Effects of Disorder on Electronic Properties of Nanocrystal Assemblies

Abstract

Structural disorder is one of the major obstacles for band-like transport in semiconductor nanocrystal (NC) solids. However, systematic study of the effect of structural disorder on electronic properties is lacking. Here, we use a tightbinding model that explicitly incorporates energetic and positional disorders to evaluate quantitatively the effects of disorder on electron transport in semiconductor NC assemblies. The density of states and wave function delocalization are analyzed to identify the mobility edges, if they exist. We summarize the results in phase diagrams of the localization− delocalization transition for different superlattice structures. We find that the critical disorder at the transition point mainly depends on short-range order, not the long-range order of the superlattice. We also use an effective-mass model to calculate the coupling energy between adjacent nanocrystals. The calculation reveals that a red-shift of the excitonic peak in the optical absorption spectrum is not directly related to the magnitude of electronic coupling. For the specific case of PbSe nanocrystals coupled by ethanedithiol ligands, we conclude that the disorder is too large to allow delocalized states. Lastly, we analyze the effects of valley degeneracy and intervalley coupling, both within a NC and across adjacent NCs, using a phenomenological model. The impact of multiple valleys on charge transport depends on the exact nature of the coupling of distinct valleys, which is unknown. We estimate the magnitude of the coupling and conclude that the presence of multiple valleys has a minor impact on the wave function delocalization. Overall, we expect that band-like transport should be attainable with suitable ligands and proper surface passivation.

Efficient, non-stochastic, Monte-Carlo-like-accurate method for the calculation of the temperature-dependent mobility in nanocrystal films

“…5(A), show a decreasing behaviour with increasing temperature and separation, in qualitative agreement with experiment 5,6 . The latter is found to be exponential and reflects the dependence exhibited by the calculated wave function overlaps (i.e., coupling) between neighbouring dots 24,25 : Increasing the separation by 3 Å reduces the mobility by three orders of magnitude. This large reduction is due to the fact that our model assumes the dots to be surrounded by vacuum.…”

Metal-Insulator Transition in Nanoparticle Solids: Insights from Kinetic Monte Carlo Simulations

“…However, neither of them was capable of taking into account size disorder effects, and typically the NPs were small. Yet others focused on model Hamiltonian approaches and built a qualitative understanding of NP FETs 58 , photoconductors 59 and studied the effect of disorder in NP films providing some initial insights into the mechanism of MIT 60 – 63 . After all these efforts, a comprehensive transport theory of the MIT in NP solids is still not available and is very much needed.…”

Resilient Pathways to Atomic Attachment of Quantum Dot Dimers and Artificial Solids from Faceted CdSe Quantum Dot Building Blocks

“…[32][33][34] Much like their bulk counterparts, dislocations in imperfectly attached NCs likely introduce undesirable mid-gap trap states. 35,36 Furthermore, disorder in the attached NC superlattice structure, which can prevent carrier delocalization, 37 may also be introduced by dislocations. Thus, the realization of many of the theorized collective properties of attached NC arrays 38,39 will remain elusive until strategies to sufficiently reduce the density of electronically deleterious defects are developed.…”