Anindarupa Chunder

Position dependent photodetector from large area reduced graphene oxide thin films

We fabricated large area infrared photodetector devices from thin film of chemically reduced graphene oxide (RGO) sheets and studied their photoresponse as a function of laser position. We found that the photocurrent either increases, decreases, or remain almost zero depending upon the position of the laser spot with respect to the electrodes. The position sensitive photoresponse is explained by Schottky barrier modulation at the RGO film-electrode interface. The time response of the photocurrent is dramatically slower than single sheet of graphene possibly due to disorder from the chemical synthesis and interconnecting sheets.

High yield fabrication of chemically reduced graphene oxide field effect transistors by dielectrophoresis

We demonstrate high yield fabrication of field effect transistors (FET) using chemically reduced graphene oxide (RGO) sheets. The RGO sheets suspended in water were assembled between prefabricated gold source and drain electrodes using ac dielectrophoresis. With the application of a backgate voltage, 60% of the devices showed p-type FET behavior, while the remaining 40% showed ambipolar behavior. After mild thermal annealing at 200 degrees C, all ambipolar RGO FET remained ambipolar with increased hole and electron mobility, while 60% of the p-type RGO devices were transformed to ambipolar. The maximum hole and electron mobilities of the devices were 4.0 and 1.5 cm(2) V( - 1) s( - 1) respectively. High yield assembly of chemically derived RGO FET will have significant impact in scaled up fabrication of graphene based nanoelectronic devices.

Space charge limited conduction with exponential trap distribution in reduced graphene oxide sheets

We elucidate on the low mobility and charge traps of the chemically reduced graphene oxide (RGO) sheets by measuring and analyzing temperature dependent current-voltage characteristics. The RGO sheets were assembled between source and drain electrodes via dielectrophoresis. At low bias voltage the conduction is Ohmic while at high bias voltage and low temperatures the conduction becomes space charge limited with an exponential distribution of traps. We estimate an average trap density of 1.75×1016 cm−3. Quantitative information about charge traps will help develop optimization strategies of passivating defects in order to fabricate high quality solution processed graphene devices.

Reduced Graphene Oxide/Poly(3‐hexylthiophene) Supramolecular Composites

Poly(3-hexylthiophene) (P3HT) supramolecular structures are fabricated on P3HT-dispersed reduced graphene oxide (RGO) monolayers and surfactant-free RGO monolayers. P3HT is able to disperse RGO in hot anisole/N,N-dimethylformamide solvents, and forms nanowires on RGO surfaces through a RGO induced crystallization process. The TEM and AFM investigation of the resultant P3HT/RGO composites shows that P3HT nanowires grow from RGO, and connect individual RGO monolayers. Raman spectroscopy confirms the interaction between P3HT and RGO, which allows the manipulation of the RGO electrical properties. Such a bottom-up approach provides interesting graphene-based composites for nanometer-scale electronics.

Fabrication of anti-reflection coatings on plastics using the spraying layer-by-layer self-assembly technique

— Anti-reflection (AR) coatings on plastic substrates have been extensively investigated with the development of large-area LCD and LED displays. A robust AR coating on plastics requires strong adhesion to the substrate, precise thickness and refractive index, and abrasion resistance. In this paper, abrasion-resistant AR coatings were fabricated on polycarbonate substrates using the layer-by-layer spraying deposition of poly(allylamine hydrochloride) (PAH) and silica nanoparticles. The adhesion between the substrates and coatings was enhanced by treating the polycarbonate surfaces with aminopropyltrimethoxylsilane (APTS). The porous low-refractive-index PAH/silica-nanoparticles multilayers were constructed by the layer-by-layer spraying of PAH and silica-nanoparticles aqueous solutions onto the functionalized substrates. The subsequent treatment of the porous coatings with tetrahydroxylsilane leads to stable abrasion-resistant AR coatings. The resultant AR coatings can reduce the reflection from 5 to 0.3%. The reported technique provides a cost-effective method for large-scale production of AR coatings on plastic substrates.

Patterning of Diverse Mammalian Cell Types in Serum Free Medium with Photoablation

Integration of living cells with novel microdevices requires the development of innovative technologies for manipulating cells. Chemical surface patterning has been proven as an effective method to control the attachment and growth of diverse cell populations. Patterning polyelectrolyte multilayers through the combination of layer‐by‐layer self‐assembly technique and photolithography offer a simple, versatile, and silicon compatible approach that overcomes chemical surface patterning limitations, such as short‐term stability and low‐protein adsorption resistance. In this study, direct photolithographic patterning of two types of multilayers, PAA (poly acrylic acid)/PAAm (poly acryl amide) and PAA/PAH (poly allyl amine hydrochloride), were developed to pattern mammalian neuronal, skeletal, and cardiac muscle cells. For all studied cell types, PAA/PAAm multilayers behaved as a cytophobic surface, completely preventing cell attachment. In contrast, PAA/PAH multilayers have shown a cell‐selective behavior, promoting the attachment and growth of neuronal cells (embryonic rat hippocampal and NG108‐15 cells) to a greater extent, while providing little attachment for neonatal rat cardiac and skeletal muscle cells (C2C12 cell line). PAA/PAAm multilayer cellular patterns have also shown a remarkable protein adsorption resistance. Protein adsorption protocols commonly used for surface treatment in cell culture did not compromise the cell attachment inhibiting feature of the PAA/PAAm multilayer patterns. The combination of polyelectrolyte multilayer patterns with different adsorbed proteins could expand the applicability of this technology to cell types that require specific proteins either on the surface or in the medium for attachment or differentiation, and could not be patterned using the traditional methods.

An analytical model for the wettability switching characteristic of a nanostructured thermoresponsive surface

The applications of thermoresponsive surfaces require the development of a rigorous mathematical treatment for these surfaces to understand and improve their behavior. We propose an analytical model to describe the transfer characteristics (variation in contact angle versus temperature) of a unique nanostructured thermosensitive surface, consisting of silica nanoparticles and a hydrophilic/hydrophobic thermoresponsive polymer, poly(N-isopropylacrylamide). Three different thermo-sensitive platforms were fabricated and the contact angle change of a water droplet on the surface with varying surface temperature was analytically modeled.

39.1: Invited Paper: Fabrication of Antireflection Coatings for Displays

Antireflection and antifogging coatings were fabricated on polycarbonate substrates using the layer-by-layer self-assembly of polyelectrolytes and silica nanoparticles. Antireflective coatings were also built on glass substrates by alternating spraying polyelectrolyte and silica nanoparticle solutions. The transmittance of the sample with both sides coated using the dipping method is above 99 % while the coating built through spraying can reduce the one side reflection from 4% to 1%. Such technology provides a simple approach to fabricate antireflection coatings on displays.

A Passive Microfluidic Valve using Superhydrophobic/Hydrophilic Nanostructures for Lab-on-A-Chip (LOC) Systems

This paper describes a passive microfluidic valve, which has an integrated superhydrophobic surface in one of the channels to selectively inhibit the flow of water based reagents and pass aqueous solutions containing surfactants. The microchannels were fabricated by standard photolithography and wet etching techniques. The superhydrophobic polymer patch was fabricated using the layer-by-layer (LBL) deposition technique, in which multiple layers of polyelectrolytes were coated on a channel wall followed by silica nanoparticle treatment. The channels were sealed with a polydimethylsiloxane (PDMS) slab by treating under oxygen plasma. The fabricated microfluidic valve was tested with liquids flowing in the microchannels under capillary action. It is shown that the valve selectively regulates the flow of test samples. The water sample stops at the front of superhydrophobic patch and the 2-isopropanol flows through the channels without any resistance.

A Nanostructured Thermosensitive Smart Surface With Integrated Microheater for Wettability Control

This paper describes the design and fabrication of a switchable thermosensitive polymer with an integrated microheater as a smart surface platform for wettabilty control. The thermoresponsive surface is synthesized on a glass substrate using the polymer poly(N-isopropylacrylamide) (PNIPAAm) which can change its wettability when subjected to change in temperature. PNIPAAm is hydrophilic when the surface temperature is less than the lower critical solution temperature (LCST) range of about 28–33 °C and is hydrophobic above the LCST range. The PNIPAAm surface is heated by spiral gold microheaters which are fabricated on the lower side of the glass substrate. The contact angle change with change in temperature is tested using a standard goniometer. Time response analysis of the surface is presented. This smart surface can be used as an active or adaptive component for microflow regulation and can be potentially integrated into large scale lab-on-chip (LOC) systems.Copyright