Electronic state calculation for hydrogenated graphene with atomic vacancy

1. Electronic state calculation for hydrogenated graphene with atomic vacancy Electronic state calculation of hydrogenated graphene and hydrogenated graphene vacancy Kusakabe Lab. M1 Gagus Ketut Sunnardianto

2. Contents 1. Introduction • - What is graphene? • - Unique Properties of graphene • - How to get graphene and graphene vacancy? • Motivation • Research scopes • Research objectives 2. Results and Discussion - Calculation (DFT+Löwdin) - Simulation condition - Charge transfer value - DOS (Density of states) 3. Summary

3. Graphene Atomic nature Crystal nature Spectrum of carbon atom Bonding & hybridized energy bands of graphene http://invsee.asu.edu/nmodules/carbonmod/bonding.html

4. Unique properties of graphene • High electron mobility (electronic properties) • 2. Robust but also very stretchable (mechanical properties) • 3. Can adsorb and desorb various atoms and molecules • (chemical properties) • 4. The thinnest material (one atom thick -> nearly transparent) C. Lee, X. Wei, J. W. Kysar, & J. Hone, Science 321, 385 (2008) R. R. Nair, P. Blake, A. N. Grigorenko, K. S. Novoselov, T. J. Booth, T. Stauber, N. M. R. Peres, & A. K. Geim , Science 320, 1308 (2008).

5. How to get Graphene….? Monolayer graphene produced by Mechanical exfoliation. Large sample With length of 1mm on Si/SiO2 http://nobelprize.org/nobel_prizes/physics/laureates/2010/press.html Daniel R.Cooper et al, ISRN Condensed matter physics, 2012

6. Graphene vacancy Hydrogenated graphene vacancy http://newsdesk.umd.edu/uniini/release.cfm?ArticleID=2390 Prof. Fuhrer(University of Maryland): Graphene vacancy acts as tiny magnets, open the possibility of “Defect engineering” for spintronic application

7. DOS of Pure graphene and graphene vacancy DOS of pure graphene DOS of graphene vacancy

8. Motivation “It is not possible to determine the charge transfer value per hydrogen adsorption directly from our experiment because the sticking coefficient on graphene is unknown.”

9. Motivation 1. Pristine 2. Hydrogenation (cleaning) 3. Annealing 4. Ar Sputtering 5. Hydrogenation 6. Annealing 7. 2nd Ar Sputtering 8. 2ndHydrogenation

10. Motivation • Experimentally, Capaz et.al1 observed the charge transfer from hydrogen to graphene around 0.161. In a recent experiment by Kudo et al2 @TITECH, they found a value around 0.6 per vacancy. • The most promising materials suggested as a potential hydrogen storage media is carbon based materials such as graphene (Durgun et al, Zhao et al) • Graphene is a revolution material for hydrogen storage, Keyvan3. [1]. APCTP-POSTECH-AMS WORKSHOP, Pohang, September 3, 2010 [2] Kudo, et al. 27aXJ-3, Spring Meeting of JPS (2013). [3] Inside Rensselaer Volume 4, Number 3, February 19, 2010

11. Research scopes This study carried out calculation for hydrogenated graphene sheet consisting of 24 carbon atoms and hydrogenated graphene vacancy consisting of 63 carbon atoms within the framework of DFT The present study just focuses on charge transfer and the evolution of the density of states to understand the change in the character of hydrogenated graphene and hydrogenated graphene vacancy Research objectives The objectives of this research are to calculate the atomic charge in hydrogenated graphene by Löwdin charge analysis to know the charge transfer and to understand the evolution of the electronic structure through density of states upon hydrogenation

12. Method Calculation • Based on Density Functional Theory (DFT) • Generalized Gradient Approximation (GGA) • VASP code (https://www.vasp.at) • Quantum espresso code (Löwdin charge analysis) Simulation condition • Force convergen criterion : F ≤ 1.0 x 10-5 [Ry/a.u] • PAW potentials to describe ionic potentials • the energy cut off of 36.75 Ry for the plane wave expansion • K-points mesh 16X16X1 for scf calculation • Charge transfer calculated using Löwdin analysis

13. Hydrogenated graphene RESULT Initial structure Initial structure Initial structure Optimized structure Optimized structure Optimized structure

14. Löwdin charge analysis Graphene+H v A Graphene+2H A B Graphene+3H A B C

15. Density of States (DOS) Pristine Fermi level Graphene+H Dirac point

16. Graphene+H Fermi level 22 19 16 17 13 10 7

17. Hydrogenated graphene vacancy Initial structure Initial structure Initial structure Optimized structure Optimized structure Optimized structure

18. Löwdin charge analysis v Graphene_Vacancy+H A Graphene_Vacancy+2H B A Graphene_Vacancy+3H C B A

19. Density of States (DOS) Graphene_Vacancy Fermi level Graphene_Vacancy+H

20. 36 29 18 21 20 10 Fermi level 9 13 Graphene_Vacancy+H

21. Summary • Our simulation show the value of charge transfer calculated by lowdin analysis was around 0.2e per hydrogen adsorbed and 0.5e per vacancy which was approximately comparable with experimental result by Kudo et al. • As for the DOS of hydrogenated graphene the Fermi level is shifted upward because of electrons doped from hydrogen to graphene structure, the sharp peak close to Fermi level is arise from pz orbital • As for the DOS of hydrogenated graphene vacancy, after monomer hydrogenation the value DOS at the Fermi level come from localized states of dangling bond is  decrease.