Chapter 1 Molecular Electronics: Challenges and Perspectives

Paolo Lugli, Simone Locci, Christoph Erlen, and Gyorgy Csaba

Abstract Molecular electronics has lately attracted increasing attention due to some appealing features such as possibly very higher integration capabilities, their low production cost, .exibility in the substrate choice, and possibility for large-area deployment. Two parallel approaches characterize this .eld: on one side molecules can be contacted and their transport characteristics exploited to achieve electronic functionalities; on the other side existing device structures, as well as novel ones, can be realized using organic layers instead of or together with inorganic materi-als. While in the latter case theoretical investigations on such devices can be carried out on adapting conventional simulators to the new materials and physics involved, completely new tools have to be developed in the former case. In this chapter, the operational principles of molecular systems will be presented based on a series of theoretical results obtained from our groups. Challenges and perspectives are also discussed.



Introduction

Molecular electronics has witnessed increased interest in recent years, triggered by the forecast that silicon technology might reach its scalability limits in a few years [1–4]. In order for molecular electronics to become a valuable alternative to sili-con technology, it will not be suf.cient to fabricate molecular electronic devices with outstanding characteristics, but appropriate circuit and architectural solutions will also be needed. While a lot of effort has been dedicated to the demonstration of electronic functionalities of single molecules and organic .lms, research at the circuit and system level is still in its infancy [5–9].

Investigation on single molecules or nanotube-based devices promises to keep Moore’s law alive once miniaturization of silicon-based structures becomes

P. Lugli (B)

Institute for Nanoelectronics, Technische Universit.t München, Arcistrasse 21, D80333 Munich,

Germany

e-mail: lugli@tum.de



A. Korkin et al. (eds.), Nanotechnology for Electronics, Photonics, and Renewable

Energy, Nanostructure Science and Technology, DOI 10.1007/978-1-4419-7454-9_1,



. Springer Science+Business Media, LLC 2010

impractical. As .rst proposed by Aviram and Ratner [10], one can imagine to squeeze entire nonlinear circuit elements (such as diodes or transistors) into single molecules. In principle, such devices could be signi.cantly faster and smaller than end-of-the-roadmap solid-state electron devices. Despite an enormous progress on the experimental characterization of single-molecule conduction in the last years, only few device concepts have emerged and it is still unclear whether individual molecular devices could be integrated into a larger-scale computing circuit. In any case, we can be sure that molecular circuits and architectures will be very dif-ferent from what we are used to in today’s systems. Several architectures, which would be suitable for the realization of electronic logic circuits or memory cells based on molecular devices, have been proposed. One possibility is to synthesize complex molecules whose arms can be separately contacted to provide the same electrical input/output of conventional logic gates. Such fascinating idea, suggested by Ellenbogen and Love [11], is unfortunately extremely challenging from the chemical-synthesis point of view, and it has not been realized up to now. Another possibility is to create a programmable interconnected network of nanoparticles and molecular entities (“nanocells”) [12]: conducting metallic nanoparticles are randomly deposited on a substrate (which can be a silicon one) and subsequently bridged via molecular connections, to create electrical pathways between previously patterned metallic leads. The molecular linkers should exhibit nonlinear IV charac-teristics, in the form of negative differential resistance or hysteretic behavior and the network is connected to a limited number of input/output pins at the edges of the nanocell, which could then be accessed, con.gured, and programmed from the edges. Simulations have demonstrated the capability of the nanocell to act as logic gate [12, 13].

Integration with CMOS technology will certainly be the .rst step for devices based on single molecules, with solutions that follow a road that has been called “More than Moore” [14], in an attempt to extend the standard chip functional-ities already offered by silicon technology. Such hybrid systems would bene.t from the speed and reliability of CMOS, while offering at the same time the versatility and intrinsically nanometer footprint of molecular devices. Two architec-tures which are in principle compatible with silicon technology are the “Quantum Cellular Automata” [15] and the “cross bar” [16, 17]. While the latter employs solu-tions based on standard interconnections, the former rel