Photobase-Triggered Formation of 3D Epitaxially Fused Quantum Dot Superlattices with High Uniformity and Low Bulk Defect Densities

Abstract

Highly ordered epitaxially fused colloidal quantum dot (QD) superlattices (epi-SLs) promise to combine the size-tunable photophysics of QDs with the efficient charge transport of bulk semiconductors. However, current epi-SL fabrication methods are crude and result in structurally and chemically inhomogeneous samples with high concentrations of extended defects that localize carriers and prevent the emergence of electronic mini-bands. Needed fabrication improvements are hampered by inadequate understanding of the ligand chemistry that causes epi-SL conversion from the unfused parent SL. Here we show that epi-SL formation by the conventional method of amine injection into an ethylene glycol subphase under a floating QD film occurs by deprotonation of glycol by the amine and subsequent exchange of oleate by glycoxide on the QD surface. By replacing the amine with hydroxide ion, we demonstrate that any Brønsted–Lowry base that creates a sufficient dose of glycoxide can produce the epi-SL. We then introduce an epi-SL fabrication method that replaces point injection of a base with contactless and uniform illumination of a dissolved photobase. Quantitative mapping of multilayer (3D) films shows that our photobase-made epi-SLs are chemically and structurally uniform and have much lower concentrations of bulk defects compared to the highly inhomogeneous and defect-rich epi-SLs produced by amine point injection. The structural–chemical uniformity and structural perfection of photobase-made epi-SLs make them leading candidates for achieving emergent mini-band charge transport in a self-assembled mesoscale solid.

Photopolymerization activated by photobase generators and applications: from photolithography to high-quality photoresists

“…n-type SWCNT sheets achieved extremely long air stability above 100 days. In another work, Qian et al 96 suggested that the PBG, (E)-1-piperidino-3-(2-hydroxyphenyl)-2-propen-1-one, could be regarded as an efficient base source to fabricate highly homogeneous films epitaxially fused quantum dot (QD) superlattices (epi-SLs) from assembled oleate-capped superlattices, which can be further performed in photopatterning for optoelectronic devices. In photobase illumination method, the PBG releases piperidine under UV light irradiation, then reacts with ethylene glycol to produce glycoxide to trigger by glycoxideoleate ligand exchange (see Fig.…”

High-Mobility Hole Transport in Single-Grain PbSe Quantum Dot Superlattice Transistors

“…The FFT image of the entire channel yields a spot pattern that was indexed to the major (red) and minor (yellow) epi-SL grains (Figure 2c), while numerous FFTs of regions within the channel show an average inter-QD distance of 6.86 nm along [100] SL and a planar QD density of 17,390 QDs/μm 2 per QD monolayer, consistent with our previous reports. 11,12 Structural analysis of seven additional devices is provided in the Supporting Information, along with the corresponding charge transport data (Figures S5−S18). Devices 1−7 are single-grain FETs, while Device 8 is multicrystalline and serves as a control for the effect of grain boundaries on charge transport.…”

“…3D (multilayer) epi-SL films were fabricated by self-assembly of 6.9 nm diameter oleate-capped PbSe QDs (Figure S1) on the surface of liquid ethylene glycol (EG), followed by injection of 1,2-ethylenediamine to trigger epitaxial fusion (necking) of the QDs via glycoxide–oleate ligand exchange , and stamp transfer of the resulting polycrystalline epi-SLs to custom-made device substrates (see Methods in the Supporting Information and Figure a). This procedure yields epi-SLs with a rhombohedrally-distorted simple cubic unit cell ( a ≈ 6.9 nm, α ≈ 99°) in which the QDs are necked across their {100} facets and the epi-SL grains are oriented predominantly with their (011̅) SL planes parallel to the substrate surface (Figure b) .…”

“…Short localization lengths indicate that carriers remain localized within small clusters of QDs, likely as a result of the disorder in epi-SL neck size (including missing necks) and electronic coupling. , Surface/interface states, QD positional and orientational disorder, and point charges within the films can also disrupt the periodicity of the energy landscape and thereby localize charge carriers. Ongoing efforts to incorporate more perfect epi-SLs ,,, into smaller FETs (<0.5 μm channels) combined with strategies to eliminate or fill carrier traps should soon enable electrical measurements of individual delocalized domains within epi-SL grains. Such small devices will require particularly low-resistance electrical contacts.…”

“…Epitaxially-fused colloidal PbX (X = S, Se) quantum dot (QD) superlattices (epi-SLs)arrays of QDs interconnected by homoepitaxial neckscombine extraordinary spatial order with strong inter-QD electronic coupling and are predicted to exhibit electronic minibands that enable high-mobility, bandlike charge transport. − However, the epi-SLs studied thus far are too structurally defective (disordered) to support miniband formation, and electrical measurements of epi-SLs have demonstrated only localized carriers and hopping transport. , Ongoing efforts to eliminate intragrain defects are essential to trigger the emergence of minibands, − but it is also important to remove intergrain defects (grain boundaries and cracks) from the transport path in order to study the inherent properties of individual epi-SL grains (monocrystals) . A robust process for making measurements at the single-grain limit (monocrystalline devices) would enable systematic studies of miniband physics as a function of epi-SL structural order, surface chemistry, orientation, device size, and other parameters.…”