Objective Owing to high porosity and large specific surface areas, nanoporous metal materials have attracted considerable attention for applications in sensors, energy storage, etc. With the rapid development of flexible sensors and energy storage devices, the fabrication of flexible nanoporous metal materials is becoming more and more important in recent years. However, nanoporous metal materials prepared by dealloying, spark plasma sintering, and heat pressing exhibit an extremely high elastic modulus and yield strength that do not meet the requirements of flexible devices. To overcome these problems, herein, flexible nanoporous metal materials were prepared by irradiating Ag nanowires (NWs) using a femtosecond (fs) laser.

Ag NWs are ideal nanomaterials for fabricating flexible nanoporous metal materials because of their good flexibility, conductivity, and oxidation resistance. The fs laser offers several advantages such as ultrashort pulse width and ultrahigh peak power. Flexible nanoporous metal materials prepared by irradiating Ag NWs using the fs laser show ultralow yield strength, which can meet the requirements of flexible devices. In this study, a novel method for preparing flexible nanoporous silver materials is reported and the influence of fs laser power on the yield strength of the resulting flexible nanoporous Ag materials is revealed.

Methods The following steps are involved in the fabrication process of flexible nanoporous Ag materials. The as-synthesized Ag NW solution and deionized (DI) water were pipetted into a centrifuge tube using a pipettor. Then, the centrifuge tube containing the Ag NWs solution and DI water was placed in an ultrasonic cleaner to strip the polyvinylpyrrolidone (PVP) coating the surface of Ag NWs. Subsequently, the centrifuge tube was centrifuged to obtain highly concentrated Ag NW pastes. After adding DI water, ultrasound and centrifugation were performed several times to remove excess PVP. The Ag NW paste was dropped onto a silicon substrate and completely dried at room temperature. Then, the dried Ag NWs were irradiated using the fs laser to fabricate the flexible nanoporous Ag materials. Nanoindentation experiments were performed to analyze the mechanical properties of the as-prepared flexible nanoporous Ag material. X-ray diffractometer (XRD) and high-resolution transmission electron microscopy were used to analyze the crystalline structure of the flexible nanoporous Ag materials. Additionally, the yield strength of nanoporous metal materials fabricated using different methods was compared.

Results and Discussions The fs laser irradiation of nanowires can induce localized surface plasmon resonance on the Ag NW surface. When the intensity of the fs laser is low, nanojoining occurs owing to the melting of the Ag NWs at the contact region. By increasing the intensity of the fs laser, the ends of the Ag NWs begin to melt and the linear ends of Ag NWs gradually transform into a spherical structure (Fig. 2). When the fs laser power is 60 mW, nanojoining occurs at the gaps between the Ag NWs because of local fusion. The volume and number of nanojoining increase with an increase in the fs laser power, and spherical structures are observed at the end of Ag NWs (Fig. 3). The yield strength of flexible nanoporous Ag materials is determined using depth-sensing nanoindentation equipped with a Berkovich indenter. Different peak loads (150, 300, and 450 μN) are investigated. Plastic deformation is found to only occur at the residual indentation impression, and no nanoporous material deformation adjacent to the contact impression is observed. This result indicates the occurrence of significant densification of the flexible nanoporous Ag materials under the Berkovich indenter (Fig. 4). With increasing fs laser power, the elastic modulus and yield strength of flexible nanoporous Ag materials increase, while their grain size decreases (Fig. 6). The yield strength of flexible nanoporous metal materials prepared using different methods is compared in this study. The experimental data clearly show that nanoporous Ag materials prepared using the fs laser exhibit the smallest yield strength at the same grain size (Fig. 7).

Conclusions The flexible nanoporous Ag materials are prepared by irradiating Ag NWs using the fs laser. The mechanical properties and grain size of the synthesized flexible nanoporous Ag materials are evaluated using a nanoindenter and XRD, respectively. Based on the experimental results, several important conclusions can be drawn.

1) The elastic modulus and yield strength of nanoporous Ag materials increase with an increase in the fs laser power. At a peak load of 450 μN and when the fs laser power is increased from 60 to 100 mW, the elastic modulus and yield strength of the flexible nanoporous Ag materials increase from 88.4 to 235.2 MPa and from 1102.0 to 2737.0 MPa, respectively.

2) The grain size of the flexible nanoporous Ag materials decreases with an increase in the fs laser power. When the fs laser power is increased from 60 to 100 mW, the grain size decreases from 44.6 to 41.5 nm.

3) Compared with Ag materials prepared by hot pressing, plasma sintering, and dealloying, flexible nanoporous Ag materials prepared by irradiating Ag NWs using the fs laser exhibit the lowest yield strength at the same grain size.