Direct laser writing of high aspect ratio nanochannels for nanofluidics

Nanochannels with high width-to-height aspect ratios are desirable for many applications, particularly those requiring optical access, but remain challenging to fabricate. In this work, the direct laser writing of such channels between diamond films and glass substrates is introduced. As previously reported, laser light can transform a portion of diamond film into a nanostrip. The strip induces delamination of the surrounding film, causing the formation of two nanochannels with triangular cross-sections. Here, it is demonstrated that nanochannels with rectangular cross-sections and width-to-height aspect ratios exceeding fifty can form between pairs of nanostrips. With atomic force microscopy, the maximum strip spacing that produces these nanochannels is investigated, and it is demonstrated that the reflectance of the channels can be measured by microspectrophotometry. The microstructure of the nanochannels, including nanostrips, and processes that occur during laser writing are inferred from transmission electron microscopy and electron energy loss spectroscopy. By fabricating a nanofluidic device and using microspectrophotometry, it is found that the nanochannels fill with water through capillary action, are resistant to clogging, and are mechanically stable against water filling. A versatile platform for producing high-aspect-ratio nanochannels that are optically accessible and fluidically functional is presented, thereby expanding opportunities for advanced applications.

💡 Research Summary

In this paper the authors present a novel method for fabricating high‑aspect‑ratio nanochannels (width‑to‑height ratio exceeding 50) directly between polycrystalline diamond (PCD) films and glass substrates using femtosecond laser writing. Building on their earlier work, where a single laser‑induced nanostrip caused delamination of the surrounding diamond film and produced two triangular nano‑slits, they now demonstrate that writing two parallel nanostrips can generate a single rectangular nanochannel sandwiched between them.

The experimental procedure involves scanning a focused femtosecond laser beam across the PCD/glass interface while varying the pulse energy (E). Atomic force microscopy (AFM) shows that both the nanostrip width and the resulting channel height increase monotonically with E. By measuring the maximum centre‑to‑centre spacing (dM) that still yields a continuous rectangular channel, the authors find a linear relationship between dM and the average channel height ho: essentially dM ≈ ho. When the spacing exceeds this critical value, the two delamination fronts evolve independently into separate wedge‑shaped cavities, and the rectangular geometry collapses into two triangular slits. This behaviour is interpreted as a competition between the elastic energy released by film relaxation and the interfacial adhesion energy, mediated by a laser‑induced transformation of diamond into a non‑diamond carbon phase (predominantly amorphous carbon) near the substrate interface.

Transmission electron microscopy (TEM) combined with electron‑energy‑loss spectroscopy (EELS) reveals the detailed microstructure of the nanostrips and channels. The nanostrips consist of a core of crystalline diamond surrounded by a thick layer of amorphous carbon. This amorphous layer, especially pronounced at the strip‑substrate interface, reduces the Young’s modulus locally, facilitating delamination, while simultaneously providing mechanical support to the channel walls during fluid filling.

Optical characterization is performed with microspectrophotometry. The reflectance (R) of a flat PCD/glass bilayer (no channel) is about 0.12, whereas channels with heights above ~300 nm exhibit reflectance values between 0.25 and 0.35. The increase is attributed to multiple internal reflections and interference within the channel cavity; finite‑difference simulations reproduce the measured spectra, confirming that the channels are optically accessible.

To assess fluidic functionality, the authors integrate the channels into a simple nanofluidic device and introduce water. Capillary action instantly fills the channels, and repeated filling‑draining cycles (≥10) show no evidence of clogging. The amorphous carbon coating appears to suppress particle adhesion, while the high Young’s modulus and thermal conductivity of the surrounding diamond film maintain structural integrity under the hydrostatic pressure of the water column.

Overall, the study establishes a practical design rule (dM ≈ ho) for producing rectangular nanochannels with aspect ratios far beyond those achievable with conventional lithography or focused‑ion‑beam techniques. The use of inexpensive, scalable materials (CVD‑grown diamond and standard glass) and a single‑step laser process makes the approach attractive for large‑scale production. Potential applications include optofluidic sensing, DNA stretching experiments, thermal management in micro‑electronics, and energy‑harvesting platforms where simultaneous optical access and robust fluid handling are required.