Electron and X‑ray Focused Beam-Induced Cross-Linking in Liquids: Toward Rapid Continuous 3D Nanoprinting and Interfacing using Soft Materials

Modern additive fabrication of three-dimensional (3D) micron to centimeter size constructs made of polymers and soft materials has immensely benefited from the development of photocurable formulations suitable for optical photolithography,holographic,and stereolithographymethods. Recent implementation of multiphoton laser polymerization and its coupling with advanced irradiation schemes has drastically improved the writing rates and resolution, which now approaches the 100 nm range. Alternatively, traditional electron beam lithography and its variations such as electron-beam chemical lithography, etc. rely on tightly focused electron beams and a high interaction cross-section of 0.1−10 keV electrons with the matter and have been routinely used for complex patterning of polymer resists, self-assembled monolayers, and dried gel films with up to a few nanometers accuracy.
Similarly, a significant progress has been made in deep X-ray lithography, direct writing with zone plate focused X-ray beams for precise, and chemically selective fabrication of high aspect ratio microstructures. Reduced radiation damage within the so-called “water window” has spurred wide biomedical X-ray spectroscopy, microscopy, and tomography research including material processing, for example, gels related controlled swelling and polymerization inside live systems, particles encapsulations,and high aspect ratio structures fabrication.The potential of focused X-rays for additive fabrication through the deposition from gas-phase precursors or from liquid solutions is now well recognized and is becoming an active area of research.
A group of researchers of the National Institute of Standards and Technology – USA coordinated by Andrei Kolmakov has used electron microscopy and the nanofocused X-ray beam of the Escamicroscopy beamline at Elettra to demonstrate that polymerization and cross-link chemical reactions can be initiated locally inside the liquid with focused beams in an addressable way. The beam writing has been done through the thin electron/X-ray transparent membrane separating high-vacuum equipment from the volatile precursor solution. Using a hydrogel as a model soft material, the researchers were able to perform in-liquid direct writing with a sub-100 nm resolution.
Fluidic (and closed) chambers equipped with 30−50 nm thin silicon nitride (SiN) membranes that isolate the liquid solution containing polymers from the vacuum of the microscope have been designed to deliver focused electron or soft X-ray beams to vacuum incompatible liquid solutions and for patterning and imaging in liquids. The crosslinking of the polymer in the solution and the control of the printing proces have been investigated by changing several parameters such as the photon/electron density, energy, dose, the electron/X-ray beam size, etc. Fig. 1-a), b) show exemplary 3D structures printed using in-liquid cross-linking by soft X-rays. The effect of the beam intensity and writing sequence on the linear feature size and morphology can be seen in the inset of Figure 1-c). The feature size rapidly increases with the beam intensity and then saturates. This is a result of the increase and saturation of the cross-link density inside the excitation volume. The feature size can be controlled effectively via printing with different X-ray photon energies just below and above the element-specific absorption edge. 
Compared to printing with electrons, the aspect ratio of the X-ray-induced structures can be appreciably larger, similar to the deep X-ray lithography results on solid films. In addition, the cross-link density of X-ray printed hydrogel structures was noticeably lower compared to their e-beam counterparts reflecting significant surface rippling of the features upon drying.
The technology proposed in this work can be implemented in any high-vacuum, environmental SEMs, atmospheric SEMs, synchrotrons, or laboratory-based X-ray microscopes. The tunability of X-ray energy at synchrotrons offers an additional opportunity to conduct element-specific 3D gel printing in solutions relevant to biomedical, soft microrobotics, electrochemical, and other applications. Moreover, the combination of this method with the recently proposed implosive fabrication techniquecan, in principle, result in nanometer-scale 3D printing.
 

Figure 1.    (a) Elettra printed with 536 eV 150 nm wide X-ray beam with 25 ms dwell time. Photon flux ca. 2 × 107ph/nm2s and 100 nm step size. (b) Dices printed with two photon energies: 13 μm base squares, 536 eV, and small 2.5 μm squares, 526 eV. (c) dwell time, and step size for different energies of electrons (blue) and X-rays (red) beams. The insets show the SEM image of gel structure written with variable beam current, dwell time along its length using 536 eV X-rays.

This research was conducted by the following research team:

Tanya Gupta^1,2, Evgheni Strelcov^1,2, Glenn Holland¹, Joshua Schumacher¹, Yang Yang¹, Mandy B. Esch¹, Vladimir Aksyuk¹, Patrick Zeller³, Matteo Amati³, Luca Gregoratti³ and Andrei Kolmakov¹

¹ National Institute of Standards and Technology, Gaithersburg, Maryland, USA; 
² Maryland NanoCenter, University of Maryland, Maryland, USA.
³ Elettra – Sincrotrone Trieste ScpA, Trieste, Italy.

Contact persons:

Andrei Kolmakov, email: