Reflectance anisotropy spectroscopy (RAS) has been largely used to investigate organic compounds: Langmuir-Blodgett and Langmuir-Schaeffer layers, the organic molecular beam epitaxy growth in situ ...and in real time, thin and ultrathin organic films exposed to volatiles, in ultra-high vacuum (UHV), in controlled atmosphere and even in liquid. In all these cases, porphyrins and porphyrin-related compounds have often been used, taking advantage of the peculiar characteristics of RAS with respect to other techniques. The technical modification of a RAS spectrometer (CD-RAS: circular dichroism RAS) allows us to investigate the circular dichroism of samples instead of the normally studied linear dichroism: CD-RAS measures (in transmission mode) the anisotropy of the optical properties of a sample under right and left circularly polarized light. Although commercial spectrometers exist to measure the circular dichroism of substances, the "open structure" of this new spectrometer and its higher flexibility in design makes it possible to couple it with UHV systems or other experimental configurations. The importance of chirality in the development of organic materials (from solutions to the solid state, as thin layers deposited-in liquid or in vacuum-on transparent substrates) could open interesting possibilities to a development in the investigation of the chirality of organic and biological layers. In this manuscript, after the detailed explanation of the CD-RAS technique, some calibration tests with chiral porphyrin assemblies in solution or deposited in solid film are reported to demonstrate the quality of the results, comparing curves obtained with CD-RAS and a commercial spectrometer.
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°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°C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600°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).
Real‐time low energy electron microscopy observations reveal the growth dynamics and stability of Ge quantum dots formed on Si(111) at temperatures between 450°C and 600°C. A mix of metastable, flat islands and tall, faceted islands are produced at 550°C and below, whereas growth at 600°C yields uniform large, triangular islands.
The morphology of the film created by the aggregation of corrole molecules from ethanol solution onto the surface of gold substrate was studied by scanning tunneling microscopy both: (i) in situ, ...namely in quasi-equilibrium (wet sample), and (ii) ex situ, i.e., removing the substrate from the fluid cell and leaving the residual amount of solution on the substrate to desiccate (dry sample). Despite the different conditions, the morphology of the corrole overlayer formed on the film of molecules is reminiscent, in both cases, of the leopard skin; the “speckles” are actually areas where the molecular overlayer of flat molecules is not completed. We quantitatively compared these two similar morphologies by studying the power spectral density of the images, the fractal dimension D P of the hole perimeters, and the cumulative distribution function of the hole areas. We found that the last layer of both films, in spite of very different formation conditions, differed from each other only by a scale factor.
Corrole derivatives have been recently employed in many applications at the solid–liquid interface. Therefore, the structural arrangement of the molecular layers in direct contact with the liquid is ...of fundamental interest. We investigated in solution the deposition of molecular layers of the previously prepared water‐soluble phosphorus complex of a 2‐sulfonato‐10‐(4‐sulfonatophenyl)‐5,15‐dimesitylcorrole see synthesis in our previous paper, M. Naitana et al., Chem. Eur. J. 2017, 23, 905–916. The layer formation of P corroles onto the Au(111) surface was monitored by STM in situ, that is, with the substrate immersed in the solution. Marked differences in the morphology between the organic layer formed on the substrate and that deposited after solvent evaporation (drop casting) are reported. In particular, the coating of gold was more effective and stable in the presence of liquid. Preservation of functionality of the corrole molecules after adsorption was verified. This result validates the relevance of corrole layers at the solid–liquid interface to exploit the peculiar properties of these molecules in real‐world applications.
Monitoring of layer formation of a phosphorus corrole onto the Au(111) surface was performed by STM in situ, that is, with the substrate immersed in the solution. Preservation of the functionality of corrole molecules after adsorption was verified.
The synthesis, spectroscopic, and optical properties of the water‐soluble phosphorus complex of a 2‐sulfonato‐10‐(4‐sulfonatophenyl)‐5,15‐dimesitylcorrole have been investigated. The compound was ...prepared by adopting a novel strategy for the corrole sulfonation, leading to the regioselective isomer in an almost quantitative yield. The phosphorus coordination has a key role in determining the corrole substitution pattern, limiting the formation of poly‐substituted species, which affected the reaction of the corrole free base. The resulting complex shows excellent optical properties in terms of emission quantum yield, also in polar protic solvents, including water. 31P NMR spectroscopy in CD3OD indicates that the P sulfonate complex has been isolated in a hexacoordinated geometry with two different ligands (L1=‐OH, L2=‐OCH3), and it is prone to axial ligand exchange with methanol, with no evidence of intermediate pentacoordinated species. The morphological characterization of thin layers of the P corrole deposited onto an Au(111) surface showed that the addition of an intermediate layer of reduced graphene oxide allows for a better control of corrole aggregation, inducing also transformation of the Au(111) reconstructed surface.
Phosphorus is needed: The regioselective sulfonation of a triaryl corrole has been carried out with a novel strategy, where phosphorus coordination plays a key role in determining the corrole substitution pattern. The complex shows a high brightness in water, making it promising for sensor applications.
Here we show a new effect due to the arsenic flux in the molecular beam epitaxy growth of InAs quantum dots on GaAs(001) at temperatures higher than 500 °C and high As/In flux ratio. We show that, by ...changing and tuning the direction of the As flux on a rippled substrate, a selective growth can be obtained where the dots form only on some appropriately orientated slopes of a sequence of mounds elongated along the 1̅10 surface direction. Since the relative As flux intensity difference over the two opposite mound slopes is very small (2–5%), the observed large effect cannot be explained simply as a pure shadowing effect and reveals instead that As, whose contribution to the modeling of growth has often been ignored or underestimated, probably for a lack of knowledge, plays a fundamental role at these growth conditions. To explain our experiment, we have developed a kinetic model that explicitly takes into account the coupling between cations (In) and anions (As) and found that the very small surface gradient in the anion flux, due to the oblique evaporation on the mounded surface, is responsible for a massive drain of cations toward the surface anion-rich areas, thus generating the selective growth of quantum dots. We expect a comparable behavior for the anions of other III–V and II–VI compound semiconductors.
Abstract
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°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°C, suggesting that the growth more closely follows the ideal shape. During annealing, the islands formed at 600°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).
The statistical methods exploiting the "Correlation-Functions" or the "Differential-Critical-Region" are both suitable for describing phase transformation kinetics ruled by nucleation and growth. We ...present a critical analysis of these two approaches, with particular emphasis to transformations ruled by diffusional growth which cannot be described by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory. In order to bridge the gap between these two methods, the conditional probability functions entering the "Differential-Critical-Region" approach are determined in terms of correlation functions. The formulation of these probabilities by means of cluster expansion is also derived, which improves the accuracy of the computation. The model is applied to 2D and 3D parabolic growths occurring at constant value of either actual or phantom-included nucleation rates. Computer simulations have been employed for corroborating the theoretical modeling. The contribution to the kinetics of phantom overgrowth is estimated and it is found to be of a few percent in the case of constant value of the actual nucleation rate. It is shown that for a parabolic growth law both approaches do not provide a closed-form solution of the kinetics. In this respect, the two methods are equivalent and the longstanding overgrowth phenomenon, which limits the KJMA theory, does not admit an exact analytical solution.
We outline a metal-free fabrication route of in-plane Ge nanowires on Ge(001) substrates. By positively exploiting the polishing-induced defects of standard-quality commercial Ge(001) wafers, ...micrometer-length wires are grown by physical vapor deposition in ultra-high-vacuum environment. The shape of the wires can be tailored by the epitaxial strain induced by subsequent Si deposition, determining a progressive transformation of the wires in SiGe faceted quantum dots. This shape transition is described by finite element simulations of continuous elasticity and gives hints on the equilibrium shape of nanocrystals in the presence of tensile epitaxial strain.
PACS
81.07.Gf; 68.35.bg; 68.35.bj; 62.23.Eg
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