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. 2015 Feb 1;6(2):1308-1315.
doi: 10.1039/c4sc03345k. Epub 2014 Nov 24.

Tuning of resistive memory switching in electropolymerized metallopolymeric films

Bin-Bin Cui  1 , Zupan Mao  2 , Yuxia Chen  1 , Yu-Wu Zhong  1 , Gui Yu  2 , Chuanlang Zhan  1 , Jiannian Yao  1
Affiliations

Tuning of resistive memory switching in electropolymerized metallopolymeric films

Bin-Bin Cui et al. Chem Sci. .

Abstract

A diruthenium complex capped with two triphenylamine units was polymerized by electrochemical oxidation to afford metallopolymeric films with alternating diruthenium and tetraphenylbenzidine structures. The obtained thin films feature rich redox processes associated with the reduction of the bridging ligands (tetra(pyrid-2-yl)pyrazine) and the oxidation of the tetraphenylbenzidine and diruthenium segments. The sandwiched ITO/polymer film/Al electrical devices show excellent resistive memory switching with a low operational voltage, large ON/OFF current ratio (100-1000), good stability (500 cycles tested), and long retention time. In stark contrast, devices with polymeric films of a related monoruthenium complex show poor memory performance. The mechanism of the field-induced conductivity of the diruthenium polymer film is rationalized by the formation of a charge transfer state, as supported by DFT calculations.

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Figures

Fig. 1
Fig. 1. Representative known transition metal complexes or metallopolymers as active layers for resistive memory. The thin films of these materials were prepared by spin-coating.
Scheme 1
Scheme 1. Synthesis of (a) poly-1 4+ and (b) poly-2 2+ via the oxidative electropolymerization of 1(PF6)4 and 2(PF6)2, respectively. The counteranions of the polymers are mostly ClO4 – ions, which were included from the electrolyte during the electropolymerization.
Fig. 2
Fig. 2. ORTEP drawing of the single-crystal X-ray structure of 1(PF6)4 at 30% probability. Anions and H atoms are omitted for clarity. Color code: carbon, grey; nitrogen, blue; pink, ruthenium.
Fig. 3
Fig. 3. (a) Cathodic CVs of 1(PF6)4 at a Pt disk electrode (d = 2 mm) in 0.1 M Bu4NClO4/CH2Cl2. (b) CVs recorded during repeated potential scans between +0.40 and +1.35 V. (c and d) CVs of the obtained poly-1 4+/Pt film in a clean electrolyte solution. The scan rates are 100 mV s–1.
Fig. 4
Fig. 4. (a) AFM height image of the poly-1 4+/ITO film (size: 5 μm × 5 μm). (b) Schematic representation of the memory device structure. (c) Typical I–V characteristics of the ITO/poly-1 4+/Al device with an active area of 6.0 mm2. The arrows denote switching order and direction. The thicknesses of the polymer film and Al electrode are 100 and 80 nm, respectively. (d) Plot of the ON/OFF current ratio versus voltage. The y axes of (c) and (d) are in logarithmic scale.
Fig. 5
Fig. 5. (a) Input applied voltage sequence and (b and c) output current responses during repeated write/read/erase/read (+5 V/1 V/–5 V/1 V) cycles for the ITO/poly-1 4+/Al device.
Fig. 6
Fig. 6. Retention times of the ON- and OFF-state data under a readout voltage of +1 V. The ON and OFF states were induced by +5 and –5 V, respectively.
Fig. 7
Fig. 7. (a) CVs of the poly-2 2+/Pt film in a clean electrolyte solution at 100 mV s–1. (b) Typical I–V characteristics of the ITO/poly-2 2+/Al device with a logarithmic scale for the current. The arrows denote switching order and direction.
Fig. 8
Fig. 8. Isodensity plots of the frontier molecular orbitals of the diruthenium-tetraphenylbenzidine basic structural component of poly-1 4+. DFT methods: B3LYP/LANL2DZ/6-31-G*/CPCM. Eigenvalues in eV are shown in parentheses for each energy level.
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