Seeded Rods with Ag and Pd Bimetallic TipsSpontaneous Rearrangements of the Nanoalloys on the Atomic Scale

Deposition of metal cocatalysts is a common practice to improve the activity of photocatalysts. The use of nanoalloyed nanoparticles allows the formation of diverse nanostructures, tailored for a specific application. Nevertheless, too often the spontaneous atomic scale phenomena interfere with the initial design to produce a modified structure with undesirable properties. Here, we demonstrate such a process for Pd, Ag, or their combination as metal tips mounted on seeded rods of a CdSe dot in a CdS rod (CdSe@CdS) that serve as hydrogen evolution photocatalysts. Spontaneous radial reconstruction at the metal tip brings both Pd and Ag atoms outward even when a two-stage preparation process is applied to specifically produce a core–shell structure. The diffusion of Pd outward enables hydrogen evolution even when the initial Pd tip is covered by a Ag shell, and in the opposite case, a Pd shell shows reduced activity compared with Pd-only tips, due to the surfacing of Ag atoms. In addition, we show that the tip reconstruction occurs already during synthesis; aberration-corrected high-resolution electron microscopy also reveals other processes, such as cation exchange and small clustering around the seeded rods, all quite invisible using regular TEM techniques. In addition, we studied the size effect of Pd-tipped seeded rods and showed that the %QE of seeded rods with 2.2 Pd tips is as high as 91%. These results are significant in the understanding of the structure–function relationship, as it highlights one possible hidden reconstruction pathway of nanoalloys

Revealing Growth Schemes of Nanoparticles in Atomic Resolution: Mapping Stacking Fault Formation and Distribution

Controlling the growth process of inorganic nanoparticles, especially the kinetically driven ones, is crucial for designing tailor-made nanoparticles for various applications. Specifically, controlling the formation of stacking faults in semiconductor quantum dots is necessary, since stacking faults were associated with inferior optical performance. Ensemble techniques, such as XRD powder diffraction and optical absorption, can be insensitive to the formation of stacking faults and in certain cases might produce misleading information. Using as a model the thoroughly studied CdSe system, we exploited the well-known unidirectional growth of the Wurtzite phase in order to follow the structural evolution of two different batches of CdSe nanoparticles. We were able to get insight on the crystal growth stages, step by step, employing high resolution electron microscopy and focal series reconstruction. The different kinetics of the two variants were monitored using a statistical approach. The same approach can be used to provide atomic-scale information for any system exhibiting unidirectional growth

Growth Schemes of Tunable Ultrathin CdS_<i>x</i>Se_1–<i>x</i> Alloyed Nanostructures at Low Temperatures

The new 2D colloidal semiconductor ultrathin nanosheets provide an appealing combination of properties. Controlling both their morphology and composition offers another path to control their physical properties. Homogeneously alloyed structures with tunable properties were obtained using NaBH₄ which controls the precursor reactivity. The effect of NaBH₄ on the degree of alloying, shape control, and optical properties of the alloyed colloidal nanosheets is presented here. The alloyed structures are a monolayer thicker than the pure CdS or CdSe. In relatively low Se contents, the addition of NaBH₄ produced high quality alloyed nanosheets that are uniform, in the wurtzite phase and with small thickness distribution as evident from transmission electron microscopy (TEM), X-ray diffraction (XRD), and optical characterization. Atomic resolution phase images provide evidence to both stacking faults formation across the entire width of the sheet as well as local disorder, suggesting a combined mechanism of oriented attachment of patches that are fused and extended by unidirectional growth

Transmission Electron Microscopy Methodology to Analyze Polymer Structure with Submolecular Resolution

The crystallinity of polymeric materials defines their properties, in particular, the mechanical ones. High-resolution transmission electron microscopy (TEM) imaging of polymers would be critical to address intricate polymer crystallinity, yet it is challenging due to polymer sensitivity to the electron beam. We performed high-resolution TEM imaging of polycaprolactone (PCL) thin films employing low-dose focal series reconstruction (LDFSR). LDFSR enabled submolecular resolution imaging of polymer crystals. The direct imaging study was augmented by scanning nanobeam electron diffraction (NBED) using the 4D STEM technique to map micro- and nanoscale crystalline domains. Employing LDFSR combined with 4D STEM, we directly observed interacting polymer chains in the crystal lattice, elucidating the crystal structure with a high degree of precision including lattice deformations. We also imaged PCL lamella using conventional TEM. Our methodology enables long-sought insights into the polymer structure, introducing a new tool for high resolution studies of polymer crystallinity that fills a critical gap in the structural science of polymer materials

Formation and Analysis of Core–Shell Fine Structures in Pt Bimetallic Nanoparticle Fuel Cell Electrocatalysts

An Ångstrom-scale structural and compositional investigation of a dealloyed Pt–Co core–shell nanoparticle fuel cell catalyst with characteristic diameter of 10–15 nm in an early stage of its life cycle reveals unusual self-organized compositional subsurface fine structure, that is, subsequent shells of Co depletion and enrichment. The origin of the unusual structure is rationalized by interplay of Co dissolution, Pt surface diffusion, and an inverse Kirkendall effect. A detailed picture about the chemical composition of the surface and subsurface provides a fundamental insight into the catalytically active structure of bimetallic electrocatalysts

Line Defects in Molybdenum Disulfide Layers

Layered molecular materials and especially MoS₂ are already accepted as promising candidates for nanoelectronics. In contrast to the bulk material, the observed electron mobility in single-layer MoS₂ is unexpectedly low. Here we reveal the occurrence of intrinsic defects in MoS₂ layers, known as inversion domains, where the layer changes its direction through a line defect. The line defects are observed experimentally by atomic resolution TEM. The structures were modeled and the stability and electronic properties of the defects were calculated using quantum-mechanical calculations based on the Density-Functional Tight-Binding method. The results of these calculations indicate the occurrence of new states within the band gap of the semiconducting MoS₂. The most stable nonstoichiometric defect structures are observed experimentally, one of which contains metallic Mo–Mo bonds and another one bridging S atoms

Direct Imaging of Single Au Atoms Within GaAs Nanowires

Incorporation of catalyst atoms during the growth process of semiconductor nanowires reduces the electron mean free path and degrades their electronic properties. Aberration-corrected scanning transmission electron microscopy (STEM) is now capable of directly imaging single Au atoms within the dense matrix of a GaAs crystal, by slightly tilting the GaAs lattice planes with respect to the incident electron beam. Au doping values in the order of 10^17–18 cm³ were measured, making ballistic transport through the nanowires practically inaccessible

Correlating Electron Tomography and Plasmon Spectroscopy of Single Noble Metal Core–Shell Nanoparticles

The 3D structure reconstruction of gold core–silver shell nanoparticles by electron tomography is combined with optical dark-field spectroscopy. Electron tomography allows segmentation of the particles into core and shell subvolumes and facilitates avoiding Bragg diffraction artifacts inherent in 2D images. This advantage proves essential for accurate correlation of plasmon spectra and structure. We find that for the nanoparticles of near-spherical shape studied here the plasmon resonances depend on the relative size of the core and shell, rather than on their exact shapes and concentricity. A remarkable dependence of the spectral shape on the permittivity of the surrounding medium is also demonstrated, suggesting that core–shell nanoparticles can be used as ratiometric sensors with a very high dynamic range

Nanoseashells and Nanooctahedra of MoS₂: Routes to Inorganic Fullerenes

Nanooctahedra of MoS2 are considered to be the true inorganic fullerenes, exhibiting different properties from the bulk and also other closed-cage morphologies of the same material. These structures are produced in high energy systems where the synthesis is performed far from equilibrium conditions, and the reaction mechanism involved remains unknown. Here, the discovery of two imperfect structures of nanooctahedra−the distorted octahedra and seashell structures with meander-like cross sections−is reported and studied in detail using transmission electron microscopy and quantum-mechanical methods. These nanoparticles can serve to understand the synthesis route by establishing the basic principles of their morphology and stability. The fundamental properties of the inorganic lattice are the basis for matching the projections observed in microscopy images with a suggested atomistic model. Quantum-mechanical calculations are used to estimate their stability and electronic properties. It was concluded that the production of nanooctahedra involves a high temperature stage, where lattice defects enable the formation of a closed structure without a templating particle. Thereafter at lower temperatures, the mixture of products is carried forward and the annealing contributes to the enrichment of the product with more symmetric structures

Line Defects in Molybdenum Disulfide Layers

Layered molecular materials and especially MoS₂ are already accepted as promising candidates for nanoelectronics. In contrast to the bulk material, the observed electron mobility in single-layer MoS₂ is unexpectedly low. Here we reveal the occurrence of intrinsic defects in MoS₂ layers, known as inversion domains, where the layer changes its direction through a line defect. The line defects are observed experimentally by atomic resolution TEM. The structures were modeled and the stability and electronic properties of the defects were calculated using quantum-mechanical calculations based on the Density-Functional Tight-Binding method. The results of these calculations indicate the occurrence of new states within the band gap of the semiconducting MoS₂. The most stable nonstoichiometric defect structures are observed experimentally, one of which contains metallic Mo–Mo bonds and another one bridging S atoms