Atomistic Insights on Facet-Dependent Functional Properties in CdSe Nanocrystals

Semiconductor nanocrystals (NCs) are used for various optoelectronic devices ranging from electronic displays to solar cells to quantum emitters. Owing to the large surface-to-volume ratio, photophysics is dominantly controlled by the atomistic details of NC facets. The insights into NC facet geometry, defects, and their impact on the functional properties remain largely limited due to the need for experimental probes. Here, we combine accurate first-principles computational techniques to probe into the crystal facet-specific electronic, optical, and catalytic properties of metal-rich nonstoichiometric wurtzite cadmium selenide (CdSe) NCs. The undercoordinated Se atoms on the unpassivated NC surface give rise to strongly localized trap states near the edge of occupied levels. The inorganic salts selectively bind and passivate these anionic sites, efficiently mitigating the trap states. However, excess passivation can reintroduce the trap states above the delocalized occupied states. The band edge states are primarily delocalized on the polar facets of the trap-free midsized (diameter ∼ 1.79–2.38 nm) CdSe NC. These facet states are specific to the wurtzite phase NCs as zinc blende ones exhibit mostly delocalized band edge states. The computed absorption spectra reveal the beneficial impact of precise surface passivation on the optical characteristics of CdSe NCs. The location of active atomic sites and facet polarity further play a critical role in their catalytic performance for hydrogen evolution reactions. Our comprehensive study pins down the predominant influence of facet properties on the electronic, optical, and catalytic characteristics of CdSe NCs. These atomistic details will guide the experimental efforts to control the phase, shape, size, and surface geometry of semiconductor NCs for optimum performance.