Photoacoustic spectroscopy as a non-destructive technique for optical properties measurements of nanostructures

Journal Pre-proof Photoacoustic spectroscopy as a non-destructive technique for optical properties measurements of nanostructures Ali Badawi, Waad Obedallah Al-Gurashi, Ateyyah M. Al-Baradi, F. Abdel-Wahab

PII:

S0030-4026(19)31287-2

DOI:

https://doi.org/10.1016/j.ijleo.2019.163389

Reference:

IJLEO 163389

To appear in:

Optik

Received Date:

7 January 2019

Revised Date:

10 August 2019

Accepted Date:

9 September 2019

Please cite this article as: Badawi A, Obedallah Al-Gurashi W, Al-Baradi AM, Abdel-Wahab F, Photoacoustic spectroscopy as a non-destructive technique for optical properties measurements of nanostructures, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.163389

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Photoacoustic spectroscopy as a non-destructive technique for optical properties measurements of nanostructures

Ali Badawi1,*, Waad Obedallah Al-Gurashi1, Ateyyah M. Al-Baradi1, F. Abdel-Wahab1, 2

Department of Physics, Faculty of Science, Taif University, Taif, Saudi Arabia

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Department of Physics, Faculty of Science, University of Aswan, Aswan, Egypt

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*Corresponding author: e-mail address: [email protected] (Dr. Ali Badawi); Tel: 00966543414808; Fax: 009661272556500

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Abstract

In this work, photoacoustic (PA) spectroscopy as a non-destructive technique has been

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effectively used to study the optical properties of nanostructures. Cadmium cobalt sulfide quantum dots (QDs) have been deposited onto TiO2 (Titania) electrodes using a successive ionic layer adsorption and reaction (SILAR) technique for different cycles (1 to 10). The

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surface morphology of the prepared photoanodes has been studied using a scanning electron microscope (SEM). The elemental properties have been investigated using an energy dispersive X-ray spectrometer. The optical properties have been measured using both PA and UV-Vis. spectrophotometer. The energy band gap (Eg) of the prepared photoanodes has been tuned from 3.29 eV to 2.49 eV as the number of SILAR deposition cycles increases from 1 to 10. This

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increase in Eg is mainly attributed to the increase in the QDs' size and hence the quantum confinement effect.

Keywords: Photoacoustic spectroscopy; cadmium cobalt sulfide; quantum dot; optical properties; tuning energy band gap.

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1. Introduction Recently, semiconductor nanoparticles (NPs) have been received an exceptional attention due to their novel properties. These desirable characteristics include and not limited in the optical, electrical, mechanical and magnetic properties. These features lead them to effectively participate in many optoelectronic applications such as optical switching [1], nanosensing [2], light

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emitting diodes and high-efficiency solar cells [3-5]. Semiconductors' quantum dots (QDs) could show strong confinement dependence when their size become

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less than their Bohr radii, which strongly affect their properties. For example,

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tuning the energy band gap of binary QDs can be achieved by controlling their size for different applications [6-8].

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Many preparation techniques are utilized to synthesize QDs for specific uses. Some of these methods based on mechanical processes as in milling [9], laser

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ablation[10] and sputtering [11]. Others include chemical reactions as in chemical bath deposition (CBD) and electro-deposition methods [12, 13]. In the last ones,

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the QDs' size is mainly controlled by either lowering the QDs' surface tension using specific surfactants or limiting the QDs' precursors [14]. In this study, a modified CBD technique called successive ionic adsorption layer and reaction (SILAR) is used because of its simplicity, low-cost and high coverage characters.

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In SILAR method, QDs' size could be controlled by varying the number of SILAR deposition cycles. By this approach, increasing the number of deposition cycles leads to an increase in the QDs size, hence tuning the optical properties. Several methods are used to characterize the optical properties of such materials. Some of these tools are based on photons' character of the understudied materials as UV-visible spectrophotometry [15], photoluminescence [16], 2

ellipsometry [17]. While the other unconventional and less common tools are built on phonons' character as photo-thermal based methods [3, 18]. Here in this study, we used photoacoustic (PA) spectrometer as a photothermal spectroscopy technique for optical properties measurements. PA technique has many advantages over other optical investigation techniques because of it's a non-destructive, noncontact tool without any suitable samples’ treatments [3, 19, 20]. Moreover, PA

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technique could be utilized to study the opaque, rough and high reflective samples. Many researchers have confirmed the efficiency of the PA technique to study the

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optical properties of QDs and nanocomposites [3, 18, 21]. In our previous works [18, 19], we studied the optical and thermal properties of ternary alloyed CdTexS1-x

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QDs using PA and UV-vis. spectrophotometer. The obtained results from both techniques were highly compatible.

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In this work, we synthesized cadmium cobalt sulfide (Cd0.8Co0.2S) QDs directly onto TiO2 NPs films deposited on transparent conducting substrates using SILAR

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method. The morphological, structural and optical properties of the prepared QDs photoanodes have been characterized. Specifically, the optical properties have

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been investigated using PA as a non-destructive and unconventional method. The obtained PA results are compared to those achieved by UV-visible spectrophotometer.

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2. Main processes in the photoacoustic spectroscopy The PA technique for optical properties investigation could be discussed as

follows [3, 18]. A chopped monochromatic light is incident onto the sample placed in a closed cavity of the PA cell. Figure 1 shows a schematic diagram of a cross sectional view of a cylindrical PA cell. The absorbed non-irradiative part of the incident photons' energy is periodically converted to heat, which causes a 3

temperature variation in the sample's surface. The periodic heated sample's surface leads to a periodic heating in the contact surrounding gas (air) layer. The periodic temperature changes cause pressure fluctuations in the surrounding air, which could be detected as a sound (acoustic) wave using a very sensitive microphone attached near the sample's surface. According to the RG theory, the pressure variation (δ P) that generate the PA signal due to the temperature variation in the 𝑃0 𝐼0 (𝛼𝑔 𝛼𝑠 )1/2

𝜋

𝑒𝑥𝑝 [𝑗 (𝜔𝑡 − )] 𝜎) 2

2𝜋𝑙𝑔 𝑇0 𝑘𝑠 𝑓 sinh(𝑙𝑠 𝑠

… (1)

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𝛿𝑃=

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surrounding air is given as [22, 23]:

Where P0 and T0 are the ambient pressure and temperature respectively, I0 is

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the incident light intensity, ω = 2πf, f is the modulation frequency, li (i: s, g and b denotes sample, gas (air) and backing respectively) is the thickness, αi and ki are

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the thermal diffusivity and thermal conductivity, σs (= (1+j) as) and as = (ω/2 αs)1/2 is the complex thermal diffusion coefficient of the sample. For optically opaque

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and thermally thick samples (μβ= 1/ β << ls and μs>> μβ), where μs, μβ and β are the sample thermal diffusion length, optical diffusion length and optical absorption coefficient respectively, the PA signal intensity (I) depends on the incident photons

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energy (hν) as [24]:

𝑃 = 𝑃0 exp[𝜎(ℎ𝜈 − ℎ𝜈0 )/𝑘𝑇]

… (2)

Where k is Boltzmann’s constant, T is temperature, σ and ν0 are fitting parameters related to the sample. Figure 2 shows schematic diagram of the PA set

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up for optical properties measurements. Figure 2: Schematic diagram of PA set up for optical properties measurements

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3. Experimental part 3.1 Samples preparation Fluorine doped tin oxide (FTO: purchased from Solaronix Co.) substrates were cleaned with soap, double distilled water (DDW) and acetone respectively and then dried. Titanium dioxide (TiO2 or called Titania) nanopaste (purchased

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from Solaronix Co., size ≈ 20 nm) was deposited onto FTOs using doctor blade method [25]. Then, the deposited TiO2 films were sintered at 500 oC for an hour

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at ambient conditions. To avoid cracking, the sintered film were left to cool down gradually to reach room temperature. After that, cadmium cobalt sulfide

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(Cd0.8Co0.2S) QDs were prepared onto the Titania films using SILAR technique for different cycles (1 to 10) as follows. Two separate solutions of 0.1 molarity

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(M) of cations (Cd+2 and Co+2) and 0.1 M of anions (S-2) were prepared. The first one contains a solution of 0.08 M of Cd(NO3)2.4H2O and 0.02 M Co(NO3)2.6H2O

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in 20 mL methanol/DDW (v/v; 1:1) as the cations' precursors. While the second one contains a solution of 0.1 M of Na2S.9H2O in 20 mL methanol/DDW (v/v;

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1:1) as the anions' precursors. The precursors' solution was stirred for 30 min. to achieve a homogeneous mixture. Moreover, two rinsing solutions of methanol/DDW (v/v; 1:1) were also prepared. To carry out the QDs deposition process, first the Titania film is dipped vertically into the cations' precursors (Cd+2

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and Co+2 ions) for a one min., then it removed and rinsed for the next min. Next, it immerses vertically for a third min. into the anions' precursors (S-2 ions). After that, it rinsed for a one min. in another rinsing dish. Finally, it heated at 80 oC for 10 min. to enhance pinning the formed QDs in the surface of the Titania film. The previous five steps are considered to one SILAR deposition cycle. Ten Cd0.8Co0.2S QDs photoanodes of 1 to 10 SILAR deposition cycles were prepared and labeled from S1 to S10. 5

3.2 Measurements The prepared QDs photoanodes were characterized using many tools. The morphology of the surface of the prepared films was studied using a scanning electron microscope (SEM: model: JSM-6300). The elemental composition measurements were carried out by an energy dispersive X-ray (EDX) basic unit that attached to the SEM. The optical properties measurements including the

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absorption (Abs.) and transmittance (T) were recorded using a UV-Vis.

spectrophotometer (JASCO V670) in the wavelength range from 300 nm to 900

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nm.

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The photoacoustic measurements were carried out through a designed system as shown in Figure 2. The measuring system is constructed by many

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parameters; a tungsten-halogen lamp (Model no. 66885, Newport) was used as the source of the incident light. The light is focused into a monochrometer (Model

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no. 74125, Newport) slit and chopped with a mechanical chopper (SR540) at 14 Hz frequency. Then the chopped monochromatic light is focused onto a closed

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PA cell (MTEC, model 300). This type of closed PA cell possess a built-in microphone with sensitivity of 65 mV/Pa and a frequency response range from 5 Hz to 20 kHz. The sensitive microphone is designed for high signal-to-noise ratios and acoustic/vibration isolation. The PA cell- where the sample is placed-

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converts the chopped monochromatic light energy to acoustic signal. The obtained PA signal is processed using a lock-in amplifier (Model no. SR830 DSP). The whole system was controlled and automated using a PC.

4. Results and discussion 4.1 The morphological properties 6

The morphology of the surface of the bare Titania NPs and the prepared ternary Cd0.8Co0.2S QDs / Titania photoanodes were investigated using a scanning electron microscope. Figure 3 (a) and (b) shows SEM micrographs of the bare Titania NPs and Cd0.8Co0.2S QDs photoanode (5 SILAR cycles as an example) respectively. To confirm the QDs adsorption procedure, both images were obtained under the same conditions of X20,000 magnification and 25K eV.

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Figure 3 (a) clarifies the highly porous structure of Titania NPs surface. It also shows that the surface of Titania NPs is composed of extremely connected

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granules with diverse sizes over the whole FTO. While Figure 3 (b) shows Cd0.8Co0.2S QDs pinned onto Titania NPs as small spherical grains, which

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confirms the success of the QDs deposition process. In addition, it looks less packed surface with quasi-spherical granules as compared with that of Titania

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(Figure 2 a).

Figure 3: SEM micrograph of (a) bare Titania NPs electrode and (b) ternary Cd0.8Co0.2S QDs photoanode.

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4.2 The structural properties

The elemental composition of both bare Titania electrode and the QDs photoanodes was studied using an energy dispersive X-ray (EDX) spectroscopy.

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Figure 4 (a) and (b) shows the EDX spectra of both bare Titania electrode and Cd0.8Co0.2S QDs photoanode (5 SILAR cycles as an example) respectively. Figure 3 (a) ensures the existence of Ti, Sn and O elements in the bare Titania electrode. In addition to these three elements, Figure 4 (b) confirms the presence of Cd, Co and S elements in Cd0.8Co0.2S QDs photoanode. This result ensures the success of Cd0.8Co0.2S QDs' deposition onto Titania electrode. Moreover, the atomic ratio of 7

Cd: Co: S equals 8.42: 2.08: 10.10, which indicates that the molar ratio of (Cd + Co)/S is about a unity.

Figure 4: EDX spectra of (a) bare Titania electrode and (b) Cd0.8Co0.2S QDs photoanode.

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4.3 The optical properties measurements

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The optical properties of the bare Titania electrode and Cd0.8Co0.2S QDs

photoanodes were measured using PA spectroscopy as a non-destructive and

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unconventional technique. Moreover, the measured optical properties using PA technique are compared with those obtained from a UV-visible spectrophotometer

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method. In PA technique, the sample is placed in the closed PA cell (MTEC model 300) as discussed before. The monochromatic light is chopped with a mechanical

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chopper with a constant frequency of 14 Hz. The recorded PA signal for each sample is normalized using a carbon black reference. Figure 5 (a) to (k) shows the

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normalized PA signal amplitude as a function of the incident photon energy (hν) of the bare Titania electrode and Cd0.8Co0.2S QDs photoanodes for different SILAR cycles. The obtained data is fitted to deduce the sample's optical energy band gap (Eg). The estimated Eg values of the bare Titania and different QDs photoanodes are listed in Table 1. It is clearly seen that the obtained Eg value of Titania

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electrode equals 3.45 eV, which is in good agreement with published value [26]. In addition, it is obvious that the absorption edges of the Cd0.8Co0.2S QDs photoanodes are red-shifted as the number of SILAR cycles increases from 1 to 10. The deduced Eg value of the prepared QDs photoanodes varies between 3.23 eV to 2.60 eV as the number of SILAR deposition cycles increases. This conclusion could be discussed as follows: as the number of SILAR deposition cycles 8

increases, additional amounts of Cd0.8Co0.2S are aggregated, leading to an increase in the QDs' size, causing a decrease in Eg value according to the quantum confinement effect [25, 27]. The QDs' energy band gap is inversely proportional to the square of the QDs' radius (R) as given by the effective mass approximation (EMA) model [7]: ℎ2

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1

1.8𝑒 2

8𝑅

𝑚𝑒

𝑚ℎ

4𝜋𝜀𝜀° 𝑅

2(

∗ +

∗) −

… (3)

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𝐸g (𝑄𝐷) = 𝐸g (𝐵𝑢𝑙𝑘) +

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Where, Eg (QD) and Eg (Bulk) are Eg values of the QDs and bulk material

respectively, h is Planck's constant, me* and mh* are the QDs' effective electron and material and ԑo is the permittivity of vacuum.

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hole masses, e is the electron' charge, 𝜀 is the relative dielectric constant of the

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Figure 5: Normalized PA signal amplitude of (a) bare Titania and (b) to (k) Cd0.8Co0.2S QDs photoanodes for different SILAR deposition cycles (S1 to S10) respectively. Furthermore and for comparison with PA measurements, the optical properties

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of the prepared Cd0.8Co0.2S QDs photoanodes were measured using a UV-Vis. spectrophotometer in the wavelength range from 300 nm to 800 nm. Figure 6 (a) and (b) shows the transmittance (T) and absorption (A) of the prepared photoanodes for different SILAR deposition cycles (1 to 10). It is clearly seen that

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as the number of SILAR deposition cycles increases from 1 to 10, the absorption increases and hence its' edge red-shifts to large wavelengths. Similarly, the transmittance decreases due to the increase in the absorption as shown in Figure 6 (b). This result is mainly attributed to the increase in the QDs' size and hence the decrease in the energy band gap according to the quantum confinement effect as discussed before. It is obvious that the absorption measurements obtained by UVvis. spectrophotometer are compatible with those achieved using the PA technique, 9

in spite of the difference in the physical background of both techniques. Since the PA measurements are based on the material's responses due to the phonons' interactions and their thermal waves outputs. While the UV-Vis. spectrophotometer measurements are based on the photons' interaction with the material and their outputs in electromagnetic waves forms. Figure 6: (a) The transmittance (T) and (b) absorption (A) of the Cd0.8Co0.2S

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QDs photoanodes for different SILAR deposition cycles (S1 to S10).

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Moreover, the energy band gap of the prepared photoanodes is deduced from the UV-Vis. transmittance measurements using Tauc's equation [28, 29].

1

… (4)

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𝛼ℎ𝜐 = 𝐵(ℎ𝜐 − 𝐸𝑔 )𝑛 1

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where 𝛼 = 𝑙𝑛 ( ) is the optical absorption coefficient, d is sample thickness, 𝑑 𝑇 h is Planck's constant, B is a constant depends on the mobility of the electron-hole,

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T is the transmittance measurements, and n is a value depends on the optical transition type (direct or indirect electronic transition) . ‘‘n’’ value equates 2 for

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Titania electrode since it is an indirect band gap semiconductor [30], and 1/2 for Cd0.8Co0.2S QDs photoanodes it is a well-known direct band gap semiconductor material [6, 31]. Then, the energy band gap of the Titania electrode and the prepared QDs photoanodes are estimated by plotting (αhv)0.5 and (αhv)2 versus hv respectively, and extrapolating the linear part of the plotted curves to αhv=0.

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Figure 7 (a) to (k) shows (αhv)0.5 and (αhv)2 of the Titania electrode and Cd0.8Co0.2S QDs photoanodes versus hv respectively. The deduced Eg values of the prepared samples are also listed in Table 1. It is clearly seen that Eg value increases from 3.35 eV to 3.38 eV as the number of SILAR deposition cycles increases from 1 to 10 . This increase in Eg value is mainly attributed to the increase in the QDs' size and hence the quantum confinement effect as discussed 10

before. In addition, the obtained Eg values are in good agreement with those deduced from the PA measurements. Our results emphasize that PA technique as a non-destructive method could be effectively used to study the optical properties of microstructures and semiconducting nanomaterials.

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Figure 7: (a) (αhv)0.5 of the Titania electrode and (b) to (k) (αhv)2 of Cd0.8Co0.2S QDs photoanodes versus hv at different SILAR deposition cycles (S1 to S10).

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Table 1: The energy band gap of Cd0.8Co0.2S QDs photoanodes at different SILAR deposition cycles (1 to 10).

5. Conclusions

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Ternary cadmium cobalt sulfide quantum dots (QDs) were successfully deposited onto TiO2 (Titania) electrodes using successive ionic layer adsorption

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and reaction (SILAR) technique for different cycles (1 to 10). The energy dispersive X-ray (EDX) measurements show that the atomic ratio of Cd: Co: S

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equals 8.42: 2.08: 10.10, which indicates that the molar ratio of (Cd + Co)/S is unity. The optical properties were carried out using both photoacoustic (PA) spectroscopy and UV-Vis. spectrophotometer techniques. Both techniques show that the energy band gap (Eg) of Cd0.8Co0.2S QDs decreases from 3.29 eV to 2.49

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eV as the number of SILAR deposition cycles increases from 1 to 10. This increase in Eg is mainly attributed to the increase in the QDs' size and hence the quantum confinement effect. Our results emphasize that PA technique as a nondestructive method could be effectively used to study the optical properties of nanostructures.

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6. Acknowledgments

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We wish to thank King Abdullaziz City for Science and Technology (KACST) for their financial support. The Quantum Optics group (QORG) at Taif University is also thanked for their assistance during this work.

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Figure 1: Schematic diagram of a cross sectional view of cylindrical PA cell

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Figure 2: Schematic diagram of a PA set up for optical properties measurements

Figure 3: SEM micrograph of (a) bare Titania NPs electrode and (b) ternary Cd0.8Co0.2S QDs photoanode.

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Figure 4: EDX spectra of (a) bare Titania electrode and (b) Cd0.8Co0.2S QDs photoanode.

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Figure 5: Normalized PA signal amplitude of (a) bare Titania and (b) to (k) Cd0.8Co0.2S QDs photoanodes for different SILAR deposition cycles (S1 to S10) respectively.

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of ro -p re lP ur na Jo Figure 6: (a) The transmittance (T) and (b) absorption (A) of the Cd0.8Co0.2S QDs photoanodes for different SILAR deposition cycles (S1 to S10).

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Figure 7: (a) (αhv)0.5 of the Titania electrode and (b) to (k) (αhv)2 of Cd0.8Co0.2S QDs photoanodes versus hv at different SILAR deposition cycles (S1 to S10).

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Table 1: The energy band gap of Cd0.8Co0.2S QDs photoanodes at different SILAR deposition cycles (1 to 10). No. of SILAR Eg(eV) from UVEg (eV) from Average Eg deposition Vis. PA technique (eV) cycles spectrophotometer 1 3.23 3.35 3.29 2 3.15 3.25 3.20 3 3.04 3.14 3.09 4 2.91 2.85 2.88 5 2.86 2.78 2.82 6 2.78 2.70 2.74 7 2.72 2.62 2.67 8 2.68 2.58 2.63 9 2.62 2.50 2.56 10 2.60 2.38 2.49

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