We reveal that the Ir⁡(001)−(5×1) reconstructed surface covered by Xe adatoms exhibits a band gap of 100 meV, which arises from the new periodic potential created by the reconstruction, whereas no band gap was observed on the Ir⁡(001)−(1×1) surface within the measured momentum range. The experimental results are compared with the theoretical ones obtained within density functional theory for both the clean and Xe-covered Ir⁡(001)−(5×1) reconstructed surfaces. The planar averaged charge density distributions of the IPS for both surfaces show that the Xe adsorption does not significantly alter the positions of charge density maxima, which rationalizes why the band gap arising from the substrate superlattice potential does not change significantly upon Xe adsorption. 

Phys. Rev. Research 7, 023294 (2025)

Multiphoton photoemission provides a means to investigate unoccupied electronic states via nonlinear light-matter interactions. In this work, we employ five-photon photoemission spectroscopy to identify, for the first time, the image-potential state (IPS) on the Ir(111) surface. Distinct from commonly studied noble metals such as Cu and Ag, the Ir(111) electronic structure leads to a strong sensitivity to excitation energy: a reduction from 1.57 eV to 1.49 eV significantly diminishes the signal. The theoretical analysis attributes this effect to the d-band proximity to the Fermi level, which influences the initial-state population and transition probabilities governing the multiphoton excitation pathways.

Appl. Phys. Express 18, 062001 (2025)

The reactivity of layered PdTe2 toward methanol decomposition was promoted by surface under-coordinated Pd generated by removing surface Te with controlled Ar ion bombardment. Methanol on the under-coordinated Pd sites at surface Te vacancies decomposed through competing dehydrogenation and C–O bond cleavage processes; approximately 26% of methanol was converted to CHx* and 17% to CHxO* (* denotes adspecies; x = 2 and 3) as major intermediates at 180 K, leading to a reaction probability of >40%. Our results suggest that a PdTe2 surface with under-coordinated Pd can serve as an efficient catalyst toward methanol decomposition and against carbon poisoning.

Phys. Chem. Chem. Phys. 27, 9336 - 9349 (2025)

For monolayer transition metal dichalcogenides (ML-TMDs), broken inversion symmetry and strong spin-orbit coupling result in spin-valley lock-in effects so that the valley degeneracy may be lifted by external magneticfields, potentially leading to real-space structural transformation. Here, magnetic-field-induced giant electric hysteretic responses to back-gate voltages are reported in ML-MoS2 field-effect transistors (FETs) on SiO2/Si at temperatures < 20 K. In further studies by scanning tunneling microscopy (STM), the moiré superlattice of MoS2 on highly ordered pyrolytic graphite (HOPG) at 4.5 K under magnetic field clearly show that the evolving moiré superlattices changed, which origin from the expansion of MoS2 lattice when different magnetic fields were applied. Our study is the very first discovery on this fascinating phenomenon, it not only reveals a new prospect of structural manipulation in two-dimensional materials but also give rise to considerable approaches for future applications on multiple devices.

Adv. Mater. 36, 2411393 (2024)

This study focuses on improving the quality of metal electrodes by introducing atomic H to create surface defects, using Ni on WS2 as an example. Our studies reveal that introducing point defects on the WS2 surface, results in a significant shift in Ni growth from the Volmer–Weber to a near Frank-van der Merwe mode. The origin of the change is the bond formation between the Ni and W atoms, which is expected to realize ohmic contact. The optimization of metal–TMD interfaces offers valuable insights for advanced applications. 

ACS Appl. Mater. Interfaces 16, 56336–56342  (2024)

This study investigates the extensively debated role of Fermi surface nesting in causing the CDW state in 2H-NbSe2 materials. Four NbSe2 structures are identified on the basis of the characteristics in STM images and first-principles simulations. The calculations reveal that an energetically favored filled phase corresponds to Peierls’ description with fully opened gaps at the Brillouin zone boundary, resulting in a drop at the Fermi level in the density of states and STS spectra. This comprehensive study demonstrates that the filled phase of NbSe2 can be categorized as a Peierls instability-induced CDW in two-dimensional systems. 

ACS Materials Lett. 6, 2941–2947 (2024)

Here we show a reaction probability > 90 % for adsorbed methanol (CH3OH) on under-coordinated Pt sites at surface Te vacancies, produced with Ar+ bombardment, on layered PtTe2 — approximately 60 % of the methanol decompose to surface intermediates CHxO (x = 2, 3) and 35 % to CHx (x = 1, 2), and ultimate production of gaseous molecular hydrogen, methane, water and formaldehyde. The characteristic reactivity is attributed to the triangular positioning and varied degrees of oxidation of the under-coordinated Pt at Te vacancies.

Nature Communications 15, 653 (2024)

To develop low-power, non-volatile computing-in-memory device using ferroelectric transistor technologies, ferroelectric channel materials with scaled thicknesses are required. Two-dimensional semiconductors, such as molybdenum disulfide (MoS2), equipped with sliding ferroelectricity could provide an answer. However, achieving switchable electric polarization in epitaxial MoS2 remains challenging due to the absence of mobile domain boundaries. Here we show that polarity-switchable epitaxial rhombohedral-stacked (3R) MoS2 can be used as a ferroelectric channel in ferroelectric memory transistors. We show that a shear transformation can spontaneously occur in 3R MoS2 epilayers, producing heterostructures with stable ferroelectric domains embedded in a highly dislocated and unstable non-ferroelectric matrix.

Nature Electronics 7, 29–38 (2024)

NbTiN has a high critical temperature (Tc) of up to 17 K, making it a great candidate for superconducting nanowire single-photon detectors (SNSPDs). In this study, we utilized STM/STS to investigate the inhomogeneity of superconducting properties in meandered NbTiN nanowires, showing that variations in the superconducting gap strongly correlate with the film thickness. Using time-dependent Ginzburg–Landau simulations and statistical modeling, we explored the implications of reducing the critical current and its sample-to-sample variations.

ACS Appl. Opt. Mater. 2, 68–75 (2024)

The study uses ultrafast pump-probe spectroscopy to investigate energy transfer (ET) mechanisms between organic Alq₃ molecules and silicon (Si). By introducing ultrathin SiO₂ layers between Alq₃ and Si, SiO₂-thickness-dependent relaxation dynamics of photoexcited carriers reveal ET from Alq₃ to Si, notably in the 200-350 ps time scale. The findings corroborate theoretical predictions of long-range dipole-dipole interactions, underscoring their role in enhancing optoelectronic device performance. Overall, this work sheds light on ET mechanisms crucial for future device development by combining experimental observations with theoretical models.

Nano Lett. 23, 10490-10497 (2023)

We investigate a buckled plumbene-Au Kagome superstructure formed by depositing Au on Pb(111). It exhibits an enhanced superconducting critical temperature (Tc) compared to monolayer Pb and bulk Pb substrate. The presence of a monolayer Au-intercalated low-buckled plumbene between the Au Kagome layer and Pb(111) substrate is confirmed. The study reveals enhanced superconductivity due to electron-phonon coupling. This work highlights the potential of the buckled plumbene-Au Kagome superstructure in enhancing Tc and exploring novel properties in plumbene.

Adv. Sci. 2300845, (2023)

Layered transition metal dichalcogenides (TMDs) show unique features for electronic and optoelectronic applications. However, surface defects can greatly affect device performance. This study proposes a two-step approach, including Ar ion bombardment and annealing, to reduce surface defects, mainly Te vacancies, on PtTe2 and PdTe2 surfaces by more than 99%. Defect density <1.0 × 1010 cm–2 was achieved, which cannot be done solely with annealing. A proposed mechanism behind the processes is also discussed.

ACS Appl. Mater. Interfaces, 15, 12, (2023)

US Patent 20240332013

Defects in 2D transition metal dichalcogenides can hinder development of 2D materials. Scanning tunneling microscopes can identify atomic defects, but are slow. This study proposes a deep learning-based atomic defect detection framework (DL-ADD) to detect defects in MoS2 and other TMDs, using data augmentation, color preprocessing, noise filtering, and a detection model. DL-ADD provides precise detection in MoS2 (average F2-scores is 0.86) and good generality to WS2 (average F2-scores is 0.89).

Sci Data 10, 91 (2023)

Many studies have demonstrated that molybdenum disulfide (MoS2), a member of Transition metal dichalcogenides, is promising as a channel material to fabricate field-effect transistors (FETs). However, the carrier mobility in MoS2 FET is always far lower than the theoretical prediction. In this work, by using scanning tunneling microscopy, we directly counted the defects in MoS2 FETs with different carrier mobility. We found that vacancies and impurities equally contribute to carrier mobility, and the total defect density induces a power-law decreasing tendency toward the carrier mobility of MoS2 FET. 

Appl. Phys. Lett. 121, 151601 (2022)

In 2012, silicene was first grown on Ag(111). Although various characteristics, such as electronic and atomic structures, have been revealed, the unique growth behavior of silicene on Ag(111) has almost been overlooked; although Ag(111) substrate is entirely covered by silicene at the beginning of the growth, further deposition of Si atoms leads to the re-exposure of bare Ag(111) substrate with the formation of stable bulk Si. This phenomenon is called dewetting (DW). We experimentally confirmed this unique DW by scanning tunneling microscopy (STM) measurement and successfully reproduced the DW growth by a simulation based on the kinetic Monte Carlo (KMC) method. 

Nanoscale 14, 14623 (2022)

Monolayer molybdenum disulfide (MoS2) and Platinum ditelluride (PtTe2) are semiconducting properties in two-dimensional material, offering a promising platform for developing next-generation electronics beyond graphene. We find the bandgap can be tuned by adjusting the tunneling current through scanning tunneling spectroscopy measurement. Combining Density Function Theory (DFT), further identify the mechanism of bandgap tuning can be attributed to the tip-induced electric field and tip-sample hybridization for MoS2 and PtTe2, respectively. 

ACS Nano 16, 14918–14924 (2022)

Defects in WTe2  can impact properties in both positive and negative ways. Here we report on both geometric and electronic characteristics of the defects in WTe2 identified by the combination of STM and DFT calculation. Interestingly, the defects under the surface are hardly observed but they are dressed with the noncentrosymmetric quasiparticle interference (QPI) fringes which enable us to identify them. These findings demonstrate that STM-QPI can be a feasible method to characterize the defects in layered materials.

Phys. Rev. B 106, 075428 (2022)

The measurements of vibrational properties of molecules on solid surfaces are usually performed at temperatures of liquid-He (~4K) for higher resolution. In this work, STM-based IETS reveals the vibrational spectra of 2D FePc lattice grown on Au(111) at 77K, it shows that the vibrational signals are still observable at much higher and more accessible temperatures. It could encourage further studies through this technique.

J. Phys. Chem. C 2022, 126, 31, 13327–13331(2022)

In the current progress of the semiconductor industry, searching for new materials beyond Si is always an issue. Ge exhibits higher electron and hole mobility than Si but Ge oxides are not as stable as Si oxides. The current work by STS shows that a small amount of Ti can help ultrathin Ge oxides (~0.7nm) resist the heat influence and maintain the bandgap up to 500°C fabricating temperatures.

J. Phys. D: Appl. Phys. 54 345102 (2021) 

The review collects the current investigations by several methods based on the STS technique for layered topological materials. The electronic characteristics of these layered topological materials are experimentally identified. 

J. Phys.: Condens. Matter 32 243001 (2020) 

WTe2 and MoTe2 are layered topological materials. Trough Quasipaticles Interference experiments, evidence of Weyl fermions is successfully observed.

ACS Nano 11, 11, 11459-11465 (2017) 

J. Phys.: Condens. Matter 30 105703 (2018) 

Scheme of using STM to carry out the Quantum Hall Effect Measurement (QHE) for single layer 2D materials. By applying magnetic field (up to 9 Tesla), it is able to observe quantized states named Landau levels in the STS spectra.

Phys. Rev. Lett. 110, 076801 (2013)