Time Evolution of a Cl-Terminated Si Surface at Ambient Conditions P. Chatterjee and S. Hazra* Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India ABSTRACT: The stability of Cl-terminated Si surface at ambient conditions and its evolution with time, which have immense importance for the growth of interesting nanostructures on it, were investigated using complementary methods. Wetting of water, i.e., contact angle measure- ments, which provide macroscopic level information, shows transition in the nature of Cl-Si surface from weak-hydrophilic toward weak- hydrophobic with time. Electron density profiles, obtained from X-ray reflectivity (XR) measurements, suggest that such a transition is associated with the growth of less uniform oxide layer. Structures of CTAB-silica mesostructured films on as-prepared and time-evolved Cl- Si substrates, obtained from XR and grazing incidence small-angle X-ray scattering measurements, show transition from strongly attached near circular micelles to weakly attached more elliptical micelles, confirming the transition (from weak-hydrophilic toward weak- hydrophobic) in microscopic level and growth of less homogeneous oxide layer. The critical time of such a transition is about 50 h, which actually represents the stability or the critical time of Cl desorption and oxide growth of the Cl-Si surface at ambient conditions. ■ INTRODUCTION Passivation of semiconductor surface, especially silicon surface, is extremely important for their applications in a wide range of fields, including nanotechnology, microelectronics, optoelec- tronics, biomedical, and biological sensors. 1,2 Passivation or termination of Si surface by different atoms or groups like, -OH, -H, -Cl, -Br, etc., essentially prevents contamination and surface defect states, thus controlling the electronic properties of the surface or interface. 2 It also tunes the surface free energy, polar-nonpolar (hydrophilic-hydrophobic) or electrostatic nature, and the reactivity of the surface. 3-8 Such nature of the passivated surface and its stability plays an important role in the growth of interesting nanostructures on it 5,6,9-15 and thus becomes one of the thrust area of investigation. Among different passivated surfaces, Cl-terminated Si surface, which has good Si-Cl bond stability and can be prepared easily through the wet-chemical process, 16-18 is drawing tremendous attention due to its higher reactivity compared to the H-terminated Si surface for the functionaliza- tion reactions. 19,20 Understanding the nature (hydrophilic- hydrophobic) of the Cl-Si surface and its stability is very important for its proper utilization. The nature of a surface is well evident from the growth of amphiphilic molecules 5,15,21-23 or composite materials 7,8 having both hydrophilic and hydro- phobic parts, apart from conventional contact angle (CA) or water adhesion measurements. 24-26 Recently, weak-hydro- philic 26 or hydrophilic-like nature of the Cl-Si surface, in the molecular level, is determined from the structures of the deposited CTAB-silica 2D-hexagonal mesostructured films on it, 8 thus removing contradictory or incomplete information about the nature of the surface. 27,28 The stability of the Cl-Si surface, on the other hand, is less studied. 18,29 It is known that the stability of a passivated surface depends on the environments, namely humidity of air, oxygen content in air, dissolved oxygen in water, and metal impurity on the surface. 13,14,18,21,22,29 Native oxide is normally grown on the Si surface desorbing the passivated atoms. Thus, understanding the growth of oxide layer is one way to understand the stability-instability of a passivated surface. Soft X-ray photo- electron spectroscopy (SXPS) 18 and high-resolution electron energy loss spectroscopy (HREELS) 29 were used to identify the presence of silicon oxide on the surface and its growth with time. Oxidation can change the chemical nature and/or the roughness of the surface, which in turn can change the hydrophilic-hydrophobic nature of the surface. However, not much work is carried out to understand the stability-instability information on the Cl-Si surface, from the point of view of hydrophilic-hydrophobic nature of the surface. Simple CA measurements along with the X-ray scattering measurements can monitor not only the hydrophilic-hydrophobic nature of the surface but also its quantitative evolution, i.e., its stability, which is never estimated before. Here the evolution of the Cl-Si surface with time in ambient conditions is monitored directly using CA and X-ray reflectivity (XR) techniques. 5,13,30-32 The structures of the CTAB-silica 2D-hexagonal mesostructured films 33-35 deposited at different point of time were estimated by the complementary XR and grazing incidence small-angle X-ray scattering (GISAXS) 8,36-40 techniques and were used to understand the evolution of the Received: February 11, 2014 Revised: May 2, 2014 Published: May 6, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 11350 dx.doi.org/10.1021/jp501480x | J. Phys. Chem. C 2014, 118, 11350-11356