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Field emission and explosive electron emission process in focused ion beam fabricated platinum and tungsten three-dimensional overhanging nanostructure
Nuclear Inst, and Methods in Physics Research B 425 (2018) 26–31
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Field emission and explosive electron emission process in focused ion beam fabricated platinum and tungsten three-dimensional overhanging nanostructure
T
Abhishek Kumar Singh Department of Physics, Darbhanga College of Engineering, Mabbi, P.O. Lal Sahpur, Darbhanga 846005, Bihar, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Focused ion beam Field emission Nanostructure
Three-dimensional platinum and tungsten overhanging nanogap (∼70 nm) electrodes are fabricated on a glass substrate using focused ion beam milling and chemical vapour deposition processes. Current-voltage (I-V) characteristics of the devices measured at a pressure of ∼10−6 mbar shows space-charge emission followed by the Fowler-Nordheim (F-N) field emission. After the F-N emission, the system enters into an explosive emission process, at a higher voltage generating a huge current. We observe a sharp and abrupt rise in the emission current which marks the transition from the F-N emission to the explosive emission state. The explosive emission process is destructive in nature and yields micro-/nano-size spherical metal particles. The chemical compositions and the size-distribution of such particles are performed.
1. Introduction Electrical transportation through nanoscale systems (e.g., nanowires, nanogaps, etc.) are extensively studied [1–3] due to their unique physical behaviours and amazing sensing capabilities [4,5]. Field induced electron emission phenomenon in 3D overhanging nanogap electrodes is utilized in various applications, such as in switches, connectors, micro-electromechanical system (MEMS) [6,7] etc. When high electric field is applied across nanogap electrodes, it induces electron current emission via quantum mechanical tunneling process, known as cold field emission (CFE), and governed by the Fowler-Nordheim (FN) field emission equation [8]. CFE based nanodevices are essential for emerging vacuum nano-electronics as they are utilized in surface-conduction electron-emitter displays, electron beam lithography, quantum interference, free electron femtosecond pulses, etc [9–13]. In the present work, we fabricate 3D overhanging nanostructures of platinum and tungsten using dual beam focused ion beam (FIB) system. The fabrication process involves FIB induced milling and chemical vapour deposition (FIB-CVD) techniques. Electric field is applied across the nanogap electrodes by the application of dc voltage (V) and corresponding emission current (I) is simultaneously measured. Currentvoltage (I-V) characteristics of the devices are studied inside the FIB chamber at ∼ 10−6 mbar pressure. The analysis of I-V shows the field emission at higher voltages and space-charge limited current at lower voltages. We observe that the systems enter into an explosive electron emission process at a certain value of the applied voltage. The explosive
emission process is destructive and leads to the formation of micro and nano-size spherical metallic particles. 2. Experimental 2.1. Instruments used for fabrication and measurements The experiments are performed in FEI make (Nova 600 NanoLab) dual beam focused ion beam (FIB) system. The system is equipped with a liquid metal ion source (LMIS) of Ga and sophisticated ion optics that can focus the Ga ion beam down to a beam diameter of few nanometers. The Ga ion beam can perform material milling and chemical vapour deposition (CVD) with ease, precision and control. Precursor gases of platinum ((CH3)3Pt(CpCH3)), tungsten (W(CO)6) and carbon (C10H8) are injected in the system through the gas injection systems (GIS) which are cracked by the ion beam during the CVD process. The electron beam of the dual beam system is extracted from a field emission gun and used for scanning electron microscopy. Energy dispersive X-ray spectroscopy (EDS) facility is also available in the system and utilized for chemical analysis in the present experiments. The electrical measurements are performed in situ under high vacuum condition via electrical feedthrough channels provided in the system. The current-voltage characteristics of the fabricated devices are performed by employing a Keithley (model 6430) source-meter interfaced with computer via LabView programs.
E-mail address: [email protected]. https://doi.org/10.1016/j.nimb.2018.04.004 Received 26 December 2017; Received in revised form 23 March 2018; Accepted 4 April 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst, and Methods in Physics Research B 425 (2018) 26–31
A.K. Singh
Fig. 1. Schematic representation of the (a) copper thin film deposited on glass substrate using I-shaped mask where a = 200 µm, b = 2 mm, c = 2 mm, d = 2 mm, (b) trench fabricated on the copper film using focused ion beam milling technique. Scanning electron micrograph (52° angle of view) of the (c) platinum wall deposited by the focused ion beam induced chemical vapour deposition process. The dotted lines represent the final structure to be fabricated. (d) Overhanging platinum nanogap electrodes fabricated by focused ion beam milling. Inset shows the magnified view of gap. (e) Normal (0° angle of view) view of the final structure and its different components (pixel intensity is inverted for better visibility). SEM images have been artificially coloured for the ease of demonstration.
It is worth mentioning that during the fabrication of the overhanging electrodes from the Pt wall using FIB, significant amount of redeposition [14] occurs inside the trench that reconnects the electrically isolated pads. Therefore, FIB milling process is again employed after the fabrication of the overhanging electrodes, in order to remove the redeposited material (Fig. 1(d)). Overhanging nanogap electrodes of W are also fabricated (see following discussion) by the method described above. These structures differ in shape and gap size from the one described in Fig. 1.
2.2. Fabrication of the 3-D overhanging electrodes Thin films (thickness = 150 nm) of copper are deposited via thermal evaporation process (at chamber pressure ∼10−5 mbar, and ∼1 Å s−1 deposition rate) on clean borosilicate glass substrates using an I-shape mask (Fig. 1(a)). A trench is created by focused ion beam milling of the film near the center (Fig. 1(b)) of the copper film. The energy and the current of the Ga FIB used are 30 keV and 1 nA. The width of the trench is 5 μm. A Pt wall is deposited using the focused ion beam induced chemical vapour deposition (FIB-CVD) process across the trench using 30 keV beam energy and 10 pA current (Fig. 1(c)). The Pt wall is 15 μm in length, 10 μm in height and 300 nm in thickness. The glass-substrate along with the Pt wall is transferred onto a tilted stub (45°) and mounted inside the FIB chamber. Mounting the sample on a tilted stub helps to view the Pt wall from the grazing angle, which is necessary for further processing of the device. The Pt wall is milled out by the focused ion beam from the grazing angle and a 5 µm × 8 µm window is formed which inturn creates an overhanging bridge-like structure of Pt. The bridge was milled by the FIB in order to achieve a nanogap with a pointed (apex) structure at one end (cathode). The SEM image of the resultant structure is shown in Fig. 1(d). The inset of Fig. 1(d) shows a magnified view of the two electrodes and the gap fabricated by the method described above. The distance (d) between the apex of the left electrode and the right electrode is measured to be 70 nm. Fig. 1(e) shows the SEM image (contrast inverted) of the structure from the normal view.
3. Results and discussions 3.1. I-V characteristics: space-charge limited emission and F-N emission The electrodes assembly with the gap is placed on a specially designed sample-holder for measuring current-voltage (I-V) characteristics in vacuum (at a pressure of the order of 10−6 mbar). The sampleholder consists of metallic probes tightly attached to the copper pads in one end and the other end is electrically connected to the Keithley source-meter via feed-through channels of the FIB system. Electric Voltage (V) is applied across the electrodes trough the source-meter and is increased in a step of 0.5 V per second beginning at 0 V. The current (I) associated with each voltage is simultaneously measured as represented in Fig. 2. Fig. 3(a) shows the I-V characteristics of the overhanging Pt electrodes plotted in the loglog scale. The slope of the straight-line fit (solid red line through the data) is 0.45 and thus 27
Nuclear Inst, and Methods in Physics Research B 425 (2018) 26–31
A.K. Singh
Fig. 3. Current-voltage characteristics of (a) Platinum overhanging nanostructure. Inset shows the F-N plot of the I-V data showing negative slope above 104 V of applied voltage, and (b) tungsten overhanging nanostructure. Inset shows the F-N plot showing negative slope above 57 V of applied voltage and also the SEM image of the nanogap structure. Fig. 2. Current-voltage measurement of (a) Platinum overhanging nanostructure and (b) tungsten overhanging nanostructure.
several complications, such as complexity of the electrode geometry. The variable physical quantity is often the applied voltage (V) instead of the actual electric field. Usually a field conversion factor (β) [17] is used in order to convert the applied voltage to the electric field as: E = βV. Using this relation with Eq. (1) implies that a plot of ln(I/V2) versus (1/V) should produce a straight-line with a negative slope value in the F-N regime of emission. Linear variation with a negative slope of ln(I/V2) with respect to (1/V) is a characteristic of the F-N emission process and a plot of ln(I/V2) versus (1/V) is referred as F-N plot. In the inset of Fig. 3(a) we show ln(I/V2) versus (1/V) plot for the data received from the Pt electrodes described in Fig. 1. The straight-line fit (solid green line in the inset of Fig. 3(a)) in the data showing negative slope at high-voltage regime is indicative of Fowler-Nordheim (F-N) field emission. Fig. 3(b) shows the I-V characteristics (in loglog scale) of the nanogap tungsten electrode fabricated by the method described in section 2.2. The plot shows that the emission process goes through a coherence space-charge limited regime (slope ≈ 0.52) followed by an incoherent space-charge limited regime (slope ≈ 1.94) before finally reaching the F-N regime. The straight-line fit with a negative slope in the high-voltage regime (see the FNplot in the inset of Fig. 3(b)) shows the signature of F-N emission as discussed earlier in the case of the Pt structure. The critical electric field values for the F-N Tunneling for Platinum and Tungsten overhanging nanostructure are 14.9 MV/cm and 8.0 MV/cm, respectively [18–22] which are higher than the reported value [23].
(approximately) corresponds to a I ∝ V 1/2 like variation in the emission current with respect to the applied voltage, which is known as the coherent-space-charge-limited emission (CSCLE) [15]. Upon increasing the applied voltage beyond 97 V, F-N emission is observed (see the green solid line in Fig. 3(a)). The field emission current (I) in the F-N regime is given by the generalized Fowler-Nordheim equation [16]:
I = λaϕ−1Aeff E 2exp ⎛− ⎝ ⎜
μbϕ3/2 ⎞ E ⎠ ⎟
(1)
Where, ϕ is the work function and Aeff is the effective area of the emission surface, E is the electric field strength, a and b are universal constants, and λ and μ are generalized correction factors. a and b are expressed in terms of fundamental qualities as follows: a = 1 8π (2me ) 2 3
e3 8πh
and
b= where e and me are respectively charge and mass of an eh electron, and h is the Planck’s constant. Values of a and b are approximately 1.54 × 10−6 A eV V−2 and 6.83 × 107 eV−3/2 Vcm−1. The generalized correction factors μ and λ are typically slowly varying functions of E, ϕ etc. The field emission current (I) depends on the actual value of the local electric field E. However in practical experimental conditions the actual value of E is sometimes unknown or difficult to evaluate due to 28
Nuclear Inst, and Methods in Physics Research B 425 (2018) 26–31
A.K. Singh
(V) in the ISCLE regime for the W electrodes should ideally be ∼V3/2 for Child-Langmuir emission, however the variation in the present case is ∼V2. However we do not completely understand this behaviour, such variation in the emission current may correspond to Mott-Gurney space-charge limited emission where ∼V 2 like variation in the emission current is expected [25]. 3.2. Evaluations of field conversion and enhancement factors, and effective emission area Parameters, such as field conversion factor (β) and effective emission area (Aeff) are calculated from the F-N plots shown in the insets of Fig. 3(a) and (b). The generalized correction factors (λ and μ) in Eq. (1) can be assumed to be functions of Schottky barrier lowering parameter (y) as: λ = t2(y) and μ = v(y) where y = 3.79 × 10−4 E1/2/ϕ. Eq. (1) thus can be expressed as:
aAeff
I=
ϕt 2 (y )
E 2exp ⎡ ⎢ ⎣
−bϕ3/2 v (y ))⎤ ⎥ E ⎦
2
(2) 2
Where t (y) is approximately equal to 1.1 and v(y) = 0.95-y . Eq. (2) is re-written as following:
I B ⎛ ⎞ = Aexp ⎛− ⎞ ⎝V2 ⎠ ⎝ V⎠
(3)
where,
A=
B=
Aeff aβ 2
exp ⎛⎜ 1.1ϕ ⎝
0.95bϕ3/2 β
b (1.44 × 10−7) ⎞ ⎟ ϕ1/2 ⎠
(3a)
(3b)
The values of A and B are calculated respectively from the intercept and the slope of the F-N plots shown in the inset of Fig. 3(a) and (b) The straight line fitted to the data in the F-N plot for the platinum overhanging nanostructure confirms the F-N field emission process (see, insets of Fig. 3). Using Eq. (3) and the values of the slope and the intercept obtained from the straight line, we estimate the value of β to be ∼1.1 × 106 cm−1 and the effective emission area (Aeff) is ∼1.1 × 10−15 cm2. The value of the work-function (ϕ ) of Pt is assumed to be equal to the bulk work function value (6.35 eV). The geometrical E field enhancement factor γ of the system is defined as [26]: γ = E ,
Fig. 4. Scanning electron micrograph of the a) platinum overhanging nanostructure at 60 V. Inset shows the SEM micrograph of a structure during the explosion at 113.8 V applied voltage; b) structure after explosive emission process at 115 V applied voltage. Inset shows sharp increase in the emission current indicating the transition from field emission to explosive emission process.
0
V
where E0 = d . Using the value of the β from the above analysis, the value of γ for the Pt electrodes calculated by the above relation is ∼8. The maximum field emission current density achieved in the Pt electrodes before it enters into the explosive emission process (see, subsequent discussion) is calculated to be 4.7 × 107 A cm−2. Similar analysis is performed on the I-V data obtained from the overhanging nanogap electrodes of tungsten (shown by the SEM image in the inset of Fig. 3(b)). The F-N curve in the inset of Fig. 3(b) shows two distinct linear regions which may be attributed to the change in the field enhancement factors (β). Since, the β factor is dependent on the cathode surface i.e. size and shape of the tip and protrusions (typical size is around in few nm) present on the surface. However, at higher current, protrusions burnout due to resistive heating which leads to change in the emission area of the electron as can be distinguished with two different slopes in Fig. 3(b). On substituting, ϕ = 4.7 eV as reported in the case of FIB deposited tungsten [27], the β value obtained for the linear fit of curve is 8.9 × 105 cm−1, which is equivalent to γ factor ∼6, Aeff = 7.6 × 10−17 cm2, J = 7.2 × 108 A cm−2. Notice that the γ of Pt electrodes is larger than W electrode as the cathode of Pt structure is sharper than that of the W structure. Note that the effective emission area calculated from the above method for this device is extremely small and the value of γ is relatively high, which is indicative of the fact that the emission is occurring from the apex of the electrode.
S Brimley et al. [24] also observed similar emission phenomenon, viz., CSCLE at low applied voltage which converts to ISCLE at a higher voltage and finally F-N emission, in similar but planar (2-D) structures. According to their analysis, the apices of the structures play a vital role on the occurrence of space-charge limited emission processes. If the emission of charge particles occurs near the apex of tip along the symmetrical axis, the charge particles experiences maximum field strength along the axis, confining it around the axis. This results into higher charge density between the site and anode. On the other hand, for emission site not at the apex, the charge particles are thrown away from the perpendicular by the field above the surface. As a result, the charged particles are scattered around and traverse a longer path towards the anode, leading to, lower charge density between the electrodes. Thus, we can conclude, that the space-charge limited emission at lower voltage originates from the sites near the tip apices. The following FN emissions observed, are from other sites away from the center of the gap. The variation of emission current (I) with respect to applied voltage 29
Nuclear Inst, and Methods in Physics Research B 425 (2018) 26–31
A.K. Singh
Fig. 5. a) SEM image (a) showing pathway of current after during explosion, (b) the structure after explosion, (c) micro-/nano-metallic particles. Inset shows the histogram of the particle size distribution, and (d) Energy dispersive X-ray spectroscopy study performed on of a nanoparticle particle formed during the explosion.
particles in the inset of Fig. 5(c). The histogram of the size distribution of these particles depicted in the inset of Fig. 5(c) shows a gradual decrease in the number of particles with increasing diameter. Some of the spherical particles are milled using ion beam and the cross-sectional image affirms that the particles are not hollow. We employ the energy dispersive spectroscopic studies on the particles (Fig. 5(d)) in order to investigate the chemical constituents of the spheres. It appears that the particles are mostly made up of copper. Characteristics X-ray signals corresponding to Si and O are due to the existence of the glass substrate beneath the spheres. Two types of particles (micro- and nano-size) are formed during the explosion is probably due to the simultaneous occurrence of two different processes. First process is vaporization and condensation of the metallic film as a result of sudden release of huge amount of energy leading to the formation of particle in a few nm regime of size. The important parameters that affect the size of nano-particle is the electron temperature and electron density as well as ambient conditions [28,29]. The surface morphology of the electrode material, also, plays a significant role in determining the size of the nano-particles [30]. In the second process, melting and condensation of the copper film’s material leads to the formation of micron size particles. The number of nanometer size particles is much larger than the micron size particles. We only show the size distribution of the particles in the nanometer size diameter in the inset of Fig. 5(c). This method can be potentially applied to produce high quality metallic nano-particles useful for both scientific and technological interest having wide range applications such as in catalysis, leather industry, electron detectors, and luminescence technology [31,32].
3.3. Explosive emission and formation of spherical micro-/nano- size particles The FN emission transforms into an explosive emission process at specific value of the applied voltage (113.8 V for the Pt electrode assembly). One can observe a sudden and abrupt increase in the emission current (inset of Fig. 4(b)) which corresponds and is indicative of the explosive emission process. The explosive emission process is also observed in the SEM. We recorded the video (captured by the SEM system from the normal view) of the Pt device during the current-voltage characterization in the entire range of applied voltage and relevant part (containing the explosion process) of the video is available as a Supplementary material with this article. A few frames of the video are shown in Fig. 4. Fig. 4(a) shows the SEM image (extracted from the video) of the Pt electrodes at 60 V applied voltage. At 113.8 V, the explosive emission occurs and the SEM image (inset Fig. 4(a)) turns white probably due to sudden emission of unprecedented amount of electron released during the explosion. The nanogap electrode structure is destroyed during the explosive emission process. A SEM image of the destroyed structure is shown in Fig. 5(b). The SEM image in Fig. 5(a) shows a sudden release of energy along a zigzag path extended up to several hundreds of micron distance through the copper pads originating from the location of the Pt overhanging structure. Fig. 5(b) shows a closer view captured in the vicinity of the overhanging electrodes after the explosion. One can observe spherical particles of various sizes besides the ruptured copper film and the remains of the exploded overhanging structure in Fig. 5(b). We show a magnified SEM image of the spherical particles in Fig. 5(c). We calculate and plot a histogram of the size distribution of the spherical 30
Nuclear Inst, and Methods in Physics Research B 425 (2018) 26–31
A.K. Singh
3.4. Discussion
Appendix A. Supplementary data
The explosion phenomenon in the field emitting electrodes is not comprehensively understood. The explosion is merely not a thermal phenomenon; rather a sudden release of stored energy after a critical limit is reached. We speculate that the explosion occurs due to a sudden breakdown of the electrical isolation of the electrodes after a certain condition (field) is reached as we observed an abrupt rise in the current. In a few tungsten structures, we could observe an intermediate state where we see some protrusion like growth on the cathode that eventually touches the surface of the anode and thus shortens the electrodes and the effective resistance shows Ohmic behaviour upon further increment of the applied voltage. However the voltage applied across the electrodes (owing to its large emission area and smaller gap size), is much smaller than the systems (shown in Fig. 1) discussed in the previous sections. At smaller voltage the stored energy in the electrode assembly is low and probably insufficient to destroy the electrode assembly. The protrusion like growth in the electrodes is possible due to the electric field at the surface [33]. Growth in the emission surface is also possible due to the cracking of Pt precursor gas molecules present in the system by the emitted electrons [34].
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nimb.2018.04.004. References [1] K. Takiya, Y. Tomoda, W. Kume, S. Ueno, T. Watanabe, Shirakashi, Appl. Surf. Sci. 258 (2012) 2029. [2] T. Li, W. Hu, D. Zhu, Adv. Mater. 22 (2010) 286. [3] C. Thalander, et al., Mater. Today 9 (2006) 28. [4] X. Liang, S.Y. Chou, Nano Lett. 8 (2008) 1472. [5] A.K. Wanekaya, W. Chen, N.V. Myung, A. Mulchandani, Electroanal. 18 (2006) 533. [6] P.G. Slade, E.D. Taylor, IEEE Trans. Comp. Package Technol. 25 (2002) 390. [7] D. Cruz, J.P. Chang, M.G. Blain, Appl. Surf. Sci. 86 (2005) 153501. [8] R.H. Fowler, L. Nordheim, Proc. R. Soc. Lond. A 119 (1928) 173. [9] W.S. Koh, L.K. Ang, Appl. Phys. Lett. 89 (2006) 183107. [10] Y.V. Kervennic, H.S.J. Van-der-zant, A.F. Morpurgo, L. Gurevich, L.P. Kouwenhoven, Appl. Phys. Lett. 80 (2002) 321. [11] M.D. Fischbein, M. Drndić, Appl. Phys. Lett. 88 (2006) 063116. [12] A.A.G. Driskill-Smith, D.G. Hasko, H. Ahmed, Appl. Phys. Lett. 71 (1997) 3159. [13] P. Hommelhoff, Y. Sortais, A. Aghajani-Talesh, M.A. Kasevich, Phys. Rev. Lett. 96 (2006) 077401. [14] S.K. Tripathi, N. Shukla, N.S. Rajput, V.N. Kulkarni, Nanotechnology 20 (2009) 275301. [15] L.K. Ang, T.J.T. Kwan, Y.Y. Lau, Phys. Rev. Lett. 91 (2003) 208303. [16] R.G. Forbes, J. Vac. Sci. Technol. B 17 (1999) 526. [17] C.A. Spindt, I. Brodie, L. Humphrey, E.R. Westerberg, J. Appl. Phys. 47 (1976) 5248. [18] Subhranu Samanta, Siddheswar Maikap, Anisha Roy, Surajit Jana, Jian-Tai Qiu, Adv. Mater. Interfaces 4 (2017) 1700959. [19] S. Chakrabarti, et al., Appl. Surf. Sci. 433 (2018) 51. [20] Somsubhra Chakrabarti, Siddheswar Maikap, Subhranu Samant, Surajit Jana, Anisha Roy, Jian-Tai Qiu, Phys. Chem. Chem. Phys. 19 (2017) 25938. [21] Wenjun Chen, et al., Carbon 129 (2018) 646. [22] Bipin Kumar Gupta, et al., AIP Adv. 8 (2018) 015117. [23] Chiu J. Fu-Chien, Appl. Phys. 100 (2006) 114102. [24] S. Brimley, M.S. Miller, M.J. Hagmann, J. Appl. Phys. 109 (2011) 094510. [25] W. Chandra, L.K. Ang, K.L. Pey, C.M. Ng, Appl. Phys. Lett. 90 (2007) 153505. [26] V.V. Zhirnov, C.L. Rinne, G.J. Wojak, R.C. Sanwald, J.J. Hren, J. Vac. Sci. Technol. B 19 (2001) 87. [27] R. Kometani, S. Ishihara, Sci. Technol. Adv. Mater. 10 (2009) 7. [28] M.A. Koten, S.A. Voeller, M.M. Patterson, J.E. Shield, J. Appl. Phys. 119 (2016) 114306. [29] Jerome Vernieres, et al., Adv. Funct. Mater. 27 (2017) 1605328. [30] X. Qiu, R.P. Joshi, Phys. Plasmas 25 (2018) 022109. [31] M. Saka, Metallic Micro and Nano Materials: Fabrication with Atomic Diffusion, Springer-Verlag, Berlin Heidelberg, 2011, p. 59. [32] W. Slówko, H. Prasol, Vacuum 67 (2002) 191. [33] I. Brodie, J. Appl. Phys. B 35 (1964) 2324. [34] J.T.L. Thong, C.H. Oon, Appl. Phys. Lett. 81 (2002) 4823.
4. Conclusions Platinum and tungsten overhanging nanostructures are successfully fabricated by focused ion beam induced chemical vapour deposition and milling processes. The analysis of I-V shows F-N field emission and space-charge limited emission current. The effective emission area and field enhancement factors are calculated from the F-N fit. I-V characteristic shows sharp and abrupt increase in the emission current while the transition from field emission to explosive electron emission occurs. The explosive emission process is destructive and the exploded structures form micro-/nano-size particles. Chemical analysis and size-distribution calculation of these particles are performed. Acknowledgements The Author is thankful to Dr. Amit Banerjee for helping in carrying out experiments at Ion Beam Centre, IIT Kanpur, Kanpur. Also, acknowledging his proof reading the manuscript and giving valuable suggestions. Thankful to the Ion Beam Staff, IIT Kanpur, for helping out during the experiments.
31
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Field emission and explosive electron emission process in focused ion beam fabricated platinum and tungsten three-dimensional overhanging nanostructure
T
Abhishek Kumar Singh Department of Physics, Darbhanga College of Engineering, Mabbi, P.O. Lal Sahpur, Darbhanga 846005, Bihar, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Focused ion beam Field emission Nanostructure
Three-dimensional platinum and tungsten overhanging nanogap (∼70 nm) electrodes are fabricated on a glass substrate using focused ion beam milling and chemical vapour deposition processes. Current-voltage (I-V) characteristics of the devices measured at a pressure of ∼10−6 mbar shows space-charge emission followed by the Fowler-Nordheim (F-N) field emission. After the F-N emission, the system enters into an explosive emission process, at a higher voltage generating a huge current. We observe a sharp and abrupt rise in the emission current which marks the transition from the F-N emission to the explosive emission state. The explosive emission process is destructive in nature and yields micro-/nano-size spherical metal particles. The chemical compositions and the size-distribution of such particles are performed.
1. Introduction Electrical transportation through nanoscale systems (e.g., nanowires, nanogaps, etc.) are extensively studied [1–3] due to their unique physical behaviours and amazing sensing capabilities [4,5]. Field induced electron emission phenomenon in 3D overhanging nanogap electrodes is utilized in various applications, such as in switches, connectors, micro-electromechanical system (MEMS) [6,7] etc. When high electric field is applied across nanogap electrodes, it induces electron current emission via quantum mechanical tunneling process, known as cold field emission (CFE), and governed by the Fowler-Nordheim (FN) field emission equation [8]. CFE based nanodevices are essential for emerging vacuum nano-electronics as they are utilized in surface-conduction electron-emitter displays, electron beam lithography, quantum interference, free electron femtosecond pulses, etc [9–13]. In the present work, we fabricate 3D overhanging nanostructures of platinum and tungsten using dual beam focused ion beam (FIB) system. The fabrication process involves FIB induced milling and chemical vapour deposition (FIB-CVD) techniques. Electric field is applied across the nanogap electrodes by the application of dc voltage (V) and corresponding emission current (I) is simultaneously measured. Currentvoltage (I-V) characteristics of the devices are studied inside the FIB chamber at ∼ 10−6 mbar pressure. The analysis of I-V shows the field emission at higher voltages and space-charge limited current at lower voltages. We observe that the systems enter into an explosive electron emission process at a certain value of the applied voltage. The explosive
emission process is destructive and leads to the formation of micro and nano-size spherical metallic particles. 2. Experimental 2.1. Instruments used for fabrication and measurements The experiments are performed in FEI make (Nova 600 NanoLab) dual beam focused ion beam (FIB) system. The system is equipped with a liquid metal ion source (LMIS) of Ga and sophisticated ion optics that can focus the Ga ion beam down to a beam diameter of few nanometers. The Ga ion beam can perform material milling and chemical vapour deposition (CVD) with ease, precision and control. Precursor gases of platinum ((CH3)3Pt(CpCH3)), tungsten (W(CO)6) and carbon (C10H8) are injected in the system through the gas injection systems (GIS) which are cracked by the ion beam during the CVD process. The electron beam of the dual beam system is extracted from a field emission gun and used for scanning electron microscopy. Energy dispersive X-ray spectroscopy (EDS) facility is also available in the system and utilized for chemical analysis in the present experiments. The electrical measurements are performed in situ under high vacuum condition via electrical feedthrough channels provided in the system. The current-voltage characteristics of the fabricated devices are performed by employing a Keithley (model 6430) source-meter interfaced with computer via LabView programs.
E-mail address: [email protected]. https://doi.org/10.1016/j.nimb.2018.04.004 Received 26 December 2017; Received in revised form 23 March 2018; Accepted 4 April 2018 0168-583X/ © 2018 Elsevier B.V. All rights reserved.
Nuclear Inst, and Methods in Physics Research B 425 (2018) 26–31
A.K. Singh
Fig. 1. Schematic representation of the (a) copper thin film deposited on glass substrate using I-shaped mask where a = 200 µm, b = 2 mm, c = 2 mm, d = 2 mm, (b) trench fabricated on the copper film using focused ion beam milling technique. Scanning electron micrograph (52° angle of view) of the (c) platinum wall deposited by the focused ion beam induced chemical vapour deposition process. The dotted lines represent the final structure to be fabricated. (d) Overhanging platinum nanogap electrodes fabricated by focused ion beam milling. Inset shows the magnified view of gap. (e) Normal (0° angle of view) view of the final structure and its different components (pixel intensity is inverted for better visibility). SEM images have been artificially coloured for the ease of demonstration.
It is worth mentioning that during the fabrication of the overhanging electrodes from the Pt wall using FIB, significant amount of redeposition [14] occurs inside the trench that reconnects the electrically isolated pads. Therefore, FIB milling process is again employed after the fabrication of the overhanging electrodes, in order to remove the redeposited material (Fig. 1(d)). Overhanging nanogap electrodes of W are also fabricated (see following discussion) by the method described above. These structures differ in shape and gap size from the one described in Fig. 1.
2.2. Fabrication of the 3-D overhanging electrodes Thin films (thickness = 150 nm) of copper are deposited via thermal evaporation process (at chamber pressure ∼10−5 mbar, and ∼1 Å s−1 deposition rate) on clean borosilicate glass substrates using an I-shape mask (Fig. 1(a)). A trench is created by focused ion beam milling of the film near the center (Fig. 1(b)) of the copper film. The energy and the current of the Ga FIB used are 30 keV and 1 nA. The width of the trench is 5 μm. A Pt wall is deposited using the focused ion beam induced chemical vapour deposition (FIB-CVD) process across the trench using 30 keV beam energy and 10 pA current (Fig. 1(c)). The Pt wall is 15 μm in length, 10 μm in height and 300 nm in thickness. The glass-substrate along with the Pt wall is transferred onto a tilted stub (45°) and mounted inside the FIB chamber. Mounting the sample on a tilted stub helps to view the Pt wall from the grazing angle, which is necessary for further processing of the device. The Pt wall is milled out by the focused ion beam from the grazing angle and a 5 µm × 8 µm window is formed which inturn creates an overhanging bridge-like structure of Pt. The bridge was milled by the FIB in order to achieve a nanogap with a pointed (apex) structure at one end (cathode). The SEM image of the resultant structure is shown in Fig. 1(d). The inset of Fig. 1(d) shows a magnified view of the two electrodes and the gap fabricated by the method described above. The distance (d) between the apex of the left electrode and the right electrode is measured to be 70 nm. Fig. 1(e) shows the SEM image (contrast inverted) of the structure from the normal view.
3. Results and discussions 3.1. I-V characteristics: space-charge limited emission and F-N emission The electrodes assembly with the gap is placed on a specially designed sample-holder for measuring current-voltage (I-V) characteristics in vacuum (at a pressure of the order of 10−6 mbar). The sampleholder consists of metallic probes tightly attached to the copper pads in one end and the other end is electrically connected to the Keithley source-meter via feed-through channels of the FIB system. Electric Voltage (V) is applied across the electrodes trough the source-meter and is increased in a step of 0.5 V per second beginning at 0 V. The current (I) associated with each voltage is simultaneously measured as represented in Fig. 2. Fig. 3(a) shows the I-V characteristics of the overhanging Pt electrodes plotted in the loglog scale. The slope of the straight-line fit (solid red line through the data) is 0.45 and thus 27
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Fig. 3. Current-voltage characteristics of (a) Platinum overhanging nanostructure. Inset shows the F-N plot of the I-V data showing negative slope above 104 V of applied voltage, and (b) tungsten overhanging nanostructure. Inset shows the F-N plot showing negative slope above 57 V of applied voltage and also the SEM image of the nanogap structure. Fig. 2. Current-voltage measurement of (a) Platinum overhanging nanostructure and (b) tungsten overhanging nanostructure.
several complications, such as complexity of the electrode geometry. The variable physical quantity is often the applied voltage (V) instead of the actual electric field. Usually a field conversion factor (β) [17] is used in order to convert the applied voltage to the electric field as: E = βV. Using this relation with Eq. (1) implies that a plot of ln(I/V2) versus (1/V) should produce a straight-line with a negative slope value in the F-N regime of emission. Linear variation with a negative slope of ln(I/V2) with respect to (1/V) is a characteristic of the F-N emission process and a plot of ln(I/V2) versus (1/V) is referred as F-N plot. In the inset of Fig. 3(a) we show ln(I/V2) versus (1/V) plot for the data received from the Pt electrodes described in Fig. 1. The straight-line fit (solid green line in the inset of Fig. 3(a)) in the data showing negative slope at high-voltage regime is indicative of Fowler-Nordheim (F-N) field emission. Fig. 3(b) shows the I-V characteristics (in loglog scale) of the nanogap tungsten electrode fabricated by the method described in section 2.2. The plot shows that the emission process goes through a coherence space-charge limited regime (slope ≈ 0.52) followed by an incoherent space-charge limited regime (slope ≈ 1.94) before finally reaching the F-N regime. The straight-line fit with a negative slope in the high-voltage regime (see the FNplot in the inset of Fig. 3(b)) shows the signature of F-N emission as discussed earlier in the case of the Pt structure. The critical electric field values for the F-N Tunneling for Platinum and Tungsten overhanging nanostructure are 14.9 MV/cm and 8.0 MV/cm, respectively [18–22] which are higher than the reported value [23].
(approximately) corresponds to a I ∝ V 1/2 like variation in the emission current with respect to the applied voltage, which is known as the coherent-space-charge-limited emission (CSCLE) [15]. Upon increasing the applied voltage beyond 97 V, F-N emission is observed (see the green solid line in Fig. 3(a)). The field emission current (I) in the F-N regime is given by the generalized Fowler-Nordheim equation [16]:
I = λaϕ−1Aeff E 2exp ⎛− ⎝ ⎜
μbϕ3/2 ⎞ E ⎠ ⎟
(1)
Where, ϕ is the work function and Aeff is the effective area of the emission surface, E is the electric field strength, a and b are universal constants, and λ and μ are generalized correction factors. a and b are expressed in terms of fundamental qualities as follows: a = 1 8π (2me ) 2 3
e3 8πh
and
b= where e and me are respectively charge and mass of an eh electron, and h is the Planck’s constant. Values of a and b are approximately 1.54 × 10−6 A eV V−2 and 6.83 × 107 eV−3/2 Vcm−1. The generalized correction factors μ and λ are typically slowly varying functions of E, ϕ etc. The field emission current (I) depends on the actual value of the local electric field E. However in practical experimental conditions the actual value of E is sometimes unknown or difficult to evaluate due to 28
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(V) in the ISCLE regime for the W electrodes should ideally be ∼V3/2 for Child-Langmuir emission, however the variation in the present case is ∼V2. However we do not completely understand this behaviour, such variation in the emission current may correspond to Mott-Gurney space-charge limited emission where ∼V 2 like variation in the emission current is expected [25]. 3.2. Evaluations of field conversion and enhancement factors, and effective emission area Parameters, such as field conversion factor (β) and effective emission area (Aeff) are calculated from the F-N plots shown in the insets of Fig. 3(a) and (b). The generalized correction factors (λ and μ) in Eq. (1) can be assumed to be functions of Schottky barrier lowering parameter (y) as: λ = t2(y) and μ = v(y) where y = 3.79 × 10−4 E1/2/ϕ. Eq. (1) thus can be expressed as:
aAeff
I=
ϕt 2 (y )
E 2exp ⎡ ⎢ ⎣
−bϕ3/2 v (y ))⎤ ⎥ E ⎦
2
(2) 2
Where t (y) is approximately equal to 1.1 and v(y) = 0.95-y . Eq. (2) is re-written as following:
I B ⎛ ⎞ = Aexp ⎛− ⎞ ⎝V2 ⎠ ⎝ V⎠
(3)
where,
A=
B=
Aeff aβ 2
exp ⎛⎜ 1.1ϕ ⎝
0.95bϕ3/2 β
b (1.44 × 10−7) ⎞ ⎟ ϕ1/2 ⎠
(3a)
(3b)
The values of A and B are calculated respectively from the intercept and the slope of the F-N plots shown in the inset of Fig. 3(a) and (b) The straight line fitted to the data in the F-N plot for the platinum overhanging nanostructure confirms the F-N field emission process (see, insets of Fig. 3). Using Eq. (3) and the values of the slope and the intercept obtained from the straight line, we estimate the value of β to be ∼1.1 × 106 cm−1 and the effective emission area (Aeff) is ∼1.1 × 10−15 cm2. The value of the work-function (ϕ ) of Pt is assumed to be equal to the bulk work function value (6.35 eV). The geometrical E field enhancement factor γ of the system is defined as [26]: γ = E ,
Fig. 4. Scanning electron micrograph of the a) platinum overhanging nanostructure at 60 V. Inset shows the SEM micrograph of a structure during the explosion at 113.8 V applied voltage; b) structure after explosive emission process at 115 V applied voltage. Inset shows sharp increase in the emission current indicating the transition from field emission to explosive emission process.
0
V
where E0 = d . Using the value of the β from the above analysis, the value of γ for the Pt electrodes calculated by the above relation is ∼8. The maximum field emission current density achieved in the Pt electrodes before it enters into the explosive emission process (see, subsequent discussion) is calculated to be 4.7 × 107 A cm−2. Similar analysis is performed on the I-V data obtained from the overhanging nanogap electrodes of tungsten (shown by the SEM image in the inset of Fig. 3(b)). The F-N curve in the inset of Fig. 3(b) shows two distinct linear regions which may be attributed to the change in the field enhancement factors (β). Since, the β factor is dependent on the cathode surface i.e. size and shape of the tip and protrusions (typical size is around in few nm) present on the surface. However, at higher current, protrusions burnout due to resistive heating which leads to change in the emission area of the electron as can be distinguished with two different slopes in Fig. 3(b). On substituting, ϕ = 4.7 eV as reported in the case of FIB deposited tungsten [27], the β value obtained for the linear fit of curve is 8.9 × 105 cm−1, which is equivalent to γ factor ∼6, Aeff = 7.6 × 10−17 cm2, J = 7.2 × 108 A cm−2. Notice that the γ of Pt electrodes is larger than W electrode as the cathode of Pt structure is sharper than that of the W structure. Note that the effective emission area calculated from the above method for this device is extremely small and the value of γ is relatively high, which is indicative of the fact that the emission is occurring from the apex of the electrode.
S Brimley et al. [24] also observed similar emission phenomenon, viz., CSCLE at low applied voltage which converts to ISCLE at a higher voltage and finally F-N emission, in similar but planar (2-D) structures. According to their analysis, the apices of the structures play a vital role on the occurrence of space-charge limited emission processes. If the emission of charge particles occurs near the apex of tip along the symmetrical axis, the charge particles experiences maximum field strength along the axis, confining it around the axis. This results into higher charge density between the site and anode. On the other hand, for emission site not at the apex, the charge particles are thrown away from the perpendicular by the field above the surface. As a result, the charged particles are scattered around and traverse a longer path towards the anode, leading to, lower charge density between the electrodes. Thus, we can conclude, that the space-charge limited emission at lower voltage originates from the sites near the tip apices. The following FN emissions observed, are from other sites away from the center of the gap. The variation of emission current (I) with respect to applied voltage 29
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Fig. 5. a) SEM image (a) showing pathway of current after during explosion, (b) the structure after explosion, (c) micro-/nano-metallic particles. Inset shows the histogram of the particle size distribution, and (d) Energy dispersive X-ray spectroscopy study performed on of a nanoparticle particle formed during the explosion.
particles in the inset of Fig. 5(c). The histogram of the size distribution of these particles depicted in the inset of Fig. 5(c) shows a gradual decrease in the number of particles with increasing diameter. Some of the spherical particles are milled using ion beam and the cross-sectional image affirms that the particles are not hollow. We employ the energy dispersive spectroscopic studies on the particles (Fig. 5(d)) in order to investigate the chemical constituents of the spheres. It appears that the particles are mostly made up of copper. Characteristics X-ray signals corresponding to Si and O are due to the existence of the glass substrate beneath the spheres. Two types of particles (micro- and nano-size) are formed during the explosion is probably due to the simultaneous occurrence of two different processes. First process is vaporization and condensation of the metallic film as a result of sudden release of huge amount of energy leading to the formation of particle in a few nm regime of size. The important parameters that affect the size of nano-particle is the electron temperature and electron density as well as ambient conditions [28,29]. The surface morphology of the electrode material, also, plays a significant role in determining the size of the nano-particles [30]. In the second process, melting and condensation of the copper film’s material leads to the formation of micron size particles. The number of nanometer size particles is much larger than the micron size particles. We only show the size distribution of the particles in the nanometer size diameter in the inset of Fig. 5(c). This method can be potentially applied to produce high quality metallic nano-particles useful for both scientific and technological interest having wide range applications such as in catalysis, leather industry, electron detectors, and luminescence technology [31,32].
3.3. Explosive emission and formation of spherical micro-/nano- size particles The FN emission transforms into an explosive emission process at specific value of the applied voltage (113.8 V for the Pt electrode assembly). One can observe a sudden and abrupt increase in the emission current (inset of Fig. 4(b)) which corresponds and is indicative of the explosive emission process. The explosive emission process is also observed in the SEM. We recorded the video (captured by the SEM system from the normal view) of the Pt device during the current-voltage characterization in the entire range of applied voltage and relevant part (containing the explosion process) of the video is available as a Supplementary material with this article. A few frames of the video are shown in Fig. 4. Fig. 4(a) shows the SEM image (extracted from the video) of the Pt electrodes at 60 V applied voltage. At 113.8 V, the explosive emission occurs and the SEM image (inset Fig. 4(a)) turns white probably due to sudden emission of unprecedented amount of electron released during the explosion. The nanogap electrode structure is destroyed during the explosive emission process. A SEM image of the destroyed structure is shown in Fig. 5(b). The SEM image in Fig. 5(a) shows a sudden release of energy along a zigzag path extended up to several hundreds of micron distance through the copper pads originating from the location of the Pt overhanging structure. Fig. 5(b) shows a closer view captured in the vicinity of the overhanging electrodes after the explosion. One can observe spherical particles of various sizes besides the ruptured copper film and the remains of the exploded overhanging structure in Fig. 5(b). We show a magnified SEM image of the spherical particles in Fig. 5(c). We calculate and plot a histogram of the size distribution of the spherical 30
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3.4. Discussion
Appendix A. Supplementary data
The explosion phenomenon in the field emitting electrodes is not comprehensively understood. The explosion is merely not a thermal phenomenon; rather a sudden release of stored energy after a critical limit is reached. We speculate that the explosion occurs due to a sudden breakdown of the electrical isolation of the electrodes after a certain condition (field) is reached as we observed an abrupt rise in the current. In a few tungsten structures, we could observe an intermediate state where we see some protrusion like growth on the cathode that eventually touches the surface of the anode and thus shortens the electrodes and the effective resistance shows Ohmic behaviour upon further increment of the applied voltage. However the voltage applied across the electrodes (owing to its large emission area and smaller gap size), is much smaller than the systems (shown in Fig. 1) discussed in the previous sections. At smaller voltage the stored energy in the electrode assembly is low and probably insufficient to destroy the electrode assembly. The protrusion like growth in the electrodes is possible due to the electric field at the surface [33]. Growth in the emission surface is also possible due to the cracking of Pt precursor gas molecules present in the system by the emitted electrons [34].
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.nimb.2018.04.004. References [1] K. Takiya, Y. Tomoda, W. Kume, S. Ueno, T. Watanabe, Shirakashi, Appl. Surf. Sci. 258 (2012) 2029. [2] T. Li, W. Hu, D. Zhu, Adv. Mater. 22 (2010) 286. [3] C. Thalander, et al., Mater. Today 9 (2006) 28. [4] X. Liang, S.Y. Chou, Nano Lett. 8 (2008) 1472. [5] A.K. Wanekaya, W. Chen, N.V. Myung, A. Mulchandani, Electroanal. 18 (2006) 533. [6] P.G. Slade, E.D. Taylor, IEEE Trans. Comp. Package Technol. 25 (2002) 390. [7] D. Cruz, J.P. Chang, M.G. Blain, Appl. Surf. Sci. 86 (2005) 153501. [8] R.H. Fowler, L. Nordheim, Proc. R. Soc. Lond. A 119 (1928) 173. [9] W.S. Koh, L.K. Ang, Appl. Phys. Lett. 89 (2006) 183107. [10] Y.V. Kervennic, H.S.J. Van-der-zant, A.F. Morpurgo, L. Gurevich, L.P. Kouwenhoven, Appl. Phys. Lett. 80 (2002) 321. [11] M.D. Fischbein, M. Drndić, Appl. Phys. Lett. 88 (2006) 063116. [12] A.A.G. Driskill-Smith, D.G. Hasko, H. Ahmed, Appl. Phys. Lett. 71 (1997) 3159. [13] P. Hommelhoff, Y. Sortais, A. Aghajani-Talesh, M.A. Kasevich, Phys. Rev. Lett. 96 (2006) 077401. [14] S.K. Tripathi, N. Shukla, N.S. Rajput, V.N. Kulkarni, Nanotechnology 20 (2009) 275301. [15] L.K. Ang, T.J.T. Kwan, Y.Y. Lau, Phys. Rev. Lett. 91 (2003) 208303. [16] R.G. Forbes, J. Vac. Sci. Technol. B 17 (1999) 526. [17] C.A. Spindt, I. Brodie, L. Humphrey, E.R. Westerberg, J. Appl. Phys. 47 (1976) 5248. [18] Subhranu Samanta, Siddheswar Maikap, Anisha Roy, Surajit Jana, Jian-Tai Qiu, Adv. Mater. Interfaces 4 (2017) 1700959. [19] S. Chakrabarti, et al., Appl. Surf. Sci. 433 (2018) 51. [20] Somsubhra Chakrabarti, Siddheswar Maikap, Subhranu Samant, Surajit Jana, Anisha Roy, Jian-Tai Qiu, Phys. Chem. Chem. Phys. 19 (2017) 25938. [21] Wenjun Chen, et al., Carbon 129 (2018) 646. [22] Bipin Kumar Gupta, et al., AIP Adv. 8 (2018) 015117. [23] Chiu J. Fu-Chien, Appl. Phys. 100 (2006) 114102. [24] S. Brimley, M.S. Miller, M.J. Hagmann, J. Appl. Phys. 109 (2011) 094510. [25] W. Chandra, L.K. Ang, K.L. Pey, C.M. Ng, Appl. Phys. Lett. 90 (2007) 153505. [26] V.V. Zhirnov, C.L. Rinne, G.J. Wojak, R.C. Sanwald, J.J. Hren, J. Vac. Sci. Technol. B 19 (2001) 87. [27] R. Kometani, S. Ishihara, Sci. Technol. Adv. Mater. 10 (2009) 7. [28] M.A. Koten, S.A. Voeller, M.M. Patterson, J.E. Shield, J. Appl. Phys. 119 (2016) 114306. [29] Jerome Vernieres, et al., Adv. Funct. Mater. 27 (2017) 1605328. [30] X. Qiu, R.P. Joshi, Phys. Plasmas 25 (2018) 022109. [31] M. Saka, Metallic Micro and Nano Materials: Fabrication with Atomic Diffusion, Springer-Verlag, Berlin Heidelberg, 2011, p. 59. [32] W. Slówko, H. Prasol, Vacuum 67 (2002) 191. [33] I. Brodie, J. Appl. Phys. B 35 (1964) 2324. [34] J.T.L. Thong, C.H. Oon, Appl. Phys. Lett. 81 (2002) 4823.
4. Conclusions Platinum and tungsten overhanging nanostructures are successfully fabricated by focused ion beam induced chemical vapour deposition and milling processes. The analysis of I-V shows F-N field emission and space-charge limited emission current. The effective emission area and field enhancement factors are calculated from the F-N fit. I-V characteristic shows sharp and abrupt increase in the emission current while the transition from field emission to explosive electron emission occurs. The explosive emission process is destructive and the exploded structures form micro-/nano-size particles. Chemical analysis and size-distribution calculation of these particles are performed. Acknowledgements The Author is thankful to Dr. Amit Banerjee for helping in carrying out experiments at Ion Beam Centre, IIT Kanpur, Kanpur. Also, acknowledging his proof reading the manuscript and giving valuable suggestions. Thankful to the Ion Beam Staff, IIT Kanpur, for helping out during the experiments.
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