Net energy up-conversion processes in CdSe/CdS (core/shell) quantum dots: A possible pathway towards optical cooling

An investigation of the possibility of optical refrigeration (OR) on zinc-blende cadmium selenide/cadmium sulfide (CdSe/CdS) core/shell structure quantum dots (QDs) has been carried out. Quality samples were synthesized in our laboratory, and significant energy up-conversion photoluminescence (UCPL) was observed in these samples, showing the potential of generating net cooling effects. To better understand and predict the UCPL characteristics of the QDs, a semiempirical model has been developed, showing good agreement with our experimental results. The model takes into account the corresponding quantum yield and cooling efficiency, predicting the possibility of realizing optical refrigeration on a CdSe QD system.

Figure 1

Schematic plot of the off-axis collecting system: A: polarizer, B: focusing lens, C: sample holder, D: collection objective, E: analyzer, F: highly reflective mirror, G: adapting objective.

Figure 2

(a) Absorption spectrum of the sample. (b) High-energy excitation PL spectrum in logarithmic scale (${\varepsilon }_{\text{ex}}=3.06\text{eV}$). (c) Time-dependent radiative decay intensity curve of the QD sample. The curve was fitted with a single exponential curve with a decay lifetime $\tau =24$ ns. All the data shown were collected with sample 3.

Figure 3

(a) Excitation-energy-dependent PL spectra (PL intensities were normalized with respect to the observed laser signals) of sample 3. (b) Excitation intensity dependence of the PL intensity of sample 4 at ${\varepsilon }_{\text{ex}}=1.941\text{eV}$. The line is a linear fit to the data.

Figure 4

(a) HPL spectra of CdSe/2CdS (right) and CdSe/4CdS (left) samples. The intensities are adjusted to match each other for easy comparison; both samples were not finished with CdFt. (b) PL spectra of CdSe/2CdS (top) and CdSe/4CdS (bottom) samples excited at 1.956 eV (91 meV lower than the HPL peak energy) and 1.918 eV (89 meV lower than its HPL peak energy), respectively. The PL intensities were normalized by their excitation intensities.

Figure 5

PL spectra of sample 3 with excitation energies at 3.05 eV, 1.956 eV, 1.946 eV, 1.940 eV, 1.935 eV, 1.926 eV, and 1.918 eV (from right to left). For ease of comparison spectral intensities were adjusted to have the same maximum value. The energy of the HPL maximum is marked by the vertical line.

Figure 6

Scheme of excitonic processes involved in the UCPL in CdSe/CdS QDs. CB: bottom of the QDs' intrinsic conduction band. VB: top of the QDs' intrinsic valence band. SET: shallow surface electron trapping states.

Figure 7

Simulated PL spectra with (a) $k_{\text{r}}/k_{\text{s}}=1$ and (b) $k_{\text{r}}/k_{\text{s}}=0.5$, where an increased intensity indicates a larger iteration step. (c) Fully iterated PL spectra with $k_{\text{r}}/k_{\text{s}}=1$ and $k_{\text{r}}/k_{\text{s}}=0.1$. (d) and (e) show simulation results (thin lines) fitted to the experimental data (thick lines, obtained from sample 3 with${\varepsilon }_{\text{ex}}=1.935\text{eV}$) with (d) $n=3, {\varepsilon }_{\text{s}}=1.976\text{eV}$, and ${\sigma }_{\text{s}}=38$ meV ($\mathrm{FWHM}=89\text{meV}$) and (e) $n=2, {\varepsilon }_{\text{s}}=2.000\text{eV}$, and ${\sigma }_{\text{s}}=34\text{meV}$ ($\mathrm{FWHM}=80\text{meV}$). In both simulations $\beta /k_{\text{r}}=0.01$ (ensuring the excitonic density is less than 1).

Figure 8

(a) Simulated PL spectra (ninth iteration) with ${\varepsilon }_{\text{ex}}=1.956\text{eV}$ (bottom), 1.919 eV (middle) and 1.919 eV (top); the inset is a close-up view of the small rectangular area. (b) PL spectra of the sample with ${\varepsilon }_{\text{ex}}=1.956\text{eV}$, 1.945 eV, 1.938 eV, 1.929 eV and 1.919 eV (from top to bottom).

Figure 9

Estimated cooling efficiency of sample 2 as a function of ${\varepsilon }_{\text{ex}}$. (a) From top to bottom: $\eta =1$, 0.997, 0.994, and 0.991. (b) From top to bottom: $\frac{{\alpha }_{\text{b}}}{\alpha }=100$, 500, 1000, and 1500 ppm. $\alpha$ is obtained from the absorption spectrum.