3D Laser Writing of Low-Loss Cross-Section-Variable Type-I Optical Waveguide Passive/Active Integrated Devices in Single Crystals.

Optical waveguides fabricated in single crystals offer crucial passive/active optical components for photonic integrated circuits. Single crystals possess inherent advantages over their amorphous counterpart, such as lower optical losses in visible-to-mid-infrared band, larger peak emission cross-section, higher doping concentration. However, the writing of Type-I positive refractive index modified waveguides in single crystals using femtosecond laser technology presents significant challenges. Herein, this work introduces a novel femtosecond laser direct writing technique that combines slit-shaping with an immersion oil objective to fabricate low-loss Type-I waveguides in single crystals. This approach allows for precise control of waveguide shape, size, mode-field, and refractive index distribution, with a spatial resolution as high as 700 nm and a high positive refractive index variation on the order of 10-2, introducing new degrees of freedom to design and fabricate passive/active optical waveguide devices. As a proof-of-concept, this work successfully produces a 7 mm-long circular-shaped gain waveguide (≈10 µm in diameter) in an Er3+-doped YAG single crystal, exhibiting a propagation loss as low as 0.23 dB cm-1, a net gain of ≈3 dB and a polarization-insensitive character. The newly-developed technique is theoretically applicable to arbitrary single crystals, holding promising potential for various applications in integrated optics, optical communication, and photonic quantum circuits.

Lever arm on bracket vs. lever arm on archwire: A 3D finite element method study of mechanics of miniscrew-supported lingual en-masse retraction of maxillary anterior teeth.

OBJECTIVE: To compare the deformation of the main archwire and 3D movements of maxillary anterior teeth during miniscrew-supported en-masse retraction with the lever arm on the archwire and on the brackets in lingual orthodontic treatment in finite element analysis (FEM) simulation. MATERIAL AND METHODS: A 3D dental-alveolar model with bonded 0.018×0.025-inch slot lingual brackets and a 0.017×0.025-inch dimension stainless-steel archwire was created. Four FEM models were created based on a 3D dental-alveolar model: in Models A and C, the lever arms were attached to the lingual bracket, while in Models B and D, the lever arms were attached to the archwire. Meanwhile, in Models A and B, the miniscrews were placed in between the molars, while in Models C and D, the miniscrews were positioned on the palatal roof. After a 1.5N retraction force was applied from the miniscrew to the end of the lever arm, the initial movements in the sagittal, transversal, and vertical planes were recorded and analysed for maxillary anterior teeth. RESULTS: In Models B and D, smaller deformation of the main archwire and less prominent bowing effect were noticed in both sagittal and vertical directions compared to their counter groups. In Models C and D, the central incisors showed less torque loss in the sagittal direction and more canine intrusion vertically. CONCLUSIONS: For the same lever arm-miniscrew retraction configuration, the lever arm on the bracket showed less deformation of the main archwire and more body movement of the teeth than the lever arm on the archwire group. With the same level arm height, the transverse and vertical bowing effect is reduced when the lever arm was placed distal to the central incisor and the miniscrews placed next to the palatal suture.

Revealing Surface Restraint-Induced Hexagonal Pd Nanocrystals via In Situ Transmission Electron Microscopy.

Achieving metal nanocrystals with metastable phase draws much attention due to their anticipated fascinating properties, wheras it is still challenging because their polymorphism nature and phase transition mechanism remain elusive. Here, phase stability of face-centered cubic (fcc) Pd nanocrystals was studied via in situ spherical aberration (Cs)-corrected transmission electron microscopy (TEM). By constructing a well-defined Pd/C composite structure, Pd nanocrystals encapsulated by graphite, the dispersion process of fcc Pd was observed through a nucleation and growth process. Interestingly, Cs-corrected scanning TEM analysis demonstrated that the newly formed Pd nanocrystals could adopt a metastable hexagonal phase, which was considered challenging to obtain. Accordingly, formation mechanism of the hexagonal Pd nanocrystals was proposed, which involved the combined effect of two factors: (1) templating of graphite and (2) size effect. This work is expected to offer new insight into the polymorphism of Pd nanocrystals and pave the way for the future design of metastable metal nanomaterials.

Surface faceting and compositional evolution of Pd@Au core-shell nanocrystals during in situ annealing.

Bimetallic core-shell nanoparticles have received considerable attention for their unique optical, magnetic and catalytic properties. However, these properties will be dramatically modified under ambient conditions by their structure and/or composition change. Thus, it is of primary importance to study the complex transformation pathway of core-shell nanoparticles at an elevated temperature. In this work, by using an aberration-corrected scanning transmission electron microscope equipped with an energy dispersive X-ray mapping system, the complete transformation process from a well-designed Pd@Au core-shell nanoparticle to a uniform alloy particle was visualized. It is revealed that this transformation process went through three steps, i.e., surface refacetting, particle resphering and complete alloying. Combining with a developed atomic kinetic Monte Carlo simulation, we found that surface energy is the driving force for shape variation, and the different atomic activation barriers of surface diffusion and bulk migration result in the multistep transformation pathway. Our results offered important information for understanding the structure evolution of bimetallic core-shell nanoparticles, which is beneficial for the rational design of nanoparticles with kinetic stability.

Highly Efficient Nanoscale Analysis of Plant Stomata and Cell Surface Using Polyaddition Silicone Rubber.

Stomata control gas exchange and water transpiration and are one of the most important physiological apparatuses in higher plants. The regulation of stomatal aperture is closely coordinated with photosynthesis, nutrient uptake, plant growth, development, and so on. With advances in scanning electron microscopy (SEM), high-resolution images of plant stomata and cell surfaces can be obtained from detached plant tissues. However, this method does not allow for rapid analysis of the dynamic variation of plant stomata and cell surfaces in situ under nondestructive conditions. In this study, we demonstrated a novel plant surface impression technique (PSIT, Silagum-Light as correction impression material based on A-silicones for all two-phase impression techniques) that allows for precise analysis of plant stomata aperture and cell surfaces. Using this method, we successfully monitored the dynamic variation of stomata and observed the nanoscale microstructure of soybean leaf trichomes and dragonfly wings. Additionally, compared with the analytical precision and the time used for preparing the observation samples between PSIT and traditional SEM, the results suggested that the analytical precision of PSIT was the same to traditional SEM, but the PSIT was more easy to operate. Thus, our results indicated that PSIT can be widely applied to the plant science field.

Unique size-dependent nanocatalysis revealed at the single atomically precise gold cluster level.

Atomically precise metal clusters have attracted increasing interest owing to their unique size-dependent properties; however, little has been known about the effect of size on the catalytic properties of metal clusters at the single-cluster level. Here, by real-time monitoring with single-molecule fluorescence microscopy the size-dependent catalytic process of individual Au clusters at single-turnover resolution, we study the size-dependent catalytic behaviors of gold (Au) clusters at the single-cluster level, and then observe the strong size effect on the catalytic properties of individual Au clusters, in both catalytic product formation and dissociation processes. Surprisingly, indicated by both experiments and density functional theory (DFT) calculations, due to such a unique size effect, besides observing the different product dissociation behaviors on different-sized Au clusters, we also observe that small Au clusters [i.e., Au15(MPA)13; here, MPA denotes 3-mercaptopropionic acid] catalyze the product formation through a competitive Langmuir-Hinshelwood mechanism, while those relatively larger Au clusters [e.g., Au18(MPA)14 and Au25(MPA)18] or nanoparticles catalyze the same process through a noncompetitive Langmuir-Hinshelwood mechanism. Such a size effect on the nanocatalysis could be attributed intrinsically to the size-dependent electronic structure of Au clusters. Further analysis of dynamic activity fluctuation of Au clusters reveals more different catalytic properties between Au clusters and traditional Au nanoparticles due to their different size-dependent structures.

A Bifunctional Highly Efficient FeNx /C Electrocatalyst.

Herein, a type of Fe, N-codoped carbon electrocatalyst (FeNx /C, Fe-N-BCNT#BP) containing bamboo carbon nanotubes and displaying bifunctional high catalytic efficiency for both oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR) is reported. It shows high electrocatalytic activity and stability for both the ORR process with onset potential of 1.03 VRHE in alkaline and the CO2RR to CO with high faradic efficiency up to 90% and selectivity of about 100% at low overpotential of 0.49 V. For CO2RR to CO, it is revealed that Fe3 C is active but the activity of FeNx centers is lower than that of C-N-based centers, contrary with that observed for ORR. Due to its low cost and high electrocatalytic performance for these two reduction reactions, the obtained catalyst is very promising for extensive application in future. The revealed huge activity difference of the same types of active sites for different reactions can efficiently guide the synthesis of advanced materials with multifunction.

Multimetallic AuPd@Pd@Pt core-interlayer-shell icosahedral electrocatalysts for highly efficient oxygen reduction reaction.

Incorporating Pt with core metals into Pt-based core-shell catalysts is regarded as a promising strategy to substantially enhance the catalytic properties towards oxygen reduction reaction (ORR) in fuel cells due to the synergetic effect between distinct metals. In this wok, ultrathin Pt skins with two atomic layers were epitaxially coated on as-prepared icosahedral Au50Pd50, Au60Pd40 and Au66Pd34 nanocrystal seeds, which are constructed with alloyed cores and Pd shells with different thickness. Through electron microscopic characterizations, Pd interlayers with tunable thickness of 3, 6, and 12 atomic layers can be found in the Au66Pd34@Pt, Au60Pd40@Pt and Au50Pd50@Pt icosahedra, respectively. These icosahedral AuPd@Pd@Pt nanocrystals show substantially enhanced activities and durabilities in electrocatalytic measurements towards ORR compared to Au75Pd25@Pt icosahedra without Pd interlayer and commercial Pt/C catalysts. Specifically, Au60Pd40@Pt icosahedra with 6 atomically thick Pd interlayer display the best electrocatalytic performances, whose mass activities before and after durability tests of 50,000 cycles are 11.6 and 30.2 times, respectively, as high as that of the commercial Pt/C.

In†Situ Observation of Hydrogen-Induced Surface Faceting for Palladium-Copper Nanocrystals at Atmospheric Pressure.

Nanocrystal (NC) morphology, which decides the number of active sites and catalytic efficiency, is strongly determined by the gases involved in synthesis, treatment, and reaction. Myriad investigations have been performed to understand the morphological response to the involved gases. However, most prior work is limited to low pressures, which is far beyond realistic conditions. A dynamic morphological evolution of palladium-copper (PdCu) NC within a nanoreactor is reported, with atmospheric pressure hydrogen at the atomic scale. In situ transmission electron microscopy (TEM) videos reveal that spherical PdCu particles transform into truncated cubes at high hydrogen pressure. First principles calculations demonstrate that the surface energies decline with hydrogen pressure, with a new order of γH-001 <γH-110 <γH-111 at 1 bar. A comprehensive Wulff construction based on the corrected surface energies is perfectly consistent with the experiments. The work provides a microscopic insight into NC behaviors at realistic gas pressure and is promising for the shaping of nanocatalysts by gas-assisted treatments.

High Catalytic Activity and Chemoselectivity of Sub-nanometric Pd Clusters on Porous Nanorods of CeO2 for Hydrogenation of Nitroarenes.

Sub-nanometric Pd clusters on porous nanorods of CeO2 (PN-CeO2) with a high Pd dispersion of 73.6% exhibit the highest catalytic activity and best chemoselectivity for hydrogenation of nitroarenes to date. For hydrogenation of 4-nitrophenol, the catalysts yield a TOF of ∼44059 h(-1) and a chemoselectivity to 4-aminophenol of >99.9%. The superior catalytic performance can be attributed to a cooperative effect between the highly dispersed sub-nanometric Pd clusters for hydrogen activation and unique surface sites of PN-CeO2 with a high concentration of oxygen vacancy for an energetically and geometrically preferential adsorption of nitroarenes via nitro group. The high concentration of surface defects of PN-CeO2 and large Pd dispersion contribute to the enhanced catalytic activity for the hydrogenation reactions. The high chemoselectivity is mainly governed by the high Pd dispersion on the support. The catalysts also deliver high catalytic activity and selectivity for nitroaromatics with various reducible substituents into the corresponding aminoarenes.

Broadly defining lasing wavelengths in single bandgap-graded semiconductor nanowires.

Designing lasing wavelengths and modes is essential to the practical applications of nanowire (NW) lasers. Here, according to the localized photoluminescence spectra, we first demonstrate the ability to define lasing wavelengths over a wide range (up to 119 nm) based on an individual bandgap-graded CdSSe NW by forward cutting the NW from CdSe to CdS end. Furthermore, free spectral range (FSR) and modes of the obtained lasers could be controlled by backward cutting the NW from CdS to CdSe end step-by-step. Interestingly, single-mode NW laser with predefined lasing wavelength is realized in short NWs because of the strong mode competition and increase in FSR. Finally, the gain properties of the bandgap-graded NWs are investigated. The combination of wavelength and mode selectivity in NW lasers may provide a new platform for the next generation of integrated optoelectronic devices.

Electrophoretic motion of ideally polarizable particles in a microchannel.

The induced-charge electrophoretic (ICEP) motion of ideally polarizable particles in a microchannel is numerically studied in this paper. A complete 3-D multi-physics model is set up to simulate the transient ICEP motion of spherical ideally polarizable particles in a microchannel. The study shows that a non-uniform distribution of induced surface charge occurs when an ideally polarizable particle is immersed in an externally applied electric field, resulting in a varying slipping (EOF) velocity along the particle's surface and hence producing micro vortexes in the liquid. The numerical results verify that the steady-state ICEP velocity of an ideally polarizable particle does not differ from the electrophoretic velocity of a non-conducting particle, although the flow field near the particle does. A strong wall-repelling effect of ICEP is found when the polarizable particle is placed close to the channel wall. This is due to the lifting effect generated from the interaction between the induced micro vortexes and the channel wall and depends on the electric field and the particle size. The wall effects on ICEP motion can be used for focusing particles and for separation of particle by density.

Continuous particle separation by size via AC-dielectrophoresis using a lab-on-a-chip device with 3-D electrodes.

In this paper, we present a novel, simple lab-on-a-chip device for continuous separation of particles by size. The device is composed of a straight rectangular microchannel connecting two inlet reservoirs and two exit reservoirs. Two asymmetric, 3-D electrodes are embedded along the channel wall to generate a non-uniform electrical field for dielectrophoresis. Particles with different sizes are collected at the different exit reservoirs. Main flow is induced by pressure difference between the inlet and the exit reservoirs. The device is used successfully for the separation of the 5 and 10 mum latex particles and for the separation of yeast cells and white blood cells. A numerical simulation based on Lagrangian tracking method is used to simulate the particle motion and the results showed a good agreement with the experimental data.