Optical Materials 46 (2015) 522–525

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Optical Materials

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Preparation and optical studies of PbS nanoparticles

http://dx.doi.org/10.1016/j.optmat.2015.05.0170925-3467/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: z_amin@bk.ru (Z.Q. Mamiyev).

Zamin Q. Mamiyev a,⇑, Narmina O. Balayeva a,b

a Institute of Physics, Azerbaijan National Academy of Sciences, H. Javid pr. 131, AZ1143 Baku, Azerbaijanb Department of Chemistry, Baku State University, Z. Khalilov str. 23, AZ1148 Baku, Azerbaijan

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 January 2015Received in revised form 31 March 2015Accepted 5 May 2015Available online 27 May 2015

Keywords:PbSOptical propertiesNanoparticlesAtomic Force Microscopy

In the present report formation of nano-sized PbS in MA/octene-1 copolymer matrix at 80 �C temperature isbeing reported. A size and distribution of particles were observed in AFM results and the images are corre-lated with the results on X-ray diffraction measurements. The structure and phase of the PbS nanoparticleswere characterized by X-ray diffraction (XRD). XRD studies reveal that as-synthesized PbS nanoparticles arein single phase cubic structure and the grain size have been calculated 10–15 nm from XRD results. The sizedistribution was further supported by UV/Vis absorption and photoluminescence (PL) spectroscopy of thecolloid nanoparticles. The obtained nanocomposites show an emission peak at 418 nm.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor nanocrystals (NCs) have attracted great interestdue to their size-tunable optical properties arising from the effectof quantum confinement. Over the past decades, polymer matrixand semiconductor NCs based nanocomposites studied extensivelyas a new alternative to conventional polymer materials. Ananocomposite is a composite material, which one of the compo-nents has at least one dimension that is nanoscopic size that isaround 1–100 nm. Lead sulfide (PbS) is an important binary IV–VI a unique semiconductor materials. The size controlled PbS NCscan be potentially used in preparing optical switches solid-statelaser, solar cells, solar absorbers, photographs, lasers, LED devices,telecommunications, detectors, optical switches, optical amplifica-tion, electro-luminescent devices such as light emitting diodes [1–3]. PbS nanocrystals are in interest because of the large excitonBohr radius that gives strong quantum confinement of both elec-trons and holes. PbS NP can also show multiple exciton generation(MEG), in which the impact of a single photon produces two ormore excitons. This phenomenon has raised the possibility ofquantum dot based solar cells with photo-conversion efficiencyof as much as 66% [4–6]. Consequently, the synthesis of PbSnanocrystals with different morphologies and the correspondingeffects on material properties is of great importance in the searchfor novel applications in electroluminescent devices such as lightemitting diodes [7]. When the size of the nanoparticles (NP) arebelow Bohr radius, its lead to the quantum confinement effect. In

order to reduce the size of the particles below Bohr radius, passiva-tion of the surface of the particles at the time of formation is essen-tial [8]. PbS band gap can be widened to the visible region byforming nanocrystals. PbS is an important direct band gap semi-conductor material with a small band gap (0.41 eV) and findsapplications in near-IR communications. It has a large excitonicBohr radius (18 nm) and high dielectric constant [9]. These proper-ties lead to a third order nonlinear optical response some 1000times that of PbS nanoparticles for particles of similar size. Thesefeatures make PbS nanoparticles particularly suitable in opticaland photonic device applications. Quantum-sized PbS nanoparti-cles may have single electron transistors and field-effectthin-film transistors and highspeed switching, solar cells, gas sen-sors and other optoelectronic devices also IR detectors [10–12]. Byvarying the size and shape from bulk material to nanoparticles, it ispossible to change the optical band gap from 0.41 eV to the valuesup to 5.2 eV. Therefore, it is possible to build optical sensors withadjustable properties [13,14]. Various methods for the preparationof nanocomposites (N/C) are known in literature such as ultrasonicmethod, microbial method, microwave-assisted heating, layer bylayer, ultraviolet irradiation, solvothermal method, electrodeposi-tion [15–17]. One important feature of our approach is the stabiliz-ing possibility of NP at the growing time.

2. Experimental

2.1. Materials

Maleic Anhydride C2H2(CO)2-O, Octene-1, azobisisobutyloni-trile (AIBN), dioxane C4H8O2, lead acetate Pb(CH3COO)2, thiourea

Fig. 1. Powder XRD pattern for PbS nanoparticles embedded in MA/octene-1copolymer matrix.

Z.Q. Mamiyev, N.O. Balayeva / Optical Materials 46 (2015) 522–525 523

(NH)2CS, Potassium hydroxide KOH, N,N-dimethylformamide (DMF),double-distilled water was used in all experiments. All of the initialchemicals from commercial source and analytical grade pure.

2.2. Instrumental

The synthesized material was studied with different techniques.The morphologies of the prepared PbS-copolymer N/C were char-acterized by the A-IST-NT’S SMART SPM™ Atomic ForceMicroscopy, Bruker D8 Advance X-ray diffractometer was used tomeasure the size and shape of particles accurately. Optical proper-ties of the N/C were studied by SPECORD 250 PLUS-223G1020 UV–Vis and Cary Eclipse Spectrofluorometer.

2.3. Synthesis procedures

It is well known that the chemical in-situ technique is the mostconvenient and frequently used to grow PbS NCs in polymermatrix. It has been found that the properties of chemically synthe-sis PbS NP depend strongly on the growth conditions. Until now,stabilized PbS colloids with fine structured absorption spectra havebeen obtained in aqueous and methanolic solutions [18–20]. In thisresearch work we have used MA/octane-1 copolymer matrix to sta-bilize PbS NP. The copolymer was synthesized in the laboratoryfrom the relevant monomers with radical polymerization reaction[21]. The supposed molecule structure for obtained MA/octene-1copolymer is given bellow.

The preparation of PbS/copolymer N/C material was carried out usingthe following procedure: After optimization of synthesis conditions,the PbS precursors were prepared by dissolving of 0.69 g of lead acetateand 1 g white color powder copolymer in 100 ml DMF in the threemouth flask and the pH of the solution was maintained at 10 with KOH.

The process was carried out by mixing at 80 �C, after 4 h 0.16 gof thiourea was added rapidly to the flask with syringe and thenthe reaction was continued for another hour. At the end of reactionstrong white clear solution turn to brownish black color whichindicated the formation of PbS nanoparticles. The in-situ is basedon sequential reaction at the substrate surface. The formation ofPbS may involve the following steps. In a DMF solution lead acetatereleases Pb2+ ions as following.

PbðCH3COOÞ2 ! Pb2þ þ 2CH3COO�

The formed Pb2+ cations join to oxygen-containing functionalgroups with weak bonds. In alkaline medium, dissociation of(NH2)2CS takes place and give rise S2� anions. By impact of gener-ated anions weak bonds break and PbS form with reaction betweencations and anions.

Sjj

NH2� C� NH2 þ OH� ! CH2N2 þH2Oþ SH�

SH� þ OH� ! S2� þH2O

Pb2þ þ S2� ! PbS #

Stabilization of particles becomes possible through oxygenatoms and double bonds in the copolymer molecule. If the agglom-eration process is not prevented at the growing time, then owing toVander Waals attraction and Ostwald ripening between particles,they get agglomerated and further growth of the particles leadsto form bulk particles. For avoiding the agglomeration of thenanoparticles they can be stabilized electrostatically and stericallyby stabilizer at appropriate stages in order to get size selective syn-thesis during precipitation reaction.

The initial factor responsible for the final morphology of theproducts is the crystallographic phase of the seed formed duringthe nucleation process which depends on the nature of the mate-rial and on the environmental conditions [22].

3. Results and discussions

3.1. X-ray diffraction characterization

Structural identification of powder PbS NCs was carried outwith X-ray diffraction in the range of angle 2h between 20� and65�. Fig. 1 shows the XRD pattern for obtained NCs, which werenanocrystalline in nature.

Diffraction peaks at 2h values 26�, 29�, 43�, 53� and 60� areobserved. All diffraction peaks can be corresponding to the purecubic phase of PbS (JCPDS card No. 05-0592) and the peaks areoriginating from (111), (200), (220), (222), (400) planes respec-tively. The highest intensity of the peak shows that the grains arepreferentially oriented along (111) direction. The estimated aver-age value of the cell constant a was calculated 5.896 Å using thefollowing relationship (For. (1)) for the cubic phase structure whichmatch perfectly with the standard data. Where h, k and l are theMiller indices; and d is the interplanar space.

a ¼ dðh2 þ k2 þ l2Þ1=2

ð1Þ

The dominant and sharp peaks indicate that PbS nanocrystalsare highly crystalline. Looking at the peak broadening, it can besaid that nanosized crystallites have been formed. This was con-firmed by crystallite size calculation using Scherer’s For. (2) [23].

D ¼ akb cos h

ð2Þ

where k is the X-ray wavelength, a is the shape factor and equalto 0.89, D is the average diameter of the crystals, h is the diffractionangle in degree and b is the line broadening of FWHM in radian.

Fig. 2. AFM images of the obtained PbS/MA-Octene1 N/C.

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The average size of PbS nanoparticles has been calculated about10–15 nm. The absence of any other phase from XRD pattern indi-cates the purity of sample.

3.2. AFM characterization

Fig. 2 shows surface characterization of the PbS-copolymer N/C.For measured on AFM the obtained N/C thin film deposited on theglass substrate from solution in DMF. The AFM images were cap-tured in the Semi Contact Mode using an e-scanner with a maxi-mum scan dimension of 10g/10g, 1g/1g and 0.8g/0.8g inambient. From AFM results the average grain size of nanoparticlesis revealed up to 15 nm. AFM analysis showed that the grain sizesof PbS nanocrystals were in good agreement with XRD results.

3.3. Optical characterization

UV–Visible spectrum has been widely used to characterize thesemiconductor nanoparticles. As the particle size decrease, absorp-tion wavelength (kmax) will be shifted to shorter wavelength, sincethe band gap increases for the nano sized particles. This is thequantum confinement effect of the semiconductor nanoparticles.As shown in Fig. 3 there is a small and finite separation betweenenergy levels.

The PL originates from the recombination of surface states.Absorption of light by PbS increases by increasing its surface area.Both surface area and band gap can be increased when the particlesize reduces to nano dimensions. At nanometric sizes, quantumconfinement effects can come into play and affect most notablythe electronic properties. Therefore, the perceptible attention hasbeen paid to prevent the agglomeration of PbS particles in orderto improve their optical properties.

UV–Vis absorption spectrum was recorded for chemically syn-thesized PbS nanoparticles at room temperature is shown in Fig. 4.

It can be seen that obtained PbS/copolymer N/C exhibit absorp-tion peak at around �368 nm, which is fairly blue-shifted from theabsorption edge of the bulk PbS (3020 nm) [25]. In the presentwork optical band gap for synthesized PbS NCs is calculated bythe Tauc relation which is described as bellow (For. (3)) [26].

a ¼ Aðhm� EgÞn

hmð3Þ

where hm denotes the energy of the incoming photon in electronvolt, a is the absorption coefficient of the material, Eg is the bandgap of the materials, B is a constant and n is exponent which canhave different values (2, 1/2, 2/3 and 1/3) corresponding to directallowed, indirect allowed, direct forbidden and indirect forbiddentransitions respectively [27]. The band gap is determined from theintercept of the plot of (ahm)2 versus hm as shown in Fig. 5. The opti-cal band gap at this condition is calculated as Eg = 3.36 eV which ishigher than the bulk band gap value (Eg = 0.41 eV at 300 K). This isattributed to the crystallite size-dependant properties of the Eg.

It is reported that the band gap of a specific material does notonly depend on its structure but the size also has a controlling fac-tor. Once the particle reaches nano-meter size, quantum effectscome into play and the effective band gap increases [28].

Another unique property associated with semiconductornanoparticles is their luminescence characteristics, with the speci-fic emission wavelengths dependent upon the nature of the semi-conductors. Photoluminescence (PL) takes place after theabsorption of a photon [29].

Fig. 6 depicts room temperature PL spectrum of theas-synthesized PbS nanoparticles. The excitation wavelength was345 nm. The PL spectrum of the PbS nanoparticles was dominatedby very strong and broad emission peak spanning over a large partof visible range (350–500 nm). Photoluminescence measurements

show that the sample emits a strong as well as stable bluish lightand broad luminescent peak estimated at 418 nm, which is eluci-dated in accordance with the transition of electrons from the con-duction band edges to holes, trapped at surface states located inthe forbidden gap [30].

Fig. 3. Energy dispersion for the (left) bulk semiconductor case compared to that ofthe (right) Quantum Dot case [24].

Fig. 4. Absorption spectra of neat MA/octene-1 (a) and MA-Octene1/PbS nanocom-posite (b).

Fig. 5. Energy band gap determination of the obtained PbS nanoparticles.

Fig. 6. Photoluminescence spectrum of as synthesized MA-Octene1/PbSnanocomposite.

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The obtained PbS nanoparticles exhibit a blue shift in the PLspectrum comparing to its bulk material, caused by the particlesize effect.

4. Conclusion

In this study, we have described a simple route and cheapchemical in-situ method to obtain PbS nanoparticles in the copoly-mer matrix. In this work, it is observed that MA/octane-1 acts asvery good role for stabilizing of PbS NCs. XRD patterns were

specified as only pure phase of cubic PbS structure, with grains sizearound 13 nm. PL measurement indicated emission peak at418 nm due to the recombination of electrons from the conductionband edge to holes trapped at the Pb2+ interstitial sites. The opticalabsorption study reveals an absorption maximum at 368 nm.Optical band gap for synthesized PbS nanoparticles is calculated3.36 eV and a significant blue-shift of the band gap energy isobserved from the bulk PbS. The yield, grain size and otherobserved characteristic properties indicate that the used techniqueis suitable and economical for obtaining PbS NP.

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