Distinct electronic switching behaviors of triphenylamine-containing polyimide memories with different bottom electrodes
Qisheng Liu,1,2 Kejian Jiang,1 Lihua Wang,1 Yongqiang Wen,1 Jingxia Wang,1 Ying Ma,1,2
and Yanlin Song1,*
1Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Organic Solids, Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
2Graduate School of Chinese Academy of Science, Beijing, 100049, China
1. Synthesis of copolymer MTPA-PI
The synthetic procedures are as follows (scheme 1):
Scheme S1. Synthetic route for the MTPA-PI.
Equivalent moles of 2,2′-bis-(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6F) and 4,4′-hydroxy-3,3′-diaminobiphenyl (HAB) were dissolved in dry N-methylpyrrolidone (NMP) with isoquinoline as the catalyst. After the mixture solution was heated to 70 ℃ and stirred for 2 h, the temperature of the solution was improved to 200 ℃ gently and kept it for 5 h under this condition. Thereafter, the solution was cooled below 70 ℃ and poured into the large quantity of a mixture of methanol and water (vol.: 1:1) under vigorous stirring. The precipitate was filtrated and washed several times with methanol and dried in vacuum giving poly(4,4-dihydroxy-3,3-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB PI) as a pale yellow powder.
6F-HAB PI (1 molar equivalent, abbreviation: 1M equiv.) and (4-(diphenylamino)phenyl)-
-methanol (DPPM, 4M equiv/mol of the polymeric repeat unit) and triphenyl phosphine (4M equiv./mol) were dissolved in dry THF under nitrogen atmosphere. During the stirring, the diisopropylazodicarboxylate (4M equiv./mol) was added dropwise. After stirred for 10 h at room temperature, the solution was poured into hot methanol under vigorous stirring, yielding MTPA-PI precipitate. The precipitate was filtrated and washed with methanol and dried under vacuum, giving target product as yellow powder.
2. Fabrication of polymer memory devices
ITO/MTPA-PI/Al (Aluminum): A 10 mg/ml cyclohexanone solution of MTPA-PI was spin-coated onto the precleaned ITO substrates at 2000 rpm for 40 s, followed by solvent removal in a vacuum chamber at 10−5 Torr and at 100 ℃ for 10 h. The Al electrodes were deposited on to the polymer films at pressure below 10−7 Torr by means of vacuum thermal evaporation with a mask. The thickness of top Al electrodes is about 300 nm with sizes 2.0 × 2.0 mm2.
Al/MTPA-PI/Al: Then Al layers (300 nm) were deposited onto pre-cleaned silicon wafers using the above method, and MTPA-PI polymer solution was spun on to Al bottom electrode at 2000 rpm for 40 s. The Al layers (300 nm) as top electrodes were deposited on to polymer films with sizes 2.0 × 2.0 mm2.
I-V measurements were carried out using a Keithley 4200 semiconductor analyzer whose maximum current compliance is 0.105 A. The measurement processes were performed at room temperature under air ambient condition. The glass transition temperature Tg was measured using a Seiko differential scanning calorimeter (DSC-6200) with a ramping rate of 10.0℃/min. The thermal stability was determined with a ramping rate of 20.0℃ min−1 under a nitrogen atmosphere by using a thermal analyzer (TGA Q50). FTIR spectra were achieved on Bruker Euinnox 55 Fourier transform infrared spectroscopy. 1H NMR spectra were obtained at room temperature with a 400 MHZ Bruker Avance 400 spectrometer. Absorption spectra were measured using a Hitachi U-4100 UV-vis spectrophotometer. CV measurements were carried out in a 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAHFP) in acetonitrile using an electrochemical workstation (IM6ex impedance analyzer) with a platinum gauze counter electrode and an Ag/AgCl (3.8 M KCl) reference electrode and PI was coated Au bottom electrode on Si wafer. A scan rate of 100 mV/s was used. The repeated units of the PI-SP were carried out with the quantum chemical calculations by using the Hybrid Hartree-Fock/density functional theory (HF/DTT) at B3LYP/6-31G* level.
4. Analysis of the HOMO and LUMO for the MTPA-PI polymer
FIG. S1 (a) UV-vis spectrum and (b) CV response of the MTPA-PI films fabricated with an Au electrode supported by a silicon substrate.
Fig. S1 (a) and (b) show the UV-vis spectrum and cylic voltammetry (CV) data. The UV-vis absorption edge of MTPA-PI extends to about 360 nm, from which the band gap of this polymer is determined to be about 3.49 eV (Eg (band gap) = 3.49 eV). From the CV data, we can see the copolymer exhibits an onset of oxidation peak at 0.97 V (EOX(onset) = 0.97 V). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels can be calculated using the following formulas: 1−2
HOMO = − [( EOX(onset) − EFOC) + 4.8 ] (eV)
LUMO = HOMO + Eg (band gap) (eV)
Here EFOC is the potential of external standard, ferrocene/ferricenium ion (Foc/Foc+) couple, measured under the same conditions. The values of EFOC are 0.38 V. Therefore, the HOMO and LUMO energy levels of MTPA-PI are −5.39 eV and −1.90 eV respectively.
1J. L. Bredas, R.Silbey, D. S. Boudreaux, and R. R. Chance, J. Am. Chem. Soc. 105, 6555 (1983).
2Y. Z. Lee, X. W. Chen, S. A. Chen, P. K. Wei, and W. S. Fann, J. Am. Chem. Soc. 123, 2296 (2001).
Figures and Scheme
FIG. S2. Repeated test results of the different devices: (a) in the ITO/6F-HAB PI/Al devices: in every device, four voltage sweeps were performed. The first sweep was performed from 0 to –2.5 V to switch on the device (curve 1). The second sweep was testing the ON state with the same sweep range (curve 2). The third sweep was carried out from 0 to 3.5 V to switch off the device (curve 3). The fourth sweep was convincing the OFF state (curve 4). (b) in the Al/6F-HAB PI/ device: in every device, three voltage sweeps were performed. The first sweep was performed with a current compliance of 0.01 A from 0 to 3.5 V to switch on the device (curve 1). The second sweep was testing the ON state with the same a current compliance (curve 2). The third sweep was carried out with a current compliance of 0.1 A from 0 to 2.2 V to switch off the device (curve 3).
FIG. S3 (a) HOMO and (b) LUMO of the monomer units of the MTPA-PI obtained by simulation.
FIG. S4. LUMO and HOMO energy levels of the MTPA-PI and the work function of electrodes.
FIG. S5. Voltage-current plots of the ITO/MTPA-PI/Al device, which fit with (a) the space charge limited current (SCLC: < −0.9V) model in the OFF state, and (b) the Ohmic model in the ON state.
FIG. S6. Typical I-V curves for the ITO/6F-HAB PI/Al device: (a) the first and second sweeps were performed from 0 to –3.0 V, (b) the first and second sweeps were performed from 0 to 3.0 V; typical I-V curves for the Al/MTPA-PI/Al device: (c) the first and second sweeps were performed from 0 to –4.0 V, (d) the first and second sweeps were performed from 0 to 4.0 V.
FIG. S7. Current of the ON state as a function of temperature for the ITO/6F-HAB PI/Al device. Temperature sweep was performed by a step of 10 K. From the plot, no obvious decrease of the ON state current was observed as the temperature increased. So Ohmic conduction resulting from the metal filament can be excluded.