materials science notes, Study notes of Earth science

Download materials science notes and more Study notes Earth science in PDF only on Docsity!

Crystalline Structure

material constant. Quantum confinement in semiconductors causes size-dependent band gaps, with smaller particles having larger band gaps and shifted optical absorption/emission (basis of quantum dots). Mechanical properties also change: smaller grains can strengthen materials by the Hall–Petch relationship, σᵧ = σ₀ + k·d⁻¹/², where σᵧ is yield strength, d is grain size, and k is a constant.Grain Boundaries: Polycrystalline nanomaterials contain grains separated by boundaries that act as defects. Grain boundaries disrupt periodic atomic arrangements, increase resistivity (electron scattering), and serve as diffusion pathways. Smaller grain size increases boundary density, generally enhancing strength (Hall–Petch effect). However, at very small nanograins (<10 nm), boundaries may become weak links, leading to “inverse Hall–Petch” softening due to grain-boundary sliding. Grain boundaries also influence corrosion, sintering, and catalytic activity. Polymorphism is when a specific material can have more than one crystal structure. Allotropy is polymorphism for elemental solids.The concept of a crystal system is used to classify crystal structures on the basis of unit cell geometry—that is, unit cell edge lengths and interaxial angles. There are seven crystal systems: cubic, tetragonal, hexagonal, orthorhombic, rhombohedral (trigonal), monoclinic, and triclinic. Anisotropy is the directionality dependence of properties. For isotropic materials, properties are independent of the direction of measurement. X-ray diffractometry is used for crystal structure and interplanar spacing determinations. A beam of x-rays directed on a crystalline material may experience diffraction (constructive interference) as a result of its interaction with a series of parallel atomic planes. Bragg’s law specifies the condition for diffraction of x-rays. Chemistry of Surfaces: Materials at the nanoscale have a high surface-area-to-volume ratio, meaning more atoms are exposed at surfaces, making them more reactive. Surface energy is the excess energy associated with atoms at the surface compared to bulk atoms; it drives phenomena like adsorption and catalysis. Surface tension refers to the cohesive forces at a liquid’s surface that resist deformation. The wetting angle (contact angle) measures how a liquid interacts with a solid surface—small angles indicate good wetting (hydrophilic behavior), while large angles indicate poor wetting (hydrophobic behavior). Hydrophilic surfaces attract water, improving spreading and adhesion, while hydrophobic surfaces repel water, leading to droplet formation. Surfactants are molecules with hydrophilic and hydrophobic regions that reduce surface tension and stabilize dispersions or emulsions. Nanomaterials Synthesis and Fabrication: Synthesis can follow top-down or bottom-up approaches. Top-down methods break down bulk materials into nanoscale structures and include mechanical milling (grinding into nanoparticles), sputtering (ion bombardment ejects atoms), chemical etching (removing layers selectively), and lithography (patterning materials with light, electrons, or ions). Bottom-up methods assemble structures atom-by-atom or molecule-by-molecule, often yielding more precise control. Examples include sol-gel (solution-to-gel transition to produce ceramics or glass), chemical vapor deposition (CVD, where gaseous precursors form thin films), self-assembly (spontaneous organization of molecules), and molecular beam epitaxy (MBE, layer-by-layer crystal growth under vacuum). Characterization: X-ray diffraction (XRD) reveals crystal structure and lattice spacing using Bragg’s law. Light and electron microscopes (optical, TEM, SEM) provide imaging at increasing resolutions, with TEM capable of atomic-scale visualization. Scanning probe microscopy (e.g., AFM, STM) maps surfaces by physically scanning with a probe. Spectroscopic methods provide information about composition and properties: UV-visible spectroscopy measures absorption/transmission, photoluminescence reveals electronic transitions, energy dispersive spectroscopy (EDS) identifies elemental composition, and mass spectrometry determines molecular mass and structure. Physical & Mechanical Properties: Key bulk and nanoscale properties include melting temperature (which can decrease at nanoscale due to surface effects), density (mass per unit volume), hardness (resistance to deformation), strength (ability to withstand stress), and elastic modulus (stiffness). Stress is force per unit area, strain is deformation per original length, and elastic modulus is stress/strain within the elastic region. Adhesion describes attractive forces between surfaces, critical in thin films and coatings, while wear refers to material loss due to friction or contact. Optical Properties: Nanoscale materials strongly interact with light. Scattering occurs when light is redirected by particles; absorption removes energy from incident light, producing color; reflection and refraction describe light bouncing off or bending at interfaces; and transmission is light passing through a material. At advanced levels, luminescence (light emission not due to heat), fluorescence (fast light emission after excitation), and plasmons (collective oscillations of electrons at surfaces) are relevant. Quantum confinement (when particle size approaches electron wavelength) alters optical properties, shifting absorption/emission spectra in nanomaterials like quantum dots. Thermal, Electrical, & Magnetic Properties: Thermal conductivity (heat transport) decreases in nanomaterials due to increased surface scattering of phonons. Electrical conductivity depends on band structure and can be altered by surface scattering, defects, or reduced dimensions; nanomaterials often display dielectric (insulating) properties or semiconductor behavior with size-tunable band gaps. At advanced levels, magnetic phenomena become important: magnetic fields interact with nanostructures differently, saturation magnetization defines the maximum alignment of spins, and types of magnetism (ferromagnetic, paramagnetic, superparamagnetic) depend on particle size and structure. Quantum confinement also affects electronic and optical properties, leading to discrete energy levels instead of continuous bands. Types and ApplicationsZero-dimensionalNanoparticles ( 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 nanoparticlesApplications: 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 QDsApplications: 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, SnO 2 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 nanorodsApplications: Catalysis, solar cells Two-dimensionalGraphene is a single, atomic layer of carbon atoms in an sp^2 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 MX 2 (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 Mn+1XnTx (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-dimensionalNanoporous 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 carbonsApplications: 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 FabricationTop-downMechanical ApproachesMechanical 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 TransferWet 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-upChemical 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. Nanomaterials made by CVD often have different properties compared to bulk materials. Why is that? Nanomaterials have following properties that can affect final property of the material being deposited: (1) Size effects : at nanoscale, materials have much higher surface to volume ratio, leading to enhanced reactivity and adsorption (2) Quantum confinement: at nanoscale, electrons are confined, changing electron interaction dynamics (3) Crystallinity and defects: CVD often produces highly crystalline, defect-controlled structures. Nanomaterials can stabilize unusual crystal phases that don’t exist in bulk. (4) Grain boundaries: Nanocrystalline films grown by CVD have many grain boundaries which affect mechanical strength, conductivity and diffusion. (5) Surface chemistry: CVD-grown nanomaterials often have unique surface terminations (dangling bonds, functional groups). These surfaces strongly influence catalysis, adhesion, and interaction with other molecules. (6) Controlled growth: CVD allows precise control over thickness, orientation, and morphology. Nanomaterials can be grown as monolayers, nanotubes, or nanowires , which inherently behave differently than bulk. 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. CharacterizationMicroscopyLight MicroscopyLight 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