How Clean Is Clean? Recipes for van der Waals Heterostructure Cleanliness Assessment.

Van der Waals (vdW) heterostructures, artificial stacks of two-dimensional materials, present an exciting platform to explore new physical phenomena and unique applications. An important and increasingly recognized factor limiting the electrical and optical performance of heterostructure samples is the presence of interfacial contamination. In published work reporting various heterostructure fabrication methods, evidence for the cleanliness of samples is often presented as optical and atomic force microscopy images, typically exhibiting a completely flat topography. In this work, we demonstrate that such samples may nonetheless contain a uniformly thin layer of contaminants at the heterostructure interface. As alternatives, we propose two robust visualization methods that are highly sensitive to such residues, based on photoluminescence mapping and on selective solvent diffusion. The detection capability and straightforward implementation of these two imaging techniques make them powerful tools to assess and improve the cleanliness of a wide variety of fabrication techniques for heterostructures comprising any combination of vdW materials.

Vibrational monitor of early demineralization in tooth enamel after in vitro exposure to phosphoridic liquid.

The Raman spectroscopic method has been applied to quantitatively assess the in vitro degree of demineralization in healthy human teeth. Based on previous evaluations of Raman selection rules (empowered by an orientation distribution function (ODF) statistical algorithm) and on a newly proposed analysis of phonon density of states (PDOS) for selected vibrational modes of the hexagonal structure of hydroxyapatite, a molecular-scale evaluation of the demineralization process upon in vitro exposure to a highly acidic beverage (i.e., CocaCola™ Classic, pH=2.5) could be obtained. The Raman method proved quite sensitive and spectroscopic features could be directly related to an increase in off-stoichiometry of the enamel surface structure since the very early stage of the demineralization process (i.e., when yet invisible to other conventional analytical techniques). The proposed Raman spectroscopic algorithm might possess some generality for caries risk assessment, allowing a prompt non-contact diagnostic practice in dentistry.