Thin Films, Bright Future
Page 2: A plethora of promising materials
In parallel with the nanotube projects, Högele‘s research group has begun to explore ultrathin materials made of elements other than carbon. In terms of their possible use in digital applications, 2D carbon-based systems have one great disadvantage. Graphene is not a semiconductor, and therefore cannot be used to build transistors, which are the key components of conventional electronics. This explains why researchers have turned to other 2D materials, which promise to be more accommodating in this respect. The crucial factor here is the so-called bandgap. In the quantum mechanical picture of condensed-matter systems, this term refers to the difference in energy levels between the so-called valence band and the conduction band in which electrons are mobile.
In recent years, the search for ultrathin 2D materials has taken on the character of a gold rush. In particular, compounds containing so-called transition metals, such as molybdenum or tungsten, have aroused great interest. Researchers explore the technological potential of some 500 materials belonging to this class. Many of these are highly reactive or unstable in the presence of air or moisture. Most importantly, when they are investigated as single or stacked two-dimensional layers of atoms, these materials exhibit quite different properties than do the more familiar bulk samples of the same composition. “This is a new research field in solid-state physics,” says Högele.
And in this field, he is now working with materials similar to molybdenum disulfide (MoS2), a new miracle material which can be used to make transistors. Many substances which, like MoS2, belong to the class of transition-metal dichalcogenides, interact strongly with light, and have great potential for applications in opto-electronics. In many cases, the physical bases for these characteristics are not fully understood. “Even after years of research we are still discovering unexpected phenomena,” Högele says. And many of these could be of practical use.
Here again, expectations are high. The prospect of quantum-based opto-electronic information processing has sparked a veritable explosion in research. Many of the effects recently characterized in 2D transition-metal-based systems may serve useful purposes in the future, although Högele cautions that there is a long way to go. He then cites one such unexpected effect, and plunges into the fundamentals of quantum physics. The electrons in 2D MoS2 can be raised to a higher energy level by excitation with polarized light. “Circularly polarized light generates charge carriers that exhibit either right- or left-handed circular motion,” Högele explains. “Their associated angular momentum is quantized, and is described by the so-called valley index, which can be detected as so-called valley polarization.” The valley index thus constitutes a further degree of freedom that could be used to encode information, and it may even prove to be a useful resource for quantum computing. The temporal evolution of quantum states is another phenomenon, which could perhaps be exploited for parallel processing of information.“ A whole research field is now devoted to finding ways to make quantum information processing technically feasible,” says Högele. Different systems compete with one another to serve as platforms for the implementation of the necessary processing operations, including some based on atoms or ions trapped in optical lattices.
But the emergence of an exciting new area of basic research for physicists represents only the first tentative step. In the world of practical applications, extensive tests are essential to ensure that novel materials behave in precisely predictable ways. This issue is underlined by recent work on the valley index. Independent research groups have measured different levels of valley polarization in what were ostensibly identical semiconducting devices. The deviations are attributed to varying levels of surface defects in the crystals used, resulting from variations in fabrication conditions. “Whether or not fascinating physical phenomena such as valley polarization can be utilized in quantum technologies will depend crucially on the ability to produce sufficiently pure and defect-free crystals,” Högele says.
It is not always easy to discern where new discoveries in basic research might lead us. This also holds for another innovative field centered on ultrathin 2D materials. The approximately 500 such materials so far described include not only semiconductors, but also insulators, ferromagnetic and even superconducting materials – in other words the whole spectrum of properties known from work with three-dimensional counterparts. However, because they are so thin, 2D ferromagnetic compounds, semiconductors and superconductors have the considerable advantage that they can be combined with each other at will. By stacking two-dimensional atomic crystals on top of one another, one obtains so-called Van der Waals crystals. These “heterostructures” are held together by very weak forces and, as each new layer is added, the physical properties of the stack can change dramatically. The precise mechanisms behind such effects are not well understood, which opens up another new playground for theorists and experimentalists. “Sometimes attention becomes focused on a particular phenomenon quite by accident,” Högele says. His attention has now been captured by tungsten-containing compounds, such as its disulfide and diselenide (WoS2 and WoSe2, respectively) and by hexagonal boron nitride (BN). “At the moment, everyone is playing around, just like Nobel Laureates Geim and Novoselov did with graphene.” The enormous variety of intriguing molecules promises to open up a plethora of technological possibilities. If one can only find the right combination of materials, perhaps one can build components like transistors or even whole circuits in nanoformat. The motivation for these efforts is the hope of reaching the ultimate in miniaturization. “It is astounding to realize how much progress is now being made in quite elementary ways.” It is now possible to assemble entirely novel composite materials that have never existed in nature. “There are lots of exciting developments to look forward to,” Högele predicts. But then he adds a cautionary tale: The properties of silicon had been investigated in the laboratory for decades before the element became the basis for today’s mass-produced electronics. “By that standard, our field is still relatively young.”
Dr. Alexander Högele heads a research group at the Chair for Condensed Matter Physics at LMU. Högele (b. 1975) studied Physics at Heidelberg University and at LMU, where he obtained his PhD. He then held a postdoctoral position in the Institute of Quantum Electronics at the ETH Zürich, before returning to LMU as Junior Professor in 2008. He received a highly endowed Starting Grant from the European Research Council (ERC) in 2013, and has now been awarded an ERC Consolidator Grant.