High Axial Ratio Nanostructures Dimension-Properties Interplay Carbon allotropes Brilliant, Transparent Mohs Hardness 10 20 W/cmK High Melting point Metallic lusture Opaque Black, Fibrous 1-2 1-1.2 25 6000 Lubricant Unusual Electrical Behaviour Black Shiny Crystals Superconductor (10-40 K) 2 Role of Dimensionality 3 Role of Dimensionality 2 r 3 D: E = 2D: E = 2m 2m n 2 ,2 K 1 D: E = 2m í OD: E = 2m í 7t L \ J n «,=1,2,3.. ly,^ =1,2,3 n^nj^ =1,2,3 4 ID Nanostructures Poly {ethylene oxide) Collagen Fibrils 5 Potential of Nanowires Electron Transport 'Nano-cables' Core-shell Superlattice AFM & STM Tips Surface Modification Potential applications Interconnects Novel Probes Multifunctional Hierarchical alignment Building blocks for devices 6 Effect of Confinement 6 1 10 100 1000 10000 100000 Diameter (A) The band gap increases with decreasing diameter (quantum confinement) Carbon Nanotubes • (Re)discovered by Iijima (1991, NEC) • 1952 Russians • Rolled up sheet of graphene • Capped at the ends with half a fullerene Carbon Nanotubes Single Walled Nanotube ( SWNT) • Single atomic layer wall • Diameter of 0.7 - 5 nm • Length several microns to centimeters Double Walled Nanotube ( SWNT) • exactly two concentric CNT • the outer wall selectively functionalized while maintaining an intact inner-tube Multi Walled Nanotube ( MWNT) • Concentric tubes ca. 50 in number, separation 0.34 nm • Inner diameters : 1.5-15 nm • Outer diameters : 2.5 - 150 nm Lengths: micrometers to centimeters Aspect Ratio: up to 107 SWCNT diameter from Raman spectroscopy • RBM (Radial Breathing Mode): 100 to 300 cm-1, vibration at which the nanotube diameter contracts and expands • D-band: vicinity of 1350 cm-1, defect-derived peak • G-band: vicinity of 1550 -1605 cm-1, in-plane vibration of graphite • G-band: 2700 cm-1, overtone of D-band 1.25 1 0 Instrument HoloLab Serie S5D00 .........7 1 I 184 cm i Itl^t Vtlt 00 ■j=-»0 ▼ RBM \ 1 X. CO J r-n -i—1 0 250 500 750 1000 1250 1500 1750 2000 1 (532nm, x50,10sec) 1/cm The wavenumber of RBM is inversely proportional to the tube diameter D D (nm) = 248/(0 = 248/184 = 1.3 (nm) 10 CNTs: Properties and Potential Electronic: Bandgap Eg~ 1/d Ballistic conductivity in metallic CNTs, the highest current density 109 A/cm2 (Cu only 106 A/cm2) SWNT - metallic or semiconducting, MWCNT - metallic Magnetic: Anisotropic magn. susceptibility X-L>> Xll Mechanical: Young's Modulus 1.8 TPa (SWNT, axial), 0.95 TPa (MWNT) (Steel: 230 GPa) tensile strength above 100 GPa (steel: 1-2 GPa) the highest known Thermal: Conductivity theor. 6600 W/m K axial, 1.5 perpendicular , 3500 experim. (Diamond 3000, Cu 400 W/m K) 300 W/m K bulk SWCNTs, 3000 W/m K individual MWCNTs Thermal stability 650 °C (SW)-800 °C (MW) in air, 2800 °C in Ar (anealing to graphitize defects), 320 °C with metal oxides on the surface - O vacancies, Mars-van Krevelen catatlytic mechanism Synthetic routes to CNT • DC arc discharge: MWCNTs and SWCNTs (with catalyst), easy design, few structural defects, short tubes, low yield, low purity, random diameters • Laser ablation: primarily SWCNTs few defects, good control over diameter, most costly method, poor scalability, requires Class 4 lasers • Molten salt: primarily MWCNTs simple process, used for filling CNTs, low yield and crystallinity, poor controllability • Chemical vapor deposition: both types, high yields, easy scalability, long tubes, alignment and pattern growth, some defects, medium purity Synthetic routes to CNT DC Arc discharge NTs observed in carbon soot of graphite electrodes during arc discharge (during production of fullerenes) The most used method of synthesis in early 1990's Carbon (+catalyst) contained in negative electrode sublimes thanks to high temperatures of the electric discharge Yield up to 30 %wt, produces both SWNTs, MWNTs Length up to 50 urn, few structural defects Deposition Cathode Inert Gas - Anode + 13 Synthetic routes to CNT Laser ablation Pulsed laser vaporizes graphite target in a high-temperature reactor filled with inert gas (650 mbar, Ar, N2) CNTs develop on the cooler surfaces of reactor as the carbon condenses Pure graphite - MWNTs Graphite + metal catalyst particles (Co + Ni) - SWNTs Yield up to 70%wt, few defects Controllable diameter of SWNTs by changing p, T More expensive than arc discharge, CVD Synthetic routes to CNT Molten salt LiCl, LiBr, 600 °C, graphite electrodes Cathode exfoliates and graphite sheet wraps MWCNTs Yield up to 30%wt, low purity Large number of defects, amorphous carbon impurity, salt encapsulating Synthetic routes to CNT CVD (Chemical Vapor Deposition) Substrate + metal catalyst particles (cobalt, nickel, iron) Distribution of metal catalyst and the size of the particles influence the diameter of NTs Patterned (or masked) deposition of metal, annealing, plasma etching Substrate is heated Two gasses are bled into the reactor - process gas (ammonia, nitrogen, hydrogen) and carbon-containing gas (acetylene, methane, ethylene) Carbon-containing gas is broken apart at the surface of the metal catalyst particle, carbon is transported to the edges of the particle, where it forms the NT Catalyst is removed by acid treatment Resulting NTs are randomly oriented 16 Synthetic routes to CNT CVD (Chemical Vapor Deposition) Plasma Enhanced CVD Plasma is generated by the application of strong electric field during growth Growing NTs follow the direction of the electric field With the correct use of reactor geometry, vertically aligned (perpendicular to substrate) NTs can be grown CDV shows the best promise for industrial manufacturing of CNTs Better price/unit ratio NTs grown on desired substrates Synthetic routes to CNT Super-growth CVD New methods of CVD using different substrates, catalysts Activity and lifetime of catalyst can be enhanced by adding water into the reactor Growing CNTs then form „forests" up to several mm high, aligned normaly Improved efficiency, reaction time and purity of CNTs (more than 99,9%) Hata, K.; Futaba, DN; Mizuno, K; Namai, T; Yumura, M; Iijima, S (2004). "Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes". Science 306 (5700): 1362-1365. doi: 10.1126/science. 1104962. PMID 15550668 Synthetic routes to CNT ■*-K ^-*■ }-2 cwn _ 2-25 nm _ Defect-free (n,m) SWNTs with open ends A bundle of (10,10) nanotubes held together with strong 7i-7i-stacking interactions 20 Defect-free (n,m) SWNTs with open ends Chiral vector (n,m) «999999»^^ ****** j***^*** a * J * J * J * J * J * J * J L **j*j*j*j*j*j*j*J******J^JjJ^J^J^ :i >*************« [*-************** .* ^ ******************* *~* ********* MHMH ***J" HBBBSfa ************ * ^* ^* ***** ** *** £999999******* *** J s* ***************** ^*^*^* ************ *********** ******** SKSKSKSKKS E999B999999I fth armchair ******** *^^^*~^~^~J-J-i-i-,i-***** j n n ra ra n ra ra ^* « ift lft # # # ^ ^ ^ ^ # # ****** ******* (n,n) armchair (n,0) zigzag (n,m) chiral TEM of a chiral CNT 21 Defect-free (n,m) SWNTs with open ends A) Armchair - an achiral metallic conducting (10,10) tube B) Chiral - semiconducting (12,7) tube C) Zigzag - an achiral conducting (15,0) tube All the (n,n) armchair tubes are metallic Chiral or zigzag tubes are metallic only if (n-m)/3 is a whole number, otherwise, they are semiconductors Roll-up of (n,m) SWNTs A 2D graphite layer the lattice vectors ax and a2 The roll-up vector Ch= na1+ma2 Achiral tubes exhibit roll-up vectors derived from (n,0) (zigzag) or (n,n) (armchair). The translation vector T is parallel to the tube axis and defines the ID unit cell. The rectangle represents an unrolled unit cell, defined by T and Ch (n,m) = (4,2) 23 Roll-up of (n,m) SWNTs Chiral vector: Ch= nax + ma 2 i . ;•• • ■■■■■ • ~~ ;;;.rv ' : i.:::; i f 1- tiiiii si Roll-up of (n,m) SWNTs ( and 0 < m < n ) iTube diameter 4 = C h a o /72 +nm + m2 ) = a0 = 0.249 mn ť9 = tan1 \J3mj{m + 2n) a = 1.42 V3=2.49Á ň = 0-^0° d(Csp2-Csp2) = 1.42 Á 26 Defects in SWNTs Atomic vacancies - reduction of tensile strength, electrical and thermal conductivity Topological (Stone Wales) defect - rearrangement of bonds into pentagonic and heptagonic pair (connected, no other types of rings known) Defects lead to phonon scattering - increased phonon relaxation rate -reduction of mean free path (reduction of ballistic conductivity) leads to reduced thermal conductivity 27 Defects in SWNTs Separation of CNTs Semiconducting CNTs - Separation by surfactants, (octadecylamine), a strong affinity Metallic CNTs - Separation by diazonium reagents, biomolecules, DNA - AC dielectrophoresis - 10 MHz, induced dipole, causes the two types of CNTs to migrate along the electric field gradient in opposite directions 29 Doping of CNTs Intercalation CNTs - Between walls of MWCNT - during synthesis or posttreatment On-wall substitution CNTs - N or B substitute for C - In-situ - element-containing precursor - Ex-situ - removal of C atom - graphite (n) or pyridine (n or p) type of group 30 Functionalization possibilities for SWNTs A) defect-group functionalization B) covalent sidewall functionalization C) noncovalent exohedral functionalization with surfactants -wrapping D) noncovalent exohedral functionalization with polymers E) Endohedral functionalization with C60 (C60@CNT, "peapods) 31 Functionalization possibilities for SWNTs 4000 3500 3000 2500 2000 1500 1000 50( Wawenumbers {cm1) Functionalization possibilities for CNTs reactions will occur first at the end caps, then on the surface, at structural defects 33 Functionalization possibilities for CNTs Ti02 and Si02 on acid-treated CNTs via ALD SEM image for the case of Si02 TEM image of vertically grown CNT coated with Ru02 both outside and inside. 34 Interconnect IBM Transistor I-.IllL-^l- 7r 3 iE-ti BE _l_■_I_I_I_I_I_Ú_■_I_I_■_I_I_I_I_I_■_I_I_I_h_I_l_ Bio-Sensor ivy JM *m ........ Chemical Sensor 35 Assembly of CNTs CNT applications: Ultra-hard Composites Nanopipettes Field Emission Transistor Nanomanipulator CNT applications CNTs as photosensitizers: (a) electron injection into the conduction band of Ti02 (b) electron back-transfer to CNTs with the formation of a hole in the valence band of Ti02 and reduction of the hole by oxidation of adsorbed OH" species OH* 37 Reduction 38 Ballistic vs. diffusive ttansporl Metallic CNTs Carbon Nanotubes Difficult to obtain in pure form (SWNT, MWNT, Cx, soot etc.) As-synthesized CNTs are a mixture of conducting, semiconducting and insulating ones Not stable under oxidizing conditions Little manufacturing control over tube diameter Nanowires Good transport properties - Single crystalline nature Mechanically robust - Defect free Flexibility in composition Doping possible to create p- and n-type nanowires Nanowires-based FETs and basic logic circuits demonstratedin the laboratory. Techniques for mass manufacture 41 Transport in Nanowires Conductance Quantization: The Landauer equation G = (2e2/h)N, N = no. of conduction channels When NW diameter is smaller than the Fermi wavelength, conductance changes in steps of 2e2/h 42 Synthetic Routes to Nanowires Epitaxial growth Catalytic VLS growth Catalytic base growth Defect nucleation Templated growth Arrested growth Assembly of nanoparticles 43 Epitaxial growth Active surface Masked surface Vapor-Liquid-Solid (VLS) Growth Metal catalyst nanoparticle (Au) Feed another element (Ge vapor, GeH4 1400- i i i i AuSi ——--- : or SiH4) _ 1200-_g 0 S iooo-u / AuGe__I Gaseous precursor feedstock is 3 g g, 800-§ absorbed in Au 600-400- I \ / 360 Form a liquid droplet of a eutectic ?nn -Au Si(Ge)— when heated 0 20 40 60 80 100 Weight percent Si (Ge) The droplet becomes supersaturated Eutectic 360 °C Au(1064 °C) Excess material is precipitated out to Si (1410 °C) form solid NWs beneath the droplet Ge (937 °C) 45 Vapor-Liquid-Solid (VLS) Growth Au/Ge Eutectic Liquid Alloy Au Particles Nucleation of NWs H(Ge)/% 1200 10 20 —- 30 40 50 60 70 80 90100 ■ i A ■ i ■ i*i ■ i. h i ..Id ii i ■! I ■ ■ f. i . J.-i| ■>.1 ■ 1.11 500 1000 soo 600 400 200 1064.43"C (Au) i (Au) + L . 361 *C i Atoms Liquid Eutectic Substrate NW Growth 46 In situ TEM images recorded during 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 and eventually forms a wire (f) 47 Si Nanowire Growth SiH4-> Si + 2H2 Mass transport in the gas phase Chemical reaction at the V-L interface Diffusion in molten catalyst Incorporation of material in the crystal lattice Si Nanowires Size Control Metal particle acts as a soft template to control the diameter of the nanowire Au 10 nm o 20 nm O 30 nm Q InP Laser Ablation a GZ ] 50 Catalytic base growth Ge-Fe catalytic nucleus Ge wire Precursor supply Fe substrate 51 Templated growth 1. Pores filled with material by CVD 2. Alumina matrix dissolved 3. Wires separated I 200 nm 52 Arrested growth Precursor supply Selective binding of a compound to certain crystal faces CdTe, TOPO blocks (111) Alivistos 53 SLS-growth mechanism Solution (Liquid1 By-prod ucts Solid Growth direction Metallo-organic Catalyst precursors particle Crystalline semiconductor Molecular rods l.2ff-BuLi 2CvCl2 3. HiO+ & ■ |2|6 I' ' 1-1» Hit MH ■Q- J^-H l*#3+H ~^>- H-^H HH^H -H H 5 ||= 11-i |®>® ■Collector variables 10 kV I- bending and axisymetrical instability Rayleigh instability thickeness Adsorption of polymer molecules electric layer Electrospinning electrospraying electrospinning Solution (viscosity, conductivity, surface tension) Instruments (voltage, distance b/w electrodes, collector shape) Ambient (temperature, humidity, atmosphere) 59 Electrospinning Left: Photograph of a jet of PEO solution during electrospinning. Right: High-speed photograph of jet instabilities. 60 Taylor cone Viscosity Concentration(%w/w) Volume charge density 2a: 1.23 Coulomb/! iler 2b: 1.77 Coulomb/liter 2c: 3.03 Coulomb/1 iter JAMS 2d: 6.57 Coulomb/1 iter 2e: S.67 Coulomb/Itter 2f: 28.8 Coidomb/liter 64 Needle-collector distance PA fibers, electrode distance 2 cm (a) and 0.5 cm (b) 65 Conductivity Morphology of fibers as a function of electric current (a) 20 hm.% PU (b) 20 hm.% PU with addition of 1.27% TEAB 66 Relative humidity i—■—i—1—i—1—i—'—i—'—i—1—i- 0 10 20 30 40 50 60 Relative Humidity (%) PEO fiber diameter as a function of relative humidity 67 69 Multijet electrospinning Needle-less spinning Inorganic fibers Th(acac)4; PVP; EtOH; acetone Electro spinning ThQ2 72