3415 © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com Emissive ZnO@Zn 3 P 2 Nanocrystals: Synthesis, Optical, and Optoelectrochemical Properties Shiding Miao,* Ting Yang, Stephen G. Hickey,* Vladimir Lesnyak, Bernd Rellinghaus, Jinzhang Xu,* and Alexander Eychmüller In recent years, research into semiconductor nanocrys- tals (NCs) or quantum dots (QDs) with tunable band-gap positions has been intensively pursued owing to their pos- sible technological applications and fundamental scien- tific importance. By using the quantum confinement effect, the absorption and emission spectra of QDs can be tuned over a wide energy range simply by changing the par- ticle dimensions. Zinc phosphide (Zn 3 P 2 ) is an important II–V compound semiconductor with a direct optical band- gap of approximately 1.5 eV, which coincides well with the theoretical optimum for solar energy conversion under air mass conditions. [1] Zn 3 P 2 has a large absorption coefficient (ca. 10 4 –10 5 cm -1 ) and a long minority carrier-diffusion length of 4–13 μm, [2–5] which permits efficient current col- lection and optimum conversion efficiency for terrestrial photoelectronics. [1,4] These properties make zinc phosphide a promising p-type semiconductor for a variety of applica- tions such as low-cost solar cells, infrared (IR) and ultra- violet (UV) sensors, lasers, light-emitting devices, and as an anode material for use in lithium-ion batteries. [6–10] In addi- tion to Zn 3 P 2 , another zinc-based semiconductor, namely ZnO, is also an attractive material for numerous applications because of its thermal/chemical stability, optical transpar- ency, large exciton binding energy, and biocompatibility. [11] More importantly, the wide band-gap of ZnO and its much higher carrier mobility is favorable for the collection of pho- toinduced electrons and thus reduces the recombination of electrons, by comparison to TiO 2 . [12] Thus, in some aspects ZnO is believed to be a superior alternative material to TiO 2 and can be readily fabricated into optoelectronic devices, especially when combined with a high-absorption efficiency material such as Zn 3 P 2 , to which it also imparts a higher degree of environmental stability. In the past decade ZnO QDs and composites thereof have been explored because the energy positions of the conduction- and valance-band levels of ZnO coincide almost exactly with those of TiO 2 . [13] It has been reported that Zn 3 P 2 and ZnO have the correct energy-level offsets to form a type-II heterostructure and consequently could potentially find use in a number of pho- tovoltaic devices. Moreover, the small absorption cross-sec- tion that ZnO has for the solar spectrum allows it to serve as a “window” in photovoltaic devices based on a Zn 3 P 2 /ZnO heterostructure junction, which is desirable to obtain higher conversion efficiencies. [7] Presently, the strategies reported for the preparation of Zn 3 P 2 or Zn 3 P 2 -based composites usually require high tem- peratures and harsh reaction conditions, [14] and methods thus far employed for their synthesis include photo-orga- nometallic chemical vapor deposition, [15] thermal-assisted pulsed laser ablation, [7] hot-wall epitaxy, [16] radio-frequency sputtering, [17] vacuum evaporation, [18,19] carbon reduction, [20] and electrochemical deposition. [21] To date, most of the work published concerns itself with the preparation and physical properties of Zn 3 P 2 -composite materials, [7,22–25] which include InP/Zn 3 P 2 , [26] Mg/Zn 3 P 2 , [27] Zn 3 P 2 /ZnSe, [15] indium tin oxide (ITO)/Zn 3 P 2 , [28] Zn 3 P 2 /ZnS, [29] and ZnO/Zn 3 P 2 , [30,31] the latter being prepared by the sputter deposition of ZnO onto Zn 3 P 2 substrates. However, the synthesis of zinc phos- phide or ZnO/Zn 3 P 2 colloidal NCs via a solution-based route has barely been reported. [32] Solution-based syntheses offer many advantages for the preparation of NCs such as ensuring homogenous mixing of the precursors, enabling a uniform distribution of dopants, and allowing one to tune the particle size (band-gap) and shape by subtle changes in the synthesis conditions such as reaction temperature, reac- tion time, etc., which in turn paves the way to tune the novel electronic and/or optical properties of semiconductor QDs of complex structure. [33,34] Up to now only three methods for the synthesis of Zn 3 P 2 colloidal nanoparticles via organome- tallic routes have been reported, and these methods remain far from satisfactory from the point of view of operational simplicity, cost effectiveness, environmental control of the toxicity, and up-scaling of the synthesis. [35,36] As reported by Reiss and co-workers, [34] the use of phosphine gas as a labile phosphorus source in the synthesis of phosphorus-containing semiconductor QDs can drastically reduce their production costs as well as provide a continuous-flow procedure when Quantum Dots DOI: 10.1002/smll.201203023 Dr. S. D. Miao, T. Yang, Prof. J. Z. Xu Hefei University of Technology Tunxi Road. 193, Hefei, 230009, Anhui Prov., China E-mail: miaosd@iccas.ac.cn S.M. xujz@hfut.edu.cn J.Z.X. Dr. S. G. Hickey, Dr. V. Lesnyak, [+] Prof. A. Eychmüller Physical Chemistry/Electrochemistry TU Dresden, Bergstr. 66b, Dresden, D-01062, Germany E-mail: s.hickey@chemie.tu-dresden.de S.G.H. Dr. B. Rellinghaus IFW, Helmholtzstr. 20, Dresden, D-01069, Germany [+] Present Address: Istituto Italiano di Technologia, Via Morego 30, 16163 Genova, Italy. small 2013, 9, No. 20, 3415–3422