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On-Surface Synthesis and Characterization of Conjugated Oligomers Using Low Temperature Scanning Tunneling
Microscopy and Spectroscopy
Guowen KUANG
Supervisor: Prof. Nian LIN
Department of Physics, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong
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Outline
Introduction • molecular electronics • scanning tunneling microscopy and spectroscopy • experimental setup
Synthesis of molecular wires • oligomers comprising porphyrins and phenyls
Characterization of molecular wires
i. resonant charge transport ii. negative differential conductance iii. spin-spin coupling iv. spin crossover
Summary
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Introduction: molecular electronics
http://www.asdn.net/asdn/electronics/molecular_electronics.shtml
Advantages:
Bottom-up fabrication Multiple functionality Mechanical flexibility Environment-friendly … …
Challenges:
Production? Synthesize molecules with specific functionality Visualization? Probe structural details at atomic resolution Characterization? Measure electronic structure and charge transport property … …
Molecular electronics is the study and application of
molecular building blocks for the fabrication of electronic components.
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Introduction: STM / STS
Morphology
STM
Electronic structure
STS
Transport property
Manipulation + STS
Molecular orbital Conductance
1 nm
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Experimental setup
Metal Molecule
Ultrahigh vacuum
Substrate
Ultrahigh vacuum
77K @ LN2
4.8K @ LHe
Low-temperature
Sample preparation:
Organic molecule source Metal atom source
Characterization:
STM: structural details STS: molecular orbital
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Outline
Introduction • molecular electronics • scanning tunneling microscopy and spectroscopy • experimental setup
Synthesis of molecular wires • oligomers comprising porphyrins and phenyls
Characterization of molecular wires
i. resonant charge transport ii. negative differential conductance iii. spin-spin coupling iv. spin crossover
Summary
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Synthesis: introduction
On-surface synthesis Oligomer 2
Oligomer 1
Oligomer 0
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Synthesis: oligomer 2
Oligomer 2
Br2-TPP @ Au(111)
1 nm
Ullmann coupling reaction @ 180oC annealing
10 nm
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Synthesis: oligomer 1
Co-deposition and
180oC annealing
Br2-TPP Br2-DPP
@ Au(111)
1 nm
Oligomer 1 Oligomer 1
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Steric hindrance
Synthesis: oligomer 0
On surface:
In solution:
Synthesize in solution then deposit on surface
Br2-2DPP
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Synthesis: oligomer 0
Synthesis of Br2-2DPP in
solution
160oC
@ Au(111) 200oC
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Synthesis: on-surface metalation
Oligomer 0, 1 and 2 can be partially or fully metalated by Fe
Metalation by Fe, Ni, Co, Zn, Mg, Mn, Ce …
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Molecular wires Metalation derivatives
Oligomer 0
Oligomer 1
Oligomer 2
Synthesis: summary
Fe
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Outline
Introduction • molecular electronics • scanning tunneling microscopy and spectroscopy • experimental setup
Synthesis of molecular wires • oligomers comprising porphyrins and phenyls
Characterization of molecular wires
i. resonant charge transport ii. negative differential conductance iii. spin-spin coupling iv. spin crossover
Summary
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Off-resonance tunneling
Charge transport: mechanism
Saturated: 0.8 Å-1
Conjugated: 0.2 Å-1
N. J. Tao, Nature Nanotech. 1, 173 (2006).
L. Lafferentz, F. Ample, H. Yu, S. Hecht, C. Joachim, and L. Grill, Science 323, 1193 (2009).
R. L. McCreery, H. J. Yan, and A. J. Bergren, Phys. Chem. Chem. Phys. 15, 1065 (2013).
L. Xiang, J. L. Palma, C. Bruot, V. Mujica, M. A. Ratner, and N. Tao, Nature Chem. 7, 221 (2015).
K. H. Khoo, Y. Chen, S. Li, and S. Y. Quek, Phys. Chem. Chem. Phys. 17, 77 (2015).
G. Sedghi, L. J. Esdaile, H. L. Anderson, S. Martin, D. Bethell, S. J. Higgins, and R. J. Nichols, Adv. Mater. 24, 653 (2012).
Thermally activated hopping: a power law decay
Resonant transport: nearly length-independent
Delocalized molecular orbital
Long distance ?
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Charge transport: electronic property of oligomer 2
HOMO localized LUMO delocalized
-0.8 V 1.6 V
dI/dV along molecular wire
1 nm
HOMO
LUMO
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Charge transport: conductance measurement
STM vertical manipulation
V
I
ΔZ
-3.0 -2.5 -2.0 -1.5 -1.0
0.0
0.2
0.4
0.6
0.8
Curr
ent (n
A)
Bias (V)
-3.0 -2.5 -2.0 -1.5 -1.0
0.0
0.2
0.4
dI/dV
(a.u
.)
Bias (V)
I (V) and dI/dV (V) Before manipulation
After manipulation
2 nm
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Charge transport: different contacts
At lower tip height:
Strong contact vs. weak contact
ΔZ = 1.3nm
Strong contact Weak contact
Z displacement: > 5nm (19 %) < 3nm (81 %)
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Charge transport: first-principle simulation
NEGF – simulation of transmission:
Conclusion : weak contact physical adsorption strong contact covalent bonding electrons transfer through delocalized LUMO
S
Weak:
Strong:
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Charge transport: long-range resonant transport
At higher tip height:
Simulation:
1.2 nm
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Charge transport: Fe-metalated oligomer 2
At higher tip height:
Resonant transport through delocalized molecular orbital magnetic impurities do not affect transport
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Charge transport: attenuation factor
1 2 3 4 5 6 7 8 9
1
10
100
1000
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
1
10
100
1000
Oligomer 2 Fe-metalated oligomer 2
dI/
dV
(n
S)
dI/
dV
(n
S)
β = 0.03A-1 β < 0.001A-1 β = 0.02A-1 β < 0.001A-1
The magnitude of the first peak
Nearly length-independent
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Charge transport: branched molecular wires
Br2-TPP molecules @ Au(111) @ 250oC
T-wire
L-wire
40 × 40 nm
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Charge transport: T-wire exhibiting NDC
• Lift up T-wire by terminal S • Stretched after manipulation • Resonant charge transport • Negative differential conductance
S
5 nm
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Charge transport: T-wire exhibiting NDC
• Lift up T-wire by terminal B • Flipped after manipulation • Resonant charge transport • NDC
B
5 nm
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Charge transport: why NDC ?
• Multi-pathway for current ? • Structural kink ?
S B
T-shape Straight
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Charge transport: multi-pathway ?
K
Multi-pathway
• 90o-kinked L-wire • Lift up by terminal K • No NDC
2 nm
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Charge transport: 90o kink ?
• 90o-kinked L-wire • Lift up by terminal T • 67 % with NDC
T
2 nm 2 nm
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Charge transport: 120o kink ?
120o kink
2 nm
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Charge transport: mechanism
Redox reaction and resonance transport
Polaron induced charge storage
Changes in molecule-electrode coupling
Alignment and misalignment of delocalized molecular orbital
NDC
N. Kang, A. Erbe, and E. Scheer, Appl. Phys. Lett. 96, 023701 (2010) J. Zhou, S. Samanta, C. Guo, J. Locklin, and B. Xu, Nanoscale 5, 5715 (2013) M. L. Perrin et al., Nat. Nanotechnol. 9, 830 (2014) E. D. Mentovich, I. Kalifa, A. Tsukernik, A. Caster, N. Rosenberg-Shraga, H. Marom, M. Gozin, and S. Richter, Small 4, 55 (2008)
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Charge transport: first-principle simulation
HOMO-1 becomes less delocalized at high bias
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Charge transport: summary
Resonant charge transport
Negative differential conductance
Oligomer 2
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Outline
Introduction • molecular electronics • scanning tunneling microscopy and spectroscopy • experimental setup
Synthesis of molecular wires • oligomers comprising porphyrins and phenyls
Characterization of molecular wires
i. resonant charge transport ii. negative differential conductance iii. spin-spin coupling iv. spin crossover
Summary
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Spin-spin coupling: introduction
Direct coupling • overlap of orbitals with non-zero magnetic moment • dipole-dipole coupling
Indirect coupling • interacting through a certain medium • RKKY – mediated by conduction electrons • exchange – mediated by organic molecule
Interaction between spins
1.73 nm 0.89 nm 1.31 nm
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Spin-spin coupling: Fe-TPP
Spin-excitation spectroscopy @ 4.8K Spin signature of individual atoms
Fe-TPP monomer
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Spin-spin coupling: Fe-metalated oligomer 2 and 1
Spins in the oligomers exhibit the same behavior as isolated spins: neighboring spins are decoupled
Oligomer 2 Oligomer 1
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Spin-spin coupling: Fe-metalated oligomer 0
Oligomer 0
Spin-excitation quenched neighboring spins are coupled Exchange interaction mediated by molecular backbone
a
a
b c d e f
b
c
d
e
f
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Spin-spin coupling: conclusion
Spin-spin indirect interaction mediated by molecular backbone
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Outline
Introduction • molecular electronics • scanning tunneling microscopy and spectroscopy • experimental setup
Synthesis of molecular wires • oligomers comprising porphyrins and phenyls
Characterization of molecular wires
i. resonant charge transport ii. negative differential conductance iii. spin-spin coupling iv. spin crossover
Summary
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Spin crossover: introduction
Spin crossover
High-spin Low-spin switching
External stimulus: • Temperature • Light • Charge flow • Pressure • Electric field • … …
Top Curr Chem (2004) 233:1–47
Assemble of molecules Single-molecule level ?
Switches in Molecular electronics
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Spin crossover: vertical manipulation + STS
Bottom to top: 1.2 to 3.4 nm
Kondo effect @ 4.8 K
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Spin crossover: width of Fano resonance
Reversible control
FWHM of Fano resonance:
--- --
------- ------- --
---- --
--- ------- ------- ---- -- -- --
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S = 1 S = 2 S = 2 S = 1 S = 1
Spin crossover: DFT simulation
unpolarized unpolarized
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Spin crossover: magnetic moment & Fe-N bond length
ρJ spin density
Porphyrin conformation:
Saddle planar
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iii.
S = 2
S = 1 iv.
ii.
i.
Summary
i. Resonant charge transport ii. Negative differential conductance iii. Spin-spin coupling iv. Spin crossover
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Publications
1. Kuang G, Rubio-Verdu C, et al. Indirect Spin-Spin Coupling through Molecular Backbone. in preparation. 2. Kuang G, Yan L, et al. Negative Differential Conductance Observed in Poly-Porphyrin Oligomers with Junction Morphology. in preparation. 3. Kuang G, Zhang Q, Lin T, Pang R, Shi X, Xu H, et al. Mechanically-Controlled Reversible Spin Crossover of Single Fe-Porphyrin Molecules. ACS Nano 2017, 11(6): 6295-6300. 4. Zhang Q, Zheng X, Kuang G, Wang W, Zhu L, Pang R, et al. Single-Molecule Investigations of Conformation Adaptation of Porphyrins on Surfaces. J Phys Chem Lett 2017, 8(6): 1241-1247. 5. Zhao Y-L, Wang W, Qi F, Li J-F, Kuang G, Zhang R-Q, et al. Donor/Acceptor Properties of Aromatic Molecules in Complex Metal–Molecule Interfaces. Langmuir 2017, 33(2): 451-458. 6. Yan L, Kuang G, Zhang Q, Shang XS, Liu PN, Lin N. Self-Assembly of a Binodal Metal-Organic Framework Exhibiting a Demi-Regular Lattice. Faraday Discuss 2017. 7. Kuang G, Chen S-Z, Wang W, Lin T, Chen K, Shang X, et al. Resonant Charge Transport in Conjugated Molecular Wires beyond 10 nm Range. J Am Chem Soc 2016, 138(35): 11140-11143. 8. Lyu G, Zhang Q, Urgel JI, Kuang G, Auwarter W, Ecija D, et al. Tunable lanthanide-directed metallosupramolecular networks by exploiting coordinative flexibility through ligand stoichiometry. Chem Commun 2016, 52(8): 1618-1621. 9. Zhang Q, Kuang G, Pang R, Shi X, Lin N. Switching Molecular Kondo Effect via Supramolecular Interaction. ACS Nano 2015, 9(12): 12521-12528. 10. Wang W, Pang R, Kuang G, Shi X, Shang X, Liu PN, et al. Intramolecularly resolved Kondo resonance of high-spin Fe(II)-porphyrin adsorbed on Au(111). Phys Rev B 2015, 91(4): 045440. 11. Kuang G, Zhang Q, Li DY, Shang XS, Lin T, Liu PN, et al. Cross-Coupling of Aryl-Bromide and Porphyrin-Bromide on an Au(111) Surface. Chem Eur J 2015, 21(22): 8028-8032. 12. Lin T, Kuang G, Wang W, Lin N. Two-Dimensional Lattice of Out-of-Plane Dinuclear Iron Centers Exhibiting Kondo Resonance. ACS Nano 2014, 8(8): 8310-8316. 13. Lin T, Kuang G, Shang XS, Liu PN, Lin N. Self-assembly of metal-organic coordination networks using on-surface synthesized ligands. Chem Commun 2014, 50(97): 15327-15329. 14. Wang S, Wang W, Tan LZ, Li XG, Shi Z, Kuang G, et al. Tuning Two-Dimensional Band Structure of Cu(111) Surface-State Electrons that Interplay with Artificial Supramolecular Architectures. Phys Rev B 2013, 88(24): 245430.
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Acknowledgements
• Prof. Lin Nian (supervisor)
• Prof. Jose Ignacio Pascual (CIC nanoGUNE, San Sebastian, Spain)
• Prof. Chen Keqiu (Hunan University)
• Prof. Liu Peinian (East China University of Science and Technology)
Former members:
• Dr. Wang Weihua
• Dr. Dong Lei
• Dr. Wang Shiyong
• Dr. Lin Tao
• Dr. Zhao Wei
• Mr. Chen Cheng
Current members:
• Dr. Yan Linghao
• Dr. Liu Jing
• Mr. Lyu Guoqing
• Mr. Zhang Qiushi
• Ms. Zhang Ran
• Mr. Gao Zi’ang
• Mr. Xia Bowen
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Acknowledgements
Thesis Examination Committee:
• Prof. Sin Kwok Raymond WONG (Chairperson)
• Prof. Wei XU
• Prof. Michael Scott ALTMAN
• Prof. Ophelia K. C. TSUI
• Prof. Yongli MI
• Prof. Nian LIN (Supervisor)
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Thanks For Your Attention