Patent Publication Number: US-2023136993-A1

Title: Light-emitting element and method for manufacturing light-emitting element

Description:
TECHNICAL FIELD 
     The disclosure relates to a light-emitting element and a method for manufacturing the light-emitting element. 
     BACKGROUND ART 
     A process step that uses ferritins in a stacking step in a light-emitting element is disclosed in Patent Literature 1. A light-emitting layer of this light-emitting element is an InGaN layer having undergone crystal growth through MOCVD. A thin film is formed firstly, iron-containing ferritins are dispersed into the thin film, and the ferritins are evaporated through heating to leave only iron. Etching is performed using the iron as a mask to process the InGaN film into a quantum-dot shape. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 11-45990 (published on Feb. 16, 1999) ([0048], [0049]) 
     SUMMARY 
     Technical Problem 
     Light-emitting elements including quantum dots need to urgently improve light emission efficiency. It is important to render quantum dots, constituting a light-emitting layer, dense in order to improve the light emission efficiency. 
     Further, in quantum-dot light-emitting diodes (QLEDs), efficient carrier injection into the quantum dots affects light emission efficiency considerably as well; thus, it is important that the quantum dots are close to each other. 
     Furthermore, improvement in in-plane unevenness in a light-emitting layer is important in large-area applications, including displays. 
     Solution to Problem 
     To solve the above problem, a light-emitting element according to one aspect of the disclosure includes at least one light-emitting layer, wherein the light-emitting layer includes a plurality of quantum dots covered by shells containing a ferritin. 
     To solve the above problem, a method for manufacturing a light-emitting element according to one aspect of the disclosure includes a mixture step of mixing together an apoferritin, a first metal, and a second metal different from the first metal within an alkaline solution to form a quantum dot covered by a ferritin. 
     Advantageous Effect of Disclosure 
     These aspects of the disclosure can provide a light-emitting element with light emission efficiency enhanced by densifying quantum dots and can provide a method for manufacturing such a light-emitting element. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    illustrates a schematic configuration of a light-emitting element according to a first embodiment. 
         FIG.  2    is a sectional view of an electron transportation layer, a light-emitting layer, and a hole transportation layer of the light-emitting element. 
         FIG.  3    is a perspective view of a ferritin containing a quantum dot formed in the light-emitting layer. 
         FIG.  4    illustrates the relationship between ferritins containing the quantum dot and a cationic metal. 
         FIG.  5    is a sectional view for describing a mixture step in a method for manufacturing the light-emitting element. 
         FIG.  6    is another sectional view for describing the mixture step in the method for manufacturing the light-emitting element. 
         FIG.  7    is further another sectional view for describing the mixture step in the method for manufacturing the light-emitting element. 
         FIG.  8    is still another sectional view of the mixture step in the method for manufacturing the light-emitting element. 
         FIG.  9    is a photograph for describing the mixture step in the method for manufacturing the light-emitting element. 
         FIG.  10    is another photograph for describing the mixture step in the method for manufacturing the light-emitting element. 
         FIG.  11    is a sectional view for describing a stacking step in the method for manufacturing the light-emitting element. 
         FIG.  12    is another sectional view for describing the stacking step in the method for manufacturing the light-emitting element. 
         FIG.  13    is a sectional view for describing a transfer step in the method for manufacturing the light-emitting element. 
         FIG.  14    is another sectional view for describing the transfer step in the method for manufacturing the light-emitting element. 
         FIG.  15    is a graph showing the relationship between the composition of the light-emitting element and light emission wavelength. 
         FIG.  16    illustrates a schematic configuration of a light-emitting element according to a second embodiment. 
         FIG.  17    is a perspective view of a ferritin containing a quantum dot formed in a light-emitting layer of the light-emitting element. 
         FIG.  18    is a perspective view of a ferritin containing a quantum dot formed in another light-emitting layer of the light-emitting element. 
         FIG.  19    is a perspective view of a ferritin containing a quantum dot formed in further another light-emitting layer of the light-emitting element. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG.  1    illustrates a schematic configuration of a light-emitting element  1  according to a first embodiment. The light-emitting element  1  includes at least one light-emitting layer  13  and may be used as a light source of electronic equipment (e.g., a display device). Among the components included in the light-emitting element  1 , the description of components irrelevant to the embodiments will be omitted. It may be understood that these omitted components are similar to publicly known components. Further, it should be noted that each drawing schematically illustrates the shape and structure of each component, and the positional relationship between the components, and that they are not necessarily drawn in conformance with their scales. 
     Configuration of Light-Emitting Element  1   
     In the light-emitting element  1 , the light-emitting layer  13  is provided between a first electrode  11  and a second electrode  16 , disposed opposite the first electrode  11 . In this embodiment, the first electrode  11  is a negative electrode (cathode), and the second electrode  16  is a positive electrode (anode). To be more specific, the light-emitting element  1  includes the first electrode  11 , an electron transportation layer (ETL)  12 , the light-emitting layer  13 , a hole transportation layer (HTL)  14 , a hole injection layer (HIL)  15 , the second electrode  16 , and a substrate  17  in this order from an upward direction to a downward direction. In the Description, the direction from the second electrode  16  toward the first electrode  11  will be referred to as an upward direction. Further, the direction opposite to the upward direction will be referred to as a downward direction. 
       FIG.  2    is a sectional view of the electron transportation layer  12 , light-emitting layer  13 , and hole transportation layer  14  of the light-emitting element  1 .  FIG.  3    is a perspective view of a ferritin protein  22  containing a quantum dot  21  formed in the light-emitting layer  13 .  FIG.  4    illustrates the relationship between ferritin proteins  22  containing quantum dots  21  and  21   b  and a cationic metal. 
     The light-emitting layer  13  includes a plurality of quantum dots  21 . Each quantum dot  21  is covered by a shell containing the ferritin protein  22  (hereinafter, also merely referred to as a ferritin). 
     The quantum dot  21  is a light-emitting material that has a valence band level and a conduction band level, and that emits light upon rejoining of holes at the valence band level and electrons at the conduction band level together. To be more specific, electrons injected from the electron transportation layer  12  into the quantum dot  21  and holes injected from the hole transportation layer  14  into the quantum dot  21  join together, thus causing the quantum dot  21  to emit light of a predetermined wavelength band. 
     The quantum dot  21  is preferably a particle with low dispersibility of visible light. The quantum dot  21  is preferably at least one semiconductor material selected from the group consisting of CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InN, InP, InAs, InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, MgS, MgSe, and MgTe and of combinations thereof. To be more specific, nanosized crystals of the foregoing semiconductor material (semiconductor crystals) are used as the material of the quantum dot  21 . 
     The quantum dot  21  may be a binary-core type, a tertiary-core type, a quaternary-core type, a core-shell type, or a core-multi shell type. Further, the quantum dot  21  may be a doped quantum dot or a slanted quantum dot. 
     The ferritin protein  22  is in the form of a substantially spherical shell and has a basket shape incorporating the quantum dot  21  into the spherical shell. The spherical shell of the ferritin protein  22  incorporates a joining site joined with the element contained in the quantum dot  21  by electrostatic attractive force. 
     A specific example of the ferritin protein  22  is a recombinant horse-origin ferritin. Other examples include human- and rat-origin ferritins. The fundamental structure of the ferritin proteins  22  is almost common, with an outer diameter of 12 nm and an inner diameter of 7 nm. The inner diameter of the ferritin protein  22  is characteristically determined by the kind of the ferritin protein  22  and has an inner diameter of 4 to 8 nm inclusive. It is noted that a small protein called  Listeria  bacteria-origin Dps protein is classified as one of the ferritin protein  22  and has an outer diameter of 9 nm and an inner diameter of 4.5 nm. 
     The thickness of the ferritin protein  22 , which is a value obtained by subtracting its inner-diameter size from its outer-diameter size, measures about 5 nm. The ferritin protein  22  is only about 5 nm thick, as described above, and thus, electrons and holes are injected into the quantum dot  21  contained in the ferritin protein  22 , through a tunnel effect, hopping conduction, or other things. 
     The ferritin protein  22  is a protein that voluntarily forms an order structure through interaction. Thus, the quantum dot  21  contained in the ferritin protein  22  is regularly disposed within the light-emitting layer  13  by the self-assembly of the ferritin protein  22 . Consequently, the quantum dot  21  included in the light-emitting layer  13  is rendered more dense than a known quantum dot  21  with coordinating ligands. Further, the quantum dot  21  can be prevented from overlapping in the stacking direction within the light-emitting layer  13 , thereby improving the efficiency of electron and hole injection into the quantum dot  21 , thus improving the light emission efficiency of the quantum dot  21 . Further, the quantum dot  21  is disposed regularly in a plane direction, thus improving the in-plane unevenness of light emitted from the light-emitting layer  13 . 
     Further, the light-emitting layer  13  may contain a bivalent cationic metal. The light-emitting layer  13  contains a bivalent cationic metal, thus, with regard to the two quantum dots  21   a  and  21   b , a shell  22   a , covering one of the quantum dots  21   a , and a shell  22   b , covering the other quantum dot  21   b , are joined together via a bivalent cationic metal ion, as illustrated in  FIG.  4   . There is a joining site also outside each of the shell  22   a  and  22   b  containing the ferritin protein  22 . When a bivalent cationic metal ion is joined to this joining site, a thing like a salt bridge is formed between the ferritin proteins  22 , thus enabling the adjacent ferritin proteins  22  to be coupled together further firmly. This enables the quantum dot  21  within the light-emitting layer  13  to be dense. Achieving the quantum dot  21  having high density can improve the color purity of light emitted from the light-emitting layer  13 . 
     The metal element constituting the bivalent cationic metal is selected from Cd, Zn, Ca, and Mg. 
     The wavelength of light emitted from the light-emitting layer  13  is controlled by the composition or particle diameter of the quantum dot  21 . The particle diameter of the quantum dot  21  is controlled by changing the composition of the ferritin protein  22  and controlling the inner diameter of the spherical shell of the ferritin protein  22 . 
     At least one of the first electrode  11  and the second electrode  16  is a light-transparency electrode that allows light emitted from the light-emitting layer  13  to pass. The substrate  17  may be composed of a light-transparency material or may be composed of a light-reflective material. 
     In the first embodiment, the first electrode  11  is composed of an indium tin oxide (ITO) for instance. That is, the first electrode  11  is composed as a light-transparency electrode. The second electrode  16  is composed of aluminum (Al) for instance. That is, the second electrode  16  is composed as a reflective electrode that reflects light emitted from the light-emitting layer  13 . 
     This configuration enables light emitted from the light-emitting layer  13  and traveling downward to be reflected by the second electrode  16 . This enables light reflected by the second electrode  16  to travel toward the first electrode  11  (upward). As such, the light-emitting element  1  enables light emitted from the light-emitting layer  13  to exit mainly upward. That is, the light-emitting element  1  is configured as a top-emission type. 
     However, the light-emitting element  1  may be configured to allow light emitted from the light-emitting layer  13  to exit mainly downward. For instance, the first electrode  11  may be configured as a reflective electrode, and the second electrode  16  may be configured as a light-transparency electrode. As such, the light-emitting element  1  is configured as a bottom-emission type. When the light-emitting element  1  is a bottom-emission type, the substrate  17  is composed of a light-transparency material. Alternatively, the light-emitting element  1  may be configured to allow light emitted from the light-emitting layer  13  to exit both upward and downward. That is, the light-emitting element  1  may be configured as a dual-light-emission type. 
     The electron transportation layer  12  contains a material with a high electron transport capability. The electron transportation layer  12  also preferably inhibits hole transport. The electron transportation layer  12  contains ZnO nanoparticles for instance. The electron transportation layer  12  may serve also as an electron injection layer (EIL) that promotes electron injection from the first electrode  11  into the light-emitting layer  13 . 
     The hole injection layer  15  promotes hole injection from the second electrode  16  into the light-emitting layer  13 . The hole injection layer  15  contains poly (3,4-ethylenedioxythiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS) for instance. 
     The hole transportation layer  14  contains a material with a high hole transport capability. The hole transportation layer  14  contains a substance, such as polyvinylcarbazole (PVK) or nickel oxide. The hole transportation layer  14  may serve also as a hole injection layer. 
     Applying a forward voltage between the second electrode  16  (anode) and the first electrode  11  (cathode) by using a power source  18  (rendering the second electrode  16  as higher potential than the first electrode  11 ) enables (i) electron supply from the first electrode  11  to the light-emitting layer  13  and (ii) hole supply from the second electrode  16  to the light-emitting layer  13 . This enables the light-emitting layer  13  to generate light upon hole and electron joining. Voltage application using the power source  18  may be controlled by a thin-film transistor (TFT) not shown. 
     Method for Manufacturing Light-Emitting Element  1   
       FIG.  5    is a sectional view for describing a mixture step in a method for manufacturing the light-emitting element  1 .  FIG.  6    is another sectional view for describing the mixture step.  FIG.  7    and  FIG.  8    are further other sectional views for describing the mixture step.  FIG.  9    is a photograph for describing the mixture step.  FIG.  10    is another photograph for describing the mixture step. It is noted that these drawings are cited from http://www.science-plaza.or.jp/about/sankangaku/forum13/yamasita.pdf. 
     The method for manufacturing the light-emitting element  1  includes a mixture step, i.e., mixing together an apoferritin  25 , a first metal, and a second metal, different from the first metal, within an alkaline solution to form the quantum dot (QD phosphor particle)  21  covered by the ferritin protein  22 . 
     The first metal is preferably Cd. The second metal preferably contains at least one selected from Zn, Se, and Te. 
     In the mixture step, a sample containing 0.5 mg/ml of the apoferritin  25 , which is a ferritin source, 1 to 3M of CH 3 COOH, which is a Cd source, 40 mM of selenourea, which is a Se source, an ammonia aqueous solution, which is a solvent, and 40 mM of CH 3 COONH 4 , which is a Cd 2 +complex ion forming agent is put into a beaker and stirred with a stirrer all night. Accordingly, the ferritin protein  22  containing the quantum dot  21  consisting of CdSe is synthesized. 
       FIG.  11    is a sectional view for describing a stacking step in the method for manufacturing the light-emitting element  1 .  FIG.  12    is another sectional view for describing the stacking step. 
     In the stacking step, the ferritin proteins  22  containing the quantum dots  21  undergo arrangement through a polypeptide method (PBLH method). 
     Firstly, the stacking step includes a process step of forming a first mixed liquid by mixing, into a liquid, the quantum dot  21  covered by the shell  22   a  or  22   b  containing the ferritin protein  22  obtained in the mixture step. To be specific, 20-40 μg/ml of the CdSe-containing ferritin obtained in the mixture step, a Mg ion, which is a bivalent cation, 20 mM of a NaCl in the form of a liquid, and 2-(4-Morpholino) ethanesulfonic acid (MES) (a pH of 5.8, 20 mM), which is a buffer material, are mixed together in a Teflon (registered trademark) trough  23 , as illustrated in  FIG.  11   . 
     Next, poly-1-benzyl-L-histidine (PBLH) in the form of a protein solution is injected into the interface of this mixed solution to obtain a second mixed liquid. PBLH loses 3D conformation due to the surface tension of an air-liquid interface to form, as illustrated in  FIG.  12   , a PBLH film  24  composed of a denatured protein film. 
     Thereafter, a burning step at about 38° C. can efficiently obtain a two-dimensional arrangement  27  of the ferritin proteins  22 , as illustrated in  FIG.  13   , through the electrostatic interaction between the PBLH film  24  and the ferritin protein  22  while producing two-dimensional crystals through a self-assembly ability inherently provided with the ferritin protein  22 . To be specific, the ferritin protein  22  has a portion called a metal-joining site, where upon joining of a bivalent cationic ion, a thing like a salt bridge is formed between the ferritin proteins  22 , thus enabling the ferritin proteins  22  to be joined together firmly. 
     Then, as illustrated in  FIG.  14   , a substrate rendered hydrophobic with the hole transportation layer  14  stacked on the second electrode  16  (anode) is prepared in advance and floated onto a solution with the PBLH film  24  thereon, to attach the PBLH film  24  with the ferritin protein  22  thereon onto the substrate, thus stacking the ferritin protein  22 . Alternatively, only the ferritin protein  22  may be stacked onto the hole transportation layer  14  by preparing, in advance, another substrate rendered hydrophobic and then floating the other substrate onto a solution with the PBLH film  24  thereon, to attach the PBLH film  24  with the ferritin protein  22  thereon onto the other substrate, followed by a transfer step, where the ferritin protein  22  on the PBLH film  24  undergoes transfer onto a substrate with the hole transportation layer  14  on the top through a transfer method using a van der Waals force or other methods. 
     Stacking, with a known method, the electron transportation layer  12  and the first electrode  11  onto the light-emitting layer  13  produced through the foregoing steps can manufacture the light-emitting element  1 . 
     The quantum dot  21  may be formed without Cd. The ferritin protein  22  containing the quantum dot  21  consisting of ZnSe can be produced by using a slow chemical reaction. To be specific, the ferritin protein  22  containing the quantum dot  21  consisting of ZnSe is obtained by coordinating ammonia with a Zn ion to form a tetraammine complex, followed by adding selenourea to supply a Se ion. 
     Next, the light-emitting layer  13  is formed by using the ferritin protein  22  containing the quantum dot  21 . To be specific, a PBLH method can form a two-dimensional crystalline film. A solution with the ferritin protein  22 , a Ca ion, which is a bivalent cation, and NaCl mixed into MES, which is a buffer agent, is filled into a Teflon (registered trademark) trough. Then, forming a thin film by dropping poly-1-benzyl-L-histidine (PBLH), which is artificial polypeptide, onto this solution, thus allowing the ferritin protein  22  to electrostatically adhere directly under the PBLH film. The in-plan direction of the ferritin protein  22  enables a ferritin two-dimensional crystalline film to be obtained by joining together of metal-joining sites. Thereafter, transfer onto a substrate with a stack of an electrode and a carrier transportation layer via another substrate obtains the light-emitting layer  13  (similar to that in  FIG.  1   ). Alternatively, the light-emitting layer  13  may be rendered as a stack of multiple layers. 
     Modification 
     A display device according to a modification of the disclosure may be a display device having a display region in which the light-emitting element in the first embodiment that emits red light (hereinafter, a red light-emitting element), the light-emitting element in the first embodiment that emits green light (hereinafter, a green light-emitting element), and the light-emitting element in the first embodiment that emits blue light (hereinafter, a blue light-emitting element) are arranged in matrix. 
     A red light-emitting layer  13 R included in the red light-emitting element, a green light-emitting layer  13 G included in the green light-emitting element, and a blue light-emitting layer  13 B included in the blue light-emitting element include quantum dots  21 R,  21 G and  21 B corresponding to the respective colors of emitted light and include the ferritin protein  22  ( FIG.  2    and  FIG.  3   ). 
     When the ferritin protein  22  is a recombinant horse-origin ferritin for instance, CdZnSe-based, CdSSe-based, and CdZnTe-based quantum dot materials can be used for the quantum dots  21 R and the quantum dot  21 G, and further, CdZnSe-based and CdSSe-based quantum dot materials can be used for the quantum dot  21 B. 
     Table 1 below shows, for each colored light-emitting layer, specific example combinations of the composition of the ferritin protein  22  and the composition and particle diameter of each of the quantum dots  21 R,  21 G, and  21 B. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Calculations 
               
               
                   
                 (For 
               
               
                   
                 reference) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 R light- 
                 QD 
                 Composition 
                 Cd0.9Zn0.1Se 
                 635 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                   
                 G light- 
                 QD 
                 Composition 
                 Cd0.5Zn0.5Se 
                 528 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                   
                 B light- 
                 QD 
                 Composition 
                 Cd0.25Zn0.75Se 
                 468 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                 Example 2 
                 R light- 
                 QD 
                 Composition 
                 Cd0.85Zn0.15Te 
                 633 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter 7 nm) 
               
               
                   
                 G light- 
                 QD 
                 Composition 
                 Cd0.3Zn0.7Te 
                 536 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                   
                 B light- 
                 QD 
                 Composition 
                 Cd0.25Zn0.75Se 
                 468 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                 Example 3 
                 R light- 
                 QD 
                 Composition 
                 CdS0.15Se0.85 
                 −630 nm  
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                   
                 G light- 
                 QD 
                 Composition 
                 CdS0.65Se0.35 
                 −532 nm  
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                   
                 B light- 
                 QD 
                 Composition 
                 CdS0.95Se0.05 
                 −467 nm  
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                 Example 4 
                 R light- 
                 QD 
                 Composition 
                 Cd0.85Zn0.15Te 
                 633 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                   
                 G light- 
                 QD 
                 Composition 
                 Cd0.3Zn0.7Te 
                 536 nm 
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7nm) 
               
               
                   
                 B light- 
                 QD 
                 Composition 
                 CdS0.95Se0.05 
                 −467 nm  
               
               
                   
                 emitting 
                   
                 Particle 
                 7 nm 
               
               
                   
                 layer 
                   
                 diameter 
               
               
                   
                   
                 Ferritin 
                 Composition 
                 Recombinant horse-origin ferritin 
               
               
                   
                   
                   
                   
                 (Inner diameter of 7 nm) 
               
               
                   
               
            
           
         
       
     
       FIG.  15    is a graph showing the relationship between the composition of the light-emitting element  1  and light emission wavelength. The lateral axis denotes the composition, x, of the quantum dot  21  included in the light-emitting layer  13 , and the longitudinal axis denotes the wavelength of light emitted by the light-emitting layer  13  including the quantum dot  21  of the composition x. Wavelength W 1  indicates the wavelength of red light, wavelength W 2  indicates the wavelength of green light, and wavelength W 3  indicates the wavelength of blue light. 
     Line L 1  denotes the relationship between the composition x of the quantum dot  21 , which is included in the light-emitting layer  13 , expressed by Cd x Zn (1-x) Se and the wavelength of light emitted by the light-emitting layer  13  and corresponds to Expression (1), Expression (2), and Expression (3), which will be described later on. Line L 2  denotes the relationship between the composition x of the quantum dot  21  expressed by Cd x Zn (1-x) Te and the wavelength of light emitted by the light-emitting layer  13  and corresponds to Expression (7) and Expression (8), which will be described later on. Line L 3  denotes the relationship between the composition x of the quantum dot  21  expressed by CdS (1-x) Se, and the wavelength of light emitted by the light-emitting layer  13  and corresponds to Expression (4), Expression (5) and Expression (6), which will be described later on. 
     Provided are reference calculations of the light emission wavelength with the quantum dot  21  having a diameter of 7 nm. To be specific, the quantum levels of conduction and valence bands are calculated by solving the single-band Schrödinger equation. CdS (1-x) Se x , denoted by the line L 3 , is calculated using the interpolation line between two points and is assumed to be able to undergo liner interpolation similarly to the other materials. 
     To be more specific, the quantum dot  21 R based on CdZnSe is expressed by Expression (1), the quantum dot  21 G based on CdZnSe is expressed by Expression (2), and the quantum dot  21 B based on CdZnSe is expressed by Expression (3). 
       Cd x1 Zn (1-x1) Se  (1)
 
       Cd x2 Zn (1-x2) Se  (2)
 
       Cd x3 Zn (1-x3) Se  (3)
 
     Here, 0.1&lt;x3&lt;x2&lt;x1&lt;1.0 is preferably satisfied, furthermore, 0.4&lt;x2&lt;x1&lt;1.0 is preferably satisfied, and 0.1&lt;x3&lt;0.4 is preferably satisfied. 
     To be more specific, the quantum dot  21 R based on CdSSe is expressed by Expression (4), the quantum dot  21 G based on CdSSe is expressed by Expression (5), and the quantum dot  21 B based on CdSSe is expressed by Expression (6). 
       CdS (1-x1) Se x1   (4)
 
       CdS (1-x2) Se x2   (5)
 
       CdS (1-x3) Se x3   (6)
 
     Here, 0&lt;x3&lt;x2&lt;x1&lt;1.0 is preferably satisfied, furthermore, 0.2&lt;x2&lt;x1&lt;1.0 is preferably satisfied, and 0&lt;x3&lt;0.2 is preferably satisfied. 
     To be more specific, the quantum dot  21 R based on CdZnTe is expressed by Expression (7), and the quantum dot  21 G based on CdZnTe is expressed by Expression (8). 
       Cd x1 Zn (1-x1) Te  (7)
 
       Cd x2 Zn (1-x2) Te  (8)
 
     Here, 0.1&lt;x2&lt;x1&lt;1.0 is preferably satisfied. 
     Alternatively, the colors of light emitted from the respective red light-emitting layer  13 R, green light-emitting layer  13 G and blue light-emitting layer  13 B may be controlled by changing the inner diameter of the ferritin protein  22  contained in the individual red light-emitting layer  13 R, green light-emitting layer  13 G and blue light-emitting layer  13 B. 
     Further, the inner diameter of the ferritin protein  22  contained in the red light-emitting layer  13 R is preferably larger than at least one of the inner diameter of the ferritin protein  22  contained in the green light-emitting layer  13 G and the inner diameter of the ferritin protein  22  contained in the blue light-emitting layer  13 B. Further, the inner diameter of the ferritin protein  22  contained in the green light-emitting layer  13 G is preferably larger the inner diameter of the ferritin protein  22  contained in the blue light-emitting layer  13 B. 
     It is noted that the outer diameter of the ferritin protein  22  is denoted as D 1  shown in  FIG.  8   , and the inner diameter of the ferritin protein  22  is denoted as D 2  shown in  FIG.  8   . It is noted that the quantum dot  21 , which grows to the inside of the spherical shell of the ferritin protein  22 , has a particle diameter approximately close to the inner diameter D 2  of the ferritin  5  protein  22 . 
     Second Embodiment 
     The first embodiment has disclosed the light-emitting element  1 , which includes the light-emitting layer  13  that emits light through injection-type electric-field light emission, whereas a second embodiment discloses a light-emitting element  1 A, which includes a light-emitting layer that emits light through electric-field light emission and includes a plurality of wavelength conversion layers that are individually disposed adjacently to the light-emitting layer from which light exits, and that convert the wavelength of light emitted from the light-emitting layer. It is noted that the detailed description of configurations common to those of the light-emitting element  1  in the first embodiment will be omitted. 
       FIG.  16    illustrates a schematic configuration of the light-emitting element  1 A in the second embodiment.  FIG.  17    is a perspective view of a ferritin protein  22  containing a quantum dot  21 B formed in a blue light-emitting layer  13 B of the light-emitting element  1 A.  FIG.  18    is a perspective view of a ferritin protein  22  containing a quantum dot  21 R formed in a red light-emitting layer  13 R of the light-emitting element  1 A.  FIG.  19    is a perspective view of a ferritin protein  22  containing a quantum dot  21 G formed in a green light-emitting layer  13 G of the light-emitting element  1 A. The description of components included in the light-emitting element  1 A that are irrelevant to the second embodiment and the description of which overlaps that in the first embodiment will be omitted. 
     Configuration of Light-Emitting Element  1 A 
     In the light-emitting element  1 A, the blue light-emitting layer  13 B is provided between a first electrode  11  and each of second electrodes  16 R,  16 G and  16 B. The second electrodes  16 R,  16 G, and  16 G are spaced from each other by edge covers  19   a  and  19   b  and disposed on a substrate  17 . The first electrode  11  functions as a cathode, and the second electrodes  16 R,  16 G, and  16 B each function as an anode. A sealing layer  10  is stacked over the first electrode  11 . Furthermore, over the sealing layer  10 , the red light-emitting layer  13 R and the green light-emitting layer  13 G are individually stacked on the sealing layer  10  as wavelength conversion layers. 
     The blue light-emitting layer  13 B includes the quantum dot  21 B that is covered with the ferritin protein  22  and that emits blue light. This quantum dot  21 B, which emits blue light, can be composed of the quantum dot  21 B that is disclosed in the modification of the first embodiment and that emits blue light. 
     The sealing layer  10  is stacked on the first electrode  11  and is made of a material that allows light emitted by the blue light-emitting layer  13 B to pass. The material constituting the sealing layer  10  is selected as appropriate from materials that are common in the field. 
     The red light-emitting layer  13 R is stacked in a position being adjacent to the blue light-emitting layer  13 B from which light exits, and overlapping the blue light-emitting layer  13 B above the sealing layer  10 . The red light-emitting layer  13 R converts the optical wavelength of blue light emitted from the blue light-emitting layer  13 B into red and includes the quantum dot  21 R that emits red light. This quantum dot  21 R can be composed of the quantum dot  21 R that is disclosed in the modification of the first embodiment and that emits red light. The quantum dot  21 R may be covered with the ferritin protein  22 . 
     The green light-emitting layer  13 G is stacked in a position overlapping the blue light-emitting layer  13 B above the sealing layer  10  positioned adjacently to the blue light-emitting layer  13 B from which light exits. The green light-emitting layer  13 G converts the optical wavelength of blue light emitted from the blue light-emitting layer  13 B into green and includes the quantum dot  21 G that emits green light. This quantum dot  21 G can be composed of the quantum dot  21 G that is disclosed in the modification of the first embodiment and that emits green light. The quantum dot  21 G may be covered with the ferritin protein  22 . 
     Upon voltage application between the first electrode  11  and the second electrode  16 R, the quantum dot  21 B included in part of the blue light-emitting layer  13 B overlapping the second electrode  16 R undergoes excitation to emit blue light through electric-field light emission. The blue light emitted from the part of the blue light-emitting layer  13 B passes through the sealing layer  10  and enters the red light-emitting layer  13 R on the sealing layer  10 . Then, the quantum dot  21 R included in the red light-emitting layer  13 R undergoes optical excitation by the blue light entered the red light-emitting layer  13 R, to thus emit red light. 
     Upon voltage application between the first electrode  11  and the second electrode  16 G, the quantum dot  21 B included in part of the blue light-emitting layer  13 B overlapping the second electrode  16 G undergoes excitation to emit blue light through electric-field light emission. The blue light emitted from the part of the blue light-emitting layer  13 B passes through the sealing layer  10  and enters the green light-emitting layer  13 G on the sealing layer  10 . Then, the quantum dot  21 G included in the green light-emitting layer  13 G undergoes optical excitation by the blue light entered the green light-emitting layer  13 G, to thus emit green light. 
     Upon voltage application between the first electrode  11  and the second electrode  16 B, the quantum dot  21 B included in part of the blue light-emitting layer  13 B overlapping the second electrode  16 B undergoes excitation to emit blue light through electric-field light emission. The blue light emitted from the part of the blue light-emitting layer  13 B passes through the sealing layer  10  and exits from a part of the surface of the sealing layer  10  overlapping the second electrode  16 B. 
     The blue light-emitting layer  13 B, the red light-emitting layer  13 R, and the green light-emitting layer  13 G respectively include the quantum dots  21 B,  21 R, and  21 G containing the ferritin protein  22 ; accordingly, the self-assembly function of the ferritin protein  22  renders the quantum dots  21 R,  21 G, and  21 B close to each other within each of the red light-emitting layer  13 R, the green light-emitting layer  13 G, and the blue light-emitting layer  13 B. As a result, the red light-emitting layer  13 R, the green light-emitting layer  13 G, and the blue light-emitting layer  13 B are rendered denser than a known configuration having ligands and quantum dots  21 , thus improving the light emission efficiency of the light-emitting element  1 A. Further, the above configuration improves in-plane unevenness in the red light-emitting layer  13 R, the green light-emitting layer  13 G, and the blue light-emitting layer  13 B further than a known configuration having ligands and QD phosphor particles  21 . 
     Further, the red light-emitting layer  13 R, the green light-emitting layer  13 G, and the blue light-emitting layer  13 B may each contain the foregoing bivalent cationic metal, like those in the first embodiment. 
     It is noted that a color filter for emitting light of a desired wavelength may be provided individually above the red light-emitting layer  13 R and above the green light-emitting layer  13 G as well as above a part of a surface of the sealing layer  10  from which blue light is emitted, depending on various intentions. 
     Method for Manufacturing Light-Emitting Element  1 A A method for manufacturing the light-emitting element  1 A includes the manufacturing method disclosed in the first embodiment and includes thereafter, preparing a stack of the substrate  17  through the first electrode  11 , and further stacking the sealing layer  10  over the first electrode  11  using a known method. 
     Thereafter, a process step of staking the ferritin protein  22  onto the sealing layer  10  by transferring the ferritin proteins  22  containing the respective quantum dots  21  from a PBLH film  24  onto the surface of the sealing layer  10 , as disclosed in the first embodiment, is repeated, and the red light-emitting layer  13 R and the green light-emitting layer  13 G are individually stacked onto the sealing layer  10  sequentially. 
     Modification of Second Embodiment 
     Further, although the second embodiment has disclosed a configuration where all the red light-emitting layer  13 R, green light-emitting layer  13 G, and blue light-emitting layer  13 B include the ferritin protein  22 , in another embodiment, any of the red light-emitting layer  13 R, green light-emitting layer  13 G and blue light-emitting layer  13 B may be a light-emitting layer  13  including the ferritin protein  22  containing the quantum dot  21 . For instance, in the second embodiment, a replacement of the blue light-emitting layer  13 B including the ferritin protein  22  with a blue light-emitting layer including an organic compound that emits blue light through electric-field light emission, like one typically used in the field of organic electro-luminescence diodes (OLEDs, organic light-emitting diodes), is also included in the technical scope of the disclosure. 
     The disclosure is not limited to the foregoing embodiments and can be modified in various manners within the scope of the claims; moreover, an embodiment that is obtained by combining, as appropriate, the technical means disclosed in the respective embodiments is also included in the technical scope of the disclosure. Furthermore, combining the technical means disclosed in the respective embodiments can form a new technical feature.