How does particle size affect nanoparticle ink performance?

In a nanoparticle ink, particle size is not just a physical measurement. It affects how the ink formulation flows, how stable the nanoparticle dispersion stays over time, how well the coating absorbs light and what the final result looks like on the substrate. For metal boride nanoparticle inks like LaB₆, controlling particle size is one of the most important steps in producing a functional coating that performs consistently.

What is a nanoparticle ink?

A nanoparticle ink is a liquid ink formulation in which nanoparticles are dispersed in a carrier solvent. The particles stay suspended in the liquid and are deposited onto a substrate when the ink is applied and the solvent evaporates. The result is a thin functional coating.

Nanoparticle inks can be applied through inkjet printing, spray coating, dip coating or spin coating. They sit at the intersection of printed electronics and functional coatings: the same inkjet printing technology used in additive manufacturing can deposit a layer that absorbs infrared radiation, conducts electricity or resists wear. The performance of a nanoparticle ink depends on the material the particles are made of, the concentration in the ink formulation, the stability of the dispersion and, most importantly, the size and size distribution of the particles.

How particle size affects nanoparticle dispersion

Nanoparticle dispersion is the distribution of particles throughout the liquid carrier. A stable dispersion means the particles are evenly spread through the ink without clumping or settling. Poor dispersion leads to uneven coatings, clogged inkjet nozzles and inconsistent performance.

Particle size directly affects how easy it is to achieve a stable dispersion. Smaller particles have a higher surface area relative to their volume, which makes them more reactive and more prone to clumping together through agglomeration. To prevent this, smaller nanoparticles need surface treatment with ligands or surfactants that keep the particles separated.

For LaB₆ nanoparticle inks, particles between 5 and 20 nm are used. At this size range, the particles can be stabilised through ligand engineering, which keeps them evenly distributed in the ink formulation and prevents settling. This is what allows the ink to stay stable for several months at concentrations up to 50 mg/mL, even at room temperature.

Particle size distribution and why it matters

Particle size distribution describes the spread of particle sizes within a batch of nanoparticles. A narrow size distribution means most particles are close to the same size. A broad distribution means there is a wide range of particle sizes present.

For nanoparticle inks, a narrow particle size distribution is important for consistent performance. If particle sizes vary widely, different particles absorb at different wavelengths, which reduces the sharpness of the absorption peak and makes the coating less predictable. In inkjet printing, large particles in a broad distribution can clog nozzles even if the average particle size is within the acceptable range. Controlling particle size distribution during synthesis is therefore as important as controlling the average particle size. For the LaB₆ ink platform, nanocrystals are synthesised within a defined size range and verified analytically before formulation.

How particle size affects optical properties

For infrared absorbing nanoparticle inks, particle size has a direct effect on the absorption spectrum. Near-infrared absorption in materials like LaB₆ is driven by localised surface plasmon resonance, a collective oscillation of the free electrons in the nanoparticle triggered by incoming light. The resonance frequency, and therefore the wavelength at which the nanoparticle absorbs most strongly, depends on the size of the particle. Larger particles shift the absorption peak toward longer wavelengths. Smaller particles shift it toward shorter wavelengths. By controlling particle size during synthesis, the absorption properties of the ink formulation can be tuned for a specific wavelength range.

For LaB₆, the near-infrared absorption range of 800 – 1400 nm is achieved at nanocrystal sizes between 5 and 20 nm. Outside this size range, the absorption peak shifts and the infrared shielding effect weakens.

How particle size affects coating quality

When a nanoparticle ink is applied and dried, the particle size determines how smooth and uniform the final coating is. Larger particles cause surface roughness and uneven film formation, particularly in thin coatings where individual particles make up a significant fraction of the total film thickness.

For inkjet printing, particle size also determines whether the ink can pass through the print head without clogging. Most inkjet systems need particles smaller than 1/100th of the nozzle diameter. At 5 – 20 nm, LaB₆ nanocrystals are well within this range, which allows stable inkjet printing without nozzle blockages.

For flexible substrates, a smooth, uniform particle size distribution is especially important. Flexible films bend and flex during use, and a coating with uneven particle distribution is more likely to crack or delaminate under mechanical stress.

Particle size and processing temperature

One of the main advantages of nanoparticle inks over bulk materials is the ability to process coatings at low temperatures, including near room temperature drying. This advantage is partly linked to particle size.

At the nanoscale, the melting point of a material decreases compared to its bulk form, a phenomenon known as melting point depression. For metal boride nanoparticles, this means the nanocrystals can form a connected, adherent coating at much lower temperatures than bulk LaB₆, which has a melting point of 2210°C as a bulk material.

The smaller the particle, the more pronounced this effect becomes. Keeping nanocrystals in the 5 – 20 nm range is what makes it possible for LaB₆ nanoparticle ink to dry and form a functional coating below 90°C, on flexible substrates and heat-sensitive materials that conventional coating methods cannot reach.

Particle size analysis of nanoparticles

Controlling particle size starts with measuring it accurately. Several techniques are used for particle size analysis of nanoparticles:

• Dynamic light scattering (DLS) measures the hydrodynamic size of particles in suspension. It is fast and non-destructive, making it useful for routine quality control of nanoparticle ink formulations.
• Laser diffraction measures particle size distribution across a wider scale than DLS. It covers particles from the nanometre to the millimetre scale and gives a complete picture of the sizes present in a batch.
• Transmission electron microscopy (TEM) provides direct images of the nanoparticles and measures core particle size independently of the surrounding ligand shell. It is more time-consuming but gives the most precise size information.
• X-ray diffraction (XRD) measures crystallite size through peak broadening analysis, which is relevant for confirming the nanocrystal size of metal boride particles.

For the LaB₆ ink platform, particle size is controlled during synthesis and verified through these methods for consistent performance batch to batch.

Nanoparticle inks and printed electronics

The growth of printed electronics and additive manufacturing has increased interest in nanoparticle inks as functional materials. In printed electronics, conductive inks based on silver nanoparticles or gold nanoparticles are used to print circuits and electrodes directly onto flexible substrates. Research published in MDPI Materials confirms that particle size distribution directly affects dispersion stability, nozzle clogging and conductivity in inkjet-printed nanoparticle inks. The same principles that govern particle size in conductive inks apply to functional inks like LaB₆: particle size affects dispersion stability, printability and the performance of the final layer.

LaB₆ nanoparticle ink is not used as a conductive ink for printed electronics, but it uses the same ink formulation and printing technology. Where silver nanoparticle inks aim for high electrical conductivity, LaB₆ ink aims for infrared absorption and hardness. Both depend on precise control over particle size and particle size distribution to deliver consistent results.

Summary

Particle size affects every aspect of nanoparticle ink performance: dispersion stability, optical absorption, coating quality and processing temperature. A narrow particle size distribution is as important as the average particle size, particularly for inkjet printing and coatings on flexible substrates. For LaB₆ nanoparticle inks, nanocrystals between 5 and 20 nm deliver stable dispersion, strong near-infrared absorption in the 800 – 1400 nm range and the ability to form a functional coating below 90°C. Particle size analysis through DLS, laser diffraction and TEM is used to verify consistent performance batch to batch.