Kunal S. Mali

Surface-induced symmetry breaking enables the on-surface synthesis of boroxine-linked chiral 2D covalent polymers with enantiomorphic domains.

We demonstrate the feasibility of covalently functionalizing graphitic surfaces, including graphene, relying on the redox-triggered electrografting of a viologen diazonium salt, abbreviated as g-DBV. This compound is in situ prepared from 1,1′-bis(4-aminobenzyl)-4,4′-bipyridinium dibromide (a-DBV). Dediazotized g-DBV moieties covalently graft onto graphitic surfaces. The efficiency of this grafting approach was revealed by employing a state-of-the-art toolbox of characterization techniques, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). Triggering the redox states of the g-DBV under electrochemical control enables functionalization of graphitic surfaces by either random (g-DBV2+), dimer phase (g-DBV•+), or stripe (g-DBV0) patterns. Importantly, the g-DBV0 striped graphene surface reveals n-doping caused by the charge transfer from the electron-rich uncharged g-DBV0 species to graphene. This finding opens a new way to nanopatterned functionalization and doping of graphene and other two-dimensional (2D) substrates by electrochromic molecules.

A 2D moiré superlattice forms when two or more atomically thin layers, such as graphene or transition metal dichalcogenides (TMDs), are stacked with a slight twist or lattice mismatch. This results in a moiré pattern with a periodicity much larger than the original lattice constants, giving rise to novel quantum properties. Moiré superlattices have revealed a range of exotic quantum phenomena, including flat electronic bands, the anomalous quantum Hall effect, unconventional superconductivity, and ferromagnetism. These discoveries underscore their transformative potential for future quantum technologies, as they provide a tunable platform bridging superconducting and insulating states.

Chirality-induced spin selectivity (CISS) has been predominantly demonstrated in enantiopure systems, while racemic mixtures are considered spin-inactive due to the cancellation of opposite enantiomeric contributions. Here, we demonstrate that surface-assisted symmetry breaking activates CISS in racemic systems. A racemic bis-helical [1]benzothieno[3,2-b]benzothiophene (BTBT) derivative (Rac-1) spontaneously segregates into homochiral (M,M) and (P,P) domains, forming a two-dimensional (2D) racemic conglomerate directly visualized by scanning tunneling microscopy (STM). Scanning tunneling spectroscopy (STS) reveals enantiospecific magnetoconductance asymmetry (EMA) within individual homochiral domains at room temperature, reaching +(40 ± 5)% for (P,P) domains and -(35 ± 10)% for (M,M) domains when the ferromagnetic Co layer is magnetized perpendicular to the substrate plane. Magnetization-direction-dependent STS measurements show that EMA is reduced from ∼35% for out-of-plane to <6% for in-plane Co layer orientation. Distance-dependent STS and control measurements on Au(111)/Mica without a ferromagnetic layer further confirm that EMA is intrinsic to CISS and independent of junction artifacts. Despite surface-induced planarization of the molecular backbone upon adsorption, as revealed by DFT calculations, EMA persists, driven by the collective 2D chiral packing and the resulting chiral electrostatic potential at the molecule-surface interface. These results establish that bulk homochirality is not a prerequisite for room-temperature CISS, identifying racemic conglomerates as a synthetically accessible platform for investigating CISS at the nanoscale.

Chiral induction by the solvent phase offers a promising route to achieve homochirality in self-assembled molecular networks. In this work, we examine solvent-driven chiral induction at the solid–liquid interface, where an enantiopure solvent biases the handedness of two-dimensional (2D) networks formed by achiral molecules physisorbed on the basal plane of highly oriented pyrolytic graphite. Using scanning tunneling microscopy at the liquid–solid interface, we visualize the 2D organization of the achiral monomer 10,12-pentacosadiynoic acid in chiral liquid environments. We compare the induction efficiencies of several enantiopure citronellyl-derived solvents that differ in their capacities for specific solute–solvent interactions. Complementary molecular dynamics simulations provide molecular-level insight into the possible mechanisms of solvent-mediated chiral induction.

Weak dipole–dipole interactions drive the formation of 2D honeycomb porous networks at interfaces. STM and simulations uncover how subtle molecular design and interfacial forces control stability, switching behavior, and guest inclusion.

Combining surface chirality and photoresponsiveness can result in interfaces with dynamic stereochemical properties. Such materials offer unique opportunities in the context of tunable chiral recognition at interfaces. Here, we present a strategy to achieve dynamically chiral surfaces using self-assembled molecular networks (SAMNs) of overcrowded alkene-based molecular motors on highly oriented pyrolytic graphite. The chiroptical properties and the dynamic behavior of the molecular motors are maintained when confined in the SAMN, as shown using scanning tunneling microscopy (STM) with in situ irradiation. Light triggers the transition from a homochiral assembly of the stable state of a molecular motor (RP chirality) to the heterochiral coassembly between the stable and the metastable state (RP/RM chirality). The RP/RM coassembly is a kinetic phase that disappears once the external stimulus is removed─highlighting the reversibility of the system. These results demonstrate how molecular motion can be harnessed to encode reversible chirality into surfaces.

The crystallinity and stacking order of two-dimensional covalent organic frameworks (2D COFs) critically govern their physical properties and performance. However, direct molecular-resolution insight into the growth of layered 2D COFs remains elusive, particularly for imine-linked systems. Herein, an imine-linked bilayer 2D COF is constructed at the solid-liquid interface, enabling time-dependent scanning tunneling microscopy (STM) visualization of interlayer growth. STM imaging reveals a layered growth mechanism involving defect-mediated nucleation, lattice-guided directional propagation, cross-defect growth, and oriented domain coalescence. This growth pathway gives rise to local ordered-on-disordered stacking embedded within predominantly AA stacking domains. These findings provide direct molecular-level insights into layered growth under surface-confined conditions and offer a conceptual perspective for considering stacking disorder in bulk imine-linked COFs.

Monolayer covalent organic frameworks (COFs) featuring reversible linkages have emerged as promising platforms for dynamic structural reconfiguration. However, achieving programmable depolymerization and reconstruction of robust imine COFs remains a significant challenge. Here we present an "electric-field-mediated molecular scalpel" strategy that combines a localized electric field with bias-responsive boronic acid molecules to achieve localized C═N bond cleavage and nanoscale reconstruction, enabling precise patterning of in-plane imine/boroxine COF hybrid framework. In situ STM reveals a gradual depolymerization mechanism. Kinetic analysis, liquid-phase atomic force microscopy (AFM), nuclear magnetic resonance (NMR), and molecular dynamics (MD) simulations collectively validate a new electric-field-mediated interfacial reconfiguration mechanism of "adsorption-activation-transformation". In this process, boronic acid molecules compete for surface sites, destabilizing the imine lattice and promoting its depolymerization. This work provides the first (sub)molecular-level insight into the dynamic depolymerization of imine COFs and, with nanometer precision, enables the construction of in-plane hybrid framework between imine and boroxine COFs, thereby establishing a generalizable framework for electric-field-mediated nanoscale molecular engineering.

Semiconductor nanocrystals with uniform morphology and composition are expected to show consistent responses during light-matter interactions. However, microscopy reveals significant variations in their photoluminescence blinking patterns, even under identical experimental conditions. This discrepancy arises from differences in crystal defects and nonradiative trap states. As a result, heterogeneous blinking patterns serve as valuable indicator of material quality, uncovering several concealed features through statistical analysis of large datasets. Nonetheless, efficient segregation and analysis of numerous blinking trajectories remain a challenge due to laborious calculations, computational bottlenecks, and manual intervention. In this study, we introduce a robust unsupervised machine learning (UML) assisted module to cluster high-dimensional blinking patterns in near-real-time, while calculating category-wise power spectral densities (PSD) to investigate active traps. Furthermore, we explore the impact of data preprocessing on clustering performance. The ‘clustering-segregation-analysis’ (UML-PSD) methodology demonstrates versatility, paving a way to advance contemporary (micro)spectroscopy, specifically for rapid and cost-effective optical characterization of semiconductor nanocrystals.