Reversible, erasable, and rewritable nanorecording on an H2 rotaxane thin film.

Reversible, erasable, and rewritable nanoscale recording on organic thin films is of practical importance in ultrahigh density information storage. The scanning tunneling microscope (STM) is a powerful tool that enables nanorecording on organic films by inducing conductance transitions of the organic molecules.1-6 Rotaxane molecules have shown significant potential to be used as the building blocks for molecular electronics.7,8 Writing highconductance nanoscale marks on rotaxane H1 thin films was realized using STM in our previous studies, while, erasing of the marks showed some difficulties.9 In this Communication, we report on the reversible, erasable, and rewritable nanorecording on the rotaxane H2 Langmuir-Blodgett (LB) thin films. The dots written with ∼3 nm feature size show significant stability in air at room temperature. Figure 1a shows the structure of the H2 molecule. It is a variation of the H1 molecule.10 An H2 molecule consists of a π-electrondeficient ring cyclobis(paraquat-p-phenylene) (CBPQT4+) and a dumbbell-shaped component. The dumbbell component has two π-electron-rich recognition sites (TTF and DNP; see Figure 1a) and is terminated by bulky “stoppers”. The ring encircles part of the dumbbell, making them mechanically interlocked with each other. The ring can move back and forth between the two different π-electron-rich recognition sites in response to the external stimuli, resulting in the switching between the two stable structures. Recent theoretical calculations11-13 have revealed that the movement of the CBPQT4+ ring is accompanied by a change in molecular electronic structure. The difference between H1 and H2 molecules lies in the spacer between TTF and DNP. In H2, a rigid cyclohexyl spacer replaces the soft alkyl spacer (-(CH2)5-) in H1.10 For STM recording, the H2 rotaxane thin films with ∼20 nm thickness were prepared on highly oriented pyrolytic graphite (HOPG) substrates using the LB technique; For macroscopic I-V characterization, the films with a thickness of ∼70 nm were prepared on the indium tin oxide (ITO) coated glass substrates; And for the micro-Raman studies, the thin films were also prepared on ITO coated glass substrates. The thickness of the film is about 100 nm, with some protuberant islands of ∼400 nm in height. The micro-Raman spectra were acquired on these plateaus with an excitation wavelength of 632.8 nm. By applying voltage pulses onto the H2 thin films through the STM tip, we realized the repeatable and rewritable nanorecording (Figure 1b). Nanoscale dots can be written repeatedly with the voltage pulses (∼2.0 V, 0.1-10 ms). This case is similar to that of the H1 thin films.9 What is more interesting is that the marks written on the H2 thin films can be erased, re-recorded, and re-erased on the same site. In the whole recording process, the dots remain a size of ∼3 nm and are stable in air at room temperature for more than 12 h. The marks are directly visible in the current image in conductive contact AFM characterizations, but invisible in the topographic image [see Figure 1c], suggesting that appearance of dots are indeed due to conductance transitions of the H2 molecules. Our further studies show that the H2 films are competent for the reversible nanorecording even after 1 month of being stored in air at room temperature. To verify the origin of the conductance transitions of the H2 molecules, we performed the macroscopic I-V measurements on the H2 films using a standard I-V characterization system (Keithley, model 4200SCS). The ITO substrate served as one electrode. We used a freshly cleaved HOPG plate as another electrode, which was pressed against the H2 thin film to ensure a good contact. Figure 2 shows the I-V characteristic of the H2 film, which manifests an electrical bi-stability with a threshold of 1.4 V. The film is initially in the high-impedance state of 108 Ω‚cm, while at 1.4 V, the film abruptly switches to a low-impedance † Institute of Physics. ‡ Institute of Chemistry. Figure 1. (a) Molecular structure of the H2 molecule. (b) Frames 1-3 show bright marks written one by one using STM; frames 4-6 show the erasing, rewriting, and re-erasing on the same recording site, with Vb ) 0.8 V, It ) 0.05 nA. The voltage pulse for recording was 2 V for 3 ms, the pulse for erasing was -2 V for 3 ms. Scale bar is 6 nm. (c) Topographic (left) and current (right) AFM images of two marks written by conductive contact AFM on a 8 nm thick H2 films. Scale bar is 10 nm. Published on Web 02/02/2007