These authors contributed equally to this work. 1. Structural analysis of as-deposited PbS quantum dots by Atomic Layer Deposition (ALD)

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Timothy Conley
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1 Supporting information for: Atomic Layer Deposition of Lead Sulfide Quantum Dots on Nanowire Surfaces Neil P. Dasgupta 1,*,, Hee Joon Jung 2,, Orlando Trejo 1, Matthew T. McDowell 2, Aaron Hryciw 3, Mark Brongersma 3, Robert Sinclair 2 and Fritz B. Prinz 1,2 1 Department of Mechanical Engineering, Stanford University, CA Department of Materials Science and Engineering, Stanford University, CA Geballe Laboratory for Advanced Materials, Stanford University, CA * dasgupta@stanford.edu, (T), (F) These authors contributed equally to this work 1. Structural analysis of as-deposited PbS quantum dots by Atomic Layer Deposition (ALD) Selected area diffraction (SAD) pattern analysis was performed by TEM in order to verify the crystal structure of PbS quantum dots deposited by ALD. First, SAD pattern analysis was conducted on single crystal silicon in order to accurately measure the reciprocal space scale as shown in Fig. S1-a. Next, SAD pattern analysis was conducted on PbS quantum dots which were deposited after 40 cycles of ALD using the 1 st and 2 nd smallest SAD apertures, which define different sizes of selected area for diffraction data collection, as shown in Fig. S1b-1 & 2. From the measurement of each R hkl (the distance from center spot to a diffraction spot or ring), we can verify the crystal structure of PbS, because R is proportional to h + k + l and each crystal structure shows specific diffraction rings which satisfy the Bragg condition. The R hkl /R 111 ratios from these measurements are consistent with 1

$Indexed diffraction spots (111, 200, 220, 311, 222, 400 ) obtained by these TEM experiments are identical to previously$ $published X-Ray Diffraction (XRD) results 1.$

2 theoretical R hkl /R 111 ratios for the rock-salt structure as shown in Fig. S1-d. Indexed diffraction spots (111, 200, 220, 311, 222, 400 ) obtained by these TEM experiments are identical to previously published X-Ray Diffraction (XRD) results 1. Based on the indices and Supplementary Equation 1: 2 R hkl d hkl =L λ (1) the average lattice parameter (a= Å) can be obtained, which is consistent with the reference value (a = 5.936Å) from JCPDS card within a 0.1% error. 2

3 Figure S1. SAD patterns from (a) Si and (b) PbS with two different sized SAD apertures, (c) TEM images using different-size SAD apertures and (d) summary of measurements. Also, the random orientation of PbS quantum dots can be verified, because the SAD pattern does not show a specific diffraction spot array (single crystalline case) or any missing diffractions (preferential out-of-plane orientation case) as seen in Fig. S1-b. Furthermore, when we select a larger population of PbS quantum dots with 2 nd smallest aperture (Fig. S1c-2) versus the smallest aperture (Fig. S1c-1), we can see greater continuity of the diffraction ring (Fig. S1b-2), which provides further evidence of random grain orientation. 2. Size measurements of PbS quantum dots To measure the grain size from plane-view TEM images, contrast-based definition of each PbS quantum dot individually from bright field TEM images was conducted manually using Adobe Photoshop CS3 v.10 with the Magic Wand tool and Lasso tool. This manual method can avoid defining certain dark contrast areas as grains when using automatic grain-defining software, which may give inaccurate size analysis. After counting the number of grains, the area of each grain was calculated using the appropriate conversion of the number of pixels of each defined PbS quantum dot from the contrast-extracted images as shown in Fig. S2. Finally, average size (= Area ) and standard deviation were obtained. 3

4 Figure S2. Images illustrating the technique of (left) defining individual quantum dots, (center) counting the individual dots, and (right) calculating the area of each dot using the defined pixels, on bright field TEM images. 3. Temperature of PbS quantum dots during in situ e-beam irradiation The lattice parameter of a nanocrystal was measured in a series of snapshots at different time intervals using an in situ HRTEM video in order to measure thermal expansion by local heating of the specimen during e-beam irradiation. The detailed TEM settings for e-beam irradiation are gun lens=3, spot size=3, acceleration voltage=200 kv, extraction voltage=4200 V, emission current=50 μa and TEM chamber pressure= Torr with the following aperture settings: C1=4, C2=3, objective and SAD apertures are not inserted in. An in-situ video was taken at 255,000x magnification with the e- beam concentrated in a 3 cm-diameter circle on the fluorescent screen, which corresponds to a diameter of ~118 nm in the specimen plane. Therefore, the actual irradiated area is around 1.0~ nm 2. 4

5 Figure S3. (a) Series of snapshots from an in situ HRTEM video (b) reference plot for lattice parameter vs. surface temperature (c) experimental plot of thermally expanded lattice parameter vs. e-beam irradiation time. A previous publication presented lattice parameter data for PbS as a function of temperature3. From this data, a linear extrapolation was obtained, as shown in reference plot of Fig. S3a. This 5

6 extrapolation was utilized as a standard which gives the relationship between lattice parameter and surface temperature. Snapshots from the in situ HRTEM video obtained in this study were taken after 3, 20, 57, 101, 160 and 178 sec of e-beam irradiation, as shown in Fig. S3b. One nanocrystal was aligned along the (001) zone-axis, which provided clear lattice fringes from {200} type planes. The measurement of the interplanar spacing (d 200 ) between {200} planes indicated that thermal expansion was occurring within grain during e-beam irradiation, since the interplanar spacing (d 200 ) increased with time from Å. The corresponding lattice parameter and extrapolated surface temperature is shown in Fig. S3c. From these data, the nanocrystal temperature was estimated to increase from 300K to 850K during an irradiation period of 180 sec. 4. Statistical analysis of particle size distribution during e-beam irradiation During e-beam irradiation, coalescence and sublimation are simultaneously occurring, causing an evolution of the average value and standard deviation of particle size as a function of annealing time. Also, the shape transformation from flat disks to dome shaped QDs causes a reduction in average diameter of the dots during annealing. To quantify this effect the procedure described in section 2 of this supplementary information was applied to the images from figure 2a in the manuscript text. The results are shown in Fig S4 below. 6

7 Figure S4. Evolution of particle size as a function of e-beam irradiation time. (a) Total area coverage of PbS on SiO 2 grid vs. irradiation time (b) # of particles vs. irradiation time (c) average diameter of particles vs. irradiation time, with standard deviation indicated (d) table to summarize statistical results. Fig. S4a shows the total area of the SiO 2 TEM grid which is covered by PbS during annealing. It can be seen that the coverage steadily decreases during annealing, due to coalescence, shape transformation, and sublimation. Fig S4b shows that the total number of PbS particles on the surface also decreases with annealing time. This is due to both a reduction in particle number due to coalescence, and complete sublimation of smaller dots, reducing the overall particle number. It can be seen that the slope of the curve increases after around 360 seconds, which we speculate is due to the 7

8 non-linear sublimation behavior as particle size decreases. As the surface-to-volume ratio increases for smaller dots the kinetics of sublimation will increase, and the required temperature for sublimation will decrease, as discussed in the manuscript text. Fig S4c shows the average particle diameter as a function of irradiation time. The average diameter initially increases due to coalescence of smaller particles into larger particles, but this quickly decrease as sublimation begins to dominate. The shape transformation from flat islands to domes during annealing also contributes to a reduction in average particle diameter. The standard deviation in particle diameter is also affected by annealing. Initially, the standard deviation increases slightly due to random coalescence on the surface. However, as the average particle size decreases due to sublimation and shape transformation, the standard deviation also decreases. At these longer annealing times, coalescence and shape transformation are no longer having a strong effect on the size profile, and sublimation is the principal contributor to size reduction. This shows that annealing is a powerful technique to modify both the average dot size, and the standard deviation in size, which will in turn affect the optical properties of the QD ensebles. 5. Crystalline bridging and re-orientation of PbS quantum dots during e-beam irradiation Reorientation of the nanocrystal lattice is observed to occur during e-beam irradiation, which allows for the alignment of two interacting dots during coalescence. Fig. S5a shows three neighboring dots on the TEM grid surface after 8 min. 30 sec. of irradiation. Quantum dot a and b are joined by crystalline bridge of aligned planes, shown by lattice fringes along the axis between the dots. After 9 min. irradiation (30 sec later), quantum dot b and c experience a similar interaction with crystalline bridging, while the bridge (lattice fringes) between quantum dot a and b is lost, as shown in Fig. S5b. Accordingly, the crystalline orientation of quantum dot b is changed from being aligned with quantum dot a (at 8 min 30 sec) to quantum dot c (at 9 min) as seen in the magnified views of quantum dot b. The angle change of the lattice fringe alignment inside quantum dot b was measured to be ~115. Crystalline bridging and re-orientation of PbS quantum dots are frequently observed during e-beam irradiation to facilitate coalescence of neighboring dots. 8

9 Figure S5. High resolution TEM images during e-beam exposure after (a) 8min 30sec and (b) 9min. A crystalline bridge is initially formed between QD-b and a, which is subsequently lost when QD-b is reoriented to form an aligned bridge with QD-c. The change in alignment of lattice fringes in QD-b during this process was measured to be ~115. REFERENCES (1) Ni., Y.; Liu, H.; Wang, F.; Liang, Y.; Hong, J.; Ma, X.; Xu, Z. Cryst. Res. Technol. 2004, 39, (2) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy: A Textbook for Materials Science, Springer Press: New York, 1998; Ch. 9 & 18. (3) Noda, K.; Masumoto, K.; Ohba, S.; Saito, Y.; Toriumi, K.; Iwata, Y.; Shibuya, I. Acta. Cryst. 1987, C43,

AP5301/ Name the major parts of an optical microscope and state their functions.

AP5301/ Name the major parts of an optical microscope and state their functions. Review Problems on Optical Microscopy AP5301/8301-2015 1. Name the major parts of an optical microscope and state their functions. 2. Compare the focal lengths of two glass converging lenses, one with