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Abstract
The metal nanoarray can couple the excitation light energy to the surface, resulting in local electromagnetic field enhancement due to the resonance effect. This is beneficial to the generation of nonlinear optical processes which depend on electromagnetic resonance enhancement, taking advantage of the field enhancement properties of metal nanoarray. Here, silver nanoperiodic arrays are integrated with AgInS2 quantum dots (QDs) to enhance the second harmonic generation (SHG) process of QDs. The experimental results show that the enhancement factor of SHG is 8.8-fold in the condition of surface plasmon resonance. In contrast, the second harmonic emission from pure quantum dots is very weak. The simulation reveals that the second harmonic enhancement is caused by the resonance between the incident laser and the Ag nanoarray. The experimental results show that it is feasible to generate a highly efficient nonlinear optical process of QDs assisted by metal nanoarray. This is beneficial for extending the nonlinear applications of quantum dots.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As a kind of quasi-zero-dimensional nanomaterial, quantum dots have been widely studied and applied in life sciences and semiconductor devices such as optical sensing, image display, nano drug loading, etc [1–10], due to their good photoluminescence properties and easy preparation processes [11–15]. However, the applications of nonlinear optical characteristics of quantum dots, especially harmonics, are limited because of low conversion efficiency. In fact, some semiconductor nanomaterials also have similar limitations. For quantum dots, in addition to the defects of the material itself, even if it meets the condition of anisotropic, the eigen absorption of most quantum dots is biased to the position of short wavelength, which is unfavorable for the generation of high-frequency harmonic signals. Even if the structure of the material is changed and the surface defects of the material are reduced, the improvement of harmonic conversion efficiency is very limited. Because nonlinear optical processes are supposed to be more dependent on the resonance enhancement of electromagnetic fields
In recent years, the regulation of light field based on two-dimensional nanoarray (or metasurface) has been widely studied [16–19]. According to the different structural parameters of microunits, the incident electromagnetic field energy of a specific wavelength can be coupled to the structural surface under the condition of localized surface plasmon resonance (LSPR). It increases the strength of the local electromagnetic field by orders of magnitude. Recently, some applications have been reported on the integration of nanostructures (nanoarrays or metasurfaces) based on micro-nano fabrication with nanomaterials. In particular, the use of nanoarrays can significantly enhance the fluorescence emission [20–25] and infrared absorption [26–28] of materials. They all take advantage of the strong coupling effect of nanometer array to the external electromagnetic field, and significantly improve the optical properties of materials by increasing the probability of effective carrier transition. This is also applicable to improve the nonlinear optical properties of QDs. Although QDs’ eigen absorption [29–31] is adverse to high-frequency nonlinear effects [32], it does not mean that their nonlinear optical applications are prohibited. In addition, nonlinear emission enhancement of halogen perovskite quantum dots based on array structure has also been reported [33]. This indicates that it is feasible to enhance the nonlinear emission of QDs by using the resonance of electromagnetic field (Fig. 1). I-III-VI ternary AgInS2 semiconductor quantum dots have excellent optical properties but do not contain toxic heavy metals. It is a kind of new nanomaterial which is friendly to environment. Although it has been applied in biological research [34,35], the research based on its nonlinearity is still absent. Therefore, the integration of AgInS2 quantum dots into nanoarrays can make full use of its nonlinear absorption and optical properties of the second harmonic enhanced emission, and expand the application range of its nonlinear. It is meaningful for the design of environmentally friendly nonlinear optoelectronic devices and biosensors with good prospects.
In this paper, we fabricated near infrared absorption silver nanoperiodic array metasurface by electron beam etching, and visualized its nonlinear characteristics by microscopic imaging method. The spin coating method was used to form the QDs film and integrate it into the nanoarray, and SHG process was studied. Experimental results show that the conversion efficiency of the second harmonic is significantly improved under the condition of local surface plasmonic resonance.
2. Experimental section
2.1 Theoretical simulation of Ag nanoarrays
We designed a periodic Ag nanoarray whose LSPR response wavelength locates in the near-infrared band, and simulated its light field characteristics by using the finite-difference time-domain method (FDTD). The structural parameters are shown in Fig. 2(b). The designed structure has a period of T = 800 nm, and the light source is a plane wave with wavelength of 800 nm. The polarization direction of the light source is in the Y-axis direction (See in Fig. 4(a)). As shown in Fig. 2(c), we simulate the transmittance of silver nanoarray structures from 300 nm to 1100 nm. A resonant dip can be seen at 800 nm, and it is formed by the surface plasmon resonance of nanostructure and the interference between Ag nanostructure. In addition to the optical field, there will be a radiation field of interaction between each nanounit, due to the existence of the oscillating dipole moment.
Fig. 2. FDTD theoretical simulation results. (a) Ag nanoarray under the action of femtosecond light is simulated theoretically (The polarization direction of the light source is in the Y-axis direction (See in Fig. 4(a)). (b) The lattice constants of Ag nanometer arrays are period T=800 nm, length L=240 nm, width w = 200 nm, and height is 30 nm. (c) The transmittance curve of the nanoarray. (d) Electric and magnetic field components of Ag nanostructures at 800 nm.
The corresponding local field intensity can be expressed as
here, Edip(ω) is the radiation field generated by the oscillating dipole moment of the adjacent nanounit. For the designed nanoarray structure [36], where ε0 is the permittivity in a vacuum, and P(ω) is the polarization intensity. There is no strong resonance near 400 nm (Fig. 2(c)), and this illustrates that the effect of field enhancement is mainly the result of fundamental wave resonance. This enhancement effect is associated with the tight field confinement of the pump between the nanostructural units supporting electric/magnetic dipolar resonant modes. Figure 2(d) shows the electric and magnetic field components of Poynting vector corresponding to the wavelength of 800 nm, in which the electric dipole resonance is obviously enhanced. Here, we aim to achieve local field enhancement using the electric dipole mode, because the effective excitation of the magnetic dipole mode inside the Ag unit is very weak.2.2 Preparation of the sample
According to the results of the theoretical simulation, we designed Ag nanoarrays operating in near infrared region and fabricated it on sapphire substrate using the high resolution Electron Beam Lithography (EBL) process. Ag nanoarrays have an area of 500 µm × 500 µm, and a period of about 810 nm. The L and W of a single Ag nanometer unit are about 239.4 nm and 203.1 nm respectively (Fig. 3(b)). Silver indium sulfide (AgInS2) with Ag/In molar ratio of 1:1 were synthesized via a hot-injection method reported earlier [37]. The integration of nanoarrays and QDs was achieved by a simple spin-coating method. Specifically, 10 µL AgInS2/chloroform solution (5×10−3 M) was dropped onto the sapphire substrate, followed by spinning at 300 rpm for 60 s and repeated the procedure three times after the surface liquid evaporates. The preparation process was conducted in the glovebox under Ar gas at room temperature.
Fig. 3. Characterization image of AgInS2 quantum dots and Ag nanoarrays. (a) TEM images of AgInS2 quantum dots. The average particle size of the quantum dots is about 5 nm. (b) Absorption and emission spectra of AgInS2 quantum dots. (c) SEM image of Ag nanoarray. Ag nanometer arrays are period T=810 nm, length L=239.4 nm, width w = 203.1 nm. (d) The experimentally measured transmission spectra.
3. Results
3.1 Nonlinear optical characterization of Ag nanoarrays
To study the nonlinear optical properties with corresponding visualization, we have constructed an optical microscopy system with spectral imaging function. The light source is a broadly tunable (680-1300 nm) femtosecond laser (80 MHz, 100 fs pulse duration, Chameleon Discovery, Coherent. See Fig. 4(a)). The optical system consists of imaging module and spectral module. A dichromatic mirror (DM, LP: 680 nm) was used to separate the strong excitation light from the signal light, and two PMTs were used for imaging at different wavelength channels. Spectral signals were collected by adjusting the filter (FB 400/10 nm) and the external spectrometer. As shown in Fig. 4(a), the samples were irradiated with 800 nm femtosecond pulses in the normal direction, and the two-photon fluorescence emission (TPFE) and SHG imaging were performed in backward detection mode.
Fig. 4. Spectral imaging system and optical properties of Ag nanoarrays. (a) The spectral imaging system consists of two modules: imaging and spectrum acquisition. The imaging module is composed of shortwave (SW)-PMT and longwave (LW)-PMT respectively, which can image signals of different wavelengths. The spectral module consists of an external spectrometer. The objective lens is switchable between 20× and 60×. (b) Nonlinear optical characterization of nanoarrays. Nanoarrays correspond to two-photon and second harmonic generation imaging under a 20× objective lens. The nanoarray period was 500 µm × 500 µm, and the field of view was 410 µm × 410 µm using a 20× objective lens. The 60× objective corresponds to the surface plasma (hot spot) imaging of the nanoarray. The “hot spot” image (corresponding to the three modes in Figure above) is obtained using a 60× objective lens.
The results of imaging and spectrum analysis show that the SHG signal of pure Ag nanoarray is relatively weak. In contrast, the signal of two-photon imaging is strong (Fig. 4(b)). Meanwhile, three channels including SHG, two-photon fluorescence (TPE mode including short wave (SW) channel TPE and long wave (LW) channel TPE) were used to image the hot spots of silver nanoarray using 60× magnification, NA=1.4 objective (Fig. 4(b)). The hot spot is the result of the coherent interaction between Ag nano unit. The formation of surface plasmons indicates that the energy of the excitation light is effectively coupled to the surface of the medium. In general, a medium with a higher surface concentration of free electrons, such as a metal, is more efficient in energy coupling. Surface plasmons increase the concentration of carrier between energy level transitions, which directly affects the improvement of nonlinear conversion efficiency. In addition, the uniformity of the field enhancement distribution is demonstrated by the spatial distribution of hot spots. It can be seen from the nonlinear imaging characterization that the prepared nanoarray structure basically meets the design requirements.
3.2 Nonlinear emission enhancement of QDs
SHG spectrum of the prepared sample was measured using the system shown in Fig. 4(a). A femtosecond pulse of 800 nm was used to excite the sample and the spectrometer picked up the signal at 400 nm (Fig. 5(a)). Specifically, the fitting power slope in log-log graph is about 2, clearly demonstrating its nonlinear characteristic (Fig. 5(b)). After changing the excitation wavelength, the signals were collected at the corresponding 2ω, and these results proved the SHG (Fig. 5(c)). As a control group, the pure silver film (non-array) integrated quantum dots were tested. Under the same excitation conditions, the patterned Ag nanoarray structure can limit the light for a longer time than the simple silver film, which causes the incident light to accumulate on the surface, resulting in the increase of the local electric field amplitude. As a contrast experiment, we test the second harmonics of pure quantum dots and quantum dots integrated with silver film (without nanoarray structure). The experimental results show that the second harmonic signals under the two conditions are consistent (Fig. 5(d)). The enhancement of the SHG signal is limited because the pure silver film cannot effectively couple the incident light energy to the structural surface. From the above experimental results, it can be seen that the SHG of AgInS2 QDs is significantly enhanced after being integrated with Ag nanoarray.
Fig. 5. Second harmonic experimental results. (a) Second Harmonic Spectra of AgInS2 quantum dots integrated by Ag Nanoarray (The power variation is achieved by using a continuous plate runner). (b) The integrated intensity of photoluminescence as a function of incident power. Here the wavelength is fixed at 800 nm. (c) Second harmonic spectrum with different excitation wavelengths (800 nm, 840 nm, 880 nm, and 920 nm, respectively). (d) SHG spectra of AgInS2 quantum dots spun-coating on sapphire substrates. (e) The relationship between SHG&TPFE enhancement factor and excitation wavelength (the excitation power is kept constant at 2.7 mW when the wavelength is changed). (f) The integrated photoluminescence intensity from the QDs solution and film; (e) is obtained by normalizing the emission from metasurface with the red line.
For a second order nonlinear process, its amplitude is proportional to the square of the electric field amplitude, and the enhancement of the nonlinear signal strength is expressed as ${E_n} \propto {|{{E_{ave}}} |^4}/{|{{E_{in}}} |^4}$. In the experiment, SHG signal was also enhanced at non-resonant wavelengths compared with QDs alone. Compared with LSPR condition, the enhancement of non-resonance field is much weaker, which is determined by the structural parameters of the Ag nanoarray, so the influence of non-resonance is not considered here. Two-photon fluorescence emission (TPFE) is a three-order process
As mentioned above, local field enhancement of Ag nanoarrays enhances nonlinear processes of QDs. At the resonant wavelength, the two-photon fluorescence emission of QDs is also significantly enhanced due to the local field enhancement. This is exactly what we have observed in the experiment. The QDs only show weak photoluminescence when excited by off-resonant incident laser. In order to better explain the experimental results, the field enhancement factor was defined as $F = {I_{Array@QDs}}/{I_{QDs}}$, and the enhancement factor is normalized to the emission from pure QDs films. We measured both SHG and TPFE intensity information of QDs film at different excitation wavelengths (the same power density) before and after its integration with Ag nanoarray (See Fig. 5(f) for the normalized information of spectral lines). The experimental results are shown in Fig. 5(e). Compared with TPFE, the enhancement factor of SHG is only 8.8, which is due to the intrinsic absorption of QDs, which leads to the suppression of the intensity of the second harmonic. The TPFE process is always below the intrinsic absorption band gap, so the effect of local field enhancement is more prominent. In addition, compared with QDs, the SHG signal of pure silver nanoarray is very weak due to “heat loss”, so it is not considered here. These experimental results are in agreement with the theory, which indicates that the local field enhancement is effective in increasing the optical conversion efficiency of QDs, and enhances the potential of optical nonlinear applications of QDs.
Here, AgInS2 “green” QDs were integrated with Ag nanoarray, and SHG emission of QDs was enhanced by the enhancement of electromagnetic field caused by LSPR of the nanoarray. Moreover, AgInS2 QDs have harmonic signals in a certain band range, and the wavelength of SHG signal is completely determined by Ag nanoarray. To some extent, it is also meaningful for the practical application of metal nanoarray.
4. Conclusion
In summary, we have shown that the SHG process can be enhanced by integrating Ag nanoarrays with AgInS2 QDs. The enhanced factor of SHG at 800 nm is 8.8-fold. The physical mechanism is that the Ag nanometer metal array plasma periodically enhances the near-infrared broadband absorption resulting in local electric field enhancement. Both the Ag nanoarrays and QDs can be fabricated/synthesized in a facile method, and the effective resonance wavelength of the metal array can be regulated by changing the period. For non-central symmetry nanomaterials, they can be integrated with metal nanocrystals to improve the nonlinear conversion efficiency. This could make sense for generating high frequency coherent light sources and biosensors.
Funding
National Natural Science Foundation of China (61620106016, 61835009, 61935012, 61961136005); Shenzhen Key projects (JCYJ20200109105404067); Shenzhen Technical Project (JCYJ20180305124902165); Shenzhen International Cooperation Research Project (GJHZ20190822095420249).
Disclosures
The authors declare no conflicts of interest.
Data availability
All data generated and analyzed are included in this paper. The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
1. Y. Y. Zhong, Y. L. Shao, F. K. Ma, Y. Z. Wu, B. B. Huang, and X. P. Hao, “Band-gap-matched CdSe QD/WS2 nanosheet composite: size-controlled photocatalyst for high-efficiency water splitting,” Nano Energy 31, 84–89 (2017). [CrossRef]
2. P. Emre O, M. Gabriel, N. Ivan, P. Eric, G. Teresa, G. Shuchi, M. Marc, P. J. José, B. Maryse, D. Turgut, K. Gerasimos, G. Stijn, and K. Frank, “Flexible graphene photodetectors for wearable fitness monitoring,” Sci. Adv. 5(9), eaaw7846 (2019). [CrossRef]
3. K. T. Yeon, P. Sungho, K. B. Jun, H. S. Been, Y. J. Hun, S. J. Seung, H. Jong-Am, K. Beom-Su, K. Y. Duck, P. Yongsup, and K. S. Jun, “Dual-functional quantum-dots light emitting diodes based on solution processable vanadium oxide hole injection layer,” Sci. Rep. 11(1), 1700 (2021). [CrossRef]
4. D. Hannaneh, D. Mahboubeh, Z. Armin, A. Farid, R. Ali, M. Samiye, and Y. Reza, “High-speed and high-precision PbSe/PbI2 solution process mid-infrared camera,” Sci. Rep. 11(1), 1533 (2021). [CrossRef]
5. Z. Mei-Xia and Z. Bing-Jie, “the research and applications of quantum dots as nano-carriers for targeted drug delivery and cancer therapy,” Nanoscale Res. Lett. 11(1), 207 (2016). [CrossRef]
6. S. Y. Zhong, Z. Nan, S. Y. Dan, Z. W. Wei, Y. Deju, X. J. Juan, and C. H. Yuan, “Activatable QD-based near-infrared fluorescence probe for sensitive detection and imaging of DNA,” ACS Appl. Mater. Interfaces 9(30), 25107–25113 (2017). [CrossRef]
7. E. Alexander L, D. James B, H. Alan L, M. Igor L, B. Mladen, and H. Timothy D, “Evaluating the potential of using quantum dots for monitoring electrical signals in neurons,” Nat. Nanotechnol. 13(4), 278–288 (2018). [CrossRef]
8. J. Kwon, S. W. Jun, S. l. Choi, X. Mao, J. Kim, E. K. Koh, Y.-H. Kim, S.-K. Kim, D. Y. Hwang, C.-S. Kim, and J. Lee, “FeSe quantum dots for in vivo multiphoton biomedical imaging,” Sci. Adv. 5(12), eaay0044 (2019). [CrossRef]
9. F. J. Hui, L. F. Ying, L. X. Jian, R. Xiang, F. D. Wei, W. Dan, M. H. Min, D. Bin, Z. Nuo, and W. Qin, “An amplification label of core–shell CdSe@CdS QD sensitized GO for a signal-on photoelectrochemical immunosensor for amyloid β-protein,” J. Mater. Chem. B 7(7), 1142–1148 (2019). [CrossRef]
10. S. L. Zhang, L. Liu, S. Ren, Z. Li, Y. Zhao, Z. Yang, R. Hu, and J. Qu, “Recent advances in nonlinear optics for bio-imaging applications,” Opto-Electron. Adv. 3(10), 200003 (2020). [CrossRef]
11. K. D. Wegner and N. Hildebrandt, “Quantum dots: bright and versatile in vitro and in vivo fluorescence imaging biosensors,” Chem. Soc. Rev. 44(14), 4792–4834 (2015). [CrossRef]
12. C. R. Kagan, E. Lifshitz, E. H. Sargent, and D. V. Talapin, “Building devices from colloidal quantum dots,” Science 353(6302), aac5523 (2016). [CrossRef]
13. J. M. Pietryga, Y.-S. Park, J. Lim, A. F. Fidler, W. K. Bae, S. Brovelli, and V. I. Klimov, “Spectroscopic and device aspects of nanocrystal quantum dots,” Chem. Rev. 116(18), 10513–10622 (2016). [CrossRef]
14. M. Alizadeh-Ghodsi, M. Pourhassan-Moghaddam, A. Zavari-Nematabad, B. Walker, N. Annabi, and A. Akbarzadeh, “State-of-the-art and trends in synthesis, properties, and application of quantum dots-based nanomaterials,” Part. Part. Syst. Char. 36(2), 1800302 (2019). [CrossRef]
15. V. Bharathi M, N. Roy, P. Moharana, K. Ghosh, and P. Paira, “Green synthesis of highly luminescent biotin-conjugated CdSe quantum dots for bioimaging applications,” New J. Chem. 44(39), 16891–16899 (2020). [CrossRef]
16. C. Cui, C. Zhou, S. Yuan, X. Qiu, L. Zhu, Y. Wang, Y. Li, J. Song, Q. Huang, Y. Wang, C. Zeng, and J. Xia, “Multiple Fano resonances in symmetry breaking silicon metasurface for manipulating light emission,” ACS Photonics 5(10), 4074–4080 (2018). [CrossRef]
17. X. Zhang, Z. Zhang, Q. Wang, S. Zhu, and H. Liu, “Controlling thermal emission by parity-symmetric fano resonance of optical absorbers in metasurfaces,” ACS Photonics 6(11), 2671–2676 (2019). [CrossRef]
18. Y. Yang, W. Wang, A. Boulesbaa, I. I. Kravchenko, D. P. Briggs, A. Puretzky, D. Geohegan, and J. Valentine, “Nonlinear Fano-resonant dielectric metasurfaces,” Nano Lett. 15(11), 7388–7393 (2015). [CrossRef]
19. H. Liu, C. Guo, G. Vampa, J. L. Zhang, T. Sarmiento, M. Xiao, P. H. Bucksbaum, J. Vučković, S. Fan, and D. A. Reis, “Enhanced high-harmonic generation from an all-dielectric metasurface,” Nat. Phys. 14(10), 1006–1010 (2018). [CrossRef]
20. A. Vaskina, R. Kolkowskia, A. F. Koenderinka, and I. Staudea, “Light-emitting metasurfaces,” Nanophotonics 8(7), 1151–1198 (2019). [CrossRef]
21. Y. Park, H. Kim, J. Lee, W. Ko, K. Bae, and K. Cho, “Direction control of colloidal quantum dot emission using dielectric metasurfaces,” Nanophotonics 9(5), 1023–1030 (2020). [CrossRef]
22. Q. Yuan, L. Fang, H. Fang, J. Li, T. Wang, W. Jie, J. Zhao, and X. Gan, “Second harmonic and sum-frequency generations from a silicon metasurface integrated with a two-dimensional material,” ACS Photonics 6(9), 2252–2259 (2019). [CrossRef]
23. A. Chu, H. He, Z. Yin, R. Peng, H. Yang, X. Gao, D. Luo, R. Chen, and G. Xing, “Plasmonically enhanced upconversion luminescence via holographically formed silver nanogratings,” ACS Appl. Mater. Interfaces 12(1), 1292–1298 (2020). [CrossRef]
24. Z. Wang, Y. Wang, G. Adamo, J. Teng, and H. Sun, “Induced optical chirality and circularly polarized emission from achiral CdSe/ZnS quantum dots via resonantly coupling with plasmonic chiral metasurfaces,” Laser Photonics Rev. 13(3), 1800276 (2019). [CrossRef]
25. J. S. Pang, I. G. Theodorou, A. Centeno, P. K. Petrov, N. M. Alford, M. P. Ryan, and F. Xie, “Tunable three-dimensional plasmonic arrays for large near infrared fluorescence enhancement,” ACS Appl. Mater. Interfaces 11(26), 23083–23092 (2019). [CrossRef]
26. O. Limaj, D. Etezadi, N. J. Wittenberg, D. Rodrigo, D. Yoo, S.-H. Oh, and H. Altug, “Infrared plasmonic biosensor for real-time and label-free monitoring of lipid membranes,” Nano Lett. 16(2), 1502–1508 (2016). [CrossRef]
27. A. Tittl, A. John-Herpin, A. Leitis, E. R. Arvelo, and H. Altug, “Metasurface-based molecular biosensing aided by artificial intelligence,” Angew. Chem. 58(42), 14810–14822 (2019). [CrossRef]
28. R. Semenyshyn, M. Hentschel, C. Huck, J. Vogt, F. Weiher, H. Giessen, and F. Neubrech, “Resonant plasmonic nanoslits enable in vitro observation of singlemonolayer collagen-peptide dynamics,” ACS Sens. 4(8), 1966–1972 (2019). [CrossRef]
29. R. H. Hurt, M. Monthioux, and A. Kane, “Toxicology of carbon nanomaterials: Status, trends, and perspectives on the special issue,” Carbon 44(6), 1028–1033 (2006). [CrossRef]
30. Nanoparticles and Nanodevices in Biological Applications, S. Bellucci, ed. (Springer Berlin Heidelberg, 2009).
31. C. B. Murray, D. J. Noms, and M. G. Bawendi, “Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites,” J. Am. Chem. Soc. 115(19), 8706–8715 (1993). [CrossRef]
32. Fundamentals of Nonlinear Optics, Peter E. Powers, ed. (Chemical Rubber Company, 2011).
33. Y. Fan, Y. Wang, N. Zhang, W. Sun, Y. Gao, C. Qiu, Q. Song, and S. Xiao, “Resonance-enhanced three-photon luminesce via lead halide perovskite metasurfaces for optical encoding,” Nat. Commun. 10(1), 2085 (2019). [CrossRef]
34. I. C. Carvalho, A. A. P. Mansur, S. M. Carvalho, R. M. Florentino, and H. S. Mansur, “L-cysteine and poly-L-arginine grafted carboxymethyl cellulose/Ag-In-S quantum dot fluorescent nanohybrids for in vitro bioimaging of brain cancer cells,” Int. J. Biol. Macromol. 133, 739–753 (2019). [CrossRef]
35. Y. Li, Z. Li, W. Ye, and S. Zhao, “Gold nanorods and graphene oxide enhanced BSA-AgInS2 quantum dot-based photoelectrochemical sensors for detection of dopamine,” Electrochim. Acta 295, 1006–1016 (2019). [CrossRef]
36. C. Kittel, Introduction to Solid State Physics, 5th ed. (Wiley, 1976).
37. T. Uematsu, K. Wajima, D. K. Sharma, S. Hirata, T. Yamamoto, T. Kameyama, M. Vacha, T. Torimoto, and S. Kuwabata, “Narrow band-edge photoluminescence from AgInS2 semiconductor nanoparticles by the formation of amorphous III–VI semiconductor shells,” NPG Asia Mater. 10(8), 713–726 (2018). [CrossRef]
