Digital photocorrosion of quantum semiconductor microstructures: a method for structural diagnostics and sensing of electrically charged molecules

La fabrication de dispositifs à base de structures multicouches de semi-conducteurs exige une mesure de routine des épaisseurs et de la localisation des interfaces des couches formées. Ceci est souvent réalisé en utilisant des techniques coûteuses et compliquées telles que la microscopie à force atomique (AFM) ou la spectroscopie de photoélectrons à rayons X (XPS). Dans ce travail, la métrologie à température ambiante dans un environnement aqueux a été développée pour des tests de post-croissance des nano-hétérostructures (NHs) semi-conductrices. La méthode utilise le procédé de photocorrosion numérique (DIP) et la sensibilité de l'émission de photoluminescence (PL) aux états de surface révélés pendant la photocorrosion. Le processus de photocorrosion des NHs semi-conducteurs GaAs/AlGaAs a été étudié en présence d'une excitation la bande interdite d'échantillons immergés dans différentes solutions aqueuses. Une photo-excitation de faible intensité au-dessus de la bande interdite (<105 mW/cm2) a été appliquée en mode pulsé caractérisée par un duty cycle (DC) donné par TON/(TON + TOFF). Ceci a produit des vitesses moyennes de gravure du matériau enleves à la précision de la sous-monocouche pendant chaque cycle DIP. En utilisant les techniques d’AFM et de XPS, il a été démontré que l'émission de la PL d'une NH GaAs/AlGaAs au cours de la DIP oscille en raison des couches de GaAs et d’AlGaAs révélées. Ces oscillations sont causées par la sensibilité de l'émission PL à la vitesse de recombinaison de surface des porteurs, qui diffère considérablement pour GaAs et AlGaAs. Le processus DIP a révélé une épaisseur de 1 nm de GaAs dans une structure de GaAs/AlGaAs, mais cela ne semble pas être une limite de résolution de cette approche. Le potentiel de circuit ouvert (OCP) mesuré au cours du DIP diffère entre les jonctions d'électrolyte-GaAs et électrolyte-AlGaAs formées au cours du processus de la photocorrosion. Les différences de OCP sont interprétées comme pouvant provenir des photo-oxydes superficiels qui portent la charge électrique. Le dipôle formé par ces oxydes superficiels définit l'OCP mesuré. L'oscillation de l’OCP pourrait également être utilisée pour la métrologie des NHs. Cela ouvre la perspective d'étendre la métrologie DIP aux NHs semi-conducteurs avec un signal PL non existant ou négligeable. Enfin, les entités chargées proches du voisinage d'une surface semi-conductrice affectent le taux de DIP. Cette propriété a été utilisée pour détecter la Legionella pneumophila qui est normalement chargée négativement au pH> 4. Les mesures de FTIR ont indiqué que les monocouches auto-assemblées (SAM) d’alkanethiol restent sur la surface semi-conductrice pendant le DIP. Cela a permis la détection de la Legionella pneumophila vivante à une concentration de 105 CFU/mL avec une architecture simple à base d'anticorps. Une discussion a été proposée suggérant des protocoles de biocapteurs possibles pour atteindre des limites de détection améliorées avec le biocapteur DIP.Fabrication of devices based on semiconductor multilayer structures demands routine measurement of thicknesses and location of the interfaces of the constituent layers. This is often achieved using expensive and complicated techniques such as scanning electron microscopy, secondary ion mass spectroscopy, atomic force microscopy (AFM) or x-ray photoelectron spectroscopy (XPS). In this work, room temperature metrology in water environment has been developed for post-growth testing of semiconductor nanoheterostructures (NHs). The method utilizes the process of digital photocorrosion (DIP) and the sensitivity of photoluminescence (PL) emission to surface states revealed during photocorrosion. The photocorrosion process of GaAs/AlGaAs semiconductor NHs has been investigated in the presence of above bandgap excitation of samples immersed in different aqueous solutions. In order to achieve precise control over photocorrosion rates, the NH samples were placed in a flow cell with controlled aqueous environment. A low intensity above-bandgap photoexcitation (< 105 mW/cm2) was incident in a pulse mode and characterized by a duty cycle (DC) given by TON/(TON + TOFF). This has produced average etch rates of material removed at sub-monolayer precision during each DIP cycle. Using AFM and XPS, it has been demonstrated that the PL emission from a GaAs/AlGaAs NH during DIP oscillates owing to revealed GaAs and AlGaAs layers. These oscillations are caused by the sensitivity of the PL emission to the carrier surface recombination velocity, which drastically differs for GaAs and AlGaAs. The DIP process has revealed a 1 nm thick GaAs in a GaAs/AlGaAs NH structure, but this does not seem to be the resolution limit of this approach. Open circuit potential (OCP) measured during DIP differs amongst GaAs-electrolyte and AlGaAs-electrolyte junctions formed during the photocorrosion process. The OCP oscillations were found in-phase with the PL oscillations measured during DIP. The differences of OCP are theorized to originate from the surficial photo-oxides that carry electric charge. The dipole formed by these surficial oxides define the measured OCP. The OCP oscillation could also be used for metrology of NHs. This opens the prospect of extending the DIP metrology to semiconductor NHs with non-existing or negligible PL signal. Lastly, charged entities near the vicinity of a semiconductor surface affects the rate of DIP. This property has been utilized to detect Legionella pneumophila that normally are negatively charged at pH > 4. Fourier transform infrared spectroscopy measurements have indicated that alkanethiol self-assembled monolayers (SAMs) remain on the semiconductor surface during DIP. This has allowed for the detection of live Legionella pneumophila at 105 CFU/mL with a simple antibody-based architecture. A discussion has been provided suggesting possible biosensing protocols for achieving enhanced detection limits with the DIP biosensor

SARS-CoV-2 detection with aptamer-functionalized gold nanoparticles

A rapid detection test for SARS-CoV-2 is urgently required to monitor virus spread and containment. Here, we describe a test that uses nanoprobes, which are gold nanoparticles functionalized with an aptamer specific to the spike membrane protein of SARS-CoV-2. An enzyme-linked immunosorbent assay confirms aptamer binding with the spike protein on gold surfaces. Protein recognition occurs by adding a coagulant, where nanoprobes with no bound protein agglomerate while those with sufficient bound protein do not. Using plasmon absorbance spectra, the nanoprobes detect 16 nM and higher concentrations of spike protein in phosphate-buffered saline. The time-varying light absorbance is examined at 540 nm to determine the critical coagulant concentration required to agglomerates the nanoprobes, which depends on the protein concentration. This approach detects 3540 genome copies/μl of inactivated SARS-CoV-2

Open circuit potential monitored digital photocorrosion of GaAs/AlGaAs quantum well microstructures

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Photoluminescence monitored photocorrosion for metrology of GaAs/AlGaAs heterostructures at the nanoscale: digital etching of GaAs: a 30-year (almost) perspective

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Photocorrosion metrology of photoluminescence emitting GaAs/AlGaAs heterostructures

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Photoluminescence monitored photocorrosion for metrology of GaAs/AlGaAs heterostructures at the nanoscale: digital etching of GaAs: a 30-year (almost) perspective (Conference Presentation)

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Biginelli Compound Functionalized Biosensors: GaAs/AlGaAs Biochips for Enhanced Detection of Bacteria

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Biginelli Compound Functionalized Biosensors: GaAs/AlGaAs Biochips for Enhanced Detection of Bacteria

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Modeling of the photoluminescence monitored photocorrosion of GaAs/AlGaAs nano-heterostructures

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Fibroblasts Accelerate Formation and Improve Reproducibility of 3D Cellular Structures Printed with Magnetic Assistance

Fibroblasts (mouse, NIH/3T3) are combined with MDA-MB-231 cells to accelerate the formation and improve the reproducibility of 3D cellular structures printed with magnetic assistance. Fibroblasts and MDA-MB-231 cells are cocultured to produce 12.5 : 87.5, 25 : 75, and 50 : 50 total population mixtures. These mixtures are suspended in a cell medium containing a paramagnetic salt, Gd-DTPA, which increases the magnetic susceptibility of the medium with respect to the cells. A 3D monotypic MDA-MB-231 cellular structure is printed within 24 hours with magnetic assistance, whereas it takes 48 hours to form a similar structure through gravitational settling alone. The maximum projected areas and circularities, and cellular ATP levels of the printed structures are measured for 336 hours. Increasing the relative amounts of the fibroblasts mixed with the MDA-MB-231 cells decreases the time taken to form the structures and improves their reproducibility. Structures produced through gravitational settling have larger maximum projected areas and cellular ATP, but are deemed less reproducible. The distribution of individual cell lines in the cocultured 3D cellular structures shows that printing with magnetic assistance yields 3D cellular structures that resemble in vivo tumors more closely than those formed through gravitational settling. The results validate our hypothesis that (1) fibroblasts act as a “glue” that supports the formation of 3D cellular structures, and (2) the structures are produced more rapidly and with greater reproducibility with magnetically assisted printing than through gravitational settling alone. Printing of 3D cellular structures with magnetic assistance has applications relevant to drug discovery, lab-on-chip devices, and tissue engineering