Fig. 1. Schematic view and TEM characterizations of MoS₂ monolayer and CdSe/ZnS QDs. (a) Schematic view of mixed-dimensional QD/MoS₂ heterostructures on SiO₂/Si substrate under the excitation of a laser. (b) High-resolution TEM (HRTEM) image of MoS₂ monolayer with the scale bar of 5 nm. The inset shows the corresponding selected area electron diffraction (SAED) pattern, where the diffraction points are arranged in a hexagonal structure. (c) Enlarged FFT image of the marked area in a rectangular region in (b). The lattice spacing is 2.78 Å. (d) The AFM image of MoS₂ monolayer with clear surface, and the height of MoS₂ is observed to be 0.75 nm. Low-resolution (e) and high-resolution (f) TEM images of QD/MoS₂ heterostructures, where scale bars are 100 nm and 20 nm, respectively. The red arrows and circles in (e) are QDs on MoS₂ monolayer, and the orange arrows in (f) are the wrinkles of MoS₂ flakes. (g) The HRTEM image of QD/MoS₂ structures with a scale bar of 2 nm. Inset shows two sets of SAED patterns.

Fig. 2. Optical and spectral characteristics of QDs, MoS₂ monolayer and mixed-dimensional vdWHs. (a) The UV-visible absorption spectrum (orange) of CdSe/ZnS QDs in 0.2 mg·L⁻¹ solution. PL spectra of QDs in solution (blue) and on SiO₂/Si substrate (dotted line). (b) Raman spectra of MoS₂ monolayer and QD/MoS₂ heterostructures. E¹_2g and A_1g modes are 384.2 cm⁻¹ and 404.3 cm⁻¹ for MoS₂ monolayer, respectively, and the E¹_2g peak shifts blue to 383.3 cm⁻¹ in heterostructures. Insets present the optical microscopy images of MoS₂ monolayer and heterostructure. The scale bars are 6 μm. (c) The absorption (black) and PL (red) spectra of MoS₂ monolayer. The PL intensity maps of MoS₂ monolayer are measured at the spectral ranges of (d) 665−750 nm and (e) 640−660 nm. The PL intensity maps of heterostructures with 0.025 mg·L⁻¹ QDs are measured at the spectral ranges of (f) 665−750 nm and (g) 640−660 nm. All the scale bars in mapping images are 5 μm. (h) Time-resolved photoluminescence (TRPL) spectra of QDs and QD/MoS₂ heterostructures. The average lifetimes of QDs and heterostrutures are 498.6 ps and 98.9 ps, respectively. (i) The schematic view of energy band for QD/MoS₂ heterostructures under the light excitation.

Fig. 3. Analysis of PL spectral evolution of heterostructures at various doping densities. (a) Normalized PL spectra of QD/MoS₂ heterostructures with different QD concentrations. The total PL spectra (red) can be fitted well with several components, involving QD exciton (dark blue), MoS₂ exciton A (blue), trion (pink) and exciton B (green). (b) Schematic view of the three-energy-levels including exciton, trion and ground states, where the trion is generated from the exciton. (c) Diagram of PL intensity of total intensity (I_Total), exciton (I_A) and trion (I_A-). Fitting curves match well with experimental results. (d) The weight ratio of exciton (I_A/I_Total) as a function of concentration. (e) Calculations of charge density (n_e) based on the law of mass action model. The electron density of charge transfer (Δn_e) is determined as 3.9×10¹³ cm⁻².

Fig. 4. Laser-power dependent spectral modulation of QDs, MoS₂ and their heterostructures. PL spectra change of (a) MoS₂ monolayer and (b) QDs at various laser powers. (c) Analysis of PL spectral shapes in QD/MoS₂ heterostructure with the excitation of various laser powers. The total PL peak consists of QD exciton (green), MoS₂ exciton (A, blue), trion (A^-, purple) and B exciton, and they can be fitted well in the shape of Lorentzian curves. (d) Plots of normalized PL intensity of exciton and trion as a function of laser powers. The relationship of PL intensity (I) and laser power (P) can be fitted as I=(P)^m. (e) Schematic of exciton generation and emission at low and high laser powers.

Fig. 5. Optoelectronic performance of the vdWHs phototransistor compared with a pristine MoS₂ phototransistor. (a) Schematic view of the phototransistor device, and the inset is an optical image of the as-prepared device with the scale bar of 10 μm. (b) Transfer characteristics of two devices in the dark and under the illuminations (from 0.11 nW to 34.75 nW) at V_ds=1 V. (c) Transfer characteristics in dark (dotted line) and under the illumination (solid line, P_eff = 34.7 nW) at V_g=0 V. (d) Photoresponsivities and (e) specific detectivities of two devices as a function of effective powers at V_g=60 V, respectively. (f) Time-dependent photoresponses under the pulsed illumination powers (from 0.11 nW to 19.78 nW) over multiple cycles at V_ds=1 V. (g) The rise and fall times in photocurrent extracted from figure (f) under the illumination of 0.11 nW. (h) The photocurrent curves of vdWHs device with various QDs concentrations.