The challenge the researchers set out to solve is one of the most stubborn in condensed matter physics and device engineering. A conventional transistor is not made of one material but of several fundamentally different ones working in intimate contact. A semiconductor carries the current, a dielectric insulator controls it through an applied gate voltage, and metallic or semimetallic contacts allow charge to enter and exit. As silicon devices shrink toward their physical scaling limits, atomically thin two-dimensional semiconductors have been heralded as natural successors. Yet for more than two decades, the obstacle to practical 2D electronics has rarely been the performance of the semiconductor itself. It has been the integration problem: how to attach the other components without destroying the delicate atomic sheet they must sit upon.

The surfaces of two-dimensional semiconductors are chemically inert in a troublesome way. They contain few dangling bonds or reactive sites where other materials can anchor, which makes it extraordinarily difficult to deposit a gate dielectric uniformly across them. Conventional dielectric deposition tends to produce patchy, leaky films on such surfaces. Direct metal deposition is equally problematic, since the energetic process can damage the atomically thin semiconductor or create large contact barriers that choke off the flow of charge. Mechanical transfer methods, in which separate 2D materials are peeled, aligned, and stacked one by one, offer an alternative, but they are slow, difficult to scale uniformly over large areas, and poorly suited to the demands of mass production.

The IBS team’s answer was to stop thinking of the crystal as just the semiconductor and start treating it as the raw material for everything. They began with multilayer tungsten diselenide, a two-dimensional semiconductor whose electrical character can be altered by replacing the atoms surrounding the tungsten layer. By preserving the tungsten framework while selectively swapping those surrounding atoms, the researchers changed the electronic identity of individual regions without rebuilding the crystal from scratch. The upper layers were first oxidized into insulating tungsten oxide, and that oxide could then be further converted through vapor-phase reactions into either tungsten disulfide, an n-type semiconductor, or tungsten ditelluride, a semimetal. Regions left untouched retained the original tungsten diselenide semiconductor behavior. In this way, semiconducting, insulating, and semimetallic functions were created directly within different layers and locations of the same parent crystal.

The precision of the method rests on a remarkably elegant physical principle: the tiny gaps between neighboring atomic layers act as natural barriers that confine the chemical reaction. By tuning the reaction temperature and gas pressure, the researchers could halt the conversion after a chosen number of layers, achieving depth control of roughly 0.7 nanometers per tungsten diselenide monolayer. That is the thickness of a single atomic sheet, meaning the process can be steered with a resolution measured in individual atomic planes. This layer-by-layer chalcogen substitution, as the technique is known, effectively turns a crystal-growth problem into a controllable chemical editing problem, one atomic layer at a time.

The most consequential demonstration involved converting the upper layers of tungsten diselenide into tungsten oxide directly above the semiconductor channel. The resulting oxide served as the gate dielectric without any need to deposit a separate high-permittivity material, forming an insulating layer with an effective thickness below one nanometer while maintaining strong insulating performance. This is a significant achievement, because the gate dielectric is where many would-be 2D devices fail. A dielectric that is too thick or too leaky wastes voltage and allows current to slip through when the transistor should be off. A dielectric grown from the channel material itself, atom-for-atom matched to the surface beneath it, sidesteps the interface problems that have plagued conventional deposition.

The electrical performance of the resulting devices came astonishingly close to theoretical ideals. Using this self-grown oxide, the researchers fabricated tungsten diselenide transistors in which a change of only 61.3 millivolts in gate voltage increased the current tenfold. That figure sits very near the theoretical room-temperature limit of about 60 millivolts per decade, the so-called Boltzmann limit that defines the most efficient switching any conventional transistor can achieve. The devices also achieved an on/off current ratio exceeding ten million, meaning the difference between the transistor’s conducting and blocking states spans more than seven orders of magnitude. For low-power electronics, where every wasted millivolt translates into wasted energy, these numbers represent the kind of switching efficiency that device engineers spend careers pursuing.

The contact side of the problem was addressed with equal precision. When the upper layers were converted into tungsten ditelluride, a semimetal, the resulting contacts reduced the barrier for charge injection to about one-third of what was observed with direct platinum contacts. Lower injection barriers mean charge carriers can enter the semiconductor more freely, cutting the parasitic resistance that has historically limited the performance of 2D transistors. The semimetal-to-semiconductor junction created by chemical conversion is also atomically clean by construction, since it forms within the same crystal lattice rather than across the boundary between two separately grown materials.

To show that the method extends beyond individual devices, the team fabricated n-type molybdenum disulfide transistors using the same tungsten oxide dielectric, and then stacked p-type and n-type transistors vertically to construct a complementary van der Waals inverter, a basic building block of digital logic. The successful voltage inversion demonstrated that the technique is not merely a laboratory curiosity for single transistors but a plausible foundation for more highly integrated circuitry. Vertical stacking of complementary devices is particularly attractive for future electronics, because it multiplies functional density without shrinking lateral dimensions further, a direction the industry finds increasingly costly.

The implications reach well beyond one material system. The study combines fundamental materials science based on atomic-scale manipulation with modern semiconductor technology, addressing what Jo Moon-Ho described as the central integration challenge in two-dimensional semiconductor research: not simply finding better semiconductors, but securing equally suitable dielectrics and electrical contacts. By directly converting selected regions of a two-dimensional crystal into different electronic functions, the approach could provide a new route toward ultralow-power, ultrascaled, and vertically integrated semiconductor devices. If the method can be scaled to wafer dimensions with the same layer-by-layer precision demonstrated here, the era in which a complete transistor is grown rather than assembled may be closer than the industry dared expect.

Subject of Research: Layer-by-layer chemical conversion of a single 2D crystal into semiconducting, insulating, and semimetallic regions for integrated transistors

Article Title: Turning single 2D crystal into a complete high-performance transistor

Article References: Turning single 2D crystal into a complete high-performance transistor. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: two-dimensional materials, tungsten diselenide, transistors, gate dielectric, chalcogen substitution, van der Waals heterostructures, semiconductors, tungsten ditelluride, low-power electronics, integrated circuits, Institute for Basic Science, atomic-scale fabrication

The core idea is to shatter graphite’s usual exfoliation statistics by hitting it with a thermal shock at cryogenic temperatures. Conventional mechanical exfoliation, the Scotch-tape method that won the Nobel Prize in 2010, tends to tear crystals along unpredictable paths and frequently produces flakes riddled with stacking faults, regions where the layer registry flips from rhombohedral to Bernal order. Those faults are not cosmetic blemishes. In rhombohedral multilayer graphene, the flat bands that enable correlated physics live only in regions of pristine ABC stacking, so a single fault can carve a promising flake into a patchwork of useless domains. By cooling the crystal and subjecting it to a rapid shock, the Santa Barbara team coaxes the graphite to cleave in ways that preserve and expose large, uniformly rhombohedral terraces.

The results are striking by any measure in the field. The method yields rhombohedral graphene flakes with uniform areas exceeding 1,300 square micrometers, vast territories on the scale of two-dimensional devices, and device fabrication yields of roughly ninety percent. For comparison, researchers working with naturally occurring rhombohedral graphite have typically contended with usable rhombohedral domains orders of magnitude smaller, often just a few square micrometers. The team combined the shock exfoliation with a low-pressure van der Waals assembly technique, in which flakes are picked up using a suspended polymer membrane stretched over a microcavity, a process fast enough that the entire pickup takes roughly half a second to a second. That speed matters, because slow assembly gives strained multilayers time to relax into the energetically favored Bernal stacking, destroying the very order the researchers worked so hard to obtain.

Proving that such large flakes are genuinely uniform required imaging tools capable of seeing stacking order at fine resolution. The team turned to scanning nanoSQUID-on-tip microscopy, a technique in which a superconducting quantum interference device is fabricated at the apex of a hollow quartz pipette and rastered across the sample with nanoscale precision. Sensitive to magnetic fields from individual electron spins, the nanoSQUID revealed uniform spin magnetism across the entire central ten-by-ten-micrometer region of the finished devices. Infrared and scanning microwave impedance microscopy provided complementary maps, distinguishing bright non-Bernal regions from the darker rhombohedral background and confirming that the exfoliated flakes retained their stacking order over unprecedented areas.

Uniformity is only half the story; the other half is cleanliness, and here the devices set a new benchmark. Using transverse magnetic focusing, a technique in which a perpendicular magnetic field bends electron trajectories into curved arcs that land on a voltage probe, the researchers measured a disorder-limited mean free path exceeding 200 micrometers at low temperatures. That means an electron can travel the length of two hundred thousand atoms, bouncing across the device hundreds of times, before scattering off an impurity. The signature is unmistakable in the data: at low temperatures, the devices show negative bend resistance over large portions of the phase diagram, a hallmark of ballistic electrons that arc directly into the wrong voltage contact rather than diffusing randomly. The resistance only turns positive once the mean free path shrinks below the device size as temperature rises.

With transport this clean, the electrons stop behaving like a gas of independent particles and start behaving like a fluid. At intermediate temperatures, where electron-electron collisions become frequent but disorder remains weak, the electrons in rhombohedral multilayer graphene flow collectively, and the team could watch this hydrodynamic regime directly. Using the nanoSQUID to image current flow in real space, they observed a size-driven crossover from Poiseuille flow, the smooth parabolic profile familiar from water moving through a pipe, to a porous flow regime in which the current squeezes through the device in a more fragmented pattern. Numerical simulations of current flow, varying the momentum-relaxing and electron-electron scattering lengths independently, matched the measured current distributions and pinned down an electron-electron scattering length of roughly 50 nanometers, alongside momentum-relaxing lengths ranging from tens to hundreds of micrometers depending on carrier density.

The physics on display here is the payoff of two decades of work on van der Waals materials. Rhombohedral multilayer graphene’s flat surface bands amplify electron-electron interactions to the point where the material spontaneously develops magnetic phases, superconducting pockets and quantized anomalous Hall states, all switchable with electrostatic gates. Recent years have delivered a torrent of discoveries: superconductivity in rhombohedral trilayer graphene, orbital multiferroicity in pentalayer graphene, fractional quantum anomalous Hall effects in multilayer graphene superlattices, and signatures of chiral superconductivity. But each of those experiments was constrained by the same materials problem, with device sizes limited by the patchy rhombohedral domains available from natural graphite. By making large, uniform rhombohedral areas routine, cryogenic shock exfoliation transforms these fragile quantum phases from laboratory curiosities into candidates for real two-dimensional nanoelectronics.

The method also carries practical lessons for how the field builds devices. The low-pressure assembly technique, using a suspended membrane over a soft microcavity, achieved transfer yields around ninety percent for incorporating rhombohedral multilayer graphene without relaxation to Bernal stacking, and it worked equally well for the subsequent stacking of dielectric layers and graphite gates in a single sequence. The team even tracked crystallographic orientation through the exfoliation process, aligning flakes to the underlying hexagonal boron nitride encapsulant, which is known to further enhance electronic quality. In one hexalayer device aligned to hBN, the researchers observed superconducting pockets and symmetry-broken phases matching those reported in the literature, along with a transition between Chern numbers two and one across a moiré-related resistance peak, demonstrating that the new fabrication route reproduces and extends the correlated phenomenology of earlier, smaller devices.

What makes the advance genuinely viral-worthy is the combination of brute-force simplicity and precision outcomes. Cryogenic shock exfoliation is, at heart, a clever way of breaking crystals, yet it delivers electron mean free paths that rival the best devices ever made in any two-dimensional material, while scaling to areas that make mesoscopic physics experiments practical. The observation of hydrodynamic electron flow, complete with a Poiseuille-to-porous crossover driven simply by device geometry, adds a beautiful confirmation of theoretical predictions about how electrons with polygonal Fermi surfaces behave as viscous fluids. It also hints at exotic possibilities such as hydrodynamic flow regimes that evade the fundamental Landauer-Sharvin contact resistance, phenomena that only become accessible when disorder is pushed this far down.

The work, supported by the Department of Energy, the National Science Foundation and the Gordon and Betty Moore Foundation, with high-quality hexagonal boron nitride crystals supplied by Takashi Taniguchi and Kenji Watanabe in Japan, marks a turning point for correlated graphene research. The experimental data and simulation code are openly available through Zenodo, and the authors have released videos of both the shock exfoliation process and the rapid low-pressure pickup, inviting labs worldwide to adopt the recipe. If the technique proves as reproducible outside Santa Barbara as it is within, the coming years could see rhombohedral graphene devices grow from micrometer-scale specks into centimeter-scale platforms, where superconducting, magnetic and topological phases interweave in designer electronic circuits. The bottleneck that held back the most exciting phase of graphene physics for half a decade may finally be behind it.

Subject of Research: Scalable cryogenic shock exfoliation for large-area, ultrahigh-mobility rhombohedral multilayer graphene devices

Article Title: Cryogenic shock exfoliation for ultrahigh mobility rhombohedral graphite nanoelectronics

Article References: Holleis, L., Choi, Y., Zhang, C., Farrell, J. H., Bargas, G., Hsu, A., Chen, Z., Sackin, I., Zhou, W., Guo, Y., Charpentier, T., Jiang, Y., Foutty, B. A., Keough, A., Huber, M. E., Taniguchi, T., Watanabe, K., Lucas, A., & Young, A. F. (2026). Cryogenic shock exfoliation for ultrahigh mobility rhombohedral graphite nanoelectronics. Nature Nanotechnology. https://doi.org/10.1038/s41565-026-02283-1

Image Credits: AI Generated

DOI: 10.1038/s41565-026-02283-1

Keywords: rhombohedral graphene, cryogenic shock exfoliation, flat bands, electron hydrodynamics, superconductivity, quantum anomalous Hall effect, nanoSQUID imaging, transverse magnetic focusing, van der Waals heterostructures, mean free path, two-dimensional materials, correlated electrons

Hyperbolic phonon polaritons, often abbreviated HPhPs, are among the most remarkable optical excitations known to materials science. They arise when infrared electromagnetic fields couple strongly to the natural lattice vibrations, or optical phonons, of certain crystals. The resulting hybrid waves behave in ways that ordinary light cannot: they can squeeze electromagnetic energy into volumes thousands of times smaller than the wavelength of the light itself, guiding it coherently along a material’s surface or through its bulk. This extreme confinement makes them attractive candidates for nanoscale light manipulation, high-sensitivity molecular spectroscopy, and the miniaturization of optical circuits to dimensions that rival modern microelectronics.

Hexagonal boron nitride, or hBN, a layered van der Waals crystal famous for its role as a protective encapsulant in two-dimensional electronics, is a particularly powerful host for these waves. Within specific infrared frequency bands, hBN behaves as a hyperbolic medium, meaning that its dielectric response is positive along one crystal axis and negative along another. In this regime, the polariton wavefronts take on hyperbolic shapes, and the waves can propagate with wavelengths dramatically shorter than free-space light, carrying electromagnetic energy along the crystal with remarkable confinement. But harnessing such waves in practical devices requires more than passive propagation; engineers need the ability to switch, steer, and reshape them on demand, and to do so at speeds fast enough to matter for information processing.

That is precisely where the experimental challenge has always lain. Hyperbolic phonon polaritons are strongly dispersive, which means their wavelength changes substantially depending on the frequency of the light that excites them. Femtosecond laser pulses, the only tools capable of resolving dynamics on the relevant timescales, are inherently broadband: a single pulse contains a wide spread of frequencies. When such a pulse strikes a sample, it excites polaritons of many different wavelengths simultaneously. In a real-space image, the interference fringes produced by all these waves overlap and average together, washing out the very propagation patterns scientists want to see. The obvious remedy, narrowing the spectrum of the pulse before it reaches the sample, comes at a cruel price: by the fundamental rules of Fourier optics, a narrower spectrum demands a longer pulse, which destroys the femtosecond time resolution that motivated the experiment in the first place.

A research team led by Kazuki Kamada of the Institute for Molecular Science (IMS) and Osaka Metropolitan University, together with Jun Nishida, Assistant Professor at IMS, and Takashi Kumagai, Associate Professor at IMS, has now found an elegant way around this trade-off. Their solution was to leave the excitation pulse untouched and instead perform the frequency selection after the fact, during detection. By inserting a diffraction grating into the detection path of an ultrafast infrared near-field optical microscope, the team spectrally separated the broadband infrared light scattered from the sharp metallic tip of an atomic force microscope before it reached the detector. Because each frequency component could then be imaged independently, the overlapping interference patterns were cleanly disentangled without ever lengthening the excitation pulse.

The resulting instrument is a genuine tour de force of measurement engineering. It combines nanoscale spatial resolution, inherited from the near-field interaction between the microscope tip and the sample, with a temporal resolution of approximately 150 femtoseconds and a spectral resolution of roughly 10 inverse centimeters. In practical terms, the technique allows researchers to select a single narrow slice of the infrared spectrum, watch the polariton associated with that frequency propagate across the sample in a snapshot lasting only a few hundred atomic vibrations of time, and then repeat the measurement at successive pump-probe delays to assemble a full movie of the wave’s evolution. No previous near-field imaging scheme had brought all three of these capabilities together for strongly dispersive polaritons.

With the new microscope in hand, the team turned it on van der Waals heterostructures composed of tungsten disulfide (WS2) and hexagonal boron nitride. The experimental geometry was a classic pump-probe arrangement: a pulse of visible light generated photoexcited charge carriers, mobile electrons and holes, within the WS2 layer, while a time-delayed infrared near-field pulse probed how those carriers affected the hyperbolic phonon polaritons propagating in the adjacent hBN. Because the two atomically thin materials lie in intimate contact across the van der Waals interface, any change in the optical properties of one should, in principle, be felt by the waves traveling in the other. The question was whether the effect would be fast enough and strong enough to see directly.

The answer was a clear yes, and the details proved richer than a simple on-off switching picture. In heterostructures containing a thin WS2 layer, the researchers observed a transient modulation dominated by a change in the electric-field amplitude of the polaritons, as if the photoexcited carriers were momentarily absorbing or damping the wave. More strikingly, in structures containing a relatively thick WS2 layer, the team resolved a change in the polariton’s wavelength itself. This is a qualitatively different and more powerful form of control: it demonstrates that optical excitation can modify not merely how strongly the wave is present but how it propagates, effectively rewriting the dispersion relation of the polariton on the fly. Electromagnetic simulations confirmed that both observations could be explained by a transient, carrier-induced change in the dielectric response of WS2, which then modifies the HPhPs propagating in the neighboring hBN through the shared interface.

The physical mechanism is worth appreciating in detail. When visible light excites electron-hole pairs in WS2, the free carriers alter the material’s dielectric function, particularly at infrared frequencies relevant to the hBN polaritons. Since the hyperbolic phonon polariton is a hybrid mode whose electromagnetic field extends across both layers of the heterostructure, this sudden change in the WS2 dielectric environment is imprinted directly onto the wave. The carriers then relax and recombine on their own timescale, and the polariton returns to its unperturbed state. The entire cycle, from optical switching to recovery, unfolds within a few hundred femtoseconds, which is exactly the regime the new imaging technique was designed to capture. In effect, the researchers have demonstrated an all-optical gate for nanoscale light, in which one beam of light controls another through the mediation of photoexcited carriers and a van der Waals interface.

The implications extend well beyond this particular material system. The study, published online in Nano Letters on July 27, 2026, establishes a general platform for observing and controlling nanoscale light propagation on femtosecond timescales, and the same imaging strategy should apply to other strongly dispersive polaritons, including surface plasmon polaritons in metals and phonon polaritons in other hyperbolic crystals. As the authors note, the results open new opportunities for ultrafast nanophotonics, a field in which devices might one day modulate, route, and process signals encoded in confined light fields at speeds approaching the fundamental limits set by the optical oscillations themselves. The work was supported by JSPS KAKENHI, the JST FOREST and CREST programs, the MEXT World Premier International Research Center Initiative, and several other Japanese research foundations, and it involved collaborators from the National Institute for Materials Science, the Fritz Haber Institute in Berlin, and Osaka Metropolitan University. For now, the immediate contribution is conceptual and methodological: a way to see, frame by femtosecond frame, how light confined to the nanoscale can be commanded to change its shape, its speed, and its very wavelength, all at the whim of a flash of visible light.

Subject of Research: Ultrafast nano-imaging and optical control of hyperbolic phonon polaritons in van der Waals heterostructures

Article Title: Capturing fleeting changes in “nanoscale light”—femtosecond nano-imaging reveals ultrafast optical control of phonon polariton

Article References: Capturing fleeting changes in “nanoscale light”—femtosecond nano-imaging reveals ultrafast optical control of phonon polariton. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: phonon polaritons, hexagonal boron nitride, tungsten disulfide, femtosecond imaging, near-field microscopy, nanophotonics, van der Waals heterostructures, ultrafast optics, pump-probe spectroscopy, dielectric response, Nano Letters, light-matter coupling

The choice of ingredients is deliberate. Multi-walled carbon nanotubes are concentric cylinders of graphene that combine excellent electrical conductivity, mechanical robustness, thermal stability, and a broadband optical response. Their one-dimensional electronic structure gives rise to van Hove singularities in the density of states, sharp peaks where optical absorption is dramatically enhanced, and their optical behavior depends strongly on whether the electric field of incoming light is polarized along the tube axis or around its circumference. Gallium selenide, meanwhile, is a layered III-VI semiconductor with a quasi-direct bandgap of roughly 2.0 electronvolts, strong excitonic effects, and pronounced optical anisotropy arising from its van der Waals crystal structure. Combining the two, the researchers reasoned, could produce light-matter interactions unavailable in either material alone.

Synthesis relied on pulsed laser ablation in liquid, a technique prized for producing high-purity nanomaterials without surfactants, reducing agents, or high-temperature chemical treatments that can contaminate delicate interfaces. The team used an Nd:YAG laser operating at 1064 nanometers, with pulses roughly 10 nanoseconds long carrying about 135 millijoules of energy at a 10-hertz repetition rate, reaching a power density of approximately 12 megawatts per square centimeter. The beam was focused through a lens onto targets immersed in distilled water within a quartz cuvette. When the intense pulses strike the target, rapid energy deposition evaporates and ejects atoms, clusters, and plasma species, which then cool and condense into nanoscale particles and hybrid structures. The surrounding liquid confines the expanding plasma plume far more than vacuum or gas would, generating extreme local temperatures and pressures that shape nucleation and growth dynamics.

The experimental procedure unfolded in two stages. First, multi-walled carbon nanotubes from Sigma-Aldrich, with carbon purity above 98 percent, outer diameters of roughly 6 to 13 nanometers, and lengths of 2.5 to 20 micrometers, were ablated in distilled water for about ten minutes to form a colloidal dispersion. Then freshly cleaved plates of GaSe single crystals, grown by the Bridgman technique and verified by X-ray diffraction and Raman spectroscopy, were immersed in that colloid and subjected to identical laser irradiation. This second pass promoted surface functionalization and laser-induced nonequilibrium interactions that welded the two materials into a hybrid interface, all without a single chemical additive.

Structural characterization confirmed the marriage. X-ray diffraction of pristine nanotubes showed characteristic features at 2-theta angles of 15.09, 17.81, and 26.91 degrees, the last corresponding to the graphitic (002) reflection of ordered sp2-bonded carbon layers. After composite formation, the diffraction profile broadened considerably and lost intensity, signaling reduced long-range crystalline order, a common signature of nanocomposites synthesized under pulsed laser irradiation. Debye-Scherrer analysis of the peak broadening yielded coherent crystallite-domain sizes of roughly 7 to 15 nanometers. Energy-dispersive X-ray spectroscopy detected carbon alongside gallium and selenium in nearly equal atomic percentages, 7.60 and 7.99 respectively, indicating the GaSe phase retained its approximately 1:1 stoichiometry through the laser process. Scanning electron microscopy revealed an entangled network of nanotubes decorated with GaSe-containing clusters, and elemental maps showed substantial spatial overlap of gallium and selenium throughout the carbon network.

Transmission electron microscopy pushed the resolution further and delivered the study’s most visually compelling evidence. High-resolution imaging distinguished the multilayer graphitic walls of a representative nanotube, with an outer diameter near 32 nanometers, an inner diameter near 16 nanometers, and about 7 to 9 walls separated by roughly 0.34 nanometers, the classic spacing of graphitic carbon. In the GaSe-containing regions, well-resolved lattice fringes with interplanar spacings of approximately 0.33 to 0.334 nanometers confirmed crystallinity, and critically, the images captured direct nanoscale contact between crystalline GaSe domains and the graphitic nanotube walls, proof that a genuine hybrid interface had formed. A selected-area electron diffraction pattern with rings and discrete reflections rounded out the structural case.

The optical consequences were striking. Ultraviolet-visible absorption and Tauc analysis showed the optical absorption edge shifting dramatically after hybridization: the estimated optical gap fell from about 2.15 electronvolts for pristine nanotubes to 1.39 electronvolts for the composite, a redshift of roughly 0.76 electronvolts. Photoluminescence measurements, excited by the second harmonic of an Nd:YAG laser at 2.34 electronvolts, showed pristine GaSe emitting near 576 nanometers and pristine nanotubes near 887 nanometers. The composite, however, produced a broadband emission spanning approximately 576 to 887 nanometers, covering both parent emission regions in a single continuous profile, with an approximately fivefold enhancement in relative steady-state intensity. The authors are careful to note this figure reflects relative intensity under their conditions, not an absolute quantum-yield increase, and that steady-state spectra alone cannot pin down the exact recombination mechanism.

To explain what was happening at the atomic scale, the team turned to DFT calculations using the QuantumATK package with the local density approximation and Perdew-Zunger parameterization. Because the experimental material contains nanotubes of varying diameters, wall numbers, and orientations, the theorists built an idealized model: an armchair double-walled nanotube, DWCNT(5,5)@(10,10), laid with its axis parallel to a crystalline GaSe surface. After structural optimization, the minimum interfacial distances settled at about 2.31 to 2.53 angstroms. The calculations tracked the absorption coefficient and reflectivity along three polarization directions, xx, yy, and zz, revealing how quantum confinement and electronic symmetry govern direction-dependent light absorption.

The simulated spectra told a coherent story. Pristine GaSe displayed strong anisotropy, with a sharp zz-polarized absorption peak near 1.3 electronvolts exceeding 80,000 per centimeter, a yy peak near 2.5 electronvolts, and a nearly negligible xx response, a direct consequence of its layered bonding. The pristine double-walled nanotube showed the opposite pattern: strong transverse absorption between 1.5 and 4.5 electronvolts from interband transitions between van Hove singularities, with weaker axial response, and a circumferential reflectivity peak of about 0.38 at 3.5 electronvolts. In the hybrid, everything changed. Absorption was strongly suppressed, broadened, and redistributed toward lower energies, with the xx component becoming dominant and new low-energy features appearing below 1 electronvolt. Reflectivity flipped as well, with the xx direction reaching about 0.20 near 0.3 electronvolts while the other directions fell below 0.02, suggesting the hybrid is highly reflective to radially polarized light, a property absent in either constituent.

Electronic-structure analyses filled in the mechanism. The total density of states of the hybrid showed finite electronic states at and around the Fermi level, indicating metallic or strongly conducting character rather than a conventional semiconducting gap, with a degree of spin asymmetry between channels. Electron difference density maps revealed spatially nonuniform redistribution of charge concentrated at the interface, and Mulliken population analysis quantified it: approximately 0.17 elementary charges migrate from the nanotube toward the GaSe component upon interface formation, with gallium atoms accumulating electrons and selenium atoms donating them in a strongly atom-dependent pattern. Together, these results establish that interfacial electronic coupling and charge redistribution, not merely physical mixing, drive the modified optical response. The authors emphasize that the polarization-dependent anisotropy remains a theoretical prediction awaiting direct experimental test, since the colloidal measurements average over randomly oriented nanotubes. Future work with aligned nanotube arrays, single-nanotube spectroscopy, time-resolved photoluminescence, and polarization-resolved measurements on oriented films could confirm the predicted directional behavior. If validated, the combination of broadband emission, a tunable absorption edge, and engineered polarization selectivity positions these laser-forged hybrids as intriguing candidates for polarization-sensitive photodetectors, optical filters, directional mirrors, and infrared photonic devices.

Subject of Research: Synthesis and anisotropic optical properties of multi-walled carbon nanotube/gallium selenide nanocomposites

Article Title: Laser-ablation synthesis and DFT investigation of anisotropic optical properties in multi-walled carbon nanotube/GaSe nanocomposites

Article References: Aliyeva, A., Jafarova, V. N., Hasanova, K. A., Sh., A. S., Guliyeva, A. A., Salmanov, V., Asad, J. H., Mamedov, R., & AlShaikh Mohammad, N. F. (2026). Laser-ablation synthesis and DFT investigation of anisotropic optical properties in multi-walled carbon nanotube/GaSe nanocomposites. Results in Optics, 25, Article 101181. https://doi.org/10.1016/j.rio.2026.101181

Image Credits: AI Generated

DOI: 10.1016/j.rio.2026.101181

Keywords: carbon nanotubes, gallium selenide, nanocomposites, laser ablation in liquids, density functional theory, optical anisotropy, photoluminescence, van der Waals heterostructures, polarization-sensitive optoelectronics, charge transfer, photodetectors, band gap engineering

The standard remedy is to pair each memory element with a selector, a two-terminal device that conducts strongly only when a sufficient voltage is applied across it and remains nearly insulating otherwise. The quality of a selector is judged by its nonlinearity, the ratio between the current it passes in the on-state and the current it leaks in the off-state at the same operating conditions. High nonlinearity suppresses sneak paths, but achieving it without sacrificing drive current, switching speed or endurance has proved remarkably difficult. Threshold switches, ovonic devices, mixed-ionic-electronic-conduction devices and transistor-based access elements each bring trade-offs in variability, fabrication complexity or compatibility with the memory stack above them.

Against this backdrop, a News and Views commentary by Yuting Sun and Min Zhu of Shanghai Jiao Tong University, published in Nature Electronics, highlights recent progress that may reshape the selector landscape. The commentary discusses work demonstrating tunnel junction selectors built from five vertically stacked layers of two-dimensional materials, a device architecture that delivers high nonlinearity, robust endurance, fast switching speeds and low device-to-device variability. The authors suggest that such devices could potentially function as a common access element across multiple memory technologies, a prospect that would simplify the design of hybrid memory systems considerably.

Two-dimensional materials, the family of crystals that can be isolated in layers only a few atoms thick, have long been touted for electronic applications because their surfaces are free of the dangling bonds that plague conventional semiconductors at scaled dimensions. In a vertical stack, dissimilar two-dimensional layers can be assembled atom by atom, allowing engineers to engineer the band alignment across the junction with a precision that bulk heterostructures cannot easily match. A tunnel junction exploits this control directly: the barrier formed between the layers governs quantum-mechanical tunnelling, so a modest change in the voltage across the stack can swing the current through many orders of magnitude. That voltage-dependent tunnelling is precisely the behaviour a selector needs.

The five-layer architecture is central to the performance reported. By sandwiching tunnelling barriers between conducting and semiconducting two-dimensional sheets, the device creates an energy landscape in which current transport is strongly suppressed at low bias but rises steeply once the applied voltage exceeds a threshold. The steepness of this transition determines the nonlinearity, and the atomically thin layers mean the effective barrier width is set with atomic precision rather than by the statistical roughness that afflicts amorphous oxide films. Because the layers are crystalline and their interfaces are abrupt, device-to-device variation, historically the Achilles heel of selector technology, is kept low. Variability matters enormously in practice: a crossbar array may contain millions or billions of cells, and the array can only be read reliably if the statistical spread of selector behaviour remains narrow across the entire population.

Endurance and speed are the other pillars of the achievement. A selector in a working memory must withstand billions of switching cycles without degradation, since it is cycled every time the adjacent memory cell is written or read. It must also switch quickly enough not to become the bottleneck of the memory operation. The commentary notes that the two-dimensional tunnel junction selectors exhibit robust endurance and fast switching speeds, indicating that the physical mechanism underlying the switching, governed by electronic tunnelling rather than by the movement of ions or the formation and dissolution of conductive filaments, is inherently less prone to the wear mechanisms that limit other selector types. Filamentary devices, for example, rely on stochastic ionic processes that both slow the switch and gradually rearrange the material, whereas a tunnelling barrier responds on the timescale of electronic transport itself.

The significance of the work extends beyond any single memory technology. Resistive random-access memory, phase-change memory, conductive-bridge memory and ferroelectric memory all require selectors, but each has historically demanded a selector tailored to its particular voltage window, current range and integration scheme. A universal selector, one device that can serve as the access element across several memory platforms, would allow foundries to standardise a single back-end-of-line module and would give memory architects freedom to mix and match storage elements within a system without redesigning the array infrastructure. Sun and Zhu point to this cross-technology compatibility as the potentially transformative aspect of the two-dimensional tunnel junction approach, framing it as a common access element for multiple memory technologies.

The commentary situates the new devices within a broader arc of two-dimensional materials research. Since the isolation of graphene opened the field, researchers have assembled van der Waals heterostructures in which layers are stacked without the lattice-matching constraints of epitaxial growth. Reviews of the field have emphasised that this assembly freedom, combined with the diversity of available two-dimensional crystals, from semiconducting transition-metal dichalcogenides to insulating hexagonal boron nitride, enables device concepts that are difficult or impossible in conventional material systems. The tunnel junction selector is a compelling example of that promise translated into a device metric, nonlinearity, that the memory industry can measure and value directly.

Challenges remain before such selectors can leave the laboratory. Integrating two-dimensional layers into large-area manufacturing remains an unsolved problem at scale, since the growth and transfer of atomically thin films over full wafers must achieve uniformity comparable to that of deposited oxides. Contact resistance between the two-dimensional stack and conventional metal interconnects, thermal stability during back-end processing, and the yield of defect-free multilayer stacks are all active areas of investigation. The commentary’s authors, who are themselves affiliated with a school of integrated circuits and a national key laboratory of micro and nano manufacture technology, are well placed to appreciate the distance between a demonstrated device and a manufacturable module.

Even so, the demonstration marks a notable convergence of two research communities. Memory engineers have spent two decades searching for a selector that combines the nonlinearity of a diode with the bidirectional operation and simple two-terminal structure that crossbar arrays require, while two-dimensional materials researchers have sought applications in which atomically thin layers provide a decisive advantage rather than an incremental one. The tunnel junction selector, with its high nonlinearity, robust endurance, fast switching and low variability, offers evidence that this convergence can produce devices that matter to the industry. If the approach survives the rigours of large-area integration, the humble selector, long an afterthought in memory design, may become the element that finally unlocks the full density promise of three-dimensional crossbar memories.

Subject of Research: Two-dimensional-material tunnel junction selectors for crossbar memory arrays

Article Title: Universal selectors from two-dimensional materials

Article References: Sun, Y., & Zhu, M. (2026). Universal selectors from two-dimensional materials. Nature Electronics. https://doi.org/10.1038/s41928-026-01723-z

Image Credits: AI Generated

DOI: 10.1038/s41928-026-01723-z

Keywords: two-dimensional materials, tunnel junction, selector device, crossbar arrays, memory technology, nonlinearity, endurance, switching speed, device variability, van der Waals heterostructures, Nature Electronics, resistive memory

The distinction between stacking geometries sounds arcane, but it is the entire story. In the most common bilayer arrangement, known as 2H stacking, the top sulfur plane sits rotated by 180 degrees relative to the bottom layer, which restores inversion symmetry and drives the valence band maximum away from the valleys where the conduction band minimum resides. The 3R, or rhombohedral, arrangement keeps the layers aligned in the same orientation and breaks that symmetry, enabling ferroelectric behaviour through interlayer sliding. The 1H geometry, the one the Taiwanese-led team targeted, places the second layer in perfectly commensurate vertical alignment with the first in a specific relative orientation that keeps the band extrema aligned in momentum space. Achieving this configuration in a scalable, crystal-growth setting rather than by manually transferring and stacking exfoliated flakes is what makes the new work remarkable.

The researchers accomplished it with a modified two-step chemical vapour deposition scheme in which growth temperature is modulated between the nucleation of the first layer and the growth of the second. Their Extended Data analysis traces the mechanism to the diffusion behaviour of molybdenum adatoms landing on the completed monolayer. Depending on temperature, arriving adatoms follow either edge diffusion pathways, hopping along equivalent crystallographic directions to attach at the flake boundary, or surface diffusion pathways, migrating across the terrace of the underlying monolayer. An Arrhenius analysis of the hopping-rate ratio shows how the balance between these two regimes selects the resulting stacking polytype: 1H, 3R or 2H. Under the conditions favouring 1H growth, molybdenum adatoms form ribbons along preferred crystallographic directions that branch and eventually coalesce into seamless bilayer triangles, a process confirmed by bright-field and dark-field transmission electron microscopy showing fully stitched, unidirectional bilayer nanoribbons.

Structural verification was exhaustive. High-angle annular dark-field scanning transmission electron microscopy, cross-sectional STEM prepared by focused ion beam milling, and selected-area diffraction patterns were compared against simulated diffraction patterns for each candidate polytype. The examined regions consistently showed the 1H stacking arrangement with atomic-scale uniformity, and interlayer spacings measured at ten separate locations in the cross-sectional images matched the expected commensurate geometry. Second-harmonic generation microscopy and atomic force microscopy provided additional, wafer-scale confirmation that the resulting flakes were structurally distinct from the 3R and 2H bilayers grown under the alternative temperature conditions of the same process.

With the structure nailed down, the team turned to the electronic structure. Angle-resolved photoemission spectroscopy performed with a photoelectron momentum microscope at the Taiwan Photon Source probed the valence bands directly, and the experimental spectra were compared with density functional theory calculations. In conventional bilayer MoS2, the valence band maximum at the K valley sits measurably higher in energy than at the Gamma point only in monolayers; in bilayers the Gamma point wins and the gap becomes indirect. For the 1H bilayers, the measured energy separation between the valence band at K and at Gamma remained consistent with a direct gap, and the extracted value for a monolayer reference in the same experiment, 146 millielectronvolts, agreed with published literature, providing an internal calibration for the measurement.

The optical consequences followed immediately. Photoluminescence mapping across the 1H bilayer flakes revealed intensified excitonic emission, and, crucially, the spectra lacked the low-energy indirect-gap emission features that usually betray bilayer character. In ordinary 2H bilayers, most electron-hole recombination funnels through the indirect transition and emits weakly at longer wavelengths; the 1H bilayers showed no such signature, indicating that radiative recombination proceeds through the direct, momentum-conserving channel. In other words, the extra layer adds carrier capacity and mobility without exacting the usual optical penalty.

Perhaps the most surprising result concerns valley physics. Monolayer MoS2 owes its valleytronics credentials to broken inversion symmetry, which couples the spin and valley degrees of freedom and allows circularly polarized light to selectively populate one of two inequivalent valleys. Bilayers with 2H stacking restore inversion symmetry and destroy this valley contrast. The 1H bilayers, however, not only preserved valley-selective circular polarization but actually exhibited stronger valley polarization than monolayers under both resonant and nonresonant excitation. The authors attribute this enhancement primarily to suppressed intervalley scattering in the top layer of the stack, meaning that once excitons are injected into a valley they are less likely to relax into the opposite valley before recombining. Robust polarization under off-resonant excitation is particularly valuable for practical devices, since it relaxes the demanding requirement for exactly resonant optical pumping.

The implications reach across several device families. Bilayer transition metal dichalcogenides are already attractive for next-generation transistors because they offer higher carrier mobility than monolayers and, in suitably stacked forms, electrically switchable polarity. They also underpin emerging sliding ferroelectric devices, in which an interlayer displacement toggles a polarization state. The demonstration that a specific, growth-accessible stacking order can restore a direct band gap means engineers may no longer have to choose between the electronic merits of two layers and the optical merits of one. Light-emitting transistors, valleytronic logic and integrated optoelectronic circuits built on MoS2 all become more plausible when the same material platform supports charge transport, light emission and valley polarization simultaneously.

There are also broader lessons for the rapidly growing field of stacking-engineered quantum materials. The explosion of interest in twisted and commensurate bilayers of graphene and transition metal dichalcogenides has shown that interlayer registry, not chemistry alone, dictates electronic behaviour. Most such studies, however, rely on mechanical assembly of exfoliated flakes, which is artisanal, slow and poorly suited to manufacturing. The two-step CVD approach reported here shows that a desired polytype can be selected during synthesis by tuning adatom diffusion kinetics, and that adjacent bilayer ribbons can stitch together seamlessly into larger crystals. If the same kinetic control can be extended to other materials and other stacking targets, it would move stacking engineering closer to wafer-scale production.

Caveats and open questions remain, as with any single study. The photoemission measurements probed multiple crystallographic orientations of the grown flakes within the probing area, requiring careful angular slicing of the momentum images, and the reported direct-gap character rests on the agreement between experiment and density functional theory rather than on a direct measurement of the conduction band. Whether the enhanced valley polarization survives at elevated temperatures, in encapsulated device structures, and across wafer-scale films will need to be established. Still, the core message stands: the indirect band gap of bilayer MoS2 is not an immutable fact of nature but a consequence of stacking geometry, and that geometry can now be grown on demand. As the authors put it in their abstract, the results establish 1H MoS2 as a model system for stacking-engineered quantum materials, and they underscore its potential for valleytronic and optoelectronic applications that the field has been pursuing since monolayer MoS2 first announced its direct gap sixteen years ago.

Subject of Research: Stacking-controlled electronic and optical properties of CVD-grown bilayer molybdenum disulfide

Article Title: Stacking-induced direct band gap in CVD-grown 1H MoS2 bilayers

Article References: Yang, T. H., Chen, I.-T., Zhang, M.-J., Huang, J.-Y., Kuo, T.-H., Chen, S.-Y., Li, H.-Y., Chao, Y.-C., Hennighausen, Z. B., Dien, V. K., Wei, H.-W., Wu, M.-C., Yen, H.-W., Chuang, T.-H., Wei, D.-H., Kong, J., Lu, T.-H., Lin, K.-I., & Lan, Y.-W. (2026). Stacking-induced direct band gap in CVD-grown 1H MoS2 bilayers. Nature. https://doi.org/10.1038/s41586-026-11069-3

Image Credits: AI Generated

DOI: 10.1038/s41586-026-11069-3

Keywords: MoS2, transition metal dichalcogenides, two-dimensional materials, band gap engineering, chemical vapour deposition, stacking polytypes, photoluminescence, angle-resolved photoemission spectroscopy, valleytronics, excitons, optoelectronics, density functional theory

The material at the heart of the study is chromium thiophosphate, CrPS4, an air-stable, layered magnetic semiconductor that has become a workhorse of two-dimensional magnetism research. On its own, CrPS4 behaves conventionally depending on thickness: few-layer samples can show ferromagnetic order, while bulk crystals adopt an antiferromagnetic arrangement. Neither of these ground states, however, exhibits the hallmark of altermagnetism, which is a finite splitting between electronic states of opposite spin that does not arise from spin-orbit coupling but instead from the symmetry of the crystal and its magnetic arrangement. The researchers reasoned that if two CrPS4 flakes were stacked with their crystal axes rotated by ninety degrees, the interlayer symmetry would be altered in precisely the way theory predicts should generate an altermagnetic state.

The idea traces back to theoretical work suggesting that twisted magnetic van der Waals bilayers constitute an ideal platform for altermagnetism. In an ordinary antiferromagnet, time-reversal symmetry combined with a lattice translation protects the degeneracy of spin-up and spin-down bands, so the electronic structure shows no net spin splitting. When layers are rotated relative to one another, that combined symmetry operation is broken, and the spin degeneracy is lifted in a momentum-dependent fashion. The result is a material whose spins cancel macroscopically, avoiding the stray fields that plague ferromagnetic devices, yet whose electronic bands are split in a way that can carry spin-polarized currents. For spintronics, this combination is extraordinarily attractive: fast switching, dense packing, and robust spin information without the crosstalk that limits conventional magnetic memory.

To test the prediction, the team fabricated orthogonally twisted CrPS4/CrPS4 homostructures, stacking four-layer flakes rotated by ninety degrees relative to each other. They then interrogated the samples with a battery of optical probes, beginning with polarized Raman spectroscopy. In the twisted configuration, the Raman spectra revealed a splitting of phonon modes that is absent in untwisted controls. This vibrational fingerprint, the researchers show, arises from the interlayer coupling unique to the altermagnetic arrangement: the same symmetry breaking that splits the electronic bands also modifies the lattice dynamics, producing a spectroscopic signature that distinguishes the twisted state from any simple superposition of ferromagnetic and antiferromagnetic layers.

The more decisive evidence came from magneto-optical spectroscopy. Measuring the photoluminescence of the twisted homostructure under an applied magnetic field, the researchers tracked the degree of circular polarization of the emitted light. The twisted sample displayed a magnetic-field dependence that resembles the response of a ferromagnet, a striking result given that the underlying spin order is compensated. Even more telling was the observation of a pronounced Zeeman splitting in the photoluminescence spectra, with the sigma-plus and sigma-minus emission peaks separating as the field increased. Crucially, neither ferromagnetic nor antiferromagnetic CrPS4 samples show this splitting in the same configuration. Its emergence only in the twisted geometry indicates that the stack has entered a genuinely distinct magnetic ground state, one that carries the fingerprints of altermagnetism.

First-principles calculations reinforced the interpretation. Using density functional theory with an on-site Coulomb correction to treat the correlated chromium d electrons, the team computed the electronic structure of the orthogonally twisted bilayer and found large spin-split bands in an antiferromagnetic configuration. The pattern of the splitting matches the theoretical expectations for altermagnets: opposite-spin bands separate in momentum space in a way dictated by crystal rotation symmetry, while the net magnetization remains zero. The agreement between the computed band structure and the optical measurements provides a coherent picture in which twisting acts as a symmetry-breaking knob that switches on spin splitting without introducing any net magnetic moment.

The significance of the result extends beyond confirming a theoretical prediction. Van der Waals homostructures, in which the same material is stacked with controlled twist angles, have already revolutionized research on graphene and transition metal dichalcogenides, giving rise to moiré physics, flat bands, and correlated electronic phases. The present work shows that the same stacking engineering can serve as a design principle for magnetism itself. Rather than searching for new chemical compounds that happen to be altermagnets, researchers can now, in principle, manufacture altermagnetic states from well-characterized magnetic layers by choosing the twist angle. Because the twist angle is a continuously tunable parameter, it opens a route to systematically exploring how altermagnetic spin splitting evolves with interlayer symmetry, something no fixed crystal structure can offer.

The experimental signatures reported here also add to a growing toolbox for identifying altermagnets. Earlier confirmations of altermagnetic band splitting relied on techniques such as spin-resolved and angle-resolved photoemission spectroscopy, x-ray magnetic circular dichroism, and magneto-optical Kerr effect measurements in compounds like CrSb, MnTe, and RuO2. The CrPS4 study demonstrates that circularly polarized photoluminescence and Raman spectroscopy, both accessible table-top optical methods, can detect altermagnetism in atomically thin devices. This accessibility matters: optical probes can be applied to microscopic samples inside cryostats, under magnetic fields, and across device geometries, accelerating the pace at which candidate altermagnetic structures can be screened and characterized.

For applications, the appeal of altermagnets lies in their potential to combine the speed and stability of antiferromagnetic spintronics with the readout convenience of ferromagnets. Antiferromagnetic memory elements are immune to external fields and can in principle switch at terahertz frequencies, but their vanishing net moment makes them hard to read. Altermagnets solve the readout problem because their spin-split bands allow spin-polarized transport and magneto-optical signals even without net magnetization. A twisted van der Waals altermagnet adds another dimension: the state exists in an atomically thin, air-stable semiconductor that can be integrated into heterostructures with other two-dimensional materials, potentially enabling spin filters, tunnel junctions, and valleytronic devices in which spin and momentum are locked by design.

Challenges remain before such devices materialize. The reported signatures are spectroscopic rather than transport-based, and future work will need to demonstrate electrical readout and manipulation of the altermagnetic state, quantify the magnitude and temperature stability of the spin splitting, and explore how different twist angles and layer numbers tune the effect. Still, the demonstration that a simple ninety-degree rotation of identical CrPS4 flakes produces a magnetic phase absent from either constituent marks a conceptual milestone. It establishes twisted van der Waals homostructures as an experimental platform for altermagnetism, transforming a theoretical proposal into a tangible, measurable state of matter and handing the spintronics community a new material class to engineer.

Subject of Research: Experimental observation of altermagnetism in orthogonally twisted CrPS4 van der Waals homostructures

Article Title: Altermagnetism in twisted van der Waals homostructures

Article References: Chen, J., Xie, X., Li, S., Zhang, S., Hou, S., Zhang, X., He, J., Liu, Z., Wang, J.-T., & Liu, Y. (2026). Altermagnetism in twisted van der Waals homostructures. Nature Physics. https://doi.org/10.1038/s41567-026-03440-y

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03440-y

Keywords: altermagnetism, CrPS4, van der Waals heterostructures, twisted bilayers, spintronics, magneto-optical spectroscopy, Zeeman splitting, Raman spectroscopy, 2D magnets, spin splitting, antiferromagnetism, first-principles calculations

The underlying problem is well known to memory engineers. Crossbar arrays, in which memory elements sit at the intersections of perpendicular word lines and bit lines, offer the most compact possible memory layout and can be stacked in three dimensions. But in a passive crossbar without a selector, current sneaks through neighboring unselected cells along paths known as sneak currents, corrupting the readout of the intended cell. Selectors are two-terminal devices placed in series with each memory element—forming one-selector-one-resistor or one-selector-one-capacitor cells—to suppress these parasitic currents. An ideal selector must be extremely nonlinear, passing large current only at its selected operating voltage while blocking current at half or a fraction of that voltage, yet it must also survive trillions of switching cycles, respond in nanoseconds, remain stable across temperatures and operate identically from device to device. No single selector technology has satisfied all of these requirements simultaneously.

The USC-led team’s answer is a graded tunnel barrier realized entirely within a van der Waals heterostructure. Instead of a single uniform insulating barrier, the device stacks materials of different band alignments so that the effective barrier profile changes with applied voltage. At low voltages, the barrier remains thick and high, strangling leakage current to negligible levels. As the voltage across the device rises toward the read or write condition, the barrier is thinned and lowered in a controlled fashion, allowing electrons to tunnel through with high efficiency. The result is an exponential increase in current over a small voltage window—precisely the nonlinearity that crossbar selectors demand.

To move beyond trial and error, the researchers developed a theoretical model that predicts the electrical characteristics of selectors with graded tunnel barriers. Guided by this model, they designed and fabricated two specific device structures. In the first, the primary tunnel barrier is hexagonal boron nitride, the wide-bandgap insulator often called white graphene. In the second, the barrier is gallium sulfide, a layered semiconductor with a smaller bandgap. Both variants sandwich the barrier between two layers of molybdenum disulfide, with graphene sheets serving as the top and bottom electrodes, forming the complete five-layer stack graphene/molybdenum disulfide/barrier/molybdenum disulfide/graphene.

The performance of the hexagonal boron nitride device is striking. It exhibits a nonlinearity exceeding 10 million, meaning the current at the operating voltage is more than ten million times larger than the leakage current at reduced voltages—among the highest values reported for any two-terminal selector. Perhaps more impressive is its endurance: the device survived more than a trillion switching cycles without failure, a figure that dwarfs the lifetimes of many competing selector technologies and approaches what commercial memory products require. Switching occurs in less than 20 nanoseconds, the current-voltage characteristics show minimal temperature dependence, and the variation from one device to another is low—critical attributes for manufacturing, where billions of cells must behave nearly identically.

The gallium sulfide variant trades some nonlinearity for a dramatically reduced operating voltage. Because gallium sulfide has a smaller barrier height than hexagonal boron nitride, the device can deliver a nonlinearity above one million while operating at only 2.5 volts. Low-voltage operation matters enormously for modern memory, which must integrate with silicon circuitry whose supply voltages continue to shrink. A selector that requires high voltages to turn on forces the surrounding periphery circuitry to handle elevated stress; a selector that switches at 2.5 volts eases that burden and reduces overall energy consumption during write operations.

Beyond single devices, the team demonstrated that their selectors can be integrated into functional memory cells. They built one-selector-one-resistor cells by pairing the selectors with memristive devices, including a hafnium-oxide-based memristor stack with palladium electrodes, and they also demonstrated one-selector-one-capacitor configurations. This dual compatibility with both resistive nonvolatile memory and capacitive volatile memory suggests the selector technology is genuinely universal, applicable across different memory families rather than tied to a single cell type. The researchers benchmarked their devices against the broad landscape of existing selector technologies—including ovonic threshold switching chalcogenides, niobium oxide threshold devices, metal-insulator-metal tunnel diodes and mixed-ionic-electronic-conduction access devices—and found that the van der Waals approach uniquely combines high selectivity with trillion-cycle endurance, nanosecond speed, temperature stability and low variability.

Choosing van der Waals materials is central to the achievement. These layered crystals are held together by weak interlayer forces, so each atomic plane can be exfoliated and restacked like molecular building blocks without the dangling bonds and interfacial defects that plague conventional three-dimensional semiconductors. Atomically sharp interfaces mean the tunnel barrier thickness is controlled layer by layer, giving designers reproducible, deterministic barrier profiles. Graphene electrodes contribute their own advantages, providing chemically inert, highly conductive contacts that do not interdiffuse with the underlying layers. The same properties that have made van der Waals heterostructures a playground for condensed matter physics here translate directly into manufacturable device metrics: negligible cycling variation, minimal temperature dependence and good interdevice uniformity all flow from the crystalline perfection of the interfaces.

The implications extend toward the long-sought goal of three-dimensional memory stacking. Because the entire selector is built from vertically stacked two-dimensional layers, it is inherently stackable, unlike selectors that rely on complex oxide growth or electroforming processes that are difficult to repeat layer upon layer. Paired with scalable nonvolatile memories such as memristors and phase-change or resistive cells, these selectors could enable dense three-dimensional crossbar memories for storage-class memory, in-memory computing and neuromorphic artificial intelligence hardware, where analog crossbar arrays perform vector-matrix multiplications directly in hardware. The research team, whose work was supported by the Army Research Office, the Air Force Office of Scientific Research and the National Science Foundation, has also released the experimental and simulation data through the Harvard Dataverse, giving the broader community the tools to build on the design framework. As the demand for data storage and energy-efficient computation continues its relentless climb, devices that tame sneak currents with atomic precision may prove to be one of the quiet enablers of the next memory generation.

Subject of Research: Van der Waals tunnel-junction selector devices with graded tunnel barriers for high-density memory crossbar arrays

Article Title: High-performance tunnel-junction selectors with graded tunnel barriers based on five-layer van der Waals heterostructures

Article References: Liu, H., Liao, H.-T., Li, M., Yang, N., Hsu, T.-H., Zhao, R., Xu, Y., Kim, S. J., Ni, Y., Chenna, V., Mahalingam, K., Bowers, C., Ganguli, S., Roy, A. K., Watanabe, K., Taniguchi, T., Hashemi, H., Guo, J., Dubey, M., … Joshua Yang, J. (2026). High-performance tunnel-junction selectors with graded tunnel barriers based on five-layer van der Waals heterostructures. Nature Electronics. https://doi.org/10.1038/s41928-026-01704-2

Image Credits: AI Generated

DOI: 10.1038/s41928-026-01704-2

Keywords: van der Waals heterostructures, tunnel-junction selector, memory crossbar arrays, molybdenum disulfide, hexagonal boron nitride, gallium sulfide, memristors, nonlinearity, endurance, three-dimensional memory, nanoelectronics, Nature Electronics

Polaritons arise when photons couple strongly to vibrations of the crystal lattice, forming so-called phonon polaritons. In certain van der Waals materials, these waves propagate hyperbolically, meaning their wavefronts sweep outward in open arcs rather than closed ellipses, a property that allows light to be confined and steered with extraordinary precision. Alpha-MoO3 has become a star of this field because its in-plane optical response is naturally anisotropic: light propagates differently along different crystallographic directions, producing hyperbolic dispersion in the mid-infrared range. Over the past several years, researchers have twisted stacked flakes of such crystals against one another to create topological transitions in polariton propagation, discovering everything from photonic magic angles to ghost hyperbolic surface waves. But all of these approaches share a fundamental limitation: the optical response of a natural material is fixed by its crystal symmetry, so reconfiguration has typically required physically restacking, rotating, or chemically modifying the sample.

Conventional strategies for controlling polaritons rely on refractive-index engineering. By placing a polariton-bearing crystal on top of another optical medium, engineers can coax the waves in the two layers to hybridize, blending their dispersion relations into something new. Yet this coherent hybridization demands strict wavevector matching between the coupled modes, a condition that is difficult to satisfy and sensitive to sample geometry, thickness, and frequency. It also ties the resulting behavior inseparably to the passive optical properties of the underlying layer. The team behind the new study, led by researchers at Shanghai Jiao Tong University and the National Center for Nanoscience and Technology, asked a different question: what if, instead of matching wavevectors, one simply filtered out the waves one did not want?

The answer came in the form of an unlikely partner for the molybdenum oxide flake: a film of aligned carbon nanotubes. These dense arrays of nanotubes, all pointing in the same direction, are electrically conductive along their axis but effectively insulating across it, making them a naturally anisotropic conductor. Crucially, the researchers found that the carrier dynamics within the nanotube film are overdamped, meaning that charge carriers lose their coherent momentum too quickly to sustain any resonant optical response of their own. As a result, the nanotubes refuse to hybridize with the polaritons in the overlying crystal. Instead of coherently mixing, the two layers interact through a purely resistive, proximity-induced coupling—essentially, the polaritons leaking into the nanotube film are simply dissipated as heat.

That dissipation, however, is anything but random. Because the nanotubes conduct only along their alignment axis, they absorb polariton energy selectively depending on the direction in which the wave propagates relative to the nanotube axis. The aligned carbon nanotube layer thus acts as what the researchers call a momentum-space loss filter: in the reciprocal space that describes the polariton dispersion, wavevectors aligned with the nanotube axis are strongly attenuated while others survive. The consequence in real space is dramatic. The intrinsic hyperbolic dispersion of the alpha-MoO3 flake, with its elegant pairs of symmetric arcs, is reshaped into a symmetry-broken dispersion in which the energy flow—the Poynting vector—is inhibited in some directions and concentrated in others. Full-wave electromagnetic simulations confirmed that an effective medium description of the heterostructure reproduces the observed field patterns, with the asymmetry gradually diminishing as the crystal flake becomes thicker and the resistive coupling weaker.

The experimental evidence came from scanning near-field optical microscopy, a technique that drags a nanoscale antenna across the sample surface to image polariton fringes with resolution far below the diffraction limit. By preparing heterostructures in which the crystal’s [100] axis was rotated at various angles relative to the nanotube axis—from zero degrees through 90 degrees—the team observed a smooth, continuous transformation of the polariton wavefronts. At certain twist angles the familiar symmetric hyperbolas remained essentially intact, while at others the fringes warped into highly asymmetric, shear-like patterns reminiscent of the hyperbolic shear polaritons previously seen only in exotic low-symmetry crystals such as beta-gallium oxide or monoclinic semiconductors. Here, however, the shear was not baked into the crystal lattice; it was imposed by an external, deliberately engineered dissipative layer whose orientation could be chosen freely at fabrication time.

The most striking result is that the entire transformation is electrically reversible. Because the dissipative loss in the nanotube film is governed by its Drude response—the same free-carrier absorption that limits the conductivity of any metal—applying a gate voltage changes the carrier density in the nanotubes and thereby tunes the strength of the momentum-space filter. In the experiments, sweeping the gate voltage from positive to negative values continuously steered the polariton propagation from symmetric hyperbolic wavefronts to strongly sheared, asymmetric ones and back again. Unlike approaches that require physically altering the sample, this means the polariton topology can be reprogrammed in real time, with a single device cycling through a whole family of optical responses that a static crystal could never display.

The work also carries conceptual weight for the broader field of non-Hermitian physics. In standard quantum and optical systems, the Hamiltonian describing the dynamics is Hermitian, guaranteeing energy conservation; loss must then be treated as a nuisance or, at best, compensated with gain. Physicists have increasingly recognized that deliberately engineered loss—described by non-Hermitian formalisms—can be a resource in its own right, enabling exceptional points, parity-time symmetry effects, and non-Hermitian skin phenomena. The new study extends that philosophy into topological nanophotonics in a particularly clean way: rather than balancing gain and loss or exploiting fine-tuned couplings, the researchers show that a simple, gate-tunable anisotropic resistor is enough to break symmetry and reshape topology, establishing dissipation itself as a programmable degree of freedom in device design.

The implications reach across several technologies. Mid-infrared polaritons are already being explored for molecular sensing, thermal management, and sub-diffraction imaging, and recent demonstrations of hyperbolic electroluminescence suggest routes toward electrically driven polariton sources. A platform in which the directionality, asymmetry, and topology of those waves can be switched by a voltage opens the door to dynamically reconfigurable nanophotonic circuits, electrically steered thermal emitters, modulators, and beam-shaping elements operating at wavelengths far below what conventional optics allows. The aligned carbon nanotube films used in the study are compatible with wafer-scale growth and high-performance electronics, hinting that the integration of polaritonic and electronic functionality on a single chip may be practical rather than aspirational. More broadly, the study suggests that the road to programmable nanophotonics may run not only through what materials let light do, but through what they quietly prevent it from doing—a reminder that in modern optics, sometimes the most powerful design tool is controlled loss.

Subject of Research: Electrically programmable polariton symmetry breaking via non-Hermitian dissipation engineering in van der Waals heterostructures

Article Title: Electrically programmable polariton symmetry breaking via non-Hermitian dissipation engineering

Article References: Chen, N., Teng, H., Sun, Y., Yang, Y., Xue, Z., You, O., Chen, K., Jiang, C., Wang, J., Zhou, S., Liu, X., Wang, C., Li, Z.-Z., Meng, S., Zhu, M., Hu, H., & Dai, Q. (2026). Electrically programmable polariton symmetry breaking via non-Hermitian dissipation engineering. Nature Materials. https://doi.org/10.1038/s41563-026-02734-z

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02734-z

Keywords: phonon polaritons, alpha-MoO3, aligned carbon nanotubes, non-Hermitian photonics, dissipation engineering, symmetry breaking, hyperbolic dispersion, shear polaritons, electrical gating, nanophotonics, van der Waals heterostructures, topological photonics

The Hall effect, discovered by Edwin Hall in 1879, is one of the foundational measurement tools of condensed matter physics. When a current flows through a conductor in the presence of a perpendicular magnetic field, the Lorentz force pushes charge carriers to one side, producing a voltage across the material at right angles to the current. That transverse voltage encodes the density and sign of the mobile charges and, in its quantized variants, has underpinned decades of research into exotic quantum states. Extending the Hall effect to composite quasiparticles such as trions is therefore more than a technical curiosity: it provides a direct probe of the transport behavior of particles that carry both charge and internal structure.

Trions occupy a fascinating middle ground between ordinary excitons and free electrons. An exciton is electrically neutral, a bound electron–hole pair that can move through a semiconductor much like an atom of light, while a trion—often written X− or X+ depending on whether the extra particle is an electron or a hole—carries net charge equal to that of a single electron. Because trions combine a finite charge with a substantial optical dipole moment, they can be created and detected optically yet still respond to electrical and magnetic forces. This dual character makes them attractive candidates for optoelectronic applications and for fundamental studies of many-body physics, but it also makes their transport properties notoriously difficult to isolate from those of the surrounding charge sea.

The experimental platform used in the new study was an electron–hole double layer, a structure in which two sheets of charge carriers of opposite polarity are separated by an insulating or semiconducting barrier. Electrons reside in one layer and holes in the other, each able to move freely within its own plane, while the Coulomb attraction between the layers binds them into excitons and, in the presence of excess carriers, into trions. Double layers of this kind, typically realized in atomically thin van der Waals heterostructures built from transition metal dichalcogenides or in carefully engineered quantum well systems, allow researchers to tune the balance between free carriers and bound complexes simply by adjusting gate voltages.

When the researchers applied a perpendicular magnetic field to this double layer system, they observed that the trion fluid—collectively, the gas of charged three-particle complexes drifting through the device—developed a transverse voltage characteristic of the Hall effect. Crucially, the measured Hall response could not be explained by the motion of free electrons or free holes alone. Its sign and magnitude tracked the optical signatures of trion formation, indicating that the composite particles themselves were participating in the transport. In effect, the experiment showed that an electrically charged but internally complex quasiparticle can behave, at the level of Hall physics, as a well-defined carrier with its own effective Hall coefficient.

Technically, the observation is a considerable feat of disentanglement. In an electron–hole double layer, the Hall voltage measured in either layer receives contributions from several populations: the free majority carriers, the minority carriers of opposite sign, the neutral excitons, and the charged trions. The theory developed alongside the experiments accounts for this mixture using a multi-component transport formalism, in which each species contributes its own conductivity tensor, modified by inter-particle scattering and by the mutual attraction between the layers. The Hall response of the trion component depends on its charge, its effective mass—which is dominated by the heavy bound complex rather than the light bare electron—and the statistics of the excess carriers that stabilize it. The agreement between the measured Hall coefficients and these predictions provides quantitative evidence that trions transport as coherent entities rather than as transient associations of separately drifting particles.

The result settles a question that has lingered since trions were first identified in semiconductor optical spectra decades ago. Because a trion is only weakly bound compared with an atom, and because its lifetime in a photoexcited sample is short, many researchers doubted whether trions could sustain the kind of steady-state drift required for a Hall measurement. The new work demonstrates that, in the right regime, a trion fluid behaves hydrodynamically and electrically much like an ordinary charged gas. The effective temperature and density of the trion population can be controlled through optical pumping and electrostatic gating, and under suitable conditions the composite particles establish their own drift velocity in response to an applied electric field, exactly as required for a Hall signal to develop.

Beyond its fundamental significance, the trion Hall effect opens practical avenues. Trions are central to proposals for excitonic and optoelectronic devices in which information is carried by light-matter bound states rather than bare electrons, promising lower-energy operation and new ways to couple optical and electrical signals. A measurable Hall response provides a purely electrical handle on trion populations, complementing optical spectroscopy and enabling faster readout of composite-particle dynamics. It could also serve as a diagnostic in quantum emitter and single-photon technologies, where trion states often mediate the optical response of defect centers and quantum dots. In van der Moiré heterostructures, where flat electronic bands amplify correlation effects, a trion Hall measurement might even reveal interaction-driven phases that are invisible to conventional transport.

The demonstration also connects to broader themes in modern condensed matter research. Hall effects have become a versatile language for characterizing quasiparticles of every description, from Weyl fermions in topological semimetals to strange metals and superconductors. Each new carrier class that exhibits a Hall response adds a fingerprint that can be used to identify and study it. Trions, being charged composites with internal degrees of freedom, sit in an unusual position in this landscape: their Hall coefficient reflects not only their charge-to-mass ratio but also the correlated motion of the constituent electron and hole, potentially encoding information about binding energies, scattering channels and the inter-layer coupling strength. The theoretical framework validated here offers a template for extracting such details in other layered systems where excitonic complexes proliferate.

Looking ahead, the researchers and their colleagues anticipate several extensions. Applying stronger magnetic fields may drive the trion fluid toward quantum Hall regimes, where quantized Hall signatures of composite particles could emerge. Studying how the trion Hall effect evolves as the layer separation shrinks toward the atomic limit could illuminate the crossover from loosely bound three-body states to the strongly correlated exciton fluids seen in recent experiments on monolayer semiconductors. And because the double layer geometry is compatible with contactless detection methods, the approach may generalize to systems where fabricating electrodes is impractical. For now, the observation stands as a clean confirmation of a decade-old theoretical expectation and a reminder that even the most familiar effects in physics can acquire new life when the carriers doing the drifting are as unconventional as charged pairs of light and matter.

Subject of Research: The Hall effect in a trion fluid formed within coupled electron–hole layers of a semiconductor device

Article Title: Trion Hall effect in electron–hole double layers

Article References: Chaturvedi, R., Nguyen, P. X., Knüppel, P., Watanabe, K., Taniguchi, T., Mak, K. F., & Shan, J. (2026). Trion Hall effect in electron–hole double layers. Nature Physics. https://doi.org/10.1038/s41567-026-03444-8

Image Credits: AI Generated

DOI: 10.1038/s41567-026-03444-8

Keywords: trion, Hall effect, exciton, electron–hole double layer, condensed matter physics, quasiparticles, semiconductor, van der Waals heterostructures, quantum Hall physics, optoelectronics, magnetic field, transport phenomena