Proton-driven patterning of bulk transition metal dichalcogenides

At the few-atom-thick limit, transition metal dichalcogenides (TMDs) exhibit a host of attractive electronic optical, and structural properties. The possibility to pattern these properties has a great impact on applied and fundamental research. Here, we demonstrate spatial control over the light emission, lattice deformation, and hydrogen storage in bulk TMDs. By low-energy proton irradiation, we create uniquely favorable conditions for the production and accumulation of molecular hydrogen just one or few monolayers beneath the crystal basal plane of bulk WS2, WSe2, WTe2, MoSe2, and MoS2 samples. H2 therein produced coalesces to form bubbles, which lead to the localized swelling of one X-M-X plane prevalently. This results eventually in the creation of atomically thin domes filled with molecular hydrogen at 10 atm. The domes emit light strongly well above room temperature and can store H2 indefinitely. They can be produced with the desired density, well-ordered positions, and size tunable from the nanometer to the micrometer scale, thus providing a template for the manageable and durable mechanical and electronic structuring of two-dimensional materials

The Interaction of Hydrogen with the van der Waals Crystal γ-InSe

The emergence of the hydrogen economy requires development in the storage, generation and sensing of hydrogen. The indium selenide (γ-InSe) van der Waals (vdW) crystal shows promise for technologies in all three of these areas. For these applications to be realised, the fundamental interactions of InSe with hydrogen must be understood. Here, we present a comprehensive experimental and theoretical study on the interaction of γ-InSe with hydrogen. It is shown that hydrogenation of γ-InSe by a Kaufman ion source results in a marked quenching of the room temperature photoluminescence signal and a modification of the vibrational modes of γ-InSe, which are modelled by density functional theory simulations. Our experimental and theoretical studies indicate that hydrogen is incorporated into the crystal preferentially in its atomic form. This behaviour is qualitatively different from that observed in other vdW crystals, such as transition metal dichalcogenides, where molecular hydrogen is intercalated in the vdW gaps of the crystal, leading to the formation of "bubbles" for hydrogen storage

Synthesis, structure, and characterization of 4,4′-(Anthracene-9,10-diylbis(ethyne-2,1-diyl))bis(1-methyl-1-pyridinium) Bismuth Iodide (C30H22N2)3Bi4I18, an air, water, and thermally stable 0D hybrid Perovskite with high photoluminescence ffficiency

4,4'-(Anthracene-9,10-diylbis(ethyne-2,1-diyl))bis(1-methyl-1-pyridinium) bismuth iodide (C30H22N2)3Bi4I18 (AEPyBiI) was obtained as a black powder by a very simple route by mixing an acetone solution of BiI3 and an aqueous solution of C30H22N2I2. This novel perovskite is air and water stable and displays a remarkable thermal stability up to nearly 300 °C. The highly conjugated cation C30H22N2 2+ is hydrolytically stable, being nitrogen atoms quaternarized, and this accounts for the insensitivity of the perovskite toward water and atmospheric oxygen under ambient conditions. The cation in aqueous solution is highly fluorescent under UV irradiation (emitting yellow-orange light). AEPyBiI as well is intensely luminescent, its photoluminescence emission being more than 1 order of magnitude greater than that of high-quality InP epilayers. The crystal structure of AEPyBiI was determined using synchrotron radiation single-crystal X-ray diffraction. AEPyBiI was extensively characterized using a wide range of techniques, such as X-ray powder diffraction, diffuse reflectance UV-vis spectroscopy, Fourier transform infrared (FTIR) and Raman spectroscopies, thermogravimetry-differential thermal analysis (TG-DTA), elemental analysis, electrospray ionization mass spectroscopy (ESI-MS), and photoluminescence spectroscopy. AEPyBiI displays a zero-dimensional (0D) perovskite structure in which the inorganic part is constituted by binuclear units consisting of two face-sharing BiI6 octahedra (Bi2I9 3- units). The C30H22N2 2+ cations are stacked along the a-axis direction in a complex motif. Considering its noteworthy light-emitting properties coupled with an easy synthesis and environmental stability, and its composition that does not contain toxic lead or easily oxidable Sn(II), AEPyBiI is a promising candidate for environmentally friendly light-emitting devices

Extraordinary second harmonic generation modulated by divergent strain field in pressurized monolayer domes

The most prominent form of nonlinear optical (NLO) frequency conversion is second harmonic generation (SHG), where incident light interacts with a nonlinear medium producing photons at double the input frequency, which has vast applications in material and biomedical science. Emerging two-dimensional nonlinear optical materials led by transition metal dichalcogenides (TMDs) have fascinating optical and mechanical properties and are highly anticipated to overcome the technical limitations imposed by traditional bulky NLO materials. However, the atomic scale interaction length and low conversion efficiency in TMD materials prevent their further implementation in NLO applications. While some uniaxial strain-engineering studies intensively investigated the anisotropic SHG response in TMDs, they did not realize giant SHG enhancement by exploiting the opto-mechanical characteristics. Herein, we employ proton (H+) irradiation to successfully fabricate large pressurized monolayer TMD domes (d ≥ 10 μm) and conduct a comprehensive investigation and characterization of their SHG performance enhancement. We show that the intensity of SHG is effectively enhanced by around two orders of magnitude at room temperature. Such giant enhancement arises from the distinct separation distance induced by capped pressurized gas and the hemi-spherical morphology, enabling constructive optical interference. Moreover, the unique divergent strain field in TMD domes promotes the first experimental study on the anisotropic nonlinear optical behavior based on biaxial strain conditions in terms of varying strain orientation and relative weights. Our work demonstrates a promising system with enhanced NLO performance and well-preserved biocompatibility, paving a way toward the future nano-scaled quantum optics design and biomedical applications

Localisation-to-delocalisation transition of moir\'{e} excitons in WSe2_2/MoSe2_2 heterostructures

Moir\'{e} excitons (MXs) are electron-hole pairs localised by the periodic (moir\'{e}) potential forming in two-dimensional heterostructures (HSs). MXs can be exploited, $e.g.$, for creating nanoscale-ordered quantum emitters and achieving or probing strongly correlated electronic phases at relatively high temperatures. Here, we studied the exciton properties of a WSe$_2$/MoSe$_2$ HS from $T$=6 K to room temperature using time-resolved and continuous-wave micro-photoluminescence, also under magnetic field. The exciton dynamics and emission lineshape evolution with temperature show clear signatures that MXs de-trap from the moir\'{e} potential and turn into free interlayer excitons (IXs) at $T\gtrsim$120 K. The MX-to-IX transition is also apparent from the exciton magnetic moment reversing its sign when the moir\'{e} potential is not capable to localise excitons at elevated temperatures. Concomitantly, the exciton formation and decay times reduce drastically. Thus, our findings establish the conditions for a truly confined nature of the exciton states in a moir\'{e} superlattice with increasing temperature

Excitons and trions in WSSe monolayers

The possibility of almost linear tuning of the band gap and of the electrical and optical properties in monolayers (MLs) of semiconducting transition metal dichalcogenide (S-TMD) alloys opens up the way to fabricate materials with on-demand characteristics. By making use of photoluminescence spectroscopy, we investigate optical properties of WSSe MLs with a S/Se ratio of 57/43 deposited on SiO$_2$/Si substrate and encapsulated in hexagonal BN flakes. Similarly to the $"parent"$ WS$_2$ and WSe$_2$ MLs, we assign the WSSe MLs to the ML family with the dark ground exciton state. We find that, in addition to the neutral bright A exciton line, three observed emission lines are associated with negatively charged excitons. The application of in-plane and out-of-plane magnetic fields allows us to assign undeniably the bright and dark (spin- and momentum-forbidden) negative trions as well as the phonon replica of the dark spin-forbidden complex. Furthermore, the existence of the single photon emitters in the WSSe ML is also demonstrated, thus prompting the opportunity to enlarge the wavelength range for potential future quantum applications of S-TMDs.Comment: 6 pages, 5 figures, +ES

Hole and electron effective masses in single InP nanowires with a Wurtzite-Zincblende homojunction

The formation of wurtzite (WZ) phase in III–V nanowires (NWs) such as GaAs and InP is a complication hindering the growth of pure-phase NWs, but it can also be exploited to form NW homostructures consisting of alternate zincblende (ZB) and WZ segments. This leads to different forms of nanostructures, such as crystal-phase superlattices and quantum dots. Here, we investigate the electronic properties of the simplest, yet challenging, of such homostructures: InP NWs with a single homojunction between pure ZB and WZ segments. Polarization-resolved microphotoluminescence (μ-PL) measurements on single NWs provide a tool to gain insights into the interplay between NW geometry and crystal phase. We also exploit this homostructure to simultaneously measure effective masses of charge carriers and excitons in ZB and WZ InP NWs, reliably. Magneto-μ-PL measurements carried out on individual NWs up to 29 T at 77 K allow us to determine the free exciton reduced masses of the ZB and WZ crystal phases, showing the heavier character of the WZ phase, and to deduce the effective mass of electrons in ZB InP NWs (me= 0.080 m0). Finally, we obtain the reduced mass of light-hole excitons in WZ InP by probing the second optically permitted transition Γ7C ↔ Γ7uV with magneto-μ-PL measurements carried out at room temperature. This information is used to extract the experimental light-hole effective mass in WZ InP, which is found to be mlh = 0.26 m0, a value much smaller than the one of the heavy hole mass. Besides being a valuable test for band structure calculations, the knowledge of carrier masses in WZ and ZB InP is important in view of the optimization of the efficiency of solar cells, which is one of the main applications of InP NWs

Immune microenvironment dynamics in breast cancer during pregnancy: impact of gestational age on tumor-infiltrating lymphocytes and prognosis

BackgroundBreast cancer during pregnancy (PrBC) is a rare condition known for its aggressive clinical behavior. The presence of tumor-infiltrating lymphocytes (TILs) has been shown to have a significant impact on the prognosis of these patients. Despite some biological characteristics of the tumor that may differ depending on the gestational age, little is known about the dynamics of the immune landscape within the tumor microenvironment (TME) in PrBC. Therefore, in this study, our objective was to gain comprehensive insights into the relationship between gestational age at breast cancer diagnosis and the composition of the TME.Methodsn = 108 PrBC were selected from our institutional registry and categorized based on the gestational age by trimester. For all cases, TILs were profiled according to the International TILs Working Group recommendations, and subtyped by CD4, CD8, and forkhead box P3 (FOXP3) immunohistochemistry. PD-L1 was tested according to the combined positive score (CPS) using the IHC 22C3 pharmDx assay, with a cutoff value of ≥10 for positivity. The statistical approach encompassed Fisher’s and Chi-squared tests, with appropriate adjustments for multiple comparisons, logistic regression models, and survival analyses based on the Kaplan–Meier method.ResultsThe proportion of patients with poorly differentiated (G3) neoplasms increased as the gestational age advanced (first trimester, n = 25, 56.8%; second trimester, n = 27, 69.2%; third trimester, n = 21, 87.5%; p = 0.03). The histologic subtypes as well as the hormone receptor (HR) and HER2 status did not show significant changes across different pregnancy trimesters. In the HR+/HER2– subtype, there was a higher proportion of tumors with high/moderate TILs in the early phases of pregnancy, similar to FOXP3 expression (TILs: first trimester, n = 10, 35.7%; second trimester, n = 2, 10.5%; third trimester, n = 0; p = 0.02; FOXP3: first trimester, n = 10, 40%; second trimester, n = 3, 15.8%; third trimester, n = 0; p = 0.03). The median follow-up for our cohort was 81 months. Patients who relapsed after a breast cancer diagnosis during the first trimester were more frequently PD-L1-negative, unlike those with no disease recurrence (n = 9, 100% vs. n = 9, 56.3%; p = 0.03; hormone therapy and n = 9, 100% vs. n = 7, 53.9%; p = 0.02; chemotherapy). No statistically significant differences were seen among the three trimesters in terms of survival outcome.ConclusionThe TME dynamics of HR+/HER2− PrBC vary based on gestational age, suggesting that immune tolerance expression during later gestational age could explain the increased aggressiveness of tumors diagnosed at that stage

Dome-shaped two-dimensional crystals: a playground for the study of the crystal mechanical and optoelectronic properties

Graphene -a single layer of carbon atoms tightly packed into a 2D honeycomb lattice- was first isolated in 2004 by A. K. Geim and K. S. Novoselov. The ground-breaking discovery that graphene can be isolated, previously thought to be impossible, opened the doors of Flatland to the condensed matter physics community. Since then, the family of 2D systems has grown rapidly, as many other crystals have been found to be characterised by a layered structure akin to graphite, with different layers bound together by weak van der Waals (vdW) forces. Among them, graphene features a semi-metallic nature and is characterised by exceptionally high carrier mobilities; hexagonal boron nitride (hBN) is an extremely good insulator and dielectric with a large bandgap; and the family of transition metal dichalcogenides (TMDs, such as MoS2 WS2, MoSe2, WSe2, MoTe2, WTe2, NbSe2, etc.) is richly varied, as it comprises superconducting materials with charge density waves and Weyl semimetal properties, as well as several semiconducting materials, with bandgaps ranging from the visible to the near infrared spectral region. In the single layer limit, semiconducting TMDs are characterised by extremely efficient light emission, which makes them ideal candidates for the realisation of innovative, flexible optoelectronic devices. Aside from the possibility of exploring the effects of lower dimensionality on the properties of atomically thin crystals, the existence of these crystals in stable form opens new avenues to materials engineering. Indeed, the inherent all-surface nature of these systems entails a higher sensitivity to external perturbations, which can in turn be exploited to modify the material properties. Among all possible external perturbations, the incredible mechanical flexibility and robustness of 2D crystals have offered the possibility to subject them to high mechanical deformations, engendering strains larger than 10 %. Such strains are able to induce major modifications in the electronic, optical, magnetic, transport and chemical properties of 2D materials, leading to the observation of a plethora of intriguing phenomena---ripe with new physics and novel opportunities. In the past decade, great attention has been thus devoted to the development of methods to mechanically deform 2D crystals and on the study of the effects of strain on these materials. This thesis will be focused on the development of an original strategy to induce strain fields in 2D crystals, and on the study of the effect of strain on the peculiar properties of the material. The thesis will be articulated as follows: The Prologue will briefly introduce the reader to the 2D world, especially highlighting the peculiar properties of 2D TMDs and hBN, thanks to which a flourishing interest in these materials has arisen. The final part of the Prologue will instead provide the reader with an overview of the field of strain engineering of 2D crystals. Chapter 1 will present the innovative method to induce strain in TMDs and hBN pioneered by the candidate and her group. It will be discussed how, by irradiating bulk flakes of these materials with low-energy hydrogen ions, it is possible to induce on the flake surface the formation of domes with thickness of one-to-few layers and filled with pressurised hydrogen. The basic properties of these structures will be discussed. This Chapter will also discuss the effects of hydrogen-ion irradiation of other crystals, where the formation of domes was not achieved but other interesting phenomenologies were observed. Chapter 2 will present a characterisation of the vibrational properties of the domes, highlighting their link with the strain distribution. This chapter will focus in particular on Raman studies of TMD domes and on Raman and infrared (IR) characterisations of hBN domes. The observed huge shifts and splittings of the vibrational modes will be correlated with the strain magnitude and character, which will be estimated by numerical calculations. Chapter 3 will focus on the possibility to engineer the domes. Lithography-based approaches will be used to achieve control over their size and position, and eventually over the strain magnitude. Chapter 4 will investigate from a fundamental point of view the morphology and mechanics of the system. In particular, an analytical method to describe the system will be presented. The model, coupled to morphological and mechanical experimental measurements, allows one to obtain precious information on the elastic properties of the membrane and on the adhesion energy between the monolayer and the bulk crystal. Chapter 5 will discuss how strain affects the optoelectronic properties of TMDs. In particular, this chapter will present steady-state and time-resolved photoluminescence (PL) studies aimed at highlighting the effect of strain on the free excitons. Such measurements highlight intriguing behaviours, such as strain-induced direct-to-indirect exciton crossovers, that deeply affect the emitted light intensity and decay time. Chapter 6 will present a characterisation of direct and indirect excitons when subjected to high magnetic fields. Indeed, magnetic fields induce a Zeeman effect in TMD MLs, which is promising for their utilisation for valleytronics. The effect of strain on the Zeeman effect has however not been investigated so far. The results presented in this chapter shed light on this, and highlight an unexpected behaviour, that unveils hybridisation phenomena between nearly resonant direct and indirect excitons. Chapter 7 will demonstrate the possibility to exploit strained 2D materials for quantum applications. In particular, this chapter will discuss the observation of single photon emitters at cryogenic temperatures in hBN-capped TMD domes. Chapter 8 will investigate a novel perspective: that of exploiting selective strain engineering in van der Waals heterostructures. Specifically, we will here focus on heterostructures made of a TMD dome and of an InSe unstrained layer, showing how strain is able to modify the electronic properties of the heterostructure