Influence of Surface Chemistry on the Binding and Electronic Coupling of CdSe Quantum Dots to Single Crystal TiO₂Surfaces

Abstract

Sensitization of mesoporous nanocrystalline TiO(2) solar cells with quantum confined semiconductor nanocrystals (QDs) has some advantages over organic dyes or inorganic complex sensitizers, yet the reported efficiencies of laboratory devices are not currently competitive with those of dye sensitized cells. Several methods previously utilized to bind CdSe QDs to mesoporous TiO(2) films were investigated using low index faces of both anatase and rutile TiO(2) polytypes as model systems. The in situ ligand exchange method, where 3-mercaptopropionic acid (MPA) covered TiO(2) crystal surfaces are treated with trioctylphosphine (TOP)/trioctylphosphine oxide (TOPO) (TOP/TOPO)-capped CdSe QDs, resulted in very irreproducible and usually low sensitized photocurrents. The ex situ ligand exchange method, whereby MPA-capped QDs are synthesized and directly adsorbed onto bare TiO(2) single crystals, resulted in both reproducible sensitized photocurrents and surface coverages that are verified with atomic force microscopy (AFM). Purification of the nanocrystals and adjustment of the pH of the sensitization solution to >10.2 was found to prevent QD agglomeration and takes advantage of the dual chemical functionality of MPA to directly link the QDs to the TiO(2) surface. The spectral response of the incident photon to current efficiencies of CdSe QDs was directly compared to the commonly used sensitizer cis-di(thiocyanato)-bis(4,4;-dicarboxy-2,2'-bipyridine)ruthenium(II) (N3) on the same single crystals.

Fluorescence Intermittency and Energy Transfer in Small Clusters of Semiconductor Quantum Dots

“…[9][10][11][12][13] QDs have been studied extensively via both ensemble [14][15][16][17] and single-molecule spectroscopic techniques, 4,[18][19][20][21][22][23][24] such that the optical and electronic properties of isolated QDs are reasonably well understood. However, many applications involve coupling QDs to other species, such as fluorophores, 3,[25][26][27] electron donor/acceptors, 8,26,28,29 or other QDs. 20,[30][31][32][33][34][35][36][37] This coupling may render the optical and electronic properties of the QDs partially or entirely altered.…”

Linker-Assisted Assembly and Interfacial Electron-Transfer Reactivity of Quantum Dot−Substrate Architectures

“…QDs have size-dependent band gaps, large oscillator strengths, and high cross sections for multiphoton absorption. [1][2][3] The energy, bandwidth, and quantum yield of emission from QDs vary greatly with surface functionalization. Thermalization of photogenerated electrons within QDs can be slowed by the phonon bottleneck.…”

“…3 Adsorption of free capping groups or other molecular impurities may interfere with desired surface attachment reactions. 3 The concentration of free capping groups in dispersions of QDs depends on the postsynthesis washing procedure and is highly variable. Competitive adsorption may be particularly significant for TDPA-capped QDs and nominally TOPO-capped QDs, which contain phosphonic acids as impurities and as adsorbed capping groups.…”

Infrared Colloidal Quantum Dot Photovoltaics via Coupling Enhancement and Agglomeration Suppression

“…As shown schematically in Figure 3a (top), exposing the surface of adjacent CQDs during solid state ligand exchange can lead to fusing, creating larger nanoparticles with smaller bandgaps. [33][34][35][36] The fusing of nanocrystal surfaces has been used to affect the electronic properties of CQD solids and induce ordering in films, particularly in the case of large nanoparticles with well-defined facets. [37][38][39][40] While transport is improved in fused dots, the quantum confinement is reduced and the sharpness of the bandedge is lost.…”