© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 4151 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Jun Pan, Muhammad Iqbal Bakti Utama, Qing Zhang, Xinfeng Liu, Bo Peng, Lai Mun Wong, Tze Chien Sum, Shijie Wang, and Qihua Xiong* Composition-Tunable Vertically Aligned CdS x Se 1-x Nanowire Arrays via van der Waals Epitaxy: Investigation of Optical Properties and Photocatalytic Behavior DOI: 10.1002/adma.201104996 Utilization of semiconductor ternary alloys – including the pseu- dobinary instances when the mixture is composed of two binary semiconductors with a common element – offers the benefit of continuous tunability of bandgap to suit the requirement of specific photonic devices of interest. The bandgap of a pseudo- binary alloy can also be tuned conveniently simply by adjusting the stoichiometry of its binary constituents, which otherwise have discrete and limited bandgap values. However, the accom- panying changes of the cell parameters of the material with respect to the composition create a difficulty in achieving well- crystalized growth of bulk alloys with complete composition tunability epitaxially, despite the strong importance of epitaxy for applications demanding a reliable and reproducible device fabrication. At some compositions, expansion or contraction of the in-plane lattice of the alloy will worsen the lattice mismatch with the substrate, which may then promote nucleation of mismatch-related dislocations and defects that are deleterious to the optical and electrical properties of the alloy. [1] To resolve the issue of lattice mismatch and the ensuing defects, growth of epi- taxial alloy in vertically aligned nanowire array geometry has been proposed [2] due to the possibility of nanowires to relieve strain in the lateral direction owing to their finite diameter. [3] In addi- tion, augmented device performance and functionality may also be realized by using nanowire arrays as a consequence of their minute size and anisotropy, [4] thus enhancing the attractiveness of assuming nanowire array geometry for a broader applications of semiconductor alloy. Nevertheless, while the strain relieving property of nanowires is superior than that of bulk, it is still lim- ited and is constrained by lattice matching, [3,5] thus hindering the epitaxial growth of alloys accross the whole compositional range when the range of the in-plane lattice is too broad. CdS x Se 1-x (with 0 < x < 1, also often written only as CdSSe) is among the alloys that have been produced in various architec- tures (e.g., bulk films, nanoparticles, nanobelts, and nanowires) and intensively studied due to their prospective applications in optoelectronics, [6] particularly due to its wide-ranged tunable bandgap which spans from visible ( ∼2.42 eV for CdS) to near IR ( ∼1.73 eV for CdSe). [7] However, to our knowledge, there have been no reports on the template-free bottom-up produc- tion of CdSSe nanowire array. Moreover, as most of the studies on CdSSe had focused on the optical properties, many other potentially interesting material properties of the alloy, such as its photocatalytic behavior for solar energy harvesting applica- tions, remain unexploited. Here we report the controlled growth of composition-tunable CdSSe nanowire array with homogenous structure, which is achieved simply by tuning the S:Se molar ratio of the source powder used in our catalyst-free vapor transport synthesis. We used the layered muscovite mica substrate to manifest the exceptional properties of van der Waals epitaxy. [8] The van der Waals epitaxy has been reported to enable epitaxial growth of materials without the necessity of lattice matching to the sub- strate, [9] thus making the strategy very appealing for growing alloys with varying stoichiometry. Additionally, we also had elu- cidated the van der Waals epitaxy on muscovite mica for the growth of nanowire arrays from pure binary compounds, [10] including attribution for CdS [11] and CdSe, [11,12] such that the application of the epitaxy to nanowire array growth from CdSSe alloy is an immediate experimental advance. [13] We will then discuss the optical properties of the sample in detail and report the photocatalytic behavior of CdS x Se 1-x nanowire arrays which is found to be strongly defects-driven. The scanning electron miscroscopy (SEM) images of the CdS x Se 1-x nanowire arrays grown from the representative S:Se molar ratios ( Figure 1a–c) show that the nanowires are strongly oriented and can be grown at high density, good uniformity, and with very well-defined morphology. In addition, the cross- sectional images reveal that the nanowires have uniform diameter down to their base, which is directly interfaced to the substrate without a buffer/seed layer or any intermediate structures. The absence of buffer layer, which is usually highly defective and often formed spontaneously as a strain relieving mechanism in Dr. J. Pan, [+] M. I. B. Utama, [+] Dr. Q. Zhang, [+] Dr. X. Liu, Dr. B. Peng, Prof. T. C. Sum, Prof. Q. H. Xiong Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371 E-mail: Qihua@ntu.edu.sg L. M. Wong, Dr. S. Wang Institute of Materials Research and Engineering Agency for Science Technologies and Research Singapore 117602 Prof. Q. H. Xiong Division of Microelectronics School of Electrical and Electronics Engineering Nanyang Technological University Singapore 639798 [ +] These authors contributed equally to this work. Adv. Mater. 2012, 24, 4151–4156