Conspectus: Surfaces -and interfaces- are ubiquitous at the nanoscale. Their relevance to nanoscience and nanotechnology is therefore inherent. Colloidal inorganic nanocrystals (NCs), which can show more than a half of their atoms at the surface, are paradigmatic of the role of surfaces in determining materials' form and functions. Therefore, colloidal NCs may be regarded as soluble surfaces, allowing convenient study of ensemble structure and properties in the solution phase.Colloidal NCs commonly bear chemical species at their surface. Such species (generally referred to as ligands) are introduced already in the synthetic procedures and are added postsynthesis in surface chemistry modification (ligand exchange) reactions. Ligands (i) affect the reactivity and diffusion of the synthetic precursors, (ii) mediate NC interactions with the surroundings, and (iii) contribute to the overall electronic structure. In principle, a vast amount of ligands, as large as our imagination, could be used to coordinate the surface of colloidal NCs. In practice and despite the plethora of studies on NC surface chemistry, a relatively limited number of ligands have been explored. In addition, the importance of designing a set of ligands with tailored features (a ligand library), which may permit comprehensive discussion and explanation of the role of surfaces in the NC structure and properties, is often overlooked. Ligand libraries may also foster heuristic access to novel, unexpected observations.Here, the rational design of ligand libraries is discussed, suggesting that it may be a general method to advance knowledge on colloidal NCs and nanomaterials at large.First, a general ligand framework is introduced. The main subunits are identified: Ligands are constituted by a binding group and a pendant moiety, bearing functional substituent groups. On this basis, ligand binding at the NC surface is discussed borrowing concepts from coordination chemistry. Dynamic equilibria at the NC surface are highlighted, revealing the compromise between forming and breaking bonds at interfaces and its intricate interplay with the surroundings. Tailoring of the ligand subunits may impart functions to the whole ligand, eventually transposable to the ligated NC.On these bases, it is shown how ligand design may be exploited to (i) exert control on the size and shape of the NCs, (ii) determine NCs' dispersibility in a solvent and affect their self-assembly, and (iii) tune the NCs' optical and electronic properties. These observations point to a description of colloidal NCs as un-decomposable species: Ligands may be conceived as an integral part of the overall chemical and electronic structure of the colloidal NC and should not be considered as mere appendages that weakly perturb the inorganic core features.Finally, a perspective on the ligand library design is given. Function-oriented design of the ligand subunits is foreseen as an effective strategy to explore the chemical diversity space. High-throughput screening processes by using computation may represent a valuable tool for such an exploration. The whole ligand features, which depend on the subunits, can be implemented in the final NCs, providing feedback for refined design, toward a priori materials design. Ligand libraries can be fundamental to enabling colloidal NCs as reliable luminophores and (photo)catalysts.

Chemical species at the surface (ligands) of colloidal inorganic semiconductor nanocrystals (QDs) markedly impact the optoelectronic properties of the resulting systems. Here, post-synthesis surface chemistry modification of colloidal metal chalcogenide QDs is demonstrated to induce both broadband absorption enhancement and band gap reduction. A comprehensive library of chalcogenol(ate) ligands is exploited to infer the role of surface chemistry on the QD optical absorption: the ligand chalcogenol(ate) binding group mainly determines the narrowing of the optical band gap, which is attributed to the np occupied orbital contribution to the valence band edge, and mediates the absorption enhancement, which is related to the ?-conjugation of the ligand pendant moiety, with further contribution from electron donor substituents. These findings point to a description of colloidal QDs that may conceive ligands as part of the overall QD electronic structure, beyond models derived from analogies with core/shell heterostructures, which consider ligands as mere perturbation to the core properties. The enhanced light absorption achieved via surface chemistry modification may be exploited for QD-based applications in which an efficient light-harvesting initiates charge carrier separation or redox processes.

Surface chemistry modification of as-synthesized colloidal inorganic semiconductor nanocrystals (QDs), commonly referred to as ligand exchange, is mandatory toward effective QD-based optoelectronic and photocatalytic applications. The widespread recourse to ligand exchange procedures on metal chalcogenide QDs often narrows the optical band gap, although little consensus exists on explanation of this experimental evidence. This work attempts at providing a comprehensive description of such a phenomenon by exploiting rationally designed thiol ligands at the surface of colloidal PbS QDs, as archetype of material in the strong quantum confinement regime: the thiol(ate)-induced QD optical band gap reduction almost linearly scales with the inorganic core surface-to-volume ratio and mainly depends on the sulfur binding atom, which is here suggested to contribute occupied 3p orbitals to the valence band edge of the QDs. As opposed to QD models based on the analogy with core/shell heterostructures, the indecomposable character of ligand/core adducts (the colloidal QDs themselves) arises.

Surface traps are ubiquitous to nanoscopic semiconductor materials. Understanding their atomistic origin and manipulating them chemically have capital importance to design defect-free colloidal quantum dots and make a leap forward in the development of efficient optoelectronic devices. Recent advances in computing power established computational chemistry as a powerful tool to describe accurately complex chemical species and nowadays it became conceivable to model colloidal quantum dots with realistic sizes and shapes. In this Perspective, we combine the knowledge gathered in recent experimental findings with the computation of quantum dot electronic structures. We analyze three different systems: namely, CdSe, PbS, and CsPbI as benchmark semiconductor nanocrystals showing how different types of trap states can form at their surface. In addition, we suggest experimental healing of such traps according to their chemical origin and nanocrystal composition.