Time Evolution and Spatial Hierarchy of Crack Patterns.

Cracks generated due to desiccation of wet colloidal systems are ubiquitous, examples being nanomaterial films, painted walls, cemented floors, mud fields, river beds, and even giant rocks. In all such cases, crack patterns are often appreciably similar but for the length and time scales, which can be widely differing. In this work, we have examined the crack formation more closely to see if there exists some generality with regard to the length scale of parameters and the formation time. Specifically, using a commonly used colloidal dispersion and optimized conditions to form polygonal network patterns rather than isolated cracks (films of subcritical thickness), we have studied the time evolution of the pattern parameters, the area occupied by the cracks, their lengths, and the widths. As is well known, initially, a network of cracks forms, which we term as the primary generation, followed by interconnecting cracks inside the polygonal regions (secondary) and, later, cracks spreading in local regions (tertiary). We find that the area and the width increase nearly linearly with time with the change in the slope corresponding to the change in the generation. When normalized with respect to the final values, the trends obtained for different film thicknesses overlap, the only exception being the pattern containing unconnected cracks. Thus, the time evolution of cracks is shown to be predictable based on width filtering. Including the angle between cracks as further input into the recursive model, the possibility of identifying the hierarchy of crack segments is also shown. The approach may be useful in determining the age, authenticity, and details of old paintings, understanding the stress profile of geological rocks, and analyzing various natural and manmade hierarchical structures.

Transparent and Flexible Supercapacitors with Networked Electrodes.

Transparent and flexible energy storage devices have received immense attention due to their suitability for innovative electronics and displays. However, it remains a great challenge to fabricate devices with high storage capacity and high degree of transmittance. This study describes a simple process for fabrication of supercapacitors with ≈75% of visible transparency and areal capacitance of ≈3 mF cm-2 with high stability tested over 5000 cycles of charging and discharging. The electrodes consist of Au wire networks obtained by a simple crackle template method which are coated with MnO2 nanostructures by electrodeposition process. Importantly, the membrane separator itself is employed as substrate to bring in the desired transparency and light weight while additionally exploiting its porous nature in enhancing the interaction of electrolyte with the active material from both sides of the substrate, thereby enhancing the storage capacity. The method opens up new ways for fabricating transparent devices.

Fabrication of Oxidation-Resistant Metal Wire Network-Based Transparent Electrodes by a Spray-Roll Coating Process.

Roll and spray coating methods have been employed for the fabrication of highly oxidation resistant transparent and conducting electrodes (TCEs) by a simple solution process using crackle lithography technique. We have spray-coated a crackle paint-based precursor to produce highly interconnected crackle network on PET roll mounted on a roll coater with web speed of 0.6 m/min. Ag TCE with a transmittance of 78% and sheet resistance of ∼20 Ω/□ was derived by spraying Ag precursor ink over the crackle template followed by lift-off and annealing under ambient conditions. The Ag wire mesh was stable toward bending and sonication tests but prone to oxidation in air. When electrolessly coated with Pd, its robustness toward harsh oxidation conditions was enhanced. A low-cost transparent electrode has also been realized by using only small amounts of Ag as seed layer and growing Cu wire mesh by electroless method. Thus, made Ag/Cu meshes are found to be highly stable for more than a year even under ambient atmosphere.

Solution-processed soldering of carbon nanotubes for flexible electronics.

We report a simple lithography-free, solution-based method of soldering of carbon nanotubes with Ohmic contacts, by taking specific examples of multi-walled carbon nanotubes (MWNTs). This is achieved by self-assembling a monolayer of soldering precursor, Pd(2+) anchored to 1,10 decanedithiol, onto which MWNTs could be aligned across the gap electrodes via solvent evaporation. The nanosoldering was realized by thermal/electrical activation or by both in sequence. Electrical activation and the following step of washing ensure selective retention of MWNTs spanning across the gap electrodes. The soldered joints were robust enough to sustain strain caused during the bending of flexible substrates as well as during ultrasonication. The estimated temperature generated at the MWNT-Au interface using an electro-thermal model is ∼150 °C, suggesting Joule heating as the primary mechanism of electrical activation. Further, the specific contact resistance is estimated from the transmission line model.

Field effect transistors and photodetectors based on nanocrystalline graphene derived from electron beam induced carbonaceous patterns.

We describe a transfer-free method for the fabrication of nanocrystalline graphene (nc-graphene) on SiO(2) substrates directly from patterned carbonaceous deposits. The deposits were produced from the residual hydrocarbons present in the vacuum chamber without any external source by using an electron beam induced carbonaceous deposition (EBICD) process. Thermal treatment under vacuum conditions in the presence of Ni catalyst transformed the EBIC deposit into nc-graphene patterns, confirmed using Raman and TEM analysis. The nc-graphene patterns have been employed as an active p-type channel material in a field effect transistor (FET) which showed a hole mobility of ~90 cm(2) V(-1) s(-1). The nc-graphene also proved to be suitable material for IR detection.

Large-area ohmic top contact to vertically grown nanowires using a free-standing Au microplate electrode.

A method of electrically contacting vertically grown nanowires of uneven heights, a common scenario among as-grown nanowires, is reported here using a chemically synthesized single-crystalline Au microplate as top electrode. The contact is electrically activated and the contact formation is predominantly due to electromigration. With this approach, the electrode could ohmically contact several thousand nanowires at once.

Field-effect transistors based on thermally treated electron beam-induced carbonaceous patterns.

Electron beam-induced carbonaceous deposition (EBICD) derived from residual hydrocarbons in the vacuum chamber has many fascinating properties. It is known to be chemically complex but robust, structurally amorphous, and electrically insulating. The present study is an attempt to gain more insight into its chemical and electrical nature based on detailed measurements such as Raman, XPS, TEM, and electrical. Interestingly, EBIC patterns are found to be blue fluorescent when excited with UV radiation, a property which owes much to sp(2) carbon clusters amidst sp(3) matrix. Temperature-dependent Raman and electrical measurements have confirmed the graphitization of the EBICD through the decomposition of functional groups above 300 °C. Finally, graphitized EBIC patterns have been employed as active p-type channel material in the field-effect transistors to obtain mobilities in the range of 0.2-4 cm(2)/V s.

Metal anion-alkyl ammonium complexes as direct write precursors to produce nanopatterns of metals, nitrides, oxides, sulfides, and alloys.

The study explores the possibility of using metal anions complexed with tetraoctylammonium bromide (ToABr) as single-source direct write precursors in e-beam and soft lithography processes to obtain micro- and nanoscale patterns of various metals, i.e., Au, Pd, Pt, Ag, Pb and Cu, as well as of their alloys (AuCu), oxides (Co(3)O(4), ZnO), nitrides (CoN, InN, GaN), and sulfides (Ag(2)S). The extraction efficiency of ToABr for different metal anions is found to be varied (40-90%), but the obtained precursors are easily processable as they have reasonable solubility in common solvents and are obtainable as smooth films, both being important for high-resolution patterning. The e-resist action of the precursors originates from the extreme e-beam sensitivity of the hydrocarbon chain present in ToABr, while direct micromolding has been possible due to easy flow of the precursor solutions in capillaries. The interaction of the anion and ToABr being mainly electrostatic enables easy removal of the hydrocarbon from patterned regions by thermolysis on a hot plate in the ambient or in controlled atmosphere to form the desired product. This method can be easily generalized.

Flexible and semitransparent strain sensors based on micromolded Pd nanoparticle-carbon µ-stripes.

Flexible resistive strain sensors have been fabricated by micromolding Pd alkanethiolate on polyimide substrates and subjecting to thermolysis in air. Thus produced stripes were ∼1 µm wide with spacing of ∼0.5 µm and contained Pd nanoparticles in carbon matrix. The nanoparticle size and the nature of carbon are much dependent on the thermolysis temperature as is also the resistance of the microstripes. Generally, lower thermolysis temperatures (<230 °C) produced stripes containing small Pd nanoparticles with significant fraction of carbon from the precursor decomposition. The stripes were poorly conducting yet interestingly, exhibited change of resistance under tensile and compressive strain. Particularly noteworthy are the stripes produced from 195 °C thermolysis, which showed a high gauge factor of ∼390 with strain sensitivity, 0.09%. With molding at 230 °C, the stripes obtained were highly conducting, and amazingly did not change the resistance with strain even after several bending cycles. The latter are ideal as flexible conduits and interconnects. Thus, the article reports a method of producing flexible sensitive strain sensors on one hand and on the other, flexible conduits with unchanging resistance, merely by fine-tuning the precursor decomposition under the molding conditions.

CNT manipulation: inserting a carbonaceous dielectric layer beneath using electron beam induced deposition.

Electron beam induced carbonaceous deposition has been carried out in the presence of water vapor at 0.4 torr pressure amidst residual hydrocarbons present in the SEM chamber. When performed at a CNT location on a Si substrate with low e beam energy (10 kV), the deposition was taking place beneath the CNT. While higher beam energy (25 kV) causing the deposition on the top surface of the CNT, in agreement with the earlier reports. The insertion of dielectric carbonaceous layer beneath the CNT allowed us to measure the I-V data along the length of the nanotube using CAFM.

Charge storage in mesoscopic graphitic islands fabricated using AFM bias lithography.

Electrochemical oxidation and etching of highly oriented pyrolytic graphite (HOPG) has been achieved using biased atomic force microscopy (AFM) lithography, allowing patterns of varying complexity to be written into the top layers of HOPG. The graphitic oxidation process and the trench geometry after writing were monitored using intermittent contact mode AFM. Electrostatic force microscopy reveals that the isolated mesoscopic islands formed during the AFM lithography process become positively charged, suggesting that they are laterally isolated from the surrounding HOPG substrate. The electrical transport studies of these laterally isolated finite-layer graphitic islands enable detailed characterization of electrical conduction along the c-direction and reveal an unexpected stability of the charged state. Utilizing conducting-atomic force microscopy, the measured I(V) characteristics revealed significant non-linearities. Micro-Raman studies confirm the presence of oxy functional groups formed during the lithography process.

Direct micromolding of Pd micro-stripes for electronic applications.

A simple, one-step direct micromolding process has been realised to produce highly conducting Pd micro-stripes over large areas on various substrates including flexible polyimide. Under a PDMS micromold, Pd octanethiolate served as a precursor at 250 degrees C, a temperature at which the precursor gets neatly metallised. Thus produced micro-stripes are robust under bending and can be utilised for flexible electronics. Hydrogen sensing by Pd micro-stripes is demonstrated. By electrolessly depositing Cu on the stripes, they can be made to peel off to form free standing Cu-Pd micro-ribbons.

Metal nanowire grating patterns.

Metal nanowire patterning in the form of grating structures has been carried out using a wide range of lithography techniques, and many hybrid methods derived from them. The challenge is to achieve sub-100 nm linewidths with controllable spacing and thickness over large areas of substrates with high throughput. In particular, the patterns with linewidth and spacing of a few tens of nm offer properties of great interest in optoelectronics and plasmonics. Crossbar grating structures--two gratings patterned perpendicular to each other--will play an important role as ultra-high density electrode grids in memristive devices for non-volatile memory.

Local anodic oxidation patterning of Au deposited Si surfaces.

Nanopatterning of Si(100) surfaces deposited with Au films from physical and chemical methods, has been carried out using a AFM set up mounted with a conducting tip. At a tip bias of -12 V, the LAO patterns drawn on various Au/SiOx surfaces have been compared with those on bare Si. The height of the oxide patterns is several times higher in the case of Au covered Si surfaces compared to patterns on bare Si surface. The enhancement in LAO is related to the catalytic activity of Au nanoparticulates at SiOx interface.

Electrical and hydrogen-sensing characteristics of field effect transistors based on nanorods of ZnO and WO2.72.

Top-gated field effect transistors (FETs) using Au-gap (5 microm) electrodes on glass substrate and SiO2/Si as gate have been fabricated with undoped and doped nanorods of ZnO as well as with WO2.72 nanorods as active semiconductor elements. The I-V characteristics at different gate voltages show that the nanorods are n-type semiconductors and the derived transfer characteristics show that the FET devices function in the depletion mode. Al-doping (3 at%) enhances the carrier mobility of ZnO nanorods to 128.6 cm2/V x s as against to 0.009 cm2/V x s estimated in the case of the undoped nanorods. Doping with Cd and Mg (3 at%) as well as N (approximately 1 at%) similarly increases the mobility although to a smaller extent. The Cd-doped ZnO nanorods exhibit the high sensitivity (defined as the ratio of the resistance in air to that in the hydrogen) (20) for 1000 ppm of hydrogen. Application of gate voltage decreases the recovery times of the nanorod sensors. FETs based on WO2.72 nanorods also show the depletion mode type characteristics and a carrier mobility of 8.38 cm2/V x s is obtained. The WO2.72 based FETs exhibit good sensitivity (approximately 10) for 1000 ppm hydrogen.

Femtoliter silver cups as surface enhanced Raman scattering active containers.

Femtoliter capacity Ag cups formed by the pulsed laser ablation of an Ag foil have been tried out as substrates for surface enhanced Raman scattering (SERS) measurements. The cups are formed as the impinging droplets from the laser plume undergo a flow pattern before freezing into cup-like structures, resulting in a surface roughness (approximately 35 nm) that makes them ideal for SERS studies. The internal volume of the cups is in the femtoliter (10(-15) l) range, well suited for small-scale reactions, particularly in biological studies. The cups exhibit enhancement factors of the order of 10(6) with the analyte molecule thiophenol. Individual cups have been dosed attoliter quantities (10(-18) l) of the analyte and detected.

Electron resist behavior of Pd hexadecanethiolate examined using X-ray photoelectron spectroscopy with nanometric lateral resolution.

Electron resist behavior of Pd hexadecanethiolate is studied by varying the e-dosage from 2-280 muC.cm(-2). The e-beam exposed resist is characterized using energy dispersive spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy with nanometric lateral resolution. Electron beam exposure causes defects in the alkyl chain of the thiolate, giving the required solubility contrast during the developing step, thus qualifying the precursor as an e-beam resist. On exposure to the e-beam, the reduction of Pd(2+) to Pd(0) is observed, and the reduction increases with increasing e-dosage. The resist is highly sensitive, with the estimated sensitivity being 32 muC.cm(-2). Thermolysis at 250 degrees C leads to the formation of Pd nanoparticles, demonstrating the essential feature of a direct write resist for conducting patterns.

Dewetting assisted patterning of polystyrene by soft lithography to create nanotrenches for nanomaterial deposition.

Micromolding in capillaries of polystyrene has been carried out using a poly(dimethylsiloxane) stamp, derived from a compact disk (CD) as master, while heating above the glass transition temperature of polystyrene. The resulting pattern contained a replica of the parallel channels but with an important difference that trenches of width approximately 30 nm were found in between. The nanotrenches in polystyrene could be filled with metals by physical vapor deposition and electroless plating. This method finds potential applications in nanoelectronics and nanofluidics.

Functionalized Au22 clusters: synthesis, characterization, and patterning.

We synthesized fluorescent, porphyrin-anchored, Au(22) clusters in a single step, starting from well-characterized Au(25) clusters protected with glutathione (-SG) by a combined core reduction/ligand exchange protocol, at a liquid-liquid interface. The prepared cluster was characterized by UV/vis, photoluminescence, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, elemental analysis, and matrix-assisted laser desorption ionization mass spectrometry. The absence of a 672 nm intraband transition of Au(25) and the simultaneous emergence of new characteristic peaks at 520 and 635 nm indicate the formation of the Au(22) core. An increase in the binding energy of 0.4 eV in Au 4f core-level peaks confirmed the presence of a reduced core size. Quantitative XPS confirmed the Au/S ratio. The presence of a free base, tetraphenylporphyrin (H(2)TPPOAS-), on the Au(22) core was confirmed by fluorimetric titrations with Cu(2+) and Zn(2+) ions. From all of these, the composition of the cluster was determined to be Au(22)[(-SG)(15)(-SAOPPTH(2))(2)], which was supported by mass spectrometry and elemental analysis. We utilized the fluorescence nature of these water-soluble clusters for the fabrication of fluorescent patterns by soft lithography. The patterns were studied using tapping-mode atomic force microscopy and confocal fluorescence imaging.

Effects of changes in the interparticle separation induced by alkanethiols on the surface plasmon band and other properties of nanocrystalline gold films.

Effects of changing the interparticle separation on the surface plasmon bands of ultrathin films of gold nanoparticles have been investigated by examining the interaction of alkanethiols of varying chain length on nanocrystalline gold films generated at the organic-aqueous interface. Adsorption of alkanethiols causes blue-shifts of the surface plasmon adsorption band, the magnitude of the shift being proportional to the chain length. The disordered nanocrystals thus created (lambdamax, 530 m) are in equilibrium with the ordered nanocrystals in the film (lambdamax, 700 m) as indicated by an isosbestic point around 600 nm. Long chain thiols disintegrate or disorder the gold films more effectively, as demonstrated by the increased population of the thiol-capped gold nanocrystals in solution. The rate of interaction of the thiols with the film decreases with the decreasing chain length. The effect of an alkanethiol on the spectrum of the gold film is specific, in that the effects with long and short chains are reversible. The changes in the plasmon band of gold due to interparticle separation can be satisfactorily modeled on the basis of the Maxwell-Garnett formalism. Spectroscopic studies, augmented by calorimetric measurements, suggest that the interaction of alkanethiols involves two steps, the first step being the exothermic gold film-thiol interaction and the second step includes the endothermic disordering process followed by further thiol capping of isolated gold particles.