Fig. 3: Enhancement of tunable, narrowband X-ray peaks by increasing material thickness into the bulk regime.

a Demonstrates the enhancement of tunable X-ray intensity from graphite as a function of material thickness. X-ray spectra collected from four distinct graphite samples under a 25 keV electron beam are shown. Solid dots with error bars represent experimental data, while translucent bands and solid lines indicate theoretical predictions. The translucent bands account for intensity variations due to uncertainties in graphite film thickness under experimental conditions. The inset highlights the tunable X-ray intensity trend with increasing thickness at 25 keV for a non-tilted crystal. Solid dots represent simulation results based on the established theory. The solid line represents the fitted curve, while four pentagrams mark the specific thicknesses at which experimental spectra were measured. b Compares tunable X-rays with bremsstrahlung background for graphite thin films and bulk graphite under 25 keV electron beams. Solid dots with error bars denote experimental tunable X-ray data, obtained using an energy-dispersive X-ray spectroscopy (EDS) detector alongside bremsstrahlung background. Solid lines depict theoretical tunable X-ray peaks convoluted with the EDS detector resolution, which closely match the experimental data. Dashed lines beneath the tunable X-ray peaks indicate bremsstrahlung background. Sharp dotted lines represent theoretical tunable X-ray peaks calculated without considering the EDS detector resolution. c Displays the comparison between tunable X-ray intensity and the bremsstrahlung. Solid round dots represent tunable X-ray intensity values derived from the established theory, validated by experimental results. Hollow squares indicate bremsstrahlung intensity, calculated using the Monte Carlo method^{96,97,98,99,100}. d Highlights the enhancement of tunable X-ray intensity with increasing material thickness under a 300 keV electron beam. The definition of the error bars is provided in the Supplementary Information (SI) Section S3. All spectra and intensity data correspond to a solid angle of 0.066 steradians, in accordance with experimental conditions.