1 Nanoscopic Materials NANO - particles, crystals, powders - films, patterned films - wires, whiskers, rods, tubes - dots Nanostructured materials = nonequilibrium character  Good sinterability  High catalytic activity  Difficult handling  Adsorption of gases and impurities  Poor compressibility Unique Features of the Nano-scale 2 Smallness: physical size Size compatibility with the basic biological structures (cells, liposomes, enzymes…) Delivery vehicles for medical applications Surface chemistry – functionalization Quantum size – new physical phenomena Smallness: surface versus bulk forces A large surface-to-volume ratio Bulk forces - gravity - unimportant for nanoparticles Surface forces - Brownian motion - colloidal particles never settle Unique Features of the Nano-scale 3 Smallness: surface versus bulk atom properties Increasing surface to bulk atom number ratio with decreasing size enhances the role of surface (boundary) • surface phonon scattering • surface electron scattering • surface atom electric charge distribution • surface atom spins in ferromagnetic, ferrimagnetic, and antiferromagnetic materials - transition to superparamagnetic state Unique Features of the Nano-scale 4 Chemical bonding in nanostructures Trigonal bonding of C in graphite and graphene – sp2 Tetrahedral bonding of C in diamond – sp3 Single wall carbon nanotubes (SWCNT) C bonding in SWCNT is contorted sp2  sp3 Chirality - variable amounts of twisting Unique Features of the Nano-scale 5 Self-assembly Combination of particles, atoms, or molecules, self-assemble into predetermined new materials and structures (micelles, SAM, MOF, DNA, proteins, superlattices....) Unique Features of the Nano-scale 6 Self-assembly of NP superlattices Synthetic routes to superlattices Unique Features of the Nano-scale 7 Self-assembly of NP superlattices • Polyhedral (3D) superlattices • Film (2D) superlattices Space filling  Unique Features of the Nano-scale 8 Self-assembly of NP superlattices Interdigitating hydrocarbon coronas between a nanocrystal pair core radius R, ligand length L, effective radius Reff Optimal packing model (OPM) Close Packed Atoms/Nanocrystals 9 Close Packed Atoms/Nanocrystals 10 Quasi-Spherical Nanocrystals 11 [110]-oriented bcc packing of 2.2 nm Au NPs capped with C18-thiol (L/R ≈ 1) [100]-oriented hcp of 4.5 nm Au nanocrystals with the same ligands (L/R ≈ 0.5) - Small softness values L/R Hard spheres = FCC or HCP - Borderline at L/R = 0.7 - Large L/R values Soft spheres = BCC The assembly of Au nanocrystal superlattices depends on the softness parameter L/R The fcc/hcp deforms surface hydrocarbon chains more significantly than that of bcc Rod-/Platelet-Shaped Nanocrystals 12 Liquid crystalline phases of rods and disks Au nanorods passivated with CTAB bilayers Smectic Rods Disks Nematic (random position, fixed orientation) Smectic (fixed position in a plane, fixed orientation) Discotic columnar (fixed position along one axis, fixed orientation) Polyhedral Nanocrystals 13 Large NPs - preference for face-to-face contacts – max. interparticle cohesion 500 nm Ag NPs densest polyhedron packings Small NPs - preference for tip-to-tip contacts – min. interligand repulsion Superlattice of 10 nm tetrahedral CdSe NPs capped with oleic acid ligands Modeled structure with contacts between tetrahedron tips Large + Small Sphere Nanocrystal Mixtures 14 Tetrahedral Holes (T)Octahedral Holes (O) Z = 4 number of atoms in the CCP cell (N) T = 8 number of tetrahedral holes (2N) Z = 4 number of atoms in the CCP cell (N) O = 4 number of octahedral holes (N) The structures derived from simple rules for construction of binary lattices of cubic and hexagonal close packed spheres N close packed large nanocrystals in a lattice cell N Octahedral + 2N Tetrahedral holes can be filled by small nanocrystals Large + Small Sphere Nanocrystal Mixtures 15 More than 20 unique binary nanocrystal superlattices A mixture of spherical nanocrystals with two sizes produces a wide range of binary superlattices Effective size ratio DA (LA) and DB (LB) are the core diameters (effective ligand thicknesses) of large and small nanocrystals Unique Features of the Nano-scale 16 Regioselectivity SEM STEM-EDS Regioselectivity - difference between a nanocrystal’s vertex and face Face atoms have more bonds to neighboring atoms and are less reactive than vertex atoms Markovnikov Pd cubes or octahedra as seeds Heterogeneous nucleation of Au Breaking the original Oh symmetry Anisotropic crystals Crystallographically nonequivalent sites – mismatch different lattice constants Unique Features of the Nano-scale 17 Chemoselectivity Chemically different phases A chemoselective reaction occurs much more quickly at one of the phases Au – thiol Fe3O4 – 3,4-dihydroxybenzoic acid STEM-EDS Janus nanoparticles Paracetamol Unique Features of the Nano-scale 18 Chemoselectivity STEM-EDS Janus Pd-Ag nanoparticles Galvanic replacement with [AuCl4] The formation of a large void in Ag and a AgAu alloyed shell Ag diffused elsewhere Pd does not change Unique Features of the Nano-scale 19 Quantum confinement and tunneling Electron quantum confinement - the spatial restrictions of nanoscale structures confine electrons resulting in the presence of energy levels whose values and spacing depend on the degree of confinement = particle size Quantum tunneling (the opposite of confinement) - an electron wave function leaks across classically forbidden energy barriers of nanoscale size Unique Features of the Nano-scale 20 Quantum confinement and tunneling Electron in a box - an infinitely deep 3D box the difference between two energetically adjacent electron energy levels, n: h is Planck’s constant, me is the electron mass, L x L x L is the confining volume Decreasing L increases the inter-level spacing E Nanoscale - quantization of energy due to confinement Micro- and larger scales - E very small Energy appears as a continuum Unique Features of the Nano-scale 21 Wave-particle duality Quantum interference between electron waves that are scattered off the boundaries of a nanostructure thereby forming a standing wave 48 Fe adatoms arranged on a Cu(111) surface at 4 K form a corral (radius 71.3 Å) confining the valence electrons - an electron trapped in a round two-dimensional box The probability density image determined by the wave function distribution captured by STM - wave function leakage into a positively biased scanning probe - discrete resonances = size quantization Unique Features of the Nano-scale 22 Relativistic phenomena at the nano-scale In 2D materials - graphene - mass-less Dirac electrons Mass-less behavior can produce • Ballistic (collision-free) charge transport • Unusual Hall effects • Enormously high carrier mobilities • Topologically dependent phases Unique Features of the Nano-scale 23 Electromagnetic interactions with nanostructures Plasmonic mode of a metal nanoparticle excited by the electric field of an incoming light wave - a cooperative excitation of free valence electrons Relaxation - reradiation of photons from the nanoparticle - collisions of oscillating valence electrons within the particle The electric field distribution of the metal nanoparticle • radiating far-field component = the emitted photons • a near-field component around the nanoparticle Unique Features of the Nano-scale 24 Fluctuations Thermodynamic fluctuations - a system gets smaller, fluctuations away from the thermodynamic equilibrium distribution become important The statistics of huge numbers of particles breaking down Quantum fluctuations - the small separation distances between objects at the nano-scale The temporary change ΔE in the amount of energy (or mass of particles) that can occur in a region for a time Δt The fluctuation time - conservation of energy is violated during the fluctuation time Unique Features of the Nano-scale 25 Fluctuations Casimir force (theor. 1948, exp. 1996) – a quantum phenomenon A pressure that pushes objects of a nano-scale separation together Vacuum energy, fluctuating electromagnetic waves, restricted wavelengths of standing waves between nanoobjects = lower energy of vacuum between nanoobjects = pressure form outside The quantum vacuum fluctuations - space is not empty but is filled with spontaneously appearing and disappearing particles The Casimir force affects friction and results in striction (the permanent adhesion of surfaces) - a critical problem for moving systems at the nano-scale = nanomotors - the force increases with decreasing distance 26 Synthesis Methods Top-down: from bulk to nanoparticles Bottom-up: from atoms to nanoparticles 27 Bottom-up Synthesis: Atom Up Coordination chemistry – molecular cages [(Pd)6(L)8]12+ diam = 19 Å 28 Bottom-up Synthesis: Atom Up Coordination chemistry – Metal-organic frameworks (MOFs) Polytopic Ligands Organic spacers Flexible or rigid Variable length Directionality Metal centers Coordinative bonds Coordination numbers 3-6 Bond angles Secondary Building Units Metal-Organic Frameworks (MOFs) 29 • organic-inorganic hybrids • crystalline • porous A regular array of positively charged metal ions (nodes ) surrounded by organic molecules (linkers) A repeating cage-like structures An extraordinarily large internal surface area 30 Atom Aggregation Methods GEM – gas evaporation method  evaporation by heating – resistive, laser, plasma, electron beam, arc discharge  the vapor nucleates homogeneously owing to collisions with the cold gas atoms  condensation - in an inert gas (He, Ar, 1 kPa) on a cold finger and walls metals, intermetallics, alloys, SiC, C60 - in a reactive gas O2 - oxides TiO2, MgO, Al2O3, Cu2O N2, NH3 - nitrides - in an organic solvent matrix- metals, carbides SMAD – the solvated metal atom dispersion 1–2 g of a metal, 100 g of solvent, cooled with liquid N2 more polar solvent (more strongly ligating) gives smaller particles Ni powder: THF < toluene < pentane = hexan Carbide formation 77–300 K 450 K Ni (g) + pentane  NixCyHz  Ni3C 31 Bottom-up Synthesis Thermal or Sonocative Decomposition of Precursors Fe(CO)5  nc-Fe + 5 CO sonolytic decomposition [Co(en)3]WO4  nc-WC – 23% Co thermolysis PhSi(OEt)3 + Si(OEt)4 + H2O  gel  -SiC (in Ar, 1500 C) (CH3SiHNH)n (l)  Si3N4 + SiC laser pyrolysis M(BH4)4 (g)  borides MB2+x (300–400 C, M = Ti, Zr, Hf) Si(OEt)4 + Ag+ or Cu2+ + H2O  SiO2/Ag+/Cu2+ metal-impregnated gel SiO2/Ag+/Cu2+ + H2  SiO2/Ag/Cu (550 C) metal NPs embedded in xerogel 32 Bottom-up Synthesis Thermometer Vacuum/N2 Stopper/Septum 230 °C Solution of precursors in OLA M(acac)n acetylacetonates Thermal decomposition of precursors Hot-injection OLA oleylamine ODE octadecene MO O O O O O Me Me Me Me Me Me 33 Thermal Decomposition of Precursors Fe(CO)5 oleic acid trioctylamine 350 o C, 1 h 350 o C, 1 h Fe Me3NO Fe2O3 6 nm Separation of nucleation and growth Fe(CO)5 thermal decomposition at 100 oC contributes to nucleation Fe(oleate) thermal decomposition at 350 oC contributes to growth OH O 34 Thermal Decomposition of Precursors 200 °C in dodecylamine Phase Control [NnBu4]2[Fe4S4(SPh)4] pyrrhotite Fe7S8 greigite Fe3S4 thiospinel, the sulfide analogue of magnetite 180 °C in octylamine Thermolysis Surface Modification 35 A nanoparticle of 5 nm core diameter with different hydrophobic ligand molecules both drawn to scale The particle is idealized as a smooth sphere • trioctylphosphine oxide (TOPO) • triphenylphosphine (TPP) • dodecanethiol (DDT) • tetraoctylammonium bromide (TOAB) • oleic acid (OA) 5 nm 36 LaMer Mechanism Hot-injection synthesis 1) Monomer formation 2) Supersaturated solution 3) Burst of nucleation 4) Depletion of monomer 5) Slow growth of particles without additional nucleation Separation of nucleation and growth - monodisperse 2 1 3 4 5 Bottom-up Synthesis 37 Crystallization free energy S > 1 supersaturation Bottom-up Synthesis 38 Nucleation rate Experimentally controllable parameters: a) level of supersaturation b) temperature c) surface free energy Vm = 3.29 × 10−5 m3 mol−1 (CdSe) 39 Bottom-up Synthesis Hot-injection N = the number concentration of the nanocrystals σ( r ) = relative standard deviation of their radii r < r > = mean radius dN / dt = nucleation rate 40 Bottom-up Synthesis Heat-up N = the number concentration of the nanocrystals σ( r ) = relative standard deviation of their radii r < r > = mean radius dN / dt = nucleation rate 41 Other Mechanisms Digestive Rippening The conversion of polydisperse NPs into monodisperse ones The etching of large NPs by dissolution of clusters/atoms by digestive ripening agents - strongly coordinating ligands Clusters/atoms redeposited on small NPs = the growth of smaller NPs Narrowing of the particle size distribution = monodisperse system A thermodynamic equilibrium size of the NPs is usually obtained Depends on the specific ligand and the reaction temperature Watzky-Finke Mechanism Slow continuous nucleation - Fast autocatalytic surface growth Seed-mediated Mechanism Au nanoclusters as seeds - Bi, Sn, In, Au, Fe, Fe3O4 Continuous Synthesis of Inorganic Nanoparticles 42 Rapid mixing of two precursor solutions and the fast removal of the nuclei from the reaction environment A very short mixing time Transport from the reactor to a tubing for the particle growth The length of tubing up to the collection vessel influences the particle growth Bottom-up Synthesis 43 Sonocative decomposition of precursors Cavity interior Filled with gases and vapors 5 000 – 20 000 C / 500 – 1500 bar Surrounding liquid layer 2000 C Bulk liquid shock waves shear forcesCavitation creation, growth, and implosive collapse of bubbles in a liquid Bottom-up Synthesis 44 Sonocative decomposition of precursors Fe2O3 amorphous Fe2O3 maghemite Fe2O3 hematite ))))) 300 o C 340 o C linear dynam/isothermal Fe(acac)3 TG MO O O O O O Me Me Me Me Me Me Particle size 20 - 30 nm Spherical shape Uniform size distribution 45 Borohydride Reduction Reduction of Metal Ions Manhattan Project Aqueous, under Ar 2 Co2+ + 4 BH4 - + 9 H2O  Co2B + 12.5 H2 + 3 B(OH)3 Under air 4 Co2B + 3 O2  8 Co + 2 B2O3 Nonaqueous Co2+ + BH4 - + diglyme  Co + H2 + B2H6 TiCl4 + 2 NaBH4  TiB2 + 2 NaCl + 2 HCl + H2 MXn + n NR4[BEt3H]  M + NR4X + n BEt3 + n/2 H2 M = group 6 to 11, Bi, Sn, ; n = 2,3; X = Cl, Br, NO3, OAc, OOC-R, acac, O-R Solvents: Diethylenglycol, Oleylamine, …. Surfactant Mixed-metal particles AgNi, AgCu, BiNi, …. NaBH4 BH3NH2tBu NR4[BEt3H] Ni‐Sn alloy 46 Borohydride Reduction SH Two-dimensional array of thiol-derivatised Au NPs (mean diam 4.2 nm) Au colloidal particles HAuCl4 + NaBH4 in toluene/H2O system, TOABr as a phase transfer agent, Au particles in the toluene layer, their surface covered with Br, addition of RSH gives stable Au colloid 47 K + K+ Mg Mg Solvents: dry anaerobic diglyme, THF, ethers, xylene NiCl2 + 2 K  Ni + 2 KCl AlCl3 + 3 K  Al + 3 KCl Reduction by Glycols or Hydrazine “Organically solvated metals” Alkali Metal Reduction 48 Bottom-up Synthesis Alkalide Reduction 13 K+(15-crown-5)2Na + 6 FeCl3 + 2CBr4 THF 30 °C 2 Fe3C (nano) + 13 K(15-crown-5)2Cl0.43Br0.57 + 13 NaCl Anealed at 950 °C / 4 h Fe3C: 2 – 15 nm 49 Reactions in Porous Solids Zeolites, Mesoporous materials Ion exchange in solution, reaction with a gaseous reagent inside the cavities: M2+ + Na-Y  M-Y + 2 Na+ M2+ + H2E  ME M = Cd, Pb; E = S, Se Ship-in-the-Bottle Synthesis Ru3+ + Na-Y  Ru(III)-Y + 3 Na+ Ru(III)-Y + 3 bpy  Ru(bpy)3 2+ reduction of Ru(III) Conducting carbon nanowires Acrylonitrile introduced into MCM-41 (3 nm diam. channels) Radical polymerization Pyrolysis gives carbon filaments 50 Bottom-up Synthesis Sol-Gel Methods Sol drying Aerogels, supercritical drying Aerosol Spray Pyrolysis Aqueous solution, nebulization, droplet flow, solvent evaporation, chemical reaction, particle consolidation, up to 800 C 3 Gd(NO3)3 + 5 Fe(NO3)3  Gd3Fe5O12 + 6 O2 + 24 NO2 MnCl2 + 2 FeCl3 + 4 H2O  MnFe2O4 + 8 HCl 51 H2O + octane H2O H2O Cd2+ Se2- CdSe CdSe PhSeSiMe3 Bottom-up Synthesis Inverse micelles Size distribution histogram 52 Rapid Expansion of Supercritical Fluid Solution Polymeric Nanoparticles Polymer in CO2 53 Spinning Disc Processing SDP A rapidly rotating disc (3003000 rpm) Ethanolic solutions of Zn(NO3)2 and NaOH, polyvinylpyrrolidone (PVP) as a capping agent Very thin films of fluid (1 to 200 m) on a surface Synthetic parameters = temperature, flow rate, disc speed, surface texture Influence on the reaction kinetics and particle size Intense mixing, accelerates nucleation and growth, affords monodispersed ZnO nanoparticles with controlled particle size down to a size of 1.3 nm and polydispersities of 10% 54 Electrospinning ThO2 from PVA/Th(NO3)4 Average diameter: 76 ± 25 nm Parameters • Solution – precursor + polymer + solvent (viscosity, conductivity, surface tension) • Instruments (voltage, distance b/w electrodes, collector shape) • Ambient (temperature, humidity, atmosphere) Vapor-Liquid-Solid (VLS) Growth 55 Synthesis of nanowires NW Metal catalyst nanoparticles - Au(s) – (1) Feed another element (Ge vapor, GeH4 or SiH4) at an elevated temperature (440-800 C/ultra- high-vacuum) Gaseous precursor feedstock is absorbed/dissolved in Au(s) till the solid solubility limit is reached (2) A liquid phase appears (3), melts to a droplet The droplet becomes supersaturated with Ge When the solubility limit is reached (4), an excess material is precipitated out to form solid NWs beneath the droplet Eutectic 360 C Au (mp 1064 C) Si (mp 1410 C) Ge (mp 938 C) Vapor-Liquid-Solid (VLS) Growth 56 1 2 3 4 57 Vapor-Liquid-Solid (VLS) Growth Au NPs Au/Ge Eutectic Liquid Alloy Nucleation of NWs NW Growth 1 2 3 4 1 2 3 4 GeH4 58 In-situ TEM images of the VLS process In-situ TEM images recorded during the process of nanowire growth: (A) Au nanoclusters in solid state at 500 °C (B) Alloying initiated at 800 °C, at this stage Au exists mostly in solid state (C) Liquid Au/Ge alloy (D) The nucleation of Ge nanocrystal on the alloy surface (E) Ge nanocrystal elongates with further Ge condensation (F) Ge forms a wire Combustion-based Methods 59 – Solution-combustion synthesis (SCS) of nanosized powders initial reaction medium is an aqueous solution – Salt-assisted combustion reaction (SACR) of nanomaterials initial reactants are in a solid state (condensed phase combustion) The solution-combustion synthesis involves a self-sustained reaction in a homogeneous solution of different oxidizers (e.g., metal nitrates) and fuels (e.g., urea, glycine and hydrazides) Depending on the type of the precursors, as well as on conditions used for the process organization, SCS may occur by either volume or layer-by- layer propagating combustion modes Combustion-based Methods 60 A thermocouples B data logger C combustion reactor D initiated by an electrically heated Ni-Cr wire The reaction by-products are leached out using HCl-water Salt-assisted combustion reaction Top-down Synthesis: Bulk Down 61  Introduction of Crystal Defects (Dislocations, Grain Boundaries) • High-Energy Ball Milling - final size only down to 100 nm (contamination issues) • Extrusion, Shear, Wear • High-Energy Irradiation • Detonative Treatment  Crystallization from Unstable States of Condensed Matter • Crystallization from Glasses • Precipitation from Supersaturated Solid or Liquid Solutions 62 Top-down Synthesis: Bulk Down Ball Milling WO3 + 3 Mg → W + 3 MgO A vibratory ball mill (Spex 8000 mixer-mill) under Ar at r.t. Carbon steel balls (diameter: about 8 mm) A ball-to-powder weight ratio of 24:1 Leached using 2.0 M HCl, 2 h stirred 63 Top-down Synthesis: Bulk Down Lithographic Techniques electron beam and focused ion beam (FIB) lithography Top-down Synthesis: Bulk Down 64 Lithographic Techniques electron beam and focused ion beam (FIB) lithography Top-down Synthesis 65 Laser ablation synthesis in solution SEM image of Pt target after ablation at 355 nm for 15 min at 14 J/cm2 Top-down Synthesis 66 Laser ablation synthesis in solution HRTEM images of AgNP (left) and AuNP (right) obtained by LASiS in DMF and water, respectively Exploding Wire Method 67 Thin wire: Au, Al, Fe, Pt (diam.  0.5 mm) The capacitor - an energy consumption 25 kWh/kg A pulse of current density 104  106 A/mm2 Temperatures ~100 000 K Time 10−8 to 10−5 seconds Exploding Wire Method 68 • A current, supplied by a capacitor, is carried across a wire • The current heats up the wire - ohmic heating • The metal melts to form a broken series of imperfect spheres – unduloids • The current rises fast - the liquid metal has no time to move out of the way • The unduloids vaporize, the metal vapor creates a lower resistance path, allowing an even higher current to flow • An electric arc is formed - turns the vapor into plasma – a bright flash of light • The plasma is allowed to expand freely, creating a shock wave • Electromagnetic radiation is released in tandem with the shock wave • The shock wave pushes liquid, gaseous, and plasmatic metal outwards, breaking the circuit and ending the process Exploding Wire Method 69 Exploding Wire Method 70 The electrical explosion chamber, 1 – lid, 2 – stainless steel cylindrical container, 3 – copper electrodes, 4 – insulation blocks, 5 – distilled water, 6 – high purity graphite sticks Synthesis of carbon nanodots, graphite nanoflakes, few-layer and multilayer graphene Exploding Wire Method 71 The synthesis of W NPs An explosion chamber, a powder collector and an electric circuit W wire, 0.27 mm in diameter The explosion by a pulsed electric plasma in an Ar atmosphere A high-voltage source (5–30 kV) A current pulse of several thousands Amps The surface of W nanopowder was passivated at r.t. in Ar gas by air (0.1 vol.%) XRD analysis showed three phases: -W, -W and W3O The particles have a spherical shape and a diameter between 20 and 200 nm Dictionary of Used Terms 72 Galvanic replacement = cementace Interdigitating = prostupující, propletené Corral = ohrada Unduloid =