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1.
Nat Commun ; 11(1): 3141, 2020 Jun 19.
Artigo em Inglês | MEDLINE | ID: mdl-32561835

RESUMO

Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

2.
ACS Nano ; 2020 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-32459460

RESUMO

Förster resonant energy transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as an optoelectronic design element to transport energy. However, so far, nanocrystal (NC) systems supported only diffusion lengths of 30 nm, which are too small to be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (CsPbBr3 PNCs). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs' high PL quantum yield, large absorption cross section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites' optical absorption depth, thus enabling the design of device architectures with improved performances and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices.

3.
Nat Mater ; 18(11): 1172-1176, 2019 11.
Artigo em Inglês | MEDLINE | ID: mdl-31548631

RESUMO

Miniaturized lasers are an emerging platform for generating coherent light for quantum photonics, in vivo cellular imaging, solid-state lighting and fast three-dimensional sensing in smartphones1-3. Continuous-wave lasing at room temperature is critical for integration with opto-electronic devices and optimal modulation of optical interactions4,5. Plasmonic nanocavities integrated with gain can generate coherent light at subwavelength scales6-9, beyond the diffraction limit that constrains mode volumes in dielectric cavities such as semiconducting nanowires10,11. However, insufficient gain with respect to losses and thermal instabilities in nanocavities has limited all nanoscale lasers to pulsed pump sources and/or low-temperature operation6-9,12-15. Here, we show continuous-wave upconverting lasing at room temperature with record-low thresholds and high photostability from subwavelength plasmons. We achieve selective, single-mode lasing from Yb3+/Er3+-co-doped upconverting nanoparticles conformally coated on Ag nanopillar arrays that support a single, sharp lattice plasmon cavity mode and greater than wavelength λ/20 field confinement in the vertical dimension. The intense electromagnetic near-fields localized in the vicinity of the nanopillars result in a threshold of 70 W cm-2, orders of magnitude lower than other small lasers. Our plasmon-nanoarray upconverting lasers provide directional, ultra-stable output at visible frequencies under near-infrared pumping, even after six hours of constant operation, which offers prospects in previously unrealizable applications of coherent nanoscale light.


Assuntos
Lasers , Nanotecnologia , Fenômenos Ópticos , Fótons , Temperatura
4.
Nat Nanotechnol ; 14(5): 420-425, 2019 05.
Artigo em Inglês | MEDLINE | ID: mdl-30833691

RESUMO

Electron microscopy has been instrumental in our understanding of complex biological systems. Although electron microscopy reveals cellular morphology with nanoscale resolution, it does not provide information on the location of different types of proteins. An electron-microscopy-based bioimaging technology capable of localizing individual proteins and resolving protein-protein interactions with respect to cellular ultrastructure would provide important insights into the molecular biology of a cell. Here, we synthesize small lanthanide-doped nanoparticles and measure the absolute photon emission rate of individual nanoparticles resulting from a given electron excitation flux (cathodoluminescence). Our results suggest that the optimization of nanoparticle composition, synthesis protocols and electron imaging conditions can lead to sub-20-nm nanolabels that would enable high signal-to-noise localization of individual biomolecules within a cellular context. In ensemble measurements, these labels exhibit narrow spectra of nine distinct colours, so the imaging of biomolecules in a multicolour electron microscopy modality may be possible.


Assuntos
Corantes Fluorescentes/química , Microscopia Eletrônica de Transmissão , Nanopartículas/química
5.
Nat Commun ; 9(1): 3082, 2018 08 06.
Artigo em Inglês | MEDLINE | ID: mdl-30082844

RESUMO

Multiphoton imaging techniques that convert low-energy excitation to higher energy emission are widely used to improve signal over background, reduce scatter, and limit photodamage. Lanthanide-doped upconverting nanoparticles (UCNPs) are among the most efficient multiphoton probes, but even UCNPs with optimized lanthanide dopant levels require laser intensities that may be problematic. Here, we develop protein-sized, alloyed UCNPs (aUCNPs) that can be imaged individually at laser intensities >300-fold lower than needed for comparably sized doped UCNPs. Using single UCNP characterization and kinetic modeling, we find that addition of inert shells changes optimal lanthanide content from Yb3+, Er3+-doped NaYF4 nanocrystals to fully alloyed compositions. At high levels, emitter Er3+ ions can adopt a second role to enhance aUCNP absorption cross-section by desaturating sensitizer Yb3+ or by absorbing photons directly. Core/shell aUCNPs 12 nm in total diameter can be imaged through deep tissue in live mice using a laser intensity of 0.1 W cm-2.

6.
Nat Nanotechnol ; 13(7): 572-577, 2018 07.
Artigo em Inglês | MEDLINE | ID: mdl-29915271

RESUMO

Reducing the size of lasers to microscale dimensions enables new technologies1 that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors2 to live brain tissue3. However, reduced cavity sizes increase optical losses and require greater input powers to reach lasing thresholds. Multiphoton-pumped lasers4-7 that have been miniaturized using nanomaterials such as lanthanide-doped upconverting nanoparticles (UCNPs)8 as lasing media require high pump intensities to achieve ultraviolet and visible emission and therefore operate under pulsed excitation schemes. Here, we make use of the recently described energy-looping excitation mechanism in Tm3+-doped UCNPs9 to achieve continuous-wave upconverted lasing action in stand-alone microcavities at excitation fluences as low as 14 kW cm-2. Continuous-wave lasing is uninterrupted, maximizing signal and enabling modulation of optical interactions10. By coupling energy-looping nanoparticles to whispering-gallery modes of polystyrene microspheres, we induce stable lasing for more than 5 h at blue and near-infrared wavelengths simultaneously. These microcavities are excited in the biologically transmissive second near-infrared (NIR-II) window and are small enough to be embedded in organisms, tissues or devices. The ability to produce continuous-wave lasing in microcavities immersed in blood serum highlights practical applications of these microscale lasers for sensing and illumination in complex biological environments.


Assuntos
Lasers , Nanopartículas/química , Nanotecnologia/instrumentação , Túlio/química , Animais , Bovinos , Desenho de Equipamento , Luz , Substâncias Luminescentes/química , Microesferas , Poliestirenos/química , Soro/química
7.
Energy Environ Sci ; 11(10): 2846-2852, 2018 Oct 01.
Artigo em Inglês | MEDLINE | ID: mdl-30713582

RESUMO

Perovskite solar cells and light-emission devices are yet to achieve their full potential owing in part to microscale inhomogeneities and defects that act as non-radiative loss pathways. These sites have been revealed using local photoluminescence mapping techniques but the short absorption depth of photons with energies above the bandgap means that conventional one-photon excitation primarily probes the surface recombination. Here, we use two-photon time-resolved confocal photoluminescence microscopy to explore the surface and bulk recombination properties of methylammonium lead halide perovskite structures. By acquiring 2D maps at different depths, we form 3D photoluminescence tomography images to visualise the charge carrier recombination kinetics. The technique unveils buried recombination pathways in both thin film and micro-crystal structures that aren't captured in conventional one-photon mapping experiments. Specifically, we reveal that light-induced passivation approaches are primarily surface-sensitive and that nominal single crystals still contain heterogeneous defects that impact charge-carrier recombination. Our work opens a new route to sensitively probe defects and associated non-radiative processes in perovskites, highlighting additional loss pathways in these materials that will need to be addressed through improved sample processing or passivation treatments.

8.
Sci Rep ; 7(1): 16938, 2017 12 05.
Artigo em Inglês | MEDLINE | ID: mdl-29209000

RESUMO

Evaluating and tuning the properties of two-dimensional (2D) materials is a major focus of advancing 2D science and technology. While many claim that the photonic properties of a 2D layer provide evidence that the material is "high quality", this may not be true for electronic performance. In this work, we deconvolute the photonic and electronic response of synthetic monolayer molybdenum disulfide. We demonstrate that enhanced photoluminescence can be robustly engineered via the proper choice of substrate, where growth of MoS2 on r-plane sapphire can yield >100x enhancement in PL and carrier lifetime due to increased molybdenum-oxygen bonding compared to that of traditionally grown MoS2 on c-plane sapphire. These dramatic enhancements in optical properties are similar to those of super-acid treated MoS2, and suggest that the electronic properties of the MoS2 are also superior. However, a direct comparison of the charge transport properties indicates that the enhanced PL due to increased Mo-O bonding leads to p-type compensation doping, and is accompanied by a 2x degradation in transport properties compared to MoS2 grown on c-plane sapphire. This work provides a foundation for understanding the link between photonic and electronic performance of 2D semiconducting layers, and demonstrates that they are not always correlated.

9.
Phys Rev Lett ; 119(8): 087401, 2017 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-28952768

RESUMO

Optoelectronic excitations in monolayer MoS_{2} manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena-critical to both many-body physics exploration and device applications-presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. Here, optical detection of bound- and free-carrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies. The results explicitly disentangle the competing effects and highlight longstanding theoretical predictions of large carrier-induced band gap and exciton renormalization in two-dimensional semiconductors.

10.
ACS Nano ; 11(2): 2115-2123, 2017 02 28.
Artigo em Inglês | MEDLINE | ID: mdl-28117983

RESUMO

Broadband optoelectronics such as artificial light harvesting technologies necessitate efficient and, ideally, tunable coupling of excited states over a wide range of energies. In monolayer MoS2, a prototypical two-dimensional layered semiconductor, the excited state manifold spans the visible electromagnetic spectrum and is comprised of an interconnected network of excitonic and free-carrier excitations. Here, photoluminescence excitation spectroscopy is used to reveal the energetic and spatial dependence of broadband excited state coupling to the ground-state luminescent excitons of monolayer MoS2. Photoexcitation of the direct band gap excitons is found to strengthen with increasing energy, demonstrating that interexcitonic coupling across the Brillouin zone is more efficient than previously reported, and thus bolstering the import and appeal of these materials for broadband optoelectronic applications. Narrow excitation resonances that are superimposed on the broadband photoexcitation spectrum are identified and coincide with the energetic positions of the higher-energy excitons and the electronic band gap as predicted by first-principles calculations. Identification of such features outlines a facile route to measure the optical and electronic band gaps and thus the exciton binding energy in the more sophisticated device architectures that are necessary for untangling the rich many-body phenomena and complex photophysics of these layered semiconductors. In as-grown materials, the excited states exhibit microscopic spatial variations that are characteristic of local carrier density fluctuations, similar to charge puddling phenomena in graphene. Such variations likely arise from substrate inhomogeneity and demonstrate the possibility to use substrate patterning to tune local carrier density and dynamically control excited states for designer optoelectronics.

11.
Adv Mater ; 29(3)2017 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-27862364

RESUMO

Using two-photon tomography, carrier lifetimes are mapped in polycrystalline CdTe photovoltaic devices. These 3D maps probe subsurface carrier dynamics that are inaccessible with traditional optical techniques. They reveal that CdCl2 treatment of CdTe solar cells suppresses nonradiative recombination and enhances carrier lifetimes throughout the film with substantial improvements particularly near subsurface grain boundaries and the critical buried p-n junction.

12.
J Am Chem Soc ; 138(41): 13551-13560, 2016 Oct 19.
Artigo em Inglês | MEDLINE | ID: mdl-27700081

RESUMO

Recently developed all-organic emitters used in display applications achieve high brightness by harvesting triplet populations via thermally activated delayed fluorescence. The photophysical properties of these emitters therefore involve new inherent complexities and are strongly affected by interactions with their host material in the solid state. Ensemble measurements occlude the molecular details of how host-guest interactions determine fundamental properties such as the essential balance of singlet oscillator strength and triplet harvesting. Therefore, using time-resolved fluorescence spectroscopy, we interrogate these emitters at the single-molecule level and compare their properties in two distinct glassy polymer hosts. We find that nonbonding interactions with aromatic moieties in the host appear to mediate the molecular configurations of the emitters, but also promote nonradiative quenching pathways. We also find substantial heterogeneity in the time-resolved photoluminescence of these emitters, which is dominated by static disorder in the polymer. Finally, since singlet-triplet cycling underpins the mechanism for increased brightness, we present the first room-temperature measurement of singlet-triplet equilibration dynamics in this family of emitters. Our observations present a molecular-scale interrogation of host-guest interactions in a disordered film, with implications for highly efficient organic light-emitting devices. Combining a single-molecule experimental technique with an emitter that is sensitive to triplet dynamics, yet read out via fluorescence, should also provide a complementary approach to performing fundamental studies of glassy materials over a large dynamic range of time scales.

13.
Nat Nanotechnol ; 9(4): 300-5, 2014 Apr.
Artigo em Inglês | MEDLINE | ID: mdl-24633523

RESUMO

Imaging at the single-molecule level reveals heterogeneities that are lost in ensemble imaging experiments, but an ongoing challenge is the development of luminescent probes with the photostability, brightness and continuous emission necessary for single-molecule microscopy. Lanthanide-doped upconverting nanoparticles overcome problems of photostability and continuous emission and their upconverted emission can be excited with near-infrared light at powers orders of magnitude lower than those required for conventional multiphoton probes. However, the brightness of upconverting nanoparticles has been limited by open questions about energy transfer and relaxation within individual nanocrystals and unavoidable tradeoffs between brightness and size. Here, we develop upconverting nanoparticles under 10 nm in diameter that are over an order of magnitude brighter under single-particle imaging conditions than existing compositions, allowing us to visualize single upconverting nanoparticles as small (d = 4.8 nm) as fluorescent proteins. We use advanced single-particle characterization and theoretical modelling to find that surface effects become critical at diameters under 20 nm and that the fluences used in single-molecule imaging change the dominant determinants of nanocrystal brightness. These results demonstrate that factors known to increase brightness in bulk experiments lose importance at higher excitation powers and that, paradoxically, the brightest probes under single-molecule excitation are barely luminescent at the ensemble level.


Assuntos
Proteínas Luminescentes/química , Nanopartículas/química , Imagem Óptica/métodos
14.
Nat Commun ; 4: 2095, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-23817445

RESUMO

Non-periodic arrangements of nanoscale light scatterers allow for the realization of extremely effective broadband light-trapping layers for solar cells. However, their optimization is challenging given the massive number of degrees of freedom. Brute-force, full-field electromagnetic simulations are computationally too time intensive to identify high-performance solutions in a vast design space. Here we illustrate how a semi-analytical model can be used to quickly identify promising non-periodic spatial arrangements of nanoscale scatterers. This model only requires basic knowledge of the scattering behaviour of a chosen nanostructure and the waveguiding properties of the semiconductor layer in a cell. Due to its simplicity, it provides new intuition into the ideal amount of disorder in high-performance light-trapping layers. Using simulations and experiments, we demonstrate that arrays of nanometallic stripes featuring a limited amount of disorder, for example, following a quasi-periodic or Fibonacci sequence, can substantially enhance solar absorption over perfectly periodic and random arrays.

15.
Sci Rep ; 3: 2098, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-23807197

RESUMO

Accurately measuring the bulk minority carrier lifetime is one of the greatest challenges in evaluating photoactive materials used in photovoltaic cells. One-photon time-resolved photoluminescence decay measurements are commonly used to measure lifetimes of direct bandgap materials. However, because the incident photons have energies higher than the bandgap of the semiconductor, most carriers are generated close to the surface, where surface defects cause inaccurate lifetime measurements. Here we show that two-photon absorption permits sub-surface optical excitation, which allows us to decouple surface and bulk recombination processes even in unpassivated samples. Thus with two-photon microscopy we probe the bulk minority carrier lifetime of photovoltaic semiconductors. We demonstrate how the traditional one-photon technique can underestimate the bulk lifetime in a CdTe crystal by 10× and show that two-photon excitation more accurately measures the bulk lifetime. Finally, we generate multi-dimensional spatial maps of optoelectronic properties in the bulk of these materials using two-photon excitation.

16.
Nano Lett ; 11(10): 4265-9, 2011 Oct 12.
Artigo em Inglês | MEDLINE | ID: mdl-21879729

RESUMO

We perform spectrally resolved cathodoluminescence (CL) imaging nanoscopy using a 30 keV electron beam to identify the resonant modes of an ultrathin (20 nm), laterally tapered plasmonic Ag nanostrip antenna. We resolve with deep-subwavelength resolution four antenna resonances (resonance orders m = 2-5) that are ascribed to surface plasmon polariton standing waves that are confined on the strip. We map the local density of states on the strip surface and show that it has contributions from symmetric and antisymmetric surface plasmon polariton modes, each with a very different mode index. This work illustrates the power of CL experiments that can visualize hidden modes that for symmetry reasons have been elusive in optical light scattering experiments.

17.
Nat Nanotechnol ; 6(9): 588-93, 2011 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-21857687

RESUMO

An increasing number of photonics applications make use of nanoscale optical antennas that exhibit a strong, resonant interaction with photons of a specific frequency. The resonant properties of such antennas are conventionally characterized by far-field light-scattering techniques. However, many applications require quantitative knowledge of the near-field behaviour, and existing local field measurement techniques provide only relative, rather than absolute, data. Here, we demonstrate a photodetector platform that uses a silicon-on-insulator substrate to spectrally and spatially map the absolute values of enhanced fields near any type of optical antenna by transducing local electric fields into photocurrent. We are able to quantify the resonant optical and materials properties of nanoscale (∼50 nm) and wavelength-scale (∼1 µm) metallic antennas as well as high-refractive-index semiconductor antennas. The data agree well with light-scattering measurements, full-field simulations and intuitive resonator models.

18.
ACS Nano ; 5(8): 6449-57, 2011 Aug 23.
Artigo em Inglês | MEDLINE | ID: mdl-21732686

RESUMO

We demonstrate top-down synthesis of monodisperse plasmonic nanoparticles designed to contain internal Raman hot spots. Our Raman-active nanoparticles are fabricated using nanoimprint lithography and thin-film deposition and are composed of novel internal structures with sublithographic dimensions: a disk-shaped Ag core, a Petri-dish-shaped SiO(2) base whose inner surface is coated with Ag film, and a sub-10 nm scale circular gap between the core and the base. Confocal Raman measurements and electromagnetic simulations show that Raman hot spots appear at the inside perimeter of individual nanoparticles and serve as the source of a 1000-fold improvement of minimum molecular detection level that enables detection of signals from a few molecules near hot spots. A multimodality version of these nanoparticles, which includes the functionality offered by magnetic multilayers, is also demonstrated. These results illustrate the potential of direct fabrication for creating exotic monodisperse nanoparticles, which combine engineered internal nanostructures and multilayer composite materials, for use in nanoparticle-based molecular imaging and detection.


Assuntos
Nanopartículas/química , Nanotecnologia/métodos , Fenômenos Magnéticos , Rodaminas/química , Dióxido de Silício/química
19.
Phys Rev Lett ; 104(24): 243902, 2010 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-20867303

RESUMO

We demonstrate the existence of electromagnetically-induced-transparency (EIT-)like spectral response in a system of nanoscale plasmonic resonator antennas coupled by means of a single-mode silicon waveguide. Our proposed scheme exploits the phase of the coupling between the antennas in contrast with the existing plasmonic approaches that rely on the strength of direct, near-field coupling of nanometallic elements. Quality factors of over 100 and group indices of over 10 are readily achieved at near-infrared frequencies by a single unit in ≈1 µm2 of total device footprint, representing a more than two orders size reduction over corresponding dielectric EIT structures. By obviating the need for a near-field interaction, the phase-coupling scheme also facilitates an improved access to the coupling medium between the resonators thereby paving the way toward dynamic control of their sharp EIT-like spectral response.

20.
Nano Lett ; 10(7): 2649-54, 2010 Jul 14.
Artigo em Inglês | MEDLINE | ID: mdl-20507083

RESUMO

Empowering silicon (Si) with optical functions constitutes a very important challenge in photonics. The scalable fabrication capabilities for this earth-abundant, environmentally friendly material are unmatched in sophistication and can be unleashed to realize a plethora of high-performance photonic functionalities that find application in information, bio-, display, camouflage, ornamental, and energy technologies. Nanofashioning represents a general strategy to turn Si into a useful optical material and Si structures have already been engineered to enable light emission, optical cloaking, waveguiding, nonlinear optics, enhanced light absorption, and sensing. Here, we demonstrate that a wide spectrum of colors can be generated by harnessing the strong resonant light scattering properties of Si nanostructures under white light illumination. The ability to engineer such colors in a predetermined fashion through a choice of the structure size, dielectric environment, and illumination conditions opens up entirely new applications of Si and puts this material in a new light.

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