Research ArticlePHYSICS

Electrically control amplified spontaneous emission in colloidal quantum dots

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Science Advances  25 Oct 2019:
Vol. 5, no. 10, eaav3140
DOI: 10.1126/sciadv.aav3140
  • Fig. 1 Optical properties of CQD materials under increasing electric field in a capacitor device.

    (A) Normalized PL and absorbance spectra of diluted CQDs in toluene. Left inset: Schematics of the core-shell-shell CQDs. Right inset: Transmission electron microscopy image of the CQDs indicating the shape distribution and the crystalline structure. Scale bar, 10 nm. (B) Schematics of the sandwich-like device structure. A sinusoidal-function bias (128 Hz) is applied to induce the electric field, and the device is optically pumped at a wavelength of 532 nm using a subnanosecond pulsed laser (500 ps/100 Hz). ITO, indium tin oxide. (C) Scanning electron microscopy image of a device’s cross section. Scale bar, 100 nm. (D) Electric field–dependent PL spectra of CdSe/CdS/ZnS CQDs with the pump fluence of 272 μJ/cm2. These emission spectra exhibit intensity decease, redshift, and broadening with increasing electric field. a.u., arbitrary units. (E) Decrease in integrated PL intensity as a function of electric field, which was normalized by the zero-field integrated PL intensity. The solid line is a guide to the eye. It shows ~18% intensity drop in the maximum electric field of 264 kV/cm. (F) Electric field–dependent peak wavelength shift and PL spectrum broadening. The solid lines are guides to the eye. Only slight changes are observed here (~1.5-nm redshift and ~2-nm broadening under the maximum electric field).

  • Fig. 2 Electric field–dependent ASE characteristics.

    (A) Emission spectra as a function of electric field with the pump fluence of 1605 μJ/cm2, indicating the “increases first and decreases later” trend of the emission intensity. (B) Two-dimensional (2D) spectra map normalized by the maximum peak ASE intensity. The maximum peak intensity occurred in an electric field of ~20 kV/cm. (C) Peak wavelength of ASE as a function of electric field. The stimulated emission peaks exhibit a blueshift of ~1.2 nm under the maximum electric field of 264 kV/cm. The solid line is a guide to the eye. (D) Integrated emission intensity (log-log plotting) and the FWHM of emission profile (semilog plotting) as a function of electric field. Turning point of the integrated emission intensity and the FWHM implies different dominant recombination processes as electric field increases. (E) 2D spectra map normalized by the peak ASE intensity at each electric field, showing spectrum width reduces first and increases afterward. (F) Light input–light output curves with increasing electric field. The straight lines are guides to the eyes. It shows clear evidence of controlling and tuning the ASE by electric field.

  • Fig. 3 Quantification of the charging level in CQDs.

    (A) Visible absorbance spectra of CQDs under different electric fields. Inset: Absorbance spectra difference of the first excitonic features, which is defined as the CQD absorbance under electric field subtracted by the CQD absorbance under no electric field. (B) An example of deconvolution of the absorbance spectrum into the sum of four Gaussian fittings. Inset: The lowest three allowed interband transitions of CQDs. (C) Normalized integrated area of A1 and A2 as a function of electric field, suggesting the electron occupation of the 1S(e) state of the conduction band. Inset: Relationship between the relative bleach of the first and the second transitions. The solid line with the slope of ~1.05 is the linear fitting of the data (A10, A20 denote A1 and A2 under no electric field, respectively). (D) Average electron number per CQD, <ne>, with respect to electric field. The red circles are <ne> extracted from the absorbance bleach of the 1S(e)-1S3/2(h) transition via Gaussian deconvolution. The blue squares are <ne> calculated from the integrated absorbance difference, which is normalized by the maximum <ne> obtained by Gaussian deconvolution. The solid line shows the linear fitting to the data. The turning point (21.22 kV/cm) is consistent with the E-field threshold (20.13 kV/cm) defined in Fig. 2.

  • Fig. 4 Evolution of decay lifetime in CQDs with respect to the applied electric field.

    (A) trPL decays with increasing electric fields. Dots are the experimental results, and solid lines are the fitting curves. The dark yellow decay curve is the result of bare CQD film on glass without electric field, while the black one is the decay of CQD film in the device structure without electric field. (B) Lifetimes as a function of the electric field extracted from the fitting of the decay curves with three exponential components. X, X1−, and X2− denote the neutral CQDs, singly charged CQDs, and doubly charged CQDs, respectively. Inset: Normalized population of X, X1−, and X2− as a function of the electric field. (C) Compare average charge per CQD, <ne>, extracted from absorption spectra and PL decays. The consistence here supports our previous explanations in Figs. 2 and 3. (D) Auger and radiative lifetimes of different electron-hole states. XX denotes the biexciton of neutral CQDs. The biexciton lifetime used here is cited from (33) for a similar CQD structure. In our device structure, Auger lifetime of the biexciton is quite close to Auger lifetime of the doubly charged trion (0.75 ns compared with 0.79 ns).

  • Fig. 5 Modeling of the ASE threshold using the kinetic equations and experimental parameter inputs.

    (A) Schematics of the ASE process in neutral and charged CQDs. Blue hollow circles represent holes in the valence band. Black solid circles represent photogenerated electrons, while red solid circles denote excess electrons induced by the electric field. XX, X1−, and X2− are the same denotations as previously described in Fig. 4. (B and C) Emission intensity decay map normalized by the peak value of the photon density under each electric field at the pump fluence of 800 and 1000 μJ/cm2, respectively. At 800 μJ/cm2, the abrupt acceleration of emission (ASE) only occurs under the E-field threshold (20.13 kV/cm). The white dash line locates at the E-field threshold. At 1000 μJ/cm2, the ASE can be observed under the electric field smaller than 149.2 kV/cm (white dash line). (D) Integrated photon density (log scale) map as a function of both the applied electric field (linear scale) and the pump fluence (log scale). The white dash line indicates the E-field threshold, and the blue dash line indicates the threshold behavior under different electric fields. (E) Simulated and experimental ASE threshold behavior as a function of the applied electric field.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaav3140/DC1

    Section S1. Electric field and current in the CQD device

    Section S2. ASE in the CQD device without electric field

    Section S3. Steady-state absorption measurement

    Section S4. PL dynamics analysis

    Section S5. Kinetics model

    Fig. S1. Schematics of the circuit.

    Fig. S2. Thickness measurement conducted by surface profiler (DektakXT surface profiler).

    Fig. S3. Permittivity measurement result.

    Fig. S4. Mutual corroboration to calibrate the voltage drop.

    Fig. S5. Energy schematic of the device.

    Fig. S6. Measured current flowing through the circuit.

    Fig. S7. Edge emission as a function of pump fluence without electric field under 500-ps laser pulse excitation (532 nm).

    Fig. S8. Measurement setup for electric field–dependent absorbance.

    Fig. S9. Measuring the transmittance of the CQD layer.

    Fig. S10. Steady-state absorbance measurement.

    Fig. S11. Gaussian fitting (red dashed line) of absorbance spectra (black solid line).

    Fig. S12. Integrated area of A3 as a function of electric field.

    Fig. S13. Fluence-dependent lifetime probe at peak wavelength.

    Fig. S14. Fitting results of the subtraction of PL dynamics at different electric fields.

    Fig. S15. Optical transition processes in neutral CQDs (X), singly charged CQDs (X1−), and doubly charged CQDs (X2−).

    Fig. S16. COMSOL simulation to determine the coupling efficiency.

    Fig. S17. Photon emission density as a function of pumping intensity.

    Fig. S18. The 2d emitted photon decay map under each electric field at the pump fluence of 800 μJ/cm2 and 1000 μJ/cm2.

    Fig. S19. Population dynamics under an electric field of ~21 kV/cm and with a pump fluence of 100 μJ/cm2.

    Fig. S20. Population dynamics under an electric field of ~21 kV/cm and with a pump fluence of 1000 μJ/cm2.

    References (3945)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Electric field and current in the CQD device
    • Section S2. ASE in the CQD device without electric field
    • Section S3. Steady-state absorption measurement
    • Section S4. PL dynamics analysis
    • Section S5. Kinetics model
    • Fig. S1. Schematics of the circuit.
    • Fig. S2. Thickness measurement conducted by surface profiler (DektakXT surface profiler).
    • Fig. S3. Permittivity measurement result.
    • Fig. S4. Mutual corroboration to calibrate the voltage drop.
    • Fig. S5. Energy schematic of the device.
    • Fig. S6. Measured current flowing through the circuit.
    • Fig. S7. Edge emission as a function of pump fluence without electric field under 500-ps laser pulse excitation (532 nm).
    • Fig. S8. Measurement setup for electric field–dependent absorbance.
    • Fig. S9. Measuring the transmittance of the CQD layer.
    • Fig. S10. Steady-state absorbance measurement.
    • Fig. S11. Gaussian fitting (red dashed line) of absorbance spectra (black solid line).
    • Fig. S12. Integrated area of A3 as a function of electric field.
    • Fig. S13. Fluence-dependent lifetime probe at peak wavelength.
    • Fig. S14. Fitting results of the subtraction of PL dynamics at different electric fields.
    • Fig. S15. Optical transition processes in neutral CQDs (X), singly charged CQDs (X1−), and doubly charged CQDs (X2−).
    • Fig. S16. COMSOL simulation to determine the coupling efficiency.
    • Fig. S17. Photon emission density as a function of pumping intensity.
    • Fig. S18. The 2d emitted photon decay map under each electric field at the pump fluence of 800 μJ/cm2 and 1000 μJ/cm2.
    • Fig. S19. Population dynamics under an electric field of ~21 kV/cm and with a pump fluence of 100 μJ/cm2.
    • Fig. S20. Population dynamics under an electric field of ~21 kV/cm and with a pump fluence of 1000 μJ/cm2.
    • References (3945)

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