Paul Fons

The high contact resistivity at metal/semiconductor interfaces in transition metal dichalcogenides (TMDCs), particularly MoS ₂ , severely limits device performance and remains an urgent challenge. Recently, semimetal contacts have been reported to reduce contact resistivity, however, they suffer from poor thermal stability. Here, we propose metallic amorphous chalcogenide (MAC) as a solution and demonstrate amorphous TiS ₂ (a-TiS ₂ ) as a promising electrode material. Molecular dynamics (MD) simulations using machine learning potential reveal the spontaneous formation of a van der Waals (vdW) gap at the a-TiS ₂ /MoS ₂ interface. Subsequent density functional theory (DFT) calculations show that the quasi-vdW interface suppresses the formation of metal-induced gap states (MIGS). The suppression of MIGS has the potential to reduce contact resistivity. In parallel, experimental characterization of sputtered a-TiS ₂ thin films by X-ray diffraction confirms thermal stability up to 500 ℃. The amorphous nature facilitates omnidirectional contact formation, a critical attribute for next-generation all-around transistors.

Transition metal nitrides such as CrN are promising for thermoelectric applications due to their high stability and tunable electronic properties, yet they have been largely limited to n-type conduction, restricting device design. Here, we report the successful synthesis of p-type CrN films via controlled residual oxygen incorporation during RF magnetron sputtering regulated through N₂ gas flow adjustment, without introducing any additional oxygen source. A low N₂ gas flow rate (f N₂ ) produces N-deficient CrN_1−δ(O) films with n-type conduction dominated by nitrogen vacancies, while higher f N₂ (≥4 sccm) stabilizes Cr vacancies in over-stoichiometric Cr_1−δN(O) films, resulting in p-type hole conduction. Cr K-edge X-ray absorption fine structure (XAFS) reveals Cr–Cr bond elongation, reduced coordination, and enhanced Cr3d–O/N2p hybridization, indicative of localized hole states. Temperature-dependent transport measurements confirm the mechanisms, leading to a room-temperature power factor (PF) up to 0.105 mW m⁻¹ K⁻² (n-type) and 0.053 mW m⁻¹ K⁻² (p-type). The structural similarity between the n- and p-type films enables the creation of p–n homojunctions, highlighting a straightforward route for CrN-based thermoelectric devices.

The integration of light and materials technology is key to the creation of innovative sensing technologies. Sensing of electric and magnetic fields, and temperature with high spatio-temporal resolution is a critical task for the development of the next-generation of nanometer-scale quantum devices. Color centers in diamonds are attractive for potential applications owing to their characteristic quantum states, although they require metallic contacts for the introduction of external microwaves. Here, we build an ultrafast diamond nonlinear photonic sensor to assess the surface electric field; an electro-optic sensor based on nitrogen-vacancy centers in a diamond nanotip breaks the spatial-limit of conventional pump-probe techniques. The 10-fs near-infrared optical pulse modulates the surface electric field of a 2D transition metal dichalcogenide and we monitor the dynamics of the local electric field at nanometer-femtosecond spatio-temporal resolutions. Our nanoscopic technique will provide new horizons to the sensing of advanced nano materials.

TiO2 nanotube arrays (NTA) have attracted much attention among photocatalysts because of their large specific surface area and easy surface transfer of excited electrons, and in recent years, attempts have been made to further improve their properties by forming Z-schemes when they are composited with other photocatalysts. However, as the spacing within and between nanotubes is only a few nanometers, the formation of heterojunctions is extremely difficult when TiO2-NTA is composited with other photocatalytic materials with larger grain sizes. Creating nanoparticle photocatalysts with dimensions smaller than those of the nanotube system is thus required to effectively form heterojunctions. We have constructed an original vacuum chemical vapor deposition (CVD) system with fine temperature control, an attribute that we believe is necessary for the preparation of small nanoparticles. Using this system, it is possible to greatly reduce the polymerization rate of melamine, the precursor of the carbon nitride (g-C3N4) photocatalyst, which offers the benefits of increased reduction power and a metal-free composition. As a result, g-C3N4 small nanoparticles with particle sizes of about 10 nm were successfully prepared, and heterojunctions could be formed even inside TiO2-NTA. The fabricated TiO2-NTA/g-C3N4 composite structure exhibited significantly improved redox power and photocatalytic hydrogen production compared to TiO2-NTA and g-C3N4 alone. In addition, while the hydrogen production rates for TiO2-NTA and g-C3N4 were almost constant, TiO2-NTA/g-C3N4 showed a rapid increase in the hydrogen production rate after a certain period of light irradiation, which was presumably caused by oxygen desorption from g-C3N4. The results of this study provide a method for supporting small nanoparticle materials on nanotube substrates and their importance in improving photocatalytic properties, and will also make a significant contribution not only to the field of photocatalysis but also to other fields requiring small nanoparticle materials.

Polarons can control carrier mobility and can also be used in the design of quantum devices. Although much effort has been directed into investigating the nature of polarons, observation of defect-related polarons is challenging due to electron-defect scattering. Here we explore the polaronic behavior of nitrogen-vacancy (NV) centers in a diamond crystal using an ultrafast pump-probe technique. A 10-fs optical pulse acts as a source of high electric field exceeding the dielectric breakdown threshold, in turn exerting a force on the NV charge distribution and polar optical phonons. The electronic and phononic responses are enhanced by an order of magnitude for a low density of NV centers, which we attribute to a combination of cooperative polaronic effects and scattering by defects. First-principles calculations support the presence of dipolar Fröhlich interaction via non-zero Born effective charges. Our findings provide insights into the physics of color centers in diamonds.

Phase-change materials such as Ge-Sb-Te (GST) exhibiting amorphous and crystalline phases can be used for phase-change random-access memory (PCRAM). GST-based PCRAM has been applied as a storage-class memory; however, its relatively low ON/OFF ratio and the large Joule heating energy required for the RESET process (amorphization) significantly limit the storage density. This study proposes a phase-change nitride, CrN, with a much wider programming window (ON/OFF ratio more than 10⁵) and lower RESET energy (one order of magnitude reduction from GST). High-resolution transmission electron microscopy revealed a phase-change from the low-resistance cubic CrN phase into the highly resistive hexagonal CrN₂ phase induced by the Soret-effect. The proposed phase-change nitride could greatly expand the scope of conventional phase-change chalcogenides and offer a strategy for the next-generation of PCRAM, enabling a large ON/OFF ratio (∼10⁵), low switching energy (∼100 pJ), and fast operation (∼30 ns).

We investigate the ultrafast spin dynamics of the prototypical topological insulator Bi2Se3 using time-resolved Kerr rotation (polarization-change) measurements across near-infrared wavelengths. The Kerr rotation angle ΔθK of Bi2Se3 was found to significantly depend on the photon energy around a resonance transition (∼1.0 eV) of bulk states, as well as the ellipticity of the pump light, in the presence of spin excitation. The observed photon-energy dependence of ΔθK can be well simulated by assuming spin-dependent refractive-index changes in the presence of bandgap renormalization and state-filling effect upon photoexcitation. Our study delivers comprehensive insights into the opto-spintronic properties of bulk Bi2Se3 and the fundamental physical processes underlying polarization changes. These findings are expected to be crucial in developing ultrafast magneto-optical memory devices, which can perform read-and-write operations in the terahertz regime.

Layered transition-metal dichalcogenides (TMDs) are model systems to explore ultrafast many-body interactions and various nonlinear optical phenomena. For the application of TMD-based optoelectronic devices capable of ultrafast response, it is essential to understand how characteristic electron–hole and electron–phonon couplings modify ultrafast electronic and optical properties under photoexcitation. Here, we investigate the sub-picosecond optical responses of layered semiconductor 2H–MoTe2 in the presence of an electron–hole (e–h) plasma and a long-lived coherent phonon. Transient reflectivity measurements depending on photon energy reveal that the optical response for short-time delays (< 1ps) was significantly modified by band-gap renormalization and state filling due to the presence of the e–h plasma. Furthermore, octave, sum, and difference phonon frequencies transiently appeared for the early time delays (< 2ps). The emergent multiple phonon frequencies can be described as higher-order optical modulations due to deformation-potential electron–phonon coupling under resonant photoexcitation conditions. This work provides comprehensive insights into fundamental physics and the application of non-equilibrium quasiparticle generations on TMDs under time-periodic phonon driving forces.

The burgeoning field of optoelectronic devices necessitates a mechanism that gives rise to a large contrast in the electrical and optical properties. A SmTe film with a NaCl-type structure demonstrates significant differences in resistivity (over 10⁵) and band gap (approximately 1.45 eV) between as-deposited and annealed films, even in the absence of a structural transition. The change in the electronic structure and accompanying physical properties is attributed to a rigid-band shift triggered by a valence transition (VT) between Sm²⁺ and Sm³⁺. The stress field within the SmTe film appears closely tied to the mixed valence state of Sm, suggesting that stress is a driving force in this VT. By mixing the valence states, the formation energy of the low-resistive state decreases, providing nonvolatility. Moreover, the valence state of Sm can be regulated through annealing and device-operation processes, such as applying voltage and current pulses. This investigation introduces an approach to developing semiconductor materials for optoelectrical applications.