Synthesis, Structural, and Optical Properties of Stable ZnS:Cu,Cl Nanocrystals † Carley Corrado, ‡ Yu Jiang, § Fadekemi Oba, ‡ Mike Kozina, § Frank Bridges,* ,§ and Jin Z. Zhang* ,‡ Department of Chemistry and Biochemistry and Department of Physics, UniVersity of California, Santa Cruz, California 95064 ReceiVed: NoVember 1, 2008; ReVised Manuscript ReceiVed: December 9, 2008 Stable water-suspendable Cu + -doped ZnS nanocrystals (NCs) have been synthesized with mercaptopropionic acid (MPA) as a capping molecule. The nanocrystals have been characterized using a combination of experimental techniques including UV-vis and photoluminescence (PL) spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma (ICP), and extended X-ray absorption fine structure (EXAFS). The UV-vis electronic absorption spectrum shows an excitonic peak at 310 nm, characteristic of quantum-confined ZnS NCs. This excitonic peak does not change noticeably with Cu + doping. XRD confirms the formation of ZnS nanocrystals, and the average size of the NCs has been determined to be around 6 nm by TEM. The incorporation of Cu + into the ZnS is manifested as a substantial red-shift of the emission band in the PL spectra upon addition of Cu 2+ that was reduced into Cu + during the synthesis reaction. EXAFS data were obtained to confirm copper doping as well as determine the local structure about Cu + and Zn 2+ in the NCs. Fitting to the EXAFS data for Cu + suggests that most Cu + ions are located near the surface within the ZnS NCs and that a significant fraction may be in the form of CuS as found in bulk material. These combined optical and structural studies have provided important new insight into the relevant electronic energy levels and their correlation to the optical and structural properties of ZnS:Cu,Cl NCs. This has important implications in potential applications of this phosphor material for solid state lighting, imaging, and other photonic devices. 1. Introduction In recent years, semiconductor nanocrystals (NCs) have drawn significant attention due to their unique structural, electronic, and optical properties originating from their large surface-to- volume (S/V) ratio and quantum confinement effect. 1-13 An important subset of semiconductor NCs are those doped with a small percentage of dopants to alter their electronic, magnetic, and optical properties for various desired applications. 14-25 One of the promising applications of doped semiconductor NCs is solid state lighting based on AC electroluminescent (EL) devices that are expected to have high electrical-to-light conversion efficiency. 26-32 Cubic ZnS with a bulk bandgap of 3.7 eV is a common and attractive choice as a host semiconductor for doping to produce nanophosphors in EL applications due to its stability, low cost, and low toxicity. 26,27,30,32-36 A number of metal ions, such as Mn 2+ , Cu + , Pb 2+ , Ag + , and Eu 2+ , have been successfully doped into ZnS to produce PL or EL emission in different regions of the visible spectrum. 37-48 Compared to bulk or micrometer scale powders, these nanoscale materials are anticipated to have some unique properties that are potentially useful to improve their AC EL performance, for example, in terms of efficiency and required voltage. 20 In EL applications, AC is preferred since the AC voltage required is usually 2 orders of magnitude lower than for DC voltages. Currently, one major limitation in AC EL devices is that the material degrades when subjected to high AC electric fields (∼10 6 V/m), but the degradation mechanism is still not well understood. 32 There are also unresolved fundamental questions regarding the structure of the nanophospors and the lumines- cence mechanism. For example, in bulk ZnS:Cu,Cl, the mech- anism for light emission proposed by Fischer involves localized electron and hole injection near conducting CuS precipitates which form for Cu concentrations > 400 ppm. 49 In these materials Cu is only in the +1 valence state unless excited optically. 50 The emission is caused by recombination of an electron and a hole, associated with Cl - and Cu + levels, respectively. However, there have been different models or explanations about the AC EL mechanism and the associated electronic energy levels of the host (ZnS) and dopant ions. 51 Traditionally, these AC EL devices have been made using micrometer-sized phosphor particles. Recently, there has been considerable interest in using nanophosphors for EL application since they can potentially allow fabrication of thinner, smaller, and ideally more efficient devices. Since the electric field is inversely proportional to the thickness, d, of the device (E ) V/d), an equivalent large electric field could be achieved for a thin device for a low applied voltage. Control of NC size and shape as well as doping could be used to engineer devices that could withstand higher electric fields without degradation. The first step toward this goal is to have a detailed understanding of the electronic energy levels of the dopant, Cu + , and their relation to the structural and optical properties in the host semiconductor, ZnS. Before detailed AC EL studies are conducted, it is often helpful to study the PL properties since a PL measurement is more convenient to carry out and provides important information about the energy levels relevant to EL. In conjunction with structural studies, PL studies allow us to gain insight into the † Part of the “George C. Schatz Festschrift”. * Corresponding authors. E-mail: bridges@physics.ucsc.edu, zhang@ chemistry.ucsc.edu. ‡ Department of Chemistry and Biochemistry. § Department of Physics. J. Phys. Chem. A 2009, 113, 3830–3839 3830 10.1021/jp809666t CCC: $40.75  2009 American Chemical Society Published on Web 01/26/2009