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The most developed approaches for the synthesis of InAs nanocrystals (NCs) rely on pyrophoric, toxic and not readily available tris-trimethylsilyl or tris-trimethylgermil arsine (TMS-As) precursors. Alternative less toxic and commercially-available chemicals, such as dimethylamino-arsine (amino-As), emerged as alternative As precursors. Nevertheless, InAs NCs made with such compounds need to be further optimized, in terms of size distribution and optical properties, in order to meet the standard reached with TMS-As. To this aim, in this work we investigated the role of ZnCl2 used as an additive in the synthesis of InAs NCs with amino-As and alane N,N-dimethylethylamine. We discovered that ZnCl2 helps not only to improve the size distribution of InAs NCs, but also to passivate their surface acting as a Z-type ligand. The presence of ZnCl2 on the surface of the NCs and the excess of Zn precursor used in the synthesis enable the subsequent in-situ growth of a ZnSe shell, which is realized by simply adding the Se precursor to the crude reaction mixture. The resulting InAs@ZnSe core@shell NCs exhibit photoluminescence emission at ~860 nm with quantum yields as high as 42%, which is a record for such heterostructures, given the relatively high mismatch (6%) between InAs and ZnSe. Such bright emission was ascribed to the formation of an In-rich intermediate layer inbetween the core and the shell, as indicated by X-ray photoelectron spectroscopy and elemental analyses, which helps to release the strain between these two materials.

Simultaneously achieving both broad absorption and sharp emission in the near-infrared (NIR) is challenging. Coupling of an efficient absorber such as lead halide perovskites to lanthanide emissive species is a promising way to meet the demands for visible to NIR spectral conversion. However, lead based perovskite sensitizers suffer from relatively narrow absorption in the visible range, poor stability, and toxicity. Herein, we introduce a downshifting configuration based on lead-free cesium manganese bromide nanocrystals acting as broad visible absorbers coupled to sharp emission in the NIR-I and NIR-II spectral regions. To achieve this, we synthesized CsMnBr3 and Cs3MnBr5 nanocrystals and attempted to dope them with a series of lanthanides, achieving success only with CsMnBr3. The correlation of the lanthanide emission to the CsMnBr3 visible absorption was confirmed with steady-state excitation spectra and time-resolved photoluminescence measurements, whereas the mechanism of downconversion from the CsMnBr3 matrix to the lanthanides was understood by density functional theory (DFT) calculations. This study shows that lead-free metal halides with an appropriate phase are an effective sensitizer for lanthanides and a route to efficient downshifting applications.

Various strategies have been proposed to engineer the band gap of metal halide perovskite nanocrystals (NCs) while preserving their structure and composition and thus ensuring spectral stability of the emission color. An aspect that has only been marginally investigated is how the type of surface passivation influences the structural/color stability of AMX3 perovskite NCs composed of two different M2+ cations. Here, we report the synthesis of blue-emitting Cs-oleate capped CsCdxPb1–xBr3 NCs, which exhibit a cubic perovskite phase containing Cd-rich domains of Ruddlesden–Popper phases (RP phases). The RP domains spontaneously transform into pure orthorhombic perovskite ones upon NC aging, and the emission color of the NCs shifts from blue to green over days. On the other hand, postsynthesis ligand exchange with various Cs-carboxylate or ammonium bromide salts, right after NC synthesis, provides monocrystalline NCs with cubic phase, highlighting the metastability of RP domains. When NCs are treated with Cs-carboxylates (including Cs-oleate), most of the Cd2+ ions are expelled from NCs upon aging, and the NCs phase evolves from cubic to orthorhombic and their emission color changes from blue to green. Instead, when NCs are coated with ammonium bromides, the loss of Cd2+ ions is suppressed and the NCs tend to retain their blue emission (both in colloidal dispersions and in electroluminescent devices), as well as their cubic phase, over time. The improved compositional and structural stability in the latter cases is ascribed to the saturation of surface vacancies, which may act as channels for the expulsion of Cd2+ ions from NCs.

It is widely accepted that the properties of most semiconductor nanocrystals can be tuned by their core size, shape and material. In covalent semiconductor nanocrystal materials, such as silicon, germanium or carbon, certain degree of tunability of the properties can be also achieved by the surface ligands. In particular, covalently bonded ligand species on the surface of such a nanocrystal (i) contribute to the density of states of the core via orbital delocalization; (ii) might introduce strain via ligand-to-ligand steric hindrance and (iii) will cause charge transfer from/to the core. In this work we study all these effects on silicon nanocrystals (SiNCs). We analyze geometrically optimized ~ 2 nm SiNCs with electronegative organic ligands using density functional theory (DFT) simulations. We show that the radiative rate is enhanced by electronegative alkyl and fluorocarbon with respect to what is expected from quantum confinement effect, while bandgap remains unchanged. Also, we show that tensile strain caused by the ligand steric hindrance is detrimental to the rate enhancement, contrary to the positive effects of the more homogeneous tensile strain induced in pressure cell.

We devised a colloidal approach for the synthesis of CsPbBr3 nanocrystals (NCs) in which the only ligands employed are alkyl phosphonic acids. Compared to more traditional syntheses of CsPbBr3 NCs, the present scheme delivers NCs with the following distinctive features: (i) The NCs do not have cubic but truncated octahedron shape enclosed by Pb-terminated facets. This is a consequence of the strong binding affinity of the phosphonate groups toward Pb2+ ions. (II) The NCs have near unity photoluminescence quantum yields (PLQYs), with no need of postsynthesis treatments, indicating that alkyl phosphonic acids are effectively preventing the formation of surface traps. (III) Unlike NCs coated with alkylammonium or carboxylate ligands, the PLQY of phosphonate coated NCs remains constant upon dilution, suggesting that the ligands are tightly bound to the surface.

Charge trapping is an ubiquitous process in colloidal quantum-dot solids and a major limitation to the efficiency of quantum dot based devices such as solar cells, LEDs, and thermoelectrics. Although empirical approaches led to a reduction of trapping and thereby efficiency enhancements, the exact chemical nature of the trapping mechanism remains largely unidentified. In this study, we determine the density of trap states in CdTe quantum-dot solids both experimentally, using a combination of electrochemical control of the Fermi level with ultrafast transient absorption and time-resolved photoluminescence spectroscopy, and theoretically, via density functional theory calculations. We find a high density of very efficient electron traps centered similar to 0.42 eV above the valence band. Electrochemical filling of these traps increases the electron lifetime and the photoluminescence quantum yield by more than an order of magnitude. The trapping rate constant for holes is an order of magnitude lower that for electrons. These observations can be explained by Auger-mediated electron trapping. From density functional theory calculations we infer that the traps are formed by dicoordinated Te atoms at the quantum dot surface. The combination of our unique experimental determination of the density of trap states with the theoretical modeling of the quantum dot surface allows us to identify the trapping mechanism and chemical reaction at play during charge trapping in these quantum dots.

We present a density functional theory (DFT) study aimed at understanding the injection and recombination processes that occur at the interface between PbS QDs and TiO2 oxide nanoparticles with different morphologies. The calculated injection rates fall in the picosecond timescale in good agreement with the experiments. In addition, our simulations show that the (101) facet of TiO2 more favourably accommodates the QD, resulting in stronger electronic couplings and faster electron injections than the (001) surfaces. Despite this, the (101) slab is also more prone to faster electron recombination with the valence band of the QD, which can lead to overall lower injection efficiencies than the (001) surface.

Poster presented at CHAINS 2016 Symposium, The Netherlands

Modern research based on computational quantum chemistry typically involves various types of interdependent calculations, on series of molecular systems and/or conformations and are performed with different computational chemistry software. Such extensive computational studies require great effort by the researcher to prepare, submit, run and analyze the simulations and is usually done through shell scripts that try to automate these tasks. Such scripts are difficult to maintain and extend, requiring a significant programming expertise to work with them. In this communication we present a workflow solution under development that aims to: automate input generation, job distribution, failure detection and recovery, dependency resolution, etc. We also present the preliminary results obtained using a test set of transition state optimizations.

Research on modern computational quantum chemistry relies on a set of computational tools to carry out calculations. The complexity of the calculations usually requires intercommunication between the different simulation tools, such communication is usually done through shell scripts that try to automate input/output actions like: write some input for a target quantum chemistry code; submit the calculation to a supercomputer using some sort of queue system like Slurm or Torque; resume in case of a recoverable failure; analyze the output data both manually or with some kind of script; and finally perform several post-processing steps over the raw data. Such scripts are difficult to maintain and extend, requiring both a significant programming expertise to work with them and constant user intervention, resulting in a sub-optimal use of the valuable computational resources. Also as the workflows complexity increase, the manual approach is impractical due to the among of data that must be analysed. Being then desirable a set of automatic and extensible tools that allows to perform complex simulations in heterogeneous hardware platforms. In this work, we present a Python Software to carry out complex simulations in an extensible and automatic way. We also present its application to the simulation of the nonadiabatic molecular dynamics of quantum dots.

Understanding and controlling charge transfer between different kinds of colloidal quantum dots (QDs) is important for devices such as light-emitting diodes and solar cells and for thermoelectric applications. Here we study photoinduced electron transfer between CdTe and CdSe QDs in a QD film. We find that very efficient electron trapping in CdTe QDs obstructs electron transfer to CdSe QDs under most conditions. Only the use of thiol ligands results in somewhat slower electron trapping; in this case the competition between trapping and electron transfer results in a small fraction of electrons being transferred to CdSe. However, we demonstrate that electron trapping can be controlled and even avoided altogether by using the unique combination of electrochemistry and transient absorption spectroscopy. When the Fermi level is raised electrochemically, traps are filled with electrons and electron transfer from CdTe to CdSe QDs occurs with unity efficiency. These results show the great importance of knowing and controlling the Fermi level in QD films and open up the possibility of studying the density of trap states in QD films as well as the systematic investigation of the intrinsic electron transfer rates in donor-acceptor films.

The results of a combined spectroscopic and computational study of lanthanide hydrides with the general formula MHx(H2)y, where M = La, Ce, Pr, Nd, Sm, Eu, and Gd, x = 1−4, and y = 0−6 are reported. To understand the nature of the dihydrogen complexes formed with lanthanide metal hydride molecules, we have first identified the binary MHx species formed in the ablation/deposition process and then analyzed the dihydrogen supercomplexes, MHx(H2)y. Our investigation shows that the trihydrides bind dihydrogen more weakly than the dihydrides and that the interaction between the central lanthanide and the H2 molecules occurs via a 6s electron transfer from the lanthanide to the H2 molecules. Evidence is also presented for the SmH and EuH diatomic molecules and the tetrahydride anions in solid hydrogen.

International audience; The Free-Electron Laser for Infrared Experiments (FELIX) was used to study the wavelength-resolved multiple photon photodissociation of discrete, gas-phase uranyl (UO$_2$$^{2+}$) complexes containing a single anionic ligand (A), with or without ligated solvent molecules (S). The uranyl antisymmetric and symmetric stretching frequencies were measured for complexes with general formula [UO$_2$A(S)$_n$]$^+$, where A was hydroxide, methoxide, or acetate; S was water, ammonia, acetone, or acetonitrile; and n = 0−3. The values for the antisymmetric stretching frequency for uranyl ligated with only an anion ([UO$_2$A]$^+$) were as low or lower than measurements for [UO$_2$]$^{2+}$ ligated with as many as five strong neutral donor ligands and are comparable to solution-phase values. This result was surprising because initial DFT calculations predicted values that were 30−40 cm$^{-1}$ higher, consistent with intuition but not with the data. Modification of the basis sets and use of alternative functionals improved computational accuracy for the methoxide and acetate complexes, but calculated values for the hydroxide were greater than the measurement regardless of the computational method used. Attachment of a neutral donor ligand S to [UO$_2$A]$^+$ produced [UO$_2$AS]$^+$, which produced only very modest changes to the uranyl antisymmetric stretch frequency, and did not universally shift the frequency to lower values. DFT calculations for [UO$_2$AS]$^+$ were in accord with trends in the data and showed that attachment of the solvent was accommodated by weakening of the U-anion bond as well as the uranyl. When uranyl frequencies were compared for [UO$_2$AS]$^+$ species having different solvent neutrals, values decreased with increasing neutral nucleophilicity.

Here we present a colloidal approach to synthesize bismuth chalcohalide nanocrystals (BiEX NCs, in which E=S, Se and X=Cl, Br, I). Our method yields orthorhombic elongated BiEX NCs, with BiSCl crystallizing in a previously unknown polymorph. The BiEX NCs display a composition-dependent band gap spanning the visible spectral range and absorption coefficients exceeding 105 cm-1 . The BiEX NCs show chemical stability at standard laboratory conditions and form colloidal inks in different solvents. These features enable the solution processing of the NCs into robust solid films yielding stable photoelectrochemical current densities under solar-simulated irradiation. Overall, our versatile synthetic protocol may prove valuable in accessing colloidal metal chalcohalide nanomaterials at large and contributes to establish metal chalcohalides as a promising complement to metal chalcogenides and halides for applied nanotechnology.

Colloidal quantum dots (QDs) stand among the most attractive light-harvesting materials to be exploited for solution-processed optoelectronic applications. To this aim, quantitative replacement of the bulky electrically insulating ligands at the QD surface coming from the synthetic procedure is mandatory. Here we present a conceptually novel approach to design light-harvesting nanomaterials demonstrating that QD surface modification with suitable short conjugated organic molecules permits us to drastically enhance light absorption of QDs, while preserving good long-term colloidal stability. Indeed, rational design of the pendant and anchoring moieties, which constitute the replacing ligand framework leads to a broadband increase of the optical absorbance larger than 300% for colloidal PbS QDs also at high energies (>3.1 eV), which could not be predicted by using formalisms derived from effective medium theory. We attribute such a drastic absorbance increase to ground-state ligand/QD orbital mixing, as inferred by density functional theory calculations; in addition, our findings suggest that the optical band gap reduction commonly observed for PbS QD solids treated with thiol-terminating ligands can be prevalently ascribed to 3p orbitals localized on anchoring sulfur atoms, which mix with the highest occupied states of the QDs. More broadly, we provide evidence that organic ligands and inorganic cores are inherently electronically coupled materials thus yielding peculiar chemical species (the colloidal QDs themselves), which display arising (opto)electronic properties that cannot be merely described as the sum of those of the ligand and core components.\nColloidal quantum dots (QDs) stand among the most attractive light-harvesting materials to be exploited for solution-processed optoelectronic applications. To this aim, quantitative replacement of the bulky electrically insulating ligands at the QD surface coming from the synthetic procedure is mandatory. Here we present a conceptually novel approach to design light-harvesting nanomaterials demonstrating that QD surface modification with suitable short conjugated organic molecules permits us to drastically enhance light absorption of QDs, while preserving good long-term colloidal stability. Indeed, rational design of the pendant and anchoring moieties, which constitute the replacing ligand framework leads to a broadband increase of the optical absorbance larger than 300% for colloidal PbS QDs also at high energies (>3.1 eV), which could not be predicted by using formalisms derived from effective medium theory. We attribute such a drastic absorbance increase to ground-state ligand/QD orbital mixing, as inferred by density functional theory calculations; in addition, our findings suggest that the optical band gap reduction commonly observed for PbS QD solids treated with thiol-terminating ligands can be prevalently ascribed to 3p orbitals localized on anchoring sulfur atoms, which mix with the highest occupied states of the QDs. More broadly, we provide evidence that organic ligands and inorganic cores are inherently electronically coupled materials thus yielding peculiar chemical species (the colloidal QDs themselves), which display arising (opto)electronic properties that cannot be merely described as the sum of those of the ligand and core components.