The 4.2 ka event is widely presumed to be a globally widespread aridity event and has been linked to several episodes of societal changes across the globe. Whether this climate event impacted the cultural development in south-central China remains uncertain due to a lack of regional paleorainfall records. We present here stalagmite stable carbon isotope and trace element–based reconstruction of hydroclimatic conditions from south-central China. Our data reveal a sub–millennial scale (~5.6 to 4.3 ka) drying trend in the region followed by a gradual transition to wetter conditions during the 4.2 ka event (4.3–3.9 ka). Together with the existing archaeological evidence, our data suggest that the drier climate before 4.3 ka may have promoted the Shijiahe culture, while the pluvial conditions during the 4.2 ka event may have adversely affected its settlements in low-lying areas. While military conflicts with the Wangwan III culture may have accelerated the collapse of Shijiahe culture, we suggest that the joint effects of climate and the region's topography also played important causal roles in its demise.

The widespread occurrence of snow-ice formation on the pack ice plays a critical role in the mass balance of Antarctic sea ice. The stable isotope composition, ice texture and salinity of eight ice cores, obtained from the Amundsen Sea during the Oden Southern Ocean 2010/11 expedition from late December 2010 to January 2011, were investigated to illustrate the snow-ice growth process and its contribution to sea-ice development. Most previous research has utilized δ18O as an index tracer to determine the percentages of core length that contain meteoric water, i.e. snow ice. However, this standard practice of snow-ice identification might be biased due to normally low-resolution isotopic measurements and mixing/diffusion processes between the snow ice and underlying ice layers. Snow-ice contributions in these ice cores based instead on an updated isotope mixing model are also presented. Depth profiles of ice texture and salinity are described to serve as representations of the structures of these ice cores. Our isotope mixing model produced an average of 15.9% snow-ice contribution for pack ice in the Amundsen Sea, and meteoric water occupying 40% of snow-ice mass for all ice stations. These results are compared to previous investigations of snow-ice occurrence around Antarctica.

The energy level alignment that occurs at the interfaces in planar-hetero structured perovskite photovoltaic devices strongly influences the charge transport across the interface, and thus plays a crucial role in overall device performance. To directly observe the energy level alignment requires pristine homogeneous surfaces that are free of contamination including adventitious carbon. Co-evaporation offers the ability to grow perovskite thin films in-situ, and the method involves thermally evaporating the perovskite precursors such as PbI2 and CH3NH3I. Early reports have shown that the perovskite film formation and stoichiometry are problematic at ultralow coverages. In particular, it was reported that there was excessive PbI2 and a deficiency in CH3NH3I. Using photoemission spectroscopy, we investigated the perovskite precursor PbI2 on gold and highly oriented pyrolytic graphite (HOPG) surfaces. Results show that the nature of the surface and the deposition conditions can strongly influence the film formation. Excessive iodine observed in the initial evaporation stages appears to be substrate dependent, and this may influence the overall energy level alignment.

Lead halide perovskites have proven their great power conversion efficiency (PCE) in the last few years and attracted more and more attentions. Evaporation is an important method to get high quality perovskite films, especially for surface and interface investigation, which is important for the solar cell performance. In this paper, we present our investigations on growing PbI2 and CH3NH3I films by evaporation, and then CH3NH3PbI3 films by co-evaporation. X-ray photoemisson spectroscopy (XPS) was used to characterize the films. The results showed that CH3NH3I film was not stable in vacuum. Both N and I decreased in vacuum with time elapsing. PbI2 and CH3NH3PbI3 films are quite stable. The atomic ratio of CH3NH3PbI3 films (C: N: Pb: I =1.29:1.07:1.00:2.94) is very close to the ideal CH3NH3PbI3, which indicates that evaporation is a good method to get high quality perovskite films with accurate atomic ratio.

Methylammonium lead halide perovskites have been developed as highly promisingmaterials to fabricate efficient solar cells in the past few years. We haveinvestigated degradation of co-evaporatedCH3NH3PbI3 films in ambient air, oxygen andwater respectively using x-ray photoelectron spectroscopy (XPS), small anglex-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electronmicroscopy (SEM). The CH3NH3PbI3 film has anexcellent atomic ratio and crystallinity. XPS results indicate that the film isnot sensitive to oxygen and dry air, while ambient and water exposures achievesimilar effects. XRD further indicates a structural conversion toPbI2 and a drastic morphology change from smooth to rough is revealedby AFM and SEM. The experiment indicated that H2O plays a dominatedrole in the degradation of CH3NH3PbI3 films.The degradation can be characterized by almost complete removal of N,substantial reduction of I, residual of PbI2, C, O, and I compoundson the surface.

Wehave investigatedthe thickness dependence of air exposure of copper phthalocyanine (CuPc) layerson molybdenum trioxide (MoO3) with ultraviolet photoemission spectroscopy (UPS). It was found that after the air exposure, the WF of MoOx dropped severely. Meanwhile, with ∼10 ÅCuPcthin films covered on the top, there is no big change of the MoOx WF before and after the air exposure.

We have investigated the counter intuitive phenomenon of inserting a metal oxide layer to improve hole injection or extraction in organic semiconductor devices using ultraviolet photoemission, x-ray photoemission, and inverse photoemission spectroscopy (UPS, XPS and IPES). We observed that metal oxides, such as MoO3, substantially increase the work function when deposited on indium-tin-oxide (ITO). The increase lifts up the highest occupied molecular orbital (HOMO) of the hole transport layer, therefore reduces the energy barrier between the HOMO and the Fermi level of the anode. The uplift creates an interface band bending region that results in a drift electric field that encourages the collection of holes at the anode. The optimum thickness for the oxide layer is estimated to be 2 nm. We have also investigated the effects of ambient or O2 exposure of MoO3. We observed that while most of the electronic energy levels of the oxide remained largely intact, the work function reduction was significant. This opens a way for optimal energy level alignment by modifying the work function through exposure. Furthermore, we observed that the work function reduction by exposure could be reversed by proper annealing of the sample in vacuum. The investigations therefore point to manipulate the interface electronic structure and charge injection/extraction by thin metal oxide films.

A low-resistance back contact for n-CdS/p-CdTe solar cells has been developed, which utilizes a thermally evaporated MoOx thin film as the buffer layer between the p-CdTe and the back electrode. The low-resistance behavior of back contact is attributed to the high work function of MoOx, which reportedly is as high as 6.8 eV, and thus adequately matches that of p-CdTe. With MoOx as the buffer, a variety of common metals, even those with a low work function such as Al, have been found to be useful as the electrode in forming the back contact. Other advantages of the MoOx buffer include dry application by vacuum deposition, and thus it is particularly suitable for the fabrication of ultra-thin CdTe solar cells without introducing additional shorting defects. Surface cleaning of CdTe films prior to MoOx deposition has also been studied. The cell stability has been evaluated through thermal annealing tests. Thermal degradation has been explained in terms of oxidation of the metal electrodes. CdTe cells with high efficiency and good stability have been demonstrated with MoOx as the back contact buffer and Ni as the electrode.

We investigated 0 to 300 Å thick stepped molybdenum trioxide (MoO3) inter-layer between in-situ oxygen plasma treated conducting indium tin oxide (ITO) and chloro-aluminum pthalocyanine (AlPc-Cl) layer-by-layer evaporated up to 228 Å, with ultra-violet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The MoO3 inter-layers were observed to increase the surface workfunction. The workfunction increase was observed to saturate at 20 Å of MoO3 coverage. The increased surface workfunction causes hole accumulation and band bending in the subsequently deposited AlPc-Cl. A possible explanation of reduction in series resistance by the insertion of the MoO3 insulating layer is discussed based on these observations and energy level alignment.

We have investigated the evolution of both the occupied and unoccupied states for alkali metal (Cs and Na) doped Copper-Phthalocyanine (CuPc) with photoemission and inverse photoemission spectroscopy. As the doping ratio increases, the lowest unoccupied molecular orbital (LUMO) of CuPc shifts downward, reaching the Fermi level. After the saturation, the LUMO intensity decreases monotonically, while a gap state grows in the valence spectra, which gives direct evidence for the origin of the doping-induced gap state in CuPc molecules.

The electronic structure of the interfaces between rubrene and various metals, including Au, Ag, Al, and Ca, have been investigated with photoemission and inverse photoemission spectroscopy. The formation of the interface dipole is observed at all interfaces.The Fermi level shifts linearly within the band gap as a function of metal workfunction, until it is pinned at the lowest unoccupied molecular orbital (LUMO) by Ca. Strong interactions take place at the interface between rubrene and Ca, evidenced by the evolution of the valence features.

Spin injection from GaAs(100) to organic semiconductor copper phthalocyanine (CuPc) has been investigated experimentally with spin-resolved two-photon photoemission (SR-2PPE) spectroscopy. With SR-2PPE, the dynamics of both electron and spin relaxation have been studied with femtosecond time resolution. The spin-polarized electrons are originally generated in GaAs through optical pumping and injected into CuPc. We observed an enhancement in spin polarization at the interface after initial CuPc deposition. This demonstrates that interface spin scattering is insignificant, which is similar to our previous result of spin injection at CuPc/Co interface. The spin polarization dropped when the CuPc film became thick, an effect attributed to bulk attenuation in CuPc. The lifetime of the unoccupied orbits in CuPC was also studied with red-blue excitation of photon energy 1.56 eV and 3.12 eV, respectively. There was a strong asymmetry in the time-resolved spectra, and an unexpected long lifetime when the lower unoccupied orbital was excited. A simple explanation of this phenomenon will be discussed.

Pentacene is widely used in organic thin film transistors due to relatively large mobilities. It has been suggested that the functional behavior in these organic thin film transistors occurs within the first few molecular layers of the device at the interfaces between the organic and the metals and dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation using photoemission spectroscopy to examine layer by layer growth of pentacene on Au and Ag and vice versa. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition. On the other hand, more complex material intermixing takes place during metal on organic deposition and as a result, the electronic structure of the interface differs from that of organic on metal deposition.

Tris(8-hydroxyquinoline) aluminum (Alq3) based organic light emission diodes (OLED) have been a focus of material research in recent years. One of the key issues in searching for a better device performance and fabricating conditions is suitable electron-injection materials. We have investigated the energy alignment and the interface formation between different metals and Alq3 using X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The interface is formed by depositing the target cathode material, such as Ca, Al or Al/LiF, onto an Alq3 film in a stepwise fashion in an ultrahigh vacuum environment. While the UPS results show the work function and vacuum level changes during interfaces formation, implying a possible surface dipole layer, XPS results show a more detailed and complex behavior. When a low work function metal such as Ca is deposited onto an Alq3 surface, a gap state is observed in UPS. At the same time, a new peak can be observed in the N 1s core level at a lower binding energy. These results can be characterized as charge transfer from the low work function metal to Alq3. The shifting of core levels are also observed, which may be explained by doping from metal atoms or charge diffusion. These interfaces are drastically different than the Al/Alq3 interface, which has very poor electron injection. At the Al/Alq3 interface there is a destructive chemical reaction and much smaller core level shifts are observed. Based on detailed analysis, energy level diagrams at the interface are proposed.

Current-voltage characteristics of organic semiconductor pentacene were studied as a function of applied tip force using conducting probe atomic force microscope. I-V measurements were performed on 150Å and 300Å pentacene films. No electrical contact was observed until 20 nN of tip force for the 150Å film coverage, whereas electrical contact was established easily with a few nN of tip force values for the 300Å film coverage. On both films, once an electrical contact was obtained, the conductance was observed to increase with increasing load. At about 50 nN and higher loads we lost electrical contact in both cases in a reproducible manner. While I-V on 150Å pentacene film showed a rectifying behavior, I-V on 300Å pentacene looked like typical I-V curves of tunneling phenomenon. From these measurements, we have estimated a hole injection barrier of about 0.76 eV and a band gap of about 2.00 eV for pentacene. These results are in agreement with those in the literature.

We report the interface formation between silver (Ag) and pentacene, an organic material used as an active material in organic thin-film transistors, using x-ray and ultraviolet photoemission spectroscopy (XPS and UPS). XPS results indicate that silver does not chemically react with pentacene regardless of the deposition order. We see that neither pentacene nor silver exhibited simple layer by layer growth when deposited on the other. UPS results show that a dipole forms as pentacene is deposited on silver.