Recently, scientists have successfully used hydrogen molecules as quantum sensors to measure the chemical properties of materials at the highest spatio-temporal resolution.
A 3D rendering of a hydrogen molecule, image from iStock
These techniques can also be applied to the analysis of two-dimensional materials that will play a role in energy systems, electronics, and quantum computers.
This time, physicists at the University of California, Irvine (UCI) successfully used hydrogen molecules as quantum sensors in scanning tunneling microscopes (STM) equipped with terahertz (i.e., 10 to the 12th power of hertz) lasers. The results were published in Science.
The team placed two bound hydrogen atoms between the silver tip of the microscope (Ag Tip) and a flat surface sample composed of copper nitride arrangement, excited the hydrogen molecules through laser pulses lasting trillionths of a second, and detected their quantum state changes in the instrument's ultra-low temperature and ultra-high vacuum environment, obtaining a time-lapse image of the sample at the atomic scale.
Schematic diagram of the experimental apparatus, image from the University of California, Irvine (UCI)
"The project presents advances in measurement techniques and the use of this method to explore scientific problems." Wilson Ho, professor of physics, astronomy and chemistry at UCI and co-author of the paper, said, "Quantum microscopes that rely on detecting coherent superpositions in two-level systems are more sensitive than existing instruments that are not based on the principles of quantum physics." ”
The hydrogen molecule is a typical dichoony system because its orientation can be switched between two positions, i.e. up and down and slightly horizontally inclined. With laser pulses, scientists can induce the system to periodically enter the excited state from the ground state and superimpose the two states. The duration of this cyclic oscillation is very short, lasting only a few tens of picoseconds (1 picosecond is a trillionth of a second), but by measuring this "decoherence time" and cycle period, scientists were able to understand how hydrogen molecules interact with the environment.
"Hydrogen molecules have become part of quantum microscopy. Because no matter where the microscope scans, hydrogen is between the tip and the sample," Wilson Ho said, "it's a very sensitive detector that allows us to see changes below 0.1 angstroms, or 0.01 nanometers." At this resolution, we can see how the charge distribution on the sample changes. ”
There is only about 0.6 nanometers of space between the STM microscope tip and the sample. The STM device assembled by the team can detect tiny currents flowing in this space and produce spectral readings that attest to the presence of hydrogen molecules and sample elements. The researchers say the experiment shows for the first time a chemically sensitive spectrum of rectified currents based on terahertz induction passing through individual molecules.
University of California, Irvine (UCI) research team, image from the University of California, Irvine
Based on the quantum coherence of hydrogen, the ability to characterize materials at this level of precision can play a large role in catalytic science and engineering, and the function of these materials often depends on the surface defects of single atoms. "As long as hydrogen can be adsorbed to the material, in principle, hydrogen can be used as a sensor to characterize the material itself by observing its electrostatic field distribution." Lead author Likun Wang, a graduate student in physics and astronomy at UCI, said.
The project is supported by the U.S. Department of Energy's Office of Basic Energy Science (DOE-BES).