Passivation

In plain words

Treating a surface so that it stops reacting with its surroundings and stops trapping electrons – by covering it with a thin protective layer, or by attaching atoms that tie up its loose bonds. Aluminium does it by itself: a skin of oxide a few nanometres thick is why aluminium window frames do not corrode away. A is all surface, so passivating it can decide whether it glows brightly, conducts well or survives in air at all.

Going deeper

Three ways a surface is made harmless: it starts with loose bonds that grab molecules and trap charge, the bonds can be capped one by one, or the whole surface can be sealed under a thin insulating cover.

Why surfaces need taming

The silicon industry was built on a good passivation. A bare silicon surface is covered in that trap charge; growing a layer of silicon dioxide on it and then annealing in hydrogen ties up nearly all of them, leaving only about one trap for tens of thousands of surface atoms. Without that interface the would not work.

The flat face of a layered crystal has no dangling bonds to begin with, which is one of its attractions. What needs passivating in a 2D material is everything else: in the lattice, edges, , and the interface with whatever oxide or metal is put on top of it.

Healing a monolayer

The most striking example is optical. As-made MoS₂ give back less than one percent of the light they absorb as emission; a treatment with an organic superacid raised that to more than 95 percent, by shutting down the routes through which excited electrons lose their energy without giving off light. Molecules that bind to , and sealing between layers of boron nitride, act in the same direction.

For devices the aim is a stable, thin cover. Oxides grown by need something to bond to, and a clean 2D surface offers nothing, so a seed layer or a surface treatment comes first – itself a small passivation problem.

Passivation with 2D layers

A 2D material can also be the passivation. In perovskite , a layer of 2D perovskite a few nanometres thick on the absorber’s surface ties up its defects and keeps moisture out, and it has become a common feature of the most efficient and stable cells. Boron nitride plays the same part for delicate crystals. Graphene has been tested as an atom-thin barrier against corrosion, with a caveat: where it is cracked, it can speed the corrosion of the metal underneath rather than slow it.

For specialists

The chemical or physical deactivation of a surface or interface: saturating dangling bonds and trap states (hydrogen on silicon, sulfur on III–V surfaces), compensating defects (superacid or thiol treatment of vacancies), or sealing with a such as Al₂O₃ or hBN. In 2D it raises yield, lowers and , and slows oxidation; in perovskite solar cells a thin 2D perovskite layer passivates the surface of the 3D absorber.

Where this comes from

Near-unity photoluminescence quantum yield in MoS₂ Amani et al. · Science 350, 1065 (2015) cited by 1,242

Dangling bond

A bond left with nothing on the other end, on an atom at a surface that has lost the neighbour it was bonded to inside the crystal. Such bonds grab stray molecules and trap electrons, which is why ordinary surfaces are messy – and why layered crystals, whose sheets keep every bond inside themselves, split into faces that stay clean down to a single layer.

An unsaturated valence orbital at a surface, edge or defect where a covalent bond has been cut. In a bulk semiconductor such as silicon these states lie in the gap, trap charge and pin the Fermi level unless passivated by hydrogen or a grown oxide. The basal plane of a layered crystal has none, which is what allows van der Waals stacking without lattice matching and interfaces with few traps – and also why atomic layer deposition nucleates poorly on it. Edges, grain boundaries and vacancies do carry them, and are where 2D materials are chemically active.

Full explanation

Point defect

A flaw in a crystal that involves a single atomic site: an atom missing (a vacancy), an atom of the wrong kind in a site (a substitution, or an antisite when the crystal’s own two kinds of atom swap places), or an extra atom squeezed in between (an interstitial). Every real crystal has some. In a sheet a few atoms thick each one sits at the surface, so a handful of them can change how the material conducts, glows or reacts far more than in a thick crystal.

A defect confined to one or a few lattice sites – vacancy, interstitial, substitutional impurity, antisite or adatom – or a small complex of them. What it does is set by the levels it introduces: shallow levels dope, deep levels trap carriers and act as non-radiative recombination centres, and an isolated deep level in a wide gap can emit single photons. Formation energies and charge states depend on the chemical conditions of growth and on the Fermi level. In 2D materials every defect is a surface defect, less screened and more exposed to the environment, and densities of 10¹² to 10¹³ per cm² are typical of TMDC monolayers.

Full explanation

Metal, semimetal and insulator

The three basic ways a solid treats electricity, set by how its electrons fill the energies open to them. In a metal they fill a band of energies only partly, so the smallest push sets them moving, and it conducts at any temperature. In an insulator they fill a band completely and a wide gap separates it from the next, so nothing can move. A semimetal sits in between: two bands just touch or overlap a little, leaving a small number of mobile electrons and holes. NbSe₂ is a metal, graphene a semimetal, hBN an insulator.

A classification by where the Fermi level falls in the band structure. In a metal it crosses a partly filled band, leaving a Fermi surface, a carrier density of order one per atom (about 10²² cm⁻³) and a resistivity that falls on cooling. In a band insulator it lies in a gap of several electronvolts; a semiconductor is an insulator whose gap is small enough to be doped or gated. A semimetal has a slight overlap between conduction and valence bands (graphite, bismuth, bulk WTe₂) or bands that touch at isolated points (graphene), giving low and nearly equal densities of electrons and holes. Interactions can open gaps that band theory does not predict: Mott insulators from half-filled bands, excitonic insulators from semimetals.

Full explanation

Atomic layer deposition (ALD)

A way to coat a surface one layer of atoms at a time. Two chemicals take turns: the first sticks to every free spot on the surface but not to itself, the excess is pumped away, and the second reacts with what stuck, leaving one thin layer of, say, aluminium oxide. Repeating the pair builds the film with a precision of a fraction of a nanometre, even over deep, steep structures. It is how the gate insulators of modern computer chips are made – and on 2D materials it runs into a problem: their clean surfaces give the first chemical nothing to stick to.

Thin-film growth by alternating, self-limiting surface reactions of two precursors separated by purges – for Al₂O₃ trimethylaluminium and water, for HfO₂ a hafnium amide or chloride with water or ozone – typically at 100 to 300 °C, giving about 0.1 nm per cycle with sub-nanometre control and conformal coverage. It is the standard route to high-κ gate dielectrics in silicon CMOS. On the dangling-bond-free basal planes of graphene, TMDCs and hBN the first precursor finds no reactive sites, so nucleation starts only at steps, defects and residues, giving islands and pinholes. Seed layers (evaporated and oxidised Al, molecular layers), ozone or plasma functionalisation, low-temperature physisorption, or transferred and native-oxide dielectrics are used instead, each at a cost in thickness, interface traps or channel damage.

Full explanation

Dielectric

An electrical insulator used for what it does in an electric field rather than for carrying current – the thin layer inside a capacitor, or the insulator that separates a transistor’s gate from its channel. A good dielectric blocks current completely, withstands a strong field without breaking down and, for gates, has a high permittivity, so that a voltage on the gate pulls many charges into the channel. For 2D transistors the dielectric is as hard to get right as the 2D layer itself.

An insulator characterised by its permittivity κ, band gap and band offsets to the channel, breakdown field, leakage, and density of fixed charge and traps at the interface and in the bulk (border traps). Silicon technology moved from SiO₂ (κ ≈ 3.9) to hafnium-based high-κ oxides (κ ≈ 20–25) to keep gate control at sub-nanometre equivalent oxide thickness. For 2D channels, hBN gives clean, trap-poor interfaces but a low κ of about 3–4 and is hard to grow at wafer scale; ALD oxides nucleate poorly on basal planes; alternatives include crystalline CaF₂, native oxides such as Bi₂SeO₅ on Bi₂O₂Se, and molecular crystals. Reliability – hysteresis, bias-temperature instability and time-dependent breakdown – is the yardstick for any dielectric meant for products.

Full explanation