WO2003018884A1

WO2003018884A1 - Method for producing optically transparent and electroconductive fibres and the sensor of scanning probe microscope made of this fibre

Info

Publication number: WO2003018884A1
Application number: PCT/EE2002/000007
Authority: WO
Inventors: Tanel TÄTTE; Tea Avarmaa; Rünno LÖHMUS; Uno MÄEORG; Mats-Erik Pistol; Ilmo Sildos; Ants LÕHMUS
Original assignee: University Of Tartu
Priority date: 2001-08-24
Filing date: 2002-08-22
Publication date: 2003-03-06
Also published as: EE04785B1; EE200100450A

Abstract

A method for producing optically transparent and electro-conductive fibres consists of the heating of four valent tin alkoxide at 100-200°C temperatures and 0.1-20 mmHg pressure in an anhydrous environment until 1-2 Pa.s viscosity is reached, and of subsequent addition of special dopant. The fibres are made of the mixture and inside a gaseous environment at 5-100 % relative humidity their structure becomes gel-like. The fibres are subsequently heated up to 300-1500°C, which transforms their structure to Sn1-yZyO2, where Z is a metal of V group and in which concentration is 0.3 parts per mol. A sensor of scanning probe microscope that can operate simultaneously as a STM probe and SNOM probe differs because it is made from the optically transparent and electroconductive fibre made by the method described in this invention and has diameter of 5-100 sg(m)m and length of 0.1.500 mm whereas at least one end of the fibre has the shape of a cone with 1-60° tip angle 0.5-250 nm tip radius.

Description

METHOD FOR PRODUCING OPTICALLY TRANSPARENT AND

ELECTROCONDUCTIVE FIBRES AND THE SENSOR OF SCANNING PROBE MICROSCOPE MADE OF THIS FIBRE

TECHNICAL FIELD

The invention of an optically transparent and el ectroconductive fibre that can be used as a scanning probe microscope ( SPM) tip belongs to the field of chemical technology and instrument design. In addition it can be used as gas analyser e.g. alcohol vapour detector.

BACKGROUND ART

Scanning tunnelling microscope ( STM) tip must be electrically conductive enough to initiate and sustain the tunnelling current between the tip and the object. The tip of a scanning optical near-field microscope ( SNOM) must be optically transparent. A combination of STM and SNOM requires a tip that is simultaneously optically transparent and el ectroconductive. Several semi -conductive oxides (doped Sn0₂, ln₂0₃, indium-tin oxide, etc.) and boron-doped diamond meet this requirement.

Simultaneous operation of the scanning optical near-field microscope and the scanning tunnelling microscope is important because the use of both modes makes it possible to obtain information on the sample from exactly the same location at the same time.

The optically transparent and electro -conductive Sn0₂ exists as a crystal or as a thin film on a solid surface. There are several methods for the production of el ectroconductive films on a solid surface by the sol -gel technology using tin (IV) butoxide (e.g. J. R. Gonzalez- Oliver, I. Kato, Non-Cryst. Solids, 82 (1986) 400), and SnCl₂-H₂0 (C. Terrier et al, Thin Solid Films, 263 (1995) 37) as the starting compound, also by the spray-pyrolysis where conductivity is due to antimony doping implanted during the spraying ( B. Correa -Lo zano et al J. Appl. Chem. , 26 (1996) 83). None of the above-mentioned methods can be applied for the production of Sn0₂ fibres and the SPM tips.

An epitaxial growth of boron -doped diamond films on a solid surface ( E. Oesterschul ze, et al Appl. Phys. Lett. 70 (1997) 435) or deposition of boron-doped diamond crystals on a metal surface ( N. Liu et al 12(3) (1994) 1712) are used for production of el ectroconductive tips. These methods produce tips of superior hardness for scanning atomic force microscopes and STMs. However, they cannot be used in SNOM devices because their shapes and sizes prohibit their optical excitation and registration of optical signals from their surface. Another method for producing el ectroconductive tips is to polish a boron -doped diamond that is soldered into titanium cell ( R. Kaneko, S. Oguchi, J. Appl. Phys 29(9) (1990) 1854). The description of their methods, however, lacks data about optical properties of the sample. The method for producing Snι-_yZ_y0₂ fibres, where Z stands for a V group metal (T. Sode, K. Kaisha, US patent 5330833, D02G 3/00) is the closest method to the present invention. Z doping is added to increase the electric conductivity of the material. The concentration of it does not exceed 0.3 parts per mol (ppmol) and it forms a solid solution with the host substance. The material consists of a small fraction of oxygen vacancies. 0.02-0.5 ppmol of SnCl₂, SnBr₂ or SnCl₂- H₂0 is dissolved in alcohol solution and treated thermally between 150 - 1550 ^*C, preferably between 300 - 1500 °C. The disadvantage of the method is its non -applicability for the production of conductive and transparent tips with necessary shapes and sizes for STM and SNOM devices. A STM tip that can also be applied as a SNOM tip must be el ectroconductive and optically transparent. In addition, it has to be sharp and hard enough for the STM operation. It has to maintain its properties also at low temperatures if applied in low -temperature SNOM devices.

The prototype of the present invention is the tip described in "Low-temperature tunneling -el ectron luminescence microscopy using tip collection", T. Murashita, J. Electron Microscopy, 46 (3) (1997) 199-205. The optically transparent and el ectroconductive tip can emit tunnel electrons and collect luminescence photons from the surface of an object.

A silicon oxide optical fibre was used because of its applicability in the ultra high vacuum and in the large temperature range of 10 - 450 K that is suitable in most cases.

The detector end of the fibre is polished and coated with an anti -reflecting coating. The sample end of the fibre is sharpened by chemical etching and mechanical strain. The flame-shape tip has its tip angle of about 60° and the tip radius of about 100 nm. The spatial accuracy of collected luminescence depends on the shape and radius of the tip (in the case of an obtuse tip, the tunnel electron current collimates because of the quantum dot effect) . It is necessary to coat the tip with a transparent and conductive layer because quartz is an insulator. Two - layer In₂0₃/Au coating is used. In₂0₃ layer is optically transparent and has low resistance. Its structure consists of loosely connected nanometer size clusters that decrease the spatial resolution of the tip and is extremely fragile. For the protection, the layer is coated with an ultra thin layer of gold. Total transparency of the layers is over 75% between 600-900 nm and surface resistance 100 Ω/cm which is sufficient for the STM tunnelling current.

However, the method is very complicated and demands highly complex and expensive apparatus. The tip consists of three components, manufacturing of which demands extreme preciseness.

DISCLOSURE OF THE INVENTION

The aim of the invention is to offer a method for producing an optically transparent and el ectroconductive fibre and a SPM tip made of this material. The core of the invention - the optically transparent and el ectroconductive fibre - is made of a gel -like compound of tin which is inside of a gaseous atmosphere at the relative humidity of 5-100%. The tin compound is made by heating four-valent tin alkoxide at the temperature of 100 -200 °C and at the pressure of 0.1-20 mmHg in a dry environment until viscosity of 1-2 Pa*s is reached and by subsequent implanting of dopants.

The optically transparent and el ectroconductive SPM -SNOM tip is made of fibre with 5-100 μm diameter and 0.1-500 mm length. One (or both) end(s) of the tip is converging, having, for example, the shape of a cone with the tip angle of 1-60° and with the tip radius of 0.5-250 nm. The method is based on the sol -gel technique, which is a well known method in thin film technologies. The doped Sn0₂ is used because of its is optical transparency and electroconductivity, hardness (6-7 on the Mohs' s scale), and its usability at low temperatures (4 K and lower). The fibre is made of four-valent tin alkoxide which is heated up to 100 -200 "C until the desired viscosity of 1-2 Pa-s is reached. The choice of the alkoxide is determined by its ability to polymerize and reach necessary viscosity in vacuum at the temperatures of 100-200° C. If temperatures higher than 200 °C are used, the viscosity of the material remains too low (less than 1 Pa-s). If temperatures lower than 100 'C are used, the necessary viscosity will be reached in more than 3 hours. Viscosity higher than 2 Pa-s results in tip shapes unsuitable for SPM operation. The heating is performed in the 0.1-20 mmHg vacuum, without any addition of water. Higher vacuum would demand a more expensive pumping equipment. However, at lower vacuum, too large quantities of the solvents remain in the mixture, thus lowering the viscosity to unwanted values. The temperature range of 100 -200 °C is necessary for removing the solvents and other unwanted end products from the material at 0.1-20 mmHg pressure. Alternatively, necessary viscosity can be reached at the temperature range of -100 - 100°C. Below - 100"C the material becomes too hard and above 100 'C the lowest suitable viscosity of 1 Pa-s cannot be reached. The material can be repeatedly dissolved and subsequently dried in organic anhydrous solvents without affecting its properties. This property is important for doping procedures. To accelerate the process of doping procedures, the material is dissolved in an organic solvent (heptane, hexane, butanol, etc.). The possible solvents must dissolve both the host material and the dopant and should be removed by vacuum evaporation process. The dopant is implanted as a pure substance, solution of a salt, alkoxide, or another compound in an organic solvent like alcohol. The mixture is treated until it becomes homogeneous. Usually 0.1-2 % per mol of SbCl₃ is used as the doping. Higher concentrations (above 2%) result in more fragile and less transparent material. At lower concentrations (lower than 0,1 %) the conductivity drops below the critical value for the tunnelling current. The solvents are removed by evaporation process, (e.g. a rotary evaporator) until viscosity of 1-5 Pa-s, necessary for the making of the fibres, is reached. The fibres are spun -off from the initial mixture into a gaseous atmosphere, at room temperature and at 5-100% relative humidity. The latter is necessary for a surface gelation process to occur at speeds that allow application of mechanical treatment of the fibre, which can be, for example, inserting a rod into the material and its subsequent pull into the gel -forming atmosphere that sharpens the tip. Pushing the material through a thin hole and the subsequent sharpening of the tip can be an alternative method for the production of the fibres. Also, the chemical etching with mineral acids or alkalis can be applied. Besides the mechanical and chemical treatment, the tips can also be sharpened by a light beam that forms a 1-60° angle with the axe of the fibre and that can be produced e.g. by an excimer laser with a pulse duration up to 10^"7 sec. Lower humidity results in too slow surface gelation process. The mechanical technology consists of spinning the viscous and gel - forming material off into the humid environment where the fibres stay for at least 48 h which is necessary for completing the polymerization, hydrolysis and the removal of evaporating organic substances that can reduce the transparency after the heating procedure.

Thereafter the fibres are heated up to the temperatures of 350-800°C, raising the temperature at 1-25 "C/h from room temperature up to 180 'C and after that at 10-100°C/h till the final temperature. Temperatures below this range do not ensure sufficient conductivity. At higher temperatures, however, the material becomes opaque, and brakes. During the heating, the material crystallizes and becomes transparent and conductive. Faster heating may cause breakage of the fibres. However, thinner fibres (diameters less than 70 μm) can sustain faster changes and higher temperatures. Slower heating and cooling procedures are not practical.

If the material jet brakes during the spin-off procedure, the end of the fibre takes proper shape for a SPM tip.

The other end of the fibre stays thick enough for registering optical signals from the fibre surface or with an optical system connected to the fibre.

DESCRIPTION OF EMBODIMENT

Four valent tin butoxide was heated up to 130 ± 10 °C for production of transparent and conductive fibre till the decomposition of butoxide and till viscosity of 1.2 Pa-s due to polymerisation of decomposition products was reached. The heating was carried out at 5 ± 1 mmHg pressure without any addition of water. The material was subsequently dissolved in heptane and doped with crystalline anhydrous SbCl₃ where the dopant concentration was 0.5±0.2 % per mol. The mixture was made homogeneous in an ultrasonic bath and the solvent was removed with a rotary evaporator at 5±1 mmHg pressure and 80 "C temperature.

The fibres were spun -off from the mixture into a gaseous environment (50 + 10 % relative humidity and 20 °C (room temperature)) with a glass rod where they were left for four days, after which the temperature was raised up to 180°C at 5±l°C/h and up to 550°C at 30±5 °C/h. During the heating, the fibres crystallised and became optically transparent and el ectroconductive. During the spin-off, the fibres broke and a sharp tip was formed at the end of the fibre, whereas the other end of the fibre remained thick enough to collect optical signal either directly from the surface or with help of an optical fibre connected to the fibre.

The resistance of the 50 ±20 μm diameter fibre was 2-5 kΩ/mm at room temperature and 50-100 kC at 6±1K. Its tug strength was 50-100 N/mm².

Applications of the invention are not restricted to the example described above and variations are possible within the following claims.

Claims

1. The method for producing optically transparent and electro -conductive fibres by spinning off the fibre from a mixture of tin compounds and dopants into a gaseous environment at room temperature, and subsequent heating up to 300-1500 °C that forms the fibres of Snι__yZ_y0₂ where Z is a metal of V group and which concentration is 0.3 parts per mol, c h a r a c t e r i z e d in that the fibres are made from the mixture of tin compounds that forms a gel -like structure in a gaseous environment with 5-100% relative humidity and that is made by heating four valent tin alkoxide at 100-200 °C temperatures at 0.1-20 mmHg pressure in a anhydrous environment until 1-2 Pa-s viscosity is reached and by subsequent addition of special dopant up to 10% concentration of relative to Sn0₂ (below called initial mixture).

2. The method according to claim 1, c h a r a c t e r i z e d in that the fibres are made by heating four valent tin butoxide.

3. The method according to claim 1, c h a r a c t e - r i z e d in that the fibres are made by inserting a rod into the initial mixture and the subsequent pull out into a humid environment, i. e. spinning the fibre off in the humid environment.

4. The method according to claim 1, c h a r a c t e - r i z e d in that the fibres are made by pushing the initial mixture through a hole.

5. The method according to claim 1, c h a r a c t e r i z e d in that the fibres are made at the temperatures of -100 -100°C from the initial mixture at lower viscosity than 1 Pa-s.

6. The sensor of scanning probe microscope that can operate simultaneously as a STM probe and SNOM probe c h a r a c t e r i z e d in that it consists of optically transparent and el ectroconductive fibre with the diameter of 5-100 μm and length of 0.1-500 mm, whereas at least one of the ends of the fibre has the shape of a cone with 1-60° tip angle and 0.5-250 nm tip radius.

7. The sensor of scanning probe microscope according to claim 6 c h a r a c t e r i z e d in that it consists of the fibre which is made by inserting a rod into the initial mixture and pulling it into a humid environment, i.e. spinning -off the fibre in the humid environment.

8. The sensor of scanning probe microscope according to claim 6 c h a r a c t e r i z e d in that it consists of mechanically sharpened fibre.

9. The sensor of scanning probe microscope according to claim 6 c h a r a c t e r i z e d in that it consists of chemically etched fibre.

10. The sensor of scanning probe microscope according to claim 6 c h a r a c t e r i z e d in that it consists of the fibre that has been sharpened by a light.