Despite being only one-atom thick, defect-free graphene is considered to be completely impermeable to all gases and liquids [1][2][3][4][5][6][7][8][9][10] . This conclusion is based on theory 3-8 and supported by experiments 1,9,10 that could not detect gas permeation through micrometre-size membranes within a detection limit of 10 5 to 10 6 atoms per second. Here, using small monocrystalline containers tightly sealed with graphene, we show that defect-free graphene is impermeable with an accuracy of eight to nine orders of magnitude higher than in the previous experiments. We could discern permeation of just a few helium atoms per hour, and this detection limit is also valid for all other tested gases (neon, nitrogen, oxygen, argon, krypton and xenon), except for hydrogen. Hydrogen shows noticeable permeation, even though its molecule is larger than helium and should experience a higher energy barrier. The puzzling observation is attributed to a two-stage process that involves dissociation of molecular hydrogen at catalytically active graphene ripples, followed by adsorbed atoms flipping to the other side of the graphene sheet with a relatively low activation energy of about 1.0 electronvolt, a value close to that previously reported for proton transport 11,12 . Our work provides a key reference for the impermeability of two-dimensional materials and is important from a fundamental perspective and for their potential applications.From a theory standpoint, monolayer graphene imposes a very high energy barrier for penetration of atoms and molecules. Density-functional-theory calculations predict that the barrier E is at least several eV 2-6 , which should prohibit any gas permeation under ambient conditions. Indeed, one can estimate that at room temperature T it would take longer than the lifetime of the universe to find an atom energetic enough to pierce a defect-free membrane of any realistic size. These expectations agree with experiments that reported no detectable gas permeation through mechanically-exfoliated graphene. The highest sensitivity was achieved using micrometersize wells etched in oxidized silicon wafers, which were sealed with graphene 1,9,10 . In those measurements, a pressurized gas (e.g., helium) could permeate along the SiO 2 layer and gradually fill the microcontainers making so-called 'nanoballoons'. Their consecutive deflation in air could be monitored using atomic force microscopy (AFM), and it was shown that the leakage occurred only along the SiO 2 surface, within several minutes but independently of the number of graphene layers used for the sealing 1 . These studies allowed a conclusion that graphene membranes were impermeable to all gases, at least with the achieved accuracy of 10 5 -10 6 atoms s -1 . This was further corroborated by creating individual atomic-scale defects in graphene nanoballoons, which resulted in their relatively fast deflation/inflation and confirmed the high sensitivity of the method 9,10 .

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy has been recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This issue inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Waals interaction. To do this, we made correlative far- and near-field characterizations validated by first-principle calculations that reveal a huge birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this remarkable anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.

We report a comprehensive experimental study of optical and electrical properties of thin polycrystalline gold films in a wide range of film thicknesses (from 20 to 200 nm). Our experimental results are supported by theoretical calculations based on the measured morphology of the fabricated gold films. We demonstrate that the dielectric function of the metal is determined by its structural morphology. Although the fabrication process can be absolutely the same for different films, the dielectric function can strongly depend on the film thickness. Our studies show that the imaginary part of the dielectric function of gold, which is responsible for optical losses, rapidly increases as the film thickness decreases for thicknesses below 80 nm. At the same time, we do not observe a noticeable dependence of optical constants on the film thickness for thicker samples. These findings establish design rules for thin-film plasmonic and nanophotonic devices.

Capillary condensation of water is ubiquitous in nature and technology. It routinely occurs in granular and porous media, can strongly alter such properties as adhesion, lubrication, friction and corrosion, and is important in many processes employed by microelectronics, pharmaceutical, food and other industries [1][2][3][4] . The century-old Kelvin equation 5 is commonly used to describe condensation phenomena and shown to hold well for liquid menisci with diameters as small as several nm [1][2][3][4][6][7][8][9][10][11][12][13][14] . For even smaller capillaries that are involved in condensation under ambient humidity and so of particular practical interest, the Kelvin equation is expected to break down because the required confinement becomes comparable to the size of water molecules . Here we take advantage of van der Waals assembly of two-dimensional crystals to create atomic-scale capillaries and study condensation inside. Our smallest capillaries are less than 4 Å in height and can accommodate just a monolayer of water. Surprisingly, even at this scale, the macroscopic Kelvin equation using the characteristics of bulk water is found to describe accurately the condensation transition in strongly hydrophilic (mica) capillaries and remains qualitatively valid for weakly hydrophilic (graphite) ones. We show that this agreement is somewhat fortuitous and can be attributed to elastic deformation of capillary walls [23][24][25] , which suppresses giant oscillatory behavior expected due to commensurability between atomic-scale confinement and water molecules 20,21 . Our work provides a much-needed basis for understanding of capillary effects at the smallest possible scale important in many realistic situations.The Kelvin equation predicts that capillaries become spontaneously filled with water at the relative humidity RHK = exp (-2σ/kBTdρN)where σ ≈ 73 mJ m -2 is the surface tension of water at room temperature T, ρN ≈ 3.3×10 28 m -3 is the number density of water, kB is the Boltzmann constant, and d is the diameter of the meniscus curvature. For a two-dimensional (2D) confinement created by parallel walls separated by a distance h, d = h/cos(θ) where θ is the contact angle of water on the walls' material. For capillary condensation to occur at relative humidity (RH) considerably below 100%, equation 1 dictates that d must be comparable to 2σ/kBTρN ≈ 1.1 nm. For example, under typical ambient RH of 40-50%, water is expected to condense in slits with h < 1.5 nm and cylindrical pores with diameters < 3 nm, if θ is close to zero. Even stronger confinement is required for capillaries involving less hydrophilic materials. So far, a broad consensus has been reached that the Kelvin equation remains accurate for menisci with d ≥ 8 nm [1][2][3][4][6][7][8][9][10][11] and can also describe condensation phenomena in hydrophilic pores as small as 4 nm in diameter 12-14 . To achieve agreement with the experiments at this scale, the Kelvin equation is usually modified to account for so-called wetting films that are adsorbed on...

Layered semiconductors such as transition metal dichalcogenides (TMDs) offer endless possibilities for designing modern photonic and optoelectronic components. However, their optical engineering is still a challenging task owing to multiple obstacles, including the absence of a rapid, contactless, and the reliable method to obtain their dielectric function as well as to evaluate in situ the changes in optical constants and exciton binding energies. Here, we present an advanced approach based on ellipsometry measurements for retrieval of dielectric functions and the excitonic properties of both monolayer and bulk TMDs. Using this method, we conduct a detailed study of monolayer MoS 2 and its bulk crystal in the broad spectral range (290-3300 nm). In the near-and midinfrared ranges, both configurations appear to have no optical absorption and possess an extremely high dielectric permittivity making them favorable for lossless subwavelength photonics. In addition, the proposed approach opens a possibility to observe a previously unreported peak in the dielectric function of monolayer MoS 2 induced by the use of perylene-3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) seeding promoters for MoS 2 synthesis and thus enables its applications in chemical and biological sensing. Therefore, this technique as a whole offers a state-of-the-art metrological tool for next-generation TMD-based devices.

The development of sensing interfaces can significantly improve the performance of biological sensors. Graphene oxide provides a remarkable immobilization platform for surface plasmon resonance (SPR) biosensors due to its excellent optical and biochemical properties. Here, we describe a novel sensor chip for SPR biosensors based on graphene-oxide linking layers. The biosensing assay model was based on a graphene oxide film containing streptavidin. The proposed sensor chip has three times higher sensitivity than the carboxymethylated dextran surface of a commercial sensor chip. Moreover, the demonstrated sensor chips are bioselective with more than 25 times reduced binding for nonspecific interaction and can be used multiple times. We consider the results presented here of importance for any future applications of highly sensitive SPR biosensing.

films are also the key element of plasmonic waveguides [10,11] and hyperbolic metamaterials. [12,13] Growth of continuous and ultrathin gold films on different substrates, such as glass, silicon oxide, silicon nitride, graphene etc. is notoriously difficult due to the poor wetting of gold to these substrates. [14,15] The growth kinetics of metal films is generally determined by the adsorption and diffusion behavior of metal adatoms on the substrate. A small ratio of the adsorption energy of metal adatoms on the substrate to the bulk cohesive energy of the metal and low diffusion barrier for an adatom favor the 3D island growth behavior also known as the Volmer-Weber growth mode. [16] Within the framework of this growth model, the formation of a metal film is associated with the following stages: nucleation of islands, island growth, island impingement and coalescence, percolation, and channel filling to finally form a continuous thin film. To reduce the percolation threshold of ultrathin gold films, adhesion or seed layers of Ti, Cr, Ni, Pt, or Ge are commonly used. However, these adhesion layers significantly affect the optical and electrical properties of ultrathin metal nanostructures. [17][18][19][20][21][22] Recently, the organosilane-based adhesion layers (mercaptosilanes and aminosilanes) were used for the deposition of sub-10 nm thick continuous Au films on silicon and glass surfaces. [23][24][25][26][27] However, organosilanes are not compatible with nonoxidized silicon surfaces and poorly compatible with standard lift-off procedures, that imposes severe limitations to their use as adhesion layers. [21] Adhesion layers based on organosilanes are also inefficient for the deposition of atomically thin metal films [14] and does not move us closer to the deposition of 2D layers from bulk plasmonic metals. Actually, the latter seems now as impossible as the deposition of atomically thin carbon films had been considered before 2004. [28] In the present paper, we propose the use of MoS 2 monolayer as an entirely new type of "universal" (i.e., it can be transferred to any arbitrary substrate) [29][30][31] adhesion layer for ultrathin (<10 nm) high-quality continuous gold films. To test the feasibility of this idea, we deposited ultrathin gold films of different thicknesses onto monolayer MoS 2 , grown on silicon dioxide substrates (Figure 1a), and studied their structural and optical properties.An electron beam evaporator Nano Master NEE-4000 was used to deposit Au films on top of atmospheric pressure CVD (APCVD)-grown full area coverage MoS 2 monolayers on silicon wafers with a 285 nm thick SiO 2 coating (from 2D semiconductors Inc.). The deposition was performed at Sub-10 nm continuous metal films are promising candidates for flexible and transparent nanophotonics and optoelectronic applications. In this article, it is demonstrated that monolayer MoS 2 is a perspective adhesion layer for the deposition of continuous conductive gold films with a thickness of only 3-4 nm. Optical properties of continuous ultrath...

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy was recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Walls interaction. To do this, we carried out a correlative far- and near-field characterization validated by first-principle calculations that reveals an unprecedented birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this outstanding anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.