arXiv subjects

Parivash Moradifar

Publications and source records attributed to Parivash Moradifar.

12 recordsLinked to original sources

arXiv2026

Alkaline-Earth Rare-Earth Fluoride Nanoparticle Superlattices for Ultrafast, Radiation Stable Scintillators

Radioluminescent nanostructures provide a pathway to the fabrication of next-generation scintillators with tunability in composition, size, and morphology, and spectral and temporal properties, as well as scalable processing. Here we create a 3D millimeter-scale solid-state scintillators from SrLuF Ce3+, Pr3+ (SrLuF) core-shell nanostructures, integrating nanoscale building blocks into self-assembled macroscopic crystals. These scintillators exhibit single-digit nanosecond decay times, linear response, resistance to radiation-induced degradation, and optical emission yields within an order of magnitude of YAG Ce3+. We select a SrLuF host lattice owing to its high effective atomic number, wide band gap, and low phonon energy, which together support efficient 4f-5d radiative transitions from Ce3+ and Pr3+ activators while suppressing afterglow. We create a library of core-shell nanoscintillators with undoped SrLuF shells and cores spanning compositions from undoped SrLuF to fully doped SrCeF or SrPrF. Time-resolved and steady-state X-ray excited optical luminescence (XEOL) reveal broadband emission at 310 nm (Ce3+) and 335 nm (Pr3+) with biexponential decays in the sub-nanosecond (100-500 ps) and sub-15 ns (4-13 ns) regimes, demonstrating tunable radiative efficiency and ultrafast dynamics. Ensemble performance of the mm-scale superlattices is characterized under both continuous-wave and femtosecond high-intensity excitation, revealing high light yield, linear response, and radiation hardness under extreme irradiation of ultrafast 50fs X-ray pulses up to 5mJ per mm2 corresponding to a peak intensity of 1013 W per cm2. Together, these results establish a design framework for stable, bright, and tunable scintillation platforms with applications in precision health, space exploration and hard X-ray imaging at next-generation free-electron laser facilities.

cond-mat.mtrl-sci↗

arXiv2026

$Q$ Factors Exceeding $10^{4}$ in Wavelength-to-Subwavelength-Scale Free-Space Resonators

Free-space-addressable optical resonators that combine long photon lifetimes (high $Q$ factors) with strong spatial localization of optical fields (small mode volumes, $V_m$) enhance light-matter interactions with facile far-field excitation. The Purcell factor governing spontaneous emission enhancement scales as $Q\,V_m^{-1}$. Periodically asymmetric resonators, in which perturbations convert bound modes into radiating modes, offer a route to free-space resonances, with the radiative $Q$ factor tuned by the geometric and optical strength of the asymmetry-inducing perturbations. However, free-space resonators that simultaneously achieve high $Q$ and small $V_m$ have remained rare. This limitation arises in part because existing designs do not tailor geometric and optical asymmetries concurrently, thus limiting access to high-$Q$ regimes. Here, we show that jointly tuning geometric and optical asymmetries unlocks a biaxial radiative landscape with iso-$Q$ contours that connect disparate perturbations with equivalent $Q$ factors. We demonstrate this framework with very-large-scale-integrated single-crystalline Si nanoantenna pixels (VINPix) with out-of-plane perturbations of 35-150 nm amorphous Si, SiN$_x$, and SiO$_2$. We experimentally establish biaxial $Q$ factor control in air and achieve $Q$ factors up to $76,000$ at wavelength-scale mode volumes ($V_m \sim 1.7\,\lambda_0^3\,n_{\mathrm{eff}}^{-3}$) in simultaneously imaged arrays of $>80$ resonators in water. Furthermore, we computationally demonstrate 50-nm-wide slotted VINPix that reach $Q$ factors of $10^6$ at subwavelength mode volumes ($V_m \sim 0.2\,\lambda_0^3\,n_{\mathrm{eff}}^{-3}$) with 20 nm SiO$_2$ perturbations, yielding Purcell factors as high as $5 \times 10^5$ in an all-dielectric free-space resonator.

physics.optics↗

arXiv2026

In situ Gas-Cell Electron Microscopy Reveals Pressure-Selected Restructuring Pathways in AuRu Ammonia Catalysts

Bimetallic catalysts provide new routes toward sustainable ammonia synthesis, but the nanoscale structural dynamics under reaction-relevant conditions remain poorly understood. Here, we combine in situ gas-cell and multimodal electron microscopy to determine how temperature, gas pressure, and chemistry select among distinct restructuring pathways in AuRu nanocrystal catalysts. Initially, the AuRu nanocrystals form polycrystalline face-centered cubic (FCC) alloys with Au/Ru intermixing. Elevated temperature ($\geq 350~^\circ$C) induces intraparticle phase segregation into distinct Au-rich (FCC) and Ru-rich hexagonal close-packed (HCP) domains that exhibit localized plasmonic modes. Atmospheric-pressure 3:1 H$_2$:N$_2$ gas unlocks a distinct restructuring regime absent at lower pressures, characterized by pronounced faceting and nanovoid formation. Systematic gas variation identifies hydrogen as the dominant driver. Density functional theory-trained machine-learning interatomic potentials and grand-canonical Monte Carlo simulations reveal that H-Ru interactions enhance the Au/Ru diffusivity mismatch, promoting vacancy accumulation and nanovoid formation. Together, these results show that, rather than simply accelerating the thermally driven phase segregation observed at lower pressures, atmospheric-pressure H$_2$:N$_2$ gas redirects restructuring toward faceting and nanovoid formation through a gas-mediated Kirkendall-type mechanism.

cond-mat.mtrl-sci↗

arXiv2025

Channel-last gate-all-around nanosheet oxide semiconductor transistors

As we move beyond the era of transistor miniaturization, back-end-of-line-compatible transistors that can be stacked monolithically in the third dimension promise improved performance for low-power electronics. In advanced transistor architectures, such as gate-all-around nanosheets, the conventional channel-first process involves depositing dielectrics directly onto the channel. Atomic layer deposition of gate dielectrics on back-end-of-line compatible channel materials, such as amorphous oxide semiconductors, can induce defects or cause structural modifications that degrade electrical performance. While post-deposition annealing can partially repair this damage, it often degrades other device metrics. We report a novel channel-last concept that prevents such damage. Channel-last gate-all-around self-aligned transistors with amorphous oxide-semiconductor channels exhibit high on-state current ($>$ 1 mA/$\mu$m) and low subthreshold swing (minimum of 63 mV/dec) without the need for post-deposition processing. This approach offers a general, scalable pathway for transistors with atomic layer deposited channel materials, enabling the future of low-power three-dimensional electronics.

cond-mat.mtrl-sci↗

arXiv2025

Mechanosensitive polymer matrices of biologically-relevant compliance based on upconverting nanoparticles

Upconverting nanoparticles (UCNPs) are promising optical biomechanical force sensors due to their near infrared excitation, low toxicity, photostability, and linear colorimetric sensitivity to micronewtons of force. Recently, a composite force sensor based on UCNPs embedded in a polystyrene microbead enabled the first real time measurement of feeding forces in living nematodes. However, the comparatively large stiffness of polystyrene only makes it relevant to biomedical application in a small subset of biological tissue. To facilitate deployment of UCNPs into biological tissues with a range of mechanical properties, we expand upon polymer UCNP composite systems by embedding UCNPs in three polymer matrices with varying stiffnesses (epoxy resin, polydimethylsiloxane, and alginate hydrogels). Furthermore, to enhance these composites mechanosensitivity, we methodically investigate using two different core-shell architectures of SrLuF based UCNPs doped with ytterbium, erbium, and varying manganese concentrations. We calibrate polymer UCNP composite optical force sensitivity with colocalized atomic force and confocal microscopy. Using the red to green emission ratio (Delta Percent IRed:IGreen) as the force read-out, we determine that SrLuF:Yb0.28Er0.025Mn0.013 with SrYF inert shell dispersed in epoxy resin exhibits the greatest emission color change (12 Delta Percent IRed:IGreen per microNewton). Finally, we map forces in the epoxy UCNP composite on the macroscale between the joint of a chicken wing bone using a commercially available wide field microscope, thereby demonstrating its ability to optically measure pressures in situ. This work establishes the utility and modularity of the UCNP polymer composite system for force sensing in geometrically and mechanically diverse biological systems.

physics.app-ph↗

arXiv2024

High-throughput antibody screening with high-quality factor nanophotonics and bioprinting

Empirical investigation of the quintillion-scale, functionally diverse antibody repertoires that can be generated synthetically or naturally is critical for identifying potential biotherapeutic leads, yet remains burdensome. We present high-throughput nanophotonics- and bioprinter-enabled screening (HT-NaBS), a multiplexed assay for large-scale, sample-efficient, and rapid characterization of antibody libraries. Our platform is built upon independently addressable pixelated nanoantennas exhibiting wavelength-scale mode volumes, high-quality factors (high-Q) exceeding 5000, and pattern densities exceeding one million sensors per square centimeter. Our custom-built acoustic bioprinter enables individual sensor functionalization via the deposition of picoliter droplets from a library of capture antigens at rates up to 25,000 droplets per second. We detect subtle differentiation in the target binding signature through spatially-resolved spectral imaging of hundreds of resonators simultaneously, elucidating antigen-antibody binding kinetic rates, affinity constant, and specificity. We demonstrate HT-NaBS on a panel of antibodies targeting SARS-CoV-2, Influenza A, and Influenza B antigens, with a sub-picomolar limit of detection within 30 minutes. Furthermore, through epitope binning analysis, we demonstrate the competence and diversity of a library of native antibodies targeting functional epitopes on a priority pathogen (H5N1 bird flu) and on glycosylated therapeutic Cetuximab antibodies against epidermal growth factor receptor. With a roadmap to image tens of thousands of sensors simultaneously, this high-throughput, resource-efficient, and label-free platform can rapidly screen for high-affinity and broad epitope coverage, accelerating biotherapeutic discovery and de novo protein design.

physics.optics↗

arXiv2024

Very-Large-Scale-Integrated High-$Q$ Nanoantenna Pixels (VINPix)

Metasurfaces provide a versatile and compact approach to free-space optical manipulation and wavefront shaping. Comprised of arrays of judiciously-arranged dipolar resonators, metasurfaces precisely control the amplitude, polarization, and phase of light, with applications spanning imaging, sensing, modulation, and computing. Three crucial performance metrics of metasurfaces and their constituent resonators are the quality factor ($Q$-factor), mode-volume ($V_m$), and the ability to control far-field radiation. Often, resonators face a trade-off between these parameters: a reduction in $V_m$ leads to an equivalent reduction in $Q$, albeit with more control over radiation. Here, we demonstrate that this perceived compromise is not inevitable $-$ high-$Q$, subwavelength $V_m$, and controlled dipole-like radiation can be achieved, simultaneously. We design high-$Q$, very-large-scale integrated silicon nanoantenna pixels $-$ VINPix $-$ that combine guided mode resonance waveguides with photonic crystal cavities. With optimized nanoantennas, we achieve $Q$-factors exceeding 1500 with $V_m$ less than 0.1 $(λ/n_{\text{air}})^3$. Each nanoantenna is individually addressable by free-space light, and exhibits dipole-like scattering to the far-field. Resonator densities exceeding a million nanoantennas per $\text{cm}^2$ can be achieved, as demonstrated by our fabrication of an 8 mm x 8 mm VINPix array. As a proof-of-concept application, we demonstrate spectrometer-free, spatially localized, refractive-index sensing utilizing a VINPix array. Our platform provides a foundation for compact, densely multiplexed devices such as spatial light modulators, computational spectrometers, and in-situ environmental sensors.

physics.optics↗

arXiv2023

Solution-phase single-particle spectroscopy for probing multi-polaronic dynamics in quantum emitters at femtosecond resolution

The development of many optical quantum technologies depends on the availability of solid-state single quantum emitters with near-perfect optical coherence. However, a standing issue that limits systematic improvement is the significant sample heterogeneity and lack of mechanistic understanding of microscopic energy flow at the single emitter level and ultrafast timescales. Here we develop solution-phase single-particle pump-probe spectroscopy with photon correlation detection that captures sample-averaged dynamics in single molecules and/or defect states with unprecedented clarity at femtosecond resolution. We apply this technique to single quantum emitters in two-dimensional hexagonal boron nitride, which suffers from significant heterogeneity and low quantum efficiency. From millisecond to nanosecond timescales, the translation diffusion, metastable-state-related bunching shoulders, rotational dynamics, and antibunching features are disentangled by their distinct photon-correlation timescales, which collectively quantify the normalized two-photon emission quantum yield. Leveraging its femtosecond resolution, spectral selectivity and ultralow noise (two orders of magnitude improvement over solid-state methods), we visualize electron-phonon coupling in the time domain at the single defect level, and discover the acceleration of polaronic formation driven by multi-electron excitation. Corroborated with results from a theoretical polaron model, we show how this translates to sample-averaged photon fidelity characterization of cascaded emission efficiency and optical decoherence time. Our work provides a framework for ultrafast spectroscopy in single emitters, molecules, or defects prone to photoluminescence intermittency and heterogeneity, opening new avenues of extreme-scale characterization and synthetic improvements for quantum information applications.

cond-mat.mes-hall↗

arXiv2022

Accelerating quantum materials development with advances in transmission electron microscopy

Quantum materials are driving a technology revolution in sensing, communication, and computing, while simultaneously testing many core theories of the past century. Materials such as topological insulators, complex oxides, quantum dots, color center hosting semiconductors, and other types of strongly correlated materials can exhibit exotic properties such as edge conductivity, multiferroicity, magnetoresistance, single photon emission, and optical-spin locking. These emergent properties arise and depend strongly on the materials detailed atomic scale structure, including atomic defects, dopants, and lattice stacking. In this review, after introduction of different classes of quantum materials and quantum excitations, we describe how progress in the field of electron microscopy, including in situ and in operando EM, can accelerate advances in quantum materials. Our review describes EM methods including: i) principles and operation modes of EM, ii) EM spectroscopies, such as electron energy loss spectroscopy, cathodoluminescence, and electron energy gain spectroscopy, iii) 4D scanning transmission electron microscopy, iv) dynamic and ultrafast EM, v) complimentary ultrafast spectroscopies, and vi) atomic electron tomography. We discuss how these methods inform structure function relations in quantum materials down to the picometer scale and femtosecond time resolution, and how they enable high resolution manipulation of quantum materials. Among numerous results, our review highlights how EM has enabled identification of the 3D structure of quantum defects, measuring reversible and metastable dynamics of quantum excitations, mapping exciton states and single photon emission, measuring nanoscale thermal transport and coupled excitation dynamics, and measuring the internal electric field of quantum heterointerfaces, all at the quantum materials intrinsic atomic and near atomic-length scale.

cond-mat.mtrl-sci↗

arXiv2022

Thermally Induced Structural Evolution and Nanoscale Interfacial Dynamics in Layered Metal Chalcogenides

Layered chalcogenides including Bi2Te3, Sb2Te3, Bi-Sb-Te ternary alloys and heterostructures are known as great thermoelectric, topological insulators and recently highlighted as plasmonic building blocks beyond noble metals. Here, we conduct a joint in situ transmission electron microscopy (in situ TEM) and density functional theory (DFT) calculations to investigate the temperature dependent nanoscale dynamics, interfacial properties and further identifying the role of native defects and edge configurations in anisotropic sublimations of Bi2Te3-Sb2Te3 in-plane heterostructure and Sb2-xBixTe3 alloy. Structural dynamics including edge evolution, formation, expansion, and coalescence of thermally induced polygonal nanopores are reported. The nanopores appear to be initiated by preferential dissociation of chalcogenide species (Te) from the center, heterointerface and edges in the heterostructure and only from the outer edges in the alloy counterpart. This results in a reduced thermal stability and significantly different sublimation pathways of the heterostructure. Furthermore, triangular and quasi hexagonal configurations are observed to be the dominant nanopores configurations in the heterostructure. Additionally, our DFT calculations provide a mechanistic understanding on the role of native defects and edge formation energies, revealing the antisite defects TeBi to be the dominant native defect in a Te-rich condition and playing a key role on the defects assisted sublimation. These findings significantly impact our understanding of controlling the nanoscale sublimation dynamics and can ultimately assist us in designing tunable low-dimensional chalcogenides.

cond-mat.mtrl-sci↗

arXiv2022

Rapid genetic screening with high quality factor metasurfaces

Genetic analysis methods are foundational to advancing personalized and preventative medicine, accelerating disease diagnostics, and monitoring the health of organisms and ecosystems. Current nucleic acid technologies such as polymerase chain reaction (PCR), next-generation sequencing (NGS), and DNA microarrays rely on fluorescence and absorbance, necessitating sample amplification or replication and leading to increased processing time and cost. Here, we introduce a label-free genetic screening platform based on high quality (high-Q) factor silicon nanoantennas functionalized with monolayers of nucleic acid fragments. Each nanoantenna exhibits substantial electromagnetic field enhancements with sufficiently localized fields to ensure isolation from neighboring resonators, enabling dense biosensor integration. We quantitatively detect complementary target sequences using DNA hybridization simultaneously for arrays of sensing elements patterned at densities of 160,000 pixels per cm$^2$. In physiological buffer, our nanoantennas exhibit average resonant quality factors of 2,200, allowing detection of two gene fragments, SARS-CoV-2 envelope (E) and open reading frame 1b (ORF1b), down to femtomolar concentrations. We also demonstrate high specificity sensing in clinical nasopharyngeal eluates within 5 minutes of sample introduction. Combined with advances in biomarker isolation from complex samples (e.g., mucus, blood, wastewater), our work provides a foundation for rapid, compact, amplification-free and high throughput multiplexed genetic screening assays spanning medical diagnostics to environmental monitoring.

physics.optics↗

arXiv2019

Full orientation control of epitaxial MoS2 on hBN assisted by substrate defects

Inversion asymmetry in two-dimensional materials grants them fascinating properties such as spin-coupled valley degrees of freedom and piezoelectricity, but at the cost of inversion domain boundaries if the epitaxy of the grown 2D layer -- on a polar substrate -- cannot adequately distinguish what are often near-degenerate 0° and 180° orientations. We employ first-principles calculations to identify a method to lift this near-degeneracy: the energetic distinction between eclipsed and staggered configurations during nucleation at a point defect in the substrate. For monolayer MoS2 grown on hexagonal boron nitride, the predicted defect complex can be more stable than common MoS2 point defects because it is both a donor-acceptor pair and a Frenkel pair shared between adjacent layers of a 2D heterostack. Orientation control is verified in experiments that achieve ~90% consistency in the orientation of as-grown triangular MoS2 flakes on hBN, as confirmed by aberration-corrected scanning/transmission electron microscopy. This defect-enhanced orientational epitaxy could provide a general mechanism to break the near-degeneracy of 0/180° orientations of polar 2D materials on polar substrates, overcoming a long-standing impediment to scalable synthesis of single-crystal 2D semiconductors.

cond-mat.mes-hall↗