WO2006075317A2

WO2006075317A2 - Nanostructures of cesium oxide and device used in handling such structures

Info

Publication number: WO2006075317A2
Application number: PCT/IL2006/000035
Authority: WO
Inventors: Reshef Tenne; Ana Albu-Yaron
Original assignee: Yeda Research And Development Company Ltd.
Priority date: 2005-01-11
Filing date: 2006-01-11
Publication date: 2006-07-20
Also published as: WO2006075317A3

Abstract

The present invention provides forms of cesium compounds, more specifically cesium oxides, with higher stability and ease of handling, suitable to be used in electronic devices, e.g. photocathodes and NEA devices. The invention also provides a novel environmental chamber for use with an electron microscope.

Description

NANOSTRUCTURES OF CESIUM OXIDE AND DEVICE USED IN HANDLING SUCH STRUCTURES

FIELD OF THE INVENTION

This invention is in the field of nanomaterials having fullerene-like or tubular structure.

LIST OF REFERENCES The following references are considered to be pertinent for the purpose of understanding the background of the present invention.

1. A.H. Sommer, Photoemissive Materials, Robert E. Krieger Publ. Com., Huntington, New York 1980; see Chs.lO (pp. 132-166) and 11 (pp. 167-17¹A).

2. Pickett, W.E. Negative electron affinity and low work function surface: cesium on exygenated diamond (100) Phys. Rev. Lett. 73, 1664-1667 (1994).

3. You, Z., Balint, I. & Aika, K.-I. Catalytic combustion of methane over microemulsion-derived MnO_x-Cs₂O-Al₂O₃ nanocomposites, Appl Catal. B: Environmental 53, 233-244 (2004).

4. van Setten, B.A.A.L., Spitters, C.G.M., Bremmer, J., Mulders, A.M.M., Makkee, M. & Molijn, J. A. Stabiity of catalytic foam diesel-soot filters based on

Cs₂O, MoO₃ and Cs₂SO₄ molten-salt catalysts Appl Catal. B: Environmental 42, 337-347 (2003). 5. Tenne, R., Margulis, L., Genut, M. & Hodes G. Polyhedral and cylindrical structures of tungsten disulphide Nature 360, 444-446 (1992).

6. Margulis, L., Salitra, G., Tenne, R. & Talianker, M. Nested fuUerene- like structures Nature 365, 113-114 (1993). 7. Chopra, N. G., Luyken, J., Cherry, K., Crespi, V. H., Cohen, M. L.,

Louie, S. G. & Zettl, A. Boron-nitride nanotubes Science 269, 966-967 (1995).

8. Rao, C. N. R. & Nath, M. Inorganic nanotubes Dalton Trans. 1-25 (2003).

9. Rapoport, L., Bilik, Yu., Feldman, Y., Homyonfer, M., Cohen, S. R. & Tenne, R. Hollow nanoparticles of WS₂ as potential solid-state lubricants Nature

387, 791-793 (1997).

10. Frimer, A.I. & Gerasimova, A.M. Electron-microscopic investigation of the structure of photoelectric cathodes Soviet Phys.-Tech. Phys. 1, 705-713 (1956). 11. Zhu, Y. Q, Sekine, T., Brigatti, K.S., Firth, S., Tenne, T., Rosentsveig,

R., Kroto, H. W. & Walton, D.R.M. Shock-wave resistance of WS₂ nanotubes J. Am. Chem. Soc. 125, 1329-1333 (2003).

12. Gemming, S., Seifert, G., Mϋhle, C, Jansen, M., Albu-Yaron, A., Arad, T. & Tenne, R. Electron microscopy, spectroscopy and first-principles calculations Of Cs₂O J. Solid State. Chem., in press.

13. Zak, A., Feldman, Y., Lyakhovitskaya, V., Leitus, G., Popovitz-Biro, R., Wachtel, E., Cohen, H., Reich, S., and Tenne, R. Alkali metal intercalated fullerene-like MS₂ (M=W, Mo) nanoparticles and their properties J. Am. Chem. Soc. 124, 4747-4758 (2002).

The above publications will be referenced bellow by indicating their number from the above list. BACKGROUND OF THE INVENTION

Films of cesium oxides with approximately 2:1 Cs to O ratio (in addition to Ag) are applied on the surface of photocathodes, e.g. S-I photocathodes¹, negative electron affinity (NEA) devices², and also discharge lamps, television cameras, lasers, etc. These films reduce the work-function of the electrode increasing thereby the electron emission currents and the long wavelength response of these devices. Another major application for Cs_1+xO is in catalytic converters for trapping emission gases³'⁴, like NO_x and for glasses and optical fibers, where it is alloyed with other elements. The stoichiometry of Cs_1+xO films, which are deposited on photoemissive and NEA devices, can not be easily controlled and they are generally poorly crystalline. Furthermore, these films are highly reactive and consequently photocathodes and NEA devices must be made, stored, moved and assembled into devices in a continuous ultrahigh vacuum environment. Furthermore, the lifetime of the above devices is determined to a large extent by the aging-process of the Cs₂O films.

The literature¹'¹⁰ highlights the great difficulties in fabricating and handling photocathodes having photosensitive cesium oxide layer because, as mentioned above, of the high reactivity and instability of cesium oxide films. The most detailed study of the Ag-O-Cs cathode is probably that of Frimer and Gerasimova¹⁰ who used an indirect method to probe Cs₂O crystals using scanning electron microscope (SEM) technique. These references also show that the highest photoemission (and thermionic emission) currents are reached when the ratio of Cs to O in the photocathode is 2: 1¹.

Although the synthesis of various oxides of cesium was reported already in the beginning of the last century, the actual structural characterization Of Cs₂O and Cs₃O was described only 50 years later, probably due to the great difficulty in handling these very reactive materials. When exposed to the ambient, Cs₂O reacts violently with CO₂, forming the stable cesium carbonate. Cs₂O crystallizes in the 3R anti-CdCl₂ structure and is the only known binary alkali-metal oxide with layered structure. The unit cell consists of three Cs₂O molecular layers in a rhombohedral arrangement (R3-MH) having the following unit cell parameters: a=b=4.256 A; c=l 8.990 A. In each molecular sheet the layer of oxygen atoms is placed in the center and is sandwiched between two layers of cesium atoms arranged in octahedral configuration around a central O atom.

In the past, closed cage (fullerene-like) nanoparticles and nanotubes of various layered compounds have been reported^5"8. Their closed nature enhances their chemical inertness and their mechanical properties making them suitable as e.g. superior solid lubricants⁹ and for impact resistant materials¹¹.

SUMMARY OF THE INVENTION

There is, therefore, a widely recognized need for cesium oxide forms with higher stability and ease of handling, suitable to be used in electronic devices, e.g. photocathodes and any other NEA devices. The present invention thus attends to this need by providing a new cesium oxide form that is devoid of the above mentioned drawbacks of cesium oxide films.

The new cesium oxide form of the invention consists of nanoparticles having a closed structure, of a substantially circular or polygonal (facetted) cross- section, with a cross-sectional size (diameter) of about 50nm to about 200nm.

Thus, according to a first aspect, the present invention provides closed cage fullerene-like or nanotube structure, also termed hereinafter "IF" or "IF nanostructure " or "IF structure ", of a cesium compound. The term "IF" or "IF nanostructure" or "IF structure" is used herein to denote a closed cage inorganic fullerene-like structure or inorganic nanotube structure.

The cesium compound comprises a compound selected from cesium oxides (e.g. Cs₂O), cesium suboxides, cesium peroxide (e.g. Cs₂O₂), cesium superoxide (e.g. CsO₂), alloys of cesium oxides, cesium peroxide, cesium superoxide or cesium suboxides with another metal, layered structures of cesium oxides with intercalated metals between the layers, and mixtures of such cesium compounds. In a preferred embodiment of the invention, the cesium compound is a cesium oxide where the Cs to O ratio is substantially 2:1, thus having the molecular structure Cs₂O. The cesium oxides nanostructures of the invention are also termed in the present application "IF cesium oxides". In a particular example of the preferred embodiment of cesium oxides bearing the formula Cs₂O, the terminology hereinafter would be "ZP- Cs₂O".

More specifically, the IF structure of the cesium compounds may comprise various arrangements such as single layer or multi layer fullerene-like structure, single layer or multi layer nanotube structure, nested layers fullerene- like structure, nested layers nanotube structure, faceted type and quasi-spherical type.

In another aspect, the present invention provides a method for preparing a closed cage fullerene-like or nanotube structure of cesium oxide, comprising (i) providing material comprising cesium or cesium compound in a closed system and under vacuum conditions; (ii) exposing the cesium or cesium compound to heat for a time period and temperature sufficient to obtain vapors of said cesium or cesium compound; and (iii) either concomitant or subsequent to step (ii), reacting said vapor with oxygen gas so as to obtain upon cooling fullerene-like or nanotube structure of cesium oxide. The cesium or cesium compound provided in step (i) of the method of the invention may be in powder form, as a film deposited on a substrate or in any other form suitable for carrying out the subsequent heating step. In a preferred embodiment, the present invention provides a method for preparing closed cage fullerene-like or nanotube structure of cesium oxide comprising (i) providing, in a closed system and under vacuum conditions, a substrate carrying a film of material comprising cesium oxide; and (ii) exposing the film to heat for a time period and temperature sufficient to allow the evaporation of the cesium oxide film and formation, upon cooling, of a fullerene-like or nanotube structure of cesium oxide. Optimal duration and temperatures of heating to be used would be such as to cause volatilization of the starting cesium oxide. In a preferred embodiment, the heating is made with focused laser beam, for example Nd: YAG pulsed laser,

CO₂ laser, ArF excimer laser, etc. that causes ablation of the cesium oxide layer deposited in step (i) and deposition of the desired nanostructures.

Further according to the present invention, there are provided electrical devices such as photocathodes, negative electron affinity (NEA) devices, discharge lamps, television cameras, lasers, etc that make use of the novel fullerene-like or tubular form of cesium oxide of the invention. More specifically, the present invention also provides a photocathode comprising cesium oxide (typically as Ag-O-Cs layer formed of elementary silver and Cs₂O) on an electrically conductive substrate (metal or semiconductor), the cesium oxide layer comprising cesium oxide nanoparticles having fullerene-like or nanotube structure. Electron guns comprising such photocathode are also within the scope of the present invention.

In still another aspect, the present invention provides environmental chamber (cell) for use with TEM, for handling air-sensitive materials. The provision of such an environmental chamber is associated with the following:

Electron microscopes are fundamental and indispensable instruments to investigate and examine microstructures, both in life and material science and techno-logy, and are widely used in nearly all the research laboratories concerned. Microscopes are powerful machines that combine images and diffraction with spectroscopy, and are primary tools for research in physics, biology, chemistry, geology and art preservation. Material science is concerned with the inter-relation and control of properties and microstructure of materials, for the development of new and improved materials. An important area of materials research concerns the fabrication and characterization of advanced materials based on nanostructures, including design and synthesis of novel inorganic fullerene and nanotubes materials, novel nano-composite materials, optoelectronic materials, etc., as well as in-situ fabrication of nanoscale materials under simultaneous TEM observations. In order to understand and control the synthesis of these materials, it is essential that they be characterized at very close to the atomic scale.

While most of the materials studied are quite stable in the environmental atmosphere, some of them are very unstable in air and must be studied under strictly inert or vacuum environment. It became evident that the existing facilities based on HRTEM instruments, while routinely capable of atomic scale spatial imaging, could not provide a sufficient high-inert environment device for loading of such specimens into the microscope, for both the high spatial resolution and chemical characterization requirements.

The environmental cell of the present invention configured to be attachable to a TEM meets the major requirement for safe manipulations, mounting and transferring of a number of rather very reactive materials into the TEM. This novel environmental cell will underpin many of its research projects in the field of advanced materials and will enable the previous hidden world of their respective microstructure to be revealed on a range of scales extending down to atomic dimensions. This unique installation can be used in studies of reactive oxides and other systems. The demanding specifications of ever increasing stringency and sophistication, triggered the development of such an environmental-cell attachment to the TEM, using innovative approaches to many technical difficulties that had to be overcome in maintaining a very high level of perfectly O₂/CO₂/H₂O-free environment, over long periods of time, as well as many other features, making and offering continual improvements in design and instrument versatility. At a time when the conventional electron microscopy has become a mature and widely used technique, and the sophisticated latest developments will provide even greater analytical power, as well as truly atomic scale resolution for both structural and chemical characterization, the design, development and setting-up of such an environmental-cell-controlled-inert atmosphere facility, attached to the HRTEM, will underpin much of its advances. Thus, according to yet another aspect of the invention, there is provided an environmental chamber for use with a Transmission Electron Microscope (TEM), the chamber being configured to be mountable onto /attached to a column of the TEM, and comprising a port configured to match a geometry of a sample holder entry of the column, and at least one port for material feeding into the chamber, thereby enabling safe manipulations, mounting and transferring of very reactive materials during the TEM operation.

BRIEF DESCRIPTION OF THE DRAWINGS In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non- limiting example only, with reference to the accompanying drawings, in which:

Fig. Ia shows a schematic configuration of the laser ablation experimental set-up. Fig. IB is the image of an environmental chamber of the present invention hermetically attached to the CompuStage entry of the TEM.

Fig. 2a-b shows TEM images of typical closed-cage nested /F-Cs₂O nanoparticles obtained by laser-ablated 3R-Cs₂O powder: a - faceted type IF nanoparticle -70-90 nm in size; b - quasi- spherical type IF nanoparticles -70- 140 nm in diameter.

Fig. 2c shows SAED from the nanoparticle in Fig. 2a. Here reflections from both (001) and the (hkO) plans are seen. A TEM-Philips CM120 (120 kV) with EDS system (EDAX Phoenix) was used.

Fig. 2D shows TEM image of a Cs₂O nanotube. Fig. 3a is HRTEM of a part of the closed and hollow Cs₂O nanoparticle shown in Fig. 2a.

Fig. 3b displays a computer simulation of the magnified image shown in Fig. 3a, showing a good agreement between the simulated and experimentally observed image. A line profile of the framed area in Fig. 3 a is shown in Fig. 3c. Fast Fourier transform (FFT) of the framed region in Fig. 3a is shown in Fig. 3d.

Fig. 4a shows HRTEM earned out on a particle, examined after being about one hour in the ambient laboratory atmosphere. The layer spacing, which is obtained from the FFT (Fig. 4b) and the line profile (Fig. 4c) (framed area in Fig. 4a) is 6.8 A in this part. The EELS spectrum (Fig. 4d) obtained from the GIF contains Cs and oxygen only. A field-emission gun HRTEM model FEI Tecnai F-30 (300 kV) was used.

Fig. 5A exemplifies the environmental chamber installation onto the TEM column.

Figs. 5B and 5C illustrate more specifically an example of the configuration of the environmental chamber.

DETAILED DESCRIPTION OF THE INVENTION The novel Cs₂O nanoparticles of the invention are characterized by closed-cage (fullerene-like-iF) structure. Those nanoparticles exhibit remarkable stability, even when exposed temporarily to plain air, making them potentially very useful for enhancing the performance and lifetime of photoemissive devices, negative electron affinity devices and catalysts. The Cs₂O nanoparticles of the invention were synthesized from 3R-Cs₂O powder which consists of a structure where 3 repeat layers of Cs₂O are arranged in rhombohedral structure. The 3R-Cs₂O powder was synthesized by reacting measured amounts of cesium metal and oxygen at 180 ⁰C for 3 days¹². The product was subsequently sealed in evacuated quartz ampoules. The orange- tinted cesium oxide powder was carefully characterized by X-ray powder diffraction (XRD) and Raman spectroscopy. The 103 cm^"1 Raman peak can be assigned to the A_lg mode.

While in the ampoule, the Cs₂O powder was ablated with Nd-YAG laser operated in the second harmonic mode (λ-532 nm). Fig. IA delineates an experimental arrangement. A beam 10 of a pulsed Nd-YAG laser HA (Continuum model 6050: λ=532nm; repetition rate- 5 OHz; average power-2W; pulse duration ~8ns) was directed (e.g., via mirrors) to a lens HB (focal length ~30cm) to be focused with a spot size of 0.1mm² onto a sample (Cs₂O powder in a quartz ampoule 13). A threshold power of the laser beam (>2.3 W) was required for a successful ablation process. Cooling with liquid N₂ vapor was provided by forced transfer of the vapors from a pressurized liquid N₂ cylinder through copper tubing in the form of a solenoid 12 which wrapped part of the ampoule surface. To avoid overheating of the powder (sample), the incident laser beam was manually scanned, hitting a new position every one minute for a total duration of 30-60min. During the ablation, a deep colored powder accumulated on the walls of the quartz ampoule 13 at the cooled side thereof (cooled by liquid nitrogen).

To transfer and load the samples into a transmission electron microscope (TEM) without risking their exposure to the ambient atmosphere, the inventors have developed a special home-built environmental chamber, which was purged with argon gas. As shown in Fig. IB, this hermetically sealed chamber 14 (environmental cell) was attached as a matching counterpart to the CompuStage entry of the TEM column 16. The construction and operation of this chamber will be described more specifically further below with reference to Figs. 5A-5C. The quartz ampoule 13 was broken inside the chamber 14, and a small amount of the laser ablated material was transferred onto a gold grid for imaging by the TEM and for analysis by selected area electron diffraction (SAED) and energy dispersive X-ray analysis (EDS). Extreme care was practiced to avoid any exposure of the analyzed grid to the ambient atmosphere.

Reference is made to Fig. 2a and 2b showing two examples, respectively, of closed cage nested nanoparticles of Cs₂O typically observed in the laser ablated material. While the one shown in Fig. 2a is faceted, the layers of the IF- Cs₂O nanoparticles shown in Fig. 2b are evenly folded, forming quasi-spherical, closed nanoparticles. Here, each fringe represents a molecular sheet of Cs₂O. The interlayer distance between the Cs₂O fringes is 6.35 A. The faceted nanoparticle in a is ~70-90nm in size, while the quasi-spherical nanoparticles shown in b are ~70-140nm in diameter and consist of at least 30 layers. Although the nanoparticles cores appear quite dark, the somewhat brighter contrast suggests that they are hollow in the center. The folded and closed nature of both nanoparticles can be verified by tilting experiments. In contrast to 3R-Cs₂O platelets which loose contrast and become featureless upon slight tilting the grid, here the "onion-like" pattern remained always in focus even if tilted 40° in each direction. SAED from the nanoparticle in Fig. 2a, which is shown in Fig. 2c confirms also its folded and closed nature, as both reflections of the (001) plan and the hexagonal pattern of the (hkO) plans are observed. EDS analysis confirmed that only cesium and oxygen were present in the nanoparticle with an approximate 2:1 ratio. The TEM of a cesium oxide nanotube is showed in Fig. 2d.

Fig. 3a displays an exploded view of a part of the faceted polyhedral Cs₂O nanoparticle shown in Fig. 2a. A computer simulation of the magnified image is shown in Fig. 3b, showing a good agreement between the simulated and experimentally observed images. A line profile of the framed area in Fig. 3a is shown in Fig. 3c confirming the interlayer spacing of 6.37 A, which is in a very well agreement with the value of c/3 of Cs₂O. Fast Fourier transform (FFT) of the framed region in Fig. 3 a is shown in Fig. 3d, and the distance calculated from it (6.35 A) agrees well with the c/3 distance of Cs₂O.

The /F-Cs₂O nanoparticles were found to be very stable under the electron beam of the TEM, while the surrounding amorphous material, which consisted of cesium and oxygen, was boiling during the TEM inspection. Even platelets of 3R-Cs₂O boiled and degraded after a few seconds under the beam. In fact, the IF- Cs₂O nanoparticles can be only observed when they are relatively isolated from the rest of the cesium-oxide materials, since otherwise the entire product boils under the beam, which can not be focused on one single tiny spot. More strikingly, when the specimen was taken out and reinserted into the TEM after two minutes in the ambient atmosphere, almost no damage occurred to the inspected fullerene-like Cs₂O nanoparticles, while its surroundings transformed into a volatile species and partially evaporated. Subsequent high resolution transmission electron microscopy (HRTEM) experiments carried out on a grid transferred from TEM after roughly one hour in the ambient laboratory atmosphere (see Fig. 4a) revealed that the pristine Cs₂O nanoparticle with fullerene-like structure was only partially damaged. Starting from the kinks, between the facets, exfoliation and amorphization of the outer molecular sheets of Cs₂O took place, advancing progressively inwards. The interlayer distance in the remaining layers increased from about 6.4 A in the pristine nanoparticle to 6.8 A and in some places even to 8.5 A. In general, however, about 20 layers where not fully damaged and the inner eight layers of the /F-Cs₂O nanoparticles, were found to be less distorted and showed smaller expansion. The layer spacing, which is obtained from the FFT (Fig. 4b) and the line profile (Fig. 4c) of the framed region in Fig. 4a is 6.8 A. Electron energy loss spectroscopy (EELS) and imaging using the Gatan energy filter (GIF), showed that the exposed nanoparticle consisted of Cs and oxygen only (Fig. 4d). Nonetheless, this analysis showed excess oxygen in the nanoparticle which could be attributed to a reaction with water, oxygen or CO₂. In keeping with previous observations with alkali metal-doped WS₂ and MoS₂ nanoparticles, water uptake into the van der Waals gap between the layers is common in layered compounds. Careful analysis of the sample with the GIF indicated that the amorphous material in the grid reacted with the ambient CO₂ to form cesium carbonate, while the IF nanoparticle reacted slowly with water, which could be possibly removed by mild heating treatment or prolonged evacuation in the HRTEM column. Furthermore, in analogy to the alkali metal intercalated /F-WS₂ ¹³, evidence for partial recovery of the damaged nanoparticles was obtained after a few days in the HRTEM. The partially recovered Cs₂O layers were less faceted than the original ones. This observation indicates that, in contrast to the presently used cesium oxide films in photocathodes, which suffer irreversible damage under low vacuum, the IF nanoparticles of the invention, if damaged, could be possibly recovered, by mild heating under vacuum conditions.

The unprecedented relative stability of the IF-Cs₂O nanoparticles both under the electron beam and in plain air provides many benefits for the work- function lowering and photoemission from various surfaces. Addition of other metals, like silver and bismuth that are known to enhance the quantum yield of photocathodes, may in fact be related to their possible catalytic effect in stimulating the synthesis of IF-Cs₂O nanoparticles. Alloying the IF-Cs₂O with other metals, as for example Rb, could be beneficial for photoemission or NEA devices.

As indicated above, the present invention provides a novel environmental chamber (cell) for use with a TEM. This environmental chamber can operate operable with both the room-temperature and the cryo holders of the CM120 TEM (commercially available from Philips). In the present example, aimed at preparing closed cage fullerene-like or nanotube structure of cesium oxide, the environmental cell is configured to operate with the CM120 Philips TEM facility, but it should be understood that the present invention is not limited to the type of the TEM with which the environmental cell is used. Fig. 5A illustrates the environmental chamber 14 of the present invention installed onto the TEM column 16, which in the present example is the CM 120 instrument, a 120 kilovolt TEM microscope (commercially available from Philips). As shown, the chamber 14 has a portable stand 17, which may be replaceable and may be of a varying height. In the present example, the chamber 14 is made of 0.8cm thick transparent polycarbonate, having a 5Kg weight, and of about 66cm length, 40cm width and 30cm height.

Figs. 5B and 5C illustrate more specifically the configuration of the chamber 14. The chamber 14 has a housing 15 which on its front surface 15 A and on its side surface 15B is formed with three glove ports, generally at 20 (e.g., of a diameter of about 8"): two such spaced-apart ports 20 on the front surface 15A₅ and one such port on the side surface 15B. Further provided are two-part connector arrangements 18 (only one such arrangement being shown in Fig. 5C) that fit onto the ports 20. The connector arrangement 18 includes sleeve and glove parts 18A and 18B coupled to each other by a plastic connector 19. The two-part sleeve-glove arrangement provides the convenience of quick, easy changing different glove sizes and types, for different operators, without changing the sleeve 18A, that remains attached to the housing 15. Sleeves 18A fit three chamber ports 20; sleeves 18A and gloves 18B, joined by the plastic connector 19, ensure an air-tight seal. Optionally provided in the chamber 14 are glove-port-plugs 22, fitting tightly into the arm holes from inside the housing 15, to make an airtight seal when the gloves 18B are being changed or while the chamber is not in use. When ready to operate, the glove-port-plugs 22 are unscrewed, pushed into the housing 15 and placed into their support racks 24. Also optionally provided in the chamber, aimed for use in the strict anaerobic atmosphere required, is a vacuum purge system 26, which allows for vacuum out existing atmosphere in the sleeves 18 A, and replacing it with any inert gas atmosphere desired, prior to removing the arm-port-plugs 22 (entering the chamber). This permits the operator's arms to enter the chamber (housing) without compromising the anaerobic chamber atmosphere.

The opposite side surface 15C of the housing 15 is formed with an opening (port) 28 (of about 10" diameter) which is configured and accommodated so as to match the geometry of the CompuStage entry (for the specimen holder location) of the microscope. It should be noted that the pre- pumped airlock (which ensures that air/Ar, introduced with a holder 30, is pumped away before the airlock is opened to the microscope column), is to be several times purged with Ar before the specimen holder is loaded in the CompuStage. The chamber-port-plug 22 (of about 10" diameter) fits tightly (screwing) from inside the chamber. When the grid is mounted in the holder and is ready to be inserted into the pre-pumped airlock of the microscope, its handle, effortlessly unscrew the plug 22 into the chamber, placing it into the support rack 24 on the top side of the chamber.

Also shown in Fig. 5C is an accordion-like sleeve 32, which fits tightly, at one end, the chamber port 28 (e.g., of 10" diameter), while at the other end, fits the air-lock area of the microscope. This allows the safe loading of the microscope holder carrying the grid into the CompuStage of the microscope.

Two steel spring tension clamps 40 of uniform pressure c are used for closing in place the equipment entry (located opposite the specimen holder entry into the CompuStage of the microscope), after installing equipment in the chamber. Also provided in the chamber 14 are: a multiple electrical outlet strip 34; two purging gas valves 36 and 38, the flow meter 36 enabling to establish and monitor a constant flow of Ar at lower volumes. Optionally provided is an automatic atmosphere control (O₂, CO₂, H₂O) while working with oxygen sensitive materials. Thus, the present invention provides closed cage fullerene-like or nanotube structure of a cesium compound, and a method for preparing such structures.

The present invention also provides the design and construction (as well as estimation of the experimental performances) of the environmental cell to be attached to the TEM microscope (e.g., Philips 120C TEM), which can be used for preparation and loading specimens of reactive materials. The environmental cell is capable of maintaining a very high level of perfectly O₂/CO₂/H₂O-free environment and provides for the high spatial resolution and high sensitivity EDX potential of the TEM microscope, for obtaining atomic images and analysis of individual nanoparticles of very reactive materials. Various factors should be considered when designing the elements of the chamber. Considering the specific example of preparation of closed cage fullerene-like or nanotube structure of a cesium compound, the following factors were taken into consideration: maintaining proper anaerobic gas mix; using a two-piece sleeve/glove combination to provide the convenience of quick, easy glove replacement while leaving the sleeves attached to the chamber, sleeves and gloves being joined by a plastic connector, which ensures an air-tight seal; adequate space, which is important for the possibility of accommodating heating, cooling, stirring, sonnication, as well as the conventional or the cryo-specimen holders; ease of use and reability (to provide the cell as a multi-user facility).

Claims

CLAIMS:

1. Closed cage fullerene-like or nanotube structure of a cesium compound.

2. The fullerene-like or nanotube structure according to claim 1 wherein said cesium compound comprises a compound selected from cesium oxides, cesium suboxides, cesium peroxide, cesium superoxide, alloys of cesium oxides, cesium peroxide, cesium superoxide or cesium suboxides with another metal, layered structure of cesium oxides with intercalated metals between the layers, and mixtures of such cesium compounds.

3. A fullerene-like structure of a cesium compound comprising cesium oxide.

4. A nanotube structure of a cesium compound comprising cesium oxide.

5. The structure of claim 3 where the Cs to O ratio is substantially 2:1.

6. The structure of claim 4 where the Cs to O ratio is substantially 2:1.

7. The structure of claim 1 having a substantially circular or facetted cross- section.

8. The structure of claim 1 which is a member selected from single or multi layer fullerene-like structure, single or multi layer nanotube structure, nested layers fullerene-like structure, nested layers nanotube structure, faceted type and quasi-spherical type.

9. Method for preparing a closed cage fullerene-like or nanotube structure of cesium oxide, comprising (i) providing material comprising cesium or cesium compound in a closed system and under vacuum conditions; (ii) exposing the cesium or cesium compound to heat for a time period and temperature sufficient to obtain vapors of said cesium or cesium compound; and (iii) either concomitant or subsequent to step (ii), reacting said vapor with oxygen gas so as to obtain upon cooling closed cage fullerene-like or nanotube structure of cesium oxide.

10. The method of claim 9 wherein said cesium or cesium compound provided in step (i) may be in powder form or as a film deposited on a inert substrate.

11. A method for preparing a closed cage fullerene-like or nanotube structure of cesium oxide comprising (i) providing, in a closed system and under vacuum conditions, a substrate carrying a film of material comprising cesium oxide; and (ii) exposing the film to heat for a period of time and at a temperature sufficient to allow the evaporation of the cesium oxide film and formation, upon cooling, of closed cage fullerene-like or nanotube structure of cesium oxide.

12. The method of claim 11 wherein said heating is by laser source.

13. The method of claim 11 wherein said heating causes ablation of cesium oxide deposited in step (i).

14. A photocathode comprising a cesium oxide layer on an electrically- conductive substrate, said cesium oxide layer comprising cesium oxide nanoparticles having closed cage fullerene-like or nanotube structure according to claim 1.

15. An electron gun comprising at least one laser and the photocathode according to claim 14.

16. A negative electron affinity (NEA) device having a coating in the form of a film comprising fullerene-like or nanotube structure according to claim 1.

17. A device for use with a Transmission Electron Microscope (TEM), the device comprising an environmental chamber, which is configured to be mountable onto a column of the TEM, and comprises a port configured to match a geometry of a sample holder entry of the column, and at least one port for material feeding into the chamber, thereby enabling safe manipulations, mounting and transferring of very reactive materials during the TEM operation.

18. The device of claim 17, wherein the environmental chamber comprises a housing which is on its one side formed with said port configured to match the geometry of the sample holder entry of the column to thereby enable appropriate mounting of the chamber onto the TEM column; said at least one glove port arranged on the housing; and at least one connector arrangement configured to fit onto said at least one glove port, respectively, the connector arrangement being configured to ensure an air-tight seal.

19. The device of claim 18, comprising at least one glove port plug on the housing configured to fit tightly into at least one respective hole from inside the housing to provide an airtight seal when said at least one connector or at least part thereof is being changed or when the chamber is not in use.