Publications

No abstract is provided for this article.

Screen-printed graphene near field communication (NFC) tag antenna is integrated with inkjet-printed WS 2 photodetector on paper substrate to fabricate battery-free wireless photosensor. A sequential multi-stack printing is employed for the wireless photosensor fabrication: the NFC tag antenna is first screen-printed with graphene conductive ink and then the photodetector is inkjet-printed with transition metal dichalcogenides (TMDs) WS 2 ink as photoactive component. High responsivity and sensitivity are observed for the WS 2 photodetector, which acts as photoactive thermistor of the NFC sensor IC chip. The highly conductive graphene nanoflakes ink enables the screen-printed graphene NFC tag antenna to withdraw sufficient power wirelessly from the reader to power the sensor IC chip. This work demonstrates a prospective approach to manufacture 2D materials enabled electronics where the electronic circuits (normally having a large size) can be realized by mass production screen printing and the sensing component (normally having a small size) can be produced by inkjet printing, enabling low cost and simple fabrication methods, compatible with flexible substrates such as paper.

No abstract is provided for this article.

Optical imaging beyond the diffraction limit was one of the primary motivations for negative-index metamaterials, resulting in Pendry's perfect lens and the more attainable superlens. While these approaches offer sub-diffractional resolution, they do not provide a mechanism for magnification of the image. Hyperbolic (or indefinite-permittivity) metamaterials have been theoretically considered and experimentally demonstrated to provide simultaneously subdiffractive imaging and magnification; however, they are plagued with low efficiency and complex fabrication. In this work, we present theoretical and experimental studies of near-field optical imaging through a flat slab of the low-loss, natural hyperbolic material, hexagonal boron nitride (hBN). This thin hBN layer exhibits wavelength-dependent multifunctional operations, offering both an enhanced near-field imaging of single buried objects with down to lambda/32 resolution (0.4 um at lambda=12.8 um), as well as enabling an enlarged reconstruction of the geometric outline of the investigated objects. Both the excellent resolution and the multifunctional operation can be explained based on the volume-confined, wavelength-dependent propagation angle of Type I hyperbolic polaritons. Our results provide both the understanding of near-field imaging performance through this natural hyperbolic media, as well as inspire their exciting potential for guiding and sensing of light at an extreme sub-diffractional scale.

We present infrared spectra (0.1--1 eV) of electrostatically gated bilayer graphene as a function of doping and compare it with tight-binding calculations. All major spectral features corresponding to the expected interband transitions are identified in the spectra: a strong peak due to transitions between parallel split-off bands and two onset-like features due to transitions between valence and conduction bands. A strong gate voltage dependence of these structures and a significant electron-hole asymmetry are observed that we use to extract several band parameters. The structures related to the gate-induced band gap are less pronounced in the experiment than predicted by the tight-binding model that uses parameters obtained from previous experiments on graphite and recent self-consistent band-gap calculations.

The development of nanophotonics is hindered by a fundamental trade-off between a material's refractive index (n) and its electronic bandgap (Eg), which severely restricts the choice of materials for short-wavelength applications. This challenge is particularly acute in the visible and ultraviolet (UV) spectra, where high-performance devices require materials that are simultaneously highly refractive and transparent. Here, we report on the van der Waals (vdW) crystal cadmium phosphorus trisulfide (CdPS3) as a solution to this long-standing problem. Through comprehensive optical and structural characterization, we show that CdPS3 possesses an anomalously high in-plane refractive index across the visible spectrum approaching 3 in the near-UV, combined with a wide indirect bandgap. This combination of properties, which circumvents the empirical Moss's law, is validated by first-principles calculations and direct near-field imaging of highly confined waveguide modes. These findings establish CdPS3 as a leading material for UV-visible photonics, opening a new pathway for the development of high-density integrated circuits and metasurfaces.

No abstract is provided for this article.

We use an atomic force microscope (AFM) to manipulate graphene films on a nanoscopic length scale. By means of local anodic oxidation with an AFM we are able to structure isolating trenches into single-layer and few-layer graphene flakes, opening the possibility of tabletop graphene based device fabrication. Trench sizes of less than 30 nm in width are attainable with this technique. Besides oxidation we also show the influence of mechanical peeling and scratching with an AFM of few layer graphene sheets placed on different substrates.

No abstract is provided for this article.

Crystallographic alignment between two-dimensional crystals in van der Waals heterostructures brought a number of profound physical phenomena, including observation of Hofstadter butterfly and topological currents, and promising novel applications, such as resonant tunnelling transistors. Here, by probing the electronic density of states in graphene using graphene-hexagonal boron nitride-graphene tunnelling transistors, we demonstrate a structural transition of bilayer graphene from incommensurate twisted stacking state into a commensurate AB stacking due to a macroscopic graphene self-rotation. This structural transition is accompanied by a topological transition in the reciprocal space and by pseudospin texturing. The stacking transition is driven by van der Waals interaction energy of the two graphene layers and is thermally activated by unpinning the microscopic chemical adsorbents which are then removed by the self-cleaning of graphene.

Two-dimensional crystals are promising building blocks for the new generation of energy materials due to their low volume, high surface area, and high transparency. Electrochemical behavior of these crystals determines their performance in applications such as energy storage/conversion, sensing, and catalysis. Nevertheless, the electrochemistry of an isolated monolayer of molybdenum disulfide, which is one of the most promising semiconducting crystals, has not been achieved to date. We report here on photoelectrochemical properties of pristine monolayer and few-layer basal plane MoS2, namely the electron transfer kinetics and electric double-layer capacitance, supported by an extensive physical and chemical characterization. This enables a comparative qualitative correlation among the electrochemical data, MoS2 structure, and external illumination, although the absolute magnitudes of the electron transfer and capacitance are specific to the redox mediator and electrolyte used in these measurements ([Ru(NH3)6]3+/2+ and LiCl, respectively). Our work shows a strong dependence of the electrochemical properties on the number of MoS2 layers and illumination intensity and proves that an effective interlayer charge transport occurs in bulk MoS2. This highlights the exciting opportunities for tuning of the electrochemical performance of MoS2 through modification of its structure, external environment, and illumination.

Due to their extraordinary quality factor and extreme sensitivity to surface perturbations, Bloch surface waves (BSW) have been widely investigated for sensing applications so far. Over the last few years, on-chip control of optical signals through BSW has experienced a rapidly-expanding interest in the scientific community, attesting to BSW’s position at the forefront towards on-chip optical operations. The backbone of on-chip optical devices requires the choice of integrated optical sources with peculiar optic/optoelectronic properties, the efficient in-plane propagation of the optical signal and the possibility to dynamic manipulate the signal through optical or electrical driving. In this paper, we discuss our approach in addressing these requirements. Regarding the optical source integration, we demonstrate the possibility to couple the MoS2 mono- and bi-layers emission—when integrated on top of a 1D photonic crystal—to a BSW. Afterward, we review our results on BSW-based polariton systems (BSWP). We show that the BSWPs combine long-range propagation with energy tuning of their dispersion through polariton–polariton interactions, paving the way for logic operations.

No abstract is provided for this article.

Water molecules at the solid–liquid interface display intricate behaviors sensitive to small changes. The presence of different interfacial components, such as cations or functional groups, shapes the physical and chemical properties of the hydrogen-bond network. Understanding such interfacial hydrogen-bond networks is essential for a large range of applications and scientific questions. To probe the interfacial hydrogen-bond network, atmospheric water capture is a powerful tool. Here, we experimentally observe that a calcium ion on a calcium-intercalated graphene oxide aerogel (Ca-GOA) surface captures 3.2 times more water molecules than in its freestanding state. From experimental Van’t Hoff estimation and density functional theory (DFT) calculations, we uncover the synergistically enhanced hydrogen-bond network of the calcium ion–epoxide complex due to significantly larger polarizations and hydrogen bond enthalpies. This study reveals valuable insights into the interfacial water hydrogen-bond network on functionalized carbon–cation complexed surfaces and potential pathways for future atmospheric water generation technologies.