The Chemical Vapor Deposition of Nickel Phosphide or Selenide Thin Films from a Single Precursor Arunkumar Panneerselvam, ² Mohammad. A. Malik, ² Mohammad Afzaal, ² Paul O’Brien,* ,² and Madeleine Helliwell ‡ The School of Chemistry and the School of Materials, The School of Chemistry, The UniVersity of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom Received October 26, 2007; E-mail: paul.obrien@manchester.ac.uk Nickel phosphide and nickel selenide semiconductors are po- tential materials for photoelectrochemical solar cells. 1,2 They also have interesting electrical and magnetic properties and have promising applications as catalysts 3 and in sensors. 4 Nickel phos- phide is an n-type semiconductor with a band gap of 1.0 eV, whereas the selenide is a p-type with a band gap of 2.0 eV. There are only few reports on the deposition of nickel phosphide films which include by magnetron sputtering, 5 electrodeposition, 6 elec- troless deposition, 7 or the reaction of orthophosphoric acid on a nickel substrate. 1 Nickel selenide films were prepared by elec- trodeposition, 2 solution growth, 8 reactive diffusion, 9 or chemical vapor deposition (CVD) methods. 10 As far as we know there is no report on the deposition of a Ni 0.85 Se/Ni 2 P heterostructure. Single source precursor (SSP) chemistry has attracted considerable interest for the growth of semiconductor thin films and nanoparticles. 11 Herein we report the synthesis and characterization of imido-bis- (diisopropylthioselenophosphinate) nickel(II), Ni[ i Pr 2 P(S)NP(Se) i - Pr 2 ] 2 , an interesting complex used as SSP for the growth of nickel phosphide (Ni 2 P) or nickel selenide (Ni 0.85 Se) and in sequence for Ni 0.85 Se/Ni 2 P layers. There are reports for the formation of different phases of the same material from a SSP 12 but to best of our knowledge there are no reports for the deposition of phosphide and selenide materials from the same precursor. The SSP was synthesized by the deprotonation of the ligand [ i - Pr 2 P(S)NHP(Se) i Pr 2 ] 13 using sodium methoxide to form the anion which is subsequently reacted with nickel(II) nitrate hexahydrate in methanol to produce a dark-red precipitate. Recrystallization of the complex from toluene gave red crystals. X-ray crystallographic studies reveal that a nickel atom is tetrahedrally coordinated through the sulfur and selenium atoms (Figure 1). The crystal structure shows the presence of independent monomeric units which are separated by normal van der Waals distances. The sulfur and selenium atoms are disordered as observed for the Pt[ i Pr 2 P(S)- NHP(Se) i Pr 2 ] 13 complex and are refined with equal occupancies for both atoms. The six-membered NiSSeP 2 N ring adopts puckered pseudo-boat conformation. Decomposition was studied by thermogravimetric analysis (TGA) (N 2 atmosphere at 10 °C min -1 ) which reveals a single-step decomposition between 300 and 368 °C. Low-pressure metal- organic (LP-MOCVD) experiments were carried out using a custom-built cold-walled low-pressure reactor tube which has been described elsewhere. 14 Deposition was carried out on a glass substrate for 60 min at temperatures between 475 and 375 °C, and the precursor temperature was kept constant at 300 °C. Nickel phosphide films were deposited at temperatures of 475, 450, and 425 °C, whereas nickel selenide films were deposited at temper- atures of 400 or 375 °C. X-ray diffraction pattern (XRD) of the as-deposited films at 475 °C and 450 °C showed hexagonal Ni 2 P (JCPDS 74-1385), with preferred orientation along the (111) plane (Figure 2a), whereas as-deposited films at 425 °C show a mixture of both Ni 2 P and Ni 5 P 4 . The scanning electron microscopy (SEM) images of the films grown at 475 °C reveal the morphology composed of convoluted wires with granules randomly attached to them (Figure 2b). Energy dispersive X-ray analysis (EDX) of these films shows that they are composed of only nickel phosphide without any major contamination of sulfur or selenium (475 and 450 °C). The analyses on wires show 54% of nickel and 46% of phosphorus, whereas on granules show 45% of nickel and 55% phosphorus. The same precursor can deposit nickel selenide films instead of nickel phosphide at temperatures 400 or 375 °C. XRD pattern of the as-deposited films show hexagonal Ni 0.85 Se (JCPDS 18-0888), with preferred orientation along (101) plane (Figure 2c) with traces of NiSe 2 (JCPDS-18-0886) at 2θ values 28.58 and 33.72. Traces ² The School of Chemistry and the School of Materials. ‡ The School of Chemistry. Figure 1. X-ray single-crystal structure showing one component of the disordered Ni[ i Pr2P(S)NP(Se) i Pr2]2 complex with 50% probability level ellipsoids and H atoms on the isopropyl rings omitted for clarity. Figure 2. XRD pattern (a) and SEM image (b) of nickel phosphide films deposited at 475 °C; XRD pattern (c) and SEM image (d) of nickel selenide films deposited at 400 °C; /) Ni2P peaks. Published on Web 02/01/2008 2420 9 J. AM. CHEM. SOC. 2008, 130, 2420-2421 10.1021/ja078202j CCC: $40.75 © 2008 American Chemical Society