Fundamentals of MOF Thin Film Growth via Liquid-Phase Epitaxy: Investigating the Initiation of Deposition and the Influence of Temperature

Abstract

Thin films can integrate the versatility and great potential found in the emerging field of metal-organic frameworks directly into device architectures. For fabrication of smart interfaces containing surface-anchored metal-organic frameworks, it is important to understand how the foundational layers form to create the interface between the underlying substrate and porous framework. Herein, the formation and morphology of the first ten cycles of film deposition are investigated for the well-studied HKUST-1 system. Effects of processing variables, such as deposition temperature and substrate quality, are studied. Sequences of scanning probe microscopy images collected after cycles of alternating solution-phase deposition reveal the formation of a discontinuous surface with nucleating and growing crystallites consistent with a Volmer-Weber growth mechanism. Quantitative image analysis determines surface roughness and surface coverage as a function of deposition cycles, producing insight regarding growth and structure of foundational film layers. For carboxylic acid terminated self-assembled monolayers on gold, preferred crystal orientation is influenced by deposition temperature with crystal growth along [100] observed at 25 °C and [111] favored at 50 °C. This difference in crystal orientation results in reduced surface roughness and increased surface coverage at 50 °C. To properly fabricate and fully determine the potential of this material for industrial applications, fundamental understanding of film formation is crucial.

Mechanistic Insights into Growth of Surface‐Mounted Metal‐Organic Framework Films Resolved by Infrared (Nano‐) Spectroscopy

“…AFM measurements indicated a highly heterogeneous film with crystal sizes significantly exceeding theoretical limits of the LbL synthesis (Figure 5 b). This result is in accordance with other reports in literature where authors noticed both (100) and (111) oriented crystallites 20, 21, 22. Remarkably, only reduced immersion times in both reactants and ethanol (2 min for both CuAc 2 and btc, with a 20 s washing step, see Experimental Section, (fast) synthesis procedure 2) have led to (100) oriented films, (Figure 5 a, middle, CuAc 2 fast synthesis) 13.…”

“…This growth rate largely exceeds the growth rates predicted from the epitaxial‐like LbL synthesis (ca. 1.3 nm per cycle) and agrees well with previous AFM studies 21, 22. Based on the AFM profile in Figure 3 a, we estimate 2.5 times larger amount of material (see Experimental Details) after five cycles than expected from the LbL stoichiometry.…”

“…have detected an increasing mass uptake with each step of the LbL synthesis, reaching 50 % of additional mass after five cycles 20. Ohnsorg et al 21. were the first to suggest that the linear growth rates observed with QCM13 can be interpreted with the growth of large MOF islands via the Volmer‐Weber mechanism,21, 22 rather than a controlled LbL growth.…”

“…Ohnsorg et al 21. were the first to suggest that the linear growth rates observed with QCM13 can be interpreted with the growth of large MOF islands via the Volmer‐Weber mechanism,21, 22 rather than a controlled LbL growth. We additionally find that the fast local growth rates are the consequence of the solution‐mediated strong interactions between loosely attached (physisorbed) CuAc 2 and btc, most likely taking place at the solid–liquid interface.…”

“…Quartz crystal microbalance (QCM) investigations, measuring the mass uptake of individual SURMOF precursor species as a function of time, indicated a non‐stoichiometric increase in the uptake after each deposition step 20. Scanning probe microscopy studies have found that [Cu 3 (btc) 2 ] SURMOF films can grow significantly faster than expected from the LbL model,21, 22 whereas the orientation of the obtained films was not always as expected from the termination of SAMs 20, 21, 22. Importantly, the LbL SURMOF synthesis is (relatively) time consuming.…”