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Research Focus and Experiments

Two topics central to Leone’s research findings throughout his career are quantum dots and attosecond laser technology. The ability to observe electrons using attosecond (one quintillionth of a second) light pulses is necessary since femtosecond (one quadrillionth of a second)-level measurements can only provide information regarding the effects that the electrons have on the nucleus’ motion in an atom. The attosecond scale provides much greater detail on the dynamics of singular electrons. His contribution to the study of quantum dots included experimentally proving their dependence on the excitation wavelength above the band gap of the individual particles.

In 2008, Leone was an integral part of a group of scientists including Thomas Pfeifer, Mark J. Abel,  Phillip M. Nagel, Aurélie Jullien, Zhi-Heng Loh, M. Justine Bell, and Daniel M. Neumark, which was aiming to understand and explore the pioneering methods of spectroscopy on the attosecond scale. This group ultimately came to the conclusion that these attosecond measurements of quantum electron dynamics could be extremely helpful in providing information to observers relating to the sublevels of an atom, including the core and valence electronic shells, as well as relations between atoms and molecules. However, at this period in time, the scale of the spectroscopic methods could only be brought to a resolution of approximately a few hundreds of attoseconds, and the spectroscopic measurement of rare-gas atoms was only hypothetical, unable to be proven by concrete experimentation.

(2) Since these initial findings and projections were published, a great deal of progress has been made with this technology, allowing Leone to reduce the scale of the attosecond pulses from a few hundred attoseconds to as low as 67 attoseconds. These lasers allow for much more precise, sub-femtosecond measurements in the extreme ultraviolet range, as well as in the infrared and visible regions. Leone has also been involved in the development and applications of attosecond transient absorption (ATA) spectroscopy, which allows for experiments to be measured over a wide range of spectra. This ATA spectroscopy has several applications, as it can be used to evaluate polyatomic molecules, inert gases, and other electron sources.

The research that Leone took part in concerning quantum dots focused on these particles’ patterns of “blinking,” which can be described as complicated and unpredictable intervals of fluorescence. Prior to this study, it was accepted that the distance of the band gap would have an affect on blinking, but Leone and his associates were able to prove that the intermittency of quantum dots also depends on the degree to which the wavelength is excited. Their work with CdSe (Cadmium selenide) and ZnS (Zinc sulfide) nanocrystals led to evidence of an energy threshold, since a step-like rather than a smooth transition between two blinking states (one very close to the band gap, and one far above the band gap) was experimentally observed. The quantum dots seemed to randomly switch “on” and “off,” as their fluorescence was abruptly interrupted. When looking at the "on" statistics only, the researchers involved were able to determine that the probability distribution at a wavelength of 575 nm was of power law form for the entire time period, while wavelengths of 525 nm, which correspond to higher energy above the band gap, led to a probability distribution similar to that of 575 nm for a short period of time, followed by a period of exponential decay according to the following equation:

$P(t_{on})=A{t_{on}}^{-\alpha _{on}}e^{-t_{on}/t_{c}}$

Researchers on the project concluded that the “off” states were not influenced by a change in wavelength, while the “on” states were definitely affected by a wavelength excitation.