Unravelling Nanocrystal Surface Structures with Relativistic DFT Calculations of Cadmium and Selenium Solid-State NMR Spectra of CdSe Nanocrystal Surfaces

Semiconductor nanocrystals (NCs) offer novel tunable electronic and optical properties that depend on their size, shape and surface passivation. Semiconductor NCs exhibit multiple exciton generation and are appealing for next-generation solar cells. Surface ligands and surface sites on NCs control their nucleation, growth, colloidal stability, chemical reactivity and optical/electronic properties. However direct experimental probes of the molecular structure of NC surfaces are lacking. Solid-state nuclear magnetic resonance (SSNMR) is a powerful tool to determine the structure of NC surfaces and sites on the surface where ligands can bind.<br/>Using CdSe as a prototypical example, SSNMR spectra have been measured to probe the structure of these nanocrystals and develop detailed models of their surface structures. DFT-optimized cluster models that represent probable molecular structures of carboxylate coordinated surface sites have been proposed. However, to the best of our knowledge, ¹¹³Cd and ⁷⁷Se chemical shifts have not been calculated for these surface models. We performed relativistic DFT calculations of cadmium and selenium magnetic shielding tensors on model compounds with previously measured solid-state NMR spectra, with (i) the 4-component Dirac-Kohn-Sham (DKS) Hamiltonian, and (ii) the scalar and (iii) spin-orbit levels within the ZORA Hamiltonian. Molecular clusters with Cd and Se sites in varying bonding environments were used to model CdSe (100) and CdSe (111) surfaces capped with carboxylic acid ligands. Our calculations identify the observed ¹¹³Cd isotropic chemical shifts δ(iso) of –465 ppm, –318 ppm and –146 ppm arising from CdSeO₃, CdSe₂O₂, and CdSe₃O surface groups respectively, with very good agreement with experimental measurements. The ¹¹³Cd chemical shifts linearly decrease with the number of O-neighbors. The calculated spans (δ₁₁ – δ₃₃) encompass the experimental values for CdSe₃O and CdSe₂O₂ clusters but are slightly larger than the measured value for CdSeO₃ clusters. Relativistic DFT calculations predicted a one-bond ¹¹³Cd-⁷⁷Se scalar coupling of 258 Hz, in good agreement with the experimental values of 250 Hz. With a dense coverage of carboxylic acid ligands, the CdSe (100) surface shows a distribution of Cd-Se bond lengths and J-couplings. Relativistic DFT simulations aid in interpretation of NMR spectra of CdSe nanocrystals and related nanomaterials, and offer new insights into the complex structures at nanocrystal surfaces.<br/>Supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. The research was performed at Ames Laboratory, which is operated for the U.S. DOE by Iowa State University under contract DE-AC02-07CH11358. We acknowledge the use of computational resources at the National Energy Research Scientific Supercomputing Center (NERSC) which is supported by the Office of Science of the U.S. DOE under contract no. DE-AC02-05CH11231.