The Templated Growth Of Silicon Nanowires Biology Essay

Published:

This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.

Nanotechnology is a promising field that encompasses almost all disciplines of science and engineering. Generally, nanotechnology defines the structures with at least one dimension from 1 to 100 nanometers [1]. Nanomaterials have unique physical, optical and chemical properties because at the nanoscale surface properties become dominant over bulk properties due to the higher surface area to volume ratio and often properties of nanomaterials are extremely promising for various emerging technological applications, such as photonics, biosensors, memory devices, communication, energy conversion, environmental protection, and space exploration [2]. Nano-sized materials can also be used as catalysts for the growth of specific nanostructures such as nanowires, nanorods, nanotubes etc.

Silicon nanowires (SiNWs) have attracted huge interest recently due to their intrinsic properties and their possible applications in nano-sensors [3], solar cells [4] and lithium batteries [5]. SiNWs field-effect transistor (SiNW-FET) arrays show ultra high sensitivity for biosensing applications [6]. In addition, being able to tune the SiNW properties makes them highly suitable for sensing devices [7].

Several growth 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 growth [11]. Till now, VLS has been proven to be the most successful mechanism to grow nanowires, because in this process gaseous reactant (e.g. gaseous Si) can easily absorbed into the liquid catalyst and this process produces the nanowires at eutectic temperature of alloy (e.g. Si-Au), which has lower temperature than individual melting point of catalyst and reactant [12]. These growth mechanisms can be employed in various fabrication techniques, such as chemical vapour deposition (CVD) [13], plasma enhanced chemical vapour deposition (PECVD) [14], laser ablation [15], and molecular beam epitaxy [16]. In the VLS mechanism, the catalyst can be a thin layer or a nano-particle. However nano-particles are used often for diameter controlled growth of nanowires; for example gold nano-particles (AuNPs) are widely used as a catalyst to grow SiNWs, because gold (Au) has many advantages such as high chemical stability, non toxicity and it does not oxidize [17]. In this work, attempts to control the diameter of SiNWs are made utilising the VLS mechanism in PECVD and AuNPs as a catalyst.

AuNPs are one of the most widely investigated nanomaterials because of their unique chemical and physical properties [18]. Au NPs offer a surface chemistry which is helpful in the self-assembly of organic molecule layers [19]. They also have several advantages over other nanomaterials, such as they are chemically stable, non-toxic and simple to functionalize for different applications [20]. They can also be easily conjugated with DNA, enzymes, antibodies and polymers, without affecting their characteristics in many cases [21]. Biomolecule conjugated Au NPs offer highly sensitive and selective biosensors for diagnosis of diseases and they have high surface area which is beneficial in drug delivery vehicles [22]. A major advantage of Au NPs is that they can be synthesized in different shapes and sizes according to requirement. These attractive properties render AuNPs useful for various applications including plasmonics, photonics, drug delivery, biological and chemical sensing, and catalysis [23].

Several methods are available to synthesize AuNPs and control their size, such as the aerosol generation [24], electron beam lithography [26], annealing of thin film [27], and colloidal synthesis [28]. However colloidal synthesis is further divided into different methods depending upon the use of different reducing agent such as the Burst method [29], sonolysis [30] and the Perroult method [31]. Among them citrate reduction, also known as the Turkevich method, is widely used, because it is cheap, simple and well investigated method [28]. In this method, the tetrachrohydroauric acid (HAuCl4) is reduced to AuNPs by the reduction reaction with trisodium citrate. The Au NPs grow by the agglomeration of gold atoms and also by coagulation with other Au particles.

In this study, the main focus is on the citrate reduction method to synthesize AuNPs, for which the "Kinetically controlled seeded synthesis of citrate stabilized gold nano-particles of up to 200nm: size focusing versus Ostwald ripening (Langmuir publication) [32] is taken as a reference and use these AuNPs for growth of SiNWs. First Au seed particles are synthesised for the various initial concentrations of HAuCl4 and then these seed particles are used for the further growth of AuNP known as seed-mediated process. Our effort is to develop different sized AuNP through successive growth steps of Au seed particles.

The overall aim of this project is to synthesis Au NPs of various diameters for the growth of SiNWs using PECVD. To achieve this, the project is divided into four main objectives:

Synthesis of gold nanoparticles by citrate reduction process.

Investigation of the effect of change in reaction temperature on size of AuNPs.

Deposition of AuNPs on Silicon (Si) substrate through poly-l-lysine coating.

Growth of SiNWs by PECVD using different sized AuNPs.

In this regard chapter 1 provides an overview on various synthesis methods of Au NPs such as the Brust method, the Perroult method, sonolysis, and citrate reduction. The main focus will be on the citrate reduction method and its advantages. This chapter also provides information on various growth methods of SiNWs and the benefits of the VLS mechanism and Au as a metal catalyst for SiNW growth.

Chapter 2 explains the methodology for the controlled growth of AuNPs by citrate reduction at different process parameters such as the concentration of gold precursor, pH and temperature. It also describes the deposition of AuNPs on Si substrates through ploy-l-lysine coating and the synthesis of SiNWs by PECVD. Various characterisation techniques such as UV-Vis Spectroscopy, SEM, and AFM are also explained.

Chapter 3 details the results of the AuNPs fabrication process and the morphological studies of the SiNWs are also included.

Finally, the conclusions are described and suggestions are made for any future work.

CHAPTER 1 LITERATURE REVIEW

1.1 Synthesis of AuNPs

The highly controlled NW growth has been reported from various methods such as aerosol-generated AuNPs [24, 25], annealing of Au thin films [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, position, surface density and purity. A certain type of method could be useful in one way but may be less useful from another point of view or characteristics, and therefore it is not possible to say that which method is best. In this section the different methods of AuNP synthesis are discussed.

1.1.1 Aerosol Generated Particles

The aerosol method is used to produce agglomerate gold particles through evaporation of gold [24]. This method requires a high temperature furnace to evaporate Au directly from a solid piece of Au. The evaporated Au is carried away by a carrier gas (nitrogen) and initial particles are formed by homogenous nucleation, which further grow by successive coagulation of particles. The size of AuNPs can be controlled by changing the temperature of the furnace, which may increases or decreases the agglomeration of particles and results in bigger diameter and smaller diameter of NPs respectively [25].

The main disadvantages of aerosol method are that it is very expensive setup and limited size control of particles. It has another disadvantage that it gives random positioning of particles on the substrate. On the other hand, this method has several advantages such as extremely pure AuNPs can be produced and deposited with a very controlled surface density. Moreover, 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 film

The common process for synthesis of AuNPs is thermal evaporation of thin Au film, directly onto the substrates [26]. Normally the thickness of the film can vary between 0.1 nm and a few nanometers. In this method, the substrate is heated up to elevated temperature in order to melt the film and split up into NPs. When these NPs are used as NWs seed particles the heating step occurs inside the NW growth reactor. The temperature at heating stage slightly affects particle distribution with higher temperatures gives larger particles and lower particle surface densities [].

The disadvantage of this method is that it has extremely poor controllability over particle size and surface density and particle positioning. However this method has also advantages such as it is a simple and reasonably cheap method.

1.1.3 Electron Beam Lithograph (EBL)

The formation of AuNP by EBL is quite similar to thin film evaporation method, since generation of NP through EBL also requires evaporation of a thin gold film and heating of substrate [27]. However, before deposition of Au film, a pattern has been defined onto the substrate that determines diameter, surface density and position of the NPs. Figure 2 is a schematic of the EBL particle synthesis process. First deposit a resist layer over substrate the substrate. Then expose the substrate by the electron beam to get the desired pattern and deposit a thin Au film by thermal evaporation onto the substrate. After that removes the resist and the Au layer over resist by liftoff process. In last only desired patterned Au film is left over substrate. After heating the substrate the patterned Au film transforms into the AuNP.

The major advantage of EBL process is that it is highly controlled synthesis process in terms of size, shape and surface density; however the exact position of AuNPs can also be controllable. The main drawback with EBL is that it cannot be processed over substrates that are too thin and brittle. Other drawbacks of EBL are that it is expensive, complicated and time taking process.

Figure-1.1: Schematic of the EBL process. (a) The substrate coated with an electron beam sensitive resist that to define. (b) The desired pattern after exposed by the electron beam. (c) thermally evaporated thin Au film onto the substrate. (d) After Liftoff a well defined gold disk. (e) Upon heating the Au disk is transformed into an AuNP [ Source: 27].

1.1.4 Colloidal synthesis of AuNPs

In this review, a detailed overview is presented on the synthesis of size controlled spherical AuNPs and the use of these AuNPs as a template to grow the Si NWs. Usually Au NPs are prepared by chemical reduction of an appropriate gold precursor (commonly HAuCl4), with a reducing agent such as organic acids, sugars, aldehydes, alcohols and other strong reducing agents (e.g. NH2NH2 and NaBH4) [33]. Extensive research has been carried out to synthesize monodispersed AuNPs by various methods such as the burst [29] and citrate reduction [28]. The focus will be on citrate reduction as a method for the preparation of monodispersed spherical gold nanoparticles, and will be the method applied in this work. However, other fabrication techniques are also described.

Burst Method

This method was discovered by Brust and Schiffrin in 1990 and it is used to form AuNPs in organic liquids that are usually not miscible with water [29]. In this method a solution of HAuCl4 and a solution of tetraoctylammonium bromide (TOAB) in toluene are mixed with sodium borohydride (NaBH4), which results in 4-6 nm-sized AuNPs being formed. NaBH4 is the reducing agent and TOAB is use as a catalyst as well as stabilizing agent and AuNPs aggregate slowly in the solution because TOAB does not bind to the AuNPs strongly, so the whole process takes approximately two weeks. To reduce the process time, a stronger binding agent, like a thiol (in particular, alkanethiols) can be added in the solution, which will covalently bind to the Au atoms.

Perroult Method

This method was discovered by Perrault and Chan in 2009 [31]. In this method hydroquinone is used to reduce HAuCl4 in an aqueous solution of Au seeds. Perroult method is similar to the photographic film development process, in which silver grains (nano particles) within the film grow through the adhesion of reduced silver atoms onto their surface. Similarly, hydroquinone reduces HAuCl4 into ionic gold. These gold atoms then attach to the surface of Au seed NPs.. The Perroult method can produce particles of between 30-250 nm [31].

Sonolysis

Sonolysis is another process which is used for the synthesis of AuNPs. In this process the reduction reaction takes place between an aqueous solution of HAuCl4 and glucose. In reaction, hydroxyl radicals and sugar pyrolysis radicals are reducing agents. Through this process ribbon shaped AuNPs are obtained and the width of these nanoribbons can be 30 -50 nm width and lengths of several micrometers [30]. Spherical AuNPs can be prepared by this method, when glucose is replaced by cyclodextrin (a glucose oligomer) because glucose is crucial in directing the morphology towards a ribbon like structure.

Citrate reduction method

In 1951 Turkevich et al. produced AuNPs, based on single phase aqueous reduction of HAuCl4 by sodium citrate and in 1973 Frens et al. showed the effect of changing concentration of sodium citrate on AuNPs size through this method [28]. It has been reported that the amount of reducing agent and gold precursor can control the size and shape of Au NPs during the seed-mediated growth [29].

In this method the gold precursor (HAuCl4) is reduced by trisodium citrate. The Au3+ ions of HAuCl4 are reduced to neutral Au atoms. These atoms start to bind together and form the Au seed particles; this is called the nucleation process. At the same time, negatively charged citrate ions, liberated during the reduction process, attach to the surface of AuNPs and prevent them from agglomerating. When the total surface area of gold particles is covered by citrate ions, then particle will no longer grow in size. If more citrate ions are available, then they will be able to cover a larger surface area and smaller AuNPs can be produced. So the final size and shape of the particles would depend on the size of seed particles, the kind and concentration of reducing agent and the amount of the precursor added [34].

The main advantage of Turkevich method is that it is well understood proved that at it produces spherical AuNPs with controlled size. A comparison between various methods of AuNP synthesis is shown in table-1.

Methods

Diameter range

Diameter

control

Position control

cleanliness

Simplicity

Cost

Aerosol

Broad

Very good

Very limited

Very clean

Reasonable

High

Colloidal

Very broad

Very good

Uncontrolled

Contaminated

Very simple

Low

EBL

Limited

Very good

Very good

clean

Reasonable

high

Annealing

Broad

limited

Uncontrolled

Contaminated

Very simple

Medium

Table- 1.1: comparison between various methods of AuNP synthesis. [Source: 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 depositing polymer and the AuNP over substrate, various methods such as Poly-L-Lysine coating and immersion 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 particles which make the surface adhesive for negatively charged nanoparticles. Due to electrostatic bonding NPs are absorbed onto the substrate by immersing the substrate in an aqueous AuNP solution. This adsorption was controlled by the time for which the substrate was immersed in to the AuNP solution. This process has disadvantages in that it requires a large amount of solutions for proper immersion of substrate.

1.2.2 Poly-L-Lysine coating

Through this process a thin layer of Poly-L-Lysine (PLL) is absorbed onto the substrate by spin coating. PLL is used because it has positively charged particles which makes binding with negatively charged NPs. For spin coating the substrate is placed over spin coater and drop 0.2ml of PLL and rotate it between 500 to 3000rpm. After that a droplet of Au colloid solution is placed over the substrate and the substrate is coated again. PLL have positively charged particles and AuNPs have negative charge, so the AuNPs attaches to the substrate by electrostatic interactions [35].

In this work, focus is on Poly-L-Lysine coating because it is advantageous over immersion, it requires very small amount of solutions (0.3-0.5 ml of PLL and AuNPs ) for AuNPs deposition.

1.3 Different mechanisms of SiNWs growth

In recent years research on SiNW fabrication has increased rapidly due to their potential applications in opto-electronics, biosensors and solar cells. There are mainly two common approaches to fabricate NWs; called "bottom-up" and "top-down" approach. Top-down fabrication method is consisting of deposition, patterning and etching of bulk material for eg. electron beam lithography and attrition. However, Bottom up approach refers to the buildup of a material from atom by atom or molecule by molecule. Both approaches have their own advantages and disadvantages and play very important role in nano technology. The main disadvantage of top down approach is the defect in surface structure and significant crystallographic damage to the processed patterns. Other hands the bottom-up method fabricate NWs with less defects. But through this method the control of NW position is difficult. Some of bottom-up methods are discussed below:

1.3.1 Solution-Liquid-Solid (SLS) Process

Buhro et al. [36] have proposed a low temperature SLS method for the synthesis of crystalline nanowires of group III-V semiconductors. In this process, a metal with low melting point (e.g. In, Sn, Bi) is used as a catalyst, and through the decomposition of organo-metallic precursors a desired material is produced. Nanowires of InP, InAs and GaAs have been prepared by low-temperature (<2030C) solution phase reactions [37].

1.3.2 Solvothermal Synthesis

Solvothermal method is exclusively a solution based process to produce nanowires and nanorods [38]. In this process, metal precursors and solvent are mixed and placed the mixed solution in an autoclave for crystal growth and the assembly process. This methodology has enabled the synthesis of crystalline nanowires of semiconductors and other materials.

1.3.3 Oxide assisted synthesis (OAS)

Oxide-assisted growth synthesis for nanowire growth was first proposed by Lee et al. [39] and does not require any metal catalyst. They found that the growth of Si nanowires is improved when SiOx made targets are used. In this method growth of the Si NW is assisted by the SiOx. Through thermal evaporation or laser ablation SiOx decomposes into Si clusters by following reaction:

Si clusters (NPs) acts as a nucleation or catalyst site. These NPs are in molten form which enhances the absorption and diffusion and grows the SiNWs.

1.3.4 Vapour- Liquid-Solid (VLS) growth and its advantages

The VLS mechanism for Si NW growth was proposed by Wagner and Ellis [8] in March 1964. The name VLS mechanism explains the path of Si, which starts from the vapour phase, then diffuses through the liquid Au-Si droplet and ends up as a solid Si NW. The Au-Si binary phase diagram (fig.2) shows that the melting point of the Au-Si alloy mainly depends on composition. Au-Si alloy which has 81 atom % Au and 19 atom % Si, melts at 363 °C. It is clear that the melting temperature of Au-Si alloy is about 700 K lower than the melting point of pure Au and more than 1000 K lower than the melting point of pure Si.

Figure-1.2: Schematic phase diagram of Au-Si alloy. [Source: Ref. 16]

Therefore, heating Au above 363 °C (460 °C) in the presence of Si precursor gas forms liquid droplets of Au-Si, schematically depicted in Figure 3b. Exposing these Au-Si alloy droplets to a silicon precursor such as silane (SiH4) starts diffusion of precursor atoms (Si) in to the surface of these droplets. At equilibrium (supersaturation) only limited amount of Si atoms diffuse into the Au-Si droplets. The further supply of Si from the gas phase forces the droplets to find a way to remove the excess Si atoms. This leads to the growth of SiNWs with a Au-Si droplet at their tip, as shown in Figure 3d.

Figure- 1.3: VLS growth mechanism of nanowire. [ Source: Ref. 40]

Several methods of nanowires growth have discussed and all of them have some advantages and disadvantages. But commonly VLS is used because of following advantages:

The VLS mechanism can be used for other materials eg Ge, InP, GaAs etc.

VLS grows the NWs at eutectic temperature of alloy (material-catalyst).

VLSworks well for various catalysts (e.g. Au, Ga, Ag, Al etc.).

VLS can be used for the growth of different nanostructures (e.g. Nanowire, nanotube etc.)

VLS mechanism can provide the size (e.g. diameter) controlled growth of NWs by using pre-determined sized NPs of catalyst.

1.4 Various techniques for SiNW synthesis

The different NW growth techniques mainly differ in the way Si is supplied. There are two ways; either NW growth is done through direct Si atoms or through Si compound. It is clear that if Si is fed through Si compound then a chemical reaction has to take place and Si atoms decomposes after that the catalyst particle absorbs the Si and initiate SiNW growth. Depending on availability of oxygen or absence of oxygen, NW growth results may differ. It therefore turns out to be suitable to differentiate between the use of oxygen and oxygen-free Si precursors. This section gives detailed information on different NW growth techniques and their advantages and disadvantages.

1.4.1 Chemical vapour deposition (CVD)

CVD is high temperature and oxygen free precursor technique for SiNWs growth. The most commonly used precursor gases are silane (SiH4), disilane (Si2H6), silicon dichloride (SiH2Cl2) and silicon tetrachloride (SiCl4). For SiCl4 growth temperatures typically range from about 800°C to 1000°C [41] compared to SiNW grown in the presence of SiH4, which requires temperatures of about 400-600 °C [42]. Schematic diagram of CVD process is shown in Figure 3, usually hydrogen (H2) or H2/inert gas mixtures is flows through an externally heated quartz tube then supplying precursor gas to the reactor. Suppose if a Si sample with a layer/ nanoparticles of metal catalyst has been placed in the hot zone of the reactor, SiNWs will starts growing by VLS mechanism.

Figure-1.4: Schematic diagram of CVD process. [Source:16]

The main advantage of high temperature CVD is that it provides much larger range of possible VLS catalyst materials. For e.g. Au [8] and Cu [43] gives excellent results at temperatures above 850 °C; however Pt and Ni are more suitable for higher temperatures [41, 44]. However CVD has also drawbacks, due to the higher temperature the NW growth velocities increases and restricts for the controllability of SiNW length [13]. Another effect due to the higher temperature is the agglomeration of metal clusters, islands, or droplets on substrate surface. During growth of SiNWs via the VLS mechanism, due to higher temperature NPs are agglomerate and it becomes more difficult to grow NW with well-defined diameters, because the NP size does not stay constant during high temperature processing.

1.4.2 Plasma Enhanced Chemical Vapour Deposition (PECVD)

The limitations of CVD can be preventing by using radio frequency (RF) power as energy element in PECVD. RF power provides high energy for creating highly reactive radicals of precursor gases. In place of thermal energy, RF power is more prominent in PECVD and only heating is required for substarte, which make it suitable for materials those cannot sustain at higher temperature. The precursor gas decomposes in active radicals and absorbed in catalyst droplets and grows NWs by VLS method, Fig-4 shows the schematic diagram of PECVD.

The main advantage of PECVD process is suitability for catalyst those are not sustain the high temperature processing. PECVD provides higher surface diffusion even at low temperature (<3000C) [14], because of high kinetic energy (RF power). But it has also drawback that it does not give pure growth, NW may have lots of H incorporation.

Figure-1.5: Schematic diagram of PECVD process.

1.5 Various catalyst materials for SiNW growth

A large number of catalysts have been used to grow 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 large numbers of catalyst materials are available. So the qualities of the SiNW as well as the required growth conditions differ from one catalyst to another. On the basis of their corresponding metal-Si binary phase diagrams, the catalysts are divided into three different categories (only related to SiNW growth):

High Si solubility metal catalyst: These types of catalysts have only one eutectic point in their phase diagram. The eutectic point is positioned at a Si composition of more than 10 atom % Si. And they also do not hold any metal-silicide phases [16]. In this category only three metals have exits Al, Ag, and Au.

Low Si solubility metal catalyst: These metals have also a single dominant eutectic point and no silicide phases like type-A catalysts. But the eutectic point is situated at lower Si concentrations, less than 1 atom % Si [16]. In, Ga and Zn belong to this category.

Silicide forming metal catalyst: Their phase diagram shows the eutectic points at above 800 °C and one or more metal-silicide phases [16]. Cu, Pt and Ti Are examples of this category.

1.6 Why gold is more suitable as a Catalyst?

Au has been the most widely used catalyst material for SiNW growth. There are several reasons which make Au as an ideal candidate:

The main advantage of Au is its high chemical stability. Au does not oxidize in air, which is advantageous for the pre growth sample preparation so it has no necessity of in situ deposition. The high chemical stability also reduces the growth requirements, particularly the tolerable oxygen background pressure.

The other advantage of Au is that it is nontoxic, which is suitable from a work safety point of view.

Many of the precursor gases or materials used for NW growth are soluble in Au.

AuNP do not react with the carrier gasses such as hydrogen and nitrogen.

Au-Si alloy has no any metal silicide phases. The eutectic point is positioned at a Si composition of more than 19 atom % Si and a temperature of 363 °C which shows a significant reduction in melting temperature and allows the SiNWs growth at lower temperature [13].

Chapter 2: Methodology

2.1 Synthesis of AuNPs

Size and shape controlled synthesis of AuNPs have been studied intensively because of their size and shape dependent physical and chemical properties [46]. The synthesis of citrate-stabilized AuNPs through reduction of HAuCl4 by sodium citrate was first developed by Turkevich in 1951 [8]. Through is method the size of AuNPs can be controlled from 5nm to 150nm by varying the reaction conditions such as concentration of sodium citrate and HAuCl4 [8], pH of solution [32] and temperature [32]. However the monodispersity is quite poor and shapes are non-uniform such as spherical, quasi-spherical, rod and triangles [47]. Now, it has been reported that the seed-mediated growth of Au NPs under the mild condition of reducing agent and HAuCl4, could be the efficient method to control the size and shape of Au NPs.

In this work, spherical AuNPs are prepared by seed mediated growth, based on work published in Langmuir "Kinetically controlled seeded synthesis of citrate -stabilized gold nano-particles of up to 200nm: size focusing versus Ostwald ripening [32]. For seed growth, AuNPs are formed and then these are used as seed particles for the successive growth of large AuNPs by adding gold precursors and reducing agents. Seed mediated growth inhibits the secondary nucleation during growth process and only allows the enlargement of pre-synthesised Au seed particles.

The main aim of producing AuNPs of different size is to use these NPs as catalyst for the controlled growth of SiNWs by VLS mechanism.

2.2 Materials

HAuCl4 (30% wt diluted in HCl) , trisodium citrate and PLL (purchased from Sigma -Aldrich, U.K.). Milli-Q water is used for all experiments. All glassware is cleaned by acetone and plastic equipment by I.P.A. and with D.I. water.

2.3 Synthesis of gold seeds

A solution of 2.2mM sodium citrate in Milli-Q water (150ml) was heated in a beaker over hot plate through a water bath for 15 min. under continuous stirring. A plate was placed over the beaker to prevent the evaporation of solvent. After boiling (1000C), 1 ml of HAuCl4 (25mM) was injected in the solution. The colour of the solution changed from yellow to bluish gray and then to light pink in 10 min. The resulting seed particles were coated with negatively charged citrate ions and hence well suspended in water.

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, then 1ml of sodium citrate (60mM) solution and 1ml of HAuCl4 (25mM) were sequentially added with 2 min. time delay as shown in fig.2.1. This process (sequential addition of 1ml sodium citrate (60mM) solution and after 2 min addition of 1ml of HAuCl4 (25mM)), was repeated 5 times after each 30 min. The concentration of each generation of AuNP was approximately same as seed solution.

Figure-2.1: Synthesis of AuNPs with continuous addition of S.C and HAuCl4 in seed solution at 900C.

(G0 represent the step of seed synthesis and G1, G2, G3, G4, G4 and G5 represent the consecutive growth steps of AuNPs after addition of S.C. and HAuCl4 with 30 min. time delay). [Modified from ref: 32]

2.3.2 Seeded growth of AuNPs with dilution of seed solution

After the synthesis of Au seed particles (S0 step) 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 process was repeated twice. After that, the solution was diluted by extracting 55ml of solution and adding 53ml of milli-Q water and 2ml of sodium citrate (60mM) solution as shown in step S1 of figure- 2.2. Then used this solution as a seed solution for further growth of AuNPs and repeat the process 8 times. After each successive growth step the extracted solution was used for characterisation.

Figure-2.2: Successive steps of AuNPs growth by dilution of seed solution with MQ water at 900C.

(S0 represents the seed synthesis and S1, S2, S3, S4, S5, S6, S7, S8 represent the consecutive growth steps of AuNPs with 1 hr. time delay). [Modified from ref: 32]

2.3.3 Synthesis of AuNPs at different temperature

To analyze the effect of temperature on morphology of AuNPs, the process of Au seed formation was performed at different temperature (1000C, 900C, 800C, 700C & 600C). First a solution of 2.2mM sodium citrate (with 150ml Milli-Q) in round beaker was heated through water bath over a hot plate for 15 min. with continuous stirring. After boiling (1000C) the sodium citrate solution, 1ml of HAuCl4 (25mM) was added into it. The colour of the solution changed from light yellow to bluish grey and then to pink in 10 min. and the Au seed particles were formed into the solution. This process was repeated at different temperatures 900C, 800C, 700C & 600C respectively.

2.4 Adsorption of AuNP by Poly-L-Lysine coating

After synthesis of AuNPs, the main aim was to deposit these AuNPs over substrate. For depositing the NPs on substrate placed the sample wafer in spin coater and drop 0.2 ml of PLL over sample and rotate it at 1000rpm speed for 1 min. then leaved the sample for 15 min to dried out naturally. After that dropped the 0.2 ml of AuNPs solution over sample and again rotated it at 1000 rpm and dried it. Repeated the whole process 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 part is not completed)

2.6 Characterization techniques

After synthesis of controlled sized AuNPs and SiNWs they were characterised by UV-Vis Spectroscopy, Atomic force microscopy (AFM) and Scanning electron microscopy (SEM).

2.6.1 UV-Vis Spectroscopy

The UV-Vis spectrometer [Evolution 300 UV-VIS, Company: Thermo Scientific] was used to measure the optical properties of AuNPs. Initially two milli-Q water filled vessels are placed in spectrometer, used as a reference sample to calibrate the UV-Vis Spectrometer. Then computer sets at absorption mode, start wavelength at 300 nm, stop wavelength 800 nm and data interval at 5 nm and proceed. The graph between wavelength and absorption is shown on screen. Then removed one D.I water filled vessel and place all samples (different sized AuNP solution) one by one and saved the readings.

2.6.3 Atomic force microscopy (AFM)

AFM [ company: … ] is used for morphological analysis. First placed the sample of 30nm average sized AuNPs on the holder and set the AFM tip for non contact measurement. The desired pics of samples are collected by computer connected to AFM. Repeat the whole process with different samples of AuNPs and SiNWs.

2.6.3 Scanning electron microscopy (SEM)

The actual size and shape 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 directly by the software and save the images of all samples. Repeat the SEM analysis for SiNWs.

Chapter 3 Result and discussion

3.1 Analysis of AuNPs without dilution of seed solution

The effect of continuous addition of HAuCl4 and sodium citrate in to Au seed solution is observed by UV-Vis spectroscopy. The average AuNP size can be estimated from the peak absorbance wavelength. The relationship between particle diameter and peak wavelength (absorbance) can be calculated by:

5 x 10-5d3 - 0.0066 d2 + 0.6722 d + 510.16 eq-1

Where d is the average AuNP diameter and λm is the peak absorbance wavelength. For this work, above equation is used iteratively to estimate the AuNP diameter which closely approximates the respective peak absorbance wavelength. The average size and surface plasmon resonance (SPR) peak of AuNP without dilution of seed solution is shown in table-3.1.

Growth step

SPR peak (nm)

Diameter (nm)

G0 (Seed)

517

15

G1

520

18

G2

523

24

G3

525

30

G4

525

30

G5

525

30

Table-3.1: Summary of size and optical properties of AuNPs after continuous addition of HAuCl4 and sodium citrate.

From the table -2 depicted that the size of AuNPs are increased from 15nm to 30nm in four consecutive steps. The reason for this growth is mainly depends upon the temperature. After the formation of Au seed at 1000C the temperature decreases to 900C and successive growth is done at same temperature. The decrease in temperature slow down the reaction rate, which inhibits the new nucleation of AuNPs and facilitate the increase in size by the agglomeration of AuNPs [32].

Figure- 3.1 Absorption spectra of AuNPs obtained after different growth (without dilution) steps.

After four successive growth steps the diameter of AuNPs does not increases, because after 30nm the transparency limit occurs. The colour of Au solution after five steps was red-purple and became transparent after 3-4 hours, because the Au colloid was not stable and get precipitated. Thus further growth of AuNP is not possible by adding sodium citrate and HAuCl4.

3.2 Analysis of AuNPs with dilution of seed solution

The larger size of the particles is getting by successive growth of AuNPs with dilution of seed solution. The table-3.2 shows all results after each step. The growth of AuNP is finished after getting 105nm size.

Growth step

SPR peak (nm)

Diameter (nm)

S0 (Seed)

525

30

S1

525

30

S2

525

30

S3

535

55

S4

540

65

S5

545

77

S6

550

85

S7

560

100

S8

565

105

Table-3.2 Summary of size and optical properties of AuNPs after dilution of seed solution.

Figure- 3.2 Absorption spectra of AuNPs obtained after different growth (with dilution of seed solution) steps.

3.3 Effect of temperature on AuNP size

3.4 Characterization of deposited AuNP by AFM

3.5 Analysis of morphology of SiNWs

Chapter 4: Conclusion and Future Work

4.1 Conclusion

4.2 Future Work

Writing Services

Essay Writing
Service

Find out how the very best essay writing service can help you accomplish more and achieve higher marks today.

Assignment Writing Service

From complicated assignments to tricky tasks, our experts can tackle virtually any question thrown at them.

Dissertation Writing Service

A dissertation (also known as a thesis or research project) is probably the most important piece of work for any student! From full dissertations to individual chapters, we’re on hand to support you.

Coursework Writing Service

Our expert qualified writers can help you get your coursework right first time, every time.

Dissertation Proposal Service

The first step to completing a dissertation is to create a proposal that talks about what you wish to do. Our experts can design suitable methodologies - perfect to help you get started with a dissertation.

Report Writing
Service

Reports for any audience. Perfectly structured, professionally written, and tailored to suit your exact requirements.

Essay Skeleton Answer Service

If you’re just looking for some help to get started on an essay, our outline service provides you with a perfect essay plan.

Marking & Proofreading Service

Not sure if your work is hitting the mark? Struggling to get feedback from your lecturer? Our premium marking service was created just for you - get the feedback you deserve now.

Exam Revision
Service

Exams can be one of the most stressful experiences you’ll ever have! Revision is key, and we’re here to help. With custom created revision notes and exam answers, you’ll never feel underprepared again.