1. Introduction

1-1 Developing bio-sensor for application in medical diagnosis

Recently, bio-molecular analysis has become one of the most important tools in

medical diagnosis. Highly sensitive and accurate methods for molecular diagnosis are

urgently needed. Currently, commercial diagnostic kits are not efficient for

quantitative analysis and their applications are frequently limited by their sensitivities.

Recent development in biochip technology has greatly reduced the size (from ml to µl) of samples to be examined. At the same time, the sensitivity of the sensor chip needs to be significantly enhanced. Applications of nanoscale sensor chip are a major research direction to produce semiconductor sensor with the required sensitivity for a

biosensor. Over the past few years, a lot of researches focused on specific properties

of carbon nanotube (CNT) (Odom et al., 1998), particularly on the nano-scale

structure and on the one-dimensional electronic conduction, rendering it became

appropriate in a variety of nano-electronic devices. One (Tans et al., 1998) proposed that a promising application is on field effect transistors (FET). Since the first fabrication of carbonnanotube field effect transistor (CNT FET) was developed,

performance of this mannered device has been significantly improved in many aspects,

e.g., the contact between the CNT and the metal electrodes, on-off ratio of channel

current, and hole mobility. In recent years, the highly charge sensitive property of

1 CNT FET rendered it an excellent candidate of being a nano-scale sensing device.

Kong et al. (2000) were the first that built a single-walled nanotube (SWNT) chemical

sensor for the detection of NO2 and NH3 gas. Electrochemical gating was studied on

single multi-walled nanotube (MWNT) by Krüger et al. (2000) and SWNT by

Rosenblatt et al.(Rosenblatt et al., 2002). Related reports have demonstrated a few highly sensitive biosensors fabricated by distinct configurations, procedures, and biomolecule-coupling techniques (Besteman et al., 2003).

Generally, biological applications are performed in aqueous solution where an electronic device may exhibit an obvious leakage current across the two electrodes, namely the source and drain, in the FET device. In sensing circumstances, this leakage current contributes to background interference (or named background current)

(Someya et al., 2003) of a sensor hampering detections toward a lower sensing limit and a higher sensitivity, crucial parameters of any sensor, and limiting the effective working range. This significant background signal, primarily ascribed to the electrochemical electrolysis of aqueous solution on the electrode surface, dominated the conduction across the two electrodes. Furthermore, the significant background current may degrade the thin-film metal electrode under a high applied bias across the source and drain. Therefore, a useful solution effectively diminishing the leakage current is valuable and worth to be developed. In this study we propose a method

2 solving this concern and investigate the effect of leakage current of the CNT device

the biological sensing application. Further, we explore the relationship between

deionized water (resistivity～18.2 ΜΩ－cm) and NaCl solution (1x10-4M) by the passivated CNT device. Currents through the CNTs are obviously different between two liquid samples: deionized water and NaCl solution. We also examine the characters in different concentration of NaCl solution (from 1x10-6M to 1M). It also indicates that the device can distinguish the differentiation of concentration of NaCl solution by high voltage of liquid-gate. Furthermore, the result indicates that the presented CNTs devices at certain bias conditions can be environmentally sensitive.

The result also indicates that the CNT device is a potential candidate for developing a bio-sensor.

1-2 Discovery of the carbon nanotube (CNT)

Crystalline (diamond and graphite) and amorphous (carbon black, pyrocarbon,

etc.) forms of carbon have pride of place among technological materials – think of the

abrasive properties of diamonds, the lubricating properties of graphite, and the performance of carbon fibers, with a micro-graphite structure, used in many applications. Two new forms of carbon have recently been discovered and application.

Fullerenes were discovered in 1985, by an Anglo-American team (Kroto et al., 1985), and nanotubes in 1991, by a Japanese team (Iijima et al., 1991). A nanotube is a

3 cylinder with a graphite structure (curved, like a roll of chicken wire) which closed at

both ends by a fullerene type cap (Odom et al., 1998). These cylinders can be a few

microns or even millimeters long, with a diameter of the order of a nanometer (10-9 m) – hence their name. They were discovered by a Japanese electron microscopist

Sumio Iijima, who was studying the material deposited on the cathode during the arc-evaporation synthesis of fullerenes (Iijima et al., 1991). He found that the central core of the cathodic deposit contained a variety of closed graphitic structures including nanoparticles and nanotubes, of a type which had never been observed previously. A short time later, Thomas Ebbesen and Pulickel Ajayan, from Iijima's laboratory, showed how nanotubes could be produced in bulk quantities by changing the arc-evaporation conditions. This paved the way to a research explosion into the physical and chemical properties of carbon nanotubes in laboratories all over the world.

1-3 Types and properties of CNT

Carbon nanotubes are cylindrical molecules of ~1 nm in diameter and 1-100

microns in length (Odom et al., 1998). They are constituted of carbon atoms only, and

can essentially be thought of as a sheet of graphite (a hexagonal lattice of carbon)

rolled into a cylinder. There are two kinds of nanotube –MWNT and SWNT. Most of

these tubular fullerene molecules consisting of multiple shells are arranged in a

4 coaxial fashion. Individual MWNTs can be easily resolved. MWNTs, however, have

many unexplained features, e.g. the multi-layered structure affects electrical

conduction in the MWNT devices. On the other hand, SWNTs, discovered in 1993,

serve as a model system for theoretical calculation and some key experiments because

of their simple and well defined structure. Nevertheless, the SWNTs tend to form a

bundled structure, which contains many SWNTs of various lengths. It is quite difficult

to resolve the SWNT bundle into individual SWNT and it will be an important technique to isolate the bundle into individual SWNT effectly.

Single-walled carbon nanotubes exist in a variety of structures corresponding to the many ways a sheet of graphite can be wrapped into a seamless tube. Each structure has a specific diameter and chirality, or wrapping angle. The “armchair” nanotubes, with a = 30°, have metallic characteristic. The “zigzag” nanotubes, for which a = 0°, can be either semimetallic or semiconductive, depending on the specific diameter. “Helical” nanotubes with chiral angles intermediate between 0 and 30° include both semimetals and semiconductors. (“Armchair” and “zigzag” refer to the pattern of carbon–carbon bonds along a tube’s circumference.)

Therefore, carbon nanotubes, depending on their structure, can be used as metals or semiconductors (Mintmire et al., 1992; Saito et al., 1992). Understanding the

5 electronic properties of the graphene sheet helps to understand the electronic

properties of carbon nanotubes. Graphene is a zero-gap semiconductor; for most

directions in the graphene sheet, there is a band-gap, and electrons are not free

flowing along those directions unless they are given extra energy. However, in certain

special directions graphene is metallic, and electrons flow easily along those

directions. This property is not obvious in bulk graphite, since there is always a

conducting metallic path which can connect any two points, and hence graphite

conducts electricity. However, when graphene is rolled up to make the nanotube, a

special direction is selected along the axis of the nanotube. Sometimes it exhibits

metallic property, and sometimes is semiconducting property. Therefore, both metals

and semiconductors can be made from the same all-carbon system

In addition to their interesting electronic structure, nanotubes have several other useful properties. Nanotubes are incredibly stiff and tough mechanically - the world's strongest fibers. Nanotubes conduct heat as well as diamond at room temperature.

Nanotubes are very sharp, and thus can be used as probe tips for scanning-probe

microscopes, and field-emission electron sources for lamps and displays.

The above characteristics have generated strong interest in their possible use in

nano-electronic and nano-mechanical devices. For example, they can be used as

6 nano-wires or as active components in electronic devices such as the field-effect transistor (Martel et al., 1998; Tans et al., 1998).

Thus, carbon nanotubes are a novel material system whose unique properties offer intriguing possibilities for the fabrication of nanometre-scale molecular electronic device. Carbon nanotubes can be thought of as naturally occurring nano size bricks that could act as base components for nano-electronics. By building up the nanobricks, nano scale device structures, which cannot be fabricated from three-dimensional bulk materials, can be constructed. Prior to construction, there are a number of issues to be addressed. Thus, nanotubes are ideal candidates in molecular electronic technology.

1-4 Application of CNT

The discovery of carbon nanotubes prompted a great interest in the electronic and mechanical properties of these novel one-dimensional materials (Wilder et al.,

1998). Band-structure calculations show that the conductive properties of nanotubes depend strongly on the tube diameter as well as on the helicity of the hexagonal carbon lattice along the tube.

Today, carbon nanotubes are driving scientific research. This field has several important directions in basic research, including chemistry, electronic transport,

7 mechanical and field emission properties (Chen et al., 2001; Jeong et al., 2004).

Furthermore, the perspective applications are very challenging and exciting. The main avenues of potential applications of carbon nanotubes are: ultimate reinforcement fibers for composites (high strength, high aspect ratio, high thermal and chemical stability), conducting nanowires, field emitters (individual nanotube field emitters, large area flat panel displays (Saito et al., 2003)) and nanotools (tips for scanning tunneling, atomic force, magnetic resonance force and scanning nearfield optical, chemical/biological force microscope tips, nanomanipulators, nanotweezers)

(Woolley et al., 2000).

1-5 Production of CNTs

(1) Electric Arc Discharge

The arc discharge technique involves the generation of an electric arc between two graphite electrodes, one of which is usually filled with a catalyst metal powder

(eg. iron, nickel, cobalt), in a Helium atmosphere(Keidar et al., 2004).

(2) Laser Ablation

The laser ablation method uses a laser to evaporate a graphite target which is usually filled with a catalyst metal powder too (Puretzky et al., 2002).

(3) Chemical Vapor Deposition

The basic ingredients needed for CVD growth of nanotubes are small catalyst

8 particles (typically iron or iron/molybdenum) and a hot environment of

carbon-containing gas (ex: CH4 or C2H4). The metal particles catalyze the decomposition of the carbon-containing gases, and the carbon dissolves in the catalyst particles. Once the catalyst particles are supersaturated with carbon, it extrudes out the excess carbon in the form of a tube. One catalyst particle of a few nanometers in diameter can produce a nanotube millimeters in length, about 1 million times the diameter of the particle (Cheung et al., 2000).

1-6 Development of CNT FET

Field effect transistors (FETs) made from semiconducting single-walled carbon

nanotubes (SWNTs) have been intensely investigated since they were first made by

Tans et al. (1998). In these and other early studies, an individual single-walled CNT

bridged two noble-metal electrodes prefabricated by lithography on an oxidized

(oxide thickness, 100–200 nm) silicon wafer. The CNT played the role of the FET

channel, while the two metal electrodes functioned as the source and drain electrodes,

respectively. The heavily doped silicon wafer was used as the gate (back gate), and

the thick SiO layer as the gate insulator.

1-7 Fabrication of CNT FET

The basic FET structure involves two metal electrodes designated as “source”

9 and “drain” connected by a semiconducting channel (Tans et al., 1998). In

conventional devices, the channel is made of Si. However, in CNT FET, the channel is

replaced by an SWNT. A third electrode, the “gate,” is separated from the channel by

a thin insulator film. Normally, if no charge is placed on the gate, no charge flows

through the channel. In a p-type (i.e., hole conducting) FET, when a negative charge is

placed on the gate and the applied voltage VGS exceeds a certain threshold, VTh, then a

hole current flows through it. Similarly, for an n-type FET, an electron current flows

when a positive charge is on the gate and the voltage exceeds the threshold.

1-8 P, N-type CNT FET

It is well known that obtaining both p- and n-type materials and controlling their

charge carrier densities are crucial to the current microelectronics. With SWNTs, an

interesting phenomenon has been that tube-FETs under ambient conditions are always

p-type with holes as the majority carriers (Collins et al., 2000; Kong et al., 2000). This

has been recently revealed to be due to electron withdrawing by O2 molecules adsorbed on SWNTs. The ability to tune SWNTs into both n and p-type should be

important to their applications in molecular electronics. A classical approach to n-type

electron-rich carbon materials is via charge-transfer doping with alkali metals

(Derycke et al., 2002). Indeed, potassium doping has led to n-type SWNT FETs and

enabled the derivation of more complex devices such as intra-tube p-n junctions with

10 interesting electronic functions. However, alkali dopants suffer from immediate

degradation upon exposure to air, rendering them undesirable for n-doping of SWNTs in practical device application

1-9 Application of CNT FET

The reported properties of SWNT transistors have varied widely due to

variations in the quality of the nanotube material, the device geometry, and the

contacts. Optimizing their properties is crucial for applications in both electronics and

in chemical and biological sensing. For electronic applications, a number of

parameters dictate the performance of an FET, such as mobility and transconductance.

For sensing, the ability to work in the appropriate environment (e.g., salty water for

biological applications) is critical. Their interesting electronic structure makes carbon

nanotubes ideal candidates for novel molecular devices. Metallic NTs, for example,

were utilized as Coulomb islands in single-electron transistors and, recently, Tans and

coworkers (1998) built a molecular FET with a semiconducting nanotube.

Carbon nanotube FET (CNT FET) is suitable for sensor applications since they

are known to exhibit charge-sensitive conductance (Bachtold et al., 2001; Collins et

al., 2001; Franklin et al., 2002; Javey et al., 2002). Nanotube FET has been used in

gaseous phase sensing by Kong et al. (2000). Many sensor applications, such as those

11 involving biological analytes, require aqueous solution (Krüger et al., 2001;

Rosenblatt et al., 2002). In their experiments, an electrolyte solution was used to form

the gate electrodes. However, water is often detrimental to semiconductor devices.

The leakage current will affect the result of measurement in aqueous solution. In this

work, we fabricated a passivated CNT FET. The passivation layer effectively

decreased the quantity of leakage current. Furthermore, the current obtained in

aqueous solution with various concentration of NaCl (1x10-8～1M) was evaluated.

The results demonstrated that the passivated CNT FET is capable of determining the ion concentration in aqueous solution under an additional voltage for inducing the separation of charges. It also implied the CNT FET was feasibly employed to detect the ions produced from specifically biological reactions.

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