Ramya Chulliyil | IIT Bombay (original) (raw)

Papers by Ramya Chulliyil

Pseudo-halide (SCN-) doped MAPbI3 Perovskites: A Few Surprises

The Journal of Physical Chemistry Letters, 2015

Pseudo-halide thiocyanate (SCN-) anion has been used as a dopant in methyl ammonium lead tri-iodi... more Pseudo-halide thiocyanate (SCN-) anion has been used as a dopant in methyl ammonium lead tri-iodide (MAPbI3) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN- pseudo-halide anion, as verified using FT-IR spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI3-x(SCN)x as compared to MAPbI3, suggestive of suppression of non-radiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI3-x(SCN)x micro-crystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics between the grain boundaries and the bulk of the doped material. Spatially-resolved emission spectroscopy on individual micro-crystals of MAPbI3-x(SCN)x unveils the optical bandgap and density of states at various (local) nanodomains are also non-uniform. Surprisingly, several (local) emissive regions within MAPbI3-x(SCN)x micro-crystals are found to be optically unstable under photo irradiation, and display unambiguous temporal intermittency in emission(blinking), which is extremely unusual and intriguing. We find diverse blinking behaviours for the undoped MAPbI3 crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials.

Europium-doped LaF 3 nanocrystals with organic 9-oxidophenalenone capping ligands that display visible light excitable steady-state blue and time-delayed red emission

Dalton Trans., 2015

Visible light excitable and color tunable ∼5% Eu(3+)-doped LaF3 nanocrystals (NCs), containing 9-... more Visible light excitable and color tunable ∼5% Eu(3+)-doped LaF3 nanocrystals (NCs), containing 9-oxidophenalenone ligands bound to the surface as visible light sensitizers for Eu(3+) dopants, have been synthesized by a facile solution-based method. The crystalline phase structure, size, composition, morphology and luminescence properties of the NCs are characterized using X-ray diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and steady-state and time-resolved emission spectroscopy. The results show that these NCs are very small in size (<10 nm), display high degree of crystallinity and have pure tysonite structure of LaF3 with P3[combining macron]c1 space group. The visible light excitation of the capping ligands triggers an alternate display of steady-state, short-lived blue emission (τ < 1 ns) and time-delayed, long-lived sensitized red Eu(3+) emission (τ = 0.41 ms), allowing photoluminescence chromacity tuning as a function of delay time within a specific inorganic composition. The visible light sensitization of the dopant Eu(3+) sites proves more efficient than direct excitation of 5% Eu(3+)-doped LaF3 NCs capped by citrate ligands. The dopant Eu(3+) ions are well protected from non-radiative deactivation through high-energy vibrations of the organic capping ligands which is proved by the long lifetime of the sensitized Eu(3+) emission. The time-resolved emission spectra collected over a period of several milliseconds reveal that the dopant Eu(3+) ions occupy at least three different sites in the NC host. It is further inferred that the sensitized Eu(3+) emission primarily comes from surface dopant sites and sites just underneath the surface of the NCs. We propose that some of the interior Eu(3+) sites also display sensitized emission, which are indirectly populated via Eu(3+) → Eu(3+) energy migration from surface-sensitized Eu(3+) sites of the NCs.

Traditional CdSe-based colloidal quantum dots (cQDs) have interesting photoluminescence (PL) prop... more Traditional CdSe-based colloidal quantum dots
(cQDs) have interesting photoluminescence (PL) properties.
Herein we highlight the advantages in both ensemble and
single-nanocrystal PL of colloidal CsPbBr3 nanocrystals
(NCs) over the traditional cQDs. An ensemble of colloidal
CsPbBr3 NCs (11 nm) exhibits ca. 90% PL quantum yield
with narrow (FWHM=86 meV) spectral width. Interestingly,
the spectral width of a single-NC and an ensemble are almost
identical, ruling out the problem of size-distribution in PL
broadening. Eliminating this problem leads to a negligible
influence of self-absorption and Fçrster resonance energy
transfer, along with batch-to-batch reproducibility of NCs
exhibiting PL peaks within 1 nm. Also, PL peak positions do
not alter with measurement temperature in the range of 25 to
100 8C. Importantly, CsPbBr3 NCs exhibit suppressed PL
blinking with ca. 90% of the individual NCs remain mostly
emissive (on-time >85%), without much influence of excitation
power.

Pseudo-halide (SCN-) doped MAPbI3 Perovskites: A Few Surprises

The Journal of Physical Chemistry Letters, 2015

Pseudo-halide thiocyanate (SCN-) anion has been used as a dopant in methyl ammonium lead tri-iodi... more Pseudo-halide thiocyanate (SCN-) anion has been used as a dopant in methyl ammonium lead tri-iodide (MAPbI3) framework, aiming for its use as an absorber layer for photovoltaic applications. The substitution of SCN- pseudo-halide anion, as verified using FT-IR spectroscopy, results in a comprehensive effect on the optical properties of the original material. Photoluminescence measurements at room temperature reveal a significant enhancement in the emission quantum yield of MAPbI3-x(SCN)x as compared to MAPbI3, suggestive of suppression of non-radiative channels. This increased intensity is attributed to a highly edge specific emission from MAPbI3-x(SCN)x micro-crystals as revealed by photoluminescence microscopy. Fluoresence lifetime imaging measurements further established contrasting carrier recombination dynamics between the grain boundaries and the bulk of the doped material. Spatially-resolved emission spectroscopy on individual micro-crystals of MAPbI3-x(SCN)x unveils the optical bandgap and density of states at various (local) nanodomains are also non-uniform. Surprisingly, several (local) emissive regions within MAPbI3-x(SCN)x micro-crystals are found to be optically unstable under photo irradiation, and display unambiguous temporal intermittency in emission(blinking), which is extremely unusual and intriguing. We find diverse blinking behaviours for the undoped MAPbI3 crystals as well, which leads us to speculate that blinking may be a common phenomenon for most hybrid perovskite materials.

Europium-doped LaF 3 nanocrystals with organic 9-oxidophenalenone capping ligands that display visible light excitable steady-state blue and time-delayed red emission

Dalton Trans., 2015

Visible light excitable and color tunable ∼5% Eu(3+)-doped LaF3 nanocrystals (NCs), containing 9-... more Visible light excitable and color tunable ∼5% Eu(3+)-doped LaF3 nanocrystals (NCs), containing 9-oxidophenalenone ligands bound to the surface as visible light sensitizers for Eu(3+) dopants, have been synthesized by a facile solution-based method. The crystalline phase structure, size, composition, morphology and luminescence properties of the NCs are characterized using X-ray diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy, and steady-state and time-resolved emission spectroscopy. The results show that these NCs are very small in size (<10 nm), display high degree of crystallinity and have pure tysonite structure of LaF3 with P3[combining macron]c1 space group. The visible light excitation of the capping ligands triggers an alternate display of steady-state, short-lived blue emission (τ < 1 ns) and time-delayed, long-lived sensitized red Eu(3+) emission (τ = 0.41 ms), allowing photoluminescence chromacity tuning as a function of delay time within a specific inorganic composition. The visible light sensitization of the dopant Eu(3+) sites proves more efficient than direct excitation of 5% Eu(3+)-doped LaF3 NCs capped by citrate ligands. The dopant Eu(3+) ions are well protected from non-radiative deactivation through high-energy vibrations of the organic capping ligands which is proved by the long lifetime of the sensitized Eu(3+) emission. The time-resolved emission spectra collected over a period of several milliseconds reveal that the dopant Eu(3+) ions occupy at least three different sites in the NC host. It is further inferred that the sensitized Eu(3+) emission primarily comes from surface dopant sites and sites just underneath the surface of the NCs. We propose that some of the interior Eu(3+) sites also display sensitized emission, which are indirectly populated via Eu(3+) → Eu(3+) energy migration from surface-sensitized Eu(3+) sites of the NCs.

Traditional CdSe-based colloidal quantum dots (cQDs) have interesting photoluminescence (PL) prop... more Traditional CdSe-based colloidal quantum dots
(cQDs) have interesting photoluminescence (PL) properties.
Herein we highlight the advantages in both ensemble and
single-nanocrystal PL of colloidal CsPbBr3 nanocrystals
(NCs) over the traditional cQDs. An ensemble of colloidal
CsPbBr3 NCs (11 nm) exhibits ca. 90% PL quantum yield
with narrow (FWHM=86 meV) spectral width. Interestingly,
the spectral width of a single-NC and an ensemble are almost
identical, ruling out the problem of size-distribution in PL
broadening. Eliminating this problem leads to a negligible
influence of self-absorption and Fçrster resonance energy
transfer, along with batch-to-batch reproducibility of NCs
exhibiting PL peaks within 1 nm. Also, PL peak positions do
not alter with measurement temperature in the range of 25 to
100 8C. Importantly, CsPbBr3 NCs exhibit suppressed PL
blinking with ca. 90% of the individual NCs remain mostly
emissive (on-time >85%), without much influence of excitation
power.