M. Kovalenko | Swiss Federal Institute of Technology (ETH) (original) (raw)

Papers by M. Kovalenko

Research paper thumbnail of Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I)

Nano letters, Jan 28, 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 anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX3, 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 CsPbCl3, CsPbBr3, and CsPbI3 nanocrystals in appropriate ratios.

Research paper thumbnail of Origin of the increased open circuit voltage in PbS–CdS core–shell quantum dot solar cells

J. Mater. Chem. A, 2015

Lead sulfide quantum dots (PbS QDs) show great potential for efficient, low cost photovoltaic app... more Lead sulfide quantum dots (PbS QDs) show great potential for efficient, low cost photovoltaic applications.

Research paper thumbnail of Photophysics of PbS Quantum Dot Films Capped with Arsenic Sulfide Ligands

Advanced Energy Materials, 2014

ABSTRACT PbS quantum dots (QDs) of different sizes capped with short (NH4)3AsS3 inorganic ligands... more ABSTRACT PbS quantum dots (QDs) of different sizes capped with short (NH4)3AsS3 inorganic ligands are produced via ligand exchange processes from oleate-capped 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 (NH4)3AsS3 complexes and relaxation is dominated by efficient 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 efficient carrier trapping at shallow ligand states, which appears more intense under excitation above the (NH4)3AsS3 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.

Research paper thumbnail of Surface modification of inorganic nanocrystals with oxo-, polyoxometalate ligands

Research paper thumbnail of Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut

Nano letters, Jan 2, 2015

Metal halides perovskites, such as hybrid organic-inorganic CH3NH3PbI3, are newcomer optoelectron... more Metal halides perovskites, such as hybrid organic-inorganic CH3NH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as solution-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. Herein we demonstrate a new avenue for halide perovskites by designing highly luminescent perovskite-based colloidal quantum dot materials. We have synthesized monodisperse colloidal nanocubes (4-15 nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of ...

Research paper thumbnail of Monodisperse colloidal gallium nanoparticles: Synthesis, low temperature crystallization, surface plasmon resonance and Li-ion storage

We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable i... more We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable in the range of 12−46 nm and with excellent size distribution as small as 7− 8%. When stored under ambient conditions, Ga nanoparticles remain stable for months due to the formation of native and passivating Ga-oxide layer (2−3 nm). The mechanism of Ga nanoparticles formation is elucidated using nuclear magnetic resonance spectroscopy and with molecular dynamics simulations. Size-dependent crystallization and melting of Ga nanoparticles in the temperature range of 98−298 K are studied with X-ray powder diffraction, specific heat measurements, transmission electron microscopy, and X-ray absorption spectroscopy. The results point to delta (δ)-Ga polymorph as a single low-temperature phase, while phase transition is characterized by the large hysteresis and by the large undercooling of crystallization and melting points down to 140−145 and 240−250 K, respectively. We have observed size-tunable plasmon resonance in the ultraviolet and visible spectral regions. We also report stable operation of Ga nanoparticles as anode material for Li-ion batteries with storage capacities of 600 mAh g −1 , 50% higher than those achieved for bulk Ga under identical testing conditions.

Research paper thumbnail of Atomistic Description of Thiostannate-Capped CdSe Nanocrystals: Retention of Four-Coordinate SnS4 Motif and Preservation of Cd-Rich Stoichiometry

Journal of the American Chemical Society, 2015

Colloidal semiconductor nanocrystals (NCs) are widely studied as building blocks for novel solid-... more Colloidal semiconductor nanocrystals (NCs) are widely studied as building blocks for novel solid-state materials. Inorganic surface functionalization, used to displace native organic capping ligands from NC surfaces, has been a major enabler of electronic solid-state devices based on colloidal NCs. At the same time, very little is known about the atomistic details of the organic-to-inorganic ligand exchange and binding motifs at the NC surface, severely limiting further progress in designing all-inorganic NCs and NC solids. Taking thiostannates (K4SnS4, K4Sn2S6, K6Sn2S7) as typical examples of chalcogenidometallate ligands and oleate-capped CdSe NCs as a model NC system, in this study we address these questions through the combined application of solution (1)H NMR spectroscopy, solution and solid-state (119)Sn NMR spectroscopy, far-infrared and X-ray absorption spectroscopies, elemental analysis, and by DFT modeling. We show that through the X-type oleate-to-thiostannate ligand exchange, CdSe NCs retain their Cd-rich stoichiometry, with a stoichiometric CdSe core and surface Cd adatoms serving as binding sites for terminal S atoms of the thiostannates ligands, leading to all-inorganic (CdSe)core[Cdm(Sn2S7)yK(6y-2m)]shell (taking Sn2S7(6-) ligand as an example). Thiostannates SnS4(4-) and Sn2S7(6-) retain (distorted) tetrahedral SnS4 geometry upon binding to NC surface. At the same time, experiments and simulations point to lower stability of Sn2S6(4-) (and SnS3(2-)) in most solvents and its lower adaptability to the NC surface caused by rigid Sn2S2 rings.

Research paper thumbnail of High Infrared Photoconductivity in Films of Arsenic-Sulfide-Encapsulated Lead-Sulfide Nanocrystals

ACS Nano, 2014

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are repo... more Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2×10(13) Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.

Research paper thumbnail of Energy transfer in close-packed PbS nanocrystal films

Physical Review B, 2008

We study the emission properties of close-packed films of PbS nanocrystals that show emission in ... more We study the emission properties of close-packed films of PbS nanocrystals that show emission in the infrared. In time resolved photoluminescence measurements, we observe a fast decay time of ˜400ps and a slow component between 20 and 80ns , depending on the temperature, which are attributed to decay from core and surface states, respectively. Photoluminescence excitation and temperature-dependent photoluminescence measurements

Research paper thumbnail of High performance photoelectrochemical hydrogen generation and solar cells with a double type II heterojunction

Physical Chemistry Chemical Physics, 2014

We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption ... more We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption of colloidal QDs on mesoporous TiO 2 followed by 3-mercaptopropionic acid (MPA) ligand exchange. High efficiency photoelectrochemical hydrogen generation is demonstrated by means of these electrodes. The deposition of ZnS on TiO 2 /CdSe further improves the external quantum efficiency from 63% to 85% at 440 nm under À0.5 V vs. SCE. Using the same photoelectrodes, solar cells with the internal quantum efficiency approaching 100% are fabricated. The ZnS deposition increases the photocurrent and chemical stability of the electrodes. Investigation of the carrier dynamics of the solar cells shows that ZnS enhances the exciton separation rate in CdSe nanocrystals, which we ascribe to the formation of a type II heterojunction between ZnS and CdSe QDs. This finding is confirmed by the dynamics of the CdSe photoluminescence, which in the presence of ZnS becomes noticeably faster. † Electronic supplementary information (ESI) available: Electrochemical reaction in photoanodes and cathodes, the effects of MPA ligand exchange for QDSSCs, absorbance, transmittance and reflectance spectra, photoluminescence of samples with different sequences of CdSe and ZnS on TiO 2 , electrochemical impedance spectroscopy (EIS) measurements, equivalent circuit fitting results of EIS and other parameters of cells, and TRPL fitting results. See

Research paper thumbnail of Hybrid inorganic–organic tandem solar cells for broad absorption of the solar spectrum

Physical Chemistry Chemical Physics, 2014

We report the first hybrid tandem solar cell with solution processable active layers using colloi... more We report the first hybrid tandem solar cell with solution processable active layers using colloidal PbS quantum dots (QDs) as the front subcell in combination with a polymer-fullerene rear subcell. Al/WO 3 is introduced as an interlayer, yielding an open circuit voltage (V OC ) equal to about 92% of the sum of the V OC of the subcells. The device exhibits a power conversion efficiency of 1.8%. Optical simulations of various tandem configurations show that combining PbS QDs with small-bandgap polymers is a promising strategy to obtain tandem solar cells with a very broad absorption range and a high short circuit current. † Electronic supplementary information (ESI) available: Complex refractive indices of active layers; simulated current densities of the subcells and maximum theoretical J SC values of various tandem device architectures. See

Research paper thumbnail of 596 TARGETING OF LYSYL OXIDASE-LIKE-2 (LOXL2) PROMOTES REVERSAL OF LIVER FIBROSIS VIA INHIBITION OF COLLAGEN CROSS-LINKING AND FIBROTIC MATRIX STABILIZATION

Journal of Hepatology, 2013

POSTERS (72±19% vs. 52±11%; p = 0.040) rats. Protein expression of VEGFR2 was significantly reduc... more POSTERS (72±19% vs. 52±11%; p = 0.040) rats. Protein expression of VEGFR2 was significantly reduced in splanchnic tissue (p = 0.049) and in the liver (p = 0.006) of BDL animals. No significant differences in splanchnic or hepatic protein expression were observed for VEGF (p = 0.304, p = 0.118), TNFa (p = 0.215, p = 0.164) or PDGFb (p = 0.43, p = 0.18). In BDL rats, splanchnic (p = 0.007) and hepatic (p = 0.048) PlGF mRNA expression was significantly reduced by LENA treatment. In BDL, a reduction of VEGF-mRNA levels in splachnic tissue (p = 0.012) and the liver (p = 0.082) was observed. LENA treatment did not influence TNFa or PDGFb mRNA expression in BDL rats, neither in splanchnic tissue nor in the liver. In PPVL animals splanchnic protein expression of CD31 was significantly decreased (p = 0.003), while TNFa was increased (p = 0.044). Conclusion: Lenalidomide ameliorates portal hypertensive syndrome in cirrhotic and non-cirrhotic portal hypertensive rats by decreasing proinflammatory and antiangiogenic signaling. Therefore, lenalidomide has some potential as a novel therapeutic option in cirrhotic patients with portal hypertension.

Research paper thumbnail of 5.2% efficient PbS nanocrystal Schottky solar cells

Energy & Environmental Science, 2013

ABSTRACT The impact of post-synthetic treatments of nanocrystals (NCs) on the performance of Scho... more ABSTRACT The impact of post-synthetic treatments of nanocrystals (NCs) on the performance of Schottky solar cells, where the active PbS nanocrystal layer is sandwiched directly between two electrodes, is investigated. By monitoring the amount of ligands on the surface of the nanocrystals through Fourier Transform Infrared (FTIR) measurements, we find that optimized processing conditions can lead to high current density and thus to an increase in overall efficiency. Our devices reach an efficiency of 5.2%, which is the highest reported using a PbS nanocrystal Schottky junction. These results demonstrate that even by using the simplest device architecture, accurate post-synthetic treatments result in substantial improvements in the performance. By drawing a direct correlation between ligand-to-NC ratio in the starting PbS solution and the device parameters, we provide important insights on how to gain experimental control for the fabrication of efficient PbS solar cells.

Research paper thumbnail of Facile Droplet-based Microfluidic Synthesis of Monodisperse IV–VI Semiconductor Nanocrystals with Coupled In-Line NIR Fluorescence Detection

Chemistry of Materials, 2014

We describe the realization of a droplet-based microfluidic platform for the controlled and repro... more We describe the realization of a droplet-based microfluidic platform for the controlled and reproducible synthesis of lead chalcogenide (PbS, PbSe) nanocrystal quantum dots (QDs). Monodisperse nanocrystals were synthesized over a wide range of experimental conditions, with real-time assessment and fine-tuning of material properties being achieved using NIR fluorescence spectroscopy. Importantly, we show for the first time that real-time monitoring of the synthetic process allows for rapid optimization of reaction conditions and the synthesis of high quality PbS nanocrystals, emitting in the range of 765−1600 nm, without any post-synthetic processing. The segmented-flow capillary reactor exhibits stable droplet generation and reproducible synthesis of PbS nanocrystals with high photoluminescence quantum yields (28%) over extended periods of time (3−6 h). Furthermore, the produced NIR-emitting nanoparticles were successfully used in the fabrication of Schottky solar cells, exhibiting a power conversion efficiency of 3.4% under simulated AM 1.5 illumination. Finally, the droplet-based microfluidic platform was used to synthesize PbSe nanocrystals having photoluminescence peaks in the range of 860−1600 nm, showing the exceptional control and stability of the reactor.

Research paper thumbnail of Hybrid Solar Cells Using HgTe Nanocrystals and Nanoporous TiO2 Electrodes

Advanced Functional Materials, 2006

Research paper thumbnail of Unraveling the Core–Shell Structure of Ligand-Capped Sn/SnO x Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mössbauer, and X-ray Absorption Spectroscopies

ACS Nano, 2014

A particularly difficult challenge in the chemistry of nanomaterials is the detailed structural a... more A particularly difficult challenge in the chemistry of nanomaterials is the detailed structural and chemical analysis of multicomponent nano-objects. This is especially true for the determination of spatially resolved information. In this study, we demonstrate that dynamic nuclear polarization surface-enhanced solid-state NMR spectroscopy (DNP-SENS), which provides selective and enhanced NMR signal collection from the (near) surface regions of a sample, can be used to resolve the coreÀshell structure of a nanoparticle. Li-ion anode materials, monodisperse 10À20 nm large tin nanoparticles covered with a ∼3 nm thick layer of native oxides, were used in this case study. DNP-SENS selectively enhanced the weak 119 Sn NMR signal of the amorphous surface SnO 2 layer. Mössbauer and X-ray absorption spectroscopies identified a subsurface SnO phase and quantified the atomic fractions of both oxides. Finally, temperature-dependent X-ray diffraction measurements were used to probe the metallic β-Sn core and indicated that even after 8 months of storage at 255 K there are no signs of conversion of the metallic β-Sn core into a brittle semiconducting R-phase, a phase transition which normally occurs in bulk tin at 286 K (13°C). Taken together, these results indicate that Sn/SnO x nanoparticles have core/shell1/shell2 structure of Sn/ SnO/SnO 2 phases. The study suggests that DNP-SENS experiments can be carried on many types of uniform colloidal nanomaterials containing NMR-active nuclei, in the presence of either hydrophilic (ion-capped surfaces) or hydrophobic (capping ligands with long hydrocarbon chains) surface functionalities.

Research paper thumbnail of ja312604r si 001

S2 Complete references 7, 10, 13 and 16 of the Main Text with full author list: Ref. 7: Yarema, M... more S2 Complete references 7, 10, 13 and 16 of the Main Text with full author list: Ref. 7: Yarema, M.; Pichler, S.; Kriegner, D.; Stangl, J.; Yarema, O.; Kirchschlager, R.; Tollabimazraehno, S.; Humer, M.; Häringer, D.; Kohl, M.; Chen, G.; Heiss, W. ACS Nano 2012, 6, 4113. Ref. 10: Ciuculescu, D.; Amiens, C.; Respaud, M.; Falqui, A.; Lecante, P.; Benfield, R. E.; Jiang, L.; Fauth, K.; Chaudret, B. Chem. Mater. 2007, 19, 4624. Ref. 13: Yarema, M.; Pichler, S.; Sytnyk, M.; Seyrkammer, R.; Lechner, R. T.; Fritz-Popovski, G.; Jarzab, D.; Szendrei, K.; Resel, R.; Korovyanko, O.; Loi, M. A.; Paris, O.; Hesser, G.; Heiss, W. ACS Nano 2011, 5, 3758. Ref. 16: Indris, S.; Scheuermann, M.; Becker, F.; Feldmann, C.; Ulrich, A. S.; Hahn, H.

Research paper thumbnail of Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I)

Nano letters, Jan 28, 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 anion-exchange in highly luminescent semiconductor nanocrystals of cesium lead halide perovskites (CsPbX3, 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 CsPbCl3, CsPbBr3, and CsPbI3 nanocrystals in appropriate ratios.

Research paper thumbnail of Origin of the increased open circuit voltage in PbS–CdS core–shell quantum dot solar cells

J. Mater. Chem. A, 2015

Lead sulfide quantum dots (PbS QDs) show great potential for efficient, low cost photovoltaic app... more Lead sulfide quantum dots (PbS QDs) show great potential for efficient, low cost photovoltaic applications.

Research paper thumbnail of Photophysics of PbS Quantum Dot Films Capped with Arsenic Sulfide Ligands

Advanced Energy Materials, 2014

ABSTRACT PbS quantum dots (QDs) of different sizes capped with short (NH4)3AsS3 inorganic ligands... more ABSTRACT PbS quantum dots (QDs) of different sizes capped with short (NH4)3AsS3 inorganic ligands are produced via ligand exchange processes from oleate-capped 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 (NH4)3AsS3 complexes and relaxation is dominated by efficient 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 efficient carrier trapping at shallow ligand states, which appears more intense under excitation above the (NH4)3AsS3 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.

Research paper thumbnail of Surface modification of inorganic nanocrystals with oxo-, polyoxometalate ligands

Research paper thumbnail of Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut

Nano letters, Jan 2, 2015

Metal halides perovskites, such as hybrid organic-inorganic CH3NH3PbI3, are newcomer optoelectron... more Metal halides perovskites, such as hybrid organic-inorganic CH3NH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as solution-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. Herein we demonstrate a new avenue for halide perovskites by designing highly luminescent perovskite-based colloidal quantum dot materials. We have synthesized monodisperse colloidal nanocubes (4-15 nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of ...

Research paper thumbnail of Monodisperse colloidal gallium nanoparticles: Synthesis, low temperature crystallization, surface plasmon resonance and Li-ion storage

We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable i... more We report a facile colloidal synthesis of gallium (Ga) nanoparticles with the mean size tunable in the range of 12−46 nm and with excellent size distribution as small as 7− 8%. When stored under ambient conditions, Ga nanoparticles remain stable for months due to the formation of native and passivating Ga-oxide layer (2−3 nm). The mechanism of Ga nanoparticles formation is elucidated using nuclear magnetic resonance spectroscopy and with molecular dynamics simulations. Size-dependent crystallization and melting of Ga nanoparticles in the temperature range of 98−298 K are studied with X-ray powder diffraction, specific heat measurements, transmission electron microscopy, and X-ray absorption spectroscopy. The results point to delta (δ)-Ga polymorph as a single low-temperature phase, while phase transition is characterized by the large hysteresis and by the large undercooling of crystallization and melting points down to 140−145 and 240−250 K, respectively. We have observed size-tunable plasmon resonance in the ultraviolet and visible spectral regions. We also report stable operation of Ga nanoparticles as anode material for Li-ion batteries with storage capacities of 600 mAh g −1 , 50% higher than those achieved for bulk Ga under identical testing conditions.

Research paper thumbnail of Atomistic Description of Thiostannate-Capped CdSe Nanocrystals: Retention of Four-Coordinate SnS4 Motif and Preservation of Cd-Rich Stoichiometry

Journal of the American Chemical Society, 2015

Colloidal semiconductor nanocrystals (NCs) are widely studied as building blocks for novel solid-... more Colloidal semiconductor nanocrystals (NCs) are widely studied as building blocks for novel solid-state materials. Inorganic surface functionalization, used to displace native organic capping ligands from NC surfaces, has been a major enabler of electronic solid-state devices based on colloidal NCs. At the same time, very little is known about the atomistic details of the organic-to-inorganic ligand exchange and binding motifs at the NC surface, severely limiting further progress in designing all-inorganic NCs and NC solids. Taking thiostannates (K4SnS4, K4Sn2S6, K6Sn2S7) as typical examples of chalcogenidometallate ligands and oleate-capped CdSe NCs as a model NC system, in this study we address these questions through the combined application of solution (1)H NMR spectroscopy, solution and solid-state (119)Sn NMR spectroscopy, far-infrared and X-ray absorption spectroscopies, elemental analysis, and by DFT modeling. We show that through the X-type oleate-to-thiostannate ligand exchange, CdSe NCs retain their Cd-rich stoichiometry, with a stoichiometric CdSe core and surface Cd adatoms serving as binding sites for terminal S atoms of the thiostannates ligands, leading to all-inorganic (CdSe)core[Cdm(Sn2S7)yK(6y-2m)]shell (taking Sn2S7(6-) ligand as an example). Thiostannates SnS4(4-) and Sn2S7(6-) retain (distorted) tetrahedral SnS4 geometry upon binding to NC surface. At the same time, experiments and simulations point to lower stability of Sn2S6(4-) (and SnS3(2-)) in most solvents and its lower adaptability to the NC surface caused by rigid Sn2S2 rings.

Research paper thumbnail of High Infrared Photoconductivity in Films of Arsenic-Sulfide-Encapsulated Lead-Sulfide Nanocrystals

ACS Nano, 2014

Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are repo... more Highly photoconductive thin films of inorganic-capped PbS nanocrystal quantum dots (QDs) are reported. Stable colloidal dispersions of (NH4)3AsS3-capped PbS QDs were processed by a conventional dip-coating technique into a thin homogeneous film of electronically coupled PbS QDs. Upon drying at 130 °C, (NH4)3AsS3 capping ligands were converted into a thin layer of As2S3, acting as an infrared-transparent semiconducting glue. Photodetectors obtained by depositing such films onto glass substrates with interdigitate electrode structures feature extremely high light responsivity and detectivity with values of more than 200 A/W and 1.2×10(13) Jones, respectively, at infrared wavelengths up to 1400 nm. Importantly, these devices were fabricated and tested under ambient atmosphere. Using a set of time-resolved optoelectronic experiments, the important role played by the carrier trap states, presumably localized on the arsenic-sulfide surface coating, has been elucidated. Foremost, these traps enable a very high photoconductive gain of at least 200. The trap state density as a function of energy has been plotted from the frequency dependence of the photoinduced absorption (PIA), whereas the distribution of lifetimes of these traps was recovered from PIA and photoconductivity (PC) phase spectra. These trap states also have an important impact on carrier dynamics, which led us to propose a kinetic model for trap state filling that consistently describes the experimental photoconductivity transients at various intensities of excitation light. This model also provides realistic values for the photoconductive gain and thus may serve as a useful tool to describe photoconductivity in nanocrystal-based solids.

Research paper thumbnail of Energy transfer in close-packed PbS nanocrystal films

Physical Review B, 2008

We study the emission properties of close-packed films of PbS nanocrystals that show emission in ... more We study the emission properties of close-packed films of PbS nanocrystals that show emission in the infrared. In time resolved photoluminescence measurements, we observe a fast decay time of ˜400ps and a slow component between 20 and 80ns , depending on the temperature, which are attributed to decay from core and surface states, respectively. Photoluminescence excitation and temperature-dependent photoluminescence measurements

Research paper thumbnail of High performance photoelectrochemical hydrogen generation and solar cells with a double type II heterojunction

Physical Chemistry Chemical Physics, 2014

We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption ... more We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption of colloidal QDs on mesoporous TiO 2 followed by 3-mercaptopropionic acid (MPA) ligand exchange. High efficiency photoelectrochemical hydrogen generation is demonstrated by means of these electrodes. The deposition of ZnS on TiO 2 /CdSe further improves the external quantum efficiency from 63% to 85% at 440 nm under À0.5 V vs. SCE. Using the same photoelectrodes, solar cells with the internal quantum efficiency approaching 100% are fabricated. The ZnS deposition increases the photocurrent and chemical stability of the electrodes. Investigation of the carrier dynamics of the solar cells shows that ZnS enhances the exciton separation rate in CdSe nanocrystals, which we ascribe to the formation of a type II heterojunction between ZnS and CdSe QDs. This finding is confirmed by the dynamics of the CdSe photoluminescence, which in the presence of ZnS becomes noticeably faster. † Electronic supplementary information (ESI) available: Electrochemical reaction in photoanodes and cathodes, the effects of MPA ligand exchange for QDSSCs, absorbance, transmittance and reflectance spectra, photoluminescence of samples with different sequences of CdSe and ZnS on TiO 2 , electrochemical impedance spectroscopy (EIS) measurements, equivalent circuit fitting results of EIS and other parameters of cells, and TRPL fitting results. See

Research paper thumbnail of Hybrid inorganic–organic tandem solar cells for broad absorption of the solar spectrum

Physical Chemistry Chemical Physics, 2014

We report the first hybrid tandem solar cell with solution processable active layers using colloi... more We report the first hybrid tandem solar cell with solution processable active layers using colloidal PbS quantum dots (QDs) as the front subcell in combination with a polymer-fullerene rear subcell. Al/WO 3 is introduced as an interlayer, yielding an open circuit voltage (V OC ) equal to about 92% of the sum of the V OC of the subcells. The device exhibits a power conversion efficiency of 1.8%. Optical simulations of various tandem configurations show that combining PbS QDs with small-bandgap polymers is a promising strategy to obtain tandem solar cells with a very broad absorption range and a high short circuit current. † Electronic supplementary information (ESI) available: Complex refractive indices of active layers; simulated current densities of the subcells and maximum theoretical J SC values of various tandem device architectures. See

Research paper thumbnail of 596 TARGETING OF LYSYL OXIDASE-LIKE-2 (LOXL2) PROMOTES REVERSAL OF LIVER FIBROSIS VIA INHIBITION OF COLLAGEN CROSS-LINKING AND FIBROTIC MATRIX STABILIZATION

Journal of Hepatology, 2013

POSTERS (72±19% vs. 52±11%; p = 0.040) rats. Protein expression of VEGFR2 was significantly reduc... more POSTERS (72±19% vs. 52±11%; p = 0.040) rats. Protein expression of VEGFR2 was significantly reduced in splanchnic tissue (p = 0.049) and in the liver (p = 0.006) of BDL animals. No significant differences in splanchnic or hepatic protein expression were observed for VEGF (p = 0.304, p = 0.118), TNFa (p = 0.215, p = 0.164) or PDGFb (p = 0.43, p = 0.18). In BDL rats, splanchnic (p = 0.007) and hepatic (p = 0.048) PlGF mRNA expression was significantly reduced by LENA treatment. In BDL, a reduction of VEGF-mRNA levels in splachnic tissue (p = 0.012) and the liver (p = 0.082) was observed. LENA treatment did not influence TNFa or PDGFb mRNA expression in BDL rats, neither in splanchnic tissue nor in the liver. In PPVL animals splanchnic protein expression of CD31 was significantly decreased (p = 0.003), while TNFa was increased (p = 0.044). Conclusion: Lenalidomide ameliorates portal hypertensive syndrome in cirrhotic and non-cirrhotic portal hypertensive rats by decreasing proinflammatory and antiangiogenic signaling. Therefore, lenalidomide has some potential as a novel therapeutic option in cirrhotic patients with portal hypertension.

Research paper thumbnail of 5.2% efficient PbS nanocrystal Schottky solar cells

Energy & Environmental Science, 2013

ABSTRACT The impact of post-synthetic treatments of nanocrystals (NCs) on the performance of Scho... more ABSTRACT The impact of post-synthetic treatments of nanocrystals (NCs) on the performance of Schottky solar cells, where the active PbS nanocrystal layer is sandwiched directly between two electrodes, is investigated. By monitoring the amount of ligands on the surface of the nanocrystals through Fourier Transform Infrared (FTIR) measurements, we find that optimized processing conditions can lead to high current density and thus to an increase in overall efficiency. Our devices reach an efficiency of 5.2%, which is the highest reported using a PbS nanocrystal Schottky junction. These results demonstrate that even by using the simplest device architecture, accurate post-synthetic treatments result in substantial improvements in the performance. By drawing a direct correlation between ligand-to-NC ratio in the starting PbS solution and the device parameters, we provide important insights on how to gain experimental control for the fabrication of efficient PbS solar cells.

Research paper thumbnail of Facile Droplet-based Microfluidic Synthesis of Monodisperse IV–VI Semiconductor Nanocrystals with Coupled In-Line NIR Fluorescence Detection

Chemistry of Materials, 2014

We describe the realization of a droplet-based microfluidic platform for the controlled and repro... more We describe the realization of a droplet-based microfluidic platform for the controlled and reproducible synthesis of lead chalcogenide (PbS, PbSe) nanocrystal quantum dots (QDs). Monodisperse nanocrystals were synthesized over a wide range of experimental conditions, with real-time assessment and fine-tuning of material properties being achieved using NIR fluorescence spectroscopy. Importantly, we show for the first time that real-time monitoring of the synthetic process allows for rapid optimization of reaction conditions and the synthesis of high quality PbS nanocrystals, emitting in the range of 765−1600 nm, without any post-synthetic processing. The segmented-flow capillary reactor exhibits stable droplet generation and reproducible synthesis of PbS nanocrystals with high photoluminescence quantum yields (28%) over extended periods of time (3−6 h). Furthermore, the produced NIR-emitting nanoparticles were successfully used in the fabrication of Schottky solar cells, exhibiting a power conversion efficiency of 3.4% under simulated AM 1.5 illumination. Finally, the droplet-based microfluidic platform was used to synthesize PbSe nanocrystals having photoluminescence peaks in the range of 860−1600 nm, showing the exceptional control and stability of the reactor.

Research paper thumbnail of Hybrid Solar Cells Using HgTe Nanocrystals and Nanoporous TiO2 Electrodes

Advanced Functional Materials, 2006

Research paper thumbnail of Unraveling the Core–Shell Structure of Ligand-Capped Sn/SnO x Nanoparticles by Surface-Enhanced Nuclear Magnetic Resonance, Mössbauer, and X-ray Absorption Spectroscopies

ACS Nano, 2014

A particularly difficult challenge in the chemistry of nanomaterials is the detailed structural a... more A particularly difficult challenge in the chemistry of nanomaterials is the detailed structural and chemical analysis of multicomponent nano-objects. This is especially true for the determination of spatially resolved information. In this study, we demonstrate that dynamic nuclear polarization surface-enhanced solid-state NMR spectroscopy (DNP-SENS), which provides selective and enhanced NMR signal collection from the (near) surface regions of a sample, can be used to resolve the coreÀshell structure of a nanoparticle. Li-ion anode materials, monodisperse 10À20 nm large tin nanoparticles covered with a ∼3 nm thick layer of native oxides, were used in this case study. DNP-SENS selectively enhanced the weak 119 Sn NMR signal of the amorphous surface SnO 2 layer. Mössbauer and X-ray absorption spectroscopies identified a subsurface SnO phase and quantified the atomic fractions of both oxides. Finally, temperature-dependent X-ray diffraction measurements were used to probe the metallic β-Sn core and indicated that even after 8 months of storage at 255 K there are no signs of conversion of the metallic β-Sn core into a brittle semiconducting R-phase, a phase transition which normally occurs in bulk tin at 286 K (13°C). Taken together, these results indicate that Sn/SnO x nanoparticles have core/shell1/shell2 structure of Sn/ SnO/SnO 2 phases. The study suggests that DNP-SENS experiments can be carried on many types of uniform colloidal nanomaterials containing NMR-active nuclei, in the presence of either hydrophilic (ion-capped surfaces) or hydrophobic (capping ligands with long hydrocarbon chains) surface functionalities.

Research paper thumbnail of ja312604r si 001

S2 Complete references 7, 10, 13 and 16 of the Main Text with full author list: Ref. 7: Yarema, M... more S2 Complete references 7, 10, 13 and 16 of the Main Text with full author list: Ref. 7: Yarema, M.; Pichler, S.; Kriegner, D.; Stangl, J.; Yarema, O.; Kirchschlager, R.; Tollabimazraehno, S.; Humer, M.; Häringer, D.; Kohl, M.; Chen, G.; Heiss, W. ACS Nano 2012, 6, 4113. Ref. 10: Ciuculescu, D.; Amiens, C.; Respaud, M.; Falqui, A.; Lecante, P.; Benfield, R. E.; Jiang, L.; Fauth, K.; Chaudret, B. Chem. Mater. 2007, 19, 4624. Ref. 13: Yarema, M.; Pichler, S.; Sytnyk, M.; Seyrkammer, R.; Lechner, R. T.; Fritz-Popovski, G.; Jarzab, D.; Szendrei, K.; Resel, R.; Korovyanko, O.; Loi, M. A.; Paris, O.; Hesser, G.; Heiss, W. ACS Nano 2011, 5, 3758. Ref. 16: Indris, S.; Scheuermann, M.; Becker, F.; Feldmann, C.; Ulrich, A. S.; Hahn, H.