Technology readiness level (TRL)

In plain words

A score from 1 to 9 for how close a technology is to real use: 1 means the basic idea has been observed, 9 means it is proven in products working in the field.

Going deeper

The nine technology readiness levels, grouped into research, development and deployment. Most published 2D-material devices sit on the lower steps; the middle of the staircase, where a lab result has to become reliable, reproducible and manufacturable, is where most technologies stop.

Where the scale comes from

NASA introduced technology readiness levels in the 1970s to judge whether a technology was mature enough to fly, and the scale settled on nine levels. It spread to defence agencies, to the European Space Agency and to the European Union’s research programmes, and was standardised for space systems as ISO 16290.

The levels run from basic principles observed (1), a technology concept formulated (2) and an experimental proof of concept (3), through validation in the laboratory (4), validation in a relevant environment (5) and demonstration in a relevant environment (6), to a prototype demonstrated in an operational environment (7), a complete and qualified system (8) and an actual system proven in operation (9).

Applying it to 2D materials

A readiness level belongs to a technology in a particular application, not to a material. Graphene as such has no TRL. Graphene as a filler in , conductive or is already in commercial products, while graphene logic remain research. Two-dimensional transistors are being developed on industrial pilot lines, roughly in the middle of the scale, whereas a typical paper reporting one excellent device is level 3 or 4.

The hardest climb is from about 4 to 7. Here a result has to work across a whole rather than on one , with reproducible contacts and , acceptable yield and variability, lasting reliability, and a process compatible with existing fabs. This stretch is often called the of death.

Reading readiness claims

Levels are usually self-assessed, and the words “relevant environment” carry much of the weight. A useful check is to ask which application is meant, in which environment it was tested, on how many devices, and for how long.

A system is only as ready as its least mature part: a transistor channel at level 5 does not help if the contacts, the or the are still at level 3. TRL also says nothing about cost, supply or market. Production maturity has its own manufacturing readiness levels, and a technology can be ready in principle yet uneconomic in practice.

For specialists

A 1–9 scale describing maturity from basic principles (1) to proven operational use (9).

Where this comes from

ISO 16290:2013 — Space systems: definition of the technology readiness levels (TRL) and their criteria of assessment ISO · International Organization for Standardization (2013)
Technology readiness assessments: a retrospective Mankins · Acta Astronautica 65, 1216 (2009) cited by 723

Composites and coatings

The biggest market for 2D materials so far: flakes of graphene or other layered materials mixed in small amounts into plastics, rubber, concrete, paints or battery electrodes, to make them stiffer, tougher, more conductive or less permeable. Sold by the tonne, these products use cheap flakes made in bulk rather than perfect single layers. Whether a few percent of flakes really helps depends on how well they are spread out, how they line up and how firmly they hold on to the material around them.

Bulk uses in which exfoliated or chemically derived flakes – graphene nanoplatelets, reduced graphene oxide, hBN, MXenes, clays – are dispersed in a matrix or applied as a film. In polymer composites a few weight percent can raise stiffness and toughness, cross the percolation threshold for electrical conduction – as low as about 0.1 percent by volume for thin, wide flakes – and raise thermal conductivity; aligned flakes in barrier coatings lengthen the diffusion path of water, oxygen and ions and slow corrosion; flakes used as conductive additives replace part of the carbon black in battery electrodes; MXene films give strong electromagnetic shielding. Performance depends on flake size and thickness, dispersion without restacking, alignment and bonding to the matrix, and on powders whose properties vary widely between suppliers.

Full explanation

Dispersions and inks

Flakes of a 2D material floating in a liquid, and that liquid used like paint or ink. Stirring or shaking a layered crystal in the right liquid splits it into flakes that stay suspended; spinning away the thick ones leaves a dispersion of thin flakes, which can be sprayed, printed or coated onto almost anything. It is the cheapest way to make 2D materials by the kilogram, but the flakes are small and overlap in a film, so they never match a perfect continuous sheet.

Colloidal suspensions of nanosheets made by liquid-phase exfoliation of layered crystals in solvents whose surface energy matches the crystal (NMP, DMF, cyclohexanone) or in water with surfactants, by electrochemical exfoliation, or – for graphene oxide and MXenes – by chemical routes that leave hydrophilic surfaces. Centrifugation sorts flakes by size and thickness (liquid cascade centrifugation), and optical spectra give mean thickness and lateral size in the liquid. Formulated as inks with tuned viscosity and surface tension, they are inkjet-, screen- or spray-printed or coated into films limited by flake–flake junctions, porosity, residual solvent or surfactant, and oxidation in storage. Powders and dispersions sold as graphene range from monolayers to graphite, which ISO/TS 21356-1 addresses by defining how they are characterised.

Full explanation

Thermal conductivity

How well a material carries heat from a hot place to a cold one. Graphene and boron nitride carry heat along their sheets better than copper, which is why they are used to spread heat away from hot spots – but across the sheets, and across every interface, heat moves far less easily.

The coefficient κ relating heat flux to temperature gradient, q = −κ∇T, carried in insulators and semiconductors almost entirely by phonons. Layered crystals are extremely anisotropic: graphite conducts about 2000 W/m·K along its layers and a few W/m·K across them; suspended graphene reaches several thousand, graphene on silicon dioxide about 600. Low κ – from heavy atoms, anharmonic bonding and interfaces, as in SnSe – is the goal for thermoelectrics, while the thermal boundary resistance at each interface often limits how fast heat leaves a 2D device.

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

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