Effect of Electrode Distance on the Electron Field Emission Property for CdS Nanofibers

Sk. Faruque Ahmed1, a), Mohibul Khan1, Nillohit Mukherjee2

1Nanoscience Laboratory, Department of Physics, Aliah University, IIA/27, New Town, Kolkata - 700160, India.

2Center of Excellence for Green Energy and Sensor Systems, Indian Institute of Engineering Science and Technology, Howrah 711103, India

a) Corresponding author: faruquekist@gmail.com

Abstract. Cadmium sulfide (CdS) nanofiber in thin films form have been prepared on flexible ITO coated Polyethylene Terephthalate (PET) substrate via chemical bathe deposition technique. X-Ray Diffractometer used for the structural analysis of the CdS nanofiber thin films and the topographical image characterized by using an Atomic Force Microscope (AFM). AFM studies showed that the average diameter of the CdS nanofiber ~ 250 nm with length several micrometers. The electron field emission property of CdS nanofiber thin films have been carried out for different anode to cathode distance. It was found that the threshold field decreases from 4.2 - 2.5 V/m when the anode to sample i.e., nanofibers thin film distance increased from 100 to 200 m. The emission current density with anode to sample distance were calculated and tried to explain the mechanism.

Keywords: CdS Nanofiber; XRD; AFM; Electron field emission.

INTRODUCTION

The flexible, light-weight display technology have recently drawn tremendous interest in the display industry for the development of highly demanding future product applications in efficient portable devices such as smart cards, pagers, personal digital assistants, cell phones digital camera, camcorders, remote control circuits and the future electronic papers [1-10]. These flexible substrates have several advantages over the hard substrate such as glass and silicon in display technology, which include robustness, light weight, flexibility (for varying the device shape to optimize visibility and thus suppress reflections), durability, thinness (providing wide viewing angles), easy scaling-up to a large format for a large volume roll-to-roll production etc [7-11]. The different substrate materials such as polycarbonate (PC), polyarylate (PAR), polyestersulfone (PES), polyimide (PI), polyethylene terepthalate (PET), thermoplastic, poly-dimenthylsiloxane (PDMS), polymethyl methacrylate (Perpex, Plexiglas) and polyethylene napthalate, commonly used for the said applications [9-18].

Cadmium Sulfide (CdS), an well-known n-type direct band gap II-VI semiconductor with a band gap energy 2.42 eV, has been found to be the best suited material for many applications such as thin film solar cells, light emitting diodes, thin film field effect transistors, thin film filters, laser materials optical wave guide, optical switches, nonlinear integrated optical devices, photocatalysts for hydrogen production by visible light etc., [19-24]. Nanostructure CdS have been synthesized through different routes such as RF Sputtering, dip coating, chemical vapor deposition, vacuum evaporation, spray technique, screen printing, close space sublimation, thermal evaporation, molecular beam epitaxy, metal organic chemical vapor deposition, pulsed laser deposition, photo chemical deposition, sol-gel and electrochemical deposition [25-31]. The structural, morphological, electrical, optical and electron filed emission properties of CdS thin films have been reported by different researchers on hard substrate such as glass, silicon.

We have synthesized CdS nanofiber on flexible substrate (ITO coated PET) using chemical bath deposition (CBD) technique which is very low-cost effective compared to other technique as mentioned above. The CBD technique is the most successful method used in the production of uniform, adherent, and reproducible large-area thin films for optical application. The structural analysis of the deposited CdS nanofiber thin films was characterized by using an XRD and surface morphology analyzed by using AFM which show that the CdS nanofiber forms on PET substrate.

In electron filed emission process the current-voltage characteristics depends on morphology, structural parameter, such as inter-electrode distance between thin film and anode, which knowledge could be helpful for the design of a flat panel display. Although electron field emission property of CdS thin films has been studied

but the field emission experiments performed for the CdS thin films were grown on hard substate such as ITO coated glass, silicon etc. [23,24]. In the present study, we have performed electron field emission form CdS nanofiber thin films on flexible substrate such as ITO coated PET for different distance between anode and CdS nanofiber thin films. The effects of anode to sample distance i.e., inter-electrode distance on the electron field emission property of CdS nanofiber thin films have been studied in details. The threshold fields were calculated and tried to explain the emission mechanism.

EXPERIMENTAL DETAILS

Synthesis of CdS Nanofiber Thin Films

Commercial ITO coated PET substrate (MERCK, Resistivity 8-12 Ω/sq, thickness 0.175 mm) of size 30 mm x 20 mm was used for deposition of CdS thin film by using CBD technique. At first, all the ITO coated PET substrates were ultrasonically cleaned for 10 minutes at temperature 300C and rinsed thoroughly with distilled water 2-3 times continuously then the substrates were dried in air. The CdS nanofibers thin films were deposited by CBD technique on ITO coated PET substrate at 70 0C from a solution containing cadmium acetate and thiourea in the alkaline medium to yield cadmium and sulphur ions. The precursors for the synthesis of CdS nanofibers thin films were analytical grade cadmium acetate (Sigma-Aldrich, 99.99%), thiourea and Monoethanolamine. Monoethanolamine was added to the cadmium acetate solution as a stabilizer. The deposition of CdS nanofiber thin films was done in a reactive solution prepared in a 50 ml beaker by the sequential addition of 5 ml of 0.5 M Cadmium acetate, 5 ml of 2 M sodium hydroxide, 6 ml of 1 M thiourea and 2 ml of 1M monoethanolamine. The solution was added with ethanol to make the total volume of 50 ml. The solution was continuously stirred with a magnetic stirrer, keeping the temperature of the solution at 70 0C and pH of the solutions was maintained at ~8 approximately. After 30 min the solution colour turned into yellow and then proper cleaned ITO coated PET substrates were vertically immersed into the bath containing reaction mixture for the deposition of CdS nanofiber thin film with suitable holder. Deposition has been carried out for another 40 min. The deposited film was taken out from chemical bath and dried in a vacuum hot oven at 70°C in Argon atmosphere for 10 minutes and the resultant films are homogeneous and well adhered to the substrate. The reaction for the formation of CdS nanofiber thin films as follows:

Cd(CH3COO)2 3H2O + 2NaOH → Cd(OH)2 + 2CH3COONa

Cd(OH)2 → Cd2+ + 2OH-

CS(NH2)2 + 3(OH-) → (CO3)2- + S2- + 7H

Cd2+ + S2- → CdS

Characterizations of CdS Nanofiber Thin Films

The structural characterization of the deposited CdS nanofiber thin films was studied by X-ray Diffractometer (Bruker, D-8 Advance). The X-ray Diffraction patterns of the CdS nanofiber thin films were recorded between the range 200 to 700 using Cu-Kα radiation of wavelength λ = 1.5406 Å, where the source tension and current were kept as 40 kV and 40 mA respectively. The surface morphology of CdS nanofiber on ITO coated PET surface have been analyzed by Atomic Force Microscope (AFM, Concept Scientific Instruments, France, CS-S-SPM-00001) contact mode. In a high vacuum (~ 10-7 mbar) chamber, the electron field emission measurement was performed. In Fig. 1 shows the schematic diagram of the electron field emission equipment. The ITO coating on the PET substrate act as the back metallic contact for field emission measurement. In a diode type configuration, the electron field emission measurements have been performed. The CdS nanofiber thin films used as a cathode and a conical shape stainless steel tip with 1 mm tip diameter used as an anode respectively. The distance between anode (conical shape stainless steel tip) and cathode (CdS nanofiber thin films) was varied manually by a spherometric arrangement with screw-pitch of 10 m. The anode and cathode i.e., CdS nanofiber thin films distance was set at a particular value by rotating the micrometer screw. An optical microscope used to observe the just touching the anode tip and the CdS nanofiber thin films. The electron field emission experiment performed using a high-voltage power DC supply (H.T.) and a high precession Keysight make digital multimeter (Model: 34450A) which were controlled by the computer interface.

.

FIGURE 1. Schematic diagram of the electron field emission measurement equipment.

RESULTS AND DISCUSSION

The XRD patterns of the CdS nanofiber thin films deposited on PET substrate is shown in Fig. 2. Generally, CdS thin films have two types of crystal orientation such as Cubic and hexagonal or mixed phase depending on various preparation procedures. The XRD pattern shows clear intensity of the CdS hexagonal (101) peak at an angle 28.150 which confirms that the hexagonal CdS phase form for the deposited nanofiber thin films [32,33].

FIGURE 2. XRD pattern of CdS nanofiber thin films on ITO coated PET substrate.

Figure 3 shows the topography images of the CdS nanofiber thin films. AFM micrographs of the deposited thin films, showed the existence of CdS nanofibers of closely packed and randomly oriented within the thin films. The average diameters of the CdS nanofibers are ~ 250 nm and few micrometers in length.

FIGURE. 3 (a) 2D and (b) 3D AFM images of CdS nanofibers thin films on ITO coated PET substrate.

Figure 4(a) shows the macroscopic field (E) versus emission current density (J) graph for CdS nanofiber thin films for different anode-cathode (CdS nanofiber thin films) distance (d). The external applied voltage applied (V), divided by the anode to cathode i.e., sample distance (d) gives the value of macroscopic field (E). Field emission characteristics of the films were analyzed using the modified Fowler-Nordheim (F-N) theory [34-38]. The F-N equation for the local current density J at some point on the emitting surface given by

ln { JE2} = ln { a φ−1 β2 }−

( s bφ3

2 β−1 )E (1)

where, f is the local work-function, b is the field enhancement factor, a is the first F- N Constant (1.541434 x 10-6 A eV V-2), b is the second F-N Constant (6.83089 x 109 eV-3/2 V m-1), s is the slope correction factor. Generally, the value of s is of the order of unity and also s is relatively slowly varying functions of 1/E, so a F-N plot i.e., ln (J/E2) verses 1/E will be expected a straight line.

FIGURE 4. (a) J-E curves for the CdS nanofiber thin films for different anode to sample separation ‘d’ and (b) corresponding Fowler-Nordheim graph.

The threshold fields of CdS nanofiber thin films decreases from 4.7 to 3.6 V/m with the increase of anode to cathode distance from 100 to 200 m respectively. A parallel shift observed in the J-E graph (Fig. 4(a)), with respect to anode to cathode distance (d) i.e., for a particular electric field the current density increases with increasing the anode to cathode separation. For example, at an electric field of 5 V/m, the J values were found to be 0.2 mA/cm2 (for d = 100 m), 1.2 mA/cm2 (for d = 150 m) and 3.7 mA/cm2 (for d = 200 m) respectively. Similar type of observation was also reported by Zhou et al. [39] for their bSiC nanorods. This type of shift observed in the CdS nanofiber thin films is due to the change in the effective emission area of the sample for different anode to cathode distance. A conical shape anode with tip diameter 1 mm used for the measurement of electron field emission, therefore the lines of force immerging from the edge of the stainless steel anode tip and terminating to the CdS nanofiber thin films i.e., cathode surface are diverging in nature as shown in schematic diagram (Fig. 1), whereas the lines of force immerging from the flat surface of the tip are parallel in nature. Hence, the effective emission area of the sample increases with increasing anode to CdS nanofiber thin film distance and as a result the current density (J) increases. Au et al. [40] reported field emission of silicon nanowires using a spherical shaped stainless-steel probe with a tip diameter of 1 mm as an anode. They also found a parallel shift in their I-V curve. Okano et al. [41] observed that macroscopic current density for diamond films was independent of the anode cathode separation. Their field emission apparatus consisted of a parallel plate arrangement of the anode and sample, separated by spacers. So, the electric lines of force between the anode and the sample were parallel in nature, hence effective emission area remained independent of the anode to sample distance.

CONCLUSION

Cadmium sulfide (CdS) nanofiber in thin films form have been prepared on flexible ITO coated Polyethylene Terephthalate (PET) substrates via chemical bathe deposition technique. X-Ray Diffractometer used for the structural analysis of the CdS nanofiber thin films and the topographical image characterized by using an Atomic Force Microscope (AFM). AFM studies showed that the average diameter of the CdS nanofiber ~ 250 nm with length several micrometers. It was observed that the threshold field of CdS nanofiber thin films decreases from 4.2 - 2.5 V/m with increasing of anode to cathode i.e., CdS nanofibers thin film distance from 100 to 200 m respectively. The above study shows that the CdS nanofiber thin films become good candidates for low-threshold flexible field emitter among other applications.

ACKNOWLEDGEMENT

The authors wish to acknowledge the University Grants Commission, New Delhi, India for the financial support under Maulana Azad National Fellowship (MANF) scheme.

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