Probing the local structure of dilute Cu dopants in fluorescent ZnS nanocrystals using EXAFS Brad Car, a Scott Medling, * a Carley Corrado, b Frank Bridges a and Jin Z. Zhang b Received 31st May 2011, Accepted 17th July 2011 DOI: 10.1039/c1nr10556f A local structure study of ZnS nanocrystals, doped with very low concentrations of Cu, was carried out using the EXAFS technique to better understand how Cu substitutes into the host lattice and forms Cu luminescence centers. We show that a large fraction of the Cu have three nearest neighbor S atoms and the Cu–S bond is significantly shortened compared to Zn–S, by 0.08  A. In addition, the second neighbor Cu–Cu peak is extremely small. We propose that Cu occupies an interior site next to a S 2 vacancy, with the Cu displaced towards the remaining S 2 and away from the vacancy; such a displacement immediately explains the lack of a significant Cu–Cu peak in the data. There is no evidence for interstitial Cu sites (Cu i ), indicating that no more than 2% of the Cu are Cu i. This study provides new insights into the local structure of the Cu dopant in ZnS without the presence of CuS nanoprecipitates that are present at higher Cu doping levels. 1 Background ZnS phosphors, co-doped with Cu and other co-activators (Cl, Br, Al), have been studied extensively 1,2 since they were found to exhibit AC electroluminescence (EL) 3 at relatively low applied AC electric fields. In the last few years, these materials have received renewed attention for both bulk and nano materials. 4–13 Both EL and photoluminescence (PL) arise from electron-hole recombinations of charges excited (either electrically or optically) into trap states; the dominant emission in both cases is a strong blue band which is observed in both bulk and nanocrystal (NC) ZnS:Cu. 1,9,13 Cl  , Br  , and Al +3 form the electron trap sites while dilute Cu centers form the hole trap sites. Cu is only slightly soluble in ZnS (up to roughly 0.04% Cu in ZnS) and, when the Cu concentration is above the solubility threshold, conducting CuS precipitates form as the crystal cools. 2,14 These conducting needle-like precipitates enhance the local electric fields about the precipitate tips when the electric field is reversed in AC operation and lead to the low voltage AC EL. 14 However, the actual transitions that lead to the blue lines are still under debate. As we noted earlier for bulk materials, 13 there are three blue lines at similar energies: a blue line in Cl  (or Br  ) doped (and some undoped) materials with the hole trap associ- ated with a Zn vacancy complex 15 (V Zn , Cl  s ,3S 2 ), a blue line for Cu doped (no Cl  ), and a similar line with both Cl and Cu doping associated with a Cu complex hole trap. In contrast, recent studies have suggested that the blue PL emission of undoped ZnS NCs arises from V S donor traps in the host lattice, but the hole trap is not defined. 4,16–21 Indirect evidence of V S has been shown by energy-dispersive X-ray spectroscopy (EDS) showing an excess of Zn over S in ZnS NCs, 18,21 as well as inductively- coupled plasma (ICP) yielding the same result. 4 Because of the large surface area in NCs, the excess of Zn over S could also arise because of the excess Zn on the surface of the NCs, rather than V S. This possibility is not unlikely since the capping ligands are often negatively charged and generally bind to Zn. Without an excess of Zn, the surface is not as well protected, leading to quenched emission. For this reason, the excess of Zn over S cannot unequivocally be assigned to S vacancies. Similarly, the blue PL emission in Cu-doped ZnS NCs has been attributed to electronic transition from V S donor trap states below the conduction band to deep trap states above the valence band created by Cu. 19–21 Most of both the old and recent literature agree that the V S donor state is about 0.6–0.8 eV below the conduction band. Then to have the blue line emission at  2.8 eV, the deep trap state must be close to the valence band, at least 3.5 eV below the conduction band. While this is inconsistent with the  1 eV splitting between the e g and t 2g energy levels for Cu centers in bulk ZnS with t 2g about 1.3 eV above the valence band, 22,23 it might be partially due to quantum confinement increasing the bandgap of the ZnS, plus a shift of the Cu levels in ZnS NCs. Thus, a long-standing question about ZnS:Cu phosphors still remains unanswered: what is the local structure of the lumines- cent emission centers that are associated with the low concen- tration Cu sites? Several models have been proposed but local structure details are limited. Because of the low solubility of Cu in ZnS, definitive studies need to be done at very low Cu concentrations where precipitates have not formed. In addition, the evidence for V S trap states is also indirect, and there may be isolated V S as well as complex centers, with V S associated with a Physics Department, UC Santa Cruz. E-mail: smedling@ucsc.edu b Chemistry Department, UC Santa Cruz 4182 | Nanoscale, 2011, 3, 4182–4189 This journal is ª The Royal Society of Chemistry 2011 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2011, 3, 4182 www.rsc.org/nanoscale PAPER