Application of silicon nanowire in latent fingerprint development

Abstract

The invention provides a preparation method of a silicon nanowire array. The preparation method comprises the following steps: taking clean raw materials; 100 gt; the preparation method comprises the following steps: placing an N-type silicon wafer with crystal orientation in a hot H2SO4 / H2O2 solution for processing, cleaning, then placing in a mixed solution of HF and AgNO3 for etching, then removing an Ag film and an oxide layer, and carrying out post-processing to obtain the silicon nanowire array. The invention also provides application of the silicon nanowire obtained by the preparation method in latent fingerprint development. The specific surface area of the silicon nanowire can reach 323.47 m < 2 > / g, the silicon nanowire has an adjustable luminescence characteristic and a surface fluorescence effect, signals of trace fingerprint residues can be enhanced through photoluminescence, high-sensitivity imaging under different laser intensities of 350-700 nm can be realized, latent fingerprints on complex objects (such as leather) can be clearly displayed, the silicon nanowire is safe and environment-friendly, chemical pretreatment on a sample is not needed, and the preparation method is simple and convenient. And the requirements of different scenes and different objects can be met.

Description

Technical Field

[0001] The present invention relates to the technical field of latent fingerprint visualization, and in particular to the application of silicon nanowires in latent fingerprint visualization. Background Art

[0002] Existing latent fingerprint visualization technologies primarily include chemical methods (such as the ninhydrin method and the silver nitrate method), physical methods (such as the powder brush method), and optical methods (such as laser excitation). These traditional methods suffer from common technical flaws, including insufficient sensitivity: they are ineffective for revealing latent fingerprints on old or complex backgrounds or on special materials (such as leather); high operational hazards: some chemical reagents (such as ethyl cyanoacrylate) are toxic, and the dust or gas generated during the process is harmful to the human body; and the risk of damaging physical evidence: some methods may contaminate or damage fingerprint residues, affecting subsequent DNA analysis.

[0003] Silicon nanowires are one-dimensional semiconductor materials with a diameter of less than 100 nm and unlimited length, prepared from single-crystal silicon. Due to their slender nanoscale diameter, they have unique physical and chemical properties. For example, due to the size effect, they may have unique absorption peaks in multiple bands, so they can be used in applications such as optical sensors. Silicon nanowires can be prepared using methods such as chemical vapor deposition (CVD) or laser ablation. For example, Chinese invention patent application CN 107634005A discloses a method for preparing silicon nanowire arrays based on metal-assisted chemical etching technology. The method deposits Ag nanoparticles on a silicon wafer, then etches it in a mixed solution of hydrofluoric acid and hydrogen peroxide. After removing the remaining Ag particles on the silicon surface, the oxide layer on the silicon wafer surface is removed, and finally disordered silicon nanowires with a length of 2 to 14 μm and a width of 50 to 150 nm are obtained. However, according to the records of the prior art, silicon nanowires can be used in optical cells or optical sensors (for example, Li et al. produced a silicon nanowire array with a length of 219.4 μm). 2 ·g -1 High surface area silicon nanowires can be used in lithium battery anodes (Li, Xiaopeng et al. Stable Silicon Anodes for Lithium-Ion Batteries Using Mesoporous Metallurgical Silicon. Advanced Energy Materials, 5(4), n / a–n / a.doi:10.1002 / aenm.201401556), but there is no report on their application in revealing latent fingerprints or other latent traces.

[0004] However, the inventors discovered that due to the nanometer-scale size of silicon nanowires, quantum confinement effects are significant, leading to an increased band gap and partially suppressing the indirect band gap characteristics, thereby improving photoluminescence efficiency. Furthermore, although the high surface area to volume ratio makes surface states non-radiative recombination centers, which reduces luminescence intensity, surface passivation can reduce surface defects and enhance luminescence. Therefore, there is an urgent need to improve the preparation method to obtain a new silicon nanowire and verify its application in the field of latent trace visualization. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects of the prior art and apply silicon nanowires to latent fingerprint revealing technology.

[0006] The idea of ​​the present invention is to improve the preparation method and control the oxidation steps and etching parameters in the preparation process of silicon nanowire arrays to prepare silicon nanowires with larger specific surface area and stronger photoluminescence efficiency.

[0007] In order to achieve the above object, the present invention provides a method for preparing a silicon nanowire array, the method comprising the following steps:

[0008] (1) Cleaning

[0009] Will <100> The N-type silicon wafers were ultrasonically cleaned in acetone, ethanol and deionized water respectively.

[0010] (2) Oxidation

[0011] The cleaned silicon wafer was placed in a 96°C H2SO4 / H2O2 solution for 1-2 minutes, and then the silicon wafer was removed and rinsed thoroughly with deionized water; the H2SO4 / H2O2 solution contained 97% H2SO4 and 30% H2O2 at a volume ratio of 3:1;

[0012] (3) Etching

[0013] In a polytetrafluoroethylene-lined autoclave, the silicon wafer obtained in step (2) is placed in a mixed solution of HF and AgNO3, and then reacted in a dark environment at 20-50°C for 20-900 minutes to obtain an etched silicon substrate;

[0014] (4) Removal of Ag film

[0015] The etched silicon substrate was immersed in HNO3 solution to fully soak it to remove the excess Ag film on the silicon substrate, and then the silicon substrate was thoroughly rinsed with deionized water;

[0016] (5) Removal of oxide layer

[0017] The obtained silicon substrate is transferred to an HF solution and reacted for 20-100 seconds to remove the oxide layer and control the length and diameter of the etched silicon nanowires;

[0018] (6) Post-processing

[0019] The silicon substrate was fully rinsed with ethanol and deionized water respectively, and then vacuum dried to obtain a silicon nanowire array.

[0020] In the present invention, the resistivity of the N-type silicon wafer is generally 0.001-0.005 Ω·cm.

[0021] In the present invention, before performing the cleaning in step (1), the N-type silicon wafer can be cut into small pieces of 2.0 × 2.0 cm in size, and then ultrasonically cleaned. Typically, the cleaning is performed three times, each in acetone and ethanol for 5 minutes, followed by a 5-minute treatment in deionized water (resistivity 18.25 MΩ·cm).

[0022] The thickness of the N-type silicon wafer used in the present invention is 500 μm.

[0023] In the present invention, placing the silicon wafer in a heated H2SO4 / H2O2 solution in step (2) can form a uniform oxide layer on the surface of the silicon wafer, which plays a role in unifying the etching rate of the silicon nanowires in the subsequent steps. The uniform etching rate is conducive to the formation of relatively uniform silicon nanowires.

[0024] In the present invention, during etching in step (3), the etching solution is filled to 80%-85% of the total volume of the autoclave, and then placed in an electric heated forced air drying oven to react in a dark environment at 20-50°C. During the reaction, a thick gray-green Ag film can be observed covering the surface of the etched silicon substrate.

[0025] The reaction time of the etching step affects the average length of the silicon nanowires. If the etching time is too short, the average length of the resulting product is too small or even unattainable. However, the inventors have found that the relationship between etching time and silicon nanowire length is not linear. Generally, controlling the etching reaction time to 20 to 90 minutes is beneficial for obtaining silicon nanowires with an average length of 2 to 5 μm.

[0026] In addition, the temperature of the etching reaction also affects the average length of the resulting product, but the relationship between temperature and length is also nonlinear. Although it is possible to obtain silicon nanowires with an average length of 2-5μm at higher temperatures, for safety, energy saving, and environmental considerations, the reaction is carried out in a dark environment at 20-50°C to ensure that the average length of silicon nanowires is 2-5μm.

[0027] According to a preferred embodiment, the mixed solution of HF and AgNO3 contains 4.6 M / L HF and 0.025 M / L AgNO3. The HNO3 solution contains HNO3 and deionized water in a volume ratio of 1:1. The mass concentration of the HF solution in step (5) is 5%. Those skilled in the art may also appropriately adjust the concentration of the solution used in the preparation method according to the teachings of the prior art.

[0028] Preferably, in step (5), the obtained silicon substrate is transferred to the HF solution and reacted for 20 to 100 seconds, preferably 30 to 75 seconds. Those skilled in the art can control the length and diameter of the etched silicon nanowires by controlling the reaction time.

[0029] Transmission electron microscopy showed that the silicon nanowires prepared by the above preparation method had a length of 2 to 5 μm and a diameter of 40 nm to 1 μm.

[0030] Furthermore, the present invention also provides application of silicon nanowires in latent fingerprint visualization.

[0031] Furthermore, the present invention also provides a method for revealing latent fingerprints using silicon nanowires, the method comprising the following steps:

[0032] (1) Attaching silicon nanowires to an object with latent fingerprints;

[0033] (2) Exciting light is used to illuminate the area on the object where the silicon nanowires are attached to obtain a latent fingerprint image.

[0034] In the present invention, the objects include plastic material objects, silicon material objects, wooden material objects, metal material objects or paper material objects, such as tape, glass, wooden tabletop, knife surface, banknotes, and leather surface can all be objects of the present invention.

[0035] In the present invention, the wavelength of the excitation light used to excite the silicon nanowires is 350nm-700nm, especially 350-370nm ultraviolet light, 400-420nm violet light, 440-460nm blue light, 525-535nm green light, or 400-700nm white light.

[0036] In addition, the specific surface area of ​​the obtained silicon nanowires was determined to be 323.47 m 2 / g. Due to their high specific surface area, tunable luminescence properties, and surface fluorescence, the silicon nanowires of this invention offer an innovative solution for latent fingerprint visualization. Leveraging their surface oxide layer and high specific surface area, the silicon nanowires of this invention enhance the signal from trace fingerprint residues through photoluminescence, enabling high-sensitivity imaging at varying laser intensities from 350nm to 700nm.

[0037] Because the luminescence properties of silicon nanowires are dominated by quantum confinement at low laser intensities, thermal effects and nonlinear processes significantly affect performance at high intensities. This invention modulates their luminescence behavior by controlling the length and diameter of the silicon nanowires and oxidizing the material surface to achieve a micro-nanostructure.

[0038] Compared with existing latent fingerprint visualization technologies, the method of the present invention has higher sensitivity and can clearly visualize latent fingerprints on complex objects (such as leather). The resulting image clearly shows detailed features such as papillary lines and fingerprint triangles. The method of the present invention is safe and environmentally friendly, and does not require chemical pretreatment of the sample, thus avoiding contamination of physical evidence or harm to the human body. The method of the present invention is simpler to operate and only requires a laser light source with adjustable parameters to meet the needs of different scenarios and different objects. The method of the present invention has better compatibility, for example, it can be seamlessly connected with existing physical evidence discovery equipment (such as laser full survey instruments), and is widely practical and easy to promote. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the method of the present invention;

[0040] Figure 2 is a morphology diagram of silicon nanowires, where a is a top view of the silicon nanowire array, b is a side view of the silicon nanowire array, and c is a single nanowire image under a transmission electron microscope;

[0041] Figure 3 : The imaging results of the tape sample of Example 3 under different wavelength excitation light, where ae is excited by 400-460nm blue light, 525-535nm green light, 350-370nm ultraviolet light, 400-420nm violet light and 400-700nm white light, respectively; fj is the excitation of the tape sample after covering the silicon nanowires with 440-460nm blue light, 525-535nm green light, 350-370nm ultraviolet light, 400-420nm violet light and 400-700nm white light, respectively;

[0042] Figure 4 These are the imaging results of different silicon nanowire samples in Example 5, where the upper image is for an etching time of 20 minutes, and the lower image is for an etching time of 900 minutes. DETAILED DESCRIPTION

[0043] The following examples are used to illustrate the technical solutions of the present invention in a non-limiting manner.

[0044] Example 1 Preparation of silicon nanowires

[0045] Take a 500 μm thick <100> N-type silicon wafers were cut into 2.0 × 2.0 cm slices. The wafers were ultrasonically treated in acetone (5 minutes once), ethanol (5 minutes once), and deionized water (5 minutes three times) to remove impurities.

[0046] Remove the silicon wafer and blow dry it with nitrogen. Then, place the silicon wafer in a 96°C H2SO4 / H2O2 solution for 1 minute to oxidize it, forming a uniform oxide layer on the surface of the silicon wafer. After removing the silicon wafer, rinse it with deionized water three times.

[0047] The cleaned silicon wafer was placed in a mixed solution of 4.6M / L HF and 0.025M / L AgNO3, and etched in a stainless steel autoclave with a polytetrafluoroethylene liner. The etching solution was filled to 80% of the total volume of the autoclave, and then placed in an electric forced air drying oven. The reaction was carried out at 50°C in the dark for 2, 5, 10, 20, 60, 90, 120, 300, 480, 900, and 1080 minutes to investigate the size of the resulting silicon nanowires.

[0048] After removing the silicon substrates, they were immersed in a 1:1 by volume HNO₃ solution in deionized water for 4 hours to remove the dendritic Ag film. The wafers were then rinsed thoroughly with deionized water. The wafers were then transferred to a 5% HF solution for 30 seconds to remove newly oxidized SiO₂ from the surface.

[0049] Finally, the silicon substrate was fully rinsed with deionized water and ethanol, and vacuum dried at 70°C for 8 hours to obtain the desired silicon nanowire array.

[0050] The nanowire arrays on the surface of each silicon wafer were scraped off to obtain a yellow powder. The average length of the powder was measured and the average value was obtained after repeating this process five times. The lengths of the samples obtained at different etching times are shown in Table 1:

[0051] Table 1 Different etching times and etched mesoporous silicon nanowire lengths

[0052]

[0053]

[0054] As can be seen from Table 1, when the etching time is too short (for example, 2 minutes), silicon nanowires cannot even be obtained. When the etching time is 5 to 10 minutes, although silicon nanowire powder can be harvested, its average length is relatively short. When the etching time is 20 to 900 minutes, silicon nanowires with an average length of 2.900 to 76.746 μm can be obtained. When the etching time is further extended to 1080 minutes, it is observed that the silicon nanowires basically disappear, that is, silicon nanowires cannot be obtained. The experimental results show that although the reaction time of the etching step is related to the average length of the silicon nanowires, the relationship between the two is not linear. Finally, an etching time of 20 to 90 minutes is selected to obtain silicon nanowires with an average length of 2 to 5 μm.

[0055] Select a silicon nanowire array with an etching time of 20 minutes and place it under an electron microscope for observation. Figure 1 As shown in (a, b), vertical arrangement of silicon nanowires can be seen on the silicon substrate, with a length of about 2 μm.

[0056] The nanowire powder sample hanging from the surface of the silicon wafer was placed under a TEM electron microscope for observation. Figure 1 As shown in (c), its diameter is about 40 to 50 nm.

[0057] The specific surface area of ​​the powder sample was measured by BET test method / equipment, and the specific surface area reached 323.47m 2 / g.

[0058] Example 2 Investigating the Effect of Different Etching Temperatures on the Length of Silicon Nanowires

[0059] The same process as in Example 1 was performed, except that the etching time was selected as 20 minutes. The average length of the obtained product was observed when the etching temperature was 20, 30, 50, 60, and 90°C, as shown in Table 2.

[0060] Table 2 Different etching temperatures and etched mesoporous silicon nanowire lengths (20 minutes)

[0061]

[0062] As shown in Table 2, etching temperature affects silicon nanowire length, but the relationship is not linear. Silicon nanowires with average lengths of 2.818 to 5.505 μm can be obtained at etching temperatures of 20°C to 50°C and 90°C. Therefore, 20°C to 50°C is the preferred etching temperature.

[0063] Example 3: Investigation of the effect of silicon nanowires on latent fingerprints under various wavelengths of excitation light

[0064] Adhesive tape, a common tool used by criminals, was used to create a tape sample with multiple fingerprints left on its smooth surface. A small amount of silicon nanowire powder prepared in Example 1 (etching time: 20 minutes) was evenly applied to the fingerprint area. The latent fingerprint area was illuminated with 440-460nm blue light, 525-535nm green light, 350-370nm ultraviolet light, 400-420nm violet light, and 400-700nm white light, and photographed. A tape sample not coated with silicon nanowire powder served as a control.

[0065] The experimental results are as follows Figure 3 shown.

[0066] The results showed that compared with the control group using only multi-band light source, due to the strong fluorescence of silicon nanowires, after the fingerprint was revealed, a small amount of silicon nanowires were adsorbed on the fingerprint, and a clear fingerprint image was presented under the illumination of multi-band light source, and the interference of the object background was relatively easy to remove. For the tape sample, the blue fingerprint ( Figure 3 h) The clearest, the fingerprint has a large contrast with the color background, the fingerprint lines are obvious, there is no line adhesion phenomenon, with excellent imaging quality, can clearly identify such as combination, divergence and small sticks ( Figure 3 Detailed fingerprint features such as the red circle in h are conducive to information recognition.

[0067] Example 4 investigates the effects of lasers of different wavelengths on the appearance of latent fingerprints on the surfaces of different materials

[0068] Using the same method as in Example 3, fingerprints were left on a glass sheet, a wooden tabletop, a metal knife surface, and a banknote surface to prepare samples. The latent fingerprints on the surfaces of the different samples were examined when 400-460 nm blue light, 525-535 nm green light, 350-370 nm ultraviolet light, 400-420 nm violet light, and 400-700 nm white light were used as excitation light. The results are shown in Table 1.

[0069] Table 1 Latent fingerprint appearance on different sample surfaces under different lasers

[0070]

[0071]

[0072] Example 5: Investigating the effect of silicon nanowires prepared with different etching times on the latent fingerprint

[0073] Using the same method as in Example 3, fingerprints were left on the surface of banknotes to prepare samples, and the imaging effects of the silicon nanowire powder samples obtained in Example 1 were examined.

[0074] It was observed that some fingerprint features could be observed in the silicon nanowire powders obtained with different etching times under excitation light, but the imaging of silicon nanowires with an average length of 2 to 5 μm (corresponding to etching time of 20 to 90 minutes) was clearer and the effect was more prominent.

[0075] like Figure 4 The imaging effects of the powder samples obtained with etching times of 20 minutes and 900 minutes respectively.

[0076] In summary, the silicon nanowires of the present invention can be applied to different object surfaces, significantly providing a latent fingerprint revealing effect on the object surface, and have good sensitivity and low background interference.

Claims

1. A method for preparing a silicon nanowire array, comprising the following steps: (1) Cleaning Will <100> The N-type silicon wafer with the same crystal orientation was ultrasonically cleaned in acetone, ethanol and deionized water respectively; (2) Oxidation The cleaned silicon wafer was placed in a 96°C H2SO4 / H2O2 solution for 1-2 minutes, and then the silicon wafer was removed and rinsed thoroughly with deionized water; the H2SO4 / H2O2 solution contained 97% H2SO4 and 30% H2O2 at a volume ratio of 3:1; (3) Etching In a polytetrafluoroethylene-lined autoclave, the silicon wafer obtained in step (2) is placed in a mixed solution of HF and AgNO3, and then reacted in a dark environment at 20-50°C for 20-900 minutes to obtain an etched silicon substrate; (4) Removal of Ag film The etched silicon substrate was taken out and immersed in HNO3 solution to remove the excess Ag film on the silicon substrate, and then the silicon substrate was thoroughly rinsed with deionized water; (5) Removal of oxide layer The obtained silicon substrate is transferred to an HF solution for reaction to remove the oxide layer; (6) Post-processing The silicon substrate was thoroughly rinsed with ethanol and deionized water, respectively, and then vacuum dried to obtain a silicon nanowire array.

2. The preparation method according to claim 1, wherein The mixed solution of HF and AgNO 3 contains 4.6 M / L HF and 0.025 M / L AgNO 3 .

3. The preparation method according to claim 1, wherein The HNO 3 solution contains HNO 3 and deionized water in a volume ratio of 1:

1.

4. The preparation method according to claim 1, characterized in that The mass concentration of the HF solution in step (5) is 5%.

5. The preparation method according to claim 1, characterized in that In the step (5), the obtained silicon substrate is transferred into an HF solution and reacted for 20-100 seconds.

6. The silicon nanowire prepared by the preparation method according to any one of claims 1 to 5, characterized in that The silicon nanowire has a length of 2 to 40 μm and a diameter of 40 nm to 1 μm.

7. Use of the silicon nanowires according to claim 6 in latent fingerprint visualization.

8. A method for revealing latent fingerprints using silicon nanowires, the method comprising the following steps: (1) Attaching silicon nanowires to an object with latent fingerprints; (2) Exciting light is used to illuminate the area on the object where the silicon nanowires are attached to obtain a latent fingerprint image.

9. The method according to claim 8, characterized in that The object includes a plastic material object, a silicon material object, a wooden material object, a metal material object or a paper material object.

10. The method according to claim 8, characterized in that The wavelength of the excitation light is 350nm to 700nm.

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