Molecular Layer Deposition of Organic–Inorganic Hybrid Materials
Nanoscience and nanotechnology are playing an ever-increasing importance in our society. Scientists and engineers have been striving to manipulate atoms and molecules precisely at will to develop ideal nanosized materials with exceptional properties. 1 To this end, various techniques have been devised for nanofabrication, such as mechanochemistry,2’3 wet chemistry,-1 physical vapor deposition (PVD),5 chemical vapor deposition (CVD),6 atomic layer deposition (ALD),7 and molecular layer deposition (MLD).8 Among all the methods, recently MLD has been attracting more and more attention for growing pure polymeric and hybrid films.8-10
MLD was first coined in 1991 by Yoshimura and co-workers" exclusively for nanoscale films of organic materials, especially pure polymers and metal-based hybrid polymers.912-15 This unique controllable technique was first demonstrated for synthesizing polyimides." Subsequently, more polymeric films were developed via MLD, including polyazomethines,16-'9 polyureas,20-24 polyamides,25-28 poly (3.4-ethylenedi- oxythiophene)29-30 polyimide-polyamides,31 polythioureas,32 polyethylene terephthal- ate,33 and some others.34-35 It was in 2008 when the first metal-based hybrid polymer was reported, which was an aluminum alkoxide (the so-called “alucone”).36 Thereafter, many more alucones13-37-45 have been developed by MLD and also ignited research interests on other metalcones, resulting in mangancone45 zincones,46-53 zircones,54-55 titanicones,56-59 hafnicones,60 and vanadicone.61 This greatly extended our capabilities in searching for advanced materials in a controllable mode. In this regard, some excellent review papers8-'012 have well documented MLD processes and their capabilities. Owing to its unlimited possibilities for new nanoscale polymeric films, MLD has exhibited great potentials for a large variety of applications, such as microelectronics,62 catalysis,63 energy conversion and storage,64 organic magnets,62 luminescent devices,14 surface engineering,10-65 and many others.15 Recently, there has been an increasing interest in MLD polymeric and hybrid films for addressing issues in rechargeable batteries. In this context, organic-inorganic hybrid materials are particularly intriguing, ascribed to then- desirable properties unattainable with the conventional materials.
This chapter focuses on introducing recent MLD research progresses on organic- inorganic hybrid materials, featuring their surface chemistry, growth characteristics, and film properties. Following this introductory section, we present some general basics commonly shared by MLD processes, including surface chemistry and growth characteristics, compared to those of ALD processes. The third part summarizes MLD processes of metalcones and the fourth part gives an account of other hybrid materials. In the last part, we conclude this chapter and give some outlook on future studies.
The Basics of MLD
MLD and ALD are two highly similar vapor-phase techniques for nanofabrication. They share the same operational principle to realize accurate controls over materials growth. They both commonly rely on alternative self-limiting surface reactions for materials growth. The former produces organic materials while the latter results in inorganic materials. They both grow materials accurately in a layer-by-layer mode. Figure 3.1a illustrates an ALD process for growing binary inorganic materials, while Figure 3. lb displays an MLD process for growing pure polymers using two homobifunctional precursors. In terms of surface reactions, for example, the model ALD process of ALO, using trimethylaluminum (TMA, Al(CH,)j) and FLO can be described in Equations 3.1 A and 3.1 В as follows66:
FIGURE 3.1 Illustrations of (a) ALD and MLD processes for growing (b) pure polymeric films and (c) organic-inorganic hybrid films.
where “I” indicates substrate surfaces while “(g)” signifies gas phases. The surface chemistry of the ALD A1,0, is based on ligand exchanges between -OH and -СН, to arrange atoms accurately in a layer-by-layer mechanism. In addition to the ligand exchange mechanism as illustrated in Equations 3.1 A and 3.1B, there are other mechanisms for ALD surface chemistry as well, such as dissociation and association.67 The four steps in Figure 3. la constitute one ALD cycle and they can repeat to build up films for desired thicknesses. The growth rate of ALD is described by growth per cycle (GPC), typically having a GPC of ~ 1 А/cycle.12 In the case of MLD of pure polymers (Figure 3.1b), one precursor first reacts with surface reactive groups via a corresponding linking chemistry to add a molecular layer on the substrate surface with new reactive sites.62 Following a thorough purge, another precursor reacts with the new reactive sites with the production of another molecular layer and recovers the surface back to the initial reactive groups. Another full purge is performed to finish one MLD cycle. Through repeating the afore-discussed four steps, MLD can realize polymeric film growth accurately at the molecular level. The growth rate of MLD also is described by GPC. Using adipoyl chloride (AC) and 1,6-hexanediamine (HD) as precursors,25-26 for example, an MLD process has been developed for growing nylon films linearly and the surface chemistry is described as follows:
The АС-HD MLD process could realize a GPC of 19 А/cycle at 62 °C.26 It is apparent that the molecular layers of -CO(CH2)4CO- and -NH(CH2)6NH- during the
MLD-nylon are much larger than the atomic layers of-Al- and -O- in the ALD-ALO,. This underlies the higher GPC of the MLD nylon process of АС-HD. Additionally, many more pure polymeric materials via MLD recently have been summarized in literature.8
In addition to fabricating pure polymeric films as illustrated in Figure 3.1b, MLD also enables organic-inorganic hybrid materials by adopting an ALD precursor and an MLD precursor (Figure 3.1c), such as metal alkoxide materials (i.e., metalcones), in which diols can be used to couple with a metal precursor. Using TMA and ethylene glycol (EG. HOCFLCFLOH, a homobifunctional diol precursor), for instance, George’s group first reported a metal-based hybrid polymer, an aluminum alkoxide (i.e., alucone) of AKOCFLCFLO)., with the following surface chemistry36:
Apparently, the molecular fragment of -OCFLCFLO- attached in the MLD-alucone is far much larger than the atomic part of -O- in the ALD-ALO,. This well explains that the resultant alucone has grown much faster than the ALD AbO,, accounting for 4 А/cycle at 85 °C for the MLD alkoxide36 versus 1.3 А/cycle for the ALD ALO, at 80 °C.66 Through smartly selecting precursors for their functional groups and backbones, MLD enables different metalcones or hybrid materials with desired properties. Substituting EG with the aromatic hydroquinone (HQ, HOC6H4OH), for example, another alucone has been deposited and has its surface chemistry as follows38:
This TMA-HQ MLD process exhibits a GPC of 4.1 А/cycle at 150 °C.38 Alucones with different backbones are expected to exhibit different properties. The aromatic backbone of HQ is expected to provide structural stability and contribute largely to the electrical properties of the resultant polymer films. To date, many more metalcones have been reported, including alucones,l3-36-45-53-65-68-84 zincones,47-53-85-86 titani- cones,46-53-59-85-86 vanadicones,61 zircones,54-55 hafnicones,60 mangancones,45 metal quinolones,87-88 and some other hybrid materials.89-104
To investigate the underlying mechanism of the many MLD processes, a suite of in situ techniques have been employed in previous studies. Fourier transform infrared spectroscopy (FTIR)24-36-38-41^4-78 and quartz crystal microbalance (QCM)36-38-39-42-44-54-56-93 are two widely utilized in situ instruments. They are very helpful to get insightful information on surface chemistry of MLD processes. FTIR spectra can clearly identify a molecular fingerprint after each surface reaction. QCM can definitively detect any molecular deposition in mass uptake and demonstrate a linear growth for any feasible MLD processes. On the other hand, quadrupole mass spectrometry (QMS)69 is also very useful to detect any byproducts resulted from MLD surface reactions. All the data collected by the three in situ tools jointly help construct the underlying surface chemistry during a MLD process.
Like ALD processes, MLD processes also are subject to three parameters, that is, precursor, temperature, and substrate. MLD precursors should be able to produce sufficient vapors easily. They should also be chemically stable at deposition temperatures and highly reactive to their coupled precursors. As discussed above using EG and HQ for alucones, MLD films are highly related to the precursors adopted, such as their structures, properties, and GPCs. On the other hand, substrates have impacts on MLD growth. In some cases, substrates should be pretreated for functionalization in order to initiate the growth of some polymeric films. On the contrary, sometimes substrates should be pretreated with a protective layer in order to resist any film growth.105 106 Furthermore, deposition temperature often is critical for MLD film growth and it is worth noting that most of the MLD processes reported to date show a decreasing growth tendency with temperature. Ascribed to its unique growth mechanism, MLD produces uniform and conformal coatings over any shaped substrates.
MLD Processes for Organic–Inorganic Hybrid Metalcones
Hybrid materials are very promising in a large variety of applications such as optics, electronics, mechanics, membranes, new energies, catalysis, and surface engineering. As recently summarized in our review article,8 MLD has fabricated a variety of hybrid materials through adopting one typical ALD metal-containing precursor as the metal source and one MLD organic precursor. In addition, MLD processes can proceed with multiple precursors as well. The metal-containing ALD precursors have been collected in Figure 3.2 while the organic precursors are summarized in Figures
3.3 and 3.4 for growing hybrid materials. In this chapter, we focus on discussing MLD processes for growing metalcones. In terms of metal elements, there to date have been reported seven types of MLD metalcones, including alucones, titanicone, zincone, zircone, hafnicone, mangancone, and vanadicone.