**Abstract**

Nanoparticles of noble metals have unique properties including large surface energies, surface plasmon excitation, quantum confinement effect, and high electron accumulation. Among these nanoparticles, silver (Ag) nanoparticles have strong responses in visible light region due to its high plasmon excitation. These unique properties depend on the size, shape, interparticle separation and surrounded medium of Ag nanoparticles. Indium tin oxide (ITO) is widely used as an electrode for flat panel devices in such as electronic, optoelectronic and sensing applications. Nowadays, Ag nanoparticles were deposited on ITO to improve their optical and electrical properties. Plasma-assisted hot-filament evaporation (PAHFE) technique produced high-density of crystalline Ag nanoparticles with controlling in the size and distribution on ITO surface. In this chapter, we will discuss about the PAHFE technique for the deposition of Ag nanoparticles on ITO and influences of the experimental parameters on the physical and optical properties, and electronic structure of the deposited Ag nanoparticles on ITO.

**Keywords:** silver nanoparticles, plasma-assisted hot-filament evaporation, properties, indium tin oxide, electronic structure

## **1. Introduction**

Noble metallic nanoparticles, which are described as metals in the nanoscale with dimensions within size range from 1 to 100 nm, recently received significant attention in optoelectronic, biosensing and photocatalysts applications [1–4]. This is due to their unique properties compared to the bulk materials such as large surface energies, surface plasmon excitation, quantum confinement effect, and high electron accumulation. The bulk material has constant physical properties regardless of their size and shape, however, these properties of the nanoparticles are a function of their size, shape, distribution and surrounded medium. Among these nanoparticles, silver (Ag) nanoparticles have particularly strong responses in the visible light region due to its high plasmon excitation at threshold energy of around 3.9 eV (318 nm). A specific phenomenon of the nanoparticles is localized surface plasmon resonance (LSPR) which results from the collective oscillations of the free

electrons on the metallic nanoparticle surfaces. Thus, the LSPR of Ag nanoparticles can be tuned to any wavelength in the visible light region. This is a highly desirable characteristic enabling the usage of Ag nanoparticles in optoelectronic devices mainly in solar cell and light emitting diode devices [5, 6]. The LSPR wavelength position can be tuned by varying the size, shape, particle spacing and compositions of the nanoparticles as well as a surrounding environment such as an insulating surface or presence of a dielectric layer [7]. Oxide layer can be form around the nanoparticles and acts as dielectric substances leading to formation of metal–metal oxide core-shell nanoparticles. These core-shell nanoparticles have been reported to produce wide SPR bands compered to pure metallic nanoparticle [8, 9]. Thus, the wide range of the LSPR existing through the metal oxide layer could be better than increasing in the nanoparticles size that may significantly lead to reduction in light scattering. On the other hand, indium tin oxide (ITO) is a transparent conducting oxide that has high transparency in visible light regions, low sheet resistance, and high work function. Moreover, ITO is widely used as anode material for optoelectronic devices as a hole injection layer in the devices [10, 11]. Thus, deposition of Ag nanoparticles layer on ITO suggests a feasible approach to enhance the flexibility, luminescent efficiency, electrical conductivity, and adhesion to device layers.

Ag nanoparticles layer are widely synthesized using evaporation-condensation, electron beam irradiation, and radio frequency plasma-assisted thermal evaporation, which show a good surface adhesion with the dielectric surface [12–14]. However, these physical deposition methods generally involve complicated structures, surface treatments, and high reaction temperatures up to several thousand °C in a plasma jet and 400°C for thermal annealing purposes [13, 15, 16]. The high reaction temperatures usually lead to the destruction of the device layers during the deposition process [13, 15, 16]. This issue can be avoided using thermal evaporation by hot-filament, as it provides a rapid evaporation of metallic nanoparticles source in high purity and the most important is it involving low substrate temperatures (usually below 400°C) [17, 18]. Moreover, plasma-driven deposition controls the transportation and deposition of the evaporated metallic adatoms, which directly leads to better size and uniformity of the deposition [19]. Nevertheless, pre-plasma treatment on the substrate surface proved to be more effective in removing organic contaminations that impede the particle mobility on the deposited surface [20, 21]. Thus, plasma-assisted hot filament evaporation technique is expected to deposition of Ag nanoparticles layer in uniform size and distribution at low substrate temperatures.

### **2. Plasma-assisted hot-filament evaporation**

Evaporation is a common method for deposition of thin film from their source materials in a vacuum as a physical vapor deposition (PVD) technique. The source materials are evaporated using evaporation source such as metal boat or coiled wire. Tungsten is a metal and has very high melting temperature about 3422°C. A tungsten wire can be coiled to form spiral shape for using as a hot-filament. This hot-filament is preferred to use for deposition very thin film compared to a tungsten boat. However, the deposition rate of the thin film on substrates using PVD technique is very low. Other common technique, chemical vapor deposition (CVD) is a technique for deposition nanostructured thin film on substrates with very high temperature using precursor gases [22]. Insertion of hot-filament in to the CVD technique helps to deposit nanocrystalline of nanostructured thin film at lower substrate temperature as hot-filament chemical vapor deposition (HFCVD) technique [18, 23–27]. The HFCVD process employs the heated filament to decompose

*Deposition of Silver Nanoparticles on Indium Tin Oxide Substrates by Plasma-Assisted… DOI: http://dx.doi.org/10.5772/intechopen.94456*

the precursor species and deposit nanostructured film on the substrate. On other hand, plasma can include electron, ions, free radicals, photons and neutrals that can generate reactive chemical species for enhancing the thin film deposition. The most recent technique was used is plasma-enhanced chemical vapor deposition (PECVD) [28–31]. However, PECVD requires a long time deposition and produces non-purity thin film with existence of toxic and explosive gases in plasma stream [32]. To avoid the existence of the toxic gases and contamination on the film, hydrogen plasma is used to remove the contaminations and provide purity thin film deposition without any toxic gases production. Therefore, hybrid of hot-filament with hydrogen plasma as plasma-assisted hot-filament evaporation (PAHFE) is a promise technique for deposition of purity metal nanoparticles layer on the substrate at low substrate temperature. Thus, the features of this technique are: (1) control of particle size, shape and interparticle separation, (2) enhancement of the nanoparticles crystallity, (3) stabilization in the physical and structure properties, (4) high deposition rate, (5) production of purity nanoparticles, (6) avoid particles aggregation, (7) low substrate temperature, and (8) fast deposition rate.

#### **2.1 Structure of PAHFE**

**Figure 1(a)** shows the real picture of the PAHFE system. The PAHFE consists to three main parts; reaction chamber, vacuum pumping system and supply units. The schematic diagram of the home-built of the reaction chamber is shown in **Figure 1(b)**. A radio frequency (RF) electrode was used as a plasma generation source. The tungsten wire with a diameter of 1 mm can be coiled to approximately 30 coils, generating coils with a diameter of 2 mm and length of 3 cm. This formed a tungsten coil filament as the hot-filament which the Ag source (Ag wire) was hanged on the filament coils for evaporation purpose. This hot-filament was placed below the RF electrode at a distance of 2 cm. Two copper electrodes were used to hang the tungsten wire, as shown in **Figure 1(b)**. Substrates which the nanoparticles deposited on were put on a substrate holder. The substrate holder was placed below the filament at a distance of 10 cm. After that, the substrates can be heated using a heater rod inserted into the substrate

**Figure 1.**

*(a) Real picture of PAHFE system. (b) Schematic diagram of the home-built PAHFE chamber [33].*

holder for achieving a desired substrate temperature. A thermocouple was used to measure the substrate temperature and connected to a temperature controller. Finally, in order to control the evaporation time, a shutter was placed between the filament and substrate holder. There are four supply units for PAHFE technique. RF power supply of 13.56 MHz was connected to the RF electrode by a matching impedance for plasma generation in the reaction chamber. Filament heating power supply is used for heating the filament to a desired temperature in purpose of Ag wire evaporation. The two copper electrodes were connected to the filament heating power supply. Substrate heating power supply was connected to the heater rod to heat the substrate for achieving a particular substrate temperature. Finally, a hydrogen gas was supplied to the reaction chamber through ¼ inch SS tube and ball valve. The gas flow is precisely controlled by a mass flow-controller. Furthermore, the vacuum pumping system is used to evacuate the reaction chamber before and after deposition processes, and control the pressure as well.
