Figure 1

Time-resolved luminescence of CdS nanorods exhibiting long-lived dual emission fed by trap states. (a) Prompt (0–2-ns integration window) and 10-μs-delayed (10–10.1-μs integration window) optical emission spectra of nanorods confined in a thin polymer matrix film after 0.7-ns pulsed excitation from a 355-nm diode laser at 21 K. Prompt emission is dominated by band-edge exciton recombination, whereas the broad, red luminescence channel appearing at longer delay times is attributed to deep-level chemical defect states. The light blue (light gray) and red (dark gray) regions denote the spectral bandwidth of collection filters used for luminescence channel isolation in the pODMR experiments. Vertical blue- and red-dashed lines mark the spectral positions used to demonstrate the existence of long-lived trap states feeding each emission channel, as is evident by the long power-law-like PL decays given in panel (b). The detection gate integration window of the ICCD used for each step in time delay was: 2 ns for 0–510 ns; 20 ns for 0.51–2.0 μs; 100 ns for 2.0–10 μs; 1.0 μs for 10–20 μs. The inset shows a transmission electron micrograph illustrating the high quality of CdS nanocrystalline nanorods.Reuse & Permissions

Figure 2

Correlating trap states with spectrally dependent ODMR. (a) Optical excitation near the band edge populates the lowest exciton state. This exciton either recombines radiatively, localizes to shallow trap states (light gray lines), or dissipates to the chemical defect (thick black lines inside CB and VB). Long carrier trapping lifetimes allow for use of ESR in changing the mutual spin configuration of trapped charge pairs, modulating optically “bright” and “dark” population ratios and thereby affecting the resultant PL intensity from each of the two emission channels. (b) Spin resonance mapping and (c) resonance spectrum for shallow trap states affecting band-edge exciton emission; (d) and (e), the same for defect emission. Multiple resonances (i.e., optically active carrier states) are observed through each emission channel. Two resonances ($g_1\approx 2.00$ and $g_3\approx 1.85$) are found to be common to both emission channels, indicating that both band-edge excitons and emissive chemical defects interact with the same species of shallow band-edge trap states. (f) and (g) These two resonances arise due to a coupled carrier pair (i.e., electron and hole), as evidenced by the correlations in resonance peak areas and temporal dynamics for each emission channel. Peaks $g_1$ and $g_3$ have the same areas and exhibit identical dynamics. The dynamics of the broad-resonance $g_2$ [band-edge emission, black line in panel (f)] and the superimposed resonances $g_4$ and $g_5$ [defect emission, black line in panel (g)] differ from those of the respective carrier pairs (resonances $g_1$ and $g_3$, blue and red/dark gray dotted lines, respectively). Data were acquired under steady-state optical excitation after application of a single microwave pulse of approximately 800 ns duration. The quoted precision of the $g$-factors arises from the Gaussian fit to the resonance lineshape.Reuse & Permissions

Figure 3

Rabi oscillations as a probe of carrier interactions, detected in the low-energy defect emission: evidence for dipolar coupling. (a) Coherent Rabi oscillations are driven by a microwave field, which reversibly nutates the spin pair between optically bright and dark mutual spin configurations, enabling read-out of the spin state through the emission intensity. (b) and (c) Fourier transform of the Rabi oscillations of the emissive defect resonances $g_1$ and $g_3$ [as identified in Fig. 2e] and of the convoluted resonances $g_4$ and $g_5$. The insets show the measured Rabi oscillations in the time domain. The frequency components observed in panel (b) indicate the occurrence of dipolar coupling, whereas those in panel (c) are difficult to assign because of the convolution of multiple resonances. (d) The half-field resonance confirms the existence of dipolar coupling for at least one of the full-field signals.Reuse & Permissions

Figure 4

Hahn spin-echoes revealing slow dephasing, detected in the low-energy defect PL. (a) The conventional Hahn-echo pulse sequence is modified for pODMR to place the remaining state polarization into an optically observable state. (b) The decay of the spin echo recorded in defect emission for the center resonance feature [i.e. the convolution of peaks $g_4$ and $g_5$, as labeled in Fig. 2e] is biexponential, suggesting the involvement of two independently resonant carriers under the same resonance condition, but with distinct dephasing pathways. The very long coherence time of one of these carriers in effect allows probing of the corresponding chemical environment, leading to an electron-spin echo envelope modulation (ESEEM) of the signal. (c) ESEEM signal with the biexponential decay removed; (d) corresponding Fourier transform.Reuse & Permissions