Andreas Kyritsakis

External electric fields influence surface diffusion, field evaporation and nanoscale morphology, but their atomistic simulation remains computationally demanding. We introduce SURFIELD, a physics-guided machine-learning framework for metallic surfaces under external electric fields. The framework learns local dipole moments, polarizabilities and zero-field potential energies, then combines them with response theory to calculate field-induced and total forces. Directional smooth overlap of atomic positions (D-SOAP) augments SOAP with four quaternion components to encode environmental orientation. For lithium and tungsten surfaces, test normalised root-mean-square errors are 3.7–3.8% for dipole moments, 3.4–5.6% for polarizabilities and 6.7–10.7% for field-induced forces. DFT-derived forces on a flat lithium slab reproduce analytical Maxwell stress, while finite-field calculations support the linear-response approximation over the investigated range. Under the reported benchmark conditions, SURFIELD is approximately 389 times faster than DFT, providing an efficient approach to modelling the surface-normal electric-field response of metallic surfaces.

The theoretical understanding of thermal-field emission (TFE) from semiconductors has been limited to 1D and 2D models. This can be attributed to the complex and interdependent phenomena involved in TFE from semiconductors, which makes the calculations cumbersome. Such limitations result in a partial understanding of the underlying physics of the surface of a semiconductor under a high electrical field—which requires the addition of the temporal dimension (4D) to yield a realistic model of the electro-thermal dynamics. Here, we develop a 3D model of TFE from semiconductors that covers arbitrary geometries and doping levels. Our model successfully reproduces the characteristic saturation plateau of some semiconductors, as well as their temperature dependence. The model gives good qualitative agreement with experimental data from n-type germanium. We propose this model as a platform for future extensions into the full 4D framework, incorporating temporal dynamics for a more coherent explanation of thermal-field emission from semiconductors.

Semiconducting field emitters present some interesting features (e.g., self-limited electron emission) for both scientific interest and industrial applications. The analysis of experimental results and device design has been restrained by the lack of accurate 3D models for the simulation of thermal-field emission from semiconductors. Here, we review and correct the equations of field emission from semiconductors and include them to expand GETELEC (General Tool for Electron Emission Calculations). Our method covers all electron emission regimes (field, thermal, and intermediate), aiming to maximize the calculation accuracy while minimizing the computational cost. GETELEC-2.0 is able to reproduce the characteristic non-linear I-V curves in Fowler–Nordheim coordinates obtained from semiconductors, giving insights about their nature, as well as providing an explanation to the lack of experimental observation of the field emitted valence band electrons from semiconductors.

Microfluidics is a technology that manipulates liquids on the scales ranging from microliters to femtoliters. Such low volumes require precise control over pressures that drive their flow into the microfluidic chips. This article describes a custom-built pressure controller for driving microfluidic chips. The pressure controller features piezoelectrically controlled pressure regulation valves. As an open-source system, it offers high customizability and allows users to modify almost every aspect. The cost is roughly a third of what similar, alternative, commercially available piezoelectrically controlled pressure regulators could be purchased for. The measured output pressure values of the device vary less than 0.7% from the device’s reported pressure values when the requested pressure is between −380 and 380 mbar. Importantly, the output pressure the device creates fluctuates only ±0.2 mbar when the pressure is cycled between 10 and 500 mbar. The pressure reading accuracy and stability validation suggest that the device is highly feasible for many advanced (low-pressure) microfluidic applications. Here, we compare the main features of our device to commercially and non-commercially available alternatives and further demonstrate the device’s performance and accessibility in successful microfluidic hydrodynamic focusing (MHF)-based synthesis of large unilamellar vesicles (LUVs).

In advanced drug delivery, versatile liposomal formulations are commonly employed for safer and more accurate therapies. Here we report a method that allows a straightforward production of synthetic monodisperse (~ 100 μm) giant unilamellar vesicles (GUVs) using a microfluidic system. The stability analysis based on the microscopy imaging showed that at ambient conditions the produced GUVs had a half-life of 61 ± 2 h. However, it was observed that ~ 90% of the calcein dye that was loaded into GUVs was transported into a surrounding medium in 24 h, thus indicating that the GUVs may release these small dye molecules without distinguishable membrane disruption. We further demonstrated the feasibility of our method by loading GUVs with larger and very different cargo objects; small soluble fluorescent proteins and larger magnetic microparticles in a suspension. Compared to previously reported microfluidics-based production techniques, the obtained results indicate that our simplified method could be equally harnessed in creating GUVs with less cost, effort and time, which could further benefit studying closed membrane systems.

Nano-protrusion (NP) on metal surface and its inevitable contamination layer under high electric field is often considered as the primary precursor that leads to vacuum breakdown, which plays an extremely detrimental effect for high energy physics equipment and many other devices. Yet, the NP growth has never been experimentally observed. Here, we conduct field emission (FE) measurements along with in-situ Transmission Electron Microscopy (TEM) imaging of an amorphous-carbon (a-C) coated tungsten nanotip at various nanoscale vacuum gap distances. We find that under certain conditions, the FE current-voltage (I-V) curves switch abruptly into an enhanced-current state, implying the growth of an NP. We then run field emission simulations, demonstrating that the temporary enhanced-current I-V is perfectly consistent with the hypothesis that a NP has grown at the apex of the tip. This hypothesis is also confirmed by the repeatable in-situ observation of such a nano-protrusion and its continued growth during successive FE measurements in TEM. We tentatively attribute this phenomenon to field-induced biased diffusion of surface a-C atoms, after performing a finite element analysis that excludes the alternative possibility of field-induced plastic deformation.

High electric fields affect the diffusion dynamics of atoms on a metal surface, causing biased surface diffusion that possibly leads to the growth of intensively field emitting protrusions and consequent vacuum breakdown (VBD). The scientific understanding of this process, as well as other fundamental VBD initiation mechanisms, is far from complete. Here we investigate the exact atomic behaviour of metal surfaces exposed to extremely high electric fields using density functional theory (DFT). Previous theories describe the field-surface dynamics in terms of the effective dipole moments and polarizability of surface atoms, disregarding higher-order (hyperpolarizability) terms. The validity of this approximation has been evaluated only for electric fields up to 3 GV/m, due to computational limitations of the plane-wave DFT basis used in previous works. In this work, we test the validity of this approximation for a much wider field range, relevant for VBD and field emission (FE), using Cu nanoparticles as our test structures. We find that although such high fields can change the entire structure of Cu nanoparticles, their energetics are described very precisely by the permanent dipole moment and polarizability terms. Thus, we show that neglecting the hyperpolarizability terms is valid even for field values that exceeds the range that is relevant for intense FE and VBD. This work lays a solid foundation for further developing atomic-level simulation models for electric field-induced surface diffusion on metal surfaces and its effects on protrusion growth and VBD initiation.