Bruno Schuler and his young team are starting a critical research project: they will selectively create defects in thin semiconductors and try to measure and control their properties using picosecond time resolution and atomic precision. The resulting knowledge should form the foundational knowledge for future quantum computers.
The term molybdenum disulfide may be familiar to some drivers and mechanics. No wonder: the substance, discovered by the American chemist Alfred Sonntag in the 1940s, is still used today as a high-performance lubricant in engines and turbines, but also for bolts. This is because of the unique chemical structure of this hard, individual layers of things that are easily moved to talk to each other.
However, molybdenum disulfide (chemical MoS2) not only lubricates well, but it is possible to remove an atomic layer of this material or grow it synthetically on a wafer scale. The extraction of MoS2 monolayer was accomplished only a few years ago, but it is already considered a breakthrough in materials science with great technological potential.
The Empa team now wants to work well with this class. The arrangement of individual atomic layers makes this material attractive to scientists looking for the foundation for next-generation nanocomputers.
MoS2 – and its chemical relatives called transition metal dichalcogenides (TMDs) – is one of the main “shooting stars” in two-dimensional (2D) materials. TMDs are 2D semiconductors and have a direct bandgap, but only in a single layer, which makes them particularly attractive for ultimate miniaturized integrated circuits or optical detectors.
The robust quantum mechanical properties of 2D materials are also being intensively explored for use in quantum metrology, quantum cryptography and quantum information technology. But not only the point is important, but especially can control the defects in it: analogous to the chemical doping of “pure” semiconductors and integrated circuits or foreign ions in semiconductor lasers, atomic defects are “like icing on the cake”, especially in 2D objects, said Schuler.
A third important quantum computer?
An Empa researcher wants to characterize atomic dots in TMDs using new types of materials and study their suitability as quantum emitters. Quantum carriers form an interface between two worlds: electron spin – the quantum mechanical analogue of electron torque – which is suitable for processing quantum information, and photons, i.e. light, which can be used to transmit quantum information over long distances without loss.
2D materials offer the great advantage that the relevant energy scales are much larger than for 3D materials, so it is expected that the technology can be used above cryogenic environments – ideally even at room temperature. Additionally, defects must be located on the surface of the 2D material, which makes them much easier to find and manipulate. But first, the defects on both sides of MoS2 must be identified and their electronic and optical properties must be thoroughly studied. Accurate, in this case, means that the area being studied is measured with an accuracy of one angstrom.
For comparison: 1 angstrom is one meter which is 4 cm at the Earth-Moon distance (400,000 km). In the picture used to record the electronic excitation of the quantum dot must be accurate to the piksecond (ps) – 1 ps is as small as a fraction of a second like 2 days compared to the age of the Earth (five billion years). These short, precise simulations provide a comprehensive overview of the complex processes that occur at the atomic scale and the factors that influence these processes.
The device has two halves
The equipment of the experiments will already take place in the basement of the Empa laboratory in Dübendorf – the most stable ground. “We have invested more than a year and a half of preparation and development work to complete the test device,” explains Bruno Schuler. “In October 2022, we connected the two parts of our system and were able to measure the arrival of heat waves for the first time. The principle works! A big milestone in this project.”
The two areas that Schuler’s team will now work on are, respectively, the scanning tunneling microscope (STM). The atomic surface of the sample is analyzed using ultrathin powder. Scientists have to put together the surface of the defective site, that is the hole or atom “foreign” in the structure.
Then, the second half of the system, established by Schuler’s employee Jonas Allerbeck, comes into play: a 50-watt infrared laser that fires an ultrashort laser beam with a non-existent lithium niobate crystal. This results in a stable electromagnetic pulse in the range of terahertz. This pulse is just one long light oscillation that can be divided with special optics into a pair of pump and probe pulses – which follow each other with different delays and can measure electrondynamics stroboscopically.
Electrons “jump” to the dot site
Both pulses are sent through the STM and directed to the probe node. The first charge removes an electron from the tip, which “jumps” to the dot site of the double layer of MoS2 and excites the electrons there. “It could be an electric charge, an exciton, a lattice vibration, or an electron gap that we’ve created there,” Schuler explains. “With the second pulse, we look a few picoseconds later how our dot site reacts to the excitation and from this we can study the process of adaptation and energy transfer to the substrate.”
In this way, Schuler is one of the few experts in the world who combines the resolution of a short picosecond time with a system that can “see” atoms. This team uses local area and 2D object systems to keep the stimulus in one place for as long as it is detected. “The fast light scanning microscope enables interesting new insights into the quantum structure at the atomic level, and 2D objects provide a unique platform for creating these states in a controlled way,” says the Empa researcher.
Source: EMPA
