No abstract

Since the discovery of surface enhanced Raman scattering (SERS), the choice of SERS-active materials has been limited mainly to metals, especially gold and silver in the visible spectrum. Although non-metals can also be SERS-active by forming nanostructures or composite structures with SERS-active materials, the mechanism behind it is still unclear and there is no perfect technique to study it. In this work, by constructing a SERS structure on a flexible polydimethylsiloxane film, we provide a way to study the effect of non-metallic nanostructures on Raman enhancement by attaching the above film onto flat and nanostructured surfaces. It was found that a nanoporous silicon surface contributes to an additional, up to five times, Raman enhancement. The pore depth and pore size also influence the observed Raman enhancement. These findings will help us not only to understand the mechanism of SERS involving non-metallic nanostructures, but also to design more efficient SERS structures for various applications.

Graphene oxide (GO), the oxidized state of graphene, has a similar chemical structure to graphene, except for the existence of many defects and oxygen-containing functional groups. GO could be photoreduced under light irradiation, and it is crucial to study and understand the changes of chemical structure and photoluminescence during this photoreduction process. Herein, by using single-molecule optical microscopy, the photoreduction of three different GO sheets were monitored in situ. The photoluminescence from highly oxidized GO sheets changed continuously under light irradiation, and many blinking spots showed up on this GO sheet surface. The appearance of these blinking events is highly related to the stacking of the GO sheets. Much less blinking could be observed on stacked GO sheets. It was found that the reduction speed of GO in water is much higher than that in air, and much less blinking could be observed when GO was reduced in water. Moreover, this is also highly related to the chemical structures of the GO sheets used for photoreduction. No blinking could be observed on less oxidized GO sheets. This work provides an effective approach for in situ study of the optical properties of GO sheets, boosting a deeper understanding of GO photoreduction. Moreover, this method could also be extended for in situ studying the optical properties of other materials.

The morphology of nanocrystals is crucial in modulating their properties including optical, chemical and catalytic ones. For the intensively studied sheet-like structures, it is also important to engineer their detailed nanostructures on sheet surfaces. In this work, micron-sized bismuth oxychloride (BiOCl) thin square sheets in tetragonal matlockite-like phase and the corner-truncated ones were successfully synthesized by using a simple wet-chemical method. These square sheets could be partially etched, forming surface cracks with the size and density easily tuned. Two different types of cracks were observed on the surfaces of the corner un-truncated and truncated BiOCl square sheets. Surprisingly, radial cracks formed on the corner-truncated BiOCl sheets, which is attributed to the existence of physical stress during the etching and is highly related to the original nanostructure of the BiOCl sheets. This work provides a wet-chemical method for engineering surface cracks, especially radial cracks, on the surfaces of BiOCl nanosheets, which may be expanded to surface engineering of other nanosheets and will show a great potential in various fields. Nanocrystals, especially semiconductor ones, usually possess unique chemical, electrical, optical and magnetic properties compared to their bulk counterparts. [1][2][3][4][5][6] These properties are highly dependent on the chemical composition, size, shape and surface state of the nanocrystals. [3,[7][8][9][10][11] Therefore, controlled synthesis of nanocrystals is becoming a hot topic in many fields, and lots of efforts have been made to precisely control the morphology and surface state of the nanocrystals. [9,[12][13][14][15][16][17] Among various shaped nanocrystals, the sheet-like ones show promising properties and applications due to their structural simplicity, large surface area, easy charge separation, etc. [18][19][20][21] Sheet-like [a] R.