Organic molecules and semiconductors have been proposed as active part of a large variety of nonvolatile memory devices, including resistors, diodes and transistors. In this review, we focus on electrically reprogrammable nonvolatile memories. We classify several possible devices according to their operation principle and critically review the role of the π-conjugated materials in the device operation. We propose specifications for applications for organic nonvolatile memory and review the state of the art with respect to these target specifications. Conclusions are drawn regarding further work on materials and device architectures.

Polymer and Organic Nonvolatile Memory Devices

Flexible non-volatile memories have attracted tremendous attentions for data storage for future electronics application. From device perspective, the advantages of flexible memory devices include thin, lightweight, printable, foldable and stretchable. The flash memories, resistive random access memories (RRAM) and ferroelectric random access memory/ferroelectric field-effect transistor memories (FeRAM/FeFET) are considered as promising candidates for next generation non-volatile memory device. Here, we review the general background knowledge on device structure, working principle, materials, challenges and recent progress with the emphasis on the flexibility of above three categories of non-volatile memories.

Towards the development of flexible non-volatile memories.

The flexible nonvolatile organic memory devices were developed on the plastic substrates based on the organic thin-film transistors embedding self-assembled gold nanoparticles (Au(NP)). The organic memory devices exhibited good programmable memory characteristics with respect to the program/erase operations, resulting in controllable and reliable threshold voltage shifts. Additionally, the endurance, data retention, and bending cyclic measurements confirmed that the flexible memory devices exhibited good electrical reliability as well as mechanical stability. The memory devices were composed of the solution-processed organic dielectric layers/metallic nanoparticles and the low-temperature processed organic transistors. Therefore, this approach could potentially be applied to advanced flexible/plastic electronic devices as well as integrated organic device circuits.

Flexible organic transistor memory devices.

Harnessing the energy from hot charge carriers is an emerging research area with the potential to improve energy conversion technologies.1−3 Here we present a novel plasmonic photoelectrode architecture carefully designed to drive photocatalytic reactions by efficient, nonradiative plasmon decay into hot carriers. In contrast to past work, our architecture does not utilize a Schottky junction, the commonly used building block to collect hot carriers. Instead, we observed large photocurrents from a Schottky-free junction due to direct hot electron injection from plasmonic gold nanoparticles into the reactant species upon plasmon decay. The key ingredients of our approach are (i) an architecture for increased light absorption inspired by optical impedance matching concepts,4 (ii) carrier separation by a selective transport layer, and (iii) efficient hot-carrier generation and injection from small plasmonic Au nanoparticles to adsorbed water molecules. We also investigated the quantum efficiency of hot elect...

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Direct Plasmon-Driven Photoelectrocatalysis

DOI: 10.1002/adma.201201195 Flash memories have recently attracted considerable attention as promising next-generation nonvolatile memories owing to their higher chip density, multi-bit per cell storage properties, and compatibility with the current complementary metal– oxide–semiconductor (CMOS) process. [ 1–4 ] The fl ash memories based on fi eld-effect transistors (FETs) with a fl oating gate architecture work on the variation of the threshold voltage ( V th ) by trapping/detrapping the charge carriers of the semiconductor under external gate bias. To meet the requirements of manipulating the trap levels (work functions of metal) and trap sites (directly related to memory window which can be controlled by varying the density of nanoparticles), employment of nanocrystals fl oating gate is considered to be an alternative approach to replace the planar fl oating gate for information storage. [ 5 , 6 ] Gold nanoparticles (Au NPs) are appropriate material to serve as fl gate due to their chemical stability and high work function. [ 7 ] The Au NPs charge trapping layer can be created by several methods including thermal evaporation, [ 8 , 9 ] chemical process such as electrostatic self-assembly

Microcontact Printing of Ultrahigh Density Gold Nanoparticle Monolayer for Flexible Flash Memories

Controlled gold nanoparticle (AuNP)-based nonvolatile memory devices were developed based on pentacene organic transistors and polymethylmethacrylate (PMMA) insulator layers. The memory device had the following configuration: n+Si gate/SiO2 blocking oxide/polyelectrolytes/AuNP/PMMA tunneling dielectric layer/Au source-drain. According to the programming/erasing operations, the memory device showed good programmable memory characteristics with a large memory window. In addition, good reliability was confirmed by the data retention characteristics. The fabrication procedures for the charge trapping and tunneling layers were based on simple solution processes (by dipping and spin-coating) and the maximum processing temperature was <100 °C, so this method has potential applications in plastic/flexible electronics.

Nonvolatile nano-floating gate memory devices based on pentacene semiconductors and organic tunneling insulator layers

Organic nanofloating gate memory devices were developed based on ink-jet printed 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene thin-film transistors (TFTs) embedding gold nanoparticles. The programming/erasing operations showed that the organic memory devices exhibited good programmable memory characteristics that resulted in a gate-voltage controlled reliable threshold voltage shift of the programmed/erased states. The data retention and endurance measurements also showed the reliable nonvolatile memory properties. Solution processes were used for synthesis of the charge trapping elements and TIPS-pentacene TFTs were made by the ink-jet printing technique at low temperatures. Therefore, these processes can readily be adopted in all-printed organic memory devices on flexible substrates.

High-performance organic charge trap flash memory devices based on ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene transistors

Nanoscale resistive switching memory cells with controlled cell sizes in the range of 25 to 90 nm were successfully fabricated using anodized aluminum oxide templates, and their electrical properties were directly measured using a conductive atomic force microscope. The size of the memory cells was systematically controlled by controlling the pore size of the nanoscale masks. The devices exhibited controllable and reliable resistive switching characteristics suitable for programmable memory applications. The reported approach provides new opportunities for the preparation of nanostructured nonvolatile memory devices with continued device scaling.

Highly scalable resistive switching memory cells using pore-size-controlled nanoporous alumina templates

Oxygen evolution reaction (OER) plays a key role in many renewable energy technologies such as water splitting and metal-air batteries. Metal-organic frameworks (MOFs) are appealing to design efficient OER electrocatalysts, however, their intrinsic poor conductivity strongly hinders the activity. Here, we show a strategy to boost the OER activity of poor-conductive MOFs by confining them between graphene multilayers. The resultant NiFe-MOF//G gives a record-low overpotential of 106 mV to reach 10 mA cm-2 and retains the activity over 150 h, which is in significant contrast to 399 mV of the pristine NiFe-MOF. We use X-ray absorption spectroscopy (XAS) and computations to demonstrate that the nanoconfinement from graphene multilayers not only forms highly reactive NiO6-FeO5 distorted octahedral species in MOF structure but also lowers limiting potential for water oxidation reaction. We also demonstrate that the strategy is applicable to other MOFs of different structures to largely enhance their electrocatalytic activities. 

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Si-Hoon Lyu

Papers

Nonvolatile nano-floating gate memory devices based on pentacene semiconductors and organic tunneling insulator layers

High-performance organic charge trap flash memory devices based on ink-jet printed 6,13-bis(triisopropylsilylethynyl) pentacene transistors

Highly scalable resistive switching memory cells using pore-size-controlled nanoporous alumina templates

Exceptional catalytic activity of oxygen evolution reaction via two-dimensional graphene multilayer confined metal-organic frameworks

Rational design on photoelectrodes and devices to boost photoelectrochemical performance of solar-driven water splitting: a mini review