SiO₂ Electret Formation by Electrode-Free Electrochemical Nanolithography: A Chemophysical Surface Functionalisation

1. Introduction

SiO₂ is one of the most widely used dielectric materials in modern technology, serving both passive and active roles. Beyond its established use as an insulating layer in silicon-based electronics, SiO₂ can acquire active functionalities by surface functionalization. These include charge trapping [1,2,3], optical sensing [4], enhancing charge transport in organic semiconductors [5], regenerable resistive switching [6] and controlled material growth or assembly [7]. These capabilities extend its relevance from microelectronics to emerging fields including organic electronics [5], memories [6] and surface sensing [4].

Here, we demonstrate the formation of SiO₂ electrets through a novel approach that enables precise submicrometre patterning.

Electrets, known for over a century, are often considered the electrostatic analogues of permanent magnets. [8] Their ability to produce persistent electric fields has enabled a broad spectrum of applications [9,10], including microphones [11], nanoxerography, [12] magnetic [13] and pressure [14] photo sensors [15], actuators, [16] energy harvesting systems [17], soft electronics, [18] and silicon-based photovoltaic technologies [10,19]. Many of these applications rely on precise spatial control of charge localisation. Common inorganic electrets include SiO₂ [1,3,19,20], Si₃N₄ [21], and transition metal oxides [22], while organic electrets are typically polymers or molecular compounds [18]. However, despite their technological importance, electret materials are poorly studied, and the diversity of materials and methods for fabricating electrets remains limited. Conventional approaches rely on strong electric fields to induce dipole alignment or structural changes, such as local amorphisation, reduction, or doping [20]. Spatial control is usually achieved using techniques like scanning probe lithography [22] or electrified microcontact printing [18].

In this work, we introduce a novel strategy for the formation of SiO₂ electrets introducing Electrode-Free Electrochemical Nanolithography. This method enables precise submicrometer patterning without direct electrode contact by applying alternating current via capacitive coupling between an electrified probe and a conductive thin film on an insulating substrate. The resulting chemophysical surface functionalisation was characterised using Kelvin Probe Force Microscopy [23] and Electric Force Microscopy [24], confirming the successful creation of patterned electret regions. This approach opens new avenues for scalable, high-resolution electret fabrication and broadens the functional versatility of SiO₂ in advanced electronic and sensing applications.

2. Results and Discussion

2.1. Method

To functionalise SiO₂ films, we used Electrode-Free Electrochemical Nanolithography (EFEN). EFEN was originally introduced as a scanning-probe-based technique, called Electrode-Free Anodic Oxidation Nanolithography [25], for electrochemically assisted etching of graphene deposited on insulating substrates. EFEN operates by applying a high-frequency alternating current (>10 kHz) via capacitive coupling between a conductive probe and the substrate, triggering electrochemical reactions at the probe-surface interface. This setup removes the need for direct electrical contact, enabling localised processing of insulating materials, although it does not achieve the same level of control as traditional electrochemical nanolithography (e.g., precise local oxidation or reduction) [26,27].

In this study, EFEN was adapted for parallel processing to enable large-area patterning. Instead of an AFM probe, a conductive stamp composed of parallel lines was used. Under high-humidity conditions (~95% RH), a nanometric water meniscus naturally forms between the stamp protrusions and the SiO₂ surface, creating a confined nano-electrochemical cell. When an AC bias is applied through the conductive stamp, localised electrochemical reactions occur only beneath the protrusions, reproducing the stamp geometry on the substrate. Beyond spatial control, EFEN allows fine-tuning of local effects by adjusting the applied bias, process duration, or repeating the treatment in targeted regions. Figure 1 shows the scheme of the process.

To demonstrate EFEN’s effectiveness for electret fabrication, experiments were conducted on thermally grown SiO₂ films deposited on highly doped silicon substrates, commonly used in organic electronics. SiO₂ is a well-known electret material. The treatment involved applying an AC bias of 15 V at 50 kHz for varying durations under 95% relative humidity. The stamp used was a polymer replica of a blank compact disc coated with a 100 nm gold layer (Figure 2a). The stamp consisted of parallel lines with a thickness of 220 nm and a spacing of 1.5 µm.

For short treatments (<1 hour), EFEN leaves the SiO₂ morphology essentially unaltered. Prolonged exposure (> 1 hour) induces only minor roughness changes (RMS < 0.2 nm), within the range of instrumental uncertainty.

To assess the impact of EFEN on the electronic properties of SiO₂, we used Electric Force Microscopy (EFM) and Kelvin Probe Force Microscopy (KPFM).

2.2. Electric Force Microscopy

EFM is highly sensitive to nanoscale variations in surface charge distribution, whereas KPFM maps surface potential (SP), providing insight into changes in work function and local charge states.

EFM operates in a two-pass mode, detecting electrostatic forces between the oscillating probe and the sample [24]. These forces generate a gradient that induces a phase shift in the cantilever oscillation. For small oscillation amplitudes, the interaction can be decomposed into capacitive forces (quadratically dependent on the applied EFM voltage, (V_EFM) and Coulombic forces, linearly dependent on VEFM due to static charges or multipoles on the surface [28]:

$$∆\varphi \propto F^{\prime }\approx {\displaystyle \frac{1}{2}}{C}_{s-t}^{\prime }{V}_{EFM}^2+E_s{C}_{s-t}^{\prime \prime }{V}_{EFM}$$

(1)

where Δϕ is the Phase shift, F’ is Force gradient C’_s-t and C’’_s-t are the first and second derivative capacitances with respect to the tip–sample distance, respectively and E_S is Electric field caused by static charge.

Although EFEN did not produce significant morphological changes, EFM imaging revealed clear contrasts replicating the stamp pattern. Figure 3a shows a representative EFM phase image of a patterned SiO₂ film. Voltage-dependent EFM measurements (Figure 3b) reveal that pristine SiO₂ responds with a purely quadratic phase–bias dependence, characteristic of neutral dielectrics. In contrast, EFEN-treated regions show a pronounced linear component and a negative shift of the phase maximum, demonstrating the presence of trapped negative charges.

2.2. Kelvin Probe Force Microscopy

KPFM measurements further demonstrated a decrease in surface potential (SP) within patterned regions (Figure 4a). The SP contrast increases approximately linearly with treatment duration up to 60 min up to -1.7 V (Figure 4b), beyond which local film degradation or increased roughness affect measurement accuracy. The trapped charge density (σ) was estimated using a parallel-plate capacitor model:

$$\mathsf{\Delta }SP={\displaystyle \frac{\sigma h_{\mathrm{e}\mathrm{f}\mathrm{f}}}{{\epsilon }_0{\epsilon }_r}}$$

(2)

where: ΔSP is the surface potential contrast, h_eff is the effective dielectric thickness of the SiO₂ layer (~200 nm), and ε_r ≈ 3 is its relative permittivity. Using these parameters, extended EFEN treatments yield surface-change densities up to σ ≈ 300 nC·cm⁻², fully consistent with reported values for SiO₂-based electrets. This model provides a reliable first-order estimate because the tip radius (≈20 nm) is significantly larger than the patterned features, ensuring that the local electrostatic interaction can be approximated as a macroscopic capacitive geometry.

Finally, EFEN processing can be repeated within the same region to increase charge density, expand coverage, or create complex patterns (Figure 5).

4. Conclusions

We demonstrated a parallel implementation of Electrode-Free Electrochemical Nanolithography that can produce patterned electrets over areas up to 1 cm² using standard laboratory equipment. The method maintains a slightly lower precision than scanning-probe techniques but enables high-throughput processing, which is essential for technological applications. Applied to thermally grown SiO₂, EFEN achieves high-quality charge patterning and chemical surface functionalisation, highlighting its potential for integration in advanced electronic and sensing devices. Because the mechanism relies on capacitive coupling rather than direct electrical contact, the method is compatible with a wide range of inorganic and organic substrates. EFEN therefore offers a versatile and scalable route to engineered electrets and functionalised surfaces.