Bromley, ST

Flikkema, E

Maschmeyer, T

Zwijnenburg, MA

English

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Bromley et al. Reply: In our Letter [1] we explicitlydemonstrate that defect-terminated SiO2 chain clustersmay be stabilized as fully coordinated rings, and specu-late that ‘‘...we envisage...fully connected clusters, suchas our molecular rings, as materials building blocks.’’ Intheir Comment [2], Sun et al. propose SiO2 clusters pos-sessing terminating defects but lower energies as buildingblocks. While never claiming that SiO2 rings have par-ticularly low energies, we hold that the suitability of acluster/molecule to act as a material building block is notprimarily determined by its energy, but by other criteria:(i) structural stability, (ii) thermodynamic accessibility,(iii) formation selectivity, and (iv) resistance to coales-cence.We stress that there is no requirement for a cluster’sstructural stability to be dependent on its energy. Si12O24rings, as tested by molecular dynamics (T  1000 K), forexample, are particularly resistant to collapse or rupture.The energy of a cluster/molecule is also often a poorguide to the ease with which it may be formed. In recentplasma experiments, for example, ground state clustersare not necessarily, or even usually, formed, but, rather,metastable clusters [3]. For thermodynamic accessibility,(ii), it is thus not sufﬁcient that a potential building blockhas relatively low energy. For our SiO2N rings, forma-tion could be envisaged by deformations of long two-ringchains, or linking smaller chains, both thermodynami-cally downhill processes [1]. As it has long been estab-lished that two-ring chains of arbitrary length can besynthesized in the formation of silica-W [4], and thattwo-ring chain clusters are formed in (Si,O)-plasma re-actions [5], the manipulation of such sources indicatesviable routes to ring formation.We note that the proposedlow energy SiO2 building blocks of Sun et al. are also notground state clusters [see Figs. 1(a) and 1(b)], nor is anyroute to their formation obvious to us. Building blockscan be utilized only if made in large quantities, thus,ideally, one would like them to be selectively identiﬁable.Our rings have IR bands at 886, 902, and 933 cm1. Jenaet al. pointed out that their Si12O24 cluster also hassimilar bands, seemingly making identiﬁcation difﬁcult[2].We note, however, that their structure also has numer-ous other signiﬁcant bands (e.g., 827, 951 cm1),which are not present in the spectra of the rings [6]. Fur-thermore, both the rings and the fully coordinated cage[see Fig. 1(b)] have particularly high ionization potentialsfacilitating their separation as neutral species from anionizing environment. Finally, criterion (iv) provides, wefeel, the deﬁning property of a cluster-based, or molecu-lar material, i.e., being an assembly of interacting butdiscrete structurally stable units. High relative reactivestability is thus of great importance when bringing clus-ters together to form a material.Terminated SiO2 clustersare particularly reactive to each other and are thus proneto coalesce [2], losing their discrete structural integrity.Coalescence is inherently avoided with fully coordinatedclusters, which also often display correspondingly large"LUMO-"HOMO gaps. Furthermore, although the curvatureand bonding strain of fully coordinated clusters can playan adverse energetic role, with increasing SiO2N clustersize this effect becomes progressively smaller.We show inFig.1(b) that such stabilization isnot onlyobserved inourSiO2 rings, with a fully coordinated Si24O48 cage-likecluster over 10 eV lower in energy than the terminatedSi24O48 cluster proposed by Sun et al.Many materials formed by the assembly of metastablediscrete units are known (e.g., C60 derivatives, conductingorganics, molecular magnets), often with ring-like build-ing units. Based on the properties of the chain-basedsilica-W [4], we speculate that a SiO2 ring-based materialwouldbeordered, have a low refractiveindex(RI  1:41),density (   1:97 g=cm3), and dielectric constant (k).The weak ring-ring interactions and their disklike formalso raise the possibility of liquid crystalline behaviorwith transitions between various (dis)ordered states withrespective changes in RI,  ,a n dk.S.T. Bromley, M. A. Zwijnenburg, E. Flikkema, andTh. MaschmeyerLaboratory of Applied Organic Chemistry and CatalysisDelftChemTechTechnical University of DelftDelft 2628 BL, The NetherlandsReceived 8 July 2003; published 23 January 2004DOI: 10.1103/PhysRevLett.92.039602PACS numbers: 61.46.+w[1] S.T. Bromley, M. A. Zwijnenburg, and Th. Maschmeyer,Phys. Rev. Lett. 90, 035502 (2003).[2] Q. Sun, Q.Wang, and P. Jena, preceding Comment, Phys.Rev. Lett. 92, 039601 (2004).[3] L. Kronik, R. Fromherz, E. Ko, G. Gantefo ¨r, and J. R.Chelikowsky, Nature Mater. 1, 49 (2002).[4] A. Weiss, Z. Anorg. Allg. Chem. 276, 95 (1954).[5] L-S. Wang, J. B. Nicholas, M. Dupuis, H. Wu, and S.S.Colson, Phys. Rev. Lett. 78, 4450 (1997).[6] S.T. Bromley, M. A. Zwijnenburg, and Th. Maschmeyer,Surf. Sci. 539, L554 (2003).[7] M.J. Frisch et al., GAUSSIAN 98, Gaussian Inc.,Pittsburgh, PA, 1998.FIG. 1 (color online). (a) A Si12O24 cluster 2.51 eV lower inenergy [O:6-311  Gd;p, Si:6-31G=B3LYP, GAUSSIAN 98[7]] than the low energy Si12O24 cluster by Sun et al. (b) Afully coordinated Si24O48 cage 10.35 eV lower in energy andwith a HOMO-LUMO gap 0.78 eV (IPv  9:39 eV) larger thanthe terminated Si24O48 cluster by Sun et al. (6-31G=B3LYP).PHYSICAL REVIEW LETTERS week ending23 JANUARY 2004 VOLUME 92, NUMBER 3039602-1 0031-9007=04=92(3)=039602(1)$22.50  2004 The American Physical Society 039602-1

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