This article describes GaN/AlN heterostructures for ultraviolet-C (UVC) emitters with multiple (up to 400 periods) two-dimensional (2D)-quantum disk/quantum well structures with the same GaN nominal thicknesses of 1.5 and 16 ML-thick AlN barrier layers, which were grown by plasma-assisted molecular-beam epitaxy in a wide range of gallium and activated nitrogen flux ratios (Ga/N2*) on c-sapphire substrates. An increase in the Ga/N2* ratio from 1.1 to 2.2 made it possible to change the 2D-topography of the structures due to a transition from the mixed spiral and 2D-nucleation growth to a purely spiral growth. As a result, the emission energy (wavelength) could be varied from 5.21 eV (238 nm) to 4.68 eV (265 nm) owing to the correspondingly increased carrier localization energy. Using electron-beam pumping with a maximum pulse current of 2 A at an electron energy of 12.5 keV, a maximum output optical power of 50 W was achieved for the 265 nm structure, while the structure emitting at 238 nm demonstrated a power of 10 W.

Harvesting hybrid mechanical and solar ambient energy with one small device remains a challenge. Here, we report on producing electric current using a Schottky type metal-oxide-semiconductor structure formed by an n-InP layer covered with native oxide and an atomic force microscope (AFM) probe with a conductive coating. The tip’s sliding reciprocating motion during AFM scanning in contact mode produces a direct current signal in the probe-sample circuit. Two electric power generation mechanisms exist. A strong current was detected under sample illumination because of a photovoltaic effect with efficiency of 7% at the Si/InP heterojunction. Having the sample set in complete darkness, we observed current pulses of the opposite polarity, which suggests the existence of another mechanism not connected to photogeneration. This dark current originates from the tunneling of triboelectrically induced charge redistribution on the metal/oxide interface. The current polarity corresponds to electronic quantum mechanical tunneling through the oxide layer from the metal tip into InP. The current density exceeded 15 kA/m2. This is 2 and more than 4 orders greater than that in silicon- and polymer-based triboelectric nanogenerators, respectively. The open-circuit voltage value was 15 mV, and output electric power reached 110 W/m2. Understanding of triboelectric phenomena in photovoltaic semiconductor materials will allow creation of a new type of high-current hybrid energy devices that combine triboelectric nanogenerators and solar cells.

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The electronic and optoelectronic properties of van der Waals heterostructures are strongly affected by heterobarrier heights at the interfaces between monolayers forming the structure and the environment. In this work, properties of MoSe 2 /graphene/6H-SiC heterostructures are studied by scanning probe microscopy and photoluminescence spectroscopy in various ambient conditions. To improve our understanding of the electronic processes, a work function of the monolayer MoSe 2 and its dependence on the number of layers were determined by Kelvin probe microscopy. A nonuniform distribution of the photoluminescence intensity from the MoSe 2 monolayer was observed because of bilayer graphene (BLG) inclusions in the monolayer graphene (MLG) covering the 6H-SiC substrate. The measured values of the MoSe 2 work function allow the construction of band energy diagrams for MoSe 2 /MLG and MoSe 2 /BLG heterobarriers, which show an increased barrier height for MLG with respect to BLG. This relatively high potential barrier at MoSe 2 /MLG leads to quenching of the photoluminescence because of separation of the electron-hole pairs at the barrier before the formation of the exciton. Moreover, selective photodegradation of the MoSe 2 layers on MLG due to photooxidation during light illumination was observed. The photooxidation was promoted by accumulation of excess holes at the MoSe 2 /MLG interface, which further participate in splitting of a surface water film. Generation of the oxygen at the heterojunction interface leads to the oxidation of the MoSe 2 . The dependence of the photoluminescence and photodegradation of MoSe 2 on the graphene layers shows the importance of the substrate selection for good device stability based on van der Waals heterostructures with MoSe 2 .

We investigated multilayer plates made by exfoliation from a high-quality MoS2 crystal and reveal that they represent a new object – van der Waals homostructure consisting of a bulk core...