Review on Experimental and Computational Studies of Zns Nanostructures Said Hamad, Scott Marcus Woodley, Richard Catlow

Review on experimental and computational studies of ZnS nanostructures Said Hamad, Scott Marcus Woodley, Richard Catlow To cite this version: Said Hamad, Scott Marcus Woodley, Richard Catlow. Review on experimental and computational studies of ZnS nanostructures. Molecular Simulation, Taylor & Francis, 2009, 35 (12-13), pp.1015- 1032. 10.1080/08927020903015346. hal-00530454 HAL Id: hal-00530454 https://hal.archives-ouvertes.fr/hal-00530454 Submitted on 29 Oct 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. For Peer Review Only Review on experimental and computational studies of ZnS nanostructures Journal: Molecular Simulation/Journal of Experimental Nanoscience Manuscript ID: GMOS-2009-0017.R1 Journal: Molecular Simulation Date Submitted by the 18-Apr-2009 Author: Complete List of Authors: Hamad, Said; Institute of Materials Science of Seville, CSIC - University of Seville Woodley, Scott; University College London, Computational Materials Chemistry, Department of Chemistry Catlow, Richard; University College London, Computational Materials Chemistry, Department of Chemistry Keywords: ZnS, nanoparticles, clusters, global minimisation, nanotubes http://mc.manuscriptcentral.com/tandf/jenmol Page 1 of 73 1 2 3 4 5 Review on experimental and computational studies 6 7 8 9 of ZnS nanostructures. 10 11 12 a* b b 13 Said Hamad , Scott M. Woodley and C. Richard A. Catlow 14 For Peer Review Only 15 a) Instituto de Ciencia de Materiales de Sevilla, CSIC-Universidad de Sevilla, Calle Américo 16 17 Vespucio, nº 49, 41092, Seville, Spain. 18 19 20 b) Computational Materials Chemistry, Department of Chemistry, University College London, 21 22 Kathleen Lonsdale Building, Gower Street, London, U.K. 23 24 25 26 27 28 We review the experimental and computational studies of nano-particulate ZnS, a system that has 29 received much attention recently. We describe in detail how the nano-particle structures evolve 30 31 with increasing size. The results of the computational studies reveal intriguing families of 32 33 structures based on spheroids, which have the greater stability for clusters with less than 50 ZnS 34 35 pairs. More complex structures are predicted for larger nano-particles. 36 37 38 Keywords: ZnS, nanoparticles, clusters, nanotubes, global minimisation. 39 40 41 42 43 44 45 46 47 48 49 50 1 51 52 53 54 55 56 57 58 59 60 http://mc.manuscriptcentral.com/tandf/jenmol Page 2 of 73 1 2 1. Introduction 3 4 The rapid growth of nanotechnology during the last decade has spawned an increasing interest in 5 6 the search of methods to control the properties of nanoscale materials. Quantum size effects 7 8 become large in materials with sizes in the range of the nanometre, which makes the properties of Field Code Changed 9 1,2 10 these materials dependent to a great extent on their size and structure . It is therefore of key 11 12 importance to understand the kinetic and thermodynamic factors that determine the size and 13 structure of nanoparticles. Since the study of kinetic processes is complex, there have been great 14 For Peer Review Only 15 efforts directed towards the study of the thermodynamic factors that control the arrangement of the 16 17 atoms or molecules that form the nanoparticles. The basic idea behind these efforts is the 18 19 assumption that the nanoparticle will adopt the structure that minimises its energy. When dealing 20 21 with crystals in the macroscopic scale, it is possible to obtain the morphology of a given crystal by 22 Field Code Changed 23 employing the Wulff construction 3, which yields the equilibrium shape of a crystal knowing the 24 25 energies of the surfaces that might appear in the crystal. This approach is not appropriate for 26 27 nanoparticles; but we can still assume that the most likely structure that a given nanoparticle will 28 29 adopt is that which minimises its energy. Thus, in order to predict the structure of a nanoparticle 30 31 we can employ global minimisation techniques, which yield the most stable structures. In this 32 33 article, we review the use of computational techniques in the study of an important semiconductor 34 35 compound, ZnS, paying particular attention to the global optimisation techniques that provide 36 37 information about the most stable nanostructures. 38 39 40 41 42 43 44 45 46 47 48 49 50 2 51 52 53 54 55 56 57 58 59 60 http://mc.manuscriptcentral.com/tandf/jenmol Page 3 of 73 1 2 3 4 2. Experimental studies on ZnS nanostructures 5 6 7 8 ZnS belongs to the family of II-VI semiconductor compounds, which have a large number 9 10 of applications in nanotechnology, as it is relatively easy to tailor the optical and electronic Field Code Changed 11 4 12 properties of the materials by controlling the shape of the nanostructures . The shape of II-VI 13 14 nanoparticles might beFor controlled byPeer several method s:Review one is by manipulating theOnly growth kinetics, 15 injecting precursor molecules into a hot surfactant, which yields either monodispersive, nearly 16 Field Code Changed 17 spherical quantum dots (QDs) or nanorods with different aspect ratios 5. This ability to produce 18 19 QDs routinely with controlled sizes and optical properties has found many applications in 20 Field Code Changed 21 biological imaging 6-9 and their photosensitizing properties are being investigated for their potential 22 Field Code Changed 23 applications in cancer treatments 10,11 , where UV irradiation might be absorbed by QDs and 24 25 transferred selectively to cancer cells. The materials are also being studied for their potential uses 26 27 in new electronic devices: a single CdSe QD can be optically excited in close proximity to a silver 28 Field Code Changed 29 nanowire 12 , which induces the QD to emit radiation coupled directly to guided plasmons in the 30 31 nanowire, causing the wire’s end to light up. 32 33 34 ZnS, which is amongst the most studied II-VI compounds, is a wide band-gap (3.7eV) 35 36 semiconductor and is one of the most important materials in optoelectronic applications, due to its Field Code Changed 37 13 14 38 good photoluminescent properties; it is also used as a photocatalyst . It can adopt two crystal Field Code Changed Field Code Changed 39 15 40 structures , namely sphalerite (cubic or zinc blende phase) and wurtzite (hexagonal phase). 41 42 Sphalerite is the most stable form at room temperature, while the less dense wurtzite is stable 43 above 1020ºC at atmospheric pressure and is metastable (as a macroscopic phase) under ambient 44 45 conditions. Of course, the stability of the two phases is influenced by the morphology of the 46 47 material: thermodynamic analysis, which makes use of surface energy data, shows that smaller 48 49 50 3 51 52 53 54 55 56 57 58 59 60 http://mc.manuscriptcentral.com/tandf/jenmol Page 4 of 73 1 Field Code Changed 16 2 wurtzite nanoparticles are more thermodynamically stable than sphalerite . For particles as small 3 4 as 7nm the temperature for the transformation from sphalerite to wurtzite is only 25ºC. The origin 5 6 of the change in the order of stability of the two crystal structures is the free energy γA (the 7 8 product of the surface free energy and the surface area). For macrocrystalline phases, this term is 9 10 negligible and the stability is determined by the difference in energy between the two phases. But 11 for nanostructures the surface to bulk ratio is large enough to make the term significant. In this 12 13 case, the difference between the surface energies of the two phases plays a key role. Changing the 14 For Peer Review Only Field Code Changed Field Code Changed 15 17 surface energies therefore induces changes in the crystallinity of the nanoparticles . This change 16 Field Code Changed Deleted: Figure 2 17 in surface energies might be achieved, for example, by changing the solvent in which the particles 18 Deleted: Figure 2 19 are immersed. When 3nm ZnS nanoparticles synthesised in methanol are dried out, they adopt a Formatted: English (U.K.) 20 Formatted: English (U.K.) 21 very disordered structure. But when water is adsorbed on the surface, they undergo a reversible Formatted: English (U.K.) 22 Formatted: English (U.K.) 23 18 water-driven structural transformation to the sphalerite phase, as shown in Figure 1. Field Code Changed 24 25 Field Code Changed 26 Using the appropriate synthesis conditions, it is possible to tune the morphology of ZnS Field Code Changed 27 Deleted: Figure 2 19 28 structures in order to create nanobelts ( Figure 2a) with the wurtzite polymorph, which is the Formatted: English (U.K.) 29 Deleted: Figure 2 30 more desirable polymorph for its optical properties. Due to their high aspect ratio, ultrafine ZnS Formatted: English (U.K.) 31 Formatted: English (U.K.) nanobelts 20 are being extensively studied as field emitters and highly useful in novel nanoscale 32 Formatted: English (U.K.) 33 21 22 Field Code Changed 34 electric and optoelectronic devices . Other exotic structures such as nanosheets ( Figure 2d) and Deleted: Figure 2 35 23 Formatted: English (U.K.) 36 nanoflowers ( Figure 2b), which show optical properties that can be finely tuned, might also be 37 Formatted: English (U.K.) 38 obtained by solution synthesis routes.

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