Nanostrukturphysik. Prof. Yong Lei & Dr. Yang Xu Fachgebiet 3D-Nanostrukturierung, Institut für Physik

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1 Nanostrukturphysik Prof. Yong Lei & Dr. Yang Xu Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: (3748), (4902) Office: Gebäude V202, Unterpörlitzer straβe 38 Vorlesung: Wednesday 9:00 10:30, C 110 Übung: Friday (G) 9:00 10:30, C 110

2 Carbon nanotubes (CNTs): introduction, fabrications, and applications

3 Carbon nanotubes (CNTs) and fullerenes Carbon nanostructures: CNTs, fullerenes (buckyballs (Buckminster Fuller): C 60, C 70, etc.), graphene. Direct nanotechnology to a new dimension Carbon (atomic weight 12.0): one of the most abundant elements in the universe. Until 1985, two bulk solid phases of carbon were known: graphite and diamond. The structure of fullerenes was discovered in 1985, which are recognized as a different phase of carbon from the graphite and diamond. After discovery of fullerene, scientists found a number of different carbon nanostructures with similar architecture: CNTs, carbon nanorods, and 2-D graphene.

4 History of carbon nanotubes and fullerenes 1953, W. R. Davis: an unusual form of carbon from carbon monoxide at 450 o C, but TEM can t reveal the architectures of such an unusual form of carbon.

5 1960, R. Bacon: growth of graphite fibers in direct current (DC) carbon arc furnace under a pressure of 92 ATM of argon at 3900 K.

6 1985, H. Kroto, R. Smalley, R. Curl: discovered fullerenes (C 60 ), won the Nobel Prize of Chemistry in 1996 for the discovery of fullerenes (buckyballs).

7 1991, a breakthrough in research of carbon nanostructures came: Iijima reported arc-discharge synthesis and high-resolution TEM characterization of such helical microtubules. These microtubules, later known as carbon nanotubes (CNTs), are molecular-scale fibers with structures related to fullerenes.

8 The allotropes of carbon: hardest naturally occurring substance, diamond one of the softest known substances, graphite : fullerenes (C 60 ); 1991: CNTs : graphene Allotropes of carbon: a) diamond; b) graphite; c) lonsdaleite; d f) fullerenes (C 60, C 540, C 70 ); g) amorphous carbon; h) carbon nanotube.

9 Structure of bucky balls C 60 is a truncated icosahedron (20 planes), resembles a soccer ball (20 hexagons and 12 pentagons), with a carbon atom at the vertices (tops) of each polygon and a bond along each polygon edge. Bucky ball is the smallest fullerene molecule in which no two pentagons share an edge. The diameter of a C 60 molecule is about nm. The C 60 molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon).

10 Synthesis(discover) of C60 C60 in solution The experimental set-up used to discover C60. The graphite disk is evaporated with a Nd:YAG laser and the evaporated carbon plasma is cooled by a stream of helium coming from a pulsed valve. The clusters of carbon are produced in the integration cup and are expanded into vacuum. The ions are detected by time of flight mass spectrometry.

11 Top: collision of high energy ions on C60 results in the addition of C56 to C70. Bottom: mass spectrum of a laser evaporated C60 film showing coalescence of fullerenes. Mass peaks are seen at (C60)n.

12 Structure of carbon nanotubes CNTs (hollow 1-D carbon nanostructures) can exist as single tubes (called single-walled nanotubes, SWCNT) or in the form of concentric tubes (termed multi-walled nanotubes, MWCNT). A SWCNT is a hollow cylinder of graphite sheet, a MWCNT is a group of coaxial SWCNTs. SWCNTs were discovered in 1991, two years after the discovery of MWCNTs. The length of CNTs can vary from a few hundred nanometers to several hundred micrometers. The diameter can vary from about 0.37 nm to 100 nm.

13 Parallel dark lines correspond to the (002) lattice imges of graphite. CNTs consisting of (a) 5 graphitic sheets, diameter 6.7 nm; (b) 2 sheets, diamter 5.5 nm; (c) 7 sheets, diameter 6.5 nm. First HRTEM images of CNTs (Iijima) in a paper on Nature in The paper was cited for 7771 times so far. (Iijima S., Nature 1991). The nanometer-scale size and hollow cylindrical shape CNTs suggests many applications (molecular sieves, nano-test-tubes, and hydraulic actuators).

14 The architecture of three different forms of carbon: (a) graphite, (b) diamond and (c) carbon nanotubes. (Bera D., 2005) The bonding of CNTs is sp 2, although it is mixed with an extent of sp 3 feature because of its high curvature. In comparison with graphite, CNTs show significantly different properties, i.e. greater interplanar distance, smaller work function, steeper Fermi edge, negative core-level shift and stronger plasma excitation - mechanically stronger, electrically more conductive, and chemically and biologically more active.

15 High resolution TEM image Graphene is a 1-atom thick sheets of sp 2 -bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene is easily visualized as an atomicscale wire made of carbon atoms and their bonds. Graphite consists of many graphene sheets stacked together. (

16 Zigzag carbon nanotube

17 Armchair carbon nanotube

18 A SWCNT can be visualized by rolling up graphene. The structure is one dimensional with axial symmetry showing a spiral conformation called chirality. Schematic of 2D graphene sheet showing lattice vector a 1 and a 2, and roll-up vector Ch = n(a1) + m(a2). The cases of (n, 0) zigzag and (n,n) armchair tubes are indicated with dashes lines, chiral (n,m). The nanotubes axis is indicated by the vector T. (Lieber CM. Solid State Commun., 1998) SWCNTs exhibit two distinct behaviors as a metal or as a semiconductor depending on the chirality.

19 (a) A typical HRTEM image of a SWNT with enhanced contrast of zigzag chain (inset). (b) Moire pattern formed by roll up graphene layer shows the two intense dark lines correspond to wall of CNT. (c) A best-fit model of SWNT with the determined chiral index (13, 8). (d) and (e) A cross-sectional view of a defect on SWNT. A pentagon heptagon pair is responsible for defect structure and generates a serial junction of two zigzag nanotubes. (f) A simulated image for SWNT with the defect rotated by 90. Scale bar: 2 nm. (Hashimoto A., Nature 2004)

20 Synthesis processes for CNTs Carbon nanostructures can be synthesized by methods such as: arc-discharge in vacuum arc-discharge in solution (ADS) laser ablation chemical vapor deposition (CVD) Manipulation of the process conditions (electrode materials, catalyst, other experimental parameters including pressure and temperature), leads to a particular shape and size of the carbon nanostructure. Although CNTs can be synthesized using different techniques, 2 methods are mainly used for commercial and bulk production of CNTs: chemical vapor deposition (CVD) and laser ablation.

21 Laser ablation Using a laser vaporization unit with a graphite sample put in an oven, closedended MWCNTs were produced in gas phase through homogeneous carbonvapor condensation in a hot argon atmosphere. The laser-produced MWCNTs are relatively short (~ 300 nm), the number of layers (4 24), the inner diameter ( nm), similar to arc-produced MWCNTs. A 70 90% conversion of graphite to SWCNTs was reported in a heated tube operating at 1200 o C. Schematic illustration of CNT growth setup using laser ablation method (Tamir S. et al., Appl Surf Sci 2006)

22 Chemical vapor deposition Chemical vapor deposition (CVD) is a reaction process to synthesize CNTs from different precursors. Compared to arc-discharge and laser ablation methods, the main advantages of this process are: (1) Production to industrial level (approximately a pound per day synthesized by Carbon Nanotechnology Inc., Houston, Texas). (2) Control over growth of desirable (diameter, length and position) CNTs which is more important for electronic applications. Dissociation of precursors at the catalytic particle surface is the key for CNT growth. Chemical vapor deposition (CVD) can be sorted depending on the energy sources: plasma-enhanced CVD (PECVD), thermal CVD, etc.

23 Thermal CVD When a conventional heat source (e.g., a furnace) is used, the technique is called thermal CVD. The thermal CVD system for CNT growth is very simple: It consists of a quartz tube inserted into a tube furnace. Hydrocarbons or CO are used as precursor. A typical growth process involves: 1 st : purge reactor with inert gas; 2 nd : gas flow is switched for specified growth period; 3 rd : gas flow is switched back to inert gas while the reactor cools down. For growth on substrates, catalysts need to be applied on substrate before loading it inside reactor. Typical temperatures for catalytic CVD in CNT growth are in the range of K. Aligned CNTs 40 micron Schematic illustration of the CVD process (Dupuis AC. Prog Mater Sci 2005)

24 Thermal CVD and VLS growth

25 PECVD The plasma-enhanced CVD (PECVD) first emerged in microelectronics to avoid high temperatures of thermal CVD, which is important for devices. It has emerged as a key growth technique to produce vertically aligned CNTs. The ability to grow CNTs with a high degree of uniformity is necessary to various technologically important applications. Such structural uniformity of CNTs, including specific arrays and single-standing nanotubes can be achieved through this technique.

26 (a) Highly aligned CNTs were grown from a 100 nm wide, 7 nm thick Ni catalyst line. (b) CNTs grown from dots of Ni catalyst with 7 nm thickness. The diameter (standard deviation 4.1%) and height (6.3%) of CNTs were found to be very precise in the case of single isolated growth. (Teo KBK et al., Nanotechnology 2003)

27 Biological applications, chemical properties and toxicity Cell viability in the presence of CNTs is an important subject: The main reasons of toxicity of CNTs in cells: presence of metallic catalyst residue; a complete insolubility of CNTs in all types of solvents. To avoid insolubility of CNTs, functionalization using larges functional groups is required. Many oligomers and polymeric compounds have been used in the functionalization of CNTs for their solubility in common solvents including water. Functionalized CNTs are highly soluble in biological liquids, and functionalized CNTs have almost no harmful effect on cells: only functionalized SWCNTs and MWCNTs are used in biological applications: drug delivery, gene delivery, antigen delivery, cancer cell destruction, ion channel blocking. Chemical modification of nanotubes through thermal oxidation, followed by subsequent esterification or amidization of the carboxyl groups Small 2005, 1, No. 2, Chemically Functionalized Carbon Nanotubes

28 Schematic of various SWCNT conjugates. (2) 5-(5-aminopentyl) thioureidyl fluorescein; (3) With a protein (biotin-lc-peo-amine); (4) With a protein (streptavidin) and fluorescein. Confocal images of HL60 cells after incubation in solutions of SWCNT conjugates: (a) after incubation in 2, (b) after incubation in a mixture of 4 (green due to SA) and the red endocytosis marker FM 4-64 at 37 o C (image shows fluorescence in the green region only), (c) same as b with additional red fluorescence shown due to FM 4-64 stained endosomes, (d) same as b after incubation at 4 o C. (Kam NWS, JACS, 2004) Carbon nanotubes can be served as an excellent biocompatible transporter

29 Sensors (gas, chemcial and bio-) The recent large interests to create new sensors to detect gas, chemicals, and bio-molecules using nanostructures with the motivation to environmental monitoring and counter-terrorism. Enhancement of selectivity and efficiency of sensors could be achieved by tuning the size, structure, and shape of nanomaterial. Sensing properties of many kinds of nanomaterials have been investigated. CNTs-based sensors have been used for the detection of chemical and biological molecules with excellent performance, fine accuracy and high precision.

30 Detection of gases: It is found that electronic properties of CNTs are largely affected by exposure to some gases: electronic properties of CNTs are extremely sensitive to presence of oxygen molecules due to formation of charge transfer complex between C and O. At RT, CNTs show a very high sensitivity to O 2, NO 2, ammonia, CO, and CO 2. Most CNT-based sensors reported so far in the literature are basically FETbased. 70% of such sensors are developed using semiconductor CNTs prepared using CVD processes. Electrons are withdrawn when CNT-based FET is exposed to NO 2 ; however, electrons are donated to CNTs in the presence of ammonia. To NO 2 gases, sensor response was found to be linear with the concentration in the range from sub-ppm to hundreds of ppm. Example: multiple functionalized CNT sensors with high sensitivity and gas selectivity, detect NO 2 at a concentration less than 1 ppb.

31 High Sensitivity (a) An array of multiple SWCNT devices. (b) A device: width of patterned catalysts was large (100 µm) allowing for the growth of a large number of SWCNTs (20-30) across source and drain electrodes. (c) Some nanotubes bridge over electrodes. (d) I-V g curve of a SWCNT device: in each device appeared p-type: an overall electrical conductance decrease by a factor of 2-3 when V g was swept from -10 to 10 V. A significant advantage of the multiple-tube devices over 1 tube was lower electrical noise, desired for sensing. 1% fluctuations for MT devices and 10% fluctuations for individual nanotubes. The lower noise level for MT devices was attributed to the larger number of tubes in the devices. (e) I-V g curve recorded for device after PEI functionalization. The PEI (polyethylenimine)-coated MT devices evolved into n-type. (Qi PF, Nano Letters, 2003)

32 The PEI functionalized n-type MT devices were ultrasensitive to NO 2, responding to as low as 100 ppt (parts per trillion) of NO 2.

33 Catalysis properties CNTs decorated with elements and compounds exhibit fascinating properties to serve as nano-scale chemical reactors or catalyst. After CNTs were discovered in 1991, attempts were undertaken to introduce materials into the central cavities. CNTs filled with solids for tens of nanometers. (Ajayan and Iijima, Nature 1993)

34 Hydrogen storage Hydrogen storage in CNTs has been a subject of intense research since its discovery. The storage potential of CNTs must exceed 8 wt% of hydrogen (as a reactor) to power electric vehicles effectively. Recent experimental and modeling studies confirm that hydrogen uptake in CNTs is up to 20 wt%. Hydrogen storage values using CNTs at thermodynamic conditions. The large variation in experimental values was interpreted in two ways: (1) CNTs are always associated with different impurities. The amount of impurities is largely different depending on synthesis methods. (2) Distribution in tube diameter is another major issue to influence the results.

35 A device with metal electrodes for measuring the electrical properties of a CNT. (Kim UJ, J. Phys. Chem. B, 2005) Electronic properties When a graphite sheet rolls up to a tube, confinement of electrons around CNTs is observed. Electronic properties of CNTs depend on the microstructure of the tube, extent of functionalization and doping. A nanotube is metallic (at RT) if roll-up vector is an integer. Otherwise, it is semiconductor with band gap Eg = 0.9 ev band gap of semiconducting CNTs is inversely proportional to tube diameter. The peculiar and remarkable electronic properties of CNTs make it possible of a band gap engineering by control the microstructure.