A bandpass filter is an electronic component that allows EM waves to pass within a specific frequency band while preventing their transmission outside the specified range. They have a wide range of applications in different fields, including 5G communications, smart home systems, medical electronics, IoTs, security monitoring and automotive electronics, as shown in
Fig. 5a [
42,
43,
44]. A UWB bandpass filter is constructed based on the superior EM response of MXene/TiO
2. Figure S10 shows the structure of the filter, including a copper transmission line topology on the upper surface, a MXene/TiO
2 dielectric substrate in the center, and a copper grounding plate on the lower surface. Accurate tailoring of the EM response of MXene/TiO
2 is critical for manipulating the performance of bandpass filters. The MT-5 bandpass filter offers the best overall performance, including high return loss (
|S11|, ≥ 10 dB in the passband), low insertion loss (|
S21|, up to 1.82 dB), a wide passband (5.44 GHz, 2.55-7.99 GHz, and covering virtually the entire S and C bands), and strong suppression outside the band (53.4 dB) (
Fig. 5b). These excellent properties can be attributed to the low dielectric loss of the MT-5 substrate. However, the MT-4 bandpass filter has a power attenuation of more than 44% due to its higher conductivity and dielectric loss, resulting in a larger insertion loss (
|S21|> 5 dB) in the 2-8 GHz range, which prevents the formation of an effective passband as shown in
Fig. 5c. For the MT-3 and MT-2 bandpass filters, the tight conductivity network formed by MXene/TiO
2 attenuated most of the EM waves, resulting in the weakening of the signal strength, as shown in
Fig. 5d, e. The material, thickness of the dielectric substrate and metal patch jointly affect the radiation characteristics and impedance matching of the filter. Among them, the thickness of the dielectric substrate has a significant impact on the S-parameter of the filter. An increase in thickness will cause the center frequency of the passband to shift slightly toward lower frequencies, because the thickness affects the phase of microwave propagation. In addition, excessive thickness will increase the quality factor, thereby reducing the passband bandwidth. Within an appropriate range, increasing the thickness can enhance the return loss within the passband, but at the same time, the insertion loss within the stopband will also be smaller. Therefore, in order to achieve good overall performance, it is necessary to comprehensively regulate the permittivity, dielectric loss and thickness of the dielectric substrate.