IE911565A1 - Time-stable labeling of individual atoms or groups of atoms¹in the surface of a solid, and the storage of information¹units in the atomic range - Google Patents

Abstract

The invention relates to a method for the temporally stable marking of individual atoms or atom groups of a solid surface by a variable structural or electronic configuration as well as utilisation of this method for storing information.

Description

-1IE 911565 - laO.Z. 0050/41616 Time-stable labeling of individual atoms or groups of atoms in the surface of a solid, and the storage of information units in the atomic range The present invention relates to a process for the time-stable labeling of individual atoms or atomic groups in the surface of a solid by means of a modified structural or electronic configuration, and the use of this process for information storage. he storage of information, in particular image 10 and data signals, currently takes place predominantly using magnetic or optical recording media. The information density which can be achieved using the latter is determined by the smallest information units which can be written and read again by the process. In conventional magnetic storage media, these units are determined by the size of the magnetic domains (Weifl regions), from a mechanical point of view by the head gap of the read/write heads used and by the distance of the read/write unit from the actual information carrier. In information carriers where the stored information is produced by a change in optical properties, the limit is the wavelength of the light used. The smallest information units here can thus not be smaller than about half the wavelength of the light. An increase in storage density in optical recording carriers of this type has in the meantime also been achieved through optical nearfield microscopy, where the optical read unit is only a few nanometers above the information-carrying surface. The best information densities achieved here are in the order of about 20 nm.

A further increase in the information density is only possible by using close-field techniques with a resolution in the subnanometer range. Suitable methods for this purpose are scanning probe techniques, including the scanning tunneling microscope and the atomic force microscope. These methods allow imaging of surfaces on an atomic scale. It has therefore been proposed to produce O.Z. 0050/41616 information storage media having the highest possible density, namely in the range of individual atoms or molecules. Success in developing these media would result in information densities in the terabyte/cm2 range.

A number of proposals have been made for storing information in the nanometer range on inorganic or organic surfaces, including M.A. McCord et al., J. Vac. Sci. Technol. B4, (1986), 86-88, R.M. Silver et al., Appl. Phys. Lett. 51 (1987), 247-249 and U. Staufer et al., J. Vac. Sci. Technol. A6 (1988), 537-539. The deposition of individual atoms has also been reported (R.S. Becker et al., Nature 325 (1987), 415-421).

However, all the proposals hitherto for the provision of maximum-resolution information storage media which also have, in particular, long-term stability are unsatisfactory. Whereas organic storage media are incapable of producing line widths of <10 nm, inorganic systems, which can reproduce structures down to 3 nm, are unstable over relatively long periods, ie. from minutes to hours. In the case of the long-term stable structures in silicon which have previously been disclosed (Van Loenen et al., Appl. Phys. Lett. 55 (1989), 1312-1314), by contrast, the atomic structure is locally destroyed, ie. ths^j^omic order is lost. A process of this type is therefore· only suitable for producing non-erasable storage media.

It is therefore an object of the present invention to provide a process for the stable labeling of atoms or atomic groups which facilitates, in particular, stable storage of information without the destruction of the local atomic lattice.

We have found that this object is achieved by a process foff the stable labeling of atoms or groups of atoms in the surface of a solid when individual atoms or groups of atoms in the surface are converted into a configuration which is modified compared with the adjacent atoms without significantly modifying the atomic lattice - 3 - O.Z. 0050/41616 structure parallel to the surface and without the participation of foreign atoms.

The process according to the invention can be carried out, in particular, by effecting the structural or electronic change in configuration in the surface of a semiconducting laminate material by applying an external electrical or magnetic field for a limited time and over a limited area.

In the process according to the invention, the 10 modification in configuration is particularly advantageously achieved by producing the local geometric, structural or electronic reconfiguration by generating metastable, excited states in a double- or multiwell potential.

The process according to the invention for stable labeling of individual atoms or groups of atoms can be particularly advantageously employed for storing information units. This provides a way of storing information in the atomic range and thus achieving a correspondingly high information density. However, the process according to the invention can be used not only for information storage, but also for erasing stored information. Thus, information units stored by the process according to the invention can be erased again through relaxation by supplying energy, thus restoring the original state. For this purpose, the supply of thermal energy by heating the entire surface or by laser treatment of the entire surface or of points or exposing the surface to light is particularly suitable.

The process according to the invention proceeds from the surface of a solid, in particular of a layered semiconductor, usually comprising a chalcogenide, eg. WSe2. The atomic labeling is carried out in the surface of a substance of this type using the near-field technique, eg. by means of a needle-shaped electrode of a surfacesensitive scanning probe, for example a scanning tunneling microscope or a scanning atomic force microscope, by - 4 - O.Z. 0050/41616 applying a short-duration electrical or magnetic field. Since the area of the maximum electrical field of a scanning probe of this type is preferably from 10 nm to 0.1 nm on the surface of the semiconducting layered material, the atoms or atomic groups can thus be affected. This local supply of energy means that the atoms or atomic groups affected are apparently raised into a stable configuration above the mean level of the surrounding atoms. An essential feature here is that the procedure can be carried out under normal ambient conditions, ie., for example, in air and at room temperature.

The aim of erasing the stored information again can be achieved , for example, by thermal treatment of the surface, as is possible, inter alia, by laser ir15 radiation. The atoms or atomic groups apparently raised above the mean level of the surrounding atoms are thus arranged back into the previous structure.

The near-field technique used for writing the information can be a conventional scanning tunneling microscopy or atomic force microscopy process. The arrangement of these near-field techniques for characterizing surfaces is known and has been described (Y. Kuk et al., Rev. Sci. Instrum. 60(2) (1989), 165-180).

The process according to the invention is des25 cribed in illustrative terms below: The surface of a tungsten diselenide sample was first imaged with atomic resolution using a scanning tunneling microscope (STM). During the scan of the tunneling tip over the sample, voltage pulses having an amplitude of from 0.8 to 10 volts were then applied, superimposed on the tunneling voltage, between the tunneling tip and the sample by means of a pulse generator. Subsequent scanning of the sample gives structures on the surface with an extension increasing with the level of the voltage pulses.

The switching of individual atoms or atomic groups is shown in Fig. 2; Fig. 1 (image size O.Z. 0050/41616 approximately 100 x 100 A) shows the STM image of a grown tungsten diselenide surface, and Fig. 2 (image size approximately 300 x 300 A) shows the image of the same surface immediately after application of approximately 1.5 volt pulses. Groups comprising three atoms were specifically modified here, the relative position corresponding exactly to the positions of the tunneling tip at the time of the pulse. It was in this way possible to write patterns onto the sample; each voltage pulse labels, in a defined position, atoms for which the spatial or electronic configuration has been modified.

There was no problem in writing more than 100 structures using the same tip and subsequently imaging these with atomic resolution. There were normally no variations in imaging quality of the tip as a consequence of the pulsing.

Both writing and reading can be carried out under normal conditions, ie., in particular, without using an inert gas, vacuum or low temperatures.

In addition, it was also possible to show, by means of further experiments, that the use of a high vacuum has no observable effect on the writing or reading operation.

In order to test the stability of the labels with time, certain arrangements of labeled points, for example in triangles, in squares or as parallelograms, were written specifically, the resultant structures were imaged, and their relative positions to one another were recorded; the structures were found again after two days in unchanged shape and in the same arrangement. In particular, it was also possible to show that they are stable not only in vacuo or under an inert gas, but also in air.

For application as a method for information storage, it is essential that the reading operation does not modify the stored information. To this end, both large and small structures (Fig. 2) on various tungsten - 6 - O.Z. 0050/41616 diselenide samples were scanned using the STM over several hours, during which time they were imaged up to 500 times. In no case was a modification due to the imaging process (= reading process when used as a data carrier) observed.

Structures on tungsten diselenide surfaces were most easily erased by heating the sample surface at about 600°C for about 40 minutes. Surfaces covered with structures, but still ordered on an atomic level become completely flat and structureless again on an atomic level due to this operation.

For use as an information storage device, the functions reading, writing and erasing have thus been demonstrated and the processes used to achieve them have been indicated; for the structures generated in Fig. 2, a data density of about 10 terabytes/cm2 is produced, exceeding the storage density of magnetic hard disks or magnetooptical disks by several orders of magnitude.

Claims (7)

1. 1. We claim: 1. A process for the time-stable labeling of individual atoms or groups of atoms in the surface of a solid, which comprises converting individual atoms or groups of atoms in the surface into a structural or electronic configuration which is modified relative to the initial state without significantly modifying the atomic lattice structure parallel to the surface and without the participation of foreign atoms.

2. A process as claimed in claim 1, wherein the structural or electronic change in configuration is achieved by applying an external electrical or magnetic field for a limited time and over a limited area.

3. A process as claimed in claim 1, wherein the structural or electronic change in configuration is achieved by applying an external electrical or magnetic field for a limited time and over a limited area, which limited duration and area are generated by the tip of a surface-sensitive scanning probe.

4. A process as claimed in claim 1 or 2 or 3, wherein the surface employed is a semiconducting layered material.

5. A method of using a process for the stable labeling of individual atoms or atomic groups as claimed in claim 1 for storing information units in the atomic range.

6. A process according to claim 1 for the timestable labeling of individual atoms or groups of atoms in the surface of a solid, substantially as hereinbefore described.

7. A time-stable labelled individual atom or group of atoms in the surface of a solid whenever obtained by a process claimed in any one of claims 1-4 or 6.