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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.