Direct laser writing of high aspect ratio nanochannels for nanofluidics

TL;DR

This work tackles the fabrication challenge of high width-to-height ratio nanochannels that are optically accessible for nanofluidics. It introduces a direct femtosecond-laser writing approach to form rectangular nanochannels between parallel nanostrips in a polycrystalline diamond film on glass, achieving aspect ratios greater than $50$ and enabling capillary water filling. Comprehensive TEM/EELS characterization reveals an amorphous carbon layer that supports delaminated regions, while microspectrophotometry and transfer-matrix simulations connect channel height to reflectance and demonstrate water-filled state detection. The resulting platform offers a scalable, cleanroom-free route to integrated optofluidic devices with robust mechanical stability for lab-on-a-chip applications.

Abstract

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.

Figures (8)

• Figure 1: Introduction.a Schematic illustrating the direct formation of nanochannels between a polycrystalline diamond (PCD) film and a glass substrate by laser writing Janssens2023. Laser light locally transforms a portion of the sample into a nanostrip that induces delamination of the surrounding film. The strip consists of non-diamond carbon that supports the delaminated film and originates from diamond through an allotropic transformation. The nanochannels have cross sections that resemble triangular slits and are several micrometers wide. b Schematic of the laser writing process that we investigate in this work. By writing two nanostrips parallel to each other, we demonstrate the formation of nanochannels with rectangular cross-sections and high aspect ratios. c Schematic of the nanofluidic device that we use to demonstrate that the channels can fill with water by capillary action.
• Figure 2: Nanochannel shape.a Scanning electron microscopy image of a rectangular nanochannel formed between two nanostrips. Each nanostrip is flanked by a triangular nanochannel. The orthonormal basis of a rectangular Cartesian coordinate system with axes $x$, $y$, and $z$ and origin $O$ is formed by vectors $\bm{\imath}$, $\bm{\jmath}$, and $\bm{k}$. Quantity $E$ denotes laser pulse energy, and $O$ is located at the film--air interface before laser writing. b-- d Reflected light microscopy image of nanochannels. e-- g Profile height $h$ of the structures in ( b-- d), respectively, obtained by averaging atomic force microscopy (AFM) data over $x$. The gray regions indicate the $y$-positions of the nanostrips, and a scale bar located at $y \leq 0$ coincides with the width of a corresponding triangular nanochannel.
• Figure 3: Maximum nanostrip spacing. Maximum spacing $d_{\text{M}}$ between the center lines of two adjacent nanostrips that still allows the formation of rectangular nanochannels, plotted as a function of the overall profile height $h_{\text{o}}$ of the resulting nanochannel. The height $h_{\text{o}}$ is determined by averaging the local channel height $h$ across the region between the center lines of the two nanostrips. Corresponding values of $E$ associated with each $h_{\text{o}}$ value are indicated next to the respective data points. The uncertainty in $d_{\text{M}}$ is the step size ($1~\upmu\text{m}$) used to measure that quantity, while the uncertainty in $h_{\text{o}}$ is the root mean square roughness of the PCD film (measured by AFM), which is significantly larger than other sources of uncertainties.
• Figure 4: Microstructure.a, b Panoramic transmission electron microscopy (TEM) images of a representative nanochannel fabricated with $E = 35$ nJ. c Enlarged view of the area within the dashed rectangle in ( a). d Electron energy loss spectroscopy (EELS) map of the area in ( c), obtained by investigating K edges. e-- g Representative EELS spectra of the regions in ( d). By comparing these spectra with those in the literature Chu2006Ayoola2020, we infer that regions 1, 2, and 3 contain diamond, amorphous carbon, and oxygen (substrate), respectively. The peak around 290 eV in ( e) is a signature of diamond, and the absence of peaks in ( f) corresponds to amorphous carbon. The peak around 540 eV in ( g) is characteristic of oxygen.
• Figure 5: Reflectance versus channel profile height.a Specular reflectance $R$ measured by microspectrophotometry for a $286\pm10$ nm thick PCD film on glass ($h=0$) and for nanochannels ($h>0$). The inset is a reflected light microscopy image of a rectangular nanochannel above which a mirror is placed. The mirror is the black square and is used to collect reflected light for the measurement of $R$, which is performed at the center of a channel. b Simulated values of $R$ for channel heights equal to $h$. The inset is a schematic of the model used for the calculations. The arrow indicates the propagation direction of the incident light beam. c Overall reflectance $R_\text{o}$ of $R$. The orange band represents the deviation in film thickness, which corresponds to the root mean square surface roughness of the film. For the experimental data, the uncertainty in $h$ is the surface roughness, and the uncertainty in $R_\text{o}$ is a standard deviation calculated from three independent measurements. The inset is a dark-field microscopy image of a rectangular nanochannel.