Crystallographic Effects of GaN Nanostructures in Photoelectrochemical Reaction.

Tuning the surface structure of the photoelectrode provides one of the most effective ways to address the critical challenges in artificial photosynthesis, such as efficiency, stability, and product selectivity, for which gallium nitride (GaN) nanowires have shown great promise. In the GaN wurtzite crystal structure, polar, semipolar, and nonpolar planes coexist and exhibit very different structural, electronic, and chemical properties. Here, through a comprehensive study of the photoelectrochemical performance of GaN photocathodes in the form of films and nanowires with controlled surface polarities we show that significant photoelectrochemical activity can be observed when the nonpolar surfaces are exposed in the electrolyte, whereas little or no activity is measured from the GaN polar c-plane surfaces. The atomic origin of this fundamental difference is further revealed through density functional theory calculations. This study provides guideline on crystal facet engineering of metal-nitride photo(electro)catalysts for a broad range of artificial photosynthesis chemical reactions.

Monolithic integration of multicolor InGaN LEDs with uniform luminescence emission.

We report the demonstration of monolithic integration of multicolor LEDs with highly spatially uniform emission wavelength. LEDs with colors ranging from green to orange are realized in a single selective area epitaxy process, and pronounced emission peak with very narrow spectral linewidth from photonic crystal effect is also achieved simultaneously. The In contents and emission colors are tuned by precisely controlling the nanowire emitter diameter and spacing. The emission wavelengths exhibit small variations of only a few nanometers among countless individual nanowire emitters over a sub-mm2 area region.

Influence of Semifluorinated Alkane Surface Domains on Phase Behavior and Linear and Nonlinear Viscoelasticity of Phospholipid Monolayers.

Semifluorinated alkanes self-assemble into 30-40 nm-large surface domains (hemimicelles) at the air/water interface. They have been drawing increasing attention to stabilize microbubbles coated with lipids, which are used for enhancing the contrast in sonographic imaging. Although previous studies suggested that semifluorinated alkanes increase the stability of phospholipid membranes, little is known about how semifluorinated alkanes influence phase behaviors and mechanical properties of lipid-coated microbubbles. As a well-defined model of microbubble surfaces, we prepared monolayers consisting of a mixture of phospholipids and semifluorinated alkanes at the air/water interface and investigated the influence of hemimicelles of semifluorinated alkanes on the phase behavior and interfacial viscoelastic properties of phospholipid monolayers. Hemimicelles are phase-separated from phospholipids and accumulate at the phase boundary, which strongly modulates the correlation between solid phospholipid domains. Intringuingly, we found that the mixed monolayer of semifluorinated alkanes and phospholipids possesses linear and nonlinear viscoelastic properties comparable to those of phospholipid monolayers. Since the mixing of semifluorinated alkanes and phospholipids enables one to overcome the intrinsically low stability of pure semifluorinated alkanes against the change in the surface area of microbubbles through the partial dissolution of gas into the aqueous phase, this is a promising strategy for the stable coating of microbubbles in ultrasound diagnosis.

Photonic crystal tunnel junction deep ultraviolet light emitting diodes with enhanced light extraction efficiency.

We report on the demonstration of top emitting AlGaN tunnel junction deep ultraviolet (UV) light emitting didoes (LEDs) operating at ∼267 nm. We show, both theoretically and experimentally, that the light extraction efficiency can be enhanced by nearly a factor of two with the incorporation of AlGaN nanowire photonic crystal structures. A peak wall-plug efficiency (WPE) ∼3.5% and external quantum efficiency (EQE) ∼5.4% were measured for AlGaN LEDs directly on-wafer without any packaging. This work demonstrates a viable path for achieving high efficiency deep UV LEDs through the integration of AlGaN planar and nanoscale structures.

Ultrahigh Q microring resonators using a single-crystal aluminum-nitride-on-sapphire platform.

Aluminum-nitride-on-sapphire has recently emerged as a novel low-loss photonics platform for a variety of on-chip electro-optics as well as linear and nonlinear optics applications. In this Letter, we demonstrate ultrahigh quality factor (Qint) microring resonators using single-crystal aluminum nitride grown on a sapphire substrate with an optimized design and fabrication process. A record high intrinsic Qint up to 2.8×106 at the wavelength of 1550 nm is achieved with a fully etched structure, indicating a low propagation loss less than 0.13 dB/cm. Such high Qint aluminum-nitride-on-sapphire resonators with their wide bandgap and electro-optical and nonlinear optical properties is promising for a wide range of low-power and high-power compact on-chip applications over a broad spectral range.

Influence of Perfluorohexane-Enriched Atmosphere on Viscoelasticity and Structural Order of Self-Assembled Semifluorinated Alkanes at the Air-Water Interface.

Semifluorinated alkanes FnHm self-assemble into nanometer-sized surface micelles at the air-water interface. In this study, we investigated how an atmosphere enriched with perfluorohexane (PFH) influences the interfacial viscoelasticity and structural order of a monolayer of FnHm by the combination of dilational rheology and grazing-incidence small-angle X-ray scattering (GISAXS). The monolayers behaved predominantly elastic which can be attributed to the strong dipole repulsions of the surface domains. Enrichment of the atmosphere with PFH lead to an increase of the compressibility and a decrease of the elastic modulus without altering the structural ordering of the FnHm molecules into highly correlated nanodomains, suggesting the adsorption of PFH molecules to the free spaces between the domains. The capability of FnHm domains to retain the structural integrity in the presence of PFH gas is promising for the fabrication of stable microbubbles for sonographic imaging.

Long-Range Lateral Correlation between Self-Assembled Domains of Fluorocarbon-Hydrocarbon Tetrablocks by Quantitative GISAXS.

The structure and lateral correlation of fluorocarbon-hydrocarbon tetrablock di(F10Hm) domains at the air/water interface have been determined by quantitative analysis of grazing incidence small-angle X-ray scattering (GISAXS) data. The measured GISAXS signals can be well represented by the full calculation of the form and structure factors. The form factor suggests that di(F10Hm) domains take a hemiellipsoid shape. Both major and minor axes of the hemiellipsoids monotonically increased in response to the elongation of the hydrocarbon blocks, which can be explained by the concominant increase in van der Waals interaction. The structure factor calculated from the GISAXS signals suggests that the domains take an orthorhombic lattice. Remarkably, the lateral correlation can reach over a distance that is more than 14 times longer than the distance to the nearest neighbors. Our data suggest that quantitative GISAXS enables the optimal design of mesoscopic self-assemblies at the air/water interface by fine-tuning of the structures of molecular building blocks.

Fluorocarbon Exposure Mode Markedly Affects Phospholipid Monolayer Behavior at the Gas/Liquid Interface: Impact on Size and Stability of Microbubbles.

Although most phospholipid-shelled microbubbles (MBs) investigated for medical applications are stabilized by a fluorocarbon (FC) gas, information on the interactions between the phospholipid and FC molecules at the gas/water interface remains scarce. We report that the procedure of introduction of perfluorohexane (F-hexane), that is, either in the gas phase above dimyristoylphosphatidylcholine (DMPC) or dipalmitoylphosphatidylcholine (DPPC) Langmuir monolayers, or in the aqueous subphase, radically affects the compression isotherms. When introduced in the gas phase, F-hexane is rapidly incorporated in the interfacial film, but is also readily desorbed upon compression and eventually totally expelled from the phospholipid monolayers. By contrast, when introduced in the aqueous phase, F-hexane remains trapped at the interface. These dissimilar outcomes demonstrate that the phospholipid monolayer acts as a barrier that effectively hinders the transfer of the FC across the interfacial film. F-hexane was also found to significantly accelerate the adsorption kinetics of the phospholipids at the gas/water interface and to lower the interfacial tension, as assessed by bubble profile analysis tensiometry. The extent of these effects is more pronounced when F-hexane is provided from the gas phase. The size and stability characteristics of DMPC- and DPPC-shelled microbubbles were also found to depend on how the FC is introduced. As compared to reference MBs prepared under nitrogen only, introduction of F-hexane always causes a decrease in MB mean radius. However, while for DMPC this decrease depends on the F-hexane introduction procedure, it is independent from the procedure and most pronounced (from ∼2.0 µm to ∼1.0 µm) for DPPC. Introducing the FC in the gas phase has the strongest effect on MB half-life (t1/2 = ∼1.8 and 6.8 h for DMPC and DPPC, respectively), as compared to when it is delivered through the aqueous phase (∼0.8 and ∼1.7 h). Fluorocarbonless reference DMPC and DPPC bubbles had a half-life of ∼0.5 and 0.8 h, respectively. The effects of F-hexane on MB characteristics are discussed with regard to the interactions between phospholipids and F-hexane and monolayer fluidization effect, as revealed by the Langmuir and tensiometric studies.

2D Spherulites of a Semi-Fluorinated Alkane: Controlled Access to Either Radial Or Ring-Banded Morphologies.

Thin films of a semi-fluorinated alkane cast onto solid substrates consist of well-formed two-dimensional non-birefringent ring-banded and/or radial spherulites. Controlling the experimental conditions allows orientation of the crystallization toward either radial-only or ring-banded-only morphologies. Intermediate states were also captured in which both radial and ring-banded spherulites coexist. Monitoring of the formation of these intermediate states brought evidence for a first crystallization mode that sweeps radially outwards from a central nucleus until the propagating front edge experiences a second crystallization mode that proceeds through a diffusion-controlled rhythmic crystallization mechanism that leads to high (≈2 µm) concentric ridges. These 2D spherulites were investigated by optical and atomic force microscopies, interferometric profilometry, and off-specular neutron scattering.

Two-Dimensional Radial or Ring-Banded Nonbirefringent Spherulites of Semifluorinated Alkanes Coexistent with Close-Packed Self-Assembled Surface Nanodomains.

A series of semifluorinated alkanes (C nF2 n+1C mH2 m+1 diblocks, F n H m, n = 6, 8, 10; m = 16, 18, 20), when cast as films onto solid substrates, were found to form ring-banded or radial spherulites when heated above their isotropic temperature and subsequently cooled down to room temperature, demonstrating that the formation of two-dimensional (2D) spherulites is a general feature of molecular fluorocarbon-hydrocarbon diblocks. These spherulites are not birefringent, a seldom encountered feature for such structures (never, so far, for spherulites made of small molecules). They also provide examples of fluorinated 2D spherulites. Film morphology was analyzed by optical microscopy, interferometric profilometry, atomic force microscopy (AFM), and scanning electron microscopy. Increasing the length of the Fn segment favors the formation of ring-banded spherulites, whereas short Fn segments tend to favor extended radial stripes. Variation of the cooling rate provides control over the size and morphology of the spherulites: slow cooling promotes fibers and radial spherulites, whereas fast cooling fosters ring-banded spherulites. The AFM studies of F10 H16 films revealed that the latter consist of stacks of regularly spaced lamellae. We also observed that, remarkably, stacked lamellae (repeating distance ∼6 nm) can coexist with a layer of close-packed monodisperse circular self-assembled surface nanodomains of Fn Hm diblocks (∼30 nm in diameter); the latter are known to form from such diblocks at interfaces at room temperature. Substrates partially covered with F10 H16 contain incomplete ring-banded spherulites and smaller objects in which the lamellae and circular nanodomains coexist.

Emergence of Strong Nonlinear Viscoelastic Response of Semifluorinated Alkane Monolayers.

Viscoelasticity of monolayers of fluorocarbon/hydrocarbon tetrablock amphiphiles di(FnHm) ((CnF2n+1CH2)(Cm-2H2m-3)CH-CH(CnF2n+1CH2)(Cm-2H2m-3)) was characterized by interfacial dilational rheology under periodic oscillation of the moving barriers at the air/water interface. Because the frequency dispersion of the response function indicated that di(FnHm) form two-dimensional gels at the interface, the viscosity and elasticity of di(FnHm) were first analyzed with the classical Kelvin-Voigt model. However, the global shape of stress response functions clearly indicated the emergence of a nonlinearity even at very low surface pressures (π ≈ 5 mN/m) and small strain amplitudes (u0 = 1%). The Fourier-transformed response function of higher harmonics exhibited a clear increase in the intensity only from odd modes, corresponding to the nonlinear elastic component under reflection because of mirror symmetry. The emergence of strong nonlinear viscoelasticity of di(FnHm) at low surface pressures and strain amplitudes is highly unique compared to the nonlinear viscoelasticity of other surfactant systems reported previously, suggesting a large potential of such fluorocarbon/hydrocarbon molecules to modulate the mechanics of interfaces using the self-assembled domains of small molecules.

Selective area epitaxy of AlGaN nanowire arrays across nearly the entire compositional range for deep ultraviolet photonics.

Semiconductor light sources operating in the ultraviolet (UV)-C band (100-280 nm) are in demand for a broad range of applications but suffer from extremely low efficiency. AlGaN nanowire photonic crystals promise to break the efficiency bottleneck of deep UV photonics. We report, for the first time, site-controlled epitaxy of AlGaN nanowire arrays with Al incorporation controllably varied across nearly the entire compositional range. It is also observed that an Al-rich AlGaN shell structure is spontaneously formed, significantly suppressing nonradiative surface recombination. An internal quantum efficiency up to 45% was measured at room-temperature. We have further demonstrated large area AlGaN nanowire LEDs operating in the UV-C band on sapphire substrate, which exhibit excellent optical and electrical performance, including a small turn-on voltage of ~4.4 V and an output power of ~0.93 W/cm2 at a current density of 252 A/cm2. The controlled synthesis of AlGaN subwavelength nanostructures with well-defined size, spacing, and spatial arrangement and tunable emission opens up new opportunities for developing high efficiency LEDs and lasers and promises to break the efficiency bottleneck of deep UV photonics.

Size, Shape, and Lateral Correlation of Highly Uniform, Mesoscopic, Self-Assembled Domains of Fluorocarbon-Hydrocarbon Diblocks at the Air/Water Interface: A GISAXS Study.

The shape and size of self-assembled mesoscopic surface domains of fluorocarbon-hydrocarbon (FnHm) diblocks and the lateral correlation between these domains were quantitatively determined from grazing incidence small-angle X-ray scattering (GISAXS). The full calculation of structure and form factors unravels the influence of fluorocarbon and hydrocarbon block lengths on the diameter and height of the domains, and provides the inter-domain correlation length. The diameter of the domains, as determined from the form factor analysis, exhibits a monotonic increase in response to the systematic lengthening of each block, which can be attributed to the increase in van der Waals attraction between molecules. The pair correlation function in real space calculated from the structure factor implies that the inter-domain correlation can reach a distance that is over 25 times larger than the domain's size. The full calculation of the GISAXS signals introduced here opens a potential towards the hierarchical design of mesoscale domains of self-assembled small organic molecules, covering several orders of magnitude in space.

Self-assembled mesoscopic surface domains of fluorocarbon-hydrocarbon diblocks can form at zero surface pressure: tilting of solid-like hydrocarbon moieties compensates for cross-section mismatch with fluorocarbon moieties.

At low molecular areas, fluorocarbon-hydrocarbon diblocks (CnF2n+1CmH2m+1, FnHm), when spread as Langmuir monolayers on water, form organized monodisperse circular self-assembled domains, one molecule high and tens of nanometers in diameter. Whether such domains form at high molecular areas (low surface pressures) could until now not be established. Furthermore, the common assumption was that the inner core hydrocarbon chains within these domains were in the liquid state in order to compensate for the difference in the cross-section area between the perfluoroalkyl (∼30 Å2) and alkyl (∼20 Å2) chains. Our IRRAS investigation of F8H16 now establishes (1) that these diblock surface domains do exist at the air/water interface at large molecular areas (zero surface pressure), (2) that they remain essentially unchanged throughout film compression, and (3) that the H16 moieties are actually stretched in an all-trans configuration and tilted by ∼30° with respect to the normal to the monolayer in order to satisfy the greater space requirement of the F8 moieties. Consequently, the core of the domains is in an ordered, crystalline-like state, and the domains can be visualized as solid particles at the air/water interface.

Existence of Two-Dimensional Physical Gels even at Zero Surface Pressure at the Air/Water Interface: Rheology of Self-Assembled Domains of Small Molecules.

Films of mesoscopic domains self-assembled from fluorocarbon/hydrocarbon diblock copolymers (FnHm) at the air/water interface were found to display highly elastic behavior. We determined the interfacial viscoelasticity of domain-patterned FnHm Langmuir monolayers by applying periodic shear stresses. Remarkably, we found the formation of two-dimensional gels even at zero surface pressure. These monolayers are predominantly elastic, which is unprecedented for surfactants, exhibiting gelation only at high surface pressures. Systematic variation of the hydrocarbon (n=8; m=14, 16, 18, 20) and fluorocarbon (n=8, 10, 12; m=16) block lengths demonstrated that subtle changes in the block length ratio significantly alter the mechanics of two-dimensional gels across one order of magnitude. These findings open perspectives for the fabrication of two-dimensional gels with tuneable viscoelasticity via self-assembly of mesoscale, low-molecular-weight materials.

Formation and nature of InGaN quantum dots in GaN nanowires.

InGaN/GaN disk-in-nanowire heterostructures on silicon substrates have emerged as important gain media for the realization of visible light sources. The nature of quantum confinement in the disks is largely unknown. From the unique nature of the measured temperature dependence of the radiative lifetime and direct transmission electron microscopy, it is evident that such self-organized islands (disks) behave as quantum dots. This is confirmed by the observation of single photon emission from a single disk-in-nanowire and the presence of a sharp minimum in the line width enhancement factor of edge emitting lasers having the InGaN disks as the gain media.

Surface-plasmon-polariton whispering-gallery mode analysis of the graphene monolayer coated InGaAs nanowire cavity.

In this article, we proposed and numerically studied the surface plasmon polariton whispering gallery mode properties of the graphene coated InGaAs nanowire cavity. The quality factor and the mode area were investigated as a function of the chemical potential, the cavity radius and the wavelength. A high cavity quality factor of 235 is predicted for a 5 nm radius cavity, accompanied by a mode area as small as3.75×10(-5)(λ(0))(2), when the chemical potential is 1.2 eV. The proposed structure offers a potential solution to high density integration of the nanophotonic devices with an ultra-compact footprint.

High χ P2PFBEMA-b-P2VP Block Copolymers Forming 6-8 nm Domains for Semiconductor Lithography.

We leveraged the potential of high χ-low N block copolymer (BCP), namely, poly[2-(perfluorobutyl) ethyl methacrylate]-block-poly(2-vinylpyridine) (P2PFBEMA-b-P2VP), and demonstrated its utility in next-generation nanomanufacturing. By combining molecular dynamics simulations with experiments, the χ value was calculated to be as high as 0.4 (at 150 °C), surpassing similar structures. Highly ordered features suitable for application were observed, ranging in periods from 19.0 nm down to 12.1 nm, with feature sizes as small as 6 nm. Transmission electron microscopy images of the BCP solutions indicated that preformed micelles in the solution facilitated the self-assembly process of the thin film. In addition, the vertical or parallel orientation of the cylindrical structure was determined by manipulating the solvent, substrate, and annealing conditions. Finally, guided by a wide topographical template, nearly defect-free directed self-assembly (DSA) lines with a resolution of 8 nm were achieved, highlighting its potential practical application in DSA lithography technology.

Efficient oral delivery of water-soluble CT contrast agent using an W1/O/W2 alginate hydrogel matrix.

HYPOTHESIS: Iohexol (IOH) is a commonly used second-generation nonionic iodinated contrast agent. However, low gastrointestinal mucosal adherence and high downstream speed limit its application in intestinal Computed tomography (CT) imaging. We hypothesize that oral IOH delivery carriers composed of environmentally responsive materials enable the intestinal targeted delivery and prolong the intestinal residence time of IOH, enhancing the intestinal disease detection efficiency. EXPERIMENTS: An emulsion-filled alginate hydrogel system was developed as the intestinal targeting vehicle for IOH. The formulation optimization was determined by response surface analysis. After a thorough study of the physicochemical properties of this hydrogel matrix, the pH sensitivity and the ability to control release were investigated, followed by a vitro cell experiment evaluating its bioactivity and CT imaging capability. FINDINGS: This alginate hydrogel matrix was sparsely structured and rapidly released IOH at pH 7.4. Meanwhile the swelling degree was 4.4 times higher than that at pH 1.2, indicating a good selective responsiveness to the gastrointestinal simulated environment. It improved the CT visual contrast of A549 cells without affecting cell morphology, suggesting that it would be an effective oral administration for water-soluble nonionic contrast agents and a potential candidate for intestinal disease detection tools.

Investigation on the scintillation reduction of elliptical vortex beams propagating in atmospheric turbulence.

We numerically investigate the scintillation index of an elliptical vortex beam in both modest turbulence and strong turbulence. Numerical simulations are realized with random phase screen scheme. It is shown that the on-axis scintillation index can be effectively reduced by an elliptical vortex beam if crucial parameters are properly chosen. The mechanisms of scintillation reduction in turbulence of different strengths are different. We find that the topological charge and the ratio of minor axis to major axis of an elliptical vortex beam are important in determining the on-axis scintillation index. Our simulation results indicate that using an elliptical vortex beam is a promising strategy to alleviate atmospheric influence on free space optical communication link.