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Surface reactivity enhancement on a Pd/Bi2Te3 heterostructure through robust topological surface states.

He QL, Lai YH, Lu Y, Law KT, Sou IK - Sci Rep (2013)

Bottom Line: We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure.The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation.A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

View Article: PubMed Central - PubMed

Affiliation: William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, SAR China.

ABSTRACT
We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure. The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation. The surface reactivity of the adsorbed tellurium on this heterostructure is also intensified possibly benefitted from the effective transfer of the bath electrons. A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

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XPS core-level spectra of S#3.(a) Te 3d peaks. Spectra (3a-1) and (3a-2) are Te 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3a-3) and (3a-4) are the corresponding spectra obtained from S#3B, respectively. The inset displays the structure of S#3. (b) Pd 3d peaks. Spectra (3b-1) and (3b-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3b-3) and (3b-4) are the corresponding spectra obtained from S#3B, respectively. (c) and (d) are the fitting results of Te 3d5/2 peaks in Spectra (3a-3) and (3a-4).
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f3: XPS core-level spectra of S#3.(a) Te 3d peaks. Spectra (3a-1) and (3a-2) are Te 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3a-3) and (3a-4) are the corresponding spectra obtained from S#3B, respectively. The inset displays the structure of S#3. (b) Pd 3d peaks. Spectra (3b-1) and (3b-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3b-3) and (3b-4) are the corresponding spectra obtained from S#3B, respectively. (c) and (d) are the fitting results of Te 3d5/2 peaks in Spectra (3a-3) and (3a-4).

Mentions: Sample #3 [S#3, inset of Fig. 3(a) shows its structure] was fabricated by in situ thermally evaporating Te with nominal thickness of 1.3 nm onto the entire surface of a structure identical to that of S#1. Te is a congener of O and thus can also act as an oxidizing agent for the metallic Pd. The findings obtained from the studies on S#1 and S#2 as described above indicate that the Pd layer in the region W/BT enjoys a stronger surface reactivity as compared with its counterpart in the region W/O BT. Thus, it is expected that more Te will be deposited on the Pd layer in the region W/BT of S#3. One piece of S#3 was cut and transferred to the XPS system right after the sample was unloaded from the MBE system (S#3A), while another piece was exposed in dry air for 3 days before being loaded into the XPS system (S#3B). A quantitative XPS analysis performed on S#3A shows that the surface concentration of Te and the concentration ratio of Te/Pd in the region W/O BT are 33.74% and 0.519. The corresponding data for the region W/BT can be obtained by subtracting the Te contribution of the underlying BT thin film from the apparent XPS data. Using the detected Bi concentration in the region W/BT of S#3A (3.55%) and the detected Bi/Te ratio of S#2A (0.72), the Te contributed from the underlying BT thin film of S#3A was estimated to be 4.93%. With this subtraction carried out, the actual surface concentration of Te and the concentration ratio of Te/Pd in the region W/BT of S#3A are determined to be 42.73% and 0.726, which are indeed substantially higher than those data obtained from the region W/O BT. The chemical shifts of the core-level peaks of Te 3d and Pd 3d resulted from the XPS spectra of S#3A act as another evidence that the surface reactivity of Pd is enhanced by the underlying BT thin film. In Fig. 3(a), Spectra (3a-1) and (3a-2) show the Te 3d core-level spectra in the region W/O BT and W/BT of S#3A, while in Fig. 3(b), Spectra (3b-1) and (3b-2) show the Pd 3d core-level spectra in the corresponding regions, respectively. As shown in Spectra (3a-1) and (3a-2), the Te 3d core-levels in the region W/BT of S#3A negatively shift (corresponding to a stronger reduction for non-metals) by 0.35 eV in reference to those in the region W/O BT, while the corresponding Pd 3d core-levels positively shift (corresponding to a stronger oxidation for metals) by 0.10 eV. The results just described above for S#3A together with the findings obtained from the studies of S#1 and S#2 addressed in the previous two paragraphs all demonstrate that the surface reactivity of Pd in the region W/BT is enhanced by the underlying BT thin film, which is believed to be attributed to the electron bath effect. This effect is mainly due to the facilitation of delocalized surface electrons originated from the TSSs of BT as described in Chen et al. 's theoretical model12. As the adsorption begins, the surface electrons migrate to the absorbates due to their topological delocalization, which promotes O2 molecules or Ten clusters toward dissociative adsorption on Pd.


Surface reactivity enhancement on a Pd/Bi2Te3 heterostructure through robust topological surface states.

He QL, Lai YH, Lu Y, Law KT, Sou IK - Sci Rep (2013)

XPS core-level spectra of S#3.(a) Te 3d peaks. Spectra (3a-1) and (3a-2) are Te 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3a-3) and (3a-4) are the corresponding spectra obtained from S#3B, respectively. The inset displays the structure of S#3. (b) Pd 3d peaks. Spectra (3b-1) and (3b-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3b-3) and (3b-4) are the corresponding spectra obtained from S#3B, respectively. (c) and (d) are the fitting results of Te 3d5/2 peaks in Spectra (3a-3) and (3a-4).
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f3: XPS core-level spectra of S#3.(a) Te 3d peaks. Spectra (3a-1) and (3a-2) are Te 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3a-3) and (3a-4) are the corresponding spectra obtained from S#3B, respectively. The inset displays the structure of S#3. (b) Pd 3d peaks. Spectra (3b-1) and (3b-2) are Pd 3d spectra obtained from the region W/O BT and W/BT of S#3A, while Spectra (3b-3) and (3b-4) are the corresponding spectra obtained from S#3B, respectively. (c) and (d) are the fitting results of Te 3d5/2 peaks in Spectra (3a-3) and (3a-4).
Mentions: Sample #3 [S#3, inset of Fig. 3(a) shows its structure] was fabricated by in situ thermally evaporating Te with nominal thickness of 1.3 nm onto the entire surface of a structure identical to that of S#1. Te is a congener of O and thus can also act as an oxidizing agent for the metallic Pd. The findings obtained from the studies on S#1 and S#2 as described above indicate that the Pd layer in the region W/BT enjoys a stronger surface reactivity as compared with its counterpart in the region W/O BT. Thus, it is expected that more Te will be deposited on the Pd layer in the region W/BT of S#3. One piece of S#3 was cut and transferred to the XPS system right after the sample was unloaded from the MBE system (S#3A), while another piece was exposed in dry air for 3 days before being loaded into the XPS system (S#3B). A quantitative XPS analysis performed on S#3A shows that the surface concentration of Te and the concentration ratio of Te/Pd in the region W/O BT are 33.74% and 0.519. The corresponding data for the region W/BT can be obtained by subtracting the Te contribution of the underlying BT thin film from the apparent XPS data. Using the detected Bi concentration in the region W/BT of S#3A (3.55%) and the detected Bi/Te ratio of S#2A (0.72), the Te contributed from the underlying BT thin film of S#3A was estimated to be 4.93%. With this subtraction carried out, the actual surface concentration of Te and the concentration ratio of Te/Pd in the region W/BT of S#3A are determined to be 42.73% and 0.726, which are indeed substantially higher than those data obtained from the region W/O BT. The chemical shifts of the core-level peaks of Te 3d and Pd 3d resulted from the XPS spectra of S#3A act as another evidence that the surface reactivity of Pd is enhanced by the underlying BT thin film. In Fig. 3(a), Spectra (3a-1) and (3a-2) show the Te 3d core-level spectra in the region W/O BT and W/BT of S#3A, while in Fig. 3(b), Spectra (3b-1) and (3b-2) show the Pd 3d core-level spectra in the corresponding regions, respectively. As shown in Spectra (3a-1) and (3a-2), the Te 3d core-levels in the region W/BT of S#3A negatively shift (corresponding to a stronger reduction for non-metals) by 0.35 eV in reference to those in the region W/O BT, while the corresponding Pd 3d core-levels positively shift (corresponding to a stronger oxidation for metals) by 0.10 eV. The results just described above for S#3A together with the findings obtained from the studies of S#1 and S#2 addressed in the previous two paragraphs all demonstrate that the surface reactivity of Pd in the region W/BT is enhanced by the underlying BT thin film, which is believed to be attributed to the electron bath effect. This effect is mainly due to the facilitation of delocalized surface electrons originated from the TSSs of BT as described in Chen et al. 's theoretical model12. As the adsorption begins, the surface electrons migrate to the absorbates due to their topological delocalization, which promotes O2 molecules or Ten clusters toward dissociative adsorption on Pd.

Bottom Line: We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure.The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation.A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

View Article: PubMed Central - PubMed

Affiliation: William Mong Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Hong Kong, SAR China.

ABSTRACT
We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure. The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation. The surface reactivity of the adsorbed tellurium on this heterostructure is also intensified possibly benefitted from the effective transfer of the bath electrons. A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

Show MeSH