Microscopy Characterization of rapid growth doped and undoped oxide nanostructures

Javier Piqueras, Teresa Cebriano, Paloma Fernández, Anne-Flore Mallet, Bianchi Méndez, Victor Sánchez, Ana Urbieta
Department of Materials Physics, University Complutense of Madrid
Javier Piqueras is Emeritus Professor of Materials Physics at the University Complutense of Madrid, and a former president of the Spanish Society for Microscopy. His present research interest is the synthesis, properties and applications of nanomaterials, mainly electronic nanomaterials, such as semiconducting nanowires, nanoparticles or other low dimensional nanostructures, involving the use of different microscopy characterization techniques.
Joule Heating of Mo and Zn wires in air by flowing a high electric current density has been used to grow in very short times nanoplates of MoO3 on the Mo surface and ZnO nanowires on the Zn surface. When the heating of metallic wires is carried out with the wire in contact with suitable chemical compounds, doped nanostructures are obtained. Growth of undoped and doped nanostructures takes place in times ranging from few seconds to few minutes indicating the existence of enhanced diffusion processes during the current flow. The obtained nanostructures have been characterized by SEM, EDX, EBSD, CL, PL and microRaman spectroscopy.
The authors thank MINECO for funding support with project MAT2015-65274-R-FEDER.
Corresponding author
Prof. Javier Piqueras, Dept. of Materials Physics, University Complutense of Madrid
Growth and characterization of semiconductor nanostructures such as nanowires, nanotubes, nanoplates, nanoparticles and other morphologies, is an important topic related to the application of the nanostructures in the fields of nanoelectronics, nanooptics and others.
In the particular case of nanostructures of the group of semiconducting oxides, also applications in the fields of sensing and transparent electrodes have been developed. Among the physical, or physical chemistry, methods of fabrication of oxide nanostructures, thermal methods of evaporation-deposition, with and without catalyst, based on vapor-liquid-solid and vapor-solid mechanisms are widely used.
By appropriate choice of the evaporating precursor, doped nanostructures can be obtained in some cases. In addition, nanostructures of metal oxides can be fabricated by heating the corresponding metal in an oxidizing atmosphere. Most of the reported thermal treatments to grow oxide nanostructures by these methods, extend to times in the range of hours at relatively high temperatures. In the case of evaporation-deposition the procedure is usually more complex due to the need of a gas flow to favor the transport of the evaporating species to a substrate.
A drastic reduction of the growth time of oxide nanostructures by thermal treatments is obtained by direct Joule heating of metallic wires by the flow of an electric current under ambient conditions1
In this work the Joule heating method has been applied to grow MoO3 nanoplates and ZnO nanowires in times ranging from few seconds to some minutes. It is demonstrated that the method also enables the fabrication of nanostructures containing Sn impurities, which leads to complex arrangements of nanowires, or structures doped with optically active ions, such as Er3+ and Tb3+. The nanostructures have been characterized by scanning electron microscopy techniques (SEM, EDS, EBSD, CL), atomic force microscopy (AFM), and micro-PL and micro-Raman in an optical confocal microscope. 
The starting material for the synthesis of the MoO3 nanoplates was high purity Mo wires of 0.25 mm diameter. The ends of 6 cm long wires were fixed at metal contacts and appropriate voltages were applied to obtain currents in the range from 4 to 5.5 A. During the current flow the wires reached temperatures between 400 ºC and 600 ºC, by Joule heating, which were measured with an Infratherm pyrometer. The times of treatment used to induce the growth of a high density of MoO3 nanoplates on the surface of the wires ranged between two and five minutes.
The same experimental arrangement was used to grow ZnO nanowires by resistive heating of pure Zn wires. In this case the applied currents were in the range from 2.9 to 3.3 A leading to temperatures of about 425 ºC. ZnO nanowires covered the surface of the wires after resistive heating during times ranging from few seconds to five minutes. The size of both, MoO3 plates and ZnO nanowires, tends to tends to reach their final values with the mentioned short heating treatments. 
By extending the time of treatments no significant changes in the density or size of the obtained nanostructures were observed. In order to grow doped nanostructures, the starting metal wire was covered by, or embedded in, powder of a suitable precursor containing the dopant. In particular, to obtain Er doped MoO3 nanoplates the heating of the Mo wire took place in the presence of Er2O3, while the Zn wire was in contact with SnO2 and Tb4O7 powders to grow Sn doped and Tb doped ZnO nanowires, respectively.
The morphology of the nanostructures was assessed with a FEI Inspect SEM or a Hitachi TM 3000 SEM. Also AFM was used to reveal the lamellar nature of the MoO3 nanoplates. For structural characterization an EBSD Bruker e-flash diffractometer in SEM and microRaman spectroscopy performed with a HoribaJobin-Ybon LabRam HR800 confocal microscope were used. Compositional analysis by EDS was carried out with a Bruker AXS 4010. Photoluminescence (PL) of the nanostructures was investigated with the LabRam confocal microscope under excitation with 325 nm light of a He-Cd laser and for cathodoluminescence (CL) measurements in SEM a Hamamatsu R928P photomultiplier was used.
Molybdenum Oxide
Figure 1a shows a SEM image with a general view of the Mo wire after one of the effective Joule heating treatments. The central region of the wire surface appears covered by MoO3 nano and microplates.
Figure 1. (a)-(b) SEM images of MoO3 plates formed on Mo wires. The stepped profile is clearly observed in the inset in (b). c) AFM topographic image of one of the plates in an area close to the edge, showing several steps. (d) Height profiles of lines #1 and #2 displayed in (c). The steps are around 100 nm heights.
At both ends of the wire, the temperature during the current flow is lower due to heat losses at the electric contacts and the optimum temperature for the growth of oxide nanostructures, in the range 450-500 ºC, was not reached. The plate morphology of the structures suggested that the oxide phase formed by heating was molybdenum trioxide, MoO3, along with MoS2, MoSe2 and MoTe2, which have attracted interest in past years. 
Higher magnification SEM images, as in Figure 1b, show that the plates have irregular edges, due to the growth of thin plates that coalesce to form a single one, leading to a stepped profile of the plates (inset Figure 1b). The lamellar structure is also revealed by the AFM images (Figure 1c). Two line profiles of the AFM image are displayed in Figure 1d, revealing that the height of the steps is around a hundred or few hundred nanometers high.
The layered structure is related to the existence of (010) planes of orthorhombic α-MoO3 weakly bonded by van der Waals forces. This was confirmed by EBSD characterization (not shown here) of the plates, which showed that the EBSD patterns of the plates correspond to α-MoO3 and that the crystal orientation of the plates is (010)2. Additional structural characterization of the as grown nano- and microplates was performed by microRaman spectroscopy, which provided fingerprint spectra corresponding to orthorhombic α-MoO3 (Figure 2).

Figure 2.
 Raman spectrum of a representative MoO3 plate. The peaks correspond to the α-MoO3 phase.
When the Mo wire is in contact with Er2O3 powder during Joule heating, nanoplates grow on the wire surface, under similar optimal conditions of current, temperature and time as in the absence of the erbium oxide powder. 
However, in this case the arrangement of the plates on the Mo surface is less homogeneous appearing concentrated in bundles with a flower-like appearance (Figure 3a). This can be explained by a catalytic effect of the foreign Er ions favouring the growth at specific sites as well as the growth of hierarchical structures. This effect has been previously reported for the case of doped nanostructures grown by evaporation-deposition and is attributed to local segregation of the dopant atoms inducing a catalytic effect4.
The incorporation of optically active Er3+ ions into the plates during the growth was assessed by cathodoluminescence (CL) in SEM. Figure 3b shows the CL spectrum of a doped MoO3 plate showing Er3+ characteristic lines 2H11/2 – 4I15/2 at 522-535 nm and 4S3/2 – 4I15/2 at 547-557 nm as well as red lines in the range 648-673 nm associated to 4F9/2 – 4I15/2. This result shows that during Joule heating a rapid growth of doped semiconducting MoO3 nanostructures is achieved. Due to the low Er content in the nanoplates no quantitative information of the Er at% could be obtained by EDS.     

Figure 3. (a) SEM image and (b) CL spectrum of Er doped MoO3 plates. Sharp lines correspond to Er3+ intraionic transitions.
Zinc Oxide 
After Joule heating treatments as short as few seconds with currents in the range 2.9-3.3 A, the surface of the Zn wire appears covered with a high density of ZnO nanowires and nanoneedles with lengths of about 1-2 μm and cross sections of 100 nm or few hundreds of nanometers (Figure 4). By increasing the time of the treatment some nanowires with lengths up to about 5μm are observed but the dimensions of the nanowires appear to reach their final size after several minutes of heating.
Figure 4. SEM image of ZnO nanowires after a few seconds of Joule heating of a Zn wire.
After the current flow, the starting Zn wire becomes a core-shell structure formed by a shell of ZnO containing the nanowires and a Zn core as revealed in the EDS map of Figure 5. Also, EDS maps of single nanowires show a homogeneous distribution of Zn and O along their length.
Figure 5. EDS maps of Zn (green) and O (orange) in a heated Zn wire showing a few microns thick oxidized layer.
Placing SnO2 or Tb2O7 powders in contact with the Zn wire during the current flow leads to new features in the morphology and arrangement of the resulting ZnO nanowires. The nanowires grown in the presence of SnO2 appear with a branched or hierarchical arrangement as shown in Figura 6a, due to the above-mentioned catalytic effect of the foreign elements, in this case Sn. Although Sn is detected in EDS measurements, the low content enables only approximate estimations of the atomic % content of about 0.5 %. 
Figure 6. a) ZnO nanowires grown by heating a Zn wire covered by SnO2 powder. b) Arrangements of ZnO nanowires and nanoparticles grown by heating a Zn wire in the presence of terbium oxide.
The ZnO nanostructures grown in the presence of Tb are complex necklace arrangements formed by nanowires containing nanoparticles of about 100 nm placed along the nanowire axis as shown in Figure 6b. Incorporation of optically active Tb3+ ions into the ZnO nanowires was detected by CL in SEM as shown in Figure 7 with the main Tb3+ transition at 544 nm (2.28 eV). EDS measurements averaged on the nanowires as well as in different positions of the structures show that Tb is mainly incorporated into the nanoparticles with concentrations of about 0.5 atomic %. This would explain the formation of the nanoparticles during growth as a result of enhanced growth rate in points of the wire where some segregation of Tb takes place.
Figure 7. CL spectrum of regions containing mixed ZnO nanowires – nanoparticles structures showing Tb3+ line at 2.28 eV superimposed to the ZnO bands.
Diffusion processes during growth
It is in general considered that the growth of oxide nano- and microstructures during thermal oxidation of the corresponding metal is based on diffusion processes involving outward diffusion of metal ions to the surface, e.g. by a stress driven mechanism5, and oxygen diffusion in the opposite direction. In the case of the growth by Joule heating the outward diffusion of the metal ions would be related to the thermal gradient arising from the metal wire core to the surface.
However, the fact that nanowires and nanoplates are grown by Joule heating in times much shorter than those reported for other thermal based grown methods, suggests that enhanced diffusion processes of the metal ions take place during the flow of electric current. In particular, in the electromigration effect atomic diffusion takes place during flow of a high density direct current in a conductor. 
A recent work on electromigration in Cu6 has demonstrated that for high density electric currents the electron flow induced strain and atomic diffusion related to local stress relaxation. In this work, high current densities of about 104 A/cm-2 were applied so that electromigration related diffusion could contribute to the observed rapid growth of the nanostructures.
Orthorhombic α-MoO3 lamellar nanoplates have been grown by Joule heating of Mo wires during the flow of a high density electric direct current.
After times ranging from few seconds to some minutes the oxide plates cover most of the surface of the starting metal wire. Er doped MoO3 nanoplates are obtained when the Joule heating is carried out in contact with erbium oxide. Incorporation of Er into the plates is assessed by CL measurements in SEM.
By performing similar heating treatments on Zn wires a high density of ZnO nanowires is obtained in few seconds on the wire surface. Joule heating of the Zn wire in the presence of Sn leads to hierarchical arrangement of ZnO nanowires while when the treatment takes place with the wire in contact with terbium oxide, Tb doped ZnO nanowires are obtained. In both cases, MoO3 plates and ZnO nanowires, the structures grow by Joule heating of the metal in times much shorter in comparison with other thermal based techniques. The rapid growth indicates that Joule heating of the metal wire involves enhanced diffusion processes. It is suggested that, in addition to radial thermal gradients in the wire, electromigration effects can contribute to diffusion processes favouring the observed rapid growth of the structures.
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