Hannah J. Joyce

Reliable pH sensing in liquid environments underpins applications in energy storage, biosensing, and process monitoring, yet conventional graphene-based electrical devices introduce ohmic contacts, encapsulation layers, and electrochemical artifacts that can obscure intrinsic behavior. These approaches also fail to capture the strong spatial variability of commercial CVD graphene. Here, we present a fully contactless methodology for quantifying pH-dependent conductivity of large-area CVD graphene using through-substrate reflection terahertz time-domain spectroscopy (THz-TDS). A customized liquid cell with a THz transparent window preserves unrestricted access to the graphene–electrolyte interface while enabling centimeter-scale conductivity mapping and in-operando single-point monitoring during sequential pH modulation. By integrating spatially registered Raman mapping with automated quality-metric filtering, we extract statistically robust trends across thousands of pixels rather than isolated device regions. We show that pristine monolayer graphene exhibits the highest baseline conductivity and strongest pH sensitivity, whereas O2-plasma-introduced defects systematically suppress both. This scalable, remote, and inert probing approach establishes THz-TDS as a powerful platform for high-throughput characterization and robust liquid-phase sensing in thin films and 2D systems.

Semiconducting InAs nanowires are promising building blocks for optoelectronics, but their performance is often complicated by an unusual negative photoconductivity (NPC) originating from surface charge traps. Harnessing the competition between this NPC and conventional positive photoconductivity (PPC) is key to unlocking novel device functionalities. Here, we systematically investigate and control the photoresponse of InAs nanowire transistors by varying the excitation wavelength, intensity, and nanowire diameter. We demonstrate precise, wavelength-controlled switching between a strong NPC, achieving a giant negative responsivity of ∼–8.9 × 103 AW–1 under UV light, and conventional PPC in the near-infrared. To explain this competing behavior, we employ a quantitative model based on hot-carrier trapping dynamics. The model reveals that the characteristic effective trap-filling time can be tuned by over three orders of magnitude by changing the excitation wavelength from UV to near-infrared, providing a simple but powerful mechanism to control charge trapping. This work provides a predictive framework for the rational design of advanced InAs nanowire devices, including high-performance photodetectors, optical memory, and neuromorphic optoelectronics.

Inorganic colloidal quantum dot light-emitting diodes could be used to build next-generation electroluminescent displays due to their colour properties and electrical stability. However, to create high-resolution and large-area displays, a pixel integration method is required, which can deposit quantum dot arrays on an active-matrix backplane and maintain uniformity and precision, without colour cross-contamination. Here we report a cracking-assisted transfer printing technology that can be used to pattern high-resolution full-colour pixel arrays over large areas. The technology uses a controlled cracking process to fracture interparticle cohesive bonds between quantum dots. This facilitates subsequent pick-up and transfer to a thin-film transistor backplane with high precision. With the technology, we achieve pixels down to a size of 600 nm with electroluminescent emission and uniform pixelization over areas up to 4 inches. We create a cadmium-free full-colour active-matrix display with a resolution of 341 pixels per inch, as well as a blue active-matrix display with a flexible form factor. Furthermore, the cracking-assisted transfer printing can improve electroluminescence performance—with higher maximum luminance and operational lifetime than other quantum dot patterning techniques—through precise nano-interface control and high quantum dot packing density.

Tungsten diselenide is a prominent candidate for nanoscale electronics and complementary metal–oxide–semiconductor technology due to its controllable p-type and n-type doping that is achievable through selective oxidation. However, characterizing the resulting oxide quality often relies on destructive or slow metrology. In this work, we demonstrate spectroscopic imaging ellipsometry (SIE) as a noninvasive technique to characterize the oxidation state of two-dimensional materials, providing spatially resolved measurements of layer thickness and uniformity. We apply this method to compare few-layer WSe2 after thermal or plasma-based oxidation treatments. We show that plasma oxidation enables a highly controllable conversion of individual WSe2 layers into uniform sub-stoichiometric tungsten oxide (WOx), whereas thermal oxidation in atmospheric conditions yields a rough interface characterized by non-uniform oxide formation and blistering. Using SIE, we quantify a layer-by-layer conversion ratio, where a single WSe2 monolayer transforms into ∼1.6 nm of WOx during plasma oxidation. These findings, corroborated by atomic force microscopy, highlight the capability of SIE to resolve dielectric evolution and interface quality in layered semiconductors.

We introduce a flexible, analytically grounded framework for designing continuous transmission filters for spectral imaging, based on principal component analysis (PCA) on the coefficients of material reflectance spectra when represented by orthogonal polynomial bases. This design strategy yields a closed-form solution that remains continuous, essential for the physical realization of optical transmission filters. This method exposes several tunable parameters ( α , β , N _terms ) that jointly control filter smoothness, endpoint weighting, and spatial frequency content. This enables filter sets to be optimized not only for spectral reconstruction accuracy but also for manufacturability. A coarse parameter search using the USGS mineral database reveals an interesting result: the Legendre polynomial case ( α = β =0) minimizes reconstruction root-mean-square error (RMSE) under low-noise conditions ( σ <0.1) with up to nine filters. In higher-noise regimes or under tighter filter-count constraints, alternative Jacobi parameters are required to minimize RMSE of reconstruction, demonstrating the importance of exploring the full polynomial design space. Through the application of PCA in the filter design process, we find a method of reconstruction that retrieves the principal scores of the measured reflectance spectra, thus likening the measurement through these designed transmission filters to an optical computation. These retrieved principal scores can then be used in spectral reconstruction, demonstrating significantly (around an order-of-magnitude) lower RMSE than when the traditional matrix-inversion method is applied for the same filter.

Spinel materials offer excellent physical, optical, and biomedical properties, particularly in epitaxial form. However, spinel electrode materials for these epitaxial films are limited. While NiCo2O4 (NCO) is a high-performing conductive spinel, it loses conductivity above 400 °C, making it unsuitable as an electrode for overgrown functional spinel layers, which typically require growth above 600 °C. Here, we demonstrate the stabilization of conducting NCO through the overgrowth of a functional spinel CoFe2O4 (CFO) layer. Notably, when the NCO is capped with 625 °C-grown CFO, it retains most of the conductivity of 350 °C-grown films. The overgrowth approach of this work shows a path for NCO to be used as a conducting electrode in epitaxial spinel-based epitaxial devices.

Terahertz (THz) spectroscopy is a non-contact technique well-suited for probing the ultrafast electrical conductivity of semiconductor nanostructures, where conventional methods are often impractical. However, geometric resonances in these nanostructures can distort the measured THz response, complicating the extraction of the intrinsic material properties. Here, we use THz spectroscopy to study GaAs nanowires of varying lengths and observe that the resonant frequency increases as the nanowire length decreases. Further measurements on a model system of well-defined GaAs microsquares (ranging in size from 500 × 500 to 7 × 7 μm2) confirmed the relationship between structure size and resonant frequency. We verify that a plasmon-based model successfully separates the geometric response from the intrinsic charge-carrier response. Thus, the model allows for the accurate evaluation of critical material properties in nanostructured semiconductors, such as electron mobility. Fluence-dependent measurements and finite-difference time-domain simulations confirm the plasmonic nature of the resonances.

III–V semiconductor nanowires have the potential to play a key role in compact and flexible electronic devices for low-power applications. Here, we integrate pairs of InAs nanowires in an inverter configuration for NMOS logic circuit applications. By controlling the nanowire diameter and channel length with a combination of growth and device design, we can control the threshold voltage of each transistor of the device. Smaller diameter (<50 nm) nanowires operate in enhancement mode whereas larger diameters operate in depletion mode. Using these controllable transistor characteristics we fabricate inverter circuits with pairs of nanowires. We optimise the inverter voltage transfer characteristics by controlling the channel and gate geometries and by reducing the gate dielectric thickness. The gate length determines the inverter’s switching voltage, which can be made symmetric about zero volts. The optimised inverter device can achieve a full rail-to-rail output voltage swing and switching ratio of 99.3%, with a resistive–capacitive-limited frequency response. The gain value and extracted high and low noise margins of 42% and 32%, respectively, satisfy the requirements for the inverter to be used in cascaded logic circuits.

Conventional spectrometer designs necessitate a compromise between their resolution and sensitivity, especially as device and detector dimensions are scaled down. Here, we report on a miniaturizable spectrometer platform where light throughput onto the detector is instead enhanced as the resolution is increased. This planar, CMOS-compatible platform is based around metasurface encoders designed to exhibit photonic bound states in the continuum, where operational range can be altered or extended simply through adjusting geometric parameters. This system can enhance photon collection efficiency by up to two orders of magnitude versus conventional designs; we demonstrate this sensitivity advantage through ultralow-intensity fluorescent and astrophotonic spectroscopy. This work represents a step forward for the practical utility of spectrometers, affording a route to integrated, chip-based devices that maintain high resolution and SNR without requiring prohibitively long integration times.

Nanoscale material systems are central to next-generation optoelectronic and quantum technologies, yet their development remains hindered by limited characterization tools, particularly at terahertz (THz) frequencies. Far-field THz spectroscopy techniques lack the sensitivity for investigating individual nanoscale systems, whereas in near-field THz nanoscopy, surface states, disorder, and sample-tip interactions often mask the response of the entire nanoscale system. Here, we present a THz resonance-amplified near-field spectroscopy technique that can detect subtle conductivity changes in isolated nanoscale systems-such as a single InAs nanowire-under ultrafast photoexcitation. By exploiting the spatial localization and resonant field enhancement in the gap of a bowtie antenna, our approach enables precise measurements of the nanostructures through shifts in the antenna resonant frequency, offering a direct means of extracting the system response, and unlocking investigations of ultrafast charge-carrier dynamics in isolated nanoscale and microscale systems.