Grain boundary

Everyday term

In plain words

The seam where two crystal grains grown in different orientations meet. A film made of many grains is only as good as its seams: they scatter electrons, weaken the sheet and are where damage usually begins.

Going deeper

Grains that grow in different orientations meet along boundaries, while islands that share one orientation merge seamlessly. In graphene, boundaries are built mostly from pentagon–heptagon pairs; in TMDCs from four- and eight-membered rings, or they take the special form of mirror twin boundaries.

What a boundary is made of

Large-area films grown by start from many separate nuclei. Each island grows with its own orientation, and where two islands with different orientations meet they cannot join perfectly, leaving a line defect: the grain boundary.

In graphene, atomic-resolution showed that grains stitch together predominantly through pentagon–heptagon pairs, which accommodate the tilt between neighbouring lattices, and that the grains in early CVD films were unexpectedly small and intricately shaped. boundaries are built from different rings, often four- and eight-membered, and can carry excess or missing atoms. A special case is the , where two domains rotated by 60° meet along a well-defined line.

How boundaries change properties

For electrons a boundary acts as a barrier whose height depends on its structure. Calculations for graphene boundaries built from periodic dislocations found two distinct behaviours: some are almost fully transparent to carriers over wide energy ranges, while others reflect them completely. Measured boundaries usually add resistance and lower the of a film, by amounts that vary from one boundary to the next.

Mechanically, well-stitched boundaries retain much of the strength of a perfect sheet, while poorly connected ones are where tears begin. Chemically, boundaries are more reactive: they oxidise and first and can serve as sites. In TMDCs, emission can brighten or dim along a boundary depending on its atomic structure.

Growing films without boundaries

Making grains larger helps, but any two misaligned grains still leave a boundary when they meet. The more effective route is alignment: if every island has the same orientation, neighbours merge without a seam. Graphene islands grown on Cu(111) align in this way, and hBN on copper surfaces with carefully oriented atomic steps grows as aligned islands that merge into single crystals.

TMDC triangles are harder, because islands pointing in opposite directions share a lattice orientation but meet at mirror twin boundaries; vicinal whose steps favour one triangle orientation address this. Boundaries are mapped by dark-field electron microscopy, polarisation-resolved , and in graphene by liquid crystals or selective oxidation that make boundaries visible under an optical microscope.

For specialists

A line defect joining two domains of different orientation, built in 2D from rows of non-hexagonal rings – pentagon–heptagon pairs in graphene, dislocation cores of four- to eight-membered rings in TMDCs, with the mirror twin boundary as a special case. Its effect depends on misorientation and core structure more than on grain size: some boundaries barely scatter carriers while others block transport, which is why wafer-scale growth aims for aligned single-orientation domains rather than merely larger ones.

Where this comes from

Grains and grain boundaries in single-layer graphene atomic patchwork quilts Huang et al. · Nature 469, 389 (2011) cited by 2,032
Electronic transport in polycrystalline graphene Yazyev and Louie · Nature Materials 9, 806 (2010) cited by 897

Chalcogenide, halide and pnictide

Family names for compounds, taken from the element in them that carries the negative charge. Chalcogenides contain sulfur, selenium or tellurium; halides contain fluorine, chlorine, bromine or iodine; pnictides contain phosphorus, arsenic, antimony or bismuth. Most layered crystals belong to one of these families, and the catalogue groups many of them that way.

Compounds named for their most electronegative constituent: chalcogenides of group 16 (S, Se, Te; oxides are conventionally counted apart), halides of group 17 (F, Cl, Br, I) and pnictides of group 15 (P, As, Sb, Bi; nitrides usually keep their own name). Down each group the anion grows larger and less electronegative, so bonding turns more covalent, gaps shrink and spin–orbit coupling grows. Chalcogenides give the MX₂ semiconductors and metals, halides mostly ionic, often magnetic insulators built from edge-sharing octahedra, and pnictides many metals, semimetals and the parents of the iron-based superconductors; mixed-anion compounds such as oxychalcogenides and chalcohalides combine two.

Full explanation

Etching

Removing material on purpose, with a chemical or a plasma. In chip-making it carves patterns: a stencil of light-sensitive polymer protects some areas while a plasma eats away the rest. For 2D materials etching has three jobs: cutting flakes into shaped devices, thinning a crystal one layer at a time, and – for MXenes – making the material in the first place, by dissolving one kind of atom out of a layered ceramic so that the remaining sheets come apart.

Material removal by chemical reaction in a liquid (wet etching) or by reactive and physical processes in a plasma (dry etching, including reactive ion etching with oxygen-, fluorine- or chlorine-based chemistries, and atomic layer etching). For 2D materials it patterns channels and Hall bars through resist masks – oxygen plasma for graphene, fluorine-based plasmas for hBN and TMDCs – exposes edges in encapsulated stacks for edge contacts, thins crystals layer by layer, and selectively removes the A element, usually aluminium, from MAX phases with hydrofluoric acid or fluoride-containing acids to make MXenes. Its side effects are edge roughness and damage, residues, doping and plasma-induced defects in neighbouring layers, so the etched edge and its electronic states have to be considered in narrow devices.

Full explanation

Photocatalysis and electrocatalysis

Using light, or an applied voltage, to drive a chemical reaction at a surface – most often splitting water to make hydrogen. Thin materials are natural candidates because nearly every atom is a surface atom, and because their edges are usually far more active than their flat faces.

Surface reactions driven by photons (photocatalysis) or by electrode potential (electrocatalysis), judged by overpotential, Tafel slope and turnover per site rather than by raw current. In MoS₂ the basal plane is close to inert while the edge sites carry the hydrogen-evolution activity – the result that made layered chalcogenides catalyst candidates – so activity scales with edge length, defect density and phase, and any comparison needs an active-site count rather than a geometric area.

Full explanation

Single crystal

A piece of material in which the atoms are lined up in one unbroken pattern from one side to the other, with every row pointing the same way. Most solids are not like that: they are polycrystalline, made of many small crystals stuck together at random angles, like a pavement of tiles laid every which way. The boundaries between those grains scatter electrons, weaken the material and blur measurements, so physics experiments and the best devices want single crystals – a centimetre-sized block to peel flakes from, or a sheet grown across a whole wafer in one orientation.

A solid whose lattice is continuous and uniformly oriented throughout, without grain boundaries – unlike polycrystalline material, made of misoriented grains, or amorphous solids without long-range order. Bulk single crystals of layered compounds are grown by chemical vapour transport, flux or Bridgman methods and are the usual source of exfoliated flakes; their quality is judged by X-ray rocking-curve widths, the residual resistivity ratio and the absence of twins and stacking faults. Large single-crystal films grow either from one nucleus or from aligned nuclei that merge without boundaries, as for graphene on Cu(111) or Ge(110) and MoS₂ or hBN on vicinal sapphire or copper, where substrate steps fix one orientation and remove the twin boundaries that two antiparallel orientations would form.

Full explanation

Wafer and wafer-scale growth

A wafer is the thin, round slice of ultra-pure silicon – today 300 millimetres across – on which computer chips are made, hundreds at a time. Wafer-scale growth means making a 2D material evenly over a whole wafer rather than as scattered flakes, which is what any factory needs. For the best 2D materials that is still hard: a sheet one atom thick has to have the same thickness, quality and orientation from one edge of a dinner-plate-sized disc to the other.

The substrate format of the semiconductor industry – 200 and 300 mm silicon wafers, with sapphire, SiC and glass for particular processes – and the scale at which a material must be grown, transferred and processed to enter manufacturing. Wafer-scale growth of 2D materials uses CVD and MOCVD of TMDCs, CVD of graphene and hBN, and MBE, with targets of uniform layer number, low defect and grain-boundary density and, increasingly, a single orientation on vicinal sapphire or Cu(111). The obstacles are the growth temperature against the thermal budget of a finished wafer (growth on the back end of line needs to stay below about 400 to 450 °C), the transfer from growth substrate to device wafer, and metrology that confirms thickness and quality across the wafer at production speed.

Full explanation

Substrate

The base a thin film or flake sits on – often a polished slice of silicon, glass or sapphire. For a crystal one atom thick, the substrate is not just a table: its bumps, stray charges and vibrations reach right into the sheet and change how it conducts and glows. That is why the same material can behave differently on two substrates, and why the flattest, cleanest one – boron nitride – gives the best results.

The material beneath a 2D layer. It acts on the layer through surface roughness, charged impurities, surface optical phonons, dielectric screening, strain and charge transfer, and during growth it sets orientation through epitaxy. Replacing SiO₂ with hexagonal boron nitride, which is atomically flat and nearly free of dangling bonds and charge traps, improved the mobility and charge homogeneity of graphene by close to an order of magnitude.

Full explanation

Second-harmonic generation (SHG)

Shine intense light on a crystal and a little of it comes back at exactly twice the frequency – half the wavelength, which is how a green laser pointer turns invisible infrared into green light. It happens only if the crystal has no centre of symmetry, which makes it a quick, contactless test of symmetry, layer number and crystal orientation.

A second-order nonlinear response, forbidden in centrosymmetric media, so in 2H TMDCs it appears for odd layer numbers and vanishes for even ones. The polarisation dependence gives crystal orientation, and so the twist angle in a stack; the intensity follows layer number and strain; and it detects broken inversion symmetry in ferroelectric and magnetic layers where no other quick probe exists.

Full explanation