Ramani K. Raman

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Research

My research focuses on harnessing the power of an fs-LASER to modify material properties, and observe the resulting transformation in real time.

Modern science seeks to transform humans from passive observers into active controllers 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 femtoseconds (fs)*.  The technique I employ is termed “ultrafast electron diffraction”[i] (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 ultrafast timescale.

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 structures[iv]. 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.