Modulation of Electrokinetic Potentials Using Graphene-Based Surfaces and Variable Substrate Charge Density

Enhanced electrokinetic phenomena, manifested through the observation of a large streaming potential (Vs), were obtained in microchannels with single-layer graphene (SLG)-coated and few-layer graphene (FLG)-coated surfaces. In comparison to silicon microchannels, the Vs obtained for a given pressure difference along the channel (ΔP) was higher by 75% for the graphene-based channels, with larger values in the SLG case. Computational modeling was used to correlate the surface charge density, tuned through plasma processing, and related zeta potential to measured Vs. The implications related to deploying lower dimensional material surfaces for modulating electrokinetic flows were investigated.

Surface Composites Synthesized through the Incorporation of Atomic Layer Deposited AlOx into Nanoporous Fuzzy Tungsten

The incorporation of energetic helium gaseous species into materials such as tungsten (W) imparts intrinsic surface fragility, yielding fuzzy tungsten. To enhance the robustness of the surface layers, aluminum oxide (AlOx) was deposited by atomic layer deposition into the fuzzy W. The conformally deposited ceramic yields a new class of surface composites. Structural characterization of the fuzzy W-AlOx composites through nanoindentation testing indicated enhanced indentation modulus (Eind) and hardness (Hind) and was modeled through various rules of mixtures approaches. The distribution of AlOx in fuzzy W was explored and a systematic study of the extent of incorporation of the AlOx into the fuzzy W was carried out. The synthesized composites may be utilized for improved structural characteristics, e.g., in reducing crack initiation and fracture.

Toward the Ultimate Limit of Analyte Detection, in Graphene-Based Field-Effect Transistors

The ultimate sensitivity of field-effect-transistor (FET)-based devices for ionic species detection is of great interest, given that such devices are capable of monitoring single-electron-level modulations. It is shown here, from both theoretical and experimental perspectives, that for such ultimate limits to be approached the thermodynamic as well as kinetic characteristics of the (FET surface)-(linker)-(ion-receptor) ensemble must be considered. The sensitivity was probed in terms of optimal packing of the ensemble, through a minimal charge state/capacitance point of view and atomic force microscopy. Through the fine-tuning of the linker and receptor interaction with the sensing surface, a record limit of detection as well as specificity in the femtomolar range, orders of magnitude better than previously obtained and in excellent accord with prediction, was observed.

Graphene and Two-Dimensional Materials for Biomolecule Sensing

An ideal biosensor material at room temperature, with an extremely large surface area per unit mass combined with the possibility of harnessing quantum mechanical attributes, would be comprised of graphene and other two-dimensional (2D) materials. The sensing of a variety of sizes and types of biomolecules involves modulation of the electrical charge density of (current through) the 2D material and manifests through specific components of the capacitance (resistance). While sensitive detection at the single-molecule level, i.e., at zeptomolar concentrations, may be achieved, specificity in a complex mixture is more demanding. Attention should be paid to the influence of inevitably present defects in the 2D materials on the sensing, as well as calibration of obtained results with acceptable standards. The consequent establishment of a roadmap for the widespread deployment of 2D material-based biosensors in point-of-care platforms has the potential to revolutionize health care. Expected final online publication date for the Annual Review of Biophysics, Volume 52 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Probing interlayer van der Waals strengths of two-dimensional surfaces and defects, through STM tip-induced elastic deformations

A methodology to test the interlayer bonding strength of two-dimensional (2D) surfaces and associated one (1D)- and two (2D)- dimensional surface defects using scanning tunneling microscope tip-induced deformation, is demonstrated. Surface elastic deformation characteristics of soft 2D monatomic sheets of graphene and graphite in contrast to NbSe₂indicates related association with the underlying local bonding configurations. Surface deformation of 2D graphitic moiré patterns reveal the inter-layer van der Waals strength varying across its domains. These results help in the understanding of the comparable interlayer bonding strength of 1D grain boundary as well as the grains. Anomalous phenomena related to probing 2D materials at small gap distances as a function of strain is discussed.

Enhanced graphene surface plasmonics through incorporation into metallic nanostructures

A methodology for enhancing the surface plasmon polariton (SPP) resonance associated with graphene, through nanoscale metal-dielectric-metal (MDM) gaps, is proposed. The modulation of the resonances, in the range of 0.7 µm to 1 µm was done through tuning the carrier density in graphene and has been shown to be of potential utility for surface analyte sensing. It was shown, from finite element simulations in the frequency domain, that the related hybrid SPP modes could be clearly delineated in far field spectroscopy.

Possibility of Obtaining Two Orders of Magnitude Larger Electrokinetic Streaming Potentials, through Liquid Infiltrated Surfaces

It is shown that the magnitude of the streaming potential (V_s) can be significantly enhanced from ∼0.02 V to as much as ∼1.6 V, in electrokinetic flows through microchannels. This was done through flows on liquid-filled surfaces, where the grooves were filled with oils of viscosity in the range 30-3000 mPa·s. The presence of immiscible oils and the improved slip are both factors that could significantly increase the V_s. The analytical relationship between streaming potential and filled liquid viscosity was derived and verified through corresponding experimental results. The work yields novel insights into complex electrolyte flows and indicates avenues for more efficient energy harvesting.

Direct DNA Methylation Profiling with an Electric Biosensor

DNA methylation is one of the principal epigenetic mechanisms that control gene expression in humans, and its profiling provides critical information about health and disease. Current profiling methods require chemical modification of bases followed by sequencing, which is expensive and time-consuming. Here, we report a direct and rapid determination of DNA methylation using an electric biosensor. The device consists of a DNA-tweezer probe integrated on a graphene field-effect transistor for label-free, highly sensitive, and specific methylation profiling. The device performance was evaluated with a target DNA that harbors a sequence of the methylguanine-DNA methyltransferase, a promoter of glioblastoma multiforme, a lethal brain tumor. The results show that we successfully profiled the methylated and nonmethylated forms at picomolar concentrations. Further, fluorescence kinetics and molecular dynamics simulations revealed that the position of the methylation site(s), their proximity, and accessibility to the toe-hold region of the tweezer probe are the primary determinants of the device performance.

Iron redox pathway revealed in ferritin via electron transfer analysis

Ferritin protein is involved in biological tissues in the storage and management of iron - an essential micro-nutrient in the majority of living systems. While there are extensive studies on iron-loaded ferritin, its functionality in iron delivery is not completely clear. Here, for the first time, differential pulse voltammetry (DPV) has been successfully adapted to address the challenge of resolving a cascade of fast and co-occurring redox steps in enzymatic systems such as ferritin. Using DPV, comparative analysis of ferritins from two evolutionary-distant organisms has allowed us to propose a stepwise resolution for the complex mix of concurrent redox steps that is inherent to ferritins and to fine-tune the structure-function relationship of each redox step. Indeed, the cyclic conversion between Fe3+ and Fe2+ as well as the different oxidative steps of the various ferroxidase centers already known in ferritins were successfully discriminated, bringing new evidence that both the 3-fold and 4-fold channels can be functional in ferritin.

Interaction and hybridization of orthogonal Fabry-Pérot like surface plasmon modes in metal-dielectric grating structures

The interaction of specific surface plasmon modes in metal-dielectric-metal arrangements is investigated, motivated by their relevance to device-based configurations. The absorption spectra of the relevant nanostructures considering geometrical variation, such as the width and height of the metal or dielectric, are probed considering such interactions. Frequency domain simulations are used to study related multiple surface plasmon polariton resonance modes. It is indicated that the resonant energy level interaction due to the coupling between modes in a horizontal dielectric layer and those in a vertical groove can be engineered and understood in terms of energy level hybridization.

Tensorial Modulation of Electrokinetic Streaming Potentials on Air and Liquid Filled Surfaces

Textured surfaces, comprised of grooves filled with air, e.g., air-filled surfaces (AFS), or with liquid, e.g., liquid-filled surfaces (LFS), significantly influence fluid flows and the related electrokinetic streaming potential (V_s). Here, electroosmotic mobility related tensorial effects on the V_s were experimentally investigated. A significant modulation of the V_s, as high as 100%, due to transverse pressure gradients, was demonstrated. The study yields insights into understanding geometrical effects in electrolyte flows with implications to the establishment of local electric fields, energy generation, and biological separations.

Modulation of the Streaming Potential and Slip Characteristics in Electrolyte Flow over Liquid-Filled Surfaces

A significant enhancement in the streaming potential ( V_s) was obtained in experiments considering the flow of electrolyte over liquid-filled surfaces (LFSs), where the grooves in patterned substrates are filled with electrolyte immiscible oils. Such LFSs yield larger V_s (by a factor of 1.5) compared to superhydrophobic surfaces, with air-filled grooves, and offer tunability of electrokinetic flow. It is shown that the density, viscosity, conductivity, as well as the dielectric constant of the filling oil, in the LFS, determine V_s. Relating a hydrodynamic slip length to the obtained V_s offers insight into flow characteristics, as modulated by the liquid interfaces in the LFS.

Enhanced Environmental Stability Coupled with a 12.5% Power Conversion Efficiency in an Aluminum Oxide-Encapsulated n-Graphene/p-Silicon Solar Cell

A significant improvement in the power conversion efficiency (PCE) and the environmental stability of n-Graphene/p-Si solar cells is indicated through effective n-doping of graphene, using low work function oxide capping layers. AlO _x, deposited through atomic layer deposition, is particularly effective for such doping and in addition serves as an antireflection coating and a cell encapsulating layer. It is shown that the related charge transfer doping and interfacial engineering was crucial to achieve a record PCE of 12.5%. The work indicates a path forward, through work function engineering, for further efficiency gains in Gr-based solar cells.

Enhanced voltage generation through electrolyte flow on liquid-filled surfaces

The generation of electrical voltage through the flow of an electrolyte over a charged surface may be used for energy transduction. Here, we show that enhanced electrical potential differences (i.e., streaming potential) may be obtained through the flow of salt water on liquid-filled surfaces that are infiltrated with a lower dielectric constant liquid, such as oil, to harness electrolyte slip and associated surface charge. A record-high figure of merit, in terms of the voltage generated per unit applied pressure, of 0.043 mV Pa⁻¹ is obtained through the use of the liquid-filled surfaces. In comparison with air-filled surfaces, the figure of merit associated with the liquid-filled surface increases by a factor of 1.4. These results lay the basis for innovative surface charge engineering methodology for the study of electrokinetic phenomena at the microscale, with possible application in new electrical power sources.

Superhydrophobic surfaces are expected to increase streaming potential, but are hindered by the presence of air. Here the authors enhance streaming potential by flowing high-dielectric salt water over liquid-filled surfaces infiltrated with low-dielectric liquid, harnessing electric slip and surface charge.

Enhanced Solar Thermal Evaporation of Ethanol-Water Mixtures, through the Use of Porous Media

A significant enhancement of solar irradiation induced evaporation of water, and ethanol-water mixtures, through the use of carbon foam based porous media, is demonstrated. A relationship between the consequent rate of mass loss, with respect to the equilibrium vapor pressure, dynamic viscosity, surface tension, and density, was developed to explain experimental observations. The evaporative heat loss was parametrized through two convective heat transfer coefficients-one related to the surface and another related to the vapor external to the surface. The work promotes a better understanding of thermal processes in binary liquid mixtures with applications ranging from phase separation to distillation and desalination.

Dimensionality-Dependent Electrochemical Kinetics at the Single-Layer Graphene-Electrolyte Interface

The theories to describe the rate at which electrochemical reactions proceed, to date, do not consider explicitly the dimensionality or the discreteness and occupancy of the energy levels of the electrodes. We show experimentally that such quantum mechanical aspects are important for dimensionally confined nanostructured materials and yield unusual variation of the kinetic rate constants with applied voltage in single-layer graphene. The observed divergence from conventional electrokinetics was ascribed to the linear energy dispersion as well as a nonzero density of states at the Dirac point in the graphene. The obtained results justify the use of density of states-based rate constants and considerably add to Marcus-Hush-Chidsey kinetics.

Photoresponse of a Single Y-Junction Carbon Nanotube

We report investigation of optical response in a single strand of a branched carbon nanotube (CNT), a Y-junction CNT composed of multiwalled CNTs. The experiment was performed by connecting a pair of branches while grounding the remaining one. Of the three branch combinations, only one combination is optically active which also shows a nonlinear semiconductor-like I-V curve, while the other two branch combinations are optically inactive and show linear ohmic I-V curves. The photoresponse includes a zero-bias photocurrent from the active branch combination. Responsivity of ≈1.6 mA/W has been observed from a single Y-CNT at a moderate bias of 150 mV with an illumination of wavelength 488 nm. The photoresponse experiment allows us to understand the nature of internal connections in the Y-CNT. Analysis of data locates the region of photoactivity at the junction of only two branches and only the combination of these two branches (and not individual branches) exhibits photoresponse upon illumination. A model calculation based on back-to-back Schottky-type junctions at the branch connection explains the I-V data in the dark and shows that under illumination the barriers at the contacts become lowered due to the presence of photogenerated carriers.

An approach towards a perfect thermal diffuser

A method for the most efficient removal of heat, through an anisotropic composite, is proposed. It is shown that a rational placement of constituent materials, in the radial and the azimuthal directions, at a given point in the composite yields a uniform temperature distribution in spherical diffusers. Such arrangement is accompanied by a very significant reduction of the source temperature, in principle, to infinitesimally above the ambient temperature and forms the basis for the design of a perfect thermal diffuser with maximal heat dissipation. Orders of magnitude enhanced performance, compared to that obtained through the use of a diffuser constituted from a single material with isotropic thermal conductivity has been observed and the analytical principles underlying the design were validated through extensive computational simulations.

High-performance flexible hydrogen sensor made of WS₂ nanosheet-Pd nanoparticle composite film

We report a flexible hydrogen sensor, composed of WS2 nanosheet-Pd nanoparticle composite film, fabricated on a flexible polyimide substrate. The sensor offers the advantages of light-weight, mechanical durability, room temperature operation, and high sensitivity. The WS2-Pd composite film exhibits sensitivity (R 1/R 2, the ratio of the initial resistance to final resistance of the sensor) of 7.8 to 50,000 ppm hydrogen. Moreover, the WS2-Pd composite film distinctly outperforms the graphene-Pd composite, whose sensitivity is only 1.14. Furthermore, the ease of fabrication holds great potential for scalable and low-cost manufacturing of hydrogen sensors.

Graphene oxide as a p-dopant and an anti-reflection coating layer, in graphene/silicon solar cells

It is shown that coating graphene-silicon (Gr/Si) Schottky junction based solar cells with graphene oxide (GO) improves the power conversion efficiency (PCE) of the cells, while demonstrating unprecedented device stability. The PCE has been shown to be increased to 10.6% (at incident radiation of 100 mW cm(-2)) for the Gr/Si solar cell with an optimal GO coating thickness compared to 3.6% for a bare/uncoated Gr/Si solar cell. The p-doping of graphene by the GO, which also serves as an antireflection coating (ARC) has been shown to be a main contributing factor to the enhanced PCE. A simple spin coating process has been used to apply GO with thickness commensurate with an anti-refection coating (ARC) and indicates the suitability of the developed methodology for large-scale solar cell assembly.

Enhanced Power Conversion Efficiency of Graphene/Silicon Heterojunction Solar Cells Through NiO Induced Doping

We report a doping strategy, where nickel oxide (NiO) nanoparticle film coating is employed for graphene/Si heterojunction solar cells to improve the power conversion efficiency (PCE). NiO doping has been shown to improve the short circuit current (J(SC)) by 12%, open circuit voltage (V(OC)) by 25% and fill factor (FF) by 145% of the cells, in turn increasing the PCE from 1.37% to 4.91%. Furthermore, NiO doped graphene/Si solar cells don't show any significant performance degradation over 10 days revealing that NiO doping can be a promising approach for practical applications of graphene in solar cells.

Electrical Contacts to Nanomaterials

The efficient passage of electrical current from an external contact to a nanomaterial is necessary for harnessing characteristics unique to the nanoscale, such as those relevant to energy quantization. However, an intrinsic resistance pertinent to dimensionality crossover and the presence of impurities precludes optimal electrical contact formation. In this review, we first discuss the relevant principles and contact resistance measurement methodologies, with modifications necessary for the nanoscale. Aspects related to the deposition of the contact material are deemed to be crucial. Consequently, the use of focused ion beam (FIB) based deposition, which relies on the ion-induced decomposition of a metallorganic precursor, and which has been frequently utilized for nanoscale contacts is considered in detail.

Bioinspired superhydrophobic surfaces, fabricated through simple and scalable roll-to-roll processing

A simple, scalable, non-lithographic, technique for fabricating durable superhydrophobic (SH) surfaces, based on the fingering instabilities associated with non-Newtonian flow and shear tearing, has been developed. The high viscosity of the nanotube/elastomer paste has been exploited for the fabrication. The fabricated SH surfaces had the appearance of bristled shark skin and were robust with respect to mechanical forces. While flow instability is regarded as adverse to roll-coating processes for fabricating uniform films, we especially use the effect to create the SH surface. Along with their durability and self-cleaning capabilities, we have demonstrated drag reduction effects of the fabricated films through dynamic flow measurements.

Solution-Processed CoFe2O4 Nanoparticles on 3D Carbon Fiber Papers for Durable Oxygen Evolution Reaction

We report CoFe2O4 nanoparticles (NPs) synthesized using a facile hydrothermal growth and their attachment on 3D carbon fiber papers (CFPs) for efficient and durable oxygen evolution reaction (OER). The CFPs covered with CoFe2O4 NPs show orders of magnitude higher OER performance than bare CFP due to high activity of CoFe2O4 NPs, leading to a small overpotential of 378 mV to get a current density of 10 mA/cm(2). Significantly, the CoFe2O4 NPs-on-CFP electrodes exhibit remarkably long stability evaluated by continuous cycling (over 15 h) and operation with a high current density at a fixed potential (over 40 h) without any morphological change and with preservation of all materials within the electrode. Furthermore, the CoFe2O4 NPs-on-CFP electrodes also exhibit hydrogen evolution reaction (HER) performance, which is considerably higher than that of bare CFP, acting as a bifunctional electrocatalyst. The achieved results show promising potential for efficient, cost-effective, and durable hydrogen generation at large scales using earth-abundant materials and cheap fabrication processes.

Modulation of the Electrostatic and Quantum Capacitances of Few Layered Graphenes through Plasma Processing

It is shown that charged defect generation, through argon ion-based plasma processing, in few layer graphene, could substantially enhance the electrical capacitance for electrochemical energy storage. Detailed consideration of the constituent space charge and quantum capacitances were used to delineate a new length scale, correlated to electrically active defects contributing to the capacitance, and was found to be smaller than a structural correlation length determined through Raman spectroscopy. The study offers insights into an industrially viable method (i.e., plasma processing) for modifying and enhancing the energy density of graphene-based electrochemical capacitors.

Ultra-high optical absorption efficiency from the ultraviolet to the infrared using multi-walled carbon nanotube ensembles

The optical absorption efficiencies of vertically aligned multi-walled (MW)-carbon nanotube (CNT) ensembles are characterized in the 350-7000 nm wavelength range where CNT site densities > 1 × 10(11) /cm(2) are achieved directly on metallic substrates. The site density directly impacts the optical absorption characteristics, and while high-density arrays of CNTs on electrically insulating and non-metallic substrates have been commonly reported, achieving high site-densities on metals has been challenging and remains an area of active research. These absorber ensembles are ultra-thin (<10 μm) and yet they still exhibit a reflectance as low as ∼0.02%, which is 100 times lower than the reference; these characteristics make them potentially attractive for high-sensitivity and high-speed thermal detectors. In addition, the use of a plasma-enhanced chemical vapor deposition process for the synthesis of the absorbers increases the portfolio of materials that can be integrated with such absorbers due to the potential for reduced synthesis temperatures. The remarkable ruggedness of the absorbers is also demonstrated as they are exposed to high temperatures in an oxidizing ambient environment, making them well-suited for extreme thermal environments encountered in the field, potentially for solar cell applications. Finally, a phenomenological model enables the determinatiom of the extinction coefficients in these nanostructures and the results compare well with experiment.

The experimental determination of the onset of electrical and thermal conductivity percolation thresholds in carbon nanotube-polymer composites

We show evidence of electrical and thermal conductivity percolation in polymer based carbon nanotube (CNT) composites, which follow power law variations with respect to the CNT concentrations in the matrix. The experimentally obtained percolation thresholds, i.e., ~ 0.074 vol % for single walled CNTs and ~ 2.0 vol % for multi-walled CNTs, were found to be aspect ratio dependent and in accordance with those determined theoretically from excluded volume percolation theory. A much greater enhancement, over 10 orders of magnitude, was obtained in the electrical conductivity at the percolation threshold, while a smaller increase of ~ 100 % was obtained in the thermal conductivity values. Such a difference is qualitatively explained on the basis of the respective conductivity contrast between the CNT filler and the polymer matrix.

Elastic response of carbon nanotube forests to aerodynamic stresses

The ability to determine static and (hydro)dynamic properties of carbon nanotubes (CNTs) is crucial for many applications. While their static properties (e.g., solubility and wettability) are fairly well understood, their mechanical responses (e.g., deflection under shear) to ambient fluid flow are to a large extent unknown. We analyze the elastic response of single-walled CNT forests, attached to the bottom wall of a channel, to the aerodynamic loading exerted by both laminar and turbulent flows. Our analysis yields analytical expressions for velocity distributions, the drag coefficient, and bending profiles of individual CNTs. This enables us to determine flexural rigidity of CNTs in wind-tunnel experiments. The model predictions agree with laboratory experiments for a large range of channel velocities.

Giant birefringence in multi-slotted silicon nanophotonic waveguides

We demonstrate record giant birefringence, nearly twice as large as has previously been achieved (Delta n(group) = 1.5 over more than 60 nm of bandwidth near lambda= 1550 nm) using a multi-slotted silicon nanophotonic waveguide. The birefringence is optimized by the use of materials with high refractive index contrast to create a compact single-mode waveguide, and the etching of deeply sub-wavelength channels within the waveguide, which are strongly coupled in the near field and separated by narrow air channels of optimum lateral width. When used as a polarization-selective delay element, the delay-bandwidth product per unit length is 46.6/mm over a bandwidth of 8.74 T Hz. We also design and demonstrate mode shaping of both the TE and TM polarizations to achieve near-identical coupling to a macroscopic external object, such as a lensed fiber or detector.

Electrical properties and applications of carbon nanotube structures

The experimentally verified electrical properties of carbon nanotube structures and manifestations in related phenomena such as thermoelectricity, superconductivity, electroluminescence, and photoconductivity are reviewed. The possibility of using naturally formed complex nanotube morphologies, such as Y-junctions, for new device architectures are then considered. Technological applications of the electrical properties of nanotube derived structures in transistor applications, high frequency nanoelectronics, field emission, and biological sensing are then outlined. The review concludes with an outlook on the technological potential of nanotubes and the need for new device architectures for nanotube systems integration.

Novel electrical switching behaviour and logic in carbon nanotube Y-junctions

Carbon-nanotube-based electronics offers significant potential as a nanoscale alternative to silicon-based devices for molecular electronics technologies. Here, we show evidence for a dramatic electrical switching behaviour in a Y-junction carbon-nanotube morphology. We observe an abrupt modulation of the current from an on- to an off-state, presumably mediated by defects and the topology of the junction. The mutual interaction of the electron currents in the three branches of the Y-junction is shown to be the basis for a potentially new logic device. This is the first time that such switching and logic functionalities have been experimentally demonstrated in Y-junction nanotubes without the need for an external gate. A class of nanoelectronic architecture and functionality, which extends well beyond conventional field-effect transistor technologies, is now possible.