Applied Science and Convergence Technology 2023; 32(2): 38-40
Published online March 30, 2023
Copyright © The Korean Vacuum Society.
Semiconductor Physics Research Center, School of Semiconductor Science and Technology, Jeonbuk National University, Jeonju 54896, Republic of Korea
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
The local surface plasmon effects of colloidal cadmium selenide (CdSe) quantum dots (QDs) were observed at a low temperature (approximately 5 K). As the wavelength between the CdSe QDs and metals is precisely tuned, the enhancement effects of CdSe photoluminescence (PL) significantly increase because of the increased coupling effects between the plasmonic fields of the metal surfaces of Au and Ag and the exciton states of a single CdSe QD. The PL enhancement effect of Au was observed to be approximately six times higher than that of Ag because of enhanced coupling. This result demonstrates the importance of wavelength matching in maximizing the surface plasmon effects and achieving enhanced PL in various device applications.
Keywords: Colloidal cadmium selenide quantum dots, Gold layers, Photoluminescence enhancement, Local surface plasmon effects
Semiconductor nanostructures such as colloidal quantum dots (QDs) have attracted substantial interest because of their significance in fundamental physics research on nanostructures and their prospective future applications [1–12]. Owing to the recent advancement of the manufacturing techniques for colloidal QDs, many research groups have focused on observing the effects of local surface plasmon, which enhances the photoluminescence (PL) of colloidal cadmium selenide (CdSe) QDs on metal surfaces. The amplified PL enables the use of colloidal QDs in diverse research areas to improve display brightness efficiency with higher resolution for biological applications such as bacteria imaging [13–17]. To realize the local surface plasmon effect, it is crucial to determine the optimal requirements for maximizing the resonance wavelengths between the colloidal CdSe QDs and metals. Surface plasmon effects have been observed between high-density colloidal QDs and metallic surfaces. Studies on the effective operation of local surface plasmonic effects at the single-QD level are limited; however, such research is necessary to design high-resolution devices using a single QD. The surface plasmon effects function effectively at the single-QD level, as shown by a comparison between the Au and Ag surfaces. First, the emitted PL of high-density CdSe QDs and singlelevel CdSe QDs were compared. The PL of the Au and Ag metal surfaces was directly observed by tuning the resonance wavelengths after selecting the single-level CdSe QDs. Subsequently, to confirm the importance of the resonant wavelengths, micro-PL measurements were obtained using a confocal micro-PL system at 5 K to avoid other effects, such as thermal vibration and phonon scattering. This study confirmed the presence of optimal resonant wavelengths that maximize surface plasmon effects via 1) single-level CdSe QD measurements and 2) a comparison of micro-PL between single CdSe QD–Ag and –Au metal surfaces. The results are essential for designing bioapplication devices and high-efficiency displays that use local surface plasmon effects.
Commercial Sigma-Aldrich CdSe QDs were used for the experiment. They were preserved in toluene and chemically scattered at regular intervals on Au and Ag metal surfaces to compare the surface plasmon effects in terms of the resonant wavelengths. To attach the CdSe QDs to the metal surface, the 2-carboxyethylphosphonic acid polymer was first stacked; then, the CdSe QDs were attached, and poly(methyl methacrylate) was subsequently used to protect against aging and abrasion. The layer-by-layer (LBL) method used for preparing CdSe QDs on metals is shown in Fig. 1. This is an effective tool that provides an inexpensive and effective method for observing the effects of local surface plasmon on single-level PL measurements at low temperatures. After the CdSe QDs were attached to the metal surfaces, optical measurements were conducted to observe the micro-PL signal from a single CdSe QD. As shown in Fig. 2, a typical micro-PL setup was used with a frequency-doubled Ti:sapphire laser (400 nm) as the excitation source. The PL of a single CdSe QD excited using a Ti:sapphire laser was observed using an objective lens, and the collection was performed using a grating spectrometer. Finally, the dispersed signal was investigated using a charge-coupled device. In this experiment, a Mai Tai laser was used to excite the CdSe QD. When the Mai Tai was pumped using a 532-nm-diode laser, the output laser pulses were 100 fs in duration and 80 MHz at 800 nm, generating an average power of 1 W. (The 800-nm-pulse laser was frequency doubled to 400 nm.)
The coherent stimulation of charged particles, such as electrons on a metal surface, significantly enhances the optical signal. Owing to the interactions between the light and electric fields on the metal surface, the wavelengths of the exciton states of CdSe QDs and the electric fields from the metal are crucial for maximizing the effects of surface plasmon absorption [8–12,18,19]. In Fig. 3, a Gaussianshaped PL peak can be observed; this peak originates from the energy transition between the valence and conduction bands. It is confirmed that this observed symmetric PL peak has the characteristics of exciton states; when the laser excitation power is increased, the integrated area of this PL peak increases linearly, as shown in Fig. 3(a). The powerdependent linear increase serves as evidence for the exciton states; otherwise, super-linear increases and increases between linear and superlinear as a function of the excitation power correspond to biexciton and charged exciton states, respectively. Therefore, the PL peak, which exhibits a linear power dependence, provides clear evidence of the exciton states of a single CdSe colloidal QD. The wavelengths of the local surface plasmon effects on the Au metal surfaces and electron–hole pairs from the CdSe QDs match; therefore, the PL enhancement with plasmon effects becomes dominant. For the Au metal surfaces, the emission wavelength of the CdSe QDs was approximately 540 nm, and the absorption of the Au metal surface was approximately 540 nm. To create this resonant local surface plasmon effect using a single CdSe QD, the emission of exciton states at 540 nm was carefully selected to assess the significance of the resonant wavelength within the absorption ranges of Au or Ag because the emission wavelengths of exciton states are adjusted based on the size of the chemically synthesized QDs.
However, the absorption of the Ag metal surface was observed at approximately 580 nm, which is far from the emission wavelengths of the electron–hole pairs from the CdSe QDs. As shown in Fig. 3(b), the observed micro-PL signal from the exciton states of a CdSe QD on Au increases seven times compared with that of a bare CdSe QD. However, a PL peak increase of only 1.2 times was observed for the CdSe QD on Ag compared with the micro-PL signal of exciton states from a bare CdSe QD, as shown in Fig. 3(c). The spectral blue shift can be observed clearly only in Fig. 3(b), where a single CdSe QD on Au is dispersed, and the resonant wavelength is matched. This spectral blue shift is a result of the local surface plasmon effect. When a single CdSe QD interacts resonantly with the absorption of the Au metal layer, the dielectric constant and thermal response are altered. A PL peak shift can be experimentally observed because of these variations caused by interactions between the exciton states of the CdSe QDs and the Au metal layer. The spectral shift is shown in Fig. 3(b). The effect of the local surface plasmon is not dominant when the resonant wavelength shifts away for the combination of the exciton states of the CdSe QD and Ag metal surfaces, as shown in Fig. 3(c). These experimental results demonstrate that the effect of the local surface plasmon is effective for a single-level CdSe QD, which can be used to obtain high-resolution emission sources. Furthermore, the conditions of resonance wavelengths are confirmed to be essential for maximizing the optical signals from a single CdSe QD, which can be used to achieve a highly secure bright image.
This study investigated the optimal condition for maximizing the local surface plasmon effects via wavelength tuning between a single CdSe QD and different metal surfaces (Au and Ag). The emission wavelength of a single CdSe QD aligned with the absorption wavelength of the Au metal surface, resulting in a seven-fold increase in the PL intensity. In addition, a spectral blue shift was observed, thus supporting the maximized local surface plasmon effects of the CdSe-QDAu- metal combined system. However, when the emission wavelength of the CdSe QD and the absorption wavelength of the Ag metal surface were mismatched, only a 1.2-fold increase was observed in the PL intensity; moreover, no spectral shift was observed. This correlation effect provides the optimal condition for designing high-resolution bright screens and biological devices, thus highlighting the importance of aligning the emission wavelengths of the exciton states of a single CdSe QD with the absorption wavelengths of the metal surface.
This research was supported by the National University Promotion Program of Jeonbuk National University in 2021.
The authors declare no conflicts of interest.