Figure 2.

3D tunability of free electron radiation in van der Waals (vdW) materials. Tunability via vdW structure tilt, electron energy, and atomic composition. a–f) Together illustrate our paradigm of a highly versatile, compact X‐ray source in which the photon energy can be tuned over a wide range by varying the structure tilt, the electron energy and the atomic composition. X‐ray spectra from WS₂ (a,b), MoSe₂ (c,d) and MoS₂ (e,f) are tailored by varying electron energy (labeled at top right corner of each panel) and structure tilt angle θ _til (labels individual curves within each panel). Filled circles represent experimental measurements and solid curves correspond to theoretical predictions. Vertical dotted lines indicate the peak photon energy predicted by Equation (2). Comparing (b) and (d), WS₂ with a smaller interlayer spacing generates harder X‐rays compared to MoSe₂ under the same conditions, showing the tunability of vdW X‐rays via atomic composition. In (a–d), θ _obs ≈ 112.5° and Δθ _obs ≈ 12°. In (e,f), θ _obs ≈ 91.5° and Δθ _obs ≈ 1°. In all cases, the intrinsic bandwidth is about a few eV, but is broadened by the energy resolution of the respective EDS detectors. The estimated standard error for the photon count number is 1%, while that for the measured photon energy is 2.5 eV.