Conventional XFNTR consists of X-ray fluorescence spectra measured for a range of wave vector transfer Qz from below to above the critical Qc for total reflection (Fig. ). The XFNTR scan is then fit as previously described (Fig. ). However, this method was unsuitable for the current work because each XFNTR scan requires at least several minutes to measure. Therefore, a single fluorescence spectrum was recorded at either Qz = 0.018 Å -1 or Qz = 0.025 Å -1 and the surface density analyzed from that spectrum, as described in Section S1 of the SI.