Introduction: Hierarchical Nanostructures for Energy Devices - Hierarchical Nanostructures for Energy Devices - page 3

problem more practically and effectively, the concept of an Energy Cycle
should be introduced and the total eciency of all energy devices involved
should be counted systematically.
The most important factor is not just a simple number, such as the
eciency of a single energy device; the balance between many energy devices
is very important. This may sound as though researchers in the energy field
should know about all different types of energy devices (generation, storage
and consumption) to increase an energy device's e ciency in the energy
cycle. However, a closer look at the various energy devices may reveal that
most of them have similar structures and requirements to make more
ecient devices. The structures usually have an active layer sandwiched
between two electrodes. The electrodes may be a transparent or non-
transparent conductor depending on the application (optoelectronic devices
need at least one transparent electrode, such as a solar cell and LED display).
Furthermore, most of the energy devices are surface devices (using an
interface) and therefore, the eciency can be increased using a larger
surface area. That is where nanomaterials can be useful. However, a larger
surface area does not always yield a highly increased eciency. Additional
smart structuring, which can lead to better carrier transport, can boost up
the eciency along with an increased surface area.

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1.3 Hierarchical Nanostructures for Ecient Energy
Devices
The study of energy device materials is a field full of opportunities for
practical and socially significant applications. 2 Many potential renewable
energy technologies in the form of solid-state devices and condensed matter
phenomena involving the conversion of energy from one form to another
exist, and some proceed with e ciency near unity. Within the last couple of
decades, there has been an increase in interest in materials with nanometre-
scale dimensions. Semiconductor nanowires, a subset of these materials,
have received exceptional attention for their unique properties and complex
structures. Many nanowire-based materials are promising candidates for
energy conversion devices.
However, eciency increases in the energy devices have been sluggish
recently and there has been a need for new groundbreaking approaches,
such as the design and fabrication of three-dimensional multifunctional
architectures from appropriate nanoscale building blocks, including the
strategic use of void space and deliberate disorder as design components to
permit a re-examination of devices that produce or store energy. 4 Recently,
the importance of nanostructured materials in energy harvesting, con-
version and storage technologies has been highlighted in several review
articles. 5-10 In particular, 3D branched nanowire structures with high
surface areas and direct transport pathways for charge carriers are especially
attractive for energy applications. 11,12 For example, 3D branched nanowires

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