Materials Science/Nanomaterials

Nanomaterials is the topic of Materials Science for the 2026 season. General information about Materials Science can be found on the main event page - this page is dedicated to additional information specific to the Nanomaterials topic.

Types and Applications

Zero-dimensional

Nanoparticles (NPs) are particles with all three dimensions <100 nm. Their extremely high surface area to volume ratio makes them highly effective in applications such as catalysis and drug delivery. Their properties often differ significantly from that of their bulk-scale counterparts.

• Examples: Gold nanoparticles (AuNPs), silver nanoparticles (AgNPs), iron oxide nanoparticles
• Applications: Catalysis, drug delivery, medical imaging

Quantum dots (QDs) are semiconductor nanocrystals. They have discrete energy levels due to quantum confinement.

• Examples: CdSe, CdTe, PbS, carbon dots, perovskite QDs
• Applications: QD-LED TVs, fluorescent imaging probes

One-dimensional

Nanowires (NWs) are crystalline wires with nanometer-scale diameters but lengths that can range up to the micrometer scale.

• Examples: Si, Ge, GaN, ZnO, InP, SnO₂
• Applications: Optoelectronics, sensors

Nanotubes are hollow cylindrical nanostructures with nanometer-scale diameters. They are classified as either single-walled or multi-walled. Nanotubes are of interest for their tensile strength, thermal conductivity and electrical properties.

• Examples: Carbon nanotubes (CNTs), boron nitride nanotubes (BNNTs)
• Applications: Electrodes, drug delivery

Nanorods are rod-shaped nanocrystals that are shorter than nanowires. They exhibit anisotropic properties. Controlling their aspect ratio allows for tuning of their optical and plasmonic behavior.

• Examples: Gold nanorods, ZnO nanorods
• Applications: Catalysis, solar cells

Two-dimensional

Graphene is a single, atomic layer of carbon atoms in an sp² hybridized lattice. It has extremely high strength, conductivity, transparency and electron mobility. Common derivatives include graphene oxide (GO) and reduced graphene oxide (rGO).

• Applications: Electrodes, flexible electronics, energy storage, biosensors, membranes

Hexagonal boron nitride (h-BN) has graphene-like structure, but with alternating B and N atoms. It has a wide bandgap and its stability and smoothness makes it a common substrate for graphene/TMD devices.

• Applications: Substrate for 2D heterostructures, lubricants, coatings

Transition metal dichalcogenides (TMDs) are 2D semiconductors with formula MX₂ (M = Mo, W; X = S, Se, Te). Each layer of M atoms is sandwiched between two layers of X atoms. Unlike the electrically conductive graphene, TMDs have an adjustable bandgap.

• Applications: Spintronics, optoelectronics

MXenes are transition metal carbides and nitrides with formula M_n+1X_nT_x (M = Ti, V, Nb; X = C, N; T = -OH, -O, -F, etc.). They are formed from selective etching of MAX phases.

• Applications: Energy storage

Three-dimensional

Nanoporous materials are solids with nanometer-scale pores. Their high surface area makes them highly effective adsorbents and catalysts.

• Examples: Zeolites, metal-organic frameworks (MOFs), porous carbons
• Applications: Catalysis, sensing, drug delivery

Nanocomposites are hybrid materials, in which a nanoscale filler is dispersed in a bulk matrix. This improves mechanical strength and thermal stability.

• Examples: Polymer (polymer + clay platelets, CNTs, graphene, etc.), metal metrix (nanoparticles or CNTs reinforcing metals), ceramic (reinforced ceramics)
• Applications: Automotive/aerospace materials, conductive plastics, flame retardants, medical implants

Hierarchical nanostructures are materials that combine multiple length scales.

• Examples: Core-shell aggregates (nanoparticles forming porous spheres), branched nanostructures (nanotubes/nanowires growing from a backbone)
• Applications: electrodes, photocatalysis, superhydrophobic coatings, tissue scaffolds

Synthesis and Fabrication

Top-down

Mechanical Approaches

Mechanical cleavage (aka the Scotch tape method) uses adhesive tape to peel thin layers off a bulk crystal. It works by weakening van der Waals interactions without breaking in-plane covalent bonds. This method was famously used by Andre Geim and Konstantin Novoselov to isolate graphene and win the 2010 Nobel Prize in Physics. It has since been used on a number of other 2D nanomaterials (e.g. BN nanosheets, TMDs).

Liquid exfoliation disperses layer materials into solvents in order to separate the constituent layers. This process may occur by:

• Mechanical force: Sonication or shear mixing;
• Ion intercalation: Insertion of ions (typically Li+) between layers, weakening van der Waals forces;
• Ion exchange: Replaces intercalated ions, resulting in further destabilization;
• Redox: Introduces defects (e.g. converting graphite into graphene oxide), enabling dispersion in solution;
• Selective etching: Chemical removal of regions to release sheets.

Lithography and Direct Writing

Photolithography works by coating a substrate with light-sensitive resist (called a photoresist), then exposing it to UV light through a photomask. The exposed or unexposed region (depending on whether a positive or negative photoresist is used) is dissolved to create a pattern. This pattern can be transferred to underlying layers via etching or deposition.

Electron beam lithography (or EBL) writes patterns directly onto a resist-coated surface with an electron beam. It is less precise than photolithography and much slower, since the beam has to scan through the pattern.

Focused ion beam (or FIB) uses a narrow beam (typically Ga+) to sputter atoms directly from a surface. This directly sculpts the material, eschewing the need for resist layers.

Nanoimprint lithography (or NIL), also known as stamp lithography, uses a patterned stamp (typically PDMS), to mechanically press nanoscale features into a resist. Soft lithography is a class of related techniques, where these stamps are used for a range of techniques:

• Microcontact printing (or uCP): A raised stamp is inked with molecules, then transferred onto a substrate, forming a self-assembled monolayer;
• Replica molding (or REM): A liquid polymer is cast using a mold with a relief pattern, cured, then peeled off the mold;
• Micromolding in capillaries (or MIMIC): A liquid polymer is run through microfluidic channels by exploiting capillary action.

Etching and Pattern Transfer

Wet etching uses solvents to dissolve material away from exposed areas of substrate. Most wet etching is isotropic, removing from all directions equally, but some can be anisotropic (in particular, KOH etching of Si).

Dry etching uses ions to remove substrate, allowing for far greater control and precision over wet etching. The most common form of dry etching is reactive ion etching (or RIE), which uses a chemically reactive plasma.

Bottom-up

Chemical vapor deposition (CVD) grows films and nanostructures by reacting gas-phase precursors (volatile compound of a material to be deposited) with other gases and produce a nonvolatile solid that deposits atomistically on a heated substrate. CVD is like baking with gases. It is the primary method of synthesizing carbon nanotubes (CNTs), graphene, and Transition Metal Dichalcogenide (TMD) monolayers.

Atomic layer deposition (ALD) is a subclass of CVD, which deposits material one monolayer at a time by alternating surface reactions. These alternating dose and purge steps provides sub-nanometer thickness control when synthesizing thin films.

Sol-gel processing converts molecular precursors (typically metal alkoxides) into networks via hydrolysis and condensation. Solvent is then removed from the resulting gel through calcination. The sol-gel method is the primary means of synthesizing ceramic nanoparticles (e.g. silica, alumina, titania, zirconia).

Hydrothermal and solvothermal methods perform reactions in sealed autoclaves. Hydrothermal methods use water, while solvothermal methods use organic solvents. The high temperature and pressure of the autoclave promotes supersaturation of starting materials, enabling crystallization of nanostructures that would be unstable under ambient conditions. Solvothermal methods are a primary method of synthesizing MOFs.

Molecular self-assembly sees molecules spontaneously organize into ordered structures through non-covalent interactions (e.g. H-bonding, pi stacking, van der Waals forces). Common systems include: self-assembled monolayers (SAMs), DNA origami and block copolymer (BCP) self-assembly.

Colloidal synthesis of nanoparticles begins by dissolving precursors within an organic solvent with ligands, which are converted to monomers that eventually rise in concentration, before exceeding a critical supersaturation. This is followed by nucleation, growth and termination steps. Colloidal synthesis can be tuned to produce particles with a narrow size distribution. Colloidal synthesis is commonly used to synthesis CdSe and PbS quantum dots, as well as various nanocrystals.

Molecular beam epitaxy (MBE) directs atomic or molecular beams (often from Knudsen cells) in ultra-high vacuum onto a substrate. It offers atomically precise growth of crystal thin films, with extremely high control over stoichiometry and doping. MBE is central to the synthesis of semiconductor layers in transistors. Growth during MBE is monitored using reflection high-energy electron diffraction (RHEED).

Vapor-liquid-solid (VLS) nanowire growth employs a catalytic droplet (often Au), which absorbs vapor precursors until supersaturated, before crystallizing into a nanowire at the liquid-solid interface. Related vapor-solid-solid (VSS) techniques use solid catalysts. VLS is the primary mode of synthesizing Si and Ge nanowires.

Characterization

Microscopy

Light Microscopy

Light microscopy or optical microscopy uses visible light to magnify samples through glass lenses. The resolution limit of a standard light microscope is given by the Rayleigh criterion. For visible light, the diffraction limit (~200 nm) makes it impossible to directly image nanoscale features using light microscopy. However, advanced forms of light microscopy allow for higher resolution.

Optical Contrast Methods

• Bright-field microscopy: Standard transmission microscopy
  • Good for stained, opaque materials
  • Very poor for transparent materials (like many nanomaterials)
  • Resolution diffraction limited
• Dark-field microscopy: Blocks direct light with a special condenser, only collecting scattered light
  • Good for unstained, transparent materials
  • Very sensitive to small features that scatter light
  • Insensitive to phase shifts (i.e. transparent regions)
  • Can visualize nanoscale structures/features, well below the diffraction limit
• Phase-contrast microscopy (PCM): Phase plate in the objective to convert phase shifts from transparent samples into intensity differences
  • Excellent for unstained, transparent materials
  • Can visualize edges without staining
  • Often produces halos and other imaging artifacts
  • Can visualize colloids, thin films, hydrogels
• Differential Interference Contrast (DIC) microscopy: Interferometry converts differences in sample index of refraction into intensity differences
  • High-contrast, sharp images, with fewer halos than PCM
  • Better than phase-contrast for thick or complex samples
  • Images appear like a 3D relief
  • Can visualize nanostructured surfaces/membranes

Fluorescence Methods

• Widefield microscopy: Entire field illuminated, all fluorophores emit simultaneously
  • Pros: Fast, simple, high-throughput
  • Cons: Blurring out of focal plane, poor depth penetration, out-of-focus light (from out-of-plane fluorophores)
• Confocal microscopy: Out-of-focus light is rejected using a laser / pinhole setup
  • Pros: Sharp images at specific depths (can be reconstructed with Z-stacking), pinhole reduces out-of-focus light
  • Cons: Photobleaching, slower
• Multiphoton microscopy: Two (or more) photons are absorbed simultaneously by the fluorophore
  • Pros: Deeper penetration, less photobleaching
  • Cons: Expensive, complicated, slower

Super-Resolution Microscopy

• Stimulated Emission Depletion Microscopy (STED): Fluorophores around the focal area are deactivated using a donut-shaped beam, shrinking the excitation area
  • Pros: Real-time imaging, no special fluorophores
  • Cons: Needs strong laser, complicated and expensive optics
  • Resolution: ~20-50 nm
• Photoactivation Localization Microscopy (PALM) & Stochastic Optical Reconstruction Microscopy (STORM): Only activate a few fluorophores at a time, but repeat many times (thousands), then reconstruct into a single image
  • Pros: Single-molecule sensitivity
  • Cons: Slow, needs special dyes
  • Resolution: ~10-20 nm
• Structured Illumination Microscopy (SIM): Multiple images are taken with different illumination patterns, generating Moire fringes that encode high-frequency information, which are reconstructed into a single image
  • Pros: Fast, relatively gentle, compatible with most fluorophores
  • Cons: Lower resolution improvement compared to STED or PALM/STORM, reconstruction is computationally expensive
  • Resolution: ~100 nm

Electron Microscopy

TEM Techniques

• Bright-field TEM (BF-TEM): Uses the transmitted electron beam to form an image
• Dark-field TEM (DF-TEM): Uses only elastically scattered electrons selected by an objective aperture
• Selected Area Electron Diffraction (SAED): Diffraction pattern from a selected region using a selected-area aperture
• High-Resolution TEM (HRTEM): Uses phase-contrast that results from interference between transmitted and diffracted beams
• Scanning TEM (STEM): Nanometer-scale probe scans across the sample, with detectors collecting signals at each point. Often combined with chemical analysis like EELS or EDS.
  • High-Angle Annular Dark Field STEM (HAADF-STEM): Detects electrons scattered at large angles

SEM Techniques

• Conventional SEM: An electron beam scans the surface, with low energy secondary electrons emitted from the surface collected by detectors.
  • Electrons are generated through either thermionic emission (W, LaB₆) or field-emission (FEG)
  • A condenser lens and objective lens controls the spot size and focuses the beam onto the sample
  • Traditional SEMs use Everhart–Thornley detectors, placed outside the column, while alternatives include in-lens detectors, placed within the electron column
• Backscattered Electron Imaging (BSE-SEM): Detects elastically backscattered electrons with energy near the primary beam energy. Used to obtain compositional information, as intensity scales with atomic number.
  • BSE displays Z contrast, meaning the contrast in the image arises from difference in atomic number, with lighter elements appearing darker
  • BSE detectors are typically solid-state semiconductors, arranged into quadrants around the beam axis.
• Field-Emission SEM (FE-SEM): Uses a field emission gun (FEG) to create a much smaller electron probe. The FEG emits electrons via quantum tunneling from a sharp tip (usually W). This results in higher resolution than traditional SEM. FEGs are divided into two classes:
  • Cold field emission guns operate at low temperatures, eliminating the thermionic contribution, resulting in high resolution
  • Schottky field emission guns are coated in ZrO₂ to facilitate thermal emission
• Low-Voltage SEM (LV-SEM): Imaging at 5 keV and below means a smaller interaction volume and penetration depth. This sharpens surface resolution and reduces beam damage.
• Environmental SEM (ESEM): SEM operated at high pressure, with gas molecules neutralizing charging and allowing imaging of wet or volatile samples.
  • The high pressure environment of ESEM is achieved through differential pumping, where multiple chambers are linked by small orifices, then each section is evacuated with its own vacuum pump.
  • ESEM detects secondary electrons using a gaseous detection device (GDD). As SEs enter the chamber of the GDD, the gas is ionized, which results in a cascade that is detected by an electrode.

Cryogenic Electron Microscopy

Cryogenic electron microscopy (cryo-EM) rapidly vitrifies aqueous or soft-matter samples, which are then analyzed while maintained at low temperatures to minimize beam damage. The 2017 Nobel Prize in Chemistry was awarded to Jacque Dubochet, Joachim Frank and Richard Henderson for cryo-EM.

• In cryo-EM, water is frozen into vitreous ice which, unlike normal ice, is not crystalline and therefore does not strongly diffract. This preserves molecular arrangements and minimizes damage.
• Vitrification is typically achieved through plunge-freezing, in which the sample is applied to a holey carbon grid, then blotted with filter paper to form a thin film, then plunge frozen into the cryogen (usually liquid ethane).
  • Grids are typically glow discharged with plasma prior to sample application, which makes the carbon film hydrophilic, allowing samples to stick to the carbon instead of aggregating.
• During sample preparation, cryo-EM samples may be subject to orientation bias or the preferred orientation problem, in which particles adopt a preferred orientation in the grid, making 3D reconstruction difficult. This is largely a result of interactions with the hydrophobic air-water interface. Some ways to ameliorate the preferred orientation problem in cryo-EM include:
  • Depositing a solid support layer over the grid for particles, physically shielding them from the air-water interface.
  • Introduction of a detergent
  • Tilting the TEM stage during data collection
• The vitrified grid is transferred into the microscope while maintained below the devitrification temperature using a cryo-transfer holder or autoloader cassette.
• Cryo-EM is typically done using high-end TEMs configured to minimize electron dose. Data collection occurs using a "low dose" workflow that cycles between three modes for every target hole in the grid:
  • Search mode: Centers stage over a broad area at low resolution
  • Focus mode: Sets the target defocus of the image away from the hole of interest
  • Exposure mode: Records a movie of the target hole
• Image processing begins by correcting for beam-induced motion that occurred during imaging, in which a stack of frames are converted into a single micrograph. Software for motion correct in cryo-EM include: MotionCor2, Unblur, CryoSPARC, RELION.
  • Motion may be global (translation of the entire micrograph) or local (non-uniform warping)
  • Since later frames accumulate radiation damage, algorithms perform dose weighting to weigh earlier frames more.
• Then, the contrast transfer function (CTF) is estimated. The CTF is a theoretical, mathematical construct that encodes all the aberrations introduced by a TEM (e.g. defocus, astigmatism, spherical aberration). Software for CTF estimation include CTFFIND4, Gctf, CryoSPARC, RELION.
  • In CTF estimation, the power spectrum of the micrograph is first computed through a fast Fourier transform.
  • Then, the power spectrum is radially averaged to reveal Thon rings, which are fit using a theoretical CTF.
• The next step is particle picking in which particles are identified in micrographs. This can be done manually, but is typically done with software. Modern software tools for particle picking leverage deep learning and include crYOLO, Topaz, Warp, CryoSPARC and RELION.
• Finally, 2D images are reconstructed into 3D images. Software iteratively refines orientations and CTF parameters. Bayesian polishing is also performed to refine trajectories.

Spectroscopy

Electron Energy-Loss Spectroscopy

Electron energy-loss spectroscopy (EELS) measures energy lost by inelastic scattering events. It is often combined with TEM or STEM and is used to perform sample elemental, chemical and bonding analysis. EELS spectra are divided into three regions:

• Zero-loss peak: Electrons that have lost ~0 eV (elastic scattering)
• Low-Loss Region (0-50 eV): Primarily used to measure band gaps
• Core-Loss Region (>50 eV): Element-specific ionization edges (K, L, M edges) for analysis
• The fine structure within the core-loss region is known as the energy-loss near-edge structure (ELNES). The ELNES is analyzed to determine the identity of elements and characterize the chemical environment.

Energy-Dispersive Spectroscopy

Energy-dispersive x-ray spectroscopy (EDS/EDX): Detects characteristic X-rays, emitted when an outer-shell electron fills an inner-shell vacancy. Often combined with STEM, EDS is used for elemental and composition analysis.

• The location of characteristic peaks in the EDS spectrum are used to determine elemental identity, with their intensities used to determine abundances.
• Bremsstrahlung radiation produces background or continuum radiation that is subtracted out during analysis.
• Because EDS instruments utilize silicon-based detectors, a small escape peak corresponding to the K-alpha X-ray appears ~1.74 keV below the true peak on all EDS spectra.

Mass Spectrometry (MS)

See: Forensics#Mass Spectrometry

X-ray Diffraction (XRD)

X-ray diffraction (XRD) is a powerful analytical technique used to characterize the crystallographic structure, phase composition, and other structural parameters of synthesized nanomaterials.

Principle of XRD: X-ray diffraction relies on the interaction of X-rays with the periodic atomic arrangements within a crystalline material. When a beam of X-rays strikes a crystal, it is scattered in many specific directions. By measuring the angles and intensities of these diffracted beams, a crystallographer can produce a three-dimensional picture of the density of electrons within the crystal. This electron density map reveals the positions of the atoms in the crystal, their chemical bonds, and various other information.

Process:

1. Preparation of Sample: The synthesized nanomaterial is finely powdered to ensure that a large number of crystallites are randomly oriented. This powder is then placed in the path of the X-ray beam. 2. X-ray Beam Interaction: An X-ray beam is directed at the sample. As the X-rays encounter the crystal planes within the nanomaterial, they are diffracted in various directions. The angles and intensities of these diffracted beams are measured using a detector. 3. Data Collection: The diffraction pattern, which is a unique fingerprint of the atomic arrangement within the material, is recorded. This pattern consists of a series of peaks corresponding to the constructive interference of the diffracted X-rays. 4. Data Analysis: The positions and intensities of these peaks are analyzed. The position of the peaks is related to the lattice dimensions of the crystal, while the intensities provide information about the atom positions within the unit cell. By applying Bragg's Law (nλ = 2d sinθ), the interplanar spacing (d) of the crystal lattice can be calculated.

Applications in Nanomaterials:

- Phase Identification: XRD can identify different crystalline phases present in a nanomaterial by comparing the diffraction pattern to standard reference patterns. - Crystallite Size Determination: The width of the diffraction peaks can be used to estimate the size of the crystallites using the Scherrer equation. This is particularly useful for nanomaterials where the crystallite size is often in the nanometer range. - Strain and Defects Analysis: Variations in peak positions and shapes can indicate internal strains, crystal defects, or the presence of dopants. - Structural Properties: Information about the symmetry, unit cell dimensions, and atomic arrangement can be deduced, providing insight into the material’s properties and potential applications.

Advantages of XRD:

- Non-destructive: The technique does not alter or destroy the sample, allowing for further analysis using other methods. - Versatile: Applicable to a wide range of materials including metals, ceramics, polymers, and composites. - Quantitative: Can provide quantitative information about phase composition and crystallite size.

Limitations: - Requires Crystalline Material: Amorphous materials do not produce distinct diffraction patterns. - Sample Preparation: Proper preparation is crucial to obtain accurate results, especially for nanomaterials. - Interpretation Complexity: Data analysis can be complex and often requires comparison with reference patterns and advanced modeling techniques.

Youtube link for understanding basics of XRD: https://youtu.be/QHMzFUo0NL8

Properties

Across optical, thermal, electrical and magnetic, there are three main principles that govern nanomaterial properties:

• Quantum confinement: Discrete states instead of bands
• Surface dominance: Interfaces dominate behavior
• Geometry control: Properties can be tuned by size and shape

External Resources

Materials Science soinc page