Method for modifying surface of purple phosphorus two-dimensional material by femtosecond laser

Abstract

The invention discloses a method for modifying the surface of a purple phosphorus two-dimensional material by using femtosecond laser, and belongs to the field of laser processing modification. The method comprises the following steps: step 1, preparing a purple phosphorus sample: stripping a purple phosphorus thin layer through a mechanical stripping method, and transferring the purple phosphorus thin layer to a quartz substrate; 2, modifying and processing the purple phosphorus thin layer by using femtosecond laser; and step 3, focusing femtosecond laser to the purple phosphorus two-dimensional material through an objective lens, then carrying out line scanning, and obtaining a modified surface of the purple phosphorus two-dimensional material by utilizing multiple times of equidistant line scanning. According to the method, the two-dimensional purple phosphorus material is irradiated by femtosecond laser, so that the surface of the two-dimensional purple phosphorus material is modified, the fluorescence intensity is improved, the fluorescence lifetime is prolonged, the surface hardness is improved, the diffusion direction of excitons is limited, and the material is patterned by utilizing an exciton directional transmission mechanism. According to the method, the sample is characterized through the fluorescence spectrum, the microscopic fluorescence imaging, the fluorescence lifetime imaging, the atomic force microscope and the transient absorption microscope, and the precision and efficiency of surface modification of the purple phosphorus two-dimensional material are improved according to the screened ablation threshold.

Description

Technical Field

[0001] The present invention belongs to the field of laser processing and modification, and relates to a method for surface modification of two-dimensional violet phosphorene materials using femtosecond lasers. Background Art

[0002] Two-dimensional semiconductor materials have attracted much attention in recent years due to their excellent optical, electrical and other properties. As a new two-dimensional allotrope of phosphorus, violet phosphorene has a tunable bandgap (1.4 - 2.1 eV) and high carrier mobility (7000 cm2 / V·s), showing great potential in the field of optoelectronic devices. However, the low fluorescence quantum yield of violet phosphorene limits its application in light-emitting devices. In the prior art, although chemical doping and plasma treatment can improve the fluorescence performance, they are prone to introduce impurities or modify the entire material in a full range. Therefore, it is crucial to develop a non-contact and high-precision surface modification method.

[0003] Femtosecond lasers have the characteristics of ultrashort pulses (in the order of 10- 15 seconds), ultra-high laser energy density and almost no heat-affected zone. Therefore, they can perform high-precision processing on surface textures. After processing, the interaction between light and the surface lattice of the material will cause changes in some material properties, making the material applicable to various fields. It has been widely used in the preparation of micro-nano structures on the material surface. Its ultrashort pulse characteristics can effectively avoid damage to the material caused by heat diffusion and achieve high-precision surface regional modification. Summary of the Invention

[0004] In order to solve the problem that the quantum yield of violet phosphorene is low, resulting in weak fluorescence, the purpose of the present invention is to provide a method for high-precision modification of the surface of two-dimensional violet phosphorene materials using femtosecond lasers. By irradiating two-dimensional violet phosphorene materials with femtosecond lasers, surface modification is promoted, the fluorescence intensity and surface hardness are improved, a new method is provided for improving the application of violet phosphorene in optoelectronic devices, and at the same time, it helps to exert its application potential in regulating the directional transport of excitons, and further contributes to the development of semiconductor materials.

[0005] The purpose of the present invention is achieved by the following technical solutions.

[0006] A method for surface modification of two-dimensional violet phosphorene materials using femtosecond lasers disclosed by the present invention includes the following steps:

[0007] Step 1: Prepare a violet phosphorene sample: Peel off a thin layer of violet phosphorene by mechanical exfoliation method and transfer it to a quartz substrate;

[0008] Step 2: Modify and process the thin layer of violet phosphorus using femtosecond laser; determine the ablation threshold of the modified violet phosphorus by changing the laser parameters and scanning the violet phosphorus in lines, that is, locate the ablation threshold between the unchanged and changed states according to whether the optical microscope and fluorescence spectrum change after laser processing.

[0009] Step 3: Focus the femtosecond laser onto the two-dimensional violet phosphorus material through an objective lens and then scan in lines to obtain the modified surface of the two-dimensional violet phosphorus material by using multiple equidistant scans.

[0010] Furthermore, surface modification of the two-dimensional violet phosphorus material is achieved by femtosecond laser processing, enhancing the fluorescence intensity, fluorescence lifetime, and surface hardness, restricting the exciton diffusion direction, and patterning the two-dimensional violet phosphorus material by using the mechanism of exciton directional transport to realize a customized transmission channel.

[0011] Furthermore, the modified surface of the processed two-dimensional violet phosphorus material is characterized by fluorescence spectrum, microscopic fluorescence imaging, fluorescence lifetime imaging, atomic force microscopy, and transient absorption microscopy, and the changes in the mechanical and optoelectronic properties of the modified surface of the two-dimensional violet phosphorus material under different processing parameters are obtained by comparison.

[0012] Furthermore, after femtosecond laser processing, the fluorescence intensity of violet phosphorus is enhanced and the exciton diffusion direction is regulated. The specific implementation method is that before femtosecond laser processing, excitons diffuse circularly in all directions, while after femtosecond laser processing, excitons only diffuse along the direction of the laser scan line, and the diffusion in the vertical direction is restricted. The fluorescence enhancement is due to the fact that excitons are blocked from diffusing and locally aggregated, and more radiate through radiative recombination, increasing the proportion of its recombination component, thereby promoting radiative emission and realizing the regulation of the enhanced fluorescence intensity of violet phosphorus and the exciton diffusion direction.

[0013] Preferably, in Step 2, femtosecond laser with a wavelength of 515 nm, a spot diameter of 370 nm, a repetition frequency of 1 kHz, and a power density of 0.19 - 0.93 kW / cm 2 is used.

[0014] Preferably, in Step 3, a processing speed of 0.05 - 0.1 μm / s, a modification range of 10 μm × 10 μm, and a scan line spacing of 0.5 - 1 μm are selected.

[0015] Furthermore, after femtosecond laser processing, the fluorescence intensity of violet phosphorus is increased by more than 5 times, the fluorescence lifetime is extended, the surface hardness of the two-dimensional violet phosphorus material is improved, and the exciton diffusion direction of violet phosphorus is regulated, thereby improving the accuracy and efficiency of the surface modification of the two-dimensional violet phosphorus material.

[0016] Beneficial effects:

[0017] 1. A method for surface modification of two-dimensional purple phosphorus materials using femtosecond laser. By irradiating two-dimensional purple phosphorus materials with femtosecond laser, surface modification occurs, enhancing fluorescence intensity, fluorescence lifetime, and surface hardness, restricting the exciton diffusion direction. Through this method, customized transmission channels can be achieved by patterning and processing materials using the mechanism of exciton directional transport, and customized light-emitting devices can be realized using the mechanisms of fluorescence enhancement and lifetime increase.

[0018] 2. A method for surface modification of two-dimensional purple phosphorus materials using femtosecond laser. The processed samples are characterized by fluorescence spectroscopy, microscopic fluorescence imaging, fluorescence lifetime imaging, atomic force microscopy, and transient absorption microscopy. The changes in the surface mechanical properties and optoelectronic properties of the samples under different processing parameters are directly compared, and the ablation threshold of purple phosphorus is determined based on the changes.

[0019] 3. A method for surface modification of two-dimensional purple phosphorus materials using femtosecond laser. Based on beneficial effect 2, according to whether the optical microscope and fluorescence spectrum change after laser processing, the ablation threshold is positioned between the unchanged and changed states. The accuracy and efficiency of surface modification of two-dimensional purple phosphorus materials by femtosecond laser are improved according to the selected ablation threshold.

[0020] 4. A method for surface modification of two-dimensional purple phosphorus materials using femtosecond laser. By adjusting femtosecond laser parameters such as femtosecond laser wavelength, power density, processing speed, and scan line spacing, the accuracy and efficiency of surface modification of two-dimensional purple phosphorus materials by femtosecond laser can be improved. Moreover, the present invention gives the preferred range of laser parameters, which helps to exert its application potential in regulating exciton directional transport and further contributes to the development of semiconductor materials. Description of the Drawings

[0021] Figure 1 Optical microscopic imaging after line filling modification of two-dimensional purple phosphorus materials by femtosecond laser.

[0022] Figure 2 Partial enlarged view of optical imaging after line filling modification of two-dimensional purple phosphorus materials using femtosecond lasers with different powers and different scan line densities.

[0023] Figure 3 (a) Fluorescence microscopic imaging and (b) fluorescence lifetime imaging of the experimental samples. According to the present invention, fluorescence enhancement and fluorescence lifetime extension are obtained for purple phosphorus after femtosecond laser processing.

[0024] Figure 4 (a) Fluorescence spectrum and (b) changes in exciton peak and trion peak extracted after double-peak fitting of the fluorescence spectrum of the experimental samples with the change of processing laser power.

[0025] Figure 5Time-resolved fluorescence spectra of the experimental samples, their fitting curves and results. Among them, (a) is the unprocessed area, and (b-j) are the processed areas, corresponding to Figure 2 the last three columns in

[0026] Figure 6 Transient absorption microscopy images of purple phosphorus before and after processing.

[0027] Figure 7 Optical microscopy images of the patterned "L" processing done to study the change in the surface mechanical properties of purple phosphorus by femtosecond laser.

[0028] Figure 8 Height images measured in the tapping mode of the atomic force microscope for the processed sample and the height comparison of the processed area and the unprocessed area intercepted.

[0029] Figure 9 Amplitude images measured in the tapping mode of the atomic force microscope for the processed sample and the amplitude comparison of the processed area and the unprocessed area intercepted.

[0030] Figure 10 Phase images measured in the tapping mode of the atomic force microscope for the processed sample and the phase comparison of the processed area and the unprocessed area intercepted. Specific implementation manner

[0031] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0032] Step 1: Prepare purple phosphorus samples by mechanical exfoliation. Specifically, crush purple phosphorus crystals, use blue film tape to adhere a small amount of samples from the crushed crystals, obtain thinner samples through multiple tearing, and then transfer the samples on the blue film tape to a quartz substrate using PDMS. Select larger and flatter samples obtained by exfoliation (with a size of about 100μm×100μm) for convenient processing. Determine that the thickness of purple phosphorus is about 70nm, that is, 60 layers, based on the comparison of the RGB values of the microscopy images.

[0033] Step 2: Use a femtosecond laser to generate 1030nm femtosecond laser, which outputs 515nm femtosecond laser after passing through a frequency doubling crystal, adjust it to the required laser power through an attenuation wheel, and focus it on the sample using a 60× objective lens. Calculate that the diameter of the femtosecond laser spot used is d = 370nm, the repetition frequency is 1kHz, and the powers used are 0.2μW, 0.4μW, 0.6μW, 0.8μW, 1.0μW, and the power density is 0.19 - 0.93kW / cm 2 . Use a halogen lamp and a camera to image the processed sample. After finding the sample to be processed, align the laser spot with the area to be processed.

[0034] Step 3: Program relevant parameters such as the processing pattern (square raster filling) and processing speed (0.1 μm / s) in the processing software, control the three-axis displacement stage through the software, thereby controlling the position of the sample stage, and control the on / off of the laser optical path through the electric trigger switch to perform controllable area processing on the sample. As Figure 1 shown in Figure 2, optical microscopy imaging was performed on the samples processed with femtosecond lasers of different powers, and the processed square size was 10 μm × 10 μm.

[0035] Furthermore, the processed sample area was characterized by a confocal / two-photon / fluorescence lifetime imaging microscope. As Figure 3 shown, the changes in the fluorescence properties of the sample surface under different processing parameters were obtained by comparison. Compared with optical imaging ( Figure 1 ), it can be found that the fluorescence intensity in the processed area is enhanced (as Figure 3 shown in Figure a), and the fluorescence lifetime is prolonged (as Figure 3 shown in Figure b). Then, fluorescence spectrum testing and time-resolved fluorescence spectrum testing were performed on the processed area. A 395 nm picosecond laser was used, and a repetition rate of 40 MHz was selected to obtain a stronger fluorescence signal. A 40× objective lens was used to focus on the sample surface, with a power of 5 μW, and an EMCCD was used to collect and process the signal. Figure 4 Figure a shows the fluorescence spectrum testing of the samples processed with femtosecond lasers of different powers. After femtosecond laser processing, purple phosphorus shows a maximum 5-fold fluorescence enhancement and a blue shift of the fluorescence peak. The changes in the exciton peak and trion peak extracted after fitting the double-peak PL spectrum with the processing laser power are as Figure 4 shown in Figure b. By performing decay fitting on the time-resolved fluorescence spectrum, it was found that compared with the original exciton lifetime of purple phosphorus (about 200 ps, as Figure 5 shown in Figure a), the processed area has an increased long-lived component (about 2 ns, as Figure 5 shown in Figures b - j).

[0036] Furthermore, the transient absorption microscope was used to test the exciton diffusion of purple phosphorus. A 450 nm laser was selected to excite the sample to generate excitons. After different time delays, the sample was detected at 570 nm of the photoinduced absorption signal corresponding to the excitons to obtain its spatial distribution at different time delays. As Figure 6 shown, before processing, the excitons diffused circularly in all directions, while after processing, the excitons only diffused along the direction of the laser scan line, and the diffusion in the vertical direction was restricted. Therefore, it can be speculated that the principle of the above fluorescence enhancement is that the exciton diffusion is hindered and locally aggregated, and more radiative recombination occurs, increasing the proportion of its recombination component, thereby promoting radiative emission.

[0037] The samples with slight ablation were characterized by atomic force microscopy. Figure 7Optical microscopy imaging of patterned "L" processing for studying the change in the surface mechanical properties of violet phosphorus by femtosecond laser. The scale bar is also marked. The atomic force microscope used operates in the tapping mode. Figure 8 Height image measured in the tapping mode of the atomic force microscope for the processed sample and height comparison of the processed area and the unprocessed area. There is a slight depression in the height of the processed area. Figure 9 Amplitude image measured in the tapping mode of the atomic force microscope for the processed sample and amplitude comparison of the processed area and the unprocessed area. The amplitude change in the processed area is significantly smaller than that in the unprocessed area, indicating that its hardness increases due to processing. It is found that the amplitude image has a relatively clear contour. In the tapping mode, when the surface height changes, the feedback system adjusts the Z position to maintain the amplitude, and the amplitude image may show the amplitude deviation during this adjustment process, which may be related to the surface topography or material hardness. A hard surface will cause rapid amplitude decay, while a soft surface may allow larger amplitude vibration. Figure 10 Phase image measured in the tapping mode of the atomic force microscope for the processed sample and phase comparison of the processed area and the unprocessed area. A harder area in the processed area may result in a smaller phase lag. Therefore, it can be judged that the material in the processed area has a depression and an increase in hardness.

[0038] The determination of the ablation threshold of femtosecond laser on violet phosphorus described in the present invention relies on the above-mentioned experimental step two. According to Figure 2 the optical imaging results, it can be determined by optical microscopy and fluorescence spectroscopy that the surface of the material does not change under the laser processing of 0.37 kW / cm 2 while the surface topography and luminescence properties of the material change after laser processing of 0.56 kW / cm 2 . Therefore, the ablation threshold of femtosecond laser on violet phosphorus is between 0.37 and 0.56 kW / cm 2 .

[0039] The above specific description further elaborates on the purpose, technical solution, and beneficial effects of the invention. It should be understood that the above is only a specific embodiment of the present invention and is not used to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser, characterized in that: The following steps are included: Step 1: Prepare purple phosphorus sample: peel off a purple phosphorus thin layer by mechanical peeling method and transfer it to a quartz substrate; Step 2: Using femtosecond laser to modify the purple phosphorus thin layer; Step 3: Focus the femtosecond laser onto the purple phosphorus two-dimensional material through an objective lens and then scan the line, and obtain the modified surface of the purple phosphorus two-dimensional material by multiple equally spaced scanning lines.

2. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser as claimed in claim 1, characterized in that: By irradiating purple phosphorus two-dimensional materials with femtosecond lasers, the surface of the purple phosphorus two-dimensional materials is modified, the fluorescence intensity, fluorescence lifetime and surface hardness are enhanced, the diffusion direction of excitons is restricted, and the mechanism of directional exciton transmission is used to pattern the purple phosphorus two-dimensional materials to realize customized transmission channels.

3. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser as claimed in claim 1, characterized in that: The modified surface of the processed purple phosphorus two-dimensional material was characterized by fluorescence spectroscopy, microfluorescence imaging, fluorescence lifetime imaging, atomic force microscopy and transient absorption microscopy, and the changes in the mechanical and photoelectric properties of the modified surface of the purple phosphorus two-dimensional material under different processing parameters were compared.

4. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser as claimed in claim 3, characterized in that: The ablation threshold of the modified purple phosphorus is measured by scanning the purple phosphorus by changing the laser parameters. That is, the ablation threshold is located between no change and change according to whether the optical microscope and fluorescence spectrum change after laser processing.

5. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser as claimed in claim 2, characterized in that: After femtosecond laser processing, the fluorescence intensity of purple phosphorus is enhanced and the diffusion direction of excitons is regulated. The specific implementation method is that before femtosecond laser processing, the excitons diffuse in a circular manner, and after femtosecond laser processing, the excitons diffuse only along the direction of the laser scanning line, while the diffusion in the vertical direction is limited. The fluorescence enhancement is that the diffusion of excitons is hindered and they gather locally, and they emit more light through radiation recombination, increasing the proportion of their recombination components, thereby promoting radiation luminescence and achieving the regulation of the fluorescence intensity enhancement of purple phosphorus and the diffusion direction of excitons.

6. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser as claimed in claim 1, characterized in that: In step 2, a femtosecond laser with a wavelength of 515 nm, a spot diameter of 370 nm, a repetition frequency of 1 kHz, and a power density of 0.19-0.93 kW / cm 2 .

7. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser as claimed in claim 6, characterized in that: In step three, the processing speed is selected to be 0.05-0.1 μm / s, the modification range is 10 μm×10 μm, and the scanning line spacing is 0.5-1 μm.

8. A method for surface modification of purple phosphorus two-dimensional material using femtosecond laser as claimed in claim 7, characterized in that: After femtosecond laser processing, the fluorescence intensity of purple phosphorus increased by more than 4.5 times, the fluorescence lifetime was extended, the surface hardness of the purple phosphorus two-dimensional material was improved, and the diffusion direction of purple phosphorus excitons was regulated, thereby improving the accuracy and efficiency of the surface modification of purple phosphorus two-dimensional materials.

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