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486 || AWSAR Awarded Popular Science Stories - 2019
to replace some 2–3 atoms with 2 different dopants from a system of 100 atoms of the host. This process of putting two dissimilar elements in a confined system of the nanometer-size regime is known as Dual Doping of Quantum Dots. We have recently addressed this issue in our manuscript entitled “Thermodynamics of Dual Doping in Quantum Dots” in the “Journal of Physical Chemistry Letters” in April 2019.
However, if we talk about the challenges associated with Dual Doping in Quantum Dots to date, it has not been viable to dual dope quantum dots deftly with reproducibility. In a general approach, the quantum dots are synthesized in the first step and different impurity dopants are added to the host thereafter. Already the space is confined, on that adding a single impurity element comes at a cost of energy. Adding two dissimilar elements with different nature (chemical and physical properties) is virtually unfeasible. They stand in need of non-
identical growth conditions.
These dissimilar dopants either phase-segregate or come
together and form clusters,
or they remain at the surface
resulting in surface doping
as shown in the first part of
the scheme. Further, when
we apply temperature for the
growth of these quantum dots,
the impurity elements try to
diffuse out of the system. This
is known as self-purification.
This is analogous to what
happens when we add on
stress or apply perturbation
to different species staying in
a confined environment. They
try to withdraw from the taxing situation and walk out. And I kept cudgeling my brain on how to make dual doping attainable. Yes, it appears to be arduous owing to the fact that it
is energetically unfavorable.
Can we play with the energy barrier by
some physical or chemical means? Is there a way out to put them in conjunction? That’s when I looked at nature and started wondering how we can prevail over all these barriers and bring two species together? And there it clicked, it is indeed feasible if we grow these species together. If we grow them together, the plausibility of conflict reduces and it is easier to administer them at the same level.
Here, we concentrate on the dual doping of cobalt (Co) and platinum (Pt) in a cadmium sulfide (CdS) semiconductor host. Co has excellent magnetic properties, and Pt helps in extending and enhancing its properties by holding a connection between Co atoms. So in order to put together Co and Pt in the host CdS, we started with only Co and Pt. We synthesized an alloy of CoPt. Dissimilar to the traditional way of doping, wherein we add
these impurities to the already synthesized host, here we start to build a CdS structure around the CoPt core. This is known as a core–shell structure. Increasing the shell thickness escalates the strain on the CoPt core that starts to diffuse out into the CdS shell. Further, high–temperature annealing assists the diffusion process. Thus, CoPt lattice disintegrates, and Co and Pt uniformly walk out into the semiconducting shell.
Now both Co and Pt unfold to trace the path of CdS. Based on their individual characteristics and properties,
we perceive that Co walks out faster compared with Pt. This effect is known as the Kirkendall effect, that is a sequel of the difference in the diffusion rates of Co and Pt. Therefore, based
If we talk about the challenges associated with Dual Doping in Quantum Dots to date, it has not been viable to dual dope quantum dots deftly with reproducibility. In a general approach, the quantum dots are synthesized in the first step and different impurity dopants are added to the host thereafter.