Ti Adatoms Research Articles

Theory of Electron Spin Resonance Spectroscopy in Scanning Tunneling Microscopy.

The integration of scanning tunneling microscopy (STM) and electron spin resonance (ESR) spectroscopy has emerged as a powerful and innovative tool for discerning spin excitations and spin-spin interactions within atoms and molecules adsorbed on surfaces. However, the origin of the STM-ESR signal and the underlying mechanisms that govern the essential features of the measured spectra have remained elusive, thereby significantly impeding the future development of the STM-ESR approach. Here, we construct a model to carry out precise numerical simulations of STM-ESR spectra for a single hydrogenated Ti adatom and a hydrogenated Ti dimer, achieving excellent agreement with experimental observations. We further develop an analytic theory that elucidates the fundamental origin of the signal as well as the essential features in the measured spectra. These new theoretical developments establish a solid foundation for the on-demand detection and manipulation of atomic-scale spin states, with promising implications for cutting-edge applications in spin sensing, quantum information, and quantum computing.

First-principles study of Ti adsorption on Al4C3 (0001) surface

Al4C3 phase is hypothesized to undergo a solid–solid phase transition into TiC, thereby facilitating the heterogeneous nucleation of α-Al. However, capturing this transition process experimentally is challenging, which undermines the credibility of this transition theory. In this study, first-principles calculations are employed to investigate the feasibility of Ti atom adsorption on the exposed (0001) surface of the Al4C3 crystal, providing theoretical support for the phase transition hypothesis. The findings indicate that Ti adatoms can effectively adsorb on the C-terminated (0001) surface, whereas adsorption on the Al-terminated surface is thermodynamically unfavorable. Upon structural optimization, Ti adatoms at bridge site B, top site T, and hollow site H1, irrespective of coverage, migrate towards the most stable hollow H2 site. Electronic structure analysis reveals that the bonding between the Ti adatom at the H2 site and the surface layer C atoms is primarily covalent, while the bonding with subsurface layer Al atoms is metallic. Additionally, Ti atom adsorption induces polarization of the bonding electron cloud between Al and C atoms. This study provides theoretical support for the Ti-induced transformation of the Al4C3 to TiC phase via surface adsorption, paving the way for the development of high-performance master alloys based on phase transition theory.

Hydrogen adsorption on titanium-decorated carbyne C12 ring: a DFT study

Abstract In the current landscape of increasing focus on green technology, hydrogen fuel emerges as a pivotal alternative energy source. While existing technology facilitates hydrogen use in fuel cells, the practicality of this fuel could be significantly enhanced with a more efficient and safer storage approach. Researchers are actively exploring one-dimensional systems as potential hydrogen storage solutions, yielding promising outcomes. A notable study delved into the hydrogen storage capacity and performance of a Ti-decorated carbyne ring using density functional theory calculations. The researchers observed a robust, non-deforming bond between the Ti adatom and the carbyne ring, displaying characteristics akin to ionic bonding. Detailed analyses of electronic properties, including density of states and band structure, highlighted a strong interaction through the alignment of p-orbitals with the Ti atom. Upon the adsorption of H2 onto the decorated carbyne ring, it was noted that the Ti-decorated systems could each adsorb up to six H2 molecules, exhibiting weak physisorption energies within the Van der Waals range. The charge density profile indicated a dipole-dipole interaction, affirming the potential of the material as a viable H2 storage medium. In conclusion, as green technology advances, hydrogen fuel, especially when stored innovatively with materials like the Ti-decorated carbyne ring, emerges as a crucial component in the pursuit of sustainable energy solutions.

Ab initio supported study on the selective adsorption-induced microstructural difference of (Ti,Al)N/TiN and (Ti,Al)N/ZrN multilayers

Interface structure is essential for the mechanical properties of multilayers. Here, the effect of deposition process on the interface structure of arc-evaporated (Ti,Al)N/TiN and (Ti,Al)N/ZrN multilayers is studied by a combination of ab initio calculations and experiments. Based on x-ray diffractions and transmission electron microscope investigations, both multilayers with close modulation ratio and period reveal face centered cubic (fcc) structure with coherent interfaces. Nevertheless, thin (Ti,Zr)N layers are formed near the interfaces in (Ti,Al)N/ZrN multilayer, but are absent in (Ti,Al)N/TiN multilayer. Ab initio calculations suggest the selective absorption of Ti and Zr adatoms onto the surface of (Ti,Al)N/ZrN multilayer during deposition, which leads to the formation of (Ti,Zr)N-rich layers by interface. However, (Ti,Al)N/TiN multilayer is predicted to have almost constant surface adsorption of different adatoms.

Investigation of unified impact of Ti adatom and N doping on hydrogen gas adsorption capabilities of defected graphene sheets

In this work, we studied the hydrogen adsorption capabilities of functionalized graphene sheets containing a variety of defects (D-G) via molecular dynamics (MD) simulations that govern the mechanisms involved in hydrogen adsorption. Specifically, the graphene sheets containing monovacancy (MV), Stone-Wales (SW), and multiple double vacancy (DV) defects were functionalized with Ti and N atoms to enhance their hydrogen adsorption capacity. We measured the adsorption capacities of the N-/D-G sheets with varying concentrations of Ti adatoms at 300 K and 77 K temperatures and various pressures. Our study revealed that the increasing concentration of Ti adatoms on the D-G sheets led to a significant improvement in the hydrogen adsorption capacity of the graphene sheets. The DV(III)-G sheets showed the maximum adsorption capacity at 300 K because the DV(III)-G sheets had a small number of large-sized pores that bind hydrogen with high binding energy. Thus, hydrogen remained adsorbed even at higher temperatures (300 K). The N doping on the D-G sheets initially reduced their hydrogen adsorption capabilities; however, the N-D-G sheets enhanced their hydrogen adsorption capacity with the increasing concentrations of Ti adatoms. Compared to all other defect types, the Ti–N-DV(III)-G sheet with a Ti concentration of 10.5% showed a hydrogen uptake of 5.5 wt% at 300 K and 100 bar pressure. Thus, the N doping and Ti implantations improved the hydrogen storage capabilities of the graphene sheets, and these findings helped design solid-state hydrogen storage systems operating at ambient conditions and moderate pressure ranges.

Effect of Formic Acid on the Outdiffusion of Ti Interstitials at TiO2 Surfaces: A DFT+U Investigation.

Ti interstitials play a key role in the surface chemistry of TiO2. However, because of their elusive behavior, proof of their participation in catalytic processes is difficult to obtain. Here, we used DFT+U calculations to investigate the interaction between formic acid (FA) and excess Ti atoms on the rutile-TiO2(110) and anatase-TiO2(101) surfaces. The excess Ti atoms favor FA dissociation, while decreasing the relative stability of the bidentate bridging coordination over the monodentate one. FA species interact significantly with the Ti interstitials, favoring their outdiffusion. Eventually, Ti atoms can emerge at the surface forming chelate species, which are more stable than monodentate FA species in the case of rutile, and are even energetically favored in the case of anatase. The presence of Ti adatoms that can directly participate to surface processes should then be considered when formic acid and possibly carboxylate-bearing species are adsorbed onto TiO2 particles.

Interplay of magnetic states and hyperfine fields of iron dimers on MgO(001)

Individual nuclear spin states can have very long lifetimes and could be useful as qubits. Progress in this direction was achieved on MgO/Ag(001) via detection of the hyperfine interaction (HFI) of Fe, Ti and Cu adatoms using scanning tunneling microscopy. Previously, we systematically quantified from first-principles the HFI for the whole series of 3d transition adatoms (Sc-Cu) deposited on various ultra-thin insulators, establishing the trends of the computed HFI with respect to the filling of the magnetic s- and d-orbitals of the adatoms and on the bonding with the substrate. Here we explore the case of dimers by investigating the correlation between the HFI and the magnetic state of free standing Fe dimers, single Fe adatoms and dimers deposited on a bilayer of MgO(001). We find that the magnitude of the HFI can be controlled by switching the magnetic state of the dimers. For short Fe-Fe distances, the antiferromagnetic state enhances the HFI with respect to that of the ferromagnetic state. By increasing the distance between the magnetic atoms, a transition toward the opposite behavior is observed. Furthermore, we demonstrate the ability to substantially modify the HFI by atomic control of the location of the adatoms on the substrate. Our results establish the limits of applicability of the usual hyperfine hamiltonian and we propose an extension based on multiple scattering processes.

Ti1–graphene single-atom material for improved energy level alignment in perovskite solar cells

Carbon-based perovskite solar cells (C-PSCs) are widely accepted as stable, cost-effective photovoltaics. However, C-PSCs have been suffering from relatively low power conversion efficiencies (PCEs) due to severe electrode-related energy loss. Herein, we report the application of a single-atom material (SAM) as the back electrode in C-PSCs. Our Ti1–rGO consists of single titanium (Ti) adatoms anchored on reduced graphene oxide (rGO) in a well-defined Ti1O4-OH configuration capable of tuning the electronic properties of rGO. The downshift of the Fermi level notably minimizes the series resistance of the carbon-based electrode. By combining with an advanced modular cell architecture, a steady-state PCE of up to 20.6% for C-PSCs is finally achieved. Furthermore, the devices without encapsulation retain 98% and 95% of their initial values for 1,300 h under 1 sun of illumination at 25°C and 60 °C, respectively. Carbon materials are promising for perovskite solar cells but suffer from poor interfacial energy level alignment. Now, Zhang et al. show that Ti atomically dispersed in reduced graphene reduces energy losses improving device performance.

Insights into NinTim clusters adsorption and diffusion on B2-NiTi phase from atomistic simulations

In this work, we aim to study the adsorption and migration of NinTim (n,m=1-4) clusters on low-index B2-NiTi surfaces (100), (110) and (111). Ni and Ti diffusion was investigated using molecular static simulations with the conjunction of the MEAM formalism to describe atomic interactions. Ni-rich clusters are found to be more mobile than Ti-rich ones on Ni- and Ti-terminated (100) surfaces. On the other hand, the activation energy of Ni diffusion on the mixed (110) is found to be 0.51 eV, while that on Ni- and Ti-terminated (111) surfaces corresponds to barriers of 0.61 eV and 0.71 eV, respectively. These results demonstrate that on the (110) surface, a high thermal activity of clusters is expected due to the lower migration barriers. In addition, the diffusion mechanisms of Ti and Ni adatoms are quite similar and are characterized by a hopping mechanism between adsorption sites. While for the small cluster diffusion such as dimers, trimers and tetramers a variety of mechanisms is noted. Using classical molecular dynamics, we found that at 800 K, the Ni-terminated (100) and Ti- terminated (111) surfaces show an important segregation effect of Ni atoms on the surface. We have also found that the homogenous Ti clusters are stable on most surfaces with slight changes of their forms. While Ni clusters are found to show less stability with a higher mobility of their dissociated atoms.

Tuning structural, electronic, and magnetic properties of black-AsP monolayer by adatom adsorptions: A first principles study

Black Arsenic-phosphorus (AsP) monolayer is a novel two-dimensional nanomaterial with the characteristics of modest direct bandgap and superhigh carrier mobility. However, little is known about how the surface adsorption affects the property of AsP monolayer. Motivated by this, we researched systematically the geometry, adsorption energy, magnetic moment and electronic structure of 11 different adatoms adsorbed on AsP monolayer using first-principles calculations. The adatoms used in this study include light nonmetallic (C, N, O) adatoms, period-3 metal (Na, Mg, Al) adatoms, and transition-metal (Ti, V, Cr, Mn, and Fe) adatoms. The adatoms cause an abundant variety of structural, magnetic and electronic properties. This study shows that AsP binds strongly with all adatoms under study and the adsorption energies in all systems are much stronger than that on graphene, SiC, BN, or MoS2. The semiconductor property of AsP is affected by the introduction of adsorbed atoms, which can induce mid-gap states or cause n-type doping. Moreover, the adatom adsorptions cause various spintronic characteristics: N-, Ti-, and Fe-adsorbed AsP become bipolar semiconductors, while the Mn-decorated AsP becomes a bipolar spin-gapless semiconductor. Our results suggest that atomic adsorption on AsP monolayers has potential application in the field of nanoelectronics and spintronics.

Enhanced Interactions of Amino Acids and Nucleic Acid Bases with Bare Black Phosphorene Monolayer Mediated by Coadsorbed Species

In this paper, we characterize amino acids and nucleic acid bases (nucleobases), such as glutamine, histidine, tyrosine, adenine, guanine, cytosine, and thymine, and examine their interaction with bare, as well as with gold cluster and Ti adatom covered, black phosphorene (α-P) monolayers using density functional theory. The binding of these amino acids and nucleobases to the bare α-P monolayer is realized generally through weak van der Waals interaction and comprises only a small amount of charge exchange. Accordingly, the electronic energy structures of adsorbates and underlying substrate are not affected significantly. However, the electronic structure of bare α-P is significantly affected upon adsorption of a gold cluster and a single Ti adatom; depending on the size of the adsorbate and the symmetry of their coverage, the energy band gap can be tuned and permanent magnetic moments can be attained. Additionally, the adsorption of amino acids or nucleobases to these adsorbates on an α-P monolayer results in enhanced binding and hence makes their sustainable fixation on α-P monolayer possible. In particular, a semiconducting Au decorated α-P monolayer undergoes a metal–insulator transition upon the adsorption of tyrosine. This and similar effects favor the α-P monolayer in biosensor applications. In contrast to the situation with adsorbates, the binding of amino acid is not enhanced when it adsorb to patterned vacancy or divacancy sites of the α-P monolayer. Our study shows that the absorbance of the bare α-P monolayer can be enhanced by coating with amino acid and nucleobases. The absorbance spectrum can be further modified by the adsorption of these molecules to gold atoms on the α-P monolayer.

Cluster adsorption and migration energetics on hcp Ti (0001) surfaces via atomistic simulations

We use the modified embedded atom method in the framework of molecular statics to investigate the energetics of Ti adatom and Tin (2 ≤ n ≤ 7) cluster adsorption and migration on hcp Ti(0001) surfaces. We have examined several parameters such as the lattice constant, elastic constants, the cohesive energy and the surface energy and we have found that the modified embedded atom method gives results in good agreement with experimental and other theoretical investigations. The Ti (0001) surface multilayer relaxation results show that the distance between the first and second layer is contracted by −5.0%, which is the usual phenomenon for low index metallic surfaces. In addition, the diffusion parameters such as adsorption energies and energy barriers for single-atom and bi-dimensional clusters were derived. These calculations show that the adsorption energy and the potential barrier increase when the cluster size increases indicating their mobility decreases. Which indicates that the mobility of the cluster decreases as the cluster grows.

First-Principle Prediction on STM Tip Manipulation of Ti Adatom on Two-Dimensional Monolayer YBr3.

Scanning tunneling microscopy (STM) is an important tool in surface science on atomic scale characterization and manipulation. In this work, Ti adatom manipulation is theoretically simulated by using a tungsten tip (W-tip) in STM based on first-principle calculations. The results demonstrate the possibility of inserting Ti adatoms into the atomic pores of monolayer YBr3, which is thermodynamically stable at room temperature. In this process, the energy barriers of vertical and lateral movements of Ti are 0.38 eV and 0.64 eV, respectively, and the Ti atoms are stably placed within YBr3 by >1.2 eV binding energy. These theoretical predictions provide an insight that it is experimentally promising to manipulate Ti adatom and form artificially designed 2D magnetic materials.

Modelling thin film growth in the Ag–Ti system

Simulations of thin film growth in the Ag–Ti system are presented using molecular dynamics combined with an adaptive kinetic Monte Carlo method (AKMC) with a modified embedded atom potential fit to ab initio data for the surface energies. For the model, atoms are assumed to deposit normally with a kinetic energy of 1–3 eV, with a typical deposition rate of around 10 monolayers per second, similar to what might be expected in a sputter deposition process. For the growth of Ti on the Ag (100) and Ag (111) surfaces, the Ti adatoms prefer to exchange with the original surface layer atoms creating a mixed Ag/Ti surface. On a silver substrate, up to four mixed layers need to be formed before a pure Ti layer is obtained. Conversely, simulations of Ag depositing onto Ti (0001) showed that in the initial phase of growth, the Ag adatoms prefer to be separated before a complete first layer of Ag was obtained in a close-packed structure. The implementation of a super-basin method within AKMC allowed the simulation of 0.4s of Ti growth on the Ag substrates, with up to 3 new layers added.

Improved wear resistance of WS2 film by LT-deposited Ti interlayer with ω phase structure

The goal of this paper was to investigate the structural/mechanical properties of arc-ion-plated Ti film deposited at ultralow temperature (166 K, LT-Ti film) and its effect as an interlayer on the wear resistance of sputtered WS2 film. The results revealed that the LT-Ti film mainly exhibited ω-phase structure due to the insufficient mobility of Ti adatoms at LT surface, while the α-phase was predominated in the Ti film deposited at room temperature (RT-Ti film). The LT-Ti film showed a higher hardness than the RT-Ti film as well as the better toughness and film-substrate adhesion on steel substrates. The wear resistance of sputtered WS2 film could be significantly improved by employing the Ti films as its interlayer, especially for the LT-Ti film.

Effects of surface vibrations on interlayer mass transport: Ab initio molecular dynamics investigation of Ti adatom descent pathways and rates from TiN/TiN(001) islands

We carry out density-functional ab initio molecular dynamics (AIMD) simulations of Ti adatom (Tiad) migration on, and descent from, TiN <100>-faceted epitaxial islands on TiN(001) at temperatures T ranging from 1200 to 2400 K. Adatom-descent energy-barriers determined via ab initio nudged-elastic-band calculations at 0 Kelvin suggest that Ti interlayer transport on TiN(001) occurs essentially exclusively via direct hopping onto a lower layer. However, AIMD simulations reveal comparable rates for Tiad descent via direct-hopping vs. push-out/exchange with a Ti island edge atom for T >= 1500 K. We demonstrate that the effect is due to surface vibrations, which yield considerably lower activation energies at finite temperatures by significantly modifying the adatom push/out-exchange reaction pathway.

Ti, Al and N adatom adsorption and diffusion on rocksalt cubic AlN (001) and (011) surfaces: Ab initio calculations

We use ab initio calculations to determine the preferred nucleation sites and migration pathways of Ti, Al and N adatoms on cubic NaCl-structure (B1) AlN surfaces, primary inputs towards a further thin film growth modelling of the TiAlN alloy system. The potential energy landscape is mapped out for both metallic species and nitrogen adatoms for two different AlN surface orientations, (001) and (110), using density functional theory. For all species, the adsorption energies on AlN(011) surface are larger than on AlN(001) surface. Ti and Al adatom adsorption energy landscapes determined at 0K by ab initio show similar features, with stable binding sites being located in, or near, epitaxial surface positions, with Ti showing a stronger binding compared to Al. In direct contrast, N adatoms (Nad) adsorb preferentially close to N surface atoms (Nsurf), thus forming strong N2-molecule-like bonds on both AlN(001) and (011). Similar to N2 desorption mechanisms reported for other cubic transition metal nitride surfaces, in the present work we investigate Nad/Nsurf desorption on AlN(011) using a drag calculation method. We show that this process leaves a Nsurf vacancy accompanied with a spontaneous surface reconstruction, highlighting faceting formation during growth.

Characteristics of hexagonal c-oriented titanium film as the template for GaN epitaxy on glass substrate by electron beam evaporation

The highly c-oriented titanium (Ti) film is deposited on glass substrates by electron beam (EB) evaporation based on the study of effects of the substrate temperature (Ts) and deposition rate (Rd) on its properties. It is found that the Ts is the critical parameter and the Rd. is also important to the crystallization and surface morphology of EB-evaporated Ti film. The crystal orientation, grain size and surface smoothness are optimized if the migration time of Ti adatoms affected by the Rd. matches to their surface mobility affected by Ts. Besides, the Ti film thickness (Tf) is also exist an optimization range to achieve hexagonal c-oriented titanium film. The highly c-orientated (002) hexagonal and smooth Ti film, whose mean grain size and root mean square roughness are approximately 250nm and 1.54nm respectively, is realized with Ts of 300°C and Rd. of approximately 0.5nm/s when the thickness is 300nm. The single crystalline characteristics of individual Ti grains are demonstrated, and GaN grown on Ti film/glass substrate by molecular beam epitaxy at 530°C has the same epitaxial relationship to Ti film. It means that Ti film deposited by EB evaporation with appropriate Ts and Rd. can acts as the template of GaN epitaxy on glass substrate under the suitable thickness.

High-efficient physical adsorption and detection of formaldehyde using Sc- and Ti-decorated graphdiyne

In sensitive analysis, the ultimate limit is to achieve reliable detection on trace amount of molecules. In this work, Sc- and Ti-decorated graphdiyne were proposed as promising materials for high-efficient molecular detection. Using density functional theory calculations, we investigated the electronic response of single Sc- and Ti-atom-decorated graphdiyne to HCHO (as a typical air pollutant). Thermodynamic analysis predicted that Sc or Ti adatom could be stabilized on the corner sites of single-layer graphdiyne sheet, with migration barriers high enough to prevent Sc or Ti adatom aggregation. The adsorption of HCHO on Sc- or Ti-decorated graphdiyne was found stronger than on pristine graphene or graphdiyne, which provides a prerequisite for molecular sensing. The electronegativity of HCHO leads to strong electronic attraction from Sc or Ti adatom to HCHO, resulting in a remarkable decrease of carrier density in graphdiyne. On Ti-decorated graphdiyne, the electronic attraction of HCHO appears to be stronger than on Sc-decorated graphdiyne and changes the system from metal to n-doped semiconductor. Quantum transport calculations show a decrease of current caused by the adsorbed HCHO. The results systematically exhibit the electronic response of Sc- or Ti-decorated graphdiyne to HCHO and suggest them as promising materials for molecule detection.

Development of an empirical interatomic potential for the Ag–Ti system

Two interatomic potential mixing rules for the Ti–Ag system were investigated based on the embedded-atom method (EAM) elemental potentials. First principles calculations were performed using SIESTA for various configurations of the Ti–Ag system to see which model best fitted the ab initio results. The results showed that the surface energies, especially that of Ti, were not well fitted by either model and the surface binding energies differed from the ab initio calculations. As a result, the modified embedded-atom method (MEAM) was investigated. In contrast to the other models, surface energies for pure Ti calculated by MEAM were in good agreement with the experimental data and the ab initio results. The MEAM mixing rule was used to investigate Ag ad-atoms on Ti and Ti ad-atoms on Ag. The results showed good agreement with SIESTA after parameter optimisation.