Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Abstract

Nuclear reaction analysis (NRA) via the resonant (1)H((15)N,αγ)(12)C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a (15)N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the (1)H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~10(13) cm(-2) (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~10(18) cm(-3) (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal (15)N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, (1)H((15)N,αγ)(12)C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of (15)N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

Nuclear Dynamics and Electronic Effects of Hydrogen on Solid Surfaces

“…We have developed NRA via 1 H( 15 N, αγ ) 12 C at the beam line of the tandem‐type Van de Graaff accelerator at the MALT facility, University of Tokyo,, where well‐characterized sample surfaces can be prepared in UHV before NRA measurements. Figure demonstrates the ability of H depth profiling with NRA.…”

Adsorption geometry of methyl chloride weakly interacting with Ag(111)

“…The surface was exposed to high purity CH 3 Cl at a typical sample temperature of 86 K. The coverage of CH 3 Cl was evaluated from the area of TPD spectra. TPD spectra were recorded with a shielded quadrupole mass spectrometer (QMS), which is described elsewhere [17], positioned 1 mm away from the sample. The QMS signal intensity was calibrated to gas phase CH 3 Cl with a relative ion gauge sensitivity factor of 2.4 [18,19].…”

Cross section of 15N-2D nuclear reactions from 3.3 to 7.0 MeV for simultaneous hydrogen and deuterium quantitation in surface layers with 15N ion beams

“…The 2 D( 15 N,p) 16 N and 2 D( 15 N,n) 16 O cross section measurements between 3.3 and 7.0 MeV were performed with 15 N 2+ ion beams of 10-20 nA provided by the 5-MV van-de-Graaff tandem accelerator (NEC) in an ultra-high vacuum (UHV) chamber at the MALT microanalysis laboratory of the University of Tokyo [5]. The system is described in detail in Refs.…”

“…The system is described in detail in Refs. [3][4][5]. In order to exclude the possibility of 2 D loss from the samples due to thermal desorption, the UHV chamber (base pressure if baked for typically 12-24 h at 150-200 o C: 610 -9 Pa) was not subjected to bake-out in the present study, resulting in a residual gas pressure of ~110 -5 Pa during the NRA measurements, which were conducted with the targets at room temperature.…”

“…15 N-1 H NRA quantitatively reveals the 1 H depth distribution in the analyzed target by registering the emitted -rays as a function of the 15 N incidence energy. At typical beam sizes of a few mm 2 , 1 H( 15 N,) 12 C NRA depth-selectively determines the 1 H content in electrical conductors as well as in insulating samples with a sensitivity of ~110 13 cm -2 for 1 H at surfaces and ~110 18 cm -3 (~100 ppm) 1 H in the bulk within several tens of minutes, irrespective of the chemical binding state of 1 H [5]. The technique is most elegantly performed with a scintillation detector positioned behind the target outside of the analyzing vacuum system, which provides accessibility for in-situ sample characterization and manipulation through surface science instrumentation [3][4][5].…”