Exploring Thermochromic Behavior of Hydrated Hybrid Perovskites in Solar Cells

Highly reproducible and reversible thermochromic nature of dihydrated methylammonium lead iodide is found. A wide bandgap variation of the material (∼2 eV) is detected between room temperature and 60 °C under ambient condition as a result of phase transition caused by moisture absorption and desorption. In situ X-ray diffraction and Fourier transform infrared spectroscopy studies are performed to understand the mechanistic behavior during the phase transition. This thermochromic property is further explored as absorber material in mesostructured solar cells. Temperature-dependent reversible power conversion efficiency greater than 1% under standard test conditions is demonstrated; revealing its potential applicability in building integrated photovoltaics

Molecular Layer Etching of Metalcone Films Using Lithium Organic Salts and Trimethylaluminum

Advances in semiconductor device manufacturing are limited by our ability to precisely add and remove thin layers of material in multistep fabrication processes. Recent reports on atomic layer etching (ALE) have provided the means for the precise removal of inorganic thin films deposited by atomic layer deposition (ALD), opening new avenues for nanoscale device design. Here, we report on a new technique for the precise removal of metal–organic thin films deposited by molecular layer deposition (MLD), which we term molecular layer etching. This etching process employs sequential exposures of lithium organic salt (LOS) and trimethylaluminum (TMA) precursors to produce self-limiting etching behavior. We employ quartz crystal microbalance experiments to demonstrate (i) etching of alucone films preloaded with LOS upon TMA exposures and (ii) layer-by-layer etching of alucone films using alternating exposures of LOS and TMA. We also identify the selectivity of these etching mechanisms. We probe the mechanism for the layer-by-layer etching of alucone using a quartz crystal microbalance and Fourier transform infrared spectroscopy and identify that the etching proceeds via heterolytic cleaving of Al–O bonds in alucone upon LOS exposure followed by methylation to produce volatile species upon TMA exposure. The etching process results in the removal of 0.4 nm/cycle of alucone at 160 °C and up to 3.6 nm/cycle of alucone at 266 °C in ex situ etching experiments on silicon wafers. This halogen-free etching process enables etching of MLD films and provides new fabrication pathways for the control of material geometries at the nanoscale

Atomic Layer Deposited Molybdenum Nitride Thin Film: A Promising Anode Material for Li Ion Batteries

Molybdenum nitride (MoN_<i>x</i>) thin films are deposited by atomic layer deposition (ALD) using molybdenum hexacarbonyl [Mo­(CO)₆] and ammonia [NH₃] at varied temperatures. A relatively narrow ALD temperature window is observed. <i>In situ</i> quartz crystal microbalance (QCM) measurements reveal the self-limiting growth nature of the deposition that is further verified with <i>ex situ</i> spectroscopic ellipsometry and X-ray reflectivity (XRR) measurements. A saturated growth rate of 2 Å/cycle at 170 °C is obtained. The deposition chemistry is studied by the <i>in situ</i> Fourier transform infrared spectroscopy (FTIR) that investigates the surface bound reactions during each half cycle. As deposited films are amorphous as observed from X-ray diffraction (XRD) and transmission electron microscopy electron diffraction (TEM ED) studies, which get converted to hexagonal-MoN upon annealing at 400 °C under NH₃ atmosphere. As grown thin films are found to have notable potential as a carbon and binder free anode material in a Li ion battery. Under half-cell configuration, a stable discharge capacity of 700 mAh g⁻¹ was achieved after 100 charge–discharge cycles, at a current density of 100 μA cm^–2

Thermal Atomic Layer Etching of MoS₂ Using MoF₆ and H₂O

Two-dimensional (2D) layered materials offer unique properties that make them attractive for continued scaling in electronic and optoelectronic device applications. Successful integration of 2D materials into semiconductor manufacturing requires high-volume and high-precision processes for deposition and etching. Several promising large-scale deposition approaches have been reported for a range of 2D materials, but fewer studies have reported removal processes. Thermal atomic layer etching (ALE) is a scalable processing technique that offers precise control over isotropic material removal. In this work, we report a thermal ALE process for molybdenum disulfide (MoS2). We show that MoF6 can be used as a fluorination source, which, when combined with alternating exposures of H2O, etches both amorphous and crystalline MoS2 films deposited by atomic layer deposition. To characterize the ALE process and understand the etching reaction mechanism, in situ quartz crystal microbalance (QCM), Fourier transform infrared (FTIR), and quadrupole mass spectrometry (QMS) experiments were performed. From temperature-dependent in situ QCM experiments, the mass change per cycle was −5.7 ng/cm2 at 150 °C and reached −270.6 ng/cm2 at 300 °C, nearly 50× greater. The temperature dependence followed Arrhenius behavior with an activation energy of 13 ± 1 kcal/mol. At 200 °C, QCM revealed a mass gain following exposure to MoF6 and a net mass loss after exposure to H2O. FTIR revealed the consumption of Mo–O species and formation of Mo–F and MoFxO species following exposures of MoF6 and the reverse behavior following H2O exposures. QMS measurements, combined with thermodynamic calculations, supported the removal of Mo and S through the formation of volatile MoF2O2 and H2S byproducts. The proposed etching mechanism involves a two-stage oxidation of Mo through the ALE half-reactions. Etch rates of 0.5 Å/cycle for amorphous films and 0.2 Å/cycle for annealed films were measured by ex situ ellipsometry, X-ray reflectivity, and transmission electron microscopy. Precisely etching amorphous films and subsequently annealing them yielded crystalline, few-layer MoS2 thin films. This thermal MoS2 ALE process provides a new mechanism for fluorination-based ALE and offers a low-temperature approach for integrating amorphous and crystalline 2D MoS2 films into high-volume device manufacturing with tight thermal budgets

Atomic-Scale Structure of Chemically Distinct Surface Oxygens in Redox Reactions

During redox reactions, oxide-supported catalytic systems undergo structural and chemical changes. Improving subsequent catalytic properties requires an understanding of the atomic-scale structure with chemical state specificity under reaction conditions. For the case of 1/2 monolayer vanadia on α-TiO2(110), we use X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy to follow the redox induced atomic positional and chemical state changes of this interface. While the resulting XSW 3D composite atomic maps include the Ti and O substrate atoms and V surface atoms, our focus in this report is on the previously unseen surface oxygen species with comparison to density functional theory predictions

Formation of Unsaturated Hydrocarbons and Hydrogen: Surface Chemistry of Methyltrioxorhenium(VII) in ALD of Mixed-Metal Oxide Structures Comprising Re(III) Units

We present the full investigation of the atomic layer deposition (ALD) of a mixed rhenium–aluminum oxide, namely ReAl2O3CH3, a material with tunable resistance, comprising the building unit of conductive rhenium oxides, ReOx. The deposition, involving methyl­trioxorhenium­(VII) (MeReO3, MTO) and trimethyl­aluminum (TMA), was analyzed by employing complementary in situ diagnostic quartz-crystal microbalance (QCM), Fourier-transform infrared (FT-IR) spectroscopy, and quadrupole mass spectrometry (QMS) to explore and reveal the underlying growth mechanism of this material. A proposed mechanism includes reductive elimination steps, thereby creating a stable Re­(III)-containing thin film, making this ALD process unique regarding its growth. In addition, as proven by QMS, the surface reactions include the formation of hydrogen and unsaturated hydrocarbons. From this straightforward process, an extraordinarily high growth rate of 4.5 Å cycle–1 at temperatures as low as 150 °C was obtained. This material was found to exhibit highly promising electrical properties in terms of low thermal coefficient of resistance (TCR) in combination with high resistivity. By blending thin films of ReAl2O3CH3 with additional layers (1, 2, or 3) of Al2O3, we were able to fine-tune the electrical resistivity in the range of 3.9 × 106–1.5 × 1011 Ω·cm. Simultaneously, the TCR was lowered to about −0.014 °C–1, making this material highly resistive over a broad temperature range and a promising candidate for advanced detector applications, e.g., multichannel plates (MCPs)

Atomic-Scale View of Redox Induced Changes for Monolayer MoO_<i>x</i> on α‑TiO₂(110) with Chemical-State Sensitivity

Supported molybdenum oxide (MoOx) plays an important role in catalytic transformations from alcohol dehydrogenation to transesterification. During these reactions, molybdenum and oxygen surface species undergo structural and chemical changes. A detailed, chemical-state specific, atomic-scale structural analysis of the catalyst under redox conditions is important for improving catalytic properties. In this study, a monolayer of Mo grown on α-TiO2(110) by atomic-layer deposition is analyzed by X-ray standing wave (XSW) excited X-ray photoelectron spectroscopy (XPS). The chemical shifts for Mo 2p3/2 and O 1s peaks are used to distinguish Mo6+ from Mo4+ and surface O from bulk O. Excitation of XPS by XSW allows pinpointing the location of these surface species relative to the underlying substrate lattice. Measured 3D composite atomic density maps for the oxidized and reduced interfaces compare well with our density functional theory models and collectively create a unique view of the redox-driven dynamics for this complex catalytic structure