Porous Si Partially Filled with Water Molecules—Crystal Structure, Energy Bands and Optical Properties from First Principles

Abstract

The paper reports the results on first-principles investigation of energy band spectrum and optical properties of bulk and nanoporous silicon. We present the evolution of energy band-gap, refractive indices and extinction coefficients going from the bulk Si of cubic symmetry to porous Si with periodically ordered square-shaped pores of 7.34, 11.26 and 15.40 Å width. We consider two natural processes observed in practice, the hydroxylation of Si pores (introduction of OH groups into pores) and the penetration of water molecules into Si pores, as well as their impact on the electronic spectrum and optical properties of Si superstructures. The penetration of OH groups into the pores of the smallest 7.34 Å width causes a disintegration of hydroxyl groups and forms non-bonded protons which might be a reason for proton conductivity of porous Si. The porosity of silicon increases the extinction coefficient, k, in the visible range of the spectrum. The water structuring in pores of various diameters is analysed in detail. By using the bond valence sum approach we demonstrate that the types and geometry of most of hydrogen bonds created within the pores manifest a structural evolution from distorted hydrogen bonds inherent to small pores (∼7 Å) to typical hydrogen bonds observed by us in larger pores (∼15 Å) which are consistent with those observed in a wide database of inorganic crystals.

Second harmonic generation on crystalline organic nanoclusters under extreme nanoconfinement in functionalized silica–benzil composites

“…As heterogeneous multiphase formations, they can be fabricated combining appropriate organic and inorganic materials in various nanoscale morphologies such as e.g., carbon nanotubes 1 – 4 and nanoribbons 5 – 7 , graphene nanoplatelets 8 – 10 , carbon, perovskite and semiconductor quantum dots 11 – 13 , nanoparticles and nanospheres 14 – 16 , nanowires 17 – 21 , organic dyes 22 , nanocrystals 23 – 25 , isotropic liquids 26 or anisotropic liquid crystal materials 27 – 30 being incorporated into organic or inorganic liquids or host solid media such as e.g., polymers or inorganic nanoporous materials. Among the latter ones, mesoporous silica, p SiO (hereafter PS ) 31 , likewise the mesoporous silicon Si 32 – 34 and alumina p Al O (anodic aluminum oxide, AAO) 35 may be considered as most advanced host matrices for many nanocomposite technologies. Specific morphology of PS membranes results from the technology of their fabrication based on unidirectional etching process of p -doped silicon with its subsequent oxidation.…”

Nanoarhitectonics of Inorganic–Organic Silica–Benzil Composites: Synthesis, Nanocrystal Morphology and Micro-Raman Analysis

“…There is a wide range of nanocomposite materials, i.e., multi-phase formations in which at least one of the phases has one, two, or three dimensions smaller than 100 nanometers. Guest nano-objects of various morphologies, including carbon nanotubes [ 3 , 4 , 5 ], graphene nanoplates [ 6 ], nanowires [ 7 , 8 , 9 ], nanoparticles and nanospheres [ 10 , 11 , 12 ], quantum dots [ 13 ], and nanocrystals [ 14 , 15 , 16 ], may be embedded into a composite host matrix, such asa polymer, gel, or nanoporous crystalline medium. Often, the filling materials bring about the main functional properties of the newly created substance, which are not characteristic of the unfilled host matrices.…”