Ravi F. Saraf

High-Resolution Thin-Film Device to Sense Texture by Touch

Touch (or tactile) sensors are gaining renewed interest as the level of sophistication in the application of minimum invasive surgery and humanoid robots increases. The spatial resolution of current large-area (greater than 1 cm2) tactile sensor lags by more than an order of magnitude compared with the human finger. By using metal and semiconducting nanoparticles, a ∼100-nm-thick, large-area thin-film device is self-assembled such that the change in current density through the film and the electroluminescent light intensity are linearly proportional to the local stress. A stress image is obtained by pressing a copper grid and a United States 1-cent coin on the device and focusing the resulting electroluminescent light directly on the charge-coupled device. Both the lateral and height resolution of texture are comparable to the human finger at similar stress levels of ∼10 kilopascals.

Tactile Devices To Sense Touch on a Par with a Human Finger

Our sense of touch enables us to recognize texture and shape and to grasp objects. The challenge in making an electronic skin which can emulate touch for applications such as a humanoid robot or minimally invasive and remote surgery is both in mimicking the (passive) mechanical properties of the dermis and the characteristics of the sensing mechanism, especially the intrinsic digital nature of neurons. Significant progress has been made towards developing an electronic skin by using a variety of materials and physical concepts, but the challenge of emulating the sense of touch remains. Recently, a nanodevice was developed that has achieved the resolution to decipher touch on a par with the human finger; this resolution is over an order of magnitude improvement on previous devices with a sensing area larger than 1 cm(2). With its robust mechanical properties, this new system represents an important step towards the realization of artificial touch.

Tuning the Energy Level Offset between Donor and Acceptor with Ferroelectric Dipole Layers for Increased Efficiency in Bilayer Organic Photovoltaic Cells

Ultrathin ferroelectric polyvinylidene fluoride (70%)-tetrafluoroethylene (30%) copolymer film is inserted between the poly3(hexylthiophene) (P3HT) donor and [6,6]-phenyl-C61-butyric acid methylester (PCBM) acceptor layers as the dipole layer to tune the relative energy levels, which can potentially maximize the open circuit voltage of bilayer organic solar cells. In this work, the power conversion efficiency of P3HT/PCBM bilayer solar cells is demonstrated to be doubled with the inserted dipoles.

Evaluation of contact resistance for isotropic electrically conductive adhesives

Electrically conductive adhesives are discussed and studied with ever-increasing interest as an alternative to solder interconnection in microelectronics circuit packaging. A similar level of scrutiny that is used to evaluate contact resistance performance for interconnections made with solder and separable connectors is necessary for electrically conductive adhesives. Experience with solder interconnection and separable connectors shows low initial contact resistance of less than 10 m/spl Omega/ when bulk conductor material is minimized in the measurement scheme. Stability is typically determined to be less than a 5-10 m/spl Omega/ change as a function of stress. The main intent of this study is to characterize the electrical contact resistance performance of joints made with isotropic electrically conductive adhesives. A copper comb pattern test vehicle was designed and fabricated using 0.25-mm thick lead frame material. The plating finishes that were applied to the copper substrate included a palladium alloy, gold, tin, and nickel. Test samples were made with several electrically conductive adhesives. Samples consisted of two comb patterns bonded to each other making a gang of 40 lap joints. Variables from circuit packaging such as coefficient of thermal expansion mismatches are purposely avoided in this study. Contact resistance measurements were made initially and as a function of time during environmental tests. Stresses included thermal cycling, thermal aging, and temperature and humidity conditioning. The stability of electrical contact resistance is shown to be influenced by both plating metallurgy and the conductive adhesive itself. Contact resistance equivalent to solder is possible with some electrically conductive adhesives on appropriate metallurgical finishes. Mechanically, adhesive joints are less robust than solder joints, and therefore care must be taken to eliminate or minimize the effects of mechanical loading. >

Selective assembly of nanoparticles on block copolymer by surface modification

We have developed a method to selectively deposit nanoparticles on the ordered nanoscale elements of PS–PI–PS (polystyrene–polyisoprene–polystyrene) block copolymer film. The process utilizes reactive ion plasma to selectively modify the PS surface with amine groups to electrostatically attract negatively charged Au nanoparticles. In spite of the strong interparticle Coulombic repulsion, the deposition on PS domains is significantly high. It is observed that the deposition at the edges of the domain is particularly high, forming a percolating nanoparticle necklace. The latter may lead to interesting avenues to fabricate electronic devices.

Laser‐Assisted Seeding for Electroless Plating on Polyimide Surfaces

Excimer laser pulses with wavelengths of 248 and 308 nm were used to selectively seed Pd on polyimide (PI) surfaces, making them suitable for electroless plating. This novel seeding process for insulating materials is accomplished with the sample immersed in the seeding solution, occurs only on the areas of the substrate that are illuminated (through the liquid) by the laser light, and does not require prior treatment of the surface. The seeding solution is transparent to the laser light and the metal deposition occurs as a consequence of the photoabsorption in the solid. This leads to electron transfer from the solid film into the solution and reduction of the Pd ions in contact with the surface. The Pd content of the seeded samples increased with the number of pulses, but was independent of repetition rate. The deposition rate of Pd did not exhibit a significant dependence on wavelength, in agreement with UV absorption spectra of PI and a single photon absorption process for electron excitation to allowed unoccupied states. As for the PD distribution, the deposits consisted of islands with distributions that depended on surface properties as well as on laser-material interactions. Sufficient PD seeds for uniform electroless plating of Cu and Co were attained after 3000 pulses at fluences ≃30 mJ/cm 2 . Although these fluences are much lower than those used for ablation of PI under water, distinct kinds of surface roughness were observed depending on the laser light and on the different types of PI

Self-assembled nanoparticle necklaces network showing single-electron switching at room temperature and biogating current by living microorganisms.

A network of one-dimensional (1D) Au nanoparticle necklaces is synthesized and shown to exhibit electronic switching, that is, gating, by the metabolic activity of yeast cells deposited on the structure. Without the cells, the network exhibits the Coulomb blockade effect at room temperature with a sharp threshold voltage, V(T) of approximately 0.45 V, which corresponds to a switching energy of approximately 20 kT. Although the enhancement in V(T) from approximately 70 mV for a single (10 nm) Au particle to >1 V is well-known for a 2D array, the uniqueness of the network topology is the relatively weak dependence of V(T) on temperature that leads to room temperature switching behavior, in contrast to an array where the blockade effect vanishes at ambient temperatures. The coupling between the biochemical process of the cell and the electronics of the network has potential applications for making electrodes for biofuel cells and highly sensitive biosensors using the cell as the specific sensing moiety.

SPONTANEOUS PLANARIZATION OF NANOSCALE PHASE SEPARATED THIN FILM

Structure of complex fluid at mesoscales is influenced by interfacial effects. We describe the dynamic response in such films to sudden change in interfacial tension. In a self-assembled block copolymer film, the monolayer of 15 nm diam cylindrical discrete phases close to the surface commence to sink at an average rate of 0.16 nm/day in response to the interfacial tension change. Surprisingly, this spontaneous planarization occurs, even though the cylinders are covalently stitched to the matrix. A simple model explains the observed behavior. The observation may lead to approaches to tailor the structure of mesoscale thin films of complex fluids for long-range order that are desirable for nanoscale device fabrication.

Amic acid doping of polyaniline: Characterization and resulting blends

Abstract Amic acids are found to be dopants for polyaniline (Pani). In particular, polyamic acids, the precursors to polyimides, react with Pani to form an all-polymer conducting matrix composed of protonated Pani and the polyamate counteranion. The acid/base interaction provides the driving force for molecular miscibility. The doping reaction is probed further by using a model system based on a monomeric diamic acid. It is found that the conductivity in the all-polymer matrix is limited by geometric constraints between the two polymers. Upon imidization of the Pani/polyamic acid blend, the doping reaction is reversed resulting in a non-conducting Pani/polyimide blend. However, the miscibility between the two polymers is still retained forming a “frustrated” blend in which the two polymers are frozen into a homogeneous, non-equilibrium state unable to phase separate as a result of physical restrictions.

Chemistry, physics, and engineering of electrically percolating arrays of nanoparticles: a mini review

Nanoparticles and their arrays do not obey Ohms law, even when the particles are made of metal, because of their small size. The non-Ohmic behavior is due to their low capacitance that allows (local) storage of charge at the single electron level to pose a substantial barrier to the passage of current at bias, V, below a threshold voltage, VT. The VT is inversely proportional to the size of the particle. For a typical <10 nm particle, the electrostatic barrier energy of the particle due to charging by a single electron is significantly larger compared to the thermal energy, affecting a substantial Coulomb blockade. The signature of the single electron effect is a highly non-linear current-bias behavior characterized by a critical point at VT with a subsequent rise in current that scales as (V/VT − 1)ζ, where ζ ≥ 1 is the critical exponent. We review the fabrication chemistry, device physics, and engineering applications of nanoparticle-based Single Electron Devices with architectures ranging from a single nanoparticle to arrays in one and two dimensions. The arrays are particularly interesting due to their natural integrability with microelectronic circuitry and robust single electron behavior at room temperature. These features open doors to a broad range of potential applications, such as chemical sensors, biomedical devices, data storage devices, and energy devices.