Bulk Silicon Crystals with the High Boron Content, Si1âxBx: Two

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Bulk Silicon Crystals with the High Boron Content, Si1xBx: Two Semiconductors Form an Unusual Metal Sergey V Ovsyannikov, Huiyang Gou, Alexander E Karkin, Vladimir V Shchennikov, Richard Wirth, Vladimir P. Dmitriev, Yoichi Nakajima, Natalia Dubrovinskaia, and Leonid S. Dubrovinsky Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm502083v • Publication Date (Web): 25 Aug 2014 Downloaded from http://pubs.acs.org on September 1, 2014

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Chemistry of Materials

Bulk Silicon Crystals with the High Boron Content, Si1-xBx: Two Semiconductors Form an Unusual Metal Sergey V. Ovsyannikov,†,* Huiyang Gou,†,¥** Alexander E. Karkin,‡ Vladimir V. Shchennikov,‡ Richard Wirth,§ Vladimir Dmitriev,∥ Yoichi Nakajima,† Natalia Dubrovinskaia,⊥ and Leonid S. Dubrovinsky† †

Bayerisches Geoinstitut, Universität Bayreuth, Universitätsstrasse 30, D-95447, Bayreuth, Germany Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China ‡ Institute of Metal Physics, Russian Academy of Sciences, Urals Division, GSP-170, 18 S. Kovalevskaya Str., Yekaterinburg 620990, Russia ¥

§

GFZ German Research Centre for Geosciences 3.3, Telegrafenberg, D-14473 Potsdam, Germany

∥European

Synchrotron Research Facility, Swiss Norwegian Beam Lines, F-38043 Grenoble, France

⊥

Materialphysik und Technologie, Lehrstuhl für Kristallographie, Physikalisches Institut, Universität Bayreuth, D-95440 Bayreuth, Germany KEYWORDS boron-doped silicon, high-pressure high-temperature synthesis, electronic transport, Raman and Infrared spectroscopy ABSTRACT: Silicon is a key technological material, and its controlled doping is one of simple and effective ways which are applied for creation of new advanced materials with tunable optoelectronic properties. Boron was known to be a dopant that can dramatically change the properties of silicon. However, a limited solubility of boron atoms in silicon matrix strongly restricted creation of bulk diamond-type structured Si-B alloys with the high boron content exceeding 0.5-1 at. %. In this work we show that bulk Si1-xBx alloys with a rather high boron content (e.g., 2.4 at.%) may be fabricated by alloying of boron and silicon at high temperatures above the melting point of silicon and high pressure. We extensively investigated the electronic transport and optical properties of these alloys using several techniques, including electrical resistivity, Hall effect, magnetoresistance, Raman, IR and optical spectroscopy, and X-ray diffraction. We found that Si1-xBx solid solutions are metals that possess very unusual optical properties, e.g., they demonstrate the antiresonant Raman spectra and the loss of the reflectivity in the near-IR range. Our work indicates new perspectives in creation and applications of Si1-xBx solid solutions with the diamond-type structure.

■ INTRODUCTION Silicon remains the most prominent technological material because of a combination of its unique and tunable semiconducting properties and low-cost methods of its production. Modifying the electronic properties of silicon by different methods one can uncover new directions of its potential applications. For instance, nanometric silicon becomes a highperformance thermoelectrics,[1,2] a minor doping can turn silicon to materials with a colossal positive magnetoresistance effect,[3-5] a spin polarization control in silicon can turn it to promising materials for spintronics,[6] and heavy doping can lead to metallization of silicon matrix.[7-9] Since silicon is one of the key industrial materials, the methods of fabrication of its different forms or composites are

of immediate technological interest.[10-12] Controllable doping remains the main method to modify the electronic properties of semiconducting materials. Combining ‘simple’ silicon and ‘enigmatic’ boron for creation of new advanced silicon-based materials with tunable optoelectronic properties was one of the early original strategies in the field. A solubility of small boron atoms in the silicon matrix is surprisingly low (usually below 0.5 at.%).[13-61] Thus, creating Si1-xBx alloys with the diamond-type structure is technically challenging. In addition, there are difficulties in quantitative determination of boron content in Si1-xBx by conventional chemical and microscopic techniques. There are several earlier reports or mentions in the literature that Si1-xBx alloys could potentially accommodate several atomic percentage of boron; however, no solid evidences of that were provided. For instance, a pioneer work by

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Pearson and Bardeen claimed the preparation of bulk Si1-xBx alloys with x that was variable up to 2.6 at.% and investigated their properties.[13] Meanwhile, a reanalysis of these data in the light of further experimental studies[14] unambiguously demonstrated that the bona fide boron amount in these samples did not exceed 1 at. %. Thus, development of new methods of doping of silicon with boron can open wide technological possibilities, and for this reason the numerous studies were devoted to investigations of properties boron-doped silicon as well as to solubility limits of boron in the silicon matrix at different conditions.[1361] Conventional methods of silicon doping, such as molecular beam epitaxy, cannot provide doping of active boron beyond the solubility limit of 0.5 at.%, and at the higher concentrations the boron atoms begin to form electrically inactive pairs and clusters.[27] Recently, a new technique of ‘gas immersion laser doping’ that employs the high-power lasers to melt the surface layer of material covered by potential dopant, has been developed.[62,63] Using this technique the surface layer of silicon may be enriched with several percentage of boron. Similarly to a case of boron-doped diamond, C1-xBx,[64-66] a strong boron doping of silicon leads to the appearance of superconductivity of which critical temperature, Tc ~0.3 K rises with the boron content.[67-72] However, this method is technically very challenging, and so far it permitted to fabricate only nonuniform and nano-sized films on the surface. Hence, alternative methods of manufacturing bulk boron-enriched silicon crystals are still demanded. Previous studies indicated that external stimuli, like applied stresses,[19-22] can noticeably enhance the efficiency of boron doping of silicon. In the present work we treated Si:B samples at high pressure high temperature (HP-HT) conditions (Figure 1) and found a rather simple technique to produce bulk Si1-xBx alloys with x as high as 0.024 (Figure 2). We established that the electrical characteristics of these Si1-xBx samples correspond to metals (Figure 3), but with unusual optical properties (Figures 4 and 5) that could lead to new applications of silicon-based materials in optoelectronics, e.g., in new types of photonic crystals (Figure 6). ■ EXPERIMENTAL SECTION Samples preparation: The samples of Si:B were prepared from silicon (Aldrich Chem. Corp. Inc., 99.9999% purity) and β-boron crystals (Chempur Inc., 99.5% purity) by highpressure high-temperature synthesis using both PistonCylinder apparatuses and Multi-Anvil Presses at Bayerisches Geoinstitut (Bayreuth). In the both cases we employed standard assemblies, including h-BN capsules for samples, heaters (made of graphite or LaCrO3), and thermocouples (PtPt10%Rh or W3Re/W25Re) for temperature determination. We tried two methods of synthesis. For syntheses in the Piston-Cylinder apparatuses we used an assembly consisted of relatively large single crystal silicon packed inside boron power (Figure 1). For syntheses in the Multi-Anvil Presses we prepared uniform mixtures of silicon and boron powders. Typical synthesis times in the both cases were about several hours. The synthesis procedures were similar to those described before.[73] The samples were firstly pressurized up to a target pressure, and then were heated up to a target temperature. The heated samples were quenched at ambient temperature by prompt switching off the power supply. A summary of sam-

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Figure 1. Synthesis of bulk boron-rich silicon crystals at high temperatures and high pressures. (a) Starting materials: single crystalline silicon (Si) inside boron (B) powder. (b) Sintering of silicon with boron: overheated well above the melting point silicon attacks boron power, captures boron atoms and clusters and gradually mixes them inside its matrix. 1 – sample capsule (e.g., may be made of BN), 2 – insulating cylinders (may be made of e.g., MgO), 3 – cylindrical heater (e.g., graphite, LaCrO3).

ples prepared is given in Table 1 in Supporting Information (SI). Chemical and microstructural characterization: The morphology and chemical composition of the samples were probed by scanning electron microscopy (SEM) at a LEO1530 instrument and by microprobe analysis at a JEOL JXA8200 electron microscope. TEM investigations were performed with a TECNAI F20 XTWIN transmission electron microscope operating at 200 kV with a field emission gun electron source. X-ray Diffraction: The crystal structure of the samples was examined by powder X-ray diffraction (XRD) studies at a high-brilliance Rigaku diffractometer (λ = 0.7108 Å). The high-quality XRD patterns of selected samples were collected at SNBL (The Swiss-Norwegian Beam Line, Grenoble, France) with a wavelength of 0.69658 Å. Electronic transport experiments: Electrical and galvanomagnetic properties of the samples were measured by a conventional Montgomery method (a modification of the Van der Pauw method) using an Oxford Instruments setup.[74] The measurements were performed at temperatures from 1.4 to 375 K and in magnetic fields up to 13.6 T. Optical experiments: Raman spectra of the samples were investigated on three different setups operating different laser lines. The spectra excited with the 632.8 nm, 514.5 nm, and 325 nm laser lines were collected in back-scattering geometry using LabRAM, Jobin Yvon – Spex and LabRAM HP spectrometers, respectively.[75] Near-infrared (NIR) and optical reflectance and absorption spectra were investigated on double-side polished samples (of thickness ~ 15 µm) at Bruker IFS 120 Fourier transform spectrometer coupled to an allreflecting Bruker microscope.[76]


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Figure 2. Microstructure characterization of several boron-silicon samples treated at high temperatures and high pressures. (a) SEM image of sample MA-5 giving an example of no reaction between non-melted Si and B. (b) SEM image of sample MA-22 in which boron inclusions remained unsolved inside the Si matrix after its melting. In this case the Si matrix was moderately and non-uniformly doped with boron. (c) and (d) SEM images of sample MA-21 subjected to melting that show the formation of boron silicide inclusions (B-Si) (dark grey grains) at the boundary between the Si matrix (light grey matrix) and boron (black grains). Plot (d) also shows the formation of a high-pressure boron polymorph having needle-like shape of grains (γ-B).[79-81] (e)-(h) microstructure characterization of sample PC-1 obtained by the method shown in Figure 1. (e) and (f) SEM images of entire cross-section and its part of sample PC-1 (grey matrix is Si uniformly doped with B, dark grains at the external border are unreacted β-B, dark clusters inside the Si:B matrix are amorphous boron inclusions (a-B). (g) Example of boron distribution maps in boron-doped silicon (light grey regions in (e) and (f)). (h) High Resolution Electron Microscopy (HREM) image of the boundary between the Si:B matrix and a-B inclusion.

■ RESULTS AND DISCUSSION We have performed more than a dozen of HP-HT synthesis runs at different HP-HT conditions below and above the melting curve of silicon. At ambient pressure silicon melts at 1687 K and its melting temperature linearly decreases with pressure to ~1000 K at 11 GPa and then weakly increases with further pressurization.[77,78] The results of some of these experiments are summarized in Table S1 in SI. We found that samples treated well above the melting point of Si at high pressures of several GPa demonstrate very unusual Raman spectra (Figure 4a,b), that are similar but much more spectacular than those reported in several previous works for heavily borondoped silicon.[55-61] Meanwhile, the samples treated at moderate high temperatures below the melting point of silicon, showed conventional Raman spectra of crystalline Si, i.e., the same as the spectrum collected on silicon single crystalline wafer (Si standard in Figure 4b). Synthesis of samples at much

higher pressures about 20 GPa and high temperatures about 1700 °C led to formation of the known high-pressure polymorph of boron (γ-B28)[79-81] and boron silicide complexes (Figure 2c and d). Previous studies of Si:B established that profound variations in the Raman spectra is one of sensitive indicators of boron doping.[55-58] Therefore, we used Raman spectroscopy technique for examination of homogeneity of the synthesized samples. This Raman probing showed that samples PC-1 and PC-2 synthesized at lower pressures as of 3 GPa in the pistoncylinder apparatus from a bulk cylindrical-like silicon crystal of several mm in sizes embedded inside a fine boron powder, are characterized by a rather high and uniform boron distribution (Figure 2e-g). Likewise, those synthesized at higher pressures, such as 7-12 GPa in the multi-anvil presses (samples labeled MA-17, -18, -20, -22, Table S1 in SI) from a melt of mixture of Si and β-B (about 5-8 at.%) powders, contained unreacted boron inclusions (Figure 2b) and demonstrated



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Figure 3. Characterization of structural (a) and electronic transport properties (b-d) of boron-doped silicon. (a) Comparative X-ray diffraction patterns for two Si1-xBx samples (PC-1 and MA-17 – the upper and the middle patterns, respectively) and a silicon standard collected at ambient conditions. The upper inset plots available literature data (Refs. 13-18) on the lattice strain in Si:B induced by Si/B substitution. The big star symbol and the vertical and horizonatal dashed lines indicate the maximal values achieved in our samples. The temperature dependencies of electrical resistivity ρ (b), the Hall constant RH (c), and carrier concentration and mobility (d) for PC2 sample. (b): The inset shows a magnetic-field, B dependencies at 4.2 K of the magnetoresistance effect (∆ρ/ρ0) for two experiments. The carrier concentration data approximately correspond to the boron content in Si which weakly depends on temperature (e.g., x~2.4 at.% in Si1-xBx at 300 K).

highly volatile Raman spectral imaging (Figure 4a) indicating at non-uniform distribution of boron inside the silicon matrix. This might be related to significant increase in activation energy of boron diffusion in silicon subjected to large strains, as reported in earlier works.[20-22] At the moment, there are three common experimental approaches to determine the composition of Si1-xBx alloys with the diamond-type structure. (i) The first method is based on the Vegard‘s law, i.e., on a linear dependence of the lattice parameter of Si1-xBx alloy on boron concentrtaion.[13-18] (ii) The second way is based on assumption that nearly all boron atoms in silicon are activated, and, hence, boron concentration may be found directly from the Hall effect. (iii) In the third method the boron concentration is aprroximately estimated from electrical resistivity value;[82] this approach is based on available experimental data suggesting that both carrier concentration and mobility in Si1-xBx alloys monotonically vary with the boron content. In order to estimate the boron concentration in our samples we investigated their structural and electronic transport properties. X-ray diffraction studies confirmed that the synthesized bulk samples of Si1-xBx alloys adopt the cubic diamond-type structure with a slightly reduced lattice parameter (Figure 3a). In the best samples PC-1 and PC-2 that are characterized by the largest and highly homogeneous boron distribution even at the nanometer scale (Figure 2g), we established the lattice parameter to be as low as 5.3967(1) Å versus 5.4305(3) Å in the original undoped Si single crystals. A Vegard’s law (a linear extrapolation of previous data)[13-18] suggests that the maximal lattice strain found in our samples ~ 0.0063 would

correspond to the boron concentrations slightly higher than ~1021 cm-3 (inset in Figure 3a). We measured temperature dependencies of electrical resistivity (ρ) and Hall effect (RH) in these PC-1 and PC-2 samples and confirmed the metallic character of the electrical conduction (Figure 3b,c). The positive sign of the Hall effect (Figure 3c) indicates the p-type conductivity as expected for boron doping. We estimated the carrier concentration (n) and mobility (µ) in these Si1-xBx samples of Si1-xBx at temperatures between 1.4 and 375 K (Figure 3d). In particular, we determined the carrier concentration at ambient conditions as n ~ 1.2 × 1021 cm-3 that corresponds to 2.4 at.% of boron (Figure 3d). According to the ‘resistivity calculator’ for boron-doped silicon[82] at such concentrations the resistivity value should decrease below ~100 µΩ cm; this well agrees with our finding (Figure 3b). These Si1-xBx samples also showed a moderate positive magnetoresistance effect that seemed to be a conventional parabolic function of the field, ∆ρ/ρ0 ~ (µB)2 (inset in Figure 3b). Thus, in this work we prepared the diamond-structured bulk Si1-xBx alloys with x as high as 0.024. This seems to be the highest boron content ever achieved in bulk Si1-xBx alloys (inset in Figure 3a). According to previous works, the boron concentration in Si1-xBx as high as about x~0.024 can induce a superconductivity with Tc well below than 1 K.[68,69,72] But this hardly presents any practical interest, and hence, investigation of superconductivity was beyond the scope of this work.


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Figure 4. Raman spectroscopy of boron-doped silicon at ambient conditions. (a) Samples MA-17 and MA-18 with non-uniform boron distribution. This plot qualitatively shows that increase in the boron content (from spectrum 1 to 3) suppresses the main silicon peak at ~521 cm-1 and enhances the background. The inset shows an enlarged section of the spectrum 3 in vicinity of peaks related to the Si-B vibrations. (b) Sample PC-2 with high and uniform boron distribution shows that parameters of the Fano interference strongly depend on the excitation energy. The peaks near about 300 and 950 cm-1 also present in spectra of conventional crystalline Si but their intensities are much less in comparison with the main peak at 520 cm-1. The corresponding parts of the spectrum of undoped crystalline Si are shown with a ×200 magnification. (c) The intrinsic phonon frequency (Ω) in Si:B vs boron content. The fitting curve is Ω ≈ 4.3x2 – 22.6x + 521.5 cm-1. (d) Examples of determination of the critical (intrinsic) frequency (Ω) in Si:B using the Fano’s theory.[86,55] These curves are the sections of the spectra shown in plot (b).

We examined the optical properties of bulk Si1-xBx alloys by means of Raman, near-IR and visual spectroscopy. As mentioned above the Raman spectra collected from our bulk Si1-xBx samples (Figure 4a,b) with the diamond-type structure qualitatively agreed with those reported earlier for heavily Bdoped Si.[55-58] The changes in the Raman spectra in vicinity of the first-order Raman phonon mode of Si at 520 cm-1 were addressed in the literature to a Fano-type interference between the discrete phonon line of Si and continuum of electronic excitations related to the B dopants (Figure 4a,b).[55-61] Our spectra demonstrated much stronger effects. For instance, sample PC-2 with x~2.4 at. % shows a distinct anti-resonance near 500 cm-1 instead of the typical phonon peak (a spectrum excited with 632 nm in Figure 4b). Thus, in our sample this interference, in fact, resulted in suppression of the Raman intensity for certain wave vectors. The Si1-xBx alloys normally show weak peaks related to Si-B vibrations at 618 at 640 cm-1 for 11B and 10B isotopes, respectively.[57-59] The natural abundances of these two stable boron isotopes, 11B and 10B are about 80% and 20%, respectively. This explains the apparent difference in the intensities between these two peaks (Figure

4a). We can clearly see that these both peaks also demonstrated the same anti-resonance effects, e.g., doublets in sample MA-21 (Figure 4a) or one combined anti-resonant peak in sample PC-2 with the higher boron concentration (Figure 4b). The other peaks near 300 and 950 cm-1 are the known overtones of silicon assigned to 2TA and 2TO modes, respectively (Figure 4b).[83-85] In order to determine the intrinsic phonon frequency from the Raman spectra of the Si1-xBx samples we used the Fano’s theory[86] in which the Raman Intensity (I) in vicinity of the resonance is determined as: I ~ (q+ε)2/(1+ε2), where ε=(ω – Ω)/Γ, ω – measured frequency, Ω – critical (intrinsic) frequency, and q and Γ are fitting parameters characterizing the transition probability (Γq2) and real change in phonon self-energy owing to electron-phonon interaction (Γ) (Figure 4d).[55] We found the critical frequencies as Ω ~491 and 495 cm-1 for the spectra excited with the 632.5 and 514.5 nm laser lines, respectively. This minor divergence potentially may be related to small fluctuations in the boron content.



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Figure 5. IR and visual reflectance spectra, dielectric functions and refractive index of boron-doped silicon (sample PC-2) at ambient conditions. (a) The IR and visual reflectance spectra of Si1-xBx and undoped single crystalline silicon wafer collected in the same conditions. The inset shows IR absorption spectrum of Si1-xBx. Using the known Kramers-Kronig transformation and RefFIT software[89,90] from these spectra we calculated the dielectric functions (b) and refractive indexes (c) of Si1-xBx. The refractive indexes of undoped single crystalline silicon wafer are given for comparison in plot (c).

Thus, the frequency of the first-order Raman mode in Si1was greatly softened comparing with the peak of undoped single-crystalline silicon at ~521.5 cm-1 (Figure 4c). Notice, that a moderate decrease in the frequency, e.g., to 505-510 cm1 may be realized in nanosized Si because of quantum conferment,[87] however, more sizable lowering is possible only upon structural deconfinement into amorphous state. Doping of silicon with the heavier elements, like Ge can also lower the phonon frequency, e.g., doping with ~ 30-40 at.% of Ge should decrease the frequency of this phonon down to ~ 491 cm-1.[88] Thus, a colossal tuning of frequency (energy) of the main vibration mode of silicon by boron doping is a new important feature of heavily boron-doped silicon crystals that emphasizes their dramatic difference with undoped or lightlydoped silicon. This effect could have potential applications in optoelectronic and other devices. Our findings together with those reported earlier by Cerdeira et al.[55] suggest one more simple and rather accurate method for determination of boron concentration in diamondtype structured Si1-xBx alloys for x < ~0.025 (Figure 4c). Fitting these data by a quadratic function (Ω ≈ 4.3x2 – 22.6x + 521.5 cm-1) and applying that to the spectra 1, 2, and 3 with Ω~502-512 cm-1 collected on other Si1-xBx samples, MA-17 and MA-18 synthesized at higher pressures of 7-8 GPa (Figure 4a), we can estimate the boron concentrations as ~0.5, 0.84, and 1.1 at.%, respectively. The comparative reflectance spectroscopy studies on Si1xBx samples with x = 0.024 and undoped single-crystalline silicon wafers discovered a striking difference between them xBx

(Figure 5a). Previous studies on Si:B having the less boron content also documented deep dips in the reflectivity, e.g., near ~ 0.3 eV[23] or ~0.5 эВ.[25] In order to figure out the dielectric functions and refractive indexes (n + ik) of Si1-xBx (Figure 5b,c) we applied the Kramers-Kronig (K-K) transformation using RefFIT software.[89,90] We found that in the visible spectral range the real part of the refractive index, n of Si1xBx is appreciably higher than that in pure Si, and its imaginary part, k exceeds 2 (Figure 5c). This picture is fully consistent with the metallic character of the electrical conduction of Si1-xBx (Figure 3b), i.e. the higher free carrier concentration leads to the higher reflection and higher absorption. However, both n and k indexes of Si1-xBx strongly decrease in the IR range (Figure 5c). These data point out at i) resonance-like transparency of Si1-xBx film for energies about 0.6 eV (i.e. for wavelengths of ~2 µm) (Figure 5a,c), and ii) optical impermeability below ~ 0.25 eV, i.e. for wavelengths above ~5 µm. The IR absorption spectra of a thin film of Si1-xBx showed an apparent dip near 0.55 eV (inset in Figure 5a). Together with the low k vales (Figure 5c) this also confirms that the strong dip in the reflectance spectra near 0.6 eV was not related to resonant absorption. The drastic difference in the refractive indexes between the undoped and heavily boron-doped silicon suggests that combined together into one matrix they could form unique photonic crystals. A simple model of a 2D photonic crystal, that could be prepared, e.g, by the gas immersion laser doping technique,[62,63] is shown in Figure 6. Theoretical investigations predicted that periodic diamond-type cubic structure is


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Chemistry of Materials (research project No. 14-02-00622a) and the Russian Academy of Sciences (code “Flow”) for the financial support.

ACKNOWLEDGMENT The authors thank Prof. H. Keppler (BGI) for assistance in the optical experiments.

REFERENCES

Figure 6. A simple model of a 2D photonic crystal that could be prepared from silicon substrate by embedding or fusing the Si:B blocks.

very promising for creation of photonic band gaps.[91] Experimental studies showed that certain silicon-based materials (e.g., Si-infiltrated opals)[92-94] are, in fact, promising photonic crystals. In addition, it was recently demonstrated that strong Fano-type anti-resonance effects can allow for the propagation of otherwise forbidden modes, and such materials could show negative refractive indexes.[95] Thus, heavily boron-doped silicon and related composite materials are certainly promising for industrial applications and further investigations of their properties are needed. ■ CONCLUSION In summary, we have demonstrated that a hightemperature synthesis above the melting point of silicon under applied moderate high pressures can produce bulk high-quality diamond-type structured silicon crystals with a high boron content (up to ~2.4 at. %). We have showed that these crystals are metals posessing extraordinary optical properties, including strong Fano-type interference effects and resonancelike transparency for wavelengths of ~2 µm. The unique properties of heavily boron-doped silicon make it a promising material for new industrial applications. We have offered a rather simple and low-cost method of fabrication of bulk diamond-type structured Si1-xBx solid solutions, and it seems that this method could play a crucial role in promotion of Si1xBx for industrial use.

ASSOCIATED CONTENT Supporting Information. A summary of Si-B samples synthesized at HP-HT conditions. This material is available free of charge via the Internet at http://pubs.acs.org.”.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], [email protected] ** E-mail: [email protected]

Funding Sources S.V.O. acknowledges the financial support of Deutsche Forschungsgemeinschaft (DFG). H.G. gratefully acknowledges the ﬁnancial support of Alexander von Humboldt Foundation. A.E.K. and V.V.S. thank the Russian Foundation for Basic Research

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SYNOPSIS TOC



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847x619mm (120 x 120 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment


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Fig. 3


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Fig. 4



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Fig. 5


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Fig. 6 779x735mm (120 x 120 DPI)


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