Download presentation

Presentation is loading. Please wait.

Published byMary Chard Modified over 2 years ago

1
ARMENIA2010 Ab-initio calculations of electronic and optical properties of graphane and related 2-D systems Olivia Pulci European Theoretical Spectroscopy Facilty (ETSF), and CNR-INFM, Dipartimento di Fisica Università di Roma Tor Vergatahttp://www.fisica.uniroma2.it/~cmtheo-grouphttp://www.etsf.eu olivia.pulci@roma2.infn.it

2
Everything started with graphene 3D: stacked in graphite 2D: graphene 1D: rolled in nanotubes 0D: wrapped in fullerens Unique physical properties: H igh carrier mobility Ambipolar field effect RT quantum Hall Single molecule detection Special mechanical properties ………………… Novoselov et al. Science 2004 For a review see for example: Castro et al. Rev. Mod. Phys. 81, 109 (2009) Allen et al. Chem. Rev. 110, 132 (2010)

3
Semi-metal E(eV) Functionalizing graphene Graphene+H->Graphane

4
OUTLINE Ab-initio: Theoretical Approaches Functionalizing Graphene with H: graphane Other exotic 2D systems (Si, Ge, SiC) conclusions

5
OUTLINE Ab-initio: Theoretical Approaches Functionalizing Graphene with H: graphane Other exotic 2D systems (Si, Ge, SiC) conclusions

6
AB-INITIO methods TDDFT vv DFTGW BSE c c h c h W EXC ground state Band structure, I, A Optical properties MBPT v cv

7
AB-INITIO methods TDDFT v v DFTGW BSE c c h c h W EXC 1) 2)3) MBPT v cv

8
G: single particle Green’s function W: screened Coulomb interaction (Step 2) Lars Hedin 1965

9
For optical properties we need to go beyond: Bethe Salpeter Equation TDDFT v v DFTGW BSE c c h c h W EXC 1) 2)3) MBPT v cv

10
Step 3: calculation of optical spectra within the Bethe Salpeter Equation Absorption spectra A photon excites an electron from an occupied state to a conduction state e h Bethe Salpeter Equation (BSE) GW BSE Kernel: e-h exchange bound excitons c v h

11
0-D 1-D 2-D 3-D Nanoclusters bulks Biological systems Generality, transferability 0D-3D Detailed physical informations Predictivity Complex theory+large comp.cost Ab-initio applicable to: Ab-initio applicable to: Nanowires Surfaces

12
functionalizing graphene: Top view Side view Top view + atomic H graphene graphane Elias et al. Science 2009 Ryu et al. Nanolett. 2008 reversible! 1.42 A-> 1.52 A (like C bulk) Theoretically predicted in 2007 (Sofo et al PRB2007), synthesized in 2008

13
Electron affinity A=electron affinity A=E (vacuum) -E (CBM) E(vacuum) A E (CBM) Especially interesting when A<0 Technological applications (cold cathod emitters,…..) I I= E (vacuum) -E (TVB) I=Ionization potential

14
C(111):H NEA (1x1) bulk-like No states into the gap A=E (vacuum) -E (CBM) =-1.4 eV (GW) (-0.6 eV in DFT) Exp:-1.27 eV (J.B. Cui et al PRL1998) E(vacuum) A E (CBM)

15
Electronegativity plays a role!

16
graphane A(DFT)=1.27 eV; A(GW)=0.4 eV >0!! Egap DFT: 3.5eV GW: 6.1 eV!! graphene A(DFT)=4.21 eV metallic metal---> insulator transition

17
WHY?? Side view d up d down compensating dipoles + _ _ +

18
Graphane HomoLumo+1 NFES Lumo Nearly free electron states

19
Graphane: optical properties DFT-RPA with H without H Dramatic changes in the optical absorption spectrum!

20
Graphane optical properties: excitonic effects From Cudazzo et al. PRL 104 226804 (2010)

21
Other exotic 2-d materials? Graphene graphane Silicene(*) (?) polysilane Germene (?) germane (?) polygermyne ……..? (*) Ag(110):Si Guy Le Lay and coworkers : P. De Padova APL 2010 B. Aufray APL 2010 H H H 22 toys models in Sahin et al. PRB2009

22
Silicon-based 2-D +H Silicene Top view Silicene Side viewPolysilane Side view Polysilane top view Not planar!!! Si larger atomic radii =0.44 Angstrom =0.70 A

23
Si-based 2-D Metallic!Wide gap semiconductor quasi-direct gap DFT gap: 2.36 eV GW gap: 4.6 eV Massless Dirac fermions at K

24
Ge-based 2-D Germane Side view Germane Top view Germene Top view Germene Side view +H Not planar!!! = 0.63 = 0.73 Å Å

25
Ge-sheets Gap at DFT gap: 1.34 eV GW gap: 3.55 eV Metallic! semiconductor Massless Dirac fermions at K

26
NFES

27
What can we learn? graphene Graphane (H) silicene Polysilane (H) germene Germane (H) gapno yes DFT:3.5 eV GW: 6.1 eV no yes M DFT:2.36 eV GW:4.6 eV no yes DFT:1.34 eV GW:3.5 eV Buckl (Å) No (0) sp2 yes (0.46) sp3 yes (0.44) sp3 yes (0.70) sp3 yes (0.63) sp3 yes (0.73) sp3 d (Å)1.421.542.282.392.352.39 NFESyes Affinity>>0~0.4 eV >>0

28
Beyond single particle approach: EXCITONIC EFFECTS c v h OPTICAL PROPERTIES

29
Excitonic effects Large Exciton binding energies!!! 2-D confinement + expected trend

30
Further possible (?) 2D materials Side view Topview SILICONGRAPHaNE SiC:H SILICONGRAPHeNE SiC Si+C!!!!

31
SiC based 2-D On one side the affinity is smaller!!! With H GAP EXISTS!

32
SiC:H Top and bottom semi-spaces have different ionization potential h h e-e- e-e- 2 eV

33
Conclusions H on graphene (graphane): metal->insulator transition; electron affinity decreases by factor 10 2-d systems (C, Si, Ge) show strong excitonic effects, with bound excitons SiC:H presents 2 different ionization potentials! (possible technological applications??)

34
Thanks to: Paola Gori (CNR-ISM, Roma) Margherita Marsili (Roma2) Viviana Garbuio (Roma2) Ari P. Seitsonen (Zurich) Friedhelm Bechstedt (IFTO Jena, Germany) Rodolfo Del Sole (Roma2) Antonio Cricenti (CNR-ISM, Roma)

35
Development of theory training Research Development of codes Undergraduates PhD Students Post Docs Other colleagues exp + Industry! Distribution: ABINIT FHI OCTOPUS Yambo DP+EXC TOSCA Carrying on Projects for users

36
BEAMLINES: Optics (O. Pulci) EELS (F. Sottile) X-ray (J. Rehr) Transport (P. Bokes) Time-resolved excitations (M. Marques) Photoemission (C. Verdozzi) Raman (G. Rignanese) new

37
http://www.etsf.eu Next call for projects: deadline 26 October Thank you for your attention olivia.pulci@roma2.infn.it

39
From Dirac’s equation: Si-C 1.79 Angstrom

40
BEAMLINES: Optics (O. Pulci) EELS (F. Sottile) X-ray (J. Rehr) Transport (P. Bokes) Time-resolved excitations (M. Marques) Photoemission (C. Verdozzi) Raman (G. Rignanese) new

41
G: single particle Green’s function W: screened Coulomb interaction (Step 2) Lars Hedin 1965

42
Optical properties (DFT)

43
Optical properties

44
Comparison… Large oscillators strength in Si and Ge-sheets!!!

47
0-D 1-D 2-D 3-D Hamiltonian of N-electron system: Nanoclusters Nanowires Surfaces bulks Biological systems... not possible to solve it!

48
Silicongraphane sandwich geometry NFE state C side

49
GROUND-STATE 1964: Density Functional Theory E=E n 1998 Nobel Prize to Kohn n EXCITED STATES Many Body Perturbation Theory Green’s function method GW + Bethe Salpeter Equation (1965-->today) Time Dependent DFT (TDDFT) (Gross 1984) G n(t)

50
C(001):H NEA Negative electron affinity A=E (vacuum) -E (CBM) =-1.5 eV (-0.7 eV in DFT) E(vacuum) A E (CBM) Exp: -1.3 eV (F. Maier et al PRB2001)

51
?????

52
Vertex function Polarization Screened Coulomb interaction Self-Energy (Hedin 1964) G: single particle Green’s function W: screened Coulomb interaction

53
Optical properties… Large oscillators strength in Si and Ge-sheets!!!

Similar presentations

Presentation is loading. Please wait....

OK

Theory of Intersubband Antipolaritons Mauro F

Theory of Intersubband Antipolaritons Mauro F

© 2017 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on adivasis of india Open ppt on ipad free Ppt on gsm based home automation Ppt on stepping stone to success Ppt on power sharing in democracy sovereign Download ppt on coordinate geometry for class 9th chemistry Ppt on eisenmenger syndrome vsd How to use ppt on ipad Ppt on history of cricket Ppt on vertically opposite angles are equal