Structural and electronic switching of a single crystal 2D metal-organic framework prepared by chemical vapor deposition

Abstract

The incorporation of metal-organic frameworks into advanced devices remains a desirable goal, but progress is hindered by difficulties in preparing large crystalline metal-organic framework films with suitable electronic performance. We demonstrate the direct growth of large-area, high quality, and phase pure single metal-organic framework crystals through chemical vapor deposition of a dimolybdenum paddlewheel precursor, Mo₂(INA)₄. These exceptionally uniform, high quality crystals cover areas up to 8600 µm² and can be grown down to thicknesses of 30 nm. Moreover, scanning tunneling microscopy indicates that the Mo₂(INA)₄ clusters assemble into a two-dimensional, single-layer framework. Devices are readily fabricated from single vapor-phase grown crystals and exhibit reversible 8-fold changes in conductivity upon illumination at modest powers. Moreover, we identify vapor-induced single crystal transitions that are reversible and responsible for 30-fold changes in conductivity of the metal-organic framework as monitored by in situ device measurements. Gas-phase methods, including chemical vapor deposition, show broader promise for the preparation of high-quality molecular frameworks, and may enable their integration into devices, including detectors and actuators.

Fig. 1. Gas-phase synthesis of large-area MOF crystals and their integration into devices.

a Schematic outlining the process for CVD growth of high-quality MOF single crystals with minimal defects and single-crystal character. A MOF precursor, such as Mo₂(INA)₄, sublimes under vacuum, is swept by a carrier gas along a thermal gradient, and deposits at lower temperatures to form high-quality MOF single crystals on a range of substrates. b Concerted deposition and coordination of Mo₂(INA)₄ clusters leads to single MOF crystals covering large areas of the substrate. c The large-area single crystals are integrated into devices (left), which are used to monitor the electronic response of the MOF to induced structural changes (middle) and to optical stimulation (right).

Fig. 2. High-quality Mo₂(INA)₄ MOF single crystals from gas-phase synthesis.

a Top: Photograph of the quartz tube reaction chamber of our CVD system. The reaction chamber is divided into three independent thermal zones. Reactants flow from left to right through the quartz tube. The red and yellow boxed regions denote the positions of the precursor and deposition substrate, respectively, during the reaction. Bottom left: Photograph of the ceramic crucible containing Mo₂(INA)₄ powder. Bottom right: Photograph of a red film of 1 deposited on a glass substrate. The left-hand side of the substrate is nearest the precursor boat. b Top: Optical image of a single deposit of 1. Scale: 10 μm. Bottom: Scanning electron micrograph of a corner of one deposit of 1. Scale: 1 µm. c 2D AFM topographical map of a single thin deposit of 1. The purple box and blue line denote approximate regions of interest investigated further in (d). A 10 µm × 10 µm region of this AFM map was used to calculate the root mean square surface roughness reported in the text. Scale: 10 μm. d Top: 3D AFM topographical map encompassing the 25 µm² area annotated by the purple box in (c). These data exemplify the exceptionally low surface roughness of the CVD-grown deposits. Bottom: AFM height profile along the path annotated by the blue line in (c). e Raman spectra of the Mo₂(INA)₄ powder precursor (red) and of a single deposit of 1 (yellow). The gray band highlights the prominent peak at 383 cm^–1 attributable to $\tilde{\nu }$(Mo–Mo). f Crystal packing of 1 viewed down the c axis. The pink line highlights one of the 1D zig-zag coordinated chains. g Crystal packing of 1 viewed down the a axis showing its layered structure. Hydrogen atoms have been omitted for clarity. Displacement ellipsoids are given at 50% probability.

Fig. 3. Reversible structural switching between 1 and 2.

a Photographs of a single crystal of 1 exposed over time to dimethylacetamide (DMA) vapor. b Single-crystal structures of 1 (left) and of 2 (right) showing how exposure to DMA and methanol can facilitate interconversion between the two structures. Solvent molecules have been omitted for clarity. c Simulated and experimental powder XRD scans of bulk samples of 1 and 2 used to demonstrate reversible conversion between the MOF structures.

Fig. 4. Stimuli responsive conductivity of Mo₂(INA)₄ single-crystal devices.

a Optical images collected during measurement of a device fabricated on a single crystal of 1. The crystal boundary is highlighted by the dashed gold box and the bright diffraction-limited laser spot is evident in the right image. Scale: 5 µm. b Current–voltage characteristics of a single-crystal device of 1 (yellow) and of a single-crystal device of 2 (blue) prepared through exposure to DMA. c Photocurrent generated by a single-crystal device of 1 when illuminated with a 526 nm laser at 1.29, 1.99, 2.76, 3.45, and 4.14 mW. The device source-drain bias was held constant at 20 V. Error bars denote standard deviation. d Photocurrent switching behavior of a single-crystal device of 1 subjected to periodic laser illumination at 5-s intervals (green highlighted regions). Device switching behavior was assessed at 1.29 mW (red), 1.99 mW (orange), 2.76 mW (green), 3.45 mW (blue), and 4.14 mW (purple).

Fig. 5. STM topography images of a Mo₂(INA)₄ monolayer on a Au(111) surface.

a STM image (bias V = 0.05 V, tunneling current I = 5 pA) of Mo₂(INA)₄ clusters deposited on Au(111) before annealing. Scale: 2.0 nm. b STM image (V = 2 V, I = 10 pA) of a 2D Mo₂(INA)₄ MOF monolayer on Au(111) after annealing (crystal structure 2 is superimposed). Scale: 2.0 nm.