Dependence of Carrier Mobility on Nanocrystal Size and Ligand Length in PbSe Nanocrystal Solids

Abstract

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.

PbSe Quantum Dot Field-Effect Transistors with Air-Stable Electron Mobilities above 7 cm2 V–1 s–1

“…We previously attributed such I D transients to screening of the applied gate field, possibly by charge trapped on the first layer of QDs. 17,18 Others have since observed similar I D transients in PbX QD FETs. 42,43 The transients are suppressed by ALD perhaps because the alumina coating passivates many of the surface traps responsible for gate screening; alternatively, the ALD matrix may inhibit whatever ligand or QD motion causes the transients.…”

“…The linear hole mobility (V SD = −10 V) calculated from eq 1 for a series of identical FETs is 0.1−0.15 cm 2 V −1 s −1 , which is 3−5 times larger than the hole mobility of EDT-treated FETs using the same QD size. 18 The larger hole mobility of sulfide-treated FETs may be due to the smaller interdot distance and better electronic coupling afforded by the smaller sulfide ligand (d S 2− ≈ 3.5 Å, d EDT 2− ≈ 8 Å; see Figure 2). 40 After ALD infilling and overcoating, the FET becomes ambipolar with a dominant nchannel and weak p-channel, decent on−off ratios (>1000), and nearly ideal I−V curves (Figure 4c,d).…”

An Enhanced UV–Vis–NIR an d Flexible Photodetector Based on Electrospun ZnO Nanowire Array/PbS Quantum Dots Film Heterostructure

“…As‐fabricated solution of QDs are typically capped with long ligands (oleic acid or oleylamine) which are necessary to maintain colloidal stable in the organic solvents,59 therefore, it is not easy to attach the QDs onto nanowires. Basically, ZnO NWs should be decorated with positive charged functional groups such as 3‐aminopropyltrimethoxysilane60 or treated the NWs by oxygen plasma61 to facilitate the attachment of QDs on NWs, then an EDT or TBAI solution was used to exchange the long ligand.…”

Controlled Chemical Doping of Semiconductor Nanocrystals Using Redox Buffers

“…[1][2][3] These solids consist of macroscopic arrays of colloidally synthesized nanoparticles, wherein the band gap and the electronic properties of the composite array can be tuned by varying the size 4 and surface chemistry 5 of the constitutive elements, respectively. However, colloidally synthesized nanocrystals typically exhibit low impurity concentrations, 6 requiring post-synthetic treatments to effectively dope the nanocrystal solid.…”

“…Over this range, the conductivity exhibits a steady monotonic increase of ~20 times, while the linear field-effect mobility decreases by a similar magnitude, leading to carrier concentrations extracted from Ohm's law displaying an increase of two orders of magnitude. The precise origin of the decrease in field effect mobility is unclear at present, but may be due to increasing energetic site disorder 5 or partial screening of the gate field due to counterion mobility within the film. 25 X-ray photoelectron spectroscopy (XPS) failed to detect PF 6 − in the treated film ( Figure S5), in line with the low observed doping levels: less than one free carrier per 100 nanoparticles for the highest doping level in Figure 5 (see SI for detailed calculation).…”