units, atoms, molecules and nanoclusters, and assembled into complex structures
[ 27 – 30 ]. The synthesized nanomaterials have been demonstrated as active com-
ponents for high-performance electronics, sensors and patterned on virtually any
substrate and in 3D space. Importantly, the size of these synthetic nanomaterials is
comparable to, or even smaller than, thestate-of-the-artnanoelectronic units in
industry [ 31 ]. These properties offlexible nanoelectronics such as ultraflexibility,
nanoscale feature sizes, high performances, etc. offer a great promise for building
bioelectronics and biomedical devices for living cellular and tissue system interface.
1.1 Synthesis of Nanowires as Nanoelectronic Units
Of all nanoelectronic building blocks for macroelectronics, semiconductor nano-
wires have the mostflexible yet controllable structures and electronic properties for
the following reasons: (1) Through rational design of catalyst and precursor, vir-
tually all kinds of semiconductor nanowires can be formed [ 32 – 39 ]; (2) nanowire
structure and doping can be rationally modulated to meet different requirements,
which is very difficult to achieve by traditional fabrication technology [ 40 – 44 ] and
(3) high-performance electronics can be realized by synthetic nanowire circuits
[ 45 – 48 ].
The most general process for the synthesis of high quality nanowires is the
nanocluster-catalyzed vapor-liquid-solid (VLS) growth [ 49 – 53 ]. In this process,
metal nanoclusters are heated to form a liquid solution. The presence of a
vapor-phase source of the semiconductor results in nucleation sites for the crystal-
lization. The solid-liquid interface forms as the growth interface attracting a con-
tinued incorporation of precursor gas and precipitation of semiconductor atoms into
the lattice, leading to a preferential one-dimensional growth. Different methods have
been explored to grow semiconductor nanowires. The chemical vapor deposition
(CVD) process, in which the metal nanocluster serves as a catalyst, is one of the most
popular techniques for VLS growth. In the case of Si nanowire growth, Au
nanoparticles serve as catalyst sites where the gaseous precursor silane decomposes
to provide semiconductor reactants [ 26 , 50 ]. With appropriate selection of nan-
ocluster catalyst diameter, reactant gases, pressure and temperature, one can easily
design nanowire structures de novo and synthesize these structures with different
modulations of composition, doping defects and geometry [ 44 ]. Based on reports in
the literature [ 18 , 44 , 54 , 55 ], virtually all electronic units can be synthesized and
implemented into single-nanowire structures via the bottom-up paradigm. For
example, nanowires involving p-i-n dopant modulation in axial and coaxial
geometries have been explored to synthesize nanowire photovoltaics [ 54 ]. Branched
nanowires containing nanowire heterostructures, including single-crystalline semi-
conductor groups IV, III–V and II–VI and metals, have been explored to synthesize
nanowire light-emitting diodes (LEDs),field-effect transistors (FET) and biosensors
[ 55 ]. Kinked nanowires, with precise geometry design and dopant modulation in the
axial direction, have been utilized in the design of localized FET detectors [ 18 , 44 ].
2 1 Introduction