The ability to exfoliate layered materials down to the single layer limit has presented the opportunity to understand how a gradual reduction in dimensionality affects the properties of bulk materials. Here we use this top–down approach to address the problem of superconductivity in the two-dimensional limit. The transport properties of electronic devices based on 2H tantalum disulfide flakes of different thicknesses are presented. We observe that superconductivity persists down to the thinnest layer investigated (3.5 nm), and interestingly, we find a pronounced enhancement in the critical temperature from 0.5 to 2.2 K as the layers are thinned down. In addition, we propose a tight-binding model, which allows us to attribute this phenomenon to an enhancement of the effective electron–phonon coupling constant. This work provides evidence that reducing the dimensionality can strengthen superconductivity as opposed to the weakening effect that has been reported in other 2D materials so far.

and (A.C-G.) andres.castellanos@imdea.org KEYWORDS. Black phosphorus, strain engineering, uniaxial strain, local strain, periodic deformation, quantum confinement, optical absorption. This is the post-peer reviewed version of the following article: J. Quereda et al. "Strong modulation of optical properties in black phosphorus through strain-engineered rippling" Nano Letters (2016) DOI:10.1021/acs.nanolett.5b04670 Which has been published in final form at: http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b04670 2 ABSTRACT Controlling the bandgap through local-strain engineering is an exciting avenue for tailoring optoelectronic materials. Two-dimensional crystals are particularly suited for this purpose because they can withstand unprecedented non-homogeneous deformations before rupture: one can literally bend them and fold them up almost like a piece of paper. Here, we study multi-layer black phosphorus sheets subjected to periodic stress to modulate their optoelectronic properties. We find a remarkable shift of the optical absorption band-edge of up to ~0.7 eV between the regions under tensile and compressive stress, greatly exceeding the strain tunability reported for transition metal dichalcogenides. This observation is supported by theoretical models which also predict that this periodic stress modulation can yield to quantum confinement of carriers at low temperatures. The possibility of generating large strain-induced variations in the local density of charge carriers opens the door for a variety of applications including photovoltaics, quantum optics and two-dimensional optoelectronic devices. TEXT.The recent isolation of black phosphorus has unleashed the interest of the community working on 2D materials because of its interesting electronic and optical properties: narrow intrinsic gap, ambipolar field effect and high carrier mobility. [1][2][3][4][5][6][7][8][9][10][11][12] Black phosphorus is composed of phosphorus atoms held together by strong bonds forming layers that interact through weak van der Waals forces holding the layers stacked on top of each other. This structure, without surface dangling This is the post-peer reviewed version of the following article: J. Quereda et al. "Strong modulation of optical properties in black phosphorus through strain-engineered rippling" Nano Letters (2016) DOI:10.1021/acs.nanolett.5b04670 Which has been published in final form at: http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b04670 3 bonds, allows black phosphorus susceptible to withstand very large localized deformations without breaking (similarly to graphene and MoS2). [13][14][15] Its outstanding mechanical resilience makes black phosphorus a prospective candidate for strain engineering, i.e. the modification of a material's optical/electrical properties by means of mechanical stress. 16 This is in contrast to conventional 3D semiconductors that tend to break for moderate deformations. Very recent theoretical works explore the effect of strain on the band structure and optical properties of black phosp...

The possibility of spatially resolving the optical properties of atomically thin materials is especially appealing as they can be modulated at the micro- and nanoscale by reducing their thickness, changing the doping level or applying a mechanical deformation. Therefore, optical spectroscopy techniques with high spatial resolution are necessary to get a deeper insight into the properties of two-dimensional (2D) materials. Here we study the optical absorption of single- and few-layer molybdenum disulfide (MoS2) in the spectral range from 1.24 eV to 3.22 eV (385 nm to 1000 nm) by developing a hyperspectral imaging technique that allows one to probe the optical properties with diffraction limited spatial resolution. We find hyperspectral imaging very suited to study indirect bandgap semiconductors, unlike photoluminescence which only provides high luminescence yield for direct gap semiconductors. Moreover, this work opens the door to study the spatial variation of the optical properties of other 2D systems, including non-semiconducting materials where scanning photoluminescence cannot be employed.

In monolayer transition metal dichalcogenides helicity-dependent charge and spin photocurrents can emerge, even without applying any electrical bias, due to circular photogalvanic and photon drag effects. Exploiting such circular photocurrents (CPCs) in devices, however, requires better understanding of their behavior and physical origin. Here, we present symmetry, spectral, and electrical characteristics of CPC from excitonic interband transitions in a MoSe2 monolayer. The dependence on bias and gate voltages reveals two different CPC contributions, dominant at different voltages and with different dependence on illumination wavelength and incidence angles. We theoretically analyze symmetry requirements for effects that can yield CPC and compare these with the observed angular dependence and symmetries that occur for our device geometry. This reveals that the observed CPC effects require a reduced device symmetry, and that effects due to Berry curvature of the electronic states do not give a significant contribution.

This is the post-peer reviewed version of the following article: J. Quereda et al. "Strong quantum confinement effect in the optical properties of ultrathin α-In2Se3".Advanced The isolation of atomically thin semiconductors has attracted the interest of the nanoscientists as these materials can complement graphene in those applications where its lack of an electronic band gap hinders its use. [1][2][3][4][5] In fact, two dimensional semiconductors have been recently employed in the fabrication of field effect transistors, [6,7] photodetectors [8][9][10][11][12][13][14] and solar cells. [15][16][17][18][19] Inherent to their reduced out-of-plane dimension, atomically thin semiconductors present a marked thickness-dependent band structure due to the quantum confinement of the charge carriers. For instance, the band gap in Mo-and W-based transition metal dichalcogenides increases from its bulk value (~1.3 eV, indirect gap) up to 1.9 eV (direct gap) in the single-layer limit. [20,21] More recently, black phosphorus has also shown a pronounced thickness-dependent band gap ranging from ~0.3 eV (direct gap) for bulk to ~1.75 eV (direct gap) for single-layer black phosphorus. [22] This thickness-dependent band gap can be very advantageous for applications as photodetectors as one can select the sensing photon energy window by simply selecting the right material thickness for the photodetector device. Up to now, however, there is a broad spectral window from 2.0 eV to 3.0 eV uncovered by 2D materials that could be very relevant for applications requiring 'solar blind' This is the post-peer reviewed version of the following article: J. Quereda et al. "Strong quantum confinement effect in the optical properties of ultrathin α-In2Se3". In this work we demonstrate a very strong quantum confinement effect in the optical properties of atomically thin α-In 2 Se 3 crystals. We observe a marked thickness-dependent shift in the optical absorption spectra acquired on mechanically exfoliated Figure S1). In order to study the optical absorption of the In 2 Se 3 crystals we used a home-made hyperspectral imaging setup, described in detail elsewhere.[25] The hyperspectral imaging is carried out by sweeping the excitation wavelength in steps and acquiring transmission mode images of the In 2 Se 3 crystals for each wavelength.The collected data is then arranged in a three-dimensional matrix, being the first two matrix According to Tauc et al. [26] , near the absorption edge the absorption coefficient of a direct-gap semiconductors iswhere A is a material-dependent constant and E g opt is the optical energy gap of the material.Equation (2) can be rewritten asThis is the post-peer reviewed version of the following article: J. Quereda et al. "Strong quantum confinement effect in the optical properties of ultrathin α-In2Se3".Advanced The usual method for determining the value of E g opt involves representing (αℏω) 2 versus the photon energy, ℏω, and fitting the absorption band edge to a linear function. According to Equation (3), the int...