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1.
Adv Mater ; 32(23): e1906478, 2020 Jun.
Artigo em Inglês | MEDLINE | ID: mdl-32347620

RESUMO

Above-equilibrium "hot"-carrier generation in metals is a promising route to convert photons into electrical charge for efficient near-infrared optoelectronics. However, metals that offer both hot-carrier generation in the near-infrared and sufficient carrier lifetimes remain elusive. Alloys can offer emergent properties and new design strategies compared to pure metals. Here, it is shown that a noble-transition alloy, Aux Pd1- x , outperforms its constituent metals concerning generation and lifetime of hot carriers when excited in the near-infrared. At optical fiber wavelengths (e.g., 1550 nm), Au50 Pd50 provides a 20-fold increase in the number of ≈0.8 eV hot holes, compared to Au, and a threefold increase in the carrier lifetime, compared to Pd. The discovery that noble-transition alloys can excel at hot-carrier generation reveals a new material platform for near-infrared optoelectronic devices.

2.
Nanoscale ; 12(17): 9661-9668, 2020 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-32319509

RESUMO

Colloidal CdSe nanoplatelets, with the electronic structure of quantum wells, self-assemble into lamellar stacks due to large co-facial van der Waals attractions. These lamellar stacks are shown to display coherent acoustic phonons that are detected from oscillatory changes in the absorption spectrum observed in infrared pump, electronic probe measurements. Rather than direct electronic excitation of the nanocrystals using a femtosecond laser, impulsive transfer of heat from the organic ligand shell, excited at C-H stretching vibrational resonances, to the inorganic core of individual nanoplatelets occurs on a time-scale of <100 ps. This heat transfer drives in-phase longitudinal acoustic phonons of the nanoplatelet lamellae, which are accompanied by subtle deformations along the nanoplatelet short axes. The frequencies of the oscillations vary from 0.7 to 2 GHz (3-8 µeV and 0.5-1 ns oscillation period) depending on the thickness of the nanoplatelets-but not their lateral areas-and the temperature of the sample. Temperature-dependence of the acoustic phonon frequency conveys a substantial stiffening of the organic ligand bonds between nanoplatelets with reduced temperature. These results demonstrate a potential for acoustic modulation of the excitonic structure of nanocrystal assemblies in self-assembled anisotropic semiconductor systems at temperatures at or above 300 K.

3.
J Am Chem Soc ; 142(1): 233-241, 2020 Jan 08.
Artigo em Inglês | MEDLINE | ID: mdl-31815456

RESUMO

Self-assembled peptide micelles and fibers demonstrate unique control over the photophysical properties of the bound, light-activated chromophore, zinc protoporphyrin IX, (PPIX)Zn. Micelles encapsulate either a mixture of uncoordinated and coordinated (PPIX)Zn or all coordinated depending on the ratio of peptide/porphyrin. As the ratio increases toward a 1:1 micelle/porphyrin ratio, providing the chromophore with a discrete coordination environment reminiscent of unstructured proteins, the micelles favor triplet formation. Fibers, however, promote a linear array of porphyrin molecules that dictates exciton hopping and excimer formation at ratios as high as 60:1, peptide/porphyrin. However, even in fibers, the formation of the triplet species increases with increasing peptide/porphyrin ratio due to increased spatial separation between neighboring chromophores facilitating intersystem crossing. Full characterization of the micelles structures and comparison to the fibers lead to the comparison with natural systems and the ability to control the excited populations that have utility in photocatalytic processes. In addition, the incorporation of a second chromophore, heme, yields an electron transfer pathway in both micelles and fibers that highlights the utility of the peptide assemblies when engineering multichromophore arrays as inspired by natural, photosynthetic proteins.

4.
Nat Commun ; 10(1): 4511, 2019 10 04.
Artigo em Inglês | MEDLINE | ID: mdl-31586067

RESUMO

Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30-200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900-1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.

5.
Nano Lett ; 19(11): 8155-8160, 2019 11 13.
Artigo em Inglês | MEDLINE | ID: mdl-31603685

RESUMO

Thermal transport across interfaces depends on the matching of vibrational structure at the interface. This work examines the transfer of thermal excitation from an organic ligand coating to either all-inorganic cesium lead tribromide (CsPbBr3) nanocrystals or hybrid organic-inorganic formamidinium lead tribromide (FAPbBr3) nanocrystals using selective infrared optical excitation. These two semiconductors are directly compared because they (or similar semiconductors) are currently envisioned as strong candidates in many optoelectronic technologies and they differ due to the presence of an organic or inorganic cation, which introduces substantial differences in the phonon density of states in otherwise quite similar semiconductors. Infrared excitation of C-H vibrations of surface-bound ligands generates a temperature gradient between the organic ligand shell and nanocrystal core, which results in heat flow, measured by probing changes of the semiconductor band gap. Heat transfer to both perovskite compositions of comparable sizes is similar (25-30 ps), due to fast intramolecular vibrational relaxation and similar matching of low-energy phonons with the organic ligand, but FAPbBr3 samples show a slow bleaching kinetic on the order of 1 ns. This slow, heat-induced change in the semiconductor band gap is attributed not to interfacial heat transfer but instead to thermal equilibration between the organic and inorganic sublattices of FAPbBr3. Ab initio molecular dynamics simulations support the hypothesis that low-energy inorganic sublattice phonon modes are populated initially in the heat transfer process, with a slow thermal population of the higher-energy phonon modes associated primarily with the organic cation. Slow thermal equilibration of FAPbBr3 is likely to substantially impact the time-dependent response of optoelectronic devices that heat the semiconductor active layer and provide further evidence that the poor bulk thermal transport of hybrid perovskite materials extends to microscopic thermal processes.

6.
J Phys Chem Lett ; 10(18): 5623-5628, 2019 Sep 19.
Artigo em Inglês | MEDLINE | ID: mdl-31502463

RESUMO

Intraband relaxation in polycrystalline films of hybrid perovskites methylammonium lead tribromide and methylammonium lead triiodide are studied by transient absorption spectroscopy from 80 K to >350 K. This temperature range spans the transitions of these materials from the high-temperature cubic phases, intermediate tetragonal phases, and low-temperature orthorhombic phases. The organic cation undergoes a distinct transition from an ordered lattice in the orthorhombic phase to a plastic crystal in cubic and tetragonal phases, which reportedly influences many optoelectronic properties. The much larger exciton binding energy of orthorhombic MAPbI3 (compared to cubic or tetragonal phases) or MAPbBr3 substantially changes the transient spectral responses of the materials by reducing the number of free carriers. However, for these measurements at low fluences, both MAPbBr3 and MAPbI3 exhibit subpicosecond intraband relaxation over the entire temperature range studied. Intraband relaxation becomes somewhat faster at higher temperatures, but freezing of organic cations are not accompanied by a discontinuity of the intraband relaxation time. These results suggest that configurational freedom of organic cations does not screen carriers from electron-phonon coupling.

7.
J Am Chem Soc ; 141(34): 13487-13496, 2019 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-31379152

RESUMO

In contrast to molecular systems, which are defined with atomic precision, nanomaterials generally show some heterogeneity in size, shape, and composition. The sample inhomogeneity translates into a distribution of energy levels, band gaps, work functions, and other characteristics, which detrimentally affect practically every property of functional nanomaterials. We discuss a novel synthetic strategy, colloidal atomic layer deposition (c-ALD) with stationary reactant phases, which largely circumvents the limitations of traditional colloidal syntheses of nano-heterostructures with atomic precision. This approach allows for significant reduction of inhomogeneity in nanomaterials in complex nanostructures without compromising their structural perfection and enables the synthesis of epitaxial nano-heterostructures of unprecedented complexity. The improved synthetic control ultimately enables bandgap and strain engineering in colloidal nanomaterials with close to atomic accuracy. To demonstrate the power of the new c-ALD method, we synthesize a library of complex II-VI semiconductor nanoplatelet heterostructures. By combining spectroscopic and computational studies, we elucidate the subtle interplay between quantum confinement and strain effects on the optical properties of semiconductor nanostructures.

8.
ACS Appl Mater Interfaces ; 11(33): 30154-30162, 2019 Aug 21.
Artigo em Inglês | MEDLINE | ID: mdl-31353888

RESUMO

Here, we propose a simple approach for the design of highly porous multicomponent heterostructures by infiltration of block-co-polymer templates with inorganic precursors in swelling solvents followed by gas-phase sequential infiltration synthesis and thermal annealing. This approach can prepare conformal coatings, free-standing membranes, and powders consisting of uniformly sized metal or metal oxide nanoparticles (NPs) well dispersed in a porous oxide matrix. We employed this new, versatile synthetic concept to synthesize catalytically active heterostructures of uniformly dispersed ∼4.3 nm PdO nanoparticles accessible through three-dimensional pore networks of the alumina support. Importantly, such materials reveal high resistance against sintering at 800 °C, even at relatively high loadings of NPs (∼10 wt %). At the same time, such heterostructures enable high mass transport due to highly interconnected nature of the pores. The surface of synthesized nanoparticles in the porous matrix is highly accessible, which enables their good catalytic performance in methane and carbon monoxide oxidation. In addition, we demonstrate that this approach can be utilized to synthesize heterostructures consisting of different types of NPs on a highly porous support. Our results show that swelling-based infiltration provides a promising route toward the robust and scalable synthesis of multicomponent structures.

9.
ACS Nano ; 13(8): 8589-8596, 2019 Aug 27.
Artigo em Inglês | MEDLINE | ID: mdl-31251582

RESUMO

Colloidal, two-dimensional semiconductor nanoplatelets (NPLs) exhibit quantum confinement in only one dimension, which results in an electronic structure that is significantly altered compared to that of other quantum-confined nanomaterials. Whereas it is often assumed that the lack of quantum confinement in the lateral plane yields a spatially extended exciton, reduced dielectric screening potentially challenges this picture. Here, we implement absorption spectroscopy in pulsed magnetic fields up to 60 T for three different CdSe NPL thicknesses and lateral areas. Based on diamagnetic shifts, we find that the exciton lateral extent is comparable to NPL thickness, indicating that the quantum confinement and reduced screening concomitant with few-monolayer thickness strongly reduces the exciton lateral extent. Atomistic electronic structure calculations of the exciton size for varying lengths, widths, and thicknesses support the substantially smaller in-plane exciton extent.

10.
Annu Rev Phys Chem ; 70: 353-377, 2019 Jun 14.
Artigo em Inglês | MEDLINE | ID: mdl-31112459

RESUMO

This article reviews thermal properties of semiconductor and emergent plasmonic nanomaterials, focusing on mechanisms through which hot carriers and phonons are produced and dissipated as well as the related impacts on optoelectronic properties. Elevated equilibrium temperatures, of particular relevance for implementation of nanomaterials in devices, affect absorptive and radiative transitions as well as emission efficiency that can present reversible and irreversible changes with temperature. In noble metal or doped semiconductor/insulator nanomaterials, hot carriers and lattice heating can substantially influence localized surface plasmon resonances and yield large ultrafast changes in transmission or strongly oscillatory coherences. Transient optical and diffraction characterizations enable nonequilibrium investigations of phonon dynamics and cooling such as lattice expansion and crystal phase stability. Timescales of nanoparticle thermalization with surroundings and transport of heat within films of such materials are also discussed.

11.
Nanoscale ; 11(17): 8204-8209, 2019 Apr 25.
Artigo em Inglês | MEDLINE | ID: mdl-30972391

RESUMO

Ligand-to-nanocrystal heating and subsequent cooling to the environmental medium is investigated with infrared pump, electronic probe (IPEP) spectroscopy. Compared to solid films, solvated nanocrystals show faster ligand-to-nanocrystal heat equilibration (c. 11 ps versus c. 17 ps). Solvated nanocrystals also display more cooling of the hot ligand-nanocrystal complex on the experimentally measured time-scale, emphasizing the thermally insulating nature of semiconductor nanocrystal solids. Although heating transfer rates among solvents are all between 150 ps and 330 ps, cooling of the nanocrystal-ligand complex is slower, on average, in chlorinated solvents (c. 315 ps) compared to deuterated hydrocarbon solvents (c. 215 ps). Differences between chlorinated and hydrocarbon solvents show the importance of matching the vibrational energies of the solvent and the ligands for increasing the rate of heat transfer. Increases in the cooling time for poorer hydrocarbon solvents, in which nanocrystals aggregated, such as toluene, compared to better solvents, like methylcyclohexane, indicate that penetration of solvent into the ligand layer facilitates improved heat transfer to the matrix.

12.
J Am Chem Soc ; 141(13): 5092-5096, 2019 Apr 03.
Artigo em Inglês | MEDLINE | ID: mdl-30882213

RESUMO

Zero-dimensional PbSe quantum dots are heterogeneously nucleated and grown onto two-dimensional zincblende CdSe nanoplatelets. Electron microscopy shows ad-grown dots predominantly decorate edges and corners of the nanoplatelets. Spectroscopic characterizations relate type I electronic alignment as demonstrated via photoluminescence excitation spectroscopy enhancement of near-infrared emission. Transient photoluminescence and absorption convey ultrafast transfer of excitons to the lower energy semiconductor dots. These structures combine benefits of large absorption cross sections of nanoplatelets and efficient near-infrared emission of PbSe with quantum confinement tuning of energy gap.

13.
Nano Lett ; 19(4): 2322-2328, 2019 04 10.
Artigo em Inglês | MEDLINE | ID: mdl-30901222

RESUMO

Temperature-dependent photoluminescence lifetimes of electron-hole pairs (excitons) in CdSe nanocrystals are governed by the energetic ordering and spacing of slowly emitting, spin-forbidden "dark" exciton states and rapidly emitting "bright" states. Here, infrared pulses that are resonant with hydrocarbon surface ligand vibrational transitions are shown to offer a route to manipulate the instantaneous emission rate of CdSe nanocrystals at cryogenic temperature. Transient heating of the inorganic nanocrystal core is achieved via resonant excitation of ligand vibrations, followed by heat flow to the nanocrystal lattice. Heating of the nanocrystal core is demonstrated using transient absorption spectroscopy, which shows a time-dependent red-shift of the quantum dot electronic absorption resonances, consistent with heating. Transient heating of the nanocrystal above the bath temperature increases the instantaneous radiative rate of the nanocrystals via a combination of thermal occupation of bright states as well as phonon-assisted emission. The lifetime of this infrared-pumped, fast-emitting sample condition is dictated by particle thermalization, which is multiple orders of magnitude shorter lived than the dark exciton state. This work demonstrates the feasibility of using heat control pulses to manipulate electronic recombination rates of excitons.

14.
Nanoscale ; 11(12): 5412-5421, 2019 Mar 21.
Artigo em Inglês | MEDLINE | ID: mdl-30855041

RESUMO

To take peptide materials from predominantly structural to functional assemblies, variations in cofactor binding sites must be engineered and controlled. Here, we have employed the peptide sequence c16-AHX3K3-CO2H where X3 represents the aliphatic structural component of the peptide design that dictates ß-sheet formation and upon self-assembly yields a change in the overall microenvironment surrounding the Zn protoporphyrin IX ((PPIX)Zn) binding site. All peptides studied yield ß-sheet rich nanofibers highlighting the materials' resiliency to amino acid substitution. We highlight that the (PPIX)Zn binding constants correlate strongly with amino acid side chain volume, where X = L or I yields the lowest dissociation constant values (KD). The resulting microenvironment highlights the materials' ability to control interchromophore electronic interactions such that slip-stacked cofacial arrangements are observed via exciton splitting in UV/visible and circular dichroism spectroscopy. Steady state and time-resolved photoluminescence suggests that greater interchromophore packing yields larger excimer populations and corresponding longer excimer association lifetimes (τA) which directly translates to shorter exciton diffusion lengths. In comparison to synthetic porphyrin molecular assemblies, this work demonstrates the ability to employ the peptide assembly to modulate the degree of cofactor arrangement, extent of excimer formation, and the exciton hopping rates all while in a platform amenable for producing polymer-like materials.


Assuntos
Nanofibras/química , Peptídeos/química , Protoporfirinas/química , Sítios de Ligação , Dicroísmo Circular , Microscopia Eletrônica de Transmissão , Ligação Proteica , Conformação Proteica em Folha beta , Espectrofotometria
15.
Nat Commun ; 10(1): 504, 2019 01 30.
Artigo em Inglês | MEDLINE | ID: mdl-30700706

RESUMO

Significant interest exists in lead trihalides that present the perovskite structure owing to their demonstrated potential in photovoltaic, lasing, and display applications. These materials are also notable for their unusual phase behavior often displaying easily accessible phase transitions. In this work, time-resolved X-ray diffraction, performed on perovskite cesium lead bromide nanocrystals, maps the lattice response to controlled excitation fluence. These nanocrystals undergo a reversible, photoinduced orthorhombic-to-cubic phase transition which is discernible at fluences greater than 0.34 mJ cm-2 through the loss of orthorhombic features and shifting of high-symmetry peaks. This transition recovers on the timescale of 510 ± 100 ps. A reversible crystalline-to-amorphous transition, observable through loss of Bragg diffraction intensity, occurs at higher fluences (greater than 2.5 mJ cm-2). These results demonstrate that light-driven phase transitions occur in perovskite materials, which will impact optoelectronic applications and enable the manipulation of non-equilibrium phase characteristics of the broad perovskite material class.

16.
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.

17.
Nano Lett ; 18(12): 7863-7869, 2018 12 12.
Artigo em Inglês | MEDLINE | ID: mdl-30431280

RESUMO

The transfer of thermal energy from the ligand passivating layer to the inorganic core of colloidal nanocrystals is observed using infrared-pump, electronic-probe (IPEP) spectroscopy. Inorganic nanocrystals are excellent model systems for organic-inorganic hybrid interfaces as they have much larger surface-to-volume ratios than bulk solids, which facilitate spectroscopic measurements of weak signals. Such interfaces between disparate materials are challenging to probe by traditional methods. Here, resonant excitation of the hydrocarbon ligand vibrational absorptions results in a transient red-shift of the CdSe nanocrystal excitonic features consistent with heating, as demonstrated by steady-state absorption measurements, which provide a calibration of the pump-induced temperature rise. The time constant associated with heating ranges from 10 to 30 ps depending on the sample morphology, static temperature, input fluence, and environment, all of which are studied in this work. Heat transfer speeds up and the magnitude of nanocrystal heating decreases at higher temperatures. Unlike chemical modulation of electrical conductivity, ligand exchange for several common organic ligands does not dramatically change the interfacial conductivity of the nanocrystal-ligand interface. However, changes in the medium (e.g., solvent) do change the rate of heat outcoupling from the nanocrystal-ligand complex. Although applied here to nanocrystals to measure interfacial heat transfer, IPEP spectroscopy is readily applicable for any heterogeneous system in which one component has spectrally isolated molecular vibrations or lattice phonons.

18.
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.

19.
ACS Nano ; 12(10): 10008-10015, 2018 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-30226751

RESUMO

The optoelectronic properties of semiconductor nanocrystals (NCs) have led to efforts to integrate them as the active material in light-emitting diodes, solid-state lighting, and lasers. Understanding related high carrier injection conditions is therefore critical as resultant thermal effects can impact optical properties. The physical integrity of NCs is indeed questionable as recent transient X-ray diffraction studies have suggested that nanoscopic particles reversibly lose crystalline order, or melt, under high fluence photoexcitation. Informed by such studies, here, we examine CdSe NCs under elevated fluences to determine the impact of lattice disordering on optical properties. To this end, we implement intensity-dependent transient absorption using both one- and two-pump methods where the latter effectively subtracts out the NC optical signatures associated with lower fluence photoexcitation, especially band-edge features. At elevated fluences, we observe a long-lived induced absorption at a lower energy than the crystalline-NC bandgap across a wide range of sizes that follows power-dependent trends and kinetics consistent with the prior transient X-ray measurements. NC photoluminescence studies provide further evidence that melting influences optical properties. These methods of characterizing bandgap narrowing caused by lattice disordering could facilitate routes to improved optical amplification and band-edge emission at high excitation density.

20.
Nanotechnology ; 29(49): 495703, 2018 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-30211698

RESUMO

Inorganic nanoporous materials with highly accessible pores are of great interest for the design of efficient catalytic, purification and detection systems. Limited access to the pores is a common problem associated with traditional approaches for the synthesis of porous materials, affecting the functionality of the low-density structure. Recently, infiltration of a nanoporous polymer template with inorganic precursors followed by oxidative annealing was proposed as a new and efficient approach to creating porous inorganic structures with controlled thickness, composition and pore sizes. Here, we report an ultra-high accessibility of the pores in porous films prepared via polymer-swelling-assisted sequential infiltration synthesis (SIS). Using a quartz crystal microbalance technique, we show the increased solvent adsorbing capabilities of highly porous alumina films as a result of high interconnectivity of the pores in such structures. The directionality and highly interconnected nature of the pores are demonstrated in experiments with the partial blocking of pore access by the deposition of a single-layer graphene that is not transparent to solvent. 60% of the pores remain accessible when only 20% of the surface is exposed to solvent. Using humidity detection as an example, we also show that highly porous alumina produced by polymer-swelling-assisted SIS is a promising candidate for sensing applications.

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