JP7485280B2 - Nanosheets and methods for producing nanosheets - Google Patents

Description

Article 30, paragraph 2 of the Patent Act applies. Publisher: The Japan Society of Applied Physics Publication name: 67th Spring Meeting of the Japan Society of Applied Physics, 2020 [Proceedings of the 67th Meeting of the Japan Society of Applied Physics] Publication date: February 28, 2020 (Reiwa 2)

The present invention relates to nanosheets and a method for producing nanosheets.

Currently, waste heat from factories and other sources is reused for heating and other purposes, but waste heat below 200°C, such as that generated by electronic devices, is difficult to control compared to higher temperature heat, and so there is a problem in that it is not effectively utilized. Waste heat below 200°C accounts for the majority of all waste heat, and if this waste heat could be controlled, it could become a new energy resource. In addition, the impact of the heat generated by electronic devices on their performance and lifespan is increasing as electronic devices become smaller and more waterproof, causing problems such as reduced performance and shorter lifespans.

Therefore, in order to solve such problems, micro heat engines have been developed that use MEMS to control the flow of heat generated by electronic devices, etc. (see, for example, Non-Patent Document 1).

The present inventors have been conducting research on so-called spin thermal conductive materials that have thermal conductivity due to spin (see, for example, Non-Patent Documents 2 or 3). In spin thermal conductive materials, adjacent spins have a strong bond via ^O2- called superexchange interaction, which gives them high thermal conductivity. In addition, spin thermal conductive materials have the characteristic that the superexchange interaction is broken by doping with holes, and the thermal conductivity is reduced. For this reason, it is considered that the direction and amount of heat flow can be controlled by hole doping due to the electric field effect in spin thermal conductive materials.

One of such spin thermal conductive materials is La ₅ Ca ₉ Cu ₂₄ O ₄₁ as shown in FIG. 8. The crystal of La ₅ Ca ₉ Cu ₂₄ O ₄₁ has a structure in which CuO ₂ layers and Cu ₂ O ₃ layers are alternately laminated with Ca and La layers interposed therebetween, as shown in FIG. 8. The CuO ₂ layers form a chain layer in which Cu and O are bonded in a chain shape, and the Cu ₂ O ₃ layers form a ladder layer in which Cu and O are bonded in a ladder shape (for example, see Non-Patent Document 4). The crystal of La ₅ Ca ₉ Cu ₂₄ O ₄₁ produced by the solid-state reaction method has a thickness of the order of micrometers in the lamination direction. In addition, in _{the La5Ca9Cu24O41 crystal} , superexchange interaction occurs only in _the c _- axis direction of _{the Cu2O3} layer. As a result, at room temperature, the thermal conductivity in the c-axis direction is approximately 100 W/(m·K), while the thermal conductivity in the a-axis and b-axis directions is small, approximately 2 W/(m·K), and the crystal has thermal conductivity anisotropy.

Conventional methods for peeling graphene from graphite or peeling each layer from a layered material include the Scotch tape method and the rubbing method (see, for example, Non-Patent Document 5), but these methods make it difficult to mass-produce thin layers.

J. H. Cho, L. W. Weiss, C. D. Richards, D. F. Bahr and R. F. Richards, “Power production by a dynamic micro heat engine with an integrated thermal switch”, J. Micromech. Microeng., 2007, 17, S217-S223 N. Terakado, R. Takahashi, Y. Takahashi and T. Fujiwara, “Line patterning of anisotropic spin chains by polarized laser for application in micro-thermal management”, Appl. Phys. Lett., 2017, 110, 191902 N. Terakado, K. Watanabe, T. Kawamata, Y. Yokochi, Y. Takahashi, Y. Koike and T. Fujiwara, “Fabrication of glass-ceramics containing spin-chain compound SrCuO2 and its high thermal conductivity”, Appl. Phys. Lett., 2015, 106, 141902 T. Vuletic, B. Korin-Hamzic, T. Ivek, S. Tomic, B. Gorshunov, M. Dressel, J. Akimitsu, “The spin-ladder and spin-chain system (La,Y,Sr,Ca)14Cu24O41: Electronic phases, charge and spin dynamics”, Physics Reports, 2006, 428, p.169-258 K. S. Novoselov, D. Jiang, F. Schedin, T. J. Booth, V. V. Khotkevich, S. V. Morozov, and A. K. Geim, “Two-dimensional atomic crystals”, PNAS, July 26, 2005, vol. 102, no. 30, p.10451-10453

The MEMS-based heat engine described in Non-Patent Document 1, which can control heat flow, requires a relatively large space to house the drive unit and other components, and has the problem that it is difficult to incorporate it into electronic devices that are becoming increasingly miniaturized.

The present invention was made with a focus on these issues, and aims to provide a nanosheet that can be easily incorporated into increasingly miniaturized electronic devices and that can control heat flow, as well as a method for producing the nanosheet.

In order to achieve the above-mentioned object, the nanosheet of the present invention has a copper oxide layer having a chain layer in which Cu and O are bonded in a chain shape and a ladder layer in which Cu and O are bonded in a ladder shape, the chain layer and the ladder layer being alternately stacked with an intermediate layer interposed therebetween, and is characterized in that the nanosheet has a thickness of 100 nm or less.

The nanosheet according to the present invention is thin, with a thickness of 100 nm or less, and can be easily incorporated into miniaturized electronic devices. The nanosheet according to the present invention has a copper oxide layer formed by bonding Cu and O in a chain and/or ladder shape, and therefore has thermal conductivity anisotropy in which the thermal conductivity in the thickness direction of the copper oxide layer is different from the thermal conductivity in the direction along the surface, and can be configured to change the thermal conductivity by applying an electric field or the like. This makes it possible to change the thermal conductivity of the copper oxide layer and control the heat flow through the copper oxide layer. Furthermore, since the nanosheet according to the present invention is thin, the overall thermal conductivity can be easily controlled by applying an electric field or the like.

The nanosheet of the present invention has a chain layer in which Cu and O are bonded in a chain shape, and a ladder layer in which Cu and O are bonded in a ladder shape, and the chain layer and the ladder layer are alternately stacked with intermediate layers interposed therebetween, so that the thermal conductivity of the chain layer and/or the ladder layer, which are copper oxide layers, can be changed to control heat flow.

In this case, the nanosheet according to the _present invention can be manufactured _{using a} LaCaCuO-based _{polycrystalline} material such as _{polycrystalline La5Ca9Cu24O41} as a raw material. When _{polycrystalline La5Ca9Cu24O41} is used as a raw material, the chain layer is made of _CuO2 , the ladder layer is made of _Cu2O3 , and the intermediate layer is made of a layer containing Ca and/or _La .

The nanosheet according to the present invention preferably has thermal conductivity due to spin and has a thermal conductivity anisotropy in which the thermal conductivity in the direction along the surface of the copper oxide layer is greater than the thermal conductivity in the thickness direction of the copper oxide layer. In particular, it is preferable that the thermal conductivity in the direction along the surface of the copper oxide layer can be changed by applying an electric field. In this case, it is believed that the thermal conductivity changes when an electric field is applied because holes are doped due to the field effect. By controlling the thermal conductivity in the direction along the surface of the copper oxide layer with an electric field, the heat flow through the copper oxide layer can be easily controlled.

The nanosheet manufacturing method of the present invention is a nanosheet manufacturing method for producing the nanosheet of the present invention, and is characterized in that a powder of a polycrystalline raw material having a chain layer in which Cu and O are bonded in a chain shape and a ladder layer in which Cu and O are bonded in a ladder shape is added to an aqueous solution in which a hydroxide is dissolved and stirred to obtain the nanosheet having the chain layer and/or the ladder layer.

The nanosheet manufacturing method according to the present invention can suitably manufacture the nanosheet according to the present invention. The nanosheet manufacturing method according to the present invention can manufacture nanosheets easily and at low cost by a liquid phase process using an aqueous solution.

In the method for producing nanosheets according to the present invention, the raw material is preferably formed by alternately laminating the chain layers and the ladder layers with intermediate layers interposed therebetween, and one or more of the intermediate layers are converted into a compound layer containing hydroxide ions by stirring the aqueous solution containing the raw material, and the nanosheet is obtained by peeling at the compound layer. In this case, the bond between the chain layer and the ladder layer is destabilized by the compound layer containing hydroxide ions, so that peeling is easy at the compound layer. Therefore, nanosheets containing chain layers and ladder layers can be easily peeled off from the raw material.

In the method for producing a nanosheet according to the present invention, it is preferable that the raw material has thermal conductivity due to spin. In this case, a nanosheet having thermal conductivity anisotropy can be easily produced. Furthermore, it is preferable that the raw material is made of a LaCaCuO-based polycrystalline material. In this case, a nanosheet capable of changing thermal conductivity can be easily produced. In this case, for example, the raw material may be made of La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystalline material, and the hydroxide may be made of a material capable of ion exchange between La ³⁺ and Ca ²⁺ when the aqueous solution containing the raw material is stirred.

The present invention provides a nanosheet that can be easily incorporated into miniaturized electronic devices and the like, and that can control heat flow, as well as a method for manufacturing the nanosheet.

1A shows XRD spectra of samples at various stirring times, and (b) shows XRD spectra of a sample stirred for 360 minutes and polycrystalline La ₅ Ca ₉ Cu ₂₄ O ₄₁ produced by a nanosheet production method according to an embodiment of the present invention. This is a transmission electron microscope (TEM) image of a sample that was stirred for 360 minutes and was produced by a nanosheet production method according to an embodiment of the present invention. FIG. 3 is a side view showing a model of the crystal structure of the striped pattern portion of the TEM image shown in FIG. 2 . 1A is a scanning probe microscope (SPM) image of a sample that was stirred for 360 minutes and was produced by a nanosheet production method according to an embodiment of the present invention, and FIG. 1B is an SPM image of an enlarged portion of FIG. (a) An SPM image of a magnified one particle of a sample that was stirred for 360 minutes and was produced by a nanosheet production method according to an embodiment of the present invention, (b) a cross-sectional image at the straight line position in (a), (c) an SPM image of a magnified other particle, and (d) a cross-sectional image at the straight line position in (c). 1 is a transmission electron microscope (TEM) image of a sample stirred for 360 minutes, produced by a nanosheet production method according to an embodiment of the present invention, and a magnified TEM image (inset) of a portion of the sample. FIG. _{1 is an explanatory diagram showing the nanosheet formation process in the nanosheet manufacturing method according to an embodiment of the present invention, illustrating (a) the raw material La5Ca9Cu24O41 polycrystal before reaction} , (b) the state during reaction in an aqueous solution, and (c) the structure of the material obtained after the reaction. FIG. 1 is a perspective view showing the structure of La ₅ Ca ₉ Cu ₂₄ O ₄₁ crystal.

Hereinafter, the embodiment of the present invention will be described based on examples.
1 to 7 show a nanosheet and a method for producing a nanosheet according to an embodiment of the present invention.
The nanosheet of the embodiment of the present invention is suitably produced by the nanosheet production method of the embodiment of the present invention.

In the method for producing a nanosheet according to the embodiment of the present invention, first, a powder of a polycrystalline raw material having a chain layer in which Cu and O are bonded in a chain shape and a ladder layer in which Cu and O are bonded in a ladder shape is placed in an aqueous solution in which a hydroxide is dissolved. Here, the raw material is preferably a LaCaCuO _- based polycrystalline material such as _{La5Ca9Cu24O41 polycrystalline} material. In addition, in this case, the hydroxide is preferably sodium hydroxide. Since the effective _ionic radius of sodium hydroxide when the coordination number of Na ⁺ is 6 is very close to that of La3 ⁺ and Ca2 ⁺ , it is considered that sodium hydroxide is ion-exchanged with the La ⁺ and Ca ⁺ of the raw material in the aqueous solution.

Next, the aqueous solution containing the raw material is stirred for a predetermined time, and then filtered to extract the powdered material. The extracted material is thinly peeled off to a thickness of 100 nm or less to obtain a nanosheet according to an embodiment of the present invention having a chain layer and/or ladder layer.

A nanosheet was produced by the nanosheet production method according to the embodiment of the present invention using La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystal as the raw material and sodium hydroxide as the hydroxide. The raw material La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystal was produced by a solid-phase reaction method. As shown in FIG. 8, the La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystal has a structure in which a CuO ₂ layer of a chain layer in which Cu and O are bonded in a chain shape and a Cu ₂ O ₃ layer of a ladder layer in which Cu and O are bonded in a ladder shape are alternately laminated through Ca and La layers. It also has thermal conductivity and thermal conduction anisotropy due to spin.

The sodium hydroxide was prepared so that the concentration of the aqueous solution was 5.0 mol/L. A magnetic stirrer was used for stirring at room temperature and pressure at 300 rpm. The stirring time was 30 minutes, 60 minutes, 240 minutes, or 360 minutes.

The materials obtained at each mixing time were filtered, dried, and ground in a mortar. Various measurements were performed on each sample after grinding. First, each sample was analyzed by X-ray diffraction (XRD). For X-ray diffraction, a Bruker D8 ADVANCE was used, the radiation source was Cu Kα (1.5416 Å), and the θ-2θ method was used. The measurement range was 2θ = 10° to 80°, the time constant was t = 0.5 s, the measurement interval was 0.02°, and a non-reflective Si holder was used. The XRD spectrum of the sample at each mixing time is shown in Figure 1 (a). The XRD spectrum of the sample stirred for 360 minutes is shown in Figure 1 (b) alongside the XRD spectrum of the raw material La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystal. In addition, "LCCO" in FIG. 1(a) indicates the peak position of the source material, La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystal.

As shown in Figures 1 (a) and (b), it was confirmed that the _{La5Ca9Cu24O41} peak was smaller than _that of the _raw material, and peaks of Ca(OH) ₂ , La(OH) ₃ , and CuO appeared in the sample stirred for 360 _minutes . Since the Ca(OH) ₂ and La(OH) ₃ peaks appeared, it is considered that Na ⁺ , La3 ⁺ , and Ca2 ⁺ performed ion exchange, and some of the intermediate layers became compound layers of Ca(OH) ₂ and La(OH) ₃ . In addition, since CuO appeared, it is considered that some of the _CuO2 in the chain layer and _{the Cu2O3} in the _ladder layer collapsed and became CuO. In addition, since _{the La5Ca9Cu24O41} peak remained _, it is considered that the unreacted part of the raw material remained. In addition, since the _CuO2 in the chain layer and _{the Cu2O3} in the ladder layer are thin, they do not appear in the XRD spectrum.

1(a), in the case of stirring _for 30 to 240 minutes _, peaks of Ca(OH) ₂ , La(OH) ₃ , and _CuO appeared, but it was confirmed that the peak of _{La5Ca9Cu24O41} was large. From this, it can be said that the reaction did not progress as much in the case of stirring for 30 to 240 minutes as in the case of stirring for 360 minutes.

A transmission electron microscope (TEM) image of the sample stirred for 360 minutes is shown in FIG. 2. As shown in FIG. 2, a striped pattern with a period of about 0.28 nm was confirmed. This striped pattern is considered to be due to the stacked chain layers, intermediate layers, and ladder layers. A model of the crystal structure of this striped pattern part is shown in FIG. 3. As shown in FIG. 3, the striped pattern part is considered to be a _stack of chain layers and ladder layers alternately stacked with intermediate layers interposed therebetween, and is considered to be derived from _{the La5Ca9Cu24O41 polycrystal} of the raw material.

Next, the surface shape of the sample that had been stirred for 360 minutes was measured using a scanning probe microscope (SPM). The SPM measurement used DFM mode, with an operating range of 20 x 20 μm and data points of 256 (X direction) x 256 (Y direction). The sample was placed on a Si substrate and the measurement was performed. The results of the surface shape measurement using the SPM are shown in Figure 4, and the surface roughness analysis results of the two particles in Figure 4 are shown in Figure 5.

As shown in Figures 4 and 5, many thin particles were observed, for example, particles with a horizontal length of 1.1 μm and a thickness of 2.7 nm were confirmed. Since this particle has a platform-like step structure, it is considered to be a thin nanosheet. Considering Figures 1 to 3 together, it is considered that the bond between the chain layer and the ladder layer became unstable due to the compound layer of Ca(OH) ₂ and La(OH) ₃ formed from the intermediate layer, and therefore, by crushing, the raw material La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystal was thinly peeled off at the compound layer, forming a nanosheet including the chain layer and the ladder layer.

4 and 5, particles with lengths and thicknesses of submicrometers (several hundreds of nm or more) were also confirmed, such as a particle with _a horizontal length of 660 nm and a thickness of ₃₁₇ nm. These particles are considered to be the result of the raw material _{La5Ca9Cu24O41 polycrystal} not being thinned to 100 nm or less.

Next, the sample stirred for 360 minutes was observed with a transmission electron microscope (TEM). In the TEM observation, an accelerating voltage of 200 kV was used, and a Ni holder was used. The results of the observation using the TEM are shown in FIG. 6. As shown in FIG. 6, it was confirmed that the edges of the particles were bent and rolled (see the enlarged view in the figure). It is believed that the edges of these particles are rolled because they are very thin. Considering this together with FIGS. 1 to 5, it is believed that these particles are also nanosheets formed by thinly peeling off La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystals by pulverization.

The formation process of the nanosheet and the structure of the obtained material, which can be considered from the XRD analysis and other measurement results, are shown in FIG. 7. As shown in FIG. 7(a), the raw material La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystal (thickness: 1 μm or more) has a CuO ₂ surface corresponding to the chain layer, a Cu ₂ O ₃ surface corresponding to the ladder layer, and a La/Ca surface corresponding to the La and Ca layers. When this is stirred in an aqueous sodium hydroxide solution, as shown in FIG. 7(b), NaOH penetrates into some layers, and ion exchange between Na ⁺ and La ³⁺ and Ca ²⁺ progresses. As a result, as shown in FIG. 7(c), it is considered that a nanosheet (for example, thickness: 10 nm or less) including a CuO ₂ layer of the chain layer, a Cu ₂ O ₃ layer of the ladder layer, a Ca and La layer, a compound layer of Ca(OH) ₂ and La(OH) ₃ , and a CuO layer are formed. It is also believed that La ₅ Ca ₉ Cu ₂₄ O ₄₁ crystals containing remaining raw materials are also present and are thin (e.g., thickness: several tens of nm to 100 nm or less). The nanosheets formed are thin and do not appear in the XRD spectrum (see FIG. 1). The La ₅ Ca ₉ Cu ₂₄ O ₄₁ crystals containing remaining raw materials have a broad peak in the XRD spectrum (see FIG. 1) because of their small particle size.

From the above various measurement results, it can be said that nanosheets having chain layers and ladder layers can be obtained by stirring for 360 minutes or more. The obtained nanosheets are made of thinly peeled La ₅ Ca ₉ Cu ₂₄ O ₄₁ polycrystals having thermal conductivity and thermal conductivity anisotropy due to spin, and have copper oxide layers of chain layers and ladder layers, so it is considered that the thermal conductivity in the direction along the surface of the copper oxide layer can be changed by hole doping due to the field effect, and the heat flow flowing through the copper oxide layer can be easily controlled. In addition, since it is thin, it is considered that the overall thermal conductivity can be easily controlled by an electric field, etc. In addition, since the obtained nanosheets are thin, with a thickness of 100 nm or less, they can be easily incorporated into electronic devices with advanced miniaturization.

As shown in the above examples, the nanosheet manufacturing method according to the embodiment of the present invention can produce a nanosheet in which chain layers and ladder layers are alternately stacked with intermediate layers interposed therebetween. The nanosheet manufacturing method according to the embodiment of the present invention can produce nanosheets easily and at low cost by using a liquid phase process that uses an aqueous solution.

The nanosheet manufacturing method according to the embodiment of the present invention forms a compound layer containing hydroxide ions, and destabilizes the bonds between the chain layer and ladder layer with the compound layer, making it possible to easily peel off the nanosheet containing the chain layer and ladder layer from the raw material. This makes it possible to produce a large amount of nanosheets at once, compared to the conventional Scotch tape method or rubbing method.

The nanosheet manufacturing method according to the embodiment of the present invention can form a thinner nanosheet by promoting the reaction of the intermediate layer to increase the proportion of the compound layer containing hydroxide ions, and therefore can form, for example, a nanosheet consisting of only one chain layer or a nanosheet consisting of only one ladder layer.

Claims (9)

A nanosheet having a copper oxide layer having a chain layer in which Cu and O are bonded in a chain shape and a ladder layer in which Cu and O are bonded in a ladder shape, the chain layer and the ladder layer being alternately stacked with an intermediate layer interposed therebetween, and having a thickness of 100 nm or less. The nanosheet according to claim 1 , characterized in that the intermediate layer contains Ca and/or La. The nanosheet according to claim 1 or 2, characterized in that it has thermal conductivity due to spin and has thermal conductivity anisotropy in which the thermal conductivity in a direction along the surface of the copper oxide layer is greater than the thermal conductivity in the thickness direction of the copper oxide layer. 4. The nanosheet according to claim 3 , characterized in that the thermal conductivity in a direction along the surface of the copper oxide layer can be changed by applying an electric field. A method for producing the nanosheet according to any one of claims 1 to 4, comprising the steps of:
A method for producing a nanosheet, characterized in that a powder of a polycrystalline raw material having a chain layer in which Cu and O are bonded in a chain shape and a ladder layer in which Cu and O are bonded in a ladder shape is placed in an aqueous solution in which a hydroxide is dissolved, and stirred to obtain the nanosheet having the chain layer and/or the ladder layer. The raw material is formed by laminating the chain layers and the ladder layers alternately with an intermediate layer interposed therebetween,
The method for producing a nanosheet according to claim 5, characterized in that one or more of the intermediate layers are converted into a compound layer containing hydroxide ions by stirring the aqueous solution containing the raw materials, and the nanosheet is obtained by peeling off the compound layer. The method for producing a nanosheet according to claim 5 or 6 , characterized in that the raw material has thermal conductivity due to spin. The method for producing a nanosheet according to any one of claims 5 to 7, characterized in that the raw material is made of a LaCaCuO-based polycrystalline material. The source material _is made of _{polycrystalline La5Ca9Cu24O41 ;}
The method for producing a nanosheet according to any one of claims 5 to 8, characterized in that the hydroxide is capable of ion exchange with La3 ⁺ and Ca2 ⁺ when the aqueous solution containing the raw materials is stirred.