Nanoscale Corrugation Research Articles

Deep subwavelength slotted photonic crystal nanobeam in a monolithic silicon photonics foundry.

We report the design and experimental realization of a deep subwavelength-engineered slotted photonic crystal fabricated using a commercial monolithic silicon photonics process with a minimum feature size near 40 nm. The deep subwavelength design includes a corrugated, slotted unit cell shape that leverages electromagnetic interface conditions to localize optical energy in low refractive index regions, achieving a four-fold enhancement of the electric field energy compared to an equivalent slotted photonic crystal without the nanoscale corrugations. This demonstration establishes a basis for future study of commercially fabricated, subwavelength-engineered photonic structures where intense light-matter interaction and manipulation of optical properties on-chip is critical, including biosensing and optical trapping applications.

Strongly Enhanced Electronic Bandstructure Renormalization by Light in Nanoscale Strained Regions of Monolayer MoS2/Au(111) Heterostructures.

Understanding and controlling the photoexcited quasiparticle (QP) dynamics in monolayer (ML) transition metal dichalcogenides (TMDs) lays the foundation for exploring the strongly interacting, nonequilibrium two-dimensional (2D) QP and polaritonic states in these quantum materials and for harnessing the properties emerging from these states for optoelectronic applications. In this study, scanning tunneling microscopy/spectroscopy (STM/scanning tunneling spectroscopy) with light illumination at the tunneling junction is performed to investigate the QP dynamics in ML MoS2 on an Au(111) substrate with nanoscale corrugations. The corrugations on the surface of the substrate induce nanoscale local strain in the overlaying ML MoS2 single crystal, which result in energetically favorable spatial regions where photoexcited QPs, including excitons, trions, and electron-hole plasmas, accumulate. These strained regions exhibit pronounced electronic bandstructure renormalization as a function of the photoexcitation wavelength and intensity as well as the strain gradient, implying strong interplay among nanoscale structures, strain, and photoexcited QPs. In conjunction with the experimental work, we construct a theoretical framework that integrates nonuniform nanoscale strain into the electronic bandstructure of a ML MoS2 lattice using a tight-binding approach combined with first-principle calculations. This methodology enables better understanding of the experimental observation of photoexcited QP localization in the nanoscale strain-modulated electronic bandstructure landscape. Our findings illustrate the feasibility of utilizing nanoscale architectures and optical excitations to manipulate the local electronic bandstructure of ML TMDs and to enhance the many-body interactions of excitons, which is promising for the development of nanoscale energy-adjustable optoelectronic and photonic technologies, including quantum emitters and solid-state quantum simulators for interacting exciton polaritons based on engineered periodic nanoscale trapping potentials.

Resonant Raman scattering on graphene: SERS and gap-mode TERS.

Nanoscale deformations and corrugations occur in graphene-like two-dimensional materials during their incorporation into hybrid structures and real devices, such as sensors based on surface-enhanced Raman scattering (SERS-based sensors). The structural features mentioned above are known to affect the electronic properties of graphene, thus highly sensitive and high-resolution techniques are required to reveal and characterize arising local defects, mechanical deformations, and phase transformations. In this study, we demonstrate that gap-mode tip-enhanced Raman Scattering (gm-TERS), which offers the benefits of structural and chemical analytical methods, allows variations in the structure and mechanical state of a two-dimensional material to be probed with nanoscale spatial resolution. In this work, we demonstrate locally enhanced gm-TERS on a monolayer graphene film placed on a plasmonic substrate with specific diameter gold nanodisks. SERS measurements are employed to determine the optimal disk diameter and excitation wavelength for further realization of gm-TERS. A significant local plasmonic enhancement of the main vibrational modes in graphene by a factor of 100 and a high spatial resolution of 10 nm are achieved in the gm-TERS experiment, making gm-TERS chemical mapping possible. By analyzing the gm-TERS spectra of the graphene film in the local area of a nanodisk, the local tensile mechanical strain in graphene was detected, resulting in a split of the G mode into two components, G+ and G-. Using the frequency split in the positions of G+ and G- modes in the TERS spectra, the stress was estimated to be up to 1.5%. The results demonstrate that gap-mode TERS mapping allows rapid and precise characterization of local structural defects in two-dimensional materials on the nanoscale.

Role of Curvature in Stabilizing Boron-Doped Nanocorrugated Graphene.

Boron-doped carbon nanostructures have attracted great interest recently because of their remarkable electrocatalytic performance comparable to or better than that of conventional metal catalysts. In a previous work (Carbon 123, 605 (2017)), we reported that along with significant performance improvement, B doping enhances the oxidation resistance of few-layer graphene (FLG) that provides increased structural stability for intermediate-temperature fuel-cell electrodes. In general, detailed characterization of the atomic and electronic structure transformations that occur in B-doped carbon nanostructures during fuel-cell operation is lacking. In this work, we use aberration-corrected scanning transmission electron microscopy, nanobeam electron diffraction, and electron energy-loss spectroscopy (EELS) to characterize the atomic and electronic structures of B-doped FLG before and after fuel-cell operation. These data point to the nanoscale corrugation of B-doped FLGs as the key factor responsible for increased stability and high corrosion resistance. The similarity of the 1s to π* and σ* transition features in the B K-edge EELS to those in B-doped carbon nanotubes provides an estimate for the curvature of nanocorrugation in B-FLG.

Proton transport through nanoscale corrugations in two-dimensional crystals

Defect-free graphene is impermeable to all atoms1–5 and ions6,7 under ambient conditions. Experiments that can resolve gas flows of a few atoms per hour through micrometre-sized membranes found that monocrystalline graphene is completely impermeable to helium, the smallest atom2,5. Such membranes were also shown to be impermeable to all ions, including the smallest one, lithium6,7. By contrast, graphene was reported to be highly permeable to protons, nuclei of hydrogen atoms8,9. There is no consensus, however, either on the mechanism behind the unexpectedly high proton permeability10–14 or even on whether it requires defects in graphene’s crystal lattice6,8,15–17. Here, using high-resolution scanning electrochemical cell microscopy, we show that, although proton permeation through mechanically exfoliated monolayers of graphene and hexagonal boron nitride cannot be attributed to any structural defects, nanoscale non-flatness of two-dimensional membranes greatly facilitates proton transport. The spatial distribution of proton currents visualized by scanning electrochemical cell microscopy reveals marked inhomogeneities that are strongly correlated with nanoscale wrinkles and other features where strain is accumulated. Our results highlight nanoscale morphology as an important parameter enabling proton transport through two-dimensional crystals, mostly considered and modelled as flat, and indicate that strain and curvature can be used as additional degrees of freedom to control the proton permeability of two-dimensional materials.

Tamm Plasmon Resonance as Optical Fingerprint of Silver/Bacteria Interaction.

The incorporation of responsive elements into photonic crystals is an effective strategy for fabricating active optical components to be used as sensors, actuators, and modulators. In particular, the combination of simple multilayered dielectric mirrors with optically responsive plasmonic materials has proven to be successful. Recently, Tamm plasmon (TP) modes have emerged as powerful tools for these purposes. These modes arise at the interface between a distributed Bragg reflector (DBR) and a plasmonic layer and can be excited at a normal incidence angle. Although the TP field is located usually at the DBR/metal interface, recent studies have demonstrated that nanoscale corrugation of the metal layer permits access to the TP mode from outside, thus opening exciting perspectives for many real-life applications. In this study, we show that the TP resonance obtained by capping a DBR with a nanostructured layer of silver is responsive to Escherichia coli. Our data indicate that the modification of the TP mode originates from the well-known capability of silver to interact with bacteria, within a process in which the release of Ag+ ions leaves an excess of negative charge in the metal lattice. Finally, we exploited this effect to devise a case study in which we optically differentiated between the presence of proliferative and nonproliferative bacteria using the TP resonance as a read-out. These findings make these devices promising all-optical probes for bacterial metabolic activity, including their response to external stressors.

Shear Transformation Zone and Its Correlation with Fracture Characteristics for Fe-Based Amorphous Ribbons in Different Structural States

Fe-based amorphous alloys often exhibit severe brittleness induced by annealing treatment, which increases the difficulties in handling and application in the industry. In this work, the shear transformation zone and its correlation with fracture characteristics for FeSiB amorphous alloy ribbons in different structural states were investigated. The results show that the bending strain decreases sharply with the annealing temperature increase, accompanied by decreased shear band density and the induced plastic deformation zone. Furthermore, the microscopic fracture surface features transform from a micron-scale dimple pattern to nano-scale dimples and periodic corrugations. According to nano-indentation results, the strain rate sensitivity and shear transformation zone volume change significantly upon annealing treatment, which is responsible for the deterioration of bending ductility and the transition of microscopic fracture surface features.

Dispersion of Hydrophilic "Hedgehog" Microparticles in Liquid CO2 Mixtures.

Liquid and supercritical CO2 are nontoxic and nonflammable reaction media with pressure-variable physical properties. These states of CO2 also have high solubility limits for gas and liquid hydrocarbons, making them good candidates for "green" hydrophobic solvents in sustainable chemical technologies. However, the dispersion of hydrophilic colloidal nanoscale and microscale particles in CO2 is challenging due to the tendency of polar particles to aggregate in nonpolar media, limiting their available surface area and catalytic efficiencies. Here we show that native hydrophilic semiconductor particles can be effectively dispersed in a liquid CO2 mixture with acetonitrile (ACN) without additional chemical or mechanical dispersion techniques. Using surface corrugation as a method to prevent aggregation, we find that geometrically complex particles with a halo of stiff nanoscale spikes disperse and remain suspended longer in liquid CO2 than those without or with less prominent nanoscale corrugation. For the particles of this size and liquid CO2 mixtures, individual particle mass remains a prominent factor determining particle sedimentation rate even in the absence of aggregation. Particle dispersion and structural stability are confirmed using a combination of UV-vis spectroscopy, finite-difference time-domain modeling, and electron microscopy. The necessity of the cosolvent (ACN) indicates that particle behavior in liquid CO2 is vastly different than in traditional liquid-phase solvents and highlights the need for future studies to understand the wetting behavior of hydrophilic particles in high-pressure nonpolar environments.

Graphene Oxide Chemistry Management via the Use of KMnO4/K2Cr2O7 Oxidizing Agents.

In this paper, we propose a facile approach to the management of graphene oxide (GO) chemistry via its synthesis using KMnO4/K2Cr2O7 oxidizing agents at different ratios. Using Fourier Transformed Infrared Spectroscopy, X-ray Photoelectron Spectroscopy, and X-ray Absorption Spectroscopy, we show that the number of basal-plane and edge-located oxygenic groups can be controllably tuned by altering the KMnO4/K2Cr2O7 ratio. The linear two-fold reduction in the number of the hydroxyls and epoxides with the simultaneous three-fold rise in the content of carbonyls and carboxyls is indicated upon the transition from KMnO4 to K2Cr2O7 as a predominant oxidizing agent. The effect of the oxidation mixture’s composition on the structure of the synthesized GOs is also comprehensively studied by means of X-ray diffraction, Raman spectroscopy, transmission electron microscopy, atomic-force microscopy, optical microscopy, and the laser diffraction method. The nanoscale corrugation of the GO platelets with the increase of the K2Cr2O7 content is signified, whereas the 10–100 μm lateral size, lamellar, and defect-free structure is demonstrated for all of the synthesized GOs regardless of the KMnO4/K2Cr2O7 ratio. The proposed method for the synthesis of GO with the desired chemistry opens up new horizons for the development of graphene-based materials with tunable functional properties.

Bending behavior and fracture surface characters for FeSiB amorphous ribbons in different free volume state

The FeSiB amorphous ribbons with monolithic amorphous structure in different free volume state were prepared by annealing the as-cast ribbons at different temperature. The results show that a fairly large amount of free volume for S25 sample, the medium amount of free volume for S250 sample, and the free volume were almost annealed out in S380 sample, and the free volume has great influence on bending behavior and the resultant fracture surface. With the decrease of free volume, the amount of shear bands on the bent crease, bending strain decrease from 1 to 0, critical shear displacement on fracture surface decrease form 3.32 mm decline to ~ 203 nm. Meanwhile, the size of micro-scale vein pattern decreases, the size of nano-scale periodic corrugations increases. The bending strain, shear band density and fracture surface characteristics were discussed via free volume content and shear transform zone size.

Large scale chemical functionalization of locally curved graphene with nanometer resolution

Anchoring various functional groups to graphene is the most versatile approach for tailoring its functional properties. To date, one must use a special tunneling microscope for attaching a molecule at a specific position on the graphene with resolution better than several hundred nanometers, however, achieving this resolution is impossible on a large scale. We demonstrate for the first time that chemical functionalization can be achieved with nanometer resolution by introducing strain with nanometer scale modulation into a graphene layer. The spatial distribution of the strain has been achieved by transferring a single-layer graphene (SLG) onto a substrate decorated by a few nm large nanoparticles (NPs). By changing the number of NPs on the substrate, the amount of locally strained SLG increases, as confirmed by atomic force microscopy (AFM) and Raman spectroscopy investigations. We further carried out hydrogenation and fluorination on the SLG with increasing amount of nanoscale corrugations. Raman spectroscopy, AFM and X-ray photoelectron spectroscopy revealed unambiguously that the level of functionalization increases proportionally with the number of NPs, which means an increasing number of the locally strained SLG. Our approach thus enables control of the amount and the position of functional groups on graphene with nanometer resolution.

NANOSCALE DIMPLES AND PERIODIC CORRUGATIONS ON FRACTURE SURFACE OF Zr-BASED BULK METALLIC GLASS COMPOSITE

Nanoscale dimples and periodic corrugations are observed on the fracture surface of Zr-based bulk metallic glass composite (BMGC). The nanoscale periodic corrugations display a curved shape, which is different from that observed in previous works. In addition, the crystallization behavior of [Formula: see text][Formula: see text][Formula: see text][Formula: see text] BMG was investigated. The second crystallization event of Zr-Cu-Ni-Al BMG can be controlled by annealing or tuning cooling rate. The in situ Zr-based BMGC was prepared via lowering cooling rate. The Zr-based BMGC displays completely brittleness.

Method combining scanning tunneling microscopy and atom probe tomography for observing inside of materials

In this paper, we demonstrate the feasibility of a method combining scanning tunneling microscopy (STM) and atom probe tomography (APT). In this method, an atomically smooth or flat surface for STM observation is prepared by field evaporation using an APT system. The chemical composition of the surface obtained by APT can be used to analyze the results of STM. By developing this method, it is expected that local and very fine structures inside materials can be observed with true atomic resolution. We also demonstrate that nanoscale corrugation, which has a strong effect on the reconstruction procedure in the analysis of APT data, can be observed by STM.

A three dimensional numerical quantum mechanical model of electronic field emission from metallic surfaces with nanoscale corrugation

The effects on the electronic emission of the presence of nanoscale steps on a tungsten surface are investigated for the first time using three dimensional quantum mechanical models. The plane wave periodic version of the density functional theory is used to obtain the electronic wavefunctions and potentials for flat and corrugated structures. Local and averaged emitted current densities are obtained from them using time dependent perturbation theory. The orders of magnitude of the averaged current densities resulting from these calculations are similar for both flat and corrugated cases; however, strong enhancements are observed on the local current densities near the edges of the steps. These numerical results are compared with those of the analytical Fowler-Nordheim type models. The slopes of the Fowler-Nordheim plots are in good agreement for both numerical and analytical models, but the magnitudes of the emitted currents are significantly different. This is related to weaknesses in the description of the electronic structure of the metal in the analytical models.

Focus Article: Theoretical aspects of vapor/gas nucleation at structured surfaces.

Heterogeneous nucleation is the preferential means of formation of a new phase. Gas and vapor nucleation in fluids under confinement or at textured surfaces is central for many phenomena of technological relevance, such as bubble release, cavitation, and biological growth. Understanding and developing quantitative models for nucleation is the key to control how bubbles are formed and to exploit them in technological applications. An example is the in silico design of textured surfaces or particles with tailored nucleation properties. However, despite the fact that gas/vapor nucleation has been investigated for more than one century, many aspects still remain unclear and a quantitative theory is still lacking; this is especially true for heterogeneous systems with nanoscale corrugations, for which experiments are difficult. The objective of this focus article is analyzing the main results of the last 10-20 years in the field, selecting few representative works out of this impressive body of the literature, and highlighting the open theoretical questions. We start by introducing classical theories of nucleation in homogeneous and in simple heterogeneous systems and then discuss their extension to complex heterogeneous cases. Then we describe results from recent theories and computer simulations aimed at overcoming the limitations of the simpler theories by considering explicitly the diffuse nature of the interfaces, atomistic, kinetic, and inertial effects.

The Cassie-Wenzel transition of fluids on nanostructured substrates: Macroscopic force balance versus microscopic density-functional theory.

Classical density functional theory is applied to investigate the validity of a phenomenological force-balance description of the stability of the Cassie state of liquids on substrates with nanoscale corrugation. A bulk free-energy functional of third order in local density is combined with a square-gradient term, describing the liquid-vapor interface. The bulk free energy is parameterized to reproduce the liquid density and the compressibility of water. The square-gradient term is adjusted to model the width of the water-vapor interface. The substrate is modeled by an external potential, based upon the Lennard-Jones interactions. The three-dimensional calculation focuses on substrates patterned with nanostripes and square-shaped nanopillars. Using both the force-balance relation and density-functional theory, we locate the Cassie-to-Wenzel transition as a function of the corrugation parameters. We demonstrate that the force-balance relation gives a qualitatively reasonable description of the transition even on the nanoscale. The force balance utilizes an effective contact angle between the fluid and the vertical wall of the corrugation to parameterize the impalement pressure. This effective angle is found to have values smaller than the Young contact angle. This observation corresponds to an impalement pressure that is smaller than the value predicted by macroscopic theory. Therefore, this effective angle embodies effects specific to nanoscopically corrugated surfaces, including the finite range of the liquid-solid potential (which has both repulsive and attractive parts), line tension, and the finite interface thickness. Consistently with this picture, both patterns (stripes and pillars) yield the same effective contact angles for large periods of corrugation.

Tomonaga-Luttinger liquid theory for metallic fullurene polymers

We investigate the low energy behavior of local density of states in metallic C$_{60}$ polymers theoretically. The multichannel bosonization method is applied to electronic band structures evaluated from first principles calculation, by which the effects of electronic correlation and nanoscale corrugation in the atomic configuration are fully taken into account. We obtain a closed-form expression for the power law anomalies in the local density of states, which successfully describes the experimental observation on the \fu polymers in a quantitative manner. An important implication from the closed-form solution is the existence of an experimentally unobserved crossover at nearly a hundred milli-electron volts, beyond which the power law exponent of the \fu polymers should change significantly.

Wetting and cavitation pathways on nanodecorated surfaces.

In this contribution we study the wetting and nucleation of vapor bubbles on nanodecorated surfaces via free energy molecular dynamics simulations. The results shed light on the stability of superhydrophobicity in submerged surfaces with nanoscale corrugations. The re-entrant geometry of the cavities under investigation is capable of sustaining a confined vapor phase within the surface roughness (Cassie state) both for hydrophobic and hydrophilic combinations of liquid and solid. The atomistic system is of nanometric size; on this scale thermally activated events can play an important role ultimately determining the lifetime of the Cassie state. Such a superhydrophobic state can break down by full wetting of the texture at large pressures (Cassie-Wenzel transition) or by nucleating a vapor bubble at negative pressures (cavitation). Specialized rare event techniques show that several pathways for wetting and cavitation are possible, due to the complex surface geometry. The related free energy barriers are of the order of 100kBT and vary with pressure. The atomistic results are found to be in semi-quantitative accord with macroscopic capillarity theory. However, the latter is not capable of capturing the density fluctuations, which determine the destabilization of the confined liquid phase at negative pressures (liquid spinodal).

P‐172L: Late‐News Poster: Enhanced Efficiency and Low Haze in Organic Light‐Emitting Diodes by Nanoscale Corrugation

Organic light emitting diodes (OLEDs) with nanoscale corrugation for light extraction (NCLE) enhanced the current efficiency and power efficiency. The haze of the nanoscale corrugation for light extraction is comparable to that of bare glass. Furthermore, the OLEDs with NCLE exhibited angle‐stable electroluminescence (EL).

Nano-Scale Corrugations in Graphene: A Density Functional Theory Study of Structure, Electronic Properties and Hydrogenation

Graphene rippled at the nanoscale level is gathering attention for advanced applications, especially in the field of nanoelectronics and hydrogen storage. Convexity enhanced reactivity toward H was demonstrated on naturally corrugated graphene grown by Si evaporation on SiC, which makes this system a platform for fundamental studies on the effects of rippling. In this work, we report a density functional theory study on a model system specifically designed to mimic graphene on SiC. We first study the supercell geometry and configuration that better reproduce the corrugated monolayer. The relatively low computational cost of this model system allows a systematic study of the dependence of stability, structure, and electronic properties of graphene subject to different levels of stretching and corrugation. The most representative structure is then progressively hydrogenated, imitating the exposure to atomic hydrogen, and stability, structural and electronic properties are evaluated as a function of hydrogen...