The central obstacle is thermodynamic. Gold’s surface free energy is roughly 1.12 joules per square metre, an order of magnitude higher than that of graphene under ambient conditions. When gold atoms are deposited onto ordinary substrates such as silicon dioxide, this high surface energy drives them to clump together into three-dimensional islands rather than spreading out into a flat layer, a behaviour known as Volmer-Weber growth. The result is a discontinuous scatter of metallic droplets rather than the uniform, layer-by-layer Frank-van der Merwe film needed for a usable ultrathin conductor. Previous attempts to force flat growth relied on seed layers or adhesion promoters that permanently bonded the gold to its growth substrate, destroying the transferability that device integration demands, or produced films whose roughness was inherited from an imperfect underlying surface.

The team’s solution begins with the growth template itself. Instead of evaporating copper onto a support, where the columnar grain structure of the deposited metal imposes its own roughness, the researchers created a freestanding copper foil by template stripping. A 500-nanometre copper layer was first evaporated onto a single-crystal silicon (111) wafer, electroplated to a self-supporting thickness of 30 to 60 micrometres, and then peeled away. The gold-facing surface of the foil inherits the atomic flatness of the silicon wafer beneath it. Atomic force microscopy revealed terraced, essentially featureless copper with a root-mean-square roughness of just 0.24 nanometres over a four-micrometre scan, dropping to 0.16 nanometres at higher magnification. Because the deposited gold faithfully replicates the copper relief, this suppression of template corrugation proved to be the decisive factor enabling continuous gold films down to 1.5 nanometres, compared with a previous lower bound of 3.5 nanometres.

The choice of copper is not merely a matter of smoothness but of energetics. The adsorption energy of gold atoms on copper, approximately 7.78 joules per square metre, comfortably exceeds twice the surface free energy of gold, satisfying the thermodynamic criterion for two-dimensional, layer-by-layer growth. On silicon dioxide, by contrast, the adsorption energy is far too low, and islanding is inevitable. Molecular dynamics simulations made this contrast vivid: gold atoms deposited onto the copper template self-assembled into a uniform, highly crystalline monolayer even under deliberately anisotropic deposition conditions, while the same atoms landing on silicon coalesced into disordered, discrete islands. High-resolution electron microscopy of three-nanometre films confirmed the picture experimentally, showing a continuous, featureless surface on copper-derived films against percolated networks on molybdenum disulfide and classic island morphologies on bare oxide.

Direct evidence for layer-by-layer growth came from a strikingly simple test. The researchers deposited a nominal one-nanometre gold layer, a regime where island formation is most pronounced on conventional substrates, and re-examined the surface. The roughness barely changed, rising only from 0.16 to 0.18 nanometres, with no discrete islands visible. After transfer to silicon dioxide substrates, films ranging from two to eight nanometres in thickness remained continuous and uniform, with roughness below 0.4 nanometres and only rare pinholes in the thinnest sample. Transmission electron microscopy of a suspended three-nanometre membrane revealed a polycrystalline structure with crystalline domains often exceeding 20 nanometres laterally, several times larger than the film itself, and lattice fringes spaced about 0.24 nanometres apart, the signature of close-packed gold (111) planes.

The optical and electrical performance places these films among the best ultrathin transparent conductors yet reported. Peak transmittance climbed monotonically as thickness decreased, reaching 86.6 percent for the 1.5-nanometre film, up from 80 percent in the team’s earlier work, while sheet resistance for the thicker films fell to 14.9 ohms per square at five nanometres and 7.8 ohms per square at eight nanometres, approaching the theoretical limits for bulk crystalline gold. Spectroscopic ellipsometry showed that the dielectric function of films from three to eight nanometres closely tracks that of bulk gold, with a plasma frequency within six percent of the bulk value, quantitative evidence of a near-bulk free-electron density and minimal defects. Benchmarked against previous ultrathin gold, multilayer chemical-vapour-deposition graphene and commercial indium tin oxide, the films occupy a leading position on the standard transparency-conductivity figure of merit.

Electrical transport in the thinnest films is governed by the physics of reduced dimensionality. Because the film thickness is far smaller than the roughly 40-nanometre electron mean free path in bulk gold, surface and grain-boundary scattering dominate the resistivity. A combined Fuchs-Sondheimer and Mayadas-Shatzkes model, with physically reasonable parameters for specular surface scattering and grain-boundary reflection, matched the measured data across the two-to-eight-nanometre range. Flexibility testing showed that a five-nanometre film on plastic survived 1,000 bending cycles at a radius of about three millimetres with only a minimal increase in sheet resistance, a resilience that underpins the device demonstrations that followed.

Those demonstrations span photonics, thermal management and biomedicine. An eight-nanometre gold film served as the transparent anode in solution-processed organic light-emitting diodes, which outperformed identical devices built on commercial indium tin oxide, likely because gold presents a lower energy barrier of 0.1 electron-volts to the hole-injection layer, and because the atomically smooth anode eliminates the protrusions that can short-circuit the roughly 100-nanometre organic stack. The team also fabricated an eight-by-eight matrix-addressable transparent OLED display on flexible plastic at six-inch scale. For thermal camouflage, the same films exploit a dual property: high visible transparency preserves the object’s appearance, while high infrared reflectivity suppresses thermal emission, in accordance with Kirchhoff’s law. A five-nanometre gold coating reduced the apparent temperature seen by an infrared camera to about a third of the actual background temperature, a roughly 30 percent improvement over the team’s earlier rougher films, and a gold-polymer laminate concealed a satellite-shaped object from thermal imaging while remaining visually transparent.

Perhaps the most striking application is on human skin. Transferred as electronic tattoos, five- and twenty-nanometre gold electrodes conformed to the forearm and maintained stable impedance across repeated measurements, with values at 10 kilohertz of 20.3 and 10.5 kilohms respectively, comparable to state-of-the-art graphene electronic tattoos. When used to record electrocardiograms and electromyograms, the gold tattoos achieved signal-to-noise ratios of 14.9 and 69.5 decibels, exceeding the 12.7 and 62.4 decibels obtained from clinical silver-silver-chloride gel electrodes. The researchers attribute this advantage to the exceptional mechanical conformability of the ultrathin films, which yields intimate skin contact, minimises motion artefacts and outweighs the electrodes’ higher intrinsic impedance. Looking forward, the team notes that the same template-stripping and transfer paradigm should extend to other quasi-two-dimensional metals such as silver, palladium and platinum, while single-crystal copper templates and alternative stabilising interfaces based on sulfur-containing materials may push continuous metallic films toward the one-nanometre threshold, opening a broad new library of atomically thin metals for next-generation technologies.

Subject of Research: Wafer-scale synthesis and transfer of atomically thin continuous gold films for optoelectronics, thermal management and bio-interfaces

Article Title: Wafer‐Scale Atomically‐Thin Gold: Transferable Platform for Flexible Optoelectronics, Thermal Management and Bio‐Interfaces

Article References: Yakubovsky, D., Mironov, M., Ermolaev, G., Khrebtov, A., Slavich, A., Narute, P., Chowdhury, P., Kirtaev, R., Khramtsov, I., Minnekhanov, A., Tselikov, G., Grudinin, D., Sidorova, M., Pak, N., Maslov, P., Vyshnevyy, A., Zhou, H., Kravets, V. G., Grigorenko, A. N., … Volkov, V. (2026). Wafer‐Scale Atomically‐Thin Gold: Transferable Platform for Flexible Optoelectronics, Thermal Management and Bio‐Interfaces. Advanced Science, 13(55), Article e76495. https://doi.org/10.1002/advs.76495

Image Credits: AI Generated

DOI: 10.1002/advs.76495

Keywords: ultrathin gold, 2D materials, template stripping, transparent electrodes, flexible electronics, OLEDs, thermal camouflage, electronic tattoos, epidermal electronics, Frank-van der Merwe growth, sheet resistance, wafer-scale fabrication

Zinc oxide has long been considered a leading candidate to replace ITO because it is abundant, non-toxic, and possesses excellent optical transparency. Yet, pure zinc oxide films often struggle to achieve the low electrical resistance required for high-efficiency solar cells. To overcome this limitation, the research team integrated carbon nanotubes into the zinc oxide matrix. Carbon nanotubes are renowned for their exceptional electrical conductivity and mechanical strength. By weaving these nanoscale tubes into the zinc oxide structure, the researchers aimed to create a hybrid material that leverages the transparency of the oxide and the conductivity of the carbon. This composite approach represents a strategic shift in materials engineering, moving from single-component solutions to synergistic multi-material systems that address multiple performance metrics simultaneously.

The fabrication process employed in this study utilized a spin-coating technique, a method widely used in semiconductor manufacturing to deposit thin, uniform layers of material onto substrates. The researchers prepared solutions containing carbon nanotubes and zinc oxide at varying concentrations of one, two, and three milligrams per milliliter. These solutions were then deposited onto glass substrates at different rotational speeds of 1,000, 2,000, and 3,000 revolutions per minute. The spin-coating speed is a critical parameter in this process, as it determines the thickness and density of the resulting film. Higher speeds generally produce thinner films, which can enhance transparency but may compromise conductivity if the material becomes too sparse. By systematically varying both the concentration and the speed, the team was able to map out the optimal conditions for achieving the best balance between light transmission and electrical performance.

To evaluate the quality of the fabricated electrodes, the researchers employed a suite of advanced characterization techniques. Ultraviolet-visible spectroscopy was used to measure the optical transmittance, determining how much visible light could pass through the material. A four-point probe system was utilized to measure the sheet resistance, a key indicator of how easily current can flow across the surface of the electrode. Additionally, scanning electron microscopy and atomic force microscopy provided detailed images of the surface morphology, allowing the team to observe the distribution of carbon nanotubes within the zinc oxide matrix. These microscopic insights are crucial for understanding how the physical structure of the composite influences its macroscopic electrical and optical properties.

The results of the study revealed that the composite electrodes exhibited transmittance values ranging from 83.98 to 87.30 percent, which is highly competitive with standard ITO films. The sheet resistance values varied between 48.01 and 91.83 ohms per square, indicating good electrical conductivity. The researchers calculated a figure of merit for each sample, a metric that combines transmittance and resistance to provide a single value representing overall performance. The highest figure of merit recorded was 48.95, achieved at a concentration of two milligrams per milliliter and a spin-coating speed of 2,000 revolutions per minute. This specific combination yielded the optimal balance, demonstrating that there is a sweet spot in the fabrication parameters where the material performs best.

Interestingly, the conditions for maximum transmittance and minimum resistance did not align perfectly. The highest transmittance of 87.30 percent was observed at a concentration of two milligrams per milliliter and a speed of 3,000 revolutions per minute. Conversely, the lowest sheet resistance of 48.01 ohms per square was found at a concentration of one milligram per milliliter and a speed of 2,000 revolutions per minute. This trade-off is typical in materials science, where optimizing one property often requires compromising another. The researchers also calculated the normalized utilization factor, a measure of how effectively the material uses its optical and electrical capabilities. The lowest value, indicating the most efficient use of the material’s potential, was 4.72 times 10 to the power of negative three, again occurring at the two milligrams per milliliter and 2,000 revolutions per minute condition. This consistency in the optimal condition for the figure of merit and the utilization factor reinforces the reliability of the findings.

The integration of carbon nanotubes into the zinc oxide matrix appears to play a pivotal role in enhancing the electrical properties of the electrode. The nanotubes likely form a percolating network within the oxide film, providing pathways for electron transport that reduce overall resistance. Meanwhile, the zinc oxide maintains the high transparency necessary for solar cell applications. The surface morphology analysis likely showed that the carbon nanotubes were well-dispersed within the zinc oxide, preventing aggregation that could scatter light or create insulating barriers. This uniform distribution is essential for achieving the high performance metrics reported in the study. The ability to control this dispersion through the spin-coating parameters highlights the importance of process optimization in nanomaterial fabrication.

The implications of this research extend beyond the specific laboratory results to the broader landscape of renewable energy technology. As the demand for solar energy continues to grow, the need for scalable and cost-effective manufacturing processes becomes increasingly urgent. The use of solution-processed materials like the carbon nanotube-zinc oxide composite offers several advantages over traditional vacuum-based deposition methods. Solution processing is generally less energy-intensive and can be applied to flexible substrates, opening up possibilities for lightweight and bendable solar cells. Furthermore, the avoidance of indium reduces the environmental impact associated with mining and processing this rare metal. This aligns with global efforts to develop sustainable materials that minimize resource depletion and ecological harm.

Despite the promising results, the study acknowledges that further optimization is required to fully compete with established ITO technologies. The sheet resistance values, while acceptable, are higher than those of high-performance ITO films, which can achieve resistances in the single digits of ohms per square. Future research may focus on increasing the density of the carbon nanotube network or introducing additional dopants to enhance conductivity without sacrificing transparency. Additionally, the long-term stability of these composite electrodes under environmental conditions such as humidity and temperature fluctuations needs to be thoroughly investigated. Solar cells are exposed to harsh outdoor environments for decades, and any degradation in the electrode performance over time could significantly impact the overall efficiency and lifespan of the device.

In conclusion, the development of carbon nanotube-zinc oxide transparent conductive electrodes represents a significant step forward in the search for ITO alternatives. By carefully optimizing the concentration and spin-coating speed, researchers have demonstrated that it is possible to achieve favorable optical and electrical properties suitable for thin-film solar cells. The highest figure of merit and lowest normalized utilization factor obtained at specific conditions provide a clear roadmap for future fabrication efforts. As the renewable energy sector continues to expand, innovations in electrode materials will play a crucial role in reducing costs and improving efficiency. This study contributes valuable data to the field, highlighting the potential of hybrid nanomaterials to meet the demands of next-generation photovoltaic technologies. The work underscores the importance of interdisciplinary approaches that combine materials science, physics, and engineering to solve complex energy challenges.

Subject of Research: Development of carbon nanotube-zinc oxide composite transparent conductive electrodes for photovoltaic applications

Article Title: Fabrication of carbon nanotube–zinc oxide transparent conductive electrodes and evaluation for thin-film solar cells

Article References: Adio, A. B., Ajao, A. M., & Atere, A. D. (2026). Fabrication of carbon nanotube–zinc oxide transparent conductive electrodes and evaluation for thin-film solar cells. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13772-y

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13772-y

Keywords: carbon nanotubes, zinc oxide, transparent conductive electrodes, thin-film solar cells, spin-coating, optoelectronics, renewable energy, materials science, sheet resistance, optical transmittance, Fabrication, carbon