Carbon nanotubes ( CNTs ) are found to be promising stuff for nanoelectronics due to its extremist little size and alone belongingss. Therefore, people have focused on researching its applications in nanoelectronics, such as replacing the conventional transistors, obstructionists and detectors, etc. In recent old ages, monitoring and controlling of pH has become an of import facet of many industrial procedures. Micro and nano stuffs, such as Carbon nanotubes ( CNTs ) , are good campaigners to fabricate micro or nano electronic devices. These devices which have a additive relationship of I-V feature could do operational magnifying circuits unneeded. Comparing with other traditional pH detectors, Nano pH detector with CNTs can supply more benefits due to their alone metallic and high current denseness belongingss which are presented in this paper.
After the find of CNT in early 1990s, many research workers focus in utilizing C nanotube ( CNT ) to construct assorted nanoelectronic devices. A batch of people have tried to analyze the belongingss of CNT late [ 1-4 ] . Advantages of Carbon Nanotube ( CNT ) electrodes for biosensors include high electrical conduction, a chemically inert electrode, high mechanical strength of a little investigation, the ability to turn the nanotube array on different substrates and in different forms, and nanoscale size of the electrode with a high facet ratio [ 5 ] . Carbon Nanotubes ( CNTs ) closely resemble hollow black lead fibres that exist in embroiled packages of 10s to 100s [ 6 ] . These come in two different signifiers: multi walled C nanotubes ( MWCNT ) and individual walled C nanotubes ( SWCNT ) . SWCNTs and MWCNTs scope in diameter from 1-10 nanometers and 10-50 nanometer severally. About 70-80 % of SWCNT tend to incorporate semiconducting belongingss, whereas 70-80 % of MWCNT tend to incorporate metallic belongingss [ 7 ] . Metallic CNTs can be used as connecting wires for Micro-Electro-Mechanical Systems ( MEMS ) and Nano-Electro-Mechanical Systems ( NEMS ) because of their size and low opposition, while semi-conducting CNTs can be used for nano transistors [ 8 ] .
The first type of individual pH detector [ 11 ] developed in the 70s old ages was a pH trial band based on the soaking up of pH index covalently immobilized on the cellulose matrix. In 1980 Peterson presented the first optical pH detector. The detector made used of the optical density dye phenol ruddy and was applied for rating of blood pH in-vivo and in-vitro. The dye was immobilized into polystyrene microspheres. Recently largely used absorbance-based pH indexs were phenol ruddy, bromothymol blue and other. In 1982 Saari reported the first fluorescence-based pH detector where fluoresceinamin was covalently immobilized on cellulose. The most often used pH indexs are hydroxypyrene trisulfonic acid Na salt ( HPTS ) , carbxyfluorescein derived functions ( e.g. mono- , dichlorocarboxyfluorescein ) , seminaphthorhodafluor ( SNARF ) and hydroxycoumarins [ 11 ] .
Biomedical applied scientists have exploited chiefly the possibilities of the bit engineering to develop silicon-based detectors. Compared to the normally piecewise-assembled detectors, the duplicability of detector features should be extremely improved due to the reproduction process of Si engineering. Therefore, in biomedical technology literature, many documents present silicon detectors such as ion detectors. Ion-Selective Field-Effect Transistor ( ISFET ) pH detector [ 9 ] is one of the most well-known illustrations. Furthermore, with more and more survey on CNTs, as mentioned in [ 10 ] CNTs with metallic belongingss have a immense potency to bring forth more compact devices for pH measuring. The possibility to hive away hydroxyl ions ( OH- ) and hydrogen ions ( H+ ) in C nanostructures has been clarified in some documents. In add-on, the strong dependance of electronic belongingss of CNT construction on chemical environment was besides reported [ 10 ] . Those consequences led to the applications of CNT as feeling stuff in pH detector application. CNT detector can change the electrical belongingss of the CNT by adsorbing the molecules on the surface of the CNT. In this paper, I will discourse the fiction of CNT based pH detector and present experiment process and demo the respond of feeling material surface to pH environment.
1. Introduction to MOSFET
A metal-oxide-semiconductor field-effect transistor ( MOSFET ) is based on the transition of charge concentration by a MOS electrical capacity between a organic structure electrode and a gate electrode located above the organic structure and insulated from all other device parts by a gate insulator bed which in the instance of a MOSFET is an oxide, such as Si dioxide [ 12 ] . If insulators other than an oxide such as Si dioxide ( frequently referred to as oxide ) are employed the device may be referred to as a metal-insulator-semiconductor FET ( MISFET ) . Compared to the MOS capacitance, the MOSFET includes two extra terminuss ( beginning and drain ) , each connected to single extremely doped parts that are separated by the organic structure part. These parts can be either p or n type, but they must both be of the same type, and of opposite type to the organic structure part. The beginning and drain ( unlike the organic structure ) are extremely doped as signified by a ‘+ ‘ mark after the type of doping.
If the MOSFET is an n-channel or nMOS FET, so the beginning and drain are ‘n+ ‘ parts and the organic structure is a ‘p ‘ part. As described above, with sufficient gate electromotive force, holes from the organic structure are driven off from the gate, organizing an inversion bed or n-channel at the interface between the p part and the oxide. This carry oning channel extends between the beginning and the drain, and current is conducted through it when a electromotive force is applied between beginning and drain.
If the MOSFET is a p-channel or pMOS FET, so the beginning and drain are ‘p+ ‘ parts and the organic structure is a ‘n ‘ part. When a negative gate-source electromotive force ( positive source-gate ) is applied, it creates a p-channel at the surface of the n part, correspondent to the n-channel instance, but with opposite mutual oppositions of charges and electromotive forces. When a electromotive force less negative than the threshold value ( a negative electromotive force for p-channel ) is applied between gate and beginning, the channel disappears and merely a really little subthreshold current can flux between the beginning and the drain.
The beginning is so named because it is the beginning of the charge bearers ( negatrons for n-channel, holes for p-channel ) that flow through the channel ; likewise, the drain is where the charge bearers leave the channel.
The device may consist a Silicon On Insulator ( SOI ) device in which a Buried OXide ( BOX ) is formed below a thin semiconducting material bed. If the channel part between the gate insulator and a Buried Oxide ( BOX ) part is really thin, the really thin channel part is referred to as an Ultra Thin Channel ( UTC ) part with the beginning and drain parts formed on either side thereof in and/or above the thin semiconducting material bed. Alternatively, the device may consist a Semiconductor On Insulator ( SEMOI ) device in which semiconductors other than Si are employed. Many alternate semiconducting material stuffs may be employed.
When the beginning and drain parts are formed above the channel in whole or in portion, they are referred to as Raised Source/Drain ( RSD ) parts
2. Introduction to ISFET
An ISFET is by and large used to mensurate ion concentrations in solutions. Actually, an ISFET ‘s beginning and drain are constructed likewise as a Metal-oxide Semiconductor Field-Effect Transistor ( MOSFET ) [ 6 ] . The basic construction of a MOSFET is formed by adding two to a great extent doped n+ parts to the MOS capacitance on p-type Si as shown in Fig.1. When the gate electromotive force VG exceeds its threshold electromotive force, so an inversion bed is formed at the SiO2/Si interface. The n+-source part can provide negatrons to the inversion part without depending on the thermic coevals rate as required for the MOS capacitance. An n+-drain part should be added so that negatrons can flux from beginning to the drain through the inversion bed when a positive drain electromotive force VD is applied. This negatron flow constitutes the drain current ID. Current into the drain is taken as a positive current and the positive gate electromotive force controls the figure of negatrons in the inversion bed and hence controls the drain current.
Fig.1. Conventional diagram of an MOSFET: 1 drain ; 2 beginning ; 3 substrate ; 4 gate ; 5 dielectric ; 6 metal contacts ; 7 inversion bed. [ 6 ]
3. Introduction to CNT based ISFET
Although an ISFET is really similar to a MOSFET, there are still some differences. As shown in Fig.2, the metal gate is replaced by the metal of a mention electrode, whilst the mark liquid in which this electrode is present makes contact with the bare gate dielectric. Both of them have the same tantamount circuit. Then, devices with this construction can be applied to pH measurement [ 13 ] . However, the aim of this paper is to heighten the inversion bed with CNTs as NANO wire to carry on negatrons between the drain and beginning, the drain current might be much greater under the same gate voltage.Fig.3 illustrates the possible application of both SWCNTS and MCNTs in an ISFET construction pH measurement device. If this is verified, so we can do these devices compact and cheap. [ 6 ]
Fig.2. Conventional diagram of a composite gate, double dielectric ISFET: 1 drain ; 2 beginning ; 3 substrate ; 4 dielectric ; 5 metal contacts ; 6 mention electrode ; 7 solution ; 8 electroactive membrane ; 9 encapsulant ; 10 inversion bed [ 6 ] .
Fig.3. CNT based ISFET: 1 N-doped drain ; 2 N-doped beginning ; 3 P-type silicone substrate ; 4 SWNT as transistor ; 5 MWCNT as nano-wire ; 6 dielectric ; 7 metal contacts ; 8 refernce electrode ; 9 solution ; 10 electroactive membrane ; 11 encapsulate [ 6 ]
4. Carbon nanotubes
Carbon nanotubes are the most studied category of nanotube/nanowire FETs [ 14 ] . A C nanotube consists chemically of a sheet of black lead rolled up into a tubing ( Fig.4 ) . Because the chemical bonds are all satisfied, there are no “ suspension ” bonds, minimising surface sprinkling and taking to high mobility conveyance. Carbon nanotubes can be single-walled ( SWNT ) or multi-walled ( MWNT ) . Typical dimensions are 1-3 nanometer for SWNTs and 20-100 nanometer for MWNTs. Clearly, for individual walled nanotubes, they realize the promise of critical dimensions smaller than any current lithographic technique. [ 15 ]
The electronic belongingss of C nanotubes depend on both their diameter and chirality ( correspondent to the figure of bends per inch of a prison guard ) . Depending on the chirality, the nanotube can be either metallic or semiconducting. While Raman dispersing can find the chirality, the common trial of whether a nanotube is semiconducting or metallic is to prove whether the opposition alterations with a backgate electromotive force. For semiconducting nanotubes, the bandgap is about 1 eV/d [ nanometer ] , where vitamin D is the diameter in nanometer. Presently, there is no technique to command chirality of the nanotube during synthesis, and precise control of the diameter is besides a challenge. One method of avoiding this job would be to develop size-sorting and chirality-sorting techniques to insulate a peculiar nanotube from a heterogenous mixture.
Fig. 4: Single walled C nanotube. [ 14 ]
The singular electrical belongingss of SWNTs root from the unusual electronic construction of the planar stuff, character. Graphene-a individual atomic bed of graphite-consists of a 2-D honeycomb construction of sp bonded C atoms, as seen in Fig. 5 ( a ) . Its set construction is rather unusual ; it has carry oning provinces at, but merely at specific points along certain waies in impulse infinite at the corners of the first Brillion zone, as is seen in Fig. 5 ( B ) . [ 14 ] It is called a zero-bandgap semiconducting material since it is metallic in some waies and semiconducting in the others. In an SWNT, the impulse of the negatrons traveling around the perimeter of the tubing is quantized, cut downing the available provinces to pieces through the 2-D set construction, is illustrated in the Fig. 5 ( B ) . This quantisation consequences in tubings that are either unidimensional metals or semiconducting materials, depending on how the allowed impulse provinces compare to the preferable waies for conductivity. Choosing the tubing axis to indicate in one of the metallic waies consequences in a tubing whose scattering is a piece through the centre of a cone [ Fig. 5 ( degree Celsius ) ] . If the axis is chosen otherwise, the allowed s takes a different conelike subdivision, such as the one shown in Fig. 5 ( vitamin D ) . The consequence is a 1-D semiconducting set construction, with a spread between the filled hole provinces and the empty negatron provinces. Nanotubes can, hence be either metals or semiconducting materials ; depending on how the tubing is rolled up. This singular theoretical anticipation has been verified utilizing a figure of measuring techniques. Possibly the most direct used scanning burrowing microscopy to image the atomic construction of a tubing and so to examine its electronic construction [ 14 ] .
Fig. 5. ( a ) Lattice construction of graphene, a honeycomb lattice of C atoms. ( B ) Energy of the carry oning provinces as a map of the negatron wavevector k. There are no carry oning provinces except along particular waies where cones of provinces exist. ( degree Celsius ) , ( vitamin D ) Graphene sheets rolled into tubings. This quantizes the allowed Kansas around the circumferential way, ensuing in 1-D pieces through the 2-D set construction in ( B ) . Depending on the manner the tubing is rolled up, the consequence can be
either ( degree Celsius ) a metal or ( vitamin D ) a semiconducting material