Nanoforging Single Layer MoSe2 Through Defect Engineering with Focused Helium Ion Beams

Abstract

Development of devices and structures based on the layered 2D materials critically hinges on the capability to induce, control, and tailor the electronic, transport, and optoelectronic properties via defect engineering, much like doping strategies have enabled semiconductor electronics and forging enabled introduction the of iron age. Here, we demonstrate the use of a scanning helium ion microscope (HIM) for tailoring the functionality of single layer MoSe2 locally, and decipher associated mechanisms at the atomic level. We demonstrate He(+) beam bombardment that locally creates vacancies, shifts the Fermi energy landscape and increases the Young's modulus of elasticity. Furthermore, we observe for the first time, an increase in the B-exciton photoluminescence signal from the nanoforged regions at the room temperature. The approach for precise defect engineering demonstrated here opens opportunities for creating functional 2D optoelectronic devices with a wide range of customizable properties that include operating in the visible region.

Figure 1

(a) AFM topography image of single layer supported MoSe₂ flake and (b) its height profile as labeled on (a). (c–e) Atomic resolution MAADF-STEM images of (c) pristine region, (d) irradiated region at 1 × 10¹⁵ He⁺/cm², and (e) 1 × 10¹⁶ He⁺/cm². (f,g) k-means clustering STEM images in (c,d) color corresponds to the number of cluster. (h) Profile of pristine MoSe₂ (c).

Figure 2. Calculated electronic band structures of single layer MoSe₂ with different Se vacancy concentrations.

All band energies are aligned to the vacuum potential for direct comparison. The vacancy induced in-gap bands are highlighted in red color. The Fermi level is set at the middle of the band gap for each system, as shown by the blue dash line.

Figure 3. Tapping mode band excitation (BE) Kelvin probe force microscopy (KPFM) of supported MoSe₂.

(a–c) Local contact potential difference (LCPD) and (d–f) capacitance gradient maps of regions irradiated by He⁺ beam with different doses: (a,d) 1 × 10¹⁴ ions/cm² (b,e) 1 × 10¹⁵ ions/cm² and (c,f) 1 × 10¹⁶ ions/cm².

Figure 4. Photoluminescence (PL) spectra of supported MoSe₂ indicating the evolution of the A-exciton (~1.55 eV) and B-exciton (~1.77 eV) peaks in undosed region (green trace) and He⁺ beam-irradiated regions corresponding to doses of 1 × 10¹⁴ ions/cm² (blue trace), 1 × 10¹⁵ ions/cm² (purple trace), and 1 × 10¹⁶ ions/cm² (red trace).

Figure 5. Nanomechanical measurements of supported MoSe₂ using contact resonance band excitation (BE) scanning probe microscopy.

(a) Storage modulus maps of supported MoSe₂ irradiated with 1 × 10¹⁴ ions/cm² He⁺ beam dose; (b) Storage modulus maps of supported MoSe₂ irradiated with 1 × 10¹⁵ ions/cm² He⁺ beam dose. (c) Young’s modulus of elasticity curves corresponding to He⁺ beam doses of 1 × 10¹⁴ ions/cm² (blue trace) and 1 × 10¹⁴ ions/cm² (red trace). (d) Calculated Young’s modulus of single layer MoSe₂ with different Se vacancy concentrations. For each vacancy concentration, two scenarios are considered: lattice constants fixed to the original ones (squares) and lattice constants re-optimized (triangles). After re-optimization, the system size is slightly reduced, suggesting vacancy-induced compression.