Method for preparing high-quality black phosphorus crystal through low-temperature gas-assisted chemical vapor transport

Abstract

The invention discloses a method for preparing a high-quality black phosphorus crystal by low-temperature gas-assisted chemical vapor transport, which comprises the following steps: placing a phosphorus source, a metal simple substance and a mineralizing agent in a quartz tube, vacuumizing, melting and sealing, and then placing in a crystal growth device; continuously introducing inert gas into the crystal growth device; and carrying out multi-stage programmed heating and cooling, and cleaning to obtain the black phosphorus crystal. Low-temperature gas is introduced into the closed heating device, so that a temperature difference area which is soft and uniform in transition can be constructed in a cavity of the closed heating device; and real-time control and temperature difference adjustment of the temperature in the cavity of the closed heating device can be realized by adjusting the gas flow. The low-temperature gas is introduced in cooperation with the single-temperature-zone heating device, and the requirements for reaction equipment and the production environment are lowered. By introducing the low-temperature gas to assist calcination, the nucleation position, crystal yield and crystal size of the black phosphorus can be accurately regulated and controlled, and the crystal crystallinity and crystal purity of the black phosphorus are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of two-dimensional materials, and particularly relates to a method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport. Background Art

[0002] As a two-dimensional layered material, black phosphorus is an extremely stable allotrope of phosphorus. Its monolayer structure has high anisotropy, and the layers are connected by van der Waals forces.

[0003] The unique structure endows black phosphorus with excellent properties such as high carrier mobility, high theoretical capacity, tunable direct bandgap, good biocompatibility, and small Young's modulus. Therefore, black phosphorus has shown broad application prospects in the fields of field-effect transistors, energy storage batteries, optoelectronic devices, biomedicine, flexible devices, etc.

[0004] Regarding the current mainstream methods for preparing black phosphorus, the chemical vapor transport method has unique advantages in preparing large-sized, highly crystalline, and highly stable black phosphorus crystals. In this method, the nucleation and growth of black phosphorus usually rely on the establishment of a temperature difference. However, currently, the method of establishing temperature difference regions through a multi-temperature zone tube furnace is likely to cause uneven heating temperature zones in the quartz tube, which may lead to problems such as difficult nucleation of black phosphorus, low crystallinity of the product, low yield, and even cause the quartz tube to bulge or burst, generating reactive phosphorus. In addition, to ensure the safety of the production process, the tube cavity of the tube furnace needs to be evacuated, and the use of a multi-temperature zone tube furnace requires the setting of a complex temperature control program. The above two points both require high equipment requirements.

[0005] Therefore, it is of great value to develop and design a chemical vapor transport method for preparing high-quality black phosphorus crystals with controllable temperature difference, uniform temperature zone transition, safety, convenience, good repeatability, and operability. Summary of the Invention

[0006] To solve the above technical problems, the purpose of the present invention is to provide a method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport with controllable temperature difference, uniform temperature zone transition, safety, convenience, good repeatability, and operability. The prepared black phosphorus crystals have a transverse size of 1 - 5 cm and a purity of 98% - 99.9%.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport, comprising the following steps:

[0009] Place a phosphorus source, a metal element, and a mineralizer in a quartz tube. After evacuating the air, perform melting and sealing, and then place it in a crystal growth device. Continuously introduce an inert gas into the crystal growth device. Perform multi-stage programmed heating and cooling, and cleaning to obtain black phosphorus crystals.

[0010] Further, the phosphorus source is red phosphorus or white phosphorus.

[0011] Further, the metal element is one or more of Cd, In, Sn, Sb, Te, Pb, and Bi, and the state is powder with a mesh size below 200.

[0012] Further, the mineralizer is one or more of CdI2, InI3, SnI2, SnI4, SbI3, TeI4, PbI2, and BiI3, and the state is powder or granule.

[0013] Further, the mass ratio of the phosphorus source, the metal element, and the mineralizer is 5:1:1 to 60:6:1.

[0014] Further, the inert gas is one or more of nitrogen, helium, argon, and neon.

[0015] Further, the temperature of the inert gas is 5°C to 30°C.

[0016] Further, the flow rate of the inert gas is controlled at 0.2 to 10 L / min.

[0017] Further, the specific process of the multi-stage programmed heating and cooling is as follows: Under room temperature conditions, heat up for 1 to 5 hours with a heating rate of 140 to 600°C / h. After the heating is completed, keep the temperature at 600 to 800°C for 1 to 5 hours. Cool down for 2 to 10 hours with a cooling rate of 10 to 50°C / h. After the cooling is completed, keep the temperature at 300 to 600°C for 2 to 10 hours. Then perform secondary cooling for 2 to 10 hours with a secondary cooling rate of 30 to 70°C / h, and cool down to 180 to 270°C.

[0018] Further, the specific steps of cleaning are as follows: Clean with excessive saturated copper sulfate solution 2 to 3 times; ultrasonically clean with ultrapure water for 5 to 10 minutes; clean with excessive alkaline solution 3 to 4 times; ultrasonically clean with ultrapure water for 2 to 6 minutes.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention discloses a method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport. By introducing low-temperature gas into a closed heating device, a gently transitional and uniform temperature difference region can be constructed within the cavity of the closed heating device. By adjusting the gas flow rate, real-time control of the temperature within the cavity of the closed heating device and adjustment of the temperature difference can be achieved. The introduction of low-temperature gas in combination with a single-temperature zone heating device reduces the requirements for reaction equipment and production environment, eliminating the need for additional multi-temperature zone tube furnaces and vacuum pumping of the heating cavity. Through the introduction of low-temperature gas-assisted calcination, the present invention can accurately control the nucleation position, crystal yield, and crystal size of black phosphorus, improving the crystal crystallinity and crystal purity of black phosphorus. The operation of the present invention is simple, the reaction conditions are controllable, and it is suitable for batch production. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings are used to provide a further understanding of the invention and constitute a part of the specification, and are used to explain the present invention together with the following specific embodiments, but do not limit the present invention. In the drawings:

[0022] Figure 1 Schematic diagram of the device connection in Example 1;

[0023] Figure 2 XRD spectrum of the black phosphorus crystal obtained in Example 1;

[0024] Figure 3 Optical picture of the black phosphorus obtained in Example 1;

[0025] Figure 4 Weighing diagram of the black phosphorus obtained in Example 1;

[0026] Figure 5 Size measurement diagram of the black phosphorus obtained in Example 1;

[0027] Figure 6 Size measurement picture of the black phosphorus obtained in Example 2;

[0028] Figure 7 Optical picture of the black phosphorus obtained in Comparative Example 1;

[0029] Figure 8 Weighing diagram of the black phosphorus obtained in Comparative Example 1;

[0030] Figure 9 Size measurement diagram of the black phosphorus obtained in Comparative Example 1;

[0031] Figure 10 Size measurement picture of the black phosphorus obtained in Comparative Example 2. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are given in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present invention more thoroughly and comprehensively understood.

[0033] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport of the present invention comprises the following steps:

[0034] 1) The black phosphorus crystal growth device used in this method includes a gas supply device, a gas cooler, a gas flow meter, a crystal growth device, a heating device and an exhaust gas treatment device connected in sequence.

[0035] 2) First weigh a certain amount of phosphorus source, metal element and mineralizer and place them in a quartz tube.

[0036] 3) Evacuate the quartz tube containing the raw materials to 5×10 -2 pa or less, and then melt-sealed and placed in a heating device of a crystal growth apparatus.

[0037] 4) Inert gas is continuously introduced into the crystal growth device through the gas supply equipment.

[0038] 5) Turn on the gas cooler to maintain the temperature of the inert gas introduced at a low temperature level.

[0039] 6) The flow rate of cryogenic inert gas is controlled by a gas flow meter.

[0040] 7) The heating equipment prepares black phosphorus through multi-stage programmed heating and cooling, and the temperature control program is: programmed heating - heat preservation - programmed cooling - heat preservation - secondary programmed cooling - natural cooling.

[0041] 8) The initially obtained black phosphorus crystals are cleaned and purified in sequence using a saturated copper sulfate solution, an alkaline solution, and ultrapure water under vacuum conditions.

[0042] Furthermore, the phosphorus source is red phosphorus or white phosphorus.

[0043] Furthermore, the metal element is one or more of Cd, In, Sn, Sb, Te, Pb and Bi, and is preferably in the form of powder with a size of less than 200 mesh.

[0044] Furthermore, the mineralizer is one or more of CdI2, InI3, SnI2, SnI4, SbI3, TeI4, PbI2 and BiI3, and is preferably in the form of powder or particles.

[0045] Furthermore, the mass ratio of the phosphorus source, the metal element and the mineralizer is 5:1:1 to 60:6:1.

[0046] Further, the gas supply device is one or more of gas cylinders, gas bags, and gas generators.

[0047] Further, the gas cooler includes one or more of a coil cooler, a shell-and-tube cooler, a spray cooler, a jacket cooler, and a partition cooler.

[0048] Further, the gas flowmeter includes one or more of a rotameter, a mass flowmeter, a vortex flowmeter, and a differential pressure flowmeter.

[0049] Further, the heating device is one of a tubular furnace, a box furnace, a rotary furnace, a pit furnace, and a crystal furnace. On the premise of simple equipment and convenient operation, a single-temperature zone heating device can meet the requirements.

[0050] Further, the inert gas is one or more of nitrogen, helium, argon, and neon.

[0051] Further, the temperature of the inert gas after cooling is maintained at 5°C to 30°C.

[0052] Further, the flow rate of the low-temperature inert gas is controlled at 0.2 to 10 L / min.

[0053] Further, the low-temperature inert gas is introduced at one end of the heating device, and the gas flow direction is parallel to the phosphorus vapor transmission direction.

[0054] Further, the multi-stage temperature rise and fall program is specifically as follows: at room temperature, it is heated for 1 to 5 hours with a heating rate of 140 to 600°C / h, and after the heating is completed, it is kept at 600 to 800°C for 1 to 5 hours; it is cooled for 2 to 10 hours with a cooling rate of 10 to 50°C / h, and after the cooling is completed, it is kept at 300 to 600°C for 2 to 10 hours; then it is cooled for a second time for 2 to 10 hours with a second cooling rate of 30 to 70°C / h until it is cooled to 180 to 270°C.

[0055] Further, during the temperature rise and fall process of continuous ventilation, the temperature difference between the two ends of the quartz tube is 1.8 to 10°C / cm.

[0056] Further, the cleaning step is specifically as follows: cleaning 2 to 3 times with excessive saturated copper sulfate; ultrasonic cleaning with ultrapure water for 5 to 10 minutes; cleaning 3 to 4 times with excessive alkali solution; ultrasonic cleaning with ultrapure water for 2 to 6 minutes.

[0057] Further, the alkali solution is one of sodium hydroxide solution or potassium hydroxide solution, and the concentration is 0.1 to 2 mol / L.

[0058] The following are specific examples.

[0059] Example 1

[0060] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport is as follows:

[0061] 1) Refer to Figure 1 , the black phosphorus crystal growth device used is successively composed of a gas supply device 1 (nitrogen gas cylinder), a coiled tube cooler 2, a mass flowmeter 3, a single-temperature zone tubular furnace 4 (heating device) and a tail gas treatment device 5 connected in sequence.

[0062] 2) Weigh 180 mg of red phosphorus, 25 mg of lead powder and 23 mg of PbI2 particles and place them in a quartz tube with a length of 15 cm.

[0063] 3) Evacuate the quartz tube containing the raw materials to below 5×10 -2 Pa, melt-seal it and then place it in a single-temperature zone tubular furnace.

[0064] 4) Continuously introduce nitrogen gas into the crystal growth device from the nitrogen gas cylinder.

[0065] 5) Turn on the coiled tube cooler to maintain the temperature of the introduced nitrogen gas at 20 °C.

[0066] 6) Control the flow rate of the low-temperature nitrogen gas to be 0.3 L / min through the mass flowmeter.

[0067] 7) The single-temperature zone tubular furnace prepares black phosphorus by multi-stage programmed heating and cooling. The temperature control program: heat from 20 °C for 2 h with a heating rate of 320 °C / h, hold at 660 °C for 2 h after the heating ends; cool for 4 h with a cooling rate of 30 °C / h, hold at 540 °C for 3 h after the cooling ends; then cool to 220 °C for the second time for 8 h with a second cooling rate of 40 °C / h, and then naturally cool to room temperature.

[0068] 8) During the heating and cooling process with continuous gas supply, the temperature difference between the two ends of the quartz tube is 4 °C / cm.

[0069] 9) The preliminarily prepared black phosphorus crystals are under vacuum conditions. First, wash them 3 times with an excessive amount of saturated copper sulfate solution to wash away potential white phosphorus, and then perform ultrasonic cleaning with ultrapure water for 6 min to remove possible copper phosphide precipitates; then wash them 3 times with an excessive amount of 0.8 mol / L potassium hydroxide solution to wash away potential red phosphorus, and then perform ultrasonic cleaning with ultrapure water for 5 min to remove potassium hydroxide and other impurities, obtaining high-quality black phosphorus crystals.

[0070] Figure 2 XRD pattern of the black phosphorus crystals obtained in Example 1. From Figure 2It can be seen that the black phosphorus sample conforms to the JCDPS standard substance card of black phosphorus, and there are almost no impurity peaks, indicating that the purity of the black phosphorus sample is relatively high. The peak pattern of the spectrum is thin and sharp, indicating that the crystallinity of the black phosphorus sample is extremely high.

[0071] Figure 3 This is an optical picture of the black phosphorus obtained in Example 1. From Figure 3 it can be seen that the residual end of the mineralizer and the growth end of the black phosphorus crystal in the calcined quartz tube are clearly demarcated, and the middle part of the quartz tube is clean and transparent, indicating a relatively high conversion rate of red phosphorus.

[0072] Figure 4 This is the weighing picture of the black phosphorus obtained in Example 1. The calculated conversion rate of red phosphorus is about 90.6%.

[0073] Figure 5 This is the size measurement picture of the black phosphorus obtained in Example 1. From Figure 5 it can be seen that the lateral size of the black phosphorus can reach 1.2 cm.

[0074] Example 2

[0075] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport is as follows:

[0076] 1) The black phosphorus crystal growth device is successively composed of an argon gas cylinder, a jacketed cooler, a rotameter, a box furnace and a tail gas treatment device connected.

[0077] 2) Weigh 1.4 g of red phosphorus, 150 mg of tin powder and 100 mg of tin tetraiodide and place them in a quartz tube with a length of 20 cm.

[0078] 3) Vacuum the quartz tube filled with raw materials to below 5×10 -2 Pa, melt-seal it and place it in a box furnace.

[0079] 4) Continuously introduce argon gas from the argon gas cylinder into the crystal growth device.

[0080] 5) Turn on the jacketed cooler to maintain the temperature of the introduced argon gas at 10°C.

[0081] 6) Control the flow rate of low-temperature nitrogen gas to be 4 L / min through the rotameter.

[0082] 7) The box furnace prepares black phosphorus by multi-stage programmed heating and cooling. The temperature control program: heat up from 10°C at a rate of 230°C / h for 3 h, keep the temperature at 700°C for 2.5 h after the heating is completed; cool down for 6 h at a rate of 20°C / h, keep the temperature at 580°C for 6.5 h after the cooling is completed; then cool down to 180°C for the second time for 10 h, the second cooling rate is 40°C / h, and then naturally cool to room temperature.

[0083] 8) During the heating and cooling process with continuous ventilation, the uniform temperature difference between the two ends of the quartz tube is 5 °C / cm.

[0084] 9) Under vacuum conditions, the initially prepared black phosphorus crystals are first washed 3 times with an excessive amount of saturated copper sulfate solution to remove potential white phosphorus, and then ultrasonically cleaned with ultrapure water for 10 min to remove possible copper phosphide precipitates; then they are washed 3 times with an excessive amount of 1.5 mol / L potassium hydroxide solution to remove potential red phosphorus, and then ultrasonically cleaned with ultrapure water for 5 min to remove potassium hydroxide and other impurities.

[0085] Figure 6 It is the size measurement diagram of the black phosphorus obtained in Example 2. As can be seen from Figure 6 it, the growth position (i.e., the nucleation position) of the black phosphorus is at the bottom of the quartz tube, and its transverse size can reach 2.3 cm.

[0086] Example 3

[0087] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport is as follows:

[0088] 1) The black phosphorus crystal growth device is successively composed of a helium gas cylinder, a spray cooler, a differential pressure flowmeter, a crystal furnace, and an exhaust gas treatment device connected.

[0089] 2) Weigh 2.3 g of red phosphorus, 50 mg of antimony powder, and 47 mg of antimony iodide and place them in a quartz tube with a length of 25 cm.

[0090] 3) The quartz tube filled with raw materials is evacuated to below 5×10 -2 Pa, melted and sealed, and then placed in the crystal furnace.

[0091] 4) Continuously introduce helium gas from the helium gas cylinder into the crystal growth device.

[0092] 5) Turn on the spray cooler to maintain the temperature of the introduced helium gas at 30 °C.

[0093] 6) Control the flow rate of the low-temperature helium gas to be 8 L / min through the differential pressure flowmeter.

[0094] 7) The crystal furnace prepares black phosphorus by multi-stage programmed heating and cooling. The temperature control program: heat from 30 °C at a rate of 248 °C / h for 2.5 h, keep the temperature at 650 °C for 5 h after the heating ends; cool for 8 h at a rate of 10 °C / h, keep the temperature at 570 °C for 8 h after the cooling ends; then cool to 240 °C for the second time for 5 h at a rate of 66 °C / h, and then naturally cool to room temperature.

[0095] 8) During the heating and cooling process with continuous ventilation, the temperature difference between the two ends of the quartz tube is 1.8 °C / cm.

[0096] 9) The initially obtained black phosphorus crystals are, under vacuum conditions, first washed 3 times with an excessive amount of saturated copper sulfate solution to remove potential white phosphorus, and then ultrasonically washed with ultrapure water for 8 min to remove possible precipitates of copper phosphide; then washed 3 times with an excessive amount of 1.2 mol / L sodium hydroxide solution to remove potential red phosphorus, and then ultrasonically washed with ultrapure water for 6 min to remove sodium hydroxide and other impurities.

[0097] Comparative Example 1 is black phosphorus crystals prepared by the traditional chemical vapor transport method without gas assistance, used to compare with the high-quality black phosphorus crystals prepared by the low-temperature gas-assisted chemical vapor transport method.

[0098] Example 4

[0099] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport is as follows:

[0100] 1) The black phosphorus crystal growth device is successively composed of a neon gas cylinder, a spray cooler, a differential pressure flowmeter, a crystal furnace, and a tail gas treatment device connected.

[0101] 2) Weigh 1000 mg of white phosphorus, 50 mg of Cd powder, and 50 mg of CdI2 and place them in a quartz tube with a length of 10 cm.

[0102] 3) The quartz tube containing the raw materials is evacuated to below 5×10 -2 Pa, melted and sealed, and then placed in the crystal furnace.

[0103] 4) Continuously introduce helium gas into the crystal growth device from the helium gas cylinder.

[0104] 5) Turn on the spray cooler to maintain the temperature of the introduced helium gas at 5°C.

[0105] 6) Control the flow rate of low-temperature helium gas to be 0.2 L / min through the differential pressure flowmeter.

[0106] 7) The crystal furnace prepares black phosphorus by multi-stage programmed temperature rise and fall. The temperature control program: heat up from 5°C at a rate of 600°C / h for 1 h, hold at 605°C for 1 h after the heating ends; cool down for 4.1 h at a rate of 50°C / h, hold at 400°C for 10 h after the cooling ends; then cool down for a second time for 4 h at a rate of 32.5°C / h, cool down to 270°C, and then naturally cool to room temperature.

[0107] 8) During the temperature rise and fall process with continuous gas supply, the temperature difference between the two ends of the quartz tube is 8°C / cm.

[0108] 9) The initially obtained black phosphorus crystals are, under vacuum conditions, first washed 3 times with an excessive amount of saturated copper sulfate solution to remove potential white phosphorus, and then ultrasonically washed with ultrapure water for 8 min to remove possible copper phosphide precipitates; then washed 3 times with an excessive amount of 2 mol / L sodium hydroxide solution to remove potential red phosphorus, and then ultrasonically washed with ultrapure water for 4 min to remove sodium hydroxide and other impurities.

[0109] Example 5

[0110] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport is as follows:

[0111] 1) The black phosphorus crystal growth device is successively composed of a helium gas cylinder, a spray cooler, a differential pressure flowmeter, a crystal furnace, and an exhaust gas treatment device connected together.

[0112] 2) Weigh 3000 mg of red phosphorus, 1500 mg of In powder, and 50 mg of InI3 and place them in a quartz tube with a length of 30 cm.

[0113] 3) The quartz tube containing the raw materials is evacuated to below 5×10 -2 pa, melted and sealed, and then placed in the crystal furnace.

[0114] 4) Continuously introduce helium gas from the helium gas cylinder into the crystal growth device.

[0115] 5) Turn on the spray cooler to maintain the temperature of the introduced helium gas at 20°C.

[0116] 6) Control the flow rate of low-temperature helium gas to be 10 L / min through the differential pressure flowmeter.

[0117] 7) The crystal furnace prepares black phosphorus by multi-stage programmed heating and cooling. The temperature control program: heat up from 20°C at a rate of 260°C / h for 3 h, hold at 800°C for 3 h after the heating ends; cool down for 10 h at a rate of 20°C / h, hold at 600°C for 2 h after the cooling ends; then cool down for a second time for 10 h at a rate of 42°C / h, cool down to 180°C, and then naturally cool to room temperature.

[0118] 8) During the heating and cooling process with continuous gas flow, the temperature difference between the two ends of the quartz tube is 2.5°C / cm.

[0119] 9) The initially obtained black phosphorus crystals are, under vacuum conditions, first washed 3 times with an excessive amount of saturated copper sulfate solution to remove potential white phosphorus, and then ultrasonically washed with ultrapure water for 10 min to remove possible copper phosphide precipitates; then washed 3 times with an excessive amount of 1 mol / L sodium hydroxide solution to remove potential red phosphorus, and then ultrasonically washed with ultrapure water for 2 min to remove sodium hydroxide and other impurities.

[0120] Example 6

[0121] A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport is as follows:

[0122] 1) The black phosphorus crystal growth device is successively composed of a helium gas cylinder, a spray cooler, a differential pressure flowmeter, a crystal furnace, and a tail gas treatment device connected together.

[0123] 2) Weigh 250 mg of red phosphorus, 3000 mg of Bi powder, and 50 mg of BiI3 and place them in a quartz tube with a length of 15 cm.

[0124] 3) Evacuate the quartz tube containing the raw materials to below 5×10 -2 pa, perform melting and sealing on it, and then place it in the crystal furnace.

[0125] 4) Continuously introduce helium gas from the helium gas cylinder into the crystal growth device.

[0126] 5) Turn on the spray cooler to maintain the temperature of the introduced helium gas at 25°C.

[0127] 6) Control the flow rate of the low-temperature helium gas to be 3 L / min through the differential pressure flowmeter.

[0128] 7) The crystal furnace prepares black phosphorus by multi-stage programmed heating and cooling. The temperature control program: heat from 25°C at a heating rate of 140°C / h for 5 h, hold at 725°C for 5 h after the heating ends; cool for 5 h at a cooling rate of 44°C / h, hold at 505°C for 5 h after the cooling ends; then cool for a second time for 4 h at a second cooling rate of 70°C / h, cool down to 225°C, and then naturally cool to room temperature.

[0129] 8) During the heating and cooling process with continuous gas supply, the temperature difference between the two ends of the quartz tube is 10°C / cm.

[0130] 9) The preliminarily prepared black phosphorus crystals are under vacuum conditions. First, wash them 2 times with an excessive saturated copper sulfate solution to wash away potential white phosphorus, and then perform ultrasonic cleaning with ultrapure water for 5 min to remove possible copper phosphide precipitates; then wash them 4 times with an excessive 1.2 mol / L sodium hydroxide solution to wash away potential red phosphorus, and then perform ultrasonic cleaning with ultrapure water for 6 min to remove sodium hydroxide and other impurities.

[0131] Comparative Example 1

[0132] A method for preparing black phosphorus crystals by a traditional chemical vapor transport method without gas assistance is as follows:

[0133] 1) Weigh 180 mg of red phosphorus, 25 mg of lead powder, and 23 mg of PbI2 particles and place them in a quartz tube with a length of 15 cm.

[0134] 2) Evacuate the quartz tube containing the raw materials to 5×10 -2 pa or less, and then melt-sealed and placed in a closed dual-temperature zone tubular furnace.

[0135] 3) The dual-zone tubular furnace is divided into a hot zone and a cold zone. Black phosphorus is prepared by multi-stage program heating and cooling, and the temperature difference between the hot zone and the cold zone is set to 60°C. The temperature control program of the hot zone: heating from 20°C for 2h, the heating rate is 335°C / h, and then kept at 690°C for 2h after the heating is completed; cooling for 4h, the cooling rate is 30°C / h, and then kept at 570°C for 3h after the cooling is completed; then cooling for a second time to 250°C for 8h, the second cooling rate is 40°C / h, and then naturally cooled to room temperature. Temperature control program in cold zone: heating from 20℃ for 2h at a heating rate of 305℃ / h, keeping at 630℃ for 2h after heating; cooling for 4h at a cooling rate of 30℃ / h, keeping at 510℃ for 3h after cooling; cooling for a second time to 190℃ for 8h at a cooling rate of 40℃ / h, and then naturally cooling to room temperature.

[0136] 4) The initially prepared black phosphorus crystals were first washed three times with an excess of saturated copper sulfate solution under vacuum conditions to remove potential white phosphorus, and then ultrasonically cleaned with ultrapure water for 6 minutes to remove possible copper phosphide precipitates; then washed three times with an excess of 0.8 mol / L potassium hydroxide solution to remove potential red phosphorus, and then ultrasonically cleaned with ultrapure water for 5 minutes to remove potassium hydroxide and other impurities.

[0137] Figure 7 This is an optical picture of the black phosphorus crystal obtained in Comparative Example 1. Figure 7 It can be seen that, unlike Example 1, the quartz tube after calcination has large pieces of red phosphorus and mineralizer residues, and the black phosphorus growth site has more impurities, indicating that the red phosphorus conversion rate is low.

[0138] Figure 8 This is a weighing diagram of the black phosphorus obtained in Comparative Example 1. The red phosphorus conversion rate was calculated to be 51.1%.

[0139] Figure 9 This is a diagram showing the size of the black phosphorus obtained in Comparative Example 1. Figure 9 It can be seen that the lateral size of black phosphorus is only 0.7cm.

[0140] Comparative Example 2

[0141] A traditional chemical vapor transport method without gas assistance to prepare black phosphorus crystals, used to compare the difference between high-quality black phosphorus crystals prepared by low-temperature gas-assisted chemical vapor transport method:

[0142] 1) Weigh 1.4 g of red phosphorus, 150 mg of tin powder and 100 mg of tin tetraiodide and place them in a quartz tube with a length of 20 cm.

[0143] 2) Evacuate the quartz tube containing the raw materials to below 5×10 -2 Pa, melt-seal it and then place it in a box furnace.

[0144] 3) The two-temperature-zone box furnace is divided into a hot zone and a cold zone. Black phosphorus is prepared by multi-stage programmed heating and cooling. Set the temperature difference between the hot zone and the cold zone to 100 °C. Temperature control program for the hot zone: Heat from 10 °C at a heating rate of 247 °C / h for 3 h, hold at 750 °C for 2.5 h after the heating ends; Cool at a cooling rate of 20 °C / h for 6 h, hold at 630 °C for 6.5 h after the cooling ends; Then cool to 230 °C for a second time at a cooling rate of 40 °C / h for 10 h, and then cool naturally to room temperature. Temperature control program for the cold zone: Heat from 10 °C at a heating rate of 213 °C / h for 3 h, hold at 650 °C for 2.5 h after the heating ends; Cool at a cooling rate of 20 °C / h for 6 h, hold at 530 °C for 6.5 h after the cooling ends; Then cool to 130 °C for a second time at a cooling rate of 40 °C / h for 10 h, and then cool naturally to room temperature.

[0145] 4) Under vacuum conditions, the initially prepared black phosphorus crystals are first washed 3 times with an excessive amount of saturated copper sulfate solution to remove potential white phosphorus, and then ultrasonically washed with ultrapure water for 10 min to remove possible copper phosphide precipitates; Then they are washed 3 times with an excessive amount of 1.5 mol / L potassium hydroxide solution to remove potential red phosphorus, and then ultrasonically washed with ultrapure water for 5 min to remove potassium hydroxide and other impurities.

[0146] Figure 10 It is an optical picture of the black phosphorus obtained in Comparative Example 2. As can be seen from Figure 10 it, black phosphorus has multiple nucleation sites, which makes the black phosphorus unable to grow concentratedly at the bottom, affecting the lateral size of the black phosphorus crystals (the maximum is only 1.4 cm).

[0147] From the comparison between Example 1 and Comparative Example 1 and Comparative Example 2, it can be seen that the conversion rate of red phosphorus during the preparation of black phosphorus by the traditional chemical vapor transport method is only about 50%, and there are more impurities mixed in the growth site of black phosphorus. From the phenomenon that there is obvious red phosphorus residue in the quartz tube after calcination, it can be seen that the quartz tube in Comparative Example 1 has experienced extremely uneven temperature transition in the two-temperature zone tube furnace, that is, the temperature difference region established by the two-temperature zone tube furnace cannot well meet the needs of black phosphorus crystal growth. In contrast, the method of preparing black phosphorus crystals by low-temperature gas-assisted chemical vapor transport can effectively adjust the temperature distribution in the heating chamber, construct a temperature difference region with uniform transition, and significantly improve the conversion rate of red phosphorus. The black phosphorus crystals prepared by this method have excellent crystallinity, large size and high purity. Therefore, the low-temperature gas-assisted chemical vapor transport method adopted in the present invention has positive significance for the preparation of high-quality black phosphorus crystals.

[0148] The above is only an illustration of the best embodiment of the present invention, but it should not be construed as a limitation of the claims. The present invention is not limited to the above embodiments, and its specific structure is allowed to change. Any changes made within the protection scope of the independent claims of the present invention are within the protection scope of the present invention.

[0149] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this invention belongs. The terms used in the description of the present invention herein are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the related listed items.

Claims

1. A method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport, characterized in that: The following steps are involved: A phosphorus source, a metal element and a mineralizer are placed in a quartz tube, which is evacuated and melt-sealed, and then placed in a crystal growth device; an inert gas is continuously introduced into the crystal growth device; the temperature is raised and lowered in multiple stages, and the device is cleaned to obtain black phosphorus crystals.

2. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The phosphorus source is red phosphorus or white phosphorus.

3. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The metal element is one or more of Cd, In, Sn, Sb, Te, Pb and Bi, and is in the form of powder with a mesh size of less than 200.

4. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The mineralizer is one or more of CdI2, InI3, SnI2, SnI4, SbI3, TeI4, PbI2 and BiI3, and is in the form of powder or granules.

5. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The mass ratio of the phosphorus source, the metal element and the mineralizer is 5:1:1 to 60:6:

1.

6. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The inert gas is one or more of nitrogen, helium, argon and neon.

7. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The temperature of the inert gas is 5°C to 30°C.

8. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The flow rate of the inert gas is controlled at 0.2 to 10 L / min.

9. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The multi-stage programmed heating and cooling are specifically as follows: under room temperature, heating for 1 to 5 hours at a heating rate of 140 to 600°C / h, and keeping at 600 to 800°C for 1 to 5 hours after the heating is completed; cooling for 2 to 10 hours at a cooling rate of 10 to 50°C / h, and keeping at 300 to 600°C for 2 to 10 hours after the cooling is completed; and cooling for a second time for 2 to 10 hours at a cooling rate of 30 to 70°C / h to a temperature of 180 to 270°C.

10. The method for preparing high-quality black phosphorus crystals by low-temperature gas-assisted chemical vapor transport according to claim 1, characterized in that: The specific cleaning steps are: cleaning with excess saturated copper sulfate for 2 to 3 times; ultrasonic cleaning with ultrapure water for 5 to 10 minutes; cleaning with excess alkali solution for 3 to 4 times; and ultrasonic cleaning with ultrapure water for 2 to 6 minutes.

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