Seminar Series - Al-Amin Dhirani - UofT St. George

Nanostructures - building blocks for designer materials exhibiting exotic quantum behaviour

Al-Amin Dhirani
Department of Chemistry and Department of Physics, University of Toronto
LM 254 - Lash Miller Chemical Laboratories, 80 St. George Street, Toronto, ON, M5S 3H6
a.dhirani@utoronto.ca

Abstract:
A theme of our research is to employ a bottom-up, building block strategy to fabricate nanostructured
materials exploiting the tunability of nanostructures. This approach leads to a possibility of
nanoengineering materials (NEMs) with desired behaviours and potentially realizing emergent quantum
phenomena (q-NEMS). In this talk, I will present two examples.
The first employes an extension of Langmuir’s method that enables for the first time fabrication of
molecularly cross-linked 2-D nanoparticle sheets (X-NS). X-NS are macroscopic (cm – scale laterally),
one nanoparticle thick and highly ordered. Remarkably, X-NS fabricated with gold nanoparticles and
semiconducting “molecular wires” linkers (oligophenylene dithiol, HS-(C6H4)n-SH, with 1 ≤ n ≤ 3)
exbibit conductivity that decreases with increasing molecule length (the opposite trend of tunneling) and
strongly enhanced molecular optical absorption compared with that of unlinked molecules in solution.
Modelling and control measurements exclude strong local fields present in X-NS as the cause of the
enhancement; rather, they point to strong changes to the local complex dielectric constant upon cross-
linking, suggesting quantum hybridization at a molecule – band (HMB) level. The tunability of the
electrical properties of X-NS via length of molecular wires linkers demonstrates an outstanding goal of
molecular electronics to realize molecular circuits whose behaviour is tunable through molecular
structure.
In the second example, we have shown that self-assembled gold nanostructures (nanoparticle, NP and
nanoshell, NS) + molecule cross-linkers (butanedithiol, BDT) exhibit multiple hallmark signatures of the
Kondo effect. In this quantum effect, delocalized s- electrons in a metal film hybridize with localized,
unpaired impurity d-electrons. Such interactions are of intense interest in condensed matter as they play
critical roles in exotic quantum effects, including high Tc superconductivity. Remarkably, NP + NS
films, exhibit signatures of the Kondo effect up to > 220 K i.e., > 10-fold higher than in NP films. Based
on these and magnetic measurements, we propose a mechanism whereby varying nanobuilding-blocks
can modify underlying electron-electron interactions to such a large degree.