Dissociation of Ethoxysilane and Methoxysilane on Si(001)‑2 × 1 and Si(111)‑7 × 7 at Room Temperature: A Comparative Study Using Synchrotron Radiation Photoemission

The adsorption of tetraethoxysilane and tetramethoxysilane (TEOS, Si­[OC₂H₅]₄ and TMOS, Si­[OCH₃]₄) on the Si(001)-2 × 1 and Si(111)-7 × 7 surface at 300 K was studied by synchrotron radiation X-ray photoelectron spectroscopy (XPS). On Si(001)-2 × 1, and for both alkoxysilanes, two adsorption regimes are present. The initial one corresponds to the full dissociation via Si–O bond breaking that leads to the grafting of ethoxy moieties and the release of a silicon monomer. This regime is superseded by a second mechanism involving the breaking of C–O bonds leading to the attachment of alkyl moieties to the surface. We propose that C–O bond breaking occurs on the silicon monomers produced during the initial regime. On Si(111)-7 × 7, although the surface reconstruction is different from that of Si(001), we observe the same products as those seen on Si(001)-2 × 1 and the same trends (Si–O bond breaking predominates over C–O bond breaking at low coverage)

Charge Transfer and Energy Level Alignment at the Interface between Cyclopentene-Modified Si(001) and Tetracyanoquinodimethane

We examine how the electronic structure (via synchrotron radiation XPS, UPS, and NEXAFS) and the molecular orientation (via NEXAFS) of a strong acceptor molecule, tetra­cyano­quino­dimethane (TCNQ), change as a function of thickness when it is deposited on the cyclo­pentene-covered Si(001)-2×1 substrate. XPS shows that the monomolecular cyclo­pentene layer acts as an efficient chemical protective barrier. All spectroscopies indicate that anionic TCNQ is formed at (sub)­monolayer coverage. However, the transfer should only concern those CN moieties pointing toward the Silicon surface. At higher thicknesses, neutral TCNQ is observed. We do not observe the upward bending of the silicon bands associated with electron transfer from the substrate to the acceptor molecular that one would expect for an unpinned Fermi level interface. In fact, donor levels are likely created within the cyclo­pentene layer or at its interface with silicon. The formation of TCNQ^– is associated with a strong increase in the work function. The attained value (∼5.7 eV) is independent of the work function of the cyclo­pentene-modified Si(001) surface (that varies with Si doping), in agreement with the integral charge transfer model. Therefore, ultrathin layers of TCNQ can be used to improve the hole-injection properties of this alkene-modified silicon surface

Ene-like Reaction of Cyclopentene on Si(001)-2 × 1: An XPS and NEXAFS Study

The control and the understanding of single-molecule covalent coatings on silicon surfaces is increasingly important in designing nanoscale electrical elements, such as organic/inorganic semiconductor hybrid structures. In this respect, ordered arrays of cyclopentene deposited on Si(001)-2 × 1 appear as promising buffer layers for further molecular crystal growth on the substrate. In this work, we examine the adsorption of cyclopentene on Si(001)-2 × 1 at 130 and 280 °C by means of C 1s XPS and NEXAFS. Until now cryogenic and room-temperature adsorption studies tended to prove that cyclopentene adsorption results from a formal cycloaddition ([2 + 2]-like) reaction of the CC double bond with a silicon dimer. Our XPS/NEXAFS study reveals that an ene-like reaction competes with the [2 + 2]-like reaction channel, leading to the formation of products bearing a CC bond, already at deposition temperature as low as ∼130 °C. This work helps to determine the optimum conditions leading to optimal chemical order in view of applications of cyclopentene as a buffer layer

van der Waals Epitaxy of GaSe/Graphene Heterostructure: Electronic and Interfacial Properties

Stacking two-dimensional materials in so-called van der Waals (vdW) heterostructures, like the combination of GaSe and graphene, provides the ability to obtain hybrid systems that are suitable to design optoelectronic devices. Here, we report the structural and electronic properties of the direct growth of multilayered GaSe by molecular beam epitaxy on graphene. Reflection high-energy electron diffraction images exhibited sharp streaky features indicative of a high-quality GaSe layer produced <i>via</i> a vdW epitaxy. Micro-Raman spectroscopy showed that, after the vdW heterointerface formation, the Raman signature of pristine graphene is preserved. However, the GaSe film tuned the charge density of graphene layer by shifting the Dirac point by about 80 meV toward lower binding energies, attesting to an electron transfer from graphene to GaSe. Angle-resolved photoemission spectroscopy (ARPES) measurements showed that the maximum of the valence band of the few layers of GaSe are located at the Γ point at a binding energy of about −0.73 eV relative to the Fermi level (p-type doping). From the ARPES measurements, a hole effective mass defined along the ΓM direction and equal to about <i>m</i>*/<i>m</i>₀ = −1.1 was determined. By coupling the ARPES data with high-resolution X-ray photoemission spectroscopy measurements, the Schottky interface barrier height was estimated to be 1.2 eV. These findings allow a deeper understanding of the interlayer interactions and the electronic structure of the GaSe/graphene vdW heterostructure

Silicon Monomer Formation and Surface Patterning of Si(001)‑2 × 1 Following Tetraethoxysilane Dissociative Adsorption at Room Temperature

The adsorption of tetraethoxysilane (TEOS, Si­[OC₂H₅]₄) on the Si(001)-2 × 1 surface at 300 K is studied through a joint experimental and theoretical approach, combining scanning tunneling microscopy (STM) and synchrotron radiation X-ray photoelectron spectroscopy (XPS) with first-principles simulations within the density functional theory (DFT). XPS shows that all Si–O bonds within the TEOS molecules are broken upon adsorption, releasing one Si atom per dissociated molecule, while the ethoxy (−OC₂H₅) groups form new Si–O bonds with surface Si dimers. A comparison between experimental STM images and DFT adsorption configurations shows that the four ethoxy groups bind to two second-neighbor silicon dimers within the same row, while the released silicon atom is captured as a monomer on an adjacent silicon dimer row. Additionally, the surface displays alternate ethoxy- and Si adatom-covered rows as TEOS coverage increases. This patterning, which spontaneously forms upon TEOS adsorption, can be used as a template for the nanofabrication of one-dimensional self-organized structures on Si(001)-2 × 1

Material Perspective on HgTe Nanocrystal-Based Short-Wave Infrared Focal Plane Arrays

After the use of nanocrystals as light downconverters, infrared sensing appears to be one of the first market applications where they can be used while being both electrically and optically active. Over recent years, tremendous progress has been achieved, leading to an apparent rise in the technological-readiness level (TRL). So far, the efforts have been focused on PbS nanocrystals for operation in the near-infrared. Here, we focus on HgTe since its narrower band gap offers more flexibility to explore the extended short-wave and midwave infrared. We report a photoconductive strategy for the design of short-wave infrared focal plane arrays with enhanced image quality. An important aspect often swept under the rug at an early stage is the material stability. It appears that HgTe remains mostly unaffected by oxidation under air operation. The evaporation of Hg, a potentially dramatic aging process, only occurs at temperatures far beyond the focal plane array’s standard working temperature. The main bottleneck appears to be the particle sintering resulting from joule heating of focal plane arrays. This suggests that a cooling system is required, whose first role is to prevent the material from sintering even before targeting dark current reduction