- For the first time, scientists have created a 3D image of excited quantum using scanning tunneling microscopy.
- The technique can be used to explore other nanostructures, like photocatalytic metal clusters and carbon nanotubes.
Quantum dots, nanometer-sized semiconductor particles, are evolving very fast in research development and emerging applications like thin-sheet solar cells, high-speed data transfer, better-quality LCD TVs and active/passive targeting in vivo conditions and fluorescent labeling in biomedical applications.
Scientists still don’t fully understand the development of quantum dots and their behavior. Faults occur during the production of semiconductor materials, thus two or more identical dots could have different composition from one another.
In material science, defects are usually seen as a hassle, but in quantum dot applications, they’re explicitly generated by doping other materials. This is done to alter the semi-conductivity, electronic structure, and other properties of nanoparticles.
In order to better understand these defects, researchers at University of Washington and University of Illinois have, for the first time, developed three dimensional imaging of an excited quantum dots at different orientations. The study will advance the engineering of quantum dots’ states.
How Did They Do It?
To obtain a 3D image, researchers have used a methodology known as single molecule absorption scanning tunneling microscopy (SMA-STM). It combines the laser’s spectral resolution with high-spatial-resolution of a scanning tunneling microscope.
The technique enables us to extract the image of individual nanoparticles in a laser beam, so that electronically excited quantum dots can be visualized. The researchers rolled the laser-excited electronic structure one the surface (using a sharp, thin metallic wire tip of the scanning tunneling microscope) to obtain image slices at multiple orientations.
These slices then attached together to reconstruct a three dimensional image of excited quantum dot.
Getting Into Details
The researchers have combined 2 capabilities of scanning tunneling microscopy – manipulating quantum dots on a surface by rolling and imaging excited electronic states. This allowed them to visualize an electronically excited state electron density from different orientations.
Schematic quantum dot rolling with localized electronic excitation (green) through STM tip
Reference: Journal of Chemical Physics | doi:abs/10.1063/1.5012784
They showed excitation of individual quantum dots on a surface how single molecule absorption scanning tunneling microscopy pictures a localized defect. Then they rolled quantum dots on the surface w.r.t a surface marker, which showed how the appearance of the state changes.
They further used density functional theory measurements of a disordered model quantum dot to represent that various excited state projections observed in the experiment can be modeled mathematically.
The technique used in this research is limited to cadmium selenide and lead sulfide quantum dots, however, it can be expanded to other compositions. Moreover, the same technique can be used to explore other nanostructures, like photocatalytic metal clusters and carbon nanotubes.
The research team is also working to advance this technique into a single-particle tomography method, but before that they need to make sure that rolling and scanning doesn’t harm the nanoparticle while it’s being reoriented (or repeated manipulation). Also, quantitative modeling of excited states will require measurement of quantum dots’ size with several defects for comparison with experiment.
Instead of regenerating a mediocre 3D picture that merges several particles, single-particle tomography would give us a clearer image.