Real-time characterization of uptake kinetics of glioblastoma vs. astrocytes in 2D cell culture using microelectrode array.

Extracellular measurement of uptake/release kinetics and associated concentration dependencies provides mechanistic insight into the underlying biochemical processes. Due to the recognized importance of preserving the natural diffusion processes within the local microenvironment, measurement approaches which provide uptake rate and local surface concentration of adherent cells in static media are needed. This paper reports a microelectrode array device and a methodology to measure uptake kinetics as a function of cell surface concentration in adherent 2D cell cultures in static fluids. The microelectrode array simultaneously measures local concentrations at five positions near the cell surface in order to map the time-dependent concentration profile which in turn enables determination of surface concentrations and uptake rates, via extrapolation to the cell plane. Hydrogen peroxide uptake by human astrocytes (normal) and glioblastoma multiforme (GBM43, cancer) was quantified for initial concentrations of 20 to 500 µM over time intervals of 4000 s. For both cell types, the overall uptake rate versus surface concentration relationships exhibited non-linear kinetics, well-described by a combination of linear and Michaelis-Menten mechanisms and in agreement with the literature. The GBM43 cells showed a higher uptake rate over the full range of concentrations, primarily due to a larger linear component. Diffusion-reaction models using the non-linear parameters and standard first-order relationships are compared. In comparison to results from typical volumetric measurements, the ability to extract both uptake rate and surface concentration in static media provides kinetic parameters that are better suited for developing reaction-diffusion models to adequately describe behavior in more complex culture/tissue geometries. The results also highlight the need for characterization of the uptake rate over a wider range of cell surface concentrations in order to evaluate the potential therapeutic role of hydrogen peroxide in cancerous cells.

Evidence of Universal Temperature Scaling in Self-Heated Percolating Networks.

During routine operation, electrically percolating nanocomposites are subjected to high voltages, leading to spatially heterogeneous current distribution. The heterogeneity implies localized self-heating that may (self-consistently) reroute the percolation pathways and even irreversibly damage the material. In the absence of experiments that can spatially resolve the current distribution and a nonlinear percolation model suitable to interpret them, one relies on empirical rules and safety factors to engineer these materials. In this paper, we use ultrahigh resolution thermo-reflectance imaging, coupled with a new imaging processing technique, to map the spatial distribution ΔT(x, y; I) and histogram f(ΔT) of temperature rise due to self-heating in two types of 2D networks (percolating and copercolating). Remarkably, we find that the self-heating can be described by a simple two-parameter Weibull distribution, even under voltages high enough to reconfigure the percolation pathways. Given the generality of the phenomenological argument supporting the distribution, other percolating networks are likely to show similar stress distribution in response to sufficiently large stimuli. Furthermore, the spatial evolution of the self-heating of network was investigated by analyzing the spatial distribution and spatial correlation, respectively. An estimation of degree of hotspot clustering reveals a mechanism analogous to crystallization physics. The results should encourage nonlinear generalization of percolation models necessary for predictive engineering of nanocomposite materials.

Single-Layer Graphene as a Barrier Layer for Intense UV Laser-Induced Damages for Silver Nanowire Network.

Single-layer graphene (SLG) has been proposed as the thinnest protective/barrier layer for wide applications involving resistance to oxidation, corrosion, atomic/molecular diffusion, electromagnetic interference, and bacterial contamination. Functional metallic nanostructures have lower thermal stability than their bulk forms and are therefore susceptible to high energy photons. Here, we demonstrate that SLG can shield metallic nanostructures from intense laser radiation that would otherwise ablate them. By irradiation via a UV laser beam with nanosecond pulse width and a range of laser intensities (in millions of watt per cm(2)) onto a silver nanowire network, and conformally wrapping SLG on top of the nanowire network, we demonstrate that graphene "extracts and spreads" most of the thermal energy away from nanowire, thereby keeping it damage-free. Without graphene wrapping, the radiation would fragment the wires into smaller pieces and even decompose them into droplets. A systematic molecular dynamics simulation confirms the mechanism of SLG shielding. Consequently, particular damage-free and ablation-free laser-based nanomanufacturing of hybrid nanostructures might be sparked off by application of SLG on functional surfaces and nanofeatures.

Effect of diameter variation on electrical characteristics of Schottky barrier indium arsenide nanowire field-effect transistors.

The effect of diameter variation on electrical characteristics of long-channel InAs nanowire metal-oxide-semiconductor field-effect transistors is experimentally investigated. For a range of nanowire diameters, in which significant band gap changes are observed due to size quantization, the Schottky barrier heights between source/drain metal contacts and the semiconducting nanowire channel are extracted considering both thermionic emission and thermally assisted tunneling. Nanowires as small as 10 nm in diameter were used in device geometry in this context. Interestingly, while experimental and simulation data are consistent with a band gap increase for decreasing nanowire diameter, the experimentally determined Schottky barrier height is found to be around 110 meV irrespective of the nanowire diameter. These observations indicate that for nanowire devices the density of states at the direct conduction band minimum impacts the so-called branching point. Our findings are thus distinctly different from bulk-type results when metal contacts are formed on three-dimensional InAs crystals.

Interface studies of N2 plasma-treated ZnSnO nanowire transistors using low-frequency noise measurements.

Due to the large surface-to-volume ratio of nanowires, the quality of nanowire-insulator interfaces as well as the nanowire surface characteristics significantly influence the electrical characteristics of nanowire transistors (NWTs). To improve the electrical characteristics by doping or post-processing, it is important to evaluate the interface characteristics and stability of NWTs. In this study, we have synthesized ZnSnO (ZTO) nanowires using the chemical vapor deposition method, characterized the composition of ZTO nanowires using x-ray photoelectron spectroscopy, and fabricated ZTO NWTs. We have characterized the current-voltage characteristics and low-frequency noise of ZTO NWTs in order to investigate the effects of interface states on subthreshold slope (SS) and the noise before and after N2 plasma treatments. The as-fabricated device exhibited a SS of 0.29 V/dec and Hooge parameter of ~1.20 × 10(-2). Upon N2 plasma treatment with N2 gas flow rate of 40 sccm (20 sccm), the SS improved to 0.12 V/dec (0.21 V/dec) and the Hooge parameter decreased to ~4.99 × 10(-3) (8.14 × 10(-3)). The interface trap densities inferred from both SS and low-frequency noise decrease upon plasma treatment, with the highest flow rate yielding the smallest trap density. These results demonstrate that the N2 plasma treatment decreases the interface trap states and defects on ZTO nanowires, thereby enabling the fabrication of high-quality nanowire interfaces.

Utilizing the unique properties of nanowire MOSFETs for RF applications.

Nanostructures have attracted a great deal of attention because of their potential usefulness for high density applications. More importantly, they offer excellent avenues for improved scaling beyond conventional approaches. Less attention has been paid to their intrinsic potential for distinct circuit applications. Here we discuss how a combination of 1-D transport, operation in the quantum capacitance limit, and ballistic transport can be utilized for certain RF applications. In particular this work explores how the above transport properties can provide a high degree of transconductance linearity at the device level. The article also discusses how device characteristics can be interpreted and analyzed in terms of device linearity if the above conditions are not ideally fulfilled. Using aggressively scaled silicon nanowire field-effect transistors as an example device in this work provides new insights toward the proper choice of channel material to improve linearity through the above-mentioned transport conditions. According to this study, a high degree of linearity occurs feasible while operating at low supply voltages making low-dimensional systems, and here in particular nanowires, an interesting candidate for portable RF applications.

Wavelength-dependent absorption in structurally tailored randomly branched vertical arrays of InSb nanowires.

Arrays of semiconductor nanowires are of potential interest for applications including photovoltaic devices and IR detectors/imagers. While nominally uniform arrays have typically been studied, arrays containing nanowires with multiple diameters and/or random distributions of diameters could allow tailoring of the photonic properties of the arrays. In this Letter, we demonstrate the growth and optical properties of randomly branched InSb nanowire arrays. The structure mentioned can be approximated as three vertically stacked regions, with average diameters of 20, 100, and 150 nm within the respective layers. Reflectance and transmittance measurements on structures with different average nanowire lengths have been performed over the wavelength range of 300-2000 nm, and absorbance has been calculated from these measurements. The structures show low reflectance over the visible and IR regions and wavelength-dependent absorbance in the IR region. A model considering the diameter-dependent photonic coupling (at a given wavelength) and random distribution of nanowire diameters within the regions has been developed. The diameter-dependent photonic coupling results in a roll-off in the absorbance spectra at wavelengths well below the bulk cutoff of ∼7 µm, and randomness is observed to broaden the absorbance response. Varying the average diameters would allow tailoring of the wavelength dependent absorption within various layers, which could be employed in photovoltaic devices or wavelength-dependent IR imagers.

Effect of nitrogen plasma on the surface of indium oxide nanowires.

The change in the atomic nitrogen concentration on a semiconducting nanowire's surface and the consequent changes in the electrical characteristics of a nanowire transistor were investigated by exposing In(2)O(3) nanowires to nitrogen (N(2)) plasma. After plasma was applied at N(2) flow rates of 20, 40, and 70 sccm with a fixed source power of 50 W, the In(2)O(3) nanowire transistor exhibited changes in the threshold voltage (V(th)), subthreshold slope (SS), and on-current (I(on)). In particular, after treatment at an N(2) flow rate of 40 sccm, V(th) shifted positively by ~2.3 V, the SS improved by ~0.24 V/dec, and I(on) increased by ~0.8 µA on average. The changes are attributed to the combination of nitrogen ions produced by the plasma with oxygen vacancies or indium interstitials on the nanowires. Optimization of the plasma treatment conditions is expected to yield desirable device characteristics by a simple, nondestructive process.

Understanding the impact of Schottky barriers on the performance of narrow bandgap nanowire field effect transistors.

Semiconductor nanowires have been explored as alternative electronic materials for high performance device applications exhibiting low power consumption specs. Electrical transport in III-V nanowire (NW) field-effect transistors (FETs) is frequently governed by Schottky barriers between the source/drain and the NW channel. Consequently the device performance is greatly impacted by the contacts. Here we present a simple model that explains how ambipolar device characteristics of NW-FETs and in particular the achievable on/off current ratio can be analyzed to gain a detailed idea of (a) the bandgap of the synthesized NWs and (b) the potential performance of various NW materials. In particular, we compare the model with our own transport measurements on InSb and InAs NW-FETs as well as results published by other groups. The analysis confirms excellent agreement with the predictions of the model, highlighting the potential of our approach to understand novel NW based materials and devices and to bridge material development and device applications.

Role of self-assembled monolayer passivation in electrical transport properties and flicker noise of nanowire transistors.

Semiconductor nanowires have achieved great attention for integration in next-generation electronics. However, for nanowires with diameters comparable to the Debye length, which would generally be required for one-dimensional operation, surface states degrade the device performance and increase the low-frequency noise. In this study, single In(2)O(3) nanowire transistors were fabricated and characterized before and after surface passivation with a self-assembled monolayer of 1-octadecanethiol (ODT). Electrical characterization of the transistors shows that device performance can be enhanced upon ODT passivation, exhibiting steep subthreshold slope (~64 mV/dec), near zero threshold voltage (~0.6 V), high mobility (~624 cm(2)/V·s), and high on-currents (~40 µA). X-ray photoelectron spectroscopy studies of the ODT-passivated nanowires indicate that the molecules are bound to In(2)O(3) nanowires through the thiol linkages. Device simulations using a rectangular geometry to represent the nanowire indicate that the improvement in subthreshold slope and positive shift in threshold voltage can be explained in terms of reduced interface trap density and changes in fixed charge density. Flicker (low-frequency, 1/f) noise measurements show that the noise amplitude is reduced following passivation. The interface trap density before and after ODT passivation is profiled throughout the band gap energy using the subthreshold current-voltage characteristics and is compared to the values extracted from the low-frequency noise measurements. The results indicate that self-assembled monolayer passivation is a promising optimization technology for the realization of low-power, low-noise, and fast-switching applications such as logic, memory, and display circuitry.

Control of current saturation and threshold voltage shift in indium oxide nanowire transistors with femtosecond laser annealing.

Transistors based on various types of nonsilicon nanowires have shown great potential for a variety of applications, especially for those that require transparency and low-temperature substrates. However, critical requirements for circuit functionality, such as saturated source-drain current and matched threshold voltages of individual nanowire transistors in a way that is compatible with low temperature substrates, have not been achieved. Here we show that femtosecond laser pulses can anneal individual transistors based on In(2)O(3) nanowires, improve the saturation of the source-drain current, and permanently shift the threshold voltage to the positive direction. We applied this technique and successfully shifted the switching threshold voltages of NMOS-based inverters and improved their noise margin, in both depletion and enhancement modes. Our demonstration provides a method to trim the parameters of individual nanowire transistors, and suggests potential for large-scale integration of nanowire-based circuit blocks and systems.

DC modeling and the source of flicker noise in passivated carbon nanotube transistors.

DC and intrinsic low-frequency noise properties of p-channel depletion-mode carbon nanotube field effect transistors (CNT-FETs) are investigated. To characterize the intrinsic noise properties, a thin atomic layer deposited (ALD) HfO(2) gate dielectric is used as a passivation layer to isolate CNT-FETs from environmental factors. The ALD HfO(2) gate dielectric in these high-performance top-gated devices is instrumental in attaining hysteresis-free current-voltage characteristics and minimizes low-frequency noise. Under small drain-source voltage, the carriers in the CNT channel are modulated by the gate electrode and the intrinsic 1/f noise is found to be correlated with charge trapping/detrapping from the oxide substrate as expected. When thermionic emission is the dominant carrier transport mechanism in CNT-FETs under large drain-source voltages, the excess 1/f noise is attributed to the noise stemming from metal-CNT Schottky barrier contacts as revealed by the measurements.

Global public health implications of a mass gathering in Mecca, Saudi Arabia during the midst of an influenza pandemic.

BACKGROUND: Every year millions of pilgrims from around the world gather under extremely crowded conditions in Mecca, Saudi Arabia to perform the Hajj. In 2009, the Hajj coincided with influenza season during the midst of an influenza A (H1N1) pandemic. After the Hajj, resource-limited countries with large numbers of traveling pilgrims could be vulnerable, given their limited ability to purchase H1N1 vaccine and capacity to respond to a possible wave of H1N1 introduced via returning pilgrims. METHODS: We studied the worldwide migration of pilgrims traveling to Mecca to perform the Hajj in 2008 using data from the Saudi Ministry of Health and international air traffic departing Saudi Arabia after the 2008 Hajj using worldwide airline ticket sales data. We used gross national income (GNI) per capita as a surrogate marker of a country's ability to mobilize an effective response to H1N1. RESULTS: In 2008, 2.5 million pilgrims from 140 countries performed the Hajj. Pilgrims (1.7 million) were of international (non-Saudi) origin, of which 91.0% traveled to Saudi Arabia via commercial flights. International pilgrims (11.3%) originated from low-income countries, with the greatest numbers traveling from Bangladesh (50,419), Afghanistan (32,621), and Yemen (28,018). CONCLUSIONS: Nearly 200,000 pilgrims that performed the Hajj in 2008 originated from the world's most resource-limited countries, where access to H1N1 vaccine and capacity to detect and respond to H1N1 in returning pilgrims are extremely limited. International efforts may be needed to assist resource-limited countries that are vulnerable to the impact of H1N1 during the 2009 to 2010 influenza season.

Oxygen plasma exposure effects on indium oxide nanowire transistors.

In(2)O(3) nanowire transistors are fabricated with and without oxygen plasma exposure of various regions of the nanowire. In two-terminal devices, exposure of the channel region results in an increased conductance of the channel region. For In(2)O(3) nanowire transistors in which the source/drain regions are exposed to oxygen plasma, the mobility, on-off current ratio and subthreshold slope, are improved with respect to those of non-exposed devices. Simulations using a two-dimensional device simulator (MEDICI) show that improved device performance can be quantified in terms of changes in interfacial trap, shifts in fixed charge densities and the corresponding reduction in Schottky barrier height at the contacts.

Device considerations for development of conductance-based biosensors.

Design and fabrication of electronic biosensors based on field-effect-transistor (FET) devices require understanding of interactions between semiconductor surfaces and organic biomolecules. From this perspective, we review practical considerations for electronic biosensors with emphasis on molecular passivation effects on FET device characteristics upon immobilization of organic molecules and an electrostatic model for FET-based biosensors.

Electronic transport through ruthenium-based redox-active molecules in metal-molecule-metal nanogap junctions.

Electronic transport through ruthenium-based redox-active organometallic molecules is measured by self-assembling diruthenium(III) tetra(2-anilinopyridinate)-di(4-thiolphenylethynyl) (trans-Ru2(ap)4(C'CC6H4S-)2 (A) and trans- Ru2(ap)4((C'CC6H4)2S-)2 (B) molecules in nanogap molecular junctions. Voltage sweeps at a high scan rate show low bias current peaks (at +/-0.35 +/- 0.05 V for A and +/-0.27 +/- 0.05 V for B), which change to plateaus in slow bias scans and a second conductance peak at approximately +/-1.05 +/- 0.15 V. The peaks/plateaus are not observed in the return bias sweeps, possibly due to charge storage in the molecules. The energy states for the molecular orbitals of these molecules as estimated from the conductance peaks are in close agreement with the respective energy values from voltammetric measurements in solution.

Mixed adlayer of alkanethiol and peptide on GaAs(100): quantitative characterization by X-ray photoelectron spectroscopy.

Homogeneous and mixed adlayers composed of an alkanethiol (1-octadecanethiol, ODT) and a peptide (CGISYGRKKRRQRRR) on GaAs(100) were formed in two different solvent systems: phosphate-buffered saline (PBS) and N,N-dimethylformamide (DMF). The chemical composition of each adlayer was characterized by X-ray photoelectron spectroscopy (XPS). The data showed that the makeup of the adlayer and its stability largely depends on the solvent used. Angle-resolved XPS also revealed that the adlayer thickness and tilt angles were different from values obtained from ellipsometry measurements and vastly varied between the two solvents used. The coverage data extracted from the XPS measurements indicated that homogeneous adlayers of peptide in PBS buffer form a multilayered film. Homogeneous alkanethiol adlayers exhibited monolayer coverage under all solvent treatments. Coadsorbed layers containing both alkanethiol and peptide have fractional monolayer coverage in both solvents.

Probing molecules in integrated silicon-molecule-metal junctions by inelastic tunneling spectroscopy.

Molecular electronics has drawn significant attention for nanoelectronic and sensing applications. A hybrid technology where molecular devices are integrated with traditional semiconductor microelectronics is a particularly promising approach for these applications. Key challenges in this area include developing devices in which the molecular integrity is preserved, developing in situ characterization techniques to probe the molecules within the completed devices, and determining the physical processes that influence carrier transport. In this study, we present the first experimental report of inelastic electron tunneling spectroscopy of integrated metal-molecule-silicon devices with molecules assembled directly to silicon contacts. The results provide direct experimental confirmation that the chemical integrity of the monolayer is preserved and that the molecules play a direct role in electronic conduction through the devices. Spectra obtained under varying measurement conditions show differences related to the silicon electrode, which can provide valuable information about the physics influencing carrier transport in these molecule/Si hybrid devices.