In this work, a systematic first-principles study of the quasi-band structure of silicon nanocrystals (Si-NCs) is provided, focusing on bandgap engineering by combining quantum confinement of the electronic states with OH surfacefunctionalization. A mapping between the bandgap, Si-NC diameter, and the degree of hydroxide coverage is provided, which can be used as a guideline for bandgap engineering. Complementary to first-principles calculations, the photoluminescence (PL) wavelength of Si-NCs in the quantum-confinement regime is measured with well-defined diameters between 1 and 4 nm. The Si-NCs are prepared by means of a microplasma technique, which allows a surfactant-free engineering of the Si-NCs surface with OH groups. The microplasma treatment technique allows us to gradually change the degree of OH coverage, enabling us, in turn, to gradually shift the emitted light in the PL spectra by up to 100 nm to longer wavelengths. The first-principles calculations are consistent with the experimentally observed dependence of the wavelengths on the OH coverage and show that the PL redshift is determined by the charge transfer between the Si-NC and the functional groups, while on the other hand surface strain plays only a minor part.

Organometal trihalide perovskite solar cells have attracted monumental attention in recent years. Today's best devices, based on a three-dimensional perovskite structure of corner-sharing PbI octahedra, are unstable, toxic, and display hysteresis in current-voltage measurements. We present zero-dimensional organic-inorganic hybrid solar cells based on methylammonium iodo bismuthate (CHNH)(BiI) (MABI) comprising a BiI bioctahedra and observe very low hysteresis for scan rates in the broad range of 150 mV s to 1500 mV s without any interfacial layer engineering. We confirm good stability for devices produced and stored in open air without humidity control. The MABI structure can also accommodate silicon nanocrystals, leading to an enhancement in the short-circuit current. Through the material MABI, we demonstrate a promising alternative to the organometal trihalide perovskite class and present a model material for future composite third-generation photovoltaics.

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Herein, we demonstrate the customized, environmentally friendly tailoring of nanoparticles and their surface chemistry by short pulsed laser irradiation in liquids. This process allows for the formation of crystalline spherical particles exceeding several hundreds of nanometers in water from colloids of electrochemically etched silicon nanocrystals (Si-NCs), which exhibit quantum confinement effects and room-temperature stable luminescence. In particular, nanosecond (ns) pulsed laser irradiation of the Si-NC/water colloids causes the selective heating of the Si-NCs accompanied by the formation of spherical particles. In contrast, femtosecond (fs) laser pulsed irradiation induces the formation of colloidal Si-NCs with peculiar surface chemistry; in particular, fs pulses generate short-lived plasmas with more ionized species in water, which enable the surface engineering of quantum confined Si-NCs, thus limiting Si-NC agglomeration and enhancing their photoluminescent properties.