Langmuir-Schaefer deposition of quantum dot multilayers Supporting Information I. AFM, UV-VIS and TEM characterization of LS layers S1 Low-magnification TEM images of Q-CdSe layers, deposited on a carbon-coated copper TEM grid at a surface pressure of 0 mn.m -1 (a) and 12 mn.m -1 (b). The black lines represent the copper grid. The dark grey areas in image a are Q-CdSe monolayer islands on carbon, whereas the light grey areas are blank carbon film. Image b only has one grey tone, indicating a homogeneous Q-CdSe monolayer. S2 AFM image (a) and profile (b) of a Q-CdSe LS monolayer deposited on a HF-etched silicon substrate. Langmuir-Schaefer deposition of quantum dot multilayers 1
S3 TEM overview of a Q-CdSe monolayer deposited at 12 mn.m -1, showing a coverage of 97.7%. S4 Absorption spectra of a Q-CdSe tetralayer and suspension, normalized at the first absorption peak. Langmuir-Schaefer deposition of quantum dot multilayers 2
II. Dewetted LS layers a H 2 O Si Si b H 2 O Si S5 Dewetting during LS deposition: no dewetting occurs on hydrophobic substrates (e.g. HF-etched Si), resulting in water-qd and QD-Si interfaces with identical areas (a). Dewetting on hydrophilic substrates (e.g. Si with native oxide) means the creation of a water-substrate interface, whereas the water-qd interface area shrinks (b). S6 AFM images and corresponding height profiles of dewetted Q-CdSe monolayers on glass before (a) and after (b) immersion in toluene. Immersion in toluene leads to resuspension of all nanocrystals except those directly in contact with the substrate, as indicated by the different height of the features. Langmuir-Schaefer deposition of quantum dot multilayers 3
S7 - UV-VIS absorption spectra of glass slides containing dewetted Q-CdSe Langmuir- Schaefer mono- and multilayers (a). The absorption scales linearly with the number of deposited layers (b) Langmuir-Schaefer deposition of quantum dot multilayers 4
S8 AFM images and corresponding height profiles of dewetted LS Q-CdSe mono- (a), bi- (b) and trilayers (c) on glass. Cell bottoms are either fully coated or not at all, suggesting that both conformal coating and further dewetting may occur. Some of the cells include smaller dewetting structures. Thicker cell walls are observed in the cells where further dewetting has occurred. Langmuir-Schaefer deposition of quantum dot multilayers 5
III. Calculation of particle density and concentration S9 TEM image of a LS CdSe monolayer (a) and detail of the corresponding Fourier Transform (b). The layer contains local hexagonal order with a mean interparticle distance of 5.87 nm. For a voidless monolayer with long-range hexagonal order this means a particle density of 33400 particles/µm². For the determination of the particle density with UV-VIS absorption data, we use the Beer-Lambert law: A. c. l (1) With l the path length and A the measured absorbance and the extinction coefficient at a certain wavelength. For a dilute suspension of small particles, the absorption crosssection can be calculated using the Maxwell Garnet effective medium theory [1] and formula derived by Ricard et al. [2]: 2 n s f LF 2 2nk 4 3 r 3 (2) Langmuir-Schaefer deposition of quantum dot multilayers 6
With 4 2 9ns f LF (3) 2 2 2 2 2 n k 2n 4n k 2 s n s is the refractive index of the solvent, r the particle radius, n and k the real and imaginary refractive index components of the bulk material at wavelength, and f LF denotes the local field factor. The absorption cross section is related to the extinction coefficient : N A ln(10) (4) Where N A is Avogadro s number. Using the n and k values for bulk cubic CdSe at 340 nm (n=2.96 and k=0.87), and together with the UV-VIS data for the tetralayer we find: at 340 nm for 3.41 nm particles in chloroform (L.mol -1.cm -1 ) Abs tetralayer at 340 nm Particle concentration tetralayer (M) Particle concentration monolayer (M) Corresponding number of particles/µm² Theoretical number of particles/µm² (hexagonal order, no voids) 816827 0.046992 5.74E-08 1.44E-08 86450 33400 ICP-MS measurements confirm the value for the extinction coefficient [3]. To obtain the particle density for the monolayer, we divided the value for the tetralayer by 4 (this is justified by the fact that the absorbance increases linearly with the number of layers). Langmuir-Schaefer deposition of quantum dot multilayers 7
To calculate the corresponding number of particles per µm², we multiplied c (M) by the number of particles of a 1M suspension in a rectangular prism with dimensions 1µm*1µm*1cm (6.02*10 12 particles). The obtained value is significantly higher than the expected value for a monolayer with hexagonal order. References [1] A. Sihvola, J. Electromagn. Wave, 2001, 15, 715 [2] D. Ricard, M. Ghanassi, M.C. Schanne-klein, Opt. Commun., 1994, 108, 311 [3] R. Čapek, I. Moreels, K. Lambert, D. De Muynck, Q. Zao, A. Van Tomme, F. Vanhaecke, Z. Hens, Optical Properties of Zincblende Cadmium Selenide Quantum Dots in preparation Langmuir-Schaefer deposition of quantum dot multilayers 8