Highly-reproducible nonvolatile memristive devices based on polyvinylpyrrolidone: Graphene quantum-dot nanocomposites

The properties of nonvolatile memristive devices (NMD) fabricated utilizing organic/inorganic hybrid nanocomposites were investigated due to their superior advantages such as mechanical flexibility, low cost, low-power consumption, simple technological process in fabrication and high reproducibility. The current-voltage (I-V) curves for the Al/polyvinylpyrrolidone (PVP): graphene quantum-dot (GQD)/indium-tin-oxide (ITO) memristive devices showed current bistability characteristics at 300 K. The window margins corresponding to the high-conductivity (ON) state and the low-conductivity (OFF) state of the devices increased with increasing concentration of the GQDs. The ON/OFF ratio of the optimized device was 1 × 10⁴, which was the largest memory margin among the devices fabricated in this research. The endurance number of ON/OFF switching was above 1 × 10² cycles, and the retention time was relatively constant, maintaining a value above 10⁴ s. The devices showed high reproducibility with the writing voltage being distributed between −0.5 and −1.5 V and the erasing voltage being distributed between 2 and 3 V. The ON state currents remained between 0.02 and 0.03 A, and the OFF state currents stayed between 10⁻⁶ and 10⁻⁴ A. The carrier transport mechanisms are illustrated by using both the results obtained by fitting the I-V curves and the energy band diagrams of the devices.     

One-pot surface modification of poly(ethylene-alt-maleic anhydride) gate insulators for low-voltage DNTT thin-film transistors

This study investigates the one-pot surface modification of poly(ethylene-alt-maleic anhydride) (PEMA) gate insulators crosslinked with 1,5-naphthalenediamine (1,5-NDA) for enhancing the device performance of low-voltage dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) organic thin-film transistors (OTFTs). Surface properties of the PEMA gate insulator could be easily modified by adding poly(maleic anhydride-alt-1-octadecene) (PMAO) to the coating solution. The surface energy of the gate insulator is strongly correlated with the growth of organic semiconductors and the charge carrier transport at the interface between the semiconductor and gate insulator. The results indicate that the device performance of low-voltage DNTT OTFTs can be improved by one-pot surface modification of the PEMA gate insulator.     

Scanning probe microscope analysis of microstructures in optically variable devices

The fine-scale features of optically variable devices (OVDs) fabricated in resist by electron beam lithography have been examined using scanning probe microscopy (SPM). These features have included patterns of gratings, micro-text and geometrical images. Scanning probe microscopy has provided information on the groove angle, depth of profile and spatial frequency of the features as determined by the details of processing of the image. The OVD patterns formed in EBR-9 and X-AR-P 7400 resists exhibited a more rounded profile with a lower side-wall angle than in ZEP-7000 and PMMA resists.     

Phase Segregation in Thin Films of Conjugated Polyrotaxane– Poly(ethylene oxide) Blends: A Scanning Force Microscopy Study

Scanning force microscopy (SFM) is used to study the surface morphology of spin‐coated thin films of the ion‐transport polymer poly(ethylene oxide) (PEO) blended with either cyclodextrin (CD)‐threaded conjugated polyrotaxanes based on poly(4,4′‐diphenylene‐vinylene) (PDV), β‐CD–PDV, or their uninsulated PDV analogues. Both the polyrotaxanes and their blends with PEO are of interest as active materials in light‐emitting devices. The SFM analysis of the blended films supported on mica and on indium tin oxide (ITO) reveals in both cases a morphology that reflects the substrate topography on the (sub‐)micrometer scale and is characterized by an absence of the surface structure that is usually associated with phase segregation. This observation confirms a good miscibility of the two hydrophilic components, when deposited by using spin‐coating, as suggested by the luminescence data on devices and thin films. Clear evidence of phase segregation is instead found when blending PEO with a new organic‐soluble conjugated polymer such as a silylated poly(fluorene)‐alt‐poly(para‐phenylene) based polyrotaxane (THS–β‐CD–PF–PPP). The results obtained are relevant to the understanding of the factors influencing the interfacial and the intermolecular interactions with a view to optimizing the performance of light‐emitting diodes, and light‐emitting electrochemical cells based on supramolecularly engineered organic polymers.     

Materials and process aspect of cross-point RRAM (invited)

A survey of non-volatile and highly scalable cross-point memory in nanoscale resistive switching device is introduced. We present the basic operation of bipolar switching memory using combination between switching layer (HfO_x/PCMO) and oxygen reservoir layer (ZrO_x/AlO_x) and discuss the crucial issue for cross-point ReRAM. Based on the results, the applications of cross-point structure without any selection device were introduced. To evaluate the feasibility of cross-point ReRAM, read-out margin was calculated using PSPICE simulation. In addition, by the device scaling, three phenomena can be confirmed: (1) reset current reduction, (2) local heating effect and (3) significant improvement of uniformity.     

Macro-scale atomic force microscope: An experimental platform for teaching precision mechatronics

A macro-scale atomic force microscope (macro-AFM) has been designed and used for teaching precision mechatronics. The macro-AFM uses a novel electromagnetic self-sensing self-actuating probe. It operates in frequency-modulation AFM (FM-AFM) mode with intermittent contact. The AFM has an imaging volume of 250 mm × 40 mm × 1 mm with a resolution of about 1 μm at 1 Hz measurement bandwidth. The macro-AFM is simple and relatively inexpensive to build. It is scaled to make motion visible to the eye and is robust for a lab environment, all of which make it an affordable and effective educational tool. The macro-AFM is used in Mechatronics (2.737), a graduate level course at MIT, where in a series of 11 laboratory exercises, the students assembled and programmed the AFM system. This article provides the design and theory used for making and controlling the macro-AFM, as well as experimental results.     

Temperature Dependence of Epitaxial Graphene Formation on SiC(0001)

The formation of epitaxial graphene on SiC(0001) surfaces is studied using atomic force microscopy, Auger electron spectroscopy, electron diffraction, Raman spectroscopy, and electrical measurements. Starting from hydrogen-annealed surfaces, graphene formation by vacuum annealing is observed to begin at about 1150°C, with the overall step-terrace arrangement of the surface being preserved but with significant roughness (pit formation) on the terraces. At higher temperatures near 1250°C, the step morphology changes, with the terraces becoming more compact. At 1350°C and above, the surface morphology changes into relatively large flat terraces separated by step bunches. Features believed to arise from grain boundaries in the graphene are resolved on the terraces, as are fainter features attributed to atoms at the buried graphene/SiC interface.     

Inside Front Cover: Quantitative Measurement of the Local Surface Potential of π‐Conjugated Nanostructures: A Kelvin Probe Force Microscopy Study (Adv. Funct. Mater. 11/2006)

Kelvin probe force microscopy provides quantitative insight into the electronic properties of thin molecular layers, as shown by the results of P. Samorì and co‐workers on p. 1407. In the cartoon shown in the inside front cover, a scanning charged tip probes the local surface potential of a self‐assembled layer, inducing charge polarization into a nanoscale “effective area”. These measurements make it possible to unravel the interplay between structural and electronic properties of molecule‐based materials and devices. We describe a systematic study on the influence of different experimental conditions on the Kelvin probe force microscopy (KPFM) quantitative determination of the local surface potential (SP) of organic semiconducting nanostructures of perylene‐bis‐dicarboximide (PDI) self‐assembled at surfaces. We focus on the effect of the amplitude, frequency, and phase of the oscillating voltage on the absolute surface potential value of a given PDI nanostructure at a surface. Moreover, we investigate the role played by the KPFM measuring mode employed and the tip–sample distance in the surface potential mapping by lift‐mode KPFM. We define the ideal general conditions to obtain a reproducible quantitative estimation of the SP and we find that by decreasing the tip–sample distance, the area of substrate contributing to the recorded SP in a given location of the surface becomes smaller. This leads to an improvement of the lateral resolution, although a more predominant effect of polarization is observed. Thus, quantitative SP measurements of these nanostructures become less reliable and the SP signal is more unstable. We have also devised a semi‐quantitative theoretical model to simulate the KPFM image by taking into account the interplay of the different work functions of tip and nanostructure as well as the nanostructure polarizability. The good agreement between the model and experimental results demonstrates that it is possible to simulate both the change in local SP at increasing tip–sample distances and the 2D potential images obtained on PDI/highly oriented pyrolytic graphite samples. These results are important as they make it possible to gain a quantitative determination of the local surface potential of π‐conjugated nanostructures; thus, they pave the way towards the optimization of the electronic properties of electroactive nanometer‐scale architectures for organic (nano)electronic applications.     

A Morphological Model for the Solvent‐Enhanced Conductivity of PEDOT:PSS Thin Films

The well‐known enhanced conductivity of poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) thin films that is obtained by addition of high‐boiling solvents like sorbitol to the aqueous dispersion used for film deposition is shown to be associated with a rearrangement of PEDOT‐rich clusters into elongated domains, as evidenced from STM and AFM. Consistently, temperature dependent conductivity measurements for sorbitol‐treated films reveal that charge transport occurs via quasi 1D variable range hopping (VRH), in contrast to 3D VRH for untreated PEDOT:PSS films. The typical hopping distance of 60–90 nm, extracted from the conductivity measurements is consistent with hopping between the 30–40 nm sized grains observed with scanning probe microscopy.     

Low-order continuous-time robust repetitive control: Application in nanopositioning

A low-order repetitive control (RC) design in continuous-time for nanopositioning applications is presented. It focuses on achieving high performance and sufficient robustness to uncertainties. The design is mainly applicable to analog implementation, but due to the exceptionally low order, it also results in a fast and efficient digital implementation. Experimental results for an analog implementation using a bucket brigade device (BBD), as well as a digital implementation, is presented. RC can provide fast and accurate tracking of periodic reference signals, which is useful in many scanning probe microscopy and nanofabrication applications.     

Metal–Organic chemical vapour deposition (mocvd) growth utilising cu(acac)2 (acac  pentane-3, 5-dionato) as a source for copper-containing materials: Influence of carrier gas on surface morphology

The unsubstituted bis-β-diketonato complex of copper, Cu(acac)₂ (acac ? pentane-3, 5-dionato), has been used to deposit both elemental copper and copper oxide thin films by metal–organic chemical vapour deposition (MOCVD). For all Cu(II) bis-β-diketonates, growth of oxygen-free layers requires the breakage of four copper–oxygen bonds present in the precursor. The influence of carrier gas composition on deposit morphology has been examined for six parameter sets: both hydrous and anhydrous streams, each for reducing (H₂), inert (Ar) and oxidising (O₂) environments.     

Molecular‐Level Dispersion of Graphene into Poly(vinyl alcohol) and Effective Reinforcement of their Nanocomposites

Despite great recent progress with carbon nanotubes and other nanoscale fillers, the development of strong, durable, and cost‐efficient multifunctional nanocomposite materials has yet to be achieved. The challenges are to achieve molecule‐level dispersion and maximum interfacial interaction between the nanofiller and the matrix at low loading. Here, the preparation of poly(vinyl alcohol) (PVA) nanocomposites with graphene oxide (GO) using a simple water solution processing method is reported. Efficient load transfer is found between the nanofiller graphene and matrix PVA and the mechanical properties of the graphene‐based nanocomposite with molecule‐level dispersion are significantly improved. A 76% increase in tensile strength and a 62% improvement of Young's modulus are achieved by addition of only 0.7 wt% of GO. The experimentally determined Young's modulus is in excellent agreement with theoretical simulation.     

Structural and electrical characterization of gold nanoclusters in thin SiO2 films: Realization of a nanoscale tunnel rectifier

Conductive atomic force microscopy has been used to measure the I-V characteristics of nanometric Au clusters embedded in a SiO₂ film prepared by sputter deposition and low temperature annealing. Highly local asymmetric rectifier I-V characteristics were evidenced and modelled in terms of electrical transport through an asymmetric double barrier tunnel junction SiO₂/Au cluster/SiO₂. The threshold voltage depends strongly on the cluster size and barrier thickness according to the model given.     

Fabrication of a 3-dimensional microstructure by sequential anodic oxidation (SAO)

Scanning probe lithography (SPL) has considerable potential for producing small features (<100 nm) with a high spatial precision, and would be useful for fabricating 2-dimensional (2D) structures. However, it has not been used successfully in the fabrication of 3-dimensional (3D) structures due to the low aspect ratio of the resulting feature. Herein, we describe a simple 3D pattering method with repeated SPL, in which precise layer-by-layer alignment is not needed. Results and processes of the pattern can be readily observed in real-time. Using the proposed method, we successfully fabricated a 3D pyramidal structure. Additional growth for repeated oxidation was observed due to the superposition of energy absorbed on the pre-oxidized species.     

High photo‐conversion efficiency in double‐graded Cu(In,Ga)(S,Se)2 thin film solar cells with two‐step sulfurization post‐treatment

Sulfur is extensively used to increase the bandgap of Cu(In,Ga)(S,Se)₂ (CIGSSe) solar cells and to improve the open circuit voltage (V_OC ) in order to optimize the characteristics of the devices. This study uses a sulfurization process to obtain a double‐graded bandgap profile. Selenization was carried out on Cu(In,Ga) precursors, followed by one sulfurization process or two consecutive sulfurization processes on top of the CIGSe absorber layer surface. The optimum two‐step sulfurization process provides an increase of V_OC of 0.05 V and an improvement of conversion efficiency of 1.17%. The efficiency of the 30 × 30 cm² monolithic module, which has 64 CIGS cells connected in series (aperture area: 878.6 cm²), is 15.85%. The optical and electrical properties of the phase and the work function distribution were investigated using the depth profiles of the absorber layer as a function of the sulfurization conditions. The CIGSSe thin film formed by two‐step sulfurization with a high sulfur concentration exhibits a single work function peak, better crystallinity, and higher conversion efficiency than those of the thin film formed by two‐step sulfurization at low sulfur concentration. In terms of the Raman spectra depth profile, the phase areas for the CIGSSe thin film that underwent the optimized high sulfur concentration two‐step‐sulfurization appeared to have less of Cu_2‐xSe phase than that with low sulfur concentration. Consequently, surface and interface phase analysis is an essential consideration to improve cell efficiency. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.     

Photo-patternable polyimide gate insulator with fluorine groups for improving performance of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiopene (C10-BTBT) thin-film transistors

Surface properties of gate insulators strongly affect the device performance of organic thin-film transistors (OTFTs). To improve the performance of OTFTs, we have developed photo-sensitive polyimide gate insulator with fluorine groups. The polyimide gate insulator film could be easily patterned by selective UV exposure without any photoinitiator. The polyimide gate insulator film, fabricated at 130 °C, has a dielectric constant of 2.8 at 10 kHz, and leakage current density of <1.6 × 10^?10 A/cm² while biased from 0 to 90 V. To investigate the potential of the polyimide with fluorine groups as a gate insulator, we fabricated C₁₀-BTBT TFTs. The field-effect mobility and the on/off current ratio of the TFTs were measured to be 0.76 ± 0.09 cm²/V s and >10⁶, respectively.     

Probing Bias‐Dependent Electrochemical Gas–Solid Reactions in (LaxSr1–x)CoO3–δ Cathode Materials

Spatial variability of bias‐dependent electrochemical processes on a (La_0.5Sr_0.5)₂CoO_4±δ modified (La_xSr_1–x)CoO_3–δ surface is studied using first‐order reversal curve method in electrochemical strain microscopy (ESM). The oxygen reduction/evolution reaction (ORR/OER) is activated at voltages as low as 3–4 V with respect to bottom electrode. The degree of bias‐induced transformation as quantified by ESM hysteresis loop area increases with applied bias. The variability of electrochemical activity is explored using correlation analysis and the ORR/OER is shown to be activated in grains at relatively low biases, but the final reaction rate is relatively small. At the same time, at grain boundaries, the onset of reaction process corresponds to larger voltages, but limiting reactivity is much higher. The reaction mechanism in ESM of mixed electronic‐ionic conductor is further analyzed. These studies both establish the framework for probing bias‐dependent electrochemical processes in solids and demonstrate rich spectrum of electrochemical transformations underpinning catalytic activity in cobaltites.