X-ray absorption spectroscopy of the cubic and hexagonal polytypes of zinc sulfide B. Gilbert, 1, * B. H. Frazer, 1,2 H. Zhang, 3 F. Huang, 3 J. F. Banfield, 3 D. Haskel, 4 J. C. Lang, 4 G. Srajer, 4 and G. De Stasio 1 1 University of Wisconsin, Department of Physics, and Synchrotron Radiation Center, 3731 Schneider Drive, Stoughton, Wisconsin 53589 2 Institut de Physique Applique ´e, Ecole Polytechnique Fe ´de ´ral de Lausanne, CH-1015 Lausanne, Switzerland 3 Department of Geology and Geophysics, University of Wisconsin-Madison, Wisconsin 4 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439 Received 18 June 2002; published 26 December 2002 We investigate the sensitivity of x-ray absorption spectroscopy to cubic-hexagonal polytypism in which nearest-neighbor positions are virtually unchanged. Experimental spectra and multiple-scattering calculations are reported at the major absorption edges in the zinc-blende and wurtzite modifications of ZnS. We demon- strate that d-like unoccupied bands are preferentially affected, as only L-edge absorption is sensitive to this structural transition. The results allow near-edge x-ray absorption spectroscopy to be evaluated as a detection method for crystal structure modifications in nano-scale systems. DOI: 10.1103/PhysRevB.66.245205 PACS numbers: 71.20.Nr, 78.70.Dm I. INTRODUCTION X-ray absorption spectroscopy XAS is widely used to investigate bonding, coordination number, and valence, but no systematic work has appeared in the literature that inves- tigates the sensitivity of x-ray absorption near-edge structure XANES to subtle structural modifications in which the nearest-neighbor environment is unchanged. Many tetrahe- drally bonded crystalline materials are found to be stable in two or more stacking arrangements, that is, polytypes, in which structural units are combined in alternative ways. Sev- eral important classes of semiconducting materials exhibit cubic-hexagonal polytypism, which can be very close in total energy, 1 yet possess different structural and electrical prop- erties such as density, electronic band gap, and luminescence efficiency. 2 ZnS is of interest as a phosphor and electrolumi- nescent material, and is a system in which electrical and structural properties are being investigated as a function of particle size. 3 Sphalerite, the cubic zinc blende form of ZnS is stable at room temperature, while wurtzite, the less dense hexagonal form, is stable above 1020 °C at atmospheric pres- sure and metastable as a macroscopic phase under ambient conditions. The relative stability of these phases is modified in both synthetic and natural ZnS nanoparticles. 4,5 As a short- range structural probe, XAS is likely to be valuable in de- tecting crystal structure modifications in nanoscale systems, and additionally describes unoccupied electronic states, im- portant in excited-state processes, such as optical absorption, luminescence, and photocatalysis. Some XANES studies on sulfides have included one ZnS polytype, or both but without the resolution to distinguish the fine structure at any absorp- tion edge. 6–10 A. Crystal structure of cubic and hexagonal ZnS Two illustrations of the crystal structures of sphalerite cubic or zinc blende phase and wurtzite hexagonal are given in Fig. 1. When the comparison of cubic, along the 111 axis, to hexagonal, along the 1000 axis, is made, as in Fig. 1, a simple description of the difference between the structures is in the stacking order of ZnS layers- respectively ABCA and ABA. 11 B. XAS as a probe of crystal structure In x-ray absorption spectroscopy, a bound electron ab- sorbs an x-ray photon and is consequently excited to an un- occupied state of the photoexcited system. Transitions to ei- ther bound or free final states which satisfy energy conservation, h  =E f -E i , are permitted, subject to dipole selection rules. Transitions to bound states can be considered probes of unoccupied atomic or molecular orbitals; transi- tions to propagating electron final states can be considered probes of the local geometry, which gives sensitivity to crys- tal structure. A propagating spherical photoelectron wave scatters coherently from surrounding atoms, and interference effects in the vicinity of the absorbing atom lead to modula- tions in the absorption cross section that vary with the pho- ton energy. This is the basis for real-space photoelectron FIG. 1. Structures of zinc blende ZB and wurtzite W modi- fications of ZnS. From the central zinc atom, the second nearest neighbor shell is displayed. The crystals are aligned so the 111 axis ZB and the c axis W are parallel to the z axis indicated. Along these directions, the polytypes can be simply described as alternate stacking sequences of Zn,S layers following an ABA pattern in wurtzite and an ABCA pattern in zinc blende. The top and bottom S 3 triangles indicated in the figure are eclipsed staggered in wurtzite zinc blende. PHYSICAL REVIEW B 66, 245205 2002 0163-1829/2002/6624/2452056/$20.00 ©2002 The American Physical Society 66 245205-1