Tip-Induced Chemistry via STM/AFM

• Tip-induced chemistry is a process that uses STM/AFM tips to precisely initiate and control chemical reactions at the atomic scale via electronic excitation, local electric fields, and mechanical interactions.
• It enables atom-by-atom engineering of molecules and defects in 2D materials, offering deterministic synthesis and in situ characterization of reaction intermediates.
• The technique facilitates real-time imaging, quantitative kinetic measurements, and functionalization of probe tips for advanced quantum and spin-resolved studies.

Tip-induced chemistry refers to the targeted initiation, control, and characterization of chemical reactions at the atomic or molecular scale using the tip of a scanning tunneling microscope (STM) or atomic force microscope (AFM). These SPM-driven processes leverage the tunneling electrons, local electric fields, and mechanical forces delivered by the tip-sample junction to modulate activation barriers, drive electronic/structural transitions, and rearrange bonds with single-atom spatial precision. Tip-induced chemistry has become a cornerstone in the study of quantum matter, molecular electronics, surface reaction mechanisms, defect engineering in 2D materials, and atomistic synthesis of designer nanostructures.

1. Mechanisms of Tip-Induced Chemical Processes

At the core, a tip positioned within the quantum tunneling regime (typically at sub-nanometer separations and at low temperature) can deliver highly localized energy—either via inelastic tunneling electrons, intense electric fields, or piconewton-scale mechanical interactions. Three principal routes dominate:

• Electronic Excitation: Inelastic electron tunneling through the tip can impart vibrational energy to targeted bonds, promote transient excited electronic states, and drive selective bond cleavage or rearrangement. The action often depends critically on the applied bias and the alignment of tip, molecular, and substrate electronic states. In one reported case, tip-induced β-hydrogen dissociation in surface-bound –CH_2CH_3 on Si(001) is accomplished via one-electron excitation into substrate dangling-bond states above +1.5 V, accompanied by thermal activation to achieve the required reactive conformation (Adamkiewicz et al., 2021).
• Local Electric Field Effects: The intense field at the tip apex (up to ~10 V/nm in specialized pulsing regimes) can distort molecular potential energy surfaces, reduce reaction barriers (e.g., in skeletal rearrangements), and polarize reaction intermediates.
• Mechanical Action: Precise vertical or lateral manipulation by the SPM tip allows for atom-by-atom assembly, defect incorporation, or direct assistance in reaction coordinate evolution by exerting forces of 100–200 pN. In the case of vacancy engineering in MoS₂, the mechanical removal of S atoms via a Fe-adatom-assisted pull with the tip provides a reproducible route to vacancy creation (Jansen et al., 2024).

These mechanisms can operate singly or in concert, enabling unprecedented selectivity, control, and reproducibility unobtainable in macroscopic or solution-phase chemistry.

2. Atom-by-Atom and Single-Molecule Engineering

A hallmark of tip-induced chemistry is the deterministic creation or modification of atomic-scale features—defects, adatoms, or whole molecular frameworks—on surfaces under UHV and cryogenic conditions.

• Defect Engineering in 2D Materials: By centering the STM tip above isolated Fe adatoms on MoS₂ and applying a specific voltage/current protocol, single top-layer S atoms are extracted (with a formation energy ∆E_vac ≈ 2–3 eV), leaving well-defined, neutral S vacancies (V_S^0) (Jansen et al., 2024). Subsequent tip-induced charging reveals bistable Jahn–Teller-distorted configurations, confirmed via DFT and orbital-resolved dI/dV mapping.
• Skeletal Editing of Organic Molecules: Stepwise removal of heteroatoms or ring contractions in polycyclic adsorbates is achieved via voltage pulses (e.g., 4.9–5.1 V) in AFM/STM on NaCl/Cu(111) (Mishra et al., 15 Sep 2025). In Mishra et al.'s work, individual dinaptho[1,8-bc:1′,8′-ef]oxepine (DNO) molecules are converted into perylene derivatives through controlled O-atom deletion and intramolecular ring reorganization, mapped at sub-Å resolution. Notably, such reactions can be monitored in situ at every step with STM/AFM.
• Spin-Active Center Generation: Controlled removal of azide groups from triazido-s-heptazine via tip-induced N₂-extrusion yields mono-, di-, and trinitrene centers, with each nitrene revealed by bond-resolved AFM and high-spin state characterized by STM and theoretical analysis (Lieske et al., 5 Jun 2025). This enables the deterministic engineering of localized magnetic functionality at the molecular level.

3. Functionalized Probes and Quantum Impurities

Tip-induced chemistry is not limited to sample engineering but extends to deliberate functionalization of the STM/AFM tip, creating “designer probes” with bespoke electronic/magnetic properties:

• Superconducting and Magnetic Functionalization: Vertical manipulation enables transfer of TCNQ molecules from graphene/Ir(111) onto a Nb STM tip, generating a charge-transferred S=½ radical at the tip apex (Ayani et al., 2022). This functionalized tip displays Yu–Shiba–Rusinov (YSR) states, with sub-gap bound-state energies E_{YSR} related to the tip’s superconducting gap Δ and exchange coupling J via:

$$E_{YSR} = \Delta \frac{1-\alpha^2}{1+\alpha^2}, \quad \alpha = \frac{\pi \rho J S}{2}$$

By tuning the tip-sample distance, the exchange interaction is reversibly modulated, effecting a controllable transition between “free spin” and “screened singlet” quantum impurity regimes. Quenching tip superconductivity by a magnetic field transforms the spectroscopic signature from YSR peaks to a Kondo zero-bias resonance, enabling in situ switching and the study of competing many-body ground states.

4. Quantitative Kinetics and Activation Regimes

Statistical and mechanistic characterization of tip-induced reactions provides kinetic laws and insight into activation processes:

• Single-Electron and Thermally Moderated Pathways: In tip-induced β-hydrogen dissociation on Si(001), conversion probability at 300 K shows a strictly linear dependence on tunneling current, indicating a one-electron process with reaction probability per electron of ∼1×10⁻¹⁰ (Adamkiewicz et al., 2021). The observed Arrhenius-type temperature dependence at fixed current demonstrates the necessity of thermal energy for conformational adjustment, while the suppression at 50 K indicates that pure tunneling excitation is insufficient unless coupled with thermal motion.
• Activation Barriers in SPM-Induced Reactions: DFT calculations for skeletal editing reactions (e.g., O-deletion and ring contraction in DNO) yield specific electronic activation energies (ΔE^‡ up to 3.85 eV for the highest barriers), closely matching the energy delivered by inelastic tunneling events in high-bias STM pulses (Mishra et al., 15 Sep 2025). On-surface reaction intermediates can be stabilized or diverted relative to the gas phase due to substrate coordination (e.g., Na⁺–O interactions).

5. Imaging, Characterization, and Electronic Structure Mapping

STM and AFM provide atomically resolved maps of both nuclear and electronic structure evolution before, during, and after a tip-induced reaction:

• Bond and Orbital Resolution: CO-functionalized AFM tips acquire Laplace-filtered images capable of directly discriminating single, double, and triple bonds within reaction products (Mishra et al., 15 Sep 2025), as well as imaging molecular orbitals associated with defect or product states across a range of tunneling biases. Features such as threefold- or C_s-symmetric orbital shapes corresponding to Jahn–Teller distortions become apparent in dI/dV mapping (Jansen et al., 2024).
• Spin-State Determination: The combination of STM/AFM imaging and theoretical probe-particle, DFT, and configuration-interaction (CASPT2, DDCI) simulation allows detailed assignment of spin multiplicities (e.g., septet vs sextet for trinitreno-s-heptazine under different charge states), as well as quantification of exchange couplings among localized and delocalized spins (Lieske et al., 5 Jun 2025).

6. Comparative Perspective: Tip-Induced Chemistry Versus Traditional Routes

Unlike solution-phase methods that typically require molecular ensembles, high temperature, or catalysts, tip-induced chemistry proceeds at cryogenic temperatures, under UHV, and at the level of individual molecules or atomic sites. Reactions are site-selective, yield can be deterministic per event, and short-lived intermediates are directly observable (Mishra et al., 15 Sep 2025).

Bulk synthetic routes to oxygen deletion or nitrene formation generally involve transition-metal catalysis and radical initiators, providing sequence selectivity but lacking atomic precision. Tip-based approaches complement these strategies by enabling:

• In situ tracking of pathways, activation barriers, and intermediates,
• Deterministic assembly of designed defect patterns, nanolattices, or spin arrays,
• Quantitative, real-time spectro-microscopy correlated with theoretical modeling.

Tip-induced chemistry in STM/AFM thus furnishes an atomically precise, highly versatile platform for studying and steering chemical dynamics, quantum phenomena, and functional material assembly on the ultimate scale of single bonds and spins (Mishra et al., 15 Sep 2025, Jansen et al., 2024, Ayani et al., 2022, Adamkiewicz et al., 2021, Lieske et al., 5 Jun 2025).