We report an N2 based annealing treatment to passivate interface traps (Dit) in n- and p-type 4H-SiC. The process has the potential to replace the commonly used hazardous and expensive gas nitric oxide (NO). N2 postoxidation annealing reduces Dit in both the upper and lower halves of the 4H-SiC bandgap, with a greater impact at the valence band edge. N2 annealing at 1500 °C is observed to be more effective in passivating traps and positive fixed charges than NO annealing for p-type devices, whereas for n-type devices, the opposite is true. The breakdown voltages for these devices are found to be lower than that of NO annealed devices. X-ray photoelectron spectroscopy has been performed to estimate the nitrogen areal density at the interface. Dit is measured as a function of nitrogen areal densities in the near interfacial regions for the different processes. Theoretical analysis through density functional theory is consistent with the measured Dit profiles by showing the generation of additional states near the valence band edge due to increased nitrogen concentration. In addition to reporting the effect of N2 annealing on both n- and p-type 4H-SiC, this work correlates the nitrogen areal densities at the interface to the Dit and explains the difference in Dit characteristics with nitrogen areal density between n- and p-type interfaces.

The rapidly increasing solar conversion efficiency (PCE) of hybrid organic-inorganic perovskite (HOIP) thin-film semiconductors has triggered interest in their use for direct solar-driven water splitting to produce hydrogen. However, application of these low-cost, electronic-structure-tunable HOIP tandem photoabsorbers has been hindered by the instability of the photovoltaic-catalyst-electrolyte (PV+E) interfaces. Here, photolytic water splitting is demonstrated using an integrated configuration consisting of an HOIP/n+silicon single junction photoabsorber and a platinum (Pt) thin film catalyst. An extended electrochemical (EC) lifetime in alkaline media is achieved using titanium nitride (TiN) on both sides of the Si support to eliminate formation of insulating silicon oxide, and as an effective diffusion barrier to allow high-temperature annealing of the catalyst/TiO2-protected-n+silicon interface necessary to retard electrolytic corrosion. Halide composition was examined in the (Cs1-xFAx)PbI3 system with a bandgap suitable for tandem operation. A fill factor (FF) of 72.5% was achieved using a Spiro-OMeTAD-hole-transport-layer (HTL)-based HOIP/n+Si solar cell, and a high photocurrent density of -15.9 mA/cm2 (at 0V vs reversible hydrogen electrode) was attained for the HOIP/n+Si/Pt photocathode in 1M NaOH under simulated one-sun illumination. While this thin-film design creates stable interfaces, the intrinsic photo- and electro-degradation of the HOIP photoabsorber remains the main obstacle for future HOIP/Si tandem PEC devices.

Most metallic elements possess one relatively close packed structure of body-centered cubic (BCC), face-centered cubic (FCC) and hexagonal close packed (HCP) structures under ambient conditions. Access to these structures as a function of temperature and pressure varies for different groups in the periodic table. Under low pressures, the copper group elements (copper (Cu), silver (Ag) and gold (Au)) only have a FCC solid structure regardless of temperatures. Recently, BCC Au was detected under extreme compressive pressures. The structural preference for a given metal element is still not understood. In this work, we surprisingly observed formation of BCC Au from amorphous Au as well as FCC Au in vacuum at room temperature within a Au nanoparticle with electron self-transfer enabled by our synthesized zirconium hydroxide nitrate tube support. We propose a theory behind this unexpected phase formation: the increase in atomic electron cloud densities of Au atoms resulting from the electron transfer contributes more free electrons over the Au nanoparticle except its interfacial part, enhancing metallic bonding strength to hold Au atoms closer to form a BCC structure that can store denser electron states within a Fermi sphere in first Brillouin zone as compared to FCC Au. It can also explain formation of BCC Au under compression. Our theory here can be a supplement to the metallic bonding theory and our strategy via building heterogenous structures can guide how to expand the structure world of metals and their alloys under ambient conditions for beneficial applications.

Decomposition of natural gas hydrocarbons using thermal plasma produced by arc discharge is a promising pathway for large-scale production of hydrogen in combination with high-value carbon nanotubes (CNTs). Metal(s) evaporating from an electrode and condensing to form seed nanoparticles that nucleate and grow CNTs. However, lack of understanding of the mechanisms underlying the CNT nucleation and growth on metal nanocatalysts in such a process makes control of the diameter, chirality, length and yield of CNTs difficult. We debundled and separated CNTs from soots produced using iron (Fe) nanocatalysts, and distributed the CNTs on a monolayer graphene support for high-resolution transmission electron microscopy (HRTEM) imaging to gain mechanistic insights into how Fe nanocatalysts nucleated and grew CNTs in a thermal plasma. Full graphene encapsulation was found for relatively small Fe nanoparticles that were molten at high temperatures. Zigzag single-wall CNTs (SWCNTs) or zigzag double-wall CNTs (DWCNTs) appeared to have grown out directly from the graphene covering on the conical or cylindrical bodies of small molten Fe nanodroplets with high curvature. Also, single-wall carbon nanocones (SWCNCs) were observed on conical Fe nanoparticles, but were likely etched by H or O atoms in the reactor causing growth of a more stable SWCNT. TEM images indicated that a SWCNT or DWCNT could also be generated from the cracked opening of the graphene covering on a face-centered cubic (FCC) Fe nanoparticle. A simple, plausible pathway is proposed for growth of an open, H-passivated, zigzag SWCNT involving reaction of CH2 and CH radicals at high temperatures.