An integrated and multi-purpose microscope for the characterization of atomically thin optoelectronic devices

2D Hybrid Perovskite Sensors for Environmental and Healthcare Monitoring

Layered perovskites, a novel class of two-dimensional (2D) layered materials, exhibit versatile photophysical properties of great interest in photovoltaics and optoelectronics. However, their instability to environmental factors, particularly water, has limited their utility. In this study, we introduce an innovative solution to the problem by leveraging the unique properties of natural beeswax as a protective coating of 2D-fluorinated phenylethylammonium lead iodide perovskite. These photodetectors show outstanding figures of merit, such as a responsivity of >2200 A/W and a detectivity of 2.4 × 1018 Jones. The hydrophobic nature of beeswax endows the 2D perovskite sensors with an unprecedented resilience to prolonged immersion in contaminated water, and it increases the lifespan of devices to a period longer than one year. At the same time, the biocompatibility of the beeswax and its self-cleaning properties make it possible to use the very same turbidity sensors for healthcare in photoplethysmography and monitor the human heartbeat with clear systolic and diastolic signatures. Beeswax-enabled multipurpose optoelectronics paves the way to sustainable electronics by ultimately reducing the need for multiple components.

Confocal Raman microscope with versatile dual polarization snapshot acquisition

In this paper we propose a new strategy towards simultaneous co- and cross-polarized measurements of Raman spectra in a confocal microscope. One of the advantages of this strategy is that it is immune to polarization-dependent efficiency of diffraction gratings. It is shown via linear angle-resolved and circular polarization measurements that the accuracy of these snapshot polarization measurements on solid and liquid samples are in good agreement with available models and data. The interest of simultaneous acquisition of the total Raman response and the degree of polarization is discussed as well.

Improved Stability of Organic Photovotlaic Devices With FeCl3 Intercalated Graphene Electrodes

In this paper, we present the first organic photovoltaic (OPV) devices fabricated with FeCl3 intercalated few layer graphene (i-FLG) electrodes. i-FLG electrodes were first fabricated and characterized by electrical and spectroscopic means, showing enhanced conductive properties compared to pristine graphene. These electrodes were then used in the fabrication of OPV devices and tested against devices made with commercially available Indium Tin Oxide (ITO) electrodes. Both types of device achieved similar efficiencies, while the i-FLG based device exhibited superior charge transport properties due to the increase in work function characterizing i-FLG. Both types of device underwent a stability study using both periodic and continuous illumination measurements, which revealed i-FLG based OPVs to be significantly more stable than those based on ITO. These improvements are expected to translate to increased device lifetimes and a greater total energy payback from i-FLG based photovoltaic devices. These results highlight the potential benefits of using intercalated graphene materials as an alternative to ITO in photovoltaic devices.

Engineering Dielectric Screening for Potential-well Arrays of Excitons in 2D Materials

Tailoring of the band gap in semiconductors is essential for the development of novel devices. In standard semiconductors, this modulation is generally achieved through highly energetic ion implantation. In two-dimensional (2D) materials, the photophysical properties are strongly sensitive to the surrounding dielectric environment presenting novel opportunities through van der Waals heterostructures encompassing atomically thin high-κ dielectrics. Here, we demonstrate a giant tuning of the exciton binding energy of the monolayer WSe₂ as a function of the dielectric environment. Upon increasing the average dielectric constant from 2.4 to 15, the exciton binding energy is reduced by as much as 300 meV in ambient conditions. The experimentally determined exciton binding energies are in excellent agreement with the theoretical values predicted from a Mott-Wannier exciton model with parameters derived from first-principles calculations. Finally, we show how texturing of the dielectric environment can be used to realize potential-well arrays for excitons in 2D materials, which is a first step toward exciton metamaterials.

Low Operating Voltage Carbon-Graphene Hybrid E-textile for Temperature Sensing

Graphene-coated polypropylene (PP) textile fibers are presented for their use as temperature sensors. These temperature sensors show a negative thermal coefficient of resistance (TCR) in a range between 30 and 45 °C with good sensitivity and reliability and can operate at voltages as low as 1 V. The analysis of the transient response of the temperature on resistance of different types of graphene produced by chemical vapor deposition (CVD) and shear exfoliation of graphite (SEG) shows that trilayer graphene (TLG) grown on copper by CVD displays better sensitivity due to the better thickness uniformity of the film and that carbon paste provides good contact for the measurements. Along with high sensitivity, TLG on PP shows not only the best response but also better transparency, mechanical stability, and washability compared to SEG. Temperature-dependent Raman analysis reveals that the temperature has no significant effect on the peak frequency of PP and expected effect on graphene in the demonstrated temperature range. The presented results demonstrate that these flexible, lightweight temperature sensors based on TLG with a negative TCR can be easily integrated in fabrics.

Laser-writable high-k dielectric for van der Waals nanoelectronics

Similar to silicon-based semiconductor devices, van der Waals heterostructures require integration with high-k oxides. Here, we demonstrate a method to embed and pattern a multifunctional few-nanometer-thick high-k oxide within various van der Waals devices without degrading the properties of the neighboring two-dimensional materials. This transformation allows for the creation of several fundamental nanoelectronic and optoelectronic devices, including flexible Schottky barrier field-effect transistors, dual-gated graphene transistors, and vertical light-emitting/detecting tunneling transistors. Furthermore, upon dielectric breakdown, electrically conductive filaments are formed. This filamentation process can be used to electrically contact encapsulated conductive materials. Careful control of the filamentation process also allows for reversible switching memories. This nondestructive embedding of a high-k oxide within complex van der Waals heterostructures could play an important role in future flexible multifunctional van der Waals devices.

Strain-Engineering of Twist-Angle in Graphene/hBN Superlattice Devices

The observation of novel physical phenomena such as Hofstadter's butterfly, topological currents, and unconventional superconductivity in graphene has been enabled by the replacement of SiO₂ with hexagonal boron nitride (hBN) as a substrate and by the ability to form superlattices in graphene/hBN heterostructures. These devices are commonly made by etching the graphene into a Hall-bar shape with metal contacts. The deposition of metal electrodes, the design, and specific configuration of contacts can have profound effects on the electronic properties of the devices possibly even affecting the alignment of graphene/hBN superlattices. In this work, we probe the strain configuration of graphene on hBN in contact with two types of metal contacts, two-dimensional (2D) top-contacts and one-dimensional edge-contacts. We show that top-contacts induce strain in the graphene layer along two opposing leads, leading to a complex strain pattern across the device channel. Edge-contacts, on the contrary, do not show such strain pattern. A finite-elements modeling simulation is used to confirm that the observed strain pattern is generated by the mechanical action of the metal contacts clamped to the graphene. Thermal annealing is shown to reduce the overall doping while increasing the overall strain, indicating an increased interaction between graphene and hBN. Surprisingly, we find that the two contact configurations lead to different twist-angles in graphene/hBN superlattices, which converge to the same value after thermal annealing. This observation confirms the self-locking mechanism of graphene/hBN superlattices also in the presence of strain gradients. Our experiments may have profound implications in the development of future electronic devices based on heterostructures and provide a new mechanism to induce complex strain patterns in 2D materials.

Graphene-Based Light Sensing: Fabrication, Characterisation, Physical Properties and Performance

Graphene and graphene-based materials exhibit exceptional optical and electrical properties with great promise for novel applications in light detection. However, several challenges prevent the full exploitation of these properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors, the lack of efficient generation and extraction of photoexcited charges, the smearing of photoactive junctions due to hot-carriers effects, large-scale fabrication and ultimately the environmental stability of the constituent materials. In order to overcome the aforementioned limits, different approaches to tune the properties of graphene have been explored. A new class of graphene-based devices has emerged where chemical functionalisation, hybridisation with light-sensitising materials and the formation of heterostructures with other 2D materials have led to improved performance, stability or versatility. For example, intercalation of graphene with FeCl 3 is highly stable in ambient conditions and can be used to define photo-active junctions characterized by an unprecedented LDR while graphene oxide (GO) is a very scalable and versatile material which supports the photodetection from UV to THz frequencies. Nanoparticles and quantum dots have been used to enhance the absorption of pristine graphene and to enable high gain thanks to the photogating effect. In the same way, hybrid detectors made from stacked sequences of graphene and layered transition-metal dichalcogenides enabled a class of devices with high gain and responsivity. In this work, we will review the performance and advances in functionalised graphene and hybrid photodetectors, with particular focus on the physical mechanisms governing the photoresponse, the performance and possible future paths of investigation.

Strain-engineered inverse charge-funnelling in layered semiconductors

The control of charges in a circuit due to an external electric field is ubiquitous to the exchange, storage and manipulation of information in a wide range of applications. Conversely, the ability to grow clean interfaces between materials has been a stepping stone for engineering built-in electric fields largely exploited in modern photovoltaics and opto-electronics. The emergence of atomically thin semiconductors is now enabling new ways to attain electric fields and unveil novel charge transport mechanisms. Here, we report the first direct electrical observation of the inverse charge-funnel effect enabled by deterministic and spatially resolved strain-induced electric fields in a thin sheet of HfS₂. We demonstrate that charges driven by these spatially varying electric fields in the channel of a phototransistor lead to a 350% enhancement in the responsivity. These findings could enable the informed design of highly efficient photovoltaic cells.

The application of strain to semiconducting materials can be used to engineer electric fields through a varying energy gap. Here, the authors observe an inverse charge-funnel effect in atomically thin HfS₂, enabled by strain-induced electric fields.

Ce-Doped bundled ultrafine diameter tungsten oxide nanowires with enhanced electrochromic performance

Cerium (Ce)-doped tungsten oxide nanostructures were synthesised using a simple solvothermal method from cerium chloride salt (CeCl₃·7H₂O) and tungsten hexachloride (WCl₆) precursors. The as-prepared samples were thoroughly characterised using electron microscopies, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The electrochromic performance of different samples was evaluated using a custom-built UV-VIS spectrometer and an electrochemistry technique. The results showed that the as-prepared samples underwent morphological evolution with the increase in the Ce/W molar ratio, from long, thin and bundled nanowires, through shorter and thicker nanowires to mixed nanowire bundles and nanoparticle agglomerates. From electrochemical testing, we found that the Ce-doped tungsten oxides exhibited higher optical contrasts of 44.3%, 49.7% and 39.4% for the 1 : 15, 1 : 10 and 1 : 5 Ce/W ratios respectively, compared with 37.4% for the pure W₁₈O₄₉ nanowires. The Ce/W = 1 : 15 samples presented an improved colouration efficiency of 67.3 cm² C^-1 against 62 cm² C^-1 for pure W₁₈O₄₉. This work demonstrated that the Ce-doped W₁₈O₄₉ nanowires are very promising candidate materials for the design and construction of electrochemical chromic devices with largely improved efficiency, contrast and stability. The results from this work suggested that smart electrochromic devices based on current Ce-doped WO_x nanomaterials could be further developed for future energy-related applications.

Novel circuit design for high-impedance and non-local electrical measurements of two-dimensional materials

Two-dimensional materials offer a novel platform for the development of future quantum technologies. However, the electrical characterisation of topological insulating states, non-local resistance, and bandgap tuning in atomically thin materials can be strongly affected by spurious signals arising from the measuring electronics. Common-mode voltages, dielectric leakage in the coaxial cables, and the limited input impedance of alternate-current amplifiers can mask the true nature of such high-impedance states. Here, we present an optical isolator circuit which grants access to such states by electrically decoupling the current-injection from the voltage-sensing circuitry. We benchmark our apparatus against two state-of-the-art measurements: the non-local resistance of a graphene Hall bar and the transfer characteristic of a WS₂ field-effect transistor. Our system allows the quick characterisation of novel insulating states in two-dimensional materials with potential applications in future quantum technologies.

Optical contrast and refractive index of natural van der Waals heterostructure nanosheets of franckeite

We study mechanically exfoliated nanosheets of franckeite by quantitative optical microscopy. The analysis of transmission-mode and epi-illumination-mode optical microscopy images provides a rapid method to estimate the thickness of the exfoliated flakes at first glance. A quantitative analysis of the optical contrast spectra by means of micro-reflectance allows one to determine the refractive index of franckeite over a broad range of the visible spectrum through a fit of the acquired spectra to a model based on the Fresnel law.