Single-atom Impurities Research Articles

Formation energy prediction of neutral single-atom impurities in 2D materials using tree-based machine learning

Abstract We applied tree-based machine learning algorithms to predict the formation energy of impurities in 2D materials, where adsorbates and interstitial defects are investigated. Regression models based on random forest, gradient boosting regression, histogram-based gradient-boosting regression, and light gradient-boosting machine algorithms are employed for training, testing, cross validation, and blind testing. We utilized chemical features from fundamental properties of atoms and supplemented them with structural features from the interaction of the added chemical element with its neighboring host atoms via the Jacobi–Legendre (JL) polynomials. Overall, the prediction accuracy yields optimal MAE ≈ 0.518 , RMSE ≈ 1.14 , and R 2 ≈ 0.855 . When trained separately, we obtained lower residual errors RMSE and MAE, and higher R 2 value for predicting the formation energy in the adsorbates than in the interstitial defects. In both cases, the inclusion of the structural features via the JL polynomials improves the prediction accuracy of the formation energy in terms of decreasing RMSE and MAE, and increasing R 2. This work demonstrates the potential and application of physically meaningful features to obtain physical properties of impurities in 2D materials that otherwise would require higher computational cost.

Enhancing surface activity in MoTe2 monolayers through p-block doping: A comprehensive DFT investigation

Molybdenum ditelluride (MoTe2), a member of the transition metal dichalcogenides (TMDs), has recently garnered significant attention in the fast growing fields of two-dimensional electronics. However, despite its advantages, the intrinsic properties of MoTe2, like the low chemical activity of its basal plane, also resulted in several technological challenges. To overcome these limitations, several methods have been explored, with single atom doping emerging as a particularly promising approach. In this study, we employed density functional theory (DFT) to investigate the influence of single atom impurities on the chemical activity of MoTe2. A total of 22 dopants were selected from the p-block of the periodic table, ranging from boron to bismuth. Specifically, we examined the adsorption of oxygen molecules (O2) on the doped structures to assess their impact on layer chemical activity. Our findings revealed that doping was energetically favorable for all investigated atoms, and it had a significant effect on surface activity. Notably, doping with dopants from groups 13–15, especially those with low atomic number, results in significant increased adsorption strength, leading to weakening of the molecular bonding in O2 by up 5.72 eV, hinting at the potential use as catalyst. Additionally, we identified certain molecules, primarily from group 17, with a remarkably high adsorption energy to charge transfer ratio. This leads to excellent sensing characteristics, where the response to adsorption in their carrier concentration is increased 100-fold over the pristine MoTe2, while sensor recovery is estimated between 0.01 and 2 s.

Central vacancy creation in icosahedral nanoparticles induced by the displacement of large impurities

We employ metadynamics simulations at room temperature to study the diffusion of large single-atom impurities within otherwise pure icosahedral nanoparticles, for different bimetallic systems (Au-Co, Ag-Co, Ag-Ni, Au-Pt and Au-Rh) and icosahedral sizes. Our simulations reveal that the displacement of the impurity induces the formation of a vacancy in the central part of the structure, as already observed for small icosahedra of Au-Co and Ag-Cu, therefore confirming the generality of this peculiar diffusion process. At the largest size, a new collective displacement mechanism is identified alongside the diffusion of the impurity. For Au-Pt clusters, different diffusion pathways are observed, which are mediated by a partial deformation of the nanoparticle surface. This is due to the lack of stability of fivefold veritices in Pt icosahedral clusters. We prove that the stability of the icosahedral surface is an essential condition for the occurrence of the combined process of impurity diffusion and internal vacancy formation.

Impurity diffusion in magic-size icosahedral clusters.

Atomic diffusion is at the basis of chemical ordering transformations in nanoalloys. Understanding the diffusion mechanisms at the atomic level is therefore a key issue in the study of the thermodynamic behavior of these systems and, in particular, of their evolution from out-of-equilibrium chemical ordering types often obtained in the experiments. Here, the diffusion is studied in the case of a single-atom impurity of Ag or Au moving within otherwise pure magic-size icosahedral clusters of Cu or Co by means of two different computational techniques, i.e., molecular dynamics and metadynamics. Our simulations reveal unexpected diffusion pathways, in which the displacement of the impurity is coupled with the creation of vacancies in the central part of the cluster. We show that the observed mechanism is quite different from the vacancy-mediated diffusion processes identified so far, and we demonstrate that it can be related to the presence of non-homogeneous compressive stress in the inner part of the icosahedral structure.

Nonadiabatic Effects on Defect Diffusion in Silicon-Doped Nanographenes.

Single atom impurities in graphene, substitutional silicon defects in particular, have been observed to diffuse under electron beam irradiation. However, the relative importance of elastic and inelastic scattering in facilitating their mobility remains unclear. Here, we employ excited-state electronic structure calculations to explore potential inelastic effects, and find an electronically nonadiabatic excited-state silicon diffusion pathway involving "softened" Si-C bonding that presents an ∼2 eV lower diffusion barrier than the ground-state pathway. Beam-induced transition rates to this state indicate that the excited-state pathway is accessible through irradiation of the defect site. However, even in the limit of fully elastic scattering, upward nonadiabatic transitions are also possible along the diffusion coordinate, increasing the diffusion barrier and further demonstrating the potential for electronic nonadiabaticity to influence beam-induced atomic transformations in materials. We also propose some experimentally testable signatures of such excited-state pathways.

Single-atom vibrational spectroscopy in the scanning transmission electron microscope.

Single-atom impurities and other atomic-scale defects can notably alter the local vibrational responses of solids and, ultimately, their macroscopic properties. Using high-resolution electron energy-loss spectroscopy in the electron microscope, we show that a single substitutional silicon impurity in graphene induces a characteristic, localized modification of the vibrational response. Extensive ab initio calculations reveal that the measured spectroscopic signature arises from defect-induced pseudo-localized phonon modes-that is, resonant states resulting from the hybridization of the defect modes and the bulk continuum-with energies that can be directly matched to the experiments. This finding realizes the promise of vibrational spectroscopy in the electron microscope with single-atom sensitivity and has broad implications across the fields of physics, chemistry, and materials science.

Seeing single silicon atom vibrations

Electron Microscopy Vibrational spectroscopy can achieve high energy resolution, but spatial resolution of unperturbed vibrations is more difficult to realize. Hage et al. show that a single-atom impurity in a solid (a silicon atom in graphene) can give rise to distinctive localized vibrational signatures. They used high-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope to detect this signal. An experimental geometry was chosen that reduced the relative elastic scattering contribution, and repeated scanning near the silicon impurity enhanced the signal. The experimental vibration frequencies are in agreement with ab initio calculations. Science , this issue p. [1124][1] [1]: /lookup/doi/10.1126/science.aba1136

Universality of defect-skyrmion interaction profiles

Magnetic skyrmions are prime candidates for future spintronic devices. However, incorporating them as information carriers hinges on their interaction with defects ubiquitous in any device. Here we map from first-principles, the energy profile of single skyrmions interacting with single-atom impurities, establishing a generic shape as function of the defect’s electron filling. Depending on their chemical nature, foreign 3d and 4d transition metal adatoms or surface implanted defects can either repel or pin skyrmions in PdFe/Ir(111) thin films, which we relate to the degree of filling of bonding and anti-bonding electronic states inherent to the proximity of the non-collinear magnetic structure. Similarities with key concepts of bond theories in catalysis and surface sciences imbue the universality of the shape of the interaction profile and the potential of predicting its interaction. The resulting fundamental understanding may give guidance for the design of devices with surface implanted defects to generate and control skyrmions.

Single atom impurity in a single molecular transistor

The influence of an impurity atom on the electrostatic behaviour of a Single Molecular Transistor was investigated through Ab-initio calculations in a double-gated geometry. The charge stability diagram carries unique signature of the position of the impurity atom in such devices which together with the charging energy of the molecule could be utilised as an electronic fingerprint for the detection of such impurity states in a nano-electronic device. The two gated geometry allows additional control over the electrostatics as can be seen from the total energy surfaces (for a specific charge state), which is sensitive to the positions of the impurity. These devices which are operational at room temperature can provide significant advantages over the conventional silicon based single dopant devices functional at low temperature. The present approach could be a very powerful tool for the detection and control of individual impurity atoms in a single molecular device and for applications in future molecular electronics.

Monochromated STEM with a 30 meV-wide, atom-sized electron probe

The origins and the recent accomplishments of aberration correction in scanning transmission electron microscopy (STEM) are reviewed. It is remembered that the successful correction of imaging aberrations of round lenses owes much to the successful correction of spectrum aberrations achieved in electron energy loss spectrometers 2-3 decades earlier. Two noteworthy examples of the types of STEM investigation that aberration correction has made possible are shown: imaging of single-atom impurities in graphene and analyzing atomic bonding of single atoms by electron energy loss spectroscopy (EELS). Looking towards the future, a new all-magnetic monochromator is described. The monochromator uses several of the principles pioneered in round lens aberration correction, and it employs stabilization schemes that make it immune to variations in the high voltage of the microscope and in the monochromator main prism current. Tests of the monochromator carried out at 60 keV have demonstrated energy resolution as good as 12 meV and monochromated probe size of ∼1.2 Å. These results were obtained in separate experiments, but they indicate that the instrument can perform imaging and EELS with an atom-sized probe <30 meV wide in energy, and that an improvement in energy resolution to 10 meV and beyond should be possible in the future.

Metallic proximity effect in ballistic graphene with resonant scatterers

We study the effect of resonant scatterers on the local density of states in a rectangular graphene setup with metallic leads. We find that the density of states in the vicinity of the Dirac point acquires a strong position dependence due to both metallic proximity effect and impurity scattering. This effect may prevent uniform gating of weakly-doped samples. We also demonstrate that even a single-atom impurity may essentially alter electronic states at low-doping on distances of the order of the sample size from the impurity.

Virtual states and decoherence in single impurity atom solid state quantum device

When a single donor atom is embedded in solid state nanodevice, a rich variety of phenomena in the proximity of T = 0 emerges due to the long phase coherence times Such phenomena are probed by means of charge sensing via tunneling currents. Some aspects connected to the transition from quantum to classical in quantum nanodevices are overviewed: the realization of transport through virtual states, and the definition of the temperature T of a microscopic/mesoscopic system as a key parameter required in the master equation when decoherence is considered. I also discuss the opportunity to study the transition from quantum to classical offered by single atom impurity devices on a time scale below the decoherence time, with a particular emphasis on the experimental test of the Leggett-Garg inequalities.

Vortex Patterns in Layered Magnets with Nonmagnetic and Magnetic Impurities

Effects of a single-atom impurity on a small vortex-like soliton are studied in the two-dimensional (2D) Heisenberg model of layered magnets. In the 41 x 41 spin lattice with exchange parameters corresponding to the 2D easy-plane copper ferromagnet and the easy-axis manganese antiferromagnet a vortex-like structure and its energy are calculated in the presence of a single nonmagnetic or magnetic impurity. It is found that the in-plane and out-of-plane vortices are attracted to and centered on the impurity in the easy-plane magnet up to the almost isotropic limit. Layered Cu-halide compounds are found to be good candidates for experimental observation of stable, fixed vortex-antivortex patterns.