Unit-cell-thick zeolitic imidazolate framework films for membrane application

Zeolitic imidazolate frameworks (ZIFs) are a subset of metal–organic frameworks with more than 200 characterized crystalline and amorphous networks made of divalent transition metal centres (for example, Zn2+ and Co2+) linked by imidazolate linkers. ZIF thin films have been intensively pursued, motivated by the desire to prepare membranes for selective gas and liquid separations. To achieve membranes with high throughput, as in ångström-scale biological channels with nanometre-scale path lengths, ZIF films with the minimum possible thickness—down to just one unit cell—are highly desired. However, the state-of-the-art methods yield membranes where ZIF films have thickness exceeding 50 nm. Here we report a crystallization method from ultradilute precursor mixtures, which exploits registry with the underlying crystalline substrate, yielding (within minutes) crystalline ZIF films with thickness down to that of a single structural building unit (2 nm). The film crystallized on graphene has a rigid aperture made of a six-membered zinc imidazolate coordination ring, enabling high-permselective H2 separation performance. The method reported here will probably accelerate the development of two-dimensional metal–organic framework films for efficient membrane separation.

Supplementary Note 1. Index of electron diffraction data from 2DZIF@substrate films.
Both electron diffraction data of 2DZIF@Graphene and 2DZIF@Au were indexed by the same way, where d spacing and angle of/between each diffraction dot were read and compared to theoretical data.Thus, according to the index, the orientations between 2DZIF and substrates were identified.
Theoretical d spacing value were calculated by the following equations, For graphene, which are two-dimensional hexagonal system, For Au, which is cubic system, Theoretical angles between lattice planes were calculated by the following equations, For graphene, which is two-dimensional hexagonal system, For Au, which is cubic system, The equations for the calculations of theoretical d spacing value and angles between lattice planes of 2DZIF were the same as that in XRD data.

iii. Collection of diffraction data
For the measurement of 2DZIF@Graphene, only the diffraction data of 2DZIF was collected where grazing incidence was applied at 0.04° to collect the diffraction of 2DZIF@Graphene, and collecting time was 60 s, with the distance of 200 mm from the sample to the detector.
For the measurements of 2DZIF@Sapphire and 2DZIF@Quartz samples.Both the diffraction data from 2DZIF and substrates were collected.Grazing incidence was applied at 0.04° to collect the diffraction of 2DZIF@Sapphire and 2DZIF@Quartz, and collecting time was 60 s, with the distance of 200 mm from the sample to the detector.Single-crystal X-ray diffractions were performed to collect the diffraction of substrates with ω of 10° and 15° for Sapphire and Quartz, respectively.Collecting time was 20 s, and the distance from the sample to the detector was 200 mm and 100 mm for sapphire and quartz, respectively.

iv. Analyses of X-ray diffraction data.
For the index of diffraction data, the q value and angle between each diffraction dot were read first, which provided corresponding d spacing value and angle.Then, theoretical d spacing value and angles of all lattice planes of the samples were calculated.Last, comparisons of experimental data with theoretical data were made and corresponding Miller indices was confirmed.Thus, according to this index, the orientation of sample was identified.
Experimental q value was calculated by the following equation,

𝑞 = 4πsin(𝜃) 𝜆
Experimental d spacing value was calculated by the following equation, Theoretical d spacing value were calculated by the following equations, For sapphire and quartz, which are trigonal system, For 2DZIF, which is orthorhombic system, Theoretical angles between lattice planes were calculated by the following equations, For sapphire and quartz, which are trigonal system, For 2DZIF, which is orthorhombic system, " !Supplementary Note 5. Syntheses and patterning application of amorphous ZIF films.
We could obtain macroscopically smooth, continuous, and uniform aZIF films on Si/SiO2 wafers.AFM of one of these films, prepared using 2 mM Zn 2+ and 16 mM 2-mIm and deposition time of 10 s, indicated that the film is smooth with thickness near 8 nm (Supplementary Fig. 13c-e).Ellipsometry of several ZIF films on Si/SiO2 wafers, prepared by varying the synthesis time, indicated that the film thickness could be tuned in the range of 8-18 nm consistent with the corresponding AFM data (Supplementary Fig. 13).
Amorphous MOFs exhibit unique physical and chemical properties due to the absence of anisotropy and crystalline grains 2 .On one hand, they may not have the well-defined pore structures of crystalline MOFs required for certain molecular sieving applications, but at the same time, they do not exhibit grain boundaries and structural anisotropies of crystalline MOFs, which can create film non-uniformities.A potential use of organic-inorganic films is in next generation resists for photolithography in place of currently used polymeric resists, and, for this application, MOF-inspired metal-organic clusters have been proposed for high resolution patterning (ref.13 in main text).
As a demonstration of the potential of our deposition method in this emerging application, an aZIF film was deposited on a silicon nitride support and subsequently exposed to a direct-write electron beam using 1:1 line-and space-patterns ranging from 10 to 40 nm in line width (or half pitch) (Supplementary Fig. 29a).The aZIF films behave similarly to ZIF-L crystals, for which e-beam treatment can induce contrast in water dissolution behavior based on framework densification and disintegration of the ligand molecular structure [3][4][5] .After development in water, the irradiated area was preserved while the non-irradiated area was dissolved (Supplementary Fig. 29b), confirming aZIF as a negative-tone resist.The thickness of the remaining aZIF structure was determined to be ~25 nm by AFM (Supplementary Fig. 29c and   29d).The resolution of the resulting pattern, as exemplified by the well-resolved lines at 20 nm half pitch, is comparable to the state-of-the-art metal-containing resists 6 , which are an emerging class of material that hold promise in extreme ultraviolet lithography and electron beam lithography [7][8][9][10] .aZIF can also be patterned in positive-tone mode by a vapor phase ligand pretreatment.The asdeposited aZIF is exposed to the sublimated vapor of 4,5-dichloroimidazole (dcIm) at 75 °C for 1.5 h, during which the aZIF matrix is partially exchanged or infiltrated with dcIm ligand.
The dcIm-treated film is then exposed to a direct-write electron beam.After development in As for the index of lattice orientation of the sapphire, since it was rotated 15° of the measurement of sapphire, and was identified by the projection as shown in panel c and d, only when the incidence was located in between a* and b* lattice axis of sapphire, the diffraction in panel b can be observed, which evidenced the lattice orientation as showed in panel e.While for the the index of lattice orientation of the 2DZIF, the sample was almost horizontally positioned.It was identified by the projection as shown in panel g and h, where (101) lattice plane was parallel to the detector plane.It is worth noting that, most of these dots in panel f can be indexed from one single orientation, where only a few, (5 J 22), (1 J 33), (012) and (312), are not belonging to this grain, as showed in panel h.As shown in panel j, the shading area of X-ray is 200 μm × {40 μm/[sin(0.04°)]}= 200 μm × 57 mm.The size of sample was 5 mm × 5 mm (l), which means the area of 2DZIF involved in the diffraction is 200 μm × 5 mm.Supplementary Fig. 10.SEM (a) and AFM (b) images of a 2DZIF film on single-crystal quartz, that is used for the GIXRD measurement.(c) Height profile and (d) height distribution from AFM, acquired from line and square area labelled in (b).Reaction condition, 2 mM Zn 2+ ; 16 mM 2-mIm; reaction time of 2 min.As for the index of lattice orientation of the quartz, since it was rotated 10° of the measurement of sapphire, and was identified by the projection as shown in panel c and d.Only when the incidence was located in between a* and b* lattice axis of quartz, the diffraction in panel b can be observed, which evidenced the lattice orientation as showed in panel e.While for the the index of lattice orientation of the 2DZIF, the sample was almost horizontally positioned.Since the in-plane orientation of 2DZIF film was not indexed, here [0k0] lattice axis from 2DZIF was assumed to be parallel to [hk0] lattice axis from SiO2 (j), the same as that observed in 2DZIF@Sapphire sample (panel k, Supplementary Fig.9).As shown in panel h, the shading area of X-ray is 200 μm × {40 μm/[sin(0.04°)]}= 200 μm × 57 mm.The size of sample was 1 cm × 1cm, which means the area of 2DZIF involved in the diffraction is 200 μm × 1 cm.

Table 1 .
Comparison of synthesis conditions for ZIF-L and 2DZIF.

Table 3 .
Lattice parameters for 2DZIF@graphene extracted from SAED and GIXRD as well as relaxed DFT structure and compared to those of ZIF-L.

Table 4 .
Comparisons of lattice parameters for c-orientated sapphire, quartz, gold and 2DZIF on different substrates extracted from SAED and GIXRD.

Table 6 .
Lattice parameters for 2DZIF@graphene compared to those after water etching.

Table 7 .
Average single gas permeance and ideal selectivity of various membranes.

Table 8 .
Gas permeance and ideal selectivity of our membranes compared to the state-of-art membranes in the literature.

Table 9 .
Thickness and separation performance of 2DZIF membranes compared to the state-of-art MOF membranes in the literature.

Table 10 .
Single gas permeance and ideal selectivity of centimeter-scale membrane.
*Intrinsic performance of 2DMOF layer is calculated based on the resistance-in-series model (see Supplementary Note 4).

Table 11 .
Sensitivity of representative commercial and metal-containing resists for electron beam lithography.