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1.
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.

2.
Phys Rev Lett ; 123(7): 076801, 2019 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-31491121

RESUMO

Structural defects in 2D materials offer an effective way to engineer new material functionalities beyond conventional doping. We report on the direct experimental correlation of the atomic and electronic structure of a sulfur vacancy in monolayer WS_{2} by a combination of CO-tip noncontact atomic force microscopy and scanning tunneling microscopy. Sulfur vacancies, which are absent in as-grown samples, were deliberately created by annealing in vacuum. Two energetically narrow unoccupied defect states followed by vibronic sidebands provide a unique fingerprint of this defect. Direct imaging of the defect orbitals, together with ab initio GW calculations, reveal that the large splitting of 252±4 meV between these defect states is induced by spin-orbit coupling.

3.
ACS Nano ; 13(9): 10520-10534, 2019 Sep 24.
Artigo em Inglês | MEDLINE | ID: mdl-31393700

RESUMO

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step toward impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS2) grown by chemical vapor deposition using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (CrW) and molybdenum (MoW) at a tungsten site, oxygen at sulfur sites in both top and bottom layers (OS top/bottom), and two negatively charged defects (CD type I and CD type II). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. CrW forms three deep unoccupied defect states, two of which arise from spin-orbit splitting. The formation of such localized trap states for CrW differs from the MoW case and can be explained by their different d shell energetics and local strain, which we directly measured. Utilizing a tight-binding model the electronic spectra of the isolectronic substitutions OS and CrW are mimicked in the limit of a zero hopping term and infinite on-site energy at a S and W site, respectively. The abundant CDs are negatively charged, which leads to a significant band bending around the defect and a local increase of the contact potential difference. In addition, CD-rich domains larger than 100 nm are observed, causing a work function increase of 1.1 V. While most defects are electronically isolated, we also observed hybrid states formed between CrW dimers. The important role of charge localization, spin-orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS2 as reported here will guide future efforts of targeted defect engineering and doping of TMDs.

4.
Sci Adv ; 5(5): eaav8141, 2019 May.
Artigo em Inglês | MEDLINE | ID: mdl-31172026

RESUMO

One-dimensional (1D) nanomaterials with highly anisotropic optoelectronic properties are key components in energy harvesting, flexible electronics, and biomedical imaging devices. 3D patterning methods that precisely assemble nanowires with locally controlled composition and orientation would enable new optoelectronic device designs. As an exemplar, we have created and 3D-printed nanocomposite inks composed of brightly emitting colloidal cesium lead halide perovskite (CsPbX3, X = Cl, Br, and I) nanowires suspended in a polystyrene-polyisoprene-polystyrene block copolymer matrix. The nanowire alignment is defined by the programmed print path, resulting in optical nanocomposites that exhibit highly polarized absorption and emission properties. Several devices have been produced to highlight the versatility of this method, including optical storage, encryption, sensing, and full-color displays.

5.
Science ; 363(6432): 1199-1202, 2019 03 15.
Artigo em Inglês | MEDLINE | ID: mdl-30872520

RESUMO

A variety of optical applications rely on the absorption and reemission of light. The quantum yield of this process often plays an essential role. When the quantum yield deviates from unity by significantly less than 1%, applications such as luminescent concentrators and optical refrigerators become possible. To evaluate such high performance, we develop a measurement technique for luminescence efficiency with sufficient accuracy below one part per thousand. Photothermal threshold quantum yield is based on the quantization of light to minimize overall measurement uncertainty. This technique is used to guide a procedure capable of making ensembles of near-unity emitting cadmium selenide/cadmium sulfide (CdSe/CdS) core-shell quantum dots. We obtain a photothermal threshold quantum yield luminescence efficiency of 99.6 ± 0.2%, indicating nearly complete suppression of nonradiative decay channels.

6.
Sci Rep ; 9(1): 2768, 2019 Feb 26.
Artigo em Inglês | MEDLINE | ID: mdl-30808883

RESUMO

Materials for nanophotonic devices ideally combine ease of deposition, very high refractive index, and facile pattern formation through lithographic templating and/or etching. In this work, we present a scalable method for producing high refractive index WS2 layers by chemical conversion of WO3 synthesized via atomic layer deposition (ALD). These conformal nanocrystalline thin films demonstrate a surprisingly high index of refraction (n > 3.9), and structural fidelity compatible with lithographically defined features down to ~10 nm. Although this process yields highly polycrystalline films, the optical constants are in agreement with those reported for single crystal bulk WS2. Subsequently, we demonstrate three photonic structures - first, a two-dimensional hole array made possible by patterning and etching an ALD WO3 thin film before conversion, second, an analogue of the 2D hole array first patterned into fused silica before conformal coating and conversion, and third, a three-dimensional inverse opal photonic crystal made by conformal coating of a self-assembled polystyrene bead template. These results can be trivially extended to other transition metal dichalcogenides, thus opening new opportunities for photonic devices based on high refractive index materials.

7.
ACS Nano ; 13(2): 1284-1291, 2019 Feb 26.
Artigo em Inglês | MEDLINE | ID: mdl-30645100

RESUMO

Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. Here, we utilize correlative, nanoscale photoemission spectroscopy to visualize how local introduction of defects modifies electronic and excitonic properties of two-dimensional materials at the nanoscale. As a model system, we study chemical vapor deposition grown monolayer WS2, a prototypical, direct gap, two-dimensional semiconductor. By cross-correlating nanoscale angle-resolved photoemission spectroscopy, core level spectroscopy, and photoluminescence, we unravel how local variations in defect density influence electronic structure, lateral band alignment, and excitonic phenomena in synthetic WS2 monolayers.

8.
ACS Appl Mater Interfaces ; 10(41): 35129-35136, 2018 Oct 17.
Artigo em Inglês | MEDLINE | ID: mdl-30230810

RESUMO

Chemical transformations that occur on photoactive materials, such as photoelectrochemical water splitting, are strongly influenced by the surface properties as well as by the surrounding environment. Herein, we elucidate the effects of oxygen and water surface adsorption on band alignment, interfacial charge transfer, and charge-carrier transport by using complementary Kelvin probe measurements and photoconductive atomic force microscopy on bismuth vanadate. By observing variations in surface potential, we show that adsorbed oxygen acts as an electron-trap state at the surface of bismuth vanadate, whereas adsorbed water results in formation of a dipole layer without inducing interfacial charge transfer. The apparent change of trap state density under dry or humid nitrogen, as well as under oxygen-rich atmosphere, proves that surface adsorbates influence charge-carrier transport properties in the material. The finding that oxygen introduces electronically active states on the surface of bismuth vanadate may have important implications for understanding functional characteristics of water splitting photoanodes, devising strategies to passivate interfacial trap states, and elucidating important couplings between energetics and charge transport in reaction environments.

9.
Beilstein J Nanotechnol ; 9: 1381-1389, 2018.
Artigo em Inglês | MEDLINE | ID: mdl-29977672

RESUMO

Background: Encased cantilevers are novel force sensors that overcome major limitations of liquid scanning probe microscopy. By trapping air inside an encasement around the cantilever, they provide low damping and maintain high resonance frequencies for exquisitely low tip-sample interaction forces even when immersed in a viscous fluid. Quantitative measurements of stiffness, energy dissipation and tip-sample interactions using dynamic force sensors remain challenging due to spurious resonances of the system. Results: We demonstrate for the first time electrostatic actuation with a built-in electrode. Solely actuating the cantilever results in a frequency response free of spurious peaks. We analyze static, harmonic, and sub-harmonic actuation modes. Sub-harmonic mode results in stable amplitudes unaffected by potential offsets or fluctuations of the electrical surface potential. We present a simple plate capacitor model to describe the electrostatic actuation. The predicted deflection and amplitudes match experimental results within a few percent. Consequently, target amplitudes can be set by the drive voltage without requiring calibration of optical lever sensitivity. Furthermore, the excitation bandwidth outperforms most other excitation methods. Conclusion: Compatible with any instrument using optical beam deflection detection electrostatic actuation in encased cantilevers combines ultra-low force noise with clean and stable excitation well-suited for quantitative measurements in liquid, compatible with air, or vacuum environments.

10.
Nat Commun ; 9(1): 2597, 2018 07 16.
Artigo em Inglês | MEDLINE | ID: mdl-30013111

RESUMO

The performance of energy materials hinges on the presence of structural defects and heterogeneity over different length scales. Here we map the correlation between morphological and functional heterogeneity in bismuth vanadate, a promising metal oxide photoanode for photoelectrochemical water splitting, by photoconductive atomic force microscopy. We demonstrate that contrast in mapping electrical conductance depends on charge transport limitations, and on the contact at the sample/probe interface. Using temperature and illumination intensity-dependent current-voltage spectroscopy, we find that the transport mechanism in bismuth vanadate can be attributed to space charge-limited current in the presence of trap states. We observe no additional recombination sites at grain boundaries, which indicates high defect tolerance in bismuth vanadate. These findings support the fabrication of highly efficient bismuth vanadate nanostructures and provide insights into how local functionality affects the macroscopic performance.


Assuntos
Bismuto/química , Nanoestruturas/química , Energia Solar , Vanadatos/química , Água/química , Condutividade Elétrica , Eletrodos , Microscopia de Força Atômica , Processos Fotoquímicos , Semicondutores , Análise Espectral/métodos
11.
ACS Nano ; 11(6): 5836-5843, 2017 06 27.
Artigo em Inglês | MEDLINE | ID: mdl-28599108

RESUMO

Gap plasmonic nanostructures are of great interest due to their ability to concentrate light into small volumes. Theoretical studies, considering quantum mechanical effects, have predicted the optimal spatial gap between adjacent nanoparticles to be in the subnanometer regime in order to achieve the strongest possible field enhancement. Here, we present a technology to fabricate gap plasmonic structures with subnanometer resolution, high reliability, and high throughput using collapsible nanofingers. This approach enables us to systematically investigate the effects of gap size and tunneling barrier height. The experimental results are consistent with previous findings as well as with a straightforward theoretical model that is presented here.

12.
Phys Rev Lett ; 118(8): 087401, 2017 Feb 24.
Artigo em Inglês | MEDLINE | ID: mdl-28282210

RESUMO

Ultrafast pump-probe measurements of plasmonic nanostructures probe the nonequilibrium behavior of excited carriers, which involves several competing effects obscured in typical empirical analyses. Here we present pump-probe measurements of plasmonic nanoparticles along with a complete theoretical description based on first-principles calculations of carrier dynamics and optical response, free of any fitting parameters. We account for detailed electronic-structure effects in the density of states, excited carrier distributions, electron-phonon coupling, and dielectric functions that allow us to avoid effective electron temperature approximations. Using this calculation method, we obtain excellent quantitative agreement with spectral and temporal features in transient-absorption measurements. In both our experiments and calculations, we identify the two major contributions of the initial response with distinct signatures: short-lived highly nonthermal excited carriers and longer-lived thermalizing carriers.

13.
Chem Commun (Camb) ; 52(26): 4753-6, 2016 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-26864502

RESUMO

Peptoid nanosheets can be broadly functionalized for a variety of applications. However, they are susceptible to degradation when exposed to chemical or mechanical stress. To improve their strength, photolabile monomers were introduced in order to crosslink the nanosheet interior. Photo-crosslinking produced a more robust material that can survive sonication, lyophilization, and other biochemical manipulations.


Assuntos
Nanoestruturas/química , Peptoides/química , Reagentes para Ligações Cruzadas/química , Eletroforese em Gel de Poliacrilamida , Interações Hidrofóbicas e Hidrofílicas , Microscopia de Fluorescência , Processos Fotoquímicos , Análise Espectral Raman , Estresse Mecânico , Difração de Raios X
14.
J Phys Chem Lett ; 6(15): 2919-23, 2015 Aug 06.
Artigo em Inglês | MEDLINE | ID: mdl-26267181

RESUMO

Exciton mobility is crucial to organic photovoltaic (OPV) efficiency, but accurate, quantitative measures and therefore precise understanding of this process are currently lacking. Here, we exploit the unique capabilities of femtosecond stimulated Raman spectroscopy (FSRS) to disentangle the signatures of the bulk and interfacial donor response in a bulk heterojunction composed of poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and phenyl-C61-butyric acid methyl ester (PCBM). Surprisingly, we find that donor excitons are very mobile for the first ∼300 fs following excitation (before thermalization) even though their overall lifetime is significantly longer (170 ps). A sharp decrease in mobility occurs after the system relaxes out of the Franck-Condon (FC) region. From this observation we predict that any polymer lacking a significant resonance Raman effect and fluorescence Stokes shift, indicating slow FC relaxation and small reorganization energy, will make an efficient OPV material.

15.
Adv Mater ; 27(38): 5805-12, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26032973

RESUMO

Gallium-nitride-based light-emitting diodes have enabled the commercialization of efficient solid-state lighting devices. Nonplanar nanomaterial architectures, such as nanowires and nanowire-based heterostructures, have the potential to significantly improve the performance of light-emitting devices through defect reduction, strain relaxation, and increased junction area. In addition, relaxation of internal strain caused by indium incorporation will facilitate pushing the emission wavelength into the red. This could eliminate inefficient phosphor conversion and enable color-tunable emission or white-light emission by combining blue, green, and red sources. Utilizing the waveguiding modes of the individual nanowires will further enhance light emission, and the properties of photonic structures formed by nanowire arrays can be implemented to improve light extraction. Recent advances in synthetic methods leading to better control over GaN and InGaN nanowire synthesis are described along with new concept devices leading to efficient white-light emission.

16.
Adv Mater ; 27(38): 5778-84, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26016695

RESUMO

Complex materials are defined as nanostructured materials with combinations of structure and/or composition that lead to performance surpassing the sum of their individual components. There are many methods that can create complex materials; however, atomic layer deposition (ALD) is uniquely suited to control composition and structural parameters at the atomic level. The use of ALD for creating complex insulators, semiconductors, and conductors is discussed, along with its use in novel structural applications.

17.
J Phys Chem A ; 118(37): 8393-401, 2014 Sep 18.
Artigo em Inglês | MEDLINE | ID: mdl-24592863

RESUMO

The Nonlinear Optical Localization using Electromagnetic Surface fields (NOLES) imaging technique was used to generate optical images in which the position of a chiral object could be determined with nanometer precision. Asymmetric gold bowtie nanostructures were used as a model system with 2D chirality. The bowties functioned as a chiral nonlinear medium that converted the fundamental of a Ti:sapphire laser to its second harmonic frequency. The bowties consisted of two lithographically prepared equilateral triangles (base = 75 nm, height = 85 nm, thickness = 25 nm) separated by a 20 nm gap. Asymmetric bowties were formed by lateral displacement of one triangle by 10 nm, yielding C2 point group symmetry. The chirality of the bowtie nanostructures was confirmed via nonzero second-harmonic generation circular dichroism (SHG-CDR) ratios, which came from single-particle SHG measurements. The SHG-CDR ratios were validated using numerical finite difference time domain simulations that quantified the relative magnitudes of gap-localized electromagnetic fields at the harmonic frequency resulting from excitation by left and right circularly (LCP and RCP) and linearly polarized fundamental waves. The relative electric dipolar and magnetic dipolar contributions to the SHG responses were determined using single-particle continuous polarization variation (CPV) SHG measurements. The spatial localization precision obtainable for individual chiral nanostructures was determined by statistical analysis of the SHG image point spread function. Our results demonstrated that both the chiral image contrast, which resulted from LCP and RCP excitation, and the corresponding localization precision was dependent upon the relative magnetic dipole/electric dipole ratio (G/F). A localization precision of 1.13 ± 0.13 nm and left-to-right image enhancements of 400% were obtained for bowties with the highest G/F ratios using 5 s frame exposure times. The polarization dependence and magnetic dipole amplification confirmed here demonstrate that the NOLES imaging technique is a powerful method for studying chiral specimens with high spatial precision.

18.
Rev Sci Instrum ; 84(10): 104906, 2013 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-24182150

RESUMO

A new technique of high-resolution micro-Raman thermometry using anatase TiO2 microparticles (0.5-3 µm) is presented. These very high spatial resolution measurements (280 nm) reveal temperature gradients even within individual microparticles. Potential applications of this technique are demonstrated by probing the temperature distribution of a micro-fabricated heater consisting of a thin silicon nitride (Si-N) membrane with a gold coil on top of the membrane. Using TiO2 microparticle micro-Raman thermometry, the temperature from the outer edge of the coil to the inner portion was measured to increase by ~40 °C. These high spatial resolution microscopic measurements were also used to measure the temperature gradient within the 20 µm wide Si-N between the gold heating coils. 2D numerical simulations of the micro heater temperature distribution are in excellent agreement with the experimental measurements of the temperatures. These measurements illustrate the potential to extend applications of micro-Raman thermometry to obtain temperature details on a sub-micrometer spatial resolution by employing microparticles.

19.
Nano Lett ; 11(8): 3258-62, 2011 Aug 10.
Artigo em Inglês | MEDLINE | ID: mdl-21714493

RESUMO

Hollow gold nanospheres (HGNs) were excited with ultrashort laser pulses, and the coherent vibrational response was examined using femtosecond time-resolved transient absorption. The results indicated that HGNs support an isotropic mode, resulting in periodic modulation of the surface plasmon differential absorption. Two different categories of coherent acoustic vibrations, which depend on particle dimensions, were observed for HGNs. Further, the vibration launching mechanism was dependent upon the dimensions of the HGN. Coherent vibrations in HGNs characterized by small outer radii (<10 nm) and low cavity-radius-to-outer-shell radius aspect ratios (<0.5) were excited by a direct mechanism, whereas the vibrations observed for the larger particles (>25 nm OR) with higher aspect ratios (>0.5) resulted from an indirect mechanism. These findings may be significant for developing a predictive understanding of nanostructure optical and mechanical properties.


Assuntos
Acústica , Ouro , Nanopartículas Metálicas , Ressonância de Plasmônio de Superfície , Vibração
20.
Opt Express ; 19(9): 8903-11, 2011 Apr 25.
Artigo em Inglês | MEDLINE | ID: mdl-21643143

RESUMO

Broadband white light is of great spectroscopic value and would be a powerful tool for nanoscale spectroscopy, however, generation and direction of white light on this length scale remains challenging. Here, we demonstrate the generation of broadband white light in sub-wavelength diameter Gallium Nitride (GaN) wires by coincident one- and two-photon absorption mediated via defect states. This generation of broadband, "white" light enables single-nanowire interferometric measurements of the nanowires themselves via analysis of the Fabry-Pérot fringes that overlay the entirety of the emission spectrum. The quality factor and finesse of individual nanowire cavities were measured and calculated to be 186 ± 88 and 3.05 ±0.6 respectively, averaged over 20 individual wires. This work presents a new, simple approach for the generation and direction of broad band white light at sub-diffraction limit length scales, ideal for translating classical white light spectroscopies to higher spatial resolutions then previously achieved.


Assuntos
Iluminação/instrumentação , Nanotecnologia/instrumentação , Nanotubos/química , Dispositivos Ópticos , Desenho de Equipamento , Análise de Falha de Equipamento , Luz , Nanotubos/efeitos da radiação
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