(Nanowerk Spotlight Guest Writer Series) 1-D nanostructured boron and boron-related compounds have attracted considerable attention to exploit their potential use in high temperature electronics, thermoelectrics, and photovoltaics due to their unique chemical and physical properties, such as chemical inertness, hardness, and adjustable energy bandgap. Moreover, the recent discovery of superconductivity above 40 K in MgB2 has triggered an intensive interest in those materials.
To date individual semiconductor nanowires have already been configured as device structures, but they have proven to be difficult to assemble. In order to make nanodevices feasible the synthesis of self-assembled semiconductor nanowire junction arrays should be achieved.
Recently, we demonstrated that boron nanowire junction arrays could be tailored by adopting our proposed ‘the so-called oxide-assisted vapor-liquid-solid growth’. In specific, two types of boron nanowire junction arrays, boron Y-junctions and boron-silicon alloy lateral junctions, were synthesized in a self-assembled manner by simple chemical vapor transfer method.
Left: SEM image with EDS line scan of single boron Y-junction. Right: SEM image of boron-silicon alloy lateral junction array. (Source: Sang H. Yun, Royal Institute of Technology)
Each boron Y-junction was formed by fusing two inclined nanowires, separating by thin insulating oxide layer. The lateral junction consists of a row of knots and necks. Each knot and neck reveals different chemical composition ratio of boron to silicon.
Most importantly, we clarified the growth mechanisms for fine control of such structures. The clarified growth mechanisms will play crucial role for the synthesis of such junction structures in other materials. The ability to synthesize self-assembled nanowire junction arrays opens up new opportunities in a number of potential applications due to the possible self-integration without post-growth modifications.
However, overall structure of those junctions is amorphous, though some junctions reveal single crystalline. Thus, more efforts for the growth of single crystalline junctions are obviously demanded. The results were published in Nano Letters ("Self-Assembled Boron Nanowire Y-Junctions") and Applied Surface Science ("Growth of boron nano-junctions").
Motivated by these unique structures of the boron junctions, my group in Material Physics at Royal Institute of Technology, Sweden is currently concentrated on two research topics; The growth of MgB2 superconductor nanowire Y-junction by diffusing Mg on the precursor boron Y-junctions is the first target, which can be used as superconductor-insulator-superconductor junction devices.
Second, the lateral alloy junction arrays is served as nanostructured photoelectrodes for the realization of eta (extremely thin absorber)-based solar cells, in which enable to cover in a wide range of solar spectrum due to their inherently different composition ratio of boron to silicon (the coexistence of various optical bandgaps) within the lateral junctions, leading to enhanced energy conversion efficiency.
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