Shining a laser on a material and measuring the tiny fraction of light that comes back at slightly different colours. The shifts reveal how the atoms vibrate, which tells researchers how many layers a has and whether it is strained or damaged.
Going deeper
Left: most scattered light leaves with the laser’s own colour (Rayleigh), but a tiny fraction gives some of its energy to a lattice vibration and comes out slightly redder (Stokes); the energy difference is the Raman shift. Right: a schematic MoS2 spectrum built from its two main peaks, which sit about 19 cm−1 apart in a monolayer and about 25 cm−1 apart in the bulk – one of the quickest ways to count layers.
Light that comes back with a different colour
When laser light hits a material, almost all of the scattered light has exactly the colour it came in with. Only a very small fraction – of the order of one in a million or fewer – is scattered inelastically: it either leaves some energy behind in a vibration of the crystal lattice and emerges slightly redder, or picks up energy from an existing vibration and emerges slightly bluer. The energy difference, quoted as the Raman shift in inverse centimetres, equals the energy of that vibration.
Because vibrational frequencies depend on the masses of the atoms, the stiffness of their bonds and the symmetry of the structure, a Raman spectrum is a fingerprint of a material’s structure. It needs no vacuum and no contacts, takes seconds to minutes, and works on a spot about a micrometre across, which is why it is the everyday characterisation tool for 2D flakes.
What it tells you about a 2D sample
Layer number is the classic use. In MoS2 the in-plane and out-of-plane A′1 modes move apart as layers are added, and in graphene the shape of the distinguishes one, two and more layers. The same peaks shift with , which softens or stiffens bonds, and with , which changes how electrons screen the vibrations. shows up directly: graphene’s D band appears only where the lattice is broken, so its size relative to the G band estimates defect density. Low-frequency shear and , in which whole layers move against each other, report and interlayer coupling.
Pitfalls
The laser that probes the sample can also change it: a tightly focused beam heats a suspended flake, shifts its peaks and can oxidise or damage materials, so power should be kept low and stated. Thin-film interference in the modulates peak intensities with oxide thickness, which makes intensity ratios unreliable across substrates. When the laser energy matches an electronic transition, resonance effects change relative intensities and can add extra features. Peak positions should be calibrated – the silicon peak at 520.7 cm−1 is the usual reference – and the excitation wavelength always reported.
For specialists
Inelastic light scattering that measures energies. In peak positions, widths and intensity ratios – graphene’s G and 2D bands, or the E′ and A′1 modes of – report layer number, strain, doping and defect density; laser heating and substrate interference must be controlled.