Nanometer-scale Channel Research Articles

Sponge-phase Lipid Droplets as Synthetic Organelles: An Ultrafast Study of Hydrogen Bonding and Interfacial Environments.

Bottom-up design of biomimetic organelles has gained recent attention as a route towards understanding the transition between non-living matter and life. Despite various artificial lipid membranes being developed, the specific relations between lipid structure, composition, interfacial properties, and morphology are not currently understood. Sponge-phase droplets contain dense, nonlamellar lipid bilayer networks that capture the complexities of the endoplasmic reticulum (ER), making them ideal artificial models of such organelles. Here, we combine ultrafast two-dimensional infrared (2D IR) spectroscopy and molecular dynamics simulations to investigate the interfacial H-bond networks in sponge-phase droplets composed of glycolipid and nonionic detergents. In the sponge phase, the interfacial environments are more hydrated and water molecules confined to the nanometer-scale aqueous channels in the sponge phase exhibit dynamics that are significantly slower compared to bulk water. Surfactant configurations and microscopic phase separation play a dominant role in determining membrane curvature and slow dynamics observed in the sponge phase. The studies suggest that H-bond networks within the nanometer-scale channels are disrupted not only by confinement but also by the interactions of surfactants, which extend 1-2 nm from the bilayer surface. The results provide a molecular-level description for controlling phase and morphology in the design of synthetic lipid organelles.

Ion filling of a one-dimensional nanofluidic channel in the interaction confinement regime.

Ion transport measurements are widely used as an indirect probe for various properties of confined electrolytes. It is generally assumed that the ion concentration in a nanoscale channel is equal to the ion concentration in the macroscopic reservoirs it connects to, with deviations arising only in the presence of surface charges on the channel walls. Here, we show that this assumption may break down even in a neutral channel due to electrostatic correlations between the ions arising in the regime of interaction confinement, where Coulomb interactions are reinforced due to the presence of the channel walls. We focus on a one-dimensional channel geometry, where an exact evaluation of the electrolyte's partition function is possible with a transfer operator approach. Our exact solution reveals that in nanometer-scale channels, the ion concentration is generally lower than in reservoirs and depends continuously on the bulk salt concentration, in contrast to the conventional mean-field theory that predicts an abrupt filling transition. We develop a modified mean-field theory taking into account the presence of ion pairs that agrees quantitatively with the exact solution and provides predictions for experimentally relevant observables, such as the ionic conductivity. Our results will guide the interpretation of nanoscale ion transport measurements.

Disentangling 1/f noise from confined ion dynamics.

Ion transport through biological and solid-state nanochannels is known to be a highly noisy process. The power spectrum of current fluctuations is empirically known to scale like the inverse of frequency, following the long-standing yet poorly understood Hooge's law. Here, we report measurements of current fluctuations across nanometer-scale two-dimensional channels with different surface properties. The structure of fluctuations is found to depend on the channel's material. While in pristine channels current fluctuations scale like 1/f1+a with a = 0-0.5, the noise power spectrum of activated graphite channels displays different regimes depending on frequency. Based on these observations, we develop a theoretical formalism directly linking ion dynamics and current fluctuations. We predict that the noise power spectrum takes the form 1/f × Schannel(f), where 1/f fluctuations emerge in fluidic reservoirs on both sides of the channel and Schannel describes fluctuations inside it. Deviations to Hooge's law thus allow direct access to the ion transport dynamics of the channel - explaining the entire phenomenology observed in experiments on 2D nanochannels. Our results demonstrate how current fluctuations can be used to characterize nanoscale ion dynamics.

Cation-Gated Ion Transport at Nanometer Scale for Tunable Power Generation.

Gated ion channels in biological cell membranes allow efficient tuning of cross-membrane ion transport with enhanced permeation and selectivity, converting ionic signals into various forms of electrical signals and energies on demands, which functionalities though are still difficult to achieve in artificial membranes. Here, we report cation-gated ion transport through synthesized porous aromatic films containing nanometer-scale ionic channels together with -NH2 groups at interiors. Ion selectivity and permeability is greatly tuned by gating cations, up to 2 orders of magnitude, and as a consequence, the membrane efficiently produces switchable electricity output from salinity gradients. The results are attributed to positively charged cations binding at -NH2 groups, which screens the intrinsic negative surface charge at channels' interiors and inverts charge polarity there. Our work adds understanding to ion gating effects at nanoscale and offers strategies of developing smart membranes and their heterostructures for separation, energy conversion, cell membrane mimics, and related technologies.

Exceedingly High Performance Top-Gate P-Type SnO Thin Film Transistor with a Nanometer Scale Channel Layer.

Implementing high-performance n- and p-type thin-film transistors (TFTs) for monolithic three-dimensional (3D) integrated circuit (IC) and low-DC-power display is crucial. To achieve these goals, a top-gate transistor is preferred to a conventional bottom-gate structure. However, achieving high-performance top-gate p-TFT with good hole field-effect mobility (μFE) and large on-current/off-current (ION/IOFF) is challenging. In this report, coplanar top-gate nanosheet SnO p-TFT with high μFE of 4.4 cm2/Vs, large ION/IOFF of 1.2 × 105, and sharp transistor’s turn-on subthreshold slopes (SS) of 526 mV/decade were achieved simultaneously. Secondary ion mass spectrometry analysis revealed that the excellent device integrity was strongly related to process temperature, because the HfO2/SnO interface and related μFE were degraded by Sn and Hf inter-diffusion at an elevated temperature due to weak Sn–O bond enthalpy. Oxygen content during process is also crucial because the hole-conductive p-type SnO channel is oxidized into oxygen-rich n-type SnO2 to demote the device performance. The hole μFE, ION/IOFF, and SS values obtained in this study are the best-reported data to date for top-gate p-TFT device, thus facilitating the development of monolithic 3D ICs on the backend dielectric of IC chips.

Composite Solid Absorbent for Simultaneous Desulfurization and Denitrification of Low-Temperature Flue Gas

ABSTRACT The composite solid absorbent was developed for simultaneous desulfurization and denitrification of low-temperature flue gas. The efficiency of this absorbent was confirmed by a pilot experiment in a coke oven. The surface and cross-section morphologies, elemental compositions and distributions, specific surface areas and pore size distributions, and phase compositions of fresh and used absorbents were measured to investigate the enhanced performance mechanism of absorbent and its removal mechanism for SO2 and NOx. The results showed that the absorbent had an efficient desulfurization and denitrification performance with about 99.9% SO2 removal and more than 80% NOx removal. The narrow pore distribution of absorbent produced more specific surface area as well as more active sites, which significantly enhanced the reactive activity of the absorbent greatly. Furthermore, the nanometer-scale channels and pores on the surface of absorbent promoted the diffusion of flue gas and the reaction of absorbent. SO2 reacted with residual O2 to produce SO3, which formed CaSO4 in the absorbent ultimately. NO reacted with residual O2 and CO to form N2 under the action of absorbent, which could be promoted at low temperature. For the CO-uncontained flue gas, NO will be removed by reacting with residual O2 and the absorbent to produce Ca(NO3)2, which is beneficial for the solidified absorption of SO2.

Dislocations as channels for the fabrication of sub-surface porous GaN by electrochemical etching

Porosification of nitride semiconductors provides a new paradigm for advanced engineering of the properties of optoelectronic materials. Electrochemical etching creates porosity in doped layers while leaving undoped layers undamaged, allowing the realization of complex three-dimensional porous nanostructures, potentially offering a wide range of functionalities, such as in-distributed Bragg reflectors. Porous/non-porous multilayers can be formed by etching the whole, as-grown wafers uniformly in one simple process, without any additional processing steps. The etch penetrates from the top down through the undoped layers, leaving them almost untouched. Here, atomic-resolution electron microscopy is used to show that the etchant accesses the doped layers via nanometer-scale channels that form at dislocation cores and transport the etchant and etch products to and from the doped layer, respectively. Results on AlGaN and non-polar GaN multilayers indicate that the same mechanism is operating, suggesting that this approach may be applicable in a range of materials.

Nanoconfinement of the Low-Temperature Dark Conglomerate: Structural Control from Focal Conics to Helical Nanofilaments.

The helical nanofilament (HNF) and low-temperature dark conglomerate (DC) liquid-crystal (LC) phases of bent-core molecules show the same local layer structure but present different bulk morphologies. The DC phase is characterized by the formation of nanoscale toric focal conics, whereas the HNF phase is constructed of bundles of twisted layers. Although the local layer structure is similar in both phases, materials that form these phases tend to form one morphology in preference to the other. Targeted control of the nanostructures would provide pathways to potential applications and insight into how conditions drive a specific phase formation. Here, W624, a compound known to form the DC phase is confined in nanometer scale channels of porous anodized aluminum oxide (AAO) membranes. Within each nanochannel, the DC phase is suppressed forming the HNF structure instead, indicating the nanoscale spatial limitation can control the phase structure of the DC phase.

Sub-10 nm Monolayer MoS2 Transistors Using Single-Walled Carbon Nanotubes as an Evaporating Mask.

Transition-metal dichalcogenides are promising challengers to conventional semiconductors owing to their remarkable electrical performance and suppression of short-channel effects (SCEs). In particular, monolayer molybdenum disulfide has exhibited superior suppression of SCEs owing to its atomic thickness, high effective carrier mass, and low dielectric constant. However, difficulties still remain in large-scale stable fabrication of nanometer-scale channels. Herein, a method to fabricate electrodes with sub-10 nm gaps was demonstrated using horizontally aligned single-walled carbon nanotubes as an evaporation mask. The widths of the nanogaps exhibit robust stability to various process parameters according to the statistical results. Based on these nanogaps, ultrashort-channel length monolayer MoS2 field-effect transistors were produced. Monolayer MoS2 devices with a 7.5 nm channel length and a 10 nm thick HfO2 dielectric layer exhibited excellent performances with an ON/OFF ratio up to 107, a mobility of 17.4 cm2/V·s, a subthreshold swing of about 120 mV/dec, and a drain-induced barrier lowering of about 140 mV/V, all of which suggest a superior suppression of SCEs. This work provides a universal and stable method for large-scale fabrication of ultrashort-channel 2D-material transistors.

Maximizing Electrokinetic Energy Conversion via the Intersecting Asymptotes Method

It has been shown in earlier studies that the maximum electrokinetic conversion efficiency between flow and electric work (e.g., electrokinetic power generation) occurs when electric double-layer (λ) overlaps and there is no electroneutral zone in a nanometer-scale channel. This result has been shown through cumbersome and lengthy numerical and theoretical studies. The case is made here that complications associated with solving the coupled set of governing equations i.e. Poisson, Nernst-Planck, and Navier-Stokes (PNPNS) could be drastically reduced to a two-step solution by method of intersecting asymptotes.

Single-Walled Carbon Nanotube Short-Channel Transistors Operating at Ultra-Low Voltages

Given that power consumption and short-channel effects exist in a trade-off relationship, there has been much effort to investigate short-channel transistors which enable relative reductions in the operating voltage. Recently, the feasibility of transistors operating at ultra-low voltages with a nanometer-scale channel length has been extensively investigated for future nanoelectronics. Here, we demonstrate solution-processed single-walled carbon nanotubes-based thin-film transistors (SWCNT-TFTs) with a sub-20-nm-channel operating at ultra-low voltages which utilize an electromigration technique that induces a broken bridge of nanoparticles. By employing the post-treatment process of thermal annealing in a vacuum, the charge transport of the short-channel SWCNT-TFTs was improved. The origin of such improvements is presumed to reduce the charge impurities, including organic/inorganic residues as defect states, as well as to improve electric contact between the SWCNT and the metal electrode, which strongly affects the one-dimensional charge transport.

Hierarchical-structure induced adjustable deformation of super carbon nanotubes with radial shrinkage up to 66%

Based on the molecular dynamic (MD) simulations, radial-deformation capabilities of super carbon nanotubes (SCNTs) are investigated under uniaxial tension. The investigations uncover the excellent reversible and adjustable radial-shrinkage characteristics of SCNTs, which are caused by the hierarchical assembly of carbon nanotubes (CNTs). MD simulations show that reversible-deformation ranges of SCNTs can be as much as 50%, depending on the atomistic topology in junction areas. By adopting the geometry design of the hierarchical structure, radial shrinkages of SCNTs can be adjusted in a considerable range of 10%–66%. Chirality is the major factor influencing the radial-shrinkage capability, which brings more than 30% difference. For given requirements of the diameter, shrink capability of SCNT can be enhanced by increasing the length of CNTs or choosing specific chirality. Poisson's ratios of SCNTs are found to be strain-dependent including two variation types, i.e. the monotonic decreasing type and the increase-decrease waving type. By analyzing the structural evolution of Y-junctions, the shrinkage mechanism is revealed to rely on the deformation modes of SCNTs and affected by geometric settings of the hierarchical structure. As a nanometer-scale channel structure with superior reversible and controllable characteristics, SCNTs exhibit great potentials in the applications of nano-pipelines and nano-switchers.

Estimation of charge effects of ultrafine channel utilizing junctionless transistor with nanodot-type floating gate

Metal nanoparticles (NPs) embedded in junctionless field-effect transistors (JL-FETs) with a channel length of about sub-10-nm are fabricated and demonstrated. The anisotropic wet etching of a silicon-on-insulator (SOI) substrate was utilized to form V-grooves and define a nanometer-scale channel. Metal NPs are selectively placed onto the bottom of a V-groove using a bio nano process (BNP). A JL-FET is applied to a floating gate memory and used to study the impacts of charges close to the short channel. Low-voltage operation and memory behavior of broad threshold voltage appear. It is estimated by simulation that positive and negative charges equivalent to approximately 10 electrons are accumulated in one NP. It is expected that the JL-FETs can overcome the scaling limitations of floating gate memories.

Pseudopotential-based electron quantum transport: Theoretical formulation and application to nanometer-scale silicon nanowire transistors

We present a formalism to treat quantum electronic transport at the nanometer scale based on empirical pseudopotentials. This formalism offers explicit atomistic wavefunctions and an accurate band structure, enabling a detailed study of the characteristics of devices with a nanometer-scale channel and body. Assuming externally applied potentials that change slowly along the electron-transport direction, we invoke the envelope-wavefunction approximation to apply the open boundary conditions and to develop the transport equations. We construct the full-band open boundary conditions (self-energies of device contacts) from the complex band structure of the contacts. We solve the transport equations and present the expressions required to calculate the device characteristics, such as device current and charge density. We apply this formalism to study ballistic transport in a gate-all-around (GAA) silicon nanowire field-effect transistor with a body-size of 0.39 nm, a gate length of 6.52 nm, and an effective oxide thickness of 0.43 nm. Simulation results show that this device exhibits a subthreshold slope (SS) of ∼66 mV/decade and a drain-induced barrier-lowering of ∼2.5 mV/V. Our theoretical calculations predict that low-dimensionality channels in a 3D GAA architecture are able to meet the performance requirements of future devices in terms of SS swing and electrostatic control.

Nonlinear transport in ionic liquid gated strontium titanate nanowires

Measurements of the current-voltage (I–V) characteristics of ionic liquid gated nanometer scale channels of strontium titanate have been carried out. At low gate voltages, the I–V characteristics exhibit a large voltage threshold for conduction and a nonlinear power law behavior at all temperatures measured. The source-drain current of these nanowires scales as a power law of the difference between the source-drain voltage and the threshold voltage. The scaling behavior of the I–V characteristic is reminiscent of collective electronic transport through an array of quantum dots. At large gate voltages, the narrow channel acts as a quasi-1D wire whose conductance follows Landauer's formula for multichannel transport.

Ultra-short channel junctionless transistor with a one-dimensional nanodot array floating gate

The electrical properties of a junctionless field-effect transistor with a sub-10-nm scale channel and FeOx nanoparticles (NPs) were studied. The anisotropic wet etching of a silicon-on-insulator substrate was used to form V-grooves and define the nanometer-scale channel. The NPs were selectively placed on the bottom of the V-groove using the bio-nano process. Low-voltage operation and a wide threshold voltage (Vth) shift as memory behavior were confirmed in a device with a 3.6-nm channel length. These results indicate that the Vth is controlled by the single-nanodot floating gate along the channel length direction.

A New Slit-Type Vacuum-Channel Transistor

A new vacuum-channel transistor with a carbon nanotube cathode and nanometer-scale channel length-called a slit-type vacuum-channel transistor is proposed and investigated. The suggested device structure features distinguishable cutoff, linear, and saturation regions with a negligible gate leakage current. Its channel length is almost the same as the mean free path of carriers in air, which suggests that the device can operate not only in vacuum but also in air, without any performance degradation. Because of its geometrical characteristics, it is possible for this device to be operated when the anode bias is almost the same as the gate bias with negligible oxide leakage. Therefore, the device can be used as an elemental device component in digital integrated circuits.

Lg = 100 nm In0.7Ga0.3As quantum well metal-oxide semiconductor field-effect transistors with atomic layer deposited beryllium oxide as interfacial layer

In this study, we have fabricated nanometer-scale channel length quantum-well (QW) metal-oxide-semiconductor field effect transistors (MOSFETs) incorporating beryllium oxide (BeO) as an interfacial layer. BeO has high thermal stability, excellent electrical insulating characteristics, and a large band-gap, which make it an attractive candidate for use as a gate dielectric in making MOSFETs. BeO can also act as a good diffusion barrier to oxygen owing to its small atomic bonding length. In this work, we have fabricated In0.53Ga0.47As MOS capacitors with BeO and Al2O3 and compared their electrical characteristics. As interface passivation layer, BeO/HfO2 bilayer gate stack presented effective oxide thickness less 1 nm. Furthermore, we have demonstrated In0.7Ga0.3As QW MOSFETs with a BeO/HfO2 dielectric, showing a sub-threshold slope of 100 mV/dec, and a transconductance (gm,max) of 1.1 mS/μm, while displaying low values of gate leakage current. These results highlight the potential of atomic layer deposited BeO for use as a gate dielectric or interface passivation layer for III–V MOSFETs at the 7 nm technology node and/or beyond.

Physical origin of dynamic ion transport features through single conical nanopores at different bias frequencies

Ionic transport through nanometer scale channels or interfaces is the physical origin of the detection signals in stochastic single molecular sensing, DNA sequencing and nano-structured electrodes. Dynamic transport regulated by systematically varying the bias frequency has not been explored. In this report, ion transport through single conical nanopore platforms is studied by applying an alternating electrical field at a wide range of scan rates. Rich frequency-dependent features of the measured transport current are discovered. The complete transition of characteristic transport features from low to high scan rates or bias frequencies is demonstrated experimentally. Key dynamic features include: multiple hysteresis loops separated by one or two non-zero cross points in the I–V curves, shifts in cross point potentials at different scan rates, and growth and diminishment in the hysteresis loops with normal and negative phase shifts. Combined theoretical and experimental studies reveal different processes contributing to and dominating the total current responses on different time scales. The respective contributions of each type of transport process to the overall measured current signals are quantitatively separated based on the fundamental insights gained. Although these exciting experimental observations are successfully simulated using an optimized model by solving PNP equations, the experimental studies at this nanoscale dimension suggest substantial deviations from a continuum regime. Inputs from molecular theory are needed to further validate the proposed physical mechanism.

(Invited) Extremely Short Channel Si-MOSFETs Prepared on SOI Substrates Using Anisotropic Wet Etching

Electrical properties of extremely short channel Si-MOSFETs are examined. Nanometer-scale channels are fabricated by the V-shape groove formation on SOI substrate using anisotropic wet etching technique. The channel length, defined as the width of the Vgroove bottom, is as short as 3 nm. Source, channel, and drain regions are uniformly highly doped by the same dopant species. Excellent transistor characteristics with threshold voltages that are optimal for low-power operation are obtained for both n-FETs and p-FETs when the thicknesses of both the channel and gate dielectric film thickness are reduced to 1 nm. Contributions of quantum effects in the nanometer-scale channel are discussed.