Ondrej Jasek

Methods for carbon nanotubes synthesis—review

Carbon nanotubes (CNTs) have been under scientific investigation for more than fifteen years because of their unique properties that predestine them for many potential applications. The field of nanotechnology and nanoscience push their investigation forward to produce CNTs with suitable parameters for future applications. It is evident that new approaches of their synthesis need to be developed and optimized. In this paper we review history, types, structure and especially the different synthesis methods for CNTs preparation including arc discharge, laser ablation and chemical vapour deposition. Moreover, we mention some rarely used ways of arc discharge deposition which involves arc discharge in liquid solutions in contrary to standard used deposition in a gas atmosphere. In addition, the methods for uniform vertically aligned CNTs synthesis using lithographic techniques for catalyst deposition as well as a method utilizing a nanoporous anodized aluminium oxide as a pattern for selective CNTs growth are reported too.

Sensing properties of multiwalled carbon nanotubes grown in MW plasma torch: electronic and electrochemical behavior, gas sensing, field emission, IR absorption.

Vertically aligned multi-walled carbon nanotubes (VA-MWCNTs) with an average diameter below 80 nm and a thickness of the uniform VA-MWCNT layer of about 16 μm were grown in microwave plasma torch and tested for selected functional properties. IR absorption important for a construction of bolometers was studied by Fourier transform infrared spectroscopy. Basic electrochemical characterization was performed by cyclic voltammetry. Comparing the obtained results with the standard or MWCNT‐modified screen-printed electrodes, the prepared VA-MWCNT electrodes indicated their high potential for the construction of electrochemical sensors. Resistive CNT gas sensor revealed a good sensitivity to ammonia taking into account room temperature operation. Field emission detected from CNTs was suitable for the pressure sensing application based on the measurement of emission current in the diode structure with bending diaphragm. The advantages of microwave plasma torch growth of CNTs, i.e., fast processing and versatility of the process, can be therefore fully exploited for the integration of surface-bound grown CNTs into various sensing structures.

Synthesis of carbon nanotubes by plasma-enhanced chemical vapor deposition in an atmospheric-pressure microwave torch

There are many different techniques for the synthesis of carbon nanotubes (CNTs), and plasma technologies experience a significant competitor in thermal chemical vapor deposition (CVD) processes. A particular process is, therefore, selected according to the specific requirements of an application, which clearly differ for the development of composites as compared to nanoelectronics, field emission, displays, sensors, and the like. This paper discusses the method for the synthesis of CNTs using an atmospheric-pressure microwave (MW) torch. It was successfully applied in the fast deposition of multiwalled nanotubes (MWNTs) on a substrate without the necessity of any vacuum or heating equipment. Dense straight-standing nanotubes were prepared on Si substrates with and also without barrier SiOx layer. Therefore, it was possible to produce CNTs directly on conductive Si and to use them as an electron-emitting electrode of the gas pressure sensor. The CNTs grown in MW torch were also used to create a gas sensor based on the changes of electrical resistance measured between two planar electrodes connected by the CNTs.

Nanopatterned Working Electrode with Carbon Nanotubes Improving Electrochemical Sensors

This paper deals with problem of mercury drop electrodes replacement in classical polarography by more ecological solid electrodes. The screen-printed thick-film amperometric sensor was prepared for modification by nanostructures. Carbon nanotubes were grown on a working electrode. The process of nanotubes growing was improved to create homogenous and high density carbon nanotubes layer. The modified electrode is very perspective in heavy metal detection using electrochemical methods because of improvement detection properties. We were able to determine units of mumol/L of cadmium.

MEMS Carbon Nanotubes Field Emission Pressure Sensor With Simplified Design: Performance and Field Emission Properties Study

The pressure sensor application gained recently substantial interest in many fields of basic and applied research and applications. In this paper, microelectromechanical system (MEMS)-based pressure sensor contains nanostructured electrode consisting of carbon nanotube (CNT) array. CNTs are directly grown on such electrode by plasma-enhanced chemical vapor deposition method using microwave plasma torch at atmospheric pressure. This growth method enables us to use a simple electrode structure without need of buffer layer and time-consuming lithography process. Combination of CNTs field emission and MEMS membrane mechanical properties make possible to enhance sensitivity of the sensor. Field emission properties of CNTs are measured by newly developed system enabling us precise measurement of expecting properties, such as dependence on diaphragm (upper electrode) distance, applied voltage, and stability of the sensor. Measured values are compared with a numerical modeling of the membrane system in CoventorWare software by finite-element method. We also suggest encapsulating the sensor using glass frit bonding because such method is more suitable for high vacuum requirements of the field emission operation.

Carbon composite micro- and nano-tubes-based electrodes for detection of nucleic acids

The first aim of this study was to fabricate vertically aligned multiwalled carbon nanotubes (MWCNTs). MWCNTs were successfully prepared by using plasma enhanced chemical vapour deposition. Further, three carbon composite electrodes with different content of carbon particles with various shapes and sizes were prepared and tested on measuring of nucleic acids. The dependences of adenine peak height on the concentration of nucleic acid sample were measured. Carbon composite electrode prepared from a mixture of glassy and spherical carbon powder and MWCNTs had the highest sensitivity to nucleic acids. Other interesting result is the fact that we were able to distinguish signals for all bases using this electrode.

Investigation of pristine graphite oxide as room-temperature chemiresistive ammonia gas sensing material

Graphite oxide has been investigated as a possible room-temperature chemiresistive sensor of ammonia in a gas phase. Graphite oxide was synthesized from high purity graphite using the modified Hummers method. The graphite oxide sample was investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, thermogravimetry and differential scanning calorimetry. Sensing properties were tested in a wide range of ammonia concentrations in air (10–1000 ppm) and under different relative humidity levels (3%–65%). It was concluded that the graphite oxide–based sensor possessed a good response to NH3 in dry synthetic air (ΔR/R0 ranged from 2.5% to 7.4% for concentrations of 100–500 ppm and 3% relative humidity) with negligible cross-sensitivity towards H2 and CH4. It was determined that the sensor recovery rate was improved with ammonia concentration growth. Increasing the ambient relative humidity led to an increase of the sensor response. The highest response of 22.2% for 100 ppm of ammonia was achieved at a 65% relative humidity level.

High-Performance Ammonia Gas Sensors Based on Plasma Treated Carbon Nanostructures

Sensors based on multi-walled carbon nanotubes were functionalized by oxygen plasma treatment and plasma co-polymerization of maleic anhydride and acetylene, obtaining core-shell carbon nanopartices covered by functional groups. The active nanostructured carbonaceous material was investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. It was confirmed that plasma treatment increases the content of surface functional groups. The sensing properties of the functionalized material were measured in the range of 10–1000 ppm of NH3 at room temperature. The plasma treated sensor showed an extremely high response of 22.5%, 27.9%, and 31.4% to 100 ppm, 250 ppm, and 500 ppm of NH3, respectively. It was shown that the nature of NH3 adsorption on the surface of functionalized nanostructured carbon material is physical adsorption.

Carbon Nanostructures in MEMS Applications

A new field emission pressure sensor based on carbon nanotubes (CNTs) emitting array is described in this paper. The pressure sensor is based on the principle that the field emission current is correlated with the electrical field intensity, i.e. the variable anode-emitter distance caused by pressure changes when the applied voltage is fixed. The sensor consists of two main parts, silicon etched membrane anode and CNTs emitting array cathode. The silicon single crystal (Si) wafer with <100> orientation is doped by phosphorus to obtain low resistant value. The CNTs were grown by plasma enhanced chemical vapour deposition by using iron as a catalyst in atmospheric pressure microwave torch. The field emission device has high emission current density, low threshold voltage, resistance to radiation, and quick response.

Gas Pressure Sensor Based on PECVD Grown Carbon Nanotubes

In this paper the development and fabrication of the pressure sensor based on electron field emission from carbon nanotubes (CNTs) was described. The sensor consisted of two parts: a silicon membrane as an anode; and multi-walled CNTs on a silicon cathode, creating a vacuum micro-chamber. Both electrodes were fabricated from the silicon single crystal (Si) wafer of an orientation 100 doped by phosphorus. The CNTs were grown by a plasma enhanced CVD using an iron catalyst in an atmospheric pressure microwave torch. The catalyst was patterned into an area corresponding to the membrane dimensions. The thin CNTs with a diameter of about 80 nm were standing vertically perpendicular to the substrate due to a crowding effect. In order to find the threshold current, the emission characteristics of prepared sensors were measured.