Recently, semiconductor nanocrystals (NCs) have attracted substantial attention in fundamental research and applications [1–3]. The chemically synthesized methods for semiconductor CdSe NCs showed amazing advantages for size homogeneity, temperature stability, size tunability, and comparably low fabrication costs relative to epitaxially grown CdSe QDs [4–7]. Specifically, the sizes of CdSe NCs can be controlled by wet chemical methods, and the energy gap which is separated between the conduction band and valence band can be altered significantly. As the size of CdSe NCs decreases, the energy gap increases and energy peaks shift to shorter wavelengths (spectral blueshift). The emission color of CdSe NCs can be covered in the whole visible range depending on size variations. With these interesting properties, CdSe NCs can become an ideal candidate for optoelectronics such as light-emitting diodes, laser, and bio-sensors [8–15]. In particular, CdSe NCs obtained a higher density of electronic states near the edges of valence and conduction bands compared to bulk semiconductors. Therefore, higher densities of carriers possibly contribute to the emission processes. Moreover, as the sizes of CdSe NCs become smaller, the exciton binding energy is also enhanced and the separation of energy level becomes larger than the bulk.

In this study, we fabricate high quality CdSe NCs using chemical synthesis methods and perform time-integrated photoluminescence (PL) measurements to observe the transition energy levels of size dependent CdSe NCs at a low temperature (10 K). The time-resolved PL shows that the radiative exciton decay times are slower as the growth times are longer; in other words, the radiative carrier life time is longer as the sizes of CdSe NCs are increased. Our result is directly related to size-dependent transition energy levels and carrier dynamics without other effects, such as strain fields from epitaxial growth methods.

For a synthesis of CdSe NCs via a hot injection approach, a cadmium precursor solution was prepared by first heating a mixture of 128 mg of cadmium oxide and 1.58 mL of oleic acid in 20 mL of 1- octadecene at 120 °C to obtain a clear solution, and the mixture was degassed at 120 °C. After 1 hour of degassing, the mixture was further heated to 250 °C under nitrogen atmosphere and allowed to cool to injection temperature. The selenium injection solution was prepared by dissolving 78.9 mg selenium and 1 mL trioctylphosphine in 2 mL of 1-octadecene, and 3 mL of as-prepared selenium injection solution was rapidly injected into the cadmium precursor solution. The reaction mixture was maintained at 205 °C and extracted at different time intervals (5, 10, 20, 30, and 60 min). The resultant CdSe NCs were purified through precipitation with acetone and methanol. Finally, the precipitates were dispersed in toluene. The PL emission from the chemically grown CdSe NCs was gathered and then dispersed by a spectrometer and observed by a cooled charge-coupled detector. A time-correlated single-photon counting system was used to obtain TRPL data with a time resolution of 30 picoseconds.

Figure 1 shows the normalized PL spectra as a function of CdSe NCs sizes at 10 K. As the reaction mixture time of CdSe NCs growth is increased NC(1): 5 min, NC(2): 10 min, NC(3): 20 min, NC(4): 30 min, and NC(5): 60 min, the dominant PL peaks of each CdSe NCs showed exciton PL peaks as a result of the transition process from ground electron and hole. The shortest mixture time of CdSe NCs corresponds to the 440 nm center PL peak that can be seen in Fig. 1, and the PL peaks indicate a gradual redshift to longer wavelengths with increasing reaction mixture times. This redshift of transition PL peaks with increasing reaction mixture times can be attributed to increased sizes of CdSe NCs. To observe the optical properties of CdSe NCs, we investigated power-dependent PL measurements for NC(1) (center peak ~440 nm), NC(3) (center peak ~515 nm), and NC(5) (center peak ~560 nm), respectively. The selected PL spectra of CdSe NCs as the excitation laser power was increased from 0.2 to 1.3 mW showed increased PL intensities as a function of excitation laser powers in Fig. 2. However, power-dependent PL peaks from five different fabricated CdSe NCs showed no dominant variations in the peak positions, shapes, or widths as a function of excitation powers between 0.2 and 1.3 mW. These interesting results of the power-dependent PL spectra confirm that only PL intensity is increasing, without other factors such as power dependent state filling effects or energy transitions between defect states, which were previously reported at epitaxially grown CdSe QDs. The PL intensity increased linearly when the excitation power was increased, as shown in Fig. 3. In the x-axis, the laser excitation power was increased from 0.2 to 1.3 mW, where we could see that the selected excitation power magnitude and the corresponding PL intensity for excitation powers showed clearly linear increasing properties. This result strongly supports that our measured PL peaks corresponded to ground excitonic transitions of confined electron-hole pairs from different CdSe NCs.

Figure 1. PL spectra of CdSe NCs grown by chemical methods for different growth times among (a) 5, (b) 10, (c) 20, (d) 30, and (e) 60 min. As the growth times of CdSe NCs are increased, the PL energy peak shifts to longer wavelengths, which means the sizes of CdSe NCs are increased.

Figure 2. PL spectra of the exciton states from the synthesized CdSe NCs as a function of laser excitation powers from 0.2 to 1.3mW for (a) CdSe NC(1), (b) CdSe NC(3), and (c) CdSe NC(5).

Figure 3. Integrated PL intensity as a function of laser excitation powers. The results showed a clearly linearly increasing PL integrated intensity for higher excitation powers at different chemically growth times of (a) 5, (b) 20, and (c) 60 min.

Time-resolved PL measurements are conducted to observe carrier dynamics in CdSe NCs using time-correlated single photon counting with a femtosecond pulse laser. In Fig. 4(a), the exciton decay times of CdSe NCs are compared as a function of reaction mixture times, and double exponential fitting was used to analyze the lifetimes of differently sized CdSe NCs. Specifically, the PL emission of CdSe NCs shows periodic regularity because the excitation of Ti:Sapphire laser emits a pulsed signal that corresponds to the repetition rate. The observed times of the photon signals from CdSe NCs are measured by multiple pulses of Ti:Sapphire laser to make a statistical diagram for analysis. We can obtain the probabilities of detection photons during specific times. In this experiment, a commercial Becker & Hickl SPC- 630 single photon counting card which provides two inputs (one for the excitation laser source and another one for the detected PL signal from the photomultiplier tube) was used to observe the double exponential decay of exciton states in CdSe NCs.

Figure 4. (a) Time-resolved PL spectra at 10 K for different sizes of synthesized CdSe NCs that correspond to different center peak wavelengths from 440 to 560 nm. (b) As the sizes of CdSe NCs are increased, the longer radiative decay times are also increased from 2.1 to 3.5 ns. The longer decay processes correspond to radiative decay processes from total decay components.

The mixturetimes of CdSe NCs increased from 5 up to 60 min. As we discussed before, the sizes of CdSe NCs increased with increasing mixture times. Consequently, the decay times of CdSe NCs increased as a function of the sizes of CdSe NCs in Fig. 4(a). The results of timeresolved PL were carefully selected using a reconvolution process with instrument response function. In general, fast and slow components can be attributed to the non-radiative decay process and the radiative decay process, respectively. In Fig. 4(b), the slow decay components to understand radiative decay process are compared as a function of reaction mixture times. The radiative decay times (slow decay component) are observed as 2.1 ns for 5 min, and they are gradually increased to 3.5 ns for 60 min. When the reaction mixture time is longer, the corresponding radiative decay components also increase gradually. The variations of the size dependent decay components can be attributed to a reduction of the exciton oscillator strengths from larger CdSe NCs.

We investigated the transition of PL emissions and carrier dynamics in terms of the size differences of CdSe NCs using chemical growth methods at low temperature (10 K). The main peak of time-integrated PL corresponds to the transition process between ground electron and hole and shifts to longer wavelengths as the sizes of synthesized CdSe NCs become larger. We conducted power-dependent PL measurements, where the PL intensities are linearly increased as a function of laser excitation powers. These linearly increased PL intensities result from the excitonic properties of confined states in CdSe NCs. The decay times of size-dependent CdSe NCs showed both fast components and slow components from double exponential fittings. The longer components correspond to radiative decay times, and this radiative decay process is gradually increased from 2.1 ns for the smallest CdSe NCs to 3.5 ns for the largest CdSe NCs. As the sizes of CdSe NCs become larger, the oscillator strengths are reduced, thus resulting in increased radiative decay processes.

This research was supported by National University Promotion Program at Jeonbuk National University in 2021.