Surface-enhanced infrared absorption (SEIRA) has been gaining substantial attention by using plasmonic nanoantennas to amplify near-field intensities so that it can extend IR spectroscopy to zeptomolar quantities and ultimately to the sigle-molecule level. Here we report a new nanoantenna for SEIRA detection, consisting of a fan-shaped Au structure positioned at a well-specified distance above a reflective plane with an intervening silica spacer layer. This antenna can be easily tuned to overlap vibrational modes within a broad spectral range from the near-IR into terahertz regimes. Our finite difference time domain (FDTD) simulations reveal a maximum SEIRA enhancement factor of 105 in the antenna junction area, which is corresponding to the experimental detection of 20-200 zeptomoles of octadecanethiol, using a standard commercial FTIR spectrometer. Our optimized antenna exhibits an order of magnitude greater SEIRA sensitivity than previous record-setting designs, which opens new opportunities for using infrared spectroscopy to analyze exceptionally small quantities of molecules.