Owing to its high carrier mobility and thickness-tunable direct band gap, black phosphorus emerges as a promising component of optoelectronic devices. Here, we evaluate the device characteristics of p-n heterojunction diodes wherein thin black phosphorus layers are interfaced with an underlying, highly n-doped GaAs substrate. The p-n heterojunctions exhibit close-to-ideal diode behavior at low bias, while under illumination they display a photoresponse that is evenly distributed over the entire junction area, with an external quantum efficiency of up to 10% at zero bias. Moreover, the observed maximum open circuit voltage of 0.6 V is consistent with the band gap estimated for a black phosphorus sheet with a thickness on the order of 10 nm. Further analysis reveals that the device performance is limited by the structural quality of the black phosphorus surface.

Arsenate ions are incorporated in amorphous cobalt oxide catalysts at the periphery of the lattice or substituting cobalt ions.

The requirements for beneficial materials restructuring into a higher performance oxygen evolution reaction (OER) electrocatalyst are still a largely open question. Here erythrite (Co3(AsO4)2·8H2O) is used as a Co‐based OER electrocatalyst to evaluate its catalytic properties during in situ restructuring into an amorphous Co‐based catalyst in four different electrolytes at pH 7. Using diffraction, microscopy, and spectroscopy, a strong effect in the restructuring behavior is observed depending of the anions in the electrolyte. Only carbonate electrolyte can activate the catalyst material, which is related to its slow restructuring process. While the catalyst turnover frequency (TOF) undesirably reduces by a factor of 28, the number of redox active sites continuously increases to a factor of 56, which results in an overall twofold increase in current of the restructured catalyst after 800 cycles. The activation is attributed to an adequate local order, a high Co oxidation state close to 3+, and a high number of redox‐active Co ions. These three requirements for beneficial restructuring provide new insights into the rational design of high‐performance OER catalysts by electrochemical restructuring.

Chemical functionalization of graphene is achieved by hyperthermal reaction with azopyridine molecular ions. The one-step, room temperature process takes place in high vacuum (10(-7) mbar) using an electrospray ion beam deposition (ES-IBD) setup. For ion surface collisions exceeding a threshold kinetic energy of 165 eV, molecular cation beams of 4,4'-azobis(pyridine) covalently attach to chemical vapor deposited (CVD) graphene. A covalent functionalization degree of 3% of the carbon atoms of graphene is reached after 3-5 h of ion exposure of 2 × 10(14) azopyridinium/cm(2) of which 50% bind covalently. This facile approach for the controlled modification of graphene extends the scope of candidate species that would not otherwise react via existing conventional methods.

The electric field is an important parameter to vary in a single-molecule experiment, because it can directly affect the charge distribution around the molecule. Yet, performing such an experiment with a well-defined electric field for a model chemical reaction at an interface has proven to be extremely difficult. Here, by combining a graphene field-effect transistor and a gate-tunable scanning tunneling microscope (STM), we reveal how this strategy enables the intramolecular H atom transfer of a metalfree macrocycle to be controlled with an external field. Experiments and theory both elucidate how the energetic barrier to tautomerization decreases with increasing electric field. The consistency between the two results demonstrates the potential in using electric fields to engineer molecular switching mechanisms that are ubiquitous in nanoscale electronic devices.

transport barrier. [ 16,23,24 ] Owing to the low transit time of the tunneling electrons, this type of device is expected to operate over a much wider frequency range than the graphene-semiconductor junction. Here, we demonstrate that for two-terminal GrIM diodes, which lack an external gate as third terminal, barrier height modulation can occur solely based upon the applied bias and lead to excellent device performance down to the smallest insulator thickness. This task requires a suitable choice of insulator in order to allow for sizable electron transport between the graphene and the metal. Moreover, our study of Gr-TiO x -Ti diodes reveals the importance of balancing the tunneling and thermionic contributions to the charge transport.To fabricate the Gr-TiO x -Ti diodes, as a fi rst step the graphene bottom electrode was mechanically exfoliated from highly oriented pyrolytic graphite onto a Si substrate coated with a 300 nm thick SiO 2 layer. The graphene was then provided with a Ti/Au contact through standard e-beam lithography, followed by thermal evaporation of the metals. Subsequently, an ultrathin TiO x layer of was formed on top through three subsequent cycles of thermal evaporation of nominally 2 nm Ti, with an oxidation step performed under ambient oxidation at the end of each cycle. The thickness of the resulting oxide was determined by atomic force microscopy (AFM) measurement to be ≈6 nm. Finally, 25 nm thick Ti top electrodes were lithographically defi ned on top of the TiO x insulator. The work functions of graphene and Ti are 4.5 [ 25 ] and 4.2 eV, [ 26 ] respectively. Titanium thus fulfi lls the above-mentioned requirement that the work function of the metal contact (M2) is smaller than that of graphene. The electron affi nity of TiO 2 is ≈4.0 eV, [ 27,28 ] which ensures the trapezoid shape of the tunneling barrier depicted in Figure 1 a The Dirac point shift of the graphene channel from +5 to −40 V (see Figure S1, Supporting Information for the measurement setup) after deposition of the Ti top electrode ( Figure S2, Supporting Information) indicates appreciable n-type doping by the Ti layer. We furthermore determined the work function difference between Ti and graphene by Kelvin force microscopy, as illustrated in Figure 1 b. The detected potential difference of ≈200 meV is in reasonable agreement with the expected value of 0.3 eV. The somewhat smaller experimental value can be explained by the n-type doping of the graphene by the Ti.The I -V characteristics of a Gr-TiO x -Ti diode (red curve in Figure 1 c), acquired between the four layer-thick graphene sheet and the Ti electrode, displays a diode-like behavior with a high current under negative bias and a low current under positive bias. Moreover, the graphene channel resistance measured as a function of bias applied to the Ti electrode (blue curve in Figure 1 c) signifi es a change of graphene's work function (Figure S2, Supporting Information). The plot in Figure 1 d reveals a maximum asymmetry of 1800 at 1.2 V (red curve)Metal-insulator-me...

Hot carriers in semiconductor or metal nanostructures are relevant, for instance, to enhance the activity of oxide-supported metal catalysts or to achieve efficient photodetection using ultrathin semiconductor layers. Moreover, rapid collection of photoexcited hot carriers can improve the efficiency of solar cells, with a theoretical maximum of 85%. Because of the long lifetime of secondary excited electrons, graphene is an especially promising two-dimensional material to harness hot carriers for solar-to-electricity conversion. However, the photoresponse of thus far realized graphene photoelectric devices is mainly governed by thermal effects, which yield only a very small photovoltage. Here, we report a Gr-TiO-Ti heterostructure wherein the photovoltaic effect is predominant. By doping the graphene, the open circuit voltage reaches values up to 0.30 V, 2 orders of magnitude larger than for devices relying upon the thermoelectric effect. The photocurrent turned out to be limited by trap states in the few-nanometer-thick TiO layer. Our findings represent a first valuable step toward the integration of graphene into third-generation solar cells based upon hot carrier extraction.

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