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Lone Pair Engineering for Multi-Functional Polar Semiconductors

Aron Walsh
November 26, 2018

Lone Pair Engineering for Multi-Functional Polar Semiconductors

Invited presentation at Fall Meeting of the Materials Research Society (#f18mrs). In "Symposium EP01—New Materials and Applications of Piezoelectric, Pyroelectric and Ferroelectric Materials"

Aron Walsh

November 26, 2018
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  1. Materials Research Society (Fall Meeting 2018)
    Lone Pair Engineering for Multi-
    Functional Polar Semiconductors
    Materials Design Group: https://wmd-group.github.io @lonepair
    Prof. Aron Walsh
    Imperial College London, UK
    Yonsei University, Korea

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  2. Lone Pairs in Functional Materials
    Unique properties from combination of off-centric
    coordination environments and high polarisability
    Piezoelectric, Ferroelectric, Multiferroic, Ion Transport,
    Gas Sensing, Photocatalytic, Photovoltaic, Thermoelectric
    SnSe Pb3
    O4
    BiFeO3

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  3. Lone Pair Engineering
    A. Electronic and Crystal Structure
    B. Carrier Capture and Recombination

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  4. Inert Pair Effect
    Unusual coordination environments of certain
    post-transition metals are linked to stereochemical
    activity of an s2 lone pair
    N. V. Sidgwick (1929); L. E. Orgel, J. Chem. Soc. 0, 3815 (1959)
    “s2–p0 mixing results in an
    instability with respect to
    antisymmetric distortions”

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  5. Oxidation State Competition
    Heavy post-transition metals can adopt the group
    oxidation state N or N-2
    Oxidation states and ionicity, Nature Materials 17, 958 (2018)
    5s0p0: Sn(IV)O2
    Wide bandgap n-type
    semiconductor
    5s2p0: Sn(II)O
    Small bandgap p-type
    semiconductor

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  6. Stereochemical Activity
    For a metal in the same oxidation state, the
    lone pair may be active or inactive: anion effect
    A. Walsh and G. W. Watson, J. Sol. Stat. Chem. 178, 1422 (2005)
    6s2p0: Pb(II)O
    Layered litharge structure
    6s2p0: Pb(II)S
    Rocksalt structure
    Self-consistent
    electron density
    (DFT/PBE)
    [from my PhD
    thesis !]

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  7. Lone Pair Electronic Structure
    Metal s2 band lies deep in the valence band with
    anti-bonding combination close to the Fermi level
    A. Walsh and G. W. Watson, J. Sol. Stat. Chem. 178, 1422 (2005)
    Partial electron density slices (E relative to highest occupied band)
    (Pb 6s + O 2p) O 2p (Pb 6s + O 2p)*
    - 7 eV - 3 eV - 1 eV
    Pb
    O
    O
    O
    O

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  8. Lone Pair Electronic Structure
    Validation using a range of X-ray spectroscopies
    (XPS, XES, XAS) led by Russ Edgell (Oxford)
    D. J. Payne et al, Phys. Rev. Lett. 96, 157403 (2006)
    DFT/PBE
    PbO – valence band
    O 2p
    O 2p
    Pb 6s
    (Pb 6s + O 2p)
    (Pb 6s + O 2p)*
    Note: Interaction strength depends
    on metal s–anion p separation

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  9. Band Engineering of Metal Oxides
    Conduction Band – Metal s0 or p0
    Valence Band – Oxygen 2p6
    Eg
    IP

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  10. Band Engineering of Metal Oxides
    Conduction Band – Metal s0 or p0
    Valence Band – Oxygen 2p6
    Lone pair bonding
    Lone pair anti-bonding
    Eg
    IP
    Lone pair effects:
    1. Reduce bandgap
    2. Lower ionisation potential
    3. Enhance conductivity (lower m*h
    )

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  11. Band Engineering of Metal Oxides
    Conduction Band – Metal s0 or p0
    Valence Band – Oxygen 2p6
    Eg
    EA

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  12. Band Engineering of Metal Oxides
    Conduction Band – Metal s0 or p0
    Valence Band – Oxygen 2p6
    Eg
    EA
    Metal d0

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  13. Band Engineering of Metal Oxides
    Conduction Band – Metal s0 or p0
    Valence Band – Oxygen 2p6
    Lone pair bonding
    Lone pair anti-bonding
    Metal d0
    s2 and d0 cations: (Sn, Sb, Pb, Bi)(Ti, W, V, Nb, Ta)Ox

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  14. New Materials for Solar Fuels
    P. Kamat, J. Phys. Chem. Lett 6, 1917 (2015)

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  15. P. Kamat, J. Phys. Chem. Lett 6, 1917 (2015)
    New Materials for Solar Fuels

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  16. Photoactive Oxide: Bi3+V5+O4
    A. Walsh et al, Chemistry of Materials 21, 547 (2009)
    Bonding
    Anti-bonding

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  17. Photoactive Oxide: Sn2+
    2
    Ti4+O4
    L. A. Burton et al, J. Solid State Chemistry 196, 157 (2012)

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  18. Lone Pairs in Bi(III) Oxyhalides
    Chemistry of Materials 28, 1980 (2016); Chemical Science 7, 4832 (2016)
    Stable and active BiOBr
    photocatalyst (with Parkin
    and Scanlon groups, UCL)

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  19. Crystal Structure Variation
    A. F. Wells, Structural Inorganic Chemistry (5th Edition, 1984)
    Group 14 Group 15 Group 16
    Metal
    Oxides
    5s2
    SnO: Litharge
    Layered, 4
    coordinate
    Sb2
    O3
    :
    Valentinite
    Double O-Sb-O
    chains
    TeO2
    : Tellurite,
    Layered, 4
    coordinate
    6s2 PbO: Litharge
    Layered, 4
    coordinate
    Bi2
    O3
    : ⍺,β,"
    Highly distorted
    coordination
    PoO2
    : Flourite
    No lone pair
    activity
    Metal
    Chalcogenides
    5s2
    SnS/SnSe
    Distorted black P
    structure
    Sb2
    S3
    /Sb2
    Se3
    Sibnite structure
    with Sb2
    X3
    sheets
    TeS2
    /TeSe2
    Unknown
    6s2
    PbS/PbSe Rocksalt
    with no static lone
    pair activity
    Bi2
    S3
    /Bi2
    Se3
    Sibnite/layered
    structure
    PoS2
    /PoSe2
    Unknown

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  20. Polar Semiconductor: Bi2
    WO6
    “fluorite”
    (Bi2
    O2
    )2+
    “perovskite”
    (WO4
    )2- −100
    −80
    −60
    −40
    −20
    0
    20
    40
    0 20 40 60 80 100
    Polarization [µC/cm2]
    Distortion [%]
    61 µC/cm2 along <100>
    Berry phase polarization
    (PBEsol/PAW/VASP)
    Lattice distortion from parent Fmmm
    Aurivillius phase – Low temperature polar (P21
    ab)
    and high-temperature non-polar (C2/m) structures

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  21. Polar Semiconductor: Bi2
    WO6
    Scalable thin-film deposition using aerosol–
    assisted chemical vapor deposition (AA-CVD)
    Experiments by Andreas Kafizas and team (ICL, Chemistry)
    Analysis and optimisation of
    photoactivity is on-going

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  22. Lone Pair Engineering
    A. Electronic and Crystal Structure
    B. Carrier Capture and Recombination

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  23. SRH: Shockley & Read, Phys. Rev. 87, 835 (1952); Hall, Phys. Rev. 87, 387 (1952)
    Carrier Capture in Optoelectronics
    SRH analysis: mid-gap defects are most active
    Beyond: defects levels are not fixed, but vary with the charge state. Non-
    radiative recombination is a multi-level phonon-emission process

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  24. Structural relaxation (electron-phonon coupling)
    is a critical component of carrier capture
    Non-Radiative Capture Process
    Q = configuration coordinate [change in local structure with charge state]
    Huang & Rhys, Proc. RS 204, 406 (1950); Henry & Lang, Phys. Rev. 15, 989 (1977)
    Radiative recombination
    [Defect luminescence]
    Defect in charge
    states E1
    and E2
    Non-radiative recombination
    [Phonon emission]

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  25. Traits of a Carrier Killer
    A. M. Stoneham, Rep. Prog. Phys. 44 1251 (1981)
    What characteristics give rise to efficient carrier
    trapping and recombination?
    1. Cascade: Closely-spaced bound
    electronic states (e.g. Ni in GaP)
    2. Resonance: Favourable
    vibrational states (e.g. C2
    in Si)
    3. Coupling: strong electron-lattice
    interaction (e.g. vacancies)

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  26. Lone Pairs Enhance Carrier Capture
    S. Kim et al, ACS Energy Letters 3, 496 (2018); arXiv 1810.11259 (2018)
    In Sn(IV) containing semiconductors, electron
    capture to Sn(III) and Sn(II) is efficient
    Multi-valency: Carrier localisation and
    large amplitude lattice distortion
    Lone pair state of Sn
    associated with S
    loss in Cu2
    ZnSnS4
    solar cells

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  27. Lone Pairs Enhance Carrier Capture
    Sunghyun Kim (Unpublished)
    Strong “lone pair” recombination centres – limit
    photovoltaic efficiency, but other applications?
    Photothermal catalysis, switches, memory…
    SRH limit S-Q limit
    JSC
    28.97
    mA/cm2
    28.97
    mA/cm2
    VOC
    0.80 V 1.20 V
    FF 85.10% 89.99%
    Efficiency 20.1% 31.0%
    Lone pair effect on theoretical
    Cu2
    ZnSn(IV)S4
    solar cell efficiency
    CdS
    CZTS
    d(1+/0)
    d(2+/1+)
    2
    1
    0
    1
    Energy (eV)
    200 100 0
    Position (nm)
    ϵ(2+/1+)
    E
    F
    ϵ(1+/0)
    Neutral
    trap
    Repulsive
    trap
    Giant
    trap
    V
    S
    -Cu
    Zn
    1+ V
    S
    2+
    Sn
    Zn
    1+
    Sn
    Zn
    2+
    Cu Sn
    1−
    1000/T (1/K)
    σn (cm2)
    0 2 4 6 8 10
    10−30
    10−27
    10−24
    10−21
    10−18
    10−15
    10−12
    (a ) (b)
    Giant capture cross-section traps
    Lone-pair
    related defects

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  28. Conclusions: Solid-State Lone Pairs
    • Band engineering: bandgaps, band
    energies, conductivity (effective masses)
    • No global understanding of structure
    control, e.g. polar transitions, emphanisis
    (symmetry breaking), or carrier capture
    Slides: https://speakerdeck.com/aronwalsh
    Thanks to group, in particular Kazuki Morita, Sunghyun Kim,
    Liam Harnett; collaborators including Andreas Kafizas, James
    Durrant (ICL), Russ Egdell (Oxford), Ram Seshadri (UCSB)

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