We measure the room-temperature electron and hole field-effect mobilities (micro(FE)) of a series of alkanedithiol-treated PbSe nanocrystal (NC) films as a function of NC size and the length of the alkane chain. We find that carrier mobilities decrease exponentially with increasing ligand length according to the scaling parameter beta = 1.08-1.10 A(-1), as expected for hopping transport in granular conductors with alkane tunnel barriers. An electronic coupling energy as large as 8 meV is calculated from the mobility data. Mobilities increase by 1-2 orders of magnitude with increasing NC diameter (up to 0.07 and 0.03 cm(2) V(-1) s(-1) for electrons and holes, respectively); the electron mobility peaks at a NC size of approximately 6 nm and then decreases for larger NCs, whereas the hole mobility shows a monotonic increase. The size-mobility trends seem to be driven primarily by the smaller number of hops required for transport through arrays of larger NCs but may also reflect a systematic decrease in the depth of trap states with decreasing NC band gap. We find that carrier mobility is independent of the polydispersity of the NC samples, which can be understood if percolation networks of the larger-diameter, smaller-band-gap NCs carry most of the current in these NC solids. Our results establish a baseline for mobility trends in PbSe NC solids, with implications for fabricating high-mobility NC-based optoelectronic devices.

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PbSe quantum dot (QD) field effect transistors (FETs) with air-stable electron mobilities above 7 cm 2 V −1 s −1 are made by infilling sulfide-capped QD films with amorphous alumina using lowtemperature atomic layer deposition (ALD). This high mobility is achieved by combining strong electronic coupling (from the ultrasmall sulfide ligands) with passivation of surface states by the ALD coating. A series of control experiments rule out alternative explanations. Partial infilling tunes the electrical characteristics of the FETs. KEYWORDS: Quantum dots, nanocrystals, lead selenide, field-effect transistors, solar cells T he recent introduction of metal chalcogenide complexes (MCCs) as ligands for colloidal quantum dots (QDs) 1 has triggered a flurry of research into inorganic ligands for fabricating high-performance all-inorganic QD solids for optoelectronic applications. 2−4 In addition to several demonstrations of MCC efficacy by Talapin and co-workers, 5−8 a variety of metal-free inorganic ions including chalcogenides, 9,10 halides, 11 thiocyanate, 12−14 and trialkyl oxonium 15 have been shown in initial studies to provide generally better performance in CdX and PbX (X = S, Se, Te) QD field-effect transistors (FETs) 6,7,12−14 and solar cells 11 than the small molecules, such as hydrazine 16 and 1,2-ethanedithiol (EDT), 17,18 traditionally used to replace the long-chain insulating organic ligands inherited from QD synthesis. Ionic inorganic ligands offer several key advantages over neutral molecular ligands. First, many inorganic ligands are ultrasmall and enable strong electronic coupling between QDs in films, which favors highmobility transport. Second, inorganic ions can quantitatively replace native long-chain ligands on the QD surface to produce charge-stabilized colloidal QD suspensions in polar media, in principle allowing the direct formation of conductive QD films from solution without the need for postassembly chemical or thermal treatments that can inhibit charge transport by increasing spatial and energetic disorder in the films. In practice, however, thermal treatments (150−300°C) are typically needed to achieve good transport in all-inorganic QD solids. Also, solution-phase exchange has so far failed to yield stable all-inorganic PbX QD colloids except with select hydrazine-free MCCs or mixed chalcogenide ions, 5,13 so postassembly ("solid state") ligand exchange has been employed instead to make PbX QD devices. 10−13,15 A third advantage of inorganic ligands is that they decompose, evaporate, or assimilate into the QDs at relatively low temperatures to create functional inorganic matrices (e.g., with MCCs) or direct QD−QD contact and partial QD necking/fusion (e.g., with S 2− and SCN − ). Despite the large site energy disorder induced by such annealing, 7,14,19 the electronic properties of films made with this approach may be adequate for many applications, including high-efficiency solar energy conversion. Recent reports of record mobilities for electrons in CdSe, 7,14 holes in PbX, 12,1...

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In this contribution we demonstrate the effective separation of single-wall carbon nanotube (SWCNT) species with diameters larger than 1 nm through multistage aqueous two-phase extraction (ATPE), including isolation at the near-monochiral species level up to at least the diameter range of SWCNTs synthesized by electric arc synthesis (1.3-1.6 nm). We also demonstrate that refined species are readily obtained from both the metallic and semiconducting subpopulations of SWCNTs and that this methodology is effective for multiple SWCNT raw materials. Using these data, we report an empirical function for the necessary surfactant concentrations in the ATPE method for separating different SWCNTs into either the lower or upper phase as a function of SWCNT diameter. This empirical correlation enables predictive separation design and identifies a subset of SWCNTs that behave unusually as compared to other species. These results not only dramatically increase the range of SWCNT diameters to which species selective separation can be achieved but also demonstrate that aqueous two-phase separations can be designed across experimentally accessible ranges of surfactant concentrations to controllably separate SWCNT populations of very small (∼0.62 nm) to very large diameters (>1.7 nm). Together, the results reported here indicate that total separation of all SWCNT species is likely feasible by the ATPE method, especially given future development of multistage automated extraction techniques.