Abstract:
To provide a ring crystalline body, which is a ring crystalline body with a small diameter and formed with a thin line and capable of providing electric conduction along the ring, and to provide a production method of the ring crystalline body. A droplet is stuck to a surface of a substrate and then the droplet is evaporated to a discontinuous underlayer ring having an ultrafine three-dimensional structure on the substrate surface. After that, when a transition metal dichalcogenide, a transition metal trichalcogenide, or a low-dimensional conductor as raw material gas is evaporated, a ring crystalline body comprising the raw material is grown along the underlayer ring.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a ring crystalline body comprising a circularly continuous crystalline material and a production method thereof and particularly relates to a finely ring crystalline body impossible to be produced by a conventional etching method and a production method thereof.  
           [0003]    2. Description of the Related Art  
           [0004]    A transition metal dichalcogenide (MX2) or a transition metal trichalcogenide (MX 3 ), which is a compound of a transition metal (M) and a chalcogen (X=S, Se, Te), has drawn attention since it has been confirmed that the substance has a variety of electric and magnetic properties of from an insulator, a semiconductor, a semimetal, to a metal and from ferromagnetic, anti-ferromagnetic properties to Pauli paramagnetism.  
           [0005]    Further, corresponding to the anisotropy of the crystal structure and the electron structure of the substance, a low-dimensional conductor which is a compound showing a high electric conductivity in the one-dimensional or two-dimensional direction, has the following characteristics: its property is transformed (Peiers transition) from a metallic property to an insulator at a low temperature; the maximum electric resistivity appears at a low temperature and owing to it, super period lattice strain (electric charge density wave) is caused: and Kohn anomaly, that the modulus of elasticity is considerably lowered by lattice vibration in specified cycles, takes place. A low-dimensional conductor having these characteristic physical properties, which a general three-dimensional conductor of such as a metal does not have, has been expected to be applied for a variety of fields.  
           [0006]    Further, the crystalline body comprising a continuously circular crystalline ring is used as, for example, a superconducting quantum interference device (SQUID). The SQUID is a superconductor-based magnetic sensor with high sensitivity and is capable of measuring extremely ultrafine magnetic flux density as extremely low as 10 −15  to 10 −12  T by using Josephson device in which a superconductor is weakly bonded and the SQUID is used for measuring the magnetic field generated following the activities of living bodies.  
           [0007]    The ring crystalline body to be used for the SQUID has conventionally been produced by forming a thin film comprising a three-dimensional conductor with a prescribed size on the surface of a substrate by a conventional thin film production method and etching the thin film in ring-like shape.  
           [0008]    In the production of SQUID, it is made possible to form a fine ring with the diameter of about several μm owing to the progress of the etching technique. However, even although the present etching technique makes it possible to narrow the diameter of the ring, it can make the line width of the ring at thinnest about 100 nm and further thinner width has therefore been expected. Also, the method of etching the thin film has a problem that the method is accompanied with much waste to make it difficult to reduce the cost.  
           [0009]    Further, it has been well known that a superconductive material such as NbSe 3  which is one of the above described transition metal trichalcogenide and one-dimensional conductor, NbSe 2  which is one of the transition metal dichalcogenide and two-dimensional conductor, and the like can be obtained in forms of thin-film like or whisker-like crystals. In the case that a ring crystalline body is produced by forming a thin film comprising NbSe 3  or NbSe 2  and then etching the film, the obtained body is a low-dimensional conductor, so that electric conductivity along the ring cannot be obtained and it means that the obtained ring crystalline body is impossible to be used for a SQUID, for which electric conductivity along a ring is essential. Consequently, a ring crystalline body comprising a three-dimensional conductor has solely been used for a SQUID.  
           [0010]    However, taking the capabilities as a superconductor into consideration, a one-dimensional conductor as well as a two-dimensional conductor is preferable as compared with a three-dimensional conductor. If it is possible to obtain a ring crystalline body of such a low-dimensional conductor having electric conductivity along the ring, it is expected that the capabilities of a SQUID are further improved. Moreover, a one-dimensional conductor, a two-dimensional conductor, and a three-dimensional conductor respectively have different manners to have superconductivity and a ring crystalline body comprising a low-dimensional conductor is therefore supposed to be useful for purposes other than a SQUID.  
         SUMMARY OF THE INVENTION  
         [0011]    Hence, an object of the present invention is to obtain a ring crystalline body made of an ultra thin line with a fine diameter, which a conventional etching technique could not provide, and to obtain a novel production method thereof. Another object of the invention is to provide a ring crystalline body capable of reliably providing electric conductivity along the ring even if it is a low-dimensional conductor and a production method thereof.  
           [0012]    In order to achieve these objects, a first aspect of the present invention provides a ring crystalline body comprising circularly continuous crystalline transition metal dichalcogenide or transition metal trichalcogenide.  
           [0013]    Further, a second aspect of the present invention provides a ring crystalline body comprising a circularly continuous crystalline low-dimensional conductor.  
           [0014]    The foregoing transition metal dichalcogenide or transition metal trichalcogenide includes, for example, NbSe 3 , NbSe 2 , TaSe 3 , TaSe 2 , TaS 2 , MoS 2 , and the like. These NbSe 3 , NbSe 2 , TaSe 3 , TaSe 2 , TaS 2 , MoS 2 , and the like are also low-dimensional conductors.  
           [0015]    The ring crystalline body comprising these materials has a totally novel structure body. Such a transition metal dichalcogenide, a transition metal trichalcogenide, or a low-dimensional conductor includes a superconductive material. A ring crystalline body comprising a superconductive material or the like may be used, for example, as an element of a SQUID in an optional shape. In the case of a circular shape, the ring crystalline body makes it possible to precisely measure a scarce signal emitted out a living thing and a living body and is thus extremely useful.  
           [0016]    Further, when a large number of the ring crystalline bodies are partly cut and joined in a spring-like shape, a nano-spring can be obtained and further, since the ring crystalline body made of the above described materials can generate an intense magnetic field in a narrow region, if a plurality of the ring crystalline bodies are joined to be a coil-like shape, a nano-actuator can be produced. Besides, a nano-ball bearing can be produced by disposing the ring crystalline bodies double.  
           [0017]    In addition to those, the ring crystalline body can be used for a quantum computer, an ultrafine battery utilizing inter-current function, a memory based on permanent current and the like and applicable to a wide range of the application fields and remarkably useful in the industrial sphere.  
           [0018]    The shape of the ring crystalline body is approximately circular or elliptical and the size is preferably approximately 0.1 to 10 μm in the diameter in the case of the circular shape and in the major axis in the case of the elliptical shape and several to several tens nm in the line width.  
           [0019]    Preferably, the crystallinity is a single crystal. Even in the case the ring crystalline body is of a low-dimensional conductor, if the crystallinity is a single crystal, electric conductivity is obtained along the ring, so that the ring crystalline body can be used as an element of a SQUID and its application fields are widened.  
           [0020]    In order to produce such an ultrafine ring crystalline body, another aspect of the invention provides a novel production method of the ring crystalline body comprising steps of forming a continuous or discontinuous underlayer ring having an ultrafine three-dimensional structure in the substrate surface and circularly growing a crystal along the underlayer ring.  
           [0021]    Usable for the material of the substrate is glass, quartz, silicon, diamond, sapphire, and the like.  
           [0022]    Further, the underlayer ring is to be a point of starting the growth of the crystal and preferably a ring composed of solely continuously circular particles of elements composing the crystal to be grown thereafter or a ring composed of a large number of fine particles comprising the elements arranged discontinuously and circularly. Alternatively, the particles and ultrafine particles may be of elements completely different from those composing the crystal. Even in the case of ultrafine particles of elements different from those of the crystal, these ultrafine particles are not diffused in the crystal and do not become contaminant substances in the crystal.  
           [0023]    According to such a method, it is made possible to produce a ring crystalline body with a size as extremely fine as 0.1 to 10 μm diameter and 10 nm line width, which is impossible to be produced by a conventional method. Further, the production speed is high and mass production at a high efficiency on the bases of industrial scale can be also possible. Moreover, since no costly apparatus is required, an economical product can be provided.  
           [0024]    Further, regarding the formed ring crystalline body, those with any optional diameter size can be produced by changing the size of the underlayer ring, and the line width and the thickness of a ring crystalline body can optionally be adjusted by controlling the growth of the crystal. Moreover, a ring crystalline body can be formed into a tubular shape by increasing the thickness of the ring crystalline body.  
           [0025]    Further, since the ring crystalline body is made of a single crystal, even if the crystal is, for example, a low-dimensional conductor, electric current flows along the ring and therefore, the ring crystalline body can be used as a material for a SQUID.  
           [0026]    Incidentally, this method can make it possible to produce not only a ring crystalline body comprising the transition metal dichalcogenides, transition metal trichalcogenide, and low-dimensional conductors, but also a ring crystalline body comprising other metal materials and organic materials.  
           [0027]    Preferably, the underlayer ring is formed by sticking droplets of an underlayer ring material to the substrate surface and evaporating the droplets.  
           [0028]    The method for sticking droplets to the substrate surface includes a method sticking droplets of an underlayer material by evaporation and a method comprising the steps of putting an ultrafine underlayer material on the substrate surface by atomic tweezers, liquefying the underlayer material by heating the substrate, and evaporating the liquefied droplets.  
           [0029]    Such droplets become approximately perfectly circular in the substrate surface and are evaporated by heating to leave ultrafine particles in the circumferential parts of the approximately perfectly circular shape. The materials of the droplets may properly be selected taking their wettability to the substrate and their surface tension into consideration.  
           [0030]    The size of the underlayer ring is determined by the size of the droplets and for example, in the case droplets of an underlayer material are stuck to the substrate surface by evaporation, the size of the droplets can be controlled by the supply amount of the underlayer material to be evaporated and the temperature of the substrate. The diameter of the droplets is preferably set to be within a range from 0.1 to 10 μm.  
           [0031]    As the underlayer material, constituent elements of a crystal to be grown can be used, however it is not limited to them. For example, in the case of producing a ring crystalline body comprising a transition metal dichalcogenide or a transition metal trichalcogenide, chalcogen elements which are contained in the crystalline body or chalcogen elements which are not contained in the crystalline body may be used for the underlayer material. Further other materials, for example, an inorganic material such as Al 2 O 3 , MoS 2  and the like, and an organic material can be used for the underlayer material.  
           [0032]    Incidentally, in the case the underlayer ring is required to be approximately elliptical shape, for example, by using a Ni type magnetic material as the underlayer material and applying an electric field or a magnetic field to the droplets, the upper face shapes of the droplets can be formed to be elliptical and by evaporating the droplets in such a state, an approximately elliptical underlayer ring can be produced.  
           [0033]    Further, if a plurality of droplets are stuck to the substrate surface while neighboring one another, underlayer rings with a shape composed of a plurality of circles adjoined to one another can be formed to make it possible to produce a plurality of ring crystalline bodies adjoined to each other along the underlayer rings.  
           [0034]    Also preferably, the crystal is grown by evaporation or by sputtering.  
           [0035]    Means for, for example, thermal CVD and plasma CVD may be employed for the evaporation and a technique conventionally well-known as a film formation technique in the semiconductor fabrication can be employed.  
           [0036]    Various conditions at the time of crystal growth, the temperature of the substrate, the flow rate of raw materials and the like in the case of evaporation are controlled, so that the line width and thickness of the ring crystalline bodies to be obtained can freely be controlled and further the crystal structure can also be freely controlled and ring crystalline bodies of a single crystal can be formed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]    [0037]FIGS. 1A and 1B schematically show a production method of an underlayer ring in a production method of a ring crystalline body according to the present invention.  
         [0038]    [0038]FIGS. 2A and 2B schematically show a production method of an underlayer ring in the production method of a ring crystalline body according to the present invention.  
         [0039]    [0039]FIG. 3 is an electron microscopic photograph of a ring crystalline body of NbSe 3 , which is a preferable example of the present invention.  
         [0040]    [0040]FIG. 4 is an electron microscopic photograph of a ring crystalline body in an 8-shape form, which is another preferable example of the present invention.  
         [0041]    [0041]FIG. 5 is an electron microscopic photograph of a ring crystalline body in a tubular form, which is the other preferable example of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    Hereinafter, a production method of a ring single crystal body according to an embodiment of the present invention will be specifically described with reference to the accompanying drawings.  
         [0043]    [0043]FIG. 1 and FIG. 2 are illustrations schematically showing a production method of an underlayer ring.  
         [0044]    At first, droplets  2  of the underlayer material is evaporated on a surface  1   a  of a substrate  1  and stuck to it. The side view of the droplet  2  at that time and a top plan view thereof are respectively illustrated in FIG. 1A and in FIG. 1B. Since the surface tension of the droplets  2  is changed according to the temperature of the substrate  1 , the diameter size of the droplets  2  can be controlled by the temperature of the substrate  1 . The pressure at that time is so controlled to be at the vacuum degree at which the droplets and the equilibrium vapor pressure are kept.  
         [0045]    Further, the droplets  2  are heated by increasing the temperature of the substrate  1  bearing the droplets  2  to evaporate droplets  2 . A side view of the substrate  1  after the evaporation is illustrated in FIG. 2A and a top plan view thereof is illustrated in FIG. 2B. When the droplets  2  of the underlayer material are evaporated, the underlayer material becomes an inorganic polymer during the melting and after melting and a large number of fine particles  3 a are discontinuously circularly arranged along the circumferences of the droplets  2  to form an underlayer ring  3 .  
         [0046]    The substrate  1  having the underlayer ring  3  formed is then subjected to a conventionally known treatment such as evaporation or sputtering to produce a ring crystalline body with crystalline properties along the underlayer ring  3 . At that time, the conditions of evaporation or sputtering are properly set, so that a ring crystalline body of a single crystal or a polycrystal can be obtained.  
         [0047]    Hereinafter, the present invention will be described according to specific examples given below.  
       EXAMPLE 1  
       [0048]    A droplet of Se as an underlayer material was stuck to the surface of a glass substrate by evaporation. The substrate temperature was set to be about 300° C. and the vacuum degree was set to be about 133 to 1330 Pa at that time. When the temperature of the substrate was heated up to 600° C. to evaporate the droplet, an approximately perfectly circular underlayer ring with the diameter of 300 nm to 500 μm was formed.  
         [0049]    When the substrate having the underlayer ring formed in such a manner was heated in a tubular quartz furnace at 700 to 800° C. while raw material gas (NbSe 3 ) being passed through the furnace to carry out evaporation, a crystal of NbSe 3  was grown circularly along the underlayer ring and a ring crystalline body was produced. An electron microscope photograph is shown in FIG. 3.  
         [0050]    The ring crystalline body of NbSe 3  was observed by x-ray diffraction and electron beam diffraction to find it was a single crystal. Further, an investigation carried out into the electric conduction made it clear that electric conduction along the ring was obtained. Further, in the case the ring crystalline body of NbSe 3  was further converted to be a superconductor and used as a SQUID element, it was supposed to be possible to obtain a highly capable SQUID.  
         [0051]    Further, in the case of using S type TaS 3  and NbS 3  and Te type NbTe 3 , and TaTe 3  in place of the Se for the underlayer material to produce an underlayer ring in the same manner as that in case of using Se and to carry out evaporation of NbSe 3  on the underlayer ring in the above described conditions, a ring crystalline body of the same single crystal as that of the case using Se was obtained.  
       EXAMPLE 2  
       [0052]    In the same evaporation conditions as those of the example  1 , droplets of Se and Ni were stuck to the substrate surface as underlayer materials. After that, if the droplets were evaporated in the same conditions as those of the example 1 while an electric field or a magnetic field being applied to the droplets, an underlayer ring with approximately elliptical shape was formed. When NbSe 3  was evaporated in the underlayer ring with the elliptical shape, a ring crystalline body with an approximately elliptical shape was obtained. The observation of the crystalline body by an x-ray diffraction and an electron beam diffraction made it clear that the obtained crystalline body was of a single crystal. Further, the electric conduction along the elliptical shape was obtained.  
       EXAMPLE 3  
       [0053]    Two droplets of Se as a underlayer raw material were stuck to the surface of a glass substrate while being adjoined to each other in the same evaporation conditions as those of the example 1. After that, an underlayer ring in an approximately 8-shape form in which two circles are adjoined to each other was formed by carrying out evaporation of droplets in the same conditions as those of the example 1. While the underlayer ring being floated in vacuum in form of a twisted approximately 8-shape, NbSe 3  was evaporated to obtain a continuous crystalline body composed of two circular crystalline bodies adjoined to each other in form of a twisted approximately 8-shape. An electron microscopic photograph of the crystalline body is shown in FIG. 4. The observation of the crystalline body by x-ray diffraction and electron beam diffraction made it clear that the crystalline body with 8-shaped form was of a solely single crystal. Further, the electric conduction continuous along the 8-shaped form was obtained.  
       EXAMPLE 4  
       [0054]    An underlayer ring of Se in an approximately perfect circle shape of 300 nm to 500 μm diameter was formed on a glass substrate in the same conditions as those of the example  1 . When the substrate having the underlayer ring formed was heated in a tubular quartz furnace at 700 to 800° C. while NbSe 2  being passed through the furnace to carry out evaporation, a crystal of NbSe 2  was grown circularly along the underlayer ring and a ring crystalline body was produced.  
         [0055]    The observation of the crystalline body of NbSe 2  by x-ray diffraction and electron beam diffraction made it clear that the crystalline body was a single crystal. Further, the investigation into the electric conduction made it clear that the electric conduction along the circular shape was obtained. Further, in the case the ring crystalline body of NbSe 2  was further converted to be a superconductor and used as a SQUID element, it was supposed to be possible to obtain a highly capable SQUID.  
       EXAMPLE 5  
       [0056]    An underlayer ring in an approximately perfect circle shape was formed on a glass substrate in the same conditions as those of the example 1 and when the substrate having the underlayer ring formed was heated in a tubular quartz furnace at 700 to 800° C. while raw material gas (NbSe 3 ) being passed through the furnace to carry out evaporation, a crystal of NbSe 3  was grown circularly along the underlayer ring and a ring crystalline body was produced. When the NbSe 3  gas evaporation was continued, a tubular ring crystalline body was obtained. An electron microscopic photograph of the tubular ring crystalline body is shown in FIG. 5.  
         [0057]    The observation of the tubular crystalline body of NbSe 3  by x-ray diffraction and electron beam diffraction made it clear that the tubular crystalline body of NbSe 3  was a single crystal. Further, the investigation into the electric conduction made it clear that the electric conduction along the circular shape was obtained.  
         [0058]    The present invention was not at all limited to the above described examples and crystalline bodies in a twisted  8 -shape form just like so-called Mobius&#39;s strip and in a coil-like form can be obtained by changing the various conditions at the time of the underlayer ring formation and evaporation.  
         [0059]    Further, besides the above described transition metal dichalcogenide (MX 2 ) or transition metal trichalcogenide (MX 3 ), a variety of combinations of M and X wherein M=Nb, Ta, and Mo and X=S, Se, and Te are possible. Further, the ring crystalline bodies comprising a variety of types of organic materials can be produced.