*3.2. In-situ Helium Implantation and Annealing in H2*

Helium bubble evolution was observed and characterized in-situ during implantation in an H2 environment at <sup>−</sup>100 ◦C. Figure 3a shows initial observation of 4He bubbles after implantation to a peak concentration of 6 at.%. All reported concentration values assume no escape of He from the TEM foil. Bubbles initially had low areal-density and were approximately 1.2 nm in diameter, similar to the microstructure that was observed in the preliminary experiments at room temperature. Larger bubbles were observed in some areas, but bubble nucleation was generally homogenously distributed and uniform in size throughout the implantation. Bubble density visibly increased with increasing implantation dose as shown in Figure 3b–f. Figure 4a shows the measured bubble diameter and density changes as a function of He concentration. Bubble size remained constant during the implantation, but areal bubble density was observed to increase with implantation time, starting at 8 <sup>×</sup> <sup>10</sup><sup>11</sup> bubbles/cm2 in Figure 3a at 6 at.%, and reaching a density of 5 <sup>×</sup> 10<sup>12</sup> bubbles/cm<sup>2</sup> in Figure 3f at 23 at.%. Bubble

density change as a function of implantation dose was determined using a linear fit, where the slope = 2.7 <sup>×</sup> <sup>10</sup><sup>11</sup> bubbles/cm2/at.% and the intercept <sup>=</sup> <sup>−</sup>1.2 <sup>×</sup> 1012 bubbles/cm2.

**Figure 3.** BF in-situ TEM images showing He bubble evolution during implantation at −100 ◦C under H2 gas flow from peak concentrations of (**a**) 6 to (**f**) 23 at.%. The images were taken in Fresnel under-focus imaging condition, so bubbles appear as small white circles.

**Figure 4.** Bubble size and density changes (**a**) during in-situ implantation at −100 ◦C in H2 gas, and (**b**) during annealing from −100 ◦C to −60 ◦C. Data points in (**a**) were measured from the images in Figure 3. Bubble diameter is shown as solid green circles, and bubble density is shown as empty black circles. Linear fits are shown for the data in (**a**).

After implantation, the sample was heated in-situ from −100 ◦C to 0 ◦C in H2 gas. According to Figure 1a, most H2 should be desorbed from the sample during this heating process. Bubble size and density were characterized during the annealing, when possible, and are summarized in Figure 4b. The sample was annealed to −60 ◦C in 390 s (0.10 ◦C/s), held at −60 ◦C for approximately 10 min, then ramped from −60 ◦C to 0 ◦C in approximately 3 min. Images recorded from −60 ◦C to 0 ◦C had too much dynamic contrast evolution for bubble size analysis, so two post-annealing images were analyzed and the results averaged to obtain a final density of 7 <sup>×</sup> 1012 bubbles/cm2 and a final diameter of 1.3 nm. No significant changes in bubble diameter were observed during annealing up to 0 ◦C. No significant changes in bubble density were observed under annealing from −100 ◦C to −60 ◦C, but the bubble density did appear to increase from 6 <sup>×</sup> <sup>10</sup><sup>12</sup> bubbles/cm2 to 7 <sup>×</sup> 1012 bubbles/cm2 after

annealing from −60 ◦C to 0 ◦C (not shown in figure). Uncertainty is not provided for bubble density measurements because a single density value was calculated from each image.

#### **4. Discussion**

Helium bubble diameter was observed to remain constant during the cryogenic in-situ implantation in the presence of H2, but bubble density increased as a function of time. Even though β-phase hydride formation could not be identified within experimental error, the lower concentration α-phase hydride likely formed and could impact bubble evolution. Although H2 may influence the nucleation and growth of He bubbles in Pd, the H concentration is very low in α-phase (H/Pd = 0.03), so cryogenic implantation temperature is thought to be a more-likely dominating factor. Very little literature exists on He implantation in a H2 environment or on cryogenic He implantation, so this section will discuss, in relation to this work, (1) a comparison of this work with previous studies on He bubble evolution in Pd, (2) theory on He bubble growth at cryogenic temperature, and (3) how H-He interactions may affect the results.
