Covalent assembly of gold nanoparticles for nonvolatile memory applications

This work reports a versatile approach for enhancing the stability of nonvolatile memory devices through covalent assembly of functionalized gold nanoparticles. 11-mercapto-1-undecanol functionalized gold nanoparticles (AuNPs) with a narrow size distribution and particle size of about 5 nm were synthesized. Then, the AuNPs were immobilized on a SiO(2) substrate using a functionalized polymer as a surface modifier. Microscopic and spectroscopic techniques were used to characterize the AuNPs and their morphology before and after immobilization. Finally, a metal-insulator-semiconductor (MIS) type memory device with such covalently anchored AuNPs as a charge trapping layer was fabricated. The MIS structure showed well-defined counterclockwise C-V hysteresis curves indicating a good memory effect. The flat band voltage shift was 1.64 V at a swapping voltage between ±7 V. Furthermore, the MIS structure showed a good retention characteristic up to 20,000 s. The present synthetic route to covalently immobilize gold nanoparticles system will be a step towards realization for the nanoparticle-based electronic devices and related applications.     

Nanocrystal nonvolatile memory devices

In this paper we present an overview of nanocrystal memories - a nascent nonvolatile memory technology that promises to extend the scaling of more conventional charge storage devices to nanometer-scale dimensions     

Characterization of gold nanoparticle pentacene memory device with polymer dielectric layer

We report on the electrical behavior of gold nanoparticles (Au NPs) intervened metal-pentacene-insulator-semiconductor structures. The structure adopts polyvinyl alcohol (PVA) and pentacene as gate insulator and semiconductor, respectively. On the PVA (250 nm) film which was spin-coated and UV cross-linked, 3-aminopropyl triethoxysilane was functionalized for self assembling of the Au NPs monolayer. The devices exhibited clockwise hysteresis in their capacitance-voltage characteristics, with a memory window depending on the range of the voltage sweep. A relatively large memory window of about 4.7 V, which was deduced from control devices, was achieved with voltage sweep of (−/+)7 V. Formation of the monolayered Au NPs was confirmed by field effect scanning electron microscopy and atomic force microscopy.     

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.     

CMOS compatible nanoscale nonvolatile resistance switching memory

We report studies on a nanoscale resistance switching memory structure based on planar silicon that is fully compatible with CMOS technology in terms of both materials and processing techniques employed. These two-terminal resistance switching devices show excellent scaling potential well beyond 10 Gb/cm2 and exhibit high yield (99%), fast programming speed (5 ns), high on/off ratio (10(3)), long endurance (10(6)), retention time (5 months), and multibit capability. These key performance metrics compare favorably with other emerging nonvolatile memory techniques. Furthermore, both diode-like (rectifying) and resistor-like (nonrectifying) behaviors can be obtained in the device switching characteristics in a controlled fashion. These results suggest that the CMOS compatible, nanoscale Si-based resistance switching devices may be well suited for ultrahigh-density memory applications.     

Preparation of carbon nanotube/chitosan/gold nanoparticle composite microspheres

The electrostatic layer-by-layer (LbL) assembly of acid-modified multi-walled carbon nanotubes (MWNTs) and biopolymer chitosan (CHIT) is realized on planar substrates and polystyrene (PS) microsphere templates, respectively. The successful stepwise growing process of the composite films on planar substrates is investigated and confirmed by scanning electron microscopy and UV-vis spectroscopy. The transfer of the LbL assembly of MWNTs and CHIT to spherical PS microspheres leads to novel (MWNT/CHIT)PS core-shell structure, on which the gold nanoparticles (GNPs) are deposited to fabricate GNP(MWNT/CHIT)PS composite microspheres. The glass carbon electrodes modified with such (MWNT/CHIT)PS or GNP(MWNT/CHIT)PS composites exhibit satisfactory electrocatalytic activities for biomolecule dopamine.     

Polyaniline nanofiber composites with amines: Novel materials for phosgene detection

Several orders of magnitude of change in resistance are observed upon chemical doping and dedoping of the conducting polymer polyaniline. This large conductivity range can be utilized to make sensitive chemical sensors. Polyaniline, in its nanofiber form, has even greater sensing capabilities due to the small fiber diameters, high surface area, and porous nanofiber network that enhances gas diffusion into the fibers. Polyaniline nanofibers have been synthesized using a rapid mixing method and dispersed in water allowing them to be easily modified with water soluble agents, making new composite materials. Polyaniline nanofiber composite materials can be used to enhance detection of analytes that unmodified polyaniline would not otherwise be able to detect. The detection mechanism involves the reaction of an additive with the analyte to generate a strong acid that is easily detected by polyaniline, resulting in orders of magnitude changes in resistance. The reaction of the additive alone with the analyte produces no electrical response, however. In this paper, an array of amine-polyaniline nanofiber composite materials is investigated for the detection of phosgene gas. The influence of environmental conditions such as humidity and temperature are examined and a detection mechanism is presented.      

Fabrication and program/erase characteristics of 30-nm SONOS nonvolatile memory devices

In this paper, we have fabricated nanoscale silicon-oxide-nitride-oxide-silicon (SONOS) nonvolatile memory devices by means of the sidewall patterning technique. The fabricated SONOS devices have a 30-nm-long and 30-nm-wide channel with 2.3/12/4.5-nm-thick oxide/nitride/oxide film on fully depleted-silicon-on-insulator (FD-SOI) substrate. The short channel effect is well suppressed though devices have very short channel length and width. Also, the fabricated SONOS devices guarantee good retention and endurance characteristics. In 30-nm SONOS devices, channel hot electron injection program mechanism is inefficient and 2-b operation based on localized carrier trapping in the nitride film is difficult. The erase speed is improved by means of band-to-band (BTB) assisted hole injection mechanism. In 30-nm SONOS devices, program and erase operation can be performed efficiently with improved erase speed by combination of Fowler-Nordheim (F-N) tunneling program and BTB assisted hole injection erase mechanism because the entire channel region programmed by F-N tunneling can be covered by two-sided hole injection from source and drain.     

pH-Dependent gold nanoparticle self-organization on functionalized Si/SiO2 surfaces

The self-organization of citrate- and acrylate-stabilized gold nanoparticles onto SiO₂/hydroxyl-, amino- and nitro-terminated surfaces was investigated as a function of pH. Bare clean Si/SiO₂ substrates were used as the SiO₂/hydroxyl-terminated surfaces and self-assembled monolayers (SAM) of (3-aminopropyl)trimethoxysilane (APTMS) and 3-(4-nitrophenoxy)-propyltrimethoxysilane (NPPTMS) on Si/SiO₂ were employed as the amino- and nitro-terminated surfaces, respectively. All the surfaces were fully characterized by contact angle, atomic force microscopy (AFM), ellipsometry and X-ray photoelectron spectroscopy (XPS). Citrate- and acrylate-stabilized gold nanoparticle stability was also investigated as a function of pH by UV–visible absorption spectroscopy and Z-potentiometry. The gold nanoparticle surface coverage of the substrates was independently estimated by AFM and XPS. The results show that colloid deposition on bare SiO₂/OH surfaces and on NPPTMS monolayers is negligible with the exception of acrylate-stabilized gold nanoparticles which were found to be immobilized on nitro-terminated surfaces at pH lower than 3.5. Nevertheless, APTMS monolayers interact strongly with citrate- and acrylate-stabilized gold nanoparticles exhibiting a dependence of the surface coverage from the pH of the colloidal solution.     

Concept of nonvolatile memory based on multiwall carbon nanotubes

In this study, a novel concept is proposed for molecular electronics that uses vertically aligned multiwall carbon nanotubes as nonvolatile memory elements. Nanotubes grown on a patterned substrate may be opened by partially deleting the outer molecular layer or layers, so that the inner core is able to move along the vertical tube axis. Mounting another dielectric plate above the nanotube forest at a specific distance from the tube caps can provide two stable van der Waals states of the inner core, providing for nonvolatile data storage. A device built using this architecture can function as a two-dimensional memory array. At each cross point in the array, a multiwall carbon nanotube exists in either the separated off-state or in the contact on-state, and can be switched between these states by applying voltage pulses at the corresponding electrodes. A theoretical memory density as high as 10(13) memory elements per square centimetre is possible, with an operation frequency exceeding 100?GHz. Significant physical characteristics of such a device are bi-stability and reversibility. Such a device can function both as nonvolatile random access memory and as terabit solid-state storage.     

Graphene oxide thin films for flexible nonvolatile memory applications

There has been strong demand for novel nonvolatile memory technology for low-cost, large-area, and low-power flexible electronics applications. Resistive memories based on metal oxide thin films have been extensively studied for application as next-generation nonvolatile memory devices. However, although the metal oxide based resistive memories have several advantages, such as good scalability, low-power consumption, and fast switching speed, their application to large-area flexible substrates has been limited due to their material characteristics and necessity of a high-temperature fabrication process. As a promising nonvolatile memory technology for large-area flexible applications, we present a graphene oxide based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance. The microscopic origin of the bipolar resistive switching behavior was elucidated and is attributed to rupture and formation of conducting filaments at the top amorphous interface layer formed between the graphene oxide film and the top Al metal electrode, via high-resolution transmission electron microscopy and in situ X-ray photoemission spectroscopy. This work provides an important step for developing understanding of the fundamental physics of bipolar resistive switching in graphene oxide films, for the application to future flexible electronics.     

Formation of NiSi2/SiNX compound nanocrystal for nonvolatile memory application

In this paper, the NiSi₂/SiN_X compound NCs (CNCs) structure is studied to further improve the retention. To introduce the nitride based traps, NiSi₂ was also sputtered in the mixture gas of Ar (50 sccm) and NH₃ (10 sccm) at room temperature, and the NiSi₂/SiN_X CNCs can be easily formed after rapid thermal annealing. In addition, standard memory devices with single and double NiSi₂ nanocrystal were also prepared for comparison. By XPS analyses, the nanocrystals fabricated in the ambiance of NH₃ can be confirmed to be composited of NiSi₂ and SiN_X compound. According to memory characteristics results, better retention characteristic of device with single-layer NiSi₂/SiN_X compound nanocrystal NVMs can be observed after 10⁴ s, raises from 50% to 72% in comparison with the control sample, even better than the double-layer NiSi₂ nanocrystal, 58%. Indeed, the formation of NiSi₂/SiN_X CNCs can improve the retention characteristics remarkably due to the additional tunnel barrier and deep traps in the nitride.     

Studies on nonvolatile resistance memory switching in ZnO thin films

Six decades of research on ZnO has recently sprouted a new branch in the domain of resistive random access memories. Highly resistive and c-axis oriented ZnO thin films were grown by us using d.c. discharge assisted pulsed laser deposition on Pt/Ti/SiO₂/Si substrates at room temperature. The resistive switching characteristics of these films were studied in the top-bottom configuration using current-voltage measurements at room temperature. Reliable and repeated switching of the resistance of ZnO thin films was obtained between two well defined states of high and low resistance with a narrow dispersion and small switching voltages. Resistance ratios of the high resistance state to low resistance state were found to be in the range of 2–5 orders of magnitude up to 20 test cycles. The conduction mechanism was found to be dominated by the Ohmic behaviour in low resistance states, while Poole-Frenkel emission was found to dominate in high resistance state. The achieved characteristics of the resistive switching in ZnO thin films seem to be promising for nonvolatile memory applications.     

Paper-based bioassays using gold nanoparticle colorimetric probes

The majority of bioassays utilize thermosensitive reagents (e.g., biomolecules) and laboratory conditions for analysis. The developing world, however, requires inexpensive, simple-to-perform tests that do not require refrigeration or access to highly trained technicians. To address this need, paper-based bioassays using gold nanoparticle (AuNP) colorimetric probes have been developed. In the two prototype DNase I and adenosine-sensing assays, blue (or black)-colored DNA-cross-linked AuNP aggregates were spotted on paper substrates. The addition of target DNase I (or adenosine) solution dissociated the gold aggregates into dispersed AuNPs, which generated an intense red color on paper within one minute. Both hydrophobic and (poly(vinyl alcohol)-coated) hydrophilic paper substrates were suitable for this biosensing platform; by contrast, uncoated hydrophilic paper caused "bleeding" and premature cessation of the assay due to surface drying. The assays are surprisingly thermally stable. During preparation, AuNP aggregate-coated papers can be dried at elevated temperatures (e.g., 90 degrees C) without significant loss of biosensing performance, which suggests the paper substrate protects AuNP aggregate probes from external nonspecific stimuli (e.g., heat). Moreover, the dried AuNP aggregate-coated papers can be stored for at least several weeks without loss of the biosensing function. The combination of paper substrates and AuNP colorimetric probes makes the final products inexpensive, low-volume, portable, disposable, and easy-to-use. We believe this simple, practical bioassay platform will be of interest for use in areas such as disease diagnostics, pathogen detection, and quality monitoring of food and water.     

Distance-dependent plasmon resonant coupling between a gold nanoparticle and gold film

We present an experimental analysis of the plasmonic scattering properties of gold nanoparticles controllably placed nanometers away from a gold metal film. We show that the spectral response of this system results from the interplay between the localized plasmon resonance of the nanoparticle and the surface plasmon polaritons of the gold film, as previously predicted by theoretical studies. In addition, we report that the metal film induces a polarization to the single nanoparticle light scattering, resulting in a doughnut-shaped point spread function when imaged in the far-field. Both the spectral response and the polarization effects are highly sensitive to the nanoparticle-film separation distance. Such a system shows promise in potential biometrology and diagnostic devices.     

Organic nonvolatile memory devices with charge trapping multilayer graphene film

We fabricated an array-type organic nonvolatile memory device with multilayer graphene (MLG) film embedded in polyimide (PI) layers. The memory devices showed a high ON/OFF ratio (over 10(6)) and a long retention time (over 10(4)?s). The switching of the Al/PI/MLG/PI/Al memory devices was due to the presence of the MLG film inserted into the PI layers. The double-log current-voltage characteristics could be explained by the space-charge-limited current conduction based on a charge-trap model. A conductive atomic force microscopy found that the conduction paths in the low-resistance ON state were distributed in a highly localized area, which was associated with a carbon-rich filamentary switching mechanism.     

Self-healing and self-organized gold nanoparticle films at a water/organic solvent interface

Gold nanoparticles (5 nm and 20 nm) have been synthesized and stabilized with mercaptoundecanol. These particles, although insoluble in water or common organic solvents, spread as a thin film at the liquid-liquid interface between a water phase and an organic phase. Films of these gold nanoparticles have been observed both by conventional transmission electron microscopy of deposited samples and by cryo-transmission electron microscopy of plunge-frozen samples. The film can be monolayered and extend over centimeter-sized areas. The particle films spontaneously re-assemble and self-organize at the interface when disrupted. This self-healing capacity of the film should make it possible to build a device for continuous production and deposition of the film.     

Bipolar resistance switching characteristics in TiN/ZnO:Mn/Pt junctions developed for nonvolatile resistive memory application

As conventional flash memory is approaching its fundamental scaling limit, there is an urgent demand for an alternative nonvolatile memory technology at present. Resistance-switching random access memory has attracted extensive interests due to its nonvolatile nature, good scalability, and simple structure. In this work, TiN/ZnO:Mn/Pt junctions, which employ a conductive compound TiN as the top electrode to replace regular metal electrodes, were fabricated and investigated for nonvolatile resistive memory applications. These junctions exhibit bistable resistance state at room temperature, and the devices can be reproducibly switched between the two resistance states by applying bidirectional voltage biases. Moreover, both resistance states are demonstrated to retain for more than 10(4) s without electrical power, demonstrating a nonvolatile nature of the memory device. The mechanism of resistance switching effects in TiN/ZnO:Mn/Pt junctions is interpreted in terms of the drift of oxygen vacancies and the resultant formation/annihilation of local conductive channels through ZnO:Mn/Pt Schottky barrier.