Sacrificial Template Synthesis and Properties of 3D Hollow-Silicon

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Sacrificial template synthesis and properties of 3-D hollow-silicon nano- and microstructures Iris Hölken, Gero Neubüser, Vasile Postica, Lars Bumke, Oleg Lupan, Martina Baum, Yogendra Kumar Mishra, Lorenz Kienle, and Rainer Adelung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06387 • Publication Date (Web): 18 Jul 2016 Downloaded from http://pubs.acs.org on July 20, 2016

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ACS Applied Materials & Interfaces

Sacrificial template synthesis and properties of 3-D hollow-silicon nano- and microstructures Iris Hölken, 1 Gero Neubüser,1 Vasile Postica,2 Lars Bumke,1 Oleg Lupan,1,* Martina Baum,1 Yogendra Kumar Mishra1, Lorenz Kienle,1,* Rainer Adelung1,* 1

Institute for Materials Science, Kiel University, Kaiser Str. 2, D-24143, Kiel, Germany

2

Department of Microelectronics and Biomedical Engineering, Technical University of Moldova, 168 Stefan cel Mare Av., MD-2004 Chisinau, Republic of Moldova

E-mail: [email protected], [email protected] , [email protected], [email protected]

*Corresponding authors Assoc. Prof. Lupan, Prof. Kienle E-mails: [email protected], [email protected], [email protected] Institute for Materials Science, Kiel University, Germany

Prof. Adelung [email protected]

KEYWORDS: Hollow nano- and microstructures, tetrapods, silicon, sacrificial templating, transmission electron microscopy

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ABSTRACT: Novel 3-D hollow aero-silicon nano- and microstructures, namely Sitetrapods (Si-T) and Si-spheres (Si-S) were synthesized by a sacrificial template approach for the first time. The new Si-T and Si-S architectures were found as most temperature stable hollow nanomaterials, up to 1000 °C, ever reported. The synthesized aero-silicon or aero-gel was integrated into sensor structures based on 3-D networks. A single microstructure Si-T was employed to investigate electrical and gas sensing properties. The elaborated hollow microstructures open up new possibilities and a wide area of perspectives in the field of nanoand microstructure synthesis by sacrificial template approaches. The enormous flexibility and variety of the hollow Si- structures is provided by the special geometry of the sacrificial template material, ZnO-tetrapods (ZnO-T). A Si layer was deposited onto the surface of ZnOT networks by plasma enhanced chemical vapor deposition. All samples demonstrated p-type conductivity, i.e., the resistance of the sensor structure increased after introducing the reducing gases in the test chamber. These hollow structures and their unique and superior properties can be advantageous in different fields, such as NEMS/MEMS, batteries, dyesensitized solar cells, gas sensing in harsh environment and biomedical applications. This method can be extended for synthesis of other types of hollow nanostructures.


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1. INTRODUCTION

Controlling materials morphologies, shape, replication and formation of hollow 3-D structures at nanoscale dimensions have been very important aspects within nanostructuring communities as well as in advanced nanotechnological applications.1-6 Strategies for the synthesis of hollow nano- and microstructures are categorized into four main topics, namely conventional hard-templating and soft-templating approaches, as well as sacrificial templating and template–free synthesis.7 The main advantage of the sacrificial template synthesis, is the possibility to mimic the exact shape of the initial structure by using various morphologies.8 Until now, there have been no reports on a comparably cost-effective production method for template based variously shaped nano- or microstructure - architectures. Multiple morphology variations of zinc oxide9-11 inspired us to continue the evolution in sacrificial templating processes to synthesize hollow structures from others semiconductors as advanced materials. Zinc oxide is a wide bandgap (3.37 eV) n-type semiconducting ceramic material which has been under continuous research within the last couple of decades.12 Of special interest are ZnO micro- and nanostructures, as they open new opportunities in the electronic and photonic field of nanotechnology.13 As already published in many articles, most fabrication processes enable the production of a huge variety of structures such as nanowires, nanorings, nanobelts, tetrapods or other architectures of ZnO.14-16 Within the area of morphologies, tetrapodal ZnO (ZnO-T) particles play a major role as they possess an unique 3-D shape due to their four arms pointing in different directions. At the Kiel University, group of Functional Nanomaterials, a simple and solvent free flame transport synthesis (FTS) was established which, besides the possibility to fabricate various different morphologies, enables their production on a large and commercial scale.17-18 Detailed information on the growth process of T-ZnO during the flame transport synthesis and some sensor application was reported recently.17 3 Environment ACS Paragon Plus


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Using ZnO tetrapods as sacrificial template, the formation of highly flexible hollow nanoand microstructures has been realized. The advantageous properties of hollow nano- and microstructures like increased surface-to-volume ratio and porosity were already described in several works,19 and have been the basis for promising progresses within the fields of sensing, data or gas storage, catalysis, energy conversion, drug delivery, etc.19-23 One example for a promising potential application is the field of biomedicine. While blood and urine analysis for clinical diagnosis have been rapidly developing within the last decades, diagnostics based on breath analysis is still poorly developed and therefore not widely used yet.24-27 On the other hand, being a non-invasive method of diagnostics, breath analysis is the principal candidate for clinical and home diagnosis.24-26 In this context, diabetes mellitus is a large and fast growing problem which can be sensed by breath analysis, because it is accompanied by two metabolic changes: increase in glucose concentrations in the blood and intensive lipolysis.24, 28 The World Health Organization estimated that by the year 2030 the number of persons diagnosed with diabetes mellitus would be 366 million and diabetes will be the 7th leading cause of death.24 Due to these facts a reliable and easy to handle method for diagnosis of this disease is necessary. Thus, introducing nanotechnology has great potential to find solutions and to develop nanodevices for this field. One potential and specific biomarker for the detection of diabetics is the identification of gaseous acetone.24-25, 29 Clinical data shows that the concentration of exhaled acetone from diabetes patients exceed 1.8 ppm, while for healthy people only 0.3–0.9 ppm can be found.25,

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Traditional methods for the detection of acetone in the exhaled air, like ion mobility spectrometry30 and mass spectrometry (MS) detection31, need bulky, sophisticated and costintensive instrumentations combined with skilled operators. Therefore these methods are not suitable for real time, everyday, out of the laboratories and high-throughput diagnosis.24, 29 Therefore, a selective and highly sensitive gas sensor for the fast detection of acetone at room 4 Environment ACS Paragon Plus

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temperature is desirable for the non-invasive detection of metabolic products caused by diabetes.24-25, 29 In this context, the main advantage of silicon based sensors is the possibility to be integrated into complementary metal-oxide-semiconductor (CMOS) systems.32 Due to an existing set of fabrication and processing technologies, silicon plays a central role in semiconductor industry.33 Additionally, many nanodevices based on silicon nanowires have been elaborated for the detection of chemical compounds and biologically relevant molecules.32, 34-41 However, recent studies demonstrated fascinating sensing performances of nanostructured networks, especially for those based on 3-D nanostructures, due to specific gas and UV sensing mechanism based on properties, e.g. surface/volume ratio, etc., whose impact are increasing in nanoscopic dimensions.17, 42-44 Thus, 3-D networks of tetrapodal structures play an important role in the development of high-performance sensor structures, due to the enormous porosity of the sensing layer and anti-agglomeration properties of the structures.17, 42

Since, the morphology has a great influence on the performances of nanostructured

materials and therefore also on nanodevices.7, 45-46 The goal of this work is the formation of innovative 3-D morphologies/architectures of silicon based nano- and microstructured materials with advanced functional properties by using sacrificial template based nanotechnology and their further integration in functional nanodevices. We report for the first time the synthesis of 3-D hollow aero-silicon microstructures, i.e. tetrapods (T) and spheres (S) by a sacrificial template synthesis. Morphological and structural investigations of aero-silicon, Si-T and Si-S, were performed in detail. The high temperature stability of 3-D hollow structures was studied. Gas sensing experiments revealed a selectivity dependency on the morphology of the microstructures. 3-D networks of hollow Si-T demonstrated high sensitivity and selectivity towards acetone vapor, while Si-S microspheres demonstrated higher sensitivity to NH3 vapor. This elaborated synthesis method is applicable to the growth of other types of hollow nano- and microstructures with different morphologies by the selection of the initial shape of the sacrificial template. This opens up a wide range of 5 Environment ACS Paragon Plus


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possibilities for further performance improvements in the field of batteries, NEMS, MEMS, gas sensing, biomedical applications as well as in clinical diagnosis.

2. EXPERIMENTAL SECTION Tetrapodal and spherical ZnO as template material In this work, ZnO tetrapods with arm length of 15-20 µm were fabricated by FTS, whereby spherical particles (several µm in diameter) purchased from Sigma Aldrich (Westerhausen, Germany) were used as reference in order to investigate the influence of particle morphology. SEM images of FTS ZnO-T used as sacrificial template can be found in previous works.17-18 Nanotechnology of hollow silicon nano- and microarchitectures For the production of hollow silicon nano- and microarchitectures, a suspension of 100 mg ZnO and 50 ml isopropanol was mixed and drop-casted onto an 8 inch silicon wafer. After complete evaporation of the isopropanol, the remaining ZnO layer was covered by a thin silicon layer by plasma enhanced chemical vapor deposition (PECVD) using a SENTECH SI 500 PPD device. Detailed information on the deposition of silicon thin films by PECVD was recently published work.47 The parameters used for the deposition are listed in Table 1. After the deposition process, the Si-coated ZnO nano- and microarchitectures were peeled off the substrate. In order to remove the ZnO templates, the coated ZnO microstructures were immersed into a 3:1 etching solution of de-ionized water and hydrochloric acid (HCl, 37%) for 3 h at room temperature. For the completion of the process, the solution was filtered by a Whatmann filter with a pore size of ~11 µm followed by cleaning with isopropanol. After complete evaporation of the isopropanol, a powder of hollow silicon nano- and microarchitectures (Si-T and Si-S) was obtained.

Preparation/Instrumentation 6 Environment ACS Paragon Plus

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The surface morphology and elemental microanalysis of the Si-T and Si-S nano-structures were examined by a scanning electron microscope (SEM, Zeiss Ultra Plus, 7 kV) with an Energy Dispersive X-ray (EDX) spectrometer. For detailed investigation of chemical composition and morphology, transmission electron microscopy (TEM) has been utilized. All measurements have been conducted with a FEI Tecnai F30 STwin microscope (300 kV, field emission gun, spherical aberration coefficient Cs=1.2 mm). Applied techniques were selected area electron diffraction (SAED) to examine the crystallinity, EDX in scanning mode (STEM-EDX), electron energy loss spectroscopy (EELS) for chemical characterization and high resolution TEM (HRTEM) to analyze the atomic structure. The sample has been placed onto a lacey carbon supported copper TEM grid. High temperature stability has been examined via an in-situ heating experiment (T ranging from room temperature up to 1000 °C. Here a molybdenum TEM grid was selected due to better temperature stability. Gas sensing investigations were performed using the techniques as reported in previous works.48-49 The concentration of all tested gases in these investigations was 100, 100, 50 and 500 ppm for ethanol, acetone, NH3 vapor and H2 gas. All samples demonstrated p-type gas response, i.e., the resistance of the sensor structure increased after introducing the reducing gases in the test chamber.

3. RESULTS AND DISCUSSION 3.1. Morphological studies of hollow silicon structures Figure 1 shows the morphology of the hollow Si-T and Si-S. The Si-T samples represent the hollow silicon tetrapod 3-D networks (see Figure 1a,b and Figure S1). The Si-T particles typically do have a diameter of 3-5 µm at their base and a diameter of 0.5-2 µm at the distal end of their arms (Figure S2a-d). The length of the Si-T arms is in the range of 15 – 20 µm. Tetrapods are hollow inside, and no remaining ZnO particles, like solid regions inside the 7 Environment ACS Paragon Plus


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tetrapod arms or at their base can be observed (see Figure 1a,b and Figure S2e-f). Single SiT structures transferred to SiO2/Si substrate with pre-patterned Au/Cr electrical contacts are presented in Figure S2a,b. As the Si-T 3-D structures may be partially destroyed during handling, some arms were broken during the transferring process (Figure S2c-f). Figure S3a,b demonstrate a hollow Si-T arm in cross-section at higher and lower magnifications, which resembles the hexagonal shape of the ZnO tetrapodal template. Figure 1c and S3c demonstrate that the thickness of the Si wall is about 100-200 nm. In dependence of deposition time it can be grown to much thinner membranes of Si (20-100 nm), which are presented in TEM part below. SEM images of hollow Si-T 3-D networks at higher magnifications are presented in Figure S4a,b, and show that some arms of the tetrapods are separated after the etching process. In most tetrapods a partial cave-in or small openings in the wall have been detected. As depicted in Figure 1c, S3c, and S4c, the columnar morphology of the Si walls, and no pores can be observed, which implies continuous formation of the Silayer with a uniform thickness. Figure S5 shows SEM images of the: (a) Si-T hollow tetrapods forming networks at different magnifications (a-b) used for sensor fabrication; (c) different shaped hollow Si structures; (d) zoomed-in hollow “airplane”-like structure. All of these structures prove that any shape can be replicated by this developed nanotechnology presented in the current work. In Figure S6 SEM images of a single Si-T arm in different magnifications and regions are presented to indicate that the formed structures reproduce perfectly the initial shape and even the smallest morphological details of the ZnO-T arm. This proves the high efficiency of the sacrificial template synthesis presented in this work. In Figure 1d SEM images of ZnO structures with a hexagonal shape with different diameters and morphologies in the range of 200 – 800 nm are presented. After the deposition of Si and the subsequent etching process to remove the ZnO template, the hollow Si-S nanostructures show a spherical morphology (see Figure 1e,f), with diameters in practically the same range as the initially ZnO nanostructures (Figure 1d). The partial cave-in or small openings in the 8 Environment ACS Paragon Plus

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wall were not observed for Si-S hollow microspheres, at least from the top view in SEM images. However, the porosity of the initial ZnO structures remained after the etching process (see Figure S7). Also in Figure S7d a single Si-S microsphere transferred to SiO2/Si substrate with Au/Cu electrical contacts is shown.

3.2. Structural study of hollow silicon structures TEM analysis showed two morphological variants of pure silicon, i.e. hollow spherical and hollow tetrapodal shaped nano- and microparticles. Spherical particles of Si-S with dimensions between 0.2 µm and 1 µm are present in agglomerations (Figure 2a). Whereby, their wall thickness bears strong variations (20-100 nm, Figure 2b) probably depending on the nucleation and subsequent growth duration during the fabrication process. The electron diffraction (ED) pattern (Figure 2c) contains defined rings of diffuse intensities revealing the nanocrystalline character of the microspheres. The experimentally determined diameters of the diffraction rings correlates well with the silicon structure. The oxygen content determined by energy dispersive X-ray (EDX) analysis is about 10 at%. According to the absence of SiO2 related intensity in ED the oxygen arises from an amorphous surface contamination (SiOx). The morphology of hollow tetrapodal Si-T particles differs from the Si-S spherical ones. The tetrapods’ arm length is in the range of some micrometer (Figure 2d) and the diameter is about 0.5 to 2 µm. In tomography series of a single tetrapod arm (see suppl. Video S1) its cylindrical shape is demonstrated as well as the completely intact surface that keeps a slightly faceted profile (see Figure 2d) due to the ZnO template. Energy-filtered electron diffraction (EF-ED, Figure 2f) at a slit width of 2 mm displays blurred rings that as combined with highresolution transmission electron microscopy (HRTEM) (not shown) revealed an amorphous character of Si-T. Images at higher magnifications (Figure 2e) showed that many tetrapods Si-T are covered by small Si particles with diameters of about 20 nm arising from a former homogenous nucleation of silicon on the ZnO template. The minimal observed wall thickness 9 Environment ACS Paragon Plus


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of the tetrapods is 20 nm (Figure 2e) and the maximal one is about 100 nm. Additionally by applying the log-ratio method50 to an electron energy loss (EEL) spectrum the thickness has been determined experimentally to 18 nm. The excerpt of the spectrum (Figure S8a) includes the zero-loss peak and a plasmon peak at 17 eV, which is characteristic for pure silicon. The absence of a peak at 24 eV (plasmon peak of SiO2, see Figure S8b) indicates that the amount of silicon oxide is insignificant. According to EDX data the concentration of oxygen is about 3 at% originating from minor surface contamination. An EDX-line scan (Figure S8c) displays the oxygen distribution measured perpendicular to a Si-T arm. These data confirms that pure Si-T has been fabricated as the oxygen concentration has been determined to about 5 at%. The profile lines from the detector signal and Si match and feature a step-like trend that originates from the arms´ faceted shape. The signal to noise ratio for oxygen is low compared to Si, but it correlates to the latter one. At position 3 3.5 µm the oxygen count rate is relatively reduced, probably due to absorption and shadowing effects of low-energy X-rays. During the heating experiment a structural transformation of the Si-T tetrapod has been observed, but no morphological changes were detected. At room temperature (RT) Si-T is entirely amorphous (Figure 2g), and spontaneous crystallization occurred at a temperature of 780 °C which matches to previous studies of amorphous Si thin films.51 Crystallites with sizes in the range of 50 nm (Figure 2h) were formed. The corresponding SAED pattern (Figure 2h) features sharp diffraction rings that match those formed by Si. Subsequent heating induces grain coarsening (see Figure 2i), but the general shape and architecture of the Si-T tetrapod is very stable until the end point of the heating experiment (T = 1000 °C). These results suggest that our nanotechnology allows the synthesis of the most stable hollow nanoand microstructures at high temperatures. It may be of great interest for various applications in different areas as mentioned above.


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3.3. Gas sensing response of networks based on hollow silicon structures A schematic of single hollow Si-T synthesis by sacrificial templating approach is represented in Figure 3a and is described in the Experimental Section in detail. Information on the fabrication of sensor structures by using new hollow Si-T 3-D networks and Si-S microspheres is similar to the one presented in our previous work17. It is shown schematically in Figure 3b (only for Si-T). Glass substrates with pre-patterned Au/Cr pads (170 nm/10 nm) sputtered in vacuum were used as templates. The gap between the gold pads was ~100 µm. The Figures S1 and S7 present SEM images at different magnifications (a-c) of the Si-T hollow tetrapods and Si-S hollow spheres forming networks used for sensor fabrication in our experiments. The respective current–voltage characteristics for both types of samples are presented in Figure S9a,b demonstrating a quasi-linear behavior. As can be observed from Figure 3c, the sensor structures based on hollow Si-T 3-D networks, Si-T, are highly selective to acetone vapor. The optimal operating temperature for acetone vapor sensing is 200 °C, with gas response Sacetone = 26. While at the same operating temperature the gas responses to other gases are 9.1, 2.0 and 8.1 for ethanol vapour, H2 gas and NH3 vapor, respectively (see Figure S10). Thus, the respective selectivity factors at 200 °C operating temperature are Sacetone/Sethanol ≈ 2.9, Sacetone/SH2 ≈ 13 and Sacetone/SNH3 ≈ 3.2. In the case of the Si-S hollow microspheres, Si-S, the gas response is higher to NH3 vapour, with SNH3 = 9.3, at optimal operating temperature of 175 °C (Figure 3d). At this temperature the other gas responses are 4.6, 2.1 and 2.6 for ethanol vapor, H2 and acetone vapour, respectively (Figure S10). Thus, the respective selectivity factors at 175 °C operating temperature are SNH3/Sethanol ≈ 2, SNH3/SH2 ≈ 4.4 and SNH3/Sacetone ≈ 3. As can be observed from Figure 3c,d, the optimal operating temperature for all four tested gases are the same, thus we can deduce that the optimal operating temperature depends only on the type of material (in this case Si) and not on the morphology of the samples.



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In Figure 3e dynamic responses of the sensors based on the Si-T samples to acetone vapour at 200 °C and of the sensor based on Si-S samples to NH3 vapour at 175 °C are presented. The calculated response and recovery times (τr and τd, respectively) are 16 s and 7 s for Si-T, 18 s and 9 s for Si-S. This relatively low rapidity of the sensors can be explained by the porosity of the samples. Discussions about the dependency of the selectivity on the morphology are presented in Supporting Information Text T1.

3.4. Sensing properties of a single hollow Si tetrapod arm For further insights into the sensing properties of aero-silicon, a nanodevice based on a single Si-T arm was fabricated by the method reported earlier by Lupan et al.48-49, 52 Aerosilicon was transferred to SiO2/Si substrates with pre-patterned Au/Cr electrical contacts by direct touching and further gradual dispersion to a lower density (Figure 4a-c). A SEM image and a schematic representation of the fabricated nanodevice based on a single Si-T arm is presented in Figure 4d and e, respectively. Both open ends of the Si-T arm were connected to electrical contacts by Pt complex, as reported previously.48-49 The diameter of the investigated Si-T is ~ 2 µm with ~ 15 µm length. Nanodevices fabricated in this way demonstrated a linear behavior of current – voltage characteristic (see Figure S11) and were tested to 100 ppm and 10 ppm of acetone at room temperature. The corresponding dynamic responses of the nanodevice based on a single Si-T arm to 100 ppm acetone at room temperature are presented in Figure 4f and Figure S12. The calculated gas response to 100 ppm and 10 ppm of acetone are ~ 60 and ~ 4.1, respectively, demonstrating improved gas sensing properties compared to 3-D networks of Si-T with faster response time, but with lower recovery time (see Table S1). Silicon is an important semiconductor for gas sensing applications. Sensors for the detection of acetone and ethanol vapor, sub-ppm level detection of NH3 were elaborated based on porous silicon embedded with metal oxides32,

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and silicon nanowires.40,

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performed by a single Si nanowire sensor.34 In Table 2 and Table S2 data on acetone and NH3 sensors are generalized to compare. As can be seen from Table 2, our sensors based on hollow Si-T 3-D networks have one of the lowest operating temperatures reported in literature, comparable with those of copper oxide.43-44 However, our gas response is much higher and the recovery time is shorter. Ni-doped SnO2 nanofibers55 and Au/In2O3 inverse opal25 have higher gas responses, but the operating temperature is much higher (340 ºC). Next, the response of hollow Si-S microspheres is compared to Si NWs networks and poly-silicon Si NWs resistors,38 where response and recovery times are much shorter. Te-modified Si NWs41 have higher rapidity at RT, but much lower response. The gas sensing mechanism is proposed in Supporting information Text T2.

4. CONCLUSIONS In summary, the successful cost-effective synthesis of 3-D hollow aero-silicon nano- and microstructures, namely tetrapods (Si-T) and spheres (Si-S) by a sacrificial templating nanotechnology was reported for the first time. A hollow advanced material was discovered as the most stable nanomaterial at high temperatures up to 1000 °C. 3-D ZnO-T networks and ZnO microspheres were used as sacrificial template materials. Morphological and structural studies were performed in detail and demonstrated a high efficiency of the elaborated synthesis method. This nanotechnology allows the reproduction and thereby the replication of the exact shape of the sacrificial ZnO-T or ZnO-S surfaces, including even the smallest features of their nano-architecture. Hollow Si nano- and microstructures can be used for batteries, dye-sensitized solar cells, NEMS/MEMS and other micromachine fabrication, including micro- and nanosensors for various applications. For that reason, gas sensing investigations of the aero-silicon was performed and revealed an excellent selectivity to acetone vapor of the tetrapodal hollow silicon structures. A response (Sacetone) of about 26 to 100 ppm was obtained at 200 ºC operating temperature. In the case of the hollow Si-S 13 Environment ACS Paragon Plus


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microspheres a good selectivity towards NH3 vapor with SNH3 = 9.3 (to 50 ppm) at optimal operating temperature of 175 °C was obtained. Further increase in acetone vapor response of the tetrapodal microstructures was obtained by integration of a single Si-T arm into the nanodevice, opening the possibility to detect acetone vapor at room temperature with a higher response (Sacetone = 77). The presented data is a significant step forward into the field of amorphous and crystalline hollow nano- and microstructures synthesis, especially by sacrificial template method, as it can be extended to the fabrication/growth of other type of materials (e.g. carbon, GaN, etc) with different morphologies by adequate template selection. More details on gas and biosensing properties of the aero-silicon based sensors and nanosensors will be reported in our upcoming works.

■ ASSOCIATED CONTENT Supporting Information: Video, Figures showing SEM images, EEL spectrum and EDXline scan profile of different hollow silicon micro- and nanostructures. Also, current – voltage characteristics of the sensor structures made from different type of silicon structures and corresponding gas response diagram to acetone are presented. The same characteristics are presented for device made on a single Si tetrapod arm. Also discussions on dependence of selectivity on morphology and gas sensing mechanism proposed for Si tetrapods are presented. Two tables showing calculated parameters for sensor structures and parameters of other NH3 sensors based are presented. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.

■ AUTHOR INFORMATION 14 Environment ACS Paragon Plus

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Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS Dr. Lupan acknowledges the Alexander von Humboldt Foundation for the research fellowship for experienced researchers 3-3MOL/1148833 STP at the Institute for Materials Science, Kiel University, Germany. This research was partly supported by the project Institutional 45inst15.818.02.29A funded by the Government of the Republic of Moldova. This research was sponsored partially by the German Research Foundation (DFG) under the schemes PAK 902 (FOR20193 & KI 1263/14-1 & AD 183/12-1 & AD 183/17-1).

■ REFERENCES (1)

Zhao, L.; Liao, K.; Pynenburg, M.; Wong, L.; Heinig, N.; Thomas, J. P.; Leung, K. T. Electro-oxidation of Ascorbic Acid by Cobalt Core–Shell Nanoparticles on a HTerminated Si(100) and by Nanostructured Cobalt-Coated Si Nanowire Electrodes. ACS Appl. Mater. Interfaces 2013, 5, 2410-2416.

(2)

Halpern, J. M.; Wang, B.; Haick, H. Controlling the Sensing Properties of Silicon Nanowires via the Bonds Nearest to the Silicon Nanowire Surface. ACS Appl. Mater. Interfaces 2015, 7, 11315-11321.

(3)

Paska, Y.; Haick, H. Interactive Effect of Hysteresis and Surface Chemistry on Gated Silicon Nanowire Gas Sensors. ACS Appl. Mater. Interfaces 2012, 4, 2604-2617.

(4)

Wang, Y.; Zhang, X.; Gao, P.; Shao, Z.; Zhang, X.; Han, Y.; Jie, J. Air Heating Approach for Multilayer Etching and Roll-to-Roll Transfer of Silicon Nanowire Arrays



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as SERS Substrates for High Sensitivity Molecule Detection. ACS Appl. Mater. Interfaces 2014, 6, 977-984. (5)

Sun, Y.; Xia, Y. Triangular Nanoplates of Silver: Synthesis, Characterization, and Use as Sacrificial Templates For Generating Triangular Nanorings of Gold. Adv. Mater. 2003, 15, 695-699.

(6)

Miao, R.; Mu, L.; Zhang, H.; She, G.; Zhou, B.; Xu, H.; Wang, P.; Shi, W. Silicon Nanowire-Based Fluorescent Nanosensor for Complexed Cu2+ and its Bioapplications. Nano Lett. 2014, 14, 3124-3129.

(7)

Lou, X. W. D.; Archer, L. A.; Yang, Z. Hollow Micro‐/Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987-4019.

(8)

Miao, J.-J.; Jiang, L.-P.; Liu, C.; Zhu, J.-M.; Zhu, J.-J. General Sacrificial Template Method for the Synthesis of Cadmium Chalcogenide Hollow Structures. Inorg. Chem. 2007, 46, 5673-5677.

(9) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Nanobelts of Semiconducting Oxides. Science 2001, 291, 1947-1949. (10) Wang, Z. L. Nanostructures of Zinc Oxide. Mater. Today 2004, 7, 26-33. (11) Zhu, C.; Saito, G.; Akiyama, T. A New CaCO3-Template Method to Synthesize Nanoporous Manganese Oxide Hollow Structures and Their Transformation to HighPerformance LiMn2O4 Cathodes for Lithium-Ion Batteries. J. Mate. Chem. A 2013, 1, 7077-7082. (12) Charinpanitkul, T.; Nartpochananon, P.; Satitpitakun, T.; Wilcox, J.; Seto, T.; Otani, Y. Facile Synthesis of Tetrapodal ZnO Nanoparticles by Modified French Process and its Photoluminescence. J. Ind. Eng. Chem. 2012, 18, 469-473. (13) Ronning, C.; Shang, N.; Gerhards, I.; Hofsass, H.; Seibt, M. Nucleation Mechanism of the Seed of Tetrapod ZnO Nanostructures. J. Appl. Phys. 2005, 98, 34307-34307.


Page 16 of 28

Page 17 of 28

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


(14) Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys. Condens. Matter. 2004, 16, R829. (15) Chow, L.; Lupan, O.; Heinrich, H.; Chai, G. Self-assembly of Densely Packed and Aligned Bilayer ZnO Nanorod Arrays. Appl. Phys. Lett. 2009, 94, 163105. (16) Lupan, O.; Chow, L.; Chai, G.; Heinrich, H. Fabrication and Characterization of Zn– ZnO Core–Shell Microspheres from Nanorods. Chem. Phys. Lett. 2008, 465, 249-253. (17) Mishra, Y. K.; Modi, G.; Cretu, V.; Postica, V.; Lupan, O.; Reimer, T.; Paulowicz, I.; Hrkac, V.; Benecke, W.; Kienle, L. Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing. ACS Appl. Mater. Interfaces 2015, 7, 14303-14316. (18) Mishra, Y. K.; Kaps, S.; Schuchardt, A.; Paulowicz, I.; Jin, X.; Gedamu, D.; Freitag, S.; Claus, M.; Wille, S.; Kovalev, A. Fabrication of Macroscopically Flexible and Highly Porous 3D Semiconductor Networks From Interpenetrating Nanostructures by a Simple Flame Transport Approach. Part. Part. Syst. Charact. 2013, 30, 775-783. (19) Lee, J.-H. Gas Sensors using Hierarchical and Hollow Oxide Nanostructures: Overview. Sens. Actuators B 2009, 140, 319-336. (20) Lian, J.; Anggara, K.; Lin, M.; Chan, Y. Formation of Hollow Iron Oxide Tetrapods via a Shape‐Preserving Nanoscale Kirkendall Effect. Small 2014, 10, 667-673. (21) Ye, J.; Zhang, H.; Yang, R.; Li, X.; Qi, L. Morphology-Controlled Synthesis of SnO2 Nanotubes by Using 1D Silica Mesostructures as Sacrificial Templates and Their Applications in Lithium-Ion Batteries. Small 2010, 6, 296-306. (22) Yuan, C.; Hou, L.; Feng, Y.; Xiong, S.; Zhang, X. Sacrificial Template Synthesis of Short Mesoporous NiO Nanotubes and Their Application in Electrochemical Capacitors. Electrochim. Acta 2013, 88, 507-512. (23) Bianco, A.; Kostarelos, K.; Prato, M. Applications of Carbon Nanotubes in Drug Delivery. Curr. Opin. Chem. Biol. 2005, 9, 674-679. 17 Environment ACS Paragon Plus


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

(24) Cao, W.; Duan, Y. Breath Analysis: Potential for Clinical Diagnosis and Exposure Assessment. Clin. Chem. 2006, 52, 800-811. (25) Xing, R.; Li, Q.; Xia, L.; Song, J.; Xu, L.; Zhang, J.; Xie, Y.; Song, H. Au-Modified Three-Dimensional In2O3 Inverse Opals: Synthesis and Improved Performance for Acetone Sensing Toward Diagnosis of Diabetes. Nanoscale 2015, 7, 13051-13060. (26) Kim, K.-H.; Jahan, S. A.; Kabir, E. A Review of Breath Analysis for Diagnosis of Human Health. TrAC-Trends Anal. Chem. 2012, 33, 1-8. (27) Mazzone, P. J.; Wang, X.-F.; Xu, Y.; Mekhail, T.; Beukemann, M. C.; Na, J.; Kemling, J. W.; Suslick, K. S.; Sasidhar, M. Exhaled Breath Analysis with a Colorimetric Sensor Array for the Identification and Characterization of Lung Cancer. J. Thorac. Oncol. 2012, 7, 137-8. (28) Kalapos, M. Possible Physiological Roles of Acetone Metabolism in Humans. Med. Hhypotheses 1999, 53, 236-242. (29) Bai, X.; Ji, H.; Gao, P.; Zhang, Y.; Sun, X. Morphology, Phase Structure and Acetone Sensitive Properties of Copper-Doped Tungsten Oxide Sensors. Sens. Actuators B 2014, 193, 100-106. (30) Lord, H.; Yu, Y.; Segal, A.; Pawliszyn, J. Breath Analysis and Monitoring by Membrane Extraction with Sorbent Interface. Anal. Chem. 2002, 74, 5650-5657. (31) Phillips, M.; Herrera, J.; Krishnan, S.; Zain, M.; Greenberg, J.; Cataneo, R. N. Variation in Volatile Organic Compounds in the Breath of Normal Humans. J. Chromatogr. B 1999, 729, 75-88. (32) Lewis, S. E.; DeBoer, J. R.; Gole, J. L.; Hesketh, P. J. Sensitive, Selective, and Analytical Improvements to a Porous Silicon Gas Sensor. Sens. Actuators B 2005, 110, 54-65. (33) Ma, D.; Lee, C.; Au, F.; Tong, S.; Lee, S. Small-Diameter Silicon Nanowire Surfaces. Science 2003, 299, 1874-1877. 18 Environment ACS Paragon Plus

Page 18 of 28

Page 19 of 28

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


(34) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Nanowire Nanosensors for Highly Sensitive and Selective Detection of Biological and Chemical Species. Science 2001, 293, 12891292. (35) Zheng, G.; Gao, X. P.; Lieber, C. M. Frequency Domain Detection of Biomolecules Using Silicon Nanowire Biosensors. Nano Lett. 2010, 10, 3179-3183. (36) Gao, X. P.; Zheng, G.; Lieber, C. M. Subthreshold Regime Has the Optimal Sensitivity for Nanowire FET Biosensors. Nano Lett. 2009, 10, 547-552. (37) Li, M.; Hu, M.; Zeng, P.; Ma, S.; Yan, W.; Qin, Y. Effect of Etching Current Density on Microstructure and NH3-Sensing Properties of Porous Silicon with Intermediate-Sized Pores. Electrochim. Acta 2013, 108, 167-174. (38) Demami, F.; Ni, L.; Rogel, R.; Salaun, A.-C.; Pichon, L. Silicon Nanowires Based Resistors as Gas Sensors. Sens. Actuators B 2012, 170, 158-162. (39) Moshnikov, V. A.; Gracheva, I.; Lenshin, A. S.; Spivak, Y. M.; Anchkov, M. G.; Kuznetsov, V. V.; Olchowik, J. M. Porous Silicon with Embedded Metal Oxides for Gas Sensing Applications. J. Non-cryst. Solids 2012, 358, 590-595. (40) Peng, K.-Q.; Wang, X.; Lee, S.-T. Gas Sensing Properties of Single Crystalline Porous Silicon Nanowires. Appl. Phys. Lett. 2009, 95, 243112. (41) Yang, L.; Lin, H.; Zhang, Z.; Cheng, L.; Ye, S.; Shao, M. Gas Sensing of TelluriumModified Silicon Nanowires to Ammonia and Propylamine. Sens. Actuators B 2013, 177, 260-264. (42) Gedamu, D.; Paulowicz, I.; Kaps, S.; Lupan, O.; Wille, S.; Haidarschin, G.; Mishra, Y. K.; Adelung, R. Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors. Adv. Mater. 2014, 26, 1541-1550. (43) Paulowicz, I.; Hrkac, V.; Kaps, S.; Cretu, V.; Lupan, O.; Braniste, T.; Duppel, V.; Tiginyanu, I.; Kienle, L.; Adelung, R. Three‐Dimensional SnO2 Nanowire Networks



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

for Multifunctional Applications: From High‐Temperature Stretchable Ceramics to Ultraresponsive Sensors. Adv. Electron. Mater. 2015, 1. (44) Lupan, O.; Postica, V.; Cretu, V.; Wolff, N.; Duppel, V.; Kienle, L.; Adelung, R. Single and Networked CuO Nanowires for Highly Sensitive p‐type Semiconductor Gas Sensor Applications. Phys. Status Solidi RRL 2016, 10, 260-266. (45) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K.; Schatz, G. C.; Zheng, J. Photoinduced Conversion of Silver Nanospheres to Nanoprisms. Science 2001, 294, 1901-1903. (46) Baker, G. A., Book Review: Nanowires and Nanobelts. By Zhong Lin Wang (Ed.). Wiley Online Library: 2005. (47) Hölken, I.; Schröder, S.; Adelung, R. In Characterisation of Silicon Nanolayers Deposited by Plasma Enhanced Chemical Vapor Deposition on 3-D ZnO Templates for Hollow Silicon Microstructures, 3rd International Conference on Nanotechnologies and Biomedical Engineering, Springer: 2016; pp 30-34. (48) Lupan, O.; Cretu, V.; Postica, V.; Ahmadi, M.; Cuenya, B. R.; Chow, L.; Tiginyanu, I.; Viana, B.; Pauporté, T.; Adelung, R. Silver-doped Zinc Oxide Single Nanowire Multifunctional Nanosensor with a Significant Enhancement in Response. Sens. Actuators B 2016, 223, 893-903. (49) Lupan, O.; Chai, G.; Chow, L. Novel Hydrogen Gas Sensor Based on Single ZnO Nanorod. Microelectron. Eng. 2008, 85, 2220-2225. (50) Malis, T.; Cheng, S. C.; Egerton, R. F. EELS Log-Ratio Technique for SpecimenThickness Measurement in the TEM. J. Electron Microsc. Tech. 1988, 8, 193-200. (51) Zacharias, M.; Bläsing, J.; Veit, P.; Tsybeskov, L.; Hirschman, K.; Fauchet, P. M. Thermal Crystallization of Amorphous Si/SiO2 Superlattices. Appl. Phys. Lett. 1999, 74, 2614-2616. (52) Lupan, O.; Cretu, V.; Deng, M.; Gedamu, D.; Paulowicz, I.; Kaps, S. r.; Mishra, Y. K.; Polonskyi, O.; Zamponi, C.; Kienle, L. Versatile Growth of Freestanding Orthorhombic 20 Environment ACS Paragon Plus

Page 20 of 28

Page 21 of 28

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α-Molybdenum Trioxide Nano-and Microstructures by Rapid Thermal Processing for Gas Nanosensors. J. Phys. Chem. C 2014, 118, 15068-15078. (53) Zhou, X.; Hu, J.; Li, C.; Ma, D.; Lee, C.; Lee, S. Silicon Nanowires as Chemical Sensors. Chem. Phys. Lett. 2003, 369, 220-224. (54) Jie, J.; Zhang, W.; Peng, K.; Yuan, G.; Lee, C. S.; Lee, S. T. Surface‐Dominated Transport Properties of Silicon Nanowires. Adv. Funct. Mater. 2008, 18, 3251-3257. (55) Cheng, J.; Wang, B.; Zhao, M.; Liu, F.; Zhang, X. Nickel-doped Tin Oxide Hollow Nanofibers Prepared by Electrospinning for Acetone Sensing. Sens. Actuators B 2014, 190, 78-85. (56) Qin, L.; Xu, J.; Dong, X.; Pan, Q.; Cheng, Z.; Xiang, Q.; Li, F. The Template-Free Synthesis of Square-Shaped SnO2 Nanowires: the Temperature Effect and Acetone Gas Sensors. Nanotechnology 2008, 19, 185705. (57) Qi, Q.; Zhang, T.; Liu, L.; Zheng, X.; Yu, Q.; Zeng, Y.; Yang, H. Selective Acetone Sensor Based on Dumbbell-like ZnO with Rapid Response and Recovery. Sens. Actuators B 2008, 134, 166-170. (58) Zeng, Y.; Zhang, T.; Yuan, M.; Kang, M.; Lu, G.; Wang, R.; Fan, H.; He, Y.; Yang, H. Growth and Selective Acetone Detection Based on ZnO Nanorod Arrays. Sens. Actuators B 2009, 143, 93-98. (59) Barreca, D.; Comini, E.; Gasparotto, A.; Maccato, C.; Sada, C.; Sberveglieri, G.; Tondello, E. Chemical Vapor Deposition of Copper Oxide Films and Entangled Quasi1D Nanoarchitectures as Innovative Gas Sensors. Sens. Actuators B 2009, 141, 270-275. (60) Zhou, L.-J.; Zou, Y.-C.; Zhao, J.; Wang, P.-P.; Feng, L.-L.; Sun, L.-W.; Wang, D.-J.; Li, G.-D. Facile Synthesis of Highly Stable and Porous Cu2O/CuO Cubes with Enhanced Gas Sensing Properties. Sens. Actuators B 2013, 188, 533-539.



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(61) Zhou, X.; Li, X.; Sun, H.; Sun, P.; Liang, X.; Liu, F.; Hu, X.; Lu, G. NanosheetAssembled ZnFe2O4 Hollow Microspheres for High-Sensitive Acetone Sensor. ACS Appl. Mater. Interfaces 2015, 7, 15414-15421. (62) Li, X.; Wang, C.; Guo, H.; Sun, P.; Liu, F.; Liang, X.; Lu, G. Double-Shell Architectures of ZnFe2O4 Nanosheets on ZnO Hollow Spheres for High-Performance Gas Sensors. ACS Appl. Mater. Interfaces 2015, 7, 17811-17818.


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Figure 1. SEM images of the hollow Si tetrapod 3-D networks, Si-T (a,b). (c) High magnification SEM image demonstrating columnar growth of the Si-T. SEM images of: (d) ZnO nanostructures; and (e,f) Si spherical nanostructures, Si-S, at different magnifications.



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Figure 2. (a) TEM brightfield image of an agglomeration of spherical Si particles. (b) Single particle showing inhomogenous wall thickness. (c) ED pattern of the particle wall, indices refer to the structure of silicon.[38] (d) TEM brightfield image of two adjacent tetrapods. (e) Enlarged section from a fractioned tetrapod. (f) EF-ED pattern of a tetrapod wall. Crystallization and grain coarsening during in-situ heating (g) amorphous Si at RT; (h) spontaeous crystallization at 780 °C; (i) Grain coarsening at 920 °C. Insets display corresponding SAED patterns.


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Figure 3. (a) Schematic of the hollow Si-T synthesis process. In the first step the Si layer is deposited on ZnO-T by PECVD. After removing the ZnO-T template by chemical etching, hollow Si-T remains. For each step a cross-section of a tetrapod arm is represented. (b) Integration of hollow Si-T 3-D networks in the sensor structure. Gas response versus operating temperature of the samples from: (c) Si-T; (d) Si-S. (e) Dynamic response of the sensor based on Si-T samples to 100 ppm acetone at 200 °C and of the sensor based on Si-S samples to 50 ppm NH3 at 175 °C.



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Figure 4. (a) SEM images of the Si-T transferred to SiO2/Si substrate with pre-patterned Au/Cr electrical contacts by direct touching and further gradual dispersion to a lower density (b,c) for further fabrication of nanodevices. (d) SEM image of the fabricated nanodevice based on a single Si-T arm. (e) Schematic representation of the nanodevice. Both ends of the arm were connected to Au/Cr contacts by a Pt complex. Connection for the electrical measurements is also presented. (f) Dynamic response of the nanodevice based on single Si-T arm to 100 ppm acetone at room temperature.


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Table 1. Process parameters for the deposition of silicon films on ZnO templates. Temperature Pressure

Power

Time

SiH4 in Ar flow

Ar flow

200 °C

15 W

15 min

120 sccm

480 sccm

86.65 Pa

Table 2. Acetone sensors based on silicon and semiconducting oxides nanostructures Acetone conc. (ppm)

Gas response (Rg/Ra)a) or (Ra/Rg)b)

Operating temp. (°°C)

Response time (s)

Recovery time (s)

Ref.

SnO2 NWs

50

~8

290

~7

~ 10

56

Ni-doped SnO2 nanofibers

50

~ 46

340

7

30

55

Dumbbell-like ZnO

50

~ 16

300

1.5

3

57

ZnO nanorod thin film

50

~ 15

300

5

15

58

CuO nanostructured films

10

~4

200

150

490

59

Cu2O /CuO cubes

500

~ 10

150

1

25

60

Au/In2O3 inverse opal

5

~ 42

340

~ 11

14

25

Cu-doped WO3 fibers

20

~7

300

5

20

29

ZnFe2O4 microspheres

20

~ 11

215

~ 11

200

61

ZnO/ ZnFe2O4 composites

100

~ 17

250

1

33

62

Hollow Si microspheres, SiS

100

~ 26

200

16

7

This work

Single Si-Tetrapod Arm, SiT

100 10

~ 60 ~ 4.1

RT

3.1 4.45

54.8 43.4

Morphology and properties of sensing material

a)

This work

Gas response for p-type semiconducting oxides; b) Gas response for n-type semiconducting oxides;



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ToC The table of contents entry. By a sacrificial template synthesis, formation of hollow silicon nano- and microstructures is successfully demonstrated. Any shape can be reproduced and thereby replicated by the developed technological approach using a large variety of morphologies of ZnO architectures, which serves as sacrificial template. The new Si-T and Si-S were found as most stable hollow nanomaterials at high temperatures, up to 1000 °C.

Iris Hölken, 1 Gero Neubüser,1 Vasile Postica,2 Lars Bumke,1 Oleg Lupan,1,* Martina Baum, 1 Yogendra Mishra,1,* Lorenz Kienle,1,* Rainer Adelung1,* *

Corresponding Author* Dr. Lupan

Sacrificial template synthesis and properties of high-temperature stable 3-D hollow-silicon microstructures ToC figure


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Sacrificial Template Synthesis and Properties of 3D Hollow-Silicon

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