  In this paper, we model the evolution and self-assembly of randomly oriented
carbon nanotubes (CNTs), grown on a metallic substrate in the form of a thin
film for field emission under diode configuration. Despite high output, the
current in such a thin film device often decays drastically. The present paper
is focused on understanding this problem. A systematic, multiphysics based
modelling approach is proposed. First, a nucleation coupled model for
degradation of the CNT thin film is derived, where the CNTs are assumed to
decay by fragmentation and formation of clusters. The random orientation of the
CNTs and the electromechanical interaction are then modeled to explain the
self-assembly. The degraded state of the CNTs and the electromechanical force
are employed to update the orientation of the CNTs. Field emission current at
the device scale is finally obtained by using the Fowler-Nordheim equation and
integration over the computational cell surfaces on the anode side. The
simulated results are in close agreement with the experimental results. Based
on the developed model, numerical simulations aimed at understanding the
effects of various geometric parameters and their statistical features on the
device current history are reported.
