Elsevier

Surface Science

Volume 606, Issues 15–16, August 2012, Pages 1195-1202
Surface Science

Stability of small chemical groups on hexagonal-SiC(0001) surfaces: A theoretical study

https://doi.org/10.1016/j.susc.2012.03.022Get rights and content

Abstract

Density functional theory (DFT) calculations are used to investigate the stability on SiC(0001) surfaces of different chemical groups -NH2, -NO2, -CH3, -OH, -SH and -CN. The adsorption stability decreases in the order -NO2 >  OH >  NH2 >  SH >  CN >  CH3. The stability of the single molecule-substrate bond is strongly influenced by the polarizability, which in turn depends on different parameters such as the electronegativity, atomic size and chemical environment. In a further step, methyl (− ACH3) and phenyl (− AC6H5) substituted groups are also considered and similar behaviour is observed. The inductive effect of the -CH3 or -C6H5 groups modifies the polarization of the Si adatom-molecule bond and the steric hindrance due to their size influences the molecular orientation. These two parameters affect the calculated adsorption energy, and are more important for –C6H5 substituent. This study provides clear tendencies that can be applied to more complex systems. Comparison of the adsorption of two large molecules, H2Pc (metal-free phthalocyanine) and PTCDI (perylene tetracarboxylic diimide) on the SiC(0001) surface is presented as an example.

Highlights

► Investigate the stability on SiC(0001) surfaces of different chemical groups -NH2, -NO2, -CH3, -OH, -SH and –CN. ► Influence of the electronegativity, atomic size and chemical environment. ► Methyl (− ACH3) and phenyl (− AC6H5) substituted groups are also considered. ► Study provides clear tendencies that can be applied to more complex systems.

Introduction

Organic functionalization on biocompatible wide band gap SiC semiconductor surfaces is of important technological interest for the development of biosensors or optoelectronic devices [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Recently, it was shown that large organic molecules, such as H2Pc (metal-free phthalocyanine) and PTCDI (perylene tetracarboxylic diimide), can be anchored on a hexagonal SiC(0001)-3x3 substrate [11], [12], [13], [14]. The adsorption of PTCDI on SiC(0001)3x3 is interesting because the PTCDI molecule forms chemical bonds with the Si adatoms, while the HOMO and LUMO states of the molecule are electronically decoupled. An insulating contact between a molecule and a surface provides new opportunities for single molecule opto-electronics because the molecule retains its optical properties. However, the adsorption mechanism of these large molecules is complex, being comparable to a pseudo-Diels-Aldler [10 + 2] (or [12 + 2]) cyclo-addition reaction through either two Si-N bonds or two Si-O bonds for the H2Pc and PTCDI molecules, respectively.

So far, only few studies have been devoted to the functionalization of different hexagonal-SiC(0001) surface reconstructions. Preuss et al. have investigated the adsorption of pyrrole molecules [15] using a combination of low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) experiments, and density functional theory (DFT) calculations. The study showed that N bonding to the SiC substrate via N-H pyrrole dissociation is more favourable than physisorption of the un-dissociated molecule.

Other studies have focused on developing an appropriate method of doping SiC for electronic applications [16], [17], [18]. Indeed, during growth of nitrogen-doped SiC using chemical vapour deposition (CVD) nitrogen-containing molecules are expected to be present in the gas phase. The adsorption of these molecules onto the SiC(0001) surface has been studied theoretically using DFT calculations [18]. On the other hand, due to the importance of dielectric layers in SiC device technology a number of studies have focused only on the initial oxidation of the SiC(0001) surfaces [19], [20], [21], [22], [23], [24], [25]. For instance, Jia Mie Soon et al. used first-principle calculations to show that the first oxygen molecule reacts at the Si adatom without any energy barrier in the initial adsorption step [19]. However, in the context of the current development of Atomic Layer Deposition techniques, guidelines for choosing the appropriate molecular precusor are needed.

In this paper, we present a systematic DFT study of the energetic stability of different chemical groups containing either N, O, S or C atoms adsorbed on the hexagonal SiC(0001)√3x√3R30° and 3x3 reconstructions. We have selected the most common groups: -NH2, -NO2, -OH, -SH, -CH3, and -CN (hereafter noted –A, unless otherwise specified). The aim is to provide clear tendencies as a function of different parameters such as the electronegativity, atomic size and chemical environment that could then be applied to more complex systems. We focus on the relative thermodynamic stability of the chemical groups based on electronic calculations and adsorption energy. A study of the kinetic parameters is beyond the scope of our present paper. Our study allows us to make an initial comparison of our results with the adsorption of large molecules as PTCDI or H2Pc on SiC(0001)-3x3 surface as already observed by scanning tunnelling microscopy (STM) [11], [12], [13], [14]. Further experiments on molecules with the specific chemical groups would be required to confirm the tendencies produced by our theoretical calculations.

The calculation method and the surface reconstruction models used in this work are presented in section II. The energetic of the –A groups adsorbed on SiC surfaces is then investigated (section III) and chemical groups such as –ACH3 and –AC6H5 are considered in section IV and V respectively. The importance of the different parameters on the stability of the chemical bonding on SiC(0001) surfaces are summarized in the conclusion.

Section snippets

Method and Models

DFT calculations are performed in the framework of the Vienna ab-initio simulation package (VASP code) [26], [27], [28]. The generalized gradient approximation (GGA), PW91 functional developed by Perdew-Wang [29] for the exchange and correlation energy is used for the geometry optimizations and the electron-ion interaction is described within the projector-augmented-wave (PAW) pseudopotentials [30], [31]. The cut-off energy used for the plane-wave basis set is 400 eV.

The SiC surfaces are

–A group on a SiC(0001)√3x√3R30° surface involving four adatoms per unit cell

We first investigate the adsorption of different -A chemical groups on the SiC(0001)√3x√3R30° surface where -A corresponds to -OH, -SH, -NH2, -CH3, -CN and -NO2 (Fig. 2). The slab used in this section is composed of four silicon adatoms per surface unit cell (Fig. 1a). One -A group forms a chemical bond with only one silicon adatom, thus leaving three Si dangling bonds unsaturated, corresponding to a 0.25 monolayer coverage (ML). We can consider that the –A chemical groups are isolated from one

Application To Large Organic Molecules

In this paper, we have confined our investigation to the adsorption of small chemical groups (− NO2, -NH2, -CH3, -CN, -OH and -SH), adsorbed via a single chemical bond on the SiC(0001) surfaces. In particular, we have shown that O is more strongly bound than N and that the presence of -CH3 or -C6H5 groups in place of a hydrogen results in a stronger binding energy (with the exception of -NHC6H5 for steric reasons). To verify the trends deduced from the calculations, we can make an initial

Conclusions

We have focused on a selection of common chemical groups containing O, N, S, and C atoms. DFT calculations allow us to elucidate some trends in the influence of three main parameters; the electronegativity, the atomic size, and the chemical environment, on the adsorption energies of the chemical groups. This study provides insights useful for understanding the adsorption of more complex molecules containing Si-O, Si-N, Si-C and Si-S bonds on the SiC(0001)-3x3 or SiC(0001)-√3x√3R30° surfaces, as

Acknowledgements

This work was supported by the ANR project MOLSIC (Contract No. ANR-08-NANO-058) and was performed using HPC resources from GENCI-IDRIS (Grant 2010–096459). We thank X. Bouju and A. Gourdon for fruitful discussions.

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