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Heteroepitaxial growth is a process of profound fundamental importance as well as an avenue to realize nanostructures such as Ge/Si quantum dots (QDs), with appealing properties for applications in opto- and nanoelectronics. However, controlling the Ge/Si QD size, shape, and composition remains a major obstacle to their practical implementation. Here, Ge nanostructures on Si(111) were investigated in situ and in real-time by low energy electron microscopy (LEEM), enabling the observation of the transition from wetting layer formation to 3D island growth and decay. The island size, shape, and distribution depend strongly on the growth temperature. As the deposition temperature increases, the islands become larger and sparser, consistent with Brownian nucleation and capture dynamics. At 550 degrees C, two distinct Ge/Si nanostructures are formed with bright and dark appearances that correspond to flat, atoll-like and tall, faceted islands, respectively. During annealing, the faceted islands increase in size at the expense of the flat ones, indicating that the faceted islands are thermodynamically more stable. In contrast, triangular islands with uniform morphology are obtained from deposition at 600 degrees C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600 degrees C initially show no change in morphology and size and then rupture simultaneously, signaling a homogeneous chemical potential of the islands. These observations reveal the role of dynamics and energetics in the evolution of Ge/Si QDs, which can serve as a step towards the precise control over the Ge nanostructure size, shape, composition, and distribution on Si(111).

Temperature mapping of the different molecular phases of tribromoterthienobenzene on (111) coinage metals., Ullmann coupling of halogenated aromatics is widely used in on-surface synthesis of two-dimensional (2D) polymers and graphene nanoribbons. It stands out among other reactions for regioselectively connecting aromatic monomers into 1D and 2D π-conjugated polymers, whose final structure and properties are determined by the initial building blocks. Thanks to their exceptional electronic properties, thiophene-containing monomers are frequently used for the synthesis of various conjugated materials. On the other hand, their use in on-surface polymerization is hampered by the possibility of ring opening when adsorbed on metal surfaces. In the present work, we mapped the temperature regime for these two competing reactions by investigating the adsorption of a thiophene-based prochiral molecule using scanning tunneling microscopy, X-ray photoelectron spectroscopy and density functional theory calculations. We followed the formation of organometallic (OM) networks, their evolution into covalent structures and the competition between C–C coupling and thiophene ring opening. The effect of surface reactivity was explored by comparing the adsorption on three (111) coinage metal substrates, namely Au, Ag and Cu. While outlining strategies to minimize the ring opening reaction, we found that the surface temperature during deposition is of paramount importance for the preparation of 2D OM networks, greatly enhancing the overall ordering of the product by depositing on hot Ag surface. Notably, the same protocol permits the creation of OM structures on the air-stable Au surface, thereby allowing the synthesis and application of 2D OM networks outside the ultra-high vacuum environment.

There are different material candidates for efficient industry compatible luminescence solar concentrators to be used in electricity generating glass applications. Among them, divalent thulium (Tm2+) doping has been shown to have a high potential. It has been reported to have 65% absorption of the solar spectral range when in di-halide hosts, while showing no self-absorption and a relatively neutral colour [1]. These characteristics are possible because Tm2+ has a strong absorption from the UV up to about 900 nm. This absorption is generated by intense 4f 13 → 4f 125d 1 electronic transitions and a single emission line at 1140 nm, credited to the 2F5/2 → 2F7/2 transition of the 4f 13 configuration. However, this specific oxidation state of thulium is rather anomalous: so far it was only stabilized on di-halide crystals. These crystals are very hygroscopic in nature, therefore not sufficiently attractive for scaling up in industry standard thin film growth techniques. Thus, it is necessary to research on the possible extension in the list of materials able to host such a promising optically active element. However, the intrinsic complex nature of the lanthanide’s electronic structure has prevented a detailed understanding of the valence stability of its elements. Therefore, designing industry compatible hosts ensuring that such exotic ion is stable and capable of efficiently emitting NIR light is a challenging task. In this work, I show the progress made so far towards unravelling the conditions for stabilization of the Tm2+ in industry compatible thin film materials – more specifically, thin film materials of the SiAlON family. I report recent results on Tm doped SiAlON thin films grown by combinatorial magnetron reactive sputtering. The gradient characterization method [2], developed at the TU Delft’s luminescence materials research group, was used in order to relate position dependent film composition – determined by energy dispersive x-ray spectroscopy – with local transmission, photoluminescence emission and excitation data. This singular approach makes it possible to retrieve a wide composition range in a smaller number of different films. These results are discussed upon the valence stability model of lanthanides of Dorenbos [3], and the model of impurity ionization adopted in solid state statistics. References [1] O. M. ten Kate, K. W. Krämer, and E. van der Kolk, “Efficient luminescent solar concentrators based on self-absorption free, Tm 2+ doped halides,” Sol. Energy Mater. Sol. Cells, vol. 140, pp. 115–120, Sep. 2015. [2] E. P. J. Merkx and E. van der Kolk, “Method for the Detailed Characterization of Cosputtered Inorganic Luminescent Material Libraries,” ACS Comb. Sci., vol. 20, no. 11, pp. 595–601, Nov. 2018. [3] P. Dorenbos, “Valence stability of lanthanide ions in inorganic compounds,” Chem. Mater., vol. 17, no. 25, pp. 6452–6456, Dec. 2005.

Several parameters, including specific surface area, morphology, crystal size, and dopant concentration, play a significant role in improving the photocatalytic performance of ZnO. However, it is still unclear which of these parameters play a significant role in enhancing the photocatalytic activity. Herein, undoped and Mn-, Co-, and Cu-doped platelet-like zinc oxide (ZnO) nanostructures were synthesized via a facile microwave synthetic route, and their ultraviolet (UV) and visible-light-induced photocatalytic activities, by monitoring the gaseous acetaldehyde (CH3CHO) degradation, were systematically investigated. Both the pure and doped ZnO nanostructures were found to be UV-active, as the CH3CHO oxidation photocatalysts with the Cu-doped ZnO one being the most UV-efficient photocatalyst. However, upon visible light exposure, all ZnO-nanostructured samples displayed no photocatalytic activity except the Co-doped ZnO, which showed a measurable photocatalytic activity. The latter suggests that Co-doped ZnO nanostructures are potent candidates for several indoor photocatalytic applications. Various complementary techniques were utilized to improve the understanding of the influence of Mn-/Co-/Cu-doping on the photocatalytic performance of the ZnO nanostructures. Results showed that the synergetic effects of variation in morphology, surface defects, that is, VO, high specific surface areas, and porosity played a significant role in modulating the photocatalytic activity of ZnO nanostructures. © 2019 American Chemical Society.

Silicon solar cells have captured a large portion of the total market of photovoltaic devices mostly due to their relatively high efficiency. However, Silicon exhibits limitations in ultraviolet absorption because high-energy photons are absorbed at the surface of the solar cell, in the heavily doped region, and the photo-generated electron-hole pairs need to diffuse into the junction region, resulting in significant carrier recombination. One of the alternatives to improve the absorption range involves the use of down-shifting nano-structures able to interact with the aforementioned high energy photons. Here, as a proof of concept, we use downshifting CdSe/CdS quantum dots to improve the performance of a silicon solar cell. The incorporation of these nanostructures triggered improvements in the short circuit current density (Jsc, from 32.5 to 37.0 mA/cm2). This improvement led to a ∼13% increase in the power conversion efficiency (PCE), from 12.0 to 13.5%. Our results demonstrate that the application of down-shifting materials is a viable strategy to improve the efficiency of Silicon solar cells with mass-compatible techniques that could serve to promote their widespread utilization.

We have followed by scanning tunneling microscopy (STM) the Stranski–Krastanov (SK) growth of thin Ge films obtained by reactive deposition epitaxy on a Si(111) surface. For Ge thickness smaller than 1.2 ML, STM images show large flat regions (reconstructed 7×7) without protrusions. With increasing thickness Ge-Si 2D islands (reconstructed 5×5) start nucleating, while the Si substrate retains its original 7×7 reconstruction. The islands are flat and have a triangular shape with a lateral size which increases progressively with deposition. We have studied the growth law of the average dimensions of the islands and of the average number of islands per unit surface as a function of coverage. STM images suggest a sizeable process of intermixing at the Ge/Si interface, which has been directly confirmed by XAFS (X-ray absorption fine structure) spectra on Ge/Si(111) samples. We have followed the evolution of the wetting layer up to its completion, both after and during deposition. The appearance of a percolated structure is observed when a critical fraction of the surface (about 70%) is covered.

Fair and meaningful device per- formance comparison among luminescent solar concentrator- photovoltaic (LSC-PV) reports cannot be realized without a gen- eral consensus on reporting stan- dards in LSC-PV research. There- fore, it is imperative to adopt standardized characterization protocols for these emerging types of PV devices that are consistent with other PV devices. This commentary highlights several common limitations in LSC literature and summarizes the best practices moving for- ward to harmonize with standard PV reporting, considering the greater nuances present with LSC-PV. Based on these prac- tices, a checklist of actionable items is provided to help stan- dardize the characterization/re- porting protocols and offer a set of baseline expectations for au- thors, reviewers, and editors. The general consensus combined with the checklist will ultimately guide LSC-PV research towards reliable and meaningful ad- vances.

We have provided direct evidence for the presence of considerable Si-Ge intermixing in strained and unstrained Ge quantum dots deposited on Si(001) and Si(111). The local structure around Ce was probed by using Ge K-edge X-ray absorption spectroscopy; complementary evidence for intermixing was provided by AFM and STM studies. These results implied that the strain energy in the dots was reduced by Si atoms diffusing into the dots, resulting in a modified form of Stranski-Krastanov growth. (C) 2000 Elsevier Science B.V. All rights reserved.

(Na0.5Bi0.5TiO3)1-x(BiAlO3)x (0 ≤ x ≤ 0.08) solid solutions were prepared by solid state reaction and their electrical properties established by ac impedance spectroscopy and electromotive force transport number measurements. Incorporation of BiAlO3 (BA) decreases the electrical conductivity of Na0.5Bi0.5TiO3 (NBT) and with increasing x sequentially changes the conduction mechanism from predominant oxide-ion conduction, to mixed ionic-electronic conduction and finally to predominant electronic conduction. The suppressed oxide-ion conduction by BA incorporation significantly reduces the dielectric loss at elevated temperatures and produces excellent high-temperature dielectric materials for high BA contents. Possible reasons for the suppressed oxide-ion conduction in the NBT-BA solid solutions are discussed and we propose the local structure, especially trapping of the oxygen vacancies by Al3+ on the B-site, plays a key role in the oxide-ion conduction in these apparently ‘stoichiometric’ NBT-based solid-solution perovskite materials.

Surface-confined polymerization via Ullmann coupling is a promising route to create one- and two-dimensional covalent ?-conjugated structures, including the bottom-up growth of graphene nanoribbons. Understanding the mechanism of the Ullmann reaction is necessary to provide a platform for rationally controlling the formation of these materials. We use fast X-ray photoelectron spectroscopy (XPS) in kinetic measurements of epitaxial surface polymerization of 1,4-dibromobenzene on Cu(110) and devise a kinetic model based on mean field rate equations, involving a transient state. This state is observed in the energy landscapes calculated by nudged elastic band (NEB) within density functional theory (DFT), which assumes as initial and final geometries of the organometallic and polymeric structures those observed by scanning tunneling microscopy (STM). The kinetic model accounts for all the salient features observed in the experimental curves extracted from the fast-XPS measurements and enables an enhanced understanding of the polymerization process, which is found to follow a nucleation-and-growth behavior preceded by the formation of a transient state. ? 2016 American Chemical Society.

Upon adsorption on the (111) facet of Ag, 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid (PVBA) self-assembles into a highly ordered, chiral twin chain structure at submonolayer coverages with domains that can extend for micrometers in one dimension. Using polarization-dependent measurements of C and N K-shell excitations in near-edge X-ray absorption fine structure (NEXAFS) spectra, we determine the binding geometry of single PVBA molecules within this unique ensemble for both low and high coverage regimes. At submonolayer coverage, the molecule is twisted to facilitate the formation of hydrogen bonds. The gas-phase planarity is gradually recovered as the coverage is increased, with complete planarity coinciding with loss of order in the overlayer. Thermal treatment of the PVBA film results in deprotonation of the carboxyl tail of the molecule, but despite the suppression of the stabilizing hydrogen-bonds, the overlayer remains ordered.

In this contribution we present a study of the Ge-Si intermixing process that arises during the growth of Ge/Si(111) self-assembled islands. The samples, grown by means of a molecular beam epitaxy (MBE)-like technique, have been characterized by in-situ scanning tunneling microscopy (STM) and atomic force microscopy (AFM) complemented by ex-situ X-ray absorption fine structure (XAFS). We have observed by STM a change in the island morphological evolution, from truncated tetrahedra to atoll-like islands, that can be related to the misfit the reduction effect induced by the intermixing process. We have evaluated the intermixing by measuring the average coordination numbers of Si and Ge around a Ge atom by XAFS. We show that the Si content in the nominally pure Ge wetting layer reaches 50% while in the three-dimensional (3-D) islands it is about 25%, and that the intermixing increases with increasing deposition temperature in the 450-530 degreesC range.

A large fraction of processes which are at the foundation of our technological society involve physical and chemical properties of surfaces. Catalytic reactions and semiconductor devices production are two of the most important ones. This paper describes a sample of some of the most relevant surface science experiments which have been recently performed, in order to understand elementary surface processes of model catalytic reactions and in semiconductor technology at the atomic level. The focus is on experiments performed with scanning tunneling microscopy and atomic force microscopy which have represented, in some cases, real breakthroughs in our understanding of these phenomena. We then present an overview of possible experimental technique developments that can be foreseen for the future and that may give us a more in-depth understanding of the elementary processes which form the basis of important complex surface phenomena. Finally, some of the challenging tasks that lie ahead for surface scientists and the collateral opportunities are discussed.

Exploiting Ge K-edge x-ray absorption spectroscopy we provide direct evidence of Si-Ge intermixing in self-organized strained and unstrained Ge quantum dots on Si, and provide a quantitative measurement of the average composition. For Ge/Si(001) dots with equivalent thickness in the range 5.8-38 nm and morphology ranging from that typical of coherently strained to that associated with relaxed dots we find that the average Si composition is approximately 30%. For Ge/Si(111), we find that the wetting layer has a Si composition near 50%. We discuss these results in terms of the energetics of dot formation and argue that strain-enhanced diffusion of Si into Ge should be considered as an important factor in minimizing the strain energy of the system.