Ramani K. Raman
My research focuses on harnessing the power of an fs-LASER to modify material properties, and observe the resulting transformation in real time.
science seeks to transform humans from passive observers into active
of nature. Controlling material properties for use in next generation
technology requires control over the motion of atoms within the
material, which occur on an unimaginably small time scale of a few
The technique I employ is termed “ultrafast
(UED), and can be loosely
compared to the task of making a movie. In this, first an fs-laser
illuminates the sample under investigation, kicking the sample into a
non-equilibrium state, much like a movie director yelling
“action”. Immediately thereafter, a train of fs electron
pulses is directed at the sample, and the resulting electron
diffraction pattern recorded, where each pulse is analogous to one
frame in a motion picture sequence. These diffraction patterns contain
within them, signatures of the instantaneous atomic structure, frozen
in time, and consequently, analysis of these diffraction patterns
yields information on how the sample structure evolves on the
Fig 1. Ultrafast Electron Diffraction (UED) Concept
I am particularly interested in using UED to
observe the transformation between graphite and diamond,
both of which are made up of carbon atoms stacked up
in different configurations. Normally, converting graphite into
diamond requires subjecting the graphite to extreme measures such as
very high temperature and pressure. However, recent studies have
hypothesized that a similar effect might be possible through something
as simple as shining fs-laser light onto graphite[ii].
Using UED, we have observed for the first time[iii],
the emergence of a novel, diamond-like structure upon fs-laser
illumination of graphite, lasting for about 14 picoseconds (ps)§,
before its eventual disappearance by 50 ps. The transformation
is driven not by heat, but by creation of charges that cause a
compressive stress within the graphite layers. These results have been
confirmed by other groups who have also observed similar transient
However, the disappearance of this transient structure at long times
indicates its insustainability within the larger graphite motif. As a
result, we, in collaboration with Prof. Lawrence Drzal in the MSU
Engineering department, are currently extending this work to observe
similar transformations in ultrathin sheets of graphite, which are
about 20 – 30 nanometers (nm)Ä
thick. The ultrathin nature of these sheets have several advantages.
First, the lack of an extended graphite lattice will likely reduce the
system’s “inertia” towards complete transformation into diamond.
Secondly, these ultrathin samples are amenable to investigation in a
transmission electron microscope (TEM). The TEM on MSU campus at the
Center for Electron Optics is equipped with powerful characterization
techniques such as atomic scale imaging, electron energy loss
spectroscopy and electron Nanodiffraction, all of which will be
employed in an effort to characterize these intermediate graphitic
structures and also identify the route to complete diamondization.
Technologically, this work could have relevance in the emerging field
of carbon nanoelectronics[v],
in that, entire circuits could be conceivably fabricated on a graphite
or carbon nanotube substrate, with laser generated diamonds forming
the insulating barriers.
Along the same theme of material modification, I am also exploring the
effect of fs-lasers on metal nanoparticles, silver in
particular, which have been observed to explode and fragment into
smaller particles under fs-laser illumination. In addition to
its application in material science and nanosynthesis, this phenomenon
has also been proposed recently as a means of killing abnormal or
cancerous cells by delivering these nanoparticles to the cells and
using them as “nano-bombs”[vi].
We have also observed such laser induced fragmentation of large 40
nm silver nanoparticles into much smaller 2 – 5 nm
particles, despite the low laser power used. These transformations are
thus, not driven by heat or extreme thermal shock, which one would
associate with a strong laser, but are a result of intricate mutual
interaction of the light with the atoms and electrons in the
nanoparticle. Using Reverse Monte-Carlo[vii]
structure modeling of our UED data we have identified the creation of
laser induced atomic defects within the silver lattice, and their
subsequent growth and percolation as the mediators of this
electronically driven process[viii].
Further studies will focus on identifying the key parameters required
to achieve control and manipulation of such processes.
1 ps = 10-12 s = 0.000,000,000,001 s
Ä 1 nm = 10-9 m = 0.000,000,001 m
[i] A. H. Zewail. 4D ultrafast
electron diffraction, crystallography, and microscopy. Annu.
Rev. Phys. Chem. 57, 65 (2006)
[ii] H. Nakayama & H. Katayama-Yoshida. Direct conversion of graphite into diamond through electronic excited states. J. Phys. Condens. Matter 15, R1077 (2003).
[iii] R. K. Raman et al., Direct observation of optically induced transient structures in graphite using ultrafast electron crystallography. Phys. Rev. Lett. 101, 077401 (2008)
[iv] J. Kanasaki et al., Formation of sp3-bonded carbon nanostructures by femtosecond laser excitation of graphite. Phys. Rev. Lett. 102, 087402 (2009)
[v] P. Avouris et al., Carbon-based electronics. Nature Nanotech. 2, 605 (2007)
[vi] R. R. Letfullin et al., Laser-induced explosion of gold nanoparticle: potential role for nanophotothermolysis of cancer. Nanomed. 1, 473 (2006)
[vii] R. L. McGreevy. Reverse Monte Carlo modeling. J. Phys.: Condens. Matter 13, R877-R913 (2001).
[viii] R. K. Raman et al., Electronically driven photo-fragmentation of silver nanocrystals revealed by ultrafast diffraction. (Submitted)
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