Linking atomic structure and light emission in quantum dots

Semiconductor quantum dots (QDs) are objects on the scale of nanometers that are small enough to exhibit quantum mechanical effects and their current and potential applications range from quantum computers to biological imaging. Electronic and optical properties of these systems are drastically different from those observed in the bulk form and depend strongly on the size, shape and surface conditions. For semiconductors these differences can be explained in terms of the quantum confinement effect - a condition where the geometric size decisively affects a variety of physical properties. The concept of quantum confinement is elegant, but is not easy to probe directly in many realistic experimental cases due to the difficulties in observing QDs in an idealised state that can be readily compared with a corresponding theoretical or a computational model. A good example of this is porous nanocrystalline Si (pSi), that captured imagination of scientists at the end of the 20th century due to the unusual optical properties and a variety of promising applications. The intense visible photoluminescence (PL) observed in pSi was originally attributed to the quantum confinement effect, but almost immediately another point of view was voiced, suggesting the effect was due to the surface effects. Over two decades of intense research followed that could not provide unequivocal evidence to support either model and eventually led to the development of a model of Si QDs that includes a core, surface, and interfacial regions. Crucially, none of the approaches used so far have been able to provide direct evidence of a connection between the light emission and the underlying atomic structure.

Germanium is a close structural and electronic analogue to Si and there has been significant interest in understanding the optical properties of the light emitting Ge QDs. However, the research into Ge QDs seemed to be suffering from all the problems encountered with pSi and no direct connection has been made between the light emission and underlying atomic structure. Establishing such a connection is crucial not only for developing advanced methods of preparation of QDs, but also for testing theoretical models of these systems. 

We used a combination of optically-detected x-ray absorption spectroscopy (a synchrotron based technique) with molecular dynamics simulations to explore the origins of light emission in small Ge nanoparticles. Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces. We show that optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to reveal the different origins of light emission in these two sets of samples. We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission. In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence. Using a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy we show that these disordered regions correspond to the disordered layer a few Å thick at the surface of simulated nanoparticles. 

This work was possible  because of the close QMUL engagement with the  Diamond Light Source - a UK national synchrotron light source located at Rutherford Laboratory in Oxfordshire. Furthermore, partners from QMUL and the University of Kent have been supported by The South East Physics Network though a PhD studentship. The work has been published in "Structural origin of light emission in germanium quantum dots", W. Little, A. Karatutlu, D. Bolmatov, K. Trachenko, A. V. Sapelkin, G. Cibin, R. Taylor, F. Mosselmans, A. J. Dent and G. Mountjoy, Scientific Reports 4, 2014, Article number: 7372 doi:10.1038/srep07372

QM Staff: A. Sapelkin and K. Trachenko