The surface science of nanocrystals

Abstract

All nanomaterials share a common feature of large surface-to-volume ratio, making their surfaces the dominant player in many physical and chemical processes. Surface ligands - molecules that bind to the surface - are an essential component of nanomaterial synthesis, processing and application. Understanding the structure and properties of nanoscale interfaces requires an intricate mix of concepts and techniques borrowed from surface science and coordination chemistry. Our Review elaborates these connections and discusses the bonding, electronic structure and chemical transformations at nanomaterial surfaces. We specifically focus on the role of surface ligands in tuning and rationally designing properties of functional nanomaterials. Given their importance for biomedical (imaging, diagnostics and therapeutics) and optoelectronic (light-emitting devices, transistors, solar cells) applications, we end with an assessment of application-targeted surface engineering.

Real-Space Mapping of Surface Trap States in CIGSe Nanocrystals Using 4D Electron Microscopy

“…[1][2][3][4][5][6][7][8][9][10] However, due to high surface-to-volume ratio of these NCs, the large number of unpassivated atoms on their surfaces lead to the formation of highly dense trap states, which serve as undesirable, non-radiative deactivation channels for photo-generated charge carriers. [11][12][13][14][15] This often acts as a bottleneck in the use of these NCs for photoactive applications. Typically, multinary metal chalcogenides possess many desirable attributes, such as high absorption coefficient and high photo-stability for optoelectronic applications, [16][17][18][19][20][21] and a record power conversion efficiency of 21.7% has been obtained in the bulk form following physical deposition techniques, 22 which is close to the efficiency of polycrystalline silicon solar cells.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…2.2 above) or with suitable ligands that form strong bonds with the surface atoms, thereby shifting the energies of the surface states away from the HOMO-LUMO gap of the NC [16,123]. Other ligands may in fact generate localized interfacial states or mid-gap states that trap one of the carriers and induce PL quenching (e.g., hole trapping by alkanethiols on CdSe QDs [82,123,124]), or directly shift the NC electronic states due to electrostatic or orbital mixing effects [64,123,[125][126][127]. The capping ligand shell can be viewed as a self-assembled monolayer (SAM) at the surface of the NC [16,123].…”

“…Other ligands may in fact generate localized interfacial states or mid-gap states that trap one of the carriers and induce PL quenching (e.g., hole trapping by alkanethiols on CdSe QDs [82,123,124]), or directly shift the NC electronic states due to electrostatic or orbital mixing effects [64,123,[125][126][127]. The capping ligand shell can be viewed as a self-assembled monolayer (SAM) at the surface of the NC [16,123]. The internal structure of this SAM can also affect the PL of the NCs, either positively, by fostering surface reconstruction that eliminates trap states [16], or negatively, by imposing disorder to the surface [16,128].…”

“…They provide ''trap states'' for charge carriers, i.e., energy levels within the bandgap where the charge carrier is spatially localized. Indeed, the quantum efficiency of NC emission improves when the NC surface is covered with a protective shell of high-bandgap material [13,15,16,61,74,[243][244][245][246], or when ligands saturate chemical bonds on the surface [16,123,[247][248][249][250][251].…”

“…Panels d and e were adapted from Ref. [123] suppressed in each individual NC, but rather because the fraction of completely dark NCs in the ensemble is smaller. This is schematically depicted in Fig.…”