ORNL's Center for Nanophase Materials Sciences and its talented microscopy specialists are uniquely positioned to analyze promising 2-D nanomaterials, from properties such as their electronic conductivity and ability to harvest light to the exact arrangement of their individual atoms. One promising material is gallium selenide, which is produced in flat, triangular crystals that are a hair's breadth wide and as thin as four atoms tall.
Here are a few of the cutting-edge microscopes ORNL uses to analyze and manipulate gallium selenide, along with specific tests they are able to perform.
Optical Microscopes
These microscopes show researchers regions of crystals down to a tenth of the width of a human hair.
- Raman spectroscopy is a technique that uses shifts in the color of a laser to measure the molecular vibrations of a material, allowing them to reveal the structure of atoms within a layer or how layered materials interact, depending on how they're stacked.
- Photoluminescence spectroscopy uses a laser to excite the electrons within a material and captures the light the material emits. This emitted light reveals the characteristic energy bands of the crystal that must be understood for optoelectronics, as well as the crystal's quality. The instrument can be chilled to 4°K (–452°F) to resolve these electronic bands and any defects by freezing the molecular vibrations.
- Pump-probe ultrafast laser spectroscopy splits an ultrafast laser pulse of light into two parts to measure very fast phenomena such as the creation and motion of electrons through the crystal. The first pulse of light creates the electrons, while the second bounces around a set of mirrors before arriving at the sample shortly thereafter—acting like a flashbulb to capture how the crystal responds to the excited electrons. The absorption of the second pulse is used to measure events as fast as 40 millionths of a billionth of a second long.
Scanning Transmission Electron Microscopes
Aberration-corrected STEM instruments focus electrons into a beam that is smaller than the size of an atom and then scan the beam on the sample to show its atomic structure, including defects (holes in the lattice) and dopants (atoms of a different element).
- Electron energy loss spectroscopy can be used during STEM imaging to identify the element corresponding to each atom and the effects of bonding, dopants, or defects by the loss of transmitted electron energy as measured by an electron energy analyzer.
Helium Ion Microscope
This microscope uses helium ions instead of electrons to image materials and is much better at imaging surfaces. It can also be used to cut materials with extremely high resolution or create defects in a sample that can then be examined with a STEM.
Scanning Tunneling Microscopes
STMs bring a conducting probe tip very near the sample and inject electrons that tunnel through the vacuum between the instrument and through the sample to a conducting substrate. STMs can scan across the sample to also measure atomic positions and are especially good at mapping the electronic structure of a material and defect states in materials.
See also: