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Abstract
The performance of electrodes is a key factor affecting the development of smart fabrics. The preparation of common fabric flexible electrodes has defects such as high cost, complicated preparation, and complex patterning that limit the development of fabric-based metal electrodes. Therefore, this paper presented a simple fabrication method for preparing Cu electrodes using selective laser reduction of CuO nanoparticles. By optimizing laser processing power, scanning speed, and focusing degree), we prepared a Cu circuit with an electrical resistivity of ∼ 5.53 µΩ.m. Based on the photothermoelectric properties of Cu electrodes, a white light photodetector is developed. The detectivity of the photodetector reaches ∼2.14 mA/W at a power density of 10.01 mW/cm2. This method is instructive for preparing metal electrodes or conductive lines on the surface of fabrics, and provides specific techniques for manufacturing wearable photodetectors.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
With the continuous development of optoelectronic devices [1–5], the market has high expectations for wearable devices, especially smart textiles [6–8]. One of the biggest advantages is that they could monitor the sense environmental information and human body condition in real time. Therefore, research on smart textiles and multifunctional integrated attracted growing attention [9]. Higher standards are proposed for smart textiles and their related materials to adapt to the evolution of wearable electronic devices and suit the expectations of users in the new era. Developing high-performance smart fabric electrodes is crucial because it serves as the fundamental material for producing smart textiles [10,11]. There are many traditional fabrication techniques for smart fabric electrodes. Weaving techniques [12–14], inkjet printing [15–17], screen printing [18,19], and sputtering of metal particles [20–22] are some standard methods for fabricating conductive electrodes on fabric surfaces. Still there are some disadvantages, such as time-consuming, high cost, need for masks, unique environment, etc. Direct laser writing is a technology that uses a laser to engrave patterns on the surface of a material [23–25]. This technique is often used to develop electronic devices due to its time-saving, low-cost, mask-free, simple patterning, and direct fabrication under ambient conditions [26,27]. It has been reported that laser direct writing technology can prepare electrodes by reducing metal oxides on the surface of flexible materials [28]. However, the research on the preparation of electrodes by reducing metal oxides on the surface of fabrics by laser direct writing technology is still in its early stages.
This study describes a method using selective laser reduction to create copper electrodes on lignin fibers. Due to the outstanding conductivity and affordable pricing, copper is chosen as the electrode among the common high-conductivity metal materials. When the CuO nanoparticle ink is spin-coated on the surface of the lignin fiber membrane, the CuO nanoparticles has strong adhesion due to the high permeability of the fiber membrane. The Cu electrode is effectively formed on the surface of the lignin fiber membrane after optimizing the direct laser writing conditions, and the resistivity is ∼5.53 µΩ.m. Studies have shown that Cu and lignin fiber membranes are so strongly adherent that bending them does not have a noticeable effect on their performance, nor does it cause them to detach from the electrode. Additionally, selective laser reduction has the benefits of low cost, quick manufacturing, no mask, simple operation, and no special requirements for the manufacturing environment. The preparation of conductive electrodes on the surface of fiber membranes by selective laser reduction provides some guidance for fabricating smart fabric electrodes.
2. Experimental
The experimental part describes the required materials, fabrication process, material characterization, and applications of Cu electrodes prepared by laser reduction. The lignin electrospinning, heat treatment, copper oxide coating, and laser reduction processes are mainly listed. The materials characterization equipment required for the fabricated devices are listed. As an application to Cu electrodes, a white light photodetector is fabricated.
2.1 Materials
Lignin (L195713) and polyacrylonitrile (PAN, 181315) were purchased from Aldrich Corporation. N, N Dimethylformamide (DMF, G76259B) was purchased from General-reagent. Ethylene glycol (from KESHI Co.), CuO nanoparticles (from Thermo Scientific Co.), and polyvinylpyrrolidone (PVP, average molecular weight: 1300000) were purchased from Aladdin Company. All chemicals were of analytical grade and used without further purification.
2.2 Preparation and characterization of the copper pattern
Lignin, PAN, and DMF were mixed in a mass ratio of 3:2:16 [29]. The lignin spinning dope was obtained after stirring the mixture in an oil bath at 50 °C for 5 hours. Fill the spinning solution into a 10 mL plastic syringe equipped with a 21-gauge needle and perform electrospinning at 20 kV. The drum collector speed, feed speed, and collection distance were 80 r/min, 0.05 mL/min, and 15 cm, respectively. After 7 hours of electrospinning at ambient temperature, lignin fiber membranes with a thickness of ∼200 µm were obtained (Fig. 1(a)). The lignin fiber membrane was placed in a 2 × 10−4 Pa vacuum tube furnace and heated to 450°C. The heating, holding, and cooling time were 800 minutes, 450 minutes, and 180 minutes, respectively (Fig. 1(b)). The Fig. 1(b1) shows the surface morphology of the heat-treated fiber. After the heating treatment, lignin fibers will be converted into stable structures that can be processed at high temperatures [30]. CuO nanoparticles, PVP, and ethylene glycol were mixed in a ratio of 40:13:27 [31]. The CuO ink was prepared under the joint action of oil bath stirring and ultrasonic vibration. CuO ink was evenly spin-coated on the heat-treated lignin fiber membrane (Fig. 1(c)). The as-prepared sample was placed in an oven and heated at 80°C for half an hour to prepare a CuO nanoparticle coating. The Fig. (c1) is the surface morphology after coating CuO, showing a densely packed granular microstructure. Figure 1(e) shows the CuO particle layer on the heat-treated lignin fiber surface. The membrane was directly irradiated with a 450 nm semiconductor laser, and then the CuO nanoparticles were reduced due to the instantaneous high temperature (Fig. 1(d)).
Fig. 1. The schematic diagram of the process of preparing copper patterns on the surface of lignin fibers by selective laser reduction.
The morphology of the samples was characterized by a field emission transmission electron microscope (TEM, Thermo Scientific Talos F200i), a scanning electron microscope (SEM, FEI Apreo S HiVoc, 10 kV), and an optical microscope (Keyence VHX-600). The chemical composition of the samples was checked by energy-dispersive spectrometry (EDS, FEI Apreo S HiVoc), X-ray diffractometry (XRD, Rigaku Ultima IV, Cu Kα radiation), and x-ray photoelectron spectroscopy (XPS, Kratos AXIS Supra). The resistance of the sample was measured by a SourceMeter (Keithley DMM6500).
2.3 Preparation and characterization of the photodetector
We fabricated white-light photodetectors based on lignin fiber membranes to further explore the application of Cu electrodes prepared by laser reduction. Photodetectors were fabricated by patterning laser-induced reduction CuO layers on heat-treated lignin fiber membranes. The i-t and i-v curves of the photodetectors were measured by an electrochemical workstation (Chenhua, CHI660E). The testing light source of the photodetector is a white light source (from Xuyu Optoelectronics Co.). The temperature characteristics of the device with irradiation time were tested by a thermal imaging temperature sensor (Haiman HTPA 32 × 32dR2L5.0).
3. Results and discussion
Different laser parameters during the laser reduction of CuO have varying influence on the reduction outcomes. The parameters of laser reduction power, scanning speed, and focusing degree are therefore explored in this section. The optimum laser reduction parameters are established after exploration. Next, the Cu electrode was expanded for use in a white light detector. The performance of a white-light photodetector is evaluated after it is manufactured.
3.1 Material characterization and process parameter optimization
The morphology of CuO ink is observed by TEM, and its size is between 80 nm - 200 nm (Fig. 2(a)). Figure 2(b-d) display the XRD patterns of lignin fiber, CuO ink and Cu prepared by laser reduction, respectively. Both lignin fibers and CuO nanoparticles are pure and unadulterated materials (Fig. 2(b) and Fig. 2(c)). XRD pattern of laser irradiated CuO nanoparticles in Fig. 2(d) show major diffraction peaks at 43.08°, 50.48° and 74.16°, which can be assigned to the (111), (200) and (220) planes of the structure of pure Cu [32].
Fig. 2. (a) TEM image of CuO ink, XRD patterns of (b) lignin fiber, (c) CuO and (d) Cu generated by laser reduction of CuO.
Responsivity (R), detectivity (D*), and noise equivalent power (NEP) are the key parameters to evaluate the photodetector [25,33,34]. Then R is calculated by the following equations:
Where Ii and Id represent the current under laser irradiation and dark conditions. P, Ee and A are the laser power, irradiation power density and effective channel area, respectively. Therefore, the D* and NEP can be calculated by the following equations: where e is the charge of electrons. D* and NEP curves are the functions of Ee. Different currents Ii will be detected at different radiant energy densities [35].During the preparation of the Cu layer, the parameters of the laser irradiation play a critical role. Figure 3 explores the optimal scanning speed and power for laser writing. Figure 3(a) shows the SEM images of Cu layer at different laser scanning speeds and powers, and the minimum resistivity at each scanning speed is plotted in Fig. 3(b). After confirming that the minimum resistivity is achieved at a scanning speed of 1000 mm/min, the characteristics of EDS and XPS are performed. A 500× magnification of a 25 mm2 square copper pattern fabricated by laser reduction of CuO is shown in Fig. 3(a). The copper pattern is reduced with the change in laser irradiation power and scanning speed. The figure shows that the laser energy irradiated on the CuO surface is insufficient at low power or high speed, making it challenging to convert CuO to Cu. Excessively high laser energy causes intense laser-induced photothermochemical reduction at high power and low scanning speed. The surface of the material and even the internal fibers are destroyed. Therefore, a Cu layer with a higher reduction effect can be obtained with a suitable combination of laser power and scanning speed. Figure 3(b) illustrates the power value corresponding to the minimum resistivity measured at each scanning speed. Therefore, three typical power values at a scanning speed of 1000 mm/min are characterized in Fig. 3(c–e). The results of the EDS measurement reveal that the copper produced is only a thin line at a laser power of 0.13 W. When the power is 0.22 W, the copper layer is all over the entire surface. While CuO is destroyed and the inner lignin fibers are exposed at a laser power of 0.32 W (Fig. 3(c)). The corresponding ratios of C, Cu, and O elements under the three laser powers (Fig. 3(d)) show the same conclusion. The proportion of copper reaches 88% at the laser power of 0.22 W, which is the highest among the three laser powers. The C, O, and Cu peaks in the XPS measurement (Fig. 3(e)) show a similar pattern to Fig. 3(d).
Fig. 3. Determination of optimum laser power and scan rate. (a) Microscope images under different scan speeds and powers; (b) The scan rate-resistivity diagram of the power with the smallest resistance at different scan rates; (c) The EDS diagrams of three different powers and (d) the ratio of C, N, O at a scanning speed of 1000 mm/min; (e) XPS diagrams of three different powers at a scanning speed of 1000 mm/min.
After obtaining the power and speed parameters for the best reduction effect, the extent of focus is concerned with affecting the concentration of laser energy. Therefore, optimizing the laser processing parameters by combining the extent of focus to fabricate highly conductive, high-quality Cu patterns is necessary. The extent of focus can assume values of 0 (on-focus condition), positive (under-focus condition), or negative (over-focus condition). The SEM images of the extent of focus between −3 mm and 2 mm are shown in Fig. 4(a). It caused the large spacing between lines because of the concentration of laser energy at the focal point. At under-focus and over-focus positions, flat surfaces and small gaps result from the gentle laser radiation. The copper layer thickness at different focus positions was also tested (Fig. 4(b)), and the copper layer thickness was the thickest at the focus point. As the degree of deviation increases, the thickness of the copper layer decreases. After the resistivity measurement, the minimum resistivity is obtained at the point where the focus position is −1, which is ∼5.53 µΩ.m. This position has more uniform pyrolysis while maintaining the degree of laser reduction.
Fig. 4. Determination of the optimal degree of laser focus. (a) SEM images, (b) thickness and (c) resistivity of copper layers under different focusing degrees.
After optimizing the ideal laser power, scanning speed and extent of focus, the interdigitated electrode pattern fabricated with the best parameters of 0.22 W, 1000 mm/min, and −1 mm over-focus is shown in Fig. 5(a-b). In the enlarged SEM image, Cu exists in the form of aggregated particles (Fig. 5(c)), and the pores between the particles are caused by the volatilization of secondary products such as diacetyl and water during the laser reduction of CuO [31]. The EDS diagrams of C, O and Cu are displayed in Fig. 5(d-f), and their contents are 3%, 4% and 90%, respectively. In addition, the permeability of CuO ink in lignin fibers resulted in good adhesion of the Cu layer. However, as a flexible electronic device, the resistivity and adhesion of the Cu layer are worth exploring after the device is bent. As shown in Fig. 5(g), the device is bent to different degrees. The resistivity of the device increases from the initial value of ∼5.53 µΩ.m to ∼16.63 µΩ.m as the bending radius decreases. This corresponds to a slight increase in the size and number of cracks on the device surface (white dashed lines outline cracks) (Fig. 5(h)). A few cracks are within an acceptable range, and the Cu layer does not fall off on the surface of the device.
Fig. 5. (a) Physical picture and (b) Microscope image of Cu electrode, (c) SEM image and (d-f) EDS image under optimal parameters; (g) SEM image and (h) resistivity curves under different bending states.
3.2 Photodetector performance characterization
As for applying copper electrodes, a photodetector is fabricated by patterned laser irradiation, as shown in Fig. 6(a), and its area is ∼1.7 cm2. The detection mechanism of photodetectors prepared by laser reduction of copper oxide is based on the photothermoelectric effect, which combines photothermal and thermoelectric impact [36–38]. When white light irradiates the photodetector, the distribution of light on the surface of the device is uneven. The material absorbs the photons and strengthens the lattice vibrations inside the material, increasing the temperature of the device. The temperature difference between the hot end with high incident light intensity and the cold end with low incident light intensity is caused by the non-uniform light intensity distribution. This creates a temperature gradient inside the device, resulting in a carrier concentration difference between the cold and hot ends of the material. The carrier concentration difference drives the carriers to move directionally from the hot end to the cold end and accumulates to form a built-in potential difference that generates a photocurrent when an external wire is connected.
Fig. 6. Application: photodetector. (a) Device diagram, the inset is a photo of the Cu photodetector; (b) The curve of the surface temperature of the photodetector as a function of time; (c) The i-v curves under different irradiation times (power density of 336.96 mW/cm2); (d) The temperature distribution of the device surface as a function of position under white light irradiation; (e) The i-t curve of the optical switch with a power density of 502.5 mW/cm2; (f) Temporal response of the photodetector with a rise and fall time of 0.2 s; (g) Comparison of carbonized lignin photodetectors and Cu photodetectors at a power density of 336.96 mW/cm2.
The device surface temperature is analyzed as a function of the light exposure time (Fig. 6(b)). The results show that the temperature of the device surface increases continuously with the increase of irradiation time. The temperature of the device surface rises to 50.1 °C after 149 s of irradiation. Photodetectors exhibit a good, positive, linear relationship between current and voltage. It indicates that a stable carrier movement process is formed inside the device with the increase of the incident time. In addition, it also shows a good ohmic contact between the active layer and the electrodes. According to Fig. 6(c), as the irradiation time increase, the photocurrent increases, and the device's resistance decreases. That is, an increase in slope represents a decrease in the resistance of the device. It has been demonstrated that an increase in irradiation time causes the device's temperature to rise steadily, that the thermal energy causes more electrons in the valence band to be excited into the conduction band, and that an increase in carriers leads to a decrease in the resistance of the device and an increase in the photocurrent. To further demonstrate the PET mechanism of the device, the temperature rise at different locations on the device surface was tested under white light irradiation (Fig. 6(d)). Due to the inhomogeneity of illumination, there is an obvious temperature gradient in the device, resulting in a difference in carrier concentration, driving the directional movement of carriers, and then generating photocurrent.
At a 0 V bias voltage, the optical switching characteristics of the device under white light irradiation of 502.5 mW/cm2 are shown in Fig. 6(e). The photodetector depicts stable and repeatable photocurrents with on- and off-curves. According to the expression of responsivity R in formula (1), high power density (Ee) illumination causes high Ii, and low power density illumination causes low Id. Changes in Ii and Ee lead to changes in R, reflecting the responsivity performance. In general, photoresponse time is an important parameter to evaluate the performance of a photodetector, including the rise and fall times. The rise and fall times of the device are approximately 0.17 and 0.09 s, respectively (Fig. 6(f)). At a power density of 336.96 mW/cm2, the photocurrent of the Cu photodetector prepared by laser irradiation reduction is ∼35 times higher than that of the laser carbonized lignin photodetector, and the photocurrents of the two devices are ∼0.456 mA and ∼0.013 mA, respectively (Fig. 6(g)).
To further probe the photodetector's photoelectric properties, the device's optical switching photocurrent was measured at different power densities (Fig. 7(a)). The black curve in Fig. 7(b) describes the photocurrent of the device. The device's photocurrent increases with the white light power density. As the photothermal effect explains, the light-induced temperature gradient becomes more significant as the incident light power density increases, thus increasing the photocurrent. The red curve in Fig. 7(b) shows that the responsivity of the photodetector decreases with the increase in optical power density. The highest responsivity of the device reaches ∼2.14 mA/W at a power density of 10.01 mW/cm2. Figure 7(c) reveals the curves of detectivity (D*) and noise equivalent power (NEP) as a function of optical power density. NEP represents the sensitivity of the photodetector, that is, the ability to detect weak light. D* and NEP are reciprocals of each other [39]. D* and power density are negatively correlated, and NEP is positively correlated with power density.
Fig. 7. Application: photodetector. (a) Optical switching i-t curves of devices at different power densities; (b) Photocurrent and responsivity (R) of devices at different power densities, (c) Detectivity (D*) and noise equivalent power (NEP); (d) The i-t diagram of the optical switch under different bending radii (power density of 64.56 mW/cm2); (e) The i-t curve of the cyclic optical switch at 2000 s and (f) the partially enlarged view at a power density of 40.12 mW/cm2.
As a flexible device, it is necessary to test the bending characteristics of the device. Figure 7(d) describes the optical switching characteristics of the device as a decrease in bending radius. Figure 7(e) depicts the i-t light switching curve for a 2000 s cycle, and the enlarged plot shows the regular i-t light switching diagram (Fig. 7(f)). The curves show that bending does not significantly impact the photocurrent, proving that the flexible photodetector has good flexibility. In addition, the smooth cycle curve indicates the stability of the photodetector.
4. Conclusions
In this paper, we proposed a method of fabricating Cu electrodes on the surface of fabrics by laser reduction and applied the electrodes to photodetectors. The preparation process is simple, fast, easy to pattern, and does not require a mask. CuO nano-ink was coated on the surface of lignin, and Cu was reduced by laser irradiation. After optimization of the laser parameters, it was determined that the highest resistivity of Cu was ∼5.53 µΩ.m at a scan speed of 1000 mm/min, a power of 0.22 W, and an extent of focus of −1 mm. The prepared copper electrode was applied to a photodetector with a responsivity of ∼2.14 mA/W (at a power density of 10.01 mW/cm2). This fabrication method can be used for various smart textile devices’ electrodes and conductive lines. This approach has excellent flexibility and can guide the development of flexible fabrics.
Funding
Sichuan Province Science and Technology Support Program (2022YFG0361); National Natural Science Foundation of China (61905168, 61905169, 61975137).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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