Characterization of Molecular QCA Cells by Q-Controlled Enhanced Amplitude Modulation Atomic Force Microscopy

Abstract

Major technical challenges for reduction of device sizes for computation and memory are the interconnection and power dissipation problems. The molecular quantum-dot cellular automata (QCA) approach offers an attractive chemistry-based alternative in which binary information is encoded in the configuration of charge among redox-active molecular sites [1]. Since the Coulomb interaction between neighboring molecules provides element-to-element communication, no current flow between molecules is required for device operation. Theoretical studies have shown that the electric fields associated with flipping a single molecular dipole are sufficient to drive neighboring dipoles. Power dissipation can be greatly reduced and true power gain is possible. Silicon phthalocyanine (SiPc) is a possible candidate for a molecular QCA element. The formation of oriented, self-assembled monolayers (SAMs) of SiPc on gold surfaces has been demonstrated and studied by XPS and SERS [2]. The measurement of surface electric charge distribution of individual molecules can be performed by scanning surface potential microscopy (SSPM) if the molecule of interest can be first resolved by atomic force microscopy (AFM) [3].We report an investigation of the imaging mechanisms and interaction forces between the tip and SiPc dimer by quality factor control (Q-control) enhanced amplitude modulation AFM. SiPc dimers were synthesized and characterized by NMR and derivatization. This dimer is expected to be stable and has a molecular weight of 2510. The SiPc dimers were oriented and immobilized on Au{111} by the sulfur-gold bond. The Au{111} thin film was epitaxially grown on a mica substrate at 300 °C. The AFM was operated in the amplitude modulation mode (tapping mode) at 0% relative humidity in an ultrapure nitrogen environment either with or without Q-control. A silicon cantilever with resonance frequency 263.42 kHz and quality factor 1544 (Q-control off) was used to image the SiPc dimers, as shown in FIG. 1. The average height of the SiPc dimmer is ~ 3.4 nm and the width is ~20.4 nm. When the quality factor was increased to 5403, as shown in FIG. 2, the average height increased to ~6.5 nm and the width is ~26.9 nm. The number of SiPc dimers per unit area observed by AFM also increased with increasing the Q factor. Theoretical simulations using the point-mass description of the AFM, demonstrated that Q enhancement reduced the force exerted from the tip to the sample surface [4]. Our results, consistent with theory, demonstrated that the image force was reduced and a greater height was measured. However, a further increase of the Q factor above 5403 resulted in false engagement of the AFM. This may be due to the fact that capillary forces are greater than van der Waals forces at the greater tip-sample distance when the AFM is operated in as extremely high Q noncontact state [5].