Method of producing a freestanding thin film of nano-crystalline graphite

Abstract

A freestanding thin film of nano-crystalline graphite is described, as well as a method of producing a freestanding thin film of nano-crystalline graphite including: providing a freestanding thin film of amorphous carbon, heating the freestanding thin film to a high temperature in an inert atmosphere or in a vacuum; and allowing the freestanding thin film to cool down,
as a result of which a freestanding thin film of nano-crystalline graphite is formed. The films can be used, for example, as phase plates in a Transmission Electron Microscope.

Claims

1. A method of producing a freestanding thin film of nano-crystalline graphite, the method comprising the steps of: providing a freestanding thin film of amorphous carbon, locally heating the freestanding thin film to a high temperature in an inert atmosphere or in a vacuum, allowing the freestanding thin film to cool down; and as a result of which a freestanding thin film of nano-crystalline graphite is formed.

2. The method of claim 1 in which the local heating is performed by irradiating the freestanding thin film with a laser beam.

3. The method of claim 2 in which the wavelength of the laser, the power of the laser, the size of the irradiated area and the thickness of the thin film are such that the free-standing thin film absorbs locally between 0.1 MW/m.sup.2 and 20 MW/m.sup.2.

4. The method of claim 1 in which during local heating the temperature of the freestanding thin film rises locally to between 1000 K and 3700 K.

5. The method of claim 1 in which the freestanding thin film is locally heated for at least 1 second.

6. The method of claim 1 in which the freestanding thin film has a thickness of less than 1 m.

7. The method of claim 1 in which the freestanding thin film is allowed to cool down in an environment of less than 500 K.

8. The method of claim 1 in which the freestanding thin film is allowed to cool down to a temperature below 1000 K in less than 1 ms.

9. The method of claim 1 in which the freestanding thin film is supported by a TEM grid.

10. The method of claim 1, the method further comprising the additional step of forming one or more holes in the film for passing beams of undiffracted or diffracted electrons, as a result of which a phase plate or phase mask for a transmission electron microscope is formed.

11. A phase plate for a transmission electron microscope, the phase plate comprising or made of a freestanding thin film of nano-crystalline graphite, wherein the freestanding thin film of nano-crystalline graphite comprises a multiplicity of layers.

12. A transmission electron microscope comprising the phase plate of claim 11.

13. The method of claim 10, wherein the one or more holes are formed by a method selected from the group of ion beam milling, gas-assisted electron beam etching, and laser beam irradiation.

14. The method of claim 1, wherein, after processing, the presence of a diffraction ring at 0.334 nm due to stacked layers, characteristic of normal graphitic carbon, is substantially reduced or eliminated.

15. The method of claim 1, wherein the film is cleaned before processing by pre-heating in a vacuum, preferably to a temperature of approximately 250 C.

16. The phase plate of claim 11, wherein the crystal size in the nano-crystalline graphite is less than 100 nm.

17. A method of observing a sample in a transmission electron microscope, the method comprising: forming a freestanding nano-crystalline graphite film in accordance with claim 1; and directing a portion of an electron beam through the nano-crystalline graphite film, the nano-crystalline graphite film shifting the phase of the portion of the electron beam, wherein the freestanding thin film of nano-crystalline graphite comprises a multiplicity of layers.

18. The method of claim 17, in which directing a portion of an electron beam through the nano-crystalline graphite film comprises directing a portion of an electron beam through the nano-crystalline graphite film in a diffraction plane of the transmission microscope, or an image of the diffraction plane.

19. A phase plate for a transmission electron microscope made in accordance with the method of claim 1, wherein the freestanding thin film of nano-crystalline graphite comprises a multiplicity of layers.

20. The phase plate of claim 11 further comprising one or more holes in the free stranding film of nano-crystalline graphite for passing beams of undiffracted electrons.

21. The phase plate of claim 19 wherein the freestanding thin film of nano-crystalline graphite has a thickness of between about 19.9 nm and 30.8 nm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is now elucidated using figures, in which identical reference numerals refer to corresponding features.

(2) To that end:

(3) FIG. 1 schematically shows a part of a transmission electron microscope,

(4) FIG. 2 schematically shows a Zernike phase plate,

(5) FIG. 3 schematically shows a layer of nano-crystalline graphite,

(6) FIG. 4A shows the diffraction pattern of a thin film of amorphous carbon,

(7) FIG. 4B shows the diffraction pattern of a treated thin film,

(8) FIG. 5A shows an amorphous carbon film with footprints, and

(9) FIG. 5B shows a laser treated film where footprints are absent.

DETAILED DESCRIPTION OF SAMPLE EMBODIMENTS

(10) FIG. 1 schematically shows a part of a transmission electron microscope.

(11) A parallel beam 102 of electrons with a selectable energy of, for example, between 40 and 400 keV, travelling along an optical axis 100. The beam of electrons is formed by an electron source (not shown) and turned in a parallel beam by condenser optics (not shown). The condenser optics can vary the diameter of the beam. The beam then impinges on the sample 104, that is positioned in the objective lens 108 by positioning unit 106. Due to the wave nature of electrons part of the electrons are scattered by the sample in beams 112A and 112B. These beams, and the central beam of unscattered electrons is focused in the diffraction plane 114. In the diffraction plane thus a Fourier representation of the sample is available. The diffraction plane is imaged by the diffraction lens 110, forming an enlarged image of the sample in image plane 116.

(12) FIG. 2 schematically shows a Zernike phase plate.

(13) The Zernike phase plate consists of a thin film 200 of a material with a small hole. The diameter of the hole is typically 1 m or less, and the material typically carbon. The wavelength of the electrons while passing through the carbon is shorter than when passing through vacuum. As a result the beams of electrons 112A and 112B, after passing through the carbon film, are phase shifted compared to the beam 102 passing through the hole. When now a beam of hardly scattered electrons (beam 112A and 112B close to the hole) are made to interfere in the image plane 116 with the beam of unscattered electrons 102, the contrast transfer function is high, opposed to the situation that would occur when the phase shift was not introduced: then the contrast transfer function would be zero, assuming there are no optical aberrations in the system.

(14) It is noted that the phase plate is positioned in the diffraction plane on a high precision holder. Alternatively the phase plate is not placed in the diffraction plane, but in an image of that plane, preferably an enlarged image, thereby making it easier to position the phase plate.

(15) FIG. 3 schematically shows a layer of nano-crystalline graphite.

(16) The lattice of NCG is built from hexagons of carbon atoms, the atoms here represented as dots. The atoms are in one plane (graphite has many planes stacked on top of each other). The most important lattice constants are d.sub.1=0.213 nm and d.sub.2=0.123 nm. The smaller distance of d.sub.1 results in a smaller circle 402 in the diffraction pattern shown in FIGS. 4A and 4B, while the smaller distance of d.sub.2 results in the larger circle 404 in the diffraction pattern shown in FIGS. 4A and 4B.

(17) It is noted that normal graphitic carbon also shows substantial peaks/rings at 0.334 nm due to the spacing between various stacked layers. Apparently this 3D layering is less prominently present or even completely absent in NCG: the absence of the associated ring in the diffractograms of FIG. 4B is proof that there is no stacking of carbon films on top of each other, as occurs in graphite.

(18) It is noted that in Diamond-like amorphous carbon, J. Robertson, Materials Science and Engineering R37, 129-281 (2002) [-5-], specifically at page 154, it is mentioned that the 0.334 nm ring is related to inter-layer scattering, and this is only seen if there is a clear layer ordering in a graphitic structure. Note that Roberson uses structure factor S(k)=(2)/d, and therefore 1.9/Angstrom (19/nm) corresponds to 3.3 Angstrom (0.33 nm). From this it can be understood that the absence of this ring implies that no such ordering is present.

(19) FIG. 4A shows the diffraction pattern of an amorphous thin film. There is a large central spot 400, representing the unscattered and slightly scattered electrons. A diffuse ring 402 represents electrons that are scattered at structures with a mutual distance of approximately 0.22 nm. Ring 402 is not sharply defined, indicating that there is a relative large variation on the mutual distance of these structures, as can be expected for amorphous material: there is no or little order. That the ring does not show points or arcs with larger intensity implies that there is no rotational preference of the scattering, another feature of amorphous material. Ring 404 represents the smaller lattice distance d.sub.2 of approximately 0.122 nm. Object 406 is (the image of) the intercept needle blocking the central beam so as to avoid over-illumination and/or damage to the image sensor.

(20) FIG. 4B shows the diffraction pattern of the thin film after treatment.

(21) The rings are much better defined. Careful analysis of the diffraction pattern showed rings corresponding to lattice distances of 0.211 nm (100%), 0.122 nm (50%), 0.107 nm (7%), and peaks at 0.081, 0.071 and 0.062 nm (all at less than 5%).

(22) FIG. 5A shows an amorphous carbon film with footprints.

(23) FIG. 5A shows an amorphous carbon film 500 with perforations 502. On the film a beam of electrons has impinged, resulting in footprints 504. These footprints are believed to result from a (temporary) change in electronic structure in the carbon film, and disappear in approximately 1 day.

(24) FIG. 5B shows a laser treated carbon film.

(25) The film shown in FIG. 5B is in origin identical to the film shown in FIG. 5A, but this film is laser treated. No footprints are visible. However, the thermal treatment induced some mechanical deformation.

(26) Compared to the diffraction pattern of FIG. 4A of amorphous carbon the rings and central spot are much better defined. The rings 402 and 404 do not show any granularity, proving that many crystals are present with a random orientation with respect to each other. Experiments showed that this is the case even for a beam diameter of 100 nm, even for a beam diameter as small as 100 nm.

(27) Experiments were made in which an amorphous carbon film with a thickness of 20 nm was placed in the evacuated sample chamber of a Scanning Electron Microscope. Such a foil was first inspected in a TEM, and the diffraction pattern shown in FIG. 4A was thus recorded.

(28) The foil was heated with a laser beam having a power of 10 mW and a wave length of 785 nm heating a spot with a diameter of between 25 m to 100 m. It is estimated that, at these conditions and film thickness, the foil absorbed 10% of this power, resulting in an absorbed power density of between 0.78 MW/m.sup.2 (spot diameter 100 m) and 12 MW/m.sup.2 (spot diameter 25 m). After heating the foil for several seconds, the laser beam was turned off and the foil was allowed to cool down in the sample chamber (the chamber at room temperature) to room temperature. Afterwards the condition of the foil was inspected in a TEM, and the diffraction pattern of FIG. 4B was recorded.

(29) It is noted that preferably the film is cleaned before heating, as a film that is not heated is contaminated. A carbon contamination with carbon in another structure than amorphous carbon may results in a film with varying thickness, and it is suggested that a non-amorphous carbon film is also transformed into non-NCG carbon film. It is believed that a focused laser beam will also crack mobile contaminants on the film surface, causing a large build-up of some kind of amorphous carbon (but not of the type wanted), and also leading a large local change in thickness. It appears that heating to 250 C. (the temperature of the heating holder) is optimal. Heating is thus preferably performed by pre-heating in vacuum to a temperature of approximately 250 C.

(30) It is noted that plasma cleaning is possible, but can easily change the thickness of the film through etching. This makes plasma cleaning a less preferred option for cleaning, but a candidate for fine-tuning the thickness of the thin film.

(31) It is noted that during experiments a phase plate of NCG is used for an extended period of time at a temperature of slightly more than 500K (250 C.) without losing the nano-crystalline form taking place, or any other deterioration. Therefore it is believed that a cooling down (quenching) of the film from 2000K to 2500K to 1000K within 0.25 ms is sufficiently fast. This can be achieved by turning off the heating laser and allowing the film to cool down in an environment at room temperature, or slightly above (for example less than 500 K).

CITED NON-PATENT LITERATURE

(32) [-1-] Phase Contrast Enhancement with Phase Plates in Biological Electron Microscopy, K. Nagayama et al., Microscopy Today, Vol. 18 No. 4 (July 2010), pp. 10-13. [-2-] Internet brochure http://www.emgrid.com.au/pdf/ACF-Metals-Products.pdf, created Aug. 14, 2007, Arizona Carbon Foil Co., Inc, Tucson, Ariz., USA more specifically paragraph 1, most specifically paragraph 1.1. [-3-] Page of internet sales brochure http://www.agarscientific.com/ultra-smooth-carbon-foils.html, Agar Scientific, Stansted, Essex, CM24 8GF, United Kingdom. [-4-] Electron Vortex Beams with High Quanta of Orbital Angular Momentum, B. J. McMorran et al., Science 331, 192 (2011), pp. 192-195. [-5-] Diamond-like amorphous carbon, J. Robertson, Materials Science and Engineering R37, 129-281 (2002).