1. Introduction

Tuning absorption and emission band of optical nanoemitters has prospective applications in optical information processing, such as optical modulating and switching [1]. Semiconductor quantum dots with tunable energy levels are the solid state “artificial atoms” and the building blocks for quantum information [2–4]. Strong surface plasmon resonance (SPR) of metallic nanostructures largely enhances the local field at the excitation and/or emission frequency of optical nanoemitters [5–7], which offers a powerful tool to tune optical properties of optical nanoemitters [8–11]. SPR could effectively modulate emission spectrum of multilevel nanoemitters by enhancing resonant transitions and suppressing nonresonant transitions [12,13]. Moreover, near-field coherent coupling between plasmon and exciton forms plexciton and generates hybridized absorption band [14–17].

Quantum-confined Stark effect in low-dimensional semiconductor materials due to strong confinement of electron and hole in a bound exciton leads to a prominent power-dependent absorption band shift [18–27]. Plasmon-enhanced Stark shifts in Au/CdSe core/shell nanoparticles have been demonstrated by Zhang et al. [28]. On the one hand, by tuning gap distance of gold nanoparticle dimer to selectively enhance spontaneous emission rate of the favor transitions, shaping emission spectra of multilevel fluorescent molecules is demonstrated by Ringler et al. [12]. On the other hand, by tuning nanoparticle size to overlap the favor Raman and plasmon modes, plasmonic control of the Raman spectrum of a single molecule in a gold-silver core-shell nanoparticle dimer is reported by Dadosh et al. [13]. Interestingly, spectral narrowing and red-shifting of CdSe/ZnS semiconductor quantum dots (SQDs) on the Ag island film is observed by Soganci et al. [29], but the detailed mechanism of spectral narrowing and shifting is unclear.

In this paper, we investigate power-dependent red-shift of two-photon luminescence (TPL) of CdSe/ZnS SQDs in the proximity of Ag nanorods (AgNRs) array, optimize spectral red-shift by tuning longitudinal surface plasmon resonances of the AgNRs array, then discuss the underlying physics processes, i.e. plasmon-enhanced quantum-confined Stark effect.

2. Synthesis and characterization

Our arrayed AgNRs were grown by using anodic aluminum oxide (AAO). AAO templates were fabricated by using a two-step anodization process [30]. The anodization voltage was stepwise reduced from 19 V down to 6 V to decrease the thickness of Al₂O₃ barrier layer. Silver nanorods were deposited in the pores of AAO templates by alternating current electrolysis (50 Hz, 5 V ac) in an electrolyte containing AgNO₃ (0.03 M) and H₂SO₄ acid (0.03 M) with a Pt counter electrode. The CdSe/ZnS core-shell SQDs were purchased from Invitrogen Corporation and drop-coated onto the barrier layer of AAO template. The diameter of SQDs is 13.4 ± 0.7 nm [31].

The AAO templates loaded with AgNRs were etched by a precise Ar ion polishing system (Gatan PIPS Model 691) and examined by scanning electron microscopy (SEM). The SEM was performed by using a FEG SEM Sirion 200 operated at an accelerating voltage of 25.0 kV. The TEM was performed by using a JEOL 2010HT operated at 100 kV. The absorption spectra were recorded by a UV-VIS-NIR spectrophotometer (Varian Cary 5000) by using a p-polarized source with an incident angle of approximate 80°. The excitation source for the measurements of SQDs TPL was generated by a mode lock Ti:sapphire laser (Mira 900, Coherent) with a pulse width approximate 3 ps and a repetition rate of 76 MHz, and the excitation wavelength was set to 804 nm. The laser scattering noise was blocked by a band pass filter. A lens with 70 mm focal-length was used to focus laser beam onto the sample. The incident angle of laser beam was approximately 80° and the area of focus spot on the sample was about 1.32 × 10⁻³ mm². The TPL from the sample was collected by reflection measurement, filtered by a couple filters and recorded by spectrometry (Spectrapro 2500i, Acton) with a liquid-nitrogen-cooled CCD (SPEC-10, Princeton). All the TPL measurements in the report were carried out at room temperature (~25°C).

3. Results and discussion

Figure 1 presents the nanostructures and absorption spectra of samples. Figure 1(a) illustrates the side view sketch of the AgNR array and CdSe/ZnS quantum dots nanocomplex. The thickness of alumina barrier layer (Δ) is about 7 nm. Figure 1(b) shows the SEM image of the AAO template loaded with AgNRs at the barrier side, with the thin Al₂O₃ barrier layer being removed by a precise Ar ion polishing system (Gatan PIPS Model 691). The diameter d_Ag and period of the AgNRs is about 18 ± 3 nm and 45 ± 4 nm. The TEM image of the AgNRs with λ_LSPR = 700 nm is shown in Fig. 1(c), and the length l_Ag is measured to be about 90 nm. The length of AgNRs is adjusted conveniently by varying the growth time t_g, and as a consequence the different LSPR peaks are obtained in the AgNR array nanosystem [30]. Figure 1(d) presents the absorption spectra of the AgNR arrays with λ_LSPR = 650, 670, 700, 710, 755 and 795 nm. The recorded incident angle (θ_in) is approximately 80°, and the oscillations in the absorption spectra are attributed to the surface interferences of the AAO templates.

 

Fig. 1 Nanostructures and absorption spectra of samples. (a) Schematic of SQD-AgNR nanocomplex. (b) SEM image of the AAO template loaded with AgNRs at the barrier layer side. (c) TEM image of the AgNRs with λ_LSPR = 700 nm. (d) Absorption spectra (θ_in = 80°) of AgNR arrays with λ_LSPR = 650, 670, 700, 710, 755 and 795 nm.

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Figure 2 shows normalized TPL spectra of CdSe/ZnS SQDs coated on the Ag nanorod array at excitation power P_exc = 5, 15, 20, 25, 30, 35, 50 and 65 mW. The LSPR wavelength λ_LSPR of the AgNRs is 670 nm, and the excitation wavelength λ_laser is 804 nm. The excitation laser is p-polarized and the electric field along the long axis of AgNR array is E_|| = E₀ sinθ_in / n_AAO, where θ_in is the incident angle and n_AAO is the refractive index of AAO template. From Fig. 2, one can see that the TPL peak red-shifts as the excitation power increases. The total re-shift reaches about 33 nm = 88.4 meV when P_exc increases from 5 mW to 65 mW, which is slightly larger than the full width at half-maximum (FWHM) of the emission spectrum of the CdSe/ZnS SQDs. The PL spectra of pure AgNRs mainly shows scattering laser light under the used excitation power. A typical shift rate of less than 0.1 nm/K was observed in the temperature-dependence PL spectra of the SQDs in the range of 5-80 °C. This result accords with previous report [32], and indicates the thermal effect is not the dominant physical process for our observed red-shifted luminescence.

 

Fig. 2 TPL spectra of SQD-AgNR nanocomplex with λ_LSPR = 670 nm and P_exc = 5, 15, 20, 25, 30, 35, 50 and 65 mW. The emission peak shifts from ~665 nm to ~698 nm as the excitation power increases from 5 mW to 65 mW. The edge ~740 nm in TPL spectra is caused by the band-pass filter.

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Figure 3 gives the normalized TPL spectra of the SQD-AgNR nanocomplex recorded at a fixed excitation power 65 mW. The LSPR wavelengths of the AgNR arrays are 555, 577, 610, 650, 670, 700, 710, 755, and 795 nm, respectively. It clearly shows that the TPL peak is tuned from 665 nm to 710 nm by adjusting LSPR of the AgNRs array from 555 nm to 795 nm with a fixed excitation power.

 

Fig. 3 TPL spectra of SQD-AgNR nanocomplex with λ_LSPR = 555, 577, 610, 650, 670, 700, 710, 755, and 795 nm, the excitation power is fixed at 65 mW.

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Power-dependent TPL peak wavelength of the SQD-AgNR nanocomplex are plotted in Fig. 4 , which clearly shows that the spectral redshift of SQD-AgNR nanocomplex is approximately proportional to the excitation power P_exc when P_exc is not too strong. The nanocomplex with shorter AgNRs (smaller λ_LSPR and far off-resonance to laser wavelength λ_laser) has a smaller spectral shift. The spectral shift becomes saturated at strong excitation region (P_exc > 35 mW) in the nanocomplex with λ_LSPR = 795 nm, which is very close to excitation laser wavelength (λ_laser = 804 nm), and the emission peak changes from 664 nm (1.867 eV) to 702 nm (1.764 eV) with a shift as large as about 38 nm (101 meV). Also, it is observed that the emission peak of SQDs on AAO template without AgNRs almost remains stable (shifts about 1.6 nm).The observed spectral shift in the weak excitation region are approximately summarized by the relationship

 

Fig. 4 Peak emission wavelength of the SQD-AgNR nanocomplex as a function of excitation power. Here, LSPR wavelengths of the AgNR arrays are 577, 610, 650, 670, 700 and 795 nm.

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We turn to theoretical analysis of the underlying physics mechanism of LSPR-dependent spectral shift in our SQD-AgNR nanocomplex. The Stark shift of spectral lines is enhanced by bound excitons in semiconductor nanostructures due to quantum confinement. The electron and hole confined in bound excitons are pulled in the opposite direction by the applied electric field. Strong light-matter coupling could also leads to Stark shift, and the amount of shift ΔE_Stark is proportional to the local light intensity |E_local|² [18,28,35]. Plasmon resonance significantly enhances quantum-confined Stark effect due to strong enhancement of local field. Moreover, strong plasmon-exciton coupling may result in boexcitonic interaction which can induce the spectral redshift [36–39]; the formula of Stark shift is modified as

where $\text{1/[(}{\omega }_{\text{Laser}}-{\omega }_{\text{LSPR}}{\text{)}}^{\text{2}}+{\gamma }^2]$ represents the dispersion behavior of the local field enhancement factor |f|², $\gamma$ is a damping factor, ${\omega }_{\text{Laser}}$ and ${\omega }_{\text{LSPR}}$ are the optical frequency of laser pulses and LSPR of the AgNR array, respectively. Equation (2) well explains observed behaviors of the spectral shift: (i) Δλ_Stark (= Δλ_Stark – Δλ₀) is proportional to excitation power P_exc; and (ii) Δλ_Stark saturates when ${\omega }_{\text{LSPR}}$ is close to ${\omega }_{\text{Laser}}$. The spectral shifting rate can be calculated from the relation

The experimental data and theoretical fitting curve of R_Shift as a function of λ_LSPR of AgNRs array are plotted in Fig. 5 , in which, the fitting parameters γ = 0.445 meV. One can see that the theoretical fitting coincide with the experimental data very well. Based on these experimental observations and theoretical analysis, we think that the giant spectral red-shift in SQD-AgNR nanocomplex is mainly attributed to the plasmon-enhanced quantum-confined effect.

 

Fig. 5 Spectral shifting rate R_Shift of the SQD-AgNR nanocomplex as a function of LSPR wavelength of the AgNRs array.

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4. Conclusion

In summary, we found giant dynamically red-shifting of TPL of CdSe/ZnS SQDs closely-packed in the proximity of Ag nanorods array. By tuning LSPR of AgNR arrays to close to the excitation laser wavelength, the spectral red-shift reaches approximately 101 meV, which is slightly larger than the FWHM of the emission spectrum of the CdSe/ZnS SQDs. LSPR-dependent shifting rate R_Shift of the SQD-AgNR nanocomposite is well explained by plasmon-enhanced quantum-confined Stark effect. These observations could find the applications in plasmon-based information processing nanodevices, such as optical modulating and switching.