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

Summary

We illustrate the application of ¹H(¹⁵N,αγ)¹²C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO₂/Si(100) stacks is described.

Abstract

Nuclear reaction analysis (NRA) via the resonant ¹H(¹⁵N,αγ)¹²C 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 ¹⁵N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the ¹H isotope in depths up to about 2 μm from the target surface. Surface H coverages are measured with a sensitivity in the order of ~10¹³ cm^-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~10¹⁸ cm^-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal ¹⁵N 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, ¹H(¹⁵N,αγ)¹²C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of ¹⁵N 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).

Introduction

The ubiquity of hydrogen as an impurity or as a constituent of a vast variety of materials and the wealth of hydrogen-induced interaction phenomena make revealing the hydrogen distribution in the near-surface region and at buried interfaces of solids an important task in many areas of engineering and fundamental material science. Prominent contexts include studies of hydrogen absorption in storage and purification materials for hydrogen energy applications, fuel cell, photo-, and hydrogenation catalysis, hydrogen retention and embrittlement in nuclear and fusion reactor engineering, hydrogen-induced surfactant effects in epitaxial growth fabrication and hydrogen-relat....

Protocol

1. Planning of Experiments

1. Identify the MALT accelerator beam line of interest depending on the measurement task (BL-1E for surface hydrogen, BL-2C for bulk or interfacial hydrogen). Contact the assisting scientist (currently M.W. or K.F.) to discuss details of the NRA measurements and their necessary preparations.
2. Download a beam time application form and observe the submission deadline on the MALT website³¹.
   Note: The MALT facility invites new project proposals each March and September for the Summer (April-September) and Winter (October-March) half year terms, respectively.
3. Write the beam time proposal and submit as i....

Results

Figure 4 shows near-surface NRA H profiles of H₂-exposed Pd(110) measured in the BL-1E UHV system at a sample temperature of 90 K under a H₂ background pressure of 1.33×10^-6 Pa. The ¹⁵N ion incidence energy has been converted into probing depth using the stopping power of Pd (S = 3.90 keV/nm). The open symbol profile was obtained after pre-exposing the Pd(110) sample to 2,000 L H₂ at 145 K to induce absorptio.......

Discussion

Figure 4 demonstrates the efficient distinction and quantitation of surface-adsorbed from bulk-absorbed hydrogen through ¹⁵N NRA at the example of a Pd(110) single crystal in the BL-1E UHV system. The high reproducibility of the surface H peak in the three profiles attests to the reliability of the in-situ UHV sample preparation and to the non-destructive nature of the NRA measurement. The quantitative agreement of the determined H coverage with the expected atomic saturation density .......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Material
Pd single crystal	SPL (Surface Preparation Laboratory), http://www.spl.eu/products.html, or any other suitable supplier	Order made to specification	Disk, 9 mm diam., (110) oriented, aligned to < 0.5 degree or less, one side polished to < 0.3 mm roughness, self-prepared specimen
H2 gas	Joutou Gas Corporation, Ltd., Japan, http://www.jyotougas.co.jp/item/gas.html		(99.9995%), or any other suitable supplier
O2 gas	Joutou Gas Corporation, Ltd., Japan, http://www.jyotougas.co.jp/item/gas.html		(99.99%), or any other suitable supplier
Ar gas	Joutou Gas Corporation, Ltd., Japan, http://www.jyotougas.co.jp/item/gas.html		(99.99995%), or any other suitable supplier
Tantalum / Wire	The Nilaco Corporation, http://nilaco.jp/en/order.php	TA-411325	(99.95%), 0.3 mm diam., or any other suitable supplier
Alumel / Wire	The Nilaco Corporation, http://nilaco.jp/en/order.php	851266	0.2 mm diam., or any other suitable supplier
Chromel / Wire (Chromel)	The Nilaco Corporation, http://nilaco.jp/en/order.php	861266	0.2 mm diam., or any other suitable supplier
Equipment
3 keV Raster Ion Bombardment Gun and Control	VARIAN, http://www.eurovac.se/docs/varian1.htm	981-2046 Power Supply, 981-2043 Ion Gun	or equivalent product of any other suitable manufacturer
LEED-AUGER Optics	OCI, http://www.ocivm.com/spectrometer_bdl800ir.html	BDL600IR	or equivalent product of any other suitable manufacturer
Quadrupole Mass Spectrometer	Pfeiffer Vacuum, http://www.pfeiffer-vacuum.com/	Prisma QMS 200	or equivalent product of any other suitable manufacturer
Palladium Hydrogen Purifier	Power + Energy Inc., http://www.powerandenergy.com	PE-3001	99.9999999% purity; P+E H2 purifiers are now business of SAES Pure Gases Inc., http://www.saespuregas.com/Products/Gas-Purifier/Hydrogen/Palladium-Membrane/Palladium-Purifier-PE2100.html