Ir Substrate Research Articles

Strong Electron-Vibration Signals in Weakly Coupled Molecular Junctions: Activation of Spin-Crossover.

Manipulating individual molecular spin states with electronic current has the potential to revolutionize quantum information devices. However, it is still unclear how a current can cause a spin transition in single-molecule devices. Here, we propose a spin-crossover (SCO) mechanism induced by electron-phonon coupling in an iron(II) phthalocyanine molecule situated on a graphene-decoupled Ir(111) substrate. We performed simulations of both elastic and inelastic electron tunneling spectroscopy (IETS), which reveal current-induced Fe-N vibrations and an underestimation of established electron-vibration signals. Going beyond standard perturbation theory, we examined molecules in various charge and spin states using the Franck-Condon framework. The increased probability of spin switching suggests that notable IETS signals indicate SCO triggered by the inelastic vibrational excitation associated with Fe-N stretching.

Selective B(3)-H Activation Affording Multinuclear Ir(III) Complexes with (o-Carboranyl)dithioester Ligands.

A method was developed to link two or three o-carborane moieties to form a series of carboranyl dithioester bridging ligands via in situ substitution of haloalkanes by tetraphenylphosphonium carboranyldithiocarboxylates. Based on these ligands, direct B-H activation without the assistance of Ag(I) and alkali was successfully achieved with half-sandwich Ir(III) substrate [Cp*IrCl2]2 to yield corresponding bimetallic or trimetallic complexes. Single crystal structure analyses of the B-H activated complexes and corresponding SnCl2-inserted derivatives confirm the selective B(3)-H activation in these complexes.

Ultrasensitive Plasmon-Enhanced Infrared Spectroelectrochemistry.

IR spectroelectrochemistry (EC-IR) is a cutting-edge operando method for exploring electrochemical reaction mechanisms. However, detection of interfacial molecules is challenged by the limited sensitivity of existing EC-IR platforms due to the lack of high-enhancement substrates. Here, we propose an innovative plasmon-enhanced infrared spectroelectrochemistry (EC-PEIRS) platform to overcome this sensitivity limitation. Plasmonic antennae with ultrahigh IR signal enhancement are electrically connected via monolayer graphene while preserving optical path integrity, serving as both the electrode and IR substrate. The [Fe(CN)6 ]3- /[Fe(CN)6 ]4- redox reaction and electrochemical CO2 reduction reaction (CO2 RR) are investigated on the EC-PEIRS platform with a remarkable signal enhancement. Notably, the enhanced IR signals enable a reconstruction of the electrochemical curve of the redox reactions and unveil the CO2 RR mechanism. This study presents a promising technique for boosting the in-depth understanding of interfacial events across diverse applications.

Ultrasensitive Plasmon‐Enhanced Infrared Spectroelectrochemistry

AbstractIR spectroelectrochemistry (EC‐IR) is a cutting‐edge operando method for exploring electrochemical reaction mechanisms. However, detection of interfacial molecules is challenged by the limited sensitivity of existing EC‐IR platforms due to the lack of high‐enhancement substrates. Here, we propose an innovative plasmon‐enhanced infrared spectroelectrochemistry (EC‐PEIRS) platform to overcome this sensitivity limitation. Plasmonic antennae with ultrahigh IR signal enhancement are electrically connected via monolayer graphene while preserving optical path integrity, serving as both the electrode and IR substrate. The [Fe(CN)6]3−/[Fe(CN)6]4− redox reaction and electrochemical CO2 reduction reaction (CO2RR) are investigated on the EC‐PEIRS platform with a remarkable signal enhancement. Notably, the enhanced IR signals enable a reconstruction of the electrochemical curve of the redox reactions and unveil the CO2RR mechanism. This study presents a promising technique for boosting the in‐depth understanding of interfacial events across diverse applications.

Unidirectional Nano-modulated Binding and Electron Scattering in Epitaxial Borophene.

A complex interplay between the crystal structure and the electron behavior within borophene renders this material an intriguing 2D system, with many of its electronic properties still undiscovered. Experimental insight into those properties is additionally hampered by the limited capabilities of the established synthesis methods, which, in turn, inhibits the realization of potential borophene applications. In this multimethod study, photoemission spectroscopies and scanning probe techniques complemented by theoretical calculations have been used to investigate the electronic characteristics of a high-coverage, single-layer borophene on the Ir(111) substrate. Our results show that the binding of borophene to Ir(111) exhibits pronounced one-dimensional modulation and transforms borophene into a nanograting. The scattering of photoelectrons from this structural grating gives rise to the replication of the electronic bands. In addition, the binding modulation is reflected in the chemical reactivity of borophene and gives rise to its inhomogeneous aging effect. Such aging is easily reset by dissolving boron atoms in iridium at high temperature, followed by their reassembly into a fresh atomically thin borophene mesh. Besides proving electron-grating capabilities of the boron monolayer, our data provide comprehensive insight into the electronic properties of epitaxial borophene which is vital for further examination of other boron systems of reduced dimensionality.

DFT study on ORR catalyzed by bimetallic Pt-skin metals over substrates of Ir, Pd and Au

Bimetallic Pt-skin catalyst is a class of near-surface alloy (NSA) that owns a high degree of control over composition. Herein, density functional theory (DFT) is used to calculate the energetics of oxygen reduction reaction (ORR) on Pt-skin over Ir, Pd and Au substrates. A Brønsted-Evans-Polanyi (BEP) relationship can be determined for the oxygen molecule dissociation. The binding energy of both atomic oxygen and hydroxyl radical is found to correlate well with the d band center of Pt-skin atoms. Their catalytic activities show the volcano relationship as the positions of each substrate in the periodic table. The effect of surface strain, band structure and charge transfer on the d band center is well studied, and it can be found that the surface strain effect plays a dominant role for all Pt-skin catalysts. Ir substrate makes the d band center of Pt-skin go far away from the Fermi level, while Au substrate makes it move towards the Fermi level. Being different from both Ir and Au, Pd substrate makes the d band center of Pt-skin comparable with the monometallic Pt.

Intercalation of hafnium oxide between epitaxially-grown monolayer graphene and Ir(111) substrate

Intercalation of insulating materials between epitaxial graphene and the metal substrates is highly demanded to restore the intrinsic properties of graphene, and thus essential for the graphene-based devices. Here we demonstrate a successful solution for the intercalation of hafnium oxide into the interface between full-layer graphene and Ir(111) substrate. We first intercalate hafnium atoms beneath the epitaxial graphene. The intercalation of the hafnium atoms leads to the variation of the graphene moiré superstructure periodicity, which is characterized by low-energy electron diffraction (LEED) and low-temperature scanning tunneling microscopy (LT-STM). Subsequently, we introduce oxygen into the interface, resulting in oxidization of the intercalated hafnium. STM and Raman’s characterizations reveal that the intercalated hafnium oxide layer could effectively decouple the graphene from the metallic substrate, while the graphene maintains its high quality. Our work suggests a high-k dielectric layer has been successfully intercalated between high-quality epitaxial graphene and metal substrate, providing a platform for applications of large-scale, high-quality graphene for electronic devices.

The influence of adatom diffusion on the formation of skyrmion lattice in sub-monolayer Fe on Ir(111)

Room temperature grown Fe monolayer (ML) on the Ir(111) single crystal substrate has attracted great research interests as nano-skyrmion lattice can form under proper growth conditions. The formation of the nanoscale skyrmion, however, appears to be greatly affected by the diffusion length of the Fe adatoms on the Ir(111) surface. We made this observation by employing spin-polarized scanning tunneling microscopy to study skyrmion formation upon systematically changing the impurity density on the substrate surface prior to Fe deposition. Since the substrate surface impurities serve as pinning centers for Fe adatoms, the eventual size and shape of the Fe islands exhibit a direct correlation with the impurity density, which in turn determines whether skyrmion can be formed. Our observation indicates that skyrmion only forms when the impurity density is below 0.006/nm2, i.e., 12 nm averaged spacing between the neighboring defects. We verify the significance of Fe diffusion length by growing Fe on clean Ir(111) substrate at low temperature of 30 K, where no skyrmion was observed to form. Our findings signify the importance of diffusion of Fe atoms on the Ir(111) substrate, which affects the size, shape and lattice perfection of the Fe islands and thus the formation of skyrmion lattice.

One-Dimensional Periodic Buckling at a Symmetry-Incompatible Heterointerface of the NaCl(001) Monolayer on Ir(111)

Monolayer and multilayer NaCl(001) films are grown on an Ir(111) substrate, and strain relaxation modes are investigated by scanning tunneling microscopy (STM) and density functional theory calculations. We report that monolayer NaCl(001) exhibits an apparent one-dimensional periodic buckling along the [010] direction, which separates one-dimensional domains of pristine lattices. An atomic model of the monolayer NaCl(001)-(2 × 6) lattice on an Ir(111)-(4 × 73) substrate is proposed, and related simulation reproduces all the STM features. The calculations suggest that the 4–6 symmetry incompatibility of the NaCl(001)/Ir(111) interface leads to the formation of domain boundaries with alternately arranged top-sited and bridge-sited adatom lines along [100], resulting in one-dimensional atomic buckling. We also observe that the atomic buckling is veiled as the NaCl(001) increases to bilayer and above, suggesting that this relaxation mode is finely tuned by the crystal thickness.

Surface microstructures and oxygen evolution properties of cobalt oxide deposited on Ir(111) and Pt(111) single crystal substrates

AbstractWe investigated the oxygen evolution reaction (OER) activity changes of cobalt oxide (CoOx) thin films on Ir(111) and Pt(111) substrates by repeated OER measurements in 0.1 M KOH. Atomic force microscopy and X‐ray photoelectron spectroscopy analysis of the as‐prepared CoOx/Ir(111) and CoOx/Pt(111) showed similar surface morphologies of the CoOx thin films and almost the same OER overpotentials, which were estimated to be around 430 mV. However, after three OER measurements, the overpotential of CoOx/Ir(111) decreased by 70 mV, whereas that of CoOx/Pt(111) increased slightly. Structural analysis showed that CoOx/Ir(111) revealed the island‐like nanostructures of CoOx dispersed on Ir(111) surface, accompanied by the generation of CoOOH. In contrast, for CoOx/Pt(111), the Pt(111) substrate remains covered by the CoOx thin film. The results suggest that the interaface at CoOx (CoOOH) nano‐islands and Ir(111) substrate are responsible for reducing the OER overpotential.

Multiplet effects in the electronic correlation of one-dimensional magnetic transition metal oxides on metals

We use the constrained random phase approximation (cRPA) method to calculate the Hubbard $U$ parameter in four one-dimensional magnetic transition metal atom oxides of composition XO$_2$ (X = Mn, Fe, Co, Ni) on Ir(100). In addition to the expected screening of the oxide, i.e., a significant reduction of the $U$ value by the presence of the metal substrate, we find a strong dependence on the electronic configuration (multiplet) of the X($d$) orbital. Each particular electronic configuration attained by atom X is dictated by the O ligands, as well as by the charge transfer and hybridization with the Ir(100) substrate. We find that MnO$_2$ and NiO$_2$ chains exhibit two different screening regimes, while the case of CoO$_2$ is somewhere in between. The electronic structure of the MnO$_2$ chain remains almost unchanged upon adsorption. Therefore, in this regime, the additional screening is predominantly generated by the electrons of the neighboring metal surface atoms. The screening strength for NiO$_2$/Ir(100) is found to depend on the Ni($d$) configuration in the adsorbed state. The case of FeO$_2$ shows an exceptional behavior, as it is the only insulating system in the absence of metallic substrate and, thus, it has the largest $U$ value. However, this value is significantly reduced by the two mentioned screening effects after adsorption.

Analysis of Surface Microstructures Formed on Ir Substrate under Different Bias Conditions by Microwave Plasma Chemical Vapor Deposition

Analysis of Surface Microstructures Formed on Ir Substrate under Different Bias Conditions by Microwave Plasma Chemical Vapor Deposition

Size-limited high-density nanopore formation in two-dimensional moiré materials

Patterns formed in monolayer graphene (Gr) and hexagonal boron nitride (h-BN) upon ion irradiation in the temperature range from 950 to 1530 K using 500 eV ${\mathrm{He}}^{+}$ at normal or 500 eV ${\mathrm{Xe}}^{+}$ at grazing incidence and fluences up to $2.5\ifmmode\times\else\texttimes\fi{}{10}^{19}$ $\mathrm{ions}/{\mathrm{m}}^{2}$ are investigated by room-temperature scanning tunneling microscopy. Subnanometer pores are created and are distributed with the periodicity of the moir\'e which results from the interaction of the monolayers with their Ir(111) substrate. The moi\ifmmode \acute{r}\else \'{r}\fi{}e contains trapping sites for vacancies within each moir\'e unit cell as is investigated in detail for the case of h-BN with the help of ab initio calculations. The density of the nanopores is extremely high in the order of ${10}^{17}$ $\mathrm{pores}/{\mathrm{m}}^{2}$ and their size distribution is limited by the size of the vacancy traps causing excess vacancies to be expelled from the array and to anneal at preexistent defects. Successful delamination of a perforated Gr monolayer from the substrate is demonstrated and makes the material accessible for membrane research.

Analysis of Surface Microstructures Formed on Ir Substrate under Different Bias Conditions by Microwave Plasma Chemical Vapor Deposition

To understand the process window of bias‐enhanced nucleation of diamond, it is important to study surface modifications of Ir substrates that occur under different conditions. Herein, various microstructures formed on Ir substrates are reported. The first microstructure contains rectangular particles with a size of several hundred nanometers obtained at a relatively high pressure of 12.5 kPa and a high temperature of 880 °C; these particles consist of epitaxially grown Ir3Si species. The second microstructure includes particles oriented along the [110] direction; these particles, however, consist of etched Ir pillars with a height of 400 nm when the substrate temperature increases to 1110 °C. Both microstructures form because of Ir sputtering due to the bombardment of energetic charged ions and the transport of Ir surface atoms. This study demonstrates two types of surface modifications under different bias‐enhanced nucleation conditions and provides a method for preparing an epitaxial Ir–Si compound that can be applied in various silicon‐based electronic devices.

First principles study of hafnium intercalation between graphene and Ir(111) substrate

The intercalation of heteroatoms between graphene and metal substrates is a promising method for integrating epitaxial graphene with functional materials. Various elements and their oxides have been successfully intercalated into graphene/metal interfaces to form graphene-based heterostructures, showing potential applications in electronic devices. Here we theoretically investigate the hafnium intercalation between graphene and Ir(111). It is found that the penetration barrier of Hf atom is significantly large due to its large atomic radius, which suggests that hafnium intercalation should be carried out with low deposition doses of Hf atoms and high annealing temperatures. Our results show the different intercalation behaviors of a large-size atom and provide guidance for the integration of graphene and hafnium oxide in device applications.

Carbon-rehybridization-induced templated growth of metal nanoclusters on graphene moiré patterns

Templated growth of monodisperse metal nanoclusters has successfully been realized through graphene moiré patterns on transition metal substrates, but the underlying mechanism is still unclear. Herein, based on first-principles calculations, we systematically study the growth mechanism of Pt and Au nanoclusters on graphene-covered Ir(111) substrate. Our results indicate that the induced sp2-sp3 rehybridization of C atoms plays a key role for the templated growth. From the center of the template to the outside, such rehybridization is gradually suppressed due to the enlarged horizontal distance between C atoms and the adjacent Ir atoms, which causes weaker bonding of Pt atoms to larger clusters and thus the perfect templated growth. Meanwhile, negligible sp2-sp3 rehybridization is observed due to the weak adsorption of Au atoms on graphene, which leads to the failure of the template. This mechanism is then extended to other metal nanoclusters successfully. We believe that it will help to understand the structure-activity relationship of more two-dimensional templates.

Virtues of Ir(1 0 0) substrate on diamond epitaxial growth: First-principle calculation and XPS study

The heteroepitaxial growth of single crystal diamond requires substrate of high quality, and Ir(100) is an ideal substrate which can meet the standard perfectly. However, the reason of its outstanding performance still remains unclear. In this paper, the interaction mechanism between carbon atoms and Ir (100) surface at the initial stage of nucleation and its uniqueness caused by distinctive energy changes compared with (100) faces of Cu and Ni are explained by first-principle calculation, which acts as a dominating factor in the transformation from epitaxial films to single crystal wafers. The energy driving force of the dissolution-precipitation process is described, which can completely accord with bias-enhanced nucleation(BEN) method to illustrate the effect of voltage on diamond nucleation. The precipitation of carbon atoms and appearance of primary nuclei with the increase of carbon concentration are discussed, which can effectively explain the special phenomenon of the “domain” formation. A carbon-rich pre-treatment before primary nucleation on Ir (100) surface shows that the initial nucleation process can form supersaturated solid solution of carbon in Ir (100), which can greatly accelerate the subsequent dissolution-precipitation circulation.

Intercalation of germanium oxide beneath large-area and high-quality epitaxial graphene on Ir(111) substrate* *Project supported by the National Key Research & Development Program of China (Grant Nos. 2019YFA0308500, 2016YFA0202300, and 2018YFA0305800), the National Natural Science Foundation of China (Grant Nos. 61888102, 61925111, and 21661132006), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB30000000 and XDB28000000), and

Epitaxial growth on transition metal surfaces is an effective way to prepare large-area and high-quality graphene. However, the strong interaction between graphene and metal substrates suppresses the intrinsic excellent properties of graphene and the conductive metal substrates also hinder its applications in electronics. Here we demonstrate the decoupling of graphene from metal substrates by germanium oxide intercalation. Germanium is firstly intercalated into the interface between graphene and Ir(111) substrate. Then oxygen is subsequently intercalated, leading to the formation of a GeO x layer, which is confirmed by x-ray photoelectron spectroscopy. Low-energy electron diffraction and scanning tunneling microscopy studies show intact carbon lattice of graphene after the GeO x intercalation. Raman characterizations reveal that the intercalated layer effectively decouples graphene from the Ir substrate. The transport measurements demonstrate that the GeO x layer can act as a tunneling barrier in the fabricated large-area high-quality vertical graphene/GeO x /Ir heterostructure.

Magnetic response and electronic states of well defined Graphene/Fe/Ir(111) heterostructure

We investigate a well defined heterostructure constituted by magnetic Fe layers sandwiched between graphene (Gr) and Ir(111). The challenging task to avoid Fe-C solubility and Fe-Ir intermixing has been achieved with atomic controlled Fe intercalation at moderate temperature below 500 K. Upon intercalation of a single ordered Fe layer in registry with the Ir substrate, an intermixing of the Gr bands and Fe $d$ states breaks the symmetry of the Dirac cone, with a downshift in energy of the apex by about 3 eV, and well-localized Fe intermixed states induced in the energy region just below the Fermi level. First principles electronic structure calculations show a large spin splitting of the Fe states, resulting in a majority spin channel almost fully occupied and strongly hybridized with Gr $\ensuremath{\pi}$ states. X-ray magnetic circular dichroism on the Gr/Fe/Ir heterostructure reveals an ordered spin configuration with a ferromagnetic response of Fe layer(s), with enhanced spin and orbital configurations with respect to the bcc-Fe bulk values. The magnetization switches from a perpendicular easy magnetization axis when the Fe single layer is lattice matched with the Ir(111) surface to a parallel one when the Fe thin film is almost commensurate with graphene.

Graphene and graphene oxide on Ir(111) are transparent to wetting but not to icing

Anti-icing coatings reduce the freezing onset temperature for water by changing the chemical and physical environment at the water-substrate interface to prevent ice nucleation and growth. Graphene oxide has several attributes that make it attractive as an anti-icing coating and it has been theoretically predicted that graphene oxide has a lower freezing onset temperature than pristine graphene. Here, we test this hypothesis using carefully prepared, well-characterized graphene oxide substrates. We compare the water contact angle for graphene and graphene oxide coatings, both prepared on iridium(111) surfaces. The results show both materials to be transparent to wetting, but indicate a lower freezing onset temperature for graphene oxide than for pristine graphene. The measured water contact angles are dominated by the properties of the underlying Ir(111) substrate while the freezing onset temperature is dictated by the functional groups present on the graphene basal plane. We suggest that the lowering of the freezing onset temperature is caused by the formation of a viscous water layer on the surface. Scanning tunneling microscopy and x-ray photoelectron spectroscopy data are used to evaluate the robustness of the coating material and suggest ways to improve the long-term performance, namely by advancing strategies to avoid water intercalation.