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1.
Science ; 365(6454): 672-675, 2019 08 16.
Artigo em Inglês | MEDLINE | ID: mdl-31416959

RESUMO

Colloidal semiconductor quantum dots (QDs) are attractive materials for realizing highly flexible, solution-processable optical gain media, but they are difficult to use in lasing because of complications associated with extremely short optical-gain lifetimes limited by nonradiative Auger recombination. By combining compositional grading of the QD's interior for hindering Auger decay with postsynthetic charging for suppressing parasitic ground-state absorption, we can reduce the lasing threshold to values below the single-exciton-per-dot limit. As a favorable departure from traditional multi-exciton-based lasing schemes, our approach should facilitate the development of solution-processable lasing devices and thereby help to extend the reach of lasing technologies into areas not accessible with traditional, epitaxially grown semiconductor materials.

2.
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.

3.
Nat Commun ; 10(1): 3253, 2019 Jul 22.
Artigo em Inglês | MEDLINE | ID: mdl-31332181

RESUMO

Semiconductor quantum rings are topological structures that support fascinating phenomena such as the Aharonov-Bohm effect and persistent current, which are of high relevance in the research of quantum information devices. The annular shape of quantum rings distinguishes them from other low-dimensional materials, and enables topologically induced properties such as geometry-dependent spin manipulation and emission. While optical transition dipole moments (TDMs) in zero to two-dimensional optical emitters have been well investigated, those in quantum rings remain obscure despite their utmost relevance to the quantum photonic applications of quantum rings. Here, we study the dimensionality and orientation of TDMs in CdSe quantum rings. In contrast to those in other two-dimensional optical emitters, we find that TDMs in CdSe quantum rings show a peculiar in-plane linear distribution. Our theoretical modeling reveals that this uniaxial TDM originates from broken rotational symmetry in the quantum ring geometries.

4.
Nano Lett ; 18(8): 4647-4652, 2018 08 08.
Artigo em Inglês | MEDLINE | ID: mdl-29985629

RESUMO

Many important light-matter coupling and energy-transfer processes depend critically on the dimensionality and orientation of optical transition dipoles in emitters. We investigate individual quasi-two-dimensional nanoplatelets (NPLs) using higher-order laser scanning microscopy and find that absorption dipoles in NPLs are isotropic in three dimensions at the excitation wavelength. Correlated polarization studies of the NPLs reveal that their emission polarization is strongly dependent on the aspect ratio of the lateral dimensions. Our simulations reveal that this emission anisotropy can be readily explained by the electric field renormalization effect caused by the dielectric contrast between the NPLs and the surrounding medium, and we conclude that emission dipoles in NPLs are isotropic in the plane of the NPLs. Our study presents an approach for disentangling the effects of dipole degeneracy and electric field renormalization on emission anisotropy and can be adapted for studying the intrinsic optical transition dipoles of various nanostructures.


Assuntos
Luminescência , Substâncias Luminescentes/química , Nanopartículas/química , Semicondutores , Anisotropia , Compostos de Cádmio/química , Simulação por Computador , Campos Eletromagnéticos , Luz , Modelos Químicos , Tamanho da Partícula , Fenômenos Físicos , Compostos de Selênio/química , Sulfetos/química , Propriedades de Superfície
5.
J Phys Chem Lett ; 9(2): 286-293, 2018 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-29283580

RESUMO

Elevated temperature optoelectronic performance of semiconductor nanomaterials remains an important issue for applications. Here we examine 2D CdSe nanoplatelets (NPs) and CdS/CdSe/CdS shell/core/shell sandwich NPs at temperatures ranging from 300 to 700 K using static and transient spectroscopies as well as in situ transmission electron microscopy. NPs exhibit reversible changes in PL intensity, spectral position, and emission line width with temperature elevation up to ∼500 K, losing a factor of ∼8 to 10 in PL intensity at 400 K relative to ambient. Temperature elevation above ∼500 K yields thickness-dependent, irreversible degradation in optical properties. Electron microscopy relates stability of the core-only NP morphology up to 555 and 600 K for the four and five monolayer NPs, respectively, followed by sintering and evaporation at still higher temperatures. Reversible PL loss, based on differences in decay dynamics between time-resolved photoluminescence and transient absorption, results primarily from hole trapping in both NPs and sandwich NPs.

6.
Nano Lett ; 17(11): 6900-6906, 2017 11 08.
Artigo em Inglês | MEDLINE | ID: mdl-28994296

RESUMO

Nonradiative Auger recombination limits the efficiency with which colloidal semiconductor nanocrystals can emit light when they are subjected to strong excitation, with important implications for the application of the nanocrystals in light-emitting diodes and lasers. This has motivated attempts to engineer the structure of the nanocrystals to minimize Auger rates. Here, we study Auger recombination rates in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells. Using time-resolved photoluminescence measurements, we show that the rate of biexcitonic Auger recombination has a nonmonotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2-4 monolayers, and then increasing with further increases in the shell thickness. This nonmonotonic behavior has not been observed previously for biexcitonic recombination in quantum dots, most likely due to inhomogeneous broadening that is not present for the nanoplatelets.

7.
ACS Nano ; 11(9): 9119-9127, 2017 09 26.
Artigo em Inglês | MEDLINE | ID: mdl-28787569

RESUMO

Quasi-two-dimensional nanoplatelets (NPLs) possess fundamentally different excitonic properties from zero-dimensional quantum dots. We study lateral size-dependent photon emission statistics and carrier dynamics of individual NPLs using second-order photon correlation (g(2)(τ)) spectroscopy and photoluminescence (PL) intensity-dependent lifetime analysis. Room-temperature radiative lifetimes of NPLs can be derived from maximum PL intensity periods in PL time traces. It first decreases with NPL lateral size and then stays constant, deviating from the electric dipole approximation. Analysis of the PL time traces further reveals that the single exciton quantum yield in NPLs decreases with NPL lateral size and increases with protecting shell thickness, indicating the importance of surface passivation on NPL emission quality. Second-order photon correlation (g(2)(τ)) studies of single NPLs show that the biexciton quantum yield is strongly dependent on the lateral size and single exciton quantum yield of the NPLs. In large NPLs with unity single exciton quantum yield, the corresponding biexciton quantum yield can reach unity. These findings reveal that by careful growth control and core-shell material engineering, NPLs can be of great potential for light amplification and integrated quantum photonic applications.

8.
Nat Commun ; 8(1): 143, 2017 07 26.
Artigo em Inglês | MEDLINE | ID: mdl-28747633

RESUMO

Colloidal semiconductor nanocrystals have emerged as promising active materials for solution-processable optoelectronic and light-emitting devices. In particular, the development of nanocrystal lasers is currently experiencing rapid progress. However, these lasers require large pump powers, and realizing an efficient low-power nanocrystal laser has remained a difficult challenge. Here, we demonstrate a nanolaser using colloidal nanocrystals that exhibits a threshold input power of less than 1 µW, a very low threshold for any laser using colloidal emitters. We use CdSe/CdS core-shell nanoplatelets, which are efficient nanocrystal emitters with the electronic structure of quantum wells, coupled to a photonic-crystal nanobeam cavity that attains high coupling efficiencies. The device achieves stable continuous-wave lasing at room temperature, which is essential for many photonic and optoelectronic applications. Our results show that colloidal nanocrystals are suitable for compact and efficient optoelectronic devices based on versatile and inexpensive solution-processable materials.Colloidal nanocrystals are a promising material for easy-to-fabricate nanolasers, but suffer from high threshold powers. Here, the authors combine colloidal quantum wells with a photonic-crystal cavity into a stable, continuous-wave room-temperature nanolaser with a threshold below one microwatt.

9.
Science ; 357(6349): 385-388, 2017 07 28.
Artigo em Inglês | MEDLINE | ID: mdl-28751606

RESUMO

Photolithography is an important manufacturing process that relies on using photoresists, typically polymer formulations, that change solubility when illuminated with ultraviolet light. Here, we introduce a general chemical approach for photoresist-free, direct optical lithography of functional inorganic nanomaterials. The patterned materials can be metals, semiconductors, oxides, magnetic, or rare earth compositions. No organic impurities are present in the patterned layers, which helps achieve good electronic and optical properties. The conductivity, carrier mobility, dielectric, and luminescence properties of optically patterned layers are on par with the properties of state-of-the-art solution-processed materials. The ability to directly pattern all-inorganic layers by using a light exposure dose comparable with that of organic photoresists provides an alternate route for thin-film device manufacturing.

10.
J Am Chem Soc ; 138(35): 11109-12, 2016 09 07.
Artigo em Inglês | MEDLINE | ID: mdl-27518932

RESUMO

We report measurements of electron transfer rates for four isoenergetic donor-acceptor pairs comprising a molecular electron acceptor, methylviologen (MV), and morphology-controlled colloidal semiconductor nanoparticles of CdSe. The four nanoparticles include a spherical quantum dot (QD) and three differing lateral areas of 4-monolayer-thick nanoplatelets (NPLs), each with a 2.42 eV energy gap. As such, the measurements, performed via ultrafast photoluminescence, relate the dependence of charge transfer rate on the spatial extent of the initial electron-hole pair wave function explicitly, which we show for the first time to be related to surface area in this regime that is intermediate between homogeneous and heterogeneous charge transfer as well as 2D to 0D electron transfer. The observed nonlinear dependence of rate with surface area is attributed to exciton delocalization within each structure, which we show via temperature-dependent absorption measurements remains constant.

11.
J Am Chem Soc ; 138(31): 9771-4, 2016 08 10.
Artigo em Inglês | MEDLINE | ID: mdl-27441723

RESUMO

Semiconductor quantum rings are of great fundamental interest because their non-trivial topology creates novel physical properties. At the same time, toroidal topology is difficult to achieve for colloidal nanocrystals and epitaxially grown semiconductor nanostructures. In this work, we introduce the synthesis of luminescent colloidal CdSe nanorings and nanostructures with double and triple toroidal topology. The nanorings form during controlled etching and rearrangement of two-dimensional nanoplatelets. We discuss a possible mechanism of the transformation of nanoplatelets into nanorings and potential utility of colloidal nanorings for magneto-optical (e.g., Aharonov-Bohm effect) and other applications.

12.
ACS Nano ; 10(6): 5769-81, 2016 06 28.
Artigo em Inglês | MEDLINE | ID: mdl-27203222

RESUMO

Semiconductor nanorods can emit linear-polarized light at efficiencies over 80%. Polarization of light in these systems, confirmed through single-rod spectroscopy, can be explained on the basis of the anisotropy of the transition dipole moment and dielectric confinement effects. Here we report emission polarization in macroscopic semiconductor-polymer composite films containing CdSe/CdS nanorods and colloidal CdSe nanoplatelets. Anisotropic nanocrystals dispersed in polymer films of poly butyl-co-isobutyl methacrylate (PBiBMA) can be stretched mechanically in order to obtain unidirectionally aligned arrays. A high degree of alignment, corresponding to an orientation factor of 0.87, was achieved and large areas demonstrated polarized emission, with the contrast ratio I∥/I⊥ = 5.6, making these films viable candidates for use in liquid crystal display (LCD) devices. To some surprise, we observed significant optical anisotropy and emission polarization for 2D CdSe nanoplatelets with the electronic structure of quantum wells. The aligned nanorod arrays serve as optical funnels, absorbing unpolarized light and re-emitting light from deep-green to red with quantum efficiencies over 90% and high degree of linear polarization. Our results conclusively demonstrate the benefits of anisotropic nanostructures for LCD backlighting. The polymer films with aligned CdSe/CdS dot-in-rod and rod-in-rod nanostructures show more than 2-fold enhancement of brightness compared to the emitter layers with randomly oriented nanostructures. This effect can be explained as the combination of linearly polarized luminescence and directional emission from individual nanostructures.

14.
J Phys Chem Lett ; 6(6): 1032-6, 2015 Mar 19.
Artigo em Inglês | MEDLINE | ID: mdl-26262865

RESUMO

Using time-resolved photoluminescence spectroscopy, we show that two-exciton Auger recombination dominates carrier recombination and cooling dynamics in CdSe nanoplatelets, or colloidal quantum wells. The electron-hole recombination rate depends only on the number of electron-hole pairs present in each nanoplatelet, and is consistent with a two-exciton recombination process over a wide range of exciton densities. The carrier relaxation rate within the conduction and valence bands also depends only on the number of electron-hole pairs present, apart from an initial rapid decay, and is consistent with the cooling rate being limited by reheating due to Auger recombination processes. These Auger-limited recombination and relaxation dynamics are qualitatively different from the carrier dynamics in either colloidal quantum dots or epitaxial quantum wells.

15.
ACS Nano ; 9(10): 9475-85, 2015 Oct 27.
Artigo em Inglês | MEDLINE | ID: mdl-26302368

RESUMO

There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue-emitting NPLs, the ASE threshold is 50 µJ/cm(2), lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights the qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.


Assuntos
Compostos de Cádmio/química , Coloides/química , Pontos Quânticos/química , Compostos de Selênio/química , Substâncias Luminescentes/química , Medições Luminescentes , Pontos Quânticos/ultraestrutura
17.
Nat Mater ; 14(5): 484-9, 2015 May.
Artigo em Inglês | MEDLINE | ID: mdl-25774956

RESUMO

Fluorescence resonance energy transfer (FRET) enables photosynthetic light harvesting, wavelength downconversion in light-emitting diodes (LEDs), and optical biosensing schemes. The rate and efficiency of this donor to acceptor transfer of excitation between chromophores dictates the utility of FRET and can unlock new device operation motifs including quantum-funnel solar cells, non-contact chromophore pumping from a proximal LED, and markedly reduced gain thresholds. However, the fastest reported FRET time constants involving spherical quantum dots (0.12-1 ns; refs 7-9) do not outpace biexciton Auger recombination (0.01-0.1 ns; ref. 10), which impedes multiexciton-driven applications including electrically pumped lasers and carrier-multiplication-enhanced photovoltaics. Few-monolayer-thick semiconductor nanoplatelets (NPLs) with tens-of-nanometre lateral dimensions exhibit intense optical transitions and hundreds-of-picosecond Auger recombination, but heretofore lack FRET characterizations. We examine binary CdSe NPL solids and show that interplate FRET (∼6-23 ps, presumably for co-facial arrangements) can occur 15-50 times faster than Auger recombination and demonstrate multiexcitonic FRET, making such materials ideal candidates for advanced technologies.

18.
J Am Chem Soc ; 136(32): 11228-31, 2014 Aug 13.
Artigo em Inglês | MEDLINE | ID: mdl-25066770

RESUMO

Colloidal nanomaterials represent an important branch of modern chemistry. However, we have very little understanding of molecular processes that occur at the nanocrystal (NC) surface during synthesis and post-synthetic modifications. Here we show that potentiometry can be used to study the surface of colloidal NCs under realistic reaction conditions. Potentiometric titrations of CdSe and InP nanostructures provide information on the active surface area, the affinity of ligands to the NC surface, and the surface reaction kinetics. These studies can be carried out at different temperatures in polar and nonpolar media for NCs of different sizes and shapes. In situ potentiometry can provide real-time feedback during synthesis of core-shell nanostructures.


Assuntos
Coloides/química , Eletroquímica/métodos , Nanoestruturas/química , Nanotecnologia/métodos , Potenciometria/métodos , Compostos de Cádmio/química , Desenho de Equipamento , Ligantes , Nanopartículas/química , Pontos Quânticos , Compostos de Selênio/química , Solventes/química , Propriedades de Superfície , Temperatura
19.
Nano Lett ; 14(5): 2772-7, 2014 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-24773282

RESUMO

The use of colloidal semiconductor nanocrystals for optical amplification and lasing has been limited by the need for high input power densities. Here we show that colloidal nanoplatelets produce amplified spontaneous emission with thresholds as low as 6 µJ/cm(2) and gain as high as 600 cm(-1), both a significant improvement over colloidal nanocrystals; in addition, gain saturation occurs at pump fluences 2 orders of magnitude higher than the threshold. We attribute this exceptional performance to large optical cross-sections, slow Auger recombination rates, and narrow ensemble emission line widths.

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