[en] Terahertz field induced photocurrents in graphene were studied experimentally and by microscopic modeling. Currents were generated by cw and pulsed laser radiation in large area as well as small-size exfoliated graphene samples. We review general symmetry considerations leading to photocurrents depending on linear and circular polarized radiation and then present a number of situations where photocurrents were detected. Starting with the photon drag effect under oblique incidence, we proceed to the photogalvanic effect enhancement in the reststrahlen band of SiC and edge-generated currents in graphene. Ratchet effects were considered for in-plane magnetic fields and a structure inversion asymmetry as well as for graphene with non-symmetric patterned top gates. Lastly, we demonstrate that graphene can be used as a fast, broadband detector of terahertz radiation. (copyright 2017 by WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim)

[en] Complementary to the gapless material graphene, the transition-metal dichalcogenide MoS₂ is a promising two-dimensional layered semiconductor for future ultrathin nanoelectronic and optoelectronic devices. Subnanometer thickness, large bandgap in the visible range and ultrafast carrier dynamics make it interesting for devices like transistors, ultrafast optical switches or photovoltaic applications. Our monolayer MoS₂ flakes were prepared by the well-known mechanical cleavage method. With a μPL experimental setup, we can perform photoluminescence spectroscopy at temperatures from 4 K up to room temperature. Under different external influences like temperature, magnetic fields or circular polarisation of the exciting laser light, we investigated the behavior of the A and B excitons, arising from transitions from the spin-orbit split valence band to the conduction band at the K-point of the Brillouin zone. Thereby, we could gather information about charged and neutral excitons and a possible valley polarisation. Furthermore, we could produce monolayer regions out of few-layer flakes via intense focussed laser radiation.

[en] The layered transition-metal dichalcogenide MoS₂ has attracted great interest because of good optical properties and as alternative to graphene for nanoelectronic applications. With the transparent tape liftoff method, single- and few-layer MoS₂ flakes were prepared. By annealing the samples in vacuum, photoluminescence peak intensity positions varying by about 20 meV at room temperature, were made uniform. Low temperature photoluminescence measurements on single-layer MoS₂ flakes show an additional low-energy peak. It can be attributed to a surface-bound exciton, because samples with a HfO₂ or Al₂O₃ coating do not show the low-energy peak. In Raman-measurements, we have identified an interlayer shear mode at 30 cm^-1 in bulk material. We observed a decrease in wavenumber with decreasing layer number. By scanning an area on the sample, we can distinguish regions of different layer numbers by mapping the spectral position of the shear mode.

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[en] Two-dimensional carbon nanomaterials ranging from single-layer graphene to defective structures such as chemically reduced graphene oxide were studied with respect to their use in electrodes and sensors. Their electrochemical properties and utility in terms of fabrication of sensing devices are compared. Specifically, the electrodes have been applied to reductive amperometric determination of hydrogen peroxide. Low-defect graphene (SG) was obtained through mechanical exfoliation of natural graphite, while higher-defect graphenes were produced by chemical vapor deposition (CVDG) and by chemical oxidation of graphite and subsequent reduction (rGO). The carbonaceous materials were mainly characterized by Raman microscopy. They were applied as electrode material and the electrochemical behavior was investigated by chronocoulometry, cyclic voltammetry, electrochemical impedance spectroscopy and amperometry and compared to a carbon disc electrode. It is shown that the quality of the graphene has an enormous impact on the amperometric performance. The use of carbon materials with many defects (like rGO) does not result in a significant improvement in signal compared to a plain carbon disc electrode. The sensitivity is 173 mA · M"−"1 · cm"−"2 in case of using CVDG which is about 50 times better than that of a plain carbon disc electrode and about 7 times better than that of rGO. The limit of detection for hydrogen peroxide is 15.1 μM (at a working potential of −0.3 V vs SCE) for CVDG. It is concluded that the application of two-dimensional carbon nanomaterials offers large perspectives in amperometric detection systems due to electrocatalytic effects that result in highly sensitive detection. (author)