first-principles study of 2d van der waals heterojunctions · aa ab ac ac stacking is the most...
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First-principles Study of 2D van der Waals Heterojunctions
Jinlong Yang
Email: [email protected]
Hefei National Laboratory for Physical Sciences at Microscale,University of Science and Technology of China
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Outline
Introduction Our works
Graphene/Silicene g ZnO BP MoS Graphene/Silicene, g-ZnO, BP, MoS2
g-C3N4/MoS2, C2N Phosphorene nanoflake heterojunctions
Conclusions Conclusions
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Introduction From 3D to 2D materials
monocrystalline silicon graphene
Free electron DOS (per volume) in 2D and 3Dn(E) n(E)
E E0 0
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Superiority of graphene
Nature 2005, 438, 7065 Ph R L tt7065; Phys. Rev. Lett. 2006, 96, 136806; Nature 2007, 448, 571; Nature Mater. 5 ; atu e ate2007, 6, 183; Science 2008, 320, 5881; Science 2008, 321 385 N L tt321, 385; Nano Lett. 2008, 8, 902; Phys. Rev. B 2009, 80, 245406; Adv. Funct. ;Mater. 2009, 19, 3077; Science 2013, 340, 6139.
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Other 2D materials
1 Phys. Rev. Lett. y2012, 108, 155501;2 Nature Nanotech. 2014, 9, 372;3 Phys. Rev. Lett. 2007, 99, 026102;4 Phys. Rev. Lett. 2010, 105, 136805;5 Nature Mater. 2009, 8, 76;6 N t M t6 Nature Mater. 2004, 3, 404.
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Why 2D van der Waals heterojunctions?
Simplest junction. Interface is clean and controllable Junction -> element of devices
Extend properties of 2D materials Bandgap opening, electric field response
New phenomena and applications Schottky and ohmic contacts, tunable self-doping Water splitting, solar cells
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Classification
Metal/Metal Graphene/Silicene
Metal/Semiconductor Metal/Semiconductor Graphene/Phosphorene, Graphene/g-ZnO,
G h /M SGraphene/MoS2
Semiconductor/Semiconductor/ g-C3N4/MoS2, Bi-layer blue phosphorus Phosphorene Nanoflake Heterojunctions Phosphorene Nanoflake Heterojunctions
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Metal/Metal
E’FEF
Charge transfer Self-doping
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Metal/Semiconductor
CBMEF
VBM
S h k h iCh t f Schottky or ohmic contactCharge transfer
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Semiconductor/Semiconductor
CBMCBM
CBMCBM CBM
CBM
VBM
CBM
VBMCBM
VBM
Energy levelVBM
Type I
VBM
Type II
VBM
Type III
Energy level alignment
Type I Type II Type III
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Our works
Graphene/Silicene, g-ZnO, BP, MoS2Graphene/Silicene, g ZnO, BP, MoS2 Tuning properties of Graphene
C N /M S C N g-C3N4/MoS2, C2N Visible light-driven water splitting
Phosphorene nanoflake heterojunctionsHighly efficient solar cells Highly efficient solar cells
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I. Tuning properties of graphene
Graphene/SiliceneG phene/g ZnO Graphene/g-ZnO
Graphene/Black phosphorus Graphene/MoS2
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Graphene/Silicene
Silicene Graphene Silicene p type dopingSiliceneWf = 4.6 eV
GrapheneWf = 4.3 eV
Silicene p-type dopingGrpahene n-type doping
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Graphene/Silicene
Tunable self-dopingTunable self doping with the interfacial distance increases:
Silicenep-type -> n-typep yp yp
Graphenen type > p typen-type -> p-type
Doping charge carrier concentrations as a function of interfacial distance D (Å).
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Graphene/g-ZnO
Graphene/Pristine g-ZnO
Fermi level remains in Fermi level remains in the induced gap
No charge transfer
Graphene g-ZnO Graphene/g-ZnO
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Graphene/Al doped g-ZnO
Graphene/Al doped g-ZnO
Al doped n-type g-ZnO
Al d d G h /
Al doped n-type g-ZnO Dirac point below the
Fermi level Electron doping in
Graphene Al dopedg-ZnO
Graphene/Al doped g-ZnO
Electron doping in graphene
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Graphene/Li doped g-ZnO
Graphene/Li doped g-ZnO
Li d d t Z O
Li d d G h /
Li doped p-type g-ZnO Dirac point above the
Fermi levelGraphene Li doped
g-ZnOGraphene/
Li doped g-ZnO Hole doping in graphene
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Graphene/PhosphoreneTunable Schottky barriers with the interlayer distance increases:
ÅD < 3.5 Å, p-type Schottky contactD > 3.5 Å, n-type Schottky contact
S h k bΦBn: n-type Schottky barrierΦBp: p-type Schottky barrier
D = 2.8 3.0 3.5 4.0 4.5 (Å)
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Graphene/MoS2
Applying electric fields perpendicular to graphene/MoS2, tunable p-type doping ofp type doping of graphene is very easy to achieve.
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Graphene/MoS2/Graphene
Applying electric fields perpendicular toperpendicular to graphene/MoS2/graphene, tunable p-type doping of
h i tgraphene is very easy to achieve, while graphene on the other side is weak n-type doped.
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Summary Tunable self-doping can be obtained in
graphene/silicene bygraphene/silicene by Interlayer distance
Tuning properties of graphene can be achieved in graphene/semiconductor composites bygraphene/semiconductor composites by Doping type of semiconductor Interlayer distance Interlayer distance Vertical electric fields
R i H Y C M S i 112 518 (2016)Review: Hu-Yang, Comput. Mat. Sci. 112, 518 (2016)
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II. Visible light-driven water splitting
g-C3N4/MoS2
C N /C N g-C3N4/C2N
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BackgroundEnergy and
environment problemsClean energy
(H )environment problems (H2)
+ Co catalystsMetal-free + Co-catalystsg-C3N4
High H2 production activity
Poor H2 production activity
Challenge: Separation of electron and hole
yy
Wang, Maeda, Thomas, et al., Nat. Mater. 2008, 8, 76; Maeda, Wang, Nishihara, et al., J. Phys. Chem. C 2009, 113, 4940; Di, Wang, Thomas, Anonietti, ChemCatChem 2010, 2, 834; Hou, Laursen, Zhang, et al., Angew. Chem. Int. Ed. 2013, 52, 3621.
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Solution
CBM e-
g-C3N4 MoS2+
CBM
VBMh+
Could g-C3N4 and VBM
Type II
h+g 3 4
MoS2 form Type II alignment??
Type II
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Monolayer of g-C3N4
dC-N1 = 1.47 Å, dC-N2 = 1.33 Å
Direct band gap of 2.7 eV
VBM is dominated by N2 VBM is dominated by N2
CBM is contributed by C and N2
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Monolayer of MoS2
dMo-S = 2.42 Å, dS-S = 3.13 Å
Direct band gap of 2.0 eV
VBM and CBM are all mainly VBM and CBM are all mainly contributed by Mo 4d orbitals and S 3p
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Geometry and band structure
Interlayer distance is 2.97 Åy Direct band gap of 1.89 eV at the Γ points
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Total and partial DOS
VBM is mainly contributed by g-C3N4
CBM is dominated by MoS2
Type II heterostructure is
constructed.
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Charge transfer
Charge transfer from g C N to MoSg-C3N4 to MoS2
Red and blue regions represent charge accumulation andaccumulation and depletion, respectively.
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Optical absorption
g-C3N4/MoS2 nano-composite exhibits morecomposite exhibits more effective UV absorption and enhanced low-energy i ibl li htvisible light response
than g-C3N4 monolayer.
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Carrier transfer and separation
The VB offset is 0.15 eV
h C ff 0 83 The CB offset is 0.83 eV
Electrons transfer from CB of g-C3N4 to CB of MoS2
Holes transfer from VB of o es a s e o oMoS2 to VB of g-C3N4
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Monolayer of C2N
structure
Direct band gap of monolayer C2N is 2 47 eV
band structure
Direct band gap of monolayer C2N is 2.47 eV
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g-C3N4/C2N nanocomposite
g-C3N4
C2N
AA AB AC
AC stacking is the most stable The buckling of g C N The buckling of g-C3N4
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Charge redistribution & band alignment g-C3N4 layer induces electron-
rich and hole-rich regions in th C N lthe C2N layer
VBM is mainly contributed by g-C3N4
CBM is dominated by C2N
Type II band alignment
before contact after contact
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Charge separation & Optical absorption
Three paths of separation: 1) photo-generated electrons in CBM of g-C3N4 migrate to CBM of C2N, 2) photo-generated holes in VBM of C2N transfer to VBM of g-C3N4, 3) electrons in VBM of g-C3N4 can be directly excited to CBM of C2N due to interlayer coupling.
g-C3N4/C2N nanocomposite exhibits significantly enhanced UV and visible light absorption than g-C3N4 monolayer.
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Summary The g-C3N4/MoS2 (C2N) nanocomposite
t t t II b d li tconstructs type II band alignment heterojunction.
With the benefit of type II heterojunction the With the benefit of type II heterojunction, the separation of electron and hole will be more efficient.efficient.
Wang-Guan-Huang-Li-Yang, J. Mater. Chem. A 2, 7960 (2014)Wang-Li-Yang, ChemPhysChem, 10.1002/cphc.201600209(2016)g g, y , p ( )
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III. Highly efficient solar cells
Phosphorene nanoflake heterojunctions
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Edge-modified PNF heterobilayer
Type-II band alignmentEdge dipole induced potential shift
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Edge states and energy gaps
No edge states in HOMO/LUMO in PNFs Edge decoration has little effect on gaps
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HOMO/LUMO energy
Edge decoration has strong effect
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Hybrid molecular orbital diagram
P-H and P-F bonds are more stable
HOMO/LUMO states are from inner P atoms
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HPNF/FPNF heterobilayer
Small HOMO/LUMO energy gaps Separated HOMO/LUMO states Enhanced light absorptionEnhanced light absorption
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PNF heterojunction solar cells
Power conversion efficiency ~ 20%Energy gap (1.5 eV) and conduction band offset (0.15 eV)
Phosphorene/MoS2 (16-18%)Phosphorene/MoS2 (16 18%) PCBM/CBN (10-20%)
g SiC (12 20%) g-SiC2 (12-20%)
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Summary
Propose edge-modified PNFs as donor and acceptor heterojunction solar cells
No edge states in HOMO/LUMO in PNFsg / Edge decoration has little effect on gaps Small HOMO/LUMO energy gapsSmall HOMO/LUMO energy gaps Separated HOMO/LUMO states Enhanced light absorption Enhanced light absorption
Hu-Lin-Yang-Dai-Yang, Nano Lett. 16, 1675 (2016)
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Conclusions
Weak interlayer van der Waals interactions in 2D heterojunctions can induce new properties and phenomena:
Bandgap opening charge transfer and new optical absorption; Bandgap opening, charge transfer and new optical absorption;
Self-doping, Schottky and ohmic contacts and electric field response.
With excellent structural, electronic, electrical and optical properties combined, 2D van der Waals heterojunctions are p p , jexpected to be applied in efficient electronic, electrochemical, photocatalytic, photovoltaic, photoresponsive and memory devices.
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AcknowledgementCollaborators:
W Hu T Wang J Wang Z Guan L Hu W. Hu, T. Wang, J. Wang, Z. Guan, L. Hu, H. Wang
f h h Profs. Zhenyu Li, Jin Zhao, Qunxiang Li
Supports: NSFC MOST MOE CAS NSFC, MOST, MOE, CAS USTCSCC, SCCAS, Tianjin and Shanghai
Supercomputer CentersSupercomputer Centers
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THANK YOU!