Atmospheric Plasma Synthesized and Surface Engineered Silicon and Graphene Nanocrystals in Energy Applications
Authors: Svrcek Vladimir 
Published: February 2020

Environmental and abundant silicon (Si) and carbon (C) play primary roles in current photovoltaic technologies. Si- and C- nanomaterials exhibit unique properties exhibited used in many field of application (e.g. solar cells, biology, optoelectronics). For instance, Si due to indirect energy band gap is poor in absorption and photoluminescence emission properties which can be enhanced at quantum confinement size due to electronic state and energy band engineering at quantum confinement size. On the other hand C-nanostructures based on sp2 hybridization reflect high strength, high electrical conductivities and enhanced photoluminescence.
In this contribution we demonstrate that confined atmospheric plasma interactions through liquid is suitable either for synthesis and or surface treatment of Si- and C- quantum nanomaterials. Two deferent plasma-liquid sources are applied for synthesis and surface engineering. Namely, femtosecond (fs) laser beam (100 fs) is used to generate a plasma in the liquid and an atmospheric pressure direct-current (dc) microplasma generated under ambient air condition and coupled to the colloid via a counter electrode. We show that graphene nanocrystals with sizes around 5 nm can be synthesized using both plasma sources. We show that the graphene nanocrystals with different room temperature photoluminescence properties covering spectral range from 400nm to 850 nm can be formed as a function of molecular precursors. Furthermore whereby incorporated into a solar cell an high open-circuit voltage of 1.8 V is achieved. On the other hand, we show that during the plasma processing the Si- nanocrystals can be successfully used to tune OH surface functionalization. We discuss how the microplasma processing allows us to gradually change the degree of OH coverage, enabling us, in turn, to gradually shift the emitted light in the photoluminescence spectra by up to 100 nm to longer wavelengths at room temperature. The first-principles calculations are consistent with the experimentally observed dependence of the photoluminescence wavelengths on the OH coverage and show that the photoluminescence 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. Moreover, such a functionalized Si- nanocrystals can better interact with host matrix or can be introduced within. We present study on organic-inorganic hybrid based on methylammonium iodo bismuthate (CH3NH3)3(Bi2I9) (MABI) thin films. Due to the large proportion of localized excitons coupled with delocalized excitons from intercluster energy transfer an anisotropic photoluminescence is observed. Nonlinear photoluminescence properties at excitation energy above twice band gap indicate a carrier multiplication due to the low dimensionality Bi2I9 clusters. We show that the MABI structure can also accommodate plasma surface engineered Si nanocrystals that enhance the excitons dissociation.

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