Fig. 1. Solution-based synthesis of CQDs and their assembly into semiconductor optoelectronic thin films. (a) Hot-injection synthesis of CQD; (b) diagram of typical structure of single CQD with inorganic quantum dot core and organic long-chain ligands^[30]; (c) integration of CQD film to the CMOS readout circuit and its use as infrared sensor in a focal plane array^[17]; (d) scanning electron microscope (SEM) cross-sectional image of a thin film made of densely packed CQDs^[41]

Fig. 2. Quantum confinement size effect and heterostructure of CQD. (a) Electronic energy levels in a bulk semiconductor and spherical CQDs made of the same material but of different size^[45]; (b) absorption spectra of PbS CQD with size ranging from 4.3 nm to 8.4 nm^[46]; (c) band alignments of different heterostructured CQDs

Fig. 3. Applications corresponding to different bands in electromagnetic waves, transparency window of the silicon-based optoelectronic platform, and the spectrum range covered by different CQDs

Fig. 4. Complex refractive index of CQD film. Refractive indexes n and extinction coefficients k of PbS CQD films capped with (a) oleic acid, (b) 1,2-ethanedithiol (EDT), and (c) I^-[51]; (d) refractive indexes and linear absorption coefficients of different-sized CdSe QD films^[56]; (e) complex refractive index of self-assembled CQW film; (f) cross-sectional TEM image of the quasi-symmetric waveguide^[62]

Fig. 5. Optical-gain model of CQD and the methods to measure its optical gain characteristics. In the (a) ground state, (b) single-exciton state, and (c) biexciton state, the CQD absorbs, transmits or magnifies an incident photon^［67］; (d)(e) TA measurement^[68] and (f)(g) VSL measurement^[69] of CQD optical gain characteristics

Fig. 6. Diagrams of ligand exchange process^[82]. (a) Solid state ligand exchange; (b) phase transfer ligand exchange

Fig. 7. Effect of CQD surface ligand on the energy level position and the doping of the semiconductor thin film. (a) Energy level positions of various ligand-exchanged PbS CQD thin films^[87]; (b) the tuning of doping type and magnitude during the phase shift ligand exchange of HgTe CQDs^[91]

Fig. 8. Methods to pattern CQD film. (a) Photolithography^[94]; (b) inkjet-printing^[96]; (c) electrohydrodynamic jet printing^[97]; (d) 3D printing^[98]; (e) transfer printing^[100]

Fig. 9. Plasmonic waveguide-integrated CQD photodetector^[109]. (a) Schematic of the plasmonic-silicon hybrid waveguide system. The arrows show the light propagation direction. Inset is the cross-section of the MIM waveguide coated with HgTe CQD; (b) the schematic of the cross-section of HgTe CQD-coated MIM waveguide and the simulated electric field distribution; SEM images of (c) the TE apodized silicon grating coupler and (d) photoconductive detector region after depositing CQD; (e) I-V curves under dark conditions and different light input powers at 2.3 µm; (f) net photocurrent and responsivity as functions of input light power under 2 V bias

Fig. 10. SiN waveguide-integrated CQD photodiode detector^[110]. (a) Device structure of waveguide coupled PbS CQD photodiode; (b) photon-generated carrier concentration distribution on PbS-PbI₂ absorption layer of the evanescent coupled photodiode; (c) I-V curves of photodiode under dark conditions and illumination at 1275 nm; (d) top view of the integrated spectrometer with a CQD photodiode array and arrayed waveguide grating. Light is injected from the right grating coupler; (e) normalized photocurrent of eight channels under zero bias

Fig. 11. Waveguide-integrated CQD LEDs^[112]. (a) Top-view of the device layout, with the dashed line indicating the location of the cross-section; (b) high-resolution SEM of a focused ion beam (FIB) cross-section; (c) optical microscope image of the sample; (d) band-alignment of the LED; (e) photoluminescence and electroluminescence spectra of CQD; (f) output optical power in a single-mode waveguide varying with LED current

Fig. 12. On-chip integration of self-assembled CQD lasers. (a) Water droplet induced CQD film splitting process; (b) optical image of the CQD microplate and the grating coupling waveguide; (c) the emission spectrum from the CQD microplate laser (P1) and the scattered spectrum from the waveguide (P2)^[115]; (d) illustration of the CQD emulsion droplets nucleating into superparticle; (e) microscope and (f) CCD image of superparticle located beside the SU8 waveguide^[121]

Fig. 13. Template-assisted on-chip CQD lasers. (a) Laplace pressure balances the pinning force of CQD and substrate, allowing for continuous moving of liquid front and regulated CQDs assembly; (b)(c) SEM image and the emission spectra above lasing threshold of the microring-waveguide coupling system fabricated by method in Fig. 13(a)^[101]; (d)-(g) EBL-assisted fabrication process of CQD microcavity and waveguide, the optical image and the false-colored SEM image of the cross-section at the tangency point, and the emission spectra of the coupled system at different location when pumped at 1.3 times of lasing threshold; (h)(i) optical image of single mode CQD laser integrated with a curved waveguide, and its output spectra above the lasing threshold; (j) optical image of the complex photonic integrated system above the pump threshold, which comprises CQD microring laser, MZ interferometer, Y-splitter, straight waveguides, bending waveguides, and couplers^[58]

Fig. 14. Plasmonic waveguide integrated on-chip CQD laser. (a) Schematic of the CQD microcavity coupled with the silver nanowire; (b) optical image of the CQD microcavity coupled with a silver nanowire. The inset is the false-colored SEM image of the cross section at the tangency point; (c) emission spectra of the tangency point (black) and the two ends (red and blue) of the silver nanowire at a pump fluence above lasing threshold; (d) peak intensities and linewidths under different pump fluences^[113]; (e) top-view SEM image of a spaser; (f) emission spectra of plasmons scattered at the left reflector edge and the right tip, confirming that the laser signal is guided and focused^[57]

Fig. 15. On-chip CQD lasers fabricated by subtractive manufacturing. (a) Fabrication diagram of low loss SiN/CQD/SiN hybrid waveguide^[124]; (b) diagram of vertical coupling of SiN/CQD/SiN microdisk laser and SiN waveguide; (c) optical microscopy image and false-color SEM image of cross section of waveguide coupled microdisk laser; (d) output emission of 7 μm diameter disk laser under different fs pump powers; (e) output intensity as a function of pump intensity, showing a clear threshold of 27 μJ·cm^-2[102]; (f) diagram of on-chip DFB laser after EBL and RIE etching; (g) top view image and cross section image (inset) of SEM image of DFB laser; (h) spectra measured from unpatterned waveguide and DFB lasers with different grating periods; (i) input-output curve, indicating a lasing threshold around 270 μJ·cm^-2[114]