Bubbles and wrinkles

In plain words

Flaws that appear when a thin layer is laid on a surface or on another layer. Water and dirt trapped underneath gather into small bubbles that push the layer up, and a layer with more length than the surface beneath it folds into wrinkles. Both stretch or bend the layer where they sit and change how it conducts and glows there, so careful stacking keeps them away from the part that is measured.

As the site uses it

Best-quality 2D layers are usually grown at temperatures above what finished circuitry can tolerate, then moved onto the target wafer in a wet transfer process that tears, wrinkles and dirties them.

What 2D materials are good for

Peeling the film off its catalyst and floating it onto another surface introduces tears, wrinkles, polymer residue and trapped bubbles, and it is the single biggest gap between laboratory graphene and industrial graphene.

What is a 2D material?

Going deeper

Trapped contamination gathers into bubbles and leaves clean interface around them; a wrinkle is a fold where a layer has more length than the surface under it. Both strain the layer and change it locally.

Why dirt gathers into bubbles

Two flat crystals pressed together attract each other over their whole area. Water and hydrocarbons trapped between them hold the layers apart, so the attraction squeezes them sideways until they collect in pockets, and the interface in between is left clean to the atom – cross-sections imaged in the electron microscope showed exactly that. , or stacking at a higher temperature as in the , lets the contamination travel further, so it gathers into fewer, larger bubbles or is pushed out of the stack altogether.

Bubbles are easy to find: they show up in images and in dark-field optical microscopy, and good practice is to map them before deciding where the contacts go.

Strain and pressure inside a bubble

The shape of a bubble is a balance between the adhesion pulling the layer down and the stiffness of the layer resisting the bulge. Measured by AFM on graphene, hBN and MoS₂, bubbles of every size turned out to have nearly the same shape for a given pair of materials, and the substance trapped in submicron bubbles sits at pressures of tens of megapascals.

The bulge stretches the layer, which shifts its and peaks. In WSe₂ such strained spots can trap and emit single ; in graphene the uneven of nanobubbles acts on the electrons like a magnetic field of more than 300 tesla.

Wrinkles

A wrinkle forms where a layer has more length than the surface under it. Transfer leaves them when a film does not lie down evenly, and growth leaves them too: copper shrinks much more than graphene as the foil cools after growth, so the graphene has to buckle. In large-area films wrinkles are one of the flaws that lower , and current that has to cross them meets extra resistance.

For specialists

Blisters of trapped adsorbates – mainly hydrocarbons and water – between or stacked layers, from nanometres to micrometres across, and ridges where compressive strain from transfer or thermal-expansion mismatch is relieved out of plane. adhesion squeezes contamination into bubbles, leaving atomically clean interfaces between them, and the bubbles take a universal shape set by adhesion and in-plane stiffness. Both strain the layer locally, shifting band edges, Raman modes and emission, and both scatter carriers.

Where this comes from

Strain-induced pseudo-magnetic fields greater than 300 tesla in graphene nanobubbles Levy et al. · Science 329, 544 (2010)
Cross-sectional imaging of individual layers and buried interfaces of graphene-based heterostructures and superlattices Haigh et al. · Nature Materials 11, 764 (2012)
Structure and electronic transport in graphene wrinkles Zhu et al. · Nano Letters 12, 3431 (2012)
Universal shape and pressure inside bubbles appearing in van der Waals heterostructures Khestanova et al. · Nature Communications 7, 12587 (2016)

Annealing

Heating a material for a while, well below its melting point, so that its atoms can move a little. It drives off water and dirt, removes some of the plastic left behind when a layer is transferred, and lets defects heal or a film settle into a more ordered crystal. For 2D materials it is a double-edged tool: a gentle anneal cleans a device, but too much heat can rotate stacked layers, oxidise air-sensitive crystals or pull atoms out of the surface.

Thermal treatment below the melting or decomposition temperature – in vacuum, inert gas, forming gas (Ar/H₂) or a chalcogen or oxygen atmosphere – used to desorb adsorbates, decompose polymer residues after transfer, lower contact resistance, heal vacancies and recrystallise grown films. Post-transfer anneals typically run at 200–400 °C; twist-angle relaxation, oxidation, chalcogen loss and the back-end thermal budget set the upper limit.

Full explanation

Photon

The smallest possible amount of light – one indivisible packet of it. The packets of a given colour all carry the same energy, and blue ones carry more than red: that is why ultraviolet light gives you sunburn and red light, however bright, does not. A semiconductor absorbs only photons with enough energy to cross its band gap and gives light back at about the gap’s energy – which is why LEDs of different colours are made from different semiconductors.

The quantum of the electromagnetic field, with energy E = hν and momentum h/λ but no mass or charge. Visible photons carry about 1.7–3.1 eV, comparable to the band gaps of semiconducting TMDCs, but a momentum far smaller than the crystal momenta across the Brillouin zone, so optical transitions are vertical and an indirect gap needs a phonon to make up the difference. Single-photon emission is established by antibunching in photon-correlation measurements.

Full explanation

Layer transfer

Moving an atomically thin layer from where it was made to where it is needed – from the copper foil it grew on to a silicon wafer, or from a flake on tape onto another flake. The layer is too thin to pick up on its own, so it is carried on a polymer film or a soft stamp, put down, and the carrier removed. Each step risks tears, wrinkles, trapped bubbles and a film of leftover polymer, which is why transfer is often the dirtiest step in making a 2D device.

The step that moves a 2D layer from its growth substrate or exfoliation source to a target substrate or stack. Wet transfer coats the layer with a supporting polymer, usually PMMA, detaches it by etching the growth substrate or by electrochemical bubbling, and floats it onto the target before the polymer is dissolved. Dry transfer picks layers up with a viscoelastic PDMS stamp or a polymer film such as polycarbonate, often using the van der Waals adhesion of a top flake, and releases them by heating. The costs are cracks and wrinkles, trapped water, hydrocarbons and polymer residue, strain and doping, and – at wafer scale – yield that compounds with every transfer, which is why growth directly on the target and transfer-free routes are pursued.

Full explanation