Vol.:(0123456789)1 3

J Mater Sci: Mater Electron DOI 10.1007/s10854-017-6869-7

A chemical synthesized Al-doped PbS nanoparticles hybrid composite for optical and electrical response

Suresh Sagadevan1 · Kaushik Pal2 · Enamul Hoque3 · Zaira Zaman Chowdhury4 

Received: 2 February 2017 / Accepted: 28 March 2017 © Springer Science+Business Media New York 2017

the Al-doped PbS nanoparticles were found to be 1.94 and 2.04  eV respectively. The dielectric properties of the Al-doped PbS nanoparticle composites typical response e.g. dielectric constant, dielectric loss, and AC conductivity were analyzed at various frequencies and temperatures.

1 Introduction

Nanotechnology/materials science is an intense area of scientific research because of its extensive diversity of potential applications particularly in optoelectronic and biological field. The knowledge about semiconduc-tor nanoparticles in powdered form has pulled in much consideration in the recent years. This was done because of their novel optical and electronic properties [1]. The novel optical and electronic properties of these nanopar-ticles emerge particularly from their higher surface-to-volume ratio and owing to their increased rate of atoms at the grain boundaries. Transition metal chalcogenides in nano regime have also pulled an extensive enthusi-asm in the field of chemistry, physics, materials science, biotechnology, etc. [2]. Along with the metal chalcoge-nide Nanomaterials, PbS is a IV–VI group semiconduc-tor. PbS show strong quantization effects owing to the small effective masses of its electrons and holes and its high dielectric constant [3]. Emerging from the impact of quantum confinement semiconductor nanocrystals (NCs) have attracted an incredible attention because of their size-tunable optical properties. Over the previous decades, polymer matrix and semiconductor NCs based Nanocomposite were examined widely as another con-trasting option to conventional polymer materials. These Nanocomposite are a composite material, where one of the elements has no less than one dimension that is of

Abstract Structural, morphological, optical and electri-cal investigations of pure and Al-doped lead sulfide (PbS) nanoparticles hybrid composite was synthesized by simple chemical route. The detail analysis of the nanoparticle mor-phology of hybrid composites through optical investigation, phase purity and crystalline size had been characterized by using X-ray diffraction (XRD), scanning electron micros-copy (SEM), transmission electron microscope (TEM), ultraviolet spectroscopy (UV), photoluminescence (PL). The lower angle XRD results confirmed that the phase purity and average crystalline size of the pure and Al doped PbS nanoparticles were determined by using the Debye–Scherrer’s formula. The average grain sizes of the pure and the Al-doped PbS nanoparticles were calculated and found to be 22 and 16 nm respectively. Surface morphology analysis was carried out by using SEM and TEM analysis. The surface morphology of pure and Al doped PbS nano-particles is without any pinholes or cracks and hence they appear to be densely packed with spherical shaped grains. The optical properties of pure and Al-doped PbS analyzed using UV–Vis. absorption spectroscopy and Photoluminis-cence (PL) spectra. The band gap values for the pure and

* Suresh Sagadevan drsureshnano@gmail.com

1 Department of Physics, AMET University, Kanathur, Chennai 603112, India

2 Engineering and Nanoscience Building, Wuhan University, 8 East Lake South Road, Wuchang 430072, People’s Republic of China

3 Department of Biomedical Engineering, King Faisal University, Al-Hofuf, Al-Ahsa 31982, Kingdom of Saudi Arabia

4 Nanotechnology and Catalysis Research Centre, University of Malaya, 50603 Kula Lumpur, Malaysia

J Mater Sci: Mater Electron

1 3

nanoscopic size of around 1–100  nm. Semiconductor nanoparticles, or so-called quantum dots, have unique optical and electronic properties on their quantum size and dielectric confinement effects [4]. These properties can be varied by changing the size and morphological structure of the particles. These days, by allowing the synthesis of adequate materials for advanced technolo-gies, the size and morphology is controlled so as to achieve the best condition. Lead sulfide (PbS) is a unique semiconductor material which is an essential binary IV–VI group semiconductor. The size controlled PbS NCs are conceivably utilized in manufacturing optical switches, solid-state laser, solar cells, solar absorbers, photographs, lasers, LED devices, telecommunications, detectors, optical switches, optical amplification and electro-luminescent devices like light emitting diodes (LEDs) [5–7]. By forming nanocrystals, the band gap of PbS is increased to the visible region. PbS is an essen-tial direct band gap semiconductor material. It has a small band gap of about 0.41 eV. It finds application in near-IR communications and has a substantial excitonic Bohr radius of 18 nm with high dielectric constant [8]. The possibility to tune the optical absorption in a large domain by decreasing the size to the nanometric scale is done owing to the predominantly narrow band gap of PbS. If the experimental route permits controlling the particle size a large blue-shift is expected because of the impact of the quantum confinement on the charge car-riers. A few techniques have been used to develop lead sulfide nanoparticles in various environments such as zeolites, glasses, polymers, inverse micelles, or in col-loidal state. Novelty of work in this article deals with the preparation of Al-doped PbS nanoparticles and their structural, surface morphology, optical and dielectric studies have been reported. The schematic representa-tion of Al-doped PbS nanoparticles confined geometry in ITO cells and corresponding to the energy band dia-gram of electron–hole heterojunction, which is compara-ble to that of metal-dielectric interfaces, depicted in the Fig. 1.

2 Experimental procedure

Precursor salts such as lead nitrate [Pb(NO3)2], thiourea (CH4N2S) and aluminium trichloride [AlCl3] was utilized to synthesize pure and Al-doped PbS nanopowders. 0.2 M of Lead nitrate and thiourea was dissolved in 200 ml of water. As synthesized Al-doped PbS nanopowder, with Al-chloride as Al-precursor. 0.0667  g of Al-chloride was dissolved in 20  ml methanol under nitrogen such a typical way that sol molarity was 0.5 M/l and the molar ratio Al/PbS = 1  mol%. Liquid ammonia of about 50  ml was added to this solution while the pH value is main-tained at ten. The solution was thoroughly stirred using a magnetic stirrer for 3  h, after which the solution was kept undisturbed for 5  h. The precipitate thus obtained was isolated. It is washed twice in de-ionized water and the product was permitted to dry in a muffle furnace. The product thus obtained was calcined at 300 °C for 1 h. It is later crushed with a mortar to obtain black colored pure and Al doped PbS nanopowder.The crystalline size and the structure of the A1-doped PbS nanoparticles were analyzed by X-ray diffraction (XRD) by using a pow-der X-ray diffractometer (Schimadzu model: XRD 6000 with CuKα (λ = 0.154 nm) radiation and with a diffraction angle between 20° and 80°). The surface morphology of the Al-doped PbS nanoparticles was observed with the help of scanning electron microscope (SEM) using JEOL; JSM-67001. Transmission Electron Microscope (TEM) with an H-800 TEM (Hitachi, Japan) and an accelerat-ing voltage of 100 kV was used to determine the size of the synthesized particle. Absorption spectrum was ana-lyzed through the ELICO SL-159 UV–Visible conven-tional spectrophotometer in the range of 300–1100  nm. The photoluminescence (PL) spectrum was recorded using the Perkin-Elmer lambda 900 spectrophotometer with a Xe lamp as the excitation light source. By using an HIOKI 3532-50 LCR HITESTER in the frequency ranges of 50 Hz to 5 MHz. The dielectric constant and the die-lectric loss of the pellets of Al-doped PbS nanoparticles in disk form were studied at various temperatures.

Fig. 1 Schematic representa-tion of Al-doped PbS nanopar-ticle confined geometry in ITO cells, corresponding energy level diagram of electron–hole pair nanoparticles

J Mater Sci: Mater Electron

1 3

3 Results and discussion

3.1 X-ray diffraction analysis

Figure 2 shows the XRD patterns of the pure and Al-doped PbS nanopowder. The diffraction peaks identified in the XRD patterns were indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), (4 2 0) and (4 2 2) planes with respect to face-centered cubic structure of pure PbS. No other impurity peaks identified to Al are distinguished even in the heavily doped samples demonstrating their pure nature. From the prominent peak the average grain size of Al-doped PbS nanoparticles was analysed in the XRD using the Scherrer formula,

where λ is the X-ray wavelength (λ = 0.154  nm), θ is the Bragg diffraction angle, and β is the full width at half maxi-mum (FWHM) of the XRD peak appearing at the diffrac-tion angle θ. The average grain sizes of the pure and the Al-doped PbS nanoparticles were calculated and found to be 22 and 16 nm respectively.

3.2 SEM and TEM analysis

The SEM images of the pure and Al-doped PbS nano-powders are displayed in Fig. 3. The surface of sample is without any pinholes or cracks and hence they appear to be densely packed with spherical shaped grains. Alongside few scattered grains pure PbS surface comprises of equally sized grains. For Al doping concentration the grains seems to be completely agglomerated. The surface was formed with grains and no agglomeration of grains is visible for the PbS nanopowders doped Al concentration. The average

(1)D =0.9λ

β cos θ

grain sizes of the pure and the Al-doped PbS nanoparticles were found to be 18 and 16  nm respectively. Spherically shaped nano grains are unmistakably visible in both the pure and Al doped PbS nanopowders from the TEM images (Fig.  4). The grain size appears to be minimized with Al doping which strongly supports the reduced crystallite size value obtained from the XRD analysis. The average parti-cle sizes of the pure and the Al-doped PbS nanoparticles were found to be 32 and 28 nm respectively.

3.3 Optical properties

3.3.1 UV–Vis absorption spectra analysis

Figure  5 demonstrates the absorbance spectra of syn-thesized Al doped PbS nanoparticles. It is noted that Al

Fig. 2 XRD pattern of pure and Al-doped PbS nanocomposites

Fig. 3 SEM images of pure and Al-doped PbS nanoparticle compos-ites

J Mater Sci: Mater Electron

1 3

doped PbS exhibit a blue shift in their absorption edge which indicates an increment in their band gap ener-gies [9]. The optical band gap (Eg) estimations of the Al doped PbS nanoparticles were evaluated from the transmittance values and from the plots of the (αhν)2 versus photon energy (hυ) (Fig.  6). An increase in the Eg value from 1.91  eV acquired for the PbS sample to 2.04  eV with increase in Al doping PbS was observed. Higher band gap values observed with doping is fea-tured to quantum confinement effect [10]. As indicated by quantum confinement, the shift of absorption thresh-old to shorter wavelength due to individual confinement

of electrons and holes, increased band gap is observed at smaller crystallites [11].

3.3.2 Photoluminesence (PL) analysis

Figure  7 demonstrates the PL spectra of the pure and Al doped PbS nanopowders. In PbS semiconductor crystal-lites the peak at 408 nm can be featured to a higher transi-tion level. It is generally associated with the transition of electrons to holes trapped at interstitial Pb2+ sites from the

Fig. 4 TEM images of pure and Al doped PbS

Fig. 5 UV–Vis absorption spectrum of pure and Al-doped PbS

Fig. 6 The optical band gap energy of pure and Al-doped PbS nano-composites

J Mater Sci: Mater Electron

1 3

conduction band [12]. Because, the recombination of impu-rity bound exciton, the peaks occur at 485 nm. The green band at 523 nm might be attributed to interstitial sulfur [13, 14].

3.4 Electrical properties

At various temperatures the pure and Al-doped PbS nano-particles pellets in disk form were analyzed. The dielectric constant and the dielectric loss of the pure and A1-doped PbS nanoparticles were also determined at varying temper-atures at a frequency between 50 Hz–5 MHz. The dielectric constant was evaluated by the following equation,

where d and A being the thickness and the area of the sam-ple respectively. The variation in dielectric constant with respect to the frequency and temperature for pure PbS nan-oparticles is shown in Fig. 8. At every temperature and at a low frequency the dielectric constant of pure PbS nano-particles was found to be high. It decreases rapidly with the applied frequency. The dielectric constant was measured at various temperatures for the A1-doped PbS nanoparticles, as a function of the frequency is shown in Fig. 9. It is obvi-ous that the strength of frequency dependence is at lower frequency region. With an increase in frequency the die-lectric constant decreases and subsequently it has become a constant at higher frequencies [15]. Porosity and grain structure might be the inhomogeneties that are present in

(2)�r =Cd

�0A

the system. The polarization was low with an increase in frequency and then it achieved a constant value [16]. This occurs owing to the fact that, beyond a certain frequency of the external field the hopping between various metal ions could not follow an alternating field.

The variations of dielectric loss of pure PbS with fre-quency and temperature are shown in Fig. 10. This shows that the dielectric loss reduces with an increase in fre-quency and at every temperature of higher frequencies the loss angle has practically similar value. A small quan-tity of the electric energy is utilized for revolutions of dipolar molecules and various types of molecular trans-fer from one position onto the next, which involves loss

Fig. 7 Photoluminiscence spectra of pure and Al-doped PbS nano-composites

Fig. 8 Temperature dependent frequency dispersion dielectric con-stant of PbS variation

Fig. 9 The variation of dielectric constant versus frequency of Al-doped PbS

J Mater Sci: Mater Electron

1 3

of energy. In nanophase materials, the inter phase layers create an absorption current in the inhomogeneities that are identified with the defects and space charge arrange-ment thus resulting in dielectric loss [17]. Figure  11 demonstrates the curve of dielectric loss versus log f at different temperatures of Al doped PbS nanoparticles. It can be seen that at every temperature the dielectric loss decreases with an increase in frequency. This may be achieved because of the space charge polarization [18]. It is also understood that the dielectric loss was decreased by increasing the frequency and it turned out to be low at high frequency region. It demonstrates the capability of these materials which were used in high frequency device applications [19].

3.4.1 AC conductivity measurement

The ac conductivity plot of pelletized form of pure and Al-doped PbS nanoparticles was analyzed. As an increase in frequency improves the electron hopping frequency, it can be observed that the ac conductivity gradually attains higher value with an increase in frequency of the applied ac field. The ac conductivity of pure and Al-doped PbS nano-particles can be calculated by the following relation:

where ε0 is permittivity in free space, εr is dielectric con-stant, f is the frequency, and tan δ is the loss factor. Fig-ure 12 shows the variation of ac conductivity of pure PbS nanoparticles at various frequencies and temperatures.

(3)�ac = 2��0�rf tan �

Fig. 10 Variation of dielectric loss of pure PbS

Fig. 11 Variation of dielectric loss of Al-doped PbS

Fig. 12 Variation of AC conductivity of PbS

Fig. 13 Frequency variation of AC conductivity of Al-doped PbS

J Mater Sci: Mater Electron

1 3

Figure 12 depicts that higher the frequency, higher the ac conductivity. For an increase in the frequency, there is a small increase in the electrical conductivity of the nanoma-terial at the low frequency range. But this value remains the same at all temperatures. Figure 13 shows the ac conduc-tivity of pure and Al-doped PbS nanoparticles. There is an increase in the electrical conductivity of the nanomaterial at low frequency region with increase in frequency which remains the same for all temperatures [20]. It is observed that the frequency independent behavior at lower frequency region demonstrates the dc conductivity and the disper-sion at higher frequency region demonstrates the frequency dependent behavior [21]. It is constant with an increase in temperature which results in enormous carrier hopping that causes an increase in the conductivity.

4 Conclusions

In summery, we experimentally prepared Al-doped PbS nanoparticles by simple cost-effective chemical assisted route. The formation of Al-doped PbS nanoparticles was confirmed by X-ray diffraction. From the XRD analysis, it was identified that the pure and Al-doped PbS exhibit face-centered cubic structure. The average grain sizes of the pure and the Al-doped PbS nanoparticles were found to be 22 and 16 nm by using the Debye–Scherrer’s formula. The SEM observation clearly showed the fact that the particles were strongly aggregated. Spherical shaped nanograins are visible from the TEM images of both the pure and doped nanopowder. The estimated band gap values for the pure and the Al-doped PbS nanoparticles were found to be 1.94 and 2.04  eV respectively. The Photoluminescence studies reveal that the Al doped PbS sample has the green shift emission bands were observed. The variations of the die-lectric constant, the dielectric loss, and the AC conductivity of Al doped PbS nanoparticles were studied. The dielectric studies revealed the fact that both the dielectric constant and the dielectric loss decreased with an increase in the frequency. The dielectric constant suddenly dropped at lower frequencies. On account of the interface/space polar-ization, the Al dopant concentration was increased more by decreasing the dielectric polarization, which in turn decreased the dielectric constant. Dielectric loss of the pure and Al-doped PbS samples were found to decrease with the increase in frequency. It could be possible because of the

interface/space charge polarization effect. The AC electri-cal conductivity was found to increase with an increase in the temperature and the frequency. The findings reported here open up the way towards the use of all-organic radical molecules in optoelectronic and spintronics devices. From a more fundamental point of view, we believe that such a simple and controllable electrochemical assembly protocol will provide a new pathway for the production of various nanomaterials containing hybrid composites with unique structures for catalysis, sensors, and energy storage and conversions.

References

1. X. Lu, I. Li, W. Zhang, C. Wang, Nanotechnology 16, 2233 (2005)

2. Z.I. Wang, Ann. Rev. Phys. Chem. 55, 159 (2004) 3. C. Rajashree, A.R Balu, V.S Nagarethinam, J. Mater. Sci. Mater.

Electron. 27, 5070 (2016) 4. A.H. Souici, N. Keghouche, J.A. Delaire, H. Remita, A. Etch-

eberry, M. Mostafavi, J. Phys. Chem. C 113, 8050–8057 (2009) 5. J.D. Patel, F. Mighri, Mater. Sci. Appl. 3, 125–130 (2012) 6. X. Zhao, I. Gorelikov, J. Langmuir. 21, 1086–1090 (2005) 7. L.F. Koao, F.B. Dejene, H.C. Swart, Int J. Electrochem. Sci. 9,

1747–1757 (2014) 8. G.I. Koleilat, L. Levina, ACS Nano 2, 833–840 (2008) 9. A. Dumbrava, D. Berger, G. Prodan, F. Moscalu, A. Diacon,

Mater. Chem. Phys. 173, 70 (2016) 10. T. Sivaraman, V. Narasimman, V.S. Nagarethinam, A.R. Balu,

Prog. Nat. Sci. Mater. Int. 25, 392 (2015) 11. T. Sivaraman, V.S. Nagarethinam, A.R. Balu, K. Usharani, J.

Mater. Sci. Mater. Electron. 27, 1158 (2016) 12. S.B. Pawar, J.S. Shaikh, R.S. Devan, Y.R. Ma, D. Heranath, P.N.

Bhosale, P.S. Patil, Appl. Surf. Sci. 258, 1869 (2011) 13. Y. Al-Douri, Q. Khasawneh, S. Kiwan, U. Hasim, S.B. Abd

Hamid, A.H. Reshak, A. Bouhemadou, M. Ameri, R. Khenata, Energy Convers. Manage. 82, 238 (2014)

14. S.J. Gnanamuthu, I.K. Punithavathy, S.J. Jeyakumar, K. Usha-rani, A.R. Balu, Mater. Res. Innov. 20, 398 (2016)

15. S. Sagadevan, I. Das, J. Podder, J. Mater. Sci. 27, 13016–13021 (2016)

16. S. Sagadevan, K. Pal, P. Koteeswari, A. Subashini, J. Mater. Sci. (2017). doi:10.1007/s10854-017-6488-3

17. D.M.D.M. Prabaharan, K. Sadaiyandi, M. Mahendran, S. Sagadevan, Mater. Res. 19, 478–482 (2016)

18. S. Sagadevan, I. Das, P. Singh, J. Podder, J. Mater. Sci. 28, 1136–1141 (2017)

19. S. Sagadevan, I. Das, K. Pal, P. Murugasen, P. Singh, J. Mater. Sci. (2016). doi:10.1007/s10854-016-6180-z

20. S. Sagadevana, J. Podder, Mater. Res. 19(2), 420–425, (2016) 21. R. Sharma, P. Pahuja, R.P. Tandon, Ceram. Int. 40, 9027–9036

(2014)