In field-effect transistors (FETs) with two-dimensional (2D) transition metal dichalcogenide channels, the dependence of field-effect mobility on atomic layer thickness has been studied and interpreted in terms of interface scattering and interlayer coupling resistance (Rint). However, a model for 1/f noise, such as in MoS2 and in MoSe2 FETs, for various contact metals and layer number thicknesses has not been reported. In this work, we have experimentally studied current–voltage and 1/f noise on MoS2 and MoSe2 FETs with source and drain contacts of high and low work function metals to understand both the mobility and the noise behavior. We have developed a noise model incorporating layer number dependent Hooge parameters and Rint. The noise and mobility models utilize screening lengths for charge, mobility, and Hooge parameter to describe the variation of these quantities with a layer number. Using our single model topology with appropriate fitting parameters for each material and each contact metal, the model captures the experimentally observed layer thickness dependence of the Hooge parameter. Our noise analysis is fully comprehensive and, hence, could be applied to any 2D layered systems.

MoS2 has great interest for nanoscale electronic devices, including transistors and sensors. Defect‐free structure, clean interface with the gate dielectric, and metals with suitable metal–semiconductor junctions are key for reliable devices. Molybdenum is shown as an excellent electrode for MoS2 field‐effect transistors (FETs), but the correlation among device current, field‐effect mobility, and the low‐frequency noise (LFN) with number of atomic layers is not studied. Furthermore, the impact of Mo/MoS2 Schottky barrier on electronic injection and scattering in the device needs to be understood. Herein, by studying the FETs with channels of different numbers of atomic‐layer thicknesses, it is shown that a molybdenum metal contact to MoS2 provides reliable current injection with low Schottky barrier height, low contact resistance, low electron–phonon scattering, and negligible impurity scattering. The mobility and LFN are optimal at a channel thickness of ≈12 atomic layers, which is consistent with the “mobility maximum window” in MoS2 transistors with prior studies using other contact metals.

While current personal monitoring devices for mining industry are optimized for monitoring microscale particles, a higher resolution technique is required to detect sub-micron and nanoscale particulate matters (PM) due to smaller volume and mass of the particles. In this work, an interdigitated capacitive sensor is designed for monitoring sub-micron and nanoscale particle concentration in real-time for mining environment and other applicable environments. The 2 mm ×12 mm disposable sensor chip fits generic air sampling cassette housing. The re-usable readout board uses resistance-capacitance (RC) delay time constant for monitoring capacitance shift and data is recorded in real-time. The sensor showed clear response with respect to a commonly used test dust, and positive capacitance shift is observed after test. Among the collected particles, about 77 % are sub-micron and most of microparticles are agglomerates of sub-micron particles. A simulation study showed that our sensor response is in a region that is proportional to the particle volume collected on sensor. A comparison with gravimetric method showed that less than 1/1000 of collected particle mass on the sampler is responsible for sensor response due to radial distribution of particle deposition and sensor location. The sensor response is converted into a standard airborne mass particle concentration gm 3 to demonstrate a continuous monitoring of particle concentration. An incorporated microheater improved stable capacitive sensor reading under air flow and various humidity.

Low-frequency noise studies in transition metal dichalcogenides have considered the layer-thickness dependence but generally do not separate channel contributions from those of contact/access regions. Here, we study the voltage-dependent 1/f noise and the resistance correlation in MoS2 FETs with an ∼142 atomic layer-thickness channel and three different lengths. The gate-voltage dependence of noise can be separated into a channel contribution, with a comparable Hooge parameter for the three devices and a contact/access region contribution. Separation of these contributions allows the evaluation of the channel noise mechanism and can be used to explain the length-dependence of the transition region between contact- and channel-dominated regimes.

High-heat-flux removal is critical for the next-generation electronic devices to reliably operate within their temperature limits. A large portion of the thermal resistance in a traditional chip package is caused by thermal resistances at interfaces between the device, heat spreaders, and the heat sink; embedding the heat sink directly into the heat-generating device can eliminate these interface resistances and drastically reduce the overall thermal resistance. Microfluidic cooling within the embedded heat sink improves the heat dissipation, with two-phase operation offering the potential for dissipation of very high heat fluxes while maintaining moderate chip temperatures. To enable multichip stacking and other heterogeneous packaging approaches, it is important to densely integrate all fluid flow paths into the device; volumetric heat dissipation emerges as a performance metric in this new heat sinking paradigm. In this paper, a compact hierarchical manifold microchannel design is presented that utilizes an integrated multilevel manifold distributor to feed coolant to an array of microchannel heat sinks. The flow features in the manifold layers and microchannels are fabricated in silicon wafers using deep reactive-ion etching. The heat source is simulated via Joule heating using thin-film platinum heaters. The on-chip spatial temperature measurements are made using four-wire resistance temperature detectors. The individual manifold layers and the microchannel-bearing wafers are diced and bonded into a sealed stack via thermocompression bonding using gold layers at the mating surfaces. Thermal and hydrodynamic testing is performed by pumping the dielectric fluid HFE-7100 through the device at a known flow rate, temperature, and pressure at different levels of chip heat input. A volumetric heat density of up to 2870 W/cm3 is dissipated at a chip temperature less than 112 °C and microchannel pressure drop less than 27 kPa. The overall pressure drop is governed by flowing through the manifold, rather than the microchannels, in this compact heat sink that occupies envelope of 5 mm $\times 5$ mm $\times2.3$ mm including all the functional flow features.

A hierarchical manifold microchannel heat sink array is fabricated and experimentally characterized for uniform heat flux dissipation over a footprint area of 5 mm x 5 mm. A 3 x 3 array of heat sinks is fabricated into the silicon substrate containing the heaters for direct intrachip cooling, eliminating the thermal resistances typically associated with the attachment of a separate heat sink. The heat sinks are fed in parallel using a hierarchical manifold distributor that delivers flow to each of the heat sinks. Each heat sink contains a bank of high-aspect-ratio microchannels; five different channel geometries with nominal widths of 15 mu m and 33 um and nominal depths between 150 mu M and 470 mu m are tested. The thermal and hydraulic performance of each heat sink array geometry is evaluated using HFE-7100 as the working fluid, for mass fluxes ranging from 600 kg/m(2) s to 2100 kg/m(2) s at a constant inlet temperature of 59 degrees C. To simulate heat generation from electronics devices, a uniform background heat flux is generated with thin-film serpentine heaters fabricated on the silicon substrate opposite the channels; temperature sensors placed across the substrate provide spatially resolved surface temperature measurements. Experiments are also conducted with simultaneous background and hotspot heat generation; the hotspot heat flux is produced by a discrete 200 mu m x 200 mu m hotspot heater. Heat fluxes up to 1020 W/cm(2) are dissipated under uniform heating conditions at chip temperatures less than 69 degrees C above the fluid inlet and at pressure drops less than 120 kPa. Heat sinks with wider channels yield higher wetted-area heat transfer coefficients, but not necessarily the lowest thermal resistance; for a fixed channel depth, samples with narrower channels have increased total wetted areas owing to the smaller fin pitches. During simultaneous background and hotspot heating conditions, background heat fluxes up to 900 W/cm(2) and hotspot fluxes up to 2700 W/cm(2) are dissipated. The hotspot temperature increases linearly with hotspot heat flux; at hotspot heat fluxes of 2700 W/cm(2), the hotspot experiences a temperature rise of 16 degrees C above the average chip temperature. (C) 2018 Elsevier Ltd. All rights reserved.

Graphene-nanowire hybrid films have recently shown excellent performance as transparent conducting electrodes. However, due to their fundamental nonhomogeneity (grain boundaries, nanowire-nanowire junctions), self-heating induces hotspots along the co-percolating electrical conduction pathways. Although the steady-state characteristics of the hotspot temperature and location have been reported, the temporal response of the hotspots has not been studied in detail. In this paper, we use transient thermore-flectance imaging with high temporal resolution to quantify the transient characteristics of the hotspots. At local hotspots as well as nearby intermediate temperature regions, the temperature response in both heating and cooling phases exhibits two distinct time constants. We quantitatively determine the thermal time constants and associated amplitudes and show dependence of these parameters on distance from the contact. Based on solutions to heat diffusion equations, the short-time constant is attributed to the local self-heating within the hotspot region, and the long-time constant to diffusion of heat through the channel region. The knowledge of time evolution of hotspots and hence a more detailed understanding of the electrothermal conduction process through co-percolating networks could enable more reliable, advanced transparent conductors as well as one-dimensional/two-dimensional hybrid materials for other applications.

Nanostructured transparent conducting electrodes (TCEs) may be suitable replacements for transparent conductive oxides due to their optical, electrical, and mechanical properties. Because nanowire (NW) nanowire or tube-tube junctions are the transport bottlenecks in network-based conductors, understanding the properties of these junctions and their connectivity within the network is crucial to understanding and controlling electrical conduction through these networks. Quantifying local self-heating within the network can provide information on the coupled electrothermal response, local conduction pathways, and potential reliability. In this study, self-heating thermal transients within a silver NW network are characterized using high-resolution transient thermoreflectance imaging that provides high temporal (similar to 200 ns) and spatial (similar to 200 nm) resolution. The self-heating induced by an applied voltage pulse results in distinct temperature changes at microscopic hotspots formed at individual NW-NW junctions. For both heating and cooling cycles, thermal time constants less than 1 mu s are observed at various hotspots. For a representative hotspot, line scans along two crossing NWs, taken at different time instants ranging from 0 to 2 mu s, show the temporal and spatial evolution of the temperature profile. We estimate the van der Waals force (similar to-4.0244 N), contact width (similar to 5 nm), and interface thermal resistance (similar to 1.6 x 10(5) K/W) between NWs and the underlying substrate. A heat transfer model that considers local power generated at a hotspot, local coupling between the NWs and substrate, heat conduction along the NWs and heat transfer into the substrate, is developed and used to interpret the experimental data. The heat transfer model and experimental temperature profile help to quantify the local power generated at the hotspot and the fraction of this power propagating along each wire. The ability to resolve the local self-heating with such temporal and spatial resolution uniquely enables understanding of electrothermal response and current pathways in the distributed conductors.

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.

Two-Dimensional Transition Metal Dichalcogenides (2D TMDCs) such as MoS2, MoSe2, WS2, and WSe2 with van der Waal's type interlayer coupling are being widely explored as channel materials in a Schottky Barrier Field Effect Transistor (SB FET) configuration. While their excellent electrostatic control and high onloff ratios have been identified, a clear correlation between electronic transport and the lowfrequency noise with different atomic-layer thickness is missing. For multilayer channels in MoS2 FETs, the effects of interlayer-coupling resistance on device conductance and mobility have been studied, but no systematic study has included interlayer effects in consideration of the intrinsic (channel) and extrinsic (total device) noise behavior. Here, we report the 1/f noise properties in MoSe2 FETs with varying channel thicknesses (3-40 atomic layers). Contributions of channel vs access/contact regions are extracted from current-voltage (transport) and 1/f noise measurements. The measured noise amplitude shows a direct crossover from channel- to contact-dominated noise as the gate voltage is increased. The results can be interpreted in terms of a Hooge relationship associated with the channel noise, a transition region, and a saturated high-gate-voltage regime whose characteristics are determined by a voltage-independent conductance and noise source associated with the metallurgical contact and the interlayer resistance. Both the channel Hooge coefficient and the channel/access noise amplitude decrease with increasing channel thickness over the range of 3-15 atomic layers, with the former remaining approximately constant and the latter increasing over a range of 20-40 atomic layers. The analysis can be extended to devices based on other TMDCs.

Potential two-dimensional (2D) van der Waals crystals with mechanical flexibility, transparency, and low cost are viable material platforms for future nanodevices. Resistive switching behavior in 2D layered Sb2Te3 nanosheets is demonstrated. Nearly three orders of magnitude switch in sheet resistance were realized for more than 20 cycles. The observed hysteretic behavior is due to the change between crystalline and amorphous phases under a melt-quench-recrystallization mechanism. More importantly, the energy required to amorphize the nanosheets decreases exponentially with layer thickness reduction. It is expected that scaling to the ultimate two-dimensional limit in chalcogenide nanosheet-based phase change devices may meet or even exceed the energy efficiency of neurobiological architectures.

Two-Dimensional Transition Metal Dichalcogenides (2D TMDCs) such as MoS2, MoSe2, WS2, and WSe2 with van der Waal’s type interlayer coupling are being widely explored as channel materials in a Schottky Barrier Field Effect Transistor (SB FET) configuration. While their excellent electrostatic control and high on/off ratios have been identified, a clear correlation between electronic transport and the lowfrequency noise with different atomic-layer thickness is missing. For multilayer channels in MoS2 FETs, the effects of interlayer-coupling resistance on device conductance and mobility have been studied, but no systematic study has included interlayer effects in consideration of the intrinsic (channel) and extrinsic (total device) noise behavior. Here, we report the 1/f noise properties in MoSe2 FETs with varying channel thicknesses (3–40 atomic layers). Contributions of channel vs access/contact regions are extracted from current-voltage (transport) and 1/f noise measurements. The measured noise amplitude shows a direct crossover from channelto contact-dominated noise as the gate voltage is increased. The results can be interpreted in terms of a Hooge relationship associated with the channel noise, a transition region, and a saturated high-gate-voltage regime whose characteristics are determined by a voltage-independent conductance and noise source associated with the metallurgical contact and the interlayer resistance. Both the channel Hooge coefficient and the channel/access noise amplitude decrease with increasing channel thickness over the range of 3–15 atomic layers, with the former remaining approximately constant and the latter increasing over a range of 20–40 atomic layers. The analysis can be extended to devices based on other TMDCs.

Amperometric microelectrode arrays (MEAs) interrogate the concentration at multiple positions simultaneously and with sufficient sampling rates, thus being able to capture fast transient gradients. However, sensitivity variability issues in amperometric MEAs degrade the reliability of the measurements, particularly at the small concentration scales found in physiological studies. This paper describes the development of on-chip platinum amperometric MEAs and in situ transient calibration for reliable measurement of physiological transient concentration gradients. The designed MEA geometry facilitates positioning near a 2D cell culture setup, and the proposed in situ transient calibration minimizes the effects of sensitivity variability, thus allowing for calculation of gradients based on concentration differences between closely spaced electrodes. The effectiveness of the MEA and the in situ transient calibration was evaluated by measuring controllably-generated gradients, and then calculating the difference between experimental and simulated data using normalized time analysis. Gradients generated by periodic uptake intervals as fast as 150 ms followed by recovery intervals of 60 s were measured over a spatial range of 70 tim, with spatial resolution of 35 mu m, and sampling time and measurement time of 10 ms. Transient gradients of hydrogen peroxide were also measured above the surface of a 2D cell culture of human astrocytes, thus demonstrating the approach in actual physiological measurements. (C) 2018 Elsevier B.V. All rights reserved.

High-heat-flux removal is necessary for next-generation microelectronic systems to operate more reliably and efficiently. Extremely high heat removal rates are achieved in this work using a hierarchical manifold microchannel heat sink array. The microchannels are imbedded directly into the heated substrate to reduce the parasitic thermal resistances due to contact and conduction resistances. Discretizing the chip footprint area into multiple smaller heat sink elements with high-aspect-ratio microchannels ensures shortened effective fluid flow lengths. Phase change of high fluid mass fluxes can thus be accommodated in micron-scale channels while keeping pressure drops low compared to traditional, microchannel heat sinks. A thermal test vehicle, with all flow distribution components heterogeneously integrated, is fabricated to demonstrate this enhanced thermal and hydraulic performance. The 5 mm x 5 mm silicon chip area, with resistive heaters and local temperature sensors fabricated directly on the opposite face, is cooled by a 3 x 3 array of microchannel heat sinks that are fed with coolant using a hierarchical manifold distributor. Using the engineered dielectric liquid HFE-7100 as the working fluid, experimental results are presented for channel mass fluxes of 1300, 2100, and 2900 kg/m(2) s and channel cross sections with nominal widths of 15 mu m and nominal depths of 35 mu m, 150 mu m, and 300 mu m. Maximum heat flux dissipation is shown to increase with mass flux and channel depth and the heat sink with 15 mu m x 300 mu m channels is shown to dissipate base heat fluxes up to 910 W/cm(2) at pressure drops of less than 162 kPa and chip temperature rise under 47 degrees C relative to the fluid inlet temperature. (C) 2017 Elsevier Ltd. All rights reserved.

Understanding biological processes such as neurotransmitter release and reuptake in neurons, glucose transport for adapted cell metabolism in cancer cells involves measuring dynamic concentration gradients with diffusion time constants varying from a few hundred milliseconds to a few minutes. [1] Many prior electrochemical sensor reports utilize single electrodes, and achieve spatial information using techniques such as the self-referencing approach, which employs a moving probe. These approaches suffer from setup complexity, poor temporal and spatial resolution and poor suitability for portable, multi-analyte, high throughput applications. [2] In this work, an array of individually addressable platinum micro-electrodes was employed for amperometric measurement of transients and gradients with fast response time and good spatial resolution. The target analyte, hydrogen peroxide (H2O2), is the secondary species generated in enzymatic reactions (e.g. glucose with glucose oxidase), and is responsible for the redox reaction that lead to electrode current in such systems. Since the reaction constant for oxidation of H2O2 is large, the associated current response approximates the inherent response time of the sensor electrodes. In order to generate local gradients/transients, and eventually to mimic cellular function, a large platinum disk electrode (LPE) was used as a bias-controlled H2O2 sink [3], with time constants as low as 1s.

Although transparent conductive oxides such as indium tin oxide (ITO) are widely employed as transparent conducting electrodes (TCEs) for applications such as touch screens and displays, new nanostructured TCEs are of interest for future applications, including emerging transparent and flexible electronics. A number of two-dimensional networks of nanostructured elements have been reported, including metallic nanowire networks consisting of silver nanowires, metallic carbon nanotubes (m-CNTs), copper nanowires or gold nanowires, and metallic mesh structures. In these single-component systems, it has generally been difficult to achieve sheet resistances that are comparable to ITO at a given broadband optical transparency. A relatively new third category of TCEs consisting of networks of 1D-1D and 1D-2D nanocomposites (such as silver nanowires and CNTs, silver nanowires and polycrystalline graphene, silver nanowires and reduced graphene oxide) have demonstrated TCE performance comparable to, or better than, ITO. In such hybrid networks, copercolation between the two components can lead to relatively low sheet resistances at nanowire densities corresponding to high optical transmittance. This review provides an overview of reported hybrid networks, including a comparison of the performance regimes achievable with those of ITO and single-component nanostructured networks. The performance is compared to that expected from bulk thin films and analyzed in terms of the copercolation model. In addition, performance characteristics relevant for flexible and transparent applications are discussed. The new TCEs are promising, but significant work must be done to ensure earth abundance, stability, and reliability so that they can eventually replace traditional ITO-based transparent conductors.

A hierarchical manifold microchannel heat sink is used to dissipate heat from a small hotspot region superposed on a larger region of uniform background heat flux. A 5 mm × 5 mm overall chip footprint area is cooled using a 3 × 3 array of intrachip silicon microchannel heat sinks fed in parallel using a manifold distributor. Each heat sink consists of a bank of 25 high-aspect-ratio microchannels that are nominally 30 μm wide and 300 μm deep. The uniform background heat flux is generated with a 3 × 3 array of thin-film heaters fabricated on the chip; temperature sensors placed in each of these nine heating zones provide spatially resolved chip surface temperature measurements. An individually powered 200 μm × 200 μm hotspot heater is centered on the chip. The heat sink thermal and hydraulic performance is evaluated using HFE-7100 as the working fluid and for mass fluxes ranging from 600 kg/m2s to 2070 kg/m2s at a constant inlet temperature of 60°C and outlet pressure of 122 kPa. Background heat fluxes up to 450 W/cm2 and hotspot fluxes of greater than 2500 W/cm2 are simultaneously dissipated. The chip temperature uniformity and maximum temperature rise during hotspot heating are assessed. For the case with the highest simultaneous background and hotspot heat fluxes, the measured heat sink pressure drop is ~75 kPa and the average chip temperature is ~30°C above the fluid inlet temperature.

Hydrogen peroxide (H2O2) is commonly known as a toxic reactive oxidative species (ROS) for cells. Recent studies have found evidence that H2O2 is also an important cellular signalling molecule. Quantifying cellular influx of H2O2 will contribute to researchers’ understanding of the role H2O2 plays in healthy cells and cells involved in the progression of cancers and degenerative diseases. This work utilizes an assay kit and fluorescence techniques to evaluate cell lines and conditions to create a model biological system for measuring cellular H2O2 consumption. Pancreatic beta cells (MIN6), astrocytes, and glioblastoma cells (GBM43 and GBAM1) were cultured and placed in 10 μM and 20 μM H2O2 solutions for up to 5 hours. The consumption of H2O2 by these cells over time was measured using an Amplex Red Hydrogen Peroxide/Peroxidase Assay Kit (Molecular Probes/Invitrogen). GBAM1 cells exposed to 20 μM H2O2 displayed the fastest rate of H2O2 consumption (4.8 ± 1.2 nmol H2O2/min/10 cells), followed by GBM43 cells (1.5 ± 0.46), astrocytes (1.1 ± 0.24), and MIN6 cells (0.29 ± 0.075). Additionally, the rate of consumption increased with increases in H2O2 concentration. In the future, an on-chip micro-electrode array (MEA) will be used for real-time electrochemical experiments to measure influx of H2O2 by astrocytes and GBAM1 cells with spatio-temporal resolution that the current techniques lack. The results from the electrochemical experiments will be compared to results from the assay kit to determine the ability of the MEA to accurately measure H2O2 concentration and flux. The MEA can be extended to a wide variety of cellular environments for analysis of additional real-time biological events.