Fig. 4: Characterisation of quantum dots.
a Normalised distribution of average widths of >1000 PbS QDs measured from TEM data taken after drop-casting QDs on to a TEM grid. μ = 5.0 nm, σ = 0.8 nm. b TEM image showing some of the QDs after drop-casting. c Calibrated image where the QD crystal structures can be seen. d, e, f Packing density of QDs assessed by replicating the self-assembly on a quartz-crystal microbalance (QCM) sample. This shows a C6S2 number density ~5.19 molecules/nm2 and a QD number density ~5.0 × 10−3 nm−2: the former is consistent with recorded values for a saturated alkanethiol SAM on Au49. The QD number density corresponds to an average QD centre-to-centre distance ~14.1 nm, and a surface-to-surface distance ~9.2 nm. d SEM micrograph showing ~50 nm Au grains with QDs assembled on top. e, f Self-assembly resolved using a contact-mode AFM on a template-stripped Au substrate for reduced surface roughness. g, h Energy-dispersive X-ray spectroscopy to identify elements in different device areas. Sections where QDs are expected (Au/QD/SLG) compared with areas where they are not (Au/SLG). The PbLα1:AuLα1 peak-count ratio, containing the PbLα1 line at 10.5515 keV and the AuLα1 line at 9.7133 keV, is measured many times and averaged for comparison. The ratio is 0.027 where QDs are present and 0.0034 where they are not. These numbers are compatible with the presence of a PbS SAM of QDs. g SEM micrograph of the region containing Au, QD and SLG, showing with a circle the Au/QD/SLG point at which EDX is carried out. h Corresponding EDX data showing PbLα1 and AuLα1 lines. i AFM image in (f) with low frequencies removed, and 2d Gaussian smoothing applied. Counting the peaks gives a NP density ~1.7 × 10−3 nm−2, three times smaller than that calculated from the QCM. This is an underestimate, as many NPs are probably not visible in this filtered picture. Even with this spacing, the voids between the irregularly arranged NPs are small enough that they should be bridged by SLG.
