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Study of H-passivation of a Bi 2 Te 3 (111) surface

Study of H-passivation of a Bi 2 Te 3 (111) surface. Jesse Maassen Physics department McGill University. First principles calculations are utilized to help answer the following questions: Will H bind to a Bi 2 Te 3 surface?

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Study of H-passivation of a Bi 2 Te 3 (111) surface

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  1. Study of H-passivation of a Bi2Te3(111) surface Jesse Maassen Physics department McGill University • First principles calculations are utilized to help answer the following questions: • Will H bind to a Bi2Te3 surface? • If so: How many H atoms per surface atom? With what configuration? What are the properties of the H-terminated surface?

  2. Bi Bi Te(2) Te(1) Te(2) The system under consideration Our focus : Since it is too computationally expensive to consider the bonding of H on Bi2Te3 slabs of varying thickness, we focus only on the case of 3 quintuple layers (QL). 1QL Primitive cell

  3. Energetics of H-passivation Our plan : Calculate the total energy of the H-bonded Bi2Te3 slab and compare this energy to that of the isolated H / Bi2Te3 system. The configuration which minimizes the total energy will correspond to the most stable case (i.e., most likely to exist). Procedure : We consider the possibility of 1H, 2H and 3H per Te surface atom. The initial positions of the H atoms are chosen such that they replicate the bond directions of the middle Te(1) atom with a bond length of 1.5Å. There are 3 such directions, that we label A, B and C. (next page…)

  4. Energetics of H-passivation Procedure (cont.): Structural relaxations are then performed to ensure each atom has a force < 0.01 eV/Å. The final step is to compute the total energy of the relaxed system. Simulation technique : The Vienna Ab Initio Simulation Package (VASP) is utilized for all calculations. The energy cutoff for the plane wave basis is set to 400 eV. The core potential is treated within the projector-augmented wave (PAW) method. A k-mesh of 13131, generated using the Monkhorst-Pack scheme, is used.

  5. Structural relaxations • 3 H atoms / surface Te • 2 H atoms / surface Te • 1 H atom / surface Te 2. Total energy calculations 3. Properties of H-passivated Bi2Te3 Outline of study

  6. Structural relaxations • 3 H atoms / surface Te • 2 H atoms / surface Te • 1 H atom / surface Te 2. Total energy calculations 3. Properties of H-passivated Bi2Te3 Outline of study

  7. Initial Final After relaxation Relaxation (case 1) : 3 H atoms / Te After relaxation, only 1 H atom remains bonded to the surface Te. The other 2 H atoms form an H2 molecule and move away from the surface. There is significant distortion to the slab. Conclusion: The 3 H atom system converts to the case of a single H.

  8. Initial Final After relaxation Relaxation (case 2) : 2 H atoms / Te After relaxation, the 2 H atoms form an H2 molecule and move away from the surface. Conclusion: The 2 H atoms desorb from the surface leaving the Bi2Te3 slab bare.

  9. Initial Final After relaxation Relaxation (case 3) : 1 H atoms / Te The single H atom is found to remain bonded to the surface Te. The presence of the H results in the distortion of the Bi2Te3 lattice. Conclusion: A single H atom bonds to the surface of Bi2Te3. Note that for the case of 1 H / Te, I tested 9 different initial configurations corresponding to the top & bottom H atoms each at the sites A, B and C.

  10. Relaxation (summary) • Starting with initial configurations including 1H, 2H and 3H atoms per surface Te, the structural relaxations show that only two cases are relevant: • Bi2Te3 slab with 1H per surface Te • Bare Bi2Te3 slab

  11. Structural relaxations • 3 H atoms / surface Te • 2 H atoms / surface Te • 1 H atom / surface Te 2. Total energy calculations 3. Properties of H-passivated Bi2Te3 Outline of study

  12. Total energy This table shows the total energy of a Bi2Te3 slab + 2 H atoms (bonded vs. unbonded). A, B and C indicate the initial bond direction. The numbers in () corresponds to the difference relative to the bare+H2 case. First letter: top H Second letter: bottom H. A Bi2Te3 slab without H is always energetically favorable by ~ 2 eV. Bonded

  13. Total energy (summary) • A bare Bi2Te3 is always energetically favorable. • H-passivated Bi2Te3 surfaces are meta-stable states that cost at minimum 2 eV. • The fact that a Bi2Te3 surface does not want to interact with a molecular adsorbate is consistent with previous experiments*. • * Physical Review 119, 567 (1960).

  14. Structural relaxations • 3 H atoms / surface Te • 2 H atoms / surface Te • 1 H atom / surface Te 2. Total energy calculations 3. Properties of H-passivated Bi2Te3 Outline of study

  15. Bi Bi Te(2) Te(1) Te(2) Properties of Bi2Te3:H (bonding picture; review) Pure Bi2Te3 Bi [5e–] : 3 Te(1) + 3 Te(2) neighbors Te(1) [6e–] : 6 Bi neighbors Te(2) [6e–] : 3 Bi neighbors It is believed* that Te(2)’s 4 p-type e– bond to the 3 neighboring Bi atoms, and the 2 s-type e– form a lone pair. This results in fully satisfied Te(2) atoms such that the QLs interact via Van der Waals (VdW) forces. The Te(1) and Bi atoms have nearly octahedral coordination, indicating that both s- and p-type e– are used in the bonding. *See, for example: J. Phys. Chem. Solids 5, 142 (1958), Physical Review 119, 567 (1960), Physics Letters A 135, 223 (1989).

  16. Properties of Bi2Te3:H (charge density) Pure Bi2Te3 Surface Te(2) charge density appears very similar to that of the Te(2) in the middle QL. Small non-zero charge in between QLs, thus not purely VdW interaction. Te(2) atoms share more charge with Bi than Te(1) with Bi.

  17. 1.71 Å 3.09 Å 3.09 Å 3.86 Å 3.13 Å 3.12 Å 3.09 Å Properties of Bi2Te3:H (bond lengths & angles) Pure Bi2Te3 Bi2Te3 : H For pure Bi2Te3, the surface Te atom shares the same bond length with all 3 Bi nearest neighbors. All the bond angles are identical and equal to 92.9. These nearly 90 angles indicate that the bonding is largely p-type. With H, the surface Te atom moves such that it breaks a bond with 1 Bi while bonding to H and conserving the 2 other Bi bonds. The bond angles between the 2 Bi atoms and the H atoms are slightly different and roughly 90; similar to the case of no H.

  18. Properties of Bi2Te3:H (charge transfer) Bi2Te3 : H Pure Bi2Te3

  19. Properties of Bi2Te3:H (charge transfer) Bi2Te3 : H Pure Bi2Te3 Charge per bond for a surface Te(2) atom = 0.37 e–/ 3 Bi nearest neighbors  0.12 e– / bond Charge transferred from Bi to the surface Te(2) + H = 0.06 e– + 0.17 e– = 0.23 e– This value equals the charge of 2 bonds.  The surface Te(2) atom broke 1 of it’s bonds with the 3 Bi to bind with the H.

  20. Properties of Bi2Te3:H (magnetic moments) Bi2Te3 : H Pure Bi2Te3 Total magnetic moment = 1.07 B Large |M| localized to surface Bi, due to dangling bond. Total magnetic moment = 0.00 B

  21. E - EF (eV) E - EF (eV) K M K M   Properties of Bi2Te3:H (bandstructure) Pure Bi2Te3 Bi2Te3 : H • Electronic bands of a 3QL Bi2Te3 slab. • All states are doubly degenerate. • Large change in the bands with H. • States are now spin-split due to the dangling bonds of the surface Bi atoms.

  22. E - EF (eV) M K  Properties of Bi2Te3:H (bandstructure) Bi2Te3 : H • The contribution of H to the bands are shown in black. • The H states are located at roughly -4 eV, and hence play no direct role in the bands at EF.

  23. Conclusion • A first principles study was performed to determine whether H atoms will bind to the surface of a 3QL Bi2Te3 slab. • Structural relaxations show that only important cases correspond to 0 H per surface Te or 1 H per surface Te. • Total energy calculations indicate that the H-termined surface is a meta-stable state with ~2 eV higher energy than a bare slab. • With H, the bands are significantly altered (due to large charge transfer) and result in spin non-degenerate states and a net magnetic moment. • An analysis of the charge density, charge transfers and bond lengths and angles, it is confirmed that the surface Te atom breaks 1 of 3 bonds in order to bind to H. This explains the high energetic cost of binding to H, and hence the unlikely occurrence of atomic adsorption.

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