Resonance-Enhanced Four-Wave Mixing Imaging for Mapping Defect Regions in Vanadium-Doped WS2 Monolayers

Abstract

Defect engineering is crucial for tuning 2D transition metal dichalcogenide properties for quantum and optoelectronic applications. While conventional photoluminescence (PL) and Raman spectroscopies are important characterization tools, their mapping in large area samples can be time-consuming and lacks direct sensitivity for comprehensive defect characterization. Here, we introduce resonance-enhanced four-wave mixing (FWM) imaging for precise imaging and characterization of vanadium-induced defect states in WS2 monolayers. Our multi-modal investigation, integrating hyperspectral PL, Raman, and supported by density functional calculations, reveals nanoscale doping inhomogeneities, their influence on excitonic and vibrational properties. We observe resonance-enhanced FWM signals correlating with vanadium-induced defect regions, evidencing their unique nonlinear optical response. This work establishes FWM as an essential platform for high-resolution, defect-sensitive imaging, advancing defect-engineered excitonic devices and enabling novel nonlinear quantum photonics.

Figures (5)

• Figure 1: Optical imaging and PL mapping of pristine and vanadium-doped WS$_2$ monolayers. Optical images of a. pristine, b. 0.4 at%, and c. 2.0 at% V-doped WS$_2$ monolayers. Scale bars represent 10 $\mu$m. Single PL spectra acquired from monolayer (1L, black curve) and defect line (red curve) regions for d. pristine, e. 0.4 at%, and f. 2.0 at% V-doped samples. PL intensity maps (integrated area) of $P_2$ peak for: g. pristine, h. 0.4 at%, and i. 2.0 at% V-doped. PL energy maps of $P_2$ peak for: j. pristine, k. 0.4 at%, and l. 2.0 at% V-doped. Map area: 20$\times$20 $\mu$m$^2$.
• Figure 2: Raman hyperspectral maps of pristine and vanadium-doped WS$_2$ monolayers.a,c.$E^{\prime}$ and b,d.$A^{\prime}_{1}$ frequency maps for pristine and 2.0 at% V-doped WS$_2$, respectively. All maps were acquired using a 2.71 eV excitation over a 20$\times$20 $\mu$m$^2$ area. e. Raman spectra comparing monolayer (1L, black circles) and the defect line (red squares) regions for pristine and 2.0 at% V-doped samples. All Raman data were normalized to the Si peak at 521.6 cm$^{-1}$.
• Figure 3: FWM imaging of pristine and vanadium-doped WS$_2$ monolayers.a. Schematic of the FWM experimental setup. Further details are in the Methods section. The inset shows energy diagrams of the FWM generation process, illustrating off-resonance (left) and on-resonance (right) conditions. b. FWM images of pristine, 0.4 at%, and 2.0 at% V-doped WS$_2$ monolayers with FWM signal at 2.03 eV (off-resonance, top row) and 1.82 eV (on-resonance, bottom row), respectively. Scale bars are 20 $\mu$m.
• Figure 4: Resonance-enhanced FWM response of pristine and vanadium-doped WS$_2$ monolayers. FWM excitation profiles for pristine, 0.4 at%, and 2.0 at% doped WS$_2$ samples acquired from the monolayer (red squares) and defect line (blue circles) regions. Intensities are normalized by the quartz reference signal at each excitation energy.
• Figure 5: Density functional theory calculations of vanadium-doped WS$_2$ monolayers.a. Unfolded band structure of a $7\times7$ WS$_2$ supercell with a single V substitution (2% concentration). Vanadium induces in-gap states and breaks valley symmetry. Color intensity indicates the primitive-cell spectral weight (Bloch character), while the small circles show the projection onto V states, with their size proportional to the spectral function weight. b. Top panel: Evolution of the unfolded electronic band structures, from semiconducting to metallic behavior, revealing the emergence of mid-gap defect states with increasing vanadium concentration. Bottom panel: Atomic structures of the $5\times5$ WS$_2$ supercells, showing the spatial distribution of vanadium dopants at varying concentrations (0, 1, 2, 4, and 8 V dopants). c. Calculated 2D optical absorption spectra for pristine, one V, and eight V cases, showing the redshift in the onset of absorption and the emergence of anisotropy at high doping. The incident light is polarized along the zigzag (XX) and armchair (YY) directions, represented by solid and dashed lines, respectively.