skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Attention:

The DOI auto-population feature in the Public Access Repository (PAR) will be unavailable from 4:00 PM ET on Tuesday, July 8 until 4:00 PM ET on Wednesday, July 9 due to scheduled maintenance. We apologize for the inconvenience caused.


Title: Chemical and Electronic Structures of Cobalt Oxynitride Films Deposited by NH3 vs. N2 Plasma: Theory vs. Experiment
The chemical structures of Co oxynitrides – in particular, interactions among N and O atoms bonded to the same cobalt – are of great importance for an array of catalytic and materials applications. X-ray diffraction (XRD), core and valence band X-ray photoelectron spectroscopy (XPS) and plane wave density functional theory (DFT) calculations are used to probe chemical and electronic interactions of nitrogen-rich CoO1-xNx (x > 0.7) films deposited on Si(100) using NH3 or N2 plasma-based sputter deposition or surface nitridation. Total energy calculations indicate that the zincblende (ZB) structure is energetically favored over the rocksalt (RS) structure for x > ~ 0.2, with an energy minimum observed in the ZB structure for x ~ 0.8 - 0.9. This is in close agreement with XPS-derived film compositions when corrected for surface oxide/hydroxide layers. XRD data indicate that films deposited on Si (100) at room temperature display either a preferred (220) orientation or no diffraction pattern, and are consistent with either rocksalt (RS) or zincblende (ZB) structure. Comparison between experimental and calculated X-ray excited valence band densities of states – also similar for all films synthesized herein – demonstrates a close agreement with a ZB, but not an RS structure. Core level XPS spectra exhibit systematic differences between films deposited in NH3 vs N2 plasma environments. Films deposited by N2 plasma magnetron sputtering exhibit greater O content as evidenced by systematic shifts in N 1s binding energies. Excellent agreement with experiment for core level binding energies is obtained for DFT calculations based on the ZB structure, but not for the RS structure. The agreement between theory and experiment demonstrates that these N-rich Co oxynitride films exhibit the ZB structure, and forms the basis of a predictive model for understanding how N and O interactions impact the electronic, magnetic and catalytic properties of these materials.  more » « less
Award ID(s):
1757946
PAR ID:
10198314
Author(s) / Creator(s):
; ; ; ; ; ; ;
Date Published:
Journal Name:
Physical Chemistry Chemical Physics
ISSN:
1463-9076
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract Carbon-doped silicon oxide (CDO) thin films as low dielectric constant materials were deposited on both n-type silicon (Si) (100) and indium tin oxide coated polyethylene naphthalate (ITO/PEN) substrates, using the plasma-enhanced chemical vapor deposition of tetrakis(trimethylsilyoxy)silane (TTMSS) precursor. Chemical structures of the CDO films were analyzed by using FTIR (Fourier transformation infrared) spectroscopy and XPS (X-ray photoelectron spectroscopy). The chemical bonds related with hydrocarbon and Si–O were the main characteristics of the CDO films. The prominent peaks from the FTIR spectra included Si–O–Si stretching, Si–CH3 bending, Si–(CH3)x stretching, and CHx stretching modes. XPS spectra composed of the O1s, C1s, and Si2p electron orbitals were used to quantitatively analyze the elemental composition of the CDO films. The growth mechanisms of CDO films were dependent on the substrate type. For the ITO/PEN substrate, the lack of Si atoms on the ITO surface made difficulty in forming initial Si–O bonds, resulting in insufficient Si–O–Si structure. In comparison, the CDO films could grow easily on Si substrates due to pre-existing Si–O bonds on the surface. The chemical structures of the CDO films are expected to affect electrical and mechanical performances. 
    more » « less
  2. null (Ed.)
    Electroreduction of N2 to NH3 is an energy- and environmentally-friendly alternative to the Haber-Bosch process. Little is known, however, about reactive sites for electrochemical nitrogen reduction reaction (NRR) at Earth-abundant oxide or oxynitride surfaces. Here, we report N-free VIII/IV-oxide films, created by O2 plasma oxidation of polycrystalline vanadium, exhibiting N2 reduction at neutral pH with an onset potential of −0.16 V vs Ag/AgCl. DFT calculations indicate that N2 scission from O-supported V-centers is energetically favorable by ~18 kcal mol−1 compared to N-supported sites. Theory and experiment yield fundamental insights concerning the effect of metal oxophilicity towards design of earth-abundant NRR electrocatalysts. 
    more » « less
  3. We report a comparative spectroscopic study on the thin films of epitaxial aluminum nitride (AlN) on basal plane sapphire (Al2O3) substrates grown in hydrogen (H2) and nitrogen (N2) gas reaction environments. AlN films of similar thicknesses (~3.0 µm) were grown by metal-organic chemical vapor deposition (MOCVD) for comparison. The impact of the gas environment on the AlN epilayers was characterized using high-resolution X-ray diffraction (HR-XRD), X-ray photoelectron spectroscopy (XPS), Raman scattering (RS), secondary ion mass spectroscopy (SIMS), cathodoluminescence (CL), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The study showed that AlN layers grown in a N2 environment have 50% less stress (~0.5 GPa) and similar total dislocation densities (~109/cm2) as compared to the films grown in a H2 environment. On the other hand, AlN films grown in a H2 gas environment have about 33% lesser carbon and 41% lesser oxygen impurities than films grown in a N2 growth environment. The possible mechanisms that influenced the structural quality and impurity incorporation for two different gas environments to grow AlN epilayers in the MOCVD system on sapphire substrates were discussed. 
    more » « less
  4. We report the growth of nanoscale hafnium dioxide (HfO2) and zirconium dioxide (ZrO2) thin films using remote plasma-enhanced atomic layer deposition (PE-ALD), and the fabrication of complementary metal-oxide semiconductor (CMOS) integrated circuits using the HfO2 and ZrO2 thin films as the gate oxide. Tetrakis (dimethylamino) hafnium (Hf[N(CH3)2]4) and tetrakis (dimethylamino) zirconium (IV) (Zr[N(CH3)2]4) were used as the precursors, while O2 gas was used as the reactive gas. The PE-ALD-grown HfO2 and ZrO2 thin films were analyzed using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The XPS measurements show that the ZrO2 film has the atomic concentrations of 34% Zr, 2% C, and 64% O while the HfO2 film has the atomic concentrations of 29% Hf, 11% C, and 60% O. The HRTEM and XRD measurements show both HfO2 and ZrO2 films have polycrystalline structures. n-channel and p-channel metal-oxide semiconductor field-effect transistors (nFETs and pFETs), CMOS inverters, and CMOS ring oscillators were fabricated to test the quality of the HfO2 and ZrO2 thin films as the gate oxide. Current-voltage (IV) curves, transfer characteristics, and oscillation waveforms were measured from the fabricated transistors, inverters, and oscillators, respectively. The experimental results measured from the HfO2 and ZrO2 thin films were compared. 
    more » « less
  5. The production of ammonia for agricultural and energy demands has accelerated research for more environmentally-friendly synthesis options, particularly the electrocatalytic reduction of molecular nitrogen (nitrogen reduction reaction, NRR). Catalyst activity for NRR, and selectivity for NRR over the competitive hydrogen evolution reaction (HER), are critical issues for which fundamental knowledge remains scarce. Herein, we present results regarding the NRR activity and selectivity of sputter-deposited titanium nitride and titanium oxynitride films for NRR and HER. Electrochemical, fluorescence and UV absorption measurements show that titanium oxynitride exhibits NRR activity under acidic conditions (pH 1.6, 3.2) but is inactive at pH 7. Ti oxynitride is HER inactive at all these pH values. In contrast, TiN – with no oxygen content upon deposition – is both NRR and HER inactive at all the above pH values. This difference in oxynitride/nitride reactivity is observed despite the fact that both films exhibit very similar surface chemical compositions – predominantly Ti IV oxide – upon exposure to ambient, as determined by ex situ X-ray photoelectron spectroscopy (XPS). XPS, with in situ transfer between electrochemical and UHV environments, however, demonstrates that this Ti IV oxide top layer is unstable under acidic conditions, but stable at pH 7, explaining the inactivity of titanium oxynitride at this pH. The inactivity of TiN at acidic and neutral pH is explained by DFT-based calculations showing that N 2 adsorption at N-ligated Ti centers is energetically significantly less favorable than at O-ligated centers. These calculations also predict that N 2 will not bind to Ti IV centers due to a lack of π-backbonding. Ex situ XPS measurements and electrochemical probe measurements at pH 3.2 demonstrate that Ti oxynitride films undergo gradual dissolution under NRR conditions. The present results demonstrate that the long-term catalyst stability and maintenance of metal cations in intermediate oxidation states for pi-backbonding are critical issues worthy of further examination. 
    more » « less