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1.
ACS Nano ; 2020 Jul 15.
Artigo em Inglês | MEDLINE | ID: mdl-32644773

RESUMO

Colloidal metal halide perovskite nanocrystals (NCs) with chiral ligands are outstanding candidates as a circularly polarized luminescence (CPL) light source due to many advantages such as high photoluminescence quantum efficiency, large spin-orbit coupling, and extensive tunability via composition and choice of organic ligands. However, achieving pronounced and controllable polarized light emission remains challenging. Here, we develop strategies to achieve high CPL responses from colloidal formamidinium lead bromide (FAPbBr3) NCs at room temperature using chiral surface ligands. First, we show that replacing a portion of typical ligands (oleylamine) with short chiral ligands ((R)-2-octylamine) during FAPbBr3 NC synthesis results in small and monodisperse NCs that yield high CPL with average luminescence dissymmetry g-factor, glum = 6.8 × 10-2. To the best of our knowledge, this is the highest among reported perovskite materials at room temperature to date and represents around 10-fold improvement over the previously reported colloidal CsPbClxBryI3-x-y NCs. In order to incorporate NCs into any optoelectronic or spintronic application, the NCs necessitate purification, which removes a substantial amount of the chiral ligands and extinguishes the CPL signals. To circumvent this issue, we also developed a postsynthetic ligand treatment using a different chiral ligand, (R-/S-)methylbenzylammonium bromide, which also induces a CPL with an average glum = ±1.18 × 10-2. This postsynthetic method is also amenable for long-range charge transport since methylbenzylammonium is quite compact in relation to other surface ligands. Our demonstrations of high CPL and glum from both as-synthesized and purified perovskite NCs at room temperature suggest a route to demonstrate colloidal NC-based spintronics.

2.
Nat Commun ; 10(1): 129, 2019 01 10.
Artigo em Inglês | MEDLINE | ID: mdl-30631053

RESUMO

Recently the hybrid organic-inorganic trihalide perovskites have shown remarkable performance as active layers in photovoltaic and other optoelectronic devices. However, their spin characteristic properties have not been fully studied, although due to the relatively large spin-orbit coupling these materials may show great promise for spintronic applications. Here we demonstrate spin-polarized carrier injection into methylammonium lead bromide films from metallic ferromagnetic electrodes in two spintronic-based devices: a 'spin light emitting diode' that results in circularly polarized electroluminescence emission; and a 'vertical spin valve' that shows giant magnetoresistance. In addition, we also apply a magnetic field perpendicular to the injected spins orientation for measuring the 'Hanle effect', from which we obtain a relatively long spin lifetime for the electrically injected carriers. Our measurements initiate the field of hybrid perovskites spin-related optoelectronic applications.

3.
J Phys Chem Lett ; 10(1): 13-19, 2019 Jan 03.
Artigo em Inglês | MEDLINE | ID: mdl-30556395

RESUMO

The temperature dependence of absorption and photoluminescence (PL) spectroscopies were used to study the optical properties of 2D perovskite films, including n = 1 and 3 of (PEA)2(CH3NH3) n-1[Pb nI3 n+1] (PEA = C6H5(CH2)2NH3). In (PEA)2[Pb1I4] (PEPI), excitons coupling to optical phonons with an average energy of ∼30 meV dominate the photophysics of absorption and PL. (PEA)2(CH3NH3)2[Pb3I10] (shortened as PMPI3), nominally prepared as n = 3, actually was a mixture of multiple layered perovskites with various n. In absorption, a PMPI3 film presents respective n materials' excitonic features, coupling to phonons with an average energy of ∼30 meV; in analyzing PL peaked singly at ∼1.6 eV and its width as a function of temperature, we found that PMPI3 behaves like PEPI at around 80 K but like 3D perovskite near room temperature, with three times larger electron-phonon interaction strength compared to that in PEPI.

4.
J Phys Condens Matter ; 30(48): 484003, 2018 Dec 05.
Artigo em Inglês | MEDLINE | ID: mdl-30418948

RESUMO

Conducting polymers, where pristine polymers are doped by active dopants, have been used in a variety of flexible optoelectronic device applications due to their tunable conductivity values. Charge transport in these materials has been intensively studied for over three decades. However, spin transport properties in these compounds have remained elusive. Here, we studied two polaron-dominated and trap-dominated spin transport processes in two types of PEDOT:PSS polymers that are lightly and heavily doped, respectively. Using pulsed spin-pumping and spin-injection techniques, we found the sign of inverse spin Hall effect and magnetoresistance obtained from the lightly doped PEDOT:PSS film can reverse its polarity as a function of temperature and applied bias, in contrast to that in the heavily doped PEDOT:PSS film. Our work provides an alternative approach for studying the spin transport in conducting polymer films.

5.
Nat Commun ; 8(1): 1328, 2017 11 06.
Artigo em Inglês | MEDLINE | ID: mdl-29105661

RESUMO

Controlling and modulating terahertz signals is of fundamental importance to allow systems level applications. We demonstrate an innovative approach for controlling the propagation properties of terahertz (THz) radiation, through use of both the excitation optical wavelength (colour) and intensity. We accomplish this using two-dimensional (2D) layered hybrid trihalide perovskites that are deposited onto silicon substrates. The absorption properties of these materials in the visible range can be tuned by changing the number of inorganic atomic layers in between the organic cation layers. Optical absorption in 2D perovskites occurs over a broad spectral range above the bandgap, resulting in free carrier generation, as well as over a narrow spectral range near the bandedge due to exciton formation. We find that only the latter contribution gives rise to photo-induced THz absorption. By patterning multiple 2D perovskites with different optical absorption properties onto a single device, we demonstrate both colour selective modulation and focusing of THz radiation. These findings open new directions for creating active THz devices.

6.
Sci Adv ; 3(7): e1700704, 2017 07.
Artigo em Inglês | MEDLINE | ID: mdl-28782030

RESUMO

Two-dimensional (2D) layered hybrid organic-inorganic halide perovskite semiconductors form natural "multiple quantum wells" that have strong spin-orbit coupling due to the heavy elements in their building blocks. This may lead to "Rashba splitting" close to the extrema in the electron bands. We have used a plethora of ultrafast transient, nonlinear optical spectroscopies and theoretical calculations to study the primary (excitons) and long-lived (free carriers) photoexcitations in thin films of 2D perovskite, namely, (C6H5C2H4NH3)2PbI4. The density functional theory calculation shows the occurrence of Rashba splitting in the plane perpendicular to the 2D barrier. From the electroabsorption spectrum and photoinduced absorption spectra from excitons and free carriers, we obtain a giant Rashba splitting in this compound, with energy splitting of (40 ± 5) meV and Rashba parameter of (1.6 ± 0.1) eV·Å, which are among the highest Rashba splitting size parameters reported so far. This finding shows that 2D hybrid perovskites have great promise for potential applications in spintronics.

7.
Angew Chem Int Ed Engl ; 55(16): 5071-5, 2016 Apr 11.
Artigo em Inglês | MEDLINE | ID: mdl-26990250

RESUMO

The key to utilizing quantum dots (QDs) as lasing media is to effectively reduce non-radiative processes, such as Auger recombination and surface trapping. A robust strategy to craft a set of CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS core/graded shell-shell QDs with suppressed re-absorption, reduced Auger recombination rate, and tunable Stokes shift is presented. In sharp contrast to conventional CdSe/ZnS QDs, which have a large energy level mismatch between CdSe and ZnS and thus show strong re-absorption and a constrained Stokes shift, the as-synthesized CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS QDs exhibited the suppressed re-absorption of CdSe core and tunable Stokes shift as a direct consequence of the delocalization of the electron wavefunction over the entire QD. Such Stokes shift-engineered QDs with suppressed re-absorption may represent an important class of building blocks for use in lasers, light emitting diodes, solar concentrators, and parity-time symmetry materials and devices.

8.
Philos Trans A Math Phys Eng Sci ; 373(2044)2015 Jun 28.
Artigo em Inglês | MEDLINE | ID: mdl-25987576

RESUMO

We used steady-state photoinduced absorption (PA), excitation dependence (EXPA(ω)) spectrum of the triplet exciton PA band, and its magneto-PA (MPA(B)) response to investigate singlet fission (SF) of hot excitons into two separated triplet excitons, in two luminescent and non-luminescent π-conjugated polymers. From the high energy step in the triplet EXPA(ω) spectrum of the luminescent polymer poly(dioctyloxy)phenylenevinylene (DOO-PPV) films, we identified a hot-exciton SF (HE-SF) process having threshold energy at E≈2E(T) (=2.8 eV, where ET is the energy of the lowest lying triplet exciton), which is about 0.8 eV above the lowest singlet exciton energy. The HE-SF process was confirmed by the triplet MPA(B) response for excitation at E>2E(T), which shows typical SF response. This process is missing in DOO-PPV solution, showing that it is predominantly interchain in nature. By contrast, the triplet EXPA(ω) spectrum in the non-luminescent polymer polydiacetylene (PDA) is flat with an onset at E=E(g) (≈2.25 eV). From this, we infer that intrachain SF that involves a triplet-triplet pair state, also known as the 'dark' 2A(g) exciton, dominates the triplet photogeneration in PDA polymer as E(g)>2E(T). The intrachain SF process was also identified from the MPA(B) response of the triplet PA band in PDA. Our work shows that the SF process in π-conjugated polymers is a much more general process than thought previously.

9.
Phys Rev Lett ; 114(11): 116601, 2015 Mar 20.
Artigo em Inglês | MEDLINE | ID: mdl-25839297

RESUMO

We studied the ultrafast transient response of photoexcitations in two hybrid organic-inorganic perovskite films used for high efficiency photovoltaic cells, namely, CH(3)NH(3)PbI(3) and CH(3)NH(3)PbI(1.1)Br(1.9) using polarized broadband pump-probe spectroscopy in the spectral range of 0.3-2.7 eV with 300 fs time resolution. For CH(3)NH(3)PbI(3) with above-gap excitation we found both photogenerated carriers and excitons, but only carriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in CH(3)NH(3)PbI(1.1)Br(1.9). Surprisingly, we also discovered in CH(3)NH(3)PbI(3), but not in CH(3)NH(3)PbI(1.1)Br(1.9), transient photoinduced polarization memory for both excitons and photocarriers, which is also reflected in the steady state photoluminescence. From the polarization memory dynamics we obtained the excitons diffusion constant in CH(3)NH(3)PbI(3), D≈0.01 cm(2) s(-1).

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