Suspended Single Layer Research Articles

Electronic band structure of Two-Dimensional WS2 /Graphene van der Waals Heterostructures

Combining single-layer two-dimensional semiconducting transition metal dichalcogenides (TMDs) with graphene layer in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these heterostructures. Here, we report the electronic and structural properties of transferred single layer WS2 on epitaxial graphene using micro-Raman spectroscopy, angle-resolved photoemission spectroscopy measurements (ARPES) and Density Functional Theory (DFT) calculations. The results show good electronic properties as well as well-defined band arising from the strong splitting of the single layer WS2 valence band at K points, with a maximum splitting of 0.44 eV. By comparing our DFT results with local and hybrid functionals, we find the top valence band of the experimental heterostructure is close to the calculations for suspended single layer WS2. . Our results provide an important reference for future studies of electronic properties of WS2 and its applications in valleytronic devices.

Large total area membrane of suspended single layer graphene for water desalination

Among carbon nanomaterials, graphene has emerged as promising novel membrane material for enhancing desalination performance. However, realistic prospects for industrial-scale production of graphene membranes has been limited. Here we take steps forward in exploring methods to increase the size and enhance the mechanical stability of the single layer graphene membranes. Using Transmission Electron Microscopy (TEM) grids as porous mechanical support and oxygen plasma treatment for creating nanopores, freestanding graphene with large total area was fabricated. 1m column of salty water was used as the driving force for water desalination measurements in these membranes. High water permeation (1.64×107Lm−2h−1bar−1) and acceptable NaCl rejection rate (~76%) was achieved over an active area of 2.77×104μm2. Without an ideal porous support, large area freestanding graphene with significant number of cracks, wrinkles, pinholes and defects cannot withstand pressures (~50–80bar) required for water desalination. Fine mesh grids with small holes as inexpensive mechanical supports can be utilized to increase the rupture strength of large area suspended graphene for many purposes including desalination and filtration of multivalent ions or larger molecules.

Creating nanoporous graphene with swift heavy ions

We examine swift heavy ion-induced defect production in suspended single layer graphene using Raman spectroscopy and a two temperature molecular dynamics model that couples the ionic and electronic subsystems. We show that an increase in the electronic stopping power of the ion results in an increase in the size of the pore-type defects, with a defect formation threshold at 1.22–1.48 keV/layer. We also report calculations of the specific electronic heat capacity of graphene with different chemical potentials and discuss the electronic thermal conductivity of graphene at high electronic temperatures, suggesting a value in the range of 1 Wm−1 K−1. These results indicate that swift heavy ions can create nanopores in graphene, and that their size can be tuned between 1 and 4 nm diameter by choosing a suitable stopping power.

Electron Thermionic Emission from Graphene and a Thermionic Energy Converter

In this paper, we propose a model to investigate the electron thermionic emission from a single-layer graphene (ignoring the effects of substrate) and to explore its application as the emitter of thermionic energy convertor (TIC). An analytical formula has been derived, which is a function of temperature, work function and Fermi energy level. The formula is significantly different from the traditional Richardson-Dushman (RD) law for which it is independent of mass to account for the supply function of the electrons in the graphene behaving like massless Fermion quasiparticles. By comparing with a recent experiment [Kaili Jiang et al., Nano Research 7, 553 (2014)] measuring electron thermionic emission from a suspended single layer graphene, our model predicts that the intrinsic work function of a single-layer graphene is about 4.514 eV with a Fermi energy level of 0.083 eV. For a given work function, a new scaling of $T^{3}$ is predicted, which is different from the traditional RD scaling of $T^2$. If the work function of the graphene is lowered to 2.5 to 3 eV, and the Fermi energy level is increased to 0.8 to 0.9 eV, it is possible to design a graphene cathode based TIC operating at around 900 K or lower, as compared with the metal-based cathode TIC (operating at about 1500 K). With a graphene based cathode (work function = 4.514 eV) at 900 K, and a metallic based anode (work function = 2.5 eV) like LaB$_6$ at 425 K, the efficiency of our proposed-TIC is about 45$\%$

Large theoretical thermoelectric power factor of suspended single-layer MoS2

We have calculated the semi-classical thermoelectric power factor of suspended single-layer (SL)- MoS2 utilizing electron relaxation times derived from ab initio calculations. Measurements of the thermoelectric power factor of SL-MoS2 on substrates reveal poor power factors. In contrast, we find the thermoelectric power factor of suspended SL-MoS2 to peak at ∼2.8 × 104 μW/m K2 at 300 K, at an electron concentration of 1012 cm−2. This figure is higher than that in bulk Bi2Te3, for example. Given its relatively high thermal conductivity, suspended SL-MoS2 may hold promise for in-plane thin-film Peltier coolers, provided reasonable mobilities can be realized.

Observation of optical second harmonic generation from suspended single-layer and bi-layer graphene

We have experimentally investigated the optical second harmonic generation (SHG) on suspended single-layer and bi-layer graphene sheets. By shining normally incident 800-nm light with polarization along the sample planes, the SHG intensities of single-layer and bi-layer graphene are found to be comparable to the one of polar GaAs with large second order susceptibility, which is unexpected because both have the centrosymmetric property. Our experimental results reveal that the strong SHG is not due to the defects breaking the symmetry. Instead, we suggest that the SHG signals result from the long-range curvature fluctuations on the suspended single-layer and bi-layer graphene sheets.

Experimental observation of a suspended single layer graphene film on Cu foil grown via chemical vapor deposition method

The authors present the blue-shift of Raman 2D and G peak of a single layer graphene film, which is fabricated via chemical vapor deposition, on the relatively even Cu foil domain with nearly uniform thickness. This can be interpreted that rather mechanically compressive strain is exerted between graphene and the even Cu domain compared to the uneven. We observe that relaxing compressive strain to minimize the perturbation from Cu surface to the graphene film by virtue of the uneven Cu domain has a critical role in forming a suspended graphene. Furthermore, mechanical strain influence derived from the underlying substrate morphology can be suggested as one of the crucial elements in evaluating high quality graphene film considering suspended shape.