- Email: [email protected]
Field emission study of urchin like nanostructured cobalt oxide films prepared by pulsed laser deposition
Journal of Alloys and Compounds 744 (2018) 281e288
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Field emission study of urchin like nanostructured cobalt oxide films prepared by pulsed laser deposition H. Jadhav a, *, S. Suryawanshi b, M.A. More b, S. Sinha a a b
Laser & Plasma Surface Processing Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Department of Physics, University of Pune, Pune 411007, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2017 Received in revised form 2 February 2018 Accepted 8 February 2018 Available online 10 February 2018
Cobalt oxide (Co3O4) urchin structures have been grown via Pulsed Laser Deposition (PLD) technique followed by thermal treatment in air. Morphological and structural analysis of as synthesized nanostructures has been carried out using Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD) technique. Chemical nature of the films have been revealed using X-ray Photoelectron Spectroscopy (XPS). Field emission measurements were carried out at a base pressure of 1 108 mbar. Turn-on field corresponding to an emission current density of 10 mA/cm2 was found to be 3 V/mm and a high field enhancement factor of ~3900 was estimated, both of which can be attributed to radially grown nanowires of long length with sharp tips. Overall good field emission current stability was observed for Co3O4 urchin structured films. Dependence of FE properties such as, current density and current stability on morphology and density of emitting sites has been observed and analyzed systematically. The observed FE results demonstrate Co3O4 urchin structures as potential candidate for applications in various vacuum micro/nanoelectronic devices. © 2018 Published by Elsevier B.V.
Keywords: 3D urchin structure Pulsed laser deposition Field emission
1. Introduction Nanostructured materials have been extensively studied as their unique properties make them highly desirable for a wide range of potential applications. Both, one dimensional (1D), as well as, multidimensional (2D and 3D) nanostructures have demonstrated their significant role as, photo catalysts [1], chemical sensors [2], field emitters [3,4], to name a few. In addition to 1D nanowires and nanotips, multidimensional (2D and 3D) nanosheets and nanoparticles, unique nanostructures such as, 3D urchins have also been reported [5]. 3D urchin like structures have been attracting significant attention since such structures offer both advantages of nano and micro/sub micrometer sized structures. Urchin structures of CuO, Fe2O3, Co3O4 and ZnO have been reported for various applications [5e8]. Nanostructure based field emitters have been widely studied due to their great commercial potential in field emission (FE) applications. High aspect ratio of 1D nanowires (NWs) provide high enhancement of electric field at the tip of the NWs, whereas, 2D
* Corresponding author. E-mail address: [email protected] (H. Jadhav). https://doi.org/10.1016/j.jallcom.2018.02.074 0925-8388/© 2018 Published by Elsevier B.V.
nanosheets (such as graphene nanosheets) lead to extremely high enhancement of the field strength at its edges due to the very low thickness of such nanosheets. 3D urchin like structures are composed of 1D sharp tips pointing in various directions which is beneficial for enhancing FE properties. Emission current density and field enhancement in case of FE effect depend on several parameters including length and radii of the emitters, inter tip distance and density of nanotips and nanostructures. Although, FE properties of Fe2O3, CuO and ZnO urchin like structures have been reported [5,9,10], FE study of Co3O4 urchin structure has not been reported till date. In comparison to metals and carbon nanotubes (CNT) which are susceptible to oxidation, stable oxides are preferred as field emitters for efficient and long term operation with limited degradation of FE current. CNT's decorated with metal oxides to reduce high work function have also been reported [11]. Therefore, studying FE properties of oxide material having different nanostructures has been highly recommended. Co3O4 is an important p-type semiconductor and it is the most stable phase belonging to the family of cobalt oxides. Chemical stability of Co3O4 over a wide temperature range and high mechanical strength makes it particularly suitable for FE applications. Nanowires and nanowalls of Co3O4 synthesized by heating cobalt foils in presence of oxygen gas have been reported for FE [12].
282
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
Urchin like structures of CuO and Fe2O3 have been reported synthesized by one step thermal oxidation of Cu and Fe microspheres in air at various temperatures [5,9]. Fe2O3 urchin structures have also been synthesized by hydrothermal synthesis, a well known technique to grow various nanostructures [13]. Thermal evaporation is another method used to synthesize ZnO core shell urchin structure and studied for photoelectrochemical properties [14]. To grow urchin like Co3O4 structures, hydrothermal method followed by thermal treatment in air has been reported [15]. Very recently R. Edla et al. have reported Co3O4 urchin like structured films deposited by Pulsed Laser Deposition (PLD) technique followed by thermal treatment in air [7]. PLD is a preferred technique for its easy operation, reproducibility of the coating and stoichiometric transfer of the ablated target material to the substrate. By controlling the deposition parameters in PLD technique, properties of the deposited film can be tuned easily. In the present study we report synthesis of urchin like nanostructured films of Co3O4 and their performance as efficient field emitters. Co-B films deposited by PLD on Si substrates were annealed in air at selected temperature for growth of urchin like Co3O4 nanostructures. Field Emission Scanning Electron Microscopy (FESEM) has been used to study morphology of the heat treated films deposited by PLD. X-ray Diffraction measurement was carried out to study the crystalline structure of these films. FE measurements were performed in vacuum at a pressure of 108 mbar. FE properties such as field emission current density and stability of current delivered by these urchin films over extended period of time have also been investigated.
2. Experimental Cobalt boride (Co-B) powder which was synthesized using chemical reduction method [16] was cold pressed at a pressure of ~1500 kg/cm2 to form circular discs of 25 mm diameter and ~3e4 mm thickness. The ratio of Co/B was 1.61 in the Co-B powder as reported in our previous work [16]. This circular disc served as a target to carry out PLD runs with Si used as substrate. Edges of Si samples were smoothened to avoid emission from the edges during FE measurements. PLD of Co-B films were performed using Nd-YAG laser operating at a second harmonic wavelength of 532 nm at a pulse repetition rate of 10 Hz. The distance between Co-B target and Si substrate was kept ~4 cm for each PLD run. The target was mounted on a rotating target holder to avoid crater formation on the target surface due to incident laser radiation whereas substrate was kept static on a steady substrate holder. Typical average values of laser fluence of 20 and 30 J/cm2 were selected for Co-B deposition based on our previous experiments [16]. The ratio of Co/B in
the PLD deposited Co-B films is 1.81 [16]. The area of the deposited Co-B films is 0.6 cm2. All PLD runs were carried out at room temperature under vacuum condition at a background pressure of 105 mbar. In order to grow 3D urchin like structure, deposited CoB films were subsequently annealed in air at different temperatures (400 C - 600 C) with a heating rate of 10 C/min. Annealing time was also varied to optimize growth of urchin like structure. Surface morphology of the films has been studied using Field Emission Scanning Electron Microscopy (FESEM). Crystalline phase of the films was determined by X-ray diffractometer with Cu Ka radiation (l ¼ 1.5414 A). Grazing incidence XRD (GIXRD) measurements were performed at room temperature for PLD deposited and annealed films at a grazing angle of incidence of 0.8 . XRD measurements were performed over the 2q range of 30-70 . X-ray photoelectron spectroscopy (XPS) was used to determine the surface electronic states of the films. XPS analysis of the films was performed using a VG make model CLAM-2 hemispherical analyser with Al Ka source (1486.6eV) having a step size of 0.3 eV. Field Emission measurements of the films were carried out in a vacuum chamber at a pressure of 108 mbar at room temperature. A phosphor coated transparent metal screen served as an anode which was located ~1 mm away from the PLD Co3O4 urchin film which served as the cathode. The macroscopic surface electric field on the sample was estimated by Fm ¼ V/d, where V is the applied voltage and d is the distance between anode and cathode. The emission current density (Jm) was evaluated using (Jm ¼ I/A) where I is the total emission current and A is the cathode surface area which was 0.6 cm2 in present FE runs. The field emission current was estimated by measuring voltage drop across a fixed resistance of 102.5 kU and stability of the emission current was continuously recorded using a data logger which recorded data every 10 s.
3. Results and discussions Fig. 1a shows FESEM image of Co-B film deposited by PLD technique. Spherical particulates with smooth surface are observed on the surface of the film. Particles of size of few nm to 1e3 mm were clearly evident in Fig. 1a, suggesting a wide particle size distribution. Such morphology of the films deposited using PLD technique can be attributed to phase explosion phenomena which occurs when laser ablation of target material is carried out using high laser power density [16]. Fig. 1b shows SEM image of Co-B film synthesized by PLD followed by thermal treatment in air at a temperature of 600 C for 8 h. The spherical particulates of Co-B transformed into urchin structures consisting of a core and numerous nanowires which radially grow on the surface and project outwards post thermal treatment in air, as observed in
Fig. 1. FESEM images of (a) Co-B film deposited by PLD, (b) Co-B film after annealing at 600 C for 8 h taken at 10 KX magnification and (c) single urchin particle.
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
Fig. 1b. Diameter of the urchin particulates is found to be in the range of ~500 nm to ~4.5 mm as measured from image shown in Fig. 1b. Densely packed NWs are also clearly visible on the surface of the film with higher number density as compared to the urchin particles (Fig. 1b). These NWs with diameter much smaller than their length have a high aspect ratio which is a favorable condition for good FE effect. FESEM image of a single urchin particle is shown in Fig. 1c. Well grown extended NWs are clearly observed on the spherical cores which act as active emission sites for good FE performance. Similar urchin structured morphology has been reported in literature when Co-B films were annealed at 600 C for 4 h and the probable mechanism responsible for urchin structures has been attributed to stress induced growth [7]. We have annealed the Co-B films at 600 C for 4 h but formation of complete urchin structure was not observed. We have observed formation of small nanowires on some of the Co-B particles for 4 h annealing time. Hence we have extended the annealing time to 8 h to get complete urchin structure. Fig. 2 shows the effect of annealing temperature on the growth mechanism of urchin structure. After heat treatment at 400 C for 8 h, smooth spherical particulates in as deposited Co-B film appear deformed with formation of small nanoprotrusions on the surface of these particulates as shown in Fig. 2a. Further increase in temperature to 500 C initiates evolution of very small NWs from the deformed spherical particle as shown in Fig. 2b. However, complete urchin like structure with well grown NWs is evident only when the annealing temperature was raised to 600 C.
283
XRD spectra of Co-B film deposited in vacuum is shown in Fig. 3a. The as deposited Co-B film exhibits a broad peak centered at around 2q ¼ 45 which can be assigned to amorphous state of Co-B. Additional peak observed at 2q ¼ 55 is due to Si substrate. Fig. 3b shows XRD spectra recorded for Co-B film when annealed at 600 C for 8 h in air. As shown in Fig. 3b XRD peaks centered at 2q ¼ 31.4 , 36.9 and 44.8 can be assigned to the cubic structure of Co3O4 as confirmed against JCPDS data file 76-1802. These diffraction peaks marked in Fig. 3b correspond to reflections from (220), (311) and (400) planes, respectively. An additional small peak detected at 2q ¼ 42.5 indicates the presence of CoO phase according to JCPDS file 78-0431. The diffraction peak appearing at 2q ¼ 51.5 is due to the silicon substrate. The average crystallite size was calculated by measuring the width of the most prominent peaks corresponding to reflections from (311) and (400). Crystallites size was calculated using Scherrer's equation as given below,
d ¼ kl=b cosq
(1)
where, d is the average crystallite size, k is the Scherrer's constant (0.9), b is the full width at half maxima, l is the wavelength of the Xray (0.154 nm) and q is the Bragg angle. The average crystallite size was determined to be ~10 nm for (311) plane and ~13 nm for (400) plane. Absence of boron oxide phase in the XRD pattern confirms that the heat treated film is composed of oxides of cobalt, mainly Co3O4 phase. Our observations are in agreement with reported data on annealed Co-B films where decomposition of Co-B phase
Fig. 2. FESEM images of Co-B film when annealed at (a) 400 C, (b) 500 C and (c) 600 C for 8 h in air.
Fig. 3. (a) XRD spectra of Co-B film and (b) Co-B film heat treated at 600 C for 8 h in air.
284
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
occurred at 400 C and crystalline Co3O4 was formed when annealed at 600 C [7]. B. Varghese et al. also reported similar XRD pattern for Co3O4 nanowires and nanowalls when a Co foil was annealed at 450 C in a vacuum chamber in presence of oxygen [12]. XPS has been used to study the surface chemical composition of these films and the results are reported in Fig. 4. XPS spectra of Co (Fig. 4a) shows two prominent peaks at BEs of about 781 eV and 796.5 eV which corresponds to 2p3/2 and 2p1/2 states, respectively. Energy difference between the 2p3/2 and 2p1/2 peaks is ~15.5 eV which is characteristic of Co3O4 phase. The major peak of Co2p3/2 is deconvoluted into two peaks centered at 780.2 eV and 782.2 eV attributed to Co3þ and Co2þ species respectively, in Co3O4 phase [17,18]. Fig. 4b shows XPS spectra of O which shows two major peaks centered at 530.1 eV and 531.8 eV corresponding to oxygen in Co3O4 crystal lattice and OH group attached to Co respectively [17,18]. Presence of OH group on the surface of the films is due to ex-situ experimental conditions. Both spectra of Co and O reveal that surface composition of the urchin structured film is mainly Co3O4. XPS peak positions estimated for our urchin structured films matched well with reported values of Co3O4. The probable mechanism responsible for the formation of urchin structure in present work is mainly attributed to difference in thermal expansion coefficients of two elements (B and Co). In as deposited Co-B coating, the particulates are of core shell structure (as observed in TEM images of Co-B film deposited by PLD in our previous study) [16] where, the contrast between core and shell indicate the presence of different elements in each part, mainly core is composed of Co and shell is of Co, B and O as reported in Ref. [7]. During oxidation at high temperatures stress between core and shell increases due to difference in thermal expansion
coefficients of Co (13 106 K1) and B (5 106 K1). This stress increases as a function of temperature and at a particular temperature (600 C in present case) stress is released in the form of NWs due to outward diffusion of metallic element. Similar growth of urchin was not observed when we annealed pure Co film deposited by PLD for identical annealing conditions (600 C, 8 h). Stress induced mechanism has also been reported for growth of different nanostructures of Fe2O3 when Fe2O3 PLD film was annealed at 600 C [19]. These PLD films having urchin like Co3O4 structure were investigated for their field emission properties. Macroscopic current density (Jm) versus applied macroscopic electric field (Fm) plots and Fowler-Nordheim (F-N) plot along with emission current stability plot of Co3O4 urchin structured film are shown in Fig. 5. Fig. 5a shows variation of Jm as a function of Fm and corresponding F-N plot is shown in Fig. 5b. The emission current stability of urchin structured Co3O4 film is shown in Fig. 5c. Here, Fm is defined as the ratio of applied voltage (V) across the two electrodes which were kept at a distance of ~1000 mm (d) from each other. Emission current density (Jm) is defined as total emission current per unit area of film (~0.6 cm2). The Co3O4 films were used as cathodes having an area of 0.6 cm2 and a phosphor coated transparent metal screen served as an anode. The turn-on field defined as the applied field required to draw an emission current density of 10 mA/cm2 was measured to be about 3 V/mm. FE properties investigated in present study have been compared with urchin structures of other metal oxides such as Fe2O3, CuO and ZnO and presented in Table 1. Comparison of other nanostructures like nanowires and nanorods with the urchin structure has also been given in Table 1.
Fig. 4. XPS spectra of Co-B film annealed at 600 C for 8 h in air.
Fig. 5. Plot of (a) current density as a function of applied electric field (b) F-N plot and (c) emission current stability of Co3O4 urchin structured films.
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288 Table 1 Some reported values of turn on field for urchin structures and other structures of different metal oxides. Material
Turn-on Field (V/mm)
Reference
Fe2O3 urchin CuO urchin ZnO urchin CuO nanowires Co3O4 nanorods Co3O4 urchin
2.8 2.94 3.7 5.31 2.8 3
[9] [5] [10] [3] [20] Present study
We measured a highest achievable current density of ~480 mA/ cm2 at 6.1 V/mm for these Co3O4 urchin structured film. Good FE behavior exhibited by these urchin structured films is mainly attributed to surface morphology of the films. Radially grown nanowires from the spherical particles enhances local field due to their pointed tips. It has also been reported that crystalline nature positively affects FE performance of films [21,22]. Improved FE properties of NiFe2O4 nanocrystallites have been reported with the improvement in crystallinity [22]. Linearly fitted F-N plot is shown in Fig. 5b. Slope (S) of the F-N plot was estimated using linear fit to the experimental data points. F-N equation was employed to analyze the FE properties of urchin structured Co3O4 films which can be expressed as [23].
. Jm ¼ lm $a$f1 $ðgc $Fm Þ2 exp nF $b$f3=2 ðgc $Fm Þ
(2)
Here, Jm and Fm are macroscopic emission current density and macroscopic applied electric field respectively, whereas, lm is macroscopic pre-exponential factor which provides a measure of the fraction of actual cathode area contributing towards emission, a and b are first and second F-N constants (1.541434 mAeVV2) and (6.830890 eV-3/2Vnm1), respectively and 4 is work function of the field emitter (4.5 eV for Co3O4), gc is macroscopic field enhancement factor and nF is potential barrier form correction factor. The macroscopic field enhancement factor has been estimated by following equation [24].
gc ¼ sm b
f3=2 S
(3)
where, sm and S are generalized slope correction factor and slope of the linearly fitted F-N plot, respectively. Table 2 presents these values of slope correction factors and slope along with turn on field and threshold field values estimated for the urchin structured Co3O4 films. The field enhancement factor ~3900 as estimated from our data provides a quantitative estimate for the degree of electric field enhancement on the emitter surface for urchin structured films. High field enhancement factor is attributed to long length and sharp tips of the NWs of the urchin structure. Along with urchin structures, densely packed NWs as observed on film surface (Fig. 1b and c) also contribute towards FE performance. These NWs serve as active emission sites which experience enhanced local field on account of their high aspect ratio. Table 2 List of estimated FE parameters for urchin structured Co3O4 film B. Sr.No.
Parameters
Value for Co3O4 urchin film
1. 2.
Turn on field (for 10 mA/cm2 current density) Threshold field (for 100 mA/cm2 current density) Slope of F-N plot (S) Macroscopic field enhancement factor (gc) Generalized slope correction factor (sm)
3 V/mm 4.6 V/mm
3. 4. 5.
1.39 107 Np Vm1 3900 0.85
285
The emission current stability of urchin like Co3O4 was also investigated. Fig. 5c shows the current stability monitored over a period of ~2 h. The initial emission current was ~8 mA which increased after ~30 min to 13 mA and then was constant showing fluctuations of ±2 mA. No marked degradation of emission current was observed in the next ~2 h. Adsorbed layer of residual gas molecules on the surface of the cathode and failure of nanostructures (emission sites) due to joule heating and/or mechanical failure on application of electric field could have resulted in initial fluctuations observed in emission current. However, once adsorbed impurities had been sputtered out from electron emitting tips the emission current improved and remained stable for an extend period of ~2 h. Therefore, in our present study, Co3O4 urchin films have shown good FE stability which can be attributed to mechanical and chemical stability of Co3O4 phase. Field emission performance critically depends on both, extrinsic properties i.e. shape and size of emitters and intrinsic material properties including work function. Several parameters, such as length and radii of the emitters, inter-tip distance and density of nanostructures determine the field emission current density, field enhancement factor and turn on field. Density of nanoparticles deposited via the process of pulsed laser deposition has been known to strongly depend on the laser fluence used for ablation of the target. Hence, to study the effect of areal density of nanostructures on FE properties, we compared performance of Co-B films deposited at two levels of laser fluence, 20 J/cm2 (film A) and 30 J/cm2 (film B), all other deposition parameters remaining same. Films deposited at 20 J/cm2 were annealed at same temperature of 600 C for 8 h as in the case of films deposited using higher fluence of 30 J/cm2. Typical FESEM images of films (A) and (B) are shown in Fig. 6a and Fig. 6b, respectively. FESEM images of single urchin particle of film (A) and film (B) are shown in Fig. 6c and d, respectively. Wide particle size distribution ranging from ~500 nm to ~2e3 mm has been observed for both films as shown in Fig. 6a and b. However, formation of densely crowded NWs along with urchin like particles was clearly evident for films (B) (Fig. 6b). These NWs were rarely observed for films (A) deposited at lower laser fluence (Fig. 6a). Higher magnification images of single urchin particulate of film A and B have been shown in Fig. 6c and d, respectively, which also confirms formation of dense NWs for film B. As the deposition was carried out for same time duration in case of both films A and B, films deposited at high laser fluence are expected to be thicker as compared to those deposited at low fluence. Number density and length of the NWs observed for film (B) is higher as compared to film (A). Our observations are in agreement with the results reported in Ref. [25], where, population density and length of the NWs formed after thermal treatment were observed to increase with increase in film thickness. As these films are composed of Co-B phase and the difference between thermal expansion coefficients of Co and B being large, during heat treatment stress level in the films increases as a function of temperature [7]. Also, during the thermal process, diffusion of oxygen molecules in the Co-B film causes volume expansion in all three dimensions. However, only upward direction is available for a film to release the thermal stress induced during thermal treatment in air. This can be the probable cause of formation of NWs on the surface of film [25]. In our present study annealing time and temperature were maintained same for both films. Thickness of film A being less oxidation of this film could have been completed much before 8 h, while oxidation continued for film B till 8 h allowing formation of longer and denser NWs. Hence, termination of oxidation mechanism can be a function of thickness of the film, and this can determine the density and length of NWs formed on annealing. Our observations are in agreement with the results reported in Ref. [25], where, effect of thickness of the films on the population density and length of the
286
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
Fig. 6. FESEM images of Co-B films deposited by PLD at laser fluence (a) 20 J/cm2 and (b) 30 J/cm2 both films heat treated in air at 600 C for 8 h and (c) and (d) are the corresponding FESEM images of single urchin particulate.
NWs formed after thermal treatment was reported to have increased with increase in films thickness. FE properties of films deposited at 20 J/cm2 (film A) were studied and compared with films deposited at 30 J/cm2 (film B) and these results are shown in Fig. 7. The observed value of turn on field for films B was ~3 V/mm. Furthermore, at an applied electric field of 6 V/mm a much higher emission current density of ~480 mA/cm2 could be drawn for films B in comparison to ~100 mA/cm2 for films deposited at 20 J/cm2. The observed higher emission current density at relatively low applied field for films B can be attributed to the presence of densely packed NWs which acted as potential emission sites. Thus better FE performance observed for film B could be correlated to presence of larger number of emitters with sharp tips oriented perpendicular to the substrate and large number of nanowires having high aspect ratio. Emission current stability for both films was also recorded and compared. The emission current versus time plot corresponding to
preset current values of ~1.5 mA and ~13 mA recorded for 2 h for films deposited at 20 J/cm2 and 30 J/cm2, respectively is shown in Fig. 7. Emission current remained fairly constant over the entire duration of 2 h without degradation, which indicates good physical and chemical stability of these Co3O4 urchin structured films. Fig. 8 shows SEM image of Co3O4 urchin structured film B recorded before (Fig. 8a and c) and after (Fig. 8b and d) measurement of FE properties. In some regions of the emission area NWs are observed to have become shorter in length (Fig. 8b and d). Some of the sharp tips and spikes associated with the urchin structure have also been lost (Fig. 8d). Intense field experienced at the tips during extended FE measurements may be the likely cause for this damage. Therefore, a surface morphology with homogeneous distribution of emitters would be preferable since uniform degradation would occur across the entire emitter surface rather than the selective and predominant degradation of the high aspect ratio features as observed in Fig. 8.
Fig. 7. Field emission characteristics of Co3O4 urchin structured films (a) plot of emission current density versus applied field, (b) F-N plot and (c) current stability plot.
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
287
Fig. 8. FESEM images of urchin structured film before FE measurement (a and c) and after FE measurement (b and d). Images (a and b) taken at 10 KX and (c and d) are at 50 KX magnification.
4. Conclusion Co3O4 urchin like structures was synthesized by PLD technique followed by thermal treatment in air at selected temperatures. Morphological study revealed the formation of well grown urchin structures along with NWs on the film surface. Structural investigation by XRD confirms the formation of Co3O4 phase. Good field emission behavior with low turn on field of 3 V/mm and maximum emission current density of 480 mA/cm2 was recorded for films deposited at laser fluence of 30 J/cm2. The promising FE characteristics of Co3O4 films could largely be attributed to well grown urchin structures having radially grown long NWs as well as densely packed NWs observed on the film surface. Acknowledgement This work was supported by collaboration PhD program between University of Mumbai and Bhabha Atomic Research Centre, Mumbai, India. Authors gratefully acknowledge Prof. S Basu, BARC for experimental support provided for X-ray diffraction measurements. References [1] M. Guo, M. Fung, F. Fang, X. Chen, A. Ng, A. Djurisic, W. Chan, ZnO and TiO2 1D nanostructures for photocatalytic applications, J. Alloys Compd. 509 (2011) 1328e1332. [2] S. Varghese, S. Lonkar, K. Singh, S. Swaminathan, A. Abdala, Recent advances in graphene based gas sensors, Sens. Actuators, B 218 (2015) 160e183. [3] C. Tang, X. Liao, W. Zhong, H. Yu, Z. Liu, Electric field assisted growth and field emission properties of thermally oxidized CuO nanowires, RSC Adv. 7 (2017) 6439e6447. [4] G. Patil, P. Baviskar, V. Bagal, R. Ladhe, A. Deore, M. More, B. Sankapal, P. Chavan, Aligned 2D CuSCN nanosheets: a high performance field emitter, RSC Adv. 6 (2016) 71958e71963. [5] T. Chang, C. Hsu, H. Lin, K. Chang, Y. Li, Formation of urchin- like CuO structure through thermal oxidation and its field emission application, J. Alloys Compd. 644 (2015) 324e333.
[6] H. Yu, L. Hsu, T. Chang, Y. Li, A 3D a-Fe2O3 nanoflake urchin-like structure for electromagnetic wave absorption, Dalton Trans. 41 (2012) 723e726. [7] R. Edla, N. Patel, M. Orlandi, N. Bazzanella, V. Bello, C. Maurizio, G. Mattei, P. Mazzoldi, A. Miotello, Highly photo-catalytically active hierarchial 3D porous/urchin nanostructured Co3O4 coating synthesized by Pulsed Laser Deposition, Appl. Catal., B 166e167 (2015) 475e484. [8] H. Hieu, N. Vuong, H. Jung, D. Jang, D.H. Kim, S. Hong, Optimization of a zinc oxide urchin-like structure for high performance gas sensing, J. Mater. Chem. 22 (2012) 1127e1135. [9] L. Hsu, H. Yu, T. Chang, Y. Li, Formation of three-dimensional urchin like aFe2O3 structure and its field emission application, ACS Appl. Mater. Interfaces 3 (2011) 3084e3090. [10] H. Jiang, J. Hu, F. Gu, C. Li, Stable field emission performance from urchin-like ZnO nanostructures, Nanotechnology 20 (2009) 055706e055711. [11] S. Sridhar, C. Tiwary, S. Vinod, J. Tijerina, S. Sridhar, K. Kalaga, B. Sirota, A. Hart, S. Ozden, R. Sinha, H. Vajtai, W. Choi, K. Kordas, P. Ajayan, Field emission with ultralow turn on voltage from metal decorated carbon nanotubes, ACS Nano 8 (2014) 7763e7770. [12] B. Varghese, T. Hoong, Z. Yanwu, M. Reddy, B. Chowdari, A. Wee, T. Vincent, C. Lim, C. Sow, Co3O4 nanostructures with different morphologies and their field emission properties, Adv. Funct. Mater. 17 (2007) 1932e1939. [13] F. Beshkar, H. Jahangiri, M. Kamazani, Dendritic a-Fe2O3 nanostructures: facile hydrothermal synthesis, characterization and microwave absorption, J. Mater. Sci. Mater. Electron. 27 (2016) 12869e12875. [14] D. Tang, G. Liu, F. Li, J. Tan, C. Liu, G. Lu, H. Cheng, Synthesis and photo electrochemical property of urchin like Zn/ZnO core-shell structures, J. Phys. Chem. C 113 (2009) 11035e11040. [15] X. Rui, H. Tan, D. Sim, W. Liu, C. Xu, H. Hng, R. Yazami, T. Lim, Q. Yan, Template free synthesis of urchin like Co3O4 hollow spheres with good lithium storage properties, J. Power Sources 222 (2013) 97e102. [16] H. Jadhav, A.K. Singh, N. Patel, R. Fernandes, S. Gupta, D.C. Kothari, A. Miotello, S. Sinha, Pulsed laser deposition of nanostructured Co-B-O thin films as efficient catalyst for hydrogen production, Appl. Surf. Sci. 387 (2016) 358e365. [17] J. Yang, H. Liu, W. Martens, R. Frost, Synthesis and characterization of cobalt hydroxide, cobalt oxyhydroxide and cobalt nanodiscs, J. Phys. Chem. C 114 (2010) 111e119. [18] M. Biesinger, B. Payne, A. Grosvenor, L. Lau, A. Gerson, R. Smart, Resolving surface chemical states in XPS analysis of first row transition metals: Cr, Mn, Fe, Co and Ni, Appl. Surf. Sci. 257 (2011) 2717e2730. [19] R. Edla, N. Patel, M. Orlandi, N. Bazzanella, V. Bello, C. Maurizio, G. Mattei, P. Mazzoldi, A. Miotello, Highly photo-catalytically active hierarchical 3Dporous/urchin nanostructured Co3O4 coating synthesized by Pulsed Laser Deposition, Appl. Catal. B Environ. 166e167 (2015) 475e484.
288
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
[20] L. He, Z. Li, Z. Zhang, Rapid low temperature synthesis of single crystalline Co3O4 nanorods on silicon substrates on a large scale, Nanotechnology 19 (2008) 155606e155610. [21] S. Jung, S. Jo, H. Moon, J. Kim, D. Zang, C. Lee, Improved crystallinity of doublewalled carbon nanotubes after a high-temperature thermal annealing and their enhanced field emission properties, J. Phys. Chem. C 111 (2007) 4175e4179. [22] S. Bhosale, S. Suryawanshi, S. Bhoraskar, M. More, D. Joag, V. Mathe, Influence of morphology and crystallinity on field emission properties of NiFe2O4
nanoparticles grown by high-temperature vapor phase condensation route, Mater. Res. Express 2 (2015) 095001e095013. [23] R. Forbes, Extraction of emission parameters for large area field emitters using a technically complete Fowler-Nordheim type equation, Nanotechnology 23 (2012) 095706e095718. [24] H. Jadhav, S. Surywanshi, M. More, S. Sinha, Pulsed laser deposition of tin oxide thin films for field emission studies, Appl. Surf. Sci. 419 (2017) 764e769. [25] L. Hsu, Y. Li, C. Hsiao, Synthesis, electrical measurement and field emission properties of a-Fe2O3 nanowires, Nano Res. Lett. 3 (2008) 330e337.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Field emission study of urchin like nanostructured cobalt oxide films prepared by pulsed laser deposition H. Jadhav a, *, S. Suryawanshi b, M.A. More b, S. Sinha a a b
Laser & Plasma Surface Processing Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Department of Physics, University of Pune, Pune 411007, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2017 Received in revised form 2 February 2018 Accepted 8 February 2018 Available online 10 February 2018
Cobalt oxide (Co3O4) urchin structures have been grown via Pulsed Laser Deposition (PLD) technique followed by thermal treatment in air. Morphological and structural analysis of as synthesized nanostructures has been carried out using Field Emission Scanning Electron Microscopy (FESEM) and X-ray Diffraction (XRD) technique. Chemical nature of the films have been revealed using X-ray Photoelectron Spectroscopy (XPS). Field emission measurements were carried out at a base pressure of 1 108 mbar. Turn-on field corresponding to an emission current density of 10 mA/cm2 was found to be 3 V/mm and a high field enhancement factor of ~3900 was estimated, both of which can be attributed to radially grown nanowires of long length with sharp tips. Overall good field emission current stability was observed for Co3O4 urchin structured films. Dependence of FE properties such as, current density and current stability on morphology and density of emitting sites has been observed and analyzed systematically. The observed FE results demonstrate Co3O4 urchin structures as potential candidate for applications in various vacuum micro/nanoelectronic devices. © 2018 Published by Elsevier B.V.
Keywords: 3D urchin structure Pulsed laser deposition Field emission
1. Introduction Nanostructured materials have been extensively studied as their unique properties make them highly desirable for a wide range of potential applications. Both, one dimensional (1D), as well as, multidimensional (2D and 3D) nanostructures have demonstrated their significant role as, photo catalysts [1], chemical sensors [2], field emitters [3,4], to name a few. In addition to 1D nanowires and nanotips, multidimensional (2D and 3D) nanosheets and nanoparticles, unique nanostructures such as, 3D urchins have also been reported [5]. 3D urchin like structures have been attracting significant attention since such structures offer both advantages of nano and micro/sub micrometer sized structures. Urchin structures of CuO, Fe2O3, Co3O4 and ZnO have been reported for various applications [5e8]. Nanostructure based field emitters have been widely studied due to their great commercial potential in field emission (FE) applications. High aspect ratio of 1D nanowires (NWs) provide high enhancement of electric field at the tip of the NWs, whereas, 2D
* Corresponding author. E-mail address: [email protected] (H. Jadhav). https://doi.org/10.1016/j.jallcom.2018.02.074 0925-8388/© 2018 Published by Elsevier B.V.
nanosheets (such as graphene nanosheets) lead to extremely high enhancement of the field strength at its edges due to the very low thickness of such nanosheets. 3D urchin like structures are composed of 1D sharp tips pointing in various directions which is beneficial for enhancing FE properties. Emission current density and field enhancement in case of FE effect depend on several parameters including length and radii of the emitters, inter tip distance and density of nanotips and nanostructures. Although, FE properties of Fe2O3, CuO and ZnO urchin like structures have been reported [5,9,10], FE study of Co3O4 urchin structure has not been reported till date. In comparison to metals and carbon nanotubes (CNT) which are susceptible to oxidation, stable oxides are preferred as field emitters for efficient and long term operation with limited degradation of FE current. CNT's decorated with metal oxides to reduce high work function have also been reported [11]. Therefore, studying FE properties of oxide material having different nanostructures has been highly recommended. Co3O4 is an important p-type semiconductor and it is the most stable phase belonging to the family of cobalt oxides. Chemical stability of Co3O4 over a wide temperature range and high mechanical strength makes it particularly suitable for FE applications. Nanowires and nanowalls of Co3O4 synthesized by heating cobalt foils in presence of oxygen gas have been reported for FE [12].
282
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
Urchin like structures of CuO and Fe2O3 have been reported synthesized by one step thermal oxidation of Cu and Fe microspheres in air at various temperatures [5,9]. Fe2O3 urchin structures have also been synthesized by hydrothermal synthesis, a well known technique to grow various nanostructures [13]. Thermal evaporation is another method used to synthesize ZnO core shell urchin structure and studied for photoelectrochemical properties [14]. To grow urchin like Co3O4 structures, hydrothermal method followed by thermal treatment in air has been reported [15]. Very recently R. Edla et al. have reported Co3O4 urchin like structured films deposited by Pulsed Laser Deposition (PLD) technique followed by thermal treatment in air [7]. PLD is a preferred technique for its easy operation, reproducibility of the coating and stoichiometric transfer of the ablated target material to the substrate. By controlling the deposition parameters in PLD technique, properties of the deposited film can be tuned easily. In the present study we report synthesis of urchin like nanostructured films of Co3O4 and their performance as efficient field emitters. Co-B films deposited by PLD on Si substrates were annealed in air at selected temperature for growth of urchin like Co3O4 nanostructures. Field Emission Scanning Electron Microscopy (FESEM) has been used to study morphology of the heat treated films deposited by PLD. X-ray Diffraction measurement was carried out to study the crystalline structure of these films. FE measurements were performed in vacuum at a pressure of 108 mbar. FE properties such as field emission current density and stability of current delivered by these urchin films over extended period of time have also been investigated.
2. Experimental Cobalt boride (Co-B) powder which was synthesized using chemical reduction method [16] was cold pressed at a pressure of ~1500 kg/cm2 to form circular discs of 25 mm diameter and ~3e4 mm thickness. The ratio of Co/B was 1.61 in the Co-B powder as reported in our previous work [16]. This circular disc served as a target to carry out PLD runs with Si used as substrate. Edges of Si samples were smoothened to avoid emission from the edges during FE measurements. PLD of Co-B films were performed using Nd-YAG laser operating at a second harmonic wavelength of 532 nm at a pulse repetition rate of 10 Hz. The distance between Co-B target and Si substrate was kept ~4 cm for each PLD run. The target was mounted on a rotating target holder to avoid crater formation on the target surface due to incident laser radiation whereas substrate was kept static on a steady substrate holder. Typical average values of laser fluence of 20 and 30 J/cm2 were selected for Co-B deposition based on our previous experiments [16]. The ratio of Co/B in
the PLD deposited Co-B films is 1.81 [16]. The area of the deposited Co-B films is 0.6 cm2. All PLD runs were carried out at room temperature under vacuum condition at a background pressure of 105 mbar. In order to grow 3D urchin like structure, deposited CoB films were subsequently annealed in air at different temperatures (400 C - 600 C) with a heating rate of 10 C/min. Annealing time was also varied to optimize growth of urchin like structure. Surface morphology of the films has been studied using Field Emission Scanning Electron Microscopy (FESEM). Crystalline phase of the films was determined by X-ray diffractometer with Cu Ka radiation (l ¼ 1.5414 A). Grazing incidence XRD (GIXRD) measurements were performed at room temperature for PLD deposited and annealed films at a grazing angle of incidence of 0.8 . XRD measurements were performed over the 2q range of 30-70 . X-ray photoelectron spectroscopy (XPS) was used to determine the surface electronic states of the films. XPS analysis of the films was performed using a VG make model CLAM-2 hemispherical analyser with Al Ka source (1486.6eV) having a step size of 0.3 eV. Field Emission measurements of the films were carried out in a vacuum chamber at a pressure of 108 mbar at room temperature. A phosphor coated transparent metal screen served as an anode which was located ~1 mm away from the PLD Co3O4 urchin film which served as the cathode. The macroscopic surface electric field on the sample was estimated by Fm ¼ V/d, where V is the applied voltage and d is the distance between anode and cathode. The emission current density (Jm) was evaluated using (Jm ¼ I/A) where I is the total emission current and A is the cathode surface area which was 0.6 cm2 in present FE runs. The field emission current was estimated by measuring voltage drop across a fixed resistance of 102.5 kU and stability of the emission current was continuously recorded using a data logger which recorded data every 10 s.
3. Results and discussions Fig. 1a shows FESEM image of Co-B film deposited by PLD technique. Spherical particulates with smooth surface are observed on the surface of the film. Particles of size of few nm to 1e3 mm were clearly evident in Fig. 1a, suggesting a wide particle size distribution. Such morphology of the films deposited using PLD technique can be attributed to phase explosion phenomena which occurs when laser ablation of target material is carried out using high laser power density [16]. Fig. 1b shows SEM image of Co-B film synthesized by PLD followed by thermal treatment in air at a temperature of 600 C for 8 h. The spherical particulates of Co-B transformed into urchin structures consisting of a core and numerous nanowires which radially grow on the surface and project outwards post thermal treatment in air, as observed in
Fig. 1. FESEM images of (a) Co-B film deposited by PLD, (b) Co-B film after annealing at 600 C for 8 h taken at 10 KX magnification and (c) single urchin particle.
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
Fig. 1b. Diameter of the urchin particulates is found to be in the range of ~500 nm to ~4.5 mm as measured from image shown in Fig. 1b. Densely packed NWs are also clearly visible on the surface of the film with higher number density as compared to the urchin particles (Fig. 1b). These NWs with diameter much smaller than their length have a high aspect ratio which is a favorable condition for good FE effect. FESEM image of a single urchin particle is shown in Fig. 1c. Well grown extended NWs are clearly observed on the spherical cores which act as active emission sites for good FE performance. Similar urchin structured morphology has been reported in literature when Co-B films were annealed at 600 C for 4 h and the probable mechanism responsible for urchin structures has been attributed to stress induced growth [7]. We have annealed the Co-B films at 600 C for 4 h but formation of complete urchin structure was not observed. We have observed formation of small nanowires on some of the Co-B particles for 4 h annealing time. Hence we have extended the annealing time to 8 h to get complete urchin structure. Fig. 2 shows the effect of annealing temperature on the growth mechanism of urchin structure. After heat treatment at 400 C for 8 h, smooth spherical particulates in as deposited Co-B film appear deformed with formation of small nanoprotrusions on the surface of these particulates as shown in Fig. 2a. Further increase in temperature to 500 C initiates evolution of very small NWs from the deformed spherical particle as shown in Fig. 2b. However, complete urchin like structure with well grown NWs is evident only when the annealing temperature was raised to 600 C.
283
XRD spectra of Co-B film deposited in vacuum is shown in Fig. 3a. The as deposited Co-B film exhibits a broad peak centered at around 2q ¼ 45 which can be assigned to amorphous state of Co-B. Additional peak observed at 2q ¼ 55 is due to Si substrate. Fig. 3b shows XRD spectra recorded for Co-B film when annealed at 600 C for 8 h in air. As shown in Fig. 3b XRD peaks centered at 2q ¼ 31.4 , 36.9 and 44.8 can be assigned to the cubic structure of Co3O4 as confirmed against JCPDS data file 76-1802. These diffraction peaks marked in Fig. 3b correspond to reflections from (220), (311) and (400) planes, respectively. An additional small peak detected at 2q ¼ 42.5 indicates the presence of CoO phase according to JCPDS file 78-0431. The diffraction peak appearing at 2q ¼ 51.5 is due to the silicon substrate. The average crystallite size was calculated by measuring the width of the most prominent peaks corresponding to reflections from (311) and (400). Crystallites size was calculated using Scherrer's equation as given below,
d ¼ kl=b cosq
(1)
where, d is the average crystallite size, k is the Scherrer's constant (0.9), b is the full width at half maxima, l is the wavelength of the Xray (0.154 nm) and q is the Bragg angle. The average crystallite size was determined to be ~10 nm for (311) plane and ~13 nm for (400) plane. Absence of boron oxide phase in the XRD pattern confirms that the heat treated film is composed of oxides of cobalt, mainly Co3O4 phase. Our observations are in agreement with reported data on annealed Co-B films where decomposition of Co-B phase
Fig. 2. FESEM images of Co-B film when annealed at (a) 400 C, (b) 500 C and (c) 600 C for 8 h in air.
Fig. 3. (a) XRD spectra of Co-B film and (b) Co-B film heat treated at 600 C for 8 h in air.
284
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
occurred at 400 C and crystalline Co3O4 was formed when annealed at 600 C [7]. B. Varghese et al. also reported similar XRD pattern for Co3O4 nanowires and nanowalls when a Co foil was annealed at 450 C in a vacuum chamber in presence of oxygen [12]. XPS has been used to study the surface chemical composition of these films and the results are reported in Fig. 4. XPS spectra of Co (Fig. 4a) shows two prominent peaks at BEs of about 781 eV and 796.5 eV which corresponds to 2p3/2 and 2p1/2 states, respectively. Energy difference between the 2p3/2 and 2p1/2 peaks is ~15.5 eV which is characteristic of Co3O4 phase. The major peak of Co2p3/2 is deconvoluted into two peaks centered at 780.2 eV and 782.2 eV attributed to Co3þ and Co2þ species respectively, in Co3O4 phase [17,18]. Fig. 4b shows XPS spectra of O which shows two major peaks centered at 530.1 eV and 531.8 eV corresponding to oxygen in Co3O4 crystal lattice and OH group attached to Co respectively [17,18]. Presence of OH group on the surface of the films is due to ex-situ experimental conditions. Both spectra of Co and O reveal that surface composition of the urchin structured film is mainly Co3O4. XPS peak positions estimated for our urchin structured films matched well with reported values of Co3O4. The probable mechanism responsible for the formation of urchin structure in present work is mainly attributed to difference in thermal expansion coefficients of two elements (B and Co). In as deposited Co-B coating, the particulates are of core shell structure (as observed in TEM images of Co-B film deposited by PLD in our previous study) [16] where, the contrast between core and shell indicate the presence of different elements in each part, mainly core is composed of Co and shell is of Co, B and O as reported in Ref. [7]. During oxidation at high temperatures stress between core and shell increases due to difference in thermal expansion
coefficients of Co (13 106 K1) and B (5 106 K1). This stress increases as a function of temperature and at a particular temperature (600 C in present case) stress is released in the form of NWs due to outward diffusion of metallic element. Similar growth of urchin was not observed when we annealed pure Co film deposited by PLD for identical annealing conditions (600 C, 8 h). Stress induced mechanism has also been reported for growth of different nanostructures of Fe2O3 when Fe2O3 PLD film was annealed at 600 C [19]. These PLD films having urchin like Co3O4 structure were investigated for their field emission properties. Macroscopic current density (Jm) versus applied macroscopic electric field (Fm) plots and Fowler-Nordheim (F-N) plot along with emission current stability plot of Co3O4 urchin structured film are shown in Fig. 5. Fig. 5a shows variation of Jm as a function of Fm and corresponding F-N plot is shown in Fig. 5b. The emission current stability of urchin structured Co3O4 film is shown in Fig. 5c. Here, Fm is defined as the ratio of applied voltage (V) across the two electrodes which were kept at a distance of ~1000 mm (d) from each other. Emission current density (Jm) is defined as total emission current per unit area of film (~0.6 cm2). The Co3O4 films were used as cathodes having an area of 0.6 cm2 and a phosphor coated transparent metal screen served as an anode. The turn-on field defined as the applied field required to draw an emission current density of 10 mA/cm2 was measured to be about 3 V/mm. FE properties investigated in present study have been compared with urchin structures of other metal oxides such as Fe2O3, CuO and ZnO and presented in Table 1. Comparison of other nanostructures like nanowires and nanorods with the urchin structure has also been given in Table 1.
Fig. 4. XPS spectra of Co-B film annealed at 600 C for 8 h in air.
Fig. 5. Plot of (a) current density as a function of applied electric field (b) F-N plot and (c) emission current stability of Co3O4 urchin structured films.
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288 Table 1 Some reported values of turn on field for urchin structures and other structures of different metal oxides. Material
Turn-on Field (V/mm)
Reference
Fe2O3 urchin CuO urchin ZnO urchin CuO nanowires Co3O4 nanorods Co3O4 urchin
2.8 2.94 3.7 5.31 2.8 3
[9] [5] [10] [3] [20] Present study
We measured a highest achievable current density of ~480 mA/ cm2 at 6.1 V/mm for these Co3O4 urchin structured film. Good FE behavior exhibited by these urchin structured films is mainly attributed to surface morphology of the films. Radially grown nanowires from the spherical particles enhances local field due to their pointed tips. It has also been reported that crystalline nature positively affects FE performance of films [21,22]. Improved FE properties of NiFe2O4 nanocrystallites have been reported with the improvement in crystallinity [22]. Linearly fitted F-N plot is shown in Fig. 5b. Slope (S) of the F-N plot was estimated using linear fit to the experimental data points. F-N equation was employed to analyze the FE properties of urchin structured Co3O4 films which can be expressed as [23].
. Jm ¼ lm $a$f1 $ðgc $Fm Þ2 exp nF $b$f3=2 ðgc $Fm Þ
(2)
Here, Jm and Fm are macroscopic emission current density and macroscopic applied electric field respectively, whereas, lm is macroscopic pre-exponential factor which provides a measure of the fraction of actual cathode area contributing towards emission, a and b are first and second F-N constants (1.541434 mAeVV2) and (6.830890 eV-3/2Vnm1), respectively and 4 is work function of the field emitter (4.5 eV for Co3O4), gc is macroscopic field enhancement factor and nF is potential barrier form correction factor. The macroscopic field enhancement factor has been estimated by following equation [24].
gc ¼ sm b
f3=2 S
(3)
where, sm and S are generalized slope correction factor and slope of the linearly fitted F-N plot, respectively. Table 2 presents these values of slope correction factors and slope along with turn on field and threshold field values estimated for the urchin structured Co3O4 films. The field enhancement factor ~3900 as estimated from our data provides a quantitative estimate for the degree of electric field enhancement on the emitter surface for urchin structured films. High field enhancement factor is attributed to long length and sharp tips of the NWs of the urchin structure. Along with urchin structures, densely packed NWs as observed on film surface (Fig. 1b and c) also contribute towards FE performance. These NWs serve as active emission sites which experience enhanced local field on account of their high aspect ratio. Table 2 List of estimated FE parameters for urchin structured Co3O4 film B. Sr.No.
Parameters
Value for Co3O4 urchin film
1. 2.
Turn on field (for 10 mA/cm2 current density) Threshold field (for 100 mA/cm2 current density) Slope of F-N plot (S) Macroscopic field enhancement factor (gc) Generalized slope correction factor (sm)
3 V/mm 4.6 V/mm
3. 4. 5.
1.39 107 Np Vm1 3900 0.85
285
The emission current stability of urchin like Co3O4 was also investigated. Fig. 5c shows the current stability monitored over a period of ~2 h. The initial emission current was ~8 mA which increased after ~30 min to 13 mA and then was constant showing fluctuations of ±2 mA. No marked degradation of emission current was observed in the next ~2 h. Adsorbed layer of residual gas molecules on the surface of the cathode and failure of nanostructures (emission sites) due to joule heating and/or mechanical failure on application of electric field could have resulted in initial fluctuations observed in emission current. However, once adsorbed impurities had been sputtered out from electron emitting tips the emission current improved and remained stable for an extend period of ~2 h. Therefore, in our present study, Co3O4 urchin films have shown good FE stability which can be attributed to mechanical and chemical stability of Co3O4 phase. Field emission performance critically depends on both, extrinsic properties i.e. shape and size of emitters and intrinsic material properties including work function. Several parameters, such as length and radii of the emitters, inter-tip distance and density of nanostructures determine the field emission current density, field enhancement factor and turn on field. Density of nanoparticles deposited via the process of pulsed laser deposition has been known to strongly depend on the laser fluence used for ablation of the target. Hence, to study the effect of areal density of nanostructures on FE properties, we compared performance of Co-B films deposited at two levels of laser fluence, 20 J/cm2 (film A) and 30 J/cm2 (film B), all other deposition parameters remaining same. Films deposited at 20 J/cm2 were annealed at same temperature of 600 C for 8 h as in the case of films deposited using higher fluence of 30 J/cm2. Typical FESEM images of films (A) and (B) are shown in Fig. 6a and Fig. 6b, respectively. FESEM images of single urchin particle of film (A) and film (B) are shown in Fig. 6c and d, respectively. Wide particle size distribution ranging from ~500 nm to ~2e3 mm has been observed for both films as shown in Fig. 6a and b. However, formation of densely crowded NWs along with urchin like particles was clearly evident for films (B) (Fig. 6b). These NWs were rarely observed for films (A) deposited at lower laser fluence (Fig. 6a). Higher magnification images of single urchin particulate of film A and B have been shown in Fig. 6c and d, respectively, which also confirms formation of dense NWs for film B. As the deposition was carried out for same time duration in case of both films A and B, films deposited at high laser fluence are expected to be thicker as compared to those deposited at low fluence. Number density and length of the NWs observed for film (B) is higher as compared to film (A). Our observations are in agreement with the results reported in Ref. [25], where, population density and length of the NWs formed after thermal treatment were observed to increase with increase in film thickness. As these films are composed of Co-B phase and the difference between thermal expansion coefficients of Co and B being large, during heat treatment stress level in the films increases as a function of temperature [7]. Also, during the thermal process, diffusion of oxygen molecules in the Co-B film causes volume expansion in all three dimensions. However, only upward direction is available for a film to release the thermal stress induced during thermal treatment in air. This can be the probable cause of formation of NWs on the surface of film [25]. In our present study annealing time and temperature were maintained same for both films. Thickness of film A being less oxidation of this film could have been completed much before 8 h, while oxidation continued for film B till 8 h allowing formation of longer and denser NWs. Hence, termination of oxidation mechanism can be a function of thickness of the film, and this can determine the density and length of NWs formed on annealing. Our observations are in agreement with the results reported in Ref. [25], where, effect of thickness of the films on the population density and length of the
286
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
Fig. 6. FESEM images of Co-B films deposited by PLD at laser fluence (a) 20 J/cm2 and (b) 30 J/cm2 both films heat treated in air at 600 C for 8 h and (c) and (d) are the corresponding FESEM images of single urchin particulate.
NWs formed after thermal treatment was reported to have increased with increase in films thickness. FE properties of films deposited at 20 J/cm2 (film A) were studied and compared with films deposited at 30 J/cm2 (film B) and these results are shown in Fig. 7. The observed value of turn on field for films B was ~3 V/mm. Furthermore, at an applied electric field of 6 V/mm a much higher emission current density of ~480 mA/cm2 could be drawn for films B in comparison to ~100 mA/cm2 for films deposited at 20 J/cm2. The observed higher emission current density at relatively low applied field for films B can be attributed to the presence of densely packed NWs which acted as potential emission sites. Thus better FE performance observed for film B could be correlated to presence of larger number of emitters with sharp tips oriented perpendicular to the substrate and large number of nanowires having high aspect ratio. Emission current stability for both films was also recorded and compared. The emission current versus time plot corresponding to
preset current values of ~1.5 mA and ~13 mA recorded for 2 h for films deposited at 20 J/cm2 and 30 J/cm2, respectively is shown in Fig. 7. Emission current remained fairly constant over the entire duration of 2 h without degradation, which indicates good physical and chemical stability of these Co3O4 urchin structured films. Fig. 8 shows SEM image of Co3O4 urchin structured film B recorded before (Fig. 8a and c) and after (Fig. 8b and d) measurement of FE properties. In some regions of the emission area NWs are observed to have become shorter in length (Fig. 8b and d). Some of the sharp tips and spikes associated with the urchin structure have also been lost (Fig. 8d). Intense field experienced at the tips during extended FE measurements may be the likely cause for this damage. Therefore, a surface morphology with homogeneous distribution of emitters would be preferable since uniform degradation would occur across the entire emitter surface rather than the selective and predominant degradation of the high aspect ratio features as observed in Fig. 8.
Fig. 7. Field emission characteristics of Co3O4 urchin structured films (a) plot of emission current density versus applied field, (b) F-N plot and (c) current stability plot.
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
287
Fig. 8. FESEM images of urchin structured film before FE measurement (a and c) and after FE measurement (b and d). Images (a and b) taken at 10 KX and (c and d) are at 50 KX magnification.
4. Conclusion Co3O4 urchin like structures was synthesized by PLD technique followed by thermal treatment in air at selected temperatures. Morphological study revealed the formation of well grown urchin structures along with NWs on the film surface. Structural investigation by XRD confirms the formation of Co3O4 phase. Good field emission behavior with low turn on field of 3 V/mm and maximum emission current density of 480 mA/cm2 was recorded for films deposited at laser fluence of 30 J/cm2. The promising FE characteristics of Co3O4 films could largely be attributed to well grown urchin structures having radially grown long NWs as well as densely packed NWs observed on the film surface. Acknowledgement This work was supported by collaboration PhD program between University of Mumbai and Bhabha Atomic Research Centre, Mumbai, India. Authors gratefully acknowledge Prof. S Basu, BARC for experimental support provided for X-ray diffraction measurements. References [1] M. Guo, M. Fung, F. Fang, X. Chen, A. Ng, A. Djurisic, W. Chan, ZnO and TiO2 1D nanostructures for photocatalytic applications, J. Alloys Compd. 509 (2011) 1328e1332. [2] S. Varghese, S. Lonkar, K. Singh, S. Swaminathan, A. Abdala, Recent advances in graphene based gas sensors, Sens. Actuators, B 218 (2015) 160e183. [3] C. Tang, X. Liao, W. Zhong, H. Yu, Z. Liu, Electric field assisted growth and field emission properties of thermally oxidized CuO nanowires, RSC Adv. 7 (2017) 6439e6447. [4] G. Patil, P. Baviskar, V. Bagal, R. Ladhe, A. Deore, M. More, B. Sankapal, P. Chavan, Aligned 2D CuSCN nanosheets: a high performance field emitter, RSC Adv. 6 (2016) 71958e71963. [5] T. Chang, C. Hsu, H. Lin, K. Chang, Y. Li, Formation of urchin- like CuO structure through thermal oxidation and its field emission application, J. Alloys Compd. 644 (2015) 324e333.
[6] H. Yu, L. Hsu, T. Chang, Y. Li, A 3D a-Fe2O3 nanoflake urchin-like structure for electromagnetic wave absorption, Dalton Trans. 41 (2012) 723e726. [7] R. Edla, N. Patel, M. Orlandi, N. Bazzanella, V. Bello, C. Maurizio, G. Mattei, P. Mazzoldi, A. Miotello, Highly photo-catalytically active hierarchial 3D porous/urchin nanostructured Co3O4 coating synthesized by Pulsed Laser Deposition, Appl. Catal., B 166e167 (2015) 475e484. [8] H. Hieu, N. Vuong, H. Jung, D. Jang, D.H. Kim, S. Hong, Optimization of a zinc oxide urchin-like structure for high performance gas sensing, J. Mater. Chem. 22 (2012) 1127e1135. [9] L. Hsu, H. Yu, T. Chang, Y. Li, Formation of three-dimensional urchin like aFe2O3 structure and its field emission application, ACS Appl. Mater. Interfaces 3 (2011) 3084e3090. [10] H. Jiang, J. Hu, F. Gu, C. Li, Stable field emission performance from urchin-like ZnO nanostructures, Nanotechnology 20 (2009) 055706e055711. [11] S. Sridhar, C. Tiwary, S. Vinod, J. Tijerina, S. Sridhar, K. Kalaga, B. Sirota, A. Hart, S. Ozden, R. Sinha, H. Vajtai, W. Choi, K. Kordas, P. Ajayan, Field emission with ultralow turn on voltage from metal decorated carbon nanotubes, ACS Nano 8 (2014) 7763e7770. [12] B. Varghese, T. Hoong, Z. Yanwu, M. Reddy, B. Chowdari, A. Wee, T. Vincent, C. Lim, C. Sow, Co3O4 nanostructures with different morphologies and their field emission properties, Adv. Funct. Mater. 17 (2007) 1932e1939. [13] F. Beshkar, H. Jahangiri, M. Kamazani, Dendritic a-Fe2O3 nanostructures: facile hydrothermal synthesis, characterization and microwave absorption, J. Mater. Sci. Mater. Electron. 27 (2016) 12869e12875. [14] D. Tang, G. Liu, F. Li, J. Tan, C. Liu, G. Lu, H. Cheng, Synthesis and photo electrochemical property of urchin like Zn/ZnO core-shell structures, J. Phys. Chem. C 113 (2009) 11035e11040. [15] X. Rui, H. Tan, D. Sim, W. Liu, C. Xu, H. Hng, R. Yazami, T. Lim, Q. Yan, Template free synthesis of urchin like Co3O4 hollow spheres with good lithium storage properties, J. Power Sources 222 (2013) 97e102. [16] H. Jadhav, A.K. Singh, N. Patel, R. Fernandes, S. Gupta, D.C. Kothari, A. Miotello, S. Sinha, Pulsed laser deposition of nanostructured Co-B-O thin films as efficient catalyst for hydrogen production, Appl. Surf. Sci. 387 (2016) 358e365. [17] J. Yang, H. Liu, W. Martens, R. Frost, Synthesis and characterization of cobalt hydroxide, cobalt oxyhydroxide and cobalt nanodiscs, J. Phys. Chem. C 114 (2010) 111e119. [18] M. Biesinger, B. Payne, A. Grosvenor, L. Lau, A. Gerson, R. Smart, Resolving surface chemical states in XPS analysis of first row transition metals: Cr, Mn, Fe, Co and Ni, Appl. Surf. Sci. 257 (2011) 2717e2730. [19] R. Edla, N. Patel, M. Orlandi, N. Bazzanella, V. Bello, C. Maurizio, G. Mattei, P. Mazzoldi, A. Miotello, Highly photo-catalytically active hierarchical 3Dporous/urchin nanostructured Co3O4 coating synthesized by Pulsed Laser Deposition, Appl. Catal. B Environ. 166e167 (2015) 475e484.
288
H. Jadhav et al. / Journal of Alloys and Compounds 744 (2018) 281e288
[20] L. He, Z. Li, Z. Zhang, Rapid low temperature synthesis of single crystalline Co3O4 nanorods on silicon substrates on a large scale, Nanotechnology 19 (2008) 155606e155610. [21] S. Jung, S. Jo, H. Moon, J. Kim, D. Zang, C. Lee, Improved crystallinity of doublewalled carbon nanotubes after a high-temperature thermal annealing and their enhanced field emission properties, J. Phys. Chem. C 111 (2007) 4175e4179. [22] S. Bhosale, S. Suryawanshi, S. Bhoraskar, M. More, D. Joag, V. Mathe, Influence of morphology and crystallinity on field emission properties of NiFe2O4
nanoparticles grown by high-temperature vapor phase condensation route, Mater. Res. Express 2 (2015) 095001e095013. [23] R. Forbes, Extraction of emission parameters for large area field emitters using a technically complete Fowler-Nordheim type equation, Nanotechnology 23 (2012) 095706e095718. [24] H. Jadhav, S. Surywanshi, M. More, S. Sinha, Pulsed laser deposition of tin oxide thin films for field emission studies, Appl. Surf. Sci. 419 (2017) 764e769. [25] L. Hsu, Y. Li, C. Hsiao, Synthesis, electrical measurement and field emission properties of a-Fe2O3 nanowires, Nano Res. Lett. 3 (2008) 330e337.