The Stem Group, Condensed Matter Division, ORNL
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REVERSIBLE, NANOMETER-SCALE CONDUCTANCE TRANSITIONS IN AN ORGANIC COMPLEX1

H. J. Gao,1,2 K. Sohlberg,1 Z. Q. Xue,2 H.Y. Chen,2 S. M. Hou,2 L. P. Ma,2 X.W. Fang,3 S. J. Pang,2 S. J. Pennycook1

Physical Review Letters, 84, 1780 (2000)
and Physical Review Focus,
http://focus.aps.org/v5/st7.html

         The possibility of using a scanning tunneling microscope (STM) for high density, erasable data storage has attracted much attention in recent years.  Epitaxial ferroelectric thin films are attractive because of their non-volatility, but are limited to bit densities of ~ 1010 bits/cm2 because of the finite size of the domains.  We have demonstrated erasable data storage using an organic charge transfer molecular complex.  Such materials have aroused much attention for their novel structures, properties and potential application to electronic devices and molecular computers.  They also have the significant advantage of not requiring an epitaxial technology. 

         A crystalline thin film of two conjugated organic compounds, 3-nitrobenzal malonitrile (NBMN) and 1,4-phenylenediamine (pDA) was evaporated onto highly oriented pyrolytic graphite.  A typical image of the film is shown in Fig. 1(a), where the dimensions match the unit cell determined by X-ray diffraction.  In the as-deposited state, the films show high resistivity, but application of a voltage pulse from the STM tip induced a local conductance transition to a low resistivity state, as demonstrated by a marked change in local current voltage characteristics.  This enabled the transformed regions to be imaged in constant height mode, as shown in (b).  Furthermore, application of a reverse polarity voltage pulse for a longer time enabled the marks to be individually erased, as shown in (d) and (e).  The resolution approached the size of the molecular complex, ~ 1 nm, as demonstrated in (f), where two marks are separated by ~ 1.7 nm.  The mechanism of the writing and erasing was shown to involve a change in phase from a crystalline (insulating) phase to an amorphous (conducting) phase. The NBMN-pDA system has a permanent electric dipole, so that application of an electric field pulse would lead to molecular reorientation and disorder. The reverse polarity would tend to reverse the reorientation, while longer times allow local heating and recrystalization.

         The results demonstrated her correspond to a data density 1014 bits/cm2, about one million times the density of a typical CD-ROM and sufficient to store the Library of Congress on a single disc.  However, substantial engineering hurdles remain concerning data lifetime and writing speed. 


Figure 1 

Figure 1: STM images of the NBMN-pDA film on HOPG. (a) An image of the surface of the film showing crystalline order, image size 6 nm x 3 6 nm; (b) a 3 x 3 3 nm array formed by voltage pulses of 4 V, 1 ms; (c) an "A" pattern formed by pulses of 3.5 V, 2 ms; (d ) and (e) STM images after erasing marks one at a time with reverse-polarity voltage pulses of –4.5 V, 50 ms; (f ) resolution test using pulses of 4.2 V, 10 ms. The distance between neighboring marks is 1.7 nm. Scan conditions are bias voltage 0.1 V, tip current 0.4 nA for (a); and 0.19 V, 0.19 nA for all others, constant height mode.

                                                     

1.           Solid State Division, ORNL

2.           Beijing Laboratory of Vacuum Physics, Chinese Academy of Sciences, Beijing, China

3.           University of Chicago, Chicago, Illinois

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