Elsa Reichmanis

Additive-free hollow-structured Co3O4 nanoparticle Li-ion battery: the origins of irreversible capacity loss.

Origins of the irreversible capacity loss were addressed through probing changes in the electronic and structural properties of hollow-structured Co3O4 nanoparticles (NPs) during lithiation and delithiation using electrochemical Co3O4 transistor devices that function as a Co3O4 Li-ion battery. Additive-free Co3O4 NPs were assembled into a Li-ion battery, allowing us to isolate and explore the effects of the Co and Li2O formation/decomposition conversion reactions on the electrical and structural degradation within Co3O4 NP films. NP films ranging between a single monolayer and multilayered film hundreds of nanometers thick prepared with blade-coating and electrophoretic deposition methods, respectively, were embedded in the transistor devices for in situ conduction measurements as a function of battery cycles. During battery operation, the electronic and structural properties of Co3O4 NP films in the bulk, Co3O4/electrolyte, and Co3O4/current collector interfaces were spatially mapped to address the origin of the initial irreversible capacity loss from the first lithiation process. Further, change in carrier injection/extraction between the current collector and the Co3O4 NPs was explored using a modified electrochemical transistor device with multiple voltage probes along the electrical channel.

Solvent Based Hydrogen Bonding: Impact on Poly(3-hexylthiophene) Nanoscale Morphology and Charge Transport Characteristics

We demonstrate that supramolecular assembly and subsequent enhancement of charge transport characteristics of conjugated polymers can be facilitated simply by adding small amounts of a more volatile poor solvent, which can hydrogen bond with the majority solvent. Addition of up to 2 vol % acetone to a precursor solution of poly(3-hexylthiophene) (P3HT) in chloroform leads to approximately a 4-fold increase in P3HT field-effect mobility. The improvement is associated with hydrogen bonding interactions between acetone and chloroform which decrease the evaporation rate of the mixed solvent. P3HT is less soluble in the binary solvent than in the more readily vaporized chloroform component, and this characteristic enables the supramolecular assembly of P3HT chains at the nanoscale. Two-dimensional molecular ordering of the polymer film was controlled by varying the quantity of poor solvent added to the precursor solution, and the correlation between field-effect mobility and molecular ordering was investigated. Hansen solubility parameters were used to systematically understand how the solvent mixture enhances the alignment and assembly of polymer chains and influences subsequent thin film properties. The value of the relative energy difference (RED) of the solvent with respect to P3HT increased from less than 1 to more than 1 during film formation, which indicates that the solvent characteristics are initially those of a good solvent but transform into those of a poor dissolution medium. A mechanistic illustration of the molecular ordering process during film formation is postulated.

Plastic electronic devices: From materials design to device applications

Whether molecular solids, oligomers, or polymers, organic materials have been shown to be attractive candidates for both passive and active roles in electronic devices because of their compatibility with high-throughput, low-cost processing techniques and their capability to be precisely functionalized through the techniques of organic synthesis to afford desired performance attributes. Structure at both molecular- and nano-scale will impact attributes such as morphology (surface roughness, grain size), adhesion, mechanical integrity, solubility, and chemical and environmental stability. These factors, in turn, will affect device performance, notably electrical performance (mobility, conductivity, on/off ratio, threshold voltage). The challenges associated with the development of practical organic materials and associated device technologies for “plastic electronics” will be described and the relationships between materials structure and process performance will be discussed.

Evaluation of cycloolefin-maleic anhydride alternating copolymers as single-layer photoresists for 193-nm photolithography

We have developed a fundamentally new class of photoresist matrix resins for use in 193 and 248 nm lithography based on cycloolefin-maleic anhydride alternating copolymers. When used in three-component formulations with cholate-based dissolution inhibitions (DIs) and conventional photoacid generators, these copolymers afford positive-tone resists with potential sub-0.25 micrometer image fidelity. The resists exhibit high contrast (3 - 5.5) and high sensitivity (3 - 5 mJ/cm2 at 248 nm, depending on exact formulation) with low loadings (ca. 1 wt%) of triphenylsulfonium salt photoacid generators. These formulations are sufficiently transparent to be used at 193 nm without further modification.

Imparting Chemical Stability in Nanoparticulate Silver via a Conjugated Polymer Casing Approach

Only limited information is available on the design and synthesis of functional materials for preventing corrosion of metal nanostructures. In the nanometer regime, even noble metals are subject to chemical attack. Here, the corrosion behavior of noble metal nanoparticles coated with a conjugated polymer nanolayer was explored for the first time. Specifically, electrochemical corrosion and sulfur tarnishing behaviors were examined for Ag-polypyrrole (PPy) core-shell nanoparticles using potentiodynamic polarization and spectrophotometric analysis, respectively. First, the Ag-PPy nanoparticles exhibited enhanced resistance to electrochemically induced corrosion compared to their exposed silver counterparts. Briefly, a neutral PPy shell provided the highest protection efficiency (75.5%), followed by sulfate ion- (61.3%) and dodecylbenzenesulfonate ion- (53.6%) doped PPy shells. However, the doping of the PPy shell with chloride ion induced an adverse effect (protection efficiency, -120%). Second, upon exposure to sulfide ions, the Ag-PPy nanoparticles preserved their morphology and colloidal stability while the bare silver analog underwent significant structural deformation. To further understand the function of the PPy shell as a protection layer for the silver core, the catalytic activity of the nanostructures was also evaluated. Using the reduction of 4-nitrophenol as a representative example of a catalytic reaction, the rate constant for that reduction using the PPy encased Ag nanoparticles was found to be 1.1 × 10(-3) s(-1), which is approximately 33% less than that determined for the parent silver. These results demonstrate that PPy can serve as both an electrical and chemical barrier for mitigating undesirable chemical degradation in corrosive environments, as well as provide a simple physical barrier to corrosive substances under appropriate conditions.

Ultrasound-induced ordering in poly(3-hexylthiophene): role of molecular and process parameters on morphology and charge transport.

Facile methods for controlling the microstructure of polymeric semiconductors are critical to the success of large area flexible electronics. Here we explore ultrasonic irradiation of solutions of poly(3-hexylthiophene) (P3HT) as a simple route to creating ordered molecular aggregates that result in a one to two order of magnitude improvement in field effect mobility. A detailed investigation of the ultrasound induced phenomenon, including the role of solvent, polymer regioregularity (RR) and film deposition method, is conducted. Absorption spectroscopy reveals that the development of low energy vibronic features is dependent on both the regioregularity as well as the solvent, with the latter especially influential on the intensity and shape of the band. Use of either higher regioregular polymer or ultrasonic irradiation of lower regioregular polymer solutions results in high field effect mobilities that are nearly independent of the dynamics of the film formation process. Surprisingly, no distinct correlation between thin-film morphology and macroscopic charge transport could be ascertained. The relationships between molecular and process parameters are very subtle: modulation of one effects changes in the others, which in turn impact charge transport on the macroscale. Our results provide insight into the degree of control that is required for the development of reproducible, robust materials and processes for advanced flexible electronics based on polymeric materials.

Solvent Evaporation Induced Liquid Crystalline Phase in Poly(3-hexylthiophene)

We report on the evolution of the chain orientation of a representative π-conjugated polymer, poly(3-hexylthiophene) (P3HT), during the solution-casting process, as monitored using polarized Raman spectroscopy. These measurements point to the formation of a liquid-crystalline phase of P3HT solutions within a specific time period during solvent evaporation, which leads to a conducting channel. These conclusions are based on the angular dependence of polarized Raman scattering peaks, the anisotropy in the fluorescence background signal, analysis of the scattering-peak shape, and direct observations of the three-phase contact line in an optical microscope under crossed polarizers. These results shed new light on the evolution of chain alignment and thus materials nanostructure, specifically in solution-processed P3HT and more generally in π-conjugated systems. They may further enable the design of improved materials and processes for this important class of polymers.

A novel approach to o‐nitrobenzyl photochemistry for resists

We have developed a two‐component (resin‐solution inhibitor) resist system having good photosensitivity in the 230–300 nm range. The resin is an optically transparent methyl methacrylate‐methacrylic acid copolymer that is soluble in aqueous alkali. The second component is one of a family of o‐nitrobenzyl carboxylates that are initially insoluble in the alkaline developer but are cleaved to soluble components upon irradiation. A number of o‐nitrobenzyl alcohol esters have been prepared and examined for use in this two‐component system. They have proven useful in the development of sensitive high‐resolution short‐wavelength photoresists which are described.

Radiation chemistry of polymeric materials: novel chemistry and applications for microlithography

In the last two decades, major advances in fabricating very large scale integration (VLSI) electronic devices have placed increasing demands on microlithography, the technology used to generate todays integrated circuits. In 1970, state-of-the-art devices contained several thousand transistors with minimum features of 10-12 μm. Today, they have several million transistors and minimum features of less than 0.3 μm. Within the next 10-15 years, a new form of lithography will be required that routinely produces features of less than 0.2 μm. Short-wavelength (deep-UV) photolithography and scanning and projection electron-beam and X-ray lithography are the possible alternatives to conventional photolithography. The consensus candidate for the next generation of lithography tools is photolithography using 193 nm light. At this wavelength, the opacity of traditional materials precludes their use, and major research efforts to develop alternative materials are currently underway. Notably, the materials being developed for these short UV wavelengths are demonstrating compatibility with the more advanced electron-beam technologies. Materials properties must be carefully tailored to maximize lithographic performance with minimal sacrifice of other performance attributes, eg adhesion, solubility and RF plasma etching stability.

Electrical contact properties between the accumulation layer and metal electrodes in ultrathin poly(3-hexylthiophene)(P3HT) field effect transistors.

Probing contact properties between an ultrathin conjugated polymer film and metal electrodes in field effect transistors (FETs) is crucial not only to understanding charge transport properties in the accumulation layer but also in building organic sensors with high sensitivity. We investigated the contact properties between gold electrodes and poly(3-hexylthiophene) (P3HT) as a function of film thickness using gated four-point sheet resistance measurements. In an FET with a 2 nm thick P3HT film, a large voltage drop of 1.9 V (V(D) = -3 V) corresponding to a contact resistance of 2.3 × 10(8) Ω was observed. An effective FET mobility of 1.4 × 10(-3) cm(2)/(V s) was calculated when the voltage drop at the contacts was factored out, which is approximately a factor of 3 greater than the two-contact FET mobility of 5.5 × 10(-4) cm(2)/(V s). A sharp decrease in the ratio of the contact resistance to the channel resistance was observed with increasing film thickness up to a thickness of approximately 6 nm, separating a contact limited regime from a charge transport limited regime. The origin of the large contact resistance observed in the device prepared with an ultrathin P3HT film is discussed in light of results from X-ray diffraction (XRD) and atomic force microscopy (AFM) studies.