Molecular dynamics simulations of nanomemory element based on boron-nitride nanotube-to-peapod transition

Abstract

We investigated a nonvolatile nanomemory element based on boron-nitride nanopeapods using molecular dynamics simulations. The suggested system was composed of two boron-nitride nanotubes filled Cu electrodes and fully ionized endo-fullerenes. The two boron-nitride nanotubes were placed face to face and the endo-fullerenes came and went between the two boron-nitride nanotubes under alternatively applied force fields. Since the endo-fullerenes encapsulated in the boron-nitride nanotubes hardly escape from the boron-nitride nanotubes, the proposed system can be considered to be a nonvolatile memory device. Several switching processes were investigated for external force fields using molecular dynamics simulations.

Introduction

The nanostructure and nanoparticles are of great interest in materials science and technology because of their interesting structures, especially their size. However, it is hard to control their physical and chemical properties. Fullerene-related materials have unique physical and chemical properties. Fullerene-related materials have attracted considerable attention due to their unique physical, chemical, optical, mechanical, magnetic, and electronic properties this last decade [1], [2]. Compared to other nanostructures, fullerenes have found promising applications in a wide variety of very important technological processes such as in designing electronic devices, super-fibers, catalytic materials, etc. [3]. Especially the large empty space (particularly inside carbon nanotubes (CNTs)) has also opened new applications such as storage materials with high capacity and stability [4]. These cavities are large enough to accommodate a wide variety of atomic and molecular species, the presence of which can significantly influence the properties of the materials. In particular, a new type of self-assembled hybrid structures called “nanopeapods”, consisting of fullerene arrays inside single-walled CNTs, have recently been reported [5], [6], [7], [8], [9], [10]. The application of nanopeapods ranges from nanometer-sized containers of chemical reactant [8] to data storage [11] and high temperature superconductor [12]. The encapsulation of fullerenes (such as C₆₀) in nanotubes is favorable on energetic grounds and occurs rapidly by exposing nanotubes to sublimed fullerenes. Mickelson et al. [13] reported how to pack C₆₀ in boron-nitride nanotubes (BNNTs). Moreover, Goldberg et al. [14] researched metal fillings inside BNNTs. Okada et al. [15] showed the reaction energy of a BN nanopeapod such as (10, 10) BNNT + C₆₀ → C₆₀@(10, 10) BNNT + 1.267 (eV). Although the quantity of the exothermic energy, 1.267 eV, obtained from the calculations of Okada et al. [15] has been doubtful [16], C₆₀@(10, 10) BNNT is obviously more stable than the structure that the (10, 10) BNNT is infinitely separate with a C₆₀ molecule. Kwon et al. [11] reported that multi-walled nanotubes called “bucky shuttle” [17] were synthesized from elemental carbon under specific conditions, and investigated bucky shuttle memory device, which acted as nanometer-sized memory element, using molecular dynamics (MD) simulations. In order to be applied as a device, the shuttle memory system needed contacts or electrodes and covered wires. Though CNTs are a representative nanostructure material, CNTs synthesized by experiments can be a semiconductor or a metal conductor by their chirality [2], because it is hard to control the chirality of CNTs. However since most of bandgap of BNNTs are about 5.5 eV, they are electrically insulators [18].

For the bucky shuttle memory element suggested by Kwon et al. [11], the bit classifications must be very complicated to be defined and detected by the connected electrodes, because the shuttle media are always inside the same nanocapsule. For the bucky shuttle memory by Kwon et al., the stored data can be measured by the current pulse in the connecting wires, caused by the motion of the charged molecules due to the applied probing voltage. Therefore, to read the stored data, K⁺@C₆₀ the should be shuttled. For example, when the K⁺@C₆₀ settles in the position of ‘bit 1’, K⁺@C₆₀ has to move toward the position of ‘bit 0’ during the data reading processes and then ‘bit 1’ can be measured. After the data reading, K⁺@C₆₀ should be returned to the position of ‘bit 1’ to maintain its original data. Therefore, the data reading processes have to include the data erasing/writing processes, and then, both power consumption and data processing time increase. However, when K⁺@C₆₀ comes and goes between separate nanocapsules, simpler data reading processing can be achieved. For the tube-to-peapod transition device proposed in this work, nonvolatility and data writing/erasing processes are the same as those for the bucky shuttle device by Kwon et al. However, for the data reading processes, our proposed system is different from that by Kwon et al., and this will be discussed in the next section.

If some processes, such as nanolithography, BNNT etching or cutting, C₆₀ intercalation control, and the metal filling of electrodes, are used appropriately to fabricate aligned nanopeapods, the aligned bucky shuttle elements of separate nanocapsules can be synthesized. In this paper, we present the schematics of nanomemory devices based on the nanotube-to-nanopeapod transition. We expect that a memory element based on BN nanopeapods can be realized by nanotechnology based on nanosciences. Therefore, using classical MD simulations, we investigate the operations and the properties of a nanomemory element based on BN nanopeapods, in which data storage can be controlled by the transport of the ionized fullerenes using external force fields.