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1.
J Am Chem Soc ; 2020 Jul 23.
Artigo em Inglês | MEDLINE | ID: mdl-32650641

RESUMO

The synthetic tunability, flexibility, and rich spin physics of semiconductor quantum dots (QDs) make them promising candidates for quantum information science applications. However, the rapid spin relaxation observed in colloidal quantum dots limits their functionality. In the current work, we demonstrate a method to harness photoexcited spin states in QDs to produce long-lived spin polarization on an appended organic ligand molecule. We present a system composed of CdSe/CdS core/shell QDs, covalently linked to naphthalenediimide (NDI) electron-accepting molecules. The electron transfer dynamics from photoexcited QDs to the appended NDI ligands is explored as a function of both shell thickness and number of NDIs per QD. Transient EPR spectroscopy shows that the photoexcited QDs strongly spin polarize the NDI radical anion, which is interpreted in the context of both the radical pair and the triplet mechanisms of spin polarization. This work serves as an initial step toward using photoexcited QDs to strongly spin polarize organic radicals having long spin relaxation times to serve as spin qubits in quantum information science applications.

2.
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.

3.
Proc Natl Acad Sci U S A ; 116(17): 8178-8183, 2019 Apr 23.
Artigo em Inglês | MEDLINE | ID: mdl-30948629

RESUMO

Singlet fission (SF) is a photophysical process in which one of two adjacent organic molecules absorbs a single photon, resulting in rapid formation of a correlated triplet pair (T1T1) state whose spin dynamics influence the successful generation of uncorrelated triplets (T1). Femtosecond transient visible and near-infrared absorption spectroscopy of a linear terrylene-3,4:11,12-bis(dicarboximide) dimer (TDI2), in which the two TDI molecules are directly linked at one of their imide positions, reveals ultrafast formation of the (T1T1) state. The spin dynamics of the (T1T1) state and the processes leading to uncoupled triplets (T1) were studied at room temperature for TDI2 aligned in 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal. Time-resolved electron paramagnetic resonance spectroscopy shows that the (T1T1) state has mixed 5(T1T1) and 3(T1T1) character at room temperature. This mixing is magnetic field dependent, resulting in a maximum triplet yield at ∼200 mT. The accessibility of the 3(T1T1) state opens a pathway for triplet-triplet annihilation that produces a single uncorrelated T1 state. The presence of the 5(T1T1) state at room temperature and its relationship with the 1(T1T1) and 3(T1T1) states emphasize that understanding the relationship among different (T1T1) spin states is critical for ensuring high-yield T1 formation from singlet fission.

4.
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.

5.
Nano Lett ; 18(11): 6948-6953, 2018 11 14.
Artigo em Inglês | MEDLINE | ID: mdl-30244582

RESUMO

Excimers, a portmanteau of "excited dimer", are transient species that are formed from the electronic interaction of a fluorophore in the excited state with a neighbor in the ground state, which have found extensive use as laser gain media. Although common in molecular fluorophores, this work presents evidence for the formation of excimers in a new class of materials: atomically precise two-dimensional semiconductor nanoplatelets. Colloidal nanoplatelets of CdSe display two-color photoluminescence resolved at low temperatures with one band attributed to band-edge fluorescence and a second, red band attributed to excimer fluorescence. Previously reasonable explanations for two-color fluorescence, such as charging, are shown to be inconsistent with additional evidence. As with excimers in other materials systems, excimer emission is increased by increasing nanoplatelet concentration and the degree of cofacial stacking. Consistent with their promise as low-threshold gain media, amplified spontaneous emission emerges from the excimer emission line.

6.
J Phys Chem Lett ; 9(16): 4481-4487, 2018 Aug 16.
Artigo em Inglês | MEDLINE | ID: mdl-30011208

RESUMO

We report femtosecond stimulated Raman spectroscopy (FSRS) measurements on dispersions of CdSe semiconductor nanocrystals (NCs) as a function of particle size and pump fluence. Upon photoexcitation, we observe depletion of stimulated Raman gain corresponding to generation of longitudinal optical (LO) phonons followed by recovery on picosecond timescales. At higher fluences, production of multiple excitons slows recovery of FSRS signals, which we attribute to sustained increases of LO phonon populations due to multiexcitonic Auger heating. Owing to the discretized electronic structure of these NCs, such heating cannot be readily monitored via electronic spectroscopic analysis of high-energy band tails as has been performed for higher-dimensional materials. Notably, recovery timescales exceed those of the biexcitonic Auger recombination process and as such reveal overall thermalization timescales likely owing to an acoustic phonon thermalization bottleneck that dictates the cooling timescale.

7.
Nano Lett ; 18(8): 4771-4776, 2018 08 08.
Artigo em Inglês | MEDLINE | ID: mdl-29944381

RESUMO

Films containing mixtures of zero- or two-dimensional nanostructures (quantum dots or nanoplatelets) were prepared in order to investigate the impacts of dimensionality on electronic interactions. Electron transfer from CsPbBr3 to CdSe was observed in all of the mixtures, regardless of particle dimensionality, and characterized via both static and transient absorption and photoluminescence spectroscopies. We find that mixtures containing nanoplatelets as the electron acceptor (CdSe) undergo charge transfer more rapidly than those containing quantum dots. We believe the faster charge transfer observed with nanoplatelets may arise from the extended spatial area of the CdSe nanoplatelets and/or the continuous density of acceptor states that are present in nanoplatelets. These results bolster the use of one- or two-dimensional nanomaterials in the place of zero-dimensional quantum dots in the design of related optoelectronic devices such as solar cells, light-emitting diodes, and photocatalysts and further offer the prospect of fewer required hopping events to transport carriers due to the larger spatial extent of the particles.

8.
Nanoscale ; 8(38): 16841-5, 2016 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-27345192

RESUMO

High refractive index sensitivity (RIS) of branched Au-Pd nanocrystals (NCs) is engineered through lowering the dielectric dispersion at the NC resonant wavelength with internal or external atomic % Pd. To our knowledge, these NCs display the highest ensemble RIS measurement for colloids with LSPR maximum band positions ≤900 nm, and these results are corroborated with FDTD computations.

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