PHOTOELECTRIC CONVERSION ELEMENT INCLUDING TRANSITION METAL DICHALCOGENIDE THIN FILM AND LIGHT-RECEIVING ELEMENT INCLUDING THE PHOTOELECTRIC CONVERSION ELEMENT

Abstract

A photoelectric conversion element including a thin film composed of a transition metal dichalcogenide and formed on a base material. In the present invention, a surface of the thin film composed of the transition metal dichalcogenide is modified with at least any nanorod particles of Au nanorod particles composed of Au and Ag nanorod particles composed of Ag. An average aspect ratio of the Au nanorod particles is 3.0 or more and 12.0 or less, and an average aspect ratio of the Ag nanorod particles is 3.0 or more and 13.0 or less. In the invention, the sensitivity to light having wavelengths in the near-infrared region improves by a sensitizing action attributed to localized surface plasmon resonance that is developed by the Au and Ag nanorod particles. The photoelectric conversion element exhibits sensitivity even to light having wavelengths in the near-infrared region near a wavelength range of 1600 nm.

Claims

1. A photoelectric conversion element comprising: a base material; and a thin film formed on the base material and composed of a transition metal dichalcogenide, wherein a surface of the thin film composed of the transition metal dichalcogenide is modified with at least any nanorod particles of Au nanorod particles composed of Au and Ag nanorod particles composed of Ag, an average aspect ratio, i.e. major axis/minor axis of the Au nanorod particles is 3.0 or more and 12.0 or less, and an average aspect ratio, i.e. major axis/minor axis of the Ag nanorod particles is 3.0 or more and 13.0 or less.

2. The photoelectric conversion element according to claim 1, wherein, in an observation of the surface of the thin film composed of the transition metal dichalcogenide and modified with the nanorod particles, an area ratio of the nanorod particles in an observation field region is 0.1% or more and 1% or less.

3. The photoelectric conversion element according to claim 1, wherein the transition metal dichalcogenide is a chalcogenide of Pt or Pd.

4. The photoelectric conversion element according to claim 1, wherein a chalcogenide element of the transition metal dichalcogenide is any of sulfur, selenium and tellurium.

5. The photoelectric conversion element according to claim 1, wherein an average of the minor axes of the nanorod particles is 10 nm or more and 60 nm or less.

6. The photoelectric conversion element according to claim 1, wherein the base material is composed of any of glass, quartz, silicon, carbon, ceramic or metal.

7. A light-receiving element comprising the photoelectric conversion element defined in claim 1.

8. The photoelectric conversion element according to claim 2, wherein the transition metal dichalcogenide is a chalcogenide of Pt or Pd.

9. The photoelectric conversion element according to claim 2, wherein a chalcogenide element of the transition metal dichalcogenide is any of sulfur, selenium and tellurium.

10. The photoelectric conversion element according to claim 3, wherein a chalcogenide element of the transition metal dichalcogenide is any of sulfur, selenium and tellurium.

11. The photoelectric conversion element according to claim 2, wherein an average of the minor axes of the nanorod particles is 10 nm or more and 60 nm or less.

12. The photoelectric conversion element according to claim 3, wherein an average of the minor axes of the nanorod particles is 10 nm or more and 60 nm or less.

13. The photoelectric conversion element according to claim 4, wherein an average of the minor axes of the nanorod particles is 10 nm or more and 60 nm or less.

14. The photoelectric conversion element according to claim 2, wherein the base material is composed of any of glass, quartz, silicon, carbon, ceramic or metal.

15. The photoelectric conversion element according to claim 3, wherein the base material is composed of any of glass, quartz, silicon, carbon, ceramic or metal.

16. The photoelectric conversion element according to claim 4, wherein the base material is composed of any of glass, quartz, silicon, carbon, ceramic or metal.

17. The photoelectric conversion element according to claim 5, wherein the base material is composed of any of glass, quartz, silicon, carbon, ceramic or metal.

18. A light-receiving element comprising the photoelectric conversion element defined in claim 2.

19. A light-receiving element comprising the photoelectric conversion element defined in claim 3.

20. A light-receiving element comprising the photoelectric conversion element defined in claim 4.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a TEM photograph showing the external appearance of Au nanorod particles manufactured in First Embodiment;

[0042] FIG. 2 is a view showing the measurement results of absorption wavelengths of the Au nanorod particles manufactured in First Embodiment;

[0043] FIG. 3 is a SEM image showing the surface morphology of a photoelectric conversion element (Au nanorod particles/PtSe.sub.2) manufactured in First Embodiment;

[0044] FIG. 4 is a graph showing the relationship between the concentration and area ratio of Au nanorod particles in a Au nanorod solution manufactured in First Embodiment;

[0045] FIG. 5 is a view showing the IR response characteristics of photoelectric conversion elements of First Embodiment and a comparative example;

[0046] FIG. 6 is a graph showing the relationship between the concentration of an applied nanorod solution and the photocurrent regarding the photoelectric conversion elements of First Embodiment and the comparative example;

[0047] FIG. 7 is a TEM photograph showing the external appearance of Ag nanorod particles manufactured in Second Embodiment;

[0048] FIG. 8 is a view showing the measurement result of absorption wavelengths of the Ag nanorod particles manufactured in Second Embodiment;

[0049] FIG. 9 is a SEM image showing the surface morphology of a photoelectric conversion element (Ag nanorod particles/PtSe.sub.2) manufactured in Second Embodiment;

[0050] FIG. 10 is a view showing the IR response characteristics of photoelectric conversion elements of Second Embodiment and a comparative example; and

[0051] FIG. 11 is a view showing the measurement results of the absorption wavelengths of Au nanorod particles (aspect ratio: 12.0) and Ag nanorod particles (aspect ratio: 13.0) manufactured with reference to Second Embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0052] First Embodiment: Hereinafter, an embodiment of the present invention will be described. In the present embodiment, a thin film composed of PtSe.sub.2 as a transition metal dichalcogenide was formed on a base material. In addition, Au nanorod particles were manufactured as nanorod particles, and the surface of the thin film was modified with these, thereby producing a photoelectric conversion element. Regarding the manufactured photoelectric conversion element, the photoresponsivity to near-infrared rays was evaluated and the surface morphology was studied.

[Manufacturing of Photoelectric Conversion Element]

Formation of Transition Metal Dichalcogenide Thin Film

[0053] A Si/SiO.sub.2 substrate (surface 280 nm oxidized, dimensions: 20?20, thickness: 1.5 mm) was prepared as a base material, and a PtSe.sub.2 thin film was formed on this base material by a CVD method (thermal CVD). First, the substrate was set in a CVD device (hot wall-type horizontal CVD device), and 5 g of a selenium powder was disposed upstream of the substrate. The substrate temperature and the temperature of the selenium powder were each made controllable. The inside of a reactor was purged with an argon gas (200 sccm) before film formation.

[0054] In addition, a platinum complex (dimethyl(N,N-dimethyl-3-butene-1-amine-N)platinum (DDAP)) was used as a thin film raw material, the platinum complex was heated and vaporized and, together with a carrier gas, introduced into the reactor, the platinum complex vaporized on the substrate was decomposed, and platinum and selenium were reacted with each other to precipitate PtSe.sub.2 on the substrate, thereby forming a PtSe.sub.2 thin film. Film formation conditions are as described below. The film thickness of the PtSe.sub.2 thin film formed on the base material in this film formation step was 4 nm. [0055] Raw material heating temperature: 67? C. [0056] Carrier gas: Argon/10 sccm [0057] Flow gas: Argon/200 sccm [0058] Substrate temperature/selenium powder heating temperature: 400? C./220? C. [0059] Film formation time: 15 minutes

Manufacturing of Au Nanorod Particles

[0060] Au nanorod particles were manufactured by a colloid particle growth method. In the present embodiment, it was intended to manufacture Au nanorod particles having a maximum absorption wavelength of 940 nm (aspect ratio: 6.0). First, Au colloid particles acting as seed particles were manufactured. 60 ?L of 10 mM sodium borohydride, which is a reducing agent, was added to a solution mixture of 0.5 mL of a 0.5 mM chloraurate aqueous solution and 0.5 mL of a 200 mM CTAB aqueous solution and intensively stirred with a Vortex for two minutes, thereby manufacturing Au colloid particles. A Au colloid solution manufactured above, which acted as a seed particle solution, was stored at 30? C. until being used in a manufacturing step of nanorod particles, which is a subsequent step.

[0061] A solution mixture of 0.5 mL of a 1 mM chloraurate aqueous solution, 7 ?L of a 100 mM silver nitrate aqueous solution and 0.5 mL of a 200 mM CTAB aqueous solution was prepared as a growth liquid. 50 ?L of 100 mM hydroquinone was added to this aqueous solution, and, furthermore, 32 ?L of the Au colloid particle solution, i.e. seed particle solution manufactured above was added to and mixed with the aqueous solution. Subsequently, the solution mixture was left to stand at 30? C. overnight (16 hours) to generate Au nanorod particles (AuNRs).

[0062] Next, a dispersion liquid containing the Au nanorod particles manufactured above was purified. In a purification step of the present embodiment, the dispersion liquid containing the Au nanorod particles manufactured above was centrifuged with a centrifuge under conditions of 12000 g and 10 minutes, pelleted, collected and dispersed in purified water. This was repeated twice, and a dispersion medium was substituted into purified water. Furthermore, a nanorod aqueous solution, for which the dispersion medium had been substituted as described above, was made to account for 10% of the volume, a 100 mM CTAB solution was made to account for 1% of the volume, a 500 mM benzyldimethylammoniumchloride (BDAC) solution was made to account for 40% of the volume, and purified water was added thereto to make the volume 100%. After this solution was left to stand at 30? C. overnight (16 hours), a supernatant containing a by-product was removed, and Au nanorod particles that settled on the bottom surface were collected. Substitution into purified water by centrifugation was repeated twice in the same manner as described above.

[0063] FIG. 1 is a TEM photograph showing the external appearance of the Au nanorod particles manufactured in the present embodiment. Regarding these Au nanorod particles, the major axis and minor axis lengths were measured, and the aspect ratios were calculated. In this measurement, 200 nanorod particles were arbitrarily extracted from the image, the major axis and minor axis of each nanorod particle were measured, the aspect ratio was calculated, and the average values of the aspect ratios, the major axis lengths and the minor axis lengths were calculated. In the Au nanorod particles manufactured in the present embodiment, the average value of the aspect ratios was 5.9. In addition, the average values of the major axis and minor axis lengths were 97.5 nm and 16.9 nm, respectively. In addition, FIG. 2 shows the measurement result of an absorption spectrum measured with an ultraviolet-visible-near-ultraviolet spectrophotometer regarding the Au nanorod particles manufactured in the present embodiment. For the Au nanorod particles of the present embodiment, it was confirmed that the maximum absorption wavelength was present near approximately 950 nm.

Modification of PtSe.sub.2 Thin Film with Au Nanorod Particles and Preparation of Light-Receiving Element

[0064] Next, regarding the PtSe.sub.2 thin film formed above, the surface of the thin film was modified with the Au nanorod particles. In the present embodiment, purified water was used as a dispersion medium, and dispersion liquids in which the (mass-based) concentration of the Au nanorod particles in the dispersion liquid was adjusted to 0.5 nM, 1.0 nM, 5.0 nM, 10 nM, 25 M or 50 nM were prepared. These six nanorod solutions were applied, thereby manufacturing modified transition metal dichalcogenide thin films. The Au nanorod particle dispersion liquids were applied with a spin coater under application conditions where 1 ?L of the nanorod solution was applied at 2000 rpm for 30 seconds.

[0065] The PtSe.sub.2 thin film was modified with the Au nanorod particles, washed and dried, then, a comb-shaped electrode was formed on the thin film surface, thereby manufacturing a light-receiving element, which is a photoelectric conversion element. The comb-shaped electrode was formed by sequentially patterning a Ti film (film thickness: 5 nm) and a Au film (film thickness: 40 nm) in a comb shape on the surface of the PtSe.sub.2 thin film modified with the Au nanorod particles.

[0066] Comparative Example: As a comparative example with respect to the above-described embodiment, a photoelectric conversion element in which a PtSe.sub.2 thin film was not modified with Au nanorod particles was manufactured. In the above-described embodiment, after the formation of the PtSe.sub.2 thin film on the substrate, modification with the Au nanorod particles was not performed, and the comb-shaped electrode was formed, thereby producing a light-receiving element.

[Measurement of Area Ratio of Nanorod Particles]

[0067] An example of a SEM image of the Au nanorod particles on the transition metal dichalcogenide thin film surface in the light-receiving element of the present embodiment is shown in FIG. 3. In addition, the area ratio of the nanorod particles was obtained based on the SEM image. In the measurement of the area ratio of the nanorod particles, the ratio of the area of the nanorod particles in an observation region in the image was calculated in percentage (%). This calculation was performed with image analysis software (product name: ImageJ), as analysis conditions, the image was converted to 8 bits and binarized, and the area ratio was obtained from the area of all nanorod particles and the area of the measurement region.

[0068] FIG. 4 is a view showing the area ratio of the Au nanorod particles on the transition metal dichalcogenide thin film surface modified with the nanorod solution having each concentration. From this figure, it is possible to confirm that there is a proportional relationship between the concentration of the nanorod solution, which is applied to the thin film, and the area ratio of the nanorod particles on the thin film surface and the area ratio can be adjusted with the concentration of the solution.

[Evaluation of Photoresponsivity of Light-Receiving Element]

[0069] Regarding the light-receiving elements of the present embodiment and the comparative example prepared above, the photoresponsivity to near-infrared rays was measured. As a measuring method, each light-receiving element was irradiated with near-infrared rays, and a photocurrent was measured at room temperature using a multimeter. The wavelengths of the near-infrared rays used for the irradiation were 740 nm, 850 nm and 940 nm (three patterns), and the response characteristic at each wavelength was measured. Regarding the irradiation with the near-infrared rays, the light-receiving element was continuously irradiated for 20 seconds at 40-second intervals. In a four-terminal method, 0.5 V was loaded as a bias voltage.

[0070] The evaluation result of the photoresponsivity is shown in FIG. 5. In FIG. 5, regarding the measurement result of each light-receiving element, the drawing is created by setting an offset every 400 nA. In addition, the relationship between the concentration of the nanorod solution and the maximum photocurrent calculated based on this result is shown in FIG. 6.

[0071] From this measurement result of the response characteristic, it was confirmed that the transition metal dichalcogenide thin films (PtSe.sub.2 thin films) modified with the Au nanorod particles were capable of generating higher photocurrents in the near-infrared region of 740 nm to 940 nm than thin films not modified with Au nanorod particles. Particularly, in the PtSe.sub.2 thin film modified with the nanorod solution having a concentration of 5 nM (the area ratio of the nanorod particles: 0.3%), a photocurrent five or more times higher is generated compared with those in unmodified PtSe.sub.2 thin films.

[0072] However, when the result of the PtSe.sub.2 thin film modified with the nanorod solution having a concentration of 50 nM (the area ratio: 2.5%) is seen, the photocurrent value becomes lower than those of the unmodified PtSe.sub.2 thin films. Therefore, it is deemed that excessive modification with the Au nanorod particles is also not preferable.

[0073] Second Embodiment: In the present embodiment, a PtSe.sub.2 transition metal dichalcogenide thin film was formed on a base material in the same manner as in First Embodiment, and this thin film surface was modified with Ag nanorod particles as nanorod particles, thereby producing a photoelectric conversion element.

[Manufacturing of Photoelectric Conversion Element]

Formation of Transition Metal Dichalcogenide Thin Film

[0074] A PtSe.sub.2 thin film was formed on the same SiO.sub.2 glass substrate as in First Embodiment by a CVD method. The raw material and method for manufacturing this transition metal dichalcogenide thin film were the same as those in First Embodiment, and the film thickness of the PtSe.sub.2 thin film was 4 nm.

Manufacturing of Ag Nanorod Particles

[0075] Ag nanorod particles were manufactured by a colloid particle growth method. In the present embodiment, it was intended to manufacture Ag nanorod particles having a maximum absorption wavelength of 940 nm (aspect ratio: 5.0). First, Ag colloid particles acting as seed particles were manufactured. 50 ?L of 50 mM sodium citrate, 3 ?L of 0.5 mM polyvinylpyrrolidone, 5 ?L of 5 mM L-arginine and 20 ?L of silver nitrate were added to 700 ?L of water and stirred well. 8 ?L of 100 mM sodium borohydride, which is a reducing agent, was added to this solution mixture and intensively stirred with a Vortex for two minutes, thereby manufacturing Ag colloid particles (seed particles). A Ag colloid particle solution, which was this seed particle solution, was stored at 30? C. for 15 minutes until being used in a manufacturing step of nanorod particles, which is a subsequent step.

[0076] Upon the manufacturing of the Ag nanorod particles, first, the seed particle solution prepared above was irradiated with a blue diode array lamp for 16 hours to generate regular decahedron silver nanoparticles. After that, 1 mL of a generated regular decahedron silver nanoparticle solution was centrifuged for 10 minutes at 12000 g to remove a supernatant and dispersed in 500 ?L of water to be purified.

[0077] 1 mL of water, 200 ?L of sodium citrate and 33 ?L of 0.5 mM polyvinylpyrrolidone were put into a 20 mL glass vial and preheated with an output of 100 W for one minute in a microwave, thereby preparing a growth liquid. 500 ?L of the regular decahedron silver nanoparticle solution purified above and 200 ?L of 10 mM silver nitrate were added to and mixed with this aqueous solution and further heated for 15 minutes, thereby obtaining Ag nanorod particles.

[0078] FIG. 7 is a TEM photograph showing the external appearance of the Ag nanorod particles manufactured in the present embodiment. Regarding these Ag nanorod particles, the major axis and minor axis lengths were measured, and the aspect ratios were calculated in the same manner as in First Embodiment. In the Ag nanorod particles manufactured in the present embodiment, the average value of the aspect ratios was 4.9. In addition, the average values of the major axis and minor axis lengths were 187.9 nm and 37.7 nm, respectively. In addition, FIG. 8 shows the measurement results of absorption wavelengths measured in the same manner as in First Embodiment regarding the Ag nanorod particles. For the Ag nanorod particles of the present embodiment, it was confirmed that the maximum absorption wavelength was present near approximately 950 nm.

Modification of PtSe.sub.2 Thin Film with Ag Nanorod Particles and Preparation of Light-Receiving Element

[0079] Next, regarding the PtSe.sub.2 thin film formed above, the surface of the thin film was modified with the Ag nanorod particles. In the present embodiment, purified water was used as a dispersion medium, and the concentration of the Ag nanorod particles was adjusted so that the absorbance of a dispersion liquid reached 2.5, thereby preparing the dispersion liquid (the concentration of the Ag nanorod particles: 1 nM). The Ag nanorod particle dispersion liquid was applied with a spin coater under application conditions where 1 ?L of the nanorod solution was applied at 2000 rpm for 30 seconds.

[0080] The PtSe.sub.2 thin film was modified with the Ag nanorod particles, washed and dried, then, a comb-shaped electrode was patterned and formed on the thin film surface in the same manner as in First Embodiment, thereby manufacturing a light-receiving element, which is a photoelectric conversion element. An example of a SEM image of the Ag nanorod particles on the PtSe.sub.2 thin film surface in the light-receiving element manufactured in the present embodiment is shown in FIG. 9. As a result of measuring the area ratio of the Ag nanorod particles base on this SEM image in the same manner as in First Embodiment, the area ratio was 0.98%.

[Evaluation of Photoresponsivity of Light-Receiving Element]

[0081] Regarding the light-receiving element of Second Embodiment produced above, the photoresponsivity to near-infrared rays was measured. The measurement method is the same as in First Embodiment, and the response characteristic to near-infrared rays at each wavelength of 740 nm, 850 nm and 940 nm was measured. In the present embodiment as well, the measurement was performed on a light-receiving element not modified with the Ag nanorod particles as a comparative example.

[0082] The evaluation result of the photoresponsivity is shown in FIG. 10. From this measurement result of the response characteristic, it was confirmed that the transition metal dichalcogenide thin films (PtSe.sub.2 thin films) modified with the Ag nanorod particles were also capable of generating higher photocurrents in the near-infrared region of 740 nm to 940 nm than thin films not modified with Ag nanorod particles.

[0083] From the results of First Embodiment (Au nanorod particles) and Second Embodiment (Ag nanorod particles) described above, the sensitizing actions of the transition metal dichalcogenide thin films by these were confirmed. For these Au/Ag nanorod particles, it is possible to adjust the aspect ratios by changing manufacturing conditions. Therefore, the Au/Ag nanorod particles show optical characteristics in accordance with the aspect ratios.

[0084] As an example of the manufacturing condition of the nanorod particles for aspect ratio adjustment, the amount and concentration of the seed particle solution (that is, the number of the seed particles) at the time of adding the seed particle solution to the growth liquid are prepared. For example, in the manufacturing of the Au nanorod particles of First Embodiment, 32 ?L of the seed particle solution is mixed with the growth solution to obtain Au nanorod particles having aspect ratios of 5.9. In order to make the aspect ratios of the Au nanorod particles larger, the amount of the seed particle solution added may be smaller than that in First Embodiment. Conversely, in order to obtain Au nanorod particles having smaller aspect ratios than those in First Embodiment, the amount of the seed particle solution added may be larger than that in First Embodiment.

[0085] In addition, the aspect ratios of the nanorod particles can also be adjusted with the heating time (reaction time) after the addition of the seed particle solution to the growth liquid. In the manufacturing of the Ag nanorod particles of Second Embodiment, the regular decahedron silver nanoparticles were used as the seed particles, and these were mixed with silver nitrate and heated for 15 minutes, thereby manufacturing Ag nanorod particles having an average aspect ratio of 4.9. When this heating time is extended, it is possible to make the aspect ratios of the Ag nanorod particles larger than those in Second Embodiment.

[0086] Therefore, based on First and Second Embodiments, the amount of the seed particle solution added and the heating time were prepared, and Au nanorod particles having an aspect ratio (average value) of 12.0 and Ag nanorod particles having an aspect ratio (average value) of 13.0 were manufactured. Regarding each of the manufactured nanorod particles, the absorption wavelength was measured in the same manner as in First Embodiment.

[0087] FIG. 11 is a view showing the measurement results of the absorption wavelengths of the Au nanorod particles (aspect ratio: 12.0) and the Ag nanorod particles (aspect ratio: 13.0). The maximum absorption wavelength of the Au nanorod particles having an aspect ratio of 12.0 reaches approximately 1680 nm. The maximum absorption wavelength of the Ag nanorod particles having an aspect ratio of 13.0 reaches approximately 1780 nm. As such, it is found that, regarding the Au and Ag nanorod particles, it is possible to optimize the light absorption characteristic by preparing the aspect ratios. Therefore, it was confirmed that the Au and Ag nanorod particles act on light having wavelengths in the near-infrared region up to near 1600 nm.

INDUSTRIAL APPLICABILITY

[0088] As described above, the photoelectric conversion element to which the transition metal dichalcogenide thin film according to the invention is applied has an improved photoresponsivity in the near-infrared region compared with conventional transition metal dichalcogenide thin films. The present invention can provide a high-sensitivity photoelectric conversion element while utilizing the preferable conditions of transition metal dichalcogenide thin films such as low manufacturing cost or adaptability at room temperature. The present invention is particularly useful as a light-receiving element that deals with the near-infrared region and can be expected to contribute to additional improvement in the measurement accuracy of LIDAR as a material for light-receiving elements in the LIDAR application.