Superlattices of epitaxially connected nanocrystals (NCs) are model systems to study electronic and optical properties of NC arrays. Using elemental analysis and structural analysis by in situ X-ray fluorescence and grazing-incidence small-angle scattering, respectively, we show that epitaxial superlattices of PbSe NCs keep their structural integrity up to temperatures of 300 °C; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices between 75 and 150 °C induces a marked red shift of the NC bandedge transition. In fact, the post-annealing band-edge reflects theoretical predictions on the impact of charge carrier delocalization in these epitaxial superlattices. In addition, we observe a pronounced enhancement of the charge carrier mobility and a reduction of the hopping activation energy after mild annealing. While the superstructure remains intact at these temperatures, structural defect studies through X-ray diffraction indicate that annealing markedly decreases the density of point defects and edge dislocations. This indicates that the connections between NCs in as-synthesized superlattices still form a major source of grain boundaries and defects, which prevent carrier delocalization over multiple NCs and hamper NC-to-NC transport. Overcoming the limitations imposed by interfacial defects is therefore an essential next step in the development of high-quality optoelectronic devices based on NC solids.

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Quantum dots (QDs) have attracted considerable attention in the development of various optoelectronic applications. The scalable heterogeneous integration of high quality, yet stable QD films is required for low-cost devices based on these materials. Here, we demonstrate the transfer printing of microscale patterns of Al2O3-passivated PbS QD films to realize large-scale integrated photodetector arrays with a 1 st excitonic absorption peak at 2.1 µm wavelength. The process provides a facile approach to selectively pick-and-print passivated QDs assemblies on device structures with high precision. Transfer-printed photoconductor devices were realized and characterized at low bias voltage and optical power. Under 10 nW surface normal illumination at 2.1 µm wavelength, the responsivity of our devices obtained at 1 V bias reached a maximum value of 25 A/W and 85 A/W for PbS QD films of 88 nm and 140 nm thick, respectively. Introduction:The short-wave infrared (SWIR: 0.9 -3µm) spectral range enables a wide range of applications, including hyperspectral imaging, sensing based on spectral signatures derived from molecular vibrations, night time surveillance, communications, etc. Detector arrays based on III-V semiconductors, such as InGaAs, are currently driving these applications due to their high quantum efficiency and low dark current at room temperature 2 . Unfortunately, the high material and fabrication cost per unit area prohibit the penetration of the technology into consumer applications.Moreover, monolithic integration on low-cost Si electronics is difficult due to the lattice mismatch between IR materials such as InGaAs and silicon 3 , requiring a hybrid integration through flip-chip bonding. Ideally, a good photodetector has to address the challenge of mass fabrication by realizing a relatively cheap material with high photoresponse and ease of integration on commercial silicon read-out integrated circuits.Colloidal quantum dots (CQDs) are a promising, new material for optical sensing applications due to their unique optical properties. The broad addressable spectrum with a sharp and tunable absorption onset based on size control, the high quantum efficiency and photostability make CQDs a suitable alternative to epitaxially grown semiconductors 4 . Moreover, colloidal synthesis is a lowcost fabrication approach that enables the QD size, shape and surface chemistry to be tuned, thereby optimizing the opto-electronic properties of the QDs for the envisioned application 5,6 .

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Superlattices of epitaxially connected nanocrystals (NCs) are model systems to study electronic and optical properties of NC arrays. Using elemental analysis and structural analysis by in situ X-ray fluorescence and grazing-incidence small-angle scattering, respectively, we show that epitaxial superlattices of PbSe NCs keep their structural integrity up to temperatures of 300 °C; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices between 75 and 150 °C induces a marked red shift of the NC bandedge transition. In fact, the post-annealing band-edge reflects theoretical predictions on the impact of charge carrier delocalization in these epitaxial superlattices. In addition, we observe a pronounced enhancement of the charge carrier mobility and a reduction of the hopping activation energy after mild annealing. While the superstructure remains intact at these temperatures, structural defect studies through X-ray diffraction indicate that annealing markedly decreases the density of point defects and edge dislocations. This indicates that the connections between NCs in as-synthesized superlattices still form a major source of grain boundaries and defects, which prevent carrier delocalization over multiple NCs and hamper NC-to-NC transport. Overcoming the limitations imposed by interfacial defects is therefore an essential next step in the development of high-quality optoelectronic devices based on NC solids.