Hexagonal Mesoporous Germanium

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Science  11 Aug 2006:
Vol. 313, Issue 5788, pp. 817-820
DOI: 10.1126/science.1130101


The blending of mesoporosity with the properties of semiconductors promises new types of multifunctional nanomaterials. It would be particularly interesting to combine the shape selectivity of a mesoporous oxide with the electronic and photonic characteristics of a useful semiconductor. We demonstrated the synthesis of a mesoporous germanium semiconductor using liquid-crystals-templated chemistry. The template removal was achieved by a two-step ion-exchange thermal procedure. This semiconductive mesoporous form of germanium possesses hexagonal pore ordering with very high surface area and exhibits strongly size-dependent optical properties as well as photoluminescence.

The physical and chemical properties of mesoporous (pore size from 20 to 500 Å) solids arise from a well-defined pore structure, high internal surface area, and their framework composition. Mesoporous silicates and transition metal oxides have been extensively studied for their adsorption, separation, catalytic, and magnetic applications (13). Mesoporous carbons, noble metals, and mesostructured organic-inorganic hybrid chalcogenides materials have also been reported (412). The pores of a mesostructured material may or may not be accessible. When the pores become accessible through template removal or otherwise, the system is then defined to be mesoporous. Similar to the silicates, the mesostructured chalcogenides exhibit long-range pore order; however, they have not been rendered porous, and in general attempts to remove the template from the pores result in framework decomposition. Mesoporous semiconductors with well-defined pore structure are relatively unknown materials. Recently, crystalline microporous (pore size < 20 Å) chalcogenides (13) and porous chalcogenide aerogels (14) that possess high porosity have been reported. Porous semiconducting frameworks could exhibit unique properties such as a combination of quantum-confinement effects, coupled electronic properties, high internal surface area, and size-selective absorption (15).

Germanium, like silicon, is an important semiconductor of broad fundamental and technological interest (16). We recently showed that a well-ordered cubic gyroid mesophase of allgermanium semiconductor (MSU-Ge-1) can be synthesized (17). This mesostructure could not be rendered porous, because attempts to remove the surfactant caused the framework to collapse, partly due to its very thin walls (∼1 nm). Here, we describe a different synthetic strategy that leads to mesoporous germanium with very high internal surface area. This mesoporous germanium (MSU-Ge-2) exhibits a considerably blue-shifted energy gap relative to the bulk germanium as well as photoluminescence.

A family of mesoporous all-germanium semiconductors was prepared with the use of templating Ge polymerization by liquid-crystalline phase according to the metathesis reaction of Eq. 1. Namely, linking Ge94– clusters (Fig. 1 inset) in formamide solution with germanium tetrachloride (GeCl4) in the presence of self-assembled surfactant resulted in formation of a well-ordered mesostructure. Presumably, the surfactant creates an organized lyotropic liquid crystalline phase to template the inorganic Ge framework (1): Embedded Image(1) Embedded Image Embedded Image where R equals CnH2n+1 and n equals 18, 20, or 22.

Fig. 1.

(A) EDS spectrum of as-prepared MSU-Ge-2 semiconductor. The carbon and oxygen related peaks are attributed to the sample (surfactant) and the grid. (Inset) Molecular structure drawing of the deltahedral Ge94– cluster (B) TGA profiles (solid lines) performed in nitrogen of as-prepared MSU-Ge-2 (black) and heat-treated NH4+-exchanged material (red). The temperatures of weight loss are indicated in the corresponding differential thermogravimetric (DTG) curves (dashed lines). (C) FT-IR spectra of (a) as-prepared MSU-Ge-2, (b) NH4+-exchanged, and (c) heat-treated NH4+-exchanged materials. The absorptions in the 1900 to 2250 cm–1 region are due to Ge–H bonds. (D) Low-angle powder XRD pattern of as-prepared MSU-Ge-2 (black) and heat-treated NH4+-exchanged mesoporous Ge (red).

While this manuscript was under review, the preparation of a mesoporous germanium with hexagonal pore symmetry was reported (18). This material was prepared from the oxidative coupling of polymeric Embedded Image species without a linking ion.

The chemical composition of the product, MSU-Ge-2 with N-eicosane-N,N-dimethyl-N-(2-hydroxyethyl) ammonium bromide (EDMHEAB) surfactant, is (EDMHEAB)0.8Ge10, as determined by elemental C, H, and N and thermogravimetric analysis (TGA) (18). The purity of the inorganic framework was confirmed by energy dispersive spectroscopy (EDS), where only Ge element was observed (Fig. 1A). Potassium or halide (Cl or Br) ions were not detected, suggesting a complete metathesis of all Ge-Cl bonds.

Surfactant removal was achieved via a two-step ion-exchange thermal approach. First, the surfactants were replaced with ammonium nitrate (NH4NO3). The successful incorporation of NH4+ cations inside the Ge framework was confirmed with elemental C, H, and N analysis (19) and Fourier-transform infrared (FT-IR) spectroscopy, which showed characteristic N-H absorption bands at ∼1420 cm–1 (Fig. 1C, curve b). The subsequent removal of the NH4+ cations from the mesopores was achieved with mild heating to give hexagonal amorphous Ge with very high mesoporosity (20). After the thermal treatment to remove the NH4+ ions, FT-IR spectroscopy showed the presence of Ge-Hx stretching bands at 2141 and 2011 cm–1 without the characteristic N-H bending band (Fig. 1C, curve c). This suggests that the removal of NH4+ ions was in the form of volatile NH3 molecules, whereas the H+ ions transferred to the Ge framework to produce Ge–H bonds passivating the surface (21).

Thermal gravimetric analysis profiles of asprepared MSU-Ge-2 material show a weight loss of 5.4% (weight/weight, w/w) below 210°C, which is due to the removal of physisorbed and possibly chemisorbed formamide. Between 210° and 580°C, a gradual weight loss of 28.2% (w/w) is observed, which is attributed to the decomposition of surfactant (Fig. 1B). The inorganic residue at 600°C of both asprepared and template-removed materials was primarily crystalline Ge. The formation of elemental Ge supports the notion that the framework in MSU-Ge-2 is in fact reduced.

The long-range periodicity of the pore structure of as-prepared MSU-Ge-2 and template-removed material was confirmed with powder x-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD patterns exhibit a broad strong peak at low scattering angles (2θ < 3°) and a very weak feature at 2θ ∼4°, which, according to TEM, could be attributed to hexagonal p6mm symmetry (Fig. 1D). The d spacing of the first strong reflection gives a pore periodicity of ∼4.8 and ∼4.6 nm, respectively, for as-prepared MSU-Ge-2 (containing surfactant) and template-removed material (22). The absence of Bragg reflections at high angles (2θ > 10°) in the XRD pattern indicates the aperiodic nature of the inorganic framework and the absence of crystalline Ge impurity.

TEM images and the corresponding fast-Fourier transform (FFT) patterns of as-prepared MSU-Ge-2 and template-removed mesoporous Ge are depicted in Fig. 2. The TEM and FFT analysis confirm the presence of the well-defined long-range hexagonal arrangement of the pore channels in the as-prepared material (Fig. 2, A and B). The images also confirm that the hexagonal pore ordering is retained even after NH4+ ion exchange and heat treatment of the sample (Fig. 2, C and D). TEM analysis indicates average pore-pore distances of ∼4.0 and ∼3.8 nm (22) and pore diameters of ∼2.3 and ∼2.0 nm, respectively. This gives a framework wall thickness of ∼1.8 nm.

Fig. 2.

TEM and (insets) FFT images. (A and B) As-prepared MSU-Ge-2. (C and D) Mesoporous MSU-Ge-2 obtained by heat-treating the NH4+-exchanged material. The images shows a periodic hexagonal array along the [100] direction (A and C) and a periodic array of parallel pore channels along the [110] direction (B and D).

Clear evidence of the mesoporosity of MSU-Ge-2 after template removal was obtained with nitrogen physisorption measurement. The adsorption-desorption isotherms show a type IV adsorption branch associated with a well-defined capillary condensation step at P/Po ∼0.13, characteristic in uniform mesopores (Fig. 3A). The very small hysteresis between the adsorption and desorption branches (at 0.4 < P/Po < 0.8) is consistent with the parallel arrangement of uniform pore channels (pore size < 40 Å) (23). This is in agreement with the TEM images of Fig. 2. The adsorption data indicate a very high Brunauer-Emmett-Teller (BET) surface area of 363 m2 g–1 and pore volume of 0.23 cm3 g–1. Because the all-germanium mesostructure is much heavier than a corresponding silica one, this surface area is comparable to a silica with surface area of 1316 m2 g–1 (24). The pore size distribution calculated by the Barrett-Joynes-Halenda (BJH) method from the adsorption branch is quite narrow, indicating a uniform pore structure with a pore diameter of ∼2.1 nm (Fig. 3B). On the basis of the pore-pore spacing of ∼4.0 nm determined from XRD, we obtained a framework wall thickness of ∼1.9 nm, in good agreement with that observed from TEM. MSU-Ge-2 materials obtained with different surfactant chain lengths, C18H37- and C22H45-, also exhibited hexagonal pore symmetry and gave mesostructures with surface areas of 109 and 364 m2 g–1, pore volumes of 0.09 and 0.40 cm3 g–1, and narrow pore size distributions at ∼1.7 and ∼2.2 nm, respectively (fig. S1).

Fig. 3.

(A) Nitrogen adsorption-desorption isotherms at 77 K of heat-treated NH4+-exchanged MSU-Ge-2 (solid circles, adsorption data; open circles, desorption data). The hysteresis observed at P/Po > 0.8 is due to the interparticle voids between the agglomerated particles. (B) BJH pore size distribution calculated from the adsorption branch of the isotherm.

Pair distribution function (PDF) analysis was used to probe the local structure of the germanium framework (25). This technique allows the observation of first, second, and third neighbors in the structure. Figure 4A shows the PDF plots versus the interatomic distances in the as-prepared and mesoporous MSU-Ge-2 material, K4Ge9 precursor, and polycrystalline Ge. The PDFs of MSU-Ge-2 show strong interatomic correlation vectors at 2.4 and 4.1 Å, corresponding to Ge-Ge nearest neighbor and Ge···Ge next nearest neighbor distances, respectively. The well-resolved small peak at 5.8 Å in the template-removed MSU-Ge-2 sample suggests better ordering in the framework after surfactant removal. The lack of atomic pair correlation vectors at distances beyond 6 Å indicates a short-range order of -Ge9-clusters in the mesoporous framework. To detect whether the -Ge9-clusters were present in the framework, we also determined the PDFs of the K4Ge9 precursor and the crystalline Ge for comparison. As indicated in Fig. 4A, up to 6 Å the PDF profile of MSU-Ge-2 is more similar to that of K4Ge9 than to that of crystalline or amorphous Ge (25). The PDF of K4Ge9 shows characteristic peaks at 2.5 and 4.0 Å, which correspond to Ge-Ge first neighbors and Ge···Ge second neighbors inside the deltahedral (Ge9)4– structure, respectively. The similarity of the PDF profiles between MSU-Ge-2 and K4Ge9 implies that the -Ge9-cluster remains intact in the inorganic framework of the former. Further evidence for the existence of -Ge9-cluster inside the MSU-Ge-2 framework was obtained by measuring the relative probability between the first Ge-Ge and the second Ge···Ge neighbor interatomic vectors, determined by the ratio of integrals of the first and second peaks in the PDF. For both as-prepared and mesoporous MSU-Ge-2, the ratios of 1.60 and 1.51, correspondingly, are close to the deltahedral (Ge9)4– ratio of 1.46 and are very different from the value of 0.34 obtained from the local tetrahedral geometry of Ge atoms in bulk germanium.

Fig. 4.

(A) Reduced atomic pair distribution function G(r) of (a) as-prepared MSU-Ge-2 (black) and heat-treated NH4+-exchanged mesoporous material (red), (b) crystalline K4Ge9, and (c) crystalline Ge. (B) XPS spectra of MSU-Ge-2 (solid circles), element Ge, and GeO2 solids (dashed lines) for the Ge 3d signal. The peaks positions are from curve fitting the experimental data. (C) Optical absorption spectra of as-prepared (blue) and mesoporous MSU-Ge-2 (red) and bulk polycrystalline Ge (dashed line). (Inset) Size dependence of the energy band gap of MSU-Ge-2 as a function of framework wall thickness. Adjusting the reaction conditions of the MSU-Ge-2, the Ge-network demonstrates control of the wall thickness, which is reflected in the size of the energy band gap. (D) Room temperature photoluminescence spectrum of as-prepared MSU-Ge-2 with excitation wavelength at 240 nm. a.u., arbitrary units.

The oxidation state of Ge in MSU-Ge-2 was probed with x-ray photoelectron spectroscopy (XPS). The XPS spectrum showed Ge 3d peaks with binding energy at 29.85 and 31.9 eV (Fig. 4B). This suggests at least two sites in the structure, one with low valence for Ge atoms, probably from the Ge9 clusters, and a higher valence site possibly due to the Ge linking atoms. The Ge 3d spectra obtained from polycrystalline Ge (29.2 eV) and GeO2 (33.9 eV) are very different from those of MSU-Ge-2, consistent with the fact that the latter is not a mixture of Ge and GeO2 phases.

The optical absorption spectrum of as-prepared MSU-Ge-2 semiconductor shows a well-defined energy gap transition at 1.70 eV. This band gap is preserved in mesoporous material after surfactant removal (1.71 eV) (Fig. 4C). These energy gaps are much wider than that of crystalline or amorphous bulk Ge (0.66 eV). The enormous blue shift in energy gap compared with that of bulk Ge is due to the substantial dimensional reduction of wall thickness (∼19 Å) in MSU-Ge-2 and is reminiscent of the quantum-confinement-induced blue shifts in energy gap observed in Ge nanocrystals (26). We found that, by adjusting the reaction conditions during the synthesis of MSU-Ge-2, the pore wall thickness can be varied from ∼1.9 to ∼1.6 nm. Evidence for this was obtained from N2 sorption and XRD experiments (fig. S2). We also found that the energy gap decreases with an increase of wall thickness, which is consistent with a quantum confinement effect exhibited by the mesoporous framework (Fig. 4C inset). MSU-Ge-2 also showed photoluminescence (PL) at ∼610 nm at room temperature when excited by photons with high energy (Fig. 4D). The origin of this photoluminescence is not yet understood.

The synthetic chemistry using Group IV Zintl clusters and their possibility for doping using different linker centers can lead to a variety of well-ordered mesoporous semiconductors. Furthermore, the generality of the synthesis method for MSU-Ge-2 will lead to a variety of elemental mesoporous semiconductors, such as Sn and Si and solid solutions thereof, where the composition and pore geometry can be controlled to enable the variation of their optoelectronic and mass transport properties.

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