Nanoscale Gaps Research Articles

Refractive Index Tuning of All-Inorganic TiO2 Nanocrystal-Based Films and High Aspect Ratio Nanostructures Using Atomic Layer Deposition: Implications for High-Throughput Fabrication of Metalenses

High refractive index (RI) components and nanostructures are of great interest for compact optics, waveguides, photonics, and metamaterials. Herein, we demonstrate that atomic layer deposition (ALD) can be used as a short postfabrication step to dramatically increase the RI of nanocrystal (NC)-based films and patterned nanostructures in conjunction with a single-step, direct nanoimprint lithography (NIL). The initial RI of TiO2 NC-based films was n = 1.95 at 543 nm which can then be increased to 2.00 by post-calcination and further up to 2.15 by TiO2 ALD. Fifteen cycles of ALD were sufficient to achieve this rapid increase of RI, and any intermediate RI value can be tuned by adjusting the number of ALD cycles accordingly. Nanoscale interstitial gaps between TiO2 NCs allowed a uniform diffusion of ALD precursors, resulting in a much denser structure and the RI increase. An array of 4 mm-sized metalenses were fabricated to demonstrate the effects of the RI tuning on the performance of optical devices. The focusing efficiencies of the as-imprinted metalenses (smallest dimension ∼ 80 nm, highest aspect ratio ∼ 8) were 61% on average, but the post-treatments including calcination and ALD significantly increased the average efficiency to 67% and up to 75% for the best-performing lens. This approach combines the fast, scalable, and versatile solvent-assisted NIL method to pattern optical nanostructures with a short post-patterning deposition and densification step that significantly enhances optical performance.

Self‐Powered Nanofluidic Pressure Sensor with a Linear Transfer Mechanism

AbstractThe transfer functions of the widely used pressure sensors do not exhibit the desired linearity, which limits their practicability in many fields, such as the Internet of Things and artificial intelligence. Herein, MXene/cellulose nanofiber composite membrane‐based linear nanofluidic pressure sensors are demonstrated. The nanoscale gaps between MXene laminates restrict the movement of electrolyte and realize the selective transport of ions, based on which mechanical signals can be converted into electric energy for self‐powering. In particular, the generated voltage and current are directly proportional to the applied pressure. The introduction of high‐strength cellulose nanofibers not only expands the detection range of the sensor but also achieves continuous adjustment of the nano‐gap between MXene laminates, which optimizes the sensitivity of the device. The feasibility of further optimization through the modulation of surface functional groups, electrolyte concentration, and device assembly method is proposed. This 2D nanofluid pressure sensor provides an important approach to manufacture portable and wearable electronic devices for applications in many fields.

Micro-gap thermo-photo-thermionics: An alternative approach to harvesting thermo-photons and its comparison with thermophotovoltaics

This work investigates an alternative to thermophotovoltaics for harvesting thermal and optical energy via photon coupling and thermionic energy conversion. In this device, a heat source is radiatively coupled to a thermionic electron emitter through a nanoscale gap and the electron emitter is coupled to the collector through a microscale gap. The analysis using fluctuational electrodynamics and finite-time thermodynamics shows that for identical thermal radiator and photon-to-electron conversion materials, the output power density in the thermionic device can be more than twice that of the thermophotovoltaic device; the thermionic mechanism can also provide more than 30% improvement in the energy conversion efficiency compared to the thermophotovoltaic device. Moreover, the maximum-power-point voltage in the thermionic device is shown to significantly exceed the conversion material’s band gap, which determines the theoretical upper limit of the open-circuit voltage in a thermophotovoltaic cell. Therefore, the results of this study indicate that thermionic energy harvesting via thermo-photon coupling can be a promising alternative to thermophotovoltaics.

Experimental and theoretical study on hybrid thermionic-photovoltaic energy converters with graphene/semiconductor Schottky junction

Both thermionic and thermophotovoltaic solid-state energy converters are being actively explored due to high output potential. Recently, hybrid thermionic-photovoltaic (TIPV) approaches was proposed to utilize both electrons and photons emitted by a high temperature cathode, whereas conventional two-terminal configuration is constrained by current-match. In this work, we develop a four-terminal TIPV converter with a dispenser cathode, a transparent indium-tin-oxide anode and tandem graphene/GaAs Schottky junction photovoltaics. The TI and PV sub-devices can operate independently without meeting the current-match constraint. The TIPV prototype yields a power density of 962 W/m2 at the cathode temperature of 1373 K. In addition, a three-terminal TIPV converter with Nano-scale electrode gap size is modelled with advantages of surface plasmon and evanescent wave tunnelling. Results demonstrate that Super-Planckian near-field enhancement guarantees a > 4 MW/m2 output power density for the converter. We also discuss the effects of electrode gap size and the cathode temperature. This work validates the TIPV converters with graphene/semiconductor Schottky junction and also paves a way to promising developments for the Super-Planckian near-field configuration.

FIB fabrication and electron emission characteristics of overhanging tungsten and platinum nanostructures

The recent advancement in nanofabrication technology has enabled realization of vacuum electron devices with nanogap spacing. These devices have advantages over semiconductor counterparts due to possible operation under hard conditions and some unique functionalities. The challenging task, however, lies with the full understanding of their current–voltage (I–V) characteristics, resulting from various electron emission mechanisms. The reliable extraction of device parameters is, therefore, vital for its potential applications. An attempt has, therefore, been made here to fabricate two three-dimensional overhanging electrodes of tungsten and platinum with a nanoscale gap of 70–100 nm on glass substrates using chemical vapor deposition and focused ion beam milling. Their (I–V) characteristics measured in situ at ∼10−6 mbar are shown to follow the Child–Langmuir law and Fowler–Nordheim field emission at low and high-bias conditions, respectively. The extraction of parameters with a simple procedure suggested earlier yields an effective emission area of ∼3510 Å2, work function of ∼2.5 eV, and field enhancement factor (β) of ∼ 1.8 for tungsten; the values for platinum are 12.5 Å2, 3.0 eV, and 5.0, respectively. The higher β in the case of platinum can be attributed to the formation of a comparatively rough emitter surface with some fine protrusions. The nanostructure gives a current spike at high voltages, which marks its transition to an explosive emission state, breakdown, and dispersion of spherical metal particles over the substrate.

Transitions between field emission and vacuum breakdown in nanoscale gaps

The continuing reduction in device size motivates a more fundamental understanding of breakdown and electron emission for nanoscale gaps. While prior experiments have separately studied breakdown and electron emission in vacuum gaps, no study has comprehensively examined the transitions between these mechanisms. In this study, we measure the current-voltage (I−V) curves for electrodes with different emitter widths for 20–800 nm gaps at vacuum (∼1 μTorr) to measure breakdown voltage and assess electron emission behavior. The breakdown voltage Vb increases linearly with increasing gap distance from ∼15 V at 20 nm to ∼220 V at 300 nm and remains nearly constant for larger gaps; Vb does not depend strongly on the emitter width. Breakdown can proceed directly from the field emission regime. Nexus theory, which predicts transitions between space-charge limited current (SCLC) and field emission (FE), shows that the experimental conditions are in the Fowler–Nordheim regime and within a factor of 0.7 to the FE-SCLC transition. We also present the results of electrode damage by emission current-induced heating to explain the flattening of Vb at larger gaps that was absent in previous experiments for similar gap distances at atmospheric pressure.

High-Sensitivity Amperometric Dual Immunoassay Using Two Cascade Reactions with Signal Amplification of Redox Cycling in Nanoscale Gap.

Here, we report a high-sensitivity dual immunoassay using Lumulus amebocyte lysate (LAL) and blood coagulation cascade reactions with redox cycling in a nanoscale-gap electrode. Endotoxin and factor XIa were used as the label molecules for the immunoassay of two types of analytes to induce the LAL and coagulation cascade reactions, respectively, when each corresponding analyte existed in the sample solution. In addition to the signal amplification by the cascade reactions, we employed redox cycling in a nanoscale gap to achieve a highly sensitive assay. The nanoscale-gap electrode amplifies the amperometric signals from p-aminophenol liberated from artificial substrates in the final steps of the cascade reactions. First, the cross reaction between the LAL and coagulation cascade reactions was investigated. The results indicated that these cascade reactions did not efficiently proceed in a single solution owing to the cross reaction. Therefore, we selected to induce two cascade reactions in different solutions by bisecting the beads after the immunocomplex formation on the beads. The cross reactions of factor XIa with the LAL cascade reaction and of endotoxin with the coagulation cascade reaction were investigated. The effects of these cross reactions were revealed to be negligible by bisecting the beads before inducing the cascade reactions. Finally, a dual immunoassay for goat and human immunoglobulin G was performed, for which the limits of detection were 70 pg/mL (470 fmol/L) and 1.0 ng/mL (6.6 pmol/L), respectively. Thus, our dual immunoassay provides a sensitive platform for clinical diagnosis requiring detection of multiple analytes.

Lithiophilic and conductive framework of 2D MoN nanosheets enabling planar lithium plating for dendrite-free and minimum-volume-change lithium metal anodes

Lithium (Li) is a promising anode for high-energy rechargeable batteries but its practical implementation is impeded by uncontrollable dendrite growth and huge volume changes. Herein, we report a dendrite-free Li composite anode with minimum volume change, which is composed of 3D interpenetrating Li and lithiophilic MoN nanosheets prepared by mechanical rolling and folding. The conductive 2D MoN nanosheets boasting excellent lithiophilic affinity and small lattice mismatch with Li induce planar Li plating therefore suppressing dendrite growth along the perpendicular direction. The MoN nanosheets interwoven framwork provides abundant Li accommodation sites and the horizontally plated Li fills the nanoscale gaps between the MoN nanosheets, thus minimizing volume change during cycling. Li3N generated in situ by the surface reaction between MoN and Li enhances the ionic conductivity and interface stability. The Li-MoN anode shows a low Li plating overpotential of 15.0 mV and excellent stability for 2,500 h at 1 mA cm−2. Paired with the LiFePO4 cathode, the full cell shows 99.7 % Coulombic efficiency and 87.7 % capacity retention after 650 cycles at 170 mA g−1. The results provide insights into the design of 3D host enabling planar Li plating for dendrite-free and minimum-volume-change Li metal anodes.

A straightforward optical alignment protocol for STM-based single molecule spectroscopy

A light-accessed scanning tunneling microscope (STM) is a powerful spectroscopic tool that enables chemical analysis at the single molecular level, but it requires highly precise optical alignments to pinpoint the nano-scale tunneling gap, leaving experimental challenges. Here we present straightforward procedures to align the optical setup for STM-luminescence and STM-based tip-enhanced Raman spectroscopy (TERS) performed with a reflection geometry in an ultrahigh vacuum chamber. Observing real-space images of the metal tip apex through a spectrograph set to the zeroth-order diffraction enables “in situ” optimization of the detection path and introduction of the excitation light of TERS to the nanogap. The best spatial overlap with the nanogap can be achieved by monitoring plasmon-enhanced, low-frequency inelastic scattering of the metal. This protocol allows us to overcome such difficulties in STM-based spectroscopy and facilitates physicochemical study of single adsorbates on nontransparent substrates.

Self-Assembled Au Nanoparticle Monolayers on Silicon in Two- and Three-Dimensions for Surface-Enhanced Raman Scattering Sensing.

Gold nanoparticle/silicon composites are canonical substrates for sensing applications because of their geometry-dependent physicochemical properties and high sensing activity via surface-enhanced Raman spectroscopy (SERS). The self-assembly of gold nanoparticles (AuNPs) synthesized via wet-chemistry on functionalized flat silicon (Si) and vertically aligned Si nanowire (VA-SiNW) arrays is a simple and cost-effective approach to prepare such substrates. Herein, we report on the critical parameters that influence nanoparticle coverage, aggregation, and assembly sites in two- and three-dimensions to prepare substrates with homogeneous optical properties and SERS activity. We show that the degree of AuNP aggregation on flat Si depends on the silane used for the Si functionalization, while the AuNP coverage can be adjusted by the incubation time in the AuNP solution, both of which directly affect the substrate properties. In particular, we report the reproducible synthesis of nearly touching AuNP chain monolayers where the AuNPs are separated by nanoscale gaps, likely to be formed due to the capillary forces generated during the drying process. Such substrates, when used for SERS sensing, produce a uniform and large enhancement of the Raman signal due to the high density of hot spots that they provide. We also report the controlled self-assembly of AuNPs on VA-SiNW arrays, which can provide even higher Raman signal enhancement. The directed assembly of the AuNPs in specific regions of the SiNWs with a control over NP density and monolayer morphology (i.e., isolated vs nearly touching NPs) is demonstrated, together with its influence on the resulting SERS activity.

Ice-assisted electron-beam lithography for halide perovskite optoelectronic nanodevices

Metal halide perovskites (MHPs) have received substantial attention due to their impressive optoelectronic properties. In particular, nanoscale perovskite structures, such as nanowires (NWs) and nanoplates (NPs) are ideal building blocks for optoelectronic devices. However, metal electrodes can be hardly patterned on these materials with conventional lithographic methods due to the solvent sensitivity of perovskite crystals and alignment issues. Here, we report a solvent-free method to fabricate metal electrodes on perovskite NPs, which starts with the vapor deposition of water ice as an electron resist and ends in the sublimation of the ice followed by a “blow-off” process. The good compatibility between MHPs and ice as well as the in-situ imaging and patterning process guarantees the fabrication with high precision and resolution. Using this technique, we create metal electrodes on single-crystal MAPbBr3 NPs featuring a nanoscale gap of 296 nm and superior photodetection ability with responsivity of 653 A/W and detectivity of 3.08 × 1013 Jones. Our study helps the widely-used electron-beam lithography break down barriers in processing perovskite materials, and provides an excellent platform to fully exploit their potentials in optoelectronic devices.

Hydrothermal and photoreduction synthesis of nanostructured α-Fe2O3/Ag urchins for sensitive SERS detection of environmental samples

Semiconductor/noble-metal heterostructures as surface enhanced Raman spectroscopy (SERS) substrates have remarkable recognition due to their synergistic contribution and abundant hotspot generation. Herein, urchin-like α-Fe2O3 and α-Fe2O3/Ag heterostructures were synthesized by the hydrothermal and photoreduction methods for the SERS application. The effective photoreduction strategies avail the dense production of noble metal (Ag) on semiconductor (α-Fe2O3). The synthesized α-Fe2O3/Ag heterostructure has numerous close-neighbouring spines with nanoscale gaps, which enable the affluent formation of hotspots and the adsorption of rich analyte molecules on the α-Fe2O3/Ag heterostructure. The silver photoreduction time is optimized for utmost hotspot generation such that the Raman signal enhancement is provoked to its highest extent. The prepared α-Fe2O3/Ag heterostructure is successfully applied for the efficacious analysis of azo dyes, sunset yellow (SY) and tartrazine (TZ). The performance analysis of α-Fe2O3/Ag heterostructure presents ultra-low limit of detection of 1.23 × 10-14 M for SY and 1.10 × 10-14 M for TZ. The enhancement factors are as high as 2.79 × 1012 for SY and 3.33 × 1012 for TZ. The uniformity and reproducibility of the proposed SERS substrate show the relative standard deviation less than 10 %. Furthermore, the real-sample analysis of the α-Fe2O3/Ag based SERS substrate is performed on the SY and TZ spiked orange juice and shows satisfactory recovery values. The preceding analytical results confirm that the α-Fe2O3/Ag based SERS substrate is feasible to be applied for practical sample analysis.

The role of EM wave polarization on radiative heat transfer across a nanoscale gap

This work presents a novel study of radiative heat transfer between closely separated plates based on an extension of Planck’s spectrum of thermal radiations to systems with a steady heat flux. This extension together with electromagnetic wave theory is chosen specifically to avoid the commonly used so-called fluctuation dissipation theory, which is also limited to equilibrium systems. The spectrum of thermal radiation with a heat flux is described by the introduction of an analog of a chemical potential, which creates a bias toward the direction of heat transfer. This is the first comprehensive study of radiative heat transfer based on the generalization of Planck’s spectrum for systems with a heat flux, which eliminates contradictions arising when a heat flux is described in terms of the laws limited to equilibrium systems. The total heat flux is split into fluxes carried by waves with different frequencies, directions of propagation, and polarizations. This simplifies the analysis because due to the stochastic independence, the energy fluxes of such waves are additive, and this also reveals that the heat carrying capacity of radiation with the parallel polarization is significantly higher than that of the perpendicularly polarized radiation. This suggests that the rate of radiative heat transfer may be noticeably increased by the control of the polarization of thermal radiation.

Impact of the Nanoscale Gap Morphology on Plasmons in Doped Indium Oxide Nanostructure Dimers

Impact of the Nanoscale Gap Morphology on Plasmons in Doped Indium Oxide Nanostructure Dimers

LB1023 Deletion of bardet biedl syndrome 1 (BBS1) gene in T cells impairs wound healing in mice

Primary cilia play an important role in cell directional migration and tissue repair. Ciliary function depends on several proteins including the BBSome, a complex of eight conserved Bardet-Biedl syndrome (BBS) proteins, such as BBS1, which transport cargos to and from cilia. The immune synapse (IS), the nano-scale gap between T cells and antigen presenting cells, in non-ciliated T cells is functionally homologous to the primary cilium. During the formation of the IS, the BBSome appears to be critical for centrosome translocation toward the IS interface within T cells. Data from our lab demonstrate that BBS1 hypomorphic mice (containing a systemic homozygous knock-in missense mutation) characteristically exhibit impaired cilia function, altered fibroblast function, and reduced wound healing/tissue repair. The skin is residence for most lymphocytes in the body. To investigate the role of BBS1 in T cells in wound healing, we generated a novel transgenic mouse model in which the BBS1 gene was conditionally deleted in T cells. For in vivo wound healing, 2 mm full-thickness punch biopsy was performed on the shaved backs of 9 to 11 weeks old male and female T-BBS1-/- mice and their wild-type littermate controls and assessed for rate of wound closure by secondary intention between day 0 and day 7. The wounds formed fibrin clots at similar times in both T-BBS1-/- and littermate controls. However, male T-BBS1-/- mice demonstrated significantly delayed wound healing at day 7 compared to controls (4.061±1.79%, p=0.0342, n=10-12/group). The female mice show a trend toward reduced would healing at 7 days as well. These results demonstrate that BBS1 plays an important role in T cell mediated skin repair and that this effect may be sex specific.

Fabrication of b-oriented MFI membranes from MFI nanosheet layers by ammonium sulfate modifier for the separation of butane isomers

Fabrication of thin and oriented zeolite membrane is highly preferable and challenging, providing the advantage of less resistance and high permeation for efficient molecular separation. MFI zeolite nanosheets serve as the desirable building block for fabricating thin and preferably b-oriented MFI membranes. However, the secondary growth of deposited MFI zeolite nanosheet layer is an indispensable aspect that comes with the challenges of sealing the nanoscale/interlayer gaps while preserving the orientation and controlling the thickness of fabricated zeolite membrane for efficient gas separation. Herein, ammonium sulfate assisted hydrothermal secondary growth of MFI nanosheet slip coated on the γ-Al2O3 disc was carried out to seal the nanoscale and interlayer gaps for the separation of butane isomers. The addition of ammonium sulfate helps to preserve the b-orientation of the deposited nanosheet layer and plays a critical role in tuning the thickness of the MFI membrane. Ammonium sulfate assisted thin MFI membrane was obtained with preserved b-orientation having a thickness of about 630 nm while exhibiting a separation factor and n-butane permeance of 41 and 1.10 × 10−7 mol m−2 s−1.Pa−1, respectively for the separation of butane isomers.

High Temperature Gradient Pirani Micro-Sensor Designed and Tested for Aerodynamic Wall Pressure Measurement

This paper presents and discusses the first demonstration of aerodynamic wall pressure measurement using a thermal Pirani effect based micro-sensor. The thermal micro-sensor is designed as a suspended micro-hot-wire separated from the substrate by a nanoscale gap, and mechanically supported by perpendicular micro-bridges. The micro-sensor was calibrated for pressure measurement from 10 kPa to 800 kPa. It was then used as aerodynamic wall pressure sensor in a turbulent boundary layer wind tunnel and successfully measured the 1.1 kPa depression occurring in the wind tunnel when the flow velocity goes up to 40 m/s.

High-Throughput Direct Writing of Metallic Micro- and Nano-Structures by Focused Ga+ Beam Irradiation of Palladium Acetate Films.

Metallic nanopatterns are ubiquitous in applications that exploit the electrical conduction at the nanoscale, including interconnects, electrical nanocontacts, and small gaps between metallic pads. These metallic nanopatterns can be designed to show additional physical properties (optical transparency, plasmonic effects, ferromagnetism, superconductivity, heat evacuation, etc.). For these reasons, an intense search for novel lithography methods using uncomplicated processes represents a key on-going issue in the achievement of metallic nanopatterns with high resolution and high throughput. In this contribution, we introduce a simple methodology for the efficient decomposition of Pd3(OAc)6 spin-coated thin films by means of a focused Ga+ beam, which results in metallic-enriched Pd nanostructures. Remarkably, the usage of a charge dose as low as 30 μC/cm2 is sufficient to fabricate structures with a metallic Pd content above 50% (at.) exhibiting low electrical resistivity (70 μΩ·cm). Binary-collision-approximation simulations provide theoretical support to this experimental finding. Such notable behavior is used to provide three proof-of-concept applications: (i) creation of electrical contacts to nanowires, (ii) fabrication of small (40 nm) gaps between large metallic contact pads, and (iii) fabrication of large-area metallic meshes. The impact across several fields of the direct decomposition of spin-coated organometallic films by focused ion beams is discussed.

A method to fabricate nanoscale gaps in graphene nano-constrictions by electrical breakdown

This work reports on a method to open nanoscale gaps in h-shaped graphene nano-constrictions by electrical breakdown at room temperature and pressure below 10−5 mbar. The method was validated on 275 devices, fabricated on eight different chips, using Chemical Vapor Deposition (CVD)-grown graphene from in-house production and from two commercial sources. The gap width was estimated by fitting the I–V traces after electrical breakdown with the Simmons model for the intermediate-voltage range. The statistics on the collected data demonstrates that the method results in normally distributed nanoscale gaps in h-shaped graphene nano-constrictions, with an estimated average width centered around 1 nm and a gap fabrication yield of 95%.

Electrochemical Synthesis of Plasmonic Nanostructures.

Thanks to their tunable and strong interaction with light, plasmonic nanostructures have been investigated for a wide range of applications. In most cases, controlling the electric field enhancement at the metal surface is crucial. This can be achieved by controlling the metal nanostructure size, shape, and location in three dimensions, which is synthetically challenging. Electrochemical methods can provide a reliable, simple, and cost-effective approach to nanostructure metals with a high degree of geometrical freedom. Herein, we review the use of electrochemistry to synthesize metal nanostructures in the context of plasmonics. Both template-free and templated electrochemical syntheses are presented, along with their strengths and limitations. While template-free techniques can be used for the mass production of low-cost but efficient plasmonic substrates, templated approaches offer an unprecedented synthetic control. Thus, a special emphasis is given to templated electrochemical lithographies, which can be used to synthesize complex metal architectures with defined dimensions and compositions in one, two and three dimensions. These techniques provide a spatial resolution down to the sub-10 nanometer range and are particularly successful at synthesizing well-defined metal nanoscale gaps that provide very large electric field enhancements, which are relevant for both fundamental and applied research in plasmonics.