Category Archives: Packaging Materials

Researchers have identified a mechanism that triggers shape-memory phenomena in organic crystals used in plastic electronics. Shape-shifting structural materials are made with metal alloys, but the new generation of economical printable plastic electronics is poised to benefit from this phenomenon, too. Shape-memory materials science and plastic electronics technology, when merged, could open the door to advancements in low-power electronics, medical electronics devices and multifunctional shape-memory materials.

The findings are published in the journal Nature Communications and confirm the shape-memory phenomenon in two organic semiconductors materials.

Illinois chemistry and biomolecular engineering professor Ying Diao, right, and graduate student Hyunjoong Chung are part of a team that has identified a mechanism that triggers shape-memory in organic crystals used in plastic electronics. Credit: L. Brian Stauffer

Illinois chemistry and biomolecular engineering professor Ying Diao, right, and graduate student Hyunjoong Chung are part of a team that has identified a mechanism that triggers shape-memory in organic crystals used in plastic electronics. Credit: L. Brian Stauffer

Devices like the expandable stents that open and unblock clogged human blood vessels use shape-memory technology. Heat, light and electrical signals, or mechanic forces pass information through the devices telling them to expand, contract, bend and morph back into their original form – and can do so repeatedly, like a snake constricting to swallow its dinner. This effect works well with metals, but remains elusive in synthetic organic materials because of the complexity of the molecules used to create them.

“The shape-memory phenomenon is common in nature, but we are not really sure about nature’s design rules at the molecular level,” said professor of chemical and biomolecular engineering and co-author of the study, Ying Diao. “Nature uses organic compounds that are very different from the metal alloys used in shape-memory materials on the market today,” Diao said. “In naturally occurring shape-memory materials, the molecules transform cooperatively, meaning that they all move together during shape change. Otherwise, these materials would shatter and the shape change would not be reversible and ultrafast.”

The discovery of the shape-memory mechanism in synthetic organic material was quite serendipitous, Diao said. The team accidentally created large organic crystals and was curious to find out how they would transform given heat.

“We looked at the single crystals under a microscope and found that the transformation process is dramatically different than we expected,” said graduate student and co-author Hyunjoong Chung. “We saw concerted movement of a whole layer of molecules sweeping through the crystal that seem to drive the shape-memory effect – something that is rarely observed in organic crystals and is therefore largely unexplored.”

This unexpected observation led the team to want to explore the merger between shape-memory materials science and the field of organic electronics, the researchers said. “Today’s electronics are dependent on transistors to switch on and off, which is a very energy-intensive process,” Diao said. “If we can use the shape-memory effect in plastic semiconductors to modulate electronic properties in a cooperative manner, it would require very low energy input, potentially contributing to advancements in low-power and more efficient electronics.”

The team is currently using heat to demonstrate the shape-memory effect, but are experimenting with light waves, electrical fields and mechanical force for future demonstrations. They are also exploring the molecular origin of the shape-memory mechanism by tweaking the molecular structure of their materials. “We have already found that changing just one atom in a molecule can significantly alter the phenomenon,” Chung said.

The researchers are very excited about the molecular cooperativity aspect discovered with this research and its potential application to the recent Nobel Prize-winning concept of molecular machines, Diao said. “These molecules can change conformation cooperatively at the molecular level, and the small molecular structure change is amplified over millions of molecules to actuate large motion at the macroscopic scale.”

By Jay Chittooran, Manager, Public Policy, SEMI

International trade is one of the best tools to spur growth and create high-skill and high-paying jobs. Over 40 million American jobs rely on trade, and this is particularly true in the semiconductor supply chain. Over the past three decades, the semiconductor industry has averaged nearly double-digit growth rates in revenue and, by 2030, the semiconductor supply chain is forecast to reach $1 trillion. Trade paves the way for this growth.

Unfortunately, despite its importance to the industry, trade has been transformed from an economic issue into a political one, raising many new trade challenges to companies throughout the semiconductor industry.

GHz-ChinaChina’s investments in the industry will continue to anchor the country as a major force in the semiconductor supply chain. China’s outsized spending has spawned concern among other countries about the implications of these investments. According to SEMI’s World Fab Forecast, 20 fabs are being built in China – and construction on 14 more is rumored to begin in the near term – compared to the 10 fabs under construction in the rest of the world. China is clearly outpacing the pack.

The Trump Administration has levied intense criticism of China, citing unfair trade practices, especially related to intellectual property issues. The U.S. Trade Representative has launched a Section 301 investigation into whether China’s practice of forced technology transfer has discriminated against U.S. consumers. Even as the probe unfolds, expectations are growing that the United States will take action against China, raising fears of not only possible retaliation in time but rising animosity between two trading partners that rely deeply on each other.

A number of other open investigations also cloud the future. The Administration launched two separate Section 232 investigations into steel and aluminum industry practices by China, claiming Chinese overproduction of both items are a threat to national security. The findings from these investigations will be submitted to the President, who, in the coming weeks, will decide an appropriate response, which could include imposing tariffs and quotas.

Another high priority area is Korea. While U.S. threats to withdraw from the U.S.-Korea Free Trade Agreement (KORUS) reached a fever pitch in August, rhetoric has since tempered. Informal discussions between the countries on how best to amend the trade deal are ongoing. The number of KORUS implementation issues aside, continued engagement with Korea – instead of scrapping a comprehensive, bilateral trade deal – will be critically important for the industry.

Lastly, negotiations to modernize the North American Free Trade Agreement (NAFTA) will continue this year. The United States wants to conclude talks by the end of March, but with the deadline fast approaching and the promise of resolution waning, tensions are running high. Notably, the outcome of the NAFTA talks will inform and set the tone for other trade action.

What’s more, a number of other actions on trade will take place this year. As we wrote recently, Congress has moved to reform the Committee on Foreign Investment in the United States (CFIUS), a government body designed to review sales and transfer of ownership of U.S. companies to foreign entities. Efforts have also started to revise the export control regime – a key component to improving global market access and making international trade more equitable.

SEMI will continue its work on behalf of its members around the globe to open up new markets and lessen the burden of regulations on cross-border trade and commerce. In addition, SEMI will continue to educate policymakers on the critical importance of unobstructed trade in continuing to push the rapid advance of semiconductors and the emerging technologies they enable into the future. If you are interested in more information on trade, or how to be involved in SEMI’s public policy program, please contact Jay Chittooran, Manager, Public Policy, at [email protected].

Scientists used spiraling X-rays at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) to observe, for the first time, a property that gives handedness to swirling electric patterns – dubbed polar vortices – in a synthetically layered material.

This property, also known as chirality, potentially opens up a new way to store data by controlling the left- or right-handedness in the material’s array in much the same way magnetic materials are manipulated to store data as ones or zeros in a computer’s memory.

Researchers said the behavior also could be explored for coupling to magnetic or optical (light-based) devices, which could allow better control via electrical switching.

Chirality is present in many forms and at many scales, from the spiral-staircase design of our own DNA to the spin and drift of spiral galaxies; it can even determine whether a molecule acts as a medicine or a poison in our bodies.

A molecular compound known as d-glucose, for example, which is an essential ingredient for human life as a form of sugar, exhibits right-handedness. Its left-handed counterpart, l-glucose, though, is not useful in human biology.

“Chirality hadn’t been seen before in this electric structure,” said Elke Arenholz, a senior staff scientist at Berkeley Lab’s Advanced Light Source (ALS), which is home to the X-rays that were key to the study. The study was published online this week in the journal Proceedings of the National Academy of Sciences.

The experiments can distinguish between left-handed chirality and right-handed chirality in the samples’ vortices. “This offers new opportunities for fundamentally new science, with the potential to open up applications,” she said.

“Imagine that one could convert a right-handed form of a molecule to its left-handed form by applying an electric field, or artificially engineer a material with a particular chirality,” said Ramamoorthy Ramesh, a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and associate laboratory director of the Lab’s Energy Technologies Area, who co-led the latest study.

Ramesh, who is also a professor of materials science and physics at UC Berkeley, custom-made the novel materials at UC Berkeley.

Padraic Shafer, a research scientist at the ALS and the lead author of the study, worked with Arenholz to carry out the X-ray experiments that revealed the chirality of the material.

The samples included a layer of lead titanate (PbTiO3) and a layer of strontium titanate (SrTiO3) sandwiched together in an alternating pattern to form a material known as a superlattice. The materials have also been studied for their tunable electrical properties that make them candidates for components in precise sensors and for other uses.

Neither of the two compounds show any handedness by themselves, but when they were combined into the precisely layered superlattice, they developed the swirling vortex structures that exhibited chirality.

“Chirality may have additional functionality,” Shafer said, when compared to devices that use magnetic fields to rearrange the magnetic structure of the material.

The electronic patterns in the material that were studied at the ALS were first revealed using a powerful electron microscope at Berkeley Lab’s National Center for Electron Microscopy, a part of the Lab’s Molecular Foundry, though it took a specialized X-ray technique to identify their chirality.

“The X-ray measurements had to be performed in extreme geometries that can’t be done by most experimental equipment,” Shafer said, using a technique known as resonant soft X-ray diffraction that probes periodic nanometer-scale details in their electronic structure and properties.

Spiraling forms of X-rays, known as circularly polarized X-rays, allowed researchers to measure both left-handed and right-handed chirality in the samples.

Arenholz, who is also a faculty member of the UC Berkeley Department of Materials Science & Engineering, added, “It took a lot of time to understand the results, and a lot of modeling and discussions.” Theorists at the University of Cantabria in Spain and their network of computational experts performed calculations of the vortex structures that aided in the interpretation of the X-ray data.

The same science team is pursuing studies of other types and combinations of materials to test the effects on chirality and other properties.

“There is a wide class of materials that could be substituted,” Shafer said, “and there is the hope that the layers could be replaced with even higher functionality materials.”

Researchers also plan to test whether there are new ways to control the chirality in these layered materials, such as by combining materials that have electrically switchable properties with those that exhibit magnetically switchable properties.

“Since we know so much about magnetic structures,” Arenholz said, “we could think of using this well-known connection with magnetism to implement this newly discovered property into devices.”

A team of Russian, Czech and German researchers gained a new perspective on the properties of three materials of biological origin. Besides two reference materials with well-studied properties — serum albumin and cytochrome C — the researchers looked at the extracellular matrix of the Shewanella oneidensis MR-1 bacterium, which is used in biofuel cells. The team measured the materials’ dynamic conductivity and dielectric permittivity in a wide range of frequencies and temperatures. To interpret their findings, the researchers used theoretical approaches and concepts from condensed matter physics. The paper detailing the study was published in the journal Scientific Reports.

“So far, the formalism of condensed matter physics has only found limited use in classical biochemistry and biophysics. As a result, certain interesting effects evade our attention,” says Konstantin Motovilov, senior research scientist at the Laboratory of Terahertz Spectroscopy at Moscow Institute of Physics and Technology (MIPT). “When we do make use of this language, we acquire new ways of modeling observed phenomena and describing biological structures. In our paper, we characterize the behavior of proteins, considered as classical amorphous semiconductors, with the help of the formalism of condensed matter physics.”

Before discussing the study, here is a quick example of how solid-state physics explains the electrical properties of different materials.

There are in fact multiple mechanisms of electrical conductivity. For each, there is a corresponding theory that describes the properties of certain materials. For example, the conductivity in metals is adequately explained by the Drude theory. In the theory, there is no interaction between the conduction electrons, which are assumed to only occasionally collide with crystal lattice, impurities, and defects. Electrical conductivity is the inverse of electrical resistivity. Conductivity indicates how easy it is for an electric current to pass through a given material. Within the Drude model, this property does not depend strongly on frequency up to the frequency of the collisions between charge carriers and lattice or impurities. However, there is a large group of conductive materials that do not fit this description. Yet their behavior in an external electromagnetic field is quite interesting. Among them are glasses, ionic conductors, and amorphous semiconductors.

To qualitatively describe the electrical properties of such materials, another theory was proposed about 40 years ago by Andrzej Karol Jonscher, an English physicist. According to his theory, charge carriers — electrons, for example — can adequately be considered as free at room temperature, provided the alternating current frequency does not exceed several megahertz. Under these conditions, the Drude model is applicable and conductivity is nearly constant, i.e., it does not depend on the frequency of the external field. If, however, the frequency is higher, this description is no longer valid and there is an increase in conductivity proportional to a certain power — which is close to 0.8 — of frequency. The same effect is observed for materials that are gradually cooled, even if the frequency is kept constant.

Interestingly, different materials exhibit quite similar behavior in that regard. Moreover, if you restate the dependences — say, talk about the ratio between direct current (static) conductivity and alternating current conductivity, as opposed to conductivity as such — the relations for all materials turn out to be identical, revealing the so-called Universal Dielectric Response (UDR). This curious phenomenon was thoroughly investigated in a study that examined the conduction in glasses and other amorphous materials, offering new insights into their structure and properties.

The authors of the paper showed that Jonscher’s law for conductivity applies to three organic materials. Among them, two are well-known reference proteins: bovine serum albumin and bovine heart cytochrome C. Their structural, physical, and chemical properties have been investigated in detail, so the researchers used them as reference materials.

In addition, they examined the extracellular matrix and filaments (EMF) of the Shewanella oneidensis MR-1 bacterium, which can produce electricity in biological fuel cells. S. oneidensis has been used in many studies with a focus on alternative energy sources, so its electrical properties are of interest to both researchers and engineers. In 2010, a team of researchers based in the United States and Canada showed that the bacterium’s extracellular appendages behave a lot like p-type semiconductors. The electrical properties of S. oneidensis MR-1 have nevertheless not been studied in detail. The recently published paper is an attempt to remedy that.

The authors measured the conductivity of the materials, as well as the energy losses in a frequency range from 1 hertz to 1.5 terahertz, or trillion hertz, for temperatures from -260 to 40 degrees Celsius. (Strictly speaking, the energy losses are given by the imaginary part of the complex dielectric permittivity.) Next, the researchers measured the direct current conductivity of EMF for temperatures from zero to 40 C, as well as the temperature dependence of their heat capacity. For each of the three materials, water content and ion concentration were also determined.

To do this, the researchers pressed the substances into pellets using a 1-centimeter mold. They then applied electrodes to the faces of the pellets to pass alternating current through them in order to measure the electrical conductivity and dielectric permittivity of the materials in the 1-300 million hertz range. For higher frequencies, this approach does not work, so for the 30-1,500 gigahertz, or billion hertz, range, the team obtained the spectra of complex dielectric permittivity using quasioptical terahertz spectroscopy. No measurements were made in the intermediate frequency range.

It turned out that at room temperature, EMF conductivity is nearly constant, and when the frequency is increased above several million hertz, or several megahertz, the conductivity is proportional to a certain power — which is close to 1 — of the frequency. Cytochrome C did not exhibit such behavior unless the frequency was low and the temperature high. In the case of albumin, it was not observed at all. This suggests that different conductivity mechanisms are at play in these materials. It is likely that EMF has nearly free charges at room temperature — just like in the Drude model — whereas albumin does not have them and cytochrome C is a mixed bag.

The dependence observed by the researchers can be explained in terms of the individual properties of the materials. Both cytochrome C and albumin are regular proteins. Although these materials do have some free charges, these are not nearly as many as it would be necessary to justify the Drude model. Comparing the conductivity in EMF to that in metals (conductors) is more realistic, as free charges are more easily generated in these molecules. However, a comparison even more valid would be that with a solution of table salt, which has a high concentration of free ions.

Naturally, a complete description is more complex and would require us to take the water content of materials and other factors into account. For instance, because EMF contains significant amounts of loosely bound water, its conductivity grows quadratically at temperatures of about -250 C and frequencies on the order of 100 billion hertz (sub-terahertz terahertz range). Temperatures that low cause the bulk water in the material to freeze, and high frequencies mean that the dielectric properties resulting from water dipole dynamics become non-negligible. The other materials, too, exhibit deviations from Jonscher’s predictions, but they are not as dramatic.

The authors have thus clearly shown the powerful methodology and instrumentation of condensed matter physics to be effective for fundamental research into the electrodynamics of biological objects. The next step could involve the application to biomaterials research of the wide range of other theories and models that have been effectively used by the physics community for many decades.

Researchers at the Center for Integrated Nanostructure Physics, within the Institute for Basic Science (IBS), have shown that defects in monolayer molybdenum disulfide (MoS2) exhibit electrical switching, providing new insights into the electrical properties of this material. As MoS2 is one of the most promising 2D semiconductors, it is expected that these results will contribute to its future use in opto-electronics.

The study on 2-D molybdenum disulfide (MoS2) defects employed low frequency noise measurements and conductive atomic force microscopy (C-AFM). The enlarged image shows an AFM cantilever tip pointing to an area with one sulfur monovacancy (area shaded red). As current flows through the AFM tip and the sample, switching events between different ionization states (neutral and charged -1) are measured. With a radius of around 25 nanometers, the AFM tip covers an area that contains around 1-8 sulfur monovacancies. Credit: IBS, published on Nature Communications

The study on 2-D molybdenum disulfide (MoS2) defects employed low frequency noise measurements and conductive atomic force microscopy (C-AFM). The enlarged image shows an AFM cantilever tip pointing to an area with one sulfur monovacancy (area shaded red). As current flows through the AFM tip and the sample, switching events between different ionization states (neutral and charged -1) are measured. With a radius of around 25 nanometers, the AFM tip covers an area that contains around 1-8 sulfur monovacancies. Credit: IBS, published on Nature Communications

Defects can cause major changes in the properties of a material, leading to either desirable or unwanted effects. For example, petrochemical industry has long taken advantage of the catalytic activity of MoS2edges, characterized by the presence of a high concentration of defects, to produce petroleum products with reduced sulfur dioxide (SO2) emissions. On the other hand, having a pristine material is a must in electronics. Currently, silicon rules the industry, because it can be prepared in a virtually defect-free manner. In the case of MoS2, its suitability for electronic applications is currently limited by the presence of naturally occurring defects. So far, the precise link between these defects and the degraded properties of MoS2 has been an open question.

In IBS, a team of physicists, material scientists, and electrical engineers worked closely together to explore the electronic properties of sulfur vacancies in MoS2 monolayers, using a combination of atomic force microscopy (AFM) and noise analysis. The scientists used a metallic AFM tip to measure the noise signal, i.e., the variation of electrical current passing through a single layer of MoS2 placed on a metal substrate.

The most common defects in MoS2 are instances of missing single sulfur atoms, also known as sulfur monovacancies. In a perfect sample, each sulfur atom has two valence electrons that bind to two molybdenum electrons. However, where a sulfur atom is missing, these two molybdenum electrons are left unsaturated, defining the neutral state (0 state) of the defect. However, the team observed rapid switching events in their noise measurements, indicating the state of the vacancy switched between neutral (0 state) and charged (-1 state).

“The switching between 0 and -1 is happening continuously. While an electron resides at the vacancy for a while, it is missing from the current, such that we observe a current drop,” explains Michael Neumann, one of the co-first authors of the study. “This goes a long way towards understanding the known anomalies of MoS2, and it is very interesting that sulfur vacancies alone are enough to explain these anomalies, without requiring more complex defects.” According to the experiments and earlier calculations, two electrons can be also trapped at the vacancy (-2 state), but this does not seem to be energetically favored.

The new observation that sulfur vacancies can be charged (-1 and -2 states) sheds light on several MoS2 anomalies, including its reduced electron mobility observed in MoS2 monolayer samples: electrons move following the direction of an applied voltage, but get scattered by charged defects. “The -1 state is occupied around 50% of the time, which would lead to scattering of electrons, and thus explain why MoS2 has such poor mobility,” clarifies Neumann. Other MoS2 characteristics which can be explained by this study are the n-type doping of MoS2, and the unexpectedly large resistance at the MoS2-metal junction.

“This research opens up the possibility of developing a new noise nanospectroscopy device capable of mapping one or more defects on a nanoscale scale over a wide area of a 2D material,” concludes the corresponding author Young Hee Lee.

The full study is available on Nature Communications.

By Natalie Shim, SEMI Korea

As dynamic back-end related technologies such as TSV (Through-Silicon Vias), InFO (Integrated Fan Out), etc., enable electronic devices to downsize with higher performance, the importance of back-end processing is greater than ever. Due to this, more and more customers are requesting “quality control” by tracing raw materials to assembly and packaging companies and the need for a standard is clear.

The Korea Advanced Back-end Factory Integration Task Force, in response to the industry’s demand, has decided to revise SEMI E142-0211 (Reapproved 1016), Specification for Substrate Mapping by adding an assembly and packaging raw materials traceability method.

Standards Chart

The first ballot is open for voting in Cycle 9-2017 (Nov 29 to Dec 29, 2017), and the TF will review the feedback at the next Information and Control Korea Technical Committee Chapter meeting scheduled for February 1, 2018, in conjunction with SEMICON Korea in Seoul, Korea.

Get Involved

SEMI Standards development activities take place throughout the year in all major manufacturing regions. To get involved, join the SEMI International Standards Program at: www.semi.org/standardsmembership.

For more information regarding Korea Advanced Back-end Factory Integration Task Force activities, please contact Natalie Shim at [email protected].

Researchers from North Carolina State University have found that the transfer of triplet excitons from nanomaterials to molecules also creates a feedback mechanism that returns some energy to the nanocrystal, causing it to photoluminesce on long time scales. The mechanism can be adjusted to control the amount of energy transfer, which could be useful in optoelectronic applications.

Pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. Credit: Cedric Mongin

Pyrenecarboxylic acid-functionalized CdSe quantum dots undergo thermally activated delayed photoluminescence. Credit: Cedric Mongin

Felix N. Castellano, Goodnight Innovation Distinguished Chair of Chemistry at NC State, had previously shown that semiconductor nanocrystals could transfer energy to molecules, thereby extending their excited state lifetimes long enough for them to be useful in photochemical reactions.

In a new contribution, Castellano and Cédric Mongin, a former postdoctoral researcher currently an assistant professor at École normale supérieure Paris-Saclay in France, have shown that not only does the transfer of triplet excitons extend excited state lifetimes, but also that some of the energy gets returned to the original nanomaterial in the process.

“When we looked at triplet exciton transfers from nanomaterials to molecules, we noticed that after the initial transfer the nanomaterial would still luminesce in a delayed fashion, which was unexpected,” says Castellano. “So we decided to find out what exactly was happening at the molecular level.”

Castellano and Mongin utilized cadmium selenide (CdSe) quantum dots as the nanomaterial and pyrenecarboxylic acid (PCA) as the acceptor molecule. At room temperature, they found that the close proximity of the relevant energy levels created a feedback mechanism that thermally repopulated the CdSe excited state, causing it to photoluminesce.

Taking the experiment one step further, the researchers then systematically varied the CdSe-PCA energy gap by changing the size of the nanocrystals. This resulted in predictable changes to the resultant excited state lifetimes. They also examined this process at different temperatures, yielding results consistent with a thermally activated energy transfer mechanism.

“Depending on relative energy separation, the system can be tuned to behave more like PCA or more like the CdSe nanoparticle,” says Castellano. “It’s a control dial for the system. We can make materials with unique photoluminescent properties simply by controlling the size of the nanoparticle and the temperature of the system.”

Graphene ribbons that are only a few atoms wide, so-called graphene nanoribbons, have special electrical properties that make them promising candidates for the nanoelectronics of the future: While graphene – a one atom thin, honeycomb-shaped carbon layer – is a conductive material, it can become a semiconductor in the form of nanoribbons. This means that it has a sufficiently large energy or band gap in which no electron states can exist: it can be turned on and off – and thus may become a key component of nanotransistors.

The microscopic ribbons lie criss-crossed on the gold substrate. Credit: EMPA

The microscopic ribbons lie criss-crossed on the gold substrate. Credit: EMPA

The smallest details in the atomic structure of these graphene bands, however, have massive effects on the size of the energy gap and thus on how well-suited nanoribbons are as components of transistors. On the one hand, the gap depends on the width of the graphene ribbons, while on the other hand it depends on the structure of the edges. Since graphene consists of equilateral carbon hexagons, the border may have a zigzag or a so-called armchair shape, depending on the orientation of the ribbons. While bands with a zigzag edge behave like metals, i.e. they are conductive, they become semiconductors with the armchair edge.

This poses a major challenge for the production of nanoribbons: If the ribbons are cut from a layer of graphene or made by cutting carbon nanotubes, the edges may be irregular and thus the graphene ribbons may not exhibit the desired electrical properties.

Creating a semiconductor with nine atoms

Empa researchers in collaboration with the Max Planck Institute for Polymer Research in Mainz and the University of California at Berkeley have now succeeded in growing ribbons exactly nine atoms wide with a regular armchair edge from precursor molecules. The specially prepared molecules are evaporated in an ultra-high vacuum for this purpose. After several process steps, they are combined like puzzle pieces on a gold base to form the desired nanoribbons of about one nanometer in width and up to 50 nanometers in length.

These structures, which can only be seen with a scanning tunneling microscope, now have a relatively large and, above all, precisely defined energy gap. This enabled the researchers to go one step further and integrate the graphene ribbons into nanotransistors. Initially, however, the first attempts were not very successful: Measurements showed that the difference in the current flow between the “ON” state (i.e. with applied voltage) and the “OFF” state (without applied voltage) was far too small. The problem was the dielectric layer of silicon oxide, which connects the semiconducting layers to the electrical switch contact. In order to have the desired properties, it needed to be 50 nanometers thick, which in turn influenced the behavior of the electrons.

However, the researchers subsequently succeeded in massively reducing this layer by using hafnium oxide(HfO2) instead of silicon oxide as the dielectric material. The layer is therefore now only 1.5 nanometers thin and the “on”-current is orders of magnitudes higher.

Another problem was the incorporation of graphene ribbons into the transistor. In the future, the ribbons should no longer be located criss-cross on the transistor substrate, but rather aligned exactly along the transistor channel. This would significantly reduce the currently high level of non-functioning nanotransistors.

For the first time, physicists have developed a technique that can peer deep beneath the surface of a material to identify the energies and momenta of electrons there.

The energy and momentum of these electrons, known as a material’s “band structure,” are key properties that describe how electrons move through a material. Ultimately, the band structure determines a material’s electrical and optical properties.

The team, at MIT and Princeton University, has used the technique to probe a semiconducting sheet of gallium arsenide, and has mapped out the energy and momentum of electrons throughout the material. The results are published today in the journal Science.

By visualizing the band structure, not just at the surface but throughout a material, scientists may be able to identify better, faster semiconductor materials. They may also be able to observe the strange electron interactions that can give rise to superconductivity within certain exotic materials.

“Electrons are constantly zipping around in a material, and they have a certain momentum and energy,” says Raymond Ashoori, professor of physics at MIT and a co-author on the paper. “These are fundamental properties which can tell us what kind of electrical devices we can make. A lot of the important electronics in the world exist under the surface, in these systems that we haven’t been able to probe deeply until now. So we’re very excited — the possibilities here are pretty vast.”

Ashoori’s co-authors are postdoc Joonho Jang and graduate student Heun Mo Yoo, along with Loren Pfeffer, Ken West, and Kirk Baldwin, of Princeton University.

Pictures beneath the surface

To date, scientists have only been able to measure the energy and momentum of electrons at a material’s surface. To do so, they have used angle-resolved photoemission spectroscopy, or ARPES, a standard technique that employs light to excite electrons and make them jump out from a material’s surface. The ejected electrons are captured, and their energy and momentum are measured in a detector. Scientists can then use these measurements to calculate the energy and momentum of electrons within the rest of the material.

“[ARPES] is wonderful and has worked great for surfaces,” Ashoori says. “The problem is, there is no direct way of seeing these band structures within materials.”

In addition, ARPES cannot be used to visualize electron behavior in insulators — materials within which electric current does not flow freely. ARPES also does not work in a magnetic field, which can greatly alter electronic properties inside a material.

The technique developed by Ashoori’s team takes up where ARPES leaves off and enables scientists to observe electron energies and momenta beneath the surfaces of materials, including in insulators and under a magnetic field.

“These electronic systems by their nature exist underneath the surface, and we really want to understand them,” Ashoori says. “Now we are able to get these pictures which have never been created before.”

Tunneling through

The team’s technique is called momentum and energy resolved tunneling spectroscopy, or MERTS, and is based on quantum mechanical tunneling, a process by which electrons can traverse energetic barriers by simply appearing on the other side — a phenomenon that never occurs in the macroscopic, classical world which we inhabit. However, at the quantum scale of individual atoms and electrons, bizarre effects such as tunneling can occasionally take place.

“It would be like you’re on a bike in a valley, and if you can’t pedal, you’d just roll back and forth. You would never get over the hill to the next valley,” Ashoori says. “But with quantum mechanics, maybe once out of every few thousand or million times, you would just appear on the other side. That doesn’t happen classically.”

Ashoori and his colleagues employed tunneling to probe a two-dimensional sheet of gallium arsenide. Instead of shining light to release electrons out of a material, as scientists do with ARPES, the team decided to use tunneling to send electrons in.

The team set up a two-dimensional electron system known as a quantum well. The system consists of two layers of gallium arsenide, separated by a thin barrier made from another material, aluminum gallium arsenide. Ordinarily in such a system, electrons in gallium arsenide are repelled by aluminum gallium arsenide, and would not go through the barrier layer.

“However, in quantum mechanics, every once in a while, an electron just pops through,” Jang says.

The researchers applied electrical pulses to eject electrons from the first layer of gallium arsenide and into the second layer. Each time a packet of electrons tunneled through the barrier, the team was able to measure a current using remote electrodes. They also tuned the electrons’ momentum and energy by applying a magnetic field perpendicular to the tunneling direction. They reasoned that those electrons that were able to tunnel through to the second layer of gallium arsenide did so because their momenta and energies coincided with those of electronic states in that layer. In other words, the momentum and energy of the electrons tunneling into gallium arsenide were the same as those of the electrons residing within the material.

By tuning electron pulses and recording those electrons that went through to the other side, the researchers were able to map the energy and momentum of electrons within the material. Despite existing in a solid and being surrounded by atoms, these electrons can sometimes behave just like free electrons, albeit with an “effective mass” that may be different than the free electron mass. This is the case for electrons in gallium arsenide, and the resulting distribution has the shape of a parabola. Measurement of this parabola gives a direct measure of the electron’s effective mass in the material.

Exotic, unseen phenomena

The researchers used their technique to visualize electron behavior in gallium arsenide under various conditions. In several experimental runs, they observed “kinks” in the resulting parabola, which they interpreted as vibrations within the material.

“Gallium and arsenic atoms like to vibrate at certain frequencies or energies in this material,” Ashoori says. “When we have electrons at around those energies, they can excite those vibrations. And we could see that for the first time, in the little kinks that appeared in the spectrum.”

They also ran the experiments under a second, perpendicular magnetic field and were able to observe changes in electron behavior at given field strengths.

“In a perpendicular field, the parabolas or energies become discrete jumps, as a magnetic field makes electrons go around in circles inside this sheet,” Ashoori says.

“This has never been seen before.”

The researchers also found that, under certain magnetic field strengths, the ordinary parabola resembled two stacked donuts.

“It was really a shock to us,” Ashoori says.

They realized that the abnormal distribution was a result of electrons interacting with vibrating ions within the material.

“In certain conditions, we found we can make electrons and ions interact so strongly, with the same energy, that they look like some sort of composite particles: a particle plus a vibration together,” Jang says.

Further elaborating, Ashoori explains that “it’s like a plane, traveling along at a certain speed, then hitting the sonic barrier. Now there’s this composite thing of the plane and the sonic boom. And we can see this sort of sonic boom — we’re hitting this vibrational frequency, and there’s some jolt happening there.”

The team hopes to use its technique to explore even more exotic, unseen phenomena below the material surface.

“Electrons are predicted to do funny things like cluster into little bubbles or stripes,” Ashoori says. “These are things we hope to see with our tunneling technique. And I think we have the power to do that.”

A team of Hokkaido University researchers has developed a novel material synthesis method called proton-driven ion introduction (PDII) which utilizes a phenomenon similar to “ion billiards.” The new method could pave the way for creating numerous new materials, thus drastically advancing materials sciences.

The synthesis method is based on a liquid-free process that allows for intercalation – insertion of guest ions into a host material – and ion substitution with those in the host material by driving ions with protons. This study, led by Assistant Professor Masaya Fujioka and Professor Junji Nishii at the university’s Research Institute for Electric Science, was published in the Journal of the American Chemical Society on November 16th.

Conventionally, intercalation and ion substitution have been conducted in an ion solution, but the process is regarded as cumbersome and problematic. In a liquid-based process, solvent molecules can be inserted into the host materials along with guest ions, degrading the crystal quality. It is also difficult to homogeneously introduce ions into host materials, and some host materials are not suitable when used with liquids.

In the PDII method, a high voltage of several kilovolts is applied to a needle-shaped anode placed in atmospheric hydrogen to generate protons via the electrolytic disassociation of hydrogen. The protons migrate along the electric field and are shot into the supply source of the desired ions – similar to balls in billiards – and the ions are driven out of the source to keep it electrically neutral. Ions forced out of the source are introduced, or intercalated, into a nanometer-level gap in the host material.

In this study, by using different materials as ion supply sources, the team succeeded in homogenously introducing lithium ions (Li+), sodium ions (Na+), potassium ions (K+), copper ions (Cu+) and silver ions (Ag+) into nanometer-level gaps in tantalum (IV) sulfide (TaS2), a layered material, while maintaining its crystallinity. Furthermore, the team successfully substituted Na+ of Na3V2(PO4)3 with K+, producing a thermodynamically metastable material, which cannot be obtained using the conventional solid-state reaction method.

“At present, we have shown that hydrogen ions (H+), Li+, Na+, K+, Cu+ and Ag+ can be used to introduce ions in our method, and we expect a larger variety of ions will be usable. By combining them with various host materials, our method could enable the production of numerous new materials,” says Masaya Fujioka. “In particular, if a method to introduce negatively charged ions and multivalent ions is established, it will spur the development of new functional materials in the solid ion battery and electronics fields.”