Increasing local field enhancement factor in small clusters of emitters in close proximity

By Tom Sharp on September 7, 2017

With the emergence of vacuum nanoelectronics, field electron emission science has found applications in the low voltage regime, which includes field emission displays, electron sources of high power microwaves, or x-ray generators. In a recent JPCM letter, Thiago de Assis and Fernando Dall’Agnol field emission properties from clusters of a few emitters at close proximity using finite element analysis. Read on to find out more in the authors own words:

In arrays of ungated field emitters, its well know that the local characteristic field enhancement factor (FEF) of each emitter is expected to decrease as the distance between the emitters decreases. However, in 2015 it has been reported the possibility of the increasing of the FEF as the distance between the emitters decreases.

Figure: (a) Maximum local FEF, as a function of the separation parameter. (b) Angle of maximum local current density relative to the vertical direction, as a function of the aspect ratio for clusters with 2, 3, 4 and 7 emitters. The upper panels indicates the color map for local FEF, considering all clusters studied. Red (blue) color indicates high (low) local FEF-values. Figures from J. Phys.: Condens. Matter 29 40LT01 (2017) © IOP Publishing.

In our recent letter, we explored this non-intuitive behavior for small clusters of hemispherical on cylindrical post emitters. This electrostatic effect is pronounced for peripheral emitters in a cluster. We show that this close proximity electrostatic phenomenon is prominent in the limit of high aspect-ratio, suggesting applications for low voltage devices.

Our results also show that emitters can significantly increase the emitted current as a consequence only of the close proximity in small clusters, which is a timely result for stimulating applications in nanometric oscillators. We propose a multi-tip oscillatory system using this phenomenon as follows: (i) during field emission from a pair of emitters, we expect the electrostatic repulsion to separate emitters that are initially in contact held by van der Waals forces; (ii) this separation can decrease the FEF by several percent due to the close proximity effect, which in turn halts the emission current; (iii) the van der Waals forces then overcomes the repulsion bringing the emitters back together, restarting the cycle.

About the authors

Dr Thiago Albuquerque de Assis is Adjoint Professor of Physics at the Federal University of Bahia, Salvador, Brazil. He is a researcher in the Surface and Materials Group. Dr de Assis holds his PhD degree from Technical University of Madrid, Spain. His research interests include the investigation of reformulated standard field emission theory from the perspective of Large Area Field Emitters, and the electrostatics in small clusters of emitters. His research interests also include thin film science and he aims to understand, using statistical mechanics, the microscopic mechanisms that lead to roughening or smoothing; including the formation (at a thin film surface) of structures with characteristic lengths.

Dr Fernando Fuzinatto Dall’Agnol is an Adjoint Professor of Physics at the Federal University of Santa Catarina-Campus of Blumenau. His research focus is mainly on numerical simulations of Field Emission related phenomena. Other areas of interest are fluid dynamics, electrical and thermal fluid dynamics.