Surface interaction of selected transition metals and ...
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experimental data calc. pattern Sn 2 + calc. pattern HSn 2 + experimental data calc. pattern HOSn + calc. pattern H 2 OSn + calc. pattern H 3 OSn + experimental data calc. pattern Pb + calc. pattern HPb + experimental data calc. pattern Sn + calc. pattern HSn + calc. pattern H 3 Sn + Joshua Rieger ; Sanna Benter; Clara Markert; Kai Kroll; Hendrik Kersten; Thorsten Benter Physical & Theoretical Chemistry Wuppertal, Germany Institute for Pure and Applied Mass Spectrometry Surface interaction of selected transition metals and semiconductors with H 2 plasma generated species Methods Introduction Basic chemical assumption Plasmas are convenient sources of radiation and energy for driving chemical and physical processes. The interaction between plasma constituents and matter results in a multitude of pathways for e.g. ionization and/or the initiation of chemical reactions. The reactivity of excited hydrogen towards tin and in general elements of the 14 th group of the periodic table (Carbon group, group IV) has been described in the literature for decades. Likewise, the thermodynamic instability of the resulting metal hydrides and the catalytic effect of metal surfaces regarding their decomposition is known since long. This chemistry has recently gained importance for the modeling of high energy H 2 plasmas in the presence of tin, which requires knowledge of the neutral and ionic reaction channels and mechanisms. The initial interaction in the formation of metal hydrides is a heterogeneous gas/surface reaction of excited atomic H and metal atoms M, in which successive H-M bonds are formed. The volatile metal hydrides thus formed subsequently desorb and may subsequently adsorb on other (metal) surfaces and decompose. Conclusion / Outlook Literature Acknowledgement Mass spectra C-TOF time of flight mass spectrometer equipped with custom ion transfer stage (TOFWERK AG, Thun, Switzerland) Modified VUV lamp RF power supplies (Heraeus GmbH, Hanau, Germany ) Hydrogen 5.0 and Argon 5.0 (Messer Group GmbH, Krefeld, Germany) Gaussian 16. [5] and GaussView 6.0.16 iHR 320 (Horiba, Kyoto, Japan) Fig. 1: Schematic setup and simplified mechanism of metal hydride formation A custom low-pressure RF discharge generates the reagent ions H 3 + within a hydrogen plasma fed by an adjustable continuous gas flow (1). The plasma source consists of an electrode-less, helical coil resonator (HCR) providing the RF energy to sustain the discharge, which is classified as a high density plasma [1]. Typical operation pressures range between 0.1 and 10 mbar. The continuously generated ions are brought into contact with the solid metal samples (3) by the gas flow within the glass tube (2). The tube itself is additionally pumped by the first differentially pumped stage (0.1 – 10 mbar) of the used time of flight mass spectrometer (TOF-MS) (4). [1] H. Kersten, T. Kutsch, K. Kroll, K. Haberer, T. Benter; H 2 plasma for the generation of protonation reagents with a standard APPI power supply, 64 th ASMS Conference on Mass spectrometry and Allied Topics, Indianapolis, IN, USA, 2017. [2] D. J. Aaserud, F. W. Lampe, Journal of Physical Chemistry, 100, 10215-10222, 1996. [3] N. Faradzhev, V. Sidorkin, Journal of Vacuum Science & Technology A: Vacuum Surfaces and Films, 27, 306-314, 2009. [4] M. Berglund, M. E. Wieser, Pure Appl. Chem., 83, 2, 397-410, 2011. [5] Frisch et al., Gaussian16, Revision A.03, Gaussian, Inc., Wallingford CT. 2016. Mass spectrometer: RF power supplies: Gases: Software: UV-Vis spectrometer: MH x + M 0.1 – 10 mbar 1 – 50 sccm RF plasma MS power supply H 2 H 3 + 1 2 3 4 Experimental setup Generous support from Carl Zeiss SMT, Oberkochen, Germany, is gratefully acknowledged. 2 2 () + () ↛ 4 () 4 + () → 4 () 4 () → () +2 2 () 4 () + → () +2 2 () () + 4 () → 3 () + 2 () () + 4 () → 2 () + 3 () 3 () + 3 () → 2 () + 4 () 2 () + 4 () ⇄ 2 6 () ∗ 2 6 () ∗ → 2 4 () + 2 () 2 6 () ∗ + → 2 6 () + 2 4 () + → 2 () +2 2 () The RF discharge primarily generates electronically excited H species and H + ions. The latter react with molecular hydrogen to form H 3 + . In the presence of N 2 or synthetic air N 2 H + and also N 4 H + are readily formed. In contrast to H 2 , the adsorption of reactive H species leads to a subsequent formation of stannane (SnH 4 ), which either decomposes on the surface or desorbs into the gas phase [2,3]. Tin oxide is reduced by atomic hydrogen to tin metal and water. In the gas phase, a reaction cascade leads to a multitude of tin hydrides, which is shown in Fig. 2. In this scheme (Eley-Rideal mechanism), more H is consumed than expected by a simple recombination of adsorbed and newly penetrating H atoms. Stannane tends to spontaneously decay into the elements, a process which is accelerated by reactive surfaces (1 st order kinetics [2]). Fig.3 shows schematically the reaction channels of these tin compounds [3]. 2 → • + • → + 2 ↑ + • → 4 ↑ 4 + → + 2 ↑ Fig 2: Cascade of elementary reaction steps involving hydrogen and tin hydride compounds [2]. Fig 3: Schematic representation of the reactions channels shown in Fig. 2. In the spectra, ionic tin hydrides and oxygenated tin compounds were detected. The comparison of the recorded mass spectrum with calculated isotopic patterns [4] reveals the distinctive patterns of various ionic tin hydrides of the type SnH x + , especially SnH 3 + and SnH + , while the more unstable SnH 4 + and SnH 2 + are absent (see Fig. 6). The dimeric Sn 2 H x compounds formed are also detected but with lower intensities. However, only Sn 2 + and Sn 2 H + are present in the mass spectrum, dimers with additional hydrogen atoms were not detected or only to a very small extent (see Fig. 7). The oxygenated compounds of the type H x OSn + mainly originate from the oxidized target surface, rather than from the gas matrix oxygen, as only very high purity gases were used. Most prominently HOSn + is detected, whereas H 2 OSn + and H 3 OSn + are detected to a much lesser extent (see Fig. 8). The characteristic patterns of the Sn isotopes are fully reproduced and thus used for compound identification. The recorded mass spectra of lead (Pb) compounds are in very good accordance with the calculated isotopic patterns [4]. In addition to Pb + , only mono plumbane (PbH + ) is observed (see Fig. 9). Experiments with nickel (Ni), tantalum (Ta) and silicon (Si) did not result in detectable ionic species, probably due to i) the formation of a protective oxide surface layer, ii) low stability of the corresponding ionic hydrides, or iii) insufficient sensitivity of the experimental setup. Ab-initio calculations of molecular stabilities Fig 6: Recorded mass spectrum of H-plasma generated tin hydrides and calculated isotopic patterns. Fig 7: Recorded mass spectrum of H-plasma generated dimeric tin hydrides and calculated isotopic patterns. Fig 8: Recorded mass spectrum of H-plasma generated oxygenated tin hydrides and calculated isotopic patterns. Fig 9: Recorded mass spectrum of H-plasma generated lead hydrides and calculated isotopic patterns. Why are SnH 2 + and SnH 4 + not detectable? H α H β H γ H δ Fig.5: UV-Vis spectrum of the RF hydrogen plasma with marked transitions of the Balmer series. The Plasma discharge continuously generates a large number of excited hydrogen species. The recombination of these species becomes “visible” through the emission spectrum. The Balmer series is identified in the UV-Vis spectrum, higher transitions of the series are not unambiguously assignable. The broad continuum presumably originates from a combination of Bremsstrahlung and electron recombination emission. The large number of H atoms present causes the inherently unstable tin hydride species to react quickly, presumably forming H 2 and more stable tin hydrides, which are detected by MS. Conclusions: • It has been confirmed that plasma generated excited hydrogen species are capable of generating gas phase metal compounds via hydride formation. • The experimental data agree with the expected isotope patterns of stannane and its dimer, as well as with plumbane. • Ab initio calculations show the relative decreasing stability of the ionic tin hydrides as compared to the molecule SnH 4 . SnH 4 + and SnH 2 + appear to be extremely unstable, which is most probably the reason why the are not detected in the recorded mass spectra. • Experiments with nickel, tantalum, and silicon reveal no signals of the corresponding ions, most probably due to a strong protective oxide layer, very low stability of the ionic hydrides, or insufficient sensitivity of the employed set-up. Outlook: • Optimization of the ion transfer from formation to analyzer injection. • Measurement of neutral metal hydrides species • Images of metal surfaces with SEM before and after treatment with H-plasma and their generated species. • Further ab-initio calculations regarding the stability of metal hydrides and simulations of the formation of hydrides on metal surfaces with MD simulations. Molecule SnH 4 SnH 3 + SnH + Sn + Energy relative to stannane +24.549 eV +56.125 eV +72.480 eV The molecules are labeled with the Mullikan charges. Ab initio calculations of SnH 2 + and SnH 4 + did not lead to any stable structures – this is experimentally verified.
Transcript of Surface interaction of selected transition metals and ...
experimental datacalc. pattern Sn2
+
calc. pattern HSn2+
experimental datacalc. pattern HOSn+
calc. pattern H2OSn+
calc. pattern H3OSn+
experimental datacalc. pattern Pb+
calc. pattern HPb+
experimental datacalc. pattern Sn+
calc. pattern HSn+
calc. pattern H3Sn+
Joshua Rieger; Sanna Benter; Clara Markert; Kai Kroll; Hendrik Kersten; Thorsten BenterPhysical & Theoretical Chemistry
Wuppertal, Germany
Institute for Pure and Applied Mass Spectrometry
Surface interaction of selected transition metals and semiconductors with H2 plasma generated species
Methods
Introduction Basic chemical assumption
Plasmas are convenient sources of radiation and energy for drivingchemical and physical processes. The interaction between plasmaconstituents and matter results in a multitude of pathways for e.g.ionization and/or the initiation of chemical reactions.
The reactivity of excited hydrogen towards tin and in general elementsof the 14th group of the periodic table (Carbon group, group IV) hasbeen described in the literature for decades. Likewise, thethermodynamic instability of the resulting metal hydrides and thecatalytic effect of metal surfaces regarding their decomposition isknown since long. This chemistry has recently gained importance forthe modeling of high energy H2 plasmas in the presence of tin, whichrequires knowledge of the neutral and ionic reaction channels andmechanisms.
The initial interaction in the formation of metal hydrides is aheterogeneous gas/surface reaction of excited atomic H and metalatoms M, in which successive H-M bonds are formed. The volatile metalhydrides thus formed subsequently desorb and may subsequentlyadsorb on other (metal) surfaces and decompose.
Conclusion / Outlook
Literature
Acknowledgement
Mass spectra
C-TOF time of flight mass spectrometerequipped with custom ion transfer stage(TOFWERK AG, Thun, Switzerland)Modified VUV lamp RF power supplies(Heraeus GmbH, Hanau, Germany )Hydrogen 5.0 and Argon 5.0 (Messer GroupGmbH, Krefeld, Germany)Gaussian 16. [5] and GaussView 6.0.16iHR 320 (Horiba, Kyoto, Japan)
Fig. 1: Schematic setup and simplified mechanism of metal hydride formation
A custom low-pressure RF discharge generates the reagent ions H3+
within a hydrogen plasma fed by an adjustable continuous gas flow (1).The plasma source consists of an electrode-less, helical coil resonator(HCR) providing the RF energy to sustain the discharge, which isclassified as a high density plasma [1]. Typical operation pressuresrange between 0.1 and 10 mbar.
The continuously generated ions are brought into contact with the solidmetal samples (3) by the gas flow within the glass tube (2). The tubeitself is additionally pumped by the first differentially pumped stage (0.1– 10 mbar) of the used time of flight mass spectrometer (TOF-MS) (4).
[1] H. Kersten, T. Kutsch, K. Kroll, K. Haberer, T.Benter; H2 plasma for the generation ofprotonation reagents with a standard APPIpower supply, 64th ASMS Conference on Massspectrometry and Allied Topics, Indianapolis, IN,USA, 2017.
[2] D. J. Aaserud, F. W. Lampe, Journal of PhysicalChemistry, 100, 10215-10222, 1996.
[3] N. Faradzhev, V. Sidorkin, Journal of VacuumScience & Technology A: Vacuum Surfaces andFilms, 27, 306-314, 2009.
[4] M. Berglund, M. E. Wieser, Pure Appl. Chem.,83, 2, 397-410, 2011.
[5] Frisch et al., Gaussian16, Revision A.03,Gaussian, Inc., Wallingford CT. 2016.
Mass spectrometer:
RF power supplies:
Gases:
Software:UV-Vis spectrometer:
MHx+M
0.1 – 10 mbar1 – 50 sccm
RF plasma
MS
power supply
H2 H3+
1
2
3
4
Experimental setup
Generous support from Carl Zeiss SMT, Oberkochen, Germany, is gratefully acknowledged.
2 𝐻2 (𝑔) + 𝑆𝑛 (𝑠) ↛ 𝑆𝑛𝐻4 (𝑔)4 𝐻 𝑔 + 𝑆𝑛 (𝑠) → 𝑆𝑛𝐻4 (𝑔)
𝑆𝑛𝐻4 (𝑔) → 𝑆𝑛 (𝑠) + 2 𝐻2 (𝑔)𝑆𝑛𝐻4 (𝑔) + 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 → 𝑆𝑛 (𝑠) + 2 𝐻2 (𝑔)
𝐻 (𝑔) + 𝑆𝑛𝐻4 (𝑔) → 𝑆𝑛𝐻3 (𝑔) + 𝐻2 (𝑔)𝑆𝑛𝐻 (𝑔) + 𝑆𝑛𝐻4 (𝑔) → 𝑆𝑛𝐻2 (𝑔) + 𝑆𝑛𝐻3 (𝑔)𝑆𝑛𝐻3 (𝑔) + 𝑆𝑛𝐻3 (𝑔) → 𝑆𝑛𝐻2 (𝑔) + 𝑆𝑛𝐻4 (𝑔)𝑆𝑛𝐻2 (𝑔) + 𝑆𝑛𝐻4 (𝑔) ⇄ 𝑆𝑛2𝐻6 (𝑔)
∗
𝑆𝑛2𝐻6 (𝑔)∗ → 𝑆𝑛2𝐻4 (𝑔) + 𝐻2 (𝑔)
𝑆𝑛2𝐻6 (𝑔)∗ +𝑀 → 𝑆𝑛2𝐻6 (𝑔) +𝑀
𝑆𝑛2𝐻4 (𝑔) + 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 → 2 𝑆𝑛 (𝑠) + 2 𝐻2 (𝑔)
The RF discharge primarily generateselectronically excited H species and H+ ions.The latter react with molecular hydrogen toform H3
+. In the presence of N2 or synthetic airN2H+ and also N4H+ are readily formed. Incontrast to H2, the adsorption of reactive Hspecies leads to a subsequent formation ofstannane (SnH4), which either decomposes onthe surface or desorbs into the gas phase[2,3].Tin oxide is reduced by atomic hydrogen to tinmetal and water. In the gas phase, a reaction
cascade leads to a multitude of tin hydrides,which is shown in Fig. 2.In this scheme (Eley-Rideal mechanism), moreH is consumed than expected by a simplerecombination of adsorbed and newlypenetrating H atoms.Stannane tends to spontaneously decay intothe elements, a process which is acceleratedby reactive surfaces (1st order kinetics [2]).
Fig.3 shows schematically the reactionchannels of these tin compounds [3].
𝐻2 → 𝐻•
𝑆𝑛𝑂𝑋+𝐻• → 𝑆𝑛 + 𝐻2𝑂 ↑
𝑆𝑛 + 𝐻• → 𝑆𝑛𝐻4 ↑
𝑆𝑛𝐻4 + 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 → 𝑆𝑛 + 𝐻2 ↑
Fig 2: Cascade of elementary reaction stepsinvolving hydrogen and tin hydride compounds [2].
Fig 3: Schematic representation of thereactions channels shown in Fig. 2.
In the spectra, ionic tin hydrides and oxygenated tin compounds were detected. The comparison ofthe recorded mass spectrum with calculated isotopic patterns [4] reveals the distinctive patterns ofvarious ionic tin hydrides of the type SnHx
+, especially SnH3+ and SnH+, while the more unstable SnH4
+
and SnH2+ are absent (see Fig. 6).
The dimeric Sn2Hx compounds formed are also detected but with lower intensities. However, onlySn2
+ and Sn2H+ are present in the mass spectrum, dimers with additional hydrogen atoms were notdetected or only to a very small extent (see Fig. 7).The oxygenated compounds of the type HxOSn+ mainly originate from the oxidized target surface,rather than from the gas matrix oxygen, as only very high purity gases were used. Most prominently
HOSn+ is detected, whereas H2OSn+ and H3OSn+ are detected to a much lesser extent (see Fig. 8).The characteristic patterns of the Sn isotopes are fully reproduced and thus used for compoundidentification.The recorded mass spectra of lead (Pb) compounds are in very good accordance with the calculatedisotopic patterns [4]. In addition to Pb+, only mono plumbane (PbH+) is observed (see Fig. 9).Experiments with nickel (Ni), tantalum (Ta) and silicon (Si) did not result in detectable ionic species,probably due to i) the formation of a protective oxide surface layer, ii) low stability of thecorresponding ionic hydrides, or iii) insufficient sensitivity of the experimental setup.
Ab-initio calculations of molecular stabilities
Fig 6: Recorded mass spectrum of H-plasmagenerated tin hydrides and calculatedisotopic patterns.
Fig 7: Recorded mass spectrum of H-plasmagenerated dimeric tin hydrides and calculatedisotopic patterns.
Fig 8: Recorded mass spectrum of H-plasmagenerated oxygenated tin hydrides andcalculated isotopic patterns.
Fig 9: Recorded mass spectrum of H-plasmagenerated lead hydrides and calculatedisotopic patterns.
Why are SnH2+ and SnH4
+ not detectable?
Hα
Hβ
Hγ
Hδ
Fig.5: UV-Vis spectrum of the RF hydrogen plasma withmarked transitions of the Balmer series.
The Plasma discharge continuously generates alarge number of excited hydrogen species. Therecombination of these species becomes“visible” through the emission spectrum. TheBalmer series is identified in the UV-Visspectrum, higher transitions of the series arenot unambiguously assignable. The broadcontinuum presumably originates from acombination of Bremsstrahlung and electronrecombination emission. The large number ofH atoms present causes the inherentlyunstable tin hydride species to react quickly,presumably forming H2 and more stable tinhydrides, which are detected by MS.
Conclusions:• It has been confirmed that plasma generated
excited hydrogen species are capable ofgenerating gas phase metal compounds viahydride formation.
• The experimental data agree with the expectedisotope patterns of stannane and its dimer, as wellas with plumbane.
• Ab initio calculations show the relative decreasingstability of the ionic tin hydrides as compared tothe molecule SnH4. SnH4
+ and SnH2+ appear to be
extremely unstable, which is most probably thereason why the are not detected in the recordedmass spectra.
• Experiments with nickel, tantalum, and siliconreveal no signals of the corresponding ions, mostprobably due to a strong protective oxide layer,very low stability of the ionic hydrides, orinsufficient sensitivity of the employed set-up.
Outlook:• Optimization of the ion transfer from formation to
analyzer injection.• Measurement of neutral metal hydrides species• Images of metal surfaces with SEM before and
after treatment with H-plasma and theirgenerated species.
• Further ab-initio calculations regarding thestability of metal hydrides and simulations of theformation of hydrides on metal surfaces with MDsimulations.
Molecule SnH4 SnH3+ SnH+ Sn+
Energy relative to stannane +24.549 eV +56.125 eV +72.480 eV
The molecules are labeled with the Mullikan charges. Ab initio calculations of SnH2+ and SnH4
+ did not lead to any stable structures – this is experimentally verified.
