A main driving force behind the interest in semiconducting nanowires is the unique capability to engineer novel heterostructures for various high-performance electronic devices. In the same way, it is also possible to achieve high-quality growth despite using lattice mismatched substrate materials. This work is focused on the growth of patterned self-catalyzed GaAs nanowire arrays on silicon substrates by gas source molecular beam epitaxy (MBE), which is expected to make an excellent candidate for high-efficiency photovoltaic applications. Patterning is used to produce the controlled nanowire morphology, uniformity and areal densities necessary for optimal ensemble nanowire devices. A template of nanoscale holes can be defined in a thin (100-300 Å) oxide layer, facilitating the growth of positioned, epitaxial nanowires while avoiding accompanying parasitic film deposition.

In our work we have used electron beam lithography as the patterning method and show that the silicon oxide film may be both thermally grown or may be deposited by chemical vapor deposition. The yield and morphology of vertically aligned nanowires has been studied as a function of the pattern parameters such as hole diameter and inter-hole spacing. Using cross-sectional transmission electron microscopy (TEM) samples prepared using a focused ion beam technique, important features of the nanowire nucleation, growth and structure have been studied. In particular, we show that a linearly increasing length-radius distribution, analogous to that observed for unpatterned self-catalyzed growth on substrates with thin oxides, may be obtained even when using patterned oxide masks due to an unintended residual layer of oxide, as confirmed by TEM analysis.

We explain how a linear length-radius dependence can result from the individual NWs beginning their growth at different times, accompanied by significant radial growth. We then show how the spread in obtained NW dimensions is significantly decreased using improved etching practices which ensure the complete removal of the oxide layer. Our experimental results also show that the axial and lateral growth rates increase strongly with increasing the interhole spacing. We account for this by proposing that a significant proportion of growth material is supplied by a secondary flux of adatoms desorbing from the oxide surface between the nanowires. Shadowing of this flux by neighboring nanowires in the array may therefore have a strong effect on the overall growth rates and the subsequent nanowire morphology, which will be characterized in an accompanying growth model.

Tissue engineering is a branch of science that deals with the design and synthesis of biomaterials with properties such that promote tissue regeneration by cell proliferation and differentiation. Biocompatible polymers have been widely investigated for the development of biomaterials due to their low cytotoxicity, together with its ability to form polymeric scaffolds with morphology suitable for mechanical support of the tissue. Also, various techniques have been developed which allow functionalization of polymeric scaffolds in order to improve such mechanical and biological properties, thus yielding, high quality hybrid materials with application in the area of tissue engineering.

In this study, graphene oxide sheets (GrO) were functionalized with hydroxyapatite nanoparticles (nHAp) through a simple and effective hydrothermal treatment and a new physicochemical process. Microstructure and crystallinity of the new hybrid nanomaterial GrO/nHap were investigated by Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, X-ray diffraction (XRD) and thermo-gravimetric analysis (TGA). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed to characterize the morphology of the functionalized material.

To analyze biological properties, the composite material was functionalized using three different GrO:nHap ratios. In order to evaluate the cytotoxicity and cell proliferation the obtained materials were subjected to a MTT assay using the NIH-3T3 cell line. Polymeric scaffolds based in chitosan-polyvinyl alcohol (PVA-Ch) co-polymer were fabricated, integrating the hybrid material GrO/nHap at different concentrations (1-5 wt %), using a physical technique. The morphological characteristics of the resulting biomaterials were observed by SEM. The technique parameters were modified to increase the surface area and pore size of the biopolymer scaffolds. Confocal electron microscopy and SEM were performed to observe cell adhesion on the composite. Cell viability of the polymeric scaffolds reinforced with the hybrid materials was measured by MTT assay, using the same cell line described before.

The mechanical and physical properties will be characterized using thermal analysis (TGA and DSC) and mechanical tester. The resulting novel materials combine the biocompatibility of the nHap with the strength and physical properties of the graphene improving the properties of the polymer matrices, thereby obtaining a biomaterial with potential applications in tissue engineering.

Origami engineering—the practice of creating useful 3D structures through folding and fold-like operations on 2D building-blocks—has the potential to improve the design of engineered systems in many ways. Potential advantages include reduced manufacturing complexity (reduced part count, improved assembly via collapsible/deployable parts), the capability to create structures at small scales (microscale and nanoscale folding and self-assembly), the capability to create deployable structures that fold compactly for storage (solar arrays and other space structures), and the potential to create structures that are highly resilient through their capability to perform in-situ reconfiguration.

This talk will describe a concept for a self-folding reprogrammable sheet for use in origami engineering applications. The sheet is a laminate structure consisting of a compliant medium sandwiched between two shape memory alloy (SMA) mesh layers. The SMA layers are thermally actuated, with the final 3D structure determined by the locations, durations, and sequencing of applied heat. The direction of actuation is determined by whether heat is applied to the top or bottom SMA layer. Folds are approximated by localized deformations in the sheet. These are fully reversible, allowing the sheets to be fully reprogrammable. The talk will cover the design and analysis of the sheets and present results from fabrication, testing, and applications.

Organic electrochemical transistors (OECTs) have been targeted for a variety of and diagnostic applications owing largely to their efficient transduction of ionic to electronic signals. In these devices, the transistor drain current is modulated by de-doping of the PEDOT:PSS {poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate)} channel due to local variations of ion flux induced by, for example, neural activity. Owing to the efficient ion mobility and high capacitance in hydrated PEDOT:PSS, we are able to fabricate devices with high intrinsic amplification (transconductance), and when scaled to micron dimensions, broad-band response up to 10 kHz.

Along with facile and robust/conformal fabrication schemes, these devices show great promise for a number of neuroscience applications. By studying film morphology and by systematically varying OECT device geometry, we develop a fundamental understanding of device operation and establish design rules for practical implementation of OECTs and for both research and clinical applications. Considering the recording capabilities of common measurement techniques, and the nature (amplitude, frequency) of neural signals, we describe how a tradeoff between OECT device response time and transconductance can be navigated.

With this scheme, we demonstrate the use of OECTs to record low amplitude, low frequency neural oscillations, high amplitude epileptiform activity, and show that measurements of individual action potentials are within reach. Thus, these devices can be tailored for various applications depending on the desired or required content of neural signals.

Organic materials have enabled large scale semiconductor production on flexible substrates, but are often more mechanically fragile than their inorganic counterparts with a higher tendency for adhesive and cohesive failure. We use a thin-film adhesion technique to quantify the impact of various processing, film structure and environmental variables on the cohesion and adhesive properties of organic semiconductor materials and their interfaces. Specifically, we will compare solution processing (spin, spray and slot die coating) with thermal evaporation, and demonstrate that overall solution processing leads to improved adhesion.

We also show the importance of various film structure parameters, such as the polymer layer thickness, composition and molecular weight. We discuss how to tune key interfacial and film parameters, such as interface chemistry, bonding and morphology, by thermal annealing to improve the adhesion. For example, the P3HT:PCBM/PEDOT:PSS interface in an inverted polymer solar cell has an adhesive value of only ~1.5 to 2 J/m2, and can be significantly increased by post electrode deposition thermal annealing time and temperature.

Using near edge X-ray absorption fine structure (NEXAFS), we precisely quantified the interfacial composition at the delaminated surfaces and correlated the increase in adhesion to changes in the interfacial structure. The over 50% increase in adhesion is caused by the development and expansion of an intermixed layer of P3HT:PCBM and PEDOT:PSS at already 45C annealing. However, thermal annealing before electrode deposition and above the crystallization temperature of PCBM (120C) should be avoided to ensure device reliability. At these conditions, micrometer sized PCBM aggregates form that not only weaken the P3HT:PCBM but also decrease the device efficiency.

The structural and chemical reorganizations are correlated with glass temperature and crystallization temperature of the materials used in the structure and thus the conclusion can be generalized to other materials systems. Understanding the interlayer adhesion and developing strategies to improve the adhesion of organic semiconductors is essential to improve the overall mechanical integrity and yield general guidelines for the design and processing of reliable organic electronic devices.

Fluorenylmethyloxycarbonyl diphenylalanine (Fmoc-FF) hydrogels have biocompatibility and viscoelasticity comparable to extracellular matrices and commonly used biopolymers. As such, they are proposed as a promising new material for regenerative medicine applications. Fmoc-FF hydrogels self-assemble through molecular stacking of molecules to form three-dimensional networks of ordered fibril structures, ideal for use as tissue engineering scaffolds or biomedical device coatings. Additionally, the self-assembly process is a simple, cost effective method for manufacturing these nanomaterials on a large scale.

The existence of piezoelectric properties could facilitate the further application of Fmoc-FF fibrous networks to applications where electrical or mechanical stimuli can be used to promote tissue regeneration. For example, bone and nerve regeneration have both been identified as being sensitive to piezoelectric properties. The direct piezoelectric effect has been linked with the ability of bone to remodel in response to an applied stress.

Piezoelectricity has also been shown to promote in-vitro axonal regeneration following nerve injury. Here, we report local shear piezoelectricity in self-assembled peptide hydrogels composed of Fmoc-FF nanofibrils, measured by piezoresponse force microscopy (~1-2 pm/V - comparable to collagen ~1-2 pm/V). The nanofibrillar nature of the gel is further confirmed by scanning electron microscopy, transmission electron microscopy, and helium ion microscopy. Also, comparisons of fluorescence emission spectra measured for Fmoc-FF in solvent and in the gel phase suggest that pi-stacking interactions between Fmoc moieties facilitate nanofibril formation.

Structural analyses (circular dichroism and attenuated total reflectance-Fourier transform infrared spectroscopy) confirm the Fmoc-FF molecules within the fibrous network are predominantly in a β-sheet conformation, similar to the dominant structure observed in diphenylalanine nanotubes. Therefore, the non-centrosymmetric nature of the β-sheet is likely to be responsible for the observed piezoelectricity in the Fmoc-FF hydrogels as well.

Amorphous hydrogenated silicon carbide (a-SiC:H) and silicon-carbon alloys are of significant interest for a number of interesting applications including microelectronic, optoelectronic, MEM/NEM, and biomedical devices due to it’s large bandgap (2-3 eV), high oxidation resistance, high Young’s modulus and hardness, and biocompatibility. Most recently, plasma deposited a-SiC:H has garnered additional interest as a potential low dielectric constant (low-k) material due to the ability to dramatically reduce k through the introduction of significant amounts of nano-porosity through careful control of hydrogen and terminal methyl group content in the as deposited films.

As we will demonstrate in this report, the ability to precisely tune the hydrogen/terminal methyl group content in a-SiC:H allows a remarkable range of material properties to be observed that can be concisely explained using the Phillips-Thorpe Bond Constraint-Percolation theory originally developed for oxide and chalcogenide glasses. We will specifically demonstrate that a remarkable range in dielectric constant (< 3 - > 7), Young’s Modulus (< 5 - > 200 GPa), and thermal conductivity (0.09 - 4 W/mK) can be achieved in plasma deposited a-SiC:H films and that the range of observed properties is directly related to the average network and bond coordination of the films. We will additionally demonstrate how critical inflections points in the observed structure-property relationships can be easily explained using the Phillips-Thorpe Constraint Theory.

The long-term performance of introcortical neural probes is often complicated by a foreign body reaction that consists of accumulation of microglia, neuronal apoptosis and an insulating glial sheath around the implants. This extensive gliosis has been associated with the system impedance increase and signal deterioration and loss of the devices.

Previously, we have proposed that the in vivo polymerization of a conducting polymer, poly (3,4 ethylene dioxythiophene) (PEDOT), in living tissue may help to build a conducting pathway between the retreated neurons and the probe. The EDOT monomer can be infused into the tissue with a microcannula/electrode guide followed by electrochemical polymerization under the oxidative current through the electrodes.

Here we examined the effects of this in vivo method by polymerizing PEDOT in living rat hippocampus at different time points post initial device implantation. We found that the system impedance was decreased for all the groups regardless of scarring stage. However, there seemed to be an optimal time window for sustained impedance improvement. The tissue responses to the polymer were examined with immunohistology.

We also investigated the effects of polymerization on local neural function with a hippocampus-dependent behavior test, delayed alternation (DA). Compared to the control groups, in vivo polymerization did not cause significant deficit on the hippocampal function.

The high hole mobility in germanium has motivated a renewed interest in this semiconductor for future electronic devices. In particular, its integration as channel material in Complementary Metal Oxide Semiconductor (CMOS) architectures would improve operation speed. However, many issues still need to be addressed before Ge can be efficiently integrated in high performance MOSFETs. Among them a critical problem is to provide a high-quality interfacial layer. The recent observation by electrically detected electron spin resonance spectroscopy (EDMR) of Pb-like centers at the Ge/GeO2 interface [1-3] allowed a more systematic investigation of the passivation of this technologically relevant interface considering both the Ge(001) and the increasingly attractive Ge(111) orientations. In this work we will report on the characterization of different interfaces produced on both Ge(100) and Ge(111) substrates by Al2O3 direct growth using atomic layer deposition (ALD), and GeO2 and sulfur passivation. We will report EDMR results correlated with admittance spectroscopy of the interface traps, and deep level transient spectroscopy (DLTS) measurements.

[1] S. Baldovino, A. Molle, and M. Fanciulli, Appl. Phys. Lett. 93, 242105 (2008)

[2] S. Baldovino, A. Molle, and M. Fanciulli, Appl. Phys. Lett. 96, 222110 (2010)

[3] S. Paleari, S. Baldovino, A. Molle, M. Fanciulli, Phys Rev. Lett. 110, 206101 (2013)