DSc Irina Hussainova Department of Materials Engineering Tallinn University of Technology

Transcription

1 Introduction to Nanoscience DSc Irina Hussainova Department of Materials Engineering Tallinn University of Technology

2 Loeng1 Nanomaterjalid

3 How small are nanostructures? The diameter of a carbon nanotube is yet another 10 4 times smaller, i.e. 3 nm=3 x 10 9 m. The width of a hair 30 µm = 3 x 10 5 m

4 How big is nanometer? NANO 4

5 What is at the bottom? 5

6 Why is nanoscience attracting so much interest? The fundamental properties of matter change at the nanoscale. The properties of atoms and molecules are not governed by the same physical laws as larger objects, but by quantum mechanics. 6

7 What s interesting about nanoscale? The physical and chemical properties of nanoparticles can be quite different from those of larger particles of the same substance. Altered properties can include but are not limited to colour, solubility, material strength, electrical conductivity, magnetic behavior, mobility (within the environment and within the human body), chemical reactivity and biological activity. 7

8 What is nanoscale science? The study of objects and phenomena at a very small scale, roughly 1 to 100 nanometers (nm) Nanoscience is the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales, where properties differ significantly from those at a larger scale; and nanotechnologies is the design, characterisation, production and application of structures, devices and systems by controlling shape and size at the nanometer scale. 8

9 Nanoscience nanoscience 9

10 What is inside? Molecule Atom Matter Molecule Atom Nucleus Baryon (Hadron) Quark u 10-2 m 10-9 m m m m <10-19 m Condensed Matter/Nanoscience/Chemistry matter/nano-science/chemistry Nanoscience Atomic Physics Nuclear Physics protons, neutrons, mesons, etc. π,ω,λ... Electron (Lepton) <10-18 m top, bottom, charm, strange, up, down High Energy Physics 10

11 2001: A Nanotechnology Odyssey 11

12 Historical background The first planar integrated circuit invented in No exponential is forever.but we can delay FOREVER, G. Moore, 2003 Integrated Circuits: transistors 4004 computer; transistors transistors transistors 386 Processor transistors transistors Pentium transistors Pentium IV 12

13 Where are nanoworld boundaries? At some point, the laws of physics will make it impossible to keep downsizing microelectronics at this rate. WHY? Decrease in size results in the particles physical chemical properties changing and, consequently, the properties of nano-materials are changed dramatically and sometime cordially. 13

14 Size-Dependent Properties At the nanometer scale, properties become sizedependent. For example, (1) Chemical properties reactivity, catalysis (2) Thermal properties melting temperature (3) Mechanical properties adhesion, capillary forces (4) Optical properties absorption and scattering of light (5) Electrical properties tunneling current (6) Magnetic properties superparamagnetic effect New properties enable new applications 14

15 What is a color of gold? Size-dependent color of gold Absorption peak broadens and shifts to longer wavelengths. Reflection, leading to scattering, is weak at small sizes and increases when > 50 nm. 100 nm gold particles λabs = 575 nm Color = purple-pink 20 nm gold particles λabs = 521 nm Color = red 1 nm gold particles λabs = 420 nm Color = brown-yellow 15

16 In the quantum world, the rules are different. The classical world Impenetrable barrier The quantum world Tunnel effect 16

17 Scale Changes Everything There are enormous scale differences in our universe! At different scales Different forces dominate Different models better explain phenomena Four important ways in which nanoscale materials may differ from macroscale materials Gravitational forces become negligible and electromagnetic forces dominate Quantum mechanics is the model used to describe motion and energy instead of the classical mechanics model Greater surface area to volume ratios Random molecular motion becomes more important 17

18 Dominance of electromagnetic forces Because the mass of nanoscale objects is so small, gravity becomes negligible Gravitational force is a function of mass and distance and is weak between (low-mass) nanosized particles Electromagnetic force as a function of charge and distance is not affected by mass, so it can be very strong even when we have nanosized particles The electromagnetic force between two protons is times stronger than the gravitational force! Sources:

19 Extended internal surface 6 x 1m 2 6m 2 6 x (1/2m) 2 x 8 12m 2 6 x (1/3m) 2 x 27 18m 2 Surface to Volume Ratio Increases A greater amount of a substance comes in contact with surrounding material This results in better catalysts, since a greater proportion of the material is exposed for potential reaction 19

20 Surface means a lot Since reactions occur at the interface of two substances, when a large percentage of the particles are located on the surface, we get maximum exposed surface area, which means maximum reactivity! So nanosized groups of particles can make great catalysts. 20

21 Melting temperature Melting Point (Microscopic Definition) Temperature at which the atoms, ions, or molecules in a substance have enough energy to overcome the intermolecular forces that hold the them in a fixed position in a solid At macroscopic length scales, the melting temperature of materials is size-independent. For example, an ice cube and a glacier both melt at the same temperature. 21

22 Thermal properties Nanocrystal size decreases In contact with 3 atoms surface energy increases melting point decreases In contact with 7 atoms Surface atoms require less energy to move because they are in contact with fewer atoms of the substance Example: 3 nm CdSe nanocrystal melts at 700 K compared to bulk CdSe at 1678 K 22

23 Quantum Effects Large ZnO particles Block UV light Scatter visible light Appear white Nanosized ZnO particles Block UV light So small compared to the wavelength of visible light that they don t scatter it Appear clear The following are among the most important things that quantum mechanical models can describe (but classical models cannot): Discreteness of energy The wave-particle duality of light and matter Quantum tunneling Uncertainty of measurement Sources: owders.com/images/zno/im_zinc_oxide_particles.jpg

24 Discreteness of energy It is the fact that electrons can only exist at discrete energy levels that prevents them from spiraling into the nucleus, as classical models predict. 24

25 Historical milestones In 1901, Max Planck published an analysis that succeeded in reproducing the observed spectrum of light emitted by a glowing object. To accomplish this, Planck had to make mathematical assumption of quantized energy of the oscillators (atoms of the blackbody) that emit radiation. Einstein later proposed that it is the electromagnetic radiation itself that is quantized, and not the energy of radiating atoms. In 1905, Albert Einstein provided an explanation of the photoelectric effect, a hitherto troubling experiment that the wave theory of light seemed incapable of explaining. He did so by postulating the existence of photons, quanta of light energy with particulate qualities. 25

26 Basic of quantum mechanics Einstein postulated that the electrons can receive energy from electromagnetic field only in discrete portions (quanta that were called photons): an amount of energy E related to the frequency, f, of the light by E = h f where h is Planck's constant ( J seconds). 26

27 The Wave-Particle duality of light and matter A central concept in quantum physics is the particle-wave duality, the fact that fundamental objects in the physical world, electrons, protons, neutrons, photons and others, all have the same dual nature: they are at the same time both particles and waves. In some situations the particle aspect may be the dominant feature, in other vice versa; but the behavior of any given object can never be understood fully by ascribing only one of these aspects to it. Particles have wave-like properties and vice versa: Electrons in atoms are standing waves Electrons beams can be diffracted 27

28 Schrödinger Equation The Schrodinger equation is a differential equation that describes the time evolution of Kinetic energy of a free particle For a particle moving in a potential V(x,t) 28

29 Quantum tunneling A nanoscopic phenomenon in which a particle violates the principles of classical mechanics by penetrating a potential barrier or impedance higher than the kinetic energy of the particle. Electron tunneling is attained when a particle with lower energy is able to exist on the other side of an energy barrier with higher potential energy.

30 Go through the wall Tunneling is the penetration of an electron into a classically forbidden region. A barrier, in terms of quantum tunneling, may be a form of energy state analogous to a "hill" or incline in classical mechanics, which classically suggests that passage through or over such a barrier would be impossible without sufficient energy. 30

31 The principal of quantum tunneling Electrons exhibit wave behavior and their position is presented by a wave (probability) function. The wave function represents a finite probability of finding an electron on the other side of the potential barrier. Since the electron does not posses enough kinetic energy to overcome the potential barrier, the only way the electron can appear on the other side is by tunneling through the barrier.

32 Uncertainty of measurement Heisenberg's uncertainty principle In his work on formulating quantum mechanics, Werner Heisenberg postulated his uncertainty principle, which states: where here indicates standard deviation, a measure of spread or uncertainty; x and p are a particle's position and linear momentum respectively. ħ is the reduced Planck's constant (Planck's constant/ 2π). Heisenberg originally explained this as a consequence of the process of measuring: Measuring position accurately would disturb momentum and viceversa 32

33 How can we see nano-objects? Optical probe characterization techniques Electron probe characterization techniques Scanning probe characterization techniques Spectroscopic characterization methods Ion particle characterization techniques Thermodynamic characterization methods Bulk engineering characterization methods 33

34 What kinds of interactions? The electron is a subatomic particle carrying a negative electric charge. In general, electrons are able to penetrate solid materials to a depth proportional to E 2 34

35 How does electron interact with matter? Electron specimen interactions 35

36 The method that is needed is determined by the question to be solved: Structure (High-Resolution) Transmission Electron Microscopy Scanning Transmission Electron Microscopy(STEM) Electron diffraction (ED) Composition Energy-dispersive X-ray spectroscopy (EDXS) Electron Energy Loss Spectroscopy (EELS) Morphology Scanning Electron Microscopy (SEM) Elemental mapping Electron Spectroscopic Imaging (ESI) STEM + X-ray spectroscopy / EELS SEM + X-ray spectroscopy 36

37 Scanning probe microscopy In 1981, Gerd Binnig and Heinrich Rohrer, two IBM scientists working in Zurich, Switzerland, invented the first scanning tunneling microscope (STM). They were awarded the Nobel Prize in physics for this work, which gave birth to the development of a new family of microscopes known as scanning probe microscopes (SPM). The STM works best with conducting materials, but it is also possible to fix organic molecules on a surface and study their structures. For example, this technique has been used in the study of DNA molecules. 37

38 SPM principal tip tunneling electrons sample This scanning probe microscope, of which there are several varieties, differs from the optical and electron microscopes in that neither light nor electrons is used to form an image. The study of surfaces is an important part of physics, with particular applications in semiconductor physics and microelectronics. In chemistry, surface reactions also play an important part, for example in catalysis. 38

39 Principles of AFM Monitors the forces of attraction and repulsion between a probe and a sample surface The tip is attached to a cantilever which moves up and down in response to forces of attraction or repulsion with the sample surface Movement of the cantilever is detected by a laser and photodetector STM AFM Shading shows interaction strength 39

40 Carbon Carbon is a basic element of life Carbon is special because of its ability to bond to many elements in many different ways It is the sixth most abundant element in the universe The most known types of carbon materials: diamond; graphite; fullerenes; and carbon nanotubes 40

41 Carbon materials 2s and 2p electrons available for bonding Diamond and graphite are two allotropes of carbon: pure forms of the same element that differ in structure. 41

42 DIAMOND - chemical bonding is purely covalent - highly symmetrical unit cell - extremely hard - low electrical conductivity - high thermal conductivity (superior) - optically transparent - used as gemstones and industrial grinding, machining and cutting 42

43 GRAPHITE Layered structure with strong bonding within the planar layers and weak, van der Waals bonding between layers Easy interplanar cleavage, applications as a lubricant and for writing (pencils) Good electrical conductor Chemically stable even at high temperatures excellent thermal shock resistance Applications: Commonly used as heating elements (in non- oxidizing atmospheres), metallurgical crucibles, casting molds, electrical contacts, brushes and resistors, high temperature refractories, welding electrodes, air purification systems, etc. 43

44 Graphite Graphite is a layered compound. In each layer, the carbon atoms are arranged in a hexagonal lattice with separation of nm, and the distance between planes is nm The acoustic and thermal properties of graphite are highly anisotropic, since phonons propagate very quickly along the tightly-bound planes, but are slower to travel from one plane to another. 44

45 Graphene Graphene is a one-atom-thick planar sheet of sp 2 -bonded carbon atoms that are densely packed in a honeycomb crystal lattice. It can be viewed as an atomic-scale chicken wire made of carbon atoms and their bonds The carbon-carbon bond length in graphene is about nm. Graphene is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes.

46 Allotropes of carbon a) diamond b) graphite c) lonsdaleite (hexagonal diamond) d) - f) fullerenes (C60, C540, C70); g) amorphous carbon h) carbon nanotube 46 Wikipedia

47 What are fullerenes? The American pavilion in the Expo'67 in Montreal was designed by architect R. Buckminster Fuller. Fullerenes were accidentally discovered1985 because of strange results in mass spectra of evaporated carbon. Robert Curl, Harry Kroto, and Richard Smalley discovered C60 and C70. (Nobel prize in 1996.) Spherical cluster of 60 carbon atoms Spherical cluster of 70 carbon atoms 47

48 Fullerenes The fullerene is the reference to a family of carbon allotropes, molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, tube, or plane. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes. Graphene is an example of a planar fullerene sheet. 48

49 Geometry of fullerenes Arranged in pentagons and hexagons: each molecule composed of group of carbon atoms that are bonded to one another to form both hexagon (six carbon atoms) and pentagons (five atoms) geometrical configuration. There are 20 hexagons and 12 pentagons, which are arranged such that no two pentagons share a common side => Similar to soccer ball 49

50 Properties of fullerenes The diameter of a C 60 molecule is about 1 (nm). The nucleus to nucleus diameter of a C 60 molecule is about 0.7 nm. Resistant to high temperatures. May possess semiconductor properties. Superconducting at low temperatures. Extremely stable molecule Highest tensile strength of any known structure or element, including diamonds which have the highest tensile strength of all known 3D structures Also has the highest packing density of all known structures (including diamonds) Impenetrable to all elements under normal circumstances, even to a helium atom with an energy of 5eV. Resistant to pressure and reclaim their original shape even after experiencing very high pressure. 50

51 Variations of buckyballs The smallest fullerene C 20 dodecahedral graph The number of fullerenes C 2n grows with increasing n = 12,13,14..., roughly in proportion to n 9. For instance, there are 1812 non-isomorphic fullerenes C fullerene graph fullerene (truncated icosahedral graph) 70-fullerene graph

52 Fullerene colors The fullerenes C60 and C70 may be dispersed in water. 52

53 Modification: Endohedralfullerenes Endohedralfullerenes are fullerenes that have additional atoms, ions, or clusters enclosed within their inner spheres. Enclose a metal molecule into the structure: Fullerenes with radioactive metals encapsulated inside the cage. Example: Radioactive Holmium inside a C82 cage. Ho@C82 Endohedralfullerenes are characterised by the fact that electrons will transfer from the metal atom to the fullerene cage and that the metal atom takes a position offcenter in the cage. 53

54 One of the most fascinating and unique feature of fullerenes is that there is spherical empty space inside the carbon cage. This hollow space, ranging from 0.4 to 1.0 nm in diameter on going from C 60 to C 240 considering the van der Waals radius of carbon (0.17 nm), is nanometer-scale void and the volume may be varied with the size of fullerene. Such a characteristic of fullerene implies intuitively an idea of stuffing atoms into its empty space so as to alter the molecular and solid state properties of the fullerenes, resulting in the formation of a brand-new family endohedral fullerenes. 54

55 Carbon nanotubes -discovery In 1991, Sumio Iijima discovers multiwalled nanotubes (MWNT) using the method of Krätschmer and Huffman. In 1993, Donald Bethune makes single-walled (SWNT) nanotubes by adding transition metals. 55

56 Carbon nanotubes Carbon nanotubes(cnts) are allotropes of carbon with a cylindrical nanostructure. Most single-walled nanotubes (SWNT) have a diameter of close to 1nm, with a tube length that can be many millions of times longer. Structure is a single sheet graphite rolled into a tube. 56

57 Zigzag SWCN 57

58 Ways to roll a carbon sheet Sketch of three different SWNT structures as examples for (a) a zig-zag-type nanotube, (b) an armchair type nanotube, (c) a helical nanotube 58

59 Structure of SWNT All carbon atoms are involved in hexagonal rings only and are therefore in equivalent position, except at the nanotube tips where 6 5 = 30 atoms at each tip are involved in pentagonal rings. length diameter >

60 Properties of nanotubes Carbon nanotubes are long meshed wires of carbon Longest tubes up to 1mm long and few nanometers thick made by IBM. Property Carbon Nanotubes Comparatively Size nm in diameter Si wires at least 50nm thick Tensile Strength 150 GPa Steel alloys 0.4 GPa Resilience Bent and straightened without damage Modulus elasticity Conductiv ity Cost Metals fracture when bent and restraightened 1054 GPa Steel 208 GPa Epoxy 3.5 GPa Estimated at 10 9 A/cm 2 Cu wires burn at 10 6 A/cm 2 $2500/gram by BuckyUSA in Houston 60 Gold is $15/gram

61 Nanotubekey properties A broad range of electrical, thermal, and structural properties depending on the tube diameter, length, and chirality, or twist. Diameter : 1 30 nm Length : < 1 mm Semiconducting like silicon or a 1000 times better electrical conductors than copper. Transport heat twice as good as diamond. Tensile strength 20 times that of steel and still flexible. 61

62 Overview of NT properties 62

63 Mechanical properties of NT The tensile strength of SWNT is 20 times that of steel. It is even higher for MWNT. 63

64 Scale effect on Young's modulus 64

65 Why nanotubes are the perfect creations Superior stiffness and strength to all other materials Extraordinary electric properties Reported to be thermally stable in a vacuum up to 2800 degrees Centigrade (and we fret over CPU temps over 50 o C) Capacity to carry an electric current 1000 times better than copper wires Twice the thermal conductivity of diamonds Pressing or stretching nanotubes can change their electrical properties by changing the quantum states of the electrons in the carbon bonds They are either conducting or semi-conducting depending on the their structure 65

66 Metal or semiconductor? Electrons in carbon nanotubes can only be at the certain energy levels. A nanotube is metallic if the energy level that allows delocalized electrons to flow between atoms throughout nanotube is right above the energy level used by electrons attached to atoms. Analogy: energy bands in atoms. The differences in conducting properties are caused by the molecular structure that results in a different band structure and thus a different band gap. The differences in conductivity can be derived from the graphene sheet properties. These single crystal structures can exhibit either semiconducting or metallic behavior depending only on the diameter and angle of lattice! 66

67 Perspectives 67