Modeling the Layer-by-Layer Growth of HKUST-1 Metal-Organic Framework Thin Films

Abstract

Metal organic frameworks have emerged as an important new class of materials with many applications, such as sensing, gas separation, drug delivery. In many cases, their performance is limited by structural defects, including vacancies and domain boundaries. In the case of MOF thin films, surface roughness can also have a pronounced influence on MOF-based device properties. Presently, there is little systematic knowledge about optimal growth conditions with regard to optimal morphologies for specific applications. In this work, we simulate the layer-by-layer (LbL) growth of the HKUST-1 MOF as a function of temperature and reactant concentration using a coarse-grained model that permits detailed insights into the growth mechanism. This model helps to understand the morphological features of HKUST-1 grown under different conditions and can be used to predict and optimize the temperature for the purpose of controlling the crystal quality and yield. It was found that reactant concentration affects the mass deposition rate, while its effect on the crystallinity of the generated HKUST-1 film is less pronounced. In addition, the effect of temperature on the surface roughness of the film can be divided into three regimes. Temperatures in the range from 10 to 129 °C allow better control of surface roughness and film thickness, while film growth in the range of 129 to 182 °C is characterized by a lower mass deposition rate per cycle and rougher surfaces. Finally, for T larger than 182 °C, the film grows slower, but in a smooth fashion. Furthermore, the potential effect of temperature on the crystallinity of LbL-grown HKUST-1 was quantified. To obtain high crystallinity, the operating temperature should preferably not exceed 57 °C, with an optimum around 28 °C, which agrees with experimental observations.

Cocrystal Growth in Organic Semiconductor Thin Films: Simulation of Pentacene, Perfluoropentacene, and Their 1:1 Blend Deposited On Graphite

“…In crystalline films on smooth and defect‐free graphite surfaces, the molecular long axes of PFP and PEN are expected to be parallel to the substrate, while heating or the presence of structural defects [ 41 ] that increase surface roughness favor an organization with an upright molecular orientation, typical of acenes on other rough surfaces (e.g., SiO 2 ). [ 17–42 ] In fact, while the chemical similarity between the deposited molecules and graphite promotes a recumbent alignment, an upright one allows exposing to air a lower energy surface. The unbalance between the two energies and kinetic factors determines the outcome; however, since at low submonolayer coverages the recumbent orientation is always thermodynamically favorite, for subsequently obtaining the upright one the temperature should be high enough to allow the initially recumbent molecules to overcome a collective energy barrier for reorientation.…”

“…In this context, atomistic simulations already proved to be a valid option for the prediction and the understanding of the underlying mechanisms of the on‐surface crystal growth. [ 14–22 ] Concerning the deposition of mixed systems, although simulations of solid mixtures of organic molecules are becoming increasingly popular, in particular for better understanding the functioning of organic LEDs and solar cells at the nanoscale, [ 23,24 ] we are not aware of any theoretical investigation on the formation of organic semiconductor mixed crystals from vapor phase.…”

“…[13] In this context, atomistic simulations already proved to be a valid option for the prediction and the understanding of the underlying mechanisms of the on-surface crystal growth. [14][15][16][17][18][19][20][21][22] Concerning the deposition of mixed systems, although simulations of solid mixtures of organic molecules are becoming increasingly popular, in particular for better understanding the functioning of organic LEDs and solar cells at the nanoscale, [23,24] we are not aware of any theoretical investigation on the formation of organic semiconductor mixed crystals from vapor phase. Pentacene (PEN) was one of the first organic semiconductors employed as active material for the construction of organic transistors, [25,26] and, despite being largely investigated throughout the years, it is still considered as a worth prototype for fundamental studies of thin films growth and as a model p-type semiconductor.…”

Layer‐By‐Layer Assembly of Asymmetric Linkers into Non‐Centrosymmetric Metal Organic Frameworks: A Thorough Theoretical Treatment

“…kMC simulations have been successfully employed in simulating structure formation and reaction routes [21][22][23] in sub-atomic, atomistic, coarse-grained levels and the SURMOF assembly can also be approached via the atomistic and coarse-grained levels. [16] Here in a coarse-grained representation of the linkers, our kMC model considers a substrate with 2D lattice of binding sites, consistent with the scheme in Figure 1. In the first step of the SURMOF-forming process, the asymmetric pillar linkers can bind to the undercoordinated SBU sites with their axis orientated perpendicular to the substrate, with either an up or [13] Based on the binding energies we would expect a near perfect ordering (>90%), but the experimentally achieved up-down alignment was observed only at ≈4%.…”

“…The LbL technique for SURMOF synthesis involves alternating deposition and rinsing cycles to stack 2D layers aligned in the x-y plane upon one another, iteratively thickening the SURMOF in the z-direction to maintain high degree of alignment and defect suppression with unprecedented spatio-temporal control over the dynamic growth. [15,16] Using appropriately functionalized substrates in connection with two different types of linkers, a so-called pillar-layered MOF can be assembled, with the layer linkers (dicarboxylic acids) aligned in the xy-plane and the pillar linkers aligned along the z-direction, perpendicular to the substrate. Figure 1 depicts an idealized representation of the design goal of this investigation, where the pillar linkers and their dipole moments are perfectly aligned.…”