Abstract
Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags. Here, we report a method for visualization of nanostructures down to a few nanometers using a conventional bright-field microscope without requiring additional molecular tags such as fluorophores. The technique, Bright-field Nanoscopy, is based on the strong thickness dependent color of ultra-thin germanium on an optically thick gold film. We demonstrate the visualization of grain boundaries in chemical vapour deposited single layer graphene and the detection of single 40 nm Ag nanoparticles. We estimate a size detection limit of about 2 nm using this technique. In addition to visualizing nano-structures, this technique can be used to probe fluid phenomena at the nanoscale, such as transport through 2D membranes. We estimated the water transport rate through a 1 nm thick polymer film using this technique, as an illustration. Further, the technique can also be extended to study the transport of specific ions in the solution. It is anticipated that this technique will find use in applications ranging from single-nanoparticles resolved sensing to studying nanoscale fluid-solid interface phenomena.
Highlights
Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags
The etch rate of GeO2 has been reported to range from 10−2 nm/min to 104 nm/min depending on the oxidative state of Ge27
We have determined the etch rate in our system to be around 0.3 nm/min [Supplementary Information (SI) text, section 8]
Summary
Most methods for optical visualization beyond the diffraction limit rely on fluorescence emission by molecular tags. We present a technique to visualize nanoscale surface features based on the optical contrast generated by the differential etching of a germanium (Ge) thin film (~30 nm thick) deposited on gold in the presence of water. Such ultra-thin Ge films on gold produce a strong thickness dependent color response[10]. A simultaneous lateral etching of the Ge film amplifies the lateral dimensions of the nanoscale feature making the color contrast observable under a regular bright-field microscope without any external modifications This optical contrast can be enhanced using Differential Interference Contrast (DIC) mode as seen in the results section
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