Elimination of the bias-stress effect in ligand-free quantum dot field-effect transistors.

Field-effect transistors (FETs) made from colloidal quantum dot (QD) solids commonly suffer from current-voltage hysteresis caused by the bias-stress effect (BSE), which complicates fundamental studies of charge transport in QD solids and the use of QD FETs in electronics. Here, we show that the BSE can be eliminated in n-channel PbSe QD FETs by first removing the QD ligands with a dose of H2S gas and then infilling the QD films with alumina by atomic layer deposition (ALD). The H2S-treated, alumina-infilled FETs have stable, hysteresis-free device characteristics (total short-term stability), indefinite air stability (total long-term stability), and a high electron mobility of up to 14 cm2 V-1 s-1, making them attractive for QD circuitry and optoelectronic devices. The BSE-free devices are utilized to conclusively establish the dependence of the electron mobility on temperature and QD diameter. We demonstrate that the BSE in these devices is caused by both electron trapping at the QD surface and proton drift within the film. The H2S/alumina chemistry produces ligand-free PbSe/PbS/Al2O3 interfaces that lack the traps that cause the electronic part of the BSE, while full alumina infilling stops the proton motion responsible for the ionic part of the BSE. Our matrix engineering approach should aid efforts to eliminate the BSE, boost carrier mobilities, and improve charge transport in other types of nanocrystal solids.

PbSe quantum dot field-effect transistors with air-stable electron mobilities above 7 cm2 V(-1) s(-1).

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 low-temperature 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.

Robust, functional nanocrystal solids by infilling with atomic layer deposition.

Thin films of colloidal semiconductor nanocrystals (NCs) are inherently metatstable materials prone to oxidative and photothermal degradation driven by their large surface-to-volume ratios and high surface energies. (1) The fabrication of practical electronic devices based on NC solids hinges on preventing oxidation, surface diffusion, ripening, sintering, and other unwanted physicochemical changes that can plague these materials. Here we use low-temperature atomic layer deposition (ALD) to infill conductive PbSe NC solids with metal oxides to produce inorganic nanocomposites in which the NCs are locked in place and protected against oxidative and photothermal damage. Infilling NC field-effect transistors and solar cells with amorphous alumina yields devices that operate with enhanced and stable performance for at least months in air. Furthermore, ALD infilling with ZnO lowers the height of the inter-NC tunnel barrier for electron transport, yielding PbSe NC films with electron mobilities of 1 cm2 V(-1) s(-1). Our ALD technique is a versatile means to fabricate robust NC solids for optoelectronic devices.

The photothermal stability of PbS quantum dot solids.

We combine optical absorption spectroscopy, ex situ transmission electron microscopy (TEM) imaging, and variable-temperature measurements to study the effect of ultraviolet (UV) light and heat treatments on ethanedithiol-treated PbS quantum dot (QD) films as a function of ambient atmosphere, temperature, and QD size. Film aging occurs mainly by oxidation or ripening and sintering depending on QD size and the presence of oxygen. We can stop QD oxidation and greatly suppress ripening by infilling the films with amorphous Al(2)O(3) using room-temperature atomic layer deposition (ALD).

Colloidal iron pyrite (FeS2) nanocrystal inks for thin-film photovoltaics.

Iron pyrite (FeS2) is a promising earth-abundant semiconductor for thin-film solar cells. In this work, phase-pure, single-crystalline, and well-dispersed colloidal FeS2 nanocrystals (NCs) were synthesized in high yield by a simple hot-injection route in octadecylamine and then were subjected to partial ligand exchange with octadecylxanthate to yield stable pyrite NC inks. Polycrystalline pyrite thin films were fabricated by sintering layers of these NCs at 500−600 °C under a sulfur atmosphere.

Dependence of carrier mobility on nanocrystal size and ligand length in PbSe nanocrystal solids.

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.

p-Type PbSe and PbS quantum dot solids prepared with short-chain acids and diacids.

We show that ligand exchange with short-chain carboxylic acids (formic, acetic, and oxalic acid) can quantitatively remove oleic acid from the surface of PbSe and PbS quantum dot (QD) films to yield p-type, carboxylate-capped QD solids with field-effect hole mobilities in the range of 10(-4)-10(-1) cm(2) V(-1) s(-1). For a given chemical treatment, PbSe devices have 10-fold higher mobilities than PbS devices because of stronger electronic coupling among the PbSe QDs and possibly a lower density of surface traps. Long-term optical and electrical measurements (i) show that carboxylate-capped PbSe QD films oxidize much more gradually in air than do thiol-capped PbSe films and (ii) quantify the slower and less extensive oxidation of PbS relative to PbSe QDs. We find that whereas the hole mobility of thiol-capped samples decreases continuously with time in air, the mobility of carboxylate-capped films first increases by an order of magnitude over several days before slowly decreasing over weeks. This behavior is a consequence of the more robust binding of carboxylate ligands to the QD surface, such that adsorbed oxygen and water initially boost the hole mobility by passivating surface states and only slowly degrade the ligand passivation to establish an oxide shell around each QD in the film. The superior hole mobilities and oxidation resistance of formic- and acetic-treated QD solids may prove useful in constructing efficient, stable QD photovoltaic devices.