Qijian Zhang

Effects of terminal electron acceptor strength on film morphology and ternary memory performance of triphenylamine donor based devices

This study reports the syntheses, photophysical and electrochemical properties and memory characteristics of triphenylamine (TPA) donor based molecules with progressively weaker terminal acceptor strength (i.e., nitro, acetyl and bromine). The influence of the terminal electron acceptor strength on the film morphology and the devices storage performances was investigated. Nonvolatile ternary (“0”, “1” and “2” states) memory devices for high-density data storage could be achieved with a simple ITO/D–A molecule/Al sandwich configuration for TPA-NAP and TPA-AAP. It is noteworthy that the memory device based on TPA-AAP exhibited a better reproducibility and stability with lower operation voltages than that based on TPA-NAP, promising low-power consumption data-storage. These obtained results demonstrate that altering the terminal electron accepting strength in D–A molecules can adjust the film morphology and the device performances for the design of future advanced organic electronic devices.

Improved ternary memory performance of donor–acceptor structured molecules through cyano substitution

Organic memory devices can greatly increase the data-storage density and have attracted significant attention in recent years. Thus, two small molecules, DPHCANA and DPCNCANA, were designed and successfully synthesized to investigate the improvement of memory devices through introducing the strong electron withdrawing cyano group on the central phenyl ring. It is noteworthy that DPCNCANA with a cyano group exhibited excellent ternary memory behavior, benefiting from induced intermolecular H-bond connection and layer by layer molecular stacking in the film state, which counteracted the higher hole transport barrier lying between the film and the bottom electrode. In addition, we have also found that the additional cyano group substitution lowered the LUMO energy level, which is favorable for the stability in air ambient circumstances. We envisage that this study will be very useful for the rational design of advanced next-generation semiconductor materials.

Adjustment of ON-State Retention Ability Based on New Donor–Acceptor Imides through Structural Tailoring for Volatile Device Applications

In this study, two D-A molecules NACANA and CANACA, based on carbazole (CA) donor and naphthalimide (NA) acceptor, with different D-A arrangement (A-D-A and D-A-D) were synthesized. The photophysical and electrochemical properties, microstructure and memory behaviors of both A-D-A and D-A-D molecules were systematically investigated. The fabricated devices ITO/NACANA or CANACA layer/Al with a simple sandwich configuration both exhibited volatile nature after shutting off the external electric field. Interestingly, NACANA showed ON-state retention time of ca. 12 min, longer than that of CANACA (ca. 6 min). The difference in retention ability of the programmed states could be assigned to the difference of the D-A arrangement. This type of retention ability adjustment by varying the arrangement of donor and acceptor segments may provide a guide of structure design for future organic-based specific memory devices with tunable volatile property.

Effects of aromatic spacers on film morphology and device memory performance based on imidazole–π–triphenylamine derivatives

Two imidazole–π–triphenylamine derivatives TPAPPI and TPATPI, connected via different aromatic spacers (i.e., phenyl or thienyl), were synthesized. The photophysical and electrochemical properties, and memory behaviors of the two donor–π–acceptor molecules were comparatively investigated. The replacement of phenyl with thienyl leads to a much better nanoscale morphology after thermal treatment, as characterized by atomic force microscopy (AFM). Sandwich devices based on TPAPPI and TPATPI both exhibited the nonvolatile WORM characteristic but the TPATPI-based device showed a higher ON/OFF ratio and a lower switching voltage. Simulation results showed that the insertion of the thienyl spacer between the donor and acceptor moieties leads to smaller torsion between the imidazole ring and TPA moiety, which indicates a smaller charge transfer barrier and a higher extent of charge transfer (CT). This comparative study of tuning the properties of conjugated D–π–A molecules via aromatic π-spacers may be an alternative approach for the design and study of future high-performance memory devices based on new D–π–A type materials.

Ternary Flexible Electro-resistive Memory Device based on Small Molecules.

Flexible memory devices have continued to attract more attention due to the increasing requirement for miniaturization, flexibility, and portability for further electronic applications. However, all reported flexible memory devices have binary memory characteristics, which cannot meet the demand of ever-growing information explosion. Organic resistive switching random access memory (RRAM) has plenty of advantages such as simple structure, facile processing, low power consumption, high packaging density, as well as the ability to store multiple states per bit (multilevel). In this study, we report a small molecule-based flexible ternary memory device for the first time. The flexible device maintains its ternary memory behavior under different bending conditions and within 500 bending cycles. The length of the alkyl chains in the molecular backbone play a significant role in molecular stacking, thus guaranteeing satisfactory memory and mechanical properties.

A novel ternary memory property achieved through rational introduction of end-capping naphthalimide acceptors

Small molecule-based multilevel rewritable memory devices have recently gained extensive attention because they possess super-high storage density and can sustain the stored data without power supply and erase and rewrite electrically; however, small molecule-based multilevel flash-type memory device is extremely challenging to achieve. Herein, we designed a symmetric molecule with end-capping naphthalimide acceptors through rational tuning. This molecule showed an improved crystal size and uniform crystal orientation in the film state. The sandwich-structured device exhibited the typical WORM (write-once–read-many times) memory property from OFF to ON1 transition and encouraging flash memory behavior for the ON1/ON2 transition. This is the first report on small molecule-based ternary memory devices with rewritable memory behavior, and this study will inspire the exploration of multilevel data-storage devices with fully rewritable properties in the subsequent researches.

Effects of Single Atom N-Substitution in the Molecular Skeleton on Fabricated Film Quality and Memory Device Performance

In this paper, two conjugated organic small molecules, NITP and NITZ, were designed and synthesized. NITZ is just altered by single atom N-substitution from the NITP thiophene group to the thiazole group in the heterocyclic aryl ring of the skeleton. The morphology and intermolecular stacking style of the fabricated nanofilms were analyzed by atomic force microscopy (AFM), grazing-incidence small-angle X-ray scattering (GISAXS), and X-ray diffraction (XRD). It demonstrates that both molecules prefer highly ordered packing after annealing, and the crystallite orientation is even more superior by N-substitution. As a result, both memory devices exhibit ternary memory performance. This study highlights that the thiazole group also constitutes a good candidate for a π-bridge of organic semiconducting molecules, and it really provides a simple and refined strategy for designing multilevel memory materials by single atom substitution.

The Effect of Random and Block Copolymerization with Pendent Carbozole Donors and Naphthalimide Acceptors on Multilevel Memory Performance

Polymeric materials have been widely used in the fabrication of data-storage devices, owing to their unique advantages and defined conduction mechanisms. To date, the most-functional polymers that have been reported for memory devices were synthesized through random copolymerization, whilst there have been no reports regarding the memory effect of block polymers. Herein, we synthesized a random copolymer (PMCz8 -co-PMBNa2 ) and its corresponding block copolymer (PMCz8 -b-PMBNa2 ) to study the effect of the method of polymerization on the memory properties of the corresponding devices. Interestingly, both devices (ITO/PMCz8 -co-PMBNa2 /Al and ITO/PMCz8 -b-PMBNa2 /Al) exhibited ternary memory performance, with threshold voltages of -1.7 V/-3.3 V and -2.7 V/-3.8 V, respectively. However, based on comprehensive measurements, the memory properties of PMCz8 -co-PMBNa2 and PMCz8 -b-PMBNa2 were found to be owing to the operation of different conduction mechanisms, which resulted from different molecular stacking in the film state. Therefore, we expect that this work will be helpful for improving our understanding of the conduction mechanisms in polymer-based data-storage devices.