Room temperature X-ray photoelectron spectroscopy (XPS), inductively coupled plasma mass spectrometry (ICPMS), high resolution Rutherford backscattering spectrometry (HR-RBS), Kelvin probe method, and scanning tunneling microscopy (STM) are employed to study the properties of a freshly exfoliated surface of geological MoS2 crystals. Our findings reveal that the semiconductor 2H-MoS2 exhibits both n- and p-type behavior, and the work function as measured by the Kelvin probe is found to vary from 4.4 to 5.3 eV. The presence of impurities in parts-per-million (ppm) and a surface defect density of up to 8% of the total area could explain the variation of the Fermi level position. High resolution RBS data also show a large variation in the MoSx composition (1.8 < x < 2.05) at the surface. Thus, the variation in the conductivity, the work function, and stoichiometry across small areas of MoS2 will have to be controlled during crystal growth in order to provide high quality uniform materials for future device fabrication.

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Quasi-2D perovskites are attractive because of their improved stability compared to 3D counterparts, but they suffer from reduced performance. Here we report an efficient quasi-2D perovskite (PEA) 2 (MA) 4 Pb 5 I 16 -based optoelectronic device processed with NH 4 SCN and NH 4 Cl additives, showing a stabilized photovoltaic power conversion efficiency as high as 14.1% (average value 12.9 ± 0.8%), which is among the highestperforming quasi-2D perovskite solar cells. These additives increase the perovskite crystallinity and induce a preferred orientation with the (0k0) planes perpendicular to the substrate, resulting in improved transport properties and hence increased short-circuit current density. Furthermore, the NH 4 Cl treatment enriches the Cl − concentration near the PEDOT:PSS/ perovskite interface, which passivates the electron traps, leading to an enhanced electroluminescence external quantum efficiency (0.68% at +2.5 V bias). As a result, high open-circuit voltages of 1.21 ± 0.01 V with a record low nonradiative V OC loss of only ∼0.16 V could be achieved for the quasi-2D perovskite system.

We explore the effects of nonradiative recombination at the extracting contacts on the achievable performance of halide perovskite photovoltaic cells. First, we perform device simulations using standard drift-diffusion models with experimental semiconductor parameters matching those of methylammonium lead triiodide (MAPbI 3 ). We quantify the range of surface recombination velocities (SRVs) that would allow this archetypal perovskite to reach power conversion efficiencies of 27%. In particular, for contacts with well-aligned energy levels, SRVs of ∼1−10 cm/s should enable open-circuit voltages of 1.30 V, within 96% of the Shockley−Queisser limit. Next, we use time-resolved photoluminescence to experimentally determine the SRVs on 14 different common electron-and hole-extracting contacts, including TiO 2 , SnO 2 , ZnO, PCBM, ITIC, ICBA, TPBi, PEDOT:PSS, PTAA, PVK, NiO, MoO 3 , WO 3 , and spiro-OMeTAD. These results point the way to the selection and rational engineering of better contacts as a means to achieve higher efficiencies in perovskite solar cells.

In this work, we demonstrate the growth of HfSe2 thin films using molecular beam epitaxy. The relaxed growth criteria have allowed us to demonstrate layered, crystalline growth without misfit dislocations on other 2D substrates such as highly ordered pyrolytic graphite and MoS2. The HfSe2 thin films exhibit an atomically sharp interface with the substrates used, followed by flat, 2D layers with octahedral (1T) coordination. The resulting HfSe2 is slightly n-type with an indirect band gap of ∼ 1.1 eV and a measured energy band alignment significantly different from recent DFT calculations. These results demonstrate the feasibility and significant potential of fabricating 2D material based heterostructures with tunable band alignments for a variety of nanoelectronic and optoelectronic applications.

High performance hole transport layers are realized using room temperature solution processing of microwave‐synthesized MoOx nanoparticles. Composition of solution‐deposited MoOx nanoparticle films can be increased from 10% to 70% MoO3 using air exposure (days) and reaction with H2O2 (minutes). The increased MoO3 content correlates well with improved solar cell performanceto the level of evaporated MoO3 and PEDOT:PSS, with good air stability.

Multifunctional biodegradable inorganic theranostic nano-agents are of great interest to the field of nanomedicine. Upon lipid modification, VS nanosheets could be converted into ultra-small VS nanodots encapsulated inside polyethylene glycol (PEG) modified lipid micelles. Owing to paramagnetism, high near-infrared (NIR) absorbance, and chelator-free Tc labeling of VS , such VS @lipid-PEG nanoparticles could be used for T1-weighted magnetic resonance (MR), photoacoustic (PA),and single photon emission computed tomography (SPECT) tri-modal imaging guided photothermal ablation of tumors. Importantly, along with the gradual degradation of VS , our VS @lipid-PEG nanoparticles exhibit effective body excretion without appreciable toxicity. The unique advantages of VS nanostructures with highly integrated functionalities and biodegradable behaviors mean they are promising for applications in cancer theranostics.

Fullerene-based organic solar cells with only a minute amount of donor show a substantial photocurrent while maintaining a large open-circuit voltage. At low concentrations the donor is fully dispersed within the fullerene and no percolation pathways of holes toward the anode exist; this morphology is in contrast to bulk-heterojunction donor:acceptor blends where percolation pathways for both electrons and holes are present within their respective transport phases. Therefore, the question of how holes contribute to the photocurrent arises. Here we demonstrate that the photocurrent is readily explained by photogenerated holes transferring back to the fullerene matrix due to Coulomb repulsion and the fullerene acting as an ambipolar conductor for both electrons and holes. The two critical parameters controlling this process are the values of the highest occupied molecular orbital level difference between the donor and the acceptor and of the recombination strength; both are found to agree between experimental measurements and kinetic Monte Carlo simulations. We provide evidence that the highest occupied molecular orbital level difference between donor and acceptor is smaller in a dilute donor configuration. Successive percolation pathways toward the contactsthe reason for introducing the bulk-heterojunction configurationare not an absolute requirement to obtain substantial photocurrents in organic solar cells.