Simulation of a molecular QCA wire Azzurra Pulimeno * , Mariagrazia Graziano * , Danilo Demarchi * and Gianluca Piccinini * * Department of Electronics and Telecommunications, Politecnico di Torino, Italy Email: {azzurra.pulimeno, mariagrazia.graziano, gianluca.piccinini}@polito.it Abstract—Molecular Quantum Dot Cellular Automata (MQCA) are among the most promising emerging technologies for the expected theoretical operating frequencies (THz), the high device densities and the non-cryogenic working temperature. In this work we simulated a molecular QCA wire, based on a molecule synthesized ad-hoc for this technology. The results discussed are obtained by means of iterative steps of ab-initio calculations. I. I NTRODUCTION Among all the new emergent devices alternative to bulk MOSFET, Quantum-dot Cellular Automata (QCA) is the one that, theoretically, allows to encode binary information without current flow, reaching high operating frequency and lowering the power consumption [1]. As proposed by Lent [2], a QCA cell could be physically implemented by a molecular system with two or more redoxcenters: the charge configuration within the molecule encodes the logical state and the electrostatic repulsion provides the device-device interaction. The simplest demonstration of logic propagation that could be performed according to this paradigm is a molecular wire (see Fig. 1 (A)). Concerning the physical implementation, all the molecules proposed in literature [3], [4], [6] are ideal systems. Only few experimental attempts have been carried out on a mixed- valence complex [7], [8], even though this molecule is not suitable for real applications. In our previous works [11]–[13], we discussed the functionalities of a bis-ferrocene molecule (Fig. 1 (B)) [9], [10] as an half QCA cell. In this work we emulated a molecular wire, evaluating simultaneously the interaction between two molecules (a complete QCA cell) and the information propagation at different inter-molecule distances. II. METHODOLOGY A. The single molecule The bis-ferrocene molecule has been synthesized ad hoc for the QCA technology [9]. The structure is reported in Fig. 1 (B): the two ferrocenes represent the dots, a carbazole bridge provides the isolation between them and the thiol group allows to bind the molecule on a substrate. This molecule is very promising as candidate molecule for QCA computing for many reasons: our early results showed that its bistability properties allow to encode the digital information and that the molecule seems sensitive to a particular electric field, as write-in system [11]–[13]; this molecule is a real system that exists and has already been bonded to a gold substrate [9], [10]. In this work we considered the oxidized molecule: at the equilibrium (no external stimuli) the free positive charge is delocalized x z y Molecule 1 Molecule i-1 Molecule i Molecule n Dot1 Dot2 Dot3 Molecular Wire d Q1 Molecule i Q3 Q2 D1 D3 D2 x z y carbazole ferrocenes Dot2 Dot3 thiol 1.0 nm 1.8 nm Dot1 (A) (B) (C) Fig. 1. (A) Schematic view of a MQCA wire of bis-ferrocene molecules. (B) Bis-ferrocene molecule: two ferrocenes linked together by means of a carbazole central group; the ferrocenes represent the two dots responsible for the logic state encoding, while the carbazole acts as the central third dot. (C) Section of the wire: the aggregated charges of the (i-1)th molecule (Q1, Q2 and Q3) act as a driver for the Molecule i varying its dot charges (D1, D2 and D3). between the two main dots (the ferrocenes) and the molecule is in a neutral state. B. The molecular wire We emulated a molecular QCA wire made of bis-ferrocene molecules by means of iterative simulation steps. In particular, we forced a logic state on the first molecule of the wire (Molecule 1, as shown in Fig. 1 (A)) applying an electric field of 2 V/nm along the ferrocenes axes. Then we used the charge configuration of this molecule as stimulus for the following cell (Molecule 2). As explained in Fig. 1 (C), at a generic point of the wire we evaluated the response of Molecule i to the charge distribution of Molecule i-1, assuming that the Molecule i is in the neutral state (charge delocalized) and ready to switch [13]. We performed this analysis firstly on a wire in which the distance d between two molecules is 1.0 nm (equal brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by PORTO Publications Open Repository TOrino