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 heart of the device is a tri-layer stack of tungsten diselenide, a semiconductor only a few atoms thick that belongs to the family of transition metal dichalcogenides. Two-dimensional materials like WSe2 are prized in neuromorphic engineering because their entire volume sits at the surface, making their electronic properties exquisitely sensitive to light, electric fields and chemical modification. In their native form, however, WSe2 devices often fall short of the stable, tunable conductance states that artificial synapses demand. The researchers solved this by treating the material with benzyl viologen, a strong electron-donating molecule that donates charge to the WSe2 lattice and shifts its Fermi level, effectively engineering the carrier polarity of the channel and setting the stage for the dramatic optical behaviors that follow.

Before any synaptic claims could be made, the team had to prove the material survived the doping process intact. Atomic force microscopy confirmed the structural integrity and surface morphology of the tri-layer films, while Raman spectroscopy verified that the crystal lattice remained high quality after exposure to the benzyl viologen treatment. On the electrical side, gate-dependent transfer curves and current-voltage measurements mapped out how the doping transformed the device’s behavior, and the researchers constructed an energy band diagram showing the Fermi level shift that the molecular dopants induce. This combination of structural and electrical characterization matters because any degradation of the delicate 2D lattice would undermine the reproducibility that real neuromorphic circuits require.

The most striking results emerge when light hits the device. The BV-doped WSe2 synapse responds strongly to both visible and deep-ultraviolet illumination, but it is the deep-ultraviolet response that gives the device its memory. Under DUV light, charges become trapped within the material system, and these trapped charges persist long after the light is switched off, producing a stable conductance state that functions as a nonvolatile memory. In other words, a flash of ultraviolet light writes information into the device, and the device holds that information without power, much as a biological synapse retains the trace of a past experience. This optically programmable memory is what allows the same physical structure to act simultaneously as a photodetector and a synaptic element.

When stimulated with sequences of optical pulses rather than continuous illumination, the device reproduces hallmark behaviors of biological synapses. Paired-pulse facilitation, a form of short-term plasticity in which the second of two closely spaced signals produces a larger response than the first, appears naturally in the device’s conductance dynamics. With longer or more intense stimulation, the device exhibits long-term potentiation and long-term depression, the persistent strengthening and weakening of connection strength that neuroscientists consider the cellular basis of learning and memory. Crucially, the magnitude of the response is tunable: by controlling the duration of the deep-ultraviolet pulses, the researchers increased the device’s photoresponse from 25.2 percent to 1438.7 percent, a dynamic range wide enough to encode many distinct synaptic weights.

That tunability translates directly into computational capability. The team demonstrated that the device can perform a logic OR gate operation, outputting a high signal when either of two inputs is present, and that it can carry out selective alphabet recognition, distinguishing among written characters based on the optical signatures stored in its conductance states. These demonstrations show that the device is not merely a passive memory element but an active processing unit, capable of the kind of in-memory logic that could dramatically reduce the energy overhead of edge-computing systems, where data must be processed locally under strict power budgets rather than shipped to energy-hungry data centers.

To test whether the device’s synaptic behavior could scale to meaningful machine learning, the researchers implemented its experimentally measured synaptic weight characteristics in an artificial neural network and trained that network to identify handwritten digits from the MNIST dataset, a standard benchmark in computer vision. The simulated network achieved an accuracy of 92 percent, and the team validated the learning process through epoch-dependent learning curves and confusion matrix analysis, confirming that the network’s errors were distributed in the way one expects from a genuinely learning system rather than a lucky configuration. The result is significant because it grounds the device physics in a concrete application: the same conductance modulation that the researchers measured in the laboratory can, in principle, serve as the adjustable weights that artificial neural networks rely on.

The broader significance of the work lies in its multifunctionality. Most neuromorphic devices reported to date specialize in one role, whether as memristors, phase-change memories or electrochemical transistors, and require separate photonic or electronic circuitry to interface with the outside world. By combining optical sensing, nonvolatile charge-trapping memory, short- and long-term plasticity, logic operation and pattern recognition in a single doped 2D material platform, the WSe2 synapse points toward integrated intelligent sensors that could see, remember and compute in one package. Such devices would be natural fits for autonomous drones, biomedical implants and environmental monitors, where every milliwatt saved extends operational life.

There are, of course, hurdles between a laboratory demonstration and deployable technology. The device relies on exquisitely controlled chemical doping of atomically thin crystals, and scaling such processes to wafer-scale manufacturing while maintaining the uniformity that large neural networks demand remains an open challenge in the 2D materials community. The 92 percent MNIST accuracy, while respectable, still trails the best software-based networks, suggesting that device variability and nonlinearity in the conductance updates will need further engineering. The published version of the paper is also an early-release, peer-reviewed accepted manuscript that may undergo further edits before the final version of record appears, so specific figures could be refined.

Even so, the study adds an important entry to the growing catalog of optoelectronic synapses, and its use of benzyl viologen doping to engineer carrier polarity in tri-layer WSe2 gives other researchers a concrete chemical lever to pull. The work was supported by the Korea Basic Science Institute through a National Research Facilities and Equipment Center grant funded by the Korean Ministry of Education, by an IITP Information Technology Research Center grant from the Ministry of Science and ICT, and by the ongoing research funding program of King Saud University in Riyadh. As artificial intelligence migrates from the cloud toward the edge, devices that can be programmed by nothing more than pulses of light, and that learn the way neurons do, may prove to be exactly the kind of hardware the next generation of intelligent machines is waiting for.

Subject of Research: Optoelectronic synaptic devices based on doped two-dimensional tungsten diselenide for neuromorphic computing and logic operations

Article Title: A multifunctional optoelectronic synaptic device for logic operation and neuromorphic computing

Article References: Dastgeer, G., Nisar, S., Fatima, I., Ali, R. F., Rabani, I., Hussain, K., Alsalme, A., & Eom, J. (2026). A multifunctional optoelectronic synaptic device for logic operation and neuromorphic computing. Advanced Composites and Hybrid Materials. https://doi.org/10.1007/s42114-026-02103-z

Image Credits: AI Generated

DOI: 10.1007/s42114-026-02103-z

Keywords: neuromorphic computing, optoelectronic synapse, tungsten diselenide, WSe2, benzyl viologen doping, two-dimensional materials, memristor, synaptic plasticity, charge trapping, artificial neural network, MNIST, logic gates