Objective With the rapid development of information technology, aerospace technology, biotechnology and other fields, there are more demands for the miniaturization, integration, low power consumption and application of its equipment. Due to the unique small size effect, surface effect and quantum size effect, nanomaterials have a very broad application prospect in the new generation of semiconductor devices. As a bottom-up forming method of nanomaterials, welding and joining of nanoscale materials provide a technical mean for high-performance micro/nano devices and various cross-scale applications, such as memristors, field effect transistors, sensors and so on. Although the concept of nanowire bonding has been put forward very early, the theory of nanowire bonding has not been fully established, and the existing nanojoining methods made high demands on spatial accuracy of energy input and external environment. In view of this, we considered graphene oxide (GO) as the intermediate layer to join nanowires. In this paper, we introduce graphene oxide as an auxiliary conductive path at a fixed point in space, and use the reduction effect of femtosecond laser and the characteristics of local energy input to reduce graphene oxide locally. As a way of direct energy input, ultrafast laser can quickly adjust the energy input in space domain and time domain through optical lens and photoelectric shutter. The theoretical point contact joint was optimized as line contact and area contact, so as to improve the current level and performance. In addition, the protective effect of reduced graphene oxide (rGO) nanofilms on the structure was further studied, and a series of nanowire network device such as ultraviolet (UV) detectors and flexible transparent conductive films were prepared.
Methods SiC nanowire-GO film-SiC nanowire structure was prepared by dry transfer method (Fig. 1). The transfer process of a single nanomaterial is as follows: 10 μL of SiC nanowires suspension was placed on polydimethylsiloxane (PDMS) with a pipette gun. After evaporation of alcohol, SiC nanowires were moved to the designated position by using the nano transfer platform under the light microscope. The structure of SiC nanowire-GO film-SiC nanowire can be obtained by repeating the above steps. The reduction of GO films are achieved by femtosecond laser irradiation (50 fs pulse duration, 800 nm wavelength and 1 kHz frequency). The surface morphology and crystalline structure of SiC nanowire-GO film-SiC nanowire structures were obtained by scanning electron microscope (SEM, Zeiss Supra 55) and X-ray diffraction (XRD, Bruker D8). The electrical characterization of SiC nanowire-GO film-SiC nanowire structures was examined by a probe station (Keithley 2636B). The simulation of light field irradiation was carried out by multi-physical field finite element simulation software (COMSOL multiphysics 5.4). In the frequency domain module of wave optics, the wavelength of incident light was set to 800 nm, the intensity of incident electric field was set to 1, and the material parameters were inquired through related literature.
Results and Discussions After femtosecond laser irradiation, the current levels at both ends of the SiC nanowire-GO film-SiC nanowire structures are significantly improved (Fig. 3). The improvement of electrical properties of SiC nanowire-SiC nanowire-GO film structures is mainly due to the formation of intralayer conductive paths of rGO (Fig. 2), while the improvement of SiC nanowire-GO film-SiC nanowire structure is due to the formation of interlayer and intralayer conductive paths of rGO. After the rGO formed by laser reduction is in contact with SiC semiconductor, the Fermi energy levels of SiC and rGO will move simultaneously due to a small amount of carrier transport at the interface, and reach equilibrium. The contact barrier between rGO and SiC is significantly lowered (Fig. 5). The spatial electric field distribution shows that the electric field at the interface of graphene and nanowires will be enhanced, which further promotes the two-photon absorption of GO for femtosecond laser, and improves the reduction efficiency (Fig. 6). For the ultraviolet sensor device constructed by SiC nanowire network, the photoelectric response characteristic before femtosecond laser irradiation is less than 10 -5 A/W when it is irradiated by ultraviolet light with wavelength of 375 nm. After femtosecond laser irradiation, although the dark current of SiC nanowire network increased significantly, it showed a good response intensity and faster response speed to ultraviolet light. The responsivity of the optical sensor was about 0.11 A/W, which was improved by more than four orders of magnitude. Moreover, we used SiC nanowires and GO film to construct a transparent flexible conductive film on PDMS. After femtosecond laser scanning irradiation, the regional current of the conductive film was increased by more than five orders of magnitude (Fig. 7). This kind of flexible and transparent conductive film can be used in the fabrication of extensible flexible electrode or touch screen panel in the future.
Conclusions In this paper, the SiC nanowire-GO nanofilm-SiC nanowire structures were prepared by dry transfer method, then graphene oxide was reduced by femtosecond laser irradiation, which reduced the barrier between SiC and GO, and a wider carrier channel was formed through the way of intra layer and inter layer conduction, which significantly increased the current level of this structure. In addition, the obtained rGO nanofilms can wrap and protect the joints of SiC nanowires, which makes the joint have better radiation resistance and heat conduction, so as to improve the stability and service life of the device. Finally, the field effect transistor with low loss and high stability, UV sensor with good response and fast response, and transparent flexible conductive film were fabricated from SiC nanowire network with GO film by femtosecond irradiation.
