Unraveling The Origin of Enhanced Field Emission from Irradiated FeCo-SiO₂Nanocomposites: A Combined Experimental and First-Principles Based Study

Abstract

This work is driven by the vision of engineering planar field emitters with ferromagnetic metal-insulator nanocomposite thin films, using swift heavy ion (SHI) irradiation method. FeCo nanoparticles inside SiO2 matrix, when subjected to SHI get elongated. Using this, we demonstrate here a planar field emitter with maximum current density of 550 μA/cm(2) at an applied field of 15 V/μm. The film, irradiated with 5 × 10(13) ions/cm(2) fluence (5e13) of 120 MeV Au(9+) ions, shows very high electron emitting quantum efficiency in comparison to its unirradiated counterpart. Surface enhanced Raman spectroscopy analysis of unirradiated and 5e13 films further confirms that the field emission (FE) enhancement is not only due to surface protrusions but also depends on the properties of entire matrix. We find experimental evidence of enhanced valence band density of states (VB DOS) for 5e13 film from XPS, which is verified in the electronic structure of a model FeCo cluster from first-principles based calculations combining density functional theory (DFT) and molecular dynamics (MD) simulations. The MD temperature is selected from the lattice temperature profile inside nanoparticles as deduced from thermal spike model. Increasing the irradiation fluence beyond 5e13, results in reduced VB DOS and melting of surface protrusions, thus causing reduction of FE current density. We finally conclude from theoretical analysis that change in fluence alters the co-ordination chemistry followed by the charge distribution and spin alignment, which influence the VB DOS and concurrent FE as evident from our experiment.

Micro-structural origin of elongation in swift heavy ion irradiated Ni nanoparticles: A combined EXAFS and DFT study

“…FeCo NPs in SiO 2 ), we observed a gradual increase in particle aspect ratio with increasing fluence [14]. This was further verified with theoretical modelling of (FeCo) 8 cluster [8]. Therefore, to understand the anomaly (i.e.…”

“…Macroscopically, at this high fluence, dissolution and/or fragmentation of NPs are likely to take place. That is why we see molten clusters at 6000 K. On the contrary, as we have discussed before, in FeCo cluster the particle elongation was maximum at 6000 K [8]. Now considering the spin alignment in the 3d orbitals of FeCo and Ni, we note that total spin moment after forming the FeCo alloy is significantly higher in comparison with that of in Ni.…”

“…Experimental evidences of spherical to rod like shape transformation in Au [12], Ni [11], Co [9], Sn [10], Pt [13], FeCo [14,8] NPs are reported in literature. The changes in macro and micro structural parameters in these electronically excited metal-insulator nano-composites are studied widely in recent past using different state-of-the-art experimental approaches viz.…”

“…This field has drawn significant attention not only due to the possibilities of exploring interesting fundamental physics at the interactive interface but also it has several promising applications in various areas of materials science. The formation of equally separated large arrays of nano-crystals inside a matrix [3], change in crystallographic phases [4], alterations in easy axis of magnetization [5], tuning plasmonic properties [6], enhanced field emission [7,8] are a few to be named amongst many such applications. When exposed to swift heavy ion (SHI) irradiation [9,10], the shape of embedded Ni nanoparticles (NPs) inside SiO 2 matrix gets significantly modified [11].…”

Triggering of spin-flipping-modulated exchange bias in FeCo nanoparticles by electronic excitation

“…We noted that the shear stress in 200 MeV Iodine (I) ion irradiated Co NPs for 10 13  ions/cm 2 fluence is calculated only to be ≈ 0.19 GPa21, which is rather insufficient for producing ion-hammering effect in metal NPs. In view of this, the elongation of metal NPs (inside an insulating media) in the direction of ion beam is mostly explained by thermal spike mechanism as described in our former work22. Detailed thermal-pressure profiles are calculated for I irradiated Co/SiO 2 nanocomposites and Pb irradiated SnO 2 along with the thermal spike induced temperature distributions in refs 14 and 23 respectively.…”