nanowire surface chemically or physically. Lubricants such as octane and mineral
oils are used between the two substrates to reduce friction. During the contact
sliding process, nanowires are detached from the donor substrate by adhesive
interactions with the receiver substrate and ultimately realigned by the sliding shear
force, resulting in the direct transfer of parallel nanowires to the receiver substrate.
After further standard fabrication processes, a well aligned multiple-nanowire
device can be fabricated. Through pre-alignment and transfer, McAlpine et al. [ 66 ]
demonstrated the fabrication of chemical sensors on aflexible substrate. Timko
et al. [ 67 ] demonstrated the use of contact printing to assemble silicon nanowires on
a polymeric substrate to create nanowire FET arrays for electrical detection and
recording from chicken cardiomyocytes. Multiplexed recording from these arrays
recorded signal propagation times across the myocardium with high spatial reso-
lution. Takei et al. [ 68 ] used a contact printing technique to assembly Ge/Si
core-shell nanowire on a polyimide substrate to form fully integrated nanowire
active matrix circuitry. Integrating it with pressure-sensitive rubber, they demon-
strated this circuitry as electronic“skin”for pressure sensing with lower operation
voltages (<5 V) than its organic counterparts.
However, this technique has limitations with respect to fabrication of
high-performance single-nanowire electronics. While the process enables
large-scale and uniform assembly of nanowires, it lacks the control to precisely
integrate individual nanowires at the nanometer scale, causing uneven electronic
performance. To further extend the contact printing technique, Yao et al. have
recently reported a nanocombing assembly technique [ 69 ]. This new technique
involves defining regions of a surface that can physically or chemically anchor part
of the nanowires and then drawing them out over a region of the surface that has
little interaction with the nanowire, to stretch and align nanowires in highly oriented
arrays (Fig.1.1b). This method pushes the yield of arrays to greater than 98.5% of
the nanowires aligned, to within±1° of the combing direction. With lithography
pre-patterning chemically distinct regions, a deterministic assembly has been
demonstrated to produce a high yield of single-nanowire (20–30 nm in diameter)
devices on different substrates.
In addition, post-growth assembly of nanowires and patterning techniques allow
for the integration of electronic units through a layer-by-layer assembly process [ 64 ,
65 , 70 ], opening up new opportunities for 3D integrated circuits (3D-ICs). 3D-ICs
consisting of multiple layers of active electronic units enable more efficient inter-
connections, higher integration density, faster operation speed and lower power
consumption. Moreover, this technology allows for the integration of different
materials without the requirement of materials or processing compatibility. As an
example, Nam et al. recently demonstrated the integration of thefirst layern-InAs
nanowire with a second layerp-Ge/Si core-shell nanowire to form a vertically
interconnected 3D complementary metal–oxide–semiconductor (CMOS) inverter
by contact printing [ 65 ].
Together, these growth and assembly technologies open up new opportunities to
realize nanoelectronics on virtually any kind of substrate and 3D interconnections
to usher nanoelectronics design into a new era.
4 1 Introduction