Abstract:Black phosphorus exhibits a layered structure similar to graphene, allowing mechanical exfoliation of ultrathin single crystals. Here we demonstrate few-layer black phosphorus field effect devices on Si/SiO 2 and measure charge carrier mobility in a four-probe configuration as well as drain current modulation in a two-point configuration. We find room-temperature mobilities of up to 300 cm 2 /Vs and drain current modulation of over 10 3 . At low temperatures the on-off ratio exceeds 10 5 and the device exhibits both electron and hole conduction. Using atomic force microscopy we observe significant surface roughening of thin black phosphorus crystals over the course of 1 hour after exfoliation. Main Text:Two-dimensional crystals have gained significant attention since the rise in popularity of graphene 1 . Due to graphene's lack of a band gap the search for a two-dimensional gapped material with high mobility has been highly sought after. Other than graphene black phosphorus (BP) is the only other known monotypic van der Waals crystal 2-4 . Previous studies have shown that bulk BP has high carrier mobility combined with the presence of a direct band gap 5,6 . BP

As mechanical structures enter the nanoscale regime, the influence of van der Waals forces increases. Graphene is attractive for nanomechanical systems 1,2 because its Young's modulus and strength are both intrinsically high, but the mechanical behavior of graphene is also strongly influenced by the van der Waals force 3,4 . For example, this force clamps graphene samples to substrates, and also holds together the individual graphene sheets in multilayer samples. Here we use a pressurized blister test to directly measure the adhesion energy of graphene sheets with a silicon oxide substrate. We find an adhesion energy of 0.45 ± 0.02 J/m 2 for monolayer graphene and 0.31 ± 0.03 J/m 2 for samples containing 2-5 graphene sheets. These values are larger than the adhesion energies measured in typical micromechanical structures and are comparable to solid/liquid adhesion energies [5][6][7] . We attribute this to the extreme flexibility of graphene, which allows it to conform to the topography of even the smoothest substrates, thus making its interaction with the substrate more liquidlike than solid-like. Figure 1a shows optical images of the devices used for this study. Graphene sealed microcavities were fabricated by the mechanical exfoliation of graphene over predefined wells (diameter ~5 um) etched in a SiO 2 substrate (See Methods). Two exfoliated graphene flakes were used, yielding membranes with between 1 and 5 graphene layers, which were suspended over the wells and clamped to the SiO 2 substrate by the van der Waals force. After exfoliation the internal pressure in the microcavity, p int , is equal to the external pressure, p ext , which is atmospheric pressure. In this state the membrane is flat, adhered to the substrate, and it confines N gas molecules inside the microcavity.To create a pressure difference across the graphene membrane, we put the sample in a pressure chamber and use nitrogen gas to increase p ext to p 0 . Devices are left in the pressure chamber at p 0 for between 4 and 6 days in order for p int to equilibrate to p 0 (Fig. 1b). This is thought to take place through the slow diffusion of gas through the SiO 2 substrate 3 . We then remove the device from the pressure chamber, and the pressure difference (p int > p ext ) causes the membrane to bulge upwards and the volume of the cavity to increase (Fig. 1c). We use an atomic force microscope (AFM) to measure the shape of the graphene membrane, which we parameterize by its maximum deflection, δ, and its radius, a (Fig. 1d).This technique allows us to measure δ and a for different values of p 0 . Figure 1e shows a series of AFM line cuts through the center of a mono-layer membrane as p 0 is increased. At low p 0 , the membrane is clamped to the substrate by the van der Waals force and δ increases with increasing p 0 . At higher p 0 (e.g., p 0 > 2 MPa) in addition to an increased deflection, we also observe delamination of the graphene from the SiO 2 substrate which leads to an increase in a (Fig. 1e). In Fig. 2a, we plot δ vs. p 0 for all the bi...

Membranes act as selective barriers and play an important role in processes such as cellular compartmentalization and industrial-scale chemical and gas purification. The ideal membrane should be as thin as possible to maximize flux, mechanically robust to prevent fracture, and have well-defined pore sizes to increase selectivity. Graphene is an excellent starting point for developing size-selective membranes because of its atomic thickness, high mechanical strength, relative inertness and impermeability to all standard gases. However, pores that can exclude larger molecules but allow smaller molecules to pass through would have to be introduced into the material. Here, we show that ultraviolet-induced oxidative etching can create pores in micrometre-sized graphene membranes, and the resulting membranes can be used as molecular sieves. A pressurized blister test and mechanical resonance are used to measure the transport of a range of gases (H(2), CO(2), Ar, N(2), CH(4) and SF(6)) through the pores. The experimentally measured leak rate, separation factors and Raman spectrum agree well with models based on effusion through a small number of ångstrom-sized pores.

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The stability of the surface of in situ cleaved black phosphorus crystals upon exposure to atmosphere is investigated with synchrotron-based photoelectron spectroscopy. After 2 days atmosphere exposure a stable subnanometer layer of primarily P2O5 forms at the surface. The work function increases by 0.1 eV from 3.9 eV for as-cleaved black phosphorus to 4.0 eV after formation of the 0.4 nm thick oxide, with phosphorus core levels shifting by <0.1 eV. The results indicate minimal charge transfer, suggesting that the oxide layer is suitable for passivation or as an interface layer for further dielectric deposition.