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 From continuous bands to discrete energy levels 3D 2D ID OD 3 D: 2D: E = ti 2m ID: E = 2m h2 ,2,2 X 2m kr + n — V r L 7t + ft — OD: E = 2m V Lj 1=1,2,3 ^,,1 =1,2,3. 1,^,^ =1,2,3... ID Nanostructures Poly {ethylene oxide) Collagen Fibrils 4 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 5 Effect of Confinement 6 1 10 100 1000 10000 100000 Diameter (A) The band gap increases with decreasing diameter (quantum confinement) Mechanical properties Strain (%) WS2 Graphite Strain-stress curve of a nanotube Carbon Nanotubes • (Re)discovered by Iijima (1991, NEC) • 1952 Russians (CO + Fe hot, SEM) • 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 Raman spectroscopy Virtual energy states Vibrational energy states Infrared Rayleigh Stokes Anti-Stokes absorption scattering Raman Raman scattering scattering Raman scattering is an extremely weak process - only one in every 106 - 108 photons scatters Height ofthe peak -* Substance concentration -> Shift ofthe peak position -* stress Width ofthe peak -» crystallinity of material t The position of the peak -* molecular structure u-i c c It £ Raman effect - interaction between the electron cloud of a molecule and the external electrical field of the monochromatic light - a change in polarizability with respect to the vibrational coordinate Anti-Stokes Rarjnan scattered light Stokeis line: stronger thijn Anti-Stoke:l Rayleigh scattered light Stokes Ramian scattered light UL Wavenumberťcm"1) -looo -800 -600 -400 -200 0 200 400 600 800 looo Wavelength (nm) 505 510 516 521 526 532 538 544 550 556 562 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 catatlvtic mechanism Synthetic routes to CNT • DC arc discharge: growth on a cathode C electrode at 3000 °C, 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, Co and Ni catalyst • 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 1 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 2 carbon electrodes touch, T rises, gap, Cathod electric arch, fast e~ from cathode ionize He gas m^a* He+ attracted to cathode (-) -Carbon (with catalyst) contained in negative electrode Anode -sublimes thanks to high temperatures of the electric discharge (3000 °C) Yield up to 30 %wt, produces both SWNTs, MWNTs Length up to 50 urn, few structural defects 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 16 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 17 Synthetic routes to CNT CVD (Chemical Vapor Deposition) CH4 + metal catalyst particles (cobalt, nickel, iron) —» Fe3C/C + H2 Tip growth Base growth (a) /Catäly^ Catalysl Support Substrate (b) Fl SURE 9: Carbon nanotu be mechanistic models: (a] root-growth mechanism and [b] tip-growth mechanism. Catalyst Support Substrate 4 Catalyst w Cala yst Support Substrate C H _____ CxHy jjfCatalyst£V Catalyst Support Substrate Growth stops (Catalyst) Cata yst Support Substrate CxHy ^(^Catalys^l^ Catalyst Support CXHy Substrate TO- 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 Substrate patterned (lithography) with Fe NPs 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 22 Roll-up of (n,m) SWNTs Chiral vector (n,m) (n,n) armchair (n,0) zigzag B* * ^ * ^ ^ * * ■« »f ,( > > > tJ v J V J ,^ W ^ V V . ■J .J .J >J J J .J J ,J ,v * W W J d J J J J J J .J .JS .J ^J^J^J^, j j j j j j j j ^jjjjjjj. j.j , j , j ,. * * * j j j j j j , J v Jj Jj Jj J i J i J i J i J i J ^ ' J J J J J J J J J J J s J tJ SJ . J . J^J^J^J . J .J^J.. JAJAJA, (n,m) chiral TEM of a chiral CNT Roll-up of (n,m) SWNTs 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) 25 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 28 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 29 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 31 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 32 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) 33 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 35 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. 36 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 37 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* 39 Reduction 40 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 demonstrated in the laboratory Techniques for mass manufacture 43 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 44 Synthetic Routes to Nanowires Epitaxial growth Catalytic VLS growth Catalytic base growth Defect nucleation Templated growth Arrested growth Assembly of nanoparticles 45 Epitaxial growth Active surface Masked surface Vapor-Liquid-Solid (VLS) Growth (1) Metal catalyst nanoparticles - Au(s) 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, melts to a droplet (3) The droplet becomes supersaturated with Ge 1400 1200 S iooo- 3 0J ex 800 600- 400 200 r i > i ■ i AuSi —----- / AuGe w 360 — Au Si (Ge)^ i.i.i. 20 40 60 80 Weight percent Si (Ge) 100 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) 47 1 Vapor-Liquid-Solid (VLS) Growth 1200t 1000-3 g\600-a s H 400^ iiiiiiii]|iiiiiiiii|iiiiiiiii|iiiiiiiii|iiiiiiiii|iiiiiiiii|iiiriiiii|iiiiiiiii|iiiiiiiii|iiiiiiiLLj 1064 °C 938 °d L+Ge V (29%,320°C) |]iiMiiM|iiiiiiiii|.........|iiiiiiiii|iiiiiiiii|iiiiiiiii|iiiiiiiri|......eii|.............. 0 20 40 60 80 100 Au Ge atomic % Ge 48 Vapor-Liquid-Solid (VLS) Growth Au/Ge Eutectic Liquid Alloy Au Particles Nucleation of N Ws 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 49 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 50 Ge NWs on the bare Ge(llO) substrate Precursor Elements (vapor) a metal catalyst substrate ivst f Heat - * . ✓* t i, í */ Supersaturatlon 1D structure Í/,, ...... ■■.Vv Precipitati 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 ] 54 Catalytic base growth Ge-Fe catalytic nucleus Ge wire Precursor supply Fe substrate 55 Templated growth 1. Pores filled with material by CVD 2. Alumina matrix dissolved 3. Wires separated I 200 run 56 Arrested growth Precursor supply Selective binding of a compound to certain crystal faces CdTe, TOPO blocks (111) Alivistos 57 SLS-growth mechanism Solution (Liquid1 By-prod ucts Solid Growth direction Metallo-organic Catalyst precursors particle Crystalline semiconductor Inorganic NanoTubes INT - MoS2, WS2 B Furnace r K /J Substrate A Gas-Phase reaction nested fülle rene-like Inorganic NanoTubes INT Defect free INT-WS2; (a) SEM image, (b) TEM image (scale bar 10 nm). 60 Molecular rods l.2ff-BuLi 2CvCl2 3. HiO+ (2|n & ■ |2|6 I' ' 1-1» Hit MH ■Q- j^-h h^Qr+h ~^>- h-^h h-^fl MM MIJ -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) 65 Electrospinning Left: Photograph of a jet of PEO solution during electrospinning. Right: High-speed photograph of jet instabilities. 66 Taylor cone Viscosity 3 5 7 9 11 Concentration(%w/w) Volume charge density 2a: 1.23 Coulomb/! iler 2b: 1.77 Coulomb/liter 2c: 3.03 Coulomb/liter JAMS 2d: 6.57 Coulomb/1 iter 2e: S.67 Coulomb/Itter 2f: 28.8 Coulc-mb/liter 70 Needle-collector distance PA fibers, electrode distance 2 cm (a) and 0.5 cm (b) 71 Conductivity Morphology of fibers as a function of electric current (a) 20 hm.% PU (b) 20 hm.% PU with addition of 1.27% TEAB 72 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 73 75 Multijet electrospinning Needle-less spinning Inorganic fibers Th(acac)4; PVP; EtOH; acetone Electro spinning ThQ2 78