Sequential physical vapor deposition and chemical vapor deposition for the growth of In2O3–SnO2 radial and longitudinal heterojunctions
Introduction
After the synthesis of single crystalline oxide nanowires in 2001 [1] and the exploitation of their properties, research has moved to the development of complex heterostructures, in which nanowires of different materials are assembled together. Quasi 1-dimensional nanostructures [2] like ZnO [3], SnO2 [4], MgO [5], SiO2 [6], In2O3 [7], [8], SnO2-doped In2O3 (ITO) [9], [10] can be assembled to create heterointerfaces or to combine the properties of the different materials at a very small scale.
Various fabrication strategies have been applied to obtain the heterointerfaces: (i) selectively localized doping of a single wire [11]; (ii) heteroepitaxy of single crystalline 1D or 2D shapes in form of branched structures [12] and disks [13] on a main backbone or nanocomposites [14]; (iii) sequential growth of longitudinal nanowires of two different materials [15]; (iv) repeated heteroepitaxial growth of longitudinal structures leading to creation of a superlattice [16].
Different functional properties have been demonstrated for the heterostructures and opening the perspective of highly innovative devices, such as improved electron transport in excitonic solar cells [17], diode junction [11], whispering gallery mode luminescence emission [13], double-isotype n-type heterojunction [14], and improved lithium storage capacity in batteries [18].
We demonstrated the possibility of obtaining a double-isotype n-type heterojunction based on a SnO2 (polycrystals)–In2O3 (nanowire)–SnO2 (nanowire) structure [15]. The recent advances on the doping of oxide nanowires [19], [20] foresees the possibility of fine tuning the electrical properties of heterointerfaces, based on the composition of each material. Compositional modulation allows electronic band engineering, which is expected to influence the functional properties of the heterojunction.
Key role in fabricating the heterostructures is the ability to control the preparation conditions, as it allows the fine tuning of the morphological and structural features. On these grounds the research is pursuing the development of new methodologies, which minimize the uncertainty of the results due to rough control of the parameters of the process.
Anisotropic heterostructures of In2O3 and SnO2 in different geometries (forming longitudinal and radial heterojunctions) were successfully fabricated in previous works by means of physical chemical deposition (PVD), by sequential vapor transport and condensation mechanisms [15].
However, precise control of the nucleation and growth processes of the second phase proved difficult due to the fast growth rate of SnO2 nanowires, thus limiting the possibility of tailoring the morphological and structural features of the heterojunction. In this manuscript we report on a new approach to design In2O3–SnO2 heterostructures, based on sequential PVD and CVD processes, in which SnO2 nanowires grow by CVD at the apex of In2O3 nanowires previously grown by PVD in a first step.
We have quantitatively investigated the growth of the SnO2 nanowires in a series of systematic experiments, highlighting that the lower temperature of the CVD process and better control in the delivery of the precursors result in highly improved accuracy regarding the control of growth rate of nanowires and, accordingly, of the final morphological features of the heterostructure.
Section snippets
Experimental
In2O3 single crystalline nanowires were synthesized via a transport-and-condensation method on α-plane sapphire substrates. The experimental set-up for the oxide deposition is composed of a high temperature alumina furnace capable of activating the decomposition of the oxide precursors and to promote evaporation. Indium oxide precursor powders (purity 99.999% from Sigma-Aldrich) were located in the centre of the furnace at temperature of 1500 °C. The controlled pressure (100 mbar) and the
Results and discussion
Fig. 1 illustrates the formation of In2O3–SnO2 heterostructures based on the prolonged catalytic activity of a liquid gold droplet, which acts as condensation center for the volatiles during both the PVD and CVD processes, according to the well accepted vapor–liquid–solid (VLS) growth [25], [26], in agreement with the SEM observations shown in Fig. 2. In the VLS process, a metal promoter (commonly used catalysts are Au, Pd, Pt) induces the condensation of volatile species (both in PVD and CVD
Conclusions
In summary, In2O3–SnO2 heterostructures have been fabricated by a two steps process on α-sapphire substrates: physical vapor deposition and chemical vapor deposition. Both In2O3 and SnO2 nanowires are single crystalline and form a controlled longitudinal In2O3–SnO2 heterojunction, based on the sequential growth of In2O3 and SnO2 nanowires. The gold catalyst used in these experiments maintains its catalytic activity during the corresponding VS/VLS and VLS growth mechanisms. The relatively slow
Acknowledgements
The authors acknowledge the European Commission for partial funding under the contract WIROX no. 295216. A.V. would like to acknowledge the European Commission for partial funding under the contract F-Light Marie Curie no. 299490. S.M. is thankful to the University of Cologne for providing the necessary financial support.
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