Effects of Post-Synthesis Processing on CdSe Nanocrystals and Their Solids: Correlation between Surface Chemistry and Optoelectronic Properties

Abstract

In this work, we report the effects on CdSe nanocrystal (NC) surface chemistry of acetone and methanol when used as the antisolvents for NC washing and as the solvents for ligand exchange of NC solids with ammonium thiocyanate (NH 4 SCN). We find that NCs washed with methanol have significantly fewer remaining organic ligands and lower photoluminescence quantum yield than those washed with acetone. When used as the ligand exchange solvent, methanol leaves more organic ligands and introduces fewer bound thiocyanates on the NC surface than when acetone is used. We demonstrate the effect of these different surface chemistries on NC solid optoelectronic properties through photoconductivity measurements, showing a greater photocurrent in NC solids with greater organic ligand coverage. We also show that NC washing with methanol or ligand exchange with NH 4 SCN in methanol removes a significant number of surface Cd atoms from the NCs, creating Cd vacancies that act as traps for recombination. Independent of the wash and exchange process, the NC surface may be repaired by introducing CdCl 2 to the NC surface, enhancing the measured photocurrent.

The effects of inorganic surface treatments on photogenerated carrier mobility and lifetime in PbSe quantum dot thin films

“…Similar structural transformations in PbS QD thin films have been reported for S-enriched surfaces [38] or through solvent stripping of surface ligands and atoms [39,40]. Surface passivation has been shown to increase the lifetime of photogenerated carriers [27,41] and doping shifts the Fermi level closer to the conduction or valence band, allowing carriers to populate these higher densities of states, and leads to the higher carrier mobilities reported in field-effect transistor (FET) measurements [5,37]. 4 PbSe QDs are particularly attractive as solar cell materials because of their broad absorption that spans the solar spectrum and the promise of increased power conversion efficiencies through multiple-exciton generation [42,43].…”

“…To reduce the interparticle spacing and design QD materials for electronic applications, the insulating ligands are replaced with short organic ligands, such as 3-mercaptopropionic acid (MPA) [23,24], or compact inorganic ligands, such as chalcogenides [25], halides [13], or the pseudohalide thiocyanate [26]. The solvents used in post-synthesis purification and in thin film deposition and the ligand exchange process can strip surface atoms [27][28][29], creating vacancies at the QD surface. Unlike bulk semiconductors with well-defined conduction and valence bands separated by a forbidden energy gap, the density of electronic states for QD thin films is constructed from a high density of quantized conduction and valence band states (or LUMO and HOMO) and localized mid-gap and tail states introduced by vacancies at the QD surface and by interfacial states introduced in electronic devices used to probe carrier transport.…”

“…Infilling the QD thin film with an inorganic matrix by atomic layer deposition has been shown to passivate the QD surface and increase the carrier mobility and lifetime [30,31]. Recent introduction of surface chemical modifications have been reported to enrich the QD surface in metal and/or chalcogen or halide ions [27,[32][33][34][35][36] that serve to dope the QD thin films and passivate surface traps, controlling the charge carrier type, concentration, mobility, and lifetime. For example, PbSe QD thin films surface-enriched with Se were doped p-type, but the surfaces were unstable and susceptible to oxidation and structural transformation.…”

“…NIR spectra were taken on an Agilent Cary 5000 spectrophotometer at 2 nm spectral bandwidth in transmission mode. NIR measurements reported herein were taken on QD thin films that were encapsulated in the nitrogen glovebox using cover glass and epoxy [27]. …”

Facile in situ Synthesis of Ag‐Doped CdSe Supra‐Quantum Dots and their Characterization

“…The energies of the conduction band minima (CBMs) were estimated from the optical bandgaps of the samples (Figure E). Consequently, the positions of the E F , VBM, and CBM of each CdSe SQD film were determined and are summarized in the energy diagram in Figure C. The Cd‐rich character of our CdSe SQDs (Table ) gives rise to n‐type doping and both samples exhibit n‐type character. When compared with the undoped sample, the E F value of the Ag‐doped CdSe SQD sample is closer to the energy of the CB than that of the undoped sample with increased n‐type character.…”