WO2006087588A2

WO2006087588A2 - Apparatus and methods for growing nanofibres and nanotips

Info

Publication number: WO2006087588A2
Application number: PCT/GB2006/050040
Authority: WO
Inventors: Kenneth Boh Khin Teo; Mark Mann; William Irland Milne
Original assignee: Cambridge Enterprise Limited
Priority date: 2005-02-16
Filing date: 2006-02-15
Publication date: 2006-08-24
Also published as: GB0503139D0; WO2006087588A3

Abstract

This invention relates to heating apparatus and methods with particular applications for growing a nanofibre (exampled by nanotube or nanowire) on a metallic tip. The invention also relates to apparatus and methods for growing nanofibres on metallic tips, and to nanotips fabricated by such methods and apparatus. Embodiments of the invention are particularly useful for providing nanotips for electron gun sources and scanning probe microscopy. A heater for heating an object in the presence of an electric field, in particular incorporated in nanotip fabrication apparatus. The heater comprises: a substantially planar electrically conductive heating element configured to define at least one aperture; a support to mount the heated object such that it protrudes through said aperture; and at least one electrical connection to said heating element; whereby, in use, the heating element is biassable by said at least one electrical connection such that the electric field in the vicinity of the object is substantially perpendicular to the plane of the element.

Description

Apparatus and.Metihods.for.Growing.Nanofibres and Nanotips

This invention relates to heating apparatus and methods with particular applications for growing nanofibre-type materials such as nanotubes and nanowires on a metallic tip. The invention also relates to apparatus and methods for growing nanofibres on metallic tips, and to nanotips fabricated by such methods and apparatus. Embodiments of the invention are particularly useful for providing nanotips for electron gun sources and scanning probe microscopy.

When wishing to heat a sharp, metallic tip/wire in an electric field, the possibility of an arc discharge to that wire is risked (because of the concentration of electric field at the pointed tip which causes electrical breakdown), This is of particular concern to scientists and engineers attempting to deposit or react chemical species on the surface of a tip/wire in the presence of an electric field or plasma, as arc discharging/electrical breakdown can undesirably affect the chemistry of many chemical and physical process, in particular the growth of nanofibre-type materials such as nanotubes or nanowires. If this occurs, the tip of the metallic wire is also usually destroyed/melted.

Existing methods of heating a metallic tip/wire involve either clamping or welding the metallic tip onto a second wire, with current passing through the second wire. The second wire resistively heats up, and heat is transported to the end of the first wire by conduction. However, this has the following limitations when used for growth of nanofibre-type materials:

1. The temperature of the tip of the wire is unknown - unless expensive thermometry techniques are used.

2. The temperature is not well controlled and can change in the presence of gases due to the heat loss from the tip.

3. This cannot be operated with high voltage or high field in the presence of gases as it would cause arc discharge/electrical breakdown due to the field enhancement of the first wire. We will describe techniques which shield the tip/wire from the field enhancement at the sharp point, which causes the discharge/electrical breakdown to take place. The techniques we describe advantageously facilitate simultaneously heating of the tip/wire and also the maintenance of an electric field of a defined direction at or near the apex of the tip/wire; this field may also be maintained such that it is substantially constant. The techniques are particularly useful for the growth of an aligned nanofibre on an object.

Background prior art can be found in Chemical Physics Letters 272 (1997), 178-182, "Well-aligned graphitic nanofibres synthesized by plasma-assisted chemical vapor deposition", Yan Chen, Zhong Lin Wang, Jin Song Yin, David J. Johnson, and R.H. Prince; International Patent No. WO99/65821; US Patent No. US2002/024279 and International Patent No. WO 02/19372; US Patent No. US2002/0117951 ; European Patent No. EP 1 129990; European Patent No. EP 1046613; and Japanese Patent No. JP2002/069756. Reference may be made to these documents for detailed examples of the growth of carbon nanofibres by means of plasma assisted CVD.

According to a first aspect of the invention there is therefore provided a heater for heating an object in the presence of an electric field, the heater comprising: a substantially planar electrically conductive heating element configured to define at least one aperture; a support to mount the heated object such that it within the aperture; and at least one electrical connection to said heating element; whereby, in use, the heating element is biassable by said at least one electrical connection such that the electric field in the vicinity of the object is substantially perpendicular to the plane of the element.

Preferably the heating element is substantially fiat, or at least locally flat in the vicinity of the aperture, and preferably the heated object is supported so that it is level with or slightly protrudes through the aperture. In embodiments the heating element may comprise an electrically conductive plate or strip mounted on a ceramic support, preferably spaced away from the support to facilitate gas flow around the heated object in nanotip fabrication apparatus. The heating element may be heated directly, for example by providing a pair of electrical connections to enable the electrically conductive plate to be ohmically heated by passing a current through it. In this case the electrical conductor may comprise a somewhat resistive material such as graphite. Alternatively the electrically conductive heating element may be heated in some other way - for example it may be heated by a radiant heater such as a quartz tube heater.

Embodiments of the above described heater construction allow chemical reactions to take place in the presence of a high voltage and/or plasma without substantial electric arc discharge or electrical breakdown. This is because the fiat, planar conductive plate or strip shields the object, typically a pointed substrate such as a metallic tip or wire, from creating large electric fields which would otherwise arise from the geometry of the object in a high field or high voltage environment. Furthermore the flat, planar electrically conductive heating element constrains the electric field to be substantially perpendicular to the plane of the element. In nanotip fabrication apparatus this results in vertically aligned growth of one or more nanofibres (such as nanotubes or nanowires) on the object, which is highly desirable for a range of applications.

In some preferred embodiments the aperture has a dimension, for example a diameter in the case of a circular aperture, of less than lmm, preferably less than 0.5mm. As previously mentioned, typically the heated object comprises a wire, which may have a sharpened end/tip, or some similarly shaped pointed object, in which case a relatively small aperture assists in keeping the wire (or other object) substantially vertical. A small aperture also helps to ensure that the electrically conductive heating element and the wire/tip are at a similar or substantially the same temperature. This helps to overcome another problem with prior art techniques, where the wire temperature is generally not well controlled. By contrast in embodiments of the present apparatus the temperature of the electrically conductive element can be controlled very precisely, for example with an accuracy of order I⁰C by resistive heating, even under the flow of reactive gases. Preferably, therefore, the heater includes a thermocouple or other temperature sensing device in thermal contact with the electrically conductive heating element, for measuring (indirectly) a temperature of the object. A feedback loop for temperature control may then also be implemented. Preferably the support is adjustable to control the protrusion of the object through the aperture, and may comprise a screw. This facilitates adjustment so that a sharp end or tip of the object is level with or just slightly protrudes from the surface. Preferably the heater is arranged to electrically connect the object to the heating element, for example by direct contact between the two or indirectly via the support, This facilitates provision of a uniform, perpendicular electric field in the vicinity of the (electrically conducting) object, A power supply may be included to bias the heating element/object to control the electric field in the vicinity of the object. This may comprise, for example a dc power supply with an output voltage in the range 0.1 KV to 10KV. A complementary electrode may be provided to apply this voltage; optionally this complementary electrode may be perforated to allow the passage of gas into/through a reaction chamber in which the heater is to reside.

One particularly useful feature of the heater, especially when intended or adapted for use with nanotip fabrication apparatus, is the scalability of the design to allow multiple nanotips to be fabricated simultaneously. Thus in some preferred embodiments the electrically conductive heating element is provided with a plurality of apertures for simultaneous heating of a plurality of objects, such as a plurality of wires, within a single, common reaction chamber. This facilitates mass production of nanotips.

The invention also provides nanotip fabrication apparatus including a heater as described above.

Thus in a further aspect the invention provides nanotip fabrication apparatus for fabricating a nano fibre on a tip of an object, the apparatus comprising: a reaction chamber including a first electrode; a gas supply connection for supplying gas to the reaction chamber; a heater, the heater having an electrically conducting surface in which is provided an aperture within which the tip is able to be supported; and first and second electrode connections, said first electrode connection being connected to said first electrode, said second electrode connection being connected to said electrically conducting surface. The object on which a nanotip is fabricated is typically a pointed, electrically conducting (generally metal) object such as a tungsten wire. Preferably the apparatus is configured so that the tip can be supported within the aperture so that it is level with or protrudes slightly from the aperture. The nanotip preferably comprises a nanofibre, more particularly a carbon-based nanofibre such as a single- or multi-walled nanotube. Broadly speaking what is meant by a nanotip is an object with a nanoscale end, nanoscale meaning less than lOOOnm across, more preferably less than lOOnm, typically in the range 1 - IOnm. The aperture through which the object tip is to protrude has a lateral dimension of, in order of increasing preference, less than 5mm, lmm, 0.5mm, 0.2mm.

The first and second electrode connections may connect to the first electrode and electrically conducting surface respectively either with or without intermediary components. Preferably the apparatus includes a power supply connection for connecting a power supply to the heater although, for example, an external, radiant heater may be employed.

Preferably the electrically conducting surface is configured in such a way that when, in use, a voltage is applied between the first and second electrode connections an electric field is generated which, in the vicinity of the tip is substantially in a direction in which the tip points, that is for a wire, substantially parallel to the wire. Thus preferably the electrically conducting surface is substantially planar at least in the vicinity of the aperture, in which case the electric field is substantially perpendicular to the plane of the conducting surface. In particularly preferred embodiments the electrically conducting surface has a plurality of apertures for fabricating a plurality of nanotips simultaneously, for example by inserting a wire through each aperture so that each wire end is level with or just protrudes from the conducting surface. A single, common support or a plurality of separate supports, for example separate screws, may be provided for the plurality of apertures.

In a related aspect the invention provides a method of heating an object in an electric field, the method comprising: shielding the object from part of the electric field by mounting the object in an aperture in an electrical conductor, said conductor being substantially planar in the vicinity of said aperture; biasing said electrical conductor such that the electrical field in the vicinity of the object is primarily perpendicular to said plane; and heating the object.

Correspondingly the invention further provides a heater for heating an object in an electrical field, the heater comprising: a shield for shielding the object from part of the electric field, the shield comprising an electrical conductor defining at least one aperture; said conductor being substantially planar in the vicinity of said aperture; and an electrical connection for biasing said electrical conductor such that the electrical field in the vicinity of the object is primarily perpendicular to said plane; and a heater for heating the object.

The invention further provides a method of growing a nanofibre on the tip of a metallic object by heating at least the tip of the object in an electric field in the presence of a gaseous supply of material for fabricating the nanofibre, the method including controlling said electric field to be substantially in the direction of said tip during the growing by mounting said tip within an aperture in an electrical conductor.

Preferably the tip is mounted such that it is substantially level with or protrudes through the aperture.

Typically the gaseous supply of material comprises a plasma. Methods for generating such a plasma and are well known to those skilled in the art.

Embodiments of the described methods are particularly useful for fabricating electron gun sources (and hence electron guns) and scanning probe microscopy tips such as AFM (Atomic Force Microscopy) tips and STM (Scanning Tunnelling Microscopy) tips.

The invention further provides an object with a pointed metallic tip and having a nanofibre attached substantially at the end point of said tip. In some preferred examples the object comprises a wire such as a tungsten wire, but the skilled person will appreciate that nanofibres may be attached to other pointed metal objects, depending upon the desired application. Preferably the nanofϊbre is attached substantially at the centre of the tip, and preferably it is aligned substantially parallel to a direction which the tip (or wire) points. Preferably only a single nanofibre is attached at the end point of the tip. Objects of this type may be obtained, for example, by repeatedly fabricating nanotips as described above and then selecting those on which only a single fibre has been grown.

In preferred embodiments the nanofibre comprises a nanowire or nanolube of material such as carbon, zinc oxide, silicon or other single elements or compounds. Here, as before, the nanofibre preferably has a lateral dimension or average diameter of less than lOOOnm, more preferably less than lOOnm or less than 50nm. As previously mentioned, such an object can advantageously be employed as an electron gun source or scanning probe microscopy tip.

These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows nanolip fabrication apparatus embodying an aspect of the present invention;

Figure 2 shows a heater according to an embodiment of an aspect of the present invention;

Figures 3a and 3b show electric field lines for a sharp, metallic lip in an electric field, (a) unshielded, and (b) shielded by the heater of figure 2;

Figures 4a and 4b show, schematically, an object tip with a nanotube attached according to, respectively, a conventional method, and a method according to an embodiment of an aspect of the present invention; Figures 5a and 5b show electron microscopy photographs of actual objects corresponding to the schematic diagrams of figures 4a and 4b; and

Figures 6a and 6b show examples of an electron source and a scanning probe microscope tip incorporating the nanotip of figures 4b and 5b.

Referring first to figure 1, this shows nanotip fabrication apparatus 100 comprising a reaction chamber 102 in which plasma-enhanced chemical vapour deposition (PE-CVD) or chemical vapour deposition (CVD) in the presence of an electric field of nanofibres may be performed. Gas for growing the nanofibres enters a reaction chamber inlet 104 and exhausts to a pump through an outlet 106. In a preferred embodiment a first electrode for striking a plasma or generating the electric field in the growth environment is formed by inlet 104, which is made of metal. In the illustrated example reaction chamber 102 is also made of metal and provided with an earth connection 108; in other embodiments reaction chamber 102 may be fabricated from an electrically insulating material such as glass. In the illustrated embodiment gas inlet/electrode 104 has the form of a "showerhead", with a grill 104a to disperse the gas within the reaction chamber.

Also incoiporated within the reaction chamber 102 is a heater stage 110 supporting a filament 112 such as a wire or tip, at the end of which a nanofibre is to be grown. The heater stage 1 10 comprises a flat, planar electrical conductor 114 mounted on a support 116, preferably formed from ceramic because of the high electric fields, and spaced away from conductor 114 to facilitate circulation of the growth gas. Filament 1 12 projects through a small aperture in conductor 114, as is explained in more detail with reference to figure 2. In some particularly preferred embodiments conductor 1 14 is provided with an array of apertures so that nanofibres may be grown simultaneously on a plurality of filaments.

In one embodiment electrical connections are made to either end of conductor 114 for example by means of conducting supports a, b, electrically insulated from the reaction chamber 112 if the reaction chamber is made of metal. The electrically conducting supports 118a, b may be taken out to external connections on the reaction chamber for connection to an electrical power supply 120 for the heater; alternatively this power supply may be located within the reaction chamber. In other embodiments the electrical conductor 1 14 of the heater stage may be heated indirectly, for example radiatively It will be appreciated, however, that at least conductor 1 14 must conduct both heat and electricity. At least one external connection 122 is provided to conductor 1 14 to allow this to be connected to an external high voltage power supply 124.

Figures 2a to 2d show the sharp wire heater 110 of figure 1 in more detail. Figure 2a shows a top view of the heater, and in particular of a preferred graphite heating element 1 14 (without screws); figure 2b shows a top view of the ceramic support (without screws); figure 2c shows a bottom view of the ceramic support (without screws); and figure 2d shows a vertical cross-section through the heater assembly, with screws included. Preferably all screw holes shown in figure 2 are threaded.

In figure 2a the flat, planar graphite heater 114 has a first pair of screw holes 202 for holding screws for the heater, a second pair of holes 204 for electrical contacts to the heater, and (in this example) a single hole 206 for wire or filament 112. The ceramic support 1 16 insulates both heat and electrical current. Referring to figures 2b and 2c, holes 206 are provided for the holding screws and a hole 208 for the wire/filament 112. The lower part of the support 116 is provided with a larger, threaded hole 210 concentric with hole 208 to receive a screw 212 to support and raise/lower the height of the fi lament/wire/tip 112. Optionally a cylindrical spacer closely fitting aperture 208 may be provided above screw 212 to reduce the risk of filament 112 become stuck down the side of the screw.

Figure 2d shows the previously mentioned features in cross-section, the assembly being held together by holding screws 214 and holding nuts 216. A temperature sensor 218 such as a thermocouple may be located on or embedded in conductor 1 14, preferably adjacent filament 112 if space permits. In preferred embodiments an electrical connection is made between wire/filament 112 and heater conductor 114. This may arise because the wire is a close fit in hole 206 or, in embodiments, a direct electrical connection may be made, for example from screw 212 to one of holding screws/nuts 214, 216.

In use a typical procedure involves placing the wire 112 in its hole 206, 208 with the supporting screw 212 tightened fully. The screw is then unwound to lower the level of the wire so that the top is level with that of the heater 114. Current is passed through the strip heater 1 14, which in turn heats the wire, as it is leans against the graphite heater. In embodiments the temperature of the heater strip or plate 114 closely matches that of the tip of the wire/filament, which ameliorates problems with prior art techniques, where the gas tends to cool the tip. The temperature is measured using the thermocouple 218, and the temperature can be adjusted by altering the level of current passing through the heater using power supply 120.

An electric field or plasma is created perpendicular to the heater (and hence the tip) by biasing the heater with respect to earth. In high electric fields (typically greater than 10³Wm, generally greater than 10⁴Wm) a nano fibre grows substantially straight and vertical, whereas in prior art techniques nanofibre spaghetti is a common result.

After the process is completed, the supporting screw is tightened to raise the height of the wire, which can then be picked up by tweezers.

The method used to fabricate a nanofibre can be tailored to meet the needs of the nanofibre required. Broadly speaking any conventional PE-CVD or CVD (in the presence of an electric field) nanofibre fabrication technique may be used with the apparatus, to seek the benefits described above.

The control parameters of the method and how they affect the process are listed below:

Growth time: The height of nanolubes grown is a function of growth time. Our process typically grows nanotubes at a rate of 8 microns per hour. Catalyst thickness: We use two thin films to form the catalyst 'seed' from which the nanotube grows. The first film (the bottom film) is always the same thickness. It is a conductive buffer layer of either Indium Tin Oxide, Titanium Nitride or Tantalum Nitride, thickness -15 nm though this is not critical. This prevents the top layer from diffusing into the wire/filament (often a metal) which would result in there being no catalyst to start nanotube growth. The top layer is the catalyst, commonly nickel or iron or cobalt. The diameter of the carbon nanotube is directly affected by the thickness of the catalyst film. Catalyst thickness is typically 2-7 run.

Temperature: The higher the growth temperature, the fewer imperfections in the carbon nanotube and the faster it grows. Growth typically starts around 500 ⁰C

Pressure of gas: The higher the pressure, the higher the growth rate.

Flow rate of gas: The higher the flow rate, the higher the pressure for a fixed pumping speed.

A description of a typical experimental run

The filaments (tungsten wires etched to form a sharp tip) were coated firstly with a thin layer to act as a diffusion barrier (exampled by Indium Tin Oxide, Titanium Nitride or Tantalum Nitride). Secondly, a thin coating of catalyst metal was applied (e.g. nickel, iron, cobalt). The tips were then loaded into the heater and the reactor was pumped down to a base pressure of 10^"" mBar. The reactor was then filled with an reducing/dilution gas (e.g. ammonia) at a flow of 120sccm, corresponding to a partial pressure of 2.5mbar. The tip was then heated to 700⁰C. Upon reaching the deposition temperature, the heater was biased at -600V to initiate a d. c. glow discharge. The growth gas, normally but not exclusively acetylene, was then inlet for the growth of the nanotip (e.g. carbon nanotube), at a rate of 30 seem (cubic centimetres per minute) and with the total reactor partial pressure at 3.2mbar. The length of the carbon nanotube depends on the deposition time. Upon completion, the gases, plasma and heater are turned off and the tip is allowed to cool to room temperature.

The skilled person will recognise that nano fibres (ie. nanotubes or nanowires) of other materials, for example Zinc Oxide, may be grown with the above described apparatus. The fabrication method can be adapted according to the materials grown by selecting the gaseous feedstock and metal catalyst.

The skilled person will understand that by fabricating a heater with a plurality of apertures in heater 114 a plurality of nanofibre tips may be fabricated in parallel (ie. simultaneously). A separate supporting screw 212 may be provided for each object, object part or filament on which a tip is to be formed, or a single, common support may be employed.

Figures 3a and 3b illustrate electric Field lines at the tip of filament 112 in the absence, and presence respectively of heater conductor 114. It can be seen that when shielded by conductor 114, with the tip 1 12 and heater 114 at substantially the same potential the electric field lines are substantially perpendicular to conductor 114 (when the ground electrode is in the direction in which the tip is pointing). The electric field lines are parallel to tip 112 at its apex.

The results of growing a nanofibre on tip 112 with the electric field distribution of figures 3b are shown in figures 4b and 5b respectively. These show a nanotip 400 comprising a wire tip 1 12 at the end of which, substantially at the apex of the tip and pointing in the same direction of the tip, has been grown a single carbon nanotube 402. The results can be contrasted with the best results of prior art techniques, as shown in figures 4a and 5a, in which a nanotube is attached to the end of a tip using manipulation (figure 5(a) is taken from Niels de Jonge, Yann Lamy, Koen Schoots, Tjerk H. Oosterkamp, Nature 420, 393-395 (2002)). It can be seen that the nanotube is not attached to the end of the tip, nor at the centre of the tip, and nor does it point in a direction parallel to direction in which the tip points. Referring to Figure 5b it can be seen that, at least in some instances, carbon nanotubes (CNTs) grow substantially vertically upward. The vertical growth of the nanotubes at the apex of the wire/filament may be due to one or more of the following list of effects: 1) the electric field at the tip; 2) growth along the general direction of ions at the tip (ie vertical - ions are heavy and gain energy so that they may not much be affected by local field perturbations); 3) vertical ion bombardment/etching at the tip, although other effects may additionally or alternatively play a part. The additional nanotubes attached to the side of Filament 112 do not much affect applications such as an electron gun or scanning probe microscopy tip (in the case of an electron gun the file is much higher at the end of the nanofibre on the tip than at the ends of the nanofibres on the side of the filament).

Figures 6a and 6b show how the nanotips of the Figures 4b and 5b may be incorporated into an electron source and into a scanning probe microscopy tip respectively.

The skilled person will recognise that may variants on the above described apparatus and methods are possible. Embodiments of the heating apparatus can be used to grow nanotubes and nanowires of a variety of materials, including carbon, by placing a substrate on the heater. The aperture 206 in the conductor 114 can be modified according to the application in order to adapt the heater for heating a great variety of objects which are small but which it is desired to heat to a precise temperature, particularly in the presence of a plasma/electric Field. Applications of nanotips fabricated by the above described methods/apparatus include electron gun sources, AFM (Atomic Force Microscopy) tips, STM (Scanning Tunnelling Microscopy) tips, and a range of other structures requiring nanoscale features. For example nanotips fabricated in accordance with the above method/using the above apparatus may be employed to fabricate a Field-emission display pixel.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims

CLAIMS:

1. A heater for heating an object in the presence of an electric field, the heater comprising: a substantially planar electrically conductive heating element configured to define at least one aperture; a support to mount the heated object within said aperture; and at least one electrical connection to said heating element; whereby, in use, the heating element is biassable by said at least one electrical connection such that the electric field in the vicinity of the object is substantially perpendicular to the plane of the element.

2. A heater as claimed in claim 1 wherein said heating element is substantially flat.

3. A heater as claimed in claim 1 or 3 wherein said heating element comprises an electrically conductive plate mounted on a ceramic support.

4. A heater as claimed in claim 3 wherein said conductive plate is spaced away from said ceramic support.

5. A heater as claimed in claim 1, 2, 3 or 4 wherein said support is adjustable to control the protrusion of said object through said aperture.

6. A heater as claimed in any preceding claim wherein said heating element defines a plurality of apertures each having an associated support.

7. A heater as claimed in any preceding claim further comprising a temperature sensor in thermal contact with said heating element for measuring a temperature of said object.

8. A heater as claimed in any preceding claim further comprising a power supply to bias said heating element to control the electric field in the vicinity of the object.

9. A healer as claimed in any preceding claim for nanotip fabrication apparatus, wherein said object comprises a wire.

10. Nanotip fabrication apparatus including the heater of any preceding claim.

1 1. Nanotip fabrication apparatus for fabricating a nanofibre on a tip of an object, the apparatus comprising: a reaction chamber including a first electrode; a gas supply connection for supplying gas to the reaction chamber; a heater, the heater having an electrically conducting surface in which is provided an aperture within which the tip is able to be supported; and first and second electrode connections, said first electrode connection being connected to said first electrode, said second electrode connection being connected to said electrically conducting surface.

12. Nanotip fabrication apparatus as claimed in claim 1 1 wherein said aperture has a dimension of less than lmm.

13. Nanotip fabrication apparatus as claimed in claim 1 1 or 12 wherein said electrically conducting surface has a plurality of said apertures for fabricating a plurality of nanotips simultaneously.

14. Nanotip fabrication apparatus as claimed in claim 11, 12 or 13 wherein said electrically conducting surface is configured such that when, in use, a voltage is applied between said first and second electrode connections an electric field is generated which, in the vicinity of said tip, is substantially in a direction in which the tip points.

15. Nanotip fabrication apparatus as claimed in claim 11 , 12, 13 or 14 wherein said electrically conducting surface is substantially planar in the vicinity of said aperture.

16. A method of healing an object in an electric field, the method comprising: shielding the object from part of the electric field by mounting the object in an aperture in an electrical conductor, said conductor being substantially planar in the vicinity of said aperture; biasing said electrical conductor such that the electrical field in the vicinity of the object is primarily perpendicular to said plane; and heating the object.

17. A method as claimed in claim 16 wherein said heating comprises passing a current through said conductor to electrically heat said conductor to thereby heat said object.

18. A method of fabricating a nanotip on an object, which includes heating the object in an electric field in accordance with the method of claim 16 or 17.

19. A method of fabricating a plurality of nanotips simultaneously by fabricating each nanotip according to the method of claim 18 simultaneously using a said electrical conductor with a plurality of said apertures.

20. A method of fabricating an electron gun or electron gun source, the method comprising heating a sharp metallic object in accordance with the method of claim 18 or 19 to fabricate a nanotip on the object; and fabricating the electron gun or electron gun source using the object.

21. A method of fabricating a tip for a scanning probe microscope, the method comprising heating a sharp metallic object in accordance with the method of claim 18 or 19 to fabricate a nanotip on the object; and fabricating the scanning probe microscope tip using the object.

22. A method of growing a nano fibre on the tip of a metallic object by heating at least the tip of the object in an electric field in the presence of a gaseous supply of material for fabricating the naπofibre, the method including controlling said electric field to be substantially in the direction of said tip during the growing by mounting said tip within an aperture in an electrical conductor.

23. A method as claimed in claim 22 wherein said heating comprises heating using said electrical conductor.

24. A method as claimed in claim 22 or 23 wherein said object comprises a wire.

25. A method of growing a plurality of πanofibres on a respective plurality of object tips in a common reaction chamber, the method comprising growing each nanofibre using the method of claimed 22, 23 or 24 by mounting each tip such that it protrudes through a respective said aperture in an electrical conductor within said common reaction chamber.

26. A method of fabricating an electron gun or electron gun source comprising: growing a nanofibre on the tip of a metallic object according to the method of claim 22, 23, 24 or 25; and fabricating the electron gun or electron gun source using the object.

27. A method of fabricating a tip for a scanning probe microscope comprising: growing a nanofibre on the tip of a metallic object according to the method of claim 22, 23, 24 or 25; and fabricating the scanning probe microscope tip using the object.

28. An object having a nanotip or nanofibre fabricated by the method of any one of claims 18, 19, 22, 23, 24, or 25.

29. An object with a pointed metallic tip and having a nanofibre attached substantially at the end point of said tip.

30. An object as claimed in claim 29 wherein said nanofibre is attached substantially at the centre of said tip.

31. An object as claimed in claim 29 or 30 wherein said nanofibre is substantially parallel to a direction in which said tip points.

32. An object as claimed in any one of claims 29 to 31 wherein a single nanofibre is attached at said end point of said tip.

33. An object in any one of claims 29 to 32 wherein said naπo fibre comprises a carbon nano fibre.

34. An object as claimed in any one of claims 29 to 33 wherein said object comprises a wire.

35. A scanning probe microscope tip including the object of any one of claims 29 to

34.

36. An electron gun or electron gun source including the object of any one of claims 29 to 34.

37. A heater for heating an object in an electrical field, the heater comprising: a shield for shielding the object from part of the electric field, the shield comprising an electrical conductor defining at least one aperture; said conductor being substantially planar in the vicinity of said aperture; and an electrical connection for biasing said electrical conductor such that the electrical field in the vicinity of the object is primarily perpendicular to said plane; and a heater for heating the object.