When chemists from the Institute of Physical Chemistry of the Polish Academy of Sciences in Warsaw were starting work on yet another material designed for the efficient production of nanocrystalline zinc oxide, they didn’t expect any surprises. They were greatly astonished when the electrical properties of the changing material turned out to be extremely exotic.

The exotic transformations causes that one of the precursors of zinc oxide, initially an insulator, at approx. 300 degrees Celsius goes to a state with electrical properties typical of metals, and at ~400 degrees Celsius it becomes a semiconductor. Credit: IPC PAS

The single source precursor (SSP) approach is widely regarded as one of the most promising of the various strategies employed for the preparation of semiconductor nanocrystalline materials. However, one of the key obstacles hampering both the rational design of SSPs and their controlled transformation to the desired nanomaterials with highly controlled physicochemical properties is the scarcity of mechanistic insights during the transformation process. Scientists from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Faculty of Chemistry of Warsaw University of Technology (WUT) have revealed that in the thermal decomposition process of a pre-organized zinc alkoxide precursor the nucleation and growth of the semiconducting zinc oxide (ZnO) phase is preceded by cascade transformations involving the formation of previously unreported intermediate radical zinc oxo-alkoxide clusters with gapless electronic states. Up to now, these types of clusters have not been considered either as intermediate structures on the path to the semiconductor ZnO phase or as a potential species accounting for the various defect states of ZnO nanocrystals.

“We discovered that one of the groups of ZnO precursors that have been studied for decades, zinc alkoxide compounds, undergo previously unobserved physicochemical transformations upon thermal decomposition. Originally, the starting compound is an insulator, when heated it rapidly transforms into a material with conductor-like properties, and a further increase in temperature equally rapidly leads to its conversion into a semiconductor,” says Dr. Kamil Soko?owski (IPC PAS).

The design and preparation of well-defined nanomaterials in a controlled manner remains a tremendous challenge and is acknowledged to be the biggest obstacle for the exploitation of many nanoscale phenomena. Professor Lewiski’s (IPC PAS, PW) group has for many years been engaged in the development of effective methods of producing nanocrystalline forms of zinc oxide, a semiconductor with wide applications in electronics, industrial catalysis, photovoltaics and photocatalysis. One of the studied approaches is based on the single source precursors. The precursor molecules contain all components of the target material in their structure and only temperature is required to trigger the chemical transformation.

“We dealt with a group of chemical compounds with the general formula RZnOR, as single source pre-designed ZnO precursors. A common feature of their structure is the presence of the cubic [Zn4O4] core with alternating zinc and oxygen atoms terminated by organic groups R. When the precursor is heated, the organic parts are degraded, and the inorganic cores self-assemble, forming the final form of the nanomaterial,” explains Dr. Soko?owski.

The tested precursor had the properties of an insulator, with an energy gap of about five electronvolts. When heated, it eventually transformed into a semiconductor with an energy gap of approximately 3 eV.

“An exceptional result of our research was the discovery that at a temperature close to 300 degrees Celsius the compound suddenly transforms into almost gap-less electronic state, showing electrical properties rather more typical of metals. When the temperature rises to approximately 400 degrees, the energy gap suddenly expands to a width characteristic of semiconductor materials. Ultimately, thanks to the combination of advanced synchrotron experiments with quantum-chemical calculations, we have established all the details of these unique transformations,” says Dr. Adam Kubas (IPC PAS), who carried out the quantum-chemical calculations.

The spectroscopic measurements were carried out using methods developed by Dr. Jakub Szlachetko (Institute of Nuclear Physics PAS, Cracow) and Dr. Jacinto Sa (IPC PAS and Uppsala University) at the Swiss Light Source synchrotron facility at the Paul Scherrer Institute in Villigen, Switzerland. The material was heated in a reaction chamber, and then its electron structure was sampled using an X-ray synchrotron beam. The set-up allowed for real-time monitoring of the transformations taking place.

This detailed in situ study of the decomposition process of the zinc alkoxide precursor, supported by computer simulations, revealed that any nucleation or growth of a semiconducting ZnO phase is preceded by cascade transformations involving the formation of previously unreported intermediate radical zinc oxo-alkoxide clusters with gapless electronic states.

“In this process homolytic cleavage of the R-Zn bond is responsible for the initial thermal decomposition process. Computer simulations revealed that the intermediate radical clusters tend to dimerise though an uncommon bimetallic Zn-Zn-bond formation. The following homolytic O-R bond cleavage then leads to sub-nano ZnO clusters which further self-organise to the ZnO nanocrystalline phase,” says Dr. Kubas.

Up to now, the radical zinc oxo clusters formed have not been considered either as intermediate structures on the way to the semiconductor ZnO phase or as potential species accounting for various defect states of ZnO nanocrystals. In a broader context, a deeper understanding of the origin and character of the defects is crucial for structure-property relationships in semiconducting materials.

The research, funded by the National Science Centre and the TEAM grant of the Foundation for Polish Science co-financed by the European Union, will contribute to the development of more precise methods of controlling the properties of nanocrystalline zinc oxide. So far, with greater or lesser success, these properties have been explained with the help of various types of material defects. For obvious reasons, however, the analyses have not taken into account the possibility of forming the specific radical zinc-oxo clusters discovered by the Warsaw-based scientists in the material.

Since the 2003 discovery of the single-atom-thick carbon material known as graphene, there has been significant interest in other types of 2-D materials as well.

These materials could be stacked together like Lego bricks to form a range of devices with different functions, including operating as semiconductors. In this way, they could be used to create ultra-thin, flexible, transparent and wearable electronic devices.

However, separating a bulk crystal material into 2-D flakes for use in electronics has proven difficult to do on a commercial scale.

The existing process, in which individual flakes are split off from the bulk crystals by repeatedly stamping the crystals onto an adhesive tape, is unreliable and time-consuming, requiring many hours to harvest enough material and form a device.

Now researchers in the Department of Mechanical Engineering at MIT have developed a technique to harvest 2-inch diameter wafers of 2-D material within just a few minutes. They can then be stacked together to form an electronic device within an hour.

The technique, which they describe in a paper published in the journal Science, could open up the possibility of commercializing electronic devices based on a variety of 2-D materials, according to Jeehwan Kim, an associate professor in the Department of Mechanical Engineering, who led the research.

The paper’s co-first authors were Sanghoon Bae, who was involved in flexible device fabrication, and Jaewoo Shim, who worked on the stacking of the 2-D material monolayers. Both are postdocs in Kim’s group.

The paper’s co-authors also included students and postdocs from within Kim’s group, as well as collaborators at Georgia Tech, the University of Texas, Yonsei University in South Korea, and the University of Virginia. Sang-Hoon Bae, Jaewoo Shim, Wei Kong, and Doyoon Lee in Kim’s research group equally contributed to this work.

“We have shown that we can do monolayer-by-monolayer isolation of 2-D materials at the wafer scale,” Kim says. “Secondly, we have demonstrated a way to easily stack up these wafer-scale monolayers of 2-D material.”

The researchers first grew a thick stack of 2-D material on top of a sapphire wafer. They then applied a 600-nanometer-thick nickel film to the top of the stack.

Since 2-D materials adhere much more strongly to nickel than to sapphire, lifting off this film allowed the researchers to separate the entire stack from the wafer.

What’s more, the adhesion between the nickel and the individual layers of 2-D material is also greater than that between each of the layers themselves.

As a result, when a second nickel film was then added to the bottom of the stack, the researchers were able to peel off individual, single-atom thick monolayers of 2-D material.

That is because peeling off the first nickel film generates cracks in the material that propagate right through to the bottom of the stack, Kim says.

Once the first monolayer collected by the nickel film has been transferred to a substrate, the process can be repeated for each layer.

“We use very simple mechanics, and by using this controlled crack propagation concept we are able to isolate monolayer 2-D material at the wafer scale,” he says.

The universal technique can be used with a range of different 2-D materials, including hexagonal boron nitride, tungsten disulfide, and molybdenum disulfide.

In this way it can be used to produce different types of monolayer 2-D materials, such as semiconductors, metals, and insulators, which can then be stacked together to form the 2-D heterostructures needed for an electronic device.

“If you fabricate electronic and photonic devices using 2-D materials, the devices will be just a few monolayers thick,” Kim says. “They will be extremely flexible, and can be stamped on to anything,” he says.

The process is fast and low-cost, making it suitable for commercial operations, he adds.

The researchers have also demonstrated the technique by successfully fabricating arrays of field-effect transistors at the wafer scale, with a thickness of just a few atoms.

“The work has a lot of potential to bring 2-D materials and their heterostructures towards real-world applications,” says Philip Kim, a professor of physics at Harvard University, who was not involved in the research.

The researchers are now planning to apply the technique to develop a range of electronic devices, including a nonvolatile memory array and flexible devices that can be worn on the skin.

They are also interested in applying the technique to develop devices for use in the “internet of things,” Kim says.

“All you need to do is grow these thick 2-D materials, then isolate them in monolayers and stack them up. So it is extremely cheap — much cheaper than the existing semiconductor process. This means it will bring laboratory-level 2-D materials into manufacturing for commercialization,” Kim says.

“That makes it perfect for IoT networks, because if you were to use conventional semiconductors for the sensing systems it would be expensive.”