Dongmei Niu

Interfacial electronic structure at the CH3NH3PbI3/MoOx interface

Interfacial electronic properties of the CH3NH3PbI3 (MAPbI3)/MoOx interface are investigated using ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy. It is found that the pristine MAPbI3 film coated onto the substrate of poly (3,4-ethylenedioxythiophene) poly(styrenesulfonate)/indium tin oxide by two-step method behaves as an n-type semiconductor, with a band gap of ∼1.7 eV and a valence band edge of 1.40 eV below the Fermi energy (EF). With the MoOx deposition of 64 A upon MAPbI3, the energy levels of MAPbI3 shift toward higher binding energy by 0.25 eV due to electron transfer from MAPbI3 to MoOx. Its conduction band edge is observed to almost pin to the EF, indicating a significant enhancement of conductivity. Meanwhile, the energy levels of MoOx shift toward lower binding energy by ∼0.30 eV, and an interface dipole of 2.13 eV is observed at the interface of MAPbI3/MoOx. Most importantly, the chemical reaction taking place at this interface results in unfavorable interface ene...

From Water Oxidation to Reduction: Transformation from NixCo3–xO4 Nanowires to NiCo/NiCoOx Heterostructures

A homologous Ni-Co based nanowire catalyst pair, composed of Ni(x)Co(3-x)O4 nanowires and NiCo/NiCoO(x) nanohybrid, is developed for efficient overall water splitting. Ni(x)Co(3-x)O4 nanowires are found as a highly active oxygen evolution reaction (OER) catalyst, and they are converted into a highly active hydrogen evolution reaction (HER) catalyst through hydrogenation treatment as NiCo/NiCoO(x) heteronanostructures. An OER current density of 10 mA cm(-2) is obtained with the Ni(x)Co(3-x)O4 nanowires under an overpotential of 337 mV in 1.0 M KOH, and an HER current density of 10 mA cm(-2) is obtained with the NiCo/NiCoO(x) heteronanostructures at an overpotential of 155 mV. When integrated in an electrolyzer, these catalysts demonstrate a stable performance in water splitting.

Orientation-dependent energy level alignment and film growth of 2,7-diocty[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on HOPG

Combining ultraviolet photoemission spectroscopy, X-ray photoemission spectroscopy, atomic force microscopy, and X-ray diffraction measurements, we performed a systematic investigation on the correlation of energy level alignment, film growth, and molecular orientation of 2,7-diocty[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on highly oriented pyrolytic graphite. The molecules lie down in the first layer and then stand up from the second layer. The ionization potential shows a sharp decrease from the lying down region to the standing up region. When C8-BTBT molecules start standing up, unconventional energy level band-bending-like shifts are observed as the film thickness increases. These shifts are ascribed to gradual decreasing of the molecular tilt angle about the substrate normal with the increasing film thickness.

2D MoS2 Neuromorphic Devices for Brain‐Like Computational Systems

Hardware implementation of artificial synapses/neurons with 2D solid-state devices is of great significance for nanoscale brain-like computational systems. Here, 2D MoS2 synaptic/neuronal transistors are fabricated by using poly(vinyl alcohol) as the laterally coupled, proton-conducting electrolytes. Fundamental synaptic functions, such as an excitatory postsynaptic current, paired-pulse facilitation, and a dynamic filter for information transmission of biological synapse, are successfully emulated. Most importantly, with multiple input gates and one modulatory gate, spiking-dependent logic operation/modulation, multiplicative neural coding, and neuronal gain modulation are also experimentally demonstrated. The results indicate that the intriguing 2D MoS2 transistors are also very promising for the next-generation of nanoscale neuromorphic device applications.

Evolution of the electronic structure of C60/La0.67Sr0.33MnO3 interface

The evolution of the electronic structure at the interface between fullerene (C60) and La0.67Sr0.33MnO3 (LSMO) has been investigated with ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy. There is a 0.61 eV barrier for the electrons to be injected from LSMO to C60. The energy bands keep bending upward with increasing C60 thickness. A total energy bending of 0.72 eV is observed, changing the C60 film from n-type to p-type. The n-p transition is ascribed to the diffusion of oxygen from LSMO to C60 which subsequently strips electrons from C60, making the latter p-type. Our results suggest a buffer layer be inserted between the LSMO and C60 to lower the interface electron barrier and prevent deterioration of the C60 film in related spintronic devices.

Irreversible light-soaking effect of perovskite solar cells caused by light-induced oxygen vacancies in titanium oxide

An irreversible light-soaking effect was disclosed in perovskite solar cells using TiO2 as an electron transporting layer. The power conversion efficiency of a fresh device was improved more by twice after light soaking for 15 min and then remained 70% even though the device was recovered in the dark for 4 days. The buried mechanism was explored by shedding light on the interaction between light and titanium dioxide. Oxygen vacancies in TiO2 were found to be increased by light-soaking, especially for wavelengths shorter than 400 nm. Such vacancies enhanced the N-type doping in the semiconductor, which not only increased the conductivity of the titania film but also accelerated the charge extraction rate between perovskite crystallites and titania, and finally contributed to upgraded power conversion efficiency.

Energy Level Evolution and Oxygen Exposure of Fullerene/Black Phosphorus Interface

The heteroepitaxial growth of fullerene (C60) on single-crystal black phosphorus (BP) has been studied using low-energy electron diffraction, X-ray and ultraviolet photoelectron spectroscopy, and density functional theory simulation. The occupied orbital features from C60 observed in the photoelectron spectra for C60/BP interface are slightly broadened at higher coverages of C60 and exhibit no direct evidence of hybridization, demonstrating that the C60/BP interaction is physisorption. Oxygen exposure of interface leads to obvious oxidation of BP in which C60 bridges the large electron-transfer barrier from BP to oxygen and plays an important role for the production of O2- and oxidation of BP. Our findings suggest that C60 does not form an ideal protection layer as the other n-type semiconductors. With the assistance of density functional theory calculations, the oxidized phosphorus at the interface prevents further charge transfer from BP to C60.

Electronic Structures and Nanofilm Growth of 2,7-Dioctyl[1]Benzothieno[3,2-b]Benzothiophene on Black Phosphorus

The interfacial electronic structure and morphology of nanofilm of 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C8-BTBT) on black phosphorus (BP) was investigated with photoemission spectroscopy (PES) and atomic force microscopy (AFM). The heterojunction of C8-BTBT/BP is a straddling one with a hole injection barrier of 1.41 eV and electron injection barrier of 2.43 eV from BP to C8-BTBT. There is a 0.18 eV interface dipole pointing from BP to C8-BTBT, which means a relative weak interaction of substrate BP and the C8-BTBT molecules. Volmer-Weber growth mode of C8-BTBT nanofilm on BP was confirmed and the C8-BTBT molecules adopt standing up configuration.

Famatinite Cu3SbS4 nanocrystals as hole transporting material for efficient perovskite solar cells

To overcome the weakness of the organic hole transporting material in perovskite solar cells (PSCs), we developed inorganic copper antimony sulfide (Cu3SbS4) nanocrystals as a hole transporting material (HTM) for PSCs by a hot-injection and spray-deposition technique. We proved that the Cu3SbS4 nanocrystal layer can enhance hole injection inhibiting the charge-carrier recombination within the perovskite layer. A power conversion efficiency (PCE) of 8.7% was obtained for the Cu3SbS4-based devices with improved stability. This work offers a new route which employs copper antimony sulfide compounds as hole transporting materials for efficient and stable perovskite solar cells.

Interfacial Electronic Structures of Photodetectors Based on C8BTBT/Perovskite

Comprehensive measurements of ultraviolet photoemission spectroscopy, X-ray photoemission spectroscopy, X-ray diffraction, and atomic force microscopy are adopted to investigate the corelevance of energy level alignment, molecular orientation, and film growth of Au/C8BTBT/perovskite interfaces. A small energy offset of valence band maximum of 0.06 eV between perovskite and C8BTBT makes hole transportation feasible. About 0.65 eV upward shift of energy levels is observed with the deposition of the Au film on C8BTBT, which enhances hole transportation to the Au electrode. The observations from the interface analysis are supported by a prototype photodetector of Au (80 nm)/C8BTBT (20 nm)/perovskite (100 nm) that exhibits excellent performances whose responsivity can reach up to 2.65 A W-1, 4 times higher than the best CH3NH3PbI3 photodetectors.