Tunable Distance Research Articles

Relativistic artificial molecule of two coupled graphene quantum dots at tunable distances

In a molecule formed by two atoms, energy difference between bonding and antibonding orbitals depends on distance between the two atoms. However, exploring molecular orbitals of two natural atoms with tunable distance has remained an outstanding experimental challenge. Graphene quantum dots can be viewed as relativistic artificial atoms, thus offering a unique platform to study molecular physics. Here, through scanning tunneling microscope, we create and directly visualize the formation process of relativistic artificial molecules based on two coupled graphene quantum dots with tunable distance. Our study indicates that energy difference between the bonding and antibonding orbitals of the lowest quasibound state increases linearly with inverse distance between the two graphene quantum dots due to the relativistic nature of the artificial molecule. For quasibound states with higher orbital momenta, the coupling between these states leads to half-energy spacing of the confined states because the length of the molecular-like orbit is approximately twice that of the atomic-like orbit. Evolution from ring-like whispering-gallery modes in the artificial atoms to figure-eight orbitals in the artificial molecules is directly imaged. The ability to resolve the coupling and orbitals of the relativistic artificial molecule at the nanoscale level yields insights into the behavior of quantum-relativistic matter.

Sulfur and Wavy-Stacking Boosted Superior Lithium Storage in 2D Covalent Organic Frameworks.

2D conjugated covalent organic frameworks (c-COFs) provide an attractive foundation as organic electrodes in energy storage devices, but their storage capability is long hindered by limited ion accessibility within densely π-π stacked interlayers. Herein, two kinds of 2D c-COFs based on dioxin and dithiine linkages are reported, which exhibit distinct in-plane configurations-fully planar and undulated layers. X-ray diffraction analysis reveals wavy square-planar networks in dithiine-bridged COF (COF-S), attributed to curved C─S─C bonds in the dithiine linkage, whereas dioxin-bridged COF (COF-O) features densely packed fully planar layers. Theoretical and experimental results elucidate that the undulated stacking within COF-S possesses an expanded layer distance of 3.8 Å and facilitates effective and rapid Li+ storage, yielding a superior specific capacity of 1305 mAh g-1 at 0.5 A g-1, surpassing that of COF-O (1180 mAh g-1 at 0.5 A g-1). COF-S also demonstrates an admirable cycle life with 80.4% capacity retention after 5000 cycles. As determined, self-expanded wavy-stacking geometry, S-enriched dithiine in COF-S enhances the accessibility and redox activity of Li storage, allowing each phthalocyanine core to store 12 Li+ compared to 8 Li+ in COF-O. These findings underscore the elements and stacking modes of 2D c-COFs, enabling tunable layer distance and modulation of accessible ions.

Polyphthalonitrile as a novel precursor for carbon membranes: Tunable microstructure characteristics and gas permeation behavior

Carbon molecular sieve (CMS) membranes hold great promise for energy-efficient gas separation, contributing to mitigating greenhouse gas emissions. Exploring new types of microstructurally tunable polymeric precursors is critical for understanding the evolution of carbon microstructure arrangement and adjusting the gas permeation behavior of CMS membranes. As a precursor for CMS membranes, polyphthalonitrile (PPN) resin with both tunable intermolecular distance and π-π stacking arrangement is reported for the first time. We have demonstrated that the aforementioned two key features of the thermally crosslinked PPN network are beneficial to forming PPN-CMS membranes with enlarged intermolecular distance and small-sized, narrow-distributed ultramicropores (<7 Å), thereby improving gas permeability and ideal selectivity. This study provides new insights into the microstructure evolution of PPN-derived microporous carbon materials. Owing to its excellent thermal stability, tunable microstructure arrangement, and flexibility of chemical synthesis, PPN represents a promising class of polymeric precursor materials for fabricating CMS membranes.

Multi-Layered PtAu Nanoframes and Their Light-Enhanced Electrocatalytic Activity via Plasmonic Hot Spots.

Here, the rational design of complex PtAu double nanoframes (DNFs) for plasmon-enhanced electrocatalytic activity toward the methanol oxidation reaction (MOR) is reported. The synthetic strategy for the DNFs consists of on-demand multiple synthetic chemical toolkits, including well-faceted Au growth, rim-on selective Pt deposition, and selective Au etching steps. DNFs are synthesized by utilizing Au truncated octahedrons (TOh) as a starting template. The outer octahedral (Oh) nanoframes (NFs) nest the inner TOh NFs, eventually forming DNFs with a tunable intra-nanogap distance. Residual Au adatoms on Pt skeletons act as light entrappers and produce plasmonic hot spots between inner and outer frames through localized surface plasmon resonance (LSPR) coupling, which promotes enhanced electrocatalytic activity for the MOR. Importantly, the correlation between the gap-induced hot carriers and electrocatalytic activity is evaluated. The highest catalytic activity is achieved when the gap is the narrowest. To further harness their light-trapping capability, hierarchically structured triple NFs (TNFs) are synthesized, wherein three NFs are entangled in a single entity with a high density of hot regions, exhibiting superior electrocatalytic activity toward the MOR with a sixfold larger current density under light irradiation compared to the dark conditions.

Inner-Sphere Electron Transfer Induced Reversible Electron Reservoir Feature of Azoheteroarene Bridged Diruthenium Frameworks.

This article demonstrates the stabilization of ground- and redox-induced metal-to-ligand charge transfer excited states on coordination of azo-coupled bmpd(L4) [bmpd = (E)-1,2-bis(1-methyl-1H-pyrazol-3-yl)diazene; L4 = -N═N-] to the electron-rich {Ru(acac)2} (acac = acetylacetonate) unit in mononuclear RuII(acac)2(L4) (1) and diastereomeric dinuclear (acac)2Ru2.5(μ-L4•-)Ru2.5(acac)2 [rac, ΔΔ/ΛΛ (2a)/meso, ΔΛ (2b)] complexes, respectively. It also develops further one-step intramolecular electron transfer induced L4•- bridged isovalent higher analogue [(acac)2RuIII(μ-L4•-)RuIII(acac)2]ClO4 in diastereomeric forms, rac-[2a]ClO4/meso-[2b]ClO4. On the contrary, under identical reaction conditions electronically and sterically permuted bimpd [L5, (E)-1,2-bis(4-iodo-1-methyl-1H-pyrazol-3-yl)diazene)] delivered mononuclear RuII(acac)2(L5) (3) as an exclusive product. Further, the generation of unprecedented heterotrinuclear complex [(acac)2RuII(μ-L4)AgI(μ-L4)RuII(acac)2]ClO4 ([4]ClO4) involving unreduced L4 via the reaction of 1 and AgClO4 revealed the absence of any inner-sphere electron transfer (IET) as in precursor 1, which in turn reaffirmed an IET (at the interface of electron-rich Ru(acac)2 and acceptor L4) mediated stabilization of 2. Structural authentication of the complexes with special reference to the tunable azo distance (N═N, N-N•-, N-N2-) of L and their spectro-electrochemical events in accessible redox states including the reversible electron reservoir feature of 2 → 2+/2+ → 2 were evaluated in conjunction with density functional theory/time-dependent density functional theory calculations. The varying extent of IET as a function of heteroaromatics appended to the azo group of L (L1 = abpy = 2,2'-azobipyridine, L2 = abbt = 2,2'-azobis(benzothiazole), L3 = abim = azobis(1-methylbenzimidazole), L4 and L5, Schemes 1 & 2) in the Ru(acac)2-derived respective molecular setup has been addressed.

Rapid formation of uniformly layered materials by coupling reaction–diffusion processes with mechanical responsiveness

Straightforward manufacturing pathways toward large-scale, uniformly layered composites may enable the next generation of materials with advanced optical, thermal, and mechanical properties. Reaction-diffusion systems are attractive candidates to this aim, but while layered composites theoretically could spontaneously arise from reaction-diffusion, in practice randomly oriented patches separated by defects form, yielding nonuniformly patterned materials. A propagating reaction front can prevent such nonuniform patterning, as is the case for Liesegang processes, in which diffusion drives a reaction front to produce layered precipitation patterns. However, while diffusion is crucial to control patterning, it slows down transport of reactants to the front and results in a steady increase of the band spacing as the front advances. Here, we circumvent these diffusive limitations by embedding the Liesegang process in mechanically responsive hydrogels. The coupling between a moving reaction front and hydrogel contraction induces the formation of a self-regulated transport channel that ballistically carries reactants toward the area where patterning occurs. This ensures rapid and uniform patterning. Specifically, large-scale ([Formula: see text]5-cm) uniform banding patterns are produced with tunable band distance (d = 60 to 160 µm) of silver dichromate crystals inside responsive gelatin-alginate hydrogels. The generality and applicability of our mechanoreaction-diffusion strategy are demonstrated by forming patterns of precipitates in significantly smaller microscopic banding patterns (d = 10 to 30 µm) that act as self-organized diffraction gratings. By circumventing the inherent limitations of diffusion, our strategy unlocks the potential of reaction-diffusion processes for the manufacturing of uniformly layered materials.

Heterogeneous Component Au (Outer)-Pt (Middle)-Au (Inner) Nanorings: Synthesis and Vibrational Characterization on Middle Pt Nanorings with Surface-Enhanced Raman Scattering.

We report a synthetic approach for heterometallic (Au-Pt-Au) nanorings with intertwined triple rings (NITs), wherein three differently sized metal circular nanorings concentrically overlap in a single entity. The synthetic method allows one to control the component of core nanorings (Au or Pt) with a tunable gap distance. The narrow circular nanogaps between inner and outer Au rings strongly enhance the electromagnetic near-field via intraparticle coupling of localized surface plasmon resonance, which realizes surface-enhanced Raman scattering (SERS) at the single-particle level. Importantly, when the component of the middle ring is Pt, in situ SERS measurement for electrochemical reactions on Pt domains could be monitored with electrochemical potential variations due to the near-field focusing that is assisted by plasmonically active inner and outer Au nanorings, which is not feasible with pure Pt nanoparticle systems. The resulting NIT systems are robust and may benefit the synthesis of complicated nanostructures, giving myriad applications.

Intercalation of Small Organic Molecules into Ti3C2Tx MXene Cathodes for Flexible High‐Volume‐Capacitance Zn‐Ion Microsupercapacitor

AbstractThe delamination of 2D Ti3C2Tx MXene endows the injection of various ions and small organic molecules into its layers, thus leading to a tunable distance between layers and adjustable electrochemical properties. A suitable selection of intercalators needs to be considered according to the relevant metal‐ion‐based energy storage device because of the different radii of metal ions such as Li+, Na+, Mg2+ Zn2+, etc. Herein, the intercalation of N,N‐dimethylacetamide (DMAC), acetonitrile (ACN), dimethyl sulfoxide (DMSO), LiCl (H2O) into Ti3C2Tx cathodes and their electrochemical performance comparisons by fabricating Zn‐ion microsupercapacitors (MSCs) is reported. Studies found that an increased calculated interlayer space of 3.42, 7.47, 7.79, 8.3 Å is obtained for the H2O, DMSO, ACN, DMAC intercalated Ti3C2Tx cathodes, and a decreased calculated binding energy of −0.03, −0.78, −1.91, and −3.06 eV is obtained for the Ti3C2Tx‐H2O, Ti3C2Tx‐DMSO, Ti3C2Tx‐ACN, and Ti3C2Tx‐DMAC, respectively. The highest interlayer space, lowest binding energy, and amide groups make the DMAC intercalated Ti3C2Tx‐based MSC exhibit volumetric capacitance of 1873 F cm−3 at a scan rate of 5 mV s−1, much higher than 1103 F cm−3 for Ti3C2Tx‐H2O, 1313 F cm−3 for Ti3C2Tx‐ACN, 544 F cm−3 for Ti3C2Tx‐DMSO. The superior flexibility that results in invariable capacitance under 5000 bending cycles, together with the lighting test of the fabricated MSC, demonstrates its application in the wearable integrated system.

Electrochemical On-Site Switching of the Directional Liquid Transport on a Conical Fiber.

Directional liquid transport (DLT), especially that proceeding on a conical fiber (DLT-CF), is an important mass-transfer process widely used both by natural organisms and in practical applications. However, on-site switching of the DLT-CF remains a challenge due to the nontunable driving force imparted by the structural gradient, which greatly limits its application. Here, unprecedently, a facile electrochemical strategy is developed for reaching the on-site switchable DLT-CF, featuring in situ control and fast response. Depending on the poised electric potential, the droplet can either move directionally or be pinned at any position for a tunable duration time, exhibiting completely different moving characteristics from the traditional DLT-CF with no control. It is proposed that the surface hysteresis resistance, closely related to both the surface hydrogen-bonding network and the droplet topology on the fiber, can be largely altered electrochemically. The tunable hysteresis resistance works synergistically with the conical-structure-induced Laplace pressure to on-site tune the forces acting on the droplet, leading to various controllable DLTs-CF, including those with tunable distance and direction, array manipulation, and assembly line processing of droplets. The strategy is applicable for versatile liquids, offering a general approach for controllable liquid transport in fibrous systems.

A General Synthetic Approach of Organic Lateral Heterostructures for Optical Signal Converters in All-Color Wavelength

A General Synthetic Approach of Organic Lateral Heterostructures for Optical Signal Converters in All-Color Wavelength

Metal-organic framework engineered corn-like SERS active Ag@Carbon with controllable spacing distance for tracking trace amount of organic compounds

Probing water-soluble organic compounds via Surface-enhanced Raman scattering (SERS) technique could be helpful to prevent harmful impacts of polluted water. A key limitation of restraining SERS technique in probing these pollutants is the difficulty to control the spacing distance of plasmonic nanoparticles within 10 nm so that SERS effect can be efficiently induced. Herein, a strategy of mass-producing Ag-based SERS active material with tunable spacing distance is reported. In brevity, metal-organic framework (MOF) engineered corn-like Ag@Carbon is synthesized by simply thermal treating Ag-MOF. The thermal treatment in-situ turns Ag+ into Ag nanoparticles (NPs), resulting in Ag NPs well-dispersed on the surface of the carbonized MOF and forming ordered SERS hotspots. Due to the spatial distance of Ag+ directly depends on the molecular diameter of MOF organic ligands, spacing distance of Ag NP is fixed at around 7 nm. Theoretical analysis and experimental study confirm that the uniformly distributed Ag NPs lead to desirable SERS activity. Further study evidences the presented corn-like Ag@Carbon could be a good candidate for tacking organic compounds with satisfactory sensitivity, specificity and low detection limit (10−8 M). Conclusively, these impressive results indicate a bright future of adopting the proposed strategy to design future SERS active materials.

Spatially Shaping Waves to Penetrate Deep inside a Forbidden Gap.

It is well known that waves with frequencies within the forbidden gap inside a crystal are transported only over a limited distance-the Bragg length-before being reflected by Bragg interference. Here, we demonstrate how to send waves much deeper into crystals in an exemplary study of light in two-dimensional silicon photonic crystals. By spatially shaping the wave fronts, the internal energy density-probed via the laterally scattered intensity-is enhanced at a tunable distance away from the front surface. The intensity is up to 100× enhanced compared to random wave fronts, and extends as far as 8× the Bragg length, which agrees with an extended mesoscopic model. We thus report a novel control knob for mesoscopic wave transport that pertains to any kind of waves.

Abstractions of Varying Decentralization Degree for Reachability of Coupled Multiagent Systems

In this paper we present a decentralized abstraction framework for multiagent systems with couplings in their dynamics, which arise in their popular coordination protocols. The discrete models are based on a varying decentralization degree, namely, the agents' individual abstractions are obtained by using discrete information up to a tunable distance in their network graph. Deriving these models at the agent level is essential to address scalability issues which appear in the discretization of systems with a high state dimension. The approach builds on the appropriate discretization of the agents' state space and the selection of a transition time step, which enable the construction of a nonblocking transition system for each agent with quantifiable transition possibilities. The transitions are based on the design of local feedback laws for the manipulation of the coupling terms, which guarantee the execution of the transitions by the continuous systems. For a class of nonlinear agent interconnections, the derivation of such abstractions is always guaranteed, based on sufficient conditions which relate the agents' dynamics and the space/time quantization.

Dipolar couplings in solid polypeptides probed by 14N NMR spectroscopy

The acquisition of 14N NMR spectra in solid samples is challenging due to quadrupolar couplings with magnitudes up to several MHz. This nucleus is nonetheless important as it is involved in the formation of essential secondary structures in biological systems. Here we report the structural study of the atomic environment of amide functions in polypeptides using magic-angle spinning NMR spectroscopy of the ubiquitous 14N isotope. The cyclic undecapeptide cyclosporin, in which only four hydrogen atoms are directly bound to nitrogen atoms, is chosen for illustration. Structural details of different environments can be revealed without resorting to isotopic enrichment. The network of inter- and intra-residue dipolar couplings between amide 14N nuclei and nearby protons can be probed and mapped out up to a tunable cutoff distance. Density functional theory calculations of NMR quadrupolar interaction tensors agree well with the experimental evidence and allow the unambiguous assignment of all four non-methylated NH nitrogen sites and neighboring proton nuclei.

Astigmatism and deformation correction for a holographic head-mounted display with a wedge-shaped holographic waveguide.

In this study, a head-mounted display (HMD) system based on a wedge-shaped holographic waveguide that can present holographic virtual images with tunable distance is achieved. The compact computer-generated-hologram system using a spatial light modulator was employed to offer the dynamic image, where the probe beam for the hologram reconstruction is a convergent wave, and the DC term of the diffraction wave can be blocked by a barrier. The wedge-shaped holographic waveguide element was used as the combiner of the HMD system to generate a compact structure. A wedge with a polished surface was designed for in-coupling the image into the waveguide, and a reflection-type holographic optical element (HOE) was used for out-coupling the image from the waveguide. The astigmatism aberration and deformation of the diffraction images at various distances are analyzed and then are compensated. Finally, the virtual image can be obtained without aberration with experimental verification.

All-fiber Mach–Zehnder interferometer for tunable two quasi-continuous points’ temperature sensing in seawater

An all-fiber Mach-Zehnder interferometer (MZI) for two quasi-continuous points' temperature sensing in seawater is proposed. Based on the beam propagation theory, transmission spectrum is designed to present two sets of clear and independent interferences. Following this design, MZI is fabricated and two points' temperature sensing in seawater are demonstrated with sensitivities of 42.69pm/°C and 39.17pm/°C, respectively. By further optimization, sensitivity of 80.91pm/°C can be obtained, which is 3-10 times higher than fiber Bragg gratings and microfiber resonator, and higher than almost all similar MZI based temperature sensors. In addition, factors affecting sensitivities are also discussed and verified in experiment. The two points' temperature sensing demonstrated here show advantages of simple and compact construction, robust structure, easy fabrication, high sensitivity, immunity to salinity and tunable distance of 1-20 centimeters between two points, which may provide references for macroscopic oceanic research and other sensing applications based on MZIs.

Fabrication of Crystallized Porous Anodic Aluminum Oxide under Ultra-High Anodization Voltage

Porous anodic aluminum oxide (PAAO) films have attracted increasing attention due to their widespread applications as porous templates. In fact, PAAO films fabricated using high-voltage anodization have apparent advantages which include large-range tunable interpore distance, less impurities, and diverse microstructures. Finding the proper methods to explore the maximum anodization voltage and to analyze related mechanisms is important. Here, crystallized PAAO films with novel microstructures have been fabricated under ultra-high anodization voltage of 700–1000 V in ethanol-oxalic electrolyte system. The unique microstructure and composition of the prepared PAAO films have been studied by using characterization methods of scanniong electron microscopy and X-ray diffraction. Some important phenomena become more obvious under extreme experimental conditions, thus a better understanding of related intrinsic mechanisms of aluminum anodization can be achieved.

Manufacturing of highly ordered porous anodic alumina with conical pore shape and tunable interpore distance in the range of 550 nm to 650 nm

AbstractIn this work, highly ordered porous anodic alumina (PAA) with tapered pore structure and interpore distance (Dc) in the range of 550 nm to 650 nm were fabricated. To produce hexagonal close-packed pore structure a two-step process, combining anodization in etidronic acid electrolyte in the first step and high-concentration, high-temperature anodization in citric acid electrolyte in the second step, was applied. The Al pre-patterned surface obtained in the first anodization was used to produce regular tapered pore arrays by subsequent and alternating anodization in 20 wt.% citric acid solution and pore wall etching in 10 wt.% phosphoric acid solution. The height of the tapered pores was ranging between 2.5 μm and 8.0 μm for the PAA with Dc= 550 nm and Dc= 650 nm, respectively. The geometry of the obtained graded structure can be used for a production of efficient antireflective coatings operating in IR spectral region.

Fracture mechanisms in multilayer phosphorene assemblies: from brittle to ductile.

The outstanding mechanical performance of nacre has stimulated numerous studies on the design of artificial nacres. Phosphorene, a new two-dimensional (2D) material, has a crystalline in-plane structure and non-bonded interaction between adjacent flakes. Therefore, multi-layer phosphorene assemblies (MLPs), in which phosphorene flakes are piled up in a staggered manner, may exhibit outstanding mechanical performance, especially exceptional toughness. Therefore, molecular dynamics simulations are performed to study the dependence of the mechanical properties on the overlap distance between adjacent phosphorene layers and the number of phosphorene flakes per layer. The results indicate that when the flake number is equal to 1, a transition of fracture patterns is observed by increasing the overlap distance, from a ductile failure controlled by interfacial friction to a brittle failure dominated by the breakage of covalent bonds inside phosphorene flakes. Moreover, the failure pattern can be tuned by changing the number of flakes in each phosphorene layer. The results imply that the ultimate strength follows a power law with the exponent -0.5 in terms of the flake number, which is in good agreement with our analytical model. Furthermore, the flake number in each phosphorene layer is optimized as 2 when the temperature is 1 K in order to potentially achieve both high toughness and strength. Moreover, our results regarding the relations between mechanical performance and overlap distance can be explained well using a shear-lag model. However, it should be pointed out that increasing the temperature of MLPs could cause the transition of fracture patterns from ductile to brittle. Therefore, the optimal flake number depends heavily on temperature to achieve both its outstanding strength and toughness. Overall, our findings unveil the fundamental mechanism at the nanoscale for MLPs as well as provide a method to design phosphorene-based structures with targeted properties via tunable overlap distance and flake number in phosphorene layers.

Focusing of linearly polarized Lorentz-Gaussian beam with helical axicon

Focusing properties of linearly polarized Lorentz-Gaussian beam with helical axicon was investigated and focal evolution principle was discussed. Some kind of focal patterns come into being for different axicon parameter and topological charge. The whole focal pattern rotates in anticlockwise direction on increasing axicon parameter for case of small topological charge, otherwise, whole focal pattern distorts considerably and rotates in clockwise direction for bigger topological charge. In addition, focal pattern for case of negative axicon parameter is symmetric about one coordinate axis with positive same value of axicon parameter. All the intensity peaks shift in anti- direction by changing sign of axicon parameter. The shape of intensity peaks also changes considerably for different axicon parameter with tunable distance between peaks. Optical gradient forces distributions show the possibility of constructing optical traps from linearly polarized Lorentz-Gaussian beam with helical axicon.