Category Archives: Process Materials

 Yole Développement (Yole) expects the IGBT market to go over US$ 5 billion by 2022 with a major growth coming from IGBT power module. The high performance that SiC and GaN materials can afford is already creating a battlefield with Silicon based IGBT. To overcome this thread, Si IGBT manufacturers need to look for prompt solutions as technologically update their systems for better efficiency or to increase their IGBT portfolio offer.

How is the IGBT market evolving for different applications? How will the IGBT market face the adoption of high performance WBG based devices?… Yole’s power electronics team proposes you today a new technology & market report titled IGBT market and technology trends 2017 report. Yole’s report presents an overview of the IGBT market including detailed forecasts and a new application section focused on energy storage systems. This analysis is also showing the status of the competitive landscape.

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The IGBT market represents a very promising bet for the next few years, announces the “More than Moore” market research and strategy consulting company: its analysts invite you to discover the latest IGBT technology trends and market challenges.

“The IGBT industry will follow power electronics’ growth pattern, mainly caused by the high volume automotive market, especially for the electrification of powertrains in EV/HEV ”, asserts Dr Ana Villamor, Technology & Market Analyst, Power Electronics at Yole Développement.

The EV/HEV sector has great growth prospects because it is still an emerging market with tremendous volume potential.

Another big sector for IGBT is clearly motor drives, which keep on growing, thanks to aggressive regulation targets. Yole Développement forecasts a 4.6% CAGR for motor drives from 2016 to 2022. Photovoltaics and wind are very dynamic markets with growth from huge installations being installed during the last few years. It is worth to say that China led the solar panel implementation in 2016, with an impressive 35 GW installed.

“There will be applications for SiC which will impact the IGBT market, for example it is highly possible that it will take over the automotive market”, comments Dr Ana Villamor. “However, we forecast that IGBTs will keep a significant market share in the power electronics industry and will not be replaced completely.”

In fact, even if the IGBT has almost reached its technological limit, new designs and new materials can still be used to improve system performance to overcome the WBG devices arrival. In coming years, there will be new IGBT designs from Infineon, Fuji or ABB coming into the market. Packages are being improved by different manufacturers to decrease parasitics and improve system efficiency. A clear example is the introduction of the embedded techniques for discrete IGBTs and overmolded solutions for IGBT modules to reduce size or increase functional density.

Currently, IGBT manufacturers can have wide voltage ranges in their portfolios, going from 400 V to 6.5k V. The 400 V IGBTs will directly compete with MOSFETs, whereas IGBTs with voltages higher than 600 V will compete with SJ MOSFETs and WBG devices, which exhibit advantages over IGBTs. Lower voltage IGBTs will not be developed since they do not show any advantage compared with MOSFETs.

As IGBTs is a mature technology, the supply chain is well established, with strong partnerships and companies well positioned in each level.

“Therefore, the main IGBT manufacturers that we included in our 2015 report are still in the IGBT best sellers, except ON Semiconductor, which has become one of the top five IGBT vendors after the acquisition of Fairchild at the end of 2016”, explains Dr Ana Villamor. “However, more companies are entering the IGBT market in order to capture added value, like Littelfuse, who just announced the agreement on the acquisition of IXYS Corporation.”

A new device being developed by Washington State University physicist Yi Gu could one day turn the heat generated by a wide array of electronics into a usable fuel source.

The device is a multicomponent, multilayered composite material called a van der Waals Schottky diode. It converts heat into electricity up to three times more efficiently than silicon — a semiconductor material widely used in the electronics industry. While still in an early stage of development, the new diode could eventually provide an extra source of power for everything from smartphones to automobiles.

“The ability of our diode to convert heat into electricity is very large compared to other bulk materials currently used in electronics,” said Gu, an associate professor in WSU’s Department of Physics and Astronomy. “In the future, one layer could be attached to something hot like a car exhaust or a computer motor and another to a surface at room temperature. The diode would then use the heat differential between the two surfaces to create an electric current that could be stored in a battery and used when needed.”

Gu recently published a paper on the Schottky diode in The Journal of Physical Chemistry Letters.

A new kind of diode

In the world of electronics, Schottky diodes are used to guide electricity in a specific direction, similar to how a valve in a water main directs the flow of liquid going through it. They are made by attaching a conductor metal like aluminum to a semiconductor material like silicon.

Instead of combining a common metal like aluminum or copper with a conventional semiconductor material like silicon, Gu’s diode is made from a multilayer of microscopic, crystalline Indium Selenide. He and a team of graduate students used a simple heating process to modify one layer of the Indium Selenide to act as a metal and another layer to act as a semiconductor. The researchers then used a new kind of confocal microscope developed by Klar Scientific, a start-up company founded in part by WSU physicist Matthew McCluskey, to study their materials’ electronic properties.

Unlike its conventional counterparts, Gu’s diode has no impurities or defects at the interface where the metal and semiconductor materials are joined together. The smooth connection between the metal and semiconductor enables electricity to travel through the multilayered device with almost 100 percent efficiency.

“When you attach a metal to a semiconductor material like silicon to form a Schottky diode, there are always some defects that form at the interface,” said McCluskey, a co-author of the study. “These imperfections trap electrons, impeding the flow of electricity. Gu’s diode is unique in that its surface does not appear to have any of these defects. This lowers resistance to the flow of electricity, making the device much more energy efficient.”

Next steps

Gu and his collaborators are currently investigating new methods to increase the efficiency of their Indium Selenide crystals. They are also exploring ways to synthesize larger quantities of the material so that it can be developed into useful devices.

“While still in the preliminary stages, our work represents a big leap forward in the field of thermoelectrics,” Gu said. “It could play an important role in realizing a more energy-efficient society in the future.”

A group of international physicists, jointly with NUST MISIS researchers, have conducted a series of experiments on graphene bombardment by swift heavy ions. The experimental results show that such a bombardment allows for the creation of nanopores in graphene. The diameter of these nanopores can be adjusted in a range of 1 to 4 nanometers.

The experimental results on graphen bombardment by swift heavy ions, conducted by NUST MISIS scientists together with colleagues from the University of Helsinki and Aalto University (Finland), the University of Nottingham (the United Kingdom), the University of Duisburg-Essen (Germany), the University of Vienna (Austria), the Center of Research on Ions, Materials and Photonics CIMAP (France), Ruder Boskovic Institute (Croatia), and the Institute of Ion Beam Physics & Materials Research (Germany) have been published in Carbon journal.

The experimental results on grapheme bombardment with a large amount of ions of different masses of C, O, Si, I, Au, Ta, Xe with high-energy  (up to 91 MeV) have shown that it is possible to create nanopores with a diameter from 1 to 4 nm when changing the energy of ions. Information on the dependence of nanopores on the energy of ions brings scientists closer to a controlled obtainment of such structures.

“We have experimentally and theoretically studied the process of nanopores occurrence (pores) in graphene after interaction between graphene with ions, as well as studying the dependence of pores` sizes on the type and ions` energy, and the nature of the appearance of these defects in grapheme have been explained,” said Arkady Krasheninikov, visiting Professor at NUST MISIS, Candidate of Physical and Mathematical Sciences, research author, and head of the ‘Minimization of degradation of two-dimensional inorganic materials with the use of atomistic calculations’ project.

According to Krasheninikov, “The current development of grapheme research is connected with studies of the possibility of controlled changes of its properties, for example by introduction of defects in its structure. The creation of defects in graphene can significantly change its electronic and conductive properties, and even lead to the induction of magnetism. One of the possible ways of introducing defects into a graphene structure is a bombardment of ions of different elements.”

Krasheninikov also added that scientists have been interested in nanoporous graphene for quite a while. He believes that the obtained nanostructures can be widely used in various fields of science and technology, in particular in the capacity of materials for the purification of liquids, DNA sequencing, etc.

“One expects that with a regular arrangement of pores in graphene, its spectrum would be readjusted into a semiconductive state and that would allow us to use it in electronics,” added Krasheninikov.

 

Silicon – the second most abundant element in the earth’s crust – shows great promise in Li-ion batteries, according to new research from the University of Eastern Finland. By replacing graphite anodes with silicon, it is possible to quadruple anode capacity.

In a climate-neutral society, renewable and emission-free sources of energy, such as wind and solar power, will become increasingly widespread. The supply of energy from these sources, however, is intermittent, and technological solutions are needed to safeguard the availability of energy also when it’s not sunny or windy. Furthermore, the transition to emission-free energy forms in transportation requires specific solutions for energy storage, and lithium-ion batteries are considered to have the best potential.

Researchers from the University of Eastern Finland introduced new technology to Li-ion batteries by replacing graphite used in anodes by silicon. The study analysed the suitability of electrochemically produced nanoporous silicon for Li-ion batteries. It is generally understood that in order for silicon to work in batteries, nanoparticles are required, and this brings its own challenges to the production, price and safety of the material. However, one of the main findings of the study was that particles sized between 10 and 20 micrometres and with the right porosity were in fact the most suitable ones to be used in batteries. The discovery is significant, as micrometre-sized particles are easier and safer to process than nanoparticles. This is also important from the viewpoint of battery material recyclability, among other things. The findings were published in Scientific Reports.

“In our research, we were able to combine the best of nano- and micro-technologies: nano-level functionality combined with micro-level processability, and all this without compromising performance,” Researcher Timo Ikonen from the University of Eastern Finland says. “Small amounts of silicon are already used in Tesla’s batteries to increase their energy density, but it’s very challenging to further increase the amount,” he continues.

Next, researchers will combine silicon with small amounts of carbon nanotubes in order to further enhance the electrical conductivity and mechanical durability of the material.

“We now have a good understanding of the material properties required in large-scale use of silicon in Li-ion batteries. However, the silicon we’ve been using is too expensive for commercial use, and that’s why we are now looking into the possibility of manufacturing a similar material from agricultural waste, for example from barley husk ash,” Professor Vesa-Pekka Lehto explains.

Many next-generation electronic and electro-mechanical device technologies hinge on the development of ferroelectric materials. The unusual crystal structures of these materials have regions in their lattice, or domains, that behave like molecular switches. The alignment of a domain can be toggled by an electric field, which changes the position of atoms in the crystal and switches the polarization direction. These crystals are typically grown on supporting substrates that help to define and organize the behavior of domains. Control over the switching of domains when making crystals of ferroelectric materials is essential for any future applications.

Now an international team by Nagoya University has developed a new way of controlling the domain structure of ferroelectric materials, which could accelerate development of future electronic and electro-mechanical devices.

“We grew lead zirconate titanate films on different substrate types to induce different kinds of physical strain, and then selectively etched parts of the films to create nanorods,” says lead author Tomoaki Yamada. “The domain structure of the nanorods was almost completely flipped compared with [that of] the thin film.”

Lead zirconate titanate is a common type of ferroelectric material, which switches based on the movement of trapped lead atoms between two stable positions in the crystal lattice. Parts of the film were deliberately removed to leave freestanding rods on the substrates. The team then used synchrotron X-ray radiation to probe the domain structure of individual rods.

The contact area of the rods with the substrate was greatly reduced and the domain properties were influenced more by the surrounding environment, which mixed up the domain structure. The team found that coating the rods with a metal could screen the effects of the air and they tended to recover the original domain structure, as determined by the substrate.

“There are few effective ways of manipulating the domain structure of ferroelectric materials, and this becomes more difficult when the material is nanostructured and the contact area with the substrate is small.” says collaborator Nava Setter. “We have learned that it’s possible to nanostructure these materials with control over their domains, which is an essential step towards the new functional nanoscale devices promised by these materials.”

The article, “Charge screening strategy for domain pattern control in nanoscale ferroelectric systems,” was published in Scientific Reports at DOI:10.1038/s41598-017-05475-x

 

Two-dimensional materials are a sort of a rookie phenom in the scientific community. They are atomically thin and can exhibit radically different electronic and light-based properties than their thicker, more conventional forms, so researchers are flocking to this fledgling field to find ways to tap these exotic traits.

Applications for 2-D materials range from microchip components to superthin and flexible solar panels and display screens, among a growing list of possible uses. But because their fundamental structure is inherently tiny, they can be tricky to manufacture and measure, and to match with other materials. So while 2-D materials R&D is on the rise, there are still many unknowns about how to isolate, enhance, and manipulate their most desirable qualities.

Now, a science team at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has precisely measured some previously obscured properties of moly sulfide, a 2-D semiconducting material also known as molybdenum disulfide or MoS2. The team also revealed a powerful tuning mechanism and an interrelationship between its electronic and optical, or light-related, properties.

To best incorporate such monolayer materials into electronic devices, engineers want to know the “band gap,” which is the minimum energy level it takes to jolt electrons away from the atoms they are coupled to, so that they flow freely through the material as electric current flows through a copper wire. Supplying sufficient energy to the electrons by absorbing light, for example, converts the material into an electrically conducting state.

As reported in the Aug. 25 issue of Physical Review Letters, researchers measured the band gap for a monolayer of moly sulfide, which has proved difficult to accurately predict theoretically, and found it to be about 30 percent higher than expected based on previous experiments. They also quantified how the band gap changes with electron density – a phenomenon known as “band gap renormalization.”

“The most critical significance of this work was in finding the band gap,” said Kaiyuan Yao, a graduate student researcher at Berkeley Lab and the University of California, Berkeley, who served as the lead author of the research paper.

“That provides very important guidance to all of the optoelectronic device engineers. They need to know what the band gap is” in orderly to properly connect the 2-D material with other materials and components in a device, Yao said.

Obtaining the direct band gap measurement is challenged by the so-called “exciton effect” in 2-D materials that is produced by a strong pairing between electrons and electron “holes” ¬- vacant positions around an atom where an electron can exist. The strength of this effect can mask measurements of the band gap.

Nicholas Borys, a project scientist at Berkeley Lab’s Molecular Foundry who also participated in the study, said the study also resolves how to tune optical and electronic properties in a 2-D material.

“The real power of our technique, and an important milestone for the physics community, is to discern between these optical and electronic properties,” Borys said.

The team used several tools at the Molecular Foundry, a facility that is open to the scientific community and specializes in the creation and exploration of nanoscale materials.

The Molecular Foundry technique that researchers adapted for use in studying monolayer moly sulfide, known as photoluminescence excitation (PLE) spectroscopy, promises to bring new applications for the material within reach, such as ultrasensitive biosensors and tinier transistors, and also shows promise for similarly pinpointing and manipulating properties in other 2-D materials, researchers said.

The research team measured both the exciton and band gap signals, and then detangled these separate signals. Scientists observed how light was absorbed by electrons in the moly sulfide sample as they adjusted the density of electrons crammed into the sample by changing the electrical voltage on a layer of charged silicon that sat below the moly sulfide monolayer.

Researchers noticed a slight “bump” in their measurements that they realized was a direct measurement of the band gap, and through a slew of other experiments used their discovery to study how the band gap was readily tunable by simply adjusting the density of electrons in the material.

“The large degree of tunability really opens people’s eyes,” said P. James Schuck, who was director of the Imaging and Manipulation of Nanostructures facility at the Molecular Foundry during this study.

“And because we could see both the band gap’s edge and the excitons simultaneously, we could understand each independently and also understand the relationship between them,” said Schuck, now at Columbia University. “It turns out all of these properties are dependent on one another.”

Moly sulfide, Schuck also noted, is “extremely sensitive to its local environment,” which makes it a prime candidate for use in a range of sensors. Because it is highly sensitive to both optical and electronic effects, it could translate incoming light into electronic signals and vice versa.

Schuck said the team hopes to use a suite of techniques at the Molecular Foundry to create other types of monolayer materials and samples of stacked 2-D layers, and to obtain definitive band gap measurements for these, too. “It turns out no one yet knows the band gaps for some of these other materials,” he said.

The team also has expertise in the use of a nanoscale probe to map the electronic behavior across a given sample.

Borys added, “We certainly hope this work seeds further studies on other 2-D semiconductor systems.”

The Molecular Foundry is a DOE Office of Science User Facility that provides free access to state-of-the-art equipment and multidisciplinary expertise in nanoscale science to visiting scientists.

Researchers from the Kavli Energy NanoSciences Institute at UC Berkeley and Berkeley Lab, and from Arizona State University also participated in this study, which was supported by the National Science Foundation.

Transition metal silicides, a distinct class of semiconducting materials that contain silicon, demonstrate superior oxidation resistance, high temperature stability and low corrosion rates, which make them promising for a variety of future developments in electronic devices. Despite their relevance to modern technology, however, fundamental aspects of the chemical bonding between their transition metal atoms and silicon remain poorly understood. One of the most important, but poorly known, properties is the strength of these chemical bonds — the thermochemical bond dissociation energy.

With funding from the National Science Foundation, a team of researchers from the University of Utah has investigated this property, and in this week’s The Journal of Chemical Physics, from AIP Publishing, they present their valuable findings for a number of specific compounds. These include precise values of the bond dissociation energies of the group four and five transition metal silicide molecules: TiSi, ZrSi, HfSi, VSi, NbSi and TaSi.

“The team measured the energy at which the diatomic silicides fall apart more quickly than they can be ionized by absorption of a second photon. This amount of energy is called the predissociation threshold. It provides an upper limit to the bond dissociation energy. However, the researchers have found that for molecules with certain electron configurations, if the molecule is cold, then the observation of a sharp predissociation threshold provides an accurate value of the thermochemical bond dissociation energy, and not simply an upper limit.”

“What I’m so pleased about with this new technique that we’ve developed is that it’s not just applicable to a small set of molecules,” said Michael Morse, one of the work’s authors. “It’s based on the fact that these small transition metal molecules have a density of electronic states that increases very rapidly as you get close to the dissociation limit, and that’s key in causing the molecule to fall apart as soon as you get above that limit […] The peculiarities of the transition metals make the method broadly applicable to that entire class of molecules, which are quite difficult to investigate by other means.”

This sharp threshold observation in a dense vibronic spectrum provides a new and highly effective means of estimating the bond dissociation energy for transition metals bonded to other p-block elements. According to the researchers, the uncertainties using this new method are much smaller than those seen with previous approaches.

Along with measuring the bond dissociation values for these molecules, the researchers were also able to use the predissociation thresholds to determine other fundamental values for certain molecules using thermochemical cycles, namely enthalpies of formation and ionization energies.

The data acquired can be used by chemists to develop more accurate computational methods regarding transition metal chemical bonding, along with bettering our understanding of these bonds.

“Quantum chemists are trying to develop new, efficient and accurate means of calculating these systems, and they’ve been quite successful with main group systems, and especially organic compounds,” Morse said. “But, the transition metals are much more difficult because there are so many more ways the electrons can be arranged. Another problem is that in the past, there hasn’t been as much highly accurate data available that can be used to compare theory and experiment. Without accurate data, it’s hard to tell how good a computational method may be.”

The research team has plans to work with other diatomic molecules containing transition metals. In fact, they already have results for the bond dissociation energies of TiC, ZrC, HfC, VC, NbC, TaC, WC, WSi, WS, WSe, and WCl that are in preparation for publication. By examining series of chemically related molecules, like these studies of the metal-carbon and tungsten-halogen molecules, the team intends to develop a broad picture of chemical bonding in the transition metal molecules.

“There’s a big advantage that comes from this sort of wide-ranging, systematic study. It allows us to develop what I like to call ‘chemical intuition’ about chemical bonds,” said Morse.

IC Insights has revised its outlook for semiconductor industry capital spending and presented its new findings in the August Update to The McClean Report 2017.  IC Insights’ latest forecast is for semiconductor industry capital spending to climb 20% this year.

Figure 1 shows the steep upward trend of quarterly capital spending in the semiconductor industry since 1Q16. Although there was a slight pause in the upward trajectory in 1Q17, 2Q17 set a new record for quarterly spending outlays.   Moreover, 1H17 semiconductor industry spending was 48% greater than in 1H16.  IC Insights believes that whether industry-wide capital spending in the second half of 2017 can match the first half of the year is greatly dependent upon the level of Samsung’s 2H17 spending outlays.

Not only has Samsung Semiconductor been on a tear with regard to its semiconductor sales, surging into the number one ranking in 2Q17, but the company has also been on a tremendous capital spending spree for its semiconductor division this year.  As depicted in Figure 2, Samsung spent a whopping $11.0 billion in capital outlays for its semiconductor group in 1H17, more than 3x greater than the company spent in 1H16 and only $300 million less than the company spent in all of 2016!   In fact, Samsung’s capital expenditures in 1H17 represented 25% of the total semiconductor industry capital spending and 28% of the outlays in 2Q17.

While the company has publicly reported that it spent $11.0 billion in capital outlays for its semiconductor division in 1H17 (a $22.0 billion annual run-rate), Samsung has been very secretive about revealing its full-year 2017 budget for its semiconductor group (it might be afraid of shocking the industry with such a big number!).  In 2012, the year of Samsung’s previous first half spending surge before 1H17, the company cut its second half capital outlays by more than 50%, from $8.5 billion in 1H12 to $3.7 billion in 2H12.  Will the company follow the same pattern in 2017?  At this point, it is impossible to tell.  IC Insights believes that Samsung’s full-year 2017 capital expenditures could range from $15.0 billion to $22.0 billion!

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If Samsung spends $22.0 billion in capital outlays this year, total semiconductor industry capital spending could reach $85.4 billion, which would represent a 27% increase over the $67.3 billion the industry spent in 2016.

It is interesting to note that two of the major spenders, TSMC and Intel, are expected to move in opposite directions with regard to their 2H17 capital spending plans. TSMC spent about $6.8 billion in capital outlays in 1H17. If it sticks to its $10.0 billion budget this year, which it reiterated in its second quarter results, it would only spend about $3.2 billion in 2H17, less than half its outlays in 1H17. In contrast, Intel spent only about $4.7 billion in 1H17, leaving the company to spend about $7.3 billion in 2H17 in order to reach its stated full-year 2017 spending budget of $12.0 billion.

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Figure 2

Characterizing the thermal properties of crystalline molybdenum disulfide, an important two-dimensional (2D) material, has proven challenging. Now researchers from A*STAR have developed a simple technique that could pave the way for its use in a wide range of new applications in energy storage, optoelectronic and flexible electronic devices (Physical Review B, “Direct calculation of the linear thermal expansion coefficients of MoS2 via symmetry-preserving deformations”).

Hexagonal molybdenum disulfide (MoS2), one of the dichalcogenides — a family of semiconducting transitional metals — has attracted considerable attention as a two-dimensional (2D) material thanks to its remarkable electronic and optoelectronic properties. It is also notable for its impressive strength and flexibility, which arise from the hexagonal lattice of molybdenum atoms sandwiched between layers of sulfur atoms.

Determining the thermal characteristics of MoS2 is key to unlocking its astonishing properties, but its complex geometry and the many required calculations for phonons — the different vibrational modes of atoms in a crystal lattice — are a costly and time-consuming computational process.

Chee Kwan Gan and Yu Yang Fredrik Liu from the A*STAR Institute of High Performance Computing have now developed a numerical technique that dramatically reduces the number of calculations, allowing the thermal expansion coefficient — which determine how their shape and size change in response to changes in temperature — of MoS2 crystals to be accurately and efficiently calculated, and could also be applied to other important 2D materials.

“Think of a phonon as a particle tied to a spring, where it vibrates with a fixed pattern at a fixed frequency,” explains Gan. “There are many phonon modes in a crystal like molybdenum disulfide, and the challenge is to calculate all of them.”

By deforming a crystal of MoS2, the researchers determined the change in frequency for each phonon in the lattice structure, and by applying a numerical method, based on perturbation theory, to these altered frequencies; they were able to estimate the crystal’s thermal characteristics, known as the Grüneisen parameters. These parameters were then used to calculate the thermal expansion coefficients for hexagonal MoS2.

“Our method uses the full symmetry of the hexagonal structure to reduce the amount of computation to only four sets of phonon calculations compared with quasi-harmonic approximation — the traditional approach — that requires many more,” says Gan.

The work presents, for the first time, an accurate and simple method for determining the thermal properties of MoS2, and provides a deeper understanding of thermal conduction in 2D materials.

“Our long-term aim is to extend the approach to other technologically important semiconducting, two-dimensional materials, such as bismuth selenide,” says Gan.

Rice University researchers have learned to manipulate two-dimensional materials to design in defects that enhance the materials’ properties.

The Rice lab of theoretical physicist Boris Yakobson and colleagues at Oak Ridge National Laboratory are combining theory and experimentation to prove it’s possible to give 2-D materials specific defects, especially atomic-scale seams called grain boundaries. These boundaries may be used to enhance the materials’ electronic, magnetic, mechanical, catalytic and optical properties.

The key is introducing curvature to the landscape that constrains the way defects propagate. The researchers call this “tilt grain boundary topology,” and they achieve it by growing their materials onto a topographically curved substrate — in this case, a cone. The angle of the cone dictates if, what kind and where the boundaries appear.

The research is the subject of a paper in the American Chemical Society journal ACS Nano.

Grain boundaries are the borders that appear in a material where edges meet in a mismatch. These boundaries are a series of defects; for example, when two sheets of hexagonal graphene meet at an angle, the carbon atoms compensate for it by forming nonhexagonal (five- or seven-member) rings.

Yakobson and his team have already demonstrated that these boundaries can be electronically significant. They can, for instance, turn perfectly conducting graphene into a semiconductor. In some cases, the boundary itself may be a conductive subnanoscale wire or take on magnetic properties.

But until now researchers had little control over where those boundaries would appear when growing graphene, molybdenum disulfide or other 2-D materials by chemical vapor deposition.

The theory developed at Rice showed growing 2-D material on a cone would force the boundaries to appear in certain places. The width of the cone controlled the placement and, more importantly, the tilt angle, a crucial parameter in tuning the materials’ electronic and magnetic properties, Yakobson said.

Experimental collaborators from Oak Ridge led by co-author David Geohegan provided evidence backing key aspects of the theory. They achieved this by growing tungsten disulfide onto small cones similar to those in Rice’s computer models. The boundaries that appeared in the real materials matched those predicted by theory.

“The nonplanar shape of the substrate forces the 2-D crystal to grow in a curved ‘non-Euclidian’ space,” Yakobson said. “This strains the crystal, which occasionally yields by giving a way to the seams, or grain boundaries. It’s no different from the way a tailor would add a seam to a suit or a dress to fit a curvy customer.”

Modeling cones of different widths also revealed a “magic cone” of 38.9 degrees upon which growing a 2-D material would leave no grain boundary at all.

The Rice team extended its theory to see what would happen if the cones sat on a plane. They predicted how grain boundaries would form over the entire surface, and again, Oak Ridge experiments confirmed their results.

Yakobson said both the Rice and Oak Ridge teams were working on aspects of the research independently. “It was slow going until we met at a conference in Florida a couple of years back and realized that we should continue together,” he said. “It was certainly gratifying to see how experiments confirmed the models, while sometimes offering important surprises. Now we need to do the additional work to comprehend them as well.”