Atomic-layer-confined multiple quantum wells enabled by monolithic bandgap engineering of transition metal dichalcogenides

Abstract

Quantum wells (QWs), enabling effective exciton confinement and strong light-matter interaction, form an essential building block for quantum optoelectronics. For two-dimensional (2D) semiconductors, however, constructing the QWs is still challenging because suitable materials and fabrication techniques are lacking for bandgap engineering and indirect bandgap transitions occur at the multilayer. Here, we demonstrate an unexplored approach to fabricate atomic-layer-confined multiple QWs (MQWs) via monolithic bandgap engineering of transition metal dichalcogenides and van der Waals stacking. The WO_X/WSe₂ hetero-bilayer formed by monolithic oxidation of the WSe₂ bilayer exhibited the type I band alignment, facilitating as a building block for MQWs. A superlinear enhancement of photoluminescence with increasing the number of QWs was achieved. Furthermore, quantum-confined radiative recombination in MQWs was verified by a large exciton binding energy of 193 meV and a short exciton lifetime of 170 ps. This work paves the way toward monolithic integration of band-engineered heterostructures for 2D quantum optoelectronics.

Fig. 1.. Atomic–layer–confined MQWs based on monolithic bandgap engineering of WSe₂.

(A) Schematic illustration of MQWs with stacking of the WO_X/WSe₂ hetero-bilayer as a building block. (B) Energy band diagram for MQWs consisting of the WSe₂ QW (blue) and the WO_X quantum barrier (red). This type I structure leads to effective carrier confinement in the QW. E_C and E_V indicate the conduction band edge and valence band edge, respectively. (C) Cross-sectional bright-field (left) and corresponding high-angle annular dark-field (right) STEM (scanning transmission electron microscopy) images of the triple QWs (TQWs). Scale bar, 2 nm. (D) Schematic illustration of monolithic band engineering of the bilayer WSe₂, showing the transition of a band structure and energy band alignment via oxidation. (E) PL spectra and (F) Raman spectra of the layer-by-layer oxidized WO_X/WSe₂ hetero-bilayer and the pristine WSe₂ bilayer. The inset in (F) shows the magnified ${\mathit{B}}_{2\mathrm{g}}^1$ mode in Raman spectra. a.u., arbitrary units.

Fig. 2.. Analysis of the QW band structure of WO_X/WSe₂.

(A) PL (blue solid line) and UV-Vis absorption (green solid line) spectra of the pristine WSe₂ monolayer, exhibiting the optical bandgap at 1.65 eV. (B) Tauc plot for the WO_X directly oxidized from the WSe₂ monolayer. The estimated bandgap is 3.28 eV. (C) UPS spectra of the secondary electron edge for the pristine WSe₂ (black solid line) and the oxidized WO_X (red solid line); the extracted work functions are 4.65 and 5.15 eV, respectively. (D) UPS spectra at the low binding energy region near E_F for the pristine WSe₂ (black solid line) and the oxidized WO_X (red solid line). The energy difference of E_F − E_V is estimated from the onset of the low binding energy, which are 1.35 and 2 eV for WSe₂ and WO_X, respectively. (E) Constructed band diagrams with type I band alignment between WO_X and WSe₂ before (left) and after (right) aligning the E_F.

Fig. 3.. Optical characteristics of MQWs.

(A) Optical image (left) and spatially resolved PL map (right) of the step-like stacked MQWs. The SQW, DQWs, and TQWs areas are indicated by gray, red, and blue triangles, respectively. Scale bar, 10 μm. (B) PL spectra for the SQW (black line), DQWs (red line), and TQWs (blue line). (C) Histograms for the integrated PL intensity (left) and full width half maximum (FWHM) (right) for the SQW (gray), DQWs (red), and TQWs (blue). The integrated intensity is normalized to that of the SQW, and the values are 2.16 ± 0.96 (DQWs) and 5.31 ± 1.90 (TQWs). (D) Power-dependent PL spectra of the TQWs at 300 K. The excitation power varies from 0.09 to 120 μW. The inset shows the log scale plot of the integrated intensity as a function of power, fitted by the power law with the exponent of α = 0.96. (E) Low-frequency (left) and high-frequency (right) Raman spectra of SQW, DQWs, and TQWs.

Fig. 4.. Exciton dynamics in MQWs.

(A) Temperature-dependent PL spectra from 4 to 300 K. The red dashed line indicates the shift of the neutral exciton peak. The defect-mediated localized peak and the neutral exciton peak are denoted by Loc. and X⁰, respectively. (B) Plots of the X⁰ energy (top) and the integrated intensity (bottom) as a function of temperature. Solid black lines are fitted with the modified Varshni equation (top) and the Arrhenius equation (bottom). (C) Time-resolved PL spectra of the pristine WSe₂ monolayer (black triangles) and TQWs (blue open circles) at 300 K. The curves are fitted with the biexponential decay (red solid line) for WSe₂ and single-exponential decay function (purple solid line) for TQWs.