Direct Laser Writing of Nanochannels Between Ultra-Thin Nanocrystalline Diamond Films and Glass Substrates

The fabrication of nanofluidic devices, which have one dimension below the 100 nm cornerstone, is complex, expensive, and time-consuming due to the small length scales involved. Direct femtosecond laser writing of nanochannels might overcome the inherent challenges; however, progress in this field is limited. Still, encouraging developments have been made. For example, laser writing can produce nanocavities in thin nickel films [1]. Films with such cavities may offer a wide range of possibilities for magnetism-related research but are not transparent in the visible spectrum, which excludes optical microscopy for nanofluidic research. Laser writing can also produce nanochannels in fused silica substrates [2]. These channels are usually perpendicular to the surface of the substrates, have variable diameters, are open on one side, and are typically no more than ten micrometers long.<br/><br/>This talk presents a method for fabricating nanofluidic devices through the direct femtosecond laser writing of arbitrary long nanochannels between nanocrystalline diamond (NCD) films and glass substrates. Laser writing transforms a portion of the sample into a nanostrip, and the nanostrip is surrounded by two nanochannels generated by film delamination. For an NCD film with an as-deposited thickness of 300 nm, the mean height and the width of a nanochannel made in this way are on the order of 30 nm and 2 μm, respectively, and are tunable to a certain extent by laser pulse energy. Experiments indicate that a nanostrip consists of NCD, non-diamond carbon, and glass particles, which might be mixed with carbon due to laser ablation. It is found that the expansion of the sample material shapes a nanostrip and causes delamination and nanochannel formation. Such an expansion happens, for example, during the transformation of diamond to carbon of other forms [3]. Since the pulse energy needed to ablate the bare glass substrate is much greater than those used for nanochannel writing, one can deduce that nanostrip formation is initiated in the NCD film near the film–substrate interface. The origin of this phenomenon might be the presence of low-quality NCD at that location, which is bound to absorb more light than high-quality NCD [4]. By fabricating a nanofluidic device and performing simulations, it is shown that the nanochannels fill with water through capillary action.<br/><br/>NCD is an appealing material for applications due to its biocompatible, chemical, mechanical, electrical, optical, and quantum properties. Low-cost diamond films can be achieved by plasma-assisted chemical vapor deposition with methane gas diluted in molecular hydrogen as a precursor mixture. With this deposition technique, NCD films are commonly synthesized on substrates seeded with nanodiamonds [5]. NCD can also be doped to make it electrically conductive, paving the way for various applications [6]. Being inexpensive, transparent, and chemically inert, glass is a natural choice for research in micro- and nanofluidics. The coefficient of thermal expansion can also be tuned towards that of other materials, allowing the fabrication of structures with relatively low residual stress. Since NCD films are inert to hydrofluoric acid (HF), NCD can act as an etch stop during the HF etching of glass [7]. Here, we rely on this property for fabricating the nanofluidic device.<br/><br/>[1] V. V. Temnov et al., Nano Lett. 2020, 20, 7912–7918.<br/>[2] Y. Li et al., Adv. Photon. Nexus 2022, 1, 026004.<br/>[3] A. Courvoisier et al., Appl. Phys. Lett. 2016, 109, 031109.<br/>[4] V. V. Kononenko et al., Appl. Phys. Lett. 2019, 114, 251903.<br/>[5] S. Mandal, RSC Adv. 2021, 11, 10159–10182.<br/>[6] S. D. Janssens et al., Appl. Phys. Lett. 2014, 104, 073107.<br/>[7] S. D. Janssens et al., Appl. Phys. Lett. 2020, 116, 193702.