Attachable and flexible aluminum oxide resistive non-volatile memory arrays fabricated on tape as the substrate.

We fabricated 8 × 8 arrays of non-volatile resistive memory devices on commercially available Scotch® Magic™ tape as a flexible substrate. The memory devices consist of double active layers of Al2O3 with a structure of Au/Al2O3/Au/Al2O3/Al (50 nm/20 nm/20 nm/20 nm/50 nm) on attachable tape substrates. Because the memory devices were fabricated using only dry and low temperature processes, the tape substrate did not suffer from any physical or chemical damage during the fabrication. The fabricated memory devices were turned to the low resistance state at ∼3.5 V and turned to the high resistance state at ∼10 V with a negative differential resistance region after ∼5 V, showing typical unipolar non-volatile resistive memory behavior. The memory devices on the tape substrates exhibited reasonable electrical performances including a high ON/OFF ratio of 104, endurance over 200 cycles of reading/writing processes, and retention times of over 104 s in both the flat and bent configurations.

Analysis of the interface characteristics of CVD-grown monolayer MoS2 by noise measurements.

We investigated the current-voltage and noise characteristics of two-dimensional (2D) monolayer molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD). A large number of trap states were produced during the CVD process of synthesizing MoS2, resulting in a disordered monolayer MoS2 system. The interface trap density between CVD-grown MoS2 and silicon dioxide was extracted from the McWhorter surface noise model. Notably, generation-recombination noise which is attributed to charge trap states was observed at the low carrier density regime. The relation between the temperature and resistance following the power law of a 2D inverted-random void model supports the idea that disordered CVD-grown monolayer MoS2 can be analyzed using a percolation theory. This study can offer a viewpoint to interpret synthesized low-dimensional materials as highly disordered systems.

Analysis of noise generation and electric conduction at grain boundaries in CVD-grown MoS2 field effect transistors.

Grain boundaries in a chemical vapour deposition (CVD)-grown monolayer of MoS2 induce significant effects on the electrical and low frequency noise characteristics of the MoS2. Here, we investigated the electrical properties and noise characteristics of MoS2 field effect transistors (FETs) made with CVD-grown monolayer MoS2. The electrical and noise characteristics of MoS2 FETs were analysed and compared for the MoS2 channel layers with and without grain boundaries. The grain boundary in the CVD-grown MoS2 FETs can be the dominant noise source, and dependence of the extracted Hooge parameters on the gate voltage indicated the domination of the correlated number-mobility fluctuation at the grain boundaries. The percolative noise characteristics of the single grain regions of MoS2 were concealed by the noise generated at the grain boundary. This study can enhance understanding of the electrical transport hindrance and significant noise generation by trapped charges at grain boundaries of the CVD-grown MoS2 devices.

Highly uniform monolayer graphene synthesis via a facile pretreatment of copper catalyst substrates using an ammonium persulfate solution.

The demand for large-area, high-quality synthesis of graphene with chemical vapor deposition (CVD) has increased for the realization of next-generation transparent and flexible optoelectronic applications. In conventional CVD processes, various synthesis parameters can strongly affect the quality of the resultant graphene. In particular, surface engineering of a copper catalyst substrate is one of the most promising pathways for achieving high-quality graphene with excellent reproducibility. For this purpose, simple wet chemical etching of a catalyst substrate without toxic fume byproducts or metal ion residues is desired. Here, we suggest a facile method for preparing a pretreated copper catalyst substrate for highly uniform, large-area CVD graphene growth. This pretreatment method involves a wet copper etchant, ammonium persulfate (APS) solution, and gentle ultrasonication (100 W), which do not produce unwanted or toxic fume byproducts during their reaction. Moreover, this approach does not leave metal ion residue on the cleaned copper substrates that serves as residual nucleation sites and leads to multilayer graphene growth. To evaluate the quality of the synthesized monolayer graphene on the cleaned copper catalyst substrates, we used various characterization techniques, such as Raman spectroscopy and sheet resistance, optical transmittance, and FET characterization.

High-Performance Solution-Processed Organo-Metal Halide Perovskite Unipolar Resistive Memory Devices in a Cross-Bar Array Structure.

Resistive random access memories can potentially open a niche area in memory technology applications by combining the advantages of the long endurance of dynamic random-access memory and the long retention time of flash memories. Recently, resistive memory devices based on organo-metal halide perovskite materials have demonstrated outstanding memory properties, such as a low-voltage operation and a high ON/OFF ratio; such properties are essential requirements for low power consumption in developing practical memory devices. In this study, a nonhalide lead source is employed to deposit perovskite films via a simple single-step spin-coating method for fabricating unipolar resistive memory devices in a cross-bar array architecture. These unipolar perovskite memory devices achieve a high ON/OFF ratio up to 108 with a relatively low operation voltage, a large endurance, and long retention times. The high-yield device fabrication based on the solution-process demonstrated here will be a step toward achieving low-cost and high-density practical perovskite memory devices.

Enhanced Charge Injection Properties of Organic Field-Effect Transistor by Molecular Implantation Doping.

Organic semiconductors (OSCs) have been widely studied due to their merits such as mechanical flexibility, solution processability, and large-area fabrication. However, OSC devices still have to overcome contact resistance issues for better performances. Because of the Schottky contact at the metal-OSC interfaces, a non-ideal transfer curve feature often appears in the low-drain voltage region. To improve the contact properties of OSCs, there have been several methods reported, including interface treatment by self-assembled monolayers and introducing charge injection layers. Here, a selective contact doping of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 -TCNQ) by solid-state diffusion in poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) to enhance carrier injection in bottom-gate PBTTT organic field-effect transistors (OFETs) is demonstrated. Furthermore, the effect of post-doping treatment on diffusion of F4 -TCNQ molecules in order to improve the device stability is investigated. In addition, the application of the doping technique to the low-voltage operation of PBTTT OFETs with high-k gate dielectrics demonstrated a potential for designing scalable and low-power organic devices by utilizing doping of conjugated polymers.

Highly Reliable Superhydrophobic Protection for Organic Field-Effect Transistors by Fluoroalkylsilane-Coated TiO2 Nanoparticles.

One of the long-standing problems in the field of organic electronics is their instability in an open environment, especially their poor water resistance. For the reliable operation of organic devices, introducing an effective protection layer using organo-compatible materials and processes is highly desirable. Here, we report a facile method for the depositing of an organo-compatible superhydrophobic protection layer on organic semiconductors under ambient conditions. The protection layer exhibiting excellent water-repellent and self-cleaning properties was deposited onto organic semiconductors directly using a dip-coating process in a highly fluorinated solution with fluoroalkylsilane-coated titanium dioxide (TiO2) nanoparticles. The proposed protection layer did not damage the underlying organic semiconductors and had good resistance against mechanical-, thermal-, light-stress-, and water-based threats. The protected organic field-effect transistors exhibited more-reliable electrical properties, even when exposed to strong solvents, due to its superhydrophobicity. This study provides a practical solution with which to enhance the reliability of environmentally sensitive organic semiconductor devices in the natural environment.

Electrical Properties of Synthesized Large-Area MoS2 Field-Effect Transistors Fabricated with Inkjet-Printed Contacts.

We report the electrical properties of synthesized large-area monolayer molybdenum disulfide (MoS2) field-effect transistors (FETs) with low-cost inkjet-printed Ag electrodes. The monolayer MoS2 film was grown by a chemical vapor deposition (CVD) method, and the top-contact Ag source/drain electrodes (S/D) were deposited onto the films using a low-cost drop-on-demand inkjet-printing process without any masks and surface treatments. The electrical characteristics of FETs were comparable to those fabricated by conventional deposition methods such as photo- or electron beam lithography. The contact properties between the S/D and the semiconductor layer were also evaluated using the Y-function method and an analysis of the output characteristic at the low drain voltage regimes. Furthermore, the electrical instability under positive gate-bias stress was studied to investigate the charge-trapping mechanism of the FETs. CVD-grown large-area monolayer MoS2 FETs with inkjet-printed contacts may represent an attractive approach for realizing large-area and low-cost thin-film electronics.

Origin of multi-level switching and telegraphic noise in organic nanocomposite memory devices.

The origin of negative differential resistance (NDR) and its derivative intermediate resistive states (IRSs) of nanocomposite memory systems have not been clearly analyzed for the past decade. To address this issue, we investigate the current fluctuations of organic nanocomposite memory devices with NDR and the IRSs under various temperature conditions. The 1/f noise scaling behaviors at various temperature conditions in the IRSs and telegraphic noise in NDR indicate the localized current pathways in the organic nanocomposite layers for each IRS. The clearly observed telegraphic noise with a long characteristic time in NDR at low temperature indicates that the localized current pathways for the IRSs are attributed to trapping/de-trapping at the deep trap levels in NDR. This study will be useful for the development and tuning of multi-bit storable organic nanocomposite memory device systems.

Trap-mediated electronic transport properties of gate-tunable pentacene/MoS2 p-n heterojunction diodes.

We investigated the trap-mediated electronic transport properties of pentacene/molybdenum disulphide (MoS2) p-n heterojunction devices. We observed that the hybrid p-n heterojunctions were gate-tunable and were strongly affected by trap-assisted tunnelling through the van der Waals gap at the heterojunction interfaces between MoS2 and pentacene. The pentacene/MoS2 p-n heterojunction diodes had gate-tunable high ideality factor, which resulted from trap-mediated conduction nature of devices. From the temperature-variable current-voltage measurement, a space-charge-limited conduction and a variable range hopping conduction at a low temperature were suggested as the gate-tunable charge transport characteristics of these hybrid p-n heterojunctions. Our study provides a better understanding of the trap-mediated electronic transport properties in organic/2-dimensional material hybrid heterojunction devices.

Integration of Flexible and Microscale Organic Nonvolatile Resistive Memory Devices Using Orthogonal Photolithography.

We present the integration of flexible and microscale organic nonvolatile resistive memory devices fabricated in a cross-bar array structure on plastic substrates. This microscale integration was made via orthogonal photolithography method using fluorinated photoresist and solvents and was achieved without causing damage to the underlying organic memory materials. Our flexible microscale organic devices exhibited high ON/OFF ratio (I(ON/I(OFF) > 10(4)) under bending conditions. In addition, the ON and OFF states of our flexible and microscale memory devices were maintained for 10,000 seconds without any serious degradation.

Enhancement of photodetection characteristics of MoS2 field effect transistors using surface treatment with copper phthalocyanine.

Recently, two-dimensional materials such as molybdenum disulfide (MoS2) have been extensively studied as channel materials for field effect transistors (FETs) because MoS2 has outstanding electrical properties such as a low subthreshold swing value, a high on/off ratio, and good carrier mobility. In this study, we characterized the electrical and photo-responsive properties of MoS2 FET when stacking a p-type organic copper phthalocyanine (CuPc) layer on the MoS2 surface. We observed that the threshold voltage of MoS2 FET could be controlled by stacking the CuPc layers due to a charge transfer phenomenon at the interface. Particularly, we demonstrated that CuPc/MoS2 hybrid devices exhibited high performance as a photodetector compared with the pristine MoS2 FETs, caused by more electron-hole pairs separation at the p-n interface. Furthermore, we found the optimized CuPc thickness (∼2 nm) on the MoS2 surface for the best performance as a photodetector with a photoresponsivity of ∼1.98 A W(-1), a detectivity of ∼6.11 × 10(10) Jones, and an external quantum efficiency of ∼12.57%. Our study suggests that the MoS2 vertical hybrid structure with organic material can be promising as efficient photodetecting devices and optoelectronic circuits.

1/f Noise Scaling Analysis in Unipolar-Type Organic Nanocomposite Resistive Memory.

We studied noise characteristics of a nanocomposite of polyimide (PI) and phenyl-C61-butyric acid methyl ester (PCBM) (denoted as PI:PCBM), a composite for the organic nonvolatile resistive memory material. The current fluctuations were investigated over a bias range that covers various intermediate resistive states and negative differential resistance (NDR) in organic nanocomposite unipolar resistive memory devices. From the analysis of the 1/f(γ) type noises, scaling behavior between the relative noise power spectral density S̃ and resistance R was observed, indicating a percolating behavior. Considering a linear rate equation of the charge trapping-detrapping at traps, the percolation behavior and NDR could be understood by the modulation of the conductive phase fraction φ with an external bias. This study can enhance the understanding of the NDR phenomena in organic nanocomposite unipolar resistive memory devices in terms of the current path formation and the memory switching.

Irradiation effects of high-energy proton beams on MoS2 field effect transistors.

We investigated the effect of irradiation on molybdenum disulfide (MoS2) field effect transistors with 10 MeV high-energy proton beams. The electrical characteristics of the devices were measured before and after proton irradiation with fluence conditions of 10(12), 10(13), and 10(14) cm(-2). For a low proton beam fluence condition of 10(12) cm(-2), the electrical properties of the devices were nearly unchanged in response to proton irradiation. In contrast, for proton beam fluence conditions of 10(13) or 10(14) cm(-2), the current level and conductance of the devices significantly decreased following proton irradiation. The electrical changes originated from proton-irradiation-induced traps, including positive oxide-charge traps in the SiO2 layer and trap states at the interface between the MoS2 channel and the SiO2 layer. Our study will enhance the understanding of the influence of high-energy particles on MoS2-based nanoelectronic devices.