Impact of Surface Functionalization and Deposition Method on Cu-BDC surMOF Formation, Morphology, Crystallinity, and Stability

For direct integration into device architectures, surface–anchored metal–organic framework (surMOF) thin films are attractive systems for a wide variety of electronic, photonic, sensing, and gas storage applications. This research systematically investigates the effect of deposition method and surface functionalization on the film formation of a copper paddle-wheel-based surMOF. Solution-phase layer-by-layer (LBL) immersion and LBL spray deposition methods are employed to deposit copper benzene-1,4-dicarboxylate (Cu-BDC) on gold substrates functionalized with carboxyl- and hydroxyl-terminated alkanethiol self-assembled monolayers (SAMs). A difference in crystal orientation is observed by atomic force microscopy and X-ray diffractometry based on surface functionalization for films deposited by the LBL immersion method but not for spray-deposited films. Cu-BDC crystallites with a strong preferred orientation perpendicular to the substrate were observed for the films deposited by the LBL immersion method on carboxyl-terminated SAMs. These crystals could be removed upon testing adhesive properties, whereas all other Cu-BDC surMOF film structures demonstrated excellent adhesive properties. Additionally, film stability upon exposure to water or heat was investigated. Ellipsometric data provide insight into film formation elucidating 7 and 14 Å average thicknesses per deposition cycle for films deposited by the immersion method on 11-mercapto-1-undecanol (MUD) and 16-mercaptohexadecanoic acid (MHDA), respectively. In contrast, the films deposited by the spray method are thicker with the same average thickness per deposition cycle (21 Å) for both SAMs. While the spray method takes less time to grow thicker films, it produces similar crystallite structures, regardless of the surface functionalization. This research is fundamental to understanding the impact of deposition method and surface functionalization on surMOF film growth and to provide strategies for the preparation of high-quality surMOFs.

Collected AFM images were analyzed in the ImageJ program.To perform the analysis, images were converted to gray scale by changing the image type to 8-bit.Then, the images were adjusted to threshold by setting brightness to include all the particles on the sample.Surface coverage for each sample was obtained by selecting "analyze particles".Surface coverage was calculated for at least three 2.5 × 2.5 μm AFM images from each deposition cycle in each sample set and there were three sample replicates.The obtained data was then compiled to determine the average and standard deviation for the surface coverage associated with each deposition cycle.These data are provided in Figure S1.

IR spectra.
In the image below, two representative full IR spectra from 600 to 4000 cm −1 are provided.2] The samples after 8 cycles of LBL immersion deposition were selected because there was significant amount of film to clearly observe peaks for the MOF while still being thin enough to collect signal associated with the underlying SAM.

Figure S1 .
Figure S1.Surface coverage of Cu-BDC films deposited using LBL immersion method on MHDA SAMs as a function of number of deposition cycles.Average surface coverage and standard deviations are determined for 2.5 μm × 2.5 μm images using ImageJ analysis.

Figure S2 .
Figure S2.Ellipsometric film thicknesses, standard deviations, and corresponding linear fits are plotted as a function of deposition cycles.Films were deposited by (a) LBL immersion and (b) spray method on gold substrates functionalized with MHDA (pink or purple circles) and MUD (blue or green triangles) SAMs.This data corresponds to Figure 2b and 7b.

Figure S4 .
Figure S4.Representative atomic force microscopy images (2.5 µm x 2.5 µm) of Cu-BDC films deposited by LBL immersion on 24 Hr MHDA (a-e) and 1 Hr MUD (f-j) SAMs.Above each image, the number of LBL deposition cycles (L) completed prior to analysis is given.Below each image, the corresponding surface roughness value (Rq) is provided and is specific to the AFM image.Scale bar of 250 nm (a) is for all images.In each row all the images are set to the same zscale shown to left of images (a) and (f).For comparison, see Figure 1 in manuscript.

Figure S5 .
Figure S5.Films were deposited by LBL immersion method on gold substrates functionalized with MHDA and MUD SAMs.(a) Average thickness and corresponding standard deviation values of films deposited on 1 Hr MHDA (pink circles), 24 Hr MHDA (orange circles), 1 Hr MUD (green triangles), and 24 Hr MUD (blue triangles) are plotted as a function of deposition cycles.(b) XRD patterns of Cu-BDC films deposited on 24 Hr MHDA (orange, top set) and 1 Hr MUD (green, bottom set) SAMs.For comparison, see Figure 2 and 3 in manuscript.

Figure S6 .
Figure S6.XRD patterns of Cu-BDC films deposited by 20 LBL immersion deposition cycles on MHDA and MUD SAMs.Patterns collected for as-deposited samples are shown in black.Patterns after stability experiments are displayed in pink for samples on MHDA (a) and blue for samples on MUD (b).These XRD patterns are collected for samples with AFM images and thickness values given in Figure S7 and TableS1, respectively.

Figure S7 .
Figure S7.Representative atomic force microscopy images (2.5 µm x 2.5 µm) of Cu-BDC films deposited by 20 LBL immersion deposition cycles on MHDA (a-e) and MUD (f-j) SAMs collected before (a,f) and after (b-e, g-j) stability experiments.Below each image, the corresponding surface roughness values (Rq) are provided.Above each column of images is a brief description of the experiment.Scale bar of 250 nm (a) is for all the images.In each row all the images are set to the same z-scale shown to left of images (a) and (f).

Table S2 .
Ellipsometric thickness and associated standard deviation values for Cu-BDC films deposited by 16 and 20 LBL spray deposition cycles (L) before and after tape test.

Figure S9 .
Figure S9.Top: Atomic force microscopy images (2.5 µm x 2.5 µm) of Cu-BDC films deposited by 20 deposition cycles of LBL immersion (a, b) and LBL spray (c, d) methods on MHDA (a, c) and MUD (b, d) SAMs.These AFM images are also found in manuscript Figure 1 and 6.Bottom: SEM images (2.5 µm x 2.5 µm) of Cu-BDC films deposited by 20 deposition cycles of LBL immersion (f, g) and LBL spray (h, i) methods on MHDA (f, g) and MUD (h, i) SAMs.