Heteronanowires Fabrication And Possible Applications Biology Essay

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A nanowire is a kind of nanostructure, which has a diameter in nanometer range. In addition, the quantum mechanical effects are important at the tens of nanometres or less nanoscales. In this case, it also named quantum wires.

There are several types of nanowires, such as, insulating nanowires (e.g. SiO2 and TiO2), semiconducting nanowires (e.g. GaN, InP, and Si) and metallic nanowires (e.g. Au, Pt, and Ni). [1] Additionally, whether the nanowire perform metallic, semiconducting or insulating, the conductive properties of nanowire is mainly depends on the diameter of the nanowire. [2] In this case, a nanowire can switch itself from metallic to insulating. Furthermore, not only the conductive property of nanowire depends on the nanowire's diameter, but also the melting point of the nanowire depends on the diameter of the nanowire. [2]

There are also many methods for synthesising nanowires, such as cutting (e.g. start from thin film or multilayer, cut by saw), gas phase synthesis (e.g. vapour-liquid-solid process, VLS), template synthesis (e.g. inside carbon nanotube, around carbon nanotube with/without carbon nanotube), step edge decoration, and Ion or e-beam assisted wire deposition. [2] Additionally, the nanowires can be used in many electronics, such as, sensor and FET-transistors. [2]

Furthermore, the heterostructure nanowire is a kind of nanowire which consists of different materials. There are mainly two types of heterostructure nanowires, the one with alternative elements along the wire direction, the other one with alternative inside and outside of the rod. In this case, the heterostructure nanowires not only have the nanowires' unique properties, but also gain some chemical and physical properties from the different components. Therefore, the heterostructure nanowires are expected to have the chemical and physical properties of the components. There are several kinds of heterostructure nanowires, such as, MgO/SnO2 core-shell heteronanowires, Bi/Bi2S3 heteronanowires, and M/M2S (M=Ag, Cu) heteronanowires. In this report, the fabrication and possible applications of MgO/SnO2 core-shell heteronanowires and M/M2S (M=Ag, Cu) heteronanowires will be introduced.

MgO/SnO2 Core-Shell Heterostructure Nanowires

The magnesium oxide (MgO) is a kind of wide bandgap insulator, which can be widely used in electronics and it is also important for using in paint, toxic waste remediation, refractory and superconductors. [3]

The tin oxide (SnO2) is a kind of wide bandgap n-type semiconductor (Eg=3.6eV at 300k), which is mainly used in transistors [4], transparent conducting electrodes [5], solar cells [6], and gas sensors [7].

Due to the small dimensions and unique properties of the one-dimensional (1D) hetero-nanowires, they are promising for the applications of the nanodevices. [3] Therefore, the heterostructure nanowires that consist of both of MgO and SnO2 are expected to have the unique chemical and physical properties. Furthermore, due to the unique properties of heteronanowires, the MgO/SnO2 heteronanowires might be used in sensors, catalysts and nanoscale optoelectronics. [3] A schematic of the MgO core, SnO2 shell heteronanowire is shown below.

Schematic 1. Structure of MgO core, SnO2 shell heteronanowire.

During the experiment, the two-step evaporation technique was used for producing MgO core, SnO2 shell nanowires. [3] In detail, the pure MgB2 powders were heated at 900°C under Ar and O2 condition for preparing the core MgO nanowires. Then the resulting products were deposited on gold coated Si substrates and the Si substrates were located inside a quartz tube. The evaporation method was used with Sn powder for fabricating the MgO core, SnO2 shell nanowires. The A piece of MgO nanowires grown substrate was placed with the deposition side downwards which on the top of the alumina boat with the Sn powders. Then it was heated at 900°C under Ar and O2 condition and the percentages of Ar and O2 gas are 3% and 97%. The resulting nanowires were tested by XRD, SEM and TEM. In order to test the properties of the resulting nanowires, in the following step, the some of the resulting nanowires were placed in quartz tube and heated at 800°C for 10min and tested by XRD, SEM and TEM. Some images of MgO core, SnO2 shell nanowires is shown below. [3]

Figure 1. (a) SEM image of MgO core, SnO2 shell nanowires, (b) Elemental map of Mg for nanowires, (c) Elemental map of Sn for nanowires. [3]

Fig 1a shows the SEM image of the MgO core, SnO2 shell heterostructure nanowires. Fig 1b and 1c shows the elemental map of Mg and Sn for the heteronanowires. According to the Fig 1b and 1c, the elemental distribution of the heteronanowires shows clearly. Fig 1b shows that, the Mg element mainly distributes in the centre of the nanowires (core), and Fig 1c shows that the Sn element distributes in the outer position (shell) of the nanowires.

Additionally, after 10min annealing of the quartz tube at 800°C, the sample's SEM image and elemental map are shown below.

Figure 2. (a) SEM image of annealed MgO core, SnO2 shell heteronanowires, (b) Elemental map of Mg in heteronanowires, (c) Element map of Sn in heteronanowires. [3]

Fig 2a shows the SEM image of 10min 800°C annealed MgO core, SnO2 shell heterostructure nanowires, which represent that the surface of heteronanowires became rough by annealing. Both of the Fig 2b and 2c are the elemental map of Mg and Sn, due to the annealing of the MgO core, SnO2 shell heteronanowires, both of the Mg and Sn elements distribution not only the core of the heteronanowires, but also the outer position of the heteronanowires.

As a result, the MgO core, SnO2 shell heterostructure nanowires can be fabricated by heating the Sn powders onto the grown MgO nanowires. In addition, 10min 800°C annealing of the MgO core, SnO2 shell heteronanowires can make the SnO2 of the heteronanowires become rough and make the Mg and Sn distribute more evenly.

M/M2S (M=Ag, Cu) Heterostructure Nanowires

Since the electrons and ions reaction and migration, the complexity and structural characterisation of the mixed conductors remain unclear. [8] Thus, the microscopic mechanism remains unclear nether. Therefore, studying the mixed ionic-electronic conductors (MIECs) at the nanoscale for investigating nanoscale solid electrochemistry is important, especially for low dimensional nanostructure materials, for instance, arrays of nanowires and nanowires. In addition, the metallic contacts of MIEC, for instance, Cu or Ag can be controllable formed by applying a pulse current or voltage to the Cu2S or Ag2S crystal. Therefore, more attention were attracted to Ag2S and Cu2S like metal chalcogenides nanostructures, and the metal (Ag or Cu)/metal chalcogenides (Ag2S or Cu2S) heterostructure nanowires can be used for exploring MIEC-based nanoelectronics.[8]

The M/M2S (M=Ag, Cu) heteronanowires were fabricated by the template-confined step-electrochemical technique.[8] Additionally, the template confined step electrochemical technique is scalable, effective and low-cost. [8]

For the template synthesis, anodic oxidised aluminium oxide (AAO) membranes that with a pore diameter in a range of 20-200 nm were used. [8] The branch side of the AAO membrane were plasma sputtered with a layer of Ag or Pt. In this case, the AAO membrane can be used as a working electrode and a standard one-compartment, three-electrode cell was used for electrochemical fabrication. It also was used for the measurements with a Pt plate and a KCl saturated Ag/AgCl reference electrode as a counter electrode. [8]

Firstly, at a constant and low plating current density (0.3-0.8mA/cm2), the metal (Cu or Ag) nanowire array was electroplated into the cylinder pores of the AAO membrane from AgNO3- and CuSO4- based solutions.[8] Then, in an aqueous 0.01-M Na2S solution (pH≈12.0), the anodic polarisation was applied to grow the metal chalcogenides at a constant current density (0.2-9.5 mA/cm2). In addition, the hydrosulfide (HS-) is the main S species source in the solution. [9] The resulting products were tested by XRD, SEM and TEM, and a schematic of the template synthesis from AAO membrane is shown below.

Schematic 2. The template synthesis from AAO membrane.[8]

The SEM and TEM images of Ag/Ag2S heteronanowires and Cu/Cu2S heteronanowires are shown below.

Figure 3. (a) SEM image of Ag/Ag2S heteronanowires, (b) SEM image of Cu/Cu2S heteronanowires, (c) TEM image of Ag/Ag2S heteronanowire, (d) Schematic of Ag/Ag2S heteronanowire and Cu/Cu2S heteronanowires.

According to the Fig 3a, SEM image of Ag/Ag2S nanowires have a similar diameter of 20nm. The formed Ag phase has a face centred cubic and the formed Ag2S phase has a monoclinic structure. [8] Fig 3b shows the SEM image of Cu/Cu2S heteronanowires and all of them have a similar diameter as well. Compared with the Ag/Ag2S heteronanowires with Cu/Cu2S heteronanowires, the heteronanowires in each sample have a similar diameter and it is obvious that the Cu/Cu2S heteronanowires have a more complex structure than the Ag/Ag2S heteronanowires. Fig 3c shows the TEM image of a single Ag/Ag2S heteronanowire, it shows clearly that the heteronanowire contains of Ag and Ag2S at low resolution transmission electron microscopy. However, when it was imaged by the high resolution transmission electron microscopy, the structure of the Ag/Ag2S heteronanowire was damaged by electron beam irradiation. [8] In addition, compared with Cu/Cu2S heteronanowire, the Ag/Ag2S heteronanowire was damaged harder than Cu/Cu2S heteronanowire, when they were imaged by high resolution transmission electron microscopy. [8]

As a result, the M/M2S (M=Ag or Cu) heterostructure nanowires were fabricated successfully by the template confined step electrochemical technique, which is low cost, scalable and effective. Both of the produced Ag/Ag2S heterostructure nanowires and Cu/Cu2S heterostructure nanowires have an even diameter and were damaged by the electron beam irradiation of the high resolution TEM.

Comparison

The MgO core, SnO2 shell heterostructure nanowires can be fabricated by a two-step evaporation process. The Mg and Sn element distribute in the core and surface of the produced heterostructure nanowire. However, when the produced heteronanowire was heated at 800°C for 10min, the surface of the heteronanowire became rough and the Mg and Sn element distribute more evenly. The MgO core, SnO2 shell heterostructure nanowires can provide a wide range of applications of nanoscale optoelectronics, catalysts and sensors.

The M/M2S (M=Ag or Cu) heterostructure nanowires can be fabricated successfully by the template-confined step-electrochemical technique. In addition, the synthesis methos is low cost, scalable and effective. The heteronanowires contain both of the metal phase and metal chalcogenides. In addition, this kind of heterostructure will be damaged by the electron beam irradiation of the high resolution TEM. The M/M2S (M=Ag or Cu) heterostructure nanowires can be used in nanoelectronics devices.

Although both of the MgO core, SnO2 shell heterostructure nanowire and M/M2S (M=Ag or Cu) heterostructure nanowires have different synthesis method, they are fabricated successfully and can be used in the nanoelectronics devices.

Conclusion

In conclusion, the fabrication and possible applications of the MgO core, SnO2 shell heterostructure nanowire and M/M2S (M=Ag or Cu) heterostructure nanowires are introduced in this essay. In addition, both of the two mentioned nanowires are fabricated successfully by different methods and can be used in nanoelectronics devices.

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