Tag Archives: #silicon

Silicon With A Two-Dimensional Structure (Chemistry)


Silicon, a semi-metal, bonds in its natural form with four other elements and its three-dimensional structure takes the form of a tetrahedron. For a long time, it seemed impossible to achieve the synthesis and characterisation of a two-dimensional equivalent – geometrically speaking, a square. Now scientists from the field of Inorganic Chemistry at Heidelberg University have succeeded in producing a crystalline complex with such a configuration. PD Dr Lutz Greb from the Institute of Inorganic Chemistry underlines that it has surprising physical and chemical properties and, in the field of molecular chemistry, will open up new approaches to using the second most abundant element in the Earth’s crust for catalysis and materials research.

As a classical semi-metal, silicon possesses properties of both metals and non-metals, and belongs to the carbon group on the periodic table. Like carbon, silicon bonds with four elements. Its three-dimensional structure then corresponds to a tetrahedron, a body with four sides. Due to the high stability of a tetrahedron, other structures are not known in natural silicon with four bonds – silicon(IV) for short. Considered purely geometrically, the two-dimensional equivalent to a tetrahedron is a square. These configurations are already known for carbon but, according to Dr Greb, a square-planar structure has not yet been produced in the field of silicon(IV) chemistry, even after over 40 years of intensive effort.

Dr Greb’s working group has succeeded for the first time in synthesising and completely characterising a square-planar silicon(IV) species. It was possible to show this with the aid of X-ray crystallography. The scientists grew a monocrystal which they irradiated with a finely focused beam of X-rays. The diffraction of the X-rays when encountering the atoms of the monocrystal led to an unmistakable pattern from which it is possible to calculate the position of the atom nuclei. This measurement enabled the researchers to show that they were dealing with molecules with square-planar silicon(IV). Further studies with spectroscopic methods supported this configuration. It displays physical and chemical properties that the researchers did not expect, e.g. colour in a naturally colourless class of substances.

“Synthesising this configuration from the components we chose is comparatively simple once you have understood the key conditions,” explains Dr Fabian Ebner, who is meanwhile a postdoctoral researcher at the Institute of Inorganic Chemistry. It surprised the scientists, however, that the square-planar silicon(IV) molecule constitutes a stable, isolable compound at all. “Due to the high reactivity, there are many conceivable ways of decomposition. Still, we have always believed that it is possible to isolate this compound,” Dr Greb emphasises.

Lutz Greb received a Starting Grant from the European Research Council (ERC) for his research in the area of structurally constrained main-group elements. The Foundation of German Business supported Fabian Ebner’s work. The research results were published in the journal “Chem”.

Featured image: Changes to the natural tetrahedral structure of silicon (top left) in an unusual square planar geometry (bottom right). | © Ebner/Greb (Heidelberg)


F. Ebner, L. Greb: An isolable, crystalline complex of square-planar silicon(IV). Chem (published online 9 June 2021)

Provided by University of Heidelberg

New Form Of Silicon Could Enable Next-gen Electronic And Energy Devices (Material Science)

Novel crystalline form of silicon could potentially be used to create next-generation electronic and energy devices

A team led by Carnegie’s Thomas Shiell and Timothy Strobel developed a new method for synthesizing a novel crystalline form of silicon with a hexagonal structure that could potentially be used to create next-generation electronic and energy devices with enhanced properties that exceed those of the “normal” cubic form of silicon used today.

Their work is published in Physical Review Letters.

Silicon plays an outsized role in human life. It is the second most abundant element in the Earth’s crust. When mixed with other elements, it is essential for many construction and infrastructure projects. And in pure elemental form, it is crucial enough to computing that the longstanding technological hub of the U.S.–California’s Silicon Valley–was nicknamed in honor of it.

Like all elements, silicon can take different crystalline forms, called allotropes, in the same way that soft graphite and super-hard diamond are both forms of carbon. The form of silicon most commonly used in electronic devices, including computers and solar panels, has the same structure as diamond. Despite its ubiquity, this form of silicon is not actually fully optimized for next-generation applications, including high-performance transistors and some photovoltaic devices.

While many different silicon allotropes with enhanced physical properties are theoretically possible, only a handful exist in practice given the lack of known synthetic pathways that are currently accessible.

Strobel’s lab had previously developed a revolutionary new form of silicon, called Si24, which has an open framework composed of a series of one-dimensional channels. In this new work, Shiell and Strobel led a team that used Si24 as the starting point in a multi-stage synthesis pathway that resulted in highly oriented crystals in a form called 4H-silicon, named for its four repeating layers in a hexagonal structure.

“Interest in hexagonal silicon dates back to the 1960s, because of the possibility of tunable electronic properties, which could enhance performance beyond the cubic form” Strobel explained.

Hexagonal forms of silicon have been synthesized previously, but only through the deposition of thin films or as nanocrystals that coexist with disordered material. The newly demonstrated Si24 pathway produces the first high-quality, bulk crystals that serve as the basis for future research activities.

Using the advanced computing tool called PALLAS, which was previously developed by members of the team to predict structural transition pathways–like how water becomes steam when heated or ice when frozen–the group was able to understand the transition mechanism from Si24 to 4H-Si, and the structural relationship that allows the preservation of highly oriented product crystals.

“In addition to expanding our fundamental control over the synthesis of novel structures, the discovery of bulk 4H-silicon crystals opens the door to exciting future research prospects for tuning the optical and electronic properties through strain engineering and elemental substitution,” Shiell said. “We could potentially use this method to create seed crystals to grow large volumes of the 4H structure with properties that potentially exceed those of diamond silicon.”

Carnegie’s Li Zhu was also a member of the research team, along with Brenton Cook and Dougal McCulloch of RMIT University and Jodie Bradby of The Australian National University.

This work was supported by the National Science Foundation, Division of Materials Research.

Portions of this work were performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory. HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences. The Advanced Photon Source is a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory.

Featured image: Visualization of the structure of 4H-Si viewed perpendicular to the hexagonal axis. A transmission electron micrograph showing the stacking sequence is displayed in the background. © Image courtesy of Thomas Shiell and Timothy Strobel

Reference: Thomas B. Shiell, Li Zhu, Brenton A. Cook, Jodie E. Bradby, Dougal G. McCulloch, and Timothy A. Strobel, “Bulk Crystalline 4H-Silicon through a Metastable Allotropic Transition”, Phys. Rev. Lett. 126, 215701 – Published 25 May 2021. DOI: https://doi.org/10.1103/PhysRevLett.126.215701

Provided by Carnegie Institute of Science

Study Suggests That Silicon Could be a Photonics Game-changer (Material Science)

New research from the University of Surrey has shown that silicon could be one of the most powerful materials for photonic informational manipulation – opening up new possibilities for the production of lasers and displays.

While computer chips’ extraordinary success has confirmed silicon as the prime material for electronic information control, silicon has a reputation as a poor choice for photonics; there are no commercially available silicon light-emitting diodes, lasers or displays.

Now, in a paper published by Light: Science and Applications journal, a Surrey-led international team of scientists has shown that silicon is an outstanding candidate for creating a device that can control multiple light beams.

The discovery means that it is now possible to produce silicon processors with built-in abilities for light beams to control other beams – boosting the speed and efficiency of electronic communications.

This is possible thanks to the wavelength band called the far-infrared or terahertz region of the electromagnetic spectrum. The effect works with a property called a nonlinearity, which is used to manipulate laser beams – for example, changing their colour. Green laser pointers work this way: they take the output from a very cheap and efficient but invisible infrared laser diode and change the colour to green with a nonlinear crystal that halves the wavelength.

Other kinds of nonlinearity can produce an output beam with a third of the wavelength or be used to redirect a laser beam to control the direction of the beam’s information. The stronger the nonlinearity, the easier it is to control with weaker input beams.

The researchers found that silicon possesses the strongest nonlinearity of this type ever discovered. Although the study was carried out with the crystal being cooled to very low cryogenic temperatures, such strong nonlinearities mean that extremely weak beams can be used.

Ben Murdin, co-author of the study and Professor of Physics at the University of Surrey, said: “Our finding was lucky because we weren’t looking for it. We were trying to understand how a very small number of phosphorus atoms in a silicon crystal could be used for making a quantum computer and how to use light beams to control quantum information stored in the phosphorus atoms.

“We were astonished to find that the phosphorus atoms were re-emitting light beams that were almost as bright as the very intense laser we were shining on them. We shelved the data for a couple of years while we thought about proving where the beams were coming from. It’s a great example of the way science proceeds by accident, and also how pan-European teams can still work together very effectively.”

Reference: Dessmann, N., Le, N.H., Eless, V. et al. Highly efficient THz four-wave mixing in doped silicon. Light Sci Appl 10, 71 (2021). https://doi.org/10.1038/s41377-021-00509-6

Provided by University of Surrey

Physicists Observed Cherenkov Radiation In Beryllium For The First Time (Physics)

Uglov and Vukolov presented the new experimental results on the observation of X-ray Cherenkov radiation generated by 5.7 MeV electrons in the thin Beryllium (Be) and silicon (Si) foils. They observed Cherenkov effect from Beryllium for the first time. They also compared the experimental results with the calculations performed according to the theoretical model of transition radiation taking into account the oxide layer on the target output surface.

Figure 1. a) Scheme for XCR investigation using a multilayer mirror; b) top view of the vacuum chamber – Ch; T is a target, D is a channel-electron multiplier (CEM), M is a multilayer mirror, G is a goniometer, FC is a Faraday cup. © Uglov et al.

Currently, most of the Cherenkov phenomenon researches for the X-ray region are theoretical studies; there are only a few experiments in this area. These experiments presented convincing experimental evidence for the observation of the Cherenkov effect in the X-ray range. At the same time, not all experimental results show complete agreement with the theory.

Thus, after the first experimental observation of the Cherenkov effect in the ultra-soft X-ray range near the K-edge of carbon absorption, Dutch researchers obtained in their study an experimental confirmation of the Cherenkov effect existing near the absorption edges of elements such as titanium, vanadium, and silicon; at the same time, they failed to detect the Cherenkov effect for Ni and to confirm the Cherenkov effect in carbon. The carbon-related researches were carried out using various modifications of carbon such as diamond, amorphous carbon, graphite and carbon in organic compounds using a lower electron energy than Bazylev and colleagues considered in their study, but sufficient for the appearance of Cherenkov radiation according to the theory.

There are also disagreements between the calculations of the X-ray Cherenkov radiation (XCR) angular density and the experimental observation of the one during the sliding interaction of 75 MeV electron beam with targets. For example, the experimental Cherenkov angles for carbon and Si turned out to be the same, which does not agree with the theory.

Intensity of radiation reflected by a multilayer mirror [Mo/B4C]100 depending on 𝜃0 : a) for Be
and b) for Si targets. © Uglov et al.

But, Uglov and Vukolov’s calculations includes the effect of the oxide layer and pinholes which demonstrated an improvement in the agreement between the calculations and the experiment.

The fact of observing the Cherenkov radiation in Be may promote the development of a high-intensity, high-monochrome radiation source with the energy of emitted photons E∼ 111 eV. According to authors, the spectral-angular radiation density of such a source can be increased several times by using the sliding interaction of the electron beam with the target. Besides, the threshold nature of the XCR can be used for the development of threshold counters for the separation of the charged particles.

“These detectors are more promising for use with multiply charged ions since the XCR yield is proportional to the square of the particle charge.

— told Uglov, lead author of the study.

Reference: S. R. Uglov, A.V.Vukolov, “Observation of soft X-ray Cherenkov radiation in Be and Si foils”, ArXiv, pp. 1-13, 28 Feb 2021. https://arxiv.org/abs/2103.00579

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Hitting the Quantum ‘Sweet Spot’: Researchers Find Best Position For Atom Qubits in Silicon (Quantum)

Australian researchers have located the ‘sweet spot’ for positioning qubits in silicon to scale up atom-based quantum processors.

Researchers from the Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) working with Silicon Quantum Computing (SQC) have located the ‘sweet spot’ for positioning qubits in silicon to scale up atom-based quantum processors.

Atomic-scale image of two interacting donors in silicon. ©CQC2T

Creating quantum bits, or qubits, by precisely placing phosphorus atoms in silicon – the method pioneered by CQC2T Director Professor Michelle Simmons – is a world-leading approach in the development of a silicon quantum computer.

In the team’s research, published today in Nature Communications, precision placement has proven to be essential for developing robust interactions – or coupling – between qubits.

“We’ve located the optimal position to create reproducible, strong and fast interactions between the qubits,” says Professor Sven Rogge, who led the research.

“We need these robust interactions to engineer a multi-qubit processor and, ultimately, a useful quantum computer.”

Two-qubit gates – the central building block of a quantum computer – use interactions between pairs of qubits to perform quantum operations. For atom qubits in silicon, previous research has suggested that for certain positions in the silicon crystal, interactions between the qubits contain an oscillatory component that could slow down the gate operations and make them difficult to control.

“For almost two decades, the potential oscillatory nature of the interactions has been predicted to be a challenge for scale-up,” Prof. Rogge says.

“Now, through novel measurements of the qubit interactions, we have developed a deep understanding of the nature of these oscillations and propose a strategy of precision placement to make the interaction between the qubits robust. This is a result that many believed was not possible.”

Finding the ‘sweet spot’ in crystal symmetries

The researchers say they’ve now uncovered that exactly where you place the qubits is essential to creating strong and consistent interactions. This crucial insight has significant implications for the design of large-scale processors.

“Silicon is an anisotropic crystal, which means that the direction the atoms are placed in can significantly influence the interactions between them,” says Dr Benoit Voisin, lead author of the research.

“While we already knew about this anisotropy, no one had explored in detail how it could actually be used to mitigate the oscillating interaction strength.”

“We found that there is a special angle, or sweet spot, within a particular plane of the silicon crystal where the interaction between the qubits is most resilient. Importantly, this sweet spot is achievable using existing scanning tunnelling microscope (STM) lithography techniques developed at UNSW.”

“In the end, both the problem and its solution directly originate from crystal symmetries, so this is a nice twist.”

Using a STM, the team are able to map out the atoms’ wave function in 2D images and identify their exact spatial location in the silicon crystal – first demonstrated in 2014 with research published in Nature Materials and advanced in a 2016 Nature Nanotechnology paper.

In the latest research, the team used the same STM technique to observe atomic-scale details of the interactions between the coupled atom qubits.

“Using our quantum state imaging technique, we could observe for the first time both the anisotropy in the wavefunction and the interference effect directly in the plane – this was the starting point to understand how this problem plays out,” says Dr Voisin.

“We understood that we had to first work out the impact of each of these two ingredients separately, before looking at the full picture to solve the problem – this is how we could find this sweet spot, which is readily compatible with the atomic placement precision offered by our STM lithography technique.”

Building a silicon quantum computer atom by atom

UNSW scientists at CQC2T are leading the world in the race to build atom-based quantum computers in silicon. The researchers at CQC2T, and its related commercialisation company SQC, are the only team in the world that have the ability to see the exact position of their qubits in the solid state.

In 2019, the Simmons group reached a major milestone in their precision placement approach – with the team first building the fastest two-qubit gate in silicon by placing two atom qubits close together, and then controllably observing and measuring their spin states in real-time. The research was published in Nature.

Now, with the Rogge team’s latest advances, the researchers from CQC2T and SQC are positioned to use these interactions in larger scale systems for scalable processors.

“Being able to observe and precisely place atoms in our silicon chips continues to provide a competitive advantage for fabricating quantum computers in silicon,” says Prof. Simmons.

The combined Simmons, Rogge and Rahman teams are working with SQC to build the first useful, commercial quantum computer in silicon. Co-located with CQC2T on the UNSW Sydney campus, SQC’s goal is to build the highest quality, most stable quantum processor.

Provided by Center For Quantum Computation and Communication Technology

Researchers Polished The Hardest Known Material Without Damaging It For Electronics (Electronics / Engineering)

Silicon has been the workhorse of electronics for decades because it is a common element, is easy to process, and has useful electronic properties. A limitation of silicon is that high temperatures damage it, which limits the operating speed of silicon-based electronics. Single-crystal diamond is a possible alternative to silicon. Researchers recently fabricated a single-crystal diamond wafer, but common methods of polishing the surface–a requirement for use in electronics–are a combination of slow and damaging.

Shape of mosaic single crystal diamond substrate before and after plasma-assisted polishing. ©Osaka University.

In a study recently published in Scientific Reports, researchers from Osaka University and collaborating partners polished a single-crystal diamond wafer to be nearly atomically smooth. This procedure will be useful for helping diamond replace at least some of the silicon components of electronic devices.

Diamond is the hardest known substance and essentially does not react with chemicals. Polishing it with a similarly hard tool damages the surface and conventional polishing chemistry is slow. In this study, the researchers in essence first modified the quartz glass surface and then polished diamond with modified quartz glass tools.

“Plasma-assisted polishing is an ideal technique for single-crystal diamond,” explains lead author Nian Liu. “The plasma activates the carbon atoms on the diamond surface without destroying the crystal structure, which lets a quartz glass plate gently smooth away surface irregularities.”

The single-crystal diamond, before polishing, had many step-like features and was wavy overall, with an average root mean square roughness of 0.66 micrometers. After polishing, the topographical defects were gone, and the surface roughness was far less: 0.4 nanometers.

“Polishing decreased the surface roughness to near-atomic smoothness,” says senior author Kazuya Yamamura. “There were no scratches on the surface, as seen in scaife mechanical smoothing approaches.”

Furthermore, the researchers confirmed that the polished surface was unaltered chemically. For example, they detected no graphite–therefore, no damaged carbon. The only detected impurity was a very small amount of nitrogen from the original wafer preparation.

“Using Raman spectroscopy, the full width at half maximum of the diamond lines in the wafer were the same, and the peak positions were almost identical,” says Liu. “Other polishing techniques show clear deviations from pure diamond.”

With this research development, high-performance power devices and heat sinks based on single-crystal diamond are now attainable. Such technologies will dramatically lower the power use and carbon input, and improve the performance, of future electronic devices.

References : K. Yamamura, N. Liu et al., “Damage-free highly efficient plasma-assisted polishing of a 20-mm square large mosaic single-crystal diamond substrate,” was published in Scientific Reports at DOI: https://doi.org/10.1038/s41598-020-76430-6

Provided by Osaka University

ALMA Detected Salt, Water, Silicon Compounds and Methyl Cyanide around Two Massive Protostars (Astronomy)

Astronomers reported results of 0.”05 -resolution observations toward the O-type proto-binary system IRAS 16547–4247, located 9,500 light-years away in the constellation of Scorpius, with the Atacama Large Millimeter/submillimeter Array. They detected sodium chloride, silicon compounds, and water vapor in the circumstellar disks — as well as methyl cyanide in the circumbinary disk.

An artist’s impression of the massive proto-binary system IRAS 16547-4247. Image credit: ALMA / ESO / NAOJ / NRAO.

Sodium chloride is familiar to us as table salt, but it is not a common molecule in the Universe. This was only the second detection of sodium chloride around massive young stars. The first example was around Orion KL Source I, but that is such a peculiar source that we were not sure whether salt is suitable to see gas disks around massive stars.

Their results confirmed that salt is actually a good marker. Since baby stars gain mass through disks, it is important to study the motion and characteristics of disks to understand how the baby stars grow.

The astronomers also found that the twin circumstellar disks around IRAS 16547-4247 stars are counter-rotating. The counter-rotation of the disks may indicate that these two stars are not actual twins, but a pair of strangers which were formed in separated clouds and paired up later. Yeah, if the stars are born as twins in a large common gaseous disk, then naturally the disks rotate in the same direction.

This ALMA composite image shows the massive proto-binary system IRAS 16547-4247. Different colors show the different distributions of dust particles (yellow), methyl cyanide (red), salt (green), and water vapor (blue). Bottom insets are the close-up views of each component. Dust and methyl cyanide are distributed widely around the binary system, whereas salt and water vapor are concentrated in the disk around each protostar. In the wide-field image, the jets from one of the protostars, seen as several dots in the above image, are shown in light blue. Image credit: ALMA / ESO / NAOJ / NRAO / Tanaka et al.

The team expects that further observations will provide more dependable information on the secrets of massive binary systems’ birth.

The presence of water vapor and sodium chloride, which were released by the destruction of dust particles, suggests the hot and dynamic nature of disks around massive protostars.

Interestingly, investigations of meteorites indicate that the disk of the proto-Solar System also experienced high temperatures in which dust particles were evaporated.

These new results suggested that these “hot-disk” lines may be common in innermost disks around massive protostars, and have great potential for future research of massive star formation. They also tentatively found that the twin disks are counter-rotating, which might give a hint of the origin of the massive proto-binary system IRAS 16547–4247.

References: Kei E.I. Tanaka et al. 2020. Salt, Hot Water, and Silicon Compounds Tracing Massive Twin Disks. ApJL 900 (1), L2; doi: 10.3847/2041-8213/abadfc link: https://iopscience.iop.org/article/10.3847/2041-8213/abadfc

How Stars Fuse Their Fuels And How They die? (Astronomy)

When a star is born, it is because it has enough mass to create enough heat, gravity and pressure to sustain nuclear fusion. Fusing hydrogen atoms to helium gives off enormous amounts of energy, and the star spends its life quietly fusing away. This process takes four hydrogen atoms to fuse into one helium atom. Hydrogen has only one proton, while helium has two protons along with two neutrons. This means that two protons are missing. Matter cannot be created or destroyed—it can only be turned into something else.

In this case, the two missing protons have turned into two neutrons. This energy is what makes the star shine and give off heat. Well.. after a while, the star has built up quite a bit of helium. This helium has found its way to the star’s center to create a helium core. Since hydrogen only has one proton, and helium has two protons and two neutrons, it’s heavier. That means the star has a little more mass in its core, which generates more heat. This heat builds up more and more, until it’s hot enough and has enough pressure to start fusing helium to carbon. This process generates a little less energy than fusing hydrogen to helium, but it still produces energy.

As a guideline, a star that has about one half the mass of the sun is too small and cool to fuse helium to carbon. So it will end up as a white dwarf made of helium. Stars between one half to four times the mass of the sun are massive and hot enough to fuse carbon to oxygen. Carbon and oxygen are fused more or less at the same time, and you’ll end up with a white dwarf made out of carbon and oxygen. I want to jump off topic here for a moment and ask a basic chemistry/physics question to all you readers. What happens when you introduce large amounts of heat and pressure to carbon? ‘Diamonds’..

The star has died and it’s a white dwarf made out of carbon: a giant diamond in the sky. Stars with masses greater than four times the mass of the sun are massive and hot enough to fuse oxygen to silicon.

Order of Nuclear Fusion in Dying Stars (Source)

Stars that have earned the title of “supergiant” are so massive and so hot that they begin fusing silicon to a solid core of iron. Once the star starts fusing iron, that’s it– it’s doomed. Fusing silicon to iron takes more energy than it gives off. This means that the star is going to die soon; it is causing its own death by using more of its own energy than it is getting back from nuclear fusion.

When a star is fusing iron in its core, it’s still giving off insane amounts of energy. The helium, hydrogen, carbon, oxygen, and silicon are still there in the star in different shells. Hydrogen is at the surface, still fusing to helium; a little further down, helium fusing to carbon and oxygen; further down we have silicon until the core, where silicon fuses to iron. This is why the star still exists and doesn’t spontaneously explode the moment the first iron atom pops into existence.

At this point, the energy process is just no longer exothermic but endothermic. Iron cannot be fused into anything heavierbecause of the insane amounts of energy and force required to fuse iron atoms. The atomic structure of iron is very stable, more so than most other elements. I’m not saying all other elements are radioactive or unstable, just that iron is slightly more stable than the previous elements.

Stars this massive can turn into several things; it depends on how heavy it is. They can explode into supernova, collapse into various types of neutron stars, or even form a black hole. The iron in the star’s core isn’t the reason why the star went supernova, its overall mass made it explode. But, the iron in its core caused it to die.