A Researcher's Guide to Characterising Nanomaterials: Key Techniques and What They Tell You

Why Characterisation Is Non-Negotiable

A nanomaterial that is not well characterised is a variable, not a material. The properties that make nanomaterials interesting — quantum confinement effects, surface plasmon resonance, catalytic activity, biological interaction — are all acutely sensitive to size, shape, surface chemistry, and purity. Two batches of nominally identical "20 nm gold nanoparticles" from different suppliers (or different synthesis runs) may have peak SPR wavelengths 15 nm apart, size distributions three times as broad, and vastly different colloidal stabilities.

Rigorous characterisation protects reproducibility, enables troubleshooting when experiments fail, and is increasingly required for publication in high-impact journals and for regulatory submissions in applied contexts. This guide covers the core analytical toolkit.

Electron Microscopy: Seeing Individual Nanoparticles

Transmission Electron Microscopy (TEM)

What it measures: Direct imaging of individual nanoparticles at atomic resolution in some cases. Provides size, shape, internal structure (crystallinity, core-shell geometry, defects), and size distribution from statistical analysis of hundreds of particles.

Sample preparation: Dilute dispersion deposited on a carbon-coated TEM grid and dried. For biological samples, negative staining or cryo-TEM preparation.

Key specifications it validates: Stated particle diameter, monodispersity, aspect ratio of nanorods/tubes, shell thickness for core-shell structures, number of walls in MWCNTs.

Limitations: Measures particles in the dry state — colloidal behaviour in solution can differ. Statistical sampling requires manual measurement of many particles; automated analysis software helps but introduces artefacts. High-resolution TEM requires specialist operation.

Variants: STEM (scanning TEM) for elemental mapping via EDX; HRTEM for lattice-resolution imaging; cryo-TEM for hydrated samples.

Scanning Electron Microscopy (SEM)

What it measures: Surface morphology and topology of nanoparticles and nanostructured surfaces. Lower resolution than TEM (~1–5 nm), but larger field of view and easier sample preparation for solid samples.

Best for: Nanoparticle films, CNT mats and forests, nanostructured surfaces, catalyst supports, porous materials. Less suitable for isolated sub-20 nm particles.

Variants: FE-SEM (field emission SEM) for higher resolution; SEM-EDX for elemental analysis.

Dynamic Light Scattering (DLS): Hydrodynamic Size in Solution

What it measures: The hydrodynamic diameter — the effective size of the nanoparticle including its surface layer (stabilising ligands, protein corona in biological media, solvent layer) as it diffuses in solution. Derived from the Stokes-Einstein equation using measured diffusion coefficients.

Why it differs from TEM size: DLS measures the hydrodynamic particle, which is larger than the core. A 10 nm gold nanoparticle with a PEG-2000 coating may show a DLS size of 25–30 nm. A citrate-capped particle measured at 15 nm by TEM may show 18–22 nm by DLS. These differences are expected and informative.

Key metric: Polydispersity index (PDI) — values below 0.1 indicate narrow, monodisperse distributions; above 0.3 indicates significant polydispersity.

Also provides: Zeta potential (electrokinetic potential at the particle surface, measured by electrophoretic light scattering). Zeta potential is the key indicator of colloidal stability: |ζ| > 30 mV indicates electrostatically stabilised colloid; values near 0 suggest imminent aggregation.

Limitations: DLS reports intensity-weighted distributions that overweight large particles; a few aggregates can dominate the measurement. Not suitable for polydisperse samples without complementary TEM. Minimum particle size ~1 nm.

X-Ray Diffraction (XRD): Crystal Structure and Crystallite Size

What it measures: The crystallographic phase of the material (confirms you have what you think you have), lattice parameters, and crystallite size via the Scherrer equation applied to peak broadening.

Key applications:

• Confirming the crystal phase of metal oxide nanoparticles (anatase vs rutile TiO₂, magnetite vs maghemite iron oxide)
• Calculating crystallite size from peak width — peaks broaden as crystallite size decreases below ~100 nm
• Measuring degree of graphitisation in carbon nanomaterials (graphite d₀₀₂ spacing, G-band position)
• Confirming alloy composition in bimetallic nanoparticles

Limitations: Reports the crystallite size (coherent diffraction domain), which may differ from particle size — nanoparticles may be polycrystalline or have amorphous shells. Requires relatively pure, dry powder samples. Poor sensitivity for surface species.

Raman Spectroscopy: Vibrational Fingerprinting for Carbon Nanomaterials

What it measures: Inelastic scattering of photons from vibrational modes, producing a molecular fingerprint. Particularly powerful for carbon nanomaterials where specific Raman peaks are diagnostic.

For graphene, Raman spectroscopy provides:

• D band (~1350 cm⁻¹) — defect density indicator; high D/G ratio indicates structural disorder
• G band (~1580 cm⁻¹) — sp² carbon stretching; present in all graphitic materials
• 2D band (~2700 cm⁻¹) — highly sensitive to layer number; monolayer graphene shows a sharp, symmetric 2D peak; bilayer/few-layer shows a broader, split 2D band

For carbon nanotubes, Raman provides:

• Radial breathing mode (RBM, 100–350 cm⁻¹) — frequency inversely proportional to CNT diameter; presence confirms nanotube structure
• D/G ratio — defect and purity assessment; pure SWCNTs have very low D/G ratios
• Electronic type enrichment — metallic vs semiconducting SWCNT contributions at different laser energies

For other nanomaterials: Phase identification of TiO₂ (anatase/rutile peaks), characterisation of semiconductor QDs, identification of surface-enhanced species in SERS.

X-Ray Photoelectron Spectroscopy (XPS): Surface Elemental and Chemical State Analysis

What it measures: Elemental composition and chemical bonding state of the top 5–10 nm of a surface. Provides atom percent of each element and, crucially, distinguishes chemical states (e.g. metallic Au⁰ vs ionic Au³⁺; C-C vs C-O vs C=O in graphene oxide).

Key applications in nanomaterials:

• Quantifying C:O ratio in graphene oxide and reduced graphene oxide — confirms reduction success
• Confirming surface ligand attachment chemistry (e.g. thiol-Au bonding in functionalised gold nanoparticles)
• Measuring metal oxidation states in oxide nanoparticles
• Confirming doping (e.g. nitrogen doping of graphene via N 1s peak)
• Catalyst characterisation — active metal speciation on support

Limitations: Surface-only technique (5–10 nm depth); bulk composition may differ from surface. Requires UHV conditions; samples must be compatible. Relatively slow measurement; specialist instrument.

BET Surface Area Analysis

What it measures: Total specific surface area (m²/g) by gas adsorption isotherm analysis (Brunauer-Emmett-Teller theory). Also provides pore size distribution for porous nanomaterials.

Most relevant for: High-surface-area materials where surface area drives performance — catalysts, adsorbents, electrode materials, activated carbon, mesoporous silica, MOFs, graphene nanoplatelets.

Interpretation: Compare measured BET surface area to the theoretical surface area calculated from TEM-derived particle size assuming spherical, non-porous particles. Significantly lower BET SA than theoretical indicates aggregation or sintering; significantly higher indicates porosity or surface roughness.

Thermogravimetric Analysis (TGA)

What it measures: Mass change as a function of temperature in controlled atmosphere. Used for CNT and graphene purity assessment (carbon content), organic ligand loading on nanoparticle surfaces, and thermal stability characterisation.

For CNTs: Combustion temperature in air indicates wall quality (defective MWCNTs combust at lower temperatures than high-quality SWCNTs). Residual mass at high temperature = inorganic catalyst content.

Putting It Together: A Minimum Characterisation Set

For most published nanomaterial research, reviewers expect at minimum:

1. Morphology and size — TEM with size statistics
2. Hydrodynamic size and zeta potential in working medium — DLS
3. Crystal structure / phase confirmation — XRD or Raman (material-dependent)
4. Surface chemistry — XPS or FTIR for functionalised materials
5. Purity — TGA for carbon nanomaterials; ICP-OES for metal contaminants

When sourcing nanomaterials from suppliers — whether on NanoMani or elsewhere — request data sheets that include these measurements. Suppliers who provide complete characterisation data stand behind their materials; those who do not may not know what they are actually selling.