Mukul Kumar

Growing carbon nanotubes

The discovery of ‘fullerenes’ added a new dimension to the knowledge of carbon science 1 ; and the subsequent discovery of ‘carbon nanotubes’ (CNTs, the elongated fullerene) added a new dimension to the knowledge of technology 2 ;. Today, ‘nanotechnology’ is a hot topic attracting scientists, industrialists, journalists, governments, and even the general public. Nanotechnology is the creation of functional materials, devices, and systems through control of matter on the nanometer scale and the exploitation of novel phenomena and properties of matter (physical, chemical, biological, electrical, etc.) at that length scale. CNTs are supposed to be a key component of nanotechnology. Almost every week a new potential application of CNTs is identified, stimulating scientists to peep into this tiny tube with ever increasing curiosity.

A simple method of producing aligned carbon nanotubes from an unconventional precursor – Camphor

Abstract Vertically aligned multi-wall carbon nanotubes of diameter 20–40 nm and length ∼200 μm were grown on quartz substrate by pyrolyzing camphor with ferrocene catalyst at 900 °C in argon atmosphere at ambient pressure. Catalyst requirement with camphor was found to be low by a factor of 10 as compared to available reports of aligned nanotubes from conventional precursors. Because of the low catalyst requirement with camphor, as-grown nanotubes are least contaminated with metal particles, whereas the oxygen atom present in camphor helps in oxidizing amorphous carbon in-situ, eliminating the need of post-deposition purification. Good graphitization of the tube layers was observed by TEM, whereas high purity was confirmed by EDX analysis. The estimated yield of as-grown nanotubes is ∼90%.

Single-wall and multi-wall carbon nanotubes from camphor—a botanical hydrocarbon

Camphor ( CHO ), a botanical hydrocarbon, has been found to be a promising precursor of carbon nanotubes (CNTs). 10 16 Single-wall and multi-wall CNTs have been grown from simple pyrolysis of camphor in the temperature range 800–1050 8 Ci n argon atmosphere at normal pressure using ferrocene as a catalyst. Single-wall nanotubes and their bundles could be prepared in low yield, but of uniform diameter 1.2–1.3 nm. On the other hand, multi-wall nanotubes of uniform diameter (20–40 nm) could be produced with a yield as high as ;90%. Structural characterizations have been done by TEM, HRTEM, EDX and Raman analyses. Good crystallinity, high purity, and absence of amorphous carbon and metallic particles are the essential features of camphor-grown nanotubes. 2003 Elsevier Science B.V. All rights reserved.

Camphor–a botanical precursor producing garden of carbon nanotubes

Camphor, a botanical precursor, has been found to produce carbon nanotubes in large quantities; like a nanotube garden containing single-wall, multi-wall and aligned-nanotubes. These products grow as a result of thermal decomposition of camphor in argon atmosphere at 875(±25) °C. The novelty of this new precursor lies in the extremely low catalyst requirement for the realization of pure carbon nanotubes. Single-wall nanotubes are found in low quantity, whereas the yield of multi-wall nanotubes is as high as 90%. Moreover, vertically aligned nanotubes grow on large-area quartz substrates inserted in the reaction zone. Crystallinity of the as-grown nanotubes is fairly good, much better than that of those generally obtained by CVD using conventional hydrocarbons. Formation of amorphous carbon is almost nil, eliminating the need for post-deposition heat treatment. The presence of unwanted catalyst particles is extremely low.

Fabrication of ZnO nanospikes and nanopillars on ITO glass by templateless seed-layer-free electrodeposition and their field-emission properties.

A simple and direct electrodeposition technique is employed to fabricate ZnO nanospikes and nanopillars on indium-tin oxide glass substrates at 70 degrees C without using any template, catalyst, or seed layer. Both ZnO nanospikes and nanopillars exhibit highly crystalline ZnO wurtzite structure with a preferred (0001) plane orientation in their high-resolution transmission electron microscopic images and X-ray diffraction patterns. The corresponding Raman spectra provide evidence for the presence of defects and oxygen vacancies in these nanostructures, which could produce the photoluminescence observed in the visible region. X-ray photoelectron spectroscopy further indicates the presence of a Zn(OH)2-rich surface region in these ZnO nanostructures and that a higher Zn(OH)2 surface moiety is found for nanospikes than nanopillars. In contrast to the nanopillars with flat tops, the nanospikes with tapered tips of 20-50 nm diameter provide a favorable geometry to facilitate excellent field-emission performance, with a low turn-on electric field of 3.2 V/microm for 1.0 microA/cm(2) and a threshold field of 6.6 V/microm for 1.0 mA/cm(2). The superior field-emission property makes the nanospikes among the best ZnO field emitters fabricated on a glass substrate at low temperature.

Controllable growth of highly N-doped carbon nanotubes from imidazole: a structural, spectroscopic and field emission study

Here we report the highest N-doping level achieved so far for carbon nitride (CNx) nanotubes by using a new precursor imidazole (as a dual supplier of carbon and nitrogen) together with ferrocene (as a catalyst). A controllable growth of aligned CNx nanotubes was achieved by simple CVD in a temperature range of 700–950 °C leading to nitrogen doping from 12.1 to 25.7 at%. The highest doping level was achieved at 850 °C, which is attributed to the abundance of C–N fragments at this decomposition temperature. Nitrogen doping level and the type of N-moieties were determined by XPS and EELS analyses. HRTEM analysis suggests that N-substituted graphitic units (gN) give rise to fullerenic curvature into the cylindrical graphite layers (leading to curled side-wall morphology), whereas pyridinic units (pN) break the graphitic continuity (leading to dangling bonds on the side walls). It is observed that the abundance of N-substituted graphitic units (gN) enhances the electrical conductivity of individual nanotubes by donating additional electrons to the CNT network. The field emission (FE) characteristics show that the increase of N doping from 19.6 at% to 25.7 at% effectively reduces the turn-on field (for 10 µA cm−2) from 1.92 V µm−1 to 0.88 V µm−1, whereas the threshold field (for 10 mA cm−2) decreases from 4.05 V µm−1 to 2.32 V µm−1. However, at high growth temperature (950 °C), the N-doping level decreases to 12.1 at%, thereby degrading the FE performance.

Efficient field emission from vertically grown planar ZnO nanowalls on an ITO-glass substrate.

Vertically grown planar ZnO nanowalls, with typical dimensions of 40-80xa0nm thickness and several micrometers wide, were electrodeposited on an indium-tin-oxide (ITO)-glass substrate at 70u2009°C. X-ray photoelectron spectroscopy (XPS) studies reveal that the nanowalls consist of ZnO covered with a Zn(OH)(2) overlayer. An x-ray diffraction (XRD) study shows that these nanowalls have the wurtzite structure and are highly crystalline. The corresponding Raman and photoluminescence spectra further indicate the presence of oxygen deficiency. These ZnO nanowalls exhibit excellent field emission performance, with not only a considerably lower turn-on field of 3.6xa0Vxa0µm(-1) (at 0.1xa0µAxa0cm(-2)) but also a higher current density of 0.34xa0mAxa0cm(-2) at 6.6xa0Vxa0µm(-1) than most ZnO nanowires and other one-dimensional nanostructures reported to date.

Nitrogen-Mediated Wet-Chemical Formation of Carbon Nitride/ZnO Heterojunctions for Enhanced Field Emission

Heterojunction structures of nanocrystalline materials are of great importance in scientific and industrial research for their potential applications in nanoscale electronics and photonics. Here, we report a simple wet-chemical method of nitrogen-mediated growth of ZnO nanocrystals on carbon nitride (CNx) nanotubes. SEM and TEM analyses show self-organized ZnO nanoflowers on CNx stems. PL spectra exhibiting a blue emission at 449 nm confirms the junction formation between CNx and ZnO. The field emission (FE) properties of CNx-ZnO film are greatly improved over those of pristine CNx. The turn-on and threshold fields for bare CNx film are 1.70 and 2.95 V/v, whereas those for CNx-ZnO hybrid are found to be 0.75 and 1.3 V/microm, respectively. This significant downshift in the turn-on and threshold fields is believed to occur via lowering of the Schottky barrier at the metal-semiconductor interface. Three-dimensionally blossomed ZnO nanopetals with multiple sharp tips effectively enhance the FE performance. Moreover, this heterojunction reinforces the electron emission lifetime and protects the CNx tubes against thermal degradation.

Carbon nanotubes from camphor by catalytic cvd

Multi-wall carbon nanotubes (MWNTs) have been grown from simple pyrolysis of camphor, a botanical hydrocarbon, at 900°C for 15 min in argon atmosphere at ambient pressure using ferrocene as a catalyst. The nanotube diameter is fairly uniform (20-40 nm) and the yield is extremely high (∼90%). Structural characterization is done by SEM, TEM, HRTEM, EDX and Raman analyses. Good crystallinity, high purity, and absence of amorphous carbon and metal particles are the special features of camphor-pyrolyzed nanotubes.

VERTICALLY ALIGNED CARBON NANOTUBES AT DIFFERENT TEMPERATURES BY SPRAY PYROLYSIS TECHNIQUES

Vertically aligned arrays of multi-walled carbon nanotubes (VACNTs) were grown by spray pyrolysis of turpentine oil and ferrocene mixture at temperatures higher than 700°C. Using this simple method, we report the successful growth of vertically aligned nanotubes of ~300μm length and diameter in the range of ?20–80nm on Si(100) substrate. The ferrocene acts as an in situFe catalyst precursor, forming the nano-sized metallic iron particles for formation of VACNTs on the Si substrate. The morphological characteristics of VACNTs are confirmed by SEM, TEM and Raman spectroscopy and growth mechanism is discussed in short.