Photonic Sintering Mechanism Of Copper Nanowire Conductive Film And Its Device Applications

Conductive films are one of the most essential components in electronics and can be applied in various fields, such as display screen, touch screen, solar cell, light-emitting diode (LED) et al. Conducitive films are also expected to be used in flexble displays and wearable electronics. However, the traditional material for conductive films is indium tin oxide (ITO), which is expensive, limited supplement and fragile. Copper nanowires (CuNWs) have been considered to be one of the most promising althernatives to replace the traditional indium tin oxide (ITO) for fabrication of conductive films, and received more and more attentions due to its low price, rich storage and excellent conductivity. Thus, new technique, which is simple, fast, without application of reducing and corrosive atmosphere, is urgently needed to fabricate CuNW conductive films.This dissertation presented a novel and rapid photonic sintering technique to fabricate CuNW transparent conductive films (TCFs). The photonic sintering mechanism was proposed by analyzing the microstructure and the surface composition of the CuNW network after photonic sintering. Meanwhile, the photonic sintering technique was used to fabricate CuNW stretchable conductive films (SCFs) on polymer substrates and the bending and stretching properties of the CuNW SCFs were systemly examined. In addition, the applications of CuNW SCFs on wearable sensor and heater were realized.High-quality CuNWs were synthesized by a green hydrothermal method using octadecylamine (ODA) as surfactant. The as-prepared CuNWs were with diameters of about 40 nm and lengths up to 50μm. The growth mechanism of CuNWs by the hydrothermal method was proposed. The morphology of the CuNWs were greatly depended on the molecular weight of the amines, which was worked as surfactantin the reaction. During the hydrothermal process, a copper-amine complex was gerated by long carbon length amines and copper ions, which improved the nucleation and growth of one-dimensional CuNWs. Through the XPS (X-ray photoelectron spectroscopy) and HRTEM (High-resolution transmission electron microscope) analysis, there were copper oxides on the surface of fresh CuNWs. The oxidation on the surface of CuNWs would significantly decrease the conductivity of CuNWs.CuNW TCF with transmittance of 85% and sheet resistance of 34.1 Ohm/sq on glass substrate was successfully prepared by the fast photonic sintering technique. The mechanism of the photonic sintering of CuNW TCFs was revealed. During the process, the light energy was transferred into heat energy to sinter CuNWs into connected network and the contact areas between them were greatly increased. Due to the surface plasmon resonance (SPR) effect, the adsorption of light for CuNWs was enhanced at the cross spots, which promoted the joning between CuNWs. Moreover, ODA on the surface of CuNWs was photolyzed into reducing organics to reduce copper oxdes. The purified CuNWs leaded to more sufficient Cu-Cu contacts. Benefit from the joining between CuNWs and the photo-deoxidation of CuNWs, photonic sintering technique became a fast, simple and effective method to fabricate high performance CuNW conductive films.CuNW TCFs on flexible polyethylene terephthalate (PET) substrates were fabricated by photonic sintering method. During the fabrication process of CuNW TCF on N-PET (Normal polyethylene terephthalate) polymer substrate, the N-PET polymer was easily damaged by the high intensity light, which decreased the transmittance of CuNW films. By using surface treated C-PET (Commercial polyethylene terephthalate) polymer, the thermal damage was avoided and flexible CuNW TCF with transmittance of 84% and sheet ressitance of 53.5 Ohm/sq was successfully obtained.Photonic sintering technique was applied on fabrication of CuNW SCFs on elastic polyurethane (PU) substrates for flexible electronics. The results showed the CuNWs were embedded into the surface of PU polymer during the photonic sintering process, which was benefit for the mechanical robostness of the SCFs. The mechanical measurement indicates the CuNW/PU SCF only doubled its resistance even after 1000 cycles of stretching/releasing under 10% strain. The results of fabrication CuNW SCFs on polydimethylsiloxane (PDMS) substrates show that the conductivity of CuNW/PDMS SCFs were quickly declined due to the weak adhesion between CuNWs and PDMS even after photonic sintering. The adhesion could be improved by recoating PDMS solution on the surface of CuNW/PDMS SCFs. Afer the recoating treatment, the resistance of CuNW/PDMS SCF was increased to 1.8 times of original one after 1000 cycles stretching/releasing at 20% strain.At last, two flexible electronic devices, wearable sensor and heater, were fabricated using the stretchable CuNW/PU SCFs. CuNW/PU SCFs were attached on the joint of fingers to output different electrical signals when the finger was bended and straightened, which could be applied on gesture detection of fingers. The CuNW/PU SCFs could be used as heater and the temperature on the film achieved 46℃ when input voltage was 3 V. The success of intergration of these two devices demonstrated that CuNW conductive films could be applied in information tranfer, clothes, and medical supplies fields.