Introduction to Nanomaterials | Engineering Physics 2 Lecture

This video explains three key characterization techniques for nanomaterials: Scanning Electron Microscopy (SEM) uses an electron beam to scan specimen surfaces, detecting secondary electrons for topographical imaging with resolution up to 1 nm; Transmission Electron Microscopy (TEM) transmits electron beams through thin specimens to reveal internal structures at resolutions down to 0.1 nm; and Atomic Force Microscopy (AFM) measures atomic forces between a sharp tip and sample surface to generate 3D topographical images without requiring vacuum, making it suitable for insulating and biological samples.

Various techniques characterize nanomaterials: (1) SEM (Scanning Electron Microscopy) - shows surface morphology, (2) TEM (Transmission Electron Microscopy) - reveals internal structure, (3) AFM (Atomic Force Microscopy) - measures surface topology and roughness, (4) XRD (X-ray Diffraction) - determines crystallographic structure, (5) FTIR - identifies chemical bonds and molecular structure, (6) Raman Spectroscopy - reveals vibrational modes and composition, (7) UV-Vis Spectroscopy - studies optical properties and band gap, (8) TGA (Thermogravimetric Analysis) - measures thermal stability, (9) Zeta Potential - determines surface charge and colloidal stability, (10) XPS (X-ray Photoelectron Spectroscopy) - analyzes surface chemical composition.

Microscopy-based techniques are essential for morphological studies of nanoparticles, determining size and shape. Scanning Electron Microscopy (SEM) uses an electron beam to scan sample surfaces, providing 3D topographical images with large depth of field. Transmission Electron Microscopy (TEM) transmits electrons through thin samples for internal structure analysis. Atomic Force Microscopy (AFM) scans surfaces with atomic-scale probes, providing accurate size/shape measurements without mathematical treatment. Scanning Tunneling Microscopy (STM) uses quantum tunneling for atomic-resolution imaging. Energy Dispersive X-ray Spectroscopy (EDX) determines elemental composition by analyzing X-rays emitted when electron beams strike samples. High Resolution TEM (HRTEM) provides atomic-level crystallographic imaging.

Surface characterization techniques such as Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM) are essential tools in nanoscience and nanotechnology for visualizing and analyzing materials at the atomic and molecular level, enabling researchers to study surface structures, topographies, and interfacial interactions that determine material properties and behaviors.

Scanning Electron Microscopy (SEM) is a powerful characterization technique for nanomaterials that uses a focused electron beam (1-50 keV) to scan across a sample surface, generating three main imaging modes: secondary electron imaging (detects low-energy electrons from surface atoms for topographical contrast), backscattered electron imaging (detects high-energy electrons scattered by atomic nuclei for compositional contrast between light and heavy elements), and elemental X-ray analysis (detects characteristic X-rays from inner-shell electron transitions for chemical composition determination); SEM requires conductive samples or metal coating (gold/iron) and provides information on nanostructure morphology, size, orientation, and elemental composition.

In bulk materials, atoms are closely packed with approximately 10^23 atoms per unit cell, causing energy levels to mix and form continuous valence and conduction bands. When materials are reduced to the microscopic range (10^-6 m), containing about 1,000 atoms, energy levels become moderately discontinuous. At the nano range (10^-9 m), with only 10-15 atoms, energy levels become fully discontinuous. This quantum confinement effect causes the same material to exhibit different optical, electrical, and other properties depending on its size within the 1-100 nanometer range.

When semiconductor nanoparticles are reduced in size, quantum confinement effects cause the band gap energy to increase (blue shift) and the band levels to become quantized, leading to distinct optical absorption and emission properties compared to bulk materials; this is demonstrated through examples like gallium arsenide nanocrystals showing a blue shift from 1.43 eV to 2.36 eV and cadmium selenide nanoparticles exhibiting color changes from red to blue with decreasing particle size, while metallic nanoparticles show different behavior where surface plasmon resonance dominates and optical properties are less sensitive to size changes.

Nanomaterials (1-100 nm) exhibit enhanced properties compared to bulk materials due to two key factors: (1) increased surface-to-volume ratio leading to higher reactivity, and (2) quantum confinement effects where reducing dimensions to the Bohr radius (0.5 Å) causes energy levels to transition from continuous to discrete, altering optical, thermal, mechanical, electrical, and magnetic properties. Based on confinement dimensions, nanomaterials are classified as quantum wells (1D confinement), quantum wires (2D confinement), or quantum dots (3D confinement), each exhibiting distinct electronic and optical behaviors.

Nanomaterials demonstrate distinctive electrical, optical, and magnetic behaviors stemming from quantum confinement effects. Electrically, the transition from continuous energy bands in bulk materials to discrete energy levels in nanomaterials causes increased conductivity in nano-ceramics and magnetic composites, with quantum confinement in semiconductors increasing effective band gaps. Optically, decreasing particle size creates discrete energy levels leading to band gap increases and blue shifts in absorption/emission spectra—gold nanoparticles become transparent at 10 nm, and further size reduction causes blue color shifts. Magnetically, nanomaterials exhibit phenomena absent in bulk: ferromagnetic particles become single-domain, superparamagnetism emerges, giant magnetoresistance appears in hybrids, and antiferromagnetic materials may behave as ferromagnets. Transition metals show superparamagnetism instead of bulk ferromagnetism, while normally paramagnetic elements like sodium, potassium, and radium exhibit ferromagnetic behavior as nanoparticles. These transformations occur because reduced dimensions cause surface atoms to dominate material behavior, fundamentally altering electronic, optical, and magnetic responses.

Coal contains graphene-like structures formed geologically over millions of years. Research demonstrated that coal can be converted to graphene quantum dots through oxidation with nitric and sulfuric acid mixtures, with different coal types yielding different-sized dots (anthracite: 28 nm, bituminous: ~2.3 nm). These quantum dots exhibit size-dependent fluorescence for applications including product authentication, marijuana traceability, and fracking water tracing. Carbon nanotubes can be chemically split into graphene nanoribbons using potassium permanganate oxidation. Graphene nanoribbons enable transparent conductive films for touchscreens and anti-icing coatings for aerospace applications, where heated graphene coatings melt ice on glass surfaces without blocking radar signals.

Key nanomaterials include quantum dots (0D, invented by Alexei Ekimov), carbon nanotubes (1D, invented by Sumio Iijima), and graphene (2D, invented by Geim and Novoselov). Quantum dots emit different colors based on size, used in QLED TVs and cancer therapy. Carbon nanotubes are lightweight, superconducting, and 100 times stronger than steel, used in spacecraft wiring and energy storage. Graphene is 200 times stronger than steel, transparent, and highly conductive, used in batteries, solar cells, and optical devices. Dendrimers (3D branched structures) are used for targeted drug delivery, allowing a single smart medicine to deliver 25-30 different medications to different body locations efficiently.

Fullerenes (discovered 1985, Nobel Prize 1996) are carbon allotropes with truncated icosahedron structures (C60 buckyballs), produced by arc discharge between graphite rods in helium. Applications include targeting degenerative diseases, MRI contrast agents, LEDs, and electronic devices. Carbon nanotubes are cylindrical structures formed by rolling graphene sheets, existing as single-walled or multi-walled variants with conducting, semiconducting, or insulating properties depending on rolling angle. They exhibit extraordinary thermal conductivity, exceptional tensile strength, flexibility, and low thermal expansion, enabling applications in thermal management, field emission, energy storage, molecular electronics, and polymer composites. Nanowires are structures with diameters constrained to tens of nanometers where quantum effects dominate, including metallic (Ni, Pt, Au), semiconducting (silica), insulating (SiO2, TiO2), and molecular varieties (DNA). Quantum dots are three-dimensional nanomaterials with all dimensions in the nanoscale, enabling applications in chemical mechanical polishing, magnetic recording, sunscreens, MRI contrast agents, and biomagnetic separations.

Scientists create nanomaterials by manipulating atomic arrangements. Carbon nanotubes are cylindrical nanostructures made of carbon atoms arranged in tube-like structures. Graphene is a single layer of carbon atoms in a honeycomb pattern, so thin that 4 million sheets would make just 1 millimeter. Both are extremely strong, flexible, and excellent electrical conductors. These materials enable applications from lightweight aircraft to flexible solar panels.

Sumio Iijima synthesized carbon nanotubes in 1991—hollow cylindrical structures with nanoscale diameters but extended lengths, exhibiting exceptional electrical conductivity. Graphene, a single layer of graphite, was isolated by Geim and Novoselov in 2004 at the University of Manchester. Graphene possesses extraordinary properties: 200 times stronger than steel, one million times thinner than a human hair, highly conductive due to conjugated double bonds, and chemically inert. They received the Nobel Prize in Physics in 2010. Quantum dots, semiconductor nanoparticles discovered between 1980-1995, enable vibrant color displays through quantum confinement effects. Nature itself utilizes nanotechnology: lotus leaves have nanostructures preventing water penetration, and geckos use millions of nano-suckers for wall climbing.