Growth, structure, luminescence and mechanical resonance of Bi₂O₃ nano- and microwires

Abstract

α-Bi₂O₃ hierarchical structures formed by multibranched micro- and nanowires have been grown by an evaporation–deposition method. The morphology, composition, structure, and several physical properties of the obtained nanowires have been investigated. The relative intensities of photoluminescence bands related to Bi²⁺ ions and oxygen vacancies have been found to depend on the composition of the precursor used in the growth process. In situ scanning electron microscope measurements of the mechanical resonance frequency of the microwires have been used to determine their Young's modulus, which was found to depend on the wire dimensions, with values ranging from 11 GPa to 284 GPa. The quality factors measured suggest that Bi₂O₃ wires may have potential applications as micromechanical resonators.

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1. Introduction

Semiconductor nanowires are considered as essential building blocks for the development of future electronic and optical nanodevices.^1–3 In particular, the growth and physical properties of nanowires of certain binary oxides – such as ZnO, In₂O₃ or SnO₂ – have been often reported,^4–6 while elongated nanostructures of other oxides of technological interest have been investigated to a much lesser extent. This is the case of Bi₂O₃, a wide band gap semiconductor with reported E_g values ranging between 2.85 eV and 3.25 eV,^7,8 which shows a good photoconductive behaviour, high ionic conductivity and refractive index and can be also used as a host of optically active ions.^9–12 Bi₂O₃ has four main crystallographic polymorphs. The monoclinic α phase and the face-centred cubic δ phase are stable at room and high temperature, respectively. Two metastable phases, the tetragonal β phase and the body-centred cubic γ phase, result from the cooling of the high-temperature δ phase. Mixed phase (α and β) Bi₂O₃ nanowires have been synthesized by a thermal oxidation method,¹³ while β phase nanowires and nanorods have been grown by chemical vapour deposition^14–16 and thermal evaporation of Bi.¹⁷ The reported α-phase elongated structures include nanowires obtained by chemical routes¹⁸ as well as nano- and microribbons¹⁹ and microrods⁸ grown by vapour deposition methods.

Hierarchical growth leading to the formation of branched structures and, in a more advanced step, to the formation of larger functional units, such as nanowire networks, is a subject of interest related to the “bottom up” fabrication of nanomaterials.^20,21 The formation of branched nanostructures and networks, self-assembled during evaporation–deposition processes, has been reported for a number of oxide semiconductors. It has been found that the addition of certain metallic elements, such as In, Sn or Al, favours the growth of the mentioned complex arrangements.^22–27 Thermal growth of hierarchical or, in general, complex branched nanostructures has been reported for ZnO when small amounts of In₂O₃,²² SnO₂,^23,24 Eu₂O₃ (ref. 25) or Al₂O₃ (ref. 26) were added to the precursor material. Morphological changes have been also reported in the addition of SnO₂ to the precursor used to grow GeO₂ (ref. 27) or Ga₂O₃ (ref. 28) nanowires. The hierarchical growth reported in ref. 22 has been proposed to be related to surface segregation of Sn and this model appears to agree with the side branching observed in ref. 23 and 24. In this work, tin or erbium oxides have been incorporated by a vapour–solid method into a Bi precursor powder to grow Bi₂O₃ hierarchical nanostructures formed by rods and nanowires.

Reports on the physical properties of nanowires and other low dimensional structures of Bi₂O₃ are scarce. For this reason, in addition to the morphological, structural and compositional characterization of the grown nanowires – that were carried out by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) in the SEM and energy-dispersive X-ray microanalysis (EDX) both in the SEM and TEM – their luminescence properties and some aspects of their mechanical behaviour have been investigated. Luminescence was studied by photoluminescence (PL) in a confocal microscope, while the Young's modulus was determined by in situ SEM measurements of mechanical resonances under an applied electric field. The investigation of mechanical properties on the micro- and nanoscale is an important topic for the development and reliability improvement of nano- and microelectromechanical systems (MEMS/NEMS) and other devices.

2. Experimental

The precursors used to grow the nanowires were mixtures of Bi powder (Goodfellow, 99.997%) with either SnO₂ (Sigma-Aldrich, 99.9%) or Er₂O₃ (Strem, 99.9%) powders. Mixtures containing Bi and 2, 5 10, 20 or 30 wt.% of SnO₂ were compacted to form disk-shaped samples of about 7 mm diameter and 1 mm thickness. Five different mixtures containing 2, 5 10, 20 or 30 wt.% of Er₂O₃ were prepared following the same method. The samples were then annealed in a horizontal tube furnace, not sealed for vacuum conditions, under argon flow at 800 °C for times between 4 and 10 hours. Transport and oxidation of bismuth took place during the thermal treatments, leading to the growth of bismuth oxide nanostructures, nanowires and complex nanowire hierarchical arrangements on the tube's wall. The structures were detached from the tube and deposited on a silicon substrate for characterization. Their morphology and size were investigated with a FEI Inspect SEM. The structure of the wires was investigated by XRD with an X'Pert PRO MRD diffractometer using Cu Kα radiation and by HRTEM with a Jeol JEM 2010F and a Jeol JEM 3100F microscope. EBSD of single microwires was performed in a FEI Inspect SEM with a Bruker e-FlashHR⁺ system using an accelerating voltage of 20 kV. The composition of the samples was assessed by EDX in the SEM and TEM by using a Bruker QUANTAX and an Oxford INCA system, respectively. PL measurements were carried out at room temperature in a Horiba Jovin-Yvon LabRAM HR800 system. The samples were excited by a 325 nm He–Cd laser on an Olympus BX 41 confocal microscope. The study of mechanical resonances was carried out on Bi₂O₃ microwires with lengths ranging from 100 to 700 μm and cross-sectional dimensions of 0.5–3.5 μm. For the synthesis of these larger structures, no Er or Sn oxides were added to the precursor and the growth took place on the compacted Bi₂O₃ disk during annealing at 650 °C for 1.5 h under argon flow. In order to investigate the resonant behaviour of these samples, the disk with the microwires was glued with silver paste on a copper electrode and placed in front of a second copper electrode inside the chamber of a FEI Inspect SEM, as shown in Fig. 1. An oscillating voltage with an amplitude of 3–5 V and a frequency range of 0.1–100 kHz was applied with a Stanford Research SR830 DSP lock-in amplifier, while a 20 V constant voltage bias was simultaneously applied with a Keithley 2400 SourceMeter. The amplitude of the oscillating wire was measured, from the SEM images, as a function of the voltage frequency and the resonance frequency corresponding to the amplitude maximum was determined. The Young's modulus of the wire was then calculated from elasticity theory by using the experimental resonance frequencies.

Fig. 1 Experimental setup used for in situ mechanical resonance measurements in the SEM.

3. Results and discussion

3.1 Morphology, structure and composition

Hierarchical structures formed by a network of branched wires growing from a central axis are observed after the thermal treatment at 800 °C for 10 h of the SnO₂ containing precursor, as shown in the low-magnification SEM image in Fig. 2a. These complex architectures were not obtained for shorter treatments. A higher magnification SEM image (Fig. 2b) shows secondary parallel rods with widths of about 1 to 3 μm and smaller perpendicular branches, most of them with cross-sectional dimensions of some hundreds of nanometers. Similar hierarchical structures, as well as wool-like arrangements of thinner wires (100–200 nm diameter), are formed when the Er-containing precursor is treated also at 800 °C for 10 h (Fig. 2c–e). Mixtures with oxide contents (either SnO₂ or Er₂O₃) below 20 wt.% give rise to a lower amount of hierarchical structures, while no noticeable difference in terms of morphology or density of branched structures was observed when a mixture containing 30 wt.% of the corresponding oxide was used. In addition, when a mesh of pure Bi was used as the only precursor, just large microrods and microcrystals, without any hierarchical structures, were obtained.⁸ This shows that for Bi₂O₃, as in the case of ref. 21–27, the addition of an atomic fraction of certain cations to the precursor favours the hierarchical growth of oxide nanostructures. It is to be noticed that, contrary to the well-known catalyst-assisted vapour–liquid–solid (VLS) nanowire growth, no catalyst nanodrops at the tip of the wires were observed. In fact – as described below – according to the analytical techniques used in the present work, significant amounts of Er and Sn were not incorporated into the as-grown nanostructures. This is probably due to the big difference between our growth temperature (800 °C) and the melting and boiling temperatures of Er₂O₃ (2345 °C and 3290 °C, respectively) and SnO₂ (1630 °C and 1800 °C, respectively). Although the obtained nanowires do not show a tapered morphology or Bi droplets at their tips, as confirmed by SEM, TEM and EDX measurements, autocatalytic processes cannot be in principle completely discarded. A small thin Bi layer, even atomically flat, might be present at the growth front, which could lead to growth via a VLS mechanism. The layer could be quickly oxidized during cooling or after exposure of the nanostructures to air after the growth, and a subsequent analysis may never find this layer.²⁹ Nevertheless, in the particular case of Bi₂O₃, the formation of thin Bi layers or droplets has been reported to give rise to samples with micro- or nanoribbon morphology.¹⁹ In any case, as previously mentioned, hierarchical Bi₂O₃ arrangements were not obtained after thermal treatments of pure Bi, while a lower density of such arrangements was obtained when mixtures of Bi with low SnO₂ or Er₂O₃ contents were used as precursors. These observations indicate that the addition of both metal oxides play a critical role in the growth of our branched, hierarchical nanostructures, probably increasing the number of nucleation sites, which is related to the surface energy of the crystal, temperature, and the supersaturated ratio of vapor.³⁰ We believe that growth in the present case is governed by a vapour–solid (VS) process, in which the vapour originates from the precursor in a high-temperature zone and thereby facilitates the formation of the micro- and nanostructures in a low-temperature zone by the assistance of the Ar flow. The VS mechanism has been invoked in order to explain the formation of several hierarchical nanostructures in the presence of alloying elements.^22–26 The fact that side branching is observed only in the presence of such elements strongly suggests that surface segregation may play an important role. During the growth of the central nanowire, the alloying content is continuously segregated to the boundaries of the nanowire core. The alloy-rich layer of the boundaries became the nucleation sites for the subsequent growth of side branches.²² As stated before, this mechanism has been proposed to explain the formation of ZnO hierarchical nanostructures when small amounts of In₂O₃,²² SnO₂,^23,24 Eu₂O₃ (ref. 25) or Al₂O₃ (ref. 26) were added to the precursor material. In the present case, no segregation was detected by SEM or TEM-EDX spectroscopy in the main trunk or in the branches of the structures, but incorporation of a small concentration of metallic elements (Sn and Er) below the EDX detection limit is certainly possible. In fact, previous studies reported alloying metallic element contents below 0.5 at% in certain areas of the hierarchical nanostructures, while in others such elements were not detected.^25,26

Fig. 2 (a) SEM image of a hierarchical structure grown from the precursor containing SnO₂. (b) SEM image of the same structure showing branched secondary parallel rods. SEM images (c) and (d) show the hierarchical structures obtained after thermal treatment of the precursor with Er₂O₃, while figure (e) shows a wool-like arrangement of the nanowires obtained by using the same precursor.

The XRD spectra of all the grown samples were indexed to the monoclinic α-Bi₂O₃ phase (JCPDS card 041-1449), regardless of the precursor used and the final morphology of the structures (Fig. 3). Moreover, no diffraction maxima related to pure Sn, Er or their corresponding oxides were observed. EDX spectra in SEM (Fig. 4) or TEM reveal only peaks corresponding to Bi and O and no Sn or Er signals were detected. For these reasons, the structures obtained from both kinds of precursors were considered to be equivalent regarding crystallography and composition. Since the presence of Er₂O₃ yielded thin nanowires and a higher density of branched structures, mainly the samples obtained from the precursor mixture Bi–Er₂O₃ were further investigated.

Fig. 3 XRD pattern representative of the hierarchical structures. All the diffraction maxima can be indexed to the monoclinic α-Bi₂O₃ phase (JCPDS card 041-1449).

Fig. 4 SEM-EDX spectrum of Bi₂O₃ nanowires grown from the precursor containing Er₂O₃. Only peaks related to O and Bi can be appreciated.

Individual nanowires with diameters of several hundreds of nanometres were selected for EBSD characterization in the SEM. EBSD allows crystallographic information to be obtained from samples in the SEM by striking an electron beam on a tilted sample and obtaining the diffracted electron pattern with the aid of a fluorescent phosphor screen. The variations in the intensity of the diffracted electrons as a function of direction form patterns of Kikuchi lines. These diffraction patterns are then analyzed by computer software and used to measure the wire crystal orientation, discriminate between different materials and provide information about extended defects. EBSD was found to be particularly useful for the structural characterization of our Bi₂O₃ thicker structures, which cannot be properly investigated by TEM. Such structures usually show smooth, well-faceted faces, where EBSD measurements can be carried out. A representative example of the acquired patterns is displayed in Fig. 5. The diffraction patterns did not change when the electron beam was moved along the length of the wire. The obtained results reveal that all the wires investigated are of monoclinic structure and belong to the α-Bi₂O₃ phase, usually with a [011] zone axis. Our EBSD studies did not reveal the presence of extended defects in these samples.

Fig. 5 Indexed EBSD pattern of a 500 nm wide Bi₂O₃ wire with the symmetries of the monoclinic α phase.

Low-magnification TEM images of the secondary nanowires (those that stem from the central axis) show branched nanostructures with lengths up to 15 μm and diameters in the 40–300 nm range (Fig. 6a). A high-resolution micrograph of a 100 nm wide nanowire and the corresponding FFT pattern are shown in Fig. 6b. Indexing of the FFT pattern and analysis of this and other images confirm that the wires belong to the monoclinic α-phase, as it was already reported for Bi₂O₃ nanowires grown by a similar procedure.³¹ The mentioned image shows fringes running perpendicular to the edge of the nanowire. The fringe spacing measures 0.82 nm, which corresponds to the (010) interplanar spacing. This shows the single crystalline nature of the wire that grows along the [010] direction. A TEM image of the junction between two perpendicular nanowires is shown in Fig. 7a. HRTEM images of these regions, as that shown in Fig. 7b, revealed that the branched Bi₂O₃ nanowires have single-crystal structures with clean backbone-to-branch junctions. No extended defects were found in our HRTEM observations of similar areas, which strongly suggest that growth mechanisms driven by screw dislocations³² or twins³³ may be ruled out.

Fig. 6 (a) Low-magnification TEM image showing a group of branched nanowires. (b) HRTEM image of a 100 nm wide nanowire showing the (010) interplanar distance of α-Bi₂O₃. The inset shows the corresponding indexed FFT pattern.

Fig. 7 (a) TEM micrograph of the junction between two perpendicular nanowires. (b) HRTEM image of the same area showing single-crystal structures with no extended defects.

3.2 Luminescence

The influence of the precursor on the luminescence of the nanowires has been investigated by PL excited by 325 nm laser light. Although no Sn or Er has been detected in the analysis of the nanowires, the luminescence can provide information on the possible indirect effects of the addition of their oxides on the final defect structure and, in the case of Er, on the presence of traces of optically active ions. In fact, the addition of Al₂O₃ to a ZnS precursor used to grow ZnO structures with the same vapour–solid method used in this work has been found to induce the formation of branched nanorod networks and cathodoluminescence (CL) spectral changes without apparent Al incorporation into the structures, as measured by EDX.²⁶ These studies demonstrated that nanorod networks fabricated with an Al₂O₃-containing precursor show an emission band, attributed to oxygen excess, which is not observed in materials prepared without Al₂O₃ or other oxides. In the present work, we compare the PL of the nanowires with previously reported PL studies of Bi₂O₃ sintered ceramics³⁴ and nanostructures.^13,35Fig. 8 shows the PL spectra of the nanowires grown with SnO₂ or Er₂O₃ in the precursor and a representative spectrum of Bi₂O₃ ceramics prepared by sintering pure Bi₂O₃ powders at 500 °C for 10 h. The obtained results indicate that modifying the composition of the source materials changes not only the morphology of the grown oxide nanostructures, but also their luminescence properties. Actually, it can be observed that the three spectra have their maxima at different energies. The observed shifts cannot be attributed to quantum confinement effects or residual stresses within the Bi₂O₃ nanowires,¹³ since SEM images reveal nanostructures with dimensions above 100 nm, while TEM observations provide no evidence of lattice distortions. As explained below, the different peak positions observed in the PL spectra of nanowires grown by adding the mentioned two oxides are due to the relative weights of the different emission bands contributing to the overall spectrum in each case.

Fig. 8 PL spectra of α-Bi₂O₃ nanowires grown from different precursors and of a bulk ceramic sample sintered for comparison purposes.

The spectrum of the ceramic sample is peaked at 2.33 eV (532 nm), but previous CL investigations reveal the existence of other components at about 1.91 eV and 2.83 eV.³⁴ The 2.33 eV and the 1.95 eV emission bands have been attributed to a Bi ion in the closest environment of three oxygen ions.³⁶ Actually, trivalent bismuth cations in inorganic compounds exhibit interesting luminescence properties related to their 6s² configuration.^37,38 Above 80 K, radiative transitions are mainly assigned to ³P₁ → ¹S₀ transitions^13,35,37 or charge transfer transitions between oxygen ligands and Bi³⁺ ions,³⁸ giving rise to luminescence spectra peaked in the blue to green region under UV excitation. On the other hand, Bi²⁺ ions show PL emission in the range 2.10–1.95 eV under UV excitation. This luminescence has been attributed to ²P_3/2(1) → ²P_1/2 transitions^35,39,40 and it is extremely dependent on the local crystal field, which in turn depends on the symmetry and coordination of the surrounding oxygen atoms. Our luminescence studies of Bi₂O₃ bulk samples annealed in different atmospheres have shown that a 2.1 eV band is related to oxygen vacancies.³⁴Fig. 8 shows that the PL bands of the nanowires synthesized with precursors containing SnO₂ and Er₂O₃ appear peaked at 2.12 eV (585 nm) and 1.92 eV (646 nm), respectively. This indicates that the presence of Sn in the precursor favours the growth of nanowires with a higher concentration of oxygen vacancies. The diffusion of Bi ions has been previously considered to predominate over the diffusion of oxygen ions during the growth of complex Bi₂O₃ architectures by thermal oxidation of Bi, which resulted in the formation of a high density of oxygen vacancies.⁴¹ In the case of the samples grown from the Er-containing precursor, the site symmetry and oxygen coordination around the reduced Bi ions favour an intense PL signal related to Bi²⁺ intraionic transitions.

3.3 Young's modulus

The characterization of the mechanical properties of semiconductor nanowires and other elongated nanostructures is a subject of interest related to their applications in NEMS, micromechanical sensors and optomechanical coupling devices.⁴² The Young's modulus of elongated nanostructures of different materials has been determined by electric field-induced mechanical resonance, nanoindentation and atomic force microscopy. For reviews on mechanical characterization of nanowires, see ref. 42 and 43. In situ TEM measurements of mechanical resonance under an applied electric field has enabled the determination of the Young's modulus of ZnO nanobelts,⁴⁴ BN nanotubes,⁴⁵ carbon nanotubes,⁴⁶ or GaN nanowires.⁴⁷ Similar in situ SEM studies, which would enable one to investigate larger nanowires, have been only occasionally reported.⁴⁸ Measurements by mechanical resonance techniques and AFM have shown, in some cases, a strong dependence of the Young's modulus of elongated nanostructures on geometrical parameters, such as the diameter of nanotubes⁴³ and nanowires⁴⁸ or the width-to-thickness ratio of nanobelts.⁴⁹ In order to study such potential effects in our nanowires, the Young's modulus has been determined for a total of fourteen wires with different lengths and cross-sectional dimensions. By applying an alternating potential at the electrodes in the arrangement shown in the schematic in Fig. 1, the nanowires are deflected by the electrostatic force, leading to oscillations with the potential frequency. By measuring in the SEM screen the oscillation amplitude as a function of the frequency, an amplitude peak appears at the resonance frequency of the nanowire. In the present case, the resulting first harmonic resonance frequencies vary from 2 to 17 kHz. From elasticity theory,⁵⁰ this resonance frequency iswhere β_i is a constant for the i-th harmonic, L the length, E the Young's modulus, I the second moment of inertia (I = πr4/4 for a cylindrical wire), ρ the density (8.9 g cm⁻³ for α-Bi₂O₃) and S the cross-sectional area. From eqn (1), the Young's modulus iswhich, for the first harmonic (β₁ = 1.875) and a cylindrical wire, is

E = (νL²/0.28r)²ρ (3)

where r is the radius of the nanowire.

Fig. 9a and b, respectively, show SEM images of a 250 μm long wire of 1 μm radius at rest and vibrating under the applied electric field. The amplitude–frequency curve of a wire of 500 nm radius is shown in Fig. 10a. The resonance frequency measurements of fourteen wires of different radii and lengths yielded a noticeable dispersion of E values, which ranged from 11 to 284 GPa. The relationship of the Young's modulus to the radius of the wires has been plotted in Fig. 10b. A general trend of a lower Young's modulus for a higher radius can be appreciated. Such a trend is qualitatively similar to that reported for ZnO,⁴⁸ CuO,⁵¹ WO₃,^52,53 Ag and Pb nanowires,⁵⁴ but in the present case the effect is observed for larger radii. In some studies, the measured values of E approach the bulk value as the radius of the nanowire increases.^47,48,51 This size dependence has been explained for ZnO nanowires in terms of surface stiffening arising from the gradually shortened bond lengths from the nanostructure central region to the surface.⁴⁸ A similar trend has also been reported for CuO nanowires and attributed to the formation of an amorphous layer in the smaller nanostructures.⁵¹ However, in other cases the opposite trend has been observed, i.e., the Young's modulus approaches the bulk value as the radii of the nanowires decrease.^52,53 This has been explained by differences in the defect structure of nanowires with different diameters. Since we are not aware of previous studies reporting the Young's modulus of α-Bi₂O₃, a bulk ceramic sample of this oxide was prepared by sintering pure Bi₂O₃ powder at 750 °C for 10 h. The ceramic was mirror-polished and the corresponding Young's modulus was determined by nanoindentation.⁵⁵ The obtained value, 105 ± 4 GPa, is included in the interval shown in Fig. 10a and is also in good agreement with the 104 GPa value that can be deduced from the elastic constants calculated by Music et al.⁵⁶ for bulk δ-Bi₂O₃. These results suggest that surface effects do not play a dominant role in the observed trend of E as a function of the wire radius.

Fig. 9 SEM images of an α-Bi₂O₃ nanowire at rest (a) and vibrating under an applied alternating electric field (b).

Fig. 10 (a) Response curve for the resonance of a Bi₂O₃ microwire. The solid line is a Lorentzian fit for the experimental data, from which the Q factor is estimated to be 740. (b) Young's modulus of the microwires plotted as a function of their radii.

The quality factor Q of the microwires as mechanical resonators can be estimated from fitting of the resonance curves to Lorentzian profiles, as expected for damped harmonic vibrations, according to the relationship Q = νr/Δν, where νr is the peak resonance frequency and Δν is the full width at half maximum of the resonance peak. The inverse factor 1/Q is related to energy dissipation. Since the wires vibrate at SEM vacuum (~10⁻⁶ Torr), the energy dissipation arising from air damping can be neglected; it mainly originates from the intrinsic properties of the material. Measurements of Q in the obtained resonance curves yield values in the range 650–750. Reported quality factors for mechanical resonances of semiconducting nanowires or nanotubes extend over a wide range of values, which are in many cases comparable to those measured in this work, e.g., 120 for V₂O₅ nanowires,⁵⁷ 20–750 for SiO2 nanowires,⁵⁸ 340 for boron nitride nanotubes,⁴⁵ 500 for ZnO nanobelts⁴⁴ or 80–700 for MoS2 diaphragms.⁵⁹ These results show that Bi₂O₃ wires may have potential application as micromechanical resonators.

4. Conclusions

Hierarchical Bi₂O₃ nano- and microwires grown by a thermal evaporation–deposition method have been investigated. The addition of SnO₂ or Er₂O₃ to the main Bi precursor favours the formation of hierarchical, multibranched arrangements. The incorporation of Er or Sn into Bi₂O₃ nanostructures was not detected by SEM or TEM-EDX microanalysis, but both metallic elements may be present in concentrations below the detection limit of this analytical technique. XRD, EBSD and HRTEM measurements reveal the good crystallinity of the grown samples, the structure of which corresponds to the monoclinic α-Bi₂O₃ phase. The composition of the precursors influences not only the morphology but also the luminescence properties of the oxide nanostructures. The PL of the nanowires shows that the relative intensities of the emission bands associated with Bi²⁺ ions and oxygen vacancies depend on the composition of the precursor used to grow the nanostructures. In situ SEM mechanical resonance induced by an alternating electric field has been used to determine the Young's modulus of the microwires. A noticeable dispersion of E values has been obtained with a trend toward smaller values for increasing radius of the microwires. The quality factors measured for the microstructures suggest that Bi₂O₃ wires may have potential application as micromechanical resonators.

Acknowledgements

This work has been supported by MINECO through projects CSD2009-00013, MAT 2012-31959 and MAT 2010-16407.

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