Method for growing sub-nano-diameter semiconductor single-walled carbon nanotubes in crystal face step height confinement

Abstract

The invention relates to the field of diameter and conductive property controlled growth of single-walled carbon nanotubes, in particular to a method for growing a sub-nano diameter semiconductor single-walled carbon nanotube in a crystal face step height confinement. According to the method, a crystal face (110) of a zirconium oxide single crystal substrate is screened, the atomic step height of the crystal face (110) of the zirconium oxide single crystal substrate is regulated and controlled through heat treatment, and the small-diameter catalyst nanoparticles are prepared through steps with the small crystal face height. By adopting a normal pressure chemical vapor deposition method, the controllable growth of the diameter and the conductive property of the single-walled carbon nanotube is realized by utilizing a confinement space provided by the step. By selecting the crystal face of the substrate and regulating and controlling the step height, based on the strong interaction between the single-walled carbon nanotube and the atomic step of the substrate, the controlled preparation of the semiconductor single-walled carbon nanotube with sub-nanometer diameter and narrow chiral distribution is realized, and a new candidate single crystal substrate is provided for the controlled growth of the single-walled carbon nanotube; and a material foundation is laid for the development of carbon nanotube-based nano-electronic devices.

Description

Technical Field

[0001] The present invention relates to the field of controlled growth of single-walled carbon nanotubes with diameter and conductive properties, and specifically to a method for confining the growth of semiconductor single-walled carbon nanotubes with sub-nanometer diameters by using the step height of a crystal face. The method utilizes the step height of a single crystal substrate to confine the growth of catalyst nanoparticles with uniform diameters, thereby confining the growth of single-walled carbon nanotubes with sub-nanometer diameters and semiconductor properties. Background Art

[0002] Semiconducting single-walled carbon nanotubes can be used as channel materials for field-effect transistors, and are expected to build integrated circuits with advanced processes of 5nm and below. They are considered to be one of the most competitive low-dimensional nanomaterials in the post-Moore era. The band gap of semiconducting single-walled carbon nanotubes is inversely proportional to the diameter. When the diameter of a single-walled carbon nanotube reaches sub-nanometer, its band gap is about 0.7 to 0.9 eV, and its power consumption is lower than that of silicon. Using single-walled carbon nanotubes with narrow chirality distribution, high-performance devices with consistent on-state current, switching ratio and carrier mobility can be constructed.

[0003] The application of single-walled carbon nanotubes in nanoelectronic devices has been limited by the coexistence of metallic (~1 / 3) and semiconducting (~2 / 3) carbon nanotubes in directly prepared samples. The difference in structure and formation energy between metallic / semiconducting single-walled carbon nanotubes is very small. Therefore, it is very difficult to control the preparation of single-walled carbon nanotubes with a single conductive property. At present, the representative works of selectively growing semiconducting single-walled carbon nanotubes mainly include: selectively inhibiting or etching metallic single-walled carbon nanotubes by regulating the oxygen content in the catalyst and the carrier or the decomposition product of the carbon source to improve the purity of the semiconducting single-walled carbon nanotubes (Reference 1, Lin DW, Yu Y., Zhang J., Small, 2021, 17, 48; Reference 2, Zhang, LL; Sun, DM; Liu, C. et al.; Advanced Materials, 2017, 29, 32; Reference 3, Zhang, SC, Hu Y, Wu J. et al.; Journal of the American Chemical Society, 2015, 137, 3; Reference 4, Kang LX, Hu Y., Zhang J. et al.; Nano Letters, 2015, 15, 1; Reference 5, Qian X., Peng F, Li Y. et al.; Nano Letters, 2015, 15, 1 al.NanoLetters,2014,14,2);High-purity semiconducting single-walled carbon nanotubes were prepared by combining catalyst structure design with H2 in-situ etching of metallic single-walled carbon nanotubes (Reference 6, Zhang F.; Hou PX; Liu C.et al.; Nature Communication,2016,7,11160; Reference 7, Zhang F.; Hou PX; Liu C.et al.; Journal ofMaterials Science & Technology,2020,54,105).

[0004] After more than two decades of development, the controllable preparation of single-walled carbon nanotubes with single conductive properties, especially semiconducting ones, has made great progress. However, the controlled preparation of semiconducting single-walled carbon nanotubes still faces problems and challenges: (1) The formation energy difference between metallic and semiconducting single-walled carbon nanotubes is very small, and it is difficult to achieve that etchants such as O and H only etch metallic single-walled carbon nanotubes, and small-diameter semiconducting single-walled carbon nanotubes with higher chemical activity will also be etched away; (2) Introducing etchants during the in-situ growth process of carbon nanotubes will reduce the growth kinetics of single-walled carbon nanotubes, thereby reducing the yield, density and length of semiconducting single-walled carbon nanotubes.

[0005] Therefore, there is an urgent need to develop a simple and efficient technical method to grow small-diameter, high-quality semiconducting or even single-chirality single-walled carbon nanotubes to promote their application in the field of nanoelectronic devices. Summary of the invention

[0006] The purpose of the present invention is to provide a method for confining the growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height. By controlling the crystal plane step height, sub-nanometer diameter high-quality semiconductor or even single-chirality single-walled carbon nanotubes can be grown simply and efficiently.

[0007] The technical solution of the present invention is:

[0008] A method for confining the growth of semiconductor single-walled carbon nanotubes with subnanometer diameters by crystal face step height. The method selects the (1 1 0) crystal face of a zirconium oxide single crystal substrate, regulates the atomic step height of the (1 1 0) crystal face of the zirconium oxide single crystal substrate by heat treatment, and uses steps with smaller crystal face heights to prepare catalyst nanoparticles with small diameters. On this basis, a normal pressure chemical vapor deposition method is used to utilize the confined space provided by the steps to achieve controllable growth of the diameter and conductive properties of the single-walled carbon nanotubes.

[0009] The method for growing sub-nanometer diameter semiconductor single-wall carbon nanotubes by crystal plane step height confinement comprises heat treatment at 775-825° C. for 10-30 minutes to obtain atomic steps with small height and uniformity.

[0010] In the method for growing sub-nanometer diameter semiconductor single-wall carbon nanotubes in a confined manner with a crystal plane step height, the heat treatment atmosphere is argon or hydrogen.

[0011] In the method for confining the growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height, the atomic step height of the (1 10) crystal plane ranges from 0.25 to 0.35 nm.

[0012] The method for confining the growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal step height adopts a quasi-static chemical vapor deposition method to directly control the diameter and conductive properties of the grown single-walled carbon nanotube network. The chemical vapor deposition temperature is 825-865°C, and the carbon source is carbon monoxide with a flow rate of 25-50sccm.

[0013] The method for confining the growth of sub-nanometer diameter semiconductor single-wall carbon nanotubes by crystal plane step height is to load a catalyst precursor on the surface of a zirconium oxide single crystal substrate, and to prepare catalyst nanoparticles with small size distribution after heat treatment, and the particle size is 0.5-2.0nm.

[0014] In the method for growing sub-nanometer diameter semiconductor single-wall carbon nanotubes with crystal plane step height confined, the catalyst precursor is prepared by chemical self-assembly, spin coating, immersion or ion beam bombardment target sputtering.

[0015] In the method for confining the growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height, the catalyst is a transition metal single substance Fe, Co, Ni or an alloy thereof.

[0016] The method for growing sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height confinement can prepare single-walled carbon nanotubes with a diameter range of 0.75±0.15nm.

[0017] The method for growing sub-nanometer diameter semiconductor single-walled carbon nanotubes by confining the crystal plane step height can preferentially grow single-chirality (8,4) single-walled carbon nanotubes with a purity of more than 90%.

[0018] The design idea of ​​the present invention is:

[0019] Zirconia has a face-centered cubic structure with a lattice constant of At high temperatures, due to the generation of oxygen vacancies, the constituent atoms are displaced and the lattice is distorted, which will lead to the transformation of the crystal form of single-crystal zirconium oxide; and the higher the temperature, the greater the change in the crystal structure, and the atomic steps are generated. Therefore, the steps of ideal height can be obtained by high-temperature heat treatment of the face-centered cubic (FCC) structure single-crystal zirconium oxide substrate in a non-oxygen environment; the strong confinement effect between the steps on the crystal surface and the catalyst and the specific chiral single-walled carbon nanotubes is used to control the growth of sub-nanometer diameter single-conductive single-walled carbon nanotubes.

[0020] The present invention selects the (1 1 0) crystal plane of a specific oxide substrate, regulates the atomic step height of the substrate crystal plane by heat treatment, and confines small-diameter catalyst particles. Then, using atmospheric pressure chemical vapor deposition, semiconducting single-walled carbon nanotubes with a diameter of 0.75±0.15nm and (8,4) chirality enrichment (purity ~90%) are grown on the upper limit of the step. The present invention realizes the controlled preparation of semiconducting single-walled carbon nanotubes with subnanometer diameter and narrow chirality distribution by selecting the substrate crystal plane and regulating the step height, based on the strong interaction between the single-walled carbon nanotube and the substrate atomic step.

[0021] The advantages and beneficial effects of the present invention are:

[0022] (1) The present invention selects the (1 1 0) crystal plane of face-centered cubic zirconia with a melting point of 2715°C as a substrate, and regulates the atomic step height of the substrate crystal plane by heat treatment in an oxygen-free environment (775-825°C, 10-30 min), thereby achieving control of the step height of the crystal plane; and then, using atmospheric pressure chemical vapor deposition, sub-nanometer diameter semiconductor or even single-chirality (8,4) single-walled carbon nanotubes can be directly and simply grown on the upper limit of the single crystal substrate step, providing a new candidate single crystal substrate for the controlled growth of single-walled carbon nanotubes, and laying a material foundation for the development of carbon nanotube-based nanoelectronic devices and promoting the application of single-walled carbon nanotubes in micro-nano devices and carbon-based chips.

[0023] (2) The present invention achieves the goal of inhibiting the agglomeration of metal nanoparticles and controlling their size under high temperature conditions by limiting the catalyst through the base step.

[0024] (3) The present invention utilizes the influence of atomic step height on the diameter and conductive properties of single-walled carbon nanotubes to achieve controlled preparation of the diameter and conductive properties of single-walled carbon nanotubes, which is a new idea for substrate control of the conductive properties of single-walled carbon nanotubes.

[0025] (4) The present invention avoids the structural destruction of small-diameter semiconducting single-walled carbon nanotubes by conventional selective etching methods, and maintains the intrinsic structure of carbon nanotubes while preferentially growing small-diameter semiconducting single-walled carbon nanotubes through the substrate confinement effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 .Atomic force microscopy photos of the (1 1 0) crystal surface of the zirconia substrate before and after heat treatment: (a) without heat treatment; (b) heat treatment at 775℃ with argon for 15min; (c) step height statistics, the horizontal axis Step height is the step height (nm), and the vertical axis Counts is the count.

[0027] Figure 2 Morphology of Co catalyst nanoparticles on the (1 1 0) crystal plane of zirconia: (a) Atomic force microscope photo; (b) Particle size statistical distribution, the horizontal axis Diameter is the diameter (nm), and the vertical axis Counts is the count.

[0028] Figure 3 Typical transmission electron micrographs of zirconia (1 1 0) surface Co catalyst after heat treatment at different temperatures: (a) 855℃, (b) 865℃, (c) 875℃.

[0029] Figure 4.Sub-nanometer diameter single-walled carbon nanotubes grown on the zirconia (1 1 0) crystal plane: (a) typical atomic force microscope photo; (bd) typical transmission electron microscope photos; (e) diameter statistics based on transmission electron microscopy, with the horizontal axis Diameter being the diameter (nm) and the vertical axis Counts being the counts; (f) typical scanning electron microscope photo.

[0030] Figure 5 Multi-wavelength Raman spectra of single-walled carbon nanotubes grown on Co catalyst on (1 1 0) surface of zirconia substrate: (a) excitation wavelength is 633nm, (b) excitation wavelength is 532nm, (c) excitation wavelength is 785nm, the horizontal axis Raman Shift is the Raman shift (cm -1 ), the vertical axis Intensity is the relative intensity (au); (d) 633nm Raman breathing mode surface scan, green represents semiconducting single-walled carbon nanotubes; (e) 532nm Raman breathing mode surface scan; (f) 785nm Raman breathing mode surface scan.

[0031] Figure 6 Typical atomic force microscopy images of confined growth of subnanometer diameter single-walled carbon nanotubes on the surface of zirconia (1 1 0) substrate: (a) 855℃, (b) 865℃, (c) 875℃.

[0032] Figure 7 .Single-walled carbon nanotubes grown on a step-free surface of a (1 1 0) zirconia substrate at 865°C: (a) typical atomic force microscope photo; (b) scanning electron microscope photo; (c) breathing mode Raman spectrum with an excitation wavelength of 633 nm, (d) breathing mode Raman spectrum with an excitation wavelength of 532 nm, the abscissa Raman Shift is the Raman shift (cm -1 ), the vertical axis Intensity is the relative intensity (au).

[0033] Figure 8 . Steps on the (1 1 0) crystal plane of a spinel substrate: (a) typical atomic force microscope photo, (b) step height statistics, (1 1 0) Step height is the step height (nm). Single-walled carbon nanotubes grown on the surface of a spinel substrate: (c) scanning electron microscope photo, (de) transmission electron microscope photo, (f) particle size statistical distribution, the horizontal axis Diameter is the diameter (nm), and the vertical axis Counts is the count. Multi-wavelength Raman spectrum: (g) Raman spectrum with an excitation wavelength of 633nm, (h) Raman spectrum with an excitation wavelength of 532nm, (i) Raman spectrum with an excitation wavelength of 785nm, the horizontal axis Raman Shift is the Raman shift (cm -1), the vertical axis Intensity is the relative intensity (au).

[0034] Fig. 9 .Co catalyst prepared on the surface of silicon substrate: (a) typical transmission electron microscope photo, (b) atomic force microscope photo, (c) statistical distribution diagram of particle diameter based on transmission electron microscope and atomic force microscope, the horizontal axis Diameter is the diameter (nm), and the vertical axis Counts is the count. Single-walled carbon nanotubes prepared on the surface of silicon substrate catalyst Co: (d) scanning electron microscope photo, (e) Raman spectrum with an excitation wavelength of 532nm, (f) Raman spectrum with an excitation wavelength of 633nm, the horizontal axis Raman Shift is the Raman shift (cm -1 ), the vertical axis Intensity is the relative intensity (au).

[0035] Fig.10 .C2H2 growth of single-walled carbon nanotubes: (a) Typical atomic force microscope photo, (b) Raman spectrum with an excitation wavelength of 633nm, (c) Raman spectrum with an excitation wavelength of 532nm, the horizontal axis Raman Shift is the Raman shift (cm -1 ), the vertical axis Intensity is the relative intensity (au). C2H4 growth of single-walled carbon nanotubes: (d) Typical atomic force microscope photo, (e) Raman spectrum with an excitation wavelength of 633nm, (f) Raman spectrum with an excitation wavelength of 532nm, the horizontal axis Raman Shift is the Raman shift (cm -1 ), the vertical axis Intensity is the relative intensity (au). DETAILED DESCRIPTION

[0036] In the specific implementation process, the present invention uses single-crystal zirconia as a substrate, selects a (1 1 0) crystal plane, and obtains highly consistent and uniform atomic steps with a height of 0.25 to 0.35 nm after heat treatment at 775 to 800° C. for 15 to 30 minutes under a specific atmosphere; a metal nanoparticle catalyst is prepared on the surface of the zirconia substrate, and a chemical vapor deposition method is used to confine the growth of sub-nanometer diameter semiconductor or even single-handed single-walled carbon nanotubes on the surface of the crystal step.

[0037] The method for confining the growth of semiconductor single-walled carbon nanotubes with sub-nanometer diameters in a crystal plane step height in the present invention comprises the following specific steps:

[0038] (1) Pretreatment of zirconia substrate:

[0039] The (1 1 0) crystal plane exposed single crystal zirconium oxide was ultrasonically cleaned in acetone and isopropanol for 10 to 15 minutes, dried with a nitrogen gun and set aside. Figure 1 As shown in a, the original polished crystal surface has no obvious steps. It is placed in a quartz boat, placed in a tube furnace, heat treated at 775-800℃ for 15-30min, cooled to room temperature and taken out. Atomic force microscope photos are shown in Figure 1 As shown in b, it can be seen that the atomic steps suitable for the growth of single-walled carbon nanotubes are uniformly and orderly distributed on it, and the step height ranges from 0.25 to 0.35 nm ( Figure 1 c), with an average height of ~0.28 nm.

[0040] (2) Preparation of Catalyst

[0041] A uniform solution is obtained by heating and stirring a certain concentration of polymer and inorganic salt, and then dropped on the crystal surface of the zirconium oxide substrate at a speed of 1000-8000 rpm. The solution is then oxidized at 415-600°C and reduced with H2 at 775-900°C to obtain catalyst nanoparticles.

[0042] (3) Growth of single-walled carbon nanotubes by chemical vapor deposition

[0043] Single-walled carbon nanotubes are grown at a temperature of 850 to 865° C. with 25 to 30 sccm CO as a carbon source for 3 to 5 minutes.

[0044] In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention is described in detail below through embodiments and drawings, but this is not intended to limit the scope of protection of the present application.

[0045] Example 1

[0046] In this embodiment, a method for confining the growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes with crystal step height is provided, and the specific steps are as follows:

[0047] (1) Zirconia with exposed (1 1 0) crystal plane was selected as the substrate, and the above specific step (1) was used for pretreatment. The heat treatment temperature was 775°C, the heat treatment time was 15 min, and the heat treatment atmosphere was Ar. The atomic step height of the substrate was regulated by the heat treatment conditions. The step height distribution range of the (1 1 0) crystal plane was 0.25-0.35 nm, and the average height was 0.28 nm ( Figure 1 ).

[0048] (2) using a polymer chemical self-assembly method to assemble a Co catalyst with a particle size of 1 to 2 nm on the (1 1 0) crystal surface zirconia treated in step (1) ( Figure 2 a), and the average particle size was ~1.5 nm ( Figure 2 b).

[0049] In this embodiment, a polystyrene-b-poly 4-vinylpyridine PS-b-P4VP (such as PS2700-b-P4VP2800 produced by Polymer Source Inc.) polymer and acetylacetonate cobalt Co(acac)2 inorganic salt are heated and stirred at 80°C to obtain a uniform solution, the polymer concentration is 0.08wt%, the inorganic salt concentration is 0.4mM, and the solvent is dimethylformamide; the solution is suspended and dropped on the zirconium oxide substrate crystal surface at a speed of 1250rpm, and oxidation at 550°C and H2 reduction treatment at 865°C are carried out in sequence to obtain catalyst nanoparticles.

[0050] The Co catalyst on the zirconia (1 1 0) crystal surface substrate was reduced in H2 at 855°C, 865°C and 875°C for 3 min. Typical transmission electron micrographs of the obtained Co catalysts are shown in Figure 2. Figure 3 As shown, it can be seen that the Co catalyst can maintain uniform size and monodispersity at temperatures below 865°C ( Figure 3 ab); Co catalyst begins to aggregate at 875℃ ( Figure 3 c), indicating that the step confinement effect of the substrate can effectively inhibit agglomeration, thereby maintaining the uniformity and high-temperature stability of small-sized (~1.5nm) catalysts, which lays the foundation for the subsequent control of the diameter, conductive properties and even chirality of single-walled carbon nanotubes.

[0051] (3) Single-walled carbon nanotubes were grown at 865°C with 25 sccm CO as the carbon source for 3 min. The typical atomic force microscopy morphology of the prepared single-walled carbon nanotubes with sub-nanometer diameter is shown in Figure 2. Figure 4 As shown in a, due to the confinement effect of the step, the single-walled carbon nanotubes grow epitaxially along the step. Statistics show that their average length is less than 1 μm and the density is about 100 tubes / μm. 2 . Figure 4 bd is a typical transmission electron microscope photo of a sub-nanometer diameter single-walled carbon nanotube, showing that the tube wall of the single-walled carbon nanotube is clear and has a high degree of crystallinity. The diameters of ~200 single-walled carbon nanotubes were measured and their distribution was statistically analyzed ( Figure 4 e), it was found that 80wt% of the single-walled carbon nanotubes have a diameter of 0.75±0.15nm, which is narrower than that reported previously. A typical scanning electron microscope photo is shown in Figure 4 f, due to its high density ( Figure 4 a), The secondary electron image collected by the scanning electron microscope shows single-walled carbon nanotubes connected end to end and densely arranged, and their lengths cannot be distinguished.

[0052] In summary, on the zirconia (1 1 0) crystal plane with a step height of ~0.28nm, using monodispersed Co particles with uniform size and density as catalyst (diameter ~1.5nm) and CO as the carbon source, the controllable growth of high-quality, narrow diameter distribution, and sub-nanometer diameter single-walled carbon nanotubes was achieved under quasi-static chemical vapor deposition conditions.

[0053] Multi-wavelength Raman (532nm, 633nm, 785nm) was used to characterize the conductive properties of carbon nanotubes. Figure 5 As shown in a, the breathing mode peak is mainly concentrated at 284 cm -1 ; 532nm Raman spectrum breathing mode has no obvious excitation signal ( Figure 5 b); 785nm Raman spectrum has a small amount of -1 and 240cm -1 Respiratory mode peak ( Figure 5 c), all belong to the semiconductor range. According to the corresponding relationship between the breathing mode and the diameter, dt = 235.9 / (ω 呼吸模 -5.5), dt represents the diameter of the single-walled carbon nanotube (nm), ω 呼吸模 Represents the wave number of the breathing vibration mode of single-walled carbon nanotubes in Raman spectrum (cm -1 ), the calculated diameter of the single-walled carbon nanotubes is distributed in the range of 0.7 to 0.9 nm, which is basically consistent with the statistical results of transmission electron microscopy. In summary, the breathing mode peaks excited by the three wavelengths are all located in the semiconductor single-walled carbon nanotube range. The chirality corresponding to the breathing mode can be identified from the Kataura diagram. The 633nm breathing mode peak is mainly concentrated in (8,4)(284cm -1 ) chirality, purity of ~90% and narrow chirality distribution of semiconducting single-walled carbon nanotubes. Figure 5 df) shows that high-density (8,4) semiconducting single-walled carbon nanotubes are excited, and very few other chiral single-walled carbon nanotubes can be detected at 532nm and 785nm. The number of colors in the surface scan is counted, and the content of (8,4) narrow chiral semiconducting single-walled carbon nanotubes is greater than 90%, which is consistent with the proportion of breathing mode peaks counted in the line scan.

[0054] In summary, semiconducting single-walled carbon nanotubes with a size of 0.7±0.15 nm and a purity of 90% (8,4) chirality were prepared by confinement on the zirconium oxide (1 1 0) crystal plane with a step height of ~0.28 nm.

[0055] The interface between the catalyst and single-walled carbon nanotubes was characterized by atomic force microscopy. Figure 6. From the interface between the catalyst and the single-walled carbon nanotube marked by the red arrow, it can be seen that when growing at a high temperature of 865℃, the carbon source from the decomposition of CO continuously decomposes, adsorbs and diffuses on the catalyst surface, causing the particles to gradually grow larger; but the single-walled carbon nanotube still maintains a diameter growth that matches the substrate step, and its diameter does not change due to the increase in particle size. This indicates that the substrate step can confine the active site for the nucleation and growth of the Co catalyst, confirming the strong confinement effect of the ~0.28nm step on the growth of sub-nanometer diameter single-walled carbon nanotubes.

[0056] Comparative Example 1: Growth of Single-walled Carbon Nanotubes on a Step-free Zirconia Substrate

[0057] In order to illustrate the confinement effect of the steps of the single-crystal zirconia substrate on single-walled carbon nanotubes, the original zirconia (1 1 0) crystal surface without steps was selected as the substrate, and the methods of steps (2) and (3) of Example 1 were used to prepare catalyst nanoparticles and grow single-walled carbon nanotubes. Typical atomic force microscope and scanning electron microscope and see Figure 7 ab. It can be seen that the catalyst particles become significantly larger after growth, indicating that the step-free substrate surface loses its confinement effect on the catalyst particles, so they aggregate and grow at high temperatures. The single-walled carbon nanotubes are randomly distributed, and their length is ~5μm, which is much longer than the single-walled carbon nanotubes with a step-confined growth length of ~1μm. This shows that there is no step constraint and confinement, causing the single-walled carbon nanotubes to grow too fast. Multi-wavelength Raman spectroscopy ( Figure 7 cd) show that the breathing mode distribution of single-walled carbon nanotubes grown on a step-free substrate is relatively broad, with no enrichment in diameter and conductivity. The above results indicate that the step-free zirconia substrate has no control over the catalyst diameter, the diameter of single-walled carbon nanotubes, and the conductivity, which indirectly verifies the decisive role of the ~0.28nm step of the zirconia (1 1 0) crystal plane in the growth of single-walled carbon nanotubes with sub-nanometer diameter and narrow chirality distribution.

[0058] Comparative Example 2: Growth of single-walled carbon nanotubes on a spinel (1 1 0) substrate with a relatively large step height

[0059] In order to clarify the confinement effect of the steps on the single-crystal zirconia substrate on single-walled carbon nanotubes, the parallel steps of 0.5-1 nm formed by heat treatment of the MgAl2O4 (1 1 0) crystal surface were selected. Figure 8 ab), with an average height of 0.65 nm. Single-walled carbon nanotubes were grown using the conditions of step (3) of Example 1. Typical scanning electron microscope photos of the grown single-walled carbon nanotubes are shown in Figure 8 c, its average length is ~5μm, which is larger than the length of ~1μm of single-walled carbon nanotubes grown in the confined area of ​​zirconia step. Its typical transmission electron microscope photo ( Figure 8df) shows that the diameter of the carbon tube is relatively large. The diameters of ~200 single-walled carbon nanotubes were measured and their distribution was statistically analyzed. It was found that the diameter of the single-walled carbon nanotubes was concentrated at 1.1±0.2nm, accounting for 80%. In summary, the diameter of the single-walled carbon nanotubes (1.1±0.2nm) confined to the ~0.65nm step of the spinel substrate (1 1 0) crystal plane is significantly larger than the diameter of the single-walled carbon nanotubes (0.7±0.15nm) confined to the ~0.28nm step of the zirconia (1 1 0) crystal plane. Multi-wavelength Raman spectroscopy (532nm, 633nm, 785nm) characterizes the following: Figure 8 gi. Among them, 633nm and 785nm did not excite the breathing mode signal; the breathing mode peak of 532nm was mainly concentrated in 225~240cm -1 , and the peak intensity is high. It can be seen that the breathing mode peaks of most of the excited single-walled carbon nanotubes are located in the metallic range, indicating that the sample is enriched with metallic single-walled carbon nanotubes.

[0060] Comparative Example 3: Growth of Single-walled Carbon Nanotubes on a Step-free Silicon Substrate

[0061] Using the step-free silicon wafer commonly used in the literature as the substrate, the catalyst nanoparticles and single-walled carbon nanotubes were prepared by the methods of steps (2) and (3) of Example 1. Typical atomic force microscopy and transmission electron microscopy of Co catalyst ( Fig. 9 ab) observations show that the catalyst particles have a large diameter, ranging from 3 to 4.75 nm ( Fig. 9 c), which is much larger than the catalyst on the spinel substrate surface. This indicates that the amorphous SiO x The surface lacks the confinement effect on the Co catalyst, and obvious agglomeration occurs. A typical scanning electron microscope image of the grown single-walled carbon nanotubes ( Fig. 9 d) shows that the density of single-walled carbon nanotubes is high and the length is greater than 20 μm, which is longer than the single-walled carbon nanotubes grown on the (1 1 0) crystal plane of the zirconia substrate. The breathing mode distribution range of the Raman spectrum is relatively wide ( Fig. 9 ef), the prepared single-walled carbon nanotubes have a conductivity of about 1 / 3 of that of metallic and 2 / 3 of that of semiconductor. The above results show that the confinement effect of the stepless silicon substrate surface on the traditional catalyst Co and the grown single-walled carbon nanotubes is weak, and it cannot limit the size of the catalyst and the diameter, conductivity and growth rate of the single-walled carbon nanotubes.

[0062] Comparative Example 4: Growth of Single-walled Carbon Nanotubes Using C2H2 and C2H4 Carbon Sources

[0063] The zirconia (100) crystal surface was used as the substrate, and the substrate was treated and the catalyst was prepared by the method of steps 1 and 2 of Example (1). C2H2 (0.075 sccm C2H2 + 1000 sccm Ar + 0.5 sccm H2) and C2H4 (0.1 sccm C2H4 + 600 sccm Ar + 0.5 sccm H2) with higher decomposition rate were used as carbon sources, and single-walled carbon nanotubes were grown at 865°C for 3 minutes. Atomic force microscopy observation ( Fig.10 a, d) show that the single-walled carbon nanotubes grown by the two carbon sources are disordered. This is mainly due to the rapid decomposition rate of the above carbon sources at 865°C, which causes the single-walled carbon nanotubes to grow too fast and break away from the substrate confinement. However, its longest length is less than 1μm, indicating that the strong interaction between the zirconia substrate and the single-walled carbon nanotubes makes the growth kinetic resistance of the single-walled carbon nanotubes large. Multi-wavelength Raman spectroscopy shows that the breathing mode peak distribution of the three samples is relatively wide ( Fig.10 bc, ef), without enrichment in diameter and conductive properties.

[0064] The embodiments and comparative examples illustrate that the present invention selects zirconium oxide (1 1 0) surface as the substrate, and controls the height of the atomic steps on its surface by regulating the heat treatment conditions, thereby controlling the size of the catalyst; combined with the optimization of quasi-static CVD growth conditions, the diameter, conductive properties and chirality of the single-walled carbon nanotubes are controlled. The present invention realizes direct confined control of the conductive properties and even single chirality of single-walled carbon nanotubes for the first time through substrate step control, clarifies the mechanism of step control of conductive properties, provides a new candidate single crystal substrate for the controlled growth of single-walled carbon nanotubes, and is expected to lay a material foundation for the application of semiconductor single-walled carbon nanotubes in integrated circuit channel materials.

Claims

1. A method for confining the growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal step height, characterized in that: This method screens the (1 1 0) crystal plane of the zirconia single crystal substrate, regulates the atomic step height of the (1 1 0) crystal plane of the zirconia single crystal substrate through heat treatment, and uses steps with smaller crystal plane height to prepare small-diameter catalyst nanoparticles; on this basis, the atmospheric pressure chemical vapor deposition method is used to utilize the confined space provided by the steps to achieve controllable growth of single-walled carbon nanotubes in diameter and conductive properties.

2. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 1, characterized in that: The heat treatment is to keep the temperature at 775-825°C for 10-30 minutes to obtain atomic steps with small height and uniformity.

3. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 2, characterized in that: The atmosphere for the heat treatment is argon or hydrogen.

4. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 1 or 2, characterized in that: The atomic step height of the (1 1 0) crystal plane ranges from 0.25 to 0.35 nm.

5. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal step height according to claim 1, characterized in that: The diameter and conductive properties of the grown single-walled carbon nanotube network are directly controlled by a quasi-static chemical vapor deposition method. The chemical vapor deposition temperature is 825-865°C, and the carbon source is carbon monoxide with a flow rate of 25-50 sccm.

6. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 1, characterized in that: A catalyst precursor is loaded on the surface of a zirconium oxide single crystal substrate, and after heat treatment, catalyst nanoparticles with small size distribution are prepared, and the particle size thereof is 0.5 to 2.0 nm.

7. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 6, characterized in that: The catalyst precursor is prepared by chemical self-assembly, spin coating, dipping or ion beam bombardment target sputtering.

8. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 6 or 7, characterized in that: The catalyst is a transition metal element such as Fe, Co, Ni or an alloy thereof.

9. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 1, characterized in that: The diameter of the prepared single-walled carbon nanotubes is in the range of 0.75±0.15nm.

10. The method for confined growth of sub-nanometer diameter semiconductor single-walled carbon nanotubes by crystal plane step height according to claim 1 or 9, characterized in that: The method preferentially grows single-chirality (8,4) single-walled carbon nanotubes with a purity of more than 90%.

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