Method for manufacturing metal nanowire and metal nanowire device

Abstract

To provide a method for manufacturing a high-density metal nanowire.SOLUTION: A method for manufacturing a metal nanowire includes: a step S1 of generating a thin film in which at least one of the alumina and SiO2 to aluminum or gold on a substrate; and a step S3 of heating the thin film.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to metal nanowire manufacturing methods and metal nanowire devices. [Background technology]

[0002] Nanowires (NWs), a type of one-dimensional nanomaterial, are crystals that grow outward from the surface of a substrate in a whisker-like shape. Known methods for manufacturing semiconductor nanowires using semiconductor materials include the VLS growth method (see, for example, Non-Patent Document 1).

[0003] On the other hand, metal nanowires made from pure metals such as aluminum are expected to be one of the promising materials in nanotechnology because of their remarkable characteristics, such as excellent electrical and thermal conductivity, high adhesive ability, excellent ductility and sinterability, low strength-to-volume ratio, low environmental impact, and excellent recyclability and repairability. However, in contrast to semiconductor nanowires, it is difficult to control the one-dimensional crystal growth while providing the material in the vapor phase, and there have been only a limited number of reports of successful production of such metal nanowires (see, for example, Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Zhang Guoqiang, Takehiko Tawara, Hiroki Hibino, and Hideki Goto, "InP / InAs nanowire heterostructures grown by self-catalytic vapor-liquid-spin growth," Journal of the Japanese Society for Crystal Growth, Vol. 46, No. 2 (2019). [Non-patent document 2] Blech, IA, Petroff, PM, Tai, KL, Kumar, V.: "Whisker growth in Al thin films" Journal of Crystal Growth 32, 161.169 (1975). Summary of the Invention [Problem to be solved by the invention]

[0005] The technology disclosed in Non-Patent Document 2 uses stress-induced migration to grow aluminum nanowires in the vertical direction. However, the surface density of nanowires that can be achieved with this technology is at most 2 × 10 5 / cm 2 This is not a sufficient density for practical one-dimensional nanomaterials.

[0006] The present disclosure has been made in light of the above circumstances, and an object of the present disclosure is to provide a method for producing high-density metal nanowires. [Means for solving the problem]

[0007] In order to solve the above problem, one aspect of the present invention is a method for producing metal nanowires, which includes the steps of generating a thin film of aluminum or gold doped with at least one of alumina and SiO2 on a substrate, and heating the thin film.

[0008] In some embodiments, the method may further include forming a grain size gradient within the thin film, with the grain size increasing from the interior to the surface.

[0009] In one embodiment, forming a grain size gradient in the thin film, with the grain size increasing from the interior to the surface, may include irradiating the surface of the thin film with FIB.

[0010] In one embodiment, the thin film may be an aluminum thin film with alumina added, where the aluminum particle size may be 10 nm to 30 nm, and the thickness of the thin film may be 100 nm to 800 nm.

[0011] In one embodiment, the implant dose in the step of irradiating with FIB is 0.05 nC / μm 2 More than 0.5nC / μm 2 It may be the following:

[0012] In one embodiment, the heating temperature in the heating step may be 300°C.

[0013] In one embodiment, the heating step may involve heating the thin film from room temperature to 300° C. over 15 minutes, then maintaining the temperature at 300° C. for 3 hours, and then naturally cooling to room temperature.

[0014] Another aspect of the present invention is a metal nanowire device. This metal nanowire device includes a substrate, a thin film formed on the substrate, the thin film being made of aluminum or gold and at least one of alumina and SiO2, and metal nanowires growing from the surface layer of the thin film. A coarse-grained layer with large grains is formed on the surface side of the thin film, and a fine-grained layer with small grains is formed on the substrate side of the thin film.

[0015] In one embodiment, a metal nanowire device may be fabricated by a method including the steps of forming a thin film of aluminum or gold doped with at least one of alumina and SiO2 on a substrate and heating the thin film.

[0016] Any combination of the above components, and conversion of the present disclosure into a method, device, system, recording medium, computer program, etc., are also valid aspects of the present disclosure. [Effects of the Invention]

[0017] According to the present disclosure, high density metal nanowires can be produced. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a flowchart showing a processing procedure of a method for manufacturing metal nanowires according to a first embodiment. [Figure 2] This is an inverse pole figure crystal orientation map (IPF map) of the cross section of a thin film measured by EBSD (electron backscatter diffraction) method. [Figure 3] 2 is a schematic diagram showing the mechanism by which metal nanowires grow by the method according to the first embodiment. FIG. [Figure 4] FIG. 1 is a schematic diagram showing metal nanowires grown by a method according to a first embodiment. [Figure 5] 1 is a scanning electron microscope photograph of metal nanowires produced using the method according to the first embodiment. [Figure 6] 1 shows scanning electron microscope images of metal nanowires at different FIB doses. [Figure 7] 1 is a histogram showing the length and density of grown metal nanowires as a function of FIB etching depth δ. [Figure 8] 1 is a bubble chart showing the density of grown metal nanowires as a function of length. [Figure 9] This is a scanning transmission electron microscope image of a thin film formed by adding SiO2 to aluminum. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present disclosure will be described below based on preferred embodiments with reference to the drawings. In the embodiments and modified examples, identical or equivalent components and parts are designated by the same reference numerals, and redundant description will be omitted where appropriate. The dimensions of the parts in the drawings are enlarged or reduced as appropriate for ease of understanding. Some elements that are not important for explaining the embodiments are omitted from the drawings. Terms including ordinal numbers such as "first," "second," etc. are used to describe various components, but these terms are used only to distinguish one component from another, and do not limit the components.

[0020] [First embodiment] 1 is a flowchart showing the procedure of a method for manufacturing metal nanowires according to the first embodiment, which includes a thin film forming step S1, a grain size gradient forming step S2, and a heating step S3.

[0021] (Thin film formation step S1) In the thin film forming step S1, the method forms a thin film of metal doped with impurities on a substrate.

[0022] By way of example, the metal that makes up the thin film may be, but is not limited to, aluminum or gold.

[0023] As an example, the added impurity may be, but is not limited to, a metal oxide such as alumina (AL2O3).

[0024] For example, the thickness of the thin film may be about 100 nm, but is not limited to this. For example, according to the experiments of the present inventors, nanowires can be fabricated with a thin film thickness of 200 nm, 400 nm, or 800 nm.

[0025] For example, the grain size of the thin film may be about 10 nm or less, but is not limited to this. For example, according to experiments by the present inventors, nanowires can be created even when the grain size of the thin film is smaller than 10 nm (for example, on the order of a few nm).

[0026] As an example, the substrate may be a semiconductor substrate such as Si, but is not limited to this.

[0027] Typically, in the thin film formation step S1, a material in which aluminum and alumina are mixed in a mass ratio of 1:1 may be evaporated onto a silicon substrate using an electron beam evaporation device, etc. However, the procedure for forming the thin film is not limited to this.

[0028] (Particle size gradient formation step S2) In the grain size gradient formation step S2, the method forms a grain size gradient in the thin film formed in step S1, where the grain size increases from the inside to the surface.

[0029] As an example, the grain size gradient forming step S2 may include a step of irradiating the surface layer of the thin film formed in step S1 with a focused ion beam (FIB).

[0030] By way of example, and not limitation, the ion material of the FIB may be gallium.

[0031] For example, when irradiating a thin film made of aluminum and alumina with a thickness of 100 nm and a grain size of 10 nm with a gallium FIB, the implantation dose due to irradiation is approximately 0.05 to 0.5 nC / μm 2 However, the present invention is not limited to this.

[0032] The FIB may be irradiated onto the thin film vertically from directly above, or obliquely from above at an angle to the thin film surface layer.

[0033] The FIB may be irradiated continuously or intermittently (pulsed).

[0034] The size of the crystal grains enlarged by FIB irradiation to create nanowires has a threshold value of several tens of nanometers (e.g., about 30 nm), but this threshold value changes depending on factors such as the heating temperature.

[0035] FIB irradiation is one example of an energy application method for enlarging the grain size near the surface of a thin film to form a grain size gradient. Therefore, in practice, any other suitable energy application method (e.g., laser irradiation, heat treatment, current treatment, etc.) may be used to form a grain size gradient.

[0036] Furthermore, our experiments have shown that in the case of gold nanowires, a grain size gradient is already established at the time of film formation, and in this case, nanowires can be grown without performing the grain size gradient formation step S2 using FIB irradiation or the like.

[0037] (Heating step S3) In a heating step S3, the method heats the thin film that was irradiated with the FIB in step S2.

[0038] The heating temperature is preferably about 300°C, but is not limited to this.

[0039] As an example, the heating step S3 may involve heating the thin film irradiated with the FIB from room temperature to approximately 300°C over approximately 15 minutes, then maintaining the temperature at approximately 300°C for approximately 3 hours, and finally naturally cooling it to room temperature.

[0040] By performing the processes of steps S1 to S3, high-density metal nanowires can be grown perpendicular to the thin film. That is, according to this embodiment, a method for producing high-density metal nanowires can be provided.

[0041] [Second embodiment] 4 schematically shows a metal nanowire device 1 according to a second embodiment. The metal nanowire device 1 includes a substrate 11, a thin film 12 containing a metal and impurities formed on the substrate 11, and metal nanowires 13 growing from the surface layer of the thin film 12. A coarse-grained layer 16 is formed on the surface side of the thin film 12, and a fine-grained layer 17 is formed on the substrate side of the thin film 12.

[0042] By way of example, the metal that makes up the thin film 12 may be, but is not limited to, aluminum or gold.

[0043] By way of example, but not limitation, the added impurity may be a metal oxide such as alumina. For example, the additive may be an inorganic compound such as SiO2.

[0044] As an example, the thickness of the thin film 12 may be about 100 nm, but is not limited to this.

[0045] As an example, the grain size of the thin film 12 may be about 10 nm or less, but is not limited to this.

[0046] As an example, the grain size of the coarse grains of the thin film 12 may be about 10 to about 30 nm, but is not limited to this.

[0047] As an example, the substrate 11 may be a semiconductor substrate such as Si, but is not limited to this.

[0048] In one embodiment, the metal nanowire device 1 may be fabricated by a method (i.e., the method according to the first embodiment) including the steps of forming a thin film on a substrate, the thin film being made of aluminum or gold to which at least one of alumina and SiO2 has been added, irradiating the surface of the thin film with an FIB, and heating the thin film.

[0049] According to this embodiment, it is possible to provide a metal nanowire device including metal nanowires at high density.

[0050] [Mechanism of metal nanowire growth] Hereinafter, with reference to FIGS. 2 and 3, the theory that the method of the embodiment enables the growth of high density metal nanowires will be explained.

[0051] When an appropriate dose of FIB is applied to the surface of the thin film formed in step S1, the surface layer of the thin film (i.e., the upper side of the thin film in the figure) is scraped and etched, and the metal crystal grains near the surface of the thin film become enlarged. Meanwhile, the crystal grains inside the thin film (i.e., the lower side of the thin film in the figure) do not enlarge but remain fine-grained. Figure 2 shows an inverse pole figure (IPF) map of the cross section of the thin film measured using electron backscatter diffraction (EBSD). Figure 2a shows the thin film before FIB irradiation, and Figure 2b shows the thin film after FIB irradiation. As shown in Figure 2a, before FIB irradiation, a layer of fine grains (grain size 10 nm or less) is formed throughout the entire thickness of the thin film. In contrast, as shown in Figure 2b, after FIB irradiation, a layer of coarse grains (grain size approximately 10–30 nm) is formed near the surface of the thin film, while the interior of the thin film remains fine-grained.

[0052] In this way, when a thin film is irradiated with an FIB, a grain size gradient is formed within the thin film, with the grain size increasing from the interior to the surface. The hydrostatic stress gradient caused by this grain size gradient causes metal atoms within the thin film to rise from the interior to the surface (i.e., from bottom to top in the figure). As a result, crystal grains that serve as growth nuclei for metal nanowires accumulate near the surface of the thin film (see Figure 3a).

[0053] Next, when the thin film irradiated with FIB is heated, the thermal stress generated by the difference in linear expansion coefficients multiplied by the temperature difference triggers the diffusion of metal atoms.

[0054] At this time, gallium ions implanted by FIB and oxygen resulting from impurities (e.g., metal oxides) added during thin film deposition precipitate at the grain boundaries within the thin film. These precipitated gallium ions and oxygen "pin" the crystal grains that serve as growth nuclei. In this way, "pinned grains" are formed at the grain boundaries. When a large number of metal atoms flow into these pinned grains, they have nowhere to go and are unable to move laterally or downward within the thin film (see Figure 3b).

[0055] As a result, the metal atoms are pushed out from the surface of the thin film into the free space outside the thin film, and grow one-dimensionally upward (see Figure 3c).

[0056] At this time, if the force pushing the metal atoms upward is not sufficiently greater than the force pushing them back into the thin film, metal nanowires will not grow (see Figure 3d). In other words, the balance between the force pushing the metal atoms that have nowhere to go out of the thin film from the free surface and the relaxation that tries to return them to the thin film determines whether or not metal nanowires will grow. This balance is thought to be determined by factors such as the FIB dose and the heating temperature and heating time of the thin film.

[0057] Figure 4 is a schematic diagram of a device 1 using metal nanowires grown by the above mechanism. The metal nanowire device 1 includes a substrate 11, a thin film 12 containing metal and impurities grown on the substrate 11, and metal nanowires 13 grown from the surface layer of the thin film 12. The crystals in the thin film 12 are coarse grains 14 with large grain sizes on the surface side of the thin film 12 and fine grains 15 with small grain sizes on the substrate side of the thin film 12. As a result, a coarse grain layer 16 is formed on the surface side of the thin film 12, and a fine grain layer 17 is formed on the substrate side of the thin film 12. As a result, a grain size gradient is formed within the thin film, with the grain size increasing from the interior toward the surface.

[0058] [Verification experiment] The inventors conducted an experiment to grow metal nanowires using a method according to an embodiment. In this experiment, a thin film of metal (aluminum) doped with impurities (alumina) was grown on a semiconductor substrate (Si substrate). The grain size of the aluminum constituting the thin film was approximately 10 nm or less, and the thickness of the thin film was approximately 100 nm. A gallium ion FIB was irradiated vertically from directly above onto the surface of this thin film. In the heating step, the film was heated from room temperature to 300°C over 15 minutes, then maintained at 300°C for 3 hours, and finally naturally cooled to room temperature. In this experiment, differences in the growth of metal nanowires were observed by changing the dose of FIB irradiation.

[0059] Figure 5 is a scanning electron microscope photograph of metal nanowires produced using the method according to the first embodiment. As shown in Figure 5, it can be seen that metal nanowires have grown at high density on the thin film.

[0060] Figure 6 shows scanning electron microscope images of metal nanowires grown from a thin film at different FIB doses. In Figures 6a–h, the rectangular areas indicate the FIB irradiation areas. The FIB irradiation dose increases in the order of Figures 6a, b, …, h. As a result, the thin film surface is etched more deeply in the order of Figures 6a, b, …, h. Specifically, the etching depths δ are 7.5 nm (a), 10.0 nm (b), 12.5 nm (c), 15.0 nm (d), 17.5 nm (e), 20.0 nm (f), 22.5 nm (g), and 25.0 nm (h). As shown in Figure 6, the grown metal nanowires are long and low-density when δ is deep and shallow (e.g., Figures 6a and h). On the other hand, when δ is intermediate (e.g., Figures 6c, d, e, and f), the grown metal nanowires are short and high-density. Thus, there is a negative correlation between the length and density of the grown metal nanowires (see also Figures 7 and 8).

[0061] FIG. 7 shows histograms of the length and density of the grown metal nanowires as a function of FIB etching depth δ.

[0062] Figure 8 is a bubble chart showing the density of grown metal nanowires as a function of length. Both the horizontal and vertical axes are logarithmic. The diameter of the metal nanowires is represented by six bubble sizes in 100-nm increments, ranging from 100 nm to 600 nm.

[0063] According to this experiment, the implantation dose by FIB irradiation is approximately 0.05 to 0.5 nC / μm 2 It was found that metal nanowires grow effectively when the etching depth is approximately 5 nm to 30 nm. The reason why an appropriate value is required for the implantation dose can be explained as follows.

[0064] If the implantation dose is too low, coarse grains of sufficient size are not formed near the surface of the thin film, and therefore, in this case, the grain size gradient is insufficient, and metal nanowires do not grow.

[0065] On the other hand, if the implantation dose is too high, the thin film is excessively etched down to the substrate, and therefore, in this case too, the metal atoms necessary for growth are not supplied, and metal nanowires do not grow.

[0066] Experiments have shown that metal nanowires grow effectively when the heating temperature is about 300°C. The reason why an appropriate value for the heating temperature is required can be explained as follows.

[0067] If the heating temperature is too low, the thermal stress is small. Metal atom diffusion is a thermally activated process (temperature dependent), so the metal atoms are less likely to move. In other words, in this case, the driving force is too low and metal nanowires do not grow.

[0068] On the other hand, metal nanowires do not grow when the heating temperature is too high. There are two possible reasons for this. First, at high temperatures, the diffusion mode shifts from grain boundary diffusion, which has a faster diffusion rate, to intragranular diffusion, rapidly slowing the growth rate (this is thought to occur at around 300 °C). The other reason is related to the pinning effect. As mentioned above, considering the balance between the opposing forces of metal atoms that have nowhere to go—whether they leave the free surface or return to the interior of the thin film—it is believed that there exists a critical grain size of pinning grains that contributes to nanowire growth. In other words, nanowires do not grow unless grains grow beyond this critical grain boundary. The critical grain size is temperature-dependent, and the higher the temperature, the larger the critical grain boundary value (i.e., more grain growth is required by FIB irradiation). In this experiment, empirically, there is a limit to grain growth with FIB irradiation (a maximum of approximately 40 nm). Therefore, excessively high temperatures make it difficult to achieve the growth required to reach the critical grain size. This is thought to be why nanowire growth is difficult.

[0069] Figure 9 shows a scanning transmission electron microscope image of a thin film formed by adding SiO2 to aluminum. As can be seen from this image, a grain size gradient is formed, with the grain size increasing from the inside of the thin film toward the surface. This experimental result shows that nanowires can be grown even when inorganic compounds such as SiO2 are used as additives.

[0070] [Each aspect of the present disclosure] The following summarizes each aspect of the present disclosure: A method for producing metal nanowires according to one aspect of the present disclosure includes the steps of forming a thin film on a substrate, the thin film being made of aluminum or gold and at least one of alumina and SiO2, irradiating a surface layer of the thin film with an FIB, and heating the thin film.

[0071] According to this aspect, it is possible to provide a method for producing high density metal nanowires.

[0072] In one embodiment, the method may further include forming a grain size gradient within the thin film, with the grain size increasing from the interior to the surface.

[0073] According to this embodiment, metal nanowires can be produced at a higher density.

[0074] In one embodiment, the step of forming a grain size gradient in the thin film, where the grain size increases from the interior to the surface, may include the step of irradiating the surface of the thin film with FIB.

[0075] According to this embodiment, metal nanowires can be produced at a higher density.

[0076] In one embodiment, the thin film may be a thin film in which at least one of alumina and SiO2 is added to aluminum, where the aluminum particle size may be 10 nm or more and 30 nm or less, and the thin film thickness may be 100 nm or more and 800 nm or less.

[0077] According to this embodiment, metal nanowires can be efficiently produced at high density.

[0078] In one embodiment, the implantation dose in the step of irradiating with FIB is 0.05 nC / μm 2 More than 0.5nC / μm 2 It may be the following:

[0079] According to this embodiment, metal nanowires can be produced with a high density more efficiently.

[0080] In one embodiment, the heating temperature in the heating step may be 300°C.

[0081] According to this embodiment, metal nanowires can be produced with a high density more efficiently.

[0082] In one embodiment, the heating step may involve heating the thin film from room temperature to 300° C. over 15 minutes, then maintaining the temperature at 300° C. for 3 hours, and then allowing it to cool naturally to room temperature.

[0083] According to this embodiment, metal nanowires can be produced with a high density more efficiently.

[0084] Yet another aspect of the present invention is a metal nanowire device.

[0085] According to this aspect, the metal nanowire device includes a substrate, a thin film formed on the substrate, the thin film being made of aluminum or gold with at least one of alumina and SiO2, and metal nanowires grown from the surface layer of the thin film. A coarse-grained layer with large grains is formed on the surface side of the thin film, and a fine-grained layer with small grains is formed on the substrate side of the thin film.

[0086] According to this embodiment, a device including a high density of metal nanowires can be provided.

[0087] In one embodiment, a metal nanowire device may be fabricated by a method including the steps of forming a thin film of aluminum or gold doped with at least one of alumina and SiO2 on a substrate and heating the thin film.

[0088] According to this aspect, a manufacturing method can be embodied to provide a metal nanowire device.

[0089] The present disclosure has been described above based on examples. These examples are merely illustrative, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components and treatment processes, and that such modifications are also within the scope of the present invention.

[0090] (Variation) In the above embodiment, an example has been described in which energy is applied to the surface of a thin film by irradiating it with FIB, thereby enlarging the grain size and forming a grain size gradient. However, the present invention is not limited to this, and any suitable method such as laser irradiation, heat treatment, or current treatment may be used as a means for applying energy to form a grain size gradient.

[0091] When understanding the abstract technical ideas of the embodiments and modifications, the technical ideas should not be interpreted as being limited to the contents of the embodiments and modifications. The above-described embodiments and modifications are merely illustrative examples, and many design modifications, such as changes, additions, and deletions of components, are possible. In the embodiments, the contents in which such design modifications are possible are emphasized by adding the notation "embodiment." However, design modifications are also permitted even in contents without such notation. [Explanation of symbols]

[0092] 1··Metal nanowire devices, 11··Substrate, 12··Thin film, 13··Metal nanowires, 14...Coarse grain, 15...Fine grain, 16...coarse grain layer, 17... fine grain layer, S1··thin film generation step, S2··grain size gradient formation step, S3··Heating step.

Claims

1. A method for producing metal nanowires, comprising: Aluminum or gold with alumina and SiO ２ forming a thin film on a substrate to which at least one of the following has been added; and, and heating the thin film.

2. 2. The method of claim 1, further comprising forming a grain size gradient in the thin film, the grain size increasing from the interior to the surface.

3. 3. The method of claim 2, wherein the step of forming the grain size gradient includes the step of irradiating a surface layer of the thin film with an FIB.

4. the thin film is a thin film of aluminum doped with alumina, The particle size of the aluminum is 10 nm or more and 30 nm or less, 4. The method according to claim 3, wherein the thickness of the thin film is between 100 nm and 800 nm.

5. The implantation dose in the step of irradiating the FIB is 0.05 nC / μm ２ 0.5nC / μm or more ２ 5. The method of claim 4, wherein:

6. 5. The method of claim 4, wherein the heating step comprises heating to a temperature of 300°C.

7. 7. The method of claim 6, wherein the heating step comprises heating the thin film from room temperature to 300°C over 15 minutes, then maintaining the temperature at 300°C for 3 hours, and then allowing the thin film to cool naturally to room temperature.

8. A substrate; Alumina and SiO formed on the aluminum or gold substrate ２ a thin film containing at least one of the above; metal nanowires grown from the surface layer of the thin film; A metal nanowire device, characterized in that a coarse grain layer with large grain sizes is formed on the surface side of the thin film, and a fine grain layer with small grain sizes is formed on the substrate side of the thin film.

9. Aluminum or gold with alumina and SiO ２ forming a thin film on a substrate to which at least one of the following has been added; 9. The metal nanowire device of claim 8, which is fabricated by a method comprising the step of:

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