arXiv subjects

David S. Ginger

Publications and source records attributed to David S. Ginger.

At least 19 recordsLinked to original sources

arXiv2026

Atomic-Scale Observation of Symmetry Breaking in Altermagnetic MnTe

The recent discovery of altermagnetism has sparked growing interest in compensated magnetic systems as promising platforms for highly scalable spintronics. Altermagnetism is a distinct magnetic order where opposite spin sublattices are connected by rotation, yielding zero net magnetization but momentum-dependent spin splitting. To date, experimental verification of altermagnetic order has been achieved predominantly through bulk-sensitive techniques, including spin-dependent electronic spectra and transport responses. However, direct atomic-scale evidence that explicitly correlates crystal symmetry, local structural distortions, and magnetic ordering has remained unexplored. Here, we report the direct atomic-scale observation of coexisting polar distortions and altermagnetic order in MnTe, combining atomic resolution scanning transmission electron microscopy (STEM) imaging with electron magnetic chiral dichroism (EMCD) measurements. We reveal that MnTe is not an ideal uniform P63/mmc g-wave altermagnet at the atomic scale. Instead, it hosts ubiquitous inversion-symmetry-breaking distortions that lower the spin-space-group (SSG) symmetry, admits d-wave altermagnetic components, and in lower-symmetry regimes, even allow s-wave spin splitting (net magnetization). The coexistence of ferroelectric signatures and altermagnetic order establishes local lattice symmetry in MnTe as a control knob for altermagnetic spin splitting, spin current generation, and multiferroic memory applications.

cond-mat.mtrl-sci↗

arXiv2026

Electrochemical reactions under reverse bias create additional mobile ions that enable hole tunneling in metal halide perovskite diodes

Gradual reverse-bias breakdown in metal-halide perovskite diodes and solar cells is thought to originate from hole tunneling through steep bands in an ionic depletion region near the electron transport layer after positively charged iodine vacancies accumulate near the hole-transport layer (HTL). However, typical reported mobile ion concentrations near $1\times10^{17}$ cm$^{-3}$ are too small to quantitatively explain significant tunneling current densities and (Zener) breakdown observed near $-5$ V. Here, we show that inferred mobile ion concentrations increase by more than 100$\times$, to over $1\times10^{18}$ cm$^{-3}$, within just three minutes of reverse bias at $-6.0$ V in p-i-n perovskite diodes. We attribute the increase in mobile ion concentration to iodide oxidation and the resulting iodine vacancy creation which must be balanced by reduction reactions near the HTL. Thin and sub-optimal HTL coverage leads to direct contact between the transparent conducting electrode and perovskite and facilitates electron transfer and reduction, enabling the creation of even larger inferred mobile ion concentrations ($\sim1\times10^{19}$ cm$^{-3}$) and leading to faster degradation under reverse bias. This explains previous work that showed increased breakdown voltages and improved reverse-bias stability by implementing thick, uniform HTLs.

physics.app-ph↗

arXiv2026

Degradation Dynamics of Perovskite Solar Cells Under Fixed Reverse Current Injection

Previous studies of reverse-bias stability in perovskite solar cells have focused primarily on voltage controlled reverse-bias tests. Here we instead present an investigation of perovskite solar cell degradation under well-defined, constant reverse-current stress. We show that the choice of hole-transport layer dictates the dominant degradation pathway: cells using thick poly(triphenylamine) (PTAA) layers with better indium-doped tin oxide (ITO) coverage can tolerate high reverse bias but quickly undergo catastrophic breakdown under fixed reverse current near their one-sun maximum power-point. In contrast, cells modified with the phosphonic-acid interface layer MeO-2PACz, with poorer ITO coverage compared to PTAA, exhibit soft, gradual, and largely recoverable degradation, regardless of the shading conditions. For MeO-2PACz devices, degradation increases with both current magnitude and duration. Importantly, when normalized by injected charge (current times duration), lower currents applied over longer times cause more severe degradation than higher currents over shorter periods. Combining electrical measurements with spatially resolved photoluminescence imaging, we argue against shunt formation and instead support an ion- and charge-mediated interfacial electrochemical degradation mode.

physics.app-ph↗

arXiv2025

Trion Formation Hampers Single Quantum Dot Performance in Silane-Coated FAPbBr3 Quantum Dots

We explore silane-coated formamidinium lead bromide (FAPbBr3) quantum dots as single photon emitters and compare them to FAPbBr3 quantum dots passivated with a phosphoethylammonium derivative (PEAC8C12), which represents current state-of-the-art in zwitterionic molecular surface ligand passivation. We compare properties including single-photon purity (g2(t)), linewidth, blinking, and photostability. We find that at room temperature, these silane-coated dots perform comparably to the PEAC8C12 passivation in terms of single-photon performance metrics, while exhibiting improvements in photostability. However, we find that at 4K, silane-coated FAPbBr3 quantum dots perform worse than the PEAC8C12-passivated samples, exhibiting faster blue-shifting and photobleaching under illumination. Analysis of fluorescence lifetime intensity distributions from the photon-counting data indicates increased efficiency of fast non-radiative processes in the silane-coated quantum dots at 4K. We propose a trion related degradation pathway at low temperatures that is consistent with the observed kinetics and estimate that at 4K with 6.1 uJ/cm2, 472 nm excitation the silane-coated quantum dots build up double the trion population of their PEAC8C12-passivated counterparts.

cond-mat.mtrl-sci↗

arXiv2025

pH Regulates Ion Dynamics in Carboxylated Mixed Conductors

Coupled ionic and electronic transport underpins processes as diverse as electrochemical energy conversion, biological signaling, and soft adaptive electronics. Yet, how chemical environments such as pH modulate this coupling at the molecular scale remains poorly understood. Here, we show that the protonation state of carboxylated polythiophenes provides precise chemical control over ion dynamics, doping efficiency, solvent uptake and mechanical response. The findings establish molecular acidity as a general strategy to program ionic preference and mechanical stability, offering design principles for pH-responsive mixed conductors and soft electronic materials that couple ionic, electronic, and mechanical functionality.

cond-mat.mtrl-sci↗

arXiv2025

Comparing Machine Learning and Physics-Based Nanoparticle Geometry Determinations Using Far-Field Spectral Properties

Anisotropic metal nanostructures exhibit polarization-dependent light scattering. This property has been widely exploited to determine geometries of subwavelength structures using far-field microscopy. Here, we explore the use of variational autoencoders (VAEs) to determine the geometries of gold nanorods (NRs) such as in-plane orientation and aspect ratio under linearly polarized dark-field illumination in an optical microscope. We input polarized dark-field scattering spectra and electron microscopy images into a dual-branch multimodal VAE with a single shared latent space trained on paired spectra-image data, using a learnable linear adapter. We achieve prediction of Au NRs using only polarized dark-field scattering spectra input. We determine geometrical parameters of orientational angle and aspect ratio quantitatively via both dual-VAE and physics-based analysis. We show that orientational angle prediction by dual-VAE performs well with only a small (300 particle) training set, yielding a mean absolute error (MAE) of 14.4 and a concordance correlation coefficient (CCC) of 0.95. This performance is only marginally worse than the physics-based cos(2theta) fitting approach between the scattering intensity and the polarizing angle, which achieves MAE of 8.78 and CCC of 0.99. Aspect ratio determination is also similar for the dual-VAE and physics-based fitting comparison (MAE of 0.21 vs. 0.23 and CCC of 0.53 vs. 0.68). By learning a shared latent manifold linking spectra and morphology, the model can generate NR images with accurate orientation and aspect ratio with spectra-only input in the small-data regime (300 particles), suggesting a general recipe for inverse nano-optical problems requiring both structure and orientation information.

physics.optics↗

arXiv2025

Surface Passivation for Halide Optoelectronics: Comparing Optimization and Reactivity of Amino-Silanes with Formamidinium

Amino-silane-based surface passivation schemes are gaining attention in halide perovskite optoelectronics, with varying levels of success. We compare surface treatments using (3-aminopropyl)trimethoxysilane (APTMS) and [3-(2-aminoethylamino)propyl]trimethoxysilane (AEAPTMS), applied via room-temperature vacuum deposition, to the perovskite FA0.78Cs0.22Pb(I0.85Br0.15)3 (FA = formamidinium). Both molecules improve thin-film photoluminescence properties and photovoltaic device performance, although their effectiveness depends strongly on deposition time. We show AEAPTMS has a wider, more robust processing window and yields higher performance under optimized conditions. In contrast, over-exposure, particularly with APTMS, reduces performance, with notable reductions in photoluminescence lifetime and absorbance. To probe the underlying chemistry, we employ nuclear magnetic resonance (NMR) spectroscopy and depth-resolved time-of-flight secondary ion mass spectrometry (ToF-SIMS), demonstrating that both amino-silanes react with formamidinium (FA+) cations in solution and in the solid state. This work underscores the importance of optimizing deposition conditions to balance effective passivation with potential performance loss and elucidates previously unrecognized reactive chemistry between amino-silane passivating agents and halide perovskites.

cond-mat.mtrl-sci↗

arXiv2025

Deposition-Dependent Coverage and Performance of Phosphonic Acid Interface Modifiers in Halide Perovskite Optoelectronics

In this work, we study the effect of various deposition methods for phosphonic acid interface modifiers commonly pursued as self-assembled monolayers in high-performance metal halide perovskite photovoltaics and light-emitting diodes. We compare the deposition of (2-(3,6-diiodo-9H-carbazol-9-yl)ethyl)phosphonic acid onto indium tin oxide (ITO) bottom contacts by varying three parameters: the method of deposition, specifically spin coating or prolonged dip coating, ITO surface treatment via HCl/FeCl3 etching, and use in combination with a second modifier, 1,6-hexylenediphosphonic acid. We demonstrate that varying these modification protocols can impact time-resolved photoluminescence carrier lifetimes and quasi-Fermi level splitting of perovskite films deposited onto the phosphonic-acid-modified ITO. Ultraviolet photoelectron spectroscopy shows an increase in effective work function after phosphonic acid modification and clear evidence for photoemission from carbazole functional groups at the ITO surface. We use X-ray photoelectron spectroscopy to probe differences in phosphonic acid coverage on the metal oxide contact and show that perovskite samples grown on ITO with the highest phosphonic acid coverage exhibit the longest carrier lifetimes. Finally, we establish that device performance follows these same trends. These results indicate that the reactivity, heterogeneity, and composition of the bottom contact help to control recombination rates and therefore power conversion efficiencies. ITO etching, prolonged deposition times for phosphonic acids via dip coating, and the use of a secondary, more hydrophilic bis-phosphonic acid, all contribute to improvements in surface coverage, carrier lifetime, and device efficiency. These improvements each have a positive impact, and we achieve the best results when all three strategies are implemented.

physics.app-ph↗

arXiv2025

Free Space Few-Photon Nonlinearity in Critically Coupled Polaritonic Metasurfaces

Few-photon optical nonlinearity in planar solid-state systems is challenging yet crucial for quantum and classical optical information processing. Polaritonic nonlinear metasurfaces have emerged as a promising candidate to push the photon number down -- but have often been hindered by challenges like the poor photon-trapping efficiency and lack of modal overlap. Here, we address these issues in a self-hybridized perovskite metasurface through critical coupling engineering, and report strong polaritonic nonlinear absorption at an ultra-low incident power density of only 519 W/cm2 (2 orders of magnitude lower than the state of art in free-space planar devices), with an estimated photon number of 6.12 per cavity lifetime. Taking advantage of a quasi-bound-state-in-the-continuum design with asymmetry-controlled quality-(Q)-factor, we systematically examine the Q-dependent device nonlinearity and determine the optimal cavity critical coupling condition. With the optimized device, we demonstrate at 6 Kelvin a tunable nonlinear response from reverse saturable absorption to saturable absorption at varying pump powers, with a maximal effective nonlinear absorption coefficient up to 29.4+-5.8 cm/W (6 orders of magnitude larger than unpatterned perovskites) at 560 nm wavelength. In addition, the cavity-exciton detuning dependent device response is analyzed and well explained by a phase-space-filling model, elucidating the underlying physics and the origin of giant nonlinearity. Our study paves the way towards practical flat nonlinear optical devices with large functional areas and massive parallel operation capabilities.

physics.optics↗

arXiv2025

Multi-Output Convolutional Neural Network for Improved Parameter Extraction in Time-Resolved Electrostatic Force Microscopy Data

Time-resolved scanning probe microscopy methods, like time-resolved electrostatic force microscopy (trEFM), enable imaging of dynamic processes ranging from ion motion in batteries to electronic dynamics in microstructured thin film semiconductors for solar cells. Reconstructing the underlying physical dynamics from these techniques can be challenging due to the interplay of cantilever physics with the actual transient kinetics of interest in the resulting signal. Previously, quantitative trEFM used empirical calibration of the cantilever or feed-forward neural networks trained on simulated data to extract the physical dynamics of interest. Both these approaches are limited by interpreting the underlying signal as a single exponential function, which serves as an approximation but does not adequately reflect many realistic systems. Here, we present a multi-branched, multi-output convolutional neural network (CNN) that uses the trEFM signal in addition to the physical cantilever parameters as input. The trained CNN accurately extracts parameters describing both single-exponential and bi-exponential underlying functions, and more accurately reconstructs real experimental data in the presence of noise. This work demonstrates an application of physics-informed machine learning to complex signal processing tasks, enabling more efficient and accurate analysis of trEFM.

cond-mat.mtrl-sci↗

arXiv2025

Overcoming the surface paradox: Buried perovskite quantum dots in wide-bandgap perovskite thin films

Colloidal perovskite quantum dots (PQDs) are an exciting platform for on-demand quantum, and classical optoelectronic and photonic devices. However, their potential success is limited by the extreme sensitivity and low stability arising from their weak intrinsic lattice bond energy and complex surface chemistry. Here we report a novel platform of buried perovskite quantum dots (b-PQDs) in a three-dimensional perovskite thin-film, fabricated using one-step, flash annealing, which overcomes surface related instabilities in colloidal perovskite dots. The b-PQDs demonstrate ultrabright and stable single-dot emission, with resolution-limited linewidths below 130 {\mu}eV, photon-antibunching (g^2(0)=0.1), no blinking, suppressed spectral diffusion, and high photon count rates of 10^4/s, consistent with unity quantum yield. The ultrasharp linewidth resolves exciton fine-structures (dark and triplet excitons) and their dynamics under a magnetic field. Additionally, b-PQDs can be electrically driven to emit single photons with 1 meV linewidth and photon-antibunching (g^2(0)=0.4). These results pave the way for on-chip, low-cost single-photon sources for next generation quantum optical communication and sensing.

physics.optics↗

arXiv2025

Deterministic Printing of Single Quantum Dots

The unique optical properties of quantum dots (QDs), size-tunable emission and high quantum yield, make them ideal candidates for applications in secure quantum communication, quantum computing, targeted single-cell and molecular tagging, and sensing. Scalable and deterministic heterointegration strategies for single QDs have, however, remained largely out of reach due to inherent material incompatibilities with conventional semiconductor manufacturing processes. To advance scalable photonic quantum device architectures, it is therefore crucial to adopt placement and heterointegration strategies that can address these challenges. Here, we present an electrohydrodynamic (EHD) printing model, single particle extraction electrodynamics (SPEED) printing, that exploits a novel regime of nanoscale dielectrophoretics to print and deterministically position single colloidal QDs. Using QDs solubilized in apolar solvents, this additive, zero-waste nanomanufacturing process overcomes continuum fluid surface energetics and stochastic imprecision that limited previous colloidal deposition strategies, achieving selective extraction and deposition of individual QDs at sub-zeptoliter volumes. Photoluminescence and autocorrelation function (g(2)) measurements confirm nanophotonic cavity-QD integration and single-photon emission from single printed QDs. By enabling deterministic placement of single quantum dots, this method provides a powerful, scalable, and sustainable platform for integrating complex photonic circuits and quantum light sources with nanoscale precision.

physics.optics↗

arXiv2025

Landau-Levich Scaling for Optimization of Quantum Dot Layer Morphology and Thickness for Enhanced Quantum-Dot Light-Emitting Diode Performance

Quantum dot (QD) light-emitting diodes (QLEDs) are promising candidates for next-generation displays because of their high efficiency, brightness, broad color gamut, and solution-processability. Large-scale solution-processing of electroluminescent QLEDs poses significant challenges, particularly concerning the precise control of the active layer's thickness and uniformity. These obstacles directly impact charge transport, leading to current leakage and reduced overall efficiency. Blade-coating is a prevalent and scalable solution processing technique known for its speed and minimal waste. Additionally, it allows for continuous "roll-to-roll" processing, making it highly adaptable in various applications. In this study, we demonstrate the precise control of blade speed in the Landau-Levich regime to create a uniform QD emission layer, using a commercial CdSe/ZnS QD as a representative example. QDs assemble into different morphologies on glass and the underlying layers of the QLED device due to variations in interaction energy. The QD film thickness can be modified from monolayer to multilayer by adjusting blade speed, which can be predicted by fitting the Landau-Levich-Derjaguin theory. The optimal speed at 7 mm/s results in a QD film with a surface coverage of around 163% and low roughness (1.57 nm mean square height). The QLED external quantum efficiency (EQE) of approximately 1.5% was achieved using commercially available CdSe/ZnS QDs with low photoluminescence quantum yield (PLQY), and an EQE of around 7% has been obtained using lab-made InP/ZnSe/ZnS QDs having a solution PLQY of 74%. All-blade-coated CdSe-QLEDs are further demonstrated by adopting the optimized speed for the QD layer. This method demonstrates significant potential for developing low-cost, reproducible, and scalable QLED technologies with uniform emission characteristics and low-waste production.

physics.app-ph↗

arXiv2024

Sub-diffraction Imaging of Carrier Dynamics in Halide Perovskite Semiconductors: Effects of Passivation, Morphology, and Ion Motion

We spatially resolve photocarrier dynamics in halide perovskites using time-resolved electrostatic force microscopy (trEFM) to map surface potential equilibration during photoexcitation. Following treatment with different surface passivation agents, we show that trEFM probes dynamics directly related to surface recombination velocity and carrier lifetimes correlated with time-resolved photoluminescence. Our results reveal nanoscale variations in recombination dynamics following surface passivation. We also observe heterogeneity in surface potential equilibration times dependent on perovskite film morphology. We combine wavelength- and intensity-dependent measurements with drift-diffusion simulations to disentangle the influence of carrier recombination and ion migration on surface potential equilibration. These results demonstrate that we can use mechanical detection to image electronic carrier recombination dynamics in perovskites below the optical diffraction limit while also showing the potential for future improvements in heterogeneity of surface passivation.

cond-mat.mtrl-sci↗

arXiv2024

Diamine Surface Passivation and Post-Annealing Enhance Performance of Silicon-Perovskite Tandem Solar Cells

We show that the use of 1,3-diaminopropane (DAP) as a chemical modifier at the perovskite/electron-transport layer (ETL) interface enhances the power conversion efficiency (PCE) of 1.7 eV bandgap FACs mixed-halide perovskite single-junction cells, primarily by boosting the open-circuit voltage (VOC) from 1.06 V to 1.15 V. Adding a post-processing annealing step after C60 evaporation, further improves the fill factor (FF) by 20% from the control to the DAP + post-annealing devices. Using hyperspectral photoluminescence microscopy, we demonstrate that annealing helps improve compositional homogeneity at the top and bottom interfaces of the solar cell, which prevents detrimental bandgap pinning in the devices and improves C60 adhesion. Using time-of-flight secondary ion mass spectrometry, we show that DAP reacts with formamidinium present near the surface of the perovskite lattice to form a larger molecular cation, 1,4,5,6-tetrahydropyrimidinium (THP) that remains at the interface. Combining the use of DAP and the annealing of C60 interface, we fabricate Si-perovskite tandems with PCE of 25.29%, compared to 23.26% for control devices. Our study underscores the critical role of chemical reactivity and thermal post-processing of the C60/Lewis-base passivator interface in minimizing device losses and advancing solar-cell performance of wide-bandgap mixed-cation mixed-halide perovskite for tandem application.

cond-mat.mtrl-sci↗

arXiv2024

Emissive Surface Traps Lead to Asymmetric Photoluminescence Line Shape in Spheroidal CsPbBr3 Quantum Dots

The morphology of quantum dots plays an important role in governing their photophysics. Here, we explore the photoluminescence of spheroidal CsPbBr3 quantum dots synthesized via the room-temperature trioctlyphosphine oxide/PbBr2 method. Despite photoluminescence quantum yields nearing 100%, these spheroidal quantum dots exhibit an elongated red photoluminescence tail not observed in typical cubic quantum dots synthesized via hot injection. We explore this elongated red tail through structural and optical characterization including small-angle x-ray scattering, transmission electron microscopy and time-resolved, steady-state, and single quantum dot photoluminescence. From these measurements we conclude that the red tail originates from emissive surface traps. We hypothesize that these emissive surface traps are located on the (111) surfaces and show that the traps can be passivated by adding phenethyl ammonium bromide, resulting in a more symmetric line shape

cond-mat.mtrl-sci↗

arXiv2024

Increased Brightness and Reduced Efficiency Droop in Perovskite Quantum Dot Light-Emitting Diodes using Carbazole-Based Phosphonic Acid Interface Modifiers

We demonstrate the use of [2-($\textit{9H}$-carbazol-9-yl)ethyl]phosphonic acid (2PACz) and [2-(3,6-di-$\textit{tert}$-butyl-$\textit{9H}$-carbazol-9-yl)ethyl]phosphonic acid (t-Bu-2PACz) as anode modification layers in metal-halide perovskite quantum dot light-emitting diodes (QLEDs). Compared to conventional QLED structures with PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate)/PVK (poly(9-vinylcarbazole)) hole-transport layers, QLEDs made with phosphonic acid (PA)-modified indium tin oxide (ITO) anodes show an over 7-fold increase in brightness, achieving a brightness of 373,000 cd m$^{-2}$, one of the highest brightnesses reported to date for colloidal perovskite QLEDs. Importantly, the onset of efficiency roll-off, or efficiency droop, occurs at ~1000-fold higher current density for QLEDs made with PA-modified anodes compared to control QLEDs made with conventional PEDOT:PSS/PVK hole transport layers, allowing the devices to sustain significantly higher levels of external quantum efficiency at a brightness of >10$^{5}$ cd m$^{-2}$. Steady-state and time-resolved photoluminescence measurements indicate these improvements are due to a combination of multiple factors, including reducing quenching of photoluminescence at the PEDOT:PSS interface and reducing photoluminescence efficiency loss at high levels of current density.

cond-mat.mtrl-sci↗

arXiv2024

Ligand Equilibrium Influences Photoluminescence Blinking in CsPbBr3: A Change Point Analysis of Widefield Imaging Data

Photoluminescence intermittency remains one of the biggest challenges to realizing perovskite quantum dots (QDs) as scalable single photon emitters. We compare CsPbBr3 QDs capped with different ligands, lecithin, and a combination of oleic acid and oleylamine, to elucidate the role of surface chemistry on photoluminescence intermittency. We employ widefield photoluminescence microscopy, sampling the blinking behavior of hundreds of QDs. Using change point analysis, we achieve the robust classification of blinking trajectories, and we analyze representative distributions from large numbers of QDs (Nlecithin = 1308, Noleic acid/oleylamine =1317). We find that lecithin suppresses blinking in CsPbBr3 QDs compared to oleic acid/oleylamine. Under common experimental conditions, lecithin-capped QDs are 7.5 times more likely to be non-blinking and spend 2.5 times longer in their most emissive state, despite both QDs having nearly identical solution photoluminescence quantum yields. We measure photoluminescence as a function of dilution and show that the differences between lecithin and oleic acid/oleylamine capping emerge at low concentrations during preparation for single particle experiments. From experiment and first principles calculations, we attribute the differences in lecithin and oleic acid/oleylamine performance to differences in their ligand binding equilibria. Consistent with our experimental data, density functional theory calculations suggest a stronger binding affinity of lecithin to the QD surface compared to oleic acid/oleylamine, implying a reduced likelihood of ligand desorption during dilution. These results suggest that using more tightly binding ligands is a necessity for surface passivation and consequently, blinking reduction in perovskite QDs used for single particle and quantum light experiments.

cond-mat.mtrl-sci↗