Using molecular beam epitaxy, atomically thin 2D semiconductor HfSe2 and MoSe2/HfSe2 van der Waals heterostructures are grown on AlN(0001)/Si(111) substrates. Details of the electronic band structure of HfSe2 are imaged by in-situ angle resolved photoelectron spectroscopy indicating a high quality epitaxial layer. High-resolution surface tunneling microscopy supported by first principles calculations provides evidence of an ordered Se adlayer, which may be responsible for a reduction of the measured workfunction of HfSe2 compared to theoretical predictions. The latter reduction minimizes the workfunction difference between the HfSe2 and MoSe2 layers resulting in a small valence band offset of only 0.13 eV at the MoSe2/HfSe2 heterointerface and a weak type II band alignment.

van der Waals heterostructures of 2D semiconductor materials can be used to realize a number of (opto)electronic devices including tunneling field effect devices (TFETs). It is shown in this work that high quality SnSe2/WSe2 vdW heterostructure can be grown by molecular beam epitaxy on AlN(0001)/Si(111) substrates using a Bi2Se3 buffer layer. A valence band offset of 0.8 eV matches the energy gap of SnSe2 in such a way that the VB edge of WSe2 and the CB edge of SnSe2 are lined up, making this materials combination suitable for (nearly) broken gap TFETs.

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Molecular beam epitaxy of 2D metal TaSe2/2D MoSe2 (HfSe2) semiconductor heterostructures on epi-AlN(0001)/Si(111) substrates is reported. Electron diffraction reveals an in-plane orientation indicative of van der Waals epitaxy, whereas electronic band imaging supported by first-principles calculations and X-ray photoelectron spectroscopy indicate the presence of a dominant trigonal prismatic 2H-TaSe2 phase and a minor contribution from octahedrally coordinated TaSe2, which is present in TaSe2/AlN and TaSe2/HfSe2/AlN but notably absent in the TaSe2/MoSe2/AlN, indicating superior structural quality of TaSe2 grown on MoSe2. Apart from its structural and chemical compatibility with the selenide semiconductors, TaSe2 has a workfunction of 5.5 eV as measured by ultraviolet photoelectron spectroscopy, which matches very well with the semiconductor workfunctions, implying that epi-TaSe2 can be used for low-resistivity contacts to MoSe2 and HfSe2.

TiO 2 has high chemical stability, strong catalytic activity and is an electron transport material in organic solar cells. However, the presence of trap states near the band edges of TiO 2 arising from defects at grain boundaries significantly affects the efficiency of organic solar cells. To become an efficient electron transport material for organic photovoltaics and related devices, such as perovskite solar cells and photocatalytic devices, it is important to tailor its band edges via doping. Nitrogen p-type doping has attracted considerable attention in enhancing the photocatalytic efficiency of TiO 2 under visible light irradiation while hydrogen n-type doping increases its electron conductivity. DFT calculations in TiO 2 provide evidence that nitrogen and hydrogen can be incorporated in interstitial sites and possibly form N i H i , N i H O and N Ti H i defects. The experimental results indicate that N i H i defects are most likely formed and these defects do not introduce deep level states. Furthermore, we show that the efficiency of P3HT:IC 60 BA-based organic photovoltaic devices is enhanced when using hydrogen-doping and nitrogen/hydrogen codoping of TiO 2 , both boosting the material n-type conductivity, with maximum power conversion efficiency reaching values of 6.51% and 6.58%, respectively, which are much higher than those of the cells with the as-deposited (4.87%) and nitrogen-doped TiO 2 (4.46%).Metal oxides such as titanium dioxide (TiO 2 ) have been intensively investigated for more than four decades because of their strong catalytic activity, high chemical stability and long lifetime of photon generated carriers [1][2][3][4][5][6][7][8][9][10] . Anatase exhibits the highest photocatalytic activity of the polymorphs of TiO 2 , however, it is constrained to the limited ultraviolet range (UV irradiation is only 5%) of the solar spectrum due to its large band gap (3.2eV) 7 . For a photocatalyst to achieve high efficiency, its band gap should be around 2.0 eV, whereas the position of the band edges should be consistent with the redox potential of water 11 . A way to reduce the band gap is doping, with nitrogen (N) atom being a particularly promising p-type dopant 3,12 .Hydrogen (H) is a small atom so it can diffuse easily in inorganic compounds occupying interstitial sites. It does not induce significant structural expansion, can modify the band gap, enhance the photocatalytic activity 13 , induce insulator-to-conductor transitions 14 , provide free electrons 15 , and interact with intrinsic defects such as oxygen vacancies 16 . H can be introduced in TiO 2 during synthesis or by immersion in water 12 . Interestingly, previous theoretical studies have shown that hydrogen can substitute for oxygen (termed as substitutional H, H O ) leading to n-type conductivity 17,18 . H doping has recently been established as an effective strategy for improving the capacitive properties of TiO 2 for application in supercapacitors 19 . Additionally, the emergence of a highly H doped TiO 2 (black titania) nanomateri...