The emission line widths of semiconductor nanocrystals yield insight into the factors that give rise to their electronic structure, thereby providing a path for utilizing nanocrystals in light emissive applications. Experiment and theory in conjunction reveal the contributions to line broadening to the core and surface emission bands. As nanocrystals become small, broad emission from the surface becomes prominent. In the case of the core emission, we reveal previously unobserved vibronic contributions in addition to the already well-known electronic structure of the band-edge exciton. As the temperature decreases, broad emission from the surface becomes prominent. This surface emission also exhibits vibronic contributions albeit more strongly. Analysis of the surface emission reveals the existence of a previously unobserved electronic structure of the surface in complete parallel to that of the core. The surface is characterized by a bright and dark state as well as a spectrum of bright states.

The size-dependent optical and electronic properties of semiconductor nanocrystal (NC) have been exploited over decades for various applications. This size dependence involves a transition from the regime of bulk colloids of ∼100 nm radius to quantum dots (QDs) of ∼10 nm radius, the details of which are material specific. To understand the transition from the QD regime (∼10 nm) to the molecular cluster regime (∼1 nm) of nanocrystals, we have carefully synthesized a set of CdSe nanocrystals with sizes ranging from 0.89 to 1.66 nm in radius. As the nanocrystals become small, the surface emission strongly increases in amplitude, and the core emission broadens and red-shifts. These effects are rationalized in terms of coupling to ligands via electron transfer theory. The core emission spectra arise from increased vibrational coupling of ligands for very small NC. The surface emission amplitudes arise from a size-dependent surface free energy. The transition from the QD to the molecular cluster regime is found to be at 1.2 nm radius, in contrast to the transition from the bulk to QD transition at the Bohr radius of 5.4 nm in CdSe. These size-dependent surface electronic phenomena may be used for light emission applications.

Semiconductor nanocrystals emit light from excitons confined to their core, as well as from their surfaces. Time-resolving the emission from the core yields information on the band edge exciton, which is now well understood. In contrast, the emission from the surface is ill-characterized and remains poorly understood, especially on long time scales. In order to understand the kinetics of charge trapping to the surface and electronic relaxation within the surface, we perform time-resolved emission spectroscopy on CdSe nanocrystals with strong surface emission. The time-resolved spectra reveal a time scale of electron transfer from core to surface much slower than previously thought. These spectra also unveil electron dynamics in the surface band, which gives rise to an average lifetime spectrum. These dynamics are explained by invoking two surface states. This simple model further rationalizes the role of ligands in tuning the surface emission of nanocrystals. These experimental results provide a critical test of our understanding of the electronic structure of the surface.

Here, we report resonance Raman spectra of CdSe colloidal nanocrystals (NCs) passivated with organic ligands. In addition to the well-known longitudinal optical phonons, we observe ligand vibrations. The ligand vibrations are shown to be resonantly enhanced through electronic mixing with the states of the NC. These measurements were enabled by substituting the native ligands with thiophenol. Thiophenol serves as an ideal probe for exciton–ligand coupling as it is a widely employed Raman molecular tag and quenches background luminescence in CdSe. The ligand vibrations are shown to be resonantly enhanced through exciting NC transitions. We show that vibronic coupling is observable in CdSe with diameters from 2 to 6 nm and for both phosphonic acid and amine native ligands. The coupling is evidenced by both asymmetric and symmetric mode enhancement through Herzberg–Teller  or Franck–Condon and Herzberg–Teller mechanisms, repectively. The ligand exchange quenching strategy may be generally applicable to study exciton–ligand interactions in a variety of semiconductor NC materials and reveals information on the electronic and vibrational structure of the NC surface.

There are several contradictory accounts of the changes to the emissive behavior of semiconductor nanocrystal upon a ligand exchange from trioctylphosphine/cadmium-phosphonates passivation to N-butylamine. This communication explains the contradictory accounts of this reaction using new insights into ligand chemistry. Also, a previously unknown link between surface emission and cadmium-phosphonate (Z-type) ligands is shown.