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1.
Nat Commun ; 10(1): 482, 2019 01 29.
Artigo em Inglês | MEDLINE | ID: mdl-30696817

RESUMO

Organic-inorganic hybrid perovskites such as methylammonium lead iodide (CH3NH3PbI3) are game-changing semiconductors for solar cells and light-emitting devices owing to their defect tolerance and exceptionally long carrier lifetimes and diffusion lengths. Determining whether the dynamically disordered organic cations with large dipole moment benefit the optoelectronic properties of CH3NH3PbI3 has been an outstanding challenge. Herein, via transient absorption measurements employing an infrared pump pulse tuned to a methylammonium vibration, we observe slow, nanosecond-long thermal dissipation from the selectively excited organic mode to the inorganic sublattice. The resulting transient electronic signatures, during the period of thermal-nonequilibrium when the induced thermal motions are mostly concentrated on the organic sublattice, reveal that the induced atomic motions of the organic cations do not alter the absorption or the photoluminescence response of CH3NH3PbI3, beyond thermal effects. Our results suggest that the attractive optoelectronic properties of CH3NH3PbI3 mainly derive from the inorganic lead-halide framework.

2.
Phys Rev Lett ; 121(12): 127401, 2018 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-30296165

RESUMO

Excitations of free electrons and optical phonons are known to permit access to the negative real part of relative permittivities (ϵ^{'}<0) that yield strong light-matter interactions. However, negative ϵ^{'} arising from excitons has been much less explored. Via development of a dielectric-coating based technique described herein, we report fundamental optical properties of two-dimensional hybrid perovskites (2DHPs), composed of alternating layers of inorganic and organic sublattices. Low members of 2DHPs (N=1 and N=2) exhibit negative ϵ^{'} stemming from the large exciton binding energy and sizable oscillator strength. Furthermore, hyperbolic dispersion (i.e., ϵ^{'} changes sign with directions) occurs in the visible range, which has been previously achieved only with artificial metamaterials. Such naturally occurring, exotic dispersion stems from the extremely anisotropic excitonic behaviors of 2DHPs, and can intrinsically support a large photonic density of states. We suggest that several other van der Waals solids may exhibit similar behaviors arising from excitonic response.

3.
Nat Commun ; 9(1): 2792, 2018 07 18.
Artigo em Inglês | MEDLINE | ID: mdl-30022022

RESUMO

Hybrid organic-inorganic perovskites are emerging semiconductors for cheap and efficient photovoltaics and light-emitting devices. Different from conventional inorganic semiconductors, hybrid perovskites consist of coexisting organic and inorganic sub-lattices, which present disparate atomic masses and bond strengths. The nanoscopic interpenetration of these disparate components, which lack strong electronic and vibrational coupling, presents fundamental challenges to the understanding of charge and heat dissipation. Here we study phonon population and equilibration processes in methylammonium lead iodide (MAPbI3) by transiently probing the vibrational modes of the organic sub-lattice following above-bandgap optical excitation. We observe inter-sub-lattice thermal equilibration on timescales ranging from hundreds of picoseconds to a couple of nanoseconds. As supported by a two-temperature model based on first-principles calculations, the slow thermal equilibration is attributable to the sequential phonon populations of the inorganic and organic sub-lattices, respectively. The observed long-lasting thermal non-equilibrium offers insights into thermal transport and heat management of the emergent hybrid material class.

4.
Nat Commun ; 9(1): 2019, 2018 05 22.
Artigo em Inglês | MEDLINE | ID: mdl-29789666

RESUMO

Two-dimensional Ruddlesden-Popper organic-inorganic hybrid layered perovskites (2D RPs) are solution-grown semiconductors with prospective applications in next-generation optoelectronics. The heat-carrying, low-energy acoustic phonons, which are important for heat management of 2D RP-based devices, have remained unexplored. Here we report on the generation and propagation of coherent longitudinal acoustic phonons along the cross-plane direction of 2D RPs, following separate characterizations of below-bandgap refractive indices. Through experiments on single crystals of systematically varied perovskite layer thickness, we demonstrate significant reduction in both group velocity and propagation length of acoustic phonons in 2D RPs as compared to the three-dimensional methylammonium lead iodide counterpart. As borne out by a minimal coarse-grained model, these vibrational properties arise from a large acoustic impedance mismatch between the alternating layers of perovskite sheets and bulky organic cations. Our results inform on thermal transport in highly impedance-mismatched crystal sub-lattices and provide insights towards design of materials that exhibit highly anisotropic thermal dissipation properties.

5.
J Am Chem Soc ; 139(27): 9186-9191, 2017 07 12.
Artigo em Inglês | MEDLINE | ID: mdl-28661665

RESUMO

Ferroelectricity in organic materials remains a subject of great interest, given its potential impact as lightweight information storage media. Here we report supramolecular charge-transfer cocrystals formed by electron acceptor and donor molecules that exhibit ferroelectric behavior along two distinct crystallographic axes. The solid-state superstructure of the cocrystals reveals that a 2:1 ratio of acceptor to donor molecules assemble into nearly orthogonal mixed stacks in which the molecules are positioned for charge-transfer in face-to-face and edge-to-face orientations, held together by an extended hydrogen-bonding network. Polarization hysteresis was observed along the face-to-face and edge-to-face axes at room temperature. The noncentrosymmetric nature of the cocrystals, required to observe ferroelectric behavior, is demonstrated using second harmonic generation measurements. This finding suggests the possibility of designing supramolecular arrays in which organic molecules support multidimensional information storage.


Assuntos
Imidoésteres/química , Naftóis/química , Cristalização , Eletricidade , Ligação de Hidrogênio , Imidoésteres/isolamento & purificação , Substâncias Macromoleculares/química , Estrutura Molecular , Naftóis/isolamento & purificação , Tamanho da Partícula
6.
Nat Commun ; 7: 12892, 2016 Sep 29.
Artigo em Inglês | MEDLINE | ID: mdl-27682836

RESUMO

Nonlinear optical responses of materials play a vital role for the development of active nanophotonic and plasmonic devices. Optical nonlinearity induced by intense optical excitation of mobile electrons in metallic nanostructures can provide large-amplitude, dynamic tuning of their electromagnetic response, which is potentially useful for all-optical processing of information and dynamic beam control. Here we report on the sub-picosecond optical nonlinearity of indium tin oxide nanorod arrays (ITO-NRAs) following intraband, on-plasmon-resonance optical pumping, which enables modulation of the full-visible spectrum with large absolute change of transmission, favourable spectral tunability and beam-steering capability. Furthermore, we observe a transient response in the microsecond regime associated with slow lattice cooling, which arises from the large aspect-ratio and low thermal conductivity of ITO-NRAs. Our results demonstrate that all-optical control of light can be achieved by using heavily doped wide-bandgap semiconductors in their transparent regime with speed faster than that of noble metals.

7.
Nano Lett ; 16(9): 5639-46, 2016 09 14.
Artigo em Inglês | MEDLINE | ID: mdl-27526053

RESUMO

Active control of light is important for photonic integrated circuits, optical switches, and telecommunications. Coupling light with acoustic vibrations in nanoscale optical resonators offers optical modulation capabilities with high bandwidth and small footprint. Instead of using noble metals, here we introduce indium-tin-oxide nanorod arrays (ITO-NRAs) as the operating media and demonstrate optical modulation covering the visible spectral range (from 360 to 700 nm) with ∼20 GHz bandwidth through the excitation of coherent acoustic vibrations in ITO-NRAs. This broadband modulation results from the collective optical diffraction by the dielectric ITO-NRAs, and a high differential transmission modulation up to 10% is achieved through efficient near-infrared, on-plasmon-resonance pumping. By combining the frequency signatures of the vibrational modes with finite-element simulations, we further determine the anisotropic elastic constants for single-crystalline ITO, which are not known for the bulk phase. This technique to determine elastic constants using coherent acoustic vibrations of uniform nanostructures can be generalized to the study of other inorganic materials.

8.
J Am Chem Soc ; 137(21): 6804-19, 2015 Jun 03.
Artigo em Inglês | MEDLINE | ID: mdl-25950197

RESUMO

The synthesis and properties of the hybrid organic/inorganic germanium perovskite compounds, AGeI3, are reported (A = Cs, organic cation). The systematic study of this reaction system led to the isolation of 6 new hybrid semiconductors. Using CsGeI3 (1) as the prototype compound, we have prepared methylammonium, CH3NH3GeI3 (2), formamidinium, HC(NH2)2GeI3 (3), acetamidinium, CH3C(NH2)2GeI3 (4), guanidinium, C(NH2)3GeI3 (5), trimethylammonium, (CH3)3NHGeI3 (6), and isopropylammonium, (CH3)2C(H)NH3GeI3 (7) analogues. The crystal structures of the compounds are classified based on their dimensionality with 1­4 forming 3D perovskite frameworks and 5­7 1D infinite chains. Compounds 1­7, with the exception of compounds 5 (centrosymmetric) and 7 (nonpolar acentric), crystallize in polar space groups. The 3D compounds have direct band gaps of 1.6 eV (1), 1.9 eV (2), 2.2 eV (3), and 2.5 eV (4), while the 1D compounds have indirect band gaps of 2.7 eV (5), 2.5 eV (6), and 2.8 eV (7). Herein, we report on the second harmonic generation (SHG) properties of the compounds, which display remarkably strong, type I phase-matchable SHG response with high laser-induced damage thresholds (up to ∼3 GW/cm(2)). The second-order nonlinear susceptibility, χS(2), was determined to be 125.3 ± 10.5 pm/V (1), (161.0 ± 14.5) pm/V (2), 143.0 ± 13.5 pm/V (3), and 57.2 ± 5.5 pm/V (4). First-principles density functional theory electronic structure calculations indicate that the large SHG response is attributed to the high density of states in the valence band due to sp-hybridization of the Ge and I orbitals, a consequence of the lone pair activation.

9.
Sci Technol Adv Mater ; 16(3): 034901, 2015 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-27877798

RESUMO

Phase-pure cuprous oxide (Cu2O) crystals are difficult to grow since cupric oxide can form within the crystal as the crystal is cooled to ambient conditions. Vacancies are the solute which causes precipitation of macroscopic defects. Therefore, even when a mostly phase-pure single crystal is used as a feed rod, cupric oxide inclusions persist in the recrystallized solid. Control of the thermal profile during crystal growth, however, can improve phase-purity; a slow counter-rotation rate of the feed and seed rods results in fewer inclusions. Cupric oxide can be removed by annealing, which produces a factor of 540 ± 70 increase in phase-purity.

10.
Opt Lett ; 39(3): 618-21, 2014 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-24487880

RESUMO

The efficiency of third-harmonic generation in cuprous oxide was measured. Intensities followed a noncubic power law that indicates nonperturbative behavior. Polarization anisotropy of the harmonic generation was demonstrated and related to the third-order susceptibility. The results will influence the understanding of harmonic generation in centrosymmetric materials and are potentially relevant to device design and the interpretation of exciton behavior.

11.
Science ; 339(6118): 429-33, 2013 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-23349286

RESUMO

Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.

12.
ACS Nano ; 5(11): 9161-70, 2011 Nov 22.
Artigo em Inglês | MEDLINE | ID: mdl-22017677

RESUMO

This article reports the study of infrared plasmonics with both random and periodic arrays of indium-tin-oxide (ITO) nanorods (NR). A description is given on the synthesis, patterning, and characterization of physical properties of the ITO NR arrays. A classical scattering model, along with a 3-D finite-element-method and a 3-D finite-difference-time-domain numerical simulation method has been used to interpret the unique light scattering phenomena. It is also shown that the intrinsic plasma frequency can be varied through careful postsynthesis processing of the ITO NRs. Examples are given on how coupled plasmon resonances can be tuned through patterning of the ITO NR arrays. In addition, environment dielectric sensing has been demonstrated through the shift of the resonances as a result of index change surrounding the NRs. These initial results suggest potential for further improvement and opportunities to develop a good understanding of infrared plasmonics using ITO and other transparent conducting oxide semiconducting materials.

14.
J Chem Phys ; 134(12): 124312, 2011 Mar 28.
Artigo em Inglês | MEDLINE | ID: mdl-21456667

RESUMO

We describe surface-enhanced Raman scattering measurements from a benzenethiol monolayer adsorbed on a silver-coated film that is, in turn, deposited on an artificial opal, where the latter is a close-packed three-dimensional dielectric lattice formed from polystyrene spheres. Data for a range of sphere sizes, silver film thicknesses, and laser excitation wavelengths are obtained. Enhancement factors can be in the range of 10(7). To partially explain these large enhancements, we have performed model finite-difference time domain simulations of the position-dependent electric fields generated at the opal surfaces for several experimentally studied laser wavelengths and sphere diameters.


Assuntos
Prata/química , Análise Espectral Raman/métodos , Adsorção , Fenóis/química , Poliestirenos/química , Compostos de Sulfidrila/química
15.
Inorg Chem ; 49(20): 9098-100, 2010 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-20860364

RESUMO

The layered compounds RbAg(2)TeS(6) and CsAg(2)TeS(6) crystallize in the noncentrosymmetric space group P6(3)cm, with a = 19.15 Å, c = 14.64 Å, and V = 4648 Å(3) and a = 19.41 Å, c = 14.84 Å, and V = 4839 Å(3), respectively. The structures are composed of neutral [Ag(2)TeS(3)] layers alternating with charge-balanced salt layers containing polysulfide chains of [S(6)](2-) and alkali-metal ions. RbAg(2)TeS(6) and CsAg(2)TeS(6) are air- and water-stable, wide-band-gap semiconductors (E(g) ∼ 2.0 eV) exhibiting nonlinear-optical second-harmonic generation.

16.
J Am Chem Soc ; 132(10): 3484-95, 2010 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-20170184

RESUMO

AAsSe(2) (A = Li, Na) have been identified as a new class of polar direct-band gap semiconductors. These I-V-VI(2) ternary alkali-metal chalcoarsenates have infinite single chains of (1/infinity)[AsQ(2)(-)] derived from corner-sharing pyramidal AsQ(3) units with stereochemically active lone pairs of electrons on arsenic. The conformations and packing of the chains depend on the structure-directing alkali metals. This results in at least four different structural types for the Li(1-x)Na(x)AsSe(2) stoichiometry (alpha-LiAsSe(2), beta-LiAsSe(2), gamma-NaAsSe(2), and delta-NaAsSe(2)). Single-crystal X-ray diffraction studies showed an average cubic NaCl-type structure for alpha-LiAsSe(2), which was further demonstrated to be locally distorted by pair distribution function (PDF) analysis. The beta and gamma forms have polar structures built of different (1/infinity)[AsSe(2)(-)] chain conformations, whereas the delta form has nonpolar packing. A wide range of direct band gaps are observed, depending on composition: namely, 1.11 eV for alpha-LiAsSe(2), 1.60 eV for LiAsS(2), 1.75 eV for gamma-NaAsSe(2), 2.23 eV for NaAsS(2). The AAsQ(2) materials are soluble in common solvents such as methanol, which makes them promising candidates for solution processing. Band structure calculations performed with the highly precise screened-exchange sX-LDA FLAPW method confirm the direct-gap nature and agree well with experiment. The polar gamma-NaAsSe(2) shows very large nonlinear optical (NLO) second harmonic generation (SHG) response in the wavelength range of 600-950 nm. The theoretical studies confirm the experimental results and show that gamma-NaAsSe(2) has the highest static SHG coefficient known to date, 337.9 pm/V, among materials with band gaps larger than 1.0 eV.


Assuntos
Arseniatos/química , Compostos de Lítio/química , Compostos de Selênio/química , Semicondutores , Compostos de Sódio/química , Cristalografia por Raios X , Solubilidade , Espectrofotometria/métodos , Análise Espectral Raman , Termodinâmica
17.
J Biomed Mater Res A ; 93(3): 878-85, 2010 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-19705463

RESUMO

The preparation of hydroxyapatite (HA) coatings via a versatile right-angle magnetron sputtering (RAMS) approach for use as a biomaterial has recently been reported. RAMS coatings show some advantages over conventionally sputtered films in that room temperature deposition yields nanocrystalline and nearly stoichiometric HA coatings under appropriate conditions, thereby avoiding the troublesome post deposition annealing treatment. In this article, we present an exploratory study of the biocompatibility of RAMS HA coatings deposited on metallic substrates. RAMS HA coatings with a thickness around 500nm were prepared on various substrates. X-ray diffraction (XRD) analysis showed that the as-deposited HA coatings were polycrystalline with some strongly preferred orientations. Atomic force microscopy (AFM) results showed that the coatings were rather smooth with surface roughness on the order of 10 nm. X-ray photoelectron spectroscopy (XPS) confirmed that the surface chemistry was nearly stoichiometric. To study the biocompatibility of these coatings, murine pre-osteoblastic MC3T3-E1 cells were seeded onto various substrates. Cell density counts using fluorescence microscopy showed that the best osteoblast proliferation is achieved on an HA RAMS-coated titanium substrate. Additionally, in preliminary studies the influence of Zn, Mg, and Al incorporation in the HA crystal lattice on the in vitro behavior was also evaluated. These experiments demonstrate that RAMS is a promising coating technique for biomedical applications.


Assuntos
Materiais Revestidos Biocompatíveis/farmacologia , Durapatita/farmacologia , Teste de Materiais/métodos , Osteoblastos/citologia , Osteoblastos/efeitos dos fármacos , Alumínio/farmacologia , Animais , Adesão Celular/efeitos dos fármacos , Linhagem Celular , Proliferação de Células/efeitos dos fármacos , Fluorescência , Magnésio/farmacologia , Camundongos , Microscopia de Força Atômica , Espectroscopia Fotoeletrônica , Titânio/farmacologia , Difração de Raios X , Zinco/farmacologia
18.
J Am Chem Soc ; 132(1): 384-9, 2010 Jan 13.
Artigo em Inglês | MEDLINE | ID: mdl-20000464

RESUMO

We report that the one-dimensional polar selenophosphate compounds APSe(6) (A = K, Rb), which show crystal-glass phase-change behavior, exhibit strong second harmonic generation (SHG) response in both crystal and glassy forms. The crystalline materials are type-I phase-matchable with SHG coefficients chi((2)) of 151.3 and 149.4 pm V(-1) for K(+) and Rb(+) salts, respectively, which is the highest among phase-matchable nonlinear optical (NLO) materials with band gaps over 1.0 eV. The glass of APSe(6) exhibits comparable SHG intensities to the top infrared NLO material AgGaSe(2) without any poling treatments. APSe(6) exhibit excellent mid-IR transparency. We demonstrate that starting from noncentrosymmetric phase-change materials such as APSe(6) (A = K, Rb), we can obtain optical glass fibers with strong, intrinsic, and temporally stable second-order nonlinear optical (NLO) response. The as-prepared glass fibers exhibit SHG and difference frequency generation (DFG) responses over a wide range of wavelengths. Raman spectroscopy and pair distribution function (PDF) analyses provide further understanding of the local structure in amorphous state of KPSe(6) bulk glass and glass fiber. We propose that this approach can be widely applied to prepare permanent NLO glass from materials that undergo a phase-change process.

19.
Opt Lett ; 34(18): 2817-9, 2009 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-19756115

RESUMO

We present what we believe to be the first experimental determination of the third-order optical susceptibility chi((3)) of bulk cuprous oxide (Cu(2)O) crystals. The measured nonlinear refractive index, obtained with the Z-scan technique at 1064 nm, is n(2)=1.32x10(-10) esu, while the two-photon absorption coefficient is beta=5.0 cm/GW of Cu(2)O. We also observe strong third-harmonic generation (THG) from Cu(2)O in our detection range owing to its unique crystal and electronic structure. Considering that the first nonvanishing nonlinear term is chi((3)), this classical semiconductor could be utilized as a promising active THG medium.

20.
J Am Chem Soc ; 131(29): 9896-7, 2009 Jul 29.
Artigo em Inglês | MEDLINE | ID: mdl-19580274

RESUMO

The new cation [Sb(7)S(8)Br(2)](3+) has a double-cubane structure and forms as the [AlCl(4)](-) salt from the ionic liquid EMIMBr-AlCl(3) (EMIM = 1-ethyl-3-methylimidazolium) at 165 degrees C. The compound is noncentrosymmetric with space group P2(1)2(1)2(1) and exhibits second-harmonic and difference-frequency nonlinear optical response across a wide range of the visible and near-infrared regions.

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