Study of Solution Grown Variable BandGap Pb 1-x Mn x S Semiconductor Nanoparticle Films Rakesh K. Joshi, a,z Pushpendra Kumar, a H. K. Sehgal, a and Aloke Kanjilal b a Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India b Department of Physics and Astronomy, University of Aarhus, DK 8000 Aarhus C, Denmark Single-phase Pb 1-x Mn x S  0.03  x  0.37 nanoparticle films were prepared using an electroless solution growth technique. Face centered cubic structure with a decrease of lattice parameter for an increase of x in the alloy films was observed. Composition dependence of optical and electrical properties is discussed. The optical bandgap  E g  could be varied from 1.50 to 2.50 eV by changing the x in the range 0.03  x  0.37 for the films. Effect of the grain size and the effect of alloying on optical properties are distinguished for the ternary nanoparticle films. Variable range hopping has been identified as mechanism for conduction in the p-type semiconductor Pb 1-x Mn x S nanoparticle films. Mobility  of the majority carriers shows a decrease with increase in Mn concentration x and follows a T -0.4 temperature dependence, indicating the surface scattering of carriers in the nanoparticle films. © 2006 The Electrochemical Society. DOI: 10.1149/1.2257384 All rights reserved. Manuscript submitted April 4, 2006; revised manuscript received May 31, 2006. Available electronically August 9, 2006. Utility of a semiconductor can be enhanced by modifying its characteristics as per requirement. Ternary alloys prepared out of binary semiconductors provide a class of semiconductors in which the lattice parameter, energy bandgap, and other operational param- eters could be continuously varied within specified limits by alloy- ing appropriate binary constituents with changes in their relative concentrations. 1-5 There has been growing interest in tailoring the properties of semiconductors and, therefore, various semiconducting materials were developed, studied, and utilized for variety of applications. 6 Criteria for the selection of binaries involve their op- erational characteristics and compatibility in chemical bonding and the crystal structure. Effect of compositional disorder in these ter- nary crystalline materials on tailoring of states into the forbidden gap is so small that, in general, these materials behave as homoge- neous uniphase semiconductors. These considerations make the ter- nary semiconductors interesting and desirable candidates for device applications. PbS, a material of interest for the present study, has been alloyed with many other binary sulfides to form variable bandgap ter- nary semiconductors of the type Pb 1-x M x S  M = Mn, Sr, Sn, Hg, Zn 7-14 to achieve variation in bandgap over large values. Con- sidering that the bandgaps of PbS bulk and MnS bulk are 0.41 and 3.5 eV, respectively, a bulk ternary alloy of the type Pb 1-x Mn x S should show a variation of the bandgaps between the limiting values of 0.41 and 3.5 eV. Considering the fact that for maximizing the utility of the ternary system, the thin film route is the most obvious and that the grain size in the polycrystalline film is strongly depen- dent on growth procedure and conditions, a variety of options re- lated to semiconducting properties of the material in thin film form become possible. Recent investigations 15-23 indicated that the reduction of particle size of semiconductors below their Bohr exciton radius has a pro- found effect on optical, electrical, and structural properties of mate- rials. For example, the nanoparticles of PbS show an increase in the value of bandgap on reducing the grain size. Here, we must note the work by Nanda et al. for the electrodeposited PbS nanoparticles of 24 nm average crystallite size with a bandgap value of 1.4 eV. 24 In another work on PbS nanoparticles, an increase of the bandgap from 0.41 bulk to 5.4 eV nano as the crystallite size is reduced to 2 nm is reported. 15 It is thus obvious that the optical bandgap in ternary thin films prepared from the binary constituents does not necessarily depend on the optical bandgap values of bulk materials, but is expected to be defined by the grain size dependent values of their binary constituents. Likewise, electronic characteristics of re- sulting films are also expected to be defined by their grain size dependence. The structural, electrical, and optical properties of the flash- evaporated polycrystalline Pb 1-x Mn x S ternary films have been stud- ied in the past. 8 To study the behavior of these films in nanodimen- sions, we made an attempt to grow these films by chemical bath deposition CBD. CBD has been successfully used to grow thin films of Pb 1-x Fe x S, 25-27 Pb 1-x Co x S, 28 Pb 1-x Hg x S, 12-14 Pb 1-x Cd x S, 29 and Pb 1-x Cu x S. 30 The CBD grown Pb 1-x Mn x S nanoparticles films were studied for the dependence of their structural, optical, and elec- trical properties on the Mn content in present investigation. Experimental The experimental setup used for the growth of ternary Pb 1-x Mn x S nanoparticle films consists of a reaction bath capable of maintaining the temperature of the chemical constituents for the reaction within accuracy of ±1°C. This is achieved with the help of a contact thermometer and an electrical circuit employing a mercury relay switch. A pH meter is provided to monitor the pH of reaction bath during the film growth. Films were grown below ambient tem- peratures by keeping the reaction bath in a freezing mixture of ice and common salt; its temperature was maintained at the appropriate values below ambient by supplying power from a heater. Continuous stirring of the chemical bath was affected during growth with the help of a magnetic stirrer. Films were grown from alkaline aqueous solution mixtures of lead acetate, thiourea, and manganese chloride. Substrate preparation.—Glass microslides, single crystal Si, and quartz were used as the substrates for deposition of the films. The glass slides were first cleaned with soap solution and then heated at 75°C in chromic acid for 2 h. The slides were washed with deion- ized water and ultrasonically agitated with propanol, trichloroethyl- ene TCE, and acetone. These were again agitated with propanol followed by washing with deionized water immediately before in- serting in chemical bath before film growth. Silicon substrates were ultrasonically agitated in a 1:10 solution of H 2 SO 4 :H 2 O 2 for 10 min, washed with deionized water, and then ultrasonically agitated with propanol, TCE, and acetone. The degreased silicon substrates were ultrasonically agitated in a 1:10 solution of hydrofluoric acid and deionized water. These were washed with deionized water and im- mediately immersed in the reaction bath to prevent surface oxida- tion. Quartz substrates were washed in propanol, TCE, and acetone followed by washing with deionized water immediately before film growth. Growth of Pb 1-x Mn x S films.—Growth parameters as concentra- tions of constituent solutions, temperature, and pH of the chemical bath were varied during film growth to work out optimized param- eters for the growth of lead-manganese sulfide films. Aqueous solu- tions of lead acetate  0.04 M, MnCl 2  0.05 M, and thiourea  0.04 M, where M is mole/liter, were found to be most appropriate for the growth of ternary films with the desired compositions. Films were grown at various values of pH, temperature, and concentra- z E-mail: rjoshi77@yahoo.com Journal of The Electrochemical Society, 153 10 C707-C712 2006 0013-4651/2006/15310/C707/6/$20.00 © The Electrochemical Society C707 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 150.46.200.22 Downloaded on 2014-07-04 to IP