3 research outputs found
Rhodizonic Acid on Noble Metals: Surface Reactivity and Coordination Chemistry
A study
of the two-dimensional crystallization of rhodizonic acid
on the crystalline surfaces of gold and copper is presented. Rhodizonic
acid, a cyclic oxocarbon related to the ferroelectric croconic acid
and the antiferroelectric squaric acid, has not been synthesized in
bulk crystalline form yet. Capitalizing on surface-assisted molecular
self-assembly, a two-dimensional analogue to the well-known solution-based
coordination chemistry, two-dimensional structures of rhodizonic acid
were stabilized under ultrahigh vacuum on Au(111) and Cu(111) surfaces.
Scanning tunneling microscopy, coupled with first-principles calculations,
reveals that on the less reactive Au surface, extended two-dimensional
islands of rhodizonic acid are formed, in which the molecules interact
via hydrogen bonding and dispersion forces. However, the rhodizonic
acid deprotonates into rhodizonate on Cu substrates upon annealing,
forming magic clusters and metal–organic coordination networks
with substrate adatoms. The networks show a 2:1 distribution of rhodizonate
coordinated with 3 and 6 Cu atoms, respectively. The stabilization
of crystalline structures of rhodizonic acid, structures not reported
before, and their transition into metal–organic networks demonstrate
the potential of surface chemistry to synthesize new and potential
useful organic nanomaterials
Enhancement of Local Piezoresponse in Polymer Ferroelectrics <i>via</i> Nanoscale Control of Microstructure
Polymer ferroelectrics are flexible and lightweight electromechanical materials that are widely studied due to their potential application as sensors, actuators, and energy harvesters. However, one of the biggest challenges is their low piezoelectric coefficient. Here, we report a mechanical annealing effect based on local pressure induced by a nanoscale tip that enhances the local piezoresponse. This process can control the nanoscale material properties over a microscale area at room temperature. We attribute this improvement to the formation and growth of β-phase extended chain crystals <i>via</i> sliding diffusion and crystal alignment along the scan axis under high mechanical stress. We believe that this technique can be useful for local enhancement of piezoresponse in ferroelectric polymer thin films
Multimodal Nonlinear Optical Imaging of MoS<sub>2</sub> and MoS<sub>2</sub>‑Based van der Waals Heterostructures
van der Waals layered structures,
notably the transitional metal
dichalcogenides (TMDs) and TMD-based heterostructures, have recently
attracted immense interest due to their unique physical properties
and potential applications in electronics, optoelectronics, and energy
harvesting. Despite the recent progress, it is still a challenge to
perform comprehensive characterizations of critical properties of
these layered structures, including crystal structures, chemical dynamics,
and interlayer coupling, using a single characterization platform.
In this study, we successfully developed a multimodal nonlinear optical
imaging method to characterize these critical properties of molybdenum
disulfide (MoS<sub>2</sub>) and MoS<sub>2</sub>-based heterostructures.
Our results demonstrate that MoS<sub>2</sub> layers exhibit strong
four-wave mixing (FWM), sum-frequency generation (SFG), and second-harmonic
generation (SHG) nonlinear optical characteristics. We believe this
is the first observation of FWM and SFG from TMD layers. All three
kinds of optical nonlinearities are sensitive to layer numbers, crystal
orientation, and interlayer coupling. The combined and simultaneous
SHG/SFG-FWM imaging not only is capable of rapid evaluation of crystal
quality and precise determination of odd–even layers but also
provides <i>in situ</i> monitoring of the chemical dynamics
of thermal oxidation in MoS<sub>2</sub> and interlayer coupling in
MoS<sub>2</sub>–graphene heterostructures. This method has
the advantages of versatility, high fidelity, easy operation, and
fast imaging, enabling comprehensive characterization of van der Waals
layered structures for fundamental research and practical applications