Surface Science 608 (2013) 67–73 Contents lists available at SciVerse ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc Charge transfer at organic–inorganic interface of surface-activated PbS by DFT method Nguyen Thuy Trang a,⁎, Luu Manh Quynh a, Tran Van Nam a, Hoang Nam Nhat b a b Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received 10 July 2012 Accepted 24 September 2012 Available online October 2012 Keywords: Organic–inorganic interface PbS 4-Aminothiophenol Charge transfer DFT a b s t r a c t Electronic structure of a surface-activated PbS by a bio-active molecule 4-aminothiophenol (4-ATP) was investigated using density functional theory (DFT) The obtained results demonstrated the importance of charge transfer which accounted for the flipping of surface rumpling and the nature of the binding between the activated surface and the capping agent The influence of 4-ATP–PbS topology on bonding nature between surface atoms was discussed The capping-induced bonding nature shift was interpreted as surface core level shifts (SCLSs) of PbS © 2012 Elsevier B.V All rights reserved Introduction Hybrid nanocomposites which are composed of nanoparticles (NPs) with inorganic nanocrystalline cores and organic shells have been strongly attracting researches because of their wide range of applications from optoelectronics to biology [1–5] One of the most brilliant candidates for fabricating such kind of composites is nanocrystalline lead sulfide PbS The narrow bulk band gap Eg ~ 0.29 eV (at low temperature) [6] and strong quantum confinement effect with Bohr radius ~18 nm [7] allow to easily optimize the optical band gap as well as the absorption and emission bands of the material by controlling the particle size [8] As a result, optical properties of PbS nanocomposites have been intensively studied for photoemission elements in organic light emitting diodes (OLEDs) and photovoltaic devices The size-induced widening of absorption and emission ranges from near infrared (NIR) to visible (VIS) region of hybrid PbS NPs makes it easier to change the opticalactive region of the hybrid devices than the devices using organic molecules alone [9–11] Hence PbS-based devices are more efficient than the organic one Moreover, recent electrochemical investigations have suggested that PbS nanocomposites can act as electrochemical biosensor [12–16] Under applied voltages ~1.1 V, Pb2+ ions in PbS NCs can be oxidized to neutral Pb atoms, which are recognizable with a high peak ⁎ Corresponding author E-mail address: trangnguyenphys@gmail.com (N.T Trang) 0039-6028/$ – see front matter © 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.susc.2012.09.014 in the cycle voltammogram [12] This observation promised a high sensitivity of PbS-based bio-sensors The development of hybrid nanocomposite devices usually faces complicated interactions between different fragments including interparticle interaction, NPs–solvent interaction, intraparticle interaction and interaction between NPs and organic electron-acceptor or donor agents in solutions It has been shown via optical observations that those interactions were in close relations with each other The interparticle interaction was shown to occur between NPs via long-range Forster resonant energy transfer (FRET) which results in an enhancement of the low energy emission [17,18] In solution, it was dominated by the interaction between NPs and solvent [19] Besides, the interactions between NPs and solvent dipoles tend to increase the intra-NP charge transfer (CT) rate via a statistical mechanism which was attributed to the long time scales of CT processes relative to the time scale of molecular motion [20] Optical measurements on mixtures of PbS NPs and exposed the fact that due to the energy level alignment, the charge exchanges of PbS NPs and electron donating molecules only occur in excited state while that of PbS NPs and electron accepting molecules can occur at ground state [19] It was shown that all of the CT processes strongly depend on NP size [20,21] To clarify such complicated interactions, electronic structure aspects should be involved In this work, in the framework of density functional theory (DFT), we investigated electronic structure of a PbS — organic molecule junction at which a clear chemical bonding should occur so that the charge can be directly transferred The selected organic molecule is 4-aminothiophenol (4-ATP) (Fig 1a) because of two reasons Firstly, it has been frequently used for nanoparticle coating as efficient surface stabilizer to prevent particle aggregations and especially as bio-activator owing to its free amino group (\NH2) which is highly 68 N.T Trang et al / Surface Science 608 (2013) 67–73 Fig (a) 4-ATP molecule, the green dash line represents the molecular axis; (b), (c) supercell model for (001) PbS surface, 4-ATP−H part is embedded into vacuum slab at different geometries: S4-ATP conjugated with surface Sr atom (b) and Pbr atom (c) bio-compatible [22,23] Secondly, its thiol group (\SH) was known to bind strongly with the Pb ion of NCs via chemical bonding [18,24] Modeling details Because (001) surface is the most dominating surface for “rock-salt”, it has been chosen as calculated model A supercell composed of a vacuum slab stacked on a PbS slab along (001) crystal direction was generated as quasi-2D simulation of such surface We assumed only the interaction of a most-top layer of PbS surface with 4-ATP and therefore only atomic layers were included in the PbS slab The thickness of the vacuum slab was chosen so that the interactions between different atomic slabs vanish (about 30 Å) According to experimental observations, the hydrogen atom in thiol group (\SH) of 4-ATP molecule is able to be removed, leaving a free bond on S atom which can form a combination with Pb atoms on PbS surface We embedded the 4-ATP molecule without H atom in thiol group (\SH) into the vacuum slab (Fig 1) The capping agent — solid surface distance was changed from Å to Å The vacuum slab thickness of about 30 Å is good enough for the interaction between neighboring 4-ATP fragment and PbS slab to vanish at the longest distance In order to confirm experimental observations that the remaining S atom of (\SH) group prefers combining with Pb atoms to combining with S atoms on PbS surface, potential curves of 4-ATP–(001) PbS surface distance were produced for two topologies corresponding to S atom from (\SH) group that directly binds with Pb and S atom on PbS surface (Fig 1b, c) For convenience, (001) PbS surface atoms which were directly conjugated with 4-ATP are called root atoms and indicated with “r” index, i.e Pbr and Sr The others were called non-root surface atoms, i.e PbPbS-surface and SPbS-surface S atom from (\SH) group of 4-ATP was indicated by “4-ATP” index, i.e S4-ATP, the remaining part of 4-ATP (4-ATP without H atom) was 4-ATP−H and the one without (\SH) group was 4-ATP−(\SH) It was assumed that the axis of 4-ATP molecular (see Fig 1a) was perpendicular to the surface and the molecular was falling straight forwards to Sr (S–S conjugation) (Fig 1b) and Pbr (Pb–S conjugation) (Fig 1c) The distance between S4-ATP and root sites was changed from Å to Å All of our calculations were carried out using LDA functional with the help of Dmol code which provides atomic-like basis sets in numerical form of the size increasing from MIN to DNP type [25] Basis functions of this basis set type are generated numerically as values on an atomic-centered spherical-polar mesh The angular portion is an appropriate spherical harmonic and the radial portion is obtained by solving the atomic DFT equations numerically In our calculations, we utilized the DNP basis set which provides numerical basis functions for each occupied orbital in free atom This basis set also complemented with polarization functions, i.e functions with angular momentum one higher than that of the highest occupied orbital of free atom In our case, DNP basis set includes following basis functions for each atom: H: two 1s, 2s and 2p functions; C, N, S: two 1s, 2s, 2p, 3s, 3p and 3d functions; Pb: two 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 6s, 6p and 6d functions Core electrons were treated at all electron relativistic level (relativistic full-potential — Rel-FP) Results and discussions 3.1 Ground state charge transfer at organic–inorganic interface Removing hydrogen atom from thiol (\SH) group left one spin up hole on 4-ATP−H fragment which was primarily located on S4-ATP atom as demonstrated by the partial density of stats (DOSs) in Fig 2a This suggested a strong ground state CT when the 4-ATP−H–PbS bond formed The ground state CT was examined along the potential curves of 4-ATP−H–PbS surface distance Fig 3a shows the Pb–S and S–S conjugation potential curves drawn on the base of single point energy calculations In both cases, the minimum of the potential well was at the distance of 3.4 Å (see the inset) The Pb–S potential well which was deeper than the S–S one suggested that Pb atoms were more preferable than S atoms for the 4-ATP−H fragment to be attached to This was in agreement with experiment that thiol group (\SH) strongly binds with Pb ions on NCs [18,24] Fig 3c shows electron deformation Δρ(r), which is the difference between crystal electron density and the sum of isolated atomic electron density, of 4-ATP−H–PbS interface at some distances The electron-donating region between S4-ATP and Pbr, which was clearly observable when a distance reduced below 3.9 Å, demonstrated the ground state electron transfer between PbS surface and 4-ATP−H fragment According to the shape of this region, the electron transfer was from Pbr to S4-ATP 2pz orbital Quantitative information of such charge transfer process was represented by the distance dependences of concentrated charges (Fig 3b) In the distance range from to 4.5 Å, the positive charge of Pb r was increased while the total positive charge of the 4-ATP−H was reduced with respect to the reduction of distance This indicated two opposite electron transfer processes on the intermediate atom S4-ATP: electron transfer from Pbr to S4-ATP and from S4-ATP to 4-ATP−(\SH) A minimum of S4-ATP negative charge was observed at the bottom of the potential N.T Trang et al / Surface Science 608 (2013) 67–73 69 Fig 2a The bonding nature between non-root S and Pb atoms slightly shifted towards covalence pole 3.2 Atomic geometry and surface core level shifts of bared PbS surface In order to address the reconstruction of geometry and electronic structure due to the 4-ATP–PbS formation, it was beneficial to examine the bared PbS surface first To characterize the surface structural reconstruction, surface relaxation δz and rumpling Δ12 were defined as the following [28]: z ẳ zS1 zPb1 ị=d0 1ị 12 ẳ 1=2 zPb1 zS2 ỵ zS1 zPb2 ị=d0 : 2ị Surface core level shifts (SCLSs) Δεs were also considered to evaluate effect of the bond formation on electronic structure of the PbS surface, [28]: s ẳ s b : 3ị Another way to define SCLS was [29]: Δs ¼ s −c : Fig Partial DOS of PbS–4-ATP interface at distance of 3.4 Å (a) and 2.5 Å (b) The Fermi level was normalized to zero and denoted by the dash vertical lines Spin up and spin down DOSs were denoted by positive and negative DOS channels, respectively well, i.e at Pb r–S4-ATP distance of 3.4 Å On the right hand side of the minimum, the Pbr–S4-ATP charge transfer was dominated by the S4-ATP–4-ATP−(\SH) one then the S4-ATP charge became less negative when the distance reduced The domination of Pb r–S4-ATP electron transfer on the left hand side of the minimum gave rise to the enhancement of the negative charge of S4-ATP when the distance reduced below 3.4 Å In the distance range above Å, the 4-ATP–PbS surface binding should be supported by the ionic bond between opposite charge ions S4-ATP and Pbr Besides, the presence of 4-ATP gave insignificant effect on the atomic charge of non-root surface atoms as well as the ionic nature of bonding between them When the distance was below ~3 Å which is equal to total ionic radii of S2− RS2− = 1.84 Å [26] and Pb 2+ RPb2+ = 1.19 Å [27], the positive atomic charge of Pbr was reduced, which indicated the enhancement of covalence nature This scenario was insured by the density of states (DOS) shown in Fig 2b For the distances below Å, the spin up 2p hole on S4-ATP was filled by the overlapping of S 2p orbital and Pb orbitals Consequently, S 2p DOS became symmetrical with both spin-up and -down S 2p bands that are partially filled (Fig 2b) At the same time, there was an increasing of density of unoccupied Pb 6s states and low energy occupied 6p states So, the covalent bond was believed to originate from the overlapping between partially filled S 2pz and Pb 6pz orbitals The change in bond nature increased the electron density at Pb r site on PbS surface Enhanced Coulomb field, which was induced by the increased electron density, reduced electron density at nearest neighbor non-root surface S site and thus reduced atomic charge of S on PbS surface as seen in ð4Þ Here, z specifies Cartesian coordination of atom on the direction perpendicular to surface; S1, Pb1 are S and Pb atoms in the most-top surface layer, S2, Pb2 are S and Pb atoms in the second-top layer which was the center layer in our case; d0 is the calculated PbS bond length of bulk model; εs, εb and εc are the eigenvalues of considered state from atoms in surface layer, bulk material and center layer respectively Table 1, these parameters and SCLSs of the most-top surface layer of bared PbS surface from our all electron full potential calculation were compared with results of previous works The absence of PbS surface reconstruction and SCLS have attracted both ab initio [28–32] and experimental investigations [33–35] Despite of the divergence of surface rumpling values and average surface relaxation by different theoretical methods, all of them led to the same trends that the most-top atomic layer processes the largest surface relaxation ranging from to 9% Concerning the surface rumpling, full potential-linear augmented plan wave (FP-LAPW) method [28] and core–shell model [36] were in good agreement to predict that in the most-top atomic layer S atoms considerably shift outwards in comparison with Pb atoms (S atom at the top of the surface) but the use of pseudopotentials (PP-LAPW) predicted insignificant positive rumpling [31] or even flipped the rumpling trend with Pb atom at the top of the surface [29,32] Experiments were involved to clarify the surface structural reconstruction trends Unfortunately, X-ray standing wave (XSW) measurements on PbS at room temperature could not help to exactly estimate surface relaxation due to phonon broadening effects [33] Basing on these measurements, surface relaxation was thought to be less than 1% The better method to observe surface structure, the low energy electron diffraction (LEED), was only carried out on PbTe, an isoelectronic counterpart of PbS [37] What can be drawn from such experiment was that surface Pb atoms were experimentally confirmed to shift inwards in comparison with nonmetallic atoms Then it was believed that full potential calculations by I.G Batyrev et al [28] and our group should be more reliable than the one with pseudopotentials It should be noted that those values from literatures were corresponding to 7-atomic-layer PbS slab model, meanwhile our calculation model was only 3-atomic-layers thick so average surface relaxation and rumpling predicted by us were smaller than that ones in Ref [28] To gain a deeper insight into the rumpling effect, we recalled the simplest theory for cohesion in ideal ionic crystal which includes only inter-ionic Coulomb interaction and the strong short-range core–core repulsion due to Pauli's principle According to this, the surface 70 N.T Trang et al / Surface Science 608 (2013) 67–73 Fig (a) Potential curves of 4-ATP–PbS surface distance The insets zoom in the curves around minimum point at 3.4 Å (b) Distance dependences of the concentrated charges of different fragments and atoms at the organic–inorganic interface in case of Pb–S conjugation (c) Electron deformation at 4-ATP–PbS interface on (100) slide at Pb–S distances of 2.5 Å (left) and 3.4 Å (right) The red color denotes electron-withdrawing area while the blue color denotes electron-donating area relaxation has purely corresponded to the reduction of Madelung's constant at the surface due to the reduction of the coordination number Then, the surface relaxations at every Pb and S site should be the same That means the surface rumpling should be absent for ideal ionic crystals If one used the core–shell model additional effects were involved as the core–shell model added more short-range interactions, i.e intraionic core–shell interaction, shell–shell, core–shell interactions between first and second neighbors [36] These interactions correspond to electron polarization potential of each ionic shell and overlap potential of wave functions at different sites, which usually occurs in covalence bonding The quantitative agreement between full-potential methods and core–shell model rumpling suggested that the surface rumpling may originate from the electronic polarization of surface ions due to coordination imperfection and covalent bonding On the other hand, the softy of electron potential in pseudopotential methods seemed to underestimate the two factors Because the calculation reported in [29] failed to reproduce PbS surface rumpling, the obtained S 2p SCLS of 0.3 eV numerically coincided with experimental value given in Ref [34] The S 2p SCLS from our calculation was in the opposite trend with the experiment if it was defined in the same way as in [28] It suggested that the 3-atomic layer slab in use was not thick enough for electron density to converge with that one of much thicker samples in the experiments However, in this study, we only concentrated on the effect of capping agent on the PbS surface so the contribution of bared surface to structure deformation and electron redistribution was given for calibration purpose only 3.3 Structural and electronic structural deformation at 4-ATP–PbS interface The reconstructed structure and structure parameters of 4-ATP–PbS interface were shown in the top panel of Fig and Table During optimization process, only 4-ATP−H part and the most-top surface layer of PbS at the interface were allowed to relax, the center and surface layer on the other side were fixed at relaxed-bared-surface geometry The energy gained after relaxation from the vertical absorption geometry to the final geometry is ~0.345 eV In this final geometry, the molecular plane of 4-ATP was strongly inclined to make an angle of 23.14° with PbS surface The average surface relaxation in the presence of 4-ATP was strongly suppressed to −0.3% (only 13.6% of bared surface relaxation remained) owing to the capping agent which compensated the surface coordination number imperfection The flipping of surface rumpling corresponded with the moving up of Pb atoms to N.T Trang et al / Surface Science 608 (2013) 67–73 71 Table A comparison of surface rumpling δr1, surface relaxation Δ12 of the most-top layer (in %), SCLSs of Pb 5d and S 2p states (in eV) between difference theoretical results and experimental observations δr1 Δ12 Δεs of Pb 5d Δεs of S 2p εs − εb εs − εc εs − εb – – Theoretical methods Core–shell model (9-atomic layers) [36] Madelung potential estimation [35] 2.1 – −3.5 – – 0.26 – – Ab initio calculations on 11-atomic layers PP/Gaussian basis set/LDA [29] −3.0 −4.1 – – Ab initio calculations on 7-atomic layers PP/PW/GGA [31] PP/PW/GGA [32] FP/LAPW/GGA [28] 0.03 −1.3 2.9 −5.1 −8.4 −7.1 – – – – – – – – −0.41 0.91 −9.47 −2.20 −0.30 0.26 0.16 0.18 0.12 0.16 0.15 – – b1% – – b1% −4 – 0.0 ± 0.1 – – −0.30 ± 0.02 – 0.0 – Ab initio calculations on 3-atomic layers FP/DNP/LDA (our work for bared surface) FP/DNP/LDA (our work for 4-ATP capped surface in tilted -capping-fragment geometry) Experimental measurements XPS on PbS at low temperature T = 100 K [34] XPS on PbS at room temperature [35] XSW on PbS at room temperature [33] LEED on PbTe [37] the top of the surface δr1 = −9.47% This seemed to be the response of surface atoms against the change in their electronic dipole moments induced by charge transfer from Pb surface atoms to capping fragment In Fig 5, we show the direction of electronic dipole moments of surface atoms as inferred from the electron deformations of relaxed bared PbS surfaces (Fig 5a) and relaxed capped PbS surface (Fig 5b) and schematized electron density in the form of positive charged nuclei and their surrounding electron clouds (Fig 5c) By this, the relaxation of surface εs − εc −0.30 −0.05 −0.01 with and without 4-ATP−H fragment could be explained in terms of the relation between charge transfer and surface electronic dipole moments The SCLS of the 4ATP-capped PbS surface corresponding to the tilted geometry was also shown in Table According to this, SCLSs of both sulfur and lead was suppressed (reduced in magnitude) In order to interpret the change in SCLSs, it is worthwhile to remind that in the previous section we concluded that Pb–S conjugation is more preferable Fig Optimized structure of 4-ATP–PbS interface (upper panel) together with corresponding electron deformation (lower panel) in (110) (a) and (−110) (b) slides 72 N.T Trang et al / Surface Science 608 (2013) 67–73 Fig Schemes of electronic dipole moments of surface ions: (a) the electronic dipole moments of surface ions without capping agent and (b) with capping agent basing on analyzing electron deformation density; (c) the electronic dipole moments of surface ions are represented in terms of positive charged nuclei (circles with “+” inside) and their surrounding electron clouds (ellipses with “−” inside) 4-ATP−H fragment which is denoted by circles with “4-ATP−H” inside was put onto the surface (“relaxed, capped” panel) and after relaxation, found its stable position (“relaxed, capped and relaxed again” panel) than S–S one and Pbr–S 4-ATP bond should be ionic at distance above Å and polarized covalent when distance reduced below Å So at final atomic geometry in which SPbS–S4-ATP distance was 3.4 Å and PbPbS– S4-ATP distance was 2.976 Å, Pb–S 4-ATP ionic bonding with a weak covalence seemed to be more preferable than S–S4-ATP covalent bonding The direct binding of capping fragment to surface Pb atoms was shown above to increase electron density on those atoms but reduce electron density on surface S atoms The increasing of electron density, in turn, increased band–band Coulomb repulsion between Pb 5d core levels and higher levels, increasing binding energy of Pb core state As a result, the positive shift of surface Pb 5d band was reduced Whereas, the electron density reduction on surface S atoms reduced the repulsion on S core levels from the higher bands which, in turn, suppressed the negative sulfur SCLS Such change in SCLSs could also be interpreted as the slight covalence shift of bonding between non-root S and Pb atoms Conclusion 4-ATP capped PbS (001) surface was investigated by means of electronic structure methods in the frame work of density functional theory The capping compensated the surface imperfection of coordination number and suppressed the average surface relaxation However, the charge transfer from PbS surface to 4-ATP−H fragment induced a change in surface electronic dipole moments which in turn flipped surface rumpling of PbS The direct bonding of capping fragment to surface Pb atoms slightly shifted surface Pb–S bonding nature to covalence This shift can be interpreted as the reduction of SCLSs of both Pb and S Acknowledgments This work is financially supported by Vietnam National University, Hanoi (TRIG A project, no QGTD 10.24) One of the authors, Nguyen Thuy Trang, would like to thank TRIG A project of Hanoi University of Science, Vietnam National University, Hanoi for supporting Appendix A Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.susc.2012.09.014 References [1] W.U Huynh, J.J Dittmer, A.P Alivisatos, Science 295 (2002) 2425 [2] S Coe, W.K Woo, M Bawendi, V Bulovic, Nature 420 (2002) 800 [3] G Konstantatos, I Howard, A Fischer, S Hoogland, J Clifford, E Klem, L Levina, E.H Sargent, Nature 442 (2006) 180 [4] J.A Nozik, Phys E 14 (2002) 115 [5] M Bruchez, M Moronne, P Gin, S Weiss, A.P Alivisatos, Science 281 (1998) 2013 [6] In: O Madelung, U Rossler, M Schulz (Eds.), Semiconductors: Group IV Elements, IV–IV and III–IV Compounds, 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negative... on 4-ATP−H fragment which was primarily located on S4-ATP atom as demonstrated by the partial density of stats (DOSs) in Fig 2a This suggested a strong ground state CT when the 4-ATP−H PbS bond... indicated with “r” index, i.e Pbr and Sr The others were called non-root surface atoms, i.e PbPbS-surface and SPbS-surface S atom from (SH) group of 4-ATP was indicated by “4-ATP” index, i.e S4-ATP,