Tag: hexagonal boron nitride

• A microscope that catches the slightest hints of heat

  A superconducting transition-edge sensor (TES) is a device well-known for its extreme sensitivity to photons, the particles of light — so much so that they can count photons one by one. They also have very little noise, which makes their readings quite reliable. TESs are often used in single-photon detectors in quantum communications systems and in cryogenic bolometers (devices that measure infrared radiation) in astronomy. But for these virtues, however, engineers haven’t been able to use TES technology together with scanning-probe optics, where scientists use a physical probe to image surfaces at extremely high resolution. In atomic force microscopy, for example, a very sharp tip is mounted on a flexible cantilever over a surface to measure forces between the tip and the sample at the nanoscale. This technology gap has been important to fill because scanning-probe optics are currently limited by how sensitive detectors are to light fields just a few nanometres big. In other words, the missing piece was a device that married the sensitivity of a TES device with the ability of a scanning probe to access spatial scales of nanometres. A new effort by researchers from Singapore, Switzerland, and the US has offered to fill this gap using a bespoke new technique called bolometric superconducting optical nanoscopy (BOSON). According to the researchers, BOSON integrates a superconducting TES directly into a scanning near-field optical microscope. The findings were published in Physical Review X on July 25.

  ‘Near field’ has a simple meaning. In conventional microscopy, like the simple light microscope in a high-school biology lab, light from a sample is captured through lenses and eventually sent to the eyes of the observer. This is called far-field microscopy because the light that contains information about the sample under study travels several multiples of its wavelength before interacting with the detecting elements. In near-field microscopy, light travels much less than a single multiple of its wavelength before reaching these elements. For example, if the wavelength of the light is 500 nm, it may travel 5 cm — or 100,000-times its wavelength — before striking the lens. On the other hand, near-field microscopy, also called near-field nanoscopy, captures and analyses light that has travelled much less than 500 nm from the sample. Devices of this kind routinely use junctions made of graphene, semiconductors or metals to translate the properties of the light energy into a measurable electrical current. These technologies demand high optical power, in the milliwatt to sub-milliwatt range, as well as elaborate engineering. They also struggle to detect changes in a sample that produce weak electromagnetic fields, like vibrating atoms in some crystals. Graphene-based devices that reveal temperature changes in a sample by shifting their resistivity are also limited by the fact that graphene’s resistivity changes very weakly with temparature, limiting the devices’ usefulness in bolometry. The team behind the new study thus set about looking for a detector whose resistance would change abruptly with even a small thermal load. This was BOSON.

  At the heart of BOSON is a bridge. It’s made of niobium, a metal that becomes a superconductor at very low temperature. It’s also only 200-250 nm wide, a really small size that makes it extremely sensitive to heat. Imagine a single snowflake landing on your finger: even the gentle heat from your body suffices to melt it quickly. Similarly, even a small amount of heat will cause the niobium bridge’s temperature to rise enough to jerk it out of its superconducting state. The bridge sits between wider niobium leads. At the start of the researchers’ experiment, the team passed a constant current through the bridge. Hovering just above the bridge was the small, sharp tip of an atomic force microscope. When an infrared laser struck the probe tip, it concentrated the electromagnetic field onto the bridge. When the tip-induced field raised the electrons’ temperature by only a few millikelvin, a “hot spot” formed on the niobium bridge. In this region, the bridge resisted the flow of current enough for a voltage to register between the leads at the ends of the bridge. This voltage was the ultimate signal of interest, demonstrating that BOSON could reliably detect extremely small changes in temperature.

  The researchers also found that BOSON’s resolution is limited not by the size of the atomic force microscope’s tip (around 20 nm tip) but by the lengths across which the energy diffuses into the bridge — under 1 micrometre in the niobium bridge — and the size of the bridge itself. The researchers have written that further narrowing the bridge could further improve its spatial resolution.

  Still, to highlight BOSON’s optical reach in their study, they overlaid the bridge with a 50-nm thick flake of hexagonal boron nitride (hBN), a material known to contain an unusual kind of wave called hyperbolic phonon-polaritons when illuminated with mid-infrared light. Hyperbolic phonon-polaritons are formed from when photons interact strongly with vibrations in the grid of atoms in a crystal, especially when the vibrations are within a particular frequency range. This interaction allows light to be guided into tracks that are narrower than the diffraction limit — a very desirable ability in microscopes trying to achieve a high resolution. The team shone an infrared laser at the hBN crystal to produce hyperbolic phonon-polaritons, then monitored the niobium bridge. They found that the phonon-polaritons produced an electromagnetic field in the crystal and the bridge was sensitive to changes in this field even when the latter’s power was as feeble as 50 nanowatt — fully four orders of magnitude below the power required to draw the attention of existing near-field microscopes. According to the researchers, this dramatic advance stemmed from operating the detector exactly at its superconducting transition temperature, where the bridge’s sensitivity to temperature changes is highest. BOSON also revealed how the phonon-polaritons dispersed within the hBN crystal, found to be consistent with theoretical predictions. The team said that since the bridge width is the effective detector size, future bridges that are only tens of nanometres wide should be able to study materials like hBN with even more sensitivity.

  By combining a superconducting bolometer with a scanning probe, the team has shown that BOSON is a universal, cryogenic nano-optical detector whose sensitivity rivals the best available TES devices. The platform can reportedly detect weak shifts in the energy of a material with nanometre precision while depositing a negligible amount of energy into the sample, a feature that could prove useful in the study of quantum materials, which are typically very fragile. According to the team’s paper, an improved BOSON may in future may be able to detect single polaritons (quasiparticles each made of a photon coupled to an electric dipole) and be sensitive to electromagnetic fields with ultra-high frequencies (in the terahertz range). They’ve also speculated that thinner superconducting bridges and the use of improved techniques to sense voltage across them could make BOSON sensitive to power changes even slighter than nanowatts.

  Featured image: A schematic diagram of the experimental setup of BOSON. CP refers to ‘Cooper pairs’, which are the charge carriers in a superconductor. I_bias is the biasing current applied to the niobium bridge. Credit: Phys. Rev. X 15, 031027.

  2025.08.02

• Boron nitride, tougher than it looks

  

  During World War I, a British aeronautical engineer named A.A. Griffith noticed something odd about glass. He found that the atomic bonds in glass needed 10,000 megapascals of stress to break apart – but a macroscopic mass of glass could be broken apart by a stress of 100 megapascals. Something about glass changed between the atomic level and the bulk, making it more brittle than its atomic properties suggested.

  Griffith attributed this difference to small imperfections in the bulk, like cracks and notches. He also realised the need for a new way to explain how brittle materials like glass fracture, since the atomic properties alone can’t explain it. He drew on thermodynamics to figure an equation based on two forms of energy: elastic energy and surface energy. The elastic energy is energy stored in a material when it is deformed – like the potential energy of a stretched rubber-band. The surface energy is the energy of molecules at the surface, which is always greater than that of molecules in the bulk. The greater the surface area of an object, the more surface energy it has.

  Griffith took a block of glass, subjected it to a tensile load (i.e. a load that stretches the material without breaking it) and then etched a small crack in it. He found that the introduction of this flaw reduced the material’s elastic energy but increased its surface energy. He also found that the free energy – which is surface energy minus elastic energy – increased up to a point as he increased the crack length, before falling back down even if the crack got longer. A material fractures, i.e. breaks, when the amount of stress it is under exceeds this peak value.

  Through experiments, engineers have also been able to calculate the fracture toughness of materials – a number essentially denoting the ability of a material to resist the propagation of surface cracks. Brittle materials usually have higher strength but lower fracture toughness. That is, they can withstand high loads without breaking or deforming, but when they do fail, they fail in catastrophic fashion. No half-measures.

  If a material’s fracture characteristics are in line with Griffith’s theory, it’s usually brittle. For example, glass has a strength of 7 megapascals (with a theoretical upper limit of 17,000 megapascals) – but a fracture toughness of 0.6-0.8 megapascals per square-root metre.

  Graphene is a 2D material, composed of a sheet of carbon atoms arranged in a hexagonal pattern. And like glass, its strength: 130,000 megapascals; its fracture toughness: 4 megapascals per square-root metre – the difference arising similarly from small flaws in the bulk material. Many people have posited graphene as a material of the future for its wondrous properties. Recently, scientists have been excited about the weird behaviour of electrons in graphene and the so-called ‘magic angle’. However, the fact that it is brittle automatically limits graphene’s applications to environments in which material failure can’t be catastrophic.

  Another up-and-coming material is hexagonal boron nitride (h-BN). As its name indicates, h-BN is a grid of boron and nitrogen atoms arranged in a hexagonal pattern. (Boron nitride has two other forms: sphalerite and wurtzite.) h-BN is already used as a lubricant because it is very soft. It can also withstand high temperatures before losing its structural integrity, making it useful in applications related to spaceflight. However, since monolayer h-BN’s atomic structure is similar to that of graphene, it was likely to be brittle as well – with small flaws in the bulk material compromising the strength arising from its atomic bonds.

  But a new study, published on June 2, has found that h-BN is not brittle. Scientists from China, Singapore and the US have reported that cracks in “single-crystal monolayer h-BN” don’t propagate according to Griffith’s theory, but that they do so in a more stable way, making the material tougher.

  Even though h-BN is sometimes called ‘white graphene’, many of its properties are different. Aside from being able to withstand up to 300º C more in air before oxidising, h-BN is an insulator (graphene is a semiconductor) and is more chemically inert. In 2017, scientists from Australia, China, Japan, South Korea, the UK and the US also reported that while graphene’s strength dropped by 30% as the number of stacked layers was increased from one to eight, that of h-BN was pretty much constant. This suggested, the scientists wrote, “that BN nanosheets are one of the strongest insulating materials, and more importantly, the strong interlayer interaction in BN nanosheets, along with their thermal stability, make them ideal for mechanical reinforcement applications.”

  The new study further cements this reputation, and in fact lends itself to the conclusion that h-BN is one of the thermally, chemically and mechanically toughest insulators that we know.

  Here, the scientists found that when a crack is introduced in monolayer h-BN, the resulting release of energy is dissipated more effectively than is observed in graphene. And as the crack grows, they found that unlike in graphene, it gets deflected instead of proceeding along a straight path, and also sprouts branches. This way, monolayer h-BN redistributes the elastic energy released in a way that allows the crack length to increase without fracturing the material (i.e. without causing catastrophic failure).

  According to their paper, this behaviour is the result of h-BN being composed of two different types of atoms, of boron and nitrogen, whereas graphene is composed solely of carbon atoms. As a result, when a bond between boron and nitrogen breaks, two types of crack-edges are formed: those with boron at the edge (B-edge) and those with nitrogen at the edge (N-edge). The scientists write that based on their calculations, “the amplitude of edge stress [along N-edges] is more than twice of that [along B-edges]”. Every time a crack branches or is deflected, the direction in which it propagates is determined according to the relative position of B-edges and N-edges around the crack tip. And as the crack propagates, the asymmetric stress along these two edges causes the crack to turn and branch at different times.

  The scientists summarise this in their paper as that h-BN dissipates more energy by introducing “more local damage” – as opposed to global damage, i.e. fracturing – “which in turn induces a toughening effect”. “If the crack is branched, that means it is turning,” Jun Lou, one of the paper’s authors and a materials scientist at Rice University, Texas, told Nanowerk. “If you have this turning crack, it basically costs additional energy to drive the crack further. So you’ve effectively toughened your material by making it much harder for the crack to propagate.” The paper continues:

  [These two mechanisms] contribute significantly to the one-order of magnitude increase in effective energy release rate compared with its Griffith’s energy release rate. This finding that the asymmetric characteristic of 2D lattice structures can intrinsically generate more energy dissipation through repeated crack deflection and branching, demonstrates a very important new toughening mechanism for brittle materials at the 2D limit.

  To quote from Physics World:

  The discovery that h-BN is also surprisingly tough means that it could be used to add tear resistance to flexible electronics, which Lou observes is one of the niche application areas for 2D-based materials. For flexible devices, he explains, the material needs to mechanically robust before you can bend it around something. “That h-BN is so fracture-resistant is great news for the 2D electronics community,” he adds.

  The team’s findings may also point to a new way of fabricating tough mechanical metamaterials through engineered structural asymmetry. “Under extreme loading, fracture may be inevitable, but its catastrophic effects can be mitigated through structural design,” [Huajian Gao, also at Rice University and another member of the study], says.

  Featured image: A representation of hexagonal boron nitride. Credit: Benjah-bmm27/Wikimedia Commons, public domain.

  2021.07.19