Tuning the Electronic Properties of a Boron-Doped Si(111) Surface by

Article pubs.acs.org/JPCC

Tuning the Electronic Properties of a Boron-Doped Si(111) Surface by Self-Assembling of Trimesic Acid Farzaneh Shayeganfar* and Alain Rochefort* Engineering Physics Department and Regroupement Québécois sur les Matériaux de Pointe (RQMP), Polytechnique Montréal, Montréal, Québec H3C 3A7, Canada ABSTRACT: The influence of self-assembled trimesic acid (TMA) on the electronic properties of a heavily boron-doped silicon surface was investigated using first-principles DFT calculations. Our results demonstrate that the adsorption of isolated TMA molecules, small molecular islands, or complete monolayers is characterized by significant adsorption energy and electron charge transfer to the Si− B interface, while the bonding character of TMA to the surface remains essentially noncovalent. The stability of the adsorbed species was ensured by an attractive interaction from the Si−B interface but also through the formation of hydrogen bonds between TMA units. Beyond this significant stability of the different TMA adlayers, the weak dispersion and the energy level position of states associated with the TMA moieties observed in the band gap region of the Si−B interface suggest that the adsorbed layer can be used to tune the electronic properties of the substrate.

I. INTRODUCTION Two-dimensional (2D) self-assembling based on nanoscale organic building blocks that interconnect through noncovalent forces such as hydrogen bonding or van der Waals interactions manifests appealing physical phenomena1−3 that could lead to several fascinating technological applications.4 Multicomponent self-assembled networks on highly ordered pyrolytic graphite (HOPG) or metallic surfaces have been much investigated,5−13 but equivalent supramolecular structures have been rarely observed on silicon-based substrates14,15 mostly due to their propensity to form covalent bonds. The fundamental understanding of chemical interactions occurring between a selfassembly arrangement and a semiconductor surface consists of two complex parts where molecule−substrate and molecule− molecule interactions compete. Hence, the result of such molecular competitive interactions among the substrate and the adsorbed molecules determines the growth of the molecular networks.16,17 In order to design 2D molecular assemblies on Si-based surfaces, two major approaches have been used to limit the number of surface dangling bonds. The first approach is to saturate the dangling bonds with weakly reactive species such as Ag atoms.18 In this case, well-organized 2D molecular crystals involving weak intermolecular interactions such as hydrogen bonding19−21 or van der Waals22 have been observed. For example, Theobald et al.19 have built a large pore network of perylene tetracarboxylic di-imide (PTCDI) and melamine on Ag/Si(111)√3 × √3R30° where the supramolecular assembling was controlled by highly directional hydrogen bonds between PTCDI and melamine. The adsorption of trimesic acid (TMA) on the same Ag/Si(111) surface has led to the formation of a well-ordered adlayer but where the surface was believed to play a non-negligible role. In contrast, Suzuki et al.21 © XXXX American Chemical Society

have convincingly demonstrated that terephtalic acid (TPA) does not form any ordered assembly on Si(111)7 × 7 due to strong molecule−substrate, but ordered layers through hydrogen bonding were formed on Ag/Si(111)√3 × √3R30° where the molecule−substrate is significantly weakened. They also found that the structures of TPA on the Ag/Si(111) surface were very similar to the layers formed on Ag(111).21 The second approach is to use a Si(111) surface heavily doped with boron atoms that reconstructs upon heating into a well-ordered Si(111)−B(√3 × √3) R30° surface. Following this reconstruction, the presence of surface dangling bonds is minimized by the presence of boron atoms that occupy a stable position just below the Si adatoms.23−25 In this T4 position, the boron atom is entirely trapping the electron lone pair of Si adatoms, and this unique characteristic allows us to weaken the molecule−surface bond to favor the diffusion and growth of molecular networks on the surface.26,27 Suzuki et al.28 have studied the influence of Bi atom coverage on the formation of a self-assembled layer, and they concluded that terephthalic acid molecules form a well-organized layer once the Si(111) is saturated with B. They also concluded that the structures found on the Si−B surface are similar to those observed on the Ag/ Si(111)21 surface, hence as on the Ag(111) surface. Then, we could anticipate the structures of adlayers on a Si−B surface to be similar to the one observed on weakly reactive metallic surfaces. Finally, although the formation of supramolecular networks has been observed at several occasions on such Si−B surfaces,27,29−31 the influence of the adsorbed layers on the Received: May 5, 2015 Revised: June 11, 2015

A

DOI: 10.1021/acs.jpcc.5b04307 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

supercells used to performed the DFT calculations on isolated and networked TMA molecules on the Si−B surface will be more explicitly given below. The adsorption energy of TMA molecules on Si(111)−B surfaces, Eads, is defined by

electronic properties of the substrate remains nearly unexplored at both theoretical and experimental levels. Following our recent works on the adsorption of trimesic acid (TMA) on graphene32,33 and bilayer graphene34 in which we observed a clear influence of the adlayer of the electronic properties of the substrate, we have explored the possibility of tuning the electronic properties of the Si(111)−B(√3 × √3) R30° surface with adsorbed TMA layers. Trimesic acid is wellknown for its ability to establish a multidirectional hydrogen bonding network, and the structure of the self-assembled monolayer was investigated on numerous weakly reactive surfaces such HOPG,35−39 graphene,39 Ag,40 Cu,41 and Au.42,43 In the present study, we considered the most common structure of TMA observed by STM on a weakly reactive surface, the so-called chicken-wire, and we also studied the super-flower structure which represents the most densely packed arrangement. First-principles computations were performed to obtain a deeper understanding of the electronic properties of the adsorbed TMA monolayer on the reconstructed Si(111)−B surface. In the present study, we begin our theoretical analysis by comparing the adsorption of a single isolated TMA molecule to an isolated TMA dimer bonded through hydrogen bonds and, finally, to an isolated cyclic TMA trimer bonded by hydrogen bonds.

Eads =

1 (E(nTMA/SiB) − [E(SiB) + nE(TMA)]) n

(1)

where E(nTMA/SiB) is the total energy for optimized TMA molecules on Si(111)−B; E(SiB) is the total energy for the Si(111)−B supercell; E(TMA) is the total energy of the isolated TMA molecule; and n is the number of TMA units in the supercell.

III. RESULTS AND DISCUSSION III.1. Adsorption of TMA Monomer, Dimer, and Trimer on Si(111)−B. In order to discuss and compare electronic properties of large TMA adlayers on the Si(111)−B surface, we start with a detailed description of the optimized structures for the isolated TMA monomer (Figure 2(a)), dimer (Figure 2(b)), and trimer (Figure 2(c)). For the adsorption of isolated species, we used supercells in which the Si(111)−B(√3 × √3) R30° surfaces were containing 144 Si atoms (12 Si adatoms, 12 B atoms), 192 Si atoms (16 Si adatoms, 16 B atoms), and 240 Si atoms (20 Si adatoms, 20 B atoms) for, respectively, the adsorption of a monomer, dimer, and trimer of TMA. Similarly to the clean Si(111)−B surface, Si atoms in the bottom layer were terminated by hydrogen atoms to avoid the presence of dangling bonds. In order to maximize the interaction of TMA with the surface Si adatoms that are separated by 6.2 Å, we assumed that TMA adsorbs preferably in a hollow site that is delimited by three Si adatoms. An adsorption of TMA where the benzene ring is located on top of a Si adatom was not considered since it gives very limited overlap between COOH groups and surface atoms.32 The results of the DFT calculations are reported in Table 1 in terms of the component of the distance between the center of mass of TMA and the surface (dTMA−SiB), the distance between the Si adatom and B atom (dSi−B) near the adsorbate, the net Mulliken charge on TMA, and finally the adsorption energy (Eads). Table 1 reveals a number of differences in the optimized structural parameters obtained from the DFT calculations for the small adsorbed TMA islands on the Si(111)−B substrate considered. As listed in Table 1, the dimer and trimer assemblies show higher charge transfer and adsorption energy and a smaller intermolecular distance than the monomer. In addition, the equilibrium distance between the Si-adatom and B atom (dSi−B) slightly decreases for the dimer and trimer, while it increases for the monomer. An increasing adsorption energy is consistent with an improved electron donation from TMA to the Si−B surface and to an improved bonding between the species. The formation of in-plane hydrogen bonds with one or two neighboring TMA molecules constitutes the main difference between adsorbed species. First, the improved stability of the dimer and trimer can be easily explained since the adsorption energy calculated with eq 1 includes also the energy needed to dissociate the isolated TMA dimer or trimer. We can define the hydrogen bonding energy, EHB, by considering isolated molecular TMA islands

II. COMPUTATIONAL DETAILS To describe the structural and electronic properties of adsorbed trimesic acid on Si−B surfaces, we have used a density functional theory (DFT) technique included in the SIESTA package.44 The DFT calculations were carried out using periodic boundary conditions within the local density approximation (LDA). The computations were performed with norm-conserving Trouillier−Martins pseudopotentials and double-ζ polarized atomic basis sets. For the k-point sampling, a 12 × 12 × 1 Monkhorst−Pack grid was used for all configurations. The structural relaxations and geometry optimizations were carried out until force and total energy were less than 0.01 eV/Å and 10−5 eV, respectively. During the optimizations steps, the bottom Si layer (including H atoms) was kept frozen to the geometry optimized without adsorbates, while the top layers that include the Si−B interface and the adsorbed layer were fully optimized. We modeled the reconstructed Si(111)−B(√3 × √3) R30° surface with the supercell shown in Figure 1 which contains 144 Si atoms (whose 12 Si adatoms), 12 B atoms, and 36 H atoms. Boron atoms occupy a T4 site just below Si adatoms, and the Si atoms located in the bottom layer were saturated with hydrogen atoms in order to avoid the presence of dangling bonds. The

Figure 1. (a) Top-view representation of a Si(111)−B(√3 × √3) R30° supercell. Si adatoms located at the topmost layer are identified by the red dotted circles. (b) Side view of the supercell. Pink atoms represent boron atoms that are located just below Si adatoms. Si atoms at the bottom layer were saturated by hydrogen atoms (in white) to avoid dangling bonds.

E HB = B

1 (E − nE TMA ) n

(2) DOI: 10.1021/acs.jpcc.5b04307 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. Top view of the supercell used in DFT calculations for isolated adsorbed (a) monomer, (b) dimer, and (c) trimer of TMA on the Si(111)−B substrate. Si adatoms are identified by the dark brown atoms.

Table 1. Calculated TMA−Surface Distance (dTMA−SiB), Si−B Distance (dSi−B)b, Net Charge Transfer on TMA per Molecule, and Adsorption Energy (Eads) per TMA Molecule Adsorbed on a Si(111)−B Surfacea

monomer dimer trimer

dTMA−SiB

dSi−B

net charge

Eads

(Å)

(Å)

(|e|)

(eV)

2.93 2.85 2.82

2.33 2.17 2.13

+0.13 +0.39 +0.30

−1.73 (−1.73) −2.84 (−2.05) −2.65 (−1.91)

a

The values between parentheses are the surface contribution to the adsorption energy. bThe Si−B distance without adsorbates is 2.20 Å.

where E is the total energy of the molecular islands; ETMA is the energy of a single TMA; and n is the number of units in the island, n = 2 for the dimer and n = 3 for the trimer. Additional DFT calculations on gas-phase species indicate that EHB is, respectively, −0.79 and −0.74 eV for the dimer and the trimer. Then, when removing the contribution EHB to the total adsorption energy given in Table 1, we obtain an estimate of the contribution from the Si−B surface, Esurf, that stabilizes the TMA molecular islands. We finally find that small TMA assemblies are more stable (Esurf(dimer) = −2.05 eV, Esurf(trimer) = −1.91 eV) than a single isolated TMA molecule (Esurf = −1.73 eV). Hence, the formation of highly directional in-plane intramolecular hydrogen bonds favors the growth of the adsorbate layer into a 2D pattern. Beside the COOH groups involved in the in-plane H-bonding, the remaining COOH groups are significantly bent toward the surface atoms, and in agreement with the relatively poor lattice matching between the adsorbed TMA layer and the reconstructed Si(111)−B surface, it is very difficult to establish a clear structural relation between the number of adsorbed molecules and the resulting adsorption energy. Then, the improved adsorption energy would originate from structural deformations of the adsorbed phase that contribute to reduce the dipole− dipole interactions between COOH groups and the Si−B interface or even to reduce the repulsive π−π interactions of the TMA moiety with the surface. In both cases, this contributes to bring TMA close to the surface and to improve the net charge transfer from TMA to the surface. The small differences in the values calculated for the dimer and trimer simply come from a variation in the adsorption site on the Si(111)−B surface where the optimized TMA islands are less constrained by the boundary conditions of this large supercell. The electron charge gained by the Si(111)−B surface is mainly localized nearby the carboxyl groups of TMA as shown in the local density of states (LDOS) plots of Figure 3. The

Figure 3. Local density of states (LDOS) of the different assembled TMA islands on the Si(111)−B surface: (a) a clean Si(111)−B surface, (b) and with an adsorbed TMA monomer, (c) TMA dimer, and (d) TMA trimer. The color scales from low (blue) to high DOS (red) regions. LDOS diagrams represent an integrated of LDOS from −15 to Fermi level around −4 eV.

LDOS plots clearly indicate that carboxylic groups (COOH) of TMA donate with Si adatoms. Although a significant charge transfer from TMA to Si−B exists, the type of adsorption for TMA maintains the characteristics of a physisorbed phase since no covalent bonds are formed between the two species. A careful analysis of Figure 4 showing the variation of the total density of states (DOS) as a function of the adsorbed phases gives additional details on the electronic effects of TMA on the Si(111)−B surface. The calculated DOS for a clean Si(111)−B surface is, although the well-known LDA underestimation of the band gap, in good agreement with the experimental electronic structure reported by Lyo and coworkers23 with atom-resolved tunneling spectroscopy. First, we observe that the electronic states of Si(111)−B in the vicinity of the Fermi level are significantly perturbed by the adsorption of TMA islands. More specifically, we observed a gradual displacement of the valence band (VB) toward Fermi level region with the adsorption of one, two, and three TMA molecules. C


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presence of adsorbed species, but more so when hydrogen bonds among them are formed. As a result, we observed the appearance of new states in the band gap of Si(111)−B which have virtually no or very weak dispersion in any directions of the Brillouin zone. The weak dispersion of these states below the CB indicates a localization of the states and confirms the weak interaction of TMA with the surface and among the molecular islands. The decreasing band dispersion of the CB for the dimer and trimer can also be attributed to the presence of hydrogen bonding but also to the larger proportion of Si-adatoms of the Si−B interface that become structurally deformed by the presence of large TMA islands. The presence of structural defects in such a Si(111)−B interface was shown to have a profound impact on the electronic properties of surface states and the band dispersion of states in the conduction band.25 Finally, although the DFT calculations were performed at 0 K, we can speculate that the presence of these localized states associated with the TMA moiety in the vicinity of the Fermi level and the valence might play a significant role in the doping of the Si(111)−B surface once the temperature is raised. III.2. Adsorption of Self-Assembled TMA Monolayers on Si(111)−B. Trimesic acid may form highly directional interactions through hydrogen bonding. More specifically, the TMA dimer constitutes the building block of supramolecular networks organized into a hexagonal chicken-wire structure (see Figure 6(a)), and the TMA trimer forms a more densely packed structure called super-flower (see Figure 6(b)). These networks have been already synthesized and revealed on graphite with the help of scanning tunnelling microscopy (STM).36−38 In addition, TMA can exhibit dual acceptor and donor character upon the participation of thr π and π* orbital of the CO moiety to the bonding. This last characteristic affords to tune the electronic properties of a specific substrate on which the network is deposited. We have modeled these two TMA networks in order to verify and extend the trend observed with the adsorption of small isolated TMA islands. The supercells used to perform DFT calculations on the TMA network are shown in the inset of Figure 6(a) and (b) for, respectively, the chicken-wire and the super-flower networks. Table 2 reports the results of electronic structure calculations for the 2D self-assembly networks of TMA on Si(111)−B in

Figure 4. Total density of states (DOS) of an isolated Si(111)−B surface and after adsorption of islands of TMA molecule on the substrate. The energy positions of HOMO/LUMO of gas-phase TMA and the Fermi energy (EF) are indicated by the solid lines.

This energy shift suggests an improving electron transfer to the substrate as the number of TMA units increases, which is also supported by the net Mulliken population reported in Table 1. The variations observed below the conduction band (CB) around −3.5 to −4.0 eV are tributary to the TMA island states and more probably associated with the existence of hydrogen bonds since the shape of the DOS for the monomer in that energy region is nearly the same as the clean Si(111)−B surface. Hence, the calculated variations in the VB and CB lead to a closing of the band gap from 1.3 to 0.6 eV for adsorbed TMA islands containing from 0 to 3 TMA units. To complete the first part of this analysis of bonding, the band structures of the adsorbed TMA islands are reported in Figure 5. Consistently with previous DOS diagrams of Figure 4, the valence bands essentially move upward to the Fermi level, while the nature and dispersion of the bands more or less remain the same. In contrast, the dispersion of the states in the CB nearby the Fermi level is strongly influenced by the

Figure 5. Band structure of Si(111)−B containing (a) 0, (b) 1, (c) 2, and (d) 3 adsorbed TMA units. Dashed lines indicate the position of the Fermi level. The DOS for the neat Si−B surface and with 2 and 3 adsorbed TMA are reproduced in (e) for reference. D


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In addition, except for a few energy shifts, we do not observe a large difference in the electronic properties of adsorbed TMA networks with respect to the associated dimer and trimer islands. The LDOS (Figure 7) and the band structures (Figure 8) associated with the chicken-wire and the super-flower

Figure 6. Representation of two different assemblies: (a) a chickenwire and (b) a super-flower network based on TMA building block arrangements on Si(111)−B. The insets show the supercell used in the DFT calculations to study the TMA networks. Figure 8. Band structure of Si(111)−B containing (a) no TMA and TMAs arranged (b) in chicken-wire networ, and (c) in super-flower network. Dashed lines indicate the position of the Fermi level.

Table 2. Adsorption Energy (Eads) and Net Mulliken Charge of the Assembled TMA Network on Si(111)−Ba network structure Eads (eV/TMA) net charge (|e|/TMA)

chicken-wire

super-flower

−2.76 (−1.93) +0.27

−2.61 (−1.84) +0.23

networks support previous findings where most of the interactions occur through the participation of COOH groups of TMA and surface Si atoms. As observed for TMA island adsorption, the adsorption of TMA networks provokes the appearance of weakly dispersed states in the Si(111)−B that can be associated with the formation of hydrogen bonds between TMA units and to the presence of structural defects at the Si(111)−B interface. As it is given that the energy position of the different states is close to the Fermi level, we can anticipate that the TMA adlayer plays a role in the doping of the Si−B interface once the temperature is raised in the system. It would also be interesting to explore the potential of different functional groups in order to evaluate their capacities to control the doping level of the Si−B interface.

a

The values between parentheses are the surface contribution to the adsorption energy.

terms of adsorption energy per molecule and net Mulliken population on the TMA moiety. The interaction of TMA with the surface does not appear drastically affected by the presence of additional TMA units in the neighboring since the contribution from the surface (Esurf) to the adsorption energy of TMA slightly drops by 4−5% in the network with respect to the isolated TMA islands, while hydrogen bonding is nearly maintained. We are aware that the use of a much larger supercell would have been beneficial to better describe the long-range interactions in these large intermolecular arrays. Nevertheless, the smaller TMA network models used clearly indicate these networks constitute stable and realistic TMA arrangement where geometrical relaxations would contribute to emphasize the stability of large arrays over an agglomeration of small TMA islands. Additional calculations on a larger Si(111)−B surface model would be also necessary to better describe the influence of Si adatoms on the geometry of adsorbed TMA islands and networks.

IV. CONCLUSION By using first-principles DFT computational method, we investigated the interfacial interaction of small islands and the supramolecular network of trimesic acid on a boron-doped silicon substrate. The adsorption of small TMA islands reveals the participation of both surface and intermolecular hydrogen bonding to stabilize adsorbed TMA units. A significant charge transfer from TMA units to the Si(111)−B surface is observed upon adsorption that is mostly mediated by the COOH groups of TMA. The electronic states associated with the hydrogen

Figure 7. Local contribution on the density of states of the (a) chicken-wire and (b) super-flower TMA network/SiB complexes. The color scales from low (blue) to high DOS (red) regions. LDOS diagrams represent an integration of LDOS from −15 to Fermi level around −4 eV. E


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bonds formed in the small TMA islands are weakly dispersed and are located in the midgap of the Si(111)−B substrate. The adsorption of TMA units and the interactions between them on the surface contribute to decrease the dispersion of states more specially in the conduction band. Quite similar results were observed for the adsorption of large TMA networks. The results suggest that the self-assembling of TMA is strongly driven by the electrostatic interaction of COOH groups of TMA with surface Si-adatoms and reveal also the role of hydrogen bonding for the formation of a supramolecular network and its related electronic properties.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Ministère du Développement économique, de l’Innovation et de l’Exportation (MDEIE) through the PSR-SIIRI program. This work would not have been possible without the computational resources provided by Calcul Québec and Compute Canada.

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