Self-terminated Electrodeposition of Ni, Co and Fe Ultrathin Films

Transcription

1 Supporting Information for Self-terminated Electrodeposition of Ni, Co and Fe Ultrathin Films Rongyue Wang, Ugo Bertocci, Haiyan Tan, Leonid A. Bendersky, Thomas P. Moffat * Material Measurement Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland , United States Theiss Research, La Jolla, California 92037, United States * thomas.moffat@nist.gov

2 Film thickness calculation: The thickness of the iron group metal films was determined using XPS. The 3p peak of the iron group metals were integrated after Au substrate and background subtraction using a Shirley s algorithm. Au 4f peaks were also integrated after background subtraction also using Shirley s algorithm. The over-layer thickness of iron group metal d was calculated using the peak area (I) ratio after accounting for the attenuation length of the photoelectrons in the iron group metal over-layers (λ AL (Ni 3p ) =1.555 nm, λ AL (Co 3p ) =1.629 nm and λ AL (Fe 3p ) =1.705 nm) and the elemental sensitivity factors (sau 4f =6.25, sni 3p =0.598, sfe 3p =0.37 and sco 3p =0.447). 48,S1 [1] XPS analysis: XPS was also used to examine the chemical state of the emersed iron group films grown under self-terminating conditions. Quantitative analysis of iron group metals and their reaction products is challenging due to the intrinsic complexity of 2p peak shapes arising from multiplet splitting, shake-up and plasmon losses. Nickel and its oxides are arguably the most studied among the iron group metals and spectral analysis is facilitated by previous efforts to empirically fit Ni, NiO and Ni(OH) 2 standards. S2-S5 Electrodeposition was performed using a cell configuration with a working electrode-reference electrode separation of approximately 10 mm. Following electrodeposition the Ni films were rinsed with H 2 -saturated water and stored under Ar for transfer to the UHV system. A summary of the Ni 2p and O 1s spectra as a function of deposition time at -1.5 V SSCE is given in Figure S7. Following 1 s of deposition Ni(OH) 2 is the only species evident on the emersed Au electrode. After doubling the deposition time metallic Ni is present as the ev peak in the Ni 2p spectra while the O 1s spectra reveal oxide species along with hydroxide suggesting a Ni/NiO/Ni(OH) 2 trilayer has been formed similar to that found when a clean Ni surface is exposed to the ambient or aqueous solutions. S5,S6 After 5 s of deposition the

3 metallic state in the Ni 2p spectra is dominant and the Ni 2p and O 1s spectra become independent of deposition time. Comparison with a sputter cleaned electrodeposited Ni film (Figure S8B) indicates that high binding energy peak near ev is associated with plasmon losses in the metal while the remaining high energy Ni 2p components represent a mixture of the oxide and hydroxide species as indicated directly in the O 1s spectra. S2-S4 Survey spectra indicate negligible Cl - on the emersed surface. The Ni 2p (after metallic Ni 2p peak subtraction) and O 1s spectra were deconvolved into NiO and Ni(OH) 2 contributions with the individual components constrained according to the binding energy, area, width and ratio derived for bulk standards while the CASA S1 fitting algorithm was used to minimize the residuals as shown in Figure S8C and S8D. In this semi-quantitative analysis the depth distribution of the various components of the hydroxide/oxide/metal trilayer surface structure was not fully evaluated although the spectra are very similar to literature results for Ni exposed to wet oxidizing ambient. S5 HAADF-STEM of a Self-terminated Electrodeposited Ni Films after Aging: The combination of the excessive specimen thickness and possible surface contamination can limit effective imaging of the in-plane film structure, thus after 1 week of aging in air at room temperature the TEM specimen was further thinned by FIB and cleaned with an oxygen plasma. The resulting HAADF-STEM image, shown in Figure S10A, reveals a four-tier structure Au/Ni/NiO/Ni(OH) 2, with each layer having a distinct lattice spacing except for the outmost hydroxide layer that appears disordered. In the cross-section the Ni layer is about 1.8 nm in thickness and the NiO film approximately 1.6 nm in thickness. The (1-11) interplanar spacings for the three crystalline layers are 0.23 nm for the Au substrate, 0.20 nm for the nine Ni layers and 0.24 nm for NiO. A 1-D FFT determination of the in-plane dimensions along the [011] direction of the individual (1-11) planes in Figure S10A is summarized in Figure S10B. The nearest neighbor distance is nm, nm, and nm nm for the Au, Ni, and NiO layers, respectively. The multilayer stack is in agreement with the XPS data and consistent with

4 literature on passive films formed on Ni. S5 The atomically sharp interface between the Ni film and Au substrates is semi-coherent. Examination of the alignment of the (111) lattice planes of Au (red lines) and Ni (green lines) reveal misfit dislocations spaced 1.1 nm apart. Comparison between the original TEM specimen (Figure 5) and that after 1 week of aging in air and the O 2 plasma treatment (Figure S10) reveals that additional growth of the NiO layer has occurred at the expense of the initial Ni layer. The 180 -rotation (or antiparallel [110]) relationship between epitaxial NiO and Ni film is apparent as indicated by the white lines for NiO versus the green lines for Ni in Figure S10. Among the sections examined (not all shown), the NiO layer grows with a dominance of antiparallel NiO [110] // Ni [110] domains with a slight 3-15 o tilt of the (111) oxide planes from that of the metal. This is similar to surface x-ray scattering (SXS) and in situ STM studies of the passive film formed on Ni in aqueous media. S6,S7

5 Figure S1. Schematic illustration of the PTFE electrochemical cell designed to give a uniform primary current distribution and well defined separation between the working and reference electrode, d ref-wk that can be used to adjust the electrolyte resistance sensed by the potentiostat control loop. The calculated conductivity, σ, of the 0.1 mol/l NaCl is 9.45 ms/cm (the actual solution resistance will be slightly lower because of the addition of 5 mmol/l NiCl 2 and HCl for ph adjustment). The cross sectional area of the working specimen is equal to that of the cell channel. R can be estimated by R=ρd ref-wk /wh, where w is 0.6 cm and h is 1.5 cm and the resistivity is ρ=1/σ=105.8 Ω-cm. For d ref-wk of 0.1 cm, 1 cm and 2 cm, R is Ω, Ω and Ω and for a current of 1 ma the IR potential drop between the working and reference electrode is V, V and V, a trend that accounts for the shift in behavior evident in Figure 3B.

6 Figure S2. Voltammetry (20 mv/s) and EQCM results for Ni deposition from ph=3, 0.1 mol/l NaCl solutions containing 5, 10 or 50 mmol/l NiCl 2 (A-C); D-F, enlarged curves of results from 10 mmol/l NiCl 2 solution near the water reduction reaction potential; Dash lines in A are current densities derived from mass and the deviation evident near the current peak most likely reflects the non-uniform nature of the primary current distribution associated with conventional Ni deposition in the EQCM cell. Termination of metal deposition at negative potentials is closely associated with the onset of water reduction and the increase in dissipative losses derived from the increase in resistance in the EQCM measurement loop (A-C). An interesting inflection of the current is evident just prior to quenching of the metal deposition reaction (D-F).

7 Figure S3. Voltammetry (20 mv/s) and EQCM results for a Au electrode in ph=3, 0.1 mol/l NaCl.

8 Figure S4. EQCM response during potentiostatic polarization of Au at -1.4 V SSCE in ph=3, 0.1 mol/l NaCl followed by relaxation under open circuit conditions.

9 Figure S5. Studies of potentiostatic Co electrodeposition and relaxation of the self-terminated growth surface under open circuit conditions for stirred and stagnant solutions. When the solution is actively stirred with a magnetic bar, the dissipative losses associated with the self-terminated surface created at -1.4 V SSCE (A) attenuate quickly at open circuit and can be distinguished from metal dissolution as the resistance change ceases near 100s while the change in mass continues to evolve. In contrast, under stagnant conditions (B) the individual processes cannot be distinguished for self-terminated films grown at -1.5 V SSCE. The electrolyte was ph=3, 0.1 mol/l NaCl containing 5 mmol/l CoCl 2.

10 Figure S6. Conventional deposition and open circuit relaxation of Co films grown at -1.2 V SSCE for 1, 3 and 6s after which the potential was kept at open circuit potential. The electrolyte was ph=3, 0.1 mol/l NaCl containing 5 mmol/l CoCl 2 that was stirred with magnetic bar.

11 Figure S7. XPS spectra for Ni films (A, Ni 2p and B, O 1s) deposited on Au at -1.5 V SSCE for different times from a ph=3, 0.1 mol/l NaCl with 5 mmol/l NiCl 2 and d ref-wk =10 mm.

12 Figure S8. A. Ni (2p) XPS spectra for self-terminated Ni film deposited at -1.5 V SSCE for 10s from ph=3, 0.1 mol/l NaCl solution with 5 mmol/l NiCl 2 with d ref-wk =10 mm B. XPS spectra for Ar sputtered Ni (2p) film revealing the plasmon at higher binding. C. The Ni (2p) spectra in B is subtracted from the spectra for the self-terminated film shown in A leaving the oxidized components to be resolved based of empirical fits to reference standards for NiO and Ni(OH) 2.The correspond O (1s) spectra resolved into NiO, Ni(OH) 2 and H 2 O components. The ratio between NiO and Ni(OH) 2 are calculated to be 65.5:34.5 and 70:30 using Ni 2p peaks and O 1s peaks, respectively.

13 Figure S9. XPS derived film thickness for NiCo (A) and NiFe (B) alloys deposited at -1.5 V SSCE for different times from a ph=3, 0.1 mol/l NaCl solution with (A) 2.5 mmol/l NiCl mmol/l CoCl 2 and (B) 2.5 mmol/l NiCl mmol/l FeSO 4 with d ref-wk =10 mm.

14 Figure S10: (a) High resolution HAADF-STEM image of an aged and oxidized self-terminated Ni film grown on Au layer. (b) The 1-D FFT of the horizontal planes in the [112] shows the different atomic spacing of between the [110] column distances projected along the [011] view direction of the NiO, Ni, and Au layers.

15 Figure S11. Cross-section STEM image of electrodeposited Ni film on PVD Au film on Si. The Ni film was grown using multi-pulse deposition sequence, -1.5V SSCE -10s, -1V SSCE -600s, -1.5V SSCE -20s, -1V SSCE -600s -1.5V SSCE -20s and -1V SSCE -600s, in ph=3, 0.1 mol/l NaCl containing 5 mmol/l NiCl 2 with d ref-wk =10 mm. The process yields a non-uniform Ni overlayer with a rough surface. Nevertheless, certain regions exhibit well-defined semicoherent interefaces while in other areas a significant presence of voids is evident.

16 Figure S12. SEM images of Ni film deposited on Au at -1.0 V SSCE for 1800 s with (A) and without (B) an initial self-terminated deposition layer (-1.5 V SSCE for 20s). Voids present in (A) are highlighted with yellow arrows.

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