Gold Nanoparticles Supercharge Flexible Infrared Photodetectors Built from TiS3

Flexible electronics have long promised a world where sensors wrap comfortably around a wrist, bend across a bandage, or integrate seamlessly into clothing, yet the hardest technical problems often hide in the details of how light is converted into an electrical signal. A team of researchers from Samara National Research University, Samara State Medical University, Vellore Institute of Technology, Jamia Millia Islamia, and the University of Delhi has now reported a flexible near-infrared photodetector that combines high sensitivity with fast response times, two figures of merit that are notoriously difficult to optimize simultaneously in devices built on plastic substrates. Writing in the Journal of Materials Science, the group describes heterostructures made from titanium trisulfide nanostructures decorated with plasmonic silver or gold, assembled directly on polyethylene terephthalate, the same inexpensive, mechanically resilient polyester used in beverage bottles and flexible packaging.

The choice of titanium trisulfide, usually abbreviated TiS3, is central to the design. Unlike the better-known transition metal dichalcogenides such as molybdenum disulfide, TiS3 belongs to the family of transition metal trichalcogenides, and its crystal structure forms quasi-one-dimensional chains of titanium and sulfur. This gives the material a ribbon-like morphology and strong optical anisotropy, meaning it interacts with light differently depending on polarization and direction. TiS3 has attracted growing interest for infrared optoelectronics because its band gap sits in a range that allows absorption well into the near-infrared, a spectral region where silicon, the workhorse of conventional imaging, is essentially blind. For wearable health monitors, industrial process control, and scientific instrumentation that rely on detecting wavelengths around one micrometer, materials like TiS3 offer a route to detectors that are both thin and mechanically compliant.

Building a working device from nanoscale ribbons on a flexible sheet is far from trivial, and the team approached the assembly problem with a technique called dielectrophoresis. The researchers first patterned interdigitated chromium electrodes on the polyethylene terephthalate substrate, creating comb-like finger structures separated by narrow gaps. When a non-uniform alternating electric field is applied between such electrodes, particles suspended in a medium experience a force that depends on their polarizability relative to the surrounding liquid. By exploiting this force, the team guided TiS3 nanostructures into the electrode gaps, positioning them where they could bridge the fingers and form continuous conduction paths. Dielectrophoretic assembly is attractive for flexible substrates because it avoids the high temperatures and aggressive solvents associated with many growth techniques, and it can be scaled by simply patterning larger electrode arrays.

The decisive step, however, came after assembly. The researchers deposited controlled amounts of metal by sputtering, creating sharp metal-semiconductor interfaces on top of the TiS3 nanostructures. Here the choice of metal turned out to be everything. Gold and silver are both noble metals with strong plasmonic behavior, meaning their conduction electrons can oscillate collectively in resonance with incident light, a phenomenon known as localized surface plasmon resonance. But the two metals differ in a subtler and equally important property: their work functions, the energy required to remove an electron from the metal surface. Because gold and silver sit at different positions on this energy scale relative to the bands of TiS3, the interfaces they form with the semiconductor are fundamentally different in character.

When illuminated with 1064-nanometer light, a wavelength commonly emitted by neodymium-doped lasers and widely used in range finding, telecommunications, and medical diagnostics, the gold-functionalized device delivered striking performance. The researchers measured a responsivity of 0.93 amperes per watt, meaning nearly one ampere of photocurrent for every watt of incident optical power. The specific detectivity, a measure that combines sensitivity with noise performance, reached 2.93 times ten to the eighth Jones. Response and recovery times came in at 118 and 228 milliseconds respectively, fast enough for many real-time sensing applications. Most tellingly, the external quantum efficiency exceeded one hundred percent, reaching 108.72 percent. That figure is physically impossible for a device in which each absorbed photon generates at most one collected electron-hole pair, and its appearance signals that an internal gain mechanism is at work, multiplying the charge produced by each optical event.

The authors attribute this gain to the synergy of two effects. First, the localized surface plasmon resonance of the gold nanostructures concentrates electromagnetic energy into nanoscale volumes near the metal surface, dramatically enhancing the local field and therefore the absorption in the adjacent semiconductor. Second, the plasmon decay can inject energetic hot electrons across the metal-semiconductor boundary, and the carefully engineered Schottky barrier at the gold-TiS3 interface governs how efficiently this charge transfer occurs. A Schottky barrier is the potential energy barrier that forms wherever a metal contacts a semiconductor, and its height determines both the ease of hot-electron injection and the efficiency with which photogenerated charges are separated rather than recombining. By tuning the barrier through the choice of metal and the sputtering conditions, the team smoothed carrier transport and amplified the photocurrent beyond what simple photoconversion could deliver.

The comparison with silver makes the physics unmistakably clear. Silver nanoparticles are, if anything, even more efficient plasmonic scatterers than gold at visible and near-infrared wavelengths, so one might expect the silver-functionalized device to win. Instead, it produced a responsivity of only 7.02 times ten to the minus five amperes per watt and an external quantum efficiency of 8.2 times ten to the minus three percent, orders of magnitude below the gold device. The reason is that silver forms an Ohmic, rather than a rectifying, contact with TiS3. Without a Schottky barrier, there is no built-in electric field to separate charges efficiently and no energy barrier to gate hot-electron injection, so the plasmonic enhancement alone cannot translate into device gain. The result is a vivid demonstration that barrier height, not plasmonic strength alone, controls performance in this class of hybrid photodetectors.

Sensitivity alone does not make a practical sensor, and the team also characterized the device under conditions that matter for real deployments. The gold-functionalized photodetector maintained a high signal-to-noise ratio and exhibited a wide linear dynamic range of 74.8 decibels, meaning it responds predictably and proportionally across a broad span of light intensities without saturating at the bright end or drowning in noise at the dim end. Linear dynamic range is particularly important in complex environments such as wearable health monitoring, where ambient illumination fluctuates constantly, and in industrial settings where a sensor must handle both weak reflected signals and strong direct beams without recalibration. A detector that compresses or distorts signals outside its linear window produces measurements that are difficult to interpret, so this figure is a meaningful indicator of reliability.

The broader significance of the work lies in its fabrication philosophy. Every step, from dielectrophoretic assembly of the nanostructures to metal sputtering, is compatible with large-area processing on low-cost polymer substrates, and none requires exotic crystal growth or transfer printing of atomically thin layers. That practicality matters because the field of flexible infrared photodetectors has produced many impressive laboratory demonstrations that would be difficult to manufacture at scale. By showing that a simple combination of a quasi-one-dimensional semiconductor, interdigitated electrodes, and a judiciously chosen plasmonic metal can deliver responsivity, detectivity, speed, and dynamic range together on PET, the researchers have outlined a scalable recipe rather than a one-off curiosity. The funding for the study came from the Russian Science Foundation under grant number 25-23-00494.

Looking ahead, the findings point toward wearable and implantable infrared sensing platforms that could monitor blood oxygenation, detect laser signals in free-space optical communication, or serve as flexible eyes for soft robotics. They also sharpen a design principle for the entire field of plasmon-enhanced two-dimensional and quasi-one-dimensional photodetectors: the metal must be chosen not only for its optical resonance but for the electronic structure of the junction it creates. Gold, in this system, wins because it does two jobs at once, concentrating light and engineering the barrier that turns absorbed photons into amplified current. As researchers extend this approach to other trichalcogenides and other noble metals, the lesson from Samara and Delhi is likely to endure, that in nanoscale optoelectronics the interface is where performance is won or lost.

Subject of Research: Noble-metal-functionalized titanium trisulfide heterostructures for flexible near-infrared photodetectors

Article Title: Noble-metal-functionalized TiS3 for high-performance flexible photodetectors

Article References: Noble-metal-functionalized TiS3 for high-performance flexible photodetectors. (n.d.). https://doi.org/10.1007/s10853-026-13914-2

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13914-2

Keywords: titanium trisulfide, photodetector, flexible electronics, plasmonics, Schottky barrier, near-infrared, gold nanoparticles, dielectrophoresis, wearable technology, hot-electron injection, polyethylene terephthalate, external quantum efficiency