Electrical Transport and Grain Growth in Solution-Cast, Chloride-Terminated Cadmium Selenide Nanocrystal Thin Films

Abstract

We report the evolution of electrical transport and grain size during the sintering of thin films spin-cast from soluble phosphine and amine-bound, chloride-terminated cadmium selenide nanocrystals. Sintering of the nanocrystals occurs in three distinct stages as the annealing temperature is increased: (1) reversible desorption of the organic ligands (≤150 °C), (2) irreversible particle fusion (200–300 °C), and (3) ripening of the grains to >5 nm domains (>200 °C). Grain growth occurs at 200 °C in films with 8 atom % Cl–, while films with 3 atom % Cl– resist growth until 300 °C. Fused nanocrystalline thin films (grain size = 4.5–5.5 nm) on thermally grown silicon dioxide gate dielectrics produce field-effect transistors with electron mobilities as high as 25 cm2/(Vs) and on/off ratios of 105 with less than 0.5 V hysteresis in threshold voltage without the addition of indium.

Solvent-like ligand-coated ultrasmall cadmium selenide nanocrystals: strong electronic coupling in a self-organized assembly

“…Second, we can prevent formation of trap states and recombination sites while maintaining strong electronic coupling by using nonvolatile PEG-thiolate ligands instead of using chloride, 22 thiocyanate, 25 sulfide, [26][27][28] metal chalcogenide complexes, 13,[29][30][31] tetrafluoroborate, 23,24 hydrazine, 14,32 or pyridine. 19 Third, SNCs coated with either small molecules or various ions are susceptible to undergoing fast oxidation even at ambient conditions 19 making processing during device fabrication very difficult.…”

“…Use of this property of the SNC films would open a new frontier of scientific research by enabling the preparation of "artificial solids" with unprecedentedly high conductivity. 17 A number of methods have been developed to enhance the electronic coupling between SNCs in colloidal dispersion and/or solid films by exchanging the native bulky, hydrocarbon ligands with small organic ligands, [18][19][20][21] inorganic ions, [22][23][24][25][26][27][28] and metal chalcogenide complexes. 13,[29][30][31] The solid phase ligand exchange reaction can cause a dramatic structural rearrangement of SNCs, which results in the appearance of cracks and voids in the solid films or irreversible aggregation of SNCs inside the films.…”

“…19,32 In contrast, a solution phase exchange reaction would enable preservation of the solution dispersibility, which allows for better solution processing for more efficient film preparation. Various ligands such as chloride, 22 thiocyanate, 25 sulfide, [26][27][28] metal chalcogenide complexes, 13,[29][30][31] tetrafluoroborate, 23,24 hydrazine, 14,32 and pyridine 19 have 4 most commonly been used to perform solution phase exchange reactions. However, these newly formed ligand-coated SNCs do not show any electronic coupling in solution, which would present as a red-shift of the excitonic peaks in the absorption spectrum.…”

To Be or not to Be: Band-Like Transport in Quantum Dot Solids

“…This has invoked some skepticism as to whether the observed effects are in part due to necking and local fusion of QDs, upon which percolative pathways with greatly altered transport properties could form [27]. In such a situation, the genuine physics of QDs may not be applicable anymore such that a careful distinction between real QD solids with high mobilities and those with favorable transport properties due to macroscopic percolative pathways is important.…”

Single-step colloidal quantum dot films for infrared solar harvesting

“…Longer-chain amines as ligands have been demonstrated to aid colloidal stabilization and improve surface passivation in CdSe CQDs. 18,19 We found that hexylamine (HXA) could completely redisperse the quantum dots. However, due to its higher boiling point, the resulting films were comparable in thickness to those made from BTA.…”