Types Of Surface Terminations Research Articles

Effects of Concentration and Surface Termination on the Workfunction of Ti3C2X2 MXene (X=F,Cl,O,OH)

Titanium carbide (Ti3C2), belonging to the MXene family, has become a focal point of research due to its exceptional electronic and structural properties. As a two-dimensional transition metal carbide, Ti3C2 exhibits promising potential in a variety of applications, ranging from electronic devices to energy storage. Among the key parameters influencing its electronic behavior, the work function plays a pivotal role. This study aims to elucidate the intricate relationship between concentration, surface termination, and the work function of Ti3C2X2 through first-principles density functional theory (DFT) calculations. First-principles DFT calculations were employed to investigate the electronic structure of Ti3C2X2, focusing on the influence of varying concentrations of surface functional groups. Hydroxyl (-OH), oxygen (-O), chlorine (-Cl), and fluoride (-F) terminations were specifically chosen as representative functional groups for their prevalence in experimental studies and potential technological applications. These calculations provide a quantum-level understanding of the electronic properties and behaviors of Ti3C2 under different surface conditions. Our results indicate a notable sensitivity of the work function of Ti3C2 to changes in both concentration and the type of surface termination. The presence of functional groups induces charge redistribution within the MXene layers, significantly impacting the overall electronic landscape. The variation in charge distribution directly correlates with changes in the work function, illustrating the dynamic nature of Ti3C2 in response to surface modifications. Furthermore, an in-depth analysis of the correlation between concentration, surface termination, and the energetics of charge transfer at the Ti3C2 surface provides valuable insights. The interplay between these factors reveals intricate mechanisms that govern the observed variations in the work function. The concentration-dependent charge transfer at the surface serves as a key determinant, emphasizing the importance of carefully tailoring the composition to achieve desired electronic properties. The understanding gained from this study has significant implications for the tailored design and utilization of Ti3C2 in various technological applications. By manipulating concentration and surface termination, researchers and engineers can fine-tune the work function to meet specific requirements in electronic devices, sensors, and energy storage systems. This level of control over electronic properties enhances the versatility and applicability of Ti3C2 in emerging technologies. In conclusion, our investigation highlights the critical roles of concentration and surface termination in modulating the work function of Ti3C2. The pronounced sensitivity of this parameter to surface modifications underscores the need for a comprehensive understanding of the underlying electronic mechanisms. By elucidating the interplay between concentration, surface termination, and charge transfer, this study provides valuable insights that pave the way for the strategic design and implementation of Ti3C2 in advanced technological applications. The ability to fine-tune electronic properties opens new avenues for exploiting the full potential of Ti3C2 in diverse fields, driving innovation and progress in materials science and technology.

Layered Pt-Co alloys: Bulk, surface and nanoparticle analysis, based on DFT

Pt-based catalysts are commonly used as cathode materials in various types of fuel cells. In this work we apply DFT calculations combined with cluster expansion method to elucidate the most stable structures of Pt-Co bulk alloy across the full range of Pt (or Co) concentrations. Our calculations reveal high stability of alloy configurations with alternating layers in (001) crystallographic planes of face centered cubic (fcc) structure exclusively occupied either by Pt or Co atoms. In particular, we find that Pt1Co1, Pt2Co1 and Pt3Co1 layered structures (periodically repeated 1–3 Pt layers, separated by single Co layers) have exceptionally high stability with Pt2Co1 possessing the lowest energy of formation among all analysed alloy configurations. Employing ab initio thermodynamics approach, we discuss stability of three types of surface terminations (Miller indices (100), (110) and (111)) for these three layered structures and also for highly symmetric fcc Pt3Co1 configuration, previously identified experimentally. Applying the Wulff construction, we obtain the nanoparticle shapes as functions of chemical potentials of constituent atoms (Pt or Co), allowing for design of computational models of Pt-Co nanoparticles and subsequent analysis of their chemical properties.

Structure and Electronic Properties of [Ca24Al28O64]4+·4e– Surfaces: Opportunities for Termination-Controlled Electron Transfer

We investigate the structure, thermodynamic stability, and electronic properties of (001), (110), and (111) surfaces of mayenite (12CaO·7Al2O3 or C12A7) in its oxygen-deficient phase known as mayenite-derived electride Ca12Al14O32 using simulations based on the density functional theory. Seven types of surface terminations were identified, depending on the orientation of the lattice nanosize cages with respect to the surface plane. We find that the surface termination has a strong effect on the distribution of electron anions in the near-surface region, as demonstrated by the range of the ionic charges and binding energies of adsorbed Au atoms and dimers. Notably, in all cases, Au species acquire a negative charge and preferentially adsorb as isolated ions rather than aggregate into clusters. These results show that C12A7 electride can provide a simple, yet versatile substrate to achieve high adsorption capacity and steric selectivity for single atom catalysts.

Atomic-resolution transmission electron microscopy of electron beam-sensitive crystalline materials.

High-resolution imaging of electron beam-sensitive materials is one of the most difficult applications of transmission electron microscopy (TEM). The challenges are manifold, including the acquisition of images with extremely low beam doses, the time-constrained search for crystal zone axes, the precise image alignment, and the accurate determination of the defocus value. We develop a suite of methods to fulfill these requirements and acquire atomic-resolution TEM images of several metal organic frameworks that are generally recognized as highly sensitive to electron beams. The high image resolution allows us to identify individual metal atomic columns, various types of surface termination, and benzene rings in the organic linkers. We also apply our methods to other electron beam-sensitive materials, including the organic-inorganic hybrid perovskite CH3NH3PbBr3.

First-principles studies of polar perovskite KTaO3 surfaces: structural reconstruction, charge compensation, and stability diagram.

Polar perovskite oxides are of considerable interest for developing advanced functional materials with exceptional electronic properties for their unique polar characters. A cleavage of polar perovskite oxides along the charged layers leads to an electrostatic instability on the cleaved surfaces, and a charge compensation is required to stabilize these surfaces. In this work, we have systemically studied 25 types of surface models of polar KTaO3 perovskite oxide, including (001), (110), and (111) surfaces with various types of surface terminations, using first-principles electronic structure calculations. The surface structural reconstruction, electronic structures, and thermodynamic properties including cleavage energy and surface energy are investigated. The phase stability diagrams of the (001), (110), and (111) surfaces are constructed with respect to the chemical potentials of component elements. The KO(001), O(110), and KO2(111) terminations are more likely to be formed than other types of terminations in corresponding surfaces, consistent with experimental observations on KTaO3(001) surfaces. This work provides useful guidance for accurate control of surface morphology for tailing functional properties of polar KTaO3 perovskite oxide.

Theoretical prediction of robust and intrinsic half-metallicity in Ni2N MXene with different types of surface terminations

Three MXenes, namely Fe2N, Co2N, and Ni2N in both bare and surface-passivated forms were investigated theoretically using density functional theory. Fe2N(OH)2, Fe2NO2, Co2NO2, Ni2NF2, Ni2N(OH)2, and Ni2NO2 are found to be intrinsic half-metals, while other structures have antiferromagnetic ground states. Notably, the half-metallicity of Ni2NT2 (T=F, OH, and O) does not depend on the type of surface terminations. This is interesting because the synthesized MXenes usually have mixed surface terminations of F, OH, and O. The half-metallic band gaps calculated with the HSE06 hybrid density functional are 2.03, 1.81, and 1.01eV, respectively. This implies that thermally induced spin flip should be suppressed. The ferromagnetic coupling between Ni atoms is explained by a superexchange interaction mechanism mediated by spin polarized N atoms. Based on the Heisenberg model, large intra- and inter-layer exchange parameters were obtained. Monte Carlo simulation indicates that the Curie temperatures of Ni2NF2, Ni2N(OH)2, and Ni2NO2 are 1800, 2400, and 3300K respectively, all much higher than room temperature. Ab initio molecular dynamics simulations also indicate that the ferromagnetic Ni2NT2 compounds are stable at room temperature. Therefore, we predict that the half-metallicity of Ni2NT2 should be relatively easy to be achieved and Ni2NT2 is a good candidate for spintronics.

Surface-termination dependence of propanoic acid deoxygenation on Mo2C

Although Mo2C is used as an alternative catalyst material for biomass conversion, the dependence of its reaction on the type of surface termination has not been studied. In the present study, we performed density functional theory calculations for the deoxygenation of propanoic acid on the two types of surface terminations possible for orthorhombic Mo2C (100), namely, on Mo- and C-terminated surfaces. The reaction energetics of the three possible deoxygenation pathways, namely, those for hydrodeoxygenation, decarbonylation, and decarboxylation, were compared by calculating the activation energies for their key surface reactions, such as hydrogenation and the scission of CC, CO, and OH bonds. The Mo-terminated surface was advantageous in the case of the hydrodeoxygenation pathway because of its low kinetic energy barrier for the hydrogenation of the oxygen species, while the C-terminated surface preferred the decarboxylation pathway because of its low kinetic energy barrier for OH bond scission.

Theoretical study on the anisotropic electronic structure of antiferromagneticBaFe2As2and Co-dopedBa(Fe1−xCox)2As2as seen by angle-resolved photoemission

By means of one-step model calculations the strong in-plane anisotropy seen in angle-resolved photoemission of the well-known iron pnictide prototype compounds BaFe$_2$As$_2$ and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ in their low-temperature antiferromagnetic phases is investigated. The fully-relativistic calculations are based on the Korringa-Kohn-Rostoker-Green function approach combined with the coherent potential approximation alloy theory to account for the disorder induced by Co substitution on Fe sites in a reliable way. The results of the calculations can be compared directly to experimental spectra of detwinned single crystals. One finds very good agreement with experiment and can reveal all features of the electronic structure contributing to the in-plane anisotropy. In particular the local density approximation can capture most of the correlation effects for the investigated system without the need for more advanced techniques. In addition, the evolution of the anisotropy for increasing Co concentration $x$ in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ can be tracked almost continuously. The results are also used to discuss surface effects and it is possible to identify clear signatures to conclude about different types of surface termination.

Direct-to-indirect bandgap transitions in ⟨110⟩ silicon nanowires

The bandstructure of ⟨110⟩ silicon nano wires (SiNWs) with diameters (d) up to 6.1 nm were studied using density functional theory. Three types of surface termination were investigated: H, F, and OH; all giving quantum confinement induced direct bandgaps in the investigated size range. Comparison of the calculated results to reported experimental values showed that trends in the bandstructure behaviour were well reproduced. By studying the relative decrease of global and local minima in the conduction band minimum with increasing d, it was possible to predict a direct-to-indirect bandgap transition at d = 9.2, 9.5, and 11.4 nm for H, F, and OH terminated NWs, respectively.

Structure and stability of clean and adsorbate covered intermetallic PdGa surfaces: A first principles study

In this work, using ab initio density functional theory based calculations we have studied the structure and stability of clean and hydrogen covered low-indexed (100) and (110) surfaces of intermetallic PdGa. We find that for the clean (100) surface, the stability of the surface terminations is independent of the surface preparation condition. On the contrary, at least three different types of surface terminations can be stabilized for the (110) surface by tuning the surface preparation conditions. Upon adsorbing molecular and atomic hydrogen on these surfaces, we find that: (a) at 450K hydrogen adsorbs only at high hydrogen partial pressure and (b) the relative stability of the different surface terminations is unaffected by the presence of adsorbates.

4원 호이슬러 합금 CoFeCrZ(Z = Ga, Ge)의 (001) 표면에서의 자성과 반쪽금속성: 제일원리 계산 연구

Department of Physics, Inha University, Incheon 402-751, Korea(Received 11 March 2015, Received in final form 3 April 2015, Accepted 7 April 2015)Recently, a first-principles study led to a prediction that quaternary Heusler compounds, CoFeCrZ (Z = Ga, Ge) are excellent half-metallic ferromagnets. In this study, we investigate the electronic and the magnetic properties at the (001) surfaces of CoFeCrGa andCoFeCrGe by means of the full-potential linearized augmented plane wave (FLAPW) method within generalized gradientapproximation. We considered two types of surface termination: CoFe-terminated and CrZ-terminated surfaces, Z being either Ga orGe. From the calculated total magnetic moments and the local density of states, we found that half-metallicity is not preserved for allthe surfaces. But the calculated atomic density of states showed that CrGa-terminated surface of the CoFeCrGa is almost half-metallic.The magnetic moment of the Co, Fe, or Cr atoms at the surface or subsurface layers in each system had very different values.Keywords: half-metallicity, surface magnetism, electronic structure

Structure and transport properties of liquid alkanes in the vicinity of α-quartz surfaces

Abstract Structure and mass transport properties of several liquid n-alkanes, methane, decane and tetracosane, in the vicinity of α-quartz surfaces of three crystal planes have been investigated by using molecular dynamics simulations. Solid α-quartz surfaces were terminated with –H and –OH groups to create hydrophobic and hydrophilic surfaces, respectively. It was observed that adsorption of the liquid alkane molecules is more noticeable near the (0 0 1) crystal plane when compared with other two planes (0 1 1) and (1 0 0). For a given alkane, the number of molecules adsorbed near a α-quartz wall marginally depends upon the crystal plane and type of surface termination. Ordering parameter and radius of gyration for liquid molecules were examined in the interface region to gain the knowledge about molecular alignment and chain configuration in the interface region. Liquid molecules in the first adsorption layer near the hydrophilic and hydrophobic surfaces are more parallel to the interface in the following order (0 0 1), (0 1 1) and (1 0 0). Portions of decane and tetracosane liquid molecules tend to enter into the H-terminated side with an orientation in the parallel direction to the interface and into the OH-terminated side with an orientation in the perpendicular direction for (1 0 0) crystal plane. Molecules are flattened in the direction perpendicular to the interface and are elongated in the other direction in the interface region for decane and tetracosane. It is observed that the in-plane self-diffusion coefficient of liquids, the self-diffusion coefficient according to the migration of molecules in the directions parallel to the interface, is smaller in the vicinity of the solid surface and approaches to the bulk liquid value away from the interface. For methane, decane and tetracosane, the in-plane diffusion in the first adsorption layer near the smoother silica surface (0 0 1), which has molecularly smoother surface than the other two surfaces (0 1 1) and (1 0 0), for both of the terminations is higher as compared with the other two surfaces (0 1 1) and (1 0 0). It was found that the diffusion coefficients in the two directions of the 2-D in-plane diffusion are different significantly and this anisotropic nature is resulted by the topology of the solid surface. At the same reduced temperature, a tendency was observed that the in-plane diffusion near the H-terminated and OH-terminated surfaces for three crystal planes increases with an increase in the chain length of the liquid alkanes.

Contactless photoconductance study on undoped and doped nanocrystalline diamond films.

Hydrogen and oxygen surface-terminated nanocrystalline diamond (NCD) films are studied by the contactless time-resolved microwave conductivity (TRMC) technique and X-ray photoelectron spectroscopy (XPS). The optoelectronic properties of undoped NCD films are strongly affected by the type of surface termination. Upon changing the surface termination from oxygen to hydrogen, the TRMC signal rises dramatically. For an estimated quantum yield of 1 for sub-bandgap optical excitation the hole mobility of the hydrogen-terminated undoped NCD was found to be ∼0.27 cm(2)/(V s) with a lifetime exceeding 1 μs. Assuming a similar mobility for the oxygen-terminated undoped NCD a lifetime of ∼100 ps was derived. Analysis of the valence band spectra obtained by XPS suggests that upon oxidation of undoped NCD the surface Fermi level shifts (toward an increased work function). This shift originates from the size and direction of the electronic dipole moment of the surface atoms, and leads to different types of band bending at the diamond/air interface in the presence of a water film. In the case of boron-doped NCD no shift of the work function is observed, which can be rationalized by pinning of the Fermi level. This is confirmed by TRMC results of boron-doped NCD, which show no dependency on the surface termination. We suggest that photoexcited electrons in boron-doped NCD occupy nonionized boron dopants, leaving relatively long-lived mobile holes in the valence band.

Half-metallicity of the inverse Heusler alloy Mn2CoAl(0 0 1) surface: A first-principles study

Half-metals exhibit 100% spin polarization at the Fermi level. Thus, they are promising magnetic electrode candidates in spintronics. We investigated the electronic structures, magnetism, and half-metallicity of the inverse Heusler alloy Mn2CoAl(001) surface by the all-electron full-potential linearized augmented plane-wave method within the generalized gradient approximation. Two types of surface terminations were considered, namely, the AlMn terminated (AlMn-term) and CoMn terminated (CoMn-term) surfaces. From the calculated layer-projected density of states, we found that the half-metallicity was preserved on the surface of the AlMn-term. However, the width of the minority-spin gap was significantly reduced because of the surface states located just above the Fermi level. By contrast, the CoMn-term lost its half-metallicity because of the strong surface potential effect on the surface Co atom. The magnetic moments of Mn atoms on the surfaces of both terminations are considerably increased compared with those in the deep inner layers, while that of the surface Co atom on the CoMn-term is slightly increased.

Bulk and surface characterization of In2O3(001) single crystals

A comprehensive bulk and surface investigation of high-quality In${}_{2}$O${}_{3}$(001) single crystals is reported. The transparent-yellow, cube-shaped single crystals were grown using the flux method. Inductively coupled plasma mass spectrometry (ICP-MS) reveals small residues of Pb, Mg, and Pt in the crystals. Four-point-probe measurements show a resistivity of 2.0 \ifmmode\pm\else\textpm\fi{} 0.5 \ifmmode\times\else\texttimes\fi{} 10${}^{5}\phantom{\rule{0.28em}{0ex}}\ensuremath{\Omega}$ cm, which translates into a carrier concentration of \ensuremath{\approx}10${}^{12}$ cm${}^{\ensuremath{-}3}$. The results from x-ray diffraction (XRD) measurements revise the lattice constant to 10.1150(5) \AA{} from the previously accepted value of 10.117 \AA{}. Scanning tunneling microscopy (STM) images of a reduced (sputtered/annealed) and oxidized (exposure to atomic oxygen at 300 \ifmmode^\circ\else\textdegree\fi{}C) surface show a step height of 5 \AA{}, which indicates a preference for one type of surface termination. The surfaces stay flat without any evidence for macroscopic faceting under any of these preparation conditions. A combination of low-energy ion scattering (LEIS) and atomically resolved STM indicates an indium-terminated surface with small islands of 2.5 \AA{} height under reducing conditions, with a surface structure corresponding to a strongly distorted indium lattice. Scanning tunneling spectroscopy (STS) reveals a pronounced surface state at the Fermi level (${E}_{\mathrm{F}}$). Photoelectron spectroscopy (PES) shows additional, deep-lying band gap states, which can be removed by exposure of the surface to atomic oxygen. Oxidation also results in a shoulder at the O 1s core level at a higher binding energy, possibly indicative of a surface peroxide species. A downward band bending of 0.4 eV is observed for the reduced surface, while the band bending of the oxidized surface is of the order of 0.1 eV or less.

Oxidative trends of TiO2—hole trapping at anatase and rutile surfaces

Understanding the nature of photogenerated carriers in a photocatalyst is central to understanding its photocatalytic performance. Based on density functional theory calculation we show that for TiO2, the most popular photo-catalyst, the electron hole self-trapping leads to band gap states which position is dependent on the type of surface termination. Such variations in hole state energies can lead to differences in photocatalytic activity among rutile and anatase surface facets. We find that the calculated hole state energies correlate with photo-deposition and photo-etching rates. We anticipated that our results can aid the design of more reactive photo-catalysts based on TiO2 and our approach can be utilized for other relevant photo-catalysts as well.

Semiconducting graphene nanoribbon retains band gap on amorphous or crystalline SiO2

Electronic properties of a semiconducting armchair graphene nanoribbon on SiO2 are examined using first-principles calculations and taking into account the van der Waals interaction. Unlike semiconducting carbon nanotubes, which exhibit variations in band gap on SiO2, the nanoribbon is found to retain its band gap on SiO2, regardless of the separation distance or the dielectric’s surface type—crystalline or amorphous. The interfacial interaction leads to electron-transfer from the nanoribbon to the dielectric. Moreover, for crystalline SiO2, the quantity of electron-transfer and the binding energy depend strongly on the type of surface termination and weakly on the binding sites.

Structural Investigation of the (001) Surface of the Al9Co2Complex Metallic Alloy

We present a detailed study of the (001) surface of the Al9Co2 crystalusing both experimental methods and first-principles calculations basedon the density functional theory. Experimentally, the (1 x 1) low-energyelectron diffraction pattern combined with the composition determinedusing X-ray photoemission spectroscopy is consistent with a bulkterminated surface. In addition, scanning tunneling microscopy (STM)measurements highlights the presence of only one type of surfacetermination. Combining experimental results with electronic structurecalculations provides many arguments to correlate the observed surfacetermination with the dense pure aluminum layer present in the Al9Co2bulk structure: (i) calculations show that this termination presents alower surface energy compared to another conceivable terminationobtained also from bulk truncation, (ii) step height measurements areconsistent with calculated interlayer spacings, (iii) calculatedbroadened densities of states and simulated STM images are qualitativelyin agreement with the experimental photoelectron spectra and theexperimental STM images.

Structural investigation of the (110) surface ofγ-Al4Cu9

We present a detailed study of the (110) surface of the $\ensuremath{\gamma}{\text{-Al}}_{4}{\text{Cu}}_{9}$ crystal using both experimental methods and first-principles calculations based on density-functional theory. Our experimental approach, using low-energy electron diffraction, scanning tunneling microscopy (STM) images, and x-ray photoelectron spectroscopy highlights the presence of two types of surface terminations. Combining experimental results and simulations provides many arguments to match the two observed surfaces with the two puckered terminations built from bulk truncation: (i) calculations show that these two puckered terminations present lower surface energies compared to another conceivable flat termination obtained also from bulk truncation, (ii) step height measurements are consistent with calculated interlayer spacings and (iii) simulated STM images are in reasonable agreement with the experimental ones and mirror the experimental voltage dependence.

Chemistry at the graphene-SiO2 interface

The structure and energetics and the chemistry of graphene on SiO2 surface are studied from first-principles. It is found that the energetic preference for the graphene layer to bind on specific sites on the O-terminated surface differs substantially from that on the Si-terminated surface. Regardless of the location of binding sites on a particular surface, electrons transfer from the graphene layer to the dielectric surface and its quantity is higher for the O-terminated surface. In addition, the electron transfer strongly depends on the type of surface termination but is independent of the binding site.