Effects of inorganic seed promoters on MoS2 few-layers grown via chemical vapor deposition

In the last years, transition metal dichalcogenides (TMDs), especially at the two-dimensional (2D) limit, gained a large interest due to their unique optical and electronic properties. Among them, MoS2 received great attention from the scientific community due to its versatility, workability, and applicability in a large number of fields such as electronics, optoelectronics and electrocatalysis. To open the possibility of 2D-MoS2 exploitation, its synthesis over large macroscopic areas using cost-effective methods is fundamental. In this study, we report a method for the synthesis of large-area (~ cm2) few-layers MoS2 via liquid precursor CVD (L-CVD), where the Mo precursor (i.e. ammonium heptamolybdate AHM) is provided via a solution that is spin-coated over the substrate. Given the capability of organic and inorganic molecules, such as alkaline salts, to enhance MoS2 growth, we investigated the action of different inorganic salts as seed promoters. In particular, by using visible Raman spectroscopy, we focused on the effect of Na(OH), KCl, KI, and Li(OH) on the thickness, morphology, uniformity and degree of coverage of the grown MoS2. We optimized the process tuning parameters such as the volume of spin-coated solution, the growth temperature, and the seed promoter concentration, to synthesise the lowest possible thickness which resulted to be 2 layers (2L) of the highest quality. We witnessed that the addition of an inorganic seed promoter in the solution improves the extension of the grown MoS2 promoting lateral growth front, and therefore the degree of coverage. From this study, we conclude that, amongst the investigated seed promoters, K-based salts proved to grant the growth of high-quality two-layer MoS2 with optimal and uniform coverage of the SiO2/Si substrate surface

Large Area Growth of Silver and Gold Telluride Ultrathin Films via Chemical Vapor Tellurization

Developing a method for the growth of ultrathin metal chalcogenides, potentially targeting the two-dimensional (2D) limit, has a pivotal impact on various nanotechnological device applications. Here, we employed a vapor deposition scheme, based on tellurization, to induce the heterogenous chemical reaction between solid Ag and Au precursors, in the form of ultrathin films, and Te vapors. We characterized the morphological and structural properties of the grown tellurides by using atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction techniques. The developed tellurization methodology provides a key advancement in the picture of growing ultrathin noble metal tellurides and holds great potential for applications in different technological fields

Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets show a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of tin nanosheets therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.Comment: 14 pages, 7 figure

High‐Density Sb2Te3 Nanopillars Arrays by Templated, Bottom‐Up MOCVD Growth

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Broadband and Tunable Light Harvesting in Nanorippled MoS2 Ultrathin Films

Nanofabrication of flat optic silica gratings conformally layered with two-dimensional (2D) MoS2 is demonstrated over large area (cm2), achieving a strong amplification of the photon absorption in the active 2D layer. The anisotropic subwavelength silica gratings induce a highly ordered periodic modulation of the MoS2 layer, promoting the excitation of Guided Mode Anomalies (GMA) at the interfaces of the 2D layer. We show the capability to achieve a broadband tuning of these lattice modes from the visible (VIS) to the near-infrared (NIR) by simply tailoring the illumination conditions and/or the period of the lattice. Remarkably, we demonstrate the possibility to strongly confine resonant and nonresonant light into the 2D MoS2 layers via GMA excitation, leading to a strong absorption enhancement as high as 240% relative to a flat continuous MoS2 film. Due to their broadband and tunable photon harvesting capabilities, these large area 2D MoS2 metastructures represent an ideal scalable platform for new generation devices in nanophotonics, photo- detection and -conversion, and quantum technologies

Cancer immunotherapy is accompanied by distinct metabolic patterns in primary and secondary lymphoid organs observed by non-invasive in vivo18F-FDG-PET

Purpose: Cancer immunotherapy depends on a systemic immune response, but the basic underlying mechanisms are still largely unknown. Despite the very successful and widespread use of checkpoint inhibitors in the clinic, the majority of cancer patients do not benefit from this type of treatment. In this translational study, we investigated whether noninvasive in vivo positron emission tomography (PET) imaging using 2-[18F]fluoro-2-deoxy-D-glucose (18F-FDG) is capable of detecting immunotherapy-associated metabolic changes in the primary and secondary lymphoid organs and whether this detection enables the prediction of a successful anti-cancer immune response. Methods: RIP1-Tag2 mice with progressed endogenous insular cell carcinomas underwent a combined cancer immunotherapy consisting of CD4+ T cells plus monoclonal antibodies (mAbs) against programmed death ligand-1 (PD-L1) and lymphocyte activation gene-3 (LAG-3) or a sham treatment after radiation-mediated immune cell depletion. A second cohort of RIP1-Tag2 mice underwent exclusive checkpoint inhibitor therapy (CIT) using anti-PD-L1/LAG-3 mAbs or sham treatment without initial immune cell depletion to mimic the clinical situation. All mice were monitored by 18F-FDG-PET combined with anatomical magnetic resonance imaging (MRI). In addition, we retrospectively analyzed PET / computed tomography (CT) scans (PET/CT) regarding 18F-FDG uptake of CIT-treated metastatic melanoma patients in the spleen (n=23) and bone marrow (BM; n=20) as well as blood parameters (n=17-21). Results: RIP1-Tag2 mice with advanced insular cell carcinomas treated with combination immunotherapy exhibited significantly increased 18F-FDG uptake in the spleen compared to sham-treated mice. Histopathology of the spleens from treated mice revealed atrophy of the white pulp with fewer germinal centers and an expanded red pulp with hyperplasia of neutrophils than those of sham-treated mice. Immunohistochemistry and flow cytometry analyses of the spleens revealed a lower number of T cells and a higher number of neutrophils compared to those in the spleens of sham-treated mice. Flow cytometry of the BM showed enhanced activation of T cells following the treatment schemes that included checkpoint inhibitors. The ratio of 18F-FDG uptake at baseline to the uptake at follow-up in the spleens of exclusively CIT-treated RIP1-Tag2 mice was significantly enhanced, but the ratio was not enhanced in the spleens of the sham-treated littermates. Flow cytometry analysis confirmed a reduced number of T cells in the spleens of exclusively CIT-treated mice compared to that of sham-treated mice. A retrospective analysis of clinical 18F-FDG-PET/CT scans revealed enhanced 18F-FDG uptake in the spleens of some successfully CIT-treated patients with metastatic melanoma, but there were no significant differences between responders and non-responders. The analysis of the BM in clinical 18F-FDG-PET/CT scans with a computational segmentation tool revealed significantly higher baseline 18F-FDG uptake in patients who responded to CIT than in non-responders, and this relationship was independent of bone metastasis, even in the baseline scan. Conclusions: Thus, we are presenting the first translational study of solid tumors focusing on the metabolic patterns of primary and secondary lymphoid organs induced by the systemic immune response after CIT. We demonstrate that the widely available 18F-FDG-PET modality is an applicable translational tool that has high potential to stratify patients at an early time point

Analisi a campo ottico prossimo in modulazione di polarizzazione (PM- SNOM) di campioni metallici nanostrutturati

Lo scopo della tesi è sviluppare e utilizzare un’evoluzione della tecnica di microscopia a scansione di campo ottico prossimo, basata sulla modulazione di polarizzazione (PM-SNOM), per lo studio delle proprietà di campioni metallici nanostrutturati che mostrano attività ottica (dicroismo lineare) legata alla presenza di risonanze plasmoniche superficiali. I campioni oggetto della mia ricerca, prodotti presso l’Università di Genova (Prof. Buatier de Mongeot), sono costituiti da nanofili di oro quasi-regolari definiti su di un substrato di vetro tramite una tecnica innovativa, che prevede il bombardamento ionico del substrato e la successiva deposizione del metallo. I meccanismi di auto-organizzazione del metallo, uniti alla presenza di un pattern quasi-regolare sul substrato, conducono alla formazione di nanostrutture elongate aventi dimensioni trasversali dell’ordine del centinaio e spessori di poche decine di nanometri. Nanostrutture d’oro di questo tipo sono attese presentare risonanze plasmoniche nello spettro visibile; inoltre, a causa della loro anisotropia, l’assorbimento di radiazione collegato alla risonanza dipende fortemente dalla polarizzazione della luce incidente. Di conseguenza, i campioni presentano un notevole dicroismo lineare, così come evidenziato da misure macroscopiche di polarimetria (rapporto dicroico 0.3-0.4 a 690 nm). Nanostrutture metalliche con comportamento plasmonico suscitano attualmente un grande interesse per possibili applicazioni in diversi settori: si va dalla realizzazione di dispositivi nanofotonici, in grado di trasportare e condizionare la luce su dimensioni laterali molto minori della lunghezza d’onda, alla produzione di sensori molecolari per applicazioni biologiche attraverso effetti di aumento del campo elettromagnetico locale in prossimità della superficie delle nanostrutture. La definizione del potenziale applicativo dei campioni da me studiati richiede un’attenta e dettagliata analisi delle loro proprietà ottiche su una scala almeno paragonabile alla dimensione laterale delle strutture. Questa esigenza è particolarmente importante considerando la morfologia irregolare dei nanofili prodotti, diretta conseguenza del meccanismo di fabbricazione, che rende questi campioni molto diversi rispetto ai sistemi modello, tipicamente fabbricati per litografia elettronica, che si trovano spesso nella letteratura su plasmoni superficiali. Inoltre aggiustando opportunamente i parametri di fabbricazione è possibile modificare la morfologia dei nanofili, ad esempio il loro grado di interconnessione. Dunque un’ulteriore motivazione per la mia ricerca riguarda la caratterizzazione di campioni con differenti proprietà morfologiche. L’obiettivo di analizzare e misurare le proprietà ottiche di dicroismo con una risoluzione spaziale al di sotto del limite di diffrazione richiede lo sviluppo e l’impiego di tecniche particolari, come quelle che sfruttano il campo ottico prossimo. A questo scopo ho integrato nel microscopio SNOM disponibile in laboratorio un modulatore fotoelastico (PEM) per implementare un sistema di modulazione della polarizzazione della luce (laser a 690 nm) accoppiata nella fibra ottica che costituisce la sonda del microscopio. L’arrangiamento sperimentale produce una polarizzazione lineare la cui direzione oscilla nel tempo con una frequenza fissata (pari a 50 kHz). Inoltre ho impiegato tecniche di demodulazione sincrona dei deboli segnali ottici trasmessi dal campione interagente con il campo prossimo, allo scopo di individuare la risposta alla luce polarizzata. Rispetto a precedenti configurazioni sperimentali usate nello stesso laboratorio, l’uso del PEM presenta diversi vantaggi (maggiore flessibilità, semplicità d’uso, affidabilità, alta frequenza di modulazione, etc.). Tuttavia l’interpretazione dei risultati non è particolarmente immediata; ciò ha richiesto lo sviluppo di opportuni modelli (basati sul metodo delle matrici di Jones) che sono stati validati attraverso specifiche misure macroscopiche. Le analisi su scala sub-micrometrica sono basate sul confronto tra tre diverse mappe costruite simultaneamente durante ogni scansione SNOM: la mappa topografica, quella ottica integrata su tutti gli stati di polarizzazione e quella demodulata rispetto alla modulazione di polarizzazione. Le caratteristiche generali delle misure indicano la presenza di “domini” ottici di forma elongata, con asse maggiore corrispondente all’asse dei nanofili. Tuttavia, le mappe risultano spesso non completamente correlate fra loro nei dettagli, che la tecnica SNOM permette di evidenziare con una risoluzione spaziale migliore di 100 nm. In particolare le zone di massimo assorbimento ottico non sempre corrispondono alle regioni di massimo spessore di materiale (sommità dei nanofili) come ci si attenderebbe. Un’altra interessante osservazione suggerita dalle mappe del dicroismo lineare (quelle ottenute per demodulazione sincrona con la modulazione di polarizzazione) è che l’attività ottica è tipicamente massima fra nanofili adiacenti o nelle loro interconnessioni. La calibrazione quantitativa di queste mappe, eseguita sulla base del modello interpretativo messo a punto per questa tesi, indica valori del rapporto dicroico locale superiori rispetto a quelli misurati macroscopicamente (0.5-0.6 rispetto a 0.3-0.4). Da un lato le osservazioni compiute nella tesi testimoniano la capacità della tecnica di rivelare le proprietà ottiche genuine dei campioni, suggerendo ad esempio che eventuali artefatti topografici sono trascurabili. Dall’altro, esse indicano che l’accoppiamento locale tra campo prossimo e nanostrutture segue meccanismi particolari, probabilmente regolati dalla presenza di risonanze plasmoniche localizzate. Come confermato dall’analisi svolta di campioni con diverse morfologie, cioè prodotti con diversi parametri, il comportamento ottico presenta evidenti peculiarità soprattutto quando il campo prossimo si accoppia con difettosità locali (punte, bordi, biforcazioni, etc.) dei nanofili, dove ad esempio si verificano fenomeni di aumento locale del campo. Il ruolo che il disordine morfologico ha nel determinare le proprietà ottiche potrebbe essere in futuro sfruttato per sviluppare applicazioni specifiche, ad esempio per l’impiego dei campioni come substrati per l’analisi di fluorescenza o Raman da singole molecole

The Rise of the Xenes: From the Synthesis to the Integration Processes for Electronics and Photonics

The recent outcomes related to the Xenes, the two-dimensional (2D) monoelemental graphene-like materials, in three interdisciplinary fields such as electronics, photonics and processing are here reviewed by focusing on peculiar growth and device integration aspects. In contrast with forerunner 2D materials such as graphene and transition metal dichalcogenides, the Xenes pose new and intriguing challenges for their synthesis and exploitation because of their artificial nature and stabilization issues. This effort is however rewarded by a fascinating and versatile scenario where the manipulation of the matter properties at the atomic scale paves the way to potential applications never reported to date. The current state-of-the-art about electronic integration of the Xenes, their optical and photonics properties, and the developed processing methodologies are summarized, whereas future challenges and critical aspects are tentatively outlined

Ambient Pressure Chemical Vapor Deposition of Flat and Vertically Aligned MoS₂ Nanosheets

Molybdenum disulfide (MoS2) got tremendous attention due to its atomically thin body, rich physics, and high carrier mobility. The controlled synthesis of large area and high crystalline monolayer MoS2 nanosheets on diverse substrates remains a challenge for potential practical applications. Synthesizing different structured MoS2 nanosheets with horizontal and vertical orientations with respect to the substrate surface would bring a configurational versatility with benefit for numerous applications, including nanoelectronics, optoelectronics, and energy technologies. Among the proposed methods, ambient pressure chemical vapor deposition (AP-CVD) is a promising way for developing large-scale MoS2 nanosheets because of its high flexibility and facile approach. Here, we show an effective way for synthesizing large-scale horizontally and vertically aligned MoS2 on different substrates such as flat SiO2/Si, pre-patterned SiO2 and conductive substrates (TaN) benefit various direct TMDs production. In particular, we show precise control of CVD optimization for yielding high-quality MoS2 layers by changing growth zone configuration and the process steps. We demonstrated that the influence of configuration variability by local changes of the S to MoO3 precursor positions in the growth zones inside the CVD reactor is a key factor that results in differently oriented MoS2 formation. Finally, we show the layer quality and physical properties of as-grown MoS2 by means of different characterizations: Raman spectroscopy, scanning electron microscopy (SEM), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). These experimental findings provide a strong pathway for conformally recasting AP-CVD grown MoS2 in many different configurations (i.e., substrate variability) or motifs (i.e., vertical or planar alignment) with potential for flexible electronics, optoelectronics, memories to energy storage devices