by Professor Norman Zhou
Joining, whether at the nano-, micro- or macro-scale, has been an essential
part of manufacturing and assembly of man-made products, providing mechanical
coupling and support, electrical connection or insulation, environmental
protection, etc. This will still be true with the emerging technology of
nanojoining, that is, producing permanent unions or connections between
nanosized building blocks, which are typically manufactured using top-down
techniques such as nanolithography, or bottom-up methods such as self-assembly,
to form functional nanodevices and nanosystems1.
Nanojoining also allows integrating these nanodevices and nanosystems to the
surroundings, i.e. micro- and macro-scale devices and systems. Nanojoining is
also referred to as nanobonding, nanowelding, nanobrazing, nanosoldering,
Permanent unions or connections between building blocks or parts to be
assembled are produced mainly through the formation of primary (and on occasions
secondary) chemical bonds between faying surfaces1.
When the parts are not compatible in atomic structures, an interlayer or
intermediate material may be required. In principle, two ideal solid surfaces,
e.g., both perfectly clean and atomically flat, will bond together if brought
into intimate contact, as they will be drawn together spontaneously by
However, most engineering surfaces are characterized as rough and
contaminated, requiring some form of energy, usually heat and/or pressure, to be
applied to overcome these surface impediments to make a joint1. It is expected that these impediments will be less
significant in nanojoining because of the much reduced surface areas and special
environments used in most nanojoining processes. On the other hand, other
challenges arise because of continuing miniaturization and the associated laws
of physics. These would for example cause difficulties in manipulation of parts.
Recently, the development of nanojoining processes is attracting tremendous
efforts2-9. Various methods have been developed
for nanojoining, some of which have been at least partially successful. For
example, by in-situ e-beam exposure with a transmission electron microscope at
high temperatures, Terrone et al.2 welded two
crossing single wall carbon nanotubes (SWCNT) through the formation of covalent
C-C bonds and the creation of seven- or eight-membered carbon rings bridging two
Recently, Professor Norman Zhou and his colleagues at Centre for
Advanced Materials Joining, have successfully brazed carbon nanotube bundles
to Ni electrodes with Ti-containing brazing alloys at 900°C to 1000°C3, in which nanotubes react with Ti to form Ti-C bonds
leading to low Ohmic contacts between carbon nanotubes and Ni electrodes.
Wei et al.4 realized nanoconnection between
carbon nanotubes and tungsten leads by depositing a Ga layer with a focused Gd+
ion beam (FIB). A small contact resistance of tens to 100 Ohms was achieved.
Chen et al.5 bonded SWCNTs to Ti electrodes using an
ultrasonic bonder to obtain robust bonds with a contact resistance of a few
kilo-Ohms. Similar to conventional resistance spot welding, Hirayama et
al.6 welded two SWCNTs by applying currents through
a scanning tunnelling microscope. Obviously, these protocols are effective
either in very specific conditions and/or for a very specific material, such as,
e-beam or ion beams that require high vacuum, ultrasonic welding which provides
less spatial control of joints, Joule heating which is limited to the joining of
Conversely, two alternate methods are described in detail in the following
section which are effective for joining nanomaterials in general:
- femtosecond laser irradiation7,8 and
- low temperature solid state sintering through surface atomic diffusion
and/or partial surface melting9.
As the electron-lattice thermal coupling time (about 1 picosecond) is much
longer than the femtosecond laser pulse width, the electrons do not have enough
time to transfer energy to the lattice. The nature of interaction of femtosecond
laser pulses and materials is known as non-thermal processing.
After electrons have been excited by a femtosecond laser, the lattice
cohesion is reduced and the binding loosens due to the Coulomb repulsion. This
is accompanied by the unique effect known as ultrafast melting, which occurs
only over nano-scale dimensions compared to the conventional thermal melting.
This opens up exciting possibilities for joining of nano-scale building blocks
for micro-electromechanical devices10. By precisely
controlling the laser energy, nanojoining at the atomic level is possible.
On the other hand, the surface atomic migration is dramatically enhanced in
nanomaterials due to their high ratio of surface energy to volume condensed
energy. Near-surface atoms with a similar mobility as in the liquid state can
provide a bonding mechanism through low temperature coalescence and sintering.
Recently, we have bonded Cu wires to Cu foils using Ag nanoparticle pastes at
It is certain that nanojoining is one of the key technologies in industrial
success of nanodevices and nanosystems. As Terrones et al. 2 pointed out for CNT nanoelectronics and nanodevices,
joining "is a key issue because both electronic devices and strong
nano-mechanical systems need molecular connections among individual SWCNTs".
Nanojoining will revolutionize various ongoing nano-manufacturing
technologies for nano-mechatronics and molecular devices. These nanodevices and
nanosystems have the potential to provide distinctive properties and superior
sensitivity, and can offer improved integration and reduced operating energy
requirements for the next generation of technologies. A current example is the
welded Au/Ag nanoparticles used in surface enhanced Raman probes which promise
to provide single molecule characterization for modern medical diagnostics, drug
development and/or quantum computing7,8.
1. Y. Zhou, "Microjoining and Nanojoining". Woodhead Publishing
Ltd, Cambridge, England, CRC Press, 2008
2. M. Terrones, F. Banhart, N. Grobert, J. C. Charlier, H.
Terrones, and P. M. Ajayan, Phys. Rev. Lett. 2002, 89, 075505
3. W. Wu, A. Hu, X. Li, J. Wei, Q. Shu, K. L.
Wang, M. Yavuz, Y. Zhou, "Vacuum brazing of carbon nanotube bundles", Mater. Lett. 62
4. C. Chen, L. Yan, E. Kong,
Y. Zhang, Nanotechnology 2006, 17, 2192.
5. B. Wei, R. Spolenak, P. Kohler-Redlich, M. Ruhle, E. Arzt, Appl.
Phys. Lett. 1999, 74, 3149.
Hiyayama, Y. Kawamoto, Y. Ohshima, and K. Takayanagi, Appl. Phys. Lett. 2001,
7. Y. Zhou, A. Hu, M. I. Khan,
W. Wu, B. Tam, and M. Yavuz. "Recent progress in micro and nanojoining". J.
Phys. Conf. Ser. 2009, 165, 012021.
A. Hu, S. K. Panda, M.I. Khan, Y. Zhou, (2009) "Laser Welding, Microwelding, Nanowelding and Nanoprocessing",
Chin. J. Lasers Vol.36, no.12, 3149.
H. Alarifi, A. Hu, M. Yavuz, Y. Zhou, "Bonding of Cu wires at low temperatures
using Ag nanoparticles paste", proceeding of Materials Research Society, 2009
Fall, Boston, USA.
10. A. Hu, M. Rybachuk, Q.-B, Lu and W. W.
Duley. "Direct synthesis of sp-bonded carbon chains on graphite surface by
femtosecond laser irradiation". Appl. Phys. Lett. 2007, 91, 1319061.
Copyright AZoNano.com, Professor Norman Zhou (University of