Loredana Protesescu - Academia.edu (original) (raw)

Papers by Loredana Protesescu

Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile ... more Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX3, where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics.

The Journal of Physical Chemistry Letters, 2019

ACS Central Science, 2018

Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is... more Understanding the surface of semiconductor nanocrystals (NCs) prepared using colloidal methods is a longstanding goal of paramount importance for all their potential optoelectronic applications, which remains unsolved largely because of the lack of site-specific physical techniques. Here, we show that multidimensional 113 Cd dynamic nuclear polarization (DNP) enhanced NMR spectroscopy allows the resolution of signals originating from different atomic and magnetic surroundings in the NC cores and at the surfaces. This enables the determination of the structural perfection, and differentiation between the surface and core atoms in all major forms of size-and shape-engineered CdSe NCs: irregularly faceted quantum dots (QDs) and atomically flat nanoplatelets, including both dominant polymorphs (zinc-blende and wurtzite) and their epitaxial nanoheterostructures (CdSe/CdS core/shell quantum dots and CdSe/CdS core/crown nanoplatelets), as well as magic-sized CdSe clusters. Assignments of the NMR signals to specific crystal facets of oleate-terminated ZB structured CdSe NCs are proposed. Significantly, we discover far greater atomistic complexity of the surface structure and the species distribution in wurtzite as compared to zinc-blende CdSe QDs, despite an apparently identical optical quality of both QD polymorphs.

Science (New York, N.Y.), Nov 10, 2017

Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-film absorber m... more Semiconducting lead halide perovskites (LHPs) have not only become prominent thin-film absorber materials in photovoltaics but have also proven to be disruptive in the field of colloidal semiconductor nanocrystals (NCs). The most important feature of LHP NCs is their so-called defect-tolerance-the apparently benign nature of structural defects, highly abundant in these compounds, with respect to optical and electronic properties. Here, we review the important differences that exist in the chemistry and physics of LHP NCs as compared with more conventional, tetrahedrally bonded, elemental, and binary semiconductor NCs (such as silicon, germanium, cadmium selenide, gallium arsenide, and indium phosphide). We survey the prospects of LHP NCs for optoelectronic applications such as in television displays, light-emitting devices, and solar cells, emphasizing the practical hurdles that remain to be overcome.

ACS applied nano materials, Jan 23, 2018

Lead halide perovskites of APbX type [A = Cs, formamidinium (FA), methylammonium; X = Br, I] in t... more Lead halide perovskites of APbX type [A = Cs, formamidinium (FA), methylammonium; X = Br, I] in the form of ligand-capped colloidal nanocrystals (NCs) are widely studied as versatile photonic sources. FAPbBr and CsPbBr NCs have become promising as spectrally narrow green primary emitters in backlighting of liquid-crystal displays (peak at 520-530 nm, full width at half-maximum of 22-30 nm). Herein, we report that wet ball milling of bulk APbBr (A = Cs, FA) mixed with solvents and capping ligands yields green luminescent colloidal NCs with a high overall reaction yield and optoelectronic quality on par with that of NCs of the same composition obtained by hot-injection method. We emphasize the superiority of oleylammonium bromide as a capping ligand used for this procedure over the standard oleic acid and oleylamine. We also show a mechanically induced anion-exchange reaction for the formation of orange-emissive CsPb(Br/I) NCs.

Chemistry of Materials, 2017

Take-down policy If you believe that this document breaches copyright please contact us providing... more Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Small, 2017

The optical properties of the newly developed near‐infrared emitting formamidinium lead triiodide... more The optical properties of the newly developed near‐infrared emitting formamidinium lead triiodide (FAPbI3) nanocrystals (NCs) and their polycrystalline thin film counterpart are comparatively investigated by means of steady‐state and time‐resolved photoluminescence. The excitonic emission is dominant in NC ensemble because of the localization of electron–hole pairs. A promisingly high quantum yield above 70%, and a large absorption cross‐section (5.2 × 10−13 cm−2) are measured. At high pump fluence, biexcitonic recombination is observed, featuring a slow recombination lifetime of 0.4 ns. In polycrystalline thin films, the quantum efficiency is limited by nonradiative trap‐assisted recombination that turns to bimolecular at high pump fluences. From the temperature‐dependent photoluminescence (PL) spectra, a phase transition is clearly observed in both NC ensemble and polycrystalline thin film. It is interesting to note that NC ensemble shows PL temperature antiquenching, in contrast ...

ACS nano, Jan 25, 2017

Crystal defects in highy luminescent colloidal nanocrystals (NCs) of CsPbX3 perovskites (X = Cl, ... more Crystal defects in highy luminescent colloidal nanocrystals (NCs) of CsPbX3 perovskites (X = Cl, Br, I) are investigated. Here, using X-ray total scattering techniques and the Debye scattering equation (DSE), we provide evidence that the local structure of these NCs always exhibits orthorhombic tilting of PbX6 octahedra within locally ordered subdomains. These subdomains are hinged through a two-/three-dimensional (2D/3D) network of twin boundaries through which the coherent arrangement of the Pb ions throughout the whole NC is preserved. The density of these twin boundaries determines the size of the subdomains and results in an apparent higher-symmetry structure on average in the high-temperature modification. Dynamic cooperative rotations of PbX6 octahedra are likely at work at the twin boundaries, causing the rearrangement of the 2D or 3D network, particularly effective in the pseudocubic phases. An orthorhombic, 3D γ-phase, isostructural to that of CsPbBr3 is found here in as-s...

ACS nano, Mar 28, 2017

Colloidal nanocrystals (NCs) of APbX3-type lead halide perovskites [A = Cs(+), CH3NH3(+) (methyla... more Colloidal nanocrystals (NCs) of APbX3-type lead halide perovskites [A = Cs(+), CH3NH3(+) (methylammonium or MA(+)) or CH(NH2)2(+) (formamidinium or FA(+)); X = Cl(-), Br(-), I(-)] have recently emerged as highly versatile photonic sources for applications ranging from simple photoluminescence down-conversion (e.g., for display backlighting) to light-emitting diodes. From the perspective of spectral coverage, a formidable challenge facing the use of these materials is how to obtain stable emissions in the red and infrared spectral regions covered by the iodide-based compositions. So far, red-emissive CsPbI3 NCs have been shown to suffer from a delayed phase transformation into a nonluminescent, wide-band-gap 1D polymorph, and MAPbI3 exhibits very limited chemical durability. In this work, we report a facile colloidal synthesis method for obtaining FAPbI3 and FA-doped CsPbI3 NCs that are uniform in size (10-15 nm) and nearly cubic in shape and exhibit drastically higher robustness tha...

Journal of the American Chemical Society, 2016

Bright green emitters with adjustable photoluminescence (PL) maxima in the range of 530− 535 nm a... more Bright green emitters with adjustable photoluminescence (PL) maxima in the range of 530− 535 nm and full-width at half-maxima (fwhm) of <25 nm are particularly desirable for applications in television displays and related technologies. Toward this goal, we have developed a facile synthesis of highly monodisperse, cubic-shaped formamidinium lead bromide nanocrystals (FAPbBr 3 NCs) with perovskite crystal structure, tunable PL in the range of 470−540 nm by adjusting the nanocrystal size (5−12 nm), high quantum yield (QY) of up to 85% and PL fwhm of <22 nm. High QYs are also retained in films of FAPbBr 3 NCs. In addition, these films exhibit low thresholds of 14 ± 2 μJ cm −2 for amplified spontaneous emission.

Nano letters, Jan 25, 2016

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bri... more Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum e...

ACS Nano, 2016

Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile ... more Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX 3 , where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics.

ACS Nano, 2015

While over the last years the syntheses of colloidal quantum dots (CQDs) with core/shell structur... more While over the last years the syntheses of colloidal quantum dots (CQDs) with core/shell structures were continuously improved to obtain highly efficient emission, it has remained a challenge to use them as active materials in laser devices. Here, we report on a successful demonstration of random lasing at room temperature in films of CdSe/CdS CQDs with different core/shell band alignments and extra thick shells. Even though the lasing process is based on random scattering, we find systematic dependencies of the laser thresholds on film morphology and excitation spot size. This systematics suggests that random lasing experiments are a valuable tool for testing nanocrystal materials, providing a direct and simple feedback for the further development of colloidal gain materials towards lasing in continuous wave operation.

ACS Applied Materials & Interfaces, 2015

Here we report for the first time an H 2-evolving photocathode fabricated by a solution-processed... more Here we report for the first time an H 2-evolving photocathode fabricated by a solution-processed organic−inorganic hybrid composed of CdSe and P3HT. The CdSe:P3HT (10:1 (w/w)) hybrid bulk heterojunction treated with 1,2-ethanedithiol (EDT) showed efficient water reduction and hydrogen generation. A photocurrent of −1.24 mA/cm 2 at 0 V versus reversible hydrogen electrode (V RHE), EQE of 15%, and an unprecedented V oc of 0.85 V RHE under illumination of AM1.5G (100 mW/cm 2) in mild electrolyte were observed. Time-resolved photoluminescence (TRPL), internal quantum efficiency (IQE), and transient photocurrent measurements were carried out to clarify the carrier dynamics of the hybrids. The exciton lifetime of CdSe was reduced by one order of magnitude in the hybrid blend, which is a sign of the fast charge separation upon illumination. By comparing the current magnitude of the solid-state devices and water-splitting devices made with identical active layers, we found that the interfaces of the water-splitting devices limit the device performance. The electron/hole transport properties investigated by comparing IQE spectra upon front-and backside illumination evidenced balanced electron/hole transport. The Faradaic efficiency is 80−100% for the hybrid photocathodes with Pt catalysts and ∼70% for the one without Pt catalysts.

Nano Letters, 2015

Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions,... more Postsynthetic chemical transformations of colloidal nanocrystals, such as ion-exchange reactions, provide an avenue to compositional fine-tuning or to otherwise inaccessible materials and morphologies. While cation-exchange is facile and commonplace, anion-exchange reactions have not received substantial deployment. Here we report fast, low-temperature, deliberately partial, or complete anionexchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX 3 , X = Cl, Br, I). By adjusting the halide ratios in the colloidal nanocrystal solution, the bright photoluminescence can be tuned over the entire visible spectral region (410−700 nm) while maintaining high quantum yields of 20− 80% and narrow emission line widths of 10−40 nm (from blue to red). Furthermore, fast internanocrystal anion-exchange is demonstrated, leading to uniform CsPb(Cl/ Br) 3 or CsPb(Br/I) 3 compositions simply by mixing CsPbCl 3 , CsPbBr 3 , and CsPbI 3 nanocrystals in appropriate ratios.

Journal of Materials Chemistry C, 2015

Charge transfer between PbS QDs and PCPDTBT can only be observed in presence of short ligands.

Advanced Energy Materials, 2014

PbS quantum dots (QDs) of different sizes capped with short (NH 4) 3 AsS 3 inorganic ligands are ... more PbS quantum dots (QDs) of different sizes capped with short (NH 4) 3 AsS 3 inorganic ligands are produced via ligand exchange processes from oleatecapped PbS QDs. The solid-state photophysical properties of the control organic-capped and the inorganic-ligand-capped QDs are investigated to determine their potential for optoelectronic applications. Ultrafast transient transmission shows that in the oleate-capped QDs, carrier recombination at sub-nanosecond scales occurs via Auger recombination, traps, and surface states. At longer times, intense signals associated with radiative recombination are obtained. After ligand exchange, the QDs become decorated with (NH 4) 3 AsS 3 complexes and relaxation is dominated by effi cient carrier transfer to the ligand states on timescales as fast as ≈2 ps, which competes with carrier thermalization to the QD band edge states. Recombination channels present in the oleate-capped QDs, such as radiative and Auger recombination, appear quenched in the inorganic-capped QDs. Evidence of effi cient carrier trapping at shallow ligand states, which appears more intense under excitation above the (NH 4) 3 AsS 3 gap, is provided. A detailed band diagram of the various relaxation and recombination processes is proposed that comprehensively describes the photophysics of the QD systems studied.

ACS Nano, 2014

The Journal of Physical Chemistry Letters, 2019

ACS Central Science, 2018

Science (New York, N.Y.), Nov 10, 2017

ACS applied nano materials, Jan 23, 2018

Chemistry of Materials, 2017

Small, 2017

ACS nano, Jan 25, 2017

ACS nano, Mar 28, 2017

Journal of the American Chemical Society, 2016

Nano letters, Jan 25, 2016

ACS Nano, 2016

Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile ... more Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX 3 , where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics.

ACS Nano, 2015

ACS Applied Materials & Interfaces, 2015

Nano Letters, 2015

Journal of Materials Chemistry C, 2015

Charge transfer between PbS QDs and PCPDTBT can only be observed in presence of short ligands.

Advanced Energy Materials, 2014

ACS Nano, 2014