Quasi One Dimensional Metal Oxide Nanostructure Biology Essay

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The rapid development of nanoscience in recent years led to the versatile usages of the quasi-one-dimensional (Q1D) nano-structures. And the metal oxides are crystalline consisting of a metal cation and an oxide anion. It is very fascinating to apply the Q1D nanotechnology to the sort of structure to produce a series of materials with well constructed chemical composition, surface terminations, avoiding dislocation and other disadvantages. This material presents a very different physical property from their coarse-grained poly crystalline counterpart due to the nano geometry.

The purpose of this article is providing a review of the Q1D metal oxide semiconductors (MOS) with the focusing on the preparation, characterization and application and discusses the future challenges. It starts with the introduction of the Q1D nanostructure and metal oxide semiconductor, then describing the growth mechanisms that are important for the preparation of Q1D metal oxide. The techniques of the construction of nanowires are presented. And last the recent advanced application will be selected and discussed.

1. Introduction

Nanotechnology is a new advanced technology that is dealing with the atomic and molecule scale materials (nanomaterials). Generally we believed that nanomaterials should include two basic conditions: First of all, the scale of nanomaterials is equal to or less than 100 nanometers in one dimension or more. Secondly, the sort of materials contains different physical and chemical properties comparing to other conventional size materials. Nanomaterials are widely introduced to many new applications in material industries, electrical and biological engineering.

Over the past 10 years, people use various methods to gradually synthesize a vast range of Q1D materials, such as nanowires, nanobelts, nanoneedles, nanotubes, nonorods, nanorings, hierrchical structures, nanorings and core-shell nanowiresis (Figure 1) [1]

Figure 1 Schematic drawing of some nanostructures: (a) nanowire, (b) core-shell nanowire, (c)nanotube, (d)nanobelt, (e)hierarchical structure, (f)nanorod and (g)nanoring [1]

Q1D metal oxide nanostructures have several main advantages due to the high aspect ratio comparing the traditional materials such as higher ratio of surface-to-volume, nano-scale dimensions, superior stability regarding to the high crystallinity, simple preparation methods, possible fictionalization of their surface, modulation of the operating temperature, catalyst deposition and the possibility of field-effect transistors configuration.

This article presents a brief review of the Q1D metal nanostructures used for a range of applications. It will start with the introduction of the Q1D nanostructure and metal oxide semiconductors, investigating the growth mechanisms and pointing out the construction of the nanostructures and last presenting the applications and future challenges.

2. Quasi-one dimensional materials

With the rise of quasi-one dimensional materials, a number of studies based on this sort of material appear in the science and technology reports. One-dimensional material is a new nano-material which is in the nano-scale for the two-dimensional direction, but the marco-scale for the length. The material has nearly 30 years of history, as early as 1970, the French scientists first developed a 7 nm diameter carbon fiber. For the first time in 1991, Japan discovered the carbon nanotubes by the high resolution electron microscopy. Chinese scientists Xie Si-Shen, who achieved the growth of aligned carbon nanotubes, successfully synthesized the world´s longest carbon nanotubes (Figure 2). Carbon nanotubes promote the entire quasi-one dimensional materials. Scientists from France and Japan in 1997 and 1998, successively developed nano-coaxial cable. This cable with a nano-level core is called nanowires and the outside is coated with nano-level thickness insulating layer. This kind of geometric structure is similar to an ordinary coaxial cable.

With the growing of the family of Q1D dimensional nanostructures and the further study on the structure and the properties of Q1D materials, the establishment of new theories of Q1D structures promotes the basic application of this sort of nano-structured devices.

Figure 2 Carbon nanotube look

3. Metal oxide semiconductor (MOS)

In a conventional metal oxide semiconductor (MOS), there are generally 3 layers (Figure 3). The upper layer, functioning as an electrode, consists of conductive materials (metal). The lower layer, containing crystal silicon materials, is another conductive electrode. An insulator, always made out of some glass or silicon dioxide materials, is between the upper and lower surface.

The Q1D MOS finds a good comprise between the constraints above due to the high aspect ratio.

Figure 3 Schematic structure of a basic MOS

4. Material Growth Mechanisms

4.1 Two approaches to the production of 1D structures

There are two different approaches to produce Q1D structure: top-down and bottom up technologies.

The former approach seeks to create smaller devices by using larger ones to orient the assembly, basing upon standard micro fabrication methods with deposition, which is widely used in such as atomic force microscope (Figure 4), focused ion beams and atomic layer deposition.

We can produce the high-ordered nanowires in top-down approach but the high cost and the complicated devices are the main constrains for this approach's wide application in industry.

Figure 4 Atomic Force Microscopy

The latter one, bottom-up approach, focus on arranging smaller components into more complicated assemblies. This technology fulfills the requirement about the low cost of the experiments and the easy way of setting up the implementation environment but consisting of some troubles on the transfer and contacting on transducers. There are some applications known as DNA nanotechnology, bis-peptides.

For the production of Q1D nanowires, the best approach is the combination of the two methods.

4.2 Growth Mechanisms

The growth of Q1D nanostructures is the crucial step for the preparation of construction. There are two main methods regarding to the synthesis environment: vapor phase growth and solution phase growth. The first method is utilized most frequently to grow the metal oxide structure with the vapor-liquid-solid (VLS) process or the vapor-solid (VS) process. Meanwhile, the second approach is more flexible and costs less compared to the first method.

4.2.1 Vapor phase growth

This process requires the assistance of a thermal furnace. The thermal furnace controls the chemical and physical interaction between oxygen gas and metallic vapor directly. In this process, the vapor-solid (VS) or vapor-liquid-solid (VLS) mechanisms domain play a key pole depending on the environment.

The VS process can be observed in many catalyst free growths. For example, Figure 5 shows the a thermal furnace synthesis system. The Gas is directed to the system via O ring and then heated to vapor in the high temperature region of the system. After going through the lower temperature region, it is condensed on the growth substrate. This growth is a fairly complex process without fine quantity models. [2]

The VS process is still requiring more basic study since the mechanism for this phenomenon is not fully discovered.

Figure 5 Thermal furnace synthesis system

The vapor-liquid-solid (VLS) growth is the most important mechanism for the vapor phase growth and the preparation of Q1D structures. Regarding this reality that the growth is too slow, scientists introduce a catalytic liquid alloy phase to accelerate this process. This addition can rapidly adsorb a vapor to super-saturation levels leading to the dramatic growth. At the liquid-solid interface, this growth can subsequently continue in a form of nucleated seeds. Obviously the VLS growth is a catalyst-assisted growth. For instance, in figure 6 it presents the VLS growth of nanowires. In the substrate, the introduced catalyst and precursors exist in the liquid phase (blue) and then the precursors grow and condense to form a nanowire. [1]

Figure 6 Vapor-Liquid-Solution (VLS) growth of nanowires

There are some typical features of the VLS method: [3]

1. Lower reaction energy compared to VS growth;

The wires growth only occurs in the catalyst activated areas;

The metal catalysts can determine the size and position of wires;

The production of highly anisotropic nanowire arrays from various materials.

2.2.2 Solution Phase Growth

Many types of nanostructures can grow in solution. The growth method can produce large quantities of nanostructures by scaled-up compared to the methods that production on a surface, and only requires the normal temperature so it can also considerably reduce the costs and the complexity of fabrication. The main mechanism for the solution phase growth can be categorized into two methods: template-assisted and template-free methods.

The first method can be utilized to produce the large-area patterning of Q1D metal oxide nanowires by assisting of the template. On the other hand, without the nanomaterials inside a template, the second method directs the Q1D nanostructure growth in a liquid environment.

In the solution-based growth, a nano-scale metallic droplet catalyzes the decomposition of the precursors and the growth of crystalline nanowire. The solution-liquid-solid (SLS) mechanism plays like a catalyst that catalyzes the nanowires growth (Figure 7). Precursors are in the liquid phase and react to form the nanowire.

Figure 7 Solution-Liquid-Solid (SLS) growth of nanowires

5. Doping of Q1D metal oxide nanostructures

To change and develop the electrical properties of a semiconductor, the doping is crucial. The process can give new features to the semiconductor by adding some impurities it. Doping can change the electronic, mechanical and chemical properties of Q1D nanostructures that give the access to produce these materials with desired properties via different dopants. The heavy doping is categorized when the high order dopants are added. This is often shown as n+ for n-type doping or p+ for p-type doping. Figure 8 shows the n-type doping and p-type doping respectively. Both the high quality n- and p- type materials are indispensable due to meet the demand of potential application by metal oxides. It is very important to control doping with intrinsic or extrinsic elements to maintain their properties.

Figure 8 n-type (left) and p-type (right) doping

As an example, nanostructured zinc oxide (ZnO) is one of the most common metal oxide materials, and naturally used to produce an n-type semiconductor. Q1D ZnO nanowires and nanobelts have unique electrical and optical properties because of the natural defects such as oxygen vacancies and zinc interstitials, and they are widely used as field- effect transistors (FET). Figure 10 displays a schematic model of the growth processes for ZnO nanowire and nanobelt and presents a change in the nucleation behavior of ZnO.

Figure 9 Schematic model of the ZnO growth processes

6. Preparation of Q1D metal oxide nanostructures

As our discussed above, there are mainly several methods to grow and dope the Q1D metal oxide. In the meanwhile, the Q1D structures can be utilized as templates for the growth of heterostructured materials as well.

In Figure 10 the typical shapes of heterostructures are shown and they are (a) dendritic growth, (b)super lattice in a single nanowire, (b) polycrystals coalescence on a single backbone and (d) core-shell geometry, and due to their structures, functional properties of each shape are entirely different. VS and VLS growth mechanisms are the typical combination which enables the crystalline assembly in a predefined growth directions and modifications [1].

Figure 10 Typical shapes for heterostructures

Basically, the heterostructures are created from the spatially controlled doping of single nanowires when there is a NWs growth. Vertically grown single-crystalline ZnO Nanowires were introduced to create the nano-junctions and selective doping was done to achieve these nano-junctions. During crystal nucleation, with aluminium as donor a single section of Nanowires was doped and thus resulting in n-n+ junction [5].

Different shapes of Heterostructures made of ZnO (single material) have been obtained: 2D and 1D ZnO nanostructures, which are vertically aligned, are grown on electrically conducting, highly oriented pyrolytic graphite (HOPG) and on insulating sapphire substrates [6].

Moving a step ahead, in order to obtain the heterostructures from a single material is the multistep oxidation of metal nanowires. In this way, the most arrays of metal-metal oxide core-shell nanowires and single-crystalline metal oxide nanotubes are obtained [7]. The basics behind this process, is the kinetic control of the conversion of single-crystalline Bi nanowires to Bi-Bi2O3 core-shell nanowires through the slow oxidation method, and then the conversion to a single crystalline Bi2O3 through fast oxidation is also controlled.

7. Applications of metal oxide nanostructures

7.1 Coaxial-Field effect transistors

The schematic model of Coaxial-Field nanowire transistor is shown in Figure 11. A p-doped Si core (blue) with the layers of i-Ge, SiOx and p-Ge is presented in the cross section of the nanowire. The source S and drain D electrodes are contacted to the inner i-Ge core and the gate G, which is isolated from the core due to the SiOx layer, is attached to the outer p-Ge layer.

The capacitance enhancement is an advantage of coaxial geometry for nano-FETs. The contacts in this structure are Ohmic-contacts (low contact resistivity), which is an added advantage to it [9]. In future, the carrier mobility's can be increased in nanowire semiconductor devices by using the modulation-doped core-shell structure [8].

Figure 11: Coaxial-gated NW transistor [8].

7.2 Logic gates

Electrically switching function of Diodes and FET is the key point to produce higher order of circuits. If we take an integrated computation circuit, the key components are these logic gates. The Q1D metal oxide structure with the feature of the transistor function in the electrical transport is confirmed to produce this device. The logic units like, ''OR'', ''NOR'', ''AND'', and ''NOT'' with n-type ZnO nanorods are designed by park et al. [10].

Figure 12 ZnO nanorod logic devices

In Figure 12, the first two logic devices in (a) and (b) are fabricated using two Schottky diodes based on single and double ZnO nanorods respectively. In the same way, the next two logic devices in (c) and (d) are fabricated using FETs (single and double respectively) based on a single nanorod. The fig 12(a) shows the schematic, scanning electron microscopy image (SEM), and characteristics of a device, which uses OR logic gate. The output voltage vs the logic input configurations {Vo vs. (V1, V2): (0, 0), (1, 0), (0, 1) and (1, 1)}. For the Logic input 1 is 3 V and for the logic input 0 is 0 V. 12(b) shows the logic device AND. At 3 V, Vc is biased for this measurement. (c) shows A NOT logic gate. Vo vs. Vi, where logic inputs 0 and 1 are 0 V and 3 V, respectively. At 3 V biased is the Vc. And (d) shows a NOR gate where the Voltage bias is not different from that of the NOT gate. 2 mm is the scale bar [10].

7.3 Light emitting diodes (LED):

A LED is a common semiconductor light source which can be found as an indicator lamp in many devices. In the LED, radial heterostructured nanowires are used because of its selective peak wave length. Figure 13, Low temperature photoluminescence (PL) measurement of the nanowires of GaAs exhibits a blue shifted peak at 1.6ev (Figure 14).

Figure 13 PL spectrums [11] Figure 14 Blue LED

7.4 Solar cell

The continuous increase in energy consumption has created an immediate need for alternative/next generation technologies which are cleaner, renewable and environmental friendly. The clear solution for the above mentioned problem is solar power, from which we can get electricity directly from sunlight through a simple process, which is undoubtedly cost effective and earth friendly. Here, in solar cell system, nano-materials are used as an absorbing layer in photovoltaic solar cells. In this absorber layer, where the nanoscale materials are employed have discrete energy level which will increase efficiency of the solar conversion. The advantages of nanowires are crystallinity of higher order, very high carrier collection rate, and designs of the cells which can use non-standard electrolyte, in which the recombination rates are higher than the liquid electrolyte cell [2]. Most current research aims for enhancing the conversion efficiency of a Q1D solar cell and reducing the cost.

Figure15 ZnO nanowire array based dye-sensitized cell

7.5 Superlattice Nanowires

Superlattice nanowires have a periodic potential thanks to its mini gaps and mini bands (Figure 16). It has been confirmed theoretically that strong thermoelectric merit can be achieved in superlattice nanowires rather than conventional nanowires or superlattice thin-films. These nanowires can work as a electron filter because of its periodic potential, which only allows the electrons of certain energy to pass through. And the axial nanowire heterostructures have been used in single-electron transistors and optical barcodes [9].

Figure16. Superlattice nanowire and periodic potential

8. Conclusion

In this review on the Q1D MOX nanostructures, we found that different properties of nanowire heterostructures are related to their geometry and factors that control the mechanism like temperature and time of growth. Synthesis of Q1D structures has been well developed and the metal oxide nanowires, nanoneedles, nanobelts and nanotubes are obtained successfully. And the applications like FET, LED, and Solar Cell etc are studied. These fascinating applications imply the quasi 1D metal oxide may play major role in replacing the non-renewable energy sources and can be a ladder for new researches in the field of optoelectronic devices and nanoelectronic field.

The key issues found during the study are VLS mechanism currently is the main method for the growth of most of the nanowires and the solution-based technique is a promising approach for mass production of Q1D MOX materials. The next is achieving a p-type ZnO nanostructures and fabrication of intra-nanowire p-n junctions. The further challenges for the Q1D MOX will always be the commercial fabrication and the cost reduction.