  Conductive metal-organic frameworks (MOFs) are emerging electroactive
materials for (opto-)electronics. However, it remains a great challenge to
achieve reliable MOF-based devices via the existing synthesis methods that are
compatible with the complementary metal-oxide-semiconductor technology, as the
surface roughness of thus-far synthetic MOF films or pellets is rather high for
efficient electrode contact. Here, we develop an on-liquid-gallium surface
synthesis (OLGSS) strategy under chemical vapor deposition (CVD) conditions for
the controlled growth of two-dimensional conjugated MOF (2D c-MOF) thin films
with ten-fold improvement of surface flatness (surface roughness can reach as
low as ~2 {\AA}) compared with MOF films grown by the traditional methods.
Supported by theoretical modeling, we unveil a layer-by-layer CVD growth mode
for constructing flattening surfaces, that is triggered by the high adhesion
energy between gallium (Ga) and planar aromatic ligands. We further demonstrate
the generality of the as-proposed OLGSS strategy by reproducing such a flat
surface over nine different 2D c-MOF films with variable thicknesses (~2 to 208
nm) and large lateral sizes (over 1 cm2). The resultant ultra-smooth 2D c-MOF
films enable the formation of high-quality electrical contacts with gold (Au)
electrodes, leading to a reduction of contact resistance by over ten orders of
magnitude compared to the traditional uneven MOF films. Furthermore, due to the
efficient interfacial interaction benifited from the high-quality contacts, the
prepared van der Waals heterostructure (vdWH) of OLGSS c-MOF and MoS2 exhibits
intriguing photoluminescence (PL) enhancement, PL peak shift and large work
function modulation. The establishment of the reliable OLGSS method provides
the chances to push the development of MOF electronics and the construction of
multicomponent MOF-based heterostructure materials.
