Elsevier

Optical Materials

Volume 86, December 2018, Pages 455-459
Optical Materials

Femtosecond two-photon absorption spectroscopy of copper indium sulfide quantum dots: A structure-optical properties relationship

https://doi.org/10.1016/j.optmat.2018.10.023Get rights and content

Highlights

  • The two-photon absorption spectrum of CIS quantum dots was characterized through the femtosecond Z-scan technique.

  • The nonlinear optical spectrum was modeled through the parabolic effective-mass approximation model.

  • The excitonic transition energies were phenomenologically corrected due to the stoichiometry of the nanocrystal.

  • A good agreement (magnitude and spectral position) between the experimental and theoretical data were obtained.

Abstract

We have interpreted the two-photon absorption spectrum of water-soluble copper indium sulfide (CIS) QDs with stoichiometry 0.18 (Cu), 0.42 (In), and 2 (S) and an average diameter of approximately 2.6 nm. For that, we employed the wavelength-tunable femtosecond Z-scan technique and the parabolic effective-mass approximation model, in which the excitonic transition energies were phenomenologically corrected due to the stoichiometry of the nanocrystal. This model considers a conduction band and three valence sub-bands allowing excitonic transitions via centrosymmetric (Δl = ±1, where l is the angular momentum of the absorbing state) and non-centrosymmetric (Δl = 0) channels. In such case, this became relevant because the CIS QDs with chalcopyrite crystalline structure is a non-centrosymmetric semiconductor. Thus, our experimental results pointed out two 2 PA allowed bands located at 715 nm (2hv = 3.47 eV) and 625 nm (2hv = 3.97 eV) with cross sections of (6.3 ± 1.0) x 102 GM and (4.5 ± 0.7) x 102 GM, respectively. According to the theoretical model, these 2 PA bands can be ascribed to the 1P1/2(h3) → 1S3/2(e) (lower energy band) and 1P1/2(hheavy) → 1S3/2(e) (90%)/(10%)1P1/2(hsplit-off) → 1P3/2(e) (higher energy band) excitonic transitions. A good agreement (magnitude and spectral position) between the experimental and theoretical data were obtained. However, our experimental data suggest that the higher-energy 2 PA band may have other contributions due to the mixing between the heavy- and the light-hole bands, which the effective mass model does not take into consideration.

Introduction

The basic and applied research of the frequency-resolved nonlinear optical (NLO) properties of materials is closely associated with the development of new technologies. For example, organic and semiconductor materials have been investigated as active medium for solar cell applications [1,2], all-optical switching [3], photonic circuits [4,5], light emitting devices [6,7], RGB devices [8], field effect transistors [9,10], to name a few. For the development of these technologies, a wide knowledge of the optical response of potential materials for the implantation of optoelectronic devices is necessary. In particular, low-dimensional materials such as nanocrystals have attracted great interest of the academic and industrial community [[11], [12], [13], [14], [15], [16], [17]]. In the last years, experimental and theoretical reports have shown that semiconductor nanocrystals or quantum dots (QDs) with anisotropic and isotropic morphology have relatively higher NLO properties due to quantum confinement effects. In addition, such effect allows fine-tuning of their optical properties such as the photoluminescence and optical absorption [13]. Thus, knowing the NLO response of nanomaterials in a wide region of the electromagnetic spectrum allows obtaining information that can be used to improve the NLO response and decrease the irradiance threshold necessary to generate a specific optical effect. In this way, high-quality nanomaterials can be made and new applications can emerge.

Recently, copper indium disulfide (CIS) QDs has received a great deal of attention because of its interesting electroluminescent, biocompatible and green-chemistry features with potential application in solar cell [[18], [19], [20]], probes for near-infrared biodetection [21,22] and so on [23]. Regarding to the NLO response, CIS QDs with diameters of 2.7 nm, 3.0 nm and 3.5 nm were studied in Ref. [21] by using the femtosecond Z-scan technique. According to the authors, the CIS QDs presented two 2 PA allowed bands located at 800 nm and 1200 nm, with 2 PA cross section between 5.2 × 102 and 6.3 × 103 GM (Goppert-Mayer, 1 GM = 10−50 cm4.s−1.photon−1) for QDs with diameter of 2.7 and 3.5 nm, respectively. In addition, the linear absorption edge was observed at 600 nm, 610 nm, 630 nm, respectively, for the QDs with diameter of 2.7 nm, 3.0 nm, and 3.5 nm. With these results, it is possible to highlight two aspects observed on Ref. [21]. The first one, according to the authors, is that the 2 PA band at 1200 nm is not related to an electronic transition via two-photon. It is associated to spurious effect such as state filling, Coulomb screening and impurity absorption. The second one is that due to the low average spectral resolution (∼30 nm) in the 2 PA measurements, it is impossible to discriminate the distinct 2 PA transitions between 700 and 1000 nm. Hence, the aim of this work is to understand the structure/2 PA properties relationship on a wide spectral range of CIS QDs, employing femtosecond pulses and the parabolic effective-mass approximation model development by Fedorov et al. [24].

Section snippets

Experimental

The CIS QDs were synthesized by a methodology based in the one presented in Ref. [25]. Briefly, the synthesis was performed using a hydrothermal method, using salts as copper and indium sources, and thiourea [CS(NH2)2] as sulfur source, with Cu: In: S molar ratio of 0.5: 1: 2, pH = 10, and 150 °C for 20 h, and as surface ligand, 3-mercaptopropionic acid (MPA) was used, in order to use water as solvent [26]. Additionally, we have purified the samples by selective precipitation and used just the

Results and discussion

In Fig. 1, the diamonds show the ground-state absorption spectrum of water-soluble copper indium sulfide QDs. The band centered at 428 nm (2.90 eV) is associated with the first excitonic transition, i.e., 1S3/2(h) → 1S(e), for copper indium sulfide QDs. The molar absorptivity (ε) found at the S-S transition peak was ε = 7.5 × 104 M−1cm−1 for the CIS QDs (2.6 nm), which was estimated from the effective mass model by using the approach described in Ref. [29]. This value is within the molar

Final remarks

In conclusion, we have analyzed the 2 PA spectrum of water-soluble copper indium sulfide (0.18:0.42:2) QDs with an average diameter of approximately 2.6 nm by means of wavelength-tunable femtosecond Z-scan technique and the parabolic effective-mass approximation model. The phenomenological correction shown a good alternative to describe the 2 PA allowed bands and also the magnitude of the 2 PA cross section for the lowest energies transition and ISREE region for CIS QDs in the strong

Acknowledgments

Financial support from FAPEMIG (Fundação de Amparo à Pesquisa do Estado de Minas Gerais, APQ-01203-16), FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo - 2011/12399-0, 2015/20032-0 and 2016/20886-1), CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), LNNano (Laboratório Nacional de Nanotecnologia), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and the Air Force Office of Scientific Research (FA9550-12-1-0028 and FA9550-15-1-0521) are

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