Bottom-Up and Top-Down Approaches for MgO

In this chapter, we present an overview of synthesis of MgO nanoparticles and thin films by using top-down and bottom-up approaches. The bottom-up approaches are generally utilized to grow nanoparticles by the methods that involve chemical reactions. Sometimes, methods based on these reactions are also able to grow thin films. The top-down approaches are preferred for growing thin films where bulk material is used for depositions. The methods, which are frequently used, are radio frequency sputtering, pulsed lased deposition, and molecular beam epitaxy and e-beam evaporation. Sometimes, methods like mechanical milling and high energy ball milling are used to grow nanoparticles.


Introduction
Nanoparticles and thin films are very common form of materials for utilization in different applications [1][2][3][4]. Synthesis approaches play vital role to determine characteristics of nanoparticles [5] and thin films [6]. Thus, a number of methods are being developed to synthesize either nanoparticles [7-9] or thin films [10][11][12]. The motive behind to explore numerous methods is to look for reproducibility and cost effectiveness in terms of industrial utilization [13,14]. Researchers are also working to get deep insights of involved phenomena during growth which persists a way to optimize for particular application [15][16][17][18] [41,42] are another important parameter, which are considered during deposition. Thus, keeping in mind the necessity and challenges in the synthesis, synthesis approaches for growing nanoparticles and thin films are discussed by taking a simple inorganic system. However, magnesium oxide is known from long time [43] but recent advances in application of this  To initiate chemical reaction among the involved atoms/ions/molecules certain salts are taken as starting materials. These salts are mixed with each other to form a homogeneous solution along with a suitable chelating agent. Control of nature of solution also plays important role during synthesis process [50]. Thus, various methods are being developed by researchers to minimize annealing treatment, nature of chelating agent, pH value of solution. Some of these methods are depicted here.
Combustion synthesis is well known phenomena to synthesize nanomaterials of different kinds in its different variance [51,52]. Most of the study utilizes solution combustion process for synthesizing nanoparticles. Typically, this method involves an oxidizer and fuel to initiate the reaction [53][54][55]. The most common oxidizers are metal nitrate/hydrates, ammonium nitrate and nitric acid. However, Urea, Glycine, Sucrose, Glucose, Citric Acid, Hydrazine based organic materials and Acetylacetonce are frequently used as a fuel. The water, hydrocarbons and alcohols works as solvent for reactions involved in this synthesis [53]. Thus, combustion synthesis is able to produce nanoparticle of various materials both at research purposes as well as at industrial scale [51,[53][54][55][56][57]. Various kind of nanoparticles like titanates [58], ferrites [59], carbonates [60], hydroxide [61] and oxides [62] are grown using this approach. Combustion synthesis is utilized for growing different kind of MgO nanostructures [63,64] and its derivative [65][66][67].
Our group utilizes, this method to synthesize MgO nanoparticles using combustion synthesis while taking magnesium nitrate as an oxidizer and citric acid as fuel  Figure 2 depicts schematic diagram of synthesis process. It is clear that synthesis takes place at low temperature, which reduces cost of synthesis. Figure 3 shows representative X-ray diffraction (XRD) pattern of the nanoparticle synthesized at 500°C for 1 h. The method is able to produce nanoparticle with pure phase and no other crystalline phases are observed [70].
Green Microwave synthesis utilizes microwave radiation rather than furnace heating in order to avoid longer duration of heating to precursor [77,78]. This method was successfully applied to form MgO nanoparticles by number of researchers [79][80][81].

MgO thin films
Spin coating method is well known tool for growing thin films which utilizes chemical reaction to form materials on the given substrate [87,88]. Sol-gel chemistry is helpful to synthesize thin films of MgO of desired crystallographic orientation using spin coater [89,90]. Figure 4 shows schematic of sol-gel method utilizing a spin coater to grow thin film [91,92].
Atomic layer deposition (ALD) method allows depositions with excellent uniformity and conformality, with a cost-effective methodology [93,94]. Thickness and composition control are usually possible over large-area substrates. Thin films of MgO were deposited by atomic layer epitaxy (ALE) from bis(cyclopentadienyl) magnesium and water using soda lime glass and Si(100) as substrates [95]. In another study, MgO films have been grown by atomic layer deposition in the wide

Top-down approaches
Top-down approaches are mostly utilized to grow thin films of inorganic materials. Some of these methods are discussed here.

MgO nanoparticle
Mechanical milling/high energy ball milling is well known method which utilizes bulk counterpart as starting material and used for growing nanoparticles of different kind of materials [97,98]. Depending upon milling process, the milling machines are categories as follows: tumbler ball mills, vibratory mills, planetary mills, and attritor mills [99,100]. In the ball milling process, powder mixture or bulk powder placed in the ball mill is subjected to high-energy collision from the balls for nanoparticle synthesis. Figure 5 depicts the schematic of high energy ball milling system [101]. Though this technique is effective to synthesize oxide nanoparticles [102,103], however, no report is available for synthesizing MgO.

MgO thin films
Top-down approaches for growing MgO thin films are depicted in this section. e-Beam evaporation method involves the evaporation of material target with e-beam energy [104]. Schematic of this method is shown in Figure 6. This method is effectively used to grow MgO thin films on different type of substrates like NaCl [105], Si [106], fused quartz [107] as well as on metallic layers [108,109]. Figure 7 shows the MgO thin films on fused quartz substrate along with MgO powder. Both the films of thickness around 5 and 50 nm reveal almost amorphous   nature. Optical absorption spectra of MgO thin films exhibit onset of film formation (Figure 8). This method is also utilized to grow MgO thin films on Si substrate. Films grown on this substrate exhibits polycrystalline nature [110].  Molecular beam epitaxy (MBE) utilizes e-beam for growing thin films [111]. It provides better control over stoichiometry ratio but also helpful in epitaxial growth of MgO [112,113].
Pulsed laser deposition (PLD) as a thin film growth technique was not much popular until the late 1980s, when it has been used to grow superconducting YBa 2 Cu 3 O 7 -δ films [114]. Since then, the amount of research involving this technique has increased significantly and a number of compositions have been stabilized in thin film successfully [115][116][117][118][119][120][121][122]. The schematic of PLD system is shown in Figure 9. In PLD, a pulsed laser beam (having wavelengths in UV range) strikes the surface of the target material to be deposited. For a short duration of laser pulse (~20 nanoseconds), enormous power (~10 MW) is delivered to the target material and absorption of energy leads to ablation before the thermodynamic equilibrium. The energy from   the laser evaporates the target's surface and the ablated material forms the plasma plume which finally deposit on the substrate mounted in front of the target.
The main advantage of PLD is the stoichiometric transfer from the multicomponent target to the thin film form, which otherwise is hard to achieve with any other thin film growth technique such as thermal evaporation or sputtering. The pulsed nature of PLD allows precise control on the film growth rates. Some drawbacks of the PLD technique are small area deposition, growth of macroscopic particles (particulates) during the ablation process, defects produced during growth, etc.
Radio frequency sputtering method: At present most desired application of MgO is its utilization as a barrier for magnetic tunnel junction and rf sputtering method is preferred choice [126,127] as well as for other applications [128]. Figure 10 shows the rf sputtering setup for the fabrication of thin films.
MgO films on Si substrate are grown by number of researchers [129,130] as well as by our group [42,131]. Films grown on Si substrate are both amorphous [132,133] and crystalline [134,135] in nature depending upon the deposition time and annealing temperature. Figure 11 shows the XRD pattern of the MgO thin film grown at substrate temperature of 350°C, deposition time of 400 min and annealing temperature of 800°C for 1 h followed by 300°C and 24 h.
Apart from this, number of groups utilizes chemical deposition (CVD) method to grow MgO thin films on different substrates [136][137][138].

Conclusions
Thus, an overview of bottom-up and top-down approaches for synthesis of MgO nanoparticles and thin films is depicted in this chapter. Chemical methods are effective to grow nanoparticles, however, later is successful to grow thin films.