15.3 Nanomaterials and Their Applications

Nanomaterials

Nanomaterials are materials with at least one dimension between 1 and 100 nanometers. At this scale, materials behave differently than their bulk counterparts because of quantum confinement effects and dramatically increased surface area-to-volume ratios. These size-dependent properties make nanomaterials central to advances in electronics, medicine, catalysis, and energy technology.

Nanoparticles and Quantum Dots

Nanoparticles are particles with dimensions in the 1–100 nm range. Their high surface area-to-volume ratio means a much larger fraction of atoms sit at the surface compared to bulk materials. Surface atoms have unsatisfied coordination, which makes nanoparticles far more chemically reactive and gives them distinct optical, magnetic, and electronic behavior.

Quantum dots are semiconductor nanoparticles (typically 2–10 nm) that confine electrons in all three spatial dimensions. This confinement creates discrete, quantized energy levels rather than the continuous bands found in bulk semiconductors. The practical result: their optical properties are tunable by size.

• Smaller quantum dots have a larger band gap, so they absorb and emit higher-energy (bluer) light
• Larger quantum dots have a smaller band gap, shifting emission toward the red end of the spectrum
• Common compositions include CdSe, CdTe, and InP cores, often coated with a ZnS shell to improve quantum yield

Applications include biological fluorescence imaging (where tunable emission allows multiplexed labeling), quantum dot LED displays, and third-generation solar cells that harvest a broader range of the solar spectrum.

Carbon-Based Nanomaterials

Carbon nanotubes (CNTs) are cylindrical structures formed by rolling graphene sheets into tubes with diameters typically 1–50 nm. Their properties depend on chirality (the angle at which the graphene sheet is rolled):

• Single-walled CNTs (SWCNTs) can be metallic or semiconducting depending on their chiral vector
• Multi-walled CNTs (MWCNTs) consist of concentric tubes and tend to be metallic conductors
• CNTs have tensile strengths roughly 100× that of steel at one-sixth the density, plus thermal conductivities rivaling diamond

Graphene is a single atomic layer of $sp^2$-hybridized carbon atoms in a hexagonal lattice. It conducts electricity with extremely high electron mobility (~200,000 $\text{cm}^2/\text{V·s}$) and has a theoretical surface area of ~2630 $\text{m}^2/\text{g}$, making it attractive for supercapacitor electrodes and catalyst supports.

Applications for carbon nanomaterials include flexible and transparent electronics, lithium-ion battery anodes, lightweight structural composites, and chemical sensors.

Nanowires and Nanocomposites

Nanowires are one-dimensional nanostructures with diameters on the nanoscale but lengths that can reach micrometers or more. They can be synthesized from metals (Au, Ag), semiconductors (Si, ZnO), or metal oxides.

• Their high aspect ratio and quantum confinement in two dimensions give them unique electrical and optical properties
• Silicon nanowires, for example, are being explored as high-sensitivity field-effect transistor (FET) biosensors
• ZnO nanowires are used in piezoelectric nanogenerators that convert mechanical energy to electricity

Nanocomposites incorporate nanomaterials (nanoparticles, CNTs, clay nanoplatelets) into a bulk matrix such as a polymer, ceramic, or metal. Even small loadings (1–5 wt%) can dramatically improve the mechanical strength, thermal stability, or electrical conductivity of the host material. These are used in aerospace structural components, automotive parts, and flame-retardant coatings.

Nanoscale Phenomena

Surface Plasmon Resonance

Surface plasmon resonance (SPR) occurs when incident light couples with the collective oscillation of conduction electrons at the surface of metallic nanoparticles (most commonly Au and Ag). The resonance condition depends on particle size, shape, composition, and the dielectric environment.

At resonance, the nanoparticle strongly absorbs and scatters light at a characteristic wavelength. For gold nanoparticles around 20 nm in diameter, this produces the well-known ruby-red color (absorption near 520 nm). Changing the particle shape to nanorods introduces a second, tunable longitudinal plasmon band in the near-infrared.

SPR is exploited in:

• Biosensing — binding events at the nanoparticle surface shift the plasmon wavelength, enabling label-free detection of biomolecules
• Surface-enhanced Raman spectroscopy (SERS) — localized electromagnetic field enhancement at "hot spots" amplifies Raman signals by factors of $10^6$ or more
• Photothermal therapy — gold nanorods absorb near-IR light and convert it to localized heat for tumor ablation

Nanocatalysis

Nanocatalysts exploit the high surface area and abundance of undercoordinated surface sites on nanoparticles to enhance catalytic activity. Because a 3 nm gold nanoparticle has roughly 50% of its atoms on the surface (compared to negligible fractions in bulk gold), even traditionally "inert" metals like Au become catalytically active at the nanoscale.

• Pt and Pd nanoparticles catalyze oxygen reduction in proton-exchange membrane fuel cells
• $\text{TiO}_2$ nanoparticles serve as photocatalysts for water splitting and degradation of organic pollutants
• Selectivity can be tuned by controlling nanoparticle size, shape, and support interactions

Nanomaterial Applications

Drug Delivery and Biomedical Applications

Nanoparticle-based drug delivery systems address key limitations of conventional therapeutics: poor solubility, rapid clearance, and lack of tissue specificity.

How targeted delivery works:

1. A therapeutic agent is loaded into or onto a nanocarrier (liposome, polymeric nanoparticle, or mesoporous silica nanoparticle)
2. The nanocarrier surface is functionalized with polyethylene glycol (PEG) to evade immune clearance and extend circulation time
3. Targeting ligands (antibodies, peptides, or aptamers) on the surface bind to receptors overexpressed on diseased cells
4. Once internalized, the carrier releases the drug in response to local stimuli such as pH change, temperature, or enzymatic activity

This approach improves bioavailability, reduces systemic toxicity, and allows lower effective doses. Iron oxide nanoparticles ($\text{Fe}_3\text{O}_4$) also serve as MRI contrast agents, combining diagnostic imaging with therapy ("theranostics").

Nanoelectronics

Nanoelectronics uses nanoscale materials to push electronic devices beyond the limits of conventional silicon lithography.

• CNT-based transistors can operate at channel lengths below 10 nm with reduced short-channel effects compared to silicon
• Resistive random-access memory (ReRAM) uses metal oxide nanofilms for non-volatile data storage with fast switching speeds
• Nanowire FETs serve as ultrasensitive chemical and biological sensors, detecting single-molecule binding events through conductance changes

The overall trend is toward smaller, faster, and more energy-efficient components, though challenges remain in large-scale fabrication and reproducibility.

Nanophotonics and Advanced Materials

Nanophotonics studies how light interacts with structures at or below its own wavelength. By engineering nanostructures, you can control light propagation, confinement, and emission in ways impossible with bulk optics.

• Metamaterials are engineered nanostructured composites with optical properties not found in nature, such as negative refractive indices
• Photonic crystals have periodic nanostructures that create photonic band gaps, selectively blocking certain wavelengths of light
• Plasmonic waveguides confine light below the diffraction limit, enabling on-chip optical interconnects

Advanced nanomaterial applications extend into energy and environmental technology as well. Nanostructured $\text{TiO}_2$ and carbon-based membranes are used in water purification, while nanostructured electrodes improve charge storage in lithium-ion batteries and supercapacitors. In solar energy, perovskite nanocrystal films have pushed lab-scale photovoltaic efficiencies above 25%.