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We find evidence for the formation and relaxation of large exciton polarons in 2D organic-inorganic hybrid perovskites. Using ps-scale time-resolved photoluminescence within the phenethylammonium lead iodide family of compounds, we identify a red shifting of emission that we associate with exciton polaron formation time scales of 3-10 ps. Atomic substitutions of the phenethylammonium cation allow local control over the structure of the inorganic lattice, and we show that the structural differences among materials strongly influence the exciton polaron relaxation process, revealing a polaron binding energy that grows larger (up to 15 meV) in more strongly distorted compounds.

We report a family of two-dimensional hybrid perovskites (2DHPs) based on phenethylammonium lead iodide ((PEA)$_2$PbI$_4$) that show complex structure in their low-temperature excitonic absorption and photoluminescence (PL) spectra as well as hot exciton PL. We replace the 2-position (ortho) H on the phenyl group of the PEA cation with F, Cl, or Br to systematically increase the cation's cross-sectional area and mass and study changes in the excitonic structure. These single atom substitutions substantially change the observable number of and spacing between discrete resonances in the excitonic absorption and PL spectra and drastically increase the amount of hot exciton PL that violates Kasha's rule by over an order of magnitude. To fit the progressively larger cations, the inorganic framework distorts and is strained, reducing the Pb-I-Pb bond angles and increasing the 2DHP band gap. Correlation between the 2DHP structure and steady-state and time-resolved spectra suggests the complex structure of resonances arises from one or two manifolds of states, depending on the 2DHP Pb-I-Pb bond angle (as)symmetry, and the resonances within a manifold are regularly spaced with an energy separation that decreases as the mass of the cation increases. The uniform separation between resonances and the dynamics that show excitons can only relax to the next-lowest state are consistent with a vibronic progression caused by a vibrational mode on the cation. These results demonstrate that simple changes to the structure of the cation can be used to tailor the properties and dynamics of the confined excitons without directly modifying the inorganic framework., Comment: 39 pages

The development of a suitable catalyst for the oxygen reduction reaction (ORR), the cathode reaction of proton exchange membrane fuel cells (PEMFC), is necessary to push this technology toward widespread adoption. There have been substantial efforts to utilize bimetallic Pt-M alloys that adopt the ordered face-centered tetragonal (L1

The topological kagome magnet (TKM) Fe3Sn2 exhibits unusual topological properties, flat electronic bands, and chiral spin textures, making it an exquisite materials platform to explore the interplay between topological band structure, strong electron correlations, and magnetism. Here we report the first synthesis of high-quality epitaxial (0001) Fe3Sn2 films with large intrinsic anomalous Hall effect close to that measured in bulk single crystals. In addition, we measured a large, anisotropic anomalous Nernst coefficient Syx of 1.26 {\mu}V/K, roughly 2-5x greater than that of common ferromagnets, suggesting the presence of Berry curvature sources near the Fermi level in this system. Crucially, the realization of high-quality Fe3Sn2 films opens the door to explore emergent interfacial physics and create novel spintronic devices based on TKMs by interfacing Fe3Sn2 with other quantum materials and by nanostructure patterning., Comment: accepted by Physical Review Materials

Next-generation 'smart' nanoparticle systems should be precisely engineered in size, shape and composition to introduce multiple functionalities, unattainable from a single material. Bottom-up chemical methods are prized for the synthesis of crystalline nanoparticles, that is, nanocrystals, with size- and shape-dependent physical properties, but they are less successful in achieving multifunctionality. Top-down lithographic methods can produce multifunctional nanoparticles with precise size and shape control, yet this becomes increasingly difficult at sizes of ∼10 nm. Here, we report the fabrication of multifunctional, smart nanoparticle systems by combining top-down fabrication and bottom-up self-assembly methods. Particularly, we template nanorods from a mixture of superparamagnetic Zn

Growth of transition metal dichalcogenide (TMD) monolayers is of interest due to their unique electrical and optical properties. Films in the 2H and 1T phases have been widely studied but monolayers of some 1T'-TMDs are predicted to be large-gap quantum spin Hall insulators, suitable for innovative transistor structures that can be switched via a topological phase transition rather than conventional carrier depletion [ Qian et al. Science 2014 , 346 , 1344 - 1347 ]. Here we detail a reproducible method for chemical vapor deposition of monolayer, single-crystal flakes of 1T'-MoTe2. Atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirm the composition and structure of MoTe2 flakes. Variable temperature magnetotransport shows weak antilocalization at low temperatures, an effect seen in topological insulators and evidence of strong spin-orbit coupling. Our approach provides a pathway to systematic investigation of monolayer, single-crystal 1T'-MoTe2 and implementation in next-generation nanoelectronic devices.

A safe, scalable method for producing highly conductive aligned films of single-walled carbon nanotubes (SWNTs) from water suspensions is presented. While microfluidic assembly of SWNTs has received significant attention, achieving desirable SWNT dispersion and morphology in fluids without an insulating surfactant or toxic superacid is challenging. We present a method that uniquely produces a noncorrosive ink that can be directly applied to a device in situ, which is different from previous fabrication techniques. Functionalized SWNTs (f-SWNTs) are dispersed in an aqueous urea solution to leverage binding between the amine group of urea and the carboxylic acid group of f-SWNTs and obtain urea-SWNT. Compared with SWNTs dispersed using conventional methods (e.g., superacid and surfactants), the dispersed urea-SWNT aggregates have a higher aspect ratio with a rodlike morphology as measured by light scattering. The Mayer rod technique is used to prepare urea-SWNT, highly aligned films (two-dimensional nematic order parameter of 0.6, 5 μm spot size, via polarized Raman) with resistance values as low as 15-1700 Ω/sq in a transmittance range of 2-80% at 550 nm. These values compete with the best literature values for conductivity of SWNT-enabled thin films. The findings offer promising opportunities for industrial applications relying on highly conductive thin SWNT films.

A low temperature, aqueous synthesis of polyhedral iron oxide nanoparticles (IONPs) is presented. The modification of the co-precipitation hydrolysis method with Triton X surfactants results in the formation of crystalline polyhedral particles. The particles are herein termed iron oxide “nanobricks” (IONBs) as the variety of particles made are all variations on a simple “brick-like” rhombohedral shape as evaluated by TEM. These IONBs can be easily coated with hydrophilic silane ligands, allowing them to be dispersed in aqueous media. The dispersed particles are investigated for potential applications as hyperthermia and T2 MRI contrast agents. The results demonstrate that the IONBs perform better than comparable spherical IONPs in both applications, and show r2 values amongst the highest for iron oxide based materials reported in the literature.

The collective magnetic properties of nanoparticle (NP) solid films are greatly affected by inter-particle dipole–dipole interactions and therefore the proximity of the neighboring particles. In this study, a series of dendritic ligands (generations 0 to 3, G0–G3) have been designed and used to cover the surface of magnetic NPs to control the spacings between the NP components in single lattices. The dendrons of different generations introduced here were based on the 2,2-bis(hydroxymethyl)propionic acid (Bis-MPA) scaffold and equipped with an appropriate surface binding group at one end and several fatty acid segments at the other extremity. The surface of the NPs was then modified by partial ligand exchange between the primary stabilizing surfactants and the new dendritic wedges. It was shown that this strategy permitted very precise tuning of inter-particle spacings in the range of 2.9–5.0 nm. As expected, the increase in the inter-particle spacings reduced the dipole–dipole interactions between magnetic NPs and therefore allowed changes in their magnetic permeability. The dendron size and inter-particle distance dependence was studied to reveal the dendritic effect and identify the optimal geometry and generation.

We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer "metamolecules" as the number of nanocrystals is systematically varied. Precise, hexagonally ordered Au nanocrystal oligomers with 1-31 members are assembled via capillary forces into polygonal topographic templates defined using electron-beam lithography. The visible and near-infrared scattering response of individual oligomers is measured by spatially resolved, polarized darkfield scattering spectroscopy. The response is highly sensitive to in-plane versus out-of-plane incident polarization, and we observe an exponentially saturating red shift in plasmon resonance wavelength as the number of nanocrystals per oligomer increases, in agreement with theoretical predictions. Simulations further elucidate the modes supported by the oligomers, including electric dipole and magnetic dipole resonances and their Fano interference. The single-oligomer sensitivity of our measurements also reveals the role of positional disorder in determining the wavelength and character of the plasmonic response. The progression of oligomer metamolecule structures studied here advances our understanding of fundamental plasmonic interactions in the transition regime between few-member plasmonic clusters and extended two-dimensional arrays.

We study gate-controlled, low-temperature resistance and magnetotransport in indium-doped CdSe quantum dot field effect transistors. We show that using the gate to accumulate electrons in the quantum dot channel increases the "localization product" (localization length times dielectric constant) describing transport at the Fermi level, as expected for Fermi level changes near a mobility edge. Our measurements suggest that the localization length increases to significantly greater than the quantum dot diameter.

Self-assembly of multicomponent nanocrystal superlattices provides a modular approach to the design of metamaterials by choosing constituent nanocrystal building blocks with desired physical properties and engineering the interparticle coupling. In this work, we report the self-assembly of binary nanocrystal superlattices composed of magnetically hard CoFe2O4 nanocrystals and magnetically soft Fe3O4 nanocrystals. Both NaZn13- and MgZn2-type CoFe2O4–Fe3O4 binary nanocrystal superlattices have been formed by the liquid–air interfacial assembly approach. Exchange coupling is achieved in both types of binary superlattices after thermal annealing under vacuum at 400 °C. The exchange-coupled CoFe2O4–Fe3O4 binary nanocrystal superlattices show single-phase magnetization switching behavior and magnetoresistance switching behavior below 200 K. The NaZn13-type CoFe2O4–Fe3O4 binary nanocrystal superlattices annealed at 500 °C even exhibit bistable magnetoresistance switching behavior at room temperature constituting...

We calculate the orbital magnetic anisotropy (Δχ) of semiconducting single-walled carbon nanotubes at different carrier densities using a nearest-neighbor tight-binding model. Kataura plots of Δχ exhibit 2n + m family groupings and chiral index dependence at all carrier densities which are consistent with the trigonal warping effect. The diameter dependence of Δχ varies strongly with carrier density. We fit our data with a symmetry-restricted model to obtain approximate analytic expressions for Δχ as a function of nanotube chirality and carrier density. Our results illustrate the important role of doping on the magnetic properties of carbon nanotubes. Experimental studies of Δχ should take these effects into account for accurate interpretation of their results.

The manipulation of radiative properties of light emitters coupled with surface plasmons is important for engineering new nanoscale optoelectronic devices, including lasers, detectors and single photon emitters. However, so far the radiative rates of excited states in semiconductors and molecular systems have been enhanced only moderately, typically by a factor of 10-50, producing emission mostly from thermalized excitons. Here, we show the generation of dominant hot-exciton emission, that is, luminescence from non-thermalized excitons that are enhanced by the highly concentrated electromagnetic fields supported by the resonant whispering-gallery plasmonic nanocavities of CdS-SiO(2)-Ag core-shell nanowire devices. By tuning the plasmonic cavity size to match the whispering-gallery resonances, an almost complete transition from thermalized exciton to hot-exciton emission can be achieved, which reflects exceptionally high radiative rate enhancement of10(3) and sub-picosecond lifetimes. Core-shell plasmonic nanowires are an ideal test bed for studying and controlling strong plasmon-exciton interaction at the nanoscale and opens new avenues for applications in ultrafast nanophotonic devices.

Co-assembly of two types of nanocrystals (NCs) into binary NC superlattices (BNSLs) provides a solution-based, inexpensive way to create novel metamaterials with rationally designed properties. The fundamental challenge is to probe and understand the nature and extent of complex interparticle interactions present in BNSLs, which can lead to collective properties that differ from their dispersed constituents or phase-separated counterparts. Here, we report the growth and magnetic characterization of large-area (∼1 cm(2)) BNSL membranes self-assembled from distinct magnetic NCs at the liquid-air interface. The resulting BNSL membranes exhibit a single-phase-like magnetization alignment process, which is not observed in the phase-separated NC mixtures having the same stoichiometry. This single-phase-like magnetic behavior is attributed to the collective interparticle dipolar interactions between two NC components in BNSLs, corroborated by calculation of the random dipolar fields as well as Monte Carlo simulation. The collective magnetic properties are demonstrated in magnetic BNSL membranes having different structures (stoichiometry) and different NC combinations.

Aerogels are ultralight, highly porous materials typicallyfabricated by subjecting a wet-gel precursor to critical-point-drying (CPD) or lyophilization (freeze-drying) in order toremove background liquid without collapsing the network.Microscopically, aerogels are composed of tenuous networksof clustered nanoparticles, and the materials often haveunique properties, including very high strength-to-weight andsurface-area-to-volume ratios. To date most aerogels are fab-ricated from silica

Photoluminescence (PL) identifies spectroscopic signatures of intertube transfer of optically pumped carriers in single-walled carbon nanotube (SWNT) ensembles. Resonant photoexcitation of large band gap SWNTs produces strong PL from smaller band gap SWNTs. Magnetic alignment measurements associate the energy-transfer PL peaks with the formation of SWNT bundles, suggesting that efficient coupling results from physical contact.

We use time-integrated and time-resolved photoluminescence and absorption to characterize the low-temperature optical properties of CdSe quantum dot solids after exchanging native aliphatic ligands for thiocyanate and subsequent thermal annealing. In contrast to trends established at room temperature, our data show that at low temperature the band-edge absorptive bleach is dominated by 1S3/2h hole occupation in the quantum dot interior. We find that our ligand treatments, which bring enhanced interparticle coupling, lead to faster surface state electron trapping, a greater proportion of surface-related photoluminescence, and decreased band-edge photoluminescence lifetimes.

Large-area growth of monolayer films of the transition metal dichalcogenides is of the utmost importance in this rapidly advancing research area. The mechanical exfoliation method offers high quality monolayer material but it is a problematic approach when applied to materials that are not air stable. One important example is 1T'-WTe2, which in multilayer form is reported to possess a large non saturating magnetoresistance, pressure induced superconductivity, and a weak antilocalization effect, but electrical data for the monolayer is yet to be reported due to its rapid degradation in air. Here we report a reliable and reproducible large-area growth process for obtaining many monolayer 1T'-WTe2 flakes. We confirmed the composition and structure of monolayer 1T'-WTe2 flakes using x-ray photoelectron spectroscopy, energy-dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy and aberration corrected transmission electron microscopy. We studied the time dependent degradation of monolayer 1T'-WTe2 under ambient conditions, and we used first-principles calculations to identify reaction with oxygen as the degradation mechanism. Finally we investigated the electrical properties of monolayer 1T'-WTe2 and found metallic conduction at low temperature along with a weak antilocalization effect that is evidence for strong spin-orbit coupling.

We investigate the size- and composition-dependent ac magnetic permeability of superparamagnetic iron oxide nanocrystals for radio frequency (RF) applications. The nanocrystals are obtained through high-temperature decomposition synthesis, and their stoichiometry is determined by Mossbauer spectroscopy. Two sets of oxides are studied: (a) as-synthesized magnetite-rich and (b) aged maghemite nanocrystals. All nanocrystalline samples are confirmed to be in the superparamagnetic state at room temperature by SQUID magnetometry. Through the one-turn inductor method, the ac magnetic properties of the nanocrystalline oxides are characterized. In magnetite-rich iron oxide nanocrystals, size-dependent magnetic permeability is not observed, while maghemite iron oxide nanocrystals show clear size dependence. The inductance, resistance, and quality factor of hand-wound inductors with a superparamagnetic composite core are measured. The superparamagnetic nanocrystals are successfully embedded into hand-wound inductors to function as inductor cores.

We present scanning photoluminescence (PL) microscopy of freely suspended single-walled carbon nanotubes grown by chemically assisted vapor deposition (CVD) across micron-sized open apertures. Scans of the PL emission versus excitation position show unusual "holes"having subwavelength spatial features associated with abrupt blue shifts of the emission energy. By varying the excitation polarization, energy, intensity, and position, we demonstrate that optical switching in some nanotubes is controllable in a highly nonlinear manner by adjusting the nonequilibrium carrier density in the nanotube. Technologically important attributes include large spectral contrast between on/off states at room temperature, a dramatic response to small changes in light intensity near threshold, and the possibility that electrical charge injection could also be used to control emission energies.