Su-Ting Han

Towards the Development of Flexible Non-Volatile Memories

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.

Layer-by-Layer-Assembled Reduced Graphene Oxide/Gold Nanoparticle Hybrid Double-Floating-Gate Structure for Low-Voltage Flexible Flash Memory

Among the many possible device confi gurations of fl ash memory, fi eld-effect-transistor (FET)-based memory with a fl oating-gate architecture is considered to be a promising candidate towards the ultimate goal of fl ash memory due to its single-transistor realization, non-destructive read-out, and compatibility with complementary metal oxide–semiconductor (CMOS) devices. [ 1–5 ] Floating-gate FET memory with metal nanoparticles (NPs) embedded in the gate dielectric is a way to replace planar fl oating gates to meet the requirements of fast data access and high density for the next generation of fl ash memory. [ 6–12 ] However, poor charge-retention time induced by the thin tunneling dielectric layer is a drawback for NP fl oatinggate-memory devices. [ 13 ] Simply increasing the thickness of the tunneling dielectric layer would degrade the program/ erase speed and increase the power consumption. [ 12 ] On this regard, as an alternative approach, the double NP fl oating-gate structure has been proposed to achieve better retention properties. [ 13–15 ] In previous reports, double-NP fl oating gates have generally been made with the same materials. The retention time could be improved by preventing the trapped charge carriers leaking back to the channel through the energy barrier arising between the upper and lower fl oating gates. Nevertheless, most double-NP fl oating gates consisting of two layers of NPs fabricated by chemical synthesis or thermal evaporation could not form NP pairs in the vertical direction. Additionally, the double-NP fl oating-gate structure may result in a poor interface between the NPs and the dielectric layer, which would have an adverse impact on the overall device performance. Therefore, developing an appropriate upper-fl oating-gate material with the following properties is necessary for technological applications: i) a suitable work function to set up an energy barrier so a long retention time is obtained; ii) a large area to achieve an accurate spatial distribution on the lower fl oating gate; and iii) a fl attened surface to improve the interface quality between the double fl oating gate and the dielectric layer. As one of the thinnest materials ever known in the universe, graphene

Microcontact printing of ultrahigh density gold nanoparticle monolayer for flexible flash memories.

A uniform monolayer of alkanethiol-protected gold nanoparticle arrays with ultrahigh density have been used as microcontact-printable charge-trapping layers for the application in flexible flash memories. The new devices are compared to two reference devices with a floating gate created by thermal evaporation and electrostatic self-assembly, and show a large memory window, long retention times and good endurance properties.

An upconverted photonic nonvolatile memory

Conventional flash memory devices are voltage driven and found to be unsafe for confidential data storage. To ensure the security of the stored data, there is a strong demand for developing novel nonvolatile memory technology for data encryption. Here we show a photonic flash memory device, based on upconversion nanocrystals, which is light driven with a particular narrow width of wavelength in addition to voltage bias. With the help of near-infrared light, we successfully manipulate the multilevel data storage of the flash memory device. These upconverted photonic flash memory devices exhibit high ON/OFF ratio, long retention time and excellent rewritable characteristics.

Nanoparticle size dependent threshold voltage shifts in organic memory transistors

The performance of organic field-effect transistor (OFET) memory devices with different size of gold nanoparticles (Au NPs) as charge trapping layers has been investigated. We synthesized 15 nm, 20 nm and 25 nm of Au NPs through a citrate-reduction method and 3-aminopropyltriethoxysilane (APTES) functionalized substrates were used to form a monolayer of Au NPs. In the programming/erasing operation, we observed reversible threshold voltage (Vth) shifts and reliable memory performances. A strong size-dependent effect on Vth shifts and memory effect was observed. Effect of size dependence on the mobilities (μ), on/off current ratios, subthreshold swings (S), data retention characteristics (>105 s) and endurance performances operation (>800 cycles) of memory devices are discussed. The experimental results suggest a guideline for optimizing the size and density of Au NPs and their influence on the device properties.

Low voltage flexible nonvolatile memory with gold nanoparticles embedded in poly(methyl methacrylate)

We demonstrate air-stable low voltage flexible nonvolatile memory transistors by embedding gold nanoparticles (Au NPs) in poly(methyl methacrylate) (PMMA) as the charge storage element. The solution processability of the nanocomposite is suitable for low-cost large area processing on flexible substrates. The memory transistor exhibits a memory window of 2.1 V, long retention time ( > 10(5) s) with low operating voltage (≤5 V). The memory behavior has been tuned via varying the composition of the fillers (Au NPs), which offers relatively easy processability for different flexible electronics applications. The electrical properties of the memory devices are found to be stable under bending. These findings will be of value for low cost and low voltage advanced flexible electronics.

Silver nanosheet-coated inverse opal film as a highly active and uniform SERS substrate

The availability of well-controlled and reproducible substrates is critically important for surface-enhanced Raman spectroscopy (SERS)-based applications, but it remains a challenge at present. Herein, we report a facile strategy to prepare a new kind of SERS-active substrate, i.e., a two dimensional (2D) macroporous Ag film composed of a silver nanosheet (AgNS)-coated inverse opal film. The prepared substrate features good SERS reproducibility with a high enhancement factor (6 × 107), enabling the ultra-sensitive detection of 10 fM rhodamine 6G (R6G). Moreover, the resultant substrate can be applied in the label-free detection of DNA with a sensitivity limit as low as 5 nM. Consequently, as a high-performance SERS-active substrate, the 2D AgNS-coated inverse opal film is promising for a myriad of chemical and biochemical sensing applications.

An Overview of the Development of Flexible Sensors.

Flexible sensors that efficiently detect various stimuli relevant to specific environmental or biological species have been extensively studied due to their great potential for the Internet of Things and wearable electronics applications. The application of flexible and stretchable electronics to device-engineering technologies has enabled the fabrication of slender, lightweight, stretchable, and foldable sensors. Here, recent studies on flexible sensors for biological analytes, ions, light, and pH are outlined. In addition, contemporary studies on device structure, materials, and fabrication methods for flexible sensors are discussed, and a market overview is provided. The conclusion presents challenges and perspectives in this field.

Controlled ambipolar charge transport through a self-assembled gold nanoparticle monolayer.

An active mechanism for controlling ambipolar charge transport is developed based on self-assembled monolayers of gold nanoparticles. Electron and hole currents are manipulated by controlling the gate bias in order to overcome the intrinsic material limitations. The endurance and retention measurements confirm that this method exhibits good electrical reliability and stability. This solution process approach has potential for applications in large-area printed electronic devices.

Energy-band engineering for tunable memory characteristics through controlled doping of reduced graphene oxide.

Tunable memory characteristics are used in multioperational mode circuits where memory cells with various functionalities are needed in one combined device. It is always a challenge to obtain control over threshold voltage for multimode operation. On this regard, we use a strategy of shifting the work function of reduced graphene oxide (rGO) in a controlled manner through doping gold chloride (AuCl3) and obtained a gradient increase of rGO work function. By inserting doped rGO as floating gate, a controlled threshold voltage (Vth) shift has been achieved in both p- and n-type low voltage flexible memory devices with large memory window (up to 4 times for p-type and 8 times for n-type memory devices) in comparison with pristine rGO floating gate memory devices. By proper energy band engineering, we demonstrated a flexible floating gate memory device with larger memory window and controlled threshold voltage shifts.