Patent Publication Number: US-10763068-B2

Title: Electron emission element and method for same

Description:
TECHNICAL FIELD 
     The present invention relates to an electron emitting device and a method of producing the same. 
     BACKGROUND ART 
     The applicant has developed electron emitting devices having a novel structure, which are capable of operating in the atmospheric air (see, for example, Patent Documents 1 and 2). 
     The electron emitting device which is described in Patent Document 2 includes a semi-conductive layer which is interposed between a pair of electrodes (i.e., a substrate electrode and a surface electrode), the semi-conductive layer being composed of a dielectric material with electrically conductive nanoparticles dispersed therein. By applying a voltage on the order of several dozen volts to the semi-conductive layer, electrons can be emitted from the surface electrode (field electron emission). Therefore, unlike any conventional electron emitting device (e.g., a corona discharger) that utilizes a discharge phenomenon under a strong field, this electron emitting device has an advantage in that ozone will not be generated. 
     This electron emitting device can be suitably used as a charger device for charging a photosensitive drum of an image forming apparatus (e.g., a copier machine), for example. According to Non-Patent Document 1, an electron emitting device that includes a surface electrode of the layered structure described in Patent Document 2 may have a lifetime of about 300 hours (equivalent to approximately 300,000 sheets in the case of a medium-fast copier machine) or more. 
     CITATION LIST 
     Patent Literature 
     [Patent Document 1] Japanese Laid-Open Patent Publication No. 2009-146891 (Japanese Patent No. 4303308) 
     [Patent Document 2]Japanese Laid-Open Patent Publication No. 2016-136485 
     Non-Patent Literature 
     [Non-Patent Document 1]Tadashi IWAMATSU et al., NIHON GAZO GAKKAISHI (Journal of the Imaging Society of Japan), Vol. 56, No. 1, pp. 16-23, (2017) 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, there is a desire to improve the characteristics and/or prolong the lifetime of the aforementioned electron emitting device. Accordingly, an objective of the present invention is to provide: an electron emitting device having a novel structure, such that the characteristics of the electron emitting device can be improved and/or its lifetime can be prolonged; and a method of producing the same. 
     Solution to Problem 
     An electron emitting device according to an embodiment of the present invention comprises a first electrode, a second electrode, and a semi-conductive layer provided between the first electrode and the second electrode, wherein the semi-conductive layer includes a porous alumina layer having a plurality of pores and silver supported in the plurality of pores of the porous alumina layer. 
     In one embodiment, the first electrode is formed of an aluminum substrate or an aluminum layer, and the porous alumina layer is an anodized layer formed at a surface of the aluminum substrate or at a surface of the aluminum layer. 
     In one embodiment, the first electrode is formed of an aluminum substrate containing aluminum in an amount of not less than 99.00 mass % but less than 99.99 mass %, and the porous alumina layer is an anodized layer formed at a surface of the aluminum substrate. 
     In one embodiment, aluminum is contained in an amount of 99.98 mass % or less in the aluminum substrate. 
     In one embodiment, the porous alumina layer has a thickness which is not less than 10 nm and not more than 5 μm. 
     In one embodiment, the plurality of pores have an opening having a two-dimensional size which is not less than 50 nm and not more than 3 μm as viewed from a normal direction of a surface thereof. 
     In one embodiment, the plurality of pores of the porous alumina layer have a depth which is not less than 10 nm and not more than 5 μm. The plurality of pores of the porous alumina layer may have a depth which is not less than 50 nm and not more than 500 nm. 
     In one embodiment, a barrier layer included in the porous alumina layer has a thickness which is not less than 1 nm and not more than 1 μm. A barrier layer included in the porous alumina layer may have a thickness of 100 nm or less. 
     In one embodiment, the plurality of pores of the porous alumina layer have a stepped side surface. The plurality of pores have, along a depth direction, two or more pore subportions with mutually differing pore diameters, such that any pore subportion at a deeper position has a smaller pore diameter. 
     In one embodiment, the silver contains silver nanoparticles having an average particle size which is not less than 1 nm and not more than 50 nm. The silver may contain silver nanoparticles with an average particle size which is not less than 3 nm and not more than 10 nm. 
     In one embodiment, the second electrode includes a gold layer. The second electrode has the layered structure described in Patent Document 2. 
     A method of producing of an electron emitting device according to an embodiment of the present invention is a method of producing any of the above electron emitting devices, comprising: a step of providing an aluminum substrate or an aluminum layer supported by a substrate; a step of anodizing a surface of the aluminum substrate or the aluminum layer to form a porous alumina layer; and a step of applying silver nanoparticles in a plurality of pores of the porous alumina layer. 
     In one embodiment, the step of forming the porous alumina layer comprises an anodization step and an etching step to be performed after the anodization step. 
     In one embodiment, the step of forming the porous alumina layer comprises a further anodization step after the etching step. 
     Advantageous Effects of Invention 
     According to an embodiment of the present invention, there is provided: an electron emitting device having an a novel structure such that its characteristics can be improved and/or its lifetime can be prolonged as compared to the aforementioned conventional technique; and a method of producing the same. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A schematic cross-sectional view of an electron emitting device  100  according to an embodiment of the present invention. 
         FIG. 2 ( a ) through ( c )  are schematic cross-sectional views for describing a method of producing the electron emitting device  100  according to an embodiment of the present invention. 
         FIG. 3 ( a ) through ( c )  are schematic cross-sectional views showing examples of porous alumina layers for use as the semi-conductive layer of the electron emitting device  100 . 
         FIG. 4 ( a ) through ( c )  are schematic cross-sectional views showing differing states of silver nanoparticles in a semi-conductive layer  30 A, in an electron emitting device according to an embodiment of the present invention. 
         FIGS. 5 ( a ) and ( b )  are diagrams showing cross-sectional STEM images of a semi-conductive layer containing silver nanoparticles. 
         FIG. 6 ( a ) through ( c )  are diagrams showing results of EDX analysis in a cross section (inside open circles  6   a ,  6   b  and  6   c  in  FIG. 5( b ) ) of a semi-conductive layer. 
         FIG. 7  A diagram schematically showing a measurement system for the electron emission characteristics of the electron emitting device  100 . 
         FIG. 8  A diagram showing a result of an energization test for an electron emitting device according to Example. 
         FIG. 9  A schematic cross-sectional view showing an electron emitting device  200  according to Comparative Example. 
         FIG. 10  A diagram showing a result of an energization test for the electron emitting device of Comparative Example. 
         FIG. 11  A diagram showing a cross-sectional STEM image of a semi-conductive layer containing silver nanoparticles, in the electron emitting device of Comparative Example. 
         FIG. 12  A diagram showing a result of EDX analysis in a cross section (a region indicated with an open circle  2   a  in  FIG. 11 ) of the semi-conductive layer of the electron emitting device of Comparative Example. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, with reference to the drawings, electron emitting devices according to embodiments of the present invention and methods of producing the same will be described. Embodiments of the present invention are not to be limited to the illustrated embodiments. In the following description, constituent elements with like functions are denoted by like reference numerals, and redundant description will be avoided. 
       FIG. 1  shows a schematic cross-sectional view of an electron emitting device  100  according to an embodiment of the present invention. 
     The electron emitting device  100  includes a first electrode  12 , a second electrode  52 , and a semi-conductive layer  30  provided between the first electrode  12  and the second electrode  52 . The first electrode  12  is composed of an aluminum substrate  12  (e.g., a thickness of 0.5 mm), for example, whereas the second electrode  52  is composed of a gold (Au) layer (e.g., 40 nm thick), for example. The dielectric layer  22  may function as a device separation layer when a plurality of electron emitting devices  100  are to be produced on an aluminum substrate. The size of a single electron emitting device  100  (i.e., the size of a region surrounded by the dielectric layer  22 ) may be e.g., about 5 mm×about 5 mm (5 mm□), and the dielectric layer  22  has a width of about 5 mm. The dielectric layer  22  may be omitted when just forming a single electron emitting device  100 . However, providing the dielectric layer  22  may result in the advantages of an ability to restrain a concentrated electric field and a leakage current from occurring between the first electrode  12  and the second electrode  52 . 
     The semi-conductive layer  30  includes a porous alumina layer  32  having a plurality of pores  34  and silver (Ag)  42  that is supported in the plurality of pores  34  of the porous alumina layer  32 . 
     The plurality of pores  34  have an opening whose two-dimensional size (Dp) as viewed from the normal direction of its surface is not less than about 50 nm and not more than about 3 μm, for example. The plurality of pores  34  may have an opening whose two-dimensional size (Dp) as viewed from the normal direction of its surface is less than about 500 nm. In the present specification, an “opening” refers to an uppermost portion of a pore  34 . If a pore  34  has two or more pore subportions with mutually differing pore diameters along the depth direction, regarding the pore diameter, the pore diameter of the uppermost portion is referred to as the opening diameter. The “two-dimensional size” means an area equivalent circle diameter of the opening (pore  34 ) as viewed from the normal direction of its surface. In the following description, any reference to “two-dimensional size”, “opening diameter”, or “pore diameter” is intended to be an area equivalent circle diameter. Details of the porous alumina layer  32  will be described later with reference to  FIG. 3 . 
     The silver supported in the pores  34  may be, for example, nanoparticles of silver (hereinafter denoted as “Ag nanoparticles”). The Ag nanoparticles preferably have an average particle size of not less than 1 nm and not more than 50 nm, for example. More preferably, the Ag nanoparticles have an average particle size of not less than 3 nm and not more than 10 nm, for example. The Ag nanoparticles may be coated with an organic compound (e.g., an alcohol derivative and/or surfactant). 
     The first electrode  12  is composed of an aluminum substrate (e.g., 0.5 mm thick), for example, whereas the porous alumina layer  32  is an anodized layer formed on the surface of the aluminum substrate. Instead of an aluminum substrate, an aluminum layer which is formed on a substrate (e.g., a glass substrate) may be used. In other words, the porous alumina layer  32  may be an anodized layer which is formed at the surface of an aluminum layer that is supported by a substrate. In this case, if the substrate is a dielectric substrate such as a glass substrate, an electrically conductive layer may be formed between the aluminum layer and the substrate, and the aluminum layer and the electrically conductive layer may be utilized as electrodes. Any aluminum layer (i.e., a portion remaining after anodization) functioning as an electrode preferably has a thickness of e.g. 10 μm or more. 
     The second electrode  52  is composed of a gold (Au) layer, for example. The Au layer preferably has a thickness of not less than 10 nm and not more than 100 nm, e.g. 40 nm. Otherwise, platinum (Pt) may be used. Furthermore, as described in Patent Document 2, a layered structure of an Au layer and a Pt layer may be adopted; in this case, preferably the Au layer serves as a lower layer and the Pt layer serves as an upper layer in the layered structure (Pt layer/Au layer). In the layered structure, the Pt layer preferably has a thickness of not less than 10 nm and not more than 100 nm, e.g. 20 nm, and the Au layer preferably has a thickness of not less than 10 nm and not more than 100 nm, e.g. 20 nm. As compared to forming the second electrode  52  with an Au layer alone, the Pt layer/Au layer layered structure can provide a lifetime which is about 5 times longer. 
     Next, with reference to  FIG. 2 , a method of producing the electron emitting device  100  will be described.  FIGS. 2( a ) through ( c )  show schematic cross-sectional views for describing a method of producing the electron emitting device  100  according to an embodiment of the present invention. 
     First, as shown in  FIG. 2( a ) , an aluminum substrate  12  having a dielectric layer  22  partially formed therein is provided. As the aluminum substrate  12 , for example, JIS A1050 (thickness: 0.5 mm) may be used. The dielectric layer  22  may be formed by performing an anodization (alumite treatment) and a sealing treatment while masking a device formation region of the surface of the aluminum substrate  12 , for example. The dielectric layer  22  is formed by performing an anodization with sulfuric acid (15 wt %, 20° C.±1° C.) for 250 seconds to 300 seconds at a current density of 1 A/dm 2  to form a porous alumina layer with a thickness of 2 μm to 4 μm, and thereafter performing a sealing treatment for the porous alumina layer with distilled water (pH: 5.5 to 7.5, 90° C.) for about 30 minutes, for example. 
     As necessary, the surface of the aluminum substrate may be subjected to a pretreatment. For example, a microblasting treatment may be performed. Alternatively, after a porous alumina layer is formed through anodization, the porous alumina layer may be removed by etching. The pores in the porous alumina layer to be first formed are likely to be distributed irregularly (randomly). Therefore, in order to form a porous alumina layer with a regular array of pores, it is preferable to remove the porous alumina layer that was formed first. 
     Next, as shown in  FIG. 2( b ) , the surface of the aluminum substrate  12  is anodized in order to form the porous alumina layer  32 . As will be described later with reference to  FIG. 3 , the anodization may be followed by an etching as necessary. Anodization and etching may be alternated a plurality of times. By adjusting the conditions of anodization and etching, pores  34  with various cross-sectional shapes and sizes can be formed. 
     Next, as shown in  FIG. 2( c ) , silver (Ag)  42  is allowed to be supported in the pores  34  of the porous alumina layer  32 . In the case where Ag nanoparticles are used as Ag, a dispersion obtained by dispersing Ag nanoparticles in an organic solvent (e.g., toluene) is applied onto the porous alumina layer  32 . The Ag nanoparticles within the dispersion may be coated with an organic compound (e.g., an alcohol derivative and/or a surfactant). The content ratio of Ag nanoparticles in the dispersion is preferably e.g. not less than 0.1 mass % and not more than 10 mass %, and may be 2 mass %, for example. The method of applying the dispersion is not particularly limited. For example, spin coating, spray coating, or the like may be used. 
     Next, with reference to  FIG. 3 , the structure of the porous alumina layer  32  of the electron emitting device  100  will be described. The porous alumina layer  32  may be any of porous alumina layers  32 A,  32 B and  32 C shown in  FIGS. 3( a ), ( b ) and ( c ) , for example. Moreover, without being limited to the porous alumina layers  32 A,  32 B and  32 C, the porous alumina layer  32  admits of various modifications, as will be described below. 
     The porous alumina layer is formed by, for example, allowing the surface of an aluminum substrate (within which portions that were not anodically oxidized will become the first electrode  12 ) to undergo anodization in an acidic electrolytic solution. The electrolytic solution to be used in the step of forming the porous alumina layer may be, for example, an aqueous solution that contains an acid which is selected from the group consisting of oxalic acid, tartaric acid, phosphoric acid, chromic acid, citric acid, and malic acid. By adjusting the conditions of anodization (e.g., the kind of electrolytic solution, applied voltage), it is possible to control the opening diameter Dp, the interpore distance Dint, the pore depth Dd, the thickness tp of the porous alumina layer, and the thickness tb of the barrier layer. A porous alumina layer which is obtained through anodization may have, for example, cylindrical pores  34 B as in the porous alumina layer  32 B shown in  FIG. 3( b ) . 
     After the anodization, the porous alumina layer may be placed in contact with an etchant for alumina and subjected to a predetermined amount of etching so as to enlarge the pore diameter. With wet etching, the pore wall and the barrier layer can be etched substantially isotropically. By adjusting the kind of etchant and its concentration, as well as the etching time, it is possible to control the etching amount (that is, the opening diameter Dp, interpore distance Dint, pore depth Dd, the barrier layer thickness tb, etc.). Examples of etchants that may be used are: an aqueous solution of phosphoric acid; an aqueous solution of an organic acid, e.g., formic acid, acetic acid, or citric acid; or a chromic and phosphoric acid mixture aqueous solution. A porous alumina layer which is obtained by performing etching only once after the anodization will have cylindrical pores  34 B, as in the porous alumina layer  32 B illustrated in  FIG. 3( b ) . However, the opening diameter Dp of the pores  34 B and the thickness tb of the barrier layer  32   b  have changed through the etching. 
     For example, an anodization may be performed with oxalic acid (0.05 M, 5° C.) and a formation voltage of 80 V for about 25 minutes; thereafter, 20 minutes of etching may be performed with phosphoric acid (0.1 M, 25° C.); as a result, a porous alumina layer  32 B having a depth Dd of about 2000 nm, an opening diameter Dp of 100 nm, an interpore distance Dint of 200 nm, and a barrier layer thickness tb of about 30 nm can be obtained. 
     In another example, for example, an anodization may be performed with oxalic acid (0.05 M, 5° C.) and a formation voltage of 80 V for about 10 minutes; thereafter, 20 minutes of etching may be performed with phosphoric acid (0.1 M, 25° C.); as a result, a porous alumina layer  32 B having a depth Dd of about 700 nm, an opening diameter Dp of 100 nm, an interpore distance Dint of 200 nm, and a barrier layer thickness tb of 50 nm can be obtained. 
     After the etching step, a further anodization may be performed to grow the pores in the depth direction, and also to thicken the porous alumina layer. Since the pore growth begins from the bottoms of the pores that have already been formed, each pore will have a stepped side surface. As a result, pores  34 A having a stepped side surface are obtained, as in the pores  34 A illustrated in  FIG. 3( a ) . Along the depth direction, each pore  34 A has two pore subportions with mutually differing pore diameters, such that the pore subportion at a deeper position has a smaller pore diameter. For example, as shown in  FIG. 3( a ) , the subportion at a deeper position (depth Dd 1 , pore diameter Dp 1 ) has a smaller pore diameter Dp 1  than the opening diameter Dp. A pore  34 A having a stepped side surface is capable of catching an Ag nanoparticle(s) at the step portion(s), thus providing an advantage of being able to support many Ag nanoparticles in the pore  34 A. For example, among the plurality of pores  34 , any pore that has an opening diameter which is not less than about 100 nm and not more than about 3 μm preferably includes a pore subportion at a deeper position, this pore subportion having a pore diameter of not less than 50 nm and not more than 500 nm. 
     The porous alumina layer  32 A may be formed in the following manner, for example. An anodization may be performed with oxalic acid (0.05 M, 5° C.) and a formation voltage of 80 V for about 10 minutes; thereafter, 20 minutes of etching with phosphoric acid (0.1 M, 25° C.) is performed; thereafter again, an anodization may be performed with oxalic acid (0.05 M, 5° C.) and a formation voltage of 80 V for about 20 minutes; as a result, a porous alumina layer  32 A having a depth Dd of about 1500 nm, an opening diameter Dp of 100 nm, an interpore distance Dint of 200 nm, and a barrier layer thickness tb of 50 nm can be obtained. Herein, along the depth direction, each pore  34 A has two pore subportions with mutually differing pore diameters, such that it has a pore subportion with a depth Dd 1  of 500 nm and a pore diameter Dp 1  of about 20 nm at a deeper position. 
     Further thereafter, as necessary, the porous alumina layer may be placed in contact with an etchant for alumina in order to perform further etching, thus further enlarging the pore diameter. As the etchant, the aforementioned etchants are preferable here also. 
     By repeating anodization steps and etching steps, pores can be formed each having two or more pore subportions with mutually differing pore diameters along the depth direction, such that any pore subportion at a deeper position has a smaller pore diameter, for example. Furthermore, as in the porous alumina layer  32 C illustrated in  FIG. 3( c ) , pores  34 C each having a sloped side surface (note that sufficiently small steps will result in the appearance of a slope) can be formed. The overall shape of each pore  34 C is substantially conical (although the cone is situated upside down). The applicant has established a technique of mass-producing an antireflection film having a moth-eye structure by using a porous alumina layer having conical pores as a mold. 
     As described above, the porous alumina layer  32  may be any of the porous alumina layers  32 A,  32 B and  32 C shown in  FIGS. 3( a ), ( b ) and ( c ) , but admits of various modifications without being limited to these. Regardless of the shape of the porous alumina layer  32 , the thickness tp of the porous alumina layer  32  is not less than about 10 nm and not more than about 5 μm, for example. If it is thinner than 10 nm, enough silver (e.g., Ag nanoparticles) cannot be supported, so that a desired electron emission efficiency may be not be obtained. Although there is no upper limit for the thickness tp of the porous alumina layer  32 , the electron emission efficiency tends to be saturated even if the porous alumina layer  32  becomes any thicker; thus, from a production efficiency standpoint, there is no need for a thickness that is greater than 5 μm. 
     The depth Dd of the plurality of pores  34  in the porous alumina layer  32  may be e.g. not less than 10 nm and not more than 5 μm. The depth Dd of the plurality of pores  34  may be e.g. not less than 50 nm and not more than 500 nm. The depth Dd of the plurality of pores  34  may be set as appropriate, depending on the thickness of the porous alumina layer  32 . 
     The thickness tb of the barrier layer  32   b  of the porous alumina layer  32  is preferably not less than 1 nm and not more than 1 μm. More preferably, the thickness tb of the barrier layer  32   b  is 100 nm or less. The barrier layer  32   b  is a layer constituting the bottom of the porous alumina layer  32 . If the barrier layer  32   b  is thinner than 1 nm, short-circuiting may occur upon voltage application; on the other hand, if it is thicker than 1 μm, a sufficient voltage may not be applied to the semi-conductive layer  30 . Generally speaking, the thickness tb of the barrier layer  32   b  of the porous alumina layer  32  depends on the interpore distance Dint and the opening diameter (two-dimensional size) Dp of the pores  34  and the conditions of anodization. 
     Hereinafter, by way of experimental examples, the electron emitting device  100  according to an embodiment of the present invention will be described in more detail. 
       FIGS. 4( a ), ( b ) and ( c )  are schematic cross-sectional views showing differing states of silver nanoparticles in the semi-conductive layer  30 A, in an electron emitting device according to an embodiment of the present invention.  FIG. 4( a )  shows a state immediately after the semi-conductive layer  30 A is formed;  FIG. 4( b )  shows a state after a “forming” treatment but before being driven; and  FIG. 4( c )  shows the structure during stable operation. These are all schematic illustrations based on results of observing a cross section of a prototyped device with a scanning transmission electron microscope (hereinafter “STEM”). 
     The semi-conductive layer  30 A is obtained by allowing Ag nanoparticles  42   n  to be supported in the porous alumina layer  32 A which has been formed as described above, for example. 
     For the Ag nanoparticles, for example, an Ag nanoparticle dispersion obtained by dispersing alcohol derivative-coated Ag nanoparticles in an organic solvent (an average particle size of the alcohol derivative-coated Ag nanoparticles: 6 nm, dispersion solvent: toluene, Ag concentration: 1.3 mass %) can be used. For example, on a porous alumina layer  32 A that is formed in a region of about 5 mm×about 5 mm, 200 μL (microliters) of the aforementioned Ag nanoparticle dispersion is added dropwise; and spin coating is performed under conditions of: e.g. 500 rpm for 5 seconds and thereafter 1500 rpm for 10 seconds. Thereafter, baking is performed at 150° C. for 1 hour, for example. For enhanced dispersibility, the Ag nanoparticles are coated with an organic substance having e.g. alkoxide and/or carboxylic acid, or a derivative thereof at its terminal end. The baking step is able to remove or reduce the organic substance. 
     The semi-conductive layer  30 A which has just been formed, the Ag nanoparticles  42   n  abound in lower portions of the pores  34 A, as shown in  FIG. 4( a ) . 
     Once the “forming” treatment is performed, as shown in  FIG. 4( b ) , in some pores  34 A, the Ag nanoparticles  42   n  are arrayed along the depth direction of the pores  34 A, thereby being distributed to near the opening of the pore  34 A. Electrons will be emitted from any pore  34 A (the third pore  34 A from the left in  FIG. 4( b ) ) in which the Ag nanoparticles  42   n  are thus distributed to near the opening. Note that the “forming” treatment refers to a treatment that involves energization for realizing stable electron emission. Although depending on the structure of the semi-conductive layer  30 A, the “forming” treatment is performed by, as the voltage to be applied to the electron emitting device  100  (e.g., a driving voltage Vd as shown in  FIG. 7 ), using a rectangular wave having e.g. a frequency of 2 kHz and a duty ratio of 0.5, and increasing this voltage to about 20 V at a rate of 0.1 V/sec. In the present specification, the voltage to be applied to the electron emitting device  100  is expressed in terms of the potential of the second electrode  52  relative to the potential of the first electrode  12 . When the voltage to be applied to the electron emitting device  100  is 20 V, for example, the potentials of the first electrode and the second electrode  52  are e.g. −20 V and 0 V, respectively. However, without being limited to this example, the potential of the first electrode  12  may be the ground potential, and the potential of the second electrode  52  may be a positive value. 
     While electrons are being stably emitted, as shown in  FIG. 4( c ) , it is considered that pores  34 A in which the Ag nanoparticles  42   n  are distributed to near the opening are being consecutively formed. 
     Thereafter, a phenomenon occurs such that the porous alumina layer  32  is locally destroyed. This is presumably because of heat generation that is caused by electron emission. 
       FIGS. 5( a ) and ( b )  show example cross-sectional STEM images of the semi-conductive layer (which has not been energized yet) of the prototyped device.  FIG. 5( b )  shows an enlarged image of the region surrounded by a broken line  5   b  in  FIG. 5( a ) .  FIGS. 6( a ), ( b ) and ( c )  show results of energy dispersive X-ray analysis (hereinafter “EDX”) of regions indicated by open circles  6   a ,  6   b  and  6   c  in  FIG. 5( b )  (i.e., vicinities of dark dots that are considered to be the Ag nanoparticles). DB-Strata237 (available from Japan FEI) was used as the STEM, and Genesis2000 (available from EDAX, Inc.) was used as the EDX. Unless otherwise specified, this will also be the case hereinafter. 
     As can be seen from  FIG. 5( a ) , pores extend along the normal direction with respect to the surface. Since presence of Ag is confirmed in  FIGS. 6( a ), ( b ) and ( c ) , the dark dots in  FIG. 5( b )  are presumed to be the Ag nanoparticles. This would indicate that the Ag nanoparticles supported in the pores are sparsely dispersed. The semi-conductive layer shown in  FIGS. 5( a ) and ( b )  includes the porous alumina layer  32 A. In other words, each pore  34 A in the porous alumina layer  32 A has a stepped side surface, and has two pore subportions with mutually differing pore diameters along the depth direction. In  FIGS. 5( a ) and ( b ) , it is considered that the pore subportions at the deeper position produce darker images. 
     With reference to  FIG. 7  and  FIG. 8 , a result of evaluating the lifetime of the electron emitting device of Example will be described.  FIG. 7  schematically shows a measurement system for the electron emission characteristics of the electron emitting device  100 .  FIG. 8  shows a result of an energization test (electron emission characteristics) for the electron emitting device  100  having the semi-conductive layer illustrated in  FIGS. 5( a ) and ( b ) . 
     As shown in  FIG. 7 , on the second electrode  52  side of the electron emitting device  100 , a counter electrode  110  is disposed so as to oppose the second electrode  52 , and a current that occurs in the counter electrode  110  due to the electrons that are emitted from the electron emitting device  100  was measured. The following is assumed: a driving voltage Vd which is applied to the electron emitting device  100 ; an intra-device current Id; a voltage Ve (which may be referred to as “collection voltage”) to be applied to the counter electrode  110 ; and an emission current Ie occurring in the counter electrode  110 . The distance between the counter electrode  110  and the second electrode  52  was 0.5 mm, and the voltage Ve applied to the counter electrode  110  was 600 V. Herein, as shown in  FIG. 7 , the potential of the second electrode  52  was the ground potential, and a negative voltage was applied to the first electrode  12 . However, without being limited to this example, the potential of the second electrode  52  may only be higher than the potential of the first electrode  12  in order to allow electrons to be emitted from the second electrode  52 . 
     In  FIG. 8 , the intra-device current Id, the emission current Ie, and the emission efficiency n are plotted against energization time. The emission efficiency η is given as η=Ie/Id. The emission efficiency η needs to be 0.01% or more, and may preferably be 0.05% or more. 
     The construction of the electron emitting device  100  produced is as follows. 
     first electrode  12 : a portion of JIS A1050 (thickness 0.5 mm) excluding any anodically oxidized portion 
     porous alumina layer ( 32 A): opening diameter Dp of about 100 nm, depth Dd of about 2200 nm, interpore distance Dint of 200 nm, porous alumina layer thickness tp of 2200 nm, barrier layer thickness tb of about 50 nm
         deeper pore subportion: pore diameter Dp 1  of about 20 nm, depth Dd 1  of about 1500 nm   shallower pore subportion: pore diameter (opening diameter Dp) of about 100 nm, depth of about 700 nm       

     Ag nanoparticles  42   n : alcohol derivative-coated Ag nanoparticles contained in the aforementioned Ag nanoparticle dispersion, having an average particle size of 6 nm 
     second electrode  52 : Au layer (thickness 40 nm) 
     device size (size of the second electrode  52 ): 5 mm×5 mm 
     The porous alumina layer  32 A shown in  FIGS. 5( a ) and ( b )  was formed by: performing an anodization with oxalic acid (0.05 M, 5° C.) and a formation voltage of 80 V for about 27 minutes; thereafter performing 20 minutes of etching with phosphoric acid (0.1 M, 25° C.); and thereafter again performing an anodization with oxalic acid (0.05 M, 5° C.) and a formation voltage of 80 V for about 27 minutes. 
     After carrying out the aforementioned “forming” treatment, an energization test for the electron emitting device  100  was performed through an intermittent driving with ON periods of 16 seconds and OFF periods of 4 seconds. The driving conditions are as follows. The driving voltage Vd (pulse voltage) applied between the first electrode  12  and the second electrode  52  was a rectangular wave having a frequency of 2 kHz and a duty ratio of 0.5, and the driving voltage Vd was increased at a rate of 0.1 V/sec, until the emission current Ie reached a predefined value (which herein was 4.8 μA/cm 2 ) or greater. Thereafter, a feedback control of adjusting the driving voltage Vd was performed so that the emission current Ie as monitored with the counter electrode  110  remained constant. The driving environment was 25° C., with a relative humidity RH of 30% to 40%. 
     As can be seen from  FIG. 8 , the electron emitting device  100  of Example had a lifetime of about 50 hours. Herein, lifetime of the electron emitting device is assumed to be the length of time during which the emission current Ie maintained a certain value. Herein, assuming a usage as a charger device of a medium-fast copier machine, the length of time during which the emission current Ie maintained 4.8 μA/cm 2  was defined as the lifetime of the electron emitting device. This value (4.8 μA/cm 2 ) is estimated, given that the photosensitive drum of the medium-fast copier machine has a rotational speed of 285 mm/sec, to be an emission current that is needed to charge this photosensitive drum. As can be seen from  FIG. 8 , the emission current Ie of the electron emitting device  100  maintained 4.8 μA/cm 2  (i.e., a value indicated by a dotted line in  FIG. 8 ) for about 50 hours. 
     From the study so far, it has been found that the lifetime can be made about 5 times longer (about 160 hours) by replacing the second electrode  74  (a single Au layer with a thickness of 40 nm) of an electron emitting device  200  of Comparative Example, which will be described later with reference to  FIG. 9  (see Patent Document 2, for example), with a Pt layer/Au layer (20 nm/20 nm) layered structure. Therefore, by replacing the second electrode  52  of the electron emitting device  100  produced with the aforementioned layered structure, its lifetime will be prolonged to about 250 hours. 
     For comparison sake, a reference electron emitting device  200  was produced as shown in  FIG. 9 , and was similarly evaluated.  FIG. 10  shows a result of an energization test (electron emission characteristics) for the electron emitting device  200  of Comparative Example. In  FIG. 10 , the intra-device current Id, the emission current Ie, and the emission efficiency η are plotted against energization time. 
     The construction of the electron emitting device produced is as follows. 
     first electrode  71 : JIS A1050 (thickness: 0.5 mm) 
     dielectric layer  72 : an anodic oxidized alumina layer (a porous alumina layer subjected to a sealing treatment), having a thickness of 4 μm 
     semi-conductive layer  73 : thickness of 1 μm to 2 μm 
     insulator  73   m : silicone resin 
     Ag nanoparticles  73   n : alcohol derivative-coated Ag nanoparticles contained in the aforementioned Ag nanoparticle dispersion, having an average particle size of 6 nm and accounting for 1.5 mass % with respect to silicone resin 
     second electrode  74 : an Au layer (thickness 40 nm) 
     device size (size of the second electrode  74 ): 5 mm×5 mm 
     The dielectric layer  72  was formed by a method similar to that of the dielectric layer  22  of the electron emitting device  100  described with reference to  FIG. 2( a ) . 
     As can be seen from  FIG. 10 , the electron emitting device  200  produced as Comparative Example had a lifetime of about 50 hours. The lifetime of the electron emitting device  200  of Comparative Example was evaluated similarly to the electron emitting device  100  of Example. 
       FIG. 11  shows an example cross-sectional STEM image of the electron emitting device  200  of Comparative Example (not energized yet).  FIG. 12  is a diagram showing a result of EDX analysis in a cross section of  FIG. 11  (a region indicated with an open circle  2   a  in  FIG. 11 ). 
     As can be seen from  FIG. 11 , Ag nanoparticles are present in regions indicated by circles in  FIG. 11 , for example. Within the silicone resin, a plurality of places where Ag nanoparticles are aggregated (e.g., inside the open circle  2   a  in  FIG. 11 ) are created. The places where Ag nanoparticles are aggregated are nonuniformly distributed within the silicone resin. 
     Presumably, the distribution of Ag nanoparticles (including also a migration that may occur upon electric field application) may be somehow related to the electron emission characteristics and/or the device lifetime; however, no specific correlation has been established yet. However still, the electron emitting device according to an embodiment of the present invention allows Ag nanoparticles to be supported in the pores of the porous alumina layer, and the Ag nanoparticle distribution can be controlled by controlling the opening diameter, depth, interpore distance, etc., of the pores. Therefore, the characteristics of the electron emitting device can be improved and/or a long lifetime can be achieved. 
     Next, three kinds of electron emitting devices (test samples Nos. 1 to 3) as shown in Table 1 below were evaluated. 
     As illustrated herein, when the first electrode is formed by using a relatively rigid aluminum substrate (thickness 0.2 mm or more) containing aluminum with a purity of not less than 99.00 mass % and not more than 99.99 mass %, the aluminum substrate can be utilized as a support substrate, so that the electron emitting device can be efficiently produced. 
     Test samples Nos. 1 to 3 differ from one another with respect to the composition (e.g., aluminum content) of the aluminum substrate  12  used in forming the first electrode  12 . The construction of test sample No. 1 (thickness: 0.5 mm) and the method of production it are basically identical with those of the electron emitting device  100  described with reference to  FIG. 7  and  FIG. 8 . However, herein, the following steps were alternated to a total of three times each: a step of adding dropwise 200 μL (microliter) of the aforementioned Ag nanoparticle dispersion onto the porous alumina layer  32 A (a region which is about 5 mm×about 5 mm); and a step of thereafter performing spincoating under conditions of 500 rpm for 5 seconds, and then 1500 rpm for 10 seconds. Thereafter, heating was performed at 150° C. for 1 hour. Test samples No. 2 (thickness: 0.5 mm) and No. 3 (thickness: 0.2 mm) were identical with test sample No. 1 except for the composition of the aluminum substrate  12 . 
     Table 1 shows the main component in the composition of the respective aluminum substrate constituting the first electrode  12  of test samples Nos. 1 to 3. 
     Test sample No. 1 was produced by using JIS A1050 as the aluminum substrate  12 . JIS A1050 has the following composition (mass %). 
     Si: 0.25% or less, Fe: 0.40% or less, Cu: 0.05% or less, Mn: 0.05% or less, Mg: 0.05% or less, Zn: 0.05% or less, V: 0.05% or less, Ti: 0.03% or less, others: each 0.03% or less, Al: 99.50% or more 
     Test sample No. 2 was produced by using JIS A1100 as the aluminum substrate  12 . JIS A1100 has the following composition (mass %). 
     Si+Fe: 0.95% or less, Cu: 0.05% to 0.20%, Mn: 0.05% or less, Zn: 0.10% or less, others: each 0.05% or less and altogether 0.15% or less, Al: 99.00% or more 
     Test sample No. 3 was produced by using an aluminum base material containing aluminum in an amount of 99.98 mass % or more as the aluminum substrate  12 . The aluminum substrate of test sample No. 3 had the following composition (mass %). 
     Si: 0.05% or less, Fe: 0.03% or less, Cu: 0.05% or less, Al: 99.98% or more 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 test 
                 composition (mass %) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 sample 
                 Si 
                 Fe 
                 Cu 
                 Mn 
                 Mg 
                 Zn 
                 Ti 
                 Al 
               
               
                   
               
               
                 No. 1 
                 ≤0.25 
                 ≤0.40 
                 ≤0.05 
                 ≤0.05 
                 ≤0.05 
                 ≤0.05 
                 ≤0.03 
                 ≥99.50 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 No. 2 
                 Si + Fe: ≤0.95 
                 0.05 to 
                 ≤0.05 
                 — 
                 ≤0.10 
                 — 
                 ≥99.00 
               
               
                   
                   
                 0.20 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 No. 3 
                 ≤0.05 
                 ≤0.03 
                 ≤0.05 
                 — 
                 — 
                 — 
                 — 
                 ≥99.98 
               
               
                   
               
            
           
         
       
     
     The energization test for test samples Nos. 1 to 3 was performed basically similarly to the energization test described with reference to  FIG. 8 . However, for simplicity&#39;s sake, no feedback control for the driving voltage Vd was performed. Specifically, after performing the aforementioned “forming” treatment, the driving voltage Vd (a rectangular wave having a frequency of 2 kHz and a duty ratio of 0.5) was increased to 26 V at a rate of 0.05 V per cycle, and thereafter maintained at 26 V. Note that one cycle of intermittent driving consists of an ON period of 16 seconds and an OFF period of 4 seconds. The driving environment was 20 to 25° C., with a relative humidity RH of 30% to 40%. 
     In any of test samples Nos. 1 to 3, when the driving voltage Vd was about 10 V or more, the emission current Ie gradually increased. By confirming that the emission current Ie increased with an increasing driving voltage Vd, it was determined that it was operating as an electron emitting device. Thus, it was confirmed that each of test samples Nos. 1 to 3 was operating as an electron emitting device. 
     Table 2 shows results of determining an average value of emission current Ie for each test sample. In Table 2, “Δ” indicates that an average value of emission current Ie was not less than 0.001 μA/cm 2  but less than 0.01 μA/cm 2 ; “∘” indicates that an average value of emission current Ie was not less than 0.01 μA/cm 2  but less than 0.1 μA/cm 2 ; and “⊚” indicates that an average value of emission current Ie was not less than 0.1 μA/cm 2  but less than 4.8 μA/cm 2 . 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 test sample 
               
            
           
           
               
               
               
               
            
               
                   
                 No. 1 
                 No. 2 
                 No. 3 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 average value of 
                 ◯ 
                 ⊚ 
                 Δ 
               
               
                   
                 emission current Ie 
               
               
                   
                   
               
            
           
         
       
     
     Test sample No. 2, in which the purity (i.e., ratio of aluminum content of the aluminum substrate) was lower than that of test sample No. 1, had an average value of emission current Ie which was greater than that of test sample No. 1. On the other hand, test sample No. 3, in which the purity (ratio of aluminum content) of the aluminum substrate was higher than that of test sample No. 1, had an average value of emission current Ie which was smaller than that of test sample No. 1. Thus, as the purity (ratio of aluminum content) of the aluminum substrate decreased, the average value of emission current Ie increased. 
     However, the aforementioned energization test only illustrates exemplary driving conditions. Depending on the driving conditions of the electron emitting device, the value of emission current Ie may vary. Under operation with a large average value of emission current Ie (i.e., amount of electron emission per unit time), the duration in which operation as an electron emitting device is possible may decrease. As used herein, “the duration in which operation as an electron emitting device is possible” means the period from the moment at which operation as an electron emitting device is confirmed to the moment at which the value of emission current Ie decreases for the same driving voltage Vd; note that this definition differs from that of “lifetime” (i.e., the length of time during which the emission current Ie maintained a certain value) which was described with reference to  FIG. 8 , for example. 
     The value of emission current and the duration of operation that is expected of an electron emitting device may vary depending on the application (i.e., driving conditions). However, in applications where large emission current values are required, for example, it is preferable to use an aluminum base material with a relatively low aluminum purity (not less than 99.00 mass % and not more than 99.50 mass %). In applications where long hours of operation is highly regarded, for example, it is preferable to use an aluminum base material having a relatively high aluminum purity (not less than 99.50 mass % and not more than 99.98 mass %). 
     The exact mechanism by which the aluminum purity affects the characteristics of the electron emitting device is not clear as yet. However, as seen from Table 1, any element that is contained as an impurity in the aluminum substrate used herein, except for Mg, is an element which has a high standard electrode potential (i.e., so-called “noble”) as compared to aluminum. Therefore, an impurity element(s) (e.g., iron) that is more noble than aluminum may possibly be affecting the characteristics of the electron emitting device. 
     INDUSTRIAL APPLICABILITY 
     Embodiments of the present invention may be suitable as an electron emitting device for use in a charger device of an image forming apparatus, or a method of producing the same, for example. 
     REFERENCE SIGNS LIST 
     
         
         
           
               12 : first electrode (aluminum substrate) 
               22 : dielectric layer 
               30 ,  30 A: semi-conductive layer 
               32 ,  32 A,  32 B,  32 C: porous alumina layer 
               32   b : barrier layer 
               34 ,  34 A,  34 B,  34 C: pore 
               42 : silver (Ag) supported in pores  34   
               42   n : Ag nanoparticle 
               52 : second electrode 
               71 : first electrode 
               72 : dielectric layer 
               73 : semi-conductive layer 
               73   m : insulator 
               73   n : Ag nanoparticle 
               74 : second electrode 
               100 ,  200 : electron emitting device