Multi-level Electronic Memory Properties Of Conjugate Organic Molecules With 1,3,4-Oxadiazole Skeleton

Due to the limits of Moore‘s law, progress in enhancing the memory capacity by minishing the size of conventional inorganic silicon-based semiconductor memory devices which enable information storage through discharging and charging processes in the circuit has stalled in recent years.[1-2] According to the recent report, more and more heat would be generated as more and more silicon circuits have been integrated into a small space, the original capacity of double speed for two years has been slowly decline. In addition, there are more serious problems appeared slowly, the precision of the circuit belongs to today’s top chip maker has less than 14 nm, which is smaller than most of the virus. To such a level, electronic behavior will be limited by quantum uncertainty, the transistor will also become unreliable. Under such a prospect, the semiconductor industry whose development follows Moore’s law step-by-step also announced the research planning blueprint that will be released next month would not centered on Moore’s law.[3] Therefore, many researchers began to employ organic semiconductor materials to prepare prototype storage devices in recent years. In contrast, organic molecules with tunable properties are promising for data storage applications, due to their low cost, simplicity, speed, lower power consumption, and longer data retention times. Resistive organic memory devices implement data storage under different applied voltages, based on variation of conductivities. This works on the principle that conductivity states representing different recording data can be read out nondestructively. [4-8] However, many reported devices exhibit two conductivity states in response to the applied voltage and can only store 2-bits in a single cell,[9-10] the rapid advancement of 21 st century society demands for the development of better data storage, like high density data storage with a capacity of 3n or higher. Thus using materials with multiple conductivity states is an effective method to achieve high density data storage.Based on the previous reports, a touch of compounds containing 1,3,4-oxadiazole conjugate skeleton had been synthesized and their electronic memory characteristics had also been studied, the details are depicted as follows:(1) Effect of the different terminal electron-donating groups of the symmetric organic small molecules with oxadiazole conjugate skeleton on their multi-level electronic memory performance: Two molecules with donor-acceptor1-acceptor2-acceptor1-donor(D-A1-A2-A1-D) symmetrical structure that have different terminal electron-donating groups were synthesized through multi-step condensation reaction. Also, two compounds were employed as the organic layer to fabricate the sandwiched memory devices with ITO/organic layer/Al structures by vacuum thermal evaporate deposition. This work is focused on changing the electron-donationg groups to tailoring the interaction between adjacent molecules and molecular accumulation in thin film, thus enhancing the intermolecular charge transfer induced by electric field and leading to a multi-level storage performance. OZO-SO with methoxy group as the terminal electron donor showed an amorphous aggregation and no intermolecular charge transfer(ICT) because of the weak electron-donating ability of methoxy group and weak intermolecular interaction, only a binary write-once-read-many times(WORM) memory performance was observed, the threshold voltage is-3V, and the ON/OFF current ratio is 103.93/1, while OZA-SO containing the diethylamino group with strong electron-donating ability as the terminal electron donor exhibited ordered accumulation, leading to strong intermolecular interaction and ICT effect, a ternary WORM-type memory behavior with a stable ON1 state was observed. There are two threshold voltages of the OZA-SO-based device, the first one is-1 V and the second one is-3.5V. In addition, the ON2/ON1/OFF current ratio is 106.20/103. 38/1. Both of the devices based on two molecules have good thermal stability, they could still maintain their WORM storage performance even after annealing at 50°C or 80°C for 12 hours, also their memory behaviors do not even change when heated by a flat heater at 50°C or 80°C. Additionally, the memory behaviors of the devices fabricated by two compounds with different thickness of the organic layer were studied and the similar results came off. All the above results not only excluded filaments penetration but also depicted the stability of the memory devices. What is more, by means of density functional theory(DFT) calculations of the qualitative charge distribution, the direct evidence of conductance state transition under electric field had been provided. For OZO-SO, there was no change to the qualitative charge distribution unless a high bias voltage was applied, and changes in the current occurred only when a high bias voltage was applied, exhibiting binary memory behavior. For OZA-SO, the qualitative charge distribution changed regardless of the magnitude of bias voltage applied and the two varieties of the current were observed, leading to a ternary memory behavior.(2) Effect of the different terminal functional groups that have discrepant steric hindrance of the asymmetric organic small molecules with oxadiazole conjugate skeleton on their multi-level electronic memory performance: Three asymmetric donor-acceptor1-acceptor2-R(D-A1-A2-R) compounds with different R groups that have various steric hindrance were synthesized, and the sandwiched memory devices with ITO/organic layer/Al structure based on these compounds were fabricated by spin-coating. In addition, without changing the electron-donating and electron-withdrawing groups, only the R group in the end position were selected to tune the molecular planarity, which plays a siginificant role in their tendency to π-stack. DEAOx TB(R = tert-butylbenzene) has a large steric hindrance, also, DEAOx DB(R = diphenyl) has a non-planar structure with its two phenyl groups twisting into a dihedral angle of 36°, thereby decreasing the π-conjugated area and impeding close packing between adjacent molecules. When introducing planar naphthalene as the R group, the planarity of DEAOxNa can be significantly improved. DEAOxTB with the maximal steric hindrance stacked disordered in the solid film state, and the DEAOxTB-based device exhibited a nonvolatile binary memory behavior, the threshold voltage is-3V and the ON/OFF current ratio is 104.46/1. DEAOxDB and DEAOx Na with smaller steric hindrance packed ordered in the solid film state, and both of the memory devices based on DEAOxDB and DEAOxNa exhibited nonvolatile ternary memory behavior, the threshold voltage are-2V,-4V for DEAOxDB-based device and-1.5V,-3.5V for DEAOxNa-based device. The ON2/ON1/OFF current ratio for DEAOx DB and DEAOx Na is 107.58/103.80/1 and 107.25/104.06/1, respectively. With disparate compliance currents, there comes out the same results: the molecular accumulation of DEAOxNa with the minimum steric hindrance is better than DEAOxDB and hence the repeatability and threshold voltages of the ternary memory behavior for DEAOxNa-based device are improved. The smaller steric hindrance of the compound, the better orderliness of the molecular arrangement in the solid film state, furthermore the greater enhancement of its multi-level storage performance.(3) Effect of different temperature on the cis-trans isomerism of the introduced azo group and the multi-level electronic memory performance for the symmetric organic small molecules with oxadiazole conjugate skeleton: A V-shaped compound SOABN with D-A1-A2-A1-D symmetric structure containing elctron donors and acceptors at both ends of azo group were synthesized, and the sandwiched memory device with ITO/SOABN/Al structure was fabricated by spin-coating. Based on the conformation change of azo group from cis- to trans- form under the condition of heating, the optical and electronic properties of SOABN at different temperature had been studied. SOABN is in the cis conformation at room temperature or annealing at 80 °C for 12 hours, exhibiting a ternary WORM-type memory performance, the first and second turn-on voltage are-1V,-2.5V for device at room temperature(ON2/ON1/OFF current ratio is 105.53/ 102.03/1) and-1V,-2V for device after annealing at 80 °C for 12 hours(ON2/ON1/OFF current ratio is 106.82/102.97/1). However, when annealing at a temperature higher than the glass transition temperature(Tg) such as 200 °C for 12 hours, the cis isomers converted to the trans state, exhibiting a binary WORM-type storage performance, the threshold voltage is-2.5V, ON/OFF current ratio is 105.68/1. In addition, relying on the study that the cis isomers of azo group at ambient conditions will reconvert to the more stable trans form with lower energy slowly, 90 days after the anneal treatment of the freshly prepared devices at different temperature, their memory behaviors were characterized again, and the results are similar to the freshly prepared device with an anneal treatment at 200 °C. Through tailoring the annealing temperature, the cis-trans isomerism and storage performance of individual small organic molecule have been tuned successfully.