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

The appeal of carbazole begins with its electrons. Because the fused ring system is rigid, planar and richly π-conjugated, electron density can delocalize across all three rings, giving carbazole derivatives high fluorescence efficiency, efficient charge transport and excellent thermal stability. The nitrogen atom in the central ring further enhances charge-transfer behaviour, and derivatives that form charge-transfer complexes display red-shifted absorption and emission. These photophysical virtues explain why carbazole-containing polymers and small molecules have become staples of organic light-emitting diodes, organic field-effect transistors, photovoltaic cells and sensors. As hole-transport materials, they even suppress crystallization in small-molecule devices, extending operational lifetimes. More surprisingly, carbazole-based microporous polymers, built by exploiting the reactive hydrogen atoms at the 3- and 6-positions of the ring, can adsorb carbon dioxide at ambient temperature thanks to the electron-withdrawing nature of the nitrogen atom, opening an unexpected environmental application for this optical workhorse.

The medicinal story is equally compelling. The planar tricyclic framework facilitates π-π stacking and hydrogen bonding with DNA, enzymes and receptor proteins, allowing carbazole derivatives to interact with a striking range of biological targets. Compounds built on this scaffold have shown anti-cancer, anti-bacterial, anti-viral, anti-inflammatory and anti-oxidant activities, and strategic modification at various ring positions has tuned their pharmacokinetic and pharmacodynamic properties. It is this dual identity, drug lead and electronic material in one molecule, that makes the synthetic chemistry of carbazole worth cataloguing in exhaustive detail.

Ansari and Ali organize their review around named reactions, the eponymous transformations that serve as the shared vocabulary of synthetic organic chemistry. Just as doctors must know the names of organs, the authors argue, chemists must know their named reactions: the labels compress substrates, conditions and mechanisms into a single phrase that lets researchers think instantly about what a transformation can and cannot do. The review spans classical nineteenth-century chemistry, including the Bucherer, Graebe-Ullmann, Borsche-Drechsel and Fischer indole syntheses, alongside modern metal-catalyzed and photocatalytic methods such as the Ackermann reaction, Cadogan cyclization, Knölker synthesis and ring-closing metathesis.

The classical routes remain surprisingly relevant. The Borsche-Drechsel cyclization, first reported by Edmund Drechsel in 1888 and elaborated by Walther Borsche in 1908, condenses arylhydrazines with cyclohexanones to form hydrazones that undergo protonation, ene-amine tautomerization and a [3,3]-sigmatropic rearrangement, ultimately delivering tetrahydrocarbazoles that can be aromatized to the parent scaffold. This venerable sequence has carried chemists to an impressive roster of natural products, including murrayanine from the stem bark of Murraya koenigii, murrayacine, clausenol and the antifungal algal alkaloids Tjipanazole D and I. In a green-chemistry twist, Kotha’s team recently performed the Borsche-Drechsel cyclization in a deep eutectic solvent, a low-melting mixture of N,N’-dimethyl urea and L-(+)-tartaric acid, replacing harsh classical media with a sustainable alternative.

The Fischer indole synthesis, discovered by Emil Fischer in 1883, follows a closely related mechanistic logic: phenylhydrazines condense with aldehydes or ketones to form hydrazones, which tautomerize, rearrange sigmatropically and cyclize with loss of ammonia to deliver indole frameworks, including fused carbazoles. It was this strategy that Bonjouklian and co-workers used to reach Tjipanazole E and B, chlorinated carbazole alkaloids bearing N-glycosidic substituents that were first isolated from the blue-green alga Tolypothrix tjipanasensis. Meanwhile, the Bucherer carbazole synthesis, reported by Hans Theodor Bucherer in 1908, couples naphthols with arylhydrazines under acidic bisulfite conditions without any transition metal, proceeding through hydrogen sulfite addition, a [3,3]-sigmatropic rearrangement and final ammonia elimination to yield benzo-fused carbazoles.

On the modern side, palladium catalysis dominates. The Buchwald-Hartwig coupling, independently discovered by Stephen Buchwald and John Hartwig in 1994, forms carbon-nitrogen bonds between amines and aryl halides through the canonical catalytic cycle of oxidative addition, amine binding, deprotonation and reductive elimination. Zhou and Verkade showed that a double N-arylation of primary amines with 2,2′-dihalobiphenyls, mediated by a proazaphosphatrane ligand, delivers N-substituted carbazoles in a single operation. The method’s excellent functional-group tolerance made it the key step in the 2008 total synthesis of murrayazoline, an antiplatelet carbazole alkaloid from the genus Murraya. The related Ackermann reaction chains a Buchwald-Hartwig coupling with a direct arylation to build carbazoles from simple anilines and 1,2-dihaloarenes, a domino strategy that was used to synthesize murrayafoline A, a growth-inhibitory and fungicidal alkaloid isolated from Rutaceae plants.

Other modern methods trade metal catalysis for other tricks. The Cadogan cyclization, reported by John Cadogan in 1974, reductively cyclizes o-nitrobiphenyls with triethyl phosphite or triphenylphosphine, proceeding through nitroso intermediates and either a nitrene or a direct cyclization pathway; it underpinned the synthesis of siamenol, an anti-HIV carbazole alkaloid from Murraya siamensis. Microwave irradiation now compresses Cadogan reaction times from hours to minutes, and a visible-light variant using the organic photocatalyst 4CzIPN under blue LEDs delivers carbazoles in 40 to 87 percent yields. The Knölker synthesis, introduced by Hans-Joachim Knölker in 1999, takes a different approach entirely: a tricarbonyliron fragment protects a reactive cyclohexadiene, enforces anti-selective stereochemistry and enables hydride abstraction with triphenylmethyl tetrafluoroborate, a sequence that made possible the first total synthesis of the complete antiostatin A and B families. Ring-closing metathesis, the Nobel-recognized chemistry of Grubbs, Schrock and Chauvin, closes carbazole rings from pre-installed dienes under mild conditions and delivered murrayaquinone A, while the Wulff-Dötz benzannulation, in which a Fischer chromium carbene complex reacts with an alkyne through carbon monoxide insertion and 6π-electrocyclization, constructed the carbazole core of the Aspidospermidine alkaloids.

What emerges from the survey is not merely a catalogue but a decision framework. Each reaction carries trade-offs: the Graebe-Ullmann synthesis is reliable but demands temperatures above 200 degrees Celsius; the Cadogan cyclization is metal-free but generates stoichiometric phosphine oxide waste; the Buchwald-Hartwig coupling is modular but requires costly palladium and pre-halogenated substrates; the Knölker synthesis offers stereocontrol but demands specialized iron chemistry. The diverted Bischler-Napieralski cascade reported by Faltracco in 2021 illustrates how even well-worn reactions can surprise, delivering an unexpected carbazole instead of the anticipated dihydro-β-carboline through a spirointermediate pathway. By assembling these options side by side, with their mechanisms, substrate requirements and validated natural product targets, the review gives synthetic chemists a practical map for choosing the right tool, and gives students a masterclass in how the vocabulary of named reactions organizes more than a century of heterocyclic chemistry.

Subject of Research: Named reactions for the synthesis of carbazole and their mechanistic and applied significance

Article Title: Named reactions in Carbazole synthesis: Mechanistic insights and potential applications

Article References: Ansari, T., & Ali, R. (2026). Named reactions in Carbazole synthesis: Mechanistic insights and potential applications. Results in Chemistry, 31, Article 103913. https://doi.org/10.1016/j.rechem.2026.103913

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103913

Keywords: carbazole, named reactions, organic synthesis, heterocyclic chemistry, Buchwald-Hartwig coupling, Fischer indole synthesis, Knölker synthesis, ring-closing metathesis, natural products, OLEDs, medicinal chemistry, palladium catalysis