Tunable magnetic states in h-bn sheets

Transcription

1 Tunable magnetic states in h-bn sheets Eduardo Machado-Charry Nanosciences Foundation & Laboratoire de simulation atomistique (L Sim), SP2M, UMR-E CEA-Grenoble E-MRS 2012 FALL MEETING, September Warsaw University of Technology

2 h-bn sheets : Interesting properties Analogous to graphene Wide band gap due to partially ionic bonding More resistant to oxidation More suitable for applications at high temperatures Induced magnetism

3 Introduction Formation Energies Imposing the magnetic moment Imposing the reconstruction Conclusions Our approach Spin-polarised DFT BigDFT * wavelet code Spin-constrained calculations No periodicity in perpendicular direction AC 2 ( A ) 448 atoms Only Γ point for BZ N B PBE XC * J. Chem. Phys. 129, (2008) E. Machado-Charry (L Sim CEA-Grenoble) ZZ Free BC Tunable magnetic states in h-bn sheet Free BC

4 Defects in h-bn sheets : Electro beam irradiation + HRTEM * Triangular multivacancy structures Discrete sizes A unique orientation ZZ AC EW reconstruction ZZ AC N-terminated zigzag edge B is easier knock out than N * Jin, Lin, Suenaga and Iijima. Phys. Rev. Lett. 102, (2009)

5 Exciting things (1) Magnetism in multivacancies ( JACS 131, (2009) ) (2) Why N-terminated edges? ( PRL 102, (2009) )

6 Exciting things (1) Magnetism in multivacancies ( JACS 131, (2009) ) for S > 1: M GS = 3 S 6 M GS size (S) removed 0 B 3 B 4 9 N+3B 3N+6B N B (2) Why N-terminated edges? ( PRL 102, (2009) )

7 Exciting things (1) Magnetism in multivacancies ( JACS 131, (2009) ) (2) Why N-terminated edges? ( PRL 102, (2009) ) Threshold beam energy : B (74 KeV) N (84 KeV) * Electron beam with energy of 120 KeV Direct formation energy of mono and multivacancies Reference system N 2 : E(V B ) > E(V N ) * Maximum energy transfer B (15 ev) N (14 ev)

8 Formation Energies Size V B edge (ev) V N edge (ev) 1 N 8.17 B 6.71 N B V 1 B V 1 N

9 Formation Energies Size V B edge (ev) V N edge (ev) 1 N 8.17 B N+B N+3B N + 3B N + 6B N B V 1 B V 1 N

10 Formation Energies Size V B edge (ev) V N edge (ev) 1 N 8.17 B N+B N+3B N + 3B N + 6B N B V 1 B N V ~4 ev 2 V 1 N

11 Formation Energies Size V B edge (ev) V N edge (ev) 1 N 8.17 B N+B N+3B N + 3B N + 6B N B V 1 B N V ~4 ev 2 V 1 N B V ~6.5eV 2

12 Formation Energies Size V B edge (ev) V N edge (ev) 1 N 8.17 B N+B N+3B N + 3B N + 6B 19.93

13 A h-bn sheet with one V N+3B multivacancy N B Strong corner reconstruction Formation of a N-N bond in each corner Strong electron localisation in the corners Total magnetic moment M = 0 µ B AC ZZ

14 A h-bn sheet with one V N+3B multivacancy N B Wannier centres Strong corner reconstruction Formation of a N-N bond in each corner Strong electron localisation in the corners Total magnetic moment M = 0 µ B AC ZZ

15 Imposing the magnetic moment in V N+3B M = 0 µ B : The ground state Total reconstruction Total energy (ev) Distance N N (Å) at corners : Total magnetic moment 1.4

16 Imposing the magnetic moment in V N+3B M = 2 µ B : Partial reconstruction Spin localisation Total energy (ev) Distance N N (Å) at corners : Total magnetic moment 1.4

17 Imposing the magnetic moment in V N+3B M = 2 µ B : Partial reconstruction Spin localisation AC ZZ

18 Imposing the magnetic moment in V N+3B M = 4 µ B : 2/3 open corners Spin localisation Total energy (ev) Distance N N (Å) at corners : Total magnetic moment 1.4

19 Imposing the magnetic moment in V N+3B M = 6 µ B : No reconstruction Spin localisation Total energy (ev) Distance N N (Å) at corners : Total magnetic moment 1.4

20 Imposing the magnetic moment in V N+3B The magnetic moment dangling bonds Magnetism Reconstruction Total energy (ev) Distance N N (Å) at corners : Total magnetic moment 1.4

21 Hypothesis Reconstruction Magnetism AC tensile strain will break lower bonds M = 4 µ B ZZ tensile strain will break upper bond M = 2 µ B AC ZZ

22 Effect of tensile strain in V N+3B Uniaxial tensile strain in the armchair direction 10 Total energy (ev) M = 0 µ B M = 4 µ B E (M 4 µb - M 0 µb ) 2 AC Uniaxial deformation in the armchair direction [( c/c 0 ) 100] Evolution of total energy vs. deformation ZZ Optimized geometry and the Wannier states with M = 4 µb at 2.4 %.. Machado-Charry (L Sim CEA-Grenoble) Tunable magnetic states in h-bn sheet

23 Larger holes M GS = 3 S 6 for S > 1 Possible transformations : M = M GS + 2 (+4,+6) It is impossible to modify M in the larger holes Deformations along the edge that do not allow the creation of N-dangling bonds in the corners

24 Conclusions Magnetism Reconstruction The magnetic moment at ground state is held by dangling bonds Strong interplay between the N-N distance in the edge and the magnetic moments of the defects By stress-induced geometry modifications, we change the ground state magnetic moment of the defects Balance of the local elastic deformations and the repulsive energy between two spins on nearby atoms Tunable magnetic states in hexagonal boron nitride sheets. In press APL

25 People involved Pascal Luigi Paul Normand Pochet Genovese Boulanger Mousseau Support Nanosciences Fondation: Muscade project France: Canada: ANR-NanoSimGraphene NSERC-CRSNG, FRNT Québec, CRC Computer time: GENCI (Grant 6323), Tera-100 from CEA-DAM, and Calcul Québec

26 People involved Pascal Luigi Paul Normand Pochet Genovese Boulanger Mousseau Thanks for your attention!! See submitted version in arxiv: v1