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The rapid development of nanoscience in recent old ages led to the versatile uses 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 really absorbing to use the Q1D nanotechnology to the kind of construction to bring forth a series of stuffs with good constructed chemical composing, surface expirations, avoiding disruption and other disadvantages. This material presents a really different physical belongings from their farinaceous poly crystalline opposite number due to the nano geometry.

The intent of this article is supplying a reappraisal of the Q1D metal oxide semiconducting materials ( MOS ) with the concentrating on the readying, word picture and application and discusses the hereafter challenges. It starts with the debut of the Q1D nanostructure and metal oxide semiconducting material, so depicting the growing mechanisms that are of import for the readying of Q1D metal oxide. The techniques of the building of nanowires are presented. And last the recent advanced application will be selected and discussed.

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1. Introduction

Nanotechnology is a new advanced engineering that is covering with the atomic and molecule graduated table stuffs ( nanomaterials ) . Generally we believed that nanomaterials should include two basic conditions: First of wholly, the graduated table of nanomaterials is equal to or less than 100 nanometres in one dimension or more. Second, the kind of stuffs contains different physical and chemical belongingss comparing to other conventional size stuffs. Nanomaterials are widely introduced to many new applications in stuff industries, electrical and biological technology.

Over the past 10 old ages, people use assorted methods to bit by bit synthesise a huge scope of Q1D stuffs, such as nanowires, nanobelts, nanoneedles, nanotubes, nonorods, nanorings, hierrchical constructions, nanorings and core-shell nanowiresis ( Figure 1 ) [ 1 ]

Figure 1 Conventional drawing of some nanostructures: ( a ) nanowire, ( B ) core-shell nanowire, ( degree Celsius ) nanotube, ( vitamin D ) nanobelt, ( vitamin E ) hierarchal construction, ( degree Fahrenheit ) nanorod and ( g ) nanoring [ 1 ]

Q1D metal oxide nanostructures have several chief advantages due to the high facet ratio comparing the traditional stuffs such as higher ratio of surface-to-volume, nano-scale dimensions, superior stableness sing to the high crystallinity, simple readying methods, possible fictionalisation of their surface, transition of the operating temperature, accelerator deposition and the possibility of field-effect transistors constellation.

This article presents a brief reappraisal of the Q1D metal nanostructures used for a scope of applications. It will get down with the debut of the Q1D nanostructure and metal oxide semiconducting materials, look intoing the growing mechanisms and indicating out the building of the nanostructures and last showing the applications and future challenges.

2. Quasi-one dimensional stuffs

With the rise of quasi-one dimensional stuffs, a figure of surveies based on this kind of stuff appear in the scientific discipline and engineering studies. Unidimensional stuff is a new nano-material which is in the nano-scale for the planar way, but the marco-scale for the length. The stuff has about 30 old ages of history, every bit early as 1970, the Gallic scientists foremost developed a 7 nm diameter C fibre. For the first clip in 1991, Japan discovered the C nanotubes by the high declaration negatron microscopy. Chinese scientists Xie Si-Shen, who achieved the growing of aligned C nanotubes, successfully synthesized the world?s longest C nanotubes ( Figure 2 ) . Carbon nanotubes promote the full quasi-one dimensional stuffs. Scientists from France and Japan in 1997 and 1998, in turn developed nano-coaxial overseas telegram. This overseas telegram with a nano-level nucleus is called nanowires and the exterior is coated with nano-level thickness insulating bed. This sort of geometric construction is similar to an ordinary coaxial overseas telegram.

With the turning of the household of Q1D dimensional nanostructures and the farther survey on the construction and the belongingss of Q1D stuffs, the constitution of new theories of Q1D structures promotes the basic application of this kind of nano-structured devices.

Figure 2 Carbon nanotube expression

3. Metal oxide semiconducting material ( MOS )

In a conventional metal oxide semiconducting material ( MOS ) , there are by and large 3 beds ( Figure 3 ) . The upper bed, working as an electrode, consists of conductive stuffs ( metal ) . The lower bed, incorporating crystal Si stuffs, is another conductive electrode. An dielectric, ever made out of some glass or Si dioxide stuffs, is between the upper and lower surface.

The Q1D MOS finds a good comprise between the restraints above due to the high facet ratio.

Figure 3 Conventional construction of a basic Moment

4. Material Growth Mechanisms

4.1 Two attacks to the production of 1D constructions

There are two different attacks to bring forth Q1D construction: top-down and bottom up engineerings.

The former attack seeks to make smaller devices by utilizing larger 1s to point the assembly, establishing upon standard micro fiction methods with deposition, which is widely used in such as atomic force microscope ( Figure 4 ) , focussed ion beams and atomic bed deposition.

We can bring forth the high-ordered nanowires in top-down attack but the high cost and the complicated devices are the chief constrains for this attack ‘s broad application in industry.

Figure 4 Atomic Force Microscopy

The latter one, bottom-up attack, concentrate on set uping smaller constituents into more complicated assemblies. This engineering fulfills the demand about the low cost of the experiments and the easy manner of puting up the execution environment but consisting of some problems on the transportation and reaching on transducers. There are some applications known as DNA nanotechnology, bis-peptides.

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

4.2 Growth Mechanisms

The growing of Q1D nanostructures is the important measure for the readying of building. There are two chief methods sing to the synthesis environment: vapour stage growing and solution stage growing. The first method is utilised most often to turn the metal oxide construction with the vapor-liquid-solid ( VLS ) procedure or the vapor-solid ( VS ) procedure. Meanwhile, the 2nd attack is more flexible and costs less compared to the first method.

4.2.1 Vapor stage growing

This procedure requires the aid of a thermic furnace. The thermic furnace controls the chemical and physical interaction between O gas and metallic vapour straight. In this procedure, the vapor-solid ( VS ) or vapor-liquid-solid ( VLS ) mechanisms domain play a cardinal pole depending on the environment.

The VS procedure can be observed in many accelerators free growings. For illustration, Figure 5 shows the a thermic furnace synthesis system. The Gas is directed to the system via O ring and so heated to vapor in the high temperature part of the system. After traveling through the lower temperature part, it is condensed on the growing substrate. This growing is a reasonably complex procedure without all right measure theoretical accounts. [ 2 ]

The VS procedure is still necessitating more basic survey since the mechanism for this phenomenon is non to the full discovered.

Figure 5 Thermal furnace synthesis system

The vapor-liquid-solid ( VLS ) growing is the most of import mechanism for the vapor stage growing and the readying of Q1D constructions. Sing this world that the growing is excessively slow, scientists introduce a catalytic liquid metal stage to speed up this procedure. This add-on can quickly adsorb a vapour to super-saturation degrees taking to the dramatic growing. At the liquid-solid interface, this growing can subsequently go on in a signifier of nucleated seeds. Obviously the VLS growing is a catalyst-assisted growing. For case, in figure 6 it presents the VLS growing of nanowires. In the substrate, the introduced accelerator and precursors exist in the liquid stage ( bluish ) and so the precursors grow and condense to organize a nanowire. [ 1 ]

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

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

1. Lower reaction energy compared to VS growing ;

The wires growing merely occurs in the accelerator activated countries ;

The metal accelerators can find the size and place of wires ;

The production of extremely anisotropic nanowire arrays from assorted stuffs.

2.2.2 Solution Phase Growth

Many types of nanostructures can turn in solution. The growing method can bring forth big measures of nanostructures by scaled-up compared to the methods that production on a surface, and merely requires the normal temperature so it can besides well cut down the costs and the complexness of fiction. The chief mechanism for the solution stage growing can be categorized into two methods: template-assisted and template-free methods.

The first method can be utilized to bring forth the large-area patterning of Q1D metal oxide nanowires by helping of the templet. On the other manus, without the nanomaterials inside a templet, the 2nd method directs the Q1D nanostructure growing in a liquid environment.

In the solution-based growing, a nano-scale metallic droplet catalyzes the decomposition of the precursors and the growing of crystalline nanowire. The solution-liquid-solid ( SLS ) mechanism dramas like a accelerator that catalyzes the nanowires growing ( Figure 7 ) . Precursors are in the liquid stage and react to organize the nanowire.

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

5. Doping of Q1D metal oxide nanostructures

To alter and develop the electrical belongingss of a semiconducting material, the doping is important. The procedure can give new characteristics to the semiconducting material by adding some drosss it. Doping can alter the electronic, mechanical and chemical belongingss of Q1D nanostructures that give the entree to bring forth these stuffs with coveted belongingss via different dopants. The heavy doping is categorized when the high order dopants are added. This is frequently shown as n+ for n-type doping or p+ for p-type doping. Figure 8 shows the n-type doping and p-type doping severally. Both the high quality n- and p- type stuffs are indispensable due to run into the demand of possible application by metal oxides. It is really of import to command doping with intrinsic or extrinsic elements to keep their belongingss.

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

As an illustration, nanostructured Zn oxide ( ZnO ) is one of the most common metal oxide stuffs, and of course used to bring forth an n-type semiconducting material. Q1D ZnO nanowires and nanobelts have alone electrical and optical belongingss because of the natural defects such as O vacancies and Zn interstitials, and they are widely used as field- consequence transistors ( FET ) . Figure 10 displays a conventional theoretical account of the growing processes for ZnO nanowire and nanobelt and presents a alteration in the nucleation behaviour of ZnO.

Figure 9 Conventional theoretical account of the ZnO growing procedures

6. Preparation of Q1D metal oxide nanostructures

As our discussed supra, there are chiefly several methods to turn and dope the Q1D metal oxide. In the interim, the Q1D constructions can be utilized as templets for the growing of heterostructured stuffs every bit good.

In Figure 10 the typical forms of heterostructures are shown and they are ( a ) dendritic growing, ( B ) super lattice in a individual nanowire, ( B ) polycrystals coalescency on a individual anchor and ( vitamin D ) core-shell geometry, and due to their constructions, functional belongingss of each form are wholly different. VS and VLS growing mechanisms are the typical combination which enables the crystalline assembly in a predefined growing waies and alterations [ 1 ] .

Figure 10 Typical forms for heterostructures

Basically, the heterostructures are created from the spatially controlled doping of individual nanowires when there is a NWs growing. Vertically adult single-crystalline ZnO Nanowires were introduced to make the nano-junctions and selective doping was done to accomplish these nano-junctions. During crystal nucleation, with aluminum as giver a individual subdivision of Nanowires was doped and therefore ensuing in n-n+ junction [ 5 ] .

Different forms of Heterostructures made of ZnO ( individual stuff ) have been obtained: 2D and 1D ZnO nanostructures, which are vertically aligned, are grown on electrically carry oning, extremely oriented pyrolytic black lead ( HOPG ) and on insulating sapphire substrates [ 6 ] .

Traveling a measure in front, in order to obtain the heterostructures from a individual stuff is the multistep oxidization of metal nanowires. In this manner, the most arrays of metal-metal oxide core-shell nanowires and single-crystalline metal oxide nanotubes are obtained [ 7 ] . The rudimentss behind this procedure, is the kinetic control of the transition of single-crystalline Bi nanowires to Bi-Bi2O3 core-shell nanowires through the slow oxidization method, and so the transition to a individual crystalline Bi2O3 through fast oxidization is besides controlled.

7. Applications of metal oxide nanostructures

7.1 Coaxial-Field consequence transistors

The conventional theoretical account of Coaxial-Field nanowire transistor is shown in Figure 11. A p-doped Si nucleus ( bluish ) with the beds of i-Ge, SiOx and p-Ge is presented in the cross subdivision of the nanowire. The beginning S and run out D electrodes are contacted to the interior i-Ge nucleus and the gate G, which is isolated from the nucleus due to the SiOx bed, is attached to the outer p-Ge bed.

The electrical capacity sweetening is an advantage of coaxal geometry for nano-FETs. The contacts in this construction are Ohmic-contacts ( low contact electric resistance ) , which is an added advantage to it [ 9 ] . In future, the bearer mobility ‘s can be increased in nanowire semiconducting material devices by utilizing the modulation-doped core-shell construction [ 8 ] .

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

7.2 Logic Gatess

Electrically exchanging map of Diodes and FET is the cardinal point to bring forth higher order of circuits. If we take an incorporate calculation circuit, the cardinal constituents are these logic Gatess. The Q1D metal oxide construction with the characteristic of the transistor map in the electrical conveyance is confirmed to bring forth 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 utilizing two Schottky rectifying tubes based on individual and dual ZnO nanorods severally. In the same manner, the following two logic devices in ( degree Celsius ) and ( vitamin D ) are fabricated utilizing FETs ( individual and dual severally ) based on a individual nanorod. The fig 12 ( a ) shows the conventional, scanning negatron microscopy image ( SEM ) , and features of a device, which uses OR logic gate. The end product electromotive force vs the logic input constellations { 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 measuring. ( degree Celsius ) shows A NOT logic gate. Vo vs. Vi, where logic inputs 0 and 1 are 0 V and 3 V, severally. At 3 V biased is the Vc. And ( vitamin D ) shows a NOR gate where the Voltage prejudice is non different from that of the NOT gate. 2 millimeter is the scale saloon [ 10 ] .

7.3 Light breathing rectifying tubes ( LED ) :

A LED is a common semiconducting material visible radiation beginning which can be found as an index lamp in many devices. In the LED, radial heterostructured nanowires are used because of its selective extremum moving ridge length. Figure 13, Low temperature photoluminescence ( PL ) measuring of the nanowires of GaAs exhibits a blue shifted extremum at 1.6ev ( Figure 14 ) .

Figure 13 PL spectrums [ 11 ] Figure 14 Blue LED

7.4 Solar cell

The uninterrupted addition in energy ingestion has created an immediate demand for alternative/next coevals engineerings which are cleaner, renewable and environmental friendly. The clear solution for the above mentioned job is solar power, from which we can acquire electricity straight from sunlight through a simple procedure, which is doubtless cost effectual and earth friendly. Here, in solar cell system, nano-materials are used as an absorbing bed in photovoltaic solar cells. In this absorber bed, where the nanoscale stuffs are employed have distinct energy degree which will increase efficiency of the solar transition. The advantages of nanowires are crystallinity of higher order, really high bearer aggregation rate, and designs of the cells which can utilize non-standard electrolyte, in which the recombination rates are higher than the liquid electrolyte cell [ 2 ] . Most current research purposes for heightening the transition efficiency of a Q1D solar cell and cut downing the cost.

Figure15 ZnO nanowire array based dye-sensitized cell

7.5 Superlattice Nanowires

Superlattice nanowires have a periodic possible thanks to its mini spreads and mini sets ( Figure 16 ) . It has been confirmed theoretically that strong thermoelectric virtue can be achieved in superlattice nanowires instead than conventional nanowires or superlattice thin-films. These nanowires can work as a negatron filter because of its periodic potency, which merely allows the negatrons of certain energy to go through through. And the axial nanowire heterostructures have been used in single-electron transistors and optical barcodes [ 9 ] .

Figure16. Superlattice nanowire and periodic potency

8. Decision

In this reappraisal on the Q1D MOX nanostructures, we found that different belongingss of nanowire heterostructures are related to their geometry and factors that control the mechanism like temperature and clip of growing. Synthesis of Q1D constructions has been good 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 absorbing applications imply the quasi 1D metal oxide may play major function in replacing the non-renewable energy beginnings and can be a ladder for new researches in the field of optoelectronic devices and nanoelectronic field.

The cardinal issues found during the survey are VLS mechanism presently is the chief method for the growing of most of the nanowires and the solution-based technique is a promising attack for mass production of Q1D MOX stuffs. The following is accomplishing a p-type ZnO nanostructures and fiction of intra-nanowire p-n junctions. The farther challenges for the Q1D MOX will ever be the commercial fiction and the cost decrease.

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