Selenium

ARTICLE pubs.acs.org/JPCC

Understanding the Decomposition Pathways of Mixed Sulfur/Selenium Lead Phosphinato Complexes Explaining the Formation of Lead Selenide Javeed Akhtar,†,‡ Mohammad Afzaal,§ Mark A. Vincent,|| Neil A. Burton,|| James Raftery,|| Ian H. Hillier,*,|| and Paul O’Brien*,† †

School of Chemistry and School of Materials, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K. NanoScience & Materials Synthesis Lab, Department of Physics, COMSATS, Institute of Information Technology, Islamabad Campus, Islamabad, Pakistan § Center of Research Excellence in Renewable Energy, King Fahd University of Petroleum and Minerals, P.O. Box 1292, Dhahran, 31261 Saudi Arabia School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, U.K.

)

‡

bS Supporting Information ABSTRACT: Lead selenide (PbSe), as micro- and nanocrystals, has been produced from a mixed lead thioseleno-phosphinato compound, [Pb{(C6H5)2PSSe}2] by chemical vapor deposition. The formation of PbSe nanocrystals in the solution has also been demonstrated at room temperatrure in a mixture of oleylamine and dodecanethiol. Density functional theory calculations of the decomposition of the complex are consistent with a dominant role for thermodynamic factors rather than kinetic ones in controlling the material formed.

’ INTRODUCTION The synthesis of novel metal-molecular precursors to deposit binary1
3 or ternary phases4
6 of inorganic materials with critical dimensions of the order of nanometers have received considerable attention in the last three decades. To date, a large number of molecular precursors have been synthesized and used to grow metal sulfide, selenide, and telluride materials.7,8 One of the driving factors is to avoid using pyrophoric and especially noxious chemicals in preparing chalcogenide thin films and nanoparticles.9 Moreover, carefully designed metal precursors that provide the relevant elemental entities can suppress impurities associated with multiple source materials.10 The presence of only one precursor molecule in the supply stream potentially reduces the likelihood and extent of prereaction in CVD and can help to control intrinsic stoichiometry. Metal dithiocarbamates and xanthates are the most commonly used single-source precursors for thin films and nanoparticles.11 A number of studies have appeared reporting the decomposition products of the compounds in the presence of primary amines which promote nanoparticle formation at low temperatures.12,13 In attempts to understand the formation of metal sulfide nanoparticles in the presence of alkylamines, a recent study has r 2011 American Chemical Society

led to suggestions of a new mechanism, on the basis of the identification of isolated intermediates and the side products.14 Studies of the side products from different alkylamines at various concentrations have revealed that steric hindrance at the alkylamine is critical to the rate of decomposition of the dithiocarbamates and the competitive reaction pathways involving the intermediates. Information on the thermal properties of related mixed chalcogenide compounds are scant. Our group has been recently investigating the use of mixed-chalcogenide compounds in CVD experiments e.g., [Ni{iPr2P(E1)NP(E2)iPr2}2] (E1 = S, E2 = Se (1); E1 = E2 = S (2), and E1 = E2 = Se (3)), which lead to the deposition of nickel sulfide, selenide, or phosphide; the material deposited depending on both the temperature and method used for the deposition.15 In continuation of such efforts, we here analyze the decomposition behavior of the lead thioselenophosphinate, [Pb{(C6H5)2PSSe}2]16,17 in both the vapor and solution. The precursor decomposes in aerosol-assisted (AA)CVD Received: June 8, 2011 Revised: July 23, 2011 Published: August 04, 2011 16904

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Table 1. Crystal Data and Structural Refinement for [Pb{(C6H5)2PSSe}2] compound

C24H20P2PbS2Se2

formula weight

799.57

temperature

100(2) K

crystal system, space group

triclinic, P1

wavelength

0.710 73 Å

unit cell dimensions

a = 9.006 Å, R = 99.36° b = 10.928 Å, β = 90.01°

volume

c = 12.770 Å, γ = 98.07° 1227.4 Å3

Z, calculated density

2, 2.163 Mg/m
3

absorption coefficient

10.150 mm
1

F(000)

752

crystal size

0.15  0.15  0.05 mm

θ range for data collection

1.62
26.55°

limiting indices


11 e h e 11,
13 e k e 8,
16 e l e 15

reflections collected/unique 7081/4935 [R(int) = 0.0300] completeness to theta 96.2% refinement method

full-matrix least-squares on F2

data/restraints/parameters

4935/32/276

goodness-of-fit on F2

1.018

final R indices [I > 2σ(I)]

R1 = 0.0450, wR2 = 0.0991

R indices (all data)

R1 = 0.0653, wR2 = 0.1073

largest diff peak and hole

+1.927 and
1.863 e Å
3

experiments to deposit only scattered PbSe microcrystals as opposed to homogeneous thin films, whereas, in solution, [Pb{(C6H5)2PSSe)}2] leads to crystalline PbSe nanoparticles at room temperature in a mixture of oleylamine and dodecanethiol. The possible mechanisms involved in the formation of only PbSe materials and room temperature decomposition of the precursor are explored and discussed with the aid density functional theory calculations. From a technological perspective, PbSe is a promising material in many applications, including in laser materials,18 thermoelectric devices,19,20 near-infrared (near-IR) luminescence,21 and IR detectors.22
24 The recently discovered phenomenon of the multiple exciton generation (MEG) effect in PbE (where E = S, Se, or Te) materials could lead to an entirely new paradigm for high-efficiency and low-cost solar cell technology.20,21

’ EXPERIMENTAL SECTION a. Synthesis of [Pb{(C6H5)2PSSe)}2]. The preparation of [Pb{(C6H5)2PSSe}2] was prepared according to previously published reports.16,17 Yield: 1.86 g, 69%. Mp: 212 °C. Anal. Found: C, 35.2; H, 2.2; P, 7.3; S, 6.0, Pb, 27.3. Calcd: C, 36.0; H, 2.5; P, 7.7; S, 6.8; Pb, 26.0. IR: 2928 υ(C
H), 1472, υ(CdC), 720, υ(C
S) cm
1. 1H NMR (δ, CDCl3, 400 MHz): 7.89 (8H, m, Ar
H), 7.49 (8H, m, Ar
H), 7.40 (4H, m, Ar
H). Negative ESI-MS, [(C6H5)2PSSe]+: m/z 296 (100%). b. Deposition of Films by AACVD. Thin films composed of PbSe crystallities were deposited using a home-built AACVD Kit.25 Deposition was carried out on glass (450 and 500 °C) substrates by dissolving 0.02 M of the precursor in tetrahydrofuran (THF). Prior to deposition, the precursor was well dissolved in THF by sonication for 10 min. This precursor solution was placed in a two-necked 100 mL round-bottom flask

Figure 1. X-ray single crystal structure of [Pb{(C6H5)2PSSe}2].

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for [Pb{(C6H5)2PSSe}2] bonds

length (Å)

bonds

angles (deg)

Pb(1)
S(4)

2.812(8)

S(4)
Pb(1)
S(2)

94.1(7)

Pb(1)
S(2)

2.863(9)

S(2)
Pb(1)
S(3)

94.4(5)

Pb(1)
S(3)

2.876(6)

S(4)
Pb(1)
Se(4)

3.00(3)

Pb(1)
Se(4) Pb(1)
S(1)

2.890(3) 2.903(6)

S(2)
Pb(1)
Se(4) S(3)
Pb(1)
Se(4)

92.60(5) 75.62(19)

Pb(1)
Se(2)

2.978(2)

S(3)
Pb(1)
S(1)

154.40(2)

Pb(1)
Se(1)

3.017(5)

S(4)
Pb(1)
Se(2)

94.40(2)

Pb(1)
Se(3)

3.044(2)

P(1)
Se(1)
Pb(1)

84.00(18)

Se(2)
P(1)

2.151(6)

P(2)
S(4)
Pb(1)

89.10(3)

S(2)
P(1)

2.19(3)

S(4)
Pb(1)
S(2)

94.10(7)

with a gas inlet that allowed the carrier gas (argon) to pass into the solution to effect transport of the aerosol. This flask was connected to the reactor tube by a piece of reinforced tubing. The argon flow rate was controlled by a Platon flow gauge. Eight glass substrates (approximately 1  3 cm) were placed inside the reactor tube and inserted in a Carbolite furnace. The precursor solution in a round-bottom flask was kept in a water bath above the piezoelectric modulator of a PIFCO ultrasonic humidifier (Model No. 1077). The aerosol droplets of the precursor thus generated were transferred into the hot-wall zone of the reactor by carrier gas. Both the solvent and the precursor underwent thermolysis as the precursor vapors reached the heated substrate surface where thermally induced reactions and film deposition took place. c. Synthesis of PbSe Nanocrystals. The compound (20 mM) was dissolved in a 1:1 molar mixture of oleylamine (10 mL) and dodecanethiol (7 mL) under nitrogen and stirred at room temperature. After 15 min, dry methanol was added to the brown/black solution, resulting in the flocculation of the particles. The deposit was washed several times with methanol, dried, and stored at room temperature. d. Material Characterization. All synthetic reactions were performed under an inert atmosphere of dry nitrogen using standard Schlenk line techniques. All reagents were purchased from Sigma-Aldrich Ltd. and used as received. Solvents were distilled and dried prior to use when necessary. Electrospray mass spectra were recorded on a Micromass Platform II. 1H NMR spectra were obtained using a Bruker AC400 FT-NMR spectrometer. Infrared spectra were obtained on a Specac single reflectance ATR instrument (4000
400 cm
1). Elemental analysis was performed by the University of Manchester 16905

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Figure 2. XRPD patterns of PbSe crystallities deposited by AACVD at (a) 450 °C and (b) 500 °C. In the insets are shown the SEM images.

Microanalytical facility. Thermogravimetric measurements were carried out using a Seiko SSC/S200 thermal analyzer with a heating rate of 10 °C min
1 under nitrogen at a flow rate 10 mL/min. X-ray powder diffraction patterns were obtained using a Bruker D8 AXE diffractometer (Cu KR). The samples were scanned between 20 and 80° in a step size of 0.05 with a count rate of 9 s. Thin films were carbon coated using Edwards E-306A coating system before carrying out scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX). SEM was performed using a Philips XL 30FEG and EDX analysis were carried out using a DX4 instrument. Transmission electron microscope (TEM) samples were prepared by evaporating a dilute toluene solution of the nanoparticles on lacy carbon coated copper grids (S166-3, Agar Scientific). Philips CM20 and technai 30 TEM were used to obtain TEM images of the nanoparticles. e. X-ray Single Crystallography. Single-crystal X-ray crystallography study for the compound was carried out on a Bruker APEX diffractometer using a graphite monochromated Mo KR radiation (λ = 0.710 73 Å). The structures were solved by direct methods and refined by full-matrix least-squares on F2.26 All non-H atoms were refined anisotropically. The crystal refinement parameters are given Table 1. H atoms were placed in calculated positions by assigned isotropic thermal parameters and allowed to ride on their parent carbon atoms. All calculations were carried out by using the SHELXTL package.27 CCDC 685544 contains the supplementary crystallographic data for this paper.

Figure 3. (a) XRPD and (b) TEM image of PbSe crystals grown at room temperature in a mixture of oleylamine and dodecanethiol.

’ RESULTS AND DISCUSSION The synthesis of [Pb{(C6H5)2PSSe}2] was easily carried out by reaction between (Pb(CH3COO)2) and (NHEt3)(Ph2PSSe) at room temperature.16,17 The compound is perfectly air stable and can be stored for a long period of time. Suitable crystals for single X-ray analysis were obtained from a concentrated THF solution at room temperature. The molecular structure of the compound is shown in Figure 1, and selected bond lengths and angles are given in Table 2. The geometry at the lead atom is a distorted square pyramidal with two sulfur and two selenium atoms, forming the base of the pyramid and the lone pair occupying the axial position. The average bond lengths of Pb
S (2.86 Å) are smaller than the bond lengths of Pb
Se (2.98 Å) as expected. The average bond angle around S/Se
Pb
S/Se is 95.9° and is similar to the value in related compounds such as lead dithiocarbamates.28,29 The decomposition behavior of the compound was studied by thermogravimetric analysis (N2, 10 °C/min), which indicated a single-step weight loss between 287 and 385 °C. Calculation of the precursor efficiency to afford bulk PbSe (35.9%) as the final product, based on the residual material (37.6%) from the TGA experiments, found the sample to be within 5%. Growth of thin films was attempted at 450 and 500 °C with an argon flow rate of 160 SCCM (SCCM denotes cubic centimeter per minute) for 1 h at atmospheric pressure. The reactor was purged with argon for 10 min at the required deposition temperature before carrying out deposition to avoid any in situ oxidation. The deposited black films were found to be nonuniform and could be easily wiped off 16906

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The Journal of Physical Chemistry C the surface, indicating that the deposited material is weakly adsorbed on glass surfaces. This observation may account for Scheme 1. Possible Decomposition Routes of [Pb{(C6H5)2PSSe}2]

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the poor adhesion between the PbSe crystallities and substrates.30 X-ray powder diffraction (XRPD) (Figure 2) of deposited material corresponds to cubic PbSe (ICDD No. 06
3540) with the rock salt structure. The strong (200) peak suggests a (100) oriented growth. Scanning electron microscope (SEM) images reveal that the deposits consist of disordered cubic PbSe crystallites (∼0.7
2.2 μm in size) randomly orientated on the substrates. These results are in marked contrast to our previous studies on PbSe multifaceted microcrystals grown from lead phosphonodiselenoates in AACVD experiments.30b Energy-dispersive X-ray spectroscopy (EDS) analysis on PbSe is not fruitful because the commonly observed selenium K line overlaps with the lead M5 absorption edge. No sulfur peaks were detected. However, phosphorus accumulated during the decomposition of the precursor was found. At low deposition temperatures phosphorus contamination is 6
9% whereas at higher temperatures, lower phosphorus contents 1
2% were seen. PbSe Nanocrystals. Initial attempts were made to prepare PbSe nanocrystals at room temperature from the mixed S/Se compound. The resulting black precipitate was analyzed by XRPD, which confirmed the formation of crystalline cubic PbSe, without any impurities (Figure 3a). TEM studies indicate that the as-deposited material consists of a mixture of randomly orientated elongated rectangles, cubes, truncated cubes, and truncated octahedron structures, as shown in Figure 3b. Though they are uniform in shape, they vary significantly by size, which is found to be highly sensitive toward the reaction conditions. Computational Investigation of the Formation of PbSe from [Pb{(C6H5)2PSSe}2]. We have carried out density functional theory (DFT) calculations to study the formation of PbSe from [Pb{(C6H5)2PSSe}2], which we have observed during the thermal CVD process and also by reaction in solution with oleylamine and dodecanethiol. The calculations employed a local implementation of the M06 functional in Gaussian0331 together with the SDD (Pb) and 6-311G** (C, H, S, P, and Se) basis sets. The rigid rotor and harmonic oscillator approximations were used to obtain corrections to the calculated electronic energies, to yield free energies. We first consider a number of possible mechanisms for the thermal decomposition of [Pb((C6H5)2PSSe)2] and have calculated the corresponding reaction free energies, both at room temperature (298.15 K) and at an elevated temperature (800 K). These are shown in Scheme 1 together with the computed reaction free energies at 298 and 800 K. We have investigated a number of different fragmentation patterns that can yield the observed product, PbSe rather than PbS. The first two mechanisms considered (1, 2) involve the initial loss of a phenyl radical (C6H5) (1a), this process being highly endergonic. The next step is either the loss of SP(C6H5) (1b in Scheme 1) or of SeP(C6H5) (2b), the former reaction being the somewhat less endergonic of the two. In the scheme, we postulate the loss of a second phenyl radical (1c), which is only slightly less endergonic than the initial step. However, the subsequent decomposition of (C6H5)PSSe
Pb-Se (1d) to give the observed product, PbSe, is slightly exergonic. Thus, both reaction sequences 1a
1d and 2a
2b point to the favorable formation of PbSe. In the second set of possible mechanisms (3
5) that we have studied, the initial step is the loss of the SeSP(C6H5)2 (3a, 4a) entity to leave (C6H5)2PSSe-Pb, which, like the loss of the phenyl radical in steps 1 and 2, is also highly endergonic, but somewhat less so. The subsequent step could be the loss of either PbS (4b) or PbSe (3b) from (C6H5)2PSSe-Pb, the latter being preferred 16907

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Figure 4. (a) Reactant, (b) transition structure, and (c) product for the formation of PbSe by the decomposition of ((C6H5)2P(Se)(CH3S)S) 3 Pb 3 ((C6H5)2P(Se)S)
. Distances in Å.

on energetic grounds. An alternative route (5) to give PbSe involves a two-step decomposition of (C6H5)2PSSe-Pb, involving first the loss of a phenyl radical (5b), and the subsequent decomposition of (C6H5)PSSe-Pb, giving PbSe (5c). However, this mechanism is considerably more endergonic than the twostep mechanism giving PbSe. Thus, the conclusions of our computations is that the formation of PbSe in CVD may involve more than one mechanism but the steps that lead to of PbSe are somewhat more favorable on thermodynamic grounds, than those that lead to PbS formation. We now describe our calculations designed to understand the reaction of the compound in the liquid phase containing the amine and thiol, where the reactive species is likely to be a thioalkoxide ion, which we modeled as CH3S
. An initial calculation on the isolated complex showed the LUMO to be antibonding between the P and Se atoms of one ligand. Thus addition of electron density from one of the lone pairs of the sulfide nucleophile into this orbital will weaken the bond between the P and Se atoms and hence facilitate the formation of PbSe. Attack on either end of this bond, by CH3S
, was studied, but only when the sulfur nucleophile attacked the phosphorus could a transition state be determined. However, once the initial complex of (I) with CH3S
is formed (Figure 4)

the decomposition step ððC6 H5 Þ2 PðSeÞðCH3 SÞSÞ 3 Pb 3 ððC6 H5 Þ2 PðSeÞSÞ
f ððC6 H5 Þ2 PðCH3 SÞSÞ 3 PbSe 3 ððC6 H5 Þ2 PðSeÞSÞ
is quite facile with a barrier of only 13 kJ mol
1 (at 298 K). In this reaction CH3S
attacks the phosphorus center such that the selenium and the attacking sulfur are the axial ligands of the phosphorus and lead to the reaction being favorable by
30 kJ mol
1 (at 298 K). The product has a well-defined PbSe unit with the unreacted ligand still coordinated to the lead and the reacted ligand loosely coordinated.

’ CONCLUSION In summary, we have analyzed the decomposition behavior of mixed [Pb{(C6H5)2PSSe}2] in both the vapor and solution, leading to formation of PbSe only. The decomposition of the precursor was performed at room temperature in the presence of oleylamine and dodecanethiol. Computational studies indicate that it is easier to break the Se
P bond than the S
P bond, and thus the formation of PbSe is a result of thermodynamic factors rather than kinetic ones. 16908

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’ ASSOCIATED CONTENT

bS

Supporting Information. TGA of [Pb{(C6H5)2PSSe}2]. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: P.O., [email protected]; I.H.H., ian.hillier@ manchester.ac.uk.

’ ACKNOWLEDGMENT Financial support from EPSRC, U.K., is gratefully acknowledged. M.A. would like to thank Center of Research Excellence in Renewable Energy, KFUPM for the support. J.A. is thankful to HEC, Pakistan, for financial assistance. ’ REFERENCES (1) O’Brien, P.; Park, J.-H.; Waters, J. Thin Solid Films 2003, 431
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