Impact of Crystal Structure and Particles Shape on the Photoluminescence Intensity of CdSe/CdS Core/Shell Nanocrystals

To study the influence of the chemical and crystalline composition of core/shell NCs on their photoluminescence (PL) the mean structural profile of a large ensemble of NCs has to be retrieved in atomic resolution. This can be achieved by retrieving the chemical profile of core/shell NCs using anomalous small angle x-ray scattering (ASAXS) in combination with the analysis of powder diffraction data recorded by wide angle x-ray scattering (WAXS). In the current synchrotron based study, we investigate CdSe/CdS core/shell NCs with different core dimensions by recording simultaneously ASAXS and WAXS spectra. The CdS shells are grown epitaxial on nominal spherical CdSe cores with core diameters from around 3.5–5.5 nm. Three different CdSe shell thicknesses are realized by depositing around 4, 6, and 8 monolayers (MLs) of CdSe. We reveal that the epitaxial core/shell structure depicts a chemical sharp interface, even after a post growth annealing step. With increasing NC diameter, however, the CdSe/CdS NCs deviate significantly from a spherical shape. Instead an elliptical particle shape with pronounced surface facets for the larger core/shell NCs is found. In combination with the powder diffraction data we could relate this anisotropic shape to a mixture of crystal phases within the CdSe core. The smallest CdSe cores exhibit a pure hexagonal wurtzite crystal structure, whereas the larger ones also possess a cubic zincblende phase fraction. This mixed crystal phase fractions lead to a non-spherical shell growth with different thicknesses along specific crystallographic directions: The long axes are terminated by basal crystal faces parallel either to the a- or c-axis, the short axes by “tilted” pyramidal planes. By combining these structural data with the measured PL quantum yield values, we can clearly connect the optical output of the NCs to their shape and to their shell thickness. Above 6 ML CdS shell-thickness no further increase of the PL can be observed, but for large aspect ratio values the PL is significantly decreased. The gained understanding of the internal crystal structure on CdSe/CdS NCs is general applicable for a precise tuning of the optical properties of crystalline core/shell NCs

Notions of locality in quantum processes

In den letzten Jahren bekam der Gedanke von indefiniten kausalen Strukturen viel Aufmerksamkeit. Die Idee ist es, sehr generelle Prozesse zu beschreiben, welche lokale Labore in eine globale Struktur einbetten und dadurch die Korrelationen zwischen den Laboren bestimmen. Mittels des Prozessmatrizenformalismus wurde ein mathematischer Formalismus gefunden, der genau diese Idee, unter der Annahme, dass die Quantenmechanik lokal Gültigkeit besitzt, verwirklicht. In der Definition von Prozessmatrizen ist bereits jedem lokalen Labor ein Eingangs- und Ausgangshilbertraum zugeordnet. Im Gegenteil dazu, beinhaltet ein Zustand in der normalen Quantenmechanik keine Information bezüglich einer Faktorisierung des globalen Hilbertraumes. Daher stellt sich die Frage was es für Auswirkungen hätte, wenn man diese Annahme lockert und stattdessen bloß fordert, dass alle Labore lokal separiert sind, man jedoch keine Faktorisierung fixiert. Führt dies zu einer modifizierten Menge an erlaubten Prozessen? Diese Arbeit beantwortet diese Frage, indem gezeigt wird, dass Quantenzustände die einzigen Quantenprozesse beschreiben, welche kompatibel mit der gelockerten Annahme sind. Im zweiten Teil der Arbeit wird ein Gedankenexperiment präsentiert, welches ein Bell-artiges Szenario in Superposition von zeitlichen Abläufen beschreibt. Auf den ersten Blick scheint es, dass dieses Szenario Non-Signaling-Korrelationen zulässt, welche allgemeiner sind als Quanten-Korrelationen - sogenannte Almost-Quantum-Korrelationen. Jedoch wird für verschiedene Konfigurationen des Szenarios gezeigt, dass die ursprüngliche Vermutung nicht hält: alle Korrelationen sind Quanten-Korrelationen. Zusätzlich wird ein Theorem bewiesen, welches eine notwendige Bedingung für Bell-artige Szenarien darstellt um Almost-Quantum-Korrelationen zu generieren.In the last few years, the idea of indefinite causal structure has received an enormous amount of attention. The idea is to formulate very general processes that relate local laboratories into a global structure, producing correlations between the local actions. With the formalism of process matrices, a mathematical framework has been found that captures this idea under the assumption that quantum mechanics holds locally. In the definition of process matrices, every local laboratory has well-defined input and output Hilbert spaces. However, in ordinary quantum theory, states do not a priori include any information about the factorization of the global Hilbert space. This raises the question of what were to happen if this assumption was relaxed - that is, if the local laboratories were assumed to be spatially separated, but without fixing the factorization beforehand. Would this lead to a modified set of possible processes? This thesis answers this question, by showing that the only quantum processes compatible with the relaxed formulation are the quantum states. In the second part of the thesis, a thought experiment involving a Bell-like scenario in a superposition of temporal orders is introduced. At first sight, the scenario seems capable of allowing nonsignalling correlations that are more general than quantum - so-called “almost quantum correlations”. However, for several versions of the scenario, it is shown that this initial conjecture does not hold up: all correlations are quantum. Additionally, a theorem is proven which gives a necessary conditions for such Bell-like scenarios to generate almost quantum correlations

The bowtie-shaped deformation isotherm of superhydrophobic cylindrical mesopores

International audienceDeformation of superhydrophobic cylindrical mesopores is studied during a cycle of forced water filling and spontaneous drying by in situ small-angle neutron scattering. A high pressure set up is put forward to characterize the deformation of ordered mesoporous silanized silica up to 80 MPa. Strain isotherms of individual pores are deduced from the shift of the Bragg spectrum associated to the deformation of the hexagonal pore lattice. Due to their superhydrophobic nature, pore walls do not cover with a pre-wetting film. This peculiarity gives the ability to use a simple mechanical model to describe both filled and empty pore state without the pitfall of disjoining pressure effects. By fitting our experimental data with this model we measured both the Young's modulus and the Poisson ratio of the nanometric silica wall. The measurement of this later parameter constitutes a specificity offered by superhydrophobic nanopores with respect to hydrophilic ones

Impact of Crystal Structure and Particles Shape on the Photoluminescence Intensity of CdSe/CdS Core/Shell Nanocrystals

To study the influence of the chemical and crystalline composition of core/shell NCs on their photoluminescence (PL) the mean structural profile of a large ensemble of NCs has to be retrieved in atomic resolution. This can be achieved by retrieving the chemical profile of core/shell NCs using anomalous small angle x-ray scattering (ASAXS) in combination with the analysis of powder diffraction data recorded by wide angle x-ray scattering (WAXS). In the current synchrotron based study, we investigate CdSe/CdS core/shell NCs with different core dimensions by recording simultaneously ASAXS and WAXS spectra. The CdS shells are grown epitaxial on nominal spherical CdSe cores with core diameters from around 3.5–5.5 nm. Three different CdSe shell thicknesses are realized by depositing around 4, 6, and 8 monolayers (MLs) of CdSe. We reveal that the epitaxial core/shell structure depicts a chemical sharp interface, even after a post growth annealing step. With increasing NC diameter, however, the CdSe/CdS NCs deviate significantly from a spherical shape. Instead an elliptical particle shape with pronounced surface facets for the larger core/shell NCs is found. In combination with the powder diffraction data we could relate this anisotropic shape to a mixture of crystal phases within the CdSe core. The smallest CdSe cores exhibit a pure hexagonal wurtzite crystal structure, whereas the larger ones also possess a cubic zincblende phase fraction. This mixed crystal phase fractions lead to a non-spherical shell growth with different thicknesses along specific crystallographic directions: The long axes are terminated by basal crystal faces parallel either to the a- or c-axis, the short axes by “tilted” pyramidal planes. By combining these structural data with the measured PL quantum yield values, we can clearly connect the optical output of the NCs to their shape and to their shell thickness. Above 6 ML CdS shell-thickness no further increase of the PL can be observed, but for large aspect ratio values the PL is significantly decreased. The gained understanding of the internal crystal structure on CdSe/CdS NCs is general applicable for a precise tuning of the optical properties of crystalline core/shell NCs.ISSN:2296-264

Mapping the Atomistic Structure of Graded Core/Shell Colloidal Nanocrystals

Engineering the compositional gradient for core/shell semiconductor nanocrystals improves their optical properties. To date, however, the structure of graded core/shell nanocrystal emitters has only been qualitatively described. In this paper, we demonstrate an approach to quantify nanocrystal structure, selecting graded Ag-In-Se/ZnSe core/shell nanocrystals as a proof-of-concept material. A combination of multi-energy small-angle X-ray scattering and electron microscopy techniques enables us to establish the radial distribution of ZnSe with sub-nanometer resolution. Using ab initio shape-retrieval analysis of X-ray scattering spectra, we further determine the average shape of nanocrystals. These results allow us to generate three-dimensional, atomistic reconstructions of graded core/shell nanocrystals. We use these reconstructions to calculate solid-state Zn diffusion in the Ag-In-Se nanocrystals and the lattice mismatch between nanocrystal monolayers. Finally, we apply these findings to propose design rules for optimal shell structure and record-luminescent core/shell nanocrystals.ISSN:2045-232

In Situ Small-Angle Neutron Scattering Investigation of Adsorption-Induced Deformation in Silica with Hierarchical Porosity

Adsorption-induced deformation of a series of silica samples with hierarchical porosity has been studied by in situ small-angle neutron scattering (SANS) and in situ dilatometry. Monolithic samples consisted of a disordered macroporous network of struts formed by a 2D lattice of hexagonally ordered cylindrical mesopores and disordered micropores within the mesopore walls. Strain isotherms were obtained at the mesopore level by analyzing the shift of the Bragg reflections from the ordered mesopore lattice in SANS data. Thus, SANS essentially measured the radial strain of the cylindrical mesopores including the volume changes of the mesopore walls due to micropore deformation. A H2O/D2O adsorbate with net zero coherent neutron scattering length density was employed in order to avoid apparent strain effects due to intensity changes during pore filling. In contrast to SANS, the strain isotherms obtained from in situ dilatometry result from a combination of axial and radial mesopore deformation together with micropore deformation. Strain data were quantitatively analyzed with a theoretical model for micro-/mesopore deformation by combining information from nitrogen and water adsorption isotherms to estimate the watersilica interaction. It was shown that in situ SANS provides complementary information to dilatometry and allows for a quantitative estimate of the elastic properties of the mesopore walls from water adsorption.(VLID)440244

Quasi-epitaxial Metal-Halide Perovskite Ligand Shells on PbS Nanocrystals

Epitaxial growth techniques enable nearly defect free heterostructures with coherent interfaces, which are of utmost importance for high performance electronic devices. While high-vacuum technology-based growth techniques are state-of-the art, here we pursue a purely solution processed approach to obtain nanocrystals with eptaxially coherent and quasi-lattice matched inorganic ligand shells. Octahedral metal-halide clusters, respectively 0-dimensional perovskites, were employed as ligands to match the coordination geometry of the PbS cubic rock-salt lattice. Different clusters (CH₃NH₃⁺)_{(6–<i>x</i>)}[M^(<i>x</i>+)Hal₆]^{(6–<i>x</i>)–} (M^<i>x</i>+ = Pb­(II), Bi­(III), Mn­(II), In­(III), Hal = Cl, I) were attached to the nanocrystal surfaces <i>via</i> a scalable phase transfer procedure. The ligand attachment and coherence of the formed PbS/ligand core/shell interface was confirmed by combining the results from transmission electron microscopy, small-angle X-ray scattering, nuclear magnetic resonance spectroscopy and powder X-ray diffraction. The lattice mismatch between ligand shell and nanocrystal core plays a key role in performance. In photoconducting devices the best performance (detectivity of 2 × 10¹¹ cm Hz ^1/2/W with > 110 kHz bandwidth) was obtained with (CH₃NH₃)₃BiI₆ ligands, providing the smallest relative lattice mismatch of <i>ca</i>. −1%. PbS nanocrystals with such ligands exhibited in millimeter sized bulk samples in the form of pressed pellets a relatively high carrier mobility for nanocrystal solids of ∼1.3 cm²/(V s), a carrier lifetime of ∼70 μs, and a low residual carrier concentration of 2.6 × 10¹³ cm^–3. Thus, by selection of ligands with appropriate geometry and bond lengths optimized quasi-epitaxial ligand shells were formed on nanocrystals, which are beneficial for applications in optoelectronics