WO2015197047A1

WO2015197047A1 - Atomic carbon source

Info

Publication number: WO2015197047A1
Application number: PCT/DE2015/000324
Authority: WO
Inventors: Friedrich HUISKEN; Sergiy KRASNOKUTSKIY
Original assignee: Friedrich-Schiller-Universität Jena
Priority date: 2014-06-26
Filing date: 2015-06-24
Publication date: 2015-12-30
Also published as: DE102014009755A1

Abstract

The aim of the invention is to provide a robust, stable, and efficient source for generating atomic carbon which is as pure as possible and is largely free of carbon clusters and other impurities. According to the invention, the atomic carbon source contains a heating chamber (2) which is arranged in a vacuum (5) and which is connected to an energy source (1) for receiving a carbon-containing substance (4). At least one sub-region (3) of the heating chamber (2) consists of a thin-walled material which does not evaporate at the operating temperature of the heated heating chamber (2) and which receives carbon atoms released from the carbon-containing substance (4) when the interior of the heating chamber (2) is being heated and releases said carbon atoms largely only in the form of a gas (8) into the surrounding vacuum (5).

Description

Atomic carbon source

The invention relates to a simple, stable and clean source for producing carbon atoms.

The new carbon source can be used in many ways. So can be z. B. form a bundled carbon atom beam with the help of skimmers and diaphragms, which can be used for molecular beam epitaxy. Even without bundling, substrates with carbon atoms can be vaporized over a large area (for example, for the production of diamond-like layers and two-dimensional graphene structures). The carbon atoms generated by the proposed source are also suitable for inclusion in helium droplets, for example, to study reactions of single carbon atoms with other species at ultra-low temperatures.

Further fields of application concern the physico-chemical process engineering, which uses methods of chemical vapor deposition and molecular beam epitaxy, environmental chemistry (reactions of carbon in the gas phase, eg in

Combustion processes), nanotechnology (production of nanodiamonds and diamond-like thin films) and nanoelectronics (production of graphene for novel electronic applications). Likewise, this source can be used for the production of larger diamonds and diamond windows.

Essentially, there are currently two methods for producing carbon in the gas phase:

The methodologically simplest method is based on the evaporation of graphite powder, for example from a crucible (as KR Thompson, RL DeKock and W. Weltner: Spectroscopy of carbon molecules, J. Am. Chem. Soc., 93, 1971, 4688) or graphitic Solid bodies (eg: FM Wachi and DE Gilmartin: High-temperature mass spectrometry - 1. Free vaporization studies of graphites, Carbon 8, 1970, 141) by application of high temperatures. In this case, the heat supply by indirect heating (especially in an oven), by direct ohmic heating or by local

Irradiation with photons (laser evaporation, for example: JP S593016 A, R. I. Kaiser,

H. Hahndorf, LCL Huang, YT Lee, HF Bettinger, PV Schleyer, HF Schaefer and PR Schreiner: Crossed beam reaction of atomic carbon, C ( ³ P _j ), with d ₆ -benzene, C ₆ D ₆ (X'Ai _g ): Observation of the per-deutero-1,2-didehydrocycloheptatrienyl radical, C ₇ D ₅ (XB ² B ₂ ), J. Chem. Phys. 110, 1999, 6091), electrons or other energetic particles.

The disadvantage of these methods is that only very little carbon is transferred into the gas phase in atomic form; most of the evaporated carbon is present in molecular form as C ₂ and C ₃ molecules (clusters) and as larger carbon clusters. This condensed form of carbon is less reactive and tends to negatively impact all applications. Another disadvantage associated with the use of high-energy methods is the generation of electronically excited and ionized carbon atoms, which can also adversely affect subsequent processes.

The second basic method is based on the use of gaseous precursor or precursor molecules (preferably hydrocarbons, eg, CH ₄ ), which are characterized by

Energy supply are dissociated, so that the atomic carbon is released (US 8,617,669 Bl, US 6,269.110 Bl, JP H06139560 A, JP S598608 A, G. Dorthe, P. Caubet, T. Vias, B. Barrere, and J. Marchais: Fast flow studies of atomic carbon kinetics at room-temperature, J. Phys. Chem. 95, 1991, 5109).

The disadvantage of this method is that the carbon atoms do not arise alone but are contaminated by the other constituents of the precursor molecules. In addition, the necessary supply of energy often gives the carbon atoms quite high kinetic energies, which is also undesirable for many applications.

A particular form of the former method is described in the article by Thompson and coworkers (K.R. Thompson, R.L. DeKock and W. Weltner: Spectroscopy of carbon molecules, J. Am. Chem. Soc., 93, 1971, 4688). These use a special tantalum cylinder and fill it with a carbon sample. Subsequently, the assembly is heated with a few hundred amperes for one hour, so that the tantalum cylinder is converted into carbide. In a further step, carbon is then removed from this carbide cylinder at very high temperatures. This two-tiered

Approach is imperative because the cylinder otherwise at high

Evaporation temperatures would be structurally destroyed if it was not sufficiently carbidized; or it must by a sufficiently large carbon sample ensure that sufficient carbide is always present. Major disadvantages of this arrangement, in addition to the high cost (time and energy), a strong contamination of the evaporated carbon by carbon monoxide and tantalum, which could not be completely eliminated even after much effort. Although it succeeds C0 ₂ and H ₂ 0 by fast pumping at least largely from the

To eliminate vapor mixture, but the contamination by carbon clusters as well as the tantalum evaporation remain highly disadvantageous. In addition, the

Carbon source can not be used because the Tantalzylinder consumed by the said evaporation of the material and thus the heating is interrupted after a short time.

In summary, it should be noted that the experts are not aware of any sources with which atomic carbon can be produced in pure form and with reasonable effort.

The invention has for its object to provide a robust, stable and efficient source for generating as pure as possible and carbon clusters and other

Contaminants to create largely unloaded atomic carbon.

The task is solved by an atomic carbon source, which consists of a

Energy source, a heating chamber in communication with this energy source with at least a portion of a thin-walled (wall thickness <0.05 mm) and at the operating temperature of the heated heating chamber non-evaporating material,

preferably of tantalum or tungsten. Inside the heating chamber is carbonaceous material, e.g. Graphite powder. With the aid of the energy source, the sealed heating chamber is heated with the carbonaceous material therein, e.g. by ohmic or inductive heating, and brought to a temperature of 2000 ° C and more, so that the carbonaceous material in the heating chamber below

Development of steam consisting of carbon atoms and clusters sublimated.

The operating temperature has the function to increase the carbon solubility of the thin-walled portion of the heating chamber, with the result that carbon atoms of penetrate into the material inside and by diffusion to the outer surface of the thin-walled portion reach, from where they evaporate into a vacuum.

The evaporation rate depends on the vapor pressure of the carbon atoms, the solubility of the carbon atoms in the material, and the diffusion rate through the thin-walled portion of the heating chamber and is thus essentially determined by the temperature and the wall thickness of this subregion. The carbon clusters (ie C ₂ and C ₃ molecules) present in the interior of the heating chamber can not penetrate as such into the thin-walled subregion of the heating chamber. Instead, they are dissociated on the inner surface of said portion with catalytic assistance so that they are subsequently dissolved as carbon atoms in the material. Thus, the carbon gas emitted by the thin-walled portion is purely atomic in nature.

At the operating temperature of about 2150 ° C, the thin-walled part remains

(for example, from tantalum) still stable, so that, first, the source is preserved

(stable, solid carbon source) and secondly none

Evaporating constituents contribute to the contamination of emerging in the vacuum from the heating chamber carbon atoms.

The thinness of the subregion and the proposed heating allow an efficient and pure generation of atomic carbon, i. H. a high yield of carbon atoms emitted from the heating chamber at a comparatively low heating temperature, so that a simultaneous evaporation of the thin-walled portion is prevented. In addition, the carbon atoms diffusing into the vacuum experience a significantly lower kinetic energy compared to the known prior art, which can be advantageous or even essential for subsequent processes (for example the deposition of the C atoms on substrates or their incorporation into cryogenic matrices or helium droplets) ,

In the subclaims, further embodiments of the atomic carbon source according to the invention are mentioned.

The invention will be described below with reference to the drawings

Embodiments will be explained in more detail. Schematic structure of the carbon source according to the invention

Special schematic structure of the carbon source according to the invention in FIG

Shape of a tantalum tube

Schematic representation for producing a pure carbon atom beam Comparison of the mass spectra measured with the carbon atom source (a) according to the invention and a traditional carbon source with C-bar (b)

In Fig. 1, the general schematic structure of the atomic carbon source according to the invention is shown.

An energy source 1 is connected to a heating chamber 2, which has a thin-walled film (about 0.05 mm in thickness) of a material which is carbon-soluble at operating temperature from 2150 ° C. (for example tantalum or tungsten). Inside is a carbonaceous substance 4 (eg.

Graphite powder).

The heating chamber 2 is arranged with the power source 1 in a closed (by a chamber, not shown) vacuum 5 and is brought by this (for example, by electrical ohmic or inductive heating) to an operating temperature of 2150 ° C or more.

First, at this operating temperature, the carbonaceous substance 4 is sublimated into a vapor 6 of carbon atoms and clusters. Secondly, at this operating temperature and the specified materials of subsection 3 of the

Heating chamber 2 carbon atoms from the vapor 6 diffuse through the carbon-soluble portion 3 and evaporate on the outer surface into the surrounding vacuum 5 (symbolized by arrow 7).

Due to the design and material specific carbon-soluble portion 3 of the heating chamber 2 is at the specified operating temperature so exiting

Carbon gas 8 purely atomic nature. The portion 3 of the heating chamber 2 thus assumes the function of a membrane which is responsible for carbon atoms, but not for

Carbon cluster is permeable. In addition, at the said operating temperature of the materials of

Subregion 3 of the heating chamber 2 no significant components of the same evaporates into the surrounding vacuum 5, which would contaminate the carbon gas 8.

FIG. 2 shows an exemplary schematic structure of the atomic carbon source according to the invention, in which the heating chamber 2 with the thin-walled partial region 3 is realized, for example, by a tantalum foil wound to the tube shape (1 × ₄ -fold winding of an approximately 0.05 mm thick tantalum foil) ,

A hermetically sealed tantalum tube 9, in the interior of which the graphite powder 4 (see also FIG.

The tantalum tube 9 is taken with its ends in electrodes 10, 1 1, which for electrical heating of the tantalum tube 9 with a not shown for clarity reasons in the drawing Niederampere current source for generating a by

Arrow depiction in the electrodes 10, 11 indicated Heizstromes I are in communication. This heating current I in the amount of 40 A heats the tantalum tube 9 with the graphite powder 4 contained internally by direct ohmic heating to a temperature of about 2150 ° C and more. This sublimates the graphite powder 4 inside the

Tantalum tube 9, and there is again the vapor 6, which consists of carbon atoms and clusters.

The operating temperature has also increased the carbon solubility of the tantalum tube 9, with the result that carbon atoms (as in principle already for

Part 3 of the heating chamber 2 described in FIG. 1) penetrate from the inside into the wall of the tantalum tube 9 and reach by diffusion on the outer surface, from which they evaporate into the vacuum 5 (again symbolized by

Arrow representations 7). Another geometry of the storage container (heating chamber 2 with the portion 3, see Fig. 1) is conceivable, provided that it can be brought by direct or indirect heating to comparable temperatures and if the

Carbon atoms from the interior of the hermetically sealed container through a thin, take over the function of the tantalum tube membrane can go into vacuum without wall components of the reservoir at operating temperature evaporate to the outside in the vacuum 5 and the resulting carbon gas. 8

contaminate.

The carbon clusters (ie C ₂ and C ₃ molecules) present in the interior of the tantalum tube 9 in the vapor 6 can not penetrate into the wall of the tantalum tube 9 as such (as the said carbon atoms do). Instead, they are dissociated on the inner surface of said wall of the tantalum tube 9 with catalytic support (also not explicitly shown for reasons of clarity), so that they are subsequently dissolved as carbon atoms in the metal.

Thus, as described above, the carbon gas 8 emitted from the tantalum tube 9 is again purely atomic in nature.

The thinness of the tantalum tube 9 and the proposed heating thus allow efficient and pure generation of atomic carbon, i. H. a high yield of carbon atoms emitted by the tantalum tube 9 at a comparatively low heating temperature, so that a simultaneous evaporation of the tantalum tube 9 is prevented.

In addition, the carbon atoms emerging in the vacuum undergo a significantly lower kinetic energy in comparison with the known prior art, which is the case for various applications (for example the already mentioned coating of surfaces and the incorporation of the C atoms in cryogenic matrices)

Helium droplets) may be advantageous or even essential.

In Fig. 3 is shown schematically how the described carbon source for

Generation of a carbon atomic beam 13 can be used. This is done essentially by a suitable diaphragm arrangement 12 and by differential pumping between these beam-shaping elements. It should be emphasized that the carbon atom beam 13 is purely effusive in nature, i. H. There is no cooling of the C atoms by gas-dynamic processes, such as in the expansion of jet streams. This means that no carbon molecules and clusters can be re-formed.

In principle, the carbon atom beam 13, which was generated with an arrangement shown in Fig. 3, sent directly into a mass spectrometer and after his Components (ratio of carbon atoms to clusters, proportion of impurities) are analyzed.

In order to obtain the same information, but in addition to demonstrate that the C atoms emitted by the carbon source according to the invention can be excellently incorporated into cryogenic matrices and even into superfluid helium droplets, a different method was chosen for the analysis of the C atom beam.

For this purpose, a beam of few nanometer-sized, 0.37 K cold, superfluid helium droplets (in a source chamber, not shown) was generated

(S.A. Krasnokutski and F. Huisken: Low-temperature reactions in helium droplets:

Reactions of aluminum atom with 0 ₂ and H ₂ 0, J. Phys. Chem. A 115, 201 1, 7120).

This came through a skimmer (sharp-edged, conical aperture) in the

Main chamber in which the atomic carbon source (see Fig. 2) was installed. To concentrate the carbon atoms within a well-defined range of constant vapor pressure, the source was in a water-cooled metal housing. Through two holes in the walls, the helium beam could enter and exit. The helium nanodroplets flew through the cloud of slow carbon atoms at a speed of about 300 m / s, colliding with them and collecting them. The carbon atoms embedded in the helium nanodroplets then entered the subsequent vacuum chamber containing the quadrupole mass spectrometer. In this spectrometer, the helium droplets were ionized by electron bombardment and their charged fragments separated by mass.

As a result, both the peaks of the pure embedded carbon atoms (and, if present, carbon clusters) as well as those of the mass spectra were found in the mass spectra

Carbon helium complexes CHe _n _(m and C _n He). The presented method represents a suitable way for the cooling and removal of the carbon atoms, although it is somewhat challenging from an experimental point of view.

FIG. 4 shows mass spectra for the comparison which illustrate the yield of the atomic carbon source according to the invention (FIG. 4a, diagram above) in comparison with a known carbon source (FIG. 4b, diagram below). Since the carbon atoms and molecules are embedded in helium droplets, not only the pure carbon peaks appear in the mass spectra, but also those that Complexes with helium atoms (C _m He _n ) are assigned. The peaks generated by carbon clusters are designated (each from the left: ^1J C ₂ ^,, ^J ₃ C ₃ and C ₂ , C ₃ , C ₆ , respectively); the other peaks belong to complexes which each contain only one carbon atom (FIG. 4a) or to complexes with several carbon atoms (FIG. 4b). It can be seen that in the upper spectrum (FIG. 4 a) the proportion of carbon clusters is extremely low and amounts to only about 1%. In contrast, in the conventional source (vaporization of a C-rod), the carbon molecules C _{3 are by} far the most common. For technical reasons, the proposed carbon atom source used the carbon isotope ¹³ C, which ^{slightly shifts} the mass ^peaks from the lower spectrum (Figure 4b).

The thermal expansion of the tantalum tube 9 during heating is not negligible. In order to cope with this circumstance, it makes sense to allow at least one electrode the compensating movement in one direction. For this purpose, a metallic and thus electrically conductive spring 14 can be seen in Fig. 3 at the electrode 10.

List of used reference numbers

energy

heating chamber

Part of the heating chamber 2

carbonaceous substance

vacuum

Steam (consisting of carbon atoms and clusters) arrow depiction

Gas (consisting solely of carbon atoms)

tantalum tubes

electrode

diaphragm arrangement

Carbon atom beam

feather

I heating current

Claims

claims

1. Atomic carbon source containing a in a vacuum (5) and arranged with an energy source (1) in communication heating chamber (2) for receiving carbonaceous substance (4), wherein the heating chamber (2) at least in one

Part area (3) made of a thin-walled and heated at operating temperature

Heating chamber (2) consists of non-evaporating material which when heated in the interior of the heating chamber (2) to the operating temperature of the carbonaceous

Substance (4) absorbs released carbon atoms and largely only releases them as gas (8) in the surrounding vacuum (5).

2. Atomic carbon source according to claim 1, characterized in that the partial region (3) of the heating chamber (2) consists of tantalum.

3. Atomic carbon source according to claim 1, characterized in that the partial region (3) of the heating chamber (2) consists of tungsten.

4. atomic carbon source according to one or more of claims 1 to 3, characterized in that the partial region (3) of the heating chamber (2) is formed by a film acting as a membrane.

5. Atomic carbon source according to one or more of claims 1 to 3, characterized in that the heating chamber (2) with the portion (3) by a thin-walled tube (9) is formed.

6. Atomic carbon source according to claim 5, characterized in that the thin-walled tube (9) has a wall thickness of less than 0.06 mm.

7. Atomic carbon source according to one or more of claims 1 to 6, characterized in that as the carbonaceous substance (4) graphite powder is provided.

8. Atomic carbon source according to one or more of claims 1 to 7, characterized in that as an energy source (1) for heating the heating chamber (2) to operating temperature, an electric current source is provided which via

Electrodes (10, 11) to the heating chamber (2, 9) is connected.

9. Atomic carbon source according to one or more of claims 1 to 8, characterized in that the energy source (1) for heating the heating chamber (2) with the partial area (3) to an operating temperature of at least 2150 ° C is provided.