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A new type of memory storage on the horizon

memory storage

For those of us old enough to remember the days of the Apple II, you know that storage has exponentially increased. Even just 10 years ago 20+ gigs of data seemed huge, now my cellphone has 64 gigs. Yet we still need more data storage and we are looking for new ways to get it. Now a way to use weak molecular bonding interactions to create well-ordered and stable metal–organic monolayers with optoelectronic properties has been found. The development could form the basis for the scalable fabrication of molecular optoelectronic devices.

A variety of emerging technologies are being investigated as potential replacements or enhancements of the electrical-charge-based electronics that lie at the heart of all electronic devices. Utilizing interactions between light and charge—referred to as optoelectronics—is of particular interest to researchers and engineers. Organic molecules that change state reversibly in response to pulses of light could, for example, be used to build versatile optoelectronic memory devices with ultrahigh storage capacities.

For individual organic molecules to be used as single digital ‘bits’ in such devices, the molecules need to be arranged into highly ordered, single-molecule layers, bonded to a metal surface. However, as bonding to a metal surface also affects optical properties, preparing ordered arrays of organic molecules with the desired optoelectronic characteristics has proved challenging.

The team developed a scheme for laying ordered monolayers of diarylethene derivatives on a copper substrate. Diarylethenes have well-established photochromic properties, undergoing reversible color changes when irradiated with light. Crucially, the diarylethene derivative used by the team has an electric dipole, this means that the distribution of electric charge in the molecule causes one end to be slightly negative and the other to be slightly positive (think magnet).

When the diarylethene derivatives are deposited onto a copper surface in the presence of sodium ions, the interaction between the ions and organic molecules results in self-organization of the molecules into a precisely ordered array in which the diarylethene derivative molecules are lined up in neat, tightly packed rows.

Scanning tunneling microscopy image (background) of the ordered array of diarylethene derivative molecules. A schematic of the molecular structure is shown in the foreground. Image credit goes to: Reproduced, with permission, from ref. 1 © 2014 Wiley

Scanning tunneling microscopy image (background) of the ordered array of diarylethene derivative molecules. A schematic of the molecular structure is shown in the foreground. Image credit goes to: Reproduced, with permission, from ref. 1 © 2014 Wiley

Application of a chemical ‘annealing’ process promotes a further subtle rearrangement of each molecule into a more stable configuration on the metal surface that reinforces the conductive ‘on’ state. The use of a copper substrate with a particular crystal arrangement, known as Cu(111), also facilitates precise arrangement of the organic molecules. The arrangement 111 is just how the molecules are organized in the plane, sort of like this example on the far right labeled FCC (111) where FCC is face centered cubic.

“With homogeneous and close placing of individual molecules on a solid surface, we might be able to develop a memory device with several hundred to a thousand times the density achievable using current technology,” says Tomoko Shimizu, MD, lead author of the study.

“We now want to study ways to achieve on–off switching of individual molecules in the superstructure in a controlled manner.”

It will be interesting to see in 10 years just how much more memory storage we will have, of course there is a limit to how much data can be stored in a particular amount of space, we just haven’t gotten anywhere close to that yet.

Sources
Shimizu, T., Jung, J., Imada, H., & Kim, Y. (2014). Supramolecular Assembly through Interactions between Molecular Dipoles and Alkali Metal Ions Angewandte Chemie International Edition, 53 (50), 13729-13733 DOI: 10.1002/anie.201407555

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