semiconductor nanoparticles

Journal Pre-proof Optical nonlinearity in nanocomposites based on metal alkanoates with hybrid metal/ semiconductor and semiconductor/semiconductor nanoparticles A. Gridyakina, H. Bordyuh, G. Klimusheva, S. Bugaychuk, D. Fedorenko, D. Zhulai, T. Mirnaya, G. Yaremchuk, A. Polishchuk PII:

S0167-7322(19)34589-1

DOI:

https://doi.org/10.1016/j.molliq.2019.112042

Reference:

MOLLIQ 112042

To appear in:

Journal of Molecular Liquids

Received Date: 15 August 2019 Revised Date:

20 October 2019

Accepted Date: 30 October 2019

Please cite this article as: A. Gridyakina, H. Bordyuh, G. Klimusheva, S. Bugaychuk, D. Fedorenko, D. Zhulai, T. Mirnaya, G. Yaremchuk, A. Polishchuk, Optical nonlinearity in nanocomposites based on metal alkanoates with hybrid metal/semiconductor and semiconductor/semiconductor nanoparticles, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.112042. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Optical nonlinearity in nanocomposites based on metal alkanoates with hybrid metal/semiconductor and semiconductor/semiconductor nanoparticles A. Gridyakina1, H. Bordyuh1, G. Klimusheva2, S. Bugaychuk2, D. Fedorenko2, D. Zhulai2, T. Mirnaya3, G. Yaremchuk3, A. Polishchuk1 1

National Aviation University, Prosp. Komarova 1, Kyiv, Ukraine, Institute of Physics, National Academy of Sciences, Kyiv, Ukraine, 3 V.I. Vernadskii Institute of General and Inorganic Chemistry, National Academy of Sciences, Kyiv, Ukraine 2

Abstract

Spectral properties and the nonlinear optical response in nanocomposites of ionic liquid crystals (ILC) of cadmium octanoate containing hybrid core/shell nanoparticles (NPs) is studied. The hybrid NPs consist of either an Ag-core and a semiconductorshell, or a semiconductor1-core and a semiconductor2-shell. Two-wave mixing with excitation by nanosecond pulses laser was used to observe the generation of high diffraction orders in the dynamic regime. The coefficients of optical nonlinear refraction were determined based on the measured diffraction efficiencies. Excitation of polaritons in the hybrid NPs and their electronic type of nonlinearity are responsible for the photoinduced nonlinear response in these nanocomposites.

Key-words: polariton optical nonlinearity, nanocomposites of ionic liquid crystals, hybrid core/shell nanoparticles, two-wave mixing, nanosecond laser excitation.

1. Introduction

The ability of molten metal salts with organic ions to form ionic liquid crystals (ILC)

was established for the first time in the 70s of the last century [1]. Their general structural formula is Me+k(COOCnH2n+1)–k , which includes metal cation Me+k with the charge +k (k=1-3) and k - carboxylate anions. Already in the early studies, many unique properties of the metal alkanoates were revealed: they form a bi-layer structure of the type of the smectic A, which consists of cation-anion layers separated by layers of alkyl chains [2-4]. Metal alkanoates possess rich polymorphism properties, since they can form lyotropic ILC at the room temperature and thermotropic ILC at the temperature range of 100-180 0C; they exhibit high ionic conductivity, which is mainly directed along the cation-anion layers [5]. The ILC of the metal alkanoates have a unique amphiphilic nature, thanks to that it is possible to dissolve both organic and inorganic impurities, which are naturally embedded in the layered matrix [6]. Besides, the thermotropic ILC and their composites can form anisotropic glasses at room temperature under condition of rapid cooling of the thermotropic ILC phase (see [6] Fig. 19). In this case, the anisotropic glasses have the same smectic structure as the ILC in the thermotropic phase. Recently there was obtained that the metal alkanoats in their thermotropic liquid crystalline phase can serve as nanoreactor for chemical synthesis of various nanoparticles at the temperature range 100–1800C [7-12]. Such nanocomposites can include nanoparticles (NPs) of different nature, namely: CdS, CdSe semiconductor NPs; Ag, Au metallic NPs; and hybrid core/shell NPs of different composition [13-14]. The ILC nanocomposites with different NPs exhibit versatile electrical properties, including nonlinear electric conductivity, photoconductivity, and photovoltaic effect [15-17]. All

electric

properties

exhibit

anisotropy

of

conductivity

depending

on

measurements along the cation-anion layers or perpendicular to them, which confirms the anisotropic structure of the matrix of glassy nanocomposites. Nonlinear optical properties of the metal alkanoates have been studied already in the first composites based on ILC [18, 19]. For these researches, the traditional techniques of record dynamic gratings and laser z-scanning were used. A nonlinear optical response have been detected and studied in anisotropic glasses of ILC with

dye impurities of different types [6, 20, 21]. As well as there were detected that all nanocomposites with different NPs exhibit a nonlinear optical response over a wide range of intensities and wavelengths of excitation laser used. We have revealed various nonlinear optical mechanisms in nanocomposites of metal alkanoates with different NPs. The main mechanisms are associated with transitions to excited electronic states in photosensitive centers, and with thermal nonlinearity. There was found the optical nonlinearity connected with electronic transitions in the cobalt coordination complexes under the action of laser radiation in the nanocomposites based on the matrix of Co-alkanoates absorbed in the visible spectral range [6, 22]. In Cd-alkanoate matrix, which does not absorb light in the visible wavelength range, the optical nonlinearity of these nanocomposites is due to existing NPs [23]. A high value of the nonlinear optical response was also found in nanocomposites with CdSe NPs when excited by continuous laser radiation with the mechanical chopper due to the CdSe NPs strong absorption, and consequent thermal dissipation, which in turn, produces the photoelastic tensions in the glassy smectic matrix [24]. In this paper, we study nonlinear optical properties of cadmium octanoate nanocomposites with hybrid core/shell NPs. The NPs consist of a metal core (Ag) and a semiconductor shell of different composition (CdS, ZnS, ZnSe), or a semiconductor-1 core (CdSe) and a semiconductor-2 shell (ZnS). Also for comparative analysis, nanocomposites with homogeneous Ag NPs are investigated. We have applied the two-wave mixing technique by exciting a Q-switched Nd:YAP laser with second harmonic conversion (λ=539.8 nm) and the duration of laser pulses τ=20 ns. In the core/shell NPs, metal cores become the main absorption centers due to the resonant excitation of surface plasmons [25, 26]. The absorbed energy is partially converted into heat. At the same time, the semiconductor shell can block the distribution of heat propagating from the metal core [27, 28]. But polaritons, which represent bound photon-phonon states, can be excited in a semiconductor shell due to

the close relative arrangement of the metal core and the semiconductor shell [29, 30]. In the present work, we investigate the features of optical nonlinearity in core/shell NPs of metal alkanoate nanocomposites with an emphasis on the possibility of excitation of polaritons in the semiconductor shell of the NPs.

2. Synthesis of hybrid core/shell NPs in metal-alkanoate ILC matrices New hybrid NPs were chemically synthesized in thermotropic liquid crystal phase (TLC) of cadmium octanoate (the abbreviation CdC8) or its equimolar mixture with zinc octanoates (ZnC8) at 1500C according to the two stage procedure. During the first stage, the silver NPs were synthesized in the TLC of CdC8 using silver nitrate as a source of silver ions [23]. The reaction mixture was homogenized by bubbling it with argon and then cooled. Further synthesis of the hybrid NPs was consisted in coating of Ag-core NPs with CdS-shell, or ZnS-shell, or ZnSe-shell, respectively. The CdC8 powders with Ag NPs and one from the nanocomposites of CdC8 with CdS NPs, or with ZnS NPs, or with ZnSe NPs were used for preparing Ag/CdS, Ag/ZnS, Ag/ZnSe core/shell NPs, respectively, [11]. The total amount of nanoparticles in matrixes had been 4 mol%, the ratio of Ag/semiconductor had been 1/1. The composition mixtures were crumpled carefully on agate mortar, and were transferred to glass tube and exposed in the thermostat. Core-shell synthesis is being under thermal influence up 150 ± 50C during 4 h. The CdC8 powders with CdSe and ZnS NPs were also used for preparation CdC8 nanocomposite with CdSe/ZnS core/shell NPs [28].

3. Characteristics of nanoparticles. The shape, size and size distribution of the synthesized silver NPs in CdC8 matrix were obtained by transmission electron microscopy (TEM) [10]. The average diameter of almost spherical Ag NPs, calculated from the TEM images, was 20±3 nm. The structural characterization of the nanocomposites with Ag/semiconductor NPs was obtained by scanning electron microscope (SEM) [31]. The SEM

measurements were carried out at the LEO 1550. This device ensures high resolution of the received images (2 - 5 nm). From the SEM images, we determined the average size and the dispersion of the size of hybrid NPs. The parameters of the NPs synthesized in CdC8 matrix are presented in Table 1. A typical SEM image of the nanocomposites of cadmium octanoate with Ag/ZnS NPs is shown in Fig. 1. The structural characteristics of hybrid CdSe/ZnS NPs embedded in CdC8 were previously studied by small-angle X-ray scattering [28]. The size of the hybrid NPs was revealed as follows: the common core/shell of the NP has an average diameter of d core/shell = 14.8 nm, the diameter of the CdSe-core dcore = 6.5 nm, and the ZnS - shell consists of NPs, where the smallest average diameter was dshell = 2.7 nm. Table1. The diameter of core/shell NPs and the dispersion of size in nanocomposites based on cadmium octanoate. NPs Ag Ag/CdS Ag/ZnS Ag/ZnSe CdSe/ZnS

Diameter, (nm) 20 22.5 12 29 14.8

Dispersion of size, (nm) +3 +5.5 +5 +5 +4.5

Figure 1. SEM-image of the nanocomposite of cadmium octanoate (CdC8) with Ag/ZnS NPs. 4. Sample preparation. The anisotropic glassy nanocomposites for the study of linear and nonlinear optical properties were prepared in the following way. A powder of the nanocomposite was placed between two glass or quartz substrates and heated to high temperature (~100 0

C) to achieve a liquid crystal phase. After that, the samples were quickly cooled to

room temperature. As a result of this process, the anisotropic glassy nanocomposites are formed. The edges of the samples were glued to avoid evaporation of the material. 5. Absorption spectra of the nanocomposites. The absorption spectra of the glassy nanocomposites were measured on universal automatic spectral complex KSVU-6 (LOMO). The absorption spectrum of the nanocomposite with homogeneous Ag NPs consists of a broad absorption band with a maximum of 425 nm, which corresponds to the excitation of localized plasmons in Ag NPs [32]. The absorption spectra of nanocomposites based on CdC8 with new hybrid NPs: Ag/CdS, Ag/ZnS and Ag/ZnSe in glass substrates are shown on Fig. 2. The wide

absorption band of nanocomposite CdC8 with Ag/CdS NPs is associated with both the absorption of localized plasmons of the Ag-core NPs (in the range of 425 nm) and the absorption of localized excitons of the CdS-shell NPs (in the range of 360 nm). The absorption of nanocomposites CdC8+Ag/ZnS NPs and CdC8+Ag/ZnSe made between glass substrates are represented mainly by absorption band of Ag-core NPs with a maximum 425 nm, as semiconductor nanoparticles (ZnS-shell and ZnSeshell) absorb the light in the ultraviolet range, which do not transparent for the glass substrates.

Figure 2. Absorption spectra of nanocomposites based on CdC8 with hybrid NPs

Ag/CdS (the curve 1), Ag/ZnS (2) and Ag/ZnSe (3) made between glass substrates.

The absorption of nanocomposites made between quartz substrates have been studied as well, see Fig. 3. As can be seen from Figs. 2 and 3, the absorption of a nanocomposite CdC8+Ag/CdS due to both Ag-core and CdS-shell of the NPs takes place in the visible range of the spectrum (from 330 nm to 450 nm). In the case of the nanocomposite CdC8+Ag/ZnSe, besides the maximum of the absorption at 425 nm which refers to the Ag-core, it can be seen a band with maximum at 275 nm refers to the ZnSe-shell. Absorption of the ZnS-shell NPs of a nanocomposite CdC8+Ag/ZnS is clearly visible in the ultraviolet region with maximum 275 nm.

Figure 3. Absorption spectra of nanocomposites based on CdC8 with hybrid NPs Ag/CdS (the curve 1), Ag/ZnS (2) and Ag/ZnSe (3) made between quartz substrates. The

absorption

spectra

of

the

nanocomposites

with

homogeneous

semiconductor CdSe NPs, and with hybrid CdSe/ZnS NPs have been investigated in [28]. The both absorption spectra differ little in the visible spectral range. Two narrow absorption bands with the maxima at 430 nm and at 460 nm in the CdC8+CdSe nanocomposites are associated with the exciton electronic transitions in the CdSe NPs [24]. The ZnS-shell coating hasn’t led to significant modification of the CdSe-core NPs. In the absorption spectrum of a nanocomposite CdC8+CdSe/ZnS made between the quartz substrates, besides the absorption bands at 430 nm and at 460 nm belonging to the CdSe-core NPs, it appears the absorption band with the maximum at 275 nm, which belongs to the ZnS-shell of the NPs (see Fig. 4).

Figure 4. Absorption spectra of nanocomposite CdC8+CdS/ZnS made between quartz substrates. 6. Experiments of two-wave mixing in the nanocomposites The two-beam scheme was applied to write holographic dynamic gratings of the nanocomposites CdC8 with hybrid NPs [33]. The optical scheme of the experimental set-up is shown in Fig. 5. The second harmonic of the Q-switched Nd:YAP3+ laser (TEM00 -mode, the laser wavelength λ=539.8 nm, the pulse duration τ =20 ns, the single pulse mode) was used as a light source. The laser beam was split into two parallel beams by the semitransparent mirror (approximately 42% reflected and 54% transient) and focused only on the plane of sample. Since the polarization vectors of both beams were parallel to each other, there was a distribution of light intensity formed by the lens in its focal plane. To increase the intensity on the sample we use a

lens with the focal length F=50 cm. The period of gratings was about ∆ = 10 µm, e.g. density of fringes ~1000 per centimetre. The average energy of single pulse is E 0 = 0.75 mJ ± 15%

. The waist radius is:

w = 1 . 22

λ D

F

E

P 0 and I = πw 2 , where P = τ .

Photodiodes were used to detect an intensity of pump beam I0 and the intensity in the first diffraction order

I {1} .

Figure 5. Two-wave mixing optical scheme of the experimental set-up. 1, 2 – Qswitched Nd:YAP laser and frequency doubler; 3 – splitter ~4% for input intensity; 4 – splitter of ~50% ; 5 – mirror; 6 – lens; 7 – sample put in a focal plane of the lens; 8 – diaphragms; 9 – photodiodes, connected to oscilloscope. We determined the linear absorption coefficient

α in all investigated

nanosomposites according to the Lambert-Buger law at the laser excitation wavelength:

I = I 0 exp( −α d ) ,

where

I0

is the intensity of the input laser radiation, I

is the light intensity at the exit of the sample, d is the thickness of the sample. The investigated samples, their notation, characteristics and experimental parameters are collected in Table 2. We observed the generation of the first diffraction orders to the right and left of the main orders from the two recording beams. By measuring this intensity, well as the intensity in the input beam, for each samples:

I0

I {1} ,

as

, we determined the diffraction efficiency

η = I {1} / I 0 .

We take the average diffraction efficiency for 10

η~

. The coefficient of the nonlinear refraction n 2 in a

independent measurements,

Kerr-like medium is defined as photoinduced changes in the refractive index relative

to its stationary value

n0

:

∆n = n0 + n2 I 0 .

It can be calculated from the experimental

~

value of the diffraction efficiency η using the following formula [34]: η~ λ α π 1− T I0

n2 =

for

η~ ≤ 2

%, and by the formulas η~ = T [J12 (ζ ) + J 22 (ζ )] ;

for

(1)

η~ > 2

ζ =

2π 1 − T

λ

α

n2 I 0

(2)

%. In these formulas T is the transmission of a sample on the wavelength

of laser excitation λ , J 1 and J 2 are the Bessel functions of the first kind and of the first and second orders, respectively. The coefficient of the nonlinear optical susceptibility

χ

(3)

is found from the value of n 2 from the known expression:  cm 2  9 ⋅ 10 4

χ ( 3) [ esu ] = n 2  ⋅  W  4π

c ⋅ ε e ⋅ n 02

(3)

where ε e is the electric constant, c is the light velocity and

n0

is the linear

refractive index of the samples (for all of our nanocomposites the average value is n~ 0 ≈ 1 . 5

).

7. Results and discussions The calculated values of the nonlinear optical coefficients in the studied nanocomposites are presented in Table 2. Table 2. The nonlinear optical coefficients of the nanocomposites CdC8+core/chell NPs. η~

n2, cm2/W

χ(3), esu

59.2

, % 1.8

-4.33⋅10-11

1.85⋅10-9

403

58.4

9.9

6.69⋅10-11

2.86⋅10-9

569.5

62.6

2.5

3.81⋅10-11

1.62⋅10-9

Substrates

Thickness d, µm

α, cm-1

Ag

glass

20.6

950.1

2G

CdSe/ZnS

glass

20

3G

Ag/CdS

glass

18.6

Designation

NP

1G

I0, MW/ cm2

3Q

Ag/CdS

quartz

14.5

579.6

60.1

1.8

3.93⋅10-11

1.68⋅10-9

4G

Ag/ZnS

glass

37.2

241.5

63.0

1.2

1.52⋅10-11

0.65⋅10-9

4Q

Ag/ZnS

quartz

13.3

259.7

71.3

1.7

2.82⋅10-11

1.21⋅10-9

5G

Ag/ZnSe

glass

21.1

785.7

57.2

2.6

4.76⋅10-11

2.04⋅10-9

5Q

Ag/ZnSe

quartz

10,1

809.3

65.6

1.2

4.13⋅10-11

1.77⋅10-9

To explain the experimental results, we propose the following model. In nanocomposites with homogeneous Ag NPs the absorbed light energy causes heating of the samples, which leads to the optical nonlinearity. The thermal nature of the nonlinearity is also confirmed by our studies carried out by the method of z-scanning of the CdC8 nanocomposite with metal NPs (Ag, Au) [32]. The z-scan technique allows one to determine the sign of nonlinearity, which was negative in CdC8+Ag NPs. The negative sign indicates the appearance of a nonlinearity of a defocusing type that is typical for thermo-optical nonlinearity. But the value of thermal n2 (or χ(3)) is significantly less for the metal alkanoate nanocomposties with (χ(3))~10-9 esu) compared with any others materials containing metallic NPs or metallic nanostructures (χ(3)) ~ 10-6 – 10-7 esu) [35, 36]. This fact can be explained by the high thermal conductivity of the metal alkanoate matrices, which we have observed in all our studies. In the metal alkanoate nanocomposites with hybrid NPs, the nature of the optical nonlinearity is essential changed (see also [37, 38]). In the nanocomposites CdC8+Ag/semiconductor the laser radiation leads to the excitation of localized plasmons in Ag-core NPs with further strong absorption of light that taking place in a semiconductor-shell of NPs. In the nanocomposites CdC8+CdSe/ZnS, localized excitons are excited both in a semiconductor core and in a semiconductor shell. And the most important, the sign of the nonlinearity changes to the opposite: n2>0, i.e. that is, it corresponds to the focusing type of nonlinearity [28]. The main effect responsible for this nonlinearity is due to the excitons’ manifestation in a

semiconductor

shell

in

core(metal)/shell(semiconductor)

NPs,

or

in

the

semiconductor-core of the CdS/ZnSe NPs (note that the absorption band of the excitons of the ZnSe NPs has a maximum at 275 nm, and this exciton cannot occur in our samples made with glass substrates). At the same time, phonons are excited due to the conversion of the absorbed energy molecular vibrations. Excitons exhibit intrinsic strong luminescence [39-43], then bond photon-phonons states can form polaritons. The nonlineariaty associated with excitons in semiconductor NPs was investigated depending on the size of the NPs, as well as the type of a matrix into which the semiconductor NPs are embedded [44-46]. In previous our work we have obtained that the nonlinear refraction coefficient n2 had a negative sign, that is, the nonlinear had a defocusing type. We assume that in our case the change in n2 to a positive value is associated with the excitation of polaritons in hybrid NPs. In addition, the optical nonlinear coefficients are higher in samples made with quartz substrates compared to samples made between glass substrates, since glasses have a strong absorption band in the wavelength range of less than 300 nm, and thereby lock the transmission of a tail part of the exciton absorption band. Thus, some of the excitons can not be excited in the samples with glass substrates. A rigorous analysis of the excitation of polaritons and the related optical nonlinearity can be performed on the basis of Maxwell equations developed for confirmed inhomogeneous media (see, e.g. [47]), and of a quantum mechanical model (see, e.g. [48]). This will be done in our next research.

8. Conclusion In the present paper, we study the optical linear and nonlinear properties of new nanocomposites

of

cadmium

octanoates,

which

contain

hybrid Ag(core)/

semiconductor(shell) NPs or semiconductor1(core) / semiconductor2(shell) NPs. The hybrid core/shell NPs were synthesized in thermotropic ILC smectic phase by chemical reactions taking place in two stages. The absorption spectra of the

nanocomposites exhibit a wide band for visible wavelengths, which is responsible for the excitation of localized surface plasmons in Ag-core of the NPs, as well as a narrow band in the UV-blue spectral range, which is responsible for the excitation of localized exciton in the semiconductor shell of the NPs. While the nanocomposites containing hybrid semiconductor1/semiconductor2 NPs, show two narrow absorption bands in the visible blue spectrum, which are responsible for the excitation of localized excitons both in the core of semiconductor1 and in the shell of semiconductor2 of the NPs. The method of two-wave mixing with the excitation of nanosecond laser pulses (λ= 539.8 nm) was used to determine the nonlinear optical coefficients connected with a change in the refractive properties of the samples under illumination by strong laser excitation. Since the cadmium octanoate matrix does not absorbs light at visible wavelengths, the optical nonlinearity in the samples is due the absorption of light in NPs. Resonance absorption occurs in the Ag-core, which lead to a further transformation of the absorption energy into a heat flux. At the same time, the electronic states of localized excitons are excited due to two-photon absorption. It is important to note that homogeneous semiconductor NPs have a negative sign of the coefficient of nonlinear refraction, which corresponds to the defocusing type of nonlinearity. The same feature is observed in samples with thermal nature of the nonlinearity. While, in our samples of the nanocomposites with hybrid NPs, the coefficient of nonlinear diffraction n2 has a positive sign, that is, the nonlinearity changes its type to focusing one. The highest value of the coefficient of nonlinear refraction

was

achieved

for

the

nanocomposites

with

hybrid

semiconducor1/semiconductor2 NPs, and this value is higher compared to the nanocomposites containing homogeneous semiconductor NPs. Base of our studies, we suggest that the optical nonlinearity in our samples can be associated with the excitation of polaritons in hybrid NPs. We continue to conduct a thorough study of this interesting phenomenon inherent in hybrid NPs.

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- Core/shell NPs of different compositions are chemically synthesized in ionic LC - Spectral bands are associated with excitation of localized plasmons and localized excitons in NPs - Nonlinear optical response is detected by dynamic holographic method - Nonlinear optical response is associated with the excitation of polaritons in core/shell NPs

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Declaration of interests X The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Declarations of interest: none ☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: