High Resolution Surface Imaging Using a Carbon Nanotube Array with Pointed Height Distribution
D. Roy Mahapatra, S.V. Anand, N. Sinha, R.V.N. Melnik
Massachusetts Institute of Technology, US
Keywords: carbon nanotube, field emission, array, surface imaging
Abstract:In this paper, we discuss a new methodology to obtain high resolution electron micrographs of different materials using field emission from a pointed array of carbon nanotubes (CNTs) and a technique to map the surface of a metallic substrate. In the proposed design, we consider a pointed height distribution of the carbon nanotube array under a diode configuration with two side gates maintained at a negative potential to obtain a highly intense beam of electrons localized at the center of the array. Furthermore, we compare the subsequent performance of the pointed array with the conventionally used random and uniform arrays and prove that such an intense, focused beam of electrons can only be obtained with the proposed design. Such a phenomenon has very widespread implications in the field of electron microscopy and imaging. Currently, one of the biggest problems faced in electron microscopy is the difficulty in focusing the thermionic/field emission electrons to a very small region in order to achieve very small field of view (and thus high resolution). In the design proposed by us, the diameter of the emitted beam of electrons is comparable to the diameter of a single CNT which is of the order of a few nanometers. Such a focused beam will be able to capture the minutest of details on the surface of the materials to be imaged. A model of field emission from an array of CNTs under diode configuration was proposed and validated by experiments by the authors. Despite high output, the current in such a thin film device often decays drastically. The random orientation of the CNTs and the electromechanical interactions were modeled to explain the self-assembly. The degraded state of the CNTs and the electromechanical forces were employed to update the orientation of the CNTs. Field emission current at the device scale was finally obtained by using the Fowler-Nordheim (F-N) equation and the integration of current densities over the computational cell surfaces on the anode side was carried out. In this contribution, the anode is made of the substrate whose surface has to be mapped. The CNTs grown on a metallic substrate are considered as the cathode in this study. The cathode with the CNT array and the anode are maintained at an optimum distance in vacuum such that a field emission current is observed in the circuit. The cathode is then moved over the anode at a very slow rate in a predefined path to cover the entire surface area of the anode. As the field emission current depends on the distance between the anode and the cathode, all the irregularities on the surface of the anode will lead to a change in the field emission current time history for the entire duration of the scan. The current densities for various height distributions of CNT arrays are calculated. This surface scanning/mapping technique can be applied for surface roughness measurements with nano-scale accuracy, micro/nano damage detection, high precision displacement sensor, vibrometers and accelerometers, among other applications.