Surface Science最新文献

We report on spin-polarized density-functional theory study of adsorption of dihalogen molecules X₂ (X = F, Cl, Br and I) on the Fe/W(110) substrate, i.e., X₂/Fe/W(110) systems. We considered different molecular orientations and adsorption sites of the halogens and obtained their corresponding ground-state structures. We obtained initial molecular orientation (IMO) and initial adsorption site (IAS), i.e., IMO-IAS combinations that give the minimum energy configurations for each of the X₂/Fe/W(110) systems. Our results shows that all the molecules studied in this work are chemisorbed on the Fe surface. Also, the halogen atoms may be adsorbed dissociatively on the hollow sites in such a way that an X₂ separates into two X atoms with each of the atoms located at two nearby hollow sites. Similarly, we found IMO-IAS combinations which resulted in a non-dissociative adsorption. In the latter, the pre-relaxed IMO-IAS is maintained even after the structural relaxation. The most stable configuration for the X₂ dihalogen molecule in this case is either the top or bridge site while the halogen is in perpendicular orientation to the Fe surface. We conclude therefore that, the final relaxed configurations of the X₂ halogen depends on the IMO through which is deposited on the Fe/W(110) substrate. The trend in the adsorption energy E_A for the most stable configurations for the dissociative adsorption is E_A (F) > E_A (Cl) > E_A (Br) > E_A (I). The trend of E_A for non-dissociative adsorption is similar to that of dissociative adsorption, however, the latter is the more energetically favorable. Electronic structure calculations show hybridization between the p and d orbitals of X and Fe atoms respectively. Furthermore, we have found antiferromagnetic coupling between the interfacial W atoms and the Fe overlayer atoms while ferromagnetic coupling is found between the halogens and the Fe atoms. Our work represents a detailed study of adsorption properties of highly reactive halogens in contact with the Fe/W(100) surface.

The stretching frequency of the CO bond is a sensitive probe of the local environment of a surface-bound CO molecule, including the adorption site and density, i.e. surface coverage. In this work, we extend our analysis beyond the frequency shift due to differences in adsorption configurations. Using density functional theory (DFT) calculations, we directly explore the correlations between surface coverage and the stretching frequency of adsorbed CO on Pd surfaces. We also perform constant pressure infrared reflection absorption measurements of CO on Pd(111) and use existing relations between pressure and coverage to derive coverage dependency. Both results are compared to previously reported experimental data. Our derived correlations of peak frequency and area with surface coverage can help interpret experimental IR spectra in real time and extract time-dependent concentration data from transient kinetic experiments.

Volatile organic compounds (VOCs) cause a considerable risk to human life, and it is vital to introduce highly efficient VOC biosensors. Methanol (CH₃OH) was identified as a vital biomarker, showing significant elevation in both lung cancer and COVID-19 patients. Two-dimensional (2D) semiconductor gas sensors offer benefits such as excellent sensitivity, resistance to high temperatures and stability. In the present study, we explored methanol adsorption on the pristine and transition metal (TM)-doped (Sc, Ti, V, Cr, and Mn) C3B 2D flakes with the density functional theory (DFT) technique. Our results revealed that the V-, Cr-, and Mn-doped C3B show larger adsorption energy values as compared to the pristine C3B surface. The change of band gap energy of surfaces after methanol adsorption is obtained between 40 and 400 %. Besides, results show that methanol has a quick recovery at room temperature. The work function variation of studied flakes upon methanol adsorption has been also investigated and results show that V-, Cr-, and Mn-doped C3B systems are sensitive to methanol gas molecule. This work suggests that the C3B-based flakes can be used as a biosensor to identify VOC biomarkers such as methanol.

Localized surface plasmons (LSP) on faceted surfaces of gold nanoparticles enable carbon monoxide disproportionation to be driven at room temperature. In order to expand the known surfaces that catalyze this reaction, we explore the adsorption of carbon monoxide at top, long bridge, short bridge, and hole sites on gold (100), (110), (111), (211), and (311) faceted surfaces, as well as the reaction barriers for disproportionation at the lowest energy adsorption site on each surface and edges between two (311) surfaces and (100) and (110) surfaces. Generally, the less atomically dense, higher index facets promote both good adsorption and reactivity, and the edges show lower barriers for disproportionation. For most of the explored surfaces, adsorption directly on top of a gold atom is most favorable. The lowest activation energy for carbon monoxide disproportionation to amorphous carbon and carbon dioxide is predicted for two carbon monoxides adsorbed on top of atoms on the (311)/(311) edge.

Silicon carbide (SiC) is a promising third-generation semiconductor due to its wide bandgap. However, the high Schottky barrier and metal-induced gap states (MIGS) at the metal/SiC interface present significant challenges for device fabrication, leading to high contact resistance and poor current delivery. This study proposes the use of bismuth (Bi), with its semimetallic properties and gap-state saturation effect, as a contact buffer layer to address these issues. We conducted a systematic investigation of the chemical and electronic characteristics of the Pt/Bi/4H-SiC(0001) system, fabricated via molecular beam epitaxy (MBE), using in situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Our findings reveal weak bonding between the Bi buffer layer and the 4H-SiC(0001) surface, resulting in a slight downward band bending effect and the formation of a substantial dipole across the Bi/4H-SiC(0001) interface. Moreover, UPS spectra indicate a reduction in the work function of Pt/Bi/4H-SiC(0001), suggesting the potential for achieving low contact resistance. Notably, the Pt/Bi/4H-SiC(0001) system remains stable when exposed to 1.6×10⁹ Langmuir of oxygen at room temperature, while a bare Bi buffer layer undergoes partial oxidation. These results provide a comprehensive understanding of the Pt/Bi/4H-SiC(0001) interfaces and strategies for improving metal/SiC contacts.

High-temperature joint adsorption has been studied of three different adsorbates, Si, Be, and P on the $\left(10\overline{1}0\right)$ Re face. All three adsorbates form surface compounds with the stoichiometry ReX, where X is Si, Be, and P. In joint adsorption, one of the adsorbates is displaced into the bulk, into the dissolved state. The processes are observed at a temperature sufficient for diffusion in the bulk, 1100–1300 K. Displacement occurs in an atom-to-atom mode, and it has a “cyclic nature”: silicon displaces phosphorus, beryllium displaces silicon, and phosphorus - beryllium.

The density functional theory is employed for learning the modulation of the electronic structure and magnetic properties of monolayer SnSe₂ by an Mn atom and by co-doping an Mn atom with a halogen atom. It is found that intrinsic SnSe₂ is nonmagnetic, which is consistent with the properties of semiconductors. Following Mn atom doping, the doped system is magnetic and the magnetic moments are primarily responsible for the Mn atom. After co-doping the Mn atom with halogen atoms, the doped system's total magnetic moments are decreased. The examination of the electronic structure demonstrates that the doping of the Mn atom and the co-doping of the Mn atom with halogen atoms lead to the introduction of impurity energy levels into the doped system, which appear only in the spin-up part and do not cross the Fermi energy level. There is asymmetry between the spin-up and spin-down energy bands and the doped system exhibits magnetic semiconductor properties. The hybridization of the p-orbitals of the halogen atoms and the 3d orbitals of the Mn atom is primarily responsible for the introduction of impurity energy levels in the energy bands of the doped system. In the Mn-doped system, ionic bonds were shaped between Mn and Se. In the co-doped system, strong ionic bonds were shaped between the Mn atom and F, Cl atoms, and covalent bonds were shaped between the Mn atom and Br, I atoms.

Nucleation and growth of supported 3D metal clusters or crystallites during deposition on MoS₂, or on other weakly-adhering layered materials, can potentially produce diverse growth shapes, and even crystal structures differing from the bulk metal. For Fe deposition on MoS₂, SEM and AFM observations reveal three distinct crystallite shapes. By comparison with atomistic structure models incorporating realistic Fe-MoS₂ interface structures, we conclude that these are: triangular fcc(111) pyramids with sloped {100} side facets; bcc(110) A-frame tents with sloped {100} side facets; and bcc(110) mesas with vertical {100} and {110} side facets. The following picture is proposed for the competitive formation of clusters and crystallites with different structures: (i) small nanoclusters formed at the onset of deposition exhibit facile fluxional dynamics allowing sampling of different crystal structures and shapes; (ii) sufficient fluxionality implies a Boltzmann distribution of sampled structures, and thus coexistence of different structures follows from the demonstrated similar energies for those structures; (iii) growing clusters reach a threshold size above which the characteristic time scale for restructuring exceeds that for cluster growth. Thereafter, clusters are locked-in to a specific crystal structure and shape as revealed by imaging of larger crystallites. Despite a penalty for fcc(111) over bcc(111) pyramids based on bulk energetics, favorable surface and interface energies makes them preferable for smaller sizes.

In a recent publication [2D Materials, 8, 045033 (2021)], it was reported that the growth of a monolayer PdTe${}_2$ in ultra-high vacuum could be achieved by deposition of tellurium on a palladium (111) crystal surface and subsequent thermal annealing. By means of low-energy electron diffraction intensity (LEED-IV) structural analysis, we show that the obtained $\left(\sqrt{3}\times \sqrt{3}\right)\text{R30}°$ superstructure is in fact a TePd${}_2$ surface alloy. Attempts to produce a PdTe${}_2$ layer in ultra-high vacuum by increasing the Te content on the surface were not successful.

Although the structure of Cu(100)-($2\sqrt{2}\times \sqrt{2}$)R45°-O missing row reconstruction (MRR) has been well-established for decades, the detailed structure of its various boundaries remains an untilled area due to the difficulties in obtaining atomically resolved images. Herein, atomic arrangement of the phase boundaries existing in MRR structure was modeled on the basis of scanning tunneling microscopy (STM) investigations. By determining the periodicity and unit structure of MRR in STM images and extending them to boundary region, several types of phase boundaries were identified, resulted respectively from: (1) the mismatch between c(2 × 2)-O patches, (2) the regulation by step edges, and (3) the mismatch between Cu missing rows (MRs). With the modeled structure, it was revealed that the types of the c(2 × 2)-O mismatch induced phase boundaries (OMIPBs) are mainly dominated by the oxygen exposure and in-diffusion barrier. The step edge regulated phase boundaries (SERPBs) are always terminated with Cu-O chain and may represent an intermediate growth stage to larger MRR structure. Comparatively, Cu MRs mismatch is often reconciled by the differently oriented domains between them. As a result, the Cu MRs mismatch induced phase boundaries (CMRMIPBs) are only occasionally observed as Cu-O chains between mismatched Cu MRs that encounter shoulder-to-shoulder. For all studied boundaries, the surrounding MRR domains exhibit obvious orientation preference through inclined packing along the SP direction with the degree closely related with the width of the boundaries.