Nanotechnology is a promising field that encompasses about all subjects of scientific discipline and technology. Generally, nanotechnology defines the constructions with at least one dimension from 1 to 100 nanometres [ 1 ] . Nanomaterials have alone physical, optical and chemical belongingss because at the nanoscale surface belongingss become dominant over majority belongingss due to the higher surface country to volume ratio and frequently belongingss of nanomaterials are highly assuring for assorted emerging technological applications, such as photonics, biosensors, memory devices, communicating, energy transition, environmental protection, and infinite geographic expedition [ 2 ] . Nano-sized stuffs can besides be used as accelerators for the growing of specific nanostructures such as nanowires, nanorods, nanotubes etc.
Silicon nanowires ( SiNWs ) have attracted immense involvement late due to their intrinsic belongingss and their possible applications in nano-sensors [ 3 ] , solar cells [ 4 ] and Li batteries [ 5 ] . SiNWs field-effect transistor ( SiNW-FET ) arrays show extremist high sensitiveness for biosensing applications [ 6 ] . In add-on, being able to tune the SiNW belongingss makes them extremely suited for feeling devices [ 7 ] .
Several growing mechanisms have been used for the production of SiNWs such as, vapour-liquid- solid ( VLS ) [ 8 ] , vapour-solid-solid ( VSS ) [ 9 ] , solid-liquid-solid [ 10 ] and oxide assisted growing [ 11 ] . Till now, VLS has been proven to be the most successful mechanism to turn nanowires, because in this procedure gaseous reactant ( e.g. gaseous Si ) can easy absorbed into the liquid accelerator and this procedure produces the nanowires at eutectic temperature of metal ( e.g. Si-Au ) , which has lower temperature than single runing point of accelerator and reactant [ 12 ] . These growing mechanisms can be employed in assorted fiction techniques, such as chemical vapour deposition ( CVD ) [ 13 ] , plasma enhanced chemical vapor deposition ( PECVD ) [ 14 ] , laser extirpation [ 15 ] , and molecular beam epitaxy [ 16 ] . In the VLS mechanism, the accelerator can be a thin bed or a nano-particle. However nano-particles are used frequently for diameter controlled growing of nanowires ; for illustration gold nano-particles ( AuNPs ) are widely used as a accelerator to turn SiNWs, because gold ( Au ) has many advantages such as high chemical stableness, non toxicity and it does non oxidise [ 17 ] . In this work, efforts to command the diameter of SiNWs are made using the VLS mechanism in PECVD and AuNPs as a accelerator.
AuNPs are one of the most widely investigated nanomaterials because of their alone chemical and physical belongingss [ 18 ] . Au NPs offer a surface chemical science which is helpful in the self-assembly of organic molecule beds [ 19 ] . They besides have several advantages over other nanomaterials, such as they are chemically stable, non-toxic and simple to functionalize for different applications [ 20 ] . They can besides be easy conjugated with DNA, enzymes, antibodies and polymers, without impacting their features in many instances [ 21 ] . Biomolecule conjugated Au NPs offer extremely sensitive and selective biosensors for diagnosing of diseases and they have high surface country which is good in drug bringing vehicles [ 22 ] . A major advantage of Au NPs is that they can be synthesized in different forms and sizes harmonizing to demand. These attractive belongingss render AuNPs utile for assorted applications including plasmonics, photonics, drug bringing, biological and chemical detection, and contact action [ 23 ] .
Several methods are available to synthesise AuNPs and command their size, such as the aerosol coevals [ 24 ] , electron beam lithography [ 26 ] , tempering of thin movie [ 27 ] , and colloidal synthesis [ 28 ] . However colloidal synthesis is farther divided into different methods depending upon the usage of different cut downing agent such as the Burst method [ 29 ] , sonolysis [ 30 ] and the Perroult method [ 31 ] . Among them citrate decrease, besides known as the Turkevich method, is widely used, because it is inexpensive, simple and good investigated method [ 28 ] . In this method, the tetrachrohydroauric acid ( HAuCl4 ) is reduced to AuNPs by the decrease reaction with trisodium citrate. The Au NPs grow by the agglomeration of gold atoms and besides by curdling with other Au atoms.
In this survey, the chief focal point is on the citrate decrease method to synthesise AuNPs, for which the “ Kinetically controlled seeded synthesis of citrate stabilized gilded nano-particles of up to 200nm: size concentrating versus Ostwald maturation ( Langmuir publication ) [ 32 ] is taken as a mention and utilize these AuNPs for growing of SiNWs. First Au seed atoms are synthesised for the assorted initial concentrations of HAuCl4 and so these seed atoms are used for the farther growing of AuNP known as seed-mediated procedure. Our attempt is to develop different sized AuNP through consecutive growing stairss of Au seed atoms.
The overall purpose of this undertaking is to synthesis Au NPs of assorted diameters for the growing of SiNWs utilizing PECVD. To accomplish this, the undertaking is divided into four chief aims:
Synthesis of gold nanoparticles by citrate decrease procedure.
Probe of the consequence of alteration in reaction temperature on size of AuNPs.
Deposition of AuNPs on Silicon ( Si ) substrate through poly-l-lysine coating.
Growth of SiNWs by PECVD utilizing different sized AuNPs.
In this respect chapter 1 provides an overview on assorted synthesis methods of Au NPs such as the Brust method, the Perroult method, sonolysis, and citrate decrease. The chief focal point will be on the citrate decrease method and its advantages. This chapter besides provides information on assorted growing methods of SiNWs and the benefits of the VLS mechanism and Au as a metal accelerator for SiNW growing.
Chapter 2 explains the methodological analysis for the controlled growing of AuNPs by citrate decrease at different procedure parametric quantities such as the concentration of gold precursor, pH and temperature. It besides describes the deposition of AuNPs on Si substrates through ploy-l-lysine coating and the synthesis of SiNWs by PECVD. Assorted word picture techniques such as UV-Vis Spectroscopy, SEM, and AFM are besides explained.
Chapter 3 inside informations the consequences of the AuNPs fiction procedure and the morphological surveies of the SiNWs are besides included.
Finally, the decisions are described and suggestions are made for any future work.
Chapter 1 LITERATURE REVIEW
1.1 Synthesis of AuNPs
The extremely controlled NW growing has been reported from assorted methods such as aerosol-generated AuNPs [ 24, 25 ] , tempering of Au thin movies [ 26 ] , electron beam lithography ( EBL ) generated AuNPs [ 27 ] , and colloidal synthesis of AuNPs [ 28 ] . Synthesis and deposition of AuNP with different methods show differences in controllability of size, place, surface denseness and pureness. A certain type of method could be utile in one manner but may be less utile from another point of position or features, and therefore it is non possible to state that which method is best. In this subdivision the different methods of AuNP synthesis are discussed.
1.1.1 Aerosol Generated Atoms
The aerosol method is used to bring forth agglomerative gold atoms through vaporization of gold [ 24 ] . This method requires a high temperature furnace to vaporize Au straight from a solid piece of Au. The evaporated Au is carried off by a bearer gas ( N ) and initial atoms are formed by homogeneous nucleation, which farther grow by consecutive curdling of atoms. The size of AuNPs can be controlled by altering the temperature of the furnace, which may increases or decreases the agglomeration of atoms and consequences in bigger diameter and smaller diameter of NPs severally [ 25 ] .
The chief disadvantages of aerosol method are that it is really expensive apparatus and limited size control of atoms. It has another disadvantage that it gives random placement of atoms on the substrate. On the other manus, this method has several advantages such as highly pure AuNPs can be produced and deposited with a really controlled surface denseness. Furthermore, another advantage of the aerosol deposition method is that the AuNPs can be deposited onto any type of substrate.
1.1.2 Annealing of thin movie
The common procedure for synthesis of AuNPs is thermic vaporization of thin Au movie, straight onto the substrates [ 26 ] . Normally the thickness of the movie can change between 0.1 nanometers and a few nanometres. In this method, the substrate is heated up to elevated temperature in order to run the movie and divide up into NPs. When these NPs are used as NWs seed particles the heating measure occurs inside the NW growing reactor. The temperature at heating phase somewhat affects atom distribution with higher temperatures gives larger atoms and lower atom surface densenesss [ ] .
The disadvantage of this method is that it has highly hapless controllability over atom size and surface denseness and atom placement. However this method has besides advantages such as it is a simple and moderately inexpensive method.
1.1.3 Electron Beam Lithograph ( EBL )
The formation of AuNP by EBL is rather similar to thin movie vaporization method, since coevals of NP through EBL besides requires vaporization of a thin gold movie and warming of substrate [ 27 ] . However, before deposition of Au movie, a form has been defined onto the substrate that determines diameter, surface denseness and place of the NPs. Figure 2 is a conventional of the EBL atom synthesis procedure. First deposit a resist bed over substrate the substrate. Then expose the substrate by the negatron beam to acquire the coveted form and sedimentation a thin Au movie by thermic vaporization onto the substrate. After that removes the resist and the Au bed over resist by liftoff procedure. In last merely desired patterned Au movie is left over substrate. After heating the substrate the patterned Au movie transforms into the AuNP.
The major advantage of EBL procedure is that it is extremely controlled synthesis procedure in footings of size, form and surface denseness ; nevertheless the exact place of AuNPs can besides be governable. The chief drawback with EBL is that it can non be processed over substrates that are excessively thin and brickle. Other drawbacks of EBL are that it is expensive, complicated and clip taking procedure.
Figure-1.1: Schematic of the EBL procedure. ( a ) The substrate coated with an negatron beam sensitive resist that to specify. ( B ) The coveted form after exposed by the negatron beam. ( degree Celsius ) thermally evaporated thin Au movie onto the substrate. ( vitamin D ) After Liftoff a good defined gold disc. ( vitamin E ) Upon heating the Au disc is transformed into an AuNP [ Source: 27 ] .
1.1.4 Colloidal synthesis of AuNPs
In this reappraisal, a elaborate overview is presented on the synthesis of size controlled spherical AuNPs and the usage of these AuNPs as a templet to turn the Si NWs. Usually Au NPs are prepared by chemical decrease of an appropriate gold precursor ( normally HAuCl4 ) , with a cut downing agent such as organic acids, sugars, aldehydes, intoxicants and other strong cut downing agents ( e.g. NH2NH2 and NaBH4 ) [ 33 ] . Extensive research has been carried out to synthesise monodispersed AuNPs by assorted methods such as the explosion [ 29 ] and citrate decrease [ 28 ] . The focal point will be on citrate decrease as a method for the readying of monodispersed spherical gold nanoparticles, and will be the method applied in this work. However, other fiction techniques are besides described.
This method was discovered by Brust and Schiffrin in 1990 and it is used to organize AuNPs in organic liquids that are normally non mixable with H2O [ 29 ] . In this method a solution of HAuCl4 and a solution of tetraoctylammonium bromide ( TOAB ) in methylbenzene are assorted with sodium borohydride ( NaBH4 ) , which consequences in 4-6 nm-sized AuNPs being formed. NaBH4 is the cut downing agent and TOAB is use as a accelerator every bit good as stabilising agent and AuNPs aggregate easy in the solution because TOAB does non adhere to the AuNPs strongly, so the whole procedure takes about two hebdomads. To cut down the procedure clip, a stronger binding agent, like a thiol ( in specific, alkanethiols ) can be added in the solution, which will covalently adhere to the Au atoms.
This method was discovered by Perrault and Chan in 2009 [ 31 ] . In this method hydroquinone is used to cut down HAuCl4 in an aqueous solution of Au seeds. Perroult method is similar to the photographic movie development procedure, in which Ag grains ( nano atoms ) within the movie grow through the adhesion of reduced Ag atoms onto their surface. Similarly, hydroquinone reduces HAuCl4 into ionic gold. These gold atoms so attach to the surface of Au seed NPs.. The Perroult method can bring forth atoms of between 30-250 nm [ 31 ] .
Sonolysis is another procedure which is used for the synthesis of AuNPs. In this procedure the decrease reaction takes topographic point between an aqueous solution of HAuCl4 and glucose. In reaction, hydroxyl groups and sugar pyrolysis groups are cut downing agents. Through this procedure thread shaped AuNPs are obtained and the breadth of these nanoribbons can be 30 -50 nm breadth and lengths of several microns [ 30 ] . Spherical AuNPs can be prepared by this method, when glucose is replaced by cyclodextrin ( a glucose oligomer ) because glucose is important in directing the morphology towards a thread like construction.
Citrate decrease method
In 1951 Turkevich et Al. produced AuNPs, based on individual stage aqueous decrease of HAuCl4 by Na citrate and in 1973 Frens et Al. showed the consequence of altering concentration of Na citrate on AuNPs size through this method [ 28 ] . It has been reported that the sum of cut downing agent and gold precursor can command the size and form of Au NPs during the seed-mediated growing [ 29 ] .
In this method the gold precursor ( HAuCl4 ) is reduced by trisodium citrate. The Au3+ ions of HAuCl4 are reduced to impersonal Au atoms. These atoms start to adhere together and organize the Au seed atoms ; this is called the nucleation procedure. At the same clip, negatively charged citrate ions, liberated during the decrease procedure, attach to the surface of AuNPs and forestall them from agglomerating. When the entire surface country of gold atoms is covered by citrate ions, so atom will no longer turn in size. If more citrate ions are available, so they will be able to cover a larger surface country and smaller AuNPs can be produced. So the concluding size and form of the atoms would depend on the size of seed atoms, the sort and concentration of cut downing agent and the sum of the precursor added [ 34 ] .
The chief advantage of Turkevich method is that it is good understood proved that at it produces spherical AuNPs with controlled size. A comparing between assorted methods of AuNP synthesis is shown in table-1.
Table- 1.1: comparing between assorted methods of AuNP synthesis. [ Beginning: 27 ] .
1.2 Adsorption of AuNPs on wafer surface
AuNPs can be deposited on to a substrate by electrostatic bonding between AuNPs and the positively charged polymers such as Poly-L-Lysine ( PLL ) and Aminopropyltrimethoxysilane ( APTES ) . For lodging polymer and the AuNP over substrate, assorted methods such as Poly-L-Lysine coating and submergence are used.
1.2.1 Immersion method
In this method, the cleaned substrate is dipped in to the 10 % ethanol solution of 3- Aminopropyltrimethoxysilane ( APTES ) [ 35 ] . APTES have positively charged atoms which make the surface adhesive for negatively charged nanoparticles. Due to electrostatic bonding NPs are absorbed onto the substrate by plunging the substrate in an aqueous AuNP solution. This surface assimilation was controlled by the clip for which the substrate was immersed in to the AuNP solution. This procedure has disadvantages in that it requires a big sum of solutions for proper submergence of substrate.
1.2.2 Poly-L-Lysine coating
Through this procedure a thin bed of Poly-L-Lysine ( PLL ) is absorbed onto the substrate by spin coating. PLL is used because it has positively charged atoms which makes adhering with negatively charged NPs. For spin surfacing the substrate is placed over spin coater and bead 0.2ml of PLL and revolve it between 500 to 3000rpm. After that a droplet of Au colloid solution is placed over the substrate and the substrate is coated once more. PLL have positively charged atoms and AuNPs have negative charge, so the AuNPs attaches to the substrate by electrostatic interactions [ 35 ] .
In this work, focal point is on Poly-L-Lysine coating because it is advantageous over submergence, it requires really little sum of solutions ( 0.3-0.5 milliliter of PLL and AuNPs ) for AuNPs deposition.
1.3 Different mechanisms of SiNWs growing
In recent old ages research on SiNW fiction has increased quickly due to their possible applications in opto-electronics, biosensors and solar cells. There are chiefly two common attacks to manufacture NWs ; called “ bottom-up ” and “ top-down ” attack. Top-down fiction method is dwelling of deposition, modeling and etching of bulk stuff for eg. electron beam lithography and abrasion. However, Bottom up attack refers to the buildup of a stuff from atom by atom or molecule by molecule. Both attacks have their ain advantages and disadvantages and play really of import function in nano engineering. The chief disadvantage of top down attack is the defect in surface construction and important crystallographic harm to the processed forms. Other hands the bottom-up method fabricate NWs with less defects. But through this method the control of NW place is hard. Some of bottom-up methods are discussed below:
1.3.1 Solution-Liquid-Solid ( SLS ) Procedure
Buhro et Al. [ 36 ] have proposed a low temperature SLS method for the synthesis of crystalline nanowires of group III-V semiconducting materials. In this procedure, a metal with low runing point ( e.g. In, Sn, Bi ) is used as a accelerator, and through the decomposition of organo-metallic precursors a coveted stuff is produced. Nanowires of InP, InAs and GaAs have been prepared by low-temperature ( & lt ; 2030C ) solution stage reactions [ 37 ] .
1.3.2 Solvothermal Synthesis
Solvothermal method is entirely a solution based procedure to bring forth nanowires and nanorods [ 38 ] . In this procedure, metal precursors and dissolver are assorted and placed the assorted solution in an sterilizer for crystal growing and the assembly procedure. This methodological analysis has enabled the synthesis of crystalline nanowires of semiconducting materials and other stuffs.
1.3.3 Oxide assisted synthesis ( OAS )
Oxide-assisted growing synthesis for nanowire growing was foremost proposed by Lee et Al. [ 39 ] and does non necessitate any metal accelerator. They found that the growing of Si nanowires is improved when SiOx made marks are used. In this method growing of the Si NW is assisted by the SiOx. Through thermic vaporization or optical maser extirpation SiOx decomposes into Si bunchs by following reaction:
Si bunchs ( NPs ) acts as a nucleation or accelerator site. These NPs are in molten form which enhances the soaking up and diffusion and grows the SiNWs.
1.3.4 Vapour- Liquid-Solid ( VLS ) growing and its advantages
The VLS mechanism for Si NW growing was proposed by Wagner and Ellis [ 8 ] in March 1964. The name VLS mechanism explains the way of Si, which starts from the vapor stage, so diffuses through the liquid Au-Si droplet and ends up as a solid Si NW. The Au-Si binary stage diagram ( fig.2 ) shows that the runing point of the Au-Si metal chiefly depends on composing. Au-Si metal which has 81 atom % Au and 19 atom % Si, thaws at 363 A°C. It is clear that the liquescent temperature of Au-Si metal is about 700 K lower than the runing point of pure Au and more than 1000 K lower than the runing point of pure Si.
Figure-1.2: Conventional stage diagram of Au-Si metal. [ Beginning: Ref. 16 ]
Therefore, heating Au above 363 A°C ( 460 A°C ) in the presence of Si precursor gas signifiers liquid droplets of Au-Si, schematically depicted in Figure 3b. Exposing these Au-Si metal droplets to a Si precursor such as silane ( SiH4 ) starts diffusion of precursor atoms ( Si ) in to the surface of these droplets. At equilibrium ( supersaturation ) merely limited sum of Si atoms diffuse into the Au-Si droplets. The farther supply of Si from the gas stage forces the droplets to happen a manner to take the extra Si atoms. This leads to the growing of SiNWs with a Au-Si droplet at their tip, as shown in Figure 3d.
Figure- 1.3: VLS growing mechanism of nanowire. [ Beginning: Ref. 40 ]
Several methods of nanowires growing have discussed and all of them have some advantages and disadvantages. But normally VLS is used because of following advantages:
The VLS mechanism can be used for other stuffs eg Ge, InP, GaAs etc.
VLS grows the NWs at eutectic temperature of metal ( material-catalyst ) .
VLSworks good for assorted accelerators ( e.g. Au, Ga, Ag, Al etc. ) .
VLS can be used for the growing of different nanostructures ( e.g. Nanowire, nanotube etc. )
VLS mechanism can supply the size ( e.g. diameter ) controlled growing of NWs by utilizing pre-determined sized NPs of accelerator.
1.4 Assorted techniques for SiNW synthesis
The different NW growing techniques chiefly differ in the manner Si is supplied. There are two ways ; either NW growing is done through direct Si atoms or through Si compound. It is clear that if Si is fed through Si compound so a chemical reaction has to take topographic point and Si atoms decomposes after that the accelerator atom absorbs the Si and originate SiNW growing. Depending on handiness of O or absence of O, NW growing consequences may differ. It hence turns out to be suited to distinguish between the usage of O and oxygen-free Si precursors. This subdivision gives elaborate information on different NW growing techniques and their advantages and disadvantages.
1.4.1 Chemical vapour deposition ( CVD )
CVD is high temperature and O free precursor technique for SiNWs growing. The most normally used precursor gases are silane ( SiH4 ) , disilane ( Si2H6 ) , silicon bichloride ( SiH2Cl2 ) and silicon tetrachloride ( SiCl4 ) . For SiCl4 growing temperatures typically range from about 800A°C to 1000A°C [ 41 ] compared to SiNW grown in the presence of SiH4, which requires temperatures of about 400-600 A°C [ 42 ] . Conventional diagram of CVD procedure is shown in Figure 3, normally H ( H2 ) or H2/inert gas mixtures is flows through an externally heated vitreous silica tubing so providing precursor gas to the reactor. Suppose if a Si sample with a layer/ nanoparticles of metal accelerator has been placed in the hot zone of the reactor, SiNWs will get down turning by VLS mechanism.
Figure-1.4: Conventional diagram of CVD procedure. [ Source:16 ]
The chief advantage of high temperature CVD is that it provides much larger scope of possible VLS accelerator stuffs. For e.g. Au [ 8 ] and Cu [ 43 ] gives first-class consequences at temperatures above 850 A°C ; nevertheless Pt and Ni are more suited for higher temperatures [ 41, 44 ] . However CVD has besides drawbacks, due to the higher temperature the NW growing speeds increases and restricts for the controllability of SiNW length [ 13 ] . Another consequence due to the higher temperature is the agglomeration of metal bunchs, islands, or droplets on substrate surface. During growing of SiNWs via the VLS mechanism, due to higher temperature NPs are agglomerative and it becomes more hard to turn NW with chiseled diameters, because the NP size does non remain changeless during high temperature processing.
1.4.2 Plasma Enhanced Chemical Vapour Deposition ( PECVD )
The restrictions of CVD can be forestalling by utilizing wireless frequence ( RF ) power as energy component in PECVD. RF power provides high energy for making extremely reactive groups of precursor gases. In topographic point of thermic energy, RF power is more outstanding in PECVD and merely warming is required for substarte, which make it suited for stuffs those can non prolong at higher temperature. The precursor gas decomposes in active groups and absorbed in accelerator droplets and grows NWs by VLS method, Fig-4 shows the conventional diagram of PECVD.
The chief advantage of PECVD procedure is suitableness for accelerator those are non prolong the high temperature processing. PECVD provides higher surface diffusion even at low temperature ( & lt ; 3000C ) [ 14 ] , because of high kinetic energy ( RF power ) . But it has besides drawback that it does non give pure growing, NW may hold tonss of H incorporation.
Figure-1.5: Conventional diagram of PECVD procedure.
1.5 Assorted accelerator stuffs for SiNW growing
A big figure of accelerators have been used to turn nanostructure ; for eg. so many metals have been successfully used for the SiNW synthesis such as Au [ 8 ] , Ag [ 41 ] , Al [ 42 ] , Cu [ 43 ] , Ga [ 42 ] , In [ 44 ] , Ni [ 4 ] , Pt [ 44 ] , and Zn [ 44 ] etc.
As one can see that the big Numberss of accelerator stuffs are available. So the qualities of the SiNW every bit good as the needed growing conditions differ from one accelerator to another. On the footing of their corresponding metal-Si binary stage diagrams, the accelerators are divided into three different classs ( merely related to SiNW growing ) :
High Si solubility metal accelerator: These types of accelerators have merely one eutectic point in their stage diagram. The eutectic point is positioned at a Si composing of more than 10 atom % Si. And they besides do non keep any metal-silicide stages [ 16 ] . In this class merely three metals have issues Al, Ag, and Au.
Low Si solubility metal accelerator: These metals have besides a individual dominant eutectic point and no silicide stages like type-A accelerators. But the eutectic point is situated at lower Si concentrations, less than 1 atom % Si [ 16 ] . In, Ga and Zn belong to this class.
Silicide organizing metal accelerator: Their stage diagram shows the eutectic points at above 800 A°C and one or more metal-silicide stages [ 16 ] . Cu, Pt and Ti Are examples of this class.
1.6 Why gold is more suited as a Catalyst?
Au has been the most widely used accelerator stuff for SiNW growing. There are several grounds which make Au as an ideal campaigner:
The chief advantage of Au is its high chemical stableness. Au does non oxidise in air, which is advantageous for the pre growing sample readying so it has no necessity of in situ deposition. The high chemical stableness besides reduces the growing demands, peculiarly the tolerable O background force per unit area.
The other advantage of Au is that it is atoxic, which is suited from a work safety point of position.
Many of the precursor gases or stuffs used for NW growing are soluble in Au.
AuNP do non respond with the bearer gasses such as H and N.
Au-Si metal has no any metal silicide stages. The eutectic point is positioned at a Si composing of more than 19 atom % Si and a temperature of 363 A°C which shows a important decrease in runing temperature and allows the SiNWs growing at lower temperature [ 13 ] .
Chapter 2: Methodology
2.1 Synthesis of AuNPs
Size and form controlled synthesis of AuNPs have been studied intensively because of their size and form dependent physical and chemical belongingss [ 46 ] . The synthesis of citrate-stabilized AuNPs through decrease of HAuCl4 by Na citrate was foremost developed by Turkevich in 1951 [ 8 ] . Through is method the size of AuNPs can be controlled from 5nm to 150nm by changing the reaction conditions such as concentration of Na citrate and HAuCl4 [ 8 ] , pH of solution [ 32 ] and temperature [ 32 ] . However the monodispersity is rather hapless and forms are non-uniform such as spherical, quasi-spherical, rod and trigons [ 47 ] . Now, it has been reported that the seed-mediated growing of Au NPs under the mild status of cut downing agent and HAuCl4, could be the efficient method to command the size and form of Au NPs.
In this work, spherical AuNPs are prepared by seed mediated growing, based on work published in Langmuir “ Kinetically controlled seeded synthesis of citrate -stabilized gilded nano-particles of up to 200nm: size concentrating versus Ostwald maturing [ 32 ] . For seed growing, AuNPs are formed and so these are used as seed atoms for the consecutive growing of big AuNPs by adding gold precursors and cut downing agents. Seed mediated growing inhibits the secondary nucleation during growing procedure and merely allows the expansion of pre-synthesised Au seed atoms.
The chief purpose of bring forthing AuNPs of different size is to utilize these NPs as accelerator for the controlled growing of SiNWs by VLS mechanism.
HAuCl4 ( 30 % wt diluted in HCl ) , trisodium citrate and PLL ( purchased from Sigma -Aldrich, U.K. ) . Milli-Q H2O is used for all experiments. All glasswork is cleaned by propanone and plastic equipment by I.P.A. and with D.I. H2O.
2.3 Synthesis of gold seeds
A solution of 2.2mM Na citrate in Milli-Q H2O ( 150ml ) was heated in a beaker over hot home base through a H2O bath for 15 min. under uninterrupted stirring. A home base was placed over the beaker to forestall the vaporization of dissolver. After boiling ( 1000C ) , 1 milliliter of HAuCl4 ( 25mM ) was injected in the solution. The coloring material of the solution changed from yellow to bluish grey and so to light tap in 10 min. The ensuing seed atoms were coated with negatively charged citrate ions and therefore good suspended in H2O.
2.3.1 Synthesis of AuNPs without dilution of seed solution
Once the synthesis of Au seeds was completed the solution was cooled down to 900C temperature, so 1ml of Na citrate ( 60mM ) solution and 1ml of HAuCl4 ( 25mM ) were consecutive added with 2 min. clip hold as shown in fig.2.1. This procedure ( consecutive add-on of 1ml Na citrate ( 60mM ) solution and after 2 min add-on of 1ml of HAuCl4 ( 25mM ) ) , was repeated 5 times after each 30 min. The concentration of each coevals of AuNP was about same as seed solution.
Figure-2.1: Synthesis of AuNPs with uninterrupted add-on of S.C and HAuCl4 in seed solution at 900C.
( G0 represent the measure of seed synthesis and G1, G2, G3, G4, G4 and G5 represent the back-to-back growing stairss of AuNPs after add-on of S.C. and HAuCl4 with 30 min. clip hold ) . [ Modified from ref: 32 ]
2.3.2 Seeded growing of AuNPs with dilution of seed solution
After the synthesis of Au seed atoms ( S0 measure ) the solution cooled down at 900C. Then 1ml of HAuCl4 ( 25mM ) solution was added into the seed solution, after 30 min the reaction was finished. The procedure was repeated twice. After that, the solution was diluted by pull outing 55ml of solution and adding 53ml of milli-Q H2O and 2ml of Na citrate ( 60mM ) solution as shown in measure S1 of figure- 2.2. Then used this solution as a seed solution for farther growing of AuNPs and reiterate the procedure 8 times. After each consecutive growing step the extracted solution was used for word picture.
Figure-2.2: Consecutive stairss of AuNPs growing by dilution of seed solution with MQ H2O at 900C.
( S0 represents the seed synthesis and S1, S2, S3, S4, S5, S6, S7, S8 represent the back-to-back growing stairss of AuNPs with 1 hour. clip hold ) . [ Modified from ref: 32 ]
2.3.3 Synthesis of AuNPs at different temperature
To analyse the consequence of temperature on morphology of AuNPs, the procedure of Au seed formation was performed at different temperature ( 1000C, 900C, 800C, 700C & A ; 600C ) . First a solution of 2.2mM Na citrate ( with 150ml Milli-Q ) in unit of ammunition beaker was heated through H2O bath over a hot home base for 15 min. with uninterrupted stirring. After boiling ( 1000C ) the Na citrate solution, 1ml of HAuCl4 ( 25mM ) was added into it. The coloring material of the solution changed from light yellow to bluish Grey and so to tap in 10 min. and the Au seed atoms were formed into the solution. This procedure was repeated at different temperatures 900C, 800C, 700C & A ; 600C severally.
2.4 Adsorption of AuNP by Poly-L-Lysine coating
After synthesis of AuNPs, the chief purpose was to lodge these AuNPs over substrate. For lodging the NPs on substrate placed the sample wafer in spin coater and bead 0.2 milliliter of PLL over sample and revolve it at 1000rpm velocity for 1 min. so leaved the sample for 15 min to dried out of course. After that dropped the 0.2 milliliter of AuNPs solution over sample and once more rotated it at 1000 revolutions per minute and dried it. Repeated the whole procedure and prepared the different samples with different sized AuNPs ( e.g. 30nm, 55nm, 75nm and 100nm ) .
2.5 Synthesis of SiNWs by Plasma Enhanced Chemical Vapour Deposition
( Yet this portion is non completed )
2.6 Word picture techniques
After synthesis of controlled sized AuNPs and SiNWs they were characterised by UV-Vis Spectroscopy, Atomic force microscopy ( AFM ) and Scaning negatron microscopy ( SEM ) .
2.6.1 UV-Vis Spectroscopy
The UV-Vis spectrometer [ Evolution 300 UV-VIS, Company: Thermo Scientific ] was used to mensurate the optical belongingss of AuNPs. Initially two milli-Q H2O filled vass are placed in spectrometer, used as a mention sample to graduate the UV-Vis Spectrometer. Then computing machine sets at soaking up manner, get down wavelength at 300 nanometer, halt wavelength 800 nanometer and informations interval at 5 nanometers and continue. The graph between wavelength and soaking up is shown on screen. Then removed one D.I H2O filled vas and topographic point all samples ( different sized AuNP solution ) one by one and saved the readings.
2.6.3 Atomic force microscopy ( AFM )
AFM [ company: aˆ¦ ] is used for morphological analysis. First placed the sample of 30nm mean sized AuNPs on the holder and set the AFM tip for non contact measuring. The coveted movies of samples are collected by computing machine connected to AFM. Repeat the whole procedure with different samples of AuNPs and SiNWs.
2.6.3 Scaning negatron microscopy ( SEM )
The existent size and form of AuNP and SiNW is measured by SEM [ Carl Zeiss SMT: Evo series ] . First placed the all samples of AuNPs in the holder and set the vaccum. Measure the diameter of AuNPs straight by the package and salvage the images of all samples. Repeat the SEM analysis for SiNWs.
Chapter 3 Result and treatment
3.1 Analysis of AuNPs without dilution of seed solution
The consequence of uninterrupted add-on of HAuCl4 and Na citrate in to Au seed solution is observed by UV-Vis spectrometry. The mean AuNP size can be estimated from the peak optical density wavelength. The relationship between atom diameter and peak wavelength ( optical density ) can be calculated by:
5 ten 10-5d3 – 0.0066 d2 + 0.6722 vitamin D + 510.16 eq-1
Where vitamin D is the mean AuNP diameter and I»m is the peak optical density wavelength. For this work, above equation is used iteratively to gauge the AuNP diameter which closely approximates the several extremum optical density wavelength. The mean size and surface plasmon resonance ( SPR ) extremum of AuNP without dilution of seed solution is shown in table-3.1.
SPR extremum ( nanometer )
Diameter ( nanometer )
G0 ( Seed )
Table-3.1: Summary of size and optical belongingss of AuNPs after uninterrupted add-on of HAuCl4 and Na citrate.
From the tabular array -2 depicted that the size of AuNPs are increased from 15nm to 30nm in four back-to-back stairss. The ground for this growing is chiefly depends upon the temperature. After the formation of Au seed at 1000C the temperature decreases to 900C and consecutive growing is done at same temperature. The lessening in temperature decelerate down the reaction rate, which inhibits the new nucleation of AuNPs and ease the addition in size by the agglomeration of AuNPs [ 32 ] .
Figure- 3.1 Absorption spectra of AuNPs obtained after different growing ( without dilution ) stairss.
After four consecutive growing steps the diameter of AuNPs does non increases, because after 30nm the transparence bound occurs. The coloring material of Au solution after five stairss was reddisn-purple and became crystalline after 3-4 hours, because the Au colloid was non stable and acquire precipitated. Thus farther growing of AuNP is non possible by adding Na citrate and HAuCl4.
3.2 Analysis of AuNPs with dilution of seed solution
The larger size of the atoms is acquiring by consecutive growing of AuNPs with dilution of seed solution. The table-3.2 shows all consequences after each measure. The growing of AuNP is finished after acquiring 105nm size.
SPR extremum ( nanometer )
Diameter ( nanometer )
S0 ( Seed )
Table-3.2 Summary of size and optical belongingss of AuNPs after dilution of seed solution.
Figure- 3.2 Absorption spectra of AuNPs obtained after different growing ( with dilution of seed solution ) stairss.