support from the Russian Ministry of Education and Science (No. 948 from 21 of November 2012). REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang D; Zhang Y; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. (2) Miao, X.; Tongay, S.; Petterson, M. K.; Berke, K.; Rinzler, A. G.; Appleton, B. R.; Hebard, A. F. High Efficiency Graphene Solar Cells by Chemical Doping. Nano Lett. 2012, 12, 27452750. (3) Wang, X.; Zhi, L.; Mllen, K. Transparent, Conductive Graphene Electrodes for Dyesensitized Solar Cells. Nano Lett. 2007, 8, 323-327. (4) Blake, P.; Brimicombe, P. D.; Nair, R. R.; Booth, T. J.; Jiang, D.; Schedin, F.; Ponomarenko, L. A.; Morozov, S. V.; Gleeson, H. F.; Hill, E. W. et al. Graphene-based Liquid Crystal Device. Nano Lett. 2008, 8, 1704-1708. (5) Castro Neto, A. H. The Carbon New Age - Review Article - Materials Today. Materials Today 2010, 13, 12-17.

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