Supporting Information. Solution-Processed 2D PbS Nanoplates with Residual Cu 2 S. Exhibiting Low Resistivity and High Infrared Responsivity

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1 Supporting Information Solution-Processed 2D PbS Nanoplates with Residual Cu 2 S Exhibiting Low Resistivity and High Infrared Responsivity Wen-Ya Wu, Sabyasachi Chakrabortty, Asim Guchhait, Gloria Yan Zhen Wong, Goutam Kumar Dalapati, Ming Lin and Yinthai Chan * Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore , Singapore. chmchany@nus.edu.sg Institute of Materials Research and Engineering (IMRE), A*STAR, 2 Fusionopolis Way, Innovis, Singapore, , Singapore. *Corresponding Author: (Yinthai Chan) chmchany@nus.edu.sg Structural and optical characterization. Transmission Electron Microscopy (TEM) A JEOL JEM 1220F (100 kv accelerating voltage) microscope was used to obtain bright field TEM images of the nanoparticles. For TEM sample preparation, a drop of the nanoparticle solution was placed onto a 300 mesh copper grid covered with a continuous carbon film. Excess solution was removed by an adsorbent paper and the sample was dried at under ambient conditions. The High-Resolution TEM images and elemental analysis were carried out on a FEI Titan electron microscope (operated at 200 KV) which was equipped with an electron beam monochromator, an energy dispersive X-ray spectroscopy (EDX) and a Gatan electron 1

2 energy loss spectrometer. The probing electron beam size for point EDX measurements was around ~0.3 nm. The dwell time for each EDX spectrum was about 30 s. X-Ray Diffraction (XRD) X-ray diffraction data was obtained with a diffractometer (Bruker AXS, GADDS) using Cu-K α radiation (λ= å) in the range of 20 to 80. Thin film samples were prepared on a clean silicon wafer by placing several drops of concentrated nanoparticle samples in hexane on the silicon surface and dried at 60 C in an oven. This was repeated several times until a thin film of nanoparticles was formed on the silicon substrate. The powder form of XRD samples were prepared by several times of purification followed by drying under N 2. Atomic Force Microscope (AFM) AFM data was acquired via a Bruker Dimension FastScan AFM operating in tapping mode (FASTSCAN-A). A software titled Nanoscope Analysis was used to determine the height profile of the NPL structures which were deposited onto silicon substrates. Optical characterization Solution sample and thin film sample: The absorption spectra for solution based samples were obtained with an Agilent 8453 UV- Visible spectrophotometer using a quartz cuvette with a path length of 1 cm. The absorbance of thin films of NPLs on a quartz substrate were measured using the same spectrophotometer. The quartz substrates were mounted on a holder to ensure that the incident light was perpendicular to the plane of the substrate. It should be noted that the NPL films on quartz were prepared the same way as the NPL films used for photodetector device fabrication. 2

3 Additional Experiments Extent of Pb 2+ exchange Figure S1. Series of XRD spectra showing the evolution of crystal structure as the Cu 2 S NPLs progressively underwent Pb 2+ exchange to yield PbS NPLs. The peak labeled with * was identified to be background signal from the substrate. Table S1. Theoretically determined values for the diffusion activation barrier of Cu and Pb in Cu 2 S and PbS as reported in the literature. Diffusing element Diffusion Activation Barrier (ev) In Cu 2 S In PbS Cu Pb

4 Figure S2. TEM image of PbS NPLs with numerous holes that were synthesized via Pb 2+ exchange with ultrathin Cu 2 S NPLs that were ~2 nm thick. Figure S3. Theoretical reciprocal lattice of (a) Cu 2 S when the zone axis is <002> and (b) PbS when the zone axis is <111>. 4

5 Figure S4. (a) Absorbance spectra of Cu 2 S and PbS NPLs in chloroform. (b) Atomic force microscopy image of PbS NPLs each with a height profile corresponding to a thickness of ~3 nm. (c) Atomic force microscopy image of a PbS NPL film with a scratched region across the film. (d) Corresponding height profile of the PbS NPL film featuring a thickness of ~57 nm. 5

6 Figure S5. XPS spectra of Cu 2P peak positions for unannealed and annealed films of PbS NPLs. The Cu 2P peak positions were similar in both films. Figure S6. (a) Current-voltage characteristics of the photodetector devices fabricated with a pristine film of Cu 2 S NPLs (10 mw, 808 nm laser source). (b) Comparison of dark current for a pristine film of Cu 2 S NPLs and PbS NPLs. 6

7 a nm b Dark Light Y[µm] Current (µα µα) X[µm] nm Voltage (V) Figure S7. (a) AFM image showing the surface morphology of a film of PbS NPLs with 30% residual Cu + after thermal annealing. Analysis of the image revealed a RMS surface roughness of ~ 27 nm. (b) Current-Voltage plot of a photodetector device using these thermally annealed films of PbS NPLs under dark and illumination conditions. The excitation power was 10mW at a wavelength of 808 nm. 7

8 Figure S8. Log-log plot of responsivity versus the inverse square root of excitation light intensity at an excitation wavelength of 808 nm. The linearity of the plot suggests that bimolecular recombination is the primary pathway by which carriers recombine. Table S2. Table of sheet resistance values measured at five distinct regions of films of PbS NPLs and PbS QDs. The bulk resistivity is calculated by multiplying the average sheet resistance with the film thickness. PbS NPLs (7.8% residual Cu + ) Sheet resistance (ohm/ ) QDs Avg. Sheet resistance (ohm/ ) Thickness (nm) Resistivity (ohm-cm) nm nm

9 Figure S9. (a) Absorbance spectra of a thin film of PbS NPLs and PbS QDs with similar optical density. (b) Current-voltage characteristics of the photodetector devices fabricated using PbS NPLs with 7.8% residual Cu + (red) and PbS QDs (black) under dark conditions and under illumination at 10 mw from a 808 nm wavelength laser source. References (1) Ha, D. H.; Caldwell, A. H.; Ward, M. J.; Honrao, S.; Mathew, K.; Hovden, R.; Koker, M. K. A.; Muller, D. A.;, Hennig, R. G.; Robinson, R. D. Solid-Solid Phase Transformations Induced through Cation Exchange and Strain in 2D Heterostructured Copper Sulfide Nanocrystals. Nano Lett. 2014, 14, (2) Bloem, J.; Kroger, F. A. Presented at Rept. Meeting on Semiconductors, Rugby, Apr, (3) Simkovich, G.; Wagner, J. B. Self-Diffusion of Lead 210 in Single Crystals of Lead Sulfide as a Function of Stoichiometry and Doping Additions. J. Chem. Phys. 1963, 38,

10 (4) Anderson, M. L.; Bartelt, N. C.; Swartzentruber, B. S. The Importance of Pb-Vacancy Attractions on Diffusion in the Pb/Cu(001) Surface Alloy. Surf. Sci. 2003, 538,