Although most metal−organic frameworks (MOFs)highly porous crystalline metal complex networks with structural and functional varietiesare electrically insulating, high electrical conduction has been recently demonstrated in MOFs while retaining permanent porosity. Usability of electronically active MOFs effectively emerges when they are created in a thin-film state as required in major potential applications such as chemiresistive sensors, supercapacitors, and electrode catalysts. Thin-film morphology including crystallinity, thickness, density, roughness, and orientation sensitively influences device performance. Fine control of such morphological parameters still remains as a main issue to be addressed. Here, we report a bottom-up procedure of assembling a conductive MOF nanosheet composed of 2,3,6,7,10,11-hexaiminotriphenylene molecules and nickel ions (HITP-Ni-NS). Creation of HITP-Ni-NS is achieved by applying air/liquid (A/L) interfacial bottom-up synthesis. HITP-Ni-NS has a multilayered structure with 14 nm thickness and is endowed with high crystallinity and uniaxial orientation, demonstrated by synchrotron X-ray crystallography. Facile transferability of HITP-Ni-NS assembled at air/liquid interfaces to any desired substrate enables us to measure its electrical conductivity, recorded as 0.6 S cm −1 highest among those of triphenylene-based MOF nanosheets with a thickness lower than 100 nm.

Nanosheets of metal–organic frameworks (MOFs)—porous crystalline materials consisting of metal ions and organic ligands—are actively studied for their intrinsic chemical/physical properties attributed to the reduced dimensionality and for their potential to function as ideal components of nanodevices, especially when electrical conduction is present. Air/liquid interfacial synthesis is a promising technique to obtain highly oriented MOF nanosheets. However, rational control of size and shape combined with the aimed functionality remains an important issue to address making it necessary to research the critical factors governing nanosheet characteristics in the interfacial synthesis. Here, we investigate the influence of the solvent—methanol (MeOH) versus N,N-dimethylformamide (DMF)—used to prepare a ligand spread solution on an assembly of MOF nanosheets composed of Ni2+ and 2,3,6,7,10,11-hexaiminotriphenylene (HITP) (HITP-Ni-NS). We find that the macroscopic morphological uniformity in the micrometer scale is higher when DMF is used as the solvent. Regarding the microscopic crystalline domain, molecules of DMF with relatively high polarity and boiling point are involved in HITP-Ni-NS formation, hindering its growth and resulting in nanosheets with slightly smaller lateral size than that grown when MeOH is used. These findings provide crucial guidelines towards establishing a judicious strategy for creating desired MOF nanosheets at the air/liquid interface, thereby driving forward research on both fundamental and applied aspects of this field.

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Metal–organic frameworks (MOFs)crystalline coordination polymerswith unique characteristics such as structural designability accompanied by tunable electronic properties and intrinsic uniform nanopores have become the platform for applications in diverse scientific areas ranging from nanotechnology to energy/environmental sciences. To utilize the superior features of MOF in potential applications, the fabrication and integration of thin films are of importance and have been actively sought. Especially, downsized MOFs into nanosheets can act as ultimately thin functional components in nanodevices and potentially display unique chemical/physical properties rarely seen in bulk MOFs. Assembling nanosheets by aligning amphiphilic molecules at the air/liquid interface has been known as the Langmuir technique. By utilizing the air/liquid interface as a reaction field between metal ions and organic ligands, MOFs are readily formed into the nanosheet state. The expected features in MOF nanosheets including electrical conduction largely depend on the nanosheet characteristics such as lateral size, thickness, morphology, crystallinity, and orientation. However, their control has not been achieved as yet. Here, we demonstrate how changing the concentration of a ligand spread solution can modify the assembly of MOF nanosheets, composed of 2,3,6,7,10,11-hexaiminotriphenylene (HITP) and Ni2+ ions (HITP-Ni-NS), at the air/liquid interface. A systematic increase in the concentration of the ligand spread solution leads to the enlargement of both the lateral size and the thickness of the nanosheets while retaining their perfect alignment and preferred orientation. On the other hand, at much higher concentrations, we find that unreacted ligand molecules are included in HITP-Ni-NS, introducing disorder in HITP-Ni-NS. These findings can develop further sophisticated control of MOF nanosheet features, accelerating fundamental and applied studies on MOFs.