C8888 Nanochemistry 1 Jiri Pinkas Office C12/224 Phone 549496493 Email: jpinkas@chemi.muni.cz Ph.D. level course Prerequisite C7780 Inorganic Materials Chemistry Course grading: Select a topic concerning nanochemistry and prepare: Presentation - 20 min (20 %) Written term paper - min 7 pages (80 %) Au nanoparticles C8888 Nanochemistry Time Plan for Spring 2023 2 Lectures 1 – 4 - Think of a topic for your paper and send me a title - Send me a 1-page abstract of your paper - Final topic approval - Study and work on your paper/presentation - Hand in your term paper Presentations (C12-311) 3 Nanoscopic Materials • Chemical methods used to change physical and chemical properties – chemical composition, substituents, concentration, crystal structure…. • Size is another variable to change physical and chemical properties for constant chemical composition • Each physical property or fenomenon has a characteristic length • When particle size is comparable to the characteristic length, property starts to depend on the size 4 Nanoscopic Scales Nanomaterials 1 – 100 nm Nanoscopic Scales 5 Nanomaterials 1 – 100 nm 6 “Prey”, the novel by Michael Crichton, author of “Jurassic Park”. The horrible beasties threatening humanity in this thriller are not giant dinosaurs, but swarms of minute “nanobots” that can invade and take control of human bodies. A report issued by a Canadian environmental body called the action group on erosion, technology and concentration took a swipe at nanotechnology. It urged a ban on the manufacture of new nanomaterials until their environmental impact had been assessed. The group is better known for successfully campaigning against biotechnology, and especially against genetically modified crops. The research, led by a group at the National Aeronautics and Space Administration's Johnson Space Centre in Houston, has found in preliminary studies that inhaling vast amounts of nanotubes is dangerous. Since they are, in essence, a form of soot, this is not surprising. But as most applications embed nanotubes in other materials, they pose little risk in reality. Nanostructural Materials and Society Dictionary of Nano Terms 7 Nanoscience - fundamental scientific study of matter on the 1−100 nm scale, especially if the properties are distinct from those of bulk materials Nanotechnology - the creation of functional materials, devices, and systems through control of matter on the nanometer length scale and the exploitation of novel properties and phenomena Nanochemistry - study of chemical aspects of synthesis, properties and application of matter on the 1−100 nm scale Nanomaterials - general term for materials (polymers, semiconductors, ceramics, oxides, metals, etc.) with particle sizes in the 1−100 nm range in at least one dimension Nanoparticles (NPs) - nanomaterials that appear to be spherical, have similar sizes in all 3 dimesions Faraday’s Work on Gold Nanoparticles 1856 8 M. Faraday, Philos. Trans. R. Soc. London, 147, 145, 1857 Typical Faraday discourse in the Royal Institution in London Gold nanoparticles 10-24 nm HRTEM image of gold NP Transmission Electron Microscopy (TEM) 9 1930 Knoll a Ruska Ernst Ruska Nobel Prize in Physics 1986 SrTiO3 Gold nanoparticle in different tilt angles prozařovací Transmission Electron Microscopy (TEM) 10 HAADF- STEM images In situ real-time liquid cell electron microscopy Au−Ag galvanic replacement: AuCl4 + 3 Ag0 (s)  Au0 (s) + 3 AgCl (s) + Cl H2SO4 added to pH = 2.5 to supress reduction of AuCl4 to Au0 by radicals generated by radiolysis of water by the high-energy imaging electron beam EDS chemical maps Scanning Electron Microscopy (SEM) 11 1942 Zworykin, Hiller, Snyder SEM is a surface imaging method in which the incident electron beam scans across the sample surface and interacts with the sample to generate backscattered and secondary electrons that are used to create an image of the sample rastrovací /řádkovací 12 Scanning Tunelling Microscopy (STM) 1982 Binning and Rohrer Binning and Rohrer Nobel Prize 1986 Nanoscale Writing 13 1989 STM positioned Xe atoms on Ni (110) crystal at 4 K, 5 nm letters Manipulation atom-by-atom 40 x 40 Å constant-current STM image of Xe adsorbed on Ni(110) the tip biased +0.020 V relative to the sample, a tunneling current of 1 x 109 A Xe appears as a 1.53 ± 0.02 Å high protrusion Covalent radius 140 pm Van der Waal radius 216 pm Atomic Force Microscopy (AFM) 14 1986 Binnig, Quate, and Gerber AFM is a method allowing a variety of non-conducting surfaces to be imaged and characterized at the atomic level The detection of forces between an observed sample surface and a sharp tip located at the end of a cantilever Atomic Force Microscopy (AFM) 15 16 Nanoscale Writing Richard Feynman (1918–1988) NP in Physics 1965 “There’s Plenty of Room at the Bottom” 17 Applications of Nanomaterials Small size - can be manipulated like molecules, made by chemical reactions in solutions, injected into biological systems, selfassembled into structures, superior lithographic resolution, negligible light scattering Intense optical absorbance and emission properties - surface plasmon resonance, photothermal therapy, biosensing Quantum size effects - band gap engineering, information technology, storage media, quantum dot-sensitized solar cells High surface area - catalysts, adsorbents, energy storage, composites, molecules on the surfaces are at high local concentrations yet low in “global” concentrations, interaction of NPs with biomolecules Surface modifications - functionalized with small organics or polymers, the optical/electronic/bio properties of the inorganic core can be tuned, targeted drug delivery 18 The Nano-Family At least one dimension is between 1 - 100 nm 0-D structures (3-D confinement): • Quantum dots • Nanoparticles AFM 1 μm x 1 μm InAs on GaAs/InP Au nanoparticles CdTe nanoparticles 19 The Nano-Family 1-D structures (2-D confinement): • Nanowires • Nanorods • Nanotubes • Nanofibers • Nanowhiskers 20 The Nano-Family 2-D structures (1-D confinement): • Thin films - CVD, ALD • Planar quantum wells • Superlattices • Graphene • SAM 21 Coherence Length, d XRD patterns of iron oxide nanocrystals of 4, 6, 8, 9, 10, 11, 12, 13, and 15 nm   cos k d  Scherrer equation k = 0.89,  = wavelength, β = full width at half-maximum of a standard (Si) Nanoscopic Behavior of Materials 22 • Surface Effects • Quantum Confinement Effects Differences between bulk and nanoscale materials 23 Decreasing grain size = Increasing volume fraction of grain boundaries (50% for 3 nm particles) Surface Effects Ru particle diameter 2.9 nm 24 Surface Effects Dispersion F = the fraction of atoms at the surface F is proportional to surface area divided by volume N = total number of atoms V ~ r3 ~ N n = number of atoms at the cube edge33 2 11 Nrr r F  F 25Si . Surface Effects Properties of grain boundaries Lower coordination number of atoms Reduced atomic density (by 10–30 %) Broad spectrum of interatomic distances Experimental evidence  HREM  EXAFS, reduced number of nearest and next-nearest neighbors  Raman spectroscopy  Mössbauer spectroscopy, quadrupole splitting distribution broadened  Diffusivity enhanced by up to 20 orders of magnitude !!  Solute solubility in the boundary region Ag (fcc) and Fe (bcc) immiscible in (s) or (l), but do form solid solution as nanocrystalline alloy  EPR, nano-Si gives a sharp signal 26 Surface Effects Atoms at surfaces - fewer neighbors than atoms in the bulk = lower coordination number - stronger and shorter bonds - unsatisfied bonds - dangling bonds - surface atoms are less stabilized than bulk atoms The smaller a particle, the larger the fraction of atoms at the surface, and the higher the average binding energy per atom The melting and other phase transition temperatures scale with surface-to-volume ratio and with the inverse size Example: the melting point depression in nanocrystals 2.5 nm Au particles 930 K bulk Au 1336 K Dangling Bonds 27 • Empty orbital • 1-e orbital • 2-e orbital 28 Surface Effects A = Atoms at surfaces (one layer) – fewer neighbors, lower coordination, unsatisfied (dangling) bonds B = Atoms close to surface (several layers) – deformation of coordination sphere, distorted bond distances and angles C = Bulk atoms, regular ordering – not present in particles below 2 nm 29 Shape Factor,  Shape Factor = the ratio between a surface area of a nonspherical particle with that of a spherical one 30 Surface Effects Calculated mean coordination number as a function of inverse radius, represented by N1/3 for Mg clusters (triangles = icosahedra, squares = decahedra, diamonds = hcp Mean coordination number What is the bulk value? 31 Surface Effects Atom binding (vaporization) energies lower in nanoparticles, fewer neighbors to keep atoms from escaping Plasticity of nanocrystalline ceramics Surface Effects in Alloys 32 Alloys: • Core-shell • Janus • Random mixture • Intermetallics Au-Pt 586 atoms Surface Effects in Alloys 33 Alloys: • Core-shell • Janus • Random mixture – solid solution • Intermetallics 34 ICP-OES: Ag 50.3 mol%, EDS: Ag 62.5 mol% ICP-OES: Ag 68.8 mol%, EDS: Ag 84.2 mol% Transmission Electron Microscopy Energy Dispersive X-ray Spectroscopy Effects of Synthesis 35 AgCu 413 Ag 393 Cu 569 Localized Surface Plasmon Resonance (LSPR) 36 Faraday’s colloidal solution of gold Metal NPs Plasmon - the collective coherent oscillations of the electron gas - conduction-band electrons at the surface of nanoparticles in response to incident electromagnetic field of light The metal NP must be large enough to support a conduction band rather than discrete localized states, like a molecule Size cutoff ∼2−5 nm for most metals Localized Surface Plasmon Resonance (LSPR) 37 LSPR results in strong absorbance and scattering of light, the energy can be tailored by different metal cores with different sizes and shapes Au NPs - the visible region for spheres (Dcore 3.0 − 200.0 nm) colorimetric sensing, biological contrast agents anisotropic Au NPs - the near-infrared, a strong scatterer (in vivo imaging) or absorber (photothermal therapy) 38 Melting Point Depression Surface atoms in solids are bound by a lower number of shorter and stronger bonds Nanoparticles with a large fraction of surface atoms • Lowering of average cohesion energy • Increasing average amplitude of thermal • Increasing internal pressure Result = depression of melting point of nanoparticles 39 Melting Point and Enthalpy Depression Nanocalorimetry of Sn nanoparticles Tm bulk = 232 C Hm bulk = 58.9 J/g 40 Melting Point and Enthalpy Depression Nanocalorimetry of Sn nanoparticles 41 Melting Point Depression Homogeneous melting model Continuous Liquid Meling Melting particle is surrounded by liquid Triple point of coexisting solid and liquid nanoparticles of the same mass surrounded by vapor Thin melted layer of a constant thickness δ coexisting with solid core and vapor Liquid Skin Melting 42 Melting Point Depression Sn – 4wt%Ag – 0.5wt%Cu Nano alloy particles 𝑇 𝑟 𝑇 bulk ∆ 𝛾 𝛾 Homogeneous melting model Tm(r) = mp of the cluster with radius r Tm bulk = mp of the bulk material  sg = the interfacial energies between the s and g phases  lg = the interfacial energies between the l and g phases s and l = solid and liquid phase densities M = molar mass Hm bulk = the bulk latent heat of melting Tm bulk = 218 C 43 Gibbs–Thomson Equation rH V T TrT m sl l mol bulk m bulk mm    2)( Tm(r) = mp of the nanoparticle with radius r Tm bulk = mp of the bulk material Vmol l = the molar volume of the liquid = M/s solid?  sl = the interfacial tension between the s and l surface Hm bulk = the bulk molar enthalpy of melting, endothermic DSC In nanoparticles confined in pores bulk 𝛾 𝛾 𝛾 𝜌 ~𝜌for Continuous Liquid Meling 44 Phase Transitions Phase transitions = collective phenomena With a lower number of atoms in a cluster a phase transition is less well defined and broadened Small clusters behave more like molecules than as bulk matter bulk bulk First-Order Phase Transitions 45 Three main consequences of a size decrease on caloric curve: - The transition is shifted, usually to a lower temperature (surface atoms are less coordinated and less bound than interior atoms) - The transition temp. is no longer sharp but becomes smooth and takes place over a finite range (fluctuations in TD quantities) - The latent heat is lower than in the bulk limit 46 Surface Effects Reduction in particle size • Metal particles usually exhibit a lattice contraction • Oxide particles exhibit a lattice expansion YIG = Y3Fe5O12 47 Surface Effects Correlation between the unitcell volume (cubic) and the XRD particle size in -Fe2O3 nanoparticles The smaller the particle size the larger the unit cell volume 48 Surface Effects The inter-ionic bonding in nanoparticles has a directional character Ions in the outermost layer of unit cells possess unpaired electronic orbitals Associated electric dipole moments, aligned roughly parallel to each other point outwards from the surface The repulsive dipolar interactions increase in smaller particles, are reduced by allowing unit cell volume to increase 49 Surface Effects Metal nanocrystals A continuum elastic model The lattice contraction observed in Ag nanoclusters Interpreted as the result of hydrostatic pressure exerted by the surface stress The surface stress 6.3 N/m for free Ag NPs 1–7 nm in diameter The smaller the particle size, the smaller the unit cell volume 50 Defects in Nanocrystals Quantum Size Effects 51 Size quantization - changes in the energy-level structures of materials as the material-unit (most often a crystal) size drops below a certain size - the Bohr diameter of the electron-hole pair Semiconductors - between a few nm and several tens of nm Metals - Au clusters, approx. 1 nm (often less than 100 atoms) Size quantization - an increase in bandgap (blue-shift in optical spectra) - increasing separation of energy levels with decrease in crystal size As the energy-level structure changes continuously with change in crystal size, a material of a particular fixed chemical composition can be made with varying and tunable physical properties - basic material properties are determined by size 52 Quantum Confinement Effects Physical and chemical properties depend on the size !! The smaller the space in which the bound motion takes place, (i.e., the stronger the confinement) the larger the energy separation between the allowed energies becomes Quantum Size Effects 53 Band gap dependency on the nanoparticle size Smaller particles have a wider band gap = blue shift Quantum Size Effects 54 Room temperature optical absorption spectra of CdSe nanocrystallites dispersed in hexane and ranging in size from ∼12 to 115 Å 55 Quantum Size Effects Metals Semiconductors 56 Metal-to-Insulator Transition 57 Metal-to-Insulator Transition Metallic behavior Single atom cannot behave as a metal nonmetal to metal transition 100-1000 atoms Magnetic behavior Single domain particles large coercive field Band gap increases with decreasing size 58 Metal-to-Insulator Transition Variation of the shift, E, in the core-level binding energy (relative to the bulk metal value) of Pd with the nanoparticle diameter The increase in the core-level binding energy in small particles Poor screening of the core charge The size-induced metal-nonmetal transition in nanocrystals 59 Electrical Conductivity Particle size Bulk value Relativeconductivity 60 Photoelectron spectra of Hg clusters of nuclearity n The 6p peak moves gradually towards the Fermi level The band gap shrinks with increase in cluster size 6s HOMO 6p LUMO Hg Valence electron configuration? [Xe] 4f14 5d10 6s2 61 a) Absorption spectra of CdSe nanocrystals (at 10 K) of various diameters b) Wavelength of the absorption threshold and band gap as a function of the particle diameter for various semiconductors. The energy gap in the bulk state in parenthesis Quantum Size Effects In Semiconductors 62 Quantum Confinement Effects Fluorescence of CdSe–CdS core–shell nanoparticles with a diameter of 1.7 nm (blue) up to 6 nm (red) Smaller particles have a wider band gap 63 Bohr Radii Quantum confinement - particles must be smaller than the Bohr radius rB of the electron-hole pair (exciton) rB = the spatial separation of the electron-hole pair Quantum Confinement Effects 64 Two regimes - strong and weak The strong QC effect the size of the crystal is reduced to much smaller than the Bohr radius for the material (≈ 3 nm for WO3) This causes direct changes to the electron wavefunctions and hence significantly alters the Eg The weak QC effect the crystal size is larger than the Bohr radius This causes indirect perturbation of the electron wavefunction due to Coulomb effects and results in more subtle changes in the bandgap energy Quantum Confinement Effects 65 WO3 films by radio frequency (RF) sputtering The crystallite size is controlled by the substrate temperature (Ts) during deposition The crystallite sizes obtained ranged from 9 nm (100 °C) to 50 nm (500 °C) The reduction in Ts, and hence reduction in crystallite size, results in a blue shift of the transmission spectrum corresponding to a widening of Eg 66 Quantum Confinement Effects Optical properties nc-TiO2 is transparent - applications? Blue shift in optical spectra of TiO2 nanoparticles Blue shift Single-Electron Effects 67 Capacitance C - the ratio of the amount of electric charge stored on a conductor to a difference in electric potential (wrt ground) C = U / q Farad The energy E needed to add a charge (electron) to an isolated structure (e.g., QD) is given by the charge q divided by twice the capacitance C If E is comparable to the thermal energy kBT (26 meV at room T) - no effect on the operation of a device - thermal noise (cool to 4 K) The capacitance C of a structure is dependent on the size Semiconductors - when size is less than about 20 nm, charging by a single charge will be clearly distinguishable above the background thermal energy at room temperature Single-electron devices • Single-electron transistors • Single-electron memories Metastable Crystal Phases 68 High-pressure crystal phases that are normally unstable at atmospheric pressure can be obtained in nanocrystalline form and are stable under normal atmospheric conditions Semiconductors Gr. 13/15 Four-coordinate cubic (sphalerite) or hexagonal (wurtzite) phases High pressure - six-coordinate (rocksalt) phase CN = 4 CN = 6 CdS – wurzite at normal pressure, rock salt above 3 GPa But: Rocksalt CdS stable at normal pressures for 2–100 nm crystals Metastable Crystal Phases 69 When reducing the crystal size, phase transitions occur: BaTiO3 / PbTiO3 cubic  tetragonal Co hcp  fcc Ti hcp  bcc TiO2 rutile  anatase CdSe / CdS wurtzite  rock salt ZrO2-Y2O3 tetragonal  monoclinic • a lack of nucleation sites • the Gibbs–Thomson effect (i.e., an enhanced internal pressure as a result of high surface/interface curvature) • surface-energy differences between allotropic phases ZrO2-Y2O3 Decrease of tetragonal  monoclinic phase-transition temperature with reducing the crystal size of YSZ Metastable Crystal Phases 70 Ni fcc in bulk, stable to mp 1726 K, no phase transitions up to 65 GPa, ferromagnetic Ni bcc metastable, a film by rf-magnetron sputtering on a GaAs(100) substrate, above critical thickness 5 nm transforms to fcc Ni hcp - A film by pulse plasma evaporation at 2 x I0-6 Torr, crystallites 25 nm, transform to fcc at 250 C - A film by UHV evaporation on (001) MgO, hcp Ni islands 2.5 nm thick transform to fcc when lateral size larger than 5 nm Metastable Crystal Phases 71 Thermodynamic factors Surface-tension effects increase with decreasing crystal size, a compressive force (at atmospheric pressure) favor the high-pressure phase, the crystals try to adopt the phase with the lowest surface energy (each crystal face having a different surface energy) Kinetic factors Phase transitions tend to be initiated at defects, small nanocrystals are often defect-free, a phase transition, which would occur thermodynamically, will be kinetically hindered Metastable Crystal Phases 72 Thermodynamic factors Scattering/Interference of Light 73 Scattering of light in random or ordered nanostructures resulting in stimulated emission (lasing) The grain boundaries between nanocrystals - to confine excitons in the nanocrystals because of potential barriers at the grain boundaries - form cavity mirrors, which are caused by changes in the refractive index at grain boundaries Constructive interference in random ‘laser-ring cavities’ (closed scattering circuits between elementary particles in a secondary particle) Depletion of Charge 74 Charge depletion - removal of free electrons, based on the screening length for electrons in a material The electric fields of ions in conducting solids are reduced by the cloud of conduction electrons Dye-sensitized solar cell (DSSC) An absorber (an organometallic dye, a semiconductor) adsorbed onto a porous, high-bandgap semiconductor (TiO2), insulating, injection of electrons from the photoexcited absorber renders it conductive Electron transport takes place in the porous oxide primarily by diffusion the size of the porous oxide particles 20 nm • The high surface area of the oxide allows efficient absorption of light by even a monomolecular layer of dye, resulting in efficient electron transfer from the excited dye to the oxide • Small particle size allows complete depletion of electrons from the individual particles by the electrolyte Impurity (Dopants) Exclusion 75 Doped bulk semiconductor - averaged throughout a macroscopic volume – typical doping density 1018 cm–3 Nanoparticles - often zero impurity and defect concentrations Spherical nanoparticles of 5 nm size 1 nanoparticle with a doping density of 1018 cm–3 - less than one dopant atom on average!! The energy of the dopant in NP is larger than in a bulk pieces of the semiconductor Thermodynamically unfavorable for the dopant to remain in a NP Excluded from the semiconductor NP volume onto the particle surface surface segregation Does not apply to a solid solution, e.g., (Cd, Zn)S Defect Exclusion 76 Bulk semiconductors – can only minimize defects Defect formation is thermodynamically unfavorable for very small crystals (the upper limit from several nm to several tens of nm) Very small nanocrystals are often structurally perfect Ballistic Transport 77 Ballistic transport of electrons - transport over distances with no scattering of the electrons (distances less than the mean free electron path) Nanotubes and nanowires - the mean free path hundreds of nm greater than the dimension of devices made with these nanotubes/wires Advantages of ballistic electron transport - less energy loss (as heating) - more speed Rectification Logic gating Superparamagnetic Effect 78 The energy required to change the direction of magnetization of small magnetic nanoparticles is ≤ kT, causing random reversal of magnetization The mean critical domain diameters Elemental Fe 10–20 nm Magnetite 110–130 nm Nanoparticles with dimensions less than the mean critical domain diameter have only one magnetic domain per particle Above the blocking temperature, the magnetization of the nanoparticles thus follows the external field (negligible retentivity and coercivity) The behavior corresponds to a paramagnetic atom, but with an absolute magnetization a factor of 104 larger = “Superparamagnetism” Thermomagnetic Curves 79 M-T (a) and M−1-T (b) curves of Ni nanoparticles The horizontal axis intercept of M−1-T curve is the Curie temperature TC of ferromagnetic materials The Curie Temperature 80 The temperature at which a ferromagnetic element starts to lose its magnetism and becomes paramagnetic Fe 768 °C Co 1115 °C Ni 362 °C (631 K) Gd 19 °C Tc(D) = the Curie temp. of ferromagnetic nanomaterial with the average size D Tc0 = the Curie temp. of bulk Svib = the vibrational part of the overall melting entropy Sm (Sm  Svib) R = the ideal gas constant h = the atomic diameter Ni NPs The Curie Temperature 81 TCn = the Curie temp. of ferromagnetic nanomaterial with the average size D TCb = the Curie temp. of bulk n = number of atoms in a nanoparticle  = the shape factor (0.806 sphere) C = number of atoms in a unit cell (4 for fcc) k = ratio b/w atomic radius and cell parameter (2/4 for fcc) Ni NPs Size Dependence of Magnetic Parameters 82 Saturation magnetization MS, remanent magnetization Mr, and coercivity HC of spherical Ni nanoparticles Room-temperature hysteresis loops of Ni nanoparticles The Saturation Magnetization 83 Iron oxide Fe3O4 nanoparticles hysteresis loops Mass magnetization values at 1.5 T The saturation magnetization Ms decreases with decreasing particle size (increasing surface area) As the surface-to-volume ratio increases with decreasing particle size, the magnetically dead layer fraction increases Size Dependence of Magnetic Parameters 84 Saturation magnetization MS, remanent magnetization Mr, and coercivity HC of spherical Ni nanoparticles Dictionary of Used Terms 85 Transmission Electron Microscopy = prozařovací EM Scanning Electron Microscopy = rastrovací /řádkovací EM