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Monolayer Atomic Crystal Molecular Superlattices

The hunt is always on in the optoelectronics and semiconductor physics communities, for materials with specific properties. Carrier mobility – the ability of a semiconductor to conduct currents of charged particles, and its response to electric fields – is vitally important. The crucial part of much of the research into these materials is a tunable bandgap: in other words, the ability to alter the material structure of the substance and thereby change the energy between its conduction and valence bands.

If your device is used as an LED, this determines the frequency of light that it produces. If your device is used as a solar panel, it determines the frequency of the sun’s rays that it can absorb. For this reason, tuneable bandgaps would allow LEDs across the visible spectrum, as well as solar panels that are more attuned to the Sun’s rays and hence more efficient.

2D materials, with their unique electronic behavior, have proved fertile ground for the hunt for materials that have these useful properties and – crucially – might be cheap and easy to manufacture. In 2014, black phosphorous (BP) was rediscovered for its properties as such a candidate material, with a tunable bandgap and good carrier mobility. Until now, however, there has been a great deal of difficulty in obtaining a monolayer of this substance – phosphorene – meaning that researchers were limited to studying multi-layer flakes of the material. Due to interactions between layers, these multilayer flakes have different properties, so aren’t ideal to confirm the theoretically predicted properties of phosphorene.

In a paper published in Nature, a team of researchers – collaborating across three countries, from UCLA, the University of Texas, and the California Institute of Technology in the US, Hunan University and Hefei National Laboratory in China, and King Saud University in Saudi Arabia – have managed to examine the properties of phosphorene. They have done so with a novel technique that involves intercalcation – sliding layers of molecules between monolayers of the phosphorene crystals. These alternating layers form a new synthetic structure the researchers call 'monolayer atomic crystals molecular superlattice' or MACMS.

Motivated by exploring intrinsic monolayer phosphorene properties, we developed this universal electrochemical molecular intercalation solution to synthesize the unique monolayer atomic crystals molecular superlattice (MACMS).

More interestingly, we realized this unique MACMS platform could generally be applied to broad two-dimensional atomic crystals (2DACs) together with almost infinite functional molecules selections to tailor and tame the properties of 2DACs.

Dr. Wang - UCLA

When creating monolayers in the past, there were three traditional approaches. One was a laborious method of “exfoliation,” which is exactly what it sounds like; attempting to scrape a monolayer from a larger structure. This produces structures that are hard to reproduce, and the yield is low. Alternatively, you can try to “build up” a single layer with chemical vapor deposition onto a substrate; this works well for some substances but becomes increasingly difficult as the complexity of the lattice you want to examine grows greater.

By separating the monolayers with intercalation – in the study, they used cetyl-trimethylammonium bromide – the scientists can have the best of both worlds. Now, the separation between the layers of phosphorene is more than double what it is in black phosphorous; effectively, the layers of phosphorene are isolated from each other, so the interaction isn’t affecting the electronic properties of the phosphorene lattice that they want to study.

The study could have implications beyond what was originally planned. The team was able to fabricate transistor devices and measure the electronic properties of phosphorene – but in the process, their new method of constructing MACMS turns out to be highly applicable to other 2D atomic crystals.

As part of the paper, the team showed that “two-dimensional atomic crystals, such as molybdenum disulfide and tungsten diselenide, can be intercalated with quaternary ammonium molecules of varying sizes and symmetries to produce a broad class of superlattices with tailored molecular structures, interlayer distances, phase compositions, electronic and optical properties.”

Dr. Wang commented, "We believe this versatile MACMS platform and designed electronic/optical/magnetic properties of artificial superlattice will promote the 2DACs research community to a new level."

Beyond the device applications of the materials that can be studied using MACMS, there is the potential for fundamental physics research. Quantum condensed matter theory is a hot topic in theoretical physics at the moment; the team is well aware of the potential uses for MACMS in tests of that theory.

In a blog post describing the achievement, Wang wrote:“[This material platform could be used for fundamental physics studies] such as the topological transition under pressure due to Fermi phase and band structure change, compared with traditional Dirac semimetal behavior of black phosphorus, or the low-temperature quantum transportation studies and its relationship with lattice symmetry."

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