Graphene nanoribbons (GNRs) are highly versatile materials due to their unique electronic, magnetic, and optical properties, which can be precisely tuned by controlling their width, edge structure, and topology. We report the on-surface synthesis and characterization of a straight N = 15 armchair GNR with periodic [18]annulene nanopores (15-pGNR). It serves as a structural link between two well-established GNRs: the pristine N = 15 armchair GNR without pores (15-AGNR) and the chevron GNR (cGNR). With the addition of the 15-pGNR reported in this study, these three GNRs form a rare experimentally accessible series of ribbons, in which the evolution of electronic properties can be tracked upon progressive carving of a basic 15-AGNR: first, by creating periodic nanopores to form 15-pGNR and then by extending the pore area and producing meandering cGNR. We have designed a molecular precursor for the 15-pGNR and grown the nanoribbons on single-crystal gold substrates by on-surface synthesis in ultra-high vacuum (UHV) conditions. The atomically precise structure of 15-pGNR was confirmed by scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM). The band gap of 15-pGNR was studied by scanning tunneling spectroscopy (STS) and dI/dV mapping, and the occupied electronic levels were investigated by angle-resolved photoemission spectroscopy (ARPES). A theoretical and experimental comparison of 15-pGNRs, 15-AGNRs, and cGNRs demonstrates that the introduction of periodic nanopores into 15-AGNR leads to a more than 2-fold increase in its band gap. In contrast, the band gaps of 15-pGNR and cGNR differ only by about 15%. Such band gap increase can be qualitatively understood to arise from two combined effects, the periodic perforation of the graphene lattice and the confinement effect induced by the GNR width.

MXenes exhibit interesting structural and physicochemical properties, enabling them to be applied in a wide range of fields, including energy storage and electromagnetic shielding. However, conventional etching synthesis routes, such as hydrofluoric acid etching, Lewis-acid etching and molten-salt etching, are limited by long reaction times, high energy consumption and severe chemical hazards. Here we developed a rapid, scalable and reduced environmental impact approach for MXenes synthesis via sequential flash Joule heating-chlorination and fluorination. By precisely tuning the thermodynamic and kinetic parameters, diverse high-quality layered MXenes with excellent electrochemical performance were synthesized through the selective removal of interstitial atoms from nine different MAX phases, each within 30 s. Computational simulations elucidate the selective-etching mechanism, while high-resolution transmission electron microscopy captures atomic-scale structural evolution from MAX phase to MXene. This universal FJH-ClF strategy reduces energy and reagent usage, establishing a safe, cost-effective and sustainable method for next-generation MXene manufacturing. Flash Joule heating converts MAX phases into MXenes within tens of seconds through sequential chlorination and fluorination. By selectively removing A-layer atoms with reduced acid use, energy input and chemical waste, this approach offers a faster and more sustainable route to diverse MXenes.

MXenes exhibit exceptional structural and physicochemical properties, enabling them to be applied in a wide range of fields, including energy storage and electromagnetic shielding. However, conventional etching synthesis routes, such as hydrofluoric acid (HF) etching, Lewis acid etching, and molten-salt etching, are limited by long reaction times, high energy consumption, and severe chemical hazards. Here, we developed a rapid, scalable, and environmentally benign approach for MXene synthesis via sequential flash Joule heating-chlorination and fluorination (FJH-ClF). By precisely tuning the thermodynamic and kinetic parameters, diverse high-quality layered MXenes with excellent electrochemical performance were synthesized through the selective removal of interstitial atoms from nine different MAX phases, each within 30 s. This universal FJH-ClF strategy drastically reduces energy and reagent usage, establishing a safe, cost-effective, and sustainable method for next-generation MXene manufacturing.

We present a synthetic procedure for large Ti 2 CT x MXene monolayers with the majority of flakes having sizes of 10–15 µm and the largest ones reaching 40 µm, which are used for device fabrication and electrical measurements on a single‐flake level. We demonstrate that if exposed to ambient conditions, Ti 2 CT x monolayers oxidize in an aqueous solution or on a substrate on a time scale of hours, but multilayer flakes are more resistant to environmental degradation. The partially oxidized monolayer Ti 2 CT x flakes exhibit low electrical conductivity and electron mobility, as well as the semiconducting‐like temperature dependence of resistance with d R /d T < 0. However, the more degradation‐resistant multilayer flakes show electrical conductivity of about 3700 S cm −1 and electron mobility of about 1.6 cm 2 V −1 s −1 , which are among the highest values reported for MXene materials, as well as the metallic temperature dependence of resistance with d R /d T > 0, which is expected for Ti 2 CT x with mixed surface terminations (T x = ─F, ─OH, = O) based on prior theoretical calculations. These results correlate with the electrical measurements of Ti 2 CT x films, which showed that the thicker films exhibit better environmental stability. The characteristics of multilayer flakes suggest high intrinsic electrical conductivity of Ti 2 CT x and justify its potential for electronic applications.

In this work, the band structure of quasi-one-dimensional HfSe3 was investigated with nanospot angle-resolved photoemission spectroscopy (nano-ARPES) in both the p-and s-polarization geometries and with density functional theory calculations. HfSe3 has a rectangular surface Brillouin zone where the effective hole mass along the chain direction ( P to Y) measured with p-polarization geometry is-0.27 +/- 0.01 me, which is smaller than the effective hole mass along the direction perpendicular to the chains measured with s-polarization geometry ( P to B), -1.17 +/- 0.01 me, in agreement with the calculated hole masses of-0.25 me ( P to Y) and-1.11 me ( in the experimental band structure along P to Y. This band separation may be partly enhanced due to intrinsic spin-orbit coupling effects, as the band structure calculated with density functional theory shows a significant separation of 0.29 eV, for the two bands closest to the top of the valence band along P to Y, only when spin-orbit interactions are included. P to B ), respectively. A band separation of 0.22 +/- 0.01 eV is observed at the top of the valence band

Low-dimensional materials hold great promises for exploring emergent physical phenomena, nanoelectronics, and quantum technologies. Their synthesis often depends on catalytic metal films, from which the synthesized materials must be transferred to insulating substrates to enable device functionality and minimize interfacial interactions during quantum investigations. Conventional transfer methods, such as chemical etching or electrochemical delamination, degrade material quality, limit scalability, or prove incompatible with complex device architectures. Here, a scalable, etch-free transfer technique is presented, employing Field's metal (51% In, 32.5% Bi, and 16.5% Sn by weight) as a low-melting-point mechanical support to gently delaminate low-dimensional materials from metal films without causing damage. Anchoring the metal film during separation prevents tearing and preserves material integrity. As a proof of concept, atomically precise graphene nanoribbons (GNRs) are transferred from Au(111)/mica to dielectric substrates, including silicon dioxide (SiO_2) and single-crystalline lanthanum oxychloride (LaOCl). Comprehensive characterization confirms the preservation of structural and chemical integrity throughout the transfer process. Wafer-scale compatibility and device integration are demonstrated by fabricating GNR-based field-effect transistors (GNRFETs) that exhibit room-temperature switching with on/off current ratios exceeding 10^3. This method provides a scalable and versatile platform for integrating low-dimensional materials into advanced low-dimensional materials-based technologies.

MXenes are a rapidly expanding family of two-dimensional transition metal carbides and nitrides that hold promise for applications in energy storage, catalysis, and electronics. However, conventional solution-based fluoride etching of their layered MAX phase precursors is slow, hazardous, and often incomplete. Here we demonstrate a low-temperature molten salt (LTMS) approach that enables rapid and efficient synthesis of three MXene materials, Mo2Ti2C3Tx, Ta4C3Tx, and Cr2TiC2Tx, from their respective MAX phase precursors using molten NH4HF2 at 180 degrees C. Complete conversion to MXenes is achieved within 40-45 min, representing more than a 100fold reduction in reaction time compared to conventional HF etching of the corresponding MAX phases. The LTMS products exhibit the characteristic accordion-like morphology, high crystallinity, and uniform elemental distribution, with no residual Al detected by elemental analysis methods. Notably, Mo2Ti2C3Tx prepared by the LTMS method shows minimal oxidation relative to HF-etched samples. These results establish the LTMS approach as a rapid, scalable, and safer route for synthesizing high-quality MXenes, especially those derived from MAX phases that are poorly etchable in HF. The LTMS-synthesized MXenes may find many applications, including gas sensing, as demonstrated in this study by the fabrication of Mo2Ti2C3Tx sensors and their detection of water vapor.

This paper demonstrates a significant advance in creating ultra-flat Si(100) surfaces suitable for low thermal budget device fabrication. This is achieved by a two-step pre-flash and protect (PFP) process that locks in an atomically flat surface that survives subsequent device processing steps. The first PFP step is a high-temperature flash in ultra-high vacuum (UHV) that creates an atomically flat surface. The second PFP step is a Piranha solution treatment that preserves the surface with a thin oxide shortly after removal from UHV. This oxide can then be easily removed with buffered oxide etchant (BOE) as needed during subsequent device fabrication. Following BOE, a surface with angstrom-level flatness is recovered, obviating the need for more aggressive thermal or chemical surface flattening processes. With this new process no aggressive chemical cleaning, such as RCA cleaning, is needed and no high-temperature surface cleaning or flattening is required for nanoscale device fabrication. This method offers promising opportunities for device fabrication and other applications that require clean and atomically flat Si(100) surfaces and low thermal budget device processing.

We present a synthetic procedure for large Ti2CTx MXene monolayers with the majority of flakes having sizes of 10-15 mu m and the largest ones reaching 40 mu m, which are used for device fabrication and electrical measurements on a single-flake level. We demonstrate that if exposed to ambient conditions, Ti2CTx monolayers oxidize in an aqueous solution or on a substrate on a time scale of hours, but multilayer flakes are more resistant to environmental degradation. The partially oxidized monolayer Ti2CTx flakes exhibit low electrical conductivity and electron mobility, as well as the semiconducting-like temperature dependence of resistance with dR/dT < 0. However, the more degradation-resistant multilayer flakes show electrical conductivity of about 3700 S cm(-1) and electron mobility of about 1.6 cm(2) V-1 s(-1), which are among the highest values reported for MXene materials, as well as the metallic temperature dependence of resistance with dR/dT > 0, which is expected for Ti2CTx with mixed surface terminations (T-x = -F, -OH, = O) based on prior theoretical calculations. These results correlate with the electrical measurements of Ti2CTx films, which showed that the thicker films exhibit better environmental stability. The characteristics of multilayer flakes suggest high intrinsic electrical conductivity of Ti2CTx and justify its potential for electronic applications.

Recent advances in the exploration of two-dimensional (2D) van der Waals (vdW) ferroelectrics revealed not only a wealth of fundamentally exciting properties but also a strong potential for nanoelectronic applications facilitated by their semiconducting nature and tunable polarization-coupled physical properties. Here, using scanning probe microscopy techniques, we investigate the effects of mechanical stress and optical illumination on the transport behavior of one of the most actively studied 2D ferroelectrics, α-In2Se3. Local I–V measurements reveal a strongly asymmetric polarization-dependent conductivity of α-In2Se3, which can be continuously tuned by the tip-induced mechanical pressure. While the local conductivity increases up to two orders of magnitude for both polarization states, the upward polarization displays a much sharper change. Further enhancement of conductivity by an order of magnitude is observed under optical illumination, resulting from a cumulative modulation of the junction barrier via polarization, strain, and optical excitation. The obtained results make α-In2Se3 a promising material for application in electronic devices with optomechanical functionality.

Two-dimensional (2D) ferroelectric materials like NbOI_2 have garnered significant interest, yet their temporal response and synergetic interaction with light remain underexplored. Previous studies on the polarization of oxide ferroelectrics have relied on time-resolved optical second harmonic generation or ultrafast X-ray scattering. Here, we probe the laser-induced polarization dynamics of 2D NbOI_2 nanocrystals using ultrafast transmission electron diffraction and deflectometry. The deflection of the electron pulses is directly sensitive to the changes in the polarization, while the diffraction signal captures the structural evolution. Excited with a UV laser pulse, the polarization of NbOI_2 is initially suppressed for two picoseconds, then it recovers and overshoots, leading to a transiently enhanced polarization persisting for over 200 ps. This recovery coincides with coherent acoustic phonon generation, triggering a piezoresponse in the NbOI_2 nanocrystals. Our results offer a new method for sensing the ferroelectric order parameter in femtosecond time scales.

In this study, using a set of scanning probe microscopy techniques, we investigate the electronic properties of the domain walls in the layered ferroelectric semiconductor of the transition metal oxide dihalide family, NbOI2. Although the uniaxial ferroelectricity of NbOI2 allows only 180° domain walls, the pristine 2D flakes, where polarization is aligned in-plane, typically exhibit a variety of as-grown domain patterns outlined by the electrically neutral and charged domain walls. The electrically biased probing tip can modify the as-grown domain structures. Piezoresponse force microscopy (PFM) domain imaging along with electrical measurements by conductive atomic force microscopy (C-AFM) reveals a conductive nature of the charged domain walls. It is shown that while modification of the pristine domain structure by the biased probing tip is possible, it is hampered by the high conductivity of the charged domain walls generated during the switching process.

Conjugational defects, also known as solitons, play an important role in the electronic, magnetic, and optical properties of materials. Understanding solitons can uncover intriguing physics and provide insights for designing quantum materials with tailored band structures and electronic properties. Here, we propose a framework to create and control solitons via topological phase transitions in a class of graphene nanoribbons (GNRs) called square-root GNRs, using a transverse electric field. To demonstrate the experimental feasibility, we design and synthesize a representative GNR with a bottom-up approach, with first-principles calculations revealing topological soliton states at the domain wall induced by the electric field. The framework introduced in this Letter can potentially enable direct manipulation of solitons and provide a platform to study them systematically.

TiS3 and ZrS3 are quasi-one-dimensional materials from the family of transition metal trichalcogenides with the general composition MX3, where M is a transition metal and X is a chalcogen. Although isostructural, TiS3 and ZrS3 have very different bandgaps of approximately 1 eV and 2 eV, respectively. Consequently, Zr1-xTixS3 solid solutions are promising for achieving a composition-dependent bandgap that is tunable across a broad spectral range, from visible to infrared, for various electronic and optoelectronic applications. Previous studies demonstrated the synthesis of Zr1-xTixS3 solid solutions from elemental precursors at 800 degrees C, but only in a narrow compositional range of 0 <= x <= 0.33, while higher Ti content led to the formation of a secondary TiS2 phase. In this work, we optimized the synthetic conditions and produced the entire range of Zr1-xTixS3 solid solutions (0 <= x <= 1) via a direct reaction between Zr-Ti alloys and sulfur vapor at a lower temperature of 600 degrees C. All Zr1-xTixS3 compositions crystallized as needle-like structures within the P21/m space group, with lattice parameters increasing with the Zr content, as confirmed by X-ray diffraction analysis. The optical bandgaps of the prepared crystals were within the 1 to 2 eV range and also increased with the Zr content, demonstrating that synthesis of Zr1-xTixS3 solid solutions is a viable route for bandgap engineering in transition metal trichalcogenides.

We demonstrate the possibility of covalently bonded hybrid DNA-graphene materials by synthesizing a model DNA-polycyclic aromatic hydrocarbon (PAH) conjugate comprising a 33mer oligonucleotide containing thymine and uridine modified with bispyrenyl benzene. This DNA-PAH conjugate (T13UPAHT19; T = thymine, UPAH = PAH-modified uridine) has an atomically thin nanographene protrusion extending by only about 1 nm from the single-stranded DNA (ssDNA). We show that DNA-PAH conjugates can be characterized with high resolution by profiling with a nanopore in a monolayer MoS2 membrane. The profiling experiments provided sufficient resolution to distinguish the thymine and PAH-modified regions of T13UPAHT19 and confirm the asymmetry of the PAH attachment relative to the 3' and 5' ends of the ssDNA due to different lengths of the T13 and T19 segments. This work provides the foundation for further exploration of DNA-graphene hybrids, demonstrating an example of their synthesis and the utility of nanopore profiling for their structural characterization with an ∼1 nm resolution.

The implementation of graphene in semiconductor technology requires bandgap tuning, which can be achieved by chemical doping, geometric confinement in nanoribbons or with the insertion of nanopores and non‐hexagonal rings. The bottom‐up on‐surface synthesis approach in ultra‐high vacuum allows for the synthesis of atomically well‐defined graphene or biphenylene nanoribbons and nanoporous graphene (NPG) structures suitable for device applications. Here, a novel 2D carbon allotrope is synthesized in the form of a functional NPG with periodically spaced biphenylene segments. First, 12‐armchair porous graphene nanoribbons (12‐pGNRs) on Au(111) and Au(788) using 7,10‐dibromo‐1,4‐diphenyl‐triphenylene (DBDT) molecular precursor are grown. Low‐temperature scanning tunneling microscopy/spectroscopy (LT‐STM/STS) and non‐contact atomic force microscopy (nc‐AFM) measurements reveal the presence of high‐quality semiconducting 12‐pGNRs. Thermal annealing of densely packed 12‐pGNRs at 550 °C triggers their lateral fusion into diverse NPG structures featuring either graphene‐type or biphenylene‐type junctions. The structural and electronic properties are again characterized by LT‐STM and nc‐AFM in combination with density functional theory (DFT) calculations. DFT shows that while graphene‐type NPGs are direct bandgap semiconductors, biphenylene‐type NPGs manifest a smaller and indirect bandgap. The NPGs are stable upon oxygen and air exposure, and the nanopores feature a noticeable affinity to carbon monoxide, making them appealing systems for chemical sensors.

We demonstrate a family of molecular precursors based on 7,10-dibromo-triphenylenes that can selectively produce different varieties of atomically precise porous graphene nanomaterials through the use of different synthetic environments. Upon Yamamoto polymerization of these molecules in solution, the free rotations of the triphenylene units around the C-C bonds result in the formation of cyclotrimers in high yields. In contrast, in on-surface polymerization of the same molecules on Au(111) these rotations are impeded, and the coupling proceeds toward the formation of long polymer chains. These chains can then be converted to porous graphene nanoribbons (pGNRs) by annealing. Correspondingly, the solution-synthesized cyclotrimers can also be deposited onto Au(111) and converted into porous nanographenes (pNGs) via thermal treatment. Thus, both processes start with the same molecular precursor and end with a porous graphene nanomaterial on Au(111), but the type of product, pNG or pGNR, depends on the specific coupling approach. We also produced extended nanoporous graphenes (NPGs) through the lateral fusion of highly aligned pGNRs on Au(111) that were grown at high coverage. The pNGs can also be synthesized directly in solution by Scholl oxidative cyclodehydrogenation of cyclotrimers. We demonstrate the generality of this approach by synthesizing two varieties of 7,10-dibromo-triphenylenes that selectively produced six nanoporous products with different dimensionalities. The basic 7,10-dibromo-triphenylene monomer is amenable to structural modifications, potentially providing access to many new porous graphene nanomaterials. We show that by constructing different porous structures from the same building blocks, it is possible to tune the energy band gap in a wide range.

Monolayers of Ti3C2Tx MXene and bilayer structures formed by partially overlapping monolayer flakes exhibit opposite sensing responses to a large scope of molecular analytes. When exposed to reducing analytes, monolayer MXene flakes show increased electrical conductivity, i.e., an n-type behavior, while bilayer structures become less conductive, exhibiting a p-type behavior. On the contrary, both monolayers and bilayers show unidirectional sensing responses with increased resistivity when exposed to oxidizing analytes. The sensing responses of Ti3C2Tx monolayers and bilayers are dominated by entirely different mechanisms. The sensing behavior of MXene monolayers is dictated by the charge transfer from adsorbed molecules and the response direction is consistent with the donor/acceptor properties of the analyte and the intrinsic n-type character of Ti3C2Tx. In contrast, the bilayer MXene structures always show the same response regardless of the donor/acceptor character of the analyte, and the resistivity always increases because of the intercalation of molecules between the Ti3C2Tx layers. This study explains the sensing behavior of bulk MXene sensors based on multiflake assemblies, in which this intercalation mechanism results in universal increase in resistance that for many analytes is seemingly inconsistent with the n-type character of the material. By scaling MXene sensors down from multiflake to single-flake level, we disentangled the charge transfer and intercalation effects and unraveled their contributions. In particular, we show that the charge transfer has a much faster kinetics than the intercalation process. Finally, we demonstrate that the layer-dependent gas sensing properties of MXenes can be employed for the design of sensor devices with enhanced molecular recognition.

Graphene nanoribbons (GNRs) of precise size and shape, critical for controlling electronic properties and future device applications, can be realized via precision synthesis on surfaces using rationally designed molecular precursors. Fluorine-bearing precursors have the potential to form GNRs on nonmetallic substrates suitable for device fabrication. Here, we investigate the deposition temperature-mediated growth of a new fluorine-bearing precursor, 6,11-diiodo-1,4-bis(2-fluorophenyl)-2,3-diphenyltriphenylene (C42H24F2I2), into helically shaped polymer intermediates and chevron-type GNRs on Au(111) by combining scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory simulations. The fluorinated precursors do not adsorb on the Au(111) surface at lower temperatures, necessitating an optimum substrate temperature to achieve maximum polymer and GNR lengths. We compare the adsorption behavior with that of pristine chevron precursors and discuss the effects of C-H and C-F bonds. The results elucidate the growth mechanism of GNRs with fluorine-bearing precursors and establish a foundation for future synthesis of GNRs on nonmetallic substrates. Fluorine-bearing precursors offer new opportunities for the on-surface synthesis of graphene nanoribbons, but the growth conditions are critical for achieving optimal results. Here, the authors investigate the deposition temperature-mediated growth of helically shaped polymers and graphene nanoribbons on Au(111) from an internally fluorinated precursor.