Germanium FCC Structure from a Colloidal Crystal Template

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Germanium FCC Structure from a Colloidal Crystal Template H. Mı´guez,† F. Meseguer,*,† C. Lo´pez,† M. Holgado,† G. Andreasen,‡ A. Mifsud,§ and V. Forne´s§ Unidad Asociada CSIC-UPV, Centro Tecnolo´ gico de Ondas, Edificio Nuevos Institutos II, Universidad Polite´ cnica de Valencia, Avda Los Naranjos s/n, 46022 Valencia, Spain, Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco 28049, Madrid, Spain, Instituto de Investigaciones Fı´sico-Quı´micas Teo´ ricas y Aplicadas (CONICET), La Plata, Argentina, and Instituto de Tecnologı´a Quı´mica UPV-CSIC, Avd. de los Naranjos s/n, 46022 Valencia, Spain Received October 26, 1999. In Final Form: December 8, 1999 Here we show a method to fabricate a macroporous structure in which the pores, essentially identical, arrange regularly in a face-centered cubic (FCC) lattice. The result is a network of air spheres in a germanium medium. This structure presents the highest dielectric contrast (Ge/air ) 16) ever achieved in the optical regime in such periodic structures, which could result in important applications in photonics. We employ solid silica colloidal crystals (opals) as templates within which a cyclic germanium growth process is carried out. Thus, the three-dimensional periodicity of the host is inherited by the guest. Afterward, the silica is removed and a germanium opal replica is obtained.

There currently exists a growing interest in new methods to fabricate dielectric and metal macroporous (pore size > 50 nm) periodic structures. It is supported by the amazing applications predicted for these structures in diverse fields such as photonics,1,2 chemical sensors,3 and membranes.4 It is difficult to obtain highly ordered, submicron porous lattices using lithographic techniques,5 high and expensive technology being needed in order to obtain regularly sized and shaped pores. However, recently developed methods based on colloidal crystal templates have opened a new route in this field. Basically, they consist of the synthesis or infiltration of the desired material within the void lattice of solid colloidal crystals (artificial opals), which act as templates. Opals are made of submicrometer silica6-8 or organic (usually latex) spheres9,10 which self-arrange in a face-centered cubic (FCC) lattice.11 After infiltration, the matrix is eliminated either by thermal annealing or by chemical etching. In this way a lattice of hollow spheres in a metallic or dielectric medium (inverse opal) is formed. So far,

polymer,12 carbon,13 metal,14-16 oxide17-21 and semiconducting22 inverse opals have been built employing the general procedure briefly described above. One of the most important applications of these composites relates to the field of photonic band gap (PBG) technology.23 Under this scope, the main target is the manufacture of a full PBG material, within which a certain light frequency band is not allowed to propagate irrespective of propagation direction.1,2 To realize this, a periodic distribution of cavities in a transparent medium with a high dielectric constant () is required. However, none of the mentioned structures accomplish this prerequisite. In some cases,13-16 the medium is strongly absorbing and light cannot fully penetrate into the porous dielectric. In others,12,17-21 the dielectric contrast is not high enough so as to develop a sizable PBG, although strong PBG effects are observable in some of the structures.24 For this purpose, the fabrication of an FCC lattice of air spheres in a transparent  ) 16 medium has been proposed.25,26 Here we show a method to fabricate germanium inverse opals. Germanium is transparent to infrared radiation

* To whom correspondence should be addressed at Unidad Asociada CSIC-UPV, Centro Tecnolo´gico de Ondas. E-mail: [email protected]. † Universidad Polite ´ cnica de Valencia and Instituto de Ciencia de Materiales de Madrid (CSIC). ‡ Instituto de Investigaciones Fı´sico-Quı´micas Teo ´ ricas y Aplicadas (CONICET). § Instituto de Tecnologı´a Quı´mica UPV-CSIC.

(12) Park, S. H.; Xia, Y. Adv. Mater. 1998, 10, 1045. (13) Zakhidov, A. A.; Baughman, R. H.; Iqbal, Z.; Cui, C.; Khayrullin, I.; Dantas, S. O.; Martı´, J.; Ralchenko, V. G. Science 1998, 282, 897. (14) Jiang, P.; Cizeron, J.; Bertone, J. F.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 7957. (15) Yan, H.; Blanford, C. F.; Holland, B. T.; Parent, M.; Smyrl, W. H.; Stein, A. Adv. Mater. 1999, 11, 1003. (16) Velev, O. D.; Tessier, P. M.; Lenhoff, A. M.; Kaler, E. W. Nature 1999, 401, 548. (17) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 448. (18) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (19) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (20) Holland, B. T.; Blanford, C. F.; Do, T.; Stein, A. Chem. Mater. 1999, 11, 795. (21) Subramanian, G.; Manoharan, V. N.; Thorne, J. D.; Pine, D. J. Adv. Mater. 1999, 11, 1261. (22) Vlasov, Yu. A.; Yao, N.; Norris, D. J. Adv. Mater. 1999, 11, 165. (23) Joannopoulos, J. D.; Meade, R. D.; Winn, J. N. Photonic Crystals; Princeton University Press: Princeton, NJ, 1995. (24) Thijssen, M. S.; Sprik, R.; Wijnhoven, J. E. G. J.; Megens, M.; Narayanan, T.; Lagendijk, A.; Vos, W. L. Phys. Rev. Lett. 1999, 83, 2730. (25) So¨zu¨er, H. S.; Haus, J. W.; Inguva, R. Phys. Rev. B 1993, 45, 13962. (26) Busch, K.; John, S. Phys. Rev. E 1998, 58, 3896.

(1) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (2) John, S. Phys. Rev. Lett. 1987, 58, 2486. (3) Tierney, M. J.; Kim, H. O. L. Anal. Chem. 1993, 65, 3435. (4) Liu, C.; Martin, C. R. Nature 1991, 352, 50. (5) Joannopoulos, J. D.; Villeneuve, P. R.; Fan, S. Nature 1997, 386, 143. (6) Vlasov, Yu. A.; Astratov, V. N.; Karimov, O. Z.; Kaplyanskii, A. A.; Bogomolov, V. N.; Prokofiev, A. V. Phys. Rev. B 1997, 55, 13357. (7) Mı´guez, H.; Lo´pez, C.; Meseguer, F.; Blanco, A.; Va´zquez, L.; Mayoral, R.; Ocan˜a, M.; Forne´s, V.; Mifsud, A. Appl. Phys. Lett. 1997, 71, 1148. (8) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L.Chem. Mater. 1999, 11, 2132. (9) Park, S. H.; Qin, D.; Xia, Y. Adv Mater. 1998, 10, 1028. (10) Yamasaki, T.; Tsutsui, T.; Appl. Phys. Lett. 1998, 72, 1957. (11) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Mifsud, A.; Moya, J. S.; Va´zquez, L. Langmuir 1997, 13, 6009.

10.1021/la991412s CCC: $19.00 © 2000 American Chemical Society Published on Web 05/09/2000

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Figure 1. {111} views of a GeO2 (top) and Ge (bottom) infiltrated opal made of 850-nm diameter spheres.

and presents an extremely high dielectric constant ( ) 16). Sintered silica opals, whose structural and optical qualities have been described in detail elsewhere,7,11 are used as templates. In this work, we take advantage of the high thermal and mechanical stability of such matrixes. To demonstrate the control on the lattice parameter of the resulting opal replica, a set of opals with different lattice parameters (between 0.3 and 1.2 µm) was employed as starting materials. Methods based on natural sedimentation techniques fail to order spheres of diameter > 0.7 µm, because gravitational force prevents the particles from seeking the minimum energy sites and, therefore, assembling in a FCC structure. In this work, we make use of a recent achievement in the field of artificial opal fabrication. By means of an electrophoretically assisted settling, opals made of large spheres can be obtained as well.27 Sample thickness is ∼300 µm in all cases. In addition, the volume fraction of the interparticle voids in the starting silica opals can be controlled by sintering at different temperatures, which opens the possibility to define the porosity in advance.28 The fabrication of Ge inverse opals involves several stages. First, bare opals are placed in a hermetically sealed cell that is then placed under vacuum (10-2 Torr). In this (27) Holgado, M.; Garcı´a-Santamarı´a, F.; Blanco, A.; Ibisate, M.; Cintas, A.; Mı´guez, H.; Serna, C. J.; Molpeceres, C.; Requena, J.; Mifsud, A.; Meseguer, F.; Lo´pez C. Langmuir 1999, 15, 4701. (28) Mı´guez, H.; Meseguer, F.; Lo´pez, C.; Blanco, A.; Moya, J. S.; Requena, J.; Mifsud, A.; Forne´s, V. Adv. Mater. 1998, 10, 480.

Figure 2. From the top to the bottom: {100}, {110}, and {111} front views of a 1.2-µm lattice parameter Ge inverse opal. FFTs are shown in all cases. The underlying plane can be observed through the first layer of the Ge mesh in the first two cases. The windows interconnecting the air cavities with those in the underlying plane can be seen: four in the case of {100}, five for {110}, and three for {111} faces.

way, the whole pore volume in the opal is available to subsequently infiltrated materials; hence, a complete and homogeneous infiltration is possible. We employ Ge(OCH3)4, tetramethoxygermane (TMOG), as a germanium precursor, which easily infiltrates porous silica. The alkoxide is allowed to impregnate the bare opal. At that moment, the infiltrated opal becomes translucent as a consequence of

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Figure 3. Low-magnification micrograph of a germanium inverse opal cleft edge. Long range order is preserved in the sample after etching. Different crystalline terraces can be seen.

the dielectric constant matching that takes place when TMOG ( ≈ 1.96) fills all the empty volume of the matrix ( ≈ 2.1). Second, TMOG is hydrolyzed at room temperature by flowing a mixture of N2 and H2O vapor, which gives rise to GeO2 formation in the opal lattice. The remaining methanol, product of the hydrolysis reaction, is removed by pumping the reactor. X-ray diffraction (XRD) is used at this stage to assess the crystallinity of the product. XRD patterns of hydrolyzed samples show broad diffraction peaks. An average crystal size of 25 nm can be estimated, while scanning electron microscopy (SEM) images show an average cluster size of around 100 nm (see Figure 1). This discrepancy may be caused either by a polycrystalline composition of the clusters or by the presence of strained crystals in the sample. Oxide crystallites are homogeneously distributed throughout the template void lattice. At this point, Ge0 is formed from GeO2 by direct reduction in H2 atmosphere at 550 °C. A strong aggregation occurs during the GeO2 f Ge0 transition, as is observed by SEM and XRD techniques. The narrowing of the diffraction peaks in the XRD patterns indicates that the average crystal size hugely increases. Aggregation implies that the resulting Ge arranges in a much more compact fashion and larger clusters are formed than those of GeO2. Ge crystals are not linked to one another, revealing a poor connectivity of the guest lattice after reduction. So, the process of GeO2 reduction frees a good deal of space within the opal. To completely fill the interparticle volume and, consequently, to ensure the connectivity of the germanium lattice, opals are subjected to several rounds of the GeO2 and Ge formation processes just described. SEM pictures (see Figure 1) reveal that, after five cycles, a Ge lattice, which presents a high connectivity, is attained. From these micrographs, it can be concluded that the germanium guest lattice is filling the interparticle voids of the template. It is worth pointing out that a thermally assisted oxide/metal transition, such as the GeO2 f Ge0 one, could not be performed inside opals made of organic particles without strongly disturbing, or even completely destroying, the matrix during this process. In sintered silica opals, however, the FCC ordering remains unaltered when

infiltration or abrupt changes of volume take place at any such temperature within the void lattice. Finally, Ge infiltrated opals are chemically etched in a 1 wt % hydrofluoric acid in water solution, following a method described previously.13 In this way, we seek to remove the SiO2 spheres of the matrix and to obtain Ge inverse opals. Template sintering is responsible for the formation of necks between the SiO2 spheres, a treatment which not only confers robustness to the structure but also allows the HF solution to flow through the whole structure as the SiO2 is being removed. A detailed structural study of the 3D order of the resulting structures is made. Etched samples are carefully cleaved and studied by SEM. Chemical analysis performed by energy dispersive spectrometry allows us to check that the matrix is efficiently eliminated by the soft HF attack without damaging the guest material. No silica was detected deep inside the acid-treated sample. The percentages of elements present in the sample (Si < 0.5%, O e 3.5%, Ge g 96%) indicate that little oxidation of Ge may take place. Facets consistent with an FCC arrangement of air spheres are apparent in the fractured edges. The Ge obtained from GeO2 shows a tendency to form a mesh rather than an array of interconnected shells coating the spheres. This is clear from the SEM pictures of Figure 2, in which front views from the main crystal directions ({100}, {110}, and {111}) are displayed. This disposition could have its origin in the large energy needed to get a contact surface between Ge and oxides.29 The structural reorganization of the guest material which takes place during reduction allows the Ge to form in the energetically most favorable places. Thus, it retracts from the SiO2 spheres’ surface and agglomerates to form the mesh. This tendency implies that no additional air voids appear between the hollow spheres after SiO2 etching, as happens when chemical vapor deposition processes are employed to synthesize materials in the opal void lattice.13 In some cases, as in the {100} and {110} views shown in Figure 2, the layer underneath can be seen through the holes in (29) Tomsia, A. P.; Saiz, E.; Dalgleish, B. J.; Cannon, M. Proceedings of the 4th Japan International SAMPE Symposium, September 25-28, 1995.

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the upper one. In all observed crystalline faces of the Ge inverse opal, the windows connecting the uppermost layer cavities with those from the plane underneath can be distinguished. These are caused by the existence of necks connecting the spheres and produced by sintering of the bare opal. The number of windows relates to the number of corresponding nearest neighbors in the layer underneath. It can be established that the repeated cycle synthesis with aggregation of crystals during the reduction process furnishes the germanium that is formed within the bare opal with a high connectivity. This provided, the germanium lattice does not collapse on dilution of the silica matrix despite being a highly open structure. Therefore, the long-range FCC order inherited from the template is preserved in the Ge inverse opals (see Figure 3). After etching, samples maintain the same overall volume of the starting bare opal. Defects, such as vacancies, dislocations, and so forth, do not disturb this stability, since Ge is also filling them. Concerning its potential application as a photonic crystal, the observation of a complete band gap has been predicted between the eighth and ninth bands of the FCC lattice of air spheres in germanium.25,26 It would require the use of templates made of even larger spheres (diameter ≈ 1200 nm) in order to place the full gap in the electronic transparency region of Ge (λ J 1850 nm at 300 K). Also, SEM images show that there is still a certain amount of disorder, which might be harmful for photonic band gap formation due to light localization effects.30,31 Optimization of the template and synthesis conditions to achieve a full photonic band gap germanium inverse opal is left as a target for further research. (30) John, S. Phys. Rev. Lett. 1984, 53, 2169. (31) Wiersma, D. S.; Bartolini, P.; Lagendijk, A.; Righini, R. Nature 1997, 390, 671.

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In summary, we have developed a method to build a macroporous lattice of air spheres in a germanium medium. As far as we know, these materials present the highest dielectric constant contrast so far achieved with a tunable periodicity j 1 µm. This feature allows us to foresee the application of this system as a FCC photonic crystal in the near-infrared region of the spectrum. Moreover, sintered SiO2 opals can be thought of as mechanically stable and nonreactive matrixes, which opens the possibility to employ them as a periodic lattice of microreactors for a wide range of processes which take place under high-temperature or -pressure conditions. In fact, as an immediate extension of the method reported here, other alkoxides can be infiltrated to build semiconducting, metallic, or superconducting FCC mesoporous lattices, in which a wide variety of new phenomena are expected to occur. Acknowledgment. H.M. thanks Comunidad Auto´ noma de Madrid for a graduate FPI grant. We thank A. Cintas and M. Planes for their helpful assistance during sphere synthesis and SEM characterization, respectively. We also want to acknowledge R. Salvarezza and J. S. Moya for their useful suggestions. This work has been partially financed by the Fundacio´n Ramo´n Areces, Spanish CICyT Project MAT97-0698-C04, and European Community Project IST-1999-19009. Supporting Information Available: Experimental design of the cyclic Ge growth process carried out within the opal void lattice, optical characterization of the TMOG infiltration process, and XRD patterns of both GeO2 and Ge infiltrated opals. This material is available free of charge via the Internet at http://pubs.acs.org. LA991412S