Inorganic Chemistry
Article
Theoretical Calculation. To better understand the
memristive mechanism of the W/Ir-vio/Ag devices, related
analysis and theoretical calculations were performed (Figure
5). If the changes in resistance are field-induced, the
memristors will display an obvious switching behavior in the
millivolt range.53 Thus, switching behavior cannot be
attributed to filamentary conduction and conformational
changes because the W/Ir-vio/Ag devices do not undergo
field-induced modulation.53 Importantly, Ir-vio molecules have
channels for charge transfer because the molecular surface has
continuous positive electrostatic potentials (ESPs) along the
conjugated backbone (Figure 5a,b). In addition, the HOMO
and LUMO energy levels from Ir-vio to the vacuum level are
estimated to be −5.61 and −4.11 eV, respectively (Table S1).
The energy barriers of Al/Ir-vio LUMO and W/Ir-vio LUMO
interfaces are 0.09 and 0.39 eV (Figure S6), respectively,
indicating that the electron transfer from the LUMO level of
Ir-vio to the two electrodes is the most favorable process, so
electrons dominate the conduction process. Subsequently,
density functional theory (DFT) calculations were performed
to evaluate the charge transfers of complex Ir-vio (Figure
5c,d). The highest occupied molecular orbital (HOMO) of
complex Ir-vio is located in the C∧N ligands and iridium atom,
and its LUMO + 1 and LUMO + 2 are mainly distributed on
the 1,10-phenanthroline moiety. The LUMO stands for the
lowest unoccupied molecular orbital. MLCT (49.2%) and
LLCT (45.1%) were obtained by the HOMO → LUMO + 1
and HOMO → LUMO + 2 transitions, resulting in the
switching behavior of the W/Ir-vio/Ag devices. In addition,
considering the multistate memory behavior of the memristors,
the interconversion between different redox states plays a
crucial role. Compound Vio has no memristive behavior and
complex Ir shows only a WORM effect (Figure 3a,b), which
can further confirm that the interconversion between different
redox states of complex Ir-vio leads to the multistate
memristive behavior. Moreover, the redox states of Ir-vio
complexes were speculated and calculated theoretically based
on the redox processes of viologen (Figure 5e). All redox states
exhibit clearly visible HOMO and LUMO distributions. For
complex Ir-vio, HOMOs are distributed on the C∧N ligands
and iridium atom, while LUMOs are mainly located in the
viologen moiety. Subsequently, complex Ir-vio gains an
electron to form its radical cationic species Ir-vio+•. Complex
Ir-vio+• displays strong delocalization so that the HOMOs
clearly show electron density contribution from the C∧N
ligands and viologen unit, and LUMOs are mainly located in
the entire N∧N ligand. Finally, the neutral substances of Ir-vio+
are formed when complex Ir-vio+• gains an electron. Poor
delocalization is exhibited for complex Ir-vio+ because its
HOMOs and LUMOs are located in the viologen moiety and
1,10-phenanthroline unit, respectively. Different redox states
show different electron density distributions and intra-
molecular charge-transfer capabilities, which result in an
excellent memristive characteristic of complex Ir-vio under
the electrical stimulus. Therefore, the realization of multilevel
memristive switching and synaptic weight simulation can be
ascribed to the interconversion among different charge-transfer
and redox states under various electrical stimulus.53
The memristors with W/Ir-vio/Ag structure exhibit multilevel
storage performance, which can successfully mimic synaptic
plasticity and have great application potential in brain-inspired
computing. The memristive mechanisms have been inves-
tigated in detail and are probably due to metal-to-ligand
charge-transfer and ligand-to-ligand charge-transfer transitions
and the mutual conversion of different redox states under
electrical stimulus. To our knowledge, this is the first work on
memristors based on organometallic compounds, which
provides a new horizon for developing multilevel resistive
memory devices and neuromorphic computing.
ASSOCIATED CONTENT
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sı
* Supporting Information
The Supporting Information is available free of charge at
Synthesis; characterization; experimental information;
and additional figures and a table (PDF)
AUTHOR INFORMATION
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Corresponding Authors
Shujuan Liu − State Key Laboratory of Organic Electronics
and Information Displays & Jiangsu Key Laboratory for
Biosensors, Institute of Advanced Materials, Nanjing
University of Posts and Telecommunications, Nanjing
Yi Tong − College of Electronic and Optical Engineering &
College of Microelectronics, Institute of Flexible Electronics
(Future Technology), Jiangsu Province Engineering Research
Center for Fabrication and Application of Special Optical
Fiber Materials and Devices, Nanjing University of Posts and
Telecommunications, Nanjing 210023 Jiangsu, P. R. China;
Qiang Zhao − State Key Laboratory of Organic Electronics
and Information Displays & Jiangsu Key Laboratory for
Biosensors, Institute of Advanced Materials, Nanjing
University of Posts and Telecommunications, Nanjing
210023 Jiangsu, P. R. China; College of Electronic and
Optical Engineering & College of Microelectronics, Institute of
Flexible Electronics (Future Technology), Jiangsu Province
Engineering Research Center for Fabrication and Application
of Special Optical Fiber Materials and Devices, Nanjing
University of Posts and Telecommunications, Nanjing
Authors
Yanling Zhuang − State Key Laboratory of Organic
Electronics and Information Displays & Jiangsu Key
Laboratory for Biosensors, Institute of Advanced Materials,
Nanjing University of Posts and Telecommunications,
Nanjing 210023 Jiangsu, P. R. China
Yu Wang − College of Electronic and Optical Engineering &
College of Microelectronics, Institute of Flexible Electronics
(Future Technology), Jiangsu Province Engineering Research
Center for Fabrication and Application of Special Optical
Fiber Materials and Devices, Nanjing University of Posts and
Telecommunications, Nanjing 210023 Jiangsu, P. R. China
Yongjing Deng − State Key Laboratory of Organic Electronics
and Information Displays & Jiangsu Key Laboratory for
Biosensors, Institute of Advanced Materials, Nanjing
CONCLUSIONS
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In summary, an electroactive viologen-containing iridium(III)
complex Ir-vio was rationally designed and fully characterized
for two-terminal memristor-based synaptic weight simulation.
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Inorg. Chem. XXXX, XXX, XXX−XXX