Germanium Inverse opals

The highest dielectric contrast photonic crystal

Germanium
is transparent to infrared radiation and presents an extremely high dielectric
constant (e=16).
That makes it the best material to produce a high contrast PBG.
For this purpose we take advantage of the high thermal and mechanical stability
of opals as matrices.

Scheme. Cyclic procedure for the synthesis of Ge within the opal voids.

The
fabrication of Ge inverse opals involves several stages. First, bare opals are
placed in a hermetically sealed cell in which vacuum (10–2 torr) is
made. In this 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 germanium precursor, which easily infiltrates
porous silica. The alkoxide is let to impregnate the bare opal. At that moment,
the infiltrated opal becomes translucent as a consequence of the dielectric
constant matching that takes place when TMOG (e»1.96)
fills all the empty volume of the matrix (e»2.1).

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

Second,
TMOG is hydrolyzed at room temperature by flowing a mixture of N2 and
H2O vapour, 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 Fig. 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 550ºC. A strong aggregation occurs during the GeO2®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. In order to completely fill the
interparticle volume and, consequently, to assure the connectivity of the
germanium lattice, opals are subjected to several rounds of the GeO2
and Ge formation processes just described. SEM pictures (see Fig. 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 to point
out that a thermally assisted oxide/metal transition, such as the GeO2®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. 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>3.5%,
Ge>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.

Figure
2.
From left to right: {100}, {110}, and {111} front views of a 1.2 mm
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.

This
is clear at sight of 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.
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.
In some cases, as in the {100} and {110} views shown in Fig. 2, the
layer underneath can be seen through the holes in 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 in spite of being a highly open structure.
Therefore, the long range FCC order inherited from the template is preserved in
the Ge inverse opals (see Fig. 3). After etching, samples maintain the same
overall volume of the starting bare opal. Defects, such as vacancies,
dislocations, etc. do not disturb this stability, since Ge is also filling them.

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.