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We examine the effect of a top nanofilm of 3 nm silicon nanoparticles (Si-NPs) on the spectral features of the reflectivity of germanium (Ge). We use a 450 nm Ge layer grown by low temperature plasma enhanced chemical vapor deposition on a crystalline Si substrate. The Si-NPs are drop casted incrementally on the Ge/Si structure from a particle colloid, forming a nanofilm of increasing thickness up to 30 nm. The stack is characterized using luminescence microscopy, atomic force microscopy, transmission electron microscopy, and reflectivity spectroscopy. The reflectance of the stack is measured over UV/ visible and infrared range 4.8−0.8 eV. With the increasing thickness of the Si nanocoating, the results show a strong reduction in the reflectivity of Ge in the visible and blue-UV region of the spectrum with less effect in the infrared, as well as a difference in reflectance at the adjacent lying complex of Λ and L transitions of Ge. The strong reduction in the visible/UV range is understood in terms of a near overlap of the band to band transitions of Si-NPs and Ge. The diminished response in the infrared contributes to shifting of the bandgap of Si-NPs from the infrared to the visible due to quantum confinement. Furthermore, optical properties of Si-NP are estimated and samples are analyzed using Modified Transfer Matrix Method model. The strong reduction of reflectivity of Ge in the visible-UV region is very useful in photovoltaics, with special emphasis on Ge-based multijunction solar cells.

In this work we use conductive atomic force microscopy (cAFM) to study the charge injection process from a nanoscale tip to a single isolated bilayer 2D MoS2 flake. The MoS2 is exfoliated and bonded to ultra-thin SiO2/Si substrate. Local current–voltage (IV) measurements conducted by cAFM provides insight in charge trapping/de-trapping mechanisms at the MoS2/SiO2 interface. The MoS2 nano-flake provides an adjustable potential barrier for embedded trap sites where the charge is injected from AFM tip is confined at the interface. A window of (ΔV <1.8 V) is obtain at a reading current of 2 nA between two consecutive IV sweeps. This is a sufficient window to differentiate between the two states indicating memory behavior. Furthermore, the physics behind the charge entrapment and its contribution to the tunneling mechanisms is discussed.

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During the 2020-current COVID-19 pandemic, the importance of wearing a mask to reduce infection and spread is key. The mask works as filter for the different microorganisms. In this work the geometrical part of the filtration process of the N95 and surgical masks was studied using luminescent ultra-small silicon nanoparticles (Si-NPs) to represent the SARS-CoV-2 by spraying it on the mask using atomizer. Scanning electron microscopy (SEM), and optical microscope were used to check the mask. The obtained images show that the Si nanoparticles to are trapped by the PE fiber network, indicating its ability to filter SARS-CoV-2. This visualization using nanotechnology can help to further improve mask designs for better filtration.

Graphic Abstract Commercial polyethylene (PE) fiber-based masks are currently used as personal filters for protection against various microorganisms. Due to the coronavirus (SARS-CoV-2) pandemic of 2020, the use of masks has become the critical mechanism in reducing the spread. The PE mask filter uses a sieve (geometry) in a spider web fashion to filter out microorganisms using Van der Waals atomic forces. However, the non-geometrical part of the filtration process is not fully understood. In this work, we utilized luminescent ultra-small silicon nanoparticles, which are Si-H or/and Si-OH terminated to examine how the filter operates at a chemical level. The particles were sprayed onto the fiber network by an atomizer and we used scanning electron microscopy (SEM), optical microscope and fluorescence spectroscopy under UV radiation. The images and measurements clearly showed that the Si nanoparticles bonded to the PE fiber network. The results were analyzed in terms of chemical bonding between Si nanoparticle and fiber. Our findings suggest that the PE fibers could act as a chemical filter via hydrogen or hydrolysis–based bonding or via Si-C bonding, which is complementary to their physical filtration ability via the geometric sieve process.