Sheng-Po Fang

Room temperature multiferroic properties of (Fex, Sr1−x)TiO3 thin films

This letter reports the structural, dielectric, ferroelectric, and magnetic properties of Fe substituted SrTiO3 thin films in room temperature. The structural data obtained from x-ray diffraction indicates that (Fex,Sr1−x)TiO3, the so called FST, transforms from pseudocubic to tetragonal structures with increase of the Fe content in SrTiO3 thin films, featuring the ferroelectricity, while vibrating sample magnetometer measurements show magnetic hysteresis loops for the samples with low iron contents indicating their ferromagnetism. The characterized ferroelectricity and ferromagnetism confirms strong multiferroitism of the single phase FST thin films in room temperature. Also, an FST thin film metal-insulator-metal multiferroic capacitor has been fabricated and characterized in microwave frequencies between 10 MHz and 5 GHz. A capacitor based on Fe0.1Sr0.9TiO3 with a thickness of 260 nm shows a high electric tunability of 18.6% at 10 V and a maximum magnetodielectric value of 1.37% at 0.4 mT with a loss t...

Immersion Lithographic Patterning of Electrospun Nanofibers for Carbon Nanofibrous Microelectrode Arrays

Immersion photolithography of electrospun SU-8 nanofibers followed by carbonization has been demonstrated for 3-D carbon nanofibrous microelectrodes. The process and resultant structures offer unique advantages: 1) precise patterning of nanofibrous microstructures via refractive index matching immersion photolithography; 2) good cell/neuron adhesion characteristic of the resultant scaffold attributed to nanomorphology in 100s nanometer scale; 3) high electrical conductivity of the carbonous microelectrodes appropriate for neural stimulation and signal detection; and 4) biocompatibility of the carbon nanofibers. Immiscible refractive index matching liquid, such as oil (noil = 1.47), has been applied to the nonhomogeneous nanofiber stack consisting of SU-8 nanofibers (nSU-8 = 1.67) and air (nair = 1) before ultraviolet light exposure, which greatly suppresses optical diffraction and scattering effects, resulting in high aspect ratio 3-D nanofibrous microstructures and enhanced patterning resolution. The aspect ratio of the fabricated 3D structures is increased from 0.26 to 0.89 and the contrast ratio from 0.56 to 0.96, compared with ones from the nonimmersion process. Ray tracing simulation taking into account diffraction effects in nanofibrous media has been discussed. Microelectrode arrays consisting of integrated nanofibers and thin-film carbon structures are implemented for in vitro neuron culture experiments. Experimental results of structures surface roughness, electrical conductivity, and cell viability on them are detailed.

Nanomanufacturing of large area carbon nanofibers using tube nozzle electrospinning (TNE), lithography and carbonization processes

This paper reports on the tube nozzle electrospinning (TNE) technique for the large scale production of electrospun nanofibers with a single plug-and-play unit to be added to the conventional electrospinning setup; lithographical micropatterning of the nanofibers; and carbonization of the patterned nanofibers. Nozzle architectures with diameters of 0.2mm and 0.5mm with 1mm and 0.5mm spacing are implemented and tested for nanofiber productivity, porosity, and uniformity. Electrospun SU8 nanofibers are achieved with no beads at an operating voltage of 10kV and a tip-collector-distance (TCD) of 7.5cm. Nanofiber morphologies are characterized with different operating conditions. The diameter of nanofibers ranges from 287nm to 566nm. Nanofibers with a porosity of 29.1% are accomplished with a single two minute electrospinning cycle and an average pore size of 0.31μm2. The TNE approach incorporated with a microcontrolled stage produces a uniform deposition of electrospun SU8 nanofiber on a 6” × 8” collector substrate in a semi-automated process. Studies on nozzle diameter and spacing for maximum liquid throughput, porosity, and uniformity of deposited nanofibers are discussed. Lithographical patterning and carbonization of the electrospun nanofibers are performed as post electrospun processes to produce patterned carbon nanofibers.

Scale of Carbon Nanomaterials Affects Neural Outgrowth and Adhesion

Carbon nanomaterials have become increasingly popular microelectrode materials for neuroscience applications. Here we study how the scale of carbon nanotubes and carbon nanofibers affect neural viability, outgrowth, and adhesion. Carbon nanotubes were deposited on glass coverslips via a layer-by-layer method with polyethylenimine (PEI). Carbonized nanofibers were fabricated by electrospinning SU-8 and pyrolyzing the nanofiber depositions. Additional substrates tested were carbonized and SU-8 thin films and SU-8 nanofibers. Surfaces were O2-plasma treated, coated with varying concentrations of PEI, seeded with E18 rat cortical cells, and examined at 3, 4, and 7 days in vitro (DIV). Neural adhesion was examined at 4 DIV utilizing a parallel plate flow chamber. At 3 DIV, neural viability was lower on the nanofiber and thin film depositions treated with higher PEI concentrations which corresponded with significantly higher zeta potentials (surface charge); this significance was drastically higher on the nanofibers suggesting that the nanostructure may collect more PEI molecules, causing increased toxicity. At 7 DIV, significantly higher neurite outgrowth was observed on SU-8 nanofiber substrates with nanofibers a significant fraction of a neurons size. No differences were detected for carbonized nanofibers or carbon nanotubes. Both carbonized and SU-8 nanofibers had significantly higher cellular adhesion post-flow in comparison to controls whereas the carbon nanotubes were statistically similar to control substrates. These data suggest a neural cell preference for larger-scale nanomaterials with specific surface treatments. These characteristics could be taken advantage of in the future design and fabrication of neural microelectrodes.

Fabrication of high-aspect-ratio nanoporous high-k MTiO 3 (M= Ba, Sr, or Ba x Sr 1−x ) using anodization and hydrothermal processes

Fabrication of a nanoporous high-k material is demonstrated using anodization and hydrothermal processes. The fabrication process offers advantages of the formation of a high-aspect-ratio large surface area nanoporous structure, low temperature conformal surface conversion, and the creation of high dielectric constant perovskite ferroelectrics. The hydrothermal process is exploited to form various MTiO3 materials such as BaTiO3, SrTiO3, and BaxSr1−xTiO3 with different Ba and Sr ratios. The crystalline phase, morphology, and element composition of the nanostructured material are investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscope (EDX), to prove the formation of ordered polycrystalline MTiO3 arrays. The three dimensional high density capacitor would be realized by this fabrication method.

High magnetic field fmri compliant carbon nanofiber neural probes

In this work, a carbon nanofiber (CNF) based neural probe compliant with high magnetic field functional magnetic resonance imaging (fMRI) has been demonstrated. The CNF probe has high spatial resolution electrode patterns, which enable extracellular deep brain stimulation (DBS) and detection. As the probe is radio frequency transparent, in-situ DBS and brain imaging can be performed simultaneously. CNF shows 5 times higher neural density in-vitro. 4.7T fMRI has been performed on the CNF neural probes implanted in a rats brain ex-vivo and no image distortion has been observed under spin or gradient echo modes.

Fabrication and Characterization of Nanoporous Metallic Interconnects Using Electrospun Nanofiber Template and Electrochemical Deposition

In this work, a nanocomposite interconnect consisting of ferromagnetic and non-ferromagnetic materials is theoretically and experimentally investigated. The relative magnetic permeability of ferromagnetic nanofibers becomes negative above the ferromagnetic resonance frequency and induces eddy current in an opposite direction of that of non-ferromagnetic conductor, resulting in eddy current cancellation. This ultimately suppresses the skin effect contributing to the reduction of radio frequency resistance. For nanofiber fabrication, an alternating electrospinning technique is utilized to improve the nanofiber growth rate and increase the thickness of the nanofiber stack. Aligned nanofibers are obtained by dynamically rotating mandrel combined with alternating electrospinning. Nanoporous conductors with the composition of non-ferromagnetic and ferromagnetic conductors are realized by electrochemical deposition of copper with the ferromagnetic nanofiber stack as a plating template.

Directional through Glass Via (TGV) Antennas for Wireless Point-to-Point Interconnects in 3D Integration and Packaging

A directional Through Glass Via (TGV) antenna is designed in a glass interposer layer for W-band (75GHz – 110GHz) wireless point-to-point chip communications with a center frequency of 78 GHz. The TGV is utilized as a main radiator (monopole antenna), which is fed by the coplanar waveguide (CPW) on the top side of the glass substrate, a circular disc is loaded on the tip of the TGV for input impedance matching, and an array of TGV reflectors are placed in proximity along with the perimeter of the circular disc in one side of the monopole antenna, forming a directional radiation pattern in the opposite direction of the reflectors. The directional radiation pattern enables not only to realize point-to-point lateral wireless communications with a distance of up to a few cm in 3D System-in-Packaging (SiP), but also to serve as an electromagnetic shield, preventing the EM signals from being transmitted towards the unwanted directions. With the wireless interconnect approach, enhanced signal integrity over far distance data transmission is expected such as low latency and low cross talk. A test antenna is designed and fabricated on the glass substrate, and a return loss of 21.6 dB at 78 GHz and a 10 dB bandwidth of 16.6 % (70.75 GHz to 83.67 GHz) have been achieved. The measured and simulated radiation frequencies are matched well. The simulated front-to-back ratio and gain of the antenna is 2.3 dBi and 20.3 dB, respectively.

A carbon nanofiber (CNF) based 3-D microelectrode array for in-vitro neural proliferation and signal recording

Microelectrode arrays (MEAs) are commonly utilized for stimulating and recording extracellular electrical signals including both local field and action potentials in both in-vitro and in-vivo neural studies. This work demonstrates the feasibility of 3D microelectrode arrays using a novel carbon nanomaterial, electrospun carbon nanofiber (CNF). CNF MEAs impedance is characterized and compared to that of carbon nanotube MEAs and commercial TiN MEAs. An in-vitro culture of CNF electrodes are performed using E18 cortical neurons and analyzed for cell interaction. With these electrodes we are able to detect extracellular neural signals including action potentials from single neurons from an array of CNF electrodes embedded within the substrate of an MEA.

Glass interposer integrated millimeter wave antennas for inter-/intra chip communications

Millimeter wave antennas are integrated in the glass interposer layer for wireless inter-/intra chip/board communications. For in-plane communication, a disc loaded monopole antenna with an omni-directional radiation pattern is designed while for out-of plane communication, a similar architecture with patch mode radiation is configured. These antennas are useful for wireless interconnects in three dimensional integrated systems in package.